





Network Working Group                                    L. Wells, Chair
Request for Comments: 1795             Internetwork Technology Institute
Obsoletes: 1434                                        A. Bartky, Editor
Category: Informational                              Sync Research, Inc.
                                                              April 1995


             Data Link Switching: Switch-to-Switch Protocol
       AIW DLSw RIG: DLSw Closed Pages, DLSw Standard Version 1.0

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This RFC describes use of Data Link Switching over TCP/IP. The RFC is
   being distributed to members of the Internet community in order to
   solicit their reactions to the proposals contained in it.  While the
   issues discussed may not be directly relevant to the research
   problems of the Internet, they may be interesting to a number of
   researchers and Implementers.

   This RFC was created as a joint effort of the Advanced Peer-to-Peer
   Networking (APPN) Implementers Workshop (AIW) Data Link Switching
   (DLSw) Related Interest Group (RIG).  The APPN Implementers Workshop
   is a group sponsored by IBM and consists of representatives of member
   companies implementing current and future IBM Networking
   interoperable products. The DLSw Related Interest Group was formed in
   this forum in order to produce a single version of the Switch to
   Switch Protocol (SSP) which could be implemented by all vendors,
   which would fix documentation problems with the existing RFC 1434,
   and which would enhance and evolve the protocol to add new functions
   and features.

   This document is based on RFC 1434.  This document contains
   significant changes to RFC 1434 and therefore obsoletes that
   document.

   Any questions or comments relative to the contents of this RFC should
   be sent to the following Internet address:
   aiw-dlsw@networking.raleigh.ibm.com.

   NOTE 1: This is a widely subscribed mailing list and messages sent to
   this address will be sent to all members of the DLSw mailing list.
   For specific questions relating to subscribing to the AIW and any of



Wells & Bartky                                                  [Page 1]

RFC 1795                  Data Link Switching                 April 1995


   it's working groups send email to: appn@vnet.ibm.com

   Information regarding all of the AIW working groups and the work they
   are producing can be obtained by copying, via anonymous ftp, the file
   aiwinfo.psbin or aiwinfo.txt from the Internet host
   networking.raleigh.ibm.com, located in directory aiw.

   NOTE 2:  These mailing lists and addresses are subject to change.

1.  Introduction

   Data Link Switching (DLSw) is a forwarding mechanism for the IBM SNA
   (Systems Network Architecture) and IBM NetBIOS (Network Basic Input
   Output Services) protocols.  This memo documents the Switch-to-Switch
   Protocol (SSP) that is used between Data Link Switches.  This
   protocol does not provide full routing, but instead provides
   switching at the SNA Data Link layer (i.e., layer 2 in the SNA
   architecture) and encapsulation in TCP/IP for transport over the
   Internet.  This RFC documents the frame formats and protocols for
   multiplexing data between Data Link Switches. The initial
   implementation of SSP uses TCP as the reliable transport between Data
   Link Switches.  However, other transport connections such as OSI TP4
   could be used in the future.

   A Data Link Switch (abbreviated also as DLSw in this document) can
   support  SNA (Physical Unit (PU) 2, PU 2.1 and PU 4) systems and
   optionally NetBIOS systems attached to IEEE 802.2 compliant Local
   Area Networks, as well as SNA (PU 2 (primary or secondary) and PU2.1)
   systems attached to IBM Synchronous Data Link Control (SDLC) links.
   For the latter case, the SDLC attached systems are provided with a
   LAN appearance within the Data Link Switch (each SDLC PU is presented
   to the SSP protocol as a unique MAC/SAP address pair).  For the
   Token-Ring LAN attached systems, the Data Link Switch appears as a
   source-routing bridge.  Token-Ring Remote systems that are accessed
   through the Data Link Switch appear as systems attached to an
   adjacent ring.  This ring is a virtual ring that is manifested within
   each Data Link Switch.

1.1  Backwards Compatibility with RFC 1434

   This document defines significant changes to RFC 1434 and does not
   state details on how to interoperate with RFC 1434 or "enhanced"
   implementations (e.g., those that added enter and exit busy flow
   control).  It is up to the implementer to refer to RFC 1434 and/or
   any other vendor's documentation in order to interoperate with a
   given vendor's implementation, if interoperability with pre-AIW DLSw
   RIG standards is desired.




Wells & Bartky                                                  [Page 2]

RFC 1795                  Data Link Switching                 April 1995


2.  Overview

   Data Link Switching was developed to provide support for SNA and
   NetBIOS in multi-protocol routers.  Since SNA and NetBIOS are
   basically connection oriented protocols, the Data Link Control
   procedure that they use on the LAN is IEEE 802.2 Logical Link Control
   (LLC) Type 2.  Data Link Switching also accommodates SNA protocols
   over WAN (Wide Area Network) links via the SDLC protocol.

   IEEE 802.2 LLC Type 2 was designed with the assumption that the
   network transit delay would be predictable (i.e., a local LAN).
   Therefore the LLC Type 2 elements of procedure use a fixed timer for
   detecting lost frames.  When remote bridging is used over wide area
   lines (especially at lower speeds), the network delay is larger and
   it can vary greatly based upon congestion.  When the delay exceeds
   the time-out value LLC Type 2 attempts to retransmit.  If the frame
   is not actually lost, only delayed, it is possible for the LLC Type 2
   procedures to become confused.  And as a result, the link may be
   eventually taken down if the delay exceeds the T1 timer times N2
   retry count.

   Given the use of LLC Type 2 services, Data Link Switching addresses
   the following bridging problems:

             DLC Time-outs
             DLC Acknowledgments over the WAN
             Flow and Congestion Control
             Broadcast Control of Search Packets
             Source-Route Bridging Hop Count Limits

   NetBIOS also makes extensive use of datagram services that use
   connectionless LLC Type 1 service.  In this case, Data Link Switching
   addresses the last two problems in the above list.

   The principal difference between Data Link Switching and bridging is
   that for connection-oriented data DLSw terminates the Data Link Control
   whereas bridging does not. The following figure illustrates this
   difference based upon two end systems operating with LLC Type 2
   services.












Wells & Bartky                                                  [Page 3]

RFC 1795                  Data Link Switching                 April 1995


   Bridging
   --------

                    Bridge           Bridge
   +------+         +----+           +----+         +------+
   | End  | +-----+ |    +-----/     |    | +-----+ | End  |
   |System+-+ LAN +-+    |    /------+    +-+ LAN +-+System|
   |      | +-----+ |    |  TCP/IP   |    | +-----+ |      |
   +------+         +----+           +----+         +------+
      Info----------------------------------------------->
          <-----------------------------------------------RR


   Data Link Switching
   -------------------

   +------+         +----+           +----+         +------+
   | End  | +-----+ |    +-----/     |    | +-----+ | End  |
   |System+-+ LAN +-+DLSw|    /------+DLSw+-+ LAN +-+System|
   |      | +-----+ |    |  TCP/IP   |    | +-----+ |      |
   +------+         +----+           +----+         +------+
    Info--------------->   -------------> Info
      <---------------RR                 ------------>
                                         <------------RR

   In traditional bridging, the Data Link Control is end-to-end.  Data
   Link Switching terminates the LLC Type 2 connection at the switch.
   This means that the LLC Type 2 connections do not cross the wide area
   network.  The DLSw multiplexes LLC connections onto a TCP connection
   to another DLSw.  Therefore, the LLC connections at each end are
   totally independent of each other.  It is the responsibility of the
   Data Link Switch to deliver frames that it has received from a LLC
   connection to the other end.  TCP is used between the Data Link
   Switches to guarantee delivery of frames.

   As a result of this design, LLC time-outs are limited to the local
   LAN (i.e., they do not traverse the wide area).  Also, the LLC Type 2
   acknowledgments (RR's) do not traverse the WAN, thereby reducing
   traffic across the wide area links.  For SDLC links, polling and poll
   response occurs locally, not over the WAN.  Broadcast of search
   frames is controlled by the Data Link Switches once the location of a
   target system is discovered.  Finally, the switches can now apply
   back pressure to the end systems to provide flow and congestion
   control.

   Only one copy of an Link Protocol Data Unit (LPDU) is sent between
   Data Link Switches in SSP messages (XIDFRAME and INFOFRAME).  Retries
   of the LPDU are absorbed by Data Link Switch that receives it.  The



Wells & Bartky                                                  [Page 4]

RFC 1795                  Data Link Switching                 April 1995


   Data Link Switch that transmits the LPDU received in an SSP message
   to a local DLC, will perform retries in a manner appropriate for the
   local DLC. This may involve running a reply timer and maintaining a
   poll retry count.  The length of the timer and the number of retries
   is an implementation choice based on user configuration parameters
   and the DLC type.

   Data Link Switching uses LAN addressing to set up connections between
   SNA systems.  SDLC attached devices are defined with MAC and SAP
   addresses to enable them to communicate with LAN attached devices.
   For NetBIOS systems, Data Link Switching uses the NetBIOS name to
   forward datagrams and to set up connections for NetBIOS sessions.
   For LLC type 2 connection establishment, SNA systems send TEST (or in
   some cases, XID) frames to the null (0x00) SAP.  NetBIOS systems have
   an address resolution procedure, based upon the Name Query and Name
   Recognized frames, that is used to establish an end-to-end circuit.

   Since Data Link Switching may be implemented in multi-protocol
   routers, there may be situations where both bridging and switching
   are enabled. SNA frames can be identified by their link SAP.  Typical
   SAP values for SNA are 0x04, 0x08, and 0x0C.  NetBIOS always uses a
   link SAP value of 0xF0.





























Wells & Bartky                                                  [Page 5]

RFC 1795                  Data Link Switching                 April 1995


3.  Transport Connection

   Data Link Switches can be in used in pairs or by themselves.

   A Single DLSw internally switches one data link to another without
   using TCP (DLC(1) to DLC(2) in the figure below).  This RFC does not
   go into details on how to implement this feature and it is not a
   requirement to support this RFC.

   A paired DLSw multiplexes data links over a reliable transport using
   a Switch-to-Switch Protocol (SSP).

   +-------------------------------------------+Switch-to-Switch
   |              DLC Interfaces               | Protocol (SSP)
   |+-----------+   DLC Request  +-----------+ |
   ||   Data    |<---------------|           | |Send SSP Frame
   ||   Link    | DLC Indication |           | |-------------->
   || Control 1 |--------------->|           | |
   |+-----------+                | Data Link | |
   |+-----------+   DLC Request  |  Switch   | |
   ||   Data    |<-------------- |           | |Rec. SSP Frame
   ||   Link    | DLC Indication |           | |<-------------
   || Control 2 | -------------->|           | |
   |+-----------+                +-----------+ |
   |            Multi-Protocol Router          |
   +-------------------------------------------+

   Before Data Link Switching can occur between two routers, they must
   establish two TCP connections between them.  Each Data Link Switch
   will maintain a list of DLSw capable routers and their status
   (active/inactive).  After the TCP connection is established, SSP
   messages are exchanged to establish the capabilities of the two Data
   Link Switches.  Once the exchange is complete,  the DLSw will employ
   SSP control messages to establish end-to-end circuits over the
   transport connection.  Within the transport connection, DLSw SSP
   messages are exchanged.  The message formats and types for these SSP
   messages are documented in the following sections.

   The default parameters associated with the TCP connections between
   Data Link Switches are as follows:

   Socket Family     AF_INET        (Internet protocols)
   Socket Type       SOCK_STREAM    (stream socket)
   Read Port Number  2065
   Write Port Number 2067






Wells & Bartky                                                  [Page 6]

RFC 1795                  Data Link Switching                 April 1995


   Two or more Data Link Switches may be attached to the same LAN,
   consisting of a number of token-ring segments interconnected by
   source-routing bridges.  In this case, a TCP connection is not
   defined between bridges attached to the same LAN.  This will allow
   using systems to select one of the possible Data Link Switches in a
   similar manner to the selection of a bridge path through a source-
   routed bridged network.  The virtual ring segment in each Data Link
   Switch attached to a common LAN must be configured with the same ring
   number.  This will prevent LAN frames sent by one Data Link Switch
   from being propagated through the other Data Link Switches.









































Wells & Bartky                                                  [Page 7]

RFC 1795                  Data Link Switching                 April 1995


3.1  SSP Frame Formats

   The following diagrams show the two message header formats exchanged
   between Data Link Switches, Control and Information.  The Control
   message header is used for all messages except Information Frames
   (INFOFRAME) and Independent Flow Control Messages (IFCM), which are
   sent in Information header format.  The INFOFRAME, KEEPALIVE and IFCM
   message headers are 16 bytes long, and the control message header is
   72 bytes long.  The fields in the first sixteen bytes of all message
   headers are the same.

    CONTROL MESSAGES (72 Bytes)
    (zero based offsets below shown in decimal (xx) )
   +-----------------------------+-----------------------------+
   | (00) Version Number         | (01) Header Length (= 72)   |
   +-----------------------------+-----------------------------+
   | (02) Message Length                                       |
   +-----------------------------+-----------------------------+
   | (04) Remote Data Link Correlator                          |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (08) Remote DLC Port ID                                   |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (12) Reserved Field                                       |
   +-----------------------------+-----------------------------+
   | (14) Message Type           | (15) Flow Control Byte      |
   +-----------------------------+-----------------------------+
   | (16) Protocol ID            | (17) Header Number          |
   +-----------------------------+-----------------------------+
   | (18) Reserved                                             |
   +-----------------------------+-----------------------------+
   | (20) Largest Frame Size     | (21) SSP Flags              |
   +-----------------------------+-----------------------------+
   | (22) Circuit Priority       | (23) Message Type (see note)|
   +-----------------------------+-----------------------------+
   | (24) Target MAC Address  (non-canonical format)           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|
   |                                                           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (30) Origin MAC Address  (non-canonical format)           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -|





Wells & Bartky                                                  [Page 8]

RFC 1795                  Data Link Switching                 April 1995


   |                                                           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |            .                              .               |
   +-----------------------------+-----------------------------+
   | (36) Origin Link SAP        | (37) Target Link SAP        |
   +-----------------------------+-----------------------------+
   | (38) Frame Direction        | (39) Reserved               |
   +-----------------------------+-----------------------------+
   | (40) Reserved                                             |
   +-----------------------------+-----------------------------+
   | (42) DLC Header Length                                    |
   +-----------------------------+-----------------------------+
   | (44) Origin DLC Port ID                                   |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (48) Origin Data Link Correlator                          |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (52) Origin Transport ID                                  |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (56) Target DLC Port ID                                   |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (60) Target Data Link Correlator                          |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (64) Target Transport ID                                  |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (68) Reserved Field                                       |
   +-----------------------------+-----------------------------+
   | (70) Reserved Field                                       |
   +-----------------------------+-----------------------------+
            (Even Byte)                     (Odd Byte)










Wells & Bartky                                                  [Page 9]

RFC 1795                  Data Link Switching                 April 1995


    INFORMATION MESSAGE (16 Bytes)
   +-----------------------------+-----------------------------+
   | (00) Version Number         | (01) Header Length (= 16)   |
   +-----------------------------+-----------------------------+
   | (02) Message Length                                       |
   +-----------------------------+-----------------------------+
   | (04) Remote Data Link Correlator                          |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (08) Remote DLC Port ID                                   |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | (12) Reserved Field                                       |
   +-----------------------------+-----------------------------+
   | (14) Message Type           | (15) Flow Control Byte      |
   +-----------------------------+-----------------------------+
            (Even Byte)                    (Odd Byte)

   The first sixteen bytes of control and information message headers
   contain identical fields.  A brief description of some of the fields
   in an SSP message are shown below (if not defined below, the fields
   and/or their values are described in subsequent sections).

   The Version Number field (offset 0) is set to 0x31 (ASCII '1'),
   indicating a decimal value of 49.  This is used to indicate DLSw
   version 1.

   The Header Length field (offset 1) is 0x48 for control messages,
   indicating a decimal value of 72 bytes, and 0x10 for information and
   Independent Flow Control messages, indicating a decimal value of 16
   bytes.

   The Message Length field (offset 2) defines the number of bytes
   within the data field following the header.

   The Flow Control Byte field (offset 15)  is described in section 8.

   The Header Number field (offset 17) is 0x01, indicating a value of
   one.

   The Circuit Priority field (offset 22) is described in section 4.

   The Frame Direction field (offset 38) is set to 0x01 for frames sent
   from the origin DLSw to the target DLSw, and is set to 0x02 for
   frames sent from the target DLSw to the origin DLSw.




Wells & Bartky                                                 [Page 10]

RFC 1795                  Data Link Switching                 April 1995


   Note:  The Remote Data Link Correlator and Remote DLC Port ID are set
   equal to the Target Data Link Correlator and Target DLC Port ID if
   the Frame Direction field is set to 0x01, and are set equal to the
   Origin Data Link Correlator and Origin DLC Port ID if the Direction
   Field is set to 0x02.

   The Protocol ID field is set to 0x42, indicating a decimal value of
   66.

   The DLC Header Length is set to zero for SNA and is set to 0x23 for
   NetBIOS datagrams, indicating a length of 35 bytes.  This includes
   the Access Control (AC) field, the Frame Control (FC) field,
   Destination MAC Address (DA), the Source MAC Address (SA), the
   Routing Information (RI) field (padded to 18 bytes), the Destination
   link SAP (DSAP), the Source link SAP (SSAP), and the LLC control
   field (UI).

   NOTE:  The values for the Message Type field are defined in section
   3.5. Note that this value is specified in two different fields
   (offset 14 and 23 decimal) of the control message header.  Only the
   first field is to be used when parsing a received SSP message.  The
   second field is to be ignored by new implementations on reception.
   The second field was left in for backwards compatibility with RFC
   1434 implementations and this field may be used in future versions if
   needed.

   The SSP Flags field contains additional information related to the
   SSP message.  The flags are defined as follows (bit 7 being the most
   significant bit and bit 0 the least significant bit of the octet):

   Bit(s)
   76543210    Name    Meaning
   ---------   -----   -------
   x.......    SSPex   1 = explorer message (CANUREACH and ICANREACH)

   Reserved fields are set to zero upon transmission and should be
   ignored upon receipt.

3.2  Address Parameters

   A data link is defined as a logical association between the two end
   stations using Data Link Switching.  It is identified by a Data Link
   ID (14 bytes) consisting of the pair of attachment addresses
   associated with each end system.  Each attachment address is
   represented by the concatenation of the MAC address (6 bytes) and the
   LLC address (1 byte).  Each attachment address is classified as
   either "Target" in the context of the Destination MAC/SAP addresses
   of an explorer frame sent in the first frame used to establish a



Wells & Bartky                                                 [Page 11]

RFC 1795                  Data Link Switching                 April 1995


   circuit, or "Origin" in the context of the Source MAC/SAP addresses.
   All MAC addresses are expressed in non-canonical (Token-Ring) format.

    DATA LINK ID  (14 Bytes @ Control message offset 24 decimal)
   +-----------------------------+-----------------------------+
   | Target MAC Address                                        |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | Origin MAC Address                                        |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | Origin Link SAP             | Target Link SAP             |
   +-----------------------------+-----------------------------+


   An end-to-end circuit is identified by a pair of Circuit ID's.  A
   Circuit ID is a 64 bit number that identifies the DLC circuit within
   a single DLSw.  It consists of a DLC Port ID (4 bytes), and a Data
   Link Correlator (4 bytes).  The Circuit ID must be unique in a single
   DLSw and is assigned locally.  The pair of Circuit ID's along with
   the Data Link IDs,  uniquely identify a single end-to-end circuit.
   Each DLSw must keep a table of these Circuit ID pairs, one for the
   local end of the circuit and the other for the remote end of the
   circuit.  In order to identify which Data Link Switch originated the
   establishment of a circuit, the terms, "Origin" DLSw and "Target"
   DLSw, will be employed in this document.

    CIRCUIT ID   (8 Bytes)
   +-----------------------------+-----------------------------+
   | DLC Port ID                                               |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+
   | Data Link Correlator                                      |
   +- - - - - - - - - - - - - - -+- - - - - - - - - - - - - - -+
   |                                                           |
   +-----------------------------+-----------------------------+

   The Origin Transport ID and the Target Transport ID fields in the
   message header are used to identify the individual TCP/IP port on a
   Data Link Switch.  The values have only local significance.  However,
   each Data Link Switch is required to reflect the values contained in



Wells & Bartky                                                 [Page 12]

RFC 1795                  Data Link Switching                 April 1995


   these two fields, along with the associated values for DLC Port ID
   and the Data Link Correlator, when returning a message to the other
   Data Link Switch.

   The following figure shows the use of the addressing parameters
   during the establishment of an end-to-end connection.  The CANUREACH,
   ICANREACH, and REACH_ACK message types all carry the Data Link ID,
   consisting of the MAC and Link SAP addresses associated with the two
   end stations.  The CANUREACH and ICANREACH messages are qualified by
   the SSPex flag into CANUREACH_ex, ICANREACH_ex (explorer messages)
   and CANUREACH_cs, ICANREACH_cs (circuit start).  The CANUREACH_ex is
   used to find a remote MAC and Link SAP address without establishing
   an SSP circuit.  Upon receipt of a CANUREACH_cs message, the target
   DLSw starts a data link for each port, thereby obtaining a Data Link
   Correlator.  If the target station can be reached, an ICANREACH_cs
   message is returned to the origin DLSw containing the Target Circuit
   ID parameter.  Upon receipt, the origin DLSw starts a data link and
   returns the Origin Circuit ID to the target DLSw within the REACH_ACK
   message.  (Note for a full list of message types, see section 3.5.)

   +------------+                                +------------+
   |Disconnected|                                |Disconnected|
   +------------+   CANUREACH_cs (Data Link ID)  +------------+
       ------------------------------------------------->
         ICANREACH_cs (Data Link ID, Target Circuit ID)
       <------------------------------------------------
     REACH_ACK (Data Link ID, Origin Cir ID, Target Cir ID)
       ------------------------------------------------->
   +------------+                                +------------+
   |Circuit Est.|                                |Circuit Est.|
   +------------+                                +------------+
     XIDFRAME (Data Link ID, Origin Cir ID, Target Cir ID)
       <------------------------------------------------>
      CONTACT (Data Link ID, Origin Cir ID, Target Cir ID)
       ------------------------------------------------->
     CONTACTED (Data Link ID, Origin Cir ID, Target Cir ID)
       <-------------------------------------------------
   +------------+                                +------------+
   | Connected  |                                | Connected  |
   +------------+                                +------------+
        INFOFRAME (Remote Circuit ID = Target Circuit ID)
       ------------------------------------------------->
        INFOFRAME (Remote Circuit ID = Origin Circuit ID)
       <-------------------------------------------------

   During the exchange of the XIDFRAME, CONTACT, and CONTACTED messages,
   the pair of Circuit ID parameters is included in the message format
   along with the DATA LINK ID parameter.  Once the connection has been



Wells & Bartky                                                 [Page 13]

RFC 1795                  Data Link Switching                 April 1995


   established, the INFOFRAME messages are exchanged with the shorter
   header.  This header contains only the Circuit ID associated with the
   remote DLSw.  The Remote Data Link Correlator and the Remote DLC Port
   ID are set equal to the Data Link Correlator and the DLC Port ID that
   are associated with the origin or target Data Link Switch, dependent
   upon the direction of the packet.

3.3  Correlators

   The local use, and contents of the Data Link Correlator, Port ID and
   Transport ID fields in SSP messages is an implementation choice.
   These fields have local significance only.  The values received from
   a partner DLSw must not be interpreted by the DLSw that receives them
   and should be echoed "as is" to a partner DLSw in subsequent
   messages.  All implementations must obey the following rules in this
   section (3.3) on the assignment and fixing of these correlator fields
   for each transport connection or circuit:

   The Transport ID fields are learned from the first SSP message
   exchanged with a DLSw partner (the Capabilities exchange).  This
   field should not be varied by a DLSw after the capabilities exchange
   and must be reflected to the partner DLSw in every SSP control
   message.

   The Target Data Link Correlator, Target Port ID and Target Transport
   ID must remain the same once the Target DLSw has sent the
   ICANREACH_cs for a given circuit.  The Origin DLSw must store the
   values specified in the ICANREACH_cs and use these on all subsequent
   SSP messages for this circuit.

   The Origin DLSw must allow these fields to vary until the
   ICANREACH_cs is received.  Each SSP message issued for a circuit must
   reflect the values specified by the Target DLSw in the last SSP
   message for this circuit received by the Origin DLSw.  Binary zero
   should be used if no such message has yet been received for a given
   circuit (apart from the Target Transport ID which will have been
   learnt as specified above).

   The Origin Data Link Correlator, Origin Port ID and Origin Transport
   ID must remain the same once the Origin DLSw has issued the REACH_ACK
   for a given circuit.  The Target DLSw must store the values specified
   in the REACH_ACK and use these on all subsequent SSP messages for
   this circuit.

   The Target DLSw must allow these fields to vary until the REACH_ACK
   is received.  Each SSP message issued for a circuit must reflect the
   values specified by the Origin DLSw in the last SSP message for this
   circuit received by the Target DLSw.  Binary zero should be used if



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RFC 1795                  Data Link Switching                 April 1995


   no such message has yet been received for a given circuit (apart from
   the Origin Transport ID which will have been learnt as specified
   above).

   For the purposes of correlator exchange, explorer messages form a
   separate circuit.  Both DLSw partners must reflect the last received
   correlator values as specified above.  However correlators learned on
   explorer messages need not be carried over to a subsequent circuit
   setup attempt.  In particular, the Origin DLSw may elect to use the
   same values for the Origin Data Link Correlator and Origin Port ID
   when it issues a CANUREACH_cs after receiving an ICANREACH_ex or
   NETBIOS_NR_ex. However the Target DLSw must not assume that the
   CANUREACH_cs will specify any of the Target Data Link Correlator or
   Target Port ID that were exchanged on the explorer messages.

   Received SSP messages that require a valid Remote Circuit ID but
   cannot be associated with an existing circuit should be rejected with
   a HALT_DL_NOACK message.  This is done to prevent a situation where
   one DLSw partner has a circuit defined while the other partner does
   not. The exception would be a HALT_DL_NOACK message with an invalid
   Remote Circuit ID.  The HALT_DL_NOACK message is typically used in
   error situations where a response is not appropriate.

   The SSP messages requiring a valid Remote Circuit ID are all messages
   except the following: CANUREACH_ex, CANUREACH_cs, ICANREACH_ex,
   ICANREACH_cs, NETBIOS_NQ_cs, NETBIOS_NR_cs, DATAFRAME, NETBIOS_ANQ,
   NETBIOS_ANR, KEEPALIVE and CAP_EXCHANGE.

3.4  Largest Frame Size Field

   The Largest Frame Size (LF Size) field in the SSP Control Header is
   used to carry the LF Size bits across the DLSw connection.  This
   should be used to ensure that the two end-stations always negotiate a
   frame size to be used on a circuit that does not require the Origin
   and Target DLSw partners to re-segment frames.

   This field is valid on CANUREACH_ex, CANUREACH_cs, ICANREACH_ex,
   ICANREACH_cs, NETBIOS_NQ_ex and NETBIOS_NR_ex messages only. The
   contents of this field should be ignored on all other frames.

   Every DLSw forwarding a SSP frame to its DLSw partner must ensure
   that the contents of this frame reflect the minimum capability of the
   route to its local end-station or any limit imposed by the DLSw
   itself.

   The bit-wise definition of this field is as follows (bit 7 is the
   most significant bit, bit 0 is the least significant bit):




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RFC 1795                  Data Link Switching                 April 1995


     7   6   5   4   3   2   1   0
   +-------------------------------+
   | c | r | b | b | b | e | e | e |
   +-------------------------------+

     c   .   .   .   .   .   .   .  LF Size Control flag
                                    (significant on messages
                                    from Origin to Target
                                    DLSw only)

                                    0=fail circuit if route
                                      obtained requires a
                                      smaller LF size
                                    1=don't fail the circuit
                                      but return the LF size
                                      obtained even if it is
                                      smaller

     .   r   .   .   .   .   .   .  Reserved
     .   .   b   .   .   .   .   .  Largest Frame Bit Base
     .   .   .   b   .   .   .   .  Largest Frame Bit Base
     .   .   .   .   b   .   .   .  Largest Frame Bit Base
     .   .   .   .   .   e   .   .  Largest Frame Bit Extended
     .   .   .   .   .   .   e   .  Largest Frame Bit Extended
     .   .   .   .   .   .   .   e  Largest Frame Bit Extended

             <----- LF Bits ----->

   Refer to IEEE 802.1D Standard, Annex C for encoding of Largest Frame
   base and extended bit values.

   The Origin DLSw "Size Control" flag informs a Target DLSw that
   chooses to reply to *_cs messages on the basis of cached information
   that it may safely return a smaller LF Size on the ICANREACH_cs frame
   if it has had to choose an alternative route on which to initialize
   the circuit.  If this bit is set to 1, the Origin DLSw takes
   responsibility for ensuring that the end-stations negotiate a
   suitable frame size for the circuit. If this bit is set to 0, the
   Target DLSw must not reply to the CANUREACH_cs if it cannot obtain a
   route to the Target end station that support an LF Size at least as
   large as that specified in the CANUREACH_cs frame.

3.5  Message Types

   The following table lists the protocol data units that are exchanged
   between Data Link Switches.  All values not listed are reserved for
   potential use in follow-on releases.




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   Command          Description                       Type   flags/notes
   -------          --------                         ------  -----------
   CANUREACH_ex     Can U Reach Station-explorer      0x03   SSPex
   CANUREACH_cs     Can U Reach Station-circuit start 0x03
   ICANREACH_ex     I Can Reach Station-explorer      0x04   SSPex
   ICANREACH_cs     I Can Reach Station-circuit start 0x04
   REACH_ACK        Reach Acknowledgment              0x05
   DGRMFRAME        Datagram Frame                    0x06   (note 1)
   XIDFRAME         XID Frame                         0x07
   CONTACT          Contact Remote Station            0x08
   CONTACTED        Remote Station Contacted          0x09
   RESTART_DL       Restart Data Link                 0x10
   DL_RESTARTED     Data Link Restarted               0x11
   ENTER_BUSY       Enter Busy                        0x0C   (note 2)
   EXIT_BUSY        Exit Busy                         0x0D   (note 2)
   INFOFRAME        Information (I) Frame             0x0A
   HALT_DL          Halt Data Link                    0x0E
   DL_HALTED        Data Link Halted                  0x0F
   NETBIOS_NQ_ex    NETBIOS Name Query-explorer       0x12   SSPex
   NETBIOS_NQ_cs    NETBIOS Name Query-circuit setup  0x12   (note 3)
   NETBIOS_NR_ex    NETBIOS Name Recognized-explorer  0x13   SSPex
   NETBIOS_NR_cs    NETBIOS Name Recog-circuit setup  0x13   (note 3)
   DATAFRAME        Data Frame                        0x14   (note 1)
   HALT_DL_NOACK    Halt Data Link with no Ack        0x19
   NETBIOS_ANQ      NETBIOS Add Name Query            0x1A
   NETBIOS_ANR      NETBIOS Add Name Response         0x1B
   KEEPALIVE        Transport Keepalive Message       0x1D   (note 4)
   CAP_EXCHANGE     Capabilities Exchange             0x20
   IFCM             Independent Flow Control Message  0x21
   TEST_CIRCUIT_REQ Test Circuit Request              0x7A
   TEST_CIRCUIT_RSP Test Circuit Response             0x7B

   Note 1: Both the DGRMFRAME and DATAFRAME messages are used to carry
   information received by the DLC entity within UI frames.  The
   DGRMFRAME message is addressed according to a pair of Circuit IDs,
   while the DATAFRAME message is addressed according to a Data Link ID,
   being composed of a pair of MAC addresses and a pair of link SAP
   addresses. The latter is employed prior to the establishment of an
   end-to-end circuit when Circuit IDs have yet to be established or
   during circuit restart when Data Links are reset.

   Note 2: These messages are not used for the DLSw Standard but may be
   used by older DLSw implementations.  They are listed here for
   informational purposes.  These messages were added after publication
   of RFC 1434 and were deleted in this standard (adaptive pacing is now
   used instead).





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   Note 3: These messages are not normally issued by a Standard DLSw,
   which uses the NB_*_ex messages as shown in section 5.4.  However if
   a Standard DLSw attempts to interoperate with older DLSw
   implementations, these messages correspond to the NETBIOS_NQ and
   NETBIOS_NR messages used in RFC1434 both to locate the resource and
   to setup a circuit.  This document does not attempt to provide a
   complete specification of the use of these messages.

   Note 4:  A KEEPALIVE message may be sent by a DLSw to a partner DLSw
   in order to verify the TCP connection (or other future SSP carrying
   protocol) is still functioning.  If received by a DLSw, this message
   is discarded and ignored.  Use of this message is optional.

   For the exchange of NetBIOS control messages, the entire DLC header
   is carried as part of the message unit.  This includes the MAC
   header, with the routing information field padded to 18 bytes, and
   the LLC header. The following message types are affected:
   NETBIOS_NQ, NETBIOS_NR, NETBIOS_ANQ, NETBIOS_ANR, and DATAFRAME when
   being used by NetBIOS systems.  The routing information in the DLC
   header is not used by the remote Data Link Switch upon receiving the
   above five messages.

   Any SSP message types not defined above if received by a DLSw are to
   be ignored (i.e., no error action is to be performed).  A Data Link
   Switch should quietly drop any SSP message with a Message Type that
   is not recognized or not supported.  Receipt of such a message should
   not cause the termination of the transport connection to the message
   sender.

4.  Circuit Priority

   At circuit start time, each circuit end point will provide priority
   information to its circuit partner.  The initiator of the circuit
   will choose which circuit priority will be effective for the life of
   the circuit.  If Priority is not implemented by the Data Link Switch,
   then "Unsupported" priority is used.

4.1  Frame format

   Circuit priority will be valid in the CANUREACH_cs, ICANREACH_cs, and
   REACH_ACK frames only. The relevant header field is shown below.  The
   Circuit Priority value is a byte value at offset 22 in an SSP Control
   Message.








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RFC 1795                  Data Link Switching                 April 1995


   The following describes the format of the Circuit Priority byte.

     7   6   5   4   3   2   1   0
   +-------------------+-----------+
   |   reserved        |    CP     |
   +-------------------+-----------+

   CP: Circuit Priority bits
           000 - Unsupported       (note 1)
           001 - Low Priority
           010 - Medium Priority
           011 - High Priority
           100 - Highest Priority
           101 to 111 are reserved for future use

   Note 1: Unsupported means that the Data Link Switch that originates
   the circuit does not implement priority.  Actions taken on
   Unsupported priority are vendor specific.

4.2  Circuit Startup

   The sender of a CANUREACH_cs is responsible for setting the CP bits
   to reflect the priority it would like to use for the circuit being
   requested.  The mechanism for choosing an appropriate value is
   implementation dependent.  The sender of an ICANREACH_cs frame will
   set the CP bits to reflect the priority it would like to use for the
   circuit being requested, with the mechanism for choosing the
   appropriate value being implementation dependent.  The receiver of
   the ICANREACH_cs will select from the priorities in the CANUREACH_cs
   and ICANREACH_cs frames, and will set the value in the CP field of
   the REACH_ACK frame that follows to the value to be used for this
   circuit.  This priority will be used for the life of the circuit.  A
   CANUREACH_cs or ICANREACH_cs with the circuit priority value set to
   Unsupported (CP=000) indicates that the sender does not support the
   circuit priority function.
















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   Flow:

      DLSw A               DLSw B

   CANUREACH_cs (CP=011) ----->           Circuit initiator requests
                                          high Priority.

        <--------- ICANREACH_cs (CP=010)  Circuit target requests
                                          medium priority.

   REACH_ACK (CP=010) -------->           Circuit initiator sets
                                          the priority for this
                                          circuit to medium. The
                                          circuit initiator could
                                          choose either high or
                                          medium in this example.

5.  DLSw State Machine

   The following state tables describe the states for a single circuit
   through the Data Link Switch.  State information is kept for each
   connection.  The initial state for a connection is DISCONNECTED.  The
   steady state is either CIRCUIT_ESTABLISHED or CONNECTED.  In the former
   state, an end-to-end circuit has been established allowing the support
   of Type 1 LLC between the end systems.  The latter state exists when an
   end-to-end connection has been established for the support of Type 2 LLC
   services between the end systems.

   For SNA, LLC type 2 connection establishment is via the use of IEEE
   802.2 Test or XID  frames.  SNA devices send these frames to the null
   SAP in order to determine the source route information in support of
   bridging.  Normally SNA devices use SAP 0x04, 0x08, or 0x0C  (most SNA
   LLC2 devices that have a single PU per MAC address use a default of
   0x04).  Typically the SAP would be used to determine if the Test frames
   should be sent to the DLSw code in the router.  If both bridging and
   DLSw are enabled, this allows the product to ensure that SNA frames are
   not both bridged and switched.  Note that although typically SNA uses a
   DSAP and SSAP of 0x04, it allows for other SAPs to be configured and
   supports unequal SAPs.  This allows multiple PUs to share connections
   between two given MAC addresses (each PU to PU session uses one LLC2
   connection).

   For NetBIOS, LLC type 2 connection establishment is via the Name Query
   and Name Recognized frames.  These frames are used for both address
   resolution and source route determination.  NetBIOS devices use SAP
   0xF0.





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5.1  Data Link Switch States

   The Switch-to-Switch Protocol is formally defined through the state
   machines described in this chapter.  The following table lists the
   thirteen possible states for the main circuit FSM.  A separate state
   machine instance is employed for each end-to-end circuit that is
   maintained by the Data Link Switch.

   State Name            Description
   ----------            -----------
   CIRCUIT_ESTABLISHED   The end-to-end circuit has been
                         established.  At this time LLC Type 1
                         services are available from end-to-end.

   CIRCUIT_PENDING       The target DLSw is awaiting a REACH_ACK
                         response to an ICANREACH_cs message.

   CIRCUIT_RESTART       The DLSw that originated the reset is
                         awaiting the restart of the data link
                         and the DL_RESTARTED response to a
                         RESTART_DL message.

   CIRCUIT_START         The origin DLSw is awaiting a
                         ICANREACH_cs in response to a
                         CANUREACH_cs message.

   CONNECTED             The end-to-end connection has
                         been established thereby allowing
                         LLC Type 2 services from end-to-end
                         in addition to LLC Type 1 services.

   CONNECT_PENDING       The origin DLSw is awaiting the
                         CONTACTED response to a CONTACT
                         message.

   CONTACT_PENDING       The target DLSw is awaiting the
                         DLC_CONTACTED confirmation to a
                         DLC_CONTACT signal (i.e., DLC
                         is waiting for a UA response to
                         an SABME command).

   DISCONNECTED          The initial state with no circuit
                         or connection established, the
                         DLSw is awaiting either a
                         CANUREACH_cs, or an ICANREACH_cs.

   DISCONNECT_PENDING    The DLSw that originated the
                         disconnect is awaiting the DL_HALTED



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RFC 1795                  Data Link Switching                 April 1995


                         response to a HALT_DL message.

   HALT_PENDING          The remote DLSw is awaiting the
                         DLC_DL_HALTED indication following
                         the DLC_HALT_DL request (i.e., DLC
                         is waiting for a UA response to a
                         DISC command), due to receiving a
                         HALT_DL message.

   HALT_PENDING_NOACK    The remote DLSw is awaiting the
                         DLC_DL_HALTED indication following
                         the DLC_HALT_DL request (i.e., DLC
                         is waiting for a UA response to a
                         DISC command), due to receiving a
                         HALT_DL_NOACK message.

   RESTART_PENDING       The remote DLSw is awaiting the
                         DLC_DL_HALTED indication following
                         the DLC_HALT_DL request (i.e., DLC
                         is waiting for a UA response to a
                         DISC command), and the restart of
                         the data link.

   RESOLVE_PENDING       The target DLSw is awaiting
                         the DLC_DL_STARTED indication
                         following the DLC_START_DL request
                         (i.e., DLC is waiting for a Test
                         response as a result of sending a
                         Test command).

   The DISCONNECTED state is the initial state for a new circuit.  One
   end station starts the connection via an XID or SABME command (i.e.,
   DLC_XID or DLC_CONTACTED).  Upon receipt, the Data Link Switches
   exchange a set of CANUREACH_cs, ICANREACH_cs and REACH_ACK messages.
   Upon completion of this three-legged exchange both Data Link Switches
   will be in the CIRCUIT_ESTABLISHED state.  Three pending states also
   exist during this exchange.  The CIRCUIT_START state is entered by
   the origin Data Link Switch after it has sent the CANUREACH_cs
   message.  The RESOLVE_PENDING state is entered by the target Data
   Link Switch awaiting a Test response to a Test Command.  And lastly,
   the CIRCUIT_PENDING state is entered by the target DLSw awaiting the
   REACH_ACK reply to an ICANREACH_cs message.

   The CIRCUIT_ESTABLISHED state allows for the exchange of LLC Type 1
   frames such as the XID exchanges between SNA stations that occurs
   prior to the establishment of a connection.  Also, datagram traffic
   (i.e., UI frames)  may be sent and received between the end stations.
   These exchanges use the XIDFRAME and DGRMFRAME messages sent between



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RFC 1795                  Data Link Switching                 April 1995


   the Data Link Switches.

   In the CIRCUIT_ESTABLISHED state, the receipt of a SABME command
   (i.e., DLC_CONTACTED) causes the origin DLSw to issue a CONTACT
   message, to send an RNR supervisory frame (i.e., DLC_ENTER_BUSY) to
   the origin station, and to enter the CONNECT_PENDING state awaiting a
   CONTACTED message.  The target DLSw, upon the receipt of a CONTACT
   message, will issue a SABME command (i.e., DLC_CONTACT) and enter the
   Contact Pending state.  Once the UA response is received (i.e.,
   DLC_CONTACTED), the target DLSw sends a CONTACTED message and enters
   the CONNECTED state. When received, the origin DLSw enters the
   CONNECTED state and sends an RR supervisory frame (i.e.,
   DLC_EXIT_BUSY).

   The CONNECTED state is the steady state for normal data flow once a
   connection has been established.  Information frames (i.e., INFOFRAME
   messages) are simply sent back and forth between the end points of
   the connection.  This is the path that should be optimized for
   performance.

   The connection is terminated upon the receipt of a DISC frame or
   under some other error condition detected by DLC (i.e., DLC_ERROR).
   Upon receipt of this indication, the DLSw will halt the local data
   link, send a HALT_DL message to the remote DLSw, and enter the
   DISCONNECT_PENDING State.  When the HALT_DL frame is received by the
   other DLSw, the local DLC is halted for this data link, a DL_HALTED
   message is returned, and the DISCONNECTED state is entered.  Receipt
   of this DL_HALTED message causes the other DLSw to also enter the
   DISCONNECTED state.

   The CIRCUIT_RESTART state is entered if one of the Data Link Switches
   receives a SABME command  (i.e., DLC_RESET) after data transfer while
   in the CONNECTED state.  This causes a DM command to be returned to
   the origin station and a RESTART_DL message to be sent to the remote
   Data Link Switch. This causes the remote data link to be halted and
   then restarted.  The remote DLSw will then send a DL_RESTARTED
   message back to the first DLSw.  The receipt of the DL_RESTARTED
   message causes the first DLSw to issue a new CONTACT message,
   assuming that the local DLC has been contacted (i.e., the origin
   station has resent the SABME command).  This is eventually responded
   to by a CONTACTED message. Following this exchange, both Data Link
   Switches will return to the CONNECTED state.  If the local DLC has
   not been contacted, the receipt of a DL_RESTARTED command causes the
   Data Link Switch to enter the CIRCUIT_ESTABLISHED state awaiting the
   receipt of a SABME command (i.e., DLC_CONTACTED signal).

   The HALT_PENDING, HALT_PENDING_NOACK and RESTART_PENDING states
   correspond to the cases when the Data Link Switch is awaiting



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RFC 1795                  Data Link Switching                 April 1995


   responses from the local station on the adjacent LAN (e.g., a UA
   response to a DISC command). Also in the RESTART_PENDING state, the
   Data Link Switch will attempt to restart the data link prior to
   sending a DL_RESTARTED message.  For some implementations, the start
   of a data link involves the exchange of a Test command/response on
   the adjacent LAN (i.e., DLC_START_DL).  For other implementations,
   this additional exchange may not be required.

5.2  State Transition Tables

   This section provides a detailed representation of the Data Link
   Switch, as documented by a single state machine.  Many of the
   transitions are dependent upon local signals between the Data Link
   Switch entity and one of the DLC entities.  These signals and their
   definitions are given in the following tables.

   DLC Events:

   Event Name      Description
   ----------      -----------
   DLC_CONTACTED   Contact Indication:  DLC has received an SABME
                   command or DLC has received a UA response as a
                   result of sending an SABME command.

   DLC_DGRM        Datagram Indication:  DLC has received a UI frame.

   DLC_ERROR       Error condition indicated by DLC:  Such a
                   condition occurs when a DISC command is received
                   or when DLC experiences an unrecoverable error.

   DLC_INFO        Information Indication:  DLC has received an
                   Information (I) frame.

   DLC_DL_HALTED   Data Link Halted Indication:  DLC has
                   received a UA response to a DISC command.

   DLC_DL_STARTED  Data Link Started Indication:  DLC has
                   received a Test response from the null SAP.

   DLC_RESET       Reset Indication:  DLC has received an SABME
                   command during the time a connection is
                   currently active and has responded with DM.

   DLC_RESOLVE_C   Resolve Command Indication:  DLC has received
                   a Test command addressed to the null SAP, or an
                   XID command addressed to the null SAP.





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RFC 1795                  Data Link Switching                 April 1995


   DLC_RESOLVED    Resolve request:  DLC has received a TEST response
                   frame (or equivalent for non-LAN DLCs) but has not
                   reserved the resources required for a circuit yet.

   DLC_XID         XID Indication:  DLC has received an XID command
                   or response to a non-null SAP.

   Other Events:

   Event Name      Description
   ----------      -----------
   XPORT_FAILURE   Failure of the transport connection used by the
                   circuit.

   CS_TIMER_EXP    The CIRCUIT_START timer (started when the circuit
                   went into CIRCUIT_START state) has expired.


   DLC Actions:

   Action Name     Description
   -----------     -----------
   DLC_CONTACT     Contact Station Request:  DLC will send a SABME
                   command or a UA response to an outstanding SABME
                   command.

   DLC_DGRM        Datagram Request:  DLC will send a UI frame.

   DLC_ENTER_BUSY  Enter Link Station Busy:  DLC will send an
                   RNR supervisory frame.

   DLC_EXIT_BUSY   Exit Link Station Busy:  DLC will send an RR
                   supervisory frame.

   DLC_HALT_DL     Halt Data Link Request:  DLC will send a DISC
                   command.

   DLC_INFO        Information Request:  DLC will send an I frame.

   DLC_RESOLVE     Resolve request:  DLC should issue a TEST (or
                   appropriate equivalent for non-LAN DLCs) but need
                   not reserve the resources required for a circuit yet.

   DLC_RESOLVE_R   Resolve Response Request:  DLC will send a
                   Test response or XID response from the null SAP.

   DLC_START_DL    Start Data Link Request:  DLC will send a Test
                   command to the null SAP.



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RFC 1795                  Data Link Switching                 April 1995


   DLC_XID         XID Request:  DLC will send an XID command or an
                   XID response.


   Other Actions:

   Action Name     Description
   ----------      -----------
   START_CS_TIMER  Start the CIRCUIT_START timer.

   DLC_RESOLVE_R and DLC_START_DL actions require the DLC to reserve the
   resources necessary for a link station as they are used only when a
   circuit is about to be started.  The DLC_RESOLVE action is used for
   topology explorer traffic and does not require such resources to be
   reserved, though a DLC implementation may choose not to distinguish
   this from the DLC_START_DL action.  See section 5.4 for details of
   the actions and events for explorer frames.

   The Data Link Switch is described by a state transition table as
   documented in the following sections.  Each of the states is
   described below in terms of the events, actions, and next state for
   each transition. If a particular event is not listed for a given
   state, no action and no state transition should occur for that event.
   Any significant comments concerning the transitions within a given
   state are given immediately following the table representing the
   state.

   A separate state machine instance is maintained by the Data Link
   Switch for each end-to-end circuit.  The number of circuits that may
   be supported by each Data Link Switch is a local implementation
   option.

   The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex are
   SSP messages that are not associated with a particular circuit.  The
   processing of these messages is covered in section 5.4.
















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5.2.1  DISCONNECTED State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_cs | DLC_START_DL        | RESOLVE_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | If source route     | If CANUREACH_cs was  |
   |                      | bridged frame with  | sent:                |
   |                      | broadcast indicated:|   CIRCUIT_START      |
   |                      |   Send CANUREACH_ex |                      |
   |                      | else:               |                      |
   |                      |   Send CANUREACH_cs |                      |
   |                      |   START_CS_TIMER    |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | If NETBIOS          |                      |
   |                      | NAME_QUERY:         |                      |
   |                      |  Send NETBIOS_NQ_ex |                      |
   |                      | else:               |                      |
   |                      |  Send DATAFRAME     |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | Send CANUREACH_cs   | CIRCUIT_START        |
   +----------------------+---------------------+----------------------+

   It is assumed that each Data Link Switch will build a set of topology
   tables giving the identity of each Data Link Switch that can reach a
   specific MAC address or a specific NetBIOS name.  This table can be
   built  using the explorer frames, as per the Explorer FSM in section
   5.4.  As a consequence, the amount of search traffic can be kept to a
   minimum.

   Upon receipt of a TEST command, broadcast XID or NetBIOS NAME_QUERY,
   the Data Link Switch checks the topology table for the target MAC/SAP
   or NetBIOS name.  If there is no matching entry in the table, the
   Data Link Switch uses the explorer FSMs in section 5.4 to locate the
   target MAC/SAP or NetBIOS name.

   When the first non-broadcast XID or SABME flows,  the Data Link
   Switch issues a CANUREACH_cs to attempt to start a circuit.  The
   CANUREACH_cs message is sent to only those Data Link Switches that
   are known to be able to reach the given MAC address.  The mechanism
   by which a topology table entry is determined to be out-of-date and
   is deleted from the table is implementation specific.

   The DISCONNECTED state is exited upon the sending of a CANUREACH_cs
   by the origin DLSw or the receipt of a CANUREACH_cs message by a



Wells & Bartky                                                 [Page 27]

RFC 1795                  Data Link Switching                 April 1995


   prospective target Data Link Switch.  In the latter case, the Data
   Link Switch will issue a Test command to the target station (i.e.,
   DLC_START_DL signal is presented to DLC).

5.2.2  RESOLVE_PENDING State

   +-------------------+-----------------------+-----------------------+
   |        Event      |      Action(s)        |      Next State       |
   +-------------------+-----------------------+-----------------------+
   | Receive DATAFRAME | DLC_DGRM              |                       |
   +-------------------+-----------------------+-----------------------+
   | DLC_DL_STARTED    | If LF value of        | If LF value of        |
   |                   | DLC_DL_STARTED        | DLC_DL_STARTED        |
   |                   | is greater than or    | is greater than or    |
   |                   | equal to LF Size of   | equal to LF Size of   |
   |                   | CANUREACH_cs or LF    | CANUREACH_cs or LF    |
   |                   | Size Control bit set: | Size Control bit set: |
   |                   |   Send ICANREACH_cs   |   CIRCUIT_PENDING     |
   |                   | else:                 | else:                 |
   |                   |   Send DLC_HALT_DL    |   HALT_PENDING_NOACK  |
   +-------------------+-----------------------+-----------------------+
   | DLC_ERROR         |                       | DISCONNECTED          |
   +-------------------+-----------------------+-----------------------+
   | DLC_DGRM          | Send DATAFRAME        |                       |
   +-------------------+-----------------------+-----------------------+

   The RESOLVE_PENDING state is entered upon receipt of a CANUREACH_cs
   message by the target DLSw.  A data link is started, causing a Test
   command to be sent by the DLC.

   Several CANUREACH_cs messages can be received in the RESOLVE_PENDING
   state.  The Data Link Switch may update its topology information
   based upon the origin MAC address information in each CANUREACH_cs
   message.

   Upon the receipt of a DLC_DL_STARTED signal in the RESOLVE_PENDING
   state, the Data Link Switch may update its topology table base upon
   the remote MAC address information.  The ICANREACH_cs message must be
   returned to the first partner DLSw from which a CANUREACH_cs was
   received for this circuit, or an implementation may optionally reply
   to all partners from which the CANUREACH_cs was received.

   The RESOLVE_PENDING state is exited once the data link has been
   started (i.e., a DLC_DL_STARTED signal is received as a result of a
   Test response received by the DLC).  The target Data Link Switch then
   enters the CIRCUIT_PENDING state.





Wells & Bartky                                                 [Page 28]

RFC 1795                  Data Link Switching                 April 1995


5.2.3  CIRCUIT_START State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_cs | If origin MAC addr  | If DLC_START_DL      |
   | for circuit in       | in CANUREACH_cs is  | issued:              |
   | opposite direction   | greater than origin |   RESOLVE_PENDING    |
   |                      | MAC addr of circuit:|                      |
   |                      |   DLC_START_DL      |                      |
   |                      | else:               |                      |
   |                      |   no action taken   |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | If LF Size Control  | If LF Size Control   |
   |                      | bit set and LF Size | bit set and LF Size  |
   |                      | is not negotiable:  | is not negotiable:   |
   |                      |   Send HALT_DL_NOACK|   DISCONNECTED       |
   |                      | else:               | else if Connected:   |
   |                      |   Send REACH_ACK,   |   CONNECT_PENDING    |
   |                      |   Send appropriate  | else:                |
   |                      |   SSP message based |   CIRCUIT_ESTABLISHED|
   |                      |   on the event      |                      |
   |                      |   that generated    |                      |
   |                      |   CANUREACH_cs      |                      |
   |                      |   (see Note)        |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | CS_TIMER_EXP         |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+

   The CIRCUIT_START state is entered by the origin Data Link Switch
   when a DLC_XID or DLC_CONTACTED signal has been received from the
   DLC.

   The CIRCUIT_START state is exited upon receipt of an ICANREACH_cs
   message.  A REACH_ACK message is returned to the target Data Link
   Switch.  If the CIRCUIT_START state was entered due to a DLC_XID
   signal, an XIDFRAME message containing the XID is sent to the target
   Data Link Switch.  If the CIRCUIT_START state was entered due to a
   DLC_CONTACTED signal, a CONTACT message is sent to the target Data
   Link Switch.





Wells & Bartky                                                 [Page 29]

RFC 1795                  Data Link Switching                 April 1995


5.2.4  CIRCUIT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CONTACT      | DLC_CONTACT         | CONTACT_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive REACH_ACK    | If Connected:       | If Connected:        |
   |                      |  Send CONTACT       |  CONNECT_PENDING,    |
   |                      |                     | else:                |
   |                      |                     |  CIRCUIT_ESTABLISHED |
   +----------------------+---------------------+----------------------+
   | Receive XIDFRAME     | DLC_XID             |                      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | If UA is sent in    |                      |
   |                      | response to SABME:  |                      |
   |                      |   DLC_ENTER_BUSY    |                      |
   |                      | else:               |                      |
   |                      |   no action taken   |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | Drop or hold until  |                      |
   |                      | REACH_ACK received  |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CIRCUIT_PENDING state is entered by the target Data Link Switch
   following the sending of an ICANREACH_cs message.  In this state it
   is awaiting the reception of a REACH_ACK message from the origin Data
   Link Switch.

   If the target Data Link Switch happens to receive a SABME command
   from the target station while in the CIRCUIT_PENDING state (i.e., a
   DLC_CONTACTED signal received from the DLC), the reception of the
   REACH_ACK message causes the target Data Link Switch to enter the
   CONNECT_PENDING state and to send a CONTACT message to the origin



Wells & Bartky                                                 [Page 30]

RFC 1795                  Data Link Switching                 April 1995


   Data Link Switch.

   If no such SABME is received, the receipt of the REACH_ACK causes the
   Data Link Switch to enter CIRCUIT_ESTABLISHED state.

5.2.5  CONNECT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CONTACTED    | If UA was sent in   | CONNECTED            |
   |                      | response to SABME:  |                      |
   |                      |   DLC_EXIT_BUSY     |                      |
   |                      | else:               |                      |
   |                      |   DLC_CONTACT       |                      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESET            | Send RESTART_DL     | CIRCUIT_RESTART      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CONNECT_PENDING state is entered when a DLC_CONTACTED signal has
   been received from the DLC (i.e., a SABME command has been received).
   A CONTACT message it then  issued.  The state is exited upon the
   receipt of a CONTACTED message.  If a DLC_RESET signal is received,
   the local data link is restarted and a RESTART_DL message is sent to
   the remote DLSw.

   An ICANREACH_cs received after the transition to CONNECT_PENDING
   state indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one
   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
   circuit started by the Data Link Switch partner that originated each
   such ICANREACH_cs.



Wells & Bartky                                                 [Page 31]

RFC 1795                  Data Link Switching                 April 1995


   Note:  Some implementations will also send a Test command in order to
   restart the data link to the station that sent the SABME command
   (i.e., a DLC_START_DL will be issued).

5.2.6  CIRCUIT_ESTABLISHED State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CONTACT      | DLC_CONTACT         | CONTACT_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive XIDFRAME     | DLC_XID             |                      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | Send CONTACT        | CONNECT_PENDING      |
   |                      | If UA is sent in    |                      |
   |                      | response to SABME:  |                      |
   |                      |   DLC_ENTER_BUSY    |                      |
   |                      | else:               |                      |
   |                      |   no action taken   |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | Send XIDFRAME       |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CIRCUIT_ESTABLISHED state is entered by the origin Data Link
   Switch from the CIRCUIT_START state, and by the target Data Link
   Switch from the CIRCUIT_PENDING state.  The state is exited when a
   connection is started (i.e., DLC receives a SABME command) or CONTACT
   is received. The next state is CONTACT_PENDING or CONNECT_PENDING.

   An ICANREACH_cs received after the transition to CIRCUIT_ESTABLISHED
   state indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one



Wells & Bartky                                                 [Page 32]

RFC 1795                  Data Link Switching                 April 1995


   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
   circuit started by the Data Link Switch partner that originated each
   such ICANREACH_cs.

5.2.7  CONTACT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive RESTART_DL   | DLC_HALT_DL         | RESTART_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_CONTACTED        | Send CONTACTED      | CONNECTED            |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CONTACT_PENDING state is entered upon the receipt of a CONTACT
   message, which causes the Data Link Switch to issue a DLC_CONTACT
   signal to the DLC (i.e., DLC sends a SABME command).  This state is
   then exited upon the receipt of a DLC_CONTACTED signal from the DLC
   (i.e., a UA response received).

   If a RESTART_DL message is received, indicating that the remote Data
   Link Switch has received a DLC_RESET signal, the local Data Link
   Switch sends a DISC command frame on the adjacent LAN (i.e.,
   DLC_HALT_DL signal) and enter the RESTART_PENDING state.

   An ICANREACH_cs received after the transition to CONTACT_PENDING
   state indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one
   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the data
   link started by the Data Link Switch partner that originated this
   ICANREACH_cs.






Wells & Bartky                                                 [Page 33]

RFC 1795                  Data Link Switching                 April 1995


5.2.8  CONNECTED State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | DLC_HALT_DL         | HALT_PENDING         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive RESTART_DL   | DLC_HALT_DL         | RESTART_PENDING      |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive INFOFRAME    | DLC_INFO            |                      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | Receive XIDFRAME     | If non-activation   |                      |
   |                      | XID3:               |                      |
   |                      |   DLC_XID           |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_cs | Send HALT_DL_NOACK  |                      |
   +----------------------+---------------------+----------------------+
   | Receive ENTER_BUSY   | DLC_ENTER_BUSY      |                      |
   +----------------------+---------------------+----------------------+
   | Receive EXIT_BUSY    | DLC_EXIT_BUSY       |                      |
   +----------------------+---------------------+----------------------+
   | Rec TEST_CIRCUIT_REQ | Snd TEST_CIRCUIT_RSP|                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESET            | Send RESTART_DL     | CIRCUIT_RESTART      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_INFO             | Send INFOFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | DLC_XID              | If non-activation   |                      |
   |                      | XID3:               |                      |
   |                      |   Send XIDFRAME     |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The CONNECTED state is entered from the CONNECT_PENDING state upon
   the receipt of a CONTACTED message or from the CONTACT_PENDING state
   upon the receipt of a DLC_CONTACTED signal.




Wells & Bartky                                                 [Page 34]

RFC 1795                  Data Link Switching                 April 1995


   The CONNECTED state is exited usually under one of two conditions: a
   DLC_ERROR signal received from the DLC (e.g., a DISC command received
   by the local DLC), or a HALT_DL message received from the other Data
   Link Switch (e.g., a DISC command received by the remote DLC).

   A SABME command (i.e., a DLC_RESET signal) received by either Data
   Link Switch will also cause the two Data Link Switches to leave the
   CONNECTED state and attempt to restart the circuit.  Following the
   receipt of a SABME, the local Data Link Switch sends a RESTART_DL
   message to the other Data Link Switch and enters the CIRCUIT_RESTART
   state.  Upon the receipt of the RESTART_DL message, the remote Data
   Link Switch sends a DISC command (i.e., DLC_HALT_DL signal) and
   enters the RESTART_PENDING state.

   An ICANREACH_cs received after the transition to CONNECTED state
   indicates that more than one CANUREACH_cs was sent at circuit
   establishment time and the target station was found by more than one
   Data Link Switch partner.  A HALT_DL_NOACK is sent to halt the
   circuit started by the Data Link Switch partner that originated each
   such ICANREACH_cs.

   Note:  Some implementations will also send a Test command in order to
   restart the data link to the station that sent the SABME command
   (i.e., a DLC_START_DL will be issued).

5.2.9  CIRCUIT_RESTART State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive DL_RESTARTED | If Connected:       | If Connected:        |
   |                      |  Send CONTACT       |  CONNECT_PENDING,    |
   |                      |                     | else:                |
   |                      |                     |  CIRCUIT_ESTABLISHED |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK| DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+






Wells & Bartky                                                 [Page 35]

RFC 1795                  Data Link Switching                 April 1995


   The CIRCUIT_RESTART state is entered if a DLC_RESET signal is
   received from the local DLC.  This was caused by the receipt of a
   SABME command while a connection was currently active.  A DM response
   will be issued to the SABME command and the Data Link Switch will
   attempt to restart the end-to-end circuit.

   The CIRCUIT_RESTART state is exited through one of two transitions.
   The next state depends upon the time the local DLC has reached the
   contacted state (i.e., a DLC_CONTACTED signal is presented) relative
   to the receipt of the DL_RESTARTED message.  This signal is caused by
   the origin station resending the SABME command that initially caused
   the Data Link Switch to enter the CIRCUIT_RESTART state.  The two
   cases are as follows:

      1) DL_RESTARTED message received before the DLC_CONTACTED signal-
         In this case, the CIRCUIT_ESTABLISHED state is entered.

      2) DL_RESTARTED message received after the DLC_CONTACTED signal-
         In this case, the CONNECT_PENDING state is entered.

5.2.10  DISCONNECT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive DL_HALTED    |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL      | Send DL_HALTED      |                      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK|                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+

   The DISCONNECT_PENDING state is entered when a DLC_ERROR signal is
   received from the local DLC.  Upon receipt of this signal, a HALT_DL
   message is sent.  Once an DL_HALTED message is received, the state is
   exited, and the Data Link Switch enters the DISCONNECTED state.









Wells & Bartky                                                 [Page 36]

RFC 1795                  Data Link Switching                 April 1995


5.2.11  RESTART_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK|                     | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DGRMFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DL_HALTED        | Send DL_RESTARTED   | CIRCUIT_ESTABLISHED  |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send HALT_DL        | DISCONNECT_PENDING   |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DGRMFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        | DLC_HALT_DL         | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+

   The RESTART_PENDING state is entered upon the receipt of a RESTART_DL
   message from the remote DLSw while the local Data Link Switch is in
   either the CONTACT_PENDING state or the CONNECTED state, which causes
   the local DLSw to issue a DISC command to the DLC.  Upon the receipt
   of the UA response (DLC_DL_HALTED), the data link is restarted, a
   DL_RESTARTED message is returned to the remote DLSw, and the
   CIRCUIT_ESTABLISHED state is entered.

   Note:  Some implementations will send a Test command in order to
   restart the data link to the target station (i.e., a DLC_START_DL
   will be issued) prior to sending the DL_RESTARTED message.

5.2.12  HALT_PENDING State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive HALT_DL_NOACK|                     | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DL_HALTED        | Send DL_HALTED      | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            | Send DL_HALTED      | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+
   | XPORT_FAILURE        |                     | HALT_PENDING_NOACK   |
   +----------------------+---------------------+----------------------+




Wells & Bartky                                                 [Page 37]

RFC 1795                  Data Link Switching                 April 1995


   The HALT_PENDING state is entered upon the receipt of a HALT_DL
   message. This causes the local DLC to issue a DISC command.  Upon the
   receipt of the UA response (DLC_DL_HALTED), a DL_HALTED message is
   returned to the remote DLSw and the DISCONNECTED state is entered.

5.2.13  HALT_PENDING_NOACK State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive DATAFRAME    | DLC_DGRM            |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DL_HALTED        |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_ERROR            |                     | DISCONNECTED         |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM             | Send DATAFRAME      |                      |
   +----------------------+---------------------+----------------------+

   The HALT_PENDING_NOACK state is entered upon the receipt of a
   HALT_DL_NOACK message.  This causes the local DLC to issue a DISC
   command.  Upon the receipt of the UA response (DLC_DL_HALTED), the
   DISCONNECTED state is entered.

5.3  NetBIOS Datagrams

   The NetBIOS protocols use a number of UI frames for directory
   services and the transmission of datagrams.  Most of these frames are
   directed to a group MAC address (GA) with the routing information
   field indicating spanning tree explorer (STE) (a.k.a. Single Route
   Broadcast).  The NB_Add_Name_Response and NB_Name_Recognized frames
   are directed to a specific MAC address with the routing information
   field indicating an all routes explorer frame (ARE) (a.k.a. All
   Routes Broadcast)  The NB_Status_Response frame, is directed to a
   specific MAC address with the routing information field indicating a
   specifically routed frame (SRF). The handling of these frames is
   summarized in the following table.














Wells & Bartky                                                 [Page 38]

RFC 1795                  Data Link Switching                 April 1995


   +---------------------------+------------------+--------------------+
   |          Event            |     Action(s)    |      Comment       |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for NETBIOS      | Send NETBIOS_ANQ | Transmitted to all |
   |  group address:           |                  |   remote DLSw      |
   |   NB_Add_Name_Query       |                  |                    |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for a specific   | Send NETBIOS_ANR | Transmitted to     |
   |  address:                 |                  |   specific DLSw    |
   |   NB_Add_Name_Response    |                  |                    |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for a specific   | Send DATAFRAME   | Transmitted to all |
   |  address:                 |                  |   remote DLSw      |
   |   NB_Status_Response      |                  |                    |
   +---------------------------+------------------+--------------------+
   | DLC_DGRM for NETBIOS      | Send DATAFRAME   | Transmitted to all |
   |  group address:           |                  |   remote DLSw      |
   |   NB_Name_in_Conflict     |                  |                    |
   |   NB_Add_Group_Name_Query |                  |                    |
   |   NB_Datagram,            |                  |                    |
   |   NB_Datagram_Broadcast   |                  |                    |
   |   NB_Status_Query         |                  |                    |
   |   NB_Terminate_Trace      |                  |                    |
   +---------------------------+------------------+--------------------+

   The above actions do not apply in the following states:
   CIRCUIT_ESTABLISHED, CONTACT_PENDING, CONNECT_PENDING, CONNECTED, and
   CIRCUIT_PENDING.  The handling of the remaining two UI frames used by
   NetBIOS systems, NB_Name_Query and NB_Name_Recognized, are documented
   as part of the DLSw state machine in the previous section (i.e.,
   DISCONNECTED and RESOLVE_PENDING states).  Furthermore, the handling
   of NetBIOS datagrams (i.e., NB_Datagram) sent to a specific MAC
   address is also governed by the DLSw state machine.

   Note:  Some implementations also issue Test frames during the
   exchange of the NetBIOS, NB_Name_Query and NB_Name_Recognized.  This
   exchange of protocol data units occurs during the start of a data
   link and is used to determine the routing information.  Most other
   implementations of NetBIOS will use the
   NB_Name_Query/NB_Name_Recognized exchange to determine routes in
   conjunction with resolving the NetBIOS names. These differences are
   not reflected in the SSP protocols.









Wells & Bartky                                                 [Page 39]

RFC 1795                  Data Link Switching                 April 1995


   The handling of the NetBIOS specific SSP messages is given in the
   following table.

   +---------------+-------------------------+-------------------------+
   |     Event     |        Action(s)        |         Comment         |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_ANQ   | DLC_DGRM:               | Routed STE              |
   |               |    NB_Add_Name_Query    | (NETBIOS Group Address) |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_ANR   | DLC_DGRM:               | Routed ARE              |
   |               |    NB_Add_Name_Response | (Specific MAC Address)  |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NQ_ex | DLC_DGRM:               | Routed STE              |
   |               |    NB_Name_Query        | (NETBIOS Group Address) |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NQ_cs | DLC_DGRM:               | Routed STE              |
   |               |    NB_Name_Query        | (NETBIOS Group Address) |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NR_ex | DLC_DGRM:               | Routed ARE              |
   |               |    NB_Name_Recognized   | (Specific MAC Address)  |
   +---------------+-------------------------+-------------------------+
   | NETBIOS_NR_cs | DLC_DGRM:               | Routed ARE              |
   |               |    NB_Name_Recognized   | (Specific MAC Address)  |
   +---------------+-------------------------+-------------------------+
   | DATAFRAME     | DLC_DGRM                | If NB_Status_Response:  |
   |               |                         |  Routed ARE             |
   |               |                         |  (Specific MAC Address) |
   |               |                         | Else:                   |
   |               |                         |  Routed STE             |
   |               |                         |  (NETBIOS Group Address)|
   +---------------+-------------------------+-------------------------+

   The above actions apply to all DLSw states.  The handling of NetBIOS
   datagrams sent within DGRMFRAME messages is governed by the DLSw
   state machine.  The DGRMFRAME message type is employed instead of the
   DATAFRAME message type once the end-to-end circuit has been
   established. At that time, the message is addressed according to the
   pair of Circuit IDs in the message header instead of relying upon the
   MAC address information in the token ring header.

5.4  Explorer Traffic

   The CANUREACH_ex, ICANREACH_ex, NETBIOS_NQ_ex, and NETBIOS_NR_ex SSP
   messages explore the topology of the DLSw cloud and the networks
   attached to it.  These explorer frames are used to determine the DLSw
   partners through which a MAC or NetBIOS name can be accessed.  This
   information may optionally be cached to reduce explorer traffic in
   the DLSw cloud.



Wells & Bartky                                                 [Page 40]

RFC 1795                  Data Link Switching                 April 1995


   If a DLSw is aware from cached information that a given MAC address
   or NetBIOS name is accessible through a given partner DLSw, it should
   direct all circuit setup attempts to that partner.  If the circuit
   setup fails, or no such data is available in the MAC or name cache
   database, the DLSw may fallback to issuing the setup attempt to all
   DLSw partners on the assumption that the cached data is now out of
   date.  The mechanism for determining when to use such a fallback is
   implementation defined.

   DLSw implementations may also use a local MAC cache to enable
   responses to CANUREACH_ex requests to be issued without the need for
   TEST frame exchange (or equivalent) until the CANUREACH_cs is
   received.  Again, the fallback mechanism for determining when such
   local cache data is out-of-date is implementation defined.

   The use of either cache is an optional function in DLSw.  An
   implementation may choose to always issue explorer frames or to use
   either or both types of cache.

   The following sections describe the FSMs used for explorer frames.
   The DLC events and actions are a subset of those described in section
   5.2 for the main circuit FSM.

5.4.1  CANUREACH/ICANREACH Explorer FSM

   The FSM described below is used to handle explorer frames routed by
   MAC address.  There is one instance of this FSM for each Data Link ID
   (Target and Origin MAC/SAP pair) for which explorer traffic is
   flowing. The states in this FSM are as follows.

   State Name            Description
   ----------            -----------
   RESET                 The initial state.

   SENT_EX               Local DLSw has issued an explorer message

   RECEIVED_EX           Local DLSw has received an explorer message














Wells & Bartky                                                 [Page 41]

RFC 1795                  Data Link Switching                 April 1995


5.4.1.1  RESET State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_ex | If replying from    | If DLC_RESOLVE sent, |
   |                      | cache, send         |   RECEIVED_EX        |
   |                      | ICANREACH_ex        |                      |
   |                      | else if allowed to  |                      |
   |                      | test availability,  |                      |
   |                      | issue DLC_RESOLVE.  |                      |
   |                      | Optionally update   |                      |
   |                      | cache.              |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_ex | Optionally update   | RESET                |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESOLVE_C        | Send CANUREACH_ex   | SENT_EX              |
   +----------------------+---------------------+----------------------+

   RESET is the initial state for the CANUREACH/ICANREACH explorer FSM.
   This state is exited when a DLC_RESOLVE_C request is received from
   the DLC or a CANUREACH_ex is received from a remote DLSw.

   A DLSw implementation may optionally reply from to CANUREACH_ex
   messages on the basis of cached topology information, in which case
   the DLC_RESOLVE exchange (i.e., TEST) is not required.  If cache is
   not used, or no match is found, and the DLC permits the use of TEST,
   DLC_RESOLVE is issued to locate the target MAC and the state changes
   to RECEIVED_EX. If no cache entry is available and TEST is not
   allowed by the DLC, a received CANUREACH_ex frame is ignored.

5.4.1.2  SENT_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_ex | DLC_RESOLVE_R       | RESET                |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_RESOLVE_C        |                     | SENT_EX              |
   +----------------------+---------------------+----------------------+

   SENT_EX is entered when the DLSw has issued a CANUREACH_ex message to
   locate a MAC address.  This state is exited when a remote DLSw
   returns a matching ICANREACH_ex, or after an implementation defined
   timeout. DLC_RESOLVE events received in this state correspond to TEST



Wells & Bartky                                                 [Page 42]

RFC 1795                  Data Link Switching                 April 1995


   retries by the origin DLC station and are absorbed.

   An implementation may choose whether to handle explorer frame
   crossover either by using entirely separate FSM instances and simply
   allowing both ends to issue TEST frames, or by detecting a reverse
   CANUREACH_ex frame here and issuing an ICANREACH_ex message and
   DLC_RESOLVE_R action.

5.4.1.3  RECEIVED_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive CANUREACH_ex | Optionally update   | RECEIVED_EX          |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | Receive ICANREACH_ex |                     | RECEIVED_EX          |
   +----------------------+---------------------+----------------------+
   | DLC_RESOLVED         | Send ICANREACH_ex   | RESET                |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+

   RECEIVED_EX is entered when the DLSw has received a CANUREACH_ex from
   a remote DLSw and has issued a DLC_RESOLVE to locate the MAC address.
   This state is exited when the DLC_RESOLVED response is received, or
   after an implementation defined timeout.

   If the target MAC is located, the DLSw must reply to the first
   received CANUREACH_ex that caused the move to this state.  If
   additional CANUREACH_ex messages are received in this state from
   other remote DLSw partners, the DLSw may optionally reply to these
   messages too but it is not required to do so.

   An implementation may choose whether to handle explorer frame
   crossover either by using entirely separate FSM instances and simply
   allowing both ends to issue TEST frames, or by detecting such a
   reverse DLC_RESOLVE_C event here and issuing an ICANREACH_ex message
   and DLC_RESOLVE_R action.












Wells & Bartky                                                 [Page 43]

RFC 1795                  Data Link Switching                 April 1995


5.4.2  NETBIOS_NQ/NR Explorer FSM

   The FSM described below is used to handle explorer frames routed by
   NetBIOS names  There is one instance of this FSM for each unique
   combination of Source Name, Destination Name, Data 2 field and
   Response Correlator.

   State Name            Description
   ----------            -----------
   RESET                 The initial state.

   SENT_EX               Local DLSw has issued an explorer
                         message

   RECEIVED_EX           Local DLSw has received an explorer
                         message

   SENT_REC_EX           An explorer frame has been both sent
                         and received for the same (potential)
                         NetBIOS circuit.

5.4.2.1  RESET State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| RECEIVED_EX          |
   |                      | Optionally update   |                      |
   |                      | cache.              |                      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NR_ex| Optionally update   | RESET                |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_EX              |
   +----------------------+---------------------+----------------------+

   The RESET state is the initial state for the NETBIOS_NQ/NR explorer
   FSM. It is exited when the DLC receives either a NETBIOS_NQ_ex or a
   DLC_DGRM containing a NetBIOS NAME_QUERY frame.  If a NETBIOS_NQ_ex
   message is received, the NAME_QUERY is propagated to the DLC and this
   FSM moves to state RECEIVED_EX.  If a NetBIOS NAME_QUERY frame is
   received, the NETBIOS_NQ_ex is propagated either to the appropriate
   DLSw partners (see below), and this FSM moves to state SENT_EX.

   Unlike SNA traffic where the CANUREACH_ex/ICANREACH_ex exchange can
   be omitted if the MAC location is already cached,
   NETBIOS_NQ_ex/NETBIOS_NR_ex frames must always be issued during
   NetBIOS session setup in order that the NetBIOS session numbers are



Wells & Bartky                                                 [Page 44]

RFC 1795                  Data Link Switching                 April 1995


   exchanged correctly between the DLC end stations.  If the location of
   a NetBIOS name is known from cached data, the NETBIOS_NQ_ex need only
   be issued to the cached DLSw partners.  Otherwise the NETBIOS_NQ_ex
   should be issued to all partners that support NetBIOS.

5.4.2.2  SENT_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| SENT_REC_EX          |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NR_ex| DLC_DGRM(NAME_RECOG)| RESET                |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_EX              |
   | (different local     | Optionally update   |                      |
   |  session number than | cache               |                      |
   |  existing searches)  |                     |                      |
   +----------------------+---------------------+----------------------+

   SENT_EX is entered when the local DLSw issues a NETBIOS_NQ_ex to its
   remote DLSw partners.  This state is exited when a NETBIOS_NR_ex is
   received from a remote DLSw, or if a matching NETBIOS_NQ_ex is
   received from a remote DLSw (i.e., a NETBIOS_NQ_ex crossover case).
   If the local NetBIOS end station issues a NAME_QUERY with a different
   session number from any previous NAME_QUERY for this search, the
   NAME_QUERY is propagated to the DLSw partners to ensure that the
   exchange of NetBIOS session numbers is handled correctly.



















Wells & Bartky                                                 [Page 45]

RFC 1795                  Data Link Switching                 April 1995


5.4.2.3  RECEIVED_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| RECEIVED_EX          |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NR_ex|                     | RECEIVED_EX          |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_REC_EX          |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM (NAME_RECOG)| Send NETBIOS_NR_ex  | RESET                |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+

   RECEIVED_EX is entered when the local DLSw receives a NETBIOS_NQ_ex
   message from a remote DLSw.  This state is exited when a
   NAME_RECOGNIZED NetBIOS frame is received from the DLC, completing
   the query, or when a matching NAME_QUERY is received from DLC (i.e.,
   NAME_QUERY crossover).

5.4.2.4  SENT_REC_EX State

   +----------------------+---------------------+----------------------+
   |        Event         |      Action(s)      |      Next State      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NQ_ex| DLC_DGRM(NAME_QUERY)| SENT_REC_EX          |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | Receive NETBIOS_NR_ex| DLC_DGRM(NAME_RECOG)| RECEIVED_EX          |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM (NAME_QUERY)| Send NETBIOS_NQ_ex  | SENT_REC_EX          |
   | (different local     | Optionally update   |                      |
   |  session number than | cache               |                      |
   |  existing searches)  |                     |                      |
   +----------------------+---------------------+----------------------+
   | DLC_DGRM (NAME_RECOG)| Send NETBIOS_NR_ex  | SENT_EX              |
   |                      | Optionally update   |                      |
   |                      | cache               |                      |
   +----------------------+---------------------+----------------------+



Wells & Bartky                                                 [Page 46]

RFC 1795                  Data Link Switching                 April 1995


   This state is required if an implementation wishes to manage NQ/NR
   crossover cases from a single FSM instance by detecting 'opposite'
   NAME_QUERY attempts between the same two NetBIOS names.  If separate
   FSM instances are used instead, this state is not required and the
   transitions to it from other states can be removed.

   SENT_RCV_EX is exited when the NAME_QUERY search in either direction
   is resolved.  If the local NetBIOS end station issues a NAME_QUERY
   with a different session number from any previous NAME_QUERY it has
   issued for this search, the NAME_QUERY is propagated to the DLSw
   partners to ensure that the exchange of NetBIOS session numbers is
   correctly handled.

5.4.2.5  NetBIOS Session Numbers

   NetBIOS NAME_QUERY and NAME_RECOGNIZED frames exchange NetBIOS session
   numbers between the end stations.  For correct NetBIOS operation over
   DLSw, it is important that all SSP NETBIOS_NQ_ex frames received by a
   DLSw cause NetBIOS NAME_QUERY frames to flow on the LAN with the new
   session number from the NETBIOS_NQ_ex.  These frames cannot be replied
   to from a cache of locally available NetBIOS names in the same way that
   MAC addresses and CANUREACH_ex messages can be handled.

   Also, NAME_QUERY messages are normally retried several times on the LAN.
   The generation and absorption of such frames is outside the scope of the
   FSM defined above.

6.  Protocol Flow Diagrams

   The Switch-to-Switch Protocol is used to setup and take down circuits
   between a pair of Data Link Switches.  Once a circuit is established,
   the end stations on the local networks can employ LLC Type 1
   (connectionless UI frames) protocols end-to-end.  In addition, the end
   systems can establish an end-to-end connection for support of LLC Type 2
   (connection oriented I frames) protocols (Type 2 I frames go end-to-end,
   supervisory frames are handled locally).

   The term, Data Link, is used in this document to refer to both a
   "logical data link" when supporting Type 1 LLC services, and a "data
   link connection" when supporting Type 2 LLC services.  In both cases,
   the Data Link is identified by the Data Link ID defined in section 3.2.

   NOTE:  THIS SECTION CONTAINS EXAMPLES ONLY.  IT CANNOT AND DOES NOT SHOW
   ALL POSSIBLE VARIATIONS AND OPTIONS ON PROTOCOL FLOWS FOR SNA/SDLC, SSP,
   AND LLC PROTOCOLS.






Wells & Bartky                                                 [Page 47]

RFC 1795                  Data Link Switching                 April 1995


6.1  Connect Protocols

   The two basic startup flows from a pure FSM perspective are shown below.
   The first flow is a startup involving XIDs and the second is one without
   XIDs.

Flow #1 - DLSw Startup With XIDs
 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                    disconnected

              DLC_RESOLVE_C   CANUREACH_ex
              ----------->    ----------->
              DLC_RESOLVE_R     ICANREACH_ex
               <-----------     <-----------

              DLC_XID         CANUREACH_cs    DLC_START_DL
              ----------->    ----------->    ----------->
              circuit_start                   resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED
                                <-----------    <-----------
          circuit_established                 circuit_pending
                              REACH_ACK
                              ----------->   circuit_established

                              XIDFRAME        DLC_XID
                              ----------->    ----------->

                     DLC_XID        XIDFRAME         DLC_XID
                <-----------    <-----------    <-----------
              DLC_XID         XIDFRAME        DLC_XID
              ----------->    ----------->    ----------->

                 DLC_XIDs       XIDFRAMEs        DLC_XIDs
              <------------>  <------------>  <------------>

              DLC_CONTACTED   CONTACT         DLC_CONTACT
              ----------->    ----------->    ----------->
              connect_pending                 contact_pending





Wells & Bartky                                                 [Page 48]

RFC 1795                  Data Link Switching                 April 1995


                 DLC_CONTACT       CONTACTED    DLC_CONTACTED
                <-----------    <-----------    <-----------
                 connected                       connected

                DLC_INFOs        IFRAMEs        DLC_INFOs
              <------------>  <------------>  <------------>

   Mapping LAN events to the DLC events and actions on Flow #1 produces
   the following flows shown below:

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                    disconnected

TEST_cmd      DLC_RESOLVE_C    CANUREACH_ex               TEST_cmd
----------->  ----------->     ----------->               ---------->
   TEST_rsp   DLC_RESOLVE_R     ICANREACH_ex                 TEST_rsp
 <---------    <-----------   <-----------             <-----------
null XID      DLC_XID          CANUREACH_cs    DLC_START_DL
----------->  ----------->     ----------->    ----------->
              circuit_start                   resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED
                                <-----------    <-------------
           circuit_established                circuit_pending
                              REACH_ACK
                              ----------->  circuit_established

                              XIDFRAME         DLC_XID       null XID
                              ----------->     --------->    -------->
        XID        DLC_XID        XIDFRAME         DLC_XID          XID
  <--------   <-----------    <-----------    <-----------    <--------
    XIDs         DLC_XIDs      XIDFRAMEs        DLC_XIDs         XIDs
<---------->  <---------->  <------------>  <------------>  <--------->
SABME         DLC_CONTACTED   CONTACT         DLC_CONTACT     SABME
----------->  ----------->    ----------->    ----------->    -------->
              connect_pending                 contact_pending

          UA     DLC_CONTACT     CONTACTED    DLC_CONTACTED          UA
  <---------   <-----------   <-----------    <-----------    <--------
                  connected                        connected




Wells & Bartky                                                 [Page 49]

RFC 1795                  Data Link Switching                 April 1995


  IFRAMEs       DLC_INFOs        IFRAMEs        DLC_INFOs       IFRAMEs
<---------->  <----------->  <------------>  <------------>  <-------->

Those implementations that prefer to respond to the SABME immediately
could use the same events to do that:

SABME         DLC_CONTACTED   CONTACT         DLC_CONTACT     SABME
----------->  ----------->    ----------->    ----------->    -------->
          UA  connect_pending                 contact_pending
  <---------
RR
----------->
         RNR
  <---------

          RR    DLC_CONTACT       CONTACTED    DLC_CONTACTED          UA
  <---------   <-----------    <-----------    <-----------    <--------
                 connected                        connected

   IFRAMEs      DLC_INFOs        IFRAMEs        DLC_INFOs      IFRAMEs
<---------->  <------------>  <------------>  <------------>  <-------->

Flow #2 - DLSw Startup Without XIDs (circuit setup)

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                    disconnected

              DLC_CONTACTED   CANUREACH_cs    DLC_START_DL
              ----------->    ----------->    ----------->
              circuit_start                   resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED
                                <-----------    <-----------
          circuit_established                 circuit_pending
                              REACH_ACK
                              ----------->   circuit_established

                              CONTACT         DLC_CONTACT
                              ----------->    ----------->
              connect_pending                 contact_pending




Wells & Bartky                                                 [Page 50]

RFC 1795                  Data Link Switching                 April 1995


                 DLC_CONTACT       CONTACTED    DLC_CONTACTED
                <-----------    <-----------    <-----------
                 connected                       connected

                DLC_INFOs        IFRAMEs        DLC_INFOs
              <------------>  <------------>  <------------>

   Mapping LAN events to the DLC events and actions on Flow #2 (and
   adding a NETBIOS_NQ and NETBIOS_NR_ex) produces:

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                     disconnected

NAME_QUERY    DLC_DGRM        NETBIOS_NQ_ex   DLC_DGRM       NAME_QUERY
----------->  ----------->    ----------->    ----------->   --------->

   NAME_RECOG    DLC_DGRM      NETBIOS_NR_ex     DLC_DGRM    NAME_RECOG
 <-----------  <------------   <-----------    <-----------  <---------

SABME         DLC_CONTACTED   CANUREACH_cs    DLC_START_DL
----------->  ----------->    ----------->    ----------->
               circuit_start                 resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED
                                <-----------    <-----------
            circuit_established                circuit_pending
                              REACH_ACK
                              ----------->   circuit_established

                              CONTACT         DLC_CONTACT     SABME
                              ----------->    ----------->    --------->
             connect_pending                 contact_pending

          UA   DLC_CONTACT       CONTACTED    DLC_CONTACTED           UA
  <---------  <-----------    <-----------    <-----------    <---------
                connected                       connected

   IFRAMEs       DLC_INFOs       IFRAMEs        DLC_INFOs       IFRAMEs
<------------> <------------> <------------>  <------------>  <-------->





Wells & Bartky                                                 [Page 51]

RFC 1795                  Data Link Switching                 April 1995


   In keeping with a paradigm of 'DLSw is a big 802.2 LAN', all other
   DLC types (SDLC for now, QLLC, channel, or whatever in the future)
   would be handled by a 'DLC transformation layer' that would transform
   the specific protocol's events into the appropriate DLSw DLC events
   and DLSw DLC actions into the appropriate protocol actions.  The XIDs
   that flow in the SSP XIDFRAME should stay 802.2ish (i.e., ABM bit
   set) and leave it up to the DLC transformation layer to suit the XID
   to its particular DLC type.

   Here is an example of a leased SDLC PU 2.0 device as the origin
   station. It should use Flow #2 since it is not known if the other
   side is a LAN, a switched line or a leased line.

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                     disconnected

implementer's  DLC_RESOLVE_C   CANUREACH_ex
choice (power  ----------->    ----------->
up, configuration
change,        DLC_RESOLVE_R   ICANREACH_ex
never,          <-----------    <-----------
connect timer,etc.)

PU 2.0 is
configured
in DLSw to    DLC_XID(null)   CANUREACH_cs    DLC_START_DL
call in       ----------->    ----------->    ----------->
              circuit_start                   resolve_pending

                                ICANREACH_cs   DLC_DL_STARTED
                                <-----------   <-----------
           circuit_established                circuit_pending
                                REACH_ACK
                                ----------->   circuit_established

                              XIDFRAME        DLC_XID
                              ----------->    ----------->

                    DLC_XID        XIDFRAME         DLC_XID
respond with   <-----------    <-----------    <-----------
XID configured



Wells & Bartky                                                 [Page 52]

RFC 1795                  Data Link Switching                 April 1995


for station or
forward XID to
station and
send response  DLC_XID        XIDFRAME        DLC_XID
               ----------->   ----------->    ----------->

        SNRM    DLC_CONTACT       CONTACT      DLC_CONTACTED
  <---------   <-----------    <-----------    <------------
              contact_pending                    connect_pending

UA            DLC_CONTACTED    CONTACTED       DLC_CONTACT
---------->    ----------->    ----------->    ----------->
                connected                       connected

   IFRAMEs       DLC_INFOs        IFRAMEs        DLC_INFOs
<----------->  <------------>  <------------>  <------------>

   Here is an example of a switched SDLC PU 2.0 device as the origin
   station.

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                     disconnected

implementer's  DLC_RESOLVE_C   CANUREACH_ex
choice (power  ----------->    ----------->
up, configuration
change,        DLC_RESOLVE_R   ICANREACH_ex
never,          <-----------    <-----------
connect timer,etc.)

XID(null)     DLC_XID(null)   CANUREACH_cs    DLC_START_DL
----------->  ----------->    ----------->    ----------->
              circuit_start                   resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED
                                <-----------    <-----------
            circuit_established                 circuit_pending
                                REACH_ACK
                                ----------->   circuit_established





Wells & Bartky                                                 [Page 53]

RFC 1795                  Data Link Switching                 April 1995


                                XIDFRAME      DLC_XID
                                ----------->  ----------->
         XID        DLC_XID         XIDFRAME         DLC_XID
  <---------   <-----------     <-----------    <-----------
XID           DLC_XID         XIDFRAME        DLC_XID
--------->    ----------->    ----------->    ----------->

        SNRM    DLC_CONTACT       CONTACT      DLC_CONTACTED
  <---------   <-----------    <-----------    <-----------
              contact_pending                 connect_pending

UA            DLC_CONTACTED   CONTACTED       DLC_CONTACT
--------->    ----------->    ----------->    ----------->
                 connected                      connected

   IFRAMEs      DLC_INFOs        IFRAMEs        DLC_INFOs
<---------->  <------------>  <------------>  <------------>

   Here is an example of a leased SDLC PU 2.0 device as the target
   station.

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw       Target
 Station        partner                          partner         Station
                                                                 (SDLC)
              disconnected                    disconnected

              DLC_RESOLVE_C   CANUREACH_ex
              ----------->    ----------->   reply if virtual MAC/SAP
                                             for SDLC station is
                                             configured, if SDLC
                                             station responds to
              DLC_RESOLVE_R    ICANREACH_ex  TEST/SNRM/DISC, etc.
               <-----------    <-----------
              DLC_XID         CANUREACH_cs    DLC_START_DL    SNRM
              ----------->    ----------->    ----------->    --------->
              circuit_start                   resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED        UA
                                <-----------    <-----------    <-------
          circuit_established                 circuit_pending
                                                              RNR
                              REACH_ACK                       --------->
                              ----------->   circuit_established



Wells & Bartky                                                 [Page 54]

RFC 1795                  Data Link Switching                 April 1995


                              XIDFRAME        DLC_XID
                              ----------->    -----------> respond with
                                                           XID configured
                                                           for station
                                                           or forward
                                                           XID to
                                                           station and
                                                           send
                   DLC_XID        XIDFRAME         DLC_XID response
              <-----------    <-----------    <-----------
              DLC_CONTACTED   CONTACT         DLC_CONTACT     RR
              ----------->    ----------->    ----------->    --------->
             connect_pending                contact_pending

                 DLC_CONTACT       CONTACTED    DLC_CONTACTED
                <-----------    <-----------    <-----------
                connected                        connected

                DLC_INFOs        IFRAMEs        DLC_INFOs       IFRAMEs
              <------------>  <------------>  <------------>  <------->

   Here is an example of a switched SDLC PU 2.0 device as the target
   station.

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw       Target
 Station        partner                          partner         Station
                                                                 (SDLC)
              disconnected                    disconnected

              DLC_RESOLVE_C   CANUREACH_ex
              ----------->    ----------->    reply if virtual MAC/SAP
                                              for SDLC station is
                                              configured, if SDLC
                                              station responds to
              DLC_RESOLVE_R     ICANREACH_ex  TEST/XID/SNRM/DISC, etc.
               <-----------     <-----------
              DLC_XID         CANUREACH_cs    DLC_START_DL    XID
              ----------->    ----------->    ----------->    --------->
              circuit_start                   resolve_pending

                                ICANREACH_cs   DLC_DL_STARTED        XID
                                <-----------   <-----------    <--------
          circuit_established                 circuit_pending



Wells & Bartky                                                 [Page 55]

RFC 1795                  Data Link Switching                 April 1995


                              REACH_ACK
                              ----------->   circuit_established

                                XIDFRAME        DLC_XID
                                ----------->    -----------> respond
                                                             with XID
                                                             received
                     DLC_XID        XIDFRAME        DLC_XID  above
                <-----------    <-----------     <---------
             DLC_CONTACTED   CONTACT         DLC_CONTACT     SNRM
             ----------->    ----------->    ----------->    --------->
             connect_pending                  contact_pending

                DLC_CONTACT       CONTACTED    DLC_CONTACTED          UA
               <-----------    <-----------    <-----------    <--------
                connected                        connected

                DLC_INFOs        IFRAMEs        DLC_INFOs       IFRAMEs
              <------------>  <------------>  <------------>  <-------->

   Here is an example of an SDLC T2.1 device as the target station.
   (SDLC T2.1 origin station would look just like the LAN T2.1 origin
   station)

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

              disconnected                    disconnected

              DLC_RESOLVE_C   CANUREACH_ex
              ----------->    ----------->    implementer's choice
                                              (virtual MAC/SAP
                                               configured,
                                               check to see if station
                                               is powered up using
              DLC_RESOLVE_R     ICANREACH_ex   TEST/XID/DISC, etc.)
               <-----------     <-----------
              DLC_XID         CANUREACH_cs    DLC_START_DL    null XID
              ----------->    ----------->    ----------->    --------->
              circuit_start                   resolve_pending

                                ICANREACH_cs    DLC_DL_STARTED       XID
                                <-----------    <-----------    <-------



Wells & Bartky                                                 [Page 56]

RFC 1795                  Data Link Switching                 April 1995


          circuit_established                 circuit_pending
                              REACH_ACK
                              ----------->   circuit_established
                              XIDFRAME        DLC_XID
                              ----------->    ----------->  respond with
                                                            XID received
                     DLC_XID        XIDFRAME        DLC_XID above
                <-----------    <-----------    <----------
                 DLC_XIDs       XIDFRAMEs        DLC_XIDs         XIDs
              <------------>  <------------>  <------------>  <-------->
              DLC_CONTACTED   CONTACT         DLC_CONTACT     SNRM
              ----------->    ----------->    ----------->    --------->
              connect_pending                 contact_pending

                 DLC_CONTACT       CONTACTED    DLC_CONTACTED         UA
                <-----------    <-----------    <-----------    <-------
                connected                        connected

                DLC_INFOs        IFRAMEs        DLC_INFOs       IFRAMEs
              <------------>  <------------>  <------------>  <-------->































Wells & Bartky                                                 [Page 57]

RFC 1795                  Data Link Switching                 April 1995


6.2  Link Restart Protocols

   The following figure depicts the protocol flows that result from
   restarting the end-to-end connection.  This causes the Data Link
   Switches to terminate the existing connection and to enter the
   Circuit Established state awaiting the start of a new connection.

     Data Link   Data Link                     Data Link   Data Link
      Control     Switch                        Switch      Control
     ---------------------                     ---------------------
          +-----------+                             +-----------+
          | Connected |                             | Connected |
    SABME +-----------+                             +-----------+
   ----------->                 RESTART_DL
      DM           ------------------------------------->     DISC
   <-----------                                               -------->
                                                               UA
                         DL_RESTARTED (Case 1)              <--------
                   <-------------------------------------
          +-----------+                             +-----------+
          |Circuit Est|                             |Circuit Est|
          +-----------+                             +-----------+
                        ........... or ...........
    SABME
   ----------->           DL_RESTARTED (Case 2)
       UA          <-------------------------------------
   <-----------                                     +-----------+
                                                    |Circuit Est|
                                CONTACT             +-----------+
      RNR           ------------------------------------>
   <----------

              Figure 5.  DLSw Link Restart Message Protocols

   Upon receipt of a SABME command from the origin station, the origin
   DLSw will send a RESTART_DL message to the target DLSw.  A DM
   response is also returned to the origin station and the data link is
   restarted.

   Upon receipt of the RESTART_DL message, the target DLSw will issue a
   DISC command to the target station.  The target station is expected
   to return a UA response.  The target DLSw will then restart its data
   link and send an DL_RESTARTED message back to the origin DLSw.
   During this exchange of messages, both Data Link Switches change
   states from Connected state to Circuit Established state.

   If the origin station now resends the SABME command, the origin DLSw
   will send a CONTACT message to the target DLSw.  If the SABME command



Wells & Bartky                                                 [Page 58]

RFC 1795                  Data Link Switching                 April 1995


   is received prior to the receipt of the DL_RESTARTED message (case 2
   in the figure), the CONNECT message is delayed until the DL_RESTARTED
   message is received.  The resulting protocol flows at this point
   parallel those given above for the connect sequence.

6.3  Disconnect Protocols

   The following figure depicts the protocol flows that result from the
   end system terminating an existing connection.  Not only is the
   connection terminated, but the circuit between the Data Link Switches
   is taken down.

     Data Link  Data Link                      Data Link  Data Link
      Control    Switch                         Switch     Control
     --------------------                      --------------------
          +-----------+                             +-----------+
          | Connected |                             | Connected |
          +-----------+                             +-----------+
      DISC
   ---------->                  HALT_DL
       UA         ------------------------------------->      DISC
   <----------                                              --------->
                                                               UA
                               DL_HALTED                    <--------
                  <-------------------------------------
          +-----------+                             +-----------+
          |Disconnectd|                             |Disconnectd|
          +-----------+                             +-----------+

                          ......... or ..........

          +-----------+                             +-----------+
          | Connected |                             | Connected |
          +-----------+                             +-----------+
       DISC              TCP Connection Failure               DISC
   <--------     <------------------------------------>    --------->
        UA                                                     UA
    -------->                                               <--------
          +-----------+                             +-----------+
          |Disconnectd|                             |Disconnectd|
          +-----------+                             +-----------+

               Figure 6.  DLSw Disconnect Message Protocols

   Upon receipt of a DISC command from the origin station, the origin
   DLSw will reply with a UA response and issue a HALT_DL message to the
   target DLSw.  Upon receipt of the HALT_DL message, the target DLSw
   will send a DISC command to the target station.  The target station



Wells & Bartky                                                 [Page 59]

RFC 1795                  Data Link Switching                 April 1995


   will then respond with a UA response, causing the target DLSw to
   return a DL_HALTED message to the origin DLSw.  During this exchange
   of messages, both Data Link Switches change states from the Connected
   state to the Disconnected state.

   If the TCP connection between two Data Link Switches fails, all
   connections that are currently multiplexed on the failed TCP
   connection will be taken down.  This implies that both Data Link
   Switches will send DISC commands to all the local systems that are
   associated with the failed connections.  Upon sending the DISC
   command, the Data Link Switch will enter the DISCONNECTED state for
   each circuit.

7.0  Capabilities Exchange Formats/Protocol

   The Data Link Switching Capabilities Exchange is a special DLSw
   Switch-to-Switch control message that describes the capabilities of
   the sending data link switch. This control message is sent after the
   switch-to-switch connection is established and optionally during run
   time if certain operational parameters have changed and need to be
   communicated to the partner switch.

   The actual contents of the Capabilities Exchange is in the data field
   following the SSP message header.  The Capabilities Exchange itself
   is formatted as a single General Data Stream (GDS) Variable with
   multiple type "LT" structured subfields.

   The SSP Message Header has the following fields set for the
   Capabilities Exchange:

   Offset   Field                 Value
   ------   -----                 -----
   0x00     Version Number        0x31
   0x01     Header Length         0x48 (decimal 72)
   0x02     Message Length        same as LL in GDS Variable
   0x14     Message Type          0x20 (CAP_EXCHANGE)
   0x16     Protocol Id           0x42
   0x17     Header Number         0x01
   0x23     Message Type          0x20 (CAP_EXCHANGE)
   0x38     Direction             0x01 for CapEx request
                                  0x02 for CapEx response

   Other fields in the SSP header are not referenced and should be set
   to zero.







Wells & Bartky                                                 [Page 60]

RFC 1795                  Data Link Switching                 April 1995


   The DLSw Capabilities Exchange Request has the following overall
   format:

   +----+----+-----------------+
   | LL | ID | Control Vectors |
   +----+----+-----------------+

   0-1         Length, in binary, of the DLSw Capabilities
               Exchange
               Request GDS Variable.  The value of LL is
               the sum of the length of all fields in the
               GDS Variable (i.e., length of LL + length of ID
               + length of Control Vectors).

   2-3         GDS Id: 0x1520

   4-n         Control Vectors consisting of type LT structured
               subfields (i.e., the DLSw Capabilities Exchange
               Structured Subfields)

   Type LT structured subfields consist of a 1-byte length field (the
   "L"), a 1-byte type field (the "T") and n-bytes of data.  The length
   field includes itself as well as the structured subfield.  The
   structured subfield consists of the type field and data so the length
   is n + 2. This imposes a length restriction of 253 bytes on all data
   contained in a structured subfield.

























Wells & Bartky                                                 [Page 61]

RFC 1795                  Data Link Switching                 April 1995


7.1  Control Vector Id Range

   Control Vector identifiers (i.e., Type) in the range of 0x80 through
   0xCF are reserved for use by the Data Link Switching standard.

   Control Vector identifiers (i.e., Type) in the range of 0xD0 through
   0xFD are used for vendor-specific purposes.

   Currently defined vectors are:

   Vector Description                       Hex Value

   Vendor Id Control Vector                 0x81
   DLSw Version Control Vector              0x82
   Initial Pacing Window Control Vector     0x83
   Version String Control Vector            0x84
   Mac Address Exclusivity Control Vector   0x85
   Supported SAP List Control Vector        0x86
   TCP Connections Control Vector           0x87
   NetBIOS Name Exclusivity Control Vector  0x88
   MAC Address List Control Vector          0x89
   NetBIOS Name List Control Vector         0x8A
   Vendor Context Control Vector            0x8B
   Reserved for future use                  0x8C - 0xCF
   Vendor Specific                          0xD0 - 0xFD

7.2  Control Vector Order and Continuity

   Since their contents can greatly affect the parsing of the
   Capabilities Exchange GDS Variable, the required control vectors must
   occur first and appear in the following order:  Vendor Id, DLSw
   Version Number, Initial Pacing Window, Supported SAP List. The
   remainder of the Control Vectors can occur in any order.

   Control Vectors that can be repeated within the same message (e.g.,
   MAC Address List Control Vector and NetBIOS Name List Control Vector)
   are not necessarily adjacent.  It is advisable, but not required, to
   have the Exclusivity Control Vector occur prior to either of the
   above two vectors so that the use of the individual MAC addresses or
   NetBIOS names will be known prior to parsing them.

   Both the Vendor Context and Vendor Specific control vectors can be
   repeated.  If there are multiple instances of the Vendor Context
   control vector, the specified context remains in effect for all
   Vendor Specific control vectors until the next Vendor Context control
   vector is encountered in the Capabilities Exchange.





Wells & Bartky                                                 [Page 62]

RFC 1795                  Data Link Switching                 April 1995


7.3  Initial Capabilities Exchange

   Capabilities exchange is always the first SSP message sent on a new
   SSP connection between two DLSw switches.  This initial Capabilities
   Exchange is used to identify the DLSw version that each switch is
   running and other required information, plus details of any optional
   extensions that the switches are capable of supporting.

   If a DLSw receives an initial capabilities message that is
   incorrectly formatted or contains invalid or unsupported data that
   prevents correct interoperation with the partner DLSw, it should
   issue a Capabilities Exchange negative response.

   If a DLSw receives a negative response to its initial capabilities
   message, it should take down its TCP connections with the offended
   partner.

   Note:  Pre v1.0 DLSw implementations do not send or respond to
   capabilities messages and can be identified by the lack of
   capabilities exchange as the first message on a new SSP connnection.
   This document does not attempt to specify how to interoperate with
   back-level DLSw implementations.

7.4  Run-Time Capabilities Exchange

   Capabilities exchange always occurs when the SSP connection is
   started between two DLSw switches.  Capabilities Exchange can also
   occur at run-time, typically when a configuration change is made.

   Support for run-time Capabilities Exchange is optional.  If a node
   does not support receiving/using Run-Time Capabilities Exchange and
   receives one, it should discard it quietly (not send back a negative
   response).  If a node supports receipt of run-time capabilities, it
   should send a positive or negative response as appropriate.  The
   receiver of a negative response to a run-time capabilities message is
   not required to take down its TCP connections with the offended
   partner.

   Run-time Capabilities Exchange can consist of one or more of the
   following control vectors.  Note that the control vectors required at
   start-up are not present in a run-time Capabilities Exchange.










Wells & Bartky                                                 [Page 63]

RFC 1795                  Data Link Switching                 April 1995


        1. MAC Address Exclusivity CV,
        2. NetBIOS Name Exclusivity CV,
        3. MAC Address List CV,
        4. NetBIOS Name List CV,
        5. Supported SAP List CV,
        6. Vendor Context CV,
        7. Vendor Specific CVs

   A run-time capabilities exchange is a replacement operation.  As
   such, all pertinent MAC addresses and NetBIOS names must be specified
   in the run-time exchange. In addition, run-time changes in
   capabilities will not effect existing link station circuits.

7.5  Capabilities Exchange Filtering Responsibilities

   Recipients of the SAP, MAC, and NetBIOS lists are not required to
   actually use them to filter traffic, etc., either initially or at
   run-time.

7.6  DLSw Capabilities Exchange Structured Subfields

   The Capabilities Exchange Subfields are listed in the table below and
   are described in the following sections:

         Required                      Allowed @
    ID   @ Startup  Length  Repeatable* Runtime  Order  Content
   ====  =========  ======  ==========  =======  =====  ===============
   0x81     Y        0x05        N         N       1    Vendor ID

   0x82     Y        0x04        N         N       2    DLSw Version

   0x83     Y        0x04        N         N       3    Initial pacing
                                                        window

   0x84     N      >=0x02        N         N       5+   Version String

   0x85     N        0x03        N         Y       5+   MAC Address
                                                        Exclusivity

   0x86     Y        0x12        N         Y       4    Supported SAP
                                                        List

   0x87     N        0x03        N         N       5+   TCP Connections

   0x88     N        0x03        N         Y       5+   NetBIOS Name
                                                        Exclusivity





Wells & Bartky                                                 [Page 64]

RFC 1795                  Data Link Switching                 April 1995


   0x89     N        0x0E        Y         Y       5+   MAC Address
                                                        List

   0x8A     N      <=0x13        Y         Y       5+   NetBIOS Name
                                                        List

   0x8B     N        0x05        Y         Y       5+   Vendor Context

   0xD0     N       varies       Y         Y       5+   Vendor Specific

   *Note: "Repeatable" means a Control Vector is repeatable within a single
   message.

7.6.1  Vendor Id (0x81) Control Vector

   The Vendor Id control vector identifies the manufacturer's IEEE
   assigned Organizationally Unique Identifier (OUI) of the Data Link
   Switch sending the DLSw Capabilities Exchange.  The OUI is sent in
   non-canonical (Token-Ring) format.  This control vector is required
   and must be the first control vector.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x05   Length of the Vendor Id structured
                          subfield

      1       1    0x81   key = 0x81  that identifies this as the
                          Vendor Id structured subfield

     2-4      3           the 3-byte Organizationally Unique
                          Identifier (OUI) for the vendor
                          (non-canonical format)

7.6.2  DLSw Version (0x82) Control Vector

   The DLSw Version control vector identifies the particular version of
   the DLSw standard supported by the sending Data Link Switch.  This
   control vector is required and must follow the Vendor Id Control
   Vector.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x04   Length of the Version String structured
                          subfield

      1       1    0x82   key = 0x82  that identifies this as the
                          DLSw Version structured subfield




Wells & Bartky                                                 [Page 65]

RFC 1795                  Data Link Switching                 April 1995


      2       1           the hexadecimal value representing the
                          DLSw standard Version number of the
                          sending Data Link Switch.
                            0x01 (indicates version 1 - closed pages)

      3       1           the hexadecimal value representing the
                          DLSw standard Release number of the
                          sending Data Link Switch.
                            0x00 (indicates release 0)

7.6.3  Initial Pacing Window (0x83) Control Vector

   The Initial Pacing Window control vector specifies the initial value
   of the receive pacing window size for the sending Data Link Switch.
   This control vector is required and must follow the DLSw Version
   Control Vector.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x04   Length of the Initial Pacing Window
                          structured subfield

      1       1    0x83   key = 0x83  that identifies this
                          as the Initial Pacing Window
                          structured subfield

     2-3      2           the pacing window size, specified
                          in byte normal form..

   Note:  The pacing window size must be non-zero.

7.6.4  Version String (0x84) Control Vector

   The Version String control vector identifies the particular version
   number of the sending Data Link Switch.  The format of the actual
   version string is vendor-defined.  This control vector is optional.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0xn    Length of the Version String
                          structured subfield

      1       1    0x84   key = 0x84  that identifies
                          this as the Version String
                          structured subfield






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     2-n     n-2          the ASCII string that identifies
                          the software version for the
                          sending DLSw.

7.6.5  MAC Address Exclusivity (0x85) Control Vector

   The MAC Address Exclusivity control vector identifies how the MAC
   Address List control vector data is to be interpreted.  Specifically,
   this control vector identifies whether the MAC addresses in the MAC
   Address List control vectors are the only ones accessible via the
   sending Data Link Switch.

   If a MAC Address List control vector is specified and the MAC Address
   Exclusivity control vector is missing, then the MAC addresses are not
   assumed to be the only ones accessible via this switch.

   A node may specify that it supports no local MAC addresses by
   including in its capabilities the MAC Address List Exclusivity CV
   (with byte 2 == 0x01), and not including any instances of the MAC
   Address List CV.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x03   Length of the Exclusivity structured
                          subfield

      1       1    0x85   key = 0x85 that identifies this as the
                          MAC address Exclusivity structured
                          subfield

      2       1           an indicator of the relationship of the
                          MAC addresses to the sending Data Link
                          Switch.
                            0x00     the MAC addresses specified in
                                     this Capabilities Exchange
                                     can be accessed via this
                                     switch but are not the
                                     exclusive set (i.e., other
                                     entities are accessible in
                                     addition to the ones specified)
                            0x01     the MAC addresses specified in
                                     this Capabilities Exchange
                                     are the only ones accessible
                                     via this switch.







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7.6.6  SAP List Support (0x86) Control Vector

   The SAP List Support control vector identifies support for Logical
   Link Control SAPs (DSAPs and SSAPs) by the sending Data Link Switch.
   This is used by the DLSw that sent the SAP List Support control
   vector to indicate which SAPs can be used to support SNA and
   optionally NetBIOS traffic.  This may be used by the DLSw that
   receives the SAP list to filter explorer traffic (TEST, XID, or
   NetBIOS UI frames) from the DLSw state machine.  For SNA, a DLSw
   should set bits for all SAP values (SSAP or DSAP) that may be used
   for SNA traffic.  For NetBIOS support, the bit for SAP 0xF0 should be
   set (if not supported then the same bit should be cleared).

   Each bit in the SAP control vector data field represents a SAP as
   defined below.  This vector is required and must follow the Initial
   Pacing Window Control Vector.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1     0x12  Length of the Supported SAP List structured
                          subfield

      1       1     0x86  key = 0x86 that identifies this as the
                          Supported SAP List structured subfield

     2-17    16           the 16-byte bit vector describing all
                          even numbered SAPs enabled.

                          Each Bit within the 16 byte bit vector will
                          indicate whether an even numbered SAP is
                          enabled (b'1') or disabled (b'0').

                          Each Byte within the 16 byte bit vector
                          will be numbered from 0 - F. (Most
                          significant byte first).

                          Byte 0   1   2   3   ...   F
                               XX  XX  XX  XX  ...   XX

                          The bits in each byte indicate whether an
                          even numbered SAP is enabled (b'1') or
                          disabled (b'0'). (Most significant bit first)

                          Bits 7   6   5   4   ...   0
                          SAP  0   2   4   6   ...   E

                          By combining the byte label with the enabled
                          bits, all supported SAPs can be determined.



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                          In the following diagram, 'n' would equal 0
                          through F depending on which byte was being
                          interpreted.

                          Bit ordering is shown below with bit
                          7 being the most significant bit and bit
                          0 the least significant bit.

                          7654 3210
                          bbbb bbbb....
                          |||| ||||
                          |||| |||SAP 0xnE enabled or not
                          |||| |||
                          |||| ||SAP 0xnC enabled or not
                          |||| ||
                          |||| |SAP 0xnA enabled or not
                          |||| |
                          |||| SAP 0xn8 enabled or not
                          ||||
                          |||SAP 0xn6 enabled or not
                          |||
                          ||SAP 0xn4 enabled or not
                          ||
                          |SAP 0xn2 enabled or not
                          |
                          SAP 0xn0 enabled or not

























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   An example of using all User Definable SAPs of 0x04 to 0xEC for SNA
   Data Link Switching and SAP 0xF0 for NetBIOS Data Link Switching
   would be as follows:

   Offset  SAPs          Binary       Hex

   0       4,8,C         0010 1010    0x2A
   1       10,14,18,1C   1010 1010    0xAA
   2       20,24,28,2C   1010 1010    0xAA
   3       30,34,38,3C   1010 1010    0xAA
   4       40,44,48,4C   1010 1010    0xAA
   5       50,54,58,5C   1010 1010    0xAA
   6       60,64,68,6C   1010 1010    0xAA
   7       70,74,78,7C   1010 1010    0xAA
   8       80,84,88,8C   1010 1010    0xAA
   9       90,94,98,9C   1010 1010    0xAA
   A       A0,A4,A8,AC   1010 1010    0xAA
   B       B0,B4,B8,BC   1010 1010    0xAA
   C       C0,C4,C8,CC   1010 1010    0xAA
   D       D0,D4,D8,DC   1010 1010    0xAA
   E       E0,E4,E8,EC   1010 1010    0xAA
   F       F0            1000 0000    0x80

7.6.7  TCP Connections (0x87) Control Vector

   The TCP Connections control vector indicates the support of an
   alternate number of TCP Connections for the Data Link Switching
   traffic.  The base implementation of Data Link Switching supports two
   TCP Connections, one for each direction of data traffic.

   This control vector is optional.  If it is omitted in a DLSw
   Capabilities Exchange, then two TCP Connections are assumed.  It is
   further assumed that if a Data Link  Switch can support one TCP
   Connection, it can support two TCP Connections.

   If TCP Connections CV values agree and the number of connections is
   one, then the  DLSw with the higher IP address must tear down the TCP
   connections on its local port 2065.

   The format of the TCP Connections Control Vector is shown below:

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x03   Length of the TCP Connections structured
                          subfield

      1       1    0x87   key = 0x87  that identifies this as the
                          TCP Connections structured subfield



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      2       1           an indicator of the support for an
                          alternate number of TCP Connections by
                          the sending Data Link Switch.
                            0x01      the number of TCP Connections
                                      may be brought down to one
                                      after Capabilities Exchange
                                      is completed.
                            0x02      the number of TCP Connections
                                      will remain at two for
                                      the duration of the DLSw
                                      connection.

7.6.8  NetBIOS Name Exclusivity (0x88) Control Vector

   The NetBIOS Name Exclusivity control vector identifies how the
   NetBIOS Name List control vector data is to be interpreted.
   Specifically, this control vector identifies whether the NetBIOS
   Names in the NetBIOS Name List control vectors are the only ones
   accessible via the sending Data Link Switch.

   If a NetBIOS Name List control vector is specified and the NetBIOS
   Name Exclusivity control vector is missing, then the NetBIOS Names
   are not assumed to be the only  ones accessible via this switch.

   A node may specify that it supports no local NetBIOS names by
   including in its  capabilities the NetBIOS Name List Exclusivity CV
   (with byte 2 == 0x01), and not including any instances of the NetBIOS
   Name List CV.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x03   Length of the Exclusivity structured
                          subfield

      1       1    0x88   key = 0x88 that identifies this as the
                          NetBIOS Name Exclusivity structured
                          subfield

      2       1           an indicator of the relationship of the
                          NetBIOS Names to the sending Data Link
                          Switch.
                            0x00     the NetBIOS Names specified in
                                     this Capabilities Exchange
                                     can be accessed via this
                                     switch but are not the
                                     exclusive set (i.e., other
                                     entities are accessible in
                                     addition to the ones specified)



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                            0x01     the NetBIOS Names specified in
                                     this Capabilities Exchange
                                     are the only ones accessible
                                     via this switch.

7.6.9  MAC Address List (0x89) Control Vector

   The MAC Address List control vector identifies one or more MAC
   addresses that are accessible through the sending Data Link Switch.
   This control vector specifies a single MAC address value and MAC
   address mask value to identify the MAC address or range of MAC
   addresses.  MAC addresses and masks are in non-canonical (Token-Ring)
   format in this control vector.

   This control vector is optional and can be repeated if necessary.

   Note 1: If a particular MAC address, <mac-addr>, satisfies the
   following algorithm, then <mac-addr> is assumed to be accessible via
   the sending Data Link Switch:

   <mac-addr> & <mac-addr-mask> == <mac-addr-value>

   where:  <mac-addr-value> is the MAC Address
                            Value specified in
                            this control vector

           <mac-addr-mask>  is the MAC Address
                            Mask specified in
                            this control vector

   Note 2:  If an individual MAC Address is desired, then <mac-addr-
   value> should be the individual MAC address and <mac-addr-mask>
   should be 0xFFFFFFFFFFFF.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0x0E   Length of the MAC Address List
                          structured subfield

      1       1    0x89   key = 0x89  that identifies this as the
                          MAC Address List structured subfield

     2-7      6           the 6-byte MAC Address Value,
                          <mac-addr-value> in the above formula

     8-13     6           the 6-byte MAC Address Mask,
                          <mac-addr-mask> in the above formula




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7.6.10  NetBIOS Name List (0x8A) Control Vector

   The NetBIOS Name List control vector identifies one or more NetBIOS
   names that are accessible through the sending Data Link Switch.  This
   control vector specifies a single NetBIOS name in ASCII.  However,
   the NetBIOS name can consist of "don't care" and "wildcard"
   characters to match on a number of NetBIOS names.  If an individual
   character position in the NetBIOS name in this control vector
   contains a '?', then the corresponding character position in real
   NetBIOS name is a "don't care".  If a NetBIOS name in this control
   vector ends in '*', then the remainder of real NetBIOS names is a
   "don't care".  '*' is only considered a wildcard if it appears at the
   end of a name.

   All blanks or nulls at the end of NetBIOS names in this control
   vector are ignored.   NetBIOS names which have fewer than 16 bytes
   and which do not end with  '*' are not assumed to have a trailing
   '*'; the "wildcard" character must be explicit.

   NetBIOS group names can exist across several LANs/networks.  As such,
   NetBIOS  group names received in a NetBIOS Name List Control Vector
   can not be treated the same as NetBIOS individual names.  The
   Individual/Group Flag allows Data  Link Switches to distinguish
   between the two.

   This control vector is optional and can be repeated if necessary.

   Offset  Length  Value  Contents
   ------  ------  -----  --------
      0       1    0xn    Length of the NetBIOS Name List
                          structured subfield (maximum = 0x13)

      1       1    0x8A   key = 0x8A  that identifies this as the
                          NetBIOS Name List structured subfield

      2       1           Individual/Group Flag
                            0x00 - Individual NetBIOS Name
                            0x01 - Group NetBIOS Name

     3-n     n-3          the NetBIOS name with possible embedded
                          '?' and terminating '*'.

7.6.11  Vendor Context (0x8B) Control Vector

   The Vendor Context control vector identifies the manufacturer's IEEE
   assigned Organizationally Unique Identifier (OUI) of the Data Link
   Switch sending the DLSw Capabilities Exchange.  The OUI is sent in
   non-canonical (Token-Ring) format.



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   This control vector is optional and is used to provide the context
   for any Vendor Specific control vectors that follow in the
   Capabilities Exchange.  If there are multiple instances of the Vendor
   Context control vector, the specified context remains in effect for
   all Vendor Specific control vectors until the next Vendor Context
   control vector is encountered.

      Offset  Length  Value  Contents
      ------  ------  -----  --------
         0       1    0x05   Length of the Vendor Context structured
                             subfield

         1       1    0x8B   key = 0x8B  that identifies this as the
                             Vendor Context structured subfield

        2-4      3           the 3-byte Organizationally Unique
                             Identifier (OUI) for the vendor
                             (non-canonical format)

7.7  Capabilities Exchange Responses

   There are two kinds of DLSw Capabilities Exchange Responses: positive
   and negative.  A positive response is returned to the sending Data
   Link Switch if there were no errors encountered in the DLSw
   Capabilities Exchange Request.  A negative response is returned if
   there is at least one error encountered.

   A positive DLSw Capabilities Exchange Response has the following
   overall format:

   +----+----+
   | LL | ID |
   +----+----+

   0-1    Length, in binary, of the DLSw Capabilities
          Exchange Response GDS Variable.  The value of
          LL in this case is 0x0004.

   2-3    GDS Id: 0x1521

   A negative DLSw Capabilities Exchange Response has the following
   overall format:

   +----+----+--------+--------+
   | LL | ID | Offset | Reason |
   +----+----+--------+--------+





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   0-1    Length, in binary, of the DLSw Capabilities Exchange
          Response GDS Variable.  The value of LL is the sum of
          the length of all fields in the GDS Variable (i.e.,
          length of LL + length of ID + length of Offsets/Reasons).

   2-3    GDS Id: 0x1522

   4-5    Offset into the DLSw Capabilities Exchange Request of the
          error.  Offset should always point to the start of the
          GDS Variable or a specific control vector.

   6-7    Reason code that uniquely identifies the error.  Specific
          values for the reason code are:

            0x0001        invalid GDS length for a DLSw Capabilities
                          Exchange Request.  (The value of Offset
                          is ignored.)

            0x0002        invalid GDS id for a DLSw Capabilities
                          Exchange Request.  (The value of Offset
                          is ignored.)

            0x0003        Vendor Id control vector is missing.  (The
                          value of Offset is ignored.)

            0x0004        DLSw Version control vector is missing. (The
                          value of Offset is ignored.)

            0x0005        Initial Pacing Window control vector is
                          missing.  (The value of Offset is ignored.)

            0x0006        length of control vectors doesn't correlate
                          to the length of the GDS variable

            0x0007        invalid control vector id

            0x0008        length of control vector invalid

            0x0009        invalid control vector data value

            0x000A        duplicate control vector (for non-repeating
                          control vectors)

            0x000B        out-of-sequence control vector (for
                          repeating control vector)

            0x000C        DLSw Supported SAP List control vector is
                          missing.



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                          (The value of Offset is ignored.)

   Note:  Multiple Offset, Reason pairs can be returned with one pair
   for each error encountered.

8.  Pacing/Flow Control

   This section describes the required Pacing and Flow Control
   mechanisms used by a Data Link Switch.

   While it is beyond the scope of this document to specify a policy for
   how an implementation maps SSP flow control to the native data link
   flow control at the edges, the following paragraphs describe a
   general philosophical overview of how the mechanism is to be applied.

   There are two types of flows which are covered by the flow control
   mechanism: connection-oriented and connectionless.  In the first,
   connection-oriented flows, the implementer is to map the native flow
   control mechanism of the two data links at the boundaries to the SSP
   flow control mechanism thus presenting an end-to-end flow control
   mechanism which "pushes back" all the way to the originating station
   in either direction.

   However, in the case of connectionless traffic, this is not possible
   at the data link level because there is no native flow control
   mechanism for connectionless data links.  At first glance it is
   tempting to allow connectionless traffic to flow the DLSw cloud
   unthrottled.  However, the rationale for subjecting these flows to
   flow control within the DLSw cloud is to "push" the discarding of
   frames (should this become necessary) back to the ingress of the DLSw
   cloud.  This "early discarding" of excessive DATAGRAMs should allow
   the cloud to remain deterministic without wasting network bandwidth.

8.1  Basic Overview

   Each circuit consists of two data flows, one in each direction.  Each
   data flow has its own independent flow control mechanism.  For each
   data flow there is an entity that originates traffic, referred to as
   the sender, and a target entity which receives the traffic, referred
   to as the receiver.

   A sender may only send data when its receiver has granted explicit
   permission to send a discrete number of data units.  Data units are
   defined as either a DGRMFRAME or an INFOFRAME.

   The receiver grants permission to send data units by sending a Flow
   Control Indicator (FCIND- defined later).  The sender must
   acknowledge all FCINDs by sending a Flow Control Acknowledgment



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   (FCACK- defined later).

   A sending implementation must maintain these values:

   1. GrantedUnits - The number of units (frames) which the sender
      currently has permission to send.

   2. CurrentWindow - This is a discrete number of units, controlled by
      the receiver, which is basis for granting additional units.

   3. InitialWindowSize - Global for all circuits on a transport
      connection.  Learned in capabilities exchange when the transport
      connection is established.  It specifies an initial value for
      CurrentWindow when each circuit is established.

   A receiving implementation must maintain these values:

   1. CurrentWindow - This is a discrete number of units, controlled by
      the receiver, which is basis for granting additional units.

   2. InitialWindowSize - Global for all circuits on a transport
      connection.  Sent in capabilities exchange when the transport
      connection is established.  It specifies an initial value for
      CurrentWindow when each circuit is established.

   3. FCACKOwed - The sender owes an FCACK.  If true, no FCIND may be
      sent.

8.2  Frame Format

   The Flow control Byte is contained at offset 15 in both the
   Information and Control SSP messages.  From a flow control
   perspective, the flow control information in the two frames are
   handled identically.

   The following diagram describes the format of the Flow Control Byte
   (Bit 7 is the most significant and Bit 0 is the Least significant bit
   of the octet):

      bit   7   6   5   4   3   2   1   0
          +---+---+---+---+---+---+---+---+
          |FCI|FCA| reserved  |    FCO    |
          +---+---+---+---+---+---+---+---+

      FCI : Flow Control Indicator
      FCA : Flow Control Ack
      FCO : Flow Control Operator Bits




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            000 - Repeat Window Operator
            001 - Increment Window Operator
            010 - Decrement Window Operator
            011 - Reset Window Operator
            100 - Halve Window Operator
            101 - Reserved
            110 - Reserved
            111 - Reserved

   A frame with the FCI bit set is referred to as a Flow Control
   Indication (FCIND).  An FCIND is used to manage the flow in the
   opposite direction of the frame which bears it.

   A frame with the FCA bit set is referred to as a Flow Control
   Acknowledgment (FCACK).  An FCACK is used to manage the flow in the
   same direction of the frame which bears it.

   NOTE:  A frame may be both a FCIND and an FCACK.

   A frame bearing an FCIND or FCACK may also contain data for the flow
   in the direction it is traveling.  In such a frame, the FCIND or
   FCACK are said to be piggy-backed.  A non-piggy-backed FCIND is
   called an Independent Flow Control Indication (IFCIND) and a non-
   piggy-backed FCACK is called an Independent Flow Control
   Acknowledgment (IFCACK). IFCIND and IFCACK messages are sent in a
   Independent Flow Control SSP message (type 0x21).

   NOTE:  A frame may be both an IFCIND and an IFCACK.

   It is desirable to carry information in control messages so as to
   reduce the need to send a flow control only message.  The diagram
   below shows the messages that may carry valid flow control
   information:

 ======                            ___                           ======
 |    |        ---------        __/   \__       ---------        |    |
 |    |      __|  _|_  |__     /   IP    \    __|  _|_  |__      |    |
 ======        |   |   |      <  Network  >     |   |   |        ======
/______\       ---------       \__     __/      ---------       /______\
 Origin       Origin DLSw         \___/        Target DLSw      Target
 Station        partner                          partner        Station

   May have valid
    FCI/FCA/FCO    Data carrying

         N             N          CANUREACH_cs
                                  ----------->
         Y*            N            ICANREACH_cs



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RFC 1795                  Data Link Switching                 April 1995


                                    <-----------
         Y             N          REACH_ACK
                                  ----------->
         Y             Y            XIDFRAMEs
                                  <------------>
         Y             Y            DGRMFRAMEs
                                  <------------>
         Y             N          CONTACT
                                  ----------->
         Y             N               CONTACTED
                                    <-----------
         Y             Y             INFOFRAMEs
                                  <------------>
         Y             N          RESTART_DL
                                  ----------->
         Y             N               DL_RESTARTED
                                    <-----------
         Y             N          CONTACT
                                  ----------->
         Y             N               CONTACTED
                                    <-----------
         N             N          HALT_DL
                                  ----------->
         N             N               DL_HALTED
                                    <-----------

   *Note: ICANREACH_cs cannot carry FCA, as there could not be an
   outstanding FCI.

8.3  Granting Permission to Send Data

   A receiver grants a sender permission to send units of data by
   sending FCIND.  Each FCIND is further qualified by a flow control
   operator, which is encoded in the FCO bits of the FCIND header. With
   one exception (the Reset Window operator) all operators may be either
   piggy-backed or carried in a IFCIND.

   The five flow control operators are outlined below:

8.3.1  Repeat Window Operator

   This operator is processed as follows:

           (CurrentWindow unchanged)
           GrantedUnits += CurrentWindow






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8.3.2  Increment Window Operator

   This operator is processed as follows:

           CurrentWindow++
           GrantedUnits += CurrentWindow

8.3.3  Decrement Window Operator

   This operator is processed as follows:

           CurrentWindow--
           GrantedUnits += CurrentWindow

   NOTE:  This operator may only be sent if CurrentWindow is greater
   than one.

8.3.4  Reset Window Operator

   This operator is processed as follows:

           CurrentWindow = 0;
           GrantedUnits  = 0;

   NOTE:  This operator may only flow on an independent pacing
   indication (may NOT be piggy-backed).

   NOTE:  After sending this operator, the only legal subsequent
   operator is Increment Window.

8.3.5  Halve Window Operator

   This operator shall be processed as follows:

           IF CurrentWindow > 1 THEN
               CurrentWindow = CurrentWindow / 2
           ENDIF
           GrantedUnits += CurrentWindow

   Note:  The divide by two operation is an unsigned integer divide
   (round down) or bit shift right operation.

8.4  Acknowledging a Flow Control Operator

   Each sender must acknowledge each FCIND with an FCACK which is
   piggy-backed on the next frame in the opposite direction in all cases
   except the Reset Window Operator.




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   The receiver may have no more than one unacknowledged FCIND
   outstanding at any time with one exception:  A Reset Window Operator
   may be sent while another FCIND is pending acknowledgment.

   NOTE: The FCI and FCO bits of the FCACK are used independently by the
   flow in the opposite direction

8.4.1  Acknowledging a Reset Window Operator

   Since this operator revokes all previously granted units, the sender
   must acknowledge this FCIND using an IFCACK (Independent Flow Control
   Acknowledgment).  This is the only case where IFCACK is used.

   Should a sender receive a non-reset FCIND followed by a Reset Window
   FCIND before acknowledging the first, it only acknowledges the Reset
   Window.

   NOTE: The FCI and FCO bits on these frames are used independently by
   the flow in the opposite direction.

8.5  Capabilities Exchange Initial Window Size

   When two nodes establish a transport connection, they engage in a
   capabilities exchange (this is a requirement).  Refer to the
   Capabilities Exchange section 7 for further details.  The two nodes
   are required to exchange the following parameter:

   InitialWindowSize -  This indicates to the partner what
                        the sending flow entity initializes
                        its CurrentWindow value to for each
                        multiplexed circuit subsequently
                        established on that transport
                        connection.  This value must be
                        non-zero.

8.6  Circuit Startup

   Process as follows:

          CurrentWindow = InitialWindowSize
          GrantedUnits  = 0

   NOTE: The InitialWindow Size variable has a scope of one per DLSw
   transport connection, while CurrentWindow and Granted units are
   maintained on a per circuit basis.  At circuit startup, a sender may
   not send data units until the receiver grants explicit permission
   with an FCIND message.  This grant may be an independent FCIND
   message or the FCIND may be piggy-backed on any of the message types



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   listed in section 8.2.

8.7  Example Receiving Implementations

   The following two examples illustrate receiving implementations of
   varying degrees of complexity.  These are not meant to be complete
   implementations but rather serve to illustrate the protocol.

   NOTE: The examples are independent of the buffering model ( buffers
   may be deterministicly or statistically committed)

   NOTE: The examples assume a process model where each event processes
   to completion without being preempted by another event.

8.7.1  Fixed Pacing Example

   Consider the following variables, in addition to InitialWindowSize
   and CurrentWindow and FCACKOwed:

          GrantDelayed     - Boolean
          GrantedUnits     - Outstanding Units

   The following section describes how various events are processed in
   this example implementation:

8.7.1.1  Circuit Startup

          CurrentWindow    = InitialWindowSize
          FCACKOwed        = FALSE
          GrantDelayed     = FALSE
          GrantedUnits     = 0
          Repeat Window Operator

8.7.1.2  Check Buffers Available

   Can my implementation afford to grant CurrentWindow just now?

8.7.1.3  Buffers Become Available

          IF Check Buffers Available THEN
             Send FCIND( Repeat Window)
             GrantDelayed = FALSE
          ELSE
             Wait on buffers to become available (LIFO)
          ENDIF






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8.7.1.4  Repeat Window Operator

          IF Check Buffers Available THEN
              Send FCIND( Repeat Window)
          ELSE
             GrantDelayed = TRUE
             Wait on buffers to become available (FIFO)
          ENDIF

8.7.1.5  Send FCIND( operator)

          GrantedUnits += CurrentWindow
          FCACKOwed     = TRUE
          Encode and Transmit FCIND piggybacked or as IFCIND

8.7.1.6  A Frame Arrives from Sender

          GrantedUnits--;
          IF frame is FCACK THEN
             IF FCACKOwed THEN
                FCACKOwed = FALSE
             ELSE
                Protocol Violation
             ENDIF
          ENDIF
          IF NOT GrantDelayed THEN
             IF GrantedUnits <= CurrentWindow THEN
                 IF FCACKOwed THEN
                   Protocol Violation
                 ELSE
                   Repeat Window Operator
                 ENDIF
             ENDIF
          ENDIF

8.7.2  Adaptive Pacing Example

   The following example illustrates a receiving implementation that
   adjusts the window size and granted units based on buffer
   availability and transport utilization.

   NOTE: This example ignores other factors which might compel the
   receiving implementation to adjust the window size (i.e., Outbound
   queue length, traffic priority, ...)

   Consider the following variables, in addition to InitialWindowSize,
   CurrentWindow and FCACKOwed:




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          GrantDelayed     - Boolean
          GrantedUnits     - Outstanding Units

8.7.2.1  Circuit Startup

          CurrentWindow    = InitialWindowSize
          FCACK            = FALSE
          GrantDelayed     = FALSE
          GrantedUnits     = 0
          Repeat Window Operator

8.7.2.2  Check Buffers Available ( X)

           Can my implementation afford to grant X units just now?

8.7.2.3  Buffers Become Available

          IF Check Buffers Available THEN
             CurrentWindow--;
             Send FCIND( Decrement Window)
             GrantDelayed = FALSE
          ELSE
             Wait on buffers to become available (LIFO)
          ENDIF

8.7.2.4  Repeat Window Operator

          IF Check Buffers Available (CurrentWindow) THEN
              Send FCIND( Repeat Window)
          ELSE
             GrantDelayed = TRUE
             Wait on buffers to become available (FIFO)
          ENDIF

8.7.2.5  Increment Window Operator

          IF Check Buffers Available ( CurrentWindow + 1) THEN
              CurrentWindow++
              Send FCIND( Increment Window)
          ELSE
              Repeat Window Operator
          ENDIF

8.7.2.6  Send FCIND( operator)

          FCACKOwed     = TRUE
          GrantedUnits += CurrentWindow
          Encode and Transmit FCIND piggybacked or as IFCIND



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8.7.2.7  An FCACK Arrives from Sender

          GrantedUnits--;
          IF NOT FCACKOwed THEN
             Protocol Violation
          ENDIF
          FCACKOwed = FALSE;
          IF NOT GrantDelayed THEN
             IF GrantedUnits < CurrentWindow THEN
                 Increment Window Operator
             ELSE IF GrantedUnits == CurrentWindow THEN
                 Repeat Window Operator
             END
          ENDIF

8.7.2.8  A Non-FCACK Frame Arrives from Sender

          GrantedUnits--;
          IF NOT GrantDelayed THEN
             IF FCACKOwed THEN
                IF GrantedUnits < CurrentWindow THEN
                   Protocol Violation
                END
             ELSE
                IF GrantedUnits <= CurrentWindow THEN
                   Repeat Window Operator
                ENDIF
             ENDIF
          ENDIF






















Wells & Bartky                                                 [Page 85]

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8.8  Adaptive Pacing Example Flow Diagrams

8.8.1  Example Flows from the Above Implementation

   The following diagram illustrates the use of adaptive pacing (use of
   Halve Window, and Reset operation are shown in subsequent diagrams).

   -----SENDER-----                     ----RECEIVER----
   Granted   Window                     Window   Granted
     0         2   circuit established    2         0
     2         2   <-------- FCIND(Rpt)   2         2
     1         2   FCACK-------------->   2         1
     4         3   <-------- FCIND(Inc)   3         4
     3         3   FCACK-------------->   3         3
                          +- FCIND(Rpt)   3         6
     2         3   DATA---|----------->   3         5
     1         3   DATA---|----------->   3         4
     4         3   <------+
     3         3   FCACK-------------->   3         3
     6         3   <-------- FCIND(Rpt)   3         6
     5         3   FCACK-------------->   3         5
     4         3   DATA--------------->   3         4
     3         3   DATA--------------->   3         3
                          +- FCIND(Rpt)   3         6
     2         3   DATA---|----------->   3         5
     1         3   DATA---|----------->   3         4
     0         3   DATA---|----------->   3         3
     3         3   <------+
     2         3   FCACK-------------->   3         2
     6         4   <-------- FCIND(Inc)   4         6
     5         4   FCACK-------------->   4         5
     4         4   DATA--------------->   4         4
                                        Waiting on Buffer
                          +- FCIND(Dec)   3         7
     3         4   DATA---|----------->   3         6
     2         4   DATA---|----------->   3         5
     1         4   DATA---|----------->   3         4
     0         4   DATA---|----------->   3         3
     3         3   <------+
     2         3   FCACK-------------->   3         2
                                        Waiting on Buffer
                          +- FCIND(Dec)   2         4
     1         3   DATA---|----------->   2         3
     0         3   DATA---|----------->   2         2
     2         2   <------+
     1         2   FCACK-------------->   2         1
     4         3   <-------- FCIND(Inc)   3         4
     3         3   FCACK-------------->   3         3



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     6         3   <-------- FCIND(Rpt)   3         6
     5         3   FCACK-------------->   3         5
     4         3   DATA--------------->   3         4
     3         3   DATA--------------->   3         3
     6         3   <-------- FCIND(Rpt)   3         6

8.8.2  Example Halve Window Flow

   The following flow illustrates the use of the Halve Window Operator:

      -----SENDER-----                     ----RECEIVER----
      Granted   Window                     Window   Granted
        0         2   circuit established    2         0
        2         2   <-------- FCIND(Rpt)   2         2
        1         2   FCACK-------------->   2         1
        4         3   <-------- FCIND(Inc)   3         4
        3         3   FCACK-------------->   3         3
                                             Resource Shortage
        2         3   DATA--------------->   1         2
        1         3   DATA--------------->   1         1
        0         3   DATA--------------->   1         0
        1         1   <-------- FCIND(Hlv)   1         1
        0         1   FCACK-------------->   1         0

   NOTE: The Halve Window Operator could have been sent before the
   granted units fell to zero.  The implementer may make a choice based
   on the severity of the condition.

8.8.3  Example Reset Window Flows

   The following flow diagram illustrates the ResetWindow operation if
   the receiver has no FCIND outstanding.

   -----SENDER-----                     ----RECEIVER----
   Granted   Window                     Window   Granted
     0         2   circuit established    2         0
     2         2   <-------- FCIND(Rpt)   2         2
     1         2   FCACK-------------->   2         1
     4         3   <-------- FCIND(Inc)   3         4
     3         3   FCACK-------------->   3         3
                          +- FCIND(Rpt)   3         6
     2         3   DATA---|----------->   3         5
     1         3   DATA---|----------->   3         4
     4         3   <------+
     3         3   FCACK-------------->   3         3
     6         3   <-------- FCIND(Rpt)   3         6
     5         3   FCACK-------------->   3         5
                                          Resource shortage!



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     0         0   <-------- FCIND(Rst)   0         5 (note still
   committed)
     0         0   IFCACK------------->   0         0
                                          Condition eases
     1         1   <-------- FCIND(Inc)   1         1
     0         1   FCACK-------------->   1         0
     2         2   <-------- FCIND(Inc)   2         2
     1         2   FCACK-------------->   3         4

   The next two flows  illustrate the Reset Window operation if the
   receiver has an outstanding FCIND.

   -----SENDER-----                     ----RECEIVER----
   Granted   Window                     Window   Granted
     0         2   circuit established    2         0
     2         2   <-------- FCIND(Rpt)   2         2
     1         2   FCACK-------------->   2         1
     4         3   <-------- FCIND(Inc)   3         4
     3         3   FCACK-------------->   3         3
                          +- FCIND(Rpt)   3         6
     2         3   DATA---|----------->   3         5
                          |               Resource shortage!
                          |+-FCIND(Rst)   0         5
     1         3   DATA---||---------->   0         4
     4         3   <------+|
     3         3   FCACK---+---------->   0         3 (Not IFCACK!)
     2         3   DATA----|---------->   0         2
     0         0   <-------+
     0         0   IFCACK------------->   0         0
                                          Condition eases
     1         1   <-------- FCIND(Inc)   1         1
     0         1   FCACK-------------->   1         0
     2         2   <-------- FCIND(Inc)   2         2
     1         2   FCACK-------------->   3         4

   -----SENDER-----                     ----RECEIVER----
   Granted   Window                     Window   Granted
     0         2   circuit established    2         0
     2         2   <-------- FCIND(Rpt)   2         2
     1         2   FCACK-------------->   2         1
     4         3   <-------- FCIND(Inc)   3         4
     3         3   FCACK-------------->   3         3
                          +- FCIND(Rpt)   3         6
     2         3   DATA---|----------->   3         5
                          |               Resource shortage!
                          |+-FCIND(Rst)   0         5
     1         3   DATA---||---------->   0         4
     4         3   <------+|



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     0         0   <-------+
     0         0   IFCACK------------->   0         0
                                          Condition eases
     1         1   <-------- FCIND(Inc)   1         1
     0         1   FCACK-------------->   1         0
     2         2   <-------- FCIND(Inc)   2         2
     1         2   FCACK-------------->   3         4

8.9  Other Considerations

8.9.1  Protocol Violations

   The following events are considered protocol violations:

   1. Sender exceeds granted units or does not acknowledge FCIND on
      first frame after its receipt (the receiver can not discern the
      difference between the two).

   2. Receiver does not follow a Reset Window Operator with an Increment
      Window Operator.

   3. Receiver has two unacknowledged FCINDs ( other than Reset Window)
      outstanding.

   4. Receiver sends Decrement Window Operator with a window size of one.

   5. Receiver attempts to increment the window size beyond 0xFFFF.

   Actions taken in response to protocol violations are left to the
   implementation of the node which discovers the violation.  If an
   implementation chooses to take down the circuit on which the
   violation occurred, HALT_DL is the appropriate action.

Acknowledgments

   Original RFC 1434 Authors:

      Roy C. Dixon, IBM
      David M. Kushi, IBM

   Chair of APPN Implementers Workshop Data Link Switching Related
   Interest Group:

      Louise Herndon Wells, Internetworking Technology Institute







Wells & Bartky                                                 [Page 89]

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   Working Group Chairs (and significant contributors to this document):

      Connect/Disconnect (State Machines): Steve Klein, IBM
      Capabilities Exchange: Wayne Clark, Cisco Systems
      Flow Control (Adaptive Pacing): Shannon Nix, Metaplex
      Priority/Class of Service: Gene Cox, IBM

   Other significant contributors:

      Peter Gayek, IBM
      Paul Brittain, Data Connection Limited

References

   1) ISO 8802-2/IEEE Std 802.2 International Standard, Information
      Processing Systems, Local Area Networks, Part 2: Logical Link
      Control, December 31, 1989.

   2) IBM LAN Technical Reference IEEE 802.2 and NETBIOS Application
      Program Interfaces SC30-3587-00, December 1993.

   3) ISO/IEC DIS 10038 DAM 2, MAC Bridging, Source Routing Supplement,
      December 1991.

   4) ISO 8802-2/IEEE Std 802.1D International Standard, Information
      Processing Systems, Local Area Networks, Part 2: MAC layer
      Bridging.
























Wells & Bartky                                                 [Page 90]

RFC 1795                  Data Link Switching                 April 1995


Security Considerations

   Security issues are not discussed in this memo.

Chair's Address

   Louise Wells
   Internetwork Technology Institute
   2021 Stratford Dr.
   Milpitas, CA  95035

   EMail: lhwells@cup.portal.com

Editor's Address

   Alan K. Bartky
   Manager of Technology
   Sync Research Inc.
   7 Studebaker
   Irvine, CA 91728-2013

   Phone: 1-714-588-2070
   EMail: alan@sync.com


   Note: Any questions or comments relative to the contents of this RFC
   should be sent to the following Internet address:
   aiw-dlsw@networking.raleigh.ibm.com.

   This address will be used to coordinate the handling of responses.

   NOTE 1:  This is a widely subscribed mailing list and messages sent to
            this address will be sent to all members of the DLSw mailing
            list.  For specific questions relating to subscribing to the
            AIW and any of it's working groups send email to:
            appn@vnet.ibm.com

            Information regarding all of the AIW working groups and the
            work they are producing can be obtained by copying, via
            anonymous ftp, the file aiwinfo.psbin or aiwinfo.txt from the
            Internet host networking.raleigh.ibm.com, located in
            directory aiw.

   NOTE 2: These mailing lists and addresses are subject to change.







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