





Network Working Group                                         M. Laubach
Request for Comments: 1577                  Hewlett-Packard Laboratories
Category: Standards Track                                   January 1994


                     Classical IP and ARP over ATM

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo defines an initial application of classical IP and ARP in
   an Asynchronous Transfer Mode (ATM) network environment configured as
   a Logical IP Subnetwork (LIS) as described in Section 3.  This memo
   does not preclude the subsequent development of ATM technology into
   areas other than a LIS; specifically, as single ATM networks grow to
   replace many ethernet local LAN segments and as these networks become
   globally connected, the application of IP and ARP will be treated
   differently.  This memo considers only the application of ATM as a
   direct replacement for the "wires" and local LAN segments connecting
   IP end-stations ("members") and routers operating in the "classical"
   LAN-based paradigm. Issues raised by MAC level bridging and LAN
   emulation are beyond the scope of this paper.

   This memo introduces general ATM technology and nomenclature.
   Readers are encouraged to review the ATM Forum and ITU-TS (formerly
   CCITT) references for more detailed information about ATM
   implementation agreements and standards.

Acknowledgments

   This memo could not have come into being without the critical review
   from Jim Forster of Cisco Systems, Drew Perkins of FORE Systems, and
   Bryan Lyles, Steve Deering, and Berry Kercheval of XEROX PARC.  The
   concepts and models presented in [1], written by Dave Piscitello and
   Joseph Lawrence, laid the structural groundwork for this work. ARP
   [3] written by Dave Plummer and Inverse ARP [12] written by Terry
   Bradley and Caralyn Brown are the foundation of ATMARP presented in
   this memo.  This document could have not been completed without the
   expertise of the IP over ATM Working Group of the IETF and the ad hoc
   PVC committee at the Amsterdam IETF meeting.




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RFC 1577             Classical IP and ARP over ATM          January 1993


1. Conventions

   The following language conventions are used in the items of
   specification in this document:

   o   MUST, SHALL, or MANDATORY -- the item is an absolute requirement
       of the specification.

   o   SHOULD or RECOMMEND -- this item should generally be followed for
       all but exceptional circumstances.

   o   MAY or OPTIONAL -- the item is truly optional and may be followed
       or ignored according to the needs of the implementor.

2.  Introduction

   The goal of this specification is to allow compatible and
   interoperable implementations for transmitting IP datagrams and ATM
   Address Resolution Protocol (ATMARP) requests and replies over ATM
   Adaptation Layer 5 (AAL5)[2,6].

   Note: this memo defines only the operation of IP and address
   resolution over ATM, and is not meant to describe the operation of
   ATM networks. Any reference to virtual connections, permanent virtual
   connections, or switched virtual connections applies only to virtual
   channel connections used to support IP and address resolution over
   ATM, and thus are assumed to be using AAL5.  This memo places no
   restrictions or requirements on virtual connections used for other
   purposes.

   Initial deployment of ATM provides a LAN segment replacement for:

      1)  Local area networks (e.g., Ethernets, Token Rings and FDDI).

      2)  Local-area backbones between existing (non-ATM) LANs.

      3)  Dedicated circuits or frame relay PVCs between IP routers.

   Note: In 1), local IP routers with one or more ATM interfaces will be
   able to connect islands of ATM networks.  In 3), public or private
   ATM Wide Area networks will be used to connect IP routers, which in
   turn may or may not connect to local ATM networks.  ATM WANs and LANs
   may be interconnected.

   Private ATM networks (local or wide area) will use the private ATM
   address structure specified in the ATM Forum UNI specification [9].
   This structure is modeled after the format of an OSI Network Service
   Access Point Address.  A private ATM address uniquely identifies an



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RFC 1577             Classical IP and ARP over ATM          January 1993


   ATM endpoint.  Public networks will use either the address structure
   specified in ITU-TS recommendation E.164 or the private network ATM
   address structure.  An E.164 address uniquely identifies an interface
   to a public network.

   The characteristics and features of ATM networks are different than
   those found in LANs:

   o   ATM provides a Virtual Connection (VC) switched environment. VC
       setup may be done on either a Permanent Virtual Connection (PVC)
       or dynamic Switched Virtual Connection (SVC) basis. SVC call
       management signalling is performed via implementations of the
       Q.93B protocol [7,9].

   o   Data to be passed by a VC is segmented into 53 octet quantities
       called cells (5 octets of ATM header and 48 octets of data).

   o   The function of mapping user Protocol Data Units (PDUs) into the
       information field of the ATM cell and vice versa is performed in
       the ATM Adaptation Layer (AAL).  When a VC is created a specific
       AAL type is associated with the VC.  There are four different AAL
       types, which are referred to individually as "AAL1", "AAL2",
       "AAL3/4", and "AAL5".  (Note: this memo concerns itself with the
       mapping of IP and ATMARP over AAL5 only.  The other AAL types are
       mentioned for introductory purposes only.)  The AAL type is known
       by the VC end points via the call setup mechanism and is not
       carried in the ATM cell header.  For PVCs the AAL type is
       administratively configured at the end points when the Connection
       (circuit) is set up.  For SVCs, the AAL type is communicated
       along the VC path via Q.93B as part of call setup establishment
       and the end points use the signaled information for
       configuration.  ATM switches generally do not care about the AAL
       type of VCs.  The AAL5 format specifies a packet format with a
       maximum size of (64K - 1) octets of user data. Cells for an AAL5
       PDU are transmitted first to last, the last cell indicating the
       end of the PDU.  ATM standards guarantee that on a given VC, cell
       ordering is preserved end-to-end.  NOTE: AAL5 provides a non-
       assured data transfer service - it is up to higher-level
       protocols to provide retransmission.

   o   ATM Forum signalling defines point-to-point and point-to-
       multipoint Connection setup [9].  Multipoint-to-multipoint VCs
       are not yet specified by ITU-TS or ATM Forum.

   o   An ATM Forum ATM endpoint address is either encoded as an NSAP
       Address (NSAPA) or is an E.164 Public-UNI address [9].  In some
       cases, both an ATM endpoint address and an E.164 Public UNI
       address are needed by an ATMARP client to reach another host or



Laubach                                                         [Page 3]

RFC 1577             Classical IP and ARP over ATM          January 1993


       router.  Since the use of ATM endpoint addresses and E.164 public
       UNI addresses by ATMARP are analogous to the use of Ethernet
       addresses, the notion of "hardware address" is extended to
       encompass ATM addresses in the context of ATMARP, even though ATM
       addresses need not have hardware significance.  ATM Forum NSAPAs
       use the same basic format as U.S. GOSIP NSAPAs [11].  Note: ATM
       Forum addresses should not be construed as being U.S. GOSIP
       NSAPAs.  They are not, the administration is different, which
       fields get filled out are different, etc.

   This memo describes the initial deployment of ATM within "classical"
   IP networks as a direct replacement for local area networks
   (ethernets) and for IP links which interconnect routers, either
   within or between administrative domains. The "classical" model here
   refers to the treatment of the ATM host adapter as a networking
   interface to the IP protocol stack operating in a LAN-based paradigm.

   Characteristics of the classical model are:

    o  The same maximum transmission unit (MTU) size is used for all VCs
       in a LIS [2].  (Refer to Section 5.)

    o  Default LLC/SNAP encapsulation of IP packets.

    o  End-to-end IP routing architecture stays the same.

    o  IP addresses are resolved to ATM addresses by use of an ATMARP
       service within the LIS - ATMARPs stay within the LIS.  From a
       client's perspective, the ATMARP architecture stays faithful to
       the basic ARP model presented in [3].

    o  One IP subnet is used for many hosts and routers. Each VC
       directly connects two IP members within the same LIS.

   Future memos will describe the operation of IP over ATM when ATM
   networks become globally deployed and interconnected.

   The deployment of ATM into the Internet community is just beginning
   and will take many years to complete. During the early part of this
   period, we expect deployment to follow traditional IP subnet
   boundaries for the following reasons:

    o  Administrators and managers of IP subnetworks will tend to
       initially follow the same models as they currently have deployed.
       The mindset of the community will change slowly over time as ATM
       increases its coverage and builds its credibility.





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RFC 1577             Classical IP and ARP over ATM          January 1993


    o  Policy administration practices rely on the security, access,
       routing, and filtering capability of IP Internet gateways: i.e.,
       firewalls. ATM will not be allowed to "back-door" around these
       mechanisms until ATM provides better management capability than
       the existing services and practices.

    o  Standards for global IP over ATM will take some time to complete
       and deploy.

   This memo details the treatment of the classical model of IP and
   ATMARP over ATM. This memo does not preclude the subsequent treatment
   of ATM networks within the IP framework as ATM becomes globally
   deployed and interconnected; this will be the subject of future
   documents. This memo does not address issues related to transparent
   data link layer interoperability.

3.  IP Subnetwork Configuration

   In the LIS scenario, each separate administrative entity configures
   its hosts and routers within a closed logical IP subnetwork.  Each
   LIS operates and communicates independently of other LISs on the same
   ATM network. Hosts connected to ATM communicate directly to other
   hosts within the same LIS. Communication to hosts outside of the
   local LIS is provided via an IP router. This router is an ATM
   Endpoint attached to the ATM network that is configured as a member
   of one or more LISs.  This configuration may result in a number of
   disjoint LISs operating over the same ATM network. Hosts of differing
   IP subnets MUST communicate via an intermediate IP router even though
   it may be possible to open a direct VC between the two IP members
   over the ATM network.

   The requirements for IP members  (hosts, routers) operating in an ATM
   LIS configuration are:

   o   All members have the same IP network/subnet number and address
       mask [8].

   o   All members within a LIS are directly connected to the ATM
       network.

   o   All members outside of the LIS are accessed via a router.

   o   All members of a LIS MUST have a mechanism for resolving IP
       addresses to ATM addresses via ATMARP (based on [3]) and vice
       versa via InATMARP (based on [12]) when using SVCs.  Refer to
       Section 6 "Address Resolution" in this memo.





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RFC 1577             Classical IP and ARP over ATM          January 1993


   o   All members of a LIS MUST have a mechanism for resolving VCs to
       IP addresses via InATMARP (based on [12]) when using PVCs.  Refer
       to Section 6 "Address Resolution" in this memo.

   o   All members within a LIS MUST be able to communicate via ATM with
       all other members in the same LIS; i.e., the virtual Connection
       topology underlying the intercommunication among the members is
       fully meshed.

   The following list identifies a set of ATM specific parameters that
   MUST be implemented in each IP station connected to the ATM network:

   o   ATM Hardware Address (atm$ha). The ATM address of the individual
       IP station.

   o   ATMARP Request Address (atm$arp-req). atm$arp-req is the ATM
       address of an individual ATMARP server located within the LIS.
       In an SVC environment, ATMARP requests are sent to this address
       for the resolution of target protocol addresses to target ATM
       addresses.  That server MUST have authoritative responsibility
       for resolving ATMARP requests of all IP members within the LIS.
       Note: if the LIS is operating with PVCs only, then this parameter
       may be set to null and the IP station is not required to send
       ATMARP requests to the ATMARP server.

   It is RECOMMENDED that routers providing LIS functionality over the
   ATM network also support the ability to interconnect multiple LISs.
   Routers that wish to provide interconnection of differing LISs MUST
   be able to support multiple sets of these parameters (one set for
   each connected LIS) and be able to associate each set of parameters
   to a specific IP network/ subnet number. In addition, it is
   RECOMMENDED that a router be able to provide this multiple LIS
   support with a single physical ATM interface that may have one or
   more individual ATM endpoint addresses.  Note: this does not
   necessarily mean different End System Identifiers (ESIs) when NSAPAs
   are used.  The last octet of an NSAPA is the NSAPA Selector (SEL)
   field which can be used to differentiate up to 256 different LISs for
   the same ESI. (Refer to Section 5.1.3.1, "Private Networks" in [9].)

4.  Packet Format

   Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as
   described in [2].  LLC/SNAP encapsulation is the default packet
   format for IP datagrams.

   This memo recognizes that other encapsulation methods may be used
   however, in the absence of other knowledge or agreement, LLC/SNAP
   encapsulation is the default.



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RFC 1577             Classical IP and ARP over ATM          January 1993


   This memo recognizes the future deployment of end-to-end signalling
   within ATM that will allow negotiation of encapsulation method on a
   per-VC basis.  Signalling negotiations are beyond the scope of this
   memo.

5.  MTU Size

   The default MTU size for IP members operating over the ATM network
   SHALL be 9180 octets. The LLC/SNAP header is 8 octets, therefore the
   default ATM AAL5 protocol data unit size is 9188 octets [2].  In
   classical IP subnets, values other than the default can be used if
   and only if all members in the LIS have been configured to use the
   non-default value.

   This memo recognizes the future deployment of end-to-end signalling
   within ATM that will allow negotiation of MTU size on a per-VC basis.
   Signalling negotiations are beyond the scope of this document.

6.  Address Resolution

   Address resolution within an ATM logical IP subnet SHALL make use of
   the ATM Address Resolution Protocol (ATMARP) (based on [3]) and the
   Inverse ATM Address Resolution Protocol (InATMARP) (based on [12]) as
   defined in this memo.  ATMARP is the same protocol as the ARP
   protocol presented in [3] with extensions needed to support ARP in a
   unicast server ATM environment.  InATMARP is the same protocol as the
   original InARP protocol presented in [12] but applied to ATM
   networks.  All IP stations MUST support these protocols as updated
   and extended in this memo.  Use of these protocols differs depending
   on whether PVCs or SVCs are used.

6.1 Permanent Virtual Connections

   An IP station MUST have a mechanism (eg. manual configuration) for
   determining what PVCs it has, and in particular which PVCs are being
   used with LLC/SNAP encapsulation.  The details of the mechanism are
   beyond the scope of this memo.

   All IP members supporting PVCs are required to use the Inverse ATM
   Address Resolution Protocol (InATMARP) (refer to [12]) on those VCs
   using LLC/SNAP encapsulation.  In a strict PVC environment, the
   receiver SHALL infer the relevant VC from the VC on which the
   InATMARP request (InARP_REQUEST) or response (InARP_REPLY) was
   received.  When the ATM source and/or target address is unknown, the
   corresponding ATM address length in the InATMARP packet MUST be set
   to zero (0) indicating a null length, otherwise the appropriate
   address field should be filled in and the corresponding length set
   appropriately. InATMARP packet format details are presented later in



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RFC 1577             Classical IP and ARP over ATM          January 1993


   this memo.

   Directly from [12]: "When the requesting station receives the InARP
   reply, it may complete the [ATM]ARP table entry and use the provided
   address information.  Note: as with [ATM]ARP, information learned via
   In[ATM]ARP  may be aged or invalidated under certain circumstances."
   It is the responsibility of each IP station supporting PVCs to re-
   validate [ATM]ARP table entries as part of the aging process.  See
   Section 6.5 on "ATMARP Table Aging".

6.2 Switched Virtual Connections

   SVCs require support for ATMARP in the non-broadcast, non-multicast
   environment that ATM networks currently provide. To meet this need a
   single ATMARP Server MUST be located within the LIS. This server MUST
   have authoritative responsibility for resolving the ATMARP requests
   of all IP members within the LIS.

   The server itself does not actively establish connections.  It
   depends on the clients in the LIS to initiate the ATMARP registration
   procedure.  An individual client connects to the ATMARP server using
   a point-to-point VC. The server, upon the completion of an ATM
   call/connection of a new VC specifying LLC/SNAP encapsulation, will
   transmit an InATMARP request to determine the IP address of the
   client.  The InATMARP reply from the client contains the information
   necessary for the ATMARP Server to build its ATMARP table cache. This
   information is used to generate replies to the ATMARP requests it
   receives.

   The ATMARP Server mechanism requires that each client be
   administratively configured with the ATM address of the ATMARP Server
   atm$arp-req as defined earlier in this memo. There is to be one and
   only one ATMARP Server operational per logical IP subnet. It is
   RECOMMENDED that the ATMARP Server also be an IP station. This
   station MUST be administratively configured to operate and recognize
   itself as the ATMARP Server for a LIS. The ATMARP Server MUST be
   configured with an IP address for each logical IP subnet it is
   serving to support InATMARP requests.

   This memo recognizes that a single ATMARP Server is not as robust as
   multiple servers which synchronize their databases correctly. This
   document is defining the client-server interaction by using a simple,
   single server approach as a reference model, and does not prohibit
   more robust approaches which use the same client-server interface.







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RFC 1577             Classical IP and ARP over ATM          January 1993


6.3 ATMARP Server Operational Requirements

   The ATMARP server accepts ATM calls/connections from other ATM end
   points. At call setup and if the VC supports LLC/SNAP encapsulation,
   the ATMARP server will transmit to the originating ATM station an
   InATMARP request (InARP_REQUEST) for each logical IP subnet the
   server is configured to serve. After receiving an InATMARP reply
   (InARP_REPLY), the server will examine the IP address and the ATM
   address. The server will add (or update) the <ATM address, IP
   address> map entry and timestamp into its ATMARP table.  If the
   InATMARP IP address duplicates a table entry IP address and the
   InATMARP ATM address does not match the table entry ATM address and
   there is an open VC associated with that table entry, the InATMARP
   information is discarded and no modifications to the table are made.
   ATMARP table entries persist until aged or invalidated. VC call tear
   down does not remove ATMARP table entries.

   The ATMARP server, upon receiving an ATMARP request (ARP_REQUEST),
   will generate the corresponding ATMARP reply (ARP_REPLY) if it has an
   entry in its ATMARP table.  Otherwise it will generate a negative
   ATMARP reply (ARP_NAK).  The ARP_NAK response is an extension to the
   ARMARP protocol and is used to improve the robustness of the ATMARP
   server mechanism.  With ARP_NAK, a client can determine the
   difference between a catastrophic server failure and an ATMARP table
   lookup failure.  The ARP_NAK packet format is the same as the
   received ARP_REQUEST packet format with the operation code set to
   ARP_NAK, i.e., the ARP_REQUEST packet data is merely copied for
   transmission with the ARP_REQUEST operation code reset to ARP_NAK.

   Updating the ATMARP table information timeout, the short form: when
   the server receives an ATMARP request over a VC, where the source IP
   and ATM address match the association already in the ATMARP table and
   the ATM address matches that associated with the VC, the server may
   update the timeout on the source ATMARP table entry: i.e., if the
   client is sending ATMARP requests to the server over the same VC that
   it used to register its ATMARP entry, the server should examine the
   ATMARP requests and note that the client is still "alive" by updating
   the timeout on the client's ATMARP table entry.

   Adding robustness to the address resolution mechanism using ATMARP:
   when the server receives an ARP_REQUEST over a VC, it examines the
   source information.  If there is no IP address associated with the VC
   over which the ATMARP request was received and if the source IP
   address is not associated with any other connection, then the server
   will add the <ATM address, IP address> entry and timestamp into its
   ATMARP table and associate the entry with this VC.





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RFC 1577             Classical IP and ARP over ATM          January 1993


6.4 ATMARP Client Operational Requirements

   The ATMARP client is responsible for contacting the ATMARP server to
   register its own ATMARP information and to gain and refresh its own
   ATMARP entry/information about other IP members.  This means, as
   noted above, that ATMARP clients MUST be configured with the ATM
   address of the ATMARP server. ATMARP clients MUST:

      1. Initiate the VC connection to the ATMARP server for
         transmitting and receiving ATMARP and InATMARP packets.

      2. Respond to ARP_REQUEST and InARP_REQUEST packets received on
         any VC appropriately.  (Refer to Section 7, "Protocol Operation"
         in [12].)

      3. Generate and transmit ARP_REQUEST packets to the ATMARP server
         and to process ARP_REPLY and ARP_NAK packets from the server
         appropriately.  ARP_REPLY packets should be used to
         build/refresh its own client ATMARP table entries.

      4. Generate and transmit InARP_REQUEST packets as needed and to
         process InARP_REPLY packets appropriately.  InARP_REPLY packets
         should be used to build/refresh its own client ATMARP table
         entries.  (Refer to Section 7, "Protocol Operation" in [12].)

      5. Provide an ATMARP table aging function to remove its own old
         client ATMARP tables entries after a convenient period of time.

   Note: if the client does not maintain an open VC to the server, the
   client MUST refresh its ATMARP information with the server at least
   once every 20 minutes.  This is done by opening a VC to the server
   and exchanging the initial InATMARP packets.

6.5 ATMARP Table Aging

   An ATMARP client or server MUST have knowledge of any open VCs it has
   (permanent or switched), their association with an ATMARP table
   entry, and in particular, which VCs support LLC/SNAP encapsulation.

   Client ATMARP table entries are valid for a maximum time of 15
   minutes.

   Server ATMARP table entries are valid for a minimum time of 20
   minutes.

   Prior to aging an ATMARP table entry, an ATMARP server MUST generate
   an InARP_REQUEST on any open VC associated with that entry. If an
   InARP_REPLY is received, that table entry is updated and not deleted.



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RFC 1577             Classical IP and ARP over ATM          January 1993


   If there is no open VC associated with the table entry, the entry is
   deleted.

   When an ATMARP table entry ages, an ATMARP client MUST invalidate the
   table entry. If there is no open VC associated with the invalidated
   entry, that entry is deleted. In the case of an invalidated entry and
   an open VC, the ATMARP client must revalidate the entry prior to
   transmitting any non address resolution traffic on that VC. In the
   case of a PVC, the client validates the entry by transmitting an
   InARP_REQUEST and updating the entry on receipt of an InARP_REPLY. In
   the case of an SVC, the client validates the entry by transmitting an
   ARP_REQUEST to the ATMARP Server and updating the entry on receipt of
   an ARP_REPLY. If a VC with an associated invalidated ATMARP table
   entry is closed, that table entry is removed.

6.6 ATMARP and InATMARP Packet Format

   Internet addresses are assigned independently of ATM addresses.  Each
   host implementation MUST know its own IP and ATM address(es) and MUST
   respond to address resolution requests appropriately.  IP members
   MUST also use ATMARP and InATMARP to resolve IP addresses to ATM
   addresses when needed.

   The ATMARP and InATMARP protocols use the same hardware type
   (ar$hrd), protocol type (ar$pro), and operation code (ar$op) data
   formats as the ARP and InARP protocols [3,12].  The location of these
   fields within the ATMARP packet are in the same byte position as
   those in ARP and InARP packets.  A unique hardware type value has
   been assigned for ATMARP.  In addition, ATMARP makes use of an
   additional operation code for ARP_NAK.  The remainder of the
   ATMARP/InATMARP packet format is different than the ARP/InARP packet
   format.

   The ATMARP and InATMARP protocols have several fields that have the
   following format and values:

   Data:
     ar$hrd     16 bits  Hardware type
     ar$pro     16 bits  Protocol type
     ar$shtl     8 bits  Type & length of source ATM number (q)
     ar$sstl     8 bits  Type & length of source ATM subaddress (r)
     ar$op      16 bits  Operation code (request, reply, or NAK)
     ar$spln     8 bits  Length of source protocol address (s)
     ar$thtl     8 bits  Type & length of target ATM number (x)
     ar$tstl     8 bits  Type & length of target ATM subaddress (y)
     ar$tpln     8 bits  Length of target protocol address (z)
     ar$sha     qoctets  source ATM number
     ar$ssa     roctets  source ATM subaddress



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RFC 1577             Classical IP and ARP over ATM          January 1993


     ar$spa     soctets  source protocol address
     ar$tha     xoctets  target ATM number
     ar$tsa     yoctets  target ATM subaddress
     ar$tpa     zoctets  target protocol address

   Where:

     ar$hrd  -  assigned to ATM Forum address family and is
                19 decimal (0x0013) [4].

     ar$pro  -  see Assigned Numbers for protocol type number for
                the protocol using ATMARP. (IP is 0x0800).

     ar$op   -  The operation type value (decimal):
                ARP_REQUEST   = 1
                ARP_REPLY     = 2
                InARP_REQUEST = 8
                InARP_REPLY   = 9
                ARP_NAK       = 10

     ar$spln -  length in octets of the source protocol address. For
                IP ar$spln is 4.

     ar$tpln -  length in octets of the target protocol address. For
                IP ar$tpln is 4.

     ar$sha  -  source ATM number (E.164 or ATM Forum NSAPA)

     ar$ssa  -  source ATM subaddress (ATM Forum NSAPA)

     ar$spa  -  source protocol address

     ar$tha  -  target ATM number (E.164 or ATM Forum NSAPA)

     ar$tsa  -  target ATM subaddress (ATM Forum NSAPA)

     ar$tpa  -  target protocol address














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RFC 1577             Classical IP and ARP over ATM          January 1993


   The encoding of the 8-bit type and length value for ar$shtl,
   ar$sstl, ar$thtl, and ar$tstl is as follows:

     MSB   8     7     6     5     4     3     2     1   LSB
        +-----+-----+-----+-----+-----+-----+-----+-----+
        |  0  | 1/0 |   Octet length of address         |
        +-----+-----+-----+-----+-----+-----+-----+-----+

   Where:

     bit.8   (reserved) = 0  (for future use)

     bit.7   (type)     = 0  ATM Forum NSAPA format
                        = 1  E.164 format

     bit.6-1 (length)   = 6 bit unsigned octet length of address
                          (MSB = bit.6, LSB = bit.1)

   ATM addresses in Q.93B (as defined by the ATM Forum UNI 3.0
   signalling specification [9]) include a "Calling Party Number
   Information Element" and a "Calling Party Subaddress Information
   Element".  These Information Elements (IEs) SHOULD map to
   ATMARP/InATMARP source ATM number and source ATM subaddress
   respectively.  Furthermore, ATM Forum defines a "Called Party Number
   Information Element" and a "Called Party Subaddress Information
   Element". These IEs map to ATMARP/InATMARP target ATM number and
   target ATM subaddress respectively.

   The ATM Forum defines three structures for the combined use of number
   and subaddress [9]:

                        ATM Number      ATM Subaddress
                      --------------    --------------
        Structure 1   ATM Forum NSAPA        null
        Structure 2       E.164              null
        Structure 3       E.164         ATM Forum NSAPA

   IP members MUST register their ATM endpoint address with their ATMARP
   server using the ATM address structure appropriate for their ATM
   network connection: i.e., LISs implemented over ATM LANs following
   ATM Forum UNI 3.0 should register using Structure 1; LISs implemented
   over an E.164 "public" ATM network should register using Structure 2.
   A LIS implemented over a combination of ATM LANs and public ATM
   networks may need to register using Structure 3.  Implementations
   based on this memo MUST support all three ATM address structures.

   ATMARP and InATMARP requests and replies for ATM address structures 1
   and 2 MUST indicate a null ATM subaddress; i.e., ar$sstl.type = 1 and



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   ar$sstl.length = 0 and ar$tstl.type = 1 and ar$tstl.length = 0.  When
   ar$sstl.length and ar$tstl.length =0, the ar$tsa and ar$ssa fields
   are not present.

   Note: the ATMARP packet format presented in this memo is general in
   nature in that the ATM number and ATM subaddress fields SHOULD map
   directly to the corresponding Q.93B fields used for ATM
   call/connection setup signalling messages.  The IP over ATM Working
   Group expects ATM Forum NSAPA numbers (Structure 1) to predominate
   over E.164 numbers (Structure 2) as ATM endpoint identifiers within
   ATM LANs.  The ATM Forum's VC Routing specification is not complete
   at this time and therefore its impact on the operational use of ATM
   Address Structure 3 is undefined. The ATM Forum will be defining this
   relationship in the future.  It is for this reason that IP members
   need to support all three ATM address structures.

6.7 ATMARP/InATMARP Packet Encapsulation

   ATMARP and InATMARP packets are to be encoded in AAL5 PDUs using
   LLC/SNAP encapsulation. The format of the AAL5 CPCS-SDU payload field
   for ATMARP/InATMARP PDUs is:

               Payload Format for ATMARP/InATMARP PDUs:
               +------------------------------+
               |        LLC 0xAA-AA-03        |
               +------------------------------+
               |        OUI 0x00-00-00        |
               +------------------------------+
               |     Ethertype 0x08-06        |
               +------------------------------+
               |                              |
               |   ATMARP/InATMARP Packet     |
               |                              |
               +------------------------------+

   The LLC value of 0xAA-AA-03 (3 octets) indicates the presence of a
   SNAP header.

   The OUI value of 0x00-00-00 (3 octets) indicates that the following
   two-bytes is an ethertype.

   The Ethertype value of 0x08-06 (2 octets) indicates ARP [4].

   The total size of the LLC/SNAP header is fixed at 8-octets. This
   aligns the start of the ATMARP packet on a 64-bit boundary relative
   to the start of the AAL5 CPCS-SDU.





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   The LLC/SNAP encapsulation for ATMARP/InATMARP presented here is
   consistent with the treatment of multiprotocol encapsulation of IP
   over ATM AAL5 as specified in [2] and in the format of ATMARP over
   IEEE 802 networks as specified in [5].

   Traditionally, address resolution requests are broadcast to all
   directly connected IP members within a LIS. It is conceivable in the
   future that larger scaled ATM networks may handle ATMARP requests to
   destinations outside the originating LIS, perhaps even globally;
   issues raised by ATMARP'ing outside the LIS or by a global ATMARP
   mechanism are beyond the scope of this memo.

7.  IP Broadcast Address

   ATM does not support broadcast addressing, therefore there are no
   mappings available from IP broadcast addresses to ATM broadcast
   services. Note: this lack of mapping does not restrict members from
   transmitting or receiving IP datagrams specifying any of the four
   standard IP broadcast address forms as described in [8].  Members,
   upon receiving an IP broadcast or IP subnet broadcast for their LIS,
   MUST process the packet as if addressed to that station.

8.  IP Multicast Address

   ATM does not support multicast address services, therefore there are
   no mappings available from IP multicast addresses to ATM multicast
   services.  Current IP multicast implementations (i.e., MBONE and IP
   tunneling, see [10]) will continue to operate over ATM based logical
   IP subnets if operated in the WAN configuration.

   This memo recognizes the future development of ATM multicast service
   addressing by the ATM Forum. When available and widely implemented,
   the roll-over from the current IP multicast architecture to this new
   ATM architecture will be straightforward.

9.  Security

   Not all of the security issues relating to IP over ATM are clearly
   understood at this time, due to the fluid state of ATM
   specifications, newness of the technology, and other factors.

   It is believed that ATM and IP facilities for authenticated call
   management, authenticated end-to-end communications, and data
   encryption will be needed in globally connected ATM networks.  Such
   future security facilities and their use by IP networks are beyond
   the scope of this memo.





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   There are known security issues relating to host impersonation via
   the address resolution protocols used in the Internet [13].  No
   special security mechanisms have been added to the address resolution
   mechanism defined here for use with networks using IP over ATM.

10.  Open Issues

   o   Interim Local Management Interface (ILMI) services will not be
       generally implemented initially by some providers and vendors and
       will not be used to obtain the ATM address network prefix from
       the network [9].  Meta-signalling does provide some of this
       functionality and in the future we need to document the options.

   o   Well known ATM address(es) for ATMARP servers?  It would be very
       handy if a mechanism were available for determining the "well
       known" ATM address(es) for the client's ATMARP server in the LIS.

   o   There are many VC management issues which have not yet been
       addressed by this specification and which await the unwary
       implementor.  For example, one problem that has not yet been
       resolved is how two IP members decide which of duplicate VCs can
       be released without causing VC thrashing.  If two IP stations
       simultaneously established VCs to each other, it is tempting to
       allow only one of these VCs to be established, or to release one
       of these VCs immediately after it is established.  If both IP
       stations simultaneously decide to release opposite VCs, a
       thrashing effect can be created where VCs are repeatedly
       established and immediately released.  For the time being, the
       safest strategy is to allow duplicate VCs to be established and
       simply age them like any other VCs.

References

   [1] Piscitello, D., and J. Lawrence, "IP and ARP over the SMDS
       Service", RFC 1209, Bell Communications Research, March 1991.

   [2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
       Layer 5", RFC 1483, Telecom Finland, July 1993.

   [3] Plummer, D., "An Ethernet Address Resolution Protocol - or -
       Converting Network Addresses to 48.bit Ethernet Address for
       Transmission on Ethernet Hardware", STD 37, RFC 826, MIT,
       November 1982.

   [4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.





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   [5] Postel, J., and J. Reynolds, "A Standard for the Transmission of
       IP Datagrams over IEEE 802 Networks", STD 43, RFC 1042,
       USC/Information Sciences Institute, February 1988.

   [6] CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII,
       Geneva, 19-29 January 1993.

   [7] CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23 September
       - 2 October 1992.

   [8] Braden, R., "Requirements for Internet Hosts -- Communication
       Layers", STD 3, RFC 1122, USC/Information Sciences Institute,
       October 1989.

   [9] ATM Forum, "ATM User-Network Interface Specification Version
       3.0.", ATM Forum, 480 San Antonio Road, Suite 100, Mountain View,
       CA 94040, June 1993.

  [10] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
       1112, Stanford University, August 1989.

  [11] Colella, R., and Gardner, E., and R. Callon, "Guidelines for OSI
       NSAP Allocation in the Internet", RFC 1237, NIST, Mitre, DEC,
       July 1991.

  [12] Bradely, T., and C. Brown, "Inverse Address Resolution Protocol",
       RFC 1293, Wellfleet Communications, Inc., January 1992.

  [13] Bellovin, S., "Security Problems in the TCP/IP Protocol Suite",
       ACM Computer Communications Review, Vol. 19, Issue 2, pp. 32-48,
       1989.

Security Considerations

   Security issues are discussed in Section 9.

Author's Address

   Mark Laubach
   Hewlett-Packard Laboratories
   1501 Page Mill Road
   Palo Alto, CA 94304

   Phone: 415-857-3513
   Fax:   415-857-8526
   EMail: laubach@hpl.hp.com





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