





Network Working Group                                           J. Young
Request for Comments: 1307                                  A. Nicholson
                                                     Cray Research, Inc.
                                                              March 1992


               Dynamically Switched Link Control Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  Discussion and suggestions for improvement are requested.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This memo describes an experimental protocol developed by a project
   team at Cray Research, Inc., in implementing support for circuit-
   switched T3 services.  The protocol is used for the control of
   network connections external to a host, but known to the host.  It is
   documented here for the benefit of others who may wish to perform
   further research.

   While working with circuit-switched T3 networks, developers at Cray
   Research, Inc., defined a model wherein a host would generate control
   messages for a network switch.  This work is described in RFC 1306,
   "Experiences Supporting By-Request Circuit-Switched T3 Networks".  In
   order to simplify the model it was decided that the inconsistencies
   of switch control should be hidden from the host generating the
   control messages.  To that end, a protocol was defined and
   implemented.  This RFC documents the Dynamically Switched Link
   Control Protocol (DSLCP), which is used for creation and control of
   downstream network links by a host.

1.0  INTRODUCTION

   The Dynamically Switched Link Control Protocol (DSLCP) allows a host
   with knowledge of a special downstream network link to issue messages
   to control the status of that link.

   This document describes the functions of the DSLCP to control
   external network connections.







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1.1  Motivation

   Circuit Switched Networks are becoming available to the Internet
   community.  These networks are made available by requesting a
   connection through a switch.  Normally circuit switched network links
   are disconnected, and their prohibitive cost suggests that it is very
   costly to leave them connected at all times.

   Internet users and hosts wish to send data over a circuit switched
   networks, but only connect the network links when a transport
   connection is to be established.  While it would be possible to use
   packet routers to identify the need for switching a connection on and
   off, only the transport provider can positively identify the
   beginning and end of a transport session.  There must be a mechanism
   to activate and deactivate the link at the beginning and end of a
   transport session.

   The DSLCP assumes that a transport provider has knowledge of a
   downstream link which must be setup before data transfer may take
   place.  However, the details of link setup may vary by the type of
   link (circuit-switched or other), specific hardware, or
   administrative differences.  The DSLCP hides these details from the
   transport provider by offering a simple request/release model of link
   preparation.  The model assumes an entity in control of the link
   which handles the details of connection preparation while responding
   to the DSLCP commands of the transport provider.  This entity is
   called the link controller.

   The DSLCP allows internet hosts to dynamically change the fabric of
   the internet by sending messages through the internet in advance of
   data which is to travel across the newly created links.

1.2  Scope

   DSLCP is intended to provide an interface between transport providers
   and arbitrary network links requiring creation, control, setup, or
   conditioning before data communications may take place.

1.3  Interfaces

   There are no specific user level interfaces to DSLCP, although they
   are not precluded.  Link control is a function of the network layer,
   initiated by requests from the transport provider.

   A DSLCP transaction is defined as a transport provider communicating
   with a link controller for the duration of transport session.  A
   network path between the host providing transport services and the
   link controller must exist in advance of the DSLCP transaction.



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   Either party to an DSLCP transaction may asynchronously generate
   messages.

1.4  Operation

   The purpose of the DSLCP is to allow a transport provider to request
   the setup of a downstream network link so that data transfer may take
   place through that link.  DSLCP messages are assumed to be
   communicated between the transport provider and the link controller
   through a transport service, such as UDP or TCP, or through a network
   service such as IP.

   DSLCP provides messages for link setup and teardown.  All the details
   of link management are left to the link controller.  The transport
   provider is interested only whether the link is ready to carry data.

1.5  Transmission

   DSLCP messages are carried through the network in datagrams using
   either IP or UDP.  DSLCP is designed to not require a reliable
   transport protocol.

2.0  DSLCP Architecture

   DSLCP is used in a host environment.  Normally, transport users on
   the host will make requests of a transport provider to carry data to
   other hosts.  Some of these requests may require the preparation of a
   downstream network link.  The transport provider has knowledge of
   these special network links, and issues a request to DSLCP that the
   link be prepared to carry data.  This happens transparently to the
   transport user.

   When a transport user requests transport services, the transport
   provider will normally attempt to establish a connection.  In the
   event the transport provider discovers that the connection requires
   special link control, the transport provider will call upon DSLCP to
   send a link setup message to the link controller.  The transport
   provider does not attempt to use the connection until DSLCP informs
   the transport provider that the link is setup or that the setup
   attempt failed.  If the setup failed, then the transport provider is
   free to attempt to find another way to create a connection.

   When the transport user is finished using the services, then the
   transport provider will call DSLCP to release the link.  The
   transport provider may now assume that the link is no longer
   available.

   In general, DSLCP maintains and hides the status of link control



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   transactions from the transport provider.  This way the transport
   provider does not need to keep track of multiple DSLCP transactions.
   For example, if the transport provider requests a link be setup for a
   new transport user while another transport user has the link active,
   the DSLCP may inform the transport provider that the link is ready
   without delay, provided that the link can support multiple transport
   connections.

3.0  FUNCTIONAL SPECIFICATION

   This document specifies both a message format and a state machine for
   DSLCP protocol transactions.

3.1  Control Message Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Identifier                   |   Total length                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Function                     |   Event Status                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Endpoint 1                                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Endpoint 2                                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Message                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Body                                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Identifier: 16 bits

       The identifier is a value assigned by the DSLCP used to uniquely
       identify link setup transactions.  It is intended to be used with
       the endpoint addresses by a link controller to identify a
       transaction.

   Total length: 16 bits

       The total length, in octets, including the header of this DSLCP
       control message.

   Function: 16 bits

       The operation to be processed or being responded to.

       Functions currently defined are:



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           Bring up        value 0
           Bring down      value 1

   Event Status: 16 bits

       The state of the controlled link, relative to the last function
       request.

       The possible event states are:
           Setup request succeeded        value 2
           Setup request failed           value 3
           Teardown request succeeded     value 4
           Teardown request failed        value 5
           Asynchronous network down      value 7

   Endpoint addresses: 32 bits each

       The internet addresses of the two communicating parties for which
       the link is being prepared.

   Message body:  arbitrary length up to 65499 octets

       An ascii string which is meaningful the link controller.  When the
       requesting host is configured, the system administrator sets the
       control strings for each network link that may be accessed by the
       requesting host.

3.2  State Machine

   The transport provider is aware of only 2 possible states for the
   controlled link: up or down.  Furthermore, transport users may
   request or release transport services from the transport provider at
   any time.  Thus, there must be a state machine employed by DSLCP when
   communicating between the transport provider and the controlled link.
   This state machine hides the details of link control transactions
   from the transport provider.  The state machine has 6 possible
   states.

        Down: There is no active transport connection and the controlled
        link is not setup.

        Coming Up: A transport user has requested a connection for which
        the transport provider has given a setup request to the DSLCP.
        The DSLCP has sent a setup request to the link controller and is
        awaiting a response.

        Up: At least one transport connection is active and the
        controlled link is setup.



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        Going Down: All transport connections have been terminated and
        the transport provider has sent an equivalent number of up
        requests and down requests to the DSLCP.  The DSLCP has sent a
        teardown request to the link controller and is awaiting a
        response.

        Bring Down: While DSLCP is in the Coming Up state, the transport
        provider requested link teardown.  As soon as a response is
        received from the link controller, the DSLCP will send a
        teardown request if the link setup was successful.

        Bring Up: While in the Going Down state, the transport provider
        requested connection setup.  As soon as a response is received
        from the link controller, the DSLCP will send a setup request.





































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    DSLCP state diagram:

              ------- +----------------+
     Transport        |     Down       |<---------\
     Connect     ---->+----------------+           \
     Request    /               ^  ^                \
     -------  Setup             |  |                 \
     Send     Failed            |  |         Teardown \ Response Timeout
     Setup   /------            |  |         Success   \ ---------------
       /    /                   |  |         --------  |
       |    |                   |  |                   |
       |    |                   |  |                   |
       |    | Teardown Response |  |                   |
       |    | Success  Timeout  |  |                   |
       |    | ----------------- |  |     +----------+  |
       |    |      Send---------|--|-----| Bring Up |--|----\
       |    |      Setup        |  |     +----------+  |    | Transport
       |    |     /             |  |               ^   |    | Teardown
       |    |    /              |  |        Transport  |    | Request
       |    |   /               |  |        Connect|   |    | ---------
       |    |  /            Setup  |        Request|   |    |
       |    |  |           Failed  |        -------|   |    |
       v    |  v           ------  |               |   |    v
 +--------------+               |  |              +-------------+
 | Coming Up    |----------+----|--|--Response--->| Going Down  |
 +--------------+          ^    |  |  Timeout     +-------------+
   |    ^      |           |    |  |  --------      ^    ^
   |    |      Transport   |    |  |  Send          |    |
   | Transport Teardown    |    |  |  Teardown      |    |
   |  Connect  Request     |    |  |                /    |
   |  Request  -------     |    |  |               /     |
   |  -------  v           |    |  |              /      /
   |      \ +------------+ -    |  |             /      /
   |       -| Bring Down | ------  |            /      /
    \       +------------+ --------|--Setup-----      /
     \                             |  Success        /
      \                            |  -------       /
       \   Setup           Network |  Send         / Transport
        \  Success         Is Down |  Teardown    /  Teardown
         \ -------         ------- |             /   Request
          \                        |            /    --------
           \                       |           /     Send
            \             +---------------+   /      Teardown
             \----------->|   Up          |---
                          +---------------+






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Events and State Transitions

   The DSLCP will process three type of events:

      A link control request from the transport provider
      An DSLCP message from the link controller
      DSLCP message timeout

   The transport provider will make link setup and and teardown requests
   to the DSLCP when transport users request and release services
   requiring link control operations.  The transport provider should not
   keep track of the status of a particular link, as this is a function
   of the DSLCP.  The transport provider may be unaware of redirection
   or other processing of link setup requests performed by DSLCP, so
   this is a function best left to DSLCP.  The DSLCP will inform the
   transport provider as to the success or failure of a particular setup
   request, and transport providers may assume the success of teardown
   requests (the DSLCP will always return a success response to a
   teardown request).

   The DSLCP will engage in link control transactions with link
   controllers.  This will include accepting messages from link
   controllers in response to requests as well as unexpected messages
   from the link controller.  Unexpected messages may include redundant
   responses to redundant requests sent as a result of timeouts.

   Because of the possibility of unavailable links and link controllers,
   DSLCP should not wait indefinitely for message responses from link
   controllers to which it has sent messages.  Since DSLCP does not
   require the use of a reliable transport protocol to carry DSLCP
   messages, DSLCP must have a timeout and retransmission mechanism.
   Since we have used DSLCP in a local network context with switch
   controllers which offer a quick turnaround (on the order of 1
   second), we use a 5 second timeout with a 3 retransmit limit.  These
   figures would require adaptation to different network and link
   controller configurations, and a self-adapting algorithm would be
   most appropriate for a general solution.

   The specific events of interest to the DSLCP are:

        Transport provider link setup request
        Transport provider link teardown request

        Link setup request failed
        Link setup request succeeded
        Link teardown request succeeded
        Link teardown request failed
        Network link is down



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        Timeout waiting for DSLCP response from link controller

   The necessary processing for each event while in each state is as
   follows:

        Transport provider link setup request

            Down:
                Send setup request to link controller.
                Enter Coming Up state.
                Notify transport provider to wait until link is up.

            Coming Up:
            Bring Up:
                Notify transport provider to wait until link is up.

            Up:
                Notify transport provider that link is up.

            Bring Down:
                Enter Coming Up state.
                Notify transport provider to wait until link is up.

            Going Down:
                Enter Bring Up state.
                Notify transport provider to wait until link is up.

            Discussion:

            If the controlled link is not capable to support multiple
            transport connections, then the DSLCP must return
            appropriate errors when it detects multiple transport setup
            requests for that link.

        Transport provider link teardown request.

            Down:
            Bring Down:
            Going Down:
                Notify transport provider that link is down.

            Coming Up:
                Enter Bring Down state.
                Notify transport provider that link is down.

            Bring Down:
                Notify transport provider that link is down.




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            Up:
                Send teardown request.
                Enter Going Down state.
                Notify transport provider that link is down.

        Link setup request failed

            Down:
            Going Down:
            Bring Up:
                Unexpected message, possibly due to duplicate requests -
                ignore it.

            Up:
                Unexpected message, link controller may be refusing
                multiple setup requests sent because of timeout - ignore
                it.

            Coming Up:
            Bring Down:
                Enter down state.

        Link setup request succeeded

            Down:
                Unexpected message, possibly due to duplicate requests
                and reordering of request packets by network.
                Send teardown request.

            Going Down:
            Bring Up:
            Up:
                Unexpected message, possibly due to duplicate requests -
                ignore it.

            Coming Up:
                Enter Up state.
                Notify transport provider(s) waiting for link that it is
                available.

            Bring Down:
                Send teardown request.
                Enter Going Down state.

        Link teardown request succeeded

            Down:
            Coming Up:



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            Bring Down:
                Unexpected message, possibly due to duplicate requests -
                ignore it.

            Up:
                Unexpected message, possibly due to duplicate requests
                and reordering of request packets by network.
                Send teardown request.
                Enter Going Down state.
                Notify transport providers that link has gone down.

            Bring Up:
                Send setup request
                Enter Coming Up state

            Going Down:
                Enter Down state

            Discussion:

            If a teardown request succeeded message arrives when the
            DSLCP is in the UP state, then some error has occurred, and
            the conservative approach is to bring down the connection
            and resynchronize.  However, it may be satisfactory to
            ignore the message without ill effect.


        Link teardown request failed

            Down:
            Coming up:
            Bring Down:
            Bring Up:
            Going Down:
            Up:
                DSLCP sent a teardown request message for an invalid
                transaction.  The link controller has no
                identifier/endpoints transaction record for the request.
                Continue as if request had succeeded.

        Network link is down

            Down:
                Ignore message.

            Bring Down:
            Going Down:
                Enter Down state.



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            Coming up:
            Bring Up:
            Up:
                Enter down state.
                Notify transport provider that link is down.


        Timeout waiting for DSLCP response from controller

            Down:
            Up:
                DSLCP protocol error - fix bug, don't set timer when
                there are no outstanding requests.

            Coming Up:
            Bring Down:
                Send teardown request.
                Enter Going down state.

            Going Down:
                Enter Down state.

            Bring Up:
                Send setup request.
                Enter Coming Up state.

References

   [1]  Nicholson, et. al., "High Speed Networking at Cray Research",
        Computer Communications Review, January, 1991.

   [2]  Nicholson, A., and J. Young, "Experiences Supporting By-Request
        Circuit-Switched T3 Networks", RFC 1306, Cray Research, Inc.,
        March 1992.

Security Considerations

   Security issues are not discussed in this memo.













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Authors' Addresses

   Jeff Young
   Cray Research, Inc.
   655F Lone Oak Drive
   Eagan, MN 55123

   Phone: (612) 452-6650
   EMail: jsy@cray.com


   Andy Nicholson
   Cray Research, Inc.
   655F Lone Oak Drive
   Eagan, MN 55123

   Phone: (612) 452-6650
   EMail: droid@cray.com

































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