rfc1307.txt

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 | 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          |---
                          +---------------+






Young & Nicholson                                               [Page 7]

RFC 1307       Dynamically Switched Link Control Protocol     March 1992


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



Young & Nicholson                                               [Page 8]

RFC 1307       Dynamically Switched Link Control Protocol     March 1992


        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.




Young & Nicholson                                               [Page 9]

RFC 1307       Dynamically Switched Link Control Protocol     March 1992


            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:



Young & Nicholson                                              [Page 10]

RFC 1307       Dynamically Switched Link Control Protocol     March 1992


            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.



Young & Nicholson                                              [Page 11]

RFC 1307       Dynamically Switched Link Control Protocol     March 1992


            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.













Young & Nicholson                                              [Page 12]

RFC 1307       Dynamically Switched Link Control Protocol     March 1992


Authors' Addresses

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

   Phone: [phone]
   EMail: [e-mail]


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

   Phone: [phone]
   EMail: [e-mail]

































Young & Nicholson                                              [Page 13]