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rfc:rfc1307

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.

Young & Nicholson [Page 1] RFC 1307 Dynamically Switched Link Control Protocol March 1992

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.

Young & Nicholson [Page 2] RFC 1307 Dynamically Switched Link Control Protocol March 1992

 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

Young & Nicholson [Page 3] RFC 1307 Dynamically Switched Link Control Protocol March 1992

 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:

Young & Nicholson [Page 4] RFC 1307 Dynamically Switched Link Control Protocol March 1992

         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.

Young & Nicholson [Page 5] RFC 1307 Dynamically Switched Link Control Protocol March 1992

      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.

Young & Nicholson [Page 6] RFC 1307 Dynamically Switched Link Control Protocol March 1992

  DSLCP state diagram:
  1. —— +—————-+

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

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: (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

Young & Nicholson [Page 13]

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