rfc2892.txt

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Network Working Group                                          D. Tsiang
Request for Comments: 2892                                     G. Suwala
Category: Informational                                    Cisco Systems
                                                             August 2000


                    The Cisco SRP MAC Layer Protocol

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document specifies the MAC layer protocol, "Spatial Reuse
   Protocol" (SRP) for use with ring based media. This is a second
   version of the protocol (V2).

   The primary requirements for SRP are as follows:

   -  Efficient use of bandwidth using:
          spatial reuse of bandwidth
          local reuse of bandwidth
          minimal protocol overhead
   -  Support for priority traffic
   -  Scalability across a large number of nodes or stations attached to
      a ring
   -  "Plug and play" design without a software based station management
      transfer (SMT) protocol or ring master negotiation as seen in
      other ring based MAC protocols [1][2]
   -  Fairness among nodes using the ring
   -  Support for ring based redundancy (error detection, ring wrap,
      etc.) similar to that found in SONET BLSR specifications.
   -  Independence of physical layer (layer 1) media type.

   This document defines the terminology used with SRP, packet formats,
   the protocol format, protocol operation and associated protocol
   finite state machines.







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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


Table of Contents

    1.  Differences between SRP V1 and V2 .......................  3
    2.  Terms and Taxonomy ......................................  4
        2.1.  Ring Terminology ..................................  4
        2.2.  Spatial Reuse .....................................  5
        2.3.  Fairness ..........................................  6
        2.4.  Transit Buffer ....................................  7
    3.  SRP Overview ............................................  8
        3.1.  Receive Operation Overview ........................  8
        3.2.  Transmit Operation Overview .......................  8
        3.3.  SRP Fairness Algorithm (SRP-fa) Overview ..........  9
        3.4.  Intelligent Protection Switching (IPS) Protocol
              Overview ..........................................  9
    4.  Packet Formats .......................................... 13
        4.1.  Overall Packet Format ............................. 13
        4.2.  Generic Packet Header Format ...................... 14
             4.2.1.  Time To Live (TTL) ......................... 14
             4.2.2.  Ring Identifier (R) ........................ 15
             4.2.3.  Priority Field (PRI) ....................... 15
             4.2.4.  MODE ....................................... 15
             4.2.5.  Parity Bit (P-bit) ......................... 16
             4.2.6.  Destination Address ........................ 16
             4.2.7.  Source Address ............................. 16
             4.2.8.  Protocol Type .............................. 16
        4.3.  SRP Cell Format ................................... 16
        4.4.  SRP Usage Packet Format ........................... 17
        4.5.  SRP Control Packet Format ......................... 18
             4.5.1.  Control Ver ................................ 19
             4.5.2.  Control Type ............................... 19
             4.5.3.  Control TTL ................................ 19
             4.5.4.  Control Checksum ........................... 19
             4.5.5.  Payload .................................... 20
             4.5.6.  Addressing ................................. 20
        4.6.  Topology Discovery ................................ 20
             4.6.1.  Topology Length ............................ 22
             4.6.2.  Topology Originator ........................ 22
             4.6.3.  MAC bindings ............................... 22
             4.6.4.  MAC Type Format ............................ 22
        4.7.  Intelligent Protection Switching (IPS) ............ 23
             4.7.1.  Originator MAC Address ..................... 23
             4.7.2.  IPS Octet .................................. 24
        4.8.  Circulating packet detection (stripping) .......... 24
    5.  Packet acceptance and stripping ......................... 25
        5.1.  Transmission and forwarding with priority ......... 27
        5.2.  Wrapping of Data .................................. 28
    6.  SRP-fa Rules Of Operation ............................... 28
        6.1.  SRP-fa pseudo-code ................................ 30



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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


        6.2.  Threshold settings ................................ 32
    7.  SRP Synchronization ..................................... 32
        7.1.  SRP Synchronization Examples ...................... 33
    8.  IPS Protocol Description ................................ 34
        8.1.  The IPS Request Types ............................. 35
        8.2.  SRP IPS Protocol States ........................... 36
             8.2.1.  Idle ....................................... 36
             8.2.2.  Pass-through ............................... 36
             8.2.3.  Wrapped .................................... 36
        8.3.  IPS Protocol Rules ................................ 36
             8.3.1.  SRP IPS Packet Transfer Mechanism .......... 36
             8.3.2.  SRP IPS Signaling and Wrapping Mechanism ... 37
        8.4.  SRP IPS Protocol Rules ............................ 38
        8.5.  State Transitions ................................. 41
        8.6.  Failure Examples .................................. 41
             8.6.1.  Signal Failure - Single Fiber Cut Scenario . 41
             8.6.2.  Signal Failure - Bidirectional Fiber Cut
                     Scenario ................................... 43
             8.6.3.  Failed Node Scenario ....................... 45
             8.6.4.  Bidirectional Fiber Cut and Node Addition
             Scenarios .......................................... 47
    9.  SRP over SONET/SDH ...................................... 48
   10.  Pass-thru mode .......................................... 49
   11.  References .............................................. 50
   12.  Security Considerations ................................. 50
   13.  IPR Notice .. ........................................... 50
   14.  Acknowledgments ......................................... 50
   15.  Authors' Addresses ...................................... 51
   16.  Full Copyright Statement ................................ 52

1.  Differences between SRP V1 and V2

   This document pertains to SRP V2. SRP V1 was a previously published
   draft specification. The following lists V2 feature differences from
   V1:

   -  Reduction of the header format from 4 bytes to 2 bytes.

   -  Replacement of the keepalive packet with a new control packet that
      carries usage information in addition to providing a keepalive
      function.

   -  Change bit value of inner ring to be 1 and outer to be 0.

   -  Reduction in the number of TTL bits from 11 to 8.

   -  Removal of the DS bit.




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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   -  Change ordering of CRC transmission to be most significant octet
      first (was least significant octet in V1).  The SRP CRC is now the
      same as in [5].

   -  Addition of the SRP cell mode to carry ATM cells over SRP.

   -  Changes to the SRP-fa to increase the usage field width and to
      remove the necessity of adding a fixed constant when propagating
      usage messages.

2.  Terms and Taxonomy

2.1.  Ring Terminology

   SRP uses a bidirectional ring. This can be seen as two symmetric
   counter-rotating rings. Most of the protocol finite state machines
   (FSMs) are duplicated for the two rings.

   The bidirectional ring allows for ring-wrapping in case of media or
   station failure, as in FDDI [1] or SONET/SDH [3]. The wrapping is
   controlled by the Intelligent Protection Switching (IPS) protocol.

   To distinguish between the two rings, one is referred to as the
   "inner" ring, the other the "outer" ring. The SRP protocol operates
   by sending data traffic in one direction (known as "downstream") and
   it's corresponding control information in the opposite direction
   (known as "upstream") on the opposite ring. Figure 1 highlights this
   graphically.























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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   FIGURE 1. Ring Terminology

                                       {outer_data
                                -----   inner_ctl}
               ---------------->| N |-----------------
              |  ---------------| 1 |<--------------  |
              | |  {inner_data  -----               | |
              | |   outer_ctl}                      | |
             -----                                 -----
             | N |                                 | N |
             | 6 |                                 | 2 |
             -----                                 -----
              ^ |                                   ^ |
            o | |                                 i | |
            u | |                                 n | |
            t | |                                 n | |
            e | |                                 e | |
            r | |                                 r | |
              | v                                   | v
             -----                                 -----
             | N |                                 | N |
             | 5 |                                 | 3 |
             -----                                 -----
              | |                                   | |
              | |               -----               | |
              |  -------------->| N |---------------  |
               -----------------| 4 |<----------------
                                -----

2.2.  Spatial Reuse

   Spatial Reuse is a concept used in rings to increase the overall
   aggregate bandwidth of the ring. This is possible because unicast
   traffic is only passed along ring spans between source and
   destination nodes rather than the whole ring as in earlier ring based
   protocols such as token ring and FDDI.

   Figure 2 below outlines how spatial reuse works. In this example,
   node 1 is sending traffic to node 4, node 2 to node 3 and node 5 to
   node 6. Having the destination node strip unicast data from the ring
   allows other nodes on the ring who are downstream to have full access
   to the ring bandwidth. In the example given this means node 5 has
   full bandwidth access to node 6 while other traffic is being
   simultaneously transmitted on other parts of the ring.







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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


2.3.  Fairness

   Since the ring is a shared media, some sort of access control is
   necessary to ensure fairness and to bound latency. Access control can
   be broken into two types which can operate in tandem:

      Global access control - controls access so that everyone gets a
      fair share of the global bandwidth of the ring.

      Local access control - grants additional access beyond that
      allocated globally to take advantage of segments of the ring that
      are less than fully utilized.

   As an example of a case where both global and local access are
   required, refer again to Figure 2. Nodes 1, 2, and 5 will get 1/2 of
   the bandwidth on a global allocation basis. But from a local
   perspective, node 5 should be able to get all of the bandwidth since
   its bandwidth does not interfere with the fair shares of nodes 1 and
   2.