A MAPOS version 1 Extension - Switch-Switch Protocol
RFC 2174

Document Type RFC - Informational (June 1997; No errata)
Was draft-rfced-info-mitsuru (individual)
Authors Mitsuru Maruyama  , Ken-ichiro Murakami 
Last updated 2013-03-02
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Network Working Group                                    K. Murakami
Request for Comments: 2174                               M. Maruyama
Category: Informational                             NTT Laboratories
                                                           June 1997

          A MAPOS version 1 Extension - Switch-Switch Protocol

Status of this Memo

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

Authors' Note

   This memo documents a MAPOS (Multiple Access Protocol over SONET/SDH)
   version 1 extension, Switch Switch Protocol which provides dynamic
   routing for unicast, broadcast, and multicast. This document is NOT
   the product of an IETF working group nor is it a standards track
   document.  It has not necessarily benefited from the widespread and
   in depth community review that standards track documents receive.


   This document describes a MAPOS version 1 extension, SSP (Switch
   Switch Protocol).  MAPOS is a multiple access protocol for
   transmission of network-protocol packets, encapsulated in High-Level
   Data Link Control (HDLC) frames, over SONET/SDH. In MAPOS network, a
   SONET switch provides the multiple access capability to end nodes.
   SSP is a protocol of Distance Vector family and provides unicast and
   broadcast/multicast routing for multiple SONET switch environment.

1. Introduction

   This document describes an extension to MAPOS version 1, Switch
   Switch Protocol, for routing both unicast and broadcast/multicast
   frames.  MAPOS[1], Multiple Access Protocol over SONET (Synchronous
   Optical Network) / SDH (Synchronous Digital Hierarchy) [2][3][4][5],
   is a link layer protocol for transmission of HDLC frames over
   SONET/SDH. A SONET switch provides the multiple access capability to
   each node. SSP is a dynamic routing protocol designed for an
   environment where a MAPOS network segment spans over multiple
   switches.  It is a protocol of Distance Vector family. It provides
   both unicast and broadcast/multicast routing. First, this document
   describes the outline of SSP. Next, it explains unicast and
   broadcast/multicast routing algorithms. Then, it describes the SSP
   protocol in detail.

Murakami & Maruyama          Informational                      [Page 1]
RFC 2174                         MAPOS                         June 1997

2. Constraints in Designing SSP

   SSP is a unified routing protocol supporting both unicast and
   broadcast/multicast. The former and the latter are based on the
   Distance Vector [6][7] and the spanning tree[8] algorithm,
   respectively. In MAPOS version 1, a small number of switches is
   assumed in a segment.  Thus, unlike DVMRP(Distance Vector Multicast
   Routing Protocol)[8], TRPB(Truncated Reverse Path Broadcasting) is
   not supported for simplicity. This means that multicast frames are
   treated just the same as broadcast frames and are delivered to every

   In MAPOS version 1, there are two constraints regarding design of the
   broadcast/multicast routing algorithm;

     (1) there is no source address field in MAPOS HDLC frames

     (2) there is no TTL(Time To Live) field in MAPOS HDLC frames to
     prevent forwarding loop.

   To cope with the first issue, VRPB(Virtual Reverse Path Broadcast)
   algorithm is introduced. In VRPB, all broadcast and multicast frames
   are assumed to be generated by a node under a specific switch called
   VSS(Virtual Source Switch). VSS is the switch which has the smallest
   switch number in a MAPOS network. Each switch determine its place in
   the spanning tree rooted from VSS independently. Whenever a switch
   receives a broadcast/multicast frame, it forwards the frame to all
   upstream and downstream switches except for the one which has sent
   the frame to the local switch.

   To cope with the second issue, the forward delay timer is introduced.
   Even if a switch finds a new VSS, it suspends forwarding for a time
   period. This timer ensures that all the switches have a consistent
   routing information and that they are synchronized after a topology

3. Unicast Routing in SSP

   This section describes the address structure of MAPOS version 1 and
   the SSP unicast routing based on it.

Murakami & Maruyama          Informational                      [Page 2]
RFC 2174                         MAPOS                         June 1997

3.1 Address Structure of MAPOS version 1

   In a multiple switch environment, a node address consists of the
   switch number and the port number to which the node is connected. As
   shown in Figure 1, the address length is 8 bits and the LSB is always
   1, which indicates the end of the address field. A MSB of 0 indicates
   a unicast address. The switch and the port number fields are
   variable-length. In this document, a unicast address is represented
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