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RFC 2174

A MAPOS version 1 Extension - Switch-Switch Protocol

Pages: 22

ToP   noToC   RFC2174 - Page 1
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.
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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.
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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
   as "0 <switch-number> <port number>".  Note that a port number
   includes EA bit.

   MSB of 1 indicates multicast or broadcast. In the case of broadcast,
   the address field contains all 1s (0xff in hex). In the case of
   multicast, the remaining bits indicate a group address.  The switch
   number field is variable-length. A multicast address is represented
   as "1 <group address>".

           Switch Number(variable length)
               |      +--- Port Number
               |      |
               V      V
           | | | | | | | | |
           | |           |1|
            ^             ^
            |             |
            |             +------- EA bit (always 1)
            1 : broadcast, multicast
            0 : unicast

                        Figure 1 Address Format

   Figure 2 shows an example of a SONET LAN that consists of three
   switches.  In this configuration, two bits of a node address are used
   to indicate the switch number. Node N1 is connected to port
   0x03(000011 in binary) of the switch S2 numbered 0x2.  Thus, the node
   address is 01000011 in binary. Node N4 has an address 01101001 in
   binary since the connected switch number is 0x3 and the port number
   is 0x09.
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                        | node |
                        |  N1  |
           01000101         |0x03              |0x03       00101001
           +------+     +---+----+         +---+----+      +------+
           | node +-----+ SONET  +---------+ SONET  +------+ node |
           |  N2  | 0x05| Switch |0x09 0x05| Switch |0x09  |  N3  |
           +------+     |   S2   |         |   S1   |      +------+
                        |  (0x2) |         |  (0x1) |
                        +---+----+         +---+----+
                            |0x07              |0x07
                            |                  |
                            |                  |0x03      01101001
                            |              +---+----+     +------+
                            +--------------+ SONET  +-----+ node |
                                       0x05| Switch |0x09 |  N4  |
                                           |   S3   |     +------+
                                           |  (0x3) |

               Figure 2 Multiple SONET Switch Environment

3.2 Forwarding Unicast Frames

   Unicast frames are forwarded along the shortest path. For example, a
   frame from node N4 destined to N1 is forwarded by switch S3 and S2.
   These SONET switches forwards an HDLC frame based on the destination
   switch number contained in the destination address.

   Each switch keeps a routing table with entries for possible
   destination switches. An entry contains the subnet mask, the next hop
   to the adjacent switch along the shortest path to the destination,
   the metric measuring the total distance to the destination, and other
   parameters associated with the entry such as timers. For example, the
   routing table in switch S1 will be as shown in Table 1. The metric
   value 1 means that the destination switch is an adjacent switch. The
   value 16 means that it is unreachable. Although the values between 17
   and 31 also mean unreachable, they are special values utilized for
   split horizon with poisoned reverse [8].
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     | destination |   subnet  | next hop | metric |   other    |
     |   switch    |   mask    |   port   |        | parameters |
     |  01000000   | 11100000  | 00000101 |    1   |            |
     |  01100000   | 11100000  | 00000111 |    1   |            |

                 Table 1  An Example of a Routing Table

   When a switch receives a unicast frame, it extracts the switch number
   from the destination address. If it equals to the local switch
   number, the frame is sent to the local node through the port
   specified in the destination address.  Otherwise, the switch looks up
   its routing table for a matching destination switch number by masking
   the destination address with the corresponding subnet mask. If a
   matching entry is found, the frame is sent to an adjacent switch
   through the next hop port in the entry. Otherwise, it is silently
   discarded or sent to the control processor for its error processing.

3.4 Protocol Overview

   This subsection describes an overview of the unicast routing protocol
   and its algorithm.

3.4.1 Route Exchange

   SSP is a distance vector protocol to establish and maintain the
   routing table. In SSP, each switch sends a routing update message to
   every adjacent switches every FULL_UPDATE_TIME (10 seconds by
   default). The update message is a copy of the routing table, that is,

   When a switch receives an update message from an adjacent switch
   through a port, it adds the cost associated with the port, usually 1,
   to every metric value in the message. The result is a set of new
   metrics from the receiving switch to the destination switches. Next,
   it compares the new metrics with those of the corresponding entries
   in the existing routing table. A smaller metric means a better route.
   Thus, if the new metric is smaller than the existing one, the entry
   is updated with the new metric and next hop. The next hop is the port
   from which the update message was received. Otherwise, the entry is
   left unchanged. If the existing next hop is the same as the new one,
   the metric is updated regardless of the metric value.  If no
   corresponding route is found, a new route entry is created.
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3.4.2 Route Expiration

   Assume a route to R is advertised by a neighboring switch S. If no
   update message has been received from switch S for the period
   FULL_UPDATE_TIME * 3 (30 seconds by default) or the route is
   advertised with metric 16 by switch S, the route to R is marked as
   unreachable by setting its metric to 16. In other words, the route to
   R is kept advertised even if the route is not refreshed up-to 30

   To process this, each routing table entry has an EXPIRATION_TIMER (30
   seconds by default, that is, FULL_UPDATE_TIME *3). If another switch
   advertises a route to R, it replaces the unreachable route. Even if a
   route is marked unreachable, the entry is kept in the routing table
   for the period of FULL_UPDATE_TIME * 3.  This enables the switch to
   notify its neighbors of the unreachable route by sending update
   messages with metric 16. To process this, each routing table entry
   has a garbage collection timer GC_TIMER (30 seconds by default). The
   entry is deleted on expiration of the timer. Figure 3 shows this

         The Last Update           Expiration         Garbage Collection
               |                       |                       |
    Routing    V   T       T       T   V   T       T       T   V
    Table      +-------+-------+-------+-------+-------+-------X
    Entry             metric < 16      |       metric = 16     |

                   EXPIRATION_TIMER            GC_TIMER
                                                       Stop Advertising
    Advertised                                                 V
    Metric     --   metric <16   ------+--  metric = 16 -------X

                                                    T: FULL_UPDATE_TIME

                       Figure 3. Route Expiration

3.4.3 Slow Convergence Prevention

   To prevent slow convergence of routing information, two techniques,
   split horizon with poisoned reverse, and triggered update are
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           Sn <------------- S3 <- S2 <- S1

                   (i) Before Outage

           Sn <--    X    -- S3 <- S2 <- S1

                   (ii) After Outage

                Figure 4 An Example of Slow Convergence

   Figure 4 shows an example of slow convergence[6]. In (i), three
   switches, S1, S2, and S3, are assumed to have a route to Sn. In (ii),
   the connection to Sn has disappeared because of an outage, but S2
   continue to advertise the route since there is no means for S2 to
   detect the outage immediately and it has the route to Sn in its
   routing table. Thus, S3 misunderstand that S2 has the best route to
   Sn and S2 is the next hop. This results in a transitive loop between
   S2 and S3. S2 and S3 increments the metric of the route to Sn every
   time they advertise the route and the loop continues until the metric
   reaches 16. To suppress the slow convergence problem, split horizon
   with poisoned reverse is used.

   In split horizon with poisoned reverse, a route is advertised as
   unreachable to the next hop. The metric is the received metric value
   plus 16. For example, in Figure 4, S2 advertises the route to Sn with
   the metric unreachable only to S3. Thus, S3 never considers that S2
   is the next hop to Sn. This ensures fast convergence on disappearance
   of a route.

   Another technique, triggered update, forces a switch to send an
   immediate update instead of waiting for the next periodic update when
   a switch detects a local port failure, or when it receives a message
   that a route has become unreachable, or that its metric has
   increased. This makes the convergence faster.

4. Broadcast/multicast Routing in SSP

   This section explains VRPB algorithm and the outline of
   broadcast/multicast routing protocol.
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4.1 Virtual Reverse Path Broadcast/Multicast Algorithm

   SSP provides broadcast/multicast routing based on a spanning tree
   algorithm.  As described in Section 2, the routing is based on the
   VRPB(Virtual Reverse Path Broadcast) algorithm.  In VRPB, each switch
   assumes that all broadcast and multicast frames are generated by a
   specific switch, VSS(Virtual Source Switch). Thus, unlike DVMRP, a
   MAPOS network has only one spanning tree at any given time.

   The frames are forwarded along the reverse path by computing the
   shortest path from the VSS to all possible recipients.  VSS is the
   switch which has the lowest switch number in the network.  Because
   the routing table contains all the unicast destination addresses
   including the switch numbers, each switch can identify the VSS
   independently by searching for the smallest switch number in its
   unicast routing table.

   In Figure 2, switch S1 is the VSS.  Each switch determines its place
   in the spanning tree, relative to the VSS, and which of its ports are
   on the shortest path tree.  Thus, the spanning tree is as shown in
   Figure 5. Except for the VSS, each switch has one upstream port and
   zero or more downstream ports. VSS have no upstream port, since it is
   the root of the spanning tree. In Figure 2.  switch S2's upstream
   port is port 0x09 and it has no downstream port.

                   S1 (VSS)
                  /  \
                 /    \
                /      \
               S2      S3

                      Figure 5  VRPB Spanning Tree

   When a switch receives a broadcast/multicast frame, it forwards the
   frame to all of the upstream switch, the downstream switches, and the
   directly connected nodes. However, it does not forward to the switch
   which sent the frame to it. For that purpose, a bit mapped
   broadcast/multicast routing table may be employed.  The
   broadcast/multicast routing process marks all the bits corresponding
   to the ports to which frames should be forwarded. The forwarding
   process refers to it and broadcasts a frame to all the ports with its
   corresponding bit marked.

4.2 Forwarding Broadcast/multicast Frames

   When a switch forwards a broadcast/multicast frame, (1) it first
   decides the VSS by referring to its unicast routing table. Then, (2)
   it refers to its broadcast/multicast routing table corresponding to
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   the VSS. A cache may be used to reduce the search overhead. (3) Based
   on the routing table, the switch forwards the frame.

   Figure 6 shows an example of S2's broadcast/multicast routing table
   for the VSS S1. It is a bit map table and each bit corresponds to a
   port. The value 1 indicates that frames should be forwarded to a node
   or a switch through the port.  If no bit is marked, the frame is
   silently discarded. In the example of Figure 6, port 0x09 is the
   upstream port to its VSS, that is, S1. Other ports, ports 0x05 and
   0x03 are path to N2 and N1 nodes, respectively.

             0F  0D  0B  09  07  05  03  01   ---   port number
           | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 |  ---   1: forward
           +---+---+---+---+---+---+---+---+        0: inhibit

            Figure 6 Broadcast/Multicast Routing Table of S2

4.3 Forwarding Path Examples

   Assume that a broadcast frame is generated by N2 in Figure 2. The
   frame is received by S2.

   Then, S2 passes it to all the connected nodes except for the source
   N2. That is, only to N1. At the same time, it also forwards the frame
   to all its upstream and downstream switches. Since S2 has no
   downstream switch, S2 forwards the frame to S1 though its upstream
   port 0x09.

   S1 is the VSS and it passes the frame to all the local nodes, that
   is, only to N3. Since it has no upstream switch and S2 is the switch
   which sent the frame to S1, the frame is eventually forwarded only to
   a downstream switch S3.

   S3 passes the frame to its local node, N4. Since S3 has only an
   upstream and the frame was received through that port, S3 does not
   forward the frame to any switch.

   The resulting path is shown in Figure 7. Although this is not the
   optimal path, VRPB ,at least, ensures that broadcast/multicast frames
   are delivered all the nodes without a loop. Figures 8 and 9 show the
   forwarding path for frames generated by a node under S3 and S4,
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                             +-> N3
             N2 -> S2 +-> S1 +-> S3 -> N4
                      +-> N1

                   Figure 7  Forwarding Path from N2

                             +-> N1
             N3 -> S1 +-> S2 +-> N2
                      +-> S3 --> N4

                   Figure 8  Forwarding Path from N3

                             +-> N3
             N4 -> S3 +-> S1 +-> S2 +-> N1
                                    +-> N2

                   Figure 9  Forwarding Path from N4

4.4 Suppressing Routing Loop

   To suppress transitive routing loop, forward delay is employed. A
   switch suspends broadcast/multicast forwarding for a period after a
   new VSS is found in the routing table. This prevents transitive
   routing loop by waiting for all the switches to have the same routing
   information and become synchronized. In addition to controlling
   sending of frames by forward delay, another mechanism is employed to
   prevent transitive routing loop by controlling reception of frames.
   That is, broadcast/multicast frames received through ports other than
   the upstream and downstream ports are discarded.

4.5 Upstream Switch Discovery

   The upstream port is determined by the shortest reverse path to the
   VSS.  It is identified by referring to the next hop port of the route
   to VSS in the local unicast routing table. When a new next hop to the
   VSS is discovered, the bit corresponding to the old next hop port is
   cleared, and the bit corresponding to the new one is marked as the
   upstream port in the broadcast/multicast routing table.
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4.6 Downstream Switch Discovery

   To determine the downstream ports, split horizon with poisoned
   reverse is employed. When a switch receives a route with a metric
   poisoned by split horizon processing through a port as described in
   Section 3.4.3, the port is considered to be a downstream port. In
   Figure 2, S1 is the VSS and the route information is sent back from
   S2 to S1 with metric unreachable based on the split horizon with
   poisoned reverse. Thus, S1 knows that S2 is one of its downstreams.

4.7 Downstream Port Expiration

   When a poison reversed packet is newly received from a port, the
   local switch knows that a new downstream switch has appeared. Then,
   it marks the bit corresponding to the port and starts
   FORWARD_DELAY_TIMER (30second by default, that is, FULL_UPDATE_TIME *
   3) for the port. The forwarding of broadcast/multicast frames to the
   port is prohibited until the timer expires.  Every time the local
   switch receives a poison reversed packet through a port, it
   initializes PORT_EXPIRATION_TIMER(30 seconds by default, that is,
   FULL_UPDATE_TIME *3) corresponding to the port. A continuous loss of
   poison reversed packets or a failure of downstream port results in
   expiration of PORT_EXPIRATION_TIMER, and the corresponding bit is

               First Update               Last Update
                   |                           |
                   V T   T   T   T   T   T   T V
   A bit in
   the routing      0   0   0   1   1   1   1   1   1   1   0   0   0
   table                       ^                           ^
                    <--------->|                <--------->|
                        ^   route up                 ^ route down
                        |                            |
                  FORWARD_DELAY               PORT_EXPIRATION

                                           T: FULL_UPDATE_TIME

                       Figure 10. Port Expiration

   When a downstream switch discovers another best path to the VSS or a
   new VSS, it stops split horizon with poison reverse and sends
   ordinary update messages. Whenever the local switch receives an
   ordinary update message from its downstream switch, it SHOULD
   immediately clear the corresponding bit in the routing table and stop
   forwarding of broadcast/multicast frames.
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4.8 Node Discovery

   When a NSP[9] packet, requesting a node address from a port, is
   received, the local switch considers that a new node is connected,
   and marks the corresponding bit in the broadcast/multicast routing
   table. When the local switch detects that the port went down as
   described in [9], it clear the corresponding bit.

4.9 Invalidating The Broadcast/multicast Routing Table

   When a new VSS is discovered or when the VSS becomes unreachable, the
   entire broadcast/multicast routing table is invalidated. That is, a
   change of upstream port affects the entire broadcast/multicast
   routing. However, a change of a downstream port does not affect
   forwarding to other downstream ports, its upstream port, and nodes.

5. Detailed Protocol Operation

   This section explains SSP packet format and protocol processing in

5.1 Packet Format

   This subsection describes the packet encapsulation in HDLC frame and
   the packet format.

5.1.1 Packet Format and Its Encapsulation

   SSP packet format is designed based on RIP[6] and its successor, RIP2
   [7]. Figure 11 shows the packet format. A SSP packet is encapsulated
   in the information field of a MAPOS HDLC frame. The HDLC protocol
   field of SSP is 0xFE05 in hex as defined by the "MAPOS Version 1
   Assigned Numbers" [10]. The packet is sent encapsulated in a unicast
   packet with the destination address 0000 0001, which indicates the
   control processor of an adjacent switch.
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(MSB)                                                       (LSB)
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -----
|    Command    |   Version     |           unused              |SSP header
+---------------+---------------+-------------------------------+ -----
| Address Family Identifier     |            All 0              |
|                         HDLC Address                          | an SSP
+---------------------------------------------------------------+ route
|                         Subnet Mask                           | entry
|                         All 0                                 |
|                         Metric                                |
+---------------+---------------+-------------------------------+ ----
| Address Family Identifier     |            All 0              |

                      Figure 11 SSP packet format

   The maximum packet size is 512 octet. The first four octets is the
   SSP header. The remainder of the message is composed of 1 - 25 route
   entries. Each entry is 20 octets long.

5.1.2 SSP Header

   SSP header consists of a command field and a version field. The
   command field is one octet long and holds one of the following

     1 - request     A request to send all or part of SSP routing table.

     2 - response    A message containing all, or a part of the sender's
                     SSP routing table.  This message may be sent in
                     response to a request, or it may be an update
                     message generated by the sender.

   The Version field indicates the version of SSP being used. The
   current version number is 1.

5.1.3 SSP Route Entries

   Each entry has an address family identifier. It indicates an
   attribute of the entry. SSP routing protocol uses 2 as its identifier
   by default. The identifier 0 indicates unspecified. This value is
   used when a switch requests other switches to send the entire SSP
   routing table. A recipient of the message SHOULD ignore all entries
   with unknown value.
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   The HDLC address is a destination address. It may be a switch address
   or a node address. The subsequent subnet mask is applied to the HDLC
   address to yield the switch number portion. The field is 4 octet long
   and the address is placed in the least significant position.

   Metric indicates the distance to the destination node. That is, how
   many switches a message must go through en route to the destination
   node. The metric field must contain a value between 1 and 31. The
   metric of 16 indicates that the destination is not reachable and is
   ignored by recipients. The values between 17 and 31 are utilized for
   poisoned reverse with split horizon and also means unreachable. The
   metric 0 indicates the local switch itself.

5.2 Routing Table

   Every switch has an SSP routing table. The table is a collection of
   route entries - one for every destination. An entry consists of the
   following information;

    (1) destination : A unicast destination address.

    (2) subnet mask : A mask to extract the switch address by applying
    bitwise AND with the destination address

    (3) next hop port : The local port number connected to the adjacent
    switch along the path to the destination.

    (4) metric : Distance to the destination node. The metric of an
    adjacent switch is 1 and that of local switch is 0.

    (5) timers for unicast routing : Timers associated with unicast
    routing such as EXPIRATION_TIMER and GC_TIMER.

    (6) flags : Various flags associated with the route such as route
    change flag to indicate that the route has changed recently or it
    has timed out.

    (7) bit map routing table for broadcast/multicast : Each bit
    corresponding to the port to an upstream or a downstream switch of
    the spanning tree is marked in addition to the ports to end nodes.
    Broadcast/multicast frames are forwarded only through those ports
    with their corresponding bit set. Since only one spanning tree
    exists at a time in a network, each route entry does not necessarily
    have to have this field.
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    (8) timers for broadcast/multicast routing : Timers associated with
    broadcast/multicast routing such as FORWARD_DELAY_TIMER and
    PORT_EXPIRATION_TIMER. These timers are prepared for each bit of
    broadcast/multicast routing table.

5.3 Sending Routing Messages

5.3.1 Packet Construction

   Because of the split horizon with poisoned reverse, a routing message
   differs depending on the adjacent switch to which the message is
   being sent. The upstream switch of a route, that is next hop,
   receives a message which contains the corresponding route with a
   metric between 17 and 31. Switches that are not the upstream switch
   of any route receive the same message. Here, we assume that a packet
   for a routing message is constructed for an adjacent switch which is
   connected through the local port N.

   First, set the version field to 1, the current SSP version. Then, set
   the command to "response". Set other fields which are supposed to be
   zero to zero.  Next, start filling in entries.

   To fill in the entries, perform the following for each route. The
   destination HDLC address, netmask, and its metric are put into the
   entry in the packet.  Routes must be included in the packet even if
   their metrics are unreachable(16).  If the next hop port is N, 16 is
   added to the metric for split horizon with poisoned reverse.

   Recall that the maximum packet size is 512 bytes.  When there is no
   more space in a packet, send the current message and start a new one.
   If a triggered update is being generated, only entries whose route
   change flags are set need be included.

5.3.2 Sending update

   Sending update may be triggered in any of the following ways;

    (1) Initial Update

    When a switch first comes up, it SHOULD send to all adjacent
    switches a request asking for their entire routing tables. The
    destination address is 00000001. When a port comes on-line, the
    request packet is sent to the port. The packet, requesting the
    entire routing table, MUST have at least an entry with the address
    family identifier 0 meaning unspecified.

    When a switch receives a request packet, it first checks the version
    number of the SSP header. If it is not 1, the packet is silently
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    discarded. Otherwise, the address family identifier is examined.  If
    the value is 0, the entire SSP routing table is returned in one or
    more response packets destined to 00000001. Otherwise, the request
    is silently discarded.  Although the original RIP specification
    defines the partial routing table request, SSP routing protocol
    omits it for the sake of simplicity.

    (2) Periodic Update

    Every switch participating in the routing process sends an update
    message (response message) to all its neighbor switches once every
    FULL_UPDATE_TIME (10 seconds). For the periodic update, a response
    packet(s) is used. The destination address is always 00000001. An
    update message contains the entire SSP routing table. The maximum
    packet size is 512byte. Thus, an update message may require several
    packets to be packed.

    (3) Triggered Update

    When a route in the unicast routing table is changed or a local port
    goes down, the switch advertises a triggered update packet without
    waiting for the full update time. The difference between triggered
    update and the other update is that triggered updates do not have to
    include the entire routing table. Only changed entries should be
    included. Triggered update may be suppressed if a regular periodic
    update is due.

    Note that when a route is advertised as unreachable (metric 16) by
    an adjacent switch, update process is triggered as well as
    expiration of the route in the local switch.

    (4) On Termination

    When a switch goes down, it is desirable to advertise all the routes
    with metric 16, that is, unreachable.

5.4 Receiving Routing Messages

   When a switch receives an update, it first checks the version number.
   If it is not 1, the update packet is silently discarded. Otherwise,
   it processes the entries in it one by one.
ToP   noToC   RFC2174 - Page 17
   For each entry, the address family identifier is checked. If it is
   not 2, the entry is ignored. Otherwise, the metric is checked. The
   value should be between 0 and 31.  An entry with illegal metric is
   ignored. Next, the HDLC address and the subnet mask is checked. An
   entry with an invalid address such as broadcast is ignored. If the
   entry passed all these validation checks, it is processed according
   to the following steps;

   Step 1 - Process Poisoned Reverse

   If the metric value is between 0 and 16, it is an unicast
   information. Go ahead to Step 2.

   If the metric value is between 17 and 31, it indicates poisoned
   reverse, that the local switch has been chosen as the next hop for
   the route. However, if the corresponding entry is not included in the
   current routing table or the message is from a port connected to its
   upstream switch, the message is illegal -- ignore it and return to
   Step 1 to process the next entry. Otherwise,

      (1) Initialize the PORT_EXPIRATION_TIMER corresponding to the
          downstream port.
      (2) Operate the FORWARD_DELAY_TIMER as follows;
          (2-1) If the broadcast/multicast forwarding was already
                enabled, go to (3).
          (2-2) If the FORWARD_DELAY_TIMER corresponding to the
                downstream port was already started, increment the
                timer. If the timer expires, mark the bit in the
                broadcast/multicast routing table corresponding to the
                port and stop the timer.
          (2-2) Otherwise, start the FORWARD_DELAY_TIMER.
      (3) Return to Step 1 to process the next entry.

    Step 2 - Process Unicast Routing Information

    First, add the cost associated with the link, usually 1, to the
    metric. If the result is greater than 16, 16 is used. Then, look up
    the unicast routing table for the corresponding entry. There are two

     Case 1  no corresponding entry is found

       If the new metric is 16, return to step 1 to process the next
       entry.  Otherwise,
       (1) Create a new route entry in the routing table
       (2) Initialize EXPIRATION_TIMER and GC_TIMER
ToP   noToC   RFC2174 - Page 18
       (3) The port corresponding to the new route is the next_hop port
           for the route. Thus, mark the bit in the broadcast/multicast
           routing table corresponding to the new next_hop port and
           start FORWARD_DELAY_TIMER. If this new route is for the
           switch with the minimum switch number, select it as the VSS
           and use its broadcast/multicast routing table. (See NOTE 1.)
       (4) Set the route change flag and invoke triggered update process
       (5) Return to step 1 to process the next entry.

           [NOTE 1]
             There are two implementations;
              (1) Prepare a spanning tree for each route and use
                  only one corresponding to the current VSS. In this
                  case, each unicast route entry has a broadcast/unicast
                  routing table.
              (2) Prepare only one spanning tree corresponding to the
                  current VSS. In this case, a switch has only one
                  broadcast/multicast routing table.
              In this document, the former is assumed.

      Case 2. A corresponding entry is found

       In this case, the update message is processed differently
       according to the new metric value.

       (a) new_metric < 16 & new_metric > current_metric

          (1)If and only if the update is from the same port(next_hop
             port) as the existing one,
            (1-1) Update the entry
            (1-2) Initialize EXPIRATION_TIMER and GC_TIMER

          (2) If the corresponding bit to the port, which the update
              message is received, is marked in the broadcast/multicast
              routing table, clear the bit.
          (3) Return to Step 1 and process the next entry.

       (b) new_metric < 16 & new_metric < current_metric

          (1) Update the entry and clear the bit in the
              broadcast/multicast routing table corresponding to the old
              next_hop port.
          (2) Initialize EXPIRATION_TIMER, GC_TIMER, and
              PORT_EXPIRATION_TIMER for the new next_hop port.
          (3) Mark a bit in the broadcast/multicast routing table
              corresponding to the new next_hop port and start
ToP   noToC   RFC2174 - Page 19
          (4) Set the route change flag and invoke triggered update with
              poisoned reverse for the new next_hop.
          (5) Return to Step 1 to process the next entry.

       (c) new_metric < 16 & new_metric = current_metric

          If a new route with the same metric value as the existing
          routing table entry is received, use the old one as follows;

          (1) If the new next hop is equal to the current one,
              initialize EXPIRATION_TIMER and GC_TIMER. Otherwise,
              ignore this update.
          (2) If the bit corresponding to the port, from which the
              update message was received, is marked in the
              broadcast/multicast routing table, clear the bit.
          (3) Return to Step 1 to process the next entry.

       (d) the new metric = 16 & the new next hop = the current one

          If the current metric is not equal to 16, this is a new
          unreachable information. Then,
          (1) Update the entry and clear the bit in the
              broadcast/multicast routing table corresponding to the old
              next_hop port.
          (2) If this route is for the current VSS, select a new VSS in
              the valid routing table entries. Valid means that the
              destination is reachable.
          (3) Set the route change flag and invoke triggered update
              process to notify the unreachable route.
              do nothing and return to Step 1 to process the next entry.

       (e) the new metric = 16 & the new next hop /= the current one

          (1) If the bit corresponding to the port, from which the
              update message was received, is marked in the
              broadcast/multicast routing table, clear the bit.
          (2) Return to Step 1 to process the next entry.
ToP   noToC   RFC2174 - Page 20
5.5 Timers

   The timer routine increments the following timers and executes its
   associated process on their expiration.


    The EXPIRATION_TIMERs and GC_TIMERs of each entry in the unicast
    routing table are incremented every FULL_UPDATE_TIME (10 seconds by
    default). When a EXPIRATION_TIMER expires, the metric is changed to
    unreachable(16), update process is triggered, and GC_TIMER is
    started. When a GC_TIMER expires, the entry is deleted from the
    local routing table. EXPIRATION_TIMER and GC_TIMER are cleared every
    time a switch receives a routing update.


    FORWARD_DELAY_TIMER is completely handled in the receive process and
    has no relation to the timer routine.


    PORT_EXPIRATION_TIMERs associated with each bit in the
    broadcast/multicast routing table are incremented every
    FULL_UPDATE_TIME (10 seconds by default).  When the timer expires,
    the corresponding downstream switch is considered to be down and the
    corresponding bit in the broadcast/multicast routing table is
    cleared. This timer is cleared by the receive process every time a
    poisoned reverse packet is received from the corresponding switch.

6. Further considerations on implementation

6.1 Port State

   A switch assumes that every port is connected to a switch initially.
   Thus, it sends update packets to every port. When a node is connected
   to a port, the switch recognizes it by receiving an NSP request
   packet, and stops sending SSP packets to the port. Whenever a switch
   detects a connection failure such as loss of signal and out-of-
   synchronization, it should clear the internal state table
   corresponding of the port.

6.2 Half way connection problem

   A port consists of two channels, transmit and receive. Although it is
   easy for a node or a switch to detect a receive channel failure,
   transmit channel failure may not be detected, causing half way
   connection.  This results in a black hole.
ToP   noToC   RFC2174 - Page 21
   Thus, whenever a switch receives a SSP update packet from a port, it
   SHOULD check the status of the corresponding transmit channel.
   SONET/SDH has a feedback mechanism for that purpose. The status of
   the local transmit channel received at the remote end can be sent
   back utilizing the overhead part, FEBE(Far End Block Error) and
   FERF(Far End Receive Failure), of the corresponding receive channel.
   If the signals indicates that the transmit channel has a problem, the
   SSP packet received from the remote end should be silently discarded.
   However, some SONET/SDH services do not provide path overhead

   Although, SONET/SDH APS(Automatic Protection Switching) can be
   utilized to switch service from a failed line to a spare line, the
   function is out of scope of this protocol.

7. Security Considerations

   Security issues are not discussed in this memo.


   [1]   Murakami, K. and M. Maruyama, "MAPOS - Multiple Access Protocol
         over SONET/SDH Version 1," RFC2171, June 1997.

   [2]   CCITT Recommendation G.707: Synchronous Digital Hierarchy Bit
         Rates, 1990.

   [3]   CCITT Recommendation G.708: Network Node Interface for
         Synchronous Digital Hierarchy, 1990.

   [4]   CCITT Recommendation G.709: Synchronous Multiplexing Structure,

   [5]   American National Standard for Telecommunications - Digital
         Hierarchy - Optical Interface Rates and Formats Specification,
         ANSI T1.105-1991.

   [6]   Hedrick, C., "Routing Information Protocol", STD 34, RFC 1058,
         Rutgers University, June 1988.

   [7]   Malkin, G., "RIP Version 2 - Carrying Additional Information ",
         RFC1723, Xylogics, Inc., November 1994.

   [8]   Pusateri, T., "Distance Vector Multicast Routing Protocol",
         September 1996, Work in Progress.

   [9]   Murakami, K. and M. Maruyama, "A MAPOS version 1 Extension -
         Node Switch Protocol," RFC2173, June 1997.
ToP   noToC   RFC2174 - Page 22
   [10]  Maruyama, M. and K. Murakami, "MAPOS Version 1 Assigned
         Numbers," RFC2172, June 1997.


   The authors would like to acknowledge the contributions and
   thoughtful suggestions of John P. Mullaney, Clark Bremer, Masayuki
   Kobayashi, Paul Francis, Toshiaki Yoshida, Takahiro Sajima, and
   Satoru Yagi.

Authors' Address

             Ken Murakami
             NTT Software Laboratories
             3-9-11, Midori-cho
             Tokyo 180, Japan

             Mitsuru Maruyama
             NTT Software Laboratories
             3-9-11, Midori-cho
             Tokyo 180, Japan