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rfc:rfc2174

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.

Abstract

 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
 node.
 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
 change.

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
 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.

Murakami & Maruyama Informational [Page 3] RFC 2174 MAPOS June 1997

                      01000011
                      +------+
                      | 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) |
                                         +---+----+
                                             |0x07
             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].

Murakami & Maruyama Informational [Page 4] RFC 2174 MAPOS June 1997

   +-------------------------+----------+--------+------------+
   | 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,
 routes.
 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.

Murakami & Maruyama Informational [Page 5] RFC 2174 MAPOS June 1997

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
 seconds.
 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
 transition.
       The Last Update           Expiration         Garbage Collection
             |                       |                       |
  Routing    V   T       T       T   V   T       T       T   V
  Table      +-------+-------+-------+-------+-------+-------X
  Entry             metric < 16      |       metric = 16     |
  1. ———————→|———————→|

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
 employed.

Murakami & Maruyama Informational [Page 6] RFC 2174 MAPOS June 1997

         Sn <------------- S3 <- S2 <- S1
                 (i) Before Outage
  1. >

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.

Murakami & Maruyama Informational [Page 7] RFC 2174 MAPOS June 1997

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

Murakami & Maruyama Informational [Page 8] RFC 2174 MAPOS June 1997

 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,
 respectively.

Murakami & Maruyama Informational [Page 9] RFC 2174 MAPOS June 1997

                           +-> 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.

Murakami & Maruyama Informational [Page 10] RFC 2174 MAPOS June 1997

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
 cleared.
             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.

Murakami & Maruyama Informational [Page 11] RFC 2174 MAPOS June 1997

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
 detail.

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.

Murakami & Maruyama Informational [Page 12] RFC 2174 MAPOS June 1997

(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

+—————+—————+——————————-+ —–

Address Family Identifier All 0

+——————————-+——————————-+

HDLC Address

+—————————————————————+ route

Subnet Mask

+—————————————————————+

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
 values;
   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.

Murakami & Maruyama Informational [Page 13] RFC 2174 MAPOS June 1997

 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.

Murakami & Maruyama Informational [Page 14] RFC 2174 MAPOS June 1997

  (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

Murakami & Maruyama Informational [Page 15] RFC 2174 MAPOS June 1997

  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.

Murakami & Maruyama Informational [Page 16] RFC 2174 MAPOS June 1997

 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
  cases.
   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

Murakami & Maruyama Informational [Page 17] RFC 2174 MAPOS June 1997

     (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
            FORWARD_DELAY_TIMER.

Murakami & Maruyama Informational [Page 18] RFC 2174 MAPOS June 1997

        (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.
        Otherwise,
            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.

Murakami & Maruyama Informational [Page 19] RFC 2174 MAPOS June 1997

5.5 Timers

 The timer routine increments the following timers and executes its
 associated process on their expiration.
  (1) EXPIRATION_TIMER and GC_TIMER
  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.
  (2) FORWARD_DELAY_TIMER
  FORWARD_DELAY_TIMER is completely handled in the receive process and
  has no relation to the timer routine.
  (3) PORT_EXPIRATION_TIMER
  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.

Murakami & Maruyama Informational [Page 20] RFC 2174 MAPOS June 1997

 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
 transparency.
 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.

References

 [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,
       1990.
 [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.

Murakami & Maruyama Informational [Page 21] RFC 2174 MAPOS June 1997

 [10]  Maruyama, M. and K. Murakami, "MAPOS Version 1 Assigned
       Numbers," RFC2172, June 1997.

Acknowledgements

 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
           Musashino-shi
           Tokyo 180, Japan
           E-mail: murakami@ntt-20.ecl.net
           Mitsuru Maruyama
           NTT Software Laboratories
           3-9-11, Midori-cho
           Musashino-shi
           Tokyo 180, Japan
           E-mail: mitsuru@ntt-20.ecl.net

Murakami & Maruyama Informational [Page 22]

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