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Versions: 02 03 rfc2080                                                 
Draft-ietf-rip-ripng-02.txt                           G. Malkin/Xylogics
                                                           February 1996

                             RIPng for IPv6


   This document specifies a routing protocol for an IPv6 internet.  It
   is based on protocols and algorithms currently in wide use in the
   IPv4 Internet.

   This specification represents the minimum change to the Routing
   Information Protocol (RIP), as specified in RFC 1058 [1] and RFC 1723
   [2], necessary for operation over IPv6 [3].

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific


   This document is a modified version of RFC 1058, written by Chuck
   Hendrick [1].  The modifications reflect RIP-2 and IPv6 enhancements,
   but the original wording is his.

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                             Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Theoretical Underpinnings  . . . . . . . . . . . . . . . . .  4
   1.2   Limitations of the Protocol  . . . . . . . . . . . . . . . .  4
   2.  Protocol Specification . . . . . . . . . . . . . . . . . . . .  4
   2.1   Message Format . . . . . . . . . . . . . . . . . . . . . . .  6
   2.2   Addressing Considerations  . . . . . . . . . . . . . . . . .  7
   2.3   Timers . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   2.4   Input Processing . . . . . . . . . . . . . . . . . . . . . .  9
   2.4.1   Request Messages . . . . . . . . . . . . . . . . . . . . .  9
   2.4.2   Response Messages  . . . . . . . . . . . . . . . . . . . . 10
   2.5   Output Processing  . . . . . . . . . . . . . . . . . . . . . 13
   2.5.1   Triggered Updates  . . . . . . . . . . . . . . . . . . . . 13
   2.5.2   Generating Response Messages . . . . . . . . . . . . . . . 14
   2.6   Split Horizon  . . . . . . . . . . . . . . . . . . . . . . . 15
   3.  Control Functions  . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Security Considerations. . . . . . . . . . . . . . . . . . . . 16
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17

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1. Introduction

   This memo describes one protocol in a series of routing protocols
   based on the Bellman-Ford (or distance vector) algorithm.  This
   algorithm has been used for routing computations in computer networks
   since the early days of the ARPANET.  The particular packet formats
   and protocol described here are based on the program "routed," which
   is included with the Berkeley distribution of Unix.

   In an international network, such as the Internet, it is very
   unlikely that a single routing protocol will used for the entire
   network.  Rather, the network will be organized as a collection of
   Autonomous Systems (AS), each of which will, in general, be
   administered by a single entity.  Each AS will have its own routing
   technology, which may differ among AS's.  The routing protocol used
   within an AS is referred to as an Interior Gateway Protocol (IGP).  A
   separate protocol, called an Exterior Gateway Protocol (EGP), is used
   to transfer routing information among the AS's.  RIPng was designed
   to work as an IGP in moderate-size AS's.  It is not intended for use
   in more complex environments.  For information on the context into
   which RIP version 1 (RIP-1) is expected to fit, see Braden and Postel

   RIPng is one of a class of algorithms known as Distance Vector
   algorithms.  The earliest description of this class of algorithms
   known to the author is in Ford and Fulkerson [8].  Because of this,
   they are sometimes known as Ford-Fulkerson algorithms.  The term
   Bellman-Ford is also used, and derives from the fact that the
   formulation is based on Bellman's equation [4].  The presentation in
   this document is closely based on [5].  This document contains a
   protocol specification.  For an introduction to the mathematics of
   routing algorithms, see [1].  The basic algorithms used by this
   protocol were used in computer routing as early as 1969 in the
   ARPANET.  However, the specific ancestry of this protocol is within
   the Xerox network protocols.  The PUP protocols [7] used the Gateway
   Information Protocol to exchange routing information.  A somewhat
   updated version of this protocol was adopted for the Xerox Network
   Systems (XNS) architecture, with the name Routing Information
   Protocol [9].  Berkeley's routed is largely the same as the Routing
   Information Protocol, with XNS addresses replaced by a more general
   address format capable of handling IPv4 and other types of address,
   and with routing updates limited to one every 30 seconds.  Because of
   this similarity, the term Routing Information Protocol (or just RIP)
   is used to refer to both the XNS protocol and the protocol used by

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1.1  Theoretical Underpinnings

   An introduction to the theory and math behind Distance Vector
   protocols is provided in [1].  It has not been incorporated in this
   document for the sake of brevity.

1.2  Limitations of the Protocol

   This protocol does not solve every possible routing problem.  As
   mentioned above, it is primary intended for use as an IGP in networks
   of moderate size.  In addition, the following specific limitations
   are be mentioned:

   - The protocol is limited to networks whose longest path (the
     network's diameter) is 15 hops.  The designers believe that the
     basic protocol design is inappropriate for larger networks.  Note
     that this statement of the limit assumes that a cost of 1 is used
     for each network.  This is the way RIPng is normally configured.
     If the system administrator chooses to use larger costs, the upper
     bound of 15 can easily become a problem.

   - The protocol depends upon "counting to infinity" to resolve certain
     unusual situations (see section 2.2 in [1]).  If the system of
     networks has several hundred networks, and a routing loop was
     formed involving all of them, the resolution of the loop would
     require either much time (if the frequency of routing updates were
     limited) or bandwidth (if updates were sent whenever changes were
     detected).  Such a loop would consume a large amount of network
     bandwidth before the loop was corrected.  We believe that in
     realistic cases, this will not be a problem except on slow lines.
     Even then, the problem will be fairly unusual, since various
     precautions are taken that should prevent these problems in most

   - This protocol uses fixed "metrics" to compare alternative routes.
     It is not appropriate for situations where routes need to be chosen
     based on real-time parameters such a measured delay, reliability,
     or load.  The obvious extensions to allow metrics of this type are
     likely to introduce instabilities of a sort that the protocol is
     not designed to handle.

2. Protocol Specification

   RIPng is intended to allow routers to exchange information for
   computing routes through an IPv6-based network.  RIPng is a distance
   vector protocol, as described in [1].  RIPng should be implemented
   only in routers; IPv6 provides other mechanisms for router discovery

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   [10].  Any router that uses RIP is assumed to have interfaces to one
   or more networks, otherwise it isn't really a router.  These are
   referred to as its directly-connected networks.  The protocol relies
   on access to certain information about each of these networks, the
   most important of which is its metric.  The RIPng metric of a network
   is an integer between 1 and 15, inclusive.  It is set in some manner
   not specified in this protocol; however, given the maximum path limit
   of 15, a value of 1 is usually used.  Implementations should allow
   the system administrator to set the metric of each network.  In
   addition to the metric, each network will have an IPv6 destination
   address prefix and prefix length associated with it.  These are to be
   set by the system administrator in a manner not specified in this

   Each router that implements RIPng is assumed to have a routing table.
   This table has one entry for every destination that is reachable
   throughout the system operating RIPng.  Each entry contains at least
   the following information:

   - The IPv6 prefix of the destination.

   - A metric, which represents the total cost of getting a datagram
     from the router to that destination.  This metric is the sum of the
     costs associated with the networks that would be traversed to get
     to the destination.

   - The IPv6 address of the next router along the path to the
     destination (i.e., the next hop).  If the destination is on one of
     the directly-connected networks, this item is not needed.

   - A flag to indicate that information about the route has changed
     recently.  This will be referred to as the "route change flag."

   - Various timers associated with the route.  See section 2.3 for more
     details on timers.

   The entries for the directly-connected networks are set up by the
   router using information gathered by means not specified in this
   protocol.  The metric for a directly-connected network is set to the
   cost of that network.  As mentioned, 1 is the usual cost.  In that
   case, the RIP metric reduces to a simple hop-count.  More complex
   metrics may be used when it is desirable to show preference for some
   networks over others (e.g., to indicate of differences in bandwidth
   or reliability).

   Implementors may also choose to allow the system administrator to
   enter additional routes.  These would most likely be routes to hosts
   or networks outside the scope of the routing system.  They are

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   referred to as "static routes."  Entries for destinations other than
   these initial ones are added and updated by the algorithms described
   in the following sections.

   In order for the protocol to provide complete information on routing,
   every router in the AS must participate in the protocol.  In cases
   where multiple IGPs are in use, there must be at least one router
   which can leak routing information between the protocols.

2.1  Message Format

   RIPng is a UDP-based protocol.  Each router that uses RIPng has a
   routing process that sends and receives datagrams on UDP port number
   <TBD>, the RIPng port.  All communications intended for another
   routers's RIPng process are sent to the RIPng port.  All routing
   update messages are sent from the RIPng port.  Unsolicited routing
   update messages have both the source and destination port equal to
   the RIPng port.  Those sent in response to a request are sent to the
   port from which the request came.  Specific queries may be sent from
   ports other than the RIPng port, but they must be directed to the
   RIPng port on the target machine.

   The RIPng packet format is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |  command (1)  |  version (1)  |       must be zero (2)        |
      |                                                               |
      ~                        IPv6 prefix (16)                       ~
      |                                                               |
      |       must be zero (2)        | prefix len (1)|  metric (1)   |

      The 20-octet portion of the datagram from IPv6 prefix through
      metric, the Routing Entry (RTE), may appear one or more times.
      The maximum number of RTEs is defined below.

   Field sizes are given in octets.  Unless otherwise specified, fields
   contain binary integers, in network byte order, with the most-
   significant octet first (big-endian).  Each tick mark represents one

   Every message contains a RIPng header which consists of a command and
   a version number.  This document describes version 1 of the protocol
   (see section 2.4).  The command field is used to specify the purpose

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   of this message.  The commands implemented in version 1 are:

   1 - request    A request for the responding system to send all or
                  part of its routing table.

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

   For each of these message types, the remainder of the datagram
   contains a list of RTEs.  Each RTE in this list contains a
   destination prefix, the number of significant bits in the prefix, and
   the cost to reach that destination (metric).

   The destination prefix is the usual 128-bit, IPv6 address prefix
   stored as 16 octets in network byte order.

   The prefix length field is the number of 1-bits, starting from the
   left, in the prefix.

   The metric field contains a value between 1 and 15 inclusive,
   specifying the current metric for the destination; or the value 16
   (infinity), which indicates that the destination is not reachable.

   The maximum datagram size is limited by the MTU of the medium over
   which the protocol is being used.  Since an unsolicited RIPng update
   is never propagated across a router, there is no danger of an MTU
   mismatch.  The determination of the number of RTEs which may be put
   into a given message is a function of the medium's MTU, the number of
   octets of header information preceeding the RIPng message, the size
   of the RIPng header, and the size of an RTE.  The formula is:

               +-                                                   -+
               | MTU - sizeof(IPv6_hdrs) - UDP_hdrlen - RIPng_hdrlen |
   #RTEs = INT | --------------------------------------------------- |
               |                      RTE_size                       |
               +-                                                   -+

2.2  Addressing Considerations

   The distinction between network, subnet and host routes does not need
   to be made for RIPng because an IPv6 address prefix is unambiguous.

   The special address 0:0:0:0:0:0:0:0 (the prefix length is also zero)
   is used to designate a default route.  A default route is used when
   it is not convenient to list every possible network in the RIPng
   updates, and when one or more routers in the system are prepared to

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   handle traffic to the networks that are not explicitly listed.  These
   "default routers" use the default route as a path for all datagrams
   for which they have no explicit route.  The decision as to how a
   router becomes a default router (i.e., how a default route entry is
   created) is left to the implementor.  In general, the system
   administrator will be provided with a way to specify which routers
   should create and advertise default route entries.  If this mechanism
   is used, the implementation should allow the network administrator to
   choose the metric associated with the default route advertisement.
   This will make it possible to establish a precedence amoung multiple
   default routers.  The default route entries are handled by RIPng in
   exactly the same manner as any other destination prefix.  System
   administrators should take care to make sure that default routes do
   not propagate further than is intended.  Generally, each AS has its
   own preferred default router.  Therefore, default routes should
   generally not leave the boundary of an AS.  The mechanisms for
   enforcing this restriction are not specified in this document.

2.3  Timers

   This section describes all events that are triggered by timers.

   Every 30 seconds, the RIPng process is awakened to send an
   unsolicited Response message, containing the complete routing table
   (see section 2.6 on Split Horizon), to every neighboring router.
   When there are many routers on a single network, there is a tendency
   for them to synchronize with each other such that they all issue
   updates at the same time.  This can happen whenever the 30 second
   timer is affected by the processing load on the system.  It is
   undesirable for the update messages to become synchronized, since it
   can lead to unnecessary collisions on broadcast networks.  Therefore,
   implementations are required to take one of two precautions:

   - The 30-second updates are triggered by a clock whose rate is not
     affected by system load or the time required to service the
     previous update timer.

   - The 30-second timer is offset by a small random time (+/- 0 to 5
     seconds) each time it is set.

   There are two timers associated with each route, a "timeout" and a
   "garbage-collection time."  Upon expiration of the timeout, the route
   is no longer valid; however, it is retained in the routing table for
   a short time so that neighbors can be notified that the route has
   been dropped.  Upon expiration of the garbage-collection timer, the
   route is finally removed from the routing table.

   The timeout is initialized when a route is established, and any time

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   an update message is received for the route.  If 180 seconds elapse
   from the last time the timeout was initialized, the route is
   considered to have expired, and the deletion process described below
   begins for that route.

   Deletions can occur for one of two reasons: the timeout expires, or
   the metric is set to 16 because of an update received from the
   current router (see section 2.4.2 for a discussion of processing
   updates from other routers).  In either case, the following events

   - The garbage-collection timer is set for 120 seconds.

   - The metric for the route is set to 16 (infinity).  This causes the
     route to be removed from service.

   - The route change flag is to indicate that this entry has been

   - The output process is signalled to trigger a response.

   Until the garbage-collection timer expires, the route is included in
   all updates sent by this router.  When the garbage-collection timer
   expires, the route is deleted from the routing table.

   Should a new route to this network be established while the garbage-
   collection timer is running, the new route will replace the one that
   is about to be deleted.  In this case the garbage-collection timer
   must be cleared.

   Triggered updates also use a small timer; however, this is best
   described in section 2.5.1.

2.4  Input Processing

   This section will describe the handling of datagrams received on the
   RIPng port.  Processing will depend upon the value in the command
   field.  Version 1 supports only two commands: Request and Response.

2.4.1  Request Messages

   A Request is used to ask for a response containing all or part of a
   routers's routing table.  Normally, Requests are sent as multicasts,
   from the RIPng port, by routers which have just come up and are
   seeking to fill in their routing tables as quickly as possible.
   However, there may be situations (e.g., router monitoring) where the
   routing table of only a single router is needed.  In this case, the
   Request should be sent directly to that router from a UDP port other

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   than the RIPng port.  If such a Request is received, the router
   responds directly to the requestor's address and port.

   The Request is processed entry by entry.  If there are no entries, no
   response is given.  There is one special case.  If there is exactly
   one entry in the request, and it has a destination prefix of zero and
   a metric of infinity (i.e., 16), then this is a request to send the
   entire routing table.  In that case, a call is made to the output
   process to send the routing table to the requesting address/port.
   Except for this special case, processing is quite simple.  Examine
   the list of RTEs in the Request one by one.  For each entry, look up
   the destination in the router's routing database and, if there is a
   route, put that route's metric in the metric field of the RTE.  If
   there is no explicit route to the specified destination, put infinity
   in the metric field.  Once all the entries have been filled in,
   change the command from Request to Response and send the datagram
   back to the requestor.

   Note that there is a difference in metric handling for specific and
   whole-table requests.  If the request is for a complete routing
   table, normal output processing is done, including Split Horizon (see
   section 2.6 on Split Horizon).  If the request is for specific
   entries, they are looked up in the routing table and the information
   is returned as is; no Split Horizon processing is done.  The reason
   for this distinction is the expectation that these requests are
   likely to be used for different purposes.  When a router first comes
   up, it multicasts a Request on every connected network asking for a
   complete routing table.  It is assumed that these complete routing
   tables are to be used to update the requestor's routing table.  For
   this reason, Split Horizon must be done.  It is further assumed that
   a Request for specific networks is made only by diagnostic software,
   and is not used for routing.  In this case, the requester would want
   to know the exact contents of the routing table and would not want
   any information hidden or modified.

2.4.2  Response Messages

   A Response can be received for one of several different reasons:

   - response to a specific query
   - regular update (unsolicited response)
   - triggered update caused by a route change

   Processing is the same no matter why the Response was generated.

   Because processing of a Response may update the router's routing
   table, the Response must be checked carefully for validity.  The
   Response must be ignored if it is not from the RIPng port.  The

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   datagram's IPv6 source address should be checked to see whether the
   datagram is from a valid neighbor; the source of the datagram must be
   on a directly-connected network.  It is also worth checking to see
   whether the response is from one of the router's own addresses.
   Interfaces on broadcast networks may receive copies of their own
   multicasts immediately.  If a router processes its own output as new
   input, confusion is likely, and such datagrams must be ignored.  As
   an additional check, periodic advertisements must have their hop
   counts set to 255, and inbound, non-unicast packets must be examined
   to ensure that the hop count is 255.  This absolutely guarantees that
   a packet is from a neighbor, because any intermediate node would have
   decremented the hop count.  Queries and their responses may still
   cross intermediate nodes.

   Once the datagram as a whole has been validated, process the RTEs in
   the Response one by one.  Again, start by doing validation.
   Incorrect metrics and other format errors usually indicate
   misbehaving neighbors and should probably be brought to the
   administrator's attention.  For example, if the metric is greater
   than infinity, ignore the entry but log the event.  The basic
   validation tests are:

   - is the destination prefix valid (e.g., not a multicast prefix)
   - is the prefix length valid
   - is the metric valid (i.e., between 1 and 16, inclusive)

   If any check fails, ignore that entry and proceed to the next.
   Again, logging the error is probably a good idea.

   Once the entry has been validated, update the metric by adding the
   cost of the network on which the message arrived.  If the result is
   greater than infinity, use infinity.  That is,

      metric = MIN (metric + cost, infinity)

   Now, check to see whether there is already an explicit route for the
   destination prefix.  If there is no such route, add this route to the
   routing table, unless the metric is infinity (there is no point in
   adding a route which unusable).  Adding a route to the routing table
   consists of:

   - Setting the destination prefix and length to those in the RTE.

   - Setting the metric to the newly calculated metric (as described

   - Set the next hop address to be the address of the router from which
     the datagram came.

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   - Initialize the timeout for the route.  If the garbage-collection
     timer is running for this route, stop it (see section 2.3 for a
     discussion of the timers).

   - Set the route change flag.

   - Signal the output process to trigger an update (see section 2.5).

   If there is an existing route, compare the next hop address to the
   address of the router from which the datagram came.  If this datagram
   is from the same router as the existing route, reinitialize the
   timeout.  Next, compare the metrics.  If the datagram is from the
   same router as the existing route, and the new metric is different
   than the old one; or, if the new metric is lower than the old one; do
   the following actions:

   - Adopt the route from the datagram.  That is, put the new metric in,
     and adjust the next hop address (if necessary).

   - Set the route change flag and signal the output process to trigger
     an update.

   - If the new metric is infinity, start the deletion process
     (described above); otherwise, re-initialize the timeout.

   If the new metric is infinity, the deletion process begins for the
   route, which is no longer used for routing packets.  Note that the
   deletion process is started only when the metric is first set to
   infinity.  If the metric was already infinity, then a new deletion
   process is not started.

   If the new metric is the same as the old one, it is simplest to do
   nothing further (beyond reinitializing the timeout, as specified
   above); but, there is a heuristic which could be applied.  Normally,
   it is senseless to replace a route if the new route has the same
   metric as the existing route; this would cause the route to bounce
   back and forth, which would generate an intolerable number of
   triggered updates.  However, if the existing route is showing signs
   of timing out, it may be better to switch to an equally-good
   alternative route immediately, rather than waiting for the timeout to
   happen.  Therefore, if the new metric is the same as the old one,
   examine the timeout for the existing route.  If it is at least
   halfway to the expiration point, switch to the new route.  This
   heuristic is optional, but highly recommended.

   Any entry that fails these tests is ignored, as it is no better than
   the current route.

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2.5  Output Processing

   This section describes the processing used to create response
   messages that contain all or part of the routing table.  This
   processing may be triggered in any of the following ways:

   - By input processing, when a Request is received.  In this case, the
     Response is sent to only one destination.

   - By the regular routing update.  Every 30 seconds, a Response
     containing the whole routing table is sent to every neighboring

   - By triggered updates.  Whenever the metric for a route is changed,
     an update is triggered.

   The special processing required for a Request is described in section

   When a Response is to be sent to all neighbors (i.e., a regular or
   triggered update), a Response message is directed to the router at
   the far end of each connected point-to-point link, and is multicast
   on all connected networks that support broadcasting.  Thus, one
   Response is prepared for each directly-connected network, and sent to
   the appropriate address (direct or multicast).  In most cases, this
   reaches all neighboring routers.  However, there are some cases where
   this may not be good enough.  This may involve a network that is not
   a broadcast network (e.g., the ARPANET), or a situation involving
   dumb routers.  In such cases, it may be necessary to specify an
   actual list of neighboring routers and send a datagram to each one
   explicitly.  It is left to the implementor to determine whether such
   a mechanism is needed, and to define how the list is specified.

2.5.1  Triggered Updates

   Triggered updates require special handling for two reasons.  First,
   experience shows that triggered updates can cause excessive loads on
   networks with limited capacity or networks with many routers on them.
   Therefore, the protocol requires that implementors include provisions
   to limit the frequency of triggered updates.  After a triggered
   update is sent, a timer should be set for a random interval between 1
   and 5 seconds.  If other changes that would trigger updates occur
   before the timer expires, a single update is triggered when the timer
   expires.  The timer is then reset to another random value between 1
   and 5 seconds.  Triggered updates may be suppressed if a regular
   update is due by the time the triggered update would be sent.

   Second, triggered updates do not need to include the entire routing

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   table.  In principle, only those routes which have changed need to be
   included.  Therefore messages generated as part of a triggered update
   must include at least those routes that have their route change flag
   set.  They may include additional routes, at the discretion of the
   implementor; however, sending complete routing updates is strongly
   discouraged.  When a triggered update is processed, messages should
   be generated for every directly-connected network.  Split Horizon
   processing is done when generating triggered updates as well as
   normal updates (see section 2.6).  If, after Split Horizon processing
   for a given network, a changed route will appear unchanged on that
   network (e.g., it appears with an infinite metric), the route need
   not be sent.  If no routes need be sent on that network, the update
   may be omitted.  Once all of the triggered updates have been
   generated, the route change flags should be cleared.

   If input processing is allowed while output is being generated,
   appropriate interlocking must be done.  The route change flags should
   not be changed as a result of processing input while a triggered
   update message is being generated.

   The only difference between a triggered update and other update
   messages is the possible omission of routes that have not changed.
   The remaining mechanisms, described in the next section, must be
   applied to all updates.

2.5.2  Generating Response Messages

   This section describes how a Response message is generated for a
   particular directly-connected network:

   The IPv6 source address must be the link-local address of the address
   of the sending router's interface.  This is important because the
   source address is put into routing tables (as the next hop) in the
   routers which receive this Response.  If an incorrect source address
   is used, other routers may be unable to route datagrams.  Sometimes
   routers are set up with multiple IPv6 addresses on a single physical
   interface.  Normally, this means that several logical IPv6 networks
   are being carried over one physical medium.

   Set the version number to the current version of RIPng.  The version
   described in this document is version 1.  Set the command to
   Response.  Set the bytes labeled "must be zero" to zero.  Start
   filling in RTEs.  Recall that the maximum datagram size is limited by
   the network's MTU.  When there is no more space in the datagram, send
   the current Response and start a new one.

   To fill in the RTEs, examine each route in the routing table.  If a
   triggered update is being generated, only entries whose route change

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   flags are set need be included.  If, after Split Horizon processing,
   the route should not be included, skip it.  If the route is to be
   included, then the destination prefix, prefix length, and metric are
   put into the RTE.  Routes must be included in the datagram even if
   their metrics are infinite.

2.6  Split Horizon

   Split Horizon is a algorithm for avoiding problems caused by
   including routes in updates sent to the gateway from which they were
   learned.  The basic split horizon algorithm omits routes learned from
   one neighbor in updates sent to that neighbor.  In the case of a
   broadcast network, all routes learned from any neighbor on that
   network are omitted from updates sent on that network.

   Split Horizon with Poisoned Reverse (more simply, Poison Reverse)
   does include such routes in updates, but sets their metrics to
   infinity.  In effect, advertising the fact that there routes are not
   reachable.  This is the preferred method of operation; however,
   implementations should provide a per-interface control allowing no
   horizoning, split horizoning, and poisoned reverse to be selected.

   For a theoretical discussion of Split Horizon and Poison Reverse, and
   why they are needed, see section 2.1.1 of [1].

3. Control Functions

   This section describes administrative controls.  These are not part
   of the protocol per se; however, experience with existing networks
   suggests that they are important.  Because they are not a necessary
   part of the protocol, they are considered optional.  However, it is
   strongly recommend that at least some of them be included in every
   implementation.  These controls are intended primarily to allow RIPng
   to be connected to networks whose routing may be unstable or subject
   to errors.  Here are some examples:

   - It is sometimes desirable to restrict the routers from which
     updates will be accepted, or to which updates will be sent.  This
     is usually done for administrative, routing policy reasons.

   - A number of sites limit the set of networks that they allow in
     Response messages.  Organization A may have a connection to
     organization B that they use for direct communication.  For
     security or performance reasons A may not be willing to give other
     organizations access to that connection.  In such a case, A should
     not include B's networks in updates that A sends to third parties.

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   Here are some typical controls.  Note, however, that the RIPng
   protocol does not require these or any other controls.

   - A neighbor list which allows the network administrator to be able
     to define a list of neighbors for each router.  A router would
     accept response messages only from routers on its list of
     neighbors.  A similar list for target routers should also be
     available to the administrator.  By default, no restrictions are

   - A filter for specific destinations would permit the network
     administrator to be able to specify a list of destination prefixes
     to allow or disallow.  The list would be associated with a
     particular interface in the incoming and/or outgoing directions.
     Only allowed networks would be mentioned in Response messages going
     out or processed in Response messages coming in.  If a list of
     allowed prefixes is specified, all other prefixes are disallowed.
     If a list of disallowed prefixes is specified, all other prefixes
     are allowed.  By default, no filters are applied.

4. Security Considerations

   No security is defined for RIPng because IPv6 provides sufficient
   security to protect RIPng packets.


   [1] Hedrick, C., "Routing Information Protocol", Request For Comments
       (RFC) 1058, Rutgers University, June 1988.

   [2] Malkin, G., "RIP Version 2 - Carrying Additional Information",
       Request For Comments (RFC) 1723, Xylogics, Inc., November, 1994.

   [3] Hinden, R., "IP Next Generation Overview",
       draft-hinden-ipng-overview-00.txt, October 1994

   [4] Bellman, R. E., "Dynamic Programming", Princeton University
       Press, Princeton, N.J., 1957.

   [5] Bertsekas, D. P., and Gallaher, R. G., "Data Networks",
       Prentice-Hall, Englewood Cliffs, N.J., 1987.

   [6] Braden, R., and Postel, J., "Requirements for Internet Gateways",
       USC/Information Sciences Institute, RFC-1009, June 1987.

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   [7] Boggs, D. R., Shoch, J. F., Taft, E. A., and Metcalfe, R. M.,
       "Pup: An Internetwork Architecture", IEEE Transactions on
       Communications, April 1980.

   [8] Ford, L. R. Jr., and Fulkerson, D. R., "Flows in Networks",
       Princeton University Press, Princeton, N.J., 1962.

   [9] Xerox Corp., "Internet Transport Protocols", Xerox System
       Integration Standard XSIS 028112, December 1981.

   [10] Narten, T., Nordmark, E., Simpson, W., "Neighbor Discovery for
       IP Version 6 (IPv6)", draft-ietf-ipngwg-discovery-04.txt,
       February 1996.

Author's Address

   Gary Scott Malkin
   Xylogics, Inc.
   53 Third Avenue
   Burlington, MA 01803

   Phone:  (617) 272-8140
   EMail:  gmalkin@Xylogics.COM

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