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                                                             T. Pusateri
INTERNET DRAFT                                          Juniper Networks
Obsoletes: RFC 1075                                        February 1997
draft-ietf-idmr-dvmrp-v3-04                         Expires: August 1997

               Distance Vector Multicast Routing Protocol

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 ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


   DVMRP is an Internet routing protocol that provides an efficient
   mechanism for connection-less datagram delivery to a group of hosts
   across an internetwork. It is a distributed protocol that dynamically
   generates IP Multicast delivery trees using a technique called
   Reverse Path Multicasting (RPM) [Deer90]. This document is an update
   to Version 1 of the protocol specified in RFC 1075 [Wait88].

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1.  Introduction

   DVMRP uses a distance vector distributed routing algorithm in order
   for each router to determine the distance from itself to any IP
   Multicast traffic source.  By determining the best path back to a
   source, a router can know which interface it should expect traffic
   from that source to arrive on.  A good introduction to distance
   vector routing can be found in [Perl92].  The application of distance
   vector routing to multicast tree formulation is described in

1.1.  Reverse Path Multicasting

   Datagrams follow multicast delivery trees from a source to all
   members of a multicast group [Deer89], replicating the packet only at
   necessary branches in the delivery tree. The trees are calculated and
   updated dynamically to track the membership of individual groups.
   When a datagram arrives on an interface, the reverse path to the
   source of the datagram is determined by examining a unicast routing
   table of known source networks. If the datagram arrives on an
   interface that would be used to transmit unicast datagrams back to
   the source, then it is forwarded to the appropriate list of
   downstream interfaces.  Otherwise, it is not on the optimal delivery
   tree and should be discarded. In this way duplicate packets can be
   filtered when loops exist in the network topology. The source
   specific delivery trees are automatically pruned back as group
   membership changes or leaf routers determine that no group members
   are present.  This keeps the delivery trees to the minimum branches
   necessary to reach all of the group members. New sections of the tree
   can also be added dynamically as new members join the multicast group
   by grafting the new sections onto the delivery trees.

1.2.  IP-IP Tunnels

   Because not all IP routers support native multicast routing, DVMRP
   includes direct support for tunneling IP Multicast datagrams through
   routers. The IP Multicast datagrams are encapsulated in unicast IP
   packets and addressed to the routers that do support native multicast
   routing. DVMRP treats tunnel interfaces in an identical manner to
   physical network interfaces.

   In previous implementations, DVMRP protocol messages were sent un-
   encapsulated to the unicast tunnel endpoint address. While this was

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   more direct, it increased the complexity of firewall configuration.
   Therefore, all DVMRP protocol messages sent to tunnel endpoint
   addresses should now be encapsulated in IP protocol 4 packets just as
   multicast data packets are encapsulated. See Appendix C for backward
   compatibility issues.  More information on encapsulated tunnels can
   be found in [Perk96].

1.3.  Document Overview

   Section 2 provides an overview of the protocol and the different
   message types exchanged by DVMRP routers. Those who wish to gain a
   general understanding of the protocol but are not interested in the
   more precise details may wish to only read this section.  Section 3
   explains the detailed operation of the protocol to accommodate
   developers needing to provide inter-operable implementations.
   Included in Appendix A, is a summary of the DVMRP parameters. A
   section on DVMRP support for tracing and troubleshooting is the topic
   of Appendix B.  Finally, a short DVMRP version compatibility section
   is provided in Appendix C to assist with backward compatibility

2.  Protocol Overview

   DVMRP can be summarized as a "broadcast & prune" multicast routing
   protocol. It performs Reverse Path Forwarding checks to determine
   when multicast traffic should be forwarded to downstream interfaces.
   In this way, source-rooted shortest path trees can be formed to reach
   all group members from each source network of multicast traffic.

2.1.  Neighbor Discovery

   Neighbor DVMRP routers can be discovered dynamically by sending
   Neighbor Probe Messages on local multicast capable network interfaces
   and tunnel pseudo interfaces. These messages are sent periodically to
   the All-DVMRP-Routers IP Multicast group address. This address falls
   into the range of IP Multicast addresses that are to remain on the
   locally attached IP network and therefore are not forwarded by
   multicast routers.

   Each Neighbor Probe message should contain the list of Neighbor DVMRP
   routers for which Neighbor Probe messages have been received. In this
   way, Neighbor DVMRP routers can ensure that they are seen by each

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   other. Care must be taken to inter-operate with older implementations
   of DVMRP that do not include this list of neighbors.  It can be
   assumed that older implementations of DVMRP will safely ignore this
   list of neighbors in the Probe message.  Therefore, it is not
   necessary to send both old and new types of Neighbor Probes.

2.2.  Source Location

   When an IP Multicast datagram is received by a router running DVMRP,
   it first looks up the source network in the DVMRP routing table.  The
   interface of the next hop of packets sent back to the source of the
   datagram is called the upstream interface.  If the datagram arrived
   on the correct upstream interface, then it is a candidate for
   forwarding to one or more downstream interfaces. If the datagram did
   not arrive on the anticipated upstream interface, it is discarded.
   This check is known as a reverse path forwarding check and must be
   performed by all DVMRP routers.

   In order to ensure that all DVMRP routers have a consistent view of
   the unicast path back to a source, a unicast routing table is
   propagated to all DVMRP routers as an integral part of the protocol.
   Each router advertises the network number and mask of the interfaces
   it is directly connected to as well as relaying the routes received
   from neighbor routers. DVMRP requires an interface metric to be
   configured on all physical and tunnel interfaces. When a route is
   received, the metric of the upstream interface over which the
   datagram was received must be added to the metric of the route being
   propagated. This adjusted metric should be computed before the route
   is compared to the metric of the current next hop gateway.

   Although there is certainly additional overhead associated with
   propagating a separate unicast routing table, it does provide two
   nice features. First, since all DVMRP routers are using the same
   unicast routing protocol, there are no inconsistencies between
   routers when determining the upstream interface (aside from normal
   convergence issues related to distance vector routing protocols).  By
   placing the burden of synchronization on the protocol as opposed to
   the network manager, DVMRP reduces the risk of creating routing loops
   or black holes due to disagreement between neighbor routers on the
   upstream interface.

   Second, by propagating its own unicast routing table, DVMRP makes it
   convenient to have separate paths for unicast vs.  multicast
   datagrams. Although, ideally, many network managers would prefer to
   keep their unicast and multicast traffic aligned, tunneled multicast
   topologies may prevent this causing the unicast and multicast paths

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   to diverge.  Additionally, service providers may prefer to keep the
   unicast and multicast traffic separate for routing policy reasons as
   they experiment with IP multicast routing and begin to offer it as a

2.3.  Dependent Downstream Routers

   In addition to providing a consistent view of source networks, the
   exchange of unicast routes in DVMRP provides one other important
   feature. DVMRP uses the unicast route exchange as a mechanism for
   upstream routers to determine if any downstream routers depend on
   them for forwarding from particular source networks. DVMRP
   accomplishes this by using a technique called "Poison Reverse". If a
   downstream router selects an upstream router as the best next hop to
   a particular source network, this is indicated by echoing back the
   route to the upstream router with a metric equal to the original
   metric plus infinity.  When the upstream router receives the report
   and sees a metric that lies between infinity and twice infinity, it
   can then add the downstream router from which it received the report
   to a list of dependent routers for this source.

   This list of dependent routers per source network built by the
   "Poison Reverse" technique will provide the foundation necessary to
   determine when it is appropriate to prune back the IP source specific
   multicast trees.

2.4.  Multi-access Networks

   When two or more DVMRP routers are connected to a multi-access
   network, it is possible for duplicate packets to be forwarded on the
   network (one copy from each router). DVMRP does not require a special
   mechanism to prevent duplication. Instead, this feature is a
   consequence of the unicast route exchange. When two routers on a
   multi-access network exchange source networks, each of the routers
   will know the others metric back to each source network. Therefore,
   of all the DVMRP routers on a shared network, the router with the
   lowest metric to a source network is responsible for forwarding data
   on to the shared network. If two or more routers have an equally low
   metric, the router with the lowest IP address becomes the designated
   forwarder for the network.

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2.5.  Building Multicast Trees

   As previously mentioned, when an IP multicast datagram arrives, the
   upstream interface is determined by looking up the interface that
   would be used if a datagram was being sent back to the source of the
   datagram. If the upstream interface is correct, then a DVMRP router
   will forward the datagram to a list of downstream interfaces.

2.5.1.  Adding Leaf Networks

   Initially, the DVMRP router must consider all of the remaining IP
   multicast capable interfaces (including tunnels) on the router.  If
   the downstream interface under consideration is a leaf network (has
   no dependent downstream neighbors for the source network), then the
   IGMP local group database must be consulted. DVMRP routers can easily
   determine if a directly attached network is a leaf network by keeping
   a list of all routers from which DVMRP Router Probe messages have
   been received on the interface. Obviously, it is necessary to refresh
   this list and age out entries received from routers that are no
   longer being refreshed. The IGMP local group database is maintained
   by an elected IP multicast router on each physical, multicast capable
   network. The details of the election procedure are discussed in
   [Fen96a]. If the destination group address is listed in the local
   group database, and the router is the designated forwarder for the
   network, then the interface should be included in the list of
   downstream interfaces.  If there are no group members on the
   interface, then the interface can be removed from the outgoing
   interface list.

2.5.2.  Adding Non-Leaf Networks

   Initially, all non-leaf networks should be included in the downstream
   interface list when a forwarding cache entry is first being created.
   This allows all downstream routers to be aware of traffic destined
   for a particular (source, group) pair. The downstream routers will
   then have the option to send prunes and grafts for this (source,
   group) pair as requirements change from their respective downstream
   routers and local group members.

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2.6.  Pruning Multicast Trees

   As mentioned above, routers at the edges with leaf networks will
   remove their leaf interfaces that have no group members associated
   with an IP multicast datagram. If a router removes all of its
   downstream interfaces, it can notify the upstream router that it no
   longer wants traffic destined for a particular (source, group) pair.
   This is accomplished by sending a DVMRP Prune message upstream to the
   router it expects to forward datagrams from a particular source.

   Recall that a downstream router will inform an upstream router that
   it depends on the upstream router to receive datagrams from
   particular source networks by using the "Poison Reverse" technique
   during the exchange of unicast routes. This method allows the
   upstream router to build a list of downstream routers on each
   interface that are dependent upon it for datagrams from a particular
   source network.  If the upstream router receives prune messages from
   each one of the dependent downstream routers on an interface, then
   the upstream router can in turn remove this interface from its
   downstream interface list.  If the upstream router is able to remove
   all of its downstream interfaces in this way, it can then send a
   DVMRP Prune message to its upstream router. This continues until the
   unneeded branches are removed from the delivery tree.

   In order to remove old prune state information for (source, group)
   pairs that are no longer active, it is necessary to limit the life of
   a prune and periodically resume the flooding procedure.  Inside the
   prune message is a prune lifetime. This indicates the length of time
   that the prune should remain in effect. When the prune lifetime
   expires, the interface is joined back onto the multicast delivery
   tree. If unwanted multicast datagrams continue to arrive, the prune
   mechanism will be re-initiated and the cycle will continue.  If all
   of the downstream interfaces are removed from a multicast delivery
   tree causing a DVMRP Prune message to be sent upstream, the lifetime
   of the prune sent will be equal to the minimum of the remaining prune
   lifetimes of the downstream interfaces.

2.7.  Grafting Multicast Trees

   Once a tree branch has been pruned from a multicast delivery tree,
   packets from the corresponding (source, group) pair will no longer be
   forwarded.  However, since IP multicast supports dynamic group
   membership, new hosts may join the multicast group.  In this case,
   DVMRP routers use Grafts to undo the prunes that are in place from
   the host back on to the multicast delivery tree.  A router will send

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   a Graft message to its upstream neighbor if a group join occurs for a
   group that the router has previously sent a prune.  Separate Graft
   messages must be sent to the appropriate upstream neighbor for each
   source that has been pruned.  Since there would be no way to tell if
   a Graft message sent upstream was lost or the source simply quit
   sending traffic, it is necessary to acknowledge each Graft message
   with a DVMRP Graft Ack message.  If an acknowledgment is not received
   within a Graft Time-out period, the Graft message should be
   retransmitted. Duplicate Graft Ack messages should simply be ignored.
   The purpose of the Graft Ack message is to simply acknowledge the
   receipt of a Graft message. It does not imply that any action was
   taken as a result of receiving the Graft message. Therefore, all
   Graft messages should be acknowledged whether or not they cause an
   action on the receiving router.

3.  Detailed Protocol Operation

   This section contains a detailed description of DVMRP. It covers
   sending and receiving of DVMRP messages as well as the generation and
   maintenance of IP Multicast forwarding cache entries.

3.1.  Protocol Header

   DVMRP packets are  encapsulated in IP datagrams, with an IP protocol
   number of 2 (IGMP) as specified in the Assigned Numbers RFC [Reyn94].
   All fields are transmitted in Network Byte Order. DVMRP packets use a
   common protocol header that specifies the IGMP [Fen96a] Packet Type
   as hexadecimal 0x13 (DVMRP). A diagram of the common protocol header

                  0         8          16              31
                 | Type    |  Code   |      Checksum      |
                 |(0x13)   |         |                    |
                 |     Reserved      |  Minor   | Major   |
                 |                   | Version  |Version  |

                     Figure 1 - Common Protocol Header

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   A Major Version of 3 and a Minor Version of 0xFF should be used to
   indicate compliance with this specification.  The value of the Code
   field determines the DVMRP packet type.  Currently, there are codes
   allocated for DVMRP protocol message types as well as protocol
   analysis and troubleshooting packets.  The protocol message Codes

       Code     Packet Type                  Description
        1     DVMRP Probe       for neighbor discovery
        2     DVMRP Report      for unicast route exchange
        7     DVMRP Prune       for pruning multicast delivery trees
        8     DVMRP Graft       for grafting multicast delivery trees
        9     DVMRP Graft Ack   for acknowledging graft messages

                 Table 1 - Standard Protocol Packet Types

   There are additional codes used for protocol analysis and
   troubleshooting. These codes are discussed in Appendix B.  The
   Checksum is the 16-bit one's complement of the one's complement sum
   of the DVMRP message. The checksum of the DVMRP message should be
   calculated with the checksum field set to zero.

3.2.  Probe Messages

   When a DVMRP router is configured to run on an interface (physical or
   tunnel), it sends local IP Multicast discovery packets to inform
   other DVMRP routers that it is operational. These discovery packets
   are called DVMRP Probes and they serve three purposes.

   1. Probes provide a mechanism for DVMRP routers to locate each other.
      DVMRP sends a list of detected neighbors in the Probe message.
      This list of DVMRP neighbors provides a foundation for the
      dependent downstream neighbor list.  If no DVMRP neighbors are
      found, the network is considered to be a leaf network. A DVMRP
      router should discard all other protocol packets from a neighbor
      until it has seen its own address in the neighbors Probe list.
      (See Appendix C for exceptions.)

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   2. Probes provide a way for DVMRP routers to determine the
      capabilities of each other. This may be deduced from the major and
      minor version numbers in the Probe packet or directly from the
      capability flags.  These flags were first introduced to allow
      optional protocol features.  This specification now mandates the
      use of Generation Id's and pruning and, therefore, provides no
      optional capabilities. Other capability flags were used for
      tracing and troubleshooting and are no longer a part of the actual

   3. Probes provide a keep-alive function in order to quickly detect
      neighbor loss. DVMRP probes sent on each multicast capable
      interface configured for DVMRP SHOULD have an interval of 10
      seconds. The neighbor time-out interval SHOULD be set at 35
      seconds. This allows fairly early detection of a lost neighbor yet
      provides tolerance for busy multicast routers. These values MUST
      be coordinated between all DVMRP routers on a physical network

3.2.1.  Router Capabilities

   In the past, there have been many versions of DVMRP in use with a
   wide range of capabilities. Practical considerations require a
   current implementation to inter-operate with these older
   implementations that don't formally specify their capabilities and
   are not compliant with this specification.  For instance, for major
   versions less than 3, it can be assumed that the neighbor does not
   support pruning.  The formal capability flags were first introduced
   in an well known implementation (Mrouted version 3.5) in an attempt
   to take the guess work out which features are supported by a
   neighbor. Many of these flags are no longer necessary since they are
   now a required part of the protocol, however, special consideration
   is necessary to not confuse older implementations that expect these
   flags to be set.  Appendix C was written to assist with these and
   other backward compatibility issues.

   Three of the flags were used for actual protocol operation.  The
   other two assigned flags were used for troubleshooting purposes which
   should be documented in a separate specification. All of the bits
   marked "U" in the Figure below are now unused. They may be defined in
   the future and MUST be set to 0. Bit position 0 is the LEAF bit which
   is a current research topic.  It MUST be set to 0.  Bit positions 1,
   2, and 3 MUST be set to 1 for backward compatibility.  They were used
   to specify the PRUNE, GENID, and MTRACE bits.  The first two, PRUNE
   and GENID, are now required features. The MTRACE bit must be set so

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   existing implementations will not assume this neighbor does not
   support multicast trace-route [Fen96b]. However, since this bit is
   now reserved and set to 1, newer implementations should not use this
   bit in the Probe message to determine if multicast trace-route is
   supported by a neighbor. Instead, the M bit should only be used in a
   Neighbors2 message as described in Appendix B. The bit marked S
   stands for SNMP capable.  This bit is used by troubleshooting
   applications and should only be tested in the Neighbors2 message.

     0                           8    9    10   S    M    G    P    L
    |        Reserved          | U  | U  | U  | U  | 1  | 1  | 1  | L  |

                     Figure 2 - Probe Capability Flags

3.2.2.  Generation ID

   If a DVMRP router is restarted, it will want to learn all of the
   routes known by its neighbors without having to wait for an entire
   report interval to pass. In order for the neighbor to detect that the
   router has restarted, a non-decreasing number is placed in the
   periodic probe message called the generation ID. When a neighbor
   detects an increase in the generation ID of a router, it should re-
   send its entire unicast routing table to the router.

   If a change in generation ID is detected, any prune information
   received from the router is no longer valid and should be flushed.
   If this prune state has caused prune information to be sent upstream,
   a graft will need to be sent upstream just as though a new member has
   joined below. Once data begins to be delivered downstream, if the
   downstream router again decides to be pruned from the delivery tree,
   a new prune can be sent upstream at that time.

   A time of day clock provides a good source for a non-decreasing 32
   bit integer.

3.2.3.  Neighbor Addresses

   As a DVMRP router sees Probe messages from its DVMRP neighbors, it
   records the neighbor addresses on each interface and places them in

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   the Probe message sent on the particular interface. This allows the
   neighbor router to know that its probes have been received by the
   sending router.

   In order to minimize one-way neighbor relationships, a router MUST
   delay sending poison route reports directly to a neighbor until the
   neighbor includes the routers address in its probe messages.

   Implementations written before this specification will not wait
   before sending reports nor will they ignore reports sent.  Therefore,
   reports from these implementations SHOULD be accepted whether or not
   a probe with the routers address has been received.

3.2.4.  Probe Packet Format

   The Probe packet is variable in length depending on the number of
   neighbor IP addresses included. The length of the IP packet can be
   used to determine the number of neighbors in the Probe message.  The
   current Major Version is 3. To maintain compatibility with previous
   versions, implementations of Version 3 must include pruning and
   grafting of multicast trees. Non-pruning implementations SHOULD NOT
   be implemented at this time.

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                      7             15        23         31
              |  Type   |     Code     |      Checksum      |
              | (0x13)  |    (0x1)     |                    |
              |         |              |          |         |
              |Reserved | Capabilities |  Minor   | Major   |
              |                                             |
              |               Generation ID                 |
              |                                             |
              |           Neighbor IP Address 1             |
              |                                             |
              |           Neighbor IP Address 2             |
              |                                             |
              |                     ...                     |
              |                                             |
              |           Neighbor IP Address N             |

                   Figure 3 - DVMRP Probe Packet Format

3.2.5.  Designated Router Election

   Since it is wasteful to have more than a single router sending IGMP
   Host Membership Queries on a given physical network, a single router
   on each physical network is elected as the Designated Querier. This
   election used to be a part of DVMRP. However, this is now handled as
   a part of the IGMP vesion 2 protocol. Therefore, DVMRP Version 3
   requires the use of IGMP Version 2 or later specifying that the
   Designated Querier election is performed as a part of IGMP.

   Even though only one router will act as the designated querier, all
   DVMRP routers must listen to IGMP Host Membership Reports and keep a
   local group database.

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3.3.  Building Forwarding Cache Entries

   In order to create optimal multicast delivery trees, DVMRP was
   designed to keep separate forwarding cache entries for each (source
   network, destination group) pair.  Because the possible combinations
   of these is quite large, forwarding cache entries are generated on
   demand as data arrives at a multicast router. Since the IP forwarding
   decision is made on a hop by hop basis (as with the unicast case), it
   is imperative that each multicast router has a consistent view of the
   reverse path back to the source network.

3.3.1.  Determining the upstream interface

   When a multicast packet arrives, a DVMRP router will use the DVMRP
   unicast routing table to determine which interface leads back to the
   source. If the packet did not arrive on that interface, it should be
   discarded without further processing. Each multicast forwarding entry
   should cache the upstream interface for a particular source host or
   source network after looking this up in the DVMRP unicast routing

3.3.2.  Determining the downstream interface list

   The downstream interface list is built from the remaining list of
   multicast capable interfaces. Any interfaces designated as leaf
   networks that do not have members of the particular multicast group
   can be automatically removed from list of downstream interfaces.  The
   remaining interfaces will either have downstream DVMRP routers or
   directly attached group members. These interfaces may be removed in
   the future if it is determined that there are no group members
   anywhere along the entire tree branch.

3.4.  Unicast Route Exchange

   It was mentioned earlier that since not all IP routers support IP
   multicast forwarding, it is necessary to tunnel IP multicast
   datagrams through these routers. One effect of using these
   encapsulated tunnels is that IP multicast traffic may not be aligned
   with IP unicast traffic. This means that a multicast datagram from a
   particular source can arrive on a different (logical) interface than
   the expected upstream interface based on traditional unicast routing.

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   The unicast routing information propagated by DVMRP is used for
   determining the reverse path back to the source of multicast traffic.
   Tunnel pseudo-interfaces are considered to be distinct for the
   purpose of determining upstream and downstream interfaces.  The
   routing information that is propagated by DVMRP contains a list of
   unicast source networks and an appropriate metric. The metric used is
   a hop count which is incremented by the cost of the incoming
   interface metric. Traditionally, physical interfaces use a metric of
   1 while the metric of a tunnel interface varies with the distance and
   bandwidth in the path between the two tunnel endpoints. Users are
   encouraged to configure tunnels with the same metric in each
   direction to create symmetric routing and provide for easier problem
   determination although the protocol does not strictly enforce this.

   Implementations may wish to provide a mechanism to aggregate source
   networks to reduce the size of the unicast routing table. All
   implementations should be able to accept reports for aggregated
   source networks in accordance with Classless Inter-Domain Routing
   (CIDR) as described in [Rekh93] and [Full93].

   There are two places where aggregation is particularly useful.

   1. At organizational boundaries to limit the number of source
      networks advertised out of the organization.

   2. Within an organization to summarize non-local routing information
      by using a default (0/0) route.

3.4.1.  Route Packing and Ordering

   Since DVMRP Route Reports may need to refresh several thousand routes
   each report interval, routers MUST attempt to spread the routes
   reported across the whole route update interval. This reduces the
   chance of synchronized route reports causing routers to become
   overwhelmed for a few seconds each report interval. Since the route
   report interval is 60 seconds, it is suggested that the total number
   routes being updated be split across multiple Route Reports sent at
   regular intervals.  Their was an earlier requirement that Route
   Reports MUST contain source network/mask pairs sorted first by
   increasing network mask and then by increasing source network. This
   restriction has been lifted. Implementations conforming to this
   specification MUST be able to receive Route Reports containing any
   mixture of network masks and source networks.

   In order to pack more source networks into a route report, source
   networks are often represented by less than 4 octets. The number of

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   non-zero bytes in the mask value is used to determine the number of
   octets used to represent each source network within that particular
   mask value. For instance if the mask value of is being
   reported, the source networks would only contain 2 octets each. DVMRP
   assumes that source networks will never be aggregated into networks
   whose prefix length is less than 8. Therefore, it does not carry the
   first octet of the mask in the Route Report since, given this
   assumption, the first octet will always be 0xFF.  This means that the
   netmask value will always be represented in 3 octets. This method of
   specifying source network masks is compatible with techniques
   described in [Rekh93] and [Full93] to group traditional Class C
   networks into super-nets and to allow different subnets of the same
   Class A network to be discontinuous. In this notation, the default
   route is represented as the least three significant octets of the
   netmask [00 00 00], followed by one octet for the network number

3.4.2.  Unicast Route Metrics

   For each source network reported, a route metric is associated with
   the unicast route being reported. The metric is the sum of the
   interface metrics between the router originating the report and the
   source network. For the purposes of DVMRP, the Infinity metric is
   defined to be 32.  This limits the breadth across the whole DVMRP
   network and is necessary to place an upper bound on the convergence
   time of the protocol.

   As seen in the packet format below, Route Reports do not contain a
   count of the number of routes reported for each netmask. Instead, the
   high order bit of the metric is used to signify the last route being
   reported for a particular mask value. If a metric is read with the
   high order bit of the 8-bit value set and if the end of the message
   has not been reached, the next value will be a new netmask to be
   applied to the subsequent list of routes.

3.4.3.  Unicast Route Dependencies

   In order for pruning to work correctly, each DVMRP router needs to
   know which downstream routers depend on it for receiving datagrams
   from particular source networks.  Initially, when a new datagram
   arrives from a particular source/group pair, it is flooded to all
   downstream interfaces that have DVMRP neighbors who have indicated a
   dependency on the receiving DVMRP router for that particular source.
   A downstream interface can only be removed when it has received Prune

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   messages from each of the dependent routers on that interface. Each
   downstream router uses Poison Reverse to indicate to the upstream
   router which source networks it expects to receive from the upstream
   router. The downstream router indicates this by echoing back the
   source networks it expects to receive from the upstream router with
   infinity added to the advertised metric. This means that the legal
   values for the metric now become between 1 and (2 x Infinity - 1) or
   1 and 63. Values between 1 and 31 indicate reachable unicast source
   networks. The value Infinity (32) indicates the source network is not
   reachable. Values between 33 and 63 indicate that the downstream
   router originating the Report is depending upon the upstream router
   to provide multicast datagrams from the corresponding source network.

3.4.4.  Sending Route Reports

   All of the active routes MUST be advertised over every interface
   running DVMRP each Route Report Interval.  In addition, flash updates
   MAY be sent as needed but any given route MUST not be advertised more
   often than the Minimum Flash Update Interval (5 seconds). Flash
   updates can reduce the chances of routing loops and black holes
   occurring when source networks become unreachable through a
   particular path.  Flash updates need only contain the source networks
   that have changed. It is not necessary to report all of the source
   networks from a particular mask value when sending an update.

   Route Reports containing downstream dependent "poison" metrics should
   be sent directly to the neighbors unicast address. These reports
   should not be sent to a neighbor until a router has seen its own
   address in the neighbors Probe router list. See Appendix C for
   exceptions.  These Reports should be refreshed at the standard Route
   Update Interval.

3.4.5.  Receiving Route Reports

   After receiving a route report, a check should be made to verify it
   is from a known neighbor. Neighbors are learned via received Probe
   messages which also indicate the capabilities of the neighbor.
   Therefore, route reports from unknown neighbors are discarded.

   Each route in the report is then parsed and processed according to
   the following rules:

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   A. If the route is new and the metric is less than infinity, the
      route should be added.

   B. If the route already exists, several checks must be performed.

      1. New metric < infinity

         a. New metric > existing metric

            If the new metric is greater than the existing metric then
            check to see if the same neighbor is reporting the route. If
            so, update the metric and flash update the route.
            Otherwise, discard the route.

         b. New metric < existing metric

            Update the metric for the route and if the neighbor
            reporting the route is different, update the upstream
            neighbor in the routing table.  Flash update the route to
            downstream neighbors and if the neighbor changed, a flash
            update should be sent to the new neighbor indicating
            downstream dependence and to the existing neighbor
            withdrawing downstream dependence of the route.

         c. New metric = existing metric

            If the neighbor reporting the route is the same as the
            existing route, then simply refresh the route. If the new
            neighbor has a lower IP address, then update the route.
            Flash updates should be sent to the new and old neighbors to
            notify them of changes in downstream dependencies.

      2. New metric = infinity

         a. New next hop = existing next hop

            If the existing metric was less than infinity, the route is
            now unreachable.  Update the route and possibly update
            neighbors as well.

            If the existing metric was between infinity and 2 x
            infinity, the neighbor used to be a dependent downstream
            router but is no longer.  Note this dependency change and
            any Prunes that it may trigger.

         b. New next hop != existing next hop

            The route can be ignored since the existing next hop has a

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            metric better than or equal to this next hop.

      3. infinity < New metric < 2 x infinity

         If the receiving router is not the designated forwarder for the
         source network on the interface the report was received, the
         poison report should be ignored.  Otherwise, the neighbor
         considers the receiving router to be upstream for the route and
         is indicating it is dependent on the receiving router.

         a. Neighbor on down stream interface

            If the neighbor is considered to be on a downstream
            interface for that route, then the neighbor should be
            registered as a downstream dependent router for that route.

            If this is the first time the neighbor has indicated
            downstream dependence for this source and one or more prunes
            have been sent upstream containing this source network, then
            Graft messages will need to be sent upstream in the
            direction of the source network for each group with existing
            prune state.

         b. Neighbor not on down stream interface

            If the receiving router thinks the neighbor is on the
            upstream interface, then the indication of downstream
            dependence should be ignored.

      4. 2 x infinity <= New metric

         If the metric is greater than or equal to 2 x infinity, the
         metric is illegal and the route should be ignored.

3.4.6.  Route Hold-down

   When a route learned via a particular gateway expires, a router may
   be able to reach the source network described by the route through an
   alternate gateway. However, in the presence of complex topologies,
   often, the alternate gateway may only be echoing back the same route
   learned via a different path. If this occurs, the route will continue
   to be propagated long after it is no longer valid.

   In order to prevent this, it is common in distance vector protocols

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   to continue to advertise a route that has been deleted with a metric
   of infinity for one or more report intervals. During this time, the
   route may be learned via a different gateway and the router is
   permitted to use this new gateway. However, the router MUST NOT
   advertise this new gateway during the hold-down period.

   DVMRP begins the hold-down period after 140 seconds (2 x Route Report
   Interval + 20). After this time, a new gateway may be used but the
   route must be advertised with an infinity metric for 2 Report
   Intervals. At this point, the hold-down period is over and the new
   gateway (if one exists) can start being advertised.  In the absence
   of a new gateway, the route is simply removed.

   Route hold-down is not effective if only some of the routers
   implement it.  Therefore, it is now a REQUIRED part of the protocol.

3.4.7.  Graceful Shutdown

   During a graceful shutdown, an implementation MAY want to inform
   neighbor routers that it is terminating. Routes that have been
   advertised with a metric less than infinity should now be advertised
   with a metric equal to infinity. This will allow neighbor routers to
   switch more quickly to an alternate path for a source network if one

   Routes that have been advertised with a metric between infinity and 2
   x infinity (indicating downstream neighbor dependence) should now be
   advertised with a metric equal to infinity (canceling the downstream

3.4.8.  Route Report Packet Format

   The format of a sample Route Report Packet is shown in Figure 4
   below. The packet shown is an example of how the source networks are
   packed into a Report. The number of octets in each Source Network
   will vary depending on the mask value.  The values below are only an
   example for clarity and are not intended to represent the format of
   every Route Report.

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                    7           15           23           31
          |   Type    |    Code    |        Checksum         |
          |  (0x13)   |   (0x2)    |                         |
          |       Reserved         |   Minor    |   Major    |
          |                        |  Version   |  Version   |
          |  Mask1    |   Mask1    |   Mask1    |    Src     |
          |  Octet2   |   Octet3   |   Octet4   |   Net11    |
          |  SrcNet11(cont.)...    |  Metric11  |    Src     |
          |                        |            |   Net12    |
          |  SrcNet12(cont.)...    |  Metric12  |   Mask2    |
          |                        |            |   Octet2   |
          |  Mask2    |   Mask2    |        SrcNet21         |
          |  Octet3   |   Octet4   |                         |
          |  SrcNet21(cont.)...    |  Metric21  |   Mask3    |
          |                        |            |   Octet2   |
          |  Mask3    |   Mask3    |           ...           |
          |  Octet3   |   Octet4   |                         |

             Figure 4 - Example Route Report Packet Format

3.5.  Pruning

   DVMRP is described as a broadcast and prune multicast routing
   protocol since datagrams are initially sent out all dependent
   downstream interfaces forming a tree rooted at the source of the
   data.  But as the routers at the leafs of the tree begin to receive
   unwanted multicast traffic, they send prune messages upstream toward
   the source.  This allows the tree branches to become optimal for a
   given source network and a given set of receivers.

3.5.1.  Leaf Networks

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   Detection of leaf networks is very important to the pruning process.
   Routers at the end of a source specific multicast delivery tree must
   detect that there are no further downstream routers. This detection
   mechanism is covered above in section 3.2 titled Probe Messages.  If
   there are no group members present for a particular multicast
   datagram received, the leaf routers will start the pruning process by
   removing their downstream interfaces and sending a prune to the
   upstream router for that source.

3.5.2.  Source Networks

   It is important to note that prunes are specific to a group and
   source network. A prune sent upstream triggered by traffic received
   from a particular source applies to all sources on that network. It
   is not currently possible to remove only one or a subset of hosts on
   a source network for a particular group. All or none of the sources
   must be removed.

   Although the Prune message contains the host address of a source, the
   source network can be determined easly by a best-match lookup using
   the unicast routing table distributed as a part of DVMRP.

3.5.3.  Receiving a Prune

   When a prune is received, the following steps should be taken:

   1.  Determine if a Probe has been received from this router recently.

   2.  If not, discard prune since there is no prior state about this

   3.  If so, make sure the neighbor is capable of pruning (based on
       received Probe message).

   4.  Since Prune messages are fixed length, ensure the prune message
       contains at least the correct amount of data.

   5.  Extract the source address, group address, and prune time-out

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   6.  If there is no current state information for the (source, group)
       pair, then ignore the prune.

   7.  Verify that the prune was received from a dependent neighbor for
       the source network. If not, discard the prune.

   8.  Determine if a prune is currently active from the same dependent
       neighbor for this (source, group) pair.

   9.  If so, reset the timer to the new time-out value.  Otherwise,
       create state for the new prune and set a timer for the prune

   10. Determine if all dependent downstream routers on the interface
       from which the prune was received have now sent prunes.

   11. If so, then determine if there are group members active on the

   12. If no group members are found, then remove the interface.

   13. If all downstream interfaces have now been removed, send a prune
       to the upstream neighbor.

3.5.4.  Sending a Prune

   When sending a prune upstream, the following steps should be taken:

   1. Decide if upstream neighbor is capable of receiving prunes.

   2. If not, then proceed no further.

   3. Stop any pending Grafts awaiting acknowledgments.

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   4. Determine the prune lifetime. This value should be the minimum of
      the prune lifetimes remaining from the downstream neighbors and
      the default prune lifetime.

   5. Form and transmit the packet to the upstream neighbor for the

3.5.5.  Retransmitting a Prune

   By increasing the prune lifetime to ~2 hours, the effect of a lost
   prune message becomes more apparent. Therefore, an implementation MAY
   choose to retransmit prunes messages using exponential back-off for
   the lifetime of the prune if traffic is still arriving on the
   upstream interface.

   One way to implement this would be to send a prune, install a
   negative cache entry for 3 seconds while waiting for the prune to
   take effect. Then remove the negative cache entry. If traffic
   continues to arrive, a new forwarding cache request will be
   generated. The prune can be resent with the remaining prune lifetime
   and a negative cache entry can be installed for 6 seconds. After
   this, the negative cache entry is removed. This procedure is repeated
   while each time doubling the length of time the negative cache entry
   is installed.

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3.5.6.  Prune Packet Format

   In addition to the standard IGMP and DVMRP headers, a Prune Packet
   contains three additional fields: the source host IP address, the
   destination group IP address, and the Prune Lifetime in seconds.

   The Prune Lifetime is a derived value calculated as the minimum of
   the default prune lifetime (2 hours) and the remaining lifetimes of
   of any downstream prunes received for the same cache entry. A router
   with no downstream dependent neighbors would use the the default
   prune lifetime.

                      7           15           23           31
            |   Type    |    Code    |        Checksum         |
            |  (0x13)   |   (0x7)    |                         |
            |       Reserved         |   Minor    |   Major    |
            |                 Source Address                   |
            |                  Group Address                   |
            |                 Prune Lifetime                   |

                      Figure 5 - Prune Packet Format

3.6.  Grafting

   Once a multicast delivery tree has been pruned back, DVMRP Graft
   messages are necessary to join new receivers onto the multicast tree.
   Graft messages are sent upstream from the new receiver's first-hop
   router until a point on the multicast tree is reached.  Graft
   messages are re-originated between adjacent DVMRP routers and are not
   forwarded by DVMRP routers.  Therefore, the first-hop router does not
   know if the Graft message ever reaches the multicast tree.  To remedy
   this, each Graft message is acknowledged hop by hop. This ensures
   that the Graft message is not lost somewhere along the path between
   the receiver's first-hop router and the closest point on the
   multicast delivery tree.

   One or more Graft messages should be sent under the following

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   1. A new local member joins a group that has been pruned upstream.

   2. A new dependent downstream router appears on a pruned branch.

   3. A dependent downstream router on a pruned branch restarts (new
      Generation ID).

   4. A Graft Retransmission Timer expires before a Graft-Ack is

3.6.1.  Grafting Each Source Network

   It is important to realize that prunes are source specific and are
   sent up different trees for each source.  Grafts are sent in response
   to a new Group Member which is not source specific. Therefore,
   separate Graft messages must be sent to the appropriate upstream
   routers to counteract each previous source specific prune that was

3.6.2.  Sending a Graft

   As mentioned above, a Graft message sent to the upstream DVMRP router
   should be acknowledged hop by hop guaranteeing end-to-end delivery.
   If a Graft Acknowledgment is not received within the Graft
   Retransmission Time-out period, the Graft should be resent to the
   upstream router. The initial retransmission period is 5 seconds.  A
   binary exponential back-off policy is used on subsequent
   retransmissions.  In order to send a Graft message, the following
   steps should be taken:

   1. Verify a forwarding cache entry exists for the (source, group)
      pair and that a prune exists for the cache entry.

   2. Verify that the upstream router is capable of receiving prunes
      (and therefore grafts).

   3. Add the graft to the retransmission timer list awaiting an

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   4. Formulate and transmit the Graft packet.

3.6.3.  Receiving a Graft

   The actions taken when a Graft is received depends on the state in
   the receiving router for the (source, group) pair in the received
   Graft message. If the receiving router has prune state for the
   (source, group) pair, then it must acknowledge the received graft and
   send a subsequent graft to its upstream router.  If the receiving
   router has some removed some downstream interfaces but has not sent a
   prune upstream, then the receiving interface can simply be added to
   the list of downstream interfaces in the forwarding cache. A Graft
   Acknowledgment must also be sent back to the source of the Graft
   message.  If the receiving router has no state at all for the
   (source, group) pair, then datagrams arriving for the (source, group)
   pair should automatically be flooded when they arrive. A Graft
   Acknowledgment must be sent to the source of the Graft message.  If a
   Graft message is received from an unknown neighbor, it should be
   discarded after it is acknowledged.

3.6.4.  Graft Packet Format

   The format of a Graft packet is show below:

                      7           15           23           31
            |   Type    |    Code    |        Checksum         |
            |  (0x13)   |   (0x8)    |                         |
            |       Reserved         |   Minor    |   Major    |
            |                 Source Address                   |
            |                  Group Address                   |

                      Figure 6 - Graft Packet Format

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3.6.5.  Sending a Graft Acknowledgment

   A Graft Acknowledgment packet is sent to a downstream neighbor in
   response to receiving a Graft message. All Graft messages should be
   acknowledged. This is true even if no other action is taken in
   response to receiving the Graft to prevent the source from
   continually re-transmitting the Graft message.  The Graft
   Acknowledgment packet is identical to the Graft packet except that
   the DVMRP code in the common header is set to Graft Ack. This allows
   the receiver of the Graft Ack message to correctly identify which
   Graft was acknowledged and stop the appropriate retransmission timer.

3.6.6.  Receiving a Graft Acknowledgment

   When a Graft Acknowledgment is received, the (source, group) pair in
   the packet can be used to determine if a Graft was sent to this
   particular upstream router.  If no Graft was sent, the Graft Ack can
   simply be ignored.  If a Graft was sent, and the acknowledgment has
   come from the correct upstream router, then it has been successfully
   received and the retransmission timer for the Graft can be stopped.

3.6.7.  Graft Acknowledgment Packet Format

   The format of a Graft Ack packet (which is identical to that of a
   Graft packet) is show below:

                      7           15           23           31
            |   Type    |    Code    |        Checksum         |
            |  (0x13)   |   (0x9)    |                         |
            |       Reserved         |   Minor    |   Major    |
            |                 Source Address                   |
            |                  Group Address                   |

                    Figure 7 - Graft Ack Packet Format

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3.7.  Interfaces

   Interfaces running DVMRP will either be multicast capable physical
   interfaces or encapsulated tunnel pseudo-interfaces. Physical
   interfaces may either be multi-access networks or point-to-point
   networks.  Tunnel interfaces are used when there are non-multicast
   capable routers between DVMRP neighbors. Protocol messages and
   multicast data traffic are sent between tunnel endpoints using IP-IP
   encapsulation.  The unicast IP addresses of the tunnel endpoints are
   used as the source and destination IP addresses in the outer IP
   header. The inner IP header remains unchanged from the original

   The maximum packet length of any DVMRP message should be the maximum
   packet size required to be forwarded without fragmenting.  The use of
   Path MTU Discovery [Mogu90] is encouraged to determine this size.  In
   the absence of Path MTU, the Requirements for Internet Hosts [Brad89]
   specifies this number as 576 octets. Be sure to consider the size of
   the encapsulated IP header as well when calculating the maximum size
   of a DVMRP protocol message.

4.  IANA Considerations

   The Internet Assigned Numbers Authority (IANA) is the central
   coordinator for the assignment of unique parameter values for
   Internet protocols.  DVMRP uses IGMP [Fen96a] IP protocol messages to
   communicate between routers. The IGMP Type field is hexadecimal 0x13.

   On IP multicast capable networks, DVMRP uses the All-DVMRP-Routers
   local multicast group. This group address is

5.  Network Management Considerations

   DVMRP provides several methods for network management monitoring and
   troubleshooting. Appendix B describes a request/response mechanism to
   directly query DVMRP neighbor information. In addition, a Management
   Information Base for DVMRP is defined in [Thal96].  A protocol
   independent multicast trace-route facility is defined in [Fen96b].

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6.  Security Considerations

   Security for DVMRP follows the general security architecture provided
   for the Internet Protocol [Atk95a]. This framework provides for both
   privacy and authentication. It recommends the use of the IP
   Authentication Header [Atk95b] to provide trusted neighbor
   relationships. Confidentiality is provided by the addition of the IP
   Encapsulating Security Payload [Atk95c]. Please refer to these
   documents for the general architecture design as well as the specific
   implementation details.

7.  References

   [Atk95a]  Atkinson, R., "Security Architecture for the Internet
             Protocol", RFC 1825, August 1995.

   [Atk95b]  Atkinson, R., "IP Authentication Header", RFC 1826, August

   [Atk95c]  Atkinson, R., "IP Encapsulating Security Payload (ESP)",
             RFC 1827, August 1995.

   [Brad89]  Braden, R., "Requirements for Internet Hosts --
             Communication Layers", RFC 1122, October 1989.

   [Deer89]  Deering, S., "Host Extensions for IP Multicasting", RFC
             1112, August 1989.

   [Deer90]  Deering, S., Cheriton, D., "Multicast Routing in Datagram
             Internetworks and Extended LANs",  ACM Transactions on
             Computer Systems, Vol. 8, No. 2, May 1990, pp. 85-110.

   [Deer91]  Deering, S., "Multicast Routing in a Datagram
             Internetwork", PhD thesis, Electric Engineering Dept.,
             Stanford University, December 1991.

   [Fen96a]  Fenner, W., "Internet Group Management Protocol, Version
             2",  Work In Progress, January 1997.

   [Fen96b]  Fenner, W., Casner, S., "A "traceroute" facility for IP
             Multicast",  Work In Progress, November 1996.

   [Full93]  Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
             Inter-Domain Routing (CIDR): an Address Assignment and

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             Aggregation Strategy", RFC 1519, September 1993.

   [Mogu90]  Mogul, J., Deering, S., "Path MTU Discovery", RFC 1191,
             November 1990.

   [Perk96]  Perkins, C., IP Encapsulation within IP, RFC 2003, October

   [Perl92]  Perlman, R., Interconnections: Bridges and Routers,
             Addison-Wesley, May 1992, pp. 205-211.

   [Rekh93]  Rekhter, Y., and T. Li, "An Architecture for IP Address
             Allocation with CIDR", RFC 1518, September 1993.

   [Reyn94]  Reynolds, J., Postel, J., "Assigned Numbers", STD 0002,
             October 1994.

   [Thal96]  Thaler, D., "Distance-Vector Multicast Routing Protocol
             MIB",  Work In Progress, June 1996.

   [Wait88]  Waitzman, D., Partridge, C., Deering, S., "Distance Vector
             Multicast Routing Protocol",  RFC 1075, November 1988.

8.  Author's Address

   Thomas Pusateri
   Juniper Networks, Inc.
   3260 Jay St.
   Santa Clara, CA  95051
   Phone:    (919) 558-0700
   EMail:    pusateri@jnx.com

9.  Acknowledgments

   The author would like to acknowledge the original designers of the
   protocol, Steve Deering, Craig Partridge, and David Waitzman.
   Version 3 of the protocol would not have been possible without the
   original work of Ajit Thyagarajan and ongoing work of Bill Fenner.
   Credit also goes to Danny Mitzel for the careful review of this
   document and Dave LeRoy and Shuching Shieh for their helpful

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10.  Appendix A - Constants & Configurable Parameters

   The following table provides a summary of the DVMRP timing

                     Parameter               Value (seconds)
            Probe Interval                 10
            Neighbor Time-out Interval     35
            Minium Flash Update Interval   5
            Route Report Interval          60
            Route Replacement Time         140
            Route Expiration Time          200
            Prune Lifetime                 variable (< 2 hours)
            Prune Retransmission Time      3 with exp. back-off
            Graft Retransmission Time      5 with exp. back-off

                        Table 2 - Parameter Summary

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11.  Appendix B - Tracing and Troubleshooting support

   There are several packet types used to gather DVMRP specific
   information.  They are generally used for diagnosing problems or
   gathering topology information. The first two messages are now
   obsoleted and should not be used. The remaining two messages provide
   a request/response mechanism to determine the versions and
   capabilities of a particular DVMRP router.

         Code        Packet Type               Description
          3     DVMRP Ask Neighbors     Obsolete
          4     DVMRP Neighbors         Obsolete
          5     DVMRP Ask Neighbors 2   Request Neighbor List
          6     DVMRP Neighbors 2       Respond with Neighbor List

                     Table 3 - Debugging Packet Types

11.1.  DVMRP Ask Neighbors2

   The Ask Neighbors2 packet is a unicast request packet directed at a
   DVMRP router. The destination should respond with a unicast
   Neighbors2 message back to the sender of the Ask Neighbors2 message.

                  0         8          16              31
                 | Type    |  Code   |      Checksum      |
                 |(0x13)   | (0x5)   |                    |
                 |     Reserved      |  Minor   | Major   |
                 |                   | Version  |Version  |

                 Figure 8 - Ask Neighbors 2 Packet Format

Pusateri                                                       [Page 33]

INTERNET-DRAFT               DVMRP Version 3               February 1997

11.2.  DVMRP Neighbors2

   The format of a Neighbors2 response packet is shown below. This is
   sent as a unicast message back to the sender of an Ask Neighbors2
   message.  There is a common header at the top followed by the routers
   capabilities.  One or more sections follow that contain an entry for
   each logical interface.  The interface parameters are listed along
   with a variable list of neighbors learned on each interface.

   If the interface is down or disabled, list a single neighbor with an
   address of for physical interfaces or the remote tunnel
   endpoint address for tunnel pseudo-interfaces.

Pusateri                                                       [Page 34]

INTERNET-DRAFT               DVMRP Version 3               February 1997

           0            8              16                    31
          |   Type    |     Code     |         Checksum         |
          |  (0x13)   |    (0x6)     |                          |
          | Reserved  | Capabilities |   Minor    |    Major    |
          |           |              |  Version   |   Version   |
          |                                                     |
          |                    Local Addr 1                     |
          |           |              |            |             |
          | Metric 1  | Threshold 1  |  Flags 1   | Nbr Count 1 |
          |                                                     |
          |                       Nbr 1                         |
          |                                                     |
          |                         ...                         |
          |                                                     |
          |                       Nbr m                         |
          |                                                     |
          |                    Local Addr N                     |
          |           |              |            |             |
          | Metric N  | Threshold N  |  Flags N   | Nbr Count N |
          |                                                     |
          |                       Nbr 1                         |
          |                                                     |
          |                         ...                         |
          |                                                     |
          |                       Nbr k                         |

                   Figure 9 - Neighbors 2 Packet Format

Pusateri                                                       [Page 35]

INTERNET-DRAFT               DVMRP Version 3               February 1997

   The capabilities of the local router are defined as follows:

            Bit    Flag                Description

            0     Leaf     This is a leaf router

            1     Prune    This router understands pruning

            2     GenID    This router sends Generation Id's

            3     Mtrace   This router handles Mtrace requests

            4     Snmp     This router supports the DVMRP MIB

                    Table 4 - DVMRP Router Capabilities

Pusateri                                                       [Page 36]

INTERNET-DRAFT               DVMRP Version 3               February 1997

   The flags associated with a particular interface are:

         Bit       Flag                   Description

         0     Tunnel         Neighbor reached via tunnel

         1     Source Route   Tunnel uses IP source routing

         2     Reserved       No longer used

         3     Down           Operational status down

         4     Disabled       Administrative status down

         5     Reserved       No longer used

         6     Leaf           No downstream neighbors on interface

                      Table 5 - DVMRP Interface flags

Pusateri                                                       [Page 37]

INTERNET-DRAFT               DVMRP Version 3               February 1997

   12.  Appendix C - Version Compatibility

      There have been two previous major versions of DVMRP with
      implementations still in circulation. If the receipt of a Probe
      message reveals a major version of 1 or 2, then it can be assumed
      that this neighbor does not support pruning or the use of the
      Generation ID in the Probe message.  However, since these older
      implementations are known to safely ignore the Generation ID and
      neighbor information in the Probe packet, it is not necessary to
      send specially formatted Probe packets to these neighbors.

      There were three minor versions (0, 1, and 2) of major version 3
      that did support pruning but did not support the Generation ID or
      capability flags.  These special cases will have to be accounted

      Any other minor versions of major version 3 closely compare to
      this specification.

      In addition, cisco Systems is known to use their software major
      and minor release number as the DVMRP major and minor version
      number. These will typically be 10 or 11 for the major version
      number. Pruning was introduced in Version 11.

      Implementations prior to this specification may not wait to send
      route reports until probe messages have been received with the
      routers address listed. Reports SHOULD be sent to these neighbors
      without first requiring a received probe with the routers address
      in it as well as reports from these neighbors SHOULD be accepted.
      Although, this allows one-way neighbor relationships to occur, it
      does maintain backward compatibility.

Pusateri                                                       [Page 38]

                              Table of Contents

      1. Introduction  . . . . . . . . . . . . . . . . . . . . . . .   2
         1.1. Reverse Path Multicasting  . . . . . . . . . . . . . .   2
         1.2. IP-IP Tunnels  . . . . . . . . . . . . . . . . . . . .   2
         1.3. Document Overview  . . . . . . . . . . . . . . . . . .   3
      2. Protocol Overview . . . . . . . . . . . . . . . . . . . . .   3
         2.1. Neighbor Discovery . . . . . . . . . . . . . . . . . .   3
         2.2. Source Location  . . . . . . . . . . . . . . . . . . .   4
         2.3. Dependent Downstream Routers . . . . . . . . . . . . .   5
         2.4. Multi-access Networks  . . . . . . . . . . . . . . . .   5
         2.5. Building Multicast Trees . . . . . . . . . . . . . . .   6
         2.6. Pruning Multicast Trees  . . . . . . . . . . . . . . .   7
         2.7. Grafting Multicast Trees . . . . . . . . . . . . . . .   7
      3. Detailed Protocol Operation . . . . . . . . . . . . . . . .   8
         3.1. Protocol Header  . . . . . . . . . . . . . . . . . . .   8
         3.2. Probe Messages . . . . . . . . . . . . . . . . . . . .   9
         3.3. Building Forwarding Cache Entries  . . . . . . . . . .  14
         3.4. Unicast Route Exchange . . . . . . . . . . . . . . . .  14
         3.5. Pruning  . . . . . . . . . . . . . . . . . . . . . . .  21
         3.6. Grafting . . . . . . . . . . . . . . . . . . . . . . .  25
         3.7. Interfaces . . . . . . . . . . . . . . . . . . . . . .  29
      4. IANA Considerations . . . . . . . . . . . . . . . . . . . .  29
      5. Network Management Considerations . . . . . . . . . . . . .  29
      6. Security Considerations . . . . . . . . . . . . . . . . . .  30
      7. References  . . . . . . . . . . . . . . . . . . . . . . . .  30
      8. Author's Address  . . . . . . . . . . . . . . . . . . . . .  31
      9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . .  31
      10. Appendix A - Constants & Configurable Parameters . . . . .  32
      11. Appendix B - Tracing and Troubleshooting support . . . . .  33
         11.1. DVMRP Ask Neighbors2  . . . . . . . . . . . . . . . .  33
         11.2. DVMRP Neighbors2  . . . . . . . . . . . . . . . . . .  34
      12. Appendix C - Version Compatibility . . . . . . . . . . . .  38

Pusateri                                                        [Page i]