T. Pusateri
INTERNET DRAFT                                          Juniper Networks
Obsoletes: RFC 1075                                         October 1997
draft-ietf-idmr-dvmrp-v3-05                          Expires: April 1998

               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
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   To learn the current status of any Internet-Draft, please check the
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   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 DVMRP routing
   table of known source networks. If the datagram arrives on an
   interface that would be used to transmit 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 on that
   interface. In this way, Neighbor DVMRP routers can ensure that they

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   are seen by each 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 path back to a source, a 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 DVMRP routing table, it does provide two nice
   features. First, since all DVMRP routers are exchanging the same
   routes, 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 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 routes in DVMRP provides one other important feature.
   DVMRP uses the 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.  Designated Forwarder

   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 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. In this way, DVMRP does an implicit
   designated forwarder election for each source network on each
   downstream interface.

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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 all IP multicast routers on each physical, multicast capable
   network. The details of the election procedure are discussed in
   [Fenn97]. If the destination group address is listed in the local
   group database, and the router is the designated forwarder for the
   source, 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 network, group) pair. The downstream routers
   will then have the option to send prunes and grafts for this (source
   network, 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 network,
   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 DVMRP 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 network,
   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 network, 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

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   from the host back on to the multicast delivery tree.  A router will
   send 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 network 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 [Fenn97] Packet Type
   as hexadecimal 0x13 (DVMRP). DVMRP protocol packets should be sent
   with the Precedence field in the IP header set to Internetwork
   Control.  A diagram of the common protocol header follows:

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

                     Figure 1 - Common Protocol Header

   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 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

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   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 on each interface 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.)

   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

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   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
   existing implementations will not assume this neighbor does not
   support multicast trace-route [Fenn96]. 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.

                      7   6   5    4   3   2   1    0
                                   S   M   G   P    L
                     |0  |0  |0  | 0 | 1  |1  |1  | 0 |

                     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 router
   detects an increase in the generation ID of a neighbor, it should
   send its entire 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,

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   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
   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 to a neighbor until the neighbor
   includes the routers address in its probe messages. On point-to-point
   interfaces and tunnel pseudo-interfaces, this means that no packets
   should be forwarded onto these interfaces until two-way neighbor
   relationships have formed.

   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.  Neighbor Time-Out

   When a neighbor times out, the following steps should be taken:

   1. All routes learned by this neighbor should be immediately placed
      in holddown. Forwarding cache entries may need to be updated.

   2. All downstream dependencies by this neighbor should be canceled.
      This may trigger prunes to be sent.

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3.2.5.  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.

                      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

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3.2.6.  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 version 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.

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.  Designated Forwarder

   Initially, a DVMRP router should assume it is the designated
   forwarder for all source networks on all downstream interfaces. As it
   receives route reports, it can determine if other routers on multi-
   access networks have better routes back to a particular source
   network. A route is considered better if the received metric is less
   than the metric that it will advertise for the source network on the
   received interface or if the metrics are the same but the IP address
   of the neighbor is lower.

   If this neighbor becomes unreachable or starts advertising a worse
   metric, then the router should become the designated forwarder for
   this source network again on the downstream interface until it hears
   from a better candidate.

   If the upstream RPF interface changes, then the router should become
   the designated forwarder on the previous upstream interface (which is

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   now a potential downstream interface) until it hears from a better

3.3.2.  Determining the upstream interface

   When a multicast packet arrives, a DVMRP router will use the DVMRP
   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 routing table.

3.3.3.  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. If the router is not the designated
   forwarder on interfaces with directly attached group members, these
   interfaces can be removed as well. This prevents duplicate packets
   from arriving on multi-access networks.

3.4.  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.

   The 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 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

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   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.

3.4.1.  Source Network Aggregation

   Implementations may wish to provide a mechanism to aggregate source
   networks to reduce the size of the 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.

   If an implementation wishes to support source aggregation, it MUST
   transmit Prune and Graft messages according to the following rules:

   A. If a Prune is received on a downstream interface for which the
      source network advertised to that neighbor is an aggregate
      generated by the receiving router, then only a single Prune should
      be sent upstream (if necessary) to the router advertising the best
      matching source network component of the aggregate.

   B. If a Graft is received on a downstream interface for which the
      source network advertised to that neighbor is an aggregate
      generated by the receiving router, then Graft messages MUST be
      sent upstream (if necessary) to the neighbors advertising each of
      the source networks that were used to generate the aggregate.

3.4.2.  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

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   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.  There 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
   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

3.4.3.  Route Metrics

   For each source network reported, a route metric is associated with
   the 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

   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

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   applied to the subsequent list of routes.

3.4.4.  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
   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 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.

3.4.5.  Sending Route Reports

   All of the active routes MUST be advertised over all interfaces with
   neighbors present 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.

   When a router sees its own address in a neighbor probe packet for the
   first time, it should send an entire copy of its routing table to the
   neighbor to reduce start-up time.

   Route Reports containing downstream dependent "poison" metrics MUST
   be sent to the All-DVMRP-Routers address.  These reports should not

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   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.

3.4.6.  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:

   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.
      The new metric is defined as the metric received in the route
      report plus the metric of the received interface.

      1. New metric < infinity

         If this neighbor is a downstream dependent neighbor, the
         neighbor is now learning the route from another source. Cancel
         the downstream dependency.

         In the following cases, the designated forwarder for the source
         network on the receiving interface may need to be updated. This
         is true under the following conditions:

         -  If the new route is better and the router receiving the
            report is currently the designated forwarder.

         -  If the new route is worse than the existing route and the
            advertising router is currently the designated forwarder.

         A route is considered better when either the received metric is
         lower than the existing metric or the received metric is the
         same but the advertising router's IP address is lower.

         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

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            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 upstream interface changed,
            a flash poison update should be sent upstream indicating a
            change in downstream dependency.

         c. New metric = existing metric

            If the neighbor reporting the route is the same as the
            existing upstream neighbor, then simply refresh the route.
            If the neighbor reporting the route has a lower IP address
            than the existing upstream neighbor, then update the route.
            If the upstream interface changes, a flash poison update
            should be sent on the new interface.

            Again, the receiving router may need to update its
            designated forwarder status if the neighbor is a better

      2. Metric = infinity
            If the neighbor was considered to be the designated
            forwarder, the receiving router should now become the
            designated forwarder for the source network on this
            interface unless it knows of a better candidate.

         a. Next hop = existing next hop

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

         b. Next hop != existing next hop

            The route can be ignored since the existing next hop has a
            metric better than or equal to this next hop.

            If the neighbor was considered a downstream dependent
            neighbor, this should be cancelled. Check to determine if
            removing this neighbor triggers a Prune.

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      3. infinity < New metric < 2 x infinity

         The neighbor considers the receiving router to be upstream for
         the route and is indicating it is dependent on the receiving

         If the neighbor was considered to be the designated forwarder,
         the receiving router should now become the designated forwarder
         for the source network on this interface unless it knows of a
         better candidate.

         a. Neighbor on down stream interface

            If the sending 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.

3.4.7.  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.

   In order to minimize outages caused by flapping routes, it is
   permissible to prematurely take a route out of hold-down only if the
   route is re-learned from the SAME router with the SAME metric.

3.4.8.  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

3.4.9.  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.

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3.5.1.  Leaf Networks

   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 easily by a best-match lookup using
   the 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.  If the neighbor is unknown, discard the received prune.

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

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

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

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

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

   7.  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

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

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

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

   11. 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.

   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.

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   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
   SHOULD 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.

   The actual retransmission time should be randomized to reduce
   synchronized prune retransmissions.

   On multi-access networks, even if a prune is received correctly, data
   may still be received due to other receivers on the network.

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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 network,
      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 network, group) pair in the
   received Graft message. If the receiving router has prune state for
   the (source network, 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 network, 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 address, 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

   When multiple addresses are configured on a single interface, it is
   necessary that all routers on the interface know about the same set
   of network addresses. In this way, each router will make the same
   choice for the designated forwarder for each source.  In addition, a
   router configured with multiple addresses on an interface should
   consistently use the same address when sending DVMRP control

   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 [Fenn97] 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

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   directly query DVMRP neighbor information. In addition, a Management
   Information Base for DVMRP is defined in [Thal97].  A protocol
   independent multicast trace-route facility is defined in [Fenn96].

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.

   [Fenn96]  Fenner, W., Casner, S., "A "traceroute" facility for IP
             Multicast",  Work In Progress, November 1996.

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INTERNET-DRAFT               DVMRP Version 3                October 1997

   [Fenn97]  Fenner, W., "Internet Group Management Protocol, Version
             2",  Work In Progress, January 1997.

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

   [Thal97]  Thaler, D., "Distance-Vector Multicast Routing Protocol
             MIB",  Work In Progress, April 1997.

   [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.
   385 Ravendale Dr.
   Moutain View, CA  94043
   Phone:    (919) 558-0700
   EMail:    pusateri@juniper.net

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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 the ongoing (and seemingly
   endless) work of Bill Fenner.  Credit also goes to Danny Mitzel for
   the careful review of this document and Nitin Jain, Dave LeRoy,
   Charles Mumford, Ravi Shekhar, Shuching Shieh, and Dave Thaler for
   their helpful comments.

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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
           Minimum Flash Update Interval   5
           Route Report Interval           60
           Route Replacement Time          140
           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

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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.

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           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

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   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

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   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     Reserved       No longer used

         4     Down           Operational status down

         5     Disabled       Administrative status down

         6     Querier        Querier for interface

         7     Leaf           No downstream neighbors on interface

                      Table 5 - DVMRP Interface flags

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   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.

      Implementations that do not monitor Generation ID changes can
      create more noticeable black holes when using long prune lifetimes
      such as ~2 hours.  This happens when a long prune is sent upstream
      and then the router that sent the long prune restarts. If the
      upstream router ignores the new Generation ID, the prune received
      by the upstream router will not be flushed and the downstream
      router will have no knowledge of the upstream prune. For this
      reason, prunes sent upstream to routers that are known to ignore
      Generation ID changes should have short lifetimes.

      If the router must run IGMP version 1 on an interface for
      backwards compatibility, DVMRP must elect the DVMRP router with
      the highest IP address as the IGMP querier.

      Some implementations of tools that send DVMRP Ask Neighbors2

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      requests and receive Neighbors2 response messages require a
      neighbor address of when no neighbors are listed in the
      response packet. (Mrinfo)

Pusateri                                                       [Page 42]

                              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. Designated Forwarder  . . . . . . . . . . . . . . . . . .   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. Route Exchange  . . . . . . . . . . . . . . . . . . . . .  15
      3.5. Pruning . . . . . . . . . . . . . . . . . . . . . . . . .  23
      3.6. Grafting  . . . . . . . . . . . . . . . . . . . . . . . .  27
      3.7. Interfaces  . . . . . . . . . . . . . . . . . . . . . . .  31
      4. IANA Considerations . . . . . . . . . . . . . . . . . . . .  31
      5. Network Management Considerations . . . . . . . . . . . . .  31
      6. Security Considerations . . . . . . . . . . . . . . . . . .  32
      7. References  . . . . . . . . . . . . . . . . . . . . . . . .  32
      8. Author's Address  . . . . . . . . . . . . . . . . . . . . .  33
      9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . .  33
      10. Appendix A - Constants & Configurable Parameters . . . . .  35
      11. Appendix B - Tracing and Troubleshooting support . . . . .  36
      11.1. DVMRP Ask Neighbors2 . . . . . . . . . . . . . . . . . .  36
      11.2. DVMRP Neighbors2 . . . . . . . . . . . . . . . . . . . .  37
      12. Appendix C - Version Compatibility . . . . . . . . . . . .  41

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