T. Pusateri
INTERNET DRAFT                                          Juniper Networks
Obsoletes: RFC 1075                                          August 2000
draft-ietf-idmr-dvmrp-v3-10                    Expires: February 4, 2001

               Distance Vector Multicast Routing Protocol

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   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
   to build per-source-group multicast delivery trees.  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 [Deer91].

1.1.  Requirements Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC-2119].

1.2.  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 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.

1.3.  Tunnel Encapsulation

   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

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   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
   more direct, it increased the complexity of firewall configuration.
   The most noticeable change in this specification regarding tunnels is
   that all DVMRP protocol messages should be sent encapsulated across
   the tunnel.  Previously, protocol messages were sent un-encapsulated
   directly to the tunnel endpoint.  See Appendix C for backward
   compatibility issues.

   Note: All protocol messages sent on point-to-point links (including
   tunnels) should use a destination address of All-DVMRP-Routers. This
   change will allow the protocol messages to be forwarded across
   multicast-only tunnels without making encapsulation and decapsulation

   In practice, tunnels typically use either IP-IP [Perk96] or Generic
   Routing Encapsulation (GRE) [Han94a,Han94b], although, other
   encapsulation methods are acceptable.

1.4.  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 builds per-source broadcast trees based upon routing
   exchanges, then dynamically creates per-source-group multicast
   delivery trees by pruning (removing branches from) the source's
   truncated broadcast tree.  It performs Reverse Path Forwarding checks
   to determine when multicast traffic should be forwarded to downstream

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   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 are 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 [Reyn94] IP Multicast group address.  (See Appendix
   C for backwards compatibility issues.)  The IP TTL of these messages
   MUST be set to 1.

   Each Neighbor Probe message contains 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
   are seen by each other.

   Once you have received a Probe from a neighbor that contains your
   address in the neighbor list, you have established a two-way neighbor
   adjacency with this router.

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 on which the best route to the source of the datagram was
   received is called the upstream (also called RPF) 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 interface over which the datagram was received must be added
   to the metric of the route being advertised in the route report
   message.  This adjusted metric should be used when comparing metrics

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   to determine the best upstream neighbor.

   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 versus 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
   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 on the upstream interface
   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.

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2.4.  Designated Forwarder

   When two or more multicast routers are connected to a multi-access
   network, it could be possible for duplicate packets to be forwarded
   on the network (one copy from each router).  DVMRP prevents this
   possibility by electing a forwarder for each source as a side effect
   of its 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

2.5.  Building Multicast Trees

   As previously mentioned, when an IP multicast datagram arrives, the
   upstream interface is determined by looking up the interface on which
   the best route to the source of the datagram was received.  If the
   upstream interface is correct, then a DVMRP router will forward the
   datagram to a list of downstream interfaces.

2.5.1.  Adding Local Group Members

   The IGMP local group database is maintained by all IP multicast
   routers on each physical, multicast capable network [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 is included in the list of downstream interfaces.  If there
   are no group members on the interface, then the interface is removed
   from the outgoing interface list.

2.5.2.  Adding Interfaces with Neighbors

   Initially, all interfaces with downstream dependent neighbors should
   be included in the downstream interface list when a forwarding cache
   entry is first created.  This allows the downstream routers to be
   aware of traffic destined for a particular (source network, group)

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   pair. The downstream routers will then have the option to send prunes
   and subsequent grafts for this (source network, group) pair as
   requirements change from their respective downstream routers and
   local group members.

2.6.  Pruning Multicast Trees

   As mentioned above, routers at the edges will remove their interfaces
   that have no group members associated with an IP multicast datagram.
   If a router removes all of its downstream interfaces, it notifies 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 broadcasting procedure.
   The prune message contains a prune lifetime, indicating 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 must be equal to the minimum of the remaining
   lifetimes of the received prunes.

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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, hosts may join a multicast group at any time.  In
   this case, DVMRP routers use Grafts to cancel the prunes that are in
   place 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 using binary exponential back-
   off between retransmissions. 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 received from a neighbor with whom a
   two-way neighbor relationship has been formed 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 (hexadecimal 0xc0 for the Type of Service Octet) [Post81].  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 MUST be calculated upon
   transmission and MUST be validated on reception of a packet.  The
   checksum of the DVMRP message is calculated with the checksum field
   set to zero. See [Brad88] for more information.

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3.2.  Probe Messages

   When a DVMRP router is configured to run on an interface (physical or
   tunnel), it multicasts DVMRP Probe packets to inform other DVMRP
   routers that it is operational. Effectively, they serve three

   1. Probes provide a mechanism for DVMRP routers to locate each other.
      DVMRP sends on each interface, a Probe Message containing the list
      of the neighbors detected for that specific interface.  If no
      DVMRP neighbors are found, the network is considered to be a leaf

   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. Probes sent on each multicast capable interface
      configured for DVMRP SHOULD use 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

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   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 on transmission and ignored on
   reception. Bit position 0 is the LEAF bit which is a current research
   topic.  It MUST be set to 0 and ignored on reception.  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 [Fenn00]. 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.

   The N bit (which stands for Netmask) is defined by this
   specification.  It is used to indicate the neighbor will accept
   network masks appended to the Prune, Graft, and Graft Ack messages.
   This bit only indicates that the neighbor understands the netmask. It
   DOES NOT mean that Prune, Graft, and Graft Ack messages sent to this
   neighbor must include a netmask. Refer to the sections on Prune,
   Graft, and Graft Ack messages for more details.

   Each time a Probe message is received from a neighbor, the
   capabilities bits should be compared to the previous version for that
   neighbor in order to detect changes in neighbor capabilities.

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

                     Figure 2 - Probe Capability Flags

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3.2.2.  Generation ID

   If a DVMRP router is restarted, it will not be aware of any previous
   prunes that it had sent or received.  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 change in the 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.

   In addition, the effects of a restart can be minimized if the router
   can learn all of the routes known by its neighbors without having to
   wait for an entire report interval to pass.  When a router detects a
   change in the generation ID of a neighbor, it should send a unicast
   copy of its entire routing table to the neighbor.

   In addition to restarting, a router may also miss prune information
   while an interface has transitioned to a down state. Therefore, a
   change in the generation ID is necessary when an interface
   transitions to the up state. In order to prevent all prune state from
   being flushed on a router when a single interface transitions, a
   DVMRP router should keep seperate generation ID numbers per

   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 in response to routes advertised
   by 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.

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   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 Expiry

   When a neighbor expires, the following steps should be taken:

   1. All routes learned from this neighbor should be immediately placed
      in hold-down.  All downstream dependencies ON this neighbor should
      be removed.

   2. If this neighbor is considered to be the designated forwarder for
      any of the routes it is advertising, a new designated forwarder
      for each source network should be selected.

   3. Any forwarding cache entries based on this upstream neighbor
      should be flushed.

   4. Any outstanding Grafts awaiting acknowledgments from this router
      should be flushed.

   5. All downstream dependencies received FROM this neighbor should be
      removed.  Forwarding cache entries should be checked to see if
      this is the last downstream dependent neighbor on the interface.
      If so, and this router isn't the designated forwarder (with local
      group members present), the interface should be removed.

      It is possible as an optimization to send a prune upstream if this
      causes the last downstream interface to be removed. However, this
      prune could be unnecessary if no more traffic is arriving. It is
      also acceptable to simply wait for traffic to arrive before
      sending the prune upstream.

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

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   used to determine the number of neighbors in the Probe message.  The
   current Major Version is 3.

                      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

   Generation ID
      This field contains a non-decreasing number used to detect a
      change in neighbor state.

   Neighbor IP Address N
      This is a list of Neighbor IP addresses whom the sending router
      has received Probe messages from.

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3.2.6.  IGMP Designated Querier 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 was formerly a part of DVMRP.  However, this is now
   specified as a part of the IGMP version 2 protocol.  See Appendix C
   for details on backwards compatibility.

   Even though only one router will act as the IGMP designated querier,
   all DVMRP routers must use IGMP to learn local group memberships.

3.3.  Multicast Forwarding

   DVMRP can forward multicast packets by building the downstream
   interface list for each packet as it arrives.  However, to reduce per
   packet processing time, the result of the first lookup MAY be cached
   in a forwarding table. Then, as routes, downstream dependent
   neighbors, or group membership change, the cache forwarding table
   entries MUST be updated to reflect these changes.

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

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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 MUST 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 by starting with the list of
   non-leaf interfaces. The upstream interface MUST be removed from this
   list. Then any interfaces on the list where all of the downstream
   dependents have sent prunes upstream MUST be removed.  Next, any
   interfaces for which the router is the designated forwarder and local
   group members are present MUST be added to the list.

3.4.  Route Exchange

   The routing information propagated by DVMRP is used for determining
   the reverse path neighbor back to the source of the multicast
   traffic. The interface used to reach this neighbor is called the
   upstream interface. Tunnel pseudo-interfaces are considered to be
   distinct from the physical interface on which the packet is actually
   transmitted for the purpose of determining upstream and downstream

   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 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

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   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, then
      if a prune is sent upstream, it should only be sent for the
      contributing route based on the source address in the received

      If additional data is received for sources within the range of the
      aggregate, then this SHOULD trigger additional prunes to be sent
      upstream for these sources.

      There may be active forwarding cache entries for other
      contributing routes to the aggregate.  Prunes should not be sent
      upstream to the contributing routes that have no forwarding state.

   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) for each route that contributed to
      the aggregate that had been previously pruned.

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
   chance of synchronized route reports causing routers to become

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   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.  It does not, however, allow
   grouping class A networks into super-nets since the first octet of
   the netmask is always assumed to be 255.

   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 [00].  This special case MUST be interpreted
   as and NOT

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, a
   "Last" bit is defined as the high order bit of the octet following

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   the network address. This bit is set to signify when the last route
   is being reported for a particular mask value.  When the "Last" bit
   is set and 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

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 broadcasted 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 the router has
   received Prune messages from each of the dependent routers on that
   interface.  Each downstream router uses Poison Reverse to indicate
   for which source networks it is dependent upon 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 flash updates MUST NOT happen more
   often than the Minimum Flash Update Interval (5 seconds).  Flash
   updates 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

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

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   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

3.4.6.  Receiving Route Reports

   After receiving a route report, a check should be made to verify it
   is from a known neighbor. Two-way neighbor relationships are
   essential for proper DVMRP operation.  Therefore, route reports from
   unknown neighbors MUST be discarded.

   In the following discussion, "Metric" refers to the metric of the
   route as received in the route report. "Adjusted Metric" refers to
   the metric of the route after the incoming interface metric has been

   If the metric received is less than infinity but the Adjusted Metric
   is greater than or equal to infinity, the Adjusted Metric should be
   set to infinity.

   If the metric is greater than or equal to infinity, then no
   adjustment of the metric should be made.

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

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

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

      1. Received Metric < infinity

         If the neighbor was considered a downstream dependent neighbor,
         the dependency is canceled.

         In the following cases, the designated forwarder on one of the
         downstream interfaces should be updated:

         -  If the Metric received would cause the router to advertise a
            better metric on a downstream interface than the existing
            designated forwarder for the source network on that
            interface (or advertised metric would be the same but the
            router's IP address is lower than the existing designated
            forwarder on that interface).  Then the receiving router

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            becomes the new designated forwarder for that source network
            on that interface. If this router had sent a prune upstream
            that is still active, it will need to send a graft.

         -  If the metric being advertised by the current designated
            forwarder is worse than the receiving routers metric that it
            would advertise on the receiving interface (from learning
            the same route from a neighbor on another interface) or the
            metric is the same but the receiving router has a lower IP
            address, then the receiving router becomes the new
            designated forwarder on that interface. This may trigger a
            graft to be sent upstream.

         -  If the metric received for the source network is better than
            the metric of the existing designated forwarder, save the
            new designated forwarder and the metric it is advertising.
            It is necessary to maintain knowledge of the current
            designated forwarder for each source network in case the
            time-out value for this neighbor is reached. If the time-out
            is reached, then the designated forwarder responsibility for
            the source network should be assumed.

         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. Adjusted Metric > existing metric

            If the Adjusted Metric is greater than the existing metric,
            then check to see if the same neighbor is reporting the
            route. If so, update the route metric and schedule a flash
            update containing the route.  Otherwise, skip to the next
            route in the report.

         b. Adjusted 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.  Schedule a flash update
            containing the route to downstream neighbors and a flash
            poison update containing the route should be sent upstream
            indicating a change in downstream dependency (even if its on
            the same upstream interface).

         c. Adjusted metric = existing metric

            If the neighbor reporting the route is the same as the
            existing upstream neighbor, then simply refresh the route.

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            If the neighbor is the same and the route is in hold-down,
            it is permissible to prematurely take the route out of hold-
            down and begin advertising it with a non-infinity metric.
            If the route is taken out of hold-down, a flash update
            containing the route should be scheduled on all DVMRP
            interfaces except the one over which it was received.

            If the neighbor reporting the route has a lower IP address
            than the existing upstream neighbor, then switch to this
            neighbor as the best route.  Schedule a flash update
            containing the route to downstream neighbors and a flash
            poison update containing the route should be sent upstream
            indicating a change in downstream dependency (even if its on
            the same upstream interface).

      2. Received 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

         a. Next hop = existing next hop

            If the existing metric was less than infinity, the route is
            now unreachable.  Delete the route and schedule a flash
            update containing the route to all interfaces for which you
            are the designated forwarder or have downstream dependents.

         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 canceled.

      3. infinity < Received 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.

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         a. Neighbor on downstream interface

            If the sending neighbor is considered to be on a downstream
            interface for that route then the neighbor is to 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 MUST be sent upstream in the direction of the
            source network for each group with existing prune state.

         b. Neighbor on upstream interface

            If the receiving router thinks the neighbor is on the
            upstream interface, then the route should be treated as if
            an infinity metric was received (See Received Metric =

      4. 2 x infinity <= Received Metric

         If the Received Metric is greater than or equal to 2 x
         infinity, the Metric is illegal and the route should be

3.4.7.  Route Expiration

   A route expires if it has not been refreshed within the Route
   Expiration time. This should be set to 2 x Route Report Interval + 20
   (or 140) seconds.  Due to flash updates, routes will typically not
   expire but instead be removed in response to receiving an infinity
   metric for the route.  However, since not all existing
   implementations implement flash updates, route expiration is

3.4.8.  Route Hold-down

   When a route is deleted (because it expires, the neighbor it was
   learned from goes away, or an infinity metric is received for the
   route) 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

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   route will continue to be propagated long after it is no longer

   In order to prevent this, it is common in distance vector protocols
   to continue to advertise a route that has been deleted with a metric
   of infinity for one or more report intervals. This is called Hold-
   down.  A route MUST only be advertised with an infinity metric while
   it is in hold-down. The hold-down period is 2 Report Intervals.

   When a route goes into hold-down, all forwarding cache entries based
   on the route should be flushed.

   During the hold-down period, the route may be learned via a different
   gateway or the same gateway with a different metric. The router MAY
   use this new route for delivering to local group members. Also,
   installing a new route during the hold-down period allows the route
   to be advertised with a non-infinity metric more quickly once the
   hold-down period is over.

   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.

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

3.4.9.  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

3.4.10.  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

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   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.

                    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.  As the routers at the leaves of the tree begin to receive
   unwanted multicast traffic, they send prune messages upstream toward
   the source.  This results in the multicast tree 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 dependent 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

   By default, prunes are meant to be applied to a group and source
   network. However, it is possible to include a Netmask in the Prune
   message to alter this behavior. If no Netmask is included, a prune
   sent upstream triggered by traffic received from a particular source
   applies to all sources on that network. If a Netmask is included, it
   MUST first be validated. If the Netmask is a host mask, only that
   source address should be pruned. Otherwise, the Netmask MUST match
   the mask sent to the downstream router for that source. If it does
   not match the mask that the upstream router expected, the upstream
   router MUST ignore the prune and should log an error. When a
   aggregate source network is advertised downstream, the Netmask in the
   prune will match the mask of the aggregate route that was advertised.

   If the Prune message only contains the host address of the source
   (and not the corresponding Netmask), 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.  Ensure the prune message contains at least the correct amount of

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   3.  Copy the source address, group address, and prune time-out value.
       If it is available in the packet, copy the Netmask value.
       Determine route to which prune applies.

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

   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
       interface and if this router is the designated forwarder for the

   10. If not, then remove the interface from all forwarding cache
       entries for this group instantiated using the route to which the
       prune applies.

3.5.4.  Sending a Prune

When a forwarding cache is being used, there is a trade-off that should
be considered when deciding when Prune messages should be sent upstream.
In all cases, when a data packet arrives and the downstream interface
list is empty, a prune is sent upstream. However, when a forwarding
cache entry transistions to an empty downstream interface list it is
possible as an optimization to send a prune at this time as well.  This
prune will possibly stop unwanted traffic sooner at the expense of
sending extra prune traffic for sources that are no longer sending.
   When sending a prune upstream, the following steps should be taken:

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   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 default prune lifetime (randomized to prevent synchronization)
      and the remaining prune lifetimes of the downstream neighbors.

   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 retransmit prunes messages using binary exponential back-off
   during 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.

   In addition to using binary exponential back-off, the interval
   between subsequent retransmissions should also be randomized to
   prevent synchronization.

   On multi-access networks, even if a prune is received by the upstream
   router, data may still be received due to other receivers (i.e. group
   members or other downstream dependent routers) on the network.

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3.5.6.  Prune Packet Format

   A Prune Packet contains three required fields: the source host IP
   address, the destination group IP address, and the Prune Lifetime in
   seconds. An optional source network mask may also be appended to the
   Prune message. This mask applied to the Source Host Address will
   indicate the route that the prune applies to.  A Source Network Mask
   field should only be sent in a Prune message to a neighbor if that
   neighbor has advertised the ability to process it by setting the
   Netmask capabilities bit.  The length of the Prune message will
   indicate if the Source Network Mask has been included or not.

   The Prune Lifetime is a derived value calculated as the minimum of
   the default prune lifetime (2 hours) and the remaining lifetimes of
   any downstream prunes received for this source network and group.  A
   router with no downstream dependent neighbors would use the the
   default prune lifetime (randomized to prevent synchronization).

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

                      Figure 5 - Prune Packet Format

   Source Host Address
      The source host IP address of the datagram that triggered the

   Group Address
      The destination group IP address of the datagram that triggered
      the prune.

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   Prune Lifetime
      The number of seconds for which the upstream neighbor should keep
      this prune active.

   Source Network Mask
      The (optional) netmask of the route this prune applies to.

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 hop-by-hop from the new receiver's
   first-hop router until a point on the multicast tree is reached.
   Since there is no way to tell whether a graft message was lost or the
   source stopped sending, 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

   1. A new local member joins a group that has been pruned upstream and
      this router is the designated forwarder for the source.

   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.  Sending a Graft

   Recall that by default, Prunes are source network specific and are
   sent up different trees for each source network.  Grafts are sent in
   response to various conditions which are not necessarily source
   specific. Therefore, it may be necessary to send separate Graft
   messages to the appropriate upstream routers to counteract each
   previous source network specific prune that was sent.

   As mentioned above, a Graft message sent to the upstream DVMRP router

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   should be acknowledged hop by hop guaranteeing end-to-end delivery.
   In order to send a Graft message, the following steps should be

   1. Verify a prune exists for the source network and group.

   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

   4. Formulate and transmit the Graft packet.

   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

3.6.2.  Receiving a Graft

   1.  If the neighbor is unknown, discard the received graft.

   2.  Ensure the graft message contains at least the correct amount of

   3.  Send back a Graft Ack to the sender.

   4.  If the sender was a downstream dependent neighbor from which a
       prune had previously been received, then remove the prune state
       for this neighbor.  If necessary, any forwarding cache entries
       based on this (source, group) pair should be updated to include
       this downstream interface.

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   5.   If a prune had been sent upstream, this may trigger a graft to
        now be sent to the upstream router.

3.6.3.  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 Host Address                |
            |                  Group Address                   |
            |               Source Network Mask                |

                      Figure 6 - Graft Packet Format

   Source Host Address
      The source host IP address indicating which source host or source
      network to Graft.

   Group Address
      The destination group IP address to be grafted.

   Source Network Mask
      The (optional) netmask of the route this graft applies to.

3.6.4.  Sending a Graft Acknowledgment

   A Graft Acknowledgment packet is sent to a downstream neighbor in
   response to receiving a Graft message. All Graft messages MUST be
   acknowledged. This is true even if no other action is taken in

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   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.

3.6.5.  Receiving a Graft Acknowledgment

   When a Graft Acknowledgment is received, ensure the message contains
   at least the correct amount of data.  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.

3.6.6.  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 Host Address                |
            |                  Group Address                   |
            |               Source Network Mask                |

                    Figure 7 - Graft Ack Packet Format

   Source Host Address
      The source host IP address that was received in the Graft message.

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   Group Address
      The destination group IP address that was received in the Graft

   Source Network Mask
      The (optional) netmask of the route this Graft Ack applies to.

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 a
   standard encapsulation method [Perk96,Han94a,Han94b].  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 packet.

   Protocol messages on point-to-point links should always use a
   destination IP address of All-DVMRP-Routers for ALL message types.
   While Prune, Graft, and Graft-Ack messages are only intended for a
   single recipient, the use of a multicast destination address is
   necessary for un-numbered links and encapsulated interfaces.

   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.

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   3.7.1.  Interface transitions

   When an interface transitions to the up state, the generation ID of
   that interface should be updated so that DVMRP neighbors know to
   resend prune information.

   When an interface transitions to the down state, all neighbors on
   that interface should be expired. All actions associated with an
   expired neighbor should be taken as specified in the Neighbor Expiry

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
   directly query DVMRP neighbor information. In addition, a Management
   Information Base for DVMRP is defined in [Thal97].

   A Management Information Base for the multicast forwarding cache is
   defined in [McCl00].

   Also, a protocol independent multicast trace-route facility is
   defined in [Fenn00].

6.  Security Considerations

   Security for DVMRP follows the general security architecture provided
   for the Internet Protocol [Ken98a]. This framework provides for both
   privacy and authentication. It recommends the use of the IP
   Authentication Header [Ken98b] to provide trusted neighbor
   relationships. Confidentiality is provided by the addition of the IP

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   Encapsulating Security Payload [Ken98c].

7.  References

   [Brad88]  Braden, R., Borman, D., Partridge, C., "Computing the
             Internet Checksum", RFC 1071, September 1988.

   [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.

   [Fenn97]  Fenner, W., "Internet Group Management Protocol, Version
             2",  RFC 2236, November 1997.

   [Fenn00]  Fenner, W., Casner, S., "A "traceroute" facility for IP
             Multicast",  Work In Progress, July 2000.

   [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.

   [Han94a]  Hanks, S., Li, T, Farinacci, D., and P. Traina, "Generic
             Routing Encapsulation", RFC 1701, NetSmiths, Ltd., and
             cisco Systems, October 1994.

   [Han94b]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
             Routing Encapsulation over IPv4 networks", RFC 1702,
             NetSmiths, Ltd., cisco Systems, October 1994.

   [Ken98a]  Kent, S., Atkinson, R. "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.

   [Ken98b]  Kent, S., Atkinson, R., "IP Authentication Header", RFC
             2402, November 1998.

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   [Ken98c]  Kent, S., Atkinson, R., "IP Encapsulating Security Payload
             (ESP)", RFC 2406, November 1998.

   [McCl00]  McCloghrie, K., Farinacci, D., Thaler, D., "IP Multicast
             Routing MIB", Work In Progress, July 2000.

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

   [Perk96]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
             October 1996.

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

   [Post81]  Postel, J., "Internet Protocol", RFC 791, September, 1981.

   [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.
   1194 North Mathilda Avenue
   Sunnyvale, CA 94089 USA
   Phone:    (408) 734-7690
   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 and
   Dave Thaler for the careful review of this document and Nitin Jain,
   Dave LeRoy, Charles Mumford, Ravi Shekhar, and Shuching Shieh 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 Expiration Time           140
         Route Hold-down                 2 x Route Report Interval
         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 for.

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

   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.

   It may be necessary to form neighbor relationships based solely on
   Route Report messages. Neighbor time-out values may need to be
   configured to a value greater than the Route Report Interval for
   these neighbors.

   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

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

   When DVMRP protocol packets are sent to tunnel endpoints, some
   implementations do not accept packets addressed to the All-DVMRP-
   Routers address and then encapsulated with the tunnel endpoint
   address.  Mrouted versions 3.9beta2 and earlier are known to have
   this problem.

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13.  Intellectual Property Rights Notice

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

14.  Full Copyright Statement

   Copyright (C) The Internet Society (date). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the  purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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                             Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   2
   1.1. Requirements Terminology . . . . . . . . . . . . . . . . . .   2
   1.2. Reverse Path Multicasting  . . . . . . . . . . . . . . . . .   2
   1.3. Tunnel Encapsulation . . . . . . . . . . . . . . . . . . . .   2
   1.4. Document Overview  . . . . . . . . . . . . . . . . . . . . .   3
   2. Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .   3
   2.1. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . .   4
   2.2. Source Location  . . . . . . . . . . . . . . . . . . . . . .   4
   2.3. Dependent Downstream Routers . . . . . . . . . . . . . . . .   5
   2.4. Designated Forwarder . . . . . . . . . . . . . . . . . . . .   6
   2.5. Building Multicast Trees . . . . . . . . . . . . . . . . . .   6
   2.6. Pruning Multicast Trees  . . . . . . . . . . . . . . . . . .   7
   2.7. Grafting Multicast Trees . . . . . . . . . . . . . . . . . .   8
   3. Detailed Protocol Operation  . . . . . . . . . . . . . . . . .   8
   3.1. Protocol Header  . . . . . . . . . . . . . . . . . . . . . .   8
   3.2. Probe Messages . . . . . . . . . . . . . . . . . . . . . . .  10
   3.3. Multicast Forwarding . . . . . . . . . . . . . . . . . . . .  15
   3.4. Route Exchange . . . . . . . . . . . . . . . . . . . . . . .  16
   3.5. Pruning  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   3.6. Grafting . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   3.7. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . .  34
   4. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  35
   5. Network Management Considerations  . . . . . . . . . . . . . .  35
   6. Security Considerations  . . . . . . . . . . . . . . . . . . .  35
   7. References . . . . . . . . . . . . . . . . . . . . . . . . . .  36
   8. Author's Address . . . . . . . . . . . . . . . . . . . . . . .  37
   9. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  37
   10. Appendix A - Constants & Configurable Parameters  . . . . . .  39
   11. Appendix B - Tracing and Troubleshooting support  . . . . . .  40
   11.1. DVMRP Ask Neighbors2  . . . . . . . . . . . . . . . . . . .  40
   11.2. DVMRP Neighbors2  . . . . . . . . . . . . . . . . . . . . .  41
   12. Appendix C - Version Compatibility  . . . . . . . . . . . . .  45
   13. Intellectual Property Rights Notice . . . . . . . . . . . . .  47
   14. Full Copyright Statement  . . . . . . . . . . . . . . . . . .  47

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