One document matched: draft-ietf-idmr-dvmrp-v3-08.txt
Differences from draft-ietf-idmr-dvmrp-v3-07.txt
T. Pusateri
INTERNET DRAFT Juniper Networks
Obsoletes: RFC 1075 February 1999
draft-ietf-idmr-dvmrp-v3-08 Expires: August 31, 1999
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-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
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
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
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
difficult.
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
issues.
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
service.
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
interface.
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 [Fen97a]. 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 [Fen97a] 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
are:
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 should be 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
purposes.
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
network.
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
protocol.
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
segment.
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 [Fen97b]. 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.
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.
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.
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.
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3.2.4. Neighbor Time-Out
When a neighbor times out, 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
used to determine the number of neighbors in the Probe message. The
current Major Version is 3.
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7 15 23 31
+---------+--------------+--------------------+
| Type | Code | Checksum |
| (0x13) | (0x1) | |
+---------+--------------+----------+---------+
| | | | |
|Reserved | Capabilities | Minor | Major |
+---------+--------------+----------+---------+
| |
| Generation ID |
+---------------------------------------------+
| |
| Neighbor IP Address 1 |
+---------------------------------------------+
| |
| Neighbor IP Address 2 |
+---------------------------------------------+
| |
| ... |
+---------------------------------------------+
| |
| Neighbor IP Address N |
+---------------------------------------------+
Figure 3 - DVMRP Probe Packet Format
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.
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
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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. Building Forwarding Cache Entries
DVMRP was originally designed to forward multicast packets by
building the downstream interface list for each packet as it arrived.
However, to reduce per packet processing time, the result of the
first lookup can be cached in a forwarding table. Then, as routes,
downstream dependent neighbors, or group membership change, the cache
forwarding table entries can 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
candidate.
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
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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
are 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 are removed as well. This prevents duplicate packets from
arriving on multi-access networks. Finally, interfaces on which all
downstream dependent neighbors have sent prunes are removed as well
(if there are no group members).
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
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 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
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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 the aggregating router
should pretend as though prunes were received for each
contributing routes of the aggregated route.
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.
The downstream interface on which the aggregated prune was
received should be removed from these forwarding cache entries.
In order to be notified when data arrives, the forwarding cache
entries based on these contributing routes should be removed from
the forwarding path in order to trigger the on-demand route lookup
and prune response.
If data from another contributing route of the aggregated network
is received by the aggregating router at a later time, the
aggregating router should remember that a prune was received for
this contributing route by the downstream neighbor who previously
sent the aggregated prune.
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.
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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
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 255.255.0.0 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 0.0.0.0/0.0.0.0 and NOT 0.0.0.0/255.0.0.0.
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
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be 32. This limits the breadth across the whole DVMRP network and is
necessary to place an upper bound on the convergence time of the
protocol.
As seen in the packet format below, Route Reports do not contain a
count of the number of routes reported for each netmask. Instead, a
"Last" bit is defined as the high order bit of the octet following
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
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 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
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changed.
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.
Reports should not be sent to a neighbor until a router has seen its
own address in the neighbors Probe router list. See Appendix C for
exceptions.
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
added.
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
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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
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
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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.
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
interface.
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
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The neighbor considers the receiving router to be upstream for
the route and is indicating it is dependent on the receiving
router.
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.
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 =
infinity).
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
ignored.
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
necessary.
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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
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
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
exists.
Routes that have been advertised with a metric between infinity and 2
x infinity (indicating downstream neighbor dependence) should now be
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advertised with a metric equal to infinity (canceling the downstream
dependence).
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
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
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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.
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.
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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
data.
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
lifetime.
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.
10. If no group members are found, then remove the interface from all
forwarding cache entries instantiated using the route to which
the prune applies.
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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:
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
source.
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
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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.
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
of any downstream prunes received for the same cache entry. A router
with no downstream dependent neighbors would use the the default
prune lifetime (randomized to prevent synchronization).
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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
prune.
Group Address
The destination group IP address of the datagram that triggered
the prune.
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
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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
conditions:
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
received.
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
should be acknowledged hop by hop guaranteeing end-to-end delivery.
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
acknowledgment.
4. Formulate and transmit the Graft packet.
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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.
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
data.
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.
5. If a prune had been sent upstream, this may trigger a graft to
now be sent to the upstream router.
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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
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
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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.
Group Address
The destination group IP address that was received in the Graft
message.
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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
messages.
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 [Fen97a] IP protocol messages to
communicate between routers. The IGMP Type field is hexadecimal 0x13.
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On IP multicast capable networks, DVMRP uses the All-DVMRP-Routers
local multicast group. This group address is 224.0.0.4.
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 [McCl98].
Also, a protocol independent multicast trace-route facility is
defined in [Fen97b].
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
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.
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[Deer91] Deering, S., "Multicast Routing in a Datagram
Internetwork", PhD thesis, Electric Engineering Dept.,
Stanford University, December 1991.
[Fen97a] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[Fen97b] Fenner, W., Casner, S., "A "traceroute" facility for IP
Multicast", Work In Progress, November 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.
[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", Work in Progress, February 1998.
[Ken98b] Kent, S., Atkinson, R., "IP Authentication Header", Work in
Progress, February 1998.
[Ken98c] Kent, S., Atkinson, R., "IP Encapsulating Security Payload
(ESP)", Work in Progress, February 1998.
[McCl98] McCloghrie, K., Farinacci, D., Thaler, D., "IP Multicast
Routing MIB", Work In Progress, July 1998.
[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.
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[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.
Mountain View, CA 94043
Phone: (650) 526-8046
EMail: pusateri@juniper.net
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
parameters:
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 0.0.0.0 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
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.
It may be necessary to form neighbor relationships based solely on
Route Report messages. Neighbor timeout 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 0.0.0.0 when no neighbors are listed in the response packet.
(Mrinfo)
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
Director.
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
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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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. Building Forwarding Cache Entries . . . . . . . . . . . . . 15
3.4. Route Exchange . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. Pruning . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.6. Grafting . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.7. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . 35
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
5. Network Management Considerations . . . . . . . . . . . . . . 36
6. Security Considerations . . . . . . . . . . . . . . . . . . . 36
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
8. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38
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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