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IDMR Working Group                                     Steve Deering
INTERNET-DRAFT                                            Xerox PARC
Expires April 1993                                    Deborah Estrin
<draft-ietf-idmr-igmp-dense-00.txt>                          USC/ISI
                                                      Dino Farinacci
                                                       cisco Systems
                                                        Van Jacobsen
                                                    October 10, 1993

     IGMP Router Extensions for Routing to Dense Multicast-Groups

Status of this Memo

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

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any other
   Internet Draft.

1.0 Introduction

This specification defines a multicast routing algorithm for multicast
groups that are densely distributed across an internet. The protocol is
unicast routing protocol independent. It is based on ESL sparse-mode
[Estrin93] and employs the same packet formats. This protocol is called
dense-mode ESL. The design is based largely on foundational work by
Deering [Deering91].

2.0. Overview

Dense-mode ESL uses Reverse Path Multicasting (RPM). RPM is a technique
in which a multicast datagram is forwarded if the receiving interface is
one used to forward unicast datagrams to the source of the datagram. The
multicast datagram is then forwarded out all other interfaces.
Dense-mode ESL builds source-based acyclic trees.

Dense-mode ESL is data driven, whereby it is assumed that all downstream
systems want to receive multicast datagrams. For densely populated
groups this is optimal. If some areas of the network do not have group
members, dense-mode ESL will prune branches of the source-based tree.
When group members leave the group, branches will also be pruned.

Unlike DVMRP [DVMRP] packets are forwarded on all outgoing interfaces
(except the incoming) until pruning and truncation occurs. DVMRP makes
use of parent-child data to reduce the number of outgoing interfaces
used before pruning. In both protocols, once truncation occurs pruning
state is maintained and packets are only forwarded onto outgoing
interfaces that in fact reach downstream members.

We chose to accept additional overhead in favor of reduced dependency on
the unicast routing protocol, and reduced overall protocol complexity.

Dense-mode ESL differs from sparse-mode ESL in two essential points: 1)
there are no periodic joins transmitted, only explicit triggered prunes,
and 2) there is no Rendezvous Point (RP).

3.0. Background

Reverse Path Broadcasting (RPB) is different from RPF because duplicate
packets are avoided in the former that are sent in the latter. In
general, the number of duplicates sent on a link can be as high as the
number of routers directly connected to that link.

Reverse Path Multicasting (RPM) is different from RPF or RPB because
pruning information is propagated upstream. Leaf routers must know that
they are leaf routers so that in response to no IGMP reports for a
group, those leaf routers know to initiate the prune process.

In DVMRP there are routing protocol dependencies for a) building a
parent-child database so that duplicate packets can be eliminated, b)
eliminating duplicate packets on multi-access LANs, and c) sending "split
horizon with poison reverse" information to detect that a router is not a
leaf router (if a router does not receive any poison reverse messages
from other routers on a multi-access LAN then that router acts as a leaf
router for that LAN and knows to prune if there are not IGMP reports on
that LAN for a group G).

Dense-mode ESL will accept some duplicate packets in order to avoid
being routing protocol dependent and avoid building a child parent

We introduce a simple prune mechanism for reducing duplicates on
multi-access LANs.

We introduce an alternative leaf-router detection mechanism that does
not rely on a specific unicast routing protocol mechanism such as split
horizon with poison reverse.

These mechanisms are described below.

4.0 Protocol Description

4.1  Leaf network detection

In DVMRP poison reverse information tells a router that other routers on
the shared LAN use the LAN as their incoming interface. As a result,
even if the DR for that LAN does not hear any IGMP Reports for a group,
the DR will know to continue to forward multicast data packets to that
group, and NOT to send a prune message to its upstream neighbor.

Since dense-mode ESL does not rely on any unicast routing protocol
mechanisms, this problem is solved by using prune messages sent upstream
on a LAN. If a downstream router on a LAN determines that it has no more
downstream members for a group, then it can multicast a prune message on
the LAN.

A leaf router detects that there are no members downstream when it is the
only router on a network and there are no IGMP Host-Report messages
received from hosts. It determines there are no other routers by not
receiving ESL Router-Query messages.

When a prune message is sent on an upstream LAN, it is data link
multicast and IP addressed to the all routers group address (
The router to process the prune will be indicated by inserting its
address in the "Address" field of the message.  The address is obtained
by an RPF lookup from the unicast routing table.  When the prune message
is sent, the expected upstream router will schedule a deletion request
of the LAN from its outgoing interfaces for the (S,G) entry from the
prune list.

Note the special case for equal-cost paths. When an upstream router is
chosen by an RPF lookup there may be equal-cost paths. The higher IP
addressed system is always chosen. If the unicast routing protocol does
not store all available equal-cost paths in the routing table, the
"Address" field may contain the address of the wrong upstream router. To
avoid this situation, the "Address" field may optionally be set to
which means that all upstream routers (the ones that have the LAN as an
outgoing interface for the (S,G) entry) may process the packet.

Other routers on the LAN will hear the prune message and respond with a
join if they still expect multicast datagrams from the expected upstream
router. The ESL-Join message is data link multicast and IP addressed to
the all routers group address ( The router to process the
join will be indicated by inserting its address in the "Address" field
of the message. The address is determined by an RPF lookup from the
unicast routing table. When the expected router receives the join
message, it will cancel the deletion request.

Routers will randomly generate a join message delay timer. If a join is
heard from another router before a router sends its own, it will cancel
sending its own join. This will reduce traffic on the LAN.

If the expected upstream router does not receive any ESL-Join messages
before the schedule time for the deletion request expires, it deletes
the outgoing LAN interface from the (S,G) multicast forwarding entry.

If an (S,G) entry contains an empty outgoing interface list, a prune is
sent upstream. Prune information is flushed periodically. This (or a
loss of state) causes the packets to be sent in RPF mode again which in
turn triggers prune messages.

4.2 New members joining an existing group

If a router is directly connected to a host that wants to become a
member of a group, the router may optionally, multicast a ESL-Join
message towards known sources. This allows join latency to be reduced
below that indicated by the relatively large timeout value suggested for
prune information.

If a receiving router has an entry for (S,G), it adds the interface on
which the IGMP Report or ESL-Join was received. If the (S,G) entry
was a negative cache entry, the router sends an ESL-Join upstream
towards S. This is done for all (Si,G) entries.

If routers have no state for (*,G), they do nothing since dense-mode ESL
will deliver a multicast datagram to all interfaces when creating state
about a group.

Any routers receiving the ESL-Join that uses the received interface as
an incoming interface for any (Si,G) entry, will not add the interface
to the outgoing interface list.

The ESL-Join message is transmitted unreliably.

4.3 Protocol Scenario

A multicast datagram is sent by a source host. If a receiving router
has no forwarding cache entry for G, it creates (S,G) and (*,G)
entries.  (*,G)->incoming = NULL and (*,G)->outgoing set to all other
interfaces on the router that have either multicast hosts or routers
present.  (S,G)->incoming = interface from RPF lookup. (S,G)->outgoing
is copied from (*,G)->outgoing minus the (S,G)->incoming.

An ESL-Prune message is triggered when an (S,G) entry is built with an
empty outgoing interface list. This type of entry is called a negative
cache entry. This can occur when a leaf router has no local members for
group G or a prune message was received from a downstream router which
causes the outgoing interface list to become NULL. ESL-Prune messages
are never sent in response to a received multicast packet that is
associated with a negative cache entry.

ESL-Prune messages received on a point to point link are not delayed
before processing as they are in the LAN procedure. If the prune is
received on an interface that is in the outgoing interface list, it is
deleted immediately.  Otherwise it is ignored.

4.4 Designated Router election

The dense-mode ESL designated router (DR) election uses the same
procedure as in sparse-mode ESL. A DR is necessary for each multi-access LAN
so a single router sends IGMP Host-Query messages to solicit host group

Each ESL router connected to a multi-access LAN should transmit ESL
Router-Query messages every 30 seconds onto the LAN to support DR
election. The highest addressed router becomes the DR.  The ESL routers
discovered should be timed out after 90 seconds. If the DR goes down, a
new DR is elected.

DR election is only necessary on multi-access networks. It is not
required that ESL Query messages be sent on point-to-point links.

4.5 Parallel paths to a source

Two or more routers may receive the same multicast datagram that was
replicated upstream. In particular, if two routers have equal cost paths
to a source and are connected on a common multi-access network,
duplicate datagrams will travel downstream onto the LAN. Dense-mode ESL
will detect such a situation and will not let it persist.

If a router receives a multicast datagram on a multi-access LAN from a
source whose corresponding (S,G) outgoing interface list includes the
received interface, the packet must be a duplicate. In this case the
highest IP addressed system should be elected to be forwarder for this
(S,G) entry. When such a datagram is received, it triggers an
ESL-Assert message to be multicast to on the LAN.  Each
router that uses the LAN as an outgoing interface for (S,G) will
compare the source IP address from the message with its own. If its
own address is smaller, it will delete the interface from the outgoing
interface list for the (S,G) entry.  Otherwise, it has been elected as
forwarder and will keep the interface in the entry.

This mechanism assures that only one router will forward multicast datagrams
from S to G onto the LAN.

Interfaces that are pruned due to Assert processing should have a
shorter timer associated with it compared to the timer that is used when
an interface is pruned due to receipt of ESL-Prune message. This is
recommended so unicast routing changes upstream do not cause long lived
black holes.

Assert messages are data link multicast and IP addressed to the all
routers group address

4.6 Timing out multicast forwarding entries

Each (S,G) and (*,G) entry has timers associated with it. During this time
source-based tree state is kept in the network.

There should be multiple timers set. One for the multicast routing entry
itself and one for each interface in the outgoing interface list. The
outgoing interface stays active in the list as long as there is
multicast traffic for the entry or there is an explicit join received
on the interface. If neither occurs the interface will be deleted from
the list after 90 seconds, by default.

Once all interfaces in the outgoing interface list are not active, a
timer should be set for the (S,G) entry. During this time the entry is
known as a negative state entry at which a prune is triggered. Once the
(S,G) entry times out, it can be recreated when the next multicast
packet or join arrives.

5.0 Sparse mode compatibility

There are two issues to consider when dealing with dense-mode ESL and
sparse-mode ESL interaction.

    - Is a group part dense and part sparse (dual-mode).
    - If a group is either dense-only or sparse-only, when should it
      transition to the other mode.

5.1 Dual-mode

If a group has membership qualities where it is densely populated in
some areas of the network and sparsely populated in others, it will have
to be in both modes for the group. If a group has one or more RP
addresses associated with it, either dynamically determined or through
configuration, it will operate in sparse-mode. In addition, if any
interface is configured as a dense-mode interface for the group, the
router will also operate in dense-mode. The group is known to be in

In dual-mode, a (*,G) entry's outgoing interfaces are built from the
union of dense-mode configured interfaces and interfaces where ESL-Join
messages have been received. A (*,G) entry's incoming interface is
always set to NULL. An (S,G) entry's outgoing interfaces are always
copied from the (*,G) entry's outgoing interfaces. A (S,G) entry's
incoming interface is based on the interface determined by an RPF lookup
when a multicast packet is received for a (*,G) entry.

Outgoing interface entries are only timed out if they are not dense-mode
configured interfaces. Typically, these are interfaces that were created
from an sparse-mode ESL-Join. The interface is kept active by periodic
receipts on ESL-Join messages from downstream routers.

If an ESL-Join is received on a dense-mode configured interface, it
should be propagated upstream if there is no (S,G) or (*,G) entry.
Otherwise, it is ignored.

Periodic ESL-Join and Prune messages are sent upstream if the interface
is not configured in dense-mode. Otherwise, periodic messages are not

A router that is upstream from the RP, will send ESL-Register messages
to the RP if there is one configured for an associated group.  The
Register messages may travel on dense-mode configured interfaces.

5.2 Mode Transitioning - from one mode to another

When a router decides the mode for a group is not efficient, it may want
to change to the alternate mode or dual-mode.

1) An upstream router can change a downstream interface from sparse to
   dense without informing downstream router. This change indicates that
   the upstream router doesn't require periodic Joins/Prunes or
   Registers to keep multicast cache entries from timing out. The
   downstream router can stop sending periodic messages when told that
   the mode has changed. However, this action is not required.

2) An upstream router needs to tell a downstream router when going from
   dense to sparse so the downstream router can start sending periodic

An upstream router sends an ESL-Mode message to neighboring downstream
routers indicating that it wants to change the mode for their common
network. The mode attribute is for all (Si,G). A receiving router is required
to acknowledge the ESL-Mode message with an ESL-ModeAck.

ESL-Mode messages are initially multicast and later retransmitted as
unicast on multi-access LANs if an acknowledgement is not received.

Downstream routers that receive an ESL-Mode message switch their
incoming interface to the mode indicated in the message. Upstream
routers unilaterally control the mode.

Sparse-mode ESL RP-Reachable messages must be sent to all downstream
interfaces regardless if they are in dense or sparse mode. This allows
any downstream sparse-mode router to determine if the RP is still
reachable. An RP-Reachable message is sent downstream in response to a
received ESL-Join with the RP address in the source join list part of
the message.

6.0 Message Types

ESL messages are encapsulated in IGMP. IGMP runs on top of IP.

The following table enumerates the IGMP messages used in each mode.

                               Sparse-mode         Dense-mode
(1) Host Membership Query          Sent               Sent
(2) Host Membership Report           -                  -
(4) Router ESL
    (0) Query                  Sent/Received            -
    (1) Register               Sent/Received            -
    (2) Join/Prune             Sent/Received*      Sent/Received**
    (3) RP-Reachable           Sent/Received            -
    (4) Assert                       -             Sent/Received
    (5) Mode                   Sent/Received***    Sent/Received***
    (6) ModeAck                Sent/Received***    Sent/Received***

*   Sent periodically
**  Sent only triggered by event
*** Sent and received only if Mode Transitioning is supported

6.1 Code field addition to IGMP packet format

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |Version| Type  |     Code      |           Checksum            |
      |                            Address                            |

      A Code field has replaced the Unused field to support the
      variations of the Router ESL message. The Code should be set to
      0 and ignored on receipt for all Types other than type 4.

      Hosts must ignore the Code field.

      Code values:

      0 - Query

          "Address" is set to 0 and ignored on receipt.

      1 - Register

          Used in sparse-mode. Refer to ESL sparse-mode specification.

      2 - Join/Prune

          Regular sparse-mode/dense-mode Join/Prune list for adding or
          deleting branches to/from a source or RP-based distribution
          tree. The format is in the ESL sparse-mode specification.

          When "Address" is used by dense-mode for leaf network
          detection, it contains the IP address of the router which
          processes the Join or Prune. Otherwise, it is set to 0 and
          ignored on receipt.

      3 - RP-reachable

          Used in sparse-mode. Refer to ESL sparse-mode specification.

      4 - Assert

          This message is used for electing one of multiple parallel routers
          for downstream forwarding on a multi-access LAN. The body of
          this message is identical to the Join/Prune (2) format.

      5 - Mode

          This message is used by an upstream router to inform
          downstream neighbors its desireability to change modes.
          "Address" is set to the group address the mode is associated with.

      6 - ModeAck

          This message is sent by a downstream router to a neighboring
          upstream router that has previously sent an ESL-Mode message.
          This acknowledges the ESL-Mode message was received without
          error. "Address" is set to the group address the mode is
          associated with.

7.0 References

[Deering91] S.E. Deering. Multicast Routing in a Datagram Internetwork.
            PhD thesis, Electrical Engineering Dept., Stanford University,
            December 1991.

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

[Estrin93]  IGMP Router Extensions for Routing to Spare Multicast-Groups,
            S. Deering, D. Estrin, D. Farinacci, V. Jacobson, September 1993

[RFC1112]   Host Extensions for IP Multicasting, Network Working Group,
            RFC 1112, S. Deering, August 1989

8.0 Interoperability Issues

1) MSOPF/ESL-DM interaction
2) DVMRP/ESL-DM interaction
3) ESL over NBMA links
   - Either still do BMA procedure with data link replication
   - (S,G) entries need to have neighbor IP addresses
   - Do we IP unicast or just data link unicast


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