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RFC4605 RFC4605 RFC4605 RFC4605 RFC4605
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Network Working Group B. Fenner
Request for Comments: 4605 AT&T Research
Category: Standards Track H. He
Nortel
B. Haberman
JHU-APL
H. Sandick
Little River Elementary School
August 2006
Internet Group Management Protocol (IGMP) /
Multicast Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
In certain topologies, it is not necessary to run a multicast routing
protocol. It is sufficient for a device to learn and proxy group
membership information and simply forward multicast packets based
upon that information. This document describes a mechanism for
forwarding based solely upon Internet Group Management Protocol
(IGMP) or Multicast Listener Discovery (MLD) membership information.
1. Introduction
This document applies spanning tree multicast routing [MCAST] to an
Internet Group Management Protocol (IGMP) or Multicast Listener
Discovery (MLD)-only environment. The topology is limited to a tree,
since we specify no protocol to build a spanning tree over a more
complex topology. The root of the tree is assumed to be connected to
a wider multicast infrastructure.
Fenner, et al. Standards Track [Page 1]
RFC 4605 IGMP/MLD-Based Multicast Forwarding August 2006
1.1. Motivation
In a simple tree topology, it is not necessary to run a multicast
routing protocol. It is sufficient to learn and proxy group
membership information and simply forward multicast packets based
upon that information. One typical example of such a tree topology
can be found on an edge aggregation box such as a Digital Subscriber
Line Access Multiplexer (DSLAM). In most deployment scenarios, an
edge box has only one connection to the core network side and has
many connections to the customer side.
Using IGMP/MLD-based forwarding to replicate multicast traffic on
devices such as the edge boxes can greatly simplify the design and
implementation of those devices. By not supporting more complicated
multicast routing protocols such as Protocol Independent Multicast
(PIM) or Distance Vector Multicast Routing Protocol (DVMRP), it
reduces not only the cost of the devices but also the operational
overhead. Another advantage is that it makes the proxy devices
independent of the multicast routing protocol used by the core
network routers. Hence, proxy devices can be easily deployed in any
multicast network.
Robustness in an edge box is usually achieved by using a hot spare
connection to the core network. When the first connection fails, the
edge box fails over to the second connection. IGMP/MLD-based
forwarding can benefit from such a mechanism and use the spare
connection for its redundant or backup connection to multicast
routers. When an edge box fails over to the second connection, the
proxy upstream connection can also be updated to the new connection.
1.2. Applicability Statement
The IGMP/MLD-based forwarding only works in a simple tree topology.
The tree must be manually configured by designating upstream and
downstream interfaces on each proxy device. In addition, the IP
addressing scheme applied to the proxying tree topology SHOULD be
configured to ensure that a proxy device can win the IGMP/MLD Querier
election to be able to forward multicast traffic. There are no other
multicast routers except the proxy devices within the tree, and the
root of the tree is expected to be connected to a wider multicast
infrastructure. This protocol is limited to a single administrative
domain.
In more complicated scenarios where the topology is not a tree, a
more robust failover mechanism is desired, or more than one
administrative domain is involved, a multicast routing protocol
should be used.
Fenner, et al. Standards Track [Page 2]
RFC 4605 IGMP/MLD-Based Multicast Forwarding August 2006
1.3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document is a product of the Multicast & Anycast Group
Membership (MAGMA) working group within the Internet Engineering Task
Force. Comments are solicited and should be addressed to the working
group’s mailing list at magma@ietf.org and/or the authors.
2. Definitions
2.1. Upstream Interface
A proxy device’s interface in the direction of the root of the tree.
Also called the "Host interface".
2.2. Downstream Interface
Each of a proxy device’s interfaces that is not in the direction of
the root of the tree. Also called the "Router interfaces".
2.3. Group Mode
In IPv4 environments, for each multicast group, a group is in IGMP
version 1 (IGMPv1) [RFC1112] mode if an IGMPv1 report is heard. A
group is in IGMP version 2 (IGMPv2) [RFC2236] mode if an IGMPv2
report is heard but no IGMPv1 report is heard. A group is in IGMP
version 3 (IGMPv3) [RFC3376] mode if an IGMPv3 report is heard but no
IGMPv1 or IGMPv2 report is heard.
In IPv6 environments, for each multicast group, a group is in MLD
version 1 (MLDv1) [RFC2710] mode if an MLDv1 report is heard. MLDv1
is equivalent to IGMPv2. A group is in MLD version 2 (MLDv2) [MLDv2]
mode if an MLDv2 report is heard but no MLDv1 report is heard. MLDv2
is equivalent to IGMPv3.
2.4. Subscription
When a group is in IGMPv1 or IGMPv2/MLDv1 mode, the subscription is a
group membership on an interface. When a group is in IGMPv3/MLDv2
mode, the subscription is an IGMPv3/MLDv2 state entry, i.e., a
(multicast address, group timer, filter-mode, source-element list)
tuple, on an interface.
Fenner, et al. Standards Track [Page 3]
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