Network Working Group B. Fenner

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.

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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.

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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 [email protected] 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.

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2.5. Membership Database

The database maintained at each proxy device into which the
membership information of each of its downstream interfaces is
merged. The membership database is a set of membership records of
the form:

(multicast-address, filter-mode, source-list)

Please refer to IGMPv3/MLDv2 [RFC3376, MLDv2] specifications for the
definition of the fields "filter-mode" and "source-list". The
operational behaviors of the membership database is defined in
section 4.1.

3. Abstract Protocol Definition

A proxy device performing IGMP/MLD-based forwarding has a single
upstream interface and one or more downstream interfaces. These
designations are explicitly configured; there is no protocol to
determine what type each interface is. It performs the router
portion of the IGMP [RFC1112, RFC2236, RFC3376] or MLD [RFC2710,
MLDv2] protocol on its downstream interfaces, and the host portion of
IGMP/MLD on its upstream interface. The proxy device MUST NOT
perform the router portion of IGMP/MLD on its upstream interface.

The proxy device maintains a database consisting of the merger of all
subscriptions on any downstream interface. Refer to Section 4 for
the details about the construction and maintenance of the membership
database.

The proxy device sends IGMP/MLD membership reports on the upstream
interface when queried and sends unsolicited reports or leaves when
the database changes.

When the proxy device receives a packet destined for a multicast
group (channel in Source-Specific Multicast (SSM)), it uses a list
consisting of the upstream interface and any downstream interface
that has a subscription pertaining to this packet and on which it is
the IGMP/MLD Querier. This list may be built dynamically or cached.
It removes the interface on which this packet arrived from the list
and forwards the packet to the remaining interfaces (this may include
the upstream interface).

Note that the rule that a proxy device must be the querier in order
to forward packets restricts the IP addressing scheme used; in
particular, the IGMP/MLD-based forwarding devices must be given the
lowest IP addresses of any potential IGMP/MLD Querier on the link, in
order to win the IGMP/MLD Querier election. IGMP/MLD Querier

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election rule defines that the Querier that has the lowest IP address
wins the election. (The IGMP/MLD Querier election rule is defined in
IGMP/MLD specifications as part of the IGMP/MLD behavior.) So in an
IGMP/MLD-based forwarding-only environment, if non-proxy device wins
the IGMP/MLD Querier election, no packets will flow.

For example, the figure below shows an IGMP/MLD-based forwarding-only
environment:

LAN 1 --------------------------------------
Upstream | | Upstream
A(non-proxy) B(proxy)
Downstream |(lowest IP) | Downstream
LAN 2 --------------------------------------

Device A has the lowest IP address on LAN 2, but it is not a proxy
device. According to IGMP/MLD Querier election rule, A will win the
election on LAN 2 since it has the lowest IP address. Device B will
not forward traffic to LAN 2 since it is not the querier on LAN 2.

The election of a single forwarding proxy is necessary to avoid local
loops and redundant traffic for links that are considered downstream
links by multiple IGMP/MLD-based forwarders. This rule "piggy-backs"
forwarder election on IGMP/MLD Querier election. The use of the
IGMP/MLD Querier election process to choose the forwarding proxy
delivers similar functionality on the local link as the PIM Assert
mechanism. On a link with only one IGMP/MLD-based forwarding device,
this rule MAY be disabled (i.e., the device MAY be configured to
forward packets to an interface on which it is not the querier).
However, the default configuration MUST include the querier rule, for
example, for redundancy purposes, as shown in the figure below:

LAN 1 --------------------------------------
Upstream | | Upstream
A B
Downstream | | Downstream
LAN 2 --------------------------------------

LAN 2 can have two proxy devices, A and B. In such a configuration,
one proxy device must be elected to forward the packets. This
document requires that the forwarder must be the IGMP/MLD querier.
So proxy device A will forward packets to LAN 2 only if A is the
querier. In the above figure, if A is the only proxy device, A can
be configured to forward packets even though B is the querier.

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Note that this does not protect against an "upstream loop". For
example, see the figure below:

LAN 1 --------------------------------------
Upstream | | Downstream
A B
Downstream | | Upstream
LAN 2 --------------------------------------

B will unconditionally forward packets from LAN 1 to LAN 2, and A
will unconditionally forward packets from LAN 2 to LAN 1. This will
cause an upstream loop. A multicast routing protocol that employs a
tree building algorithm is required to resolve loops like this.

3.1. Topology Restrictions

This specification describes a protocol that works only in a simple
tree topology. The tree must be manually configured by designating
upstream and downstream interfaces on each proxy device, and the root
of the tree is expected to be connected to a wider multicast
infrastructure.

3.2. Supporting Senders

In order for senders to send from inside the proxy tree, all traffic
is forwarded towards the root. The multicast router(s) connected to
the wider multicast infrastructure should be configured to treat all
systems inside the proxy tree as though they were directly connected;
e.g., for Protocol Independent Multicast - Sparse Mode (PIM-SM)
[PIM-SM], these routers should Register-encapsulate traffic from new
sources within the proxy tree just as they would directly-connected
sources.

This information is likely to be manually configured; IGMP/MLD-based
multicast forwarding provides no way for the routers upstream of the
proxy tree to know what networks are connected to the proxy tree. If
the proxy topology is congruent with some routing topology, this
information MAY be learned from the routing protocol running on the
topology; e.g., a router may be configured to treat multicast packets
from all prefixes learned from routing protocol X via interface Y as
though they were from a directly connected system.

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RFC 4605 IGMP/MLD-Based Multicast Forwarding August 2006

4. Proxy Device Behavior

This section describes an IGMP/MLD-based multicast forwarding
device's actions in more detail.

4.1. Membership Database

The proxy device performs the router portion of the IGMP/MLD protocol
on each downstream interface. For each interface, the version of
IGMP/MLD used is explicitly configured and defaults to the highest
version supported by the system.

The output of this protocol is a set of subscriptions; this set is
maintained separately on each downstream interface. In addition, the
subscriptions on each downstream interface are merged into the
membership database.

The membership database is a set of membership records of the form:

(multicast-address, filter-mode, source-list)

Each record is the result of the merge of all subscriptions for that
record's multicast-address on downstream interfaces. If some
subscriptions are IGMPv1 or IGMPv2/MLDv1 subscriptions, these
subscriptions are converted to IGMPv3/MLDv2 subscriptions. The
IGMPv3/MLDv2 and the converted subscriptions are first preprocessed
to remove the timers in the subscriptions and, if the filter mode is
EXCLUDE, to remove every source whose source timer > 0. Then the
preprocessed subscriptions are merged using the merging rules for
multiple memberships on a single interface (specified in Section 3.2
of the IGMPv3 specification [RFC3376] and in Section 4.2 of the MLDv2
specification [MLDv2]) to create the membership record. For example,
there are two downstream interfaces, I1 and I2, that have
subscriptions for multicast address G. I1 has an IGMPv2/MLDv1
subscription that is (G). I2 has an IGMPv3/MLDv2 subscription that
has membership information (G, INCLUDE, (S1, S2)). The I1's
subscription is converted to an IGMPv3/MLDv2 subscription that has
membership information (G, EXCLUDE, NULL). Then the subscriptions
are preprocessed and merged, and the final membership record is (G,
EXCLUDE, NULL).

The proxy device performs the host portion of the IGMP/MLD protocol
on the upstream interface. If there is an IGMPv1 or IGMPv2/MLDv1
querier on the upstream network, then the proxy device will perform
IGMPv1 or IGMPv2/MLDv1 on the upstream interface accordingly.
Otherwise, it will perform IGMPv3/MLDv2.

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If the proxy device performs IGMPv3/MLDv2 on the upstream interface,
then when the composition of the membership database changes, the
change in the database is reported on the upstream interface as
though this proxy device were a host performing the action. If the
proxy device performs IGMPv1 or IGMPv2/MLDv1 on the upstream
interface, then when the membership records are created or deleted,
the changes are reported on the upstream interface. All other
changes are ignored. When the proxy device reports using IGMPv1 or
IGMPv2/MLDv1, only the multicast address field in the membership
record is used.

4.2. Forwarding Packets

A proxy device forwards packets received on its upstream interface to
each downstream interface based upon the downstream interface's
subscriptions and whether or not this proxy device is the IGMP/MLD
Querier on each interface. A proxy device forwards packets received
on any downstream interface to the upstream interface, and to each
downstream interface other than the incoming interface based upon the
downstream interfaces' subscriptions and whether or not this proxy
device is the IGMP/MLD Querier on each interface. A proxy device MAY
use a forwarding cache in order not to make this decision for each
packet, but MUST update the cache using these rules any time any of
the information used to build it changes.

4.3. SSM Considerations

To support Source-Specific Multicast (SSM), the proxy device should
be compliant with the specification about using IGMPv3 for SSM [SSM].
Note that the proxy device should be compliant with both the IGMP
Host Requirement and the IGMP Router Requirement for SSM since it
performs IGMP Host Portion on the upstream interface and IGMP Router
Portion on each downstream interface.

An interface can be configured to perform IGMPv1 or IGMPv2. In this
scenario, the SSM semantic will not be maintained for that interface.
However, a proxy device that supports this document should ignore
those IGMPv1 or IGMPv2 subscriptions sent to SSM addresses. And more
importantly, the packets with source-specific addresses SHOULD NOT be
forwarded to interfaces with IGMPv2 or IGMPv1 subscriptions for these
addresses.

5. Security Considerations

Since only the Querier forwards packets, the IGMP/MLD Querier
election process may lead to black holes if a non-forwarder is
elected Querier. An attacker on a downstream LAN can cause itself to
be elected Querier, and as a result, no packets would be forwarded.

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However, there are some operational ways to avoid this problem. It
is fairly common for an operator to number the routers starting from
the bottom of the subnet. So an operator SHOULD assign the subnet's
lowest IP address(es) to a proxy (proxies) in order for the proxy
(proxies) to win the querier election.

IGMP/MLD-based forwarding does not provide the "upstream loop"
detection mechanism described in Section 3. Hence, to avoid the
problems caused by an "upstream loop", it MUST be administratively
assured that such loops don't exist when deploying IGMP/MLD Proxying.

The IGMP/MLD information presented by the proxy to its upstream
routers is the aggregation of all its downstream group membership
information. Any access control applied on the group membership
protocol at the upstream router cannot be performed on a single
subscriber. That is, the access control will apply equally to all
the interested hosts reachable via the proxy device.

6. Acknowledgements

The authors would like to thank Erik Nordmark, Dave Thaler, Pekka
Savola, Karen Kimball, and others for reviewing the specification and
providing their comments.

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RFC 4605 IGMP/MLD-Based Multicast Forwarding August 2006

7. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.

[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.

[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.

[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.

[MLDv2] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

[SSM] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, August 2006.

8. Informative References

[MCAST] Deering, S., "Multicast Routing in a Datagram
Internetwork", Ph.D. Thesis, Stanford University, December
1991.

[PIM-SM] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.

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Authors' Addresses

Bill Fenner
AT&T Labs - Research
1 River Oaks Place
San Jose, CA 95134

Phone: +1 408 493-8505
EMail: [email protected]

Haixiang He
Nortel
600 Technology Park Drive
Billerica, MA 01821

EMail: [email protected]

Brian Haberman
Johns Hopkins University Applied Physics Lab
11100 Johns Hopkins Road
Laurel, MD 20723-6099

EMail: [email protected]

Hal Sandick
Little River Elementary School
2315 Snow Hill Road
Durham, NC 27712

EMail: [email protected]

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RFC 4605 IGMP/MLD-Based Multicast Forwarding August 2006

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