MAGMA Working Group Bill Fenner, AT&T Research
INTERNET-DRAFT Haixiang He, Nortel Networks
draft-ietf-magma-igmp-proxy-04.txt Brian Haberman, Caspian Networks
Hal Sandick, Sheperd Middle School
Expire: March, 2004 September, 2003
IGMP/MLD-based Multicast Forwarding ("IGMP/MLD Proxying")
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet Drafts are working documents of the Internet Engineering
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Abstract
In certain topologies, it is not necessary to run a multicast routing
protocol. It is sufficient to learn and proxy group membership
information and simply forward based upon that information. This
draft 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 [Deering91] to
an Internet Group Management Protocol (IGMP) or Multicast Listener
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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.
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 tree topology can
be found on an edge aggregation box such as 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 PIM or 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 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. There are no multicast
routers 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 domains are involved, a multicast routing protocol
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should be used.
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 [Bradner97].
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) [Deering89] mode if an IGMPv1 report is heard. A
group is in IGMP version 2 (IGMPv2) [Fenner97] mode if an IGMPv2
report is heard but no IGMPv1 report is heard. A group is in IGMP
version 3 (IGMPv3) [CDFKT02] 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) [DFH99] mode if a MLDv1 report is heard. MLDv1 is
equivalent to IGMPv2. A group is in MLD version 2 (MLDv2)
[VCFDFKH02] 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
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group membership on an interface. When a group is in IGMPv3/MLDv2
mode, the subscription is a an IGMPv3/MLDv2 state entry (i.e. a
(multicast address, group timer, filter-mode, source-element list)
tuple) on an interface.
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 [CDFKT02, VCFDFKH02] 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 [Deering89, Fenner97, CDFKT02] or MLD [DFH99,
VCFDFKH02] 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
which 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
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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
election rule defines that the Querier that has the lowest IP address
wins the election. 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, in an IGMP/MLD-based forwarding only environment as
shown in the figure below:
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 which are considered to be
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
Protocol Independent Multicast (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 purpose, 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 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
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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.
Note that this does not protect against an "upstream loop". For
example, as shown in 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 which 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)
[FHHK02], 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
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though they were from a directly connected system.
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 the
IGMPv3/MLDv2 specification[CDFKT02,VCFDFKH02] 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 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
[HC01]. 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
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elected Querier. An attacker on a downstream LAN can cause itself to
be elected Querier resulting in no packets being forwarded.
IGMP/MLD-based forwarding does not provide "upstream loop" detection
mechanism as 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 can not be performed on a single
subscriber. That is, the access control will apply equally to all the
interested hosts reachable via the proxy device.
Acknowledgments
The authors would like to thank Eric Nordmark, Dave Thaler, Pekka
Savola, Karen Kimball and others for reviewing the specification and
providing their comments.
Normative References
Bradner97 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119/BCP 14, Harvard
University, March 1997.
CDFKT02 Cain, B., S. Deering, B. Fenner, I. Kouvelas and A.
Thyagarajan, "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
Fenner97 Fenner, W., "Internet Group Management Protocol,
Version 2", RFC 2236, Xerox PARC, November 1997.
Deering89 Deering, S., "Host Extensions for IP Multicasting",
RFC 1112, August 1989.
DFH99 Deering, S., Fenner, W., and Haberman, B., "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
VCFDFKH02 Vida, R., Costa, L., Fdida, S., Deering, S., Fenner, B.,
Kouvelas, I., and Haberman, B., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", Work in Progress.
HC01 Holbrook, H., and Cain, B., "Using IGMPv3 for Source-
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Specific Multicast", Work in Progress.
Informative References
Deering91 Deering, S., "Multicast Routing in a Datagram
Internetwork", Ph.D. Thesis, Stanford University,
December 1991.
FHHK02 Fenner, W., Handley, M., Holbrook, H., and Kouvelas, I.,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", Work in Progress.
Author's Address:
William C. Fenner
AT&T Labs - Research
75 Willow Rd
Menlo Park, CA 94025
Phone: +1 650 330 7893
Email: fenner@research.att.com
Haixiang He
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821
Phone: +1 978 288 7482
Email: haixiang@nortelnetworks.com
Brian Haberman
Caspian Networks
One Park Drive, Suite 400
Research Triangle Park, NC 27709
Phone: +1 919 949 4828
Email: bkhabs@nc.rr.com
Hal Sandick
Sheperd Middle School
2401 Dakota St.
Durham, NC 27707
Email: sandick@nc.rr.com
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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