Internet DRAFT - draft-wu-rgmp
draft-wu-rgmp
INTERNET-DRAFT Router-port Group Management Protocol I. Wu
Type: Informational T. Eckert
Expires: April 2002 cisco Systems
Fri, Nov 1 2002
Cisco Systems
Router-port Group Management Protocol (RGMP)
<draft-wu-rgmp-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1] except that the right to
produce derivative works is not granted.
Internet-Drafts are working documents of the Internet Engineering
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Abstract
This draft documents RGMP, a protocol developed by Cisco Systems
that is used between multicast routers and switches to restrict
multicast packet forwarding in switches to those routers where the
packets may be needed.
RGMP is designed for backbone switched networks where multiple, high
speed routers are interconnected.
1. Conventions used in this document
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 RFC2119 [2].
2. Introduction
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IGMP Snooping is a popular, but not well documented mechanism to
restrict multicast traffic, in switched networks, to those ports that
want to receive the multicast traffic. It dynamically establishes
and terminates multicast group specific forwarding in switches that
support this feature.
The main limitation of IGMP Snooping is that it can only restrict
multicast traffic onto switch ports were receiving hosts are
connected directly or indirectly via other switches. IGMP Snooping
can not restrict multicast traffic to ports where at least one
multicast router is connected. It must instead flood multicast
traffic to these ports. Snooping on IGMP messages alone is an
intrinsic limitation. Through it, a switch can only learn which
multicast flows are being requested by hosts. A switch can not
learn through IGMP which traffic flows need to be received by
router ports to be routed because routers do not report these
flows via IGMP.
In situations where multiple multicast routers are connected to a
switched backbone, IGMP Snooping will not reduce multicast traffic
load. Nor will it assist in increasing internal bandwidth through
the use of switches in the network.
In switched backbone networks or exchange points, where
predominantly routers are connected with each other, a large amount
of multicast traffic may lead to unexpected congestion. It also
leads to more resource consumption in the routers because they must
discards the unwanted multicast traffic.
The RGMP protocol described in this document allows to restrict
multicast traffic to router ports. To effectively restrict traffic,
it must be supported both by the switches and the routers in the
network.
The message format of RGMP resembles that of IGMPv2 so existing
switches that are capable of IGMP Snooping can easily be enhanced
with this feature. Its messages are encapsulated in IPv4 datagrams,
with a protocol number of 2, the same as that of IGMP. All RGMP
messages are sent with TTL 1, to destination address 224.0.0.25.
This address has been assigned by IANA to carry messages from
routers to switches [3].
RGMP is designed to work in conjunction with multicast routing
protocols where explicit join/prune to the distribution tree is
performed. PIM-SM [4] is an example of such a protocol.
The RGMP protocol specifies operations only for IP version 4
multicast routing. IP version 6 is not considered.
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To keep RGMP simple, efficient and easy to implement, it is designed
for switches to expect RGMP messages from only one source per
port. For this reason, RGMP only supports a single RGMP enabled
router to be connected directly to a port of an RGMP enabled switch.
Such a topology should be customary when connecting routers to
backbone switches and thus not pose a limitation on the deployment
of RGMP.
All RGMP messages have the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The reserved field in the message MUST be transmitted as zeros and
ignored on receipt.
2.1 Type
There are four types of RGMP messages of concern to the router-
switch interaction. The type codes are defined to be the highest
values in an octet to avoid the re-use of already assigned IGMP
type codes.
0xFF = Hello
0xFE = Bye
0xFD = Join a group
0xFC = Leave a group
Unrecognized message types should be silently ignored.
Note:
RGMP and the IANA assignment of address 224.0.0.25 for it predates
RFC3228 [9]. RGMP defines Type values which in RFC3228 are assigned
to protocol testing and experimentation. This is not an operational
issue for RGMP itself because only RGMP packets use the IPv4
destination address 224.0.0.25. The Type values defined above
are thus ONLY valid in conjunction with the RGMP destination address.
2.2. Checksum
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Checksum covers the RGMP message (the entire IPv4 payload). The
algorithm and handling of checksum are the same as those for IGMP
messages as described in RFC2236 [5].
2.3. Group Address
In an RGMP Hello or Bye message, the group address field is set to
zero.
In an RGMP Join or Leave message, the group address field holds the
IPv4 multicast group address of the group being joined or left.
2.4 IPv4 header
RGMP messages are sent by routers to switches. The source IPv4 address
of an RGMP packet is the sending interface IPv4 address of the
originating router. The destination IPv4 address of an RGMP packet is
224.0.0.25. Switches supporting RGMP need to listen to packets to
this group.
3. RGMP Protocol Description
3.1 RGMP Router side Protocol Description
Backbone switches use RGMP to learn which groups are desired at each
of their ports. Multicast routers use RGMP to pass such information
to the switches. Only routers send RGMP messages. They ignore
received RGMP messages.
A Router enabled for RGMP on an interface periodically [Hello
Interval] sends an RGMP Hello message to the attached network to
indicate that it is RGMP enabled. When RGMP is disabled on a
routers interface, it will send out an RGMP Bye message on
that interface, indicating that it again wishes to receive IPv4
multicast traffic promiscuously from that interface.
When RGMP is enabled on an interface, a router sends an RGMP Join
message out this interface for each group that it wants to receive
traffic for from the interface. The router needs to periodically
[Join Interval] re-send an RGMP Join for a group to indicate its
continued desire to receive multicast traffic for it.
Routers supporting RGMP MUST NOT send RGMP Join or Leave messages for
groups 224.0.0.x (x=0...255), 224.0.1.39 and 224.0.1.40. The latter
two are known as cisco-rp-announce and cisco-rp-discovery [3].
When a router no longer needs to receive traffic for a particular
group, it sends an RGMP Leave message for the group. For robustness,
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the router MAY send more than one such message.
If IPv4 multicast packets for an undesired group are received at a
router from a switch, the router MAY send a RGMP Leave message for
that group to the switch. These messages are called data-triggered
RGMP Leave messages and the router SHOULD rate-limit them. The
router MAY suppress sending a data triggered RGMP Leave message if
it has a desired group that has the same MAC destination address as
the undesired group (See RFC1112 [6] for MAC ambiguity). Such
suppression of data triggered RGMP Leave messages SHOULD be
configurable if supported.
3.2 RGMP Switch side Protocol Description
A switch enabled for RGMP on a network consumes RGMP messages
received from ports of the network and processes them as described
below. If enabled for RGMP, the switch must NOT forward/flood
received RGMP messages out to other ports of the network.
RGMP on a switch operates on a per port basis, establishing per-group
forwarding state on RGMP enabled ports. A port reverts into an
RGMP enabled port upon receipt of an RGMP Hello message on the
port, and a timer [5 * Hello Interval] is started. This timer is
restarted by each RGMP Hello message arriving on the port. If
this timer expires or if it is removed by the arrival of an RGMP Bye
message, then the port reverts to its prior state of multicast
traffic forwarding.
Correct deployment of RGMP is one RGMP enabled router directly
connected to a port on a switch that supports RGMP. The port on the
switch MAY want to keep track of the IPv4 originator address of the
RGMP Hello and Bye messages it receives on that port. In the event
it receives multiple IPv4 originating addresses in RGMP messages
onone port, the switch MAY generate an alert to notify the
administrator. The switch MAY also have a configuration option that
will allow to disable RGMP in this situation and fall back to
flooding IPv4 multicast on that port, although this is a potentially
dangerous option:
By default, connecting two or more RGMP enabled routers to a switch
port will cause intermittent black holing of IPv4 multicast traffic
towards these routers. Black holing occurs when a RGMP Leave is
received from one router while the other router is still joined.
This malfunction is not only easily recognized by the actual users
connected through the routers, but it also adheres to the principle
of a failure situation to rather cause less traffic than more.
Reverting to flooding easily maintains the illusion that everything
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is working perfectly, only that the traffic constraining benefits
of RGMP are not realized and probably congestion happens at a much
later time than the misconfiguration and can then not easily be
correlated with the cause anymore.
Because routers supporting RGMP are not required to send RGMP Join
or Leave messages for groups 224.0.0.x (x=0...255), 224.0.1.39 and
224.0.1.40, RGMP enabled ports always need to receive traffic for
these groups. Traffic for other groups is initially not forwarded
to an RGMP enabled port.
RGMP Join and Leave messages are accepted if they arrive on an
RGMP enabled port, otherwise they will be discarded. Upon acceptance
of an RGMP Join message, the switch MUST start forwarding traffic for
the group to the port. Upon acceptance of an RGMP Leave message, the
switch SHOULD stop forwarding traffic for the group to that port. The
switches ability to stop forwarding traffic for a group may be
limited for example because of destination MAC based forwarding in
the switch, which makes it necessary for the switch to always forward
traffic for all MAC-ambiguous IPv4 multicast groups (see [6] for
MAC-ambiguity).
To stop forwarding of traffic for a group in the event of lost RGMP
Leave message(s), a switch MAY time out RGMP forwarding state on
a port for a group [5 * Join Interval] after the last RGMP Join for
that group has been received on the port.
Without any layer 2 IPv4 multicast filtering methods running, a switch
needs to flood multicast traffic to all ports. If a switch does
actually run one or more mechanisms beside RGMP to filter IPv4 multicast
traffic, such as IGMP snooping [10], then the default will not by
default flood IPv4 multicast traffic to all ports anymore. Instead,
the switch will try to determine which ports still needs to receive
all IPv4 multicast traffic by default, and which ports not:
Compliance with this specification requires that a switch MUST be able
to elect a port for flooding through the presence of PIM Hello messages
[4] arriving from the port and also through a manual configuration
option. In addition, the switch SHOULD recognize a port connected to
a router by other appropriate protocol packets or dedicated IPv4
multicast router discovery mechanisms such as MRDISC [11]. The manual
configuration is required to support routers not supporting PIM or
other methods recognized by the switch.
Further mechanisms for IPv4 multicast traffic restriction may also be
used on RGMP enabled ports. In this case, forwarding for a group on
the port must be established if either mechanism requires it, and it
must only be removed if no mechanism requires it anymore.
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4. Operational Notes
4.1. Support for networks with multiple switches
To be simple to implement on switches and resilient in face of
potential layer 2 network topology changes, RGMP does not specify how
to restrict multicast traffic on links connecting switches amongst
each other. With just RGMP being used, multicast traffic will thus
be flooded on inter-switch links within a network if at least
one router is connected to each of the switches.
This happens implicitly because the switch will not flood/forward
received RGMP messages out to the inter-switch link and thus the
switch on the other end will only recognize the port as a router
port via the PIM Hello messages flooded by the switches. Manual
configuration for inter-switch links may be required if non-PIM
routers are being used, depending on the other capabilities of the
switch.
A switch can of course - if appropriate - send out RGMP messages on
ports to make it look like an RGMP enabled router to a potential
switch at the other end of the link. This would allow to constrain IPv4
multicast traffic between switches, but this type of "RGMP Spoofing"
by the switch is outside the scope of this specification.
4.2. Interoperability with RGMP-incapable routers
Since RGMP messages received at a switch only affect the state
of their ingress ports, the traffic restriction is applied
there only. RGMP-incapable routers will receive multicast
traffic for all multicast groups.
4.3. RGMP and multicast routing protocols
One result of the simplicity of RGMP are its restrictions in
supporting specific routing protocols. The following paragraphs
list a few known restrictions.
A router running RGMP on a switched network will not receive traffic
for a multicast group unless it explicitly requests it via RGMP
Join messages (besides those group ranges specified to be
flooded in 3.1). For this reason, it is not possible to run a
protocol like PIM Dense-Mode or DVMRP across an RGMP enabled
network with RGMP enabled routers.
In Bidir-PIM, a router elected to be the DF must not be enabled
for RGMP on the network, because it unconditionally needs to forward
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traffic received from the network towards the RP. If a router is not
the DF for any group on the network, it can be enabled for RGMP on
that network.
In PIM-SM, directly connected sources on the network can not be
supported if the elected DR is running RGMP, because this DR needs
to unconditionally receive traffic from directly connected sources
to trigger the PIM-SM registering process on the DR. In PIM-SSM,
directly connected sources can be supported with RGMP enabled
routers.
Both in PIM-SM and PIM-SSM, upstream routers forwarding traffic
into the switched network need to send RGMP Joins for the group in
support of the PIM assert process.
5. List of timers and default values
5.1. Hello Interval
The Hello Interval is the interval between RGMP Hello messages sent
by an RGMP-enabled router to an RGMP-enabled switch. Default:
60 seconds.
5.2. Join Interval
The Join Interval is the interval between periodic RGMP Join
messages sent by an RGMP-enabled router to an RGMP-enabled switch
for a given group address. Default: 60 seconds.
6. Security Considerations
The RGMP protocol assumes that physical port security can be
guaranteed for switch ports from which RGMP messages are accepted.
Physical port security for RGMP means that physical measures will
ensure that such ports are dedicatedly connected to one system
which acts as an RGMP capable router. This is also the recommended
configuration to best leverage the benefits of the RGMP protocol
(eg: avoiding unwanted third-party IPv4 multicast traffic arriving
on said ports).
RGMP specific DoS attacks arise from forged RGMP messages.
If more than one system is connected to a port of the RGMP switch,
then one system may forge RGMP messages and affect the operations
of the other system(s) on the same port. This is a potential
security risk.
When physical security ensures that only one system is connected to
a RGMP capable port on a switch, then forged messages from this
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system itself can take effect. Such forged messages can always be
avoided by system local measures.
We consider the ramifications of a forged message of each type:
Hello Message:
A forged RGMP Hello message can restrict multicast data towards
a non-RGMP enabled router on the same port. This effectively
introduces a blackholing DoS attack.
Leave Message:
A forged RGMP Leave message can restrict IPv4 multicast traffic
for individual groups toward the port. The effect is a possible
blackholing DoS attack similar to an RGMP Hello Message only that
it does not affect all IPv4 multicast traffic but only that of the
groups indicated in the forged messages. It will also only affect
a port if there officially is only one RGMP enabled router connected
to it (i.e.: if the port is RGMP enabled).
Bye Message:
A forged RGMP Bye message can turn the port into being
RGMP-disabled. This indirectly could cause a DoS attack based
on the port getting overloaded with IPv4 multicast traffic if the
network bandwidth of the port was provisioned with the expectation
that RGMP will suppress unwanted IPv4 multicast messeges.
This type of DoS attack simply reestablishes a port behavior as
if RGMP was not configured, so it invalidates the benefit of RGMP,
but it does not introduce an issue that would not have been there
without RGMP in the first place.
Join Message:
A forged RGMP Join message could attract undesired multicast
packets to the port where it is received from. The effect is
similar to an RGMP Bye Message only that it does not affect all IPv4
multicast traffic but only that of the groups indicated in the
forged messages and that it will affect a port only if there
officially is only one RGMP enabled router connected to it
(i.e.: if the port is RGMP enabled).
7. References
1 Bradner, S., "The Internet Standards Process -- Revision 3",
RFC2026, October 1996.
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2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2119, March 1997.
3 Internet Multicast Addresses,
ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses
4 Estrin, D., et al, "Protocol Independent Multicast-Sparse Mode
(PIM-SM): Protocol Specification", RFC2362, June 1998
5 Fenner, W., "Internet Group Management Protocol, Version 2",
RFC2236, November 1997
6 Deering, S., "Host Extensions for IP Multicasting", RFC1112,
August 1989.
7. ANSI/IEEE Std 802.1D 1998 Edition, "Media Access Control (MAC)
Bridges", 1998.
8. Farinacci D., Tweedly D., Speakman T.,
"Cisco Group Management Protocol (CGMP)", 1996/1997
ftp://ftpeng.cisco.com/ipmulticast/specs/cgmp.txt
9. Fenner, B., "IANA Considerations for IPv4 Internet Group
Management Protocol (IGMP)", RFC3228, February 2002
10. M. Christensen, F. Solensky, "IGMP and MLD snooping switches",
Work in Progress, 2002
11. S. Biswas, B. Cain, B. Haberman, "IGMP Multicast Router
Discovery", Work In Progress, 2002
8. Acknowledgments
The authors would like to thank Gorry Fairhurst, Bill Fenner,
Giovanni Meo, Mike Norton, Pavlin Radoslavov and Alex Zinin for
their review of the document and their suggestions.
9. Author's Addresses
Ishan Wu
cisco Systems
170 West Tasman Drive
San Jose, CA 95134
Phone: (408) 526-5673
Email: iwu@cisco.com
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Toerless Eckert
cisco Systems
170 West Tasman Drive
San Jose, CA 95134
Phone: (408) 853-5856
Email: eckert@cisco.com
10. Revision History
A record of changes which will be removed before publication.
(1) Fixed typographic details as per Gorry Fairhurst.
(2) Corrected protocol behavior description
(legacy text that incorrectly made it into -01).
a) Sending of data triggered RGMP Leave messages and
their suppression due to MAC ambiguity is optional.
b) Switches maintaining per group join timers is optional.
c) Switches may want to recognize misconfiguration (connection
of more than one router to a port).
(3) The Intellectual Property Rights appendix (A.) was
modified to indicate claims.
(4) An appendix to compare RGMP against GARP/GMRP was added.
(5) An appendix to discuss possible future extensions and
a comparison against PIM Snooping was added.
(6) An acknowledgment section was added.
(7) A note about RFC3228 and RGMPs use of experimental Type
fields was added.
(8) Added security consideration about physical misconfiguration.
Version -03:
(9) Changed references to IP into IPv4.
(10) Overhauled security considertions into explicitly describing
the recommended configuration and the security risks evolving
when this recommendation is not implemented.
(11) Added reference to MRDISC as recommended router discover
mechanism
(12) Editorial fixes in appendices.
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Appendix A. Intellectual Property Rights
The IETF has been notified of intellectual property rights
claimed in regard to some or all of the specification contained
in this document. For more information consult the online list
of claimed rights.
Appendix B. Comparison with GARP/GMRP
This appendix is not part of the RGMP specification but is
provided for information only.
GARP/GMRP (defined in IEEE 802.1D [7]) is the ANSI/ISO/IEC/IEEE
protocol suite to constrain ethernet multicast traffic in bridged
ethernet networks. As such it is also a possible alternative to
RGMP for the purpose of constraining multicast traffic towards
router ports. This appendix will explain the motivation not to
rely on GARP/GMRP and how GARP/GMRP and RGMP differ.
The key factor in rolling out GARP/GMRP would have been to
completely replace IGMP Snooping. This was the design goal of
GARP/GMRP. For efficient operations, IGMP Snooping requires hardware
filtering support in the switch (to differentiate between hosts
membership reports and actual IPv4 multicast traffic). Especially in
many older switches this support does not exist. Vendors tried to
find a way around this issue to provide the benefit of constraining
IPv4 multicast traffic in a switched LAN without having to build more
expensive switch hardware. GARP/GMRP is one protocol resulting from
this. CGMP from Cisco is another one. While CGMP solves the problem
without requiring changes to the host stack software, GARP/GMRP
requires support for it by the host stack.
Up to date GARP/GMRP has so far not made significant inroads into
deployed solutions. IGMP Snooping (and CGMP) are the norm for this
environment. In result, GARP/GMRP can not necessarily be expected to
be supported by layer 2 switches. In addition, GARP/GMRP does not
address clearly the issues RGMP tries to solve. On one hand,
GARP/GMRP provides much more functionality and as such complexity as
immediately required. On the other hand, GARP/GMRP is limited by being
a standard predominantly for the Ethernet scope.
Beyond the process and applicability reasons, the main differences
between GARP/GMRP and RGMP are as follows:
o GARP/GMRP switches/systems need to send and listen/react to
GARP/GMRP messages. In RGMP, routers only need to send RGMP
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messages and switches only need to listen to them. This
protocol approach is meant to simplify implementation,
operations and troubleshooting.
o The same switch running RGMP in a backbone network will likely
see more states then running on the edge only doing IGMP Snooping,
making it preferable to keep the amount of per group processing
and memory requirements in RGMP more in bounds than possible in
IGMP Snooping and GARP/GMRP: In GARP/GMRP, a (multiple) timer
based state-machines needs to be maintained on a per ethernet
group address, in RGMP timer maintenance is completely optional
and there are only two states per group (joined or not joined).
o GARP/GMRP is an ethernet level protocol from the IEEE. It supports
to constrain traffic for ethernet addresses (groups). RGMP does
constrain traffic for IPv4 multicast groups. Today this is even
beyond the capabilities of typical switch platforms used as layer2
switches. Extensions to support further entities are likely easier
to come by through extensions to RGMP than to GARP/GMRP.
o RGMP shares the basic packet format with IGMP (version 2)
and is as such easy to add to router and switch platforms that
already support IGMP respectively IGMP Snooping. This is
especially true for switches that in hardware can differentiate
between IGMP protocol type packets and other IPv4 multicast traffic
sent to the same (or a MAC ambiguous) group. In addition, due to
the state simplicity of RGMP it is easy to integrate IGMP
Snooping and RGMP operations in a switches IPv4 multicast control
and forwarding plane.
o GARP/GMRP supports more than one system (host/router) on a
switch port which is one reason for its complexity. In RGMP,
this configuration is explicitly not supported: More than one
router per switched port is not only not a common scenario in
today's switches layer 2 networks, it is also an undesired
configuration when unwanted IPv4 multicast traffic is to be
kept away from routers.
o GARP/GMRP defines how to constrain multicast traffic between
switches, another reason for its complexity. RGMP does not
explicitly support this as part of the protocol because of
the following reasons:
o It is not necessary to include this function as part of the
RGMP protocol description because switch implementations
can transparently decide to support this function (see
4.1 about this "RGMP Spoofing").
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o Important deployments through which large amounts of IPv4
multicast are moved today are typically single switch.
MIX - Multicast Internet eXchange points.
o Avoiding congestion on inter-switch links in general is more
complex than simply constraining IPv4 multicast traffic to paths
where it is needed. With or without IPv4 multicast, the aggregate
bandwidth needed between switches can easily be the aggregate
required bandwidth to routers on either sides. For this
reason, inter-switch bandwidth is most often appropriately
over provisioned. In addition, the likelihood for receiving
routers to be only on the sources side of an inter-switch
link is in general deployments rather low. The cases where
traffic constrainment on inter-switch links is required and
helpful is thus limited and can in most cases be avoided or
worked around. Moving the network to a layer 3 routed network
is often the best solution, supporting RGMP-Spoofing (see
section 4.1) is another one.
Appendix C. Possible future extensions / comparison to PIM Snooping
This appendix is not part of the RGMP specification but is
provided for information only.
This appendix presents a discussion of possible extensions to
RGMP. Included are points on why the extensions are not included
and in addition a motivation for RGMP in comparison to (PIM)
snooping.
o Support for multiple switches
As discussed in "RGMP Spoofing", chapter 4.1 and GARP/GMRP
comparison in Appendix B.
o Support for SSM
While RGMP works with PIM-SSM, it does not have explicit messages
for the router to selectively join to (S,G) channels individually.
Instead the router must RGMP join to all (Si,G) channels by
joining to G. Extending RGMP to include (S,G) Join/Leaves is
feasible. However, currently the majority of switches do not
support actual traffic constraining on a per channel basis. In
addition, the likelihood for actual channel collision (two SSM
channels using the same group) will only become an issue when SSM
is fully deployed.
o Support for IPv6
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RGMP could easily be extended to support IPv6 by mapping
the RGMP packet format into the MLD/IPv6 packet format. This
was not done for this specification because most switches today
do not even support MLD snooping.
o Support for multiple routers per port
As discussed in Appendix B. This is probably one extension
that should be avoided. Multiple RGMP router per port
are inappropriate for efficient multicast traffic constrainment.
o Support for non-join based protocols / protocol elements
For protocols like PIM dense-mode, DVMRP or Bidir-PIM DF routers,
additional RGMP messages may be added to allow routers to indicate
that certain group (ranges) traffic need to be flooded from
(dense-mode) or to (Bidir-PIM) them.
o Support for multi-policy switching
In Multicast Exchange Points (MIXes) environments situations exist
where different downstream routers for policy reasons need to
receive the same traffic flow from different upstream routers.
This problem could be solved by actually providing an upstream
neighbor field in RGMP Join/Leave messages. The RGMP switch would
then forward traffic from one upstream router only to those
downstream routers who want to have the traffic from exactly this
upstream router. This extension would best go in hand with
changes to the layer 3 routing protocol run between the routers.
As previously mentioned, RGMP was designed to be easy to
implement and to support simple layer2 switches. Implementations
could also be applied to switches beyond layer 2. If all the above
possible future extensions were to be supported by an evolution of
RGMP, it would be questionable whether such a protocol could be any
less complex than actually snooping into the layer3 IPv4 routing
protocol run between routers in a switched LAN.
From the perspective of protocol architecture it is certainly
more appropriate to have a separate protocol like RGMP or GARP/GMRP
for this purpose. Then again, the more complex the requirements are,
the more duplication of effort is involved and snooping seems to
become a more attractive option.
Even though there exists one predominant routing Protocol, PIM, in
IPv4 multicast, routing with PIM in itself is extremely complex for a
switch to snoop into. PIM has two main versions, different modes
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INTERNET-DRAFT Router-port Group Management Protocol Nov 1 2002
- sparse, dense, bidir, ssm, join / prune / graft messages
(depending on the mode of the group), various PIM Hello options,
different versions of asserts, two dynamic mode announcement
protocols (BSR, AutoRP), and finally supports both IPv4 and IPv6.
A switch snooping into PIM is very likely to implement just a subset
of this feature set, making it very hard for the user to determine
what level of actual traffic constrainment is achieved unless a
clear specification exists for the implementation (or better the
method per se.). In addition, there is always the danger that such
a snooping implementation may break newer features of the routing
protocol that it was not designed to handle (likely because they
could not have been predicted). Exactly this, for example, happens
with switches using IGMP (v2) snooping implementations are being
subjected to IGMP version 3 messages - they break IGMPv3.
In summary, with PIM still evolving, the approach taken by RGMP
is the safest one for the immediate problems at hand, and
extensions like those listed should be considered in time for
actual demand. (PIM) snooping is a valid alternative once the
total amount of features that need to be supported makes it
an equally attractive solution (with respect to complexity) to a
dedicated protocol and if its functions are well defined to allow
predicting its effects - but always at the price of possible
incompatibilities with upcoming PIM protocol extensions unless
support for layer 2 switches is explicitly considered in moving
PIM protocols forward.
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