MBONE Deployment WG P. Savola
Internet-Draft CSC/FUNET
Expires: February 9, 2005 August 11, 2004
IPv6 Multicast Deployment Issues
draft-ietf-mboned-ipv6-multicast-issues-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This memo describes known issues with IPv6 multicast, and provides
historical reference of how some earlier problems have been resolved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Multicast-related Abbreviations . . . . . . . . . . . . . 3
2. Justification for IPv6 Inter-domain ASM . . . . . . . . . . . 3
2.1 SSM Deployment Issues . . . . . . . . . . . . . . . . . . 3
2.2 Groups of Different Non-global Scope . . . . . . . . . . . 4
3. Different Solutions to Inter-domain Multicast . . . . . . . . 5
3.1 Changing the Multicast Usage Model . . . . . . . . . . . . 5
3.2 Implementing MSDP for IPv6 . . . . . . . . . . . . . . . . 6
3.3 Implementing Another Multicast Routing Protocol . . . . . 6
3.4 Embedding the RP Address in an IPv6 Multicast Address . . 6
4. Issues with IPv6 Multicast . . . . . . . . . . . . . . . . . . 7
4.1 Issues with Embedded RP . . . . . . . . . . . . . . . . . 7
4.1.1 RP Failover with Embedded RP . . . . . . . . . . . . . 7
4.1.2 Embedded RP and Control Mechanisms . . . . . . . . . . 7
4.2 Neighbor Discovery Using Multicast . . . . . . . . . . . . 8
4.3 Functionality Like MLD Snooping . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Normative References . . . . . . . . . . . . . . . . . . . . 9
7.2 Informative References . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
There are many issues concerning the deployment and implementation,
and to a lesser degree, specification of IPv6 multicast. This memo
describes known problems to raise awareness, and documents how
previous problems have been resolved.
Section 2 describes the justifications for providing an inter-domain
multicast solution using Any Source Multicast (ASM) with IPv6.
Section 3 in turn describes which options were considered for filling
those the requirements for the IPv6 inter-domain multicast solutions.
These sections are provided for historical reference of the
discussion and consensus in the IETF MBONED working group.
Section 4 lists issues that have come up with IPv6 multicast but have
not yet been at least fully resolved, and may require raised
awareness.
1.1 Multicast-related Abbreviations
ASM Any Source Multicast
BSR Bootstrap Router
CGMP Cisco Group Management Protocol
DR Designated Router
IGMP Internet Group Management Protocol
MLD Multicast Listener Discovery
MSDP Multicast Source Discovery Protocol
PIM Protocol Independent Multicast
PIM-SM Protocol Independent Multicast - Sparse Mode
RP Rendezvous Point
SSM Source-specific Multicast
2. Justification for IPv6 Inter-domain ASM
This section documents the reasons and the discussion which led to
the agreement why a solution to IPv6 inter-domain ASM was necessary.
The main reason was that SSM [I-D.ietf-ssm-arch] was not considered
to solve all the relevant problems (e.g., many-to-many applications,
source discovery), and that SSM was not sufficiently widely deployed
and used.
2.1 SSM Deployment Issues
To be deployed, SSM requires changes to:
1. routers
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2. IGMP/MLD-snooping Ethernet switches
3. hosts
4. application programming interfaces (APIs)
5. multicast usage models
Introducing SSM support in the routers has been straightforward as
PIM-SSM is a subset of PIM-SM [I-D.ietf-pim-sm-v2-new].
IGMP-snooping Ethernet switches have been a more difficult issue
[SSMSNOOP]; some which perform IGMPv2 snooping discard IGMPv3 reports
or queries, or multicast transmissions associated to them. If MLDv1
snooping had been implemented (or is implemented in a similar
manner), this would likely have affected that as well.
Host systems require MLDv2 [RFC3810] support. The situation has
improved with respect to MLDv2 support for end systems, and
interoperability has increased after the publication of the RFC due
to the stabilization of the ICMP types used.
The multicast source filtering API specification has also been
completed [RFC3678]; its deployment is likely roughly equal (or
slightly worse) than MLDv2. The API is required for creating
(cross-platform) SSM applications.
The most difficult issue, multicast usage models, remains a problem
as of this writing. SSM is an excellent fit for one-to-many
distribution topologies, and porting such applications to use SSM
would likely be rather simple. However, a significant number of
current applications are many-to-many (e.g., conferencing
applications) which cannot be converted to SSM without significant
effort, including, for example, out-of-band source discovery. For
such applications to be usable for IPv6 at least in a short to medium
term, ASM -like techniques seem to be required.
2.2 Groups of Different Non-global Scope
Many ASM applications are used with a smaller scope than global; some
of these have a wider scope than others. However, groups of smaller
scope typically need to be in their own PIM-SM domains to prevent
inappropriate data leakage. Therefore if a site has groups of
different scopes, having multiple PIM domain borders becomes a
requirement unless inter-domain multicast is used instead; further,
configuring such nesting scopes would likely be an operational
challenge. In consequence, if these applications of non-global scope
need to be used, inter-domain multicast support is practically
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required.
In consequence, especially if multicast with different non-global
scopes is used, there will be a need for inter-domain multicast
solutions. As many applications are relying on ASM characteristics,
this further increases a need for an inter-domain ASM solution.
3. Different Solutions to Inter-domain Multicast
When ASM is used, the Internet must be divided to multiple PIM-SM for
both administrative and technical reasons, which means there will be
multiple PIM-SM RPs which need to communicate the information of
sources between themselves.
On the other hand, SSM does not require RPs and also works in the
inter-domain without such communication. Section 2 describes the
justification why Inter-domain ASM was still considered to be
required. This section describes different solutions which were
discussed to providing inter-domain multicast for IPv6.
For inter-domain multicast, there is consensus to continue using SSM,
and also use Embedded-RP as appropriate.
This section provides historical reference of the discussion and
decisions.
3.1 Changing the Multicast Usage Model
As ASM model has been found to be complex and a bit problematic, some
felt that this is a good incentive to move to SSM for good (at least
for most cases). Below two paragraphs are adapted from
[I-D.bhattach-diot-pimso]:
The most serious criticism of the SSM architecture is that it does
not support shared trees which may be useful for supporting
many-to-many applications. In the short-term this is not a
serious concern since the multicast application space is likely to
be dominated by one-to-many applications. Some other classes of
multicast applications that are likely to emerge in the future are
few-to-few (e.g. private chat rooms, whiteboards), few-to-many
(e.g., video conferencing, distance learning) and many-to-many
(e.g., large chat rooms, multi-user games). The first two classes
can be easily handled using a few one-to-many source-based trees.
The issue of many-to-many multicasting service on top of a SSM
architecture is an open issue at this point. However, some feel
that even many-to-many applications should be handled with
multiple one- to-many instead of shared trees.
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In any case, even though SSM would avoid the problems of ASM, it was
felt that SSM is not sufficiently widely available to completely
replace ASM (see Section 2.1), and that the IETF should not try to
force the application writers to change their multicast usage models.
3.2 Implementing MSDP for IPv6
In IPv4, notification of multicast sources between these PIM-SM RPs
is done with Multicast Source Discovery Protocol (MSDP) [RFC3618].
The protocol is widely considered a sub-optimal solution and even
dangerous to deploy; when it was specified, it was only meant as a
"stop-gap" measure.
The easiest stop-gap solution (to a stop-gap solution) would have
been to specify IPv6 TLV's for MSDP. This would be fairly
straightforward, and existing implementations would probably be
relatively easy to modify.
There is and has been resistance to this, as MSDP was not supposed to
last this long in the first place; there is clear consensus that
there should be no further work on it [I-D.ietf-mboned-msdp-deploy].
3.3 Implementing Another Multicast Routing Protocol
One possibility might have been to specify and/or implement a
different multicast routing protocol.
In fact, Border Gateway Multicast Protocol (BGMP)
[I-D.ietf-bgmp-spec] has been specified; however, it is widely held
to be quite complex and there have been no implementations, nor will
to make any. Lacking deployment experience and specification
analysis, it is difficult to say which problems it might solve (and
possibly, which new ones to introduce). One probable reason why BGMP
failed to attract continuing interest was it's dependance on
similarly heavy-weight multicast address allocation/assignment
protocols.
As of this writing, no other inter-domain protocols have been
specified, and BGMP is not considered a realistic option.
3.4 Embedding the RP Address in an IPv6 Multicast Address
One way to work around these problems was to allocate and assign
multicast addresses in such a fashion that the address of the RP
could be automatically calculated from the IPv6 multicast address.
Making some assumptions about how the RPs would configure Interface
Identifiers, this is can achieved as described in
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[I-D.ietf-mboned-embeddedrp]; PIM-SM implementations need to
implement the Embedded RP group-to-RP mapping mechanism which
processes this encoding.
To completely replace the need for MSDP for IPv6, a different way to
implement "Anycast RP" [RFC3446] -technique, for sharing the state
information between different RP's in one PIM-SM domain, is also
needed. One such approach is described in [I-D.ietf-pim-anycast-rp].
4. Issues with IPv6 Multicast
This section describes issues that have come up with IPv6 multicast
but have not yet been at least fully resolved.
4.1 Issues with Embedded RP
4.1.1 RP Failover with Embedded RP
Embedded RP provides a means for ASM multicast without inter-domain
MSDP. However, to continue providing failover mechanisms for RPs, a
form of state sharing, Anycast-RP, should still be supported.
Instead of MSDP, this can be achieved using a PIM-SM extension
[I-D.ietf-pim-anycast-rp].
One should note that as Embedded RP does not require MSDP peerings
between the RPs, it's possible to deploy more RPs in a PIM domain.
For example, the scalability and redundancy could be achieved by
co-locating RP functionality in the DRs: each major source, which
"owns" a group, could have its own DR act as the RP. This has about
the same redundancy characteristics as using SSM -- so there may not
be an actually very urgent need for Anycast-RP if operational methods
to include fate-sharing of the groups is followed.
In any case, a "cold failover" variant of anycast-RP, without state
sharing, applicable for long-term redundancy, is still an option. In
this mechanism, multiple routers would be configured with the RP
address, but only one would be active at the time: if the main RP
goes down, another takes its place. However, the multicast state
stored in the RP would be lost, unless it is synchronized by some
out-of-band mechanism.
4.1.2 Embedded RP and Control Mechanisms
With ASM and MSDP deployment, the ISPs can better control who is
using their RPs.
With Embedded RP, anyone could use a third-party RP to host their
groups unless some mechanisms, for example access-lists, are in place
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to control the use of the RP [I-D.ietf-mboned-embeddedrp].
Such abuse is of questionable benefit, though, as anyone with a /64
could form an RP of its own.
Whether this is a sufficiently serious problem worth designing a
(potentially complex) solution for is still under debate, as of this
writing.
4.2 Neighbor Discovery Using Multicast
Neighbor Discovery [RFC2461] uses link-local multicast in Ethernet
media, not broadcast as ARP does with IPv4. This has been seen to
cause operational problems with some equipment.
The author has seen one brand of managed Ethernet switches, and heard
reports of a few unmanaged switches, that do not forward IPv6
link-local multicast packets to other ports at all. In essence,
native IPv6 is impossible with this equipment. These problems have
likely been fixed in later revisions of the equipment, but this does
not fix the equipment on the field, and it is likely that similar
problems will surface again.
It seems likely that this may be a problem with some switches that
build multicast forwarding state based on Layer 3 information (and do
not support IPv6); state using Layer 2 information would work just
fine [I-D.ietf-magma-snoop]. Therefore the snooping swich developers
should be aware of the tradeoff of using Layer 2 vs Layer 3
information on multicast data forwarding, especially if IPv6 snooping
is not supported.
For the deployment of IPv6, it would be important to find out how
this can be fixed (e.g., how exactly this breaks specifications) and
how one can identify which equipment could cause problems like these
(and whether there are workarounds).
One workaround might be to implement a toggle in the nodes that would
use link-layer broadcast instead of multicast as a fallback solution.
However, this would have to be used in all the systems on the same
link, otherwise local communication is impaired.
4.3 Functionality Like MLD Snooping
On Ethernet, multicast frames are forwarded to every port, even
without subscribers (or IPv6 support).
Especially if multicast traffic is relatively heavy (e.g., video
streaming), it becomes particularly important to have some feature
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like Multicast Listener Discovery (MLD) snooping implemented, to
reduce the amount of flooding [I-D.ietf-magma-snoop].
In addition, some vendors have not realized which multicast addresses
(in particular, link-local addresses) MLD reports -- utilized in the
snooping -- should be generated for. The introduction of MLD
snooping could cause hosts which do not send MLD reports
appropriately to be blocked out. As specified in [RFC2461], an MLD
report must be generated for every group except all-nodes (ff02::1 --
which is forwarded to all ports); this also includes all the other
link-local groups.
Looking at the actual problem from a higher view, it is not clear
that MLD snooping is the right long-term solution. It makes the
switches complex, requires the processing of datagrams above the
link-layer, and should be discouraged
[I-D.ietf-mboned-iesg-gap-analysis]: the whole idea of L2-only
devices having to peek into L3 datagrams seems like a severe layering
violation -- and often the devices aren't upgradeable (if there are
bugs or missing features, which could be fixed later) in any way.
Better mechanisms could be having routers tell switches which
multicasts to forward where (e.g., [CGMP]) or by using some other
mechanisms [GARP].
5. Security Considerations
Only deployment and implementation issues are considered, and these
do not have any particular security considerations; security
considerations for each technology are covered in the respective
specifications.
6. Acknowledgements
Early discussions with Stig Venaas, Jerome Durand, Tim Chown et al.
led to the writing of this draft. Brian Haberman offered extensive
comments along the way. "Itojun" Hagino brought up the need for MLD
snooping in a presentation. Bill Nickless pointed out issues in the
gap analysis and provided a pointer to GARP/GMRP; Havard Eidnes made
a case for a protocol like CGMP. Leonard Giuliano pointed out a more
complete analysis of SSM with different kind of applications.
7. References
7.1 Normative References
[I-D.ietf-bgmp-spec]
Thaler, D., "Border Gateway Multicast Protocol (BGMP):
Protocol Specification", draft-ietf-bgmp-spec-06 (work in
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progress), January 2004.
[I-D.ietf-mboned-embeddedrp]
Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
draft-ietf-mboned-embeddedrp-07 (work in progress), July
2004.
[I-D.ietf-mboned-msdp-deploy]
McBride, M., "Multicast Source Discovery Protocol (MSDP)
Deployment Scenarios", draft-ietf-mboned-msdp-deploy-06
(work in progress), March 2004.
[I-D.ietf-pim-anycast-rp]
Farinacci, D., "Anycast-RP using PIM",
draft-ietf-pim-anycast-rp-02 (work in progress), June
2004.
[I-D.ietf-pim-sm-v2-new]
Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode PIM-SM):
Protocol Specification (Revised)",
draft-ietf-pim-sm-v2-new-10 (work in progress), July 2004.
[I-D.ietf-ssm-arch]
Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", draft-ietf-ssm-arch-05 (work in progress), July 2004.
[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC3446] Kim, D., Meyer, D., Kilmer, H. and D. Farinacci, "Anycast
Rendevous Point (RP) mechanism using Protocol Independent
Multicast (PIM) and Multicast Source Discovery Protocol
(MSDP)", RFC 3446, January 2003.
[RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery
Protocol (MSDP)", RFC 3618, October 2003.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
7.2 Informative References
[CGMP] "Cisco Group Management Protocol",
<http://www.javvin.com/protocolCGMP.html>.
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[GARP] Tobagi, F., Molinero-Fernandez, P. and M. Karam, "Study of
IEEE 802.1p GARP/GMRP Timer Values", 1997.
[I-D.bhattach-diot-pimso]
Bhattacharyya, S., Diot, C., Giuliano, L. and R. Rockell,
"Deployment of PIM-SO at Sprint (PIM-SO)", March 2000.
[I-D.ietf-magma-snoop]
Christensen, M., Kimball, K. and F. Solensky,
"Considerations for IGMP and MLD Snooping Switches",
draft-ietf-magma-snoop-11 (work in progress), May 2004.
[I-D.ietf-mboned-iesg-gap-analysis]
Meyer, D. and B. Nickless, "Internet Multicast Gap
Analysis from the MBONED Working Group for the IESG",
draft-ietf-mboned-iesg-gap-analysis-00 (work in progress),
July 2002.
[I-D.ietf-pim-sm-bsr]
Fenner, B., "Bootstrap Router (BSR) Mechanism for PIM",
draft-ietf-pim-sm-bsr-04 (work in progress), July 2004.
[RFC3678] Thaler, D., Fenner, B. and B. Quinn, "Socket Interface
Extensions for Multicast Source Filters", RFC 3678,
January 2004.
[SSMSNOOP]
"Operational Problems with IGMP snooping switches", March
2003, <http://www.ietf.org/proceedings/03mar/148.htm>.
Author's Address
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
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