MBONE Deployment WG P. Savola Internet-Draft CSC/FUNET Expires:
February 9,March 3, 2005 August 11,September 2, 2004 IPv6 Multicast Deployment Issues draft-ietf-mboned-ipv6-multicast-issues-00.txtdraft-ietf-mboned-ipv6-multicast-issues-01.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on February 9,March 3, 2005. 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 Abstract This memo describes known issues with IPv6 multicast, and provides historical reference of how some earlier problems have been resolved. 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 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 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 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. 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 [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,redundancy without state sharing, applicable for long-term redundancy,sharing is still an option. This does not offer any load-balancing of RPs or shared trees, but provides only long-term redundancy. In this mechanism, multiple routers would be configured with the RP address,address (with appropriate unicast metrics), but only one of them would be active at theany 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 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 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 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>. [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: email@example.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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