MobOpts Research Group Thomas C. Schmidt Internet Draft HAW Hamburg Matthias Waehlisch Expires: April 2006 FHTW Berlin October 2005 Multicast Mobility in MIPv6: Problem Statement IPR Statement 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 becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Status of this Memo 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 document is a submission of the IRTF MobOpts RG. Comments should be directed to the MobOpts RG mailing list, mobopts@irtf.org. Abstract In this document we discuss mobility extensions to current IP layer multicast solutions. Problems arising from mobile group communication in general, in the case of multicast listener mobility and for mobile Any Source Multicast as well as Source Specific Multicast senders are documented subsequently. The principal approaches to multicast mobility are introduced in brief. Schmidt, Waehlisch Expires - April 2006 [Page 1] MMCASTv6-PS October 2005 Table of Contents 1. Introduction and Motivation....................................2 2. Problem Description............................................3 2.1 Generals...................................................3 2.2 Multicast Listener Mobility................................5 2.3 Multicast Source Mobility..................................5 2.3.1 Any Source Multicast Mobility.................. ......5 2.3.2 Source Specific Multicast Mobility.............. .....6 3. Solutions......................................................7 4. Security Considerations........................................7 5. IANA Considerations............................................8 6. References.....................................................8 Acknowledgments..................................................10 Author's Addresses...............................................10 Intellectual Property Statement..................................10 Copyright Notice.................................................11 Disclaimer of Validity...........................................11 Acknowledgement..................................................11 1. Introduction and Motivation Group communication forms an integral building block of a wide variety of applications, ranging from public content distribution and streaming over voice and video conferencing, collaborative environments and gaming up to the self-organization of distributed systems. Its support by network layer multicast will be needed, whenever globally distributed, scalable, serverless or instantaneous communication is required. As broadband media delivery more and more emerges to be a typical mass scenario, scalability and bandwidth efficiency of multicast routing continuously gains relevance. Internet multicasting will be of particular importance to mobile Schmidt, Waehlisch Expires - April 2006 [Page 2] MMCASTv6-PS October 2005 environments, where users commonly share frequency bands of limited capacity. The rapidly increasing mobile reception of 'infotainment' streams may soon require a wide deployment of mobile multicast services. The fundamental approach to deal with mobility in IPv6 [2] is stated in the Mobile IPv6 RFCs [3,4]. MIPv6 [3] only roughly treats multicast mobility, in a pure remote subscription approach or through bi-directional tunneling via the Home Agent. Whereas the remote subscription suffers from slow handovers, as it relies on multicast routing to adapt to handovers, bi-directional tunneling introduces inefficient overheads and delays due to triangular forwarding. Therefore both approaches cannot be considered solutions for a deployment on large scale. A mobile multicast service for a future Internet should admit 'close to optimal' routing at predictable and limited cost, robustness combined with a service quality compliant to real-time media distribution. Intricate multicast routing procedures, though, are not easily extensible to comply with mobility requirements. Any client subscribed to a group while in motion, requires delivery branches to pursue its new location; any mobile source requests the entire delivery tree to adapt to its changing positions. Significant effort has been already invested in protocol designs for mobile multicast receivers. Only limited work has been dedicated to multicast source mobility, which poses the more delicate problem [21]. In multimedia conference scenarios each member commonly operates as receiver and as sender for multicast based group communication. In addition, real-time communication such as voice or video over IP places severe temporal requirement on mobility protocols: Seamless handover scenarios need to limit disruptions or delay to less than 100 ms. Jitter disturbances are not to exceed 50 ms. Note that 100 ms is about the duration of a spoken syllable in real-time audio. It is the aim of this document, to specify the problem scope for a multicast mobility management as to be refined in future work. The attempt is made to subdivide the various challenges according to their originating aspects and to present existing proposals for solution, as well as major bibliographic references. 2. Problem Description 2.1 Generals Multicast mobility must be considered as a generic term, which subsumes a collection of quite distinct functions. At first, multicast communication divides into Any Source Multicast (ASM) [5] Schmidt, Waehlisch Expires - April 2006 [Page 3] MMCASTv6-PS October 2005 and Source Specific Multicast (SSM) [6,7]. At second, multicast communication is asymmetric, so the roles of senders and receivers need distinction. Both individually may be mobile. Their interaction is facilitated by a multicast routing function s.a. DVMRP [8], PIM- SM/SSM [9,10] or CBT [11] and the multicast listener discovery protocol [12,13]. Any multicast mobility solution must account for all of these functional blocks. It should enable seamless continuity of multicast sessions when moving from one IPv6 subnet to another. It should preserve the multicast nature of packet distribution and approximate optimal routing. It should support per flow handover for multicast traffic, as properties and designations of flows may be of individual kind. Multicast routing dynamically adapts to session topologies, which then may change under mobility. However, routing convergence arrives at a time scale of seconds, even minutes and is far too slow to support seamless handovers for interactive or real-time media sessions. The actual temporal behavior strongly depends on the routing protocol in use and on the geometry of the current distribution tree. A mobility scheme that arranges for adjustments, i.e. partial changes or full reconstruction, of multicast trees is forced to make provision for time buffers sufficient for protocol convergence. Special attention is needed with a possible rapid movement of the mobile node, as this may occur at much higher rates than compatible with protocol convergence. IP layer multicast packet distribution is an unreliable service, which is bound to connectionless transport protocols. Packet loss thus will not be handled in a predetermined fashion. Mobile multicast handovers should not cause significant packet drops. Due to statelessness the bi-casting of multicast flows does not cause foreseeable degradations of the transport layer. Group addresses in general are location transparent, even though there are proposals to embed unicast prefixes or Rendezvous Point addresses [14]. Source addresses contributing to a multicast session are interpreted by the routing infrastructure and by receiver applications, which frequently are source address aware. Multicast therefore inherits the mobility address duality problem for source addresses, being a logical node identifier (HoA) at the one hand and a topological locator (CoA) at the other. Multicast sources in general operate decoupled from their receivers in the following sense: A multicast source submits data to a group of unknown receivers, thus operating without any feedback channel. It neither has means to inquire on properties of its delivery trees, nor will it be able to learn about the state of its receivers. In the Schmidt, Waehlisch Expires - April 2006 [Page 4] MMCASTv6-PS October 2005 event of an inter-tree handover, a mobile multicast source therefore is vulnerable to loosing receivers without taking notice. 2.2 Multicast Listener Mobility A mobile multicast listener entering a new IP subnet may encounter either one of the following conditions: The new network may not be multicast enabled or the specific multicast service in use may be unsupported or prohibited. Alternatively the requested multicast service may be supported and enabled in the new network, but the multicast groups under subscription may not be forwarded to it. Then current distribution trees for the desired groups may reside at large routing distance. It may as well occur that some or all groups under subscription of the mobile node are received by one or several local group members at the instance of arrival and that multicast streams natively flow. The problem of achieving seamless multicast listener handovers is thus threefold: o Ensure multicast reception even in visited networks without appropriate multicast support. o Expedite primary multicast forwarding to comply with a seamless timescale at handovers. o Realize native multicast forwarding whenever applicable to preserve network resources and avoid data redundancy. Additional aspects related to infrastructure remain. In changing its point of attachment a mobile receiver may not have enough time to leave groups in the previous network. Also, packet duplication and disorder may result from the change of topology. 2.3 Multicast Source Mobility 2.3.1 Any Source Multicast Mobility A node submitting data to an ASM group defines the root of either a shared or source specific delivery tree. Beside root location forwarding along this delivery tree will be bound to a topological network address due to reverse path forwarding (RPF) checks. A mobile multicast source moving away is solely enabled to either inject data into a previously established delivery tree by using its previous topologically correct source address, or to (re-)define a multicast distribution tree compliant to its new location. In pursuing the latter the mobile sender will have to proceed without control of the new tree construction due to decoupling of sender and receivers. A mobile multicast source consequently must meet address transparency at two layers: In order to comply with RPF checks, it has to use an address within the IPv6 basic header's source field, which is in Schmidt, Waehlisch Expires - April 2006 [Page 5] MMCASTv6-PS October 2005 topological accordance with the employed multicast distribution tree. For application transparency the logical node identifier, commonly the HoA, must be presented as packet's source address to the socket layer at the receiver side. Conforming to address transparency and temporal handover constraints will be the key problem for any route optimizing mobility solution. Additional issues arrive from possible packet loss and from multicast scoping. A mobile source away from home must attend scoping restrictions which arise from its home and its visited location [3]. In presence of inter- domain multicast routing a change of address must trigger the exchange of a new multicast source record. 2.3.2 Source Specific Multicast Mobility Fundamentally Source Specific Multicast has been designed for changeless addresses of multicast senders. Source addresses in client subscription to SSM groups are directly used for route identification. Any SSM subscriber is thus forced to know the topological address of its group contributors. SSM source identification invalidates, when source addresses change under mobility. Consequently source mobility for SSM packet distribution introduces a significant conceptual complexity in addition to the problems of mobile ASM. As a listener is subscribed to an (S,G) channel membership and as routers have established an (S,G)-state shortest path tree rooted at source S, any change of source addresses under mobility requests for state updates at all routers and all receivers. A moving source would have to update its change of CoA with all listeners, which subsequently had to newly subscribe and initiate corresponding source-specific trees. As the principle multicast decoupling of a sender from its receivers likewise holds for SSM, the need for client update turns into a severe problem. An SSM listener subscribing to or excluding any specific multicast source, may want to rely on the correctness of network operations. The SSM design permits trust in equivalence to the correctness of unicast routing tables. Any SSM mobility solution should preserve this degree of confidence. Binding updates for SSM sources thus should have to prove address correctness in the unicast routing sense, which is equivalent to binding update security with a correspondent node in MIPv6 [3]. All of the above severely add complexity to a robust SSM mobility solution, which should converge to optimal routes and, for the sake of efficiency, should avoid data encapsulation, as well. Like in ASM handover delays are to be considered critical. The distance of Schmidt, Waehlisch Expires - April 2006 [Page 6] MMCASTv6-PS October 2005 subsequent points of attachment, the ’step size’ of the mobile, may serve as an appropriate measure of complexity. Finally, Source Specific Multicast has been designed as a light- weight approach to group communication. In adding mobility management, it is desirable to preserve the principle leanness of SSM by minimizing additional signaling overheads. 3. Solutions Three approaches to mobility in Any Source Multicast are commonly around: o Bi-directional Tunnelling guides the mobile node to tunnel all multicast data via its home agent. This principle multicast solution hides all movement and results in static multicast trees. It transparently may be employed by mobile multicast sources, on the price of triangular routing and possibly significant performance degradations due to widely spanned data tunnels. o Remote Subscription forces the mobile node to re-initiate multicast distribution subsequent to handover, using its current Care-of Address. This approach of tree discontinuation relies on multicast dynamics to adapt to network changes. It not only results in rigorous service disruption, but leads to mobility driven changes of source addresses, and thus disregards session persistence under multicast source mobility. o Agent-based solutions attempt to balance between the previous two mechanisms. Static agents typically act as local tunnelling proxies, allowing for some inter-agent handover while the mobile node moves away. A decelerated inter-tree handover, i.e. tree walking, will be the outcome of agent-based multicast mobility, where some extra effort is needed to sustain session persistence through address transparency of mobile sources. There are proposals of agent based approaches compliant to the unicast real-time mobility infrastructure of Fast MIPv6 [15], the M- FMIPv6 [16], and of Hierarchical MIPv6 [17], the M-HMIPv6 [18], and to context transfer [19]. An approach based on dynamically negotiated inter-agent handovers is presented in [20]. It should be noted that none of the above approaches addresses SSM source mobility, except the bi-directional tunnelling. 4. Security Considerations This document discusses multicast extensions to mobility. Security issues arise from source address binding updates, specifically in the Schmidt, Waehlisch Expires - April 2006 [Page 7] MMCASTv6-PS October 2005 case of source specific multicast. Threats of hijacking unicast sessions will result from any solution jointly operating binding updates for unicast and multicast sessions. Future solutions must address the security implications. 5. IANA Considerations There are no IANA considerations introduced by this draft. 6. References Normative References 1 Bradner, S., "Intellectual Property Rights in IETF Technology", BCP 79, RFC 3979, March 2005. 2 Hinden, R. and Deering, S. "Internet Protocol Version 6 Specification", RFC 2460, December 1998. 3 Johnson, D.B., Perkins, C., Arkko, J. "Mobility Support in IPv6", RFC 3775, June 2004. 4 Arkko, J., Devarapalli, V., Dupont, F "Using IPsec to Protect Mobile IPv6 Signaling Between Mobile Nodes and Home Agents", RFC 3776, June 2004. 5 S. Deering "Host Extensions for IP Multicasting", RFC 1112, August 1989. 6 S. Bhattacharyya "An Overview of Source-Specific Multicast (SSM)", RFC 3569, July 2003. 7 H. Holbrook, B. Cain "Source-Specific Multicast for IP", draft- ietf-ssm-arch-07.txt (work in progress), October 2005. 8 D. Waitzman, C. Partridge, S.E. Deering "Distance Vector Multicast Routing Protocol", RFC 1075, November 1988. 9 D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering, M. Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC 2362, June 1998. 10 B. Fenner, M. Handley, H. Holbrook, I. Kouvelas: "Protocol Independent Multicast - Sparse Mode PIM-SM): Protocol Specification(Revised)", draft-ietf-pim-sm-v2-new-11.txt (work in progress), October 2004. Schmidt, Waehlisch Expires - April 2006 [Page 8] MMCASTv6-PS October 2005 11 A. Ballardie " Core Based Trees (CBT version 2) Multicast Routing", RFC 2189, September 1997. 12 S. Deering, W. Fenner and B. Haberman "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999. 13 R. Vida and L. Costa (Eds.) "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC3810, June 2004. 14 P. Savola, B. Haberman "Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address", RFC 3956, November 2004. 15 Koodli, R. "Fast Handovers for Mobile IPv6", RFC 4068, July 2004. 16 Suh, K., Kwon, D.-H., Suh, Y.-J. and Park, Y. "Fast Multicast Protocol for Mobile IPv6 in the fast handovers environments", Internet Draft - (work in progress, expired), February 2004. 17 Soliman, H., Castelluccia, C., El-Malki, K., Bellier, L. "Hierarchical Mobile IPv6 mobility management", RFC 4140, August 2005. 18 Schmidt, T.C. and Waehlisch, M. "Seamless Multicast Handover in a Hierarchical Mobile IPv6 Environment(M-HMIPv6)", draft-schmidt- waehlisch-mhmipv6-03.txt, (work in progress), April 2005. 19 Jonas, K. and Miloucheva, I. "Multicast Context Transfer in mobile IPv6", draft-miloucheva-mldv2-mipv6-00.txt, (work in progress), June 2005. 20 Zhang, H. et al "Mobile IPv6 Multicast with Dynamic Multicast Agent" draft-zhang-mipshop-multicast-dma-01.txt, (work in progress), September 2005. Informative References 21 Romdhani, I., Kellil, M., Lach, H.-Y. et. al. "IP Mobile Multicast: Challenges and Solutions", IEEE Comm. Surveys, 6, 1, 2004. 22 Jannetau, C., Tian, Y., Csaba, S. et al "Comparison of Three Approaches Towards Mobile Multicast", IST Mobile Summit 2003, Aveiro, Portugal, 16-18 June 2003, online http://www.comnets.rwth- aachen.de/~o_drive/publications/ist-summit-2003-IPMobileMulticast- paperv2.0.pdf. Schmidt, Waehlisch Expires - April 2006 [Page 9] MMCASTv6-PS October 2005 23 Mieghem, P., Hooghiemstra, G., Hofstad, R. "On the Efficiency of Multicast", Transactions on Networking, 9, 6, pp. 719 - 732, December 2001. 24 Schmidt, T.C. and Waehlisch, M. "Predictive versus Reactive - Analysis of Handover Performance and Its Implications on IPv6 and Multicast Mobility", Telecommunication Systems, 30:1/2/3, Springer 2005, in print. 25 Jelger, C., Noel, T. "Multicast for Mobile Hosts in IP Networks: Progress and Challenges", IEEE Wireless Comm., pp 58-64, Oct. 2002. Acknowledgments The authors would like to thank Mark Palkow (DaViKo GmbH) and Hans L. Cycon (FHTW Berlin) for valuable discussions and a joyful collaboration. Author's Addresses Thomas C. Schmidt HAW Hamburg, Dept. Informatik Berliner Tor 7 D-20099 Hamburg Phone: +49-40-42875-8157 Email: Schmidt@informatik.haw-hamburg.de Matthias Waehlisch FHTW Berlin, HRZ Treskowallee 8 D-10318 Berlin Email: mw@fhtw-berlin.de 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 Schmidt, Waehlisch Expires - April 2006 [Page 10] MMCASTv6-PS October 2005 on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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