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Internet Research Task Force (IRTF)                           T. SchmidtRequest for Comments: 5757                                   HAW HamburgCategory: Informational                                     M. WaehlischISSN: 2070-1721                                                 link-lab                                                            G. Fairhurst                                                  University of Aberdeen                                                           February 2010Multicast Mobility in Mobile IP Version 6 (MIPv6):Problem Statement and Brief SurveyAbstract   This document discusses current mobility extensions to IP-layer   multicast.  It describes problems arising from mobile group   communication in general, the case of multicast listener mobility,   and problems for mobile senders using Any Source Multicast and   Source-Specific Multicast.  Characteristic aspects of multicast   routing and deployment issues for fixed IPv6 networks are summarized.   Specific properties and interplays with the underlying network access   are surveyed with respect to the relevant technologies in the   wireless domain.  It outlines the principal approaches to multicast   mobility, together with a comprehensive exploration of the mobile   multicast problem and solution space.  This document concludes with a   conceptual road map for initial steps in standardization for use by   future mobile multicast protocol designers.  This document is a   product of the IP Mobility Optimizations (MobOpts) Research Group.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Research Task Force   (IRTF).  The IRTF publishes the results of Internet-related research   and development activities.  These results might not be suitable for   deployment.  This RFC represents the consensus of the MobOpts   Research Group of the Internet Research Task Force (IRTF).  Documents   approved for publication by the IRSG are not a candidate for any   level of Internet Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5757.Schmidt, et al.               Informational                     [Page 1]

RFC 5757                       MMCASTv6-PS                 February 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.Schmidt, et al.               Informational                     [Page 2]

RFC 5757                       MMCASTv6-PS                 February 2010Table of Contents1. Introduction and Motivation .....................................41.1. Document Scope .............................................52. Problem Description .............................................62.1. General Issues .............................................62.2. Multicast Listener Mobility ................................92.2.1. Node and Application Perspective ....................92.2.2. Network Perspective ................................102.3. Multicast Source Mobility .................................112.3.1. Any Source Multicast Mobility ......................112.3.2. Source-Specific Multicast Mobility .................122.4. Deployment Issues .........................................133. Characteristics of Multicast Routing Trees under Mobility ......144. Link Layer Aspects .............................................154.1. General Background ........................................154.2. Multicast for Specific Technologies .......................164.2.1. 802.11 WLAN ........................................164.2.2. 802.16 WIMAX .......................................164.2.3. 3GPP/3GPP2 .........................................184.2.4. DVB-H / DVB-IPDC ...................................194.2.5. TV Broadcast and Satellite Networks ................194.3. Vertical Multicast Handovers ..............................205. Solutions ......................................................205.1. General Approaches ........................................205.2. Solutions for Multicast Listener Mobility .................215.2.1. Agent Assistance ...................................215.2.2. Multicast Encapsulation ............................225.2.3. Hybrid Architectures ...............................235.2.4. MLD Extensions .....................................235.3. Solutions for Multicast Source Mobility ...................245.3.1. Any Source Multicast Mobility Approaches ...........245.3.2. Source-Specific Multicast Mobility Approaches ......256. Security Considerations ........................................267. Summary and Future Steps .......................................27Appendix A. Implicit Source Notification Options...................29   Informative References.............................................29   Acknowledgments....................................................37Schmidt, et al.               Informational                     [Page 3]

RFC 5757                       MMCASTv6-PS                 February 20101.  Introduction and Motivation   Group communication forms an integral building block of a wide   variety of applications, ranging from content broadcasting and   streaming, voice and video conferencing, collaborative environments   and massive multiplayer gaming, up to the self-organization of   distributed systems, services, or autonomous networks.  Network-layer   multicast support will be needed whenever globally distributed,   scalable, serverless, or instantaneous communication is required.   The early idea of Internet multicasting [1] soon led to a wide   adoption of Deering's host group model [2].  Broadband media delivery   is emerging as a typical mass scenario that demands scalability and   bandwidth efficiency from multicast routing.  Although multicast   mobility has been a concern for about ten years [3] and has led to   numerous proposals, there is as yet no generally accepted solution.   Multicast network support will be of particular importance to mobile   environments, where users commonly share frequency bands of limited   capacity.  Reception of "infotainment" streams may soon require wide   deployment of mobile multicast services.   Mobility in IPv6 [4] is standardized in the Mobile IPv6 RFCs [5][6],   and it addresses the scenario of network-layer changes while moving   between wireless domains.  MIPv6 [5] only roughly defines multicast   mobility for Mobile Nodes (MNs) using a remote subscription approach   or through bidirectional tunneling via the Home Agent (HA).  Remote   subscription suffers from slow handovers relying on multicast routing   to adapt to handovers.  Bidirectional tunneling introduces   inefficient overhead and delay due to triangular forwarding, i.e.,   instead of traveling on shortest paths, packets are routed through   the Home Agent.  Therefore, these approaches have not been optimized   for a large scale deployment.  A mobile multicast service for a   future Internet should provide "close-to-optimal" routing at   predictable and limited cost, offering robustness combined with a   service quality compliant to real-time media distribution.   Intricate multicast routing procedures are not easily extensible to   satisfy the requirements for mobility.  A client subscribed to a   group while performing mobility handovers requires the multicast   traffic to follow to its new location; a mobile source needs the   entire delivery tree to comply with or to adapt to its changing   position.  Significant effort has already been invested in protocol   designs for mobile multicast receivers; only limited work has been   dedicated to multicast source mobility, which poses the more delicate   problem [65].Schmidt, et al.               Informational                     [Page 4]

RFC 5757                       MMCASTv6-PS                 February 2010   In multimedia conference scenarios, games, or collaborative   environments, each member commonly operates as a receiver and as a   sender for multicast group communication.  In addition, real-time   communication such as conversational voice or video places severe   temporal requirements on mobility protocols: Typical seamless   handover scenarios are expected to limit disruptions or delay to less   than 100 - 150 ms [7].  Jitter disturbances should not exceed 50 ms.   Note that 100 ms is about the duration of a spoken syllable in real-   time audio.  This problem statement is intended to also be applicable   to a range of other scenarios with a range of delivery requirements   appropriate to the general Internet.   This document represents the consensus of the MobOpts Research Group.   It has been reviewed by the Research Group members active in the   specific area of work.  In addition, this document has been   comprehensively reviewed by multiple active contributors to the IETF   MEXT, MBONED, and PIM Working Groups.1.1.  Document Scope   This document defines the problem scope for multicast mobility   management, which may be elaborated in future work.  It is subdivided   to present the various challenges according to their originating   aspects, and identifies existing proposals and major bibliographic   references.   When considering multicast node mobility, the network layer is   complemented by some wireless access technology.  Two basic scenarios   are of interest: single-hop mobility (shown in Figure 1.a) and multi-   hop mobility (shown in Figure 1.b).  Single-hop mobility is the focus   of this document, which coincides with the perspective of MIPv6 [5].   The key issues of mobile multicast membership control and the   interplay of mobile and multicast routing will be illustrated using   this simple scenario.   Multi-hop network mobility is a subsidiary scenario.  All major   aspects are inherited from the single-hop problem, while additional   complexity is incurred from traversing a mobile cloud.  This may be   solved by either encapsulation or flooding ([8] provides a general   overview).  Specific issues arising from (nested) tunneling or   flooding, especially the preservation of address transparency,   require treatment analogous to MIPv6.Schmidt, et al.               Informational                     [Page 5]

RFC 5757                       MMCASTv6-PS                 February 2010                                       +------+           +------+                                       |  MN  |  =====>   |  MN  |                                       +------+           +------+                                          |                  .                                          |                  .                                          |                  .                                       +-------+          +-------+                                       | LAR 1 |          | LAR 2 |                                       +-------+          +-------+                                                \        /                                            ***  ***  ***  ***                                           *   **   **   **   *   +------+           +------+            *                    *   |  MN  |  =====>   |  MN  |             *  Mobile Network  *   +------+           +------+            *                    *      |                  .                 *   **   **   **   *      |                  .                  ***  ***  ***  ***      |                  .                  |                 .   +-------+          +-------+         +-------+          +-------+   | AR 1  |          | AR 2  |         | AR 1  |  =====>  | AR 2  |   +-------+          +-------+         +-------+          +-------+       |                |                   |                |       ***  ***  ***  ***                   ***  ***  ***  ***      *   **   **   **   *                 *   **   **   **   *     *                    *               *                    *      *  Fixed Internet  *                 *  Fixed Internet  *     *                    *               *                    *      *   **   **   **   *                 *   **   **   **   *       ***  ***  ***  ***                   ***  ***  ***  ***     a) Single-Hop Mobility                  b) Multi-Hop Mobility   Figure 1: Mobility Scenarios - A Mobile Node (MN) Directly Attaching   to Fixed Access Routers (ARs) or Attached via Local Access Routers   (LARs)2.  Problem Description2.1.  General Issues   Multicast mobility is a generic term, which subsumes a collection of   distinct functions.  First, the multicast communication is divided   into Any Source Multicast (ASM) [2] and Source-Specific Multicast   (SSM) [9][10].  Second, the roles of senders and receivers are   distinct and asymmetric.  Both may individually be mobile.  Their   interaction is facilitated by a multicast routing protocol such as   the Distance Vector Multicast Routing Protocol (DVMRP) [11], theSchmidt, et al.               Informational                     [Page 6]

RFC 5757                       MMCASTv6-PS                 February 2010   Protocol Independent Multicast - Sparse Mode / Source-Specific   Multicast (PIM-SM/SSM) [12][13], the Bidirectional PIM [14], or the   inter-domain multicast prefix advertisements via Multiprotocol   Extensions for BGP-4 (MBGP) [15].  IPv6 clients interact using the   multicast listener discovery protocol (MLD and MLDv2) [16][17].   Any solution for multicast mobility needs to take all of these   functional blocks into account.  It should enable seamless continuity   of multicast sessions when moving from one IPv6 subnet to another.   It is desired to preserve the multicast nature of packet distribution   and approximate optimal routing.  It should support per-flow handover   for multicast traffic because the properties and designations of   flows can be distinct.  Such distinctions may result from differing   Quality-of-Service (QoS) / real-time requirements, but may also be   caused by network conditions that may differ for different groups.   The host group model extends the capability of the network-layer   unicast service.  In common with the architecture of fixed networks,   multicast mobility management should transparently utilize or   smoothly extend the unicast functions of MIPv6 [5], its security   extensions [6][18], its expediting schemes FMIPv6 [19] and   Hierarchical Mobile IPv6 Environment (HMIPv6) [20], its context   transfer protocols [21], its multihoming capabilities [22][23],   emerging protocols like PMIPv6 [62], or future developments.  From   the perspective of an integrated mobility architecture, it is   desirable to avoid multicast-specific as well as unicast-restricted   solutions, whenever general approaches can be derived that can   jointly support unicast and multicast.   Multicast routing dynamically adapts to the network topology at the   locations of the sender(s) and receiver(s) participating in a   multicast session, which then may change under mobility.  However,   depending on the topology and the protocol in use, current multicast   routing protocols may require a time close to seconds to converge   following a change in receiver or sender location.  This is far too   slow to support seamless handovers for interactive or real-time media   sessions.  The actual temporal behavior strongly depends on the   multicast routing protocol in use, the configuration of routers, and   on the geometry of the current distribution tree.  A mobility scheme   that readjusts routing, i.e., partially changes or fully reconstructs   a multicast tree, is forced to comply with the time scale for   protocol convergence.  Specifically, it needs to consider a possible   rapid movement of the mobile node, as this may occur at much higher   rates than common protocol state updates.   The mobility of hosts using IP multicast can impact the service   presented to the higher-layer protocols.  IP-layer multicast packet   distribution is an unreliable service that is bound to aSchmidt, et al.               Informational                     [Page 7]

RFC 5757                       MMCASTv6-PS                 February 2010   connectionless transport service.  Where applications are sensitive   to packet loss or jitter, countermeasures need to be performed (loss   recovery, content recoding, concealment, etc.) by the multicast   transport or application.  Mobile multicast handovers should not   introduce significant additional packet drops.  Due to statelessness,   the bi-casting of multicast flows does not cause degradations at the   transport layer, and applications should implement mechanisms to   detect and correctly respond to duplicate datagrams.  Nevertheless,   individual application programs may not be robust with respect to   repeated reception of duplicate streams.   IP multicast applications can be designed to adapt the multicast   stream to prevailing network conditions (adapting the sending rate to   the level of congestion, adaptive tuning of clients in response to   measured delay, dynamic suppression of feedback messages, etc.).  An   adaptive application may also use more than one multicast group   (e.g., layered multicast in which a client selects a set of multicast   groups based on perceived available network capacity).  A mobility   handover may temporarily disrupt the operation of these higher-layer   functions.  The handover can invalidate assumptions about the   forwarding path (e.g., acceptable delivery rate, round-trip delay),   which could impact an application and level of network traffic.  Such   effects need to be considered in the design of multicast applications   and in the design of network-layer mobility.  Specifically, mobility   mechanisms need to be robust to transient packet loss that may result   from invalid path expectations following a handover of an MN to a   different network.   Group addresses, in general, are location transparent, even though   they may be scoped and methods can embed unicast prefixes or   Rendezvous Point addresses [24].  The addresses of sources   contributing to a multicast session are interpreted by the routing   infrastructure and by receiver applications, which frequently are   aware of source addresses.  Multicast therefore inherits the mobility   address duality problem of MIPv6 for source addresses: addresses   being a logical node identifier, i.e., the home address (HoA) on the   one hand, and a topological locator, the care-of address (CoA), on   the other.  At the network layer, the elements that comprise the   delivery tree, i.e., multicast senders, forwarders, and receivers,   need to carefully account for address duality issues, e.g., by using   binding caches, extended multicast states, or signaling.   Multicast sources, in general, operate decoupled from their receivers   in the following sense: a multicast source sends packets to a group   of receivers that are unknown at the network layer and thus operates   without a feedback channel.  It neither has means to inquire about   the properties of its delivery trees, nor the ability to learn about   the network-layer state of its receivers.  In the event of an inter-Schmidt, et al.               Informational                     [Page 8]

RFC 5757                       MMCASTv6-PS                 February 2010   tree handover, a mobile multicast source therefore is vulnerable to   losing connectivity to receivers without noticing.  (Appendix A   describes implicit source notification approaches).  Applying a MIPv6   mobility binding update or return routability procedure will   similarly break the semantic of a receiver group remaining   unidentified by the source and thus cannot be applied in unicast   analogy.   Despite the complexity of the requirements, multicast mobility   management should seek lightweight solutions with easy deployment.   Realistic, sample deployment scenarios and architectures should be   provided in future solution documents.2.2.  Multicast Listener Mobility2.2.1.  Node and Application Perspective   A mobile multicast listener entering a new IP subnet requires   multicast reception following a handover in real-time.  This needs to   transfer the multicast membership context from its old to its new   point of attachment.  This can either be achieved by   (re-)establishing a tunnel or by transferring the MLD Listening State   information of the MN's moving interface(s) to the new upstream   router(s).  In the latter case, it may encounter any one of the   following conditions:      o In the simplest scenario, packets of some, or all, of the        subscribed groups of the mobile node are already received by one        or several other group members in the new network, and thus        multicast streams natively flow after the MN arrives at the new        network.      o The requested multicast service may be supported and enabled in        the visited network, but the multicast groups under subscription        may not be forwarded to it, e.g., groups may be scoped or        administratively prohibited.  This means that current        distribution trees for the desired groups may only be re-joined        at a (possibly large) routing distance.      o The new network may not be multicast-enabled or the specific        multicast service may be unavailable, e.g., unsupported or        prohibited.  This means that current distribution trees for the        desired groups need to be re-joined at a large routing distance        by (re-)establishing a tunnel to a multicast-enabled network        node.   The problem of achieving seamless multicast listener handovers is   thus threefold:Schmidt, et al.               Informational                     [Page 9]

RFC 5757                       MMCASTv6-PS                 February 2010      o Ensure multicast reception, even in visited networks, without        appropriate multicast support.      o Minimize multicast forwarding delay to provide seamless and fast        handovers for real-time services.  Dependent on Layer 2 (L2) and        Layer 3 (L3) handover performance, the time available for        multicast mobility operations is typically bound by the total        handover time left after IPv6 connectivity is regained.  In        real-time scenarios, this may be significantly less than 100 ms.      o Minimize packet loss and reordering that result from multicast        handover management.   Moreover, in many wireless regimes, it is also desirable to minimize   multicast-related signaling to preserve the limited resources of   battery-powered mobile devices and the constrained transmission   capacities of the networks.  This may lead to a desire to restrict   MLD queries towards the MN.  Multihomed MNs may ensure smooth   handoffs by using a "make-before-break" approach, which requires a   per-interface subscription, facilitated by an MLD JOIN operating on a   pre-selected IPv6 interface.   Encapsulation on the path between the upstream router and the   receiver may result in MTU size conflicts, since path-MTU discovery   is often not supported for multicast and can reduce scalability in   networks with many different MTU sizes or introduce potential denial-   of-service vulnerabilities (since the originating addresses of ICMPv6   messages cannot be verified for multicast).  In the absence of   fragmentation at tunnel entry points, this may prevent the group from   being forwarded to the destination.2.2.2.  Network Perspective   The infrastructure providing multicast services is required to keep   traffic following the MN without compromising network functionality.   Mobility solutions thus have to face some immediate problems:      o Realize native multicast forwarding, and where applicable,        conserve network resources and utilize link-layer multipoint        distribution to avoid data redundancy.      o Activate link-multipoint services, even if the MN performs only        a L2/vertical handover.      o Ensure routing convergence, even when the MN moves rapidly and        performs handovers at a high frequency.Schmidt, et al.               Informational                    [Page 10]

RFC 5757                       MMCASTv6-PS                 February 2010      o Avoid avalanche problems and stream multiplication (n-casting),        which potentially result from replicated tunnel initiation or        redundant forwarding at network nodes.   There are additional implications for the infrastructure: In changing   its point of attachment, an exclusive mobile receiver may initiate   forwarding of a group in the new network and termination of a group   distribution service in the previous network.  Mobility management   may impact multicast routing by, e.g., erroneous subscriptions   following predictive handover operations, or slow traffic termination   at leaf nodes resulting from MLD query timeouts, or by departure of   the MN from a previous network without leaving the subscribed groups.   Finally, packet duplication and reordering may follow a change of   topology.2.3.  Multicast Source Mobility2.3.1.  Any Source Multicast Mobility   A node submitting data to an ASM group either forms the root of a   source-specific shortest path tree (SPT), distributing data towards a   rendezvous point (RP) or receivers, or it forwards data directly down   a shared tree, e.g., via encapsulated PIM Register messages, or using   bidirectional PIM routing.  Native forwarding along source-specific   delivery trees will be bound to the source's topological network   address, due to reverse path forwarding (RPF) checks.  A mobile   multicast source moving to a new subnetwork is only able to either   inject data into a previously established delivery tree, which may be   a rendezvous-point-based shared tree, or to (re-)initiate the   construction of a multicast distribution tree for its new network   location.  In the latter case, the mobile sender will have to proceed   without knowing whether the new tree has regained ability to forward   traffic to the group, due to the decoupling of sender and receivers.   A mobile multicast source must therefore provide address transparency   at two layers: To comply with RPF checks, it has to use an address   within the source field of the IPv6 basic header, which is in   topological agreement with the employed multicast distribution tree.   For application transparency, the logical node identifier, commonly   the HoA, must be presented as the packet source address to the   transport layer at the receiver side.   The address transparency and temporal handover constraints pose major   problems for route-optimizing mobility solutions.  Additional issues   arise from possible packet loss and from multicast scoping.  A mobile   source away from home must respect scoping restrictions that arise   from its home and its visited location [5].Schmidt, et al.               Informational                    [Page 11]

RFC 5757                       MMCASTv6-PS                 February 2010   Intra-domain multicast routing may allow the use of shared trees that   can reduce mobility-related complexity.  A static rendezvous point   may allow a mobile source to continuously send data to the group by   encapsulating packets to the RP with its previous topologically   correct or home source address.  Intra-domain mobility is   transparently provided by bidirectional shared domain-spanning trees,   when using bidirectional PIM, eliminating the need for tunneling to   the corresponding RP (in contrast to IPv4, IPv6 ASM multicast groups   are associated with a specific RP/RPs).   Issues arise in inter-domain multicast, whenever notification of   source addresses is required between distributed instances of shared   trees.  A new CoA acquired after a mobility handover will necessarily   be subject to inter-domain record exchange.  In the presence of an   embedded rendezvous point address [24], e.g., the primary rendezvous   point for inter-domain PIM-SM will be globally appointed, and a newly   attached mobile source can contact the RP without prior signaling   (like a new source) and transmit data in the PIM register tunnel.   Multicast route optimization (e.g., PIM "shortcuts") will require   multicast routing protocol operations equivalent to serving a new   source.2.3.2.  Source-Specific Multicast Mobility   Source-Specific Multicast has been designed for multicast senders   with static source addresses.  The source addresses in a client   subscription to an SSM group is directly used to route   identification.  Any SSM subscriber is thus forced to know the   topological address of the contributor to the group it wishes to   join.  The SSM source identification becomes invalid when the   topological source address changes under mobility.  Hence, client   implementations of SSM source filtering must be MIPv6 aware in the   sense that a logical source identifier (HoA) is correctly mapped to   its current topological correspondent (CoA).   As a consequence, source mobility for SSM requires a conceptual   treatment beyond the problem scope of mobile ASM.  A listener   subscribes to an (S,G) channel membership and routers establish an   (S,G)-state shortest path tree rooted at source S; therefore, any   change of source addresses under mobility requires state updates at   all routers on the upstream path and at all receivers in the group.   On source handover, a new SPT needs to be established that will share   paths with the previous SPT, e.g., at the receiver side.  As the   principle of multicast decoupling of a sender from its receivers   holds for SSM, the client updates needed for switching trees become a   severe burden.Schmidt, et al.               Informational                    [Page 12]

RFC 5757                       MMCASTv6-PS                 February 2010   An SSM listener may subscribe to or exclude any specific multicast   source and thereby wants to rely on the topological 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 [5].   The above methods could add significant complexity to a solution for   robust SSM mobility, which needs to converge to optimal routes and,   for efficiency, is desired to avoid data encapsulation.  Like ASM,   handover management is a time-critical operation.  The routing   distance between subsequent points of attachment, the "step size" of   the mobile from previous to next designated router, may serve as an   appropriate measure of complexity [25][26].   Finally, Source-Specific Multicast has been designed as a lightweight   approach to group communication.  In adding mobility management, it   is desirable to preserve the leanness of SSM by minimizing additional   signaling overhead.2.4.  Deployment Issues   IP multicast deployment, in general, has been slow over the past 15   years, even though all major router vendors and operating systems   offer implementations that support multicast [27].  While many   (walled) domains or enterprise networks operate point-to-multipoint   services, IP multicast roll-out is currently limited in public inter-   domain scenarios [28].  A dispute arose on the appropriate layer,   where group communication service should reside, and the focus of the   research community turned towards application-layer multicast.  This   debate on "efficiency versus deployment complexity" now overlaps the   mobile multicast domain [29].  Garyfalos and Almeroth [30] derived   from fairly generic principles that when mobility is introduced, the   performance gap between IP- and application-layer multicast widens in   different metrics up to a factor of four.   Facing deployment complexity, it is desirable that any solution for   mobile multicast does not change the routing protocols.  Mobility   management in such a deployment-friendly scheme should preferably be   handled at edge nodes, preserving a mobility-agnostic routing   infrastructure.  Future research needs to search for such simple,   infrastructure-transparent solutions, even though there are   reasonable doubts as to whether this can be achieved in all cases.Schmidt, et al.               Informational                    [Page 13]

RFC 5757                       MMCASTv6-PS                 February 2010   Nevertheless, multicast services in mobile environments may soon   become indispensable, when multimedia distribution services such as   Digital Video Broadcasting for Handhelds (DVB-H) [31][32] or IPTV   develop a strong business case for portable IP-based devices.  As IP   mobility becomes an important service and as efficient link   utilization is of a larger impact in costly radio environments, the   evolution of multicast protocols will naturally follow mobility   constraints.3.  Characteristics of Multicast Routing Trees under Mobility   Multicast distribution trees have been studied from a focus of   network efficiency.  Grounded on empirical observations, Chuang and   Sirbu [33] proposed a scaling power-law for the total number of links   in a multicast shortest path tree with m receivers (proportional to   m^k).  The authors consistently identified the scale factor to attain   the independent constant k = 0.8.  The validity of such universal,   heavy-tailed distribution suggests that multicast shortest path trees   are of self-similar nature with many nodes of small, but few of   higher degrees.  Trees consequently would be shaped tall rather than   wide.   Subsequent empirical and analytical work [34][35] debated the   applicability of the Chuang and Sirbu scaling law.  Van Mieghem et   al. [34] proved that the proposed power law cannot hold for an   increasing Internet or very large multicast groups, but is indeed   applicable for moderate receiver numbers and the current Internet   size of N = 10^5 core nodes.  Investigating self-similarity, Janic   and Van Mieghem [36] semi-empirically substantiated that multicast   shortest path trees in the Internet can be modeled with reasonable   accuracy by uniform recursive trees (URTs) [37], provided m remains   small compared to N.   The mobility perspective on shortest path trees focuses on their   alteration, i.e., the degree of topological changes induced by   movement.  For receivers, and more interestingly for sources, this   may serve as a characteristic measure of the routing complexity.   Mobile listeners moving to neighboring networks will only alter tree   branches extending over a few hops.  Source-specific multicast trees   subsequently generated from source handover steps are not   independent, but highly correlated.  They most likely branch to   identical receivers at one or several intersection points.  By the   self-similar nature, the persistent sub-trees (of previous and next   distribution tree), rooted at any such intersection point, exhibit   again the scaling law behavior, are tall-shaped with nodes of mainly   low degree and thus likely to coincide.  Tree alterations under   mobility have been studied in [26], both analytically and bySchmidt, et al.               Informational                    [Page 14]

RFC 5757                       MMCASTv6-PS                 February 2010   simulations.  It was found that even in large networks and for   moderate receiver numbers more than 80% of the multicast router   states remain invariant under a source handover.4.  Link-Layer Aspects4.1.  General Background   Scalable group data distribution has the highest potential in edge   networks, where large numbers of end systems reside.  Consequently,   it is not surprising that most LAN network access technologies   natively support point-to-multipoint or multicast services.  Wireless   access technologies inherently support broadcast/multicast at L2 and   operate on a shared medium with limited frequency and bandwidth.   Several aspects need consideration: First, dissimilar network access   radio technologies cause distinct group traffic transmissions.  There   are:      o connection-less link services of a broadcast type, which mostly        are bound to limited reliability;      o connection-oriented link services of a point-to-multipoint type,        which require more complex control and frequently exhibit        reduced efficiency;      o connection-oriented link services of a broadcast type, which are        restricted to unidirectional data transmission.   In addition, multicast may be distributed via multiple point-to-point   unicast links without the use of a dedicated multipoint radio   channel.  A fundamental difference between unicast and group   transmission arises from power management.  Some radio technologies   adjust transmit power to be as small as possible based on link-layer   feedback from the receiver, which is not done in multipoint mode.   They consequently incur a "multicast tax", making multicast less   efficient than unicast unless the number of receivers is larger than   some threshold.   Second, point-to-multipoint service activation at the network access   layer requires a mapping mechanism from network-layer requests.  This   function is commonly achieved by L3 awareness, i.e., IGMP/MLD   snooping [70] or proxy [38], which occasionally is complemented by   Multicast VLAN Registration (MVR).  MVR allows sharing of a single   multicast IEEE 802.1Q Virtual LAN in the network, while subscribers   remain in separate VLANs.  This L2 separation of multicast and   unicast traffic can be employed as a workaround for point-to-point   link models to establish a common multicast link.Schmidt, et al.               Informational                    [Page 15]

RFC 5757                       MMCASTv6-PS                 February 2010   Third, an address mapping between the layers is needed for common   group identification.  Address resolution schemes depend on framing   details for the technologies in use, but commonly cause a significant   address overlap at the lower layer (i.e., more than one IP multicast   group address is sent using the same L2 address).4.2.  Multicast for Specific Technologies4.2.1.  802.11 WLAN   IEEE 802.11 Wireless Local Area Network (WLAN) is a broadcast network   of Ethernet type.  This inherits multicast address mapping concepts   from 802.3.  In infrastructure mode, an access point operates as a   repeater, only bridging data between the Base (BSS) and the Extended   Service Set (ESS).  A mobile node submits multicast data to an access   point in point-to-point acknowledged unicast mode (when the ToDS bit   is set).  An access point receiving multicast data from an MN simply   repeats multicast frames to the BSS and propagates them to the ESS as   unacknowledged broadcast.  Multicast frames received from the ESS   receive similar treatment.   Multicast frame delivery has the following characteristics:      o As an unacknowledged service, it offers limited reliability.        The loss of frames (and hence packets) arises from interference,        collision, or time-varying channel properties.      o Data distribution may be delayed, as unicast power saving        synchronization via Traffic Indication Messages (TIM) does not        operate in multicast mode.  Access points buffer multicast        packets while waiting for a larger Delivery TIM (DTIM) interval,        whenever stations use the power saving mode.      o Multipoint data may cause congestion, because the distribution        system floods multicast, without further control.  All access        points of the same subnet replicate multicast frames.   To limit or prevent the latter, many vendors have implemented a   configurable rate limit for forwarding multicast packets.   Additionally, an IGMP/MLD snooping or proxy may be active at the   bridging layer between the BSS and the ESS or at switches   interconnecting access points.4.2.2.  802.16 WIMAX   IEEE 802.16 Worldwide Interoperability for Microwave Access (WIMAX)   combines a family of connection-oriented radio transmission services   that can operate in single-hop point-to-multipoint (PMP) or in meshSchmidt, et al.               Informational                    [Page 16]

RFC 5757                       MMCASTv6-PS                 February 2010   mode.  The latter does not support multipoint transmission and   currently has no deployment.  PMP operates between Base and   Subscriber Stations in distinguished, unidirectional channels.  The   channel assignment is controlled by the Base Station, which assigns   channel IDs (CIDs) within service flows to the Subscriber Stations.   Service flows may provide an optional Automatic Repeat Request (ARQ)   to improve reliability and may operate in point-to-point or point-to-   multipoint (restricted to downlink and without ARQ) mode.   A WIMAX Base Station operates as a full-duplex L2 switch, with   switching based on CIDs.  Two IPv6 link models for mobile access   scenarios exist: A shared IPv6 prefix for IP over Ethernet Circuit   Switched (CS) [39] provides Media Access Control (MAC) separation   within a shared prefix.  A second, point-to-point link model [40] is   recommended in the IPv6 Convergence Sublayer [41], which treats each   connection to a mobile node as a single link.  The point-to-point   link model conflicts with a consistent group distribution at the IP   layer when using a shared medium (cf.Section 4.1 for MVR as a   workaround).   To invoke a multipoint data channel, the base station assigns a   common CID to all Subscriber Stations in the group.  An IPv6   multicast address mapping to these 16-bit IDs is proposed by copying   either the 4 lowest bits, while sustaining the scope field, or by   utilizing the 8 lowest bits derived from Multicast on Ethernet CS   [42].  For selecting group members, a Base Station may implement   IGMP/MLD snooping or proxy as foreseen in 802.16e-2005 [43].   A Subscriber Station multicasts IP packets to a Base Station as a   point-to-point unicast stream.  When the IPv6 CS is used, these are   forwarded to the upstream access router.  The access router (or the   Base Station for IP over Ethernet CS) may send downstream multicast   packets by feeding them to the multicast service channel.  On   reception, a Subscriber Station cannot distinguish multicast from   unicast streams at the link layer.   Multicast services have the following characteristics:      o Multicast CIDs are unidirectional and available only in the        downlink direction.  Thus, a native broadcast-type forwarding        model is not available.      o The mapping of multicast addresses to CIDs needs        standardization, since different entities (Access Router, Base        Station) may have to perform the mapping.Schmidt, et al.               Informational                    [Page 17]

RFC 5757                       MMCASTv6-PS                 February 2010      o CID collisions for different multicast groups may occur due to        the short ID space.  This can result in several point-to-        multipoint groups sharing the same CID, reducing the ability of        a receiver to filter unwanted L2 traffic.      o The point-to-point link model for mobile access contradicts a        consistent mapping of IP-layer multicast onto 802.16 point-to-        multipoint services.      o Multipoint channels cannot operate ARQ service and thus        experience a reduced reliability.4.2.3.  3GPP/3GPP2   The 3rd Generation Partnership Project (3GPP) System architecture   spans a circuit switched (CS) and a packet-switched (PS) domain, the   latter General Packet Radio Services (GPRS) incorporates the IP   Multimedia Subsystem (IMS) [44].  The 3GPP PS is connection-oriented   and based on the concept of Packet Data Protocol (PDP) contexts.   PDPs define point-to-point links between the Mobile Terminal and the   Gateway GPRS Support Node (GGSN).  Internet service types are PPP,   IPv4, and IPv6, where the recommendation for IPv6 address assignment   associates a prefix to each (primary) PDP context [45].   In Universal Mobile Telecommunications System (UMTS) Rel. 6, the IMS   was extended to include Multimedia Broadcast and Multicast Services   (MBMS).  A point-to-multipoint GPRS connection service is operated on   radio links, while the gateway service to Internet multicast is   handled at the IGMP/MLD-aware GGSN.  Local multicast packet   distribution is used within the GPRS IP backbone resulting in the   common double encapsulation at GGSN: global IP multicast datagrams   over Generic Tunneling Protocol (GTP) (with multipoint TID) over   local IP multicast.   The 3GPP MBMS has the following characteristics:      o There is no immediate Layer 2 source-to-destination transition,        resulting in transit of all multicast traffic at the GGSN.      o As GGSNs commonly are regional, distant entities, triangular        routing and encapsulation may cause a significant degradation of        efficiency.   In 3GPP2, the MBMS has been extended to the Broadcast and Multicast   Service (BCMCS) [46], which on the routing layer operates very   similar to MBMS.  In both 3GPP and 3GPP2, multicast can be sent using   either point-to-point (PTP) or point-to-multipoint (PTM) tunnels, andSchmidt, et al.               Informational                    [Page 18]

RFC 5757                       MMCASTv6-PS                 February 2010   there is support for switching between PTP and PTM.  PTM uses a   unidirectional common channel, operating in unacknowledged mode   without adjustment of power levels and no reporting on lost packets.4.2.4.  DVB-H / DVB-IPDC   Digital Video Broadcasting for Handhelds (DVB-H) is a unidirectional   physical layer broadcasting specification for the efficient delivery   of broadband and IP-encapsulated data streams, and is published as an   ETSI standard [47] (seehttp://www.dvb-h.org).  This uses   multiprotocol encapsulation (MPE) to transport IP packets over an   MPEG-2 Transport Stream (TS) with link forward error correction   (FEC).  Each stream is identified by a 13-bit TS ID (PID), which   together with a multiplex service ID, is associated with IPv4 or IPv6   addresses [48] and used for selective traffic filtering at receivers.   Upstream channels may complement DVB-H using other transmission   technologies.  The IP Datacast Service, DVB-IPDC [31], specifies a   set of applications that can use the DVB-H transmission network.   Multicast distribution services are defined by a mapping of groups   onto appropriate PIDs, which is managed at the IP Encapsulator [49].   To increase flexibility and avoid collisions, this address resolution   is facilitated by dynamic tables, provided within the self-contained   MPEG-2 TS.  Mobility is supported in the sense that changes of cell   ID, network ID, or Transport Stream ID are foreseen [50].  A   multicast receiver thus needs to relocate the multicast services to   which it is subscribed during the synchronization phase, and update   its service filters.  Its handover decision may depend on service   availability.  An active service subscription (multicast join)   requires initiation at the IP Encapsulator / DVB-H Gateway, which   cannot be signaled in a pure DVB-H network.4.2.5.  TV Broadcast and Satellite Networks   IP multicast may be enabled in TV broadcast networks, including those   specified by DVB, the Advanced Television Systems Committee (ATSC),   and related standards [49].  These standards are also used for one-   and two-way satellite IP services.  Networks based on the MPEG-2   Transport Stream may support either the multiprotocol encapsulation   (MPE) or the unidirectional lightweight encapsulation (ULE) [51].   The second generation DVB standards allow the Transport Stream to be   replaced with a Generic Stream, using the Generic Stream   Encapsulation (GSE) [52].  These encapsulation formats all support   multicast operation.   In MPEG-2 transmission networks, multicast distribution services are   defined by a mapping of groups onto appropriate PIDs, which is   managed at the IP Encapsulator [49].  The addressing issues resembleSchmidt, et al.               Informational                    [Page 19]

RFC 5757                       MMCASTv6-PS                 February 2010   those for DVB-H (Section 4.2.4) [48].  The issues for using GSE   resemble those for ULE (except the PID is not available as a   mechanism for filtering traffic).  Networks that provide   bidirectional connectivity may allow active service subscription   (multicast join) to initiate forwarding from the upstream IP   Encapsulator / gateway.  Some kind of filtering can be achieved using   the Input Stream Identifier (ISI) field.4.3.  Vertical Multicast Handovers   A mobile multicast node may change its point of Layer 2 attachment   within homogeneous access technologies (horizontal handover) or   between heterogeneous links (vertical handover).  In either case, a   Layer 3 network change may or may not take place, but multicast-aware   links always need information about group traffic demands.   Consequently, a dedicated context transfer of multicast subscriptions   is required at the network access.  Such Media Independent Handover   (MIH) is addressed in IEEE 802.21 [53], but is relevant also beyond   IEEE protocols.  Mobility services transport for MIH are required as   an abstraction for Layer 2 multicast service transfer in an Internet   context [54] and are specified in [55].   MIH needs to assist in more than service discovery: There is a need   for complex, media-dependent multicast adaptation, a possible absence   of MLD signaling in L2-only transfers, and requirements originating   from predictive handovers.  A multicast mobility services transport   needs to be sufficiently comprehensive and abstract to initiate a   seamless multicast handoff at network access.   Functions required for MIH include:      o Service discovery.      o Service context transformation.      o Service context transfer.      o Service invocation.5.  Solutions5.1.  General Approaches   Three approaches to mobile multicast are common [56]:      o Bidirectional Tunneling, in which the mobile node tunnels all        multicast data via its home agent.  This fundamental multicast        solution hides all movement and results in static multicast        trees.  It may be employed transparently by mobile multicastSchmidt, et al.               Informational                    [Page 20]

RFC 5757                       MMCASTv6-PS                 February 2010        listeners and sources, at the cost of triangular routing and        possibly significant performance degradation from widely spanned        data tunnels.      o Remote Subscription forces the mobile node to re-initiate        multicast distribution following handover, e.g., by submitting        an MLD listener report to the subnet where a receiver attaches.        This approach of tree discontinuation relies on multicast        dynamics to adapt to network changes.  It not only results in        significant service disruption but leads to mobility-driven        changes of source addresses, and thus cannot support session        persistence under multicast source mobility.      o Agent-based solutions attempt to balance between the previous        two mechanisms.  Static agents typically act as local tunneling        proxies, allowing for some inter-agent handover when the mobile        node moves.  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.   MIPv6 [5] introduces bidirectional tunneling as well as remote   subscription as minimal standard solutions.  Various publications   suggest utilizing remote subscription for listener mobility only,   while advising bidirectional tunneling as the solution for source   mobility.  Such an approach avoids the "tunnel convergence" or   "avalanche" problem [56], which refers to the responsibility of the   home agent to multiply and encapsulate packets for many receivers of   the same group, even if they are located within the same subnetwork.   However, this suffers from the drawback that multicast communication   roles are not explicitly known at the network layer and may change   unexpectedly.   None of the above approaches address SSM source mobility, except the   use of bidirectional tunneling.5.2.  Solutions for Multicast Listener Mobility5.2.1.  Agent Assistance   There are proposals for agent-assisted handover for host-based   mobility, which complement the unicast real-time mobility   infrastructure of Fast MIPv6 (FMIPv6) [19], the M-FMIPv6 [57][58],   and of Hierarchical MIPv6 (HMIPv6) [20], the M-HMIPv6 [59], and to   context transfer [60], which have been thoroughly analyzed in   [25][61].Schmidt, et al.               Informational                    [Page 21]

RFC 5757                       MMCASTv6-PS                 February 2010   All these solutions presume the context state was stored within a   network node that is reachable before and after a move.  But there   could be cases were the MN is no longer in contact with the previous   network, when at the new location.  In this case, the network itself   cannot assist in the context transfer.  Such scenarios may occur when   moving from one (walled) operator to another and will require a   backwards compatible way to recover from loss of connectivity and   context based on the node alone.   Network-based mobility management, Proxy MIPv6 (PMIPv6) [62], is   multicast transparent in the sense that the MN experiences a point-   to-point home link fixed at its (static) Local Mobility Anchor (LMA).   This virtual home link is composed of a unicast tunnel between the   LMA and the current Mobile Access Gateway (MAG), and a point-to-point   link connecting the current MAG to the MN.  A PMIPv6 domain thereby   inherits MTU-size problems from spanning tunnels at the receiver   site.  Furthermore, two avalanche problem points can be identified:   the LMA may be required to tunnel data to a large number of MAGs,   while an MAG may be required to forward the same multicast stream to   many MNs via individual point-to-point links [63].  Future   optimizations and extensions to shared links preferably adapt native   multicast distribution towards the edge network, possibly using a   local routing option, including context transfer between access   gateways to assist IP-mobility-agnostic MNs.   An approach based on dynamically negotiated inter-agent handovers is   presented in [64].  Aside from IETF work, numerous publications   present proposals for seamless multicast listener mobility, e.g.,   [65] provides a comprehensive overview of the work prior to 2004.5.2.2.  Multicast Encapsulation   Encapsulation of multicast data packets is an established method to   shield mobility and to enable access to remotely located data   services, e.g., streams from the home network.  Applying generic   packet tunneling in IPv6 [66] using a unicast point-to-point method   will also allow multicast-agnostic domains to be transited, but does   inherit the tunnel convergence problem and may result in traffic   multiplication.   Multicast-enabled environments may take advantage of point-to-   multipoint encapsulation, i.e., generic packet tunneling using an   appropriate multicast destination address in the outer header.  Such   multicast-in-multicast encapsulated packets similarly enable   reception of remotely located streams, but do not suffer from the   scaling overhead from using unicast tunnels.Schmidt, et al.               Informational                    [Page 22]

RFC 5757                       MMCASTv6-PS                 February 2010   The tunnel entry point performing encapsulation should provide   fragmentation of data packets to avoid issues resulting from MTU-size   constraints within the network(s) supporting the tunnel(s).5.2.3.  Hybrid Architectures   There has been recent interest in seeking methods that avoid the   complexity at the Internet core network, e.g., application-layer and   overlay proposals for (mobile) multicast.  The possibility of   integrating multicast distribution on the overlay into the network   layer is also being considered by the IRTF Scalable Adaptive   Multicast (SAM) Research Group.   An early hybrid architecture using reactively operating proxy-   gateways located at the Internet edges was introduced by Garyfalos   and Almeroth [30].  The authors presented an Intelligent Gateway   Multicast as a bridge between mobility-aware native multicast   management in access networks and mobility group distribution   services in the Internet core, which may be operated on the network   or application layer.  The Hybrid Shared Tree approach [67]   introduced a mobility-agnostic multicast backbone on the overlay.   Current work in the SAM RG is developing general architectural   approaches for hybrid multicast solutions [68] and a common multicast   API for a transparent access of hybrid multicast [69] that will   require a detailed design in future work.5.2.4.  MLD Extensions   The default timer values and Robustness Variable specified in MLD   [17] were not designed for the mobility context.  This results in a   slow reaction of the multicast-routing infrastructure (including   L3-aware access devices [70]) following a client leave.  This may be   a disadvantage for wireless links, where performance may be improved   by carefully tuning the Query Interval and other variables.  Some   vendors have optimized performance by implementing a listener node   table at the access router that can eliminate the need for query   timeouts when receiving leave messages (explicit receiver tracking).   An MN operating predictive handover, e.g., using FMIPv6, may   accelerate multicast service termination when leaving the previous   network by submitting an early Done message before handoff.  MLD   router querying will allow the multicast forwarding state to be   restored in the case of an erroneous prediction (i.e., an anticipated   move to a network that has not taken place).  Backward context   transfer may otherwise ensure a leave is signaled.  A further   optimization was introduced by Jelger and Noel [71] for the special   case when the HA is a multicast router.  A Done message receivedSchmidt, et al.               Informational                    [Page 23]

RFC 5757                       MMCASTv6-PS                 February 2010   through a tunnel from the mobile end node (through a point-to-point   link directly connecting the MN, in general), should not initiate   standard MLD membership queries (with a subsequent timeout).  Such   explicit treatment of point-to-point links will reduce traffic and   accelerate the control protocol.  Explicit tracking will cause   identical protocol behavior.   While away from home, an MN may wish to rely on a proxy or "standby"   multicast membership service, optionally provided by an HA or proxy   router.  Such functions rely on the ability to restart fast packet   forwarding; it may be desirable for the proxy router to remain part   of the multicast delivery tree, even when transmission of group data   is paused.  To enable such proxy control, the authors in [71] propose   an extension to MLD, introducing a Listener Hold message that is   exchanged between the MN and the HA.  This idea was developed in [59]   to propose multicast router attendance control, allowing for a   general deployment of group membership proxies.  Some currently   deployed IPTV solutions use such a mechanism in combination with a   recent (video) frame buffer, to enable fast channel switching between   several IPTV multicast flows (zapping).5.3.  Solutions for Multicast Source Mobility5.3.1.  Any Source Multicast Mobility Approaches   Solutions for multicast source mobility can be divided into three   categories:      o Statically Rooted Distribution Trees.  These methods follow a        shared tree approach.  Romdhani et al. [72] proposed employing        the Rendezvous Points of PIM-SM as mobility anchors.  Mobile        senders tunnel their data to these "Mobility-aware Rendezvous        Points" (MRPs).  When restricted to a single domain, this scheme        is equivalent to bidirectional tunneling.  Focusing on inter-        domain mobile multicast, the authors designed a tunnel- or SSM-        based backbone distribution of packets between MRPs.      o Reconstruction of Distribution Trees.  Several authors have        proposed the construction of a completely new distribution tree        after the movement of a mobile source and therefore have to        compensate for the additional routing (tree-building) delay.  M-        HMIPv6 [59] tunnels data into a previously established tree        rooted at mobility anchor points to compensate for the routing        delay until a protocol-dependent timer expires.  The Range-Based        Mobile Multicast (RBMoM) protocol [73] introduces an additional        Multicast Agent (MA) that advertises its service range.  A        mobile source registers with the closest MA and tunnels data        through it.  When moving out of the previous service range, itSchmidt, et al.               Informational                    [Page 24]

RFC 5757                       MMCASTv6-PS                 February 2010        will perform MA discovery, a re-registration and continue data        tunneling with a newly established Multicast Agent in its new        current vicinity.      o Tree Modification Schemes.  In the case of DVMRP routing, Chang        and Yen [74] propose an algorithm to extend the root of a given        delivery tree for incorporating a new source location in ASM.        The authors rely on a complex additional signaling protocol to        fix DVMRP forwarding states and heal failures in the reverse        path forwarding (RPF) checks.5.3.2.  Source-Specific Multicast Mobility Approaches   The shared tree approach of [72] has been extended to support SSM   mobility by introducing the HoA address record to the Mobility-aware   Rendezvous Points.  The MRPs operate using extended multicast routing   tables that simultaneously hold the HoA and CoA and thus can   logically identify the appropriate distribution tree.  Mobility thus   may reintroduce the concept of rendezvous points to SSM routing.   Approaches for reconstructing SPTs in SSM rely on a client   notification to establish new router state.  They also need to   preserve address transparency for the client.  Thaler [75] proposed   introducing a binding cache and providing source address transparency   analogous to MIPv6 unicast communication.  Initial session   announcements and changes of source addresses are distributed   periodically to clients via an additional multicast control tree   rooted at the home agent.  Source tree handovers are then activated   on listener requests.   Jelger and Noel [76] suggest handover improvements employing anchor   points within the source network, supporting continuous data   reception during client-initiated handovers.  Client updates are   triggered out of band, e.g., by Source Demand Routing (SDR) / Session   Announcement Protocol (SAP) [77].  Receiver-oriented tree   construction in SSM thus remains unsynchronized with source   handovers.   To address the synchronization problem at the routing layer, several   proposals have focused on direct modification of the distribution   trees.  A recursive scheme may use loose unicast source routes with   branch points, based on a multicast Hop-by-Hop protocol.  Vida et al.   [78] optimized SPT for a moving source on the path between the source   and first branching point.  O'Neill [79] suggested a scheme to   overcome RPF check failures that originate from multicast source   address changes with a rendezvous point scenario by introducing   extended routing information, which accompanies data in a Hop-by-Hop   option "RPF redirect" header.  The Tree Morphing approach of SchmidtSchmidt, et al.               Informational                    [Page 25]

RFC 5757                       MMCASTv6-PS                 February 2010   and Waehlisch [80] used source routing to extend the root of a   previously established SPT, thereby injecting router state updates in   a Hop-by-Hop option header.  Using extended RPF checks, the elongated   tree autonomously initiates shortcuts and smoothly reduces to a new   SPT rooted at the relocated source.  An enhanced version of this   protocol abandoned the initial source routing and could be proved to   comply with rapid source movement [81].  Lee et al. [82] introduced a   state-update mechanism for reusing major parts of established   multicast trees.  The authors start from an initially established   distribution state, centered at the mobile source's home agent.  A   mobile source leaving its home network will signal a multicast   forwarding state update on the path to its home agent and,   subsequently, distribution states according to the mobile source's   new CoA along the previous distribution tree.  Multicast data is then   intended to flow natively using triangular routes via the elongation   and an updated tree centered on the home agent.  Based on Host   Identity Protocol identifiers, Kovacshazi and Vida [83] introduce   multicast routing states that remain independent of IP addresses.   Drawing upon a similar scaling law argument, parts of these states   may then be reused after source address changes.6.  Security Considerations   This document discusses multicast extensions to mobility.  It does   not define new methods or procedures.  Security issues arise from   source address binding updates, specifically in the case of source-   specific multicast.  Threats of hijacking unicast sessions will   result from any solution jointly operating binding updates for   unicast and multicast sessions.   Multicast protocols exhibit a risk of network-based traffic   amplification.  For example, an attacker may abuse mobility signaling   to inject unwanted traffic into a previously established multicast   distribution infrastructure.  These threats are partially mitigated   by reverse path forwarding checks by multicast routers.  However, a   multicast or mobility agent that explicitly replicates multicast   streams, e.g., Home Agent that n-casts data, may be vulnerable to   denial-of-service attacks.  In addition to source authentication, a   rate control of the replicator may be required to protect the agent   and the downstream network.   Mobility protocols need to consider the implications and requirements   for Authentication, Authorization, and Accounting (AAA).  An MN may   have been authorized to receive a specific multicast group when using   one mobile network, but this may not be valid when attaching to a   different network.  In general, the AAA association for an MN may   change between attachments, or may be individually chosen prior to   network (re-)association.  The most appropriate network path may beSchmidt, et al.               Informational                    [Page 26]

RFC 5757                       MMCASTv6-PS                 February 2010   one that satisfies user preferences, e.g., to use/avoid a specific   network, minimize monetary cost, etc., rather than one that only   minimizes the routing cost.  Consequently, AAA bindings may need to   be considered when performing context transfer.   Admission control issues may arise when new CoA source addresses are   introduced to SSM channels [84].  Due to lack of feedback, the   admission [85] and binding updates [86] of mobile multicast sources   require autonomously verifiable authentication.  This can be achieved   by, for instance, Cryptographically Generated Addresses (CGAs).   Modification to IETF protocols (e.g., routing, membership, session   announcement, and control) as well as the introduction of new   entities, e.g., multicast mobility agents, can introduce security   vulnerabilities and require consideration of issues such as   authentication of network entities, methods to mitigate denial of   service (in terms of unwanted network traffic, unnecessary   consumption of router/host resources and router/host state/buffers).   Future solutions must therefore analyze and address the security   implications of supporting mobile multicast.7.  Summary and Future Steps   This document is intended to provide a basis for the future design of   mobile IPv6 multicast methods and protocols by:      o providing a structured overview of the problem space that        multicast and mobility jointly generate at the IPv6 layer;      o referencing the implications and constraints arising from lower        and upper layers and from deployment;      o briefly surveying conceptual ideas of currently available        solutions;      o including a comprehensive bibliographic reference base.   It is recommended that future steps towards extending mobility   services to multicast proceed to first solve the following problems:      1. Ensure seamless multicast reception during handovers, meeting         the requirements of mobile IPv6 nodes and networks.  Thereby         addressing the problems of home subscription without n-tunnels,         as well as native multicast reception in those visited         networks, which offer a group communication service.Schmidt, et al.               Informational                    [Page 27]

RFC 5757                       MMCASTv6-PS                 February 2010      2. Integrate multicast listener support into unicast mobility         management schemes and architectural entities to define a         consistent mobility service architecture, providing equal         support for unicast and multicast communication.      3. Provide basic multicast source mobility by designing address         duality management at end nodes.Schmidt, et al.               Informational                    [Page 28]

RFC 5757                       MMCASTv6-PS                 February 2010Appendix A.  Implicit Source Notification Options   An IP multicast source transmits data to a group of receivers without   requiring any explicit feedback from the group.  Sources therefore   are unaware at the network layer of whether any receivers have   subscribed to the group, and unconditionally send multicast packets   that propagate in the network to the first-hop router (often known in   PIM as the designated router).  There have been attempts to   implicitly obtain information about the listening group members,   e.g., extending an IGMP/MLD querier to inform the source of the   existence of subscribed receivers.  Multicast Source Notification of   Interest Protocol (MSNIP) [87] was such a suggested method that   allowed a multicast source to query the upstream designated router.   However, this work did not progress within the IETF mboned working   group and was terminated by the IETF.   Multicast sources may also be controlled at the session or transport   layer using end-to-end control protocols.  A majority of real-time   applications employ the Real-time Transport Protocol (RTP) [88].  The   accompanying control protocol, RTP Control Protocol (RTCP), allows   receivers to report information about multicast group membership and   associated performance data.  In multicast, the RTCP reports are   submitted to the same group and thus may be monitored by the source   to monitor, manage and control multicast group operations.RFC 2326,   the Real Time Streaming Protocol (RTSP), provides session layer   control that may be used to control a multicast source.  However,   RTCP and RTSP information is intended for end-to-end control and is   not necessarily visible at the network layer.  Application designers   may chose to implement any appropriate control plane for their   multicast applications (e.g., reliable multicast transport   protocols), and therefore a network-layer mobility mechanism must not   assume the presence of a specific transport or session protocol.Informative References    [1]  Aguilar, L. "Datagram Routing for Internet Multicasting", In         ACM SIGCOMM '84 Communications Architectures and Protocols, pp.         58-63, ACM Press, June, 1984.    [2]  Deering, S., "Host extensions for IP multicasting", STD 5,RFC1112, August 1989.    [3]  G. Xylomenos and G.C. Plyzos, "IP Multicast for Mobile Hosts",         IEEE Communications Magazine, 35(1), pp. 54-58, January 1997.Schmidt, et al.               Informational                    [Page 29]

RFC 5757                       MMCASTv6-PS                 February 2010    [4]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)         Specification",RFC 2460, December 1998.    [5]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in         IPv6",RFC 3775, June 2004.    [6]  Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with         IKEv2 and the Revised IPsec Architecture",RFC 4877, April         2007.    [7]  ITU-T Recommendation, "G.114 - One-way transmission time",         Telecommunication Union Standardization Sector, 05/2003.    [8]  Akyildiz, I and Wang, X., "A Survey on Wireless Mesh Networks",         IEEE Communications Magazine, 43(9), pp. 23-30, September 2005.    [9]  Bhattacharyya, S., Ed., "An Overview of Source-Specific         Multicast (SSM)",RFC 3569, July 2003.   [10]  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",RFC 4607, August 2006.   [11]  Waitzman, D., Partridge, C., and S. Deering, "Distance Vector         Multicast Routing Protocol",RFC 1075, November 1988.   [12]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,         Handley, M., Jacobson, V., Liu, C., Sharma, P., and L. Wei,         "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol         Specification",RFC 2362, June 1998.   [13]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,         "Protocol Independent Multicast - Sparse Mode (PIM-SM):         Protocol Specification (Revised)",RFC 4601, August 2006.   [14]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,         "Bidirectional Protocol Independent Multicast (BIDIR-PIM)",RFC5015, October 2007.   [15]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,         "Multiprotocol Extensions for BGP-4",RFC 4760, January 2007.   [16]  Deering, S., Fenner, W., and B. Haberman, "Multicast Listener         Discovery (MLD) for IPv6",RFC 2710, October 1999.   [17]  Vida, R., Ed., and L. Costa, Ed., "Multicast Listener Discovery         Version 2 (MLDv2) for IPv6",RFC 3810, June 2004.Schmidt, et al.               Informational                    [Page 30]

RFC 5757                       MMCASTv6-PS                 February 2010   [18]  Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route         Optimization for Mobile IPv6",RFC 4866, May 2007.   [19]  Koodli, R., Ed., "Mobile IPv6 Fast Handovers",RFC 5568, July         2009.   [20]  Soliman, H., Castelluccia, C., ElMalki, K., and L. Bellier,         "Hierarchical Mobile IPv6 (HMIPv6) Mobility Management",RFC5380, October 2008.   [21]  Loughney, J., Ed., Nakhjiri, M., Perkins, C., and R. Koodli,         "Context Transfer Protocol (CXTP)",RFC 4067, July 2005.   [22]  Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.         Kuladinithi, "Analysis of Multihoming in Mobile IPv6", Work in         Progress, May 2008.   [23]  Narayanan, V., Thaler, D., Bagnulo, M.,  and H. Soliman, "IP         Mobility and Multi-homing Interactions and Architectural         Considerations", Work in Progress, July 2007.   [24]  Savola, P. and B. Haberman, "Embedding the Rendezvous Point         (RP) Address in an IPv6 Multicast Address",RFC 3956, November         2004.   [25]  Schmidt, T.C. and Waehlisch, M. "Predictive versus Reactive -         Analysis of Handover Performance and Its Implications on IPv6         and Multicast Mobility", Telecommunication Systems, 30(1-3),         pp. 123- 142, November 2005.   [26]  Schmidt, T.C. and Waehlisch, M. "Morphing Distribution Trees -         On the Evolution of Multicast States under Mobility and an         Adaptive Routing Scheme for Mobile SSM Sources",         Telecommunication Systems, 33(1-3), pp. 131-154, December 2006.   [27]  Diot, C. et al. "Deployment Issues for the IP Multicast Service         and Architecture", IEEE Network Magazine, spec. issue on         Multicasting, 14(1), pp. 78-88, 2000.   [28]  Eubanks, M.http://multicasttech.com/status/, 2008.   [29]  Garyfalos, A, Almeroth, K. and Sanzgiri, K. "Deployment         Complexity Versus Performance Efficiency in Mobile Multicast",         Intern.  Workshop on Broadband Wireless Multimedia: Algorithms,         Architectures and Applications (BroadWiM), San Jose,         California, USA, October 2004. Online:http://imj.ucsb.edu/papers/BROADWIM-04.pdf.Schmidt, et al.               Informational                    [Page 31]

RFC 5757                       MMCASTv6-PS                 February 2010   [30]  Garyfalos, A, Almeroth, K. "A Flexible Overlay Architecture for         Mobile IPv6 Multicast", IEEE Journ. on Selected Areas in Comm.,         23(11), pp. 2194-2205, November 2005.   [31]  "Digital Video Broadcasting (DVB); IP Datacast over DVB-H: Set         of Specifications for Phase 1", ETSI TS 102 468;   [32]  ETSI TS 102 611, "Digital Video Broadcasting (DVB); IP Datacast         over DVB-H: Implementation Guidelines for Mobility)", European         Standard (Telecommunications series), November 2004.   [33]  Chuang, J. and Sirbu, M. "Pricing Multicast Communication: A         Cost- Based Approach", Telecommunication Systems, 17(3),         281-297, 2001.  Presented at the INET'98, Geneva, Switzerland,         July 1998.   [34]  Van Mieghem, P, Hooghiemstra, G, Hofstad, R. "On the Efficiency         of Multicast", IEEE/ACM Trans. Netw., 9(6), pp. 719-732, Dec.         2001.   [35]  Chalmers, R.C. and Almeroth, K.C, "On the topology of multicast         trees", IEEE/ACM Trans. Netw., 11(1), 153-165, 2003.   [36]  Janic, M. and Van Mieghem, P. "On properties of multicast         routing trees", Int. J. Commun. Syst., 19(1), pp. 95-114, Feb.         2006.   [37]  Van Mieghem, P. "Performance Analysis of Communication Networks         and Systems", Cambridge University Press, 2006.   [38]  Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet         Group Management Protocol (IGMP) / Multicast Listener Discovery         (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")",RFC4605, August 2006.   [39]  Jeon, H., Jeong, S., and M. Riegel, "Transmission of IP over         Ethernet over IEEE 802.16 Networks",RFC 5692, October 2009.   [40]  Shin, M-K., Ed., Han, Y-H., Kim, S-E., and D. Premec, "IPv6         Deployment Scenarios in 802.16 Networks",RFC 5181, May 2008.   [41]  Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S.         Madanapalli, "Transmission of IPv6 via the IPv6 Convergence         Sublayer over IEEE 802.16 Networks",RFC 5121, February 2008.   [42]  Kim, S., Jin, J., Lee, S., and S. Lee, "Multicast Transport on         IEEE 802.16 Networks", Work in Progress, July 2007.Schmidt, et al.               Informational                    [Page 32]

RFC 5757                       MMCASTv6-PS                 February 2010   [43]  IEEE 802.16e-2005: IEEE Standard for Local and metropolitan         area networks Part 16: "Air Interface for Fixed and Mobile         Broadband Wireless Access Systems Amendment for Physical and         Medium Access Control Layers for Combined Fixed and Mobile         Operation in Licensed Bands", New York, February 2006.   [44]  3rd Generation Partnership Project; Technical Specification         Group Services and System Aspects; "IP Multimedia Subsystem         (IMS)"; Stage 2, 3GPP TS 23.228, Rel. 5 ff, 2002 - 2007.   [45]  Wasserman, M., Ed., "Recommendations for IPv6 in Third         Generation Partnership Project (3GPP) Standards",RFC 3314,         September 2002.   [46]  3GPP2, www.3gpp2.org, "X.S0022-A, Broadcast and Multicast         Service in cdma2000 Wireless IP Network, Rev. A.",http://www.3gpp2.org/Public_html/specs/tsgx.cfm, February 2007.   [47]  ETSI EN 302 304, "Digital Video Broadcasting (DVB);         Transmission System for Handheld Terminals (DVB-H)", European         Standard (Telecommunications series), November 2004.   [48]  Fairhurst, G. and M. Montpetit, "Address Resolution Mechanisms         for IP Datagrams over MPEG-2 Networks",RFC 4947, July 2007.   [49]  Montpetit, M.-J., Fairhurst, G., Clausen, H., Collini-Nocker,         B., and H. Linder, "A Framework for Transmission of IP         Datagrams over MPEG-2 Networks",RFC 4259, November 2005.   [50]  Yang, X, Vare, J, Owens, T. "A Survey of Handover Algorithms in         DVB-H", IEEE Comm. Surveys, 8(4), pp. 16-24, 2006.   [51]  Fairhurst, G. and B. Collini-Nocker, "Unidirectional         Lightweight Encapsulation (ULE) for Transmission of IP         Datagrams over an MPEG-2 Transport Stream (TS)",RFC 4326,         December 2005.   [52]  Fairhurst, G. and B. Collini-Nocker, "Extension Formats for         Unidirectional Lightweight Encapsulation (ULE) and the Generic         Stream Encapsulation (GSE)",RFC 5163, April 2008.   [53]  "Draft IEEE Standard for Local and Metropolitan Area Networks:         Media Independent Handover Services", IEEE LAN/MAN Draft IEEE         P802.21/D07.00, July 2007.   [54]  Melia, T., Ed., "Mobility Services Transport: Problem         Statement",RFC 5164, March 2008.Schmidt, et al.               Informational                    [Page 33]

RFC 5757                       MMCASTv6-PS                 February 2010   [55]  Melia, T., Ed., Bajko, G., Das, S., Golmie, N., and JC. Zuniga,         "IEEE 802.21 Mobility Services Framework Design (MSFD)",RFC5677, December 2009.   [56]  Janneteau, C, Tian, Y, Csaba, S. et al. "Comparison of Three         Approaches Towards Mobile Multicast", IST Mobile Summit 2003,         Aveiro, Portugal, 16-18 June 2003.   [57]  Suh, K., Kwon, D.-H., Suh, Y.-J. and Y. Park, "Fast Multicast         Protocol for Mobile IPv6 in the fast handovers environments",         Work in Progress, January 2004.   [58]  Xia, F. and B. Sarikaya, "FMIPv6 extensions for Multicast         Handover", Work in Progress, March 2007.   [59]  Schmidt, T. and M. Waehlisch, "Seamless Multicast Handover in a         Hierarchical Mobile IPv6 Environment (M-HMIPv6)", Work in         Progress, November 2005.   [60]  Miloucheva, I. and K. Jonas, "Multicast Context Transfer in         mobile IPv6", Work in Progress, June 2005.   [61]  Leoleis, G, Prezerakos, G, Venieris, I, "Seamless multicast         mobility support using fast MIPv6 extensions", Computer Comm.,         29(18), pp. 3745-3765, 2006.   [62]  Gundavelli, S., Ed., Leung, K., Devarapalli, V., Chowdhury, K.,         and B. Patil, "Proxy Mobile IPv6",RFC 5213, August 2008.   [63]  Deng, H., Chen, G., Schmidt, T., Seite, P., and P. Yang,         "Multicast Support Requirements for Proxy Mobile IPv6", Work in         Progress, July 2009.   [64]  Zhang, H., Chen, X., Guan, J., Shen, B., Liu, E., and S.         Dawkins, "Mobile IPv6 Multicast with Dynamic Multicast Agent",         Work in Progress, January 2007.   [65]  Romdhani, I, Kellil, M, Lach, H.-Y. et. al. "IP Mobile         Multicast: Challenges and Solutions", IEEE Comm. Surveys, 6(1),         pp. 18-41, 2004.   [66]  Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6         Specification",RFC 2473, December 1998.   [67]  Waehlisch, M., Schmidt, T.C. "Between Underlay and Overlay: On         Deployable, Efficient, Mobility-agnostic Group Communication         Services", Internet Research, 17(5), pp. 519-534, Emerald         Insight, Bingley, UK, November 2007.Schmidt, et al.               Informational                    [Page 34]

RFC 5757                       MMCASTv6-PS                 February 2010   [68]  J. Buford,"Hybrid Overlay Multicast Framework", Work in         Progress, February 2008.   [69]  Waehlisch, M., Schmidt, T., and S. Venaas, "A Common API for         Transparent Hybrid Multicast", Work in Progress, October 2009.   [70]  Christensen, M., Kimball, K., and F. Solensky, "Considerations         for Internet Group Management Protocol (IGMP) and Multicast         Listener Discovery (MLD) Snooping Switches",RFC 4541, May         2006.   [71]  Jelger, C, Noel, T. "Multicast for Mobile Hosts in IP Networks:         Progress and Challenges", IEEE Wirel. Comm., 9(5), pp 58-64,         Oct. 2002.   [72]  Romdhani, I, Bettahar, H. and Bouabdallah, A. "Transparent         handover for mobile multicast sources", in P. Lorenz and P.         Dini, eds, Proceedings of the IEEE ICN'06, IEEE Press, 2006.   [73]  Lin, C.R. et al. "Scalable Multicast Protocol in IP-Based         Mobile Networks", Wireless Networks, 8 (1), pp. 27-36, January,         2002.   [74]  Chang, R.-S. and Yen, Y.-S. "A Multicast Routing Protocol with         Dynamic Tree Adjustment for Mobile IPv6", Journ. Information         Science and Engineering, 20(6), pp. 1109-1124, 2004.   [75]  Thaler, D. "Supporting Mobile SSM Sources for IPv6",         Proceedings of ietf meeting, Dec. 2001.         URL: www.ietf.org/proceedings/01dec/slides/magma-2.pdf   [76]  Jelger, C. and T. Noel, "Supporting Mobile SSM sources for IPv6         (MSSMSv6)",Work in Progress, January 2002.   [77]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement         Protocol",RFC 2974, October 2000.   [78]  Vida, R, Costa, L, Fdida, S. "M-HBH - Efficient Mobility         Management in Multicast", Proc. of NGC '02, pp. 105-112, ACM         Press 2002.   [79]  A. O'Neill"Mobility Management and IP Multicast", Work in         Progress, July 2002.   [80]  Schmidt, T. C. and Waehlisch, M. "Extending SSM to MIPv6 -         Problems, Solutions and Improvements", Computational Methods in         Science and Technology, 11(2), pp. 147-152. Selected Papers         from TERENA Networking Conference, Poznan, May 2005.Schmidt, et al.               Informational                    [Page 35]

RFC 5757                       MMCASTv6-PS                 February 2010   [81]  Schmidt, T.C., Waehlisch, M., and Wodarz, M. "Fast Adaptive         Routing Supporting Mobile Senders in Source Specific         Multicast", Telecommunication Systems, 43(1), pp. 95-108, 2009,http://dx.doi.org/10.1007/s11235-009-9200-y.   [82]  Lee, H., Han, S. and Hong, J. "Efficient Mechanism for Source         Mobility in Source Specific Multicast", in K. Kawahara and I.         Chong, eds, "Proceedings of ICOIN2006", LNCS vol. 3961, pp.         82-91, Springer-Verlag, Berlin, Heidelberg, 2006.   [83]  Kovacshazi, Z. and Vida, R. "Host Identity Specific Multicast",         Third International Conference on Networking and Services ICNS,         IEEE Press, pp. 1-1, June 2007.   [84]  Kellil, M, Romdhani, I, Lach, H.-Y, Bouabdallah, A. and         Bettahar, H. "Multicast Receiver and Sender Access Control and         its Applicability to Mobile IP Environments: A Survey", IEEE         Comm. Surveys & Tutorials, 7(2), pp. 46-70, 2005.   [85]  Castellucia, C, Montenegro, G. "Securing Group Management in         IPv6 with Cryptographically Based Addresses", Proc. 8th IEEE         Int'l Symp. Comp. and Commun, Turkey, July 2003, pp. 588-93.   [86]  Schmidt, T.C, Waehlisch, M., Christ, O., and Hege, G.         "AuthoCast - a mobility-compliant protocol framework for         multicast sender authentication", Security and Communication         Networks, 1(6),  pp. 495-509, 2008.   [87]  Fenner, B., Holbrook, H., and I. Kouvelas, "Multicast Source         Notification of Interest Protocol (MSNIP)", Work in Progress,         November 2001.   [88]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,         "RTP: A Transport Protocol for Real-Time Applications", STD 64,RFC 3550, July 2003.Schmidt, et al.               Informational                    [Page 36]

RFC 5757                       MMCASTv6-PS                 February 2010Acknowledgments   Work on exploring the problem space for mobile multicast has been   pioneered by Greg Daley and Gopi Kurup within their early document   "Requirements for Mobile Multicast Clients".   Since then, many people have actively discussed the different issues   and contributed to the enhancement of this memo. The authors would   like to thank (in alphabetical order) Kevin C. Almeroth, Lachlan   Andrew, Jari Arkko, Cedric Baudoin, Hans L. Cycon, Hui Deng, Marshall   Eubanks, Zhigang Huang, Christophe Jelger, Andrei Gutov, Rajeev   Koodli, Mark Palkow, Craig Partridge, Imed Romdhani, Hesham Soliman,   Dave Thaler, and last, but not least, very special thanks to Stig   Venaas for his frequent and thorough advice.Authors' Addresses   Thomas C. Schmidt   Dept. Informatik   Hamburg University of Applied Sciences,   Berliner Tor 7   D-20099 Hamburg, Germany   Phone: +49-40-42875-8157   EMail: schmidt@informatik.haw-hamburg.de   Matthias Waehlisch   link-lab   Hoenower Str. 35   D-10318 Berlin, Germany   EMail: mw@link-lab.net   Godred Fairhurst   School of Engineering,   University of Aberdeen,   Aberdeen, AB24 3UE, UK   EMail: gorry@erg.abdn.ac.ukSchmidt, et al.               Informational                    [Page 37]