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Internet Research Task Force (IRTF)                           T. Schmidt
Request for Comments: 5757                                   HAW Hamburg
Category: Informational                                     M. Waehlisch
ISSN: 2070-1721                                                 link-lab
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                           February 2010

           Multicast Mobility in Mobile IP Version 6 (MIPv6):
                   Problem Statement and Brief Survey


   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; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 5757                       MMCASTv6-PS                 February 2010

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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.

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Table of Contents

   1. Introduction and Motivation .....................................4
      1.1. Document Scope .............................................5
   2. Problem Description .............................................6
      2.1. General Issues .............................................6
      2.2. Multicast Listener Mobility ................................9
           2.2.1. Node and Application Perspective ....................9
           2.2.2. Network Perspective ................................10
      2.3. Multicast Source Mobility .................................11
           2.3.1. Any Source Multicast Mobility ......................11
           2.3.2. Source-Specific Multicast Mobility .................12
      2.4. Deployment Issues .........................................13
   3. Characteristics of Multicast Routing Trees under Mobility ......14
   4. Link Layer Aspects .............................................15
      4.1. General Background ........................................15
      4.2. Multicast for Specific Technologies .......................16
           4.2.1. 802.11 WLAN ........................................16
           4.2.2. 802.16 WIMAX .......................................16
           4.2.3. 3GPP/3GPP2 .........................................18
           4.2.4. DVB-H / DVB-IPDC ...................................19
           4.2.5. TV Broadcast and Satellite Networks ................19
      4.3. Vertical Multicast Handovers ..............................20
   5. Solutions ......................................................20
      5.1. General Approaches ........................................20
      5.2. Solutions for Multicast Listener Mobility .................21
           5.2.1. Agent Assistance ...................................21
           5.2.2. Multicast Encapsulation ............................22
           5.2.3. Hybrid Architectures ...............................23
           5.2.4. MLD Extensions .....................................23
      5.3. Solutions for Multicast Source Mobility ...................24
           5.3.1. Any Source Multicast Mobility Approaches ...........24
           5.3.2. Source-Specific Multicast Mobility Approaches ......25
   6. Security Considerations ........................................26
   7. Summary and Future Steps .......................................27
   Appendix A. Implicit Source Notification Options...................29
   Informative References.............................................29

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1.  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].

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   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

   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.

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                                       +------+           +------+
                                       |  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

2.  Problem Description

2.1.  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], the

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   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 a

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   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-

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   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

   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 Mobility

2.2.1.  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

      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

   The problem of achieving seamless multicast listener handovers is
   thus threefold:

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      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.

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      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

2.3.  Multicast Source Mobility

2.3.1.  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].

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   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

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.

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   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.

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   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

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

   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 by

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   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 Aspects

4.1.  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

      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.

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   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 Technologies

4.2.1.  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 mesh

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   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

   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.

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      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

   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, and

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   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] (see http://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 resemble

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   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.  Solutions

5.1.  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 multicast

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        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

   None of the above approaches address SSM source mobility, except the
   use of bidirectional tunneling.

5.2.  Solutions for Multicast Listener Mobility

5.2.1.  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

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   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

   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.

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   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 received

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   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 Mobility

5.3.1.  Any Source Multicast Mobility Approaches

   Solutions for multicast source mobility can be divided into three

      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, it

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        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

   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 Schmidt

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   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 be

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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

      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.

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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.

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RFC 5757                       MMCASTv6-PS                 February 2010

Appendix 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, RFC
         1112, 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

    [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)", RFC
         5015, 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.

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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

   [20]  Soliman, H., Castelluccia, C., ElMalki, K., and L. Bellier,
         "Hierarchical Mobile IPv6 (HMIPv6) Mobility Management", RFC
         5380, 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

   [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:

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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.

   [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.

   [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")", RFC
         4605, 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.

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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.

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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)", RFC
         5677, 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.

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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

   [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,

   [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.

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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,

   [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.

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RFC 5757                       MMCASTv6-PS                 February 2010


   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
   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.uk

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