MobOpts Research Group Thomas C. Schmidt
Internet Draft HAW Hamburg
Category: Informational Matthias Waehlisch
Expires: January 2008 link-lab
July 2007

Multicast Mobility in MIPv6: Problem Statement and Brief Survey

IPR Statement

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79 [1].

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This document is a submission of the IRTF MobOpts RG. Comments should
be directed to the MobOpts RG mailing list, mobopts@irtf.org.


In this document we discuss mobility extensions to current IP layer
multicast solutions. Problems arising from mobile group communication
in general, in the case of multicast listener mobility and for mobile
Any Source Multicast as well as Source Specific Multicast senders are
documented. Characteristic aspects of multicast routing and
deployment issues for fixed IPv6 networks are summarized. The
principal approaches to the multicast mobility problems are outlined

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Table of Contents

1. Introduction and Motivation....................................3

2. Problem Description............................................4
2.1 Generals...................................................4
2.2 Multicast Listener Mobility................................6
2.3 Multicast Source Mobility..................................6
2.3.1 Any Source Multicast Mobility.........................6
2.3.2 Source Specific Multicast Mobility....................7
2.4 Deployment Issues..........................................8

3. Characteristics of Multicast Routing Trees under Mobility......9

4. Layer 2 Aspects...............................................10
4.1 General Background........................................10
4.2 Multicast for Specific Technologies.......................10
4.2.1 802.11 WLAN..........................................10
4.2.2 802.16 WIMAX.........................................11
4.2.3 3GPP.................................................12
4.2.4 DVB-H / DVB-IPDC.....................................13
4.3 Vertical Multicast Handovers..............................13

5. Solutions.....................................................13
5.1 General Approaches........................................13
5.2 Solutions for Multicast Listener Mobility.................14
5.2.1 Agent Assistance.....................................14
5.2.2 Hybrid Architectures.................................14
5.2.3 MLD Extensions.......................................15
5.3 Solutions for Multicast Source Mobility...................15
5.3.1 Any Source Multicast Mobility Approaches.............16
5.3.2 Source Specific Multicast Mobility Approaches........16

6. Security Considerations.......................................17

7. IANA Considerations...........................................18

Appendix A. Implicit Source Notification Options.................18

8. References....................................................18


Author's Addresses...............................................24

Intellectual Property Statement..................................24

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Copyright Notice.................................................25

Disclaimer of Validity...........................................25


1. Introduction and Motivation

Group communication forms an integral building block of a wide
variety of applications, ranging from public content distribution and
streaming over voice and video conferencing, collaborative
environments and (massive multiplayer) gaming up to the self-
organization of distributed systems, services or autonomous networks.
Its support by network layer multicast will be needed, whenever
globally distributed, scalable, serverless or instantaneous
communication is required. As broadband media delivery more and more
emerges to be a typical mass scenario, scalability and bandwidth
efficiency of multicast routing continuously gains relevance. The
idea of Internet multicasting already arose in the early days [2],
soon leading to Deering's fruitfully adopted host group model [3].
Its realization will be of particular importance to mobile
environments, where users commonly share frequency bands of limited
capacity. The rapidly increasing mobile reception of 'infotainment'
streams may soon require a wide deployment of mobile multicast
services. Multicast mobility consequently has been a concern for
about ten years [4] and led to innumerous proposals, but no generally
accepted solution.

The fundamental approach to deal with mobility in IPv6 [5] is stated
in the Mobile IPv6 RFCs [6,7]. MIPv6 [6] only roughly treats
multicast mobility, in a pure remote subscription approach or through
bi-directional tunneling via the Home Agent. Whereas the remote
subscription suffers from slow handovers, as it relies on multicast
routing to adapt to handovers, bi-directional tunneling introduces
inefficient overheads and delays due to triangular forwarding.
Therefore none of the approaches can be considered solutions for a
deployment on large scale. A mobile multicast service for a future
Internet should admit 'close to optimal' routing at predictable and
limited cost, robustness combined with a service quality compliant to
real-time media distribution.

Intricate multicast routing procedures, though, are not easily
extensible to comply with mobility requirements. Any client
subscribed to a group while in motion, requires delivery branches to
pursue its new location; any mobile source requests the entire

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delivery tree to adapt to its changing positions. 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 [50].

In multimedia conference scenarios, games or collaborative
environments each member commonly operates as receiver and as sender
for multicast based group communication. In addition, real-time
communication such as voice or video over IP places severe temporal
requirement on mobility protocols: Seamless handover scenarios need
to limit disruptions or delay to less than 100 ms. Jitter
disturbances are not to exceed 50 ms. Note that 100 ms is about the
duration of a spoken syllable in real-time audio.

It is the aim of this document, to specify the problem scope for a
multicast mobility management as to be refined in future work. The
attempt is made to subdivide the various challenges according to
their originating aspects and to present existing proposals for
solution, as well as major bibliographic references.

2. Problem Description

2.1 Generals

Multicast mobility must be considered as a generic term, which
subsumes a collection of quite distinct functions. At first,
multicast communication divides into Any Source Multicast (ASM) [3]
and Source Specific Multicast (SSM) [8,9]. At second, the roles of
senders and receivers are asymmetric and need distinction. Both may
individually be mobile. Their interaction is facilitated by a
multicast routing function such as DVMRP [10], PIM-SM/SSM [11,12],
Bi-directional PIM [13], CBT [14], BGMP [15] or inter-domain
multicast prefix advertisements via MBGP [16] and the multicast
listener discovery protocol [17,18].

Any multicast mobility solution must account for all of these
functional blocks. It should enable seamless continuity of multicast
sessions when moving from one IPv6 subnet to another. It should
preserve the multicast nature of packet distribution and approximate
optimal routing. It should support per flow handover for multicast
traffic, as properties and designations of flows may be of distinct

The host group model extends network layer unicast service
capabilities. In concordance with the architecture of fixed networks,
multicast mobility management should transparently utilize or
smoothly extend the unicast functions of MIPv6 [6], its security
extensions [7,19], its expediting schemes FMIPv6 [20] and HMIPv6

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[21], its context transfer protocols [22] and its multihoming
capabilities [23,24]. It is desirable to avoid multicast-specific
solutions, whenever a general approach jointly supporting unicast and
multicast can be derived.

Multicast routing dynamically adapts to session topologies, which
then may change under mobility. However, depending on the topology
and the protocol in use, routing convergence may arrive at a time
scale close to seconds, or even minutes and is far too slow to
support seamless handovers for interactive or real-time media
sessions. The actual temporal behavior strongly depends on the
routing protocol in use and on the geometry of the current
distribution tree. A mobility scheme that arranges for adjustments,
i.e., partial changes or full reconstruction of multicast trees, is
forced to comply with timing sufficiently tolerant for protocol
convergence. Special attention is needed with a possible rapid
movement of the mobile node, as this may occur at much higher rates
than compatible with protocol convergence.

IP layer multicast packet distribution is an unreliable service,
which is bound to connectionless transport protocols. Packet loss
thus will not be handled in a predetermined fashion. Mobile multicast
handovers should not cause significant packet drops. Due to
statelessness the bi-casting of multicast flows does not cause
foreseeable degradations of the transport layer.

Group addresses in general are location transparent, even though
there are proposals to embed unicast prefixes or Rendezvous Point
addresses [25]. Addresses of sources contributing to a multicast
session are interpreted by the routing infrastructure and by receiver
applications, which frequently are source address aware. Multicast
therefore inherits the mobility address duality problem for source
addresses, being a logical node identifier, i.e., the home address
(HoA) at the one hand, and a topological locator, the care-of-address
(CoA) at the other. The network layer of group members, i.e.,
multicast senders, forwarders and receivers, needs to carefully
account for address duality issues by means of binding caches,
extended multicast states or signaling.

Multicast sources in general operate decoupled from their receivers
in the following sense: A multicast source submits data to a group of
unknown receivers and thus operates without any feedback channel. It
neither has means to inquire on properties of its delivery trees, nor
will it be able to learn about the state of its receivers. In the
event of an inter-tree handover, a mobile multicast source therefore
is vulnerable to loosing receivers without taking notice. (Cf.
Appendix A for implicit source notification approaches). Applying a
MIPv6 mobility binding update or return routability procedure will
likewise break the semantic of a receiver group remaining

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unidentified by the source and thus cannot be applied in unicast

2.2 Multicast Listener Mobility

A mobile multicast listener entering a new IP subnet faces the
problem of transferring the multicast membership context 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
MN's moving interface(s) to the new access router(s). In the latter
case it may encounter either one of the following conditions: The new
network may not be multicast enabled or the specific multicast
service in use may be unsupported or prohibited. Alternatively, the
requested multicast service may be supported and enabled in the new
network, but the multicast groups under subscription may not be
forwarded to it. Then current distribution trees for the desired
groups may reside at large routing distance. It may as well occur
that data of some or all groups under subscription of the mobile node
are received by one or several local group members at the instance of
arrival and that multicast streams flow natively.

The problem of achieving seamless multicast listener handovers is
thus threefold:
o Ensure multicast reception even in visited networks without
appropriate multicast support.
o Expedite primary multicast forwarding to comply with a seamless
timescale at handovers.
o Realize native multicast forwarding whenever applicable to
preserve network resources and avoid data redundancy.

Additional implications for the infrastructure remain. In changing
its point of attachment a mobile receiver may not have enough time to
leave groups in the previous network. Also, packet duplication and
disorder may result from the change of topology.

2.3 Multicast Source Mobility

2.3.1 Any Source Multicast Mobility

A node submitting data to an ASM group either defines the root of a
source specific shortest path tree (SPT), distributing data towards a
rendezvous point or receivers, or it forwards data directly down a
shared tree, e.g., via encapsulated PIM register messages. Aside from
tunneling or shared trees, forwarding along source specific delivery
trees will be bound to a topological network address due to reverse
path forwarding (RPF) checks. A mobile multicast source moving away
is solely enabled to either inject data into a previously established
delivery tree, which may be a rendezvous point based shared tree, or
to (re-)define a multicast distribution tree compliant to its new

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location. In pursuing the latter the mobile sender will have to
proceed without control of the new tree construction due to
decoupling of sender and receivers.

A mobile multicast source consequently must meet address transparency
at two layers: In order to comply with RPF checks, it has to use an
address within the IPv6 basic header's source field, which is in
topological concordance with the employed multicast distribution
tree. For application transparency the logical node identifier,
commonly the HoA, must be presented as packet's source address to the
socket layer at the receiver side.

Conforming to address transparency and temporal handover constraints
will be major problems for any route optimizing mobility solution.
Additional issues arrive from possible packet loss and from multicast
scoping. A mobile source away from home must attend scoping
restrictions, which arise from its home and its visited location [6].

Within intra-domain multicast routing the employment of shared trees
may considerably relax mobility related complexity. Relying upon a
static rendezvous point, a mobile source may continuously submit data
by encapsulating packets with its previous topologically correct or
home source address. Constraints even weaken, when bi-directional PIM
is used. Intra-domain mobility is transparently covered by bi-
directional shared domain-spanning trees, eliminating the need for
tunneling data to reach a rendezvous point.

However, issues arise in inter-domain multicast scenarios, 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 presence of embedded rendezvous point addresses [25], e.g., for
inter-domain PIM-SM, the primary rendezvous point will be globally
appointed and the signaling requirements obsolete.

2.3.2 Source Specific Multicast Mobility

Fundamentally, Source Specific Multicast has been designed for static
addresses of multicast senders. Source addresses in client
subscription to SSM groups are directly used for route
identification. Any SSM subscriber is thus forced to know the
topological address of its group contributors. SSM source
identification invalidates, when source addresses change under
mobility. 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).

Consequently source mobility for SSM packet distribution requires a
dedicated conceptual treatment in addition to the problems of mobile

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ASM. As a listener is subscribed to an (S,G) channel membership and
as routers have established an (S,G)-state shortest path tree rooted
at source S, any change of source addresses under mobility requests
for state updates at all routers and all receivers. On source
handover a new SPT needs to be established, which partly will
coincide with the previous SPT, e.g., at the receiver side. As the
principle multicast decoupling of a sender from its receivers
likewise holds for SSM, client updates needed for switching trees
turns into a severe problem.

An SSM listener subscribing to or excluding any specific multicast
source, may want 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 [6].

All of the above severely add complexity to a robust SSM mobility
solution, which should converge to optimal routes and, for the sake
of efficiency, should avoid data encapsulation, as well. Like in ASM
handover delays are to be considered critical. The 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 [26,27].

Finally, Source Specific Multicast has been designed as a light-
weight approach to group communication. In adding mobility
management, it is desirable to preserve the principle leanness of SSM
by minimizing additional signaling overheads.

2.4 Deployment Issues

IP multicast deployment in general has been hesitant over the past 15
years, even though all major router vendors and operating systems
offer a wide variety of implementations to support multicast [28].
While many (walled) domains or enterprise networks operate multicast,
group service rollout has been largely limited in public inter-domain
scenarios [29]. 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 into the
mobile multicast domain [30]. Hereunto Garyfalos and Almeroth [31]
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.

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Facing deployment complexity it is desirable that any solution to
mobile multicast should leave routing protocols unchanged. Mobility
management in such deployment-friendly schemes should preferably be
handled at edge nodes, preserving the routing infrastructure in
mobility agnostic condition. Regarding the current state of
proposals, the urge remains open to search for such simple,
infrastructure transparent solutions, even though there are
reasonable doubts, whether the desired can be achieved in all cases.

Nevertheless, multicast services in mobile environments may soon
become indispensable, when multimedia distribution services such as
DVB-H or IPTV will develop as a strong business cases for IP
portables. As IP mobility will unfold dominance and as efficient link
utilization will show 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 well under the focus
of network efficiency. Grounded on empirical observations Chuang and
Sirbu [32] proposed a scaling power-law for the total number of links
in a multicast shortest path tree with m receivers (prop. 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 rather tall than wide.

Subsequent empirical and analytical work, cf. [33,34], debated the
applicability of the Chuang and Sirbu scaling law. Van Mieghem et al.
[33] 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 N = 10^5 core
nodes. Investigating on self-similarity Janic and Van Mieghem [35]
semi-empirically substantiated that multicast shortest path trees in
the Internet can be modeled with reasonable accuracy by uniform
recursive trees (URT) [36], 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 an outer measure for 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 the
identical receivers at one or several intersection points. By the
self-similar nature, the persistent subtrees (of previous and next

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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 [27], both analytically and by
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. Layer 2 Aspects

4.1 General Background

Scalable group data distribution admits highest potentials in leaf
networks, where large numbers of end system reside. Consequently it
is not surprising that most LAN network access technologies natively
support point-to-multipoint or multicast services. Of focal interest
to the mobility domain are wireless access technologies, which always
operate on a shared medium of limited frequencies and bandwidth.

Several aspects need consideration. At first connectionless and
connection oriented technologies both occur in radio links, the first
being bound to limited reliability, the latter causing specific
complexity and reduced efficiency on the multicast control side.

At 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 [52], 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 layer 2 separation of multicast and
unicast traffic can be employed as a workaround for point-to-point
link models to establish a common multicast link.

Thirdly, 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.

4.2 Multicast for Specific Technologies

4.2.1 802.11 WLAN

IEEE 802.11 WLAN is a broadcast network of Ethernet type, which
inherits multicast address mapping concepts from 802.3. In
infrastructure mode an access point operates as 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

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point-to-point acknowledged unicast mode (ToDS bit on). An access
point receiving multicast data from a MN simply repeats multicast
frames to the BSS and propagates them to the ESS as unacknowledged
broadcast. Multicast frames received from the ESS are treated

Multicast frame delivery is burdened with the following issues:

o As an unacknowledged service it attains limited reliability.
Frames admit increased loss probability due to interferences,
collisions, or time-varying channel properties.

o Data distribution may be delayed, as access points buffer
multicast packets while waiting for DTIM, whenever stations are using
power saving mode.

o Multipoint data may cause congestion, as the distribution system
experiences multicast as flooding. 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 limiting for multicast packets. Additionally,
IGMP/MLD snooping may be active at the bridging layer between BSS and
ESS or at switches interconnecting access points.

4.2.2 802.16 WIMAX

IEEE 802.16 WIMAX combines a family of connection oriented radio
transmission services, operating in distinguished, unidirectional
channels. The channel assignment is controlled by Base Stations,
which assign 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 (without ARQ) mode.

A WIMAX Base Station operates as L2 switch in full duplex mode, where
switching is based on CIDs. Two possible IPv6 link models for mobile
access deployment scenarios exist: Shared IPv6 prefix and point-to-
point link model [37]. The latter treats each connection to a mobile
node as a single link, which on the IP layer conflicts a consistent
group distribution via a shared medium (cf. section 4.1 for a

To invoke a multipoint data channel, the base station assigns a
common CID to all Subscriber Stations of that group. IPv6 multicast
address mapping to these 16 bit IDs is proposed for copying either
the 4 lowest bits, while sustaining the scope field, or by utilizing
the 8 lowest bits derived from Multicast on Ethernet CS [38]. For

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selecting group members, a Base Station may implement IGMP/MLD
snooping or even IGMP/MLD proxying as foreseen in 802.16e-2005.

A Subscriber Station will issue multicast data to a Base Station as
point-to-point unicast stream, which is passed on and discovered as
such at the access router. The access router may return multicast
data by feeding into a multicast service channel. On the reception
side a Subscriber Station cannot distinguish multicast from unicast

Multicast services bear the following issues:

o The mapping of multicast addresses to CIDs needs standardization,
as different entities (Access Router, Base Station) may have to
perform the mapping.

o CID collisions for different multicast groups are very likely due
to the short ID space. As a consequence, multicast data transmission
may occur in joint point-to-multipoint groups of reduced

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

The 3GPP System architecture spans a circuit switched (CS) and a
packet switched (PS) domain, the latter General Packet Radio Services
(GPRS) incorporates the Internet Multimedia Subsystem (IMS). 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 [39]. Current packet filtering practice causes inter-working
problems between Mobile IPv6 nodes connected via GPRS [40].

As of UMTS Rel. 6 the IMS has been 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 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 GTP (with multipoint TID) over local IP multicast.

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4.2.4 DVB-H / DVB-IPDC

Digital Video Broadcasting for Handhelds (DVB-H) is a physical layer
broadcasting specification for the efficient delivery of broadband,
IP-encapsulated data streams. It was formally adopted as ETSI
standard (EN 203 204, see www.dvb-h.org). DVB uses a mechanism called
multi-protocol encapsulation, which enables a transport of network
layer protocols on top of MPEG-2 transport streams and includes a
forward error correction (FEC). Thereby DVB cannot only support TV
broadcasting, but offers an IP Datacast Service. DVB-IPDC consists of
a number of individual, application layer specifications, some of
which still under development. Transport Streams (TS) form the basic
logical channels, identified by a 13 bit TS ID (PID). Multicast
distribution services are defined by a mapping of groups onto
appropriate PIDs, which is managed at the IP Encapsulator [41].
Mobility is supported in the sense that changes of cell ID, network
ID or Transport Stream ID are foreseen.

4.3 Vertical Multicast Handovers

A mobile multicast node may operate homogeneous (horizontal) or
heterogeneous (vertical) layer 2 handovers with or without layer 3
network changes. Consequently, multicast configuration context
transfer at network access' needs dedicated treatment. Media
Independent Handover (MIH) is addressed in IEEE 802.21, but continues
to admit relevance beyond IEEE protocols. Mobility services transport
for MIH naturally reside on the network layer and are currently under
preparation [42].

MIH need to assist in more than service discovery. Keeping in mind
complex, media dependent multicast adaptations, 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 the network access.

5. Solutions

5.1 General Approaches

Three approaches to mobile Multicast are commonly around [43]:

o Bi-directional Tunnelling guides the mobile node to tunnel 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 listeners and
sources, on the price of triangular routing and possibly significant
performance degradations due to widely spanned data tunnels.

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o Remote Subscription forces the mobile node to re-initiate
multicast distribution subsequent to handover by submitting an MLD
listener report within the subnet it newly attached to. This approach
of tree discontinuation relies on multicast dynamics to adapt to
network changes. It not only results in rigorous service disruption,
but leads to mobility driven changes of source addresses, and thus
disregards session persistence under multicast source mobility.

o Agent-based solutions attempt to balance between the previous two
mechanisms. Static agents typically act as local tunnelling proxies,
allowing for some inter-agent handover while the mobile node moves
away. A decelerated inter-tree handover, i.e. tree walking, will be
the outcome of agent-based multicast mobility, where some extra
effort is needed to sustain session persistence through address
transparency of mobile sources.

MIPv6 [6] introduces bi-directional tunnelling as well as remote
subscription as minimal standard solutions. Various publications
suggest utilizing remote subscription for listener mobility, only,
while advising bi-directional tunnelling as the solution for source
mobility. Such approach avoids the 'tunnel convergence' or
'avalanche' problem [43], which denotes the home agent responsibility
to multiply and encapsulate packets for many receivers of the same
group, even if they are located within the same subnetwork. However,
it suffers from the drawback that multicast communication roles are
not explicitly known at the network layer and may change or mix

It should be noted that none of the above approaches address SSM
source mobility, except the bi-directional tunnelling.

5.2 Solutions for Multicast Listener Mobility

5.2.1 Agent Assistance

There are proposals of agent assisted handovers compliant to the
unicast real-time mobility infrastructure of Fast MIPv6 [20], the M-
FMIPv6 [44,45], and of Hierarchical MIPv6 [21], the M-HMIPv6 [46],
and to context transfer [47], which have been thoroughly analyzed in
[26,48]. An approach based on dynamically negotiated inter-agent
handovers is presented in [49]. Aside from IETF work, countless
publications present proposals for seamless multicast listener
mobility, cf. [50] for a comprehensive overview.

5.2.2 Hybrid Architectures

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Stimulated by avoidance of deployment complexity at the Internet core
network, application layer and overlay proposals for (mobile)
multicast raised interest in recent times. The prospect on
integrating multicast distribution on the overlay into the network
layer is taken by the IRTF Scalable Adaptive Multicast Research Group

An early hybrid architecture of reactively operating proxy-gateways
located at the Internet edges is introduced by Garyfalos and Almeroth
in [31]. The authors present 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.

Currently SAM is developing general architectural approaches for
hybrid multicast solutions [51], which require detailed design in
future work.

5.2.3 MLD Extensions

MLD timer defaults [18] cause slow reactions of the multicast routing
infrastructure as well as of layer-3-aware access devices [52] on
client leaves, which may be disadvantageous for wireless links. This
tardy adaptation may be improved by carefully adjusting the Query
Interval. MNs operating predictive handovers may submit early exclude
reports, which allow for a possible withdrawal in case of an
erroneous prediction. A further optimisation is introduced by Jelger
and Noel [53] for the special case of the HA being a multicast
router. A leave message received through a tunnel established to a
mobile end node (in general, via a point-to-point link directly
connecting the MN) should initiate a membership query with subsequent
timeout according to the MLD standard. These steps may be suppressed
with the result of traffic reduction and significant acceleration of
the control protocol.

While away a MN may want to rely on a proxy or standby multicast
membership service, which is facilitated by a HA or proxy agent. Such
function relies 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 though transmission of group data is paused. To
enable such proxy control, the authors in [53] propose to extend MLD
by a Listener Hold message exchanged between MN and HA. This idea has
been taken up in [46] and further developed to a multicast router
attendance control, allowing for a general deployment of group
membership proxies.

5.3 Solutions for Multicast Source Mobility

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5.3.1 Any Source Multicast Mobility Approaches

Solutions for the multicast source mobility problem can be sorted in
three categories:

o Statically Rooted Distribution Trees:

Following a shared tree approach, Romdhani et al. [54] propose to
employ Rendezvous Points of PIM-SM as mobility anchors. Mobile
senders tunnel their data to these "Mobility-aware Rendezvous Points"
(MRPs), whence in restriction to a single domain this scheme is
equivalent to the bi-directional tunneling. Focusing on interdomain
mobile multicast, the authors design a tunnel- or SSM-based backbone
distribution of packets between MRPs.

o Reconstruction of Distribution Trees:

Several authors propose to construct a completely new distribution
tree after the movement of a mobile source and thereby have to
compensate routing delays. M-HMIPv6 [46] tunnels data into previously
established trees rooted at mobility anchor points to compensate for
routing delays until a protocol dependent timer expires. The RBMoM
protocol [55] introduces additional Multicast Agents (MA), which
advertise their service range. The mobile source registers with the
closest MA and tunnels its data through it. When moving out of the
previous service range, it will perform a MA discovery, a re-
registration and continue data tunneling with its newly established
Multicast Agent in its current vicinity.

o Tree Modification Schemes:

In the case of DVMRP routing, Chang and Yen [56] propose an algorithm
to extend the root of a given delivery tree for incorporating a new
source location in ASM. To fix DVMRP forwarding states and heal
reverse path forwarding (RPF) check failures, the authors rely on a
complex additional signaling protocol.

5.3.2 Source Specific Multicast Mobility Approaches

The shared tree approach of [54] has been extended to SSM mobility by
introducing the HoA address record to Mobility-aware Rendezvous
Points. These MRPs operate on extended multicast routing tables,
which simultaneously hold HoA and CoA and are thus enabled to
logically identify the appropriate distribution tree. Mobility thus
re-introduces rendezvous points to SSM routing.

Approaches of reconstructing SPTs in SSM have to rely on client
notification for initiating new router state establishment. At the
same time they need to preserve address transparency to the client.

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To account for the latter, Thaler [57] proposes to employ binding
caches and to obtain source address transparency analogous to MIPv6
unicast communication. Initial session announcements and changes of
source addresses are to be distributed periodically to clients via an
additional multicast control tree based at the home agent. Source
tree handovers are then activated on listener requests.
Jelger and Noel [58] suggest handover improvements by employing
anchor points within the source network, supporting a continuous data
reception during client initiated handovers. Client updates are to be
triggered out of band, e.g. by SDR. Receiver oriented tree
construction in SSM thus remains unsynchronized with source

To address this synchronization problem at the routing layer, several
proposals concentrate on direct modification of distribution trees.
Based on a multicast Hop-by-Hop protocol, a recursive scheme of loose
unicast source routes with branch points, Vida et al [59] optimize
SPTs for moving sources on the path between source and first
branching point. O'Neill [60] suggests a scheme to overcome RPF check
failures originating from multicast source address changes in 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 and Waehlisch
[61] uses 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. Lee et al. [62] introduce a state
update mechanism for re-using major parts of established multicast
trees. The authors start from initially established distribution
states centered at the mobile source's home agent. A mobile 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 are implemented along the
previous distribution tree. Multicast data then is intended to
natively flow in triangular routes via the elongation and updated
tree centered at the home agent. Consequently this mechanism refrains
from using shortest path trees. Unfortunately the authors do not
address the problem of RPF check failures in their paper.

6. Security Considerations

This document discusses multicast extensions to mobility. 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. Admission control issues
may arise with new CoA source addresses being introduced to SSM

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channels (cf. [63] for a comprehensive discussion). Due to lack of
feedback, admissions [64] and binding updates [65] of mobile
multicast sources require self-consistent authentication as
achievable by CGAs. Future solutions must address the security

7. IANA Considerations

There are no IANA considerations introduced by this draft.

Appendix A. Implicit Source Notification Options

A multicast source will transmit data to a group of receivers without
any option of an explicit feedback channel. There are attempts though
to implicitly obtain information on listening group members. One
approach has been dedicated to inquire designated routers on the pure
existence of receivers. Based on an extension of IGMP, the Multicast
Source Notification of Interest Protocol (MSNIP) [66] was designed to
allow for the multicast source querying its designated router.
However, work on MSNIP has been terminated by IETF.

A majority of real-time applications employ RTP [67] as its
application layer transport protocol, which is accompanied by its
control protocol RTCP. RTP is capable of multicast group distribution
and RTCP receiver reports are submitted to the same group in the
multicast case. Thus RTCP may be used to monitor, manage and control
multicast group operations, as it provides a fairly comprehensive
insight into group member statuses. However, RTCP information is
neither present at the network layer nor does multicast communication
presuppose the use of RTP.

8. References

Normative References

1 S. Bradner "Intellectual Property Rights in IETF Technology", BCP
79, RFC 3979, March 2005.

2 Aguilar, L. "Datagram Routing for Internet Multicasting", In ACM
SIGCOMM '84 Communications Architectures and Protocols, pp. 58-63,
ACM Press, June, 1984.

3 S. Deering "Host Extensions for IP Multicasting", RFC 1112, August

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4 G. Xylomenos and G.C. Plyzos "IP Multicast for Mobile Hosts", IEEE
Communications Magazine, pp. 54-58, January 1997.

5 R. Hinden and S. Deering "Internet Protocol Version 6
Specification", RFC 2460, December 1998.

6 D.B. Johnson, C. Perkins and J. Arkko "Mobility Support in IPv6",
RFC 3775, June 2004.

7 V. Devarapalli and F. Dupont "Mobile IPv6 Operation with IKEv2 and
the Revised IPsec Architecture", RFC 4877, April 2007.

8 S. Bhattacharyya "An Overview of Source-Specific Multicast (SSM)",
RFC 3569, July 2003.

9 H. Holbrook, B. Cain "Source-Specific Multicast for IP", RFC 4607,
August 2006.

10 D. Waitzman, C. Partridge, S.E. Deering "Distance Vector Multicast
Routing Protocol", RFC 1075, November 1988.

11 D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering, M.
Handley, V. Jacobson, C. Liu, P. Sharma, L. Wei "Protocol
Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification", RFC 2362, June 1998.

12 B. Fenner, M. Handley, H. Holbrook, I. Kouvelas: "Protocol
Independent Multicast - Sparse Mode PIM-SM): Protocol
Specification (Revised)", RFC 4601, August 2006.

13 M. Handley, I. Kouvelas, T. Speakman, L. Vicisano "Bi-directional
Protocol Independent Multicast (BIDIR-PIM)", draft-ietf-pim-bidir-
09.txt, (work in progress), February 2007.

14 A. Ballardie "Core Based Trees (CBT version 2) Multicast Routing",
RFC 2189, September 1997.

15 D. Thaler "Border Gateway Multicast Protocol (BGMP): Protocol
Specification", RFC 3913, September 2004.

16 T. Bates et al. "Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.

17 S. Deering, W. Fenner and B. Haberman "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.

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18 R. Vida and L. Costa (Eds.) "Multicast Listener Discovery Version
2 (MLDv2) for IPv6", RFC 3810, June 2004.

19 Arkko, J., Vogt, C., Haddad, W. "Enhanced Route Optimization for
Mobile IPv6", RFC 4866, May 2007.

20 Koodli, R. "Fast Handovers for Mobile IPv6", RFC 4068, July 2004.

21 Soliman, H., Castelluccia, C., El-Malki, K., Bellier, L.
"Hierarchical Mobile IPv6 mobility management", RFC 4140, August

22 Loughney, J., Nakhjiri, M., Perkins, C., Koodli, R. "Context
Transfer Protocol (CXTP)", RFC 4067, July 2005.

Informative References

23 Montavont, N., et al. "Analysis of Multihoming in Mobile IPv6",
draft-ietf-monami6-mipv6-analysis-02.txt, Internet Draft - (work
in progress), February 2007.

24 Narayanan, V., Thaler, D., Bagnulo, M., Soliman, H. "IP Mobility
and Multi-homing Interactions and Architectural Considerations",
draft-vidya-ip-mobility-multihoming-interactions-01.txt, Internet
Draft - (work in progress), July 2007.

25 Savola, P., Haberman, B. "Embedding the Rendezvous Point (RP)
Address in an IPv6 Multicast Address", RFC 3956, November 2004.

26 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.

27 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,
Vol. 33, No. 1-3, pp. 131-154, Berlin Heidelberg: Springer,
December 2006.

28 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.

29 Eubanks, M.: http://multicasttech.com/status/, 2007.

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30 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-

31 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.

32 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.

33 Van Mieghem, P., Hooghiemstra, G., Hofstad, R. "On the Efficiency
of Multicast", Transactions on Networking, 9, 6, pp. 719-732,
December 2001.

34 Chalmers, R.C. and Almeroth, K.C., "On the topology of multicast
trees", IEEE/ACM Trans. Netw. 11(1), 153-165, 2003.

35 Janic, M. and Van Mieghem, P. "On properties of multicast routing
trees", Int. J. Commun. Syst. 19(1), pp. 95-114, 2006.

36 Van Mieghem, P. "Performance Analysis of Communication Networks
and Systems", Cambridge University Press, 2006.

37 Shin, M. et al. "IPv6 Deployment Scenarios in 802.16 Networks",
draft-ietf-v6ops-802-16-deployment-scenarios-04, (work in
progress), April 2007.

38 Kim, S. et al. "Multicast Transport on IEEE 802.16 Networks",
draft-sekim-802-16-multicast-00, (work in progress), June 2007.

39 Wasserman, M. "Recommendations for IPv6 in Third Generation
Partnership Project (3GPP) Standards", RFC 3314, September 2002.

40 Chen, X., Rinne, J. and Wiljakka, J. "Problem Statement for MIPv6
Interactions with GPRS/UMTS Packet Filtering", draft-chen-mip6-
gprs-07.txt, (work in progress), January 2007.

41 Montpetit, M. et al. "A Framework for Transmission of IP Datagrams
over MPEG-2 Networks", RFC 4259, November 2005.

42 Melia, T. et al. "Mobility Services Transport: Problem Statement",
draft-ietf-mipshop-mis-ps-02, (work in progress), July 2007.

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43 Jannetau, C., Tian, Y., Csaba, S. et al "Comparison of Three
Approaches Towards Mobile Multicast", IST Mobile Summit 2003,
Aveiro, Portugal, 16-18 June 2003, online http://www.comnets.rwth-

44 Suh, K., Kwon, D.-H., Suh, Y.-J. and Park, Y. "Fast Multicast
Protocol for Mobile IPv6 in the fast handovers environments",
Internet Draft - (work in progress, expired), February 2004.

45 Xia, F. and Sarikaya, B. "FMIPv6 extensions for Multicast
Handover", draft-xia-mipshop-fmip-multicast-00.txt, (work in
progress), September 2006.

46 Schmidt, T.C. and Waehlisch, M. "Seamless Multicast Handover in a
Hierarchical Mobile IPv6 Environment(M-HMIPv6)", draft-schmidt-
waehlisch-mhmipv6-04.txt, (work in progress, expired), December

47 Jonas, K. and Miloucheva, I. "Multicast Context Transfer in mobile
IPv6", draft-miloucheva-mldv2-mipv6-00.txt, (work in progress,
expired), June 2005.

48 Leoleis, G., Prezerakos, G., Venieris, I. "Seamless multicast
mobility support using fast MIPv6 extensions", Computer Comm. 29,
pp. 3745-3765, 2006.

49 Zhang, H. et al "Mobile IPv6 Multicast with Dynamic Multicast
Agent", draft-zhang-mipshop-multicast-dma-03.txt, (work in
progress), January 2007.

50 Romdhani, I., Kellil, M., Lach, H.-Y. et. al. "IP Mobile
Multicast: Challenges and Solutions", IEEE Comm. Surveys, 6(1),

51 Buford, J. "Hybrid Overlay Multicast Framework", draft-irtf-sam-
hybrid-overlay-framework-01.txt, Internet Draft (work in
progress), January 2007.

52 Christensen, M., Kimball, K. and Solensky, F. "Considerations for
Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches", RFC 4541, May 2006.

53 Jelger, C., Noel, T. "Multicast for Mobile Hosts in IP Networks:
Progress and Challenges", IEEE Wirel. Comm., pp 58-64, Oct. 2002.

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54 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.

55 Lin, C.R. et al., "Scalable Multicast Protocol in IP-Based Mobile
Networks", Wireless Networks and Applications, 5, pp. 259-271,

56 Chang, R.-S. and Yen, Y.-S. "A Multicast Routing Protocol with
Dynamic Tree Adjustment for Mobile IPv6", Journ. Information
Science and Engineering 20, 1109-1124, 2004.

57 Thaler, D. "Supporting Mobile SSM Sources for IPv6", Proceedings
of ietf meeting Dec. 2001, individual.
URL: www.ietf.org/proceedings/01dec/slides/magma-2.pdf

58 Jelger, C. and Noel, T. "Supporting Mobile SSM sources for IPv6
(MSSMSv6)", Internet Draft (work in progress, expired), January

59 Vida, R., Costa, L., Fdida, S. "M-HBH - Efficient Mobility
Management in Multicast", Proc. of NGC '02, pp. 105-112, ACM Press

60 O'Neill, A. "Mobility Management and IP Multicast", draft-oneill-
mip-multicast-01.txt, (work in progress, expired), July 2002.

61 Schmidt, T. C. and Waehlisch, M. "Extending SSM to MIPv6 -
Problems, Solutions and Improvements", Computational Methods in
Science and Technology 11(2), 147-152. Selected Papers from TERENA
Networking Conference, Poznan, May 2005.

62 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.

63 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.

64 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.

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65 Christ, O., Schmidt, T.C., Waehlisch, M. "A Light-Weight
Implementation Scheme of the Tree Morphing Protocol for Mobile
Multicast Sources ", Proc. of 33rd Euromicro Conf., IEEE/CS, Sept.

66 Fenner, B. et al. "Multicast Source Notification of Interest
Protocol", draft-ietf-idmr-msnip-05.txt, (work in progress,
expired), March 2004.

67 Schulzrinne, H. et al. "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003.


The authors would like to thank Mark Palkow (DaViKo GmbH) and Hans L.
Cycon (FHTW Berlin) for valuable discussions and a joyful
collaboration. They also thank Stig Venaas (UNINETT) for many

Author's Addresses

Thomas C. Schmidt
HAW Hamburg, Dept. Informatik
Berliner Tor 7
D-20099 Hamburg, Germany
Phone: +49-40-42875-8157
Email: Schmidt@informatik.haw-hamburg.de

Matthias Waehlisch
Hoenowerstr. 35
D-10318 Berlin, Germany
Email: mw@link-lab.net

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Schmidt, Waehlisch Expires - January 2008 [Page 24]
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