wdiff draft-irtf-mobopts-mmcastv6-ps-04.txt draft-irtf-mobopts-mmcastv6-ps-05.txt

MobOpts Research Group Thomas C. Schmidt
Internet Draft HAW Hamburg
Intended Status: Informational Matthias Waehlisch
Expires: January May 2009 link-lab
Godred Fairhurst
University of Aberdeen
November 2008

Multicast Mobility in MIPv6: Problem Statement and Brief Survey
<draft-irtf-mobopts-mmcastv6-ps-04.txt>
<draft-irtf-mobopts-mmcastv6-ps-05.txt>

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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.

Abstract

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 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 roadmap for
initial steps in standardization for use by future mobile multicast
protocol designers.

Table of Contents

1. Introduction and Motivation....................................3
1.1 Document Scope..............................................4

2. Problem Description............................................5
2.1 General Issues..............................................5
2.2 Multicast Listener Mobility.................................8
2.2.1 Node & Application Perspective........................8
2.2.2 Network Perspective...................................9
2.3 Multicast Source Mobility...................................9
2.3.1 Any Source Multicast Mobility.........................9 Mobility........................10
2.3.2 Source Specific Multicast Mobility...................10 Mobility...................11
2.4 Deployment Issues..........................................11 Issues..........................................12

3. Characteristics of Multicast Routing Trees under Mobility.....12

4. Link Layer Aspects............................................13
4.1 General Background.........................................13
4.2 Multicast for Specific Technologies........................14
4.2.1 802.11 WLAN..........................................14
4.2.2 802.16 WIMAX.........................................14 WIMAX.........................................15
4.2.3 3GPP.................................................16 3GPP/3GPP2...........................................16
4.2.4 DVB-H / DVB-IPDC.....................................16 DVB-IPDC.....................................17
4.2.5 TV Broadcast and Satellite Networks..................18
4.3 Vertical Multicast Handovers...............................17 Handovers...............................18

5. Solutions.....................................................17 Solutions.....................................................19
5.1 General Approaches.........................................17 Approaches.........................................19
5.2 Solutions for Multicast Listener Mobility..................18 Mobility..................20
5.2.1 Agent Assistance.....................................18 Assistance.....................................20
5.2.2 Multicast Encapsulation..............................19 Encapsulation..............................21
5.2.3 Hybrid Architectures.................................19 Architectures.................................21
5.2.4 MLD Extensions.......................................20 Extensions.......................................21
5.3 Solutions for Multicast Source Mobility....................21 Mobility....................22
5.3.1 Any Source Multicast Mobility Approaches.............21 Approaches.............22
5.3.2 Source Specific Multicast Mobility Approaches........21 Approaches........23

6. Security Considerations.......................................22 Considerations.......................................24

7. Summary and Future Steps......................................23 Steps......................................25
8. IANA Considerations...........................................24 Considerations...........................................25

Appendix A. Implicit Source Notification Options.................24 Options.................26

9. References....................................................24

Acknowledgments..................................................31 References....................................................26

Acknowledgments..................................................33

Author's Addresses...............................................31 Addresses...............................................34

Intellectual Property Statement..................................32 Statement..................................34

Copyright Notice.................................................32 Notice.................................................35

Disclaimer of Validity...........................................33

Acknowledgement..................................................33 Validity...........................................35

Acknowledgement..................................................35

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 [2] soon lead to a wide
adoption of Deering's host group model [3]. 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 [4] 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 [5] is standardized in the Mobile IPv6 RFCs [6,7].
MIPv6 [6] only roughly defines multicast mobility for Mobile Nodes
(MN), using a remote subscription approach or through bi-directional
tunneling via the Home Agent (HA). Remote subscription suffers from
slow handovers, relying on multicast routing to adapt to handovers.
Bi-directional 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
[63].
[66].

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 requirement requirements on mobility protocols: Typical seamless
handover scenarios are expected to limit disruptions or delay to less
than 100
ms. - 150 ms [8]. Jitter disturbances should not exceed 50 ms.
Note that 100 ms is about the duration of a spoken syllable in real-time 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.

1.1Document Scope

This document defines the problem scope for multicast mobility
management, which may be elaborated in future work. It is subdivided
to present the various challenges according to their originating
aspects, and identifies existing proposals and major bibliographic
references.

When considering multicast node mobility, two basic scenarios are of
interest: Single-hop mobility (shown in figure 1.a) and multi-hop
mobile routing (figure 1.b). Single-hop mobility is the focus of this
document, which coincides with the perspective of MIPv6 [6]. 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] ([9] provides a general
overview). Specific issues arising from (nested) tunneling or
flooding, especially the preservation of address transparency,
require treatment analogous to MIPv6.

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

2. Problem Description

2.1 General Issues

Multicast mobility is a generic term, which subsumes a collection of
distinct functions. First, multicast communication is divided into
Any Source Multicast (ASM) [3] and Source Specific Multicast (SSM)
[9,10].
[10,11]. 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 DVMRP [11], [12], PIM-
SM/SSM [12,13], [13,14], Bi-directional PIM [14],or [15],or inter-domain multicast
prefix advertisements via MBGP [15]. [16]. IPv6 clients interact using the
multicast listener discovery protocol (MLD and MLDv2) [16,17]. Other
multicast routing protocols without significant current deployment
include CBT [18], BGMP [19]. [17,18].

Any multicast mobility solution must 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
should 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
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 [6], its security
extensions [7,20], [7,19], its expediting schemes FMIPv6 [21] [20] and HMIPv6
[22],
[21], its context transfer protocols [23], [22], its multihoming
capabilities [24,25], [23,24], emerging protocols like PMIPv6 [61] [63] 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, or even minutes, 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 re-adjusts 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 connectionless
transport protocols. Where applications are sensitive to packet loss
or jitter, countermeasures must 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 foreseeable degradations at the
transport layer. 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 [26]. [25]. 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
addreses.
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 unknown 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 is it able to learn about the
network-layer state of its receivers. In the event of an inter-tree
handover, a mobile multicast source therefore is vulnerable to
loosing connectivity to receivers without noticing. (Appendix A
describes implicit source notification approaches). Applying a MIPv6
mobility binding update or return routability procedure will
similarly break the semantic of a receiver group remaining
unidentified by the source and thus cannot be applied in unicast
analogy.

Despite the complexity of the requirements, multicast mobility
management should seek lightweight solutions with easy deployment.
Realistic, sample deployment scenarios and architectures should be
provided in future solution documents.

2.2 Multicast Listener Mobility

2.2.1 Node & 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 either one of the
following conditions: The new network may not be multicast-enabled
o In the simplest scenario, packets of some, or all, of the specific multicast service may be unavailable, e.g. unsupported
subscribed groups of the mobile node are already received by one
or prohibited. Alternatively, several other group members in the new network, and thus
multicast streams natively flow after the MN arrives at the new
network.
o The requested multicast service may be supported and enabled in
the visited network, but the multicast groups under subscription
may not be forwarded to it. This means that current distribution
trees for the desired groups may only be re-
joined re-joined at a large (possibly
large) routing distance.
o The simplest scenario is when
packets of some, or all, of the subscribed groups of the mobile node
are already received by one new network may not be multicast-enabled or several group members in the
destination network, and thus specific
multicast streams natively flow after service may be unavailable, e.g., unsupported or
prohibited. This means that current distribution trees for the MN arrives
desired groups need to be re-joined at the new network. a large routing distance
by (re) establishing a tunnel to a multicast-enabled network
node.

The problem of achieving seamless multicast listener handovers is
thus threefold:
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. Dependant on layer 2
and 3 handover performance, the time available for multicast
mobility operations is typically bound to a fraction of 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 can not be verified for multicast). In the absence of
fragmentation at tunnel entry points, this may prevent the group
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 layer 2 / vertical handover.
o Ensure routing convergence, even when the MN moves rapidly
and performs handovers at a high frequency.
o Avoid avalanche problems and 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 cause
initiation in the new network and termination of a group distribution
service in the previous network. Mobility management may issue
traffic directives that lead to suboptimal routing, i.e., erroneous
subscriptions following predictive handover operations, or slow
effective leaves caused by MLD querying, or by departure of the MN
from a previous network without leaving the subscribed groups.
Finally, packet duplication and re-ordering may follow a change of
topology.

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
bi-directional 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 precede 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 [6].

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 bi-directional shared domain-spanning
trees, when using bi-directional 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 [26], [25], e.g., the primary rendezvous
point for inter-domain PIM-SM will be globally appointed appointed, and a newly
attached mobile source can contact the RP without prior signaling requirements obsolete.
(like a new source) and transmit data in the PIM register tunnel.

Multicast route optimization (e.g., PIM 'shortcuts') will require
multicast routing protocol operations equivalent to serving a new
source.

2.3.2 Source Specific Multicast Mobility

Source Specific Multicast has been designed for multicast senders
with static source addresses. The source addresses in a client
subscription to an SSM group is directly used to route
identification. Any SSM subscriber is thus forced to know the
topological address of the contributor to the group it wishes to
join. The SSM source identification becomes invalid when the
topological source address changes under mobility. Hence client
implementations of SSM source filtering must be MIPv6 aware in the
sense that a logical source identifier (HoA) is correctly mapped to
its current topological correspondent (CoA).

As a consequence, source mobility for SSM requires a conceptual
treatment beyond the problem scope of mobile ASM. A listener
subscribes to an (S,G) channel membership and routers establish an
(S,G)-state shortest path tree rooted at source S, therefore any
change of source addresses under mobility requires state updates at
all routers on the upstream path and at all receivers in the group.
On source handover, a new SPT needs to be established that will share
paths with the previous SPT, e.g., at the receiver side. As the
principle of multicast decoupling of a sender from its receivers
holds for SSM, the client updates needed for switching trees become a
severe burden.

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 [6].

The above methods add significant complexity to provide a robust SSM
mobility solution, which needs to converge to optimal routes and, for
efficiency, should 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 [27,28]. [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 leanness of SSM by
minimizing additional signaling overhead.

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 implementations that support multicast [29]. [28]. While many
(walled) domains or enterprise networks operate point-to-multipoint
services, IP multicast rollout is currently limited in public inter-
domain scenarios [30]. [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 the
mobile multicast domain [31]. [30]. Garyfalos and Almeroth [32] [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.

Facing deployment complexity, it is desirable that any solution for
mobile multicast should leave routing protocols unchanged. 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, whether this can be achieved in all cases.

Nevertheless, multicast services in mobile environments may soon
become indispensable, when multimedia distribution services such as
DVB-H [33,34] [32,33] or IPTV develop a strong business case for IP
portables. As IP mobility becomes an important service and as
efficient link utilization is of a larger impact in costly radio
environments, the evolution of multicast protocols will naturally
follow mobility constraints.

3.Characteristics of Multicast Routing Trees under Mobility

Multicast distribution trees have been studied from a focus of
network efficiency. Grounded on empirical observations Chuang and
Sirbu [35] [34] 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 rather tall than
wide.

Subsequent empirical and analytical work [36,37] [35,36] debated the
applicability of the Chuang and Sirbu scaling law. Van Mieghem et al.
[36]
[35] 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 self-similarity Janic and Van Mieghem [38] [37] semi-
empirically substantiated that multicast shortest path trees in the
Internet can be modeled with reasonable accuracy by uniform recursive
trees (URT) [39], [38], provided m remains small compared to N.

The mobility perspective on shortest path trees focus 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
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 [28], [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. Link Layer Aspects

4.1 General Background

Scalable group data distribution has the highest potential in leaf
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. Of focal interest
to the mobility domain are wireless access technologies, which always
operate on a shared medium with limited frequency and bandwidth.

Several aspects need consideration: First, dissimilar network access
radio technologies cause distinct group traffic transmissions. There
are:

o connection-less link services of a broadcast type, which mostly
are bound to limited reliability;

o connection-oriented link services of a point-to-multipoint type,
which require more complex control and frequently exhibit reduced
efficiency;

o connection-oriented link services of a broadcast type, which are
restricted to unidirectional data transmission.

In addition, multicast may be distributed via multiple point-to-point
unicast links without 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 [67] [70] or proxy [40], [39], 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.

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.

4.2 Multicast for Specific Technologies

4.2.1 802.11 WLAN

IEEE 802.11 WLAN is a broadcast network of Ethernet type. This
inherits multicast address mapping concepts from 802.3. In
infrastructure mode 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 a 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.
Frames (and hence packet) loss arise 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 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 WIMAX combines a family of connection-oriented radio
transmission services, operating services that can operate in single-hop point-to-
multipoint (PMP) or in mesh 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 Stations, Station,
which assign assigns channel IDs (CIDs) within service flows to the
subscriber stations.
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 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 [41]. [40]. The latter treats each connection to
a mobile node as a single link and is recommended in the IPv6
Convergence Sublayer [42], [41], while MAC separation within a shared
prefix is applied in the IP over Ethernet CS [43]. [42]. The point-to-point
link model on the IP layer conflicts with a consistent group
distribution via a shared medium (cf. section 4.1 for MVR as a
workaround).

To invoke a multipoint data channel, the base station assigns a
common CID to all Subscriber Stations in the group. An IPv6 multicast
address mapping to these 16 bit IDs is proposed by copying either the
4 lowest bits, while sustaining the scope field, or by utilizing the
8 lowest bits derived from Multicast on Ethernet CS [44]. [43]. For
selecting group members, a Base Station may implement IGMP/MLD
snooping or proxy as foreseen in 802.16e-2005 [45]. [44].

A Subscriber Station will send multicast data to a Base Station as a
point-to-point unicast stream, which - in the presence of the IPv6 CS
- is forwarded to the upstream access router. The access router or -
in the presence of the IP over Ethernet CS - the Base Station may
return multicast data to the downstream Base Station by feeding into
a multicast service channel. On reception, a Subscriber Station
cannot distinguish multicast from unicast streams. streams on 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.

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 selectiveness.

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 3GPP/3GPP2

The 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) [46]. [45]. 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 [47]. [46]. Current packet filtering practice causes inter-working
problems between Mobile IPv6 nodes connected via GPRS [48]. [47].

In 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 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 GGSN commonly are regional, distant entities, triangular
routing and encapsulation may cause a significant degradation of
efficiency.

In 3GPP2, the MBMS has been extended to the Broadcast and Multicast
Service (BCMCS) [48], which on the routing layer operates very
similar to MBMS. In both 3GPP and 3GPP2 multicast can either be sent
using point-to-point (PTP) or point-to-multipoint (PTM) tunnels, and
there is support for switching between PTP and PTM. PTM uses an
unidirectional common channel, operating in unacknowledged 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, IP-encapsulated data streams, and published as an ETSI
standard [49] (see http://www.dvb-h.org). DVB uses a mechanism called
multi-protocol encapsulation (MPE), which enables a transport of
network layer protocols on top of a link layer built from MPEG-2
transport streams and includes link forward error correction (FEC).
In this model, DVB transmission networks not only support TV
broadcasting, but also offer an IP Datacast Service. DVB-IPDC [33] [32]
consists of a number of individual, application layer specifications,
some of which continue to be developed. Transport Streams (TS) form
the basic logical channels, identified by a 13 bit TS ID (PID). This,
together with a multiplex service ID, is associated with IPv4 or IPv6
addresses [50] and used for selective traffic filtering at receivers.
Upstream channels may complement DVB-H using alternative transmission
technologies.

Multicast distribution services are defined by a mapping of groups
onto appropriate PIDs, which is managed at the IP Encapsulator [51].
To increase flexibility and avoid collisions, this address resolution
is facilitated by dynamic tables, provided within the self-consistent
MPEG-2 TS. Mobility is supported in the sense that changes of cell
ID, network ID or Transport Stream ID are foreseen [52]. A multicast
receiver thus needs to re-locate the multicast services it is
subscribed to, which is to be done in 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.

4.3 Vertical Multicast Handovers

A mobile

4.2.5 TV Broadcast and Satellite Networks

IP multicast node may operate homogeneous (horizontal) or
heterogeneous (vertical) layer 2 handovers with or without layer 3
network changes. Consequently, a dedicated context transfer of
multicast configuration is required at network access. Media
Independent Handover (MIH) is addressed be enabled in IEEE 802.21 [53], but is
relevant TV broadcast networks, including those
specified by DVB, ATSC, and related standards [51]. These standards
are also beyond IEEE protocols. Mobility services used for one and two-way satellite IP services. Networks
based on the MPEG-2 Transport Stream may support either the multi-
protocol encapsulation (MPE) or the unidirectional lightweight
encapsulation (ULE) [53]. The second generation DVB standards allow
the Transport Stream to be replaced with a Generic Stream, using the
generic stream encapsulation (GSE) [54]. 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 [51]. The addressing issues resemble
those for DVB-H (section 4.2.4) [50]. 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 [55], but is relevant also beyond
IEEE protocols. Mobility services transport for MIH naturally reside
on the network layer and are currently in the process of
specification [54]. [56].

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 [55]: [57]:

o Bi-directional Tunnelling, 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
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 tunnelling 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 [6] introduces bi-directional tunnelling tunneling as well as remote
subscription as minimal standard solutions. Various publications
suggest utilizing remote subscription for listener mobility only,
while advising bi-directional tunnelling tunneling as the solution for source
mobility. Such an approach avoids the 'tunnel convergence' or
'avalanche' problem [55], [57], which refers to the responsibility of the
home agent to multiply and encapsulate packets for many receivers of
the same group, even if they are located within the same subnetwork.
However, this suffers from the drawback that multicast communication
roles are not explicitly known at the network layer and may change
unexpectedly.

None of the above approaches address SSM source mobility, except the
use of bi-directional tunnelling. 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 [21], [20], the M-FMIPv6 [56,57], [58,59], and of
Hierarchical MIPv6 [22], [21], the M-HMIPv6 [58], [60], and to context transfer
[59],
[61], which have been thoroughly analyzed in [27,60]. [26,62].

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, PMIPv6 [61], at its current stage
of evolution [63], is multicast transparent, as
transparent in the sense that the MN experiences a point-
to-point point-to-point
home link fixed at its local mobility anchor (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 convergence problem; future data to a large number of MAGs,
while a MAG may be required to forward the same multicast stream to
many MNs via individual point-to-point links [64]. Future
optimizations and extensions to shared links should foresee 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 [62]. [65]. Aside from IETF work, countless numerous publications
present proposals for seamless multicast listener mobility, e.g. [63] [66]
provides a comprehensive overview. 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 tunnelling tunneling in IPv6 [64] [67] using a unicast point-to-point method
will also allow multicast-agnostic domains to be transited, but does
inherit the tunnel convergence problem and may result in traffic
multiplication.

Multicast enabled environments may take advantage of point-to-
multipoint encapsulation, i.e., generic packet tunnelling 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.

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 method 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 [32]. [31]. 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 [65] [68] introduced
a mobility-agnostic multicast backbone on the overlay.

Current work in the SAM RG is developing general architectural
approaches for hybrid multicast solutions [66] [69] that will require a
detailed design in future work.

5.2.4 MLD Extensions

The default timer values specified in MLD [17] [18] were not designed for
the mobility context. This results in a slow reaction of the
multicast routing infrastructure (including layer-3-aware access
devices [67]) [70]) following a client leave. This may be a disadvantage
for wireless links, where performance may be improved by carefully
tuning the Query Interval. Some vendors have optimised optimized performance by
implementing a listener node table at the access router that can
eliminate query timeouts on leaves (explicit receiver tracking).

A 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 case of an erroneous prediction (i.e., an anticipated
move to a network that is not fulfilled). Backward context transfer
may otherwise ensure a leave is signaled. A further optimization was
introduced by Jelger and Noel [68] [71] for the special case when the HA
is a multicast router. A Done message received through a tunnel from
the mobile end node (through a point-to-point link directly
connecting the MN, in general), should not initiate regular 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, a MN may wish to rely on a proxy or 'standby'
multicast membership service, optionally provided by a 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 [68] [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 [58] [60]
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 in to into three
categories:

o Statically Rooted Distribution Trees. These methods follow a
shared tree approach. Romdhani et al. [69] [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 bi-directional tunneling. Focusing on interdomain
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 [58] [60] 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 RBMoM
protocol [70] [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 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 [71] [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 [69] [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 re-introduces the concept of rendezvous points to SSM routing.

Approaches for reconstructing SPTs in SSM rely on a client
notification to establish new router state. It also needs to preserve
address transparency for the client. Thaler [72] [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 [73] [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 SDR/SAP [74]. [77]. Receiver-oriented tree
construction in SSM thus remains unsynchronized with source
handovers.

To address the synchronization problem at the routing layer, several
proposals have focused on direct modification of the distribution
trees. A recursive scheme may use loose unicast source routes with
branch points, based on a multicast Hop-by-Hop protocol. Vida et al
[75]
[78] optimized SPT for a moving source on the path between the source
and first branching point. O'Neill [76] [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
and Waehlisch [77] [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. Lee et al. [78] [81] introduced a
state-update mechanism for re-using 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 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.

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.

Mobility protocols need to consider the implications and requirements
for AAA. AAA binding records may change between attachments, or may
be individually chosen prior to network (re-)association. The most
appropriate network path may be 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 cannot be treated at a pure level of context transfer.

Admission control issues may arise with new CoA source addresses
being introduced to SSM channels (cf. [79] [82] for a comprehensive
discussion). Due to lack of feedback, the admission [80] [83] and binding
updates [81] [84] of mobile multicast sources require self-consistent autonomously
verifiable authentication as achievable can be achieved by Cryptographically
Generated Addresses (CGAs).

Modification to IETF protocols (e.g. routing, membership, session
announcement and control) as well as the introduction of new
entities, e.g., multicast mobility agents, can introduce security
vulnerabilities and require consideration of issues such as
authentication of network entities, methods to mitigate denial of
service (in terms of unwanted network traffic, unnecessary
consumption of router/host resources and router/host state/buffers).
Future solutions must therefore analyze and address the security
implications of supporting mobile multicast.

7.Summary and Future Steps

This document is intended to provide a basis for the future design of
mobile IPv6 multicast methods and protocols by:

o providing a structured overview of the problem space that
multicast and mobility jointly generate at the IPv6 layer;

o referencing the implications and constraints arising from
lower and upper layers, and from deployment;

o briefly surveying conceptual ideas of currently available
solutions;

o including a comprehensive bibliographic reference base.

It is recommended that future steps towards extending mobility
services to multicast proceed to first solve the following problems:

1. Ensure seamless multicast reception during handovers,
meeting the requirements of mobile IPv6 nodes and networks.
Thereby address the problems of home subscription without
n-tunnels, as well as native multicast reception in those
visited networks, which offer a group communication service.

2. Integrate multicast listener support into unicast mobility
management schemes and architectural entities to define a
consistent mobility service architecture, providing equal
supporting for unicast and multicast communication.

3. Provide basic multicast source mobility by designing
address duality management at end nodes.

8. IANA Considerations

There are no IANA considerations introduced by this draft.

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 multicats multicast packets which
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) [82] [85] was such a suggested method that allowed a
multicast source to querying the upstream designated router. However,
this work did not progress within the IETF mboned working group and
was terminated by 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 Realtime Transport Protocol (RTP) [83]. [86]. The
accompanying control protocol RTCP [81] 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. The Real Time Streaming
Protocol (RTSP), (RFC 2326) 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
visble
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 protocols.

9. References

Informative 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 1989.

4 G. Xylomenos and G.C. Plyzos, "IP Multicast for Mobile Hosts",
IEEE Communications Magazine, 35(1), 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 ITU-T Recommendation, "G.114 - One-way transmission time",
Telecommunication Union Standardization Sector, 05/2003.

9 Akyildiz, I and Wang, X. X., "A Survey on Wireless Mesh Networks",
IEEE Communications Magazine, 43(9), pp. 23-30, September 2005.

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

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

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

13 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.

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

15 M. Handley, I. Kouvelas, T. Speakman, L. Vicisano, "Bidirectional
Protocol Independent Multicast (BIDIR-PIM)", RFC 5015, October
2007.

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.

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

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

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

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

20 Koodli, R. "Mobile IPv6 Fast Handovers", RFC 5268, June 2008.

21 Soliman, H, Castelluccia, C, El-Malki, K, Bellier, L.
"Hierarchical Mobile IPv6 mobility management", RFC 4140, August
2005.

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

23 Montavont, N, et al. "Analysis of Multihoming in Mobile IPv6",
draft-ietf-monami6-mipv6-analysis-05, Internet Draft (work in
progress), May 2008.

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, 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/, 2008.

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-
04.pdf.gz

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 "Digital Video Broadcasting (DVB); IP Datacast over DVB-H: Set of
Specifications for Phase 1", ETSI TS 102 468;

33 ETSI TS 102 611, "Digital Video Broadcasting (DVB); IP Datacast
over DVB-H: Implementation Guidelines for Mobility)", European
Standard (Telecommunications series), November 2004.

34 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.

35 Van Mieghem, P, Hooghiemstra, G, Hofstad, R. "On the Efficiency of
Multicast", IEEE/ACM Trans. Netw., 9, 6, pp. 719-732, Dec. 2001.

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

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

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

39 Fenner, B, He, H, Haberman, B, Sandick, H. "Internet Group
Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-
Based Multicast Forwarding ("IGMP/MLD Proxying")", RFC 4605,
August 2006.

40 Shin, M. et al. "IPv6 Deployment Scenarios in 802.16 Networks",
RFC 5181, May 2008.

41 Patil, B. et al. "Transmission of IPv6 via the IPv6 Convergence
Sublayer over IEEE 802.16 Networks", RFC 5121, February 2008.

42 Jeon, H., Riegel, M. and Jeong, S. "Transmission of IP over
Ethernet over IEEE 802.16 Networks ", draft-ietf-16ng-ip-over-
ethernet-over-802.16-06.txt, (work in progress), April 2008.

43 Kim, S. et al. "Multicast Transport on IEEE 802.16 Networks",
draft-sekim-802-16-multicast-01, (work in progress), July 2007.

44 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.

45 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.

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

47 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.

48 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.

49 ETSI EN 302 304, "Digital Video Broadcasting (DVB); Transmission
System for Handheld Terminals (DVB-H)", European Standard
(Telecommunications series), November 2004.

50 Fairhurst, G. and Montpetit, M. "Address Resolution Mechanisms for
IP Datagrams over MPEG-2 Networks", RFC 4947, July 2007.

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

52 Yang, X, Vare, J, Owens, T. "A Survey of Handover Algorithms in
DVB-H", IEEE Comm. Surveys, 8(4), 2006.

53 Fairhurst, G., and Collini-Nocker, B. "Unidirectional Lightweight
Encapsulation (ULE) for Transmission of IP Datagrams over an MPEG-
2 Transport Stream (TS)", RFC4326, December 2005.

54 Fairhurst, G., and Collini-Nocker, B. "Extension Formats for
Unidirectional Lightweight Encapsulation (ULE) and the Generic
Stream Encapsulation (GSE)", RFC5163, April 2008.

55 "Draft IEEE Standard for Local and Metropolitan Area Networks:
Media Independent Handover Services", IEEE LAN/MAN Draft IEEE
P802.21/D07.00, July 2007.

56 Melia, T. et al. "Mobility Services Transport: Problem Statement",
RFC 5164, March 2008.

57 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, online http://www.comnets.rwth-
aachen.de/~o_drive/publications/ist-summit-2003-IPMobileMulticast-
paperv2.0.pdf.

58 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.

59 Xia, F. and Sarikaya, B, "FMIPv6 extensions for Multicast
Handover", draft-xia-mipshop-fmip-multicast-01, (work in progress,
expired), March 2007.

60 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
2005.

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

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

63 Gundavelli, S, et al. "Proxy Mobile IPv6", draft-ietf-netlmm-
proxymip6-18.txt, RFC 5213, August 2008.

64 Deng, H, Schmidt, T.C., Seite, P., and Yang, P. "Multicast Support
Requirements for Proxy Mobile IPv6", draft-deng-multimob-pmip6-
requirement-01, (work in progress), May October 2008.

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

66 Romdhani, I, Kellil, M, Lach, H.-Y. et. al. "IP Mobile Multicast:
Challenges and Solutions", IEEE Comm. Surveys, 6(1), 2004.

67 Conta, A, Deering, S, "Generic Packet Tunneling in IPv6 -
Specification", RFC 2473, December 1998.

68 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.

69 Buford, J. "Hybrid Overlay Multicast Framework", draft-irtf-sam-
hybrid-overlay-framework-02.txt, Internet Draft (work in
progress), February 2008.

70 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.

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

72 Romdhani, I, Bettahar, H. and Bouabdallah, A. "Transparent
handover for mobile multicast sources", in P. Lorenz and P. Dini,
eds, Proceedings of the IEEE ICN'06, IEEE Press, 2006.

73 Lin, C.R. et al. "Scalable Multicast Protocol in IP-Based Mobile
Networks", Wireless Networks, 8 (1), pp. 27-36, January, 2002.

74 Chang, R.-S. and Yen, Y.-S. "A Multicast Routing Protocol with
Dynamic Tree Adjustment for Mobile IPv6", Journ. Information
Science and Engineering 20, 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 Noel, T. "Supporting Mobile SSM sources for IPv6
(MSSMSv6)", Internet Draft (work in progress, expired), January
2002.

77 Handley, M, Perkins, C, Whelan, E. "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 O'Neill, A. "Mobility Management and IP Multicast", draft-oneill-
mip-multicast-00.txt, (work in progress, expired), 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.

81 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.

82 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.

83 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.

81 Christ, O,

84 Schmidt, T.C, Waehlisch, M. "A Light-Weight
Implementation Scheme of the Tree Morphing Protocol M., Christ, O, and Hege, G. "AuthoCast -
a mobility-compliant protocol framework for Mobile
Multicast Sources ", Proc. of 33rd Euromicro Conf, pp. 149-156,
IEEE/CS Press, Sept. 2007.

82 multicast sender
authentication", Security and Communication Networks, 1(6), 2008.

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

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

Acknowledgments

Work on exploring the problem space for mobile multicast has been
pioneered by Greg Daley and Gopi Kurup within their early draft
"Requirements for Mobile Multicast Clients" (draft-daley-magma-
mobile).

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, Cedric
Baudoin, Hans L. Cycon, Hui Deng, Marshall Eubanks, Zhigang Huang,
Christophe Jelger, Rajeev Koodli, Mark Palkow, Imed Romdhani, Hesham
Soliman, Dave Thaler and last last, but not least least, very special thanks to
Stig Venaas for his frequent and thorough advice.

Author's Addresses

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

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

Godred Fairhurst
School of Engineering,
University of Aberdeen,
Aberdeen, AB24 3UE, UK
EMail: gorry@erg.abdn.ac.uk

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