Internet Research Task Force (IRTF) T. Schmidt
Request for Comments: 5757 HAW Hamburg
Category: Informational M. Waehlisch
ISSN: 2070-1721 link-lab
G. Fairhurst
University of Aberdeen
February 2010
Multicast Mobility in Mobile IP Version 6 (MIPv6):
Problem Statement and Brief Survey
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 problems for mobile senders using Any Source Multicast and
Source-Specific Multicast. Characteristic aspects of multicast
routing and deployment issues for fixed IPv6 networks are summarized.
Specific properties and interplays with the underlying network access
are surveyed with respect to the relevant technologies in the
wireless domain. It outlines the principal approaches to multicast
mobility, together with a comprehensive exploration of the mobile
multicast problem and solution space. This document concludes with a
conceptual road map for initial steps in standardization for use by
future mobile multicast protocol designers. This document is a
product of the IP Mobility Optimizations (MobOpts) Research Group.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the MobOpts
Research Group of the Internet Research Task Force (IRTF). Documents
approved for publication by the IRSG are not a candidate for any
level of Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5757.
Schmidt, et al. Informational [Page 1]RFC 5757 MMCASTv6-PS February 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Schmidt, et al. Informational [Page 2]
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Table of Contents
1. Introduction and Motivation .....................................4
1.1. Document Scope .............................................5
2. Problem Description .............................................6
2.1. General Issues .............................................6
2.2. Multicast Listener Mobility ................................9
2.2.1. Node and Application Perspective ....................9
2.2.2. Network Perspective ................................10
2.3. Multicast Source Mobility .................................11
2.3.1. Any Source Multicast Mobility ......................11
2.3.2. Source-Specific Multicast Mobility .................12
2.4. Deployment Issues .........................................13
3. Characteristics of Multicast Routing Trees under Mobility ......14
4. Link Layer Aspects .............................................15
4.1. General Background ........................................15
4.2. Multicast for Specific Technologies .......................16
4.2.1. 802.11 WLAN ........................................16
4.2.2. 802.16 WIMAX .......................................16
4.2.3. 3GPP/3GPP2 .........................................18
4.2.4. DVB-H / DVB-IPDC ...................................19
4.2.5. TV Broadcast and Satellite Networks ................19
4.3. Vertical Multicast Handovers ..............................20
5. Solutions ......................................................20
5.1. General Approaches ........................................20
5.2. Solutions for Multicast Listener Mobility .................21
5.2.1. Agent Assistance ...................................21
5.2.2. Multicast Encapsulation ............................22
5.2.3. Hybrid Architectures ...............................23
5.2.4. MLD Extensions .....................................23
5.3. Solutions for Multicast Source Mobility ...................24
5.3.1. Any Source Multicast Mobility Approaches ...........24
5.3.2. Source-Specific Multicast Mobility Approaches ......25
6. Security Considerations ........................................26
7. Summary and Future Steps .......................................27
Appendix A. Implicit Source Notification Options...................29
Informative References.............................................29
Acknowledgments....................................................37
Schmidt, et al. Informational [Page 3]RFC 5757 MMCASTv6-PS February 2010
1. Introduction and Motivation
Group communication forms an integral building block of a wide
variety of applications, ranging from content broadcasting and
streaming, voice and video conferencing, collaborative environments
and massive multiplayer gaming, up to the self-organization of
distributed systems, services, or autonomous networks. Network-layer
multicast support will be needed whenever globally distributed,
scalable, serverless, or instantaneous communication is required.
The early idea of Internet multicasting [1] soon led to a wide
adoption of Deering's host group model [2]. Broadband media delivery
is emerging as a typical mass scenario that demands scalability and
bandwidth efficiency from multicast routing. Although multicast
mobility has been a concern for about ten years [3] and has led to
numerous proposals, there is as yet no generally accepted solution.
Multicast network support will be of particular importance to mobile
environments, where users commonly share frequency bands of limited
capacity. Reception of "infotainment" streams may soon require wide
deployment of mobile multicast services.
Mobility in IPv6 [4] is standardized in the Mobile IPv6 RFCs [5][6],
and it addresses the scenario of network-layer changes while moving
between wireless domains. MIPv6 [5] only roughly defines multicast
mobility for Mobile Nodes (MNs) using a remote subscription approach
or through bidirectional tunneling via the Home Agent (HA). Remote
subscription suffers from slow handovers relying on multicast routing
to adapt to handovers. Bidirectional tunneling introduces
inefficient overhead and delay due to triangular forwarding, i.e.,
instead of traveling on shortest paths, packets are routed through
the Home Agent. Therefore, these approaches have not been optimized
for a large scale deployment. A mobile multicast service for a
future Internet should provide "close-to-optimal" routing at
predictable and limited cost, offering robustness combined with a
service quality compliant to real-time media distribution.
Intricate multicast routing procedures are not easily extensible to
satisfy the requirements for mobility. A client subscribed to a
group while performing mobility handovers requires the multicast
traffic to follow to its new location; a mobile source needs the
entire delivery tree to comply with or to adapt to its changing
position. Significant effort has already been invested in protocol
designs for mobile multicast receivers; only limited work has been
dedicated to multicast source mobility, which poses the more delicate
problem [65].
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In multimedia conference scenarios, games, or collaborative
environments, each member commonly operates as a receiver and as a
sender for multicast group communication. In addition, real-time
communication such as conversational voice or video places severe
temporal requirements on mobility protocols: Typical seamless
handover scenarios are expected to limit disruptions or delay to less
than 100 - 150 ms [7]. Jitter disturbances should not exceed 50 ms.
Note that 100 ms is about the duration of a spoken syllable in real-
time audio. This problem statement is intended to also be applicable
to a range of other scenarios with a range of delivery requirements
appropriate to the general Internet.
This document represents the consensus of the MobOpts Research Group.
It has been reviewed by the Research Group members active in the
specific area of work. In addition, this document has been
comprehensively reviewed by multiple active contributors to the IETF
MEXT, MBONED, and PIM Working Groups.
1.1. Document Scope
This document defines the problem scope for multicast mobility
management, which may be elaborated in future work. It is subdivided
to present the various challenges according to their originating
aspects, and identifies existing proposals and major bibliographic
references.
When considering multicast node mobility, the network layer is
complemented by some wireless access technology. Two basic scenarios
are of interest: single-hop mobility (shown in Figure 1.a) and multi-
hop mobility (shown in Figure 1.b). Single-hop mobility is the focus
of this document, which coincides with the perspective of MIPv6 [5].
The key issues of mobile multicast membership control and the
interplay of mobile and multicast routing will be illustrated using
this simple scenario.
Multi-hop network mobility is a subsidiary scenario. All major
aspects are inherited from the single-hop problem, while additional
complexity is incurred from traversing a mobile cloud. This may be
solved by either encapsulation or flooding ([8] provides a general
overview). Specific issues arising from (nested) tunneling or
flooding, especially the preservation of address transparency,
require treatment analogous to MIPv6.
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+------+ +------+
| MN | =====> | MN |
+------+ +------+
| .
| .
| .
+-------+ +-------+
| LAR 1 | | LAR 2 |
+-------+ +-------+
\ /
*** *** *** ***
* ** ** ** *
+------+ +------+ * *
| MN | =====> | MN | * Mobile Network *
+------+ +------+ * *
| . * ** ** ** *
| . *** *** *** ***
| . | .
+-------+ +-------+ +-------+ +-------+
| AR 1 | | AR 2 | | AR 1 | =====> | AR 2 |
+-------+ +-------+ +-------+ +-------+
| | | |
*** *** *** *** *** *** *** ***
* ** ** ** * * ** ** ** *
* * * *
* Fixed Internet * * Fixed Internet *
* * * *
* ** ** ** * * ** ** ** *
*** *** *** *** *** *** *** ***
a) Single-Hop Mobility b) Multi-Hop Mobility
Figure 1: Mobility Scenarios - A Mobile Node (MN) Directly Attaching
to Fixed Access Routers (ARs) or Attached via Local Access Routers
(LARs)
2. Problem Description
2.1. General Issues
Multicast mobility is a generic term, which subsumes a collection of
distinct functions. First, the multicast communication is divided
into Any Source Multicast (ASM) [2] and Source-Specific Multicast
(SSM) [9][10]. Second, the roles of senders and receivers are
distinct and asymmetric. Both may individually be mobile. Their
interaction is facilitated by a multicast routing protocol such as
the Distance Vector Multicast Routing Protocol (DVMRP) [11], the
Schmidt, et al. Informational [Page 6]RFC 5757 MMCASTv6-PS February 2010
Protocol Independent Multicast - Sparse Mode / Source-Specific
Multicast (PIM-SM/SSM) [12][13], the Bidirectional PIM [14], or the
inter-domain multicast prefix advertisements via Multiprotocol
Extensions for BGP-4 (MBGP) [15]. IPv6 clients interact using the
multicast listener discovery protocol (MLD and MLDv2) [16][17].
Any solution for multicast mobility needs to take all of these
functional blocks into account. It should enable seamless continuity
of multicast sessions when moving from one IPv6 subnet to another.
It is desired to preserve the multicast nature of packet distribution
and approximate optimal routing. It should support per-flow handover
for multicast traffic because the properties and designations of
flows can be distinct. Such distinctions may result from differing
Quality-of-Service (QoS) / real-time requirements, but may also be
caused by network conditions that may differ for different groups.
The host group model extends the capability of the network-layer
unicast service. In common with the architecture of fixed networks,
multicast mobility management should transparently utilize or
smoothly extend the unicast functions of MIPv6 [5], its security
extensions [6][18], its expediting schemes FMIPv6 [19] and
Hierarchical Mobile IPv6 Environment (HMIPv6) [20], its context
transfer protocols [21], its multihoming capabilities [22][23],
emerging protocols like PMIPv6 [62], or future developments. From
the perspective of an integrated mobility architecture, it is
desirable to avoid multicast-specific as well as unicast-restricted
solutions, whenever general approaches can be derived that can
jointly support unicast and multicast.
Multicast routing dynamically adapts to the network topology at the
locations of the sender(s) and receiver(s) participating in a
multicast session, which then may change under mobility. However,
depending on the topology and the protocol in use, current multicast
routing protocols may require a time close to seconds to converge
following a change in receiver or sender location. This is far too
slow to support seamless handovers for interactive or real-time media
sessions. The actual temporal behavior strongly depends on the
multicast routing protocol in use, the configuration of routers, and
on the geometry of the current distribution tree. A mobility scheme
that readjusts routing, i.e., partially changes or fully reconstructs
a multicast tree, is forced to comply with the time scale for
protocol convergence. Specifically, it needs to consider a possible
rapid movement of the mobile node, as this may occur at much higher
rates than common protocol state updates.
The mobility of hosts using IP multicast can impact the service
presented to the higher-layer protocols. IP-layer multicast packet
distribution is an unreliable service that is bound to a
Schmidt, et al. Informational [Page 7]
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connectionless transport service. Where applications are sensitive
to packet loss or jitter, countermeasures need to be performed (loss
recovery, content recoding, concealment, etc.) by the multicast
transport or application. Mobile multicast handovers should not
introduce significant additional packet drops. Due to statelessness,
the bi-casting of multicast flows does not cause degradations at the
transport layer, and applications should implement mechanisms to
detect and correctly respond to duplicate datagrams. Nevertheless,
individual application programs may not be robust with respect to
repeated reception of duplicate streams.
IP multicast applications can be designed to adapt the multicast
stream to prevailing network conditions (adapting the sending rate to
the level of congestion, adaptive tuning of clients in response to
measured delay, dynamic suppression of feedback messages, etc.). An
adaptive application may also use more than one multicast group
(e.g., layered multicast in which a client selects a set of multicast
groups based on perceived available network capacity). A mobility
handover may temporarily disrupt the operation of these higher-layer
functions. The handover can invalidate assumptions about the
forwarding path (e.g., acceptable delivery rate, round-trip delay),
which could impact an application and level of network traffic. Such
effects need to be considered in the design of multicast applications
and in the design of network-layer mobility. Specifically, mobility
mechanisms need to be robust to transient packet loss that may result
from invalid path expectations following a handover of an MN to a
different network.
Group addresses, in general, are location transparent, even though
they may be scoped and methods can embed unicast prefixes or
Rendezvous Point addresses [24]. The addresses of sources
contributing to a multicast session are interpreted by the routing
infrastructure and by receiver applications, which frequently are
aware of source addresses. Multicast therefore inherits the mobility
address duality problem of MIPv6 for source addresses: addresses
being a logical node identifier, i.e., the home address (HoA) on the
one hand, and a topological locator, the care-of address (CoA), on
the other. At the network layer, the elements that comprise the
delivery tree, i.e., multicast senders, forwarders, and receivers,
need to carefully account for address duality issues, e.g., by using
binding caches, extended multicast states, or signaling.
Multicast sources, in general, operate decoupled from their receivers
in the following sense: a multicast source sends packets to a group
of receivers that are unknown at the network layer and thus operates
without a feedback channel. It neither has means to inquire about
the properties of its delivery trees, nor the ability to learn about
the network-layer state of its receivers. In the event of an inter-
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tree handover, a mobile multicast source therefore is vulnerable to
losing connectivity to receivers without noticing. (Appendix A
describes implicit source notification approaches). Applying a MIPv6
mobility binding update or return routability procedure will
similarly break the semantic of a receiver group remaining
unidentified by the source and thus cannot be applied in unicast
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 and Application Perspective
A mobile multicast listener entering a new IP subnet requires
multicast reception following a handover in real-time. This needs to
transfer the multicast membership context from its old to its new
point of attachment. This can either be achieved by
(re-)establishing a tunnel or by transferring the MLD Listening State
information of the MN's moving interface(s) to the new upstream
router(s). In the latter case, it may encounter any one of the
following conditions:
o In the simplest scenario, packets of some, or all, of the
subscribed groups of the mobile node are already received by one
or several other group members in the new network, and thus
multicast streams natively flow after the MN arrives at the new
network.
o The requested multicast service may be supported and enabled in
the visited network, but the multicast groups under subscription
may not be forwarded to it, e.g., groups may be scoped or
administratively prohibited. This means that current
distribution trees for the desired groups may only be re-joined
at a (possibly large) routing distance.
o The new network may not be multicast-enabled or the specific
multicast service may be unavailable, e.g., unsupported or
prohibited. This means that current distribution trees for the
desired groups need to be re-joined at a large routing distance
by (re-)establishing a tunnel to a multicast-enabled network
node.
The problem of achieving seamless multicast listener handovers is
thus threefold:
Schmidt, et al. Informational [Page 9]RFC 5757 MMCASTv6-PS February 2010
o Ensure multicast reception, even in visited networks, without
appropriate multicast support.
o Minimize multicast forwarding delay to provide seamless and fast
handovers for real-time services. Dependent on Layer 2 (L2) and
Layer 3 (L3) handover performance, the time available for
multicast mobility operations is typically bound by the total
handover time left after IPv6 connectivity is regained. In
real-time scenarios, this may be significantly less than 100 ms.
o Minimize packet loss and reordering that result from multicast
handover management.
Moreover, in many wireless regimes, it is also desirable to minimize
multicast-related signaling to preserve the limited resources of
battery-powered mobile devices and the constrained transmission
capacities of the networks. This may lead to a desire to restrict
MLD queries towards the MN. Multihomed MNs may ensure smooth
handoffs by using a "make-before-break" approach, which requires a
per-interface subscription, facilitated by an MLD JOIN operating on a
pre-selected IPv6 interface.
Encapsulation on the path between the upstream router and the
receiver may result in MTU size conflicts, since path-MTU discovery
is often not supported for multicast and can reduce scalability in
networks with many different MTU sizes or introduce potential denial-
of-service vulnerabilities (since the originating addresses of ICMPv6
messages cannot be verified for multicast). In the absence of
fragmentation at tunnel entry points, this may prevent the group from
being forwarded to the destination.
2.2.2. Network Perspective
The infrastructure providing multicast services is required to keep
traffic following the MN without compromising network functionality.
Mobility solutions thus have to face some immediate problems:
o Realize native multicast forwarding, and where applicable,
conserve network resources and utilize link-layer multipoint
distribution to avoid data redundancy.
o Activate link-multipoint services, even if the MN performs only
a L2/vertical handover.
o Ensure routing convergence, even when the MN moves rapidly and
performs handovers at a high frequency.
Schmidt, et al. Informational [Page 10]RFC 5757 MMCASTv6-PS February 2010
o Avoid avalanche problems and stream multiplication (n-casting),
which potentially result from replicated tunnel initiation or
redundant forwarding at network nodes.
There are additional implications for the infrastructure: In changing
its point of attachment, an exclusive mobile receiver may initiate
forwarding of a group in the new network and termination of a group
distribution service in the previous network. Mobility management
may impact multicast routing by, e.g., erroneous subscriptions
following predictive handover operations, or slow traffic termination
at leaf nodes resulting from MLD query timeouts, or by departure of
the MN from a previous network without leaving the subscribed groups.
Finally, packet duplication and reordering may follow a change of
topology.
2.3. Multicast Source Mobility
2.3.1. Any Source Multicast Mobility
A node submitting data to an ASM group either forms the root of a
source-specific shortest path tree (SPT), distributing data towards a
rendezvous point (RP) or receivers, or it forwards data directly down
a shared tree, e.g., via encapsulated PIM Register messages, or using
bidirectional PIM routing. Native forwarding along source-specific
delivery trees will be bound to the source's topological network
address, due to reverse path forwarding (RPF) checks. A mobile
multicast source moving to a new subnetwork is only able to either
inject data into a previously established delivery tree, which may be
a rendezvous-point-based shared tree, or to (re-)initiate the
construction of a multicast distribution tree for its new network
location. In the latter case, the mobile sender will have to proceed
without knowing whether the new tree has regained ability to forward
traffic to the group, due to the decoupling of sender and receivers.
A mobile multicast source must therefore provide address transparency
at two layers: To comply with RPF checks, it has to use an address
within the source field of the IPv6 basic header, which is in
topological agreement with the employed multicast distribution tree.
For application transparency, the logical node identifier, commonly
the HoA, must be presented as the packet source address to the
transport layer at the receiver side.
The address transparency and temporal handover constraints pose major
problems for route-optimizing mobility solutions. Additional issues
arise from possible packet loss and from multicast scoping. A mobile
source away from home must respect scoping restrictions that arise
from its home and its visited location [5].
Schmidt, et al. Informational [Page 11]RFC 5757 MMCASTv6-PS February 2010
Intra-domain multicast routing may allow the use of shared trees that
can reduce mobility-related complexity. A static rendezvous point
may allow a mobile source to continuously send data to the group by
encapsulating packets to the RP with its previous topologically
correct or home source address. Intra-domain mobility is
transparently provided by bidirectional shared domain-spanning trees,
when using bidirectional PIM, eliminating the need for tunneling to
the corresponding RP (in contrast to IPv4, IPv6 ASM multicast groups
are associated with a specific RP/RPs).
Issues arise in inter-domain multicast, whenever notification of
source addresses is required between distributed instances of shared
trees. A new CoA acquired after a mobility handover will necessarily
be subject to inter-domain record exchange. In the presence of an
embedded rendezvous point address [24], e.g., the primary rendezvous
point for inter-domain PIM-SM will be globally appointed, and a newly
attached mobile source can contact the RP without prior signaling
(like a new source) and transmit data in the PIM register tunnel.
Multicast route optimization (e.g., PIM "shortcuts") will require
multicast routing protocol operations equivalent to serving a new
source.
2.3.2. Source-Specific Multicast Mobility
Source-Specific Multicast has been designed for multicast senders
with static source addresses. The source addresses in a client
subscription to an SSM group is directly used to route
identification. Any SSM subscriber is thus forced to know the
topological address of the contributor to the group it wishes to
join. The SSM source identification becomes invalid when the
topological source address changes under mobility. Hence, client
implementations of SSM source filtering must be MIPv6 aware in the
sense that a logical source identifier (HoA) is correctly mapped to
its current topological correspondent (CoA).
As a consequence, source mobility for SSM requires a conceptual
treatment beyond the problem scope of mobile ASM. A listener
subscribes to an (S,G) channel membership and routers establish an
(S,G)-state shortest path tree rooted at source S; therefore, any
change of source addresses under mobility requires state updates at
all routers on the upstream path and at all receivers in the group.
On source handover, a new SPT needs to be established that will share
paths with the previous SPT, e.g., at the receiver side. As the
principle of multicast decoupling of a sender from its receivers
holds for SSM, the client updates needed for switching trees become a
severe burden.
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An SSM listener may subscribe to or exclude any specific multicast
source and thereby wants to rely on the topological correctness of
network operations. The SSM design permits trust in equivalence to
the correctness of unicast routing tables. Any SSM mobility solution
should preserve this degree of confidence. Binding updates for SSM
sources thus should have to prove address correctness in the unicast
routing sense, which is equivalent to binding update security with a
correspondent node in MIPv6 [5].
The above methods could add significant complexity to a solution for
robust SSM mobility, which needs to converge to optimal routes and,
for efficiency, is desired to avoid data encapsulation. Like ASM,
handover management is a time-critical operation. The routing
distance between subsequent points of attachment, the "step size" of
the mobile from previous to next designated router, may serve as an
appropriate measure of complexity [25][26].
Finally, Source-Specific Multicast has been designed as a lightweight
approach to group communication. In adding mobility management, it
is desirable to preserve the leanness of SSM by minimizing additional
signaling overhead.
2.4. Deployment Issues
IP multicast deployment, in general, has been slow over the past 15
years, even though all major router vendors and operating systems
offer implementations that support multicast [27]. While many
(walled) domains or enterprise networks operate point-to-multipoint
services, IP multicast roll-out is currently limited in public inter-
domain scenarios [28]. A dispute arose on the appropriate layer,
where group communication service should reside, and the focus of the
research community turned towards application-layer multicast. This
debate on "efficiency versus deployment complexity" now overlaps the
mobile multicast domain [29]. Garyfalos and Almeroth [30] derived
from fairly generic principles that when mobility is introduced, the
performance gap between IP- and application-layer multicast widens in
different metrics up to a factor of four.
Facing deployment complexity, it is desirable that any solution for
mobile multicast does not change the routing protocols. Mobility
management in such a deployment-friendly scheme should preferably be
handled at edge nodes, preserving a mobility-agnostic routing
infrastructure. Future research needs to search for such simple,
infrastructure-transparent solutions, even though there are
reasonable doubts as to whether this can be achieved in all cases.
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Nevertheless, multicast services in mobile environments may soon
become indispensable, when multimedia distribution services such as
Digital Video Broadcasting for Handhelds (DVB-H) [31][32] or IPTV
develop a strong business case for portable IP-based devices. As IP
mobility becomes an important service and as efficient link
utilization is of a larger impact in costly radio environments, the
evolution of multicast protocols will naturally follow mobility
constraints.
3. Characteristics of Multicast Routing Trees under Mobility
Multicast distribution trees have been studied from a focus of
network efficiency. Grounded on empirical observations, Chuang and
Sirbu [33] proposed a scaling power-law for the total number of links
in a multicast shortest path tree with m receivers (proportional to
m^k). The authors consistently identified the scale factor to attain
the independent constant k = 0.8. The validity of such universal,
heavy-tailed distribution suggests that multicast shortest path trees
are of self-similar nature with many nodes of small, but few of
higher degrees. Trees consequently would be shaped tall rather than
wide.
Subsequent empirical and analytical work [34][35] debated the
applicability of the Chuang and Sirbu scaling law. Van Mieghem et
al. [34] proved that the proposed power law cannot hold for an
increasing Internet or very large multicast groups, but is indeed
applicable for moderate receiver numbers and the current Internet
size of N = 10^5 core nodes. Investigating self-similarity, Janic
and Van Mieghem [36] semi-empirically substantiated that multicast
shortest path trees in the Internet can be modeled with reasonable
accuracy by uniform recursive trees (URTs) [37], provided m remains
small compared to N.
The mobility perspective on shortest path trees focuses on their
alteration, i.e., the degree of topological changes induced by
movement. For receivers, and more interestingly for sources, this
may serve as a characteristic measure of the routing complexity.
Mobile listeners moving to neighboring networks will only alter tree
branches extending over a few hops. Source-specific multicast trees
subsequently generated from source handover steps are not
independent, but highly correlated. They most likely branch to
identical receivers at one or several intersection points. By the
self-similar nature, the persistent sub-trees (of previous and next
distribution tree), rooted at any such intersection point, exhibit
again the scaling law behavior, are tall-shaped with nodes of mainly
low degree and thus likely to coincide. Tree alterations under
mobility have been studied in [26], both analytically and by
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simulations. It was found that even in large networks and for
moderate receiver numbers more than 80% of the multicast router
states remain invariant under a source handover.
4. Link-Layer Aspects
4.1. General Background
Scalable group data distribution has the highest potential in edge
networks, where large numbers of end systems reside. Consequently,
it is not surprising that most LAN network access technologies
natively support point-to-multipoint or multicast services. Wireless
access technologies inherently support broadcast/multicast at L2 and
operate on a shared medium with limited frequency and bandwidth.
Several aspects need consideration: First, dissimilar network access
radio technologies cause distinct group traffic transmissions. There
are:
o connection-less link services of a broadcast type, which mostly
are bound to limited reliability;
o connection-oriented link services of a point-to-multipoint type,
which require more complex control and frequently exhibit
reduced efficiency;
o connection-oriented link services of a broadcast type, which are
restricted to unidirectional data transmission.
In addition, multicast may be distributed via multiple point-to-point
unicast links without the use of a dedicated multipoint radio
channel. A fundamental difference between unicast and group
transmission arises from power management. Some radio technologies
adjust transmit power to be as small as possible based on link-layer
feedback from the receiver, which is not done in multipoint mode.
They consequently incur a "multicast tax", making multicast less
efficient than unicast unless the number of receivers is larger than
some threshold.
Second, point-to-multipoint service activation at the network access
layer requires a mapping mechanism from network-layer requests. This
function is commonly achieved by L3 awareness, i.e., IGMP/MLD
snooping [70] or proxy [38], which occasionally is complemented by
Multicast VLAN Registration (MVR). MVR allows sharing of a single
multicast IEEE 802.1Q Virtual LAN in the network, while subscribers
remain in separate VLANs. This L2 separation of multicast and
unicast traffic can be employed as a workaround for point-to-point
link models to establish a common multicast link.
Schmidt, et al. Informational [Page 15]RFC 5757 MMCASTv6-PS February 2010
Third, an address mapping between the layers is needed for common
group identification. Address resolution schemes depend on framing
details for the technologies in use, but commonly cause a significant
address overlap at the lower layer (i.e., more than one IP multicast
group address is sent using the same L2 address).
4.2. Multicast for Specific Technologies
4.2.1. 802.11 WLAN
IEEE 802.11 Wireless Local Area Network (WLAN) is a broadcast network
of Ethernet type. This inherits multicast address mapping concepts
from 802.3. In infrastructure mode, an access point operates as a
repeater, only bridging data between the Base (BSS) and the Extended
Service Set (ESS). A mobile node submits multicast data to an access
point in point-to-point acknowledged unicast mode (when the ToDS bit
is set). An access point receiving multicast data from an MN simply
repeats multicast frames to the BSS and propagates them to the ESS as
unacknowledged broadcast. Multicast frames received from the ESS
receive similar treatment.
Multicast frame delivery has the following characteristics:
o As an unacknowledged service, it offers limited reliability.
The loss of frames (and hence packets) arises from interference,
collision, or time-varying channel properties.
o Data distribution may be delayed, as unicast power saving
synchronization via Traffic Indication Messages (TIM) does not
operate in multicast mode. Access points buffer multicast
packets while waiting for a larger Delivery TIM (DTIM) interval,
whenever stations use the power saving mode.
o Multipoint data may cause congestion, because the distribution
system floods multicast, without further control. All access
points of the same subnet replicate multicast frames.
To limit or prevent the latter, many vendors have implemented a
configurable rate limit for forwarding multicast packets.
Additionally, an IGMP/MLD snooping or proxy may be active at the
bridging layer between the BSS and the ESS or at switches
interconnecting access points.
4.2.2. 802.16 WIMAX
IEEE 802.16 Worldwide Interoperability for Microwave Access (WIMAX)
combines a family of connection-oriented radio transmission services
that can operate in single-hop point-to-multipoint (PMP) or in mesh
Schmidt, et al. Informational [Page 16]RFC 5757 MMCASTv6-PS February 2010
mode. The latter does not support multipoint transmission and
currently has no deployment. PMP operates between Base and
Subscriber Stations in distinguished, unidirectional channels. The
channel assignment is controlled by the Base Station, which assigns
channel IDs (CIDs) within service flows to the Subscriber Stations.
Service flows may provide an optional Automatic Repeat Request (ARQ)
to improve reliability and may operate in point-to-point or point-to-
multipoint (restricted to downlink and without ARQ) mode.
A WIMAX Base Station operates as a full-duplex L2 switch, with
switching based on CIDs. Two IPv6 link models for mobile access
scenarios exist: A shared IPv6 prefix for IP over Ethernet Circuit
Switched (CS) [39] provides Media Access Control (MAC) separation
within a shared prefix. A second, point-to-point link model [40] is
recommended in the IPv6 Convergence Sublayer [41], which treats each
connection to a mobile node as a single link. The point-to-point
link model conflicts with a consistent group distribution at the IP
layer when using a shared medium (cf. Section 4.1 for MVR as a
workaround).
To invoke a multipoint data channel, the base station assigns a
common CID to all Subscriber Stations in the group. An IPv6
multicast address mapping to these 16-bit IDs is proposed by copying
either the 4 lowest bits, while sustaining the scope field, or by
utilizing the 8 lowest bits derived from Multicast on Ethernet CS
[42]. For selecting group members, a Base Station may implement
IGMP/MLD snooping or proxy as foreseen in 802.16e-2005 [43].
A Subscriber Station multicasts IP packets to a Base Station as a
point-to-point unicast stream. When the IPv6 CS is used, these are
forwarded to the upstream access router. The access router (or the
Base Station for IP over Ethernet CS) may send downstream multicast
packets by feeding them to the multicast service channel. On
reception, a Subscriber Station cannot distinguish multicast from
unicast streams at the link layer.
Multicast services have the following characteristics:
o Multicast CIDs are unidirectional and available only in the
downlink direction. Thus, a native broadcast-type forwarding
model is not available.
o The mapping of multicast addresses to CIDs needs
standardization, since different entities (Access Router, Base
Station) may have to perform the mapping.
Schmidt, et al. Informational [Page 17]RFC 5757 MMCASTv6-PS February 2010
o CID collisions for different multicast groups may occur due to
the short ID space. This can result in several point-to-
multipoint groups sharing the same CID, reducing the ability of
a receiver to filter unwanted L2 traffic.
o The point-to-point link model for mobile access contradicts a
consistent mapping of IP-layer multicast onto 802.16 point-to-
multipoint services.
o Multipoint channels cannot operate ARQ service and thus
experience a reduced reliability.
4.2.3. 3GPP/3GPP2
The 3rd Generation Partnership Project (3GPP) System architecture
spans a circuit switched (CS) and a packet-switched (PS) domain, the
latter General Packet Radio Services (GPRS) incorporates the IP
Multimedia Subsystem (IMS) [44]. The 3GPP PS is connection-oriented
and based on the concept of Packet Data Protocol (PDP) contexts.
PDPs define point-to-point links between the Mobile Terminal and the
Gateway GPRS Support Node (GGSN). Internet service types are PPP,
IPv4, and IPv6, where the recommendation for IPv6 address assignment
associates a prefix to each (primary) PDP context [45].
In Universal Mobile Telecommunications System (UMTS) Rel. 6, the IMS
was extended to include Multimedia Broadcast and Multicast Services
(MBMS). A point-to-multipoint GPRS connection service is operated on
radio links, while the gateway service to Internet multicast is
handled at the IGMP/MLD-aware GGSN. Local multicast packet
distribution is used within the GPRS IP backbone resulting in the
common double encapsulation at GGSN: global IP multicast datagrams
over Generic Tunneling Protocol (GTP) (with multipoint TID) over
local IP multicast.
The 3GPP MBMS has the following characteristics:
o There is no immediate Layer 2 source-to-destination transition,
resulting in transit of all multicast traffic at the GGSN.
o As GGSNs commonly are regional, distant entities, triangular
routing and encapsulation may cause a significant degradation of
efficiency.
In 3GPP2, the MBMS has been extended to the Broadcast and Multicast
Service (BCMCS) [46], which on the routing layer operates very
similar to MBMS. In both 3GPP and 3GPP2, multicast can be sent using
either point-to-point (PTP) or point-to-multipoint (PTM) tunnels, and
Schmidt, et al. Informational [Page 18]RFC 5757 MMCASTv6-PS February 2010
there is support for switching between PTP and PTM. PTM uses a
unidirectional common channel, operating in unacknowledged mode
without adjustment of power levels and no reporting on lost packets.
4.2.4. DVB-H / DVB-IPDC
Digital Video Broadcasting for Handhelds (DVB-H) is a unidirectional
physical layer broadcasting specification for the efficient delivery
of broadband and IP-encapsulated data streams, and is published as an
ETSI standard [47] (see http://www.dvb-h.org). This uses
multiprotocol encapsulation (MPE) to transport IP packets over an
MPEG-2 Transport Stream (TS) with link forward error correction
(FEC). Each stream is identified by a 13-bit TS ID (PID), which
together with a multiplex service ID, is associated with IPv4 or IPv6
addresses [48] and used for selective traffic filtering at receivers.
Upstream channels may complement DVB-H using other transmission
technologies. The IP Datacast Service, DVB-IPDC [31], specifies a
set of applications that can use the DVB-H transmission network.
Multicast distribution services are defined by a mapping of groups
onto appropriate PIDs, which is managed at the IP Encapsulator [49].
To increase flexibility and avoid collisions, this address resolution
is facilitated by dynamic tables, provided within the self-contained
MPEG-2 TS. Mobility is supported in the sense that changes of cell
ID, network ID, or Transport Stream ID are foreseen [50]. A
multicast receiver thus needs to relocate the multicast services to
which it is subscribed during the synchronization phase, and update
its service filters. Its handover decision may depend on service
availability. An active service subscription (multicast join)
requires initiation at the IP Encapsulator / DVB-H Gateway, which
cannot be signaled in a pure DVB-H network.
4.2.5. TV Broadcast and Satellite Networks
IP multicast may be enabled in TV broadcast networks, including those
specified by DVB, the Advanced Television Systems Committee (ATSC),
and related standards [49]. These standards are also used for one-
and two-way satellite IP services. Networks based on the MPEG-2
Transport Stream may support either the multiprotocol encapsulation
(MPE) or the unidirectional lightweight encapsulation (ULE) [51].
The second generation DVB standards allow the Transport Stream to be
replaced with a Generic Stream, using the Generic Stream
Encapsulation (GSE) [52]. These encapsulation formats all support
multicast operation.
In MPEG-2 transmission networks, multicast distribution services are
defined by a mapping of groups onto appropriate PIDs, which is
managed at the IP Encapsulator [49]. The addressing issues resemble
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those for DVB-H (Section 4.2.4) [48]. The issues for using GSE
resemble those for ULE (except the PID is not available as a
mechanism for filtering traffic). Networks that provide
bidirectional connectivity may allow active service subscription
(multicast join) to initiate forwarding from the upstream IP
Encapsulator / gateway. Some kind of filtering can be achieved using
the Input Stream Identifier (ISI) field.
4.3. Vertical Multicast Handovers
A mobile multicast node may change its point of Layer 2 attachment
within homogeneous access technologies (horizontal handover) or
between heterogeneous links (vertical handover). In either case, a
Layer 3 network change may or may not take place, but multicast-aware
links always need information about group traffic demands.
Consequently, a dedicated context transfer of multicast subscriptions
is required at the network access. Such Media Independent Handover
(MIH) is addressed in IEEE 802.21 [53], but is relevant also beyond
IEEE protocols. Mobility services transport for MIH are required as
an abstraction for Layer 2 multicast service transfer in an Internet
context [54] and are specified in [55].
MIH needs to assist in more than service discovery: There is a need
for complex, media-dependent multicast adaptation, a possible absence
of MLD signaling in L2-only transfers, and requirements originating
from predictive handovers. A multicast mobility services transport
needs to be sufficiently comprehensive and abstract to initiate a
seamless multicast handoff at network access.
Functions required for MIH include:
o Service discovery.
o Service context transformation.
o Service context transfer.
o Service invocation.
5. Solutions
5.1. General Approaches
Three approaches to mobile multicast are common [56]:
o Bidirectional Tunneling, in which the mobile node tunnels all
multicast data via its home agent. This fundamental multicast
solution hides all movement and results in static multicast
trees. It may be employed transparently by mobile multicast
Schmidt, et al. Informational [Page 20]RFC 5757 MMCASTv6-PS February 2010
listeners and sources, at the cost of triangular routing and
possibly significant performance degradation from widely spanned
data tunnels.
o Remote Subscription forces the mobile node to re-initiate
multicast distribution following handover, e.g., by submitting
an MLD listener report to the subnet where a receiver attaches.
This approach of tree discontinuation relies on multicast
dynamics to adapt to network changes. It not only results in
significant service disruption but leads to mobility-driven
changes of source addresses, and thus cannot support session
persistence under multicast source mobility.
o Agent-based solutions attempt to balance between the previous
two mechanisms. Static agents typically act as local tunneling
proxies, allowing for some inter-agent handover when the mobile
node moves. A decelerated inter-tree handover, i.e., "tree
walking", will be the outcome of agent-based multicast mobility,
where some extra effort is needed to sustain session persistence
through address transparency of mobile sources.
MIPv6 [5] introduces bidirectional tunneling as well as remote
subscription as minimal standard solutions. Various publications
suggest utilizing remote subscription for listener mobility only,
while advising bidirectional tunneling as the solution for source
mobility. Such an approach avoids the "tunnel convergence" or
"avalanche" problem [56], which refers to the responsibility of the
home agent to multiply and encapsulate packets for many receivers of
the same group, even if they are located within the same subnetwork.
However, this suffers from the drawback that multicast communication
roles are not explicitly known at the network layer and may change
unexpectedly.
None of the above approaches address SSM source mobility, except the
use of bidirectional tunneling.
5.2. Solutions for Multicast Listener Mobility
5.2.1. Agent Assistance
There are proposals for agent-assisted handover for host-based
mobility, which complement the unicast real-time mobility
infrastructure of Fast MIPv6 (FMIPv6) [19], the M-FMIPv6 [57][58],
and of Hierarchical MIPv6 (HMIPv6) [20], the M-HMIPv6 [59], and to
context transfer [60], which have been thoroughly analyzed in
[25][61].
Schmidt, et al. Informational [Page 21]RFC 5757 MMCASTv6-PS February 2010
All these solutions presume the context state was stored within a
network node that is reachable before and after a move. But there
could be cases were the MN is no longer in contact with the previous
network, when at the new location. In this case, the network itself
cannot assist in the context transfer. Such scenarios may occur when
moving from one (walled) operator to another and will require a
backwards compatible way to recover from loss of connectivity and
context based on the node alone.
Network-based mobility management, Proxy MIPv6 (PMIPv6) [62], is
multicast transparent in the sense that the MN experiences a point-
to-point home link fixed at its (static) Local Mobility Anchor (LMA).
This virtual home link is composed of a unicast tunnel between the
LMA and the current Mobile Access Gateway (MAG), and a point-to-point
link connecting the current MAG to the MN. A PMIPv6 domain thereby
inherits MTU-size problems from spanning tunnels at the receiver
site. Furthermore, two avalanche problem points can be identified:
the LMA may be required to tunnel data to a large number of MAGs,
while an MAG may be required to forward the same multicast stream to
many MNs via individual point-to-point links [63]. Future
optimizations and extensions to shared links preferably adapt native
multicast distribution towards the edge network, possibly using a
local routing option, including context transfer between access
gateways to assist IP-mobility-agnostic MNs.
An approach based on dynamically negotiated inter-agent handovers is
presented in [64]. Aside from IETF work, numerous publications
present proposals for seamless multicast listener mobility, e.g.,
[65] provides a comprehensive overview of the work prior to 2004.
5.2.2. Multicast Encapsulation
Encapsulation of multicast data packets is an established method to
shield mobility and to enable access to remotely located data
services, e.g., streams from the home network. Applying generic
packet tunneling in IPv6 [66] using a unicast point-to-point method
will also allow multicast-agnostic domains to be transited, but does
inherit the tunnel convergence problem and may result in traffic
multiplication.
Multicast-enabled environments may take advantage of point-to-
multipoint encapsulation, i.e., generic packet tunneling using an
appropriate multicast destination address in the outer header. Such
multicast-in-multicast encapsulated packets similarly enable
reception of remotely located streams, but do not suffer from the
scaling overhead from using unicast tunnels.
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RFC 5757 MMCASTv6-PS February 2010
The tunnel entry point performing encapsulation should provide
fragmentation of data packets to avoid issues resulting from MTU-size
constraints within the network(s) supporting the tunnel(s).
5.2.3. Hybrid Architectures
There has been recent interest in seeking methods that avoid the
complexity at the Internet core network, e.g., application-layer and
overlay proposals for (mobile) multicast. The possibility of
integrating multicast distribution on the overlay into the network
layer is also being considered by the IRTF Scalable Adaptive
Multicast (SAM) Research Group.
An early hybrid architecture using reactively operating proxy-
gateways located at the Internet edges was introduced by Garyfalos
and Almeroth [30]. The authors presented an Intelligent Gateway
Multicast as a bridge between mobility-aware native multicast
management in access networks and mobility group distribution
services in the Internet core, which may be operated on the network
or application layer. The Hybrid Shared Tree approach [67]
introduced a mobility-agnostic multicast backbone on the overlay.
Current work in the SAM RG is developing general architectural
approaches for hybrid multicast solutions [68] and a common multicast
API for a transparent access of hybrid multicast [69] that will
require a detailed design in future work.
5.2.4. MLD Extensions
The default timer values and Robustness Variable specified in MLD
[17] were not designed for the mobility context. This results in a
slow reaction of the multicast-routing infrastructure (including
L3-aware access devices [70]) following a client leave. This may be
a disadvantage for wireless links, where performance may be improved
by carefully tuning the Query Interval and other variables. Some
vendors have optimized performance by implementing a listener node
table at the access router that can eliminate the need for query
timeouts when receiving leave messages (explicit receiver tracking).
An MN operating predictive handover, e.g., using FMIPv6, may
accelerate multicast service termination when leaving the previous
network by submitting an early Done message before handoff. MLD
router querying will allow the multicast forwarding state to be
restored in the case of an erroneous prediction (i.e., an anticipated
move to a network that has not taken place). Backward context
transfer may otherwise ensure a leave is signaled. A further
optimization was introduced by Jelger and Noel [71] for the special
case when the HA is a multicast router. A Done message received
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RFC 5757 MMCASTv6-PS February 2010
through a tunnel from the mobile end node (through a point-to-point
link directly connecting the MN, in general), should not initiate
standard MLD membership queries (with a subsequent timeout). Such
explicit treatment of point-to-point links will reduce traffic and
accelerate the control protocol. Explicit tracking will cause
identical protocol behavior.
While away from home, an MN may wish to rely on a proxy or "standby"
multicast membership service, optionally provided by an HA or proxy
router. Such functions rely on the ability to restart fast packet
forwarding; it may be desirable for the proxy router to remain part
of the multicast delivery tree, even when transmission of group data
is paused. To enable such proxy control, the authors in [71] propose
an extension to MLD, introducing a Listener Hold message that is
exchanged between the MN and the HA. This idea was developed in [59]
to propose multicast router attendance control, allowing for a
general deployment of group membership proxies. Some currently
deployed IPTV solutions use such a mechanism in combination with a
recent (video) frame buffer, to enable fast channel switching between
several IPTV multicast flows (zapping).
5.3. Solutions for Multicast Source Mobility
5.3.1. Any Source Multicast Mobility Approaches
Solutions for multicast source mobility can be divided into three
categories:
o Statically Rooted Distribution Trees. These methods follow a
shared tree approach. Romdhani et al. [72] proposed employing
the Rendezvous Points of PIM-SM as mobility anchors. Mobile
senders tunnel their data to these "Mobility-aware Rendezvous
Points" (MRPs). When restricted to a single domain, this scheme
is equivalent to bidirectional tunneling. Focusing on inter-
domain mobile multicast, the authors designed a tunnel- or SSM-
based backbone distribution of packets between MRPs.
o Reconstruction of Distribution Trees. Several authors have
proposed the construction of a completely new distribution tree
after the movement of a mobile source and therefore have to
compensate for the additional routing (tree-building) delay. M-
HMIPv6 [59] tunnels data into a previously established tree
rooted at mobility anchor points to compensate for the routing
delay until a protocol-dependent timer expires. The Range-Based
Mobile Multicast (RBMoM) protocol [73] introduces an additional
Multicast Agent (MA) that advertises its service range. A
mobile source registers with the closest MA and tunnels data
through it. When moving out of the previous service range, it
Schmidt, et al. Informational [Page 24]RFC 5757 MMCASTv6-PS February 2010
will perform MA discovery, a re-registration and continue data
tunneling with a newly established Multicast Agent in its new
current vicinity.
o Tree Modification Schemes. In the case of DVMRP routing, Chang
and Yen [74] propose an algorithm to extend the root of a given
delivery tree for incorporating a new source location in ASM.
The authors rely on a complex additional signaling protocol to
fix DVMRP forwarding states and heal failures in the reverse
path forwarding (RPF) checks.
5.3.2. Source-Specific Multicast Mobility Approaches
The shared tree approach of [72] has been extended to support SSM
mobility by introducing the HoA address record to the Mobility-aware
Rendezvous Points. The MRPs operate using extended multicast routing
tables that simultaneously hold the HoA and CoA and thus can
logically identify the appropriate distribution tree. Mobility thus
may reintroduce the concept of rendezvous points to SSM routing.
Approaches for reconstructing SPTs in SSM rely on a client
notification to establish new router state. They also need to
preserve address transparency for the client. Thaler [75] proposed
introducing a binding cache and providing source address transparency
analogous to MIPv6 unicast communication. Initial session
announcements and changes of source addresses are distributed
periodically to clients via an additional multicast control tree
rooted at the home agent. Source tree handovers are then activated
on listener requests.
Jelger and Noel [76] suggest handover improvements employing anchor
points within the source network, supporting continuous data
reception during client-initiated handovers. Client updates are
triggered out of band, e.g., by Source Demand Routing (SDR) / Session
Announcement Protocol (SAP) [77]. Receiver-oriented tree
construction in SSM thus remains unsynchronized with source
handovers.
To address the synchronization problem at the routing layer, several
proposals have focused on direct modification of the distribution
trees. A recursive scheme may use loose unicast source routes with
branch points, based on a multicast Hop-by-Hop protocol. Vida et al.
[78] optimized SPT for a moving source on the path between the source
and first branching point. O'Neill [79] suggested a scheme to
overcome RPF check failures that originate from multicast source
address changes with a rendezvous point scenario by introducing
extended routing information, which accompanies data in a Hop-by-Hop
option "RPF redirect" header. The Tree Morphing approach of Schmidt
Schmidt, et al. Informational [Page 25]RFC 5757 MMCASTv6-PS February 2010
and Waehlisch [80] used source routing to extend the root of a
previously established SPT, thereby injecting router state updates in
a Hop-by-Hop option header. Using extended RPF checks, the elongated
tree autonomously initiates shortcuts and smoothly reduces to a new
SPT rooted at the relocated source. An enhanced version of this
protocol abandoned the initial source routing and could be proved to
comply with rapid source movement [81]. Lee et al. [82] introduced a
state-update mechanism for reusing major parts of established
multicast trees. The authors start from an initially established
distribution state, centered at the mobile source's home agent. A
mobile source leaving its home network will signal a multicast
forwarding state update on the path to its home agent and,
subsequently, distribution states according to the mobile source's
new CoA along the previous distribution tree. Multicast data is then
intended to flow natively using triangular routes via the elongation
and an updated tree centered on the home agent. Based on Host
Identity Protocol identifiers, Kovacshazi and Vida [83] introduce
multicast routing states that remain independent of IP addresses.
Drawing upon a similar scaling law argument, parts of these states
may then be reused after source address changes.
6. Security Considerations
This document discusses multicast extensions to mobility. It does
not define new methods or procedures. Security issues arise from
source address binding updates, specifically in the case of source-
specific multicast. Threats of hijacking unicast sessions will
result from any solution jointly operating binding updates for
unicast and multicast sessions.
Multicast protocols exhibit a risk of network-based traffic
amplification. For example, an attacker may abuse mobility signaling
to inject unwanted traffic into a previously established multicast
distribution infrastructure. These threats are partially mitigated
by reverse path forwarding checks by multicast routers. However, a
multicast or mobility agent that explicitly replicates multicast
streams, e.g., Home Agent that n-casts data, may be vulnerable to
denial-of-service attacks. In addition to source authentication, a
rate control of the replicator may be required to protect the agent
and the downstream network.
Mobility protocols need to consider the implications and requirements
for Authentication, Authorization, and Accounting (AAA). An MN may
have been authorized to receive a specific multicast group when using
one mobile network, but this may not be valid when attaching to a
different network. In general, the AAA association for an MN may
change between attachments, or may be individually chosen prior to
network (re-)association. The most appropriate network path may be
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one that satisfies user preferences, e.g., to use/avoid a specific
network, minimize monetary cost, etc., rather than one that only
minimizes the routing cost. Consequently, AAA bindings may need to
be considered when performing context transfer.
Admission control issues may arise when new CoA source addresses are
introduced to SSM channels [84]. Due to lack of feedback, the
admission [85] and binding updates [86] of mobile multicast sources
require autonomously verifiable authentication. This can be achieved
by, for instance, Cryptographically Generated Addresses (CGAs).
Modification to IETF protocols (e.g., routing, membership, session
announcement, and control) as well as the introduction of new
entities, e.g., multicast mobility agents, can introduce security
vulnerabilities and require consideration of issues such as
authentication of network entities, methods to mitigate denial of
service (in terms of unwanted network traffic, unnecessary
consumption of router/host resources and router/host state/buffers).
Future solutions must therefore analyze and address the security
implications of supporting mobile multicast.
7. Summary and Future Steps
This document is intended to provide a basis for the future design of
mobile IPv6 multicast methods and protocols by:
o providing a structured overview of the problem space that
multicast and mobility jointly generate at the IPv6 layer;
o referencing the implications and constraints arising from lower
and upper layers and from deployment;
o briefly surveying conceptual ideas of currently available
solutions;
o including a comprehensive bibliographic reference base.
It is recommended that future steps towards extending mobility
services to multicast proceed to first solve the following problems:
1. Ensure seamless multicast reception during handovers, meeting
the requirements of mobile IPv6 nodes and networks. Thereby
addressing the problems of home subscription without n-tunnels,
as well as native multicast reception in those visited
networks, which offer a group communication service.
Schmidt, et al. Informational [Page 27]RFC 5757 MMCASTv6-PS February 2010
2. Integrate multicast listener support into unicast mobility
management schemes and architectural entities to define a
consistent mobility service architecture, providing equal
support for unicast and multicast communication.
3. Provide basic multicast source mobility by designing address
duality management at end nodes.
Schmidt, et al. Informational [Page 28]RFC 5757 MMCASTv6-PS February 2010
Appendix A. Implicit Source Notification Options
An IP multicast source transmits data to a group of receivers without
requiring any explicit feedback from the group. Sources therefore
are unaware at the network layer of whether any receivers have
subscribed to the group, and unconditionally send multicast packets
that propagate in the network to the first-hop router (often known in
PIM as the designated router). There have been attempts to
implicitly obtain information about the listening group members,
e.g., extending an IGMP/MLD querier to inform the source of the
existence of subscribed receivers. Multicast Source Notification of
Interest Protocol (MSNIP) [87] was such a suggested method that
allowed a multicast source to query the upstream designated router.
However, this work did not progress within the IETF mboned working
group and was terminated by the IETF.
Multicast sources may also be controlled at the session or transport
layer using end-to-end control protocols. A majority of real-time
applications employ the Real-time Transport Protocol (RTP) [88]. The
accompanying control protocol, RTP Control Protocol (RTCP), allows
receivers to report information about multicast group membership and
associated performance data. In multicast, the RTCP reports are
submitted to the same group and thus may be monitored by the source
to monitor, manage and control multicast group operations. RFC 2326,
the Real Time Streaming Protocol (RTSP), provides session layer
control that may be used to control a multicast source. However,
RTCP and RTSP information is intended for end-to-end control and is
not necessarily visible at the network layer. Application designers
may chose to implement any appropriate control plane for their
multicast applications (e.g., reliable multicast transport
protocols), and therefore a network-layer mobility mechanism must not
assume the presence of a specific transport or session protocol.
Informative References
[1] Aguilar, L. "Datagram Routing for Internet Multicasting", In
ACM SIGCOMM '84 Communications Architectures and Protocols, pp.
58-63, ACM Press, June, 1984.
[2] Deering, S., "Host extensions for IP multicasting", STD 5, RFC
1112, August 1989.
[3] G. Xylomenos and G.C. Plyzos, "IP Multicast for Mobile Hosts",
IEEE Communications Magazine, 35(1), pp. 54-58, January 1997.
Schmidt, et al. Informational [Page 29]RFC 5757 MMCASTv6-PS February 2010
[4] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[5] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[6] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
IKEv2 and the Revised IPsec Architecture", RFC 4877, April
2007.
[7] ITU-T Recommendation, "G.114 - One-way transmission time",
Telecommunication Union Standardization Sector, 05/2003.
[8] Akyildiz, I and Wang, X., "A Survey on Wireless Mesh Networks",
IEEE Communications Magazine, 43(9), pp. 23-30, September 2005.
[9] Bhattacharyya, S., Ed., "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, July 2003.
[10] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
RFC 4607, August 2006.
[11] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
Multicast Routing Protocol", RFC 1075, November 1988.
[12] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
Handley, M., Jacobson, V., Liu, C., Sharma, P., and L. Wei,
"Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification", RFC 2362, June 1998.
[13] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[14] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-PIM)", RFC
5015, October 2007.
[15] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.
[16] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[17] Vida, R., Ed., and L. Costa, Ed., "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
Schmidt, et al. Informational [Page 30]RFC 5757 MMCASTv6-PS February 2010
[18] Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
Optimization for Mobile IPv6", RFC 4866, May 2007.
[19] Koodli, R., Ed., "Mobile IPv6 Fast Handovers", RFC 5568, July
2009.
[20] Soliman, H., Castelluccia, C., ElMalki, K., and L. Bellier,
"Hierarchical Mobile IPv6 (HMIPv6) Mobility Management", RFC
5380, October 2008.
[21] Loughney, J., Ed., Nakhjiri, M., Perkins, C., and R. Koodli,
"Context Transfer Protocol (CXTP)", RFC 4067, July 2005.
[22] Montavont, N., Wakikawa, R., Ernst, T., Ng, C., and K.
Kuladinithi, "Analysis of Multihoming in Mobile IPv6", Work in
Progress, May 2008.
[23] Narayanan, V., Thaler, D., Bagnulo, M., and H. Soliman, "IP
Mobility and Multi-homing Interactions and Architectural
Considerations", Work in Progress, July 2007.
[24] Savola, P. and B. Haberman, "Embedding the Rendezvous Point
(RP) Address in an IPv6 Multicast Address", RFC 3956, November
2004.
[25] Schmidt, T.C. and Waehlisch, M. "Predictive versus Reactive -
Analysis of Handover Performance and Its Implications on IPv6
and Multicast Mobility", Telecommunication Systems, 30(1-3),
pp. 123- 142, November 2005.
[26] Schmidt, T.C. and Waehlisch, M. "Morphing Distribution Trees -
On the Evolution of Multicast States under Mobility and an
Adaptive Routing Scheme for Mobile SSM Sources",
Telecommunication Systems, 33(1-3), pp. 131-154, December 2006.
[27] Diot, C. et al. "Deployment Issues for the IP Multicast Service
and Architecture", IEEE Network Magazine, spec. issue on
Multicasting, 14(1), pp. 78-88, 2000.
[28] Eubanks, M. http://multicasttech.com/status/, 2008.
[29] Garyfalos, A, Almeroth, K. and Sanzgiri, K. "Deployment
Complexity Versus Performance Efficiency in Mobile Multicast",
Intern. Workshop on Broadband Wireless Multimedia: Algorithms,
Architectures and Applications (BroadWiM), San Jose,
California, USA, October 2004. Online:
http://imj.ucsb.edu/papers/BROADWIM-04.pdf.
Schmidt, et al. Informational [Page 31]RFC 5757 MMCASTv6-PS February 2010
[30] Garyfalos, A, Almeroth, K. "A Flexible Overlay Architecture for
Mobile IPv6 Multicast", IEEE Journ. on Selected Areas in Comm.,
23(11), pp. 2194-2205, November 2005.
[31] "Digital Video Broadcasting (DVB); IP Datacast over DVB-H: Set
of Specifications for Phase 1", ETSI TS 102 468;
[32] ETSI TS 102 611, "Digital Video Broadcasting (DVB); IP Datacast
over DVB-H: Implementation Guidelines for Mobility)", European
Standard (Telecommunications series), November 2004.
[33] Chuang, J. and Sirbu, M. "Pricing Multicast Communication: A
Cost- Based Approach", Telecommunication Systems, 17(3),
281-297, 2001. Presented at the INET'98, Geneva, Switzerland,
July 1998.
[34] Van Mieghem, P, Hooghiemstra, G, Hofstad, R. "On the Efficiency
of Multicast", IEEE/ACM Trans. Netw., 9(6), pp. 719-732, Dec.
2001.
[35] Chalmers, R.C. and Almeroth, K.C, "On the topology of multicast
trees", IEEE/ACM Trans. Netw., 11(1), 153-165, 2003.
[36] Janic, M. and Van Mieghem, P. "On properties of multicast
routing trees", Int. J. Commun. Syst., 19(1), pp. 95-114, Feb.
2006.
[37] Van Mieghem, P. "Performance Analysis of Communication Networks
and Systems", Cambridge University Press, 2006.
[38] Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet
Group Management Protocol (IGMP) / Multicast Listener Discovery
(MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")", RFC
4605, August 2006.
[39] Jeon, H., Jeong, S., and M. Riegel, "Transmission of IP over
Ethernet over IEEE 802.16 Networks", RFC 5692, October 2009.
[40] Shin, M-K., Ed., Han, Y-H., Kim, S-E., and D. Premec, "IPv6
Deployment Scenarios in 802.16 Networks", RFC 5181, May 2008.
[41] Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S.
Madanapalli, "Transmission of IPv6 via the IPv6 Convergence
Sublayer over IEEE 802.16 Networks", RFC 5121, February 2008.
[42] Kim, S., Jin, J., Lee, S., and S. Lee, "Multicast Transport on
IEEE 802.16 Networks", Work in Progress, July 2007.
Schmidt, et al. Informational [Page 32]RFC 5757 MMCASTv6-PS February 2010
[43] IEEE 802.16e-2005: IEEE Standard for Local and metropolitan
area networks Part 16: "Air Interface for Fixed and Mobile
Broadband Wireless Access Systems Amendment for Physical and
Medium Access Control Layers for Combined Fixed and Mobile
Operation in Licensed Bands", New York, February 2006.
[44] 3rd Generation Partnership Project; Technical Specification
Group Services and System Aspects; "IP Multimedia Subsystem
(IMS)"; Stage 2, 3GPP TS 23.228, Rel. 5 ff, 2002 - 2007.
[45] Wasserman, M., Ed., "Recommendations for IPv6 in Third
Generation Partnership Project (3GPP) Standards", RFC 3314,
September 2002.
[46] 3GPP2, www.3gpp2.org, "X.S0022-A, Broadcast and Multicast
Service in cdma2000 Wireless IP Network, Rev. A.",
http://www.3gpp2.org/Public_html/specs/tsgx.cfm, February 2007.
[47] ETSI EN 302 304, "Digital Video Broadcasting (DVB);
Transmission System for Handheld Terminals (DVB-H)", European
Standard (Telecommunications series), November 2004.
[48] Fairhurst, G. and M. Montpetit, "Address Resolution Mechanisms
for IP Datagrams over MPEG-2 Networks", RFC 4947, July 2007.
[49] Montpetit, M.-J., Fairhurst, G., Clausen, H., Collini-Nocker,
B., and H. Linder, "A Framework for Transmission of IP
Datagrams over MPEG-2 Networks", RFC 4259, November 2005.
[50] Yang, X, Vare, J, Owens, T. "A Survey of Handover Algorithms in
DVB-H", IEEE Comm. Surveys, 8(4), pp. 16-24, 2006.
[51] Fairhurst, G. and B. Collini-Nocker, "Unidirectional
Lightweight Encapsulation (ULE) for Transmission of IP
Datagrams over an MPEG-2 Transport Stream (TS)", RFC 4326,
December 2005.
[52] Fairhurst, G. and B. Collini-Nocker, "Extension Formats for
Unidirectional Lightweight Encapsulation (ULE) and the Generic
Stream Encapsulation (GSE)", RFC 5163, April 2008.
[53] "Draft IEEE Standard for Local and Metropolitan Area Networks:
Media Independent Handover Services", IEEE LAN/MAN Draft IEEE
P802.21/D07.00, July 2007.
[54] Melia, T., Ed., "Mobility Services Transport: Problem
Statement", RFC 5164, March 2008.
Schmidt, et al. Informational [Page 33]RFC 5757 MMCASTv6-PS February 2010
[55] Melia, T., Ed., Bajko, G., Das, S., Golmie, N., and JC. Zuniga,
"IEEE 802.21 Mobility Services Framework Design (MSFD)", RFC
5677, December 2009.
[56] Janneteau, C, Tian, Y, Csaba, S. et al. "Comparison of Three
Approaches Towards Mobile Multicast", IST Mobile Summit 2003,
Aveiro, Portugal, 16-18 June 2003.
[57] Suh, K., Kwon, D.-H., Suh, Y.-J. and Y. Park, "Fast Multicast
Protocol for Mobile IPv6 in the fast handovers environments",
Work in Progress, January 2004.
[58] Xia, F. and B. Sarikaya, "FMIPv6 extensions for Multicast
Handover", Work in Progress, March 2007.
[59] Schmidt, T. and M. Waehlisch, "Seamless Multicast Handover in a
Hierarchical Mobile IPv6 Environment (M-HMIPv6)", Work in
Progress, November 2005.
[60] Miloucheva, I. and K. Jonas, "Multicast Context Transfer in
mobile IPv6", Work in Progress, June 2005.
[61] Leoleis, G, Prezerakos, G, Venieris, I, "Seamless multicast
mobility support using fast MIPv6 extensions", Computer Comm.,
29(18), pp. 3745-3765, 2006.
[62] Gundavelli, S., Ed., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[63] Deng, H., Chen, G., Schmidt, T., Seite, P., and P. Yang,
"Multicast Support Requirements for Proxy Mobile IPv6", Work in
Progress, July 2009.
[64] Zhang, H., Chen, X., Guan, J., Shen, B., Liu, E., and S.
Dawkins, "Mobile IPv6 Multicast with Dynamic Multicast Agent",
Work in Progress, January 2007.
[65] Romdhani, I, Kellil, M, Lach, H.-Y. et. al. "IP Mobile
Multicast: Challenges and Solutions", IEEE Comm. Surveys, 6(1),
pp. 18-41, 2004.
[66] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
Specification", RFC 2473, December 1998.
[67] Waehlisch, M., Schmidt, T.C. "Between Underlay and Overlay: On
Deployable, Efficient, Mobility-agnostic Group Communication
Services", Internet Research, 17(5), pp. 519-534, Emerald
Insight, Bingley, UK, November 2007.
Schmidt, et al. Informational [Page 34]RFC 5757 MMCASTv6-PS February 2010
[68] J. Buford, "Hybrid Overlay Multicast Framework", Work in
Progress, February 2008.
[69] Waehlisch, M., Schmidt, T., and S. Venaas, "A Common API for
Transparent Hybrid Multicast", Work in Progress, October 2009.
[70] Christensen, M., Kimball, K., and F. Solensky, "Considerations
for Internet Group Management Protocol (IGMP) and Multicast
Listener Discovery (MLD) Snooping Switches", RFC 4541, May
2006.
[71] Jelger, C, Noel, T. "Multicast for Mobile Hosts in IP Networks:
Progress and Challenges", IEEE Wirel. Comm., 9(5), pp 58-64,
Oct. 2002.
[72] Romdhani, I, Bettahar, H. and Bouabdallah, A. "Transparent
handover for mobile multicast sources", in P. Lorenz and P.
Dini, eds, Proceedings of the IEEE ICN'06, IEEE Press, 2006.
[73] Lin, C.R. et al. "Scalable Multicast Protocol in IP-Based
Mobile Networks", Wireless Networks, 8 (1), pp. 27-36, January,
2002.
[74] Chang, R.-S. and Yen, Y.-S. "A Multicast Routing Protocol with
Dynamic Tree Adjustment for Mobile IPv6", Journ. Information
Science and Engineering, 20(6), pp. 1109-1124, 2004.
[75] Thaler, D. "Supporting Mobile SSM Sources for IPv6",
Proceedings of ietf meeting, Dec. 2001.
URL: www.ietf.org/proceedings/01dec/slides/magma-2.pdf
[76] Jelger, C. and T. Noel, "Supporting Mobile SSM sources for IPv6
(MSSMSv6)",Work in Progress, January 2002.
[77] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[78] Vida, R, Costa, L, Fdida, S. "M-HBH - Efficient Mobility
Management in Multicast", Proc. of NGC '02, pp. 105-112, ACM
Press 2002.
[79] A. O'Neill "Mobility Management and IP Multicast", Work in
Progress, July 2002.
[80] Schmidt, T. C. and Waehlisch, M. "Extending SSM to MIPv6 -
Problems, Solutions and Improvements", Computational Methods in
Science and Technology, 11(2), pp. 147-152. Selected Papers
from TERENA Networking Conference, Poznan, May 2005.
Schmidt, et al. Informational [Page 35]RFC 5757 MMCASTv6-PS February 2010
[81] Schmidt, T.C., Waehlisch, M., and Wodarz, M. "Fast Adaptive
Routing Supporting Mobile Senders in Source Specific
Multicast", Telecommunication Systems, 43(1), pp. 95-108, 2009,
http://dx.doi.org/10.1007/s11235-009-9200-y.
[82] Lee, H., Han, S. and Hong, J. "Efficient Mechanism for Source
Mobility in Source Specific Multicast", in K. Kawahara and I.
Chong, eds, "Proceedings of ICOIN2006", LNCS vol. 3961, pp.
82-91, Springer-Verlag, Berlin, Heidelberg, 2006.
[83] Kovacshazi, Z. and Vida, R. "Host Identity Specific Multicast",
Third International Conference on Networking and Services ICNS,
IEEE Press, pp. 1-1, June 2007.
[84] Kellil, M, Romdhani, I, Lach, H.-Y, Bouabdallah, A. and
Bettahar, H. "Multicast Receiver and Sender Access Control and
its Applicability to Mobile IP Environments: A Survey", IEEE
Comm. Surveys & Tutorials, 7(2), pp. 46-70, 2005.
[85] Castellucia, C, Montenegro, G. "Securing Group Management in
IPv6 with Cryptographically Based Addresses", Proc. 8th IEEE
Int'l Symp. Comp. and Commun, Turkey, July 2003, pp. 588-93.
[86] Schmidt, T.C, Waehlisch, M., Christ, O., and Hege, G.
"AuthoCast - a mobility-compliant protocol framework for
multicast sender authentication", Security and Communication
Networks, 1(6), pp. 495-509, 2008.
[87] Fenner, B., Holbrook, H., and I. Kouvelas, "Multicast Source
Notification of Interest Protocol (MSNIP)", Work in Progress,
November 2001.
[88] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
Schmidt, et al. Informational [Page 36]RFC 5757 MMCASTv6-PS February 2010
Acknowledgments
Work on exploring the problem space for mobile multicast has been
pioneered by Greg Daley and Gopi Kurup within their early document
"Requirements for Mobile Multicast Clients".
Since then, many people have actively discussed the different issues
and contributed to the enhancement of this memo. The authors would
like to thank (in alphabetical order) Kevin C. Almeroth, Lachlan
Andrew, Jari Arkko, Cedric Baudoin, Hans L. Cycon, Hui Deng, Marshall
Eubanks, Zhigang Huang, Christophe Jelger, Andrei Gutov, Rajeev
Koodli, Mark Palkow, Craig Partridge, Imed Romdhani, Hesham Soliman,
Dave Thaler, and last, but not least, very special thanks to Stig
Venaas for his frequent and thorough advice.
Authors' Addresses
Thomas C. Schmidt
Dept. Informatik
Hamburg University of Applied Sciences,
Berliner Tor 7
D-20099 Hamburg, Germany
Phone: +49-40-42875-8157
EMail: schmidt@informatik.haw-hamburg.de
Matthias Waehlisch
link-lab
Hoenower Str. 35
D-10318 Berlin, Germany
EMail: mw@link-lab.net
Godred Fairhurst
School of Engineering,
University of Aberdeen,
Aberdeen, AB24 3UE, UK
EMail: gorry@erg.abdn.ac.uk
Schmidt, et al. Informational [Page 37]
