One document matched: draft-schmidt-mobopts-mmcastv6-ps-00.txt
MobOpts Research Group Thomas C. Schmidt
Internet Draft HAW Hamburg
Matthias Waehlisch
Expires: April 2006 FHTW Berlin
October 2005
Multicast Mobility in MIPv6: Problem Statement
<draft-schmidt-mobopts-mmcastv6-ps-00.txt>
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Abstract
In this document we discuss mobility extensions to current IP layer
multicast solutions. Problems arising from mobile group communication
in general, in the case of multicast listener mobility and for mobile
Any Source Multicast as well as Source Specific Multicast senders are
documented subsequently. The principal approaches to multicast
mobility are introduced in brief.
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Table of Contents
1. Introduction and Motivation....................................2
2. Problem Description............................................3
2.1 Generals...................................................3
2.2 Multicast Listener Mobility................................5
2.3 Multicast Source Mobility..................................5
2.3.1 Any Source Multicast Mobility.................. ......5
2.3.2 Source Specific Multicast Mobility.............. .....6
3. Solutions......................................................7
4. Security Considerations........................................7
5. IANA Considerations............................................8
6. References.....................................................8
Acknowledgments..................................................10
Author's Addresses...............................................10
Intellectual Property Statement..................................10
Copyright Notice.................................................11
Disclaimer of Validity...........................................11
Acknowledgement..................................................11
1. Introduction and Motivation
Group communication forms an integral building block of a wide
variety of applications, ranging from public content distribution and
streaming over voice and video conferencing, collaborative
environments and gaming up to the self-organization of distributed
systems. Its support by network layer multicast will be needed,
whenever globally distributed, scalable, serverless or instantaneous
communication is required. As broadband media delivery more and more
emerges to be a typical mass scenario, scalability and bandwidth
efficiency of multicast routing continuously gains relevance.
Internet multicasting will be of particular importance to mobile
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environments, where users commonly share frequency bands of limited
capacity. The rapidly increasing mobile reception of 'infotainment'
streams may soon require a wide deployment of mobile multicast
services.
The fundamental approach to deal with mobility in IPv6 [2] is stated
in the Mobile IPv6 RFCs [3,4]. MIPv6 [3] only roughly treats
multicast mobility, in a pure remote subscription approach or through
bi-directional tunneling via the Home Agent. Whereas the remote
subscription suffers from slow handovers, as it relies on multicast
routing to adapt to handovers, bi-directional tunneling introduces
inefficient overheads and delays due to triangular forwarding.
Therefore both approaches cannot be considered solutions for a
deployment on large scale. A mobile multicast service for a future
Internet should admit 'close to optimal' routing at predictable and
limited cost, robustness combined with a service quality compliant to
real-time media distribution.
Intricate multicast routing procedures, though, are not easily
extensible to comply with mobility requirements. Any client
subscribed to a group while in motion, requires delivery branches to
pursue its new location; any mobile source requests the entire
delivery tree to adapt to its changing positions. Significant effort
has been already invested in protocol designs for mobile multicast
receivers. Only limited work has been dedicated to multicast source
mobility, which poses the more delicate problem [21].
In multimedia conference scenarios each member commonly operates as
receiver and as sender for multicast based group communication. In
addition, real-time communication such as voice or video over IP
places severe temporal requirement on mobility protocols: Seamless
handover scenarios need to limit disruptions or delay to less than
100 ms. Jitter disturbances are not to exceed 50 ms. Note that 100 ms
is about the duration of a spoken syllable in real-time audio.
It is the aim of this document, to specify the problem scope for a
multicast mobility management as to be refined in future work. The
attempt is made to subdivide the various challenges according to
their originating aspects and to present existing proposals for
solution, as well as major bibliographic references.
2. Problem Description
2.1 Generals
Multicast mobility must be considered as a generic term, which
subsumes a collection of quite distinct functions. At first,
multicast communication divides into Any Source Multicast (ASM) [5]
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and Source Specific Multicast (SSM) [6,7]. At second, multicast
communication is asymmetric, so the roles of senders and receivers
need distinction. Both individually may be mobile. Their interaction
is facilitated by a multicast routing function s.a. DVMRP [8], PIM-
SM/SSM [9,10] or CBT [11] and the multicast listener discovery
protocol [12,13].
Any multicast mobility solution must account for all of these
functional blocks. It should enable seamless continuity of multicast
sessions when moving from one IPv6 subnet to another. It should
preserve the multicast nature of packet distribution and approximate
optimal routing. It should support per flow handover for multicast
traffic, as properties and designations of flows may be of individual
kind.
Multicast routing dynamically adapts to session topologies, which
then may change under mobility. However, routing convergence arrives
at a time scale of seconds, even minutes and is far too slow to
support seamless handovers for interactive or real-time media
sessions. The actual temporal behavior strongly depends on the
routing protocol in use and on the geometry of the current
distribution tree. A mobility scheme that arranges for adjustments,
i.e. partial changes or full reconstruction, of multicast trees is
forced to make provision for time buffers sufficient for protocol
convergence. Special attention is needed with a possible rapid
movement of the mobile node, as this may occur at much higher rates
than compatible with protocol convergence.
IP layer multicast packet distribution is an unreliable service,
which is bound to connectionless transport protocols. Packet loss
thus will not be handled in a predetermined fashion. Mobile multicast
handovers should not cause significant packet drops. Due to
statelessness the bi-casting of multicast flows does not cause
foreseeable degradations of the transport layer.
Group addresses in general are location transparent, even though
there are proposals to embed unicast prefixes or Rendezvous Point
addresses [14]. Source addresses contributing to a multicast session
are interpreted by the routing infrastructure and by receiver
applications, which frequently are source address aware. Multicast
therefore inherits the mobility address duality problem for source
addresses, being a logical node identifier (HoA) at the one hand and
a topological locator (CoA) at the other.
Multicast sources in general operate decoupled from their receivers
in the following sense: A multicast source submits data to a group of
unknown receivers, thus operating without any feedback channel. It
neither has means to inquire on properties of its delivery trees, nor
will it be able to learn about the state of its receivers. In the
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event of an inter-tree handover, a mobile multicast source therefore
is vulnerable to loosing receivers without taking notice.
2.2 Multicast Listener Mobility
A mobile multicast listener entering a new IP subnet may encounter
either one of the following conditions: The new network may not be
multicast enabled or the specific multicast service in use may be
unsupported or prohibited. Alternatively the requested multicast
service may be supported and enabled in the new network, but the
multicast groups under subscription may not be forwarded to it. Then
current distribution trees for the desired groups may reside at large
routing distance. It may as well occur that some or all groups under
subscription of the mobile node are received by one or several local
group members at the instance of arrival and that multicast streams
natively flow.
The problem of achieving seamless multicast listener handovers is
thus threefold:
o Ensure multicast reception even in visited networks without
appropriate multicast support.
o Expedite primary multicast forwarding to comply with a seamless
timescale at handovers.
o Realize native multicast forwarding whenever applicable to
preserve network resources and avoid data redundancy.
Additional aspects related to infrastructure remain. In changing its
point of attachment a mobile receiver may not have enough time to
leave groups in the previous network. Also, packet duplication and
disorder may result from the change of topology.
2.3 Multicast Source Mobility
2.3.1 Any Source Multicast Mobility
A node submitting data to an ASM group defines the root of either a
shared or source specific delivery tree. Beside root location
forwarding along this delivery tree will be bound to a topological
network address due to reverse path forwarding (RPF) checks. A mobile
multicast source moving away is solely enabled to either inject data
into a previously established delivery tree by using its previous
topologically correct source address, or to (re-)define a multicast
distribution tree compliant to its new location. In pursuing the
latter the mobile sender will have to proceed without control of the
new tree construction due to decoupling of sender and receivers.
A mobile multicast source consequently must meet address transparency
at two layers: In order to comply with RPF checks, it has to use an
address within the IPv6 basic header's source field, which is in
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topological accordance with the employed multicast distribution tree.
For application transparency the logical node identifier, commonly
the HoA, must be presented as packet's source address to the socket
layer at the receiver side.
Conforming to address transparency and temporal handover constraints
will be the key problem for any route optimizing mobility solution.
Additional issues arrive from possible packet loss and from multicast
scoping. A mobile source away from home must attend scoping
restrictions which arise from its home and its visited location [3].
In presence of inter- domain multicast routing a change of address
must trigger the exchange of a new multicast source record.
2.3.2 Source Specific Multicast Mobility
Fundamentally Source Specific Multicast has been designed for
changeless addresses of multicast senders. Source addresses in client
subscription to SSM groups are directly used for route
identification. Any SSM subscriber is thus forced to know the
topological address of its group contributors. SSM source
identification invalidates, when source addresses change under
mobility.
Consequently source mobility for SSM packet distribution introduces a
significant conceptual complexity in addition to the problems of
mobile ASM. As a listener is subscribed to an (S,G) channel
membership and as routers have established an (S,G)-state shortest
path tree rooted at source S, any change of source addresses under
mobility requests for state updates at all routers and all receivers.
A moving source would have to update its change of CoA with all
listeners, which subsequently had to newly subscribe and initiate
corresponding source-specific trees. As the principle multicast
decoupling of a sender from its receivers likewise holds for SSM, the
need for client update turns into a severe problem.
An SSM listener subscribing to or excluding any specific multicast
source, may want to rely on the 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 [3].
All of the above severely add complexity to a robust SSM mobility
solution, which should converge to optimal routes and, for the sake
of efficiency, should avoid data encapsulation, as well. Like in ASM
handover delays are to be considered critical. The distance of
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subsequent points of attachment, the ’step size’ of the mobile, may
serve as an appropriate measure of complexity.
Finally, Source Specific Multicast has been designed as a light-
weight approach to group communication. In adding mobility
management, it is desirable to preserve the principle leanness of SSM
by minimizing additional signaling overheads.
3. Solutions
Three approaches to mobility in Any Source Multicast are commonly
around:
o Bi-directional Tunnelling guides the mobile node to tunnel all
multicast data via its home agent. This principle multicast solution
hides all movement and results in static multicast trees. It
transparently may be employed by mobile multicast sources, on the
price of triangular routing and possibly significant performance
degradations due to widely spanned data tunnels.
o Remote Subscription forces the mobile node to re-initiate
multicast distribution subsequent to handover, using its current
Care-of Address. This approach of tree discontinuation relies on
multicast dynamics to adapt to network changes. It not only results
in rigorous service disruption, but leads to mobility driven changes
of source addresses, and thus disregards session persistence under
multicast source mobility.
o Agent-based solutions attempt to balance between the previous two
mechanisms. Static agents typically act as local tunnelling proxies,
allowing for some inter-agent handover while the mobile node moves
away. A decelerated inter-tree handover, i.e. tree walking, will be
the outcome of agent-based multicast mobility, where some extra
effort is needed to sustain session persistence through address
transparency of mobile sources.
There are proposals of agent based approaches compliant to the
unicast real-time mobility infrastructure of Fast MIPv6 [15], the M-
FMIPv6 [16], and of Hierarchical MIPv6 [17], the M-HMIPv6 [18], and
to context transfer [19]. An approach based on dynamically negotiated
inter-agent handovers is presented in [20].
It should be noted that none of the above approaches addresses SSM
source mobility, except the bi-directional tunnelling.
4. Security Considerations
This document discusses multicast extensions to mobility. Security
issues arise from source address binding updates, specifically in the
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case of source specific multicast. Threats of hijacking unicast
sessions will result from any solution jointly operating binding
updates for unicast and multicast sessions. Future solutions must
address the security implications.
5. IANA Considerations
There are no IANA considerations introduced by this draft.
6. References
Normative References
1 Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3979, March 2005.
2 Hinden, R. and Deering, S. "Internet Protocol Version 6
Specification", RFC 2460, December 1998.
3 Johnson, D.B., Perkins, C., Arkko, J. "Mobility Support in IPv6",
RFC 3775, June 2004.
4 Arkko, J., Devarapalli, V., Dupont, F "Using IPsec to Protect
Mobile IPv6 Signaling Between Mobile Nodes and Home Agents", RFC
3776, June 2004.
5 S. Deering "Host Extensions for IP Multicasting", RFC 1112, August
1989.
6 S. Bhattacharyya "An Overview of Source-Specific Multicast (SSM)",
RFC 3569, July 2003.
7 H. Holbrook, B. Cain "Source-Specific Multicast for IP", draft-
ietf-ssm-arch-07.txt (work in progress), October 2005.
8 D. Waitzman, C. Partridge, S.E. Deering "Distance Vector Multicast
Routing Protocol", RFC 1075, November 1988.
9 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.
10 B. Fenner, M. Handley, H. Holbrook, I. Kouvelas: "Protocol
Independent Multicast - Sparse Mode PIM-SM): Protocol
Specification(Revised)", draft-ietf-pim-sm-v2-new-11.txt (work in
progress), October 2004.
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11 A. Ballardie " Core Based Trees (CBT version 2) Multicast
Routing", RFC 2189, September 1997.
12 S. Deering, W. Fenner and B. Haberman "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
13 R. Vida and L. Costa (Eds.) "Multicast Listener Discovery Version
2 (MLDv2) for IPv6", RFC3810, June 2004.
14 P. Savola, B. Haberman "Embedding the Rendezvous Point (RP)
Address in an IPv6 Multicast Address", RFC 3956, November 2004.
15 Koodli, R. "Fast Handovers for Mobile IPv6", RFC 4068, July 2004.
16 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.
17 Soliman, H., Castelluccia, C., El-Malki, K., Bellier, L.
"Hierarchical Mobile IPv6 mobility management", RFC 4140, August
2005.
18 Schmidt, T.C. and Waehlisch, M. "Seamless Multicast Handover in a
Hierarchical Mobile IPv6 Environment(M-HMIPv6)", draft-schmidt-
waehlisch-mhmipv6-03.txt, (work in progress), April 2005.
19 Jonas, K. and Miloucheva, I. "Multicast Context Transfer in mobile
IPv6", draft-miloucheva-mldv2-mipv6-00.txt, (work in progress),
June 2005.
20 Zhang, H. et al "Mobile IPv6 Multicast with Dynamic Multicast
Agent" draft-zhang-mipshop-multicast-dma-01.txt, (work in
progress), September 2005.
Informative References
21 Romdhani, I., Kellil, M., Lach, H.-Y. et. al. "IP Mobile
Multicast: Challenges and Solutions", IEEE Comm. Surveys, 6, 1,
2004.
22 Jannetau, C., Tian, Y., Csaba, S. et al "Comparison of Three
Approaches Towards Mobile Multicast", IST Mobile Summit 2003,
Aveiro, Portugal, 16-18 June 2003, online http://www.comnets.rwth-
aachen.de/~o_drive/publications/ist-summit-2003-IPMobileMulticast-
paperv2.0.pdf.
Schmidt, Waehlisch Expires - April 2006 [Page 9]
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23 Mieghem, P., Hooghiemstra, G., Hofstad, R. "On the Efficiency of
Multicast", Transactions on Networking, 9, 6, pp. 719 - 732,
December 2001.
24 Schmidt, T.C. and Waehlisch, M. "Predictive versus Reactive -
Analysis of Handover Performance and Its Implications on IPv6 and
Multicast Mobility", Telecommunication Systems, 30:1/2/3, Springer
2005, in print.
25 Jelger, C., Noel, T. "Multicast for Mobile Hosts in IP Networks:
Progress and Challenges", IEEE Wireless Comm., pp 58-64, Oct.
2002.
Acknowledgments
The authors would like to thank Mark Palkow (DaViKo GmbH) and Hans L.
Cycon (FHTW Berlin) for valuable discussions and a joyful
collaboration.
Author's Addresses
Thomas C. Schmidt
HAW Hamburg, Dept. Informatik
Berliner Tor 7
D-20099 Hamburg
Phone: +49-40-42875-8157
Email: Schmidt@informatik.haw-hamburg.de
Matthias Waehlisch
FHTW Berlin, HRZ
Treskowallee 8
D-10318 Berlin
Email: mw@fhtw-berlin.de
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