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Multipoint Communications in a Beyond-3G Internetwork
Elias C. Efstathiou & George C. PolyzosElias C. Efstathiou & George C. Polyzos
Mobile Multimedia LaboratoryDepartment of Informatics
Athens University of Economics and BusinessAthens 10434, Greece
http://mm.aueb.gr/Tel.: +30 10 8203 650, Fax: +30 10 8203 686
Outline
Introduction The Internet Beyond 3G Mobile Multicast: High-level Issues
IP Multicast, Mobile IP and Cellular IP Filters and Media Stream Quality Combining IP Multicast and Mobility
IETF Mobile Multicast Approach Extensions to the IETF Approach
Our Perspective IGMP Mobility Support and IGMP Assumptions Multicast Semantics and Mobility Mobile Multicast Requirements Cellular IP and Mobile Multicast The Beyond-3G Environment
Conclusions
Introduction - The Internet Beyond 3G
Diverse network technologies 2.5G and 3G networks
700 million cellular subscribers today 2 billion expected in the 2006-2007 timeframe
Digital Video Broadcasting (DVB) networks Terrestrial (DVB-T), Satellite (DVB-S) flavors 30 Mbps of shared downlink bandwidth
IEEE 802.11 networks 11 up to 54 Mbps in the ISM band New business models emerging
Traditional wired access networks Ethernet PSTN, ISDN, DSL, Cable
The Internet Beyond 3G (cont’d)
Goal: All-IP internetwork IP-over-everything, IP-under-everything Integrated services spanning network technologies
Audio, video, data Unidirectional and bidirectional
Support for Multipoint Communications? One-to-many, many-to-many
Support for Mobility? Personal mobility Network mobility (moving ships, trains, cars)
Our focus: The Mobile Multicast Problem IP-based quasi-reliable mobile multipoint communications
assuming a fixed routing infrastructure (no ad hoc networks) assuming IPv4, but taking IPv6 into account
Mobile Multicast: High-level Issues
IP Multicast Easy for some technologies only
“Native” support in Ethernet and broadcast networks… …but, point-to-point links in Cellular, PSTN/ISDN and DSL
Mobility Not supported in the original Internet design
An IP address is a subnet and interface identifier but it’s also used in packet routing
TCP connection identifiers include lower layer IP identifiers and do not allow them to change
Answer: Mobile IP One address for identification, another for routing
Hostile Wireless Environment Higher BER challenges original TCP and IP assumptions
IP Multicast
Many-to-many communication protocol Host group service model
a receiving host may join and leave a multicast group at any time all IP hosts can communicate unidirectionally with all group members
using only the group’s identifier (its class D multicast address) Routing packets and tracking membership
Global and Local mechanisms respectively
Global Routing Mechanisms Multicast routing protocols deliver a group’s packets to multicast
routers that have expressed interest in receiving packets for a particular group
DVMRP, CBT, MOSPF, PIM Graft delay when a multicast router joins the multicast tree
IP Multicast (cont’d)
Local Membership Tracking Mechanisms Multicast router: the “interface” between the local and the global
mechanism Exposes aggregate list of groups all its hosts have joined
Protocols for membership tracking IGMP (for IPv4) and MLD (for IPv6)
Soft-state principle – no explicit LEAVE_GROUP primitive IGMP assumes link-level native broadcast
Mobile IP (M-IP) Allows internetwork host mobility in a manner
transparent to the transport layer M-IP terminology
Mobile Host (MH) Correspondent Host (CH) Home Agent (HA) Foreign Agent (FA) Care-of Address (CoA – 2 types: FA CoA and co-located CoA) Tunneling (IP-in-IP encapsulation)
Registration with the HA needed Soft-state principle, re-registrations required Delays, lost packets
“Triangle routing” inefficiency Solved in M-IPv6, which specifies that all CHs can make a CoA-to-
home address binding and can also tunnel and de-tunnel packets
Cellular IP (C-IP) C-IP is a micro-mobility protocol
Unlike M-IP’s “slow” macro-mobility, C-IP assumes “fast” mobility Needed because M-IP incurs delays
FA discovery time + Registration with FA + Registration with HA M-IP can violate “the mobility assumption”
“total registration delay maybe more than the time a MH spends inside a cell [controlled by one FA]”. No packets will be delivered
C-IP assumptions Campus-wide, partially-overlapping micro-cells Two-tier architecture: C-IP will rely on M-IP
Simpler than proposed “hierarchical FA M-IP schemes” (Realistic!) C-IP gateway M-IP FA
“interface” between the M-IP and C-IP routing infrastructures Base stations IP packet forwarders
Simple routing: no tunneling, resembles MAC bridge frame forwarding with auto-learning
Cellular IP Access Network
Mobile IP Internetwork
Correspondent Host
Home Agent
Foreign Agent and Cellular IP Gateway
BS1 BS4
BS3
Mobile Host
BS2
Cellular IP Network
Filters and Media Stream Quality
Filters and transcoders Absolutely necessary for mobile multicast Help maintain a level of Perceived Quality of Service (P-QoS) “Smart” filters, “Simple” filters
Layered coding and multi-resolution layered coding A media stream is separated into more than one stream
Sub-streams can be transmitted in different multicast groups Receivers “tune into” as many as possible
Filter mobility characteristics Fixed
Usually located at the boundary between wired and wireless section Mobile
In multicast trees, they can propagate upstream, closer to the source, combine into one and serve many receivers in the same sub-tree
Combining IP Multicast and Mobility Mobile devices are fundamentally different
Limited battery life have to avoid unnecessary operations constant network traffic monitoring is impractical
Radio interface cannot assume high bandwidth nor low BER Handoffs forced disconnections
Vertical and horizontal TDMA and CDMA with power control no link-level multicast yet
IPv4 address shortage GPRS operators rely on NAT
NAT makes IP multicast more difficult
Cellular operators interpret “multicast” differently Cell-limited usually Not the IP-based multicast envisaged for the Beyond-3G Internet
IETF Mobile Multicast Approach
RFC 3220 (Mobile IPv4) proposes two methods (1) MIP-RS – “Remote subscription”
Assumption: a multicast router exists in the visited subnet MHs simply use IGMP and (re-)subscribe to any number of groups
Disadvantage 1: delay, packet losses, tree rearrangement Disadvantage 2: “get-ahead” and “lag-behind” problems
(2) MIP-BT – “Bi-directional tunneled multicast” Assumption 1: the MH’s HA is a multicast router
IGMP requests are tunneled to the HA The HA joins groups on MH’s behalf
Assumption 2: MH must decapsulate the multicast packets sent to it through the tunnel - even if it uses an FA for M-IP decapsulation
Disadvantage 1: potential packet duplication Disadvantage 2: potential tunnel convergence
Packet Duplication and Tunnel Convergence
FA
MH MH MH MH
HA HA HA
FA
MH MH
HA
Both visiting MHs belong to the same home networkand are members of the same multicast group but because of tunneling the FA has no way of knowing this
Visiting MHs belong to different home networks butthey are members of the same multicast group causingmultiple HA-FA tunnels to carry the same datagrams
Extensions to the IETF Approach Mobile Multicast (MoM) Protocol
Based on MIP-BT. Key extension: The Designated Multicast Service Provider (DMSP)
A DMSP for a group is an HA chosen by a subnet’s FA out of the many that may forward packets for a specific group there
FA chooses a DMSP and performs DMSP handoffs when needed Solves tunnel convergence Most cited alternative to MIP-BT and MIP-RS V. Chikarmane et al., “Multicast Support for Mobile Hosts Using Mobile IP:
Design Issues and Proposed Architecture,” ACM/Baltzer Mobile Networks and Applications, 3(4):365-379, Jan. 1999.
Mobile Multicast with Routing Optimization (MMROP) Based on MIP-RS. Key extension: The Mobility Agent (MA) MAs route missing packets (via tunneling) to neighboring subnets Tunnels need to be setup between FAs Solves get-ahead problem MMROP assumes packets are somehow numbered J. Lai et al., “Mobile Multicast with Routing Optimization for Recipient Mobility,”
Proceedings IEEE ICC2001, pp. 1340-1344, June 2001.
Extensions to the IETF Approach (cont’d)
Constraint Tree Migration Scheme (CTMS) Improved version of the CBT multicast routing protocol “automatically [migrates multicast trees] to better ones while maintaining the
QoS guarantees specified my mobile users” Reduces packets losses due to reconfigurations and join delays Difficult to deploy
Most multicast routers still run DVMRP K. Chen et al., “CTMS: A novel constrained tree migration scheme for
multicast services in generic wireless systems,” IEEE JSAC, 19:1998-2014, October 2001.
Multicast Scheme for Wireless Networks (MobiCast) Based on MIP-RS. Key-extension: The Domain Foreign Agent (DFA) DFAs serve many small adjacent cells Small cells are organized in one Dynamic Virtual Macrocell (DVM) Similar to hierarchical FA M-IP and to our joint M-IP/C-IP solution C. Tan and S. Pink, “MobiCast: A Multicast Scheme for Wireless Networks,”
ACM MONET, 5(4):259-271, 2000.
IGMP Mobility Support & IGMP Assumptions
IGMP was designed with Ethernet in mind IGMP is not suitable for routers with point-to-point links
IGMP queries have to be issued to each one of these links Not everyone will hear responses… … unless the router multi-unicasts them More state information needed at the router
IGMP is not suitable for mobile hosts Mobile hosts cannot constantly monitor network traffic Mobile hosts should not be forced to resend unnecessary data Solution use explicit JOIN_GROUP and LEAVE_GROUP
primitives
Multicast Semantics and Mobility
Multicast semantics require reexamination in the presence of host mobility
Example: consider two Ethernet IP subnets X and Y Some MHs from X are visiting Y and some MHs from Y are
visiting X 224.0.0.1 is the special IPv4 link-local all-hosts multicast group A packet addressed to 224.0.0.1 is sent to X – what happens?
Packet is delivered only to hosts in subnet X regardless if they are visitors or not
Packet is delivered only to hosts in subnet X that are not visitors Packet is delivered to all X hosts irrespective of location
Answer: it depends on the originating service protocol IPv6 will help: defines link local, site local and
organizational local multicast scopes
Mobile Multicast Requirements Significant vs non-significant moves
If a MH move causes the new subnet’s multicast router to subscribe to new groups, the move is significant
Non-significant moves should have no effect on the global mechanisms
Both types must appear similar from the user’s perspective Multicast packet buffering
Buffer packets until when? Disconnections due to
Handoffs Physical layer problems User intent
Mobile subnets Deal with them as one logical entity
Roaming Sophisticated authentication and pricing schemes are also required
Cellular IP and Mobile Multicast
Integrate efficient multicast mechanisms into C-IP Use C-IP in conjunction with M-IP Based on IETF’s C-IP and M-IP interoperability ideas Scalability concerns
Similar to the MobiCast scheme MobiCast DFA C-IP gateway MobiCast DVM C-IP subnet
Based on MIP-RS Closer to real-life network deployments
Campus-wide 802.11 internetworks UMTS cells DVB-T macrocells
MIP-BT tunneling not scalable MHs should first exploit resources in their immediate environment
Cellular IP and Mobile Multicast (cont’d)
“Flat” address space in C-IP Multicast addresses do not appear different from unicast addresses Simple mapping of IP identifiers to forwarder ports
C-IP has keep-alive mechanisms similar to IGMP Adapt the C-IP “route-update” packet mechanism MHs send these packets but instead of using their own IP address in
the source field, they use the multicast group address instead When, with C-IP forwarding, they reach the C-IP gateway, the
gateway (M-IP FA) may then subscribe to a group, if it’s not receiving it already.
No need for IGMP. Reuse C-IP soft state mechanisms. Multicast groups can be “virtual” C-IP hosts
C-IP forwarders should handle IP address-to-multiple ports mappings C-IP gateway must be a multicast router MHs use their IP stack in an “unconventional” way Packet duplication only when paths towards receivers diverge!
Cellular IP and Mobile Multicast (cont’d)
Global Multicast Routing Protocol
224.1.2.3 transmissionsource
Foreign Agent and Cellular IP Gateway
BS1 BS4
BS3
Mobile Host
BS2
Mobile Host
Mobile Host
The 3 MHs subscribe to 224.1.2.3 by sending route_update with 224.1.2.3 as the source IP address. BSs update tables accordingly for virtual MH “224.1.2.3”:BS1 BS2BS2 BS3 and BS4BS3 and BS4 do link-local broadcast
The FA/CIP-GW grafts to new multicast trees every time a route_update with a new group arrives. IGMP-like soft-state assured through C-IP mechanisms.
The Beyond-3G Environment
Many, superimposed, cellular technologies Even if we ignore satellites… DVB-T (1-100 Km) macrocells
DVB-T with 3G as the return channel 5-30 Mbps of shared bandwidth
3G neighborhood size cells 802.11 microcells
Devices with multiple interfaces 802.11 combined with DVB or GPRS
Mobile IP an accepted standard M-IP support in devices and networks
Improved TCP versions Filter and transcoding standards
Conclusions
The Multipoint Communications Problem Standard IETF protocols can solve it
IP multicast Mobile IP Cellular IP
The Environment Beyond-3G will be based on IP Therefore, most of these IP-based solutions will be applicable
Future Research Areas in Mobile Multicast Ad hoc networks Strong reliability Security Roaming and pricing agreements
