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CS716

Advanced Computer Networks

By Dr. Amir Qayyum

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Lecture No. 27

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Multicast

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Internetworking

• Basics of internetworking (heterogeneity)– IP protocol, address resolution, control messages …

• Routing• Global internets (scale)

– Virtual geography and addresses– Hierarchical routing

• Future internetworking: IPv6• Multicast traffic• MPLS

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Internet Multicast Outline• Motivation and challenges

• Support strategy

• IP multicast service model

• Multicast in the Internet

• Routing

– Review of ELAN techniques

– Multicast routing

• Limitations

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Multicast• Unicast: one destination• Broadcast: all destinations• Multicast: subset of destinations• When is multicast useful ?

– Send data to multiple receivers at once• Videoconferencing, video-on-demand, telecollaboration• Software update to group of customers

– Limited broadcast/self-defined multicast• Send question to unknown receiver• Resource discovery; Distributed database

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Multicast• Why not just use broadcast/unicast ?

– Broadcast not supported outside of LAN– Unicast sends multiple copies across common links

• Multicast support– Often supported by hardware in LAN’s (as broadcast, if

not multicast)– But difficult to extend in scalable manner

• Multicast challenges– Efficient distribution on an internetwork– Specification of recipient group (abstraction must

support self-definition)

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Multicast Support Strategy• IPv4 used as basis for experimental solutions

– Use class D addresses (1110 <28 bits>)– Demonstrated with MBone– Uses tunneling

• Multicast integrated into IPv6• Internet Group Management Protocol (IGMP)• Several routing/forwarding schemes:

– Distance-vector– Link-state– Protocol-independent

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IP Multicast Service Model• Each group uses a single address

– Class D addresses (1110 <28 bits>)– Some are well-known, some are dynamically assigned

• Group membership– Members located anywhere in the Internet– Number of receivers is arbitrary– Members can join/leave dynamically– Hosts can belong to more than one group

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IP Multicast Service Model

• Senders simply use group address as destination– Sender need not be in group– LAN loopback needed for sender in group

• Multicast scope– LAN (local scope)– Administrative scope (e.g. campus), may overlap, can assign

group addresses dynamically– TTL scope (no more than N hops)

• Scope is exposed to protocols and applications (by exposing IP TTL)

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IP Multicast Service Model• Multicast reception requires membership in group

– Internet Group Management Protocol (IGMP), RFC 1112

– New operations to join and leave group– LAN routers track local membership– Forwarding depends on routing scheme– Last hop typically uses LAN broadcast

• Packet reception same as IP unicast

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Internet Multicast Backbone - MBone

• Existing infrastructure for multicast in the Internet• Multicast route propagation using DVMRP• Problem: most IP routers do not support multicast• Solution: tunneling by multicast-capable routers

– Encapsulate multicast traffic in IP packets– Send to other multicast-capable routers– Recipients unpack & forward original multicast packet

• Passes through multicast-incapable areas of Internet

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ELAN Multicast Techniques• Direct support (Ethernet)

– Application subscribes to group– IP layer notifies Ethernet card to listen to

packets with group address• Support through broadcast (LANE)• Flooding in ELANs

– Each packet sent on all but incoming link– Switches must remember each packet!

• Spanning tree: every host gets one copy

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ELAN Multicast Techniques• Spanning tree selection

– Elect a leader; spanning tree is shortest path to leader (Perlman)

– Distribute topology everywhere, compute in parallel (link-state)

• Problems with spanning trees– Bandwidth wasted for groups with few receivers; Solution:

prune LAN’s with no receivers from tree– For very large ELAN’s, no single tree is efficient; Solution:

define tree per group or tree per source

• The same solutions are used in the Internet!

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Spanning Tree Tradeoffs

• Tree per group or tree per source ?

• Per group advantage– One routing entry per group

• Per source advantages– More efficient distribution

– Spreads load better across links

– Leverage unicast routing tables

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Multicast Routing in the Internet

• Multicast Open Shortest Path First (MOSPF)

• Distance-Vector Multicast Routing Protocol (DVMRP, used in MBONE)

• Protocol-Independent Multicast (PIM)– Deals with scalability issues of above protocols

– Dense Mode (PIM-DM)

– Sparse Mode (PIM-SM)

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Multicast Routing in the Internet• How do senders find receivers?

– Receivers inform all senders of interest (MOSPF)– Send to all receivers; uninterested receivers prune

(DVMRP, PIM-DM)– Agree on set of rendezvous points (PIM-SM)

• Types of distribution trees– Separate tree from each sender (DVMRP, MOSPF,

PIM-DM, PIM-SM)– Tree rooted at rendezvous point (PIM-SM)

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Link State Multicast (MOSPF)

• Each host on a LAN– Periodically announces its group memberships, via

Internet Group Management Protocol (IGMP)

• Extend LSP to include set of groups with members on a given LAN

• MOSPF routing extends OSPF– Uses Dijkstra’s algorithm– Computes shortest-path spanning tree for source-group

pairs– Forward packet on local portion of tree

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Link State Multicast (MOSPF)• Tree computation

– Can’t precompute for all source-group pairs– Compute on demand when first packet from a source S

to a group G arrives– Cache trees for active source-group pairs– Recompute when link-state changes

• Scalability limitations– Reasonable intra-AS scalability– But meaningless for inter-AS– Source-group pairs scale with sources (needs to be

hierarchical)

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Distance Vector Multicast (DVMRP)• Idea

– Graph of directed next-hop edges to a destination S form a tree

– Use reverse edges to broadcast from S• Implementation (Reverse Path Broadcast, or RPB)

– Forward multicast packet on all links– If and only if packet came from next hop for packet source

• Avoid repetition on LAN’s– Assign parent router for each LAN– Has shortest path to source, ties broken by ID– Track parenthood via vector exchanges

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RPB and RPM

MM MM

MM

MMMember of multicast group G Unicast route to S

RPB from S

RPM from S to G

Pruned

GG

SS

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RPB to RPM (reverse path multicast)• Identify leaf networks

– Only one router on network– Thus no distance packets received on interface

• Prune leaf networks– Without hosts in a group– Hosts must self-identify using IGMP

• Forward pruning information– Extend distance vector with group information– Forward packets only to interested parties– Only when multicast source active

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Distance Vector MulticastRPM Implementation

• Assume that everyone is interested

• Respond to unwanted packets with prune requests

• Prune requests– Canceled by graft request

– Time out periodically

• Need ARQ for prune or graft ?

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Distance Vector Multicast - Scalability• Packets are periodically broadcast (thus

guaranteed to reach all interested members)• High overhead for sparse groups, consider:

– Multicast group of 10 members– Scattered around the world– Packets periodically reach all routers in Internet

• High overhead for routers– All off-tree routers maintain pruning state– And periodically retransmit

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Protocol Independent Multicast (PIM)

• Approach– Define rendezvous points (RP) for each group– Need multiple RP’s to handle failures

• Two versions– Dense mode

• Explicit prune messages• Shared tree

– Sparse mode• Explicit join messages• Shared or source-specific tree

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Protocol Independent Multicast (PIM)

• Rendezvous points (RP) for each multicast group

SS

RPRP RPRPRPRP

Specific multicast

tree

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Protocol Independent Multicast

• Joins– Receiver: send packet to one RP

– Source: send to all RP’s

• Tree selection– Rooted at rendezvous points

– Shared for infrequent traffic

– Source-specific if merited by traffic level

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Limitations on Multicast

• Scalability (addressed to some extent by PIM)– Explosive growth of the Internet population– Explosive growth of multicast, multimedia

applications

• Control of network resources– Applications have different performance needs– Different resource commitments by clients and/or

organizations– Different ASs provide different QoS …