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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 customersLimited 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 LANs (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 )
Demonstrated with MBone
Uses tunneling
Multicast integrated into IPv6 Internet Group Management Protocol (IGMP)
Several routing/forwarding schemes:
Distance-vectorLink-state
Protocol-independent
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IP Multicast Service Model
Each group uses a single address
Class D addresses (1110 )
Some are well-known, some are dynamically assigned
Group membership
Members located anywhere in the Internet
Number of receivers is arbitrary
Members canjoin/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 loopbackneeded for sender in group Multicast scope
LAN (local scope)
Administrative scope (e.g. campus), may overlap, canassign 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 ingroup
Internet Group Management Protocol
(IGMP), RFC 1112
New operations to join and leave group
LAN routers track local membership
Forwarding depends on routing schemeLast 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 routersRecipients unpack & forward original multicast packet
Passes through multicast-incapable areas ofInternet
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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 linkSwitches 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 LANs with no receivers from tree
For very large ELANs, 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 LANPeriodically announces its group memberships, via
Internet Group Management Protocol (IGMP)
Extend LSP to include set of groups withmembers on a given LAN
MOSPF routing extends OSPF
Uses Dijkstras 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
Cant precompute for all source-group pairs
Compute on demand when first packet from a
source S to a group G arrivesCache 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 forma tree
Use reverse edges to broadcast from S
Implementation (Reverse Path Broadcast, orRPB)
Forward multicast packet on all linksIf and only if packet came from next hop for packet
source
Avoid repetition on LANs
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
M M
M
MMember of
multicast
group G Unicast route to S
RPB from SRPM from S to G
Pruned
G
S
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RPB to RPM (reverse path multicast)
Identify leafnetworksOnly 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 informationForward packets only to interested parties
Only when multicast source active
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Distance Vector Multicast
RPM Implementation Assume that everyone is interested
Respond to unwanted packets with
prune requests
Prune requests
Canceled by graft requestTime 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 forsparse 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 multipleRPs 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 multicastgroup
S
RPRP
RP
Specific
multicast
tree
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Protocol Independent Multicast
Joins
Receiver: send packet to one RP
Source: send to all RPs
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, multimediaapplications
Control of network resources
Applications have different performance needs
Different resource commitments by clients and/or
organizations
Different ASs provide different QoS