advanced computer networks - cs716 power point slides lecture 27

7/27/2019 Advanced Computer Networks - CS716 Power Point Slides Lecture 27

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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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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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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

advanced computer networks - cs716 power point slides lecture 27

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