CSE 401N:Computer Network

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CSE 401N:Computer Network

LECTURE-14MULTICAST ROUTING

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Chapter 4 roadmap

4.1 Introduction and Network Service Models4.2 Routing Principles4.3 Hierarchical Routing4.4 The Internet (IP) Protocol4.5 Routing in the Internet4.6 What’s Inside a Router?4.7 IPv64.8 Multicast Routing4.9 Mobility

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Multicast: one sender to many receivers

Multicast: act of sending a datagram to multiple receivers with single “transmit” operation

Question: how to achieve multicast

Multicast via unicast source sends N unicast

datagrams, one addressed to each of N receivers

multicast receiver (red)

not a multicast receiver (grey)

routersforward unicastdatagrams

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Application-layer multicast

receivers involved in multicast copy and forward unicast datagrams among themselves

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Network multicast Router actively participate

in multicast, making copies of packets as needed and forwarding towards multicast receivers

Only a single copy of a datagram will ever traverse a link

Multicastrouters (red) duplicate and forward multicast datagrams

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Multicast Groups

How to identify the receivers of a multicast datagram?

Let datagram carry the IP addresses of all the recipients Only workable with a small number of recipients Sender needs to know the addresses of all receivers Not a good solution!

Internet solution: use multicast group A multicast group identifies a set of receivers and has a

single identifier Packet sent to a multicast group are received by all

members in the group

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Internet Multicast Service Model

Multicast group in the Internet has its own Class D multicast address IP addresses 224.0.0.0 to 239.255.255.255 reserved

for multicast (IP addresses begin with 1110)

128.119.40.186

128.59.16.12

128.34.108.63

128.34.108.60

multicast group

226.17.30.197

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Internet Multicast Service Model

Host group semantics:o anyone can join/leave a multicast groupo anyone can send to a multicast groupo no network-layer identification of hosts that have joined

a multicast group Questions to answer:

How are hosts join/leave a group? A: Internet Group Management Protocol (IGMP) How do the routers deliver a multicast datagram to all

group members? A: multicast routing protocols

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IP multicasting: two-step process

local: host informs local router of desire to join a multicast group: IGMP

wide area: local router interacts with other routers to receive multicast datagrams many multicast routing protocols (e.g., DVMRP,

MOSPF, PIM)

IGMPIGMP

IGMP

wide-areamulticast

routing

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Internet Group Management Protocol (IGMP)

RFC 2236 Operate between a host and its directly

attached router Provide the means for:

A host to inform its attached router that it wants to join/leave a multicast group

A router to detect if a LAN has any members for a particular multicast group

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IGMP Messages

Three message types: membership query, membership report, leave group

General membership query Sent by a router to all nodes on the LAN Query multicast groups joined by attached hosts Multicast group address field set to 0

Specific membership query Sent by a router to all nodes on the LAN Query if a specific multicast group is joined by attached hosts Multicast group address field set to the multicast group address being

queried

Type

Max. Resp.

Time

Checksum

Multicast group address

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IGMP Messages (cont’d)

Membership report message Sent by host to all nodes on the LAN in response to a

membership query message or when first joins a multicast group

Contains the address of a group the host has joined A router only cares about whether one or more hosts

belong to a given group want to hear from only one of the hosts that belongs to each group Each membership query message includes a “maximum

response time” field A host waits a random time between 0 and maximum

response time before sending a membership report message Feedback suppression: if the host observes a membership

report message from some other host for the multicast group, discard its own pending membership report message

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IGMP Messages (cont’d) Leave group message:

sent by host to report leaving a group leave group message is optional

• Router periodically sends a membership query message

• A router infers that no hosts are joined to a group when no host responds to the query message

• Soft state protocol: state is removed via a timeout event if it is not explicitly refreshed

IGMP messages are encapsulated within an IP datagram

There is no control over who sends to the group and who joins the group

Multicast Routing: Problem Statement

Goal: Given a multicast group, find a tree connecting routers having local multicast group members shared tree: a single tree used by all group members source-based tree: different tree for each sender

Shared tree Source-based trees

Approaches for building mcast trees

group-shared tree: group uses one tree minimal cost (Steiner) tree center-based tree

source-based tree: one tree per source shortest path tree reverse path forwarding

…we first look at basic approaches, then specific protocols adopting these approaches

Group-Shared Tree: Steiner Tree All multicast packets are

routed along the same tree, regardless of the sender

Steiner Tree: minimum cost tree connecting all routers with attached group members problem is NP-complete good heuristics exist

not used in practice: information about all links needed rerun whenever link costs change not able to use unicast routing

info

Router having attached group members

Router having no attached group members

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1

11

2

22

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Group-Shared Tree: Center-based tree

single delivery tree shared by all one router identified as center of

tree to join:

router with attached group members sends unicast join message addressed to the center

join message “processed” by intermediate routers and forwarded towards center

join message either hits existing tree branch for this center, or arrives at center

path taken by join message becomes new branch of tree for this router

R1

R2

R3

R4

R5

R6 R7

2

3

1

router with attachedgroup member

router with no attachedgroup member

path order in which join messages generated

1

center

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Multicast Routing Using a Source-Based Tree

Construct a multicast routing tree for each source in the multicast group

Router maintains state for each (group, sender) pair

Shortest path tree: union of the shortest paths from source to each receiver Obtained by Dijkstra’s

algorithm

Router having attached group members

Router having no attached group members

A

B

D

E

C

F

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Steiner Tree v.s Shortest Path Tree

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1

11

2

22

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1

11

2

22

3

A A (source)

Steiner tree: minimize the sum of the link costs in the tree

Shortest path tree: minimize the cost from source to each receiver

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Compute Shortest Path Tree

Use link state algorithm: each router needs to know the state of each link in the network

Can we compute the tree without link state information?

A: use reverse path forwarding algorithm

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Reverse Path Forwarding (RPF)

Forward packet from source S to all interfaces (except the incoming one) iff the packet arrived on the link that is on its shortest path to S A router need only know

the next hop on its shortest path to the sender

result is a source-specific reverse shortest path tree

A

B

D

E

C

F

Packet that will be forwarded

Packet not forwarded beyond receiving router

G

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RPF: Pruning

forwarding tree contains subtrees with no mcast group members no need to forward

datagrams down subtree “prune” msgs sent

upstream by router with no downstream group members

• Router tells parent in the tree to stop forwarding

• Prune message propagate upstream

A

B

D

E

C

FG

Prune message

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Rejoining

If a router sends a prune message upstream, what should happen if it later needs to join the group? Send a graft message to upstream router Or, allow pruned branches to time-out and be

added again to the tree

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Issues in Wide-Area Multicast

Hosts may join and leave a multicast group dynamicallyneed to dynamically update routing tree

State information for a multicast connection must be established and maintained in routers Group-shared tree: per group state Source-based tree: per (group, source) state

Internet Multicasting Routing: DVMRP

DVMRP: distance vector multicast routing protocol, RFC1075

flood and prune: source-based tree, reverse path forwarding each router uses a distance vector algorithm to

compute the next hop on its shortest path to each source

initial datagram to mcast group flooded everywhere via RPF

routers not wanting multicast datagram: send upstream prune messages

routers store state for each (source, group) pair

DVMRP: continued…

soft state: DVMRP router periodically “forgets” branches are pruned: multicast data again flows down unpruned branch router: reprune or else continue to receive data

routers can quickly regraft to tree using graft message

odds and ends commonly implemented in commercial routers MBONE (Multicast Backbone) routing done using DVMRP

Tunneling some Internet routers are not multicast-capable Internet MBONE: a virtual network of multicast-capable routers on top of

a physical network containing a mix of unicast and multicast routers

mcast datagram encapsulated inside unicast datagram unicast IP datagram sent thru “tunnel” via regular IP unicast to

receiving mcast router receiving mcast router extracts mcast datagram

physical topology logical topology

AB

C

A B

C

PIM: Protocol Independent Multicast

not dependent on any particular underlying unicast routing protocol (works with all)

two different multicast distribution scenarios:

Dense mode: group members

densely located, many routers need to be involved in routing multicast datagrams

Sparse mode: # routers with attached

group members small wrt total # routers

Consequences of Sparse-Dense Dichotomy

Dense mode group membership by

routers assumed until routers explicitly prune

data-driven construction of mcast tree (e.g., RPF)

Sparse mode routers not involved in

multicast distribution until explicitly join a group

receiver- driven construction of mcast tree (e.g., center-based)

PIM - Dense Mode

flood-and-prune reverse path forwarding similar to DVMRP, but can interoperate with

any underlying unicast routing protocol

PIM - Sparse Mode

center-based approach router sends JOIN msg to

rendezvous point (RP) to join the tree intermediate routers update

state and forward JOIN message toward RP

PRUNE message is sent when a router leaves a group

R1

R2

R3

R4

R5

R6R7

join

join

join

rendezvouspoint

PIM - Sparse Mode

sender unicast data to RP, which multicasts down RP-rooted tree

RP can send STOP message to source if no routers are jointed to the tree “no one is listening!”

after joining RP, router can switch to source-specific tree

R1

R2

R3

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

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Switch from group-shared tree to a source-specific tree

In group-shared tree, E sends to RP, then RP sends to A

when data rate from E exceeds a threshold A sends join message directly

to E A does not send prune

message towards RP so it can continue to receive from other sources

increased performance: less traffic concentration, shorter paths

source

receiverRP

Before switch

After switch

A B C

E

D

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MOSPF: Multicast Open Shortest Path First

Multicast extension to OSPF Routers flood group membership information

with Link State Advertisements Each router independently computes source-

specific, shortest-path trees for each multicast group

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Evaluating a Multicast Routing Protocol

Scalability: the amount of state required in the routers Group-shared tree: per

group state Source-based tree: per

group, per source state Excess traffic received?

Yes: DVMRP, PIM Dense Mode

No: MOSPF, PIM Sparse Mode

Traffic Concentration Group-shared tree tends

to concentrate traffic on a smaller number of links

Source-based trees tend to distribute multicast traffic more evenly

Optimality of forwarding paths Minimum cost multicast

tree not used in practice Shortest path trees and

center based trees are used

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THANX.

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CSE 401N:Computer Network