Multicast & Multimedia

CS731Wei Tsang Ooi

Overview

• What is Multicast ?• Group Management (IGMP)• Routing Protocols (DVMRP, MOPSF,

CBT, PIM-DM, PIM-SM)• Adaptivity (DSG, RLM,

ThinStreams, SCUBA, MEGA)

What is Multicast ?

What is Multicast ?

• Sending a packet to multiple destination using a single address

Unicast

RouterRouter

Router

Router

A

B

C

SABC

Multicast

RouterRouter

Router

Router

A

B

C

SG

Multicast

RouterRouter

Router

Router

A

B

C

S

G

G

Multicast

RouterRouter

Router

Router

A

B

C

S G

G

Questions

• How does a router know if some host in its subnet is member of some group ?

• How does a router know where to forward a packet ?

• What if a packet is lost ?• What if the members have different

network capacity ?

Questions

• How does a router know if some host in its subnet is member of some group ?

• How does a router know where to forward a packet ?

• What if a packet is lost ?• What if the members have different

network capacity ?

Group Management

Group Management

• Routers maintain “local host group membership table”

• “which group has a member in my subnet ?”

IGMP v1.0

• JOIN messageA : “I want to join group G.”

• QUERY messageR : “Which group have you joined ?”

IGMP v1.0

• MEMBERSHIP Report

A : “I am a member of group G”A : “I am a member of group H”B : “I am a member of group G”

Avoiding Implosion

• Select random delay t• After time t, if nobody belongs to

the same group, send membership report.

• Resend the report after some delay just to be safe.

Leave

• If nobody tell the router that they belongs to some group G after a few query messages, G will be removed from local host membership table.

• Long leave latency (minutes)

IGMP v2.0

• LEAVE message“I want to leave group G”

• Group-Specific Query“Anybody else belongs to group G ?”

IGMP v3.0

• Group-Source Inclusion“I want to listen to S from group G”

• Group-Source Exclusion“I do not want to listen to S from group G”

Questions

• How does a router know if some host in its subnet is member of some group ?

• How does a router know where to forward a packet ?

• What if a packet is lost ?• What if the members have different

network capacity ?

Routing Protocols

Routing Protocols

• Generic Methods :– Form a tree to all routers with

members– Deliver the packets along the tree

Rounting Protocols

• DVMRP• MOSPF• CBT• PIM

Rounting Protocols

•DVMRP• MOSPF• CBT• PIM

From S to G

R P

Q

T

R P

Q

T

Is R on the shortest path to S ?

R P

Q

T

If no.. ignore the packet

R P

Q

T

If yes.. continue

Where should I

forward it to ?

R P

Q

T

R P

Q

T

Truncation

• Router checks local host membership table.

• Duplicate the packets only if there is a member in the subnet.

R P

Q

T

?

?

R P

Q

T

US

?

?

Exchanging Routing Table

• Routers periodically sends routing tables to their neighbours

• If neighbour is going to ignore my packets, don’t need to send the packets to it.

Pruning

• Default : Always send to neighbouring routers, unless told otherwise.

• Routers who received a “useless” packet send a prune message back.

“Don’t send me packet addressed to G anymore !”

R P

Q

T

R P

Q

TPRUNE

R P

Q

T

R P

Q

TGRAFT

R P

Q

T

R P

Q

T

PRUNE

Problems of DVMRP

• One tree for each pair (source, group)

• Rebuilt tree periodically• So need to exchange routing

tables periodically• First packet may visits more

routers than it needs to

Rounting Protocols

• DVMRP

•MOSPF• CBT• PIM

MOSPF

• Based on OSPF• For intra-gateway routing• Routers flood membership

infomation to all other routers

MOPSF

• All routers must have the same topological/group members information.

• Each of them compute a shortest path tree.

Problems of MOSPF

• Different minimal tree leads to wastage.

P

RQ

P

Q R

Solution

• Some convention to make sure all tree computed are the same

Problems of MOSPF

• One tree for each (source, group) pair

• Computational intensive

Solution

• Compute the tree when received the first packet.

Example of MOSPF

P

Q R

TU

P computes the tree

P

Q R

TU

Q computes the tree

P

Q R

TU

R & U compute the tree

P

Q R

TU

T computes the tree

P

Q R

TU

MOSPF vs DVMRP

• MOSPF only forward packets down the path that leads to members.

Problems with MOSPF/DVMRP

• Not scalable O(SG)• Does not work well over sparsely

distributed group • Also known as dense-mode

routing protocols

Rounting Protocols

• DVMRP• MOSPF

•CBT• PIM

Core Based Tree

• Designed for – sparse-mode– better scalability

• A router is desinated as a core (how?)

Join

core PQ

RU

V

join

Join

core PQ

RU

V

ack

Intercept Join

core PQ

RU

V

join

(G, v, core)

Intercept Join

core PQ

RU

V

ack

Send (on the tree)

core PQ

RU

V

Send (not on the tree)

core PQ

RU

V

Send (not on the tree)

core PQ

RU

V

Core Router Discoveries

• Position of core affect performance.

• No perfect solutions in choosing core.

Core Discovery - Bootstrap

• A router is elected as Bootstrap Router• Other routers send “core candidate”

message to the bootstrap router• Bootstrap router send a vector of

candidate core routers to all routers• Routers hash the group address, index

into the vector to find the core router.

Core Discovery - Manual

• Configure each routers with (core, group) table.

CBT Summaries

• One shared tree per group• Scalablity O(G)• No need to broadcast routing tables

or flood link states• Worst case delay is twice of the

shortest path tree (average 1.4 times)

• Single point of failure

Rounting Protocols

• DVMRP• MOSPF• CBT

•PIM

Protocol Independent Multicast

• Get the best of both world :– dense mode : shortest path– sparse mode : shared tree

• Independent of unicast routing protocols.

Join

P coreQ

R

join

Send (SM)

P coreQ

R

S

Switch

P coreQ

R

S

switch

Switch

P coreQ

R

S

Send (DM)

P coreQ

R

S

Questions

• How does a router know if some host in its subnet is member of some group ?

• How does a router know where to forward a packet ?

• What if a packet is lost ?• What if the members have different

network capacity ?

Questions

• How does a router know if some host in its subnet is member of some group ?

• How does a router know where to forward a packet ?

• What if a packet is lost ?• What if the members have

different network capacity ?

Multicasting over

Heterogenous

Network

Problems

• Different receivers reside on different networks with different capacities, what should the source transmit ?

ISDN

28.8KModem

T3S

Solutions

• Source decides what to send based on feedback from receivers

• Receivers decides what to receive from the source

• Network decides what the receivers will received

Source Driven

• Source decides what to send based on feedback from receivers

• Receivers decides what to receive from the source

• Network decides what the receivers will received

Jean Bolot et. al. (1994)

• Networks condition is categorized as :– Unloaded– Loaded– Congested

• Objectives : Stays in loaded region

Naive Approach

• If sender received one complain about congestions in the network, sender reduce the sending rate.

• Works in unicast case but not in the case of multicase.

A Better Solution

• If at least x% of the receivers are congested, reduce sending rate.

• If at least y% of the receivers are loaded, do nothing.

• Else increase sending rate

How to Get Feedback ?

• Sender cannot ask all receivers at once and receivers cannot all answer at once.

Bolot’s Idea

• Senders and receivers generate random 16-bit “key”

• If a the first k bit of receiver’s key match the first k bit of the sender’s, the receivers responds.

Bolot’s Idea

• First iteration, senders use the receiver responds to estimate the group size.

• Subsequently, receivers only responds if the network condition is worst than the sender thought.

Receiver Driven

• Source decides what to send based on feedback from receivers

• Receivers decides what to receive from the source

• Network decides what the receivers will received

Destination Set Grouping

S.Y. Cheung et. al. 1995• Source transmit same data in

different streams with different quality

• Receivers can feedback to the source, to adjust the rate of a stream (intra-stream), or they can move to different streams (inter-stream)

Simulcast Streams

stream 1 2 3

high

low

Problems

• Receivers might decide to join higher quality streams, but later find out that they are not able to handle it.

• Side Effects : quality of streams might be lowered.

Solutions

• inter-stream switch is by “invitation” of source only

• punishment : cannot switch again within a time inverval

RLMMcCanne et. al. 1996

• Source transmits data in different “layers”.

• One layer per group• Need better quality, add a layer• Congestion, drop a layer

Join-Experiment

• Receivers find out if they can join the next higher layer by experiments.

• Join the layer, if congested, drop it.

Join Experiment

1

2

3

4

tjoi

n

Problems

• If two or more receivers perform experiments at once, they interfere with each other.

Solutions

• Before performing an experiment, a receiver announce its intention to others.

• Others will refrain from performing the experiment.

More Problems

• New receiver needs to quickly subscribe to layers.

Solution

• join-experiments for lower layer is allowed to overlap with higher layer experiments.

• The receivers needs to compensate for the overlapping when analyzing the result of the experiment.

Problems

• Buffer space needed by router is : layerBandwidth*totalExperimentTime

• Packets lost if buffer space is not large enough

• Congestions is detected after it occurs.

ThinStreams Linda, Brian & Rosen 1997

• Each layer is thin has a fixed thickness

• Use throughput as a measurement instead of packet losses

• Use clock signal to sync join-experiment

• Enforce link-sharing by making it harder to join higher group.

SCUBA Elan Amir et. al 1997

• Reflect receiver’s interest in adapting bandwidth

• Receiver sends interest report to source.

• Source adjust bandwidth according to its weight

SCUBA Example

S1

S2R3

R2

R1

Interest Report

S1 S2R1 0.8 0.2

R2 0.8 0.2

R3 0.5 0.5

S1’s weight =( 0.8 + 0.8 + 0.5 )/ 3 = 0.7S2’s weight =( 0.2 + 0.2 + 0.5 )/ 3 = 0.3

Result

• S1 will transmit at 70% total bandwidth

• S2 will transmit at 30% total bandwidth

Others will share the rest of 5%

Problems

• Receiver Heterogeneity

Solution

• Layered Multicast

1

2

3

4

1

2

3

4

S0 S1

Layers

Network Driven

• Source decides what to send based on feedback from receivers

• Receivers decide what to receive from the source

• Network decides what the receivers will received

MeGaElan Amir et. al. 1995

• Position a application level gateway at strategic point in the network.

MeGa

S

64Kbps100Mbps

The End