Reliable and Application Layer Multicast Ghislaine Amrani - - PowerPoint PPT Presentation

9/11/2006 1

Reliable and Application Layer Multicast

Ghislaine Amrani

9/11/2006

Reliable and Application layer Multicast

2/19

[ [

Agenda

I) Introduction Introduction

g What is Multicast ? g What are the applications of Multicast ? g How does it works ?

II) Reliable Reliable Multicast Multicast

g Definition g Techniques

III) Application layer Multicast Application layer Multicast IV) Conclusion Conclusion

Agenda

9/11/2006

Reliable and Application layer Multicast

3/19

[ [

Introduction

g What is Multicast ?

g Multicast is the simultaneous delivery of one copy of the information

to a group of receivers who wants to have it.

g Advantages :

g Bandwidth is conserved on shared links : only one copy of each packet

is sent on each link.

g Unlimited number of participants. g Not limited to one single LAN (like broadcast) Multicast routing

g What are the applications of Multicast ?

g Video and audio streaming g Video conference g Internet games g … I) Introduction

• What is Multicast

and applications

• How does it works

II)Reliable Multicast III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

4/19

[ [

Introduction

g How does it works ?

g Multicast uses a different kind of IP

adresse : Class D (224.0.0.0 – 239.255.255.255).

g One address is assigned to group of

receivers instead of one address to a single receiver (unicast)

g When a host joins a group, it notifies

the nearest multicast subnet router of its presence in the group using IGMP protocol. R1 R2 R3

Sender

I) Introduction

• What is Multicast

and applications

• How does it works

II)Reliable Multicast III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

5/19

[ [

Reliable Multicast - Definition

g Definition

g Reliable multicast is a mechanism by which multicast data from a

sender is guaranteed to reach the receivers at some time.

g Multicast application requires a Reliable Multicast protocol on top of

IP Multicast :

g MFTP (Multicast File Transfer Protocol) g SRM (Scalable Reliable Multicast) for video conferences g …

g The Reliable Multicast protocol informs the upper-layer about the

failure or error-free of the delivery.

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

6/19

[ [

Reliable Multicast - Use

Multi Multi-

• Point Applications

Point Applications Real time Real time Delay Delay-

• sensitive

sensitive

Streaming media Streaming media Streaming data Streaming data Interactive Interactive Non Non-

• interactive

interactive Ex : Multi-payer Internet games, on-line trading…

Ex : Video conferencing

Ex : Internet TV…

Information content is not drastically reduced if loss occur no need for Reliable Multicast We have to be sure that packets will reach the destination Need for Reliable Multicast

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

7/19

[ [

Reliable Multicast - Challenges

g Challenges :

g g Feedback implosion

Feedback implosion : where feedback from all receivers can saturate the source and the link close to the source.

g g “

“Crying baby Crying baby” ” : where fee receivers have a high packet loss and slow down the whole multicast process by repeating transmissions.

…

ACK ACK ACK ACK ACK ACK A C K A C K

Receivers Sender

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

8/19

[ [

Reliable Multicast - Techniques

g Techniques :

g SCE : Single-Connection Emulation g NAK-based loss reporting g Distributed loss recovery g Router-assisted loss recovery g FEC (Forward Error Correction) -based

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

9/19

[ [

g g SCE : Single

SCE : Single-

• Connection Emulation

Connection Emulation

g Also called ACK-based loss reporting g The simplest Reliable Multicast protocol g The sender establish a TCP session with every receiver :

g He sends data via IP Multicast g He receives ACKs from receivers via IP unicast

g Work very well with small groups but has a bad scalability g Risks of implosion if senders send their ACKs at the same time and

“Crying baby”

Reliable Multicast - Techniques

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

10/19

[ [

Reliable Multicast - Techniques

g g Nak

Nak-

• based loss reporting

based loss reporting

g Receivers are responsible for detecting losses (by cheking the

sequence number) :

g They send NAK (negative acknowledgement) to the sender g They request retransmissions for themselves.

g NAK-based Vs ACK-based :

g generates less acknowledgement traffic g better throughput

Better reliability than ACK-based g Problem : NAK implosion

11

12 14 Sender Sender Receiver Receiver

NAK

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

11/19

[ [

g g Distributed loss Recovery

Distributed loss Recovery

g Uses multiple retransmitters to repair the packet loss of nearby

group member.

g Advantages :

g Speeds up loss recovery process g Prevents the source from overload g Supposed to solve scalability problems

g It must decides :

g Who should retransmit g Who is close to me and to whom I should send the NAK

Reliable Multicast - Techniques

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

12/19

[ [

g Two classes of distributed recovery techniques :

g Global recovery with duplicate avoidance (no explicit retransmitter)

g Any receiver may retransmit the packet g RTT (round trip time) to avoid duplicate retransmission and prevent feedback

implosion

g Reduce duplicate NAK but can’t avoid them completely

g Tree-based local recovery (explicit retransmitter designed) :

g Receivers close to each other form a subgroup and elect a DR g Only DR receive NAK and retransmit packets. g Fusion tree = hierarchy of subgroups

g When fusion tree is built, three ways to recover losses :

g Multicast with duplicate avoidance g Cascaded unicast g Hybrid

DR2 Source Router Receivers of subgroup 1 DR1

Reliable Multicast - Techniques

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

13/19

[ [

g g Router assisted recovery

Router assisted recovery

g Adds router support for fast and efficient recovery g Break with the established end-to-end principle :

g Intermediate router examine the header g It takes action : repair packet and forward it only to the host who

requested it

g reducing data redundancy

g Example of protocols :

g LMS (Light-weight Multicast) g PGM (Pragmatic General Multicast) g …

g New idea : require new hardware or at least new firmware

Reliable Multicast - Techniques

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

14/19

[ [

g g FEC

FEC-

• based recovery

based recovery

g Restores erased packets using

FEC packets.

g + : Utilization of NAKs avoided :

g Little or no feedback implosion g No crying baby problem

g - : Increase latency

1) We have k packets of data 2) Operation of FEC erasure correction adds n-k FEC

packets so the sender sends k+(n-k) = n packets

3) If loss occurs the receiver can reconstruct the original

message if the loss is < to n-k lost packets Sender Receiver

Reliable Multicast - Techniques

I) Introduction II)Reliable Multicast Definition Utilization Challenge Techniques III) Application Layer Multicast

9/11/2006

Reliable and Application layer Multicast

15/19

[ [

Application Layer Multicast – WHAT ?

g IP Multicast : data packets replicated at routers g Application-layer Multicast : data packets replicated at end

hosts.

g Overlay Network of intelligent node capable of receiving and

sending data to multiple downstream peers via TCP Unicast.

g Applications : Live Streaming, Internet TV / Radio…

I) Introduction II)Reliable Multicast III) Application Layer Multicast What is it ? Advantages Aspects

9/11/2006

Reliable and Application layer Multicast

16/19

[ [

g Effective transport

g Builds multicast transport on top of a the unicast transport g One to many reliability (IP Multicast) Multiple one to one

reliability solved by TCP (Application Layer Multicast)

g Easy deployment

g Not all networks support IP Multicast but all support IP Unicast. g Application layer multicast is bundled with the application g No additional routing state required in the routers

g Asynchronous Delivery

g Node can store the data and deliver it to the receiver when he’s ready

g Versatility

g Intelligent adaptation from sender to receiver

Application Layer Multicast – WHY ?

I) Introduction II)Reliable Multicast III) Application Layer Multicast What is it ? Advantages Aspects

9/11/2006

Reliable and Application layer Multicast

17/19

[ [

g Aspect of Application Layer Multicast

g Overlay setup : deployment of the physical overlay infrastructure

g Mesh Implementation : virtual links are specified g Non-Mesh Implementation : nodes talk to each other directly

g Tree organization

g Building a distribution tree for each multicast group g Receives maintenance to adapt to changes in the Network

g Content Distribution

g Real-time streaming : transmission relayed by all overlay nodes instantly g Latency-insensitive applications : time & speed controlled by session

management g End-User Subscription

Application Layer Multicast

I) Introduction II)Reliable Multicast III) Application Layer Multicast What is it ? Advantages Aspects

9/11/2006

Reliable and Application layer Multicast

18/19

[ [

Conclusion

g Reliable Multicast (IP Multicast) g +

g Less bandwidth consumption for a

large number of receivers

g More efficient

g -

g Need routers to maintain group state g Feedback Implosion / Crying baby g Based on multicast g

Utilization :

g Network games, data conference…

g Application Layer Multicast g +

g Based on Unicast g Easy deployment g No Implosion / Crying baby problem g Asynchronous Delivery g Versatility

g -

g High deploy cost for large scale

group

g Latency g

Utilization

g Live streaming…

I) Introduction II)Reliable Multicast III) Application Layer Multicast IV) Conclusion

9/11/2006

Reliable and Application layer Multicast

19/19

[ [

g Thank you for you attention

Any questions ?

Thank you for your attention Any question ?