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Design and Implementation of a Simple Totally-Ordered Reliable

Multicast Protocol in Java

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Introduction

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Background

– Computer networks allow geographically distributed processes to collaborate or share a common stream of information.

– Major characteristic of group communication applications is one source sends data to multiple receivers.

– Multicast is well-suited for the distributed applications that focus on group activities, compared with unicast and broadcast.

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Multicast

• Multicast aims to deliver data to a select group of hosts. A single packet is addressed to all intended hosts and the network replicates packets only as needed.

• IP Multicast is an internet protocol that enables transmission of data to a group of receivers.

• IP Multicast is unreliable.

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• Reliability– guarantees data eventually delivered to non-

faulty receivers.

Totally-ordered Reliable Multicast

• Total Ordering – all receivers observe the same order of reception

of messages.

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• Easy to build networking applications.• Portable to any systems. • Object oriented - clearer structure, supports reuse.

Java

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• session – a collaborative group, includes a collection of

processes.– members cooperate to fulfill a common goal or

share a common stream of information.

• channel– the communication pipe for session members.– encapsulates the transmission protocol to

session members.

Basic Concepts(1)

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• session name server

– a program running at a well-known address.– maintains a list of the current sessions along

with session member information.

• default channel– a special channel, created for each session when

the session is created. – the services of a session are implemented

through the default channel.

Basic Concepts(2)

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• Session()

int join(String name, int netmask)

int leave()

int query(Session_Info si)• Channel()

int join(Session s, String cname)

int send(int sid, byte[] buffer)

int receive(int sid, byte[] buffer)

int leave()

User-level API

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import TRMP.*;import java.lang.*;public class Test implements Constants{ public static void main(String[] args){ Session s = new Session(); long netmask = Long.parseLong(args[0], 16); s.join(“s1”, netmask); Channel ch = new Channel(); ch.join(s, “c1”); String str = “Hello."; byte[] bytes = str.getBytes(); ch.send(ALL,bytes); ch.leave(); s.leave(); }} * Appendix

A

Sample Testing Program

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Design

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• Reliability

• Ordering

• Flow Control

Design Considerations

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• (hybrid) sender-initiated approach.

• sender maintains status of all receivers.

• timers used for detecting ACK losses.

Reliability (sender)

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sender receiver

timeout

Reliability (sender)

ACK ACK

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• sends ACKs.

• detects out of ordered packets, and sends NAK when discontinued packet sequence number is detected.

• timers applied for the loss of NAKs.

• suppress ACKs for fragmented packets.

Reliability (receiver)

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Reliability (receiver)

sender receiver

NAK

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• a centralized scheme, one process sequences all the messages in a group.

• TCP tunneling and multicasting locally.

• large messages fragmented before multicasting and reassembled by receivers.

Ordering

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LAN 2

LAN 1LAN 3

TCP

TCP

multicast

multicast

multicast

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• sliding window mechanism.

• fixed window size for simplicity.

• bounds buffers.

Flow Control

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Implementation

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• Session owner– the oldest member in a session

– grants session membership and sequences the membership messages sent in the default channel.

• Membership database– kept on each session member.

– contains current session and channel information.

• Membership messages– contain session and channel membership updates.

Additional Concepts(1)

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• Channel leader – the oldest channel member in a channel.

– sequence the messages transmitted in the channel.

• Multicaster– the oldest channel member in one subnet of the

channel.

– multicast the messages within its subnet.

Additional Concepts(2)

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Network Layer

user application

session channelssessionnameserver

the default channel

UDP

TRMP API

per session

Architecture

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Session Name Server

• Session information is provided to processes when they join a session.

• SNS accepts four types of messages.

– joining session request.

– leaving session request

– terminating session request.

– querying sessions request.

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The Default Channel

– automatically joins when a process joins a session.

– contains all the session members. – transmits the membership messages. – session owner sequences the membership

messages. – supports reliable point to point message (TCP)

as well as multicasting.

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Example Configuration of a Session with Two Channels

SNS

channel 1

channel 2

default channel

LAN 4

LAN 3

LAN 2

LAN 1

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Session

– session and channel membership database created locally when a process joins a session.

– members update the membership database whenever they receive join and leave session, join and leave channel, and crash (leave) messages.

– provides the same views of the session and channels states to all the session members.

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Channel

– provide totally-ordered reliable multicast message delivery to channel members by TCP tunneling and local multicasting.

– multiple channels may exist simultaneously in one session.

– one session member may join more than one channel at a time.

– members of a channel divided into subnet groups according to their subnet address.

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LAN 2

LAN 1LAN 3

TCP

TCP

multicast

multicast

multicast

channel leader

multicaster

multicaster

TCP

data source

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Session and Related Classes (UML)

SessionDefaultChannel

Member ChannelInfo

InSessionThread

SessionNameSRef

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Channel and Related Classes (UML)

Channel

MCastMgr

Session ChReceiveThread

DefaultChannel

MCReceiveThread

MCSendThread MCAckReceiveThread NackThread

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Threads Synchronization

• In Java, each object has a monitor associated with it. No two threads can acquire an object's monitor at the same time. Multiple threads can synchronize access to an object with each other through the use of wait()/notify() calls.

• Multithread synchronization is heavily used in the implementation.

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Multithread Synchronizationpublic synchronized void handleNackEvent() {

while (true)

{

if ( mcrt.lostMsgs.size() == 0) wait();

//mcrt.lostMsgs is the lost message queue

...

}

public synchronized void requestResend(int seq) {

// add new element to the list with associated information

...

if ( mcrt.lostMsgs.size() == 1 ) notify();

}

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Multithread SynchronizationMCReceiveThread NackThread

NackThread()

initialization

requestResend()

update lost queue

handleNackEvent()

notify()

detect message lost

acquire lock

ask for resend to the multicaster

lost message queue is empty

wait()

receive resent message

cancelResend()

update lost queue

release lock

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Testing

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Functions Tested

– session name server support.– join and leave session, channels.– ordering.– reliability.– failure handling (message loss, host crash). – performance.

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Testing Environment

labss3a labss4alabss3d labss4d compute

paladin jet

muck

LAN1

LAN2

LAN3

session s1

channel c1 Channel c2

SNS

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Performance Test

Purpose:To compare TRMP latency with TCP (base line).

Result:

TRMP has lower latency for more than 12 receivers.

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TRMP Vs. TCP

0

24

68

10

1214

16

1 2 4 6 8 10 12 14 16 18 20

Number of receivers

Lat

ency

(ms)

TRMP

TCP

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Summary

– We have designed and implemented a totally-ordered reliable multicast protocol (TRMP) in Java.

– Simple test applications are implemented to test the functions and the performance of the protocol. The test results show that the protocol can provide totally-ordered reliable multicast with reasonable speed.

– The TRMP package contains 38 classes and 4,661 lines of Java code.

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Related Work

– Group Communication Support for Distributed Collaboration System (CCTL).

– A Reliable Multicast Framework for Light-Weight Sessions and Application Level Framing (SRM).

– Reliable Multicasting of Continuous Data Streams (RMTP).

– The Java Reliable Multicast Service: A Reliable Multicast Library(JRMS).

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Future Work

• flexible window size

• different types of quality of service

• NAK and re-transmission suppression

• performance

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Thank you !