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7/26/2001 1
Design and Implementation of a Simple Totally-Ordered Reliable
Multicast Protocol in Java
7/26/2001 3
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.
7/26/2001 4
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.
7/26/2001 5
• 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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• (hybrid) sender-initiated approach.
• sender maintains status of all receivers.
• timers used for detecting ACK losses.
Reliability (sender)
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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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• 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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• sliding window mechanism.
• fixed window size for simplicity.
• bounds buffers.
Flow Control
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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.
7/26/2001 26
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.
7/26/2001 28
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
7/26/2001 32
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.
7/26/2001 33
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();
}
7/26/2001 34
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
7/26/2001 36
Functions Tested
– session name server support.– join and leave session, channels.– ordering.– reliability.– failure handling (message loss, host crash). – performance.
7/26/2001 37
Testing Environment
labss3a labss4alabss3d labss4d compute
paladin jet
muck
LAN1
LAN2
LAN3
session s1
channel c1 Channel c2
SNS
7/26/2001 38
Performance Test
Purpose:To compare TRMP latency with TCP (base line).
Result:
TRMP has lower latency for more than 12 receivers.
7/26/2001 39
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
7/26/2001 40
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.
7/26/2001 41
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).
7/26/2001 42
Future Work
• flexible window size
• different types of quality of service
• NAK and re-transmission suppression
• performance