© Chinese University, CSE Dept. Distributed Systems / 3 - 1 Distributed Systems Topic 3: Communication Dr. Michael R. Lyu Computer Science & Engineering.

© Chinese University, CSE Dept. Distributed Systems / 3 - 1

Distributed Systems

Topic 3: Communication

Dr. Michael R. LyuComputer Science & Engineering Department

The Chinese University of Hong Kong


Outline

1 Communication Primitives

2 Client/Server Communication

3 Group Communication

4 CORBA Event Service

5 Summary


1 Communication Primitives

Application

Presentation

Transport

Network

Data link

Physical

SessionThe ISO/OSI

Reference Model:

HTTP, FTP, Telnet

CORBA IIOP

XDR, CORBA Data

Secure Sockets (SSL)

for connection-

oriented comm.

message;

TCP, UDP

packet;IP;

ATM VC

error-free trans.

PPP, CSMA/CD

ISDN, baseband

signaling

host host

PSE PSE

PSE (packet switching exchange)


1.1 ISO/OSI Transport Layer

Level 4 of ISO/OSI reference model.

Concerned with the transport of information through a network.

Two facets in UNIX networks: – TCP– UDP

Application

Presentation

Transport

Network

Data link

Physical

Session

connection oriented virtual connection w sequencing &

acknowledgement - connectionless- up to 64k bytes datagram- no seqs and acks


1.1 ISO/OSI Transport Layer (TCP)

Transmission Control Protocol (TCP) provides bi-directional stream of bytes between two distributed components.

UNIX rsh, rcp and rlogin are based on TCP.

Reliable but slow protocol.

Buffering at both sides decouples computation speeds.


1.1 ISO/OSI Transport Layer (UDP)

User Datagram Protocol (UDP) enables a component to pass a message containing a sequence of bytes to another component.

Other component is identified within message.

Unreliable but very fast protocol.

Restricted message length.

Queuing at receiver. UNIX rwho command is UDP based.


1.2 ISO/OSI Presentation Layer

At application layer: complex data types

How to transmit complex values through transport layer?

Presentation layer issues:– Complex data structures and– Heterogeneity.

Application

Presentation

Transport

Network

Data link

Physical

Session


1.2 Complex Data Structures

Marshalling: Disassemble data structures into a transmittable form

Unmarshaling: Re-assemble the complex data structure.

class Person {

private:

int dob;

char * name;

public:

char * marshal() {

char * msg;

msg=new char[strlen(name)+10];

sprintf(msg,”%d,%d,%s”, dob,

strlen(name),name);

return(msg);

};

};


1.2 Heterogeneity

Heterogeneous data representation on different hardware platforms.

Approach 1 (Example XDR): – Define a shared representation,

– For each different platform, provide mapping between common and specific representation.

Approach 2 (Example ASN):


1.3 Communication Patterns

Basic operations: send and receive messages (as in UDP).

Message delivery:– Synchronous or

– Asynchronous

Messages are used to model:– Notification and

– Request.


1.3 Synchronous Communication

Time

sendersend

receiver

blocked

Transport Layer

receiveblocked

(1) (3) (4) (5)

ackn

(2)


1.3 Communication Deadlocks

P1:

send() to P2;

receive() from P2;

P2:

send() to P1;

receive() from P1;

P1

P2

Waits-for

Components are mutually waiting for each other.

To avoid deadlocks: Waits-for relation has to be acyclic!


1.3 Asynchronous Communication

Time

sendersend

receiver

Transport Layer

receiveblocked

(1) (3) (4)(2)


1.3 Notification

Uni-directional communication Message contains marshaled

notification parameters.

send(...)

Notifier Notified

receive(...)


1.3 Request

Bi-directional communication. Request message contains marshaled parameters. Requester receives reply message. Reply message contains marshaled results.

send(...)receive(...)

Requester Provider

receive(...)request

send(...)reply

...


1.3 Reliability Issues

Unreliable message refers to message transmission without acknowledgement or retries (e.g., UDP).

A reliable delivery service may be constructed from an unreliable one by the use of ack.

Positive ack. for client-server communication and negative ack. for group multicast.

Reliable communication involves overheads. Each message should have a unique identifier.


2 Client/Server Communication

Qualities of service.

Request protocol (R).

Request reply protocol (RR).

Request reply acknowledgement protocol (RRA).


2.1 Qualities of service

Exactly once,

At most once,

At least once and

Maybe?


2.2 Request Protocol

If service – does not have out or inout parameters and

– does not have a return type

client may not want to wait for server to finish.

execution requestsend(...)

Client Server

receive(...) exec op;


2.3 Request/Reply Protocol

To be applied if client expects result from server. Client requests service execution from server through

request message. Delivery of service result in reply message.

send(...)

receive(...)

Client Server

receive(...) exec op;send(...)

request

reply


2.4 RRA Protocol

In addition to RR protocol, client sends acknowledgement after it received reply.

Acknowledgement sent asynchronously.

send(...)

receive(...)send (...)

Client Server

receive(...) exec op;send(...)receive(...)

request

reply

ackn


3 Group Communication

Client/server requests:– There is no other party involved.

– Client has to identify server.

Sometimes other properties are required:– Communication between multiple components.

– Anonymous communication.


3.1 Concepts

Broadcast: Send msg to a group.

Multicast: Send msg to subgroup only.

MNN

NNNN

MM

N

NNNN

NN

NN

NN

NN

NN

N

Useful applications:Fault toleranceObject locationBetter performanceMultiple update


3.2 Qualities of Service

Ideal: Immediate and reliable.

S

R1

R2Time

Optimal: Simultaneous and reliable.

S

R1

TimeR2



In reality: not simultaneous ...

... and not reliable

TimeS

R1

R2

TimeS

R1

R2



Problem: To achieve reliable broadcast/multicast is very expensive.

Degrees of reliability:– Best effort,

– K-reliability,

– totally ordered,

– Atomicity.

Choose the degree of reliability needed and be prepared to pay the price.


3.3 CORBA Event Management

CORBA event management service defines interfaces for different group communication models.

Events are created by suppliers (producers) and communicated through an event channel to multiple consumers.

Service does not define a quality of service (left to implementers).


3.3.1 Push Model

Consumers register with those event channel through which events they are interested in are communicated.

Event producers create a new event by invoking a push operation from an event channel.

Event channel notifies all registered consumers by invoking their push operations.


3.3.1 Push Model (Example)

Share value updated

ProducerEvent

Channel

Redisplay chart

Redisplay table

Consumer Consumer

push(...) push(...) push(...)


3.3.2 The Pull Model

Event producer registers its capability of producing events with event channel.

Consumer obtains event by invoking pull operation from event channel.

Event channel asks producer to produce event and delivers it to the consumer.


3.3.2 Pull Model (Example)

Current

value: 76.10

ProducerEvent

Channel

Current share value?

Consumer Consumer

pull(...)pull(...)


3.3.2 Event Channel

Event Channel

Direction of event transfer

Push supplier

Pull supplier

Push consumer

Pull consumer

Supported combinations:push suppliers, push consumerspush suppliers, pull consumerspull suppliers, push consumerspull suppliers, pull consumers


3.3.2 Event Channel (with proxies)

Event Channel

Direction of event transfer

Push supplier

Pull supplier

Push consumer

Pull consumer

Proxy push consumer

Proxy pull consumer Proxy pull supplier

Proxy push supplier


4 Summary

What communication primitives do we use? How are differences between application and

communication layer resolved? What quality of service do the client/server

protocols achieve that we discussed? What quality of services are involved in group

communication? CORBA Event Service for group communication. Read Textbook Chapter 3 through Chapter 4.


Homework #1

1.7 Total 10 questions, 10 points each 1.11 1.13 Due: 7/10/2003 (Tuesday) in class 2.9 2.13 3.1 3.7 3.9 4.7 4.21

© Chinese University, CSE Dept. Distributed Systems / 3 - 1 Distributed Systems Topic 3: Communication Dr. Michael R. Lyu Computer Science & Engineering.

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