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Chapter 2. Communication

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Layered Protocol

TCP/IP : de facto standard

Our Major Concern

Not always 7-layered Protocol

But some other protocols if necessary

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Client-Server Model

TCP/IP

Client-Server Model

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Basic Operations of Client-Server Model(Send and Receive Protocol)

Client … txmsg.opcode=opcode; txmsg.source=me; txmsg.param=param; sendMessage(serverID,&txmsg); waitMessage(serverID,&rxmsg); checkAndProcess(rxmsg);

Client

Client

Server

While(1) { waitMessage(FromAny,&rxmsg); source=rxmsg.source; opcode=rxmsg.opcode; param=rxmsg.params; result=functions[opcode](param); txmsg.result=result; txmsg.source=me; sendMessage(source,&txmsg);}

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Steps of RPC

1. Client procedure calls client stub in normal way

2. Client stub builds message, calls local OS3. Client's OS sends message to remote OS4. Remote OS gives message to server stub5. Server stub unpacks parameters, calls server6. Server does work, returns result to the stub7. Server stub packs it in message, calls local OS8. Server's OS sends message to client's OS9. Client's OS gives message to client stub10. Stub unpacks result, returns to client

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RPC : Remote Procedure Call

Client … txmsg.opcode=opcode; txmsg.source=me; txmsg.param=param; sendMessage(serverID,&txmsg); waitMessage(serverID,&rxmsg); checkAndProcess(rxmsg);

Server

result=funcA(param);

txmsg.opcode=opcode;txmsg.source=me;txmsg.param=param;sendMessage(serverID,&txmsg);

waitMessage(FromAny,&rxmsg);source=rxmsg.source;opcode=rxmsg.opcode;param=rxmsg.params;

result=functions[opcode](param);

funcA

STUB

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Binding a Client to a Server

Binding in DCE (Distributed Computing Environment)

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Middleware

Client

Stub for Client

Server

Stub for Server

Binding (Dynamic)

Middleware

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Middleware Protocol

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OS

Some Issues : Reliable vs. Non-Reliable

Client Server

Process Process

OS

1. Request

2. Reply

If reply message is lost?

1. Blocking2. Acknowledge3. Idempotent

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Some Issues : Blocking vs. Non-Blocking

Blocking (No Buffer, Synchronous)

Client

Server

Send

KernelTrap

ServerProcessing

Send

ReceiveBlocked

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Some Issues : Blocking vs. Non-Blocking

Non-Blocking (No Buffer, Asynchronous)

Client

Server

Send

Copy tosystembuffer

Processother task

ServerProcessing

Send

Receive

Receive

Blocked

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Asynchronous RPC

If client does not receive return value, No reason to be blocked. Asynchronous RPC

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Asynchronous RPC

Even though client does receive return value, Two asynchronous RPC

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Some Issues : Parameter Passing

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Parameter Passing for Objects

Two types of Object Parameter Passing Object Copy vs. Reference Copy Should it be transparent ?

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Remote Object Invocation

Generalization of RPC to Object Invocation

Implementation of object interface

like client stub in RPC

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Marshalling

System A

Object inMain Memory

System B

Object

Object inMain Memory

Marshalling Demarshalling

When objects in memory are to be passed across a network to another host or persisted to storage, their in-memory representation must be converted to a suitable out-of-memory format. This process is called marshalling, and converting back to an in memory representation is called demarshalling

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Extended RPC : Door for Single Machine

Client and Server are located in a single machine

RPC for processes on the same machine

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RMI : Object Binding

Implicit BindingDistr_object* obj_ref; //Declare a system-wide object referenceobj_ref = …; // Initialize the reference to a distributed objectobj_ref-> do_something(); // Implicitly bind and invoke a method

Explicit BindingDistr_object objPref; //Declare a system-wide object referenceLocal_object* obj_ptr; //Declare a pointer to local objectsobj_ref = …; //Initialize the reference to a distributed objectobj_ptr = bind(obj_ref); //Explicitly bind and obtain a pointer to the local proxyobj_ptr -> do_something(); //Invoke a method on the local proxy

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Stream-Oriented Communication

In distributed environment Continuous Media : e.g. video Discrete Media : e.g. Simple File Transfer

Temporal Aspects of Data Stream Asynchronous Transmission Synchronous Transmission Isochronous Transmission

Composition of Stream Simple Stream Complex Stream : composed of simple or complex

streams Synchronization is very important

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Data Streaming

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QoS : Quality of Service

Timeline, Volume and Reliability

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Group Communication

Send to a group

Send to a machine

Send to multiple receivers

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Group Communication : Design Issues

Closed or Open

Peer or Hierarchical Addressing

Multi-Casting or Broadcasting Uni-casting Predicate Addressing

Open to outsideClosed to outside

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Group Communication : Design Issues

Group Membership Send/Receive Protocol

How to distinguish it from single receive Atomicity Message ordering Scalability