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OS2-Sharif University of Technology - Sem1-90-91; R. Jalili
Communication
Chapter 2
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
IPC
• Inter-Process Communication is the heart of all
DSs.
• Processes on different machines.
• Always based on low-level message passing.
• In this chapter:
– RPC
– RMI
– MOM (Message Oriented MiddleWare)
– Streams (due to the advent of Multimedia DSs)
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Layered Protocols (1)• Layers, interfaces, and protocols in the OSI model.
2-1
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Layered Protocols
• Protocol
– Connection Oriented
– Connectionless
• Protocol Stack
• Description of the layers, Unit of exchange.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Layered Protocols (2)
• A typical message as it appears on the network.
2-2
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Data Link Layer
• Discussion between a receiver and a sender in the data link layer.
2-3
2
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Transport Protocols
• Makes the underlying layers usable by the
application layer.
• Provide a reliable or unreliable connection for
the upper layer.
• UDP :: TCP
• RTP for real-time systems.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Client-Server TCP
a) Normal operation of TCP.
b) Transactional TCP.2-4
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Middleware Protocols
• An adapted reference model for networked communication.
2-5
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
RPC
• PC?
• R…………….PC?
• Simple idea
• Complexity in provision
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Conventional Procedure Calla) Parameter passing in a local procedure call: the
stack before the call to read Count = read (fd, buf, nbytes);
b) The stack while the called procedure is active
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Issues
• Calling Method?
– Call by value
– Call by reference
– Call by Copy/Restore
– Call by name
3
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Client and Server Stubs
• Principle of RPC between a client and server program.
• The read stub is called on behalf of the real read procedure!
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Steps of a Remote Procedure Call
1. Client procedure calls client stub in normal way
2. Client stub builds message, calls local OS
3. Client's OS sends message to remote OS
4. Remote OS gives message to server stub
5. Server stub unpacks parameters, calls server
6. Server does work, returns result to the stub
7. Server stub packs it in message, calls local OS
8. Server's OS sends message to client's OS
9. Client's OS gives message to client stub
10. Stub unpacks result, returns to client
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Passing Value Parameters (1)
• Steps involved in doing remote computation through RPC
• It works fine, while the scenario is simple and straightforward;
but ….
2-8
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Passing Value Parameters (2)• Different character set standards (ASCII vs EBCDIC)
• Little-Endian vs Big-Endian Architecture.
a) Original message on the Pentium (L. E.)
b) The message after receipt on the SPARC (B. E.)
c) The message after being inverted. The little numbers in boxes indicate the address of each byte
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Call by Reference Parameter Passing
???
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Parameter Specification and Stub Generation
• Both sides should agree on the content of passing
data structures.
• Example in the next slide.
• The way a message including the parameters is
interpreted is the main issue!!
• Client and server should agree on the
representation of simple data structures.
• Agreement on the actual exchange of the messages
(connection-oriented or connection-less)
4
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Parameter Specification and Stub Generation
a) A procedure
b) The corresponding message.
c) Interface Definition Language � compiling into client stub and server stub
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Extended RPC Models
• RPC becoming as de facto standard for comm. in
DSs.
• Popularity due to simplicity.
• Two extensions
– Doors
– Async RPC.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Doors
• Equivalent to RPC for processes located on the
same machine.
• A door is a name for a procedure in the address
space of a server process, called by colocated
processes within the server.
• Idea was originally from the Spirit OS (1994)
• Same as LightWeight RPC.
• The server process must register a door before
use (calling door-create)
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Doors• The principle of using doors as IPC mechanism.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Asynchronous RPC (1)
a) The interconnection between client and server in a traditional RPC
b) The interaction using asynchronous RPC
2-12
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Asynchronous RPC (2)
• A client and server interacting through two
asynchronous RPCs
2-13
5
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Writing a Client and a Server• The steps in writing a client and a server in DCE RPC.
2-14
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Binding a Client to a Server
• Client-to-server binding in DCE.
2-15
OS2-Sharif University of Technology - Sem1-90-91; R. Jalili
Performing an RPC
• The whole scenario!
• Semantics
– At-most-once operation
• Idempotency
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Remote Object Invocation
• OO technology in centralized systems.
• Promoting the idea of RPC to the OO technology.
• Proxy as the client delegate == Client stub.
• Skeleton == server stub
• The object state is normally not distributed � remote object instead of distributed object
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Distributed Objects• Common organization of a remote object with
client-side proxy.
2-16
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Message-Oriented Communication
• Sometimes both RPC and RMI is not appropriate
• Synchronous nature of RPC and RMI!
• � Messaging.
6
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Berkeley Sockets (2)
• Connection-oriented communication pattern using sockets.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
The Message-Passing Interface (MPI)
• Some of the most intuitive message-passing
primitives of MPI.
Primitive Meaning
MPI_bsend Append outgoing message to a local send buffer
MPI_send Send a message and wait until copied to local or remote buffer
MPI_ssend Send a message and wait until receipt starts
MPI_sendrecv Send a message and wait for reply
MPI_isend Pass reference to outgoing message, and continue
MPI_issend Pass reference to outgoing message, and wait until receipt starts
MPI_recv Receive a message; block if there are none
MPI_irecv Check if there is an incoming message, but do not block
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Stream-Oriented Communication
• Till now, focus was on exchanging one or more independent and complete units of info.
• However, consider an audio stream, CD quality is also required � the original sound has been sampled at 44100 Hz �a sample in each 1/44100 Sec is required to re-produce the original sound.
• Time-dependent and continuous media is required :: Temporal relationship between data items are crucial.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Data Stream (1)
• Setting up a stream between two processes across a network.
• Data stream is a sequence of data units.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Transmission Modes
• Async Trans Mode: Sending regardless of time
• Synch Trans Mode: There is a max end-to-end
delay for each unit: Sensor info!
• Isochronous Trans Mode: Data units should be
transferred on time:: A max and min end-to-end
delay (bounded jitter).
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Data Stream (2)
• Setting up a stream directly between two devices.
2-35.2
7
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Data Stream (3)
• An example of multicasting a stream to several receivers.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
QoS
• Time-Dependent requirement:: QoS
• Next slide as a sample QoS specification
• Formulation based on the token bucket algorithm
• Basic idea is that tokens are generated at a
constant rate.
• Token is a fixed # of bytes, an application is
allowed to pass to the network.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Specifying QoS (2)
• The principle of a token bucket algorithm.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Setting Up a Stream
• The basic organization of RSVP (Resource reSerVation Protocol)
for resource reservation in a distributed system.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Synchronization Mechanisms (1)
• The principle of explicit synchronization on the
level data units.
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili
Synchronization Mechanisms (2)
• The principle of synchronization as supported by
high-level interfaces.
2-41
8
End of Chapter 2
OS2-Sharif University of Technology
- Sem1-90-91; R. Jalili