Communication Chapter 2
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)
Layered Protocols (1) • Layers, interfaces, and protocols in the OSI model. 2-1
Layered Protocols • Protocol – Connection Oriented – Connectionless • Protocol Stack • Description of the layers, Unit of exchange.
Layered Protocols (2) • A typical message as it appears on the network. 2-2
Data Link Layer 2-3 • Discussion between a receiver and a sender in the data link layer.
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.
Client-Server TCP a) Normal operation of TCP. 2-4 b) Transactional TCP.
Middleware Protocols • An adapted reference model for networked communication. 2-5
RPC • PC? • R…………….PC? • Simple idea • Complexity in provision
Conventional Procedure Call a) 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
Issues • Calling Method? – Call by value – Call by reference – Call by Copy/Restore – Call by name
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!
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
Passing Value Parameters (1) • Steps involved in doing remote computation through RPC 2-8 • It works fine, while the scenario is simple and straightforward; but ….
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
Call by Reference Parameter Passing ???
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)
Parameter Specification and Stub Generation a) A procedure b) The corresponding message. c) Interface Definition Language compiling into client stub and server stub
Extended RPC Models • RPC becoming as de facto standard for comm. in DSs. • Popularity due to simplicity. • Two extensions – Doors – Async RPC.
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 collocated 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)
Doors • The principle of using doors as IPC mechanism.
Asynchronous RPC (1) 2-12 a) The interconnection between client and server in a traditional RPC b) The interaction using asynchronous RPC
Asynchronous RPC (2) • A client and server interacting through two asynchronous RPCs 2-13
Writing a Client and a Server • The steps in writing a client and a server in DCE RPC. 2-14
Binding a Client to a Server • Client-to-server binding in DCE. 2-15
Performing an RPC • The whole scenario! • Semantics – At-most-once operation • Idempotency
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
Distributed Objects • Common organization of a remote object with client-side proxy. 2-16
Message-Oriented Communication • Sometimes both RPC and RMI is not appropriate • Synchronous nature of RPC and RMI! Messaging.
Berkeley Sockets (2) • Connection-oriented communication pattern using sockets.
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 bufger Send a message and wait until copied to local or remote MPI_send bufger 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 Pass reference to outgoing message, and wait until receipt MPI_issend starts MPI_recv Receive a message; block if there are none MPI_irecv Check if there is an incoming message, but do not block
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.
Data Stream (1) • Setting up a stream between two processes across a network. • Data stream is a sequence of data units.
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).
Data Stream (2) • Setting up a stream directly between two devices. 2-35.2
Data Stream (3) • An example of multicasting a stream to several receivers.
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.
Specifying QoS (2) • The principle of a token bucket algorithm.
Setting Up a Stream • The basic organization of RSVP (Resource reSerVation Protocol) for resource reservation in a distributed system.
End of Chapter 2
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