IPC. Communication. Layered Protocols. Layered Protocols (1) Data Link Layer. Layered Protocols (2)

Transcription

1 IPC Communication Chapter 2 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. Protocol Layered Protocols 2-1 Connection Oriented Connectionless Protocol Stack Description of the layers, Unit of exchange. Layered Protocols (2) Data Link Layer A typical message as it appears on the network Discussion between a receiver and a sender in the data link layer. 1

2 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. b) Transactional 2-4 TCP. Middleware Protocols An adapted reference model for networked communication. PC? RPC 2-5 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 Calling Method? Call by value Call by reference Call by Copy/Restore Call by name Issues 2

3 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 Passing Value Parameters (2) Different character set standards (ASCII vs EBCDIC) Little-Endian vs Big-Endian Architecture. 2-8 It works fine, while the scenario is simple and straightforward; but. 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) 3

4 Parameter Specification and Stub Generation a) A procedure b) The corresponding message. Extended RPC Models RPC becoming as de facto standard for comm. in DSs. Popularity due to simplicity. Two extensions Doors Async RPC. c) Interface Definition Language compiling into client stub and server stub 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) Doors The principle of using doors as IPC mechanism. Asynchronous RPC (1) 2-12 Asynchronous RPC (2) A client and server interacting through two asynchronous RPCs 2-13 a) The interconnection between client and server in a traditional RPC b) The interaction using asynchronous RPC 4

5 Writing a Client and a Server The steps in writing a client and a server in DCE RPC. Binding a Client to a Server Client-to-server binding in DCE 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. Message-Oriented Communication Sometimes both RPC and RMI is not appropriate Synchronous nature of RPC and RMI! 2-16 Messaging. 5

6 Berkeley Sockets (2) The Message-Passing Interface (MPI) Some of the most intuitive message-passing primitives of MPI. Connection-oriented communication pattern using sockets. Primitive MPI_bsend MPI_send MPI_ssend MPI_sendrecv MPI_isend MPI_issend MPI_recv MPI_irecv Meaning Append outgoing message to a local send buffer Send a message and wait until copied to local or remote buffer Send a message and wait until receipt starts Send a message and wait for reply Pass reference to outgoing message, and continue Pass reference to outgoing message, and wait until receipt starts Receive a message; block if there are none 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 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 Data Stream (2) Setting up a stream directly between two devices. 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). 6

7 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) Setting Up a Stream The basic organization of RSVP (Resource reservation Protocol) for resource reservation in a distributed system. The principle of a token bucket algorithm. Synchronization Mechanisms (1) The principle of explicit synchronization on the level data units. Synchronization Mechanisms (2) The principle of synchronization as supported by high-level interfaces

8 End of Chapter 2 8