OS2-SUT– Sem ; R. Jalili Communication Chapter 2

OS2-SUT– Sem ; 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-SUT– Sem ; R. Jalili Layered Protocols (1) Layers, interfaces, and protocols in the OSI model. 2-1

OS2-SUT– Sem ; R. Jalili Layered Protocols Protocol –Connection Oriented –Connectionless Protocol Stack Description of the layers, Unit of exchange.

OS2-SUT– Sem ; R. Jalili Layered Protocols (2) A typical message as it appears on the network. 2-2

OS2-SUT– Sem ; R. Jalili Data Link Layer Discussion between a receiver and a sender in the data link layer. 2-3

OS2-SUT– Sem ; 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-SUT– Sem ; R. Jalili Client-Server TCP a)Normal operation of TCP. b)Transactional TCP. 2-4

OS2-SUT– Sem ; R. Jalili Middleware Protocols An adapted reference model for networked communication. 2-5

OS2-SUT– Sem ; R. Jalili RPC PC? R…………….PC? Simple idea Complexity in provision

OS2-SUT– Sem ; R. Jalili 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

OS2-SUT– Sem ; R. Jalili Issues Calling Method? –Call by value –Call by reference –Call by Copy/Restore –Call by name

OS2-SUT– Sem ; 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-SUT– Sem ; 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-SUT– Sem ; 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-SUT– Sem ; 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-SUT– Sem ; R. Jalili Call by Reference Parameter Passing ???

OS2-SUT– Sem ; 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)

OS2-SUT– Sem ; 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-SUT– Sem ; 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-SUT– Sem ; 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-SUT– Sem ; R. Jalili Doors The principle of using doors as IPC mechanism.

OS2-SUT– Sem ; 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-SUT– Sem ; R. Jalili Asynchronous RPC (2) A client and server interacting through two asynchronous RPCs 2-13

OS2-SUT– Sem ; R. Jalili Writing a Client and a Server The steps in writing a client and a server in DCE RPC. 2-14

OS2-SUT– Sem ; R. Jalili Binding a Client to a Server Client-to-server binding in DCE. 2-15

OS2-SUT– Sem ; R. Jalili Performing an RPC The whole scenario! Semantics –At-most-once operation Idempotency

OS2-SUT– Sem ; 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-SUT– Sem ; R. Jalili Distributed Objects Common organization of a remote object with client-side proxy. 2-16

OS2-SUT– Sem ; R. Jalili Message-Oriented Communication Sometimes both RPC and RMI is not appropriate Synchronous nature of RPC and RMI!  Messaging.

OS2-SUT– Sem ; R. Jalili Berkeley Sockets (2) Connection-oriented communication pattern using sockets.

OS2-SUT– Sem ; R. Jalili The Message-Passing Interface (MPI) Some of the most intuitive message-passing primitives of MPI. PrimitiveMeaning MPI_bsendAppend outgoing message to a local send buffer MPI_sendSend a message and wait until copied to local or remote buffer MPI_ssendSend a message and wait until receipt starts MPI_sendrecvSend a message and wait for reply MPI_isendPass reference to outgoing message, and continue MPI_issendPass reference to outgoing message, and wait until receipt starts MPI_recvReceive a message; block if there are none MPI_irecvCheck if there is an incoming message, but do not block

OS2-SUT– Sem ; 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 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-SUT– Sem ; R. Jalili Data Stream (1) Setting up a stream between two processes across a network. Data stream is a sequence of data units.

OS2-SUT– Sem ; 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-SUT– Sem ; R. Jalili Data Stream (2) Setting up a stream directly between two devices

OS2-SUT– Sem ; R. Jalili Data Stream (3) An example of multicasting a stream to several receivers.

OS2-SUT– Sem ; 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-SUT– Sem ; R. Jalili Specifying QoS (2) The principle of a token bucket algorithm.

OS2-SUT– Sem ; R. Jalili Setting Up a Stream The basic organization of RSVP (Resource reSerVation Protocol) for resource reservation in a distributed system.

OS2-SUT– Sem ; R. Jalili Synchronization Mechanisms (1) The principle of explicit synchronization on the level data units.

OS2-SUT– Sem ; R. Jalili Synchronization Mechanisms (2) The principle of synchronization as supported by high-level interfaces. 2-41

End of Chapter 2 OS2-SUT– Sem ; R. Jalili