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 Protocol Stack 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 Normal operation of TCP. Transactional TCP. 2-4

Middleware Protocols An adapted reference model for networked communication. 2-5

RPC PC? R…………….PC? Simple idea Complexity in provision

Conventional Procedure Call Parameter passing in a local procedure call: the stack before the call to read Count = read (fd, buf, nbytes); 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 Client procedure calls client stub in normal way Client stub builds message, calls local OS Client's OS sends message to remote OS Remote OS gives message to server stub Server stub unpacks parameters, calls server Server does work, returns result to the stub Server stub packs it in message, calls local OS Server's OS sends message to client's OS Client's OS gives message to client stub Stub unpacks result, returns to client

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

Passing Value Parameters (2) Different character set standards (ASCII vs EBCDIC) Little-Endian vs Big-Endian Architecture. Original message on the Pentium (L. E.) The message after receipt on the SPARC (B. E.) 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 procedure The corresponding message. 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 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 The interconnection between client and server in a traditional RPC 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 Idempotency 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 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

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.

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. 2-41

End of Chapter 2