OS2-Sem 831, Rasool Jalili Communication Chapter 2

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Layered Protocols (1) Layers, interfaces, and protocols in the OSI model. 2-1

OS2-Sem 831, Rasool Jalili Layered Protocols Protocol –Connection Oriented –Connectionless Protocol Stack Description of the layers, Unit of exchange.

OS2-Sem 831, Rasool Jalili Layered Protocols (2) A typical message as it appears on the network. 2-2

OS2-Sem 831, Rasool Jalili Data Link Layer Discussion between a receiver and a sender in the data link layer. 2-3

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Client-Server TCP a)Normal operation of TCP. b)Transactional TCP.

OS2-Sem 831, Rasool Jalili Middleware Protocols An adapted reference model for networked communication. 2-5

OS2-Sem 831, Rasool Jalili RPC PC? R…………….PC? Simple idea Complexity in provision

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Issues Calling Method? –Call by value –Call by reference –Call by Copy/Restore –Call by name

OS2-Sem 831, Rasool 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-Sem 831, Rasool 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-Sem 831, Rasool 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-Sem 831, Rasool 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-Sem 831, Rasool 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-Sem 831, Rasool 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-Sem 831, Rasool Jalili Extended RPC Models RPC becoming as de facto standard for comm. In DSs. Popularity due to simplicity. Two extensions –Doors –Async RPC.

OS2-Sem 831, Rasool 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 with 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-Sem 831, Rasool Jalili Doors The principle of using doors as IPC mechanism.

OS2-Sem 831, Rasool Jalili Asynchronous RPC (1) a)The interconnection between client and server in a traditional RPC b)The interaction using asynchronous RPC 2-12

OS2-Sem 831, Rasool Jalili Asynchronous RPC (2) A client and server interacting through two asynchronous RPCs 2-13

OS2-Sem 831, Rasool Jalili Writing a Client and a Server The steps in writing a client and a server in DCE RPC. 2-14

OS2-Sem 831, Rasool Jalili Binding a Client to a Server Client-to-server binding in DCE. 2-15

OS2-Sem 831, Rasool Jalili Performing an RPC The whole scenario! Semantics –At-most-once operation Idempotency

OS2-Sem 831, Rasool Jalili Remote Object Invocation OO technology in centralized systems. Promoting the idea if RPC to the OO technology. Proxy as the server delegate == Client stub. Skeleton == server stub The object state is normally not distributed  remote object instead of distributed object

OS2-Sem 831, Rasool Jalili Distributed Objects Common organization of a remote object with client-side proxy. 2-16

OS2-Sem 831, Rasool Jalili Binding a Client to an Object a)(a) Example with implicit binding using only global references b)(b) Example with explicit binding using global and local references Distr_object* obj_ref;//Declare a systemwide object reference obj_ref = …;// Initialize the reference to a distributed object obj_ref-> do_something();// Implicitly bind and invoke a method (a) Distr_object objPref;//Declare a systemwide object reference Local_object* obj_ptr;//Declare a pointer to local objects obj_ref = …;//Initialize the reference to a distributed object obj_ptr = bind(obj_ref);//Explicitly bind and obtain a pointer to the local proxy obj_ptr -> do_something();//Invoke a method on the local proxy (b)

OS2-Sem 831, Rasool Jalili Object References Endpoint and registration Failure and object migration  need to invalidate all bindings by clients. Location Server Static RMI and invocation :: Interfaces of an object is known to the client before compilation. Dynamic invocation.

OS2-Sem 831, Rasool Jalili Parameter Passing The situation when passing an object by reference or by value. 2-18

OS2-Sem 831, Rasool Jalili The DCE Distributed-Object Model a)Distributed dynamic objects in DCE. b)Distributed named objects

OS2-Sem 831, Rasool Jalili Message-Oriented Communication Sometimes both RPC and RMI is not appropriate Synchronous nature of RPC and RMI!  Messaging.

OS2-Sem 831, Rasool Jalili Persistence and Synchronicity in Communication (1) General organization of a communication system in which hosts are connected through a network 2-20

OS2-Sem 831, Rasool Jalili Persistence and Synchronicity in Communication (2) Persistent communication of letters back in the days of the Pony Express.

OS2-Sem 831, Rasool Jalili Persistence and Synchronicity in Communication (3) a)Persistent asynchronous communication b)Persistent synchronous communication

OS2-Sem 831, Rasool Jalili Persistence and Synchronicity in Communication (4) c)Transient asynchronous communication d)Receipt-based transient synchronous communication

OS2-Sem 831, Rasool Jalili Persistence and Synchronicity in Communication (5) e)Delivery-based transient synchronous communication at message delivery f)Response-based transient synchronous communication

OS2-Sem 831, Rasool Jalili Berkeley Sockets (1) Socket primitives for TCP/IP. PrimitiveMeaning SocketCreate a new communication endpoint BindAttach a local address to a socket ListenAnnounce willingness to accept connections AcceptBlock caller until a connection request arrives ConnectActively attempt to establish a connection SendSend some data over the connection ReceiveReceive some data over the connection CloseRelease the connection

OS2-Sem 831, Rasool Jalili Berkeley Sockets (2) Connection-oriented communication pattern using sockets.

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Message-Queuing Model (1) Four combinations for loosely-coupled communications using queues. 2-26

OS2-Sem 831, Rasool Jalili Message-Queuing Model (2) Basic interface to a queue in a message-queuing system. PrimitiveMeaning PutAppend a message to a specified queue GetBlock until the specified queue is nonempty, and remove the first message PollCheck a specified queue for messages, and remove the first. Never block. Notify Install a handler to be called when a message is put into the specified queue.

OS2-Sem 831, Rasool Jalili General Architecture of a Message-Queuing System (1) Messages are put in local queues and got from local queues. The relationship between queue-level addressing and network- level addressing.

OS2-Sem 831, Rasool Jalili General Architecture of a Message-Queuing System (2) The general organization of a message-queuing system with routers. 2-29

OS2-Sem 831, Rasool Jalili Message Brokers The general organization of a message broker in a message-queuing system. The broker acts as an application level gateway to convert the format of incoming msgs to be understood by the destination applications, eg. records as a known messaging system

OS2-Sem 831, Rasool Jalili Example General organization of IBM's MQSeries message-queuing system. Used to access and manipulate large scale databases: finance Queues are managed by queue managers; Each queue manager is responsible for removing msgs from its send queue and forward to other queue managers. A queue manager (QM) is responsible to pick up incoming msgs from the underlying net and store in the appropriate input Q. QMs are connected (pairwise) through msg channels. A msg channel is a reliable uni-directional transport-level connection between a sending and a receiving QM. Each end os a msg channel is managed by a MCA (msg chnl Agnt)

OS2-Sem 831, Rasool Jalili Example: IBM MQSeries

OS2-Sem 831, Rasool Jalili Channels Some attributes associated with message channel agents. AttributeDescription Transport typeDetermines the transport protocol to be used FIFO deliveryIndicates that messages are to be delivered in the order they are sent Message lengthMaximum length of a single message Setup retry count Specifies maximum number of retries to start up the remote MCA Delivery retriesMaximum times MCA will try to put received message into queue

OS2-Sem 831, Rasool Jalili Message Transfer (1) The general organization of an MQSeries queuing network using routing tables and aliases.

OS2-Sem 831, Rasool Jalili Message Transfer (2) Primitives available in an IBM MQSeries MQI (MQ Interface) PrimitiveDescription MQopenOpen a (possibly remote) queue MQcloseClose a queue MQputPut a message into an opened queue MQgetGet a message from a (local) queue

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Data Stream (1) Setting up a stream between two processes across a network. Data stream is a sequence of data units.

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Data Stream (2) Setting up a stream directly between two devices

OS2-Sem 831, Rasool Jalili Data Stream (3) An example of multicasting a stream to several receivers.

OS2-Sem 831, Rasool 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-Sem 831, Rasool Jalili Specifying QoS (1) A flow specification. Characteristics of the InputService Required maximum data unit size (bytes) Token bucket rate (bytes/sec) Token bucket size (bytes) Maximum transmission rate (bytes/sec) Loss sensitivity (bytes) Loss interval (  sec) Burst loss sensitivity (data units) Minimum delay noticed (  sec) Maximum delay variation (  sec) Quality of guarantee

OS2-Sem 831, Rasool Jalili QoS Loss sensitivity (bytes) Loss interval (  sec) Burst loss sensitivity (data units): how many consecutive data items may be lost Minimum delay noticed (  sec): how long the network can delay the delivery of a data item before the receiver notices. Maximum delay variation (  sec): Maximum tolerated jitter Quality of guarantee

OS2-Sem 831, Rasool Jalili Specifying QoS (2) The principle of a token bucket algorithm.

OS2-Sem 831, Rasool Jalili Setting Up a Stream The basic organization of RSVP (Resource reSerVation Protocol) for resource reservation in a distributed system.

OS2-Sem 831, Rasool Jalili Synchronization Mechanisms (1) The principle of explicit synchronization on the level data units.

OS2-Sem 831, Rasool Jalili Synchronization Mechanisms (2) The principle of synchronization as supported by high-level interfaces. 2-41