1. Communication
Chapter 2

2. Layered Protocols (1)
Layers, interfaces, and protocols in the OSI model.

3. Layered Protocols (2)
A typical message as it appears on the network.

4. Data Link Layer
Discussion between a receiver and a sender in the data link layer.

5. Client-Server TCP

6. Middleware Protocols
An adapted reference model for networked communication.

7. Remote Procedure Call Remote Procedure Call (RPC) Problems
Definition: Information can be transported from the caller to the callee in the parameters and can come back in the procedure result.
Problems
Calling and called procedures run on different machine
Executed in different address spaces (more complications)
Passing parameters and results (especially in not identical or typos)
Machine crash or failure when execution

8. Conventional Procedure Call
fd: an integer indicating a file
buf: an array  using call-by-copy/restore
nbytes: how many bytes to read
Parameter passing in a local procedure call: the stack before the call to read
The stack while the called procedure is active

9. Client and Server Stubs
Put a different version of read in the client’s library
Client stub
Server-side equivalent of a client stub
Server stub
Principle of RPC between a client and server program.

10. 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

11. Passing Value Parameters (1/2)
Parameter Marshaling
Packing parameters into a message
Steps involved in doing remote computation through RPC

12. Passing Value Parameters (2/2)
Problems
Different char. representations
IBM mainframe: EBCDIC character code
IBM PC: ASCII character code
Different math. representations
One’s complement vs. two’s complement
Data storing way
Intel Pentium: store numbers from right to left
Sun SPARC: from left to right
How to pass pointers & references
a) Original message on the Pentium
b) The message after receipt on the SPARC
c) The message after being inverted. The little numbers in boxes indicate the address of each byte

13. Parameter Specification and Stub Generation
Defining the message format is one aspect of an RPC protocol
A standard of data structure representation
An interface consists of a collection of procedure that can be called by a client.
Interface: Interface Definition Language (IDL)
A procedure
The corresponding message.

14. Extended RPC Models Performance Concern Doors Need OS support!!
RPC message exchange will cost a lot because they reside in different machines.
IPC (local interprocess communication) can overcome this problem since they are used in the same machine.
Doors
A generic name for a procedure in the address space of a server process that can be called by processes colocated with the server.
A similar mechanism is called Lightweight RPC.
A registered door (registered to OS) can be made available to other processes
Need OS support!!

15. Asynchronous RPC (1/2)
To overcome the drawback of RPC client’s waiting.
Asynchronous RPC can be made both either concurrently.
Combining two asyn. RPCs is sometimes referred to as a deferred synchronous RPC.
Note that “one-way RPCs” don’t need server to reply an ACK back to the client.

16. Example: DCE RPC DCE (Distributed Computing Environment)
Developed by the Open Software Foundation (OSF)
A middleware system
Initially designed for UNIX, now in VMS and Windows NT
Some famous services of the DCE
Distributed file service
Directory service
Security service
Distributed time service
DCE RPC enables a client to access a remote service by simply calling a local procedure.

17. Writing a Client and a Server
Using Interface Definition Language (IDL)
IDL files contain
type definitions, constant declarations, and information needed to marshal parameters
Formal definition of what the procedures do
The syntax of the calls
Globally unique identifier for specified interface
Writing client/server DCE RPC (See next page)
First, using uuidgen program to generate a prototype IDL file
Second, editing the IDL file, filling the names of the procedures and their parameters.
Call IDL compiler to generate 3 files: the header file, the client stub, and the server stub
Finally, programmer write the client and server code.

18. Writing a Client and a Server
The steps in writing a client and a server in DCE RPC.

19. Binding a Client to a Server
How a client can call a server
It is necessary that the server be registered and prepared to accept incoming calls.
Server location announcement
1. Locate the server’s machine
2. Locate the server (i.e., the correct process) on that machine.
The client needs to know an endpoint, (also known as port) which is used by the server’s OS to distinguish incoming messages for diff. processes.
In DCE, a table of (server, endpoint)-pairs is maintained on each server machine by a process called DCE daemon.
Client-to-server binding in DCE.

20. Remote Object Invocation
Object encapsulates data, called the state, and the operations on those data called methods.
Methods are made available through an interface.
The interface and the objects can be placed in different machine.
Client binds to a distributed object (DO), an implementation of the object’s interface, called a proxy.
Skeleton, a server stub, used to unmarshal invocation requests to proper method invocations at the object’s interface at the server.

21. Compile-time vs. Runtime Objects
A class is a description of an abstract type in terms of a module with data elements and operations on that data.
Compile-time
Advantage: easy to build distributed app.
Drawbacks: the dependency on a particular PL.
Runtime
Advantage: distributed objects had been written.
It’s easy to constructing distributed app. by using objects in different language.
Persistent and Transient Objects
Persistent objects are alive on the server all the time.
Transient objects exist only as long as the server manages the object.

22. Binding a Client to an Object
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* obj_ref; //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)
(a) Example with implicit binding using only global references
(b) Example with explicit binding using global and local references

23. Static vs. Dynamic RMI Remote Method Invocation (RMI)
Static invocation
The interfaces of an object are known when the client application is being developed
If interfaces change, the client application must be recompiled.
Dynamic invocation
invoke(object, method, input_parameters, output_parameters);
object: distributed object; method: method to be invoked; input_para: data structure; output_para: data structure
For example
Static: fobject.append(int)
Dynamic: invoke(fobject, id(append), int)
has been known
returns an id for the method append

24. Parameter Passing
The situation when passing an object by reference or by value.
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25. The DCE Distributed-Object Model
Distributed dynamic objects in DCE.
Distributed named objects

26. Persistence and Synchronicity in Communication (1)
General organization of a communication system in which hosts are connected through a network
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27. Persistence and Synchronicity in Communication (2)
Persistent communication of letters back in the days of the Pony Express.

28. Persistence and Synchronicity in Communication (3)
Persistent asynchronous communication
Persistent synchronous communication
2-22.1

29. Persistence and Synchronicity in Communication (4)
2-22.2
Transient asynchronous communication
Receipt-based transient synchronous communication

30. Persistence and Synchronicity in Communication (5)
Delivery-based transient synchronous communication at message delivery
Response-based transient synchronous communication

31. Berkeley Sockets (1) Socket primitives for TCP/IP. Primitive Meaning
Create a new communication endpoint
Bind
Attach a local address to a socket
Listen
Announce willingness to accept connections
Accept
Block caller until a connection request arrives
Connect
Actively attempt to establish a connection
Send
Send some data over the connection
Receive
Receive some data over the connection
Close
Release the connection

32. Berkeley Sockets (2)
Connection-oriented communication pattern using sockets.

33. The Message-Passing Interface (MPI)
MPI is used in local communication system
Socket, in contrast with MPI, has two drawbacks
Wrong level of abstraction by supporting send and receive primitives.
Using general protocol stacks, TCP/IP
Socket is not suitable for COWs (Cluster of Workstations) or MPPs (Massively Parallel Processors)
MPI is designed for parallel applications
MPI use (groupID, processID) to replace TCP/IP.

34. 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

35. Message-Queuing Model (1/2)
Also called as Message-Oriented Middleware (MOM)
Intermediate-term storage capacity for messages, without requiring either the sender or receiver to be active during message transmission.
Difference with Berkeley socket and MPI
Applications communicate by inserting messages in specific queues.
A queue can be read only by its associated application, but it is also possible for multiple applications to share a single queue.
Loosely-coupled communication
Like the Internet system
Allow a process to install a handler as a callback function, which is automatically invoked whenever a message is put into the queue.

36. Message-Queuing Model (2/2)
Four combinations for loosely-coupled communications using queues.

37. Message-Queuing Model (2)
Basic interface to a queue in a message-queuing system.
Primitive
Meaning
Put
Append a message to a specified queue
Get
Block until the specified queue is nonempty, and remove the first message
Poll
Check 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.

38. General Architecture of a Message-Queuing System (1/3)
The relationship between queue-level addressing and network-level addressing.

39. General Architecture of a Message-Queuing System (2/3)
Source queue
Local to the sender
On the same machine or on a machine nearby such as on the same LAN.
Destination queue
Local to the receiver
Queue manager
Interacts directly with the application that is sending or receiving a message
Relays
Special queue managers that operate as routers

40. General Architecture of a Message-Queuing System (3/3)
The general organization of a message-queuing system with routers.

41. Message Brokers
The problem of integrating existing and new applications into a single, coherent distributed information system.
Acts as an application-level gateway
To convert incoming messages to a format that can be understood by the destination application.
Simple as a reformatter for messages.
The general organization of a message broker in a message-queuing system.

42. Example: IBM MQSeries
General organization of IBM's MQSeries message-queuing system.

43. Channels Some attributes associated with message channel agents.
Description
Transport type
Determines the transport protocol to be used
FIFO delivery
Indicates that messages are to be delivered in the order they are sent
Message length
Maximum length of a single message
Setup retry count
Specifies maximum number of retries to start up the remote MCA
Delivery retries
Maximum times MCA will try to put received message into queue

44. Message Transfer (1)
The general organization of an MQSeries queuing network using routing tables and aliases.

45. Message Transfer (2) Primitives available in an IBM MQSeries MQI
Description
MQopen
Open a (possibly remote) queue
MQclose
Close a queue
MQput
Put a message into an opened queue
MQget
Get a message from a (local) queue
Primitives available in an IBM MQSeries MQI

46. Data Stream (1/3) Continuous media Discrete media Data stream
The temporal relationships between different data items are fundamental to correctly interpreting what the data actually means.
Discrete media
The temporal relationships between different data items are not fundamental to correctly interpreting the data.
Data stream
Nothing but a sequence of data units.

47. Data Stream (2/3) Asynchronous transmission mode
The data item in a stream are transmitted one after the other, but there are no further timing constraints on when transmission of items should take place.
Synchronous transmission mode
There is a maximum end-to-end delay defined for each unit in a data stream
Isochronous transmission mode
It is necessary that data units are transferred on time.
Data transfer is subject to a maximum and minimum end-to-end delay, also referred to as bounded (delay) jitter.

48. Data Stream (3/3) Simple stream Complex stream
Consists of only a single sequence of data
Complex stream
Consists of several related simple stream, called substreams.
Main problem with multicast streaming is when the receivers have different requirements with respect to the quality of the stream.
Filter

49. Streams and Quality of Service (QoS)
Time-dependent (and other nonfunctional) requirements are generally expressed as
Flow specification
Bandwidth requirements, transmission rates, delays, etc.
Token bucket algorithm
How the stream will shape its network traffic

50. Specifying QoS A flow specification. Characteristics of the Input
Service Required
maximum data unit size (bytes)
Token bucket rate (bytes/sec)
Toke 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
A flow specification.

51. Setting Up a Stream Resource reSerVation Protocol (RSVP)
A transport-level control protocol for enabling resource reservation in network routers.
Does not interpret the flow specification
Receiver-initiated QoS protocol
In fact, to make RSVP work, the RSVP process will have to translate the QoS parameters of its flow specifications into things that make sense to the data link layer.

52. Synchronization Mechanisms (1)
The principle of explicit synchronization on the level data units.

53. Synchronization Mechanisms (2)
The principle of synchronization as supported by high-level interfaces.
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