Chemistry ppt

1. Chapter four
Communication

2. Objectives of the Chapter
• Review of how processes communicate in a network (the rules or the
protocols) and their structures
• Introduce most widely used communication models for distributed
systems:
• Network Protocols and Standards
• Remote Procedure Call (RPC) -which hides the details of
message passing and suitable for client-server models.
• Remote Object (Method) Invocation (RMI).invoking method from
other machine.
• Message-Oriented Middleware (MOM) -instead of the client-
server model, think in terms of messages and have a high level
message queuing model similar to e-mail
• Stream-Oriented Communication -for multimedia to support the
continuous flow of messages with timing constraints
• Multicast Communication -information dissemination for several
recipients.
• Web services - offering general services to remote applications
without immediate interactions from end users.

3. Introduction
• IPC is at the heart of all distributed systems
• communication in distributed systems is based on message
passing as offered by the underlying network as opposed to
using shared memory
• modern distributed systems consist of thousands of
processes scattered across an unreliable network such as
the Internet.

4. Network Protocols and Standards
• why communication in distributed systems? because there is
no shared memory
• two communicating processes must agree on the syntax and
semantics of messages
• a protocol is a set of rules that governs data communications,
a protocol defines :-
what is communicated,
how it is communicated,
when it is communicated

5. The key elements of a protocol are syntax, semantics, and timing
• syntax: refers to the structure or format of the data
• semantics: refers to the meaning of each section of bits
• timing: refers to when data should be sent and how fast they can
be sent.
Two computers, possibly from different manufacturers, must be
able to talk to each other due to standard model.
• The ISO-OSI model is a seven layer architecture. It defines
seven layers or levels in a complete communication system.
What is ISO-OSI protocol?
• OSI protocols are a family of standards for information
exchange. They consist of a set of rules that should
represent a standard for physical connections, cabling,
data formats, transmission models, as well as means to
ensure correction of errors and missing data.

6.  Physical: Physical characteristics of the media
Host (upper) Layers
Media (lower) Layers
 Data Link: Reliable data delivery across the link
 Network: Managing connections across the network
or routing
 Transport: End-to-end connection and reliability (handles
lost packets); TCP (connection-oriented),
UDP (connectionless), etc.
 Session: Managing sessions between applications
(dialog control and synchronization).
 Presentation: Data presentation to applications; concerned
with the syntax and semantics of the
information transmitted
 Application: Network services to applications; contains
protocols that are commonly needed by
users; FTP, HTTP, SMTP, ...

7. The interaction between layers in the OSI model

8. Physical layer
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.

9. Data link layer
The data link layer is responsible for moving
frames from one hop (node) to the next.

10. Hop-to-hop delivery

11. Network layer
The network layer is responsible for the
delivery of individual packets from the source host to the
destination host.

12. Source-to-destination delivery

13. Transport layer
The transport layer is responsible for the delivery
of a message from one process to another.

14. Reliable process-to-process delivery of a message

15. Session layer
The session layer is responsible for dialog
control and synchronization.

16. Presentation layer
The presentation layer is responsible for translation,
compression, and encryption.

17. Application layer
The application layer is responsible for
providing services to the user.

18. Summary of layers

19. discussion between a receiver and a sender in the data link layer
 a conversation occurs between a sender and a receiver at each
layer
 e.g., at the data link layer

20. normal operation of TCP
assuming no messages are lost,
 the client initiates a setup
connection using a three-way
handshake (1-3)
 the client sends its request (4)
 it then sends a message to close
the connection (5)
 the server acknowledges receipt
and informs the client that the
connection will be closed down (6)
 then sends the answer (7)
followed by a request to close the
connection (8)
 the client responds with an ack to
finish conversation (9)
Transport Protocols: Client-Server TCP

21. transactional TCP
 much of the overhead in TCP is for managing the connection
 the client sends a single message
consisting of a setup request,
service request, and information
to the server that the connection
will be closed down immediately
after receiving the answer (1)
 the server sends acceptance of
connection request, the answer,
and a connection release (2)
 the client acknowledges tear
down of the connection (3)
 combine connection setup with
request and closing connection with
answer
 such protocol is called TCP for
Transactions (T/TCP)

22. Remote Procedure Call
• in 1984, Birrel and Nelson introduced a different way of handling
communication: RPC
• it allows a program to call a procedure located on another
machine
• simple and elegant, but there are implementation problems
– the calling and called procedures run in different address
spaces
– parameters and results have to be exchanged
– what if the machines are not identical?
– what happens if both machines crash?

23. principle of RPC between a client and server program
 Client and Server Stubs
 RPC would like to make a remote procedure call look the same
as a local one; it should be transparent, i.e., the calling procedure
should not know that the called procedure is executing on a
different machine or vice versa
 when a program is compiled, it uses different versions of library
functions called client stubs
 a server stub is the server-side equivalent of a client stub

24.  Steps of a Remote Procedure Call
1. Client procedure calls client stub in the normal way message
2. Client stub builds a message and calls the local OS (packing
parameters into a message is called parameter marshaling)
3. Client's OS sends the message to the remote OS
4. Remote OS gives the message to the server stub
5. Server stub unpacks the parameters and calls the server
6. Server does the work and returns the result to the stub
7. Server stub packs it in a message and calls the local OS
8. Server's OS sends the to the client's OS
9. Client's OS gives the message to the client stub
10. Stub unpacks the result and returns to client
 hence, for the client remote services are accessed by making
ordinary (local) procedure calls; not by calling send and receive.

25. steps involved in doing remote computation through RPC
 Parameter Passing
1. Passing Value Parameters
 e.g., consider a remote procedure add(i, j), where i and j are
integer parameters

26. 2. Passing Reference Parameters
 assume the parameter is a pointer to an array
 copy the array into the message and send it to the server
 the server stub can then call the server with a pointer to this
array
 the server then makes any changes to the array and sends it
back to the client stub which copies it to the client
 this is in effect call-by-copy/restore
 optimization of the method
 one of the copy operations can be eliminated if the stub knows
whether the parameter is input or output to the server
 if it is an input to the server (e.g., in a call to write), it need not
be copied back
 if it is an output, it need not be sent over in the first place; only
send the size
 the above procedure can handle pointers to simple arrays and
structures, but difficult to generalize it to an arbitrary data
structure

27. Asynchronous RPC
 two cases
1. if there is no result to be returned
 e.g., adding entries in a database, ...
 the server immediately sends an ack promising that it will
carryout the request
 the client can now proceed without blocking
a) the interconnection between client and server in a traditional RPC
b) the interaction using asynchronous RPC

28. 2. if the result can be collected later
 e.g., prefetching network addresses of a set of hosts, ...
 the server immediately sends an ack promising that it will
carryout the request
 the client can now proceed without blocking
 the server later sends the result
a client and server interacting through two asynchronous RPCs

29. • resulted from object-based technology that has proven its
value in developing non distributed applications
• it is an expansion of the RPC mechanisms
• it enhances distribution transparency as a consequence of
an object that hides its internal from the outside world by
means of a well-defined interface
• Distributed Objects
– an object encapsulates data, called the state, and the
operations on those data, called methods
– methods are made available through interfaces.
– the state of an object can be manipulated only by invoking
methods
Remote Object (Method) Invocation (RMI)

30.  the state of an object is not distributed, only the interfaces are;
such objects are also referred to as remote objects
 the implementation of an object’s interface is called a proxy
(analogous to a client stub in RPC systems)
 it is loaded into the client’s address space when a client binds to
a distributed object.
 tasks: a proxy marshals method invocation into messages and
unmarshals reply messages to return the result of the method
invocation to the client
 a server stub, called a skeleton, unmarshals messages and
marshals replies.

31. common organization of a remote object with client-side proxy

32. • RPCs and RMIs are not adequate for all distributed system
applications
• the provision of access transparency may be good but they
have semantics that is not adequate for all applications
• example problems
– they assume that the receiving side is running at the time of
communication
– a client is blocked until its request has been processed
Message Oriented Communication

33.  Persistence and Synchronicity in Communication
general organization of a communication system in which hosts are connected
through a network
 assume the communication system is organized as a computer
network shown below.

34.  communication can be
 persistent or transient
 asynchronous or synchronous
 persistent: a message that has been submitted for transmission is
stored by the communication system as long as it takes to deliver it
to the receiver
 e.g., email delivery
persistent communication of letters back in the days
of the Pony Express

35.  transient: a message that has been submitted for
transmission is stored by the communication system only as
long as the sending and receiving applications are executing.
 asynchronous: a sender continues immediately after it has
submitted its message for transmission.
 synchronous: the sender is blocked until its message is
stored in a local buffer at the receiving host or delivered to
the receiver
 in general there are six possibilities

36. persistent asynchronous
communication
 persistent synchronous
communication

37. receipt-based transient synchronous
communication
 transient asynchronous
communication

38.  response-based transient
synchronous communication
 delivery-based transient
synchronous communication at
message delivery
 the sender is blocked until the
message is delivered to the
receiver for further processing
 strongest form; the sender is
blocked until it receives a reply
message from the receiver

39.  Stream Oriented Communication
Types of Media are
 discrete media: text, executable code, graphics,
images; temporal relationships between data items are
not fundamental to correctly interpret the data
 continuous media: video, audio, animation; temporal
relationships between data items are fundamental to
correctly interpret the data
 a data stream is a sequence of data units and can be
applied to discrete as well as continuous media
 stream-oriented communication provides facilities for the
exchange of time-dependent information (continuous
media) such as audio and video streams.

40. • timing in transmission modes
– asynchronous transmission mode: data items are
transmitted one after the other, but no timing constraints;
e.g. text transfer
– synchronous transmission mode: a maximum end-to-end
delay defined for each data unit; it is possible that data can
be transmitted faster than the maximum delay, but not
slower
– isochronous transmission mode: maximum and minimum
end-to-end delay are defined; also called bounded delay
jitter; applicable for distributed multimedia systems
• a continuous data stream can be simple or complex
– simple stream: consists of a single sequence of data; e.g.,
mono audio, video only (only visual frames)
– complex stream: consists of several related simple streams
that must be synchronized; e.g., stereo audio, video
consisting of audio and video (may also contain subtitles,

41. Unicast, Broadcast versus Multicast
• Unicast
– One-to-one
– Destination – unique
receiver host address
• Broadcast
– One-to-all
– Destination – address of
network
• Multicast
– One-to-many
– Multicast group must be
identified
– Destination – address of
group
Key:
 Unicast transfer
 Broadcast transfer
 Multicast transfer

42. Thank you!!