Transport Layer Seminar

8/7/2019 Transport Layer Seminar

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TRANSPORT LAYER

*TRANSPORT SERVICE

** TRANSPORT SERVICES PRIMITIVES


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TRANSPORT SERVICE PRIMITIVES

* to allow the user to access the transport service,

the transport layer must provide some operation

to application program (ie.,)transport interface

*Each transport service has its own interface

*The connection oriented transport service is

reliable.


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EXAMPLE

Consider two process connected by pipes in

UNIX.

They assume that the connection between them

is perfect they dont expect for the acks, lost packages,

congestion(*).

They expect only for 100% reliable connection.

Process A puts data into one end of the pipe andprocess B takes it out of the other (*)

Transport layer can also provide

unreliable(datagram) service.


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CONTS

User mainly concentrate on connection oriented

transport service

Advantages

- transport service must be convenient andeasy to use


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PRIMITIVES FOR SIMPLE TRANSPORT

SERVICE

Primitive Packet sent Meaning

LISTEN (none) Block until some

process ties to

connect

CONNECT CONNECTION REQ Actively attempt toestablish a

connection

SEND DATA Send info

RECEIVE (none) Block until DATA

packet arrives

DISCONNECT DISCONNECT

REQ

The side wants to

release the

connection


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CONTS

An application with a server and number of

remote clients.

To start with the server executes a LISTEN

primitive, typically by calling a library procedurethat makes a system call to block the server until

a client turns up.

When a client wants to talk to the server, it

executes a CONNECT primitive.

The transport entity carries out this primitive by

blocking the caller and sending a packet to the

server.


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NESTING OF TPDUS, PACKETS,AND

FRAMES


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TPDU- TRANSPORT PROTOCOL DATA UNIT

In transport layer , even in simple unidirectional

data exchange is more complicated than at the

network layer

Every data packet sent will also beacknowledged (eventually)

The packets bearing control TPDUs are also

acknowledged, implicitly, or explicitly

The acknowledged are managed by the transportentities, using the network layer protocol, and

are not visible to the transport users.

The transport entities will need to worry About

timers and retransmissions.


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CONTS

When a connection is no longer needed , it must

be released to free up table space within the two

transport entities.

Disconnection has two variants* asymmetric variants

* symmetric variants

o In asymmetric variants, either the transport

user can issue a DISCONNECT primitive , whichresult in a DISCONNECT TPDU being sent to

the remote transport entity


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In symmetric variants, Each direction is closed

separately, independently of other one .

When one side DISCONNECT, that means it has

no more idea to send but still willing to acceptdata from its partner

In this model, a connection is released when both

sides have done DISCONNECT.


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BERELEY SOCKETS

Set of transport primitives, the socket primitive s

used in Berkeley UNIX for TCP.

These primitives are widely used for internet

programming


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PRIMITIVE MEANING

SOCKET Create a new connection end

point

BIND Attach a local address to a socket

LISTEN Announces willingness to accept

connections; give queue size

ACCEPT Block the caller until a

connection attempt arrives

CONNECT Actively attempt to establish a

connection

SEND Send some data over the

connection

RECEIVE Receive some data from the

connection

CLOSE Release the connection


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The first four primitives are executed by servers.

The SOCKET primitive creates s new end point

and allocates table space for it within the

transport entity .

A newly created socket do not have network

address. These are assigned using BIND

primitive.

Once a server a bounded an address to a socket,

remote clients can connect to it.

Next listen call, which allocates space to queue

incoming for the case that several clients try to

connect at the same time .


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When TPDU asking for a connection arrives , the

transport entity create a new socket with the

same properties as the original one and returns

a file descriptor for it.

The server can then fork off a process and thread

to handle the connection on the new socket & go

back to waiting for the next connection on the

original socket

ACCEPT returns a normal file descriptor , which

can be used for reading and writing in the

standard way, the same as for files.


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In a client side a socket must first be created

using the SOCKET primitive, but BIND is not

required since the address used does not matter

to the server.

The CONNECT primitive blocks the caller and

activity starts the connection process.

When it completes the clients process is

unblocked and the connection is established


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Both sides can now use SEND and RECV to

transmit and receive the data over the full-

duplex connection .

The standard UNIX -READ and WRITE systemcalls can also be used if none of the special

options of SEND & RECV are required

Connection release with sockets is symmetric.

When both sides have executed a CLOSE

primitive , the connection is released.

Documents

Transport Layer Seminar