Page 174 - DCAP406_DCAP_207_Computer Networks

Unit 11: Transport Layer

is not important, it multiplexes several transport connections onto the same network Notes
connection, thus reducing the cost for establishing and maintaining the network
connections. When several connections are multiplexed, they call for demultiplexing at
the receiving end. In case of transport layer, the communication takes place only between
two processes and not between two machines. Hence, the communication at transport
layer is also known as peer-to-peer or process-to-process communication.

Fragmentation and re-assembly: When transport layer receives large message from session
layer, it breaks the message into smaller units depending upon the requirement. This
process is called fragmentation. Thereafter, it is passed to the network layer. Conversely,
when transport layer acts as receiving process, it reorders the pieces of message before
reassembling them into a message.

Addressing: Transport Layer deals with addressing or labeling a frame. It also differentiates
between a connection and a transaction. Connection identifiers are ports or sockets that
label each frame so the receiving device knows which process it has been sent from. This
helps in keeping track of multiple-message conversations. Ports or sockets address multiple
conservations in the same location. For example, the first line of a postal address is
analogous of port and distinguishes among several occupants of the same house. Computer
applications listen for information on their own ports and therefore more than one network-
based application may be used at the same time. The transaction identifiers deal with the
request or response frames. They are one-time events.

Self Assessment

Give one word for the following statements:
1. It is the amount of time when an acknowledgement is received from the destination
machine to which a connection is requested. Obviously, lesser is the delay, better is the
service.
2. Due to congestion in the network, lack of availability of space in table, some internal
problem etc, causes the connection not to set within the establishment delay.
3. The transport layer for creating and releasing the connections across the network includes
naming mechanism so that a process on one machine can indicate with whom it wishes to
communicate.
4. When transport layer receives large message from session layer, it breaks the message
into smaller units depending upon the requirement.

5. IETF working group had proposed the integrated services (IS) model based on outbound
bandwidth policies for predictable resources in the network.
6. A policy framework that describes the quality of a specific stream of data in terms of
bandwidth, buffer usage, priority, CPU usage, etc.
7. It is the capability of the transport layer to terminate a connection itself spontaneously in
the case of congestion.

11.3 A Simple Transport Protocol

Some of the examples are:

The Example Service Primitives
The Example Transport Entity

The Example as a Finite State Machine

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