Page 172 - DCAP406_DCAP_207_Computer Networks

Unit 11: Transport Layer

Connection establishment failure probability: Due to congestion in the network, lack of Notes
availability of space in table, some internal problem, etc., causes the connection not to set within
the establishment delay.
Throughput: It defines the number of bytes of user data transferred per second in a defined time
interval. For each communication link it is measured separately.

Transit delay: It is the time gap between a transmitted data from source machine to the reception
of the same data by the destination machine. Like, throughput, for each communication link it
is measured separately.
Residual error ratio: It is the fraction of the lost data with respect to the total data sent over the
network by source machine.

Protection priority: It is defined as the capability of the transport layer to provide Protection
against third parties who try to interfere with the data. It specifies the priority of the important
connections so that high priority connections are served before the low priority connections in
the event of congestion.
Resilience: It is the capability of the transport layer to terminate a connection itself spontaneously
in the case of congestion.
Transport layer can not always fulfill all of the parameters as mentioned above. It tries to
implement a trade off among the parameters of quality of service. This process is called option
negotiation.

11.1.3 Transport Service Primitives

They are used to access transport services by the application layer or the users. Each transport
service is defined with a unique transport primitive. The network layer provides an unreliable
service whereas the transport layer attempts to provide a reliable service on top of the unreliable
service. Some of the transport primitives are LISTEN, CONNECT, SEND, RECEIVE and
DISCONNECT.

Transport Protocol Data Unit (TPDU) is a term used for exchanging data from transport entity to
transport entity. The TPDUs are contained in the packets exchanged by the network layer. The
packets are then contained in the frames exchanged by the data link layer. At the destination
machine, when a frame arrives the data link layer processes the frame header and passes contents
of the frame payload field up to the network entity. Similar process takes place at the network
layer.
The above situation may be understood from an example, when a remote machine, say, client
machine requests another machine, say, server for connection. The client machine issues a TPDU
CONNECT to the server. The server has already transmitted a TPDU LISTEN to the network to
block the connection until a client machine turns up. On receiving the TPDU CONNECT, it
unblocks the server machine and a CONNECTION ACCEPTED TPDU is sent back to the client
machine and thus connection is established by unblocking the client machine too. After this, the
SEND and RECEIVE primitives enable exchange of data.
Following are the steps implemented by client machine to establish the connection:

Create a socket
Connect the socket to the address of the server machine

Send/Receive data
Close the socket

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