Page 178 - DCAP406_DCAP_207_Computer Networks

Unit 11: Transport Layer

Notes
Figure 11.4: The Correspondence between the UDP and IP Datagrams

Following is a brief description of each field:
Source Port: Source port specifies port number of the application relating to the user data.
Destination Port: As it name indicates, this pertains to the destination application.
Length: It describes the total length of the UDP datagram, including both data and header
information.
UDP Checksum: It gives an option of integrity checking.
At this point, it is important to understand the layering concept along with the need for headers.
The relationship between the IP and UDP has been depicted in Figure 11.4.

Notes There are a number of good reasons for choosing UDP as a data transport service.
When the amount of data being transmitted is small, UDP is considered the most efficient
choice for a transport layer protocol because of the overhead for establishing connections
and ensuring reliable delivery may be greater than the work of re-transmitting the entire
data. Applications for a query-response model also work excellent for using UDP. The
response is used as a positive acknowledgment to the query. When a response is not
received within a certain time period, the application initiates another query.

Some examples of the usage of UDP are Remote file server (NFS), name translation (DNS), intra-
domain routing (RIP), network management (SNMP), multimedia applications and telephony.

11.3.5 Transmission Control Protocol

Transmission Control Protocol (TCP) was designed to provide a reliable end-to-end data transfers
over an unreliable internetwork in which TCP adapts properties of the internetwork dynamically.
The internetwork may have different topologies, bandwidths, throughputs, delays, packet sizes,
etc for different networks building up the internetwork. Each machine supporting TCP has a
TCP entity that accepts user data streams from local processes and breaks them up into pieces
not exceeding 64k bytes to transmit each piece as a separate IP datagram.

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