Ques : Describe UDP header

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 Ans :

1) Source Port Number: A 16-bit number which defines the source port number
for a particular application program that is sending the UDP datagrams.

2) Destination Port Number: A 16-bit number that defines the destination port
number for a particular application program that is receiving the UDP datagrams.

Source port number 16 bits                           Destination port number 16 bits
               16 bits
                                              Check sum
                                                  16 bits

3) Length : The 16-bit field denotes the size of UDP header combined with payload
data. It can range between 0 to 65,535 bytes.

4) Checksum: This is 16-bit field that provides detection of errors over the entire
user datagram.

Characteristics of UDP
The basic characteristics of UDP are as follows:

  1. UDP is a connectionless service.
  2. It adds no non-reliable flow control to IP.
  3. It serves as a multiplex/demultiplexer for sending and receiving datagrams.
  4. The communication ends (end terminals) need not be synchronized.
  5. There is no provision for acknowledgement of datagrams.

    Applications of UDP
    The standard applications using UDP include:

    1. Trivial File Transfer Protocol (TFTP)
    2. Domain Name System (DNS) name server
    3. Remote Procedure Call (RPC) used by the Network File System (NFS)
    4. Simple Network Management Protocol (SNMP).

    Internet Control Message Protocol (ICMP)
    The Internet Control Message Protocol notifies the sender of IP datagrams about
    abnormal events. ICMP is important in the connectionless environment. ICMP
    messages are carried in IP packets. The most commonly employed ICMP message
    types include:

    1. Destination Unreachable: This message type indicates that a packet cannot be
    delivered because the destination host cannot be reached. The reason for the nondelivered may be that the host or network is unreachable or unknown, the protocol
    or port is unknown or unusable.

    2. Echo and Echo Reply: These two messages are used to check whether hosts are
    reachable on the network. One host sends an Echo message to the other,
    optionally containing some data and the receiving host responds with an Echo
    Reply containing the same data. These messages are the basis for the Ping

    3. Source Quench: Sent by a router to indicate that it is experiencing congestion
    and is discarding datagrams.

    4. TTL Exceeded:
    This message indicates that a datagram has been discarded
    because the TTL field reached 0 or because the entire packet was not received
    before the fragmentation timer expired.

    5. Timestamp and Timestamp Reply:
    These messages are similar to the Echo
    messages, but place a timestamp (with millisecond granularity) in the message,
    yielding a measure of how long remote systems spend buffering and processing
    datagrams and providing a mechanism so that hosts can synchronize their clocks.

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