Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 285

UDP

UDP is a connectionless, best effort transport-layer protocol. UDP uses IP protocol number 17.
Because UDP does not spend time setting up and handling connections, it generates less overhead
than TCP. UDP's benefit is that with a small header, it can send data with less overhead to the
destination. Applications that are time-sensitive, such as voice and video, tend to use UDP
transport instead of TCP. There is no need to retransmit a voice conversation.

UDP Header
The UDP header is shown in Figure 6-9.

UDP Header Figure 6-9

0 1 2 3
01234567890123456789012345678901

Source Port Destination Port

UDP Length Checksum

The following is a description of each field of the UDP header in Figure 6-9:
•

Source port —This field is 16 bits long and indicates the source port number. It is an
optional field; a value of zero is inserted if the field is not used.

•

Destination port —This field is 16 bits long and indicates the destination port number.
Some destination ports are 69 (TFTP) and 161 (SNMP).

•

UDP length —This field is 16 bits long and indicates the length in bytes of the datagram
that includes the UDP header and data.

•

Checksum —This field is 16 bits long. The checksum is performed on the UDP header,
UDP data, and IP pseudo-header. The IP pseudo-header consists of the IP source address,
IP destination address, protocol, and UDP length. Figure 6-10 shows the pseudo-header.

IP Pseudo-Header for Checksum

Figure 6-10

0 1 2 3
01234567890123456789012345678901

Source IP Address

Destination IP Address

00000000 Protocol UDP Length