32 bits

Source Port (16 bits)

DDeesstitninaatitoionnPPoortrt (16 bits)

UDP Lengthi (1(166bbitist)s)

Checksum (16 bits)

Figure 2.11 UDP frame format.

The following list describes the function of each UDP frame format field:

► Source Port—Indicates the port of the sending device.

► Destination Port—Communicates with another device listening to the destination port.

► Length—Specifies the length of the UDP header and data only.

► UDP Checksum—Represents an optional field and is set to 0 if it is not used. It is used to report errors to higher layers.

When a device wishes to talk to another, it will use a well-known UDP port number as the destination field. The source port will be a random number. For example, if a router downloads an IOS image from a TFTP server, the destination port will be 69 (see the list of UPD port numbers). Following are some common UDP port numbers:

► 19—Character generator

► 69—Trivial File Transfer Protocol (TFTP)

► 161—Simple Network Management Protocol (SNMP)


It is important to differentiate between TCP and UDP. TCP is a connection-orientated protocol, while UDP is a connectionless protocol. TCP and UDP provide services at the Transport layer (layer 4) of the OSI model.

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