RFC768 http://www.cis.ohio-state.edu/htbin/rfc/rfc768.html
The User Datagram Protocol (UDP), defined by IETF RFC768, provides a simple, but unreliable message service for transaction-oriented services. Each UDP header carries both a source port identifier and destination port identifier, allowing high-level protocols to target specific applications and services among hosts.
The UDP header structure is shown as follows:
|
16
|
32
bits
|
|
|
|
|
|
|
|
Data |
|
|
UDP
header structure
|
|
Source
port
Source
port is an optional field. When used,
it indicates the port of the sending
process and may be assumed to be the
port to which a reply should be addressed
in the absence of any other information.
If not used, a value of zero is inserted.
Destination
port
Destination
port has a meaning within the context
of a particular Internet destination
address.
Length
The length
in octets of this user datagram, including
this header and the data. The minimum
value of the length is eight.
Checksum
The 16-bit
one’s complement of the one’s complement
sum of a pseudo header of information
from the IP header, the UDP header and
the data, padded with zero octets at
the end (if necessary) to make a multiple
of two octets.
Data
UDP data
field.