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Version 0.0.0 @ 03:55/08.07.2000
Decomposition of a UDP Packet
The above is an example of an UDP Header. UDP is an unreliable protocol where no attempt is made to keep track of packets as to being in a sequence, order, or verify that they were received. Checks are made to see if the packet appears to be the same packet transmitted by evaluation of a checksum and comparison to the packets checksum. These packets are dropped if the destination calculations of the checksum differs from the packets claimed checksum.
Description of fields listed in above UDP header starting from left to right, top to bottom.
- Source Port [SP] (16 bits): When a connection is attempted, or being conducted, this specifies what port the local machine is waiting to listen for responses from the destination machine.
- Destination Port [DP] (16 bits): When a user desires to connect up to a service on a remote machine, the Application Layer program specifies what port initial connections should use. When not as part of an initial connection, this specifies what port number is going to be used for the remote machine as a packet is being sent out to its destination.
- Length [Len] (16 bits): This allows for the receiving station to know how many of the incoming bits are supposed to be part of a valid packet. The length is a calculation of how many bytes are part of the UDP packet including bytes in the header. Since UDP always has 4 fields in the header and each has 16 bits, and the data/payload of variable length, we know the length will be 8 + (number of bytes in the payload.)
- UDP Checksum [UDPCS] (16 bits):This is a checksum that covers the header and data portion of a UDP packet to allow the receiving host to verify the integrity of an incoming UDP packet. The UDP packet is loaded with a predefined number in the checksum field, and then when the checksum is computed, then the checksum is written over the previous value. When the packet arrives at the destination, the destination machine's OS looks to the 4 header field (bytes made from bits 16 through 31) and pulls them out of the packet, then re-calculates the checksum on the packet without anything in the checksum field. Then the OS compares the checksum calculated with thte one that was transmitted in the packet. If the checksum is the same, the data is fine, and it is allowed to pass on through, but if there is a difference, the UDP packet, and data are dropped, and no attempt is made by the receiving machine to get a new copy, and the sending machine will not try to send that same packet. The packet is lost forever. UDP is not reliable! For a reliable Transport Layer TCP/IP suite protocol see the TCP packet.
- Data (Variable bits): As you might expect, this is the payload, or data portion of an UDP packet. The payload may be any number of protocols (often application layer.) Some of the most commonly used UDP protocols include NFS, DNS, as well as mutliple audio and video streaming protocols. If an error occurs in a UDP packet and an error is desired to be fixed, it is left to the application layer to find the error and request its application layer "hunk" or "chunk" of data.
This ends the brief preview of an UDP packet.
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