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TCP Checksum Calculation and the TCP "Pseudo Header"
(Page 1 of 3)
The Transmission Control Protocol
is designed to provide reliable data transfer between a pair of devices
on an IP internetwork. Much of the effort required to ensure reliable
delivery of data segments is of necessity focused on the problem of
ensuring that data is not lost in transit. But there's another important
critical impediment to the safe transmission of data: the risk of errors
being introduced into a TCP segment during its travel across the internetwork.
Detecting Transmission Errors Using Checksums
If the data gets where it needs to
go but is corrupted and we do not detect the corruption, this is in
some ways worse than it never showing up at all. To provide basic protection
against errors in transmission, TCP includes a 16-bit Checksum
field in its header. The idea behind a checksum is very straight-forward:
take a string of data bytes and add them all together. Then send this
sum with the data stream and have the receiver check the sum. In TCP,
a special algorithm is used to calculate this checksum by the device
sending the segment; the same algorithm is then employed by the recipient
to check the data it received and ensure that there were no errors.
The checksum calculation used by
TCP is a bit different than a regular checksum algorithm. A conventional
checksum is performed over all the bytes that the checksum is intended
to protect, and can detect most bit errors in any of those fields. The
designers of TCP wanted this bit error protection, but also desired
to protect against other type of problems.
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