TCP Checksum Calculation and the TCP "Pseudo Header"
(Page 2 of 3)
Increasing The Scope of Detected Errors: the TCP Pseudo Header
To this end, a change was made in how the TCP checksum is computed. This special TCP checksum algorithm was eventually also adopted for use by the User Datagram Protocol (UDP).
Instead of computing the checksum over only the actual data fields of the TCP segment, a 12-byte TCP pseudo header is created prior to checksum calculation. This header contains important information taken from fields in both the TCP header and the IP datagram into which the TCP segment will be encapsulated. The TCP pseudo header has the format shown in Table 158 and Figure 217.
Once this 96-bit header has been formed, it is placed in a buffer, following which the TCP segment itself is placed. Then, the checksum is computed over the entire set of data (pseudo header plus TCP segment). The value of the checksum is placed into the Checksum field of the TCP header, and the pseudo header is discardedit is not an actual part of the TCP segment and is not transmitted. This process is illustrated in Figure 218.
When the TCP segment arrives at its destination, the receiving TCP software performs the same calculation. It forms the pseudo header, prepends it to the actual TCP segment, and then performs the checksum (setting the Checksum field to zero for the calculation as before). If there is a mismatch between its calculation and the value the source device put in the Checksum field, this indicates that an error of some sort occurred and the segment is normally discarded.
Home - Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.