The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Message Formatting and Data Transfer

TCP Message (Segment) Format 1 2 3 TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size

Advantages of the Pseudo Header Method

So, why bother with this “pseudo header”? The source and destination devices both compute the checksum using the fields in this pseudo header. This means that if, for any reason, the two devices don't use the same values for the pseudo header, the checksum will fail. Now, when we consider what's in the header, we find that this means the checksum now protects against not just errors in the TCP segment fields but also against:

• Incorrect Segment Delivery: If there is a mismatch in the Destination Address between what the source specified and what the destination that got the segment used, the checksum will fail. The same will happen if the Source Address does not match.
  
  
• Incorrect Protocol: If a datagram is routed to TCP that actually belongs to a different protocol for whatever reason, this can be immediately detected.
  
  
• Incorrect Segment Length: If part of the TCP segment has been omitted by accident, the lengths the source and destination used won't match and the checksum will fail.

What's clever about the pseudo header is that by using it for the checksum calculation, we can provide this protection without actually needing to send the fields in the pseudo header itself. This eliminates duplicating the IP fields used in the pseudo header within the TCP header, which would be redundant and wasteful of bandwidth. The drawback of the pseudo header method is that it makes checksum calculation take more time and effort (though this is not much of an issue today.)

In the context of today's modern, high-speed, highly-reliable networks, the use of the pseudo header sometimes seems “archaic”. How likely is it that a datagram will be delivered to the wrong address? Not very. At the time TCP was created, however, there was significant concern that there might not be proper “end-to-end” checking of the delivery of datagrams at the IP level. Including IP information in the TCP checksum was seen as a useful additional level of protection.

Of course, there is one interesting implication of the TCP pseudo header: it violates the architectural layering principles that the designers of TCP sought to respect in splitting TCP and IP up. For the checksum, TCP must know IP information that technically it “shouldn't”. TCP checksum calculation requires, for example, that the protocol number from the IP header be given to the TCP layer on the receiving device from the IP datagram that carried the segment. The TCP pseudo header is a good example of a case where strict layering was eschewed in favor of practicality.

Finally, TCP also supports an optional method of having two devices agree on an alternative checksum algorithm. This must be negotiated during connection establishment.

TCP Message (Segment) Format 1 2 3 TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size