Internet Core Protocols: The Definitive Guide

Header checksums

Part of this integrity-checking service is handled through the use of a checksum applied against the IP datagram’s header (but not against the data inside of the IP datagram). Every device that receives an IP datagram must examine the IP header and compare that information with the value stored in the header’s checksum field. If the values do not match, then the datagram is assumed to be corrupt and is discarded immediately.

The data portion of the IP datagram is not verified, for three reasons. First of all, a device would have to examine the entire datagram to verify the contents. This process would require additional CPU processing time, which is more often than not going to be a waste of time.

Second, the data portion of an IP datagram always consists of a higher-level datagram, such as those generated by TCP and UDP. Since these protocols provide their own error-checking routines, the recipient system will have to conduct this verification effort anyway. The theory is that datagrams will move faster if routers do not have to verify their contents, a task which will be handled by the destination system anyway.

Finally, some application protocols are capable of working with partially corrupt data. In those cases, IP would actually be performing a disservice if it were to throw away datagrams with invalid checksums, since the application protocol would never get it. Granted, most applications do not work this way, but most applications will also utilize some ...