Internet Core Protocols: The Definitive Guide

Fragmentation Problems

In addition, a variety of fragmentation-related problems can crop up that will prevent datagrams from being successfully delivered. Since IP will process only a complete datagram (and more importantly, will discard an incomplete datagram), fragmentation problems will cause a substantial number of retransmissions if an error-correcting protocol is generating the IP datagrams.

Fragmentation problems can occur in a variety of cases, although the most common cause is due to the sender attempting to detect the end-to-end MTU of a network using Path MTU Discovery, but an intermediary device does not return ICMP Error Messages back to the sending system. The result is that the sender continues trying to send packets that are too large to be fragmented, with the Don’t Fragment flag enabled. For a comprehensive discussion on Path MTU Discovery and the problems that can result, refer to Section 5.3.4 in Chapter 5.

Other fragmentation problems can occur when using infrastructure equipment that is under heavy load, or when the network itself becomes somewhat congested. In those situations, a device that is fragmenting packets for delivery of another (smaller) network is losing some of the fragments, or the network itself is losing the packets. These problems can be difficult to diagnose, since ping tests using small or normal-sized messages across the network may perform just fine.

The best way to diagnose these problems is to send large ICMP Echo Request messages to the ...