Internet Core Protocols: The Definitive Guide

Datagrams Are Corrupted or Never Sent

Sometimes, you may notice that a system does not always send the UDP datagrams that you expect it to, or that the first UDP packet from a stream of data does not get sent. Typically, this situation is a result of a lack of entries in the sender’s ARP cache for the destination system. Not having an entry, the system is forced to issue an ARP lookup for the destination system, and then wait for a response.

However, many TCP/IP implementations allow only one IP packet to be held in the ARP call-back queue for any given host. If during the act of issuing the ARP lookup the sender receives another out-bound packet for that system, then the first packet is likely to get cleared from the ARP queue in order to make room for the new packet. In this case, the first packet (the one that caused the lookup in the first place) will be destroyed.

This particular problem happens most often when multiple packets are quickly sent to a destination system, or when the size of the UDP or ICMP message exceeds the MTU of the local network, forcing IP to fragment the single message into multiple IP packets. In the latter case, IP will issue two or more ARP requests (depending on the number of fragments generated) for the same destination system, resulting in the ARP agent clearing all but the last fragment from the call-back queue immediately. The last fragment that was sent before an ARP response was received will be the first fragment to get sent on the wire.

When the ...