Internet Core Protocols: The Definitive Guide

Time Exceeded error messages

Time Exceeded error messages are used to indicate that a forwarding or reassembly operation took too long to complete and that the reporting device is discarding the data. In order to provide more-detailed reporting, the Time Exceeded message provides two different submessages (using the ICMP Message Code field as described later in Section 5.2.1.1). The submessages offered by Time Exceeded include:

Time-to-Live Exceeded in Transit

This error message is used when an IP datagram’s Time-to-Live value has reached zero but the datagram has not yet been delivered to its final destination. Since the Time-to-Live field indicates the maximum number of hops that a datagram can take, the router cannot forward a datagram with a Time-to-Live value of zero, and must destroy it instead. Since most systems use Time-to-Live values of 30 or more, the presence of this message generally indicates that a routing loop is preventing the datagram from being delivered.

However, this message is also used with the traceroute program to identify the routers in between a source and destination system. For more information on how traceroute uses this message, refer to Section 5.3.3 later in this chapter.

Fragment Reassembly Time Exceeded

This error message is used when a datagram has been fragmented but the destination system has not received all of the fragments within the allotted time (60 seconds on most Unix systems). This message generally indicates that a fragment has been lost ...