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ICMPv4 Time Exceeded Messages
(Page 3 of 3)
Applications of Time Exceeded Messages
As an ICMP error message type, ICMP
Time Exceeded messages are usually sent in response to the two
conditions described above (TTL or reassembly timer expiration).
Generally, Time To Live expiration messages are generated by
routers as they try to route a datagram, while reassembly violations
are indicated by end hosts. However, there is actually a very clever
application of these messages that has nothing to do with reporting
errors at all.
Key Concept: ICMPv4 Time Exceeded messages are sent in two different time-related circumstances. The first is if a datagrams Time To Live (TTL) field is reduced to zero, causing it to expire and the datagram to be dropped. The second is when all the pieces of a fragmented message are not received before the expiration of the recipients reassembly timer.
The TCP/IP traceroute (or
tracert) utility is used to show the sequence of devices over
which a datagram is passed on a particular route between a source and
destination, as well as the amount of time it takes for a datagram to
reach each hop in that route. This utility was originally implemented
using Time Exceeded messages by sending datagrams with successively
higher TTL values. First, a dummy datagram is sent
with a TTL value of 1, causing the first hop in the route to
discard the datagram and send back an ICMP Time Exceeded; the
time elapsed for this could be measured. Then, a second datagram is
sent with a TTL value of 2, causing the second device in the
route to report back a Time Exceeded, and so on. By continuing
to increase the TTL value we can get reports back from each hop
in the route. See
the topic describing traceroute for more details on its operation.
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Version 3.0 - Version Date: September 20, 2005
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