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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
                9  ICMP Message Types and Formats
                     9  ICMP Version 4 (ICMPv4) Error Message Types and Formats

Previous Topic/Section
ICMPv4 Source Quench Messages
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ICMPv4 Redirect Messages
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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 datagram’s 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 recipient’s 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.


Previous Topic/Section
ICMPv4 Source Quench Messages
Previous Page
Pages in Current Topic/Section
12
3
Next Page
ICMPv4 Redirect Messages
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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