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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 6 (ICMPv6) Error Message Types and Formats

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ICMPv6 Packet Too Big Messages
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ICMPv6 Time Exceeded Messages
(Page 2 of 2)

ICMPv6 Time Exceeded Message Format

The format for ICMPv6 Time Exceeded messages can be found in Table 105 and Figure 155.

Table 105: ICMPv6 Time Exceeded Message Format

Field Name

Size (bytes)




Type: Identifies the ICMPv6 message type; for Time Exceeded messages this is set to 3.



Code: Identifies the “subtype” of time error being communicated. A value of 0 indicates expiration of the Hop Limit field; a value of 1 indicates that the fragment reassembly time has been exceeded.



Checksum: 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.



Unused: 4 bytes left blank and not used.

Original Datagram Portion


Original Datagram Portion: As much of the IPv6 datagram as will fit without causing the size of the ICMPv6 error message (including its own IP header) to exceed the minimum IPv6 maximum transmission unit (MTU) of 1280 bytes.

Figure 155: ICMPv6 Time Exceeded Message Format


Key Concept: Like their ICMPv4 namesakes, ICMPv6 Time Exceeded messages are sent in two different “time-related” circumstances. The first is if a datagram’s Hop Limit 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 recipient’s reassembly timer expires.

Applications of Time Exceeded Messages

In IPv4, ICMP Time Exceeded messages are used both as an error message and in a clever application to implement the TCP/IP traceroute command. This is done by first sending a “dummy” datagram is sent with a Time To Live value of 1, causing the first hop in the route to discard the datagram and send back an ICMP Time Exceeded. Then, a second datagram is sent to the same destination with a TTL value of 2, causing the second device in the route to report back a Time Exceeded, and so on.

There is of course an IPv6 version of traceroute, sometimes called traceroute6. Due to IPv6 and its protocols and applications still being in development, I have not been able to confirm definitively that traceroute6 is implemented using ICMPv6 Time Exceeded messages in the manner described above, but I believe this is the case (and it certainly would make sense.) See the topic discussing traceroute for further information.

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Version 3.0 - Version Date: September 20, 2005

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