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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)
|
Description
|
Type
|
1
|
Type: Identifies
the ICMPv6 message type; for Time Exceeded messages this is set
to 3.
|
Code
|
1
|
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
|
2
|
Checksum: 16-bit
checksum field for the ICMP header, as described in the
topic on the ICMP common message format.
|
Unused
|
4
|
Unused:
4 bytes left blank and not used.
|
Original
Datagram Portion
|
Variable
|
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
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Key Concept: Like their ICMPv4 namesakes, ICMPv6 Time Exceeded messages are sent in two different time-related circumstances. The first is if a datagrams 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 recipients 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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