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ICMPv4 Traceroute Messages
(Page 2 of 2)
ICMPv4 Traceroute Message Format
Since the Traceroute message
was specifically designed for the traceroute utility, it was
possible to incorporate into it extra information of use to a host tracing
a route. The message format is as shown in Table 101
and Figure 151.
Table 101: ICMPv4 Traceroute Message Format
Field
Name
|
Size (bytes)
|
Description
|
Type
|
1
|
Type: Identifies
the ICMP message type, in this case 30.
|
Code
|
1
|
Code:
Set to the value 0 if the datagram the source device sent was successfully
sent to the next router, or 1 to indicate that the datagram was dropped
(meaning the traceroute failed).
|
Checksum
|
2
|
Checksum: 16-bit
checksum field for the ICMP header, as described in the
topic on the ICMP common message format.
|
ID
Number
|
2
|
ID Number:
An identification field used to match up this Traceroute message
to the original message sent by the source (the one containing the Traceroute
IP option).
|
Unused
|
2
|
Unused: Not used,
set to 0.
|
Outbound
Hop Count
|
2
|
Outbound
Hop Count: The number of routers the original message has already
passed through.
|
Return
Hop Count
|
2
|
Return Hop Count:
The number of routers the return message has passed through.
|
Output
Link Speed
|
4
|
Output
Link Speed: The speed of the link over which the Traceroute
message is being sent, in bytes per second.
|
Output
Link MTU
|
4
|
Output Link MTU:
The Maximum
Transmission Unit (MTU) of the link over
which the Traceroute message is being sent, in bytes.
|
Figure 151: ICMPv4 Traceroute Message Format
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Why Traceroute Messages Are Not Widely Used
Note that while this method of implementing
traceroute indeed has advantages over the older Time Exceeded
method, it has one critical flaw as well: it requires changes to both
hosts and routers to support the new IP option and the Traceroute
ICMP message. People aren't big on change, especially when it comes
to the basic operation of IP. For this reason, RFC 1393 never moved
beyond experimental status, and most IP devices still use
the older method of implementing traceroute. It is possible that
you may encounter ICMP Traceroute messages, however, so it's
good to know they exist.
Key Concept: ICMP Traceroute messages were designed to provide a more capable way of implementing the traceroute (tracert) utility. However, most TCP/IP implementations still use ICMP Time Exceeded messages for this task. |
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Version 3.0 - Version Date: September 20, 2005
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