Jeff West has asked me to document the printout produced by the mturoute utility. Here's the first part of the documentation.

mturoute works in two modes:

• Without the -t flag, it sends variable-lenght ICMP echo packets to the specified destination address, trying to figure out the largest packet that is successfully propagated to the destination.
• With the -t flag, it uses traceroute-like algorithm to find the hop-by-hop IP addresses (the source IP addresses of the ICMP TTL exceeded replies) and uses the same packet-size-calculating algorithm to measure the path MTU to each hop.

Today we'll focus on the non-trace mode. It tries to measure the path MTU with a bisection method varying the packet sizes between minimum MTU (92) and maximum MTU (specified with the -m parameter, default is 10000 bytes). The payload size of the first packet (without the -m flag) is thus 5046 bytes ((10000 + 92)/2).

On each iteration, the algorithm prints a “cryptic” sign indicating whether the ping with the current payload size succeeded. The following indications are given:

• '+': ICMP echo reply arrived
• '-': The ping failed (for various reasons, including exceeding the path MTU)
• 'u': ICMP destination unreachable response arrived, indicating blackhole or access-list.

ICMP unreachable is considered a successful response; at least we're measuring the path MTU up to the failure point

• '.': timeout. The ping will be retried up to three times (or the number specified with the -r option).

In the ping mode, mturoute prints the current ICMP payload size at each step, resulting in a printout similar to the one below. If you'd have specified the -d option, the printout would include detailed status codes from the IcmpSendEcho function.

$ mturoute 10.0.3.3
* ICMP Fragmentation is not permitted. *
* Maximum payload is 10000 bytes. *
- ICMP payload of 5046 bytes failed..
- ICMP payload of 2569 bytes failed..
+ ICMP payload of 1330 bytes succeeded.
- ICMP payload of 1949 bytes failed..
- ICMP payload of 1639 bytes failed..
- ICMP payload of 1484 bytes failed..
+ ICMP payload of 1407 bytes succeeded.
- ICMP payload of 1445 bytes failed..
+ ICMP payload of 1426 bytes succeeded.
+ ICMP payload of 1435 bytes succeeded.
+ ICMP payload of 1440 bytes succeeded.
+ ICMP payload of 1442 bytes succeeded.
+ ICMP payload of 1443 bytes succeeded.
+ ICMP payload of 1444 bytes succeeded.
+ ICMP payload of 1444 bytes succeeded.
Path MTU: 1472 bytes.

Note: To use the debug-enabled version of mturoute, or the version that does not need VC++ runtime, download the new ZIP archive from my web site.