I wanted to get in-depth details on how various MTU parameters interact in GRE/IPSec/MPLS environment. Before going into router configuration details, I wanted to have a tool that would reliably measure actual path MTU between the endpoints. After a while, Google gave me a usable link: supposedly the tracepath program on Linux does what I needed. As I'm a purely Windows user (for me, PCs are just a tool), I needed a Windows equivalent … and found mturoute, the utility that does exactly what I was looking for. Unfortunately, the original mturoute had an interesting bug: ICMP unreachable generated due to DF bit on oversized packet was accepted as a successful ping. The second run of the program always reported the correct MTU size (as Windows caches the maximum MTU per destination host), but I wanted to be more precise. Half a day later, after installing Windows SDK and Visual Studio Express on my PC, rediscovering my C programming skills and reading a lot of Winsock documentation, I've managed to fix the bugs and even add a “retry on ping timeout” feature. You can download the fixed version (source + exe) from the Articles area of my web site. It was compiled and tested on Windows XP, if you can test it on other platforms (2000, Vista), please let me know the results.

Here is a sample run of the program (the reduced path MTU is due to an MPLS-enabled GRE tunnel in the path):

$ mturoute -t 10.0.3.3
mturoute to 10.0.3.3, 30 hops max, variable sized packets
* ICMP Fragmentation is not permitted. *
* Maximum payload is 10000 bytes. *
 1  --+---+++-+++-++  host: 10.0.0.1  max: 1500 bytes
 2  --+---++---+++  host: 10.2.0.2  max: 1476 bytes
 3  --+---+-+++++++  host: 10.0.3.3  max: 1472 bytes