mturoute – Free utility to find the MTU Value to a host or IP Address

One of the most important thing that is ignored most of the time while troubleshooting a network problem to a host or a network is checking the MTU related issue. This makes more sense in situations like troubleshooting VPN related issues.

MTUROUTE is a small but smart utility that uses ICMP pings of various sizes in order to determine the MTU values (Maximum Transmission Unit) on the path between itself and the target system. MTUROUTE can operate in normal mode where it sends multiple ICMP packets to each hop on the path to identify the smallest MTU between the host and hop or in a Traceroute mode where it will attempt to determine the lowest MTU between the local host and each hop in the communication.

It is important to note that care has to be taken as it generates muti fold ICMP traffic as it tries to determine the MTU Values.

Mturoute sends a non-fragmentable icmp probe to the target IP address with a given payload size. Since the DF (Don't Fragment) bit is set, any network equipment that has an MTU setting which is smaller than the packet size will drop the packet. Based on the presence or absence of a response, mturoute adjusts the payload size of its next probe. Subsequent probes are sent with a packet size midway between the highest value for which a response was received and the lowest value for which a response was not received. (For the initial conditions, the low value is 0 and the high value is whatever you set with -m, defaulting to 10000 bytes).

This process is repeated, narrowing the search space by approx 50% with each cycle, terminating once the highest possible probe size has been converged upon. It then adds the 28 bits of ICMP protocol overhead before reporting results.

For example,

C:\mturoute\Release>mturoute.exe 192.168.0.1
* 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 succeeded.
+ ICMP payload of 1464 bytes succeeded.
– ICMP payload of 1474 bytes failed..
+ ICMP payload of 1469 bytes succeeded.
+ ICMP payload of 1471 bytes succeeded.
+ ICMP payload of 1472 bytes succeeded.
– ICMP payload of 1473 bytes failed..
+ ICMP payload of 1472 bytes succeeded.
+ ICMP payload of 1472 bytes succeeded.
Path MTU: 1500 bytes.

In TraceRoute mode

C:\mturoute\Release>mturoute.exe -t 192.168.0.1
mturoute to 192.168.0.1, 30 hops max, variable sized packets
* ICMP Fragmentation is not permitted. *
* Maximum payload is 10000 bytes. *
 1  –+—+++-+++-++  host: 192.168.0.1  max: 1500 bytes

For help, simply type the command with no arguements

C:\mturoute\Release>mturoute.exe
********************************************************
* mturoute.exe – written by Eli Fulkerson, June 2005   *
*                fixes by Ivan Pepelnjak, October 2007 *
* Please visit http://www.elifulkerson.com for updates.*
*       http://ioshints.blogspot.com/search?q=mturoute *
********************************************************

Usage: mturoute [-t] [-f] [-m MAX_PAYLOAD_SIZE] host
Flags:
   -t : Toggles 'traceroute' mode.  (Default is off)
   -f : Allow fragmentation.  This will return the max ping size that the
        target host will respond to, but not necessarily the MTU.
   -w : Sets the time to wait for a response to an Echo Request.
   -r : Sets the maximum number of probe retries on timeout (default = 3).
   -i : Sets the interval between two echo requests.
   -d : Increases the debugging level. Reports ICMP status/failures.
   -m : Sets a maximum payload size to test. (Default is 10000)
   -v : Prints version info and exits

Warning:  This utility generates a lot of ICMP traffic.

For more information and download, click here