MTU infront of and behind a router

Can anyone explain this,

I am getting the optimum MTU pinging iburst.co.za connected directly to the UTD via ethernet at 1372. Then I set my router (that manages my pppoe connection) at 1372 and connect as a networked computer, and then I get the optimum MTU for my pc at 1322 pinging the same site.

Maybe I am missing something?

Post your ping results so we can see what's happening.

This is pinging with my laptop connected to the router (MTU on laptop 1352, router 1352). Going any higher with the packet size and it boms out.

C:\Documents and Settings\>ping www.iburst.co.za -n 10 -l 1324 -f

Pinging www.iburst.co.za [196.30.31.120] with 1324 bytes of data:

Reply from 196.30.31.120: bytes=1324 time=269ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=225ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=430ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=282ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=402ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=263ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=247ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=311ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=201ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=313ms TTL=125

Ping statistics for 196.30.31.120:
Packets: Sent = 10, Received = 10, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 201ms, Maximum = 430ms, Average = 294ms


Now pinging directly connected to the UTD via ethernet (MTU on Laptop still 1352)


C:\Documents and Settings\>ping www.iburst.co.za -n 10 -l 1372 -f

Pinging www.iburst.co.za [196.30.31.120] with 1372 bytes of data:

Reply from 196.30.31.120: bytes=1372 time=389ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=351ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=330ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=250ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=250ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=260ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=250ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=401ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=351ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=230ms TTL=126

Ping statistics for 196.30.31.120:
Packets: Sent = 10, Received = 10, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 230ms, Maximum = 401ms, Average = 306ms



By the way earlier today pings was constantly around 60 - 80. Now it is up again? Can not understand this.

Hi Gimli, the -n 10 isn't necessary, what I would like to see is an incremental pring comparison to show exactly what MSS value (-l xxxx) causes fragmentation in both UTD attachment scenarios.

IC, those are the maximum -l XXXX values before it gives message 'Fragmentation is required but -f is set'. For both scenarios. Or does the round trip time or TTL tell you something?

Herewith incremental:

This is behind the router (linksys wrt45g): (MTU on router and laptop 1352)

C:\Documents and Settings\>ping www.iburst.co.za -l 1324 -f
Pinging www.iburst.co.za [196.30.31.120] with 1324 bytes of data:

Reply from 196.30.31.120: bytes=1324 time=314ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=253ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=288ms TTL=125
Reply from 196.30.31.120: bytes=1324 time=301ms TTL=125

Ping statistics for 196.30.31.120:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 253ms, Maximum = 314ms, Average = 289ms

C:\Documents and Settings\>ping www.iburst.co.za -l 1325 -f

Pinging www.iburst.co.za [196.30.31.120] with 1325 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 196.30.31.120:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),


And this is connected directly from laptop onto UTD



C:\Documents and Settings\>ping www.iburst.co.za -l 1372 -f

Pinging www.iburst.co.za [196.30.31.120] with 1372 bytes of data:

Reply from 196.30.31.120: bytes=1372 time=359ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=260ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=337ms TTL=126
Reply from 196.30.31.120: bytes=1372 time=260ms TTL=126

Ping statistics for 196.30.31.120:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 260ms, Maximum = 359ms, Average = 304ms

C:\Documents and Settings\>ping www.iburst.co.za -l 1373 -f

Pinging www.iburst.co.za [196.30.31.120] with 1373 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 196.30.31.120:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

Methinks it's is something to do with "TCPWindowSize".
Checkout http://www.speedguide.net/read_articles.php?id=157 and have a look at your registry on the 2 computers to c if anything has been set on either machine.
I'd be interested to know.

Now I need to admit firstly that I have not done this in nearly two years time so.... My errors were two fold.

1) I forgot that you take the maximum packet size and add 28 to account for the header. This would perfectly explain why I got 1324 pinging from my laptop..... 1324 +28 = 1352, which was my router mtu at the time.

2) When pinging from my laptop directly connected to the UTD I got 1372 which means I could setup the router at 1400.

3) Then I changed MTU on my laptop to 1400 and connected behind my router but still got 1324.... This leads to the second lesson. Always reboot your machine else the new MTU setting does not take effect.

Stupid I know but the brain has gone a bit rusty.

Now everything is quite dandy with MTU set to 1400 all round.

Now that one is solved, here is another question that is even more interesting to me. Why if I just ping www.iburst.co.za I get avarage time of 69 ms, but if I include the do not defragment flag with packet size they average at 313 ms ?


C:\Documents and Settings\>ping www.iburst.co.za

Pinging www.iburst.co.za [196.30.31.120] with 32 bytes of data:

Reply from 196.30.31.120: bytes=32 time=76ms TTL=125
Reply from 196.30.31.120: bytes=32 time=69ms TTL=125
Reply from 196.30.31.120: bytes=32 time=67ms TTL=125
Reply from 196.30.31.120: bytes=32 time=67ms TTL=125

Ping statistics for 196.30.31.120:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 67ms, Maximum = 76ms, Average = 69ms

C:\Documents and Settings\>ping www.iburst.co.za -l 1372 -f

Pinging www.iburst.co.za [196.30.31.120] with 1372 bytes of data:

Reply from 196.30.31.120: bytes=1372 time=257ms TTL=125
Reply from 196.30.31.120: bytes=1372 time=291ms TTL=125
Reply from 196.30.31.120: bytes=1372 time=331ms TTL=125
Reply from 196.30.31.120: bytes=1372 time=374ms TTL=125

Ping statistics for 196.30.31.120:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 257ms, Maximum = 374ms, Average = 313ms

uh.. bcause the packet is bigger? oh and if you're gonna ping you should rpbably ping the gateway and not the website