FTTC - wierd MTU issues

Im have a strange problem with my FTTC connection. The Openreach modem seems to drop down to almost zero through put when it has been connected for a couple of days. Web browsing no longer becomes possible however from command promt you can ping any domain name as usual. It not an issue with my router as it also does it using the modem connected directly to a PC.

When the problem occurs ive found the the maximum packet size seems to drop lower and lower, and effectively it seems like port 80 is blocked also.Power cycling the Openreach modem fixes the problem for a couple of days.

As a start point I wondered if someone with FTTC could tell me the maximum packet size they can ping to bbc.co.uk?

My connection is working well at the minute after a modem reboot and the maximum packet size is 1464. Yesterday I had throughput problems and it dropped to a max of 1422 that could be pinged.

Any thoughts?


C:\Users\Paul Brown>ping -l 1464 bbc.co.uk

Pinging bbc.co.uk [212.58.241.131] with 1464 bytes of data:
Reply from 212.58.241.131: bytes=1464 time=30ms TTL=245
Reply from 212.58.241.131: bytes=1464 time=30ms TTL=245
Reply from 212.58.241.131: bytes=1464 time=32ms TTL=245
Reply from 212.58.241.131: bytes=1464 time=30ms TTL=245

Ping statistics for 212.58.241.131:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 30ms, Maximum = 32ms, Average = 30ms

C:\Users\Paul Brown>ping -l 1465 bbc.co.uk

Pinging bbc.co.uk [212.58.241.131] with 1465 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 212.58.241.131:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

You have MTU = 1492.

Ensure MTU settings in router & PC (if you ever changed it) are 1500.

Do the ping with 'ping -f -l nnnn bbc.co.uk'. Find largest nnnn for which it doesn't need fragmentation.

Then add 28 to get MTU.

Thanks for the reply xray. Makes sense, on my Draytek router the max MTU setting is 1498 which ties in with what im currently getting then.

So moving on why would this drop so I can only send smaller packets when the problem is occuring? throughput reduces to almost zero

Edited by Ripley (Sun 15-Jul-12 23:37:12)

Don't forget that PPPoE uses 8 bytes more than PPPoA (added by the router), so the MTU on the PC should be set to 8 less than on the router.

A bit more here.

1500 on the router and 1492 on the PC should work fine...

Thanks I will do that.

Still interested to know why the maximum packet size would reduce when this problem occurs for me, then a reboot of the Openreach FTTC modem fixes the problem...

The BT routers actually have it set to 1492 ...

Quite possibly.

That may be because most BT users aren't likely to be particularly tech-savvy, but may have heard that the MTU on the PC should not be greater than on the router. So, for the user who has a little (possibly dangerous) knowledge, if it fibs a little bit everything gets set properly

In reply to a post by MHC:

The BT routers actually have it set to 1492 ...

As does my Netgear WNR1000 v 3 router as suplied by Plusnet to go with the OR Hg612 modem for my FTTC connection.

I am Running Windows 7 that apparently auto-adjusts accordingly to suit the connection.

Hmmm ?

My Buffalo is set to its default of 1500. I'm not aware of a problem, but perhaps I have one I'm not aware of?

What downside could this be causing please?

I manage to get;

mailgate:~$ ping -D -s 1468 bbc.co.uk
PING bbc.co.uk (212.58.241.131): 1468 data bytes
1476 bytes from 212.58.241.131: icmp_seq=0 ttl=246 time=21.889 ms

and then;

mailgate:~$ ping -D -s 1469 bbc.co.uk
PING bbc.co.uk (212.58.241.131): 1469 data bytes
Request timeout for icmp_seq 0
36 bytes from ae1.er01.thdow.bbc.co.uk (132.185.254.18): frag needed and DF set (MTU 1514)
Vr HL TOS Len ID Flg off TTL Pro cks Src Dst
4 5 00 d905 64cf 0 0000 37 01 42da 192.168.16.20 212.58.241.131

So the largest packet from the LAN I get a reply to from bbc.co.uk is 1468 bytes

I get responses to bigger pings still on other sites;

mailgate:~$ ping -D -s 1470 www.ox.ac.uk
PING www.ox.ac.uk (163.1.60.42): 1470 data bytes
1478 bytes from 163.1.60.42: icmp_seq=0 ttl=51 time=28.839 ms

1470 bytes is reproducibly the largest packet I can send out from the LAN and get a response although some sites won't respond at this size.

The MTU of the client (Mac Mini) I'm using is 1500, the firewall is 1500 and the router has no MTU set at all.

From the router directly I get;

caffn8me-gw#ping
Protocol [ip]:
Target IP address: www.ox.ac.uk
Repeat count [5]:
Datagram size [100]: 1498
Timeout in seconds [2]:
Extended commands [n]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 1498-byte ICMP Echos to 163.1.60.42, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/26/28 ms

This is the maximum from the router to www.ox.ac.uk but there may be sites with an even higher limit.

I believe the Openreach modem has a maximum MTU of 1600 bytes on ethernet (see here) and as far as I know the specification for BT's FTTC network doesn't set a limit for MTU. If I can find the specification I read previously I'll post a link.