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Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised

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From Andy Furniss <spam@andyfurniss.entadsl.com>
Newsgroups comp.os.linux.networking, comp.protocols.tcp-ip
Subject Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised
Date Sun, 22 May 2011 20:40:50 +0100
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Cross-posted to 2 groups.

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Mark Hobley wrote:
> On Sun, 22 May 2011 11:40:16 +0100, Andy Furniss wrote:
>
>> I see the same, but don't know what the behavior should be. If you set
>> mtu on say eth0 then advertised tcp mss will be affected (which in a way
>> is setting mtu plus mru for tcp) - maybe the route setting really just
>> does mtu alone, which in a way is what you asked for
>
> That would not be very good at all, because it breaks PMTUD. I would
> obviously want the MTU to match the route.

It doesn't break PMTUD as such - if someone sends you a larger packet 
with DF set than your ISP can send, then your ISP should send a frag 
needed. The trouble is it may never reach the sender so it's best to 
avoid the situation arising (for tcp) by advertising a smaller mss.

>
>> If you want to fix up your wan use iptables mss clamping on the gateway
>> if you can.
>
> I had a quick look through the router configuration settings and I could not
> see such an option, so I guess that this facility is not provided.

Ahh, I see now you don't have shell access. It may be clamping anyway as 
noted elsewhere. Some routers will clamp to the mtu on the wan/ppp 
connection by default, or maybe your ISP is doing some clamping.

I would see if the test upload works after you set mtu on the machines 
nic rather than the route.

>
>> Using ping to try and work out isp behavior can be misleading - you may
>> just be seeing the settings of the server/network you are pinging and
>> the pings may get fragged/re-assembled without you knowing.
>
> With PMTUD enabled, the Don't Fragment flag is set though, isn't it? So this
> would mean that we have a single datagram end to end.

A quick test on my box shows that df is only set for ping when < mtu.

If the ping is larger it gets fragged and reply is also fragged.

Assuming iputils ping you can force no frag with ping -M do .....

If you have set mtu low with route/on nic  this will just refuse to send 
 > than that, so to test you need to set higher mtu, and of course be 
aware that what you see may just depend on the settings/net of the 
target rather than your ISP.

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Thread

Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-21 14:58 +0000
  Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised "David W. Hodgins" <dwhodgins@nomail.afraid.org> - 2011-05-21 15:16 -0400
    Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-21 22:02 +0000
      Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2011-05-22 19:59 +0300
        Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 17:09 +0000
  Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-21 20:20 +0000
    Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Richard Kettlewell <rjk@greenend.org.uk> - 2011-05-21 22:47 +0100
    Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-21 22:28 +0000
      Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 01:15 +0000
        Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-22 06:01 +0000
          Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 07:28 +0000
            Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Andy Furniss <spam@andyfurniss.entadsl.com> - 2011-05-22 11:40 +0100
              Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 12:25 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Andy Furniss <spam@andyfurniss.entadsl.com> - 2011-05-22 20:40 +0100
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Andy Furniss <spam@andyfurniss.entadsl.com> - 2011-05-22 20:56 +0100
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 22:57 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 23:12 +0000
            Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-22 11:31 +0000
              Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 11:50 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-22 12:52 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 13:27 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-22 19:19 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-22 22:12 +0000
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-25 15:55 +0000
                ip route - Wrong Maximum Segment Size being advertised Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-25 17:27 +0000
                Re: Linux 2.6.39.1 - Wrong Maximum Segment Size in both directions Mark Hobley <markhobley@yahoo.donottypethisbit.co> - 2011-05-29 00:34 +0000
                Re: Linux 2.6.39.1 - Wrong Maximum Segment Size in both directions Andy Furniss <spam@andyfurniss.entadsl.com> - 2011-05-29 11:11 +0100
                Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Martijn Lievaart <m@rtij.nl.invlalid> - 2011-05-23 08:13 +0200
    Re: Linux 2.6.39-rc7 - Wrong Maximum Segment Size being advertised Barry Margolin <barmar@alum.mit.edu> - 2011-05-22 00:44 -0400

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