Re: [gentoo-user] PMTUD

[Grant <emailgrant@gmail.com>]

>> Thanks Mick.  Can you generally rely on PMTUD to set the MTU optimally
>> or should this be experimented with when changing connections?
>
> Short answer:  default Linux machine settings behave properly as network
> devices and acknowledge packets larger than their MTU value with the
> appropriate response.
>
> Longer answer:
>
> Communications between IPv4 end points use PMTUD by setting a Don't Fragment
> (DF) bit in the headers of the outgoing packet.  If a router/server along the
> path has a smaller MTU, it will drop that packet and respond with an ICMP
> 'Destination Unreachable -- Fragmentation Needed' packet including its smaller
> MTU value.  Upon receiving this smaller packet value the initiating host will
> dynamically reduce the size of the outgoing packets, until the packet arrives
> at its intended destination.  PMTUD should always be switched on in any well
> behaving network implementation, but here's the rub:  some network nodes,
> firewalls, servers are configured to never respond with *any* ICMP packets
> (because they think that this is a way to avoid DDoS problems and the like).
> Therefore, the initiating host keeps sending large packets never knowing that
> they are dropped on the way.  This network problem is known as a PMTUD black
> hole and is explained better here:
>
>   http://tools.ietf.org/html/rfc2923
>
> Some MSWindows servers were notoriously bad at this, but I think that modern
> configurations have corrected their buggy ways.  Linux machines have PMTUD
> switched on by default and behave properly.

Got it, thank you.

> If you are still troubled by the proxy connection stalling problem, have you
> tried transferring large files over the network using scp/sftp to see if you
> are also getting similar symptoms?  This would isolate it to the application
> level (squid) or if the problem remains would point to network configuration
> issues.

How can I make this determination?  I'm testing a 50MB scp over hotel
wifi from my laptop to the remote proxy server now (with squid running
in case it matters) and it seems OK.  It oscillates constantly between
0.0KB/s and 80.0KB/s.  As soon as I start browsing via the proxy
server, the upload frequently goes to "stalled" but I suppose that
could be a bandwidth issue.  Browsing still stalls before very long.

- Grant