Re: Pulling stalls before 52MB (works via netcat)

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Hi Grant,

On Sun, 4 May 2014, Grant wrote:

The first thing that comes to mind is you're over-taxing Linux's selective ack's. Try disabling with:

# echo 0 > /proc/sys/net/ipv4/tcp_sack

That didn't do it but switching to MTU 576 seems to have fixed it. I suspect the root of the problem is that I'm using an AT&T modem/router with the server. I've had trouble using a proxy server there before and I discovered that the attached modem/router doesn't send ICMP responses. The solution proposed by AT&T was to put the modem/router into bridged mode but it's remote so I haven't been able to do that.

Could this MTU discovery also point to the modem/router and could putting it into bridged mode solve the problem? What can I do in the meantime?

Is it strange that netcat works with MTU 1500 and openssh doesn't?

I don't know what model of modem/router you're using, but I have this problem with my home Technicolor TG582n. If I try to upload large amounts of data to a system with MTU < 1500 (e.g. my office on PPPoE), it stalls immediately, not after 52 MB.

It seems like a bug in the router: it doesn't forward ICMP MTU-exceeded errors received from the Internet, or does it wrongly so they're ignored by the receiving host on my LAN (which is trying to upload the data). If I replace it with an older ST516 then the problem goes away, but that router doesn't have wireless so it's less convenient. There appears to be no way to contact Technicolor to report this issue to them.

I have seen problems with corruption of packets based on certain patterns appearing in them, which are more likely to hit encrypted streams randomly, while an unencrypted stream that doesn't contain the trigger sequence will almost never hit the problem.

If you put it into bridged mode, it may help you for two reasons:

* It may reduce your MTU (if you're bridging PPPoA to PPPoE) so that the MTU discovery problem can't happen.

* It may take the modem's (suspected faulty) NAT code out of the network path.

However, it might make no difference, and without isolating the problem to a corrupt ICMP message or something that's worked around by reducing the MTU to 1492, I wouldn't bet on it.

Cheers, Chris.
--
_____ __     _
\  __/ / ,__(_)_  | Chris Wilson <chris+sig@xxxxxxxxx> Cambs UK |
/ (_/ ,\/ _/ /_ \ | Security/C/C++/Java/Ruby/Perl/SQL Developer |
\__/_/_/_//_/___/ | We are GNU : free your mind & your software |
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