[CLUG] ISP throttling

Alex Satrapa grail at goldweb.com.au
Wed Jan 12 09:56:54 GMT 2005

On 11 Jan 2005, at 23:56, freegazer wrote:

> yeah it was  a 256k link so, u guess.

Other things that an ISP can do to affect download by Linux boxen is 
change their handling of ECN (Explicit Congestion Notification). Some 
routers (eg: SOHO, or cheap ISP level) can't or won't handle ECN 
properly. So at the ISP side, maybe they just started clearing the ECN 
bit after your later complaints.

You may also have been having problems with packet sizes being too 
large, which can be solved by the ISP performing maximum segment size 
clamping on their router (this doesn't do much for UDP unfortunately) - 
they directly munge the TCP negotiation so that both ends will 
negotiate a MSS larger than the TCP over L2TP can handle (if your ISP 
recommends setting MTU on an interface to 1460 or lower, this is 
because they're using L2TP to receive the packets from Telstra's line 
terminating units).

Yes, the ISP can do something to "fix" the problem in both instances, 
but this doesn't make it their fault that the problem occurred. All 
kinds of equipment exist on the Internet - their interactions are often 
unpredictable. ISPs like to say things like, "we don't support Linux" 
not because they don't *like* Linux, not because they're trying to 
*attack* Linux, but simply because they don't understand it enough to 
help you solve problems over the phone.

Supporting Microsoft Windows is fairly easy - the ways that Microsoft 
Windows breaks are relatively well documented, to the point that an 
experienced technical support person can write a script which will 
guide the inexperienced ("level 1") helpdesk person through solving the 
problem. Since it's not so easy to diagnose problems with Linux 
(Roaring Penguin isn't the only PPPoE client for Linux, and networking 
configuration changes significantly between 2.0, 2.4 and 2.6) and then 
you've got iptables thrown into the mix.

Please don't go slinging mud at your ISP because you chose to use a 
product that they don't know how to support, and as a result something 
broke that they couldn't help you fix.

Alex Satrapa

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