Re: TCP send behaviour leads to cable modem woes

[Bryan Whitehead <driver@jpl.nasa.gov>]

Take a look at the wondershaper script.

It's helped my DSL get good rates from home both up and down...

http://lartc.org/wondershaper/

Svein Ove Aas wrote:
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> My internet connection is via a cable modem, and thereon from Telenor. (A 
> Norwegian ISP.)
> 
> In general, when I download something I can get up to 1400-1500 Kb/s, which is 
> pretty good for a 1024/256 account. (They don't appear to oversubscribe their 
> lines (yahoo!), but mine is also uncapped when there is spare capacity. Think 
> traffic-control.)
> 
> So far, so good.
> 
> My account includes 4 IP addresses, and when I actually have four computers 
> directly connected I can easily get 7-8Kb/s upload from each of them.
> Oddly, when one of them is acting as a firewall/bridge for the others or I'm 
> just uploading from one, I get 7-8Kb/s for *all* of them. (Or the one.)
> 
> This is, dare I say, *not* expected behaviour.
> I've been in contact with telenor about it, and have garnered the following 
> information.
> 
> (A)	Although the line appear to be straight Ethernet attached to a 
> packet-filtering switch (just ARP-filtering, actually), it's *actually* an 
> ATM-based line. This should come as no surprise.
> 
> (B)	Whatever they have allocating the ATM cells for transfer is doing it in 
> bursts of about 16KB. Or possibly 32KB. Well, the tech I talked to was pretty 
> sure it was a power of two, at least.
> 
> (C)	This means that while I get 8 bursts (or more) of 16KB per second on 
> download (empirically confirmed, but the cable modem will tend to space it 
> out when the line is at capacity), giving me a latency of 128-256 ms and so 
> on and so forth (which I have), I get only *two* bursts per second to upload 
> things. I think. You may want to apply a multiplier somewhere.
> 
> And, finally, (D):
> 
> This thoroughly screws up TCP/IP for uploading purposes. It *completely* 
> breaks Realtek cards, screws up uploading speeds in Linux and Windows XP (I 
> assume they think there is a lot of intermittent packet loss because of the 
> delay), and has no apparent effect on Windows 9x/2000.
> 
> The cable modem in question is manufactured by Coresma and is marked NeMo. 
> It's also supposed to have a pretty large send buffer, so if I could just 
> force Linux to send at some user-defined speed without being so paranoid 
> about overloading the line, I could get a lot more use out of it.
> 
> For the curious, if I do just that with UDP, I can indeed send at up to 30KB/s 
> without losing packets. They *do* come in bursts, though.
> 
> 
> Please, save me before I lose my mind!
> 
> - - Svein Ove Aas
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-- 
Bryan Whitehead
SysAdmin - JPL - Interferometry and Large Optical Systems
Phone: 818 354 2903
driver@jpl.nasa.gov