Re: [tcpm] [Tmrg] Increasing the Initial Window - Notes

[Joe Touch <[e-mail]>]

Hi, all,

On 11/18/2010 4:46 AM, David Borman wrote:
>
> On Nov 17, 2010, at 2:41 PM, Jakob Heitz wrote:
>
>> IMHO:
>>
>> If you think a small TCP IW is protecting any network from
>> congestion collapse, you are either living in the past
>> or under delusions of grandeur.
>
> That is my point, increasing the TCP IW may increase congestion and
> packet drops in some scenarios, but it doesn't change how TCP reacts to
> congestion ...

Actually, it does.

Consider that there are three cases:

A) connections so small the IW doesn't matter (they already fit in IW=3)

B) connections 4-20 or so segments range

C) connections much longer than 20 or so segments

(A) isn't affected by this proposed change at all, so we don't need to 
consider it.

(C) adjusts to the IW using AIMD, so even if we guess wrong, TCP will 
"react to congestion" here.

And then there's (B). (B) doesn't react to congestion. Every new 
connection uses IW=10. Even after repeated attempts - to a given 
address, or, e.g., to every address - end up dropping, e.g., half of 
those segments. (B) will sit there and beat its head against the wall.

That's the congestion-increasing case. Will this cause collapse? I don't 
know. But it's definitely NOT reacting to congestion.

So can we please get off the long-connection case? That's not the issue 
here at all*...

Joe

PS - * even long connections can be affected by the IW, e.g., when they 
do a restart after idle, if they use IW for the restart value 
repeatedly; that's worth considering, but should be considered roughly 
the same as (B) if each busy period is in the 4-20 segments range. I.e., 
we can just talk about (B), IMO.