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

[Joe Touch <[e-mail]>]

On 11/18/2010 12:32 PM, David Borman wrote:
...
>> 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.
>
> No, (B) will also react to congestion, if the congestion causes
> packet loss. For any given flow, if all the packets get through the
> first time, from it's viewpoint there is no congestion. If they don't
> all get through, that connection does normal TCP backoff and
> retransmits.

To be more clear, here's the case:

	start 1000 connections in a row.

	during the first connection, lose some packets and do
	normal TCP backoff

	so what do the other 999 connections start with?
	ans: 10 packets

The point is that subsequent connections don't do anything different. If 
you have 1000 connections, you're sending a certain amount of data into 
the network without reacting. We're tripling that.

That can easily cause congestion. At which point the *existing* 
connections will backoff, but new connections keep making problems.

Joe