Broadband Optimizer for Revolution?

[Alex Tweedly]

Dar Scott wrote:

>
> On May 31, 2005, at 12:43 PM, Mark Wieder wrote:
>
>> I'd also think twice about increasing the size of the TCP transmit
>> window in any case. The receive window should be fine, but to my
>> caffeine-deprived brain this morning I think increasing the TCP
>> transmit window without a corresponding increase in the receive window
>> at the receiving end could result in an overall slower speed.
>
>
> Well, I have had my caffeine, so I have no excuse, but I don't know 
> what transmit window is.
>
> The TCP window is controlled by the receiver.
>
Not entirely. The "Window" field in the TCP header is indeed the 
receiver's definition of the number of octets he is willing to receive, 
so he does control it; however, a TCP stack will keep track of the "send 
window" for each connection. The "recv window" as sent by the receiver 
is an upper bound on the "send window" - but but the send window will be 
decreased according to any data sent since the last ACK was received, 
and also as a result of NAKs (and slow start).   (see rfc 793, sect 3.2)

Also, it will never exceed the configured send window size; thus even if 
a remote receiver is able and willing to have the window grow to say 
64K, if the inet.sendspace was set to 32K, then the transmitter will 
never grow his send window above 32k (because he needs to buffer all 
that data within the TCp stack to allow for retransmits, and is 
unwilling to hold more than 32k).

> Is the "transmit window" the initial congestion window used by some 
> implementations for slow start?  I don't think TCP implementations 
> increase the congestion window beyond the TCP window.
>
> Am I too focused on the TCP standard and am missing the real world 
> implementations?
>
There have been TCP implementations that ignored (or at least violated) 
the receive window. For a while this was a favourite way to win 
benchmarks, held under controlled conditions where the congestion 
control was never triggered - and the ability to get more in-flight data 
was an advantage.

But I think the main issue is that the sender can configure his maximum 
send window, and this can be lower than the remote system indicates he 
is willing to receive.

-- 
Alex Tweedly       http://www.tweedly.net



-- 
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.322 / Virus Database: 267.3.0 - Release Date: 30/05/2005