Re: initial congestion window for connections in the listen queue

[David Miller <davem@davemloft.net>]

From: Octavian Purdila <opurdila@ixiacom.com>
Date: Mon, 13 Apr 2009 23:46:15 +0300

> 
> A question for the TCP wizards:
> 
>>struct sock *tcp_create_openreq_child(struct sock *sk, 
>>        struct request_sock >*req, struct sk_buff *skb)
>>{
>>...
>>         /* So many TCP implementations out there (incorrectly) count the
>>          * initial SYN frame in their delayed-ACK and congestion control
>>          * algorithms that we must have the following bandaid to talk 
>>          * efficiently to them.  -DaveM
>>          */
>>          newtp->snd_cwnd = 2;
> 
> Shouldn't the same logic from tcp_init_cwnd() be used here?
> 
>>From my traces, this seems to prevent TSO from helping short lived 
> connections.

On any standard ethernet MTU or larger, you should be getting
an initial CWND of 3 or 4 because of the logic in tcp_init_cwnd().

Don't just guesstimate what initial ->snd_cwnd value the
kernel is using by looking casually at tcpdump traces.  Add
some kernel debugging printk's and find out for sure.