Subject: Re: [PATCHv3] IPv4 TCP fails to send window scale option when window scale is zero

From: Eric Dumazet <[email-masked]>
Date: Thu, 01 Oct 2009 19:20:33 +0200

Gilad Ben-Yossef a écrit :
> From: Ori Finkelman <[email-masked]>
> 
> Acknowledge TCP window scale support by inserting the proper option in SYN/ACK 
> and SYN headers even if our window scale is zero.
> 
> This fixes the following observed behavior:
> 
> 1. Client sends a SYN with TCP window scaling option and non zero window scale 
> value to a Linux box.
> 2. Linux box notes large receive window from client.
> 3. Linux decides on a zero value of window scale for its part.
> 4. Due to compare against requested window scale size option, Linux does not to
>  send windows scale TCP option header on SYN/ACK at all.
> 
> With the following result:
> 
> Client box thinks TCP window scaling is not supported, since SYN/ACK had no 
> TCP window scale option, while Linux thinks that TCP window scaling is 
> supported (and scale might be non zero), since SYN had  TCP window scale 
> option and we have a mismatched idea between the client and server 
> regarding window sizes.
> 
> Probably it also fixes up the following bug (not observed in practice):
> 
> 1. Linux box opens TCP connection to some server.
> 2. Linux decides on zero value of window scale.
> 3. Due to compare against computed window scale size option, Linux does 
> not to set windows scale TCP  option header on SYN. 
> 
> With the expected result that the server OS does not use window scale option 
> due to not receiving such an option in the SYN headers, leading to suboptimal 
> performance.
> 
> Signed-off-by: Gilad Ben-Yossef <[email-masked]>
> Signed-off-by: Ori Finkelman <[email-masked]>

Acked-by: Eric Dumazet <[email-masked]>

Note, to reproduce the wscale set to 0 on active connections,
you have to play with /proc/sys/net settings :

echo 65535 >/proc/sys/net/core/rmem_max
echo "4096 16384 32768" >/proc/sys/net/ipv4/tcp_rmem

-> wscale 0 -> SYN packet without WSCALE option (on non patched kernels)