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

From: Eric Dumazet <[email-masked]>
Date: Tue, 29 Sep 2009 19:19:54 +0200

Gilad Ben-Yossef a écrit :
> From: Ori Finkalman <[email-masked]>
> 
> 
> Acknowledge TCP window scale support by inserting the proper option in
> SYN/ACK header
> 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.
> 
> 
> 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.
> 
> 
> Please comment and/or apply.
> 
> 
> ---
> 
> 
> Bug reported and patch written by Ori Finkalman from Comsleep Ltd. I'm
> just helping mainline it.
> 
> 
> The behavior was observed with a Windows box as the client and latest
> Debian kernel but for the best
> of my understanding this can happen with latest kernel versions and
> other client OS (probably also Linux)
> 
> as well.
> 
> 
> 
> Signed-off-by: Gilad Ben-Yossef <[email-masked]>
> Signed-off-by: Ori Finkelman <[email-masked]>
> 
> 
> Index: net/ipv4/tcp_output.c
> ===================================================================
> --- net/ipv4/tcp_output.c    (revision 46)
> +++ net/ipv4/tcp_output.c    (revision 210)
> @@ -353,6 +353,7 @@ static void tcp_init_nondata_skb(struct
> #define OPTION_SACK_ADVERTISE    (1 << 0)
> #define OPTION_TS        (1 << 1)
> #define OPTION_MD5        (1 << 2)
> +#define OPTION_WSCALE        (1 << 3)
> 
> struct tcp_out_options {
>     u8 options;        /* bit field of OPTION_* */
> @@ -417,7 +418,7 @@ static void tcp_options_write(__be32 *pt
>                    TCPOLEN_SACK_PERM);
>     }
> 
> -    if (unlikely(opts->ws)) {
> +    if (unlikely(OPTION_WSCALE & opts->options)) {
>         *ptr++ = htonl((TCPOPT_NOP << 24) |
>                    (TCPOPT_WINDOW << 16) |
>                    (TCPOLEN_WINDOW << 8) |
> @@ -530,8 +531,8 @@ static unsigned tcp_synack_options(struc
> 
>     if (likely(ireq->wscale_ok)) {
>         opts->ws = ireq->rcv_wscale;
> -        if(likely(opts->ws))
> -            size += TCPOLEN_WSCALE_ALIGNED;
> +        opts->options |= OPTION_WSCALE;
> +        size += TCPOLEN_WSCALE_ALIGNED;
>     }
>     if (likely(doing_ts)) {
>         opts->options |= OPTION_TS;
> 
> 
> 

Seems not the more logical places to put this logic...

How about this instead ?

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5200aab..b78c084 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -216,6 +216,11 @@ void tcp_select_initial_window(int __space, __u32 mss,
 			space >>= 1;
 			(*rcv_wscale)++;
 		}
+		/*
+		 * Set a minimum wscale of 1
+		 */
+		if (*rcv_wscale == 0)
+			*rcv_wscale = 1;
        }

        /* Set initial window to value enough for senders,