From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <shemminger@vyatta.com>
Subject: Re: [PATCH] Add sysctl to set the advertised TCP initial receive
 window.
Date: Thu, 10 Dec 2009 08:57:19 -0800
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On Wed, 09 Dec 2009 18:05:42 -0800
chavey@google.com wrote:

> @@ -221,6 +222,7 @@ struct tcp_options_received {
>  	u8	num_sacks;	/* Number of SACK blocks		*/
>  	u16	user_mss;  	/* mss requested by user in ioctl */
>  	u16	mss_clamp;	/* Maximal mss, negotiated at connection setup */
> +	u8	init_rcv_wnd;	/* TCP initial receive window in MSS */
>  };

You could use the hole in the structure:

                snd_wscale : 4, /* Window scaling received from sender  */
                rcv_wscale : 4; /* Window scaling to send to receiver   */
+	u8	init_rcv_wnd;	/* TCP initial receive window in MSS */
/*      SACKs data      */
        u8      num_sacks;      /* Number of SACK blocks