From 89e95a613c8a045ce0c5b992ba19f10613f6ab2f Mon Sep 17 00:00:00 2001 From: Ori Finkelman Date: Thu, 1 Oct 2009 06:41:59 +0000 Subject: [PATCH] IPv4 TCP fails to send window scale option when window scale is zero 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 Signed-off-by: Ori Finkelman Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_output.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 5200aab0ca97..fcd278a7080e 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -361,6 +361,7 @@ static inline int tcp_urg_mode(const struct tcp_sock *tp) #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_* */ @@ -427,7 +428,7 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp, TCPOLEN_SACK_PERM); } - if (unlikely(opts->ws)) { + if (unlikely(OPTION_WSCALE & opts->options)) { *ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_WINDOW << 16) | (TCPOLEN_WINDOW << 8) | @@ -494,8 +495,8 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb, } if (likely(sysctl_tcp_window_scaling)) { opts->ws = tp->rx_opt.rcv_wscale; - if (likely(opts->ws)) - size += TCPOLEN_WSCALE_ALIGNED; + opts->options |= OPTION_WSCALE; + size += TCPOLEN_WSCALE_ALIGNED; } if (likely(sysctl_tcp_sack)) { opts->options |= OPTION_SACK_ADVERTISE; @@ -537,8 +538,8 @@ static unsigned tcp_synack_options(struct sock *sk, 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; -- 2.39.5