IPv4 TCP fails to send window scale option when window scale is zero
Subsystems (3)
PATCH v2 1/1 IPv4 TCP fails to send window scale option when window scale is zero
From: Ori Finkelman <[email protected]>
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.
---
Original bug reported and patch written by Ori Finkelman from Comsleep Ltd. I've fixed the SYN header case based on feedback from Eric Dumazet and Ilpo Jarvinen, as part of trying to get the patch mainlined.
The SYN/ACK 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.
The SYN/ACK scenario was tested on a x86 system. The SYN sceanrio was only compile tested.
net/ipv4/tcp_output.c +0 -0
@@ -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;Signature · 17 lines
Gilad Ben-Yossef Chief Coffee Drinker & CTO Codefidence Ltd. Web: http://codefidence.com Cell: +972-52-8260388 Skype: gilad_codefidence Tel: +972-8-9316883 ext. 201 Fax: +972-8-9316884 Email: [email protected] Check out our Open Source technology and training blog - http://tuxology.net "Now the world has gone to bed Darkness won't engulf my head I can see by infra-red How I hate the night."
net/ipv4/tcp_output.c static inline int tcp_urg_mode(const struct tcp_sock · line 365
Quoted 12 lines · Gilad Ben-Yossef
From: Ori Finkelman <[email protected]>
net/ipv4/tcp_output.c +1
-0@@ -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)
I manually applied your patch and tested it.
So far so good, it works well.
But you'll need to find correct way to submit a patch so that your mailer doesnt mangle the content.
File Documentation/email-clients.txt contains useful tips.
Quoted 18 lines · Eric Dumazet
net/ipv4/tcp_output.c static inline int tcp_urg_mode(const struct tcp_sock · line 366
Quoted 13 lines · Gilad Ben-Yossef
From: Ori Finkelman <[email protected]>
net/ipv4/tcp_output.c +1
-0@@ -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)I manually applied your patch and tested it.
Quoted 2 lines
So far so good, it works well.
Glad to hear. Thank you both Eric and Ilpo for the review.
Quoted 7 lines
But you'll need to find correct way to submit a patch so that your mailer doesnt mangle the content.
File
Documentation/email-clients.txtcontains useful tips.
Arrggghh... I thought I have subdued Thunderbird but it tricked me. My prefs.js got re-written somehow. My sincere apologies. This is not my week with MUA.
I also noticed I put the signed-off-by in the wrong place...
Next email is version 3 - same content, hopefully correct formatting.
Thanks again.
Gilad
Signature · 17 lines
Gilad Ben-Yossef Chief Coffee Drinker & CTO Codefidence Ltd. Web: http://codefidence.com Cell: +972-52-8260388 Skype: gilad_codefidence Tel: +972-8-9316883 ext. 201 Fax: +972-8-9316884 Email: [email protected] Check out our Open Source technology and training blog - http://tuxology.net "Now the world has gone to bed Darkness won't engulf my head I can see by infra-red How I hate the night."