Re: TCP packet size and delivery packet decisions

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 06 septembre 2010 à 22:30 -0700, David Miller a écrit :


> The small 78 byte window is why the sending system is splitting up the
> writes into smaller pieces.
> 
> I presume that the system advertises exactly a 78 byte window because
> this is how large the commands are.  But this is an extremely foolish
> and baroque thing to do, and it's why you are having problems.

I am not sure why TSO added a "Bound mss with half of window"
requirement for tcp_sync_mss()

I tried with MSS=1000 and WIN=1000, and segment size chosen is 500

With WIN=78, 78/2->39 is then capped to 48 (68U - tp->tcp_header_len)

Is there a hard requirement about segment size being at most half the
window ?

Following patch solves the problem for me :

[PATCH] tcp: bound mss to window in tcp_sync_mss()

Leandro Melo de Sales noticed that if a peer announces a very small
initial tcp window (78 in his case), first sent frames have unnecessary
small lengths (48 in his case)

CLNT->SRV    [SYN] Seq=0 Win=5840 Len=0 MSS=1460
SRV->CLNT    [SYN, ACK] Seq=0 Ack=1 Win=78 Len=0 MSS=78
CLNT->SRV    [ACK] Seq=1 Ack=1 Win=5840 Len=0
CLNT->SRV    [PSH, ACK] Seq=1 Ack=1 Win=5840 Len=48
CLNT->SRV    [PSH, ACK] Seq=49 Ack=1 Win=5840 Len=30
SRV->CLNT    [ACK] Seq=1 Ack=49 Win=78 Len=0
SRV->CLNT    [RST, ACK] Seq=1 Ack=79 Win=78 Len=0

tcp_sync_mss() bounds mss to half the window, while it could use full
window:

CLNT->SRV    [SYN] Seq=0 Win=5840 Len=0 MSS=1460
SRV->CLNT    [SYN, ACK] Seq=0 Ack=1 Win=78 Len=0 MSS=78
CLNT->SRV    [ACK] Seq=1 Ack=1 Win=5840 Len=0
CLNT->SRV    [PSH, ACK] Seq=1 Ack=1 Win=5840 Len=78
SRV->CLNT    [ACK] Seq=1 Ack=79 Win=78 Len=0

Reported-by: ツ Leandro Melo de Sales <leandroal@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
---
 include/net/tcp.h     |    9 +++++++++
 net/ipv4/tcp_output.c |    2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index eaa9582..c262676 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -481,6 +481,15 @@ static inline int tcp_bound_to_half_wnd(struct tcp_sock *tp, int pktsize)
 		return pktsize;
 }
 
+/* Bound MSS / TSO packet size with the window */
+static inline int tcp_bound_to_wnd(struct tcp_sock *tp, int pktsize)
+{
+	if (tp->max_window && pktsize > tp->max_window)
+		return max(tp->max_window, 68U - tp->tcp_header_len);
+	else
+		return pktsize;
+}
+
 /* tcp.c */
 extern void tcp_get_info(struct sock *, struct tcp_info *);
 
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index de3bd84..49cdbe4 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1224,7 +1224,7 @@ unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
 		icsk->icsk_mtup.search_high = pmtu;
 
 	mss_now = tcp_mtu_to_mss(sk, pmtu);
-	mss_now = tcp_bound_to_half_wnd(tp, mss_now);
+	mss_now = tcp_bound_to_wnd(tp, mss_now);
 
 	/* And store cached results */
 	icsk->icsk_pmtu_cookie = pmtu;