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If a TCP connection has been idle for one retransmit timeout or more

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it must reset its congestion window back to the initial window.

RFC3390 has increased the initial window from 1 segment to up to
4 segments.

The initial window increase of RFC3390 wasn't reflected into the
restart window which remained at its original defaults of 4 segments
for local and 1 segment for all other connections.  Both values are
controllable through sysctl net.inet.tcp.local_slowstart_flightsize
and net.inet.tcp.slowstart_flightsize.

The increase helps TCP's slow start algorithm to open up the congestion
window much faster.

Reviewed by:	lstewart
MFC after:	1 week
This commit is contained in:
Andre Oppermann 2010-08-18 18:05:54 +00:00
parent 3ed1008b89
commit c3f0bdc66b
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=211464
2 changed files with 28 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

41
sys/netinet/tcp_output.c
View File

@ -140,7 +140,7 @@ tcp_output(struct tcpcb *tp)
{
struct socket *so = tp->t_inpcb->inp_socket;
long len, recwin, sendwin;
int off, flags, error;
int off, flags, error, rw;
struct mbuf *m;
struct ip *ip = NULL;
struct ipovly *ipov = NULL;
@ -176,23 +176,34 @@ tcp_output(struct tcpcb *tp)
idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
if (idle && ticks - tp->t_rcvtime >= tp->t_rxtcur) {
/*
* We have been idle for "a while" and no acks are
* expected to clock out any data we send --
* slow start to get ack "clock" running again.
* If we've been idle for more than one retransmit
* timeout the old congestion window is no longer
* current and we have to reduce it to the restart
* window before we can transmit again.
*
* Set the slow-start flight size depending on whether
* this is a local network or not.
* The restart window is the initial window or the last
* CWND, whichever is smaller.
*
* This is done to prevent us from flooding the path with
* a full CWND at wirespeed, overloading router and switch
* buffers along the way.
*
* See RFC5681 Section 4.1. "Restarting Idle Connections".
*/
int ss = V_ss_fltsz;
if (V_tcp_do_rfc3390)
rw = min(4 * tp->t_maxseg,
max(2 * tp->t_maxseg, 4380));
#ifdef INET6
if (isipv6) {
if (in6_localaddr(&tp->t_inpcb->in6p_faddr))
ss = V_ss_fltsz_local;
} else
#endif /* INET6 */
if (in_localaddr(tp->t_inpcb->inp_faddr))
ss = V_ss_fltsz_local;
tp->snd_cwnd = tp->t_maxseg * ss;
else if ((isipv6 ? in6_localaddr(&tp->t_inpcb->in6p_faddr) :
in_localaddr(tp->t_inpcb->inp_faddr)))
#else
else if (in_localaddr(tp->t_inpcb->inp_faddr))
#endif
rw = V_ss_fltsz_local * tp->t_maxseg;
else
rw = V_ss_fltsz * tp->t_maxseg;
tp->snd_cwnd = min(rw, tp->snd_cwnd);
}
tp->t_flags &= ~TF_LASTIDLE;
if (idle) {

2
sys/netinet/tcp_var.h
View File

@ -565,6 +565,7 @@ extern int tcp_log_in_vain;
VNET_DECLARE(int, tcp_mssdflt); /* XXX */
VNET_DECLARE(int, tcp_minmss);
VNET_DECLARE(int, tcp_delack_enabled);
VNET_DECLARE(int, tcp_do_rfc3390);
VNET_DECLARE(int, tcp_do_newreno);
VNET_DECLARE(int, path_mtu_discovery);
VNET_DECLARE(int, ss_fltsz);
@ -575,6 +576,7 @@ VNET_DECLARE(int, ss_fltsz_local);
#define V_tcp_mssdflt VNET(tcp_mssdflt)
#define V_tcp_minmss VNET(tcp_minmss)
#define V_tcp_delack_enabled VNET(tcp_delack_enabled)
#define V_tcp_do_rfc3390 VNET(tcp_do_rfc3390)
#define V_tcp_do_newreno VNET(tcp_do_newreno)
#define V_path_mtu_discovery VNET(path_mtu_discovery)
#define V_ss_fltsz VNET(ss_fltsz)