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Increase the initial CWND to 10 segments as defined in IETF TCPM

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draft-ietf-tcpm-initcwnd-05. It explains why the increased initial
window improves the overall performance of many web services without
risking congestion collapse.

As long as it remains a draft it is placed under a sysctl marking it
as experimental:
 net.inet.tcp.experimental.initcwnd10 = 1
When it becomes an official RFC soon the sysctl will be changed to
the RFC number and moved to net.inet.tcp.

This implementation differs from the RFC draft in that it is a bit
more conservative in the case of packet loss on SYN or SYN|ACK because
we haven't reduced the default RTO to 1 second yet.  Also the restart
window isn't yet increased as allowed.  Both will be adjusted with
upcoming changes.

Is is enabled by default.  In Linux it is enabled since kernel 3.0.

MFC after:	2 weeks
This commit is contained in:
Andre Oppermann 2012-10-28 19:47:46 +00:00
parent 05d43d9882
commit 09440655fe
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=242266
2 changed files with 14 additions and 0 deletions
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12
sys/netinet/tcp_input.c
View File

@ -159,6 +159,14 @@ SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, rfc3390, CTLFLAG_RW,
&VNET_NAME(tcp_do_rfc3390), 0,
"Enable RFC 3390 (Increasing TCP's Initial Congestion Window)");
SYSCTL_NODE(_net_inet_tcp, OID_AUTO, experimental, CTLFLAG_RW, 0,
"Experimental TCP extensions");
VNET_DEFINE(int, tcp_do_initcwnd10) = 1;
SYSCTL_VNET_INT(_net_inet_tcp_experimental, OID_AUTO, initcwnd10, CTLFLAG_RW,
&VNET_NAME(tcp_do_initcwnd10), 0,
"Enable draft-ietf-tcpm-initcwnd-05 (Increasing initial CWND to 10)");
VNET_DEFINE(int, tcp_do_rfc3465) = 1;
SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, rfc3465, CTLFLAG_RW,
&VNET_NAME(tcp_do_rfc3465), 0,
@ -347,6 +355,7 @@ cc_conn_init(struct tcpcb *tp)
*
* RFC5681 Section 3.1 specifies the default conservative values.
* RFC3390 specifies slightly more aggressive values.
* Draft-ietf-tcpm-initcwnd-05 increases it to ten segments.
*
* If a SYN or SYN/ACK was lost and retransmitted, we have to
* reduce the initial CWND to one segment as congestion is likely
@ -354,6 +363,9 @@ cc_conn_init(struct tcpcb *tp)
*/
if (tp->snd_cwnd == 1)
tp->snd_cwnd = tp->t_maxseg; /* SYN(-ACK) lost */
else if (V_tcp_do_initcwnd10)
tp->snd_cwnd = min(10 * tp->t_maxseg,
max(2 * tp->t_maxseg, 14600));
else if (V_tcp_do_rfc3390)
tp->snd_cwnd = min(4 * tp->t_maxseg,
max(2 * tp->t_maxseg, 4380));

2
sys/netinet/tcp_var.h
View File

@ -611,6 +611,7 @@ 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_initcwnd10);
VNET_DECLARE(int, tcp_sendspace);
VNET_DECLARE(int, tcp_recvspace);
VNET_DECLARE(int, path_mtu_discovery);
@ -623,6 +624,7 @@ VNET_DECLARE(int, tcp_abc_l_var);
#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_initcwnd10 VNET(tcp_do_initcwnd10)
#define V_tcp_sendspace VNET(tcp_sendspace)
#define V_tcp_recvspace VNET(tcp_recvspace)
#define V_path_mtu_discovery VNET(path_mtu_discovery)