[DCCP]: Use higher RTO default for CCID3

The TFRC nofeedback timer normally expires after the maximum of 4
RTTs and twice the current send interval (RFC 3448, 4.3). On LANs
with a small RTT this can mean a high processing load and reduced
performance, since then the nofeedback timer is triggered very
frequently.

This patch provides a configuration option to set the bound for the
nofeedback timer, using as default 100 milliseconds.

By setting the configuration option to 0, strict RFC 3448 behaviour
can be enforced for the nofeedback timer.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 13 insertions, 8 deletions

diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index 70ebe705eb75..99807783a22f 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -245,9 +245,10 @@ static void ccid3_hc_tx_no_feedback_timer(unsigned long data)
                 }
                 /*
                  * Schedule no feedback timer to expire in
-                 * max(4 * R, 2 * s/X)  =  max(4 * R, 2 * t_ipi)
+                 * max(t_RTO, 2 * s/X)  =  max(t_RTO, 2 * t_ipi)
+                 * See comments in packet_recv() regarding the value of t_RTO.
                  */
-                t_nfb = max(4 * hctx->ccid3hctx_rtt, 2 * hctx->ccid3hctx_t_ipi);
+                t_nfb = max(hctx->ccid3hctx_t_rto, 2 * hctx->ccid3hctx_t_ipi);
                 break;
         case TFRC_SSTATE_NO_SENT:
                 DCCP_BUG("Illegal %s state NO_SENT, sk=%p", dccp_role(sk), sk);
@@ -512,16 +513,20 @@ static void ccid3_hc_tx_packet_recv(struct sock *sk, struct sk_buff *skb)
                  */
                 sk->sk_write_space(sk);
 
-                /* Update timeout interval. We use the alternative variant of
-                 * [RFC 3448, 3.1] which sets the upper bound of t_rto to one
-                 * second, as it is suggested for TCP (see RFC 2988, 2.4). */
+                /*
+                 * Update timeout interval for the nofeedback timer.
+                 * We use a configuration option to increase the lower bound.
+                 * This can help avoid triggering the nofeedback timer too often
+                 * ('spinning') on LANs with small RTTs.
+                 */
                 hctx->ccid3hctx_t_rto = max_t(u32, 4 * hctx->ccid3hctx_rtt,
-                                                   USEC_PER_SEC            );
+                                                   CONFIG_IP_DCCP_CCID3_RTO *
+                                                   (USEC_PER_SEC/1000)       );
                 /*
                  * Schedule no feedback timer to expire in
-                 * max(4 * R, 2 * s/X)  =  max(4 * R, 2 * t_ipi)
+                 * max(t_RTO, 2 * s/X)  =  max(t_RTO, 2 * t_ipi)
                  */
-                t_nfb = max(4 * hctx->ccid3hctx_rtt, 2 * hctx->ccid3hctx_t_ipi);
+                t_nfb = max(hctx->ccid3hctx_t_rto, 2 * hctx->ccid3hctx_t_ipi);
                         
                 ccid3_pr_debug("%s, sk=%p, Scheduled no feedback timer to "
                                "expire in %lu jiffies (%luus)\n",