2 files changed, 46 insertions, 8 deletions

diff --git a/net/dccp/ccids/Kconfig b/net/dccp/ccids/Kconfig
index dac89166eb18..80f469887691 100644
--- a/net/dccp/ccids/Kconfig
+++ b/net/dccp/ccids/Kconfig
@@ -89,4 +89,37 @@ config IP_DCCP_CCID3_DEBUG
             parameter to 0 or 1.
 
             If in doubt, say N.
+
+config IP_DCCP_CCID3_RTO
+          int "Use higher bound for nofeedback timer"
+          default 100
+          depends on IP_DCCP_CCID3 && EXPERIMENTAL
+          ---help---
+            Use higher lower bound for nofeedback timer expiration.
+
+            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 option enables to set a higher lower bound for the nofeedback
+            value. Values in units of milliseconds can be set here.
+
+            A value of 0 disables this feature by enforcing the value specified
+            in RFC 3448. The following values have been suggested as bounds for
+            experimental use:
+                * 16-20ms to match the typical multimedia inter-frame interval
+                * 100ms as a reasonable compromise [default]
+                * 1000ms corresponds to the lower TCP RTO bound (RFC 2988, 2.4)
+
+            The default of 100ms is a compromise between a large value for
+            efficient DCCP implementations, and a small value to avoid disrupting
+            the network in times of congestion.
+
+            The purpose of the nofeedback timer is to slow DCCP down when there
+            is serious network congestion: experimenting with larger values should
+            therefore not be performed on WANs.
+
+
 endmenu
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",