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-rw-r--r--net/dccp/ccids/Kconfig33
-rw-r--r--net/dccp/ccids/ccid3.c21
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",