Commit 89858ad1 authored by Gerrit Renker's avatar Gerrit Renker Committed by David S. Miller
Browse files

dccp ccid-3: use per-route RTO or TCP RTO as fallback 



This makes RTAX_RTO_MIN also available to CCID-3, replacing the compile-time
RTO lower bound with a per-route tunable value.

The original Kconfig option solved the problem that a very low RTT (in the
order of HZ) can trigger too frequent and unnecessary reductions of the
sending rate.

This tunable does not affect the initial RTO value of 2 seconds specified in
RFC 5348, section 4.2 and Appendix B. But like the hardcoded Kconfig value,
it allows to adapt to network conditions.

The same effect as the original Kconfig option of 100ms is now achieved by

> ip route replace to unicast 192.168.0.0/24 rto_min 100j dev eth0

(assuming HZ=1000).

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 4886fcad

Documentation/networking/dccp.txt

+3 −0
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@@ -174,6 +174,9 @@ Per-route rto_min support 
		> ip route change 10.0.0.0/24   rto_min 250j dev wlan0

		> ip route add    10.0.0.254/32 rto_min 800j dev wlan0

		> ip route show dev wlan0

	CCID-3 also supports the rto_min setting: it is used to define the lower

	bound for the expiry of the nofeedback timer. This can be useful on LANs

	with very low RTTs (e.g., loopback, Gbit ethernet).





Notes

net/dccp/ccids/Kconfig

+0 −31
Original line number Diff line number Diff line
@@ -47,37 +47,6 @@ config IP_DCCP_CCID3_DEBUG 


	  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.



config IP_DCCP_TFRC_LIB

	def_bool y if IP_DCCP_CCID3

net/dccp/ccids/ccid3.c

+5 −6
Original line number Diff line number Diff line
@@ -460,13 +460,12 @@ done_computing_x: 
	sk->sk_write_space(sk);



	/*

	 * 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.

	 * Update timeout interval for the nofeedback timer. In order to control

	 * rate halving on networks with very low RTTs (<= 1 ms), use per-route

	 * tunable RTAX_RTO_MIN value as the lower bound.

	 */

	hc->tx_t_rto = max_t(u32, 4 * hc->tx_rtt, (CONFIG_IP_DCCP_CCID3_RTO *

						       (USEC_PER_SEC / 1000)));

	hc->tx_t_rto = max_t(u32, 4 * hc->tx_rtt,

				  USEC_PER_SEC/HZ * tcp_rto_min(sk));

	/*

	 * Schedule no feedback timer to expire in

	 * max(t_RTO, 2 * s/X)  =  max(t_RTO, 2 * t_ipi)

net/dccp/ccids/ccid3.h

+1 −1
Original line number Diff line number Diff line
@@ -42,7 +42,7 @@ 
#include "lib/tfrc.h"

#include "../ccid.h"



/* Two seconds as per RFC 3448 4.2 */

/* Two seconds as per RFC 5348, 4.2 */

#define TFRC_INITIAL_TIMEOUT	   (2 * USEC_PER_SEC)



/* In usecs - half the scheduling granularity as per RFC3448 4.6 */

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