Commit 4886fcad authored by Gerrit Renker's avatar Gerrit Renker Committed by David S. Miller
Browse files

dccp ccid-2: Share TCP's minimum RTO code 



Using a fixed RTO_MIN of 0.2 seconds was found to cause problems for CCID-2
over 802.11g: at least once per session there was a spurious timeout. It
helped to then increase the the value of RTO_MIN over this link.

Since the problem is the same as in TCP, this patch makes the solution from
commit "05bb1fad"
       "[TCP]: Allow minimum RTO to be configurable via routing metrics."
available to DCCP.

This avoids reinventing the wheel, so that e.g. the following works in the
expected way now also for CCID-2:

> ip route change 10.0.0.2 rto_min 800 dev ath0

Luckily this useful rto_min function was recently moved to net/tcp.h,
which simplifies sharing code originating from TCP.

Documentation also updated (plus minor whitespace fixes).

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 22b71c8f

Documentation/networking/dccp.txt

+20 −6
Original line number Diff line number Diff line 
DCCP protocol

============

=============





Contents

========



- Introduction

- Missing features

- Socket options

- Sysctl variables

- IOCTLs

- Other tunables

- Notes





Introduction

============



Datagram Congestion Control Protocol (DCCP) is an unreliable, connection

oriented protocol designed to solve issues present in UDP and TCP, particularly

for real-time and multimedia (streaming) traffic.
@@ -29,9 +31,9 @@ It has a base protocol and pluggable congestion control IDs (CCIDs). 
DCCP is a Proposed Standard (RFC 2026), and the homepage for DCCP as a protocol

is at http://www.ietf.org/html.charters/dccp-charter.html





Missing features

================



The Linux DCCP implementation does not currently support all the features that are

specified in RFCs 4340...42.
@@ -45,7 +47,6 @@ http://linux-net.osdl.org/index.php/DCCP_Testing#Experimental_DCCP_source_tree 


Socket options

==============



DCCP_SOCKOPT_SERVICE sets the service. The specification mandates use of

service codes (RFC 4340, sec. 8.1.2); if this socket option is not set,

the socket will fall back to 0 (which means that no meaningful service code
@@ -112,6 +113,7 @@ DCCP_SOCKOPT_CCID_TX_INFO 
On unidirectional connections it is useful to close the unused half-connection

via shutdown (SHUT_WR or SHUT_RD): this will reduce per-packet processing costs.





Sysctl variables

================

Several DCCP default parameters can be managed by the following sysctls
@@ -155,15 +157,27 @@ sync_ratelimit = 125 ms 
	sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit

	of this parameter is milliseconds; a value of 0 disables rate-limiting.





IOCTLS

======

FIONREAD

	Works as in udp(7): returns in the `int' argument pointer the size of

	the next pending datagram in bytes, or 0 when no datagram is pending.





Other tunables

==============

Per-route rto_min support

	CCID-2 supports the RTAX_RTO_MIN per-route setting for the minimum value

	of the RTO timer. This setting can be modified via the 'rto_min' option

	of iproute2; for example:

		> 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





Notes

=====



DCCP does not travel through NAT successfully at present on many boxes. This is

because the checksum covers the pseudo-header as per TCP and UDP. Linux NAT

support for DCCP has been added.

net/dccp/ccids/ccid2.c

+3 −2
Original line number Diff line number Diff line
@@ -325,8 +325,9 @@ static void ccid2_rtt_estimator(struct sock *sk, const long mrtt) 
		hc->tx_srtt = m << 3;

		hc->tx_mdev = m << 1;



		hc->tx_mdev_max = max(TCP_RTO_MIN, hc->tx_mdev);

		hc->tx_mdev_max = max(hc->tx_mdev, tcp_rto_min(sk));

		hc->tx_rttvar   = hc->tx_mdev_max;



		hc->tx_rtt_seq  = dccp_sk(sk)->dccps_gss;

	} else {

		/* Update scaled SRTT as SRTT += 1/8 * (m - SRTT) */
@@ -367,7 +368,7 @@ static void ccid2_rtt_estimator(struct sock *sk, const long mrtt) 
				hc->tx_rttvar -= (hc->tx_rttvar -

						  hc->tx_mdev_max) >> 2;

			hc->tx_rtt_seq  = dccp_sk(sk)->dccps_gss;

			hc->tx_mdev_max = TCP_RTO_MIN;

			hc->tx_mdev_max = tcp_rto_min(sk);

		}

	}

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