linux/net/dccp
Gerrit Renker 231cc2aaf1 dccp ccid-2: Replace broken RTT estimator with better algorithm
The current CCID-2 RTT estimator code is in parts broken and lags behind the
suggestions in RFC2988 of using scaled variants for SRTT/RTTVAR.

That code is replaced by the present patch, which reuses the Linux TCP RTT
estimator code.

Further details:
----------------
 1. The minimum RTO of previously one second has been replaced with TCP's, since
    RFC4341, sec. 5 says that the minimum of 1 sec. (suggested in RFC2988, 2.4)
    is not necessary. Instead, the TCP_RTO_MIN is used, which agrees with DCCP's
    concept of a default RTT (RFC 4340, 3.4).
 2. The maximum RTO has been set to DCCP_RTO_MAX (64 sec), which agrees with
    RFC2988, (2.5).
 3. De-inlined the function ccid2_new_ack().
 4. Added a FIXME: the RTT is sampled several times per Ack Vector, which will
    give the wrong estimate. It should be replaced with one sample per Ack.
    However, at the moment this can not be resolved easily, since
    - it depends on TX history code (which also needs some work),
    - the cleanest solution is not to use the `sent' time at all (saves 4 bytes
      per entry) and use DCCP timestamps / elapsed time to estimated the RTT,
      which however is non-trivial to get right (but needs to be done).

Reasons for reusing the Linux TCP estimator algorithm:
------------------------------------------------------
Some time was spent to find a better alternative, using basic RFC2988 as a first
step. Further analysis and experimentation showed that the Linux TCP RTO
estimator is superior to a basic RFC2988 implementation. A summary is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid2/rto_estimator/

In addition, this estimator fared well in a recent empirical evaluation:

    Rewaskar, Sushant, Jasleen Kaur and F. Donelson Smith.
    A Performance Study of Loss Detection/Recovery in Real-world TCP
    Implementations. Proceedings of 15th IEEE International
    Conference on Network Protocols (ICNP-07), 2007.

Thus there is significant benefit in reusing the existing TCP code.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
2010-08-23 20:13:31 -07:00
..
ccids dccp ccid-2: Replace broken RTT estimator with better algorithm 2010-08-23 20:13:31 -07:00
ackvec.c net: dccp: fix sign bug 2010-07-18 15:07:14 -07:00
ackvec.h dccp: Minimise header option overhead in setting the MPS 2009-03-02 03:07:23 -08:00
ccid.c include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h 2010-03-30 22:02:32 +09:00
ccid.h dccp: fix bug in cache allocation 2010-02-03 19:00:30 -08:00
dccp.h dccp: remove unused function argument 2010-06-25 21:33:14 -07:00
diag.c dccp_diag: LISTEN sockets don't have CCIDs 2008-12-17 16:08:01 -08:00
feat.c include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h 2010-03-30 22:02:32 +09:00
feat.h dccp: Debugging functions for feature negotiation 2009-01-21 14:34:05 -08:00
input.c dccp: make implementation of Syn-RTT symmetric 2010-06-25 21:33:15 -07:00
ipv4.c net-next: remove useless union keyword 2010-06-10 23:31:35 -07:00
ipv6.c ipv6: Refactor update of IPv6 flowi destination address for srcrt (RH) option 2010-06-02 07:08:31 -07:00
ipv6.h
Kconfig dccp: Lockless integration of CCID congestion-control plugins 2009-01-04 21:42:53 -08:00
Makefile dccp: Integrate the TFRC library with DCCP 2009-01-04 21:45:33 -08:00
minisocks.c include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h 2010-03-30 22:02:32 +09:00
options.c dccp: make implementation of Syn-RTT symmetric 2010-06-25 21:33:15 -07:00
output.c net: sock_def_readable() and friends RCU conversion 2010-05-01 15:00:15 -07:00
probe.c include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h 2010-03-30 22:02:32 +09:00
proto.c snmp: add align parameter to snmp_mib_init() 2010-06-25 21:33:17 -07:00
sysctl.c sysctl net: Remove unused binary sysctl code 2009-11-12 02:05:06 -08:00
timer.c net: sk_dst_cache RCUification 2010-04-13 01:41:33 -07:00