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>

1 files changed, 15 insertions, 5 deletions

diff --git a/net/dccp/ccids/ccid2.h b/net/dccp/ccids/ccid2.h
index 1ec6a30..b017843 100644
--- a/net/dccp/ccids/ccid2.h
+++ b/net/dccp/ccids/ccid2.h
@@ -42,7 +42,12 @@ struct ccid2_seq {
  * struct ccid2_hc_tx_sock - CCID2 TX half connection
  * @tx_{cwnd,ssthresh,pipe}: as per RFC 4341, section 5
  * @tx_packets_acked:	     Ack counter for deriving cwnd growth (RFC 3465)
- * @tx_lastrtt:		     time RTT was last measured
+ * @tx_srtt:		     smoothed RTT estimate, scaled by 2^3
+ * @tx_mdev:		     smoothed RTT variation, scaled by 2^2
+ * @tx_mdev_max:	     maximum of @mdev during one flight
+ * @tx_rttvar:		     moving average/maximum of @mdev_max
+ * @tx_rto:		     RTO value deriving from SRTT and RTTVAR (RFC 2988)
+ * @tx_rtt_seq:		     to decay RTTVAR at most once per flight
  * @tx_rpseq:		     last consecutive seqno
  * @tx_rpdupack:	     dupacks since rpseq
  */
@@ -55,11 +60,16 @@ struct ccid2_hc_tx_sock {
 	int			tx_seqbufc;
 	struct ccid2_seq	*tx_seqh;
 	struct ccid2_seq	*tx_seqt;
-	long			tx_rto;
-	long			tx_srtt;
-	long			tx_rttvar;
-	unsigned long		tx_lastrtt;
+
+	/* RTT measurement: variables/principles are the same as in TCP */
+	u32			tx_srtt,
+				tx_mdev,
+				tx_mdev_max,
+				tx_rttvar,
+				tx_rto;
+	u64			tx_rtt_seq:48;
 	struct timer_list	tx_rtotimer;
+
 	u64			tx_rpseq;
 	int			tx_rpdupack;
 	unsigned long		tx_last_cong;