dccp tfrc: Increase number of RTT samples

This improves the receiver RTT sampling algorithm so that it tries harder to get
as many RTT samples as possible. 

The algorithm is based the concepts presented in RFC 4340, 8.1, using timestamps
and the CCVal window counter. There exist 4 cases for the CCVal difference:
 * == 0: less than RTT/4 passed since last packet -- unusable;
 *  > 4: (much) more than 1 RTT has passed since last packet -- also unusable;
 * == 4: perfect sample (exactly one RTT has passed since last packet);
 * 1..3: sub-optimal sample (between RTT/4 and 3*RTT/4 has passed).

In the last case the algorithm tried to optimise by storing away the candidate
and then re-trying next time. The problem is that
 * a large number of samples is needed to smooth out the inaccuracies of the
   algorithm;
 * the sender may not be sending enough packets to warrant a "next time";
 * hence it is better to use suboptimal samples whenever possible.
The algorithm now stores away the current sample only if the difference is 0.

Applicability and background
----------------------------
A realistic example is MP3 streaming where packets are sent at a rate of less
than one packet per RTT, which means that suitable samples are absent for a
very long time.

The effectiveness of using suboptimal samples (with a delta between 1 and 4) was
confirmed by instrumenting the algorithm with counters. The results of two 20
second test runs were:
 * With the old algorithm and a total of 38442 function calls, only 394 of these
   calls resulted in usable RTT samples (about 1%), and 378 out of these were
   "perfect" samples and 28013 (unused) samples had a delta of 1..3.
 * With the new algorithm and a total of 37057 function calls, 1702 usable RTT
   samples were retrieved (about 4.6%), 5 out of these were "perfect" samples.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

net/dccp/ccids/lib/packet_history.c

@@ -427,32 +427,17 @@ int tfrc_rx_hist_init(struct tfrc_rx_hist *h, struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_init);
 
-/**
- * tfrc_rx_hist_rtt_last_s - reference entry to compute RTT samples against
- */
-static inline struct tfrc_rx_hist_entry *
-			tfrc_rx_hist_rtt_last_s(const struct tfrc_rx_hist *h)
-{
-	return h->ring[0];
-}
-
-/**
- * tfrc_rx_hist_rtt_prev_s: previously suitable (wrt rtt_last_s) RTT-sampling entry
- */
-static inline struct tfrc_rx_hist_entry *
-			tfrc_rx_hist_rtt_prev_s(const struct tfrc_rx_hist *h)
-{
-	return h->ring[h->rtt_sample_prev];
-}
-
 /**
  * tfrc_rx_hist_sample_rtt  -  Sample RTT from timestamp / CCVal
- * Based on ideas presented in RFC 4342, 8.1. Returns 0 if it was not able
- * to compute a sample with given data - calling function should check this.
+ * Based on ideas presented in RFC 4342, 8.1. This function expects that no loss
+ * is pending and uses the following history entries (via rtt_sample_prev):
+ * - h->ring[0]  contains the most recent history entry prior to @skb;
+ * - h->ring[1]  is an unused `dummy' entry when the current difference is 0;
  */
 void tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
 {
-	u32 sample = 0, delta_v;
+	struct tfrc_rx_hist_entry *last = h->ring[0];
+	u32 sample, delta_v;
 
 	/*
 	 * When not to sample:
@@ -466,47 +451,27 @@ void tfrc_rx_hist_sample_rtt(struct tfrc_rx_hist *h, const struct sk_buff *skb)
 	    tfrc_rx_hist_loss_pending(h))
 		return;
 
-	delta_v = SUB16(dccp_hdr(skb)->dccph_ccval,
-			tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
-
-	if (delta_v < 1 || delta_v > 4) {	/* unsuitable CCVal delta */
-		if (h->rtt_sample_prev == 2) {	/* previous candidate stored */
-			sample = SUB16(tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_ccval,
-				       tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
-			if (sample)
-				sample = 4 / sample *
-				         ktime_us_delta(tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_tstamp,
-							tfrc_rx_hist_rtt_last_s(h)->tfrchrx_tstamp);
-			else    /*
-				 * FIXME: This condition is in principle not
-				 * possible but occurs when CCID is used for
-				 * two-way data traffic. I have tried to trace
-				 * it, but the cause does not seem to be here.
-				 */
-				DCCP_BUG("please report to dccp@vger.kernel.org"
-					 " => prev = %u, last = %u",
-					 tfrc_rx_hist_rtt_prev_s(h)->tfrchrx_ccval,
-					 tfrc_rx_hist_rtt_last_s(h)->tfrchrx_ccval);
-		} else if (delta_v < 1) {
-			h->rtt_sample_prev = 1;
-			goto keep_ref_for_next_time;
-		}
-
-	} else if (delta_v == 4) /* optimal match */
-		sample = ktime_to_us(net_timedelta(tfrc_rx_hist_rtt_last_s(h)->tfrchrx_tstamp));
-	else {			 /* suboptimal match */
-		h->rtt_sample_prev = 2;
-		goto keep_ref_for_next_time;
-	}
+	h->rtt_sample_prev = 0;		/* reset previous candidate */
 
-	if (unlikely(sample > DCCP_SANE_RTT_MAX)) {
-		DCCP_WARN("RTT sample %u too large, using max\n", sample);
-		sample = DCCP_SANE_RTT_MAX;
+	delta_v = SUB16(dccp_hdr(skb)->dccph_ccval, last->tfrchrx_ccval);
+	if (delta_v == 0) {		/* less than RTT/4 difference */
+		h->rtt_sample_prev = 1;
+		return;
 	}
+	sample = dccp_sane_rtt(ktime_to_us(net_timedelta(last->tfrchrx_tstamp)));
 
-	h->rtt_sample_prev = 0;	       /* use current entry as next reference */
-keep_ref_for_next_time:
+	if (delta_v <= 4)		/* between RTT/4 and RTT */
+		sample *= 4 / delta_v;
+	else if (!(sample < h->rtt_estimate && sample > h->rtt_estimate/2))
+		/*
+		* Optimisation: CCVal difference is greater than 1 RTT, yet the
+		* sample is less than the local RTT estimate; which means that
+		* the RTT estimate is too high.
+		* To avoid noise, it is not done if the sample is below RTT/2.
+		*/
+		return;
 
-	h->rtt_estimate = tfrc_ewma(h->rtt_estimate, sample, 9);
+	/* Use a lower weight than usual to increase responsiveness */
+	h->rtt_estimate = tfrc_ewma(h->rtt_estimate, sample, 5);
 }
 EXPORT_SYMBOL_GPL(tfrc_rx_hist_sample_rtt);