tcp: use RFC6298 compliant TCP RTO calculation

This patch adjusts Linux RTO calculation to be RFC6298 Standard
compliant. MinRTO is no longer added to the computed RTO, RTO damping
and overestimation are decreased.

In RFC 6298 Standard TCP Retransmission Timeout (RTO) calculation the
calculated RTO is rounded up to the Minimum RTO (MinRTO), if it is
less.  The Linux implementation as a discrepancy to the Standard
basically adds the defined MinRTO to the calculated RTO. When
comparing both approaches, the Linux calculation seems to perform
worse for sender limited TCP flows like Telnet, SSH or constant bit
rate encoded transmissions, especially for Round Trip Times (RTT) of
50ms to 800ms.

Compared to the Linux implementation the RFC 6298 proposed RTO
calculation performs better and more precise in adapting to current
network characteristics. Extensive measurements for bulk data did not
show a negative impact of the adjusted calculation.

Using the RFC 6298 compliant implementation, the tcp_sock struct variable
u32 mdev_max_us becomes obsolete and consequently is being removed.

Exemplary performance comparison for sender-limited-flows:

- Rate: 10Mbit/s
- Delay: 200ms, Delay Variation: 10ms
- Time between each scheduled segment: 1s
- Amount Data Segments: 300
- Mean of 11 runs

         Mean Response Waiting Time [milliseconds]

PER [%] |   0.5      1    1.5      2      3      5      7     10
--------+-------------------------------------------------------
old     | 206.4  208.6  218.0  218.6  227.2  249.3  274.7  308.2
new     | 203.9  206.0  207.0  209.9  217.3  225.6  238.7  259.1

Detailed analysis:

https://docs.google.com/document/d/1pKmPfnQb6fDK4qpiNVkN8cQyGE4wYDZukcuZfR-BnnM/

Reasoning for historic design:

Sarolahti, P.; Kuznetsov, A. (2002). Congestion Control in Linux TCP.
Conference Paper. Proceedings of the FREENIX Track. 2002 USENIX Annual
https://www.cs.helsinki.fi/research/iwtcp/papers/linuxtcp.pdf

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: Daniel Metz <dmetz@mytum.de>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>

include/linux/tcp.h

@@ -231,7 +231,6 @@ struct tcp_sock {
 /* RTT measurement */
 	u32	srtt_us;	/* smoothed round trip time << 3 in usecs */
 	u32	mdev_us;	/* medium deviation			*/
-	u32	mdev_max_us;	/* maximal mdev for the last rtt period	*/
 	u32	rttvar_us;	/* smoothed mdev_max			*/
 	u32	rtt_seq;	/* sequence number to update rttvar	*/
 	struct rtt_meas {

net/ipv4/tcp_input.c

@@ -680,8 +680,7 @@ static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
 /* Called to compute a smoothed rtt estimate. The data fed to this
  * routine either comes from timestamps, or from segments that were
  * known _not_ to have been retransmitted [see Karn/Partridge
- * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
- * piece by Van Jacobson.
+ * Proceedings SIGCOMM 87].
  * NOTE: the next three routines used to be one big routine.
  * To save cycles in the RFC 1323 implementation it was better to break
  * it up into three procedures. -- erics
@@ -692,59 +691,21 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
 	long m = mrtt_us; /* RTT */
 	u32 srtt = tp->srtt_us;
 
-	/*	The following amusing code comes from Jacobson's
-	 *	article in SIGCOMM '88.  Note that rtt and mdev
-	 *	are scaled versions of rtt and mean deviation.
-	 *	This is designed to be as fast as possible
-	 *	m stands for "measurement".
-	 *
-	 *	On a 1990 paper the rto value is changed to:
-	 *	RTO = rtt + 4 * mdev
-	 *
-	 * Funny. This algorithm seems to be very broken.
-	 * These formulae increase RTO, when it should be decreased, increase
-	 * too slowly, when it should be increased quickly, decrease too quickly
-	 * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
-	 * does not matter how to _calculate_ it. Seems, it was trap
-	 * that VJ failed to avoid. 8)
-	 */
 	if (srtt != 0) {
-		m -= (srtt >> 3);	/* m is now error in rtt est */
-		srtt += m;		/* rtt = 7/8 rtt + 1/8 new */
-		if (m < 0) {
-			m = -m;		/* m is now abs(error) */
-			m -= (tp->mdev_us >> 2);   /* similar update on mdev */
-			/* This is similar to one of Eifel findings.
-			 * Eifel blocks mdev updates when rtt decreases.
-			 * This solution is a bit different: we use finer gain
-			 * for mdev in this case (alpha*beta).
-			 * Like Eifel it also prevents growth of rto,
-			 * but also it limits too fast rto decreases,
-			 * happening in pure Eifel.
-			 */
-			if (m > 0)
-				m >>= 3;
-		} else {
-			m -= (tp->mdev_us >> 2);   /* similar update on mdev */
-		}
-		tp->mdev_us += m;		/* mdev = 3/4 mdev + 1/4 new */
-		if (tp->mdev_us > tp->mdev_max_us) {
-			tp->mdev_max_us = tp->mdev_us;
-			if (tp->mdev_max_us > tp->rttvar_us)
-				tp->rttvar_us = tp->mdev_max_us;
-		}
-		if (after(tp->snd_una, tp->rtt_seq)) {
-			if (tp->mdev_max_us < tp->rttvar_us)
-				tp->rttvar_us -= (tp->rttvar_us - tp->mdev_max_us) >> 2;
+		m -= (srtt >> 3);	 /* m' = m - srtt / 8 = (R' - SRTT) */
+		srtt += m;		 /* srtt = srtt + m’ = srtt + m - srtt / 8 */
+		if (m < 0)
+			m = -m;
+		m -= (tp->mdev_us >> 2); /* m'' = |m'| - mdev / 4 */
+		tp->mdev_us += m;
+		tp->rttvar_us = tp->mdev_us;
+		if (after(tp->snd_una, tp->rtt_seq))
 			tp->rtt_seq = tp->snd_nxt;
-			tp->mdev_max_us = tcp_rto_min_us(sk);
-		}
 	} else {
 		/* no previous measure. */
-		srtt = m << 3;		/* take the measured time to be rtt */
-		tp->mdev_us = m << 1;	/* make sure rto = 3*rtt */
-		tp->rttvar_us = max(tp->mdev_us, tcp_rto_min_us(sk));
-		tp->mdev_max_us = tp->rttvar_us;
+		srtt = m << 3;
+		tp->mdev_us = m << 1;
+		tp->rttvar_us = tp->mdev_us;
 		tp->rtt_seq = tp->snd_nxt;
 	}
 	tp->srtt_us = max(1U, srtt);
@@ -798,6 +759,7 @@ static void tcp_update_pacing_rate(struct sock *sk)
  */
 static void tcp_set_rto(struct sock *sk)
 {
+	const u32 min_rto = tcp_rto_min_us(sk);
 	const struct tcp_sock *tp = tcp_sk(sk);
 	/* Old crap is replaced with new one. 8)
 	 *
@@ -809,17 +771,15 @@ static void tcp_set_rto(struct sock *sk)
 	 *    is invisible. Actually, Linux-2.4 also generates erratic
 	 *    ACKs in some circumstances.
 	 */
-	inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);
-
+	if (((tp->srtt_us >> 3) + tp->rttvar_us) < min_rto)
+		inet_csk(sk)->icsk_rto = usecs_to_jiffies(min_rto);
+	else
+		inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);
 	/* 2. Fixups made earlier cannot be right.
 	 *    If we do not estimate RTO correctly without them,
 	 *    all the algo is pure shit and should be replaced
 	 *    with correct one. It is exactly, which we pretend to do.
 	 */
-
-	/* NOTE: clamping at TCP_RTO_MIN is not required, current algo
-	 * guarantees that rto is higher.
-	 */
 	tcp_bound_rto(sk);
 }
 

net/ipv4/tcp_metrics.c

@@ -561,7 +561,7 @@ void tcp_init_metrics(struct sock *sk)
 		 * retransmission.
 		 */
 		tp->rttvar_us = jiffies_to_usecs(TCP_TIMEOUT_FALLBACK);
-		tp->mdev_us = tp->mdev_max_us = tp->rttvar_us;
+		tp->mdev_us = tp->rttvar_us;
 
 		inet_csk(sk)->icsk_rto = TCP_TIMEOUT_FALLBACK;
 	}