Import a clean-room implementation of the experimental H-TCP congestion control

algorithm based on the Internet-Draft "draft-leith-tcp-htcp-06.txt". It is
implemented as a kernel module compatible with the recently committed modular
congestion control framework.

H-TCP was designed to provide increased throughput in fast and long-distance
networks. It attempts to maintain fairness when competing with legacy NewReno
TCP in lower speed scenarios where NewReno is able to operate adequately. The
paper "H-TCP: A framework for congestion control in high-speed and long-distance
networks" provides additional detail.

In collaboration with:	David Hayes <dahayes at swin edu au> and
			Grenville Armitage <garmitage at swin edu au>
Sponsored by:	FreeBSD Foundation
Reviewed by:	rpaulo (older patch from a few weeks ago)
MFC after:	3 months

1 files changed, 521 insertions, 0 deletions

diff --git a/sys/netinet/cc/cc_htcp.c b/sys/netinet/cc/cc_htcp.c
new file mode 100644
index 0000000..61175ac
--- /dev/null
+++ b/sys/netinet/cc/cc_htcp.c
@@ -0,0 +1,521 @@
+/*-
+ * Copyright (c) 2007-2008
+ * 	Swinburne University of Technology, Melbourne, Australia
+ * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
+ * Copyright (c) 2010 The FreeBSD Foundation
+ * All rights reserved.
+ *
+ * This software was developed at the Centre for Advanced Internet
+ * Architectures, Swinburne University, by Lawrence Stewart and James Healy,
+ * made possible in part by a grant from the Cisco University Research Program
+ * Fund at Community Foundation Silicon Valley.
+ *
+ * Portions of this software were developed at the Centre for Advanced
+ * Internet Architectures, Swinburne University of Technology, Melbourne,
+ * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/*
+ * An implementation of the H-TCP congestion control algorithm for FreeBSD,
+ * based on the Internet Draft "draft-leith-tcp-htcp-06.txt" by Leith and
+ * Shorten. Originally released as part of the NewTCP research project at
+ * Swinburne University's Centre for Advanced Internet Architectures, Melbourne,
+ * Australia, which was made possible in part by a grant from the Cisco
+ * University Research Program Fund at Community Foundation Silicon Valley. More
+ * details are available at:
+ *   http://caia.swin.edu.au/urp/newtcp/
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <sys/kernel.h>
+#include <sys/limits.h>
+#include <sys/malloc.h>
+#include <sys/module.h>
+#include <sys/socket.h>
+#include <sys/socketvar.h>
+#include <sys/sysctl.h>
+#include <sys/systm.h>
+
+#include <net/vnet.h>
+
+#include <netinet/cc.h>
+#include <netinet/tcp_seq.h>
+#include <netinet/tcp_timer.h>
+#include <netinet/tcp_var.h>
+
+#include <netinet/cc/cc_module.h>
+
+/* Fixed point math shifts. */
+#define HTCP_SHIFT 8
+#define HTCP_ALPHA_INC_SHIFT 4
+
+#define HTCP_INIT_ALPHA 1
+#define HTCP_DELTA_L hz		/* 1 sec in ticks. */
+#define HTCP_MINBETA 128	/* 0.5 << HTCP_SHIFT. */
+#define HTCP_MAXBETA 204	/* ~0.8 << HTCP_SHIFT. */
+#define HTCP_MINROWE 26		/* ~0.1 << HTCP_SHIFT. */
+#define HTCP_MAXROWE 512	/* 2 << HTCP_SHIFT. */
+
+/* RTT_ref (ms) used in the calculation of alpha if RTT scaling is enabled. */
+#define HTCP_RTT_REF 100
+
+/* Don't trust SRTT until this many samples have been taken. */
+#define HTCP_MIN_RTT_SAMPLES 8
+
+/*
+ * HTCP_CALC_ALPHA performs a fixed point math calculation to determine the
+ * value of alpha, based on the function defined in the HTCP spec.
+ *
+ * i.e. 1 + 10(delta - delta_l) + ((delta - delta_l) / 2) ^ 2
+ *
+ * "diff" is passed in to the macro as "delta - delta_l" and is expected to be
+ * in units of ticks.
+ *
+ * The joyousnous of fixed point maths means our function implementation looks a
+ * little funky...
+ *
+ * In order to maintain some precision in the calculations, a fixed point shift
+ * HTCP_ALPHA_INC_SHIFT is used to ensure the integer divisions don't
+ * truncate the results too badly.
+ *
+ * The "16" value is the "1" term in the alpha function shifted up by
+ * HTCP_ALPHA_INC_SHIFT
+ *
+ * The "160" value is the "10" multiplier in the alpha function multiplied by
+ * 2^HTCP_ALPHA_INC_SHIFT
+ *
+ * Specifying these as constants reduces the computations required. After
+ * up-shifting all the terms in the function and performing the required
+ * calculations, we down-shift the final result by HTCP_ALPHA_INC_SHIFT to
+ * ensure it is back in the correct range.
+ *
+ * The "hz" terms are required as kernels can be configured to run with
+ * different tick timers, which we have to adjust for in the alpha calculation
+ * (which originally was defined in terms of seconds).
+ *
+ * We also have to be careful to constrain the value of diff such that it won't
+ * overflow whilst performing the calculation. The middle term i.e. (160 * diff)
+ * / hz is the limiting factor in the calculation. We must constrain diff to be
+ * less than the max size of an int divided by the constant 160 figure
+ * i.e. diff < INT_MAX / 160
+ *
+ * NB: Changing HTCP_ALPHA_INC_SHIFT will require you to MANUALLY update the
+ * constants used in this function!
+ */
+#define HTCP_CALC_ALPHA(diff) \
+((\
+	(16) + \
+	((160 * (diff)) / hz) + \
+	(((diff) / hz) * (((diff) << HTCP_ALPHA_INC_SHIFT) / (4 * hz))) \
+) >> HTCP_ALPHA_INC_SHIFT)
+
+static void	htcp_ack_received(struct cc_var *ccv, uint16_t type);
+static void	htcp_cb_destroy(struct cc_var *ccv);
+static int	htcp_cb_init(struct cc_var *ccv);
+static void	htcp_cong_signal(struct cc_var *ccv, uint32_t type);
+static int	htcp_mod_init(void);
+static void	htcp_post_recovery(struct cc_var *ccv);
+static void	htcp_recalc_alpha(struct cc_var *ccv);
+static void	htcp_recalc_beta(struct cc_var *ccv);
+static void	htcp_record_rtt(struct cc_var *ccv);
+static void	htcp_ssthresh_update(struct cc_var *ccv);
+
+struct htcp {
+	/* cwnd before entering cong recovery. */
+	unsigned long	prev_cwnd;
+	/* cwnd additive increase parameter. */
+	int		alpha;
+	/* cwnd multiplicative decrease parameter. */
+	int		beta;
+	/* Largest rtt seen for the flow. */
+	int		maxrtt;
+	/* Shortest rtt seen for the flow. */
+	int		minrtt;
+	/* Time of last congestion event in ticks. */
+	int		t_last_cong;
+};
+
+static int htcp_rtt_ref;
+/*
+ * The maximum number of ticks the value of diff can reach in
+ * htcp_recalc_alpha() before alpha will stop increasing due to overflow.
+ * See comment above HTCP_CALC_ALPHA for more info.
+ */
+static int htcp_max_diff = INT_MAX / ((1 << HTCP_ALPHA_INC_SHIFT) * 10);
+
+/* Per-netstack vars. */
+static VNET_DEFINE(uint8_t, htcp_adaptive_backoff) = 0;
+static VNET_DEFINE(uint8_t, htcp_rtt_scaling) = 0;
+#define	V_htcp_adaptive_backoff    VNET(htcp_adaptive_backoff)
+#define	V_htcp_rtt_scaling    VNET(htcp_rtt_scaling)
+
+MALLOC_DECLARE(M_HTCP);
+MALLOC_DEFINE(M_HTCP, "htcp data",
+    "Per connection data required for the HTCP congestion control algorithm");
+
+struct cc_algo htcp_cc_algo = {
+	.name = "htcp",
+	.ack_received = htcp_ack_received,
+	.cb_destroy = htcp_cb_destroy,
+	.cb_init = htcp_cb_init,
+	.cong_signal = htcp_cong_signal,
+	.mod_init = htcp_mod_init,
+	.post_recovery = htcp_post_recovery,
+};
+
+static void
+htcp_ack_received(struct cc_var *ccv, uint16_t type)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = ccv->cc_data;
+	htcp_record_rtt(ccv);
+
+	/*
+	 * Regular ACK and we're not in cong/fast recovery and we're cwnd
+	 * limited and we're either not doing ABC or are slow starting or are
+	 * doing ABC and we've sent a cwnd's worth of bytes.
+	 */
+	if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
+	    (ccv->flags & CCF_CWND_LIMITED) && (!V_tcp_do_rfc3465 ||
+	    CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
+	    (V_tcp_do_rfc3465 && ccv->flags & CCF_ABC_SENTAWND))) {
+		htcp_recalc_beta(ccv);
+		htcp_recalc_alpha(ccv);
+		/*
+		 * Use the logic in NewReno ack_received() for slow start and
+		 * for the first HTCP_DELTA_L ticks after either the flow starts
+		 * or a congestion event (when alpha equals 1).
+		 */
+		if (htcp_data->alpha == 1 ||
+		    CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh))
+			newreno_cc_algo.ack_received(ccv, type);
+		else {
+			if (V_tcp_do_rfc3465) {
+				/* Increment cwnd by alpha segments. */
+				CCV(ccv, snd_cwnd) += htcp_data->alpha *
+				    CCV(ccv, t_maxseg);
+				ccv->flags &= ~CCF_ABC_SENTAWND;
+			} else
+				/*
+				 * Increment cwnd by alpha/cwnd segments to
+				 * approximate an increase of alpha segments
+				 * per RTT.
+				 */
+				CCV(ccv, snd_cwnd) += (((htcp_data->alpha <<
+				    HTCP_SHIFT) / (CCV(ccv, snd_cwnd) /
+				    CCV(ccv, t_maxseg))) * CCV(ccv, t_maxseg))
+				    >> HTCP_SHIFT;
+		}
+	}
+}
+
+static void
+htcp_cb_destroy(struct cc_var *ccv)
+{
+
+	if (ccv->cc_data != NULL)
+		free(ccv->cc_data, M_HTCP);
+}
+
+static int
+htcp_cb_init(struct cc_var *ccv)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = malloc(sizeof(struct htcp), M_HTCP, M_NOWAIT);
+
+	if (htcp_data == NULL)
+		return (ENOMEM);
+
+	/* Init some key variables with sensible defaults. */
+	htcp_data->alpha = HTCP_INIT_ALPHA;
+	htcp_data->beta = HTCP_MINBETA;
+	htcp_data->maxrtt = TCPTV_SRTTBASE;
+	htcp_data->minrtt = TCPTV_SRTTBASE;
+	htcp_data->prev_cwnd = 0;
+	htcp_data->t_last_cong = ticks;
+
+	ccv->cc_data = htcp_data;
+
+	return (0);
+}
+
+/*
+ * Perform any necessary tasks before we enter congestion recovery.
+ */
+static void
+htcp_cong_signal(struct cc_var *ccv, uint32_t type)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = ccv->cc_data;
+
+	switch (type) {
+	case CC_NDUPACK:
+		if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
+			if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
+				/*
+				 * Apply hysteresis to maxrtt to ensure
+				 * reductions in the RTT are reflected in our
+				 * measurements.
+				 */
+				htcp_data->maxrtt = (htcp_data->minrtt +
+				    (htcp_data->maxrtt - htcp_data->minrtt) *
+				    95) / 100;
+				htcp_ssthresh_update(ccv);
+				htcp_data->t_last_cong = ticks;
+				htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
+			}
+			ENTER_RECOVERY(CCV(ccv, t_flags));
+		}
+		break;
+
+	case CC_ECN:
+		if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
+			/*
+			 * Apply hysteresis to maxrtt to ensure reductions in
+			 * the RTT are reflected in our measurements.
+			 */
+			htcp_data->maxrtt = (htcp_data->minrtt + (htcp_data->maxrtt -
+			    htcp_data->minrtt) * 95) / 100;
+			htcp_ssthresh_update(ccv);
+			CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
+			htcp_data->t_last_cong = ticks;
+			htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
+			ENTER_CONGRECOVERY(CCV(ccv, t_flags));
+		}
+		break;
+
+	case CC_RTO:
+		/*
+		 * Grab the current time and record it so we know when the
+		 * most recent congestion event was. Only record it when the
+		 * timeout has fired more than once, as there is a reasonable
+		 * chance the first one is a false alarm and may not indicate
+		 * congestion.
+		 */
+		if (CCV(ccv, t_rxtshift) >= 2)
+			htcp_data->t_last_cong = ticks;
+		break;
+	}
+}
+
+static int
+htcp_mod_init(void)
+{
+
+	htcp_cc_algo.after_idle = newreno_cc_algo.after_idle;
+
+	/*
+	 * HTCP_RTT_REF is defined in ms, and t_srtt in the tcpcb is stored in
+	 * units of TCP_RTT_SCALE*hz. Scale HTCP_RTT_REF to be in the same units
+	 * as t_srtt.
+	 */
+	htcp_rtt_ref = (HTCP_RTT_REF * TCP_RTT_SCALE * hz) / 1000;
+
+	return (0);
+}
+
+/*
+ * Perform any necessary tasks before we exit congestion recovery.
+ */
+static void
+htcp_post_recovery(struct cc_var *ccv)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = ccv->cc_data;
+
+	if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
+		/*
+		 * If inflight data is less than ssthresh, set cwnd
+		 * conservatively to avoid a burst of data, as suggested in the
+		 * NewReno RFC. Otherwise, use the HTCP method.
+		 *
+		 * XXXLAS: Find a way to do this without needing curack
+		 */
+		if (SEQ_GT(ccv->curack + CCV(ccv, snd_ssthresh),
+		    CCV(ccv, snd_max)))
+			CCV(ccv, snd_cwnd) = CCV(ccv, snd_max) - ccv->curack +
+			    CCV(ccv, t_maxseg);
+		else
+			CCV(ccv, snd_cwnd) = max(1, ((htcp_data->beta *
+			    htcp_data->prev_cwnd / CCV(ccv, t_maxseg))
+			    >> HTCP_SHIFT)) * CCV(ccv, t_maxseg);
+	}
+}
+
+static void
+htcp_recalc_alpha(struct cc_var *ccv)
+{
+	struct htcp *htcp_data;
+	int alpha, diff, now;
+
+	htcp_data = ccv->cc_data;
+	now = ticks;
+
+	/*
+	 * If ticks has wrapped around (will happen approximately once every 49
+	 * days on a machine with the default kern.hz=1000) and a flow straddles
+	 * the wrap point, our alpha calcs will be completely wrong. We cut our
+	 * losses and restart alpha from scratch by setting t_last_cong = now -
+	 * HTCP_DELTA_L.
+	 *
+	 * This does not deflate our cwnd at all. It simply slows the rate cwnd
+	 * is growing by until alpha regains the value it held prior to taking
+	 * this drastic measure.
+	 */
+	if (now < htcp_data->t_last_cong)
+		htcp_data->t_last_cong = now - HTCP_DELTA_L;
+
+	diff = now - htcp_data->t_last_cong - HTCP_DELTA_L;
+
+	/* Cap alpha if the value of diff would overflow HTCP_CALC_ALPHA(). */
+	if (diff < htcp_max_diff) {
+		/*
+		 * If it has been more than HTCP_DELTA_L ticks since congestion,
+		 * increase alpha according to the function defined in the spec.
+		 */
+		if (diff > 0) {
+			alpha = HTCP_CALC_ALPHA(diff);
+
+			/*
+			 * Adaptive backoff fairness adjustment:
+			 * 2 * (1 - beta) * alpha_raw
+			 */
+			if (V_htcp_adaptive_backoff)
+				alpha = max(1, (2 * ((1 << HTCP_SHIFT) -
+				    htcp_data->beta) * alpha) >> HTCP_SHIFT);
+
+			/*
+			 * RTT scaling: (RTT / RTT_ref) * alpha
+			 * alpha will be the raw value from HTCP_CALC_ALPHA() if
+			 * adaptive backoff is off, or the adjusted value if
+			 * adaptive backoff is on.
+			 */
+			if (V_htcp_rtt_scaling)
+				alpha = max(1, (min(max(HTCP_MINROWE,
+				    (CCV(ccv, t_srtt) << HTCP_SHIFT) /
+				    htcp_rtt_ref), HTCP_MAXROWE) * alpha)
+				    >> HTCP_SHIFT);
+
+		} else
+			alpha = 1;
+
+		htcp_data->alpha = alpha;
+	}
+}
+
+static void
+htcp_recalc_beta(struct cc_var *ccv)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = ccv->cc_data;
+
+	/*
+	 * TCPTV_SRTTBASE is the initialised value of each connection's SRTT, so
+	 * we only calc beta if the connection's SRTT has been changed from its
+	 * inital value. beta is bounded to ensure it is always between
+	 * HTCP_MINBETA and HTCP_MAXBETA.
+	 */
+	if (V_htcp_adaptive_backoff && htcp_data->minrtt != TCPTV_SRTTBASE &&
+	    htcp_data->maxrtt != TCPTV_SRTTBASE)
+		htcp_data->beta = min(max(HTCP_MINBETA,
+		    (htcp_data->minrtt << HTCP_SHIFT) / htcp_data->maxrtt),
+		    HTCP_MAXBETA);
+	else
+		htcp_data->beta = HTCP_MINBETA;
+}
+
+/*
+ * Record the minimum and maximum RTT seen for the connection. These are used in
+ * the calculation of beta if adaptive backoff is enabled.
+ */
+static void
+htcp_record_rtt(struct cc_var *ccv)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = ccv->cc_data;
+
+	/* XXXLAS: Should there be some hysteresis for minrtt? */
+
+	/*
+	 * Record the current SRTT as our minrtt if it's the smallest we've seen
+	 * or minrtt is currently equal to its initialised value. Ignore SRTT
+	 * until a min number of samples have been taken.
+	 */
+	if ((CCV(ccv, t_srtt) < htcp_data->minrtt ||
+	    htcp_data->minrtt == TCPTV_SRTTBASE) &&
+	    (CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES))
+		htcp_data->minrtt = CCV(ccv, t_srtt);
+
+	/*
+	 * Record the current SRTT as our maxrtt if it's the largest we've
+	 * seen. Ignore SRTT until a min number of samples have been taken.
+	 */
+	if (CCV(ccv, t_srtt) > htcp_data->maxrtt
+	    && CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES)
+		htcp_data->maxrtt = CCV(ccv, t_srtt);
+}
+
+/*
+ * Update the ssthresh in the event of congestion.
+ */
+static void
+htcp_ssthresh_update(struct cc_var *ccv)
+{
+	struct htcp *htcp_data;
+
+	htcp_data = ccv->cc_data;
+
+	/*
+	 * On the first congestion event, set ssthresh to cwnd * 0.5, on
+	 * subsequent congestion events, set it to cwnd * beta.
+	 */
+	if (CCV(ccv, snd_ssthresh) == TCP_MAXWIN << TCP_MAX_WINSHIFT)
+		CCV(ccv, snd_ssthresh) = (CCV(ccv, snd_cwnd) * HTCP_MINBETA)
+		    >> HTCP_SHIFT;
+	else {
+		htcp_recalc_beta(ccv);
+		CCV(ccv, snd_ssthresh) = (CCV(ccv, snd_cwnd) * htcp_data->beta)
+		    >> HTCP_SHIFT;
+	}
+}
+
+
+SYSCTL_DECL(_net_inet_tcp_cc_htcp);
+SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, htcp, CTLFLAG_RW,
+    NULL, "H-TCP related settings");
+SYSCTL_VNET_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, adaptive_backoff, CTLFLAG_RW,
+    &VNET_NAME(htcp_adaptive_backoff), 0, "enable H-TCP adaptive backoff");
+SYSCTL_VNET_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, rtt_scaling, CTLFLAG_RW,
+    &VNET_NAME(htcp_rtt_scaling), 0, "enable H-TCP RTT scaling");
+
+DECLARE_CC_MODULE(htcp, &htcp_cc_algo);