[TCP]: TCP Low Priority congestion control

 TCP Low Priority is a distributed algorithm whose goal is to utilize only
 the excess network bandwidth as compared to the ``fair share`` of
 bandwidth as targeted by TCP. Available from:
   http://www.ece.rice.edu/~akuzma/Doc/akuzma/TCP-LP.pdf

Original Author:
 Aleksandar Kuzmanovic <akuzma@northwestern.edu>

See http://www-ece.rice.edu/networks/TCP-LP/ for their implementation.
As of 2.6.13, Linux supports pluggable congestion control algorithms.
Due to the limitation of the API, we take the following changes from
the original TCP-LP implementation:
 o We use newReno in most core CA handling. Only add some checking
   within cong_avoid.
 o Error correcting in remote HZ, therefore remote HZ will be keeped
   on checking and updating.
 o Handling calculation of One-Way-Delay (OWD) within rtt_sample, sicne
   OWD have a similar meaning as RTT. Also correct the buggy formular.
 o Handle reaction for Early Congestion Indication (ECI) within
   pkts_acked, as mentioned within pseudo code.
 o OWD is handled in relative format, where local time stamp will in
   tcp_time_stamp format.

Port from 2.4.19 to 2.6.16 as module by:
 Wong Hoi Sing Edison <hswong3i@gmail.com>
 Hung Hing Lun <hlhung3i@gmail.com>
Signed-off-by: Wong Hoi Sing Edison <hswong3i@gmail.com>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -550,6 +550,16 @@ config TCP_CONG_SCALABLE
 	properties, though is known to have fairness issues.
 	See http://www-lce.eng.cam.ac.uk/~ctk21/scalable/
 
+config TCP_CONG_LP
+	tristate "TCP Low Priority"
+	depends on EXPERIMENTAL
+	default n
+	---help---
+	TCP Low Priority (TCP-LP), a distributed algorithm whose goal is
+	to utiliza only the excess network bandwidth as compared to the
+	``fair share`` of bandwidth as targeted by TCP.
+	See http://www-ece.rice.edu/networks/TCP-LP/
+
 endmenu
 
 config TCP_CONG_BIC

net/ipv4/Makefile

@@ -44,6 +44,7 @@ obj-$(CONFIG_TCP_CONG_HYBLA) += tcp_hybla.o
 obj-$(CONFIG_TCP_CONG_HTCP) += tcp_htcp.o
 obj-$(CONFIG_TCP_CONG_VEGAS) += tcp_vegas.o
 obj-$(CONFIG_TCP_CONG_SCALABLE) += tcp_scalable.o
+obj-$(CONFIG_TCP_CONG_LP) += tcp_lp.o
 
 obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
 		      xfrm4_output.o

net/ipv4/tcp_lp.c

+/*
+ * TCP Low Priority (TCP-LP)
+ *
+ * TCP Low Priority is a distributed algorithm whose goal is to utilize only
+ *   the excess network bandwidth as compared to the ``fair share`` of
+ *   bandwidth as targeted by TCP. Available from:
+ *     http://www.ece.rice.edu/~akuzma/Doc/akuzma/TCP-LP.pdf
+ *
+ * Original Author:
+ *   Aleksandar Kuzmanovic <akuzma@northwestern.edu>
+ *
+ * See http://www-ece.rice.edu/networks/TCP-LP/ for their implementation.
+ * As of 2.6.13, Linux supports pluggable congestion control algorithms.
+ * Due to the limitation of the API, we take the following changes from
+ * the original TCP-LP implementation:
+ *   o We use newReno in most core CA handling. Only add some checking
+ *     within cong_avoid.
+ *   o Error correcting in remote HZ, therefore remote HZ will be keeped
+ *     on checking and updating.
+ *   o Handling calculation of One-Way-Delay (OWD) within rtt_sample, sicne
+ *     OWD have a similar meaning as RTT. Also correct the buggy formular.
+ *   o Handle reaction for Early Congestion Indication (ECI) within
+ *     pkts_acked, as mentioned within pseudo code.
+ *   o OWD is handled in relative format, where local time stamp will in
+ *     tcp_time_stamp format.
+ *
+ * Port from 2.4.19 to 2.6.16 as module by:
+ *   Wong Hoi Sing Edison <hswong3i@gmail.com>
+ *   Hung Hing Lun <hlhung3i@gmail.com>
+ *
+ * Version: $Id: tcp_lp.c,v 1.22 2006-05-02 18:18:19 hswong3i Exp $
+ */
+
+#include <linux/config.h>
+#include <linux/module.h>
+#include <net/tcp.h>
+
+/* resolution of owd */
+#define LP_RESOL       1000
+
+/**
+ * enum tcp_lp_state
+ * @LP_VALID_RHZ: is remote HZ valid?
+ * @LP_VALID_OWD: is OWD valid?
+ * @LP_WITHIN_THR: are we within threshold?
+ * @LP_WITHIN_INF: are we within inference?
+ *
+ * TCP-LP's state flags.
+ * We create this set of state flag mainly for debugging.
+ */
+enum tcp_lp_state {
+	LP_VALID_RHZ = (1 << 0),
+	LP_VALID_OWD = (1 << 1),
+	LP_WITHIN_THR = (1 << 3),
+	LP_WITHIN_INF = (1 << 4),
+};
+
+/**
+ * struct lp
+ * @flag: TCP-LP state flag
+ * @sowd: smoothed OWD << 3
+ * @owd_min: min OWD
+ * @owd_max: max OWD
+ * @owd_max_rsv: resrved max owd
+ * @remote_hz: estimated remote HZ
+ * @remote_ref_time: remote reference time
+ * @local_ref_time: local reference time
+ * @last_drop: time for last active drop
+ * @inference: current inference
+ *
+ * TCP-LP's private struct.
+ * We get the idea from original TCP-LP implementation where only left those we
+ * found are really useful.
+ */
+struct lp {
+	u32 flag;
+	u32 sowd;
+	u32 owd_min;
+	u32 owd_max;
+	u32 owd_max_rsv;
+	u32 remote_hz;
+	u32 remote_ref_time;
+	u32 local_ref_time;
+	u32 last_drop;
+	u32 inference;
+};
+
+/**
+ * tcp_lp_init
+ *
+ * Init all required variables.
+ * Clone the handling from Vegas module implementation.
+ */
+static void tcp_lp_init(struct sock *sk)
+{
+	struct lp *lp = inet_csk_ca(sk);
+
+	lp->flag = 0;
+	lp->sowd = 0;
+	lp->owd_min = 0xffffffff;
+	lp->owd_max = 0;
+	lp->owd_max_rsv = 0;
+	lp->remote_hz = 0;
+	lp->remote_ref_time = 0;
+	lp->local_ref_time = 0;
+	lp->last_drop = 0;
+	lp->inference = 0;
+}
+
+/**
+ * tcp_lp_cong_avoid
+ *
+ * Implementation of cong_avoid.
+ * Will only call newReno CA when away from inference.
+ * From TCP-LP's paper, this will be handled in additive increasement.
+ */
+static void tcp_lp_cong_avoid(struct sock *sk, u32 ack, u32 rtt, u32 in_flight,
+			      int flag)
+{
+	struct lp *lp = inet_csk_ca(sk);
+
+	if (!(lp->flag & LP_WITHIN_INF))
+		tcp_reno_cong_avoid(sk, ack, rtt, in_flight, flag);
+}
+
+/**
+ * tcp_lp_remote_hz_estimator
+ *
+ * Estimate remote HZ.
+ * We keep on updating the estimated value, where original TCP-LP
+ * implementation only guest it for once and use forever.
+ */
+static u32 tcp_lp_remote_hz_estimator(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct lp *lp = inet_csk_ca(sk);
+	s64 rhz = lp->remote_hz << 6;	/* remote HZ << 6 */
+	s64 m = 0;
+
+	/* not yet record reference time
+	 * go away!! record it before come back!! */
+	if (lp->remote_ref_time == 0 || lp->local_ref_time == 0)
+		goto out;
+
+	/* we can't calc remote HZ with no different!! */
+	if (tp->rx_opt.rcv_tsval == lp->remote_ref_time
+	    || tp->rx_opt.rcv_tsecr == lp->local_ref_time)
+		goto out;
+
+	m = HZ * (tp->rx_opt.rcv_tsval -
+		  lp->remote_ref_time) / (tp->rx_opt.rcv_tsecr -
+					  lp->local_ref_time);
+	if (m < 0)
+		m = -m;
+
+	if (rhz != 0) {
+		m -= rhz >> 6;	/* m is now error in remote HZ est */
+		rhz += m;	/* 63/64 old + 1/64 new */
+	} else
+		rhz = m << 6;
+
+	/* record time for successful remote HZ calc */
+	lp->flag |= LP_VALID_RHZ;
+
+ out:
+	/* record reference time stamp */
+	lp->remote_ref_time = tp->rx_opt.rcv_tsval;
+	lp->local_ref_time = tp->rx_opt.rcv_tsecr;
+
+	return rhz >> 6;
+}
+
+/**
+ * tcp_lp_owd_calculator
+ *
+ * Calculate one way delay (in relative format).
+ * Original implement OWD as minus of remote time difference to local time
+ * difference directly. As this time difference just simply equal to RTT, when
+ * the network status is stable, remote RTT will equal to local RTT, and result
+ * OWD into zero.
+ * It seems to be a bug and so we fixed it.
+ */
+static u32 tcp_lp_owd_calculator(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct lp *lp = inet_csk_ca(sk);
+	s64 owd = 0;
+
+	lp->remote_hz = tcp_lp_remote_hz_estimator(sk);
+
+	if (lp->flag & LP_VALID_RHZ) {
+		owd =
+		    tp->rx_opt.rcv_tsval * (LP_RESOL / lp->remote_hz) -
+		    tp->rx_opt.rcv_tsecr * (LP_RESOL / HZ);
+		if (owd < 0)
+			owd = -owd;
+	}
+
+	if (owd > 0)
+		lp->flag |= LP_VALID_OWD;
+	else
+		lp->flag &= ~LP_VALID_OWD;
+
+	return owd;
+}
+
+/**
+ * tcp_lp_rtt_sample
+ *
+ * Implementation or rtt_sample.
+ * Will take the following action,
+ *   1. calc OWD,
+ *   2. record the min/max OWD,
+ *   3. calc smoothed OWD (SOWD).
+ * Most ideas come from the original TCP-LP implementation.
+ */
+static void tcp_lp_rtt_sample(struct sock *sk, u32 usrtt)
+{
+	struct lp *lp = inet_csk_ca(sk);
+	s64 mowd = tcp_lp_owd_calculator(sk);
+
+	/* sorry that we don't have valid data */
+	if (!(lp->flag & LP_VALID_RHZ) || !(lp->flag & LP_VALID_OWD))
+		return;
+
+	/* record the next min owd */
+	if (mowd < lp->owd_min)
+		lp->owd_min = mowd;
+
+	/* always forget the max of the max
+	 * we just set owd_max as one below it */
+	if (mowd > lp->owd_max) {
+		if (mowd > lp->owd_max_rsv) {
+			if (lp->owd_max_rsv == 0)
+				lp->owd_max = mowd;
+			else
+				lp->owd_max = lp->owd_max_rsv;
+			lp->owd_max_rsv = mowd;
+		} else
+			lp->owd_max = mowd;
+	}
+
+	/* calc for smoothed owd */
+	if (lp->sowd != 0) {
+		mowd -= lp->sowd >> 3;	/* m is now error in owd est */
+		lp->sowd += mowd;	/* owd = 7/8 owd + 1/8 new */
+	} else
+		lp->sowd = mowd << 3;	/* take the measured time be owd */
+}
+
+/**
+ * tcp_lp_pkts_acked
+ *
+ * Implementation of pkts_acked.
+ * Deal with active drop under Early Congestion Indication.
+ * Only drop to half and 1 will be handle, because we hope to use back
+ * newReno in increase case.
+ * We work it out by following the idea from TCP-LP's paper directly
+ */
+static void tcp_lp_pkts_acked(struct sock *sk, u32 num_acked)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct lp *lp = inet_csk_ca(sk);
+
+	/* calc inference */
+	if (tcp_time_stamp > tp->rx_opt.rcv_tsecr)
+		lp->inference = 3 * (tcp_time_stamp - tp->rx_opt.rcv_tsecr);
+
+	/* test if within inference */
+	if (lp->last_drop && (tcp_time_stamp - lp->last_drop < lp->inference))
+		lp->flag |= LP_WITHIN_INF;
+	else
+		lp->flag &= ~LP_WITHIN_INF;
+
+	/* test if within threshold */
+	if (lp->sowd >> 3 <
+	    lp->owd_min + 15 * (lp->owd_max - lp->owd_min) / 100)
+		lp->flag |= LP_WITHIN_THR;
+	else
+		lp->flag &= ~LP_WITHIN_THR;
+
+	pr_debug("TCP-LP: %05o|%5u|%5u|%15u|%15u|%15u\n", lp->flag,
+		 tp->snd_cwnd, lp->remote_hz, lp->owd_min, lp->owd_max,
+		 lp->sowd >> 3);
+
+	if (lp->flag & LP_WITHIN_THR)
+		return;
+
+	/* FIXME: try to reset owd_min and owd_max here
+	 * so decrease the chance the min/max is no longer suitable
+	 * and will usually within threshold when whithin inference */
+	lp->owd_min = lp->sowd >> 3;
+	lp->owd_max = lp->sowd >> 2;
+	lp->owd_max_rsv = lp->sowd >> 2;
+
+	/* happened within inference
+	 * drop snd_cwnd into 1 */
+	if (lp->flag & LP_WITHIN_INF)
+		tp->snd_cwnd = 1U;
+
+	/* happened after inference
+	 * cut snd_cwnd into half */
+	else
+		tp->snd_cwnd = max(tp->snd_cwnd >> 1U, 1U);
+
+	/* record this drop time */
+	lp->last_drop = tcp_time_stamp;
+}
+
+static struct tcp_congestion_ops tcp_lp = {
+	.init = tcp_lp_init,
+	.ssthresh = tcp_reno_ssthresh,
+	.cong_avoid = tcp_lp_cong_avoid,
+	.min_cwnd = tcp_reno_min_cwnd,
+	.rtt_sample = tcp_lp_rtt_sample,
+	.pkts_acked = tcp_lp_pkts_acked,
+
+	.owner = THIS_MODULE,
+	.name = "lp"
+};
+
+static int __init tcp_lp_register(void)
+{
+	BUG_ON(sizeof(struct lp) > ICSK_CA_PRIV_SIZE);
+	return tcp_register_congestion_control(&tcp_lp);
+}
+
+static void __exit tcp_lp_unregister(void)
+{
+	tcp_unregister_congestion_control(&tcp_lp);
+}
+
+module_init(tcp_lp_register);
+module_exit(tcp_lp_unregister);
+
+MODULE_AUTHOR("Wong Hoi Sing Edison, Hung Hing Lun");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("TCP Low Priority");