This commit marks the first formal contribution of the "Five New TCP Congestion

Control Algorithms for FreeBSD" FreeBSD Foundation funded project. More details
about the project are available at: http://caia.swin.edu.au/freebsd/5cc/

- Add a KPI and supporting infrastructure to allow modular congestion control
  algorithms to be used in the net stack. Algorithms can maintain per-connection
  state if required, and connections maintain their own algorithm pointer, which
  allows different connections to concurrently use different algorithms. The
  TCP_CONGESTION socket option can be used with getsockopt()/setsockopt() to
  programmatically query or change the congestion control algorithm respectively
  from within an application at runtime.

- Integrate the framework with the TCP stack in as least intrusive a manner as
  possible. Care was also taken to develop the framework in a way that should
  allow integration with other congestion aware transport protocols (e.g. SCTP)
  in the future. The hope is that we will one day be able to share a single set
  of congestion control algorithm modules between all congestion aware transport
  protocols.

- Introduce a new congestion recovery (TF_CONGRECOVERY) state into the TCP stack
  and use it to decouple the meaning of recovery from a congestion event and
  recovery from packet loss (TF_FASTRECOVERY) a la RFC2581. ECN and delay based
  congestion control protocols don't generally need to recover from packet loss
  and need a different way to note a congestion recovery episode within the
  stack.

- Remove the net.inet.tcp.newreno sysctl, which simplifies some portions of code
  and ensures the stack always uses the appropriate mechanisms for recovering
  from packet loss during a congestion recovery episode.

- Extract the NewReno congestion control algorithm from the TCP stack and
  massage it into module form. NewReno is always built into the kernel and will
  remain the default algorithm for the forseeable future. Implementations of
  additional different algorithms will become available in the near future.

- Bump __FreeBSD_version to 900025 and note in UPDATING that rebuilding code
  that relies on the size of "struct tcpcb" is required.

Many thanks go to the Cisco University Research Program Fund at Community
Foundation Silicon Valley and the FreeBSD Foundation. Their support of our work
at the Centre for Advanced Internet Architectures, Swinburne University of
Technology is greatly appreciated.

In collaboration with:	David Hayes <dahayes at swin edu au> and
			Grenville Armitage <garmitage at swin edu au>
Sponsored by:	Cisco URP, FreeBSD Foundation
Reviewed by:	rpaulo
Tested by:	David Hayes (and many others over the years)
MFC after:	3 months

3 files changed, 641 insertions, 0 deletions

diff --git a/sys/netinet/cc/cc.c b/sys/netinet/cc/cc.c
new file mode 100644
index 0000000..4643ca4
--- /dev/null
+++ b/sys/netinet/cc/cc.c
@@ -0,0 +1,340 @@
+/*-
+ * 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.
+ */
+
+/*
+ * This software was first released in 2007 by James Healy and Lawrence Stewart
+ * whilst working on 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/libkern.h>
+#include <sys/lock.h>
+#include <sys/malloc.h>
+#include <sys/module.h>
+#include <sys/mutex.h>
+#include <sys/queue.h>
+#include <sys/rwlock.h>
+#include <sys/sbuf.h>
+#include <sys/socket.h>
+#include <sys/socketvar.h>
+#include <sys/sysctl.h>
+
+#include <net/if.h>
+#include <net/if_var.h>
+
+#include <netinet/cc.h>
+#include <netinet/in.h>
+#include <netinet/in_pcb.h>
+#include <netinet/tcp_var.h>
+
+#include <netinet/cc/cc_module.h>
+
+/*
+ * List of available cc algorithms on the current system. First element
+ * is used as the system default CC algorithm.
+ */
+struct cc_head cc_list = STAILQ_HEAD_INITIALIZER(cc_list);
+
+/* Protects the cc_list TAILQ. */
+struct rwlock cc_list_lock;
+
+/*
+ * Set the default CC algorithm to new_default. The default is identified
+ * by being the first element in the cc_list TAILQ.
+ */
+static void
+cc_set_default(struct cc_algo *new_default)
+{
+	CC_LIST_WLOCK_ASSERT();
+
+	/*
+	 * Make the requested system default CC algorithm the first element in
+	 * the list if it isn't already.
+	 */
+	if (new_default != CC_DEFAULT()) {
+		STAILQ_REMOVE(&cc_list, new_default, cc_algo, entries);
+		STAILQ_INSERT_HEAD(&cc_list, new_default, entries);
+	}
+}
+
+/*
+ * Sysctl handler to show and change the default CC algorithm.
+ */
+static int
+cc_default_algo(SYSCTL_HANDLER_ARGS)
+{
+	struct cc_algo *funcs;
+	int err, found;
+
+	err = found = 0;
+
+	if (req->newptr == NULL) {
+		char default_cc[TCP_CA_NAME_MAX];
+
+		/* Just print the current default. */
+		CC_LIST_RLOCK();
+		strlcpy(default_cc, CC_DEFAULT()->name, TCP_CA_NAME_MAX);
+		CC_LIST_RUNLOCK();
+		err = sysctl_handle_string(oidp, default_cc, 1, req);
+	} else {
+		/* Find algo with specified name and set it to default. */
+		CC_LIST_WLOCK();
+		STAILQ_FOREACH(funcs, &cc_list, entries) {
+			if (strncmp((char *)req->newptr, funcs->name,
+			    TCP_CA_NAME_MAX) == 0) {
+				found = 1;
+				cc_set_default(funcs);
+			}
+		}
+		CC_LIST_WUNLOCK();
+
+		if (!found)
+			err = ESRCH;
+	}
+
+	return (err);
+}
+
+/*
+ * Sysctl handler to display the list of available CC algorithms.
+ */
+static int
+cc_list_available(SYSCTL_HANDLER_ARGS)
+{
+	struct cc_algo *algo;
+	struct sbuf *s;
+	int err, first;
+
+	err = 0;
+	first = 1;
+	s = sbuf_new(NULL, NULL, TCP_CA_NAME_MAX, SBUF_AUTOEXTEND);
+
+	if (s == NULL)
+		return (ENOMEM);
+
+	CC_LIST_RLOCK();
+	STAILQ_FOREACH(algo, &cc_list, entries) {
+		err = sbuf_printf(s, first ? "%s" : ", %s", algo->name);
+		if (err)
+			break;
+		first = 0;
+	}
+	CC_LIST_RUNLOCK();
+
+	if (!err) {
+		sbuf_finish(s);
+		err = sysctl_handle_string(oidp, sbuf_data(s), 1, req);
+	}
+
+	sbuf_delete(s);
+	return (err);
+}
+
+/*
+ * Initialise CC subsystem on system boot.
+ */
+void
+cc_init()
+{
+	CC_LIST_LOCK_INIT();
+	STAILQ_INIT(&cc_list);
+}
+
+/*
+ * Returns non-zero on success, 0 on failure.
+ */
+int
+cc_deregister_algo(struct cc_algo *remove_cc)
+{
+	struct cc_algo *funcs, *tmpfuncs;
+	struct tcpcb *tp;
+	struct inpcb *inp;
+	int err;
+
+	err = ENOENT;
+
+	/* Never allow newreno to be deregistered. */
+	if (&newreno_cc_algo == remove_cc)
+		return (EPERM);
+
+	/* Remove algo from cc_list so that new connections can't use it. */
+	CC_LIST_WLOCK();
+	STAILQ_FOREACH_SAFE(funcs, &cc_list, entries, tmpfuncs) {
+		if (funcs == remove_cc) {
+			/*
+			 * If we're removing the current system default,
+			 * reset the default to newreno.
+			 */
+			if (strncmp(CC_DEFAULT()->name, remove_cc->name,
+			    TCP_CA_NAME_MAX) == 0)
+				cc_set_default(&newreno_cc_algo);
+
+			STAILQ_REMOVE(&cc_list, funcs, cc_algo, entries);
+			err = 0;
+			break;
+		}
+	}
+	CC_LIST_WUNLOCK();
+	
+	if (!err) {
+		/*
+		 * Check all active control blocks and change any that are
+		 * using this algorithm back to newreno. If the algorithm that
+		 * was in use requires cleanup code to be run, call it.
+		 *
+		 * New connections already part way through being initialised
+		 * with the CC algo we're removing will not race with this code
+		 * because the INP_INFO_WLOCK is held during initialisation.
+		 * We therefore don't enter the loop below until the connection
+		 * list has stabilised.
+		 */
+		INP_INFO_RLOCK(&V_tcbinfo);
+		LIST_FOREACH(inp, &V_tcb, inp_list) {
+			INP_WLOCK(inp);
+			/* Important to skip tcptw structs. */
+			if (!(inp->inp_flags & INP_TIMEWAIT) &&
+			    (tp = intotcpcb(inp)) != NULL) {
+				/*
+				 * By holding INP_WLOCK here, we are
+				 * assured that the connection is not
+				 * currently executing inside the CC
+				 * module's functions i.e. it is safe to
+				 * make the switch back to newreno.
+				 */
+				if (CC_ALGO(tp) == remove_cc) {
+					tmpfuncs = CC_ALGO(tp);
+					/* Newreno does not require any init. */
+					CC_ALGO(tp) = &newreno_cc_algo;
+					if (tmpfuncs->cb_destroy != NULL)
+						tmpfuncs->cb_destroy(tp->ccv);
+				}
+			}
+			INP_WUNLOCK(inp);
+		}
+		INP_INFO_RUNLOCK(&V_tcbinfo);
+	}
+
+	return (err);
+}
+
+/*
+ * Returns 0 on success, non-zero on failure.
+ */
+int
+cc_register_algo(struct cc_algo *add_cc)
+{
+	struct cc_algo *funcs;
+	int err;
+
+	err = 0;
+
+	/*
+	 * Iterate over list of registered CC algorithms and make sure
+	 * we're not trying to add a duplicate.
+	 */
+	CC_LIST_WLOCK();
+	STAILQ_FOREACH(funcs, &cc_list, entries) {
+		if (funcs == add_cc || strncmp(funcs->name, add_cc->name,
+		    TCP_CA_NAME_MAX) == 0)
+			err = EEXIST;
+	}
+
+	if (!err)
+		STAILQ_INSERT_TAIL(&cc_list, add_cc, entries);
+
+	CC_LIST_WUNLOCK();
+
+	return (err);
+}
+
+/*
+ * Handles kld related events. Returns 0 on success, non-zero on failure.
+ */
+int
+cc_modevent(module_t mod, int event_type, void *data)
+{
+	struct cc_algo *algo;
+	int err;
+
+	err = 0;
+	algo = (struct cc_algo *)data;
+
+	switch(event_type) {
+	case MOD_LOAD:
+		if (algo->mod_init != NULL)
+			err = algo->mod_init();
+		if (!err)
+			err = cc_register_algo(algo);
+		break;
+
+	case MOD_QUIESCE:
+	case MOD_SHUTDOWN:
+	case MOD_UNLOAD:
+		err = cc_deregister_algo(algo);
+		if (!err && algo->mod_destroy != NULL)
+			algo->mod_destroy();
+		if (err == ENOENT)
+			err = 0;
+		break;
+
+	default:
+		err = EINVAL;
+		break;
+	}
+
+	return (err);
+}
+
+/* Declare sysctl tree and populate it. */
+SYSCTL_NODE(_net_inet_tcp, OID_AUTO, cc, CTLFLAG_RW, NULL,
+    "congestion control related settings");
+
+SYSCTL_PROC(_net_inet_tcp_cc, OID_AUTO, algorithm, CTLTYPE_STRING|CTLFLAG_RW,
+    NULL, 0, cc_default_algo, "A", "default congestion control algorithm");
+
+SYSCTL_PROC(_net_inet_tcp_cc, OID_AUTO, available, CTLTYPE_STRING|CTLFLAG_RD,
+    NULL, 0, cc_list_available, "A",
+    "list available congestion control algorithms");
diff --git a/sys/netinet/cc/cc_module.h b/sys/netinet/cc/cc_module.h
new file mode 100644
index 0000000..f3fe752
--- /dev/null
+++ b/sys/netinet/cc/cc_module.h
@@ -0,0 +1,70 @@
+/*-
+ * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
+ * All rights reserved.
+ *
+ * This software was developed by Lawrence Stewart while studying at the Centre
+ * for Advanced Internet Architectures, Swinburne University, made possible in
+ * part by a grant from the Cisco University Research Program Fund at Community
+ * Foundation Silicon Valley.
+ *
+ * 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.
+ *
+ * $FreeBSD$
+ */
+
+/*
+ * This software was first released in 2009 by Lawrence Stewart 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/
+ */
+
+#ifndef _NETINET_CC_MODULE_H_
+#define _NETINET_CC_MODULE_H_
+
+/*
+ * Allows a CC algorithm to manipulate a commonly named CC variable regardless
+ * of the transport protocol and associated C struct.
+ * XXXLAS: Out of action until the work to support SCTP is done.
+ *
+#define	CCV(ccv, what)							\
+(*(									\
+	(ccv)->type == IPPROTO_TCP ?	&(ccv)->ccvc.tcp->what :	\
+					&(ccv)->ccvc.sctp->what		\
+))
+ */
+#define	CCV(ccv, what) (ccv)->ccvc.tcp->what
+
+#define	DECLARE_CC_MODULE(ccname, ccalgo) 				\
+	static moduledata_t cc_##ccname = {				\
+		.name = #ccname,					\
+		.evhand = cc_modevent,					\
+		.priv = ccalgo						\
+	};								\
+	DECLARE_MODULE(ccname, cc_##ccname,				\
+	    SI_SUB_PROTO_IFATTACHDOMAIN, SI_ORDER_ANY)
+
+int	cc_modevent(module_t mod, int type, void *data);
+
+#endif /* _NETINET_CC_MODULE_H_ */
diff --git a/sys/netinet/cc/cc_newreno.c b/sys/netinet/cc/cc_newreno.c
new file mode 100644
index 0000000..e383510
--- /dev/null
+++ b/sys/netinet/cc/cc_newreno.c
@@ -0,0 +1,231 @@
+/*-
+ * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
+ *	The Regents of the University of California.
+ * Copyright (c) 2007-2008,2010
+ *	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, James Healy and
+ * David Hayes, 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.
+ */
+
+/*
+ * This software was first released in 2007 by James Healy and Lawrence Stewart
+ * whilst working on 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/module.h>
+#include <sys/socket.h>
+#include <sys/socketvar.h>
+#include <sys/sysctl.h>
+
+#include <net/if.h>
+#include <net/if_var.h>
+
+#include <netinet/cc.h>
+#include <netinet/in.h>
+#include <netinet/in_pcb.h>
+#include <netinet/tcp_seq.h>
+#include <netinet/tcp_var.h>
+
+#include <netinet/cc/cc_module.h>
+
+void	newreno_ack_received(struct cc_var *ccv, uint16_t type);
+void	newreno_cong_signal(struct cc_var *ccv, uint32_t type);
+void	newreno_post_recovery(struct cc_var *ccv);
+void	newreno_after_idle(struct cc_var *ccv);
+
+struct cc_algo newreno_cc_algo = {
+	.name = "newreno",
+	.ack_received = newreno_ack_received,
+	.cong_signal = newreno_cong_signal,
+	.post_recovery = newreno_post_recovery,
+	.after_idle = newreno_after_idle
+};
+
+/*
+ * Increase cwnd on receipt of a successful ACK:
+ * if cwnd <= ssthresh, increases by 1 MSS per ACK
+ * if cwnd > ssthresh, increase by ~1 MSS per RTT
+ */
+void
+newreno_ack_received(struct cc_var *ccv, uint16_t type)
+{
+	if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
+	    (ccv->flags & CCF_CWND_LIMITED)) {
+		u_int cw = CCV(ccv, snd_cwnd);
+		u_int incr = CCV(ccv, t_maxseg);
+
+		/*
+		 * Regular in-order ACK, open the congestion window.
+		 * Method depends on which congestion control state we're
+		 * in (slow start or cong avoid) and if ABC (RFC 3465) is
+		 * enabled.
+		 *
+		 * slow start: cwnd <= ssthresh
+		 * cong avoid: cwnd > ssthresh
+		 *
+		 * slow start and ABC (RFC 3465):
+		 *   Grow cwnd exponentially by the amount of data
+		 *   ACKed capping the max increment per ACK to
+		 *   (abc_l_var * maxseg) bytes.
+		 *
+		 * slow start without ABC (RFC 5681):
+		 *   Grow cwnd exponentially by maxseg per ACK.
+		 *
+		 * cong avoid and ABC (RFC 3465):
+		 *   Grow cwnd linearly by maxseg per RTT for each
+		 *   cwnd worth of ACKed data.
+		 *
+		 * cong avoid without ABC (RFC 5681):
+		 *   Grow cwnd linearly by approximately maxseg per RTT using
+		 *   maxseg^2 / cwnd per ACK as the increment.
+		 *   If cwnd > maxseg^2, fix the cwnd increment at 1 byte to
+		 *   avoid capping cwnd.
+		 */
+		if (cw > CCV(ccv, snd_ssthresh)) {
+			if (V_tcp_do_rfc3465) {
+				if (ccv->flags & CCF_ABC_SENTAWND)
+					ccv->flags &= ~CCF_ABC_SENTAWND;
+				else
+					incr = 0;
+			} else
+				incr = max((incr * incr / cw), 1);
+		} else if (V_tcp_do_rfc3465) {
+			/*
+			 * In slow-start with ABC enabled and no RTO in sight?
+			 * (Must not use abc_l_var > 1 if slow starting after
+			 * an RTO. On RTO, snd_nxt = snd_una, so the
+			 * snd_nxt == snd_max check is sufficient to
+			 * handle this).
+			 *
+			 * XXXLAS: Find a way to signal SS after RTO that
+			 * doesn't rely on tcpcb vars.
+			 */
+			if (CCV(ccv, snd_nxt) == CCV(ccv, snd_max))
+				incr = min(ccv->bytes_this_ack,
+				    V_tcp_abc_l_var * CCV(ccv, t_maxseg));
+			else
+				incr = min(ccv->bytes_this_ack, CCV(ccv, t_maxseg));
+		}
+		/* ABC is on by default, so incr equals 0 frequently. */
+		if (incr > 0)
+			CCV(ccv, snd_cwnd) = min(cw + incr,
+			    TCP_MAXWIN << CCV(ccv, snd_scale));
+	}
+}
+
+/*
+ * manage congestion signals
+ */
+void
+newreno_cong_signal(struct cc_var *ccv, uint32_t type)
+{
+	u_int win;
+
+	win = max(CCV(ccv, snd_cwnd) / 2 / CCV(ccv, t_maxseg), 2) *
+	    CCV(ccv, t_maxseg);
+
+	switch (type) {
+	case CC_NDUPACK:
+		if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
+			if (!IN_CONGRECOVERY(CCV(ccv, t_flags)))
+				CCV(ccv, snd_ssthresh) = win;
+			ENTER_RECOVERY(CCV(ccv, t_flags));
+		}
+		break;
+	case CC_ECN:
+		if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
+			CCV(ccv, snd_ssthresh) = win;
+			CCV(ccv, snd_cwnd) = win;
+			ENTER_CONGRECOVERY(CCV(ccv, t_flags));
+		}
+		break;
+	}
+}
+
+/*
+ * decrease the cwnd in response to packet loss or a transmit timeout.
+ * th can be null, in which case cwnd will be set according to reno instead
+ * of new reno.
+ */
+void
+newreno_post_recovery(struct cc_var *ccv)
+{
+	if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
+		/*
+		 * Fast recovery will conclude after returning from this
+		 * function. Window inflation should have left us with
+		 * approximately snd_ssthresh outstanding data. But in case we
+		 * would be inclined to send a burst, better to do it via the
+		 * slow start mechanism.
+		 *
+		 * 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) = CCV(ccv, snd_ssthresh);
+	}
+}
+
+/*
+ * if a connection has been idle for a while and more data is ready to be sent,
+ * reset cwnd
+ */
+void
+newreno_after_idle(struct cc_var *ccv)
+{
+	/*
+	 * We have been idle for "a while" and no acks are expected to clock out
+	 * any data we send -- slow start to get ack "clock" running again.
+	 */
+	if (V_tcp_do_rfc3390)
+		CCV(ccv, snd_cwnd) = min(4 * CCV(ccv, t_maxseg),
+		    max(2 * CCV(ccv, t_maxseg), 4380));
+	else
+		CCV(ccv, snd_cwnd) = CCV(ccv, t_maxseg) * 2;
+}
+
+
+DECLARE_CC_MODULE(newreno, &newreno_cc_algo);