Add support for TCP Selective Acknowledgements. The work for this

originated on RELENG_4 and was ported to -CURRENT.

The scoreboarding code was obtained from OpenBSD, and many
of the remaining changes were inspired by OpenBSD, but not
taken directly from there.

You can enable/disable sack using net.inet.tcp.do_sack. You can
also limit the number of sack holes that all senders can have in
the scoreboard with net.inet.tcp.sackhole_limit.

Reviewed by:	gnn
Obtained from:	Yahoo! (Mohan Srinivasan, Jayanth Vijayaraghavan)

1 files changed, 592 insertions, 0 deletions

diff --git a/sys/netinet/tcp_sack.c b/sys/netinet/tcp_sack.c
new file mode 100644
index 0000000..8dfa682
--- /dev/null
+++ b/sys/netinet/tcp_sack.c
@@ -0,0 +1,592 @@
+/*
+ * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
+ *	The Regents of the University of California.  All rights reserved.
+ *
+ * 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.
+ * 4. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
+ *
+ *	@(#)tcp_sack.c	8.12 (Berkeley) 5/24/95
+ * $FreeBSD$
+ */
+
+/*
+ * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994
+ *	The Regents of the University of California.  All rights reserved.
+ *
+ * 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.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
+ *
+ *	@@(#)COPYRIGHT	1.1 (NRL) 17 January 1995
+ *
+ * NRL grants permission for redistribution and use in source and binary
+ * forms, with or without modification, of the software and documentation
+ * created at NRL 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.
+ * 3. All advertising materials mentioning features or use of this software
+ *    must display the following acknowledgements:
+ * 	This product includes software developed by the University of
+ * 	California, Berkeley and its contributors.
+ * 	This product includes software developed at the Information
+ * 	Technology Division, US Naval Research Laboratory.
+ * 4. Neither the name of the NRL nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THE SOFTWARE PROVIDED BY NRL IS PROVIDED BY NRL 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 NRL 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.
+ *
+ * The views and conclusions contained in the software and documentation
+ * are those of the authors and should not be interpreted as representing
+ * official policies, either expressed or implied, of the US Naval
+ * Research Laboratory (NRL).
+ */
+#include "opt_ipfw.h"		/* for ipfw_fwd		*/
+#include "opt_inet.h"
+#include "opt_inet6.h"
+#include "opt_ipsec.h"
+#include "opt_tcpdebug.h"
+#include "opt_tcp_input.h"
+#include "opt_tcp_sack.h"
+
+#include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/kernel.h>
+#include <sys/sysctl.h>
+#include <sys/malloc.h>
+#include <sys/mbuf.h>
+#include <sys/proc.h>		/* for proc0 declaration */
+#include <sys/protosw.h>
+#include <sys/socket.h>
+#include <sys/socketvar.h>
+#include <sys/syslog.h>
+#include <sys/systm.h>
+
+#include <machine/cpu.h>	/* before tcp_seq.h, for tcp_random18() */
+
+#include <vm/uma.h>
+
+#include <net/if.h>
+#include <net/route.h>
+
+#include <netinet/in.h>
+#include <netinet/in_systm.h>
+#include <netinet/ip.h>
+#include <netinet/ip_icmp.h>	/* for ICMP_BANDLIM		*/
+#include <netinet/in_var.h>
+#include <netinet/icmp_var.h>	/* for ICMP_BANDLIM		*/
+#include <netinet/in_pcb.h>
+#include <netinet/ip_var.h>
+#include <netinet/ip6.h>
+#include <netinet/icmp6.h>
+#include <netinet6/nd6.h>
+#include <netinet6/ip6_var.h>
+#include <netinet6/in6_pcb.h>
+#include <netinet/tcp.h>
+#include <netinet/tcp_fsm.h>
+#include <netinet/tcp_seq.h>
+#include <netinet/tcp_timer.h>
+#include <netinet/tcp_var.h>
+#include <netinet6/tcp6_var.h>
+#include <netinet/tcpip.h>
+#ifdef TCPDEBUG
+#include <netinet/tcp_debug.h>
+
+u_char tcp_saveipgen[40]; /* the size must be of max ip header, now IPv6 */
+struct tcphdr tcp_savetcp;
+#endif /* TCPDEBUG */
+
+#ifdef FAST_IPSEC
+#include <netipsec/ipsec.h>
+#include <netipsec/ipsec6.h>
+#endif
+
+#ifdef IPSEC
+#include <netinet6/ipsec.h>
+#include <netinet6/ipsec6.h>
+#include <netkey/key.h>
+#endif /*IPSEC*/
+#include <machine/in_cksum.h>
+
+extern struct uma_zone *sack_hole_zone;
+
+/*
+ * This function is called upon receipt of new valid data (while not in header
+ * prediction mode), and it updates the ordered list of sacks.
+ */
+void
+tcp_update_sack_list(tp)
+	struct tcpcb *tp;
+{
+	/*
+	 * First reported block MUST be the most recent one.  Subsequent
+	 * blocks SHOULD be in the order in which they arrived at the
+	 * receiver.  These two conditions make the implementation fully
+	 * compliant with RFC 2018.
+	 */
+	int i, j = 0, count = 0, lastpos = -1;
+	struct sackblk sack, firstsack, temp[MAX_SACK_BLKS];
+
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	/* First clean up current list of sacks */
+	for (i = 0; i < tp->rcv_numsacks; i++) {
+		sack = tp->sackblks[i];
+		if (sack.start == 0 && sack.end == 0) {
+			count++; /* count = number of blocks to be discarded */
+			continue;
+		}
+		if (SEQ_LEQ(sack.end, tp->rcv_nxt)) {
+			tp->sackblks[i].start = tp->sackblks[i].end = 0;
+			count++;
+		} else {
+			temp[j].start = tp->sackblks[i].start;
+			temp[j++].end = tp->sackblks[i].end;
+		}
+	}
+	tp->rcv_numsacks -= count;
+	if (tp->rcv_numsacks == 0) { /* no sack blocks currently (fast path) */
+		tcp_clean_sackreport(tp);
+		if (SEQ_LT(tp->rcv_nxt, tp->rcv_laststart)) {
+			/* ==> need first sack block */
+			tp->sackblks[0].start = tp->rcv_laststart;
+			tp->sackblks[0].end = tp->rcv_lastend;
+			tp->rcv_numsacks = 1;
+		}
+		return;
+	}
+	/* Otherwise, sack blocks are already present. */
+	for (i = 0; i < tp->rcv_numsacks; i++)
+		tp->sackblks[i] = temp[i]; /* first copy back sack list */
+	if (SEQ_GEQ(tp->rcv_nxt, tp->rcv_lastend))
+		return;     /* sack list remains unchanged */
+	/*
+	 * From here, segment just received should be (part of) the 1st sack.
+	 * Go through list, possibly coalescing sack block entries.
+	 */
+	firstsack.start = tp->rcv_laststart;
+	firstsack.end = tp->rcv_lastend;
+	for (i = 0; i < tp->rcv_numsacks; i++) {
+		sack = tp->sackblks[i];
+		if (SEQ_LT(sack.end, firstsack.start) ||
+		    SEQ_GT(sack.start, firstsack.end))
+			continue; /* no overlap */
+		if (sack.start == firstsack.start && sack.end == firstsack.end){
+			/*
+			 * identical block; delete it here since we will
+			 * move it to the front of the list.
+			 */
+			tp->sackblks[i].start = tp->sackblks[i].end = 0;
+			lastpos = i;    /* last posn with a zero entry */
+			continue;
+		}
+		if (SEQ_LEQ(sack.start, firstsack.start))
+			firstsack.start = sack.start; /* merge blocks */
+		if (SEQ_GEQ(sack.end, firstsack.end))
+			firstsack.end = sack.end;     /* merge blocks */
+		tp->sackblks[i].start = tp->sackblks[i].end = 0;
+		lastpos = i;    /* last posn with a zero entry */
+	}
+	if (lastpos != -1) {    /* at least one merge */
+		for (i = 0, j = 1; i < tp->rcv_numsacks; i++) {
+			sack = tp->sackblks[i];
+			if (sack.start == 0 && sack.end == 0)
+				continue;
+			temp[j++] = sack;
+		}
+		tp->rcv_numsacks = j; /* including first blk (added later) */
+		for (i = 1; i < tp->rcv_numsacks; i++) /* now copy back */
+			tp->sackblks[i] = temp[i];
+	} else {        /* no merges -- shift sacks by 1 */
+		if (tp->rcv_numsacks < MAX_SACK_BLKS)
+			tp->rcv_numsacks++;
+		for (i = tp->rcv_numsacks-1; i > 0; i--)
+			tp->sackblks[i] = tp->sackblks[i-1];
+	}
+	tp->sackblks[0] = firstsack;
+	return;
+}
+
+/*
+ * Delete all receiver-side SACK information.
+ */
+void
+tcp_clean_sackreport(tp)
+	struct tcpcb *tp;
+{
+	int i;
+
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	tp->rcv_numsacks = 0;
+	for (i = 0; i < MAX_SACK_BLKS; i++)
+		tp->sackblks[i].start = tp->sackblks[i].end=0;
+}
+
+/*
+ * Process the TCP SACK option.  Returns 1 if tcp_dooptions() should continue,
+ * and 0 otherwise, if the option was fine.  tp->snd_holes is an ordered list
+ * of holes (oldest to newest, in terms of the sequence space).
+ */
+int
+tcp_sack_option(struct tcpcb *tp, struct tcphdr *th, u_char *cp, int optlen)
+{
+	int tmp_olen;
+	u_char *tmp_cp;
+	struct sackhole *cur, *p, *temp;
+
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	if (!tp->sack_enable)
+		return (1);
+
+	/* Note: TCPOLEN_SACK must be 2*sizeof(tcp_seq) */
+	if (optlen <= 2 || (optlen - 2) % TCPOLEN_SACK != 0)
+		return (1);
+	tmp_cp = cp + 2;
+	tmp_olen = optlen - 2;
+	tcpstat.tcps_sack_rcv_blocks++;
+	if (tp->snd_numholes < 0)
+		tp->snd_numholes = 0;
+	if (tp->t_maxseg == 0)
+		panic("tcp_sack_option"); /* Should never happen */
+	while (tmp_olen > 0) {
+		struct sackblk sack;
+
+		bcopy(tmp_cp, (char *) &(sack.start), sizeof(tcp_seq));
+		sack.start = ntohl(sack.start);
+		bcopy(tmp_cp + sizeof(tcp_seq),
+		    (char *) &(sack.end), sizeof(tcp_seq));
+		sack.end = ntohl(sack.end);
+		tmp_olen -= TCPOLEN_SACK;
+		tmp_cp += TCPOLEN_SACK;
+		if (SEQ_LEQ(sack.end, sack.start))
+			continue; /* bad SACK fields */
+		if (SEQ_LEQ(sack.end, tp->snd_una))
+			continue; /* old block */
+		if (SEQ_GT(th->th_ack, tp->snd_una)) {
+			if (SEQ_LT(sack.start, th->th_ack))
+				continue;
+		}
+		if (SEQ_GT(sack.end, tp->snd_max))
+			continue;
+		if (tp->snd_holes == NULL) { /* first hole */
+			tp->snd_holes = (struct sackhole *)
+				uma_zalloc(sack_hole_zone,M_NOWAIT);
+			if (tp->snd_holes == NULL) {
+				/* ENOBUFS, so ignore SACKed block for now*/
+				continue;
+			}
+			cur = tp->snd_holes;
+			cur->start = th->th_ack;
+			cur->end = sack.start;
+			cur->rxmit = cur->start;
+			cur->next = NULL;
+			tp->snd_numholes = 1;
+			tp->rcv_lastsack = sack.end;
+			continue; /* with next sack block */
+		}
+		/* Go thru list of holes:  p = previous,  cur = current */
+		p = cur = tp->snd_holes;
+		while (cur) {
+			if (SEQ_LEQ(sack.end, cur->start))
+				/* SACKs data before the current hole */
+				break; /* no use going through more holes */
+			if (SEQ_GEQ(sack.start, cur->end)) {
+				/* SACKs data beyond the current hole */
+				p = cur;
+				cur = cur->next;
+				continue;
+			}
+			if (SEQ_LEQ(sack.start, cur->start)) {
+				/* Data acks at least the beginning of hole */
+				if (SEQ_GEQ(sack.end, cur->end)) {
+					/* Acks entire hole, so delete hole */
+					if (p != cur) {
+						p->next = cur->next;
+						uma_zfree(sack_hole_zone, cur);
+						cur = p->next;
+					} else {
+						cur = cur->next;
+						uma_zfree(sack_hole_zone, p);
+						p = cur;
+						tp->snd_holes = p;
+					}
+					tp->snd_numholes--;
+					continue;
+				}
+				/* otherwise, move start of hole forward */
+				cur->start = sack.end;
+				cur->rxmit = SEQ_MAX(cur->rxmit, cur->start);
+				p = cur;
+				cur = cur->next;
+				continue;
+			}
+			/* move end of hole backward */
+			if (SEQ_GEQ(sack.end, cur->end)) {
+				cur->end = sack.start;
+				cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
+				p = cur;
+				cur = cur->next;
+				continue;
+			}
+			if (SEQ_LT(cur->start, sack.start) &&
+			    SEQ_GT(cur->end, sack.end)) {
+				/*
+				 * ACKs some data in middle of a hole; need to
+				 * split current hole
+				 */
+				temp = (struct sackhole *)
+					uma_zalloc(sack_hole_zone,M_NOWAIT);
+				if (temp == NULL)
+					continue; /* ENOBUFS */
+				temp->next = cur->next;
+				temp->start = sack.end;
+				temp->end = cur->end;
+				temp->rxmit = SEQ_MAX(cur->rxmit, temp->start);
+				cur->end = sack.start;
+				cur->rxmit = SEQ_MIN(cur->rxmit, cur->end);
+				cur->next = temp;
+				p = temp;
+				cur = p->next;
+				tp->snd_numholes++;
+			}
+		}
+		/* At this point, p points to the last hole on the list */
+		if (SEQ_LT(tp->rcv_lastsack, sack.start)) {
+			/*
+			 * Need to append new hole at end.
+			 * Last hole is p (and it's not NULL).
+			 */
+			temp = (struct sackhole *)
+				uma_zalloc(sack_hole_zone,M_NOWAIT);
+			if (temp == NULL)
+				continue; /* ENOBUFS */
+			temp->start = tp->rcv_lastsack;
+			temp->end = sack.start;
+			temp->rxmit = temp->start;
+			temp->next = 0;
+			p->next = temp;
+			tp->rcv_lastsack = sack.end;
+			tp->snd_numholes++;
+		}
+	}
+	return (0);
+}
+
+/*
+ * Delete stale (i.e, cumulatively ack'd) holes.  Hole is deleted only if
+ * it is completely acked; otherwise, tcp_sack_option(), called from
+ * tcp_dooptions(), will fix up the hole.
+ */
+void
+tcp_del_sackholes(tp, th)
+	struct tcpcb *tp;
+	struct tcphdr *th;
+{
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	if (tp->sack_enable && tp->t_state != TCPS_LISTEN) {
+		/* max because this could be an older ack just arrived */
+		tcp_seq lastack = SEQ_GT(th->th_ack, tp->snd_una) ?
+			th->th_ack : tp->snd_una;
+		struct sackhole *cur = tp->snd_holes;
+		struct sackhole *prev;
+		while (cur)
+			if (SEQ_LEQ(cur->end, lastack)) {
+				prev = cur;
+				cur = cur->next;
+				uma_zfree(sack_hole_zone, prev);
+				tp->snd_numholes--;
+			} else if (SEQ_LT(cur->start, lastack)) {
+				cur->start = lastack;
+				if (SEQ_LT(cur->rxmit, cur->start))
+					cur->rxmit = cur->start;
+				break;
+			} else
+				break;
+		tp->snd_holes = cur;
+	}
+}
+
+void
+tcp_free_sackholes(struct tcpcb *tp)
+{
+	struct sackhole *p, *q;
+
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	q = tp->snd_holes;
+	while (q != NULL) {
+		p = q;
+		q = q->next;
+		uma_zfree(sack_hole_zone, p);
+	}
+	tp->snd_holes = 0;
+}
+
+/*
+ * Checks for partial ack.  If partial ack arrives, turn off retransmission
+ * timer, deflate the window, do not clear tp->t_dupacks, and return 1.
+ * If the ack advances at least to tp->snd_recover, return 0.
+ */
+void
+tcp_sack_partialack(tp, th)
+	struct tcpcb *tp;
+	struct tcphdr *th;
+{
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	u_long  ocwnd = tp->snd_cwnd;
+
+	callout_stop(tp->tt_rexmt);
+	tp->t_rtttime = 0;
+	/*
+	 * Set snd_cwnd to one segment beyond acknowledged offset
+	 * (tp->snd_una has not yet been updated when this function is called.)
+	 */
+	/* 
+	 * Should really be 
+	 * min(tp->snd_cwnd, tp->t_maxseg + (th->th_ack - tp->snd_una)) 
+	 */
+	tp->snd_cwnd = tp->t_maxseg + (th->th_ack - tp->snd_una);
+	tp->t_flags |= TF_ACKNOW;
+	(void) tcp_output(tp);
+	tp->snd_cwnd = ocwnd;
+	/*
+	 * Partial window deflation.  Relies on fact that tp->snd_una
+	 * not updated yet.
+	 */
+	tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_maxseg);
+}
+
+#ifdef TCP_SACK_DEBUG
+void
+tcp_print_holes(struct tcpcb *tp)
+{
+	struct sackhole *p = tp->snd_holes;
+	if (p == 0)
+		return;
+	printf("Hole report: start--end dups rxmit\n");
+	while (p) {
+		printf("%x--%x r %x\n", p->start, p->end, p->rxmit);
+		p = p->next;
+	}
+	printf("\n");
+}
+#endif /* TCP_SACK_DEBUG */
+
+/*
+ * Returns pointer to a sackhole if there are any pending retransmissions;
+ * NULL otherwise.
+ */
+struct sackhole *
+tcp_sack_output(struct tcpcb *tp)
+{
+	struct sackhole *p;
+
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	if (!tp->sack_enable)
+		return (NULL);
+	p = tp->snd_holes;
+	while (p) {
+		if (SEQ_LT(p->rxmit, p->end)) {
+			if (SEQ_LT(p->rxmit, tp->snd_una)) {/* old SACK hole */
+				p = p->next;
+				continue;
+			}
+#ifdef TCP_SACK_DEBUG
+			if (p)
+				tcp_print_holes(tp);
+#endif
+			return (p);
+		}
+		p = p->next;
+	}
+	return (NULL);
+}
+
+/*
+ * After a timeout, the SACK list may be rebuilt.  This SACK information
+ * should be used to avoid retransmitting SACKed data.  This function
+ * traverses the SACK list to see if snd_nxt should be moved forward.
+ */
+void
+tcp_sack_adjust(struct tcpcb *tp)
+{
+	INP_LOCK_ASSERT(tp->t_inpcb);
+	struct sackhole *cur = tp->snd_holes;
+	if (cur == NULL)
+		return; /* No holes */
+	if (SEQ_GEQ(tp->snd_nxt, tp->rcv_lastsack))
+		return; /* We're already beyond any SACKed blocks */
+	/*
+	 * Two cases for which we want to advance snd_nxt:
+	 * i) snd_nxt lies between end of one hole and beginning of another
+	 * ii) snd_nxt lies between end of last hole and rcv_lastsack
+	 */
+	while (cur->next) {
+		if (SEQ_LT(tp->snd_nxt, cur->end))
+			return;
+		if (SEQ_GEQ(tp->snd_nxt, cur->next->start))
+			cur = cur->next;
+		else {
+			tp->snd_nxt = cur->next->start;
+			return;
+		}
+	}
+	if (SEQ_LT(tp->snd_nxt, cur->end))
+		return;
+	tp->snd_nxt = tp->rcv_lastsack;
+	return;
+}
+