Tail loss probe
https://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01


Index: sys/netinet/tcp_input.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_input.c,v
retrieving revision 1.347
diff -d -u -r1.347 tcp_input.c
--- sys/netinet/tcp_input.c     10 Jun 2016 13:31:44 -0000      1.347
+++ sys/netinet/tcp_input.c     25 Jul 2016 13:13:34 -0000
@@ -1998,6 +1998,11 @@
                       tp->ts_recent_age = tcp_now;
                       tp->ts_recent = opti.ts_val;
               }
+               /*
+                * Disable Tail Loss Probe timer
+                */
+               if (TCP_TIMER_ISARMED(tp, TCPT_TLP))
+                       TCP_TIMER_DISARM(tp, TCPT_TLP);

               if (tlen == 0) {
                       /* Ack prediction. */
Index: sys/netinet/tcp_output.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_output.c,v
retrieving revision 1.186
diff -d -u -r1.186 tcp_output.c
--- sys/netinet/tcp_output.c    10 Jun 2016 13:27:16 -0000      1.186
+++ sys/netinet/tcp_output.c    25 Jul 2016 13:13:34 -0000
@@ -578,6 +578,11 @@
       int sigoff = 0;
#endif
       uint64_t *tcps;
+       uint64_t flightsize;
+
+       /* but first things first - disarm the TLP timer */
+       if (TCP_TIMER_ISARMED(tp, TCPT_TLP))
+               TCP_TIMER_DISARM(tp, TCPT_TLP);

#ifdef DIAGNOSTIC
       if (tp->t_inpcb && tp->t_in6pcb)
@@ -676,6 +681,7 @@
       } else {
               if (idle && (tcp_now - tp->t_rcvtime) >= tp->t_rxtcur) {
                       /*
+                        * RFC5681 Section 4.1.
                        * 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.
@@ -1526,6 +1532,46 @@
                       }
               }

+timer:
+               flightsize = (tp->snd_nxt - tp->sack_newdata) +
+                   sack_bytes_rxmt;
+               if (tcp_tlp_enabled != 0 &&
+                   TCPS_HAVEESTABLISHED(tp->t_state) &&
+                   TCP_SACK_ENABLED(tp) &&
+                   len != 0 &&
+                   /* only if we've transmitted everything or cwnd reached */
+                   (off + len == so->so_snd.sb_cc ||
+                   tp->snd_cwnd - flightsize < tp->t_segsz) &&
+                   tp->snd_tlp_nxt != tp->snd_nxt) {
+                       int pto;
+                       /*
+                        * Arm the timer for tail loss probe
+                        * Draft dukkipati Section 2.1
+                        * FlightSize > 1: schedule PTO in max(2*SRTT, 10ms)
+                        * tp->t_srtt >> 5 = hz expressed value
+                        */
+                       if (flightsize > tp->t_segsz)
+                               pto = max(tp->t_srtt >> 4, mstohz(10));
+                       /*
+                        * FlightSize == 1:
+                        * schedule PTO in max(2*SRTT, 1.5*SRTT+WCDelAckT).
+                        */
+                       else
+                               pto = max(tp->t_srtt >> 4,
+                                   3 * (tp->t_srtt >> 6) + mstohz(200));
+                       /*
+                        * If RTO is earlier, schedule PTO in its place:
+                        * PTO = min(RTO, PTO)
+                        */
+                       if (pto >= TCP_REXMTVAL(tp) * hz / PR_SLOWHZ &&
+                           TCP_TIMER_ISARMED(tp, TCPT_REXMT)) {
+                               TCP_TIMER_DISARM(tp, TCPT_REXMT);
+                               pto = TCP_REXMTVAL(tp) * hz / PR_SLOWHZ;
+                       }
+                       tp->snd_tlp_nxt = tp->snd_nxt;
+                       TCP_TIMER_ARM_HZ(tp, TCPT_TLP, pto > 0 ? pto : 1);
+               }
+
               /*
                * Set retransmit timer if not currently set,
                * and not doing an ack or a keep-alive probe.
@@ -1534,10 +1580,10 @@
                * Initialize shift counter which is used for backoff
                * of retransmit time.
                */
-timer:
               if (TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0) {
-                       if ((sack_rxmit && tp->snd_nxt != tp->snd_max)
-                           || tp->snd_nxt != tp->snd_una) {
+                       if (((sack_rxmit && tp->snd_nxt != tp->snd_max)
+                           || tp->snd_nxt != tp->snd_una) &&
+                           TCP_TIMER_ISARMED(tp, TCPT_TLP) == 0) {
                               if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) {
                                       TCP_TIMER_DISARM(tp, TCPT_PERSIST);
                                       tp->t_rxtshift = 0;
Index: sys/netinet/tcp_seq.h
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_seq.h,v
retrieving revision 1.17
diff -d -u -r1.17 tcp_seq.h
--- sys/netinet/tcp_seq.h       25 Jul 2014 17:53:59 -0000      1.17
+++ sys/netinet/tcp_seq.h       25 Jul 2016 13:13:34 -0000
@@ -58,7 +58,8 @@

#define        tcp_sendseqinit(tp) \
       (tp)->snd_una = (tp)->snd_nxt = (tp)->snd_max = (tp)->snd_up = \
-           (tp)->snd_recover = (tp)->snd_high = (tp)->snd_fack = (tp)->iss
+           (tp)->snd_recover = (tp)->snd_high = (tp)->snd_fack = \
+           (tp)->snd_tlp_nxt = (tp)->iss

#define TCP_ISS_RANDOM_MASK 0x00ffffff /* bits of randomness in a TCP ISS */
#define TCP_ISSINCR         0x01000000 /* increment per time and per conn */
Index: sys/netinet/tcp_timer.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_timer.c,v
retrieving revision 1.90
diff -d -u -r1.90 tcp_timer.c
--- sys/netinet/tcp_timer.c     26 Apr 2016 08:44:44 -0000      1.90
+++ sys/netinet/tcp_timer.c     25 Jul 2016 13:13:34 -0000
@@ -149,6 +149,8 @@

int    tcp_maxpersistidle = 0;         /* max idle time in persist */

+u_int  tcp_tlp_enabled = 0;            /* tail loss probe */
+
/*
 * Time to delay the ACK.  This is initialized in tcp_init(), unless
 * its patched.
@@ -159,12 +161,14 @@
void   tcp_timer_persist(void *);
void   tcp_timer_keep(void *);
void   tcp_timer_2msl(void *);
+void   tcp_timer_tlp(void *);

const tcp_timer_func_t tcp_timer_funcs[TCPT_NTIMERS] = {
       tcp_timer_rexmt,
       tcp_timer_persist,
       tcp_timer_keep,
       tcp_timer_2msl,
+       tcp_timer_tlp,
};

/*
@@ -658,3 +662,61 @@
       mutex_exit(softnet_lock);
       KERNEL_UNLOCK_ONE(NULL);
}
+
+void
+tcp_timer_tlp(void *arg)
+{
+       struct tcpcb *tp = arg;
+#ifdef INET
+       struct inpcb *inp;
+#endif
+#ifdef INET6
+       struct in6pcb *in6p;
+#endif
+       struct socket *so = NULL;
+
+       mutex_enter(softnet_lock);
+       if ((tp->t_flags & TF_DEAD) != 0 ||
+           TCPS_HAVEESTABLISHED(tp->t_state) == 0) {
+               mutex_exit(softnet_lock);
+               return;
+       }
+       if (!callout_expired(&tp->t_timer[TCPT_TLP])) {
+               mutex_exit(softnet_lock);
+               return;
+       }
+
+       /* Do a tail loss probe rxmit */
+       KERNEL_LOCK(1, NULL);
+       /* check if tlp is still needed */
+       if (tp->snd_tlp_nxt == tp->snd_nxt ||
+           TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0) {
+
+               TCP_STATINC(TCP_STAT_TLP);
+
+               tcp_free_sackholes(tp);
+               tp->snd_fack = tp->snd_una;
+
+#ifdef INET
+               inp = tp->t_inpcb;
+               if (inp)
+                       so = inp->inp_socket;
+#endif
+#ifdef INET6
+               in6p = tp->t_in6pcb;
+               if (in6p)
+                       so = in6p->in6p_socket;
+#endif
+               KASSERT(so != NULL);
+               /* Check if it should rexmit */
+               if (tp->snd_tlp_nxt - tp->snd_una == so->so_snd.sb_cc)
+                       tp->snd_nxt = tp->snd_una;
+
+               tp->t_force = 1;
+               (void) tcp_output(tp);
+               tp->t_force = 0;
+       }
+
+       KERNEL_UNLOCK_ONE(NULL);
+       mutex_exit(softnet_lock);
+}
Index: sys/netinet/tcp_timer.h
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_timer.h,v
retrieving revision 1.28
diff -d -u -r1.28 tcp_timer.h
--- sys/netinet/tcp_timer.h     24 May 2011 18:37:52 -0000      1.28
+++ sys/netinet/tcp_timer.h     25 Jul 2016 13:13:34 -0000
@@ -69,12 +69,13 @@
 * Definitions of the TCP timers.  These timers are counted
 * down PR_SLOWHZ times a second.
 */
-#define        TCPT_NTIMERS    4
+#define        TCPT_NTIMERS    5

#define        TCPT_REXMT      0               /* retransmit */
#define        TCPT_PERSIST    1               /* retransmit persistance */
#define        TCPT_KEEP       2               /* keep alive */
#define        TCPT_2MSL       3               /* 2*msl quiet time timer */
+#define        TCPT_TLP        4               /* tail loss probe */

/*
 * The TCPT_REXMT timer is used to force retransmissions.
@@ -110,6 +111,9 @@
 * an ack segment in response from the peer.  If, despite the TCPT_KEEP
 * initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE
 * amount of time probing, then we drop the connection.
+ *
+ * The TCPT_TLP timer is used to trigger retransmissions on short streams
+ * that wouldn't have chance to rexmit a segment until RTO
 */

/*
@@ -141,7 +145,7 @@

#ifdef TCPTIMERS
const char *tcptimers[] =
-    { "REXMT", "PERSIST", "KEEP", "2MSL" };
+    { "REXMT", "PERSIST", "KEEP", "2MSL", "TLP" };
#endif

/*
@@ -159,6 +163,13 @@
       callout_schedule(&(tp)->t_timer[(timer)],                       \
           (nticks) * (hz / PR_SLOWHZ))

+/*
+ * Arm the timer for a specific number of hz units
+ */
+#define TCP_TIMER_ARM_HZ(tp, timer, nticks)                            \
+       callout_schedule(&(tp)->t_timer[(timer)],                       \
+           (nticks))
+
#define        TCP_TIMER_DISARM(tp, timer)                                     \
       callout_stop(&(tp)->t_timer[(timer)])

@@ -188,6 +199,7 @@
extern int tcp_maxpersistidle;         /* max idle time in persist */
extern int tcp_ttl;                    /* time to live for TCP segs */
extern const int tcp_backoff[];
+extern u_int tcp_tlp_enabled;          /* tail loss probe enabled */

void   tcp_timer_init(void);
#endif
Index: sys/netinet/tcp_usrreq.c
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_usrreq.c,v
retrieving revision 1.212
diff -d -u -r1.212 tcp_usrreq.c
--- sys/netinet/tcp_usrreq.c    26 Apr 2016 08:44:45 -0000      1.212
+++ sys/netinet/tcp_usrreq.c    25 Jul 2016 13:13:34 -0000
@@ -2184,6 +2184,13 @@
                      sysctl_tcp_keep, 0, &tcp_keepcnt, 0,
                      CTL_NET, pf, IPPROTO_TCP, TCPCTL_KEEPCNT, CTL_EOL);
       sysctl_createv(clog, 0, NULL, NULL,
+                      CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
+                      CTLTYPE_INT, "tlp",
+                      SYSCTL_DESCR("Enable tail loss probe"),
+                      NULL, 0, &tcp_tlp_enabled, 0,
+                      CTL_NET, pf, IPPROTO_TCP, TCPCTL_TLP, CTL_EOL);
+
+       sysctl_createv(clog, 0, NULL, NULL,
                      CTLFLAG_PERMANENT|CTLFLAG_IMMEDIATE,
                      CTLTYPE_INT, "slowhz",
                      SYSCTL_DESCR("Keepalive ticks per second"),
Index: sys/netinet/tcp_var.h
===================================================================
RCS file: /cvsroot/src/sys/netinet/tcp_var.h,v
retrieving revision 1.177
diff -d -u -r1.177 tcp_var.h
--- sys/netinet/tcp_var.h       14 Feb 2015 22:09:53 -0000      1.177
+++ sys/netinet/tcp_var.h       25 Jul 2016 13:13:34 -0000
@@ -235,6 +235,7 @@
       tcp_seq snd_una;                /* send unacknowledged */
       tcp_seq snd_nxt;                /* send next */
       tcp_seq snd_up;                 /* send urgent pointer */
+       tcp_seq snd_tlp_nxt;            /* next sequence when we sent tlp */
       tcp_seq snd_wl1;                /* window update seg seq number */
       tcp_seq snd_wl2;                /* window update seg ack number */
       tcp_seq iss;                    /* initial send sequence number */
@@ -710,8 +711,9 @@
#define        TCP_STAT_ECN_SHS        73      /* # of successful ECN handshakes */
#define        TCP_STAT_ECN_CE         74      /* # of packets with CE bit */
#define        TCP_STAT_ECN_ECT        75      /* # of packets with ECT(0) bit */
+#define        TCP_STAT_TLP            76      /* # of tail loss probes sent */

-#define        TCP_NSTATS              76
+#define        TCP_NSTATS              77

/*
 * Names for TCP sysctl objects.
@@ -754,7 +756,8 @@
#define        TCPCTL_DEBX             32      /* # of tcp debug sockets */
#define        TCPCTL_DROP             33      /* drop tcp connection */
#define        TCPCTL_MSL              34      /* Max Segment Life */
-#define        TCPCTL_MAXID            35
+#define        TCPCTL_TLP              35      /* Tail Loss Probe */
+#define        TCPCTL_MAXID            36

#define        TCPCTL_NAMES { \
       { 0, 0 }, \
Index: usr.bin/netstat/inet.c
===================================================================
RCS file: /cvsroot/src/usr.bin/netstat/inet.c,v
retrieving revision 1.106
diff -d -u -r1.106 inet.c
--- usr.bin/netstat/inet.c      8 Feb 2015 15:09:45 -0000       1.106
+++ usr.bin/netstat/inet.c      25 Jul 2016 13:13:36 -0000
@@ -503,6 +503,7 @@
       p(TCP_STAT_ECN_SHS, "\t%" PRIu64 " successful ECN handshake%s\n");
       p(TCP_STAT_ECN_CE, "\t%" PRIu64 " packet%s with ECN CE bit\n");
       p(TCP_STAT_ECN_ECT, "\t%" PRIu64 " packet%s ECN ECT(0) bit\n");
+       p(TCP_STAT_TLP, "\t%" PRIu64 " tail drop probe%s\n");
#undef p
#undef ps
#undef p2

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