Index: tcp_input.c
===================================================================
--- tcp_input.c
+++ tcp_input.c
@@ -1824,6 +1824,15 @@
 				tcp_clean_sackreport(tp);
 			TCPSTAT_INC(tcps_preddat);
 			tp->rcv_nxt += tlen;
+			if (tlen &&
+			    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+			    (tp->t_fbyte_in == 0)) {
+				tp->t_fbyte_in = ticks;
+				if (tp->t_fbyte_in == 0)
+					tp->t_fbyte_in = 1;
+				if (tp->t_fbyte_out && tp->t_fbyte_in)
+					tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+			}
 			/*
 			 * Pull snd_wl1 up to prevent seq wrap relative to
 			 * th_seq.
@@ -2999,6 +3008,15 @@
 			else
 				tp->t_flags |= TF_ACKNOW;
 			tp->rcv_nxt += tlen;
+			if (tlen &&
+			    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+			    (tp->t_fbyte_in == 0)) {
+				tp->t_fbyte_in = ticks;
+				if (tp->t_fbyte_in == 0)
+					tp->t_fbyte_in = 1;
+				if (tp->t_fbyte_out && tp->t_fbyte_in)
+					tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+			}
 			thflags = th->th_flags & TH_FIN;
 			TCPSTAT_INC(tcps_rcvpack);
 			TCPSTAT_ADD(tcps_rcvbyte, tlen);
Index: tcp_log_buf.h
===================================================================
--- tcp_log_buf.h
+++ tcp_log_buf.h
@@ -32,7 +32,7 @@
 
 #define	TCP_LOG_REASON_LEN	32
 #define	TCP_LOG_TAG_LEN		32
-#define	TCP_LOG_BUF_VER		(8)
+#define	TCP_LOG_BUF_VER		(9)
 
 /*
  * Because the (struct tcp_log_buffer) includes 8-byte uint64_t's, it requires
@@ -143,6 +143,7 @@
 	uint32_t	tlb_rttvar;	/* TCPCB t_rttvar */
 	uint32_t	tlb_rcv_up;	/* TCPCB rcv_up */
 	uint32_t	tlb_rcv_adv;	/* TCPCB rcv_adv */
+	uint32_t	tlb_flags2;	/* TCPCB t_flags2 */
 	uint32_t	tlb_rcv_nxt;	/* TCPCB rcv_nxt */
 	uint32_t	tlb_rcv_wnd;	/* TCPCB rcv_wnd */
 	uint32_t	tlb_dupacks;	/* TCPCB t_dupacks */
@@ -150,6 +151,8 @@
 	int		tlb_snd_numholes; /* TCPCB snd_numholes */
 	uint32_t	tlb_flex1;	/* Event specific information */
 	uint32_t	tlb_flex2;	/* Event specific information */
+	uint32_t	tlb_fbyte_in;	/* TCPCB first byte in time */
+	uint32_t	tlb_fbyte_out;	/* TCPCB first byte out time */
 	uint8_t		tlb_snd_scale:4, /* TCPCB snd_scale */
 			tlb_rcv_scale:4; /* TCPCB rcv_scale */
 	uint8_t		_pad[3];	/* Padding */
Index: tcp_log_buf.c
===================================================================
--- tcp_log_buf.c
+++ tcp_log_buf.c
@@ -1693,6 +1693,9 @@
 	COPY_STAT(snd_numholes);
 	COPY_STAT(snd_scale);
 	COPY_STAT(rcv_scale);
+	COPY_STAT_T(flags2);
+	COPY_STAT_T(fbyte_in);
+	COPY_STAT_T(fbyte_out);
 #undef COPY_STAT
 #undef COPY_STAT_T
 	log_buf->tlb_flex1 = 0;
Index: tcp_stacks/bbr.c
===================================================================
--- tcp_stacks/bbr.c
+++ tcp_stacks/bbr.c
@@ -8451,6 +8451,15 @@
 				tp->t_flags |= TF_ACKNOW;
 			}
 			tp->rcv_nxt += tlen;
+			if (tlen &&
+			    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+			    (tp->t_fbyte_in == 0)) {
+				tp->t_fbyte_in = ticks;
+				if (tp->t_fbyte_in == 0)
+					tp->t_fbyte_in = 1;
+				if (tp->t_fbyte_out && tp->t_fbyte_in)
+					tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+			}
 			thflags = th->th_flags & TH_FIN;
 			KMOD_TCPSTAT_ADD(tcps_rcvpack, (int)nsegs);
 			KMOD_TCPSTAT_ADD(tcps_rcvbyte, tlen);
@@ -8667,6 +8676,15 @@
 		tcp_clean_sackreport(tp);
 	KMOD_TCPSTAT_INC(tcps_preddat);
 	tp->rcv_nxt += tlen;
+	if (tlen &&
+	    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+	    (tp->t_fbyte_in == 0)) {
+		tp->t_fbyte_in = ticks;
+		if (tp->t_fbyte_in == 0)
+			tp->t_fbyte_in = 1;
+		if (tp->t_fbyte_out && tp->t_fbyte_in)
+			tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+	}
 	/*
 	 * Pull snd_wl1 up to prevent seq wrap relative to th_seq.
 	 */
Index: tcp_stacks/rack.c
===================================================================
--- tcp_stacks/rack.c
+++ tcp_stacks/rack.c
@@ -8756,6 +8756,15 @@
 #endif
 			rack_handle_delayed_ack(tp, rack, tlen, tfo_syn);
 			tp->rcv_nxt += tlen;
+			if (tlen &&
+			    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+			    (tp->t_fbyte_in == 0)) {
+				tp->t_fbyte_in = ticks;
+				if (tp->t_fbyte_in == 0)
+					tp->t_fbyte_in = 1;
+				if (tp->t_fbyte_out && tp->t_fbyte_in)
+					tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+			}
 			thflags = th->th_flags & TH_FIN;
 			KMOD_TCPSTAT_ADD(tcps_rcvpack, nsegs);
 			KMOD_TCPSTAT_ADD(tcps_rcvbyte, tlen);
@@ -8979,6 +8988,15 @@
 		tcp_clean_sackreport(tp);
 	KMOD_TCPSTAT_INC(tcps_preddat);
 	tp->rcv_nxt += tlen;
+	if (tlen &&
+	    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+	    (tp->t_fbyte_in == 0)) {
+		tp->t_fbyte_in = ticks;
+		if (tp->t_fbyte_in == 0)
+			tp->t_fbyte_in = 1;
+		if (tp->t_fbyte_out && tp->t_fbyte_in)
+			tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+	}
 	/*
 	 * Pull snd_wl1 up to prevent seq wrap relative to th_seq.
 	 */
Index: tcp_usrreq.c
===================================================================
--- tcp_usrreq.c
+++ tcp_usrreq.c
@@ -1170,6 +1170,16 @@
 			socantsendmore(so);
 			tcp_usrclosed(tp);
 		}
+		if (TCPS_HAVEESTABLISHED(tp->t_state) &&
+		    ((tp->t_flags2 & TF2_FBYTES_COMPLETE) == 0) &&
+		    (tp->t_fbyte_out == 0) &&
+		    (so->so_snd.sb_ccc > 0)) {
+			tp->t_fbyte_out = ticks;
+			if (tp->t_fbyte_out == 0)
+				tp->t_fbyte_out = 1;
+			if (tp->t_fbyte_out && tp->t_fbyte_in)
+				tp->t_flags2 |= TF2_FBYTES_COMPLETE;
+		}
 		if (!(inp->inp_flags & INP_DROPPED) &&
 		    !(flags & PRUS_NOTREADY)) {
 			if (flags & PRUS_MORETOCOME)
Index: tcp_var.h
===================================================================
--- tcp_var.h
+++ tcp_var.h
@@ -202,6 +202,8 @@
 
 	tcp_seq	t_rtseq;		/* sequence number being timed */
 	u_int	t_starttime;		/* time connection was established */
+	u_int	t_fbyte_in;		/* ticks time when first byte queued in */
+	u_int	t_fbyte_out;		/* ticks time when first byte queued out */
 
 	u_int	t_pmtud_saved_maxseg;	/* pre-blackhole MSS */
 	int	t_blackhole_enter;	/* when to enter blackhole detection */
@@ -434,7 +436,7 @@
 #define	TF2_ECN_SND_CWR		0x00000040 /* ECN CWR in queue */
 #define	TF2_ECN_SND_ECE		0x00000080 /* ECN ECE in queue */
 #define	TF2_ACE_PERMIT		0x00000100 /* Accurate ECN mode */
-
+#define TF2_FBYTES_COMPLETE	0x00000400 /* We have first bytes in and out */
 /*
  * Structure to hold TCP options that are only used during segment
  * processing (in tcp_input), but not held in the tcpcb.