2 * Copyright (c) 2004 Jeffrey M. Hsu. All rights reserved.
3 * Copyright (c) 2004 The DragonFly Project. All rights reserved.
5 * This code is derived from software contributed to The DragonFly Project
8 * Redistribution and use in source and binary forms, with or without
9 * modification, are permitted provided that the following conditions
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15 * documentation and/or other materials provided with the distribution.
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17 * contributors may be used to endorse or promote products derived
18 * from this software without specific, prior written permission.
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62 * @(#)tcp_output.c 8.4 (Berkeley) 5/24/95
63 * $FreeBSD: src/sys/netinet/tcp_output.c,v 1.39.2.20 2003/01/29 22:45:36 hsu Exp $
67 #include "opt_inet6.h"
68 #include "opt_tcpdebug.h"
70 #include <sys/param.h>
71 #include <sys/systm.h>
72 #include <sys/kernel.h>
73 #include <sys/sysctl.h>
75 #include <sys/domain.h>
76 #include <sys/protosw.h>
77 #include <sys/socket.h>
78 #include <sys/socketvar.h>
79 #include <sys/in_cksum.h>
80 #include <sys/thread.h>
81 #include <sys/globaldata.h>
84 #include <net/if_var.h>
85 #include <net/route.h>
86 #include <net/netmsg2.h>
87 #include <net/netisr2.h>
89 #include <netinet/in.h>
90 #include <netinet/in_systm.h>
91 #include <netinet/ip.h>
92 #include <netinet/in_pcb.h>
93 #include <netinet/ip_var.h>
94 #include <netinet6/in6_pcb.h>
95 #include <netinet/ip6.h>
96 #include <netinet6/ip6_var.h>
97 #include <netinet/tcp.h>
99 #include <netinet/tcp_fsm.h>
100 #include <netinet/tcp_seq.h>
101 #include <netinet/tcp_timer.h>
102 #include <netinet/tcp_timer2.h>
103 #include <netinet/tcp_var.h>
104 #include <netinet/tcpip.h>
106 #include <netinet/tcp_debug.h>
110 extern struct mbuf
*m_copypack();
113 int path_mtu_discovery
= 1;
114 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, path_mtu_discovery
, CTLFLAG_RW
,
115 &path_mtu_discovery
, 1, "Enable Path MTU Discovery");
117 static int avoid_pure_win_update
= 1;
118 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, avoid_pure_win_update
, CTLFLAG_RW
,
119 &avoid_pure_win_update
, 1, "Avoid pure window updates when possible");
122 * 1 - enabled for increasing and decreasing the buffer size
123 * 2 - enabled only for increasing the buffer size
125 int tcp_do_autosndbuf
= 1;
126 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, sendbuf_auto
, CTLFLAG_RW
,
127 &tcp_do_autosndbuf
, 0, "Enable automatic send buffer sizing");
129 int tcp_autosndbuf_inc
= 8*1024;
130 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, sendbuf_inc
, CTLFLAG_RW
,
131 &tcp_autosndbuf_inc
, 0, "Incrementor step size of automatic send buffer");
133 int tcp_autosndbuf_min
= 32768;
134 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, sendbuf_min
, CTLFLAG_RW
,
135 &tcp_autosndbuf_min
, 0, "Min size of automatic send buffer");
137 int tcp_autosndbuf_max
= 2*1024*1024;
138 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, sendbuf_max
, CTLFLAG_RW
,
139 &tcp_autosndbuf_max
, 0, "Max size of automatic send buffer");
141 int tcp_prio_synack
= 1;
142 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, prio_synack
, CTLFLAG_RW
,
143 &tcp_prio_synack
, 0, "Prioritize SYN, SYN|ACK and pure ACK");
145 static int tcp_idle_cwv
= 1;
146 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, idle_cwv
, CTLFLAG_RW
,
148 "Congestion window validation after idle period (part of RFC2861)");
150 static int tcp_idle_restart
= 1;
151 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, idle_restart
, CTLFLAG_RW
,
152 &tcp_idle_restart
, 0, "Reset congestion window after idle period");
154 static int tcp_do_tso
= 1;
155 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, tso
, CTLFLAG_RW
,
156 &tcp_do_tso
, 0, "Enable TCP Segmentation Offload (TSO)");
158 static int tcp_fairsend
= 4;
159 SYSCTL_INT(_net_inet_tcp
, OID_AUTO
, fairsend
, CTLFLAG_RW
,
161 "Amount of segments sent before yield to other senders or receivers");
163 static void tcp_idle_cwnd_validate(struct tcpcb
*);
165 static int tcp_tso_getsize(struct tcpcb
*tp
, u_int
*segsz
, u_int
*hlen
);
166 static void tcp_output_sched(struct tcpcb
*tp
);
169 * Tcp output routine: figure out what should be sent and send it.
172 tcp_output(struct tcpcb
*tp
)
174 struct inpcb
* const inp
= tp
->t_inpcb
;
175 struct socket
*so
= inp
->inp_socket
;
176 long len
, recvwin
, sendwin
;
178 int off
, flags
, error
= 0;
185 u_char opt
[TCP_MAXOLEN
];
186 unsigned int ipoptlen
, optlen
, hdrlen
;
191 const boolean_t isipv6
= INP_ISIPV6(inp
);
193 const boolean_t isipv6
= FALSE
;
195 boolean_t can_tso
= FALSE
, use_tso
;
196 boolean_t report_sack
, idle_cwv
= FALSE
;
197 u_int segsz
, tso_hlen
, tso_lenmax
= 0;
199 boolean_t need_sched
= FALSE
;
201 KKASSERT(so
->so_port
== &curthread
->td_msgport
);
204 * Determine length of data that should be transmitted,
205 * and flags that will be used.
206 * If there is some data or critical controls (SYN, RST)
207 * to send, then transmit; otherwise, investigate further.
211 * If we have been idle for a while, the send congestion window
212 * could be no longer representative of the current state of the
213 * link; need to validate congestion window. However, we should
214 * not perform congestion window validation here, since we could
215 * be asked to send pure ACK.
217 if (tp
->snd_max
== tp
->snd_una
&&
218 (ticks
- tp
->snd_last
) >= tp
->t_rxtcur
&& tcp_idle_restart
)
222 * Calculate whether the transmit stream was previously idle
223 * and adjust TF_LASTIDLE for the next time.
225 idle
= (tp
->t_flags
& TF_LASTIDLE
) || (tp
->snd_max
== tp
->snd_una
);
226 if (idle
&& (tp
->t_flags
& TF_MORETOCOME
))
227 tp
->t_flags
|= TF_LASTIDLE
;
229 tp
->t_flags
&= ~TF_LASTIDLE
;
231 if (TCP_DO_SACK(tp
) && tp
->snd_nxt
!= tp
->snd_max
&&
232 !IN_FASTRECOVERY(tp
))
233 nsacked
= tcp_sack_bytes_below(&tp
->scb
, tp
->snd_nxt
);
236 * Find out whether TSO could be used or not
238 * For TSO capable devices, the following assumptions apply to
239 * the processing of TCP flags:
240 * - If FIN is set on the large TCP segment, the device must set
241 * FIN on the last segment that it creates from the large TCP
243 * - If PUSH is set on the large TCP segment, the device must set
244 * PUSH on the last segment that it creates from the large TCP
249 && (tp
->t_flags
& TF_SIGNATURE
) == 0
253 struct rtentry
*rt
= inp
->inp_route
.ro_rt
;
255 if (rt
!= NULL
&& (rt
->rt_flags
& RTF_UP
) &&
256 (rt
->rt_ifp
->if_hwassist
& CSUM_TSO
)) {
258 tso_lenmax
= rt
->rt_ifp
->if_tsolen
;
269 if ((tp
->t_flags
& (TF_SACK_PERMITTED
| TF_NOOPT
)) ==
271 (!TAILQ_EMPTY(&tp
->t_segq
) ||
272 tp
->reportblk
.rblk_start
!= tp
->reportblk
.rblk_end
))
277 /* Make use of SACK information when slow-starting after a RTO. */
278 if (TCP_DO_SACK(tp
) && tp
->snd_nxt
!= tp
->snd_max
&&
279 !IN_FASTRECOVERY(tp
)) {
280 tcp_seq old_snd_nxt
= tp
->snd_nxt
;
282 tcp_sack_skip_sacked(&tp
->scb
, &tp
->snd_nxt
);
283 nsacked
+= tp
->snd_nxt
- old_snd_nxt
;
287 off
= tp
->snd_nxt
- tp
->snd_una
;
288 sendwin
= min(tp
->snd_wnd
, tp
->snd_cwnd
+ nsacked
);
289 sendwin
= min(sendwin
, tp
->snd_bwnd
);
291 flags
= tcp_outflags
[tp
->t_state
];
293 * Get standard flags, and add SYN or FIN if requested by 'hidden'
296 if (tp
->t_flags
& TF_NEEDFIN
)
298 if (tp
->t_flags
& TF_NEEDSYN
)
302 * If in persist timeout with window of 0, send 1 byte.
303 * Otherwise, if window is small but nonzero
304 * and timer expired, we will send what we can
305 * and go to transmit state.
307 if (tp
->t_flags
& TF_FORCE
) {
310 * If we still have some data to send, then
311 * clear the FIN bit. Usually this would
312 * happen below when it realizes that we
313 * aren't sending all the data. However,
314 * if we have exactly 1 byte of unsent data,
315 * then it won't clear the FIN bit below,
316 * and if we are in persist state, we wind
317 * up sending the packet without recording
318 * that we sent the FIN bit.
320 * We can't just blindly clear the FIN bit,
321 * because if we don't have any more data
322 * to send then the probe will be the FIN
325 if (off
< so
->so_snd
.ssb_cc
)
329 tcp_callout_stop(tp
, tp
->tt_persist
);
335 * If snd_nxt == snd_max and we have transmitted a FIN, the
336 * offset will be > 0 even if so_snd.ssb_cc is 0, resulting in
337 * a negative length. This can also occur when TCP opens up
338 * its congestion window while receiving additional duplicate
339 * acks after fast-retransmit because TCP will reset snd_nxt
340 * to snd_max after the fast-retransmit.
342 * A negative length can also occur when we are in the
343 * TCPS_SYN_RECEIVED state due to a simultanious connect where
344 * our SYN has not been acked yet.
346 * In the normal retransmit-FIN-only case, however, snd_nxt will
347 * be set to snd_una, the offset will be 0, and the length may
350 len
= (long)ulmin(so
->so_snd
.ssb_cc
, sendwin
) - off
;
353 * Lop off SYN bit if it has already been sent. However, if this
354 * is SYN-SENT state and if segment contains data, suppress sending
355 * segment (sending the segment would be an option if we still
356 * did TAO and the remote host supported it).
358 if ((flags
& TH_SYN
) && SEQ_GT(tp
->snd_nxt
, tp
->snd_una
)) {
361 if (len
> 0 && tp
->t_state
== TCPS_SYN_SENT
) {
362 tp
->t_flags
&= ~(TF_ACKNOW
| TF_XMITNOW
);
368 * Be careful not to send data and/or FIN on SYN segments.
369 * This measure is needed to prevent interoperability problems
370 * with not fully conformant TCP implementations.
372 if (flags
& TH_SYN
) {
379 * A negative len can occur if our FIN has been sent but not
380 * acked, or if we are in a simultanious connect in the
381 * TCPS_SYN_RECEIVED state with our SYN sent but not yet
384 * If our window has contracted to 0 in the FIN case
385 * (which can only occur if we have NOT been called to
386 * retransmit as per code a few paragraphs up) then we
387 * want to shift the retransmit timer over to the
390 * However, if we are in the TCPS_SYN_RECEIVED state
391 * (the SYN case) we will be in a simultanious connect and
392 * the window may be zero degeneratively. In this case we
393 * do not want to shift to the persist timer after the SYN
394 * or the SYN+ACK transmission.
397 if (sendwin
== 0 && tp
->t_state
!= TCPS_SYN_RECEIVED
) {
398 tcp_callout_stop(tp
, tp
->tt_rexmt
);
400 tp
->snd_nxt
= tp
->snd_una
;
401 if (!tcp_callout_active(tp
, tp
->tt_persist
))
406 KASSERT(len
>= 0, ("%s: len < 0", __func__
));
408 * Automatic sizing of send socket buffer. Often the send buffer
409 * size is not optimally adjusted to the actual network conditions
410 * at hand (delay bandwidth product). Setting the buffer size too
411 * small limits throughput on links with high bandwidth and high
412 * delay (eg. trans-continental/oceanic links). Setting the
413 * buffer size too big consumes too much real kernel memory,
414 * especially with many connections on busy servers.
416 * The criteria to step up the send buffer one notch are:
417 * 1. receive window of remote host is larger than send buffer
418 * (with a fudge factor of 5/4th);
419 * 2. hiwat has not significantly exceeded bwnd (inflight)
420 * (bwnd is a maximal value if inflight is disabled).
421 * 3. send buffer is filled to 7/8th with data (so we actually
422 * have data to make use of it);
423 * 4. hiwat has not hit maximal automatic size;
424 * 5. our send window (slow start and cogestion controlled) is
425 * larger than sent but unacknowledged data in send buffer.
427 * The remote host receive window scaling factor may limit the
428 * growing of the send buffer before it reaches its allowed
431 * It scales directly with slow start or congestion window
432 * and does at most one step per received ACK. This fast
433 * scaling has the drawback of growing the send buffer beyond
434 * what is strictly necessary to make full use of a given
435 * delay*bandwith product. However testing has shown this not
436 * to be much of an problem. At worst we are trading wasting
437 * of available bandwith (the non-use of it) for wasting some
438 * socket buffer memory.
440 * The criteria for shrinking the buffer is based solely on
441 * the inflight code (snd_bwnd). If inflight is disabled,
442 * the buffer will not be shrinked. Note that snd_bwnd already
443 * has a fudge factor. Our test adds a little hysteresis.
445 if (tcp_do_autosndbuf
&& (so
->so_snd
.ssb_flags
& SSB_AUTOSIZE
)) {
446 const int asbinc
= tcp_autosndbuf_inc
;
447 const int hiwat
= so
->so_snd
.ssb_hiwat
;
448 const int lowat
= so
->so_snd
.ssb_lowat
;
451 if ((tp
->snd_wnd
/ 4 * 5) >= hiwat
&&
452 so
->so_snd
.ssb_cc
>= (hiwat
/ 8 * 7) &&
453 hiwat
< tp
->snd_bwnd
+ hiwat
/ 10 &&
454 hiwat
+ asbinc
< tcp_autosndbuf_max
&&
455 hiwat
< (TCP_MAXWIN
<< tp
->snd_scale
) &&
456 sendwin
>= (so
->so_snd
.ssb_cc
-
457 (tp
->snd_nxt
- tp
->snd_una
))) {
458 newsize
= ulmin(hiwat
+ asbinc
, tcp_autosndbuf_max
);
459 if (!ssb_reserve(&so
->so_snd
, newsize
, so
, NULL
))
460 atomic_clear_int(&so
->so_snd
.ssb_flags
, SSB_AUTOSIZE
);
462 if (newsize
>= (TCP_MAXWIN
<< tp
->snd_scale
))
463 atomic_clear_int(&so
->so_snd
.ssb_flags
, SSB_AUTOSIZE
);
465 } else if ((long)tp
->snd_bwnd
<
466 (long)(hiwat
* 3 / 4 - lowat
- asbinc
) &&
467 hiwat
> tp
->t_maxseg
* 2 + asbinc
&&
468 hiwat
+ asbinc
>= tcp_autosndbuf_min
&&
469 tcp_do_autosndbuf
== 1) {
470 newsize
= ulmax(hiwat
- asbinc
, tp
->t_maxseg
* 2);
471 ssb_reserve(&so
->so_snd
, newsize
, so
, NULL
);
477 * - Congestion window needs validation
478 * - There are SACK blocks to report
479 * - RST or SYN flags is set
483 * Checking for SYN|RST looks overkill, just to be safe than sorry
486 if (report_sack
|| idle_cwv
|| (flags
& (TH_RST
| TH_SYN
)))
489 tcp_seq ugr_nxt
= tp
->snd_nxt
;
491 if ((flags
& TH_FIN
) && (tp
->t_flags
& TF_SENTFIN
) &&
492 tp
->snd_nxt
== tp
->snd_max
)
495 if (SEQ_GT(tp
->snd_up
, ugr_nxt
))
501 * Find out segment size and header length for TSO
503 error
= tcp_tso_getsize(tp
, &segsz
, &tso_hlen
);
508 segsz
= tp
->t_maxseg
;
509 tso_hlen
= 0; /* not used */
513 * Truncate to the maximum segment length if not TSO, and ensure that
514 * FIN is removed if the length no longer contains the last data byte.
523 if (__predict_false(tso_lenmax
< segsz
))
524 tso_lenmax
= segsz
<< 1;
527 * Truncate TSO transfers to (IP_MAXPACKET - iphlen -
528 * thoff), and make sure that we send equal size
529 * transfers down the stack (rather than big-small-
532 len
= min(len
, tso_lenmax
);
533 nsegs
= min(len
, (IP_MAXPACKET
- tso_hlen
)) / segsz
;
551 if (SEQ_LT(tp
->snd_nxt
+ len
, tp
->snd_una
+ so
->so_snd
.ssb_cc
))
554 recvwin
= ssb_space(&so
->so_rcv
);
557 * Sender silly window avoidance. We transmit under the following
558 * conditions when len is non-zero:
560 * - We have a full segment
561 * - This is the last buffer in a write()/send() and we are
562 * either idle or running NODELAY
563 * - we've timed out (e.g. persist timer)
564 * - we have more then 1/2 the maximum send window's worth of
565 * data (receiver may be limiting the window size)
566 * - we need to retransmit
572 * NOTE! on localhost connections an 'ack' from the remote
573 * end may occur synchronously with the output and cause
574 * us to flush a buffer queued with moretocome. XXX
576 * note: the len + off check is almost certainly unnecessary.
578 if (!(tp
->t_flags
& TF_MORETOCOME
) && /* normal case */
579 (idle
|| (tp
->t_flags
& TF_NODELAY
)) &&
580 len
+ off
>= so
->so_snd
.ssb_cc
&&
581 !(tp
->t_flags
& TF_NOPUSH
)) {
584 if (tp
->t_flags
& TF_FORCE
) /* typ. timeout case */
586 if (len
>= tp
->max_sndwnd
/ 2 && tp
->max_sndwnd
> 0)
588 if (SEQ_LT(tp
->snd_nxt
, tp
->snd_max
)) /* retransmit case */
590 if (tp
->t_flags
& TF_XMITNOW
)
595 * Compare available window to amount of window
596 * known to peer (as advertised window less
597 * next expected input). If the difference is at least two
598 * max size segments, or at least 50% of the maximum possible
599 * window, then want to send a window update to peer.
603 * "adv" is the amount we can increase the window,
604 * taking into account that we are limited by
605 * TCP_MAXWIN << tp->rcv_scale.
607 long adv
= min(recvwin
, (long)TCP_MAXWIN
<< tp
->rcv_scale
) -
608 (tp
->rcv_adv
- tp
->rcv_nxt
);
612 * This ack case typically occurs when the user has drained
613 * the TCP socket buffer sufficiently to warrent an ack
614 * containing a 'pure window update'... that is, an ack that
615 * ONLY updates the tcp window.
617 * It is unclear why we would need to do a pure window update
618 * past 2 segments if we are going to do one at 1/2 the high
619 * water mark anyway, especially since under normal conditions
620 * the user program will drain the socket buffer quickly.
621 * The 2-segment pure window update will often add a large
622 * number of extra, unnecessary acks to the stream.
624 * avoid_pure_win_update now defaults to 1.
626 if (avoid_pure_win_update
== 0 ||
627 (tp
->t_flags
& TF_RXRESIZED
)) {
628 if (adv
>= (long) (2 * segsz
)) {
632 hiwat
= (long)(TCP_MAXWIN
<< tp
->rcv_scale
);
633 if (hiwat
> (long)so
->so_rcv
.ssb_hiwat
)
634 hiwat
= (long)so
->so_rcv
.ssb_hiwat
;
635 if (adv
>= hiwat
/ 2)
640 * Send if we owe the peer an ACK, RST, SYN, or urgent data. ACKNOW
641 * is also a catch-all for the retransmit timer timeout case.
643 if (tp
->t_flags
& TF_ACKNOW
)
645 if ((flags
& TH_RST
) ||
646 ((flags
& TH_SYN
) && !(tp
->t_flags
& TF_NEEDSYN
)))
648 if (SEQ_GT(tp
->snd_up
, tp
->snd_una
))
651 * If our state indicates that FIN should be sent
652 * and we have not yet done so, then we need to send.
654 if ((flags
& TH_FIN
) &&
655 (!(tp
->t_flags
& TF_SENTFIN
) || tp
->snd_nxt
== tp
->snd_una
))
659 * TCP window updates are not reliable, rather a polling protocol
660 * using ``persist'' packets is used to insure receipt of window
661 * updates. The three ``states'' for the output side are:
662 * idle not doing retransmits or persists
663 * persisting to move a small or zero window
664 * (re)transmitting and thereby not persisting
666 * tcp_callout_active(tp, tp->tt_persist)
667 * is true when we are in persist state.
668 * The TF_FORCE flag in tp->t_flags
669 * is set when we are called to send a persist packet.
670 * tcp_callout_active(tp, tp->tt_rexmt)
671 * is set when we are retransmitting
672 * The output side is idle when both timers are zero.
674 * If send window is too small, there is data to transmit, and no
675 * retransmit or persist is pending, then go to persist state.
677 * If nothing happens soon, send when timer expires:
678 * if window is nonzero, transmit what we can, otherwise force out
681 * Don't try to set the persist state if we are in TCPS_SYN_RECEIVED
682 * with data pending. This situation can occur during a
683 * simultanious connect.
685 if (so
->so_snd
.ssb_cc
> 0 &&
686 tp
->t_state
!= TCPS_SYN_RECEIVED
&&
687 !tcp_callout_active(tp
, tp
->tt_rexmt
) &&
688 !tcp_callout_active(tp
, tp
->tt_persist
)) {
694 * No reason to send a segment, just return.
696 tp
->t_flags
&= ~TF_XMITNOW
;
700 if (need_sched
&& len
> 0) {
701 tcp_output_sched(tp
);
706 * Before ESTABLISHED, force sending of initial options
707 * unless TCP set not to do any options.
708 * NOTE: we assume that the IP/TCP header plus TCP options
709 * always fit in a single mbuf, leaving room for a maximum
711 * max_linkhdr + sizeof(struct tcpiphdr) + optlen <= MCLBYTES
715 hdrlen
= sizeof(struct ip6_hdr
) + sizeof(struct tcphdr
);
717 hdrlen
= sizeof(struct tcpiphdr
);
718 if (flags
& TH_SYN
) {
719 tp
->snd_nxt
= tp
->iss
;
720 if (!(tp
->t_flags
& TF_NOOPT
)) {
723 opt
[0] = TCPOPT_MAXSEG
;
724 opt
[1] = TCPOLEN_MAXSEG
;
725 mss
= htons((u_short
) tcp_mssopt(tp
));
726 memcpy(opt
+ 2, &mss
, sizeof mss
);
727 optlen
= TCPOLEN_MAXSEG
;
729 if ((tp
->t_flags
& TF_REQ_SCALE
) &&
730 (!(flags
& TH_ACK
) ||
731 (tp
->t_flags
& TF_RCVD_SCALE
))) {
732 *((u_int32_t
*)(opt
+ optlen
)) = htonl(
734 TCPOPT_WINDOW
<< 16 |
735 TCPOLEN_WINDOW
<< 8 |
736 tp
->request_r_scale
);
740 if ((tcp_do_sack
&& !(flags
& TH_ACK
)) ||
741 tp
->t_flags
& TF_SACK_PERMITTED
) {
742 uint32_t *lp
= (uint32_t *)(opt
+ optlen
);
744 *lp
= htonl(TCPOPT_SACK_PERMITTED_ALIGNED
);
745 optlen
+= TCPOLEN_SACK_PERMITTED_ALIGNED
;
751 * Send a timestamp and echo-reply if this is a SYN and our side
752 * wants to use timestamps (TF_REQ_TSTMP is set) or both our side
753 * and our peer have sent timestamps in our SYN's.
755 if ((tp
->t_flags
& (TF_REQ_TSTMP
| TF_NOOPT
)) == TF_REQ_TSTMP
&&
757 (!(flags
& TH_ACK
) || (tp
->t_flags
& TF_RCVD_TSTMP
))) {
758 u_int32_t
*lp
= (u_int32_t
*)(opt
+ optlen
);
760 /* Form timestamp option as shown in appendix A of RFC 1323. */
761 *lp
++ = htonl(TCPOPT_TSTAMP_HDR
);
762 *lp
++ = htonl(ticks
);
763 *lp
= htonl(tp
->ts_recent
);
764 optlen
+= TCPOLEN_TSTAMP_APPA
;
767 /* Set receive buffer autosizing timestamp. */
768 if (tp
->rfbuf_ts
== 0 && (so
->so_rcv
.ssb_flags
& SSB_AUTOSIZE
))
769 tp
->rfbuf_ts
= ticks
;
772 * If this is a SACK connection and we have a block to report,
773 * fill in the SACK blocks in the TCP options.
776 tcp_sack_fill_report(tp
, opt
, &optlen
);
779 if (tp
->t_flags
& TF_SIGNATURE
) {
783 * Initialize TCP-MD5 option (RFC2385)
785 bp
= (u_char
*)opt
+ optlen
;
786 *bp
++ = TCPOPT_SIGNATURE
;
787 *bp
++ = TCPOLEN_SIGNATURE
;
789 for (i
= 0; i
< TCP_SIGLEN
; i
++)
791 optlen
+= TCPOLEN_SIGNATURE
;
793 * Terminate options list and maintain 32-bit alignment.
799 #endif /* TCP_SIGNATURE */
800 KASSERT(optlen
<= TCP_MAXOLEN
, ("too many TCP options"));
804 ipoptlen
= ip6_optlen(inp
);
806 if (inp
->inp_options
) {
807 ipoptlen
= inp
->inp_options
->m_len
-
808 offsetof(struct ipoption
, ipopt_list
);
815 /* TSO segment length must be multiple of segment size */
816 KASSERT(len
>= (2 * segsz
) && (len
% segsz
== 0),
817 ("invalid TSO len %ld, segsz %u", len
, segsz
));
819 KASSERT(len
<= segsz
,
820 ("invalid len %ld, segsz %u", len
, segsz
));
823 * Adjust data length if insertion of options will bump
824 * the packet length beyond the t_maxopd length. Clear
825 * FIN to prevent premature closure since there is still
826 * more data to send after this (now truncated) packet.
828 * If just the options do not fit we are in a no-win
829 * situation and we treat it as an unreachable host.
831 if (len
+ optlen
+ ipoptlen
> tp
->t_maxopd
) {
832 if (tp
->t_maxopd
<= optlen
+ ipoptlen
) {
833 static time_t last_optlen_report
;
835 if (last_optlen_report
!= time_uptime
) {
836 last_optlen_report
= time_uptime
;
837 kprintf("tcpcb %p: MSS (%d) too "
838 "small to hold options!\n",
841 error
= EHOSTUNREACH
;
845 len
= tp
->t_maxopd
- optlen
- ipoptlen
;
852 KASSERT(max_linkhdr
+ hdrlen
<= MCLBYTES
, ("tcphdr too big"));
854 KASSERT(max_linkhdr
+ hdrlen
<= MHLEN
, ("tcphdr too big"));
858 * Grab a header mbuf, attaching a copy of data to
859 * be transmitted, and initialize the header from
860 * the template for sends on this connection.
863 if ((tp
->t_flags
& TF_FORCE
) && len
== 1)
864 tcpstat
.tcps_sndprobe
++;
865 else if (SEQ_LT(tp
->snd_nxt
, tp
->snd_max
)) {
866 if (tp
->snd_nxt
== tp
->snd_una
)
867 tp
->snd_max_rexmt
= tp
->snd_max
;
869 tcpstat
.tcps_sndsackrtopack
++;
870 tcpstat
.tcps_sndsackrtobyte
+= len
;
872 tcpstat
.tcps_sndrexmitpack
++;
873 tcpstat
.tcps_sndrexmitbyte
+= len
;
875 tcpstat
.tcps_sndpack
++;
876 tcpstat
.tcps_sndbyte
+= len
;
880 tcp_idle_cwnd_validate(tp
);
882 /* Update last send time after CWV */
883 tp
->snd_last
= ticks
;
885 if ((m
= m_copypack(so
->so_snd
.ssb_mb
, off
, (int)len
,
886 max_linkhdr
+ hdrlen
)) == NULL
) {
891 * m_copypack left space for our hdr; use it.
897 m
= m_gethdr(M_NOWAIT
, MT_HEADER
);
899 m
= m_getl(hdrlen
+ max_linkhdr
, M_NOWAIT
, MT_HEADER
,
906 m
->m_data
+= max_linkhdr
;
908 if (len
<= MHLEN
- hdrlen
- max_linkhdr
) {
909 m_copydata(so
->so_snd
.ssb_mb
, off
, (int) len
,
910 mtod(m
, caddr_t
) + hdrlen
);
913 m
->m_next
= m_copy(so
->so_snd
.ssb_mb
, off
, (int) len
);
914 if (m
->m_next
== NULL
) {
923 * If we're sending everything we've got, set PUSH.
924 * (This will keep happy those implementations which only
925 * give data to the user when a buffer fills or
928 if (off
+ len
== so
->so_snd
.ssb_cc
)
931 if (tp
->t_flags
& TF_ACKNOW
)
932 tcpstat
.tcps_sndacks
++;
933 else if (flags
& (TH_SYN
| TH_FIN
| TH_RST
))
934 tcpstat
.tcps_sndctrl
++;
935 else if (SEQ_GT(tp
->snd_up
, tp
->snd_una
))
936 tcpstat
.tcps_sndurg
++;
938 tcpstat
.tcps_sndwinup
++;
940 MGETHDR(m
, M_NOWAIT
, MT_HEADER
);
946 (hdrlen
+ max_linkhdr
> MHLEN
) && hdrlen
<= MHLEN
)
949 m
->m_data
+= max_linkhdr
;
953 * Prioritize SYN, SYN|ACK and pure ACK.
954 * Leave FIN and RST as they are.
956 if (tcp_prio_synack
&& (flags
& (TH_FIN
| TH_RST
)) == 0)
957 m
->m_flags
|= M_PRIO
;
959 m
->m_pkthdr
.rcvif
= NULL
;
961 ip6
= mtod(m
, struct ip6_hdr
*);
962 th
= (struct tcphdr
*)(ip6
+ 1);
963 tcp_fillheaders(tp
, ip6
, th
, use_tso
);
965 ip
= mtod(m
, struct ip
*);
966 th
= (struct tcphdr
*)(ip
+ 1);
967 /* this picks up the pseudo header (w/o the length) */
968 tcp_fillheaders(tp
, ip
, th
, use_tso
);
972 * Fill in fields, remembering maximum advertised
973 * window for use in delaying messages about window sizes.
974 * If resending a FIN, be sure not to use a new sequence number.
976 if (flags
& TH_FIN
&& tp
->t_flags
& TF_SENTFIN
&&
977 tp
->snd_nxt
== tp
->snd_max
)
982 * If we are doing retransmissions, then snd_nxt will
983 * not reflect the first unsent octet. For ACK only
984 * packets, we do not want the sequence number of the
985 * retransmitted packet, we want the sequence number
986 * of the next unsent octet. So, if there is no data
987 * (and no SYN or FIN), use snd_max instead of snd_nxt
988 * when filling in ti_seq. But if we are in persist
989 * state, snd_max might reflect one byte beyond the
990 * right edge of the window, so use snd_nxt in that
991 * case, since we know we aren't doing a retransmission.
992 * (retransmit and persist are mutually exclusive...)
994 if (len
|| (flags
& (TH_SYN
|TH_FIN
)) ||
995 tcp_callout_active(tp
, tp
->tt_persist
))
996 th
->th_seq
= htonl(tp
->snd_nxt
);
998 th
->th_seq
= htonl(tp
->snd_max
);
999 th
->th_ack
= htonl(tp
->rcv_nxt
);
1001 bcopy(opt
, th
+ 1, optlen
);
1002 th
->th_off
= (sizeof(struct tcphdr
) + optlen
) >> 2;
1004 th
->th_flags
= flags
;
1008 * Calculate receive window. Don't shrink window, but avoid
1009 * silly window syndrome by sending a 0 window if the actual
1010 * window is less then one segment.
1012 if (recvwin
< (long)(so
->so_rcv
.ssb_hiwat
/ 4) &&
1013 recvwin
< (long)segsz
)
1015 if (recvwin
< (tcp_seq_diff_t
)(tp
->rcv_adv
- tp
->rcv_nxt
))
1016 recvwin
= (tcp_seq_diff_t
)(tp
->rcv_adv
- tp
->rcv_nxt
);
1017 if (recvwin
> (long)TCP_MAXWIN
<< tp
->rcv_scale
)
1018 recvwin
= (long)TCP_MAXWIN
<< tp
->rcv_scale
;
1021 * Adjust the RXWIN0SENT flag - indicate that we have advertised
1022 * a 0 window. This may cause the remote transmitter to stall. This
1023 * flag tells soreceive() to disable delayed acknowledgements when
1024 * draining the buffer. This can occur if the receiver is attempting
1025 * to read more data then can be buffered prior to transmitting on
1029 tp
->t_flags
|= TF_RXWIN0SENT
;
1031 tp
->t_flags
&= ~TF_RXWIN0SENT
;
1034 th
->th_win
= htons((u_short
) (recvwin
>>tp
->rcv_scale
));
1036 if (SEQ_GT(tp
->snd_up
, tp
->snd_nxt
)) {
1037 KASSERT(!use_tso
, ("URG with TSO"));
1039 th
->th_urp
= htons((u_short
)(tp
->snd_up
- tp
->snd_nxt
));
1040 th
->th_flags
|= TH_URG
;
1044 * If no urgent pointer to send, then we pull
1045 * the urgent pointer to the left edge of the send window
1046 * so that it doesn't drift into the send window on sequence
1047 * number wraparound.
1049 tp
->snd_up
= tp
->snd_una
; /* drag it along */
1053 #ifdef TCP_SIGNATURE
1054 if (tp
->t_flags
& TF_SIGNATURE
) {
1055 tcpsignature_compute(m
, len
, optlen
,
1056 (u_char
*)(th
+ 1) + sigoff
, IPSEC_DIR_OUTBOUND
);
1058 #endif /* TCP_SIGNATURE */
1061 * Put TCP length in extended header, and then
1062 * checksum extended header and data.
1064 m
->m_pkthdr
.len
= hdrlen
+ len
; /* in6_cksum() need this */
1067 * ip6_plen is not need to be filled now, and will be
1068 * filled in ip6_output().
1070 th
->th_sum
= in6_cksum(m
, IPPROTO_TCP
,
1071 sizeof(struct ip6_hdr
),
1072 sizeof(struct tcphdr
) + optlen
+ len
);
1074 m
->m_pkthdr
.csum_thlen
= sizeof(struct tcphdr
) + optlen
;
1076 m
->m_pkthdr
.csum_flags
= CSUM_TSO
;
1077 m
->m_pkthdr
.tso_segsz
= segsz
;
1079 m
->m_pkthdr
.csum_flags
= CSUM_TCP
;
1080 m
->m_pkthdr
.csum_data
=
1081 offsetof(struct tcphdr
, th_sum
);
1083 th
->th_sum
= in_addword(th
->th_sum
,
1084 htons((u_short
)(optlen
+ len
)));
1089 * IP version must be set here for ipv4/ipv6 checking
1092 KASSERT(ip
->ip_v
== IPVERSION
,
1093 ("%s: IP version incorrect: %d",
1094 __func__
, ip
->ip_v
));
1099 * In transmit state, time the transmission and arrange for
1100 * the retransmit. In persist state, just set snd_max.
1102 if (!(tp
->t_flags
& TF_FORCE
) ||
1103 !tcp_callout_active(tp
, tp
->tt_persist
)) {
1104 tcp_seq startseq
= tp
->snd_nxt
;
1107 * Advance snd_nxt over sequence space of this segment.
1109 if (flags
& (TH_SYN
| TH_FIN
)) {
1112 if (flags
& TH_FIN
) {
1114 tp
->t_flags
|= TF_SENTFIN
;
1118 if (SEQ_GT(tp
->snd_nxt
, tp
->snd_max
)) {
1119 tp
->snd_max
= tp
->snd_nxt
;
1121 * Time this transmission if not a retransmission and
1122 * not currently timing anything.
1124 if (tp
->t_rtttime
== 0) {
1125 tp
->t_rtttime
= ticks
;
1126 tp
->t_rtseq
= startseq
;
1127 tcpstat
.tcps_segstimed
++;
1132 * Set retransmit timer if not currently set,
1133 * and not doing a pure ack or a keep-alive probe.
1134 * Initial value for retransmit timer is smoothed
1135 * round-trip time + 2 * round-trip time variance.
1136 * Initialize shift counter which is used for backoff
1137 * of retransmit time.
1139 if (!tcp_callout_active(tp
, tp
->tt_rexmt
) &&
1140 tp
->snd_nxt
!= tp
->snd_una
) {
1141 if (tcp_callout_active(tp
, tp
->tt_persist
)) {
1142 tcp_callout_stop(tp
, tp
->tt_persist
);
1145 tcp_callout_reset(tp
, tp
->tt_rexmt
, tp
->t_rxtcur
,
1147 } else if (len
== 0 && so
->so_snd
.ssb_cc
&&
1148 tp
->t_state
> TCPS_SYN_RECEIVED
&&
1149 !tcp_callout_active(tp
, tp
->tt_rexmt
) &&
1150 !tcp_callout_active(tp
, tp
->tt_persist
)) {
1152 * Avoid a situation where we do not set persist timer
1153 * after a zero window condition. For example:
1154 * 1) A -> B: packet with enough data to fill the window
1155 * 2) B -> A: ACK for #1 + new data (0 window
1157 * 3) A -> B: ACK for #2, 0 len packet
1159 * In this case, A will not activate the persist timer,
1160 * because it chose to send a packet. Unless tcp_output
1161 * is called for some other reason (delayed ack timer,
1162 * another input packet from B, socket syscall), A will
1163 * not send zero window probes.
1165 * So, if you send a 0-length packet, but there is data
1166 * in the socket buffer, and neither the rexmt or
1167 * persist timer is already set, then activate the
1175 * Persist case, update snd_max but since we are in
1176 * persist mode (no window) we do not update snd_nxt.
1180 panic("tcp_output: persist timer to send SYN");
1181 if (flags
& TH_FIN
) {
1183 tp
->t_flags
|= TF_SENTFIN
;
1185 if (SEQ_GT(tp
->snd_nxt
+ xlen
, tp
->snd_max
))
1186 tp
->snd_max
= tp
->snd_nxt
+ xlen
;
1192 if (so
->so_options
& SO_DEBUG
) {
1193 tcp_trace(TA_OUTPUT
, tp
->t_state
, tp
,
1194 mtod(m
, void *), th
, 0);
1199 * Fill in IP length and desired time to live and
1200 * send to IP level. There should be a better way
1201 * to handle ttl and tos; we could keep them in
1202 * the template, but need a way to checksum without them.
1205 * m->m_pkthdr.len should have been set before cksum
1206 * calcuration, because in6_cksum() need it.
1210 * we separately set hoplimit for every segment,
1211 * since the user might want to change the value
1212 * via setsockopt. Also, desired default hop
1213 * limit might be changed via Neighbor Discovery.
1215 ip6
->ip6_hlim
= in6_selecthlim(inp
,
1216 (inp
->in6p_route
.ro_rt
?
1217 inp
->in6p_route
.ro_rt
->rt_ifp
: NULL
));
1219 /* TODO: IPv6 IP6TOS_ECT bit on */
1220 error
= ip6_output(m
, inp
->in6p_outputopts
,
1221 &inp
->in6p_route
, (so
->so_options
& SO_DONTROUTE
),
1226 KASSERT(!INP_CHECK_SOCKAF(so
, AF_INET6
), ("inet6 pcb"));
1228 ip
->ip_len
= m
->m_pkthdr
.len
;
1229 ip
->ip_ttl
= inp
->inp_ip_ttl
; /* XXX */
1230 ip
->ip_tos
= inp
->inp_ip_tos
; /* XXX */
1232 * See if we should do MTU discovery.
1233 * We do it only if the following are true:
1234 * 1) we have a valid route to the destination
1235 * 2) the MTU is not locked (if it is,
1236 * then discovery has been disabled)
1238 if (path_mtu_discovery
&&
1239 (rt
= inp
->inp_route
.ro_rt
) &&
1240 (rt
->rt_flags
& RTF_UP
) &&
1241 !(rt
->rt_rmx
.rmx_locks
& RTV_MTU
))
1242 ip
->ip_off
|= IP_DF
;
1244 KASSERT(inp
->inp_flags
& INP_HASH
,
1245 ("inpcb has no hash"));
1246 m_sethash(m
, inp
->inp_hashval
);
1247 error
= ip_output(m
, inp
->inp_options
, &inp
->inp_route
,
1248 (so
->so_options
& SO_DONTROUTE
) |
1249 IP_DEBUGROUTE
, NULL
, inp
);
1252 KASSERT(error
!= 0, ("no error, but th not set"));
1255 tp
->t_flags
&= ~(TF_ACKNOW
| TF_XMITNOW
);
1258 * We know that the packet was lost, so back out the
1259 * sequence number advance, if any.
1261 if (!(tp
->t_flags
& TF_FORCE
) ||
1262 !tcp_callout_active(tp
, tp
->tt_persist
)) {
1264 * No need to check for TH_FIN here because
1265 * the TF_SENTFIN flag handles that case.
1267 if (!(flags
& TH_SYN
))
1272 if (error
== ENOBUFS
) {
1273 KASSERT((len
== 0 && (flags
& (TH_SYN
| TH_FIN
)) == 0) ||
1274 tcp_callout_active(tp
, tp
->tt_rexmt
) ||
1275 tcp_callout_active(tp
, tp
->tt_persist
),
1276 ("neither rexmt nor persist timer is set"));
1279 if (error
== EMSGSIZE
) {
1281 * ip_output() will have already fixed the route
1282 * for us. tcp_mtudisc() will, as its last action,
1283 * initiate retransmission, so it is important to
1286 tcp_mtudisc(inp
, 0);
1289 if ((error
== EHOSTUNREACH
|| error
== ENETDOWN
) &&
1290 TCPS_HAVERCVDSYN(tp
->t_state
)) {
1291 tp
->t_softerror
= error
;
1296 tcpstat
.tcps_sndtotal
++;
1299 * Data sent (as far as we can tell).
1301 * If this advertises a larger window than any other segment,
1302 * then remember the size of the advertised window.
1304 * Any pending ACK has now been sent.
1306 if (recvwin
> 0 && SEQ_GT(tp
->rcv_nxt
+ recvwin
, tp
->rcv_adv
)) {
1307 tp
->rcv_adv
= tp
->rcv_nxt
+ recvwin
;
1308 tp
->t_flags
&= ~TF_RXRESIZED
;
1310 tp
->last_ack_sent
= tp
->rcv_nxt
;
1311 tp
->t_flags
&= ~(TF_ACKNOW
| TF_XMITNOW
);
1312 if (tcp_delack_enabled
)
1313 tcp_callout_stop(tp
, tp
->tt_delack
);
1315 if (tcp_fairsend
> 0 && (tp
->t_flags
& TF_FAIRSEND
) &&
1316 segcnt
>= tcp_fairsend
)
1324 tcp_setpersist(struct tcpcb
*tp
)
1326 int t
= ((tp
->t_srtt
>> 2) + tp
->t_rttvar
) >> 1;
1329 if (tp
->t_state
== TCPS_SYN_SENT
||
1330 tp
->t_state
== TCPS_SYN_RECEIVED
) {
1331 panic("tcp_setpersist: not established yet, current %s",
1332 tp
->t_state
== TCPS_SYN_SENT
?
1333 "SYN_SENT" : "SYN_RECEIVED");
1336 if (tcp_callout_active(tp
, tp
->tt_rexmt
))
1337 panic("tcp_setpersist: retransmit pending");
1339 * Start/restart persistance timer.
1341 TCPT_RANGESET(tt
, t
* tcp_backoff
[tp
->t_rxtshift
], TCPTV_PERSMIN
,
1343 tcp_callout_reset(tp
, tp
->tt_persist
, tt
, tcp_timer_persist
);
1344 if (tp
->t_rxtshift
< TCP_MAXRXTSHIFT
)
1349 tcp_idle_cwnd_validate(struct tcpcb
*tp
)
1351 u_long initial_cwnd
= tcp_initial_window(tp
);
1354 tcpstat
.tcps_sndidle
++;
1356 /* According to RFC5681: RW=min(IW,cwnd) */
1357 min_cwnd
= min(tp
->snd_cwnd
, initial_cwnd
);
1360 u_long idle_time
, decay_cwnd
;
1363 * RFC2861, but only after idle period.
1367 * Before the congestion window is reduced, ssthresh
1368 * is set to the maximum of its current value and 3/4
1369 * cwnd. If the sender then has more data to send
1370 * than the decayed cwnd allows, the TCP will slow-
1371 * start (perform exponential increase) at least
1372 * half-way back up to the old value of cwnd.
1374 tp
->snd_ssthresh
= max(tp
->snd_ssthresh
,
1375 (3 * tp
->snd_cwnd
) / 4);
1378 * Decay the congestion window by half for every RTT
1379 * that the flow remains inactive.
1381 * The difference between our implementation and
1382 * RFC2861 is that we don't allow cwnd to go below
1383 * the value allowed by RFC5681 (min_cwnd).
1385 idle_time
= ticks
- tp
->snd_last
;
1386 decay_cwnd
= tp
->snd_cwnd
;
1387 while (idle_time
>= tp
->t_rxtcur
&&
1388 decay_cwnd
> min_cwnd
) {
1390 idle_time
-= tp
->t_rxtcur
;
1392 tp
->snd_cwnd
= max(decay_cwnd
, min_cwnd
);
1395 * Slow-start from scratch to re-determine the send
1396 * congestion window.
1398 tp
->snd_cwnd
= min_cwnd
;
1401 /* Restart ABC counting during congestion avoidance */
1406 tcp_tso_getsize(struct tcpcb
*tp
, u_int
*segsz
, u_int
*hlen0
)
1408 struct inpcb
* const inp
= tp
->t_inpcb
;
1410 const boolean_t isipv6
= INP_ISIPV6(inp
);
1412 const boolean_t isipv6
= FALSE
;
1414 unsigned int ipoptlen
, optlen
;
1417 hlen
= sizeof(struct ip
) + sizeof(struct tcphdr
);
1420 ipoptlen
= ip6_optlen(inp
);
1422 if (inp
->inp_options
) {
1423 ipoptlen
= inp
->inp_options
->m_len
-
1424 offsetof(struct ipoption
, ipopt_list
);
1432 if ((tp
->t_flags
& (TF_REQ_TSTMP
| TF_NOOPT
)) == TF_REQ_TSTMP
&&
1433 (tp
->t_flags
& TF_RCVD_TSTMP
))
1434 optlen
+= TCPOLEN_TSTAMP_APPA
;
1437 if (tp
->t_maxopd
<= optlen
+ ipoptlen
)
1438 return EHOSTUNREACH
;
1440 *segsz
= tp
->t_maxopd
- optlen
- ipoptlen
;
1446 tcp_output_sched_handler(netmsg_t nmsg
)
1448 struct tcpcb
*tp
= nmsg
->lmsg
.u
.ms_resultp
;
1452 lwkt_replymsg(&nmsg
->lmsg
, 0);
1455 tcp_output_fair(tp
);
1459 tcp_output_init(struct tcpcb
*tp
)
1461 netmsg_init(tp
->tt_sndmore
, NULL
, &netisr_adone_rport
, MSGF_DROPABLE
,
1462 tcp_output_sched_handler
);
1463 tp
->tt_sndmore
->lmsg
.u
.ms_resultp
= tp
;
1467 tcp_output_cancel(struct tcpcb
*tp
)
1470 * This message is still pending to be processed;
1471 * drop it. Optimized.
1474 if ((tp
->tt_sndmore
->lmsg
.ms_flags
& MSGF_DONE
) == 0) {
1475 lwkt_dropmsg(&tp
->tt_sndmore
->lmsg
);
1481 tcp_output_pending(struct tcpcb
*tp
)
1483 if ((tp
->tt_sndmore
->lmsg
.ms_flags
& MSGF_DONE
) == 0)
1490 tcp_output_sched(struct tcpcb
*tp
)
1493 if (tp
->tt_sndmore
->lmsg
.ms_flags
& MSGF_DONE
)
1494 lwkt_sendmsg(netisr_cpuport(mycpuid
), &tp
->tt_sndmore
->lmsg
);
1501 * Yield to other senders or receivers on the same netisr if the current
1502 * TCP stream has sent tcp_fairsend segments and is going to burst more
1503 * segments. Bursting large amount of segements in a single TCP stream
1504 * could delay other senders' segments and receivers' ACKs quite a lot,
1505 * if others segments and ACKs are queued on to the same hardware transmit
1506 * queue; thus cause unfairness between senders and suppress receiving
1509 * Fairsend should be performed at the places that do not affect segment
1510 * sending during congestion control, e.g.
1511 * - User requested output
1512 * - ACK input triggered output
1515 * For devices that are TSO capable, their TSO aggregation size limit could
1519 tcp_output_fair(struct tcpcb
*tp
)
1523 tp
->t_flags
|= TF_FAIRSEND
;
1524 ret
= tcp_output(tp
);
1525 tp
->t_flags
&= ~TF_FAIRSEND
;