tcp_output.c
/*-
* Copyright (c) 1982, 1986, 1988, 1990, 1993, 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_output.c 8.4 (Berkeley) 5/24/95
*/
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_mac.h"
#include "opt_tcpdebug.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/domain.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mbuf.h>
#include <sys/mutex.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <net/route.h>
#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet/ip_options.h>
#ifdef INET6
#include <netinet6/in6_pcb.h>
#include <netinet/ip6.h>
#include <netinet6/ip6_var.h>
#endif
#include <netinet/tcp.h>
#define TCPOUTFLAGS
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcpip.h>
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#ifdef IPSEC
#include <netipsec/ipsec.h>
#endif /*IPSEC*/
#include <machine/in_cksum.h>
#include <security/mac/mac_framework.h>
#ifdef notyet
extern struct mbuf *m_copypack();
#endif
int path_mtu_discovery = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, path_mtu_discovery, CTLFLAG_RW,
&path_mtu_discovery, 1, "Enable Path MTU Discovery");
int ss_fltsz = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, slowstart_flightsize, CTLFLAG_RW,
&ss_fltsz, 1, "Slow start flight size");
int ss_fltsz_local = 4;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, local_slowstart_flightsize, CTLFLAG_RW,
&ss_fltsz_local, 1, "Slow start flight size for local networks");
int tcp_do_newreno = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, newreno, CTLFLAG_RW,
&tcp_do_newreno, 0, "Enable NewReno Algorithms");
int tcp_do_tso = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, tso, CTLFLAG_RW,
&tcp_do_tso, 0, "Enable TCP Segmentation Offload");
int tcp_do_autosndbuf = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_auto, CTLFLAG_RW,
&tcp_do_autosndbuf, 0, "Enable automatic send buffer sizing");
int tcp_autosndbuf_inc = 8*1024;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_inc, CTLFLAG_RW,
&tcp_autosndbuf_inc, 0, "Incrementor step size of automatic send buffer");
int tcp_autosndbuf_max = 256*1024;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_RW,
&tcp_autosndbuf_max, 0, "Max size of automatic send buffer");
/*
* Tcp output routine: figure out what should be sent and send it.
*/
int
tcp_output(struct tcpcb *tp)
{
struct socket *so = tp->t_inpcb->inp_socket;
long len, recwin, sendwin;
int off, flags, error;
struct mbuf *m;
struct ip *ip = NULL;
struct ipovly *ipov = NULL;
struct tcphdr *th;
u_char opt[TCP_MAXOLEN];
unsigned ipoptlen, optlen, hdrlen;
#ifdef IPSEC
unsigned ipsec_optlen = 0;
#endif
int idle, sendalot;
int sack_rxmit, sack_bytes_rxmt;
struct sackhole *p;
int tso = 0;
struct tcpopt to;
#if 0
int maxburst = TCP_MAXBURST;
#endif
#ifdef INET6
struct ip6_hdr *ip6 = NULL;
int isipv6;
isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) != 0;
#endif
INP_WLOCK_ASSERT(tp->t_inpcb);
/*
* Determine length of data that should be transmitted,
* and flags that will be used.
* If there is some data or critical controls (SYN, RST)
* to send, then transmit; otherwise, investigate further.
*/
idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
if (idle && (ticks - (int)tp->t_rcvtime) >= tp->t_rxtcur) {
/*
* 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.
*
* Set the slow-start flight size depending on whether
* this is a local network or not.
*/
int ss = ss_fltsz;
#ifdef INET6
if (isipv6) {
if (in6_localaddr(&tp->t_inpcb->in6p_faddr))
ss = ss_fltsz_local;
} else
#endif /* INET6 */
if (in_localaddr(tp->t_inpcb->inp_faddr))
ss = ss_fltsz_local;
tp->snd_cwnd = tp->t_maxseg * ss;
}
tp->t_flags &= ~TF_LASTIDLE;
if (idle) {
if (tp->t_flags & TF_MORETOCOME) {
tp->t_flags |= TF_LASTIDLE;
idle = 0;
}
}
again:
/*
* If we've recently taken a timeout, snd_max will be greater than
* snd_nxt. There may be SACK information that allows us to avoid
* resending already delivered data. Adjust snd_nxt accordingly.
*/
if ((tp->t_flags & TF_SACK_PERMIT) &&
SEQ_LT(tp->snd_nxt, tp->snd_max))
tcp_sack_adjust(tp);
sendalot = 0;
off = tp->snd_nxt - tp->snd_una;
sendwin = min(tp->snd_wnd, tp->snd_cwnd);
sendwin = min(sendwin, tp->snd_bwnd);
flags = tcp_outflags[tp->t_state];
/*
* Send any SACK-generated retransmissions. If we're explicitly trying
* to send out new data (when sendalot is 1), bypass this function.
* If we retransmit in fast recovery mode, decrement snd_cwnd, since
* we're replacing a (future) new transmission with a retransmission
* now, and we previously incremented snd_cwnd in tcp_input().
*/
/*
* Still in sack recovery , reset rxmit flag to zero.
*/
sack_rxmit = 0;
sack_bytes_rxmt = 0;
len = 0;
p = NULL;
if ((tp->t_flags & TF_SACK_PERMIT) && IN_FASTRECOVERY(tp) &&
(p = tcp_sack_output(tp, &sack_bytes_rxmt))) {
long cwin;
cwin = min(tp->snd_wnd, tp->snd_cwnd) - sack_bytes_rxmt;
if (cwin < 0)
cwin = 0;
/* Do not retransmit SACK segments beyond snd_recover */
if (SEQ_GT(p->end, tp->snd_recover)) {
/*
* (At least) part of sack hole extends beyond
* snd_recover. Check to see if we can rexmit data
* for this hole.
*/
if (SEQ_GEQ(p->rxmit, tp->snd_recover)) {
/*
* Can't rexmit any more data for this hole.
* That data will be rexmitted in the next
* sack recovery episode, when snd_recover
* moves past p->rxmit.
*/
p = NULL;
goto after_sack_rexmit;
} else
/* Can rexmit part of the current hole */
len = ((long)ulmin(cwin,
tp->snd_recover - p->rxmit));
} else
len = ((long)ulmin(cwin, p->end - p->rxmit));
off = p->rxmit - tp->snd_una;
KASSERT(off >= 0,("%s: sack block to the left of una : %d",
__func__, off));
if (len > 0) {
sack_rxmit = 1;
sendalot = 1;
tcpstat.tcps_sack_rexmits++;
tcpstat.tcps_sack_rexmit_bytes +=
min(len, tp->t_maxseg);
}
}
after_sack_rexmit:
/*
* Get standard flags, and add SYN or FIN if requested by 'hidden'
* state flags.
*/
if (tp->t_flags & TF_NEEDFIN)
flags |= TH_FIN;
if (tp->t_flags & TF_NEEDSYN)
flags |= TH_SYN;
SOCKBUF_LOCK(&so->so_snd);
/*
* If in persist timeout with window of 0, send 1 byte.
* Otherwise, if window is small but nonzero
* and timer expired, we will send what we can
* and go to transmit state.
*/
if (tp->t_flags & TF_FORCEDATA) {
if (sendwin == 0) {
/*
* If we still have some data to send, then
* clear the FIN bit. Usually this would
* happen below when it realizes that we
* aren't sending all the data. However,
* if we have exactly 1 byte of unsent data,
* then it won't clear the FIN bit below,
* and if we are in persist state, we wind
* up sending the packet without recording
* that we sent the FIN bit.
*
* We can't just blindly clear the FIN bit,
* because if we don't have any more data
* to send then the probe will be the FIN
* itself.
*/
if (off < so->so_snd.sb_cc)
flags &= ~TH_FIN;
sendwin = 1;
} else {
tcp_timer_activate(tp, TT_PERSIST, 0);
tp->t_rxtshift = 0;
}
}
/*
* If snd_nxt == snd_max and we have transmitted a FIN, the
* offset will be > 0 even if so_snd.sb_cc is 0, resulting in
* a negative length. This can also occur when TCP opens up
* its congestion window while receiving additional duplicate
* acks after fast-retransmit because TCP will reset snd_nxt
* to snd_max after the fast-retransmit.
*
* In the normal retransmit-FIN-only case, however, snd_nxt will
* be set to snd_una, the offset will be 0, and the length may
* wind up 0.
*
* If sack_rxmit is true we are retransmitting from the scoreboard
* in which case len is already set.
*/
if (sack_rxmit == 0) {
if (sack_bytes_rxmt == 0)
len = ((long)ulmin(so->so_snd.sb_cc, sendwin) - off);
else {
long cwin;
/*
* We are inside of a SACK recovery episode and are
* sending new data, having retransmitted all the
* data possible in the scoreboard.
*/
len = ((long)ulmin(so->so_snd.sb_cc, tp->snd_wnd)
- off);
/*
* Don't remove this (len > 0) check !
* We explicitly check for len > 0 here (although it
* isn't really necessary), to work around a gcc
* optimization issue - to force gcc to compute
* len above. Without this check, the computation
* of len is bungled by the optimizer.
*/
if (len > 0) {
cwin = tp->snd_cwnd -
(tp->snd_nxt - tp->sack_newdata) -
sack_bytes_rxmt;
if (cwin < 0)
cwin = 0;
len = lmin(len, cwin);
}
}
}
/*
* Lop off SYN bit if it has already been sent. However, if this
* is SYN-SENT state and if segment contains data and if we don't
* know that foreign host supports TAO, suppress sending segment.
*/
if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
if (tp->t_state != TCPS_SYN_RECEIVED)
flags &= ~TH_SYN;
off--, len++;
}
/*
* Be careful not to send data and/or FIN on SYN segments.
* This measure is needed to prevent interoperability problems
* with not fully conformant TCP implementations.
*/
if ((flags & TH_SYN) && (tp->t_flags & TF_NOOPT)) {
len = 0;
flags &= ~TH_FIN;
}
if (len < 0) {
/*
* If FIN has been sent but not acked,
* but we haven't been called to retransmit,
* len will be < 0. Otherwise, window shrank
* after we sent into it. If window shrank to 0,
* cancel pending retransmit, pull snd_nxt back
* to (closed) window, and set the persist timer
* if it isn't already going. If the window didn't
* close completely, just wait for an ACK.
*/
len = 0;
if (sendwin == 0) {
tcp_timer_activate(tp, TT_REXMT, 0);
tp->t_rxtshift = 0;
tp->snd_nxt = tp->snd_una;
if (!tcp_timer_active(tp, TT_PERSIST))
tcp_setpersist(tp);
}
}
/* len will be >= 0 after this point. */
KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
/*
* Automatic sizing of send socket buffer. Often the send buffer
* size is not optimally adjusted to the actual network conditions
* at hand (delay bandwidth product). Setting the buffer size too
* small limits throughput on links with high bandwidth and high
* delay (eg. trans-continental/oceanic links). Setting the
* buffer size too big consumes too much real kernel memory,
* especially with many connections on busy servers.
*
* The criteria to step up the send buffer one notch are:
* 1. receive window of remote host is larger than send buffer
* (with a fudge factor of 5/4th);
* 2. send buffer is filled to 7/8th with data (so we actually
* have data to make use of it);
* 3. send buffer fill has not hit maximal automatic size;
* 4. our send window (slow start and cogestion controlled) is
* larger than sent but unacknowledged data in send buffer.
*
* The remote host receive window scaling factor may limit the
* growing of the send buffer before it reaches its allowed
* maximum.
*
* It scales directly with slow start or congestion window
* and does at most one step per received ACK. This fast
* scaling has the drawback of growing the send buffer beyond
* what is strictly necessary to make full use of a given
* delay*bandwith product. However testing has shown this not
* to be much of an problem. At worst we are trading wasting
* of available bandwith (the non-use of it) for wasting some
* socket buffer memory.
*
* TODO: Shrink send buffer during idle periods together
* with congestion window. Requires another timer. Has to
* wait for upcoming tcp timer rewrite.
*/
if (tcp_do_autosndbuf && so->so_snd.sb_flags & SB_AUTOSIZE) {
if ((tp->snd_wnd / 4 * 5) >= so->so_snd.sb_hiwat &&
so->so_snd.sb_cc >= (so->so_snd.sb_hiwat / 8 * 7) &&
so->so_snd.sb_cc < tcp_autosndbuf_max &&
sendwin >= (so->so_snd.sb_cc - (tp->snd_nxt - tp->snd_una))) {
if (!sbreserve_locked(&so->so_snd,
min(so->so_snd.sb_hiwat + tcp_autosndbuf_inc,
tcp_autosndbuf_max), so, curthread))
so->so_snd.sb_flags &= ~SB_AUTOSIZE;
}
}
/*
* Truncate to the maximum segment length or enable TCP Segmentation
* Offloading (if supported by hardware) and ensure that FIN is removed
* if the length no longer contains the last data byte.
*
* TSO may only be used if we are in a pure bulk sending state. The
* presence of TCP-MD5, SACK retransmits, SACK advertizements and
* IP options prevent using TSO. With TSO the TCP header is the same
* (except for the sequence number) for all generated packets. This
* makes it impossible to transmit any options which vary per generated
* segment or packet.
*
* The length of TSO bursts is limited to TCP_MAXWIN. That limit and
* removal of FIN (if not already catched here) are handled later after
* the exact length of the TCP options are known.
*/
#ifdef IPSEC
/*
* Pre-calculate here as we save another lookup into the darknesses
* of IPsec that way and can actually decide if TSO is ok.
*/
ipsec_optlen = ipsec_hdrsiz_tcp(tp);
#endif
if (len > tp->t_maxseg) {
if ((tp->t_flags & TF_TSO) && tcp_do_tso &&
((tp->t_flags & TF_SIGNATURE) == 0) &&
tp->rcv_numsacks == 0 && sack_rxmit == 0 &&
tp->t_inpcb->inp_options == NULL &&
tp->t_inpcb->in6p_options == NULL
#ifdef IPSEC
&& ipsec_optlen == 0
#endif
) {
tso = 1;
} else {
len = tp->t_maxseg;
sendalot = 1;
tso = 0;
}
}
if (sack_rxmit) {
if (SEQ_LT(p->rxmit + len, tp->snd_una + so->so_snd.sb_cc))
flags &= ~TH_FIN;
} else {
if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
flags &= ~TH_FIN;
}
recwin = sbspace(&so->so_rcv);
/*
* Sender silly window avoidance. We transmit under the following
* conditions when len is non-zero:
*
* - We have a full segment (or more with TSO)
* - This is the last buffer in a write()/send() and we are
* either idle or running NODELAY
* - we've timed out (e.g. persist timer)
* - we have more then 1/2 the maximum send window's worth of
* data (receiver may be limited the window size)
* - we need to retransmit
*/
if (len) {
if (len >= tp->t_maxseg)
goto send;
/*
* NOTE! on localhost connections an 'ack' from the remote
* end may occur synchronously with the output and cause
* us to flush a buffer queued with moretocome. XXX
*
* note: the len + off check is almost certainly unnecessary.
*/
if (!(tp->t_flags & TF_MORETOCOME) && /* normal case */
(idle || (tp->t_flags & TF_NODELAY)) &&
len + off >= so->so_snd.sb_cc &&
(tp->t_flags & TF_NOPUSH) == 0) {
goto send;
}
if (tp->t_flags & TF_FORCEDATA) /* typ. timeout case */
goto send;
if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
goto send;
if (SEQ_LT(tp->snd_nxt, tp->snd_max)) /* retransmit case */
goto send;
if (sack_rxmit)
goto send;
}
/*
* Compare available window to amount of window
* known to peer (as advertised window less
* next expected input). If the difference is at least two
* max size segments, or at least 50% of the maximum possible
* window, then want to send a window update to peer.
* Skip this if the connection is in T/TCP half-open state.
* Don't send pure window updates when the peer has closed
* the connection and won't ever send more data.
*/
if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) &&
!TCPS_HAVERCVDFIN(tp->t_state)) {
/*
* "adv" is the amount we can increase the window,
* taking into account that we are limited by
* TCP_MAXWIN << tp->rcv_scale.
*/
long adv = min(recwin, (long)TCP_MAXWIN << tp->rcv_scale) -
(tp->rcv_adv - tp->rcv_nxt);
if (adv >= (long) (2 * tp->t_maxseg))
goto send;
if (2 * adv >= (long) so->so_rcv.sb_hiwat)
goto send;
}
/*
* Send if we owe the peer an ACK, RST, SYN, or urgent data. ACKNOW
* is also a catch-all for the retransmit timer timeout case.
*/
if (tp->t_flags & TF_ACKNOW)
goto send;
if ((flags & TH_RST) ||
((flags & TH_SYN) && (tp->t_flags & TF_NEEDSYN) == 0))
goto send;
if (SEQ_GT(tp->snd_up, tp->snd_una))
goto send;
/*
* If our state indicates that FIN should be sent
* and we have not yet done so, then we need to send.
*/
if (flags & TH_FIN &&
((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == tp->snd_una))
goto send;
/*
* In SACK, it is possible for tcp_output to fail to send a segment
* after the retransmission timer has been turned off. Make sure
* that the retransmission timer is set.
*/
if ((tp->t_flags & TF_SACK_PERMIT) &&
SEQ_GT(tp->snd_max, tp->snd_una) &&
!tcp_timer_active(tp, TT_REXMT) &&
!tcp_timer_active(tp, TT_PERSIST)) {
tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
goto just_return;
}
/*
* TCP window updates are not reliable, rather a polling protocol
* using ``persist'' packets is used to insure receipt of window
* updates. The three ``states'' for the output side are:
* idle not doing retransmits or persists
* persisting to move a small or zero window
* (re)transmitting and thereby not persisting
*
* tcp_timer_active(tp, TT_PERSIST)
* is true when we are in persist state.
* (tp->t_flags & TF_FORCEDATA)
* is set when we are called to send a persist packet.
* tcp_timer_active(tp, TT_REXMT)
* is set when we are retransmitting
* The output side is idle when both timers are zero.
*
* If send window is too small, there is data to transmit, and no
* retransmit or persist is pending, then go to persist state.
* If nothing happens soon, send when timer expires:
* if window is nonzero, transmit what we can,
* otherwise force out a byte.
*/
if (so->so_snd.sb_cc && !tcp_timer_active(tp, TT_REXMT) &&
!tcp_timer_active(tp, TT_PERSIST)) {
tp->t_rxtshift = 0;
tcp_setpersist(tp);
}
/*
* No reason to send a segment, just return.
*/
just_return:
SOCKBUF_UNLOCK(&so->so_snd);
return (0);
send:
SOCKBUF_LOCK_ASSERT(&so->so_snd);
/*
* Before ESTABLISHED, force sending of initial options
* unless TCP set not to do any options.
* NOTE: we assume that the IP/TCP header plus TCP options
* always fit in a single mbuf, leaving room for a maximum
* link header, i.e.
* max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MCLBYTES
*/
optlen = 0;
#ifdef INET6
if (isipv6)
hdrlen = sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
else
#endif
hdrlen = sizeof (struct tcpiphdr);
/*
* Compute options for segment.
* We only have to care about SYN and established connection
* segments. Options for SYN-ACK segments are handled in TCP
* syncache.
*/
if ((tp->t_flags & TF_NOOPT) == 0) {
to.to_flags = 0;
/* Maximum segment size. */
if (flags & TH_SYN) {
tp->snd_nxt = tp->iss;
to.to_mss = tcp_mssopt(&tp->t_inpcb->inp_inc);
to.to_flags |= TOF_MSS;
}
/* Window scaling. */
if ((flags & TH_SYN) && (tp->t_flags & TF_REQ_SCALE)) {
to.to_wscale = tp->request_r_scale;
to.to_flags |= TOF_SCALE;
}
/* Timestamps. */
if ((tp->t_flags & TF_RCVD_TSTMP) ||
((flags & TH_SYN) && (tp->t_flags & TF_REQ_TSTMP))) {
to.to_tsval = ticks + tp->ts_offset;
to.to_tsecr = tp->ts_recent;
to.to_flags |= TOF_TS;
/* Set receive buffer autosizing timestamp. */
if (tp->rfbuf_ts == 0 &&
(so->so_rcv.sb_flags & SB_AUTOSIZE))
tp->rfbuf_ts = ticks;
}
/* Selective ACK's. */
if (tp->t_flags & TF_SACK_PERMIT) {
if (flags & TH_SYN)
to.to_flags |= TOF_SACKPERM;
else if (TCPS_HAVEESTABLISHED(tp->t_state) &&
(tp->t_flags & TF_SACK_PERMIT) &&
tp->rcv_numsacks > 0) {
to.to_flags |= TOF_SACK;
to.to_nsacks = tp->rcv_numsacks;
to.to_sacks = (u_char *)tp->sackblks;
}
}
#ifdef TCP_SIGNATURE
/* TCP-MD5 (RFC2385). */
if (tp->t_flags & TF_SIGNATURE)
to.to_flags |= TOF_SIGNATURE;
#endif /* TCP_SIGNATURE */
/* Processing the options. */
hdrlen += optlen = tcp_addoptions(&to, opt);
}
#ifdef INET6
if (isipv6)
ipoptlen = ip6_optlen(tp->t_inpcb);
else
#endif
if (tp->t_inpcb->inp_options)
ipoptlen = tp->t_inpcb->inp_options->m_len -
offsetof(struct ipoption, ipopt_list);
else
ipoptlen = 0;
#ifdef IPSEC
ipoptlen += ipsec_optlen;
#endif
/*
* Adjust data length if insertion of options will
* bump the packet length beyond the t_maxopd length.
* Clear the FIN bit because we cut off the tail of
* the segment.
*
* When doing TSO limit a burst to TCP_MAXWIN minus the
* IP, TCP and Options length to keep ip->ip_len from
* overflowing. Prevent the last segment from being
* fractional thus making them all equal sized and set
* the flag to continue sending. TSO is disabled when
* IP options or IPSEC are present.
*/
if (len + optlen + ipoptlen > tp->t_maxopd) {
flags &= ~TH_FIN;
if (tso) {
if (len > TCP_MAXWIN - hdrlen - optlen) {
len = TCP_MAXWIN - hdrlen - optlen;
len = len - (len % (tp->t_maxopd - optlen));
sendalot = 1;
} else if (tp->t_flags & TF_NEEDFIN)
sendalot = 1;
} else {
len = tp->t_maxopd - optlen - ipoptlen;
sendalot = 1;
}
}
/*#ifdef DIAGNOSTIC*/
#ifdef INET6
if (max_linkhdr + hdrlen > MCLBYTES)
#else
if (max_linkhdr + hdrlen > MHLEN)
#endif
panic("tcphdr too big");
/*#endif*/
/*
* This KASSERT is here to catch edge cases at a well defined place.
* Before, those had triggered (random) panic conditions further down.
*/
KASSERT(len >= 0, ("[%s:%d]: len < 0", __func__, __LINE__));
/*
* Grab a header mbuf, attaching a copy of data to
* be transmitted, and initialize the header from
* the template for sends on this connection.
*/
if (len) {
struct mbuf *mb;
u_int moff;
if ((tp->t_flags & TF_FORCEDATA) && len == 1)
tcpstat.tcps_sndprobe++;
else if (SEQ_LT(tp->snd_nxt, tp->snd_max) || sack_rxmit) {
tcpstat.tcps_sndrexmitpack++;
tcpstat.tcps_sndrexmitbyte += len;
} else {
tcpstat.tcps_sndpack++;
tcpstat.tcps_sndbyte += len;
}
#ifdef notyet
if ((m = m_copypack(so->so_snd.sb_mb, off,
(int)len, max_linkhdr + hdrlen)) == 0) {
SOCKBUF_UNLOCK(&so->so_snd);
error = ENOBUFS;
goto out;
}
/*
* m_copypack left space for our hdr; use it.
*/
m->m_len += hdrlen;
m->m_data -= hdrlen;
#else
MGETHDR(m, M_DONTWAIT, MT_DATA);
if (m == NULL) {
SOCKBUF_UNLOCK(&so->so_snd);
error = ENOBUFS;
goto out;
}
#ifdef INET6
if (MHLEN < hdrlen + max_linkhdr) {
MCLGET(m, M_DONTWAIT);
if ((m->m_flags & M_EXT) == 0) {
SOCKBUF_UNLOCK(&so->so_snd);
m_freem(m);
error = ENOBUFS;
goto out;
}
}
#endif
m->m_data += max_linkhdr;
m->m_len = hdrlen;
/*
* Start the m_copy functions from the closest mbuf
* to the offset in the socket buffer chain.
*/
mb = sbsndptr(&so->so_snd, off, len, &moff);
if (len <= MHLEN - hdrlen - max_linkhdr) {
m_copydata(mb, moff, (int)len,
mtod(m, caddr_t) + hdrlen);
m->m_len += len;
} else {
m->m_next = m_copy(mb, moff, (int)len);
if (m->m_next == NULL) {
SOCKBUF_UNLOCK(&so->so_snd);
(void) m_free(m);
error = ENOBUFS;
goto out;
}
}
#endif
/*
* If we're sending everything we've got, set PUSH.
* (This will keep happy those implementations which only
* give data to the user when a buffer fills or
* a PUSH comes in.)
*/
if (off + len == so->so_snd.sb_cc)
flags |= TH_PUSH;
SOCKBUF_UNLOCK(&so->so_snd);
} else {
SOCKBUF_UNLOCK(&so->so_snd);
if (tp->t_flags & TF_ACKNOW)
tcpstat.tcps_sndacks++;
else if (flags & (TH_SYN|TH_FIN|TH_RST))
tcpstat.tcps_sndctrl++;
else if (SEQ_GT(tp->snd_up, tp->snd_una))
tcpstat.tcps_sndurg++;
else
tcpstat.tcps_sndwinup++;
MGETHDR(m, M_DONTWAIT, MT_DATA);
if (m == NULL) {
error = ENOBUFS;
goto out;
}
#ifdef INET6
if (isipv6 && (MHLEN < hdrlen + max_linkhdr) &&
MHLEN >= hdrlen) {
MH_ALIGN(m, hdrlen);
} else
#endif
m->m_data += max_linkhdr;
m->m_len = hdrlen;
}
SOCKBUF_UNLOCK_ASSERT(&so->so_snd);
m->m_pkthdr.rcvif = (struct ifnet *)0;
#ifdef MAC
mac_create_mbuf_from_inpcb(tp->t_inpcb, m);
#endif
#ifdef INET6
if (isipv6) {
ip6 = mtod(m, struct ip6_hdr *);
th = (struct tcphdr *)(ip6 + 1);
tcpip_fillheaders(tp->t_inpcb, ip6, th);
} else
#endif /* INET6 */
{
ip = mtod(m, struct ip *);
ipov = (struct ipovly *)ip;
th = (struct tcphdr *)(ip + 1);
tcpip_fillheaders(tp->t_inpcb, ip, th);
}
/*
* Fill in fields, remembering maximum advertised
* window for use in delaying messages about window sizes.
* If resending a FIN, be sure not to use a new sequence number.
*/
if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
tp->snd_nxt == tp->snd_max)
tp->snd_nxt--;
/*
* If we are doing retransmissions, then snd_nxt will
* not reflect the first unsent octet. For ACK only
* packets, we do not want the sequence number of the
* retransmitted packet, we want the sequence number
* of the next unsent octet. So, if there is no data
* (and no SYN or FIN), use snd_max instead of snd_nxt
* when filling in ti_seq. But if we are in persist
* state, snd_max might reflect one byte beyond the
* right edge of the window, so use snd_nxt in that
* case, since we know we aren't doing a retransmission.
* (retransmit and persist are mutually exclusive...)
*/
if (sack_rxmit == 0) {
if (len || (flags & (TH_SYN|TH_FIN)) ||
tcp_timer_active(tp, TT_PERSIST))
th->th_seq = htonl(tp->snd_nxt);
else
th->th_seq = htonl(tp->snd_max);
} else {
th->th_seq = htonl(p->rxmit);
p->rxmit += len;
tp->sackhint.sack_bytes_rexmit += len;
}
th->th_ack = htonl(tp->rcv_nxt);
if (optlen) {
bcopy(opt, th + 1, optlen);
th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
}
th->th_flags = flags;
/*
* Calculate receive window. Don't shrink window,
* but avoid silly window syndrome.
*/
if (recwin < (long)(so->so_rcv.sb_hiwat / 4) &&
recwin < (long)tp->t_maxseg)
recwin = 0;
if (recwin < (long)(tp->rcv_adv - tp->rcv_nxt))
recwin = (long)(tp->rcv_adv - tp->rcv_nxt);
if (recwin > (long)TCP_MAXWIN << tp->rcv_scale)
recwin = (long)TCP_MAXWIN << tp->rcv_scale;
/*
* According to RFC1323 the window field in a SYN (i.e., a <SYN>
* or <SYN,ACK>) segment itself is never scaled. The <SYN,ACK>
* case is handled in syncache.
*/
if (flags & TH_SYN)
th->th_win = htons((u_short)
(min(sbspace(&so->so_rcv), TCP_MAXWIN)));
else
th->th_win = htons((u_short)(recwin >> tp->rcv_scale));
/*
* Adjust the RXWIN0SENT flag - indicate that we have advertised
* a 0 window. This may cause the remote transmitter to stall. This
* flag tells soreceive() to disable delayed acknowledgements when
* draining the buffer. This can occur if the receiver is attempting
* to read more data then can be buffered prior to transmitting on
* the connection.
*/
if (th->th_win == 0)
tp->t_flags |= TF_RXWIN0SENT;
else
tp->t_flags &= ~TF_RXWIN0SENT;
if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
th->th_urp = htons((u_short)(tp->snd_up - tp->snd_nxt));
th->th_flags |= TH_URG;
} else
/*
* If no urgent pointer to send, then we pull
* the urgent pointer to the left edge of the send window
* so that it doesn't drift into the send window on sequence
* number wraparound.
*/
tp->snd_up = tp->snd_una; /* drag it along */
#ifdef TCP_SIGNATURE
if (tp->t_flags & TF_SIGNATURE) {
int sigoff = to.to_signature - opt;
tcp_signature_compute(m, 0, len, optlen,
(u_char *)(th + 1) + sigoff, IPSEC_DIR_OUTBOUND);
}
#endif
/*
* Put TCP length in extended header, and then
* checksum extended header and data.
*/
m->m_pkthdr.len = hdrlen + len; /* in6_cksum() need this */
#ifdef INET6
if (isipv6)
/*
* ip6_plen is not need to be filled now, and will be filled
* in ip6_output.
*/
th->th_sum = in6_cksum(m, IPPROTO_TCP, sizeof(struct ip6_hdr),
sizeof(struct tcphdr) + optlen + len);
else
#endif /* INET6 */
{
m->m_pkthdr.csum_flags = CSUM_TCP;
m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
htons(sizeof(struct tcphdr) + IPPROTO_TCP + len + optlen));
/* IP version must be set here for ipv4/ipv6 checking later */
KASSERT(ip->ip_v == IPVERSION,
("%s: IP version incorrect: %d", __func__, ip->ip_v));
}
/*
* Enable TSO and specify the size of the segments.
* The TCP pseudo header checksum is always provided.
* XXX: Fixme: This is currently not the case for IPv6.
*/
if (tso) {
m->m_pkthdr.csum_flags = CSUM_TSO;
m->m_pkthdr.tso_segsz = tp->t_maxopd - optlen;
}
/*
* In transmit state, time the transmission and arrange for
* the retransmit. In persist state, just set snd_max.
*/
if ((tp->t_flags & TF_FORCEDATA) == 0 ||
!tcp_timer_active(tp, TT_PERSIST)) {
tcp_seq startseq = tp->snd_nxt;
/*
* Advance snd_nxt over sequence space of this segment.
*/
if (flags & (TH_SYN|TH_FIN)) {
if (flags & TH_SYN)
tp->snd_nxt++;
if (flags & TH_FIN) {
tp->snd_nxt++;
tp->t_flags |= TF_SENTFIN;
}
}
if (sack_rxmit)
goto timer;
tp->snd_nxt += len;
if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
tp->snd_max = tp->snd_nxt;
/*
* Time this transmission if not a retransmission and
* not currently timing anything.
*/
if (tp->t_rtttime == 0) {
tp->t_rtttime = ticks;
tp->t_rtseq = startseq;
tcpstat.tcps_segstimed++;
}
}
/*
* Set retransmit timer if not currently set,
* and not doing a pure ack or a keep-alive probe.
* Initial value for retransmit timer is smoothed
* round-trip time + 2 * round-trip time variance.
* Initialize shift counter which is used for backoff
* of retransmit time.
*/
timer:
if (!tcp_timer_active(tp, TT_REXMT) &&
((sack_rxmit && tp->snd_nxt != tp->snd_max) ||
(tp->snd_nxt != tp->snd_una))) {
if (tcp_timer_active(tp, TT_PERSIST)) {
tcp_timer_activate(tp, TT_PERSIST, 0);
tp->t_rxtshift = 0;
}
tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
}
} else {
/*
* Persist case, update snd_max but since we are in
* persist mode (no window) we do not update snd_nxt.
*/
int xlen = len;
if (flags & TH_SYN)
++xlen;
if (flags & TH_FIN) {
++xlen;
tp->t_flags |= TF_SENTFIN;
}
if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max))
tp->snd_max = tp->snd_nxt + len;
}
#ifdef TCPDEBUG
/*
* Trace.
*/
if (so->so_options & SO_DEBUG) {
u_short save = 0;
#ifdef INET6
if (!isipv6)
#endif
{
save = ipov->ih_len;
ipov->ih_len = htons(m->m_pkthdr.len /* - hdrlen + (th->th_off << 2) */);
}
tcp_trace(TA_OUTPUT, tp->t_state, tp, mtod(m, void *), th, 0);
#ifdef INET6
if (!isipv6)
#endif
ipov->ih_len = save;
}
#endif
/*
* Fill in IP length and desired time to live and
* send to IP level. There should be a better way
* to handle ttl and tos; we could keep them in
* the template, but need a way to checksum without them.
*/
/*
* m->m_pkthdr.len should have been set before cksum calcuration,
* because in6_cksum() need it.
*/
#ifdef INET6
if (isipv6) {
/*
* we separately set hoplimit for every segment, since the
* user might want to change the value via setsockopt.
* Also, desired default hop limit might be changed via
* Neighbor Discovery.
*/
ip6->ip6_hlim = in6_selecthlim(tp->t_inpcb, NULL);
/* TODO: IPv6 IP6TOS_ECT bit on */
error = ip6_output(m,
tp->t_inpcb->in6p_outputopts, NULL,
((so->so_options & SO_DONTROUTE) ?
IP_ROUTETOIF : 0), NULL, NULL, tp->t_inpcb);
} else
#endif /* INET6 */
{
ip->ip_len = m->m_pkthdr.len;
#ifdef INET6
if (tp->t_inpcb->inp_vflag & INP_IPV6PROTO)
ip->ip_ttl = in6_selecthlim(tp->t_inpcb, NULL);
#endif /* INET6 */
/*
* If we do path MTU discovery, then we set DF on every packet.
* This might not be the best thing to do according to RFC3390
* Section 2. However the tcp hostcache migitates the problem
* so it affects only the first tcp connection with a host.
*/
if (path_mtu_discovery)
ip->ip_off |= IP_DF;
error = ip_output(m, tp->t_inpcb->inp_options, NULL,
((so->so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0), 0,
tp->t_inpcb);
}
if (error) {
/*
* We know that the packet was lost, so back out the
* sequence number advance, if any.
*
* If the error is EPERM the packet got blocked by the
* local firewall. Normally we should terminate the
* connection but the blocking may have been spurious
* due to a firewall reconfiguration cycle. So we treat
* it like a packet loss and let the retransmit timer and
* timeouts do their work over time.
* XXX: It is a POLA question whether calling tcp_drop right
* away would be the really correct behavior instead.
*/
if (((tp->t_flags & TF_FORCEDATA) == 0 ||
!tcp_timer_active(tp, TT_PERSIST)) &&
((flags & TH_SYN) == 0) &&
(error != EPERM)) {
if (sack_rxmit) {
p->rxmit -= len;
tp->sackhint.sack_bytes_rexmit -= len;
KASSERT(tp->sackhint.sack_bytes_rexmit >= 0,
("sackhint bytes rtx >= 0"));
} else
tp->snd_nxt -= len;
}
out:
SOCKBUF_UNLOCK_ASSERT(&so->so_snd); /* Check gotos. */
switch (error) {
case EPERM:
tp->t_softerror = error;
return (error);
case ENOBUFS:
if (!tcp_timer_active(tp, TT_REXMT) &&
!tcp_timer_active(tp, TT_PERSIST))
tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
tp->snd_cwnd = tp->t_maxseg;
return (0);
case EMSGSIZE:
/*
* For some reason the interface we used initially
* to send segments changed to another or lowered
* its MTU.
*
* tcp_mtudisc() will find out the new MTU and as
* its last action, initiate retransmission, so it
* is important to not do so here.
*
* If TSO was active we either got an interface
* without TSO capabilits or TSO was turned off.
* Disable it for this connection as too and
* immediatly retry with MSS sized segments generated
* by this function.
*/
if (tso)
tp->t_flags &= ~TF_TSO;
tcp_mtudisc(tp->t_inpcb, 0);
return (0);
case EHOSTDOWN:
case EHOSTUNREACH:
case ENETDOWN:
case ENETUNREACH:
if (TCPS_HAVERCVDSYN(tp->t_state)) {
tp->t_softerror = error;
return (0);
}
/* FALLTHROUGH */
default:
return (error);
}
}
tcpstat.tcps_sndtotal++;
/*
* Data sent (as far as we can tell).
* If this advertises a larger window than any other segment,
* then remember the size of the advertised window.
* Any pending ACK has now been sent.
*/
if (recwin > 0 && SEQ_GT(tp->rcv_nxt + recwin, tp->rcv_adv))
tp->rcv_adv = tp->rcv_nxt + recwin;
tp->last_ack_sent = tp->rcv_nxt;
tp->t_flags &= ~(TF_ACKNOW | TF_DELACK);
if (tcp_timer_active(tp, TT_DELACK))
tcp_timer_activate(tp, TT_DELACK, 0);
#if 0
/*
* This completely breaks TCP if newreno is turned on. What happens
* is that if delayed-acks are turned on on the receiver, this code
* on the transmitter effectively destroys the TCP window, forcing
* it to four packets (1.5Kx4 = 6K window).
*/
if (sendalot && (!tcp_do_newreno || --maxburst))
goto again;
#endif
if (sendalot)
goto again;
return (0);
}
void
tcp_setpersist(struct tcpcb *tp)
{
int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;
int tt;
if (tcp_timer_active(tp, TT_REXMT))
panic("tcp_setpersist: retransmit pending");
/*
* Start/restart persistance timer.
*/
TCPT_RANGESET(tt, t * tcp_backoff[tp->t_rxtshift],
TCPTV_PERSMIN, TCPTV_PERSMAX);
tcp_timer_activate(tp, TT_PERSIST, tt);
if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
tp->t_rxtshift++;
}
/*
* Insert TCP options according to the supplied parameters to the place
* optp in a consistent way. Can handle unaligned destinations.
*
* The order of the option processing is crucial for optimal packing and
* alignment for the scarce option space.
*
* The optimal order for a SYN/SYN-ACK segment is:
* MSS (4) + NOP (1) + Window scale (3) + SACK permitted (2) +
* Timestamp (10) + Signature (18) = 38 bytes out of a maximum of 40.
*
* The SACK options should be last. SACK blocks consume 8*n+2 bytes.
* So a full size SACK blocks option is 34 bytes (with 4 SACK blocks).
* At minimum we need 10 bytes (to generate 1 SACK block). If both
* TCP Timestamps (12 bytes) and TCP Signatures (18 bytes) are present,
* we only have 10 bytes for SACK options (40 - (12 + 18)).
*/
int
tcp_addoptions(struct tcpopt *to, u_char *optp)
{
u_int mask, optlen = 0;
for (mask = 1; mask < TOF_MAXOPT; mask <<= 1) {
if ((to->to_flags & mask) != mask)
continue;
if (optlen == TCP_MAXOLEN)
break;
switch (to->to_flags & mask) {
case TOF_MSS:
while (optlen % 4) {
optlen += TCPOLEN_NOP;
*optp++ = TCPOPT_NOP;
}
if (TCP_MAXOLEN - optlen < TCPOLEN_MAXSEG)
continue;
optlen += TCPOLEN_MAXSEG;
*optp++ = TCPOPT_MAXSEG;
*optp++ = TCPOLEN_MAXSEG;
to->to_mss = htons(to->to_mss);
bcopy((u_char *)&to->to_mss, optp, sizeof(to->to_mss));
optp += sizeof(to->to_mss);
break;
case TOF_SCALE:
while (!optlen || optlen % 2 != 1) {
optlen += TCPOLEN_NOP;
*optp++ = TCPOPT_NOP;
}
if (TCP_MAXOLEN - optlen < TCPOLEN_WINDOW)
continue;
optlen += TCPOLEN_WINDOW;
*optp++ = TCPOPT_WINDOW;
*optp++ = TCPOLEN_WINDOW;
*optp++ = to->to_wscale;
break;
case TOF_SACKPERM:
while (optlen % 2) {
optlen += TCPOLEN_NOP;
*optp++ = TCPOPT_NOP;
}
if (TCP_MAXOLEN - optlen < TCPOLEN_SACK_PERMITTED)
continue;
optlen += TCPOLEN_SACK_PERMITTED;
*optp++ = TCPOPT_SACK_PERMITTED;
*optp++ = TCPOLEN_SACK_PERMITTED;
break;
case TOF_TS:
while (!optlen || optlen % 4 != 2) {
optlen += TCPOLEN_NOP;
*optp++ = TCPOPT_NOP;
}
if (TCP_MAXOLEN - optlen < TCPOLEN_TIMESTAMP)
continue;
optlen += TCPOLEN_TIMESTAMP;
*optp++ = TCPOPT_TIMESTAMP;
*optp++ = TCPOLEN_TIMESTAMP;
to->to_tsval = htonl(to->to_tsval);
to->to_tsecr = htonl(to->to_tsecr);
bcopy((u_char *)&to->to_tsval, optp, sizeof(to->to_tsval));
optp += sizeof(to->to_tsval);
bcopy((u_char *)&to->to_tsecr, optp, sizeof(to->to_tsecr));
optp += sizeof(to->to_tsecr);
break;
case TOF_SIGNATURE:
{
int siglen = TCPOLEN_SIGNATURE - 2;
while (!optlen || optlen % 4 != 2) {
optlen += TCPOLEN_NOP;
*optp++ = TCPOPT_NOP;
}
if (TCP_MAXOLEN - optlen < TCPOLEN_SIGNATURE)
continue;
optlen += TCPOLEN_SIGNATURE;
*optp++ = TCPOPT_SIGNATURE;
*optp++ = TCPOLEN_SIGNATURE;
to->to_signature = optp;
while (siglen--)
*optp++ = 0;
break;
}
case TOF_SACK:
{
int sackblks = 0;
struct sackblk *sack = (struct sackblk *)to->to_sacks;
tcp_seq sack_seq;
while (!optlen || optlen % 4 != 2) {
optlen += TCPOLEN_NOP;
*optp++ = TCPOPT_NOP;
}
if (TCP_MAXOLEN - optlen < TCPOLEN_SACKHDR + TCPOLEN_SACK)
continue;
optlen += TCPOLEN_SACKHDR;
*optp++ = TCPOPT_SACK;
sackblks = min(to->to_nsacks,
(TCP_MAXOLEN - optlen) / TCPOLEN_SACK);
*optp++ = TCPOLEN_SACKHDR + sackblks * TCPOLEN_SACK;
while (sackblks--) {
sack_seq = htonl(sack->start);
bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
optp += sizeof(sack_seq);
sack_seq = htonl(sack->end);
bcopy((u_char *)&sack_seq, optp, sizeof(sack_seq));
optp += sizeof(sack_seq);
optlen += TCPOLEN_SACK;
sack++;
}
tcpstat.tcps_sack_send_blocks++;
break;
}
default:
panic("%s: unknown TCP option type", __func__);
break;
}
}
/* Terminate and pad TCP options to a 4 byte boundary. */
if (optlen % 4) {
optlen += TCPOLEN_EOL;
*optp++ = TCPOPT_EOL;
}
/*
* According to RFC 793 (STD0007):
* "The content of the header beyond the End-of-Option option
* must be header padding (i.e., zero)."
* and later: "The padding is composed of zeros."
*/
while (optlen % 4) {
optlen += TCPOLEN_PAD;
*optp++ = TCPOPT_PAD;
}
KASSERT(optlen <= TCP_MAXOLEN, ("%s: TCP options too long", __func__));
return (optlen);
}
