FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_subr.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_subr.c  8.2 (Berkeley) 5/24/95
     * $FreeBSD: releng/5.3/sys/netinet/tcp_subr.c 146003 2005-05-08 10:23:51Z cperciva $
     */
    
    #include "opt_compat.h"
    #include "opt_inet.h"
    #include "opt_inet6.h"
    #include "opt_ipsec.h"
    #include "opt_mac.h"
    #include "opt_tcpdebug.h"
    #include "opt_tcp_sack.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/callout.h>
    #include <sys/kernel.h>
    #include <sys/sysctl.h>
    #include <sys/mac.h>
    #include <sys/malloc.h>
    #include <sys/mbuf.h>
    #ifdef INET6
    #include <sys/domain.h>
    #endif
    #include <sys/proc.h>
    #include <sys/socket.h>
    #include <sys/socketvar.h>
    #include <sys/protosw.h>
    #include <sys/random.h>
    
    #include <vm/uma.h>
    
    #include <net/route.h>
    #include <net/if.h>
    
    #include <netinet/in.h>
    #include <netinet/in_systm.h>
    #include <netinet/ip.h>
    #ifdef INET6
    #include <netinet/ip6.h>
    #endif
    #include <netinet/in_pcb.h>
    #ifdef INET6
    #include <netinet6/in6_pcb.h>
    #endif
    #include <netinet/in_var.h>
    #include <netinet/ip_var.h>
    #ifdef INET6
    #include <netinet6/ip6_var.h>
    #include <netinet6/nd6.h>
    #endif
    #include <netinet/tcp.h>
    #include <netinet/tcp_fsm.h>
    #include <netinet/tcp_seq.h>
    #include <netinet/tcp_timer.h>
    #include <netinet/tcp_var.h>
    #ifdef INET6
    #include <netinet6/tcp6_var.h>
    #endif
    #include <netinet/tcpip.h>
    #ifdef TCPDEBUG
    #include <netinet/tcp_debug.h>
    #endif
    #include <netinet6/ip6protosw.h>
    
    #ifdef IPSEC
    #include <netinet6/ipsec.h>
    #ifdef INET6
    #include <netinet6/ipsec6.h>
    #endif
    #endif /*IPSEC*/
    
   #ifdef FAST_IPSEC
   #include <netipsec/ipsec.h>
   #include <netipsec/xform.h>
   #ifdef INET6
   #include <netipsec/ipsec6.h>
   #endif
   #include <netipsec/key.h>
   #define IPSEC
   #endif /*FAST_IPSEC*/
   
   #include <machine/in_cksum.h>
   #include <sys/md5.h>
   
   int     tcp_mssdflt = TCP_MSS;
   SYSCTL_INT(_net_inet_tcp, TCPCTL_MSSDFLT, mssdflt, CTLFLAG_RW,
       &tcp_mssdflt , 0, "Default TCP Maximum Segment Size");
   
   #ifdef INET6
   int     tcp_v6mssdflt = TCP6_MSS;
   SYSCTL_INT(_net_inet_tcp, TCPCTL_V6MSSDFLT, v6mssdflt,
           CTLFLAG_RW, &tcp_v6mssdflt , 0,
           "Default TCP Maximum Segment Size for IPv6");
   #endif
   
   /*
    * Minimum MSS we accept and use. This prevents DoS attacks where
    * we are forced to a ridiculous low MSS like 20 and send hundreds
    * of packets instead of one. The effect scales with the available
    * bandwidth and quickly saturates the CPU and network interface
    * with packet generation and sending. Set to zero to disable MINMSS
    * checking. This setting prevents us from sending too small packets.
    */
   int     tcp_minmss = TCP_MINMSS;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, minmss, CTLFLAG_RW,
       &tcp_minmss , 0, "Minmum TCP Maximum Segment Size");
   /*
    * Number of TCP segments per second we accept from remote host
    * before we start to calculate average segment size. If average
    * segment size drops below the minimum TCP MSS we assume a DoS
    * attack and reset+drop the connection. Care has to be taken not to
    * set this value too small to not kill interactive type connections
    * (telnet, SSH) which send many small packets.
    */
   int     tcp_minmssoverload = TCP_MINMSSOVERLOAD;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, minmssoverload, CTLFLAG_RW,
       &tcp_minmssoverload , 0, "Number of TCP Segments per Second allowed to"
       "be under the MINMSS Size");
   
   #if 0
   static int      tcp_rttdflt = TCPTV_SRTTDFLT / PR_SLOWHZ;
   SYSCTL_INT(_net_inet_tcp, TCPCTL_RTTDFLT, rttdflt, CTLFLAG_RW,
       &tcp_rttdflt , 0, "Default maximum TCP Round Trip Time");
   #endif
   
   int     tcp_do_rfc1323 = 1;
   SYSCTL_INT(_net_inet_tcp, TCPCTL_DO_RFC1323, rfc1323, CTLFLAG_RW,
       &tcp_do_rfc1323 , 0, "Enable rfc1323 (high performance TCP) extensions");
   
   int     tcp_do_rfc1644 = 0;
   SYSCTL_INT(_net_inet_tcp, TCPCTL_DO_RFC1644, rfc1644, CTLFLAG_RW,
       &tcp_do_rfc1644 , 0, "Enable rfc1644 (TTCP) extensions");
   
   static int      tcp_tcbhashsize = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, tcbhashsize, CTLFLAG_RDTUN,
        &tcp_tcbhashsize, 0, "Size of TCP control-block hashtable");
   
   static int      do_tcpdrain = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, do_tcpdrain, CTLFLAG_RW, &do_tcpdrain, 0,
        "Enable tcp_drain routine for extra help when low on mbufs");
   
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, pcbcount, CTLFLAG_RD,
       &tcbinfo.ipi_count, 0, "Number of active PCBs");
   
   static int      icmp_may_rst = 1;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, icmp_may_rst, CTLFLAG_RW, &icmp_may_rst, 0,
       "Certain ICMP unreachable messages may abort connections in SYN_SENT");
   
   static int      tcp_isn_reseed_interval = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, isn_reseed_interval, CTLFLAG_RW,
       &tcp_isn_reseed_interval, 0, "Seconds between reseeding of ISN secret");
   
   /*
    * TCP bandwidth limiting sysctls.  Note that the default lower bound of
    * 1024 exists only for debugging.  A good production default would be
    * something like 6100.
    */
   SYSCTL_NODE(_net_inet_tcp, OID_AUTO, inflight, CTLFLAG_RW, 0,
       "TCP inflight data limiting");
   
   static int      tcp_inflight_enable = 1;
   SYSCTL_INT(_net_inet_tcp_inflight, OID_AUTO, enable, CTLFLAG_RW,
       &tcp_inflight_enable, 0, "Enable automatic TCP inflight data limiting");
   
   static int      tcp_inflight_debug = 0;
   SYSCTL_INT(_net_inet_tcp_inflight, OID_AUTO, debug, CTLFLAG_RW,
       &tcp_inflight_debug, 0, "Debug TCP inflight calculations");
   
   static int      tcp_inflight_min = 6144;
   SYSCTL_INT(_net_inet_tcp_inflight, OID_AUTO, min, CTLFLAG_RW,
       &tcp_inflight_min, 0, "Lower-bound for TCP inflight window");
   
   static int      tcp_inflight_max = TCP_MAXWIN << TCP_MAX_WINSHIFT;
   SYSCTL_INT(_net_inet_tcp_inflight, OID_AUTO, max, CTLFLAG_RW,
       &tcp_inflight_max, 0, "Upper-bound for TCP inflight window");
   
   static int      tcp_inflight_stab = 20;
   SYSCTL_INT(_net_inet_tcp_inflight, OID_AUTO, stab, CTLFLAG_RW,
       &tcp_inflight_stab, 0, "Inflight Algorithm Stabilization 20 = 2 packets");
   
   SYSCTL_NODE(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW, 0, "TCP SACK");
   int tcp_do_sack = 1;
   SYSCTL_INT(_net_inet_tcp_sack, OID_AUTO, enable, CTLFLAG_RW,
       &tcp_do_sack, 0, "Enable/Disable TCP SACK support");
   
   uma_zone_t sack_hole_zone;
   
   static struct inpcb *tcp_notify(struct inpcb *, int);
   static void     tcp_discardcb(struct tcpcb *);
   static void     tcp_isn_tick(void *);
   
   /*
    * Target size of TCP PCB hash tables. Must be a power of two.
    *
    * Note that this can be overridden by the kernel environment
    * variable net.inet.tcp.tcbhashsize
    */
   #ifndef TCBHASHSIZE
   #define TCBHASHSIZE     512
   #endif
   
   /*
    * XXX
    * Callouts should be moved into struct tcp directly.  They are currently
    * separate because the tcpcb structure is exported to userland for sysctl
    * parsing purposes, which do not know about callouts.
    */
   struct  tcpcb_mem {
           struct  tcpcb tcb;
           struct  callout tcpcb_mem_rexmt, tcpcb_mem_persist, tcpcb_mem_keep;
           struct  callout tcpcb_mem_2msl, tcpcb_mem_delack;
   };
   
   static uma_zone_t tcpcb_zone;
   static uma_zone_t tcptw_zone;
   struct callout isn_callout;
   
   /*
    * Tcp initialization
    */
   void
   tcp_init()
   {
           int hashsize = TCBHASHSIZE;
   
           tcp_ccgen = 1;
   
           tcp_delacktime = TCPTV_DELACK;
           tcp_keepinit = TCPTV_KEEP_INIT;
           tcp_keepidle = TCPTV_KEEP_IDLE;
           tcp_keepintvl = TCPTV_KEEPINTVL;
           tcp_maxpersistidle = TCPTV_KEEP_IDLE;
           tcp_msl = TCPTV_MSL;
           tcp_rexmit_min = TCPTV_MIN;
           tcp_rexmit_slop = TCPTV_CPU_VAR;
   
           INP_INFO_LOCK_INIT(&tcbinfo, "tcp");
           LIST_INIT(&tcb);
           tcbinfo.listhead = &tcb;
           TUNABLE_INT_FETCH("net.inet.tcp.tcbhashsize", &hashsize);
           if (!powerof2(hashsize)) {
                   printf("WARNING: TCB hash size not a power of 2\n");
                   hashsize = 512; /* safe default */
           }
           tcp_tcbhashsize = hashsize;
           tcbinfo.hashbase = hashinit(hashsize, M_PCB, &tcbinfo.hashmask);
           tcbinfo.porthashbase = hashinit(hashsize, M_PCB,
                                           &tcbinfo.porthashmask);
           tcbinfo.ipi_zone = uma_zcreate("inpcb", sizeof(struct inpcb),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);
           uma_zone_set_max(tcbinfo.ipi_zone, maxsockets);
   #ifdef INET6
   #define TCP_MINPROTOHDR (sizeof(struct ip6_hdr) + sizeof(struct tcphdr))
   #else /* INET6 */
   #define TCP_MINPROTOHDR (sizeof(struct tcpiphdr))
   #endif /* INET6 */
           if (max_protohdr < TCP_MINPROTOHDR)
                   max_protohdr = TCP_MINPROTOHDR;
           if (max_linkhdr + TCP_MINPROTOHDR > MHLEN)
                   panic("tcp_init");
   #undef TCP_MINPROTOHDR
           /*
            * These have to be type stable for the benefit of the timers.
            */
           tcpcb_zone = uma_zcreate("tcpcb", sizeof(struct tcpcb_mem),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);
           uma_zone_set_max(tcpcb_zone, maxsockets);
           tcptw_zone = uma_zcreate("tcptw", sizeof(struct tcptw),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);
           uma_zone_set_max(tcptw_zone, maxsockets / 5);
           tcp_timer_init();
           syncache_init();
           tcp_hc_init();
           tcp_reass_init();
           callout_init(&isn_callout, CALLOUT_MPSAFE);
           tcp_isn_tick(NULL);
           EVENTHANDLER_REGISTER(shutdown_pre_sync, tcp_fini, NULL,
                   SHUTDOWN_PRI_DEFAULT);
           sack_hole_zone = uma_zcreate("sackhole", sizeof(struct sackhole),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);
   }
   
   void
   tcp_fini(xtp)
           void *xtp;
   {
           callout_stop(&isn_callout);
   
   }
   
   /*
    * Fill in the IP and TCP headers for an outgoing packet, given the tcpcb.
    * tcp_template used to store this data in mbufs, but we now recopy it out
    * of the tcpcb each time to conserve mbufs.
    */
   void
   tcpip_fillheaders(inp, ip_ptr, tcp_ptr)
           struct inpcb *inp;
           void *ip_ptr;
           void *tcp_ptr;
   {
           struct tcphdr *th = (struct tcphdr *)tcp_ptr;
   
   #ifdef INET6
           if ((inp->inp_vflag & INP_IPV6) != 0) {
                   struct ip6_hdr *ip6;
   
                   ip6 = (struct ip6_hdr *)ip_ptr;
                   ip6->ip6_flow = (ip6->ip6_flow & ~IPV6_FLOWINFO_MASK) |
                           (inp->in6p_flowinfo & IPV6_FLOWINFO_MASK);
                   ip6->ip6_vfc = (ip6->ip6_vfc & ~IPV6_VERSION_MASK) |
                           (IPV6_VERSION & IPV6_VERSION_MASK);
                   ip6->ip6_nxt = IPPROTO_TCP;
                   ip6->ip6_plen = sizeof(struct tcphdr);
                   ip6->ip6_src = inp->in6p_laddr;
                   ip6->ip6_dst = inp->in6p_faddr;
           } else
   #endif
           {
                   struct ip *ip;
   
                   ip = (struct ip *)ip_ptr;
                   ip->ip_v = IPVERSION;
                   ip->ip_hl = 5;
                   ip->ip_tos = inp->inp_ip_tos;
                   ip->ip_len = 0;
                   ip->ip_id = 0;
                   ip->ip_off = 0;
                   ip->ip_ttl = inp->inp_ip_ttl;
                   ip->ip_sum = 0;
                   ip->ip_p = IPPROTO_TCP;
                   ip->ip_src = inp->inp_laddr;
                   ip->ip_dst = inp->inp_faddr;
           }
           th->th_sport = inp->inp_lport;
           th->th_dport = inp->inp_fport;
           th->th_seq = 0;
           th->th_ack = 0;
           th->th_x2 = 0;
           th->th_off = 5;
           th->th_flags = 0;
           th->th_win = 0;
           th->th_urp = 0;
           th->th_sum = 0;         /* in_pseudo() is called later for ipv4 */
   }
   
   /*
    * Create template to be used to send tcp packets on a connection.
    * Allocates an mbuf and fills in a skeletal tcp/ip header.  The only
    * use for this function is in keepalives, which use tcp_respond.
    */
   struct tcptemp *
   tcpip_maketemplate(inp)
           struct inpcb *inp;
   {
           struct mbuf *m;
           struct tcptemp *n;
   
           m = m_get(M_DONTWAIT, MT_HEADER);
           if (m == NULL)
                   return (0);
           m->m_len = sizeof(struct tcptemp);
           n = mtod(m, struct tcptemp *);
   
           tcpip_fillheaders(inp, (void *)&n->tt_ipgen, (void *)&n->tt_t);
           return (n);
   }
   
   /*
    * Send a single message to the TCP at address specified by
    * the given TCP/IP header.  If m == NULL, then we make a copy
    * of the tcpiphdr at ti and send directly to the addressed host.
    * This is used to force keep alive messages out using the TCP
    * template for a connection.  If flags are given then we send
    * a message back to the TCP which originated the * segment ti,
    * and discard the mbuf containing it and any other attached mbufs.
    *
    * In any case the ack and sequence number of the transmitted
    * segment are as specified by the parameters.
    *
    * NOTE: If m != NULL, then ti must point to *inside* the mbuf.
    */
   void
   tcp_respond(tp, ipgen, th, m, ack, seq, flags)
           struct tcpcb *tp;
           void *ipgen;
           register struct tcphdr *th;
           register struct mbuf *m;
           tcp_seq ack, seq;
           int flags;
   {
           register int tlen;
           int win = 0;
           struct ip *ip;
           struct tcphdr *nth;
   #ifdef INET6
           struct ip6_hdr *ip6;
           int isipv6;
   #endif /* INET6 */
           int ipflags = 0;
           struct inpcb *inp;
   
           KASSERT(tp != NULL || m != NULL, ("tcp_respond: tp and m both NULL"));
   
   #ifdef INET6
           isipv6 = ((struct ip *)ipgen)->ip_v == 6;
           ip6 = ipgen;
   #endif /* INET6 */
           ip = ipgen;
   
           if (tp != NULL) {
                   inp = tp->t_inpcb;
                   KASSERT(inp != NULL, ("tcp control block w/o inpcb"));
                   INP_INFO_WLOCK_ASSERT(&tcbinfo);
                   INP_LOCK_ASSERT(inp);
           } else
                   inp = NULL;
   
           if (tp != NULL) {
                   if (!(flags & TH_RST)) {
                           win = sbspace(&inp->inp_socket->so_rcv);
                           if (win > (long)TCP_MAXWIN << tp->rcv_scale)
                                   win = (long)TCP_MAXWIN << tp->rcv_scale;
                   }
           }
           if (m == NULL) {
                   m = m_gethdr(M_DONTWAIT, MT_HEADER);
                   if (m == NULL)
                           return;
                   tlen = 0;
                   m->m_data += max_linkhdr;
   #ifdef INET6
                   if (isipv6) {
                           bcopy((caddr_t)ip6, mtod(m, caddr_t),
                                 sizeof(struct ip6_hdr));
                           ip6 = mtod(m, struct ip6_hdr *);
                           nth = (struct tcphdr *)(ip6 + 1);
                   } else
   #endif /* INET6 */
                 {
                   bcopy((caddr_t)ip, mtod(m, caddr_t), sizeof(struct ip));
                   ip = mtod(m, struct ip *);
                   nth = (struct tcphdr *)(ip + 1);
                 }
                   bcopy((caddr_t)th, (caddr_t)nth, sizeof(struct tcphdr));
                   flags = TH_ACK;
           } else {
                   m_freem(m->m_next);
                   m->m_next = NULL;
                   m->m_data = (caddr_t)ipgen;
                   /* m_len is set later */
                   tlen = 0;
   #define xchg(a,b,type) { type t; t=a; a=b; b=t; }
   #ifdef INET6
                   if (isipv6) {
                           xchg(ip6->ip6_dst, ip6->ip6_src, struct in6_addr);
                           nth = (struct tcphdr *)(ip6 + 1);
                   } else
   #endif /* INET6 */
                 {
                   xchg(ip->ip_dst.s_addr, ip->ip_src.s_addr, n_long);
                   nth = (struct tcphdr *)(ip + 1);
                 }
                   if (th != nth) {
                           /*
                            * this is usually a case when an extension header
                            * exists between the IPv6 header and the
                            * TCP header.
                            */
                           nth->th_sport = th->th_sport;
                           nth->th_dport = th->th_dport;
                   }
                   xchg(nth->th_dport, nth->th_sport, n_short);
   #undef xchg
           }
   #ifdef INET6
           if (isipv6) {
                   ip6->ip6_flow = 0;
                   ip6->ip6_vfc = IPV6_VERSION;
                   ip6->ip6_nxt = IPPROTO_TCP;
                   ip6->ip6_plen = htons((u_short)(sizeof (struct tcphdr) +
                                                   tlen));
                   tlen += sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
           } else
   #endif
           {
                   tlen += sizeof (struct tcpiphdr);
                   ip->ip_len = tlen;
                   ip->ip_ttl = ip_defttl;
                   if (path_mtu_discovery)
                           ip->ip_off |= IP_DF;
           }
           m->m_len = tlen;
           m->m_pkthdr.len = tlen;
           m->m_pkthdr.rcvif = NULL;
   #ifdef MAC
           if (inp != NULL) {
                   /*
                    * Packet is associated with a socket, so allow the
                    * label of the response to reflect the socket label.
                    */
                   INP_LOCK_ASSERT(inp);
                   mac_create_mbuf_from_inpcb(inp, m);
           } else {
                   /*
                    * Packet is not associated with a socket, so possibly
                    * update the label in place.
                    */
                   mac_reflect_mbuf_tcp(m);
           }
   #endif
           nth->th_seq = htonl(seq);
           nth->th_ack = htonl(ack);
           nth->th_x2 = 0;
           nth->th_off = sizeof (struct tcphdr) >> 2;
           nth->th_flags = flags;
           if (tp != NULL)
                   nth->th_win = htons((u_short) (win >> tp->rcv_scale));
           else
                   nth->th_win = htons((u_short)win);
           nth->th_urp = 0;
   #ifdef INET6
           if (isipv6) {
                   nth->th_sum = 0;
                   nth->th_sum = in6_cksum(m, IPPROTO_TCP,
                                           sizeof(struct ip6_hdr),
                                           tlen - sizeof(struct ip6_hdr));
                   ip6->ip6_hlim = in6_selecthlim(tp != NULL ? tp->t_inpcb :
                       NULL, NULL);
           } else
   #endif /* INET6 */
           {
                   nth->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
                       htons((u_short)(tlen - sizeof(struct ip) + ip->ip_p)));
                   m->m_pkthdr.csum_flags = CSUM_TCP;
                   m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
           }
   #ifdef TCPDEBUG
           if (tp == NULL || (inp->inp_socket->so_options & SO_DEBUG))
                   tcp_trace(TA_OUTPUT, 0, tp, mtod(m, void *), th, 0);
   #endif
   #ifdef INET6
           if (isipv6)
                   (void) ip6_output(m, NULL, NULL, ipflags, NULL, NULL, inp);
           else
   #endif /* INET6 */
           (void) ip_output(m, NULL, NULL, ipflags, NULL, inp);
   }
   
   /*
    * Create a new TCP control block, making an
    * empty reassembly queue and hooking it to the argument
    * protocol control block.  The `inp' parameter must have
    * come from the zone allocator set up in tcp_init().
    */
   struct tcpcb *
   tcp_newtcpcb(inp)
           struct inpcb *inp;
   {
           struct tcpcb_mem *tm;
           struct tcpcb *tp;
   #ifdef INET6
           int isipv6 = (inp->inp_vflag & INP_IPV6) != 0;
   #endif /* INET6 */
           int callout_flag;
   
           tm = uma_zalloc(tcpcb_zone, M_NOWAIT | M_ZERO);
           if (tm == NULL)
                   return (NULL);
           tp = &tm->tcb;
           /*      LIST_INIT(&tp->t_segq); */      /* XXX covered by M_ZERO */
           tp->t_maxseg = tp->t_maxopd =
   #ifdef INET6
                   isipv6 ? tcp_v6mssdflt :
   #endif /* INET6 */
                   tcp_mssdflt;
   
           /* Set up our timeouts. */
           /*
            * XXXRW: Are these actually MPSAFE?  I think so, but need to
            * review the timed wait code, as it has some list variables,
            * etc, that are global.
            */
           callout_flag = debug_mpsafenet ? CALLOUT_MPSAFE : 0;
           callout_init(tp->tt_rexmt = &tm->tcpcb_mem_rexmt, callout_flag);
           callout_init(tp->tt_persist = &tm->tcpcb_mem_persist, callout_flag);
           callout_init(tp->tt_keep = &tm->tcpcb_mem_keep, callout_flag);
           callout_init(tp->tt_2msl = &tm->tcpcb_mem_2msl, callout_flag);
           callout_init(tp->tt_delack = &tm->tcpcb_mem_delack, callout_flag);
   
           if (tcp_do_rfc1323)
                   tp->t_flags = (TF_REQ_SCALE|TF_REQ_TSTMP);
           if (tcp_do_rfc1644)
                   tp->t_flags |= TF_REQ_CC;
           tp->sack_enable = tcp_do_sack;
           tp->t_inpcb = inp;      /* XXX */
           /*
            * Init srtt to TCPTV_SRTTBASE (0), so we can tell that we have no
            * rtt estimate.  Set rttvar so that srtt + 4 * rttvar gives
            * reasonable initial retransmit time.
            */
           tp->t_srtt = TCPTV_SRTTBASE;
           tp->t_rttvar = ((TCPTV_RTOBASE - TCPTV_SRTTBASE) << TCP_RTTVAR_SHIFT) / 4;
           tp->t_rttmin = tcp_rexmit_min;
           tp->t_rxtcur = TCPTV_RTOBASE;
           tp->snd_cwnd = TCP_MAXWIN << TCP_MAX_WINSHIFT;
           tp->snd_bwnd = TCP_MAXWIN << TCP_MAX_WINSHIFT;
           tp->snd_ssthresh = TCP_MAXWIN << TCP_MAX_WINSHIFT;
           tp->t_rcvtime = ticks;
           tp->t_bw_rtttime = ticks;
           /*
            * IPv4 TTL initialization is necessary for an IPv6 socket as well,
            * because the socket may be bound to an IPv6 wildcard address,
            * which may match an IPv4-mapped IPv6 address.
            */
           inp->inp_ip_ttl = ip_defttl;
           inp->inp_ppcb = (caddr_t)tp;
           return (tp);            /* XXX */
   }
   
   /*
    * Drop a TCP connection, reporting
    * the specified error.  If connection is synchronized,
    * then send a RST to peer.
    */
   struct tcpcb *
   tcp_drop(tp, errno)
           register struct tcpcb *tp;
           int errno;
   {
           struct socket *so = tp->t_inpcb->inp_socket;
   
           if (TCPS_HAVERCVDSYN(tp->t_state)) {
                   tp->t_state = TCPS_CLOSED;
                   (void) tcp_output(tp);
                   tcpstat.tcps_drops++;
           } else
                   tcpstat.tcps_conndrops++;
           if (errno == ETIMEDOUT && tp->t_softerror)
                   errno = tp->t_softerror;
           so->so_error = errno;
           return (tcp_close(tp));
   }
   
   static void
   tcp_discardcb(tp)
           struct tcpcb *tp;
   {
           struct tseg_qent *q;
           struct inpcb *inp = tp->t_inpcb;
           struct socket *so = inp->inp_socket;
   #ifdef INET6
           int isipv6 = (inp->inp_vflag & INP_IPV6) != 0;
   #endif /* INET6 */
   
           /*
            * Make sure that all of our timers are stopped before we
            * delete the PCB.
            */
           callout_stop(tp->tt_rexmt);
           callout_stop(tp->tt_persist);
           callout_stop(tp->tt_keep);
           callout_stop(tp->tt_2msl);
           callout_stop(tp->tt_delack);
   
           /*
            * If we got enough samples through the srtt filter,
            * save the rtt and rttvar in the routing entry.
            * 'Enough' is arbitrarily defined as 4 rtt samples.
            * 4 samples is enough for the srtt filter to converge
            * to within enough % of the correct value; fewer samples
            * and we could save a bogus rtt. The danger is not high
            * as tcp quickly recovers from everything.
            * XXX: Works very well but needs some more statistics!
            */
           if (tp->t_rttupdated >= 4) {
                   struct hc_metrics_lite metrics;
                   u_long ssthresh;
   
                   bzero(&metrics, sizeof(metrics));
                   /*
                    * Update the ssthresh always when the conditions below
                    * are satisfied. This gives us better new start value
                    * for the congestion avoidance for new connections.
                    * ssthresh is only set if packet loss occured on a session.
                    */
                   ssthresh = tp->snd_ssthresh;
                   if (ssthresh != 0 && ssthresh < so->so_snd.sb_hiwat / 2) {
                           /*
                            * convert the limit from user data bytes to
                            * packets then to packet data bytes.
                            */
                           ssthresh = (ssthresh + tp->t_maxseg / 2) / tp->t_maxseg;
                           if (ssthresh < 2)
                                   ssthresh = 2;
                           ssthresh *= (u_long)(tp->t_maxseg +
   #ifdef INET6
                                         (isipv6 ? sizeof (struct ip6_hdr) +
                                                  sizeof (struct tcphdr) :
   #endif
                                          sizeof (struct tcpiphdr)
   #ifdef INET6
                                          )
   #endif
                                         );
                   } else
                           ssthresh = 0;
                   metrics.rmx_ssthresh = ssthresh;
   
                   metrics.rmx_rtt = tp->t_srtt;
                   metrics.rmx_rttvar = tp->t_rttvar;
                   /* XXX: This wraps if the pipe is more than 4 Gbit per second */
                   metrics.rmx_bandwidth = tp->snd_bandwidth;
                   metrics.rmx_cwnd = tp->snd_cwnd;
                   metrics.rmx_sendpipe = 0;
                   metrics.rmx_recvpipe = 0;
   
                   tcp_hc_update(&inp->inp_inc, &metrics);
           }
   
           /* free the reassembly queue, if any */
           while ((q = LIST_FIRST(&tp->t_segq)) != NULL) {
                   LIST_REMOVE(q, tqe_q);
                   m_freem(q->tqe_m);
                   uma_zfree(tcp_reass_zone, q);
                   tp->t_segqlen--;
                   tcp_reass_qsize--;
           }
           tcp_free_sackholes(tp);
           inp->inp_ppcb = NULL;
           tp->t_inpcb = NULL;
           uma_zfree(tcpcb_zone, tp);
           soisdisconnected(so);
   }
   
   /*
    * Close a TCP control block:
    *    discard all space held by the tcp
    *    discard internet protocol block
    *    wake up any sleepers
    */
   struct tcpcb *
   tcp_close(tp)
           struct tcpcb *tp;
   {
           struct inpcb *inp = tp->t_inpcb;
   #ifdef INET6
           struct socket *so = inp->inp_socket;
   #endif
   
           tcp_discardcb(tp);
   #ifdef INET6
           if (INP_CHECK_SOCKAF(so, AF_INET6))
                   in6_pcbdetach(inp);
           else
   #endif
                   in_pcbdetach(inp);
           tcpstat.tcps_closed++;
           return (NULL);
   }
   
   void
   tcp_drain()
   {
           if (do_tcpdrain)
           {
                   struct inpcb *inpb;
                   struct tcpcb *tcpb;
                   struct tseg_qent *te;
   
           /*
            * Walk the tcpbs, if existing, and flush the reassembly queue,
            * if there is one...
            * XXX: The "Net/3" implementation doesn't imply that the TCP
            *      reassembly queue should be flushed, but in a situation
            *      where we're really low on mbufs, this is potentially
            *      usefull.
            */
                   INP_INFO_RLOCK(&tcbinfo);
                   LIST_FOREACH(inpb, tcbinfo.listhead, inp_list) {
                           if (inpb->inp_vflag & INP_TIMEWAIT)
                                   continue;
                           INP_LOCK(inpb);
                           if ((tcpb = intotcpcb(inpb)) != NULL) {
                                   while ((te = LIST_FIRST(&tcpb->t_segq))
                                       != NULL) {
                                           LIST_REMOVE(te, tqe_q);
                                           m_freem(te->tqe_m);
                                           uma_zfree(tcp_reass_zone, te);
                                           tcpb->t_segqlen--;
                                           tcp_reass_qsize--;
                                   }
                           }
                           INP_UNLOCK(inpb);
                   }
                   INP_INFO_RUNLOCK(&tcbinfo);
           }
   }
   
   /*
    * Notify a tcp user of an asynchronous error;
    * store error as soft error, but wake up user
    * (for now, won't do anything until can select for soft error).
    *
    * Do not wake up user since there currently is no mechanism for
    * reporting soft errors (yet - a kqueue filter may be added).
    */
   static struct inpcb *
   tcp_notify(inp, error)
           struct inpcb *inp;
           int error;
   {
           struct tcpcb *tp = (struct tcpcb *)inp->inp_ppcb;
   
           /*
            * Ignore some errors if we are hooked up.
            * If connection hasn't completed, has retransmitted several times,
            * and receives a second error, give up now.  This is better
            * than waiting a long time to establish a connection that
            * can never complete.
            */
           if (tp->t_state == TCPS_ESTABLISHED &&
               (error == EHOSTUNREACH || error == ENETUNREACH ||
                error == EHOSTDOWN)) {
                   return inp;
           } else if (tp->t_state < TCPS_ESTABLISHED && tp->t_rxtshift > 3 &&
               tp->t_softerror) {
                   tcp_drop(tp, error);
                   return (struct inpcb *)0;
           } else {
                   tp->t_softerror = error;
                   return inp;
           }
   #if 0
           wakeup( &so->so_timeo);
           sorwakeup(so);
           sowwakeup(so);
   #endif
   }
   
   static int
   tcp_pcblist(SYSCTL_HANDLER_ARGS)
   {
           int error, i, n, s;
           struct inpcb *inp, **inp_list;
           inp_gen_t gencnt;
           struct xinpgen xig;
   
           /*
            * The process of preparing the TCB list is too time-consuming and
            * resource-intensive to repeat twice on every request.
            */
           if (req->oldptr == NULL) {
                   n = tcbinfo.ipi_count;
                   req->oldidx = 2 * (sizeof xig)
                           + (n + n/8) * sizeof(struct xtcpcb);
                   return 0;
           }
   
           if (req->newptr != NULL)
                   return EPERM;
   
           /*
            * OK, now we're committed to doing something.
            */
           s = splnet();
           INP_INFO_RLOCK(&tcbinfo);
           gencnt = tcbinfo.ipi_gencnt;
           n = tcbinfo.ipi_count;
           INP_INFO_RUNLOCK(&tcbinfo);
           splx(s);
   
           error = sysctl_wire_old_buffer(req, 2 * (sizeof xig)
                   + n * sizeof(struct xtcpcb));
           if (error != 0)
                   return (error);
   
           xig.xig_len = sizeof xig;
           xig.xig_count = n;
           xig.xig_gen = gencnt;
           xig.xig_sogen = so_gencnt;
           error = SYSCTL_OUT(req, &xig, sizeof xig);
           if (error)
                   return error;
   
           inp_list = malloc(n * sizeof *inp_list, M_TEMP, M_WAITOK);
           if (inp_list == NULL)
                   return ENOMEM;
   
           s = splnet();
           INP_INFO_RLOCK(&tcbinfo);
           for (inp = LIST_FIRST(tcbinfo.listhead), i = 0; inp != NULL && i < n;
                inp = LIST_NEXT(inp, inp_list)) {
                   INP_LOCK(inp);
                   if (inp->inp_gencnt <= gencnt) {
                           /*
                            * XXX: This use of cr_cansee(), introduced with
                            * TCP state changes, is not quite right, but for
                            * now, better than nothing.
                            */
                           if (inp->inp_vflag & INP_TIMEWAIT)
                                   error = cr_cansee(req->td->td_ucred,
                                       intotw(inp)->tw_cred);
                           else
                                   error = cr_canseesocket(req->td->td_ucred,
                                       inp->inp_socket);
                           if (error == 0)
                                   inp_list[i++] = inp;
                   }
                   INP_UNLOCK(inp);
           }
           INP_INFO_RUNLOCK(&tcbinfo);
           splx(s);
           n = i;
   
           error = 0;
           for (i = 0; i < n; i++) {
                   inp = inp_list[i];
                   if (inp->inp_gencnt <= gencnt) {
                           struct xtcpcb xt;
                           caddr_t inp_ppcb;
   
                           bzero(&xt, sizeof(xt));
                           xt.xt_len = sizeof xt;
                           /* XXX should avoid extra copy */
                           bcopy(inp, &xt.xt_inp, sizeof *inp);
                           inp_ppcb = inp->inp_ppcb;
                           if (inp_ppcb == NULL)
                                   bzero((char *) &xt.xt_tp, sizeof xt.xt_tp);
                           else if (inp->inp_vflag & INP_TIMEWAIT) {
                                   bzero((char *) &xt.xt_tp, sizeof xt.xt_tp);
                                   xt.xt_tp.t_state = TCPS_TIME_WAIT;
                           } else
                                   bcopy(inp_ppcb, &xt.xt_tp, sizeof xt.xt_tp);
                           if (inp->inp_socket != NULL)
                                   sotoxsocket(inp->inp_socket, &xt.xt_socket);
                           else {
                                   bzero(&xt.xt_socket, sizeof xt.xt_socket);
                                   xt.xt_socket.xso_protocol = IPPROTO_TCP;
                           }
                           xt.xt_inp.inp_gencnt = inp->inp_gencnt;
                           error = SYSCTL_OUT(req, &xt, sizeof xt);
                   }
           }
           if (!error) {
                   /*
                    * Give the user an updated idea of our state.
                    * If the generation differs from what we told
                    * her before, she knows that something happened
                    * while we were processing this request, and it
                    * might be necessary to retry.
                    */
                   s = splnet();
                   INP_INFO_RLOCK(&tcbinfo);
                   xig.xig_gen = tcbinfo.ipi_gencnt;
                   xig.xig_sogen = so_gencnt;
                   xig.xig_count = tcbinfo.ipi_count;
                   INP_INFO_RUNLOCK(&tcbinfo);
                   splx(s);
                   error = SYSCTL_OUT(req, &xig, sizeof xig);
           }
           free(inp_list, M_TEMP);
           return error;
   }
   
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_PCBLIST, pcblist, CTLFLAG_RD, 0, 0,
               tcp_pcblist, "S,xtcpcb", "List of active TCP connections");
   
   static int
   tcp_getcred(SYSCTL_HANDLER_ARGS)
   {
          struct xucred xuc;
          struct sockaddr_in addrs[2];
          struct inpcb *inp;
          int error, s;
  
          error = suser_cred(req->td->td_ucred, SUSER_ALLOWJAIL);
          if (error)
                  return (error);
          error = SYSCTL_IN(req, addrs, sizeof(addrs));
          if (error)
                  return (error);
          s = splnet();
          INP_INFO_RLOCK(&tcbinfo);
          inp = in_pcblookup_hash(&tcbinfo, addrs[1].sin_addr, addrs[1].sin_port,
              addrs[0].sin_addr, addrs[0].sin_port, 0, NULL);
          if (inp == NULL) {
                  error = ENOENT;
                  goto outunlocked;
          }
          INP_LOCK(inp);
          if (inp->inp_socket == NULL) {
                  error = ENOENT;
                  goto out;
          }
          error = cr_canseesocket(req->td->td_ucred, inp->inp_socket);
          if (error)
                  goto out;
          cru2x(inp->inp_socket->so_cred, &xuc);
  out:
          INP_UNLOCK(inp);
  outunlocked:
          INP_INFO_RUNLOCK(&tcbinfo);
          splx(s);
          if (error == 0)
                  error = SYSCTL_OUT(req, &xuc, sizeof(struct xucred));
          return (error);
  }
  
  SYSCTL_PROC(_net_inet_tcp, OID_AUTO, getcred,
      CTLTYPE_OPAQUE|CTLFLAG_RW|CTLFLAG_PRISON, 0, 0,
      tcp_getcred, "S,xucred", "Get the xucred of a TCP connection");
  
  #ifdef INET6
  static int
  tcp6_getcred(SYSCTL_HANDLER_ARGS)
  {
          struct xucred xuc;
          struct sockaddr_in6 addrs[2];
          struct inpcb *inp;
          int error, s, mapped = 0;
  
          error = suser_cred(req->td->td_ucred, SUSER_ALLOWJAIL);
          if (error)
                  return (error);
          error = SYSCTL_IN(req, addrs, sizeof(addrs));
          if (error)
                  return (error);
          if (IN6_IS_ADDR_V4MAPPED(&addrs[0].sin6_addr)) {
                  if (IN6_IS_ADDR_V4MAPPED(&addrs[1].sin6_addr))
                          mapped = 1;
                  else
                          return (EINVAL);
          }
          s = splnet();
          INP_INFO_RLOCK(&tcbinfo);
          if (mapped == 1)
                  inp = in_pcblookup_hash(&tcbinfo,
                          *(struct in_addr *)&addrs[1].sin6_addr.s6_addr[12],
                          addrs[1].sin6_port,
                          *(struct in_addr *)&addrs[0].sin6_addr.s6_addr[12],
                          addrs[0].sin6_port,
                          0, NULL);
          else
                  inp = in6_pcblookup_hash(&tcbinfo, &addrs[1].sin6_addr,
                                   addrs[1].sin6_port,
                                   &addrs[0].sin6_addr, addrs[0].sin6_port,
                                   0, NULL);
          if (inp == NULL) {
                  error = ENOENT;
                  goto outunlocked;
          }
          INP_LOCK(inp);
          if (inp->inp_socket == NULL) {
                  error = ENOENT;
                  goto out;
          }
          error = cr_canseesocket(req->td->td_ucred, inp->inp_socket);
          if (error)
                  goto out;
          cru2x(inp->inp_socket->so_cred, &xuc);
  out:
          INP_UNLOCK(inp);
  outunlocked:
          INP_INFO_RUNLOCK(&tcbinfo);
          splx(s);
          if (error == 0)
                  error = SYSCTL_OUT(req, &xuc, sizeof(struct xucred));
          return (error);
  }
  
  SYSCTL_PROC(_net_inet6_tcp6, OID_AUTO, getcred,
      CTLTYPE_OPAQUE|CTLFLAG_RW|CTLFLAG_PRISON, 0, 0,
      tcp6_getcred, "S,xucred", "Get the xucred of a TCP6 connection");
  #endif
  
  
  void
  tcp_ctlinput(cmd, sa, vip)
          int cmd;
          struct sockaddr *sa;
          void *vip;
  {
          struct ip *ip = vip;
          struct tcphdr *th;
          struct in_addr faddr;
          struct inpcb *inp;
          struct tcpcb *tp;
          struct inpcb *(*notify)(struct inpcb *, int) = tcp_notify;
          tcp_seq icmp_seq;
          int s;
  
          faddr = ((struct sockaddr_in *)sa)->sin_addr;
          if (sa->sa_family != AF_INET || faddr.s_addr == INADDR_ANY)
                  return;
  
          if (cmd == PRC_QUENCH)
                  notify = tcp_quench;
          else if (icmp_may_rst && (cmd == PRC_UNREACH_ADMIN_PROHIB ||
                  cmd == PRC_UNREACH_PORT || cmd == PRC_TIMXCEED_INTRANS) && ip)
                  notify = tcp_drop_syn_sent;
          else if (cmd == PRC_MSGSIZE)
                  notify = tcp_mtudisc;
          /*
           * Redirects don't need to be handled up here.
           */
          else if (PRC_IS_REDIRECT(cmd))
                  return;
          /*
           * Hostdead is ugly because it goes linearly through all PCBs.
           * XXX: We never get this from ICMP, otherwise it makes an
           * excellent DoS attack on machines with many connections.
           */
          else if (cmd == PRC_HOSTDEAD)
                  ip = NULL;
          else if ((unsigned)cmd >= PRC_NCMDS || inetctlerrmap[cmd] == 0)
                  return;
          if (ip != NULL) {
                  s = splnet();
                  th = (struct tcphdr *)((caddr_t)ip
                                         + (ip->ip_hl << 2));
                  INP_INFO_WLOCK(&tcbinfo);
                  inp = in_pcblookup_hash(&tcbinfo, faddr, th->th_dport,
                      ip->ip_src, th->th_sport, 0, NULL);
                  if (inp != NULL)  {
                          INP_LOCK(inp);
                          if (inp->inp_socket != NULL) {
                                  icmp_seq = htonl(th->th_seq);
                                  tp = intotcpcb(inp);
                                  if (SEQ_GEQ(icmp_seq, tp->snd_una) &&
                                          SEQ_LT(icmp_seq, tp->snd_max))
                                          inp = (*notify)(inp, inetctlerrmap[cmd]);
                          }
                          if (inp != NULL)
                                  INP_UNLOCK(inp);
                  } else {
                          struct in_conninfo inc;
  
                          inc.inc_fport = th->th_dport;
                          inc.inc_lport = th->th_sport;
                          inc.inc_faddr = faddr;
                          inc.inc_laddr = ip->ip_src;
  #ifdef INET6
                          inc.inc_isipv6 = 0;
  #endif
                          syncache_unreach(&inc, th);
                  }
                  INP_INFO_WUNLOCK(&tcbinfo);
                  splx(s);
          } else
                  in_pcbnotifyall(&tcbinfo, faddr, inetctlerrmap[cmd], notify);
  }
  
  #ifdef INET6
  void
  tcp6_ctlinput(cmd, sa, d)
          int cmd;
          struct sockaddr *sa;
          void *d;
  {
          struct tcphdr th;
          struct inpcb *(*notify)(struct inpcb *, int) = tcp_notify;
          struct ip6_hdr *ip6;
          struct mbuf *m;
          struct ip6ctlparam *ip6cp = NULL;
          const struct sockaddr_in6 *sa6_src = NULL;
          int off;
          struct tcp_portonly {
                  u_int16_t th_sport;
                  u_int16_t th_dport;
          } *thp;
  
          if (sa->sa_family != AF_INET6 ||
              sa->sa_len != sizeof(struct sockaddr_in6))
                  return;
  
          if (cmd == PRC_QUENCH)
                  notify = tcp_quench;
          else if (cmd == PRC_MSGSIZE)
                  notify = tcp_mtudisc;
          else if (!PRC_IS_REDIRECT(cmd) &&
                   ((unsigned)cmd >= PRC_NCMDS || inet6ctlerrmap[cmd] == 0))
                  return;
  
          /* if the parameter is from icmp6, decode it. */
          if (d != NULL) {
                  ip6cp = (struct ip6ctlparam *)d;
                  m = ip6cp->ip6c_m;
                  ip6 = ip6cp->ip6c_ip6;
                  off = ip6cp->ip6c_off;
                  sa6_src = ip6cp->ip6c_src;
          } else {
                  m = NULL;
                  ip6 = NULL;
                  off = 0;        /* fool gcc */
                  sa6_src = &sa6_any;
          }
  
          if (ip6 != NULL) {
                  struct in_conninfo inc;
                  /*
                   * XXX: We assume that when IPV6 is non NULL,
                   * M and OFF are valid.
                   */
  
                  /* check if we can safely examine src and dst ports */
                  if (m->m_pkthdr.len < off + sizeof(*thp))
                          return;
  
                  bzero(&th, sizeof(th));
                  m_copydata(m, off, sizeof(*thp), (caddr_t)&th);
  
                  in6_pcbnotify(&tcbinfo, sa, th.th_dport,
                      (struct sockaddr *)ip6cp->ip6c_src,
                      th.th_sport, cmd, NULL, notify);
  
                  inc.inc_fport = th.th_dport;
                  inc.inc_lport = th.th_sport;
                  inc.inc6_faddr = ((struct sockaddr_in6 *)sa)->sin6_addr;
                  inc.inc6_laddr = ip6cp->ip6c_src->sin6_addr;
                  inc.inc_isipv6 = 1;
                  INP_INFO_WLOCK(&tcbinfo);
                  syncache_unreach(&inc, &th);
                  INP_INFO_WUNLOCK(&tcbinfo);
          } else
                  in6_pcbnotify(&tcbinfo, sa, 0, (const struct sockaddr *)sa6_src,
                                0, cmd, NULL, notify);
  }
  #endif /* INET6 */
  
  
  /*
   * Following is where TCP initial sequence number generation occurs.
   *
   * There are two places where we must use initial sequence numbers:
   * 1.  In SYN-ACK packets.
   * 2.  In SYN packets.
   *
   * All ISNs for SYN-ACK packets are generated by the syncache.  See
   * tcp_syncache.c for details.
   *
   * The ISNs in SYN packets must be monotonic; TIME_WAIT recycling
   * depends on this property.  In addition, these ISNs should be
   * unguessable so as to prevent connection hijacking.  To satisfy
   * the requirements of this situation, the algorithm outlined in
   * RFC 1948 is used, with only small modifications.
   *
   * Implementation details:
   *
   * Time is based off the system timer, and is corrected so that it
   * increases by one megabyte per second.  This allows for proper
   * recycling on high speed LANs while still leaving over an hour
   * before rollover.
   *
   * As reading the *exact* system time is too expensive to be done
   * whenever setting up a TCP connection, we increment the time
   * offset in two ways.  First, a small random positive increment
   * is added to isn_offset for each connection that is set up.
   * Second, the function tcp_isn_tick fires once per clock tick
   * and increments isn_offset as necessary so that sequence numbers
   * are incremented at approximately ISN_BYTES_PER_SECOND.  The
   * random positive increments serve only to ensure that the same
   * exact sequence number is never sent out twice (as could otherwise
   * happen when a port is recycled in less than the system tick
   * interval.)
   *
   * net.inet.tcp.isn_reseed_interval controls the number of seconds
   * between seeding of isn_secret.  This is normally set to zero,
   * as reseeding should not be necessary.
   *
   */
  
  #define ISN_BYTES_PER_SECOND 1048576
  #define ISN_STATIC_INCREMENT 4096
  #define ISN_RANDOM_INCREMENT (4096 - 1)
  
  u_char isn_secret[32];
  int isn_last_reseed;
  u_int32_t isn_offset, isn_offset_old;
  MD5_CTX isn_ctx;
  
  tcp_seq
  tcp_new_isn(tp)
          struct tcpcb *tp;
  {
          u_int32_t md5_buffer[4];
          tcp_seq new_isn;
  
          /* Seed if this is the first use, reseed if requested. */
          if ((isn_last_reseed == 0) || ((tcp_isn_reseed_interval > 0) &&
               (((u_int)isn_last_reseed + (u_int)tcp_isn_reseed_interval*hz)
                  < (u_int)ticks))) {
                  read_random(&isn_secret, sizeof(isn_secret));
                  isn_last_reseed = ticks;
          }
  
          /* Compute the md5 hash and return the ISN. */
          MD5Init(&isn_ctx);
          MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_fport, sizeof(u_short));
          MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_lport, sizeof(u_short));
  #ifdef INET6
          if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) {
                  MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->in6p_faddr,
                            sizeof(struct in6_addr));
                  MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->in6p_laddr,
                            sizeof(struct in6_addr));
          } else
  #endif
          {
                  MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_faddr,
                            sizeof(struct in_addr));
                  MD5Update(&isn_ctx, (u_char *) &tp->t_inpcb->inp_laddr,
                            sizeof(struct in_addr));
          }
          MD5Update(&isn_ctx, (u_char *) &isn_secret, sizeof(isn_secret));
          MD5Final((u_char *) &md5_buffer, &isn_ctx);
          new_isn = (tcp_seq) md5_buffer[0];
          isn_offset += ISN_STATIC_INCREMENT +
                  (arc4random() & ISN_RANDOM_INCREMENT);
          new_isn += isn_offset;
          return new_isn;
  }
  
  /*
   * Increment the offset to the next ISN_BYTES_PER_SECOND / hz boundary
   * to keep time flowing at a relatively constant rate.  If the random
   * increments have already pushed us past the projected offset, do nothing.
   */
  static void
  tcp_isn_tick(xtp)
          void *xtp;
  {
          u_int32_t projected_offset;
  
          projected_offset = isn_offset_old + ISN_BYTES_PER_SECOND / hz;
  
          if (projected_offset > isn_offset)
                  isn_offset = projected_offset;
  
          isn_offset_old = isn_offset;
          callout_reset(&isn_callout, 1, tcp_isn_tick, NULL);
  }
  
  /*
   * When a source quench is received, close congestion window
   * to one segment.  We will gradually open it again as we proceed.
   */
  struct inpcb *
  tcp_quench(inp, errno)
          struct inpcb *inp;
          int errno;
  {
          struct tcpcb *tp = intotcpcb(inp);
  
          if (tp != NULL)
                  tp->snd_cwnd = tp->t_maxseg;
          return (inp);
  }
  
  /*
   * When a specific ICMP unreachable message is received and the
   * connection state is SYN-SENT, drop the connection.  This behavior
   * is controlled by the icmp_may_rst sysctl.
   */
  struct inpcb *
  tcp_drop_syn_sent(inp, errno)
          struct inpcb *inp;
          int errno;
  {
          struct tcpcb *tp = intotcpcb(inp);
  
          if (tp != NULL && tp->t_state == TCPS_SYN_SENT) {
                  tcp_drop(tp, errno);
                  return (struct inpcb *)0;
          }
          return inp;
  }
  
  /*
   * When `need fragmentation' ICMP is received, update our idea of the MSS
   * based on the new value in the route.  Also nudge TCP to send something,
   * since we know the packet we just sent was dropped.
   * This duplicates some code in the tcp_mss() function in tcp_input.c.
   */
  struct inpcb *
  tcp_mtudisc(inp, errno)
          struct inpcb *inp;
          int errno;
  {
          struct tcpcb *tp = intotcpcb(inp);
          struct rmxp_tao tao;
          struct socket *so = inp->inp_socket;
          u_int maxmtu;
          u_int romtu;
          int mss;
  #ifdef INET6
          int isipv6;
  #endif /* INET6 */
          bzero(&tao, sizeof(tao));
  
          if (tp != NULL) {
  #ifdef INET6
                  isipv6 = (tp->t_inpcb->inp_vflag & INP_IPV6) != 0;
  #endif
                  maxmtu = tcp_hc_getmtu(&inp->inp_inc); /* IPv4 and IPv6 */
                  romtu =
  #ifdef INET6
                      isipv6 ? tcp_maxmtu6(&inp->inp_inc) :
  #endif /* INET6 */
                      tcp_maxmtu(&inp->inp_inc);
                  if (!maxmtu)
                          maxmtu = romtu;
                  else
                          maxmtu = min(maxmtu, romtu);
                  if (!maxmtu) {
                          tp->t_maxopd = tp->t_maxseg =
  #ifdef INET6
                                  isipv6 ? tcp_v6mssdflt :
  #endif /* INET6 */
                                  tcp_mssdflt;
                          return inp;
                  }
                  mss = maxmtu -
  #ifdef INET6
                          (isipv6 ?
                           sizeof(struct ip6_hdr) + sizeof(struct tcphdr) :
  #endif /* INET6 */
                           sizeof(struct tcpiphdr)
  #ifdef INET6
                           )
  #endif /* INET6 */
                          ;
  
                  if (tcp_do_rfc1644) {
                          tcp_hc_gettao(&inp->inp_inc, &tao);
                          if (tao.tao_mssopt)
                                  mss = min(mss, tao.tao_mssopt);
                  }
                  /*
                   * XXX - The above conditional probably violates the TCP
                   * spec.  The problem is that, since we don't know the
                   * other end's MSS, we are supposed to use a conservative
                   * default.  But, if we do that, then MTU discovery will
                   * never actually take place, because the conservative
                   * default is much less than the MTUs typically seen
                   * on the Internet today.  For the moment, we'll sweep
                   * this under the carpet.
                   *
                   * The conservative default might not actually be a problem
                   * if the only case this occurs is when sending an initial
                   * SYN with options and data to a host we've never talked
                   * to before.  Then, they will reply with an MSS value which
                   * will get recorded and the new parameters should get
                   * recomputed.  For Further Study.
                   */
                  if (tp->t_maxopd <= mss)
                          return inp;
                  tp->t_maxopd = mss;
  
                  if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
                      (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP)
                          mss -= TCPOLEN_TSTAMP_APPA;
                  if ((tp->t_flags & (TF_REQ_CC|TF_NOOPT)) == TF_REQ_CC &&
                      (tp->t_flags & TF_RCVD_CC) == TF_RCVD_CC)
                          mss -= TCPOLEN_CC_APPA;
  #if     (MCLBYTES & (MCLBYTES - 1)) == 0
                  if (mss > MCLBYTES)
                          mss &= ~(MCLBYTES-1);
  #else
                  if (mss > MCLBYTES)
                          mss = mss / MCLBYTES * MCLBYTES;
  #endif
                  if (so->so_snd.sb_hiwat < mss)
                          mss = so->so_snd.sb_hiwat;
  
                  tp->t_maxseg = mss;
  
                  tcpstat.tcps_mturesent++;
                  tp->t_rtttime = 0;
                  tp->snd_nxt = tp->snd_una;
                  tcp_output(tp);
          }
          return inp;
  }
  
  /*
   * Look-up the routing entry to the peer of this inpcb.  If no route
   * is found and it cannot be allocated, then return NULL.  This routine
   * is called by TCP routines that access the rmx structure and by tcp_mss
   * to get the interface MTU.
   */
  u_long
  tcp_maxmtu(inc)
          struct in_conninfo *inc;
  {
          struct route sro;
          struct sockaddr_in *dst;
          struct ifnet *ifp;
          u_long maxmtu = 0;
  
          KASSERT(inc != NULL, ("tcp_maxmtu with NULL in_conninfo pointer"));
  
          bzero(&sro, sizeof(sro));
          if (inc->inc_faddr.s_addr != INADDR_ANY) {
                  dst = (struct sockaddr_in *)&sro.ro_dst;
                  dst->sin_family = AF_INET;
                  dst->sin_len = sizeof(*dst);
                  dst->sin_addr = inc->inc_faddr;
                  rtalloc_ign(&sro, RTF_CLONING);
          }
          if (sro.ro_rt != NULL) {
                  ifp = sro.ro_rt->rt_ifp;
                  if (sro.ro_rt->rt_rmx.rmx_mtu == 0)
                          maxmtu = ifp->if_mtu;
                  else
                          maxmtu = min(sro.ro_rt->rt_rmx.rmx_mtu, ifp->if_mtu);
                  RTFREE(sro.ro_rt);
          }
          return (maxmtu);
  }
  
  #ifdef INET6
  u_long
  tcp_maxmtu6(inc)
          struct in_conninfo *inc;
  {
          struct route_in6 sro6;
          struct ifnet *ifp;
          u_long maxmtu = 0;
  
          KASSERT(inc != NULL, ("tcp_maxmtu6 with NULL in_conninfo pointer"));
  
          bzero(&sro6, sizeof(sro6));
          if (!IN6_IS_ADDR_UNSPECIFIED(&inc->inc6_faddr)) {
                  sro6.ro_dst.sin6_family = AF_INET6;
                  sro6.ro_dst.sin6_len = sizeof(struct sockaddr_in6);
                  sro6.ro_dst.sin6_addr = inc->inc6_faddr;
                  rtalloc_ign((struct route *)&sro6, RTF_CLONING);
          }
          if (sro6.ro_rt != NULL) {
                  ifp = sro6.ro_rt->rt_ifp;
                  if (sro6.ro_rt->rt_rmx.rmx_mtu == 0)
                          maxmtu = IN6_LINKMTU(sro6.ro_rt->rt_ifp);
                  else
                          maxmtu = min(sro6.ro_rt->rt_rmx.rmx_mtu,
                                       IN6_LINKMTU(sro6.ro_rt->rt_ifp));
                  RTFREE(sro6.ro_rt);
          }
  
          return (maxmtu);
  }
  #endif /* INET6 */
  
  #ifdef IPSEC
  /* compute ESP/AH header size for TCP, including outer IP header. */
  size_t
  ipsec_hdrsiz_tcp(tp)
          struct tcpcb *tp;
  {
          struct inpcb *inp;
          struct mbuf *m;
          size_t hdrsiz;
          struct ip *ip;
  #ifdef INET6
          struct ip6_hdr *ip6;
  #endif
          struct tcphdr *th;
  
          if ((tp == NULL) || ((inp = tp->t_inpcb) == NULL))
                  return 0;
          MGETHDR(m, M_DONTWAIT, MT_DATA);
          if (!m)
                  return 0;
  
  #ifdef INET6
          if ((inp->inp_vflag & INP_IPV6) != 0) {
                  ip6 = mtod(m, struct ip6_hdr *);
                  th = (struct tcphdr *)(ip6 + 1);
                  m->m_pkthdr.len = m->m_len =
                          sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
                  tcpip_fillheaders(inp, ip6, th);
                  hdrsiz = ipsec6_hdrsiz(m, IPSEC_DIR_OUTBOUND, inp);
          } else
  #endif /* INET6 */
          {
                  ip = mtod(m, struct ip *);
                  th = (struct tcphdr *)(ip + 1);
                  m->m_pkthdr.len = m->m_len = sizeof(struct tcpiphdr);
                  tcpip_fillheaders(inp, ip, th);
                  hdrsiz = ipsec4_hdrsiz(m, IPSEC_DIR_OUTBOUND, inp);
          }
  
          m_free(m);
          return hdrsiz;
  }
  #endif /*IPSEC*/
  
  /*
   * Move a TCP connection into TIME_WAIT state.
   *    tcbinfo is unlocked.
   *    inp is locked, and is unlocked before returning.
   */
  void
  tcp_twstart(tp)
          struct tcpcb *tp;
  {
          struct tcptw *tw;
          struct inpcb *inp;
          int tw_time, acknow;
          struct socket *so;
  
          tw = uma_zalloc(tcptw_zone, M_NOWAIT);
          if (tw == NULL) {
                  tw = tcp_timer_2msl_tw(1);
                  if (tw == NULL) {
                          tcp_close(tp);
                          return;
                  }
          }
          inp = tp->t_inpcb;
          tw->tw_inpcb = inp;
  
          /*
           * Recover last window size sent.
           */
          tw->last_win = (tp->rcv_adv - tp->rcv_nxt) >> tp->rcv_scale;
  
          /*
           * Set t_recent if timestamps are used on the connection.
           */
          if ((tp->t_flags & (TF_REQ_TSTMP|TF_RCVD_TSTMP|TF_NOOPT)) ==
              (TF_REQ_TSTMP|TF_RCVD_TSTMP))
                  tw->t_recent = tp->ts_recent;
          else
                  tw->t_recent = 0;
  
          tw->snd_nxt = tp->snd_nxt;
          tw->rcv_nxt = tp->rcv_nxt;
          tw->iss     = tp->iss;
          tw->irs     = tp->irs;
          tw->cc_recv = tp->cc_recv;
          tw->cc_send = tp->cc_send;
          tw->t_starttime = tp->t_starttime;
          tw->tw_time = 0;
  
  /* XXX
   * If this code will
   * be used for fin-wait-2 state also, then we may need
   * a ts_recent from the last segment.
   */
          /* Shorten TIME_WAIT [RFC-1644, p.28] */
          if (tp->cc_recv != 0 && (ticks - tp->t_starttime) < tcp_msl) {
                  tw_time = tp->t_rxtcur * TCPTV_TWTRUNC;
                  /* For T/TCP client, force ACK now. */
                  acknow = 1;
          } else {
                  tw_time = 2 * tcp_msl;
                  acknow = tp->t_flags & TF_ACKNOW;
          }
          tcp_discardcb(tp);
          so = inp->inp_socket;
          ACCEPT_LOCK();
          SOCK_LOCK(so);
          so->so_pcb = NULL;
          tw->tw_cred = crhold(so->so_cred);
          tw->tw_so_options = so->so_options;
          sotryfree(so);
          inp->inp_socket = NULL;
          if (acknow)
                  tcp_twrespond(tw, TH_ACK);
          inp->inp_ppcb = (caddr_t)tw;
          inp->inp_vflag |= INP_TIMEWAIT;
          tcp_timer_2msl_reset(tw, tw_time);
          INP_UNLOCK(inp);
  }
  
  /*
   * The appromixate rate of ISN increase of Microsoft TCP stacks;
   * the actual rate is slightly higher due to the addition of
   * random positive increments.
   *
   * Most other new OSes use semi-randomized ISN values, so we
   * do not need to worry about them.
   */
  #define MS_ISN_BYTES_PER_SECOND         250000
  
  /*
   * Determine if the ISN we will generate has advanced beyond the last
   * sequence number used by the previous connection.  If so, indicate
   * that it is safe to recycle this tw socket by returning 1.
   */
  int
  tcp_twrecycleable(struct tcptw *tw)
  {
          tcp_seq new_iss = tw->iss;
          tcp_seq new_irs = tw->irs;
  
          new_iss += (ticks - tw->t_starttime) * (ISN_BYTES_PER_SECOND / hz);
          new_irs += (ticks - tw->t_starttime) * (MS_ISN_BYTES_PER_SECOND / hz);
  
          if (SEQ_GT(new_iss, tw->snd_nxt) && SEQ_GT(new_irs, tw->rcv_nxt))
                  return 1;
          else
                  return 0;
  }
  
  struct tcptw *
  tcp_twclose(struct tcptw *tw, int reuse)
  {
          struct inpcb *inp;
  
          inp = tw->tw_inpcb;
          tw->tw_inpcb = NULL;
          tcp_timer_2msl_stop(tw);
          inp->inp_ppcb = NULL;
  #ifdef INET6
          if (inp->inp_vflag & INP_IPV6PROTO)
                  in6_pcbdetach(inp);
          else
  #endif
                  in_pcbdetach(inp);
          tcpstat.tcps_closed++;
          crfree(tw->tw_cred);
          tw->tw_cred = NULL;
          if (reuse)
                  return (tw);
          uma_zfree(tcptw_zone, tw);
          return (NULL);
  }
  
  int
  tcp_twrespond(struct tcptw *tw, int flags)
  {
          struct inpcb *inp = tw->tw_inpcb;
          struct tcphdr *th;
          struct mbuf *m;
          struct ip *ip = NULL;
          u_int8_t *optp;
          u_int hdrlen, optlen;
          int error;
  #ifdef INET6
          struct ip6_hdr *ip6 = NULL;
          int isipv6 = inp->inp_inc.inc_isipv6;
  #endif
  
          m = m_gethdr(M_DONTWAIT, MT_HEADER);
          if (m == NULL)
                  return (ENOBUFS);
          m->m_data += max_linkhdr;
  
  #ifdef MAC
          mac_create_mbuf_from_inpcb(inp, m);
  #endif
  
  #ifdef INET6
          if (isipv6) {
                  hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
                  ip6 = mtod(m, struct ip6_hdr *);
                  th = (struct tcphdr *)(ip6 + 1);
                  tcpip_fillheaders(inp, ip6, th);
          } else
  #endif
          {
                  hdrlen = sizeof(struct tcpiphdr);
                  ip = mtod(m, struct ip *);
                  th = (struct tcphdr *)(ip + 1);
                  tcpip_fillheaders(inp, ip, th);
          }
          optp = (u_int8_t *)(th + 1);
  
          /*
           * Send a timestamp and echo-reply if both our side and our peer
           * have sent timestamps in our SYN's and this is not a RST.
           */
          if (tw->t_recent && flags == TH_ACK) {
                  u_int32_t *lp = (u_int32_t *)optp;
  
                  /* Form timestamp option as shown in appendix A of RFC 1323. */
                  *lp++ = htonl(TCPOPT_TSTAMP_HDR);
                  *lp++ = htonl(ticks);
                  *lp   = htonl(tw->t_recent);
                  optp += TCPOLEN_TSTAMP_APPA;
          }
  
          /*
           * Send `CC-family' options if needed, and it's not a RST.
           */
          if (tw->cc_recv != 0 && flags == TH_ACK) {
                  u_int32_t *lp = (u_int32_t *)optp;
  
                  *lp++ = htonl(TCPOPT_CC_HDR(TCPOPT_CC));
                  *lp   = htonl(tw->cc_send);
                  optp += TCPOLEN_CC_APPA;
          }
          optlen = optp - (u_int8_t *)(th + 1);
  
          m->m_len = hdrlen + optlen;
          m->m_pkthdr.len = m->m_len;
  
          KASSERT(max_linkhdr + m->m_len <= MHLEN, ("tcptw: mbuf too small"));
  
          th->th_seq = htonl(tw->snd_nxt);
          th->th_ack = htonl(tw->rcv_nxt);
          th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
          th->th_flags = flags;
          th->th_win = htons(tw->last_win);
  
  #ifdef INET6
          if (isipv6) {
                  th->th_sum = in6_cksum(m, IPPROTO_TCP, sizeof(struct ip6_hdr),
                      sizeof(struct tcphdr) + optlen);
                  ip6->ip6_hlim = in6_selecthlim(inp, NULL);
                  error = ip6_output(m, inp->in6p_outputopts, NULL,
                      (tw->tw_so_options & SO_DONTROUTE), NULL, NULL, inp);
          } else
  #endif
          {
                  th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
                      htons(sizeof(struct tcphdr) + optlen + IPPROTO_TCP));
                  m->m_pkthdr.csum_flags = CSUM_TCP;
                  m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                  ip->ip_len = m->m_pkthdr.len;
                  if (path_mtu_discovery)
                          ip->ip_off |= IP_DF;
                  error = ip_output(m, inp->inp_options, NULL,
                      (tw->tw_so_options & SO_DONTROUTE), NULL, inp);
          }
          if (flags & TH_ACK)
                  tcpstat.tcps_sndacks++;
          else
                  tcpstat.tcps_sndctrl++;
          tcpstat.tcps_sndtotal++;
          return (error);
  }
  
  /*
   * TCP BANDWIDTH DELAY PRODUCT WINDOW LIMITING
   *
   * This code attempts to calculate the bandwidth-delay product as a
   * means of determining the optimal window size to maximize bandwidth,
   * minimize RTT, and avoid the over-allocation of buffers on interfaces and
   * routers.  This code also does a fairly good job keeping RTTs in check
   * across slow links like modems.  We implement an algorithm which is very
   * similar (but not meant to be) TCP/Vegas.  The code operates on the
   * transmitter side of a TCP connection and so only effects the transmit
   * side of the connection.
   *
   * BACKGROUND:  TCP makes no provision for the management of buffer space
   * at the end points or at the intermediate routers and switches.  A TCP
   * stream, whether using NewReno or not, will eventually buffer as
   * many packets as it is able and the only reason this typically works is
   * due to the fairly small default buffers made available for a connection
   * (typicaly 16K or 32K).  As machines use larger windows and/or window
   * scaling it is now fairly easy for even a single TCP connection to blow-out
   * all available buffer space not only on the local interface, but on
   * intermediate routers and switches as well.  NewReno makes a misguided
   * attempt to 'solve' this problem by waiting for an actual failure to occur,
   * then backing off, then steadily increasing the window again until another
   * failure occurs, ad-infinitum.  This results in terrible oscillation that
   * is only made worse as network loads increase and the idea of intentionally
   * blowing out network buffers is, frankly, a terrible way to manage network
   * resources.
   *
   * It is far better to limit the transmit window prior to the failure
   * condition being achieved.  There are two general ways to do this:  First
   * you can 'scan' through different transmit window sizes and locate the
   * point where the RTT stops increasing, indicating that you have filled the
   * pipe, then scan backwards until you note that RTT stops decreasing, then
   * repeat ad-infinitum.  This method works in principle but has severe
   * implementation issues due to RTT variances, timer granularity, and
   * instability in the algorithm which can lead to many false positives and
   * create oscillations as well as interact badly with other TCP streams
   * implementing the same algorithm.
   *
   * The second method is to limit the window to the bandwidth delay product
   * of the link.  This is the method we implement.  RTT variances and our
   * own manipulation of the congestion window, bwnd, can potentially
   * destabilize the algorithm.  For this reason we have to stabilize the
   * elements used to calculate the window.  We do this by using the minimum
   * observed RTT, the long term average of the observed bandwidth, and
   * by adding two segments worth of slop.  It isn't perfect but it is able
   * to react to changing conditions and gives us a very stable basis on
   * which to extend the algorithm.
   */
  void
  tcp_xmit_bandwidth_limit(struct tcpcb *tp, tcp_seq ack_seq)
  {
          u_long bw;
          u_long bwnd;
          int save_ticks;
  
          /*
           * If inflight_enable is disabled in the middle of a tcp connection,
           * make sure snd_bwnd is effectively disabled.
           */
          if (tcp_inflight_enable == 0) {
                  tp->snd_bwnd = TCP_MAXWIN << TCP_MAX_WINSHIFT;
                  tp->snd_bandwidth = 0;
                  return;
          }
  
          /*
           * Figure out the bandwidth.  Due to the tick granularity this
           * is a very rough number and it MUST be averaged over a fairly
           * long period of time.  XXX we need to take into account a link
           * that is not using all available bandwidth, but for now our
           * slop will ramp us up if this case occurs and the bandwidth later
           * increases.
           *
           * Note: if ticks rollover 'bw' may wind up negative.  We must
           * effectively reset t_bw_rtttime for this case.
           */
          save_ticks = ticks;
          if ((u_int)(save_ticks - tp->t_bw_rtttime) < 1)
                  return;
  
          bw = (int64_t)(ack_seq - tp->t_bw_rtseq) * hz /
              (save_ticks - tp->t_bw_rtttime);
          tp->t_bw_rtttime = save_ticks;
          tp->t_bw_rtseq = ack_seq;
          if (tp->t_bw_rtttime == 0 || (int)bw < 0)
                  return;
          bw = ((int64_t)tp->snd_bandwidth * 15 + bw) >> 4;
  
          tp->snd_bandwidth = bw;
  
          /*
           * Calculate the semi-static bandwidth delay product, plus two maximal
           * segments.  The additional slop puts us squarely in the sweet
           * spot and also handles the bandwidth run-up case and stabilization.
           * Without the slop we could be locking ourselves into a lower
           * bandwidth.
           *
           * Situations Handled:
           *      (1) Prevents over-queueing of packets on LANs, especially on
           *          high speed LANs, allowing larger TCP buffers to be
           *          specified, and also does a good job preventing
           *          over-queueing of packets over choke points like modems
           *          (at least for the transmit side).
           *
           *      (2) Is able to handle changing network loads (bandwidth
           *          drops so bwnd drops, bandwidth increases so bwnd
           *          increases).
           *
           *      (3) Theoretically should stabilize in the face of multiple
           *          connections implementing the same algorithm (this may need
           *          a little work).
           *
           *      (4) Stability value (defaults to 20 = 2 maximal packets) can
           *          be adjusted with a sysctl but typically only needs to be
           *          on very slow connections.  A value no smaller then 5
           *          should be used, but only reduce this default if you have
           *          no other choice.
           */
  #define USERTT  ((tp->t_srtt + tp->t_rttbest) / 2)
          bwnd = (int64_t)bw * USERTT / (hz << TCP_RTT_SHIFT) + tcp_inflight_stab * tp->t_maxseg / 10;
  #undef USERTT
  
          if (tcp_inflight_debug > 0) {
                  static int ltime;
                  if ((u_int)(ticks - ltime) >= hz / tcp_inflight_debug) {
                          ltime = ticks;
                          printf("%p bw %ld rttbest %d srtt %d bwnd %ld\n",
                              tp,
                              bw,
                              tp->t_rttbest,
                              tp->t_srtt,
                              bwnd
                          );
                  }
          }
          if ((long)bwnd < tcp_inflight_min)
                  bwnd = tcp_inflight_min;
          if (bwnd > tcp_inflight_max)
                  bwnd = tcp_inflight_max;
          if ((long)bwnd < tp->t_maxseg * 2)
                  bwnd = tp->t_maxseg * 2;
          tp->snd_bwnd = bwnd;
  }
  
  #ifdef TCP_SIGNATURE
  /*
   * Callback function invoked by m_apply() to digest TCP segment data
   * contained within an mbuf chain.
   */
  static int
  tcp_signature_apply(void *fstate, void *data, u_int len)
  {
  
          MD5Update(fstate, (u_char *)data, len);
          return (0);
  }
  
  /*
   * Compute TCP-MD5 hash of a TCPv4 segment. (RFC2385)
   *
   * Parameters:
   * m            pointer to head of mbuf chain
   * off0         offset to TCP header within the mbuf chain
   * len          length of TCP segment data, excluding options
   * optlen       length of TCP segment options
   * buf          pointer to storage for computed MD5 digest
   * direction    direction of flow (IPSEC_DIR_INBOUND or OUTBOUND)
   *
   * We do this over ip, tcphdr, segment data, and the key in the SADB.
   * When called from tcp_input(), we can be sure that th_sum has been
   * zeroed out and verified already.
   *
   * This function is for IPv4 use only. Calling this function with an
   * IPv6 packet in the mbuf chain will yield undefined results.
   *
   * Return 0 if successful, otherwise return -1.
   *
   * XXX The key is retrieved from the system's PF_KEY SADB, by keying a
   * search with the destination IP address, and a 'magic SPI' to be
   * determined by the application. This is hardcoded elsewhere to 1179
   * right now. Another branch of this code exists which uses the SPD to
   * specify per-application flows but it is unstable.
   */
  int
  tcp_signature_compute(struct mbuf *m, int off0, int len, int optlen,
      u_char *buf, u_int direction)
  {
          union sockaddr_union dst;
          struct ippseudo ippseudo;
          MD5_CTX ctx;
          int doff;
          struct ip *ip;
          struct ipovly *ipovly;
          struct secasvar *sav;
          struct tcphdr *th;
          u_short savecsum;
  
          KASSERT(m != NULL, ("NULL mbuf chain"));
          KASSERT(buf != NULL, ("NULL signature pointer"));
  
          /* Extract the destination from the IP header in the mbuf. */
          ip = mtod(m, struct ip *);
          bzero(&dst, sizeof(union sockaddr_union));
          dst.sa.sa_len = sizeof(struct sockaddr_in);
          dst.sa.sa_family = AF_INET;
          dst.sin.sin_addr = (direction == IPSEC_DIR_INBOUND) ?
              ip->ip_src : ip->ip_dst;
  
          /* Look up an SADB entry which matches the address of the peer. */
          sav = KEY_ALLOCSA(&dst, IPPROTO_TCP, htonl(TCP_SIG_SPI));
          if (sav == NULL) {
                  printf("%s: SADB lookup failed for %s\n", __func__,
                      inet_ntoa(dst.sin.sin_addr));
                  return (EINVAL);
          }
  
          MD5Init(&ctx);
          ipovly = (struct ipovly *)ip;
          th = (struct tcphdr *)((u_char *)ip + off0);
          doff = off0 + sizeof(struct tcphdr) + optlen;
  
          /*
           * Step 1: Update MD5 hash with IP pseudo-header.
           *
           * XXX The ippseudo header MUST be digested in network byte order,
           * or else we'll fail the regression test. Assume all fields we've
           * been doing arithmetic on have been in host byte order.
           * XXX One cannot depend on ipovly->ih_len here. When called from
           * tcp_output(), the underlying ip_len member has not yet been set.
           */
          ippseudo.ippseudo_src = ipovly->ih_src;
          ippseudo.ippseudo_dst = ipovly->ih_dst;
          ippseudo.ippseudo_pad = 0;
          ippseudo.ippseudo_p = IPPROTO_TCP;
          ippseudo.ippseudo_len = htons(len + sizeof(struct tcphdr) + optlen);
          MD5Update(&ctx, (char *)&ippseudo, sizeof(struct ippseudo));
  
          /*
           * Step 2: Update MD5 hash with TCP header, excluding options.
           * The TCP checksum must be set to zero.
           */
          savecsum = th->th_sum;
          th->th_sum = 0;
          MD5Update(&ctx, (char *)th, sizeof(struct tcphdr));
          th->th_sum = savecsum;
  
          /*
           * Step 3: Update MD5 hash with TCP segment data.
           *         Use m_apply() to avoid an early m_pullup().
           */
          if (len > 0)
                  m_apply(m, doff, len, tcp_signature_apply, &ctx);
  
          /*
           * Step 4: Update MD5 hash with shared secret.
           */
          MD5Update(&ctx, _KEYBUF(sav->key_auth), _KEYLEN(sav->key_auth));
          MD5Final(buf, &ctx);
  
          key_sa_recordxfer(sav, m);
          KEY_FREESAV(&sav);
          return (0);
  }
  #endif /* TCP_SIGNATURE */

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