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
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_compat.h"
    #include "opt_inet.h"
    #include "opt_inet6.h"
    #include "opt_ipsec.h"
    #include "opt_tcpdebug.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/callout.h>
    #include <sys/eventhandler.h>
    #include <sys/hhook.h>
    #include <sys/kernel.h>
    #include <sys/khelp.h>
    #include <sys/sysctl.h>
    #include <sys/jail.h>
    #include <sys/malloc.h>
    #include <sys/refcount.h>
    #include <sys/mbuf.h>
    #ifdef INET6
    #include <sys/domain.h>
    #endif
    #include <sys/priv.h>
    #include <sys/proc.h>
    #include <sys/sdt.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 <net/if_var.h>
    #include <net/vnet.h>
    
    #include <netinet/in.h>
    #include <netinet/in_fib.h>
    #include <netinet/in_kdtrace.h>
    #include <netinet/in_pcb.h>
    #include <netinet/in_systm.h>
    #include <netinet/in_var.h>
    #include <netinet/ip.h>
    #include <netinet/ip_icmp.h>
    #include <netinet/ip_var.h>
    #ifdef INET6
    #include <netinet/icmp6.h>
    #include <netinet/ip6.h>
    #include <netinet6/in6_fib.h>
    #include <netinet6/in6_pcb.h>
    #include <netinet6/ip6_var.h>
    #include <netinet6/scope6_var.h>
    #include <netinet6/nd6.h>
    #endif
    
    #ifdef TCP_RFC7413
    #include <netinet/tcp_fastopen.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>
    #include <netinet/tcp_syncache.h>
    #include <netinet/cc/cc.h>
   #ifdef INET6
   #include <netinet6/tcp6_var.h>
   #endif
   #include <netinet/tcpip.h>
   #ifdef TCPPCAP
   #include <netinet/tcp_pcap.h>
   #endif
   #ifdef TCPDEBUG
   #include <netinet/tcp_debug.h>
   #endif
   #ifdef INET6
   #include <netinet6/ip6protosw.h>
   #endif
   #ifdef TCP_OFFLOAD
   #include <netinet/tcp_offload.h>
   #endif
   
   #include <netipsec/ipsec_support.h>
   
   #include <machine/in_cksum.h>
   #include <sys/md5.h>
   
   #include <security/mac/mac_framework.h>
   
   VNET_DEFINE(int, tcp_mssdflt) = TCP_MSS;
   #ifdef INET6
   VNET_DEFINE(int, tcp_v6mssdflt) = TCP6_MSS;
   #endif
   
   struct rwlock tcp_function_lock;
   
   static int
   sysctl_net_inet_tcp_mss_check(SYSCTL_HANDLER_ARGS)
   {
           int error, new;
   
           new = V_tcp_mssdflt;
           error = sysctl_handle_int(oidp, &new, 0, req);
           if (error == 0 && req->newptr) {
                   if (new < TCP_MINMSS)
                           error = EINVAL;
                   else
                           V_tcp_mssdflt = new;
           }
           return (error);
   }
   
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_MSSDFLT, mssdflt,
       CTLFLAG_VNET | CTLTYPE_INT | CTLFLAG_RW, &VNET_NAME(tcp_mssdflt), 0,
       &sysctl_net_inet_tcp_mss_check, "I",
       "Default TCP Maximum Segment Size");
   
   #ifdef INET6
   static int
   sysctl_net_inet_tcp_mss_v6_check(SYSCTL_HANDLER_ARGS)
   {
           int error, new;
   
           new = V_tcp_v6mssdflt;
           error = sysctl_handle_int(oidp, &new, 0, req);
           if (error == 0 && req->newptr) {
                   if (new < TCP_MINMSS)
                           error = EINVAL;
                   else
                           V_tcp_v6mssdflt = new;
           }
           return (error);
   }
   
   SYSCTL_PROC(_net_inet_tcp, TCPCTL_V6MSSDFLT, v6mssdflt,
       CTLFLAG_VNET | CTLTYPE_INT | CTLFLAG_RW, &VNET_NAME(tcp_v6mssdflt), 0,
       &sysctl_net_inet_tcp_mss_v6_check, "I",
      "Default TCP Maximum Segment Size for IPv6");
   #endif /* INET6 */
   
   /*
    * 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.
    */
   VNET_DEFINE(int, tcp_minmss) = TCP_MINMSS;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, minmss, CTLFLAG_VNET | CTLFLAG_RW,
        &VNET_NAME(tcp_minmss), 0,
       "Minimum TCP Maximum Segment Size");
   
   VNET_DEFINE(int, tcp_do_rfc1323) = 1;
   SYSCTL_INT(_net_inet_tcp, TCPCTL_DO_RFC1323, rfc1323, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_do_rfc1323), 0,
       "Enable rfc1323 (high performance TCP) extensions");
   
   static int      tcp_log_debug = 0;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, log_debug, CTLFLAG_RW,
       &tcp_log_debug, 0, "Log errors caused by incoming TCP segments");
   
   static int      tcp_tcbhashsize;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, tcbhashsize, CTLFLAG_RDTUN | CTLFLAG_NOFETCH,
       &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_UINT(_net_inet_tcp, OID_AUTO, pcbcount, CTLFLAG_VNET | CTLFLAG_RD,
       &VNET_NAME(tcbinfo.ipi_count), 0, "Number of active PCBs");
   
   static VNET_DEFINE(int, icmp_may_rst) = 1;
   #define V_icmp_may_rst                  VNET(icmp_may_rst)
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, icmp_may_rst, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(icmp_may_rst), 0,
       "Certain ICMP unreachable messages may abort connections in SYN_SENT");
   
   static VNET_DEFINE(int, tcp_isn_reseed_interval) = 0;
   #define V_tcp_isn_reseed_interval       VNET(tcp_isn_reseed_interval)
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, isn_reseed_interval, CTLFLAG_VNET | CTLFLAG_RW,
       &VNET_NAME(tcp_isn_reseed_interval), 0,
       "Seconds between reseeding of ISN secret");
   
   static int      tcp_soreceive_stream;
   SYSCTL_INT(_net_inet_tcp, OID_AUTO, soreceive_stream, CTLFLAG_RDTUN,
       &tcp_soreceive_stream, 0, "Using soreceive_stream for TCP sockets");
   
   VNET_DEFINE(uma_zone_t, sack_hole_zone);
   #define V_sack_hole_zone                VNET(sack_hole_zone)
   
   VNET_DEFINE(struct hhook_head *, tcp_hhh[HHOOK_TCP_LAST+1]);
   
   VNET_DEFINE(u_char, ts_offset_secret[32]);
   #define V_ts_offset_secret      VNET(ts_offset_secret)
   
   static struct inpcb *tcp_notify(struct inpcb *, int);
   static struct inpcb *tcp_mtudisc_notify(struct inpcb *, int);
   static void tcp_mtudisc(struct inpcb *, int);
   static char *   tcp_log_addr(struct in_conninfo *inc, struct tcphdr *th,
                       void *ip4hdr, const void *ip6hdr);
   
   
   static struct tcp_function_block tcp_def_funcblk = {
           "default",
           tcp_output,
           tcp_do_segment,
           tcp_default_ctloutput,
           NULL,
           NULL,   
           NULL,
           NULL,
           NULL,
           NULL,
           0,
           0
   };
   
   int t_functions_inited = 0;
   struct tcp_funchead t_functions;
   static struct tcp_function_block *tcp_func_set_ptr = &tcp_def_funcblk;
   
   static void
   init_tcp_functions(void)
   {
           if (t_functions_inited == 0) {
                   TAILQ_INIT(&t_functions);
                   rw_init_flags(&tcp_function_lock, "tcp_func_lock" , 0);
                   t_functions_inited = 1;
           }
   }
   
   static struct tcp_function_block *
   find_tcp_functions_locked(struct tcp_function_set *fs)
   {
           struct tcp_function *f;
           struct tcp_function_block *blk=NULL;
   
           TAILQ_FOREACH(f, &t_functions, tf_next) {
                   if (strcmp(f->tf_fb->tfb_tcp_block_name, fs->function_set_name) == 0) {
                           blk = f->tf_fb;
                           break;
                   }
           }
           return(blk);
   }
   
   static struct tcp_function_block *
   find_tcp_fb_locked(struct tcp_function_block *blk, struct tcp_function **s)
   {
           struct tcp_function_block *rblk=NULL;
           struct tcp_function *f;
   
           TAILQ_FOREACH(f, &t_functions, tf_next) {
                   if (f->tf_fb == blk) {
                           rblk = blk;
                           if (s) {
                                   *s = f;
                           }
                           break;
                   }
           }
           return (rblk);
   }
   
   struct tcp_function_block *
   find_and_ref_tcp_functions(struct tcp_function_set *fs)
   {
           struct tcp_function_block *blk;
           
           rw_rlock(&tcp_function_lock);   
           blk = find_tcp_functions_locked(fs);
           if (blk)
                   refcount_acquire(&blk->tfb_refcnt); 
           rw_runlock(&tcp_function_lock);
           return(blk);
   }
   
   struct tcp_function_block *
   find_and_ref_tcp_fb(struct tcp_function_block *blk)
   {
           struct tcp_function_block *rblk;
           
           rw_rlock(&tcp_function_lock);   
           rblk = find_tcp_fb_locked(blk, NULL);
           if (rblk) 
                   refcount_acquire(&rblk->tfb_refcnt);
           rw_runlock(&tcp_function_lock);
           return(rblk);
   }
   
   
   static int
   sysctl_net_inet_default_tcp_functions(SYSCTL_HANDLER_ARGS)
   {
           int error=ENOENT;
           struct tcp_function_set fs;
           struct tcp_function_block *blk;
   
           memset(&fs, 0, sizeof(fs));
           rw_rlock(&tcp_function_lock);
           blk = find_tcp_fb_locked(tcp_func_set_ptr, NULL);
           if (blk) {
                   /* Found him */
                   strcpy(fs.function_set_name, blk->tfb_tcp_block_name);
                   fs.pcbcnt = blk->tfb_refcnt;
           }
           rw_runlock(&tcp_function_lock); 
           error = sysctl_handle_string(oidp, fs.function_set_name,
                                        sizeof(fs.function_set_name), req);
   
           /* Check for error or no change */
           if (error != 0 || req->newptr == NULL)
                   return(error);
   
           rw_wlock(&tcp_function_lock);
           blk = find_tcp_functions_locked(&fs);
           if ((blk == NULL) ||
               (blk->tfb_flags & TCP_FUNC_BEING_REMOVED)) { 
                   error = ENOENT; 
                   goto done;
           }
           tcp_func_set_ptr = blk;
   done:
           rw_wunlock(&tcp_function_lock);
           return (error);
   }
   
   SYSCTL_PROC(_net_inet_tcp, OID_AUTO, functions_default,
               CTLTYPE_STRING | CTLFLAG_RW,
               NULL, 0, sysctl_net_inet_default_tcp_functions, "A",
               "Set/get the default TCP functions");
   
   static int
   sysctl_net_inet_list_available(SYSCTL_HANDLER_ARGS)
   {
           int error, cnt, linesz;
           struct tcp_function *f;
           char *buffer, *cp;
           size_t bufsz, outsz;
   
           cnt = 0;
           rw_rlock(&tcp_function_lock);
           TAILQ_FOREACH(f, &t_functions, tf_next) {
                   cnt++;
           }
           rw_runlock(&tcp_function_lock);
   
           bufsz = (cnt+2) * (TCP_FUNCTION_NAME_LEN_MAX + 12) + 1;
           buffer = malloc(bufsz, M_TEMP, M_WAITOK);
   
           error = 0;
           cp = buffer;
   
           linesz = snprintf(cp, bufsz, "\n%-32s%c %s\n", "Stack", 'D', "PCB count");
           cp += linesz;
           bufsz -= linesz;
           outsz = linesz;
   
           rw_rlock(&tcp_function_lock);   
           TAILQ_FOREACH(f, &t_functions, tf_next) {
                   linesz = snprintf(cp, bufsz, "%-32s%c %u\n",
                       f->tf_fb->tfb_tcp_block_name,
                       (f->tf_fb == tcp_func_set_ptr) ? '*' : ' ',
                       f->tf_fb->tfb_refcnt);
                   if (linesz >= bufsz) {
                           error = EOVERFLOW;
                           break;
                   }
                   cp += linesz;
                   bufsz -= linesz;
                   outsz += linesz;
           }
           rw_runlock(&tcp_function_lock);
           if (error == 0)
                   error = sysctl_handle_string(oidp, buffer, outsz + 1, req);
           free(buffer, M_TEMP);
           return (error);
   }
   
   SYSCTL_PROC(_net_inet_tcp, OID_AUTO, functions_available,
               CTLTYPE_STRING|CTLFLAG_RD,
               NULL, 0, sysctl_net_inet_list_available, "A",
               "list available TCP Function sets");
   
   /*
    * 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     0
   #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  tcp_timer       tt;
           struct  cc_var          ccv;
           struct  osd             osd;
   };
   
   static VNET_DEFINE(uma_zone_t, tcpcb_zone);
   #define V_tcpcb_zone                    VNET(tcpcb_zone)
   
   MALLOC_DEFINE(M_TCPLOG, "tcplog", "TCP address and flags print buffers");
   MALLOC_DEFINE(M_TCPFUNCTIONS, "tcpfunc", "TCP function set memory");
   
   static struct mtx isn_mtx;
   
   #define ISN_LOCK_INIT() mtx_init(&isn_mtx, "isn_mtx", NULL, MTX_DEF)
   #define ISN_LOCK()      mtx_lock(&isn_mtx)
   #define ISN_UNLOCK()    mtx_unlock(&isn_mtx)
   
   /*
    * TCP initialization.
    */
   static void
   tcp_zone_change(void *tag)
   {
   
           uma_zone_set_max(V_tcbinfo.ipi_zone, maxsockets);
           uma_zone_set_max(V_tcpcb_zone, maxsockets);
           tcp_tw_zone_change();
   }
   
   static int
   tcp_inpcb_init(void *mem, int size, int flags)
   {
           struct inpcb *inp = mem;
   
           INP_LOCK_INIT(inp, "inp", "tcpinp");
           return (0);
   }
   
   /*
    * Take a value and get the next power of 2 that doesn't overflow.
    * Used to size the tcp_inpcb hash buckets.
    */
   static int
   maketcp_hashsize(int size)
   {
           int hashsize;
   
           /*
            * auto tune.
            * get the next power of 2 higher than maxsockets.
            */
           hashsize = 1 << fls(size);
           /* catch overflow, and just go one power of 2 smaller */
           if (hashsize < size) {
                   hashsize = 1 << (fls(size) - 1);
           }
           return (hashsize);
   }
   
   int
   register_tcp_functions(struct tcp_function_block *blk, int wait)
   {
           struct tcp_function_block *lblk;
           struct tcp_function *n;
           struct tcp_function_set fs;
   
           if (t_functions_inited == 0) {
                   init_tcp_functions();
           }
           if ((blk->tfb_tcp_output == NULL) ||
               (blk->tfb_tcp_do_segment == NULL) ||
               (blk->tfb_tcp_ctloutput == NULL) ||
               (strlen(blk->tfb_tcp_block_name) == 0)) {
                   /* 
                    * These functions are required and you
                    * need a name.
                    */
                   return (EINVAL);
           }
           if (blk->tfb_tcp_timer_stop_all ||
               blk->tfb_tcp_timer_activate ||
               blk->tfb_tcp_timer_active ||
               blk->tfb_tcp_timer_stop) {
                   /*
                    * If you define one timer function you 
                    * must have them all.
                    */
                   if ((blk->tfb_tcp_timer_stop_all == NULL) ||
                       (blk->tfb_tcp_timer_activate == NULL) ||
                       (blk->tfb_tcp_timer_active == NULL) ||
                       (blk->tfb_tcp_timer_stop == NULL)) {
                           return (EINVAL);                        
                   }
           }       
           if (blk->tfb_flags & TCP_FUNC_BEING_REMOVED) {
                   return (EINVAL);
           }
           n = malloc(sizeof(struct tcp_function), M_TCPFUNCTIONS, wait);
           if (n == NULL) {
                   return (ENOMEM);
           }
           n->tf_fb = blk;
           strcpy(fs.function_set_name, blk->tfb_tcp_block_name);
           rw_wlock(&tcp_function_lock);
           lblk = find_tcp_functions_locked(&fs);
           if (lblk) {
                   /* Duplicate name space not allowed */
                   rw_wunlock(&tcp_function_lock);
                   free(n, M_TCPFUNCTIONS);
                   return (EALREADY);
           }
           refcount_init(&blk->tfb_refcnt, 0);
           TAILQ_INSERT_TAIL(&t_functions, n, tf_next);
           rw_wunlock(&tcp_function_lock);
           return(0);
   }       
   
   int
   deregister_tcp_functions(struct tcp_function_block *blk)
   {
           struct tcp_function_block *lblk;
           struct tcp_function *f;
           int error=ENOENT;
           
           if (strcmp(blk->tfb_tcp_block_name, "default") == 0) {
                   /* You can't un-register the default */
                   return (EPERM);
           }
           rw_wlock(&tcp_function_lock);
           if (blk == tcp_func_set_ptr) {
                   /* You can't free the current default */
                   rw_wunlock(&tcp_function_lock);
                   return (EBUSY);
           }
           if (blk->tfb_refcnt) {
                   /* Still tcb attached, mark it. */
                   blk->tfb_flags |= TCP_FUNC_BEING_REMOVED;
                   rw_wunlock(&tcp_function_lock);         
                   return (EBUSY);
           }
           lblk = find_tcp_fb_locked(blk, &f);
           if (lblk) {
                   /* Found */
                   TAILQ_REMOVE(&t_functions, f, tf_next);
                   f->tf_fb = NULL;
                   free(f, M_TCPFUNCTIONS);
                   error = 0;
           }
           rw_wunlock(&tcp_function_lock);
           return (error);
   }
   
   void
   tcp_init(void)
   {
           const char *tcbhash_tuneable;
           int hashsize;
   
           tcbhash_tuneable = "net.inet.tcp.tcbhashsize";
   
           if (hhook_head_register(HHOOK_TYPE_TCP, HHOOK_TCP_EST_IN,
               &V_tcp_hhh[HHOOK_TCP_EST_IN], HHOOK_NOWAIT|HHOOK_HEADISINVNET) != 0)
                   printf("%s: WARNING: unable to register helper hook\n", __func__);
           if (hhook_head_register(HHOOK_TYPE_TCP, HHOOK_TCP_EST_OUT,
               &V_tcp_hhh[HHOOK_TCP_EST_OUT], HHOOK_NOWAIT|HHOOK_HEADISINVNET) != 0)
                   printf("%s: WARNING: unable to register helper hook\n", __func__);
           hashsize = TCBHASHSIZE;
           TUNABLE_INT_FETCH(tcbhash_tuneable, &hashsize);
           if (hashsize == 0) {
                   /*
                    * Auto tune the hash size based on maxsockets.
                    * A perfect hash would have a 1:1 mapping
                    * (hashsize = maxsockets) however it's been
                    * suggested that O(2) average is better.
                    */
                   hashsize = maketcp_hashsize(maxsockets / 4);
                   /*
                    * Our historical default is 512,
                    * do not autotune lower than this.
                    */
                   if (hashsize < 512)
                           hashsize = 512;
                   if (bootverbose && IS_DEFAULT_VNET(curvnet))
                           printf("%s: %s auto tuned to %d\n", __func__,
                               tcbhash_tuneable, hashsize);
           }
           /*
            * We require a hashsize to be a power of two.
            * Previously if it was not a power of two we would just reset it
            * back to 512, which could be a nasty surprise if you did not notice
            * the error message.
            * Instead what we do is clip it to the closest power of two lower
            * than the specified hash value.
            */
           if (!powerof2(hashsize)) {
                   int oldhashsize = hashsize;
   
                   hashsize = maketcp_hashsize(hashsize);
                   /* prevent absurdly low value */
                   if (hashsize < 16)
                           hashsize = 16;
                   printf("%s: WARNING: TCB hash size not a power of 2, "
                       "clipped from %d to %d.\n", __func__, oldhashsize,
                       hashsize);
           }
           in_pcbinfo_init(&V_tcbinfo, "tcp", &V_tcb, hashsize, hashsize,
               "tcp_inpcb", tcp_inpcb_init, NULL, 0, IPI_HASHFIELDS_4TUPLE);
   
           /*
            * These have to be type stable for the benefit of the timers.
            */
           V_tcpcb_zone = uma_zcreate("tcpcb", sizeof(struct tcpcb_mem),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
           uma_zone_set_max(V_tcpcb_zone, maxsockets);
           uma_zone_set_warning(V_tcpcb_zone, "kern.ipc.maxsockets limit reached");
   
           tcp_tw_init();
           syncache_init();
           tcp_hc_init();
   
           TUNABLE_INT_FETCH("net.inet.tcp.sack.enable", &V_tcp_do_sack);
           V_sack_hole_zone = uma_zcreate("sackhole", sizeof(struct sackhole),
               NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
   
   #ifdef TCP_RFC7413
           tcp_fastopen_init();
   #endif
   
           /* Skip initialization of globals for non-default instances. */
           if (!IS_DEFAULT_VNET(curvnet))
                   return;
   
           tcp_reass_global_init();
   
           /* XXX virtualize those bellow? */
           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;
           if (tcp_rexmit_min < 1)
                   tcp_rexmit_min = 1;
           tcp_persmin = TCPTV_PERSMIN;
           tcp_persmax = TCPTV_PERSMAX;
           tcp_rexmit_slop = TCPTV_CPU_VAR;
           tcp_finwait2_timeout = TCPTV_FINWAIT2_TIMEOUT;
           tcp_tcbhashsize = hashsize;
           /* Setup the tcp function block list */
           init_tcp_functions();
           register_tcp_functions(&tcp_def_funcblk, M_WAITOK);
           read_random(&V_ts_offset_secret, sizeof(V_ts_offset_secret));
   
           if (tcp_soreceive_stream) {
   #ifdef INET
                   tcp_usrreqs.pru_soreceive = soreceive_stream;
   #endif
   #ifdef INET6
                   tcp6_usrreqs.pru_soreceive = soreceive_stream;
   #endif /* INET6 */
           }
   
   #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
   
           ISN_LOCK_INIT();
           EVENTHANDLER_REGISTER(shutdown_pre_sync, tcp_fini, NULL,
                   SHUTDOWN_PRI_DEFAULT);
           EVENTHANDLER_REGISTER(maxsockets_change, tcp_zone_change, NULL,
                   EVENTHANDLER_PRI_ANY);
   #ifdef TCPPCAP
           tcp_pcap_init();
   #endif
   }
   
   #ifdef VIMAGE
   static void
   tcp_destroy(void *unused __unused)
   {
           int error, n;
   
           /*
            * All our processes are gone, all our sockets should be cleaned
            * up, which means, we should be past the tcp_discardcb() calls.
            * Sleep to let all tcpcb timers really disappear and cleanup.
            */
           for (;;) {
                   INP_LIST_RLOCK(&V_tcbinfo);
                   n = V_tcbinfo.ipi_count;
                   INP_LIST_RUNLOCK(&V_tcbinfo);
                   if (n == 0)
                           break;
                   pause("tcpdes", hz / 10);
           }
           tcp_hc_destroy();
           syncache_destroy();
           tcp_tw_destroy();
           in_pcbinfo_destroy(&V_tcbinfo);
           /* tcp_discardcb() clears the sack_holes up. */
           uma_zdestroy(V_sack_hole_zone);
           uma_zdestroy(V_tcpcb_zone);
   
   #ifdef TCP_RFC7413
           /*
            * Cannot free the zone until all tcpcbs are released as we attach
            * the allocations to them.
            */
           tcp_fastopen_destroy();
   #endif
   
           error = hhook_head_deregister(V_tcp_hhh[HHOOK_TCP_EST_IN]);
           if (error != 0) {
                   printf("%s: WARNING: unable to deregister helper hook "
                       "type=%d, id=%d: error %d returned\n", __func__,
                       HHOOK_TYPE_TCP, HHOOK_TCP_EST_IN, error);
           }
           error = hhook_head_deregister(V_tcp_hhh[HHOOK_TCP_EST_OUT]);
           if (error != 0) {
                   printf("%s: WARNING: unable to deregister helper hook "
                       "type=%d, id=%d: error %d returned\n", __func__,
                       HHOOK_TYPE_TCP, HHOOK_TCP_EST_OUT, error);
           }
   }
   VNET_SYSUNINIT(tcp, SI_SUB_PROTO_DOMAIN, SI_ORDER_FOURTH, tcp_destroy, NULL);
   #endif
   
   void
   tcp_fini(void *xtp)
   {
   
   }
   
   /*
    * 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(struct inpcb *inp, void *ip_ptr, void *tcp_ptr)
   {
           struct tcphdr *th = (struct tcphdr *)tcp_ptr;
   
           INP_WLOCK_ASSERT(inp);
   
   #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->inp_flow & 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 = htons(sizeof(struct tcphdr));
                   ip6->ip6_src = inp->in6p_laddr;
                   ip6->ip6_dst = inp->in6p_faddr;
           }
   #endif /* INET6 */
   #if defined(INET6) && defined(INET)
           else
   #endif
   #ifdef INET
           {
                   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;
           }
   #endif /* INET */
           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(struct inpcb *inp)
   {
           struct tcptemp *t;
   
           t = malloc(sizeof(*t), M_TEMP, M_NOWAIT);
           if (t == NULL)
                   return (NULL);
           tcpip_fillheaders(inp, (void *)&t->tt_ipgen, (void *)&t->tt_t);
           return (t);
   }
   
   /*
    * 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 th 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 th,
    * 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 th must point to *inside* the mbuf.
    */
   void
   tcp_respond(struct tcpcb *tp, void *ipgen, struct tcphdr *th, struct mbuf *m,
       tcp_seq ack, tcp_seq seq, int flags)
   {
           struct tcpopt to;
           struct inpcb *inp;
           struct ip *ip;
           struct mbuf *optm;
           struct tcphdr *nth;
           u_char *optp;
   #ifdef INET6
           struct ip6_hdr *ip6;
           int isipv6;
   #endif /* INET6 */
           int optlen, tlen, win;
           bool incl_opts;
   
           KASSERT(tp != NULL || m != NULL, ("tcp_respond: tp and m both NULL"));
   
   #ifdef INET6
           isipv6 = ((struct ip *)ipgen)->ip_v == (IPV6_VERSION >> 4);
           ip6 = ipgen;
   #endif /* INET6 */
           ip = ipgen;
   
           if (tp != NULL) {
                   inp = tp->t_inpcb;
                   KASSERT(inp != NULL, ("tcp control block w/o inpcb"));
                   INP_WLOCK_ASSERT(inp);
           } else
                   inp = NULL;
   
           incl_opts = false;
           win = 0;
           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 ((tp->t_flags & TF_NOOPT) == 0)
                           incl_opts = true;
           }
           if (m == NULL) {
                   m = m_gethdr(M_NOWAIT, MT_DATA);
                   if (m == NULL)
                           return;
                   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 if (!M_WRITABLE(m)) {
                   struct mbuf *n;
   
                   /* Can't reuse 'm', allocate a new mbuf. */
                   n = m_gethdr(M_NOWAIT, MT_DATA);
                   if (n == NULL) {
                           m_freem(m);
                           return;
                   }
   
                   if (!m_dup_pkthdr(n, m, M_NOWAIT)) {
                           m_freem(m);
                           m_freem(n);
                           return;
                   }
   
                   n->m_data += max_linkhdr;
                   /* m_len is set later */
   #define xchg(a,b,type) { type t; t=a; a=b; b=t; }
   #ifdef INET6
                   if (isipv6) {
                           bcopy((caddr_t)ip6, mtod(n, caddr_t),
                                 sizeof(struct ip6_hdr));
                           ip6 = mtod(n, struct ip6_hdr *);
                           xchg(ip6->ip6_dst, ip6->ip6_src, struct in6_addr);
                           nth = (struct tcphdr *)(ip6 + 1);
                   } else
   #endif /* INET6 */
                   {
                           bcopy((caddr_t)ip, mtod(n, caddr_t), sizeof(struct ip));
                           ip = mtod(n, struct ip *);
                           xchg(ip->ip_dst.s_addr, ip->ip_src.s_addr, uint32_t);
                           nth = (struct tcphdr *)(ip + 1);
                   }
                   bcopy((caddr_t)th, (caddr_t)nth, sizeof(struct tcphdr));
                   xchg(nth->th_dport, nth->th_sport, uint16_t);
                   th = nth;
                   m_freem(m);
                   m = n;
           } else {
                   /*
                    *  reuse the mbuf. 
                    * XXX MRT We inherit the FIB, which is lucky.
                    */
                   m_freem(m->m_next);
                   m->m_next = NULL;
                   m->m_data = (caddr_t)ipgen;
                   /* m_len is set later */
   #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, uint32_t);
                           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, uint16_t);
  #undef xchg
          }
          tlen = 0;
  #ifdef INET6
          if (isipv6)
                  tlen = sizeof (struct ip6_hdr) + sizeof (struct tcphdr);
  #endif
  #if defined(INET) && defined(INET6)
          else
  #endif
  #ifdef INET
                  tlen = sizeof (struct tcpiphdr);
  #endif
  #ifdef INVARIANTS
          m->m_len = 0;
          KASSERT(M_TRAILINGSPACE(m) >= tlen,
              ("Not enough trailing space for message (m=%p, need=%d, have=%ld)",
              m, tlen, (long)M_TRAILINGSPACE(m)));
  #endif
          m->m_len = tlen;
          to.to_flags = 0;
          if (incl_opts) {
                  /* Make sure we have room. */
                  if (M_TRAILINGSPACE(m) < TCP_MAXOLEN) {
                          m->m_next = m_get(M_NOWAIT, MT_DATA);
                          if (m->m_next) {
                                  optp = mtod(m->m_next, u_char *);
                                  optm = m->m_next;
                          } else
                                  incl_opts = false;
                  } else {
                          optp = (u_char *) (nth + 1);
                          optm = m;
                  }
          }
          if (incl_opts) {
                  /* Timestamps. */
                  if (tp->t_flags & TF_RCVD_TSTMP) {
                          to.to_tsval = tcp_ts_getticks() + tp->ts_offset;
                          to.to_tsecr = tp->ts_recent;
                          to.to_flags |= TOF_TS;
                  }
  #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
                  /* TCP-MD5 (RFC2385). */
                  if (tp->t_flags & TF_SIGNATURE)
                          to.to_flags |= TOF_SIGNATURE;
  #endif
                  /* Add the options. */
                  tlen += optlen = tcp_addoptions(&to, optp);
  
                  /* Update m_len in the correct mbuf. */
                  optm->m_len += optlen;
          } else
                  optlen = 0;
  #ifdef INET6
          if (isipv6) {
                  ip6->ip6_flow = 0;
                  ip6->ip6_vfc = IPV6_VERSION;
                  ip6->ip6_nxt = IPPROTO_TCP;
                  ip6->ip6_plen = htons(tlen - sizeof(*ip6));
          }
  #endif
  #if defined(INET) && defined(INET6)
          else
  #endif
  #ifdef INET
          {
                  ip->ip_len = htons(tlen);
                  ip->ip_ttl = V_ip_defttl;
                  if (V_path_mtu_discovery)
                          ip->ip_off |= htons(IP_DF);
          }
  #endif
          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_WLOCK_ASSERT(inp);
                  mac_inpcb_create_mbuf(inp, m);
          } else {
                  /*
                   * Packet is not associated with a socket, so possibly
                   * update the label in place.
                   */
                  mac_netinet_tcp_reply(m);
          }
  #endif
          nth->th_seq = htonl(seq);
          nth->th_ack = htonl(ack);
          nth->th_x2 = 0;
          nth->th_off = (sizeof (struct tcphdr) + optlen) >> 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;
  
  #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
          if (to.to_flags & TOF_SIGNATURE) {
                  if (!TCPMD5_ENABLED() ||
                      TCPMD5_OUTPUT(m, nth, to.to_signature) != 0) {
                          m_freem(m);
                          return;
                  }
          }
  #endif
  
          m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
  #ifdef INET6
          if (isipv6) {
                  m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
                  nth->th_sum = in6_cksum_pseudo(ip6,
                      tlen - sizeof(struct ip6_hdr), IPPROTO_TCP, 0);
                  ip6->ip6_hlim = in6_selecthlim(tp != NULL ? tp->t_inpcb :
                      NULL, NULL);
          }
  #endif /* INET6 */
  #if defined(INET6) && defined(INET)
          else
  #endif
  #ifdef INET
          {
                  m->m_pkthdr.csum_flags = CSUM_TCP;
                  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)));
          }
  #endif /* INET */
  #ifdef TCPDEBUG
          if (tp == NULL || (inp->inp_socket->so_options & SO_DEBUG))
                  tcp_trace(TA_OUTPUT, 0, tp, mtod(m, void *), th, 0);
  #endif
          TCP_PROBE3(debug__output, tp, th, m);
          if (flags & TH_RST)
                  TCP_PROBE5(accept__refused, NULL, NULL, m, tp, nth);
  
  #ifdef INET6
          if (isipv6) {
                  TCP_PROBE5(send, NULL, tp, ip6, tp, nth);
                  (void)ip6_output(m, NULL, NULL, 0, NULL, NULL, inp);
          }
  #endif /* INET6 */
  #if defined(INET) && defined(INET6)
          else
  #endif
  #ifdef INET
          {
                  TCP_PROBE5(send, NULL, tp, ip, tp, nth);
                  (void)ip_output(m, NULL, NULL, 0, NULL, inp);
          }
  #endif
  }
  
  /*
   * 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(struct inpcb *inp)
  {
          struct tcpcb_mem *tm;
          struct tcpcb *tp;
  #ifdef INET6
          int isipv6 = (inp->inp_vflag & INP_IPV6) != 0;
  #endif /* INET6 */
  
          tm = uma_zalloc(V_tcpcb_zone, M_NOWAIT | M_ZERO);
          if (tm == NULL)
                  return (NULL);
          tp = &tm->tcb;
  
          /* Initialise cc_var struct for this tcpcb. */
          tp->ccv = &tm->ccv;
          tp->ccv->type = IPPROTO_TCP;
          tp->ccv->ccvc.tcp = tp;
          rw_rlock(&tcp_function_lock);
          tp->t_fb = tcp_func_set_ptr;
          refcount_acquire(&tp->t_fb->tfb_refcnt);
          rw_runlock(&tcp_function_lock);
          if (tp->t_fb->tfb_tcp_fb_init) {
                  (*tp->t_fb->tfb_tcp_fb_init)(tp);
          }
          /*
           * Use the current system default CC algorithm.
           */
          CC_LIST_RLOCK();
          KASSERT(!STAILQ_EMPTY(&cc_list), ("cc_list is empty!"));
          CC_ALGO(tp) = CC_DEFAULT();
          CC_LIST_RUNLOCK();
  
          if (CC_ALGO(tp)->cb_init != NULL)
                  if (CC_ALGO(tp)->cb_init(tp->ccv) > 0) {
                          if (tp->t_fb->tfb_tcp_fb_fini)
                                  (*tp->t_fb->tfb_tcp_fb_fini)(tp);
                          refcount_release(&tp->t_fb->tfb_refcnt);
                          uma_zfree(V_tcpcb_zone, tm);
                          return (NULL);
                  }
  
          tp->osd = &tm->osd;
          if (khelp_init_osd(HELPER_CLASS_TCP, tp->osd)) {
                  if (tp->t_fb->tfb_tcp_fb_fini)
                          (*tp->t_fb->tfb_tcp_fb_fini)(tp);
                  refcount_release(&tp->t_fb->tfb_refcnt);
                  uma_zfree(V_tcpcb_zone, tm);
                  return (NULL);
          }
  
  #ifdef VIMAGE
          tp->t_vnet = inp->inp_vnet;
  #endif
          tp->t_timers = &tm->tt;
          TAILQ_INIT(&tp->t_segq);
          tp->t_maxseg =
  #ifdef INET6
                  isipv6 ? V_tcp_v6mssdflt :
  #endif /* INET6 */
                  V_tcp_mssdflt;
  
          /* Set up our timeouts. */
          callout_init(&tp->t_timers->tt_rexmt, 1);
          callout_init(&tp->t_timers->tt_persist, 1);
          callout_init(&tp->t_timers->tt_keep, 1);
          callout_init(&tp->t_timers->tt_2msl, 1);
          callout_init(&tp->t_timers->tt_delack, 1);
  
          if (V_tcp_do_rfc1323)
                  tp->t_flags = (TF_REQ_SCALE|TF_REQ_TSTMP);
          if (V_tcp_do_sack)
                  tp->t_flags |= TF_SACK_PERMIT;
          TAILQ_INIT(&tp->snd_holes);
          /*
           * The tcpcb will hold a reference on its inpcb until tcp_discardcb()
           * is called.
           */
          in_pcbref(inp); /* Reference for tcpcb */
          tp->t_inpcb = inp;
  
          /*
           * 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_ssthresh = TCP_MAXWIN << TCP_MAX_WINSHIFT;
          tp->t_rcvtime = 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 = V_ip_defttl;
          inp->inp_ppcb = tp;
  #ifdef TCPPCAP
          /*
           * Init the TCP PCAP queues.
           */
          tcp_pcap_tcpcb_init(tp);
  #endif
          return (tp);            /* XXX */
  }
  
  /*
   * Switch the congestion control algorithm back to NewReno for any active
   * control blocks using an algorithm which is about to go away.
   * This ensures the CC framework can allow the unload to proceed without leaving
   * any dangling pointers which would trigger a panic.
   * Returning non-zero would inform the CC framework that something went wrong
   * and it would be unsafe to allow the unload to proceed. However, there is no
   * way for this to occur with this implementation so we always return zero.
   */
  int
  tcp_ccalgounload(struct cc_algo *unload_algo)
  {
          struct cc_algo *tmpalgo;
          struct inpcb *inp;
          struct tcpcb *tp;
          VNET_ITERATOR_DECL(vnet_iter);
  
          /*
           * Check all active control blocks across all network stacks and change
           * any that are using "unload_algo" back to NewReno. If "unload_algo"
           * requires cleanup code to be run, call it.
           */
          VNET_LIST_RLOCK();
          VNET_FOREACH(vnet_iter) {
                  CURVNET_SET(vnet_iter);
                  INP_INFO_WLOCK(&V_tcbinfo);
                  /*
                   * New connections already part way through being initialised
                   * with the CC algo we're removing will not race with this code
                   * because the INP_INFO_WLOCK is held during initialisation. We
                   * therefore don't enter the loop below until the connection
                   * list has stabilised.
                   */
                  LIST_FOREACH(inp, &V_tcb, inp_list) {
                          INP_WLOCK(inp);
                          /* Important to skip tcptw structs. */
                          if (!(inp->inp_flags & INP_TIMEWAIT) &&
                              (tp = intotcpcb(inp)) != NULL) {
                                  /*
                                   * By holding INP_WLOCK here, we are assured
                                   * that the connection is not currently
                                   * executing inside the CC module's functions
                                   * i.e. it is safe to make the switch back to
                                   * NewReno.
                                   */
                                  if (CC_ALGO(tp) == unload_algo) {
                                          tmpalgo = CC_ALGO(tp);
                                          /* NewReno does not require any init. */
                                          CC_ALGO(tp) = &newreno_cc_algo;
                                          if (tmpalgo->cb_destroy != NULL)
                                                  tmpalgo->cb_destroy(tp->ccv);
                                  }
                          }
                          INP_WUNLOCK(inp);
                  }
                  INP_INFO_WUNLOCK(&V_tcbinfo);
                  CURVNET_RESTORE();
          }
          VNET_LIST_RUNLOCK();
  
          return (0);
  }
  
  /*
   * Drop a TCP connection, reporting
   * the specified error.  If connection is synchronized,
   * then send a RST to peer.
   */
  struct tcpcb *
  tcp_drop(struct tcpcb *tp, int errno)
  {
          struct socket *so = tp->t_inpcb->inp_socket;
  
          INP_INFO_LOCK_ASSERT(&V_tcbinfo);
          INP_WLOCK_ASSERT(tp->t_inpcb);
  
          if (TCPS_HAVERCVDSYN(tp->t_state)) {
                  tcp_state_change(tp, TCPS_CLOSED);
                  (void) tp->t_fb->tfb_tcp_output(tp);
                  TCPSTAT_INC(tcps_drops);
          } else
                  TCPSTAT_INC(tcps_conndrops);
          if (errno == ETIMEDOUT && tp->t_softerror)
                  errno = tp->t_softerror;
          so->so_error = errno;
          return (tcp_close(tp));
  }
  
  void
  tcp_discardcb(struct tcpcb *tp)
  {
          struct inpcb *inp = tp->t_inpcb;
          struct socket *so = inp->inp_socket;
  #ifdef INET6
          int isipv6 = (inp->inp_vflag & INP_IPV6) != 0;
  #endif /* INET6 */
          int released;
  
          INP_WLOCK_ASSERT(inp);
  
          /*
           * Make sure that all of our timers are stopped before we delete the
           * PCB.
           *
           * If stopping a timer fails, we schedule a discard function in same
           * callout, and the last discard function called will take care of
           * deleting the tcpcb.
           */
          tp->t_timers->tt_draincnt = 0;
          tcp_timer_stop(tp, TT_REXMT);
          tcp_timer_stop(tp, TT_PERSIST);
          tcp_timer_stop(tp, TT_KEEP);
          tcp_timer_stop(tp, TT_2MSL);
          tcp_timer_stop(tp, TT_DELACK);
          if (tp->t_fb->tfb_tcp_timer_stop_all) {
                  /* 
                   * Call the stop-all function of the methods, 
                   * this function should call the tcp_timer_stop()
                   * method with each of the function specific timeouts.
                   * That stop will be called via the tfb_tcp_timer_stop()
                   * which should use the async drain function of the 
                   * callout system (see tcp_var.h).
                   */
                  tp->t_fb->tfb_tcp_timer_stop_all(tp);
          }
  
          /*
           * 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 occurred on a session.
                   *
                   * XXXRW: 'so' may be NULL here, and/or socket buffer may be
                   * being torn down.  Ideally this code would not use 'so'.
                   */
                  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;
                  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 */
          tcp_reass_flush(tp);
  
  #ifdef TCP_OFFLOAD
          /* Disconnect offload device, if any. */
          if (tp->t_flags & TF_TOE)
                  tcp_offload_detach(tp);
  #endif
                  
          tcp_free_sackholes(tp);
  
  #ifdef TCPPCAP
          /* Free the TCP PCAP queues. */
          tcp_pcap_drain(&(tp->t_inpkts));
          tcp_pcap_drain(&(tp->t_outpkts));
  #endif
  
          /* Allow the CC algorithm to clean up after itself. */
          if (CC_ALGO(tp)->cb_destroy != NULL)
                  CC_ALGO(tp)->cb_destroy(tp->ccv);
  
          khelp_destroy_osd(tp->osd);
  
          CC_ALGO(tp) = NULL;
          inp->inp_ppcb = NULL;
          if (tp->t_timers->tt_draincnt == 0) {
                  /* We own the last reference on tcpcb, let's free it. */
                  TCPSTATES_DEC(tp->t_state);
                  if (tp->t_fb->tfb_tcp_fb_fini)
                          (*tp->t_fb->tfb_tcp_fb_fini)(tp);
                  refcount_release(&tp->t_fb->tfb_refcnt);
                  tp->t_inpcb = NULL;
                  uma_zfree(V_tcpcb_zone, tp);
                  released = in_pcbrele_wlocked(inp);
                  KASSERT(!released, ("%s: inp %p should not have been released "
                          "here", __func__, inp));
          }
  }
  
  void
  tcp_timer_discard(void *ptp)
  {
          struct inpcb *inp;
          struct tcpcb *tp;
          
          tp = (struct tcpcb *)ptp;
          CURVNET_SET(tp->t_vnet);
          INP_INFO_RLOCK(&V_tcbinfo);
          inp = tp->t_inpcb;
          KASSERT(inp != NULL, ("%s: tp %p tp->t_inpcb == NULL",
                  __func__, tp));
          INP_WLOCK(inp);
          KASSERT((tp->t_timers->tt_flags & TT_STOPPED) != 0,
                  ("%s: tcpcb has to be stopped here", __func__));
          tp->t_timers->tt_draincnt--;
          if (tp->t_timers->tt_draincnt == 0) {
                  /* We own the last reference on this tcpcb, let's free it. */
                  TCPSTATES_DEC(tp->t_state);
                  if (tp->t_fb->tfb_tcp_fb_fini)
                          (*tp->t_fb->tfb_tcp_fb_fini)(tp);
                  refcount_release(&tp->t_fb->tfb_refcnt);
                  tp->t_inpcb = NULL;
                  uma_zfree(V_tcpcb_zone, tp);
                  if (in_pcbrele_wlocked(inp)) {
                          INP_INFO_RUNLOCK(&V_tcbinfo);
                          CURVNET_RESTORE();
                          return;
                  }
          }
          INP_WUNLOCK(inp);
          INP_INFO_RUNLOCK(&V_tcbinfo);
          CURVNET_RESTORE();
  }
  
  /*
   * Attempt to close a TCP control block, marking it as dropped, and freeing
   * the socket if we hold the only reference.
   */
  struct tcpcb *
  tcp_close(struct tcpcb *tp)
  {
          struct inpcb *inp = tp->t_inpcb;
          struct socket *so;
  
          INP_INFO_LOCK_ASSERT(&V_tcbinfo);
          INP_WLOCK_ASSERT(inp);
  
  #ifdef TCP_OFFLOAD
          if (tp->t_state == TCPS_LISTEN)
                  tcp_offload_listen_stop(tp);
  #endif
  #ifdef TCP_RFC7413
          /*
           * This releases the TFO pending counter resource for TFO listen
           * sockets as well as passively-created TFO sockets that transition
           * from SYN_RECEIVED to CLOSED.
           */
          if (tp->t_tfo_pending) {
                  tcp_fastopen_decrement_counter(tp->t_tfo_pending);
                  tp->t_tfo_pending = NULL;
          }
  #endif
          in_pcbdrop(inp);
          TCPSTAT_INC(tcps_closed);
          if (tp->t_state != TCPS_CLOSED)
                  tcp_state_change(tp, TCPS_CLOSED);
          KASSERT(inp->inp_socket != NULL, ("tcp_close: inp_socket NULL"));
          so = inp->inp_socket;
          soisdisconnected(so);
          if (inp->inp_flags & INP_SOCKREF) {
                  KASSERT(so->so_state & SS_PROTOREF,
                      ("tcp_close: !SS_PROTOREF"));
                  inp->inp_flags &= ~INP_SOCKREF;
                  INP_WUNLOCK(inp);
                  ACCEPT_LOCK();
                  SOCK_LOCK(so);
                  so->so_state &= ~SS_PROTOREF;
                  sofree(so);
                  return (NULL);
          }
          return (tp);
  }
  
  void
  tcp_drain(void)
  {
          VNET_ITERATOR_DECL(vnet_iter);
  
          if (!do_tcpdrain)
                  return;
  
          VNET_LIST_RLOCK_NOSLEEP();
          VNET_FOREACH(vnet_iter) {
                  CURVNET_SET(vnet_iter);
                  struct inpcb *inpb;
                  struct tcpcb *tcpb;
  
          /*
           * 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
           *      useful.
           */
                  INP_INFO_WLOCK(&V_tcbinfo);
                  LIST_FOREACH(inpb, V_tcbinfo.ipi_listhead, inp_list) {
                          if (inpb->inp_flags & INP_TIMEWAIT)
                                  continue;
                          INP_WLOCK(inpb);
                          if ((tcpb = intotcpcb(inpb)) != NULL) {
                                  tcp_reass_flush(tcpb);
                                  tcp_clean_sackreport(tcpb);
  #ifdef TCPPCAP
                                  if (tcp_pcap_aggressive_free) {
                                          /* Free the TCP PCAP queues. */
                                          tcp_pcap_drain(&(tcpb->t_inpkts));
                                          tcp_pcap_drain(&(tcpb->t_outpkts));
                                  }
  #endif
                          }
                          INP_WUNLOCK(inpb);
                  }
                  INP_INFO_WUNLOCK(&V_tcbinfo);
                  CURVNET_RESTORE();
          }
          VNET_LIST_RUNLOCK_NOSLEEP();
  }
  
  /*
   * 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(struct inpcb *inp, int error)
  {
          struct tcpcb *tp;
  
          INP_INFO_LOCK_ASSERT(&V_tcbinfo);
          INP_WLOCK_ASSERT(inp);
  
          if ((inp->inp_flags & INP_TIMEWAIT) ||
              (inp->inp_flags & INP_DROPPED))
                  return (inp);
  
          tp = intotcpcb(inp);
          KASSERT(tp != NULL, ("tcp_notify: tp == NULL"));
  
          /*
           * 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)) {
                  if (inp->inp_route.ro_rt) {
                          RTFREE(inp->inp_route.ro_rt);
                          inp->inp_route.ro_rt = (struct rtentry *)NULL;
                  }
                  return (inp);
          } else if (tp->t_state < TCPS_ESTABLISHED && tp->t_rxtshift > 3 &&
              tp->t_softerror) {
                  tp = tcp_drop(tp, error);
                  if (tp != NULL)
                          return (inp);
                  else
                          return (NULL);
          } 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, m, n, pcb_count;
          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 = V_tcbinfo.ipi_count +
                      counter_u64_fetch(V_tcps_states[TCPS_SYN_RECEIVED]);
                  n += imax(n / 8, 10);
                  req->oldidx = 2 * (sizeof xig) + n * sizeof(struct xtcpcb);
                  return (0);
          }
  
          if (req->newptr != NULL)
                  return (EPERM);
  
          /*
           * OK, now we're committed to doing something.
           */
          INP_LIST_RLOCK(&V_tcbinfo);
          gencnt = V_tcbinfo.ipi_gencnt;
          n = V_tcbinfo.ipi_count;
          INP_LIST_RUNLOCK(&V_tcbinfo);
  
          m = counter_u64_fetch(V_tcps_states[TCPS_SYN_RECEIVED]);
  
          error = sysctl_wire_old_buffer(req, 2 * (sizeof xig)
                  + (n + m) * sizeof(struct xtcpcb));
          if (error != 0)
                  return (error);
  
          bzero(&xig, sizeof(xig));
          xig.xig_len = sizeof xig;
          xig.xig_count = n + m;
          xig.xig_gen = gencnt;
          xig.xig_sogen = so_gencnt;
          error = SYSCTL_OUT(req, &xig, sizeof xig);
          if (error)
                  return (error);
  
          error = syncache_pcblist(req, m, &pcb_count);
          if (error)
                  return (error);
  
          inp_list = malloc(n * sizeof *inp_list, M_TEMP, M_WAITOK);
  
          INP_INFO_WLOCK(&V_tcbinfo);
          for (inp = LIST_FIRST(V_tcbinfo.ipi_listhead), i = 0;
              inp != NULL && i < n; inp = LIST_NEXT(inp, inp_list)) {
                  INP_WLOCK(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_flags & INP_TIMEWAIT) {
                                  if (intotw(inp) != NULL)
                                          error = cr_cansee(req->td->td_ucred,
                                              intotw(inp)->tw_cred);
                                  else
                                          error = EINVAL; /* Skip this inp. */
                          } else
                                  error = cr_canseeinpcb(req->td->td_ucred, inp);
                          if (error == 0) {
                                  in_pcbref(inp);
                                  inp_list[i++] = inp;
                          }
                  }
                  INP_WUNLOCK(inp);
          }
          INP_INFO_WUNLOCK(&V_tcbinfo);
          n = i;
  
          error = 0;
          for (i = 0; i < n; i++) {
                  inp = inp_list[i];
                  INP_RLOCK(inp);
                  if (inp->inp_gencnt <= gencnt) {
                          struct xtcpcb xt;
                          void *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_flags & 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 (xt.xt_tp.t_timers)
                                          tcp_timer_to_xtimer(&xt.xt_tp, xt.xt_tp.t_timers, &xt.xt_timer);
                          }
                          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;
                          INP_RUNLOCK(inp);
                          error = SYSCTL_OUT(req, &xt, sizeof xt);
                  } else
                          INP_RUNLOCK(inp);
          }
          INP_INFO_RLOCK(&V_tcbinfo);
          for (i = 0; i < n; i++) {
                  inp = inp_list[i];
                  INP_RLOCK(inp);
                  if (!in_pcbrele_rlocked(inp))
                          INP_RUNLOCK(inp);
          }
          INP_INFO_RUNLOCK(&V_tcbinfo);
  
          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.
                   */
                  INP_LIST_RLOCK(&V_tcbinfo);
                  xig.xig_gen = V_tcbinfo.ipi_gencnt;
                  xig.xig_sogen = so_gencnt;
                  xig.xig_count = V_tcbinfo.ipi_count + pcb_count;
                  INP_LIST_RUNLOCK(&V_tcbinfo);
                  error = SYSCTL_OUT(req, &xig, sizeof xig);
          }
          free(inp_list, M_TEMP);
          return (error);
  }
  
  SYSCTL_PROC(_net_inet_tcp, TCPCTL_PCBLIST, pcblist,
      CTLTYPE_OPAQUE | CTLFLAG_RD, NULL, 0,
      tcp_pcblist, "S,xtcpcb", "List of active TCP connections");
  
  #ifdef INET
  static int
  tcp_getcred(SYSCTL_HANDLER_ARGS)
  {
          struct xucred xuc;
          struct sockaddr_in addrs[2];
          struct inpcb *inp;
          int error;
  
          error = priv_check(req->td, PRIV_NETINET_GETCRED);
          if (error)
                  return (error);
          error = SYSCTL_IN(req, addrs, sizeof(addrs));
          if (error)
                  return (error);
          inp = in_pcblookup(&V_tcbinfo, addrs[1].sin_addr, addrs[1].sin_port,
              addrs[0].sin_addr, addrs[0].sin_port, INPLOOKUP_RLOCKPCB, NULL);
          if (inp != NULL) {
                  if (inp->inp_socket == NULL)
                          error = ENOENT;
                  if (error == 0)
                          error = cr_canseeinpcb(req->td->td_ucred, inp);
                  if (error == 0)
                          cru2x(inp->inp_cred, &xuc);
                  INP_RUNLOCK(inp);
          } else
                  error = ENOENT;
          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");
  #endif /* INET */
  
  #ifdef INET6
  static int
  tcp6_getcred(SYSCTL_HANDLER_ARGS)
  {
          struct xucred xuc;
          struct sockaddr_in6 addrs[2];
          struct inpcb *inp;
          int error;
  #ifdef INET
          int mapped = 0;
  #endif
  
          error = priv_check(req->td, PRIV_NETINET_GETCRED);
          if (error)
                  return (error);
          error = SYSCTL_IN(req, addrs, sizeof(addrs));
          if (error)
                  return (error);
          if ((error = sa6_embedscope(&addrs[0], V_ip6_use_defzone)) != 0 ||
              (error = sa6_embedscope(&addrs[1], V_ip6_use_defzone)) != 0) {
                  return (error);
          }
          if (IN6_IS_ADDR_V4MAPPED(&addrs[0].sin6_addr)) {
  #ifdef INET
                  if (IN6_IS_ADDR_V4MAPPED(&addrs[1].sin6_addr))
                          mapped = 1;
                  else
  #endif
                          return (EINVAL);
          }
  
  #ifdef INET
          if (mapped == 1)
                  inp = in_pcblookup(&V_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, INPLOOKUP_RLOCKPCB, NULL);
          else
  #endif
                  inp = in6_pcblookup(&V_tcbinfo,
                          &addrs[1].sin6_addr, addrs[1].sin6_port,
                          &addrs[0].sin6_addr, addrs[0].sin6_port,
                          INPLOOKUP_RLOCKPCB, NULL);
          if (inp != NULL) {
                  if (inp->inp_socket == NULL)
                          error = ENOENT;
                  if (error == 0)
                          error = cr_canseeinpcb(req->td->td_ucred, inp);
                  if (error == 0)
                          cru2x(inp->inp_cred, &xuc);
                  INP_RUNLOCK(inp);
          } else
                  error = ENOENT;
          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 /* INET6 */
  
  
  #ifdef INET
  void
  tcp_ctlinput(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;
          struct icmp *icp;
          struct in_conninfo inc;
          tcp_seq icmp_tcp_seq;
          int mtu;
  
          faddr = ((struct sockaddr_in *)sa)->sin_addr;
          if (sa->sa_family != AF_INET || faddr.s_addr == INADDR_ANY)
                  return;
  
          if (cmd == PRC_MSGSIZE)
                  notify = tcp_mtudisc_notify;
          else if (V_icmp_may_rst && (cmd == PRC_UNREACH_ADMIN_PROHIB ||
                  cmd == PRC_UNREACH_PORT || cmd == PRC_UNREACH_PROTOCOL || 
                  cmd == PRC_TIMXCEED_INTRANS) && ip)
                  notify = tcp_drop_syn_sent;
  
          /*
           * 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) {
                  in_pcbnotifyall(&V_tcbinfo, faddr, inetctlerrmap[cmd], notify);
                  return;
          }
  
          icp = (struct icmp *)((caddr_t)ip - offsetof(struct icmp, icmp_ip));
          th = (struct tcphdr *)((caddr_t)ip + (ip->ip_hl << 2));
          INP_INFO_RLOCK(&V_tcbinfo);
          inp = in_pcblookup(&V_tcbinfo, faddr, th->th_dport, ip->ip_src,
              th->th_sport, INPLOOKUP_WLOCKPCB, NULL);
          if (inp != NULL && PRC_IS_REDIRECT(cmd)) {
                  /* signal EHOSTDOWN, as it flushes the cached route */
                  inp = (*notify)(inp, EHOSTDOWN);
                  goto out;
          }
          icmp_tcp_seq = th->th_seq;
          if (inp != NULL)  {
                  if (!(inp->inp_flags & INP_TIMEWAIT) &&
                      !(inp->inp_flags & INP_DROPPED) &&
                      !(inp->inp_socket == NULL)) {
                          tp = intotcpcb(inp);
                          if (SEQ_GEQ(ntohl(icmp_tcp_seq), tp->snd_una) &&
                              SEQ_LT(ntohl(icmp_tcp_seq), tp->snd_max)) {
                                  if (cmd == PRC_MSGSIZE) {
                                          /*
                                           * MTU discovery:
                                           * If we got a needfrag set the MTU
                                           * in the route to the suggested new
                                           * value (if given) and then notify.
                                           */
                                          mtu = ntohs(icp->icmp_nextmtu);
                                          /*
                                           * If no alternative MTU was
                                           * proposed, try the next smaller
                                           * one.
                                           */
                                          if (!mtu)
                                                  mtu = ip_next_mtu(
                                                      ntohs(ip->ip_len), 1);
                                          if (mtu < V_tcp_minmss +
                                              sizeof(struct tcpiphdr))
                                                  mtu = V_tcp_minmss +
                                                      sizeof(struct tcpiphdr);
                                          /*
                                           * Only process the offered MTU if it
                                           * is smaller than the current one.
                                           */
                                          if (mtu < tp->t_maxseg +
                                              sizeof(struct tcpiphdr)) {
                                                  bzero(&inc, sizeof(inc));
                                                  inc.inc_faddr = faddr;
                                                  inc.inc_fibnum =
                                                      inp->inp_inc.inc_fibnum;
                                                  tcp_hc_updatemtu(&inc, mtu);
                                                  tcp_mtudisc(inp, mtu);
                                          }
                                  } else
                                          inp = (*notify)(inp,
                                              inetctlerrmap[cmd]);
                          }
                  }
          } else {
                  bzero(&inc, sizeof(inc));
                  inc.inc_fport = th->th_dport;
                  inc.inc_lport = th->th_sport;
                  inc.inc_faddr = faddr;
                  inc.inc_laddr = ip->ip_src;
                  syncache_unreach(&inc, icmp_tcp_seq);
          }
  out:
          if (inp != NULL)
                  INP_WUNLOCK(inp);
          INP_INFO_RUNLOCK(&V_tcbinfo);
  }
  #endif /* INET */
  
  #ifdef INET6
  void
  tcp6_ctlinput(int cmd, struct sockaddr *sa, void *d)
  {
          struct in6_addr *dst;
          struct inpcb *(*notify)(struct inpcb *, int) = tcp_notify;
          struct ip6_hdr *ip6;
          struct mbuf *m;
          struct inpcb *inp;
          struct tcpcb *tp;
          struct icmp6_hdr *icmp6;
          struct ip6ctlparam *ip6cp = NULL;
          const struct sockaddr_in6 *sa6_src = NULL;
          struct in_conninfo inc;
          struct tcp_ports {
                  uint16_t th_sport;
                  uint16_t th_dport;
          } t_ports;
          tcp_seq icmp_tcp_seq;
          unsigned int mtu;
          unsigned int off;
  
          if (sa->sa_family != AF_INET6 ||
              sa->sa_len != sizeof(struct sockaddr_in6))
                  return;
  
          /* if the parameter is from icmp6, decode it. */
          if (d != NULL) {
                  ip6cp = (struct ip6ctlparam *)d;
                  icmp6 = ip6cp->ip6c_icmp6;
                  m = ip6cp->ip6c_m;
                  ip6 = ip6cp->ip6c_ip6;
                  off = ip6cp->ip6c_off;
                  sa6_src = ip6cp->ip6c_src;
                  dst = ip6cp->ip6c_finaldst;
          } else {
                  m = NULL;
                  ip6 = NULL;
                  off = 0;        /* fool gcc */
                  sa6_src = &sa6_any;
                  dst = NULL;
          }
  
          if (cmd == PRC_MSGSIZE)
                  notify = tcp_mtudisc_notify;
          else if (V_icmp_may_rst && (cmd == PRC_UNREACH_ADMIN_PROHIB ||
                  cmd == PRC_UNREACH_PORT || cmd == PRC_UNREACH_PROTOCOL || 
                  cmd == PRC_TIMXCEED_INTRANS) && ip6 != NULL)
                  notify = tcp_drop_syn_sent;
  
          /*
           * 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)
                  ip6 = NULL;
          else if ((unsigned)cmd >= PRC_NCMDS || inet6ctlerrmap[cmd] == 0)
                  return;
  
          if (ip6 == NULL) {
                  in6_pcbnotify(&V_tcbinfo, sa, 0,
                                (const struct sockaddr *)sa6_src,
                                0, cmd, NULL, notify);
                  return;
          }
  
          /* Check if we can safely get the ports from the tcp hdr */
          if (m == NULL ||
              (m->m_pkthdr.len <
                  (int32_t) (off + sizeof(struct tcp_ports)))) {
                  return;
          }
          bzero(&t_ports, sizeof(struct tcp_ports));
          m_copydata(m, off, sizeof(struct tcp_ports), (caddr_t)&t_ports);
          INP_INFO_RLOCK(&V_tcbinfo);
          inp = in6_pcblookup(&V_tcbinfo, &ip6->ip6_dst, t_ports.th_dport,
              &ip6->ip6_src, t_ports.th_sport, INPLOOKUP_WLOCKPCB, NULL);
          if (inp != NULL && PRC_IS_REDIRECT(cmd)) {
                  /* signal EHOSTDOWN, as it flushes the cached route */
                  inp = (*notify)(inp, EHOSTDOWN);
                  goto out;
          }
          off += sizeof(struct tcp_ports);
          if (m->m_pkthdr.len < (int32_t) (off + sizeof(tcp_seq))) {
                  goto out;
          }
          m_copydata(m, off, sizeof(tcp_seq), (caddr_t)&icmp_tcp_seq);
          if (inp != NULL)  {
                  if (!(inp->inp_flags & INP_TIMEWAIT) &&
                      !(inp->inp_flags & INP_DROPPED) &&
                      !(inp->inp_socket == NULL)) {
                          tp = intotcpcb(inp);
                          if (SEQ_GEQ(ntohl(icmp_tcp_seq), tp->snd_una) &&
                              SEQ_LT(ntohl(icmp_tcp_seq), tp->snd_max)) {
                                  if (cmd == PRC_MSGSIZE) {
                                          /*
                                           * MTU discovery:
                                           * If we got a needfrag set the MTU
                                           * in the route to the suggested new
                                           * value (if given) and then notify.
                                           */
                                          mtu = ntohl(icmp6->icmp6_mtu);
                                          /*
                                           * If no alternative MTU was
                                           * proposed, or the proposed
                                           * MTU was too small, set to
                                           * the min.
                                           */
                                          if (mtu < IPV6_MMTU)
                                                  mtu = IPV6_MMTU - 8;
                                          bzero(&inc, sizeof(inc));
                                          inc.inc_fibnum = M_GETFIB(m);
                                          inc.inc_flags |= INC_ISIPV6;
                                          inc.inc6_faddr = *dst;
                                          if (in6_setscope(&inc.inc6_faddr,
                                                  m->m_pkthdr.rcvif, NULL))
                                                  goto out;
                                          /*
                                           * Only process the offered MTU if it
                                           * is smaller than the current one.
                                           */
                                          if (mtu < tp->t_maxseg +
                                              sizeof (struct tcphdr) +
                                              sizeof (struct ip6_hdr)) {
                                                  tcp_hc_updatemtu(&inc, mtu);
                                                  tcp_mtudisc(inp, mtu);
                                                  ICMP6STAT_INC(icp6s_pmtuchg);
                                          }
                                  } else
                                          inp = (*notify)(inp,
                                              inet6ctlerrmap[cmd]);
                          }
                  }
          } else {
                  bzero(&inc, sizeof(inc));
                  inc.inc_fibnum = M_GETFIB(m);
                  inc.inc_flags |= INC_ISIPV6;
                  inc.inc_fport = t_ports.th_dport;
                  inc.inc_lport = t_ports.th_sport;
                  inc.inc6_faddr = *dst;
                  inc.inc6_laddr = ip6->ip6_src;
                  syncache_unreach(&inc, icmp_tcp_seq);
          }
  out:
          if (inp != NULL)
                  INP_WUNLOCK(inp);
          INP_INFO_RUNLOCK(&V_tcbinfo);
  }
  #endif /* INET6 */
  
  static uint32_t
  tcp_keyed_hash(struct in_conninfo *inc, u_char *key)
  {
          MD5_CTX ctx;
          uint32_t hash[4];
  
          MD5Init(&ctx);
          MD5Update(&ctx, &inc->inc_fport, sizeof(uint16_t));
          MD5Update(&ctx, &inc->inc_lport, sizeof(uint16_t));
          switch (inc->inc_flags & INC_ISIPV6) {
  #ifdef INET
          case 0:
                  MD5Update(&ctx, &inc->inc_faddr, sizeof(struct in_addr));
                  MD5Update(&ctx, &inc->inc_laddr, sizeof(struct in_addr));
                  break;
  #endif
  #ifdef INET6
          case INC_ISIPV6:
                  MD5Update(&ctx, &inc->inc6_faddr, sizeof(struct in6_addr));
                  MD5Update(&ctx, &inc->inc6_laddr, sizeof(struct in6_addr));
                  break;
  #endif
          }
          MD5Update(&ctx, key, 32);
          MD5Final((unsigned char *)hash, &ctx);
  
          return (hash[0]);
  }
  
  uint32_t
  tcp_new_ts_offset(struct in_conninfo *inc)
  {
          return (tcp_keyed_hash(inc, V_ts_offset_secret));
  }
  
  /*
   * 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.
   *
   * Locking of the global variables isn_secret, isn_last_reseed, isn_offset,
   * isn_offset_old, and isn_ctx is performed using the ISN lock.  In
   * general, this means holding an exclusive (write) lock.
   */
  
  #define ISN_BYTES_PER_SECOND 1048576
  #define ISN_STATIC_INCREMENT 4096
  #define ISN_RANDOM_INCREMENT (4096 - 1)
  
  static VNET_DEFINE(u_char, isn_secret[32]);
  static VNET_DEFINE(int, isn_last);
  static VNET_DEFINE(int, isn_last_reseed);
  static VNET_DEFINE(u_int32_t, isn_offset);
  static VNET_DEFINE(u_int32_t, isn_offset_old);
  
  #define V_isn_secret                    VNET(isn_secret)
  #define V_isn_last                      VNET(isn_last)
  #define V_isn_last_reseed               VNET(isn_last_reseed)
  #define V_isn_offset                    VNET(isn_offset)
  #define V_isn_offset_old                VNET(isn_offset_old)
  
  tcp_seq
  tcp_new_isn(struct in_conninfo *inc)
  {
          tcp_seq new_isn;
          u_int32_t projected_offset;
  
          ISN_LOCK();
          /* Seed if this is the first use, reseed if requested. */
          if ((V_isn_last_reseed == 0) || ((V_tcp_isn_reseed_interval > 0) &&
               (((u_int)V_isn_last_reseed + (u_int)V_tcp_isn_reseed_interval*hz)
                  < (u_int)ticks))) {
                  read_random(&V_isn_secret, sizeof(V_isn_secret));
                  V_isn_last_reseed = ticks;
          }
  
          /* Compute the md5 hash and return the ISN. */
          new_isn = (tcp_seq)tcp_keyed_hash(inc, V_isn_secret);
          V_isn_offset += ISN_STATIC_INCREMENT +
                  (arc4random() & ISN_RANDOM_INCREMENT);
          if (ticks != V_isn_last) {
                  projected_offset = V_isn_offset_old +
                      ISN_BYTES_PER_SECOND / hz * (ticks - V_isn_last);
                  if (SEQ_GT(projected_offset, V_isn_offset))
                          V_isn_offset = projected_offset;
                  V_isn_offset_old = V_isn_offset;
                  V_isn_last = ticks;
          }
          new_isn += V_isn_offset;
          ISN_UNLOCK();
          return (new_isn);
  }
  
  /*
   * 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(struct inpcb *inp, int errno)
  {
          struct tcpcb *tp;
  
          INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
          INP_WLOCK_ASSERT(inp);
  
          if ((inp->inp_flags & INP_TIMEWAIT) ||
              (inp->inp_flags & INP_DROPPED))
                  return (inp);
  
          tp = intotcpcb(inp);
          if (tp->t_state != TCPS_SYN_SENT)
                  return (inp);
  
          tp = tcp_drop(tp, errno);
          if (tp != NULL)
                  return (inp);
          else
                  return (NULL);
  }
  
  /*
   * When `need fragmentation' ICMP is received, update our idea of the MSS
   * based on the new value. 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.
   */
  static struct inpcb *
  tcp_mtudisc_notify(struct inpcb *inp, int error)
  {
  
          tcp_mtudisc(inp, -1);
          return (inp);
  }
  
  static void
  tcp_mtudisc(struct inpcb *inp, int mtuoffer)
  {
          struct tcpcb *tp;
          struct socket *so;
  
          INP_WLOCK_ASSERT(inp);
          if ((inp->inp_flags & INP_TIMEWAIT) ||
              (inp->inp_flags & INP_DROPPED))
                  return;
  
          tp = intotcpcb(inp);
          KASSERT(tp != NULL, ("tcp_mtudisc: tp == NULL"));
  
          tcp_mss_update(tp, -1, mtuoffer, NULL, NULL);
    
          so = inp->inp_socket;
          SOCKBUF_LOCK(&so->so_snd);
          /* If the mss is larger than the socket buffer, decrease the mss. */
          if (so->so_snd.sb_hiwat < tp->t_maxseg)
                  tp->t_maxseg = so->so_snd.sb_hiwat;
          SOCKBUF_UNLOCK(&so->so_snd);
  
          TCPSTAT_INC(tcps_mturesent);
          tp->t_rtttime = 0;
          tp->snd_nxt = tp->snd_una;
          tcp_free_sackholes(tp);
          tp->snd_recover = tp->snd_max;
          if (tp->t_flags & TF_SACK_PERMIT)
                  EXIT_FASTRECOVERY(tp->t_flags);
          tp->t_fb->tfb_tcp_output(tp);
  }
  
  #ifdef INET
  /*
   * Look-up the routing entry to the peer of this inpcb.  If no route
   * is found and it cannot be allocated, then return 0.  This routine
   * is called by TCP routines that access the rmx structure and by
   * tcp_mss_update to get the peer/interface MTU.
   */
  u_long
  tcp_maxmtu(struct in_conninfo *inc, struct tcp_ifcap *cap)
  {
          struct nhop4_extended nh4;
          struct ifnet *ifp;
          u_long maxmtu = 0;
  
          KASSERT(inc != NULL, ("tcp_maxmtu with NULL in_conninfo pointer"));
  
          if (inc->inc_faddr.s_addr != INADDR_ANY) {
  
                  if (fib4_lookup_nh_ext(inc->inc_fibnum, inc->inc_faddr,
                      NHR_REF, 0, &nh4) != 0)
                          return (0);
  
                  ifp = nh4.nh_ifp;
                  maxmtu = nh4.nh_mtu;
  
                  /* Report additional interface capabilities. */
                  if (cap != NULL) {
                          if (ifp->if_capenable & IFCAP_TSO4 &&
                              ifp->if_hwassist & CSUM_TSO) {
                                  cap->ifcap |= CSUM_TSO;
                                  cap->tsomax = ifp->if_hw_tsomax;
                                  cap->tsomaxsegcount = ifp->if_hw_tsomaxsegcount;
                                  cap->tsomaxsegsize = ifp->if_hw_tsomaxsegsize;
                          }
                  }
                  fib4_free_nh_ext(inc->inc_fibnum, &nh4);
          }
          return (maxmtu);
  }
  #endif /* INET */
  
  #ifdef INET6
  u_long
  tcp_maxmtu6(struct in_conninfo *inc, struct tcp_ifcap *cap)
  {
          struct nhop6_extended nh6;
          struct in6_addr dst6;
          uint32_t scopeid;
          struct ifnet *ifp;
          u_long maxmtu = 0;
  
          KASSERT(inc != NULL, ("tcp_maxmtu6 with NULL in_conninfo pointer"));
  
          if (inc->inc_flags & INC_IPV6MINMTU)
                  return (IPV6_MMTU);
  
          if (!IN6_IS_ADDR_UNSPECIFIED(&inc->inc6_faddr)) {
                  in6_splitscope(&inc->inc6_faddr, &dst6, &scopeid);
                  if (fib6_lookup_nh_ext(inc->inc_fibnum, &dst6, scopeid, 0,
                      0, &nh6) != 0)
                          return (0);
  
                  ifp = nh6.nh_ifp;
                  maxmtu = nh6.nh_mtu;
  
                  /* Report additional interface capabilities. */
                  if (cap != NULL) {
                          if (ifp->if_capenable & IFCAP_TSO6 &&
                              ifp->if_hwassist & CSUM_TSO) {
                                  cap->ifcap |= CSUM_TSO;
                                  cap->tsomax = ifp->if_hw_tsomax;
                                  cap->tsomaxsegcount = ifp->if_hw_tsomaxsegcount;
                                  cap->tsomaxsegsize = ifp->if_hw_tsomaxsegsize;
                          }
                  }
                  fib6_free_nh_ext(inc->inc_fibnum, &nh6);
          }
  
          return (maxmtu);
  }
  #endif /* INET6 */
  
  /*
   * Calculate effective SMSS per RFC5681 definition for a given TCP
   * connection at its current state, taking into account SACK and etc.
   */
  u_int
  tcp_maxseg(const struct tcpcb *tp)
  {
          u_int optlen;
  
          if (tp->t_flags & TF_NOOPT)
                  return (tp->t_maxseg);
  
          /*
           * Here we have a simplified code from tcp_addoptions(),
           * without a proper loop, and having most of paddings hardcoded.
           * We might make mistakes with padding here in some edge cases,
           * but this is harmless, since result of tcp_maxseg() is used
           * only in cwnd and ssthresh estimations.
           */
  #define PAD(len)        ((((len) / 4) + !!((len) % 4)) * 4)
          if (TCPS_HAVEESTABLISHED(tp->t_state)) {
                  if (tp->t_flags & TF_RCVD_TSTMP)
                          optlen = TCPOLEN_TSTAMP_APPA;
                  else
                          optlen = 0;
  #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
                  if (tp->t_flags & TF_SIGNATURE)
                          optlen += PAD(TCPOLEN_SIGNATURE);
  #endif
                  if ((tp->t_flags & TF_SACK_PERMIT) && tp->rcv_numsacks > 0) {
                          optlen += TCPOLEN_SACKHDR;
                          optlen += tp->rcv_numsacks * TCPOLEN_SACK;
                          optlen = PAD(optlen);
                  }
          } else {
                  if (tp->t_flags & TF_REQ_TSTMP)
                          optlen = TCPOLEN_TSTAMP_APPA;
                  else
                          optlen = PAD(TCPOLEN_MAXSEG);
                  if (tp->t_flags & TF_REQ_SCALE)
                          optlen += PAD(TCPOLEN_WINDOW);
  #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
                  if (tp->t_flags & TF_SIGNATURE)
                          optlen += PAD(TCPOLEN_SIGNATURE);
  #endif
                  if (tp->t_flags & TF_SACK_PERMIT)
                          optlen += PAD(TCPOLEN_SACK_PERMITTED);
          }
  #undef PAD
          optlen = min(optlen, TCP_MAXOLEN);
          return (tp->t_maxseg - optlen);
  }
  
  static int
  sysctl_drop(SYSCTL_HANDLER_ARGS)
  {
          /* addrs[0] is a foreign socket, addrs[1] is a local one. */
          struct sockaddr_storage addrs[2];
          struct inpcb *inp;
          struct tcpcb *tp;
          struct tcptw *tw;
          struct sockaddr_in *fin, *lin;
  #ifdef INET6
          struct sockaddr_in6 *fin6, *lin6;
  #endif
          int error;
  
          inp = NULL;
          fin = lin = NULL;
  #ifdef INET6
          fin6 = lin6 = NULL;
  #endif
          error = 0;
  
          if (req->oldptr != NULL || req->oldlen != 0)
                  return (EINVAL);
          if (req->newptr == NULL)
                  return (EPERM);
          if (req->newlen < sizeof(addrs))
                  return (ENOMEM);
          error = SYSCTL_IN(req, &addrs, sizeof(addrs));
          if (error)
                  return (error);
  
          switch (addrs[0].ss_family) {
  #ifdef INET6
          case AF_INET6:
                  fin6 = (struct sockaddr_in6 *)&addrs[0];
                  lin6 = (struct sockaddr_in6 *)&addrs[1];
                  if (fin6->sin6_len != sizeof(struct sockaddr_in6) ||
                      lin6->sin6_len != sizeof(struct sockaddr_in6))
                          return (EINVAL);
                  if (IN6_IS_ADDR_V4MAPPED(&fin6->sin6_addr)) {
                          if (!IN6_IS_ADDR_V4MAPPED(&lin6->sin6_addr))
                                  return (EINVAL);
                          in6_sin6_2_sin_in_sock((struct sockaddr *)&addrs[0]);
                          in6_sin6_2_sin_in_sock((struct sockaddr *)&addrs[1]);
                          fin = (struct sockaddr_in *)&addrs[0];
                          lin = (struct sockaddr_in *)&addrs[1];
                          break;
                  }
                  error = sa6_embedscope(fin6, V_ip6_use_defzone);
                  if (error)
                          return (error);
                  error = sa6_embedscope(lin6, V_ip6_use_defzone);
                  if (error)
                          return (error);
                  break;
  #endif
  #ifdef INET
          case AF_INET:
                  fin = (struct sockaddr_in *)&addrs[0];
                  lin = (struct sockaddr_in *)&addrs[1];
                  if (fin->sin_len != sizeof(struct sockaddr_in) ||
                      lin->sin_len != sizeof(struct sockaddr_in))
                          return (EINVAL);
                  break;
  #endif
          default:
                  return (EINVAL);
          }
          INP_INFO_RLOCK(&V_tcbinfo);
          switch (addrs[0].ss_family) {
  #ifdef INET6
          case AF_INET6:
                  inp = in6_pcblookup(&V_tcbinfo, &fin6->sin6_addr,
                      fin6->sin6_port, &lin6->sin6_addr, lin6->sin6_port,
                      INPLOOKUP_WLOCKPCB, NULL);
                  break;
  #endif
  #ifdef INET
          case AF_INET:
                  inp = in_pcblookup(&V_tcbinfo, fin->sin_addr, fin->sin_port,
                      lin->sin_addr, lin->sin_port, INPLOOKUP_WLOCKPCB, NULL);
                  break;
  #endif
          }
          if (inp != NULL) {
                  if (inp->inp_flags & INP_TIMEWAIT) {
                          /*
                           * XXXRW: There currently exists a state where an
                           * inpcb is present, but its timewait state has been
                           * discarded.  For now, don't allow dropping of this
                           * type of inpcb.
                           */
                          tw = intotw(inp);
                          if (tw != NULL)
                                  tcp_twclose(tw, 0);
                          else
                                  INP_WUNLOCK(inp);
                  } else if (!(inp->inp_flags & INP_DROPPED) &&
                             !(inp->inp_socket->so_options & SO_ACCEPTCONN)) {
                          tp = intotcpcb(inp);
                          tp = tcp_drop(tp, ECONNABORTED);
                          if (tp != NULL)
                                  INP_WUNLOCK(inp);
                  } else
                          INP_WUNLOCK(inp);
          } else
                  error = ESRCH;
          INP_INFO_RUNLOCK(&V_tcbinfo);
          return (error);
  }
  
  SYSCTL_PROC(_net_inet_tcp, TCPCTL_DROP, drop,
      CTLFLAG_VNET | CTLTYPE_STRUCT | CTLFLAG_WR | CTLFLAG_SKIP, NULL,
      0, sysctl_drop, "", "Drop TCP connection");
  
  /*
   * Generate a standardized TCP log line for use throughout the
   * tcp subsystem.  Memory allocation is done with M_NOWAIT to
   * allow use in the interrupt context.
   *
   * NB: The caller MUST free(s, M_TCPLOG) the returned string.
   * NB: The function may return NULL if memory allocation failed.
   *
   * Due to header inclusion and ordering limitations the struct ip
   * and ip6_hdr pointers have to be passed as void pointers.
   */
  char *
  tcp_log_vain(struct in_conninfo *inc, struct tcphdr *th, void *ip4hdr,
      const void *ip6hdr)
  {
  
          /* Is logging enabled? */
          if (tcp_log_in_vain == 0)
                  return (NULL);
  
          return (tcp_log_addr(inc, th, ip4hdr, ip6hdr));
  }
  
  char *
  tcp_log_addrs(struct in_conninfo *inc, struct tcphdr *th, void *ip4hdr,
      const void *ip6hdr)
  {
  
          /* Is logging enabled? */
          if (tcp_log_debug == 0)
                  return (NULL);
  
          return (tcp_log_addr(inc, th, ip4hdr, ip6hdr));
  }
  
  static char *
  tcp_log_addr(struct in_conninfo *inc, struct tcphdr *th, void *ip4hdr,
      const void *ip6hdr)
  {
          char *s, *sp;
          size_t size;
          struct ip *ip;
  #ifdef INET6
          const struct ip6_hdr *ip6;
  
          ip6 = (const struct ip6_hdr *)ip6hdr;
  #endif /* INET6 */
          ip = (struct ip *)ip4hdr;
  
          /*
           * The log line looks like this:
           * "TCP: [1.2.3.4]:50332 to [1.2.3.4]:80 tcpflags 0x2<SYN>"
           */
          size = sizeof("TCP: []:12345 to []:12345 tcpflags 0x2<>") +
              sizeof(PRINT_TH_FLAGS) + 1 +
  #ifdef INET6
              2 * INET6_ADDRSTRLEN;
  #else
              2 * INET_ADDRSTRLEN;
  #endif /* INET6 */
  
          s = malloc(size, M_TCPLOG, M_ZERO|M_NOWAIT);
          if (s == NULL)
                  return (NULL);
  
          strcat(s, "TCP: [");
          sp = s + strlen(s);
  
          if (inc && ((inc->inc_flags & INC_ISIPV6) == 0)) {
                  inet_ntoa_r(inc->inc_faddr, sp);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i to [", ntohs(inc->inc_fport));
                  sp = s + strlen(s);
                  inet_ntoa_r(inc->inc_laddr, sp);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i", ntohs(inc->inc_lport));
  #ifdef INET6
          } else if (inc) {
                  ip6_sprintf(sp, &inc->inc6_faddr);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i to [", ntohs(inc->inc_fport));
                  sp = s + strlen(s);
                  ip6_sprintf(sp, &inc->inc6_laddr);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i", ntohs(inc->inc_lport));
          } else if (ip6 && th) {
                  ip6_sprintf(sp, &ip6->ip6_src);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i to [", ntohs(th->th_sport));
                  sp = s + strlen(s);
                  ip6_sprintf(sp, &ip6->ip6_dst);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i", ntohs(th->th_dport));
  #endif /* INET6 */
  #ifdef INET
          } else if (ip && th) {
                  inet_ntoa_r(ip->ip_src, sp);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i to [", ntohs(th->th_sport));
                  sp = s + strlen(s);
                  inet_ntoa_r(ip->ip_dst, sp);
                  sp = s + strlen(s);
                  sprintf(sp, "]:%i", ntohs(th->th_dport));
  #endif /* INET */
          } else {
                  free(s, M_TCPLOG);
                  return (NULL);
          }
          sp = s + strlen(s);
          if (th)
                  sprintf(sp, " tcpflags 0x%b", th->th_flags, PRINT_TH_FLAGS);
          if (*(s + size - 1) != '\0')
                  panic("%s: string too long", __func__);
          return (s);
  }
  
  /*
   * A subroutine which makes it easy to track TCP state changes with DTrace.
   * This function shouldn't be called for t_state initializations that don't
   * correspond to actual TCP state transitions.
   */
  void
  tcp_state_change(struct tcpcb *tp, int newstate)
  {
  #if defined(KDTRACE_HOOKS)
          int pstate = tp->t_state;
  #endif
  
          TCPSTATES_DEC(tp->t_state);
          TCPSTATES_INC(newstate);
          tp->t_state = newstate;
          TCP_PROBE6(state__change, NULL, tp, NULL, tp, NULL, pstate);
  }

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