FreeBSD/Linux Kernel Cross Reference
sys/net/ipv4/tcp_output.c

     /*
      * INET         An implementation of the TCP/IP protocol suite for the LINUX
      *              operating system.  INET is implemented using the  BSD Socket
      *              interface as the means of communication with the user level.
      *
      *              Implementation of the Transmission Control Protocol(TCP).
      *
      * Authors:     Ross Biro
      *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
     *              Mark Evans, <evansmp@uhura.aston.ac.uk>
     *              Corey Minyard <wf-rch!minyard@relay.EU.net>
     *              Florian La Roche, <flla@stud.uni-sb.de>
     *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
     *              Linus Torvalds, <torvalds@cs.helsinki.fi>
     *              Alan Cox, <gw4pts@gw4pts.ampr.org>
     *              Matthew Dillon, <dillon@apollo.west.oic.com>
     *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
     *              Jorge Cwik, <jorge@laser.satlink.net>
     */
    
    /*
     * Changes:     Pedro Roque     :       Retransmit queue handled by TCP.
     *                              :       Fragmentation on mtu decrease
     *                              :       Segment collapse on retransmit
     *                              :       AF independence
     *
     *              Linus Torvalds  :       send_delayed_ack
     *              David S. Miller :       Charge memory using the right skb
     *                                      during syn/ack processing.
     *              David S. Miller :       Output engine completely rewritten.
     *              Andrea Arcangeli:       SYNACK carry ts_recent in tsecr.
     *              Cacophonix Gaul :       draft-minshall-nagle-01
     *              J Hadi Salim    :       ECN support
     *
     */
    
    #define pr_fmt(fmt) "TCP: " fmt
    
    #include <net/tcp.h>
    
    #include <linux/compiler.h>
    #include <linux/gfp.h>
    #include <linux/module.h>
    
    /* People can turn this off for buggy TCP's found in printers etc. */
    int sysctl_tcp_retrans_collapse __read_mostly = 1;
    
    /* People can turn this on to work with those rare, broken TCPs that
     * interpret the window field as a signed quantity.
     */
    int sysctl_tcp_workaround_signed_windows __read_mostly = 0;
    
    /* Default TSQ limit of two TSO segments */
    int sysctl_tcp_limit_output_bytes __read_mostly = 131072;
    
    /* This limits the percentage of the congestion window which we
     * will allow a single TSO frame to consume.  Building TSO frames
     * which are too large can cause TCP streams to be bursty.
     */
    int sysctl_tcp_tso_win_divisor __read_mostly = 3;
    
    int sysctl_tcp_mtu_probing __read_mostly = 0;
    int sysctl_tcp_base_mss __read_mostly = TCP_BASE_MSS;
    
    /* By default, RFC2861 behavior.  */
    int sysctl_tcp_slow_start_after_idle __read_mostly = 1;
    
    int sysctl_tcp_cookie_size __read_mostly = 0; /* TCP_COOKIE_MAX */
    EXPORT_SYMBOL_GPL(sysctl_tcp_cookie_size);
    
    static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                               int push_one, gfp_t gfp);
    
    /* Account for new data that has been sent to the network. */
    static void tcp_event_new_data_sent(struct sock *sk, const struct sk_buff *skb)
    {
            struct tcp_sock *tp = tcp_sk(sk);
            unsigned int prior_packets = tp->packets_out;
    
            tcp_advance_send_head(sk, skb);
            tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
    
            /* Don't override Nagle indefinitely with F-RTO */
            if (tp->frto_counter == 2)
                    tp->frto_counter = 3;
    
            tp->packets_out += tcp_skb_pcount(skb);
            if (!prior_packets || tp->early_retrans_delayed)
                    tcp_rearm_rto(sk);
    }
    
    /* SND.NXT, if window was not shrunk.
     * If window has been shrunk, what should we make? It is not clear at all.
     * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
     * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
     * invalid. OK, let's make this for now:
     */
    static inline __u32 tcp_acceptable_seq(const struct sock *sk)
    {
           const struct tcp_sock *tp = tcp_sk(sk);
   
           if (!before(tcp_wnd_end(tp), tp->snd_nxt))
                   return tp->snd_nxt;
           else
                   return tcp_wnd_end(tp);
   }
   
   /* Calculate mss to advertise in SYN segment.
    * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
    *
    * 1. It is independent of path mtu.
    * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
    * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
    *    attached devices, because some buggy hosts are confused by
    *    large MSS.
    * 4. We do not make 3, we advertise MSS, calculated from first
    *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
    *    This may be overridden via information stored in routing table.
    * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
    *    probably even Jumbo".
    */
   static __u16 tcp_advertise_mss(struct sock *sk)
   {
           struct tcp_sock *tp = tcp_sk(sk);
           const struct dst_entry *dst = __sk_dst_get(sk);
           int mss = tp->advmss;
   
           if (dst) {
                   unsigned int metric = dst_metric_advmss(dst);
   
                   if (metric < mss) {
                           mss = metric;
                           tp->advmss = mss;
                   }
           }
   
           return (__u16)mss;
   }
   
   /* RFC2861. Reset CWND after idle period longer RTO to "restart window".
    * This is the first part of cwnd validation mechanism. */
   static void tcp_cwnd_restart(struct sock *sk, const struct dst_entry *dst)
   {
           struct tcp_sock *tp = tcp_sk(sk);
           s32 delta = tcp_time_stamp - tp->lsndtime;
           u32 restart_cwnd = tcp_init_cwnd(tp, dst);
           u32 cwnd = tp->snd_cwnd;
   
           tcp_ca_event(sk, CA_EVENT_CWND_RESTART);
   
           tp->snd_ssthresh = tcp_current_ssthresh(sk);
           restart_cwnd = min(restart_cwnd, cwnd);
   
           while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
                   cwnd >>= 1;
           tp->snd_cwnd = max(cwnd, restart_cwnd);
           tp->snd_cwnd_stamp = tcp_time_stamp;
           tp->snd_cwnd_used = 0;
   }
   
   /* Congestion state accounting after a packet has been sent. */
   static void tcp_event_data_sent(struct tcp_sock *tp,
                                   struct sock *sk)
   {
           struct inet_connection_sock *icsk = inet_csk(sk);
           const u32 now = tcp_time_stamp;
   
           if (sysctl_tcp_slow_start_after_idle &&
               (!tp->packets_out && (s32)(now - tp->lsndtime) > icsk->icsk_rto))
                   tcp_cwnd_restart(sk, __sk_dst_get(sk));
   
           tp->lsndtime = now;
   
           /* If it is a reply for ato after last received
            * packet, enter pingpong mode.
            */
           if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
                   icsk->icsk_ack.pingpong = 1;
   }
   
   /* Account for an ACK we sent. */
   static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
   {
           tcp_dec_quickack_mode(sk, pkts);
           inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
   }
   
   /* Determine a window scaling and initial window to offer.
    * Based on the assumption that the given amount of space
    * will be offered. Store the results in the tp structure.
    * NOTE: for smooth operation initial space offering should
    * be a multiple of mss if possible. We assume here that mss >= 1.
    * This MUST be enforced by all callers.
    */
   void tcp_select_initial_window(int __space, __u32 mss,
                                  __u32 *rcv_wnd, __u32 *window_clamp,
                                  int wscale_ok, __u8 *rcv_wscale,
                                  __u32 init_rcv_wnd)
   {
           unsigned int space = (__space < 0 ? 0 : __space);
   
           /* If no clamp set the clamp to the max possible scaled window */
           if (*window_clamp == 0)
                   (*window_clamp) = (65535 << 14);
           space = min(*window_clamp, space);
   
           /* Quantize space offering to a multiple of mss if possible. */
           if (space > mss)
                   space = (space / mss) * mss;
   
           /* NOTE: offering an initial window larger than 32767
            * will break some buggy TCP stacks. If the admin tells us
            * it is likely we could be speaking with such a buggy stack
            * we will truncate our initial window offering to 32K-1
            * unless the remote has sent us a window scaling option,
            * which we interpret as a sign the remote TCP is not
            * misinterpreting the window field as a signed quantity.
            */
           if (sysctl_tcp_workaround_signed_windows)
                   (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
           else
                   (*rcv_wnd) = space;
   
           (*rcv_wscale) = 0;
           if (wscale_ok) {
                   /* Set window scaling on max possible window
                    * See RFC1323 for an explanation of the limit to 14
                    */
                   space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
                   space = min_t(u32, space, *window_clamp);
                   while (space > 65535 && (*rcv_wscale) < 14) {
                           space >>= 1;
                           (*rcv_wscale)++;
                   }
           }
   
           /* Set initial window to a value enough for senders starting with
            * initial congestion window of TCP_DEFAULT_INIT_RCVWND. Place
            * a limit on the initial window when mss is larger than 1460.
            */
           if (mss > (1 << *rcv_wscale)) {
                   int init_cwnd = TCP_DEFAULT_INIT_RCVWND;
                   if (mss > 1460)
                           init_cwnd =
                           max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);
                   /* when initializing use the value from init_rcv_wnd
                    * rather than the default from above
                    */
                   if (init_rcv_wnd)
                           *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
                   else
                           *rcv_wnd = min(*rcv_wnd, init_cwnd * mss);
           }
   
           /* Set the clamp no higher than max representable value */
           (*window_clamp) = min(65535U << (*rcv_wscale), *window_clamp);
   }
   EXPORT_SYMBOL(tcp_select_initial_window);
   
   /* Chose a new window to advertise, update state in tcp_sock for the
    * socket, and return result with RFC1323 scaling applied.  The return
    * value can be stuffed directly into th->window for an outgoing
    * frame.
    */
   static u16 tcp_select_window(struct sock *sk)
   {
           struct tcp_sock *tp = tcp_sk(sk);
           u32 cur_win = tcp_receive_window(tp);
           u32 new_win = __tcp_select_window(sk);
   
           /* Never shrink the offered window */
           if (new_win < cur_win) {
                   /* Danger Will Robinson!
                    * Don't update rcv_wup/rcv_wnd here or else
                    * we will not be able to advertise a zero
                    * window in time.  --DaveM
                    *
                    * Relax Will Robinson.
                    */
                   new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
           }
           tp->rcv_wnd = new_win;
           tp->rcv_wup = tp->rcv_nxt;
   
           /* Make sure we do not exceed the maximum possible
            * scaled window.
            */
           if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
                   new_win = min(new_win, MAX_TCP_WINDOW);
           else
                   new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
   
           /* RFC1323 scaling applied */
           new_win >>= tp->rx_opt.rcv_wscale;
   
           /* If we advertise zero window, disable fast path. */
           if (new_win == 0)
                   tp->pred_flags = 0;
   
           return new_win;
   }
   
   /* Packet ECN state for a SYN-ACK */
   static inline void TCP_ECN_send_synack(const struct tcp_sock *tp, struct sk_buff *skb)
   {
           TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
           if (!(tp->ecn_flags & TCP_ECN_OK))
                   TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
   }
   
   /* Packet ECN state for a SYN.  */
   static inline void TCP_ECN_send_syn(struct sock *sk, struct sk_buff *skb)
   {
           struct tcp_sock *tp = tcp_sk(sk);
   
           tp->ecn_flags = 0;
           if (sysctl_tcp_ecn == 1) {
                   TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
                   tp->ecn_flags = TCP_ECN_OK;
           }
   }
   
   static __inline__ void
   TCP_ECN_make_synack(const struct request_sock *req, struct tcphdr *th)
   {
           if (inet_rsk(req)->ecn_ok)
                   th->ece = 1;
   }
   
   /* Set up ECN state for a packet on a ESTABLISHED socket that is about to
    * be sent.
    */
   static inline void TCP_ECN_send(struct sock *sk, struct sk_buff *skb,
                                   int tcp_header_len)
   {
           struct tcp_sock *tp = tcp_sk(sk);
   
           if (tp->ecn_flags & TCP_ECN_OK) {
                   /* Not-retransmitted data segment: set ECT and inject CWR. */
                   if (skb->len != tcp_header_len &&
                       !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
                           INET_ECN_xmit(sk);
                           if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
                                   tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
                                   tcp_hdr(skb)->cwr = 1;
                                   skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
                           }
                   } else {
                           /* ACK or retransmitted segment: clear ECT|CE */
                           INET_ECN_dontxmit(sk);
                   }
                   if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
                           tcp_hdr(skb)->ece = 1;
           }
   }
   
   /* Constructs common control bits of non-data skb. If SYN/FIN is present,
    * auto increment end seqno.
    */
   static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
   {
           skb->ip_summed = CHECKSUM_PARTIAL;
           skb->csum = 0;
   
           TCP_SKB_CB(skb)->tcp_flags = flags;
           TCP_SKB_CB(skb)->sacked = 0;
   
           skb_shinfo(skb)->gso_segs = 1;
           skb_shinfo(skb)->gso_size = 0;
           skb_shinfo(skb)->gso_type = 0;
   
           TCP_SKB_CB(skb)->seq = seq;
           if (flags & (TCPHDR_SYN | TCPHDR_FIN))
                   seq++;
           TCP_SKB_CB(skb)->end_seq = seq;
   }
   
   static inline bool tcp_urg_mode(const struct tcp_sock *tp)
   {
           return tp->snd_una != tp->snd_up;
   }
   
   #define OPTION_SACK_ADVERTISE   (1 << 0)
   #define OPTION_TS               (1 << 1)
   #define OPTION_MD5              (1 << 2)
   #define OPTION_WSCALE           (1 << 3)
   #define OPTION_COOKIE_EXTENSION (1 << 4)
   #define OPTION_FAST_OPEN_COOKIE (1 << 8)
   
   struct tcp_out_options {
           u16 options;            /* bit field of OPTION_* */
           u16 mss;                /* 0 to disable */
           u8 ws;                  /* window scale, 0 to disable */
           u8 num_sack_blocks;     /* number of SACK blocks to include */
           u8 hash_size;           /* bytes in hash_location */
           __u8 *hash_location;    /* temporary pointer, overloaded */
           __u32 tsval, tsecr;     /* need to include OPTION_TS */
           struct tcp_fastopen_cookie *fastopen_cookie;    /* Fast open cookie */
   };
   
   /* The sysctl int routines are generic, so check consistency here.
    */
   static u8 tcp_cookie_size_check(u8 desired)
   {
           int cookie_size;
   
           if (desired > 0)
                   /* previously specified */
                   return desired;
   
           cookie_size = ACCESS_ONCE(sysctl_tcp_cookie_size);
           if (cookie_size <= 0)
                   /* no default specified */
                   return 0;
   
           if (cookie_size <= TCP_COOKIE_MIN)
                   /* value too small, specify minimum */
                   return TCP_COOKIE_MIN;
   
           if (cookie_size >= TCP_COOKIE_MAX)
                   /* value too large, specify maximum */
                   return TCP_COOKIE_MAX;
   
           if (cookie_size & 1)
                   /* 8-bit multiple, illegal, fix it */
                   cookie_size++;
   
           return (u8)cookie_size;
   }
   
   /* Write previously computed TCP options to the packet.
    *
    * Beware: Something in the Internet is very sensitive to the ordering of
    * TCP options, we learned this through the hard way, so be careful here.
    * Luckily we can at least blame others for their non-compliance but from
    * inter-operatibility perspective it seems that we're somewhat stuck with
    * the ordering which we have been using if we want to keep working with
    * those broken things (not that it currently hurts anybody as there isn't
    * particular reason why the ordering would need to be changed).
    *
    * At least SACK_PERM as the first option is known to lead to a disaster
    * (but it may well be that other scenarios fail similarly).
    */
   static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                                 struct tcp_out_options *opts)
   {
           u16 options = opts->options;    /* mungable copy */
   
           /* Having both authentication and cookies for security is redundant,
            * and there's certainly not enough room.  Instead, the cookie-less
            * extension variant is proposed.
            *
            * Consider the pessimal case with authentication.  The options
            * could look like:
            *   COOKIE|MD5(20) + MSS(4) + SACK|TS(12) + WSCALE(4) == 40
            */
           if (unlikely(OPTION_MD5 & options)) {
                   if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
                           *ptr++ = htonl((TCPOPT_COOKIE << 24) |
                                          (TCPOLEN_COOKIE_BASE << 16) |
                                          (TCPOPT_MD5SIG << 8) |
                                          TCPOLEN_MD5SIG);
                   } else {
                           *ptr++ = htonl((TCPOPT_NOP << 24) |
                                          (TCPOPT_NOP << 16) |
                                          (TCPOPT_MD5SIG << 8) |
                                          TCPOLEN_MD5SIG);
                   }
                   options &= ~OPTION_COOKIE_EXTENSION;
                   /* overload cookie hash location */
                   opts->hash_location = (__u8 *)ptr;
                   ptr += 4;
           }
   
           if (unlikely(opts->mss)) {
                   *ptr++ = htonl((TCPOPT_MSS << 24) |
                                  (TCPOLEN_MSS << 16) |
                                  opts->mss);
           }
   
           if (likely(OPTION_TS & options)) {
                   if (unlikely(OPTION_SACK_ADVERTISE & options)) {
                           *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
                                          (TCPOLEN_SACK_PERM << 16) |
                                          (TCPOPT_TIMESTAMP << 8) |
                                          TCPOLEN_TIMESTAMP);
                           options &= ~OPTION_SACK_ADVERTISE;
                   } else {
                           *ptr++ = htonl((TCPOPT_NOP << 24) |
                                          (TCPOPT_NOP << 16) |
                                          (TCPOPT_TIMESTAMP << 8) |
                                          TCPOLEN_TIMESTAMP);
                   }
                   *ptr++ = htonl(opts->tsval);
                   *ptr++ = htonl(opts->tsecr);
           }
   
           /* Specification requires after timestamp, so do it now.
            *
            * Consider the pessimal case without authentication.  The options
            * could look like:
            *   MSS(4) + SACK|TS(12) + COOKIE(20) + WSCALE(4) == 40
            */
           if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
                   __u8 *cookie_copy = opts->hash_location;
                   u8 cookie_size = opts->hash_size;
   
                   /* 8-bit multiple handled in tcp_cookie_size_check() above,
                    * and elsewhere.
                    */
                   if (0x2 & cookie_size) {
                           __u8 *p = (__u8 *)ptr;
   
                           /* 16-bit multiple */
                           *p++ = TCPOPT_COOKIE;
                           *p++ = TCPOLEN_COOKIE_BASE + cookie_size;
                           *p++ = *cookie_copy++;
                           *p++ = *cookie_copy++;
                           ptr++;
                           cookie_size -= 2;
                   } else {
                           /* 32-bit multiple */
                           *ptr++ = htonl(((TCPOPT_NOP << 24) |
                                           (TCPOPT_NOP << 16) |
                                           (TCPOPT_COOKIE << 8) |
                                           TCPOLEN_COOKIE_BASE) +
                                          cookie_size);
                   }
   
                   if (cookie_size > 0) {
                           memcpy(ptr, cookie_copy, cookie_size);
                           ptr += (cookie_size / 4);
                   }
           }
   
           if (unlikely(OPTION_SACK_ADVERTISE & options)) {
                   *ptr++ = htonl((TCPOPT_NOP << 24) |
                                  (TCPOPT_NOP << 16) |
                                  (TCPOPT_SACK_PERM << 8) |
                                  TCPOLEN_SACK_PERM);
           }
   
           if (unlikely(OPTION_WSCALE & options)) {
                   *ptr++ = htonl((TCPOPT_NOP << 24) |
                                  (TCPOPT_WINDOW << 16) |
                                  (TCPOLEN_WINDOW << 8) |
                                  opts->ws);
           }
   
           if (unlikely(opts->num_sack_blocks)) {
                   struct tcp_sack_block *sp = tp->rx_opt.dsack ?
                           tp->duplicate_sack : tp->selective_acks;
                   int this_sack;
   
                   *ptr++ = htonl((TCPOPT_NOP  << 24) |
                                  (TCPOPT_NOP  << 16) |
                                  (TCPOPT_SACK <<  8) |
                                  (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
                                                        TCPOLEN_SACK_PERBLOCK)));
   
                   for (this_sack = 0; this_sack < opts->num_sack_blocks;
                        ++this_sack) {
                           *ptr++ = htonl(sp[this_sack].start_seq);
                           *ptr++ = htonl(sp[this_sack].end_seq);
                   }
   
                   tp->rx_opt.dsack = 0;
           }
   
           if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
                   struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
   
                   *ptr++ = htonl((TCPOPT_EXP << 24) |
                                  ((TCPOLEN_EXP_FASTOPEN_BASE + foc->len) << 16) |
                                  TCPOPT_FASTOPEN_MAGIC);
   
                   memcpy(ptr, foc->val, foc->len);
                   if ((foc->len & 3) == 2) {
                           u8 *align = ((u8 *)ptr) + foc->len;
                           align[0] = align[1] = TCPOPT_NOP;
                   }
                   ptr += (foc->len + 3) >> 2;
           }
   }
   
   /* Compute TCP options for SYN packets. This is not the final
    * network wire format yet.
    */
   static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                                   struct tcp_out_options *opts,
                                   struct tcp_md5sig_key **md5)
   {
           struct tcp_sock *tp = tcp_sk(sk);
           struct tcp_cookie_values *cvp = tp->cookie_values;
           unsigned int remaining = MAX_TCP_OPTION_SPACE;
           u8 cookie_size = (!tp->rx_opt.cookie_out_never && cvp != NULL) ?
                            tcp_cookie_size_check(cvp->cookie_desired) :
                            0;
           struct tcp_fastopen_request *fastopen = tp->fastopen_req;
   
   #ifdef CONFIG_TCP_MD5SIG
           *md5 = tp->af_specific->md5_lookup(sk, sk);
           if (*md5) {
                   opts->options |= OPTION_MD5;
                   remaining -= TCPOLEN_MD5SIG_ALIGNED;
           }
   #else
           *md5 = NULL;
   #endif
   
           /* We always get an MSS option.  The option bytes which will be seen in
            * normal data packets should timestamps be used, must be in the MSS
            * advertised.  But we subtract them from tp->mss_cache so that
            * calculations in tcp_sendmsg are simpler etc.  So account for this
            * fact here if necessary.  If we don't do this correctly, as a
            * receiver we won't recognize data packets as being full sized when we
            * should, and thus we won't abide by the delayed ACK rules correctly.
            * SACKs don't matter, we never delay an ACK when we have any of those
            * going out.  */
           opts->mss = tcp_advertise_mss(sk);
           remaining -= TCPOLEN_MSS_ALIGNED;
   
           if (likely(sysctl_tcp_timestamps && *md5 == NULL)) {
                   opts->options |= OPTION_TS;
                   opts->tsval = TCP_SKB_CB(skb)->when;
                   opts->tsecr = tp->rx_opt.ts_recent;
                   remaining -= TCPOLEN_TSTAMP_ALIGNED;
           }
           if (likely(sysctl_tcp_window_scaling)) {
                   opts->ws = tp->rx_opt.rcv_wscale;
                   opts->options |= OPTION_WSCALE;
                   remaining -= TCPOLEN_WSCALE_ALIGNED;
           }
           if (likely(sysctl_tcp_sack)) {
                   opts->options |= OPTION_SACK_ADVERTISE;
                   if (unlikely(!(OPTION_TS & opts->options)))
                           remaining -= TCPOLEN_SACKPERM_ALIGNED;
           }
   
           if (fastopen && fastopen->cookie.len >= 0) {
                   u32 need = TCPOLEN_EXP_FASTOPEN_BASE + fastopen->cookie.len;
                   need = (need + 3) & ~3U;  /* Align to 32 bits */
                   if (remaining >= need) {
                           opts->options |= OPTION_FAST_OPEN_COOKIE;
                           opts->fastopen_cookie = &fastopen->cookie;
                           remaining -= need;
                           tp->syn_fastopen = 1;
                   }
           }
           /* Note that timestamps are required by the specification.
            *
            * Odd numbers of bytes are prohibited by the specification, ensuring
            * that the cookie is 16-bit aligned, and the resulting cookie pair is
            * 32-bit aligned.
            */
           if (*md5 == NULL &&
               (OPTION_TS & opts->options) &&
               cookie_size > 0) {
                   int need = TCPOLEN_COOKIE_BASE + cookie_size;
   
                   if (0x2 & need) {
                           /* 32-bit multiple */
                           need += 2; /* NOPs */
   
                           if (need > remaining) {
                                   /* try shrinking cookie to fit */
                                   cookie_size -= 2;
                                   need -= 4;
                           }
                   }
                   while (need > remaining && TCP_COOKIE_MIN <= cookie_size) {
                           cookie_size -= 4;
                           need -= 4;
                   }
                   if (TCP_COOKIE_MIN <= cookie_size) {
                           opts->options |= OPTION_COOKIE_EXTENSION;
                           opts->hash_location = (__u8 *)&cvp->cookie_pair[0];
                           opts->hash_size = cookie_size;
   
                           /* Remember for future incarnations. */
                           cvp->cookie_desired = cookie_size;
   
                           if (cvp->cookie_desired != cvp->cookie_pair_size) {
                                   /* Currently use random bytes as a nonce,
                                    * assuming these are completely unpredictable
                                    * by hostile users of the same system.
                                    */
                                   get_random_bytes(&cvp->cookie_pair[0],
                                                    cookie_size);
                                   cvp->cookie_pair_size = cookie_size;
                           }
   
                           remaining -= need;
                   }
           }
           return MAX_TCP_OPTION_SPACE - remaining;
   }
   
   /* Set up TCP options for SYN-ACKs. */
   static unsigned int tcp_synack_options(struct sock *sk,
                                      struct request_sock *req,
                                      unsigned int mss, struct sk_buff *skb,
                                      struct tcp_out_options *opts,
                                      struct tcp_md5sig_key **md5,
                                      struct tcp_extend_values *xvp,
                                      struct tcp_fastopen_cookie *foc)
   {
           struct inet_request_sock *ireq = inet_rsk(req);
           unsigned int remaining = MAX_TCP_OPTION_SPACE;
           u8 cookie_plus = (xvp != NULL && !xvp->cookie_out_never) ?
                            xvp->cookie_plus :
                            0;
   
   #ifdef CONFIG_TCP_MD5SIG
           *md5 = tcp_rsk(req)->af_specific->md5_lookup(sk, req);
           if (*md5) {
                   opts->options |= OPTION_MD5;
                   remaining -= TCPOLEN_MD5SIG_ALIGNED;
   
                   /* We can't fit any SACK blocks in a packet with MD5 + TS
                    * options. There was discussion about disabling SACK
                    * rather than TS in order to fit in better with old,
                    * buggy kernels, but that was deemed to be unnecessary.
                    */
                   ireq->tstamp_ok &= !ireq->sack_ok;
           }
   #else
           *md5 = NULL;
   #endif
   
           /* We always send an MSS option. */
           opts->mss = mss;
           remaining -= TCPOLEN_MSS_ALIGNED;
   
           if (likely(ireq->wscale_ok)) {
                   opts->ws = ireq->rcv_wscale;
                   opts->options |= OPTION_WSCALE;
                   remaining -= TCPOLEN_WSCALE_ALIGNED;
           }
           if (likely(ireq->tstamp_ok)) {
                   opts->options |= OPTION_TS;
                   opts->tsval = TCP_SKB_CB(skb)->when;
                   opts->tsecr = req->ts_recent;
                   remaining -= TCPOLEN_TSTAMP_ALIGNED;
           }
           if (likely(ireq->sack_ok)) {
                   opts->options |= OPTION_SACK_ADVERTISE;
                   if (unlikely(!ireq->tstamp_ok))
                           remaining -= TCPOLEN_SACKPERM_ALIGNED;
           }
           if (foc != NULL) {
                   u32 need = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
                   need = (need + 3) & ~3U;  /* Align to 32 bits */
                   if (remaining >= need) {
                           opts->options |= OPTION_FAST_OPEN_COOKIE;
                           opts->fastopen_cookie = foc;
                           remaining -= need;
                   }
           }
           /* Similar rationale to tcp_syn_options() applies here, too.
            * If the <SYN> options fit, the same options should fit now!
            */
           if (*md5 == NULL &&
               ireq->tstamp_ok &&
               cookie_plus > TCPOLEN_COOKIE_BASE) {
                   int need = cookie_plus; /* has TCPOLEN_COOKIE_BASE */
   
                   if (0x2 & need) {
                           /* 32-bit multiple */
                           need += 2; /* NOPs */
                   }
                   if (need <= remaining) {
                           opts->options |= OPTION_COOKIE_EXTENSION;
                           opts->hash_size = cookie_plus - TCPOLEN_COOKIE_BASE;
                           remaining -= need;
                   } else {
                           /* There's no error return, so flag it. */
                           xvp->cookie_out_never = 1; /* true */
                           opts->hash_size = 0;
                   }
           }
           return MAX_TCP_OPTION_SPACE - remaining;
   }
   
   /* Compute TCP options for ESTABLISHED sockets. This is not the
    * final wire format yet.
    */
   static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
                                           struct tcp_out_options *opts,
                                           struct tcp_md5sig_key **md5)
   {
           struct tcp_skb_cb *tcb = skb ? TCP_SKB_CB(skb) : NULL;
           struct tcp_sock *tp = tcp_sk(sk);
           unsigned int size = 0;
           unsigned int eff_sacks;
   
   #ifdef CONFIG_TCP_MD5SIG
           *md5 = tp->af_specific->md5_lookup(sk, sk);
           if (unlikely(*md5)) {
                   opts->options |= OPTION_MD5;
                   size += TCPOLEN_MD5SIG_ALIGNED;
           }
   #else
           *md5 = NULL;
   #endif
   
           if (likely(tp->rx_opt.tstamp_ok)) {
                   opts->options |= OPTION_TS;
                   opts->tsval = tcb ? tcb->when : 0;
                   opts->tsecr = tp->rx_opt.ts_recent;
                   size += TCPOLEN_TSTAMP_ALIGNED;
           }
   
           eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
           if (unlikely(eff_sacks)) {
                   const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
                   opts->num_sack_blocks =
                           min_t(unsigned int, eff_sacks,
                                 (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
                                 TCPOLEN_SACK_PERBLOCK);
                   size += TCPOLEN_SACK_BASE_ALIGNED +
                           opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
           }
   
           return size;
   }
   
   
   /* TCP SMALL QUEUES (TSQ)
    *
    * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
    * to reduce RTT and bufferbloat.
    * We do this using a special skb destructor (tcp_wfree).
    *
    * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
    * needs to be reallocated in a driver.
    * The invariant being skb->truesize substracted from sk->sk_wmem_alloc
    *
    * Since transmit from skb destructor is forbidden, we use a tasklet
    * to process all sockets that eventually need to send more skbs.
    * We use one tasklet per cpu, with its own queue of sockets.
    */
   struct tsq_tasklet {
           struct tasklet_struct   tasklet;
           struct list_head        head; /* queue of tcp sockets */
   };
   static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);
   
   static void tcp_tsq_handler(struct sock *sk)
   {
           if ((1 << sk->sk_state) &
               (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
                TCPF_CLOSE_WAIT  | TCPF_LAST_ACK))
                   tcp_write_xmit(sk, tcp_current_mss(sk), 0, 0, GFP_ATOMIC);
   }
   /*
    * One tasklest per cpu tries to send more skbs.
    * We run in tasklet context but need to disable irqs when
    * transfering tsq->head because tcp_wfree() might
    * interrupt us (non NAPI drivers)
    */
   static void tcp_tasklet_func(unsigned long data)
   {
           struct tsq_tasklet *tsq = (struct tsq_tasklet *)data;
           LIST_HEAD(list);
           unsigned long flags;
           struct list_head *q, *n;
           struct tcp_sock *tp;
           struct sock *sk;
   
           local_irq_save(flags);
           list_splice_init(&tsq->head, &list);
           local_irq_restore(flags);
   
           list_for_each_safe(q, n, &list) {
                   tp = list_entry(q, struct tcp_sock, tsq_node);
                   list_del(&tp->tsq_node);
   
                   sk = (struct sock *)tp;
                   bh_lock_sock(sk);
   
                   if (!sock_owned_by_user(sk)) {
                           tcp_tsq_handler(sk);
                   } else {
                           /* defer the work to tcp_release_cb() */
                           set_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);
                   }
                   bh_unlock_sock(sk);
   
                   clear_bit(TSQ_QUEUED, &tp->tsq_flags);
                   sk_free(sk);
           }
   }
   
   #define TCP_DEFERRED_ALL ((1UL << TCP_TSQ_DEFERRED) |           \
                             (1UL << TCP_WRITE_TIMER_DEFERRED) |   \
                             (1UL << TCP_DELACK_TIMER_DEFERRED) |  \
                             (1UL << TCP_MTU_REDUCED_DEFERRED))
   /**
    * tcp_release_cb - tcp release_sock() callback
    * @sk: socket
    *
    * called from release_sock() to perform protocol dependent
    * actions before socket release.
    */
   void tcp_release_cb(struct sock *sk)
   {
           struct tcp_sock *tp = tcp_sk(sk);
           unsigned long flags, nflags;
   
           /* perform an atomic operation only if at least one flag is set */
           do {
                   flags = tp->tsq_flags;
                   if (!(flags & TCP_DEFERRED_ALL))
                           return;
                   nflags = flags & ~TCP_DEFERRED_ALL;
           } while (cmpxchg(&tp->tsq_flags, flags, nflags) != flags);
   
           if (flags & (1UL << TCP_TSQ_DEFERRED))
                   tcp_tsq_handler(sk);
   
           if (flags & (1UL << TCP_WRITE_TIMER_DEFERRED)) {
                   tcp_write_timer_handler(sk);
                   __sock_put(sk);
           }
           if (flags & (1UL << TCP_DELACK_TIMER_DEFERRED)) {
                   tcp_delack_timer_handler(sk);
                   __sock_put(sk);
           }
           if (flags & (1UL << TCP_MTU_REDUCED_DEFERRED)) {
                   sk->sk_prot->mtu_reduced(sk);
                   __sock_put(sk);
           }
   }
   EXPORT_SYMBOL(tcp_release_cb);
   
   void __init tcp_tasklet_init(void)
   {
           int i;
   
           for_each_possible_cpu(i) {
                   struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
   
                   INIT_LIST_HEAD(&tsq->head);
                   tasklet_init(&tsq->tasklet,
                                tcp_tasklet_func,
                                (unsigned long)tsq);
           }
   }
   
   /*
    * Write buffer destructor automatically called from kfree_skb.
    * We cant xmit new skbs from this context, as we might already
    * hold qdisc lock.
    */
   static void tcp_wfree(struct sk_buff *skb)
   {
           struct sock *sk = skb->sk;
           struct tcp_sock *tp = tcp_sk(sk);
   
           if (test_and_clear_bit(TSQ_THROTTLED, &tp->tsq_flags) &&
               !test_and_set_bit(TSQ_QUEUED, &tp->tsq_flags)) {
                   unsigned long flags;
                   struct tsq_tasklet *tsq;
   
                   /* Keep a ref on socket.
                    * This last ref will be released in tcp_tasklet_func()
                    */
                   atomic_sub(skb->truesize - 1, &sk->sk_wmem_alloc);
   
                   /* queue this socket to tasklet queue */
                   local_irq_save(flags);
                   tsq = &__get_cpu_var(tsq_tasklet);
                   list_add(&tp->tsq_node, &tsq->head);
                   tasklet_schedule(&tsq->tasklet);
                   local_irq_restore(flags);
           } else {
                   sock_wfree(skb);
           }
   }
   
   /* This routine actually transmits TCP packets queued in by
    * tcp_do_sendmsg().  This is used by both the initial
    * transmission and possible later retransmissions.
    * All SKB's seen here are completely headerless.  It is our
    * job to build the TCP header, and pass the packet down to
    * IP so it can do the same plus pass the packet off to the
    * device.
    *
    * We are working here with either a clone of the original
    * SKB, or a fresh unique copy made by the retransmit engine.
    */
   static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                               gfp_t gfp_mask)
   {
           const struct inet_connection_sock *icsk = inet_csk(sk);
           struct inet_sock *inet;
           struct tcp_sock *tp;
           struct tcp_skb_cb *tcb;
          struct tcp_out_options opts;
          unsigned int tcp_options_size, tcp_header_size;
          struct tcp_md5sig_key *md5;
          struct tcphdr *th;
          int err;
  
          BUG_ON(!skb || !tcp_skb_pcount(skb));
  
          /* If congestion control is doing timestamping, we must
           * take such a timestamp before we potentially clone/copy.
           */
          if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP)
                  __net_timestamp(skb);
  
          if (likely(clone_it)) {
                  if (unlikely(skb_cloned(skb)))
                          skb = pskb_copy(skb, gfp_mask);
                  else
                          skb = skb_clone(skb, gfp_mask);
                  if (unlikely(!skb))
                          return -ENOBUFS;
          }
  
          inet = inet_sk(sk);
          tp = tcp_sk(sk);
          tcb = TCP_SKB_CB(skb);
          memset(&opts, 0, sizeof(opts));
  
          if (unlikely(tcb->tcp_flags & TCPHDR_SYN))
                  tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
          else
                  tcp_options_size = tcp_established_options(sk, skb, &opts,
                                                             &md5);
          tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
  
          if (tcp_packets_in_flight(tp) == 0) {
                  tcp_ca_event(sk, CA_EVENT_TX_START);
                  skb->ooo_okay = 1;
          } else
                  skb->ooo_okay = 0;
  
          skb_push(skb, tcp_header_size);
          skb_reset_transport_header(skb);
  
          skb_orphan(skb);
          skb->sk = sk;
          skb->destructor = (sysctl_tcp_limit_output_bytes > 0) ?
                            tcp_wfree : sock_wfree;
          atomic_add(skb->truesize, &sk->sk_wmem_alloc);
  
          /* Build TCP header and checksum it. */
          th = tcp_hdr(skb);
          th->source              = inet->inet_sport;
          th->dest                = inet->inet_dport;
          th->seq                 = htonl(tcb->seq);
          th->ack_seq             = htonl(tp->rcv_nxt);
          *(((__be16 *)th) + 6)   = htons(((tcp_header_size >> 2) << 12) |
                                          tcb->tcp_flags);
  
          if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
                  /* RFC1323: The window in SYN & SYN/ACK segments
                   * is never scaled.
                   */
                  th->window      = htons(min(tp->rcv_wnd, 65535U));
          } else {
                  th->window      = htons(tcp_select_window(sk));
          }
          th->check               = 0;
          th->urg_ptr             = 0;
  
          /* The urg_mode check is necessary during a below snd_una win probe */
          if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
                  if (before(tp->snd_up, tcb->seq + 0x10000)) {
                          th->urg_ptr = htons(tp->snd_up - tcb->seq);
                          th->urg = 1;
                  } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
                          th->urg_ptr = htons(0xFFFF);
                          th->urg = 1;
                  }
          }
  
          tcp_options_write((__be32 *)(th + 1), tp, &opts);
          if (likely((tcb->tcp_flags & TCPHDR_SYN) == 0))
                  TCP_ECN_send(sk, skb, tcp_header_size);
  
  #ifdef CONFIG_TCP_MD5SIG
          /* Calculate the MD5 hash, as we have all we need now */
          if (md5) {
                  sk_nocaps_add(sk, NETIF_F_GSO_MASK);
                  tp->af_specific->calc_md5_hash(opts.hash_location,
                                                 md5, sk, NULL, skb);
          }
  #endif
  
          icsk->icsk_af_ops->send_check(sk, skb);
  
          if (likely(tcb->tcp_flags & TCPHDR_ACK))
                  tcp_event_ack_sent(sk, tcp_skb_pcount(skb));
  
          if (skb->len != tcp_header_size)
                  tcp_event_data_sent(tp, sk);
  
          if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
                  TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
                                tcp_skb_pcount(skb));
  
          err = icsk->icsk_af_ops->queue_xmit(skb, &inet->cork.fl);
          if (likely(err <= 0))
                  return err;
  
          tcp_enter_cwr(sk, 1);
  
          return net_xmit_eval(err);
  }
  
  /* This routine just queues the buffer for sending.
   *
   * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
   * otherwise socket can stall.
   */
  static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
  {
          struct tcp_sock *tp = tcp_sk(sk);
  
          /* Advance write_seq and place onto the write_queue. */
          tp->write_seq = TCP_SKB_CB(skb)->end_seq;
          skb_header_release(skb);
          tcp_add_write_queue_tail(sk, skb);
          sk->sk_wmem_queued += skb->truesize;
          sk_mem_charge(sk, skb->truesize);
  }
  
  /* Initialize TSO segments for a packet. */
  static void tcp_set_skb_tso_segs(const struct sock *sk, struct sk_buff *skb,
                                   unsigned int mss_now)
  {
          if (skb->len <= mss_now || !sk_can_gso(sk) ||
              skb->ip_summed == CHECKSUM_NONE) {
                  /* Avoid the costly divide in the normal
                   * non-TSO case.
                   */
                  skb_shinfo(skb)->gso_segs = 1;
                  skb_shinfo(skb)->gso_size = 0;
                  skb_shinfo(skb)->gso_type = 0;
          } else {
                  skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss_now);
                  skb_shinfo(skb)->gso_size = mss_now;
                  skb_shinfo(skb)->gso_type = sk->sk_gso_type;
          }
  }
  
  /* When a modification to fackets out becomes necessary, we need to check
   * skb is counted to fackets_out or not.
   */
  static void tcp_adjust_fackets_out(struct sock *sk, const struct sk_buff *skb,
                                     int decr)
  {
          struct tcp_sock *tp = tcp_sk(sk);
  
          if (!tp->sacked_out || tcp_is_reno(tp))
                  return;
  
          if (after(tcp_highest_sack_seq(tp), TCP_SKB_CB(skb)->seq))
                  tp->fackets_out -= decr;
  }
  
  /* Pcount in the middle of the write queue got changed, we need to do various
   * tweaks to fix counters
   */
  static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
  {
          struct tcp_sock *tp = tcp_sk(sk);
  
          tp->packets_out -= decr;
  
          if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                  tp->sacked_out -= decr;
          if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
                  tp->retrans_out -= decr;
          if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
                  tp->lost_out -= decr;
  
          /* Reno case is special. Sigh... */
          if (tcp_is_reno(tp) && decr > 0)
                  tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
  
          tcp_adjust_fackets_out(sk, skb, decr);
  
          if (tp->lost_skb_hint &&
              before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
              (tcp_is_fack(tp) || (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)))
                  tp->lost_cnt_hint -= decr;
  
          tcp_verify_left_out(tp);
  }
  
  /* Function to create two new TCP segments.  Shrinks the given segment
   * to the specified size and appends a new segment with the rest of the
   * packet to the list.  This won't be called frequently, I hope.
   * Remember, these are still headerless SKBs at this point.
   */
  int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
                   unsigned int mss_now)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *buff;
          int nsize, old_factor;
          int nlen;
          u8 flags;
  
          if (WARN_ON(len > skb->len))
                  return -EINVAL;
  
          nsize = skb_headlen(skb) - len;
          if (nsize < 0)
                  nsize = 0;
  
          if (skb_cloned(skb) &&
              skb_is_nonlinear(skb) &&
              pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
                  return -ENOMEM;
  
          /* Get a new skb... force flag on. */
          buff = sk_stream_alloc_skb(sk, nsize, GFP_ATOMIC);
          if (buff == NULL)
                  return -ENOMEM; /* We'll just try again later. */
  
          sk->sk_wmem_queued += buff->truesize;
          sk_mem_charge(sk, buff->truesize);
          nlen = skb->len - len - nsize;
          buff->truesize += nlen;
          skb->truesize -= nlen;
  
          /* Correct the sequence numbers. */
          TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
          TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
          TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
  
          /* PSH and FIN should only be set in the second packet. */
          flags = TCP_SKB_CB(skb)->tcp_flags;
          TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
          TCP_SKB_CB(buff)->tcp_flags = flags;
          TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
  
          if (!skb_shinfo(skb)->nr_frags && skb->ip_summed != CHECKSUM_PARTIAL) {
                  /* Copy and checksum data tail into the new buffer. */
                  buff->csum = csum_partial_copy_nocheck(skb->data + len,
                                                         skb_put(buff, nsize),
                                                         nsize, 0);
  
                  skb_trim(skb, len);
  
                  skb->csum = csum_block_sub(skb->csum, buff->csum, len);
          } else {
                  skb->ip_summed = CHECKSUM_PARTIAL;
                  skb_split(skb, buff, len);
          }
  
          buff->ip_summed = skb->ip_summed;
  
          /* Looks stupid, but our code really uses when of
           * skbs, which it never sent before. --ANK
           */
          TCP_SKB_CB(buff)->when = TCP_SKB_CB(skb)->when;
          buff->tstamp = skb->tstamp;
  
          old_factor = tcp_skb_pcount(skb);
  
          /* Fix up tso_factor for both original and new SKB.  */
          tcp_set_skb_tso_segs(sk, skb, mss_now);
          tcp_set_skb_tso_segs(sk, buff, mss_now);
  
          /* If this packet has been sent out already, we must
           * adjust the various packet counters.
           */
          if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
                  int diff = old_factor - tcp_skb_pcount(skb) -
                          tcp_skb_pcount(buff);
  
                  if (diff)
                          tcp_adjust_pcount(sk, skb, diff);
          }
  
          /* Link BUFF into the send queue. */
          skb_header_release(buff);
          tcp_insert_write_queue_after(skb, buff, sk);
  
          return 0;
  }
  
  /* This is similar to __pskb_pull_head() (it will go to core/skbuff.c
   * eventually). The difference is that pulled data not copied, but
   * immediately discarded.
   */
  static void __pskb_trim_head(struct sk_buff *skb, int len)
  {
          int i, k, eat;
  
          eat = min_t(int, len, skb_headlen(skb));
          if (eat) {
                  __skb_pull(skb, eat);
                  skb->avail_size -= eat;
                  len -= eat;
                  if (!len)
                          return;
          }
          eat = len;
          k = 0;
          for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
                  int size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
  
                  if (size <= eat) {
                          skb_frag_unref(skb, i);
                          eat -= size;
                  } else {
                          skb_shinfo(skb)->frags[k] = skb_shinfo(skb)->frags[i];
                          if (eat) {
                                  skb_shinfo(skb)->frags[k].page_offset += eat;
                                  skb_frag_size_sub(&skb_shinfo(skb)->frags[k], eat);
                                  eat = 0;
                          }
                          k++;
                  }
          }
          skb_shinfo(skb)->nr_frags = k;
  
          skb_reset_tail_pointer(skb);
          skb->data_len -= len;
          skb->len = skb->data_len;
  }
  
  /* Remove acked data from a packet in the transmit queue. */
  int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
  {
          if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
                  return -ENOMEM;
  
          __pskb_trim_head(skb, len);
  
          TCP_SKB_CB(skb)->seq += len;
          skb->ip_summed = CHECKSUM_PARTIAL;
  
          skb->truesize        -= len;
          sk->sk_wmem_queued   -= len;
          sk_mem_uncharge(sk, len);
          sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
  
          /* Any change of skb->len requires recalculation of tso factor. */
          if (tcp_skb_pcount(skb) > 1)
                  tcp_set_skb_tso_segs(sk, skb, tcp_skb_mss(skb));
  
          return 0;
  }
  
  /* Calculate MSS. Not accounting for SACKs here.  */
  int tcp_mtu_to_mss(struct sock *sk, int pmtu)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
          const struct inet_connection_sock *icsk = inet_csk(sk);
          int mss_now;
  
          /* Calculate base mss without TCP options:
             It is MMS_S - sizeof(tcphdr) of rfc1122
           */
          mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
  
          /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
          if (icsk->icsk_af_ops->net_frag_header_len) {
                  const struct dst_entry *dst = __sk_dst_get(sk);
  
                  if (dst && dst_allfrag(dst))
                          mss_now -= icsk->icsk_af_ops->net_frag_header_len;
          }
  
          /* Clamp it (mss_clamp does not include tcp options) */
          if (mss_now > tp->rx_opt.mss_clamp)
                  mss_now = tp->rx_opt.mss_clamp;
  
          /* Now subtract optional transport overhead */
          mss_now -= icsk->icsk_ext_hdr_len;
  
          /* Then reserve room for full set of TCP options and 8 bytes of data */
          if (mss_now < 48)
                  mss_now = 48;
  
          /* Now subtract TCP options size, not including SACKs */
          mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);
  
          return mss_now;
  }
  
  /* Inverse of above */
  int tcp_mss_to_mtu(struct sock *sk, int mss)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
          const struct inet_connection_sock *icsk = inet_csk(sk);
          int mtu;
  
          mtu = mss +
                tp->tcp_header_len +
                icsk->icsk_ext_hdr_len +
                icsk->icsk_af_ops->net_header_len;
  
          /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
          if (icsk->icsk_af_ops->net_frag_header_len) {
                  const struct dst_entry *dst = __sk_dst_get(sk);
  
                  if (dst && dst_allfrag(dst))
                          mtu += icsk->icsk_af_ops->net_frag_header_len;
          }
          return mtu;
  }
  
  /* MTU probing init per socket */
  void tcp_mtup_init(struct sock *sk)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct inet_connection_sock *icsk = inet_csk(sk);
  
          icsk->icsk_mtup.enabled = sysctl_tcp_mtu_probing > 1;
          icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
                                 icsk->icsk_af_ops->net_header_len;
          icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, sysctl_tcp_base_mss);
          icsk->icsk_mtup.probe_size = 0;
  }
  EXPORT_SYMBOL(tcp_mtup_init);
  
  /* This function synchronize snd mss to current pmtu/exthdr set.
  
     tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
     for TCP options, but includes only bare TCP header.
  
     tp->rx_opt.mss_clamp is mss negotiated at connection setup.
     It is minimum of user_mss and mss received with SYN.
     It also does not include TCP options.
  
     inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.
  
     tp->mss_cache is current effective sending mss, including
     all tcp options except for SACKs. It is evaluated,
     taking into account current pmtu, but never exceeds
     tp->rx_opt.mss_clamp.
  
     NOTE1. rfc1122 clearly states that advertised MSS
     DOES NOT include either tcp or ip options.
  
     NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
     are READ ONLY outside this function.         --ANK (980731)
   */
  unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct inet_connection_sock *icsk = inet_csk(sk);
          int mss_now;
  
          if (icsk->icsk_mtup.search_high > pmtu)
                  icsk->icsk_mtup.search_high = pmtu;
  
          mss_now = tcp_mtu_to_mss(sk, pmtu);
          mss_now = tcp_bound_to_half_wnd(tp, mss_now);
  
          /* And store cached results */
          icsk->icsk_pmtu_cookie = pmtu;
          if (icsk->icsk_mtup.enabled)
                  mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
          tp->mss_cache = mss_now;
  
          return mss_now;
  }
  EXPORT_SYMBOL(tcp_sync_mss);
  
  /* Compute the current effective MSS, taking SACKs and IP options,
   * and even PMTU discovery events into account.
   */
  unsigned int tcp_current_mss(struct sock *sk)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
          const struct dst_entry *dst = __sk_dst_get(sk);
          u32 mss_now;
          unsigned int header_len;
          struct tcp_out_options opts;
          struct tcp_md5sig_key *md5;
  
          mss_now = tp->mss_cache;
  
          if (dst) {
                  u32 mtu = dst_mtu(dst);
                  if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
                          mss_now = tcp_sync_mss(sk, mtu);
          }
  
          header_len = tcp_established_options(sk, NULL, &opts, &md5) +
                       sizeof(struct tcphdr);
          /* The mss_cache is sized based on tp->tcp_header_len, which assumes
           * some common options. If this is an odd packet (because we have SACK
           * blocks etc) then our calculated header_len will be different, and
           * we have to adjust mss_now correspondingly */
          if (header_len != tp->tcp_header_len) {
                  int delta = (int) header_len - tp->tcp_header_len;
                  mss_now -= delta;
          }
  
          return mss_now;
  }
  
  /* Congestion window validation. (RFC2861) */
  static void tcp_cwnd_validate(struct sock *sk)
  {
          struct tcp_sock *tp = tcp_sk(sk);
  
          if (tp->packets_out >= tp->snd_cwnd) {
                  /* Network is feed fully. */
                  tp->snd_cwnd_used = 0;
                  tp->snd_cwnd_stamp = tcp_time_stamp;
          } else {
                  /* Network starves. */
                  if (tp->packets_out > tp->snd_cwnd_used)
                          tp->snd_cwnd_used = tp->packets_out;
  
                  if (sysctl_tcp_slow_start_after_idle &&
                      (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto)
                          tcp_cwnd_application_limited(sk);
          }
  }
  
  /* Returns the portion of skb which can be sent right away without
   * introducing MSS oddities to segment boundaries. In rare cases where
   * mss_now != mss_cache, we will request caller to create a small skb
   * per input skb which could be mostly avoided here (if desired).
   *
   * We explicitly want to create a request for splitting write queue tail
   * to a small skb for Nagle purposes while avoiding unnecessary modulos,
   * thus all the complexity (cwnd_len is always MSS multiple which we
   * return whenever allowed by the other factors). Basically we need the
   * modulo only when the receiver window alone is the limiting factor or
   * when we would be allowed to send the split-due-to-Nagle skb fully.
   */
  static unsigned int tcp_mss_split_point(const struct sock *sk, const struct sk_buff *skb,
                                          unsigned int mss_now, unsigned int max_segs)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
          u32 needed, window, max_len;
  
          window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
          max_len = mss_now * max_segs;
  
          if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
                  return max_len;
  
          needed = min(skb->len, window);
  
          if (max_len <= needed)
                  return max_len;
  
          return needed - needed % mss_now;
  }
  
  /* Can at least one segment of SKB be sent right now, according to the
   * congestion window rules?  If so, return how many segments are allowed.
   */
  static inline unsigned int tcp_cwnd_test(const struct tcp_sock *tp,
                                           const struct sk_buff *skb)
  {
          u32 in_flight, cwnd;
  
          /* Don't be strict about the congestion window for the final FIN.  */
          if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
              tcp_skb_pcount(skb) == 1)
                  return 1;
  
          in_flight = tcp_packets_in_flight(tp);
          cwnd = tp->snd_cwnd;
          if (in_flight < cwnd)
                  return (cwnd - in_flight);
  
          return 0;
  }
  
  /* Initialize TSO state of a skb.
   * This must be invoked the first time we consider transmitting
   * SKB onto the wire.
   */
  static int tcp_init_tso_segs(const struct sock *sk, struct sk_buff *skb,
                               unsigned int mss_now)
  {
          int tso_segs = tcp_skb_pcount(skb);
  
          if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
                  tcp_set_skb_tso_segs(sk, skb, mss_now);
                  tso_segs = tcp_skb_pcount(skb);
          }
          return tso_segs;
  }
  
  /* Minshall's variant of the Nagle send check. */
  static inline bool tcp_minshall_check(const struct tcp_sock *tp)
  {
          return after(tp->snd_sml, tp->snd_una) &&
                  !after(tp->snd_sml, tp->snd_nxt);
  }
  
  /* Return false, if packet can be sent now without violation Nagle's rules:
   * 1. It is full sized.
   * 2. Or it contains FIN. (already checked by caller)
   * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
   * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
   *    With Minshall's modification: all sent small packets are ACKed.
   */
  static inline bool tcp_nagle_check(const struct tcp_sock *tp,
                                    const struct sk_buff *skb,
                                    unsigned int mss_now, int nonagle)
  {
          return skb->len < mss_now &&
                  ((nonagle & TCP_NAGLE_CORK) ||
                   (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
  }
  
  /* Return true if the Nagle test allows this packet to be
   * sent now.
   */
  static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
                                    unsigned int cur_mss, int nonagle)
  {
          /* Nagle rule does not apply to frames, which sit in the middle of the
           * write_queue (they have no chances to get new data).
           *
           * This is implemented in the callers, where they modify the 'nonagle'
           * argument based upon the location of SKB in the send queue.
           */
          if (nonagle & TCP_NAGLE_PUSH)
                  return true;
  
          /* Don't use the nagle rule for urgent data (or for the final FIN).
           * Nagle can be ignored during F-RTO too (see RFC4138).
           */
          if (tcp_urg_mode(tp) || (tp->frto_counter == 2) ||
              (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
                  return true;
  
          if (!tcp_nagle_check(tp, skb, cur_mss, nonagle))
                  return true;
  
          return false;
  }
  
  /* Does at least the first segment of SKB fit into the send window? */
  static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
                               const struct sk_buff *skb,
                               unsigned int cur_mss)
  {
          u32 end_seq = TCP_SKB_CB(skb)->end_seq;
  
          if (skb->len > cur_mss)
                  end_seq = TCP_SKB_CB(skb)->seq + cur_mss;
  
          return !after(end_seq, tcp_wnd_end(tp));
  }
  
  /* This checks if the data bearing packet SKB (usually tcp_send_head(sk))
   * should be put on the wire right now.  If so, it returns the number of
   * packets allowed by the congestion window.
   */
  static unsigned int tcp_snd_test(const struct sock *sk, struct sk_buff *skb,
                                   unsigned int cur_mss, int nonagle)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
          unsigned int cwnd_quota;
  
          tcp_init_tso_segs(sk, skb, cur_mss);
  
          if (!tcp_nagle_test(tp, skb, cur_mss, nonagle))
                  return 0;
  
          cwnd_quota = tcp_cwnd_test(tp, skb);
          if (cwnd_quota && !tcp_snd_wnd_test(tp, skb, cur_mss))
                  cwnd_quota = 0;
  
          return cwnd_quota;
  }
  
  /* Test if sending is allowed right now. */
  bool tcp_may_send_now(struct sock *sk)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb = tcp_send_head(sk);
  
          return skb &&
                  tcp_snd_test(sk, skb, tcp_current_mss(sk),
                               (tcp_skb_is_last(sk, skb) ?
                                tp->nonagle : TCP_NAGLE_PUSH));
  }
  
  /* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
   * which is put after SKB on the list.  It is very much like
   * tcp_fragment() except that it may make several kinds of assumptions
   * in order to speed up the splitting operation.  In particular, we
   * know that all the data is in scatter-gather pages, and that the
   * packet has never been sent out before (and thus is not cloned).
   */
  static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
                          unsigned int mss_now, gfp_t gfp)
  {
          struct sk_buff *buff;
          int nlen = skb->len - len;
          u8 flags;
  
          /* All of a TSO frame must be composed of paged data.  */
          if (skb->len != skb->data_len)
                  return tcp_fragment(sk, skb, len, mss_now);
  
          buff = sk_stream_alloc_skb(sk, 0, gfp);
          if (unlikely(buff == NULL))
                  return -ENOMEM;
  
          sk->sk_wmem_queued += buff->truesize;
          sk_mem_charge(sk, buff->truesize);
          buff->truesize += nlen;
          skb->truesize -= nlen;
  
          /* Correct the sequence numbers. */
          TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
          TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
          TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
  
          /* PSH and FIN should only be set in the second packet. */
          flags = TCP_SKB_CB(skb)->tcp_flags;
          TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
          TCP_SKB_CB(buff)->tcp_flags = flags;
  
          /* This packet was never sent out yet, so no SACK bits. */
          TCP_SKB_CB(buff)->sacked = 0;
  
          buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
          skb_split(skb, buff, len);
  
          /* Fix up tso_factor for both original and new SKB.  */
          tcp_set_skb_tso_segs(sk, skb, mss_now);
          tcp_set_skb_tso_segs(sk, buff, mss_now);
  
          /* Link BUFF into the send queue. */
          skb_header_release(buff);
          tcp_insert_write_queue_after(skb, buff, sk);
  
          return 0;
  }
  
  /* Try to defer sending, if possible, in order to minimize the amount
   * of TSO splitting we do.  View it as a kind of TSO Nagle test.
   *
   * This algorithm is from John Heffner.
   */
  static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          const struct inet_connection_sock *icsk = inet_csk(sk);
          u32 send_win, cong_win, limit, in_flight;
          int win_divisor;
  
          if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
                  goto send_now;
  
          if (icsk->icsk_ca_state != TCP_CA_Open)
                  goto send_now;
  
          /* Defer for less than two clock ticks. */
          if (tp->tso_deferred &&
              (((u32)jiffies << 1) >> 1) - (tp->tso_deferred >> 1) > 1)
                  goto send_now;
  
          in_flight = tcp_packets_in_flight(tp);
  
          BUG_ON(tcp_skb_pcount(skb) <= 1 || (tp->snd_cwnd <= in_flight));
  
          send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
  
          /* From in_flight test above, we know that cwnd > in_flight.  */
          cong_win = (tp->snd_cwnd - in_flight) * tp->mss_cache;
  
          limit = min(send_win, cong_win);
  
          /* If a full-sized TSO skb can be sent, do it. */
          if (limit >= min_t(unsigned int, sk->sk_gso_max_size,
                             sk->sk_gso_max_segs * tp->mss_cache))
                  goto send_now;
  
          /* Middle in queue won't get any more data, full sendable already? */
          if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
                  goto send_now;
  
          win_divisor = ACCESS_ONCE(sysctl_tcp_tso_win_divisor);
          if (win_divisor) {
                  u32 chunk = min(tp->snd_wnd, tp->snd_cwnd * tp->mss_cache);
  
                  /* If at least some fraction of a window is available,
                   * just use it.
                   */
                  chunk /= win_divisor;
                  if (limit >= chunk)
                          goto send_now;
          } else {
                  /* Different approach, try not to defer past a single
                   * ACK.  Receiver should ACK every other full sized
                   * frame, so if we have space for more than 3 frames
                   * then send now.
                   */
                  if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
                          goto send_now;
          }
  
          /* Ok, it looks like it is advisable to defer.  */
          tp->tso_deferred = 1 | (jiffies << 1);
  
          return true;
  
  send_now:
          tp->tso_deferred = 0;
          return false;
  }
  
  /* Create a new MTU probe if we are ready.
   * MTU probe is regularly attempting to increase the path MTU by
   * deliberately sending larger packets.  This discovers routing
   * changes resulting in larger path MTUs.
   *
   * Returns 0 if we should wait to probe (no cwnd available),
   *         1 if a probe was sent,
   *         -1 otherwise
   */
  static int tcp_mtu_probe(struct sock *sk)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct inet_connection_sock *icsk = inet_csk(sk);
          struct sk_buff *skb, *nskb, *next;
          int len;
          int probe_size;
          int size_needed;
          int copy;
          int mss_now;
  
          /* Not currently probing/verifying,
           * not in recovery,
           * have enough cwnd, and
           * not SACKing (the variable headers throw things off) */
          if (!icsk->icsk_mtup.enabled ||
              icsk->icsk_mtup.probe_size ||
              inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
              tp->snd_cwnd < 11 ||
              tp->rx_opt.num_sacks || tp->rx_opt.dsack)
                  return -1;
  
          /* Very simple search strategy: just double the MSS. */
          mss_now = tcp_current_mss(sk);
          probe_size = 2 * tp->mss_cache;
          size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
          if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {
                  /* TODO: set timer for probe_converge_event */
                  return -1;
          }
  
          /* Have enough data in the send queue to probe? */
          if (tp->write_seq - tp->snd_nxt < size_needed)
                  return -1;
  
          if (tp->snd_wnd < size_needed)
                  return -1;
          if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
                  return 0;
  
          /* Do we need to wait to drain cwnd? With none in flight, don't stall */
          if (tcp_packets_in_flight(tp) + 2 > tp->snd_cwnd) {
                  if (!tcp_packets_in_flight(tp))
                          return -1;
                  else
                          return 0;
          }
  
          /* We're allowed to probe.  Build it now. */
          if ((nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC)) == NULL)
                  return -1;
          sk->sk_wmem_queued += nskb->truesize;
          sk_mem_charge(sk, nskb->truesize);
  
          skb = tcp_send_head(sk);
  
          TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
          TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
          TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;
          TCP_SKB_CB(nskb)->sacked = 0;
          nskb->csum = 0;
          nskb->ip_summed = skb->ip_summed;
  
          tcp_insert_write_queue_before(nskb, skb, sk);
  
          len = 0;
          tcp_for_write_queue_from_safe(skb, next, sk) {
                  copy = min_t(int, skb->len, probe_size - len);
                  if (nskb->ip_summed)
                          skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
                  else
                          nskb->csum = skb_copy_and_csum_bits(skb, 0,
                                                              skb_put(nskb, copy),
                                                              copy, nskb->csum);
  
                  if (skb->len <= copy) {
                          /* We've eaten all the data from this skb.
                           * Throw it away. */
                          TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
                          tcp_unlink_write_queue(skb, sk);
                          sk_wmem_free_skb(sk, skb);
                  } else {
                          TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
                                                     ~(TCPHDR_FIN|TCPHDR_PSH);
                          if (!skb_shinfo(skb)->nr_frags) {
                                  skb_pull(skb, copy);
                                  if (skb->ip_summed != CHECKSUM_PARTIAL)
                                          skb->csum = csum_partial(skb->data,
                                                                   skb->len, 0);
                          } else {
                                  __pskb_trim_head(skb, copy);
                                  tcp_set_skb_tso_segs(sk, skb, mss_now);
                          }
                          TCP_SKB_CB(skb)->seq += copy;
                  }
  
                  len += copy;
  
                  if (len >= probe_size)
                          break;
          }
          tcp_init_tso_segs(sk, nskb, nskb->len);
  
          /* We're ready to send.  If this fails, the probe will
           * be resegmented into mss-sized pieces by tcp_write_xmit(). */
          TCP_SKB_CB(nskb)->when = tcp_time_stamp;
          if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
                  /* Decrement cwnd here because we are sending
                   * effectively two packets. */
                  tp->snd_cwnd--;
                  tcp_event_new_data_sent(sk, nskb);
  
                  icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
                  tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
                  tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;
  
                  return 1;
          }
  
          return -1;
  }
  
  /* This routine writes packets to the network.  It advances the
   * send_head.  This happens as incoming acks open up the remote
   * window for us.
   *
   * LARGESEND note: !tcp_urg_mode is overkill, only frames between
   * snd_up-64k-mss .. snd_up cannot be large. However, taking into
   * account rare use of URG, this is not a big flaw.
   *
   * Returns true, if no segments are in flight and we have queued segments,
   * but cannot send anything now because of SWS or another problem.
   */
  static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                             int push_one, gfp_t gfp)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb;
          unsigned int tso_segs, sent_pkts;
          int cwnd_quota;
          int result;
  
          sent_pkts = 0;
  
          if (!push_one) {
                  /* Do MTU probing. */
                  result = tcp_mtu_probe(sk);
                  if (!result) {
                          return false;
                  } else if (result > 0) {
                          sent_pkts = 1;
                  }
          }
  
          while ((skb = tcp_send_head(sk))) {
                  unsigned int limit;
  
  
                  tso_segs = tcp_init_tso_segs(sk, skb, mss_now);
                  BUG_ON(!tso_segs);
  
                  if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE)
                          goto repair; /* Skip network transmission */
  
                  cwnd_quota = tcp_cwnd_test(tp, skb);
                  if (!cwnd_quota)
                          break;
  
                  if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
                          break;
  
                  if (tso_segs == 1) {
                          if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
                                                       (tcp_skb_is_last(sk, skb) ?
                                                        nonagle : TCP_NAGLE_PUSH))))
                                  break;
                  } else {
                          if (!push_one && tcp_tso_should_defer(sk, skb))
                                  break;
                  }
  
                  /* TSQ : sk_wmem_alloc accounts skb truesize,
                   * including skb overhead. But thats OK.
                   */
                  if (atomic_read(&sk->sk_wmem_alloc) >= sysctl_tcp_limit_output_bytes) {
                          set_bit(TSQ_THROTTLED, &tp->tsq_flags);
                          break;
                  }
                  limit = mss_now;
                  if (tso_segs > 1 && !tcp_urg_mode(tp))
                          limit = tcp_mss_split_point(sk, skb, mss_now,
                                                      min_t(unsigned int,
                                                            cwnd_quota,
                                                            sk->sk_gso_max_segs));
  
                  if (skb->len > limit &&
                      unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
                          break;
  
                  TCP_SKB_CB(skb)->when = tcp_time_stamp;
  
                  if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
                          break;
  
  repair:
                  /* Advance the send_head.  This one is sent out.
                   * This call will increment packets_out.
                   */
                  tcp_event_new_data_sent(sk, skb);
  
                  tcp_minshall_update(tp, mss_now, skb);
                  sent_pkts += tcp_skb_pcount(skb);
  
                  if (push_one)
                          break;
          }
  
          if (likely(sent_pkts)) {
                  if (tcp_in_cwnd_reduction(sk))
                          tp->prr_out += sent_pkts;
                  tcp_cwnd_validate(sk);
                  return false;
          }
          return !tp->packets_out && tcp_send_head(sk);
  }
  
  /* Push out any pending frames which were held back due to
   * TCP_CORK or attempt at coalescing tiny packets.
   * The socket must be locked by the caller.
   */
  void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
                                 int nonagle)
  {
          /* If we are closed, the bytes will have to remain here.
           * In time closedown will finish, we empty the write queue and
           * all will be happy.
           */
          if (unlikely(sk->sk_state == TCP_CLOSE))
                  return;
  
          if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
                             sk_gfp_atomic(sk, GFP_ATOMIC)))
                  tcp_check_probe_timer(sk);
  }
  
  /* Send _single_ skb sitting at the send head. This function requires
   * true push pending frames to setup probe timer etc.
   */
  void tcp_push_one(struct sock *sk, unsigned int mss_now)
  {
          struct sk_buff *skb = tcp_send_head(sk);
  
          BUG_ON(!skb || skb->len < mss_now);
  
          tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
  }
  
  /* This function returns the amount that we can raise the
   * usable window based on the following constraints
   *
   * 1. The window can never be shrunk once it is offered (RFC 793)
   * 2. We limit memory per socket
   *
   * RFC 1122:
   * "the suggested [SWS] avoidance algorithm for the receiver is to keep
   *  RECV.NEXT + RCV.WIN fixed until:
   *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
   *
   * i.e. don't raise the right edge of the window until you can raise
   * it at least MSS bytes.
   *
   * Unfortunately, the recommended algorithm breaks header prediction,
   * since header prediction assumes th->window stays fixed.
   *
   * Strictly speaking, keeping th->window fixed violates the receiver
   * side SWS prevention criteria. The problem is that under this rule
   * a stream of single byte packets will cause the right side of the
   * window to always advance by a single byte.
   *
   * Of course, if the sender implements sender side SWS prevention
   * then this will not be a problem.
   *
   * BSD seems to make the following compromise:
   *
   *      If the free space is less than the 1/4 of the maximum
   *      space available and the free space is less than 1/2 mss,
   *      then set the window to 0.
   *      [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
   *      Otherwise, just prevent the window from shrinking
   *      and from being larger than the largest representable value.
   *
   * This prevents incremental opening of the window in the regime
   * where TCP is limited by the speed of the reader side taking
   * data out of the TCP receive queue. It does nothing about
   * those cases where the window is constrained on the sender side
   * because the pipeline is full.
   *
   * BSD also seems to "accidentally" limit itself to windows that are a
   * multiple of MSS, at least until the free space gets quite small.
   * This would appear to be a side effect of the mbuf implementation.
   * Combining these two algorithms results in the observed behavior
   * of having a fixed window size at almost all times.
   *
   * Below we obtain similar behavior by forcing the offered window to
   * a multiple of the mss when it is feasible to do so.
   *
   * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
   * Regular options like TIMESTAMP are taken into account.
   */
  u32 __tcp_select_window(struct sock *sk)
  {
          struct inet_connection_sock *icsk = inet_csk(sk);
          struct tcp_sock *tp = tcp_sk(sk);
          /* MSS for the peer's data.  Previous versions used mss_clamp
           * here.  I don't know if the value based on our guesses
           * of peer's MSS is better for the performance.  It's more correct
           * but may be worse for the performance because of rcv_mss
           * fluctuations.  --SAW  1998/11/1
           */
          int mss = icsk->icsk_ack.rcv_mss;
          int free_space = tcp_space(sk);
          int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));
          int window;
  
          if (mss > full_space)
                  mss = full_space;
  
          if (free_space < (full_space >> 1)) {
                  icsk->icsk_ack.quick = 0;
  
                  if (sk_under_memory_pressure(sk))
                          tp->rcv_ssthresh = min(tp->rcv_ssthresh,
                                                 4U * tp->advmss);
  
                  if (free_space < mss)
                          return 0;
          }
  
          if (free_space > tp->rcv_ssthresh)
                  free_space = tp->rcv_ssthresh;
  
          /* Don't do rounding if we are using window scaling, since the
           * scaled window will not line up with the MSS boundary anyway.
           */
          window = tp->rcv_wnd;
          if (tp->rx_opt.rcv_wscale) {
                  window = free_space;
  
                  /* Advertise enough space so that it won't get scaled away.
                   * Import case: prevent zero window announcement if
                   * 1<<rcv_wscale > mss.
                   */
                  if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
                          window = (((window >> tp->rx_opt.rcv_wscale) + 1)
                                    << tp->rx_opt.rcv_wscale);
          } else {
                  /* Get the largest window that is a nice multiple of mss.
                   * Window clamp already applied above.
                   * If our current window offering is within 1 mss of the
                   * free space we just keep it. This prevents the divide
                   * and multiply from happening most of the time.
                   * We also don't do any window rounding when the free space
                   * is too small.
                   */
                  if (window <= free_space - mss || window > free_space)
                          window = (free_space / mss) * mss;
                  else if (mss == full_space &&
                           free_space > window + (full_space >> 1))
                          window = free_space;
          }
  
          return window;
  }
  
  /* Collapses two adjacent SKB's during retransmission. */
  static void tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *next_skb = tcp_write_queue_next(sk, skb);
          int skb_size, next_skb_size;
  
          skb_size = skb->len;
          next_skb_size = next_skb->len;
  
          BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);
  
          tcp_highest_sack_combine(sk, next_skb, skb);
  
          tcp_unlink_write_queue(next_skb, sk);
  
          skb_copy_from_linear_data(next_skb, skb_put(skb, next_skb_size),
                                    next_skb_size);
  
          if (next_skb->ip_summed == CHECKSUM_PARTIAL)
                  skb->ip_summed = CHECKSUM_PARTIAL;
  
          if (skb->ip_summed != CHECKSUM_PARTIAL)
                  skb->csum = csum_block_add(skb->csum, next_skb->csum, skb_size);
  
          /* Update sequence range on original skb. */
          TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
  
          /* Merge over control information. This moves PSH/FIN etc. over */
          TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
  
          /* All done, get rid of second SKB and account for it so
           * packet counting does not break.
           */
          TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
  
          /* changed transmit queue under us so clear hints */
          tcp_clear_retrans_hints_partial(tp);
          if (next_skb == tp->retransmit_skb_hint)
                  tp->retransmit_skb_hint = skb;
  
          tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));
  
          sk_wmem_free_skb(sk, next_skb);
  }
  
  /* Check if coalescing SKBs is legal. */
  static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
  {
          if (tcp_skb_pcount(skb) > 1)
                  return false;
          /* TODO: SACK collapsing could be used to remove this condition */
          if (skb_shinfo(skb)->nr_frags != 0)
                  return false;
          if (skb_cloned(skb))
                  return false;
          if (skb == tcp_send_head(sk))
                  return false;
          /* Some heurestics for collapsing over SACK'd could be invented */
          if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                  return false;
  
          return true;
  }
  
  /* Collapse packets in the retransmit queue to make to create
   * less packets on the wire. This is only done on retransmission.
   */
  static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
                                       int space)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb = to, *tmp;
          bool first = true;
  
          if (!sysctl_tcp_retrans_collapse)
                  return;
          if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
                  return;
  
          tcp_for_write_queue_from_safe(skb, tmp, sk) {
                  if (!tcp_can_collapse(sk, skb))
                          break;
  
                  space -= skb->len;
  
                  if (first) {
                          first = false;
                          continue;
                  }
  
                  if (space < 0)
                          break;
                  /* Punt if not enough space exists in the first SKB for
                   * the data in the second
                   */
                  if (skb->len > skb_availroom(to))
                          break;
  
                  if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
                          break;
  
                  tcp_collapse_retrans(sk, to);
          }
  }
  
  /* This retransmits one SKB.  Policy decisions and retransmit queue
   * state updates are done by the caller.  Returns non-zero if an
   * error occurred which prevented the send.
   */
  int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct inet_connection_sock *icsk = inet_csk(sk);
          unsigned int cur_mss;
  
          /* Inconslusive MTU probe */
          if (icsk->icsk_mtup.probe_size) {
                  icsk->icsk_mtup.probe_size = 0;
          }
  
          /* Do not sent more than we queued. 1/4 is reserved for possible
           * copying overhead: fragmentation, tunneling, mangling etc.
           */
          if (atomic_read(&sk->sk_wmem_alloc) >
              min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf))
                  return -EAGAIN;
  
          if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
                  if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                          BUG();
                  if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
                          return -ENOMEM;
          }
  
          if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
                  return -EHOSTUNREACH; /* Routing failure or similar. */
  
          cur_mss = tcp_current_mss(sk);
  
          /* If receiver has shrunk his window, and skb is out of
           * new window, do not retransmit it. The exception is the
           * case, when window is shrunk to zero. In this case
           * our retransmit serves as a zero window probe.
           */
          if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
              TCP_SKB_CB(skb)->seq != tp->snd_una)
                  return -EAGAIN;
  
          if (skb->len > cur_mss) {
                  if (tcp_fragment(sk, skb, cur_mss, cur_mss))
                          return -ENOMEM; /* We'll try again later. */
          } else {
                  int oldpcount = tcp_skb_pcount(skb);
  
                  if (unlikely(oldpcount > 1)) {
                          tcp_init_tso_segs(sk, skb, cur_mss);
                          tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
                  }
          }
  
          tcp_retrans_try_collapse(sk, skb, cur_mss);
  
          /* Some Solaris stacks overoptimize and ignore the FIN on a
           * retransmit when old data is attached.  So strip it off
           * since it is cheap to do so and saves bytes on the network.
           */
          if (skb->len > 0 &&
              (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
              tp->snd_una == (TCP_SKB_CB(skb)->end_seq - 1)) {
                  if (!pskb_trim(skb, 0)) {
                          /* Reuse, even though it does some unnecessary work */
                          tcp_init_nondata_skb(skb, TCP_SKB_CB(skb)->end_seq - 1,
                                               TCP_SKB_CB(skb)->tcp_flags);
                          skb->ip_summed = CHECKSUM_NONE;
                  }
          }
  
          /* Make a copy, if the first transmission SKB clone we made
           * is still in somebody's hands, else make a clone.
           */
          TCP_SKB_CB(skb)->when = tcp_time_stamp;
  
          /* make sure skb->data is aligned on arches that require it */
          if (unlikely(NET_IP_ALIGN && ((unsigned long)skb->data & 3))) {
                  struct sk_buff *nskb = __pskb_copy(skb, MAX_TCP_HEADER,
                                                     GFP_ATOMIC);
                  return nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
                                -ENOBUFS;
          } else {
                  return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
          }
  }
  
  int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          int err = __tcp_retransmit_skb(sk, skb);
  
          if (err == 0) {
                  /* Update global TCP statistics. */
                  TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
  
                  tp->total_retrans++;
  
  #if FASTRETRANS_DEBUG > 0
                  if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
                          net_dbg_ratelimited("retrans_out leaked\n");
                  }
  #endif
                  if (!tp->retrans_out)
                          tp->lost_retrans_low = tp->snd_nxt;
                  TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
                  tp->retrans_out += tcp_skb_pcount(skb);
  
                  /* Save stamp of the first retransmit. */
                  if (!tp->retrans_stamp)
                          tp->retrans_stamp = TCP_SKB_CB(skb)->when;
  
                  tp->undo_retrans += tcp_skb_pcount(skb);
  
                  /* snd_nxt is stored to detect loss of retransmitted segment,
                   * see tcp_input.c tcp_sacktag_write_queue().
                   */
                  TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt;
          }
          return err;
  }
  
  /* Check if we forward retransmits are possible in the current
   * window/congestion state.
   */
  static bool tcp_can_forward_retransmit(struct sock *sk)
  {
          const struct inet_connection_sock *icsk = inet_csk(sk);
          const struct tcp_sock *tp = tcp_sk(sk);
  
          /* Forward retransmissions are possible only during Recovery. */
          if (icsk->icsk_ca_state != TCP_CA_Recovery)
                  return false;
  
          /* No forward retransmissions in Reno are possible. */
          if (tcp_is_reno(tp))
                  return false;
  
          /* Yeah, we have to make difficult choice between forward transmission
           * and retransmission... Both ways have their merits...
           *
           * For now we do not retransmit anything, while we have some new
           * segments to send. In the other cases, follow rule 3 for
           * NextSeg() specified in RFC3517.
           */
  
          if (tcp_may_send_now(sk))
                  return false;
  
          return true;
  }
  
  /* This gets called after a retransmit timeout, and the initially
   * retransmitted data is acknowledged.  It tries to continue
   * resending the rest of the retransmit queue, until either
   * we've sent it all or the congestion window limit is reached.
   * If doing SACK, the first ACK which comes back for a timeout
   * based retransmit packet might feed us FACK information again.
   * If so, we use it to avoid unnecessarily retransmissions.
   */
  void tcp_xmit_retransmit_queue(struct sock *sk)
  {
          const struct inet_connection_sock *icsk = inet_csk(sk);
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb;
          struct sk_buff *hole = NULL;
          u32 last_lost;
          int mib_idx;
          int fwd_rexmitting = 0;
  
          if (!tp->packets_out)
                  return;
  
          if (!tp->lost_out)
                  tp->retransmit_high = tp->snd_una;
  
          if (tp->retransmit_skb_hint) {
                  skb = tp->retransmit_skb_hint;
                  last_lost = TCP_SKB_CB(skb)->end_seq;
                  if (after(last_lost, tp->retransmit_high))
                          last_lost = tp->retransmit_high;
          } else {
                  skb = tcp_write_queue_head(sk);
                  last_lost = tp->snd_una;
          }
  
          tcp_for_write_queue_from(skb, sk) {
                  __u8 sacked = TCP_SKB_CB(skb)->sacked;
  
                  if (skb == tcp_send_head(sk))
                          break;
                  /* we could do better than to assign each time */
                  if (hole == NULL)
                          tp->retransmit_skb_hint = skb;
  
                  /* Assume this retransmit will generate
                   * only one packet for congestion window
                   * calculation purposes.  This works because
                   * tcp_retransmit_skb() will chop up the
                   * packet to be MSS sized and all the
                   * packet counting works out.
                   */
                  if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
                          return;
  
                  if (fwd_rexmitting) {
  begin_fwd:
                          if (!before(TCP_SKB_CB(skb)->seq, tcp_highest_sack_seq(tp)))
                                  break;
                          mib_idx = LINUX_MIB_TCPFORWARDRETRANS;
  
                  } else if (!before(TCP_SKB_CB(skb)->seq, tp->retransmit_high)) {
                          tp->retransmit_high = last_lost;
                          if (!tcp_can_forward_retransmit(sk))
                                  break;
                          /* Backtrack if necessary to non-L'ed skb */
                          if (hole != NULL) {
                                  skb = hole;
                                  hole = NULL;
                          }
                          fwd_rexmitting = 1;
                          goto begin_fwd;
  
                  } else if (!(sacked & TCPCB_LOST)) {
                          if (hole == NULL && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
                                  hole = skb;
                          continue;
  
                  } else {
                          last_lost = TCP_SKB_CB(skb)->end_seq;
                          if (icsk->icsk_ca_state != TCP_CA_Loss)
                                  mib_idx = LINUX_MIB_TCPFASTRETRANS;
                          else
                                  mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
                  }
  
                  if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
                          continue;
  
                  if (tcp_retransmit_skb(sk, skb)) {
                          NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
                          return;
                  }
                  NET_INC_STATS_BH(sock_net(sk), mib_idx);
  
                  if (tcp_in_cwnd_reduction(sk))
                          tp->prr_out += tcp_skb_pcount(skb);
  
                  if (skb == tcp_write_queue_head(sk))
                          inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                                    inet_csk(sk)->icsk_rto,
                                                    TCP_RTO_MAX);
          }
  }
  
  /* Send a fin.  The caller locks the socket for us.  This cannot be
   * allowed to fail queueing a FIN frame under any circumstances.
   */
  void tcp_send_fin(struct sock *sk)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb = tcp_write_queue_tail(sk);
          int mss_now;
  
          /* Optimization, tack on the FIN if we have a queue of
           * unsent frames.  But be careful about outgoing SACKS
           * and IP options.
           */
          mss_now = tcp_current_mss(sk);
  
          if (tcp_send_head(sk) != NULL) {
                  TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_FIN;
                  TCP_SKB_CB(skb)->end_seq++;
                  tp->write_seq++;
          } else {
                  /* Socket is locked, keep trying until memory is available. */
                  for (;;) {
                          skb = alloc_skb_fclone(MAX_TCP_HEADER,
                                                 sk->sk_allocation);
                          if (skb)
                                  break;
                          yield();
                  }
  
                  /* Reserve space for headers and prepare control bits. */
                  skb_reserve(skb, MAX_TCP_HEADER);
                  /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
                  tcp_init_nondata_skb(skb, tp->write_seq,
                                       TCPHDR_ACK | TCPHDR_FIN);
                  tcp_queue_skb(sk, skb);
          }
          __tcp_push_pending_frames(sk, mss_now, TCP_NAGLE_OFF);
  }
  
  /* We get here when a process closes a file descriptor (either due to
   * an explicit close() or as a byproduct of exit()'ing) and there
   * was unread data in the receive queue.  This behavior is recommended
   * by RFC 2525, section 2.17.  -DaveM
   */
  void tcp_send_active_reset(struct sock *sk, gfp_t priority)
  {
          struct sk_buff *skb;
  
          /* NOTE: No TCP options attached and we never retransmit this. */
          skb = alloc_skb(MAX_TCP_HEADER, priority);
          if (!skb) {
                  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
                  return;
          }
  
          /* Reserve space for headers and prepare control bits. */
          skb_reserve(skb, MAX_TCP_HEADER);
          tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
                               TCPHDR_ACK | TCPHDR_RST);
          /* Send it off. */
          TCP_SKB_CB(skb)->when = tcp_time_stamp;
          if (tcp_transmit_skb(sk, skb, 0, priority))
                  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
  
          TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
  }
  
  /* Send a crossed SYN-ACK during socket establishment.
   * WARNING: This routine must only be called when we have already sent
   * a SYN packet that crossed the incoming SYN that caused this routine
   * to get called. If this assumption fails then the initial rcv_wnd
   * and rcv_wscale values will not be correct.
   */
  int tcp_send_synack(struct sock *sk)
  {
          struct sk_buff *skb;
  
          skb = tcp_write_queue_head(sk);
          if (skb == NULL || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
                  pr_debug("%s: wrong queue state\n", __func__);
                  return -EFAULT;
          }
          if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
                  if (skb_cloned(skb)) {
                          struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
                          if (nskb == NULL)
                                  return -ENOMEM;
                          tcp_unlink_write_queue(skb, sk);
                          skb_header_release(nskb);
                          __tcp_add_write_queue_head(sk, nskb);
                          sk_wmem_free_skb(sk, skb);
                          sk->sk_wmem_queued += nskb->truesize;
                          sk_mem_charge(sk, nskb->truesize);
                          skb = nskb;
                  }
  
                  TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
                  TCP_ECN_send_synack(tcp_sk(sk), skb);
          }
          TCP_SKB_CB(skb)->when = tcp_time_stamp;
          return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
  }
  
  /**
   * tcp_make_synack - Prepare a SYN-ACK.
   * sk: listener socket
   * dst: dst entry attached to the SYNACK
   * req: request_sock pointer
   * rvp: request_values pointer
   *
   * Allocate one skb and build a SYNACK packet.
   * @dst is consumed : Caller should not use it again.
   */
  struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                                  struct request_sock *req,
                                  struct request_values *rvp,
                                  struct tcp_fastopen_cookie *foc)
  {
          struct tcp_out_options opts;
          struct tcp_extend_values *xvp = tcp_xv(rvp);
          struct inet_request_sock *ireq = inet_rsk(req);
          struct tcp_sock *tp = tcp_sk(sk);
          const struct tcp_cookie_values *cvp = tp->cookie_values;
          struct tcphdr *th;
          struct sk_buff *skb;
          struct tcp_md5sig_key *md5;
          int tcp_header_size;
          int mss;
          int s_data_desired = 0;
  
          if (cvp != NULL && cvp->s_data_constant && cvp->s_data_desired)
                  s_data_desired = cvp->s_data_desired;
          skb = alloc_skb(MAX_TCP_HEADER + 15 + s_data_desired,
                          sk_gfp_atomic(sk, GFP_ATOMIC));
          if (unlikely(!skb)) {
                  dst_release(dst);
                  return NULL;
          }
          /* Reserve space for headers. */
          skb_reserve(skb, MAX_TCP_HEADER);
  
          skb_dst_set(skb, dst);
  
          mss = dst_metric_advmss(dst);
          if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
                  mss = tp->rx_opt.user_mss;
  
          if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
                  __u8 rcv_wscale;
                  /* Set this up on the first call only */
                  req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
  
                  /* limit the window selection if the user enforce a smaller rx buffer */
                  if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
                      (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
                          req->window_clamp = tcp_full_space(sk);
  
                  /* tcp_full_space because it is guaranteed to be the first packet */
                  tcp_select_initial_window(tcp_full_space(sk),
                          mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
                          &req->rcv_wnd,
                          &req->window_clamp,
                          ireq->wscale_ok,
                          &rcv_wscale,
                          dst_metric(dst, RTAX_INITRWND));
                  ireq->rcv_wscale = rcv_wscale;
          }
  
          memset(&opts, 0, sizeof(opts));
  #ifdef CONFIG_SYN_COOKIES
          if (unlikely(req->cookie_ts))
                  TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
          else
  #endif
          TCP_SKB_CB(skb)->when = tcp_time_stamp;
          tcp_header_size = tcp_synack_options(sk, req, mss,
                                               skb, &opts, &md5, xvp, foc)
                          + sizeof(*th);
  
          skb_push(skb, tcp_header_size);
          skb_reset_transport_header(skb);
  
          th = tcp_hdr(skb);
          memset(th, 0, sizeof(struct tcphdr));
          th->syn = 1;
          th->ack = 1;
          TCP_ECN_make_synack(req, th);
          th->source = ireq->loc_port;
          th->dest = ireq->rmt_port;
          /* Setting of flags are superfluous here for callers (and ECE is
           * not even correctly set)
           */
          tcp_init_nondata_skb(skb, tcp_rsk(req)->snt_isn,
                               TCPHDR_SYN | TCPHDR_ACK);
  
          if (OPTION_COOKIE_EXTENSION & opts.options) {
                  if (s_data_desired) {
                          u8 *buf = skb_put(skb, s_data_desired);
  
                          /* copy data directly from the listening socket. */
                          memcpy(buf, cvp->s_data_payload, s_data_desired);
                          TCP_SKB_CB(skb)->end_seq += s_data_desired;
                  }
  
                  if (opts.hash_size > 0) {
                          __u32 workspace[SHA_WORKSPACE_WORDS];
                          u32 *mess = &xvp->cookie_bakery[COOKIE_DIGEST_WORDS];
                          u32 *tail = &mess[COOKIE_MESSAGE_WORDS-1];
  
                          /* Secret recipe depends on the Timestamp, (future)
                           * Sequence and Acknowledgment Numbers, Initiator
                           * Cookie, and others handled by IP variant caller.
                           */
                          *tail-- ^= opts.tsval;
                          *tail-- ^= tcp_rsk(req)->rcv_isn + 1;
                          *tail-- ^= TCP_SKB_CB(skb)->seq + 1;
  
                          /* recommended */
                          *tail-- ^= (((__force u32)th->dest << 16) | (__force u32)th->source);
                          *tail-- ^= (u32)(unsigned long)cvp; /* per sockopt */
  
                          sha_transform((__u32 *)&xvp->cookie_bakery[0],
                                        (char *)mess,
                                        &workspace[0]);
                          opts.hash_location =
                                  (__u8 *)&xvp->cookie_bakery[0];
                  }
          }
  
          th->seq = htonl(TCP_SKB_CB(skb)->seq);
          /* XXX data is queued and acked as is. No buffer/window check */
          th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);
  
          /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
          th->window = htons(min(req->rcv_wnd, 65535U));
          tcp_options_write((__be32 *)(th + 1), tp, &opts);
          th->doff = (tcp_header_size >> 2);
          TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS, tcp_skb_pcount(skb));
  
  #ifdef CONFIG_TCP_MD5SIG
          /* Okay, we have all we need - do the md5 hash if needed */
          if (md5) {
                  tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
                                                 md5, NULL, req, skb);
          }
  #endif
  
          return skb;
  }
  EXPORT_SYMBOL(tcp_make_synack);
  
  /* Do all connect socket setups that can be done AF independent. */
  void tcp_connect_init(struct sock *sk)
  {
          const struct dst_entry *dst = __sk_dst_get(sk);
          struct tcp_sock *tp = tcp_sk(sk);
          __u8 rcv_wscale;
  
          /* We'll fix this up when we get a response from the other end.
           * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
           */
          tp->tcp_header_len = sizeof(struct tcphdr) +
                  (sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);
  
  #ifdef CONFIG_TCP_MD5SIG
          if (tp->af_specific->md5_lookup(sk, sk) != NULL)
                  tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
  #endif
  
          /* If user gave his TCP_MAXSEG, record it to clamp */
          if (tp->rx_opt.user_mss)
                  tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
          tp->max_window = 0;
          tcp_mtup_init(sk);
          tcp_sync_mss(sk, dst_mtu(dst));
  
          if (!tp->window_clamp)
                  tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
          tp->advmss = dst_metric_advmss(dst);
          if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
                  tp->advmss = tp->rx_opt.user_mss;
  
          tcp_initialize_rcv_mss(sk);
  
          /* limit the window selection if the user enforce a smaller rx buffer */
          if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
              (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
                  tp->window_clamp = tcp_full_space(sk);
  
          tcp_select_initial_window(tcp_full_space(sk),
                                    tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
                                    &tp->rcv_wnd,
                                    &tp->window_clamp,
                                    sysctl_tcp_window_scaling,
                                    &rcv_wscale,
                                    dst_metric(dst, RTAX_INITRWND));
  
          tp->rx_opt.rcv_wscale = rcv_wscale;
          tp->rcv_ssthresh = tp->rcv_wnd;
  
          sk->sk_err = 0;
          sock_reset_flag(sk, SOCK_DONE);
          tp->snd_wnd = 0;
          tcp_init_wl(tp, 0);
          tp->snd_una = tp->write_seq;
          tp->snd_sml = tp->write_seq;
          tp->snd_up = tp->write_seq;
          tp->snd_nxt = tp->write_seq;
  
          if (likely(!tp->repair))
                  tp->rcv_nxt = 0;
          tp->rcv_wup = tp->rcv_nxt;
          tp->copied_seq = tp->rcv_nxt;
  
          inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
          inet_csk(sk)->icsk_retransmits = 0;
          tcp_clear_retrans(tp);
  }
  
  static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
  
          tcb->end_seq += skb->len;
          skb_header_release(skb);
          __tcp_add_write_queue_tail(sk, skb);
          sk->sk_wmem_queued += skb->truesize;
          sk_mem_charge(sk, skb->truesize);
          tp->write_seq = tcb->end_seq;
          tp->packets_out += tcp_skb_pcount(skb);
  }
  
  /* Build and send a SYN with data and (cached) Fast Open cookie. However,
   * queue a data-only packet after the regular SYN, such that regular SYNs
   * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
   * only the SYN sequence, the data are retransmitted in the first ACK.
   * If cookie is not cached or other error occurs, falls back to send a
   * regular SYN with Fast Open cookie request option.
   */
  static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct tcp_fastopen_request *fo = tp->fastopen_req;
          int syn_loss = 0, space, i, err = 0, iovlen = fo->data->msg_iovlen;
          struct sk_buff *syn_data = NULL, *data;
          unsigned long last_syn_loss = 0;
  
          tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
          tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
                                 &syn_loss, &last_syn_loss);
          /* Recurring FO SYN losses: revert to regular handshake temporarily */
          if (syn_loss > 1 &&
              time_before(jiffies, last_syn_loss + (60*HZ << syn_loss))) {
                  fo->cookie.len = -1;
                  goto fallback;
          }
  
          if (sysctl_tcp_fastopen & TFO_CLIENT_NO_COOKIE)
                  fo->cookie.len = -1;
          else if (fo->cookie.len <= 0)
                  goto fallback;
  
          /* MSS for SYN-data is based on cached MSS and bounded by PMTU and
           * user-MSS. Reserve maximum option space for middleboxes that add
           * private TCP options. The cost is reduced data space in SYN :(
           */
          if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->rx_opt.mss_clamp)
                  tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
          space = tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) -
                  MAX_TCP_OPTION_SPACE;
  
          syn_data = skb_copy_expand(syn, skb_headroom(syn), space,
                                     sk->sk_allocation);
          if (syn_data == NULL)
                  goto fallback;
  
          for (i = 0; i < iovlen && syn_data->len < space; ++i) {
                  struct iovec *iov = &fo->data->msg_iov[i];
                  unsigned char __user *from = iov->iov_base;
                  int len = iov->iov_len;
  
                  if (syn_data->len + len > space)
                          len = space - syn_data->len;
                  else if (i + 1 == iovlen)
                          /* No more data pending in inet_wait_for_connect() */
                          fo->data = NULL;
  
                  if (skb_add_data(syn_data, from, len))
                          goto fallback;
          }
  
          /* Queue a data-only packet after the regular SYN for retransmission */
          data = pskb_copy(syn_data, sk->sk_allocation);
          if (data == NULL)
                  goto fallback;
          TCP_SKB_CB(data)->seq++;
          TCP_SKB_CB(data)->tcp_flags &= ~TCPHDR_SYN;
          TCP_SKB_CB(data)->tcp_flags = (TCPHDR_ACK|TCPHDR_PSH);
          tcp_connect_queue_skb(sk, data);
          fo->copied = data->len;
  
          if (tcp_transmit_skb(sk, syn_data, 0, sk->sk_allocation) == 0) {
                  tp->syn_data = (fo->copied > 0);
                  NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
                  goto done;
          }
          syn_data = NULL;
  
  fallback:
          /* Send a regular SYN with Fast Open cookie request option */
          if (fo->cookie.len > 0)
                  fo->cookie.len = 0;
          err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
          if (err)
                  tp->syn_fastopen = 0;
          kfree_skb(syn_data);
  done:
          fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
          return err;
  }
  
  /* Build a SYN and send it off. */
  int tcp_connect(struct sock *sk)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *buff;
          int err;
  
          tcp_connect_init(sk);
  
          if (unlikely(tp->repair)) {
                  tcp_finish_connect(sk, NULL);
                  return 0;
          }
  
          buff = alloc_skb_fclone(MAX_TCP_HEADER + 15, sk->sk_allocation);
          if (unlikely(buff == NULL))
                  return -ENOBUFS;
  
          /* Reserve space for headers. */
          skb_reserve(buff, MAX_TCP_HEADER);
  
          tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
          tp->retrans_stamp = TCP_SKB_CB(buff)->when = tcp_time_stamp;
          tcp_connect_queue_skb(sk, buff);
          TCP_ECN_send_syn(sk, buff);
  
          /* Send off SYN; include data in Fast Open. */
          err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
                tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
          if (err == -ECONNREFUSED)
                  return err;
  
          /* We change tp->snd_nxt after the tcp_transmit_skb() call
           * in order to make this packet get counted in tcpOutSegs.
           */
          tp->snd_nxt = tp->write_seq;
          tp->pushed_seq = tp->write_seq;
          TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
  
          /* Timer for repeating the SYN until an answer. */
          inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                    inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
          return 0;
  }
  EXPORT_SYMBOL(tcp_connect);
  
  /* Send out a delayed ack, the caller does the policy checking
   * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
   * for details.
   */
  void tcp_send_delayed_ack(struct sock *sk)
  {
          struct inet_connection_sock *icsk = inet_csk(sk);
          int ato = icsk->icsk_ack.ato;
          unsigned long timeout;
  
          if (ato > TCP_DELACK_MIN) {
                  const struct tcp_sock *tp = tcp_sk(sk);
                  int max_ato = HZ / 2;
  
                  if (icsk->icsk_ack.pingpong ||
                      (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
                          max_ato = TCP_DELACK_MAX;
  
                  /* Slow path, intersegment interval is "high". */
  
                  /* If some rtt estimate is known, use it to bound delayed ack.
                   * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
                   * directly.
                   */
                  if (tp->srtt) {
                          int rtt = max(tp->srtt >> 3, TCP_DELACK_MIN);
  
                          if (rtt < max_ato)
                                  max_ato = rtt;
                  }
  
                  ato = min(ato, max_ato);
          }
  
          /* Stay within the limit we were given */
          timeout = jiffies + ato;
  
          /* Use new timeout only if there wasn't a older one earlier. */
          if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
                  /* If delack timer was blocked or is about to expire,
                   * send ACK now.
                   */
                  if (icsk->icsk_ack.blocked ||
                      time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
                          tcp_send_ack(sk);
                          return;
                  }
  
                  if (!time_before(timeout, icsk->icsk_ack.timeout))
                          timeout = icsk->icsk_ack.timeout;
          }
          icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
          icsk->icsk_ack.timeout = timeout;
          sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
  }
  
  /* This routine sends an ack and also updates the window. */
  void tcp_send_ack(struct sock *sk)
  {
          struct sk_buff *buff;
  
          /* If we have been reset, we may not send again. */
          if (sk->sk_state == TCP_CLOSE)
                  return;
  
          /* We are not putting this on the write queue, so
           * tcp_transmit_skb() will set the ownership to this
           * sock.
           */
          buff = alloc_skb(MAX_TCP_HEADER, sk_gfp_atomic(sk, GFP_ATOMIC));
          if (buff == NULL) {
                  inet_csk_schedule_ack(sk);
                  inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
                  inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
                                            TCP_DELACK_MAX, TCP_RTO_MAX);
                  return;
          }
  
          /* Reserve space for headers and prepare control bits. */
          skb_reserve(buff, MAX_TCP_HEADER);
          tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);
  
          /* Send it off, this clears delayed acks for us. */
          TCP_SKB_CB(buff)->when = tcp_time_stamp;
          tcp_transmit_skb(sk, buff, 0, sk_gfp_atomic(sk, GFP_ATOMIC));
  }
  
  /* This routine sends a packet with an out of date sequence
   * number. It assumes the other end will try to ack it.
   *
   * Question: what should we make while urgent mode?
   * 4.4BSD forces sending single byte of data. We cannot send
   * out of window data, because we have SND.NXT==SND.MAX...
   *
   * Current solution: to send TWO zero-length segments in urgent mode:
   * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
   * out-of-date with SND.UNA-1 to probe window.
   */
  static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb;
  
          /* We don't queue it, tcp_transmit_skb() sets ownership. */
          skb = alloc_skb(MAX_TCP_HEADER, sk_gfp_atomic(sk, GFP_ATOMIC));
          if (skb == NULL)
                  return -1;
  
          /* Reserve space for headers and set control bits. */
          skb_reserve(skb, MAX_TCP_HEADER);
          /* Use a previous sequence.  This should cause the other
           * end to send an ack.  Don't queue or clone SKB, just
           * send it.
           */
          tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
          TCP_SKB_CB(skb)->when = tcp_time_stamp;
          return tcp_transmit_skb(sk, skb, 0, GFP_ATOMIC);
  }
  
  void tcp_send_window_probe(struct sock *sk)
  {
          if (sk->sk_state == TCP_ESTABLISHED) {
                  tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
                  tcp_sk(sk)->snd_nxt = tcp_sk(sk)->write_seq;
                  tcp_xmit_probe_skb(sk, 0);
          }
  }
  
  /* Initiate keepalive or window probe from timer. */
  int tcp_write_wakeup(struct sock *sk)
  {
          struct tcp_sock *tp = tcp_sk(sk);
          struct sk_buff *skb;
  
          if (sk->sk_state == TCP_CLOSE)
                  return -1;
  
          if ((skb = tcp_send_head(sk)) != NULL &&
              before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
                  int err;
                  unsigned int mss = tcp_current_mss(sk);
                  unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
  
                  if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
                          tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;
  
                  /* We are probing the opening of a window
                   * but the window size is != 0
                   * must have been a result SWS avoidance ( sender )
                   */
                  if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
                      skb->len > mss) {
                          seg_size = min(seg_size, mss);
                          TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
                          if (tcp_fragment(sk, skb, seg_size, mss))
                                  return -1;
                  } else if (!tcp_skb_pcount(skb))
                          tcp_set_skb_tso_segs(sk, skb, mss);
  
                  TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
                  TCP_SKB_CB(skb)->when = tcp_time_stamp;
                  err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
                  if (!err)
                          tcp_event_new_data_sent(sk, skb);
                  return err;
          } else {
                  if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
                          tcp_xmit_probe_skb(sk, 1);
                  return tcp_xmit_probe_skb(sk, 0);
          }
  }
  
  /* A window probe timeout has occurred.  If window is not closed send
   * a partial packet else a zero probe.
   */
  void tcp_send_probe0(struct sock *sk)
  {
          struct inet_connection_sock *icsk = inet_csk(sk);
          struct tcp_sock *tp = tcp_sk(sk);
          int err;
  
          err = tcp_write_wakeup(sk);
  
          if (tp->packets_out || !tcp_send_head(sk)) {
                  /* Cancel probe timer, if it is not required. */
                  icsk->icsk_probes_out = 0;
                  icsk->icsk_backoff = 0;
                  return;
          }
  
          if (err <= 0) {
                  if (icsk->icsk_backoff < sysctl_tcp_retries2)
                          icsk->icsk_backoff++;
                  icsk->icsk_probes_out++;
                  inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                                            min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
                                            TCP_RTO_MAX);
          } else {
                  /* If packet was not sent due to local congestion,
                   * do not backoff and do not remember icsk_probes_out.
                   * Let local senders to fight for local resources.
                   *
                   * Use accumulated backoff yet.
                   */
                  if (!icsk->icsk_probes_out)
                          icsk->icsk_probes_out = 1;
                  inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                                            min(icsk->icsk_rto << icsk->icsk_backoff,
                                                TCP_RESOURCE_PROBE_INTERVAL),
                                            TCP_RTO_MAX);
          }
  }