text: /*
   * 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 four TSO segments */
  int sysctl_tcp_limit_output_bytes __read_mostly = 262144;
  
  /* 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;
  
  /* By default, RFC2861 behavior.  */
  int sysctl_tcp_slow_start_after_idle __read_mostly = 1;
  
  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 inet_connection_sock *icsk = inet_csk(sk);
  	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;
  
  	tp->packets_out += tcp_skb_pcount(skb);
  	if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
  		tcp_rearm_rto(sk);
  
  	NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
  		      tcp_skb_pcount(skb));
  }
  
  /* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
   * window scaling factor due to loss of precision.
   * 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) ||
  	    (tp->rx_opt.wscale_ok &&
  	     ((tp->snd_nxt - tcp_wnd_end(tp)) < (1 << tp->rx_opt.rcv_wscale))))
  		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.
   */
  void tcp_cwnd_restart(struct sock *sk, s32 delta)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
  	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_jiffies32;
  	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_jiffies32;
  
  	if (tcp_packets_in_flight(tp) == 0)
  		tcp_ca_event(sk, CA_EVENT_TX_START);
  
  	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);
  }
  
  
  u32 tcp_default_init_rwnd(u32 mss)
  {
  	/* Initial receive window should be twice of TCP_INIT_CWND to
  	 * enable proper sending of new unsent data during fast recovery
  	 * (RFC 3517, Section 4, NextSeg() rule (2)). Further place a
  	 * limit when mss is larger than 1460.
  	 */
  	u32 init_rwnd = TCP_INIT_CWND * 2;
  
  	if (mss > 1460)
  		init_rwnd = max((1460 * init_rwnd) / mss, 2U);
  	return init_rwnd;
  }
  
  /* 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) = (U16_MAX << TCP_MAX_WSCALE);
  	space = min(*window_clamp, space);
  
  	/* Quantize space offering to a multiple of mss if possible. */
  	if (space > mss)
  		space = rounddown(space, 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 */
  		space = max_t(u32, space, sysctl_tcp_rmem[2]);
  		space = max_t(u32, space, sysctl_rmem_max);
  		space = min_t(u32, space, *window_clamp);
  		while (space > U16_MAX && (*rcv_wscale) < TCP_MAX_WSCALE) {
  			space >>= 1;
  			(*rcv_wscale)++;
  		}
  	}
  
  	if (mss > (1 << *rcv_wscale)) {
  		if (!init_rcv_wnd) /* Use default unless specified otherwise */
  			init_rcv_wnd = tcp_default_init_rwnd(mss);
  		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
  	}
  
  	/* Set the clamp no higher than max representable value */
  	(*window_clamp) = min_t(__u32, U16_MAX << (*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 old_win = tp->rcv_wnd;
  	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.
  		 */
  		if (new_win == 0)
  			NET_INC_STATS(sock_net(sk),
  				      LINUX_MIB_TCPWANTZEROWINDOWADV);
  		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;
  		if (old_win)
  			NET_INC_STATS(sock_net(sk),
  				      LINUX_MIB_TCPTOZEROWINDOWADV);
  	} else if (old_win == 0) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFROMZEROWINDOWADV);
  	}
  
  	return new_win;
  }
  
  /* Packet ECN state for a SYN-ACK */
  static void tcp_ecn_send_synack(struct sock *sk, struct sk_buff *skb)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  
  	TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
  	if (!(tp->ecn_flags & TCP_ECN_OK))
  		TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
  	else if (tcp_ca_needs_ecn(sk) ||
  		 tcp_bpf_ca_needs_ecn(sk))
  		INET_ECN_xmit(sk);
  }
  
  /* Packet ECN state for a SYN.  */
  static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
  	bool use_ecn = sock_net(sk)->ipv4.sysctl_tcp_ecn == 1 ||
  		tcp_ca_needs_ecn(sk) || bpf_needs_ecn;
  
  	if (!use_ecn) {
  		const struct dst_entry *dst = __sk_dst_get(sk);
  
  		if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
  			use_ecn = true;
  	}
  
  	tp->ecn_flags = 0;
  
  	if (use_ecn) {
  		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
  		tp->ecn_flags = TCP_ECN_OK;
  		if (tcp_ca_needs_ecn(sk) || bpf_needs_ecn)
  			INET_ECN_xmit(sk);
  	}
  }
  
  static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
  {
  	if (sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback)
  		/* tp->ecn_flags are cleared at a later point in time when
  		 * SYN ACK is ultimatively being received.
  		 */
  		TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
  }
  
  static 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 void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
  			 struct tcphdr *th, 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;
  				th->cwr = 1;
  				skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
  			}
  		} else if (!tcp_ca_needs_ecn(sk)) {
  			/* ACK or retransmitted segment: clear ECT|CE */
  			INET_ECN_dontxmit(sk);
  		}
  		if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
  			th->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;
  
  	tcp_skb_pcount_set(skb, 1);
  
  	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_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 */
  };
  
  /* 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-operability 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 */
  
  	if (unlikely(OPTION_MD5 & options)) {
  		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
  			       (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
  		/* 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);
  	}
  
  	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;
  		u8 *p = (u8 *)ptr;
  		u32 len; /* Fast Open option length */
  
  		if (foc->exp) {
  			len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
  			*ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
  				     TCPOPT_FASTOPEN_MAGIC);
  			p += TCPOLEN_EXP_FASTOPEN_BASE;
  		} else {
  			len = TCPOLEN_FASTOPEN_BASE + foc->len;
  			*p++ = TCPOPT_FASTOPEN;
  			*p++ = len;
  		}
  
  		memcpy(p, foc->val, foc->len);
  		if ((len & 3) == 2) {
  			p[foc->len] = TCPOPT_NOP;
  			p[foc->len + 1] = TCPOPT_NOP;
  		}
  		ptr += (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);
  	unsigned int remaining = MAX_TCP_OPTION_SPACE;
  	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(sock_net(sk)->ipv4.sysctl_tcp_timestamps && !*md5)) {
  		opts->options |= OPTION_TS;
  		opts->tsval = tcp_skb_timestamp(skb) + tp->tsoffset;
  		opts->tsecr = tp->rx_opt.ts_recent;
  		remaining -= TCPOLEN_TSTAMP_ALIGNED;
  	}
  	if (likely(sock_net(sk)->ipv4.sysctl_tcp_window_scaling)) {
  		opts->ws = tp->rx_opt.rcv_wscale;
  		opts->options |= OPTION_WSCALE;
  		remaining -= TCPOLEN_WSCALE_ALIGNED;
  	}
  	if (likely(sock_net(sk)->ipv4.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 = fastopen->cookie.len;
  
  		need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
  					       TCPOLEN_FASTOPEN_BASE;
  		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;
  			tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
  		}
  	}
  
  	return MAX_TCP_OPTION_SPACE - remaining;
  }
  
  /* Set up TCP options for SYN-ACKs. */
  static unsigned int tcp_synack_options(struct request_sock *req,
  				       unsigned int mss, struct sk_buff *skb,
  				       struct tcp_out_options *opts,
  				       const struct tcp_md5sig_key *md5,
  				       struct tcp_fastopen_cookie *foc)
  {
  	struct inet_request_sock *ireq = inet_rsk(req);
  	unsigned int remaining = MAX_TCP_OPTION_SPACE;
  
  #ifdef CONFIG_TCP_MD5SIG
  	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;
  	}
  #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_timestamp(skb) + tcp_rsk(req)->ts_off;
  		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 && foc->len >= 0) {
  		u32 need = foc->len;
  
  		need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
  				   TCPOLEN_FASTOPEN_BASE;
  		need = (need + 3) & ~3U;  /* Align to 32 bits */
  		if (remaining >= need) {
  			opts->options |= OPTION_FAST_OPEN_COOKIE;
  			opts->fastopen_cookie = foc;
  			remaining -= need;
  		}
  	}
  
  	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_sock *tp = tcp_sk(sk);
  	unsigned int size = 0;
  	unsigned int eff_sacks;
  
  	opts->options = 0;
  
  #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 = skb ? tcp_skb_timestamp(skb) + tp->tsoffset : 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 subtracted 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)) {
  		struct tcp_sock *tp = tcp_sk(sk);
  
  		if (tp->lost_out > tp->retrans_out &&
  		    tp->snd_cwnd > tcp_packets_in_flight(tp))
  			tcp_xmit_retransmit_queue(sk);
  
  		tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
  			       0, GFP_ATOMIC);
  	}
  }
  /*
   * One tasklet per cpu tries to send more skbs.
   * We run in tasklet context but need to disable irqs when
   * transferring 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;
  		smp_mb__before_atomic();
  		clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);
  
  		if (!sk->sk_lock.owned &&
  		    test_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags)) {
  			bh_lock_sock(sk);
  			if (!sock_owned_by_user(sk)) {
  				clear_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags);
  				tcp_tsq_handler(sk);
  			}
  			bh_unlock_sock(sk);
  		}
  
  		sk_free(sk);
  	}
  }
  
  #define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED |		\
  			  TCPF_WRITE_TIMER_DEFERRED |	\
  			  TCPF_DELACK_TIMER_DEFERRED |	\
  			  TCPF_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)
  {
  	unsigned long flags, nflags;
  
  	/* perform an atomic operation only if at least one flag is set */
  	do {
  		flags = sk->sk_tsq_flags;
  		if (!(flags & TCP_DEFERRED_ALL))
  			return;
  		nflags = flags & ~TCP_DEFERRED_ALL;
  	} while (cmpxchg(&sk->sk_tsq_flags, flags, nflags) != flags);
  
  	if (flags & TCPF_TSQ_DEFERRED)
  		tcp_tsq_handler(sk);
  
  	/* Here begins the tricky part :
  	 * We are called from release_sock() with :
  	 * 1) BH disabled
  	 * 2) sk_lock.slock spinlock held
  	 * 3) socket owned by us (sk->sk_lock.owned == 1)
  	 *
  	 * But following code is meant to be called from BH handlers,
  	 * so we should keep BH disabled, but early release socket ownership
  	 */
  	sock_release_ownership(sk);
  
  	if (flags & TCPF_WRITE_TIMER_DEFERRED) {
  		tcp_write_timer_handler(sk);
  		__sock_put(sk);
  	}
  	if (flags & TCPF_DELACK_TIMER_DEFERRED) {
  		tcp_delack_timer_handler(sk);
  		__sock_put(sk);
  	}
  	if (flags & TCPF_MTU_REDUCED_DEFERRED) {
  		inet_csk(sk)->icsk_af_ops->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 can't xmit new skbs from this context, as we might already
   * hold qdisc lock.
   */
  void tcp_wfree(struct sk_buff *skb)
  {
  	struct sock *sk = skb->sk;
  	struct tcp_sock *tp = tcp_sk(sk);
  	unsigned long flags, nval, oval;
  
  	/* Keep one reference on sk_wmem_alloc.
  	 * Will be released by sk_free() from here or tcp_tasklet_func()
  	 */
  	WARN_ON(refcount_sub_and_test(skb->truesize - 1, &sk->sk_wmem_alloc));
  
  	/* If this softirq is serviced by ksoftirqd, we are likely under stress.
  	 * Wait until our queues (qdisc + devices) are drained.
  	 * This gives :
  	 * - less callbacks to tcp_write_xmit(), reducing stress (batches)
  	 * - chance for incoming ACK (processed by another cpu maybe)
  	 *   to migrate this flow (skb->ooo_okay will be eventually set)
  	 */
  	if (refcount_read(&sk->sk_wmem_alloc) >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
  		goto out;
  
  	for (oval = READ_ONCE(sk->sk_tsq_flags);; oval = nval) {
  		struct tsq_tasklet *tsq;
  		bool empty;
  
  		if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
  			goto out;
  
  		nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED | TCPF_TSQ_DEFERRED;
  		nval = cmpxchg(&sk->sk_tsq_flags, oval, nval);
  		if (nval != oval)
  			continue;
  
  		/* queue this socket to tasklet queue */
  		local_irq_save(flags);
  		tsq = this_cpu_ptr(&tsq_tasklet);
  		empty = list_empty(&tsq->head);
  		list_add(&tp->tsq_node, &tsq->head);
  		if (empty)
  			tasklet_schedule(&tsq->tasklet);
  		local_irq_restore(flags);
  		return;
  	}
  out:
  	sk_free(sk);
  }
  
  /* Note: Called under hard irq.
   * We can not call TCP stack right away.
   */
  enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
  {
  	struct tcp_sock *tp = container_of(timer, struct tcp_sock, pacing_timer);
  	struct sock *sk = (struct sock *)tp;
  	unsigned long nval, oval;
  
  	for (oval = READ_ONCE(sk->sk_tsq_flags);; oval = nval) {
  		struct tsq_tasklet *tsq;
  		bool empty;
  
  		if (oval & TSQF_QUEUED)
  			break;
  
  		nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED | TCPF_TSQ_DEFERRED;
  		nval = cmpxchg(&sk->sk_tsq_flags, oval, nval);
  		if (nval != oval)
  			continue;
  
  		if (!refcount_inc_not_zero(&sk->sk_wmem_alloc))
  			break;
  		/* queue this socket to tasklet queue */
  		tsq = this_cpu_ptr(&tsq_tasklet);
  		empty = list_empty(&tsq->head);
  		list_add(&tp->tsq_node, &tsq->head);
  		if (empty)
  			tasklet_schedule(&tsq->tasklet);
  		break;
  	}
  	return HRTIMER_NORESTART;
  }
  
  /* BBR congestion control needs pacing.
   * Same remark for SO_MAX_PACING_RATE.
   * sch_fq packet scheduler is efficiently handling pacing,
   * but is not always installed/used.
   * Return true if TCP stack should pace packets itself.
   */
  static bool tcp_needs_internal_pacing(const struct sock *sk)
  {
  	return smp_load_acquire(&sk->sk_pacing_status) == SK_PACING_NEEDED;
  }
  
  static void tcp_internal_pacing(struct sock *sk, const struct sk_buff *skb)
  {
  	u64 len_ns;
  	u32 rate;
  
  	if (!tcp_needs_internal_pacing(sk))
  		return;
  	rate = sk->sk_pacing_rate;
  	if (!rate || rate == ~0U)
  		return;
  
  	/* Should account for header sizes as sch_fq does,
  	 * but lets make things simple.
  	 */
  	len_ns = (u64)skb->len * NSEC_PER_SEC;
  	do_div(len_ns, rate);
  	hrtimer_start(&tcp_sk(sk)->pacing_timer,
  		      ktime_add_ns(ktime_get(), len_ns),
  		      HRTIMER_MODE_ABS_PINNED);
  }
  
  /* 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));
  	tp = tcp_sk(sk);
  
  	skb->skb_mstamp = tp->tcp_mstamp;
  	if (clone_it) {
  		TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
  			- tp->snd_una;
  		tcp_rate_skb_sent(sk, skb);
  
  		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);
  	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 no packet is in qdisc/device queue, then allow XPS to select
  	 * another queue. We can be called from tcp_tsq_handler()
  	 * which holds one reference to sk_wmem_alloc.
  	 *
  	 * TODO: Ideally, in-flight pure ACK packets should not matter here.
  	 * One way to get this would be to set skb->truesize = 2 on them.
  	 */
  	skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
  
  	/* If we had to use memory reserve to allocate this skb,
  	 * this might cause drops if packet is looped back :
  	 * Other socket might not have SOCK_MEMALLOC.
  	 * Packets not looped back do not care about pfmemalloc.
  	 */
  	skb->pfmemalloc = 0;
  
  	skb_push(skb, tcp_header_size);
  	skb_reset_transport_header(skb);
  
  	skb_orphan(skb);
  	skb->sk = sk;
  	skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
  	skb_set_hash_from_sk(skb, sk);
  	refcount_add(skb->truesize, &sk->sk_wmem_alloc);
  
  	skb_set_dst_pending_confirm(skb, sk->sk_dst_pending_confirm);
  
  	/* Build TCP header and checksum it. */
  	th = (struct tcphdr *)skb->data;
  	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);
  
  	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);
  	skb_shinfo(skb)->gso_type = sk->sk_gso_type;
  	if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
  		th->window      = htons(tcp_select_window(sk));
  		tcp_ecn_send(sk, skb, th, tcp_header_size);
  	} else {
  		/* RFC1323: The window in SYN & SYN/ACK segments
  		 * is never scaled.
  		 */
  		th->window	= htons(min(tp->rcv_wnd, 65535U));
  	}
  #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, 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);
  		tp->data_segs_out += tcp_skb_pcount(skb);
  		tcp_internal_pacing(sk, skb);
  	}
  
  	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));
  
  	tp->segs_out += tcp_skb_pcount(skb);
  	/* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
  	skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
  	skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
  
  	/* Our usage of tstamp should remain private */
  	skb->tstamp = 0;
  
  	/* Cleanup our debris for IP stacks */
  	memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
  			       sizeof(struct inet6_skb_parm)));
  
  	err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);
  
  	if (likely(err <= 0))
  		return err;
  
  	tcp_enter_cwr(sk);
  
  	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(struct sk_buff *skb, unsigned int mss_now)
  {
  	if (skb->len <= mss_now || skb->ip_summed == CHECKSUM_NONE) {
  		/* Avoid the costly divide in the normal
  		 * non-TSO case.
  		 */
  		tcp_skb_pcount_set(skb, 1);
  		TCP_SKB_CB(skb)->tcp_gso_size = 0;
  	} else {
  		tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
  		TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
  	}
  }
  
  /* 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);
  }
  
  static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
  {
  	return TCP_SKB_CB(skb)->txstamp_ack ||
  		(skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
  }
  
  static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
  {
  	struct skb_shared_info *shinfo = skb_shinfo(skb);
  
  	if (unlikely(tcp_has_tx_tstamp(skb)) &&
  	    !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
  		struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
  		u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;
  
  		shinfo->tx_flags &= ~tsflags;
  		shinfo2->tx_flags |= tsflags;
  		swap(shinfo->tskey, shinfo2->tskey);
  		TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
  		TCP_SKB_CB(skb)->txstamp_ack = 0;
  	}
  }
  
  static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
  {
  	TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
  	TCP_SKB_CB(skb)->eor = 0;
  }
  
  /* 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, gfp_t gfp)
  {
  	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_unclone(skb, gfp))
  		return -ENOMEM;
  
  	/* Get a new skb... force flag on. */
  	buff = sk_stream_alloc_skb(sk, nsize, gfp, true);
  	if (!buff)
  		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;
  	tcp_skb_fragment_eor(skb, buff);
  
  	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;
  
  	buff->tstamp = skb->tstamp;
  	tcp_fragment_tstamp(skb, buff);
  
  	old_factor = tcp_skb_pcount(skb);
  
  	/* Fix up tso_factor for both original and new SKB.  */
  	tcp_set_skb_tso_segs(skb, mss_now);
  	tcp_set_skb_tso_segs(buff, mss_now);
  
  	/* Update delivered info for the new segment */
  	TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;
  
  	/* 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_tail(). The difference is that pulled
   * data is not copied, but immediately discarded.
   */
  static int __pskb_trim_head(struct sk_buff *skb, int len)
  {
  	struct skb_shared_info *shinfo;
  	int i, k, eat;
  
  	eat = min_t(int, len, skb_headlen(skb));
  	if (eat) {
  		__skb_pull(skb, eat);
  		len -= eat;
  		if (!len)
  			return 0;
  	}
  	eat = len;
  	k = 0;
  	shinfo = skb_shinfo(skb);
  	for (i = 0; i < shinfo->nr_frags; i++) {
  		int size = skb_frag_size(&shinfo->frags[i]);
  
  		if (size <= eat) {
  			skb_frag_unref(skb, i);
  			eat -= size;
  		} else {
  			shinfo->frags[k] = shinfo->frags[i];
  			if (eat) {
  				shinfo->frags[k].page_offset += eat;
  				skb_frag_size_sub(&shinfo->frags[k], eat);
  				eat = 0;
  			}
  			k++;
  		}
  	}
  	shinfo->nr_frags = k;
  
  	skb->data_len -= len;
  	skb->len = skb->data_len;
  	return len;
  }
  
  /* Remove acked data from a packet in the transmit queue. */
  int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
  {
  	u32 delta_truesize;
  
  	if (skb_unclone(skb, GFP_ATOMIC))
  		return -ENOMEM;
  
  	delta_truesize = __pskb_trim_head(skb, len);
  
  	TCP_SKB_CB(skb)->seq += len;
  	skb->ip_summed = CHECKSUM_PARTIAL;
  
  	if (delta_truesize) {
  		skb->truesize	   -= delta_truesize;
  		sk->sk_wmem_queued -= delta_truesize;
  		sk_mem_uncharge(sk, delta_truesize);
  		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(skb, tcp_skb_mss(skb));
  
  	return 0;
  }
  
  /* Calculate MSS not accounting any TCP options.  */
  static inline 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;
  	return mss_now;
  }
  
  /* Calculate MSS. Not accounting for SACKs here.  */
  int tcp_mtu_to_mss(struct sock *sk, int pmtu)
  {
  	/* Subtract TCP options size, not including SACKs */
  	return __tcp_mtu_to_mss(sk, pmtu) -
  	       (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
  }
  
  /* 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;
  }
  EXPORT_SYMBOL(tcp_mss_to_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);
  	struct net *net = sock_net(sk);
  
  	icsk->icsk_mtup.enabled = net->ipv4.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, net->ipv4.sysctl_tcp_base_mss);
  	icsk->icsk_mtup.probe_size = 0;
  	if (icsk->icsk_mtup.enabled)
  		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
  }
  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;
  }
  
  /* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
   * As additional protections, we do not touch cwnd in retransmission phases,
   * and if application hit its sndbuf limit recently.
   */
  static void tcp_cwnd_application_limited(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
  	    sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
  		/* Limited by application or receiver window. */
  		u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
  		u32 win_used = max(tp->snd_cwnd_used, init_win);
  		if (win_used < tp->snd_cwnd) {
  			tp->snd_ssthresh = tcp_current_ssthresh(sk);
  			tp->snd_cwnd = (tp->snd_cwnd + win_used) >> 1;
  		}
  		tp->snd_cwnd_used = 0;
  	}
  	tp->snd_cwnd_stamp = tcp_jiffies32;
  }
  
  static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
  {
  	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Track the maximum number of outstanding packets in each
  	 * window, and remember whether we were cwnd-limited then.
  	 */
  	if (!before(tp->snd_una, tp->max_packets_seq) ||
  	    tp->packets_out > tp->max_packets_out) {
  		tp->max_packets_out = tp->packets_out;
  		tp->max_packets_seq = tp->snd_nxt;
  		tp->is_cwnd_limited = is_cwnd_limited;
  	}
  
  	if (tcp_is_cwnd_limited(sk)) {
  		/* Network is feed fully. */
  		tp->snd_cwnd_used = 0;
  		tp->snd_cwnd_stamp = tcp_jiffies32;
  	} 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_jiffies32 - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
  		    !ca_ops->cong_control)
  			tcp_cwnd_application_limited(sk);
  
  		/* The following conditions together indicate the starvation
  		 * is caused by insufficient sender buffer:
  		 * 1) just sent some data (see tcp_write_xmit)
  		 * 2) not cwnd limited (this else condition)
  		 * 3) no more data to send (null tcp_send_head )
  		 * 4) application is hitting buffer limit (SOCK_NOSPACE)
  		 */
  		if (!tcp_send_head(sk) && sk->sk_socket &&
  		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
  		    (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
  			tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
  	}
  }
  
  /* Minshall's variant of the Nagle send check. */
  static bool tcp_minshall_check(const struct tcp_sock *tp)
  {
  	return after(tp->snd_sml, tp->snd_una) &&
  		!after(tp->snd_sml, tp->snd_nxt);
  }
  
  /* Update snd_sml if this skb is under mss
   * Note that a TSO packet might end with a sub-mss segment
   * The test is really :
   * if ((skb->len % mss) != 0)
   *        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
   * But we can avoid doing the divide again given we already have
   *  skb_pcount = skb->len / mss_now
   */
  static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
  				const struct sk_buff *skb)
  {
  	if (skb->len < tcp_skb_pcount(skb) * mss_now)
  		tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
  }
  
  /* Return false, if packet can be sent now without violation Nagle's rules:
   * 1. It is full sized. (provided by caller in %partial bool)
   * 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 bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
  			    int nonagle)
  {
  	return partial &&
  		((nonagle & TCP_NAGLE_CORK) ||
  		 (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
  }
  
  /* Return how many segs we'd like on a TSO packet,
   * to send one TSO packet per ms
   */
  u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
  		     int min_tso_segs)
  {
  	u32 bytes, segs;
  
  	bytes = min(sk->sk_pacing_rate >> 10,
  		    sk->sk_gso_max_size - 1 - MAX_TCP_HEADER);
  
  	/* Goal is to send at least one packet per ms,
  	 * not one big TSO packet every 100 ms.
  	 * This preserves ACK clocking and is consistent
  	 * with tcp_tso_should_defer() heuristic.
  	 */
  	segs = max_t(u32, bytes / mss_now, min_tso_segs);
  
  	return min_t(u32, segs, sk->sk_gso_max_segs);
  }
  EXPORT_SYMBOL(tcp_tso_autosize);
  
  /* Return the number of segments we want in the skb we are transmitting.
   * See if congestion control module wants to decide; otherwise, autosize.
   */
  static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
  {
  	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
  	u32 tso_segs = ca_ops->tso_segs_goal ? ca_ops->tso_segs_goal(sk) : 0;
  
  	return tso_segs ? :
  		tcp_tso_autosize(sk, mss_now, sysctl_tcp_min_tso_segs);
  }
  
  /* Returns the portion of skb which can be sent right away */
  static unsigned int tcp_mss_split_point(const struct sock *sk,
  					const struct sk_buff *skb,
  					unsigned int mss_now,
  					unsigned int max_segs,
  					int nonagle)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	u32 partial, 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;
  
  	partial = needed % mss_now;
  	/* If last segment is not a full MSS, check if Nagle rules allow us
  	 * to include this last segment in this skb.
  	 * Otherwise, we'll split the skb at last MSS boundary
  	 */
  	if (tcp_nagle_check(partial != 0, tp, nonagle))
  		return needed - partial;
  
  	return needed;
  }
  
  /* 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, halfcwnd;
  
  	/* 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 0;
  
  	/* For better scheduling, ensure we have at least
  	 * 2 GSO packets in flight.
  	 */
  	halfcwnd = max(cwnd >> 1, 1U);
  	return min(halfcwnd, cwnd - in_flight);
  }
  
  /* 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(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(skb, mss_now);
  		tso_segs = tcp_skb_pcount(skb);
  	}
  	return tso_segs;
  }
  
  
  /* 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). */
  	if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
  		return true;
  
  	if (!tcp_nagle_check(skb->len < cur_mss, tp, 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(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, gfp);
  
  	buff = sk_stream_alloc_skb(sk, 0, gfp, true);
  	if (unlikely(!buff))
  		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;
  
  	tcp_skb_fragment_eor(skb, buff);
  
  	buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
  	skb_split(skb, buff, len);
  	tcp_fragment_tstamp(skb, buff);
  
  	/* Fix up tso_factor for both original and new SKB.  */
  	tcp_set_skb_tso_segs(skb, mss_now);
  	tcp_set_skb_tso_segs(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,
  				 bool *is_cwnd_limited, u32 max_segs)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	u32 age, send_win, cong_win, limit, in_flight;
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *head;
  	int win_divisor;
  
  	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
  		goto send_now;
  
  	if (icsk->icsk_ca_state >= TCP_CA_Recovery)
  		goto send_now;
  
  	/* Avoid bursty behavior by allowing defer
  	 * only if the last write was recent.
  	 */
  	if ((s32)(tcp_jiffies32 - tp->lsndtime) > 0)
  		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 >= 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;
  	}
  
  	head = tcp_write_queue_head(sk);
  
  	age = tcp_stamp_us_delta(tp->tcp_mstamp, head->skb_mstamp);
  	/* If next ACK is likely to come too late (half srtt), do not defer */
  	if (age < (tp->srtt_us >> 4))
  		goto send_now;
  
  	/* Ok, it looks like it is advisable to defer. */
  
  	if (cong_win < send_win && cong_win <= skb->len)
  		*is_cwnd_limited = true;
  
  	return true;
  
  send_now:
  	return false;
  }
  
  static inline void tcp_mtu_check_reprobe(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct net *net = sock_net(sk);
  	u32 interval;
  	s32 delta;
  
  	interval = net->ipv4.sysctl_tcp_probe_interval;
  	delta = tcp_jiffies32 - icsk->icsk_mtup.probe_timestamp;
  	if (unlikely(delta >= interval * HZ)) {
  		int mss = tcp_current_mss(sk);
  
  		/* Update current search range */
  		icsk->icsk_mtup.probe_size = 0;
  		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, mss);
  
  		/* Update probe time stamp */
  		icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
  	}
  }
  
  /* 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 inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb, *nskb, *next;
  	struct net *net = sock_net(sk);
  	int probe_size;
  	int size_needed;
  	int copy, len;
  	int mss_now;
  	int interval;
  
  	/* Not currently probing/verifying,
  	 * not in recovery,
  	 * have enough cwnd, and
  	 * not SACKing (the variable headers throw things off)
  	 */
  	if (likely(!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;
  
  	/* Use binary search for probe_size between tcp_mss_base,
  	 * and current mss_clamp. if (search_high - search_low)
  	 * smaller than a threshold, backoff from probing.
  	 */
  	mss_now = tcp_current_mss(sk);
  	probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
  				    icsk->icsk_mtup.search_low) >> 1);
  	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
  	interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
  	/* When misfortune happens, we are reprobing actively,
  	 * and then reprobe timer has expired. We stick with current
  	 * probing process by not resetting search range to its orignal.
  	 */
  	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
  		interval < net->ipv4.sysctl_tcp_probe_threshold) {
  		/* Check whether enough time has elaplased for
  		 * another round of probing.
  		 */
  		tcp_mtu_check_reprobe(sk);
  		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. */
  	nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC, false);
  	if (!nskb)
  		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 {
  			__wsum csum = skb_copy_and_csum_bits(skb, 0,
  							     skb_put(nskb, copy),
  							     copy, 0);
  			nskb->csum = csum_block_add(nskb->csum, csum, len);
  		}
  
  		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(skb, mss_now);
  			}
  			TCP_SKB_CB(skb)->seq += copy;
  		}
  
  		len += copy;
  
  		if (len >= probe_size)
  			break;
  	}
  	tcp_init_tso_segs(nskb, nskb->len);
  
  	/* We're ready to send.  If this fails, the probe will
  	 * be resegmented into mss-sized pieces by tcp_write_xmit().
  	 */
  	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;
  }
  
  static bool tcp_pacing_check(const struct sock *sk)
  {
  	return tcp_needs_internal_pacing(sk) &&
  	       hrtimer_active(&tcp_sk(sk)->pacing_timer);
  }
  
  /* TCP Small Queues :
   * Control number of packets in qdisc/devices to two packets / or ~1 ms.
   * (These limits are doubled for retransmits)
   * This allows for :
   *  - better RTT estimation and ACK scheduling
   *  - faster recovery
   *  - high rates
   * Alas, some drivers / subsystems require a fair amount
   * of queued bytes to ensure line rate.
   * One example is wifi aggregation (802.11 AMPDU)
   */
  static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
  				  unsigned int factor)
  {
  	unsigned int limit;
  
  	limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 10);
  	limit = min_t(u32, limit, sysctl_tcp_limit_output_bytes);
  	limit <<= factor;
  
  	if (refcount_read(&sk->sk_wmem_alloc) > limit) {
  		/* Always send the 1st or 2nd skb in write queue.
  		 * No need to wait for TX completion to call us back,
  		 * after softirq/tasklet schedule.
  		 * This helps when TX completions are delayed too much.
  		 */
  		if (skb == sk->sk_write_queue.next ||
  		    skb->prev == sk->sk_write_queue.next)
  			return false;
  
  		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
  		/* It is possible TX completion already happened
  		 * before we set TSQ_THROTTLED, so we must
  		 * test again the condition.
  		 */
  		smp_mb__after_atomic();
  		if (refcount_read(&sk->sk_wmem_alloc) > limit)
  			return true;
  	}
  	return false;
  }
  
  static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
  {
  	const u32 now = tcp_jiffies32;
  	enum tcp_chrono old = tp->chrono_type;
  
  	if (old > TCP_CHRONO_UNSPEC)
  		tp->chrono_stat[old - 1] += now - tp->chrono_start;
  	tp->chrono_start = now;
  	tp->chrono_type = new;
  }
  
  void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* If there are multiple conditions worthy of tracking in a
  	 * chronograph then the highest priority enum takes precedence
  	 * over the other conditions. So that if something "more interesting"
  	 * starts happening, stop the previous chrono and start a new one.
  	 */
  	if (type > tp->chrono_type)
  		tcp_chrono_set(tp, type);
  }
  
  void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  
  	/* There are multiple conditions worthy of tracking in a
  	 * chronograph, so that the highest priority enum takes
  	 * precedence over the other conditions (see tcp_chrono_start).
  	 * If a condition stops, we only stop chrono tracking if
  	 * it's the "most interesting" or current chrono we are
  	 * tracking and starts busy chrono if we have pending data.
  	 */
  	if (tcp_write_queue_empty(sk))
  		tcp_chrono_set(tp, TCP_CHRONO_UNSPEC);
  	else if (type == tp->chrono_type)
  		tcp_chrono_set(tp, TCP_CHRONO_BUSY);
  }
  
  /* 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.
   *
   * Send at most one packet when push_one > 0. Temporarily ignore
   * cwnd limit to force at most one packet out when push_one == 2.
  
   * 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;
  	bool is_cwnd_limited = false, is_rwnd_limited = false;
  	u32 max_segs;
  
  	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;
  		}
  	}
  
  	max_segs = tcp_tso_segs(sk, mss_now);
  	tcp_mstamp_refresh(tp);
  	while ((skb = tcp_send_head(sk))) {
  		unsigned int limit;
  
  		if (tcp_pacing_check(sk))
  			break;
  
  		tso_segs = tcp_init_tso_segs(skb, mss_now);
  		BUG_ON(!tso_segs);
  
  		if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
  			/* "skb_mstamp" is used as a start point for the retransmit timer */
  			skb->skb_mstamp = tp->tcp_mstamp;
  			goto repair; /* Skip network transmission */
  		}
  
  		cwnd_quota = tcp_cwnd_test(tp, skb);
  		if (!cwnd_quota) {
  			if (push_one == 2)
  				/* Force out a loss probe pkt. */
  				cwnd_quota = 1;
  			else
  				break;
  		}
  
  		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
  			is_rwnd_limited = true;
  			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, &is_cwnd_limited,
  						 max_segs))
  				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,
  							  max_segs),
  						    nonagle);
  
  		if (skb->len > limit &&
  		    unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
  			break;
  
  		if (test_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags))
  			clear_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags);
  		if (tcp_small_queue_check(sk, skb, 0))
  			break;
  
  		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 (is_rwnd_limited)
  		tcp_chrono_start(sk, TCP_CHRONO_RWND_LIMITED);
  	else
  		tcp_chrono_stop(sk, TCP_CHRONO_RWND_LIMITED);
  
  	if (likely(sent_pkts)) {
  		if (tcp_in_cwnd_reduction(sk))
  			tp->prr_out += sent_pkts;
  
  		/* Send one loss probe per tail loss episode. */
  		if (push_one != 2)
  			tcp_schedule_loss_probe(sk);
  		is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tp->snd_cwnd);
  		tcp_cwnd_validate(sk, is_cwnd_limited);
  		return false;
  	}
  	return !tp->packets_out && tcp_send_head(sk);
  }
  
  bool tcp_schedule_loss_probe(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 rtt = usecs_to_jiffies(tp->srtt_us >> 3);
  	u32 timeout, rto_delta_us;
  
  	/* Don't do any loss probe on a Fast Open connection before 3WHS
  	 * finishes.
  	 */
  	if (tp->fastopen_rsk)
  		return false;
  
  	/* Schedule a loss probe in 2*RTT for SACK capable connections
  	 * in Open state, that are either limited by cwnd or application.
  	 */
  	if ((sysctl_tcp_early_retrans != 3 && sysctl_tcp_early_retrans != 4) ||
  	    !tp->packets_out || !tcp_is_sack(tp) ||
  	    icsk->icsk_ca_state != TCP_CA_Open)
  		return false;
  
  	if ((tp->snd_cwnd > tcp_packets_in_flight(tp)) &&
  	     tcp_send_head(sk))
  		return false;
  
  	/* Probe timeout is at least 1.5*rtt + TCP_DELACK_MAX to account
  	 * for delayed ack when there's one outstanding packet. If no RTT
  	 * sample is available then probe after TCP_TIMEOUT_INIT.
  	 */
  	timeout = rtt << 1 ? : TCP_TIMEOUT_INIT;
  	if (tp->packets_out == 1)
  		timeout = max_t(u32, timeout,
  				(rtt + (rtt >> 1) + TCP_DELACK_MAX));
  	timeout = max_t(u32, timeout, msecs_to_jiffies(10));
  
  	/* If the RTO formula yields an earlier time, then use that time. */
  	rto_delta_us = tcp_rto_delta_us(sk);  /* How far in future is RTO? */
  	if (rto_delta_us > 0)
  		timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us));
  
  	inet_csk_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout,
  				  TCP_RTO_MAX);
  	return true;
  }
  
  /* Thanks to skb fast clones, we can detect if a prior transmit of
   * a packet is still in a qdisc or driver queue.
   * In this case, there is very little point doing a retransmit !
   */
  static bool skb_still_in_host_queue(const struct sock *sk,
  				    const struct sk_buff *skb)
  {
  	if (unlikely(skb_fclone_busy(sk, skb))) {
  		NET_INC_STATS(sock_net(sk),
  			      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
  		return true;
  	}
  	return false;
  }
  
  /* When probe timeout (PTO) fires, try send a new segment if possible, else
   * retransmit the last segment.
   */
  void tcp_send_loss_probe(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  	int pcount;
  	int mss = tcp_current_mss(sk);
  
  	skb = tcp_send_head(sk);
  	if (skb) {
  		if (tcp_snd_wnd_test(tp, skb, mss)) {
  			pcount = tp->packets_out;
  			tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
  			if (tp->packets_out > pcount)
  				goto probe_sent;
  			goto rearm_timer;
  		}
  		skb = tcp_write_queue_prev(sk, skb);
  	} else {
  		skb = tcp_write_queue_tail(sk);
  	}
  
  	/* At most one outstanding TLP retransmission. */
  	if (tp->tlp_high_seq)
  		goto rearm_timer;
  
  	/* Retransmit last segment. */
  	if (WARN_ON(!skb))
  		goto rearm_timer;
  
  	if (skb_still_in_host_queue(sk, skb))
  		goto rearm_timer;
  
  	pcount = tcp_skb_pcount(skb);
  	if (WARN_ON(!pcount))
  		goto rearm_timer;
  
  	if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
  		if (unlikely(tcp_fragment(sk, skb, (pcount - 1) * mss, mss,
  					  GFP_ATOMIC)))
  			goto rearm_timer;
  		skb = tcp_write_queue_next(sk, skb);
  	}
  
  	if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
  		goto rearm_timer;
  
  	if (__tcp_retransmit_skb(sk, skb, 1))
  		goto rearm_timer;
  
  	/* Record snd_nxt for loss detection. */
  	tp->tlp_high_seq = tp->snd_nxt;
  
  probe_sent:
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
  	/* Reset s.t. tcp_rearm_rto will restart timer from now */
  	inet_csk(sk)->icsk_pending = 0;
  rearm_timer:
  	tcp_rearm_rto(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_mask(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 allowed_space = tcp_full_space(sk);
  	int full_space = min_t(int, tp->window_clamp, allowed_space);
  	int window;
  
  	if (unlikely(mss > full_space)) {
  		mss = full_space;
  		if (mss <= 0)
  			return 0;
  	}
  	if (free_space < (full_space >> 1)) {
  		icsk->icsk_ack.quick = 0;
  
  		if (tcp_under_memory_pressure(sk))
  			tp->rcv_ssthresh = min(tp->rcv_ssthresh,
  					       4U * tp->advmss);
  
  		/* free_space might become our new window, make sure we don't
  		 * increase it due to wscale.
  		 */
  		free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
  
  		/* if free space is less than mss estimate, or is below 1/16th
  		 * of the maximum allowed, try to move to zero-window, else
  		 * tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
  		 * new incoming data is dropped due to memory limits.
  		 * With large window, mss test triggers way too late in order
  		 * to announce zero window in time before rmem limit kicks in.
  		 */
  		if (free_space < (allowed_space >> 4) || 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.
  	 */
  	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.
  		 */
  		window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
  	} else {
  		window = tp->rcv_wnd;
  		/* 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 = rounddown(free_space, mss);
  		else if (mss == full_space &&
  			 free_space > window + (full_space >> 1))
  			window = free_space;
  	}
  
  	return window;
  }
  
  void tcp_skb_collapse_tstamp(struct sk_buff *skb,
  			     const struct sk_buff *next_skb)
  {
  	if (unlikely(tcp_has_tx_tstamp(next_skb))) {
  		const struct skb_shared_info *next_shinfo =
  			skb_shinfo(next_skb);
  		struct skb_shared_info *shinfo = skb_shinfo(skb);
  
  		shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
  		shinfo->tskey = next_shinfo->tskey;
  		TCP_SKB_CB(skb)->txstamp_ack |=
  			TCP_SKB_CB(next_skb)->txstamp_ack;
  	}
  }
  
  /* Collapses two adjacent SKB's during retransmission. */
  static bool 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);
  
  	if (next_skb_size) {
  		if (next_skb_size <= skb_availroom(skb))
  			skb_copy_bits(next_skb, 0, skb_put(skb, next_skb_size),
  				      next_skb_size);
  		else if (!skb_shift(skb, next_skb, next_skb_size))
  			return false;
  	}
  	tcp_highest_sack_combine(sk, next_skb, skb);
  
  	tcp_unlink_write_queue(next_skb, sk);
  
  	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;
  	TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
  
  	/* 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));
  
  	tcp_skb_collapse_tstamp(skb, next_skb);
  
  	sk_wmem_free_skb(sk, next_skb);
  	return true;
  }
  
  /* 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;
  	if (skb_cloned(skb))
  		return false;
  	if (skb == tcp_send_head(sk))
  		return false;
  	/* Some heuristics 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;
  
  		if (!tcp_skb_can_collapse_to(to))
  			break;
  
  		space -= skb->len;
  
  		if (first) {
  			first = false;
  			continue;
  		}
  
  		if (space < 0)
  			break;
  
  		if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
  			break;
  
  		if (!tcp_collapse_retrans(sk, to))
  			break;
  	}
  }
  
  /* 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, int segs)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	unsigned int cur_mss;
  	int diff, len, err;
  
  
  	/* Inconclusive 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 (refcount_read(&sk->sk_wmem_alloc) >
  	    min_t(u32, sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2),
  		  sk->sk_sndbuf))
  		return -EAGAIN;
  
  	if (skb_still_in_host_queue(sk, skb))
  		return -EBUSY;
  
  	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;
  
  	len = cur_mss * segs;
  	if (skb->len > len) {
  		if (tcp_fragment(sk, skb, len, cur_mss, GFP_ATOMIC))
  			return -ENOMEM; /* We'll try again later. */
  	} else {
  		if (skb_unclone(skb, GFP_ATOMIC))
  			return -ENOMEM;
  
  		diff = tcp_skb_pcount(skb);
  		tcp_set_skb_tso_segs(skb, cur_mss);
  		diff -= tcp_skb_pcount(skb);
  		if (diff)
  			tcp_adjust_pcount(sk, skb, diff);
  		if (skb->len < cur_mss)
  			tcp_retrans_try_collapse(sk, skb, cur_mss);
  	}
  
  	/* RFC3168, section 6.1.1.1. ECN fallback */
  	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
  		tcp_ecn_clear_syn(sk, skb);
  
  	/* Update global and local TCP statistics. */
  	segs = tcp_skb_pcount(skb);
  	TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
  	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
  		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
  	tp->total_retrans += segs;
  
  	/* make sure skb->data is aligned on arches that require it
  	 * and check if ack-trimming & collapsing extended the headroom
  	 * beyond what csum_start can cover.
  	 */
  	if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
  		     skb_headroom(skb) >= 0xFFFF)) {
  		struct sk_buff *nskb;
  
  		skb->skb_mstamp = tp->tcp_mstamp;
  		nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
  		err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
  			     -ENOBUFS;
  	} else {
  		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
  	}
  
  	if (likely(!err)) {
  		TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
  	} else if (err != -EBUSY) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
  	}
  	return err;
  }
  
  int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int err = __tcp_retransmit_skb(sk, skb, segs);
  
  	if (err == 0) {
  #if FASTRETRANS_DEBUG > 0
  		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
  			net_dbg_ratelimited("retrans_out leaked\n");
  		}
  #endif
  		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_timestamp(skb);
  
  	}
  
  	if (tp->undo_retrans < 0)
  		tp->undo_retrans = 0;
  	tp->undo_retrans += tcp_skb_pcount(skb);
  	return err;
  }
  
  /* 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 max_segs;
  	int mib_idx;
  
  	if (!tp->packets_out)
  		return;
  
  	if (tp->retransmit_skb_hint) {
  		skb = tp->retransmit_skb_hint;
  	} else {
  		skb = tcp_write_queue_head(sk);
  	}
  
  	max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
  	tcp_for_write_queue_from(skb, sk) {
  		__u8 sacked;
  		int segs;
  
  		if (skb == tcp_send_head(sk))
  			break;
  
  		if (tcp_pacing_check(sk))
  			break;
  
  		/* we could do better than to assign each time */
  		if (!hole)
  			tp->retransmit_skb_hint = skb;
  
  		segs = tp->snd_cwnd - tcp_packets_in_flight(tp);
  		if (segs <= 0)
  			return;
  		sacked = TCP_SKB_CB(skb)->sacked;
  		/* In case tcp_shift_skb_data() have aggregated large skbs,
  		 * we need to make sure not sending too bigs TSO packets
  		 */
  		segs = min_t(int, segs, max_segs);
  
  		if (tp->retrans_out >= tp->lost_out) {
  			break;
  		} else if (!(sacked & TCPCB_LOST)) {
  			if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
  				hole = skb;
  			continue;
  
  		} else {
  			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_small_queue_check(sk, skb, 1))
  			return;
  
  		if (tcp_retransmit_skb(sk, skb, segs))
  			return;
  
  		NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));
  
  		if (tcp_in_cwnd_reduction(sk))
  			tp->prr_out += tcp_skb_pcount(skb);
  
  		if (skb == tcp_write_queue_head(sk) &&
  		    icsk->icsk_pending != ICSK_TIME_REO_TIMEOUT)
  			inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
  						  inet_csk(sk)->icsk_rto,
  						  TCP_RTO_MAX);
  	}
  }
  
  /* We allow to exceed memory limits for FIN packets to expedite
   * connection tear down and (memory) recovery.
   * Otherwise tcp_send_fin() could be tempted to either delay FIN
   * or even be forced to close flow without any FIN.
   * In general, we want to allow one skb per socket to avoid hangs
   * with edge trigger epoll()
   */
  void sk_forced_mem_schedule(struct sock *sk, int size)
  {
  	int amt;
  
  	if (size <= sk->sk_forward_alloc)
  		return;
  	amt = sk_mem_pages(size);
  	sk->sk_forward_alloc += amt * SK_MEM_QUANTUM;
  	sk_memory_allocated_add(sk, amt);
  
  	if (mem_cgroup_sockets_enabled && sk->sk_memcg)
  		mem_cgroup_charge_skmem(sk->sk_memcg, amt);
  }
  
  /* Send a FIN. The caller locks the socket for us.
   * We should try to send a FIN packet really hard, but eventually give up.
   */
  void tcp_send_fin(struct sock *sk)
  {
  	struct sk_buff *skb, *tskb = tcp_write_queue_tail(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Optimization, tack on the FIN if we have one skb in write queue and
  	 * this skb was not yet sent, or we are under memory pressure.
  	 * Note: in the latter case, FIN packet will be sent after a timeout,
  	 * as TCP stack thinks it has already been transmitted.
  	 */
  	if (tskb && (tcp_send_head(sk) || tcp_under_memory_pressure(sk))) {
  coalesce:
  		TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
  		TCP_SKB_CB(tskb)->end_seq++;
  		tp->write_seq++;
  		if (!tcp_send_head(sk)) {
  			/* This means tskb was already sent.
  			 * Pretend we included the FIN on previous transmit.
  			 * We need to set tp->snd_nxt to the value it would have
  			 * if FIN had been sent. This is because retransmit path
  			 * does not change tp->snd_nxt.
  			 */
  			tp->snd_nxt++;
  			return;
  		}
  	} else {
  		skb = alloc_skb_fclone(MAX_TCP_HEADER, sk->sk_allocation);
  		if (unlikely(!skb)) {
  			if (tskb)
  				goto coalesce;
  			return;
  		}
  		skb_reserve(skb, MAX_TCP_HEADER);
  		sk_forced_mem_schedule(sk, skb->truesize);
  		/* 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, tcp_current_mss(sk), 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;
  
  	TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
  
  	/* 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);
  	tcp_mstamp_refresh(tcp_sk(sk));
  	/* Send it off. */
  	if (tcp_transmit_skb(sk, skb, 0, priority))
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
  }
  
  /* 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 || !(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)
  				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(sk, skb);
  	}
  	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
   *
   * Allocate one skb and build a SYNACK packet.
   * @dst is consumed : Caller should not use it again.
   */
  struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
  				struct request_sock *req,
  				struct tcp_fastopen_cookie *foc,
  				enum tcp_synack_type synack_type)
  {
  	struct inet_request_sock *ireq = inet_rsk(req);
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct tcp_md5sig_key *md5 = NULL;
  	struct tcp_out_options opts;
  	struct sk_buff *skb;
  	int tcp_header_size;
  	struct tcphdr *th;
  	int mss;
  
  	skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
  	if (unlikely(!skb)) {
  		dst_release(dst);
  		return NULL;
  	}
  	/* Reserve space for headers. */
  	skb_reserve(skb, MAX_TCP_HEADER);
  
  	switch (synack_type) {
  	case TCP_SYNACK_NORMAL:
  		skb_set_owner_w(skb, req_to_sk(req));
  		break;
  	case TCP_SYNACK_COOKIE:
  		/* Under synflood, we do not attach skb to a socket,
  		 * to avoid false sharing.
  		 */
  		break;
  	case TCP_SYNACK_FASTOPEN:
  		/* sk is a const pointer, because we want to express multiple
  		 * cpu might call us concurrently.
  		 * sk->sk_wmem_alloc in an atomic, we can promote to rw.
  		 */
  		skb_set_owner_w(skb, (struct sock *)sk);
  		break;
  	}
  	skb_dst_set(skb, dst);
  
  	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
  
  	memset(&opts, 0, sizeof(opts));
  #ifdef CONFIG_SYN_COOKIES
  	if (unlikely(req->cookie_ts))
  		skb->skb_mstamp = cookie_init_timestamp(req);
  	else
  #endif
  		skb->skb_mstamp = tcp_clock_us();
  
  #ifdef CONFIG_TCP_MD5SIG
  	rcu_read_lock();
  	md5 = tcp_rsk(req)->af_specific->req_md5_lookup(sk, req_to_sk(req));
  #endif
  	skb_set_hash(skb, tcp_rsk(req)->txhash, PKT_HASH_TYPE_L4);
  	tcp_header_size = tcp_synack_options(req, mss, skb, &opts, md5, foc) +
  			  sizeof(*th);
  
  	skb_push(skb, tcp_header_size);
  	skb_reset_transport_header(skb);
  
  	th = (struct tcphdr *)skb->data;
  	memset(th, 0, sizeof(struct tcphdr));
  	th->syn = 1;
  	th->ack = 1;
  	tcp_ecn_make_synack(req, th);
  	th->source = htons(ireq->ir_num);
  	th->dest = ireq->ir_rmt_port;
  	skb->mark = ireq->ir_mark;
  	/* 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);
  
  	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->rsk_rcv_wnd, 65535U));
  	tcp_options_write((__be32 *)(th + 1), NULL, &opts);
  	th->doff = (tcp_header_size >> 2);
  	__TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
  
  #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, req_to_sk(req), skb);
  	rcu_read_unlock();
  #endif
  
  	/* Do not fool tcpdump (if any), clean our debris */
  	skb->tstamp = 0;
  	return skb;
  }
  EXPORT_SYMBOL(tcp_make_synack);
  
  static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	const struct tcp_congestion_ops *ca;
  	u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
  
  	if (ca_key == TCP_CA_UNSPEC)
  		return;
  
  	rcu_read_lock();
  	ca = tcp_ca_find_key(ca_key);
  	if (likely(ca && try_module_get(ca->owner))) {
  		module_put(icsk->icsk_ca_ops->owner);
  		icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
  		icsk->icsk_ca_ops = ca;
  	}
  	rcu_read_unlock();
  }
  
  /* Do all connect socket setups that can be done AF independent. */
  static 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;
  	u32 rcv_wnd;
  
  	/* 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);
  	if (sock_net(sk)->ipv4.sysctl_tcp_timestamps)
  		tp->tcp_header_len += TCPOLEN_TSTAMP_ALIGNED;
  
  #ifdef CONFIG_TCP_MD5SIG
  	if (tp->af_specific->md5_lookup(sk, sk))
  		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));
  
  	tcp_ca_dst_init(sk, dst);
  
  	if (!tp->window_clamp)
  		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
  	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
  
  	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);
  
  	rcv_wnd = tcp_rwnd_init_bpf(sk);
  	if (rcv_wnd == 0)
  		rcv_wnd = dst_metric(dst, RTAX_INITRWND);
  
  	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,
  				  sock_net(sk)->ipv4.sysctl_tcp_window_scaling,
  				  &rcv_wscale,
  				  rcv_wnd);
  
  	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;
  	else
  		tp->rcv_tstamp = tcp_jiffies32;
  	tp->rcv_wup = tp->rcv_nxt;
  	tp->copied_seq = tp->rcv_nxt;
  
  	inet_csk(sk)->icsk_rto = tcp_timeout_init(sk);
  	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 space, err = 0;
  	struct sk_buff *syn_data;
  
  	tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
  	if (!tcp_fastopen_cookie_check(sk, &tp->rx_opt.mss_clamp, &fo->cookie))
  		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 :(
  	 */
  	tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
  
  	space = __tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) -
  		MAX_TCP_OPTION_SPACE;
  
  	space = min_t(size_t, space, fo->size);
  
  	/* limit to order-0 allocations */
  	space = min_t(size_t, space, SKB_MAX_HEAD(MAX_TCP_HEADER));
  
  	syn_data = sk_stream_alloc_skb(sk, space, sk->sk_allocation, false);
  	if (!syn_data)
  		goto fallback;
  	syn_data->ip_summed = CHECKSUM_PARTIAL;
  	memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
  	if (space) {
  		int copied = copy_from_iter(skb_put(syn_data, space), space,
  					    &fo->data->msg_iter);
  		if (unlikely(!copied)) {
  			kfree_skb(syn_data);
  			goto fallback;
  		}
  		if (copied != space) {
  			skb_trim(syn_data, copied);
  			space = copied;
  		}
  	}
  	/* No more data pending in inet_wait_for_connect() */
  	if (space == fo->size)
  		fo->data = NULL;
  	fo->copied = space;
  
  	tcp_connect_queue_skb(sk, syn_data);
  	if (syn_data->len)
  		tcp_chrono_start(sk, TCP_CHRONO_BUSY);
  
  	err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);
  
  	syn->skb_mstamp = syn_data->skb_mstamp;
  
  	/* Now full SYN+DATA was cloned and sent (or not),
  	 * remove the SYN from the original skb (syn_data)
  	 * we keep in write queue in case of a retransmit, as we
  	 * also have the SYN packet (with no data) in the same queue.
  	 */
  	TCP_SKB_CB(syn_data)->seq++;
  	TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
  	if (!err) {
  		tp->syn_data = (fo->copied > 0);
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
  		goto done;
  	}
  
  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;
  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_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB);
  
  	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
  		return -EHOSTUNREACH; /* Routing failure or similar. */
  
  	tcp_connect_init(sk);
  
  	if (unlikely(tp->repair)) {
  		tcp_finish_connect(sk, NULL);
  		return 0;
  	}
  
  	buff = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, true);
  	if (unlikely(!buff))
  		return -ENOBUFS;
  
  	tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
  	tcp_mstamp_refresh(tp);
  	tp->retrans_stamp = tcp_time_stamp(tp);
  	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;
  
  	tcp_ca_event(sk, CA_EVENT_DELAYED_ACK);
  
  	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_us) {
  			int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 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;
  
  	tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);
  
  	/* 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_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
  	if (unlikely(!buff)) {
  		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);
  
  	/* We do not want pure acks influencing TCP Small Queues or fq/pacing
  	 * too much.
  	 * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
  	 */
  	skb_set_tcp_pure_ack(buff);
  
  	/* Send it off, this clears delayed acks for us. */
  	tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0);
  }
  EXPORT_SYMBOL_GPL(tcp_send_ack);
  
  /* 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, int mib)
  {
  	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_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
  	if (!skb)
  		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);
  	NET_INC_STATS(sock_net(sk), mib);
  	return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
  }
  
  /* Called from setsockopt( ... TCP_REPAIR ) */
  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_mstamp_refresh(tcp_sk(sk));
  		tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
  	}
  }
  
  /* Initiate keepalive or window probe from timer. */
  int tcp_write_wakeup(struct sock *sk, int mib)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  
  	if (sk->sk_state == TCP_CLOSE)
  		return -1;
  
  	skb = tcp_send_head(sk);
  	if (skb && 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, GFP_ATOMIC))
  				return -1;
  		} else if (!tcp_skb_pcount(skb))
  			tcp_set_skb_tso_segs(skb, mss);
  
  		TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
  		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, mib);
  		return tcp_xmit_probe_skb(sk, 0, mib);
  	}
  }
  
  /* 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);
  	struct net *net = sock_net(sk);
  	unsigned long probe_max;
  	int err;
  
  	err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);
  
  	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 < net->ipv4.sysctl_tcp_retries2)
  			icsk->icsk_backoff++;
  		icsk->icsk_probes_out++;
  		probe_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;
  		probe_max = TCP_RESOURCE_PROBE_INTERVAL;
  	}
  	inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
  				  tcp_probe0_when(sk, probe_max),
  				  TCP_RTO_MAX);
  }
  
  int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
  {
  	const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
  	struct flowi fl;
  	int res;
  
  	tcp_rsk(req)->txhash = net_tx_rndhash();
  	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL);
  	if (!res) {
  		__TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
  		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
  		if (unlikely(tcp_passive_fastopen(sk)))
  			tcp_sk(sk)->total_retrans++;
  	}
  	return res;
  }
  EXPORT_SYMBOL(tcp_rtx_synack);
  