tcp_output.c

/*
 * INET		An implementation of the TCP/IP protocol suite for the LINUX
 *		operating system.  INET is implemented using the  BSD Socket
 *		interface as the means of communication with the user level.
 *
 *		Implementation of the Transmission Control Protocol(TCP).
 *
 * Authors:	Ross Biro
 *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *		Mark Evans, <evansmp@uhura.aston.ac.uk>
 *		Corey Minyard <wf-rch!minyard@relay.EU.net>
 *		Florian La Roche, <flla@stud.uni-sb.de>
 *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *		Linus Torvalds, <torvalds@cs.helsinki.fi>
 *		Alan Cox, <gw4pts@gw4pts.ampr.org>
 *		Matthew Dillon, <dillon@apollo.west.oic.com>
 *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *		Jorge Cwik, <jorge@laser.satlink.net>
 */

/*
 * Changes:	Pedro Roque	:	Retransmit queue handled by TCP.
 *				:	Fragmentation on mtu decrease
 *				:	Segment collapse on retransmit
 *				:	AF independence
 *
 *		Linus Torvalds	:	send_delayed_ack
 *		David S. Miller	:	Charge memory using the right skb
 *					during syn/ack processing.
 *		David S. Miller :	Output engine completely rewritten.
 *		Andrea Arcangeli:	SYNACK carry ts_recent in tsecr.
 *		Cacophonix Gaul :	draft-minshall-nagle-01
 *		J Hadi Salim	:	ECN support
 *
 */

#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;

/* This limits the percentage of the congestion window which we
 * will allow a single TSO frame to consume.  Building TSO frames
 * which are too large can cause TCP streams to be bursty.
 */
int sysctl_tcp_tso_win_divisor __read_mostly = 3;

int sysctl_tcp_mtu_probing __read_mostly = 0;
int sysctl_tcp_base_mss __read_mostly = 512;

/* By default, RFC2861 behavior.  */
int sysctl_tcp_slow_start_after_idle __read_mostly = 1;

int sysctl_tcp_cookie_size __read_mostly = 0; /* TCP_COOKIE_MAX */
EXPORT_SYMBOL_GPL(sysctl_tcp_cookie_size);


/* Account for new data that has been sent to the network. */
static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int prior_packets = tp->packets_out;

	tcp_advance_send_head(sk, skb);
	tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;

	/* Don't override Nagle indefinately with F-RTO */
	if (tp->frto_counter == 2)
		tp->frto_counter = 3;

	tp->packets_out += tcp_skb_pcount(skb);
	if (!prior_packets)//可以看到只有当prior_packets为0时才会重启定时器,而prior_packets则是发送未确认的段的个数,也就是说如果发送了很多段,如果前面的段没有确认,那么后面发送的时候不会重启这个定时器.
		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
					  inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
}

/* SND.NXT, if window was not shrunk.
 * If window has been shrunk, what should we make? It is not clear at all.
 * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
 * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
 * invalid. OK, let's make this for now:
 */
static inline __u32 tcp_acceptable_seq(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (!before(tcp_wnd_end(tp), tp->snd_nxt))
		return tp->snd_nxt;
	else
		return tcp_wnd_end(tp);
}

/* Calculate mss to advertise in SYN segment.
 * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
 *
 * 1. It is independent of path mtu.
 * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
 * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
 *    attached devices, because some buggy hosts are confused by
 *    large MSS.
 * 4. We do not make 3, we advertise MSS, calculated from first
 *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
 *    This may be overridden via information stored in routing table.
 * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
 *    probably even Jumbo".
 */
static __u16 tcp_advertise_mss(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct dst_entry *dst = __sk_dst_get(sk);
	int mss = tp->advmss;

	if (dst && dst_metric(dst, RTAX_ADVMSS) < mss) {
		mss = dst_metric(dst, RTAX_ADVMSS);
		tp->advmss = mss;
	}

	return (__u16)mss;
}

/* RFC2861. Reset CWND after idle period longer RTO to "restart window".
 * This is the first part of cwnd validation mechanism. */
static void tcp_cwnd_restart(struct sock *sk, struct dst_entry *dst)
{
	struct tcp_sock *tp = tcp_sk(sk);
	s32 delta = tcp_time_stamp - tp->lsndtime;
	u32 restart_cwnd = tcp_init_cwnd(tp, dst);
	u32 cwnd = tp->snd_cwnd;

	tcp_ca_event(sk, CA_EVENT_CWND_RESTART);

	tp->snd_ssthresh = tcp_current_ssthresh(sk);
	restart_cwnd = min(restart_cwnd, cwnd);

	while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
		cwnd >>= 1;
	tp->snd_cwnd = max(cwnd, restart_cwnd);
	tp->snd_cwnd_stamp = tcp_time_stamp;
	tp->snd_cwnd_used = 0;
}

/* Congestion state accounting after a packet has been sent. */
static void tcp_event_data_sent(struct tcp_sock *tp,
				struct sk_buff *skb, struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	const u32 now = tcp_time_stamp;

	if (sysctl_tcp_slow_start_after_idle &&
	    (!tp->packets_out && (s32)(now - tp->lsndtime) > icsk->icsk_rto))
		tcp_cwnd_restart(sk, __sk_dst_get(sk));

	tp->lsndtime = now;

	/* If it is a reply for ato after last received
	 * packet, enter pingpong mode.
	 */
	if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
		icsk->icsk_ack.pingpong = 1;
}

/* Account for an ACK we sent. */
static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
{
	tcp_dec_quickack_mode(sk, pkts);
	inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
}

/* Determine a window scaling and initial window to offer.
 * Based on the assumption that the given amount of space
 * will be offered. Store the results in the tp structure.
 * NOTE: for smooth operation initial space offering should
 * be a multiple of mss if possible. We assume here that mss >= 1.
 * This MUST be enforced by all callers.
 */
void tcp_select_initial_window(int __space, __u32 mss,
			       __u32 *rcv_wnd, __u32 *window_clamp,
			       int wscale_ok, __u8 *rcv_wscale,
			       __u32 init_rcv_wnd)
{
	unsigned int space = (__space < 0 ? 0 : __space);

	/* If no clamp set the clamp to the max possible scaled window */
	if (*window_clamp == 0)
		(*window_clamp) = (65535 << 14);
	space = min(*window_clamp, space);

	/* Quantize space offering to a multiple of mss if possible. */
	if (space > mss)
		space = (space / mss) * mss;

	/* NOTE: offering an initial window larger than 32767
	 * will break some buggy TCP stacks. If the admin tells us
	 * it is likely we could be speaking with such a buggy stack
	 * we will truncate our initial window offering to 32K-1
	 * unless the remote has sent us a window scaling option,
	 * which we interpret as a sign the remote TCP is not
	 * misinterpreting the window field as a signed quantity.
	 */
	if (sysctl_tcp_workaround_signed_windows)
		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
	else
		(*rcv_wnd) = space;

	(*rcv_wscale) = 0;
	if (wscale_ok) {
		/* Set window scaling on max possible window
		 * See RFC1323 for an explanation of the limit to 14
		 */
		space = max_t(u32, sysctl_tcp_rmem[2], sysctl_rmem_max);
		space = min_t(u32, space, *window_clamp);
		while (space > 65535 && (*rcv_wscale) < 14) {
			space >>= 1;
			(*rcv_wscale)++;
		}
	}

	/* Set initial window to value enough for senders,
	 * following RFC2414. Senders, not following this RFC,
	 * will be satisfied with 2.
	 */
	if (mss > (1 << *rcv_wscale)) {
		int init_cwnd = 4;
		if (mss > 1460 * 3)
			init_cwnd = 2;
		else if (mss > 1460)
			init_cwnd = 3;
		/* when initializing use the value from init_rcv_wnd
		 * rather than the default from above
		 */
		if (init_rcv_wnd)
			*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
		else
			*rcv_wnd = min(*rcv_wnd, init_cwnd * mss);
	}

	/* Set the clamp no higher than max representable value */
	(*window_clamp) = min(65535U << (*rcv_wscale), *window_clamp);
}

/* Chose a new window to advertise, update state in tcp_sock for the
 * socket, and return result with RFC1323 scaling applied.  The return
 * value can be stuffed directly into th->window for an outgoing
 * frame.
 */
static u16 tcp_select_window(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	u32 cur_win = tcp_receive_window(tp);
	u32 new_win = __tcp_select_window(sk);

	/* Never shrink the offered window */
	if (new_win < cur_win) {
		/* Danger Will Robinson!
		 * Don't update rcv_wup/rcv_wnd here or else
		 * we will not be able to advertise a zero
		 * window in time.  --DaveM
		 *
		 * Relax Will Robinson.
		 */
		new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
	}
	tp->rcv_wnd = new_win;
	tp->rcv_wup = tp->rcv_nxt;

	/* Make sure we do not exceed the maximum possible
	 * scaled window.
	 */
	if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
		new_win = min(new_win, MAX_TCP_WINDOW);
	else
		new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));

	/* RFC1323 scaling applied */
	new_win >>= tp->rx_opt.rcv_wscale;

	/* If we advertise zero window, disable fast path. */
	if (new_win == 0)
		tp->pred_flags = 0;

	return new_win;
}

/* Packet ECN state for a SYN-ACK */
static inline void TCP_ECN_send_synack(struct tcp_sock *tp, struct sk_buff *skb)
{
	TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_CWR;
	if (!(tp->ecn_flags & TCP_ECN_OK))
		TCP_SKB_CB(skb)->flags &= ~TCPCB_FLAG_ECE;
}

/* Packet ECN state for a SYN.  */
static inline void TCP_ECN_send_syn(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);

	tp->ecn_flags = 0;
	if (sysctl_tcp_ecn == 1) {
		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_ECE | TCPCB_FLAG_CWR;
		tp->ecn_flags = TCP_ECN_OK;
	}
}

static __inline__ void
TCP_ECN_make_synack(struct request_sock *req, struct tcphdr *th)
{
	if (inet_rsk(req)->ecn_ok)
		th->ece = 1;
}

/* Set up ECN state for a packet on a ESTABLISHED socket that is about to
 * be sent.
 */
static inline void TCP_ECN_send(struct sock *sk, struct sk_buff *skb,
				int tcp_header_len)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (tp->ecn_flags & TCP_ECN_OK) {
		/* Not-retransmitted data segment: set ECT and inject CWR. */
		if (skb->len != tcp_header_len &&
		    !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
			INET_ECN_xmit(sk);
			if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
				tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
				tcp_hdr(skb)->cwr = 1;
				skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
			}
		} else {
			/* ACK or retransmitted segment: clear ECT|CE */
			INET_ECN_dontxmit(sk);
		}
		if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
			tcp_hdr(skb)->ece = 1;
	}
}

/* Constructs common control bits of non-data skb. If SYN/FIN is present,
 * auto increment end seqno.
 */
static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
{
	skb->ip_summed = CHECKSUM_PARTIAL;
	skb->csum = 0;

	TCP_SKB_CB(skb)->flags = flags;
	TCP_SKB_CB(skb)->sacked = 0;

	skb_shinfo(skb)->gso_segs = 1;
	skb_shinfo(skb)->gso_size = 0;
	skb_shinfo(skb)->gso_type = 0;

	TCP_SKB_CB(skb)->seq = seq;
	if (flags & (TCPCB_FLAG_SYN | TCPCB_FLAG_FIN))
		seq++;
	TCP_SKB_CB(skb)->end_seq = seq;
}

static inline int tcp_urg_mode(const struct tcp_sock *tp)
{
	return tp->snd_una != tp->snd_up;
}

#define OPTION_SACK_ADVERTISE	(1 << 0)
#define OPTION_TS		(1 << 1)
#define OPTION_MD5		(1 << 2)
#define OPTION_WSCALE		(1 << 3)
#define OPTION_COOKIE_EXTENSION	(1 << 4)

//在发送数据的时候，在tcp_transmit_skb中填充  tcp_options_write
//tcp发送数据的时候究竟携带了哪些tcp选项字段可以参考tcp_syn_options和tcp_established_options，真正忘TCP头部填充选项字段在tcp_options_write
struct tcp_out_options {
	u8 options;		/* bit field of OPTION_* */
	u8 ws;			/* window scale, 0 to disable */
	u8 num_sack_blocks;	/* number of SACK blocks to include */
	u8 hash_size;		/* bytes in hash_location */
	u16 mss;		/* 0 to disable */
	__u32 tsval, tsecr;	/* need to include OPTION_TS */
	__u8 *hash_location;	/* temporary pointer, overloaded */
};

/* The sysctl int routines are generic, so check consistency here.
 */
static u8 tcp_cookie_size_check(u8 desired)
{
	int cookie_size;

	if (desired > 0)
		/* previously specified */
		return desired;

	cookie_size = ACCESS_ONCE(sysctl_tcp_cookie_size);
	if (cookie_size <= 0)
		/* no default specified */
		return 0;

	if (cookie_size <= TCP_COOKIE_MIN)
		/* value too small, specify minimum */
		return TCP_COOKIE_MIN;

	if (cookie_size >= TCP_COOKIE_MAX)
		/* value too large, specify maximum */
		return TCP_COOKIE_MAX;

	if (cookie_size & 1)
		/* 8-bit multiple, illegal, fix it */
		cookie_size++;

	return (u8)cookie_size;
}

/* Write previously computed TCP options to the packet.
 *
 * Beware: Something in the Internet is very sensitive to the ordering of
 * TCP options, we learned this through the hard way, so be careful here.
 * Luckily we can at least blame others for their non-compliance but from
 * inter-operatibility perspective it seems that we're somewhat stuck with
 * the ordering which we have been using if we want to keep working with
 * those broken things (not that it currently hurts anybody as there isn't
 * particular reason why the ordering would need to be changed).
 *
 * At least SACK_PERM as the first option is known to lead to a disaster
 * (but it may well be that other scenarios fail similarly).
 */
static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
			      struct tcp_out_options *opts)
{
	u8 options = opts->options;	/* mungable copy */

	/* Having both authentication and cookies for security is redundant,
	 * and there's certainly not enough room.  Instead, the cookie-less
	 * extension variant is proposed.
	 *
	 * Consider the pessimal case with authentication.  The options
	 * could look like:
	 *   COOKIE|MD5(20) + MSS(4) + SACK|TS(12) + WSCALE(4) == 40
	 */
	if (unlikely(OPTION_MD5 & options)) {
		if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
			*ptr++ = htonl((TCPOPT_COOKIE << 24) |
				       (TCPOLEN_COOKIE_BASE << 16) |
				       (TCPOPT_MD5SIG << 8) |
				       TCPOLEN_MD5SIG);
		} else {
			*ptr++ = htonl((TCPOPT_NOP << 24) |
				       (TCPOPT_NOP << 16) |
				       (TCPOPT_MD5SIG << 8) |
				       TCPOLEN_MD5SIG);
		}
		options &= ~OPTION_COOKIE_EXTENSION;
		/* overload cookie hash location */
		opts->hash_location = (__u8 *)ptr;
		ptr += 4;
	}

	if (unlikely(opts->mss)) {
		*ptr++ = htonl((TCPOPT_MSS << 24) |
			       (TCPOLEN_MSS << 16) |
			       opts->mss);
	}

	if (likely(OPTION_TS & options)) {
		if (unlikely(OPTION_SACK_ADVERTISE & options)) {
			*ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
				       (TCPOLEN_SACK_PERM << 16) |
				       (TCPOPT_TIMESTAMP << 8) |
				       TCPOLEN_TIMESTAMP);
			options &= ~OPTION_SACK_ADVERTISE;
		} else {
			*ptr++ = htonl((TCPOPT_NOP << 24) |
				       (TCPOPT_NOP << 16) |
				       (TCPOPT_TIMESTAMP << 8) |
				       TCPOLEN_TIMESTAMP);
		}
		*ptr++ = htonl(opts->tsval);
		*ptr++ = htonl(opts->tsecr);
	}

	/* Specification requires after timestamp, so do it now.
	 *
	 * Consider the pessimal case without authentication.  The options
	 * could look like:
	 *   MSS(4) + SACK|TS(12) + COOKIE(20) + WSCALE(4) == 40
	 */
	if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
		__u8 *cookie_copy = opts->hash_location;
		u8 cookie_size = opts->hash_size;

		/* 8-bit multiple handled in tcp_cookie_size_check() above,
		 * and elsewhere.
		 */
		if (0x2 & cookie_size) {
			__u8 *p = (__u8 *)ptr;

			/* 16-bit multiple */
			*p++ = TCPOPT_COOKIE;
			*p++ = TCPOLEN_COOKIE_BASE + cookie_size;
			*p++ = *cookie_copy++;
			*p++ = *cookie_copy++;
			ptr++;
			cookie_size -= 2;
		} else {
			/* 32-bit multiple */
			*ptr++ = htonl(((TCPOPT_NOP << 24) |
					(TCPOPT_NOP << 16) |
					(TCPOPT_COOKIE << 8) |
					TCPOLEN_COOKIE_BASE) +
				       cookie_size);
		}

		if (cookie_size > 0) {
			memcpy(ptr, cookie_copy, cookie_size);
			ptr += (cookie_size / 4);
		}
	}

	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
		*ptr++ = htonl((TCPOPT_NOP << 24) |
			       (TCPOPT_NOP << 16) |
			       (TCPOPT_SACK_PERM << 8) |
			       TCPOLEN_SACK_PERM);
	}

	if (unlikely(OPTION_WSCALE & options)) {
		*ptr++ = htonl((TCPOPT_NOP << 24) |
			       (TCPOPT_WINDOW << 16) |
			       (TCPOLEN_WINDOW << 8) |
			       opts->ws);
	}

	if (unlikely(opts->num_sack_blocks)) {
		struct tcp_sack_block *sp = tp->rx_opt.dsack ?
			tp->duplicate_sack : tp->selective_acks;
		int this_sack;

		*ptr++ = htonl((TCPOPT_NOP  << 24) |
			       (TCPOPT_NOP  << 16) |
			       (TCPOPT_SACK <<  8) |
			       (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
						     TCPOLEN_SACK_PERBLOCK)));

		for (this_sack = 0; this_sack < opts->num_sack_blocks;
		     ++this_sack) {
			*ptr++ = htonl(sp[this_sack].start_seq);
			*ptr++ = htonl(sp[this_sack].end_seq);
		}

		tp->rx_opt.dsack = 0;
	}
}

/* Compute TCP options for SYN packets. This is not the final
 * network wire format yet.
 *///syn包的tcp option是这个tcp_syn_options，不是syn包的用tcp_established_options  参考樊东东下P890有图形化解释
static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
				struct tcp_out_options *opts,
				struct tcp_md5sig_key **md5) {
	struct tcp_sock *tp = tcp_sk(sk);
	struct tcp_cookie_values *cvp = tp->cookie_values;
	unsigned remaining = MAX_TCP_OPTION_SPACE;
	u8 cookie_size = (!tp->rx_opt.cookie_out_never && cvp != NULL) ?
			 tcp_cookie_size_check(cvp->cookie_desired) :
			 0;

#ifdef CONFIG_TCP_MD5SIG
	*md5 = tp->af_specific->md5_lookup(sk, sk);
	if (*md5) {
		opts->options |= OPTION_MD5;
		remaining -= TCPOLEN_MD5SIG_ALIGNED;
	}
#else
	*md5 = NULL;
#endif

	/* We always get an MSS option.  The option bytes which will be seen in
	 * normal data packets should timestamps be used, must be in the MSS
	 * advertised.  But we subtract them from tp->mss_cache so that
	 * calculations in tcp_sendmsg are simpler etc.  So account for this
	 * fact here if necessary.  If we don't do this correctly, as a
	 * receiver we won't recognize data packets as being full sized when we
	 * should, and thus we won't abide by the delayed ACK rules correctly.
	 * SACKs don't matter, we never delay an ACK when we have any of those
	 * going out.  */
	opts->mss = tcp_advertise_mss(sk);
	remaining -= TCPOLEN_MSS_ALIGNED;

	if (likely(sysctl_tcp_timestamps && *md5 == NULL)) {
		opts->options |= OPTION_TS;
		opts->tsval = TCP_SKB_CB(skb)->when;
		opts->tsecr = tp->rx_opt.ts_recent;
		remaining -= TCPOLEN_TSTAMP_ALIGNED;
	}
	if (likely(sysctl_tcp_window_scaling)) {
		opts->ws = tp->rx_opt.rcv_wscale;
		opts->options |= OPTION_WSCALE;
		remaining -= TCPOLEN_WSCALE_ALIGNED;
	}
	if (likely(sysctl_tcp_sack)) {
		opts->options |= OPTION_SACK_ADVERTISE;
		if (unlikely(!(OPTION_TS & opts->options)))
			remaining -= TCPOLEN_SACKPERM_ALIGNED;
	}

	/* Note that timestamps are required by the specification.
	 *
	 * Odd numbers of bytes are prohibited by the specification, ensuring
	 * that the cookie is 16-bit aligned, and the resulting cookie pair is
	 * 32-bit aligned.
	 */
	if (*md5 == NULL &&
	    (OPTION_TS & opts->options) &&
	    cookie_size > 0) {
		int need = TCPOLEN_COOKIE_BASE + cookie_size;

		if (0x2 & need) {
			/* 32-bit multiple */
			need += 2; /* NOPs */

			if (need > remaining) {
				/* try shrinking cookie to fit */
				cookie_size -= 2;
				need -= 4;
			}
		}
		while (need > remaining && TCP_COOKIE_MIN <= cookie_size) {
			cookie_size -= 4;
			need -= 4;
		}
		if (TCP_COOKIE_MIN <= cookie_size) {
			opts->options |= OPTION_COOKIE_EXTENSION;
			opts->hash_location = (__u8 *)&cvp->cookie_pair[0];
			opts->hash_size = cookie_size;

			/* Remember for future incarnations. */
			cvp->cookie_desired = cookie_size;

			if (cvp->cookie_desired != cvp->cookie_pair_size) {
				/* Currently use random bytes as a nonce,
				 * assuming these are completely unpredictable
				 * by hostile users of the same system.
				 */
				get_random_bytes(&cvp->cookie_pair[0],
						 cookie_size);
				cvp->cookie_pair_size = cookie_size;
			}

			remaining -= need;
		}
	}
	return MAX_TCP_OPTION_SPACE - remaining;
}

/* Set up TCP options for SYN-ACKs. */
static unsigned tcp_synack_options(struct sock *sk,
				   struct request_sock *req,
				   unsigned mss, struct sk_buff *skb,
				   struct tcp_out_options *opts,
				   struct tcp_md5sig_key **md5,
				   struct tcp_extend_values *xvp)
{
	struct inet_request_sock *ireq = inet_rsk(req);
	unsigned remaining = MAX_TCP_OPTION_SPACE;
	u8 cookie_plus = (xvp != NULL && !xvp->cookie_out_never) ?
			 xvp->cookie_plus :
			 0;

#ifdef CONFIG_TCP_MD5SIG
	*md5 = tcp_rsk(req)->af_specific->md5_lookup(sk, req);
	if (*md5) {
		opts->options |= OPTION_MD5;
		remaining -= TCPOLEN_MD5SIG_ALIGNED;

		/* We can't fit any SACK blocks in a packet with MD5 + TS
		 * options. There was discussion about disabling SACK
		 * rather than TS in order to fit in better with old,
		 * buggy kernels, but that was deemed to be unnecessary.
		 */
		ireq->tstamp_ok &= !ireq->sack_ok;
	}
#else
	*md5 = NULL;
#endif

	/* We always send an MSS option. */
	opts->mss = mss;
	remaining -= TCPOLEN_MSS_ALIGNED;

	if (likely(ireq->wscale_ok)) {
		opts->ws = ireq->rcv_wscale;
		opts->options |= OPTION_WSCALE;
		remaining -= TCPOLEN_WSCALE_ALIGNED;
	}
	if (likely(ireq->tstamp_ok)) {
		opts->options |= OPTION_TS;
		opts->tsval = TCP_SKB_CB(skb)->when;
		opts->tsecr = req->ts_recent;
		remaining -= TCPOLEN_TSTAMP_ALIGNED;
	}
	if (likely(ireq->sack_ok)) {
		opts->options |= OPTION_SACK_ADVERTISE;
		if (unlikely(!ireq->tstamp_ok))
			remaining -= TCPOLEN_SACKPERM_ALIGNED;
	}

	/* Similar rationale to tcp_syn_options() applies here, too.
	 * If the <SYN> options fit, the same options should fit now!
	 */
	if (*md5 == NULL &&
	    ireq->tstamp_ok &&
	    cookie_plus > TCPOLEN_COOKIE_BASE) {
		int need = cookie_plus; /* has TCPOLEN_COOKIE_BASE */

		if (0x2 & need) {
			/* 32-bit multiple */
			need += 2; /* NOPs */
		}
		if (need <= remaining) {
			opts->options |= OPTION_COOKIE_EXTENSION;
			opts->hash_size = cookie_plus - TCPOLEN_COOKIE_BASE;
			remaining -= need;
		} else {
			/* There's no error return, so flag it. */
			xvp->cookie_out_never = 1; /* true */
			opts->hash_size = 0;
		}
	}
	return MAX_TCP_OPTION_SPACE - remaining;
}

/* Compute TCP options for ESTABLISHED sockets. This is not the
 * final wire format yet.
 *///syn包的tcp option是这个tcp_syn_options，不是syn包的用tcp_established_options  参考樊东东下P890有图形化解释
 //TCP选项中是否有timestamp选项字段和sack选项字段，如果有这些选项字段，则发送数据时，mss的值为1500-TCP头-IP头-选项字段
static unsigned tcp_established_options(struct sock *sk, struct sk_buff *skb,
					struct tcp_out_options *opts,
					struct tcp_md5sig_key **md5) {
	struct tcp_skb_cb *tcb = skb ? TCP_SKB_CB(skb) : NULL;
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned size = 0;
	unsigned int eff_sacks;

#ifdef CONFIG_TCP_MD5SIG
	*md5 = tp->af_specific->md5_lookup(sk, sk);
	if (unlikely(*md5)) {
		opts->options |= OPTION_MD5;
		size += TCPOLEN_MD5SIG_ALIGNED;
	}
#else
	*md5 = NULL;
#endif

	if (likely(tp->rx_opt.tstamp_ok)) {
		opts->options |= OPTION_TS;
		opts->tsval = tcb ? tcb->when : 0;
		opts->tsecr = tp->rx_opt.ts_recent;
		size += TCPOLEN_TSTAMP_ALIGNED;
	}

	eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
	if (unlikely(eff_sacks)) {
		const unsigned remaining = MAX_TCP_OPTION_SPACE - size;
		opts->num_sack_blocks =
			min_t(unsigned, eff_sacks,
			      (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
			      TCPOLEN_SACK_PERBLOCK);
		size += TCPOLEN_SACK_BASE_ALIGNED +
			opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
	}

	return size;
}

/* 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.
 */

/*
 * 通常要发送一个TCP段都是通过tcp_transmit_skb()的。该函数会给
 * 待发送的段构造TCP首部，然后调用网络层接口到IP层，最终
 * 抵达网络设备。由于在成功发送到网络设备后会释放该
 * SKB，而TCP必须要接到对应的ACK后才能真正释放数据，因此
 * 在发送前会根据参数确定是克隆还是复制一份SKB用于发送。
 */ //最终的tcp发送都会调用这个  clone_it表示发送发送队列的第一个SKB的时候，采用克隆skb还是直接使用skb，如果是发送应用层数据则使用克隆的，等待对方应答ack回来才把数据删除。如果是会送ack信息，则无需克隆
//如果不支持TSO或者GSO这里的SKB->len为mss，否则如果支持TSO并且有数据再shinfo中，则这里的SKB长度为shinfo或者拥塞窗口的最小值
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 tcp_options_size, tcp_header_size;
	struct tcp_md5sig_key *md5;
	struct tcphdr *th;
	int err;

	BUG_ON(!skb || !tcp_skb_pcount(skb));

	/* If congestion control is doing timestamping, we must
	 * take such a timestamp before we potentially clone/copy.
	 */
	 	/*
	 * 如果拥塞控制要作时间采样，则必须设置一个时间戳，
	 * 之后再克隆或复制数据包。Linux支持多达十种拥塞控制算法，
	 * 但并不是每种算法都需要作时间采样的，因此在设置
	 * 时间戳前先判断当前的拥塞控制算法是否需作时间采样。
	 */
	if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP)
		__net_timestamp(skb);

    /*
	 * 根据参数clone_it确定是否克隆待发送的数据包。
	 */
	if (likely(clone_it)) {
		if (unlikely(skb_cloned(skb)))
			skb = pskb_copy(skb, gfp_mask);
		else
			skb = skb_clone(skb, gfp_mask);
		if (unlikely(!skb))
			return -ENOBUFS;
	}

    /*
	 * 获取INET层和TCP层的传输控制块、SKB中的TCP私有控制块
	 * 以及当前TCP首部长度。
	 */
	inet = inet_sk(sk);
	tp = tcp_sk(sk);
	tcb = TCP_SKB_CB(skb);
	memset(&opts, 0, sizeof(opts));

    /*
	 * 判断当前TCP段是不是SYN段，因为有些选项只能出现在SYN段中，需作
	 * 特别处理。
	 */
	if (unlikely(tcb->flags & TCPCB_FLAG_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);

    /*
	 * 如果已发出未确认的数据包数为零，则只初始化
	 * 拥塞控制，并开始跟踪该连接的RTT。
	 */
	if (tcp_packets_in_flight(tp) == 0)
		tcp_ca_event(sk, CA_EVENT_TX_START);

	skb_push(skb, tcp_header_size);
	skb_reset_transport_header(skb);

	/*
	 * 到此为止，SKB已添加到发送队列中。但从SKB角度去看
	 * 还不知道它属于哪个传输控制块，因此此时需调用
	 * skb_set_owner_w()设置该SKB的宿主。
	 */
	skb_set_owner_w(skb, sk);

	/* Build TCP header and checksum it. */
	th = tcp_hdr(skb);
	th->source		= inet->inet_sport;
	th->dest		= inet->inet_dport;
	th->seq			= htonl(tcb->seq);
	th->ack_seq		= htonl(tp->rcv_nxt);
	*(((__be16 *)th) + 6)	= htons(((tcp_header_size >> 2) << 12) |
					tcb->flags);

    /*
	 * 设置TCP首部的接收窗口，需要分两种情况处理
	 */
	if (unlikely(tcb->flags & TCPCB_FLAG_SYN)) {
		/* RFC1323: The window in SYN & SYN/ACK segments
		 * is never scaled.
		 */
		 /*
		 * 如果是SYN段，则设置接收窗口初始值为rcv_wnd和65535U
		 * 中的最小值。
		 */
		th->window	= htons(min(tp->rcv_wnd, 65535U));
	} else {
	    /*
		 * 如果是其他段，则调用tcp_select_window()计算当前接收
		 * 窗口的大小。
		 */
		th->window	= htons(tcp_select_window(sk));
	}
	th->check		= 0;
	th->urg_ptr		= 0;

    /*
	 * 判断是否需要设置紧急指针和带外数据标志。判断条件有两个，
	 * 一是发送时是否设置了紧急方式，二是紧急指针是否在以该报文
	 * 数据序号为起始的65535范围之内，其中第二个条件主要是判断紧急
	 * 指针的合法性。
	 */
	/* 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首部调整完毕，开始构建TCP首部选项。
	 */
	tcp_options_write((__be32 *)(th + 1), tp, &opts);
	if (likely((tcb->flags & TCPCB_FLAG_SYN) == 0))
		TCP_ECN_send(sk, skb, tcp_header_size);

#ifdef CONFIG_TCP_MD5SIG
	/* Calculate the MD5 hash, as we have all we need now */
	if (md5) {
		sk_nocaps_add(sk, NETIF_F_GSO_MASK);
		tp->af_specific->calc_md5_hash(opts.hash_location,
					       md5, sk, NULL, skb);
	}
#endif

    /*
	 * 调用IPv4执行校验和接口send_check计算校验和，并设置到TCP首部中。
	 * 在TCP中send_check接口被初始化为tcp_v4_send_check。
	 */
	icsk->icsk_af_ops->send_check(sk, skb);

    /*
	 * 如果发送出去的段有ACK标志，则需要通知延时确认模块，递减
	 * 快速发送ACK段的数量，同时停止延时确认定时器。
	 */
	if (likely(tcb->flags & TCPCB_FLAG_ACK))
		tcp_event_ack_sent(sk, tcp_skb_pcount(skb));

    /*
	 * 如果发送出去的TCP段有负载，则检测拥塞窗口闲置是否超时，
	 * 并使其失效。同时记录发送TCP的时间，根据最近接受段的时间
	 * 确定本端延时确认是否进入pingpong模式。
	 */
	if (skb->len != tcp_header_size)
		tcp_event_data_sent(tp, skb, sk);

	if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
		TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
			      tcp_skb_pcount(skb));

    /*
	 * 调用发送接口queue_xmit发送报文，如果失败则返回
	 * 错误码。在TCP中该接口实现函数为ip_queue_xmit()。
	 */
	err = icsk->icsk_af_ops->queue_xmit(skb);
	if (likely(err <= 0))
		return err;

    /*
	 * 当发送失败时，类似接收到显式拥塞通知，使拥塞
	 * 控制进入CWR状态。最后，根据错误信息，返回发送
	 * 是否成功。
	 */
	tcp_enter_cwr(sk, 1);

	return net_xmit_eval(err);
}

/* This routine just queues the buffer for sending.
 *
 * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
 * otherwise socket can stall.
 */
static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);

	/* Advance write_seq and place onto the write_queue. */
	tp->write_seq = TCP_SKB_CB(skb)->end_seq;
	skb_header_release(skb);
	tcp_add_write_queue_tail(sk, skb);
	sk->sk_wmem_queued += skb->truesize;
	sk_mem_charge(sk, skb->truesize);
}

/* Initialize TSO segments for a packet. */
static void tcp_set_skb_tso_segs(struct sock *sk, struct sk_buff *skb,
				 unsigned int mss_now)
{
	if (skb->len <= mss_now || !sk_can_gso(sk) ||
	    skb->ip_summed == CHECKSUM_NONE) {
		/* Avoid the costly divide in the normal
		 * non-TSO case.
		 */
		skb_shinfo(skb)->gso_segs = 1;
		skb_shinfo(skb)->gso_size = 0;
		skb_shinfo(skb)->gso_type = 0;
	} else {
		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss_now);
		skb_shinfo(skb)->gso_size = mss_now;
		skb_shinfo(skb)->gso_type = sk->sk_gso_type;
	}
}

/* When a modification to fackets out becomes necessary, we need to check
 * skb is counted to fackets_out or not.
 */
static void tcp_adjust_fackets_out(struct sock *sk, 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, struct sk_buff *skb, int decr)
{
	struct tcp_sock *tp = tcp_sk(sk);

	tp->packets_out -= decr;

	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
		tp->sacked_out -= decr;
	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
		tp->retrans_out -= decr;
	if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
		tp->lost_out -= decr;

	/* Reno case is special. Sigh... */
	if (tcp_is_reno(tp) && decr > 0)
		tp->sacked_out -= min_t(u32, tp->sacked_out, decr);

	tcp_adjust_fackets_out(sk, skb, decr);

	if (tp->lost_skb_hint &&
	    before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
	    (tcp_is_fack(tp) || (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)))
		tp->lost_cnt_hint -= decr;

	tcp_verify_left_out(tp);
}

/* Function to create two new TCP segments.  Shrinks the given segment
 * to the specified size and appends a new segment with the rest of the
 * packet to the list.  This won't be called frequently, I hope.
 * Remember, these are still headerless SKBs at this point.
 */
int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
		 unsigned int mss_now)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *buff;
	int nsize, old_factor;
	int nlen;
	u8 flags;

	BUG_ON(len > skb->len);

	nsize = skb_headlen(skb) - len;
	if (nsize < 0)
		nsize = 0;

	if (skb_cloned(skb) &&
	    skb_is_nonlinear(skb) &&
	    pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
		return -ENOMEM;

	/* Get a new skb... force flag on. */
	buff = sk_stream_alloc_skb(sk, nsize, GFP_ATOMIC);
	if (buff == NULL)
		return -ENOMEM; /* We'll just try again later. */

	sk->sk_wmem_queued += buff->truesize;
	sk_mem_charge(sk, buff->truesize);
	nlen = skb->len - len - nsize;
	buff->truesize += nlen;
	skb->truesize -= nlen;

	/* Correct the sequence numbers. */
	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

	/* PSH and FIN should only be set in the second packet. */
	flags = TCP_SKB_CB(skb)->flags;
	TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN | TCPCB_FLAG_PSH);
	TCP_SKB_CB(buff)->flags = flags;
	TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;

	if (!skb_shinfo(skb)->nr_frags && skb->ip_summed != CHECKSUM_PARTIAL) {
		/* Copy and checksum data tail into the new buffer. */
		buff->csum = csum_partial_copy_nocheck(skb->data + len,
						       skb_put(buff, nsize),
						       nsize, 0);

		skb_trim(skb, len);

		skb->csum = csum_block_sub(skb->csum, buff->csum, len);
	} else {
		skb->ip_summed = CHECKSUM_PARTIAL;
		skb_split(skb, buff, len);
	}

	buff->ip_summed = skb->ip_summed;

	/* Looks stupid, but our code really uses when of
	 * skbs, which it never sent before. --ANK
	 */
	TCP_SKB_CB(buff)->when = TCP_SKB_CB(skb)->when;
	buff->tstamp = skb->tstamp;

	old_factor = tcp_skb_pcount(skb);

	/* Fix up tso_factor for both original and new SKB.  */
	tcp_set_skb_tso_segs(sk, skb, mss_now);
	tcp_set_skb_tso_segs(sk, buff, mss_now);

	/* If this packet has been sent out already, we must
	 * adjust the various packet counters.
	 */
	if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
		int diff = old_factor - tcp_skb_pcount(skb) -
			tcp_skb_pcount(buff);

		if (diff)
			tcp_adjust_pcount(sk, skb, diff);
	}

	/* Link BUFF into the send queue. */
	skb_header_release(buff);
	tcp_insert_write_queue_after(skb, buff, sk);

	return 0;
}

/* This is similar to __pskb_pull_head() (it will go to core/skbuff.c
 * eventually). The difference is that pulled data not copied, but
 * immediately discarded.
 */
static void __pskb_trim_head(struct sk_buff *skb, int len)
{
	int i, k, eat;

	eat = len;
	k = 0;
	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
		if (skb_shinfo(skb)->frags[i].size <= eat) {
			put_page(skb_shinfo(skb)->frags[i].page);
			eat -= skb_shinfo(skb)->frags[i].size;
		} else {
			skb_shinfo(skb)->frags[k] = skb_shinfo(skb)->frags[i];
			if (eat) {
				skb_shinfo(skb)->frags[k].page_offset += eat;
				skb_shinfo(skb)->frags[k].size -= eat;
				eat = 0;
			}
			k++;
		}
	}
	skb_shinfo(skb)->nr_frags = k;

	skb_reset_tail_pointer(skb);
	skb->data_len -= len;
	skb->len = skb->data_len;
}

/* Remove acked data from a packet in the transmit queue. */
//一个应用层接收的包可能是要分段的，也就是多次通过mtu发送，但是有可能前面一部分发送出去后都给了应答，中途某个地方应答超时，则需要移除前面已发送成功的，只发送后面的部分
int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
{
	if (skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
		return -ENOMEM;

	/* If len == headlen, we avoid __skb_pull to preserve alignment. */
	if (unlikely(len < skb_headlen(skb)))
		__skb_pull(skb, len);
	else
		__pskb_trim_head(skb, len - skb_headlen(skb));

	TCP_SKB_CB(skb)->seq += len;
	skb->ip_summed = CHECKSUM_PARTIAL;

	skb->truesize	     -= len;
	sk->sk_wmem_queued   -= len;
	sk_mem_uncharge(sk, len);
	sock_set_flag(sk, SOCK_QUEUE_SHRUNK);

	/* Any change of skb->len requires recalculation of tso
	 * factor and mss.
	 */
	if (tcp_skb_pcount(skb) > 1)
		tcp_set_skb_tso_segs(sk, skb, tcp_current_mss(sk));

	return 0;
}

/* Calculate MSS. Not accounting for SACKs here.  */
int tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
	struct tcp_sock *tp = tcp_sk(sk);
	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);

	/* Clamp it (mss_clamp does not include tcp options) */
	if (mss_now > tp->rx_opt.mss_clamp)
		mss_now = tp->rx_opt.mss_clamp;

	/* Now subtract optional transport overhead */
	mss_now -= icsk->icsk_ext_hdr_len;

	/* Then reserve room for full set of TCP options and 8 bytes of data */
	if (mss_now < 48)
		mss_now = 48;

	/* Now subtract TCP options size, not including SACKs */
	mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);

	return mss_now;
}

/* Inverse of above */
int tcp_mss_to_mtu(struct sock *sk, int mss)
{
	struct tcp_sock *tp = tcp_sk(sk);
	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;

	return mtu;
}

/* MTU probing init per socket */
void tcp_mtup_init(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);

	icsk->icsk_mtup.enabled = sysctl_tcp_mtu_probing > 1;
	icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
			       icsk->icsk_af_ops->net_header_len;
	icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, sysctl_tcp_base_mss);
	icsk->icsk_mtup.probe_size = 0;
}

/* 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)
 */
 
/*
 * tcp_sync_mss()为传输控制块中与mss相关的成员进行
 * 数据同步。
 */ //通过最新的路径发现pmtu获取最新的mss
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;

    /*
     * 如果路径MTU发现段长度上限无效，则需要更新之。
     */
    if (icsk->icsk_mtup.search_high > pmtu)
        icsk->icsk_mtup.search_high = pmtu;

    /*
     * 根据PMTU得到MSS，基本计算方法是
     * (PMTU-IP首部长度-TCP首部长度-IP选项长度-TCP选项)，
     * 其中IP选项和TCP选项的长度可以都为0.
     */
    mss_now = tcp_mtu_to_mss(sk, pmtu);
    mss_now = tcp_bound_to_half_wnd(tp, mss_now);

    /* And store cached results */
    /*
     * 保存最近更新的有效PMTU。
     */
    icsk->icsk_pmtu_cookie = pmtu;
    /*
     * 最后将得到的MSS更新到缓存中。
     */
    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;
}

/* Compute the current effective MSS, taking SACKs and IP options,
 * and even PMTU discovery events into account.
 *//*
 * 用来计算当前有效MSS，需考虑TCP首部中的SACK选项
 * 和IP选项，以及PMTU。
 */ //获取当前发送数据的mss  
unsigned int tcp_current_mss(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);

	  /*
    	 * 获取套接字的路由缓存项，用来从中
    	 * 取出PMTU。
    	 */
	struct dst_entry *dst = __sk_dst_get(sk);
	u32 mss_now;
	unsigned header_len;
	struct tcp_out_options opts;
	struct tcp_md5sig_key *md5;

	mss_now = tp->mss_cache;

    /*
	 * 如果获取到该套接字的路由，则从中取出PMTU与最近
	 * 一次更新的路径MTU比较，如果不相等，即更新
	 * icsk_pmtu_cookie和当前有效MSS。
	 */
	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); //如果带有选项字段，还要把TCP头部中带的选项字段去掉，这样mss就更少了点
	/* The mss_cache is sized based on tp->tcp_header_len, which assumes
	 * some common options. If this is an odd packet (because we have SACK
	 * blocks etc) then our calculated header_len will be different, and
	 * we have to adjust mss_now correspondingly */
	if (header_len != tp->tcp_header_len) {
		int delta = (int) header_len - tp->tcp_header_len;
		mss_now -= delta;
	}

	return mss_now;
}

/* Congestion window validation. (RFC2861) */
static void tcp_cwnd_validate(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (tp->packets_out >= tp->snd_cwnd) {
		/* Network is feed fully. */
		tp->snd_cwnd_used = 0;
		tp->snd_cwnd_stamp = tcp_time_stamp;
	} else {
		/* Network starves. */
		if (tp->packets_out > tp->snd_cwnd_used)
			tp->snd_cwnd_used = tp->packets_out;

		if (sysctl_tcp_slow_start_after_idle &&
		    (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto)
			tcp_cwnd_application_limited(sk);
	}
}

/* Returns the portion of skb which can be sent right away without
 * introducing MSS oddities to segment boundaries. In rare cases where
 * mss_now != mss_cache, we will request caller to create a small skb
 * per input skb which could be mostly avoided here (if desired).
 *
 * We explicitly want to create a request for splitting write queue tail
 * to a small skb for Nagle purposes while avoiding unnecessary modulos,
 * thus all the complexity (cwnd_len is always MSS multiple which we
 * return whenever allowed by the other factors). Basically we need the
 * modulo only when the receiver window alone is the limiting factor or
 * when we would be allowed to send the split-due-to-Nagle skb fully.
 *///以发送窗口和拥塞窗口的最小值作为分段段长。也就是比较可用拥塞窗口和tcp_sendmsg中的分散聚合I/O页 
static unsigned int tcp_mss_split_point(struct sock *sk, struct sk_buff *skb,
					unsigned int mss_now, unsigned int cwnd)
{
	struct tcp_sock *tp = tcp_sk(sk);
	u32 needed, window, cwnd_len;

	window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
	cwnd_len = mss_now * cwnd;

	if (likely(cwnd_len <= window && skb != tcp_write_queue_tail(sk)))
		return cwnd_len;

	needed = min(skb->len, window);

	if (cwnd_len <= needed)
		return cwnd_len;

	return needed - needed % mss_now;
}

/* Can at least one segment of SKB be sent right now, according to the
 * congestion window rules?  If so, return how many segments are allowed.
 */
/*
 * 当TCP发送方输出时，先判断目前是否可以立即发送。如果可以，
 * 则获取当前拥塞窗口的未使用量，用来判断拥塞窗口是否还有
 * 配额用来发送。
 * tcp_cwnd_test()函数就是根据当前的拥塞窗口和网络中正在传输的段
 * 得到现在可以发送段的数目。参数skb为当前待输出的段。
 */
static inline unsigned int tcp_cwnd_test(struct tcp_sock *tp,
					 struct sk_buff *skb)
{
	u32 in_flight, cwnd;

	/* Don't be strict about the congestion window for the final FIN.  */
	/*
	 * 如果当前发送的段存在FIN标志，则只能输出一个，因为接着
	 * 开始断开连接了，不能再发送用户数据了。
	 */
	if ((TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN) &&
	    tcp_skb_pcount(skb) == 1)
		return 1;

	/*
	 * 如果拥塞窗口大于正在网络中传输的段数，则此次可以输出
	 * 的段数为他们之间之差，否则为0.
	 */
	in_flight = tcp_packets_in_flight(tp);
	cwnd = tp->snd_cwnd;
	if (in_flight < cwnd)
		return (cwnd - in_flight);

	return 0;
}


/* Intialize 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 sock *sk, struct sk_buff *skb,
			     unsigned int mss_now)
{
	int tso_segs = tcp_skb_pcount(skb);

	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
		tcp_set_skb_tso_segs(sk, skb, mss_now);
		tso_segs = tcp_skb_pcount(skb);
	}
	return tso_segs;
}

/* Minshall's variant of the Nagle send check. */
static inline int tcp_minshall_check(const struct tcp_sock *tp)
{
	return after(tp->snd_sml, tp->snd_una) &&
		!after(tp->snd_sml, tp->snd_nxt);
}

/* Return 0, if packet can be sent now without violation Nagle's rules:
 * 1. It is full sized.
 * 2. Or it contains FIN. (already checked by caller)
 * 3. Or TCP_NODELAY was set.
 * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
 *    With Minshall's modification: all sent small packets are ACKed.
 */
static inline int tcp_nagle_check(const struct tcp_sock *tp,
				  const struct sk_buff *skb,
				  unsigned mss_now, int nonagle)
{
	return (skb->len < mss_now &&
		((nonagle & TCP_NAGLE_CORK) ||
		 (!nonagle && tp->packets_out && tcp_minshall_check(tp))));
}

/* Return non-zero if the Nagle test allows this packet to be
 * sent now.
 */
static inline int tcp_nagle_test(struct tcp_sock *tp, 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 1;

	/* Don't use the nagle rule for urgent data (or for the final FIN).
	 * Nagle can be ignored during F-RTO too (see RFC4138).
	 */
	if (tcp_urg_mode(tp) || (tp->frto_counter == 2) ||
	    (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN))
		return 1;

	if (!tcp_nagle_check(tp, skb, cur_mss, nonagle))
		return 1;

	return 0;
}

/* Does at least the first segment of SKB fit into the send window? */
static inline int tcp_snd_wnd_test(struct tcp_sock *tp, 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(struct sock *sk, struct sk_buff *skb,
				 unsigned int cur_mss, int nonagle)
{
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int cwnd_quota;

	tcp_init_tso_segs(sk, skb, cur_mss);

	if (!tcp_nagle_test(tp, skb, cur_mss, nonagle))
		return 0;

	cwnd_quota = tcp_cwnd_test(tp, skb);
	if (cwnd_quota && !tcp_snd_wnd_test(tp, skb, cur_mss))
		cwnd_quota = 0;

	return cwnd_quota;
}

/* Test if sending is allowed right now. */
int tcp_may_send_now(struct sock *sk)
{
	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_fragment1(struct sock *sk, struct sk_buff *skb, unsigned int len,
			unsigned int mss_now)
{
	struct sk_buff *buff;
	int nlen = skb->len - len;
	u8 flags;

	/* All of a TSO frame must be composed of paged data.  */
	if (skb->len != skb->data_len)
		return tcp_fragment(sk, skb, len, mss_now);

	buff = sk_stream_alloc_skb(sk, 0, GFP_ATOMIC);
	if (unlikely(buff == NULL))
		return -ENOMEM;

	sk->sk_wmem_queued += buff->truesize;
	sk_mem_charge(sk, buff->truesize);
	buff->truesize += nlen;
	skb->truesize -= nlen;

	/* Correct the sequence numbers. */
	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

	/* PSH and FIN should only be set in the second packet. */
	flags = TCP_SKB_CB(skb)->flags;
	TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN | TCPCB_FLAG_PSH);
	TCP_SKB_CB(buff)->flags = flags;

	/* This packet was never sent out yet, so no SACK bits. */
	TCP_SKB_CB(buff)->sacked = 0;

	buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
	skb_split(skb, buff, len);

	/* Fix up tso_factor for both original and new SKB.  */
	tcp_set_skb_tso_segs(sk, skb, mss_now);
	tcp_set_skb_tso_segs(sk, buff, mss_now);

	/* Link BUFF into the send queue. */
	skb_header_release(buff);
	tcp_insert_write_queue_after(skb, buff, sk);

	return 0;
}

/* 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).
 */
/*
 * tso_fragment()实现将支持TSO的TCP段剪切成len字节长，剩余的数据
 * 移动到新的SKB中，并将其添加到队列中旧SKB之后。
 * @skb:待分段的TCP段
 * @len:为分段长
 * @mss_now:当前MSS。
 * 如果len大于mss_now，则说明分出的段仍是支持TSO的段，mss_now用来
 * 设置该段的gso_size；否则分出的是普通段。
 *///后面新的buffer添加到了发送队列的尾部
static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
			unsigned int mss_now)
{
	struct sk_buff *buff;
	int nlen = skb->len - len;
	u8 flags;

	/* All of a TSO frame must be composed of paged data.  */
	/*
	 * tso_fragment()中只处理支持TSO情况下的处理，
	 * 即skb中的数据区中只包含聚合分散I/O数据，
	 * 不包括线性缓冲区数据。
	 */
	if (skb->len != skb->data_len) //说明既有线性缓冲区也有分散聚合I/O数据
		return tcp_fragment(sk, skb, len, mss_now);

    //把nlen中多余拥塞窗口的数据拷贝到这个buff中
	buff = sk_stream_alloc_skb(sk, 0, GFP_ATOMIC);
	if (unlikely(buff == NULL))
		return -ENOMEM;

	sk->sk_wmem_queued += buff->truesize;
	sk_mem_charge(sk, buff->truesize);
	buff->truesize += nlen;
	skb->truesize -= nlen;

	/* Correct the sequence numbers. */
	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;

	/* PSH and FIN should only be set in the second packet. */
	flags = TCP_SKB_CB(skb)->flags;
	TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN | TCPCB_FLAG_PSH);
	TCP_SKB_CB(buff)->flags = flags;

	/* This packet was never sent out yet, so no SACK bits. */
	TCP_SKB_CB(buff)->sacked = 0;

	buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
	skb_split(skb, buff, len); //把源skb中的len字节留在skb中，新的skb->len-len放到新的skb1中

	/* Fix up tso_factor for both original and new SKB.  */
	tcp_set_skb_tso_segs(sk, skb, mss_now);
	tcp_set_skb_tso_segs(sk, buff, mss_now);

	/* Link BUFF into the send queue. */
	skb_header_release(buff);//从skb_split拆分出来的新skb1是没用tcp ip头部信息的
	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 int tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	const struct inet_connection_sock *icsk = inet_csk(sk);
	u32 send_win, cong_win, limit, in_flight;
	int win_divisor;

	if (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN)
		goto send_now;

	if (icsk->icsk_ca_state != TCP_CA_Open)
		goto send_now;

	/* Defer for less than two clock ticks. */
	if (tp->tso_deferred &&
	    (((u32)jiffies << 1) >> 1) - (tp->tso_deferred >> 1) > 1)
		goto send_now;

	in_flight = tcp_packets_in_flight(tp);

	BUG_ON(tcp_skb_pcount(skb) <= 1 || (tp->snd_cwnd <= in_flight));

	send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

	/* From in_flight test above, we know that cwnd > in_flight.  */
	cong_win = (tp->snd_cwnd - in_flight) * tp->mss_cache;

	limit = min(send_win, cong_win);

	/* If a full-sized TSO skb can be sent, do it. */
	if (limit >= sk->sk_gso_max_size)
		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_burst(tp) * tp->mss_cache)
			goto send_now;
	}

	/* Ok, it looks like it is advisable to defer.  */
	tp->tso_deferred = 1 | (jiffies << 1);

	return 1;

send_now:
	tp->tso_deferred = 0;
	return 0;
}

/* Create a new MTU probe if we are ready.
 * MTU probe is regularly attempting to increase the path MTU by
 * deliberately sending larger packets.  This discovers routing
 * changes resulting in larger path MTUs.
 *
 * Returns 0 if we should wait to probe (no cwnd available),
 *         1 if a probe was sent,
 *         -1 otherwise
 */
static int tcp_mtu_probe(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct sk_buff *skb, *nskb, *next;
	int len;
	int probe_size;
	int size_needed;
	int copy;
	int mss_now;

	/* Not currently probing/verifying,
	 * not in recovery,
	 * have enough cwnd, and
	 * not SACKing (the variable headers throw things off) */
	if (!icsk->icsk_mtup.enabled ||
	    icsk->icsk_mtup.probe_size ||
	    inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
	    tp->snd_cwnd < 11 ||
	    tp->rx_opt.num_sacks || tp->rx_opt.dsack)
		return -1;

	/* Very simple search strategy: just double the MSS. */
	mss_now = tcp_current_mss(sk);
	probe_size = 2 * tp->mss_cache;
	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {
		/* TODO: set timer for probe_converge_event */
		return -1;
	}

	/* Have enough data in the send queue to probe? */
	if (tp->write_seq - tp->snd_nxt < size_needed)
		return -1;

	if (tp->snd_wnd < size_needed)
		return -1;
	if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
		return 0;

	/* Do we need to wait to drain cwnd? With none in flight, don't stall */
	if (tcp_packets_in_flight(tp) + 2 > tp->snd_cwnd) {
		if (!tcp_packets_in_flight(tp))
			return -1;
		else
			return 0;
	}

	/* We're allowed to probe.  Build it now. */
	if ((nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC)) == NULL)
		return -1;
	sk->sk_wmem_queued += nskb->truesize;
	sk_mem_charge(sk, nskb->truesize);

	skb = tcp_send_head(sk);

	TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
	TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
	TCP_SKB_CB(nskb)->flags = TCPCB_FLAG_ACK;
	TCP_SKB_CB(nskb)->sacked = 0;
	nskb->csum = 0;
	nskb->ip_summed = skb->ip_summed;

	tcp_insert_write_queue_before(nskb, skb, sk);

	len = 0;
	tcp_for_write_queue_from_safe(skb, next, sk) {
		copy = min_t(int, skb->len, probe_size - len);
		if (nskb->ip_summed)
			skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
		else
			nskb->csum = skb_copy_and_csum_bits(skb, 0,
							    skb_put(nskb, copy),
							    copy, nskb->csum);

		if (skb->len <= copy) {
			/* We've eaten all the data from this skb.
			 * Throw it away. */
			TCP_SKB_CB(nskb)->flags |= TCP_SKB_CB(skb)->flags;
			tcp_unlink_write_queue(skb, sk);
			sk_wmem_free_skb(sk, skb);
		} else {
			TCP_SKB_CB(nskb)->flags |= TCP_SKB_CB(skb)->flags &
						   ~(TCPCB_FLAG_FIN|TCPCB_FLAG_PSH);
			if (!skb_shinfo(skb)->nr_frags) {
				skb_pull(skb, copy);
				if (skb->ip_summed != CHECKSUM_PARTIAL)
					skb->csum = csum_partial(skb->data,
								 skb->len, 0);
			} else {
				__pskb_trim_head(skb, copy);
				tcp_set_skb_tso_segs(sk, skb, mss_now);
			}
			TCP_SKB_CB(skb)->seq += copy;
		}

		len += copy;

		if (len >= probe_size)
			break;
	}
	tcp_init_tso_segs(sk, nskb, nskb->len);

	/* We're ready to send.  If this fails, the probe will
	 * be resegmented into mss-sized pieces by tcp_write_xmit(). */
	TCP_SKB_CB(nskb)->when = tcp_time_stamp;
	if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
		/* Decrement cwnd here because we are sending
		 * effectively two packets. */
		tp->snd_cwnd--;
		tcp_event_new_data_sent(sk, nskb);

		icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
		tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
		tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;

		return 1;
	}

	return -1;
}

/* This routine writes packets to the network.  It advances the
 * send_head.  This happens as incoming acks open up the remote
 * window for us.
 *
 * LARGESEND note: !tcp_urg_mode is overkill, only frames between
 * snd_up-64k-mss .. snd_up cannot be large. However, taking into
 * account rare use of URG, this is not a big flaw.
 *
 * Returns 1, if no segments are in flight and we have queued segments, but
 * cannot send anything now because of SWS or another problem.
 *///push_one表示只发送队首的SKB还是把整个发送队列SKB都发送出去
static int tcp_write_xmit1(struct sock *sk, unsigned int mss_now, int nonagle,
			  int push_one, gfp_t gfp)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;
	unsigned int tso_segs, sent_pkts;
	int cwnd_quota;
	int result;

	sent_pkts = 0;

	if (!push_one) {
		/* Do MTU probing. */
		result = tcp_mtu_probe(sk);
		if (!result) {
			return 0;
		} else if (result > 0) {
			sent_pkts = 1;
		}
	}

	while ((skb = tcp_send_head(sk))) {
		unsigned int limit;

		tso_segs = tcp_init_tso_segs(sk, skb, mss_now);
		BUG_ON(!tso_segs);

		cwnd_quota = tcp_cwnd_test(tp, skb);
		if (!cwnd_quota)
			break;

		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
			break;

		if (tso_segs == 1) {
			if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
						     (tcp_skb_is_last(sk, skb) ?
						      nonagle : TCP_NAGLE_PUSH))))
				break;
		} else {
			if (!push_one && tcp_tso_should_defer(sk, skb))
				break;
		}

		limit = mss_now;
		if (tso_segs > 1 && !tcp_urg_mode(tp))
			limit = tcp_mss_split_point(sk, skb, mss_now,
						    cwnd_quota);

		if (skb->len > limit &&
		    unlikely(tso_fragment(sk, skb, limit, mss_now)))
			break;

		TCP_SKB_CB(skb)->when = tcp_time_stamp;

		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
			break;

		/* 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++;

		if (push_one)
			break;
	}

	if (likely(sent_pkts)) {
		tcp_cwnd_validate(sk);
		return 0;
	}
	return !tp->packets_out && tcp_send_head(sk);
}

/* This routine writes packets to the network.  It advances the
 * send_head.  This happens as incoming acks open up the remote
 * window for us.
 *
 * LARGESEND note: !tcp_urg_mode is overkill, only frames between
 * snd_up-64k-mss .. snd_up cannot be large. However, taking into
 * account rare use of URG, this is not a big flaw.
 *
 * Returns 1, if no segments are in flight and we have queued segments, but
 * cannot send anything now because of SWS or another problem.
 */
/*
 * tcp_write_xmit()将发送队列上的SKB发送出去，返回值为0表示发送成功。
 * 过程如下:
 * 1)检测当前状态是否是TCP_CLOSE
 * 2)检测拥塞窗口的大小
 * 3)检测当前段是否完全处在发送窗口内
 * 4)检测段是否使用nagle算法进行发送
 * 5)通过以上检测后将该SKB发送出去
 * 6)循环检测并发送发送队列上所有未发送的SKB。
 * 参数说明如下:
 * mss_now:当前有效的MSS
 * nonagle: 标识是否启用nonagle算法。
 */ //最终调用tcp_transmit_skb   由push_one表示是发送队列上的一个SKB还是把全部SKB一起发送出去
static int tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
			  int push_one, gfp_t gfp)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;
	unsigned int tso_segs, sent_pkts;
	int cwnd_quota;
	int result;

	sent_pkts = 0;

	if (!push_one) {
		/* Do MTU probing. */
		result = tcp_mtu_probe(sk);
		if (!result) {
			return 0;
		} else if (result > 0) {
			sent_pkts = 1;
		}
	}

	/*
	 * 如果发送队列不为空，则准备开始发送段。
	 */
	while ((skb = tcp_send_head(sk))) {
		unsigned int limit;

		/*
		 * 设置有关tso的信息，包括GSO类型、GSO分段的大小等。这些
		 * 信息是准备给软件TSO分段使用的。如果网络设备不支持TSO，
		 * 但又使用了TSO功能，则段在提交给网络设备之前，需进行
		 * 软分段，即由代码实现TSO分段。
		 */
		tso_segs = tcp_init_tso_segs(sk, skb, mss_now);
		BUG_ON(!tso_segs);

              /* 
               * 检查目前是否可以发送数据,
               * 确认当前发送窗口的大小
               */
		/*
		 * 检测拥塞窗口的大小，如果为0，则说明
		 * 拥塞窗口已满，目前不能发送。
		 */
		cwnd_quota = tcp_cwnd_test(tp, skb);
		if (!cwnd_quota)
			break;

		/*
		 * 检测当前段(包括线性区和分散聚合I/O区shinfo)是否完全处在发送窗口内，如果是
		 * 则可以发送，否则目前不能发送。
		 */
		if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
			break;

		if (tso_segs == 1) {
			/*
			 * 如果无需TSO分段，则检测是否使用Nagle算法，
			 * 并确定当前能否立即发送该段。
			 */
			if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
						     (tcp_skb_is_last(sk, skb) ?
						      nonagle : TCP_NAGLE_PUSH))))
				break;
		} else {
			/*
			 * 如果需要TSO分段，则检测该段是否应该延时发送，
			 * 如果是则目前不能发送。tcp_tso_should_defer()用来检测
			 * GSO段是否需要延时发送。在段中有FIN标志，或者
			 * 不处于Open拥塞状态，或者TSO段延时超过2个时钟
			 * 滴答，或者拥塞窗口和发送窗口的最小值大于64KB
			 * 或三倍的当前有效MSS，在这些情况下会立即发送，
			 * 而其他情况下会延时发送，这样主要为了减少软GSO
			 * 分段的次数提高性能。
			 */
			if (!push_one && tcp_tso_should_defer(sk, skb))
				break;
		}
		
		/*
		 * 根据条件，可能需要对SKB中的段进行分段处理，分段的
		 * 段包括两种:一种是普通的用MSS分段的段，另一种则是
		 * TSO分段的段。能否发送段主要取决于两个条件:一是段
		 * 需完全在发送窗口中，二是拥塞窗口未满。第一种段，
		 * 应该不会再分段了，因为在tcp_sendmsg()中创建段的SKB时已经
		 * 根据MSS处理了。而第二种段，则一般情况下都会大于MSS，
		 * 因此通过TSO分段的段有可能大于拥塞窗口剩余空间，如果
		 * 是这样，就需以发送窗口和拥塞窗口的最小值作为段长对
		 * 数据包再次分段。
		 */
		 
		/*
		 * limit为再次分段的段长，初始化为当前MSS。
		 */
		limit = mss_now;
		/*
		 * 判断当前段是不是TSO分段的段，如果是才处理。
		 */
		if (tso_segs > 1 && !tcp_urg_mode(tp))
			/*
			 * 以发送窗口和拥塞窗口的最小值作为分段段长。也就是比较可用拥塞窗口和tcp_sendmsg中的分散聚合I/O页
			 */
			limit = tcp_mss_split_point(sk, skb, mss_now,
						    cwnd_quota);

		/*
		 * 得到分段段长后，如果SKB中的数据长度大于分段
		 * 段长，则调用tso_fragment()根据该段长进行分段，如果
		 * 分段失败则目前暂不发送。
		 */
		if (skb->len > limit &&
		    unlikely(tso_fragment(sk, skb, limit, mss_now)))//后面新的buffer添加到了发送队列的尾部，等待下次tcp_sendmsg的时候发送
			break;

		/*
		 * 记录此段的发送时间，主要用于RTT的计算。
		 */
		TCP_SKB_CB(skb)->when = tcp_time_stamp;
              /* 
               * 使用地址族相关的af_sepcific->queue_xmit函数,
               * 将数据转发到网络层。IPv4使用的是
               * ip_queue_xmit
               */
		if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
			break;

		/* Advance the send_head.  This one is sent out.
		 * This call will increment packets_out.
		 */
		/* 
		 * 处理对统计量的更新。更重要的是，
		 * 它会初始化所发送TCP信息段的重传
		 * 定时器。不必对每个TCP分组都这样做，
		 * 该机制只用于已经确认的数据区之后的
		 * 第一个分组
               */
		tcp_event_new_data_sent(sk, skb);

		/*
		 * 如果发送的段小于MSS，则更新最近发送的小包的
		 * 最后一个字节序号。
		 */
		tcp_minshall_update(tp, mss_now, skb);
		/*
		 * 更新在函数中已发送的总段数。
		 */
		sent_pkts++;

		if (push_one)
			break;
	}

	/*
	 * 如果本次有数据发送，则对TCP拥塞窗口进行确认，
	 * 最后返回成功。
	 */
	if (likely(sent_pkts)) {
		tcp_cwnd_validate(sk);
		return 0;
	}
	
	/*
	 * 如果本次没有数据发送，则根据已发送但未确认的段数packets_out和
	 * sk_send_head返回，packets_out不为零或sk_send_head为空都被视为有数据发出，
	 * 因此返回成功。
	 */
	return !tp->packets_out && tcp_send_head(sk);
}


/* Push out any pending frames which were held back due to
 * TCP_CORK or attempt at coalescing tiny packets.
 * The socket must be locked by the caller.
 *///把sk发送队列中所有的skb全部发送出去        只发送队列上的第一个SKB采用tcp_push_one 最终都要调用tcp_write_xmit
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, 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.
 *//*
 * tcp_push_one()用来输出发送队列上的第一个SKB，参数mss_now
 * 为当前MSS。
 */ //发送队列上所有的SKB使用__tcp_push_pending_frames   最终都要调用tcp_write_xmit
void tcp_push_one(struct sock *sk, unsigned int mss_now)
{
	struct sk_buff *skb = tcp_send_head(sk);

	BUG_ON(!skb || skb->len < mss_now);

	tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
}

/* This function returns the amount that we can raise the
 * usable window based on the following constraints
 *
 * 1. The window can never be shrunk once it is offered (RFC 793)
 * 2. We limit memory per socket
 *
 * RFC 1122:
 * "the suggested [SWS] avoidance algorithm for the receiver is to keep
 *  RECV.NEXT + RCV.WIN fixed until:
 *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
 *
 * i.e. don't raise the right edge of the window until you can raise
 * it at least MSS bytes.
 *
 * Unfortunately, the recommended algorithm breaks header prediction,
 * since header prediction assumes th->window stays fixed.
 *
 * Strictly speaking, keeping th->window fixed violates the receiver
 * side SWS prevention criteria. The problem is that under this rule
 * a stream of single byte packets will cause the right side of the
 * window to always advance by a single byte.
 *
 * Of course, if the sender implements sender side SWS prevention
 * then this will not be a problem.
 *
 * BSD seems to make the following compromise:
 *
 *	If the free space is less than the 1/4 of the maximum
 *	space available and the free space is less than 1/2 mss,
 *	then set the window to 0.
 *	[ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
 *	Otherwise, just prevent the window from shrinking
 *	and from being larger than the largest representable value.
 *
 * This prevents incremental opening of the window in the regime
 * where TCP is limited by the speed of the reader side taking
 * data out of the TCP receive queue. It does nothing about
 * those cases where the window is constrained on the sender side
 * because the pipeline is full.
 *
 * BSD also seems to "accidentally" limit itself to windows that are a
 * multiple of MSS, at least until the free space gets quite small.
 * This would appear to be a side effect of the mbuf implementation.
 * Combining these two algorithms results in the observed behavior
 * of having a fixed window size at almost all times.
 *
 * Below we obtain similar behavior by forcing the offered window to
 * a multiple of the mss when it is feasible to do so.
 *
 * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
 * Regular options like TIMESTAMP are taken into account.
 */
u32 __tcp_select_window(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	/* MSS for the peer's data.  Previous versions used mss_clamp
	 * here.  I don't know if the value based on our guesses
	 * of peer's MSS is better for the performance.  It's more correct
	 * but may be worse for the performance because of rcv_mss
	 * fluctuations.  --SAW  1998/11/1
	 */
	int mss = icsk->icsk_ack.rcv_mss;
	int free_space = tcp_space(sk);
	int full_space = min_t(int, tp->window_clamp, tcp_full_space(sk));
	int window;

	if (mss > full_space)
		mss = full_space;

	if (free_space < (full_space >> 1)) {
		icsk->icsk_ack.quick = 0;

		if (tcp_memory_pressure)
			tp->rcv_ssthresh = min(tp->rcv_ssthresh,
					       4U * tp->advmss);

		if (free_space < mss)
			return 0;
	}

	if (free_space > tp->rcv_ssthresh)
		free_space = tp->rcv_ssthresh;

	/* Don't do rounding if we are using window scaling, since the
	 * scaled window will not line up with the MSS boundary anyway.
	 */
	window = tp->rcv_wnd;
	if (tp->rx_opt.rcv_wscale) {
		window = free_space;

		/* Advertise enough space so that it won't get scaled away.
		 * Import case: prevent zero window announcement if
		 * 1<<rcv_wscale > mss.
		 */
		if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
			window = (((window >> tp->rx_opt.rcv_wscale) + 1)
				  << tp->rx_opt.rcv_wscale);
	} else {
		/* Get the largest window that is a nice multiple of mss.
		 * Window clamp already applied above.
		 * If our current window offering is within 1 mss of the
		 * free space we just keep it. This prevents the divide
		 * and multiply from happening most of the time.
		 * We also don't do any window rounding when the free space
		 * is too small.
		 */
		if (window <= free_space - mss || window > free_space)
			window = (free_space / mss) * mss;
		else if (mss == full_space &&
			 free_space > window + (full_space >> 1))
			window = free_space;
	}

	return window;
}

/* Collapses two adjacent SKB's during retransmission. */
static void tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *next_skb = tcp_write_queue_next(sk, skb);
	int skb_size, next_skb_size;

	skb_size = skb->len;
	next_skb_size = next_skb->len;

	BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);

	tcp_highest_sack_combine(sk, next_skb, skb);

	tcp_unlink_write_queue(next_skb, sk);

	skb_copy_from_linear_data(next_skb, skb_put(skb, next_skb_size),
				  next_skb_size);

	if (next_skb->ip_summed == CHECKSUM_PARTIAL)
		skb->ip_summed = CHECKSUM_PARTIAL;

	if (skb->ip_summed != CHECKSUM_PARTIAL)
		skb->csum = csum_block_add(skb->csum, next_skb->csum, skb_size);

	/* Update sequence range on original skb. */
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;

	/* Merge over control information. This moves PSH/FIN etc. over */
	TCP_SKB_CB(skb)->flags |= TCP_SKB_CB(next_skb)->flags;

	/* All done, get rid of second SKB and account for it so
	 * packet counting does not break.
	 */
	TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;

	/* changed transmit queue under us so clear hints */
	tcp_clear_retrans_hints_partial(tp);
	if (next_skb == tp->retransmit_skb_hint)
		tp->retransmit_skb_hint = skb;

	tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));

	sk_wmem_free_skb(sk, next_skb);
}

/* Check if coalescing SKBs is legal. */
static int tcp_can_collapse(struct sock *sk, struct sk_buff *skb)
{
	if (tcp_skb_pcount(skb) > 1)
		return 0;
	/* TODO: SACK collapsing could be used to remove this condition */
	if (skb_shinfo(skb)->nr_frags != 0)
		return 0;
	if (skb_cloned(skb))
		return 0;
	if (skb == tcp_send_head(sk))
		return 0;
	/* Some heurestics for collapsing over SACK'd could be invented */
	if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
		return 0;

	return 1;
}

/* 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;
	int first = 1;

	if (!sysctl_tcp_retrans_collapse)
		return;
	if (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN)
		return;

	tcp_for_write_queue_from_safe(skb, tmp, sk) {
		if (!tcp_can_collapse(sk, skb))
			break;

		space -= skb->len;

		if (first) {
			first = 0;
			continue;
		}

		if (space < 0)
			break;
		/* Punt if not enough space exists in the first SKB for
		 * the data in the second
		 */
		if (skb->len > skb_tailroom(to))
			break;

		if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
			break;

		tcp_collapse_retrans(sk, to);
	}
}

/* This retransmits one SKB.  Policy decisions and retransmit queue
 * state updates are done by the caller.  Returns non-zero if an
 * error occurred which prevented the send.
 */
 /*
 * 通常情况下，TCP在传送超时(即输出后在超时时间内没接收到对应
 * 的ACK)，以及接收到需要分片ICMP消息这两种情况下会发送重传，无论
 * 是什么原因导致重传，最后都会调用统一的入口函数tcp_retransmit_skb()。
 */
int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);
	unsigned int cur_mss;
	int err;

	/* Inconslusive MTU probe */
	/*
	 * 如果路径MTU发现还没有结束，无论是何种
	 * 原因导致的重传，都将其标志为结束。
	 */
	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.
	 */
	 /*
	 * 说明在发送缓冲区中消耗了过多内存去做其他的一些
	 * 工作(如分片等，只有1/4的缓存才是保留给这些工作的)，
	 * 所以暂时不能重传。
	 */
	if (atomic_read(&sk->sk_wmem_alloc) >
	    min(sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2), sk->sk_sndbuf))
		return -EAGAIN;

    /*
	 * 检测重传的段，接收方是否已经收到其他部分或全部。
	 * 如果收到全部，则说明TCP的实现代码有bug。如果收到
	 * 部分，则需要调整TCP段的负载，即删除SKB存储区前部
	 * 的接收方已接收到的数据。
	 */
    ////这里的seq end_seq应该是应用层接收来的数据直接存到SKB中，这时的数据长度应该没有分段的，可能大于1500。分段在该后面几行
	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {// 若这样，说明是有一部分数据才需要重传，形如：seq---snd_una---end_seq，前面一半已收到ACK  
		if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una)) // 若这样，说明全部ACK，无需重传，BUG 
			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.
	 */
	 /*
	 * 如果接收方已经缩小接收窗口，且待重传的SKB已经
	 * 不在新窗口内，则不能再重传该SKB。但有一种情况
	 * 例外，拿就是当接收方接收窗口缩小到零，在这种
	 * 情况下，会发送零窗口探测段。
	 */
	if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
	    TCP_SKB_CB(skb)->seq != tp->snd_una) //seq序号在前面的tcp_trim_head从新初始化了
		return -EAGAIN;

    /*
	 * 如果SKB中的数据长度大于当前的MSS，则需要对该SKB分段，
	 * 这种情况一般出现在启用了路径MTU，接收到需要分片ICMP
	 * 目的不可达报文时，如果ICMP中下一跳的PMTU小于当前PMTU，
	 * 则更新当前PMTU及当前MSS。
	 */
	if (skb->len > cur_mss) {
		if (tcp_fragment(sk, skb, cur_mss, cur_mss))
			return -ENOMEM; /* We'll try again later. */
	} else {
		int oldpcount = tcp_skb_pcount(skb);

		if (unlikely(oldpcount > 1)) {
			tcp_init_tso_segs(sk, skb, cur_mss);
			tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
		}
	}

	tcp_retrans_try_collapse(sk, skb, cur_mss);

	/* Some Solaris stacks overoptimize and ignore the FIN on a
	 * retransmit when old data is attached.  So strip it off
	 * since it is cheap to do so and saves bytes on the network.
	 */
	 /*
	 * 因为某些Solaris系统的协议栈会忽略重传段上带有
	 * FIN标志的payload，所以将payload部分全部剥离，这样
	 * 可以在网络上节省一部分流量。
	 */
	if (skb->len > 0 &&
	    (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN) &&
	    tp->snd_una == (TCP_SKB_CB(skb)->end_seq - 1)) {
		if (!pskb_trim(skb, 0)) {
			/* Reuse, even though it does some unnecessary work */
			tcp_init_nondata_skb(skb, TCP_SKB_CB(skb)->end_seq - 1,
					     TCP_SKB_CB(skb)->flags);
			skb->ip_summed = CHECKSUM_NONE;
		}
	}

	/*
	 * 进行TCP段的发送以及发送后的数据调整。
	 */
	/* Make a copy, if the first transmission SKB clone we made
	 * is still in somebody's hands, else make a clone.
	 */
	/*
	 * 进行TCP段发送的时间，并将其发送出去。
	 */
	TCP_SKB_CB(skb)->when = tcp_time_stamp;

	err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);

    /*
	 * 如果发送成功，则需要更新统计数据和状态数据，如
	 * 重传总次数、重传而未得到确认的TCP段数，并为该TCP
	 * 段加上重传标志等。
	 */
	if (err == 0) {
		/* Update global TCP statistics. */
		TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);

		tp->total_retrans++;

#if FASTRETRANS_DEBUG > 0
		if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
			if (net_ratelimit())
				printk(KERN_DEBUG "retrans_out leaked.\n");
		}
#endif
		if (!tp->retrans_out)
			tp->lost_retrans_low = tp->snd_nxt;
		TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
		tp->retrans_out += tcp_skb_pcount(skb);

		/* Save stamp of the first retransmit. */
		if (!tp->retrans_stamp)
			tp->retrans_stamp = TCP_SKB_CB(skb)->when;

		tp->undo_retrans++;

		/* snd_nxt is stored to detect loss of retransmitted segment,
		 * see tcp_input.c tcp_sacktag_write_queue().
		 */
		TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt;
	}
	return err;
}

/* Check if we forward retransmits are possible in the current
 * window/congestion state.
 */
static int tcp_can_forward_retransmit(struct sock *sk)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	/* Forward retransmissions are possible only during Recovery. */
	if (icsk->icsk_ca_state != TCP_CA_Recovery)
		return 0;

	/* No forward retransmissions in Reno are possible. */
	if (tcp_is_reno(tp))
		return 0;

	/* Yeah, we have to make difficult choice between forward transmission
	 * and retransmission... Both ways have their merits...
	 *
	 * For now we do not retransmit anything, while we have some new
	 * segments to send. In the other cases, follow rule 3 for
	 * NextSeg() specified in RFC3517.
	 */

	if (tcp_may_send_now(sk))
		return 0;

	return 1;
}

/* This gets called after a retransmit timeout, and the initially
 * retransmitted data is acknowledged.  It tries to continue
 * resending the rest of the retransmit queue, until either
 * we've sent it all or the congestion window limit is reached.
 * If doing SACK, the first ACK which comes back for a timeout
 * based retransmit packet might feed us FACK information again.
 * If so, we use it to avoid unnecessarily retransmissions.
 */
void tcp_xmit_retransmit_queue(struct sock *sk)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;
	struct sk_buff *hole = NULL;
	u32 last_lost;
	int mib_idx;
	int fwd_rexmitting = 0;

	if (!tp->packets_out)
		return;

	if (!tp->lost_out)
		tp->retransmit_high = tp->snd_una;

	if (tp->retransmit_skb_hint) {
		skb = tp->retransmit_skb_hint;
		last_lost = TCP_SKB_CB(skb)->end_seq;
		if (after(last_lost, tp->retransmit_high))
			last_lost = tp->retransmit_high;
	} else {
		skb = tcp_write_queue_head(sk);
		last_lost = tp->snd_una;
	}

	tcp_for_write_queue_from(skb, sk) {
		__u8 sacked = TCP_SKB_CB(skb)->sacked;

		if (skb == tcp_send_head(sk))
			break;
		/* we could do better than to assign each time */
		if (hole == NULL)
			tp->retransmit_skb_hint = skb;

		/* Assume this retransmit will generate
		 * only one packet for congestion window
		 * calculation purposes.  This works because
		 * tcp_retransmit_skb() will chop up the
		 * packet to be MSS sized and all the
		 * packet counting works out.
		 */
		if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
			return;

		if (fwd_rexmitting) {
begin_fwd:
			if (!before(TCP_SKB_CB(skb)->seq, tcp_highest_sack_seq(tp)))
				break;
			mib_idx = LINUX_MIB_TCPFORWARDRETRANS;

		} else if (!before(TCP_SKB_CB(skb)->seq, tp->retransmit_high)) {
			tp->retransmit_high = last_lost;
			if (!tcp_can_forward_retransmit(sk))
				break;
			/* Backtrack if necessary to non-L'ed skb */
			if (hole != NULL) {
				skb = hole;
				hole = NULL;
			}
			fwd_rexmitting = 1;
			goto begin_fwd;

		} else if (!(sacked & TCPCB_LOST)) {
			if (hole == NULL && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
				hole = skb;
			continue;

		} else {
			last_lost = TCP_SKB_CB(skb)->end_seq;
			if (icsk->icsk_ca_state != TCP_CA_Loss)
				mib_idx = LINUX_MIB_TCPFASTRETRANS;
			else
				mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
		}

		if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
			continue;

		if (tcp_retransmit_skb(sk, skb))
			return;
		NET_INC_STATS_BH(sock_net(sk), mib_idx);

		if (skb == tcp_write_queue_head(sk))
			inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
						  inet_csk(sk)->icsk_rto,
						  TCP_RTO_MAX);
	}
}


/* Send a fin.  The caller locks the socket for us.  This cannot be
 * allowed to fail queueing a FIN frame under any circumstances.
 */
/*
 * tcp_send_fin()的实现比较简单，过程大致如下:
 * 1)由于发送FIN无需占用额外的负载，因此如果发送队列不空，
 *    则在发送队列的最后一个TCP段上设置FIN标志。但FIN标志会
 *    占用一个序号，因此需递增序号。
 * 2)如果发送队列为空，则需构造一个新的TCP段，但该TCP段不需要
 *    负荷，只需要TCP首部即可。设置相应的值，然后添加到
 *    发送队列中。
 * 3)最后关闭Nagle算法，立即将发送队列上未发送的段(包括FIN段)全部
 *    发送出去。
 */
void tcp_send_fin(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb = tcp_write_queue_tail(sk);
	int mss_now;

	/* Optimization, tack on the FIN if we have a queue of
	 * unsent frames.  But be careful about outgoing SACKS
	 * and IP options.
	 */
	/*
	 * 计算当前有效的MSS，即最大段长度
	 */
	mss_now = tcp_current_mss(sk);

	/*
	 * 如果发送队列不为空，则在最后一个要发送的SKB包中加入
	 * FIN标志，更新其结束序列号。更新sock结构中的最后一个加入
	 * 到发送队列的字节的序列号
	 */
	if (tcp_send_head(sk) != NULL) {
		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_FIN;
		TCP_SKB_CB(skb)->end_seq++;
		tp->write_seq++;
	} else {
		/* Socket is locked, keep trying until memory is available. */
		/*
		 * 内核为了发送FIN，做这样一个循环，没必要吧?
		 */
		for (;;) {
			skb = alloc_skb_fclone(MAX_TCP_HEADER,
					       sk->sk_allocation);
			if (skb)
				break;
			/*
			 * 主动让出处理器
			 */
			yield();
		}

		/* Reserve space for headers and prepare control bits. */
		skb_reserve(skb, MAX_TCP_HEADER);
		/* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
        /*
         * 注意在发送FIN包时，会同时设置TCPCB_FLAG_ACK和TCPCB_FLAG_FIN标志，
         * 所以FIN包中TCP首部的ack标志和fin标志都会设置。
         */
		tcp_init_nondata_skb(skb, tp->write_seq,
				     TCPCB_FLAG_ACK | TCPCB_FLAG_FIN);
		/*
		 * 将skb包加入到发送队列中
		 */
		tcp_queue_skb(sk, skb);
	}
	/*
	 * 如果发送队列不为空，并且sock状态不是TCP_CLOSE，则将发送队列中
	 * 未发送的SKB包发送出去。
	 */ //把整个发送队列中的SKB数据都发送出去
	__tcp_push_pending_frames(sk, mss_now, TCP_NAGLE_OFF);
}


/* Send a fin.  The caller locks the socket for us.  This cannot be
 * allowed to fail queueing a FIN frame under any circumstances.
 */

 /*
 * tcp_send_fin()的实现比较简单，过程大致如下:
 * 1)由于发送FIN无需占用额外的负载，因此如果发送队列不空，
 *    则在发送队列的最后一个TCP段上设置FIN标志。但FIN标志会
 *    占用一个序号，因此需递增序号。
 * 2)如果发送队列为空，则需构造一个新的TCP段，但该TCP段不需要
 *    负荷，只需要TCP首部即可。设置相应的值，然后添加到
 *    发送队列中。
 * 3)最后关闭Nagle算法，立即将发送队列上未发送的段(包括FIN段)全部
 *    发送出去。
 */ //这里说明了在发送FIN的时候，是确保前面的数据都发送出去了的
void tcp_send_fin1(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb = tcp_write_queue_tail(sk);
	int mss_now;

	/* Optimization, tack on the FIN if we have a queue of
	 * unsent frames.  But be careful about outgoing SACKS
	 * and IP options.
	 *//*
	 * 计算当前有效的MSS，即最大段长度
	 */
	mss_now = tcp_current_mss(sk);

	if (tcp_send_head(sk) != NULL) {
		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_FIN;
		TCP_SKB_CB(skb)->end_seq++;
		tp->write_seq++;
	} else {
		/* Socket is locked, keep trying until memory is available. */
		for (;;) {
			skb = alloc_skb_fclone(MAX_TCP_HEADER,
					       sk->sk_allocation);
			if (skb)
				break;
			yield();
		}

		/* Reserve space for headers and prepare control bits. */
		skb_reserve(skb, MAX_TCP_HEADER);
		/* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
		tcp_init_nondata_skb(skb, tp->write_seq,
				     TCPCB_FLAG_ACK | TCPCB_FLAG_FIN);
		tcp_queue_skb(sk, skb);
	}
	__tcp_push_pending_frames(sk, mss_now, TCP_NAGLE_OFF);
}

/* We get here when a process closes a file descriptor (either due to
 * an explicit close() or as a byproduct of exit()'ing) and there
 * was unread data in the receive queue.  This behavior is recommended
 * by RFC 2525, section 2.17.  -DaveM
 */
void tcp_send_active_reset(struct sock *sk, gfp_t priority)
{
	struct sk_buff *skb;

	/* NOTE: No TCP options attached and we never retransmit this. */
	skb = alloc_skb(MAX_TCP_HEADER, priority);
	if (!skb) {
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
		return;
	}

	/* Reserve space for headers and prepare control bits. */
	skb_reserve(skb, MAX_TCP_HEADER);
	tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
			     TCPCB_FLAG_ACK | TCPCB_FLAG_RST);
	/* Send it off. */
	TCP_SKB_CB(skb)->when = tcp_time_stamp;
	if (tcp_transmit_skb(sk, skb, 0, priority))
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);

	TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
}

/* Send a crossed SYN-ACK during socket establishment.
 * WARNING: This routine must only be called when we have already sent
 * a SYN packet that crossed the incoming SYN that caused this routine
 * to get called. If this assumption fails then the initial rcv_wnd
 * and rcv_wscale values will not be correct.
 */
int tcp_send_synack(struct sock *sk)
{
	struct sk_buff *skb;

	skb = tcp_write_queue_head(sk);
	if (skb == NULL || !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN)) {
		printk(KERN_DEBUG "tcp_send_synack: wrong queue state\n");
		return -EFAULT;
	}
	if (!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_ACK)) {
		if (skb_cloned(skb)) {
			struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
			if (nskb == NULL)
				return -ENOMEM;
			tcp_unlink_write_queue(skb, sk);
			skb_header_release(nskb);
			__tcp_add_write_queue_head(sk, nskb);
			sk_wmem_free_skb(sk, skb);
			sk->sk_wmem_queued += nskb->truesize;
			sk_mem_charge(sk, nskb->truesize);
			skb = nskb;
		}

		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_ACK;
		TCP_ECN_send_synack(tcp_sk(sk), skb);
	}
	TCP_SKB_CB(skb)->when = tcp_time_stamp;
	return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
}

/* Prepare a SYN-ACK. */
struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
				struct request_sock *req,
				struct request_values *rvp)
{
	struct tcp_out_options opts;
	struct tcp_extend_values *xvp = tcp_xv(rvp);
	struct inet_request_sock *ireq = inet_rsk(req);
	struct tcp_sock *tp = tcp_sk(sk);
	const struct tcp_cookie_values *cvp = tp->cookie_values;
	struct tcphdr *th;
	struct sk_buff *skb;
	struct tcp_md5sig_key *md5;
	int tcp_header_size;
	int mss;
	int s_data_desired = 0;

	if (cvp != NULL && cvp->s_data_constant && cvp->s_data_desired)
		s_data_desired = cvp->s_data_desired;
	skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15 + s_data_desired, 1, GFP_ATOMIC);
	if (skb == NULL)
		return NULL;

	/* Reserve space for headers. */
	skb_reserve(skb, MAX_TCP_HEADER);

	skb_dst_set(skb, dst_clone(dst));

	mss = dst_metric(dst, RTAX_ADVMSS);
	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
		mss = tp->rx_opt.user_mss;

	if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
		__u8 rcv_wscale;
		/* Set this up on the first call only */
		req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
		/* tcp_full_space because it is guaranteed to be the first packet */
		tcp_select_initial_window(tcp_full_space(sk),
			mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
			&req->rcv_wnd,
			&req->window_clamp,
			ireq->wscale_ok,
			&rcv_wscale,
			dst_metric(dst, RTAX_INITRWND));
		ireq->rcv_wscale = rcv_wscale;
	}

	memset(&opts, 0, sizeof(opts));
#ifdef CONFIG_SYN_COOKIES
	if (unlikely(req->cookie_ts))
		TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
	else
#endif
	TCP_SKB_CB(skb)->when = tcp_time_stamp;
	tcp_header_size = tcp_synack_options(sk, req, mss,
					     skb, &opts, &md5, xvp)
			+ sizeof(*th);

	skb_push(skb, tcp_header_size);
	skb_reset_transport_header(skb);

	th = tcp_hdr(skb);
	memset(th, 0, sizeof(struct tcphdr));
	th->syn = 1;
	th->ack = 1;
	TCP_ECN_make_synack(req, th);
	th->source = ireq->loc_port;
	th->dest = ireq->rmt_port;
	/* Setting of flags are superfluous here for callers (and ECE is
	 * not even correctly set)
	 */
	tcp_init_nondata_skb(skb, tcp_rsk(req)->snt_isn,
			     TCPCB_FLAG_SYN | TCPCB_FLAG_ACK);

	if (OPTION_COOKIE_EXTENSION & opts.options) {
		if (s_data_desired) {
			u8 *buf = skb_put(skb, s_data_desired);

			/* copy data directly from the listening socket. */
			memcpy(buf, cvp->s_data_payload, s_data_desired);
			TCP_SKB_CB(skb)->end_seq += s_data_desired;
		}

		if (opts.hash_size > 0) {
			__u32 workspace[SHA_WORKSPACE_WORDS];
			u32 *mess = &xvp->cookie_bakery[COOKIE_DIGEST_WORDS];
			u32 *tail = &mess[COOKIE_MESSAGE_WORDS-1];

			/* Secret recipe depends on the Timestamp, (future)
			 * Sequence and Acknowledgment Numbers, Initiator
			 * Cookie, and others handled by IP variant caller.
			 */
			*tail-- ^= opts.tsval;
			*tail-- ^= tcp_rsk(req)->rcv_isn + 1;
			*tail-- ^= TCP_SKB_CB(skb)->seq + 1;

			/* recommended */
			*tail-- ^= (((__force u32)th->dest << 16) | (__force u32)th->source);
			*tail-- ^= (u32)(unsigned long)cvp; /* per sockopt */

			sha_transform((__u32 *)&xvp->cookie_bakery[0],
				      (char *)mess,
				      &workspace[0]);
			opts.hash_location =
				(__u8 *)&xvp->cookie_bakery[0];
		}
	}

	th->seq = htonl(TCP_SKB_CB(skb)->seq);
	th->ack_seq = htonl(tcp_rsk(req)->rcv_isn + 1);

	/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
	th->window = htons(min(req->rcv_wnd, 65535U));
	tcp_options_write((__be32 *)(th + 1), tp, &opts);
	th->doff = (tcp_header_size >> 2);
	TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS, tcp_skb_pcount(skb));

#ifdef CONFIG_TCP_MD5SIG
	/* Okay, we have all we need - do the md5 hash if needed */
	if (md5) {
		tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
					       md5, NULL, req, skb);
	}
#endif

	return skb;
}

/* Do all connect socket setups that can be done AF independent. */
/* 初始化传输控制块中与连接相关的成员 */ 
static void tcp_connect_init(struct sock *sk)
{
	struct dst_entry *dst = __sk_dst_get(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	__u8 rcv_wscale;

	/* We'll fix this up when we get a response from the other end.
	 * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
	 */
	tp->tcp_header_len = sizeof(struct tcphdr) +
		(sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);

#ifdef CONFIG_TCP_MD5SIG
	if (tp->af_specific->md5_lookup(sk, sk) != NULL)
		tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
#endif

	/* If user gave his TCP_MAXSEG, record it to clamp */
	if (tp->rx_opt.user_mss)
		tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
	tp->max_window = 0;
	tcp_mtup_init(sk);
	tcp_sync_mss(sk, dst_mtu(dst));

	if (!tp->window_clamp)
		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
	tp->advmss = dst_metric(dst, RTAX_ADVMSS);
	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
		tp->advmss = tp->rx_opt.user_mss;

	tcp_initialize_rcv_mss(sk);

	tcp_select_initial_window(tcp_full_space(sk),
				  tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
				  &tp->rcv_wnd,
				  &tp->window_clamp,
				  sysctl_tcp_window_scaling,
				  &rcv_wscale,
				  dst_metric(dst, RTAX_INITRWND));

	tp->rx_opt.rcv_wscale = rcv_wscale;
	tp->rcv_ssthresh = tp->rcv_wnd;

	sk->sk_err = 0;
	sock_reset_flag(sk, SOCK_DONE);
	tp->snd_wnd = 0;
	tcp_init_wl(tp, 0);
	tp->snd_una = tp->write_seq;
	tp->snd_sml = tp->write_seq;
	tp->snd_up = tp->write_seq;
	tp->rcv_nxt = 0;
	tp->rcv_wup = 0;
	tp->copied_seq = 0;

	inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
	inet_csk(sk)->icsk_retransmits = 0;
	tcp_clear_retrans(tp);
}

/* Build a SYN and send it off. */
int tcp_connect(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *buff;

    /*
	 * 初始化传输控制块中与连接相关的成员。
	 */
	tcp_connect_init(sk);

	buff = alloc_skb_fclone(MAX_TCP_HEADER + 15, sk->sk_allocation);
	if (unlikely(buff == NULL))
		return -ENOBUFS;

	/* Reserve space for headers. */
	skb_reserve(buff, MAX_TCP_HEADER);

	tp->snd_nxt = tp->write_seq;
	tcp_init_nondata_skb(buff, tp->write_seq++, TCPCB_FLAG_SYN);
	TCP_ECN_send_syn(sk, buff);

	/* Send it off. */
	TCP_SKB_CB(buff)->when = tcp_time_stamp;
	tp->retrans_stamp = TCP_SKB_CB(buff)->when;
	skb_header_release(buff);
	__tcp_add_write_queue_tail(sk, buff);
	sk->sk_wmem_queued += buff->truesize;
	sk_mem_charge(sk, buff->truesize);
	tp->packets_out += tcp_skb_pcount(buff);
	tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);

	/* 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;
}

/* Send out a delayed ack, the caller does the policy checking
 * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
 * for details.
 */
void tcp_send_delayed_ack(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	int ato = icsk->icsk_ack.ato;
	unsigned long timeout;

	if (ato > TCP_DELACK_MIN) {
		const struct tcp_sock *tp = tcp_sk(sk);
		int max_ato = HZ / 2;

		if (icsk->icsk_ack.pingpong ||
		    (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
			max_ato = TCP_DELACK_MAX;

		/* Slow path, intersegment interval is "high". */

		/* If some rtt estimate is known, use it to bound delayed ack.
		 * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
		 * directly.
		 */
		if (tp->srtt) {
			int rtt = max(tp->srtt >> 3, TCP_DELACK_MIN);

			if (rtt < max_ato)
				max_ato = rtt;
		}

		ato = min(ato, max_ato);
	}

	/* Stay within the limit we were given */
	timeout = jiffies + ato;

	/* Use new timeout only if there wasn't a older one earlier. */
	if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
		/* If delack timer was blocked or is about to expire,
		 * send ACK now.
		 */
		if (icsk->icsk_ack.blocked ||
		    time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
			tcp_send_ack(sk);
			return;
		}

		if (!time_before(timeout, icsk->icsk_ack.timeout))
			timeout = icsk->icsk_ack.timeout;
	}
	icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
	icsk->icsk_ack.timeout = timeout;
	sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
}

/* This routine sends an ack and also updates the window. */
/*
 * tcp_send_ack()用来发送一个ACK段，同时更新窗口。
 * 1)发送ACK段时，TCP必须不在CLOSE状态
 * 2)为ACK段分配一个SKB，如果分配失败则在启动延时
 *    确认定时器后返回。
 * 3)如果分配SKB成功，则设置SKB中相关的参数，如标志
 *   和gso属性等。
 * 4)最后设置TCP序号和发送时间，调用tcp_transmit_skb()将
 *    该ACK段发送出去。
 */
void tcp_send_ack(struct sock *sk)
{
	struct sk_buff *buff;

	/* If we have been reset, we may not send again. */
	if (sk->sk_state == TCP_CLOSE)
		return;

	/* We are not putting this on the write queue, so
	 * tcp_transmit_skb() will set the ownership to this
	 * sock.
	 */
	buff = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
	if (buff == NULL) {
		inet_csk_schedule_ack(sk);
		inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
		inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
					  TCP_DELACK_MAX, TCP_RTO_MAX);
		return;
	}

	/* Reserve space for headers and prepare control bits. */
	skb_reserve(buff, MAX_TCP_HEADER);
	tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPCB_FLAG_ACK);

	/* Send it off, this clears delayed acks for us. */
	TCP_SKB_CB(buff)->when = tcp_time_stamp;
	tcp_transmit_skb(sk, buff, 0, GFP_ATOMIC);
}

/* This routine sends a packet with an out of date sequence
 * number. It assumes the other end will try to ack it.
 *
 * Question: what should we make while urgent mode?
 * 4.4BSD forces sending single byte of data. We cannot send
 * out of window data, because we have SND.NXT==SND.MAX...
 *
 * Current solution: to send TWO zero-length segments in urgent mode:
 * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
 * out-of-date with SND.UNA-1 to probe window.
 */
static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;

	/* We don't queue it, tcp_transmit_skb() sets ownership. */
	skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
	if (skb == NULL)
		return -1;

	/* Reserve space for headers and set control bits. */
	skb_reserve(skb, MAX_TCP_HEADER);
	/* Use a previous sequence.  This should cause the other
	 * end to send an ack.  Don't queue or clone SKB, just
	 * send it.
	 */
	tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPCB_FLAG_ACK);
	TCP_SKB_CB(skb)->when = tcp_time_stamp;
	return tcp_transmit_skb(sk, skb, 0, GFP_ATOMIC);
}

/* Initiate keepalive or window probe from timer. */
/*
 * tcp_write_wakeup()用来输出持续探测段。如果传输
 * 控制块处于关闭状态，则直接返回失败，否
 * 则传输持续探测段，过程如下:
 * 1)如果发送队列不为空，则利用那些待发送
 *    段来发送探测段，当然这些待发送的段至
 *     少有一部分在对方的接收窗口内。
 * 2)如果发送队列为空，则构造需要已确认，
 *    长度为零的段发送给对端。
 * 其返回值如下:
 *  0: 表示发送持续探测段成功
 *  小于0: 表示发送持续探测段失败
 *  大于0: 表示由于本地拥塞而导致发送持续探测段失败。
 */
int tcp_write_wakeup(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct sk_buff *skb;

	if (sk->sk_state == TCP_CLOSE)
		return -1;

	if ((skb = tcp_send_head(sk)) != NULL &&
	    before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
	    /*
		 * 如果发送队列中有段需要发送，并且最先
		 * 待发送的段至少有一部分在对端接收窗口
		 * 内，那么可以直接利用该待发送的段来发
		 * 送持续探测段。
		 */
		int err;
		/*
		 * 获取当前的MSS以及待分段的段长。分段得到
		 * 的新段必须在对方接收窗口内，待分段的段
		 * 长初始化为SND.UNA-SND_WND-SKB.seq.
		 */
		unsigned int mss = tcp_current_mss(sk);
		unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;

        /*
		 * 如果该段的序号已经大于pushed_seq，则需要
		 * 更新pushed_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 )
		 */
		 /*
		 * 如果待分段段长大于剩余等待发送数据，或者段长度
		 * 大于当前MSS，则对该段进行分段，分段段长取待分段
		 * 段长与当前MSS两者中的最小值，以保证只发送出一个
		 * 段到对方。
		 */
		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)->flags |= TCPCB_FLAG_PSH;
			if (tcp_fragment(sk, skb, seg_size, mss))
				return -1;
		} else if (!tcp_skb_pcount(skb))
			tcp_set_skb_tso_segs(sk, skb, mss);

        /*
		 * 将探测段发送出去，如果发送成功，
		 * 则更新发送队首等标志。
		 */
		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
		TCP_SKB_CB(skb)->when = tcp_time_stamp;
		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
		if (!err)
			tcp_event_new_data_sent(sk, skb);
		return err;
	} else {
        /*
		 * 如果发送队列为空，则构造并发送一个需要已确认、
		 * 长度为零的段给对端。如果处于紧急模式，则多发送
		 * 一个序号为SND.UNA的段给对端。
		 */
		if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
			tcp_xmit_probe_skb(sk, 1);
		return tcp_xmit_probe_skb(sk, 0);
	}
}

/* A window probe timeout has occurred.  If window is not closed send
 * a partial packet else a zero probe.
 */
 /*
 * 当持续定时器超时之后，会调用tcp_send_probe0()
 * 进行探测。
 */
void tcp_send_probe0(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	int err;

    /*
	 * 输出持续探测段。
	 */
	err = tcp_write_wakeup(sk);

    /*
	 * 如果有已发送但未确认的段，或者发送队列为空，
	 * 这两种情况都无需再发送持续探测段了，因此需要
	 * 将icsk_probes_out和icsk_backoff清零，然后返回。
	 */
	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;
	}

    /*
	 * 如果重传成功或并非由于本地拥塞而发送失败，
	 * 则更新icsk_backoff和icsk_probes_out，然后复位持续定时器。
	 */
	if (err <= 0) {
		if (icsk->icsk_backoff < sysctl_tcp_retries2)
			icsk->icsk_backoff++;
		icsk->icsk_probes_out++;
		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
					  min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
					  TCP_RTO_MAX);
	} else {
		/* If packet was not sent due to local congestion,
		 * do not backoff and do not remember icsk_probes_out.
		 * Let local senders to fight for local resources.
		 *
		 * Use accumulated backoff yet.
		 */
		 /*
		 * 如果由于本地拥塞而导致发送失败，则不需要累计
		 * icsk_probes_out，同时复位持续定时器，缩短超时时间，
		 * 尽可能争取资源。
		 */
		if (!icsk->icsk_probes_out)
			icsk->icsk_probes_out = 1;
		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
					  min(icsk->icsk_rto << icsk->icsk_backoff,
					      TCP_RESOURCE_PROBE_INTERVAL),
					  TCP_RTO_MAX);
	}
}

EXPORT_SYMBOL(tcp_select_initial_window);
EXPORT_SYMBOL(tcp_connect);
EXPORT_SYMBOL(tcp_make_synack);
EXPORT_SYMBOL(tcp_simple_retransmit);
EXPORT_SYMBOL(tcp_sync_mss);
EXPORT_SYMBOL(tcp_mtup_init);