2 * INET An implementation of the TCP/IP protocol suite for the LINUX
3 * operating system. INET is implemented using the BSD Socket
4 * interface as the means of communication with the user level.
6 * Implementation of the Transmission Control Protocol(TCP).
9 * Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
10 * Mark Evans, <evansmp@uhura.aston.ac.uk>
11 * Corey Minyard <wf-rch!minyard@relay.EU.net>
12 * Florian La Roche, <flla@stud.uni-sb.de>
13 * Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
14 * Linus Torvalds, <torvalds@cs.helsinki.fi>
15 * Alan Cox, <gw4pts@gw4pts.ampr.org>
16 * Matthew Dillon, <dillon@apollo.west.oic.com>
17 * Arnt Gulbrandsen, <agulbra@nvg.unit.no>
18 * Jorge Cwik, <jorge@laser.satlink.net>
21 #include <linux/module.h>
22 #include <linux/gfp.h>
25 int sysctl_tcp_syn_retries __read_mostly = TCP_SYN_RETRIES;
26 int sysctl_tcp_synack_retries __read_mostly = TCP_SYNACK_RETRIES;
27 int sysctl_tcp_keepalive_time __read_mostly = TCP_KEEPALIVE_TIME;
28 int sysctl_tcp_keepalive_probes __read_mostly = TCP_KEEPALIVE_PROBES;
29 int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
30 int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
31 int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
32 int sysctl_tcp_orphan_retries __read_mostly;
33 int sysctl_tcp_thin_linear_timeouts __read_mostly;
35 /*Function to reset tcp_ack related sysctl on resetting master control */
36 void set_tcp_default(void)
38 sysctl_tcp_delack_seg = TCP_DELACK_SEG;
41 /*sysctl handler for tcp_ack realted master control */
42 int tcp_proc_delayed_ack_control(struct ctl_table *table, int write,
43 void __user *buffer, size_t *length,
46 int ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
48 /* The ret value will be 0 if the input validation is successful
49 * and the values are written to sysctl table. If not, the stack
50 * will continue to work with currently configured values
55 /*sysctl handler for tcp_ack realted master control */
56 int tcp_use_userconfig_sysctl_handler(struct ctl_table *table, int write,
57 void __user *buffer, size_t *length,
60 int ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
62 if (write && ret == 0) {
63 if (!sysctl_tcp_use_userconfig)
69 static void tcp_write_err(struct sock *sk)
71 sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
72 sk->sk_error_report(sk);
75 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
78 /* Do not allow orphaned sockets to eat all our resources.
79 * This is direct violation of TCP specs, but it is required
80 * to prevent DoS attacks. It is called when a retransmission timeout
81 * or zero probe timeout occurs on orphaned socket.
83 * Also close if our net namespace is exiting; in that case there is no
84 * hope of ever communicating again since all netns interfaces are already
85 * down (or about to be down), and we need to release our dst references,
86 * which have been moved to the netns loopback interface, so the namespace
87 * can finish exiting. This condition is only possible if we are a kernel
88 * socket, as those do not hold references to the namespace.
90 * Criteria is still not confirmed experimentally and may change.
91 * We kill the socket, if:
92 * 1. If number of orphaned sockets exceeds an administratively configured
94 * 2. If we have strong memory pressure.
95 * 3. If our net namespace is exiting.
97 static int tcp_out_of_resources(struct sock *sk, bool do_reset)
99 struct tcp_sock *tp = tcp_sk(sk);
102 /* If peer does not open window for long time, or did not transmit
103 * anything for long time, penalize it. */
104 if ((s32)(tcp_time_stamp - tp->lsndtime) > 2*TCP_RTO_MAX || !do_reset)
107 /* If some dubious ICMP arrived, penalize even more. */
111 if (tcp_check_oom(sk, shift)) {
112 /* Catch exceptional cases, when connection requires reset.
113 * 1. Last segment was sent recently. */
114 if ((s32)(tcp_time_stamp - tp->lsndtime) <= TCP_TIMEWAIT_LEN ||
115 /* 2. Window is closed. */
116 (!tp->snd_wnd && !tp->packets_out))
119 tcp_send_active_reset(sk, GFP_ATOMIC);
121 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
125 if (!check_net(sock_net(sk))) {
126 /* Not possible to send reset; just close */
134 /* Calculate maximal number or retries on an orphaned socket. */
135 static int tcp_orphan_retries(struct sock *sk, bool alive)
137 int retries = sysctl_tcp_orphan_retries; /* May be zero. */
139 /* We know from an ICMP that something is wrong. */
140 if (sk->sk_err_soft && !alive)
143 /* However, if socket sent something recently, select some safe
144 * number of retries. 8 corresponds to >100 seconds with minimal
146 if (retries == 0 && alive)
151 static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
153 struct net *net = sock_net(sk);
155 /* Black hole detection */
156 if (net->ipv4.sysctl_tcp_mtu_probing) {
157 if (!icsk->icsk_mtup.enabled) {
158 icsk->icsk_mtup.enabled = 1;
159 icsk->icsk_mtup.probe_timestamp = tcp_time_stamp;
160 tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
162 struct net *net = sock_net(sk);
163 struct tcp_sock *tp = tcp_sk(sk);
166 mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
167 mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
168 mss = max(mss, 68 - tp->tcp_header_len);
169 icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
170 tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
175 /* This function calculates a "timeout" which is equivalent to the timeout of a
176 * TCP connection after "boundary" unsuccessful, exponentially backed-off
177 * retransmissions with an initial RTO of TCP_RTO_MIN or TCP_TIMEOUT_INIT if
178 * syn_set flag is set.
180 static bool retransmits_timed_out(struct sock *sk,
181 unsigned int boundary,
182 unsigned int timeout,
185 unsigned int linear_backoff_thresh, start_ts;
186 unsigned int rto_base = syn_set ? TCP_TIMEOUT_INIT : TCP_RTO_MIN;
188 if (!inet_csk(sk)->icsk_retransmits)
191 start_ts = tcp_sk(sk)->retrans_stamp;
192 if (unlikely(!start_ts))
193 start_ts = tcp_skb_timestamp(tcp_write_queue_head(sk));
195 if (likely(timeout == 0)) {
196 linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);
198 if (boundary <= linear_backoff_thresh)
199 timeout = ((2 << boundary) - 1) * rto_base;
201 timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
202 (boundary - linear_backoff_thresh) * TCP_RTO_MAX;
204 return (tcp_time_stamp - start_ts) >= timeout;
207 /* A write timeout has occurred. Process the after effects. */
208 static int tcp_write_timeout(struct sock *sk)
210 struct inet_connection_sock *icsk = inet_csk(sk);
211 struct tcp_sock *tp = tcp_sk(sk);
213 bool do_reset, syn_set = false;
215 if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
216 if (icsk->icsk_retransmits) {
217 dst_negative_advice(sk);
218 if (tp->syn_fastopen || tp->syn_data)
219 tcp_fastopen_cache_set(sk, 0, NULL, true, 0);
220 if (tp->syn_data && icsk->icsk_retransmits == 1)
221 NET_INC_STATS_BH(sock_net(sk),
222 LINUX_MIB_TCPFASTOPENACTIVEFAIL);
224 retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
227 if (retransmits_timed_out(sk, sysctl_tcp_retries1, 0, 0)) {
228 /* Some middle-boxes may black-hole Fast Open _after_
229 * the handshake. Therefore we conservatively disable
230 * Fast Open on this path on recurring timeouts with
231 * few or zero bytes acked after Fast Open.
233 if (tp->syn_data_acked &&
234 tp->bytes_acked <= tp->rx_opt.mss_clamp) {
235 tcp_fastopen_cache_set(sk, 0, NULL, true, 0);
236 if (icsk->icsk_retransmits == sysctl_tcp_retries1)
237 NET_INC_STATS_BH(sock_net(sk),
238 LINUX_MIB_TCPFASTOPENACTIVEFAIL);
240 /* Black hole detection */
241 tcp_mtu_probing(icsk, sk);
243 dst_negative_advice(sk);
246 retry_until = sysctl_tcp_retries2;
247 if (sock_flag(sk, SOCK_DEAD)) {
248 const bool alive = icsk->icsk_rto < TCP_RTO_MAX;
250 retry_until = tcp_orphan_retries(sk, alive);
252 !retransmits_timed_out(sk, retry_until, 0, 0);
254 if (tcp_out_of_resources(sk, do_reset))
259 if (retransmits_timed_out(sk, retry_until,
260 syn_set ? 0 : icsk->icsk_user_timeout, syn_set)) {
261 /* Has it gone just too far? */
268 void tcp_delack_timer_handler(struct sock *sk)
270 struct tcp_sock *tp = tcp_sk(sk);
271 struct inet_connection_sock *icsk = inet_csk(sk);
273 sk_mem_reclaim_partial(sk);
275 if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
276 !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
279 if (time_after(icsk->icsk_ack.timeout, jiffies)) {
280 sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
283 icsk->icsk_ack.pending &= ~ICSK_ACK_TIMER;
285 if (!skb_queue_empty(&tp->ucopy.prequeue)) {
288 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSCHEDULERFAILED);
290 while ((skb = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)
291 sk_backlog_rcv(sk, skb);
293 tp->ucopy.memory = 0;
296 if (inet_csk_ack_scheduled(sk)) {
297 if (!icsk->icsk_ack.pingpong) {
298 /* Delayed ACK missed: inflate ATO. */
299 icsk->icsk_ack.ato = min(icsk->icsk_ack.ato << 1, icsk->icsk_rto);
301 /* Delayed ACK missed: leave pingpong mode and
304 icsk->icsk_ack.pingpong = 0;
305 icsk->icsk_ack.ato = TCP_ATO_MIN;
308 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKS);
312 if (tcp_under_memory_pressure(sk))
316 static void tcp_delack_timer(unsigned long data)
318 struct sock *sk = (struct sock *)data;
321 if (!sock_owned_by_user(sk)) {
322 tcp_delack_timer_handler(sk);
324 inet_csk(sk)->icsk_ack.blocked = 1;
325 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
326 /* deleguate our work to tcp_release_cb() */
327 if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
334 static void tcp_probe_timer(struct sock *sk)
336 struct inet_connection_sock *icsk = inet_csk(sk);
337 struct tcp_sock *tp = tcp_sk(sk);
341 if (tp->packets_out || !tcp_send_head(sk)) {
342 icsk->icsk_probes_out = 0;
346 /* RFC 1122 4.2.2.17 requires the sender to stay open indefinitely as
347 * long as the receiver continues to respond probes. We support this by
348 * default and reset icsk_probes_out with incoming ACKs. But if the
349 * socket is orphaned or the user specifies TCP_USER_TIMEOUT, we
350 * kill the socket when the retry count and the time exceeds the
351 * corresponding system limit. We also implement similar policy when
352 * we use RTO to probe window in tcp_retransmit_timer().
354 start_ts = tcp_skb_timestamp(tcp_send_head(sk));
356 skb_mstamp_get(&tcp_send_head(sk)->skb_mstamp);
357 else if (icsk->icsk_user_timeout &&
358 (s32)(tcp_time_stamp - start_ts) > icsk->icsk_user_timeout)
361 max_probes = sysctl_tcp_retries2;
362 if (sock_flag(sk, SOCK_DEAD)) {
363 const bool alive = inet_csk_rto_backoff(icsk, TCP_RTO_MAX) < TCP_RTO_MAX;
365 max_probes = tcp_orphan_retries(sk, alive);
366 if (!alive && icsk->icsk_backoff >= max_probes)
368 if (tcp_out_of_resources(sk, true))
372 if (icsk->icsk_probes_out > max_probes) {
373 abort: tcp_write_err(sk);
375 /* Only send another probe if we didn't close things up. */
381 * Timer for Fast Open socket to retransmit SYNACK. Note that the
382 * sk here is the child socket, not the parent (listener) socket.
384 static void tcp_fastopen_synack_timer(struct sock *sk)
386 struct inet_connection_sock *icsk = inet_csk(sk);
387 int max_retries = icsk->icsk_syn_retries ? :
388 sysctl_tcp_synack_retries + 1; /* add one more retry for fastopen */
389 struct request_sock *req;
391 req = tcp_sk(sk)->fastopen_rsk;
392 req->rsk_ops->syn_ack_timeout(req);
394 if (req->num_timeout >= max_retries) {
398 /* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
399 * returned from rtx_syn_ack() to make it more persistent like
400 * regular retransmit because if the child socket has been accepted
401 * it's not good to give up too easily.
403 inet_rtx_syn_ack(sk, req);
405 inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
406 TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
410 * The TCP retransmit timer.
413 void tcp_retransmit_timer(struct sock *sk)
415 struct tcp_sock *tp = tcp_sk(sk);
416 struct inet_connection_sock *icsk = inet_csk(sk);
418 if (tp->fastopen_rsk) {
419 WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
420 sk->sk_state != TCP_FIN_WAIT1);
421 tcp_fastopen_synack_timer(sk);
422 /* Before we receive ACK to our SYN-ACK don't retransmit
423 * anything else (e.g., data or FIN segments).
427 if (!tp->packets_out)
430 WARN_ON(tcp_write_queue_empty(sk));
432 tp->tlp_high_seq = 0;
434 if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
435 !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) {
436 /* Receiver dastardly shrinks window. Our retransmits
437 * become zero probes, but we should not timeout this
438 * connection. If the socket is an orphan, time it out,
439 * we cannot allow such beasts to hang infinitely.
441 struct inet_sock *inet = inet_sk(sk);
442 if (sk->sk_family == AF_INET) {
443 net_dbg_ratelimited("Peer %pI4:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
445 ntohs(inet->inet_dport),
447 tp->snd_una, tp->snd_nxt);
449 #if IS_ENABLED(CONFIG_IPV6)
450 else if (sk->sk_family == AF_INET6) {
451 net_dbg_ratelimited("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
453 ntohs(inet->inet_dport),
455 tp->snd_una, tp->snd_nxt);
458 if (tcp_time_stamp - tp->rcv_tstamp > TCP_RTO_MAX) {
463 tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
465 goto out_reset_timer;
468 if (tcp_write_timeout(sk))
471 if (icsk->icsk_retransmits == 0) {
474 if (icsk->icsk_ca_state == TCP_CA_Recovery) {
476 mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
478 mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
479 } else if (icsk->icsk_ca_state == TCP_CA_Loss) {
480 mib_idx = LINUX_MIB_TCPLOSSFAILURES;
481 } else if ((icsk->icsk_ca_state == TCP_CA_Disorder) ||
484 mib_idx = LINUX_MIB_TCPSACKFAILURES;
486 mib_idx = LINUX_MIB_TCPRENOFAILURES;
488 mib_idx = LINUX_MIB_TCPTIMEOUTS;
490 NET_INC_STATS_BH(sock_net(sk), mib_idx);
495 if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
496 /* Retransmission failed because of local congestion,
499 if (!icsk->icsk_retransmits)
500 icsk->icsk_retransmits = 1;
501 inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
502 min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
507 /* Increase the timeout each time we retransmit. Note that
508 * we do not increase the rtt estimate. rto is initialized
509 * from rtt, but increases here. Jacobson (SIGCOMM 88) suggests
510 * that doubling rto each time is the least we can get away with.
511 * In KA9Q, Karn uses this for the first few times, and then
512 * goes to quadratic. netBSD doubles, but only goes up to *64,
513 * and clamps at 1 to 64 sec afterwards. Note that 120 sec is
514 * defined in the protocol as the maximum possible RTT. I guess
515 * we'll have to use something other than TCP to talk to the
516 * University of Mars.
518 * PAWS allows us longer timeouts and large windows, so once
519 * implemented ftp to mars will work nicely. We will have to fix
520 * the 120 second clamps though!
522 icsk->icsk_backoff++;
523 icsk->icsk_retransmits++;
526 /* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
527 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
528 * might be increased if the stream oscillates between thin and thick,
529 * thus the old value might already be too high compared to the value
530 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
531 * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
532 * exponential backoff behaviour to avoid continue hammering
533 * linear-timeout retransmissions into a black hole
535 if (sk->sk_state == TCP_ESTABLISHED &&
536 (tp->thin_lto || sysctl_tcp_thin_linear_timeouts) &&
537 tcp_stream_is_thin(tp) &&
538 icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
539 icsk->icsk_backoff = 0;
540 icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
542 /* Use normal (exponential) backoff */
543 icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
545 inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
546 if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1, 0, 0))
552 void tcp_write_timer_handler(struct sock *sk)
554 struct inet_connection_sock *icsk = inet_csk(sk);
557 if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) ||
561 if (time_after(icsk->icsk_timeout, jiffies)) {
562 sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
566 event = icsk->icsk_pending;
569 case ICSK_TIME_EARLY_RETRANS:
570 tcp_resume_early_retransmit(sk);
572 case ICSK_TIME_LOSS_PROBE:
573 tcp_send_loss_probe(sk);
575 case ICSK_TIME_RETRANS:
576 icsk->icsk_pending = 0;
577 tcp_retransmit_timer(sk);
579 case ICSK_TIME_PROBE0:
580 icsk->icsk_pending = 0;
589 static void tcp_write_timer(unsigned long data)
591 struct sock *sk = (struct sock *)data;
594 if (!sock_owned_by_user(sk)) {
595 tcp_write_timer_handler(sk);
597 /* deleguate our work to tcp_release_cb() */
598 if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
605 void tcp_syn_ack_timeout(const struct request_sock *req)
607 struct net *net = read_pnet(&inet_rsk(req)->ireq_net);
609 NET_INC_STATS_BH(net, LINUX_MIB_TCPTIMEOUTS);
611 EXPORT_SYMBOL(tcp_syn_ack_timeout);
613 void tcp_set_keepalive(struct sock *sk, int val)
615 if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
618 if (val && !sock_flag(sk, SOCK_KEEPOPEN))
619 inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
621 inet_csk_delete_keepalive_timer(sk);
625 static void tcp_keepalive_timer (unsigned long data)
627 struct sock *sk = (struct sock *) data;
628 struct inet_connection_sock *icsk = inet_csk(sk);
629 struct tcp_sock *tp = tcp_sk(sk);
632 /* Only process if socket is not in use. */
634 if (sock_owned_by_user(sk)) {
635 /* Try again later. */
636 inet_csk_reset_keepalive_timer (sk, HZ/20);
640 if (sk->sk_state == TCP_LISTEN) {
641 pr_err("Hmm... keepalive on a LISTEN ???\n");
645 if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
646 if (tp->linger2 >= 0) {
647 const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;
650 tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
654 tcp_send_active_reset(sk, GFP_ATOMIC);
658 if (!sock_flag(sk, SOCK_KEEPOPEN) ||
659 ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_SYN_SENT)))
662 elapsed = keepalive_time_when(tp);
664 /* It is alive without keepalive 8) */
665 if (tp->packets_out || tcp_send_head(sk))
668 elapsed = keepalive_time_elapsed(tp);
670 if (elapsed >= keepalive_time_when(tp)) {
671 /* If the TCP_USER_TIMEOUT option is enabled, use that
672 * to determine when to timeout instead.
674 if ((icsk->icsk_user_timeout != 0 &&
675 elapsed >= icsk->icsk_user_timeout &&
676 icsk->icsk_probes_out > 0) ||
677 (icsk->icsk_user_timeout == 0 &&
678 icsk->icsk_probes_out >= keepalive_probes(tp))) {
679 tcp_send_active_reset(sk, GFP_ATOMIC);
683 if (tcp_write_wakeup(sk, LINUX_MIB_TCPKEEPALIVE) <= 0) {
684 icsk->icsk_probes_out++;
685 elapsed = keepalive_intvl_when(tp);
687 /* If keepalive was lost due to local congestion,
690 elapsed = TCP_RESOURCE_PROBE_INTERVAL;
693 /* It is tp->rcv_tstamp + keepalive_time_when(tp) */
694 elapsed = keepalive_time_when(tp) - elapsed;
700 inet_csk_reset_keepalive_timer (sk, elapsed);
711 void tcp_init_xmit_timers(struct sock *sk)
713 inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
714 &tcp_keepalive_timer);