1TCP(7) Linux Programmer's Manual TCP(7)
2
6 tcp - TCP protocol
7
9 #include <sys/socket.h>
10 #include <netinet/in.h>
11 #include <netinet/tcp.h>
12 tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
14
16 This is an implementation of the TCP protocol defined in RFC 793,
17 RFC 1122 and RFC 2001 with the NewReno and SACK extensions. It pro‐
18 vides a reliable, stream-oriented, full-duplex connection between two
19 sockets on top of ip(7), for both v4 and v6 versions. TCP guarantees
20 that the data arrives in order and retransmits lost packets. It gener‐
21 ates and checks a per-packet checksum to catch transmission errors.
22 TCP does not preserve record boundaries.
23 A newly created TCP socket has no remote or local address and is not
25 fully specified. To create an outgoing TCP connection use connect(2)
26 to establish a connection to another TCP socket. To receive new incom‐
27 ing connections, first bind(2) the socket to a local address and port
28 and then call listen(2) to put the socket into the listening state.
29 After that a new socket for each incoming connection can be accepted
30 using accept(2). A socket which has had accept(2) or connect(2) suc‐
31 cessfully called on it is fully specified and may transmit data. Data
32 cannot be transmitted on listening or not yet connected sockets.
33 Linux supports RFC 1323 TCP high performance extensions. These include
35 Protection Against Wrapped Sequence Numbers (PAWS), Window Scaling and
36 Timestamps. Window scaling allows the use of large (> 64 kB) TCP win‐
37 dows in order to support links with high latency or bandwidth. To make
38 use of them, the send and receive buffer sizes must be increased. They
39 can be set globally with the /proc/sys/net/ipv4/tcp_wmem and
40 /proc/sys/net/ipv4/tcp_rmem files, or on individual sockets by using
41 the SO_SNDBUF and SO_RCVBUF socket options with the setsockopt(2) call.
42 The maximum sizes for socket buffers declared via the SO_SNDBUF and
44 SO_RCVBUF mechanisms are limited by the values in the
45 /proc/sys/net/core/rmem_max and /proc/sys/net/core/wmem_max files.
46 Note that TCP actually allocates twice the size of the buffer requested
47 in the setsockopt(2) call, and so a succeeding getsockopt(2) call will
48 not return the same size of buffer as requested in the setsockopt(2)
49 call. TCP uses the extra space for administrative purposes and inter‐
50 nal kernel structures, and the /proc file values reflect the larger
51 sizes compared to the actual TCP windows. On individual connections,
52 the socket buffer size must be set prior to the listen(2) or connect(2)
53 calls in order to have it take effect. See socket(7) for more informa‐
54 tion.
55 TCP supports urgent data. Urgent data is used to signal the receiver
57 that some important message is part of the data stream and that it
58 should be processed as soon as possible. To send urgent data specify
59 the MSG_OOB option to send(2). When urgent data is received, the ker‐
60 nel sends a SIGURG signal to the process or process group that has been
61 set as the socket "owner" using the SIOCSPGRP or FIOSETOWN ioctls (or
62 the POSIX.1-specified fcntl(2) F_SETOWN operation). When the SO_OOBIN‐
63 LINE socket option is enabled, urgent data is put into the normal data
64 stream (a program can test for its location using the SIOCATMARK ioctl
65 described below), otherwise it can be received only when the MSG_OOB
66 flag is set for recv(2) or recvmsg(2).
67 When out-of-band data is present, select(2) indicates the file descrip‐
69 tor as having an exceptional condition and poll [4m(2) indicates a POLLPRI
70 event.
71 Linux 2.4 introduced a number of changes for improved throughput and
73 scaling, as well as enhanced functionality. Some of these features
74 include support for zero-copy sendfile(2), Explicit Congestion Notifi‐
75 cation, new management of TIME_WAIT sockets, keep-alive socket options
76 and support for Duplicate SACK extensions.
77 Address formats
79 TCP is built on top of IP (see ip(7)). The address formats defined by
80 ip(7) apply to TCP. TCP supports point-to-point communication only;
81 broadcasting and multicasting are not supported.
82 /proc interfaces
84 System-wide TCP parameter settings can be accessed by files in the
85 directory /proc/sys/net/ipv4/. In addition, most IP /proc interfaces
86 also apply to TCP; see ip(7). Variables described as Boolean take an
87 integer value, with a nonzero value ("true") meaning that the corre‐
88 sponding option is enabled, and a zero value ("false") meaning that the
89 option is disabled.
90 tcp_abc (Integer; default: 0; Linux 2.6.15 to Linux 3.8)
92 Control the Appropriate Byte Count (ABC), defined in RFC 3465.
93 ABC is a way of increasing the congestion window (cwnd) more
94 slowly in response to partial acknowledgments. Possible values
95 are:
96 0 increase cwnd once per acknowledgment (no ABC)
98 1 increase cwnd once per acknowledgment of full sized segment
100 2 allow increase cwnd by two if acknowledgment is of two seg‐
102 ments to compensate for delayed acknowledgments.
103 tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
105 Enable resetting connections if the listening service is too
106 slow and unable to keep up and accept them. It means that if
107 overflow occurred due to a burst, the connection will recover.
108 Enable this option only if you are really sure that the listen‐
109 ing daemon cannot be tuned to accept connections faster.
110 Enabling this option can harm the clients of your server.
111 tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
113 Count buffering overhead as bytes/2^tcp_adv_win_scale, if
114 tcp_adv_win_scale is greater than 0; or bytes-
115 bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less than
116 or equal to zero.
117 The socket receive buffer space is shared between the applica‐
119 tion and kernel. TCP maintains part of the buffer as the TCP
120 window, this is the size of the receive window advertised to the
121 other end. The rest of the space is used as the "application"
122 buffer, used to isolate the network from scheduling and applica‐
123 tion latencies. The tcp_adv_win_scale default value of 2
124 implies that the space used for the application buffer is one
125 fourth that of the total.
126 tcp_allowed_congestion_control (String; default: see text; since Linux
128 2.4.20)
129 Show/set the congestion control algorithm choices available to
130 unprivileged processes (see the description of the TCP_CONGES‐
131 TION socket option). The items in the list are separated by
132 white space and terminated by a newline character. The list is
133 a subset of those listed in tcp_available_congestion_control.
134 The default value for this list is "reno" plus the default set‐
135 ting of tcp_congestion_control.
136 tcp_autocorking (Boolean; default: enabled; since Linux 3.14)
138 If this option is enabled, the kernel tries to coalesce small
139 writes (from consecutive write(2) and sendmsg(2) calls) as much
140 as possible, in order to decrease the total number of sent pack‐
141 ets. Coalescing is done if at least one prior packet for the
142 flow is waiting in Qdisc queues or device transmit queue.
143 Applications can still use the TCP_CORK socket option to obtain
144 optimal behavior when they know how/when to uncork their sock‐
145 ets.
146 tcp_available_congestion_control (String; read-only; since Linux
148 2.4.20)
149 Show a list of the congestion-control algorithms that are regis‐
150 tered. The items in the list are separated by white space and
151 terminated by a newline character. This list is a limiting set
152 for the list in tcp_allowed_congestion_control. More conges‐
153 tion-control algorithms may be available as modules, but not
154 loaded.
155 tcp_app_win (integer; default: 31; since Linux 2.4)
157 This variable defines how many bytes of the TCP window are
158 reserved for buffering overhead.
159 A maximum of (window/2^tcp_app_win, mss) bytes in the window are
161 reserved for the application buffer. A value of 0 implies that
162 no amount is reserved.
163 tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
165 The initial value of search_low to be used by the packetization
166 layer Path MTU discovery (MTU probing). If MTU probing is
167 enabled, this is the initial MSS used by the connection.
168 tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
170 Enable BIC TCP congestion control algorithm. BIC-TCP is a
171 sender-side-only change that ensures a linear RTT fairness under
172 large windows while offering both scalability and bounded TCP-
173 friendliness. The protocol combines two schemes called additive
174 increase and binary search increase. When the congestion window
175 is large, additive increase with a large increment ensures lin‐
176 ear RTT fairness as well as good scalability. Under small con‐
177 gestion windows, binary search increase provides TCP friendli‐
178 ness.
179 tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
181 Set the threshold window (in packets) where BIC TCP starts to
182 adjust the congestion window. Below this threshold BIC TCP
183 behaves the same as the default TCP Reno.
184 tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6
186 to 2.6.13)
187 Force BIC TCP to more quickly respond to changes in congestion
188 window. Allows two flows sharing the same connection to con‐
189 verge more rapidly.
190 tcp_congestion_control (String; default: see text; since Linux 2.4.13)
192 Set the default congestion-control algorithm to be used for new
193 connections. The algorithm "reno" is always available, but
194 additional choices may be available depending on kernel configu‐
195 ration. The default value for this file is set as part of ker‐
196 nel configuration.
197 tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
199 Lower limit, in bytes, of the size of socket reads that will be
200 offloaded to a DMA copy engine, if one is present in the system
201 and the kernel was configured with the CONFIG_NET_DMA option.
202 tcp_dsack (Boolean; default: enabled; since Linux 2.4)
204 Enable RFC 2883 TCP Duplicate SACK support.
205 tcp_ecn (Integer; default: see below; since Linux 2.4)
207 Enable RFC 3168 Explicit Congestion Notification.
208 This file can have one of the following values:
210 0 Disable ECN. Neither initiate nor accept ECN. This was
212 the default up to and including Linux 2.6.30.
213 1 Enable ECN when requested by incoming connections and
215 also request ECN on outgoing connection attempts.
216 2 Enable ECN when requested by incoming connections, but do
218 not request ECN on outgoing connections. This value is
219 supported, and is the default, since Linux 2.6.31.
220 When enabled, connectivity to some destinations could be
222 affected due to older, misbehaving middle boxes along the path,
223 causing connections to be dropped. However, to facilitate and
224 encourage deployment with option 1, and to work around such
225 buggy equipment, the tcp_ecn_fallback option has been intro‐
226 duced.
227 tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
229 Enable RFC 3168, Section 6.1.1.1. fallback. When enabled, out‐
230 going ECN-setup SYNs that time out within the normal SYN
231 retransmission timeout will be resent with CWR and ECE cleared.
232 tcp_fack (Boolean; default: enabled; since Linux 2.2)
234 Enable TCP Forward Acknowledgement support.
235 tcp_fin_timeout (integer; default: 60; since Linux 2.2)
237 This specifies how many seconds to wait for a final FIN packet
238 before the socket is forcibly closed. This is strictly a viola‐
239 tion of the TCP specification, but required to prevent denial-
240 of-service attacks. In Linux 2.2, the default value was 180.
241 tcp_frto (integer; default: see below; since Linux 2.4.21/2.6)
243 Enable F-RTO, an enhanced recovery algorithm for TCP retransmis‐
244 sion timeouts (RTOs). It is particularly beneficial in wireless
245 environments where packet loss is typically due to random radio
246 interference rather than intermediate router congestion. See
247 RFC 4138 for more details.
248 This file can have one of the following values:
250 0 Disabled. This was the default up to and including Linux
252 2.6.23.
253 1 The basic version F-RTO algorithm is enabled.
255 2 Enable SACK-enhanced F-RTO if flow uses SACK. The basic ver‐
257 sion can be used also when SACK is in use though in that case
258 scenario(s) exists where F-RTO interacts badly with the
259 packet counting of the SACK-enabled TCP flow. This value is
260 the default since Linux 2.6.24.
261 Before Linux 2.6.22, this parameter was a Boolean value, sup‐
263 porting just values 0 and 1 above.
264 tcp_frto_response (integer; default: 0; since Linux 2.6.22)
266 When F-RTO has detected that a TCP retransmission timeout was
267 spurious (i.e., the timeout would have been avoided had TCP set
268 a longer retransmission timeout), TCP has several options con‐
269 cerning what to do next. Possible values are:
270 0 Rate halving based; a smooth and conservative response,
272 results in halved congestion window (cwnd) and slow-start
273 threshold (ssthresh) after one RTT.
274 1 Very conservative response; not recommended because even
276 though being valid, it interacts poorly with the rest of
277 Linux TCP; halves cwnd and ssthresh immediately.
278 2 Aggressive response; undoes congestion-control measures that
280 are now known to be unnecessary (ignoring the possibility of
281 a lost retransmission that would require TCP to be more cau‐
282 tious); cwnd and ssthresh are restored to the values prior to
283 timeout.
284 tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
286 The number of seconds between TCP keep-alive probes.
287 tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
289 The maximum number of TCP keep-alive probes to send before giv‐
290 ing up and killing the connection if no response is obtained
291 from the other end.
292 tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
294 The number of seconds a connection needs to be idle before TCP
295 begins sending out keep-alive probes. Keep-alives are sent only
296 when the SO_KEEPALIVE socket option is enabled. The default
297 value is 7200 seconds (2 hours). An idle connection is termi‐
298 nated after approximately an additional 11 minutes (9 probes an
299 interval of 75 seconds apart) when keep-alive is enabled.
300 Note that underlying connection tracking mechanisms and applica‐
302 tion timeouts may be much shorter.
303 tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
305 If enabled, the TCP stack makes decisions that prefer lower
306 latency as opposed to higher throughput. It this option is dis‐
307 abled, then higher throughput is preferred. An example of an
308 application where this default should be changed would be a
309 Beowulf compute cluster.
310 tcp_max_orphans (integer; default: see below; since Linux 2.4)
312 The maximum number of orphaned (not attached to any user file
313 handle) TCP sockets allowed in the system. When this number is
314 exceeded, the orphaned connection is reset and a warning is
315 printed. This limit exists only to prevent simple denial-of-
316 service attacks. Lowering this limit is not recommended. Net‐
317 work conditions might require you to increase the number of
318 orphans allowed, but note that each orphan can eat up to ~64 kB
319 of unswappable memory. The default initial value is set equal
320 to the kernel parameter NR_FILE. This initial default is
321 adjusted depending on the memory in the system.
322 tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
324 The maximum number of queued connection requests which have
325 still not received an acknowledgement from the connecting
326 client. If this number is exceeded, the kernel will begin drop‐
327 ping requests. The default value of 256 is increased to 1024
328 when the memory present in the system is adequate or greater (>=
329 128 MB), and reduced to 128 for those systems with very low mem‐
330 ory (<= 32 MB).
331 Prior to Linux 2.6.20, it was recommended that if this needed to
333 be increased above 1024, the size of the SYNACK hash table
334 (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep
335 TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
337 and the kernel should be recompiled. In Linux 2.6.20, the fixed
339 sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.
340 tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
342 The maximum number of sockets in TIME_WAIT state allowed in the
343 system. This limit exists only to prevent simple denial-of-ser‐
344 vice attacks. The default value of NR_FILE*2 is adjusted
345 depending on the memory in the system. If this number is
346 exceeded, the socket is closed and a warning is printed.
347 tcp_moderate_rcvbuf (Boolean; default: enabled; since Linux
349 2.4.17/2.6.7)
350 If enabled, TCP performs receive buffer auto-tuning, attempting
351 to automatically size the buffer (no greater than tcp_rmem[2])
352 to match the size required by the path for full throughput.
353 tcp_mem (since Linux 2.4)
355 This is a vector of 3 integers: [low, pressure, high]. These
356 bounds, measured in units of the system page size, are used by
357 TCP to track its memory usage. The defaults are calculated at
358 boot time from the amount of available memory. (TCP can only
359 use low memory for this, which is limited to around 900
360 megabytes on 32-bit systems. 64-bit systems do not suffer this
361 limitation.)
362 low TCP doesn't regulate its memory allocation when the
364 number of pages it has allocated globally is below
365 this number.
366 pressure When the amount of memory allocated by TCP exceeds
368 this number of pages, TCP moderates its memory con‐
369 sumption. This memory pressure state is exited once
370 the number of pages allocated falls below the low
371 mark.
372 high The maximum number of pages, globally, that TCP will
374 allocate. This value overrides any other limits
375 imposed by the kernel.
376 tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
378 This parameter controls TCP Packetization-Layer Path MTU Discov‐
379 ery. The following values may be assigned to the file:
380 0 Disabled
382 1 Disabled by default, enabled when an ICMP black hole detected
384 2 Always enabled, use initial MSS of tcp_base_mss.
386 tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
388 By default, TCP saves various connection metrics in the route
389 cache when the connection closes, so that connections estab‐
390 lished in the near future can use these to set initial condi‐
391 tions. Usually, this increases overall performance, but it may
392 sometimes cause performance degradation. If tcp_no_metrics_save
393 is enabled, TCP will not cache metrics on closing connections.
394 tcp_orphan_retries (integer; default: 8; since Linux 2.4)
396 The maximum number of attempts made to probe the other end of a
397 connection which has been closed by our end.
398 tcp_reordering (integer; default: 3; since Linux 2.4)
400 The maximum a packet can be reordered in a TCP packet stream
401 without TCP assuming packet loss and going into slow start. It
402 is not advisable to change this number. This is a packet
403 reordering detection metric designed to minimize unnecessary
404 back off and retransmits provoked by reordering of packets on a
405 connection.
406 tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
408 Try to send full-sized packets during retransmit.
409 tcp_retries1 (integer; default: 3; since Linux 2.2)
411 The number of times TCP will attempt to retransmit a packet on
412 an established connection normally, without the extra effort of
413 getting the network layers involved. Once we exceed this number
414 of retransmits, we first have the network layer update the route
415 if possible before each new retransmit. The default is the RFC
416 specified minimum of 3.
417 tcp_retries2 (integer; default: 15; since Linux 2.2)
419 The maximum number of times a TCP packet is retransmitted in
420 established state before giving up. The default value is 15,
421 which corresponds to a duration of approximately between 13 to
422 30 minutes, depending on the retransmission timeout. The
423 RFC 1122 specified minimum limit of 100 seconds is typically
424 deemed too short.
425 tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
427 Enable TCP behavior conformant with RFC 1337. When disabled, if
428 a RST is received in TIME_WAIT state, we close the socket imme‐
429 diately without waiting for the end of the TIME_WAIT period.
430 tcp_rmem (since Linux 2.4)
432 This is a vector of 3 integers: [min, default, max]. These
433 parameters are used by TCP to regulate receive buffer sizes.
434 TCP dynamically adjusts the size of the receive buffer from the
435 defaults listed below, in the range of these values, depending
436 on memory available in the system.
437 min minimum size of the receive buffer used by each TCP
439 socket. The default value is the system page size.
440 (On Linux 2.4, the default value is 4 kB, lowered to
441 PAGE_SIZE bytes in low-memory systems.) This value is
442 used to ensure that in memory pressure mode, alloca‐
443 tions below this size will still succeed. This is not
444 used to bound the size of the receive buffer declared
445 using SO_RCVBUF on a socket.
446 default the default size of the receive buffer for a TCP
448 socket. This value overwrites the initial default
449 buffer size from the generic global
450 net.core.rmem_default defined for all protocols. The
451 default value is 87380 bytes. (On Linux 2.4, this
452 will be lowered to 43689 in low-memory systems.) If
453 larger receive buffer sizes are desired, this value
454 should be increased (to affect all sockets). To
455 employ large TCP windows, the net.ipv4.tcp_win‐
456 dow_scaling must be enabled (default).
457 max the maximum size of the receive buffer used by each
459 TCP socket. This value does not override the global
460 net.core.rmem_max. This is not used to limit the size
461 of the receive buffer declared using SO_RCVBUF on a
462 socket. The default value is calculated using the
463 formula
464 max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
466 (On Linux 2.4, the default is 87380*2 bytes, lowered
468 to 87380 in low-memory systems).
469 tcp_sack (Boolean; default: enabled; since Linux 2.2)
471 Enable RFC 2018 TCP Selective Acknowledgements.
472 tcp_slow_start_after_idle (Boolean; default: enabled; since Linux
474 2.6.18)
475 If enabled, provide RFC 2861 behavior and time out the conges‐
476 tion window after an idle period. An idle period is defined as
477 the current RTO (retransmission timeout). If disabled, the con‐
478 gestion window will not be timed out after an idle period.
479 tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
481 If this option is enabled, then use the RFC 1122 interpretation
482 of the TCP urgent-pointer field. According to this interpreta‐
483 tion, the urgent pointer points to the last byte of urgent data.
484 If this option is disabled, then use the BSD-compatible inter‐
485 pretation of the urgent pointer: the urgent pointer points to
486 the first byte after the urgent data. Enabling this option may
487 lead to interoperability problems.
488 tcp_syn_retries (integer; default: 5; since Linux 2.2)
490 The maximum number of times initial SYNs for an active TCP con‐
491 nection attempt will be retransmitted. This value should not be
492 higher than 255. The default value is 5, which corresponds to
493 approximately 180 seconds.
494 tcp_synack_retries (integer; default: 5; since Linux 2.2)
496 The maximum number of times a SYN/ACK segment for a passive TCP
497 connection will be retransmitted. This number should not be
498 higher than 255.
499 tcp_syncookies (Boolean; since Linux 2.2)
501 Enable TCP syncookies. The kernel must be compiled with CON‐
502 FIG_SYN_COOKIES. Send out syncookies when the syn backlog queue
503 of a socket overflows. The syncookies feature attempts to pro‐
504 tect a socket from a SYN flood attack. This should be used as a
505 last resort, if at all. This is a violation of the TCP proto‐
506 col, and conflicts with other areas of TCP such as TCP exten‐
507 sions. It can cause problems for clients and relays. It is not
508 recommended as a tuning mechanism for heavily loaded servers to
509 help with overloaded or misconfigured conditions. For recom‐
510 mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
511 and tcp_abort_on_overflow.
512 tcp_timestamps (integer; default: 1; since Linux 2.2)
514 Set to one of the following values to enable or disable RFC 1323
515 TCP timestamps:
516 0 Disable timestamps.
518 1 Enable timestamps as defined in RFC1323 and use random offset
520 for each connection rather than only using the current time.
521 2 As for the value 1, but without random offsets. Setting
523 tcp_timestamps to this value is meaningful since Linux 4.10.
524 tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
526 This parameter controls what percentage of the congestion window
527 can be consumed by a single TCP Segmentation Offload (TSO)
528 frame. The setting of this parameter is a tradeoff between
529 burstiness and building larger TSO frames.
530 tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11)
532 Enable fast recycling of TIME_WAIT sockets. Enabling this
533 option is not recommended as the remote IP may not use monotoni‐
534 cally increasing timestamps (devices behind NAT, devices with
535 per-connection timestamp offsets). See RFC 1323 (PAWS) and RFC
536 6191.
537 tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
539 Allow to reuse TIME_WAIT sockets for new connections when it is
540 safe from protocol viewpoint. It should not be changed without
541 advice/request of technical experts.
542 tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
544 Enable TCP Vegas congestion avoidance algorithm. TCP Vegas is a
545 sender-side-only change to TCP that anticipates the onset of
546 congestion by estimating the bandwidth. TCP Vegas adjusts the
547 sending rate by modifying the congestion window. TCP Vegas
548 should provide less packet loss, but it is not as aggressive as
549 TCP Reno.
550 tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
552 Enable TCP Westwood+ congestion control algorithm. TCP West‐
553 wood+ is a sender-side-only modification of the TCP Reno proto‐
554 col stack that optimizes the performance of TCP congestion con‐
555 trol. It is based on end-to-end bandwidth estimation to set
556 congestion window and slow start threshold after a congestion
557 episode. Using this estimation, TCP Westwood+ adaptively sets a
558 slow start threshold and a congestion window which takes into
559 account the bandwidth used at the time congestion is experi‐
560 enced. TCP Westwood+ significantly increases fairness with
561 respect to TCP Reno in wired networks and throughput over wire‐
562 less links.
563 tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
565 Enable RFC 1323 TCP window scaling. This feature allows the use
566 of a large window (> 64 kB) on a TCP connection, should the
567 other end support it. Normally, the 16 bit window length field
568 in the TCP header limits the window size to less than 64 kB. If
569 larger windows are desired, applications can increase the size
570 of their socket buffers and the window scaling option will be
571 employed. If tcp_window_scaling is disabled, TCP will not nego‐
572 tiate the use of window scaling with the other end during con‐
573 nection setup.
574 tcp_wmem (since Linux 2.4)
576 This is a vector of 3 integers: [min, default, max]. These
577 parameters are used by TCP to regulate send buffer sizes. TCP
578 dynamically adjusts the size of the send buffer from the default
579 values listed below, in the range of these values, depending on
580 memory available.
581 min Minimum size of the send buffer used by each TCP
583 socket. The default value is the system page size.
584 (On Linux 2.4, the default value is 4 kB.) This value
585 is used to ensure that in memory pressure mode, allo‐
586 cations below this size will still succeed. This is
587 not used to bound the size of the send buffer declared
588 using SO_SNDBUF on a socket.
589 default The default size of the send buffer for a TCP socket.
591 This value overwrites the initial default buffer size
592 from the generic global
593 /proc/sys/net/core/wmem_default defined for all proto‐
594 cols. The default value is 16 kB. If larger send
595 buffer sizes are desired, this value should be
596 increased (to affect all sockets). To employ large
597 TCP windows, the /proc/sys/net/ipv4/tcp_window_scaling
598 must be set to a nonzero value (default).
599 max The maximum size of the send buffer used by each TCP
601 socket. This value does not override the value in
602 /proc/sys/net/core/wmem_max. This is not used to
603 limit the size of the send buffer declared using
604 SO_SNDBUF on a socket. The default value is calcu‐
605 lated using the formula
606 max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
608 (On Linux 2.4, the default value is 128 kB, lowered
610 64 kB depending on low-memory systems.)
611 tcp_workaround_signed_windows (Boolean; default: disabled; since Linux
613 2.6.26)
614 If enabled, assume that no receipt of a window-scaling option
615 means that the remote TCP is broken and treats the window as a
616 signed quantity. If disabled, assume that the remote TCP is not
617 broken even if we do not receive a window scaling option from
618 it.
619 Socket options
621 To set or get a TCP socket option, call getsockopt(2) to read or set‐
622 sockopt(2) to write the option with the option level argument set to
623 IPPROTO_TCP. Unless otherwise noted, optval is a pointer to an int.
624 In addition, most IPPROTO_IP socket options are valid on TCP sockets.
625 For more information see ip(7).
626 Following is a list of TCP-specific socket options. For details of
628 some other socket options that are also applicable for TCP sockets, see
629 socket(7).
630 TCP_CONGESTION (since Linux 2.6.13)
632 The argument for this option is a string. This option allows
633 the caller to set the TCP congestion control algorithm to be
634 used, on a per-socket basis. Unprivileged processes are
635 restricted to choosing one of the algorithms in tcp_allowed_con‐
636 gestion_control (described above). Privileged processes
637 (CAP_NET_ADMIN) can choose from any of the available congestion-
638 control algorithms (see the description of tcp_available_conges‐
639 tion_control above).
640 TCP_CORK (since Linux 2.2)
642 If set, don't send out partial frames. All queued partial
643 frames are sent when the option is cleared again. This is use‐
644 ful for prepending headers before calling sendfile(2), or for
645 throughput optimization. As currently implemented, there is a
646 200 millisecond ceiling on the time for which output is corked
647 by TCP_CORK. If this ceiling is reached, then queued data is
648 automatically transmitted. This option can be combined with
649 TCP_NODELAY only since Linux 2.5.71. This option should not be
650 used in code intended to be portable.
651 TCP_DEFER_ACCEPT (since Linux 2.4)
653 Allow a listener to be awakened only when data arrives on the
654 socket. Takes an integer value (seconds), this can bound the
655 maximum number of attempts TCP will make to complete the connec‐
656 tion. This option should not be used in code intended to be
657 portable.
658 TCP_INFO (since Linux 2.4)
660 Used to collect information about this socket. The kernel
661 returns a struct tcp_info as defined in the file
662 /usr/include/linux/tcp.h. This option should not be used in
663 code intended to be portable.
664 TCP_KEEPCNT (since Linux 2.4)
666 The maximum number of keepalive probes TCP should send before
667 dropping the connection. This option should not be used in code
668 intended to be portable.
669 TCP_KEEPIDLE (since Linux 2.4)
671 The time (in seconds) the connection needs to remain idle before
672 TCP starts sending keepalive probes, if the socket option
673 SO_KEEPALIVE has been set on this socket. This option should
674 not be used in code intended to be portable.
675 TCP_KEEPINTVL (since Linux 2.4)
677 The time (in seconds) between individual keepalive probes. This
678 option should not be used in code intended to be portable.
679 TCP_LINGER2 (since Linux 2.4)
681 The lifetime of orphaned FIN_WAIT2 state sockets. This option
682 can be used to override the system-wide setting in the file
683 /proc/sys/net/ipv4/tcp_fin_timeout for this socket. This is not
684 to be confused with the socket(7) level option SO_LINGER. This
685 option should not be used in code intended to be portable.
686 TCP_MAXSEG
688 The maximum segment size for outgoing TCP packets. In Linux 2.2
689 and earlier, and in Linux 2.6.28 and later, if this option is
690 set before connection establishment, it also changes the MSS
691 value announced to the other end in the initial packet. Values
692 greater than the (eventual) interface MTU have no effect. TCP
693 will also impose its minimum and maximum bounds over the value
694 provided.
695 TCP_NODELAY
697 If set, disable the Nagle algorithm. This means that segments
698 are always sent as soon as possible, even if there is only a
699 small amount of data. When not set, data is buffered until
700 there is a sufficient amount to send out, thereby avoiding the
701 frequent sending of small packets, which results in poor uti‐
702 lization of the network. This option is overridden by TCP_CORK;
703 however, setting this option forces an explicit flush of pending
704 output, even if TCP_CORK is currently set.
705 TCP_QUICKACK (since Linux 2.4.4)
707 Enable quickack mode if set or disable quickack mode if cleared.
708 In quickack mode, acks are sent immediately, rather than delayed
709 if needed in accordance to normal TCP operation. This flag is
710 not permanent, it only enables a switch to or from quickack
711 mode. Subsequent operation of the TCP protocol will once again
712 enter/leave quickack mode depending on internal protocol pro‐
713 cessing and factors such as delayed ack timeouts occurring and
714 data transfer. This option should not be used in code intended
715 to be portable.
716 TCP_SYNCNT (since Linux 2.4)
718 Set the number of SYN retransmits that TCP should send before
719 aborting the attempt to connect. It cannot exceed 255. This
720 option should not be used in code intended to be portable.
721 TCP_USER_TIMEOUT (since Linux 2.6.37)
723 This option takes an unsigned int as an argument. When the
724 value is greater than 0, it specifies the maximum amount of time
725 in milliseconds that transmitted data may remain unacknowledged
726 before TCP will forcibly close the corresponding connection and
727 return ETIMEDOUT to the application. If the option value is
728 specified as 0, TCP will use the system default.
729 Increasing user timeouts allows a TCP connection to survive
731 extended periods without end-to-end connectivity. Decreasing
732 user timeouts allows applications to "fail fast", if so desired.
733 Otherwise, failure may take up to 20 minutes with the current
734 system defaults in a normal WAN environment.
735 This option can be set during any state of a TCP connection, but
737 is effective only during the synchronized states of a connection
738 (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, and
739 LAST-ACK). Moreover, when used with the TCP keepalive
740 (SO_KEEPALIVE) option, TCP_USER_TIMEOUT will override keepalive
741 to determine when to close a connection due to keepalive fail‐
742 ure.
743 The option has no effect on when TCP retransmits a packet, nor
745 when a keepalive probe is sent.
746 This option, like many others, will be inherited by the socket
748 returned by accept(2), if it was set on the listening socket.
749 Further details on the user timeout feature can be found in
751 RFC 793 and RFC 5482 ("TCP User Timeout Option").
752 TCP_WINDOW_CLAMP (since Linux 2.4)
754 Bound the size of the advertised window to this value. The ker‐
755 nel imposes a minimum size of SOCK_MIN_RCVBUF/2. This option
756 should not be used in code intended to be portable.
757 Sockets API
759 TCP provides limited support for out-of-band data, in the form of (a
760 single byte of) urgent data. In Linux this means if the other end
761 sends newer out-of-band data the older urgent data is inserted as nor‐
762 mal data into the stream (even when SO_OOBINLINE is not set). This
763 differs from BSD-based stacks.
764 Linux uses the BSD compatible interpretation of the urgent pointer
766 field by default. This violates RFC 1122, but is required for interop‐
767 erability with other stacks. It can be changed via
768 /proc/sys/net/ipv4/tcp_stdurg.
769 It is possible to peek at out-of-band data using the recv(2) MSG_PEEK
771 flag.
772 Since version 2.4, Linux supports the use of MSG_TRUNC in the flags
774 argument of recv(2) (and recvmsg(2)). This flag causes the received
775 bytes of data to be discarded, rather than passed back in a caller-sup‐
776 plied buffer. Since Linux 2.4.4, MSG_TRUNC also has this effect when
777 used in conjunction with MSG_OOB to receive out-of-band data.
778 Ioctls
780 The following ioctl(2) calls return information in value. The correct
781 syntax is:
782 int value;
784 error = ioctl(tcp_socket, ioctl_type, &value);
785 ioctl_type is one of the following:
787 SIOCINQ
789 Returns the amount of queued unread data in the receive buffer.
790 The socket must not be in LISTEN state, otherwise an error (EIN‐
791 VAL) is returned. SIOCINQ is defined in <linux/sockios.h>.
792 Alternatively, you can use the synonymous FIONREAD, defined in
793 <sys/ioctl.h>.
794 SIOCATMARK
796 Returns true (i.e., value is nonzero) if the inbound data stream
797 is at the urgent mark.
798 If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
800 true, then the next read from the socket will return the urgent
801 data. If the SO_OOBINLINE socket option is not set, and SIOCAT‐
802 MARK returns true, then the next read from the socket will
803 return the bytes following the urgent data (to actually read the
804 urgent data requires the recv(MSG_OOB) flag).
805 Note that a read never reads across the urgent mark. If an
807 application is informed of the presence of urgent data via
808 select(2) (using the exceptfds argument) or through delivery of
809 a SIGURG signal, then it can advance up to the mark using a loop
810 which repeatedly tests SIOCATMARK and performs a read (request‐
811 ing any number of bytes) as long as SIOCATMARK returns false.
812 SIOCOUTQ
814 Returns the amount of unsent data in the socket send queue. The
815 socket must not be in LISTEN state, otherwise an error (EINVAL)
816 is returned. SIOCOUTQ is defined in <linux/sockios.h>. Alter‐
817 natively, you can use the synonymous TIOCOUTQ, defined in
818 <sys/ioctl.h>.
819 Error handling
821 When a network error occurs, TCP tries to resend the packet. If it
822 doesn't succeed after some time, either ETIMEDOUT or the last received
823 error on this connection is reported.
824 Some applications require a quicker error notification. This can be
826 enabled with the IPPROTO_IP level IP_RECVERR socket option. When this
827 option is enabled, all incoming errors are immediately passed to the
828 user program. Use this option with care — it makes TCP less tolerant
829 to routing changes and other normal network conditions.
830
832 EAFNOTSUPPORT
833 Passed socket address type in sin_family was not AF_INET.
834 EPIPE The other end closed the socket unexpectedly or a read is exe‐
836 cuted on a shut down socket.
837 ETIMEDOUT
839 The other end didn't acknowledge retransmitted data after some
840 time.
841 Any errors defined for ip(7) or the generic socket layer may also be
843 returned for TCP.
844
846 Support for Explicit Congestion Notification, zero-copy sendfile(2),
847 reordering support and some SACK extensions (DSACK) were introduced in
848 2.4. Support for forward acknowledgement (FACK), TIME_WAIT recycling,
849 and per-connection keepalive socket options were introduced in 2.3.
850
852 Not all errors are documented.
853 IPv6 is not described.
854
856 accept(2), bind(2), connect(2), getsockopt(2), listen(2), recvmsg(2),
857 sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)
858 RFC 793 for the TCP specification.
860 RFC 1122 for the TCP requirements and a description of the Nagle algo‐
861 rithm.
862 RFC 1323 for TCP timestamp and window scaling options.
863 RFC 1337 for a description of TIME_WAIT assassination hazards.
864 RFC 3168 for a description of Explicit Congestion Notification.
865 RFC 2581 for TCP congestion control algorithms.
866 RFC 2018 and RFC 2883 for SACK and extensions to SACK.
867
869 This page is part of release 5.02 of the Linux man-pages project. A
870 description of the project, information about reporting bugs, and the
871 latest version of this page, can be found at
872 https://www.kernel.org/doc/man-pages/.
873Linux 2019-03-06 TCP(7)