1TCP(7)                     Linux Programmer's Manual                    TCP(7)
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3
4

NAME

6       tcp - TCP protocol
7

SYNOPSIS

9       #include <sys/socket.h>
10       #include <netinet/in.h>
11       #include <netinet/tcp.h>
12
13       tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
14

DESCRIPTION

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
24       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
34       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
43       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
56       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
68       When out-of-band data is present, select(2) indicates the file descrip‐
69       tor as having an exceptional condition and poll (2) indicates a POLLPRI
70       event.
71
72       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
78   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
83   /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
91       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
97              0  increase cwnd once per acknowledgment (no ABC)
98
99              1  increase cwnd once per acknowledgment of full sized segment
100
101              2  allow  increase  cwnd by two if acknowledgment is of two seg‐
102                 ments to compensate for delayed acknowledgments.
103
104       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
112       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
118              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
127       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
137       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
147       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
156       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
160              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
164       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
169       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
180       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
185       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
191       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
198       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
203       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
204              Enable RFC 2883 TCP Duplicate SACK support.
205
206       tcp_ecn (Integer; default: see below; since Linux 2.4)
207              Enable RFC 3168 Explicit Congestion Notification.
208
209              This file can have one of the following values:
210
211              0      Disable  ECN.  Neither initiate nor accept ECN.  This was
212                     the default up to and including Linux 2.6.30.
213
214              1      Enable ECN when requested  by  incoming  connections  and
215                     also request ECN on outgoing connection attempts.
216
217              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
221              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
228       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
233       tcp_fack (Boolean; default: enabled; since Linux 2.2)
234              Enable TCP Forward Acknowledgement support.
235
236       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
242       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
249              This file can have one of the following values:
250
251              0  Disabled.   This  was  the  default up to and including Linux
252                 2.6.23.
253
254              1  The basic version F-RTO algorithm is enabled.
255
256              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
262              Before Linux 2.6.22, this parameter was a  Boolean  value,  sup‐
263              porting just values 0 and 1 above.
264
265       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
271              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
275              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
279              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
285       tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
286              The number of seconds between TCP keep-alive probes.
287
288       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
293       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
301              Note that underlying connection tracking mechanisms and applica‐
302              tion timeouts may be much shorter.
303
304       tcp_low_latency (Boolean; default: disabled;  since  Linux  2.4.21/2.6;
305       obsolete since Linux 4.14)
306              If  enabled,  the  TCP  stack  makes decisions that prefer lower
307              latency as opposed to higher throughput.  It this option is dis‐
308              abled,  then  higher  throughput is preferred.  An example of an
309              application where this default should  be  changed  would  be  a
310              Beowulf  compute  cluster.   Since  Linux  4.14, this file still
311              exists, but its value is ignored.
312
313       tcp_max_orphans (integer; default: see below; since Linux 2.4)
314              The maximum number of orphaned (not attached to  any  user  file
315              handle)  TCP sockets allowed in the system.  When this number is
316              exceeded, the orphaned connection is  reset  and  a  warning  is
317              printed.   This  limit  exists only to prevent simple denial-of-
318              service attacks.  Lowering this limit is not recommended.   Net‐
319              work  conditions  might  require  you  to increase the number of
320              orphans allowed, but note that each orphan can eat up to  ~64 kB
321              of  unswappable  memory.  The default initial value is set equal
322              to the  kernel  parameter  NR_FILE.   This  initial  default  is
323              adjusted depending on the memory in the system.
324
325       tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
326              The  maximum  number  of  queued  connection requests which have
327              still  not  received  an  acknowledgement  from  the  connecting
328              client.  If this number is exceeded, the kernel will begin drop‐
329              ping requests.  The default value of 256 is  increased  to  1024
330              when the memory present in the system is adequate or greater (>=
331              128 MB), and reduced to 128 for those systems with very low mem‐
332              ory (<= 32 MB).
333
334              Prior to Linux 2.6.20, it was recommended that if this needed to
335              be increased above 1024, the  size  of  the  SYNACK  hash  table
336              (TCP_SYNQ_HSIZE) in include/net/tcp.h should be modified to keep
337
338                  TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
339
340              and the kernel should be recompiled.  In Linux 2.6.20, the fixed
341              sized TCP_SYNQ_HSIZE was removed in favor of dynamic sizing.
342
343       tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
344              The maximum number of sockets in TIME_WAIT state allowed in  the
345              system.  This limit exists only to prevent simple denial-of-ser‐
346              vice attacks.   The  default  value  of  NR_FILE*2  is  adjusted
347              depending  on  the  memory  in  the  system.   If this number is
348              exceeded, the socket is closed and a warning is printed.
349
350       tcp_moderate_rcvbuf   (Boolean;   default:   enabled;    since    Linux
351       2.4.17/2.6.7)
352              If  enabled, TCP performs receive buffer auto-tuning, attempting
353              to automatically size the buffer (no greater  than  tcp_rmem[2])
354              to match the size required by the path for full throughput.
355
356       tcp_mem (since Linux 2.4)
357              This  is  a  vector of 3 integers: [low, pressure, high].  These
358              bounds, measured in units of the system page size, are  used  by
359              TCP  to  track its memory usage.  The defaults are calculated at
360              boot time from the amount of available memory.   (TCP  can  only
361              use  low  memory  for  this,  which  is  limited  to  around 900
362              megabytes on 32-bit systems.  64-bit systems do not suffer  this
363              limitation.)
364
365              low    TCP  doesn't regulate its memory allocation when the num‐
366                     ber of pages it has allocated globally is below this num‐
367                     ber.
368
369              pressure
370                     When  the  amount of memory allocated by TCP exceeds this
371                     number of pages, TCP moderates  its  memory  consumption.
372                     This  memory  pressure state is exited once the number of
373                     pages allocated falls below the low mark.
374
375              high   The maximum number of  pages,  globally,  that  TCP  will
376                     allocate.   This value overrides any other limits imposed
377                     by the kernel.
378
379       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
380              This parameter controls TCP Packetization-Layer Path MTU Discov‐
381              ery.  The following values may be assigned to the file:
382
383              0  Disabled
384
385              1  Disabled by default, enabled when an ICMP black hole detected
386
387              2  Always enabled, use initial MSS of tcp_base_mss.
388
389       tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
390              By  default,  TCP  saves various connection metrics in the route
391              cache when the connection closes,  so  that  connections  estab‐
392              lished  in  the  near future can use these to set initial condi‐
393              tions.  Usually, this increases overall performance, but it  may
394              sometimes cause performance degradation.  If tcp_no_metrics_save
395              is enabled, TCP will not cache metrics on closing connections.
396
397       tcp_orphan_retries (integer; default: 8; since Linux 2.4)
398              The maximum number of attempts made to probe the other end of  a
399              connection which has been closed by our end.
400
401       tcp_reordering (integer; default: 3; since Linux 2.4)
402              The  maximum  a  packet  can be reordered in a TCP packet stream
403              without TCP assuming packet loss and going into slow start.   It
404              is  not  advisable  to  change  this  number.   This is a packet
405              reordering detection metric  designed  to  minimize  unnecessary
406              back  off and retransmits provoked by reordering of packets on a
407              connection.
408
409       tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
410              Try to send full-sized packets during retransmit.
411
412       tcp_retries1 (integer; default: 3; since Linux 2.2)
413              The number of times TCP will attempt to retransmit a  packet  on
414              an  established connection normally, without the extra effort of
415              getting the network layers involved.  Once we exceed this number
416              of retransmits, we first have the network layer update the route
417              if possible before each new retransmit.  The default is the  RFC
418              specified minimum of 3.
419
420       tcp_retries2 (integer; default: 15; since Linux 2.2)
421              The  maximum  number  of  times a TCP packet is retransmitted in
422              established state before giving up.  The default  value  is  15,
423              which  corresponds  to a duration of approximately between 13 to
424              30  minutes,  depending  on  the  retransmission  timeout.   The
425              RFC 1122  specified  minimum  limit  of 100 seconds is typically
426              deemed too short.
427
428       tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
429              Enable TCP behavior conformant with RFC 1337.  When disabled, if
430              a  RST is received in TIME_WAIT state, we close the socket imme‐
431              diately without waiting for the end of the TIME_WAIT period.
432
433       tcp_rmem (since Linux 2.4)
434              This is a vector of 3  integers:  [min,  default,  max].   These
435              parameters  are  used  by  TCP to regulate receive buffer sizes.
436              TCP dynamically adjusts the size of the receive buffer from  the
437              defaults  listed  below, in the range of these values, depending
438              on memory available in the system.
439
440              min    minimum size of the  receive  buffer  used  by  each  TCP
441                     socket.   The default value is the system page size.  (On
442                     Linux  2.4,  the  default  value  is  4 kB,  lowered   to
443                     PAGE_SIZE  bytes  in  low-memory systems.)  This value is
444                     used to ensure that in memory pressure mode,  allocations
445                     below  this size will still succeed.  This is not used to
446                     bound the size  of  the  receive  buffer  declared  using
447                     SO_RCVBUF on a socket.
448
449              default
450                     the  default size of the receive buffer for a TCP socket.
451                     This value overwrites the  initial  default  buffer  size
452                     from the generic global net.core.rmem_default defined for
453                     all protocols.  The default value is  87380  bytes.   (On
454                     Linux  2.4,  this  will be lowered to 43689 in low-memory
455                     systems.)  If larger receive buffer  sizes  are  desired,
456                     this  value  should be increased (to affect all sockets).
457                     To  employ  large  TCP  windows,  the   net.ipv4.tcp_win‐
458                     dow_scaling must be enabled (default).
459
460              max    the  maximum  size of the receive buffer used by each TCP
461                     socket.   This  value  does  not  override   the   global
462                     net.core.rmem_max.  This is not used to limit the size of
463                     the receive buffer declared using SO_RCVBUF on a  socket.
464                     The default value is calculated using the formula
465
466                         max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
467
468                     (On  Linux  2.4, the default is 87380*2 bytes, lowered to
469                     87380 in low-memory systems).
470
471       tcp_sack (Boolean; default: enabled; since Linux 2.2)
472              Enable RFC 2018 TCP Selective Acknowledgements.
473
474       tcp_slow_start_after_idle  (Boolean;  default:  enabled;  since   Linux
475       2.6.18)
476              If  enabled,  provide RFC 2861 behavior and time out the conges‐
477              tion window after an idle period.  An idle period is defined  as
478              the current RTO (retransmission timeout).  If disabled, the con‐
479              gestion window will not be timed out after an idle period.
480
481       tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
482              If this option is enabled, then use the RFC 1122  interpretation
483              of  the TCP urgent-pointer field.  According to this interpreta‐
484              tion, the urgent pointer points to the last byte of urgent data.
485              If  this  option is disabled, then use the BSD-compatible inter‐
486              pretation of the urgent pointer: the urgent  pointer  points  to
487              the  first byte after the urgent data.  Enabling this option may
488              lead to interoperability problems.
489
490       tcp_syn_retries (integer; default: 6; since Linux 2.2)
491              The maximum number of times initial SYNs for an active TCP  con‐
492              nection attempt will be retransmitted.  This value should not be
493              higher than 255.  The default value is 6, which  corresponds  to
494              retrying for up to approximately 127 seconds.  Before Linux 3.7,
495              the default value was 5, which (in conjunction with  calculation
496              based  on other kernel parameters) corresponded to approximately
497              180 seconds.
498
499       tcp_synack_retries (integer; default: 5; since Linux 2.2)
500              The maximum number of times a SYN/ACK segment for a passive  TCP
501              connection  will  be  retransmitted.   This number should not be
502              higher than 255.
503
504       tcp_syncookies (Boolean; since Linux 2.2)
505              Enable TCP syncookies.  The kernel must be  compiled  with  CON‐
506              FIG_SYN_COOKIES.  Send out syncookies when the syn backlog queue
507              of a socket overflows.  The syncookies feature attempts to  pro‐
508              tect a socket from a SYN flood attack.  This should be used as a
509              last resort, if at all.  This is a violation of the  TCP  proto‐
510              col,  and  conflicts  with other areas of TCP such as TCP exten‐
511              sions.  It can cause problems for clients and relays.  It is not
512              recommended  as a tuning mechanism for heavily loaded servers to
513              help with overloaded or misconfigured  conditions.   For  recom‐
514              mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
515              and tcp_abort_on_overflow.
516
517       tcp_timestamps (integer; default: 1; since Linux 2.2)
518              Set to one of the following values to enable or disable RFC 1323
519              TCP timestamps:
520
521              0  Disable timestamps.
522
523              1  Enable timestamps as defined in RFC1323 and use random offset
524                 for each connection rather than only using the current time.
525
526              2  As for the value 1,  but  without  random  offsets.   Setting
527                 tcp_timestamps to this value is meaningful since Linux 4.10.
528
529       tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
530              This parameter controls what percentage of the congestion window
531              can be consumed by  a  single  TCP  Segmentation  Offload  (TSO)
532              frame.   The  setting  of  this  parameter is a tradeoff between
533              burstiness and building larger TSO frames.
534
535       tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11)
536              Enable fast  recycling  of  TIME_WAIT  sockets.   Enabling  this
537              option is not recommended as the remote IP may not use monotoni‐
538              cally increasing timestamps (devices behind  NAT,  devices  with
539              per-connection  timestamp offsets).  See RFC 1323 (PAWS) and RFC
540              6191.
541
542       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
543              Allow to reuse TIME_WAIT sockets for new connections when it  is
544              safe  from protocol viewpoint.  It should not be changed without
545              advice/request of technical experts.
546
547       tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
548              Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is a
549              sender-side-only  change  to  TCP  that anticipates the onset of
550              congestion by estimating the bandwidth.  TCP Vegas  adjusts  the
551              sending  rate  by  modifying  the  congestion window.  TCP Vegas
552              should provide less packet loss, but it is not as aggressive  as
553              TCP Reno.
554
555       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
556              Enable  TCP  Westwood+  congestion control algorithm.  TCP West‐
557              wood+ is a sender-side-only modification of the TCP Reno  proto‐
558              col  stack that optimizes the performance of TCP congestion con‐
559              trol.  It is based on end-to-end  bandwidth  estimation  to  set
560              congestion  window  and  slow start threshold after a congestion
561              episode.  Using this estimation, TCP Westwood+ adaptively sets a
562              slow  start  threshold  and a congestion window which takes into
563              account the bandwidth used at the  time  congestion  is  experi‐
564              enced.   TCP  Westwood+  significantly  increases  fairness with
565              respect to TCP Reno in wired networks and throughput over  wire‐
566              less links.
567
568       tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
569              Enable RFC 1323 TCP window scaling.  This feature allows the use
570              of a large window (> 64 kB) on  a  TCP  connection,  should  the
571              other  end support it.  Normally, the 16 bit window length field
572              in the TCP header limits the window size to less than 64 kB.  If
573              larger  windows  are desired, applications can increase the size
574              of their socket buffers and the window scaling  option  will  be
575              employed.  If tcp_window_scaling is disabled, TCP will not nego‐
576              tiate the use of window scaling with the other end  during  con‐
577              nection setup.
578
579       tcp_wmem (since Linux 2.4)
580              This  is  a  vector  of  3 integers: [min, default, max].  These
581              parameters are used by TCP to regulate send buffer  sizes.   TCP
582              dynamically adjusts the size of the send buffer from the default
583              values listed below, in the range of these values, depending  on
584              memory available.
585
586              min    Minimum  size of the send buffer used by each TCP socket.
587                     The default value is the system  page  size.   (On  Linux
588                     2.4,  the  default value is 4 kB.)  This value is used to
589                     ensure that in memory pressure  mode,  allocations  below
590                     this  size will still succeed.  This is not used to bound
591                     the size of the send buffer declared using SO_SNDBUF on a
592                     socket.
593
594              default
595                     The  default  size  of  the send buffer for a TCP socket.
596                     This value overwrites the  initial  default  buffer  size
597                     from  the  generic global /proc/sys/net/core/wmem_default
598                     defined for all protocols.  The default value  is  16 kB.
599                     If  larger  send  buffer  sizes  are  desired, this value
600                     should be increased (to affect all sockets).   To  employ
601                     large   TCP   windows,   the  /proc/sys/net/ipv4/tcp_win‐
602                     dow_scaling must be set to a nonzero value (default).
603
604              max    The maximum size of the send  buffer  used  by  each  TCP
605                     socket.   This  value  does  not  override  the  value in
606                     /proc/sys/net/core/wmem_max.  This is not used  to  limit
607                     the size of the send buffer declared using SO_SNDBUF on a
608                     socket.  The default value is calculated using  the  for‐
609                     mula
610
611                         max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
612
613                     (On Linux 2.4, the default value is 128 kB, lowered 64 kB
614                     depending on low-memory systems.)
615
616       tcp_workaround_signed_windows (Boolean; default: disabled; since  Linux
617       2.6.26)
618              If  enabled,  assume  that no receipt of a window-scaling option
619              means that the remote TCP is broken and treats the window  as  a
620              signed quantity.  If disabled, assume that the remote TCP is not
621              broken even if we do not receive a window  scaling  option  from
622              it.
623
624   Socket options
625       To  set  or get a TCP socket option, call getsockopt(2) to read or set‐
626       sockopt(2) to write the option with the option level  argument  set  to
627       IPPROTO_TCP.   Unless  otherwise  noted, optval is a pointer to an int.
628       In addition, most IPPROTO_IP socket options are valid on  TCP  sockets.
629       For more information see ip(7).
630
631       Following  is  a  list  of TCP-specific socket options.  For details of
632       some other socket options that are also applicable for TCP sockets, see
633       socket(7).
634
635       TCP_CONGESTION (since Linux 2.6.13)
636              The  argument  for  this option is a string.  This option allows
637              the caller to set the TCP congestion  control  algorithm  to  be
638              used,   on  a  per-socket  basis.   Unprivileged  processes  are
639              restricted to choosing one of the algorithms in tcp_allowed_con‐
640              gestion_control   (described   above).    Privileged   processes
641              (CAP_NET_ADMIN) can choose from any of the available congestion-
642              control algorithms (see the description of tcp_available_conges‐
643              tion_control above).
644
645       TCP_CORK (since Linux 2.2)
646              If set, don't send  out  partial  frames.   All  queued  partial
647              frames  are sent when the option is cleared again.  This is use‐
648              ful for prepending headers before calling  sendfile(2),  or  for
649              throughput  optimization.   As currently implemented, there is a
650              200 millisecond ceiling on the time for which output  is  corked
651              by  TCP_CORK.   If  this ceiling is reached, then queued data is
652              automatically transmitted.  This option  can  be  combined  with
653              TCP_NODELAY  only since Linux 2.5.71.  This option should not be
654              used in code intended to be portable.
655
656       TCP_DEFER_ACCEPT (since Linux 2.4)
657              Allow a listener to be awakened only when data  arrives  on  the
658              socket.   Takes  an  integer value (seconds), this can bound the
659              maximum number of attempts TCP will make to complete the connec‐
660              tion.   This  option  should  not be used in code intended to be
661              portable.
662
663       TCP_INFO (since Linux 2.4)
664              Used to collect  information  about  this  socket.   The  kernel
665              returns   a   struct   tcp_info   as   defined   in   the   file
666              /usr/include/linux/tcp.h.  This option should  not  be  used  in
667              code intended to be portable.
668
669       TCP_KEEPCNT (since Linux 2.4)
670              The  maximum  number  of keepalive probes TCP should send before
671              dropping the connection.  This option should not be used in code
672              intended to be portable.
673
674       TCP_KEEPIDLE (since Linux 2.4)
675              The time (in seconds) the connection needs to remain idle before
676              TCP starts  sending  keepalive  probes,  if  the  socket  option
677              SO_KEEPALIVE  has  been  set on this socket.  This option should
678              not be used in code intended to be portable.
679
680       TCP_KEEPINTVL (since Linux 2.4)
681              The time (in seconds) between individual keepalive probes.  This
682              option should not be used in code intended to be portable.
683
684       TCP_LINGER2 (since Linux 2.4)
685              The  lifetime  of orphaned FIN_WAIT2 state sockets.  This option
686              can be used to override the  system-wide  setting  in  the  file
687              /proc/sys/net/ipv4/tcp_fin_timeout for this socket.  This is not
688              to be confused with the socket(7) level option SO_LINGER.   This
689              option should not be used in code intended to be portable.
690
691       TCP_MAXSEG
692              The maximum segment size for outgoing TCP packets.  In Linux 2.2
693              and earlier, and in Linux 2.6.28 and later, if  this  option  is
694              set  before  connection  establishment,  it also changes the MSS
695              value announced to the other end in the initial packet.   Values
696              greater  than  the (eventual) interface MTU have no effect.  TCP
697              will also impose its minimum and maximum bounds over  the  value
698              provided.
699
700       TCP_NODELAY
701              If  set,  disable the Nagle algorithm.  This means that segments
702              are always sent as soon as possible, even if  there  is  only  a
703              small  amount  of  data.   When  not set, data is buffered until
704              there is a sufficient amount to send out, thereby  avoiding  the
705              frequent  sending  of  small packets, which results in poor uti‐
706              lization of the network.  This option is overridden by TCP_CORK;
707              however, setting this option forces an explicit flush of pending
708              output, even if TCP_CORK is currently set.
709
710       TCP_QUICKACK (since Linux 2.4.4)
711              Enable quickack mode if set or disable quickack mode if cleared.
712              In quickack mode, acks are sent immediately, rather than delayed
713              if needed in accordance to normal TCP operation.  This  flag  is
714              not  permanent,  it  only  enables  a switch to or from quickack
715              mode.  Subsequent operation of the TCP protocol will once  again
716              enter/leave  quickack  mode  depending on internal protocol pro‐
717              cessing and factors such as delayed ack timeouts  occurring  and
718              data  transfer.  This option should not be used in code intended
719              to be portable.
720
721       TCP_SYNCNT (since Linux 2.4)
722              Set the number of SYN retransmits that TCP  should  send  before
723              aborting  the  attempt  to connect.  It cannot exceed 255.  This
724              option should not be used in code intended to be portable.
725
726       TCP_USER_TIMEOUT (since Linux 2.6.37)
727              This option takes an unsigned int  as  an  argument.   When  the
728              value is greater than 0, it specifies the maximum amount of time
729              in milliseconds that transmitted data may remain  unacknowledged
730              before  TCP will forcibly close the corresponding connection and
731              return ETIMEDOUT to the application.  If  the  option  value  is
732              specified as 0, TCP will use the system default.
733
734              Increasing  user  timeouts  allows  a  TCP connection to survive
735              extended periods without  end-to-end  connectivity.   Decreasing
736              user timeouts allows applications to "fail fast", if so desired.
737              Otherwise, failure may take up to 20 minutes  with  the  current
738              system defaults in a normal WAN environment.
739
740              This option can be set during any state of a TCP connection, but
741              is effective only during the synchronized states of a connection
742              (ESTABLISHED,  FIN-WAIT-1,  FIN-WAIT-2, CLOSE-WAIT, CLOSING, and
743              LAST-ACK).   Moreover,  when  used  with   the   TCP   keepalive
744              (SO_KEEPALIVE)  option, TCP_USER_TIMEOUT will override keepalive
745              to determine when to close a connection due to  keepalive  fail‐
746              ure.
747
748              The  option  has no effect on when TCP retransmits a packet, nor
749              when a keepalive probe is sent.
750
751              This option, like many others, will be inherited by  the  socket
752              returned by accept(2), if it was set on the listening socket.
753
754              Further  details  on  the  user  timeout feature can be found in
755              RFC 793 and RFC 5482 ("TCP User Timeout Option").
756
757       TCP_WINDOW_CLAMP (since Linux 2.4)
758              Bound the size of the advertised window to this value.  The ker‐
759              nel  imposes  a  minimum size of SOCK_MIN_RCVBUF/2.  This option
760              should not be used in code intended to be portable.
761
762   Sockets API
763       TCP provides limited support for out-of-band data, in the  form  of  (a
764       single  byte  of)  urgent  data.   In Linux this means if the other end
765       sends newer out-of-band data the older urgent data is inserted as  nor‐
766       mal  data  into  the  stream (even when SO_OOBINLINE is not set).  This
767       differs from BSD-based stacks.
768
769       Linux uses the BSD compatible  interpretation  of  the  urgent  pointer
770       field by default.  This violates RFC 1122, but is required for interop‐
771       erability   with   other   stacks.    It    can    be    changed    via
772       /proc/sys/net/ipv4/tcp_stdurg.
773
774       It  is  possible to peek at out-of-band data using the recv(2) MSG_PEEK
775       flag.
776
777       Since version 2.4, Linux supports the use of  MSG_TRUNC  in  the  flags
778       argument  of  recv(2)  (and recvmsg(2)).  This flag causes the received
779       bytes of data to be discarded, rather than passed back in a caller-sup‐
780       plied  buffer.   Since Linux 2.4.4, MSG_TRUNC also has this effect when
781       used in conjunction with MSG_OOB to receive out-of-band data.
782
783   Ioctls
784       The following ioctl(2) calls return information in value.  The  correct
785       syntax is:
786
787              int value;
788              error = ioctl(tcp_socket, ioctl_type, &value);
789
790       ioctl_type is one of the following:
791
792       SIOCINQ
793              Returns  the amount of queued unread data in the receive buffer.
794              The socket must not be in LISTEN state, otherwise an error (EIN‐
795              VAL)  is  returned.   SIOCINQ  is  defined in <linux/sockios.h>.
796              Alternatively, you can use the synonymous FIONREAD,  defined  in
797              <sys/ioctl.h>.
798
799       SIOCATMARK
800              Returns true (i.e., value is nonzero) if the inbound data stream
801              is at the urgent mark.
802
803              If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
804              true,  then the next read from the socket will return the urgent
805              data.  If the SO_OOBINLINE socket option is not set, and SIOCAT‐
806              MARK  returns  true,  then  the  next  read from the socket will
807              return the bytes following the urgent data (to actually read the
808              urgent data requires the recv(MSG_OOB) flag).
809
810              Note  that  a  read  never  reads across the urgent mark.  If an
811              application is informed of  the  presence  of  urgent  data  via
812              select(2)  (using the exceptfds argument) or through delivery of
813              a SIGURG signal, then it can advance up to the mark using a loop
814              which  repeatedly tests SIOCATMARK and performs a read (request‐
815              ing any number of bytes) as long as SIOCATMARK returns false.
816
817       SIOCOUTQ
818              Returns the amount of unsent data in the socket send queue.  The
819              socket  must not be in LISTEN state, otherwise an error (EINVAL)
820              is returned.  SIOCOUTQ is defined in <linux/sockios.h>.   Alter‐
821              natively,  you  can  use  the  synonymous  TIOCOUTQ,  defined in
822              <sys/ioctl.h>.
823
824   Error handling
825       When a network error occurs, TCP tries to resend  the  packet.   If  it
826       doesn't  succeed after some time, either ETIMEDOUT or the last received
827       error on this connection is reported.
828
829       Some applications require a quicker error notification.   This  can  be
830       enabled  with the IPPROTO_IP level IP_RECVERR socket option.  When this
831       option is enabled, all incoming errors are immediately  passed  to  the
832       user  program.   Use this option with care — it makes TCP less tolerant
833       to routing changes and other normal network conditions.
834

ERRORS

836       EAFNOTSUPPORT
837              Passed socket address type in sin_family was not AF_INET.
838
839       EPIPE  The other end closed the socket unexpectedly or a read  is  exe‐
840              cuted on a shut down socket.
841
842       ETIMEDOUT
843              The  other  end didn't acknowledge retransmitted data after some
844              time.
845
846       Any errors defined for ip(7) or the generic socket layer  may  also  be
847       returned for TCP.
848

VERSIONS

850       Support  for  Explicit  Congestion Notification, zero-copy sendfile(2),
851       reordering support and some SACK extensions (DSACK) were introduced  in
852       2.4.   Support for forward acknowledgement (FACK), TIME_WAIT recycling,
853       and per-connection keepalive socket options were introduced in 2.3.
854

BUGS

856       Not all errors are documented.
857
858       IPv6 is not described.
859

SEE ALSO

861       accept(2), bind(2), connect(2), getsockopt(2),  listen(2),  recvmsg(2),
862       sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)
863
864       The kernel source file Documentation/networking/ip-sysctl.txt.
865
866       RFC 793 for the TCP specification.
867       RFC 1122  for the TCP requirements and a description of the Nagle algo‐
868       rithm.
869       RFC 1323 for TCP timestamp and window scaling options.
870       RFC 1337 for a description of TIME_WAIT assassination hazards.
871       RFC 3168 for a description of Explicit Congestion Notification.
872       RFC 2581 for TCP congestion control algorithms.
873       RFC 2018 and RFC 2883 for SACK and extensions to SACK.
874

COLOPHON

876       This page is part of release 5.07 of the Linux  man-pages  project.   A
877       description  of  the project, information about reporting bugs, and the
878       latest    version    of    this    page,    can     be     found     at
879       https://www.kernel.org/doc/man-pages/.
880
881
882
883Linux                             2020-06-09                            TCP(7)
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