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


6       tcp - TCP protocol


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


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.
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.
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.
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.
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).
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.
72       Linux  2.4  introduced  a number of changes for improved throughput and
73       scaling, as well as enhanced functionality.  Some of these features in‐
74       clude  support for zero-copy sendfile(2), Explicit Congestion Notifica‐
75       tion, new management of TIME_WAIT sockets,  keep-alive  socket  options
76       and support for Duplicate SACK extensions.
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.
83   /proc interfaces
84       System-wide  TCP parameter settings can be accessed by files in the di‐
85       rectory /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.
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 acknowledgements.  Possible values
95              are:
97              0  increase cwnd once per acknowledgement (no ABC)
99              1  increase cwnd once per acknowledgement of full sized segment
101              2  allow  increase cwnd by two if acknowledgement is of two seg‐
102                 ments to compensate for delayed acknowledgements.
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.   En‐
110              abling this option can harm the clients of your server.
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
115              bytes-bytes/2^(-tcp_adv_win_scale), if tcp_adv_win_scale is less
116              than or equal to zero.
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 im‐
124              plies that the space used for  the  application  buffer  is  one
125              fourth that of the total.
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.
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.   Ap‐
143              plications  can  still  use the TCP_CORK socket option to obtain
144              optimal behavior when they know how/when to uncork  their  sock‐
145              ets.
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.
156       tcp_app_win (integer; default: 31; since Linux 2.4)
157              This  variable  defines how many bytes of the TCP window are re‐
158              served for buffering overhead.
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.
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 en‐
167              abled, this is the initial MSS used by the connection.
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.
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 be‐
183              haves the same as the default TCP Reno.
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.
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  ad‐
194              ditional choices may be available depending on kernel configura‐
195              tion.  The default value for this file is set as part of  kernel
196              configuration.
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.
203       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
204              Enable RFC 2883 TCP Duplicate SACK support.
206       tcp_ecn (Integer; default: see below; since Linux 2.4)
207              Enable RFC 3168 Explicit Congestion Notification.
209              This file can have one of the following values:
211              0      Disable  ECN.  Neither initiate nor accept ECN.  This was
212                     the default up to and including Linux 2.6.30.
214              1      Enable ECN when requested  by  incoming  connections  and
215                     also request ECN on outgoing connection attempts.
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.
221              When  enabled,  connectivity  to  some destinations could be af‐
222              fected 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.
228       tcp_ecn_fallback (Boolean; default: enabled; since Linux 4.1)
229              Enable RFC 3168, Section fallback.  When enabled,  out‐
230              going  ECN-setup  SYNs  that  time out within the normal SYN re‐
231              transmission timeout will be resent with CWR and ECE cleared.
233       tcp_fack (Boolean; default: enabled; since Linux 2.2)
234              Enable TCP Forward Acknowledgement support.
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.
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.
249              This file can have one of the following values:
251              0  Disabled.   This  was  the  default up to and including Linux
252                 2.6.23.
254              1  The basic version F-RTO algorithm is enabled.
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.
262              Before Linux 2.6.22, this parameter was a  Boolean  value,  sup‐
263              porting just values 0 and 1 above.
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:
271              0  Rate halving based; a smooth and conservative  response,  re‐
272                 sults  in  halved  congestion  window  (cwnd)  and slow-start
273                 threshold (ssthresh) after one RTT.
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.
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.
285       tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
286              The number of seconds between TCP keep-alive probes.
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.
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.
301              Note that underlying connection tracking mechanisms and applica‐
302              tion timeouts may be much shorter.
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 la‐
307              tency 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  Be‐
310              owulf  compute  cluster.   Since Linux 4.14, this file still ex‐
311              ists, but its value is ignored.
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 or‐
320              phans allowed, but note that each orphan can eat up to ~64 kB of
321              unswappable  memory.   The default initial value is set equal to
322              the kernel parameter NR_FILE.  This initial default is  adjusted
323              depending on the memory in the system.
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).
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
338                  TCP_SYNQ_HSIZE * 16 <= tcp_max_syn_backlog
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.
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  de‐
347              pending  on  the  memory  in  the system.  If this number is ex‐
348              ceeded, the socket is closed and a warning is printed.
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.
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.)
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.
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.
375              high   The maximum number of pages, globally, that TCP will  al‐
376                     locate.  This value overrides any other limits imposed by
377                     the kernel.
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:
383              0  Disabled
385              1  Disabled by default, enabled when an ICMP black hole detected
387              2  Always enabled, use initial MSS of tcp_base_mss.
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.
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.
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 re‐
405              ordering detection metric designed to minimize unnecessary  back
406              off  and retransmits provoked by reordering of packets on a con‐
407              nection.
409       tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
410              Try to send full-sized packets during retransmit.
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.
420       tcp_retries2 (integer; default: 15; since Linux 2.2)
421              The maximum number of times a TCP packet is retransmitted in es‐
422              tablished 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.
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.
433       tcp_rmem (since Linux 2.4)
434              This is a vector of 3 integers: [min, default, max].  These  pa‐
435              rameters  are used by TCP to regulate receive buffer sizes.  TCP
436              dynamically adjusts the size of the receive buffer from the  de‐
437              faults  listed below, in the range of these values, depending on
438              memory available in the system.
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.
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).
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
466                         max(87380, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
468                     (On  Linux  2.4, the default is 87380*2 bytes, lowered to
469                     87380 in low-memory systems).
471       tcp_sack (Boolean; default: enabled; since Linux 2.2)
472              Enable RFC 2018 TCP Selective Acknowledgements.
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.
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.
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.
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.
504       tcp_syncookies (integer; default: 1; since Linux 2.2)
505              Enable TCP syncookies.  The kernel must be  compiled  with  CON‐
506              FIG_SYN_COOKIES.   The  syncookies feature attempts to protect a
507              socket from a SYN flood attack.  This should be used as  a  last
508              resort, if at all.  This is a violation of the TCP protocol, and
509              conflicts with other areas of TCP such as  TCP  extensions.   It
510              can  cause  problems  for  clients and relays.  It is not recom‐
511              mended as a tuning mechanism for heavily loaded servers to  help
512              with  overloaded  or  misconfigured conditions.  For recommended
513              alternatives see  tcp_max_syn_backlog,  tcp_synack_retries,  and
514              tcp_abort_on_overflow.  Set to one of the following values:
516              0  Disable TCP syncookies.
518              1  Send  out  syncookies  when the syn backlog queue of a socket
519                 overflows.
521              2  (since Linux 3.12) Send out syncookies unconditionally.  This
522                 can be useful for network testing.
524       tcp_timestamps (integer; default: 1; since Linux 2.2)
525              Set to one of the following values to enable or disable RFC 1323
526              TCP timestamps:
528              0  Disable timestamps.
530              1  Enable timestamps as defined in RFC1323 and use random offset
531                 for each connection rather than only using the current time.
533              2  As  for  the  value  1,  but without random offsets.  Setting
534                 tcp_timestamps to this value is meaningful since Linux 4.10.
536       tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
537              This parameter controls what percentage of the congestion window
538              can  be  consumed  by  a  single  TCP Segmentation Offload (TSO)
539              frame.  The setting of this  parameter  is  a  tradeoff  between
540              burstiness and building larger TSO frames.
542       tcp_tw_recycle (Boolean; default: disabled; Linux 2.4 to 4.11)
543              Enable  fast  recycling of TIME_WAIT sockets.  Enabling this op‐
544              tion is not recommended as the remote IP may not  use  monotoni‐
545              cally  increasing  timestamps  (devices behind NAT, devices with
546              per-connection timestamp offsets).  See RFC 1323 (PAWS) and  RFC
547              6191.
549       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
550              Allow  to reuse TIME_WAIT sockets for new connections when it is
551              safe from protocol viewpoint.  It should not be changed  without
552              advice/request of technical experts.
554       tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
555              Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is a
556              sender-side-only change to TCP that  anticipates  the  onset  of
557              congestion  by  estimating the bandwidth.  TCP Vegas adjusts the
558              sending rate by modifying  the  congestion  window.   TCP  Vegas
559              should  provide less packet loss, but it is not as aggressive as
560              TCP Reno.
562       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
563              Enable TCP Westwood+ congestion control  algorithm.   TCP  West‐
564              wood+  is a sender-side-only modification of the TCP Reno proto‐
565              col stack that optimizes the performance of TCP congestion  con‐
566              trol.   It  is  based  on end-to-end bandwidth estimation to set
567              congestion window and slow start threshold  after  a  congestion
568              episode.  Using this estimation, TCP Westwood+ adaptively sets a
569              slow start threshold and a congestion window  which  takes  into
570              account  the  bandwidth  used  at the time congestion is experi‐
571              enced.  TCP Westwood+ significantly increases fairness with  re‐
572              spect to TCP Reno in wired networks and throughput over wireless
573              links.
575       tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
576              Enable RFC 1323 TCP window scaling.  This feature allows the use
577              of  a  large  window  (>  64 kB) on a TCP connection, should the
578              other end support it.  Normally, the 16 bit window length  field
579              in the TCP header limits the window size to less than 64 kB.  If
580              larger windows are desired, applications can increase  the  size
581              of  their  socket  buffers and the window scaling option will be
582              employed.  If tcp_window_scaling is disabled, TCP will not nego‐
583              tiate  the  use of window scaling with the other end during con‐
584              nection setup.
586       tcp_wmem (since Linux 2.4)
587              This is a vector of 3 integers: [min, default, max].  These  pa‐
588              rameters are used by TCP to regulate send buffer sizes.  TCP dy‐
589              namically adjusts the size of the send buffer from  the  default
590              values  listed below, in the range of these values, depending on
591              memory available.
593              min    Minimum size of the send buffer used by each TCP  socket.
594                     The  default  value  is  the system page size.  (On Linux
595                     2.4, the default value is 4 kB.)  This value is  used  to
596                     ensure  that  in  memory pressure mode, allocations below
597                     this size will still succeed.  This is not used to  bound
598                     the size of the send buffer declared using SO_SNDBUF on a
599                     socket.
601              default
602                     The default size of the send buffer  for  a  TCP  socket.
603                     This  value  overwrites  the  initial default buffer size
604                     from the generic  global  /proc/sys/net/core/wmem_default
605                     defined  for  all protocols.  The default value is 16 kB.
606                     If larger send  buffer  sizes  are  desired,  this  value
607                     should  be  increased (to affect all sockets).  To employ
608                     large  TCP   windows,   the   /proc/sys/net/ipv4/tcp_win‐
609                     dow_scaling must be set to a nonzero value (default).
611              max    The  maximum  size  of  the  send buffer used by each TCP
612                     socket.  This  value  does  not  override  the  value  in
613                     /proc/sys/net/core/wmem_max.   This  is not used to limit
614                     the size of the send buffer declared using SO_SNDBUF on a
615                     socket.   The  default value is calculated using the for‐
616                     mula
618                         max(65536, min(4 MB, tcp_mem[1]*PAGE_SIZE/128))
620                     (On Linux 2.4, the default value is 128 kB, lowered 64 kB
621                     depending on low-memory systems.)
623       tcp_workaround_signed_windows  (Boolean; default: disabled; since Linux
624       2.6.26)
625              If enabled, assume that no receipt of  a  window-scaling  option
626              means  that  the remote TCP is broken and treats the window as a
627              signed quantity.  If disabled, assume that the remote TCP is not
628              broken  even  if  we do not receive a window scaling option from
629              it.
631   Socket options
632       To set or get a TCP socket option, call getsockopt(2) to read  or  set‐
633       sockopt(2)  to  write  the option with the option level argument set to
634       IPPROTO_TCP.  Unless otherwise noted, optval is a pointer  to  an  int.
635       In  addition,  most IPPROTO_IP socket options are valid on TCP sockets.
636       For more information see ip(7).
638       Following is a list of TCP-specific socket  options.   For  details  of
639       some other socket options that are also applicable for TCP sockets, see
640       socket(7).
642       TCP_CONGESTION (since Linux 2.6.13)
643              The argument for this option is a string.   This  option  allows
644              the  caller  to  set  the TCP congestion control algorithm to be
645              used, on a per-socket basis.   Unprivileged  processes  are  re‐
646              stricted  to  choosing one of the algorithms in tcp_allowed_con‐
647              gestion_control   (described   above).    Privileged   processes
648              (CAP_NET_ADMIN) can choose from any of the available congestion-
649              control algorithms (see the description of tcp_available_conges‐
650              tion_control above).
652       TCP_CORK (since Linux 2.2)
653              If  set,  don't  send  out  partial  frames.  All queued partial
654              frames are sent when the option is cleared again.  This is  use‐
655              ful  for  prepending  headers before calling sendfile(2), or for
656              throughput optimization.  As currently implemented, there  is  a
657              200  millisecond  ceiling on the time for which output is corked
658              by TCP_CORK.  If this ceiling is reached, then  queued  data  is
659              automatically  transmitted.   This  option  can be combined with
660              TCP_NODELAY only since Linux 2.5.71.  This option should not  be
661              used in code intended to be portable.
663       TCP_DEFER_ACCEPT (since Linux 2.4)
664              Allow  a  listener  to be awakened only when data arrives on the
665              socket.  Takes an integer value (seconds), this  can  bound  the
666              maximum number of attempts TCP will make to complete the connec‐
667              tion.  This option should not be used in  code  intended  to  be
668              portable.
670       TCP_INFO (since Linux 2.4)
671              Used  to  collect information about this socket.  The kernel re‐
672              turns  a  struct  tcp_info  as  defined  in  the  file  /usr/in‐
673              clude/linux/tcp.h.   This  option should not be used in code in‐
674              tended to be portable.
676       TCP_KEEPCNT (since Linux 2.4)
677              The maximum number of keepalive probes TCP  should  send  before
678              dropping the connection.  This option should not be used in code
679              intended to be portable.
681       TCP_KEEPIDLE (since Linux 2.4)
682              The time (in seconds) the connection needs to remain idle before
683              TCP  starts  sending  keepalive  probes,  if  the  socket option
684              SO_KEEPALIVE has been set on this socket.   This  option  should
685              not be used in code intended to be portable.
687       TCP_KEEPINTVL (since Linux 2.4)
688              The time (in seconds) between individual keepalive probes.  This
689              option should not be used in code intended to be portable.
691       TCP_LINGER2 (since Linux 2.4)
692              The lifetime of orphaned FIN_WAIT2 state sockets.   This  option
693              can  be  used  to  override  the system-wide setting in the file
694              /proc/sys/net/ipv4/tcp_fin_timeout for this socket.  This is not
695              to  be confused with the socket(7) level option SO_LINGER.  This
696              option should not be used in code intended to be portable.
698       TCP_MAXSEG
699              The maximum segment size for outgoing TCP packets.  In Linux 2.2
700              and  earlier,  and  in Linux 2.6.28 and later, if this option is
701              set before connection establishment, it  also  changes  the  MSS
702              value  announced to the other end in the initial packet.  Values
703              greater than the (eventual) interface MTU have no  effect.   TCP
704              will  also  impose its minimum and maximum bounds over the value
705              provided.
707       TCP_NODELAY
708              If set, disable the Nagle algorithm.  This means  that  segments
709              are  always  sent  as  soon as possible, even if there is only a
710              small amount of data.  When not  set,  data  is  buffered  until
711              there  is  a sufficient amount to send out, thereby avoiding the
712              frequent sending of small packets, which results  in  poor  uti‐
713              lization of the network.  This option is overridden by TCP_CORK;
714              however, setting this option forces an explicit flush of pending
715              output, even if TCP_CORK is currently set.
717       TCP_QUICKACK (since Linux 2.4.4)
718              Enable quickack mode if set or disable quickack mode if cleared.
719              In quickack mode, acks are sent immediately, rather than delayed
720              if  needed  in accordance to normal TCP operation.  This flag is
721              not permanent, it only enables a  switch  to  or  from  quickack
722              mode.   Subsequent operation of the TCP protocol will once again
723              enter/leave quickack mode depending on  internal  protocol  pro‐
724              cessing  and  factors such as delayed ack timeouts occurring and
725              data transfer.  This option should not be used in code  intended
726              to be portable.
728       TCP_SYNCNT (since Linux 2.4)
729              Set  the  number  of SYN retransmits that TCP should send before
730              aborting the attempt to connect.  It cannot  exceed  255.   This
731              option should not be used in code intended to be portable.
733       TCP_USER_TIMEOUT (since Linux 2.6.37)
734              This  option  takes  an  unsigned  int as an argument.  When the
735              value is greater than 0, it specifies the maximum amount of time
736              in milliseconds that transmitted data may remain unacknowledged,
737              or bufferred data may remain untransmitted (due to  zero  window
738              size)  before  TCP will forcibly close the corresponding connec‐
739              tion and return ETIMEDOUT to the  application.   If  the  option
740              value is specified as 0, TCP will use the system default.
742              Increasing  user timeouts allows a TCP connection to survive ex‐
743              tended periods without end-to-end connectivity.  Decreasing user
744              timeouts  allows  applications  to  "fail  fast", if so desired.
745              Otherwise, failure may take up to 20 minutes  with  the  current
746              system defaults in a normal WAN environment.
748              This option can be set during any state of a TCP connection, but
749              is effective only during the synchronized states of a connection
751              LAST-ACK).   Moreover,  when  used  with   the   TCP   keepalive
752              (SO_KEEPALIVE)  option, TCP_USER_TIMEOUT will override keepalive
753              to determine when to close a connection due to  keepalive  fail‐
754              ure.
756              The  option  has no effect on when TCP retransmits a packet, nor
757              when a keepalive probe is sent.
759              This option, like many others, will be inherited by  the  socket
760              returned by accept(2), if it was set on the listening socket.
762              Further  details  on  the  user  timeout feature can be found in
763              RFC 793 and RFC 5482 ("TCP User Timeout Option").
765       TCP_WINDOW_CLAMP (since Linux 2.4)
766              Bound the size of the advertised window to this value.  The ker‐
767              nel  imposes  a  minimum size of SOCK_MIN_RCVBUF/2.  This option
768              should not be used in code intended to be portable.
770   Sockets API
771       TCP provides limited support for out-of-band data, in the  form  of  (a
772       single  byte  of)  urgent  data.   In Linux this means if the other end
773       sends newer out-of-band data the older urgent data is inserted as  nor‐
774       mal  data  into  the  stream (even when SO_OOBINLINE is not set).  This
775       differs from BSD-based stacks.
777       Linux uses the BSD compatible  interpretation  of  the  urgent  pointer
778       field by default.  This violates RFC 1122, but is required for interop‐
779       erability   with   other   stacks.    It    can    be    changed    via
780       /proc/sys/net/ipv4/tcp_stdurg.
782       It  is  possible to peek at out-of-band data using the recv(2) MSG_PEEK
783       flag.
785       Since version 2.4, Linux supports the use of MSG_TRUNC in the flags ar‐
786       gument  of  recv(2)  (and  recvmsg(2)).   This flag causes the received
787       bytes of data to be discarded, rather than passed back in a caller-sup‐
788       plied  buffer.   Since Linux 2.4.4, MSG_TRUNC also has this effect when
789       used in conjunction with MSG_OOB to receive out-of-band data.
791   Ioctls
792       The following ioctl(2) calls return information in value.  The  correct
793       syntax is:
795              int value;
796              error = ioctl(tcp_socket, ioctl_type, &value);
798       ioctl_type is one of the following:
800       SIOCINQ
801              Returns  the amount of queued unread data in the receive buffer.
802              The socket must not be in LISTEN state, otherwise an error (EIN‐
803              VAL) is returned.  SIOCINQ is defined in <linux/sockios.h>.  Al‐
804              ternatively, you can use the  synonymous  FIONREAD,  defined  in
805              <sys/ioctl.h>.
807       SIOCATMARK
808              Returns true (i.e., value is nonzero) if the inbound data stream
809              is at the urgent mark.
811              If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
812              true,  then the next read from the socket will return the urgent
813              data.  If the SO_OOBINLINE socket option is not set, and SIOCAT‐
814              MARK  returns  true, then the next read from the socket will re‐
815              turn the bytes following the urgent data (to actually  read  the
816              urgent data requires the recv(MSG_OOB) flag).
818              Note  that a read never reads across the urgent mark.  If an ap‐
819              plication is informed of the presence of  urgent  data  via  se‐
820              lect(2)  (using the exceptfds argument) or through delivery of a
821              SIGURG signal, then it can advance up to the mark using  a  loop
822              which  repeatedly tests SIOCATMARK and performs a read (request‐
823              ing any number of bytes) as long as SIOCATMARK returns false.
825       SIOCOUTQ
826              Returns the amount of unsent data in the socket send queue.  The
827              socket  must not be in LISTEN state, otherwise an error (EINVAL)
828              is returned.  SIOCOUTQ is defined in <linux/sockios.h>.   Alter‐
829              natively,  you  can  use  the  synonymous  TIOCOUTQ,  defined in
830              <sys/ioctl.h>.
832   Error handling
833       When a network error occurs, TCP tries to resend  the  packet.   If  it
834       doesn't  succeed after some time, either ETIMEDOUT or the last received
835       error on this connection is reported.
837       Some applications require a quicker error notification.   This  can  be
838       enabled  with the IPPROTO_IP level IP_RECVERR socket option.  When this
839       option is enabled, all incoming errors are immediately  passed  to  the
840       user  program.   Use this option with care — it makes TCP less tolerant
841       to routing changes and other normal network conditions.


845              Passed socket address type in sin_family was not AF_INET.
847       EPIPE  The other end closed the socket unexpectedly or a read  is  exe‐
848              cuted on a shut down socket.
850       ETIMEDOUT
851              The  other  end didn't acknowledge retransmitted data after some
852              time.
854       Any errors defined for ip(7) or the generic socket layer  may  also  be
855       returned for TCP.


858       Support  for  Explicit  Congestion Notification, zero-copy sendfile(2),
859       reordering support and some SACK extensions (DSACK) were introduced  in
860       2.4.   Support for forward acknowledgement (FACK), TIME_WAIT recycling,
861       and per-connection keepalive socket options were introduced in 2.3.


864       Not all errors are documented.
866       IPv6 is not described.


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


884       This page is part of release 5.13 of the Linux  man-pages  project.   A
885       description  of  the project, information about reporting bugs, and the
886       latest    version    of    this    page,    can     be     found     at
887       https://www.kernel.org/doc/man-pages/.
891Linux                             2021-03-22                            TCP(7)