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 (> 64K) TCP windows
37       in order to support links with high latency or bandwidth.  To make  use
38       of them, the send and receive buffer sizes must be increased.  They can
39       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.1-2001-specified  fcntl(2)  F_SETOWN  operation).   When the
63       SO_OOBINLINE socket option is enabled, urgent data is put into the nor‐
64       mal  data stream (a program can test for its location using the SIOCAT‐
65       MARK ioctl described below), otherwise it can be received only when the
66       MSG_OOB flag is set for recv(2) or recvmsg(2).
67
68       Linux  2.4  introduced  a number of changes for improved throughput and
69       scaling, as well as enhanced functionality.   Some  of  these  features
70       include  support for zero-copy sendfile(2), Explicit Congestion Notifi‐
71       cation, new management of TIME_WAIT sockets, keep-alive socket  options
72       and support for Duplicate SACK extensions.
73
74   Address formats
75       TCP  is built on top of IP (see ip(7)).  The address formats defined by
76       ip(7) apply to TCP.  TCP supports  point-to-point  communication  only;
77       broadcasting and multicasting are not supported.
78
79   /proc interfaces
80       System-wide  TCP  parameter  settings  can  be accessed by files in the
81       directory /proc/sys/net/ipv4/.  In addition, most IP  /proc  interfaces
82       also  apply  to TCP; see ip(7).  Variables described as Boolean take an
83       integer value, with a nonzero value ("true") meaning  that  the  corre‐
84       sponding option is enabled, and a zero value ("false") meaning that the
85       option is disabled.
86
87       tcp_abc (Integer; default: 0; since Linux 2.6.15)
88              Control the Appropriate Byte Count (ABC), defined in  RFC  3465.
89              ABC  is  a  way  of increasing the congestion window (cwnd) more
90              slowly in response to partial acknowledgments.  Possible  values
91              are:
92
93              0  increase cwnd once per acknowledgment (no ABC)
94
95              1  increase cwnd once per acknowledgment of full sized segment
96
97              2  allow  increase  cwnd by two if acknowledgment is of two seg‐
98                 ments to compensate for delayed acknowledgments.
99
100       tcp_abort_on_overflow (Boolean; default: disabled; since Linux 2.4)
101              Enable resetting connections if the  listening  service  is  too
102              slow  and  unable  to keep up and accept them.  It means that if
103              overflow occurred due to a burst, the connection  will  recover.
104              Enable  this option only if you are really sure that the listen‐
105              ing  daemon  cannot  be  tuned  to  accept  connections  faster.
106              Enabling this option can harm the clients of your server.
107
108       tcp_adv_win_scale (integer; default: 2; since Linux 2.4)
109              Count   buffering   overhead  as  bytes/2^tcp_adv_win_scale,  if
110              tcp_adv_win_scale    is    greater    than    0;    or    bytes-
111              bytes/2^(-tcp_adv_win_scale),  if tcp_adv_win_scale is less than
112              or equal to zero.
113
114              The socket receive buffer space is shared between  the  applica‐
115              tion  and  kernel.   TCP maintains part of the buffer as the TCP
116              window, this is the size of the receive window advertised to the
117              other  end.   The rest of the space is used as the "application"
118              buffer, used to isolate the network from scheduling and applica‐
119              tion  latencies.   The  tcp_adv_win_scale  default  value  of  2
120              implies that the space used for the application  buffer  is  one
121              fourth that of the total.
122
123       tcp_allowed_congestion_control  (String; default: see text; since Linux
124       2.4.20)
125              Show/set the congestion control algorithm choices  available  to
126              unprivileged  processes  (see the description of the TCP_CONGES‐
127              TION socket option).  The list is a subset of  those  listed  in
128              tcp_available_congestion_control.   The  default  value for this
129              list is "reno" plus the default setting  of  tcp_congestion_con‐
130              trol.
131
132       tcp_available_congestion_control   (String;   read-only;   since  Linux
133       2.4.20)
134              Show a list of the congestion-control algorithms that are regis‐
135              tered.    This   list   is  a  limiting  set  for  the  list  in
136              tcp_allowed_congestion_control.  More  congestion-control  algo‐
137              rithms may be available as modules, but not loaded.
138
139       tcp_app_win (integer; default: 31; since Linux 2.4)
140              This  variable  defines  how  many  bytes  of the TCP window are
141              reserved for buffering overhead.
142
143              A maximum of (window/2^tcp_app_win, mss) bytes in the window are
144              reserved  for the application buffer.  A value of 0 implies that
145              no amount is reserved.
146
147       tcp_base_mss (Integer; default: 512; since Linux 2.6.17)
148              The initial value of search_low to be used by the  packetization
149              layer  Path  MTU  discovery  (MTU  probing).   If MTU probing is
150              enabled, this is the initial MSS used by the connection.
151
152       tcp_bic (Boolean; default: disabled; Linux 2.4.27/2.6.6 to 2.6.13)
153              Enable BIC TCP  congestion  control  algorithm.   BIC-TCP  is  a
154              sender-side only change that ensures a linear RTT fairness under
155              large windows while offering both scalability and  bounded  TCP-
156              friendliness.  The protocol combines two schemes called additive
157              increase and binary search increase.  When the congestion window
158              is  large, additive increase with a large increment ensures lin‐
159              ear RTT fairness as well as good scalability.  Under small  con‐
160              gestion  windows,  binary search increase provides TCP friendli‐
161              ness.
162
163       tcp_bic_low_window (integer; default: 14; Linux 2.4.27/2.6.6 to 2.6.13)
164              Set the threshold window (in packets) where BIC  TCP  starts  to
165              adjust  the  congestion  window.   Below  this threshold BIC TCP
166              behaves the same as the default TCP Reno.
167
168       tcp_bic_fast_convergence (Boolean; default: enabled; Linux 2.4.27/2.6.6
169       to 2.6.13)
170              Force  BIC  TCP to more quickly respond to changes in congestion
171              window.  Allows two flows sharing the same  connection  to  con‐
172              verge more rapidly.
173
174       tcp_congestion_control (String; default: see text; since Linux 2.4.13)
175              Set  the default congestion-control algorithm to be used for new
176              connections.  The algorithm  "reno"  is  always  available,  but
177              additional choices may be available depending on kernel configu‐
178              ration.  The default value for this file is set as part of  ker‐
179              nel configuration.
180
181       tcp_dma_copybreak (integer; default: 4096; since Linux 2.6.24)
182              Lower  limit, in bytes, of the size of socket reads that will be
183              offloaded to a DMA copy engine, if one is present in the  system
184              and the kernel was configured with the CONFIG_NET_DMA option.
185
186       tcp_dsack (Boolean; default: enabled; since Linux 2.4)
187              Enable RFC 2883 TCP Duplicate SACK support.
188
189       tcp_ecn (Boolean; default: disabled; since Linux 2.4)
190              Enable RFC 2884 Explicit Congestion Notification.  When enabled,
191              connectivity to some  destinations  could  be  affected  due  to
192              older, misbehaving routers along the path causing connections to
193              be dropped.
194
195       tcp_fack (Boolean; default: enabled; since Linux 2.2)
196              Enable TCP Forward Acknowledgement support.
197
198       tcp_fin_timeout (integer; default: 60; since Linux 2.2)
199              This specifies how many seconds to wait for a final  FIN  packet
200              before the socket is forcibly closed.  This is strictly a viola‐
201              tion of the TCP specification, but required to  prevent  denial-
202              of-service attacks.  In Linux 2.2, the default value was 180.
203
204       tcp_frto (integer; default: 0; since Linux 2.4.21/2.6)
205              Enable F-RTO, an enhanced recovery algorithm for TCP retransmis‐
206              sion timeouts (RTOs).  It is particularly beneficial in wireless
207              environments  where packet loss is typically due to random radio
208              interference rather than intermediate  router  congestion.   See
209              RFC 4138 for more details.
210
211              This file can have one of the following values:
212
213              0  Disabled.
214
215              1  The basic version F-RTO algorithm is enabled.
216
217              2  Enable SACK-enhanced F-RTO if flow uses SACK.  The basic ver‐
218                 sion can be used also when SACK is in use though in that case
219                 scenario(s)  exists  where  F-RTO  interacts  badly  with the
220                 packet counting of the SACK-enabled TCP flow.
221
222              Before Linux 2.6.22, this parameter was a  Boolean  value,  sup‐
223              porting just values 0 and 1 above.
224
225       tcp_frto_response (integer; default: 0; since Linux 2.6.22)
226              When  F-RTO  has  detected that a TCP retransmission timeout was
227              spurious (i.e, the timeout would have been avoided had TCP set a
228              longer retransmission timeout), TCP has several options concern‐
229              ing what to do next.  Possible values are:
230
231              0  Rate halving  based;  a  smooth  and  conservative  response,
232                 results  in  halved  congestion  window (cwnd) and slow-start
233                 threshold (ssthresh) after one RTT.
234
235              1  Very conservative  response;  not  recommended  because  even
236                 though  being  valid,  it  interacts  poorly with the rest of
237                 Linux TCP; halves cwnd and ssthresh immediately.
238
239              2  Aggressive response; undoes congestion-control measures  that
240                 are  now known to be unnecessary (ignoring the possibility of
241                 a lost retransmission that would require TCP to be more  cau‐
242                 tious); cwnd and ssthresh are restored to the values prior to
243                 timeout.
244
245       tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
246              The number of seconds between TCP keep-alive probes.
247
248       tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
249              The maximum number of TCP keep-alive probes to send before  giv‐
250              ing  up  and  killing  the connection if no response is obtained
251              from the other end.
252
253       tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
254              The number of seconds a connection needs to be idle  before  TCP
255              begins sending out keep-alive probes.  Keep-alives are sent only
256              when the SO_KEEPALIVE socket option  is  enabled.   The  default
257              value  is  7200 seconds (2 hours).  An idle connection is termi‐
258              nated after approximately an additional 11 minutes (9 probes  an
259              interval of 75 seconds apart) when keep-alive is enabled.
260
261              Note that underlying connection tracking mechanisms and applica‐
262              tion timeouts may be much shorter.
263
264       tcp_low_latency (Boolean; default: disabled; since Linux 2.4.21/2.6)
265              If enabled, the TCP stack  makes  decisions  that  prefer  lower
266              latency as opposed to higher throughput.  It this option is dis‐
267              abled, then higher throughput is preferred.  An  example  of  an
268              application  where  this  default  should  be changed would be a
269              Beowulf compute cluster.
270
271       tcp_max_orphans (integer; default: see below; since Linux 2.4)
272              The maximum number of orphaned (not attached to  any  user  file
273              handle)  TCP sockets allowed in the system.  When this number is
274              exceeded, the orphaned connection is  reset  and  a  warning  is
275              printed.   This  limit  exists only to prevent simple denial-of-
276              service attacks.  Lowering this limit is not recommended.   Net‐
277              work  conditions  might  require  you  to increase the number of
278              orphans allowed, but note that each orphan can eat up to ~64K of
279              unswappable  memory.   The default initial value is set equal to
280              the kernel parameter NR_FILE.  This initial default is  adjusted
281              depending on the memory in the system.
282
283       tcp_max_syn_backlog (integer; default: see below; since Linux 2.2)
284              The  maximum  number  of  queued  connection requests which have
285              still  not  received  an  acknowledgement  from  the  connecting
286              client.  If this number is exceeded, the kernel will begin drop‐
287              ping requests.  The default value of 256 is  increased  to  1024
288              when the memory present in the system is adequate or greater (>=
289              128Mb), and reduced to 128 for those systems with very low  mem‐
290              ory  (<=  32Mb).   It  is  recommended  that if this needs to be
291              increased above 1024,  TCP_SYNQ_HSIZE  in  include/net/tcp.h  be
292              modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and the
293              kernel be recompiled.
294
295       tcp_max_tw_buckets (integer; default: see below; since Linux 2.4)
296              The maximum number of sockets in TIME_WAIT state allowed in  the
297              system.  This limit exists only to prevent simple denial-of-ser‐
298              vice attacks.   The  default  value  of  NR_FILE*2  is  adjusted
299              depending  on  the  memory  in  the  system.   If this number is
300              exceeded, the socket is closed and a warning is printed.
301
302       tcp_moderate_rcvbuf   (Boolean;   default:   enabled;    since    Linux
303       2.4.17/2.6.7)
304              If  enabled, TCP performs receive buffer auto-tuning, attempting
305              to automatically size the buffer (no greater  than  tcp_rmem[2])
306              to match the size required by the path for full throughput.
307
308       tcp_mem (since Linux 2.4)
309              This  is  a  vector of 3 integers: [low, pressure, high].  These
310              bounds, measured in units of the system page size, are  used  by
311              TCP  to  track its memory usage.  The defaults are calculated at
312              boot time from the amount of available memory.   (TCP  can  only
313              use  low  memory  for  this,  which  is  limited  to  around 900
314              megabytes on 32-bit systems.  64-bit systems do not suffer  this
315              limitation.)
316
317              low       TCP  doesn't  regulate  its memory allocation when the
318                        number of pages it has  allocated  globally  is  below
319                        this number.
320
321              pressure  When  the  amount  of  memory allocated by TCP exceeds
322                        this number of pages, TCP moderates  its  memory  con‐
323                        sumption.   This  memory pressure state is exited once
324                        the number of pages  allocated  falls  below  the  low
325                        mark.
326
327              high      The  maximum  number of pages, globally, that TCP will
328                        allocate.   This  value  overrides  any  other  limits
329                        imposed by the kernel.
330
331       tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
332              This parameter controls TCP Packetization-Layer Path MTU Discov‐
333              ery.  The following values may be assigned to the file:
334
335              0  Disabled
336
337              1  Disabled by default, enabled when an ICMP black hole detected
338
339              2  Always enabled, use initial MSS of tcp_base_mss.
340
341       tcp_no_metrics_save (Boolean; default: disabled; since Linux 2.6.6)
342              By default, TCP saves various connection metrics  in  the  route
343              cache  when  the  connection  closes, so that connections estab‐
344              lished in the near future can use these to  set  initial  condi‐
345              tions.   Usually, this increases overall performance, but it may
346              sometimes cause performance degradation.  If tcp_no_metrics_save
347              is enabled, TCP will not cache metrics on closing connections.
348
349       tcp_orphan_retries (integer; default: 8; since Linux 2.4)
350              The  maximum number of attempts made to probe the other end of a
351              connection which has been closed by our end.
352
353       tcp_reordering (integer; default: 3; since Linux 2.4)
354              The maximum a packet can be reordered in  a  TCP  packet  stream
355              without  TCP assuming packet loss and going into slow start.  It
356              is not advisable to  change  this  number.   This  is  a  packet
357              reordering  detection  metric  designed  to minimize unnecessary
358              back off and retransmits provoked by reordering of packets on  a
359              connection.
360
361       tcp_retrans_collapse (Boolean; default: enabled; since Linux 2.2)
362              Try to send full-sized packets during retransmit.
363
364       tcp_retries1 (integer; default: 3; since Linux 2.2)
365              The  number  of times TCP will attempt to retransmit a packet on
366              an established connection normally, without the extra effort  of
367              getting the network layers involved.  Once we exceed this number
368              of retransmits, we first have the network layer update the route
369              if  possible before each new retransmit.  The default is the RFC
370              specified minimum of 3.
371
372       tcp_retries2 (integer; default: 15; since Linux 2.2)
373              The maximum number of times a TCP  packet  is  retransmitted  in
374              established  state  before  giving up.  The default value is 15,
375              which corresponds to a duration of approximately between  13  to
376              30  minutes,  depending  on  the  retransmission  timeout.   The
377              RFC 1122 specified minimum limit of  100  seconds  is  typically
378              deemed too short.
379
380       tcp_rfc1337 (Boolean; default: disabled; since Linux 2.2)
381              Enable TCP behavior conformant with RFC 1337.  When disabled, if
382              a RST is received in TIME_WAIT state, we close the socket  imme‐
383              diately without waiting for the end of the TIME_WAIT period.
384
385       tcp_rmem (since Linux 2.4)
386              This  is  a  vector  of  3 integers: [min, default, max].  These
387              parameters are used by TCP to  regulate  receive  buffer  sizes.
388              TCP  dynamically adjusts the size of the receive buffer from the
389              defaults listed below, in the range of these  values,  depending
390              on memory available in the system.
391
392              min       minimum  size  of  the receive buffer used by each TCP
393                        socket.  The default value is the  system  page  size.
394                        (On  Linux  2.4,  the  default value is 4K, lowered to
395                        PAGE_SIZE bytes in low-memory systems.)  This value is
396                        used  to  ensure that in memory pressure mode, alloca‐
397                        tions below this size will still succeed.  This is not
398                        used  to bound the size of the receive buffer declared
399                        using SO_RCVBUF on a socket.
400
401              default   the default size of  the  receive  buffer  for  a  TCP
402                        socket.   This  value  overwrites  the initial default
403                        buffer    size     from     the     generic     global
404                        net.core.rmem_default  defined for all protocols.  The
405                        default value is 87380 bytes.   (On  Linux  2.4,  this
406                        will  be  lowered to 43689 in low-memory systems.)  If
407                        larger receive buffer sizes are  desired,  this  value
408                        should  be  increased  (to  affect  all  sockets).  To
409                        employ  large  TCP  windows,   the   net.ipv4.tcp_win‐
410                        dow_scaling must be enabled (default).
411
412              max       the  maximum  size  of the receive buffer used by each
413                        TCP socket.  This value does not override  the  global
414                        net.core.rmem_max.  This is not used to limit the size
415                        of the receive buffer declared using  SO_RCVBUF  on  a
416                        socket.   The  default  value  is calculated using the
417                        formula
418
419                            max(87380, min(4MB, tcp_mem[1]*PAGE_SIZE/128))
420
421                        (On Linux 2.4, the default is 87380*2  bytes,  lowered
422                        to 87380 in low-memory systems).
423
424       tcp_sack (Boolean; default: enabled; since Linux 2.2)
425              Enable RFC 2018 TCP Selective Acknowledgements.
426
427       tcp_slow_start_after_idle   (Boolean;  default:  enabled;  since  Linux
428       2.6.18)
429              If enabled, provide RFC 2861 behavior and time out  the  conges‐
430              tion  window after an idle period.  An idle period is defined as
431              the current RTO (retransmission timeout).  If disabled, the con‐
432              gestion window will not be timed out after an idle period.
433
434       tcp_stdurg (Boolean; default: disabled; since Linux 2.2)
435              If  this option is enabled, then use the RFC 1122 interpretation
436              of the TCP urgent-pointer field.  According to this  interpreta‐
437              tion, the urgent pointer points to the last byte of urgent data.
438              If this option is disabled, then use the  BSD-compatible  inter‐
439              pretation  of  the  urgent pointer: the urgent pointer points to
440              the first byte after the urgent data.  Enabling this option  may
441              lead to interoperability problems.
442
443       tcp_syn_retries (integer; default: 5; since Linux 2.2)
444              The  maximum number of times initial SYNs for an active TCP con‐
445              nection attempt will be retransmitted.  This value should not be
446              higher  than  255.  The default value is 5, which corresponds to
447              approximately 180 seconds.
448
449       tcp_synack_retries (integer; default: 5; since Linux 2.2)
450              The maximum number of times a SYN/ACK segment for a passive  TCP
451              connection  will  be  retransmitted.   This number should not be
452              higher than 255.
453
454       tcp_syncookies (Boolean; since Linux 2.2)
455              Enable TCP syncookies.  The kernel must be  compiled  with  CON‐
456              FIG_SYN_COOKIES.  Send out syncookies when the syn backlog queue
457              of a socket overflows.  The syncookies feature attempts to  pro‐
458              tect a socket from a SYN flood attack.  This should be used as a
459              last resort, if at all.  This is a violation of the  TCP  proto‐
460              col,  and  conflicts  with other areas of TCP such as TCP exten‐
461              sions.  It can cause problems for clients and relays.  It is not
462              recommended  as a tuning mechanism for heavily loaded servers to
463              help with overloaded or misconfigured  conditions.   For  recom‐
464              mended alternatives see tcp_max_syn_backlog, tcp_synack_retries,
465              and tcp_abort_on_overflow.
466
467       tcp_timestamps (Boolean; default: enabled; since Linux 2.2)
468              Enable RFC 1323 TCP timestamps.
469
470       tcp_tso_win_divisor (integer; default: 3; since Linux 2.6.9)
471              This parameter controls what percentage of the congestion window
472              can  be  consumed  by  a  single  TCP Segmentation Offload (TSO)
473              frame.  The setting of this  parameter  is  a  tradeoff  between
474              burstiness and building larger TSO frames.
475
476       tcp_tw_recycle (Boolean; default: disabled; since Linux 2.4)
477              Enable  fast  recycling  of  TIME_WAIT  sockets.   Enabling this
478              option is not recommended since this causes problems when  work‐
479              ing with NAT (Network Address Translation).
480
481       tcp_tw_reuse (Boolean; default: disabled; since Linux 2.4.19/2.6)
482              Allow  to reuse TIME_WAIT sockets for new connections when it is
483              safe from protocol viewpoint.  It should not be changed  without
484              advice/request of technical experts.
485
486       tcp_vegas_cong_avoid (Boolean; default: disabled; Linux 2.2 to 2.6.13)
487              Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is a
488              sender-side only change to TCP that  anticipates  the  onset  of
489              congestion  by  estimating the bandwidth.  TCP Vegas adjusts the
490              sending rate by modifying  the  congestion  window.   TCP  Vegas
491              should  provide less packet loss, but it is not as aggressive as
492              TCP Reno.
493
494       tcp_westwood (Boolean; default: disabled; Linux 2.4.26/2.6.3 to 2.6.13)
495              Enable TCP Westwood+ congestion control  algorithm.   TCP  West‐
496              wood+  is a sender-side only modification of the TCP Reno proto‐
497              col stack that optimizes the performance of TCP congestion  con‐
498              trol.   It  is  based  on end-to-end bandwidth estimation to set
499              congestion window and slow start threshold  after  a  congestion
500              episode.  Using this estimation, TCP Westwood+ adaptively sets a
501              slow start threshold and a congestion window  which  takes  into
502              account  the  bandwidth  used  at the time congestion is experi‐
503              enced.  TCP  Westwood+  significantly  increases  fairness  with
504              respect  to TCP Reno in wired networks and throughput over wire‐
505              less links.
506
507       tcp_window_scaling (Boolean; default: enabled; since Linux 2.2)
508              Enable RFC 1323 TCP window scaling.  This feature allows the use
509              of  a large window (> 64K) on a TCP connection, should the other
510              end support it.  Normally, the 16 bit window length field in the
511              TCP  header  limits  the window size to less than 64K bytes.  If
512              larger windows are desired, applications can increase  the  size
513              of  their  socket  buffers and the window scaling option will be
514              employed.  If tcp_window_scaling is disabled, TCP will not nego‐
515              tiate  the  use of window scaling with the other end during con‐
516              nection setup.
517
518       tcp_wmem (since Linux 2.4)
519              This is a vector of 3  integers:  [min,  default,  max].   These
520              parameters  are  used by TCP to regulate send buffer sizes.  TCP
521              dynamically adjusts the size of the send buffer from the default
522              values  listed below, in the range of these values, depending on
523              memory available.
524
525              min       Minimum size of the  send  buffer  used  by  each  TCP
526                        socket.   The  default  value is the system page size.
527                        (On Linux 2.4, the default value is 4K  bytes.)   This
528                        value  is used to ensure that in memory pressure mode,
529                        allocations below this size will still succeed.   This
530                        is  not  used  to  bound  the  size of the send buffer
531                        declared using SO_SNDBUF on a socket.
532
533              default   The default size of the send buffer for a TCP  socket.
534                        This  value overwrites the initial default buffer size
535                        from           the           generic            global
536                        /proc/sys/net/core/wmem_default defined for all proto‐
537                        cols.  The default value is 16K bytes.  If larger send
538                        buffer   sizes  are  desired,  this  value  should  be
539                        increased (to affect all sockets).   To  employ  large
540                        TCP windows, the /proc/sys/net/ipv4/tcp_window_scaling
541                        must be set to a nonzero value (default).
542
543              max       The maximum size of the send buffer used by  each  TCP
544                        socket.   This  value  does  not override the value in
545                        /proc/sys/net/core/wmem_max.   This  is  not  used  to
546                        limit  the  size  of  the  send  buffer declared using
547                        SO_SNDBUF on a socket.  The default  value  is  calcu‐
548                        lated using the formula
549
550                            max(65536, min(4MB, tcp_mem[1]*PAGE_SIZE/128))
551
552                        (On  Linux  2.4, the default value is 128K bytes, low‐
553                        ered 64K depending on low-memory systems.)
554
555       tcp_workaround_signed_windows (Boolean; default: disabled; since  Linux
556       2.6.26)
557              If  enabled,  assume  that no receipt of a window-scaling option
558              means that the remote TCP is broken and treats the window  as  a
559              signed quantity.  If disabled, assume that the remote TCP is not
560              broken even if we do not receive a window  scaling  option  from
561              it.
562
563   Socket options
564       To  set  or get a TCP socket option, call getsockopt(2) to read or set‐
565       sockopt(2) to write the option with the option level  argument  set  to
566       IPPROTO_TCP.   Unless  otherwise  noted, optval is a pointer to an int.
567       In addition, most IPPROTO_IP socket options are valid on  TCP  sockets.
568       For more information see ip(7).
569
570       TCP_CONGESTION (since Linux 2.6.13)
571              Get  or  set  the  congestion-control algorithm for this socket.
572              The optval argument is a pointer to a character-string buffer.
573
574              For getsockopt() *optlen specifies the amount of space available
575              in  the buffer pointed to by optval, which should be at least 16
576              bytes (defined by the kernel-internal constant TCP_CA_NAME_MAX).
577              On return, the buffer pointed to by optval is set to a null-ter‐
578              minated string containing the  name  of  the  congestion-control
579              algorithm  for this socket, and *optlen is set to the minimum of
580              its original value and TCP_CA_NAME_MAX.  If the value passed  in
581              *optlen  is  too  small,  then the string returned in *optval is
582              silently truncated, and no terminating null byte is  added.   If
583              an  empty  string  is  returned,  then  the  socket is using the
584              default congestion-control algorithm,  determined  as  described
585              under tcp_congestion_control above.
586
587              For  setsockopt() optlen specifies the length of the congestion-
588              control algorithm name contained in the  buffer  pointed  to  by
589              optval;  this length need not include any terminating null byte.
590              The algorithm "reno" is always permitted; other  algorithms  may
591              be  available,  depending  on  kernel  configuration.   Possible
592              errors from setsockopt() include: algorithm not  found/available
593              (ENOENT);  setting  this  algorithm  requires  the CAP_NET_ADMIN
594              capability (EPERM); and failure getting kernel module (EBUSY).
595
596       TCP_CORK (since Linux 2.2)
597              If set, don't send  out  partial  frames.   All  queued  partial
598              frames  are sent when the option is cleared again.  This is use‐
599              ful for prepending headers before calling  sendfile(2),  or  for
600              throughput  optimization.   As currently implemented, there is a
601              200 millisecond ceiling on the time for which output  is  corked
602              by  TCP_CORK.   If  this ceiling is reached, then queued data is
603              automatically transmitted.  This option  can  be  combined  with
604              TCP_NODELAY  only since Linux 2.5.71.  This option should not be
605              used in code intended to be portable.
606
607       TCP_DEFER_ACCEPT (since Linux 2.4)
608              Allow a listener to be awakened only when data  arrives  on  the
609              socket.   Takes  an  integer value (seconds), this can bound the
610              maximum number of attempts TCP will make to complete the connec‐
611              tion.   This  option  should  not be used in code intended to be
612              portable.
613
614       TCP_INFO (since Linux 2.4)
615              Used to collect  information  about  this  socket.   The  kernel
616              returns   a   struct   tcp_info   as   defined   in   the   file
617              /usr/include/linux/tcp.h.  This option should  not  be  used  in
618              code intended to be portable.
619
620       TCP_KEEPCNT (since Linux 2.4)
621              The  maximum  number  of keepalive probes TCP should send before
622              dropping the connection.  This option should not be used in code
623              intended to be portable.
624
625       TCP_KEEPIDLE (since Linux 2.4)
626              The time (in seconds) the connection needs to remain idle before
627              TCP starts  sending  keepalive  probes,  if  the  socket  option
628              SO_KEEPALIVE  has  been  set on this socket.  This option should
629              not be used in code intended to be portable.
630
631       TCP_KEEPINTVL (since Linux 2.4)
632              The time (in seconds) between individual keepalive probes.  This
633              option should not be used in code intended to be portable.
634
635       TCP_LINGER2 (since Linux 2.4)
636              The  lifetime  of orphaned FIN_WAIT2 state sockets.  This option
637              can be used to override the  system-wide  setting  in  the  file
638              /proc/sys/net/ipv4/tcp_fin_timeout for this socket.  This is not
639              to be confused with the socket(7) level option SO_LINGER.   This
640              option should not be used in code intended to be portable.
641
642       TCP_MAXSEG
643              The maximum segment size for outgoing TCP packets.  In Linux 2.2
644              and earlier, and in Linux 2.6.28 and later, if  this  option  is
645              set  before  connection  establishment,  it also changes the MSS
646              value announced to the other end in the initial packet.   Values
647              greater  than  the (eventual) interface MTU have no effect.  TCP
648              will also impose its minimum and maximum bounds over  the  value
649              provided.
650
651       TCP_NODELAY
652              If  set,  disable the Nagle algorithm.  This means that segments
653              are always sent as soon as possible, even if  there  is  only  a
654              small  amount  of  data.   When  not set, data is buffered until
655              there is a sufficient amount to send out, thereby  avoiding  the
656              frequent  sending  of  small packets, which results in poor uti‐
657              lization of the network.  This option is overridden by TCP_CORK;
658              however, setting this option forces an explicit flush of pending
659              output, even if TCP_CORK is currently set.
660
661       TCP_QUICKACK (since Linux 2.4.4)
662              Enable quickack mode if set or disable quickack mode if cleared.
663              In quickack mode, acks are sent immediately, rather than delayed
664              if needed in accordance to normal TCP operation.  This  flag  is
665              not  permanent,  it  only  enables  a switch to or from quickack
666              mode.  Subsequent operation of the TCP protocol will once  again
667              enter/leave  quickack  mode  depending on internal protocol pro‐
668              cessing and factors such as delayed ack timeouts  occurring  and
669              data  transfer.  This option should not be used in code intended
670              to be portable.
671
672       TCP_SYNCNT (since Linux 2.4)
673              Set the number of SYN retransmits that TCP  should  send  before
674              aborting  the  attempt  to connect.  It cannot exceed 255.  This
675              option should not be used in code intended to be portable.
676
677       TCP_WINDOW_CLAMP (since Linux 2.4)
678              Bound the size of the advertised window to this value.  The ker‐
679              nel  imposes  a  minimum size of SOCK_MIN_RCVBUF/2.  This option
680              should not be used in code intended to be portable.
681
682   Sockets API
683       TCP provides limited support for out-of-band data, in the  form  of  (a
684       single  byte  of)  urgent  data.   In Linux this means if the other end
685       sends newer out-of-band data the older urgent data is inserted as  nor‐
686       mal  data  into  the  stream (even when SO_OOBINLINE is not set).  This
687       differs from BSD-based stacks.
688
689       Linux uses the BSD compatible  interpretation  of  the  urgent  pointer
690       field by default.  This violates RFC 1122, but is required for interop‐
691       erability   with   other   stacks.    It    can    be    changed    via
692       /proc/sys/net/ipv4/tcp_stdurg.
693
694       It  is  possible to peek at out-of-band data using the recv(2) MSG_PEEK
695       flag.
696
697       Since version 2.4, Linux supports the use of  MSG_TRUNC  in  the  flags
698       argument  of  recv(2)  (and recvmsg(2)).  This flag causes the received
699       bytes of data to be discarded, rather than passed back in a caller-sup‐
700       plied  buffer.   Since  Linux 2.4.4, MSG_PEEK also has this effect when
701       used in conjunction with MSG_OOB to receive out-of-band data.
702
703   Ioctls
704       The following ioctl(2) calls return information in value.  The  correct
705       syntax is:
706
707              int value;
708              error = ioctl(tcp_socket, ioctl_type, &value);
709
710       ioctl_type is one of the following:
711
712       SIOCINQ
713              Returns  the amount of queued unread data in the receive buffer.
714              The socket must not be in LISTEN state, otherwise an error (EIN‐
715              VAL)  is  returned.   SIOCINQ  is  defined in <linux/sockios.h>.
716              Alternatively, you can use the synonymous FIONREAD,  defined  in
717              <sys/ioctl.h>.
718
719       SIOCATMARK
720              Returns true (i.e., value is nonzero) if the inbound data stream
721              is at the urgent mark.
722
723              If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
724              true,  then the next read from the socket will return the urgent
725              data.  If the SO_OOBINLINE socket option is not set, and SIOCAT‐
726              MARK  returns  true,  then  the  next  read from the socket will
727              return the bytes following the urgent data (to actually read the
728              urgent data requires the recv(MSG_OOB) flag).
729
730              Note  that  a  read  never  reads across the urgent mark.  If an
731              application is informed of  the  presence  of  urgent  data  via
732              select(2)  (using the exceptfds argument) or through delivery of
733              a SIGURG signal, then it can advance up to the mark using a loop
734              which  repeatedly tests SIOCATMARK and performs a read (request‐
735              ing any number of bytes) as long as SIOCATMARK returns false.
736
737       SIOCOUTQ
738              Returns the amount of unsent data in the socket send queue.  The
739              socket  must not be in LISTEN state, otherwise an error (EINVAL)
740              is returned.  SIOCOUTQ is defined in <linux/sockios.h>.   Alter‐
741              natively,  you  can  use  the  synonymous  TIOCOUTQ,  defined in
742              <sys/ioctl.h>.
743
744   Error handling
745       When a network error occurs, TCP tries to resend  the  packet.   If  it
746       doesn't  succeed after some time, either ETIMEDOUT or the last received
747       error on this connection is reported.
748
749       Some applications require a quicker error notification.   This  can  be
750       enabled  with the IPPROTO_IP level IP_RECVERR socket option.  When this
751       option is enabled, all incoming errors are immediately  passed  to  the
752       user  program.   Use this option with care — it makes TCP less tolerant
753       to routing changes and other normal network conditions.
754

ERRORS

756       EAFNOTSUPPORT
757              Passed socket address type in sin_family was not AF_INET.
758
759       EPIPE  The other end closed the socket unexpectedly or a read  is  exe‐
760              cuted on a shut down socket.
761
762       ETIMEDOUT
763              The  other  end didn't acknowledge retransmitted data after some
764              time.
765
766       Any errors defined for ip(7) or the generic socket layer  may  also  be
767       returned for TCP.
768

VERSIONS

770       Support  for  Explicit  Congestion Notification, zero-copy sendfile(2),
771       reordering support and some SACK extensions (DSACK) were introduced  in
772       2.4.   Support for forward acknowledgement (FACK), TIME_WAIT recycling,
773       and per-connection keepalive socket options were introduced in 2.3.
774

BUGS

776       Not all errors are documented.
777       IPv6 is not described.
778

SEE ALSO

780       accept(2), bind(2), connect(2), getsockopt(2),  listen(2),  recvmsg(2),
781       sendfile(2), sendmsg(2), socket(2), ip(7), socket(7)
782
783       RFC 793 for the TCP specification.
784       RFC 1122  for the TCP requirements and a description of the Nagle algo‐
785       rithm.
786       RFC 1323 for TCP timestamp and window scaling options.
787       RFC 1337 for a description of TIME_WAIT assassination hazards.
788       RFC 3168 for a description of Explicit Congestion Notification.
789       RFC 2581 for TCP congestion control algorithms.
790       RFC 2018 and RFC 2883 for SACK and extensions to SACK.
791

COLOPHON

793       This page is part of release 3.53 of the Linux  man-pages  project.   A
794       description  of  the project, and information about reporting bugs, can
795       be found at http://www.kernel.org/doc/man-pages/.
796
797
798
799Linux                             2013-06-21                            TCP(7)
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