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tcp(4)
TCP(4) BSD Kernel Interfaces Manual TCP(4)
NAME
tcp — Internet Transmission Control Protocol
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
int
socket(AF_INET, SOCK_STREAM, 0);
DESCRIPTION
The TCP protocol provides reliable, flow-controlled, two-way transmission
of data. It is a byte-stream protocol used to support the SOCK_STREAM
abstraction. TCP uses the standard Internet address format and, in addi‐
tion, provides a per-host collection of “port addresses”. Thus, each
address is composed of an Internet address specifying the host and net‐
work, with a specific TCP port on the host identifying the peer entity.
Sockets utilizing the TCP protocol are either “active” or “passive”.
Active sockets initiate connections to passive sockets. By default, TCP
sockets are created active; to create a passive socket, the listen(2)
system call must be used after binding the socket with the bind(2) system
call. Only passive sockets may use the accept(2) call to accept incoming
connections. Only active sockets may use the connect(2) call to initiate
connections.
Passive sockets may “underspecify” their location to match incoming con‐
nection requests from multiple networks. This technique, termed
“wildcard addressing”, allows a single server to provide service to
clients on multiple networks. To create a socket which listens on all
networks, the Internet address INADDR_ANY must be bound. The TCP port
may still be specified at this time; if the port is not specified, the
system will assign one. Once a connection has been established, the
socket's address is fixed by the peer entity's location. The address
assigned to the socket is the address associated with the network inter‐
face through which packets are being transmitted and received. Normally,
this address corresponds to the peer entity's network.
TCP supports a number of socket options which can be set with
setsockopt(2) and tested with getsockopt(2):
TCP_INFO Information about a socket's underlying TCP session may
be retrieved by passing the read-only option TCP_INFO
to getsockopt(2). It accepts a single argument: a
pointer to an instance of struct tcp_info.
This API is subject to change; consult the source to
determine which fields are currently filled out by this
option. FreeBSD specific additions include send window
size, receive window size, and bandwidth-controlled
window space.
TCP_CCALGOOPT Set or query congestion control algorithm specific
parameters. See mod_cc(4) for details.
TCP_CONGESTION Select or query the congestion control algorithm that
TCP will use for the connection. See mod_cc(4) for
details.
TCP_FUNCTION_BLK Select or query the set of functions that TCP will use
for this connection. This allows a user to select an
alternate TCP stack. The alternate TCP stack must
already be loaded in the kernel. To list the available
TCP stacks, see functions_available in the MIB
Variables section further down. To list the default
TCP stack, see functions_default in the MIB Variables
section.
TCP_KEEPINIT This setsockopt(2) option accepts a per-socket timeout
argument of u_int in seconds, for new, non-established
TCP connections. For the global default in millisec‐
onds see keepinit in the MIB Variables section further
down.
TCP_KEEPIDLE This setsockopt(2) option accepts an argument of u_int
for the amount of time, in seconds, that the connection
must be idle before keepalive probes (if enabled) are
sent for the connection of this socket. If set on a
listening socket, the value is inherited by the newly
created socket upon accept(2). For the global default
in milliseconds see keepidle in the MIB Variables sec‐
tion further down.
TCP_KEEPINTVL This setsockopt(2) option accepts an argument of u_int
to set the per-socket interval, in seconds, between
keepalive probes sent to a peer. If set on a listening
socket, the value is inherited by the newly created
socket upon accept(2). For the global default in mil‐
liseconds see keepintvl in the MIB Variables section
further down.
TCP_KEEPCNT This setsockopt(2) option accepts an argument of u_int
and allows a per-socket tuning of the number of probes
sent, with no response, before the connection will be
dropped. If set on a listening socket, the value is
inherited by the newly created socket upon accept(2).
For the global default see the keepcnt in the MIB
Variables section further down.
TCP_NODELAY Under most circumstances, TCP sends data when it is
presented; when outstanding data has not yet been
acknowledged, it gathers small amounts of output to be
sent in a single packet once an acknowledgement is
received. For a small number of clients, such as win‐
dow systems that send a stream of mouse events which
receive no replies, this packetization may cause sig‐
nificant delays. The boolean option TCP_NODELAY
defeats this algorithm.
TCP_MAXSEG By default, a sender- and receiver-TCP will negotiate
among themselves to determine the maximum segment size
to be used for each connection. The TCP_MAXSEG option
allows the user to determine the result of this negoti‐
ation, and to reduce it if desired.
TCP_NOOPT TCP usually sends a number of options in each packet,
corresponding to various TCP extensions which are pro‐
vided in this implementation. The boolean option
TCP_NOOPT is provided to disable TCP option use on a
per-connection basis.
TCP_NOPUSH By convention, the sender-TCP will set the “push” bit,
and begin transmission immediately (if permitted) at
the end of every user call to write(2) or writev(2).
When this option is set to a non-zero value, TCP will
delay sending any data at all until either the socket
is closed, or the internal send buffer is filled.
TCP_MD5SIG This option enables the use of MD5 digests (also known
as TCP-MD5) on writes to the specified socket. Outgo‐
ing traffic is digested; digests on incoming traffic
are verified. When this option is enabled on a socket,
all inbound and outgoing TCP segments must be signed
with MD5 digests.
One common use for this in a FreeBSD router deployment
is to enable based routers to interwork with Cisco
equipment at peering points. Support for this feature
conforms to RFC 2385.
In order for this option to function correctly, it is
necessary for the administrator to add a tcp-md5 key
entry to the system's security associations database
(SADB) using the setkey(8) utility. This entry can
only be specified on a per-host basis at this time.
If an SADB entry cannot be found for the destination,
the system does not send any outgoing segments and
drops any inbound segments.
Each dropped segment is taken into account in the TCP
protocol statistics.
The option level for the setsockopt(2) call is the protocol number for
TCP, available from getprotobyname(3), or IPPROTO_TCP. All options are
declared in <netinet/tcp.h>.
Options at the IP transport level may be used with TCP; see ip(4).
Incoming connection requests that are source-routed are noted, and the
reverse source route is used in responding.
The default congestion control algorithm for TCP is cc_newreno(4). Other
congestion control algorithms can be made available using the mod_cc(4)
framework.
MIB Variables
The TCP protocol implements a number of variables in the net.inet.tcp
branch of the sysctl(3) MIB.
TCPCTL_DO_RFC1323 (rfc1323) Implement the window scaling and timestamp
options of RFC 1323 (default is true).
TCPCTL_MSSDFLT (mssdflt) The default value used for the maximum seg‐
ment size (“MSS”) when no advice to the contrary is
received from MSS negotiation.
TCPCTL_SENDSPACE (sendspace) Maximum TCP send window.
TCPCTL_RECVSPACE (recvspace) Maximum TCP receive window.
log_in_vain Log any connection attempts to ports where there is
not a socket accepting connections. The value of 1
limits the logging to SYN (connection establishment)
packets only. That of 2 results in any TCP packets to
closed ports being logged. Any value unlisted above
disables the logging (default is 0, i.e., the logging
is disabled).
msl The Maximum Segment Lifetime, in milliseconds, for a
packet.
keepinit Timeout, in milliseconds, for new, non-established TCP
connections. The default is 75000 msec.
keepidle Amount of time, in milliseconds, that the connection
must be idle before keepalive probes (if enabled) are
sent. The default is 7200000 msec (2 hours).
keepintvl The interval, in milliseconds, between keepalive
probes sent to remote machines, when no response is
received on a keepidle probe. The default is 75000
msec.
keepcnt Number of probes sent, with no response, before a con‐
nection is dropped. The default is 8 packets.
always_keepalive Assume that SO_KEEPALIVE is set on all TCP connec‐
tions, the kernel will periodically send a packet to
the remote host to verify the connection is still up.
icmp_may_rst Certain ICMP unreachable messages may abort connec‐
tions in SYN-SENT state.
do_tcpdrain Flush packets in the TCP reassembly queue if the sys‐
tem is low on mbufs.
blackhole If enabled, disable sending of RST when a connection
is attempted to a port where there is not a socket
accepting connections. See blackhole(4).
delayed_ack Delay ACK to try and piggyback it onto a data packet.
delacktime Maximum amount of time, in milliseconds, before a
delayed ACK is sent.
path_mtu_discovery
Enable Path MTU Discovery.
tcbhashsize Size of the TCP control-block hash table (read-only).
This may be tuned using the kernel option TCBHASHSIZE
or by setting net.inet.tcp.tcbhashsize in the
loader(8).
pcbcount Number of active process control blocks (read-only).
syncookies Determines whether or not SYN cookies should be gener‐
ated for outbound SYN-ACK packets. SYN cookies are a
great help during SYN flood attacks, and are enabled
by default. (See syncookies(4).)
isn_reseed_interval
The interval (in seconds) specifying how often the
secret data used in RFC 1948 initial sequence number
calculations should be reseeded. By default, this
variable is set to zero, indicating that no reseeding
will occur. Reseeding should not be necessary, and
will break TIME_WAIT recycling for a few minutes.
reass.cursegments The current total number of segments present in all
reassembly queues.
reass.maxsegments The maximum limit on the total number of segments
across all reassembly queues. The limit can be
adjusted as a tunable.
reass.maxqueuelen The maximum number of segments allowed in each
reassembly queue. By default, the system chooses a
limit based on each TCP connection's receive buffer
size and maximum segment size (MSS). The actual limit
applied to a session's reassembly queue will be the
lower of the system-calculated automatic limit and the
user-specified reass.maxqueuelen limit.
rexmit_min, rexmit_slop
Adjust the retransmit timer calculation for TCP. The
slop is typically added to the raw calculation to take
into account occasional variances that the SRTT
(smoothed round-trip time) is unable to accommodate,
while the minimum specifies an absolute minimum.
While a number of TCP RFCs suggest a 1 second minimum,
these RFCs tend to focus on streaming behavior, and
fail to deal with the fact that a 1 second minimum has
severe detrimental effects over lossy interactive con‐
nections, such as a 802.11b wireless link, and over
very fast but lossy connections for those cases not
covered by the fast retransmit code. For this reason,
we use 200ms of slop and a near-0 minimum, which gives
us an effective minimum of 200ms (similar to Linux).
initcwnd_segments Enable the ability to specify initial congestion win‐
dow in number of segments. The default value is 10 as
suggested by RFC 6928. Changing the value on fly
would not affect connections using congestion window
from the hostcache. Caution: This regulates the burst
of packets allowed to be sent in the first RTT. The
value should be relative to the link capacity. Start
with small values for lower-capacity links. Large
bursts can cause buffer overruns and packet drops if
routers have small buffers or the link is experiencing
congestion.
rfc3042 Enable the Limited Transmit algorithm as described in
RFC 3042. It helps avoid timeouts on lossy links and
also when the congestion window is small, as happens
on short transfers.
rfc3390 Enable support for RFC 3390, which allows for a vari‐
able-sized starting congestion window on new connec‐
tions, depending on the maximum segment size. This
helps throughput in general, but particularly affects
short transfers and high-bandwidth large propagation-
delay connections.
sack.enable Enable support for RFC 2018, TCP Selective Acknowledg‐
ment option, which allows the receiver to inform the
sender about all successfully arrived segments, allow‐
ing the sender to retransmit the missing segments
only.
sack.maxholes Maximum number of SACK holes per connection. Defaults
to 128.
sack.globalmaxholes
Maximum number of SACK holes per system, across all
connections. Defaults to 65536.
maxtcptw When a TCP connection enters the TIME_WAIT state, its
associated socket structure is freed, since it is of
negligible size and use, and a new structure is allo‐
cated to contain a minimal amount of information nec‐
essary for sustaining a connection in this state,
called the compressed TCP TIME_WAIT state. Since this
structure is smaller than a socket structure, it can
save a significant amount of system memory. The
net.inet.tcp.maxtcptw MIB variable controls the maxi‐
mum number of these structures allocated. By default,
it is initialized to kern.ipc.maxsockets / 5.
nolocaltimewait Suppress creating of compressed TCP TIME_WAIT states
for connections in which both endpoints are local.
fast_finwait2_recycle
Recycle TCP FIN_WAIT_2 connections faster when the
socket is marked as SBS_CANTRCVMORE (no user process
has the socket open, data received on the socket can‐
not be read). The timeout used here is
finwait2_timeout.
finwait2_timeout Timeout to use for fast recycling of TCP FIN_WAIT_2
connections. Defaults to 60 seconds.
ecn.enable Enable support for TCP Explicit Congestion Notifica‐
tion (ECN). ECN allows a TCP sender to reduce the
transmission rate in order to avoid packet drops.
Settings:
0 Disable ECN.
1 Allow incoming connections to request
ECN. Outgoing connections will request
ECN.
2 Allow incoming connections to request
ECN. Outgoing connections will not
request ECN.
ecn.maxretries Number of retries (SYN or SYN/ACK retransmits) before
disabling ECN on a specific connection. This is
needed to help with connection establishment when a
broken firewall is in the network path.
pmtud_blackhole_detection
Turn on automatic path MTU blackhole detection. In
case of retransmits OS will lower the MSS to check if
it's MTU problem. If current MSS is greater than con‐
figured value to try, it will be set to configured
value, otherwise, MSS will be set to default values
(net.inet.tcp.mssdflt and net.inet.tcp.v6mssdflt).
pmtud_blackhole_mss
MSS to try for IPv4 if PMTU blackhole detection is
turned on.
v6pmtud_blackhole_mss
MSS to try for IPv6 if PMTU blackhole detection is
turned on.
pmtud_blackhole_activated
Number of times configured values were used in an
attempt to downshift.
pmtud_blackhole_activated_min_mss
Number of times default MSS was used in an attempt to
downshift.
pmtud_blackhole_failed
Number of connections for which retransmits continued
even after MSS downshift.
functions_available
List of available TCP function blocks (TCP stacks).
functions_default The default TCP function block (TCP stack).
functions_inherit_listen_socket_stack
Determines whether to inherit listen socket's tcp
stack or use the current system default tcp stack, as
defined by functions_default
). Default is true.
insecure_rst Use criteria defined in RFC793 instead of RFC5961 for
accepting RST segments. Default is false.
insecure_syn Use criteria defined in RFC793 instead of RFC5961 for
accepting SYN segments. Default is false.
ERRORS
A socket operation may fail with one of the following errors returned:
[EISCONN] when trying to establish a connection on a socket
which already has one;
[ENOBUFS] or [ENOMEM]
when the system runs out of memory for an internal
data structure;
[ETIMEDOUT] when a connection was dropped due to excessive
retransmissions;
[ECONNRESET] when the remote peer forces the connection to be
closed;
[ECONNREFUSED] when the remote peer actively refuses connection
establishment (usually because no process is listening
to the port);
[EADDRINUSE] when an attempt is made to create a socket with a port
which has already been allocated;
[EADDRNOTAVAIL] when an attempt is made to create a socket with a net‐
work address for which no network interface exists;
[EAFNOSUPPORT] when an attempt is made to bind or connect a socket to
a multicast address.
[EINVAL] when trying to change TCP function blocks at an
invalid point in the session;
[ENOENT] when trying to use a TCP function block that is not
available;
SEE ALSO
getsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4),
ip(4), mod_cc(4), siftr(4), syncache(4), setkey(8), tcp_functions(9)
V. Jacobson, R. Braden, and D. Borman, TCP Extensions for High
Performance, RFC 1323.
A. Heffernan, Protection of BGP Sessions via the TCP MD5 Signature
Option, RFC 2385.
K. Ramakrishnan, S. Floyd, and D. Black, The Addition of Explicit
Congestion Notification (ECN) to IP, RFC 3168.
HISTORY
The TCP protocol appeared in 4.2BSD. The RFC 1323 extensions for window
scaling and timestamps were added in 4.4BSD. The TCP_INFO option was
introduced in Linux 2.6 and is subject to change.
BSD August 6, 2018 BSD