Ubuntu Manpage: tcp — Internet Transmission Control Protocol

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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 addition, provides a per-host collection of “port addresses”. Thus, each address is composed of an
Internet address specifying the host and network, 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 connection 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 interface 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 milliseconds 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” section 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 milliseconds 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 window systems that send a stream of mouse events which receive no replies, this
packetization may cause significant 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 negotiation, 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 provided 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. Outgoing 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 segment 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 connection is dropped. The default
is 8 packets.

always_keepalive Assume that SO_KEEPALIVE is set on all TCP connections, 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 connections in SYN-SENT state.

do_tcpdrain Flush packets in the TCP reassembly queue if the system 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 generated 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_initial, 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 connections, 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). The initial value is
used before an RTT measurement has been performed.

initcwnd_segments Enable the ability to specify initial congestion window 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.

rfc6675_pipe Calculate the bytes in flight using the algorithm described in RFC 6675, and is also a
prerequisite to enable Proportional Rate Reduction.

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 variable-sized starting congestion
window on new connections, 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 Acknowledgment option, which allows the
receiver to inform the sender about all successfully arrived segments, allowing 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 allocated to
contain a minimal amount of information necessary 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 maximum 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 cannot 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 Notification (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 configured 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.

ts_offset_per_conn
When initializing the TCP timestamps, use a per connection offset instead of a per
host pair offset. Default is to use per connection offsets as recommended in RFC
7323.

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 network 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;

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.

Debian                                          December 1, 2019                                          TCP(4)

NAME

SYNOPSIS

DESCRIPTION

ERRORS

SEE ALSO

HISTORY