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>

       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_CONGESTION Select or query the congestion control algorithm that TCP will use for the connection.
See mod_cc(4) for details.

TCP_KEEPINIT This write-only 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 write-only 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 write-only 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 write-only 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
if the net.inet.tcp.signature_verify_input sysctl is nonzero. The current default
behavior for the system is to respond to a system advertising this option with TCP-MD5;
this may change.

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. Only IPv4 (AF_INET) sessions are supported.

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 must have an SPI of 0x1000 and can therefore only be
specified on a per-host basis at this time.

If an SADB entry cannot be found for the destination, the outgoing traffic will have an
invalid digest option prepended, and the following error message will be visible on the
system console: tcp_signature_compute: SADB lookup failed for %d.%d.%d.%d.

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).

slowstart_flightsize
The number of packets allowed to be in-flight during the TCP slow-start phase on a
non-local network.

local_slowstart_flightsize
The number of packets allowed to be in-flight during the TCP slow-start phase to local
machines in the same subnet.

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.

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).

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.

When this feature is enabled, the slowstart_flightsize and local_slowstart_flightsize
settings are not observed for new connection slow starts, but they are still used for
slow starts that occur when the connection has been idle and starts sending again.

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.

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.

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]          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.

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                                          November 19, 2012                                         TCP(4)

NAME

SYNOPSIS

DESCRIPTION

ERRORS

SEE ALSO

HISTORY