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_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).

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.

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.

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.

NAME

SYNOPSIS

DESCRIPTION

ERRORS

SEE ALSO

HISTORY