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TCP(4) | Device Drivers Manual | TCP(4) |
NAME¶
tcp — Internet Transmission Control ProtocolSYNOPSIS¶
#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 theSOCK_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_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. In the current release, only
outgoing traffic is digested; digests on incoming traffic are not
verified. 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.
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.
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.
- keepidle
- Amount of time, in milliseconds, that the connection must be idle before keepalive probes (if enabled) are sent.
- keepintvl
- The interval, in milliseconds, between keepalive probes
sent to remote machines. After
TCPTV_KEEPCNT
(default 8) probes are sent, with no response, the connection is dropped. - 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.
- newreno
- Enable TCP NewReno Fast Recovery algorithm, as described in RFC 2582.
- 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).
- inflight.enable
- Enable TCP bandwidth-delay product limiting. An attempt will be made to calculate the bandwidth-delay product for each individual TCP connection, and limit the amount of inflight data being transmitted, to avoid building up unnecessary packets in the network. This option is recommended if you are serving a lot of data over connections with high bandwidth-delay products, such as modems, GigE links, and fast long-haul WANs, and/or you have configured your machine to accommodate large TCP windows. In such situations, without this option, you may experience high interactive latencies or packet loss due to the overloading of intermediate routers and switches. Note that bandwidth-delay product limiting only effects the transmit side of a TCP connection.
- inflight.debug
- Enable debugging for the bandwidth-delay product algorithm.
- inflight.min
- This puts a lower bound on the bandwidth-delay product window, in bytes. A value of 1024 is typically used for debugging. 6000-16000 is more typical in a production installation. Setting this value too low may result in slow ramp-up times for bursty connections. Setting this value too high effectively disables the algorithm.
- inflight.max
- This puts an upper bound on the bandwidth-delay product window, in bytes. This value should not generally be modified, but may be used to set a global per-connection limit on queued data, potentially allowing you to intentionally set a less than optimum limit, to smooth data flow over a network while still being able to specify huge internal TCP buffers.
- inflight.stab
- The bandwidth-delay product algorithm requires a slightly larger window than it otherwise calculates for stability. This parameter determines the extra window in maximal packets / 10. The default value of 20 represents 2 maximal packets. Reducing this value is not recommended, but you may come across a situation with very slow links where the ping(8) time reduction of the default inflight code is not sufficient. If this case occurs, you should first try reducing inflight.min and, if that does not work, reduce both inflight.min and inflight.stab, trying values of 15, 10, or 5 for the latter. Never use a value less than 5. Reducing inflight.stab can lead to upwards of a 20% underutilization of the link as well as reducing the algorithm's ability to adapt to changing situations and should only be done as a last resort.
- 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 asSBS_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.
SEE ALSO¶
getsockopt(2), socket(2), sysctl(3), blackhole(4), inet(4), intro(4), ip(4), syncache(4), setkey(8) 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. TheTCP_INFO
option
was introduced in Linux 2.6 and is subject to change.August 16, 2008 | Debian |