TCP is the virtual circuit protocol of the Internet protocol family.
It provides reliable, flow-controlled, in order, two-way transmission of data.
It is a byte-stream protocol layered above the Internet Protocol (IP), or the Internet Protocol Version 6 (IPv6),
the Internet protocol family's internetwork datagram delivery protocol.
Programs can access TCP using the
socket interface as a SOCK_STREAM
socket type, or using the Transport Level Interface (TLI)
where it supports the connection-oriented (T_COTS_ORD)
TCP uses IP's host-level addressing and adds its own per-host collection of "port
addresses." The endpoints of a TCP
connection are identified by the combination of an IP or IPv6 address and a TCP port
number. Although other protocols, such as the User Datagram Protocol (UDP),
may use the same host and port address format, the port space of these protocols
is distinct. See inet(7P) and inet6(7p) for details on the common aspects of
addressing in the Internet protocol family.
Sockets utilizing TCP are either "active"
or "passive." Active sockets initiate connections to passive sockets.
Both types of sockets must have their local IP
or IPv6 address and TCP port number bound
with the bind(3SOCKET)
system call after the socket is created. By default, TCP sockets are active. A passive socket is created by calling the listen(3SOCKET)
system call after binding the socket with bind(). This
establishes a queueing parameter for the passive socket. After this, connections
to the passive socket can be received with the accept(3SOCKET) system call. Active
sockets use the connect(3SOCKET)
call after binding to initiate connections.
By using the special value INADDR_ANY with IP, or the unspecified address (all zeroes) with IPv6, the local IP address can be left unspecified in the bind() call by either active or passive TCP sockets. This feature is usually used if the local address is either
unknown or irrelevant. If left unspecified, the local IP or IPv6 address will be bound at connection time to the address
of the network interface used to service the connection.
Once a connection has been established, data can be exchanged using
the read(2) and write(2) system calls.
Under most circumstances, TCP sends
data when it is presented. When outstanding data has not yet been acknowledged, TCP gathers small amounts of output to be sent in
a single packet once an acknowledgement has been 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. To circumvent
this problem, TCP provides a socket-level
boolean option, TCP_NODELAY. TCP_NODELAY is defined in <netinet/tcp.h>, and is set with setsockopt(3SOCKET)
and tested with getsockopt(3SOCKET).
The option level for the setsockopt() call is the protocol
number for TCP, available from getprotobyname(3SOCKET).
Another socket level option, SO_RCVBUF, can be used to control the window that TCP advertises to the peer. IP
level options may also be used with TCP.
See ip(7P) and ip6(7p).
TCP provides an urgent data mechanism,
which may be invoked using the out-of-band provisions of send(3SOCKET). The caller may mark
one byte as "urgent" with the MSG_OOB flag to send(3SOCKET).
This sets an "urgent pointer" pointing to this byte in the TCP stream. The receiver on the other side of the
stream is notified of the urgent data by a SIGURG signal. The SIOCATMARK ioctl(2) request returns
a value indicating whether the stream is at the urgent mark. Because the system
never returns data across the urgent mark in a single read(2) call, it is possible to advance to the
urgent data in a simple loop which reads data, testing the socket with the SIOCATMARK ioctl() request,
until it reaches the mark.
Incoming connection requests that include an IP source route option are noted, and the reverse source route is
used in responding.
A checksum over all data helps TCP
implement reliability. Using a window-based flow control mechanism that makes
use of positive acknowledgements, sequence numbers, and a retransmission strategy, TCP can usually recover when datagrams are damaged,
delayed, duplicated or delivered out of order by the underlying communication
If the local TCP receives no acknowledgements
from its peer for a period of time, as would be the case if the remote machine
crashed, the connection is closed and an error is returned to the user. If
the remote machine reboots or otherwise loses state information about a TCP connection, the connection is aborted and an error
is returned to the user.
SunOS supports TCP Extensions for High
Performance (RFC 1323) which includes the window scale and time stamp options,
and Protection Against Wrap Around Sequence Numbers (PAWS). SunOS also supports
Selective Acknowledgment (SACK) capabilities (RFC 2018) and Explicit Congestion
Notification (ECN) mechanism (RFC 3168).
Turn on the window scale option in one of the following ways:
- An application can set SO_SNDBUF or SO_RCVBUF size in
the setsockopt() option to be larger than 64K. This must
be done before the program calls listen() or connect(), because the window scale option
is negotiated when the connection is established. Once the connection has
been made, it is too late to increase the send or receive window beyond the
default TCP limit of 64K.
- For all applications, use ndd(1M)
to modify the configuration parameter tcp_wscale_always.
If tcp_wscale_always is set to 1, the
window scale option will always be set when connecting to a remote system.
If tcp_wscale_always is 0, the window
scale option will be set only if the user has requested a send or receive
window larger than 64K. The default value of tcp_wscale_always
- Regardless of the value of tcp_wscale_always,
the window scale option will always be included in a connect acknowledgement
if the connecting system has used the option.
Turn on SACK capabilities in the following
- Use ndd to modify the configuration parameter tcp_sack_permitted. If tcp_sack_permitted is
set to 0, TCP will not
accept SACK or send out SACK information. If tcp_sack_permitted is set
to 1, TCP will not initiate
a connection with SACK permitted option in
the SYN segment, but will respond with SACK permitted option in the SYN|ACK segment if an incoming connection request has the SACK permitted option. This means that TCP will only accept SACK information
if the other side of the connection also accepts SACK information. If tcp_sack_permitted is set
to 2, it will both initiate and accept connections with SACK information. The default for tcp_sack_permitted is 2 (active enabled).
Turn on TCP ECN mechanism in the following
- Use ndd to modify the configuration parameter tcp_ecn_permitted. If tcp_ecn_permitted is set
to 0, TCP will not negotiate
with a peer that supports ECN mechanism. If tcp_ecn_permitted is set to 1 when initiating a connection, TCP
will not tell a peer that it supports ECN mechanism. However, it will tell
a peer that it supports ECN mechanism when
accepting a new incoming connection request if the peer indicates that it
supports ECN mechanism in the SYN segment. If tcp_ecn_permitted
is set to 2, in addition to negotiating with a peer on ECN mechanism when
accepting connections, TCP will indicate in the outgoing SYN segment that
it supports ECN mechanism when TCP makes active outgoing connections. The default for tcp_ecn_permitted is 1.
Turn on the time stamp option in the following way:
- Use ndd to modify the configuration parameter tcp_tstamp_always. If tcp_tstamp_always is 1, the time stamp option will always be set when connecting to
a remote machine. If tcp_tstamp_always is 0, the timestamp option will not be set when connecting to a remote
system. The default for tcp_tstamp_always is 0.
- Regardless of the value of tcp_tstamp_always,
the time stamp option will always be included in a connect acknowledgement
(and all succeeding packets) if the connecting system has used the time stamp
Use the following procedure to turn on the time stamp option only when
the window scale option is in effect:
- Use ndd to modify the configuration parameter tcp_tstamp_if_wscale. Setting tcp_tstamp_if_wscale
to 1 will cause the time stamp option to be set when connecting
to a remote system, if the window scale option has been set. If tcp_tstamp_if_wscale is 0, the time stamp option
will not be set when connecting to a remote system. The default for tcp_tstamp_if_wscale is 1.
Protection Against Wrap Around Sequence Numbers (PAWS) is always used
when the time stamp option is set.
SunOS also supports multiple methods of generating initial sequence
numbers. One of these methods is the improved technique suggested in RFC 1948. We HIGHLY recommend
that you set sequence number generation parameters to be as close to boot
time as possible. This prevents sequence number problems on connections that
use the same connection-ID as ones that used a different sequence number generation.
The /etc/init.d/inetinit script contains commands which
configure initial sequence number generation. The script reads the value
contained in the configuration file /etc/default/inetinit
to determine which method to use.
The /etc/default/inetinit file is an unstable interface,
and may change in future releases.
TCP may be configured to report some information
on connections that terminate by means of an RST packet.
By default, no logging is done. If the ndd(1M)
parameter tcp_trace is set to 1, then trace data is
collected for all new connections established after that time.
The trace data consists of the TCP headers and IP source and destination addresses of the last few packets sent
in each direction before RST occurred. Those packets are logged in a series
of strlog(9F) calls.
This trace facility has a very low overhead, and so is superior to such utilities
as snoop(1M) for
non-intrusive debugging for connections terminating by means of an RST.