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TTCP(4)                  BSD Kernel Interfaces Manual                  TTCP(4)

     ttcp -- Transmission Control Protocol Extensions for Transactions

     #include <&lt;sys/types.h>&gt;
     #include <&lt;sys/socket.h>&gt;
     #include <&lt;netinet/in.h>&gt;
     #include <&lt;netinet/tcp.h>&gt;

     setsockopt(sock, IPPROTO_TCP, TCP_NOPUSH, &amp;One, sizeof One);

     sendto(sock, msg, len, MSG_EOF, &amp;sin, sizeof sin);

     sendto(sock, msg, len, MSG_EOF, 0, 0);

     T/TCP refers to a set of extensions to the TCP protocol (see tcp(4))
     which permit hosts to reliably exchange a small amount of data in a two-
     packet exchange, thus eliminating the extra round-trip delays inherent in
     a standard TCP connection.  The socket interface includes modifications
     to support T/TCP, detailed here for the specific case, and in the
     socket(2) and send(2) manual pages for the protocol-independent support.
     T/TCP is defined in RFC 1644.

     The T/TCP extensions work by including certain options in all segments of
     a particular connection, which enable the implementation to avoid the
     three-way handshake for all but the first connection between a pair of
     hosts.  These same options also make it possible to more reliably recog-
     nize old, duplicate packets, which in turn reduces the amount of time the
     TCP protocol must maintain state after a connection closes.  The
     ``net.inet.tcp.rfc1644'' MIB variable can be used to disable T/TCP nego-
     tiation at run time; however, the protocol has been designed to ensure
     that attempts by non-T/TCP systems to communicate with T/TCP-enhanced
     ones automatically degenerate into standard TCP.

     The expected model of a ``transaction'' as used by T/TCP is a fairly sim-
     ple one:

     1.   A client program generates a request to be sent to the server, which
          is small enough to fit in a single TCP segment, and sends a SYN PUSH
          FIN segment with options and data to the server.

     2.   The server program accepts the request in the same manner as for
          regular TCP connections, interprets it, and generates a reply which
          may be small enough to fit in a single segment.  If it is, the reply
          is sent in a single SYN PUSH FIN ACK segment with (different)
          options and data back to the client.  If not, then the connection
          degenerates into (almost) the usual case for TCP.  The server then
          closes its socket.

     3.   The client reads the reply and closes its socket.

     Support on the client side is provided by extending the semantics of the
     sendto(2) and sendmsg(2) system calls to understand the notion of
     ``implied connect'' and ``send and shutdown''.  To send the request in a
     transaction, the sendto(2) system call is typically used, as in the fol-
     lowing example:

           char request[REQ_LEN];
           struct sockaddr_in sin;
           int sock, req_len;

           sock = socket(PF_INET, SOCK_STREAM, 0);

           /* prepare request[] and sin */

           err = sendto(sock, request, req_len, MSG_EOF,
                   (struct sockaddr *)&sin, sin.sin_len);

           /* do something if error */

           req_len = read(sock, request, sizeof request);

           /* do something with the reply */

     Note that, after the call to sendto(), the socket is now in the same
     state as if the connect(2) and shutdown(2) system calls had been used.
     That is to say, the only reasonable operations to perform on this socket
     are read(2) and close(2).  (As the client's TCP sender is already shut
     down, it is not possible to connect(2) this socket to another destina-

     There are two different options available for servers using T/TCP:

     1.   Set the TCP_NOPUSH socket option, and use normal write(2) calls when
          formulating the response.

     2.   Use sendto(2) with the MSG_EOF flag, as in the client, but with the
          destination unspecified.

     The first option is generally the appropriate choice when converting
     existing servers to use T/TCP extensions; simply add a call to
     setsockopt(sock, IPPROTO_TCP, TCP_NOPUSH, &amp;One, sizeof One) (where One is
     an integer variable with a non-zero value).  The server socket must be
     closed before any data is sent (unless the socket buffers fill up).

     The second option is preferable for new servers, and is sometimes easy
     enough to retrofit into older servers.  In this case, where the reply
     phase would ordinarily have included a call to write(), one substitutes:

           sendto(sock, buf, len, MSG_EOF, (struct sockaddr *)0, 0)

     In this case, the reply is sent immediately, but as in the client case,
     the socket is no longer useful for anything and should be immediately

     The T/TCP extensions require the ``net.inet.tcp.rfc1644'' MIB variable to
     be true in order for the appropriate TCP options to be sent.  See tcp(4)
     for more information.

     send(2), setsockopt(2), inet(4), tcp(4)

     R. Braden, T/TCP - TCP Extensions for Transactions, RFC 1644.

     Support for T/TCP first appeared in FreeBSD 2.1, based on code written by
     Bob Braden and Liming Wei at the University of Southern California,
     Information Sciences Institute, and ported by Andras Olah at the Univer-
     sity of Twente.

BSD                            January 18, 1995                            BSD