TTCP(4)

NAME
ttcp - Transmission Control Protocol Extensions for Transactions

#include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <netinet/tcp.h> int setsockopt(sock, IPPROTO_TCP, TCP_NOPUSH, &One, sizeof One)
ssize_t sendto(sock, msg, len, MSG_EOF, &sin, sizeof sin)
ssize_t sendto(sock, msg, len, MSG_EOF, 0, 0)

DESCRIPTION
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 recognize 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 negotiation 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.

TRANSACTION MODEL
The expected model of a “transaction” as used by T/TCP is a fairly simple 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.

CLIENT SUPPORT
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 following 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);
    close(sock);
    /* 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). (Because the client's TCP sender is already shut down, it is not possible to connect(2) this socket to another destination.)

SERVER SUPPORT
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, &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 lnoger useful for anything and should be immediately closed.

MIB VARIABLES
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.

SEE ALSO
send(2), setsockopt(2), inet(4), tcp(4) R. Braden, T/TCP - TCP Extensions for Transactions, RFC 1644.

HISTORY
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 University of Twente.