Unix Network Programming Episode 57

SO_KEEPALIVE Socket Option

When the keep-alive option is set for a TCP socket and no data has been exchanged across the socket in either direction for two hours, TCP automatically sends a keep-alive probe to the peer. This probe is a TCP segment to which the peer must respond. One of three scenarios results:

1.The peer responds with the expected ACK. The application is not notified (since everything is okay). TCP will send another probe following another two hours of inactivity.

2.The peer responds with an RST, which tells the local TCP that the peer host has crashed and rebooted. The socket’s pending error is set to ECONNRESET and the socket is closed.

3.There is no response from the peer to the keep-alive probe. Berkeley-derived TCPs send 8 additional probes, 75 seconds apart, trying to elicit a response. TCP will give up if there is no response within 11 minutes and 15 seconds after sending the first probe.

SO_LINGER Socket Option

This option specifies how the close function operates for a connection-oriented protocol (e.g., for TCP and SCTP, but not for UDP). By default, close returns immediately, but if there is any data still remaining in the socket send buffer, the system will try to deliver the data to the peer.

The SO_LINGER socket option lets us change this default. This option requires the following structure to be passed between the user process and the kernel. It is defined by including <sys/socket.h>.

struct linger
{
    int l_onoff;
    int l_linger;
}

Calling setsockopt leads to one of the following three scenarios, depending on the values of the two structure members:

1.If l_onoff is 0, the option is turned off. The value of l_linger is ignored and the previously discussed TCP default applies: close returns immediately.

2.If l_onoff is nonzero and l_linger is zero, TCP aborts the connection when it is closed (pp. 1019–1020 of TCPv2). That is, TCP discards any data still remaining in the socket send buffer and sends an RST to the peer, not the normal four-packet connection termination sequence (Section 2.6(See 7.2.6)). We will show an example of this in Figure 16.21(See 9.5.6). This avoids TCP’s TIME_WAIT state, but in doing so, leaves open the possibility of another incarnation of this connection being created within 2MSL seconds (Section 2.7(See 7.2.7)) and having old duplicate segments from the just-terminated connection being incorrectly delivered to the new incarnation.

3.If l_onoff is nonzero and l_linger is nonzero, then the kernel will linger when the socket is closed (p. 472 of TCPv2). That is, if there is any data still remaining in the socket send buffer, the process is put to sleep until either: (i) all the data is sent and acknowledged by the peer TCP, or (ii) the linger time expires. If the socket has been set to nonblocking (Chapter 16(See 9.5)), it will not wait for the close to complete, even if the linger time is nonzero. When using this feature of the SO_LINGER option, it is important for the application to check the return value from close, because if the linger time expires before the remaining data is sent and acknowledged, close returns EWOULDBLOCK and any remaining data in the send buffer is discarded.

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