netinet/tcp_timer.h

Keep the external definition the same for binary compatibility

#define TCPT_NTIMERS_EXT 4

Definitions of the TCP timers. The TCPT_PTO timer is used for probing for a tail loss in a send window. If this probe gets acknowledged using SACK, it will allow the connection to enter fast-recovery instead of hitting a retransmit timeout. A probe timeout will send the last unacknowledged segment to generate more acks with SACK information which can be used for fast-retransmiting the lost packets. This will fire in the order of 10ms. The TCPT_REXMT timer is used to force retransmissions. The TCP has the TCPT_REXMT timer set whenever segments have been sent for which ACKs are expected but not yet received. If an ACK is received which advances tp->snd_una, then the retransmit timer is cleared (if there are no more outstanding segments) or reset to the base value (if there are more ACKs expected). Whenever the retransmit timer goes off, we retransmit one unacknowledged segment, and do a backoff on the retransmit timer. The TCPT_DELACK timer is used for transmitting delayed acknowledgements if an acknowledgement was delayed in anticipation of a new segment. The TCPT_PERSIST timer is used to keep window size information flowing even if the window goes shut. If all previous transmissions have been acknowledged(so that there are no retransmissions in progress), and the window is too small to bother sending anything, then we start the TCPT_PERSIST timer. When it expires, if the window is nonzero, we go to transmit state. Otherwise, at intervals send a single byte into the peer's window to force him to update our window information. We do this at most as often as TCPT_PERSMIN time intervals, but no more frequently than the current estimate of round-trip packet time. The TCPT_PERSIST timer is cleared whenever we receive a window update from the peer. The TCPT_KEEP timer is used to keep connections alive. If an connection is idle (no segments received) for TCPTV_KEEP_INIT amount of time, but not yet established, then we drop the connection. Once the connection is established, if the connection is idle for TCPTV_KEEP_IDLE time (and keepalives have been enabled on the socket), we begin to probe the connection. We force the peer to send us a segment by sending: <SEQ=SND.UNA-1><ACK=RCV.NXT><CTL=ACK> This segment is (deliberately) outside the window, and should elicit an ack segment in response from the peer. If, despite the TCPT_KEEP initiated segments we cannot elicit a response from a peer in TCPT_MAXIDLE amount of time probing, then we drop the connection. The TCPT_2MSL timer is used for keeping the conenction in Time-wait state before fully closing it so that the connection 4-tuple can be reused.

#define TCPT_REXMT 0