4.6. Counters and Timers

Restart Timer

There is one special timer used by the automaton. The Restart timer is used to time transmissions of Configure-Request and Terminate-Request packets. Expiration of the Restart timer causes a Timeout event, and retransmission of the corresponding Configure-Request or Terminate-Request packet. The Restart timer MUST be configurable, but SHOULD default to three (3) seconds.

Implementation Note:

The Restart timer SHOULD be based on the speed of the link. The default value is designed for low speed (2,400 to 9,600 bps), high switching latency links (typical telephone lines). Higher speed links, or links with low switching latency, SHOULD have correspondingly faster retransmission times.

Instead of a constant value, the Restart timer MAY begin at an initial small value and increase to the configured final value. Each successive value less than the final value SHOULD be at least twice the previous value. The initial value SHOULD be large enough to account for the size of the packets, twice the round trip time for transmission at the link speed, and at least an additional 100 milliseconds to allow the peer to process the packets before responding. Some circuits add another 200 milliseconds of satellite delay. Round trip times for modems operating at 14,400 bps have been measured in the range of 160 to more than 600 milliseconds.

Max-Terminate

There is one required restart counter for Terminate-Requests. Max-Terminate indicates the number of Terminate-Request packets sent without receiving a Terminate-Ack before assuming that the peer is unable to respond. Max-Terminate MUST be configurable, but SHOULD default to two (2) transmissions.

Max-Configure

A similar counter is recommended for Configure-Requests. Max- Configure indicates the number of Configure-Request packets sent without receiving a valid Configure-Ack, Configure-Nak or Configure-Reject before assuming that the peer is unable to respond. Max-Configure MUST be configurable, but SHOULD default to ten (10) transmissions.

Max-Failure

A related counter is recommended for Configure-Nak. Max-Failure indicates the number of Configure-Nak packets sent without sending a Configure-Ack before assuming that configuration is not converging. Any further Configure-Nak packets for peer requested options are converted to Configure-Reject packets, and locally desired options are no longer appended. Max-Failure MUST be configurable, but SHOULD default to five (5) transmissions.