[tcpm] Settling Time and RFC3517bis

[Anthony Sabatini <[e-mail]>]

Settling Time is the concept of how long we wait for an out of order packet.  It has been an unnamed mainstay of IPv4 from the beginning, masquerading under the name "Fragmentation Reassembly Timer" but at the TCP level its physical existence has gone unacknowledged.

Settling Time is rather easy to calculate as it has a ceiling of .25*RTT (any number over that and you are in retransmission territory) and a floor of Fragmentation Reassembly Time * 1.25 (need to leave enough time to process reassembled packet).  Obviously the lower number is better and there are probably thousands of emails on how  to set the Fragmentation Reassembly Timer properly.

The point here is we are dancing around issues by saying we wait for multiple ACKs before a SACK block is processed, you are badly simulating reassembly delay.  Better to act on the first appearance of a hole in the sequence but delay the actual call for retransmission by Settling Time to see if the world has changed before you issue the retransmission.  Also the time represented by multiple ACKs is extremely variable,  short segments and random times for WOW, long segments and continuous for file transfer. 

Note for the record a DELAY TIMER is not in and of itself an event, it is delaying and existing event, therefore its expiration does not have any implications of its own, the event already  happened.

Tony

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