[tcpm] new-cwv: Proposed change to recovery equations.

[Gorry Fairhurst <[e-mail]>]

Prior to the last TCPM WG, we received a question about why we used a 
different adjustment of cwnd at the start and end of the recovery phase 
in 
https://datatracker.ietf.org/doc/draft-ietf-tcpm-newcwv/?include_text=1, 
section 4.4.1. We agreed to explore whether we could find a single 
approach, rather than reductions based on different terms.

The authors have now looked at various pathologies, and we think that 
the updated equations below seem appropriate. These now use the same set 
of terms, except that the first can not include the "R" term, since this 
is only discovered during recovery.

If people have comments or questions, please say. We plan to prepare an 
updated draft with this change and including other corrections received.

Gorry, Raffaello, Arjuna.

--- Excerpt of proposed new text:

    A sender that detects a packet-drop, or receives an indication of an
    ECN marked packet, MUST record the current FlightSize in the variable
    LossFlightSize and MUST calculate a safe cwnd for loss recovery using
    the method below:

            cwnd = (Max(pipeACK,LossFlightSize))/2.

...

    At the end of the recovery phase, the TCP sender MUST reset the cwnd
    using the method below:

            cwnd = (Max(pipeACK,LossFlightSize) - R)/2.

    Where, R is the volume of data that was retransmitted during the
    recovery phase.