Re: [quicwg/base-drafts] Largest acked in ACK frame MUST NOT decrease (#2205)

[Magnus Westerlund <[e-mail]>]

My concern was primarily about the clearness of this behavior in the specification text. In transport I find the following which indicates this but are not explicit. 

13.1.1:    To limit receiver state or the size of ACK frames, a receiver MAY
   limit the number of ACK Ranges it sends.

19.3:

  Largest Acknowledged:  A variable-length integer representing the
      largest packet number the peer is acknowledging; this is usually
      the largest packet number that the peer has received prior to
      generating the ACK frame. 

I think there are strong hints, with the above "usually the largest" indicating that it is not always. I would have made this explicit in 19.3, but then I prefer being more explicit than not. I think it is possible interop issue point. Anywhere else this is more explicit? (I have read github master version)

@mikkelfj I think there are strong points for allowing an ACK frame to acknowledge only a part of the range between highest received and confirmed to be have been ACKed. The two main reasons I see.
- high rate flows suffering an packet loss burst so that the number of ACK ranges becomes to large to fit a single QUIC packet. 
- Low complexity receivers ACKing a single range of received packets and doing this for reordered packets.
 

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