Re: [tcpm] Comments on draft-blanton-tcpm-3517bis-01

[Mark Allman <[e-mail]>]

> 1) IsLost (SeqNum):
> 
>       This routine returns whether the given sequence number is
>       considered to be lost.  The routine returns true when either
>       DupThresh discontiguous SACKed sequences have arrived above
>       'SeqNum' or more than (DupThresh - 1) * SMSS bytes with sequence
>       numbers greater than 'SeqNum' have been SACKed.  Otherwise, the
>       routine returns false.
> 
> Does RFC require DupThresh discontinuous sack blocks? e.g.,
> For example, if a sender sends 4 data packets
> send pkt1: 1-100
> send pkt2: 101-200
> send pkt3: 201-300
> send pkt4: 301-400
> receive ack: ack1 with one sack block 101-400.
> does IsLost(1) returns true?

Yes.

If **EITHER**:

  - You have DupThresh distinct blocks above 1 (so, e.g., if you
    continue your example a few more and you collect up SACKs for
    101-200, 301-400, 501-500 you'd declare sequence number 1 to be
    lost), 

    **OR**,

  - You have (DupThresh-1) * SMSS bytes SACKed in the scoreboard (so,
    (400-101) > (2 * 100) in the case you present)

then you declare the given sequence number to be lost.

> 2)
> If the incoming ACK is a duplicate acknowledgment and the TCP is
> 
>    not currently in loss recovery, the TCP MUST increase DupAcks by
>    one and take the following steps:
> 
>    (1) If DupAcks >= DupThresh, go to step (4).
> 
>    (2) If DupAcks < DupThresh but IsLost (SND.UNA) returns
>        true---indicating at least three segments have arrived above
>        the current cumulative acknowledgment point, which is taken
>        to indicate loss---go to step (4).
> 
> can we combine (1) and (2)? go to steps (4) if IsLost(SND.UNA) is
> true. I guess it depends on your answer of my first question.

My first hit here is that this could indeed be tightened up by just
getting rid of DupAcks and consulting IsLost (SND.UNA).  If that returns
true we enter loss recovery, if not we don't.  Do I understand that to
be what you're saying?

(I do need to go back and look at the context around this bit of the
document to make sure that is plausible, I guess.)

allman