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

[Ethan Blanton <[e-mail]>]

Yuchung Cheng spake unto us the following wisdom:
> I have two questions:
> 
> 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?

Let me clerify this a bit.  Are you positing this when SMSS >> 100
bytes?  If so, then the answer is "maybe".  The draft is written in
the context of a TCP which does not keep track of segments.   If, in
this case, SMSS >> 100 bytes, then IsLost(1) will return false for a
pure byte-counting TCP.  Note from Section 3:

   Finally, note that the algorithm in this document assumes a
   sender that is not keeping track of segment boundaries after
   transmitting a segment.  It is possible that a sender that did
   keep this extra state may be able to use a more refined and
   precise algorithm than the one presented herein, however, we
   leave this as future work.

If SMSS == 100 bytes, then yes, IsLost(1) will return true.

> 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.

No, not unless you assume a TCP which tracks segments, you assume that
all segments are SMSS-sized, or you *always* use Nagle's algorithm.
If a TCP is sending segments of size < SMSS, then counting incoming
DupAcks allows you to trigger loss recovery when 3 incoming ACKs with
new SACK information arrive, regardless of the size of the segments
triggering those ACKs.

Hope that helps.

Ethan