Re: [tcpm] Re: RFC 1323: Timestamps option

Hi Matt,

On May 30, 2007, at 10:59 AM, Matt Mathis wrote:

> I had assumed that you can "fake" paws as follows: once you receive  
> a late
> timestamp negotiation (in either role), you treat any later  
> untimestamped
> segments as though they bear the timstamp just before the earliest  
> time stamp
> seen.   I believe that this is optimal, given other assumptions.

Yes, for PAWS you only need the clock to tick at least once per  
sequence wrap, so as long as Timestamps are enabled before the first  
wrap, PAWS should work just fine this way.

>
> I think it also works well enough to treat any later untimestamped
> segments as though they bear the timestamp just before the the most  
> recently
> received timestamp.  This is actually the same if the timestamps do  
> not
> advance during the negotiation RTT.   (Well enough means that the  
> failures
> are neither frequent nor serious).

The problem is, how do you tell that it is a "later untimestamped"  
segment, and not an old segment from before we enabled timestamps?   
This would allow old packets from before we enabled timestamps to  
arrive after the sequence wrap and be treated as OK, the very thing  
that PAWS is trying to prevent.  That's one of the reasons why once  
you enable timestamps, you have to then include them in every packet  
for the duration of the connection.

			-David Borman

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