[e2e] Packet dropping

[rick jones]

On May 2, 2007, at 10:36 AM, Craig Partridge wrote:
> My reasoning may be flawed, but just to dig myself deeper.  The
> question was what's the best way for the queue to drain?
>
> So let's consider ten packets 0-9 in flight at sender, router and 
> receiver
>
> In the scenario, the router is about to receive buffer 9
>
>     Sender's buffer		Router Buffer		Receiver Buffer
>
>     012345679			012345679
>
> If it drops 9, then in one RTT we'll have
>
>     Sender's buffer		Router Buffer		Receiver Buffer
>
>     9abcdefghi			<some packets>
>
> If it drops 0, then in one RTT with fast retransmit we'll have
>
>     Sender's buffer		Router Buffer		Receiver Buffer
>     012345679			0			123456789
>
>
> In either case the router queue looks similar -- the issue is which 
> wins
> going forward.  And it wasn't immediately clear to me that fast 
> retransmit
> was better.  If we drop 0, we're in fast retransmit and about to enter
> slow start on packet a.  If we drop 9, we're about to fire off dupe 
> acks
> on a-i, and will enter fast retransmit on packet b.

At the risk of ignoring previously stated context I think it is prudent 
to _not_ assume there will be segments abcdfeghi, in which case, 
dropping 0 gives us the best chance at having fast retransmit in the 
first place rather than a retransmission timeout.

rick jones
there is no rest for the wicked, yet the virtuous have no pillows