Subject: Re: Nr in data packets

From: Vernon Schryver <[email-masked]>
Date: Sat, 22 Jan 2000 10:04:52 -0700 (MST)

> From: David Chen <[email-masked]>

> Does PPP recognized the difference of incoming packet has the 1,2,4,3,5 
> (out of sequence)
> order or 1,2,4,5 (missing packet).
> If it does not, then it does not stop PPP function correctly even the lower 
> layer does
> not guarantee the frame transmitting sequence.
> However, the performance could suffer.

It makes no sense to talk about PPP recognizing out of order packets.
Once again, for the umteenth time, the sequence 1,2,4,3,5 can break a PPP
link by confusing the state machine.

Please read RFC 1661 and understand the state machine.  Please look at
PPP packets and notice that they do not have any sequence numbers, and so
PPP cannot detect an out-of-order packet.  (The ID field in many PP packets
is not a sequence number and cannot serve as one for obvious reasons.)
Please accept the fact that there are upper layer protocols such as those
carried by PPP bridging packets that also require that their data never
be re-ordered, that cannot recognize a re-ordering and discard delayed
packets.

Yes, you could produce a garbage PPP implementation that re-ordered
packets.  It would work most of the time.  Most of the problems it would
cause would not be attributed to that bug and you might sell a lot of
it, because many people lack the skills, tools, and interesting to
determine the nature of bugs However, there are parts of the computer
industry where such garbage is not accepted.  Worse, such garbage would
eventually be figured out and laid at your doorstep (especially given
this public discussion), and then even the hordes that are not bothered
by the blue screen of death several times per day would shun your product.
Only the biggest vendors can get away with selling garbage, and even
they can't sell it forever.

Vernon Schryver    [email-masked]

---

From: David Chen <[email-masked]>
Date: Mon, 24 Jan 2000 11:11:05 -0500

The design principle of the state machine is that
if you are in state that have seen packet #1,2 and not seeing #3 then you
goto other state otherwise goto yet another state.

The rational here is that PPP state machine does not differentiate the 
difference of ....
unless the flow control is implemented in PPP.

As far as TCP, one of its design drawback is that it does not handle out of 
sequence packet.
The L2tp or any underlining layers of (TCP) are not obligated to make up 
its design deficiency.

--- David

>Vernon Schryver    [email-masked]

========================================
David Chen
Indus River Networks, Inc.
31 Nagog Park
Acton, MA 01720
U.S.A.
[email-masked]
(978) 266-8141
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