Subject: RE: Qos and L2TP (was: Do we want a new L2TP Header?)

From: [email-masked]
Date: Fri, 1 Jun 2001 19:21:27 -0400

> why having mutiple sessions for a single PPP session will help DS QoS
> since the l2tp header is not visible to PHB?
> 
> Please refer to RFC2475 and RFC2983 for PHB.

As the author of RFC2983 and a co-author of RFC-2475, here's
the answer ... some QoS differentiation is based on packet
reordering - the more important packets are forwarded before the less
important packets in the queue.  If L2TP is enforcing ordering within a
session, it will undo this re-ordering for QoS, thereby making the QoS
even worse than it was to begin with (the more important packets now
have to wait for the less important ones that got worse forwarding
treatment).
Hence one can either have ordering-based QoS differentiation or L2TP
enforcement of ordering in a single L2TP session, but not both.  Multiple
L2TP sessions allows ordering-based QoS differentiation across L2TP
sessions, and L2TP enforcement of ordering within each L2TP session.

A more detailed explanation of this can be found in Section 4.1 of RFC 2983.

--David
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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
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From: Jeff Mcadams <[email-masked]>
Date: Fri, 1 Jun 2001 21:08:25 -0400

Also sprach [email-masked]
>As the author of RFC2983 and a co-author of RFC-2475, here's the answer
>... some QoS differentiation is based on packet reordering - the more
>important packets are forwarded before the less important packets in
>the queue.  If L2TP is enforcing ordering within a session, it will
>undo this re-ordering for QoS, thereby making the QoS even worse than
>it was to begin with (the more important packets now have to wait for
>the less important ones that got worse forwarding treatment).

Not necessarily...though the potential is there.  Keep in mind that the
data sequencing is not reliable delivery.  If the QoS re-ordering moves
some higher priority packets in front of lower priority packets, the
behavior of the end system receiving these (out of order) packets is not
exactly specified.  The system MAY (assuming I'm remembering all this
correctly) buffer the packets to re-order them based on the sequence
number, in which case your theory would be right, but it *might* also
just accept the higher priority packets and then when the lower priority
packets come in, just drop them on the floor since they are out of
order.  Now...I'll also agree that this is going to result in an even
greater hit in throughput for the lower priority traffic than was
originally contemplated by re-ordering the packets for QoS reasons...but
your original theory as to what will result is not *necessarily*
correct.
-- 
Jeff McAdams                            Email: [email-masked]
Head Network Administrator              Voice: (502) 966-3848
IgLou Internet Services                        (800) 436-4456