Subject: RE: Silent Packet Loss & Header Compression

From: Ramana Gollamudi <[email-masked]>
Date: Thu, 24 May 2001 15:30:57 -0400

Okay, I should have put my statements better. I understand the requirement
for routers to make their "best effort" really, really good, else bad things
happen to TCP. I was more interested in knowing why the "really  good best
effort" of devices that handle PPP/L2TP packets are not good enough. The
replies seem to suggest that it not the devices, or characteristics, but the
way MP PPP works, and the requirement from other non IP protocols.  

Thanks,

Ramana

-----Original Message-----
From: James Carlson [mailto:[email-masked]]
Sent: Thursday, May 24, 2001 8:11 AM
To: Ramana Gollamudi
Cc: 'Ken Peirce'; [email-masked]'
Subject: RE: Silent Packet Loss & Header Compression

Ramana Gollamudi writes:
> Agreed. However, most of internet seems to work reasonably well without
> intermediate nodes doing providing any support for ensuring ordered
> delivery.

That assumption is false.  Well-designed routers take a great deal of
care to avoid reordering IP packets within a flow.  This is a
substantial concern even at high speeds with (presumably) high levels
of traffic aggregation.

It's true that reordering does sometimes occur, but it should be quite
rare and due to transient problems at the network level (i.e., route
flaps) and not due to design flaws.

> Best effort seems to enough.

"Best effort" does not mean "reorder at will."

> The argument in this mail chain
> seemed to indicate that MP PPP spread over a multiple LACs is the driver
for
> sequencing in L2TP. So, I wondered how common this usage is, and what does
> it really solve?

No.  The driver for sequencing in L2TP is two-fold.  RFC 1661 says:

1.  Introduction

   The Point-to-Point Protocol is designed for simple links which
   transport packets between two peers.  These links provide full-duplex
   simultaneous bi-directional operation, and are assumed to deliver
   packets in order.  It is intended that PPP provide a common solution
   for easy connection of a wide variety of hosts, bridges and routers
   [1].

The second reason is that some network layer protocols that run on PPP
cannot tolerate link-layer reordering of packets.  Reportedly, this is
a problem with SNA.  (I haven't worked with SNA in so many years now
that all of those brain cells have been recycled.  ;-})

-- 
James Carlson, Internet Engineering       <[email-masked]>
SUN Microsystems / 1 Network Drive         71.234W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.497N   Fax +1 781 442 1677

---

From: James Carlson <[email-masked]>
Date: Thu, 24 May 2001 15:57:42 -0400 (EDT)

Ramana Gollamudi writes:
> effort" of devices that handle PPP/L2TP packets are not good enough.

Right.  There's a difference between "really ought not reorder unless
something drastic happens" and "will never reorder."  The PPP state
machines and some network layer protocols assume the latter, and thus
require protection, even though the reordering through properly
designed, configured, and maintained equipment should be really rare.

Going through at least four providers and multiple private peerings to
a gateway machine, I see about 0.17% (3595 in 2125304, right now) of
the packets get reordered.  That figure's probably a little inflated
by the nasty persistent flaps that one of the providers (which will
remain nameless) experiences several times a day.

> The

Right.  MP is another protocol that relies heavily on RFC 1661's
promise of unduplicated and in-order delivery.