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From: Archie Cobbs <archie@whistle.com>
Message-Id: <200008181549.IAA07643@bubba.whistle.com>
Subject: Re: cvs commit: src/usr.sbin/ppp README.changes mp.c
In-Reply-To: <200008180654.e7I6sKF44766@hak.lan.Awfulhak.org> from Brian Somers
 at "Aug 18, 2000 07:54:20 am"
To: Brian Somers <brian@Awfulhak.org>
Date: Fri, 18 Aug 2000 08:49:38 -0700 (PDT)
Cc: Archie Cobbs <archie@whistle.com>,
	Brian Somers <brian@FreeBSD.ORG>, cvs-committers@FreeBSD.ORG,
	cvs-all@FreeBSD.ORG, brian@Awfulhak.orgg
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Brian Somers writes:
> > Mixing MP frames with non-MP frames is slightly dangerous. The
> > problem is that the PPP link layer is required to deliver packets
> > in order. When you go from MP encapsulating packets to not MP
> > encapsulating packets (or vice-versa) you can't guarantee that the
> > peer will deliver the packets in the same order that you sent them
> > without being careful. This is mainly a problem when going from 1
> > -> 2 links I believe...  or when mixing MP and non-MP over multiple
> > links.
> 
> It should only ever be possible to deliver out-of-order network layer 
> frames here, and even then, only a single frame can be out of order 
> at the time of the second link opening.
> 
> I think the reason behind the link layer being required to deliver in 
> order is so that the control protocol packets don't make a mess of 
> the peers state machine (out of order IPCP frames for example).
> 
> As ppp currently only does IP as an NCP, and IP packets are allowed 
> to be delivered out-of-order, I think this is ok.

If you're doing Van Jacobson header compression, for example,
with connection ID compression, out of order packets can really
screw things up.

-Archie

___________________________________________________________________________
Archie Cobbs   *   Whistle Communications, Inc.  *   http://www.whistle.com


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