Subject: [rbridge] loops in trill networks

From: Eric.Gray at marconi.com (Gray, Eric)
Date: Tue, 11 Oct 2005 16:02:40 -0300

Alia/Radia,
 
    Frame (re)ordering was a critical consideration back in the days of
specifying LAN Emulation over ATM.  There were - at that time - a few
protocols, used for example in synchronizing sales or accounting data,
that assumed in-order frame delivery. I'm not sure, but I think NetBios
may have been one of those protocols.
 
    I do not know similar protocols no longer exist and, short of under-
taking a major research project to verify that no such protocols do exist,
I believe we should assume that in-order frame delivery may still be a
requirement.
 
    In terms of critiqueing protocols in general for their (in)ability to
deal
with (re)ordering, there is an inefficiency inherent in assuming that all
protocols at all levels can handle re-ordering.  Many protocols designed
to run over an essentially wire-like media assume things can't jump over
each other on the wire.
 
    However, it's probably reasonable - at this point in technology usage -
to define protocols that are optimized for _somewhat_ order independent
frame delivery, relying on the fact that a majority of higher level
protocols
currently in use can deal with some frame re-ordering.
 
    BTW, I believe it's reasonably well-known that IP doesn't deal perfectly
with re-ordering either because there's a cost associated with having to
do so and IP - itself - is optimized for the assumption of in-order
delivery.
 
--
Eric

-----Original Message-----
From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org]On
Behalf Of Alia Atlas
Sent: Tuesday, October 11, 2005 2:18 PM
To: Developing a hybrid router/bridge.
Subject: Re: [rbridge] loops in trill networks

On 10/11/05, Radia Perlman < Radia.Perlman at sun.com
<mailto:Radia.Perlman at sun.com> > wrote: 

I'm not sure what protocols there are that need absolute ordering
guarantees.

Likewise, but I thought that Ethernet had that constraint. 

RSTP can reorder sometimes after a topology change, so apparently
occasional reordering is not considered a problem. 

It might be useful to have a small amount of text about that.

All the IETF protocols, I believe, are designed to withstand some packet
reordering (and should, in my opinion, any well designed protocol). 

Sure - anything on top of IP isn't an issue.  My concern was other stuff.

Alia 

Radia

Alia Atlas wrote:

> What about potential frame reordering
> during the routing convergence?  Is this a concern?
>
> Alia

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