Subject: Re: [Idr] AD-review comments on draft-ietf-idr-bgp-analysis-04.txt

From: Russ White <[email-masked]>
Date: Mon, 3 May 2004 07:21:21 -0400 (Eastern Daylight Time)

> >>   BGP uses an algorithm that is neither a pure distance vector
> >>   algorithm or a pure link state algorithm.  It is instead a modified
> >>   distance vector algorithm referred to as a "Path Vector" algorithm
> >>   that uses path information to avoid traditional distance vector
> >>   problems.  Each route within BGP pairs destination with path
> >>   information to that destination.  Path information (also known as
> >>   AS_PATH information) is stored within the AS_PATH attribute in BGP.
> >>   This allows BGP to reconstruct large portions of overall topology
> >>   whenever required.

> >I've always been uncomfortable with documents saying that BGP
> >reconstructs the overall topology. It doesn't really do this like the
> >link-state protocols do, for example.

> Is this sentence "This allows BGP to reconstruct large portions of
> overall topology ...." talking about using AS_PATH as topology
> information or reconstructing routes/paths?

It's saying you can use the information in the AS_PATH to reconstuct large
portions of the internetwork's overall topology. I've disagreed with this
statement for a while. In fact, I would say you can't even really
reconstruct the path to each and every destination represented by a single
prefix within an advertisement from the AS_PATH in that advertisement,
necessarily.

:-)

Russ

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From: Russ White <[email-masked]>
Date: Mon, 3 May 2004 07:21:21 -0400 (Eastern Daylight Time)

:-)

Russ

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From: Enrico Salazar <[email-masked]>
Date: Mon, 3 May 2004 13:08:56 -0700 (PDT)

Russ,
 
Being able to "reconstruct large portions of overall topology" does *not* automatically implies being able to "reconstruct the path to each and every destination represented by a single prefix within an advertisement from the AS_PATH in that advertisement". E.g., from examining AS_PATH fo type AS_SEQUENCE one could infer partial information about the inter-AS topology.
 
Enrico

:-)

Russ

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From: Enrico Salazar <[email-masked]>
Date: Mon, 3 May 2004 13:08:56 -0700 (PDT)

Russ,

Enrico

:-)

Russ

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From: Russ White <[email-masked]>
Date: Mon, 3 May 2004 16:36:10 -0400 (Eastern Daylight Time)

> Being able to "reconstruct large portions of overall topology" does *not*
> automatically implies being able to "reconstruct the path to each and
> every destination represented by a single prefix within an advertisement
> from the AS_PATH in that advertisement". E.g., from examining AS_PATH fo
> type AS_SEQUENCE one could infer partial information about the inter-AS
> topology.

Okay, if what many people read as the implication of this statement isn't
true, and it's not critical/important to the general operation of BGP, then
why are we leaving it in the draft?

:-)

Russ

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From: Russ White <[email-masked]>
Date: Mon, 3 May 2004 16:36:10 -0400 (Eastern Daylight Time)

:-)

Russ

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From: John Leslie <[email-masked]>
Date: Tue, 4 May 2004 18:38:13 -0400

Russ White <[email-masked]> wrote:
> To: Enrico Salazar <[email-masked]>
> 

   I've always assumed this was an attempt to distinguish BGP -- a true
distance-vector protocol -- from all the "distance-scalar" protocols
incorrectly called "distance-vector" protocols.

   Unlike the distance-scalar protocols, BGP allows us to reconstruct
what I'd call "likely actual paths" to be taken by packets sent to
different BGP neighbors, and make policy decision based on those
likely paths.

   Personally, I'd be much happier with language which at least hinted
at this actual difference; but I gave up trying to change this text
long before the last-call started...

--
John Leslie <[email-masked]>

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From: John Leslie <[email-masked]>
Date: Tue, 4 May 2004 18:38:13 -0400

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From: Enrico Salazar <[email-masked]>
Date: Wed, 5 May 2004 07:50:40 -0700 (PDT)

Russ,
 
How many is "many people" that you are referring to in your e-mail ? 
 
Enrico

> type AS_SEQUENCE one could infer partial information about the inter-AS

Okay, if what many people read as the implication of this statement isn't true, and it's not critical/important to the general operation of BGP, then why are we leaving it in the draft?

:-)

Russ

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From: Enrico Salazar <[email-masked]>
Date: Wed, 5 May 2004 07:50:40 -0700 (PDT)

Russ,

Enrico

:-)

Russ