Subject: [Idr] AS deletion and reording in the AS_PATH

From: Sandy Murphy <[email-masked]>
Date: Mon, 24 Jan 2005 12:47:36 -0500 (EST)

On the rpsec list, Russ White has made two interesting observations.
The first concerns deleting or reordering the AS's in an AS_PATH when
creating the AS_PATH for an UPDATE:

>On searching the document for all occurances of the words: "as_path," or: 
>"as path," I found only a few instances. The one I _believe_ is relevant is 
>here, on page 25:
>
>         b) When a given BGP speaker advertises the route to an external
>         peer, then the advertising speaker updates the AS_PATH attribute
>         as follows:
>
>            1) if the first path segment of the AS_PATH is of type
>            AS_SEQUENCE, the local system prepends its own AS number as the
>            last element of the sequence (put it in the leftmost position
>            with respect to the position of octets in the protocol mes-
>            sage). If the act of prepending will cause an overflow in the
>            AS_PATH segment, i.e. more than 255 ASs, it SHOULD prepend a
>            new segment of type AS_SEQUENCE and prepend its own AS number
>            to this new segment.
>
>When I examine this text, I find two interesting things:
>
>-- First, there is no mention of any conditions under which information may 
>be removed from the AS Path, or the AS Path is reordered. I believe, based 
>on reading of the specification (without outside reference to operational 
>or implementation experience), this means the AS Path may be reordered, or 
>information may be removed from the AS Path. Let me at least say neither of 
>these are strictly prohibited.

First: would those on the list agree that deleting or reordering the AS's
in an AS_PATH is an OK thing to do?

Second:  would those on the list agree that any manipulation other than
"prepends its own AS number" is allowed, on the basis of there being "no
mention" of the alternate manipulation?

--Sandy

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From: Curtis Villamizar <[email-masked]>
Date: Mon, 24 Jan 2005 17:18:13 -0500

> mention" of the alternate manipulation?

The text says that the AS Path is updated "as follows".  It does not
say "as follows or in any other arbitrary way".

The reality has always been that violations of the spec do occur when
someone considers it in their interest to do so and when it doesn't
mess up interoperability or cause operational problems.  The IETF
doesn't have any means of enforcement and that's probably a good thing.

Chopping the middle could create loops.  Reordering probably doesn't
have any adverse consequence unless someone else policy is based on a
sequence of AS in a particular order.  Since it has a negative
consequence and no benefit, reordering is at the very least a bad
idea.

Therefore deleting is definitely a bad thing to do.  Reordering is
also a bad thing to do.

I agree with the second point.  The intent is to prohibit any
modification of the AS path, except when aggregating, other than the
modification specificly described above.

Curtis