Subject: Re: [RPSEC] BGP Requirements Draft

From: Sandy Murphy <[email-masked]>
Date: Mon, 26 Sep 2005 04:23:53 -0400 (EDT)

>Nope. As long as your AS isn't in the AS path, the path is loop free.
>Otherwise, AS Sets would be meaningless in the first place.

No, Russ.  If your check only checks if there are valid adjacencies
in the path as represented, you are not certain that the update
passed through those routers.  So if your AS does not appear in the
AS_PATH, you are *NOT* certain that the path is loop free.

As I pointed out before.  And we've talked about cases in which
this happens.

--Sandy

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From: Paul Jakma <[email-masked]>
Date: Mon, 26 Sep 2005 10:11:43 +0100 (IST)

> As I pointed out before.  And we've talked about cases in which

It seems to me there are conflicting desires here:

a) The desire to retain ability to manipulate the AS_PATH for traffic
    engineering purposes

AS_PATH twiddling really is the only knob operators have, and there 
are various interesting things one can do, eg inserting AS numbers of 
/others/ into announcements to preclude an advertisement from 
reaching them and passing through them.

b) The desire to have AS_PATH reliably attest to the path the
    announcement has traveled through

It seems to me that one possible approach is to recognise these 
seperate goals might be best served with two seperate attributes, 
rather than try have AS_PATH meet both goals poorly.

> --Sandy

regards,
-- 
Paul Jakma	[email-masked]	[email-masked]	Key ID: 64A2FF6A
Fortune:
Individualists unite!

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From: Russ White <[email-masked]>
Date: Mon, 26 Sep 2005 08:09:15 -0400

> No, Russ.  

No, Sandy. *sigh* this is silly.

> If your check only checks if there are valid adjacencies

Reordering the AS Path is a completely different concept from removing 
information from the AS Path, first. Reordering is allowed in the 
specification, and is assumed in real life usage. All someone has to do 
is "aggregate" 10.1.1.0/24 into 10.1.1.0/24, and call the entire AS Path 
an unordered AS Set, if that's really what they want to do.

As for removal, yes, it happens, in the case of reorigination of shorter 
prefixes, as in aggregation without using the aggregate keyword. If the 
originator of a shorter prefix route happens to filter longer prefixes 
within the same block of address space, it doesn't cause a loop. This is 
what discard routes are for. The entire question comes down to something 
quite simple: How do you want to handle this concept of: "the routing 
information has traversed the AS's listed in the AS Path in the order 
listed in the AS Path," when the bundle of bits passing through a 
specific set of AS' has never been the question? The question is, and 
always has been, is the routing information reliable? Can you verify the 
transmitter has a path to the destination? Is there any way to verify 
the transmitter has authorization to use any given path to the destination?

:-)

Russ

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[email-masked] CCIE <>< Grace Alone