Re: [Idr] RFC-compliant use of Extended Length for new BGP attributes

Paul Jakma <paul@clubi.ie> Mon, 24 March 2008 10:20 UTC

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From: Paul Jakma <paul@clubi.ie>
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To: Marcelo Schmidt <mschmidt@equinix.com>
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Subject: Re: [Idr] RFC-compliant use of Extended Length for new BGP attributes
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On Thu, 13 Mar 2008, Marcelo Schmidt wrote:

> I am interested in input from the list on loose behavior we've seen 
> from various BGP specs.  The specific example in question is a case 
> we are seeing in the wild where a NEW_AS_PATH attribute has 8 data 
> bytes, but sets the Extended Length bit and uses a 2-byte length 
> field.

Post-AS4_ASPATH Quagga does this for AS_PATH and AS4_PATH.

> RFC 1771 explicitly states "Extended Length may be used only if the 
> length of the attribute value is greater than 255 octets."

> However, the superceding RFC 4271 does not have this explicit 
> language, therefore would imply that either of the following cases 
> would technically be compliant:

Right. Further, it'd be an extremely strange 1771 speaker that got 
confused by an extended length being less than 255. It'd have to go 
out of its way to enforce a fairly useless restriction.

> Furthermore, I've      seen      loose      language,      such     as
> draft-kumaki-pce-bgp-disco-attribute-00.txt  that  seems  to  imply to
> always set Ext-Len bit regardless.

That seems a good idea to me. Different length length fields is just 
asking for the odd boundary-case bug.

> Specifically, is anyone aware of any new BGP extensions that would 
> REQUIRE Ext-Len to be set, even if attribute is <= 255 bytes? Our 
> thought is that if we receive a case A above with length <= 255 
> bytes, we are free to propogate to downstream neighbors as a case B 
> (generically, for any attribute in BGP)."

There doesn't seem to be any reason you can't do that.

Any drafts that try to add their own interpretation on how the 
attributes TLV is to be used are simply over-reaching and wrong on 
that matter, and should be stripped of such language. 1771 is the 
canon here really, and must be in order, e.g., for unknown transitive 
attributes to be able to propogate.

regards,
-- 
Paul Jakma	paul@clubi.ie	paul@jakma.org	Key ID: 64A2FF6A
Fortune:
No matter how subtle the wizard, a knife in the shoulder blades will seriously
cramp his style.
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