[Idr] Available Bandwidth erratum 5486 [was: Re: AD Review of draft-ietf-idr-te-pm-bgp-14]

[John Scudder <[e-mail]>]

+lsr to the cc

Hi Alvaro,

On Nov 28, 2018, at 4:01 PM, Alvaro Retana <[e-mail]<mailto:[e-mail]>> wrote:

[major] AFAICT, Available Bandwidth is the only definition that is different between rfc7810/draft-ietf-lsr-isis-rfc7810bis and rfc7471.  The difference comes from the correction made to address this report [1].  Instead of trying to fix the definition here, I think that a similar report should be filed against rfc7471.  Please submit it and I will approve.

[1] https://www.rfc-editor.org/errata/eid5486<https://urldefense.proofpoint.com/v2/url?u=https-3A__www.rfc-2Deditor.org_errata_eid5486&d=DwMFaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=hLt5iDJpw7ukqICc0hoT7A&m=pNTkxj6RjNdyIYjBZKCUjdk9QWVKbBBhnnfj9xq2jjU&s=QvXYEMqBgaIkuM7plcuybtDVxI3JTI-4EndPcX0ier8&e=>

Maybe I'm missing something but isn't that erratum all wrong?

Here is why I think so. I agree that there is a problem with the RFC 7810 paragraph in question:


   Available Bandwidth: This field carries the available bandwidth on a
   link, forwarding adjacency, or bundled link in IEEE floating-point
   format with units of bytes per second.  For a link or forwarding
   adjacency, available bandwidth is defined to be residual bandwidth
   (see Section 4.5) minus the measured bandwidth used for the actual
   forwarding of non-RSVP-TE label switched path packets.  For a bundled
   link, available bandwidth is defined to be the sum of the component
   link available bandwidths minus the measured bandwidth used for the
   actual forwarding of non-RSVP-TE label switched path packets.  For a
   bundled link, available bandwidth is defined to be the sum of the
   component link available bandwidths.


It seems obvious that there was a cut-and-paste problem or similar, since the same sentence is duplicated with minor changes. But the erratum leaves the duplication! The erratum wants it to be:


Available Bandwidth: This field carries the available bandwidth on a
   link, forwarding adjacency, or bundled link in IEEE floating-point
   format with units of bytes per second.  For a link or forwarding
   adjacency, available bandwidth is defined to be residual bandwidth
   (see Section 4.5) minus the measured bandwidth used for the actual
   forwarding of non-RSVP-TE label switched path packets.  For a bundled
   link, available bandwidth is defined to be the sum of the component
   link (residual) bandwidths minus the measured bandwidth used for the
   actual forwarding of non-RSVP-TE label switched path packets.For a
   bundled link, available bandwidth is defined to be the sum of the
   component link available bandwidths.


So the proposed "fix" is to leave the sentence duplicated, but change "available" to "(residual)" in the first copy? I don't think that could possibly be right. Just eyeballing it, it seems to me as though the correct fix would be to change the paragraph to be:


   Available Bandwidth: This field carries the available bandwidth on a
   link, forwarding adjacency, or bundled link in IEEE floating-point
   format with units of bytes per second.  For a link or forwarding
   adjacency, available bandwidth is defined to be residual bandwidth
   (see Section 4.5) minus the measured bandwidth used for the actual
   forwarding of non-RSVP-TE label switched path packets.  For a bundled
   link, available bandwidth is defined to be the sum of the component
   link available bandwidths minus the measured bandwidth used for the
   actual forwarding of non-RSVP-TE label switched path packets.  For a
   bundled link, available bandwidth is defined to be the sum of the
   component link available bandwidths.


in which case it would match RFC 7471. Or possibly:


   Available Bandwidth: This field carries the available bandwidth on a
   link, forwarding adjacency, or bundled link in IEEE floating-point
   format with units of bytes per second.  For a link or forwarding
   adjacency, available bandwidth is defined to be residual bandwidth
   (see Section 4.5) minus the measured bandwidth used for the actual
   forwarding of non-RSVP-TE label switched path packets.  For a bundled
   link, available bandwidth is defined to be the sum of the component
   link available residual bandwidths minus the measured bandwidth used for the
   actual forwarding of non-RSVP-TE label switched path packets.  For a
   bundled link, available bandwidth is defined to be the sum of the
   component link available bandwidths.


I have no idea which of these is right, but the erratum can't be right. Naively, they look algebraically the same, it's just a matter of where in the equation you subtract the measured bandwidth. Maybe they truly are exactly equivalent or maybe there is some subtlety that makes one right and one wrong.

If the first option above is right, then RFC 7471 looks to be correct as written. If the first option is wrong, then RFC 7471 would need its own erratum as you suggest, I guess.

$0.02,

--John

P.S.: I see the defect remains in draft-ginsberg-lsr-isis-rfc7810bis-00.