Re: [Idr] AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17

Alvaro Retana <> Wed, 03 April 2019 15:22 UTC

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Date: Wed, 3 Apr 2019 08:22:16 -0700
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To: "Ketan Talaulikar (ketant)" <>
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Subject: Re: [Idr] AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17
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On March 24, 2019 at 11:43:02 AM, Ketan Talaulikar (ketant) ( wrote:



Thanks for your work on this document!

I still have a couple of comments, specially related to
Manageability/Security…and a couple of new nits.  I am starting the IETF LC
as I think these can be addressed with any other incoming comments.




316   o  Member-ASN, which contains the ASN of the confederation member, if

317      BGP confederations are used, of the local BGP node.

[major] This description matches the definition of the TLV (§4.1), but it
also matches the description of the ASN descriptor in §4.2.  What is the
difference?  What if the contents are different?  [Same question applies

[KT] With ref to rfc5065, in the case of confederations, the TLV 512
carries the AS Confederation Identifier while the TLV 517 carries the
Member-AS Number. I’ve fixed the terminologies to more accurately reflect
those from rfc5065 and also included additional reference to rfc5056 in TLV
512 use description.


I think we may need a reference to rfc7752 when TLV 512 is mentioned
because that is where it is defined.


860 9.  Manageability Considerations


868   Specifically, the malformed NLRI attribute tests for syntactic checks

869   in the Fault Management section of [RFC7752] now apply to the TLVs

870   for the BGP-LS NLRI TLVs defined in this document.  The semantic or

871   content checking for the TLVs specified in this document and their

872   association with the BGP-LS NLRI types or their associated BGP-LS

873   Attributes is left to the consumer of the BGP-LS information (e.g. an

874   application or a controller) and not the BGP protocol.

[nit] s/the malformed NLRI attribute tests/the malformed attribute tests

[KT] Fixed to read “Link-State NLRI and BGP-LS Attribute”

rfc7752 doesn’t cover the NLRI in the Fault Management section.


[minor] "...retrieving this information...over a BGP-LS session, via some
APIs or a data model (refer Section 1 and 2 of [RFC7752])."  I think that
even mentioning that an API or data model can be used instead of BGP-LS is
a stretch -- that is not how I interpret the initial sections in rfc7752
(which are just background sections), and there are no formal API/data
model definition.

[KT] How else would Alto Server or a PCE component get access to the BGP-LS
information? There would be some APIs or a model. It may not be formal but
there needs to be one?

We talked about this (rfc7752 refers to BGP-LS) while in Prague.

[major] " error detected in the NLRI descriptor TLVs would result in
that specific NLRI update being unusable and hence its update to be
discarded along with an error log."  According to rfc7752, this statement
is true only for syntactic errors, not semantic ones.

[KT] This is about the TLV validation by the consumer – which can also
perform semantic checks. This aspect is one that needs clarification in the
fault management section of RFC7752. The text on validation by consumer was
added to this draft to prevent gating it on this gap in RFC7752.

I rather that is addressed in rfc7752bis.  I think that the new text at the
end of this section covers enough.

Semantic errors ("left to the consumer", as mentioned above) means that the
BGP-LS session can be used to transport trash (trash-in-trash-out).  This
issue is bigger than this document -- but (because we don't have a current
solution) I would like to see it mentioned somewhere as a potential issue
(maybe in the Management or Security Considerations sections, your choice).

[major] " error detected in the NLRI descriptor TLVs would result in
that specific NLRI update being unusable and hence its update to be
discarded..."  This text says that an error in a TLV results in the whole
update (not just the TLV or the BGL-LS attribute) being discarded.  This
action goes beyond what is specified in rfc7752, which doesn't really
specify what do to in those cases.  If that is what is expected here, then
please make this behavior more prominent...use Normative language, etc..

[KT] I agree. As mentioned above, this was an interim text as mentioned
above. I am open to suggestions. My concern is that multiple BGP-LS
extensions don’t need to do this and I would rather have the normative
language in an update to RFC7752.

Agreed — as we discussed in Prague.  Same comment as above.



895   BGP Peering Segments are associated with the normal BGP routing

896   peering sessions.  However, the BGP peering information along with

897   these Peering Segments themselves are advertised via a distinct BGP-

898   LS peering session.  It is expected that this isolation as described

899   in [RFC7752] is followed when advertising BGP peering topology

900   information via BGP-LS.

[major] rfc7752 doesn't talk about this type of isolation (it only talks
about isolation through dedicated RRs).  See also comment in the Security
Considerations section related to isolation.

[KT] Agree – this is the outcome of the discussions in the IDR WG and
shepherd review. Ideally this should have been captured in RFC7752 and not
individual extension documents.

Right.  The major point was that the reference to rfc7752 is misleading
because this type of isolation is not discussed there.


[major] "...precaution is necessary to ensure that the BGP peering
information collected via BGP-LS is limited to specific controllers or
applications..."  This sounds as if you're referring to information that
(once collected) can be shared between controllers -- I think that case is
out of scope.

[KT] Actually the intention here is to highlight that since EPE can be used
to engineer the path out of a domain at the BGP router, this information
about the EPE SIDs should be maintained in a secure manner. The knowledge
of EPE SIDs may be used to influence traffic flows both within and
egressing outside the domain. If you still think this is out of scope and
should be taken out then I will do so.

I think it is.  The SR Architecture already talks about the use inside a
domain…so I think that could be opening a can of worms with this that we
really don’t want to open.


§1: [nit] s/Link State NLRI/Link-State NLRI

§3: [nit] s/his section/This section

[nit]: The length in Figure 2 is not needed.