Re: [mpls] Alissa Cooper's Discuss on draft-ietf-mpls-sfc-05: (with DISCUSS and COMMENT)

"Adrian Farrel" <> Thu, 07 March 2019 15:34 UTC

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From: Adrian Farrel <>
To: 'Alissa Cooper' <>
Cc: 'Datatracker on behalf of Alissa Cooper' <>, 'IESG' <>, 'IETF MPLS List' <>,,,
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Date: Thu, 07 Mar 2019 15:34:12 -0000
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Subject: Re: [mpls] Alissa Cooper's Discuss on draft-ietf-mpls-sfc-05: (with DISCUSS and COMMENT)
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Great. I think we have converged.

Thanks for the Discussion.



Comparing the requirements in RFC 8300 Section 8.1 under "Single Domain
Boundary" and the text in Section 15 of this document, it seems that the
mechanism specified in this document is not subject to the same normative
requirements as specified for the administrative boundaries of a network
MPLS is used as the transport encapsulation for NSH. What is the reasoning
that? I would have expected to see similar normative requirements here as in
RFC 8300 Section 8.1.

RFC 8300 is concerned with the forwarding plane, so (obviously) includes
normative language about packet handling.
But it is worth noting that the term used in 8.1 is "NSH domain".
However, the reference is back to the general SFC architecture in RFC 7665.
That's a more appropriate place to look for the constraints on deploying SFC

7665 defines an SFC-Enabled Domain as "A network or region of a network that
implements SFC.  An SFC-enabled domain is limited to a single network
administrative domain." We should note that an "administrative domain" is
(probably?) not the same as a single AS: an administrative domain is often
partitioned into multiple ASes.

We do say that "Discussion of the security properties of SFC networks can be
found in [RFC7665].  Further security discussion for the NSH and its use is
present in [RFC8300]." So we intend to inherit all the security and
deployment constraints.

That should be made explicit then, in particular that the normative security
requirements from those documents apply to this specification.


[AF] That's easily done.


= Section 4.5 =

The application of SR to SFC was considered in early versions of the
 SR architecture [RFC8402] and the MPLS-SR specification
 [I-D.ietf-spring-segment-routing-mpls], but has since been moved out
 of those documents.

The application of SR to SFC was considered in early versions of the
 SR architecture [RFC8402] and the MPLS-SR specification
 [I-D.ietf-spring-segment-routing-mpls], but was not ultimately adopted.

(I think this is about the ideas, not the organization of documents.)

Well, yes and no. There are, in fact, a number of drafts working their way
through the system that discuss the use of SR for SFC. Most of these
documents are not fully mature, but their proponents remain confident that
they will be adopted and become RFCs in the fullness of time. I could
- draft-filsfils-spring-srv6-network-programming
- draft-guichard-spring-nsh-sr
- draft-xuclad-spring-sr-service-programming
There are probably others.

This is not the impression the reader is left with from reading Section 4.5.

So, SFC is clearly in scope for SR and the SPRING charter explicitly says:
 o Source-routed stateless service chaining using SR-MPLS
    and SRv6 dataplanes.

We included the reference to draft-xuclad-spring-sr-service-chaining (now
replaced by draft-xuclad-spring-sr-service-programming - we need to make an
update!) and believe that and the text cover the point.

I wonder about the archival value of the second paragraph altogether, given
the multiple streams of work on this. Perhaps it would make more sense to
just add a sentence to the end of the first paragraph noting that
implementation work was ongoing at the time of this writing, and drop the
second paragraph.


[AF] Consider it done.