[Lsr] Fwd: Alvaro Retana's Discuss on draft-ietf-spring-segment-routing-mpls-19: (with DISCUSS and COMMENT) (draft-ietf-ospf-segment-routing-extensions / draft-ietf-ospf-ospfv3-segment-routing-extensions / draft-ietf-isis-segment-routing-extensions)

[Alvaro Retana <[e-mail]>]

Hi!

I just entered a DISCUSS position related
to draft-ietf-spring-segment-routing-mpls (see below).  I believe that the
issue needs to be solved in conjunction with the IGP extension drafts, so
I’m copying the authors/shepherds/chairs here.

Thanks!

Alvaro.

On April 10, 2019 at 4:25:22 PM, Alvaro Retana via Datatracker (
[e-mail]) wrote:

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DISCUSS:
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(1) This first point is a cross-document DISCUSS. In short, the assumptions
in
this document about what an MCC is responsible for are not in line with the
corresponding IGP drafts for OSPF [1][2] and IS-IS [3]. This misalignment
must
be resolved before any of these documents are published.

[Note: I'll start a thread with the corresponding WGS, Authors, Shepherds,
Chairs and ADs. Let's please discuss this point there.]

This document uses the following definition in §2: "We call "MPLS Control
Plane
Client (MCC)" any control plane entity installing forwarding entries in the
MPLS data plane. IGPs with SR extensions...are examples of MCCs."

The focus of the IGP drafts is on the transport of the SR information, and
not
on other functions (see below). Which component is responsible for what is
the
point that needs clarification -- either in this document, the IGP drafts,
or
both.

These are some specific cases:

(1.1) §2.4 (Mapping a SID Index to an MPLS label): "The following rules
MUST be
applied by the MCC when calculating the MPLS label value corresponding the
SID
index value "I"." There's nothing in the IGP extension documents that point
at
this set of rules, and only a passing reference in the OSPF documents about
outgoing labels.

(1.2) §2.5 (Incoming Label Collision) also assumes more functions from an
MCC
than what the IGP documents do. For example: "Within an MCC, apply
tie-breaking rules to select one FEC only and assign the label to it."

(1.3) §2.8 also expects work by the IGPs: "the MCC is responsible for
downloading the correct label value to FIB"...in this case not just
calculating
the label, but installing it in the FIB.

(1.4) §2.10.1: "The method by which the MCC on router "R0" determines that
PUSH
or CONTINUE operation must be applied using the SID "Si" is beyond the
scope of
this document. An example of a method to determine the SID "Si" for PUSH
operation is the case where IS-IS
[I-D.ietf-isis-segment-routing-extensions]..." Note that the IS-IS draft (or

the OSPF ones, for that matter) don't talk about how to determine the
operation
-- if that is out of scope of this document, then where is it specified?

(1.5) From §2:

An implementation SHOULD check that an IGP node-SID is not associated
with a prefix that is owned by more than one router within the same
routing domain. If so, it SHOULD NOT use this Node-SID, MAY use
another one if available, and SHOULD log an error.

rfc8402 reads (§3.2): "An IGP Node-SID MUST NOT be associated with a prefix
that is owned by more than one router within the same routing domain." The
text above is not in line with that (MUST NOT vs SHOULD). Also, how can
"SHOULD check" be Normatively enforced?

Both sentences above seem to be trying to specify a behavior for the IGPs.

[1] https://tools.ietf.org/html/draft-ietf-ospf-segment-routing-extensions
[2]
https://tools.ietf.org/html/draft-ietf-ospf-ospfv3-segment-routing-extensions

[3] https://tools.ietf.org/html/draft-ietf-isis-segment-routing-extensions