Telechat Review of draft-ietf-spring-segment-routing-13
review-ietf-spring-segment-routing-13-rtgdir-telechat-hardwick-2017-12-12-00

Hello,

I have been selected as the Routing Directorate reviewer for this draft. The
Routing Directorate seeks to review all routing or routing-related drafts as
they pass through IETF last call and IESG review, and sometimes on special
request. The purpose of the review is to provide assistance to the Routing ADs.
For more information about the Routing Directorate, please see
​http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir

Document: draft-ietf-spring-segment-routing-13.txt
Reviewer: Jon Hardwick
Review Date: 12 December 2017
IETF LC End Date: 30 November 2017
Telechat date: 14 December 2017
Intended Status: Standards Track

Summary
No issues found. This document is ready for publication.

Comments
This document is very well written and is easy to understand.  It serves as a
good introduction to, and overview of, the segment routing architecture.  It
includes a guide to the operation of the control plane and the MPLS and IPv6
data planes and gives references to the appropriate documents which standardize
the necessary control and data plane extensions.  I found no issues and believe
the document is ready to be published.  I noted one minor clarification which
the ADs may wish to make to the document before it is published – see below.

Major Issues
No major issues found.

Minor Issues
Section 3.1 discusses the prefix SID.  Section 3.1.1 introduces the concept of
an algorithm that is to be used by an SR-capable router in selecting the
correct next hop to use when executing a forwarding instruction on a Prefix
SID.  Section 3.1.2 explains how per-algorithm forwarding is to be applied in
the MPLS data plane, but section 3.1.3 does not mention per-algorithm
forwarding in the context of IPv6.  It would be better to have an explicit
statement in 3.1.3, as currently the reader may wonder whether per-algorithm
forwarding is intended to be applied in the IPv6 data plane.

Also, section 3.1.3 says “any remote IPv6 node will maintain a plain IPv6 FIB
entry for any prefix, no matter if the represent a segment or not. This allows
forwarding of packets to the node which owns the SID even by nodes which do not
support Segment Routing.”  Clearly, a node that does not understand that an
IPv6 address represents a SID will be unable to apply per-algorithm forwarding
reliably. In my opinion, section 3.1.3 could note this restriction more clearly.

Nits
Note also the nit in the section 3.1.3 text I quoted above: “no matter if the
represent a segment or not” -> “whether or not the prefix represents a segment”.