[Gen-art] Genart last call review of draft-ietf-isis-reverse-metric-15

Stewart Bryant <stewart.bryant@gmail.com> Wed, 17 October 2018 15:59 UTC

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Subject: [Gen-art] Genart last call review of draft-ietf-isis-reverse-metric-15
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Reviewer: Stewart Bryant
Review result: Ready with Issues

I am the assigned Gen-ART reviewer for this draft. The General Area
Review Team (Gen-ART) reviews all IETF documents being processed
by the IESG for the IETF Chair.  Please treat these comments just
like any other last call comments.

For more information, please see the FAQ at


Document: draft-ietf-isis-reverse-metric-15
Reviewer: Stewart Bryant
Review Date: 2018-10-17
IETF LC End Date: 2018-10-17
IESG Telechat date: 2018-11-21

Summary: Generally a well written document, but some earlier text on what a
reverse metric is and what it does would be very helpful to the reader. Also
the reader is left with the impression that the use of this gives disruption
free network changes, and yet it does not discuss microloops.

Major issues: None

Minor issues:
1.2.  Distributed Forwarding Planes

 Temporarily signaling
   the 'Reverse Metric' over this link to discourage the traffic via the

SB> I know it's always chicken and egg, but it would be useful if a
SB> clearer definition of reverse metric were provided before you
SB> explained its use.

   corresponding line-card will help to reduce the traffic loss in the
   network.  In the meantime, the remote PE routers will select a
   different set of PE routers for the BGP best path calculation or use
   a different link towards the same PE router on which a line-card is

SB> Remember that when you change paths you have to deal with
SB> microloops.


1.5.  IS-IS Reverse Metric

   This document uses the routing protocol itself as the transport
   mechanism to allow one IS-IS router to advertise a "reverse metric"
   in an IS-IS Hello (IIH) PDU to an adjacent node on a point-to-point
   or multi-access LAN link.  This would allow the provisioning to be
   performed only on a single node, setting a "reverse metric" on a link
   and have traffic bidirectionally shift away from that link gracefully
   to alternate, viable paths.

SB> Again you need to be much clearer what a reverse metric is before
SB> you cite the use cases and advantages.


3.1.  Processing Changes to Default Metric

   It is important to use the same IS-IS metric type on both ends of the
   link and in the entire IS-IS area or level.

SB> Isn't the point about links redundant given that it needs to be the
SB> same in the area/the level

   On the receiving side of
   the 'reverse-metric' TLV, the accumulated value of configured metric
   and the reverse-metric needs to be limited to 63 in "narrow" metric
   mode and to (2^24 - 2) in "wide" metric mode.
   This applies to both
   the Default Metric of Extended IS Reachability TLV and the Traffic
   Engineering Default Metric sub-TLV in LSP or Pseudonode LSP for the
   "wide" metric mode case.  If the "U" bit is present in the flags, the
   accumulated metric value is to be limited to (2^24 - 1) for both the
   normal link metric and Traffic Engineering metric in IS-IS "wide"
   metric mode.

SB> A clarifying note explaining the different usage of 2^24 - 1 and
SB> 2^24 - 2 would be helpful to the reader.

3.2.  Multi-Topology IS-IS Support on Point-to-point links

   The Reverse Metric TLV is applicable to Multi-Topology IS-IS (M-ISIS)
   [RFC5120].  On point-to-point links, if an IS-IS router is configured
   for M-ISIS, it MUST send only a single Reverse Metric TLV in IIH PDUs
   toward its neighbor(s) on the designated link.

SB> Might you not want to use this on a topology by topology basis?
SB> For example you might want to bring up important typologies first.


   inbound metric value to be the maximum and this puts the link of this
   new node in the last resort position without impacting the other IS-
   IS nodes on the same LAN.

SB> It is only down in S3.4 that you provide this simple definition of
SB> reverse metric as the "inbound metric". It would be helpful to have this
SB> earlier in the text.


It is RECOMMENDED also that the CSPF does the immediate CSPF
   re-calculation when the Traffic Engineering metric is raised to (2^24
   - 2) to be the last resort link.

SB> Again it would help the reader if "link of last resort" was earlier
SB> in the text,
   It is RECOMMENDED that implementations provide a capability to
   disable any changes by Reverse Metric mechanism through neighbor's
   Hello PDUs.

SB> Changes of what? That sentence does not seem to read very well.


   If an implementation enables this mechanism by default, it is
   RECOMMENDED that it be disabled by the operators when not explicitly
   using it.

SB> Why not RECOMMEND that it be disabled by default, or perhaps
SB> strengthen that to MUST be disabled by default.

it is highly RECOMMENDED that operators configure
 authentication of IS-IS PDUs to mitigate use of the Reverse Metric
 TLV as a potential attack vector.

SB> Not sure that you can qualify RFC2119 RECOMMENDED


>From the IANA section

SB> Why is 18 chosen in an otherwise empty registry?

Regarding Appendix A and I think Appendix B

SB> As noted earlier you really need to talk about microloops. There
SB> is no disruption free lunch available.


Nits/editorial comments:

>From ID-nits
  ** Downref: Normative reference to an Informational RFC: RFC 5443
  This is correctly dealt with in the LC

  == Outdated reference: A later version (-07) exists of
 I am sure the RFC Editor will address, but could usefully be fixed in any