[Isis-wg] Comments on draft-ietf-isis-reverse-metric-05

"Les Ginsberg (ginsberg)" <ginsberg@cisco.com> Wed, 26 April 2017 01:01 UTC

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From: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
To: "isis-wg@ietf.org" <isis-wg@ietf.org>, "draft-ietf-isis-reverse-metric.authors@ietf.org" <draft-ietf-isis-reverse-metric.authors@ietf.org>
Thread-Topic: Comments on draft-ietf-isis-reverse-metric-05
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Date: Wed, 26 Apr 2017 01:01:54 +0000
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Subject: [Isis-wg] Comments on draft-ietf-isis-reverse-metric-05
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Naiming/Michael/Shane -

Some comments on the draft.

Section 1.2.  Distributed Forwarding Planes

RFC 5306 (Restart Signaling) has already defined use of the SA bit in the Restart TLV to request that  a neighbor suppress advertisement of the adjacency thus preventing 2-way connectivity check from passing on that link. It is not clear to me why SA bit is not sufficient.

For that matter, the local system could simply not advertise the adjacency to the neighbor and achieve the same result. Why do we need any extension to handle the adjacency bringup case?

Section 1.3 Mobility Cases

I am not clear on why both ISs in such a case would not detect the change in proximity and both do metric adjustment. What is the need for use of reverse metric in this case?

Section 2

>From the description what is being advertised in the new TLV is not a metric but a metric offset i.e. you want the receiving IS to add the advertised value to its existing configured metric. Identifying the metric field as "metric offset" would make this point more clearly.

In regards to the use of sub-TLVs, I think the only use case you have is to advertise a TE metric offset - but this could easily be done as an additional fixed field in the TLV itself. Unless you foresee other sub-TLVs I think sub-TLVs could be eliminated from the TLV definition. (I also think advertising TE metric offset is unnecessary - see additional comments on TE below)

If  you want to retain sub-TLVs for future proofing you do not need both an S flag indicating the presence of sub-TLVs and a sub-TLV length field. One or the other will suffice.

Last Paragraph of Section 3.1 states:

"If the router does not understand the Reverse Metric TLV..."

I don't think this needs to be said. It is standard IS-IS behavior to silently ignore TLVs which are not understood - and if a router does not understand the new TLV it certainly would not know what it is it "should not do". :-)

The point about allowing local policy to disable processing of the Reverse Metric TLV is a good one and the security reasons for it should be emphasized.

Section 3.5

"During the period when a Reverse Metric TLV is used, IS-IS routers

   that are generating and receiving a Reverse Metric TLV MUST NOT

   change their existing IS-IS metric or Traffic Engineering parameters

   in their persistent provisioning database"

I would expect that use of Reverse Metric would often be associated with a maintenance window - in which case this is precisely the time to expect configuration changes. Because traffic has been diverted from the link this is actually the safest time to make configuration changes. Therefore I think this restriction is both unnecessary and undesirable.

Regarding the TE related text

https://www.ietf.org/id/draft-ietf-ospf-link-overload-06.txt  has highlighted that TE CSPF may not always be based on metric (IGP or TE). In which case altering the metric advertisement may not be sufficient to move TE traffic away from the link.

I think a more robust strategy would be to assign a bit in the link attributes sub-TLV defined in RFC 5029 to indicate that the state of the link is "maintenance" (or "overload") and that TE traffic should avoid this. That would be more robust than altering TE metric and would also eliminate the need to use the reverse metric to alter TE metric. Please see https://www.iana.org/assignments/isis-tlv-codepoints/isis-tlv-codepoints.xhtml#isis-tlv-codepoints-19of22 .