Subject: [OSPF] AD review of draft-ietf-ospf-segment-routing-extensions-18

From: Alia Atlas <[email-masked]>
Date: Fri, 11 Aug 2017 22:09:54 -0400

As is customary, I have done another AD review
of draft-ietf-ospf-segment-routing-extensions-18. I do appreciate the
improvements in the draft.

I do still see a few minor issues.  I would like to see a revised draft
before IETF Last Call. I expect to progress this at an IESG telechat with
the primary spring documents, when Alvaro feels they are ready.

1) In Sec 3.1, "If the SR-Algorithm TLV appears in multiple Router
   Information LSAs that have different flooding scopes, the SR-
   Algorithm TLV in the Router Information LSA with the narrowest
   flooding scope SHOULD be used.  "
   Given that the area-scope is REQUIRED - shouldn't this also prefer
   the area-scope?  Is there future-proofing being done?

2) In Sec 3.4: "For the purpose of the SRMS Preference TLV advertisement,
AS-scoped flooding is REQUIRED.  This
   is because SRMS servers can be located in a different area then
   consumers of the SRMS advertisements.  If the SRMS advertisements
   from the SRMS server are only used inside the SRMS server's area,
   area-scoped flooding may be used."

REQUIRED is like MUST - I think you mean "AS-scoped flooded SHOULD be
used.... area-scoped flooding MAY be used."

3) In Sec 4. "The Segment Routing Mapping Server, which is described in
   [I-D.ietf-spring-segment-routing-ldp-interop], is an example where we
   need a single advertisement to advertise SIDs for multiple prefixes
   from a contiguous address range."

I've read through the vastly improved section (thank you)
in draft-ietf-spring-segment-routing-ldp-interop-08 and I don't see any
explanation for why a contiguous address range is needed.

I can speculate that a primary purpose is to advertise SIDs for the
loopback addresses of routers that don't support SR - and those loopback
addresses are likely to be allocated from a contiguous range (though why
some wouldn't be supporting SR and cause gaps isn't clear).

4) Sec 5: In the end of Sec 4.2 in
draft-ietf-spring-segment-routing-ldp-interop-08, it says "Note: SR
mappings advertisements cannot set Penultimate Hop Popping.
   In the previous example, P6 requires the presence of the segment 103
   such as to map it to the LDP label 1037.  For that reason, the P flag
   available in the Prefix-SID is not available in the Remote-Binding
   SID."
However, in this draft Sec 5 gives the following rules:

"As the Mapping Server does not specify the originator of a prefix
advertisement, it is not possible to determine PHP behavior solely based on
the Mapping Server advertisement. However, PHP behavior SHOULD be done in
following cases: The Prefix is intra-area type and the downstream neighbor
is the originator of the prefix. The Prefix is inter-area type and
downstream neighbor is an ABR, which is advertising prefix reachability and
is also generating the Extended Prefix TLV with the A-flag set for this
prefix as described in section 2.1 of [RFC7684]. The Prefix is external
type and downstream neighbor is an ASBR, which is advertising prefix
reachability and is also generating the Extended Prefix TLV with the A-flag
set for this prefix as described in section 2.1 of [RFC7684].

These seem to be contradictory.

5) In Sec 7.1, it says "Multiple Mapping Servers can advertise
   Prefix-SIDs for the same prefix, in which case the same Prefix-SID
   MUST be advertised by all of them."

What is forcing this constraint?  Does it work if the Prefix-SID is an
index into an
SRGB or SRLB that is not the same value globally? I don't see it specified
in Sec 7.2 of draft-ietf-spring-segment-routing-ldp-interop-08?

Regards,
Alia

---

From: "Acee Lindem (acee)" <[email-masked]>
Date: Sun, 13 Aug 2017 23:57:40 +0000

Thanks Alia – I’ve read through the comments and I think the authors should be able to address these in this draft or the LDP interoperation draft.
Acee

From: OSPF <[email-masked]<mailto:[email-masked]>> on behalf of Alia Atlas <[email-masked]<mailto:[email-masked]>>
Date: Friday, August 11, 2017 at 10:09 PM
To: OSPF WG List <[email-masked]<mailto:[email-masked]>>, "[email-masked]<mailto:[email-masked]>" <[email-masked]<mailto:[email-masked]>>
Subject: [OSPF] AD review of draft-ietf-ospf-segment-routing-extensions-18

As is customary, I have done another AD review of draft-ietf-ospf-segment-routing-extensions-18. I do appreciate the improvements in the draft.

I do still see a few minor issues.  I would like to see a revised draft before IETF Last Call. I expect to progress this at an IESG telechat with the primary spring documents, when Alvaro feels they are ready.

2) In Sec 3.4: "For the purpose of the SRMS Preference TLV advertisement, AS-scoped flooding is REQUIRED.  This

REQUIRED is like MUST - I think you mean "AS-scoped flooded SHOULD be used.... area-scoped flooding MAY be used."

I've read through the vastly improved section (thank you) in draft-ietf-spring-segment-routing-ldp-interop-08 and I don't see any explanation for why a contiguous address range is needed.

I can speculate that a primary purpose is to advertise SIDs for the loopback addresses of routers that don't support SR - and those loopback addresses are likely to be allocated from a contiguous range (though why some wouldn't be supporting SR and cause gaps isn't clear).

4) Sec 5: In the end of Sec 4.2 in draft-ietf-spring-segment-routing-ldp-interop-08, it says "Note: SR mappings advertisements cannot set Penultimate Hop Popping.

"As the Mapping Server does not specify the originator of a prefix advertisement, it is not possible to determine PHP behavior solely based on the Mapping Server advertisement. However, PHP behavior SHOULD be done in following cases: The Prefix is intra-area type and the downstream neighbor is the originator of the prefix. The Prefix is inter-area type and downstream neighbor is an ABR, which is advertising prefix reachability and is also generating the Extended Prefix TLV with the A-flag set for this prefix as described in section 2.1 of [RFC7684]. The Prefix is external type and downstream neighbor is an ASBR, which is advertising prefix reachability and is also generating the Extended Prefix TLV with the A-flag set for this prefix as described in section 2.1 of [RFC7684].

What is forcing this constraint?  Does it work if the Prefix-SID is an index into an
SRGB or SRLB that is not the same value globally? I don't see it specified in Sec 7.2 of draft-ietf-spring-segment-routing-ldp-interop-08?

Regards,
Alia

---

From: Peter Psenak <[email-masked]>
Date: Mon, 14 Aug 2017 17:29:09 +0200

Hi Alia,

thanks for comments, please see inline:

On 12/08/17 04:09 , Alia Atlas wrote:
> before IETF Last Call. I expect to progress this at an IESG telechat
> with the primary spring documents, when Alvaro feels they are ready.
>
>

link-local scope here does not really make much sense, so the assumption 
was that it's either area or AS-scope, in which case area-scope has 
narrower flooding scope. I'll clarify that in the text.

>
> 2) In Sec 3.4: "For the purpose of the SRMS Preference TLV
> advertisement, AS-scoped flooding is REQUIRED.  This

will change to SHOULD.

> some wouldn't be supporting SR and cause gaps isn't clear).

range is an optimization similar to summarization. Instead of 
advertising each individual prefix to SID mappings, we can advertise 
single range with the starting SID. I referenced the 
I-D.ietf-spring-segment-routing-ldp-interop, because SRMS is an example 
where the range advertisements is clearly useful, although it's not 
limited to to that case. One can use SRMS as a SID provisioning tool.

> advertisement, it is not possible to determine PHP behavior solely based
> on the Mapping Server advertisement. However, PHP behavior SHOULD be
> done in following cases: The Prefix is intra-area type and the
> downstream neighbor is the originator of the prefix. The Prefix is
> inter-area type and downstream neighbor is an ABR, which is advertising
> prefix reachability and is also generating the Extended Prefix TLV with
> the A-flag set for this prefix as described in section 2.1 of [RFC7684].
> The Prefix is external type and downstream neighbor is an ASBR, which is
> advertising prefix reachability and is also generating the Extended
> Prefix TLV with the A-flag set for this prefix as described in section
> 2.1 of [RFC7684].
>

The text in draft-ietf-spring-segment-routing-ldp-interop-08 refers to 
the fact that SRMS advertisements itself can not include PHP signaling 
in the advertisement itself, like the regular SID advertisement does, 
because SRMS is not the "owner" of the prefix.

The text in the draft-ietf-ospf-segment-routing-extensions-18 describes 
how the PHP can still be done for SIDs that come from the SRMS 
adverisements, using additional information available to the protocol - 
e.g. prefix owner.

I don't believe these contradict each other.

> SRGB or SRLB that is not the same value globally?

yes, it does. The SID value for the single prefix MUST be unique though, 
otherwise we get into the conflict resolution area, that is covered by 
the draft-ietf-spring-conflict-resolution.

> I don't see it
> specified in Sec 7.2 of draft-ietf-spring-segment-routing-ldp-interop-08?

SR architecture assumes unique mapping of a SID to a prefix. If that is 
not followed, draft-ietf-spring-conflict-resolution comes into picture.

thanks,
Peter