Re: [Idr] AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17

"Ketan Talaulikar (ketant)" <ketant@cisco.com> Fri, 08 March 2019 16:05 UTC

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From: "Ketan Talaulikar (ketant)" <ketant@cisco.com>
To: Alvaro Retana <aretana.ietf@gmail.com>, "draft-ietf-idr-bgpls-segment-routing-epe@ietf.org" <draft-ietf-idr-bgpls-segment-routing-epe@ietf.org>
CC: Susan Hares <shares@ndzh.com>, "idr-chairs@ietf.org" <idr-chairs@ietf.org>, "idr@ietf. org" <idr@ietf.org>
Thread-Topic: AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17
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Date: Fri, 08 Mar 2019 16:05:19 +0000
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Subject: Re: [Idr] AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17
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Hi Alvaro,

I get the impression that your review and comments might have got truncated in the email that you sent below. It goes only till Sec 4.2 and you have referred to Sec 5 at the start of your email.

Could you please check/confirm that you have sent your entire set of review comments?

Thanks,
Ketan

-----Original Message-----
From: Ketan Talaulikar (ketant) 
Sent: 04 March 2019 11:57
To: 'Alvaro Retana' <aretana.ietf@gmail.com>; draft-ietf-idr-bgpls-segment-routing-epe@ietf.org
Cc: Susan Hares <shares@ndzh.com>; idr-chairs@ietf.org; idr@ietf. org <idr@ietf.org>
Subject: RE: AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17

Hi Alvaro,

Thanks for your detail review and feedback/comments. I will work on them along with co-authors and get back with updates.

Thanks,
Ketan

-----Original Message-----
From: Alvaro Retana <aretana.ietf@gmail.com> 
Sent: 02 March 2019 03:05
To: draft-ietf-idr-bgpls-segment-routing-epe@ietf.org
Cc: Susan Hares <shares@ndzh.com>; idr-chairs@ietf.org; idr@ietf. org <idr@ietf.org>
Subject: AD Review of draft-ietf-idr-bgpls-segment-routing-epe-17

Dear authors:

I just finished reviewing this document.  Please take a look at in-line comments below.

In general, I think that the document needs work in being consistent with terminology and its use.  Also, more details are needed in the Management and Security sections.

I have major concerns about the references to draft-ietf-spring-segment-routing-central-epe.  In the current text, draft-ietf-spring-segment-routing-central-epe is not only referenced as an example of the use of the SIDs defined here, but in a Normative way to indicate how the TLVs should be treated.  The obvious question is about the applicability of the extensions (see the text in §5 about "other use cases").  The importance given to I-D.ietf-spring-segment-routing-central-epe in this document, which should elevate it to a Normative reference, is probably more than it deserves since it just "illustrates the application of Segment Routing to solve the BGP Egress Peer Engineering (BGP-EPE) requirement" and it doesn’t contain a list of protocol requirements nor it mandates how the new TLVs should be used.  Note also that draft-ietf-spring-segment-routing-central-epe refers normatively to this document to explain the use case, so pointing Normatively back to it creates a loop....  Please treat draft-ietf-spring-segment-routing-central-epe as what is should be: a good Informative reference — I put specific comments in Sections 3 and 5 below.

I will wait for at least the major items to be addressed before starting the IETF Last Call.

Thanks!

Alvaro.


[The line numbers come from idnits.]

...
32 Requirements Language

34   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
35   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
36   document are to be interpreted as described in RFC 2119 [RFC2119].

[major] Please use the rfc8174 template.


...
103 1.  Introduction
...
113   The SR architecture [RFC8402] defines three types of BGP Peering
114   Segments that may be instantiated at a BGP node:

116   o  Peer Node Segment (Peer-Node-SID) : instruction to steer to a
117      specific peer node

119   o  Peer Adjacency Segment (Peer-Adj-SID) : instruction to steer over
120      a specific local interface towards a specific peer node

122   o  Peer Set Segment (Peer-Set-SID) : instruction to load-balance to a
123      set of specific peer nodes

[major] rfc8402 doesn't use the same names as above.  Please be consistent with the language already defined there.  Here, and across the rest of the document.


...
138   One use-case for these BGP Peering Segments is to enable computation
139   of SR paths that enable Central BGP-EPE as described in
140   [I-D.ietf-spring-segment-routing-central-epe].  This use-case
141   comprises of a centralized controller that learns the BGP Peering
142   SIDs via BGP-LS and then uses this information to program a SR policy
143   [I-D.ietf-spring-segment-routing-policy] at any node in the domain to
144   perform traffic steering via a specific BGP egress node to a specific
145   EBGP peer(s) optionally also over a specific interface.

[minor] I-D.ietf-spring-segment-routing-central-epe doesn't mention I-D.ietf-spring-segment-routing-policy anywhere.


147   This document introduces a new BGP protocol type for BGP-LS NLRI and
148   defines new BGP-LS Node and Link description TLVs to facilitate
149   advertising BGP-LS Link NLRI that represent the BGP peering topology.
150   Further, it specifies the BGP-LS Attribute TLVs for advertisement of
151   the BGP Peering Segments (i.e.  Peer Node SID, Peer Adjacency SID,
152   and Peer Set SID) to be advertised in the same BGP-LS Link NLRI.

[major] s/introduces a new BGP protocol type for BGP-LS NLRI/introduces a new BGP-LS Protocol-ID

[minor] s/Node and Link description TLVs/Node and Link Descriptor TLVs

[minor] s/Link NLRI/Link-State NLRI/g

154 2.  Segment Routing Documents

156   The main reference is the SR architecture defined in [RFC8402].

158   The SR BGP-EPE architecture and use-case is described in
159   [I-D.ietf-spring-segment-routing-central-epe].

[nit] This section doesn't add anything that hasn't been already mentioned.  Please consider removing it.

161 3.  BGP Peering Segments

163   As described in [I-D.ietf-spring-segment-routing-central-epe], a BGP-
164   EPE enabled Egress PE node MAY advertise SIDs corresponding to its
165   attached peers.  These SIDs are called BGP peering segments or BGP
166   Peering SIDs.  In case of EBGP, they enable the expression of source-
167   routed inter-domain paths.

[major] "As described in [I-D.ietf-spring-segment-routing-central-epe], a BGP-EPE enabled Egress PE node MAY advertise SIDs..."
I-D.ietf-spring-segment-routing-central-epe is just an illustrative document; it uses Normative language just to describe the requirements for the solution, and not to specify behavior (as suggested above).  In this case the "MAY" seems to be used to describe the use case:  s/MAY/may


169   An ingress border router of an AS may compose a list of SIDs to steer
170   a flow along a selected path within the AS, towards a selected egress
171   border router C of the AS, and to a specific EBGP peer.  At minimum,
172   a BGP-EPE policy applied at an ingress PE involves two SIDs: the Node
173   SID of the chosen egress PE and then the BGP Peering SID for the
174   chosen egress PE peer or peering interface.

176   Each BGP session MUST be described by a Peer Node SID.  The
177   description of the BGP session MAY be augmented by additional Peer
178   Adjacency SIDs.  Finally, multiple Peer Node SIDs or Peer Adjacency
179   SIDs MAY be part of the same group/set in order to group EPE
180   resources under a common Peer-Set SID.

[minor] Please add forward references to the sections where more details are provided.


182   When the extensions defined in this document are applied to the EPE
183   use-case defined in [I-D.ietf-spring-segment-routing-central-epe],
184   then the following BGP Peering SIDs need to be instantiated on a BGP
185   router for each of its BGP peer sessions that are enabled for EPE

[major] I-D.ietf-spring-segment-routing-central-epe is being used as Normative (below) justification for the definition of peering segments.  If possible, please describe the specification in general terms...  The text above and in §5 (where it talks about different behaviors for "other use
cases") raises the question of whether this document applies in general to any (known) use of the segments defined here, or only to BGP-EPE.  IOW, what are those "other use cases"?  Where are they described to guarantee that the specification applies to them?


187   o  One Peer-Node-SID MUST be instantiated to describe the BGP peer
188      session.

190   o  One or more Peer-Adj-SID MAY be instantiated corresponding to the
191      underlying link(s) to the directly connected BGP peer session.

193   o  A Peer-Set-SID MAY be instantiated and additionally associated and
194      shared between one or more Peer-Node-SIDs or Peer-Adj-SIDs.

196   While an egress point in a topology usually refers to EBGP sessions
197   between external peers, there's nothing in the extensions defined in
198   this document that would prevent the use of these extensions in the
199   context of IBGP sessions.  However, unlike EBGP sessions which are
200   generally between directly connected BGP routers which are also along
201   the traffic forwarding path, IBGP peer sessions may be setup to BGP
202   routers which are not in the forwarding path.  As such, when the IBGP
203   design includes sessions with route-reflectors, a BGP router SHOULD
204   NOT instantiate a BGP Peering SID for those sessions to peer nodes
205   which are not in the forwarding path since the purpose of BGP Peering
206   SID is to steer traffic to that specific peers.  Thus, the
207   applicability for IBGP peering may be limited to only those
208   deployments where the IBGP peer is also along with forwarding data
209   path.  Further details and the use-cases of BGP Peering SIDs and
210   their BGP-LS extensions to IBGP deployments are beyond the scope of
211   this document.

[nit] s/along with forwarding data path/along the forwarding data path

[major] "...details...of BGP Peering SIDs and their BGP-LS extensions to IBGP deployments are beyond the scope of this document."  The text in the same paragraph says: "nothing...would prevent the use of these extensions in the context of IBGP sessions".  I think that is a contradiction because
there are already descriptions in this document.   Solution: take out the
last sentence.

213   The BGP Peering SIDs instantiated as described above are then
214   advertised via BGP-LS Link NLRI as described in the sections below.

[minor] I think that what you meant is something like this: "Any instantiated BGP Peering SIDs are then advertised using BGP-LS, as described in the following sections."

216 4.  BGP-LS NLRI for BGP

[minor] Perhaps "BGP Protocol-ID", there's no new NLRI for BGP

[minor] s/BGP-LS NLRI/BGP-LS Link-State NLRI/g   The NLRI is called
"Link-State", not "BGP-LS".

218   This section describes the BGP-LS NLRI encodings that describe the
219   BGP peering and link connectivity between BGP routers.

221   This document specifies the advertisement of BGP peering topology
222   information via BGP-LS NLRI which requires use of a new BGP protocol
223   identifier.

[minor] s/BGP protocol identifier/BGP-LS Protocol-ID    The new definition
is for BGP-LS, not BGP.

225      Protocol-ID : BGP (codepoint 7 Early Allocation by IANA Section 8
226      from the registry "BGP-LS Protocol-IDs")

[nit] Consider using a table.


...
271         Value: 4 octet unsigned non-zero integer representing the BGP
272         Identifier as defined in [RFC4271] and [RFC6286].

[minor] rfc6286 Updated rfc4271, so there's no need to include both. s/BGP Identifier as defined in [RFC4271] and [RFC6286]./BGP Identifier [RFC6286].


...
282         Value: 4 octet unsigned non-zero integer representing the
283         Member ASN inside the Confederation [RFC5065].

[major] rfc5065 uses "Member-AS Number", please be consistent: s/Member ASN inside the Confederation [RFC5065]./Member-AS Number [RFC5065].


285 4.2.  Mandatory BGP Node Descriptors ...
293   o  Autonomous System Number, which contains the ASN or confederation
294      identifier (ASN), if confederations are used, of the local BGP
295      node.

[minor] Is this TLV new, or are you referring to TLV 512 [rfc7752]?  Please include the TLV number (and a reference, if appropriate) to avoid confusion