Re: [Idr] Shepherd writeup for draft-ietf-idr-bgp-ls-sbfd-extensions
"Ketan Talaulikar (ketant)" <ketant@cisco.com> Mon, 08 March 2021 18:35 UTC
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From: "Ketan Talaulikar (ketant)" <ketant@cisco.com>
To: Jeffrey Haas <jhaas@pfrc.org>, "draft-ietf-idr-bgp-ls-sbfd-extensions@ietf.org" <draft-ietf-idr-bgp-ls-sbfd-extensions@ietf.org>, "idr@ietf.org" <idr@ietf.org>
Thread-Topic: Shepherd writeup for draft-ietf-idr-bgp-ls-sbfd-extensions
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Date: Mon, 08 Mar 2021 18:35:26 +0000
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Subject: Re: [Idr] Shepherd writeup for draft-ietf-idr-bgp-ls-sbfd-extensions
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Hi Jeff, Thanks for your review and comments. Please check and let know if I've got your complete review (it was the same in the IDR archive) since we sometimes see truncation when emails are send out with comments embedded within IDNITs text. We've posted an update for the draft with comments addressed as clarified in detail inline below. https://www.ietf.org/archive/id/draft-ietf-idr-bgp-ls-sbfd-extensions-05.txt -----Original Message----- From: Jeffrey Haas <jhaas@pfrc.org> Sent: 20 February 2021 23:40 To: draft-ietf-idr-bgp-ls-sbfd-extensions@ietf.org; idr@ietf.org Subject: Shepherd writeup for draft-ietf-idr-bgp-ls-sbfd-extensions Authors, Please find attached the shepherd review comments for draft-ietf-idr-bgp-ls-sbfd-extensions. The line numbering is from the ID-nits tool output vs. draft version -04. Tersely: - Some suggest grammar and syntax tweaks. Changes you find contentious can be deferred to the authority of the RFC Editor. - A few changes to use the terminology as specified in the S-BFD RFC. - One item of major concern is the ordering of the S-BFD Discrimators. Please use this thread to respond to how you'd like to address that open concern. I believe once we have addressed these items we can submit this document to the IESG. A final item of discussion is we are in the middle of RFC7752-bis work. While it is my opinion that the -bis work has no implications currently on the mechanism described in this draft, the authors and Working Group should consider if they prefer to this mechanism to normatively require the -bis document. [KT] Since the bis document RFC will obsolete RFC7752 when published, would it not be ok to continue using RFC7752 until then? The concern that I have is that these minor BGP-LS extensions are not held up on the publication of the bis document. -- Jeff ----------------------------------8<--- cut here --->8--------------------------- 81 1. Introduction 83 Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] defines 84 a simplified mechanism to use Bidirectional Forwarding Detection 85 (BFD) [RFC5880] with large portions of negotiation aspects 86 eliminated, thus providing benefits such as quick provisioning as 87 well as improved control and flexibility to network nodes initiating 88 the path monitoring. 90 For monitoring of a service path end-to-end via S-BFD, the headend/ 91 initiator node needs to know the S-BFD Discriminator of the Initiator is the capitalization used in RFC 7880. [KT] Ack 92 destination/tail-end node of that service. The link-state routing Responder would be the appropriate term to use here. I would suggest dropping headend/tail end and simply use Initiator/Responder (or Reflector) as per RFC 7880. [KT] I've update the text to use both in a consistent manner so it is easier to understand from both BFD and TE/SR terminologies perspective for the use-cases under discussion. 93 protocols (IS-IS, OSPF and OSPFv3) have been extended to advertise 94 the S-BFD Discriminators. With this a initiator node can learn the With this, an Initiator can [KT] ack 95 S-BFD discriminator for all nodes within its IGP area/level or level, or [KT] ack [...] 123 3. Problem and Requirement [...] 133 boundaries. This provides a seamless MPLS transport connectivity for 134 any two service end-points across the entire domain. In order to 135 detect failures for such end to end services and trigger faster 136 protection and/or re-routing, S-BFD MAY be used for the Service Layer 137 (e.g. for MPLS VPNs, pseudowires, etc. ) or the Transport Layer 138 monitoring. This brings up the need for setting up S-BFD session Suggest "creates the need" [KT] ack 139 spanning across AS domains. 141 In a similar Segment Routing (SR) [RFC8402] multi-domain network, an 142 end to end SR Policy [I-D.ietf-spring-segment-routing-policy] path 143 may be provisioned between service end-points across domains either 144 via local provisioning or by a controller or signalled from a Path provisioning, or [KT] ack 145 Computation Engine (PCE). Monitoring using S-BFD can similarly be While PCE is being used in a very general fashion here, an informational reference to RFC 4655 may be appropriate. [KT] ack 146 setup for such a SR Policy. 148 Extending the automatic discovery of S-BFD discriminators of nodes 149 from within the IGP domain to across the administrative domain using "domain across the administrative domain" or "domain to cross an administrative domain" [KT] Ack 150 BGP-LS enables setting up of S-BFD sessions on demand across IGP "enables creating S-BFD sessions" [KT] ack 151 domains. The S-BFD discriminators for service end point nodes MAY be 152 learnt by the PCE or a controller via the BGP-LS feed that it gets 153 from across IGP domains and it can signal or provision the remote domains, and it [KT] ack 154 S-BFD discriminator on the initiator node on demand when S-BFD Initiator [KT] ack 155 monitoring is required. The mechanisms for the signaling of the 156 S-BFD discriminator from the PCE/controller to the initiator node and 157 setup of the S-BFD session is outside the scope of this document. "are outside the scope" [KT] ack 159 Additionally, the service end-points themselves MAY also learn the 160 S-BFD discriminator of the remote nodes themselves by receiving the 161 BGP-LS feed via a route reflector (RR) or a centralized BGP Speaker Suggest informational citation for RFC 4456. [KT] ack 162 that is consolidating the topology information across the domains. 163 The initiator node can then itself setup the S-BFD session to the Initiator [KT] ack 164 remote node without a controller/PCE assistance. 166 While this document takes examples of MPLS and SR paths, the S-BFD 167 discriminator advertisement mechanism is applicable for any S-BFD 168 use-case in general. 170 4. BGP-LS Extensions for S-BFD Discriminator 172 The BGP-LS [RFC7752] specifies the Node NLRI for advertisement of 173 nodes and their attributes using the BGP-LS Attribute. The S-BFD 174 discriminators of a node are considered as its node level attribute 175 and advertised as such. 177 This document defines a new BGP-LS Attribute TLV called the S-BFD 178 Discriminators TLV and its format is as follows: "TLV, and its" [KT] ack 180 0 1 2 3 181 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 | Type | Length | 184 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 185 | Discriminator 1 | 186 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 187 | Discriminator 2 (Optional) | 188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 189 | ... | 190 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 191 | Discriminator n (Optional) | 192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 194 Figure 1: S-BFD Discriminators TLV 196 where: 198 o Type: 1032 (early allocation by IANA) 200 o Length: variable. Minimum of 4 octets and increments of 4 octets 201 there on for each additional discriminator 203 o Discriminators : multiples of 4 octets, each carrying a S-BFD 204 local discriminator value of the node. At least one discriminator 205 MUST be included in the TLV. Here upon deep review, I think we have an interesting item requiring further discussion and perhaps text. RFC 7880, section 3, says the following about multiple discriminators: : When a node allocates multiple S-BFD Discriminators, how remote nodes : determine which of the discriminators is associated with a specific : entity is currently unspecified. The use of multiple S-BFD : Discriminators by a single network node is therefore discouraged : until a means of learning the mapping is defined. RFC 7752, section 3.1 says the following about sorting: : In order to compare NLRIs with unknown : TLVs, all TLVs MUST be ordered in ascending order by TLV Type. If : there are more TLVs of the same type, then the TLVs MUST be ordered : in ascending order of the TLV value within the TLVs with the same : type by treating the entire Value field as an opaque hexadecimal : string and comparing leftmost octets first, regardless of the length : of the string. All TLVs that are not specified as mandatory are : considered optional. Given the above text, it seems like it should be recommended that the list of Discriminators is sorted. [KT] The text in RFC7752 is in the context of the TLVs that go into the NLRI portion. The SBFD Discriminator TLV goes into the BGP-LS Attribute and hence these considerations do not apply for it. Where this is potentially problematic is the mapping procedures: 207 The S-BFD Discriminators TLV can be added to the BGP-LS Attribute 208 associated with the Node NLRI that originates the corresponding 209 underlying IGP TLV/sub-TLV as described below. This information is 210 derived from the protocol specific advertisements as below.. 212 o IS-IS, as defined by the S-BFD Discriminators sub-TLV in 213 [RFC7883]. 215 o OSPFv2/OSPFv3, as defined by the S-BFD Discriminators TLV in 216 [RFC7884]. Both of these RFCs extend the hand-waving that was done during the S-BFD standardization process for "how do you pick one". The answer at that point of time was "the implementation will decide", which might normally be fine. However, since RFC 7752 has an opinion about how attributes should be presented for canonicalization purposes, a simple copy-and-paste from the IGP into BGP-LS may not be appropriate. And that perhaps impacts a proprietary scheme; e.g. "pick the first one in the set". [KT] Please check response to previous comment. In the interest of keeping BGP-LS as simple and "true" to what is being advertised in the IGPs, the current text looks good to me. However, do let know if you feel that we need to handle things differently in this instance. Thanks, Ketan
- [Idr] Shepherd writeup for draft-ietf-idr-bgp-ls-… Jeffrey Haas
- Re: [Idr] Shepherd writeup for draft-ietf-idr-bgp… Ketan Talaulikar (ketant)
- Re: [Idr] Shepherd writeup for draft-ietf-idr-bgp… Jeffrey Haas
- Re: [Idr] Shepherd writeup for draft-ietf-idr-bgp… Ketan Talaulikar (ketant)