[Idr] AD Review of draft-ietf-idr-bgp-ls-app-specific-attr-08
Alvaro Retana <aretana.ietf@gmail.com> Mon, 11 April 2022 14:46 UTC
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Subject: [Idr] AD Review of draft-ietf-idr-bgp-ls-app-specific-attr-08
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Dear authors: I just finished reading this document. Thank you for the work! I've had the pleasure of processing all the BGP-LS extensions to date. A constant throughout has been the concept that BGP-LS provides transport; what to do with the information and any error checking (beyond basic checks) is left to the consumer and out of scope. However, this document includes a good number of instructions for the consumer; for example: ...the link attributes advertised within an ASLA TLV with such a zero-length application bit mask MUST NOT be considered by BGP-LS consumers for those applications for which at least one instance of ASLA TLV is present in the same advertisement with their specific application bit set. A BGP-LS consumer that supports this specification SHOULD prefer the application-specific attribute value received via sub-TLVs within the ASLA TLV over the value received via the top-level TLVs. These are two examples in which the document directly specifies what the consumer is to do with the information. Again, these types of statements have been considered out of scope. Please remove them. I commented inline about other instances but may not have caught them all. I will wait for my comments to be addressed before moving the document forward. Thanks! Alvaro. [Line numbers from idnits.] ... 78 1. Introduction [] I appreciate the background, but the objective of this document is to define a way to carry the IGP-derived information. The justification for the IGPs to carry the information and its potential use should have already been justified in [RFC8920] and [RFC8919]. It concerns me that adding this much background will raise unnecessary questions. I strongly recommend that you focus on the objective of this document and keep it simple: [RFC8920] and [RFC8919] define extensions to carry application specific attributes. This document specifies how this information is carried in BGP-LS. Ok, maybe a few more words would be good. You get the idea. ... 131 2. Application Specific Link Attributes TLV ... [major] For all the fields below, I made the same comment: the information comes from the IGPs. Instead of describing the fields independently, perhaps consider a statement like this: The semantics and values of the fields in the TLV are described in [RFC8920] and [RFC8919]. 167 o SABM Length : Standard Application Identifier Bit Mask Length in 168 octets. The values MUST be 0, 4, or 8. If the Standard 169 Application Identifier Bit Mask is not present, the value MUST be 170 set to 0. [major] BGP-LS is just the carrier of the information, the validation or use of the data by the consumer is out of scope. All the information is taken from the IGPs. The descriptions should then indicate where the information comes from: SABM Length: Standard Application Identifier Bit Mask Length as defined in [rfc8919] and [rfc8920]. 172 o UDABM Length : User-Defined Application Identifier Bit Mask Length 173 in octets. The values MUST be 0, 4, or 8. If the User-Defined 174 Application Identifier Bit Mask is not present, the value MUST be 175 set to 0. [major] Same comment as above. Note that the specifications in rfc8919/rfc8920 are not the same, which is another reason to simple point at the existing documents. 177 o Standard Application Identifier Bit Mask : of size 0, 4, or 8 178 octets as indicated by the SABML. An optional set of bits, where 179 each bit represents a single standard application. The bits are 180 defined in the IANA "IGP Parameters" registries under the "Link 181 Attribute Applications" registry [RFC8919]. [major] Same comment as above... 183 o User-Defined Application Identifier Bit Mask : of size 0, 4, or 8 184 octets as indicated by the UDABML. An optional set of bits, where 185 each bit represents a single user-defined application. The bits 186 are not managed or assigned by IANA or any other standards body 187 and definition is left to the implementation. [major] Same comment as above... 189 o sub-TLVs : BGP-LS Attribute TLVs corresponding to the Link NLRI 190 that are application-specific (as specified in Section 3) are 191 included as sub-TLVs of the ASLA TLV. [minor] s/sub-TLVs :/Link Attribute sub-TLVs : 193 An ASLA TLV with both the SABM and UDABM lengths set to 0 (i.e. 194 without any application identifier bit masks) indicates that the link 195 attribute sub-TLVs that it encloses are applicable for all 196 applications. However, the link attributes advertised within an ASLA 197 TLV with such a zero-length application bit mask MUST NOT be 198 considered by BGP-LS consumers for those applications for which at 199 least one instance of ASLA TLV is present in the same advertisement 200 with their specific application bit set. [major] This paragraph talks about what a consumer should do with the information. This is outside the scope of BGP-LS. ... 208 When the node is not running any of the IGPs but running a protocol 209 like BGP, then the link attributes for the node's local links MAY be 210 originated as part of the BGP-LS Attribute using the ASLA TLV and its 211 sub-TLVs within the Link NLRI corresponding to the local node. [major] The subject of including non-IGP information, BGP specifically, has come up in other BGP-LS-related drafts. The conclusion has been that the process is currently not specified and that draft-ietf-idr-bgp-ls-bgp-only-fabric may be the best place to do that -- at least that was the conclusion the last time we discussed it [1]. Has anything changed? If not, then please remove the paragraph above. [1] https://mailarchive.ietf.org/arch/msg/idr/7nNtfqJ6OyktdP4QETX2xHSHbTw/ 213 3. Application-Specific Link Attributes 215 Several BGP-LS Attribute TLVs corresponding to the Link NLRI are 216 defined in BGP-LS and more may be added in the future. The following 217 types of link attributes are required to be considered application- 218 specific. [major] "are required" The wording sounds like it may need to be a Normative statement ("are REQUIRED"), but that would then be an indication to the consumer of what to do. Please use "are expected" instead. 220 o those that have different values for different applications e.g., 221 a different TE metric value used for RSVP-TE than for SR Policy 223 o those that apply to multiple applications but need to be used only 224 by a specific application e.g., certain Shared Risk Link Group 225 (SRLG) values are configured on a node for LFA but the same do not 226 need to be used for RSVP-TE [major] This text doesn't describe "types of link attributes", but *uses* of the attributes. For example, 1096 (SRLG is used as an example) is not already defined to indicate "different values for different applications" nor how it should be "used only by a specific application". 228 The following table lists the currently defined BGP-LS Attributes 229 TLVs corresponding to Link NLRI that have application-specific 230 semantics. These were originally defined with semantics for RSVP-TE 231 and GMPLS applications. [minor] s/that have/that can have [] What's the value of identifying these TLVs? More specifically: given that it is up to the consumer (not BGP-LS) to decide what is appropriate, what is the importance of this information? If a TLV that is not listed (1089, for example) is included, then what? [I know there are Normative statements about that in the next section -- see more comments there.] 233 +---------------+-------------------------------+-------------------+ 234 | TLV Code | Description | Reference | 235 | Point | | Document | 236 +---------------+-------------------------------+-------------------+ 237 | 1088 | Administrative group (color) | [RFC7752] | 238 | 1092 | TE Default Metric | [RFC7752] | 239 | 1096 | Shared Risk Link Group | [RFC7752] | 240 | 1114 | Unidirectional Link Delay | [RFC8571] | 241 | 1115 | Min/Max Unidirectional Link | [RFC8571] | 242 | | Delay | | 243 | 1116 | Unidirectional Delay | [RFC8571] | 244 | | Variation | | 245 | 1117 | Unidirectional Link Loss | [RFC8571] | 246 | 1118 | Unidirectional Residual | [RFC8571] | 247 | | Bandwidth | | 248 | 1119 | Unidirectional Available | [RFC8571] | 249 | | Bandwidth | | 250 | 1120 | Unidirectional Utilized | [RFC8571] | 251 | | Bandwidth | | 252 | 1173 | Extended Administrative Group | [RFC9104] | 253 +---------------+-------------------------------+-------------------+ 255 Table 1: BGP-LS Attribute TLVs also used as sub-TLVs of ASLA TLV 257 All the BGP-LS Attribute TLVs defined in the table above are 258 RECOMMENDED to continue to be advertised at the top-level in the BGP- 259 LS Attribute for carrying attributes specific to RSVP-TE without the 260 use of the ASLA TLV. [nit] s/RECOMMENDED to continue to be advertised/RECOMMENDED to be advertised [major] When is it ok to not advertise these TLVs at the top-level? Assuming that the reason is backward compatibility (?), why isn't it a requirement? The propagation of those TLVs was defined elsewhere -- by recommending, the text is conditioning the application of those other RFCs. [major] What is the "top-level"? I guess you mean "not as a sub-TLV of the Application-Specific Link Attributes TLV", but that is not explained anywhere -- and rfc7752 doesn't talk about top-level at all. 262 When a new link attribute is introduced, it may be thought of as 263 being specific to only a single application. However, subsequently, 264 it may be also shared by other applications and/or require 265 application-specific values. In such cases, it is RECOMMENDED to err 266 on the side of caution and define such attributes as application- 267 specific to ensure flexibility in the future. [major] "new link attribute...may...require application-specific values. In such cases, it is RECOMMENDED to...define such attributes as application-specific..." What does "to...define...as application-specific" mean? The attributes listed above were not defined as "application-specific"; the ASLA TLV provides the application-specific context. Is that (ability to include the attribute as a sub-TLV) that you mean, or are you expecting something more? At least, you need to be more specific on what is expected. When would it be ok not to define a new attribute as "application-specific"? Why is it recommended and not required? 269 BGP-LS Attribute TLVs corresponding to Link NLRI that are defined in 270 the future MUST specify if they are application-specific and hence 271 are REQUIRED to be encoded within an ASLA TLV. [major] This paragraph provides more insight into what you might mean. Let me see if I understand. If a new link attribute is specified as "application-specific" it then MUST be included in the ASLA TLV. Does that mean that it can't be used as a top-level TLV? I'm assuming that if a new link attribute is not specified as "application-specific" it still can be included in the ASLA TLV, and it can also be used as a top-level TLV. Is that true? Given that the consumer decides what to do with the information received, there doesn't seem to be a strong incentive to specifying new attributes as "application-specific". Not doing it would result in maximum flexibility. 273 Only application-specific link attributes need to be advertised 274 within the ASLA TLV. Link attributes that do not have application- 275 specific semantics MUST NOT be advertised within the ASLA TLV. 276 Receivers MUST ignore any non-application-specific attribute sub-TLVs 277 within the ASLA TLV. [major] Given the requirement in the second sentence, the first sentence just adds confusion. Note also that it is still not clear to me how one can tell if an attribute has "application-specific semantics" or not. [major] "Receivers MUST ignore..." Since when is it ok to give instructions to the consumer about what to do with the BGL-LS information? 279 When the same application-specific link attributes are advertised 280 both within the ASLA TLV and as top-level TLVs in the BGP-LS 281 Attribute, the attributes advertised within the ASLA TLV take 282 precedence for the applications indicated in the ASLA TLV encoding. [major] Again, isn't this an action that the consumer decides? 284 4. Procedures 286 The procedures described in this section apply to networks where all 287 BGP-LS originators and consumers support this specification. The 288 backward compatibility aspects and operations in deployments where 289 there are some BGP-LS originators or consumers that do not support 290 this specification are described further in Section 6. [major] How can it be confirmed that "all BGP-LS originators and consumers support this specification"? I'm sure that it is left to the operator, right? Please say so. 292 The BGP-LS originator learns of the association of an application- 293 specific attribute to one or more applications from either the 294 underlying IGP protocol LSA/LSPs from which it is advertising the 295 topology information or from the local node configuration when 296 advertising attributes for the local node only. [major] "or from the local node..." See the comment in §2 about non-IGP information. 298 The association of an application-specific link attribute with a 299 specific application context when advertising attributes for the 300 local node only (e.g., when running BGP as the only routing protocol) 301 is an implementation-specific matter and outside the scope of this 302 document. [major] See the comment in §2 about non-IGP information. ... 310 A BGP-LS originator node that is advertising link-state information 311 from the underlying IGP determines the protocol encoding of 312 application-specific link attributes based on the following rules: 314 1. Application-specific link attributes received from an IGP node 315 using existing RSVP-TE/GMPLS encodings MUST be encoded using the 316 respective BGP-LS top-level TLVs listed in Table 1. [major] "MUST be encoded using...Table 1." What about a new application-specific attribute? By definition it won't be listed in Table 1. My concern is that this document is defining the general process, so a new attribute would have to Update this document to include the new attribute in Table 1 (because of the Normative pointer), or it would have to Update the process itself. The complication is that later (step F, for example) this document requires that, even if some IGP information is encoded in the corresponding TLV, the information not be encoded in the ASLA TLV. This makes the generalization beyond "Table 1" complicated. <sigh> 318 2. Application-specific link attributes received from an OSPF node 319 using ASLA sub-TLV or from an IS-IS node using either ASLA sub- 320 TLV or ASLA SRLG TLV MUST be encoded in the BGP-LS ASLA TLV as 321 sub-TLVs. [major] "MUST be encoded in the BGP-LS ASLA TLV" Step F requires that the Maximum Link Bandwidth NOT be encoded in the SLA TLV. That requirement contradicts the requirement in this step. 323 3. In the case of IS-IS, the following specific procedures are to be 324 followed: 326 A. When application-specific link attributes are received from a 327 node with the L bit set in the ASLA sub-TLV and application 328 bits other than RSVP-TE are set in the application bit masks 329 then the application-specific link attributes advertised in 330 the corresponding legacy IS-IS TLVs/sub-TLVs MUST be encoded 331 within the BGP-LS ASLA TLV as sub-TLVs with the application 332 bits, other than the RSVP-TE bit, copied from the IS-IS ASLA 333 sub-TLV. The link attributes advertised in the legacy IS-IS 334 TLVs/sub-TLVs are also advertised in BGP-LS top-level TLVs 335 listed in Table 1. Note that this is true regardless of 336 whether the RSVP-TE bit was set in the IS-IS ASLA TLV/sub- 337 TLV. The same procedure also applies for the advertisement 338 of the SRLG values from the IS-IS ASLA SRLG TLV within the 339 BGP-LS SRLG TLV (1096) both at the top-level and within the 340 BGP-LS ASLA TLV. [major] Using the ASLA TLV is required ("MUST be encoded within the BGP-LS ASLA TLV"), but announcing the information from "the legacy IS-IS TLVs/sub-TLVs" is not ("link attributes advertised in the legacy IS-IS TLVs/sub-TLVs are also advertised in BGP-LS top-level TLVs"). Why is there a difference in the treatment? Is the intent that the second action should be required or just a recommendation? Would it be clearer if Normative language is used? Also, it is not clear to me if the "link attributes advertised in the legacy IS-IS TLVs/sub-TLVs" should be advertised *only* in the top-level, or both in the top-level and the ASLA TLV. The "also" part (given the preceding sentences) is what is throwing me off. [major] "Note that this is true regardless of whether the RSVP-TE bit was set in the IS-IS ASLA TLV/sub-TLV." I find this part confusing. I guess that by "this" you mean the part about legacy TLVs/sub-TLVs, and not the initial part that explicitly says that the RSVP-TE bit is not set. Perhaps divide the text in this bullet into 2 or 3 different paragraphs to improve clarity. [major] "The same procedure also applies for the advertisement of the SRLG values from the IS-IS ASLA SRLG TLV within the BGP-LS SRLG TLV (1096) both at the top-level and within the BGP-LS ASLA TLV." My reading of that "same procedure" is that the information in the IS-IS ASLA sub-TLV "MUST be encoded within the BGP-LS ASLA TLV as sub-TLVs" -- that's it. But the text in this last sentence goes on to say "both at the top-level and within the BGP-LS ASLA TLV." IOW, the described procedure doesn't seem to contemplate the "both" part. ... 349 C. A merge of the SRLG values advertised in IS-IS SRLG ASLA TLVs 350 and the other link attributes advertised in IS-IS ASLA sub- 351 TLVs, on a per-application basis, is REQUIRED for all 352 applications that have their bit set in the SABM/UDABM in at 353 least one of the aforementioned TLV types. When performing 354 this merge, only the TLVs with the application's bit set in 355 SABM/UDABM MUST be used when such TLVs are available from 356 either TLV types. If the bit for an application is set in 357 the SABM/UDABM of only one of the TLV types, then the 358 attributes from the other TLV type with zero-length 359 application bit mask MUST be also selected for that 360 application, if such TLV is available. Such merged link 361 attributes are advertised in a per application instance of 362 the BGP-LS ASLA TLV. [major] "A merge of the SRLG values...is REQUIRED..." How is that "merge" performed? Should the router advertise all the addresses? Should it make sure there are no duplicates? Should the addresses be ordered somehow (for easier comparison in the next step)? Please be explicit. 364 D. If the resulting set of merged link attributes and SRLG 365 values is common across multiple applications, they MAY be 366 advertised in a common BGP-LS ASLA TLV instance where the 367 bits for all such applications would be set in the 368 application bit mask. 370 E. Both the SRLG values from IS-IS ASLA SRLG TLVs and the link 371 attributes from IS-IS ASLA sub-TLVs, with the zero-length 372 application bit mask, MUST be advertised into a BGP-LS ASLA 373 TLV with a zero-length application bit mask, independent of 374 the merging described above. [] I think this means that the SRLG information will be merged *and* independently advertised. 376 F. [RFC8919] allows the advertisement of the Maximum Link 377 Bandwidth within an ASLA sub-TLV even though it is not an 378 application-specific attribute. However, when originating 379 the Maximum Link Bandwidth into BGP-LS, the attribute MUST be 380 encoded only in the top-level BGP-LS Maximum Link Bandwidth 381 TLV (1089) and the receiver MUST ignore them when advertised 382 within the BGP-LS ASLA TLV. [] "MUST be encoded only in the top-level" See the comment in step 2. [major] "the receiver MUST ignore them..." This statement is requiring how the consumer handles the information it receives. 384 G. [RFC8919] also allows the advertisement of the Maximum 385 Reservable Link Bandwidth and the Unreserved Bandwidth within 386 an ASLA sub-TLV even though these attributes are specific to 387 RSVP-TE application. However, when originating the Maximum 388 Reservable Link Bandwidth and Unreserved Bandwidth into BGP- 389 LS, these attributes MUST be encoded only in the BGP-LS top- 390 level Maximum Reservable Link Bandwidth TLV (1090) and 391 Unreserved Bandwidth TLV (1091) respectively and not within 392 the BGP-LS ASLA TLV. [] "MUST be encoded only in the BGP-LS top-level..." This statement also contradicts the requirement in step 2. ... 401 A BGP-LS consumer node would normally receive all the application- 402 specific link attributes corresponding to RSVP-TE and GMPLS 403 applications as existing top-level BGP-LS TLVs while for other 404 applications they are encoded in ASLA TLV(s) with appropriate 405 applicable bit mask setting. A BGP-LS consumer that supports this 406 specification SHOULD prefer the application-specific attribute value 407 received via sub-TLVs within the ASLA TLV over the value received via 408 the top-level TLVs. [major] Giving instructions to the consumer... 410 5. Deployment Considerations [major] This section doesn't belong in this document. These considerations are on the deployment of attributes by the IGPs, not the collection of the data through BGP-LS. ... 430 6. Backward Compatibility 432 The backward compatibility aspects for BGP-LS are associated with the 433 originators (i.e., nodes) and consumers (e.g., PCE, controllers, 434 applications, etc.) of the topology information. BGP-LS 435 implementations have been originating link attributes and consuming 436 them without any application-specific scoping prior to the extensions 437 specified in this document. 439 IGP backward compatibility aspects associated with application- 440 specific link attributes for RSVP-TE, SR Policy, and LFA applications 441 are discussed in the Backward Compatibility sections of [RFC8920] and 442 [RFC8919]. While those backward compatibility aspects ensure 443 compatibility of IGP advertisements, they also serve to ensure the 444 backward compatibility of the BGP-LS advertisements used by BGP-LS 445 consumers. In deployments where the BGP-LS originators or consumers 446 do not support the extensions specified in this document, the IGPs 447 need to continue to advertise link attributes intended for use by SR 448 Policy and LFA applications using the RSVP-TE/GMPLS encodings. This 449 allows BGP-LS advertisements to be consistent with the behavior prior 450 to the extensions defined in this document [major] This section basically says that these extensions are not backwards compatible. The text is then misleading by saying that the IGP backwards compatibility aspects "ensure the backward compatibility of the BGP-LS advertisements used by BGP-LS consumers". Neither one is backwards compatible!! Instead of this section, just point at the Deployment Considerations of the IGP RFCs. 452 It is RECOMMENDED that nodes that support this specification are 453 selected as originators of BGP-LS information when advertising the 454 link-state information from the IGPs. [major] "RECOMMENDED that nodes that support this specification are selected as originators of BGP-LS" In general, this seems like a fine recommendation, but what about the case where the consumer doesn't support this specification? That seems to be the caveat of why the action is recommended and not required. Please be explicit about that case. In fact, it would be better if it wasn't Normatively recommended. 456 7. IANA Considerations 458 This document requests assignment of code-points from the registry 459 "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and 460 Attribute TLVs" based on the table below which reflects the values 461 assigned via the early allocation process. The column "IS-IS TLV/ 462 Sub-TLV" defined in the registry does not require any value and 463 should be left empty. [] NEW> IANA has assigned (early allocation) the following code point from the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry. This document requests that the allocation be made permanent. The column "IS-IS TLV/Sub-TLV" defined in the registry does not require any value and should be left empty. 465 +------------+------------------------------------------+----------+ 466 | Code Point | Description | Length | 467 +------------+------------------------------------------+----------+ 468 | 1122 | Application-Specific Link Attributes TLV | variable | 469 +------------+------------------------------------------+----------+ [nit] Please add a Table number/name. ... 486 9. Security Considerations 488 The procedures and protocol extensions defined in this document do 489 not affect the BGP security model. See the "Security Considerations" 490 section of [RFC4271] for a discussion of BGP security. Also, refer 491 to [RFC4272] and [RFC6952] for analyses of security issues for BGP. 492 Security considerations for acquiring and distributing BGP-LS 493 information are discussed in [RFC7752]. The TLVs introduced in this 494 document are used to propagate the application-specific link 495 attributes IGP extensions defined in [RFC8919] and [RFC8920]. It is 496 assumed that the IGP instances originating these TLVs will support 497 all the required security (as described in [RFC8919] and [RFC8920]) 498 in order to prevent any security issues when propagating the TLVs 499 into BGP-LS. The advertisement of the link attribute information 500 defined in this document presents no significant additional risk 501 beyond that associated with the existing link attribute information 502 already supported in [RFC7752]. [minor] I think that pointing at the security considerations of rfc7752 is enough. IOW, pointing as rfc4271, rfc4272, and rfc6952 is not necessary. [?] Is the attack surface increased my putting this information in BGP-LS? What can a rogue node do with this additional information? We should carry forward similar text to the one in the IGP RFCs. Suggestion> This document defines a new way to advertise link attributes. Tampering with the information defined in this document may have an effect on applications using it, including impacting traffic engineering, which uses various link attributes for its path computation. As the advertisements defined in this document limit the scope to specific applications, the impact of tampering is similarly limited in scope. ... 539 11.2. Informative References ... 593 [RFC8571] Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and 594 C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of 595 IGP Traffic Engineering Performance Metric Extensions", 596 RFC 8571, DOI 10.17487/RFC8571, March 2019, 597 <https://www.rfc-editor.org/info/rfc8571>. 599 [RFC9104] Tantsura, J., Wang, Z., Wu, Q., and K. Talaulikar, 600 "Distribution of Traffic Engineering Extended 601 Administrative Groups Using the Border Gateway Protocol - 602 Link State (BGP-LS)", RFC 9104, DOI 10.17487/RFC9104, 603 August 2021, <https://www.rfc-editor.org/info/rfc9104>. [major] These last two references should be Normative: you are using Normative language to refer to the contents of Table 1, where these references are used. [EoR -08]
- [Idr] AD Review of draft-ietf-idr-bgp-ls-app-spec… Alvaro Retana
- Re: [Idr] AD Review of draft-ietf-idr-bgp-ls-app-… Ketan Talaulikar
- Re: [Idr] AD Review of draft-ietf-idr-bgp-ls-app-… Alvaro Retana
- Re: [Idr] AD Review of draft-ietf-idr-bgp-ls-app-… Ketan Talaulikar
- Re: [Idr] AD Review of draft-ietf-idr-bgp-ls-app-… Alvaro Retana