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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-08) exists of draft-ietf-idr-next-hop-capability-07 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force J. G. Scudder 3 Internet-Draft K. Kompella 4 Intended status: Informational Juniper Networks 5 Expires: 30 October 2022 28 April 2022 7 BGP Entropy Label Capability, Version 2 8 draft-scudder-bgp-entropy-label-00 10 Abstract 12 RFC 6790 defined the Entropy Label Capability Attribute (ELC); RFC 13 7447 deprecated that attribute. This specification, dubbed "Entropy 14 Label Capability Attribute version 2" (ELCv2), was intended to be 15 offered for standardization, to replace the ELC as a way to signal 16 that a BGP protocol speaker is capable of processing entropy labels. 18 Although ultimately a different specification was chosen for that 19 purpose, at least one implementation of ELCv2 was shipped by Juniper 20 Networks and is currently in use in service provider networks. This 21 document is published in order to document what was implemented. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on 30 October 2022. 40 Copyright Notice 42 Copyright (c) 2022 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 47 license-info) in effect on the date of publication of this document. 48 Please review these documents carefully, as they describe your rights 49 and restrictions with respect to this document. Code Components 50 extracted from this document must include Revised BSD License text as 51 described in Section 4.e of the Trust Legal Provisions and are 52 provided without warranty as described in the Revised BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 58 2. Entropy Label Capability Path Attribute, Version 2 . . . . . 3 59 2.1. Sending the ELCv2 . . . . . . . . . . . . . . . . . . . . 4 60 2.2. Receiving the ELCv2 . . . . . . . . . . . . . . . . . . . 4 61 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 62 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 63 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 64 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 66 6.2. Informative References . . . . . . . . . . . . . . . . . 6 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 69 1. Introduction 71 [RFC6790] defines the Entropy Label Capability attribute (ELC), an 72 optional, transitive BGP path attribute. For correct operation, it 73 is necessary that any intermediate node modifying the next hop of a 74 route must remove the ELC unless the node so doing is able to process 75 entropy labels. Sadly, these requirements cannot be fulfilled with 76 the ELC as specified, because it is an optional, transitive 77 attribute: by definition, a node that does not support the ELC will 78 propagate the attribute. But such a node might be exactly the one 79 that we desire to remove it. 81 Ultimately the IDR working group adopted 82 [I-D.ietf-idr-next-hop-capability] as a proposed solution for this 83 and similar problems. However, prior to that, at least one 84 implementation of this specification was shipped, by Juniper 85 Networks. The shipping implementation uses the code point that was 86 assigned by RFC 6790, and deprecated by RFC 7447. This document 87 explains what was implemented and deployed, dubbed "Entropy Label 88 Capability Attribute version 2" (ELCv2). 90 Although [I-D.ietf-idr-next-hop-capability] uses an optional, non- 91 transitive path attribute, at the time ELCv2 was developed it was 92 decided that an optional, non-transitive solution would over- 93 constrain the deployment options available -- in many cases (for 94 example, route reflectors) it's fine that an intermediate node does 95 propagate an ELC even if it doesn't itself have the ability to 96 process entropy labels. 98 Instead, in this specification, we take the approach of carrying a 99 copy of the next hop information in the ELCv2. This allows the node 100 processing it to know if it can rely on the information carried 101 therein, while still allowing it to be propagated by all intermediate 102 nodes. 104 1.1. Requirements Language 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 108 "OPTIONAL" in this document are to be interpreted as described in BCP 109 14 [RFC2119] [RFC8174] when, and only when, they appear in all 110 capitals, as shown here. 112 2. Entropy Label Capability Path Attribute, Version 2 114 The Entropy Label Capability Path Attribute, Version 2 (ELCv2) is an 115 optional, transitive BGP attribute (for the attribute type code, see 116 Section 3). The ELCv2 has as its data a network layer address, 117 representing the next hop of the route the ELCv2 accompanies. The 118 ELCv2 signals a useful optimization, so it is desirable to make it 119 transitive; the next hop data is to ensure correctness across BGP 120 speakers that do not understand the ELCv2. 122 The Attribute Data field of the ELCv2 path attribute is encoded as 123 shown below: 125 +---------------------------------------------------------+ 126 | Address Family Identifier (2 octets) | 127 +---------------------------------------------------------+ 128 | Subsequent Address Family Identifier (1 octet) | 129 +---------------------------------------------------------+ 130 | Length of Next Hop Network Address (1 octet) | 131 +---------------------------------------------------------+ 132 | Network Address of Next Hop (variable) | 133 +---------------------------------------------------------+ 135 The meanings of the fields are as given in Section 3 of [RFC4760]. 137 When BGP [RFC4271] is used for distributing labeled Network Layer 138 Reachability Information (NLRI) as described in, for example, 139 [RFC8277], the route may include the ELCv2 as part of the Path 140 Attributes. The inclusion of this attribute with a route indicates 141 that the egress of the associated Label Switched Path (LSP) can 142 process entropy labels as an egress Label Switched Router (LSR) for 143 that route -- see Section 4.2 of [RFC6790]. Below, we refer to this 144 for brevity as being "EL-capable." 146 2.1. Sending the ELCv2 148 When a BGP speaker S has a route R it wishes to advertise with next 149 hop N to its peer, it MUST NOT include the ELCv2 attribute except if 150 it knows that the egress of the associated LSP L is EL-capable. 151 Specifically, this will be true if S: 153 * Is itself the egress, and knows itself to be EL-capable, or 155 * Is re-advertising a BGP route it received with a valid ELCv2 156 attribute, and is not changing the value of N, or 158 * Is re-advertising a BGP route it received with a valid ELCv2 159 attribute, and is changing the value of N, and knows (for example, 160 through configuration) that the router represented by N is either 161 the LSP egress and is EL-capable, or that it will process the 162 outer label(s) without processing the entropy label below, as with 163 a transit LSR, or 165 * Is redistributing a route learned from another protocol, and that 166 other protocol conveyed the knowledge that the egress of L was EL- 167 capable (for example, this might be known through the LDP ELC TLV, 168 Section 5.1 of [RFC6790]). 170 In any event, when sending an ELCv2, S MUST set the data portion of 171 the ELCv2 to be equal to N, using the encoding given in Section 2. 173 The ELCv2 MAY be advertised with routes that are labeled, such as 174 those using SAFI 4 [RFC8277]. It MUST NOT be advertised with 175 unlabeled routes. 177 We note that due to the nature of BGP optional transitive path 178 attributes, any BGP speaker that does not implement this 179 specification will propagate the ELCv2, the requirements of this 180 section notwithstanding. However, such a speaker will not update the 181 data part of the ELCv2. 183 2.2. Receiving the ELCv2 185 When a BGP speaker receives an unlabeled route that includes the 186 ELCv2, it MUST discard the ELCv2. 188 When a BGP speaker receives a labeled route that includes the ELCv2, 189 it MUST compare the ELCv2's data portion to the next hop of the 190 route. If the two are equal, the egress of the LSP supports entropy 191 labels, which implies that the receiving BGP speaker, if acting as 192 ingress, MAY insert an entropy label below the advertised label, as 193 per Section 4.2 of [RFC6790]. If the two are not equal, either some 194 intermediate router that does not implement this specification 195 modified the next hop, or some router on the path had an incorrect 196 implementation. In either case, the action taken is the same: the 197 ELCv2 MUST be discarded. The Partial bit MAY be inspected -- if it 198 is equal to zero, then the mismatch must have been caused by an 199 incorrect implementation, and the error MAY be logged. 201 When a BGP speaker receives a route that includes an ELCv2 whose 202 Attribute Length is less than 4, whose Attribute Length is not equal 203 to 4 plus the value encoded in the Length of Next Hop Network Address 204 carried in the Attribute Data, or whose Attribute Data is otherwise 205 inconsistent with the encoding specified in Section 2, it MUST 206 discard the ELCv2. 208 3. IANA Considerations 210 As per [RFC7447], IANA has deprecated BGP attribute 28. That 211 deprecated type code is used by implementations of this 212 specification. IANA is requested to update the references for 213 attribute 28 to include this specification. 215 4. Security Considerations 217 Insertion of an ELCv2 by an attacker could cause forwarding to fail. 218 Deletion of an ELCv2 by an attacker could cause one path in the 219 network to be overutilized and another to be underutilized. However, 220 we note that an attacker able to accomplish either of these (below, 221 an "on-path attacker") could equally insert or remove any other BGP 222 path attribute or message. The former attack described above denies 223 service for a given route, which can be accomplished by an on-path 224 attacker in any number of ways even absent ELCv2. The latter attack 225 defeats an optimization but nothing more; it seems dubious that an 226 attacker would go to the trouble of doing so rather than launching 227 some more damaging attack. In sum, the ELCv2 attribute creates no 228 significant issues beyond those analyzed in [RFC4272]. 230 5. Acknowledgements 232 Thanks to Alia Atlas, Bruno Decraene, Martin Djernaes, John Drake, 233 Adrian Farrell, Keyur Patel, Ravi Singh, and Jim Uttaro for their 234 discussion of this issue. Particular thanks to Kevin Wang for his 235 many valuable contributions. 237 6. References 239 6.1. Normative References 241 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 242 Requirement Levels", BCP 14, RFC 2119, 243 DOI 10.17487/RFC2119, March 1997, 244 . 246 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 247 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 248 DOI 10.17487/RFC4271, January 2006, 249 . 251 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 252 "Multiprotocol Extensions for BGP-4", RFC 4760, 253 DOI 10.17487/RFC4760, January 2007, 254 . 256 [RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and 257 L. Yong, "The Use of Entropy Labels in MPLS Forwarding", 258 RFC 6790, DOI 10.17487/RFC6790, November 2012, 259 . 261 [RFC7447] Scudder, J. and K. Kompella, "Deprecation of BGP Entropy 262 Label Capability Attribute", RFC 7447, 263 DOI 10.17487/RFC7447, February 2015, 264 . 266 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 267 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 268 May 2017, . 270 6.2. Informative References 272 [I-D.ietf-idr-next-hop-capability] 273 Decraene, B., Kompella, K., and W. Henderickx, "BGP Next- 274 Hop dependent capabilities", Work in Progress, Internet- 275 Draft, draft-ietf-idr-next-hop-capability-07, 8 December 276 2021, . 279 [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", 280 RFC 4272, DOI 10.17487/RFC4272, January 2006, 281 . 283 [RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address 284 Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017, 285 . 287 Authors' Addresses 288 John G. Scudder 289 Juniper Networks 290 Email: jgs@juniper.net 292 Kireeti Kompella 293 Juniper Networks 294 Email: kireeti@juniper.net