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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'RFC5575bis' is mentioned on line 414, but not defined ** Obsolete undefined reference: RFC 5575 (Obsoleted by RFC 8955) == Unused Reference: 'I-D.ietf-idr-rfc5575bis' is defined on line 427, but no explicit reference was found in the text == Unused Reference: 'RFC4456' is defined on line 443, but no explicit reference was found in the text == Outdated reference: A later version (-27) exists of draft-ietf-idr-rfc5575bis-19 Summary: 2 errors (**), 0 flaws (~~), 5 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Uttaro 3 Internet-Draft AT&T 4 Updates: 5575bis (if approved) J. Alcaide 5 Intended status: Standards Track C. Filsfils 6 Expires: September 9, 2020 D. Smith 7 Cisco 8 P. Mohapatra 9 Sproute Networks 10 March 8, 2020 12 Revised Validation Procedure for BGP Flow Specifications 13 draft-ietf-idr-bgp-flowspec-oid-11 15 Abstract 17 This document describes a modification to the validation procedure 18 defined in [RFC5575bis] for the dissemination of BGP Flow 19 Specifications. [RFC5575bis] requires that the originator of the 20 Flow Specification matches the originator of the best-match unicast 21 route for the destination prefix embedded in the Flow Specification. 22 This allows only BGP speakers within the data forwarding path (such 23 as autonomous system border routers) to originate BGP Flow 24 Specifications. Though it is possible to disseminate such Flow 25 Specifications directly from border routers, it may be operationally 26 cumbersome in an autonomous system with a large number of border 27 routers having complex BGP policies. The modification proposed 28 herein enables Flow Specifications to be originated from a 29 centralized BGP route controller. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at https://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on September 9, 2020. 48 Copyright Notice 50 Copyright (c) 2020 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents 55 (https://trustee.ietf.org/license-info) in effect on the date of 56 publication of this document. Please review these documents 57 carefully, as they describe your rights and restrictions with respect 58 to this document. Code Components extracted from this document must 59 include Simplified BSD License text as described in Section 4.e of 60 the Trust Legal Provisions and are provided without warranty as 61 described in the Simplified BSD License. 63 Table of Contents 65 1. Requirements Language . . . . . . . . . . . . . . . . . . . . 2 66 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 67 3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3 68 4. Revised Validation Procedure . . . . . . . . . . . . . . . . 5 69 4.1. Revision of Route Feasibility . . . . . . . . . . . . . . 5 70 4.2. Revision of AS_PATH Validation . . . . . . . . . . . . . 6 71 5. Other RFC5575bis Considerations . . . . . . . . . . . . . . . 7 72 6. Topology Considerations . . . . . . . . . . . . . . . . . . . 8 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 74 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 75 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 76 10. Normative References . . . . . . . . . . . . . . . . . . . . 9 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 79 1. Requirements Language 81 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 82 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 83 document are to be interpreted as described in RFC 2119 [RFC2119]. 85 2. Introduction 87 [RFC5575bis] defined a new BGP [RFC4271] capability that can be used 88 to distribute traffic Flow Specifications amongst BGP speakers in 89 support of traffic filtering. The primary intention of [RFC5575bis] 90 is to enable downstream autonomous systems to signal traffic 91 filtering policies to upstream autonomous systems. In this way, 92 traffic is filtered closer to the source and the upstream autonomous 93 system(s) avoid carrying the traffic to the downstream autonomous 94 system only to be discarded. [RFC5575bis] also enables more granular 95 traffic filtering based upon upper layer protocol information (e.g., 96 protocol port numbers) as opposed to coarse IP destination prefix- 97 based filtering. Flow specification NLRIs received from a BGP peer 98 are subject to validity checks before being considered feasible and 99 subsequently installed within the respective Adj-RIB-In. 101 The validation procedure defined within [RFC5575bis] requires that 102 the originator of the Flow Specification NLRI matches the originator 103 of the best-match unicast route for the destination prefix embedded 104 in the Flow Specification. This allows only BGP speakers within the 105 data forwarding path (such as autonomous system border routers) to 106 originate BGP Flow Specification NLRIs. Though it is possible to 107 disseminate such Flow Specification NLRIs directly from border 108 routers, it may be operationally cumbersome in an autonomous system 109 with a large number of border routers having complex BGP policies. 111 This document describes a modification to the [RFC5575bis] validation 112 procedure allowing Flow Specification NLRIs to be originated from a 113 centralized BGP route controller within the local autonomous system 114 that is not in the data forwarding path. While the proposed 115 modification cannot be used for inter-domain coordination of traffic 116 filtering, it greatly simplifies distribution of intra-domain traffic 117 filtering policies within an autonomous system which has a large 118 number of border routers having complex BGP policies. By relaxing 119 the validation procedure for iBGP, the proposed modification allows 120 Flow Specifications to be distributed in a standard and scalable 121 manner throughout an autonomous system. 123 3. Motivation 125 Step (b) of the validation procedure in [RFC5575bis], section 6 is 126 defined with the underlying assumption that the Flow Specification 127 NLRI traverses the same path, in the inter-domain and intra-domain 128 route distribution graph, as that of the longest-match unicast route 129 for the destination prefix embedded in the Flow Specification. 131 In the case of inter-domain traffic filtering, the Flow Specification 132 originator at the egress border routers of an AS (e.g. RTR-D and 133 RTR-E of ASN1 in figure 1) matches the eBGP neighbor that advertised 134 the longest match destination prefix (see RTR-F and RTR-G 135 respectively in figure 1). Similarly, at the ingress border routers 136 of ASN (see RTR-A and RTR-B of ASN1 in figure 1), the Flow 137 Specification originator matches the egress iBGP border routers that 138 had advertised the unicast route for the best-match destination 139 prefix (see RTR-D and RTR-E respectively in figure 1). This is true 140 even when ingress border routers select paths from different egress 141 border routers as best path based upon IGP distance. For example, in 142 figure 1: 144 RTR-A chooses RTR-D's path as best 146 RTR-B chooses RTR-E as the best path 148 / - - - - - - - - - - - - - - 149 | ASN1 | 150 +-------+ +-------+ 151 | | | | | | 152 | RTR-A | | RTR-B | 153 | | | | | | 154 +-------+ +-------+ 155 | \ / | 156 iBGP \ / iBGP 157 | \ / | 158 +-------+ 159 | | | | 160 | RTR-C | 161 | | RC | | 162 +-------+ 163 | / \ | 164 / \ 165 | iBGP / \ iBGP | 166 +-------+ +-------+ 167 | | RTR-D | | RTR-E | | 168 | | | | 169 | | | | | | 170 +-------+ +-------+ 171 | | | | 172 - - -|- - - - - - - - -|- - -/ 173 | eBGP eBGP | 174 - - -|- - - - - - - - -|- - -/ 175 | | | | 176 +-------+ +-------+ 177 | | | | | | 178 | RTR-F | | RTR-G | 179 | | | | | | 180 +-------+ +-------+ 181 | ASN2 | 182 / - - - - - - - - - - - - - - 184 Figure 1 186 It is highly desirable that the mechanisms exist to protect each ASN 187 independently from network security attacks using the BGP Flow 188 Specification NLRI for intra-domain purposes only. Network operators 189 often deploy a dedicated Security Operations Center (SOC) within 190 their ASN to monitor and detect such security attacks. To mitigate 191 attacks within a domain (AS or group of ASes), operators require the 192 ability to originate intra-domain Flow Specification NLRIs from a 193 central BGP route controller that is not within the data forwarding 194 plane. In this way, operators can direct border routers within their 195 ASN with specific attack mitigation actions (drop the traffic, 196 forward to a clean-pipe center, etc.). 198 To originate a Flow Specification NLRI, a central BGP route 199 controller must set itself as the originator in the Flow 200 Specification NLRI. This is necessary given the route controller is 201 originating the Flow Specification rather than reflecting it, and to 202 avoid the complexity of having to determine the egress border router 203 whose path was chosen as the best in each of the ingress border 204 routers. Thus, it is necessary to modify step (b) of the 205 [RFC5575bis] validation procedure such that an iBGP peer that is not 206 within the data forwarding plane may originate Flow Specification 207 NLRIs. 209 4. Revised Validation Procedure 211 4.1. Revision of Route Feasibility 213 Step (b) of the validation procedure specified in [RFC5575bis], 214 section 6 is redefined as follows: 216 a. One of the following conditions MUST hold true: 218 1. The originator of the Flow Specification matches the 219 originator of the best-match unicast route for the 220 destination prefix embedded in the Flow Specification (this 221 is the unicast route with the longest possible prefix length 222 covering the destination prefix embedded in the Flow 223 Specification). 225 2. The AS_PATH attribute of the Flow Specification does not 226 contain AS_SET and/or AS_SEQUENCE segments. 228 1. This condition SHOULD be enabled by default. This 229 default behavior should validate an empty AS_PATH. 231 2. This condition MAY be disabled by configuration on a BGP 232 speaker. 234 3. As an exception to this rule, a given AS_PATH with AS_SET 235 and/or AS_SEQUENCE segments MAY be validated by policy. 237 Explanation: 239 In this context, an empty AS_PATH means that it does not have 240 AS_SET and/or AS_SEQUENCE segments, and local domain means the 241 local AS [RFC4271] or the local confederation of ASes (in the case 242 that the local AS belongs to a confederation of ASes [RFC5065]). 243 Thus, receiving a Flow Specification with an empty AS_PATH 244 indicates that the Flow Specification was originated inside the 245 local domain. 247 With the above modification to the [RFC5575bis] validation 248 procedure, a BGP peer within the local domain that is not within 249 the data forwarding path can originate a Flow Specification. 251 Disabling the new condition above (b.2.2) may be a good practice 252 when the operator knows with certainty that there is not a Flow 253 Specification originated inside the local domain. 255 Also, policy may be useful to validate a specific set of non-empty 256 AS_PATHs (b.2.3). For example, it could validate a Flow 257 Specification whose AS_PATH contains only an AS_SEQUENCE with ASes 258 that are all known to belong to the same administrative domain. 260 4.2. Revision of AS_PATH Validation 262 [RFC5575bis] states: 264 o BGP implementations MUST also enforce that the AS_PATH attribute 265 of a route received via the External Border Gateway Protocol 266 (eBGP) contains the neighboring AS in the left-most position of 267 the AS_PATH attribute. 269 This rule prevents the exchange of BGP Flow Specification NLRIs at 270 Internet exchanges with BGP route servers. Therefore, this document 271 also redefines the [RFC5575bis] AS_PATH validation procedure 272 referenced above as follows: 274 o BGP Flow Specification implementations MUST enforce that the AS in 275 the left-most position of the AS_PATH attribute of a Flow 276 Specification route received via the External Border Gateway 277 Protocol (eBGP) matches the AS in the left-most position of the 278 AS_PATH attribute of the best-match unicast route for the 279 destination prefix embedded in the Flow Specification NLRI. 281 Explanation: 283 For clarity, the AS in the left-most position of the AS_PATH means 284 the AS that was last added to the AS_SEQUENCE. 286 This proposed modification enables the exchange of BGP Flow 287 Specification NLRIs at Internet exchanges with BGP route servers 288 while at the same time, for security reasons, prevents an eBGP 289 peer from advertising an inter-domain Flow Specification for a 290 destination prefix that it does not provide reachability 291 information for. 293 Comparing only the last ASes added is sufficient for eBGP learned 294 Flow Specification NLRIs. Requiring a full AS_PATH match would 295 limit origination of inter-domain Flow Specifications to the 296 origin AS of the best-match unicast route for the destination 297 prefix embedded in the Flow Specification only. As such, a full 298 AS_PATH validity check may prevent transit ASes from originating 299 inter-domain Flow Specifications, which is not desirable. 301 Redefinition of this AS_PATH validation rule for a Flow 302 Specification does not mean that the original rule in [RFC5575bis] 303 cannot be enforced as well. Its enforcement remains optional per 304 [RFC4271] section 6.3. That is, we can enforce the first AS in 305 the AS_PATH to be the same as the neighbor AS for any address- 306 family route (including a Flow Specification). 308 Using the new rule to validate a Flow Specification received from 309 an Internal Border Gateway Protocol (iBGP) peer is out of the 310 scope of this document. Note that in most scenarios such 311 validation would be redundant. 313 Using the new rule to validate a Flow Specification route received 314 from an External Border Gateway Protocol (eBGP) peer belonging to 315 the same local domain (in the case that the local AS belongs to a 316 confederation of ASes) is out of the scope of this document. Note 317 that although it's possible, its utility is dubious. 319 5. Other RFC5575bis Considerations 321 This section clarifies some of the terminology and rules referenced 322 in [RFC5575bis]. Namely: 324 o In the context of this document and [RFC5575bis], AS_PATH 325 attribute is defined as the reconstructed AS path information (by 326 combining AS_PATH and AS4_PATH attributes, if the BGP speaker is a 327 NEW speaker and receives the route from an OLD speaker), according 328 to section 4.2.3 of [RFC6793]. 330 o Support for two-octet AS only implementations is out of the scope 331 of this document (i.e. it's assumed that the BGP speaker supports 332 [RFC6793]). 334 6. Topology Considerations 336 [RFC5575bis] indicates that the originator may refer to the 337 originator path attribute (ORIGINATOR_ID) or (if the attribute is not 338 present) the transport address of the peer from which we received the 339 update. If the latter applies, a network should be designed so it 340 has a congruent topology. 342 With the additional second condition (b.2) in the validation 343 procedure, non-congruent topologies are supported within the local 344 domain if the Flow Specification is originated within the local 345 domain. 347 Explanation: 349 Consider the following scenarios without the second condition 350 (b.2) being added to the validation procedure: 352 1. Consider a topology with two BGP speakers with two peering 353 sessions between them, one for unicast and one for Flow 354 Specification. This is a non-congruent topology. Let's 355 assume that the ORIGINATOR_ID attribute was not received (e.g. 356 a route reflector receiving routes from its clients). In this 357 case, the Flow Specification validation procedure will fail 358 because of the first condition (b.1). 360 2. Consider a topology with a BGP speaker within a confederation 361 of ASes, inside local AS X. ORIGINATOR_ID attribute is not 362 advertised within the local domain. Let's assume the Flow 363 Specification route is received from peer A and the best-match 364 unicast route is received from peer B. Both peers belong in 365 local AS Y. Both AS X and AS Y belong to the same local 366 domain. The Flow Specification validation procedure will also 367 fail because of the first condition (b.1). 369 In the examples above, if Flow Specifications are originated in 370 the same local domain, AS_PATH will not contain AS_SET and/or 371 AS_SEQUENCE segments. When the second condition (b.2) in the 372 validation procedure is used, the validation procedure will pass. 373 Thus, non-congruent topologies are supported if the Flow 374 Specification is originated in the same local domain. 376 Even when the second condition (b.2) is used in the validation 377 procedure, a Flow Specification originated in a different local 378 domain needs a congruent topology. AS_SEQUENCE is not empty and 379 the first condition (b.1) in the validation procedure needs to be 380 evaluated. Because transport addresses for Flow Specification and 381 unicast routes are different, the validation procedure will fail. 383 This is true both across domains and within domains. Consider 384 both cases: 386 * Consider the first example. If the Flow Specification route is 387 originated in another AS, the validation procedure will fail 388 because the topology is non-congruent within the domain. 390 * Consider the second example and modify it so AS X and AS Y 391 belong to different local domains (no confederation of ASes 392 exists). The validation procedure will fail because the 393 topology is non-congruent across domains. 395 7. IANA Considerations 397 This memo includes no request to IANA. 399 8. Security Considerations 401 No new security issues are introduced by relaxing the validation 402 procedure for IBGP learned Flow Specifications. With this proposal, 403 the security characteristics of BGP Flow Specifications remain 404 equivalent to the existing security properties of BGP unicast 405 routing. 407 BGP updates learned from iBGP peers are trusted so the Traffic Flow 408 Specifications contained in BGP updates are trusted. Therefore it is 409 not required to validate that the originator of an intra-domain 410 Traffic Flow Specification matches the originator of the best-match 411 unicast route for the flow destination prefix. This proposal 412 continues to enforce the validation Procedure for eBGP learned 413 Traffic Flow Specifications, as per [RFC5575bis] rules. In this way, 414 the security properties of [RFC5575bis] are maintained such that an 415 EBGP peer cannot cause a denial-of-service attack by advertising an 416 inter-domain Flow Specification for a destination prefix that it does 417 not provide reachability information for. 419 9. Acknowledgements 421 The authors would like to thank Han Nguyen for his direction on this 422 work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen 423 and Shyam Sethuram for their review comments. 425 10. Normative References 427 [I-D.ietf-idr-rfc5575bis] 428 Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M. 429 Bacher, "Dissemination of Flow Specification Rules", 430 draft-ietf-idr-rfc5575bis-19 (work in progress), January 431 2020. 433 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 434 Requirement Levels", BCP 14, RFC 2119, 435 DOI 10.17487/RFC2119, March 1997, 436 . 438 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 439 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 440 DOI 10.17487/RFC4271, January 2006, 441 . 443 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route 444 Reflection: An Alternative to Full Mesh Internal BGP 445 (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, 446 . 448 [RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous 449 System Confederations for BGP", RFC 5065, 450 DOI 10.17487/RFC5065, August 2007, 451 . 453 [RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet 454 Autonomous System (AS) Number Space", RFC 6793, 455 DOI 10.17487/RFC6793, December 2012, 456 . 458 Authors' Addresses 460 James Uttaro 461 AT&T 462 200 S. Laurel Ave 463 Middletown, NJ 07748 464 USA 466 Email: ju1738@att.com 467 Juan Alcaide 468 Cisco 469 7100 Kit Creek Road 470 Research Triangle Park, NC 27709 471 USA 473 Email: jalcaide@cisco.com 475 Clarence Filsfils 476 Cisco 478 Email: cf@cisco.com 480 David Smith 481 Cisco 482 111 Wood Ave South 483 Iselin, NJ 08830 484 USA 486 Email: djsmith@cisco.com 488 Pradosh Mohapatra 489 Sproute Networks 491 Email: mpradosh@yahoo.com