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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-05) exists of draft-homma-sfc-forwarding-methods-analysis-01 == Outdated reference: A later version (-11) exists of draft-ietf-sfc-architecture-07 == Outdated reference: A later version (-06) exists of draft-ietf-sfc-dc-use-cases-02 == Outdated reference: A later version (-28) exists of draft-ietf-sfc-nsh-00 Summary: 0 errors (**), 0 flaws (~~), 5 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Service Function Chaining D. Dolson 3 Internet-Draft Sandvine 4 Intended status: Informational S. Homma 5 Expires: December 21, 2015 NTT 6 D. Lopez 7 Telefonica I+D 8 June 19, 2015 10 Hierarchical Service Chaining 11 draft-dolson-sfc-hierarchical-01 13 Abstract 15 Hierarchical Service Function Chaining (hSFC) is a network 16 architecture allowing an organization to compartmentalize a large- 17 scale network into multiple domains of administration. 19 The goals of hSFC are to make a large-scale network easier to reason 20 about, simpler to control and to support independent functional 21 groups within large operators. 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 http://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 December 21, 2015. 40 Copyright Notice 42 Copyright (c) 2015 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 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 59 2. Hierarchical Service Function Chaining (hSFC) . . . . . . . . 3 60 2.1. Top Level . . . . . . . . . . . . . . . . . . . . . . . . 4 61 2.2. Lower Levels . . . . . . . . . . . . . . . . . . . . . . 5 62 3. Internal Boundary Node (IBN) . . . . . . . . . . . . . . . . 6 63 3.1. IBN Path Configuration . . . . . . . . . . . . . . . . . 7 64 3.1.1. Flow-Stateful IBN . . . . . . . . . . . . . . . . . . 7 65 3.1.2. Encoding Upper-Level Paths in Metadata . . . . . . . 8 66 3.1.3. Using Unique Paths per Upper-Level Path . . . . . . . 9 67 3.2. Gluing Levels Together . . . . . . . . . . . . . . . . . 9 68 4. Sub-domain Classifier . . . . . . . . . . . . . . . . . . . . 10 69 5. Control Plane Elements . . . . . . . . . . . . . . . . . . . 10 70 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 71 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 72 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 73 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 74 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 75 9.2. Informative References . . . . . . . . . . . . . . . . . 12 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 78 1. Introduction 80 Service Function Chaining (SFC) is a technique for prescribing 81 differentiated traffic forwarding policies within the SFC domain. 82 SFC is described in detail in the SFC architecture document 83 [I-D.ietf-sfc-architecture], and is not repeated here. 85 In this document we consider the difficult problem of implementing 86 SFC across a large, geographically dispersed network comprised of 87 millions of hosts and thousands of network forwarding elements, 88 involving multiple operational teams (with varying functional 89 responsibilities). We expect asymmetrical routing is inherent in the 90 network, while recognizing that some Service Functions (SFs) require 91 bidirectional traffic for transport-layer sessions (e.g., NATs, 92 firewalls). We assume that some paths need to be selected on the 93 basis of application-specific data visible to the network, with 94 5-tuple stickiness to specific Service Function instances. 96 Note: in this document, the notion of the "path" of a packet is the 97 series of SF instances traversed by a packet. The means of 98 delivering packets between SFs (the forwarding mechanisms of the 99 underlay network) is not relevant to the current discussion. 101 Difficult problems are often made easier by decomposing them in a 102 hierarchical (nested) manner. So instead of considering an 103 omniscient SFC Control Plane that can manage (create, withdraw, 104 supervise, etc.) complete paths from one end of the network to the 105 other, we decompose the network into smaller sub-domains. Each sub- 106 domain may support a subset of the network applications or a subset 107 of the users. The criteria for determining decomposition into SFC- 108 enabled sub-domains are beyond the scope of this document. 110 Note that decomposing a network into multiple SFC-enabled domains 111 should permit end-to-end visibility of Service Functions and Service 112 Function Paths. Decomposition should also be implemented with 113 special care to ease monitoring and troubleshooting of the network as 114 a whole. 116 An example of simplifying a network by using multiple SF domains is 117 further discussed in [I-D.ietf-sfc-dc-use-cases]. 119 We assume the SF technology uses NSH [I-D.ietf-sfc-nsh] or a similar 120 labeling mechanism. 122 The "domains" discussed in this document are assumed to be under 123 control of a single organization, such that here is a strong trust 124 relationship between the domains. The intention of creating multiple 125 domains is to improve the ability to operate a network. It is 126 outside of the scope of the document to consider domains operated by 127 different organizations. 129 1.1. Requirements Language 131 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 132 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 133 document are to be interpreted as described in RFC 2119 [RFC2119]. 135 2. Hierarchical Service Function Chaining (hSFC) 137 A hierarchy has multiple levels. The top-most level encompasses the 138 entire network domain to be managed, and lower levels encompass 139 portions of the network. 141 2.1. Top Level 143 Considering the example in Figure 1, a top-level network domain 144 includes SFC components distributed over a wide area, including: 146 o classifiers (CFs), 148 o Service Function Forwarders (SFFs) and 150 o Sub-domains. 152 For the sake of clarity, components of the underlay network are not 153 shown; an underlay network is assumed to provide connectivity between 154 SFC components. 156 Top-level service function paths carry packets from classifiers 157 through a series of SFFs and sub-domains, with the operations within 158 sub-domains being opaque to the higher levels. 160 We expect the system to include a top-level control-plane having 161 responsibility for configuring forwarding and classification. The 162 top-level Service Chaining control-plane manages end-to-end service 163 chains and associated service function paths from network edge points 164 to sub-domains and configuring top-level classifiers at a coarse 165 level (e.g., based on source or destination host) to forward traffic 166 along paths that will transit appropriate sub-domains. The figure 167 shows one possible service chain passing from edge, through two sub- 168 domains, to network egress. The top-level control plane does NOT 169 configure classification or forwarding within the sub-domains. 171 At this network-wide level, the number of SFPs required is a linear 172 function of the number of ways in which a packet is required to 173 traverse different sub-domains and egress the network. Note that the 174 various paths which may be taken within a sub-domain are not 175 represented by distinct network-wide SFPs; specific policies at the 176 ingress nodes of each sub-domain bind flows to sub-domain paths. 178 Packets are classified at the edge of the network to select the paths 179 by which sub-domains are to be traversed. At the ingress of each 180 sub-domain, paths are reclassified to select the paths by which SFs 181 in the sub-domain are to be traversed. At the egress of each sub- 182 domain, packets are returned to the top-level paths. Contrast this 183 with an approach requiring the top-level classifier to select paths 184 to specify all of the SFs in each sub-domains. 186 It should be assumed that some service functions in the network 187 require bidirectional symmetry of paths (see more in Section 4). 188 Therefore the classifiers at the top level must be configured with 189 policies ensuring server-to-client packets take the reverse path of 190 client-to-server packet through sub-domains. (Recall the "path" 191 denotes the series of service functions; the precise physical network 192 path within the underlay network is not relevant here.) 194 +------------+ 195 |Sub-domain#1| 196 | in DC1 | 197 +----+-------+ 198 | 199 .---- SFF1 ------. +--+ 200 +--+ / / | \--|CF| 201 --->|CF|--/---->' | \ +--+ 202 +--+ / SC#1 | \ 203 | | | 204 | V .------>|---> 205 | / / | 206 \ | / / 207 +--+ \ | / / +--+ 208 |CF|---\ | / /---|CF| 209 +--+ '---- SFF2 ------' +--+ 210 | 211 +----+-------+ 212 |Sub-domain#2| 213 | in DC2 | 214 +------------+ 216 One path is shown from edge classifier to SFF1 to Sub-domain#1 217 (residing in data-center1) to SFF1 to SFF2 (residing in data-center 218 2) to Sub-domain#2 to SFF2 to network egress. 220 Figure 1: Network-wide view of Top Level of Hierarchy 222 2.2. Lower Levels 224 Each of the sub-domains in Figure 1 is an SFC domain. 226 Unlike the top level, however, data packets entering the sub-domain 227 are already encapsulated within SFC transport. Figure 2 shows a sub- 228 domain interfaced with a higher-level domain by means of an Internal 229 Boundary Node (IBN). It is the purpose of the IBN to remove packets 230 from the SFC encapsulation, apply Classification rules, and direct 231 the packets to the selected local service function paths terminating 232 at an egress IBN. The egress SFC Domain Gateway finally restores 233 packets to the original SFC transport and hands them off to SFFs. 235 Each sub-domain intersects a subset of the total paths that are 236 possible in the higher-level domain. An IBN is concerned with 237 higher-level paths, but only those traversing the sub-domain. A top- 238 level controller may configure the IBN as an SF (the IBN plays the SF 239 role in the top-level domain). 241 We expect each sub-domain to have a control-plane that can operate 242 independently of the top-level control-plane. The sub-domain 243 control-plane configures the classification and forwarding rules in 244 the sub-domain. The classification rules reside in the IBN, where 245 packets are moved from SFC encapsulation of the top-level domain to 246 and from SFC encapsulation of the lower-level domain. 248 +----+ +-----+ +----------------------+ +-----+ 249 | |SC#1| SFF | | IBN 1 | | SFF | 250 ->| |================* *===============> 251 | | +-----+ | # (in DC 1) # | +-----+ 252 | CF | | V # | 253 | | |+---+ +---+| Top domain 254 | | * * * * *||CF | * * * * * *|SFF|| * * * * * 255 | | * |+---+ +-+-+| * 256 +----+ * | | | | | | Sub * 257 * +-o-o--------------o-o-+ domain* 258 * SC#2 | |SC#1 ^ ^ #1 * 259 * +-----+ | | | * 260 * | V | | * 261 * | +---+ +------+ | | * 262 * | |SFF|->|SF#1.1|--+ | * 263 * | +---+ +------+ | * 264 * V | * 265 * +---+ +------+ +---+ +------+ * 266 * |SFF|->|SF#2.1|->|SFF|->|SF#2.2| * 267 * +---+ +------+ +---+ +------+ * 268 * * * * * * * * * * * * * * * * * * * * * * 270 *** Sub-domain boundary; === top-level chain; --- low-level chain. 272 Figure 2: Sub-domain within a higher-level domain 274 If desired, the pattern can be applied recursively. For example, 275 SF#1.1 in Figure 2 could be a sub-domain of the sub-domain. 277 3. Internal Boundary Node (IBN) 279 A network element termed "Internal Boundary Node" (IBN) bridges 280 packets between domains. It looks like an SF to the higher level, 281 and looks like a classifier and end-of-chain to the lower level. 283 To achieve the benefits of hierarchy, the IBN should be applying more 284 granular traffic classification rules at the lower level than the 285 traffic passed to it. This means that the number of SF Paths within 286 the lower level is greater than the number of SF Paths arriving to 287 the IBN. 289 The IBN is also the termination of lower-level SF paths. This is 290 because the packets exiting lower-level SF paths must be returned to 291 the higher-level SF paths and forwarded to the next hop in the 292 higher-level domain. 294 3.1. IBN Path Configuration 296 An operator of a lower-level SF Domain may be aware of which high- 297 level paths transit their domain, or they may wish to accept any 298 paths. 300 When packets enter the sub-domain, the Path Identifier and Path Index 301 are re-marked according to the path selected by the classifier. 303 After exiting a path in the sub-domain, packets can be restored to an 304 upper-level SF path by these methods: 306 1. Stateful per flow, 308 2. Pushing path identifier into metadata, 310 3. Using unique lower-level paths per upper-level path. 312 3.1.1. Flow-Stateful IBN 314 An IBN can be flow-aware, returning packets to the correct higher- 315 level SF path on the basis of 5-tuple of packets exiting the lower- 316 level SF paths. 318 When packets are received by the IBN on a higher-level path, the 319 encapsulated packets are parsed for IP and transport-layer (TCP or 320 UDP) coordinates. State is created, indexed by the 5-tuple of 321 {source-IP, destination-IP, source-port, destination-port and 322 transport protocol}. The state contains critical fields of the 323 encapsulating SFC header (or perhaps the entire header). 325 The simplest approach has the packets return to the same IBN at the 326 end of the chain that classified the packet at the start of the 327 chain. This is because the required 5-tuple state is rapidly 328 changing and most efficiently kept locally. If the packet is 329 returned to a different IBN for egress, 5-tuple state must be 330 synchronized between the IBNs. 332 When a packet returns to the IBN at the end of a chain, the SFC 333 header is removed, the packet is parsed for IP and transport-layer 334 coordinates, and state is retrieved by the 5-tuple of the packet. 335 The state contains the information required to forward the packet 336 within the higher-level service chain. 338 State cannot be created by packets arriving from the lower-level 339 chain; when state cannot be found for such packets, they MUST be 340 dropped. 342 This stateful approach is limited to use with SFs that retain the 343 5-tuple of the packet. This approach cannot be used with SFs that 344 modify the 5-tuple (e.g., as done by a NAT) or otherwise create 345 packets for new 5-tuples other than those received (e.g., as an HTTP 346 cache might do to retrieve content on behalf of the original flow). 347 In both cases, the fundamental problem is the inability to forward 348 packets when state cannot be found for the packet 5-tuples. 350 In the stateful approach, there are issues caused by the state, such 351 as how long the state should be maintained (it MUST time out 352 eventually), as well as whether the state needs to be replicated to 353 other devices to create a highly available network. 355 It is valid to consider the state disposable after failure, since it 356 can be re-created by each new packet arriving from the higher-level 357 domain. For example, if an IBN loses all flow state, the state is 358 re-created by an end-point retransmitting a TCP packet. 360 If an SFC domain handles multiple network regions (e.g., multiple 361 private networks), the 5-tuple may be augmented with a 6th parameter, 362 perhaps using some metadata to identify the network region. 364 In this stateful approach, it is not necessary for the sub-domain's 365 control-plane to modify paths when higher-level paths are changed. 366 The complexity of the higher-level domain does not cause complexity 367 in the lower-level domain. 369 3.1.2. Encoding Upper-Level Paths in Metadata 371 An IBN can push the upper-level service path identifier (SPI) and 372 service index (SI) (or encoding thereof) into a metadata field of the 373 lower-level encapsulation (e.g., placing upper-level path information 374 into a metadata field of NSH). When packets exit the lower-level 375 path, the upper-level SPI and SI can be restored from the metadata 376 retrieved from the packet. 378 This approach requires the SFs in the path to be capable of 379 forwarding the metadata and appropriately attaching metadata to any 380 packets injected for a flow. 382 Using new metadata may inflate packet size when variable-length 383 metadata (type 2 from NSH [I-D.ietf-sfc-nsh]) is used. 385 It is conceivable that the MD-type 1 Mandatory Context Header fields 386 of NSH [I-D.ietf-sfc-nsh] are not all relevant to the lower-level 387 domain. In this case, one of the metadata slots of the Mandatory 388 Context Header could be repurposed within the lower-level domain, and 389 restored when leaving. 391 In this metadata approach, it is not necessary for the sub-domain's 392 controller to modify paths when higher-level paths are changed. The 393 complexity of the higher-level domain does not cause complexity in 394 the lower-level domain. 396 3.1.3. Using Unique Paths per Upper-Level Path 398 In this approach, paths within the sub-domain are constrained so that 399 a path identifier (of the sub-domain) unambiguously indicates the 400 egress path (of the upper domain). This allows the original path 401 information to be restored at sub-domain egress from a look-up table 402 using the sub-domain path identifier. 404 Whenever the upper-level domain provisions a path via the lower-level 405 domain, the lower-level domain controller must provision 406 corresponding paths to traverse the lower-level domain. 408 A down-side of this approach is that the number of paths in the 409 lower-level domain is multiplied by the number of paths in the 410 higher-level domain that traverse the lower-level domain. I.e., a 411 sub-path must be created for each combination of upper Path 412 identifier and lower path. 414 3.2. Gluing Levels Together 416 The path identifier or metadata on a packet received by the IBN may 417 be used as input to reclassification and path selection within the 418 lower-level domain. 420 In some cases the meanings of the various path IDs and metadata must 421 be coordinated between domains. 423 One approach is to use well-known identifier values in metadata, 424 communicated by some organizational registry. 426 Another approach is to use well-known labels for path identifiers or 427 metadata, as an indirection to the actual identifiers. The actual 428 identifiers can be assigned by control-plane systems. For example, a 429 sub-domain classifier could have a policy, "if pathID=classA then 430 chain packet to path 1234"; the higher-level controller would be 431 expected to configure the concrete higher-level pathID for classA. 433 4. Sub-domain Classifier 435 Within the sub-domain (referring to Figure 2), after the IBN removes 436 higher-level encapsulation from incoming packets, it sends the 437 packets to the classifier, which selects the encapsulation for the 438 packet within the sub-domain. 440 One of the goals of the hierarchical approach is to make it easy to 441 have transport-flow-aware service chaining with bidirectional paths. 442 For example, it is desired that for each TCP flow, the client-to- 443 server packets traverse the same SFs as the server-to-client packets, 444 but in the opposite sequence. We call this bidirectional symmetry. 445 If bidirectional symmetry is required, it is the responsibility of 446 the control-plane to be aware of symmetric paths and configure the 447 classifier to chain the traffic in a symmetric manner. 449 Another goal of the hierarchical approach is to simplify the 450 mechanisms of scaling in and scaling out service functions. All of 451 the complexities of load-balancing among multiple SFs can be handled 452 within a sub-domain, under control of the classifier, allowing the 453 higher-level domain to be oblivious to the existence of multiple SF 454 instances. 456 Considering the requirements of bidirectional symmetry and load- 457 balancing, it is useful to have all packets entering a sub-domain to 458 be received by the same classifier or a coordinated cluster of 459 classifiers. There are both stateful and stateless approaches to 460 ensuring bidirectional symmetry. 462 5. Control Plane Elements 464 Controllers have been mentioned in this document without much 465 explanation. Although control protocols have not yet been 466 standardized, from the point of view of hierarchical service chaining 467 we have these expectations: 469 o Each control-plane instance manages a single level of hierarchy of 470 a single domain. 472 o Each control-plane is agnostic about other levels of hierarchy. 473 This aspect allows humans to reason about the system within a 474 single domain and allows control-plane algorithms to use only 475 domain-local inputs. Top-level control does not need visibility 476 to sub-domain policies, nor does sub-domain control need 477 visibility to higher-level policies. 479 o Sub-domain control-planes are agnostic about control-planes of 480 other sub-domains. This allows both humans and machines to 481 manipulate sub-domain policy without considering policies of other 482 domains. 484 Recall that the IBN acts as an SF in the higher-level domain 485 (receiving SF instructions from the higher-level control-plane) and 486 as a classifier in the lower-level domain (receiving classification 487 rules from the sub-domain control-plane). In this view, it is the 488 IBN that glues the layers together. 490 The above expectations are not intended to prohibit network-wide 491 control. A control hierarchy can be envisaged to distribute 492 information and instructions to multiple domains and sub-domains. 493 Control hierarchy is outside the scope of this document. 495 6. Acknowledgements 497 The concept of Hierarchical Service Path Domains was introduced in 498 draft-homma-sfc-forwarding-methods-analysis-01 499 [I-D.homma-sfc-forwarding-methods-analysis] as a means to improve 500 scalability of service chaining in large networks. 502 The authors would like to thank the following individuals for taking 503 the time to read and provide valuable feedback: 505 Ron Parker 507 Mohamed Boucadair 509 Christian Jacquenet 511 7. IANA Considerations 513 This memo includes no request to IANA. 515 8. Security Considerations 517 Hierarchical service chaining makes use of service chaining 518 architecture, and hence inherits the security considerations 519 described in the architecture document. 521 Furthermore, hierarchical service chaining inherits security 522 considerations of the data-plane protocols (e.g., NSH) and control- 523 plane protocols used to realize the solution. 525 The systems described in this document bear responsibility for 526 forwarding internet traffic. In some cases the systems are 527 responsible for maintaining separation of traffic in private 528 networks. 530 This document describes systems within different domains of 531 administration that must have consistent configurations in order to 532 properly forward traffic and to maintain private network separation. 533 Any protocol designed to distribute the configurations must be secure 534 from tampering. 536 All of the systems and protocols must be secure from modification by 537 untrusted agents. 539 9. References 541 9.1. Normative References 543 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 544 Requirement Levels", BCP 14, RFC 2119, March 1997. 546 9.2. Informative References 548 [I-D.homma-sfc-forwarding-methods-analysis] 549 Homma, S., Kengo, K., Lopez, D., Stiemerling, M., and D. 550 Dolson, "Analysis on Forwarding Methods for Service 551 Chaining", draft-homma-sfc-forwarding-methods-analysis-01 552 (work in progress), January 2015. 554 [I-D.ietf-sfc-architecture] 555 Halpern, J. and C. Pignataro, "Service Function Chaining 556 (SFC) Architecture", draft-ietf-sfc-architecture-07 (work 557 in progress), March 2015. 559 [I-D.ietf-sfc-dc-use-cases] 560 Surendra, S., Tufail, M., Majee, S., Captari, C., and S. 561 Homma, "Service Function Chaining Use Cases In Data 562 Centers", draft-ietf-sfc-dc-use-cases-02 (work in 563 progress), January 2015. 565 [I-D.ietf-sfc-nsh] 566 Quinn, P. and U. Elzur, "Network Service Header", draft- 567 ietf-sfc-nsh-00 (work in progress), March 2015. 569 Authors' Addresses 571 David Dolson 572 Sandvine 573 408 Albert Street 574 Waterloo, ON N2L 3V3 575 Canada 577 Phone: +1 519 880 2400 578 Email: ddolson@sandvine.com 580 Shunsuke Homma 581 NTT, Corp. 582 3-9-11, Midori-cho 583 Musashino-shi, Tokyo 180-8585 584 Japan 586 Email: homma.shunsuke@lab.ntt.co.jp 588 Diego R. Lopez 589 Telefonica I+D 590 Don Ramon de la Cruz, 82 591 Madrid 28006 592 Spain 594 Phone: +34 913 129 041 595 Email: diego.r.lopez@telefonica.com