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2 Network Working Group P. Quinn, Ed.
3 Internet-Draft Cisco Systems, Inc.
4 Intended status: Informational T. Nadeau, Ed.
5 Expires: October 19, 2014 Brocade
6 April 17, 2014
8 Service Function Chaining Problem Statement
9 draft-ietf-sfc-problem-statement-05.txt
11 Abstract
13 This document provides an overview of the issues associated with the
14 deployment of service functions (such as firewalls, load balancers)
15 in large-scale environments. The term service function chaining is
16 used to describe the definition and instantiation of an ordered set
17 of instances of such service functions, and the subsequent "steering"
18 of traffic flows through those service functions.
20 The set of enabled service function chains reflect operator service
21 offerings and is designed in conjunction with application delivery
22 and service and network policy.
24 Status of this Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF). Note that other groups may also distribute
31 working documents as Internet-Drafts. The list of current Internet-
32 Drafts is at http://datatracker.ietf.org/drafts/current/.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 This Internet-Draft will expire on October 19, 2014.
41 Copyright Notice
43 Copyright (c) 2014 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (http://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
56 Table of Contents
58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
59 1.1. Definition of Terms . . . . . . . . . . . . . . . . . . . 3
60 2. Problem Space . . . . . . . . . . . . . . . . . . . . . . . . 5
61 2.1. Topological Dependencies . . . . . . . . . . . . . . . . . 5
62 2.2. Configuration complexity . . . . . . . . . . . . . . . . . 5
63 2.3. Constrained High Availability . . . . . . . . . . . . . . 6
64 2.4. Consistent Ordering of Service Functions . . . . . . . . . 6
65 2.5. Application of Service Policy . . . . . . . . . . . . . . 6
66 2.6. Transport Dependence . . . . . . . . . . . . . . . . . . . 7
67 2.7. Elastic Service Delivery . . . . . . . . . . . . . . . . . 7
68 2.8. Traffic Selection Criteria . . . . . . . . . . . . . . . . 7
69 2.9. Limited End-to-End Service Visibility . . . . . . . . . . 7
70 2.10. Per-Service (re)Classification . . . . . . . . . . . . . . 7
71 2.11. Symmetric Traffic Flows . . . . . . . . . . . . . . . . . 8
72 2.12. Multi-vendor Service Functions . . . . . . . . . . . . . . 8
73 3. Service Function Chaining . . . . . . . . . . . . . . . . . . 9
74 3.1. Service Overlay . . . . . . . . . . . . . . . . . . . . . 9
75 3.2. Control Plane . . . . . . . . . . . . . . . . . . . . . . 9
76 3.3. Service Classification . . . . . . . . . . . . . . . . . . 9
77 3.4. Dataplane Metadata . . . . . . . . . . . . . . . . . . . . 10
78 4. Related IETF Work . . . . . . . . . . . . . . . . . . . . . . 11
79 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
80 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
81 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
82 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
83 9. Informative References . . . . . . . . . . . . . . . . . . . . 17
84 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
86 1. Introduction
88 The delivery of end-to-end services often require various service
89 functions including traditional network service functions (for
90 example firewalls and server load balancers), as well as application-
91 specific features. Service functions may be delivered within the
92 context of an isolated user group, or shared amongst many users/user
93 groups.
95 Current service function deployment models are relatively static in
96 that they are tightly coupled to network topology and physical
97 resources. The result of that static nature of existing deployments
98 greatly reduces, and in many cases, limits the ability of an operator
99 to introduce new services and/or service functions. Furthermore
100 there is a cascading effect: service changes affect other services.
102 This document outlines the problems encountered with existing service
103 deployment models for Service Function Chaining (SFC) (often referred
104 to simply as service chaining; in this document the terms will be
105 used interchangeably), as well as the problems of service chain
106 creation, deletion, modification/update, policy integration with
107 service chains, and policy enforcement within the network
108 infrastructure.
110 1.1. Definition of Terms
112 Classification: Locally instantiated policy that results in matching
113 of traffic flows for identification of appropriate outbound
114 forwarding actions.
116 Network Overlay: A logical network built, via virtual links or
117 packet encapsulation, over an existing network (the underlay).
119 Network Service: An externally visible service offered by a network
120 operator; a service may consist of a single service function or a
121 composite built from several service functions executed in one or
122 more pre-determined sequences and delivered by one or more service
123 nodes.
125 Service Function: A function that is responsible for specific
126 treatment of received packets. A Service Function can act at the
127 network layer or other OSI layers. A Service Function can be a
128 virtual instance or be embedded in a physical network element.
129 One of multiple Service Functions can be embedded in the same
130 network element. Multiple instances of the Service Function can
131 be enabled in the same administrative domain.
133 A non-exhaustive list of Service Functions includes: firewalls,
134 WAN and application acceleration, Deep Packet Inspection (DPI),
135 server load balancers, NAT44 [RFC3022], NAT64 [RFC6146], HOST_ID
136 injection [RFC6967], HTTP Header Enrichment functions, TCP
137 optimizer, etc.
139 The generic term "L4-L7 services" is often used to describe many
140 service functions.
142 Service Function Chain (SFC): A service Function chain defines an
143 ordered set of service functions that must be applied to packets
144 and/or layer-2 frames selected as a result of classification. The
145 implied order may not be a linear progression as nodes may copy to
146 more than one branch. The term service chain is often used as
147 shorthand for service function chain.
149 Service Function Path (SFP): The instantiation of a service function
150 chain in the network. Packets follow a service function path from
151 a classifier through the required instances of service functions
152 in the network.
154 Service Node (SN): Physical or virtual element that hosts one or
155 more service functions.
157 Service Overlay: An overlay network created for the purpose of
158 forwarding data along a service function path.
160 Service Topology: The service overlay connectivity forms a service
161 topology.
163 2. Problem Space
165 The following points describe aspects of existing service deployments
166 that are problematic, and that the Service Function Chaining (SFC)
167 working group aims to address.
169 2.1. Topological Dependencies
171 Network service deployments are often coupled to network topology,
172 whether it be real or virtualized, or a hybrid of the two. Such
173 dependency imposes constraints on the service delivery, potentially
174 inhibiting the network operator from optimally utilizing service
175 resources, and reduces the flexibility. This limits scale, capacity,
176 and redundancy across network resources.
178 These topologies serve only to "insert" the service function (i.e.,
179 ensure that traffic traverses a service function); they are not
180 required from a native packet delivery perspective. For example,
181 firewalls often require an "in" and "out" layer-2 segment and adding
182 a new firewall requires changing the topology (i.e., adding new
183 layer-2 segments).
185 As more service functions are required - often with strict ordering -
186 topology changes are needed before and after each service function
187 resulting in complex network changes and device configuration. In
188 such topologies, all traffic, whether a service function needs to be
189 applied or not, often passes through the same strict order.
191 The topological coupling limits placement and selection of service
192 functions: service functions are "fixed" in place by topology and
193 therefore placement and service function selection taking into
194 account network topology information is not viable. Furthermore,
195 altering the services traversed, or their order, based on flow
196 direction is not possible.
198 A common example is web servers using a server load balancer as the
199 default gateway. When the web service responds to non-load balanced
200 traffic (e.g., administrative or backup operations) all traffic from
201 the server must traverse the load balancer forcing network
202 administrators to create complex routing schemes or create additional
203 interfaces to provide an alternate topology.
205 2.2. Configuration complexity
207 A direct consequence of topological dependencies is the complexity of
208 the entire configuration, specifically in deploying service function
209 chains. Simple actions such as changing the order of the service
210 functions in a service function chain require changes to the
211 topology. Changes to the topology are avoided by the network
212 operator once installed, configured and deployed in production
213 environments fearing misconfiguration and downtime. All of this
214 leads to very static service delivery deployments. Furthermore, the
215 speed at which these topological changes can be made is not rapid or
216 dynamic enough as it often requires manual intervention, or use of
217 slow provisioning systems.
219 2.3. Constrained High Availability
221 An effect of topological dependency is constrained service function
222 high availability. Worse, when modified, inadvertent non-high
223 availability or downtime can result.
225 Since traffic reaches many service functions based on network
226 topology, alternate, or redundant service functions must be placed in
227 the same topology as the primary service.
229 2.4. Consistent Ordering of Service Functions
231 Service functions are typically independent; service function_1
232 (SF1)...service function_n (SFn) are unrelated and there is no notion
233 at the service layer that SF1 occurs before SF2. However, to an
234 administrator many service functions have a strict ordering that must
235 be in place, yet the administrator has no consistent way to impose
236 and verify the ordering of the service functions that are used to
237 deliver a given service.
239 Service function chains today are most typically built through manual
240 configuration processes. These are slow and error prone. With the
241 advent of newer service deployment models the control and policy
242 planes provide not only connectivity state, but will also be
243 increasingly utilized for the creation of network services. Such
244 control/management planes could be centralized, or be distributed.
246 2.5. Application of Service Policy
248 Service functions rely on topology information such as VLANs or
249 packet (re) classification to determine service policy selection,
250 i.e. the service function specific action taken. Topology
251 information is increasingly less viable due to scaling, tenancy and
252 complexity reasons. The topological information is often stale,
253 providing the operator with inaccurate placement that can result in
254 suboptimal resource utilization. Furthermore topology-centric
255 information often does not convey adequate information to the service
256 functions, forcing functions to individually perform more granular
257 classification.
259 2.6. Transport Dependence
261 Service functions can and will be deployed in networks with a range
262 of transports, including under and overlays. The coupling of service
263 functions to topology requires service functions to support many
264 transport encapsulations or for a transport gateway function to be
265 present.
267 2.7. Elastic Service Delivery
269 Given that the current state of the art for adding/removing service
270 functions largely centers around VLANs and routing changes, rapid
271 changes to the service deployment can be hard to realize due to the
272 risk and complexity of such changes.
274 2.8. Traffic Selection Criteria
276 Traffic selection is coarse, that is, all traffic on a particular
277 segment traverse service functions whether the traffic requires
278 service enforcement or not. This lack of traffic selection is
279 largely due to the topological nature of service deployment since the
280 forwarding topology dictates how (and what) data traverses service
281 function(s). In some deployments, more granular traffic selection is
282 achieved using policy routing or access control filtering. This
283 results in operationally complex configurations and is still
284 relatively inflexible.
286 2.9. Limited End-to-End Service Visibility
288 Troubleshooting service related issues is a complex process that
289 involve both network-specific and service-specific expertise. This
290 is especially the case when service function chains span multiple
291 DCs, or across administrative boundaries. Furthermore, the physical
292 and virtual environments (network and service), can be highly
293 divergent in terms of topology and that topological variance adds to
294 these challenges.
296 2.10. Per-Service (re)Classification
298 Classification occurs at each service function independent from
299 previously applied service functions. More importantly, the
300 classification functionality often differs per service function and
301 service functions may not leverage the results from other service
302 functions.
304 2.11. Symmetric Traffic Flows
306 Service function chains may be unidirectional or bidirectional
307 depending on the state requirements of the service functions. In a
308 unidirectional chain traffic is passed through a set of service
309 functions in one forwarding direction only. Bidirectional chains
310 require traffic to be passed through a set of service functions in
311 both forwarding directions. Many common service functions such as
312 DPI and firewall often require bidirectional chaining in order to
313 ensure flow state is consistent.
315 Existing service deployment models provide a static approach to
316 realizing forward and reverse service function chain association most
317 often requiring complex configuration of each network device
318 throughout the SFC.
320 2.12. Multi-vendor Service Functions
322 Deploying service functions from multiple vendors often require per-
323 vendor expertise: insertion models differ, there are limited common
324 attributes and inter- vendor service functions do not share
325 information.
327 3. Service Function Chaining
329 Service Function Chaining aims to address the aforementioned problems
330 associated with service deployment. Concretely, the SFC working
331 group will investigate solutions that address the following elements:
333 3.1. Service Overlay
335 Service function chaining utilizes a service specific overlay that
336 creates the service topology. The service overlay provides service
337 function connectivity and is built "on top" of the existing network
338 topology and allows operators to use whatever overlay or underlay
339 they prefer to create a path between service functions, and to locate
340 service functions in the network as needed.
342 Within the service topology, service functions can be viewed as
343 resources for consumption and an arbitrary topology constructed to
344 connect those resources in a required order. Adding new service
345 functions to the topology is easily accomplished, and no underlying
346 network changes are required.
348 Lastly, the service overlay can provide service specific information
349 needed for troubleshooting service-related issues.
351 3.2. Control Plane
353 Service aware control plane(s) provide information about the
354 available service functions on a network. The information provided
355 by the control plane includes service network location (for topology
356 creation), service type (e.g. firewall, load balancer, etc.) and,
357 optionally, administrative information about the service functions
358 such as load, capacity and operating status. The service aware
359 control plane allows for the formulation of service function chains
360 and exchanges requisite information needed to instantiate the service
361 function chains in the network.
363 Furthermore, the service aware control plane may interact with the
364 topology aware control plane (if separate) to ensure optimal
365 selection (and possibly placement) of service functions within a
366 service function path.
368 3.3. Service Classification
370 Classification is used to select which traffic enters a service
371 overlay. The granularity of the classification varies based on
372 device capabilities, customer requirements, and service offered.
373 Initial classification determines the service function chain required
374 to process the traffic. Subsequent classification can be used within
375 a given service function chain to alter the sequence of service
376 functions applied. Symmetric classification ensures that forward and
377 reverse chains are in place. Similarly, asymmetric -- relative to
378 required service function -- chains can be achieved via service
379 classification.
381 3.4. Dataplane Metadata
383 Data plane metadata provides the ability to exchange information
384 between logical classification points and service functions (and vice
385 versa) and between service functions. As such metadata is not used
386 as forwarding information to deliver packets along the service
387 overlay.
389 Metadata can include the result of antecedent classification and/or
390 information from external sources. Service functions utilize
391 metadata, as required, for localized policy decisions.
393 In addition to sharing of information, the use of metadata addresses
394 several of the issues raised in section 2, most notably the de-
395 coupling of policy from the topology, and the need for per-service
396 classification (and re-classification).
398 A common approach to service metadata creates a common foundation for
399 interoperability between service functions, regardless of vendor.
401 4. Related IETF Work
403 The following subsections discuss related IETF work and are provided
404 for reference. This section is not exhaustive, rather it provides an
405 overview of the various initiatives and how they relate to network
406 service chaining.
408 1. [L3VPN]: The L3VPN working group is responsible for defining,
409 specifying and extending BGP/MPLS IP VPNs solutions. Although
410 BGP/MPLS IP VPNs can be used as transport for service chaining
411 deployments, the SFC WG focuses on the service specific
412 protocols, not the general case of VPNs. Furthermore, BGP/MPLS
413 IP VPNs do not address the requirements for service chaining.
415 2. [LISP]: LISP provides locator and ID separation. LISP can be
416 used as an L3 overlay to transport service chaining data but does
417 not address the specific service chaining problems highlighted in
418 this document.
420 3. [NVO3]: The NVO3 working group is chartered with creation of
421 problem statement and requirements documents for multi-tenant
422 network overlays. NVO3 WG does not address service chaining
423 protocols.
425 4. [ALTO]: The Application Layer Traffic Optimization Working Group
426 is chartered to provide topological information at a higher
427 abstraction layer, which can be based upon network policy, and
428 with application-relevant service functions located in it. The
429 mechanism for ALTO obtaining the topology can vary and policy can
430 apply to what is provided or abstracted. This work could be
431 leveraged and extended to address the need for services
432 discovery.
434 5. [I2RS]: The Interface to the Routing System Working Group is
435 chartered to investigate the rapid programming of a device's
436 routing system, as well as the service of a generalized, multi-
437 layered network topology. This work could be leveraged and
438 extended to address some of the needs for service chaining in the
439 topology and device programming areas.
441 6. [ForCES]: The ForCES working group has created a framework,
442 requirements, a solution protocol, a logical function block
443 library, and other associated documents in support of Forwarding
444 and Control Element Separation. The work done by ForCES may
445 provide a basis for both the separation of SFC elements, as well
446 as provide protocol and design guidance for those elements.
448 5. Summary
450 This document highlights problems associated with network service
451 deployment today and identifies several key areas that will be
452 addressed by the SFC working group. Furthermore, this document
453 identifies four components that are the basis for service function
454 chaining. These components will form the areas of focus for the
455 working group.
457 6. Security Considerations
459 Security considerations are not addressed in this problem statement
460 only document. Given the scope of service chaining, and the
461 implications on data and control planes, security considerations are
462 clearly important and will be addressed in the specific protocol and
463 deployment documents created by the SFC WG.
465 7. Contributors
467 The following people are active contributors to this document and
468 have provided review, content and concepts (listed alphabetically by
469 surname):
471 Puneet Agarwal
472 Broadcom
473 Email: pagarwal@broadcom.com
475 Mohamed Boucadair
476 France Telecom
477 Email: mohamed.boucadair@orange.com
479 Abhishek Chauhan
480 Citrix
481 Email: Abhishek.Chauhan@citrix.com
483 Uri Elzur
484 Intel
485 Email: uri.elzur@intel.com
487 Kevin Glavin
488 Riverbed
489 Email: Kevin.Glavin@riverbed.com
491 Ken Gray
492 Cisco Systems
493 Email: kegray@cisco.com
495 Jim Guichard
496 Cisco Systems
497 Email:jguichar@cisco.com
499 Christian Jacquenet
500 France Telecom
501 Email: christian.jacquenet@orange.com
503 Surendra Kumar
504 Cisco Systems
505 Email: smkumar@cisco.com
507 Nic Leymann
508 Deutsche Telekom
509 Email: n.leymann@telekom.de
511 Darrel Lewis
512 Cisco Systems
513 Email: darlewis@cisco.com
515 Rajeev Manur
516 Broadcom
517 Email:rmanur@broadcom.com
519 Brad McConnell
520 Rackspace
521 Email: bmcconne@rackspace.com
523 Carlos Pignataro
524 Cisco Systems
525 Email: cpignata@cisco.com
527 Michael Smith
528 Cisco Systems
529 Email: michsmit@cisco.com
531 Navindra Yadav
532 Cisco Systems
533 Email: nyadav@cisco.com
535 8. Acknowledgments
537 The authors would like to thank David Ward, Rex Fernando, David
538 Mcdysan, Jamal Hadi Salim, Charles Perkins, Andre Beliveau, Joel
539 Halpern and Jim French for their reviews and comments.
541 9. Informative References
543 [ALTO] "Application-Layer Traffic Optimization (alto)",
544 .
546 [ForCES] "Forwarding and Control Element Separation (forces)",
547 .
549 [I2RS] "Interface to the Routing System (i2rs)",
550 .
552 [L3VPN] "Layer 3 Virtual Private Networks (l3vpn)",
553 .
555 [LISP] "Locator/ID Separation Protocol (lisp)",
556 .
558 [NVO3] "Network Virtualization Overlays (nvo3)",
559 .
561 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
562 Address Translator (Traditional NAT)", RFC 3022,
563 January 2001.
565 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
566 NAT64: Network Address and Protocol Translation from IPv6
567 Clients to IPv4 Servers", RFC 6146, April 2011.
569 [RFC6967] Boucadair, M., Touch, J., Levis, P., and R. Penno,
570 "Analysis of Potential Solutions for Revealing a Host
571 Identifier (HOST_ID) in Shared Address Deployments",
572 RFC 6967, June 2013.
574 Authors' Addresses
576 Paul Quinn (editor)
577 Cisco Systems, Inc.
579 Email: paulq@cisco.com
581 Thomas Nadeau (editor)
582 Brocade
584 Email: tnadeau@lucidvision.com