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Dunbar 4 Intended status: Informational Huawei Technologies 5 Expires: January 2, 2015 M. Shore 6 No Mountain Software 7 D. Lopez 8 Telefonica 9 G. Karagiannis 10 University of Twente 11 July 1, 2014 13 Virtualized Network Function (VNF) Pool Problem Statement 14 draft-zong-vnfpool-problem-statement-06 16 Abstract 18 Network functions are traditionally implemented on specialized 19 hardware rather than on general purpose servers, but there is a clear 20 trend to implement a number of network functions, such as firewall or 21 load balancer, as software on virtualized computing platforms. These 22 virtualized functions are called Virtualized Network Functions 23 (VNFs), which can be used to build network services. The use of VNFs 24 to build network services introduces additional challenges on 25 reliability, such as additional points of failure and the need to 26 coordinate various VNFs. 28 This document introduces a general idea of VNF Pool to support 29 reliable function provision by the VNFs. We then highlight the 30 reliability challenges and issues when using the VNFs to build 31 services. Related IETF works are also briefly described. 33 Status of This Memo 35 This Internet-Draft is submitted in full conformance with the 36 provisions of BCP 78 and BCP 79. 38 Internet-Drafts are working documents of the Internet Engineering 39 Task Force (IETF). Note that other groups may also distribute 40 working documents as Internet-Drafts. The list of current Internet- 41 Drafts is at http://datatracker.ietf.org/drafts/current/. 43 Internet-Drafts are draft documents valid for a maximum of six months 44 and may be updated, replaced, or obsoleted by other documents at any 45 time. It is inappropriate to use Internet-Drafts as reference 46 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on January 2, 2015. 50 Copyright Notice 52 Copyright (c) 2014 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (http://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 68 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 70 3.1. From Specialized Hardware to Virtualized Network Function 4 71 3.2. Concept of VNF Set . . . . . . . . . . . . . . . . . . . 5 72 3.3. Challenges to reliability . . . . . . . . . . . . . . . . 6 73 4. VNF Pool . . . . . . . . . . . . . . . . . . . . . . . . . . 6 74 5. Challenges and Open Issues . . . . . . . . . . . . . . . . . 8 75 5.1. Redundancy model inside VNF . . . . . . . . . . . . . . . 8 76 5.2. State synchronization inside VNF . . . . . . . . . . . . 8 77 5.3. Interaction between VNF and Service Control Entity . . . 8 78 5.4. Reliable transport . . . . . . . . . . . . . . . . . . . 9 79 5.5. Scope Considerations . . . . . . . . . . . . . . . . . . 9 80 6. Related Works . . . . . . . . . . . . . . . . . . . . . . . . 9 81 6.1. Reliable Server Pooling (RSerPool) . . . . . . . . . . . 9 82 6.2. Virtual Router Redundancy Protocol (VRRP) . . . . . . . . 10 83 6.3. Service Function Chaining (SFC) . . . . . . . . . . . . . 10 84 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 85 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 86 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 87 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 88 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 89 10.2. Informative References . . . . . . . . . . . . . . . . . 11 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 92 1. Introduction 94 Network functions such as firewall, load balancer, WAN optimizer are 95 conventionally deployed as specialized hardware servers in both 96 network operators' networks and data center networks, as the building 97 blocks of the network services. 99 A Virtualized Network Function (VNF) provides such network function 100 through its implementation as software instances running on general 101 purpose servers via a virtualization layer (i.e., hypervisor). VNFs 102 potentially offer benefits such as elastic service offering, reduced 103 operational and equipment costs [NFV-WP]. 105 There is a trend to move network functions from specialized hardware 106 servers to general purpose servers based on virtualized computing 107 platforms, in order to build network services by using VNFs. For 108 example, in Service Function Chaining (SFC), a network service can be 109 built using a set of sequentially connected VNF instances deployed at 110 different points in the network [SFC]. 112 Nevertheless, the use of VNFs can pose additional challenges on the 113 reliability of the provided services. For a VNF instance, it 114 typically would not have built-in reliability mechanisms on its host 115 (i.e., a general purpose server). Instead, there are more factors of 116 risk such as software failure at various levels including hypervisors 117 and virtual machines, hardware failure, and instance migration that 118 may make a VNF instance unreliable. 120 In order to achieve higher reliability, a VNF may adopt a pooling 121 mechanism, where a number of VNF instances with the same function can 122 be grouped as a pool to provide the function. We call such a pool a 123 VNF Pool. Conceptually, a Pool Manager is used to manage a VNF Pool, 124 e.g., selects active/standby VNF instances, and potentially interacts 125 with a Service Control Entity. A Service Control Entity is an entity 126 that combines and orchestrates a set of network functions, e.g., 127 VNFs, to build network services. The major benefit of using VNF Pool 128 is that the reliability mechanisms such as redundancy management are 129 achieved by the VNF Pool inside the VNF and thus transparent to the 130 Service Control Entity. A VNF Pool-enabled VNF still acts as a 131 normal VNF when orchestrated by the Service Control Entity. 133 We are specifically concerned with the reliability of an individual 134 VNF based on the VNF Pool managed inside the VNF. For example, how 135 to manage the redundancy model, e.g., select active/standby for a VNF 136 instance in a VNF Pool, considering the policy and the infrastructure 137 conditions? How are the service states of a VNF instance held and 138 accessed for efficient synchronization with backup instances in a VNF 139 Pool? What pool states need to be maintained to support the pooling 140 mechanism itself, and how are such states maintained? We also 141 consider the information exchanged between the VNF and Service 142 Control Entity. For example, how can a VNF Pool be addressed by the 143 Service Control Entity? After a VNF instance failover, how does the 144 Pool Manager notify the Service Control Entity of some characteristic 145 changes of the VNF, e.g., capacity change, but without disclosure of 146 the pooling procedure? 147 Note that we do not address the reliability related control or 148 routing between adjacent VNFs that can form a network service, as 149 such coordination could be done by the Service Control Entity. 151 This document introduces a general idea of VNF Pool to support 152 reliable functions provision by the VNFs. We then highlight the 153 reliability challenges and issues when using the VNFs to build 154 services. Related IETF works are also briefly described. 156 2. Terminology 158 Reliability: capability of a functional entity to consistently 159 provide its function under various dynamic and even unexpected 160 conditions such as fault, overload, etc. 162 Service Control Entity: an entity of the service provider that 163 decides how to combine and orchestrate the network functions to build 164 network services. Examples of Service Control Entity are 165 orchestrator of DC services, SFC control plane, etc. 167 Virtualized Network Function (VNF): a VNF provides the same 168 functional behavior and interfaces as the equivalent network 169 function, but is deployed as software instance(s) building on top of 170 a virtualization layer [NFV-TERM]. 172 VNF Pool: a number of VNF instances providing the same network 173 function. 175 VNF Pool Element: a VNF instance inside a VNF pool. 177 VNF Pool Manager: an entity that manages a VNF pool, and interacts 178 with the Service Control Entity to provide the network function. 180 VNF Set: a general set of VNF instances that can be grouped into 181 multiple VNF Pools, where each pool corresponds to a specific VNF and 182 different pools provide different functions. 184 3. Background 186 3.1. From Specialized Hardware to Virtualized Network Function 188 Network functions are traditionally implemented on specialized 189 hardware. There is a trend to implement a number of network 190 functions as software instances on general purpose servers, via 191 virtualized computing platforms. These virtualized functions are 192 called Virtualized Network Functions (VNFs). For example, in 193 Figure 1, virtual firewall (vFW) can be deployed as software 194 instances on general purpose servers, which could be located in Data 195 Center (DC) networks, network operators' networks, or end user 196 premises. Compared with traditional FW deployed as "standalone box" 197 built by specialized hardware and software, vFW has potential 198 advantages such as agility, scalability [NFV-WP]. 200 FW vFW vFW vFW 201 +-------------+ +-----------+ +-----------+ +-----------+ 202 | Specialized | |FW Software| |FW Software| |FW Software| ... 203 | Hardware |----\ +-----------+ +-----------+ +-----------+ 204 | + |----/ +------------------------------------------+ 205 | Software | | Virtualization Platform | 206 +-------------+ +------------------------------------------+ 207 +-----------------+ +-----------------+ 208 | General Purpose | | General Purpose | 209 | Server | | Server | ... 210 +-----------------+ +-----------------+ 212 Figure 1: Example of vFW. 214 3.2. Concept of VNF Set 216 We call a general set of VNF instances a VNF set. A VNF set can 217 include a single or multiple types of VNF, and each type of VNF may 218 have a number of instances providing the same function. The 219 following examples are all valid VNF sets. 221 1. n vFW instances: {vFW#1,vFW#2,...,vFW#n}. 223 2. m vFW instances and k virtual load balancer (vLB) instances: 224 {vFW#1,...,vFW#m,vLB#1,...,vLB#k}. 226 To be more generic, we denote VNF-A#x the xth instance of a VNF of 227 type A (e.g., vFW), VNF-B#y the yth instance of a VNF of type B 228 (e.g., vLB), and so on. 230 A VNF set can be used as part of a Service Function Chaining (SFC) 231 [SFC], where the instances of various functions are sequentially 232 connected to build a network service. A simple example is shown in 233 Figure 2. 235 Network Service 236 +----------+ +----------+ +----------+ 237 | VNF-A#x | data conn | VNF-B#y | data conn | VNF-C#z | 238 | |-----------| |-----------| | 239 +----------+ +----------+ +----------+ 241 Figure 2: A VNF set used as part of a SFC. 243 Alternatively, a VNF set can be also used merely as a set of VNFs, 244 where the instances provide network functions in a parallel way. An 245 example is shown in Figure 3. 247 +----------+ +----------+ +----------+ 248 | VNF-A#x | | VNF-B#y | | VNF-C#z | 249 +----------+ +----------+ +----------+ 250 \ | / 251 data conn \ |data /data conn 252 \ |conn / 253 \ | / 254 +---------------+ 255 | Client | 256 +---------------+ 258 Figure 3: A VNF set used as multiple VNFs. 260 Some more detailed use cases of VNFs are documented in other drafts 261 [VNFPOOL-UC1] [VNFPOOL-UC2] [VNFPOOL-UC3]. 263 3.3. Challenges to reliability 265 The use of VNFs introduces additional challenges to the reliability 266 of the provided network services. For a VNF instance, it typically 267 would not have built-in reliability mechanisms on its host (i.e., a 268 general purpose server). Instead, there are more factors of risk 269 that may make VNF instance unreliable. 271 1. Instance failure due to hardware failure or status change such 272 as server overload. 274 2. Instance failure due to software failure at various levels 275 including hypervisor, Virtual Machine (VM), VNF. 277 3. Instance migration caused by instance performance downgrade 278 caused by load (e.g., CPU, memory, disk I/O), server consolidation 279 or other service requirement changes. This is distinct from a 280 hard failure, although it may give the appearance of one. 282 4. VNF Pool 284 There are a number of existing technologies for providing reliable 285 functions, such as Reliable Server Pooling (RSerPool) [RFC5351], 286 Virtual Router Redundancy Protocol (VRRP) [RFC5798], amongst many 287 others. Both technologies provide the service with an abstract 288 object (e.g., pool handle in RSerPool, virtual router ID in VRRP) 289 representing a group of identical functional instances. The dynamic 290 mapping of such abstract object to the actual serving instance is 291 managed internally in the group to cover the failover procedure. The 292 advantage is to provide reliable functions in a transparent manner 293 for both end-hosts and service control entities. 295 We adopt the similar idea of VNF Pool to provide reliable network 296 functions, as shown in figure 4. 298 +------------------------+ 299 | Service Control Entity | 300 +------------------------+ 301 ^ ^ 302 | | 303 +-----------+ +------------+ 304 | | 305 v v 306 + - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - - + 307 | VNF-A +--------------+ | | VNF-B +--------------+ | 308 | | Pool Manager | | | | Pool Manager | | 309 | +--------------+ | | +--------------+ | 310 | + - - - - - - - - - - - - - + | | + - - - - - - - - - - - - - + | 311 | |+---------+ +---------+| | | |+---------+ +---------+| | 312 | || VNF-A#1 | ... | VNF-A#n || | | || VNF-B#1 | ... | VNF-B#m || | 313 | |+---------+ +---------+| | | |+---------+ +---------+| | 314 | | VNF-A Pool | | | | VNF-B Pool | | 315 | + - - - - - - - - - - - - - + | | + - - - - - - - - - - - - - + | 316 + - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - - + 318 Figure 4: VNF Pool Architecture. 320 In VNF Pool architecture, each VNF has a VNF Pool containing a number 321 of VNF instances (or VNF Pool Elements) providing the same function. 322 In this sense, a VNF set can be grouped into multiple VNF Pools, 323 where each pool corresponds to a specific VNF, thus different pools 324 provide different functions. Each VNF also has a Pool Manager that 325 manages the VNF instances in the VNF Pool. Pool Manager interacts 326 with the Service Control Entity to provide the network function. 328 The main benefit of using VNF Pool is that the pooling mechanisms 329 such as redundancy management are achieved by the VNF Pool inside the 330 VNF and thus transparent to the Service Control Entity. The Service 331 Control Entity simply interacts with the Pool Manager in each VNF to 332 request and orchestrate the network functions with desired 333 reliability level. In another word, a VNF Pool-enabled VNF still 334 acts as a normal VNF when orchestrated by the Service Control Entity. 336 5. Challenges and Open Issues 338 5.1. Redundancy model inside VNF 340 Before a live VNF instance fails, one or more backup instances in the 341 same VNF Pool need to be selected. How to select such backup 342 instances? Moreover, there are policies influencing the appropriate 343 selection of backup instance. For example, it should be avoided that 344 a live VNF instance and its backup instances are placed in a single 345 physical server, or locations with shared risks in the network. On 346 the other hand, it would be desirable to place the live and backup 347 instances in geographically closed locations. Information from the 348 underlying network may need to be collected via - e.g., the interface 349 with Application Layer Traffic Optimization (ALTO) [ALTO], or 350 Interface to Routing System (I2RS) [I2RS]. Various infrastructure 351 conditions may also need to be considered for appropriate placement 352 of instances. 354 5.2. State synchronization inside VNF 356 Service states related to the specific function performed by a VNF 357 instance, e.g., NAT translation table, TCP connection states, should 358 be synchronized between a live VNF instance and its backup instances 359 for stateful failover. Who is responsible for and how to collect, 360 hold, and access such service states to achieve efficient 361 synchronization? A VNF instance should provide negotiated level of 362 state sharing with the necessary performance to fulfill the service 363 requirements - e.g., state synchronization method, format of state 364 data, location and mechanism to access state data. 366 Other than service states, pool states could be operational 367 information of VNF pool itself, e.g. redundancy settings, backup 368 location/status, etc. What pool states need to be maintained to 369 support the pooling mechanism itself, and how are such states 370 maintained? 372 5.3. Interaction between VNF and Service Control Entity 374 Some information needs to be exchanged between a VNF and the Service 375 Control Entity when the Service Control Entity orchestrates a VNF 376 Pool-enable VNF. For example, how can a VNF Pool be addressed by the 377 Service Control Entity? A Pool Manager can advertise the locator 378 (e.g., IP address) of the active instance - subject to dynamic due to 379 failover. It is also possible to use a virtual address for the whole 380 VNF Pool (similar to RSerPool or VRRP), and map between virtual and 381 actual addresses. Moreover, after a VNF instance failover, how does 382 the Pool Manager notify the Service Control Entity of some 383 characteristic changes of the VNF, e.g., capacity change, but without 384 disclosure of the pooling procedure? 386 5.4. Reliable transport 388 The transport mechanism used to carry the pool control messages, 389 e.g., redundancy management, should provide reliable message 390 delivery. Transport redundancy mechanisms such as Multipath TCP 391 (MPTCP) [MPTCP] and the Stream Control Transmission Protocol (SCTP) 392 [RFC3286] will need to be evaluated for applicability. Latency 393 requirements for pool control message delivery must also be 394 evaluated. 396 5.5. Scope Considerations 398 Ideally, the reliability goal is that the network service provided by 399 the VNFs will continue throughout an interruption within the VNFs , 400 and VNF instances failure or migration will not be visible to the 401 external entities. Our work of VNF Pool initially focuses on several 402 reliability mechanisms that are mainly associated with a redundancy 403 model based on a VNF Pool. Additional mechanisms may include pool 404 state maintenance only for pooling purpose. Service state 405 synchronization is out of scope for this phase. 407 We currently assume that a VNF Pool contains the instances of same 408 functional type, e.g., FW, LB, etc. Different types of VNFs are 409 envisioned to be held in separate VNF Pools. VNF Pool composed of 410 both virtualized and non-virtualized functional instances may be 411 included after further use case and requirements study. 413 We are specifically concerned with the reliability of an individual 414 VNF based on the VNF Pool managed inside the VNF. We do not address 415 the reliability related control or routing between adjacent VNFs that 416 can form a network service, as such coordination could be done by the 417 Service Control Entity. 419 We do not intend to resolve the service availability that usually 420 involves more factors including the interruptions in various OSI 421 layers, and even user perception on service performance. 423 6. Related Works 425 6.1. Reliable Server Pooling (RSerPool) 427 RSerPool supports high availability and scalability of the 428 applications through the use of pools of servers [RFC5351]. The main 429 functions of RSerPool involve server pool management, as well as 430 receiving requests from a client to bind to a desired server. The 431 applicability and gaps of RSerPool to our work of VNF Pool are 432 described in another draft [VNFPOOL-RSP]. 434 6.2. Virtual Router Redundancy Protocol (VRRP) 436 VRRP specifies an election protocol that dynamically assigns 437 responsibility of a virtual router to one of the VRRP routers called 438 master on a LAN [RFC5798]. The election process provides dynamic 439 failover in the forwarding responsibility should the Master become 440 unavailable. The advantage of VRRP is a higher availability default 441 path without requiring configuration of dynamic routing or router 442 discovery protocols on every end-host. 444 6.3. Service Function Chaining (SFC) 446 A service chain defines an ordered set of service functions that must 447 be applied to packets [SFC]. Although the VNFs can be used as part 448 of a SFC, SFC and our work of VNF Pool have different focus. 450 As mentioned in the section of scope consideration, we mostly 451 consider the reliability of an individual VNF based on the VNF Pool 452 inside the VNF. We do not address the reliability related control or 453 routing between adjacent VNFs in the forwarding graph. Moreover, 454 according to VNF Pool architecture and principles, the VNF Pools will 455 be orthogonal to and invisible to the SFC. A VNF Pool-enabled VNF 456 still acts as a normal VNF when orchestrated by the SFC. Just like 457 the communication between any pool users and VNF Pool, the 458 information exchanged between the VNF Pool and the SFC may include 459 some operational information of the VNF Pool. 461 7. Security Considerations 463 Any technology which allows the insertion, deletion, reordering, or 464 manipulation of network functions has the potential to be subverted 465 by an attacker, with serious consequences. Distributed VNFs 466 introduce an additional attack vector, in which bad actors join 467 several VNFs of a service. Replay attacks have the potential to 468 create denials of service, reordering, adding, or removing VNFs. VNF 469 reliability technologies must provide cryptographic protections 470 against spoofing and insertion attacks as well as replay attacks, in 471 the form of client authentication, origin authentication on VNF 472 reliability management (control plane) traffic, and replay 473 protections. There may be circumstances under which an attacker 474 masquerading as a VNF manager can introduce data leakage or similar 475 attacks, and consequently server authentication would be required, as 476 well. 478 Failing over a VNF or otherwise transferring service state raises 479 issues related to the transfer of security state, including VNF 480 element identity and credentials, session-associated cryptographic 481 state, and so on. Where possible, transfer of security state should 482 be avoided as a matter of good practice, and this will require 483 particular attention as solutions are drafted. 485 8. IANA Considerations 487 This document has no actions for IANA. 489 9. Acknowledgements 491 The authors would like to thank Chidung Lac from Orange, Daniel King 492 from Lancaster University, Lingli Deng, Zhen Cao from China Mobile, 493 Richard Yang from Yale University, Hidetoshi Yokota from KDDI, 494 Mukhtiar Shaikh from Brocade, Qiang Zu from Ericsson, Marco Liebsch 495 from NEC, Kapil Sood from Intel, Adrian Farrel, and Susan Hares for 496 their valuable comments. 498 10. References 500 10.1. Normative References 502 TBD. 504 10.2. Informative References 506 [NFV-WP] NFV Whitepaper: "Network Function Virtualization", issue 1, 507 2012, http://portal.etsi.org/NFV/NFV_White_Paper.pdf. 509 [SFC] "Service Function Chaining (SFC)", 510 . 512 [NFV-TERM] ETSI GS NFV 003: "Terminology for Main Conceptional 513 Entities in NFV", Version 0.0.4, 2013. 515 [VNFPOOL-UC1] L. Xia, Q. Wu, D. King, H. Yokota, and N. Khan, 516 "Requirements and Use Cases for Virtual Network Functions", draft- 517 xia-vnfpool-use-cases-00, February 2014. 519 [VNFPOOL-UC2] D. King, M. Liebsch, P. Willis and J. Ryoo, 520 "Virtualization of Mobile Core Network Use Case", draft-king-vnfpool- 521 mobile-use-case-00, February 2014. 523 [VNFPOOL-UC3] S. Hares and K. Subramaniam, "Use Cases for Resource 524 Pools with Virtual Network Functions (VNFs)", draft-hares-vnf-pool- 525 use-case-00, January 2014. 527 [ALTO] "Application-Layer Traffic Optimization (alto)", 528 . 530 [I2RS] "Interface to the Routing System (i2rs)", 531 . 533 [MPTCP] "Multipath TCP (mptcp)", . 536 [RFC3286] L. Ong and J. Yoakum, "An Introduction to the Stream 537 Control Transmission Protocol (SCTP)", RFC3286, May 2002. 539 [NFV-REL] ETSI GS NFV REL 001: "Network Function Virtualization; 540 Resiliency Requirements", Version 0.0.7, 2014. 542 [NFV-SWA] ETSI GS NFV SWA 001: "Network Function Virtualization; SW 543 Architecture; Virtual Network Functions Architecture", Version 0.1.0, 544 2014. 546 [RFC5351] P. Lei, L. Ong, M. Tuexen and T. Dreibholz, "An 547 Overview of Reliable Server Pooling Protocols", RFC5351, September 548 2008. 550 [RFC5798] S. Nadas, "Virtual Router Redundancy Protocol (VRRP) 551 Version 3 for IPv4 and IPv6", RFC5798, March 2010. 553 [VNFPOOL-RSP] T. Dreibholz, M. Tuexen, M. Shore and N. Zong, "The 554 Applicability of Reliable Server Pooling (RSerPool) for Virtual 555 Network Function Resource Pooling (VNFPOOL)", draft-dreibholz- 556 vnfpool-rserpool-applic-00, October 2013. 558 Authors' Addresses 560 Ning Zong 561 Huawei Technologies 563 Email: zongning@huawei.com 565 Linda Dunbar 566 Huawei Technologies 568 Email: linda.dunbar@huawei.com 569 Melinda Shore 570 No Mountain Software 572 Email: melinda.shore@nomountain.net 574 Diego Lopez 575 Telefonica 577 Email: diego@tid.es 579 Georgios Karagiannis 580 University of Twente 582 Email: g.karagiannis@utwente.nl