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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group S. Kiesel, Ed. 3 Internet-Draft University of Stuttgart 4 Intended status: Informational S. Previdi 5 Expires: November 30, 2012 Cisco Systems, Inc. 6 M. Stiemerling 7 NEC Europe Ltd. 8 R. Woundy 9 Comcast Corporation 10 Y R. Yang 11 Yale University 12 May 29, 2012 14 Application-Layer Traffic Optimization (ALTO) Requirements 15 draft-ietf-alto-reqs-15.txt 17 Abstract 19 Many Internet applications are used to access resources, such as 20 pieces of information or server processes that are available in 21 several equivalent replicas on different hosts. This includes, but 22 is not limited to, peer-to-peer file sharing applications. The goal 23 of Application-Layer Traffic Optimization (ALTO) is to provide 24 guidance to applications that have to select one or several hosts 25 from a set of candidates capable of providing a desired resource. 26 This guidance shall be based on parameters that affect performance 27 and efficiency of the data transmission between the hosts, e.g., the 28 topological distance. The ultimate goal is to improve performance or 29 Quality of Experience in the application while reducing the 30 utilization of the underlying network infrastructure. 32 This document enumerates requirements for specifying, assessing, or 33 comparing protocols and implementations. 35 Status of this Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at http://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on November 30, 2012. 51 Copyright Notice 53 Copyright (c) 2012 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents 58 (http://trustee.ietf.org/license-info) in effect on the date of 59 publication of this document. Please review these documents 60 carefully, as they describe your rights and restrictions with respect 61 to this document. Code Components extracted from this document must 62 include Simplified BSD License text as described in Section 4.e of 63 the Trust Legal Provisions and are provided without warranty as 64 described in the Simplified BSD License. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 2. Terminology and Architectural Framework . . . . . . . . . . . 5 70 2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5 71 2.2. ALTO Terminology . . . . . . . . . . . . . . . . . . . . . 5 72 2.3. Architectural Framework for ALTO . . . . . . . . . . . . . 6 73 3. ALTO Requirements . . . . . . . . . . . . . . . . . . . . . . 8 74 3.1. ALTO Client Protocol . . . . . . . . . . . . . . . . . . . 8 75 3.1.1. General Requirements . . . . . . . . . . . . . . . . . 8 76 3.1.2. Host Group Descriptor Support . . . . . . . . . . . . 8 77 3.1.3. Rating Criteria Support . . . . . . . . . . . . . . . 9 78 3.1.4. Placement of Entities and Timing of Transactions . . . 11 79 3.1.5. Protocol Extensibility . . . . . . . . . . . . . . . . 13 80 3.1.6. Error Handling and Overload Protection . . . . . . . . 13 81 3.2. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 14 82 3.3. Security and Privacy . . . . . . . . . . . . . . . . . . . 15 83 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 84 5. Security Considerations . . . . . . . . . . . . . . . . . . . 18 85 5.1. High-level security considerations . . . . . . . . . . . . 18 86 5.2. Information Disclosure Scenarios . . . . . . . . . . . . . 18 87 5.2.1. Classification of Information Disclosure Scenarios . . 18 88 5.2.2. Discussion of Information Disclosure Scenarios . . . . 19 89 5.3. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 21 90 5.4. Security Requirements . . . . . . . . . . . . . . . . . . 21 91 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 92 6.1. Normative References . . . . . . . . . . . . . . . . . . . 22 93 6.2. Informative References . . . . . . . . . . . . . . . . . . 22 94 Appendix A. Contributors List and Acknowledgments . . . . . . . . 23 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 97 1. Introduction 99 The motivation for Application-Layer Traffic Optimization (ALTO) is 100 described in the ALTO problem statement [RFC5693]. 102 The goal of ALTO is to provide information which can help peer-to- 103 peer (P2P) applications to make better decisions with respect to peer 104 selection. However, ALTO may be useful for non-P2P applications as 105 well. For example, clients of client-server applications may use 106 information provided by ALTO to select one of several servers or 107 information replicas. As another example, ALTO information could be 108 used to select a media relay needed for NAT traversal. The goal of 109 these informed decisions is to improve performance or Quality of 110 Experience in the application while reducing the utilization of the 111 underlying network infrastructure. 113 Usually, it would be difficult or even impossible for application 114 entities to acquire this information by other mechanisms, e.g., using 115 measurements between the peers of a P2P overlay, because of 116 complexity or because it is based on network topology information, 117 network operational costs, or network policies, which the respective 118 network provider does not want to disclose in detail. 120 The functional entities that provide the ALTO service do not take 121 part in the actual user data transport, i.e., they do not implement 122 functions for relaying user data. These functional entities may be 123 placed on various kinds of physical nodes, e.g., on dedicated 124 servers, as auxiliary processes in routers, on "trackers" or "super 125 peers" of a P2P application, etc. 127 2. Terminology and Architectural Framework 129 2.1. Requirements Notation 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 [RFC2119]. 135 2.2. ALTO Terminology 137 This document uses the following ALTO-related terms, which are 138 defined in [RFC5693]: 140 Application, Peer, P2P, Resource, Resource Identifier, Resource 141 Provider, Resource Consumer, Transport Address, Overlay Network, 142 Resource Directory, ALTO Service, ALTO Server, ALTO Client, ALTO 143 Query, ALTO Response, ALTO Transaction, Local Traffic, Peering 144 Traffic, Transit Traffic, Application protocol, ALTO Client Protocol, 145 Provisioning protocol. 147 Furthermore, the following additional terms will be used: 149 o Host Group Descriptor: Information used to describe one or more 150 Internet hosts (such as the resource consumer that seeks ALTO 151 guidance, or one or more candidate resource providers) and their 152 location within the network topology. There can be several 153 different types of host group descriptors, for example, a single 154 IP address, an address prefix or address range that contains the 155 host(s), or an autonomous system (AS) number. Different host 156 group descriptor types may provide different levels of detail. 157 Depending on the system architecture, this may have implications 158 on the quality of the guidance ALTO is able to provide, on whether 159 recommendations can be aggregated, and on how much privacy- 160 sensitive information about users might be disclosed to additional 161 parties. 163 o Rating Criterion: The condition or relation that defines the 164 "better" in "better-than-random peer selection", which is the 165 ultimate goal of ALTO. Examples may include "host's Internet 166 access is not subject to volume based charging (flat rate)" or 167 "low topological distance". Some rating criteria, such as "low 168 topological distance", need to include a reference point, e. g., 169 "low topological distance from a given resource consumer". This 170 reference point can be described by means of a host group 171 descriptor. 173 o Host Characteristics Attribute: Properties of a host, other than 174 the host group descriptor. It may be evaluated according to one 175 or more rating criteria. This information may be stored in an 176 ALTO server and transmitted via an ALTO protocol. One example for 177 a host characteristics attribute would be a data field indicating 178 whether a host's Internet access is subject to volume based 179 charging or not (flat rate). 181 o Target-Aware Query Mode: In this mode of operation, an ALTO client 182 performs the ALTO query when the desired resource and a set of 183 candidate resource providers are already known, i. e., after 184 distributed hash table (DHT) lookups, queries to the resource 185 directory, etc. To this end the ALTO client transmits a list of 186 host group descriptors and optionally one or more rating criteria 187 to the ALTO server. The ALTO server evaluates the host group 188 descriptors according to the indicated criteria or a default 189 criterion. It returns a list of these host group descriptors to 190 the ALTO client, which is sorted according to the rating criteria 191 and/or enriched with host characteristic attributes. 193 o Target-Independent Query Mode: In this mode of operation, ALTO 194 queries are performed in advance or periodically, in order to 195 receive comprehensive guidance. The ALTO client indicates the 196 desired host characteristic attributes in the ALTO query. The 197 ALTO server answers with a list that indicates for all known host 198 group descriptors (possibly subject to the server's policies) the 199 desired host characteristic attributes. These lists will be 200 cached locally and evaluated later, when a resource is to be 201 accessed. 203 2.3. Architectural Framework for ALTO 205 There are various architectural options for how ALTO could be 206 implemented, and specifying or mandating one specific architecture is 207 out of the scope of this document. 209 In addition to the terminology (see Section 2 of [RFC5693] and 210 Section 2.2 of this document), [RFC5693] presents a figure that gives 211 a high-level overview of protocol interaction between these 212 components. 214 This document itemizes requirements for the following components: 215 ALTO client protocols, ALTO server discovery mechanisms, host group 216 descriptors, rating criteria, and host characteristics attributes. 217 Furthermore, requirements regarding the overall architecture, 218 especially with respect to security and privacy issues, are 219 presented. 221 Note that the detailed specification of such protocols and mechanisms 222 is out of the scope of this document. In fact, this document does 223 not even assume that there will be only one single specification for 224 each of these components, respectively. However, this document 225 enumerates requirements for ALTO, to be considered when specifying, 226 assessing, or comparing protocols and implementations. 228 3. ALTO Requirements 230 [*** Note to the RFC editor: before publication as an RFC, please 231 remove the draft version number from the requirements numbering, 232 i.e., change ARv15-1 to AR-1, and so on. Furthermore, remove this 233 note. ***] 235 3.1. ALTO Client Protocol 237 3.1.1. General Requirements 239 REQ. ARv15-1: The ALTO service is provided by one or more ALTO 240 servers. It may be queried by ALTO clients seeking guidance for 241 selecting appropriate resource providers. ALTO clients and ALTO 242 servers MUST implement an ALTO client protocol. An ALTO client 243 protocol MUST be able to transmit ALTO queries from an ALTO client to 244 an ALTO server, and it MUST be able to transmit the corresponding 245 ALTO replies from the ALTO server to the ALTO client. 247 The detailed specification of an ALTO client protocol is out of the 248 scope of this document. In fact, this document does not even assume 249 that there will be only one single protocol specification. However, 250 this document enumerates requirements for ALTO, to be considered when 251 specifying, assessing, or comparing protocols and implementations. 253 REQ. ARv15-2: An ALTO protocol MUST provide adequate mechanisms for 254 operations and management support, as outlined in RFC 5706 [RFC5706]. 256 3.1.2. Host Group Descriptor Support 258 The ALTO guidance is based on the evaluation of several resource 259 providers or groups of resource providers, considering one or more 260 rating criteria. The resource providers or groups of resource 261 providers are characterized by means of host group descriptors. 263 REQ. ARv15-3: The ALTO client protocol MUST support the usage of 264 multiple host group descriptor types. 266 REQ. ARv15-4: ALTO clients and ALTO servers MUST clearly identify 267 the type of each host group descriptor sent in ALTO queries or 268 responses. An ALTO protocol specification MUST provide appropriate 269 protocol elements. 271 REQ. ARv15-5: An ALTO client protocol MUST support the host group 272 descriptor types "IPv4 address prefix" and "IPv6 address prefix". 273 They can be used to specify the IP address of one host, or an IP 274 address range (in CIDR notation) containing all hosts in question. 276 REQ. ARv15-6: An ALTO client protocol MUST be extensible to enable 277 support of other host group descriptor types in future. An ALTO 278 client protocol specification MUST define an appropriate procedure 279 for adding new host group descriptor types, e.g., by establishing an 280 IANA registry. 282 REQ. ARv15-7: For host group descriptor types other than "IPv4 283 address prefix" and "IPv6 address prefix", the host group descriptor 284 type identification MUST be supplemented by a reference to a facility 285 that can be used to translate host group descriptors of this type to 286 IPv4/IPv6 address prefixes, e.g., by means of a mapping table or an 287 algorithm. 289 REQ. ARv15-8: Protocol functions for mapping other host group 290 descriptor types to IPv4/IPv6 address prefixes SHOULD be designed and 291 specified as part of an ALTO client protocol, and the corresponding 292 address mapping information SHOULD be made available by the same 293 entity that wants to use these host group descriptors within an ALTO 294 client protocol. However, an ALTO server or an ALTO client MAY also 295 send a reference to an external mapping facility, e.g., a translation 296 table to be obtained via an alternative mechanism. 298 Rationale for the previous two requirements: The preferred type of 299 host group descriptors are IPv4 and IPv6 prefixes. However, in 300 some situations one party may prefer to use another type, e.g., 301 Autonomous System (AS) numbers. Usually, applications seeking 302 ALTO guidance work with IP addresses, e.g., when establishing 303 connections. Understanding guiding information that is based on 304 other host group descriptor types, i.e., mapping from this other 305 types to IP prefixes and back, may be a non-trivial task. 306 Therefore, before a party may use other host group descriptor 307 types, they must provide a mapping mechanism to IP prefixes. 309 REQ. ARv15-9: An ALTO client protocol specification MUST define 310 mechanisms that can be used by the ALTO server to indicate that a 311 host group descriptor used by the ALTO client is of an unsupported 312 type, or that the indicated mapping mechanism could not be used. 314 REQ. ARv15-10: An ALTO client protocol specification MUST define 315 mechanisms that can be used by the ALTO client to indicate that a 316 host group descriptor used by the ALTO server is of an unsupported 317 type, or that the indicated mapping mechanism could not be used. 319 3.1.3. Rating Criteria Support 321 REQ. ARv15-11: An ALTO client protocol specification MUST define a 322 rating criterion that can be used to express and evaluate the 323 "relative operator's preference." This is a relative measure, i.e., 324 it is not associated with any unit of measurement. A more-preferred 325 rating according to this criterion indicates that the application 326 should prefer the respective candidate resource provider over others 327 with less-preferred ratings (unless information from non-ALTO sources 328 suggests a different choice, such as transmission attempts suggesting 329 that the path is currently congested). The operator of the ALTO 330 server does not have to disclose how and based on which data the 331 ratings are actually computed. Examples could be: cost for peering 332 or transit traffic, traffic engineering inside the network, and other 333 policies. 335 REQ. ARv15-12: An ALTO client protocol MUST be extensible to enable 336 support of other rating criteria types in future. An ALTO client 337 protocol specification MUST define an appropriate procedure for 338 adding new rating criteria types, e.g., by establishing an IANA 339 registry. 341 REQ. ARv15-13: ALTO client protocol specifications MUST NOT define 342 rating criteria closely related to the instantaneous network 343 congestion state, i. e., rating criteria that have the primary aim to 344 serve as an alternative to established congestion control strategies, 345 such as using TCP-based transport. 347 REQ. ARv15-14: Applications using ALTO guidance MUST NOT rely solely 348 on the ALTO guidance to avoid causing network congestion. Instead, 349 applications MUST use other appropriate means, such as TCP based 350 transport, to avoid causing excessive congestion. 352 Rationale for the previous requirement: One design assumption for 353 ALTO is that it is acceptable that the host characteristics 354 attributes, which are stored and processed in the ALTO servers for 355 giving the guidance, are updated rather infrequently. Typical 356 update intervals may be several orders of magnitude longer than 357 the typical network-layer packet round-trip time (RTT). 358 Therefore, ALTO cannot be a replacement for TCP-like congestion 359 control mechanisms. 361 REQ. ARv15-15: In the target-independent query mode, the ALTO query 362 message SHOULD allow the ALTO client to express which host 363 characteristics attributes should be returned. 365 REQ. ARv15-16: In the target-aware query mode, the ALTO query 366 message SHOULD allow the ALTO client to express which rating criteria 367 should be considered by the server, as well as their relative 368 relevance for the specific application that will eventually make use 369 of the guidance. The corresponding ALTO response message SHOULD 370 allow the ALTO server to express which rating criteria have been 371 considered when generating the response. 373 REQ. ARv15-17: An ALTO client protocol specification MUST define 374 mechanisms that can be used by the ALTO client and the ALTO server to 375 indicate that a rating criteria used by the other party is of an 376 unsupported type. 378 3.1.4. Placement of Entities and Timing of Transactions 380 With respect to the placement of ALTO clients, several modes of 381 operation exist: 383 o One mode of ALTO operation is that an ALTO client may be embedded 384 directly in the resource consumer, i.e., the application protocol 385 entity that will eventually initiate data transmission to/from the 386 selected resource provider(s) in order to access the desired 387 resource. For example, an ALTO client could be integrated into 388 the peer of a P2P application that uses a distributed algorithm 389 such as "query flooding" for resource discovery. 391 o Another mode of operation is to integrate the ALTO client into a 392 third party such as a resource directory. This third party may 393 issue ALTO queries to solicit preference on potential resource 394 providers, considering the respective resource consumer. For 395 example, an ALTO client could be integrated into the tracker of a 396 tracker-based P2P application, in order to request ALTO guidance 397 on behalf of the peers contacting the tracker. 399 REQ. ARv15-18: An ALTO client protocol MUST support the mode of 400 operation in which the ALTO client is directly embedded in the 401 resource consumer. 403 REQ. ARv15-19: An ALTO client protocol MUST support the mode of 404 operation in which the ALTO client is embedded in a third party. 405 This third party performs queries on behalf of resource consumers. 407 REQ. ARv15-20: An ALTO client protocol MUST be designed in a way 408 that the ALTO service can be provided by an entity that is not the 409 operator of the underlying IP network. 411 REQ. ARv15-21: An ALTO client protocol MUST be designed in a way 412 that different instances of the ALTO service operated by different 413 providers can coexist. 415 REQ. ARv15-22: An ALTO client protocol specification MUST specify at 416 least one query mode, either the target-aware or the target- 417 independent query mode. 419 Note that this requirements document does not assume that there will 420 be only one single protocol specification. 422 REQ. ARv15-23: An ALTO client protocol specification SHOULD specify 423 both the target-aware and the target-independent query mode. If an 424 ALTO client protocol specification specifies more than one query 425 mode, it MUST define at least one of these modes as REQUIRED to 426 implement by ALTO Clients and ALTO Servers. Furthermore, it MUST 427 specify an appropriate protocol mechanism for negotiating between 428 ALTO Client and ALTO Server, which query mode to use. 430 REQ. ARv15-24: An ALTO client protocol SHOULD support version 431 numbering, TTL (time-to-live) attributes, and/or similar mechanisms 432 in ALTO transactions, in order to enable time validity checking for 433 caching, and to enable comparisons of multiple recommendations 434 obtained through redistribution. 436 REQ. ARv15-25: An ALTO client protocol SHOULD allow the ALTO server 437 to add information about appropriate modes of re-use to its ALTO 438 responses. Re-use may include redistributing an ALTO response to 439 other parties, as well as using the same ALTO information in a 440 resource directory to improve the responses to different resource 441 consumers, within the specified lifetime of the ALTO response. The 442 ALTO server SHOULD be able to express that 444 o no re-use should occur 446 o re-use is appropriate for a specific "target audience", i.e., a 447 set of resource consumers explicitly defined by a list of host 448 group descriptors. The ALTO server MAY specify a "target 449 audience" in the ALTO response that is only a subset of the known 450 actual "target audience", e.g., if required by operator policies 452 o re-use is appropriate for any resource consumer that would send 453 (or cause a third party sending on behalf of it) the same ALTO 454 query (i.e., with the same query parameters, except for the 455 resource consumer ID, if applicable) to this ALTO server 457 o re-use is appropriate for any resource consumer that would send 458 (or cause a third party sending on behalf of it) the same ALTO 459 query (i.e., with the same query parameters, except for the 460 resource consumer ID, if applicable) to any other ALTO server that 461 was discovered (using an ALTO discovery mechanism) together with 462 this ALTO server 464 o re-use is appropriate for any resource consumer that would send 465 (or cause a third party sending on behalf of it) the same ALTO 466 query (i.e., with the same query parameters, except for the 467 resource consumer ID, if applicable) to any ALTO server in the 468 whole network 470 REQ. ARv15-26: An ALTO client protocol MUST support the transport of 471 ALTO transactions even if the ALTO client is located in the private 472 address realm behind a network address translator (NAT). There are 473 different types of NAT, see [RFC4787] and [RFC5382]. 475 3.1.5. Protocol Extensibility 477 REQ. ARv15-27: An ALTO client protocol MUST include support for 478 adding protocol extensions in a non-disruptive, backward-compatible 479 way. 481 REQ. ARv15-28: An ALTO client protocol MUST include protocol 482 versioning support, in order to clearly distinguish between 483 incompatible versions of the protocol. 485 3.1.6. Error Handling and Overload Protection 487 REQ. ARv15-29: An ALTO client protocol MUST use congestion-aware 488 transport, e.g., by using TCP. 490 REQ. ARv15-30: An ALTO client protocol specification MUST specify 491 mechanisms, or detail how to leverage appropriate mechanisms provided 492 by underlying protocol layers that can be used by an ALTO server to 493 inform clients about an impending or occurring overload situation, 494 and provide all of the following options to the server: 496 o terminate the conversation with the client, 498 o redirect the client to another ALTO server, and 500 o request the client to throttle its query rate. 502 In particular, a simple form of throttling is to let an ALTO 503 server answer a query with an error message advising the client to 504 retry the query later (e.g, using a protocol function such as 505 HTTP's Retry-After header ([RFC2616], section 14.37). Another 506 simple option is to actually answer the query with the desired 507 information, but adding an indication that the ALTO client should 508 not send further queries to this ALTO server before an indicated 509 period of time has elapsed. 511 REQ. ARv15-31: An ALTO client protocol specification MUST specify 512 mechanisms, or detail how to leverage appropriate mechanisms provided 513 by underlying protocol layers that can be used by an ALTO server to 514 inform clients about its inability to answer queries due to technical 515 problems or system maintenance, and provide all of the following 516 options to the server: 518 o terminate the conversation with the client, 520 o redirect the client to another ALTO server, and 522 o request the client to retry the query later. 524 Note: The existence of the above-mentioned protocol mechanisms does 525 not imply that an ALTO server must use them when facing an overload, 526 technical problem, or maintenance situation, respectively. Some 527 servers may be unable to use them in that situation, or they may 528 prefer to simply refuse the connection or not to send any answer at 529 all. 531 3.2. ALTO Server Discovery 533 An ALTO client protocol is supported by one or more ALTO server 534 discovery mechanisms, which may be used by ALTO clients in order to 535 determine one or more ALTO servers, to which ALTO requests can be 536 sent. This section enumerates requirements for an ALTO client, as 537 well as general requirements to be fulfilled by the ALTO server 538 discovery mechanisms. 540 REQ. ARv15-32: An ALTO server discovery mechanism MUST support 541 features allowing ALTO clients that are embedded in the resource 542 consumer to find one or several ALTO servers that can provide ALTO 543 guidance suitable for the resource consumer, using an ALTO protocol 544 version compatible with the ALTO client. This mode of operation is 545 called "resource consumer initiated ALTO server discovery". 547 REQ. ARv15-33: An ALTO server discovery mechanism MUST support 548 features allowing ALTO clients that are embedded in a resource 549 directory and perform third-party ALTO queries on behalf of a remote 550 resource consumer to find one or several ALTO servers that can 551 provide ALTO guidance suitable for the respective resource consumer, 552 using an ALTO protocol version compatible with the ALTO client. This 553 mode of operation is called "third-party ALTO server discovery". 555 REQ. ARv15-34: ALTO clients MUST be able to perform resource 556 consumer initiated ALTO server discovery, even if they are located 557 behind a network address translator (NAT). 559 REQ. ARv15-35: ALTO clients MUST be able to perform third-party ALTO 560 server discovery, even if they are located behind a network address 561 translator (NAT). 563 REQ. ARv15-36: ALTO clients MUST be able to perform third-party ALTO 564 server discovery, even if the resource consumer, on behalf of which 565 the ALTO query will be sent, is located behind a network address 566 translator (NAT). 568 REQ. ARv15-37: ALTO server discovery mechanisms SHOULD leverage an 569 existing protocol or mechanism, such as DNS, DHCP, or PPP based 570 automatic configuration, etc. A single mechanism with a broad 571 spectrum of applicability SHOULD be preferred over several different 572 mechanisms with narrower scopes. 574 REQ. ARv15-38: Every ALTO server discovery mechanism SHOULD be able 575 to return the respective contact information for multiple ALTO 576 servers. 578 REQ. ARv15-39: Every ALTO server discovery mechanism SHOULD be able 579 to indicate preferences for each returned ALTO server contact 580 information. 582 3.3. Security and Privacy 584 Note: The following requirements mandate the inclusion of certain 585 security mechanisms at a protocol specification level. Whether it 586 makes sense to enable these mechanisms in a given deployment scenario 587 depends on a threat analysis for this specific scenario. For a 588 classification of potential information disclosure risks refer to 589 Section 5.2. 591 REQ. ARv15-40: An ALTO client protocol specification MUST specify 592 mechanisms for the authentication of ALTO servers, or how to leverage 593 appropriate mechanisms provided by underlying protocol layers. 595 REQ. ARv15-41: An ALTO client protocol specification MUST specify 596 mechanisms for the authentication of ALTO clients, or how to leverage 597 appropriate mechanisms provided by underlying protocol layers. 599 REQ. ARv15-42: An ALTO client protocol specification MUST specify 600 mechanisms for the encryption of messages, or how to leverage 601 appropriate mechanisms provided by underlying protocol layers. 603 REQ. ARv15-43: An ALTO client is not required to implement 604 mechanisms or to comply with rules that limit its ability to 605 redistribute information retrieved from the ALTO server to third 606 parties. 608 REQ. ARv15-44: An ALTO client protocol MUST support different levels 609 of detail in queries and responses, in order to protect the privacy 610 of users, to ensure that the operators of ALTO servers and other 611 users of the same application cannot derive sensitive information. 613 REQ. ARv15-45: An ALTO client protocol MAY include mechanisms that 614 can be used by the ALTO client when requesting guidance to specify 615 the resource (e.g., content identifiers) it wants to access. An ALTO 616 server MUST provide adequate guidance even if the ALTO client prefers 617 not to specify the desired resource (e.g., keeps the data field 618 empty). The mechanism MUST be designed in a way that the operator of 619 the ALTO server cannot easily deduce the resource identifier (e.g., 620 file name in P2P file sharing) if the ALTO client prefers not to 621 specify it. 623 REQ. ARv15-46: An ALTO client protocol specification MUST specify 624 appropriate mechanisms for protecting the ALTO service against DoS 625 attacks, or how to leverage appropriate mechanisms provided by 626 underlying protocol layers. 628 4. IANA Considerations 630 This requirements document does not mandate any immediate IANA 631 actions. However, such IANA considerations may arise from future 632 ALTO specification documents, which try to meet the requirements 633 given here. 635 5. Security Considerations 637 5.1. High-level security considerations 639 High-level security considerations for the ALTO service can be found 640 in the "Security Considerations" section of the ALTO problem 641 statement document [RFC5693]. 643 5.2. Information Disclosure Scenarios 645 The unwanted disclosure of information is one key concern related to 646 ALTO. From a user privacy perspective, neither the ALTO server nor a 647 third party using or misusing the ALTO service should be able to 648 infer the application behavior, e.g., who is exchanging which files 649 with whom using a P2P file sharing application. Many network 650 operators, in contrast, are concerned about the amount of information 651 related to their network infrastructure (e.g., topology information, 652 number of "premium customers", or utilization statistics) that might 653 be released through ALTO. This section presents a classification and 654 discussion of information disclosure scenarios and potential 655 countermeasures. 657 5.2.1. Classification of Information Disclosure Scenarios 659 The following issues may be considered a risk for the operator of an 660 ALTO server, depending on the specific deployment scenario: 662 o (1) Excess disclosure of ALTO server operator's data to an 663 authorized ALTO client. The operator of an ALTO server has to 664 feed information, such as tables mapping host group descriptors to 665 host characteristics attributes, into the server, thereby enabling 666 it to give guidance to ALTO clients. Some operators might 667 consider the full set of this information confidential (e.g., a 668 detailed map of the operator's network topology), and might want 669 to disclose only a subset of it or somehow obfuscated information 670 to an ALTO client. 672 o (2) Disclosure of ALTO server operator's data (e.g., network 673 topology information) to an unauthorized third party. There are a 674 three sub-cases here: 676 * (2a) An ALTO server receives and answers queries originating 677 from an unauthorized ALTO client. 679 * (2b) An unauthorized party snoops on the data transmission from 680 the ALTO server to an authorized ALTO client. 682 * (2c) An authorized ALTO client knowingly forwards the 683 information it had received from the ALTO server to an 684 unauthorized party. 686 o (3) Excess retrieval of ALTO server operator's data by 687 collaborating ALTO clients. Several authorized ALTO clients could 688 ask one or more ALTO servers for guidance, possibly several times 689 during an extended period of time, and redistribute the responses 690 among each other (see also case 2c). By aggregating and 691 correlating the ALTO responses they could find out more 692 information than intended to be disclosed by the ALTO server 693 operator(s). 695 The following issues may be considered a risk for the user of an ALTO 696 client, depending on the specific deployment scenario: 698 o (4) Disclosure of the application behavior to the (authorized) 699 ALTO server. The operator of an ALTO server could infer the 700 application behavior (e.g., content identifiers in P2P file 701 sharing applications, or lists of resource providers that are 702 considered for establishing a connection) from the ALTO queries 703 sent by an ALTO client. 705 o (5) Disclosure of the application behavior to an unauthorized 706 third party. There are a three sub-cases here: 708 * (5a) An ALTO client willingly sends queries directly to an 709 untrusted or malicious ALTO server, possibly due to a forged 710 response of the ALTO server discovery mechanism. 712 * (5b) An unauthorized party snoops on the data transmission from 713 the ALTO client to an authorized ALTO server. 715 * (5c) An authorized ALTO server knowingly forwards the 716 information it had received from the ALTO client to an 717 unauthorized party. 719 o (6) One or several collaborating (see case 5c) ALTO servers could 720 try to infer the application behavior by aggregating and 721 correlating queries from one or more ALTO clients, possibly over 722 an extended period of time. 724 5.2.2. Discussion of Information Disclosure Scenarios 726 An ALTO server operator should consider: 728 o Issue (1) may be addressed by the ALTO server operator choosing 729 the level of detail of the information to be populated into the 730 ALTO server and returned in the responses. For example, by 731 specifying a broader address range (i.e., a shorter prefix length) 732 than a group of hosts in question actually uses, an ALTO server 733 operator may control to some extent how much information about the 734 network topology is disclosed. Furthermore, access control 735 mechanisms for filtering ALTO responses according to the 736 authenticated ALTO client identity might be installed in the ALTO 737 server, although this might not be effective given the lack of 738 efficient mechanisms for addressing (2c) and (3), see below. 740 o (2a) and (2b) may be addressed by authentication, access control, 741 and encryption schemes for the ALTO client protocol. However, 742 deployment of encryption schemes might not be effective given the 743 lack of efficient mechanisms for addressing (2c) and (3), see 744 below. 746 o Straightforward authentication and encryption schemes will not 747 help solving (2c) and (3), and there is no other simple and 748 efficient mechanism known. The cost of complex approaches, e.g., 749 based on digital rights management (DRM), might easily outweigh 750 the benefits of the whole ALTO solution, and therefore they are 751 not considered as a viable solution. That is, ALTO server 752 operators must be aware that (2c) and (3) cannot be prevented from 753 happening, and therefore they should feed only such data into an 754 ALTO server that they do not consider sensitive with respect to 755 (2c) and (3). 757 A user of an ALTO client should consider: 759 o Issue (4) can and needs to be addressed in several ways: If the 760 ALTO client is embedded in the resource consumer, the resource 761 consumer's IP address (or the "public" IP address of the outermost 762 NAT in front of the resource consumer) is disclosed to the ALTO 763 server as a matter of principle, because it is in the source 764 address fields of the IP headers. By using a proxy, the 765 disclosure of source addresses to the ALTO server can be avoided 766 at the cost of disclosing them to said proxy. If, in contrast, 767 the ALTO client is embedded in a third party (e.g., a resource 768 directory), which issues ALTO requests on behalf of resource 769 consumers, it is possible to hide the exact addresses of the 770 resource consumers from the ALTO server, e.g., by zeroing-out or 771 randomizing the last few bits of IP addresses. However, there is 772 the potential side effect of yielding inaccurate results. 774 The disclosure of candidate resource providers' addresses to the 775 ALTO server can be avoided by allowing ALTO clients to use the 776 target-independent query mode. In this mode of operation, guiding 777 information (e.g., "maps") is retrieved from the ALTO server and 778 used entirely locally by the ALTO client, i.e., without sending 779 host location attributes of candidate resource providers to the 780 ALTO server. In the target-aware query mode, this issue can be 781 addressed by ALTO clients through obfuscating the identity of 782 candidate resource consumers, e.g., by specifying a broader 783 address range (i.e., a shorter prefix length) than a group of 784 hosts in question actually uses, or by zeroing-out or randomizing 785 the last few bits of IP addresses. However, there is the 786 potential side effect of yielding inaccurate results. 788 o (5a) may be addressed by mandating that the ALTO server discovery 789 procedure as a whole must be secure against spoofing. 791 Note: Given that this document does not mandate a specific system 792 architecture, it is difficult to specify more details than that 793 the discovery procedure as a whole should be secure against 794 spoofing. There are many different archtectural options, e.g., 795 have an insecure discovery mechanism and use server certificates 796 to later verify its response (c.f. the DNS + HTTPS security model 797 widely used in the World Wide Web). Therefore, at this 798 requirements stage, it is not mandatory for the discovery 799 mechanism itself to be secure against spoofing attacks. 801 o (5b) may be addressed by encryption schemes for the ALTO client 802 protocol. However, the effort vs. benefit should be evaluated for 803 any specific deployment scenario, while also considering the risks 804 and solution approaches for issues (4), (5c), and (6). 806 o Straightforward authentication and encryption schemes will not 807 help solving (5c) and (6). However, potential risks can be 808 mitigated using the same approaches as used for issue (4), see 809 above. 811 These insights are reflected in the requirements in this document. 813 5.3. ALTO Server Discovery 815 See discussion of (5a) above. 817 5.4. Security Requirements 819 For a set of specific security requirements please refer to 820 Section 3.3 of this document. 822 6. References 824 6.1. Normative References 826 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 827 Requirement Levels", BCP 14, RFC 2119, March 1997. 829 [RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic 830 Optimization (ALTO) Problem Statement", RFC 5693, 831 October 2009. 833 6.2. Informative References 835 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 836 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 837 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 839 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 840 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 841 RFC 4787, January 2007. 843 [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. 844 Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, 845 RFC 5382, October 2008. 847 [RFC5706] Harrington, D., "Guidelines for Considering Operations and 848 Management of New Protocols and Protocol Extensions", 849 RFC 5706, November 2009. 851 Appendix A. Contributors List and Acknowledgments 853 The initial version of this document was co-authored by Laird Popkin. 855 The authors would like to thank 857 o Vijay K. Gurbani 859 o Enrico Marocco 861 for fostering discussions that lead to the creation of this document, 862 and for giving valuable comments on it. 864 The authors were supported by the following people, who have 865 contributed to this document: 867 o Richard Alimi 869 o Jason Livingood 871 o Michael Scharf 873 o Nico Schwan 875 o Jan Seedorf 877 The authors would like to thank the members of the P2PI and ALTO 878 mailing lists for their feedback. 880 Laird Popkin and Y. Richard Yang are grateful to the many 881 contributions made by the members of the P4P working group and Yale 882 Laboratory of Networked Systems. The P4P working group is hosted by 883 DCIA. 885 Martin Stiemerling is partially supported by the COAST project 886 (COntent Aware Searching, retrieval and sTreaming, 887 http://www.coast-fp7.eu), a research project supported by the 888 European Commission under its 7th Framework Program (contract no. 889 248036). The views and conclusions contained herein are those of the 890 authors and should not be interpreted as necessarily representing the 891 official policies or endorsements, either expressed or implied, of 892 the COAST project or the European Commission. 894 Authors' Addresses 896 Sebastian Kiesel (editor) 897 University of Stuttgart Computing Center 898 Networks and Communication Systems Department 899 Allmandring 30 900 70550 Stuttgart 901 Germany 903 Email: ietf-alto@skiesel.de 904 URI: http://www.rus.uni-stuttgart.de/nks/ 906 Stefano Previdi 907 Cisco Systems, Inc. 909 Email: sprevidi@cisco.com 911 Martin Stiemerling 912 NEC Laboratories Europe 914 Email: martin.stiemerling@neclab.eu 915 URI: http://ietf.stiemerling.org 917 Richard Woundy 918 Comcast Corporation 920 Email: Richard_Woundy@cable.comcast.com 922 Yang Richard Yang 923 Yale University 925 Email: yry@cs.yale.edu