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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: July 19, 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 January 16, 2012 14 Application-Layer Traffic Optimization (ALTO) Requirements 15 draft-ietf-alto-reqs-13.txt 17 Abstract 19 Many Internet applications are used to access resources, such as 20 pieces of information or server processes, which 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, which 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 29 (or Quality of Experience) in the application while reducing resource 30 consumption in 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 July 19, 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 . . . . . . . . . . . . . . . . . . . . . . 7 74 3.1. ALTO Client Protocol . . . . . . . . . . . . . . . . . . . 7 75 3.1.1. General Requirements . . . . . . . . . . . . . . . . . 7 76 3.1.2. Host Group Descriptor Support . . . . . . . . . . . . 7 77 3.1.3. Rating Criteria Support . . . . . . . . . . . . . . . 8 78 3.1.4. Placement of Entities and Timing of Transactions . . . 9 79 3.1.5. Protocol Extensibility . . . . . . . . . . . . . . . . 11 80 3.1.6. Error Handling and Overload Protection . . . . . . . . 11 81 3.2. ALTO Server Discovery . . . . . . . . . . . . . . . . . . 13 82 3.3. Security and Privacy . . . . . . . . . . . . . . . . . . . 14 83 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 84 5. Security Considerations . . . . . . . . . . . . . . . . . . . 17 85 5.1. High-level security considerations . . . . . . . . . . . . 17 86 5.2. Information Disclosure Scenarios . . . . . . . . . . . . . 17 87 5.2.1. Classification of Information Disclosure Scenarios . . 17 88 5.2.2. Discussion of Information Disclosure Scenarios . . . . 18 89 5.3. Security Requirements . . . . . . . . . . . . . . . . . . 19 90 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 91 6.1. Normative References . . . . . . . . . . . . . . . . . . . 20 92 6.2. Informative References . . . . . . . . . . . . . . . . . . 20 93 Appendix A. Contributors List and Acknowledgments . . . . . . . . 21 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 96 1. Introduction 98 The motivation for Application-Layer Traffic Optimization (ALTO) is 99 described in the ALTO problem statement [RFC5693]. 101 The goal of ALTO is to provide information which can help peer-to- 102 peer (P2P) applications to make better decisions with respect to peer 103 selection. However, ALTO may be useful for non-P2P applications as 104 well. For example, clients of client-server applications may use 105 information provided by ALTO to select one of several servers or 106 information replicas. As another example, ALTO information could be 107 used to select a media relay needed for NAT traversal. The goal of 108 these informed decisions is to improve performance or Quality of 109 Experience in the application while reducing resource consumption in 110 the underlying network infrastructure. 112 Usually, it would be difficult or even impossible for application 113 entities to acquire this information by other mechanisms, e.g., using 114 measurements between the peers of a P2P overlay, because of 115 complexity or because it is based on network topology information, 116 network operational costs, or network policies, which the respective 117 network provider does not want to disclose in detail. 119 The functional entities that provide the ALTO service do not take 120 part in the actual user data transport, i.e., they do not implement 121 functions for relaying user data. These functional entities may be 122 placed on various kinds of physical nodes, e.g., on dedicated 123 servers, as auxiliary processes in routers, on "trackers" or "super 124 peers" of a P2P application, etc. 126 2. Terminology and Architectural Framework 128 2.1. Requirements Notation 130 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 131 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 132 document are to be interpreted as described in [RFC2119]. 134 2.2. ALTO Terminology 136 This document uses the following ALTO-related terms, which are 137 defined in [RFC5693]: 139 Application, Peer, P2P, Resource, Resource Identifier, Resource 140 Provider, Resource Consumer, Transport Address, Overlay Network, 141 Resource Directory, ALTO Service, ALTO Server, ALTO Client, ALTO 142 Query, ALTO Response, ALTO Transaction, Local Traffic, Peering 143 Traffic, Transit Traffic, Application protocol, ALTO Client Protocol, 144 Provisioning protocol. 146 Furthermore, the following additional terms will be used: 148 o Host Group Descriptor: Information used to describe one or more 149 Internet hosts (such as the resource consumer which seeks ALTO 150 guidance, or one or more candidate resource providers) and their 151 location within the network topology. There can be several 152 different types of host group descriptors, for example, a single 153 IP address, an address prefix or address range that contains the 154 host(s), or an autonomous system (AS) number. Different host 155 group descriptor types may provide different levels of detail. 156 Depending on the system architecture, this may have implications 157 on the quality of the guidance ALTO is able to provide, on whether 158 recommendations can be aggregated, and on how much privacy- 159 sensitive information about users might be disclosed to additional 160 parties. 162 o Rating Criterion: The condition or relation that defines the 163 "better" in "better-than-random peer selection", which is the 164 ultimate goal of ALTO. Examples may include "host's Internet 165 access is not subject to volume based charging (flat rate)" or 166 "low topological distance". Some rating criteria, such as "low 167 topological distance", need to include a reference point, i. e., 168 "low topological distance from a given resource consumer". This 169 reference point can be described by means of a host group 170 descriptor. 172 o Host Characteristics Attribute: Properties of a host, other than 173 the host group descriptor. It may be evaluated according to one 174 or more rating criteria. This information may be stored in an 175 ALTO server and transmitted via an ALTO protocol. One example for 176 a host characteristics attribute would be a data field indicating 177 whether a host's Internet access is subject to volume based 178 charging or not (flat rate). 180 o Target-Aware Query Mode: In this mode of operation, an ALTO client 181 performs the ALTO query when the desired resource and a set of 182 candidate resource providers are already known, i. e., after DHT 183 lookups, queries to the resource directory, etc. To this end the 184 ALTO client transmits a list of host group descriptors and 185 optionally one or more rating criteria to the ALTO server. The 186 ALTO server evaluates the host group descriptors according to the 187 indicated criteria or a default criterion. It returns a list of 188 these host group descriptors to the ALTO client, which is sorted 189 according to the rating criteria and/or enriched with host 190 characteristic attributes. 192 o Target-Independent Query Mode: In this mode of operation, ALTO 193 queries are performed in advance or periodically, in order to 194 receive comprehensive guidance. The ALTO client indicates the 195 desired host characteristic attributes in the ALTO query. The 196 ALTO server answers with a list that indicates for all known host 197 group descriptors (possibly subject to the server's policies) the 198 desired host characteristic attributes. These lists will be 199 cached locally and evaluated later, when a resource is to be 200 accessed. 202 2.3. Architectural Framework for ALTO 204 There are various architectural options for how ALTO could be 205 implemented, and specifying or mandating one specific architecture is 206 out of the scope of this document. 208 The ALTO problem statement [RFC5693] defines a terminology (see 209 Section 2 of [RFC5693] and Section 2.2 of this document), introduces 210 several components. It presents a figure that gives a high-level 211 overview of protocol interaction between these components. 213 This document itemizes requirements for the following components: 214 ALTO client protocols, ALTO server discovery mechanisms, host group 215 descriptors, rating criteria, and host characteristics attributes. 216 Furthermore, requirements regarding the overall architecture, 217 especially with respect to security and privacy issues, are 218 presented. 220 3. ALTO Requirements 222 [*** Note to the RFC editor: before publication as an RFC, please 223 remove the draft version number from the requirements numbering, 224 i.e., change ARv13-1 to AR-1, and so on. Furthermore, remove this 225 note. ***] 227 3.1. ALTO Client Protocol 229 3.1.1. General Requirements 231 REQ. ARv13-1: The ALTO service is provided by one or more ALTO 232 servers. It may be queried by ALTO clients seeking guidance for 233 selecting appropriate resource providers. ALTO clients and ALTO 234 servers MUST implement an ALTO client protocol. An ALTO client 235 protocol MUST be able to transmit ALTO queries from an ALTO client to 236 an ALTO server, and it MUST be able to transmit the corresponding 237 ALTO replies from the ALTO server to the ALTO client. 239 The detailed specification of an ALTO client protocol is out of the 240 scope of this document. However, this document enumerates 241 requirements for ALTO, to be considered when specifying, assessing, 242 or comparing protocols and implementations. 244 3.1.2. Host Group Descriptor Support 246 The ALTO guidance is based on the evaluation of several resource 247 providers or groups of resource providers, considering one or more 248 rating criteria. The resource providers or groups of resource 249 providers are characterized by means of host group descriptors. 251 REQ. ARv13-2: The ALTO client protocol MUST support the usage of 252 multiple host group descriptor types. 254 REQ. ARv13-3: ALTO clients and ALTO servers MUST clearly identify 255 the type of each host group descriptor sent in ALTO queries or 256 responses. 258 REQ. ARv13-4: An ALTO client protocol MUST support the host group 259 descriptor types "IPv4 address prefix" and "IPv6 address prefix". 260 They can be used to specify the IP address of one host, or an IP 261 address range (in CIDR notation) containing all hosts in question. 263 REQ. ARv13-5: An ALTO client protocol MUST be extensible to enable 264 support of other host group descriptor types in future. An ALTO 265 client protocol specification MUST define an appropriate procedure 266 for adding new host group descriptor types, e.g., by establishing an 267 IANA registry. 269 REQ. ARv13-6: For host group descriptor types other than "IPv4 270 address prefix" and "IPv6 address prefix", the host group descriptor 271 type identification MUST be supplemented by a reference to a 272 facility, which can be used to translate host group descriptors of 273 that type to IPv4/IPv6 address prefixes, e.g., by means of a mapping 274 table or an algorithm. 276 REQ. ARv13-7: Protocol functions for mapping other host group 277 descriptor types to IPv4/IPv6 address prefixes SHOULD be designed and 278 specified as part of an ALTO client protocol, and the corresponding 279 address mapping information SHOULD be made available by the same 280 entity that wants to use these host group descriptors within an ALTO 281 client protocol. However, an ALTO server or an ALTO client MAY also 282 send a reference to an external mapping facility, e.g., a translation 283 table to be obtained via an alternative mechanism. 285 REQ. ARv13-8: An ALTO client protocol specification MUST define 286 mechanisms that can be used by the ALTO server to indicate that a 287 host group descriptor used by the ALTO client is of an unsupported 288 type, or that the indicated mapping mechanism could not be used. 290 REQ. ARv13-9: An ALTO client protocol specification MUST define 291 mechanisms, which can be used by the ALTO client to indicate that a 292 host group descriptor used by the ALTO server is of an unsupported 293 type, or that the indicated mapping mechanism could not be used. 295 3.1.3. Rating Criteria Support 297 REQ. ARv13-10: An ALTO client protocol specification MUST define a 298 rating criterion that can be used to express and evaluate the 299 "relative operator's preference." This is a relative measure, i.e., 300 it is not associated with any unit of measurement. A more-preferred 301 rating according to this criterion indicates that the application 302 should prefer the respective candidate resource provider over others 303 with less-preferred ratings (unless information from non-ALTO sources 304 suggests a different choice, such as transmission attempts suggesting 305 that the path is currently congested). The operator of the ALTO 306 server does not have to disclose how and based on which data the 307 ratings are actually computed. Examples could be: cost for peering 308 or transit traffic, traffic engineering inside the network, and other 309 policies. 311 REQ. ARv13-11: An ALTO client protocol MUST be extensible to enable 312 support of other rating criteria types in future. An ALTO client 313 protocol specification MUST define an appropriate procedure for 314 adding new rating criteria types, e.g., by establishing an IANA 315 registry. 317 REQ. ARv13-12: ALTO client protocol specifications MUST NOT define 318 rating criteria closely related to the instantaneous network 319 congestion state, i. e., rating criteria that have the primary aim to 320 serve as an alternative to established congestion control strategies, 321 such as using TCP-based transport. 323 One design assumption for ALTO is that it is acceptable that the 324 host characteristics attributes, which are stored and processed in 325 the ALTO servers for giving the guidance, are updated rather 326 infrequently. Typical update intervals may be several orders of 327 magnitude longer than the typical network-layer packet round-trip 328 time (RTT). Therefore, ALTO cannot be a replacement for TCP-like 329 congestion control mechanisms. 331 REQ. ARv13-13: Applications using ALTO guidance MUST NOT rely solely 332 on the ALTO guidance to avoid causing network congestion. Instead, 333 applications MUST use other appropriate means, such as TCP based 334 transport, to avoid causing excessive congestion. 336 REQ. ARv13-14: In the target-independent query mode, the ALTO query 337 message SHOULD allow the ALTO client to express which host 338 characteristics attributes should be returned. 340 REQ. ARv13-15: In the target-aware query mode, the ALTO query 341 message SHOULD allow the ALTO client to express which rating criteria 342 should be considered by the server, as well as their relative 343 relevance for the specific application that will eventually make use 344 of the guidance. The corresponding ALTO response message SHOULD 345 allow the ALTO server to express which rating criteria have been 346 considered when generating the response. 348 REQ. ARv13-16: An ALTO client protocol specification MUST define 349 mechanisms, which can be used by the ALTO client and the ALTO server 350 to indicate that a rating criteria used by the other party is of an 351 unsupported type. 353 3.1.4. Placement of Entities and Timing of Transactions 355 With respect to the placement of ALTO clients, several modes of 356 operation exist: 358 o One mode of ALTO operation is that an ALTO client may be embedded 359 directly in the resource consumer, i.e., the application protocol 360 entity that will eventually initiate data transmission to/from the 361 selected resource provider(s) in order to access the desired 362 resource. For example, an ALTO client could be integrated into 363 the peer of a P2P application that uses a distributed algorithm 364 such as "query flooding" for resource discovery. 366 o Another mode of operation is to integrate the ALTO client into a 367 third party such as a resource directory. This third party may 368 issue ALTO queries to solicit preference on potential resource 369 providers, considering the respective resource consumer. For 370 example, an ALTO client could be integrated into the tracker of a 371 tracker-based P2P application, in order to request ALTO guidance 372 on behalf of the peers contacting the tracker. 374 REQ. ARv13-17: An ALTO client protocol MUST support the mode of 375 operation in which the ALTO client is directly embedded in the 376 resource consumer. 378 REQ. ARv13-18: An ALTO client protocol MUST support the mode of 379 operation in which the ALTO client is embedded in a third party. 380 This third party performs queries on behalf of resource consumers. 382 REQ. ARv13-19: An ALTO client protocol MUST be designed in a way 383 that the ALTO service can be provided by an entity which is not the 384 operator of the underlying IP network. 386 REQ. ARv13-20: An ALTO client protocol MUST be designed in a way 387 that different instances of the ALTO service operated by different 388 providers can coexist. 390 REQ. ARv13-21: An ALTO client protocol specification MUST specify at 391 least one query mode, either the target-aware or the target- 392 independent query mode. 394 REQ. ARv13-22: An ALTO client protocol specification SHOULD specify 395 both the target-aware and the target-independent query mode. If an 396 ALTO client protocol specification specifies more than one query 397 mode, it MUST define at least one of these modes as REQUIRED to 398 implement by ALTO Clients and ALTO Servers. Furthermore, it MUST 399 specify an appropriate protocol mechanism for negotiating between 400 ALTO Client and ALTO Server, which query mode to use. 402 REQ. ARv13-23: An ALTO client protocol SHOULD support version 403 numbering, TTL (time-to-live) attributes, and/or similar mechanisms 404 in ALTO transactions, in order to enable time validity checking for 405 caching, and to enable comparisons of multiple recommendations 406 obtained through redistribution. 408 REQ. ARv13-24: An ALTO client protocol SHOULD allow the ALTO server 409 to add information about appropriate modes of re-use to its ALTO 410 responses. Re-use may include redistributing an ALTO response to 411 other parties, as well as using the same ALTO information in a 412 resource directory to improve the responses to different resource 413 consumers, within the specified lifetime of the ALTO response. The 414 ALTO server SHOULD be able to express that 416 o no re-use should occur 418 o re-use is appropriate for a specific "target audience", i.e., a 419 set of resource consumers explicitly defined by a list of host 420 group descriptors. The ALTO server MAY specify a "target 421 audience" in the ALTO response, which is only a subset of the 422 known actual "target audience", e.g., if required by operator 423 policies 425 o re-use is appropriate for any resource consumer that would send 426 (or cause a third party sending on behalf of it) the same ALTO 427 query (i.e., with the same query parameters, except for the 428 resource consumer ID, if applicable) to this ALTO server 430 o re-use is appropriate for any resource consumer that would send 431 (or cause a third party sending on behalf of it) the same ALTO 432 query (i.e., with the same query parameters, except for the 433 resource consumer ID, if applicable) to any other ALTO server, 434 which was discovered (using an ALTO discovery mechanism) together 435 with this ALTO server 437 o re-use is appropriate for any resource consumer that would send 438 (or cause a third party sending on behalf of it) the same ALTO 439 query (i.e., with the same query parameters, except for the 440 resource consumer ID, if applicable) to any ALTO server in the 441 whole network 443 REQ. ARv13-25: An ALTO client protocol MUST support the transport of 444 ALTO transactions even if the ALTO client is located in the private 445 address realm behind a network address translator (NAT). There are 446 different types of NAT, see [RFC4787] and [RFC5382]. 448 3.1.5. Protocol Extensibility 450 REQ. ARv13-26: An ALTO client protocol MUST include support for 451 adding protocol extensions in a non-disruptive, backward-compatible 452 way. 454 REQ. ARv13-27: An ALTO client protocol MUST include protocol 455 versioning support, in order to clearly distinguish between 456 incompatible versions of the protocol. 458 3.1.6. Error Handling and Overload Protection 460 REQ. ARv13-28: An ALTO client protocol MUST use TCP based transport. 462 REQ. ARv13-29: An ALTO client protocol specification MUST specify 463 mechanisms, or detail how to leverage appropriate mechanisms provided 464 by underlying protocol layers, which can be used by an ALTO server to 465 inform clients about an impending or occurring overload situation, 466 and request them to throttle their query rate. 468 In particular, a simple form of throttling is to let an ALTO server 469 answer a query with an error message advising the client to retry the 470 query later (e.g, using a protocol function such as HTTP's Retry- 471 After header ([RFC2616], section 14.37). Another simple option is to 472 actually answer the query with the desired information, but adding an 473 indication that the ALTO client should not send further queries to 474 this ALTO server before an indicated period of time has elapsed. 476 REQ. ARv13-30: An ALTO client protocol specification MUST specify 477 mechanisms, or detail how to leverage appropriate mechanisms provided 478 by underlying protocol layers, which can be used by an ALTO server to 479 inform clients about an impending or occurring overload situation, 480 and redirect them to another ALTO server. 482 REQ. ARv13-31: An ALTO client protocol specification MUST specify 483 mechanisms, or detail how to leverage appropriate mechanisms provided 484 by underlying protocol layers, which can be used by an ALTO server to 485 inform clients about an impending or occurring overload situation, 486 and terminate the conversation with the ALTO client. 488 REQ. ARv13-32: An ALTO client protocol specification MUST specify 489 mechanisms, or detail how to leverage appropriate mechanisms provided 490 by underlying protocol layers, which can be used by an ALTO server to 491 inform clients about its inability to answer queries due to technical 492 problems or system maintenance, and advise them to retry the query 493 later. 495 REQ. ARv13-33: An ALTO client protocol specification MUST specify 496 mechanisms, or detail how to leverage appropriate mechanisms provided 497 by underlying protocol layers, which can be used by an ALTO server to 498 inform clients about its inability to answer queries due to technical 499 problems or system maintenance, and redirect them to another ALTO 500 server. 502 REQ. ARv13-34: An ALTO client protocol specification MUST specify 503 mechanisms, or detail how to leverage appropriate mechanisms provided 504 by underlying protocol layers, which can be used by an ALTO server to 505 inform clients about its inability to answer queries due to technical 506 problems or system maintenance, and terminate the conversation with 507 the ALTO client. 509 Note: The existence of the above-mentioned protocol mechanisms does 510 not imply that an ALTO server must use them when facing an overload, 511 technical problem, or maintenance situation, respectively. Some 512 servers may be unable to use them in that situation, or they may 513 prefer to simply refuse the connection or not to send any answer at 514 all. 516 3.2. ALTO Server Discovery 518 An ALTO client protocol is supported by one or more ALTO server 519 discovery mechanisms, which may be used by ALTO clients in order to 520 determine one or more ALTO servers, to which ALTO requests can be 521 sent. This section enumerates requirements for an ALTO client, as 522 well as general requirements to be fulfilled by the ALTO server 523 discovery mechanisms. 525 REQ. ARv13-35: ALTO clients which are embedded in the resource 526 consumer MUST be able to use an ALTO server discovery mechanism, in 527 order to find one or several ALTO servers that can provide ALTO 528 guidance suitable for the resource consumer. This mode of operation 529 is called "resource consumer initiated ALTO server discovery". 531 REQ. ARv13-36: ALTO clients which are embedded in a resource 532 directory and perform third-party ALTO queries on behalf of a remote 533 resource consumer MUST be able to use an ALTO server discovery 534 mechanism, in order to find one or several ALTO servers that can 535 provide ALTO guidance suitable for the respective resource consumer. 536 This mode of operation is called "third-party ALTO server discovery". 538 REQ. ARv13-37: ALTO clients MUST be able to perform resource 539 consumer initiated ALTO server discovery, even if they are located 540 behind a network address translator (NAT). 542 REQ. ARv13-38: ALTO clients MUST be able to perform third-party ALTO 543 server discovery, even if they are located behind a network address 544 translator (NAT). 546 REQ. ARv13-39: ALTO clients MUST be able to perform third-party ALTO 547 server discovery, even if the resource consumer, on behalf of which 548 the ALTO query will be sent, is located behind a network address 549 translator (NAT). 551 REQ. ARv13-40: ALTO server discovery mechanisms SHOULD leverage an 552 existing protocol or mechanism, such as DNS, DHCP, or PPP based 553 automatic configuration, etc. A single mechanism with a broad 554 spectrum of applicability SHOULD be preferred over several different 555 mechanisms with narrower scopes. 557 REQ. ARv13-41: Every ALTO server discovery mechanism SHOULD be able 558 to return the respective contact information for multiple ALTO 559 servers. 561 REQ. ARv13-42: Every ALTO server discovery mechanism SHOULD be able 562 to indicate preferences for each returned ALTO server contact 563 information. 565 3.3. Security and Privacy 567 Note: The following requirements mandate the inclusion of certain 568 security mechanisms at a protocol specification level. Whether it 569 makes sense to enable these mechanisms in a given deployment scenario 570 depends on a threat analysis for this specific scenario. 572 REQ. ARv13-43: An ALTO client protocol specification MUST specify 573 mechanisms for the authentication of ALTO servers, or how to leverage 574 appropriate mechanisms provided by underlying protocol layers. 576 REQ. ARv13-44: An ALTO client protocol specification MUST specify 577 mechanisms for the authentication of ALTO clients, or how to leverage 578 appropriate mechanisms provided by underlying protocol layers. 580 REQ. ARv13-45: An ALTO client protocol specification MUST specify 581 mechanisms for the encryption of messages, or how to leverage 582 appropriate mechanisms provided by underlying protocol layers. 584 REQ. ARv13-46: An ALTO client is not required to implement 585 mechanisms or to comply with rules that limit its ability to 586 redistribute information retrieved from the ALTO server to third 587 parties. 589 REQ. ARv13-47: An ALTO client protocol MUST support different levels 590 of detail in queries and responses, in order to protect the privacy 591 of users, to ensure that the operators of ALTO servers and other 592 users of the same application cannot derive sensitive information. 594 REQ. ARv13-48: An ALTO client protocol MAY include mechanisms that 595 can be used by the ALTO client when requesting guidance to specify 596 the resource (e.g., content identifiers) it wants to access. An ALTO 597 server MUST provide adequate guidance even if the ALTO client prefers 598 not to specify the desired resource (e.g., keeps the data field 599 empty). The mechanism MUST be designed in a way that the operator of 600 the ALTO server cannot easily deduce the resource identifier (e.g., 601 file name in P2P file sharing) if the ALTO client prefers not to 602 specify it. 604 REQ. ARv13-49: An ALTO client protocol specification MUST specify 605 appropriate mechanisms for protecting the ALTO service against DoS 606 attacks, or how to leverage appropriate mechanisms provided by 607 underlying protocol layers. 609 4. IANA Considerations 611 This requirements document does not mandate any immediate IANA 612 actions. However, such IANA considerations may arise from future 613 ALTO specification documents which try to meet the requirements given 614 here. 616 5. Security Considerations 618 5.1. High-level security considerations 620 High-level security considerations for the ALTO service can be found 621 in the "Security Considerations" section of the ALTO problem 622 statement document [RFC5693]. 624 5.2. Information Disclosure Scenarios 626 The unwanted disclosure of information is one key concern related to 627 ALTO. From a user privacy perspective, neither the ALTO server nor a 628 third party using or misusing the ALTO service should be able to 629 infer the application behavior, e.g., who is exchanging which files 630 with whom using a P2P file sharing application. Many network 631 operators, in contrast, are concerned about the amount of information 632 related to their network infrastructure (e.g., topology information, 633 number of "premium customers", or utilization statistics) that might 634 be released through ALTO. This section presents a classification and 635 discussion of information disclosure scenarios and potential 636 countermeasures. 638 5.2.1. Classification of Information Disclosure Scenarios 640 o (1) Excess disclosure of ALTO server operator's data to an 641 authorized ALTO client. The operator of an ALTO server has to 642 feed information, such as tables mapping host group descriptors to 643 host characteristics attributes, into the server, thereby enabling 644 it to give guidance to ALTO clients. Some operators might 645 consider the full set of this information confidential (e.g., a 646 detailed map of the operator's network topology), and might want 647 to disclose only a subset of it or somehow obfuscated information 648 to an ALTO client. 650 o (2) Disclosure of the application behavior to the ALTO server. 651 The operator of an ALTO server could infer the application 652 behavior (e.g., content identifiers in P2P file sharing 653 applications, or lists of resource providers that are considered 654 for establishing a connection) from the ALTO queries sent by an 655 ALTO client. 657 o (3) Disclosure of ALTO server operator's data (e.g., network 658 topology information) to an unauthorized third party. There are a 659 three sub-cases here: 661 * (3a) An ALTO server sends the information directly to an 662 unauthorized ALTO client. 664 * (3b) An unauthorized party snoops on the data transmission from 665 the ALTO server to an authorized ALTO client. 667 * (3c) An authorized ALTO client knowingly forwards the 668 information it had received from the ALTO server to an 669 unauthorized party. 671 o (4) Disclosure of the application behavior to an unauthorized 672 third party. 674 o (5) Excess retrieval of ALTO server operator's data by 675 collaborating ALTO clients. Several authorized ALTO clients could 676 ask an ALTO server for guidance, and redistribute the responses 677 among each other (see also case 3c). By correlating the ALTO 678 responses they could find out more information than intended to be 679 disclosed by the ALTO server operator. 681 5.2.2. Discussion of Information Disclosure Scenarios 683 Scenario (1) may be addressed by the ALTO server operator choosing 684 the level of detail of the information to be populated into the ALTO 685 server and returned in the responses. For example, by specifying a 686 broader address range (i.e., a shorter prefix length) than a group of 687 hosts in question actually uses, an ALTO server operator may control 688 to some extent how much information about the network topology is 689 disclosed. Furthermore, access control mechanisms for filtering ALTO 690 responses according to the authenticated ALTO client identity might 691 be installed in the ALTO server, although this might not be effective 692 given the lack of efficient mechanisms for addressing (3c) and (5), 693 see below. 695 (2) can and needs to be addressed in several ways: If the ALTO client 696 is embedded in the resource consumer, the resource consumer's IP 697 address (or the "public" IP address of the outermost NAT in front of 698 the resource consumer) is disclosed to the ALTO server as a matter of 699 principle, because it is in the source address fields of the IP 700 headers. By using a proxy, the disclosure of source addresses to the 701 ALTO server can be avoided at the cost of disclosing them to said 702 proxy. If, in contrast, the ALTO client is embedded in a third party 703 (e.g., a resource directory) which issues ALTO requests on behalf of 704 resource consumers, it is possible to hide the exact addresses of the 705 resource consumers from the ALTO server, e.g., by zeroing-out or 706 randomizing the last few bits of IP addresses. However, there is the 707 potential side effect of yielding inaccurate results. 709 The disclosure of candidate resource providers' addresses to the ALTO 710 server can be avoided by allowing ALTO clients to use the target- 711 independent query mode. In this mode of operation, guiding 712 information (e.g., "maps") is retrieved from the ALTO server and used 713 entirely locally by the ALTO client, i.e., without sending host 714 location attributes of candidate resource providers to the ALTO 715 server. In the target-aware query mode, this issue can be addressed 716 by ALTO clients through obfuscating the identity of candidate 717 resource consumers, e.g., by specifying a broader address range 718 (i.e., a shorter prefix length) than a group of hosts in question 719 actually uses, or by zeroing-out or randomizing the last few bits of 720 IP addresses. However, there is the potential side effect of 721 yielding inaccurate results. 723 (3a), (3b), and (4) may be addressed by authentication, access 724 control, and encryption schemes for the ALTO client protocol. 725 However, deployment of encryption schemes might not be effective 726 given the lack of efficient mechanisms for addressing (3c) and (5), 727 see below. 729 Straightforward authentication and encryption schemes will not help 730 solving (3c) and (5), and there is no other simple and efficient 731 mechanism known. The cost of complex approaches, e.g., based on 732 digital rights management (DRM), might easily outweigh the benefits 733 of the whole ALTO solution, and therefore they are not considered as 734 a viable solution. That is, ALTO server operators must be aware that 735 (3c) and (5) cannot be prevented from happening, and therefore they 736 should feed only such data into an ALTO server, which they do not 737 consider sensitive with respect to (3c) and (5). 739 These insights are reflected in the requirements in this document. 741 5.3. Security Requirements 743 For a set of specific security requirements please refer to 744 Section 3.3 of this document. 746 6. References 748 6.1. Normative References 750 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 751 Requirement Levels", BCP 14, RFC 2119, March 1997. 753 6.2. Informative References 755 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 756 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 757 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 759 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 760 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 761 RFC 4787, January 2007. 763 [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. 764 Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, 765 RFC 5382, October 2008. 767 [RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic 768 Optimization (ALTO) Problem Statement", RFC 5693, 769 October 2009. 771 Appendix A. Contributors List and Acknowledgments 773 The initial version of this document was co-authored by Laird Popkin. 775 The authors would like to thank 777 o Vijay K. Gurbani 779 o Enrico Marocco 781 for fostering discussions that lead to the creation of this document, 782 and for giving valuable comments on it. 784 The authors were supported by the following people, who have 785 contributed to this document: 787 o Richard Alimi 789 o Zoran Despotovic 791 o Jason Livingood 793 o Saverio Niccolini 795 o Michael Scharf 797 o Nico Schwan 799 o Jan Seedorf 801 The authors would like to thank the members of the P2PI and ALTO 802 mailing lists for their feedback. 804 Laird Popkin and Y. Richard Yang are grateful to the many 805 contributions made by the members of the P4P working group and Yale 806 Laboratory of Networked Systems. The P4P working group is hosted by 807 DCIA. 809 Martin Stiemerling is partially supported by the COAST project 810 (COntent Aware Searching, retrieval and sTreaming, 811 http://www.coast-fp7.eu), a research project supported by the 812 European Commission under its 7th Framework Program (contract no. 813 248036). The views and conclusions contained herein are those of the 814 authors and should not be interpreted as necessarily representing the 815 official policies or endorsements, either expressed or implied, of 816 the COAST project or the European Commission. 818 Authors' Addresses 820 Sebastian Kiesel (editor) 821 University of Stuttgart Computing Center 822 Networks and Communication Systems Department 823 Allmandring 30 824 70550 Stuttgart 825 Germany 827 Email: ietf-alto@skiesel.de 828 URI: http://www.rus.uni-stuttgart.de/nks/ 830 Stefano Previdi 831 Cisco Systems, Inc. 833 Email: sprevidi@cisco.com 835 Martin Stiemerling 836 NEC Laboratories Europe 838 Email: martin.stiemerling@neclab.eu 839 URI: http://ietf.stiemerling.org 841 Richard Woundy 842 Comcast Corporation 844 Email: Richard_Woundy@cable.comcast.com 846 Yang Richard Yang 847 Yale University 849 Email: yry@cs.yale.edu