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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTPbis Working Group M. Nottingham 3 Internet-Draft Akamai 4 Intended status: Standards Track P. McManus 5 Expires: September 29, 2014 Mozilla 6 J. Reschke 7 greenbytes 8 March 28, 2014 10 HTTP Alternative Services 11 draft-ietf-httpbis-alt-svc-00 13 Abstract 15 This document specifies "alternative services" for HTTP, which allow 16 an origin's resources to be authoritatively available at a separate 17 network location, possibly accessed with a different protocol 18 configuration. 20 Editorial Note (To be removed by RFC Editor) 22 Discussion of this draft takes place on the HTTPBIS working group 23 mailing list (ietf-http-wg@w3.org), which is archived at 24 . 26 Working Group information can be found at 27 ; that specific to HTTP/2 are at 28 . 30 The changes in this draft are summarized in Appendix A. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at http://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on September 29, 2014. 49 Copyright Notice 51 Copyright (c) 2014 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (http://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 67 1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3 68 2. Alternative Services Concepts . . . . . . . . . . . . . . . . 4 69 2.1. Host Authentication . . . . . . . . . . . . . . . . . . . 5 70 2.2. Alternative Service Caching . . . . . . . . . . . . . . . 6 71 2.3. Requiring Server Name Indication . . . . . . . . . . . . . 6 72 2.4. Using Alternative Services . . . . . . . . . . . . . . . . 6 73 3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 7 74 3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 8 75 4. The Service HTTP Header Field . . . . . . . . . . . . . . . . 9 76 5. The 4NN Not Authoritative HTTP Status Code . . . . . . . . . . 9 77 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 78 6.1. The Alt-Svc Message Header Field . . . . . . . . . . . . . 10 79 6.2. The Service Message Header Field . . . . . . . . . . . . . 10 80 6.3. The 4NN Not Authoritative HTTP Status Code . . . . . . . . 10 81 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 82 7.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . . 11 83 7.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 11 84 7.3. Changing Protocols . . . . . . . . . . . . . . . . . . . . 12 85 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 86 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 87 9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 88 9.2. Informative References . . . . . . . . . . . . . . . . . . 13 89 Appendix A. Change Log (to be removed by RFC Editor before 90 publication) . . . . . . . . . . . . . . . . . . . . 14 91 A.1. Since draft-nottingham-httpbis-alt-svc-05 . . . . . . . . 14 93 1. Introduction 95 HTTP [HTTP-p1] conflates the identification of resources with their 96 location. In other words, "http://" (and "https://") URLs are used 97 to both name and find things to interact with. 99 In some cases, it is desirable to separate these aspects; to be able 100 to keep the same identifier for a resource, but interact with it 101 using a different location on the network. 103 For example: 105 o An origin server might wish to redirect a client to an alternative 106 when it needs to go down for maintenance, or it has found an 107 alternative in a location that is more local to the client. 109 o An origin server might wish to offer access to its resources using 110 a new protocol (such as HTTP/2, see [HTTP2]) or one using improved 111 security (such as TLS, see [RFC5246]). 113 o An origin server might wish to segment its clients into groups of 114 capabilities, such as those supporting Server Name Indication 115 (SNI, see Section 3 of [RFC6066]) and those not supporting it, for 116 operational purposes. 118 This specification defines a new concept in HTTP, "Alternative 119 Services", that allows a resource to nominate additional means of 120 interacting with it on the network. It defines a general framework 121 for this in Section 2, along with a specific mechanism for 122 discovering them using HTTP header fields in Section 3. 124 It also introduces a new status code in Section 5, so that origin 125 servers (or their nominated alternatives) can indicate that they are 126 not authoritative for a given origin, in cases where the wrong 127 location is used. 129 1.1. Notational Conventions 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 This document uses the Augmented BNF defined in [RFC5234] along with 136 the "OWS", "DIGIT", "DQUOTE", "parameter", "uri-host", "port" and 137 "delta-second" rules from [HTTP-p1], and uses the "#rule" extension 138 defined in Section 7 of that document. 140 2. Alternative Services Concepts 142 This specification defines a new concept in HTTP, the "alternative 143 service". When an origin (see [RFC6454]) has resources that are 144 accessible through a different protocol / host / port combination, it 145 is said to have an alternative service. 147 An alternative service can be used to interact with the resources on 148 an origin server at a separate location on the network, possibly 149 using a different protocol configuration. Alternative services are 150 considered authoritative for an origin's resources, in the sense of 151 [HTTP-p1], Section 9.1. 153 For example, an origin: 155 ("http", "www.example.com", "80") 157 might declare that its resources are also accessible at the 158 alternative service: 160 ("h2", "new.example.com", "81") 162 By their nature, alternative services are explicitly at the 163 granularity of an origin; i.e., they cannot be selectively applied to 164 resources within an origin. 166 Alternative services do not replace or change the origin for any 167 given resource; in general, they are not visible to the software 168 "above" the access mechanism. The alternative service is essentially 169 alternative routing information that can also be used to reach the 170 origin in the same way that DNS CNAME or SRV records define routing 171 information at the name resolution level. Each origin maps to a set 172 of these routes - the default route is derived from origin itself and 173 the other routes are introduced based on alternative-protocol 174 information. 176 Furthermore, it is important to note that the first member of an 177 alternative service tuple is different from the "scheme" component of 178 an origin; it is more specific, identifying not only the major 179 version of the protocol being used, but potentially communication 180 options for that protocol. 182 This means that clients using an alternative service will change the 183 host, port and protocol that they are using to fetch resources, but 184 these changes MUST NOT be propagated to the application that is using 185 HTTP; from that standpoint, the URI being accessed and all 186 information derived from it (scheme, host, port) are the same as 187 before. 189 Importantly, this includes its security context; in particular, when 190 TLS [RFC5246] is in use, the alternative server will need to present 191 a certificate for the origin's host name, not that of the 192 alternative. Likewise, the Host header field is still derived from 193 the origin, not the alternative service (just as it would if a CNAME 194 were being used). 196 The changes MAY, however, be made visible in debugging tools, 197 consoles, etc. 199 Formally, an alternative service is identified by the combination of: 201 o An ALPN protocol, as per [I-D.ietf-tls-applayerprotoneg] 203 o A host, as per [RFC3986] 205 o A port, as per [RFC3986] 207 Additionally, each alternative service MUST have: 209 o A freshness lifetime, expressed in seconds; see Section 2.2 211 There are many ways that a client could discover the alternative 212 service(s) associated with an origin. 214 2.1. Host Authentication 216 Clients MUST NOT use alternative services with a host other than the 217 origin's without strong server authentication; this mitigates the 218 attack described in Section 7.2. One way to achieve this is for the 219 alternative to use TLS with a certificate that is valid for that 220 origin. 222 For example, if the origin's host is "www.example.com" and an 223 alternative is offered on "other.example.com" with the "h2" protocol, 224 and the certificate offered is valid for "www.example.com", the 225 client can use the alternative. However, if "other.example.com" is 226 offered with the "h2c" protocol, the client cannot use it, because 227 there is no mechanism in that protocol to establish strong server 228 authentication. 230 Furthermore, this means that the HTTP Host header field and the SNI 231 information provided in TLS by the client will be that of the origin, 232 not the alternative. 234 2.2. Alternative Service Caching 236 Mechanisms for discovering alternative services can associate a 237 freshness lifetime with them; for example, the Alt-Svc header field 238 uses the "ma" parameter. 240 Clients MAY choose to use an alternative service instead of the 241 origin at any time when it is considered fresh; see Section 2.4 for 242 specific recommendations. 244 Clients with existing connections to alternative services are not 245 required to fall back to the origin when its freshness lifetime ends; 246 i.e., the caching mechanism is intended for limiting how long an 247 alternative service can be used for establishing new requests, not 248 limiting the use of existing ones. 250 To mitigate risks associated with caching compromised values (see 251 Section 7.2 for details), user agents SHOULD examine cached 252 alternative services when they detect a change in network 253 configuration, and remove any that could be compromised (for example, 254 those whose association with the trust root is questionable). UAs 255 that do not have a means of detecting network changes SHOULD place an 256 upper bound on their lifetime. 258 2.3. Requiring Server Name Indication 260 A client must only use a TLS-based alternative service if the client 261 also supports TLS Server Name Indication (SNI) ([RFC6066], Section 262 3). This supports the conservation of IP addresses on the 263 alternative service host. 265 2.4. Using Alternative Services 267 By their nature, alternative services are optional; clients are not 268 required to use them. However, it is advantageous for clients to 269 behave in a predictable way when they are used by servers (e.g., for 270 load balancing). 272 Therefore, if a client becomes aware of an alternative service, the 273 client SHOULD use that alternative service for all requests to the 274 associated origin as soon as it is available, provided that the 275 security properties of the alternative service protocol are 276 desirable, as compared to the existing connection. 278 When a client uses an alternate service, it MUST emit the Service 279 header field (Section 4) on every request using that alternate 280 service. 282 The client is not required to block requests; the origin's connection 283 can be used until the alternative connection is established. 284 However, if the security properties of the existing connection are 285 weak (e.g. cleartext HTTP/1.1) then it might make sense to block 286 until the new connection is fully available in order to avoid 287 information leakage. 289 Furthermore, if the connection to the alternative service fails or is 290 unresponsive, the client MAY fall back to using the origin. Note, 291 however, that this could be the basis of a downgrade attack, thus 292 losing any enhanced security properties of the alternative service. 294 3. The Alt-Svc HTTP Header Field 296 An HTTP(S) origin server can advertise the availability of 297 alternative services (see Section 2) to clients by adding an Alt-Svc 298 header field to responses. 300 Alt-Svc = 1#( alternative *( OWS ";" OWS parameter ) ) 301 alternative = DQUOTE protocol-id DQUOTE "=" port 302 protocol-id = 304 For example: 306 Alt-Svc: "http2"=8000 308 This indicates that the "http2" protocol on the same host using the 309 indicated port (in this case, 8000). 311 Alt-Svc MAY occur in any HTTP response message, regardless of the 312 status code. 314 Alt-Svc does not allow advertisement of alternative services on other 315 hosts, to protect against various header-based attacks. 317 It can, however, have multiple values: 319 Alt-Svc: "h2c"=8000, "h2"=443 321 The value(s) advertised by Alt-Svc can be used by clients to open a 322 new connection to one or more alternative services immediately, or 323 simultaneously with subsequent requests on the same connection. 325 Intermediaries MUST NOT change or append Alt-Svc values. 327 Finally, note that while it may be technically possible to put 328 content other than printable ASCII in a HTTP header field, some 329 implementations only support ASCII (or a superset of it) in header 330 field values. Therefore, this field SHOULD NOT be used to convey 331 protocol identifiers that are not printable ASCII, or those that 332 contain quote characters. 334 [[syntax: The header field syntax is both misleading (use of double 335 quotes although not a quoted string) and incomplete (does not support 336 all values). Alternate proposal in .]] 339 3.1. Caching Alt-Svc Header Field Values 341 When an alternative service is advertised using Alt-Svc, it is 342 considered fresh for 24 hours from generation of the message. This 343 can be modified with the 'ma' (max-age) parameter; 345 Alt-Svc: "h2"=443;ma=3600 347 which indicates the number of seconds since the response was 348 generated the alternative service is considered fresh for. 350 ma = delta-seconds 352 See Section 4.2.3 of [HTTP-p6] for details of determining response 353 age. 355 For example, a response: 357 HTTP/1.1 200 OK 358 Content-Type: text/html 359 Cache-Control: 600 360 Age: 30 361 Alt-Svc: "h2c"=8000; ma=60 363 indicates that an alternative service is available and usable for the 364 next 60 seconds. However, the response has already been cached for 365 30 seconds (as per the Age header field value), so therefore the 366 alternative service is only fresh for the 30 seconds from when this 367 response was received, minus estimated transit time. 369 When an Alt-Svc response header field is received from an origin, its 370 value invalidates and replaces all cached alternative services for 371 that origin. 373 See Section 2.2 for general requirements on caching alternative 374 services. 376 Note that the freshness lifetime for HTTP caching (here, 600 seconds) 377 does not affect caching of Alt-Svc values. 379 4. The Service HTTP Header Field 381 The Service HTTP header field is used in requests to indicate the 382 identity of the alternate service in use, just as the Host header 383 field identifies the host and port of the origin. 385 Service = uri-host [ ":" port ] 387 Service is intended to allow alternate services to detect loops, 388 differentiate traffic for purposes of load balancing, and generally 389 to ensure that it is possible to identify the intended destination of 390 traffic, since introducing this information after a protocol is in 391 use has proven to be problematic. 393 When using an Alternate Service, clients MUST include a Service 394 header in all requests. 396 For example: 398 GET /thing 399 Host: origin.example.com 400 Service: alternate.example.net 401 User-Agent: Example/1.0 403 5. The 4NN Not Authoritative HTTP Status Code 405 The 4NN (Not Authoritative) status code indicates that the current 406 origin server (usually, but not always an alternative service; see 407 Section 2) is not authoritative for the requested resource, in the 408 sense of [HTTP-p1], Section 9.1. 410 Clients receiving 4NN (Not Authoritative) from an alternative service 411 MUST remove the corresponding entry from its alternative service 412 cache (see Section 2.2) for that origin. Regardless of the 413 idempotency of the request method, they MAY retry the request, either 414 at another alternative server, or at the origin. 416 4NN (Not Authoritative) MAY carry an Alt-Svc header field. 418 This status code MUST NOT be generated by proxies. 420 A 4NN response is cacheable by default; i.e., unless otherwise 421 indicated by the method definition or explicit cache controls (see 422 Section 4.2.2 of [HTTP-p6]). 424 [[code: We should decide on the status code before Last Call.]] 426 6. IANA Considerations 428 6.1. The Alt-Svc Message Header Field 430 This document registers Alt-Svc in the Permanent Message Header 431 Registry [RFC3864]. 433 o Header Field Name: Alt-Svc 435 o Application Protocol: http 437 o Status: standard 439 o Author/Change Controller: IETF 441 o Specification Document: [this document] 443 o Related Information: 445 6.2. The Service Message Header Field 447 This document registers Alt-Svc in the Permanent Message Header 448 Registry [RFC3864]. 450 o Header Field Name: Service 452 o Application Protocol: http 454 o Status: standard 456 o Author/Change Controller: IETF 458 o Specification Document: [this document] 460 o Related Information: 462 6.3. The 4NN Not Authoritative HTTP Status Code 464 This document registers the 4NN (Not Authoritative) HTTP Status code 465 ([HTTP-p2], Section 8.2). 467 o Status Code: 4NN 469 o Short Description: Not Authoritative 471 o Specification: [this document], Section 5 473 7. Security Considerations 475 Identified security considerations should be enumerated in the 476 appropriate documents depending on which proposals are accepted. 477 Those listed below are generic to all uses of alternative services; 478 more specific ones might be necessary. 480 7.1. Changing Ports 482 Using an alternative service implies accessing an origin's resources 483 on an alternative port, at a minimum. An attacker that can inject 484 alternative services and listen at the advertised port is therefore 485 able to hijack an origin. 487 For example, an attacker that can add HTTP response header fields can 488 redirect traffic to a different port on the same host using the Alt- 489 Svc header field; if that port is under the attacker's control, they 490 can thus masquerade as the HTTP server. 492 This risk can be mitigated by restricting the ability to advertise 493 alternative services, and restricting who can open a port for 494 listening on that host. 496 7.2. Changing Hosts 498 When the host is changed due to the use of an alternative service, it 499 presents an opportunity for attackers to hijack communication to an 500 origin. 502 For example, if an attacker can convince a user agent to send all 503 traffic for "innocent.example.org" to "evil.example.com" by 504 successfully associating it as an alternative service, they can 505 masquerade as that origin. This can be done locally (see mitigations 506 above) or remotely (e.g., by an intermediary as a man-in-the-middle 507 attack). 509 This is the reason for the requirement in Section 2.1 that any 510 alternative service with a host different to the origin's be strongly 511 authenticated with the origin's identity; i.e., presenting a 512 certificate for the origin proves that the alternative service is 513 authorized to serve traffic for the origin. 515 However, this authorization is only as strong as the method used to 516 authenticate the alternative service. In particular, there are well- 517 known exploits to make an attacker's certificate appear as 518 legitimate. 520 Alternative services could be used to persist such an attack; for 521 example, an intermediary could man-in-the-middle TLS-protected 522 communication to a target, and then direct all traffic to an 523 alternative service with a large freshness lifetime, so that the user 524 agent still directs traffic to the attacker even when not using the 525 intermediary. 527 As a result, there is a requirement in Section 2.2 to examine cached 528 alternative services when a network change is detected. 530 7.3. Changing Protocols 532 When the ALPN protocol is changed due to the use of an alternative 533 service, the security properties of the new connection to the origin 534 can be different from that of the "normal" connection to the origin, 535 because the protocol identifier itself implies this. 537 For example, if a "https://" URI had a protocol advertised that does 538 not use some form of end-to-end encryption (most likely, TLS), it 539 violates the expectations for security that the URI scheme implies. 541 Therefore, clients cannot blindly use alternative services, but 542 instead evaluate the option(s) presented to assure that security 543 requirements and expectations (of specifications, implementations and 544 end users) are met. 546 8. Acknowledgements 548 Thanks to Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin, 549 Erik Nygren, Paul Hoffman, Adam Langley, Will Chan and Richard Barnes 550 for their feedback and suggestions. 552 The Alt-Svc header field was influenced by the design of the 553 Alternative-Protocol header field in SPDY. 555 9. References 557 9.1. Normative References 559 [HTTP-p1] Fielding, R., Ed. and J. Reschke, 560 Ed., "Hypertext Transfer Protocol 561 (HTTP/1.1): Message Syntax and 562 Routing", 563 draft-ietf-httpbis-p1-messaging-26 564 (work in progress), February 2014. 566 [HTTP-p6] Fielding, R., Ed., Nottingham, M., 567 Ed., and J. Reschke, Ed., "Hypertext 568 Transfer Protocol (HTTP/1.1): 570 Caching", 571 draft-ietf-httpbis-p6-cache-26 (work 572 in progress), February 2014. 574 [I-D.ietf-tls-applayerprotoneg] Friedl, S., Popov, A., Langley, A., 575 and S. Emile, "Transport Layer 576 Security (TLS) Application Layer 577 Protocol Negotiation Extension", 578 draft-ietf-tls-applayerprotoneg-05 579 (work in progress), March 2014. 581 [RFC2119] Bradner, S., "Key words for use in 582 RFCs to Indicate Requirement 583 Levels", BCP 14, RFC 2119, 584 March 1997. 586 [RFC3986] Berners-Lee, T., Fielding, R., and 587 L. Masinter, "Uniform Resource 588 Identifier (URI): Generic Syntax", 589 STD 66, RFC 3986, January 2005. 591 [RFC5234] Crocker, D. and P. Overell, 592 "Augmented BNF for Syntax 593 Specifications: ABNF", STD 68, 594 RFC 5234, January 2008. 596 [RFC6066] Eastlake, D., "Transport Layer 597 Security (TLS) Extensions: Extension 598 Definitions", RFC 6066, 599 January 2011. 601 [RFC6454] Barth, A., "The Web Origin Concept", 602 RFC 6454, December 2011. 604 9.2. Informative References 606 [HTTP-p2] Fielding, R., Ed. and J. Reschke, 607 Ed., "Hypertext Transfer Protocol 608 (HTTP/1.1): Semantics and Content", 609 draft-ietf-httpbis-p2-semantics-26 610 (work in progress), February 2014. 612 [HTTP2] Belshe, M., Peon, R., and M. 613 Thomson, Ed., "Hypertext Transfer 614 Protocol version 2", 615 draft-ietf-httpbis-http2-10 (work in 616 progress), February 2014. 618 [RFC3864] Klyne, G., Nottingham, M., and J. 619 Mogul, "Registration Procedures for 620 Message Header Fields", BCP 90, 621 RFC 3864, September 2004. 623 [RFC5246] Dierks, T. and E. Rescorla, "The 624 Transport Layer Security (TLS) 625 Protocol Version 1.2", RFC 5246, 626 August 2008. 628 Appendix A. Change Log (to be removed by RFC Editor before publication) 630 A.1. Since draft-nottingham-httpbis-alt-svc-05 632 This is the first version after adoption of 633 draft-nottingham-httpbis-alt-svc-05 as Working Group work item. It 634 only contains editorial changes. 636 Authors' Addresses 638 Mark Nottingham 639 Akamai 641 EMail: mnot@mnot.net 642 URI: http://www.mnot.net/ 644 Patrick McManus 645 Mozilla 647 EMail: mcmanus@ducksong.com 648 URI: https://mozillians.org/u/pmcmanus/ 650 Julian F. Reschke 651 greenbytes GmbH 653 EMail: julian.reschke@greenbytes.de 654 URI: http://greenbytes.de/tech/webdav/