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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4366 (ref. '4') (Obsoleted by RFC 5246, RFC 6066) ** Obsolete normative reference: RFC 5246 (ref. '5') (Obsoleted by RFC 8446) -- Obsolete informational reference (is this intentional?): RFC 2818 (ref. '7') (Obsoleted by RFC 9110) -- Obsolete informational reference (is this intentional?): RFC 4474 (ref. '9') (Obsoleted by RFC 8224) Summary: 3 errors (**), 0 flaws (~~), 2 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIP V. Gurbani 3 Internet-Draft Bell Laboratories, Alcatel-Lucent 4 Updates: RFC3261 S. Lawrence 5 (if approved) Avaya, Inc. 6 Intended status: Standards Track A. Jeffrey 7 Expires: September 23, 2010 Bell Laboratories, Alcatel-Lucent 8 March 22, 2010 10 Domain Certificates in the Session Initiation Protocol (SIP) 11 draft-ietf-sip-domain-certs-06 13 Abstract 15 This document describes how to construct and interpret certain 16 information in a X.509 PKIX-compliant certificate for use in a 17 Session Initiation Protocol (SIP) over Transport Layer Security (TLS) 18 connection. More specifically, this document describes how to encode 19 and extract the identity of a SIP domain in a certificate and how to 20 use that identity for SIP domain authentication. As such, this 21 document is relevant both to implementors of SIP and to issuers of 22 certificates. 24 Status of this Memo 26 This Internet-Draft is submitted to IETF in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF), its areas, and its working groups. Note that 31 other groups may also distribute working documents as Internet- 32 Drafts. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 The list of current Internet-Drafts can be accessed at 40 http://www.ietf.org/ietf/1id-abstracts.txt. 42 The list of Internet-Draft Shadow Directories can be accessed at 43 http://www.ietf.org/shadow.html. 45 This Internet-Draft will expire on September 23, 2010. 47 Copyright Notice 48 Copyright (c) 2010 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the BSD License. 61 This document may contain material from IETF Documents or IETF 62 Contributions published or made publicly available before November 63 10, 2008. The person(s) controlling the copyright in some of this 64 material may not have granted the IETF Trust the right to allow 65 modifications of such material outside the IETF Standards Process. 66 Without obtaining an adequate license from the person(s) controlling 67 the copyright in such materials, this document may not be modified 68 outside the IETF Standards Process, and derivative works of it may 69 not be created outside the IETF Standards Process, except to format 70 it for publication as an RFC or to translate it into languages other 71 than English. 73 Table of Contents 75 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 76 1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 4 77 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 78 3. Problem statement . . . . . . . . . . . . . . . . . . . . . . 4 79 4. SIP domain to host resolution . . . . . . . . . . . . . . . . 6 80 5. The need for mutual interdomain authentication . . . . . . . . 7 81 6. Certificate usage by a SIP service provider . . . . . . . . . 8 82 7. Behavior of SIP entities . . . . . . . . . . . . . . . . . . . 9 83 7.1. Finding SIP Identities in a Certificate . . . . . . . . . 9 84 7.2. Comparing SIP Identities . . . . . . . . . . . . . . . . . 10 85 7.3. Client behavior . . . . . . . . . . . . . . . . . . . . . 11 86 7.4. Server behavior . . . . . . . . . . . . . . . . . . . . . 12 87 7.5. Proxy behavior . . . . . . . . . . . . . . . . . . . . . . 13 88 7.6. Registrar behavior . . . . . . . . . . . . . . . . . . . . 13 89 7.7. Redirect server behavior . . . . . . . . . . . . . . . . . 13 90 7.8. Virtual SIP Servers and Certificate Content . . . . . . . 13 91 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 92 8.1. Connection authentication using Digest . . . . . . . . . . 14 93 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 94 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 95 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 96 11.1. Normative References . . . . . . . . . . . . . . . . . . . 15 97 11.2. Informative References . . . . . . . . . . . . . . . . . . 16 98 Appendix A. Editorial guidance (non-normative) . . . . . . . . . 16 99 A.1. Additions . . . . . . . . . . . . . . . . . . . . . . . . 17 100 A.2. Changes . . . . . . . . . . . . . . . . . . . . . . . . . 17 101 A.2.1. 26.3.1 . . . . . . . . . . . . . . . . . . . . . . . . 17 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 104 1. Terminology 106 1.1. Key Words 108 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 109 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 110 document are to be interpreted as described in RFC 2119 [1]. 112 Additional definition(s): 114 SIP domain identity: An identity (e.g., "sip:example.com") contained 115 in an X.509 certificate bound to a subject that identifies the 116 subject as an authoritative SIP server for a domain. 118 2. Introduction 120 RFC 5246 [5] Transport Layer Security (TLS) is available in an 121 increasing number of Session Initiation Protocol (SIP) RFC 3261 [2] 122 implementations. In order to use the authentication capabilities of 123 TLS, certificates as defined by the Internet X.509 Public Key 124 Infrastructure RFC 5280 [6] are required. 126 Existing SIP specifications do not sufficiently specify how to use 127 certificates for domain (as opposed to host) authentication. This 128 document provides guidance to ensure interoperability and uniform 129 conventions for the construction and interpretation of certificates 130 used to identify their holders as being authoritative for the domain. 132 The discussion in this document is pertinent to an X.509 PKIX- 133 compliant certificate used for a TLS connection; this document does 134 not define use of such certificates for any other purpose (such as 135 S/MIME). 137 3. Problem statement 139 TLS uses RFC 5280 [6] X.509 Public Key Infrastructure to bind an 140 identity or a set of identities, to the subject of a X.509 141 certificate. While RFC3261 provides adequate guidance on the use of 142 X.509 certificates used for S/MIME, it is relatively silent on the 143 use of such certificates for TLS. With respect to certificates for 144 TLS, RFC3261 (Section 26.3.1) says: 146 "Proxy servers, redirect servers and registrars SHOULD possess a 147 site certificate whose subject corresponds to their canonical 148 hostname." 150 The security properties of TLS and S/MIME as used in SIP are 151 different: X.509 certificates for S/MIME are generally used for end- 152 to-end authentication and encryption, thus they serve to bind the 153 identity of a user to the certificate and RFC3261 is sufficiently 154 clear that in certificates used for S/MIME, the subjectAltName field 155 will contain the appropriate identity. On the other hand, X.509 156 certificates used for TLS serve to bind the identities of the per-hop 157 domain sending or receiving the SIP messages. However, the lack of 158 guidelines in RFC3261 on exactly where to put identities -- in the 159 subjectAltName field or carried as a Common Name (CN) in the Subject 160 field -- of a X.509 certificates created ambiguities. Following the 161 accepted practice of the time, legacy X.509 certificates were allowed 162 to store the identity in the CN field of the certificate instead of 163 the currently specified subjectAltName extension. Lack of further 164 guidelines on how to interpret the identities, which identity to 165 choose if more than one identity is present in the certificate, the 166 behavior when multiple identities with different schemes were present 167 in the certificate, etc. lead to ambiguities when attempting to 168 interpret the certificate in a uniform manner for TLS use. 170 This document shows how the certificates are to be used for mutual 171 authentication when both the client and server possess appropriate 172 certificates, and normative behavior for matching the DNS query 173 string with an identity stored in the X.509 certificate. 174 Furthermore, a certificate can contain multiple identities for the 175 subject in the subjectAltName extension (the "subject" of a 176 certificate identifies the entity associated with the public key 177 stored in the public key field.) As such, this document specifies 178 appropriate matching rules to encompass various subject identity 179 representation options. And finally, this document also provides 180 guidelines to service providers for assigning certificates to SIP 181 servers. 183 The rest of this document is organized as follows: the next section 184 provides an overview of the most primitive case of a client using DNS 185 to access a SIP server and the resulting authentication steps. 186 Section 5 looks at the reason why mutual inter-domain authentication 187 is desired in SIP, and the lack of normative text and behavior in 188 RFC3261 for doing so. Section 6 outlines normative guidelines for a 189 service provider assigning certificates to SIP servers. Section 7 190 provides normative behavior on the SIP entities (user agent clients, 191 user agent servers, registrars, redirect servers, and proxies) that 192 need perform authentication based on X.509 certificates. Section 8 193 includes the security considerations. 195 4. SIP domain to host resolution 197 Routing in SIP is performed by having the client execute RFC 3263 [8] 198 procedures on a URI, called the "Application Unique String (AUS) 199 (c.f. Section 8 of RFC 3263 [8]). These procedures take as input a 200 SIP AUS (the SIP URI), extract the domain portion of that URI for use 201 as a lookup key, and query the Domain Name Service (DNS) to obtain an 202 ordered set of one or more IP addresses with a port number and 203 transport corresponding to each IP address in the set (the "Expected 204 Output"). If the transport indicates the use of TLS, then a TLS 205 connection is opened to the server on a specific IP address and port. 206 The server presents an X.509 certificate to the client for 207 verification as part of the initial TLS handshake. 209 The client extracts identifiers from the Subject and any 210 subjectAltName extension in the certificate (see Section 7.1) and 211 compares these values to the domain part extracted from the original 212 sip URI (the AUS). If any identifier match is found, the server is 213 considered to be authenticated and subsequent signaling can now 214 proceed over the TLS connection. Matching rules for X.509 215 certificates and the normative behavior for clients is specified in 216 Section 7.3. 218 As an example, consider a request that is to be routed to the SIP 219 address "sips:alice@example.com". This address requires a secure 220 connection to the SIP domain "example.com" (the 'sips' scheme 221 mandates a secure connection). Through a series of DNS 222 manipulations, the domain name is mapped to a set of host addresses 223 and transports. The entity attempting to create the connection 224 selects an address appropriate for use with TLS from this set. When 225 the connection is established to that server, the server presents a 226 certificate asserting the identity "sip:example.com". Since the 227 domain part of the SIP AUS matches the subject of the certificate, 228 the server is authenticated (see Section 7.2 for the normative rules 229 that govern this comparison) 231 SIPS borrows this pattern of server certificate matching from 232 HTTPS. However, RFC 2818 [7] prefers that the identity be 233 conveyed as a subjectAltName extension of type dNSName rather than 234 the common practice of conveying the identity in the CN field of 235 the Subject field. Similarly, this document recommends that the 236 SIP domain identity be conveyed as a subjectAltName extension of 237 type uniformResourceIdentifier (c.f. Section 6, Section 7.1). 239 A domain name in an X.509 certificates is properly interpreted 240 only as a sequence of octets to be compared to the URI used to 241 reach the host. No inference can be made based on the DNS name 242 hierarchy. For example, a valid certificate for "example.com" 243 does not imply that the owner of that certificate has any 244 relationship at all to "subname.example.com". 246 5. The need for mutual interdomain authentication 248 Consider the SIP trapezoid shown in Figure 1. 250 Proxy-A.example.com Proxy-B.example.net 251 +-------+ +-------+ 252 | Proxy |--------------------| Proxy | 253 +----+--+ +---+---+ 254 | | 255 | | 256 | | 257 | +---+ 258 0---0 | | 259 /-\ |___| 260 +---+ / / 261 +----+ 262 alice@example.com bob@example.net 264 Figure 1: SIP Trapezoid 266 An user, alice@example.com, invites bob@example.net for a multimedia 267 communication session. Alice's outbound proxy, Proxy-A.example.com, 268 uses normal RFC 3263 [8] resolution rules to find a proxy -- Proxy- 269 B.example.net -- in the example.net domain that uses TLS. Proxy-A 270 actively establishes an interdomain TLS connection with Proxy-B and 271 each presents a certificate to authenticate that connection. 273 RFC 3261 [2] section 26.3.2.2 "Interdomain Requests" states that when 274 a TLS connection is created between two proxies: 276 "Each side of the connection SHOULD verify and inspect the 277 certificate of the other, noting the domain name that appears in 278 the certificate for comparison with the header fields of SIP 279 messages." 281 However, RFC3261 is silent on whether to use the subjectAltName or CN 282 of the certificate to obtain the domain name, and which takes 283 precedence when there are multiple names identifying the holder of 284 the certificate. 286 The authentication problem for Proxy-A is straightforward: in the 287 certificate Proxy-A receives from Proxy-B, Proxy-A looks for an 288 identity that is a SIP URI ("sip:example.net") or a DNS name 289 ("example.net") that asserts Proxy-B's authority over the example.net 290 domain. Normative behavior for a TLS client like Proxy-A is 291 specified in Section 7.3. 293 The problem for Proxy-B is slightly more complex since it accepted 294 the TLS request passively. Thus, Proxy-B does not possess an 295 equivalent AUS that it can use as an anchor in matching identities 296 from Proxy-A's certificate. 298 RFC 3261 [2] section 26.3.2.2 only tells Proxy-B to "compare the 299 domain asserted by the certificate with the 'domainname' portion 300 of the From header field in the INVITE request." The difficulty 301 with that instruction is that the domainname in the From header 302 field is not always that of the domain from which the request is 303 received. 305 The normative behavior for a TLS server like Proxy-B that passively 306 accepts a TLS connection and requires authentication of the sending 307 peer domain is provided in Section 7.4. 309 6. Certificate usage by a SIP service provider 311 It is possible for service providers to continue the practice of 312 using existing certificates for SIP usage with the identity conveyed 313 only in the Subject field, but should carefully consider the 314 following advantages of conveying identity in the subjectAltName 315 extension field: 317 o The subjectAltName extension can hold multiple values, so the same 318 certificate can identify multiple servers or sip domains. 320 o There is no fixed syntax specified for the Subject field, so 321 issuers vary in how the field content is set. This forces a 322 recipient to use heuristics to extract the identity, again 323 increasing opportunities for misinterpretation. 325 Because of these advantages, service providers are strongly 326 encouraged to obtain certificates which contain the identity or 327 identities in the subjectAltName extension field. 329 When assigning certificates to authoritative servers, a SIP service 330 provider MUST ensure that the SIP domain used to reach the server 331 appears as an identity in the subjectAltName field, or for 332 compatibility with existing certificates, the Subject field of the 333 certificate. In practice, this means that a service provider 334 distributes to its users SIP URIs whose domain portion corresponds to 335 an identity for which the service provider has been issued a 336 certificate. 338 7. Behavior of SIP entities 340 This section normatively specifies the behavior of SIP entities when 341 using X.509 certificates to determine an authenticated SIP domain 342 identity. 344 The first two subsections apply to all SIP implementations that use 345 TLS to authenticate the peer: Section 7.1 describes how to extract a 346 set of SIP identities from the certificate obtained from a TLS peer, 347 and Section 7.2 specifies how to compare SIP identities. The 348 remaining subsections provide context for how and when these rules 349 are to be applied by entities in different SIP roles. 351 7.1. Finding SIP Identities in a Certificate 353 Implementations (both clients and server) MUST determine the validity 354 of a certificate by following the procedures described in RFC 5280 355 [6]. 357 As specified by RFC 5280 [6] section 4.2.1.12, implementations MUST 358 check for restrictions on certificate usage declared by any 359 extendedKeyUsage extensions in the certificate. The SIP Extended Key 360 Usage (EKU) document [12] defines an extendedKeyUsage for SIP. 362 Given an X.509 certificate that the above checks have found to be 363 acceptable, the following describes how to determine what SIP domain 364 identity or identities the certificate contains. A single 365 certificate can serve more than one purpose - that is, the 366 certificate might contain identities not acceptable as SIP, domain 367 identities and/or might contain one or more identities that are 368 acceptable for use as SIP domain identities. 370 1. Examine each value in the subjectAltName field. The 371 subjectAltName field and the constraints on its values are 372 defined in Section 4.2.1.6 of RFC 5280 [6]. The subjectAltName 373 field can be absent or can contain one or more values. Each 374 value in the subjectAltName has a type; the only types acceptable 375 for encoding a SIP domain identity SHALL be: 377 URI If the scheme of the URI is not "sip", then the 378 implementation MUST NOT accept the value as a SIP domain 379 identity. 381 If the scheme of the URI value is "sip", and the URI value 382 that contains a userpart (there is an '@'), the implementation 383 MUST NOT accept the value as a SIP domain identity (a value 384 with a userpart identifies an individual user, not a domain). 386 If the scheme of the URI value is "sip", and there is no 387 userinfo component in the URI (there is no '@'), then the 388 implementation MUST accept the hostpart as a SIP domain 389 identity. 391 Note: URI scheme tokens are always case insensitive 393 DNS An implementation MUST accept a domain name system 394 identifier as a SIP domain identity if and only if no other 395 identity is found that matches the "sip" URI type described 396 above. 398 2. If and only if the subjectAltName does not appear in the 399 certificate, the implementation MAY examine the CN field of the 400 certificate. If a valid DNS name is found there, the 401 implementation MAY accept this value as a SIP domain identity. 402 Accepting a DNS name in the CN value is allowed for backward 403 compatibility, but when constructing new certificates, consider 404 the advantages of using the subjectAltName extension field (see 405 Section 6). 407 The above procedure yields a set containing zero or more identities 408 from the certificate. A client uses these identities to authenticate 409 a server (see Section 7.3) and a server uses them to authenticate a 410 client (see Section 7.4). 412 7.2. Comparing SIP Identities 414 When an implementation (either client or server) compares two values 415 as SIP domain identities: 417 Implementations MUST compare only the DNS name component of each 418 SIP domain identifier; an implementation MUST NOT use any scheme 419 or parameters in the comparison. 421 Implementations MUST compare the values as DNS names, which means 422 that the comparison is case insensitive as specified by RFC 4343 423 [3]. Implementations MUST handle Internationalized Domain Names 424 (IDNs) in accordance with Section 7.2 of RFC 5280 [6] . 426 Implementations MUST match the values in their entirety: 428 Implementations MUST NOT match suffixes. For example, 429 "foo.example.com" does not match "example.com". 431 Implemenations MUST NOT match any form of wildcard, such as a 432 leading "." or "*." with any other DNS label or sequence of 433 labels. For example, "*.example.com" matches only 434 "*.example.com" but not "foo.example.com". Similarly, 435 ".example.com" matches only ".example.com", and does not match 436 "foo.example.com." 438 RFC 2818 [7] (HTTP over TLS) allows the dNSName component to 439 contain a wildcard; e.g., "DNS:*.example.com". RFC 5280 440 [6], while not disallowing this explicitly, leaves the 441 interpretation of wildcards to the individual specification. 442 RFC 3261 [2] does not provide any guidelines on the presence 443 of wildcards in certificates. Through the rule above, this 444 document prohibits such wildcards in certificates for SIP 445 domains. 447 7.3. Client behavior 449 A client uses the domain portion of the SIP AUS to query a (possibly 450 untrusted) DNS to obtain a result set, which is one or more SRV and A 451 records identifying the server for the domain (see Section 4 for an 452 overview.) 454 The SIP server, when establishing a TLS connection, presents its 455 certificate to the client for authentication. The client MUST 456 determine the SIP domain identities in the server certificate using 457 the procedure in Section 7.1. Then, the client MUST compare the 458 original domain portion of the SIP AUS used as input to the RFC 3263 459 [8] server location procedures to the SIP domain identities obtained 460 from the certificate. 462 o If there were no identities found in the server certificate, the 463 server is not authenticated. 465 o If the domain extracted from the AUS matches any SIP domain 466 identity obtained from the certificate when compared as described 467 in section Section 7.2, the server is authenticated for the 468 domain. 470 If the server is not authenticated, the client MUST close the 471 connection immediately. 473 7.4. Server behavior 475 When a server accepts a TLS connection, the server presents its own 476 X.509 certificate to the client. Servers that wish to authenticate 477 the client will ask the client for a certificate. If the client 478 possesses a certificate, that certificate is presented to the server. 479 If the client does not present a certificate, the client MUST NOT be 480 considered authenticated. 482 Whether or not to close a connection if the client does not 483 present a certificate is a matter of local policy, and depends on 484 the authentication needs of the server for the connection. Some 485 currently deployed servers use Digest authentication to 486 authenticate individual requests on the connection, and choose to 487 treat the connection as authenticated by those requests for some 488 purposes (but see Section 8.1). 490 If the local server policy requires client authentication for some 491 local purpose, then one element of such a local policy might be to 492 allow the connection only if the client is authenticated. For 493 example, if the server is an inbound proxy that has peering 494 relationships with the outbound proxies of other specific domains, 495 the server might allow only connections authenticated as coming 496 from those domains. 498 When authenticating the client, the server MUST obtain the set of SIP 499 domain identities from the client certificate as described in 500 Section 7.1. Because the server accepted the TLS connection 501 passively, unlike a client, the server does not possess an AUS for 502 comparison. Nonetheless, server policies can use the set of SIP 503 domain identities gathered from the certificate in Section 7.1 to 504 make authorization decisions. 506 For example, a very open policy could be to accept a X.509 507 certificate and validate the certificate using the procedures in RFC 508 5280 [6] . If the certificate is valid, the identity set is logged. 510 Alternatively, the server could have a list of all SIP domains the 511 server is allowed to accept connections from; when a client presents 512 its certificate, for each identity in the client certificate, the 513 server searches for the identity in the list of acceptable domains to 514 decide whether or not to accept the connection. Other policies that 515 make finer distinctions are possible. 517 The decision of whether or not the authenticated connection to the 518 client is appropriate for use to route new requests to the client 519 domain is independent of whether or not the connection is 520 authenticated; the connect-reuse [10] draft discusses this aspect in 521 more detail. 523 7.5. Proxy behavior 525 A proxy MUST use the procedures defined for a User Agent Server (UAS) 526 in Section 7.4 when authenticating a connection from a client. 528 A proxy MUST use the procedures defined for a User Agent Client (UAC) 529 in Section 7.3 when requesting an authenticated connection to a UAS. 531 If a proxy adds a Record-Route when forwarding a request with the 532 expectation that the route is to use secure connections, the proxy 533 MUST insert into the Record-Route header a URI that corresponds to an 534 identity for which the proxy has a certificate; if the proxy does not 535 insert such a URI, then creation of a secure connection using the 536 value from the Record-Route as the AUS will be impossible. 538 7.6. Registrar behavior 540 A SIP registrar, acting as a server, follows the normative behavior 541 of Section 7.4. When the SIP registrar accepts a TLS connection from 542 the client, the SIP registrar presents its certificate. Depending on 543 the registrar policies, the SIP registrar can challenge the client 544 with HTTP Digest. 546 7.7. Redirect server behavior 548 A SIP redirect server follows the normative behavior of a UAS as 549 specified in Section 7.4. 551 7.8. Virtual SIP Servers and Certificate Content 553 In the "virtual hosting" cases where multiple domains are managed by 554 a single application, a certificate can contain multiple subjects by 555 having distinct identities in the subjectAltName field as specified 556 in RFC 4474 [9]. Clients seeking to authenticate a server on such a 557 virtual host can still follow the directions in Section 7.3 to find 558 the identity matching the SIP AUS used to query DNS. 560 Alternatively, if the TLS client hello "server_name" extension as 561 defined in RFC 4366 [4] is supported, the client SHOULD use that 562 extension to request a certificate corresponding to the specific 563 domain (from the SIP AUS) that the client is seeking to establish a 564 connection with. 566 8. Security Considerations 568 The goals of TLS (when used with X.509 certificates) include the 569 following security guarantees at the transport layer: 571 Confidentiality: packets tunneled through TLS can be read only by 572 the sender and receiver. 574 Integrity: packets tunneled through TLS cannot be undetectably 575 modified on the connection between the sender and receiver. 577 Authentication: each principal is authenticated to the other as 578 possessing a private key for which a certificate has been issued. 579 Moreover, this certificate has not been revoked, and is verifiable 580 by a certificate chain leading to a (locally configured) trust 581 anchor. 583 We expect appropriate processing of domain certificates to provide 584 the following security guarantees at the application level: 586 Confidentiality: SIPS messages from alice@example.com to 587 bob@example.net can be read only by alice@example.com, 588 bob@example.net, and SIP proxies issued with domain certificates 589 for example.com or example.net. 591 Integrity: SIPS messages from alice@example.com to bob@example.net 592 cannot be undetectably modified on the links between 593 alice@example.com, bob@example.net, and SIP proxies issued with 594 domain certificates for example.com or example.net. 596 Authentication: alice@example.com and proxy.example.com are mutually 597 authenticated; moreover proxy.example.com is authenticated to 598 alice@example.com as an authoritative proxy for domain 599 example.com. Similar mutual authentication guarantees are given 600 between proxy.example.com and proxy.example.net and between 601 proxy.example.net and bob@example.net. As a result, 602 alice@example.com is transitively mutually authenticated to 603 bob@example.net (assuming trust in the authoritative proxies for 604 example.com and example.net). 606 8.1. Connection authentication using Digest 608 Digest authentication in SIP provides for authentication of the 609 message sender to the challenging UAS. As commonly deployed, digest 610 authentication provides only very limited integrity protection of the 611 authenticated message, and has no provision for binding the 612 authentication to any attribute of the transport. Many existing SIP 613 deployments have chosen to use the Digest authentication of one or 614 more messages on a particular transport connection as a way to 615 authenticate the connection itself - by implication, authenticating 616 other (unauthenticated) messages on that connection. Some even 617 choose to similarly authenticate a UDP source address and port based 618 on the digest authentication of another message received from that 619 address and port. This use of digest goes beyond the assurances that 620 the Digest Authentication mechanism was designed to provide. A SIP 621 implementation SHOULD NOT use the Digest Authentication of one 622 message on a TCP connection or from a UDP peer to infer any 623 authentication of any other messages on that connection or from that 624 peer. Authentication of the domain at the other end of a connection 625 SHOULD be accomplished using TLS and the certificate validation rules 626 described by this specification instead. 628 9. IANA Considerations 630 This memo does not contain any considerations for IANA. 632 10. Acknowledgments 634 The following IETF contributors provided substantive input to this 635 document: Jeroen van Bemmel, Michael Hammer, Cullen Jennings, Paul 636 Kyzivat, Derek MacDonald, Dave Oran, Jon Peterson, Eric Rescorla, 637 Jonathan Rosenberg, Russ Housley. Special acknowledgement goes to 638 Stephen Kent for extensively reviewing draft versions and suggesting 639 invaluable feedback, edits, and comments. 641 Paul Hoffman, Eric Rescorla and Robert Sparks provided much valuable 642 WGLC comments. 644 11. References 646 11.1. Normative References 648 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 649 Levels", RFC 2119, March 1997. 651 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 652 Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: 653 Session Initiation Protocol", RFC 3261, June 2002. 655 [3] Eastlake, D., "Domain Name System (DNS) Case Insensitivity 656 Clarification", RFC 4343, January 2006. 658 [4] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and 659 T. Wright, "Transport Layer Security (TLS) Extensions", 660 RFC 4366, April 2006. 662 [5] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) 663 Protocol Version 1.2", RFC 5246, August 2008. 665 [6] Cooper, D., Santesson, S., Farrell, S., Boyen, S., Housley, R., 666 and W. Polk, "Internet X.509 Public Key Infrastructure 667 Certificate and Certificate Revocation List (CRL) Profile", 668 RFC 5280, May 2008. 670 11.2. Informative References 672 [7] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 674 [8] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol 675 (SIP): Location SIP Servers", RFC 3263, June 2002. 677 [9] Peterson, J. and C. Jennings, "Enhancements for Authenticated 678 Identity Management in the Session Initiation Protocol (SIP)", 679 RFC 4474, August 2006. 681 [10] Mahy, R., Gurbani, V., and B. Tate, "Connection Reuse in the 682 Session Initiation Protocol", 683 draft-ietf-sip-connect-reuse-14.txt (work in progress), 684 August 2009. 686 [11] Drage, K., "A Process for Handling Essential Corrections to the 687 Session Initiation Protocol (SIP)", 688 draft-drage-sip-essential-correction-03.txt (work in progress), 689 July 2008. 691 [12] Lawrence, S. and V. Gurbani, "Using Extended Key Usage (EKU) 692 for Session Initiation Protocol (SIP) X.509 Certificates", 693 draft-ietf-sip-eku-08.txt (work in progress), October 2009. 695 Appendix A. Editorial guidance (non-normative) 697 This document is intended to update RFC 3261 in accordance with the 698 SIP Working Group procedures described in [11] or its successor. 700 This appendix provides guidance to the editor of the next 701 comprehensive update to RFC 3261 [2] on how to incorporate the 702 changes provided by this document. 704 A.1. Additions 706 The content of sections Section 4 through Section 7 inclusive can be 707 incorporated as subsections within a section that describes SIP 708 domain authentication. 710 The contents of Section 8.1 can be incorporated into the Security 711 Considerations section of the new document. 713 All normative references from this document can be carried forward to 714 the successor document. 716 A.2. Changes 718 The following subsections describe changes in specific sections of 719 RFC 3261 [2] that need to be modified in the successor document to 720 align them with the content of this document. In each of the 721 following, the token is a reference to the 722 section added as described in Appendix A.1. 724 A.2.1. 26.3.1 726 The current text says: 728 Proxy servers, redirect servers and registrars SHOULD possess a 729 site certificate whose subject corresponds to their canonical 730 hostname. 732 The suggested replacement for the above is: 734 Proxy servers, redirect servers, registrars, and any other server 735 that is authoritative for some SIP purpose in a given domain 736 SHOULD possess a certificate whose subjects include the name of 737 that SIP domain. 739 Authors' Addresses 741 Vijay K. Gurbani 742 Bell Laboratories, Alcatel-Lucent 743 1960 Lucent Lane 744 Room 9C-533 745 Naperville, IL 60566 746 USA 748 Phone: +1 630 224-0216 749 Email: vkg@alcatel-lucent.com 750 Scott Lawrence 751 Avaya, Inc. 752 600 Technology Park 753 Billerica, MA 01821 754 USA 756 Phone: +1 978 288 5508 757 Email: xmlscott@gmail.com 759 Alan S.A. Jeffrey 760 Bell Laboratories, Alcatel-Lucent 761 1960 Lucent Lane 762 Room 9C-533 763 Naperville, IL 60566 764 USA 766 Email: ajeffrey@alcatel-lucent.com