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(The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (October 15, 2018) is 2013 days in the past. Is this intentional? Checking references for intended status: Best Current Practice ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'RFCThis' is mentioned on line 487, but not defined ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 6962 (Obsoleted by RFC 9162) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Peterson 3 Internet-Draft Neustar 4 Intended status: Best Current Practice R. Barnes 5 Expires: April 18, 2019 Mozilla 6 R. Housley 7 Vigil Security 8 October 15, 2018 10 Best Practices for Securing RTP Media Signaled with SIP 11 draft-ietf-sipbrandy-rtpsec-06 13 Abstract 15 Although the Session Initiation Protocol (SIP) includes a suite of 16 security services that has been expanded by numerous specifications 17 over the years, there is no single place that explains how to use SIP 18 to establish confidential media sessions. Additionally, existing 19 mechanisms have some feature gaps that need to be identified and 20 resolved in order for them to address the pervasive monitoring threat 21 model. This specification describes best practices for negotiating 22 confidential media with SIP, including both comprehensive protection 23 solutions which bind the media to SIP-layer identities as well as 24 opportunistic security solutions. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on April 18, 2019. 43 Copyright Notice 45 Copyright (c) 2018 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 3. Security at the SIP and SDP layer . . . . . . . . . . . . . . 3 63 3.1. Comprehensive Protection . . . . . . . . . . . . . . . . 3 64 3.2. Opportunistic Security . . . . . . . . . . . . . . . . . 4 65 4. STIR Profile for Endpoint Authentication and Verification 66 Services . . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 4.1. Credentials . . . . . . . . . . . . . . . . . . . . . . . 5 68 4.2. Anonymous Communications . . . . . . . . . . . . . . . . 6 69 4.3. Connected Identity Usage . . . . . . . . . . . . . . . . 7 70 4.4. Authorization Decisions . . . . . . . . . . . . . . . . . 8 71 5. Media Security Protocols . . . . . . . . . . . . . . . . . . 9 72 6. Relayed Media and Conferencing . . . . . . . . . . . . . . . 9 73 7. ICE and Connected Identity . . . . . . . . . . . . . . . . . 10 74 8. Best Current Practices . . . . . . . . . . . . . . . . . . . 10 75 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 76 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 77 11. Security Considerations . . . . . . . . . . . . . . . . . . . 11 78 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 79 12.1. Normative References . . . . . . . . . . . . . . . . . . 11 80 12.2. Informative References . . . . . . . . . . . . . . . . . 13 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 83 1. Introduction 85 The Session Initiation Protocol (SIP) [RFC3261] includes a suite of 86 security services, ranging from Digest authentication for 87 authenticating entities with a shared secret, to TLS for transport 88 security, to S/MIME (optionally) for body security. SIP is 89 frequently used to establish media sessions, in particular audio or 90 audiovisual sessions, which have their own security mechanisms 91 available, such as Secure RTP [RFC3711]. However, the practices 92 needed to bind security at the media layer to security at the SIP 93 layer, to provide an assurance that protection is in place all the 94 way up the stack, rely on a great many external security mechanisms 95 and practices, and require a central point of documentation to 96 explain their optimal use as a best practice. 98 Revelations about widespread pervasive monitoring of the Internet 99 have led to a reevaluation of the threat model for Internet 100 communications [RFC7258]. In order to maximize the use of security 101 features, especially of media confidentiality, opportunistic measures 102 must often serve as a stopgap when a full suite of services cannot be 103 negotiated all the way up the stack. This document explains the 104 limitations that may inhibit the use of comprehensive protection, and 105 provides recommendations for which external security mechanisms 106 implementers should use to negotiate secure media with SIP. It 107 moreover gives a gap analysis of the limitations of existing 108 solutions, and specifies solutions to address them. 110 Various specifications that user agents must implement to support 111 media confidentiality are given in the sections below; a summary of 112 the best current practices appears in Section 8. 114 2. Terminology 116 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 117 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 118 "OPTIONAL" in this document are to be interpreted as described in BCP 119 14 RFC 2119 [RFC2119] and RFC 8174 [RFC8174] when, and only when, 120 they appear in all capitals, as shown here. 122 3. Security at the SIP and SDP layer 124 There are two approaches to providing confidentiality for media 125 sessions set up with SIP: comprehensive protection and opportunistic 126 security (as defined in [RFC7435]). 128 3.1. Comprehensive Protection 130 Comprehensive protection for media sessions established by SIP 131 requires the interaction of three protocols: SIP, the Session 132 Description Protocol (SDP) [RFC4566], and the Real-time Protocol 133 (RTP) [RFC3550], in particular its secure profile Secure RTP (SRTP) 134 [RFC3711]. Broadly, it is the responsibility of SIP to provide 135 integrity protection for the media keying attributes conveyed by SDP, 136 and those attributes will in turn identify the keys used by endpoints 137 in the RTP media session(s) that SDP negotiates. Note that this 138 framework does not apply to keys that also require confidentiality 139 protection in the signaling layer, such as the SDP "k=" line, which 140 MUST NOT be used in conjunction with this profile. In that way, once 141 SIP and SDP have exchanged the necessary information to initiate a 142 session, media endpoints will have a strong assurance that the keys 143 they exchange have not been tampered with by third parties, and that 144 end-to-end confidentiality is available. 146 To establishing the identity of the endpoints of a SIP session, this 147 specification uses STIR [RFC8224]. The STIR Identity header has been 148 designed to prevent a class of impersonation attacks that are 149 commonly used in robocalling, voicemail hacking, and related threats. 150 STIR generates a signature over certain features of SIP requests, 151 including header field values that contain an identity for the 152 originator of the request, such as the From header field or P- 153 Asserted-Identity field, and also over the media keys in SDP if they 154 are present. As currently defined, STIR only provides a signature 155 over the "a=fingerprint" attribute, which is a key fingerprint 156 utilized by DTLS-SRTP [RFC5763]; consequently, STIR only offers 157 comprehensive protection for SIP sessions, in concert with SDP and 158 SRTP, when DTLS-SRTP is the media security service. The underlying 159 PASSporT [RFC8225] object used by STIR is extensible, however, and it 160 would be possible to provide signatures over other SDP attributes 161 that contain alternate keying material. A profile for using STIR to 162 provide media confidentiality is given in Section 4. 164 3.2. Opportunistic Security 166 Work is already underway on defining approaches to opportunistic 167 media security for SIP in [I-D.johnston-dispatch-osrtp], which builds 168 on the prior efforts of [I-D.kaplan-mmusic-best-effort-srtp]. The 169 major protocol change proposed by that specification is to signal the 170 use of opportunistic encryption by negotiating the AVP profile in 171 SDP, rather than the SAVP profile (as specified in [RFC3711]) that 172 would ordinarily be used when negotiating SRTP. 174 Opportunistic encryption approaches typically have no integrity 175 protection for the keying material in SDP. Sending SIP over TLS hop- 176 by-hop between user agents and any intermediaries will reduce the 177 prospect that active attackers can alter keys for session requests on 178 the wire. However, opportunistic confidentiality for media will 179 prevent passive attacks of the form most common in the threat of 180 pervasive monitoring. 182 4. STIR Profile for Endpoint Authentication and Verification Services 184 STIR [RFC8224] defines the Identity header field for SIP, which 185 provides a cryptographic attestation of the source of communications. 186 This profile of STIR assumes that a STIR verification service will 187 act in concert with an SRTP media endpoint to ensure that the key 188 fingerprints, as given in SDP, match the keys exchanged to establish 189 DTLS-SRTP. To satisfy this condition, the verification service 190 function would in this case be implemented in the SIP UAS, which 191 would be composed with the media endpoint. If the STIR 192 authentication service or verification service functions are 193 implemented at an intermediary rather than an endpoint, this 194 introduces the possibility that the intermediary could act as a man 195 in the middle, altering key fingerprints. As this attack is not in 196 STIR's core threat model, which focuses on impersonation rather than 197 man-in-the-middle attacks, STIR offers no specific protections 198 against such interference. 200 The SIPBRANDY deployment profile of STIR for media confidentiality 201 thus shifts these responsibilities to the endpoints rather than the 202 intermediaries. While intermediaries MAY provide the verification 203 service function of STIR for SIPBRANDY transactions, intermediaries 204 supporting this specification MUST NOT block or otherwise redirects 205 calls if they do not trust the signing credential. The SIPBRANDY 206 profile is based on an end-to-end trust model, so it is up to the 207 endpoints to determine if they support signing credentials, not 208 intermediaries. 210 In order to be compliant with best practices for SIP media 211 confidentiality with comprehensive protection, user agent 212 implementations MUST implement both the authentication service and 213 verification service roles described in [RFC8224]. STIR 214 authentication services MUST signal their compliance with this 215 specification by adding the "msec" header element defined in this 216 specification to the PASSporT header. Implementations MUST provide 217 key fingerprints in SDP and the appropriate signatures over them per 218 [RFC8225]. 220 When generating either an offer or an answer [RFC3264], compliant 221 implementations MUST include an "a=fingerprint" attribute containing 222 the fingerprint of an appropriate key (see Section 4.1). 224 4.1. Credentials 226 In order to implement the authentication service function in the user 227 agent, SIP endpoints will need to acquire the credentials needed to 228 sign for their own identity. That identity is typically carried in 229 the From header field of a SIP request, and either contains a 230 greenfield SIP URI (e.g. "sip:alice@example.com") or a telephone 231 number, which can appear in a variety of ways (e.g. 232 "sip:+17004561212@example.com;user=phone"). [RFC8224] Section 8 233 contains guidance for separating the two, and determining what sort 234 of credential is needed to sign for each. 236 To date, few commercial certificate authorities issue certificates 237 for SIP URIs or telephone numbers; though work is ongoing on systems 238 for this purpose (such as [I-D.ietf-acme-telephone]) it is not mature 239 enough to be recommended as a best practice. This is one reason why 240 the STIR standard is architected to permit intermediaries to act as 241 an authentication service on behalf of an entire domain, just as in 242 SIP an proxy server can provide domain-level SIP service. While 243 certificate authorities that offered proof-of-possession certificates 244 similar to those used in the email world could be offered for SIP, 245 either for greenfield identifiers or for telephone numbers, this 246 specification does not require their use. 248 For users who do not possess such certificates, DTLS-SRTP [RFC5763] 249 permits the use of self-signed keys. This profile of STIR therefore 250 relaxes the authority requirements of [RFC8224] to allow the use of 251 self-signed keys for authentication services that are composed with 252 user agents, by generating a certificate (per the guidance of 253 [RFC8226]) with a subject corresponding to the user's identity. Such 254 a credential could be used for trust on first use (see [RFC7435]) by 255 relying parties. Note that relying parties SHOULD NOT use 256 certificate revocation mechanisms or real-time certificate 257 verification systems for self-signed certificates as they will not 258 increase confidence in the certificate. 260 Users who wish to remain anonymous can instead generate self-signed 261 certificates as described in Section 4.2. 263 Generally speaking, without access to out-of-band information about 264 which certificates were issued to whom, it will be very difficult for 265 relying parties to ascertain whether or not the signer of a SIP 266 request is genuinely an "endpoint." Even the term "endpoint" is a 267 problematic one, as SIP user agents can be composed in a variety of 268 architectures and may not be devices under direct user control. 269 While it is possible that techniques based on certificate 270 transparency [RFC6962] or similar practices could help user agents to 271 recognize one another's certificates, those operational systems will 272 need to ramp up with the certificate authorities that issue 273 credentials to end user devices going forward. 275 4.2. Anonymous Communications 277 In some cases, the identity of the initiator of a SIP session may be 278 withheld due to user or provider policy. Per the recommendations of 279 [RFC3323], this may involve using an identity such as 280 "anonymous@anonymous.invalid" in the identity fields of a SIP 281 request. [RFC8224] does not currently permit authentication services 282 to sign for requests that supply this identity. It does however 283 permit signing for valid domains, such as "anonymous@example.com," as 284 a way of implementation an anonymization service as specified in 285 [RFC3323]. 287 Even for anonymous sessions, providing media confidentiality and 288 partial SDP integrity is still desirable. This specification 289 RECOMMENDS using one-time self-signed certificates for anonymous 290 communications, with a subjectAltName of 291 "sip:anonymous@anonymous.invalid". After a session is terminated, 292 the certificate SHOULD be discarded, and a new one, with new keying 293 material, SHOULD be generated before each future anonymous call. As 294 with self-signed certificates, relying parties SHOULD NOT use 295 certificate revocation mechanisms or real-time certificate 296 verification systems for anonymous certificates as they will not 297 increase confidence in the certificate. 299 Note that when using one-time anonymous self-signed certificates, any 300 man in the middle could strip the Identity header and replace it with 301 one signed by its own one-time certificate, changing the "mkey" 302 parameters of PASSporT and any "a=fingerprint" attributes in SDP as 303 it chooses. This signature only provides protection against non- 304 Identity aware entities that might modify SDP without altering the 305 PASSporT conveyed in the Identity header. 307 4.3. Connected Identity Usage 309 STIR [RFC8224] provides integrity protection for the SDP bodies of 310 SIP requests, but not SIP responses. When a session is established, 311 therefore, any SDP body carried by a 200 class response in the 312 backwards direction will not be protected by an authentication 313 service and cannot be verified. Thus, sending a secured SDP body in 314 the backwards direction will require an extra RTT, typically a 315 request sent in the backwards direction. 317 The problem of providing "Connected Identity" for the original 318 RFC4474 was explored in [RFC4916], which uses a provisional or mid- 319 dialog UPDATE request in the backwards direction to convey an 320 Identity header for the recipient of an INVITE. The procedures in 321 that specification are largely compatible with the revision of the 322 Identity header in [RFC8224]. However, the following updates to 323 [RFC4916] are required: 325 The UPDATE carrying signed SDP with a fingerprint in the backwards 326 direction MUST be sent during dialog establishment, following the 327 receipt of a PRACK after a provisional 1xx response. 329 For use with this STIR Profile for media confidentiality, the UAS 330 that responds to the INVITE request MUST act as an authentication 331 service for the UPDATE sent in the backwards direction. 333 The text in RFC4916 Section 4.4.1 regarding the receipt at a UAC 334 of error codes 428, 436, 437 and 438 in response to a mid-dialog 335 request RECOMMENDS treating the dialog as terminated. [RFC8224] 336 allows the retransmission of requests with repairable error 337 conditions (see section 6.1.1) in a way that can override that 338 SHOULD in RFC4916. In particular, an authentication service MAY 339 retry a mid-dialog as [RFC8224] allows rather than treating the 340 dialog as terminated, though note that only one such retry is 341 permitted. 343 The examples in RFC4916 are based on the original RFC4474, and 344 will not match signatures using [RFC8224]. 346 Future work may be done to revise RFC4916 for STIR; that work should 347 take into account any impacts on the profile described in this 348 document. The use of RFC4916 has some further interactions with ICE; 349 see Section 7. 351 4.4. Authorization Decisions 353 [RFC8224] grants STIR verification services a great deal of latitude 354 when making authorization decisions based on the presence of the 355 Identity header field. It is largely a matter of local policy 356 whether an endpoint rejects a call based on absence of an Identity 357 header field, or even the presence of a header that fails an 358 integrity check against the request. 360 For this profile, however, a compliant verification service that 361 receives a dialog-forming SIP request containing an Identity header 362 with a PASSporT type of "msec", after validating the request per the 363 steps described in [RFC8224] Section 6.2, MUST reject the request if 364 there is any failure in that validation process with the appropriate 365 status code per Section 6.2.2. If the request is valid, then if a 366 terminating user accepts the request, it MUST then follow the steps 367 in Section 4.3 to act as an authentication service and send a signed 368 request with the "msec" PASSPorT type in its Identity header as well, 369 in order to enable end-to-end bidirectional confidentiality. 371 For the purposes of this profile, the "msec" PASSporT type can be 372 used by authentication services in one of two ways: as a mandatory 373 request for media security, or as a merely opportunistic request for 374 media security. As any verification service that receives an 375 Identity header in a SIP request with an unrecognized PASSporT type 376 will simply ignore that Identity header, an authentication service 377 will know whether or not the terminating side supports "msec" based 378 on whether or not its user agent receives a signed request in the 379 backwards direction per Section 4.3. If no such requests are 380 received, the UA may do one or two things: shut down the dialog, if 381 the policy of the UA requires that "msec" be supported by the 382 terminating side for this dialog; or, if policy permits, allow the 383 dialog to continue without media security. 385 5. Media Security Protocols 387 As there are several ways to negotiate media security with SDP, any 388 of which might be used with either opportunistic or comprehensive 389 protection, further guidance to implementers is needed. In 390 [I-D.johnston-dispatch-osrtp], opportunistic approaches considered 391 include DTLS-SRTP, security descriptions [RFC4568], and ZRTP 392 [RFC6189]. 394 Support for DTLS-SRTP is REQUIRED by this specification. 396 The "mkey" claim of PASSporT provides integrity protection for 397 "a=fingerprint" attributes in SDP, including cases where multiple 398 "a=fingerprint" attributes appear in the same SDP. 400 6. Relayed Media and Conferencing 402 Providing end-to-end media confidentiality for SIP is complicated by 403 the presence of many forms of media relays. While many media relays 404 merely proxy media to a destination, others present themselves as 405 media endpoints and terminate security associations before re- 406 originating media to its destination. 408 Centralized conference bridges are one type of entity that typically 409 terminates a media session in order to mux media from multiple 410 sources and then to re-originate the muxed media to conference 411 participants. In many such implementations, only hop-by-hop media 412 confidentiality is possible. Work is ongoing to specify a means to 413 encrypt both the hop-by-hop media between a user agent and a 414 centralized server as well as the end-to-end media between user 415 agents, but is not sufficiently mature at this time to make a 416 recommendation for a best practice here. Those protocols are 417 expected to identify their own best practice recommendations as they 418 mature. 420 Another class of entities that might relay SIP media are back-to-back 421 user agents (B2BUAs). If a B2BUA follows the guidance in [RFC7879], 422 it may be possible for those devices to act as media relays while 423 still permitting end-to-end confidentiality between user agents. 425 Ultimately, if an endpoint can decrypt media it receives, then that 426 endpoint can forward the decrypted media without the knowledge or 427 consent of the media's originator. No media confidentiality 428 mechanism can protect against these sorts of relayed disclosures, or 429 trusted entities that can decrypt media and then record a copy to be 430 sent elsewhere (see [RFC7245]). 432 7. ICE and Connected Identity 434 Providing confidentiality for media with comprehensive protection 435 requires careful timing of when media streams should be sent and when 436 a user interface should signify that confidentiality is in place. 438 In order to best enable end-to-end connectivity between user agents, 439 and to avoid media relays as much as possible, implementations of 440 this specification must support ICE [RFC8445]. To speed up call 441 establishment, it is RECOMMENDED that implementations support trickle 442 ICE [I-D.ietf-mmusic-trickle-ice-sip]. 444 Note that in the comprehensive protection case, the use of Connected 445 Identity [RFC4916] with ICE entails that the answer containing the 446 key fingerprints, and thus the STIR signature, will come in an UPDATE 447 sent in the backwards direction a provisional response and 448 acknowledgment (PRACK), rather than in any earlier SDP body. Only at 449 such a time as that UPDATE is received will the media keys be 450 considered exchanged in this case. 452 Similarly, in order to prevent, or at least mitigate, the denial-of- 453 service attack described in Section 19.5.1 of [RFC8445], this 454 specification incorporates best practices for ensuring that 455 recipients of media flows have consented to receive such flows. 456 Implementations of this specification MUST implement the STUN usage 457 for consent freshness defined in [RFC7675]. 459 8. Best Current Practices 461 The following are the best practices for SIP user agents to provide 462 media confidentiality for SIP sessions. 464 Implementations MUST support the STIR endpoint profile given in 465 Section 4, and signal that in PASSporT with the "msec" header 466 element. 468 Implementations MUST follow the authorization decision behavior in 469 Section 4.4. 471 Implementations MUST support DTLS-SRTP for key-management, as 472 described in Section 5. 474 Implementations MUST support the ICE, and the STUN consent freshness 475 mechanism, as specified in Section 7. 477 9. Acknowledgments 479 We would like to thank Eric Rescorla, Adam Roach, Andrew Hutton, and 480 Ben Campbell for contributions to this problem statement and 481 framework. 483 10. IANA Considerations 485 This specification defines a new values for the PASSporT Type 486 registry called "msec," and the IANA is requested to add that to the 487 registry with a value pointing to [RFCThis]. 489 11. Security Considerations 491 This document describes the security features that provide media 492 sessions established with SIP with confidentiality, integrity, and 493 authentication. 495 12. References 497 12.1. Normative References 499 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 500 Requirement Levels", BCP 14, RFC 2119, 501 DOI 10.17487/RFC2119, March 1997, 502 . 504 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 505 A., Peterson, J., Sparks, R., Handley, M., and E. 506 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 507 DOI 10.17487/RFC3261, June 2002, 508 . 510 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 511 with Session Description Protocol (SDP)", RFC 3264, 512 DOI 10.17487/RFC3264, June 2002, 513 . 515 [RFC3323] Peterson, J., "A Privacy Mechanism for the Session 516 Initiation Protocol (SIP)", RFC 3323, 517 DOI 10.17487/RFC3323, November 2002, 518 . 520 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 521 Jacobson, "RTP: A Transport Protocol for Real-Time 522 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 523 July 2003, . 525 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 526 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 527 RFC 3711, DOI 10.17487/RFC3711, March 2004, 528 . 530 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 531 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 532 July 2006, . 534 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 535 Description Protocol (SDP) Security Descriptions for Media 536 Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006, 537 . 539 [RFC4916] Elwell, J., "Connected Identity in the Session Initiation 540 Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June 541 2007, . 543 [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework 544 for Establishing a Secure Real-time Transport Protocol 545 (SRTP) Security Context Using Datagram Transport Layer 546 Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May 547 2010, . 549 [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an 550 Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 551 2014, . 553 [RFC7675] Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M. 554 Thomson, "Session Traversal Utilities for NAT (STUN) Usage 555 for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675, 556 October 2015, . 558 [RFC7879] Ravindranath, R., Reddy, T., Salgueiro, G., Pascual, V., 559 and P. Ravindran, "DTLS-SRTP Handling in SIP Back-to-Back 560 User Agents", RFC 7879, DOI 10.17487/RFC7879, May 2016, 561 . 563 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 564 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 565 May 2017, . 567 [RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, 568 "Authenticated Identity Management in the Session 569 Initiation Protocol (SIP)", RFC 8224, 570 DOI 10.17487/RFC8224, February 2018, 571 . 573 [RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion 574 Token", RFC 8225, DOI 10.17487/RFC8225, February 2018, 575 . 577 [RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity 578 Credentials: Certificates", RFC 8226, 579 DOI 10.17487/RFC8226, February 2018, 580 . 582 [RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive 583 Connectivity Establishment (ICE): A Protocol for Network 584 Address Translator (NAT) Traversal", RFC 8445, 585 DOI 10.17487/RFC8445, July 2018, 586 . 588 12.2. Informative References 590 [I-D.ietf-acme-telephone] 591 Peterson, J. and R. Barnes, "ACME Identifiers and 592 Challenges for Telephone Numbers", draft-ietf-acme- 593 telephone-01 (work in progress), October 2017. 595 [I-D.ietf-mmusic-trickle-ice-sip] 596 Ivov, E., Stach, T., Marocco, E., and C. Holmberg, "A 597 Session Initiation Protocol (SIP) Usage for Incremental 598 Provisioning of Candidates for the Interactive 599 Connectivity Establishment (Trickle ICE)", draft-ietf- 600 mmusic-trickle-ice-sip-18 (work in progress), June 2018. 602 [I-D.johnston-dispatch-osrtp] 603 Johnston, A., Ph.D., D., Hutton, A., Liess, L., and T. 604 Stach, "An Opportunistic Approach for Secure Real-time 605 Transport Protocol (OSRTP)", draft-johnston-dispatch- 606 osrtp-02 (work in progress), February 2016. 608 [I-D.kaplan-mmusic-best-effort-srtp] 609 Audet, F. and H. Kaplan, "Session Description Protocol 610 (SDP) Offer/Answer Negotiation For Best-Effort Secure 611 Real-Time Transport Protocol", draft-kaplan-mmusic-best- 612 effort-srtp-01 (work in progress), October 2006. 614 [RFC6189] Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP: 615 Media Path Key Agreement for Unicast Secure RTP", 616 RFC 6189, DOI 10.17487/RFC6189, April 2011, 617 . 619 [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate 620 Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, 621 . 623 [RFC7245] Hutton, A., Ed., Portman, L., Ed., Jain, R., and K. Rehor, 624 "An Architecture for Media Recording Using the Session 625 Initiation Protocol", RFC 7245, DOI 10.17487/RFC7245, May 626 2014, . 628 [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection 629 Most of the Time", RFC 7435, DOI 10.17487/RFC7435, 630 December 2014, . 632 Authors' Addresses 634 Jon Peterson 635 Neustar, Inc. 637 Email: jon.peterson@team.neustar 639 Richard Barnes 640 Mozilla 642 Email: rlb@ipv.sx 644 Russ Housley 645 Vigil Security, LLC 647 Email: housley@vigilsec.com