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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MMUSIC Working Group C. Holmberg 3 Internet-Draft I. Sedlacek 4 Intended status: Standards Track Ericsson 5 Expires: April 21, 2014 G. Salgueiro 6 Cisco 7 October 18, 2013 9 UDP Transport Layer (UDPTL) over Datagram Transport Layer Security 10 (DTLS) 11 draft-ietf-mmusic-udptl-dtls-00 13 Abstract 15 This document specifies how the UDP Transport Layer (UDPTL) protocol 16 can be transported over the Datagram Transport Layer Security (DTLS) 17 protocol, how the usage of UDPTL over DTLS is indicated in the 18 Session Description Protocol (SDP), and how UDPTL over DTLS is 19 negotiated in a session established using the Session Initiation 20 Protocol (SIP). 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on April 21, 2014. 39 Copyright Notice 41 Copyright (c) 2013 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 58 3. Secure Channel . . . . . . . . . . . . . . . . . . . . . . . 5 59 3.1. Secure Channel Establishment . . . . . . . . . . . . . . 5 60 3.2. Secure Channel Usage . . . . . . . . . . . . . . . . . . 5 61 4. Miscellaneous Considerations . . . . . . . . . . . . . . . . 5 62 4.1. Anonymous Calls . . . . . . . . . . . . . . . . . . . . . 6 63 4.2. Middlebox Interaction . . . . . . . . . . . . . . . . . . 6 64 4.3. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 6 65 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 66 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 67 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 68 8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 7 69 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 70 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 71 9.2. Informative References . . . . . . . . . . . . . . . . . 9 72 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 9 73 A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9 74 A.2. Basic Message Flow with Identity . . . . . . . . . . . . 10 75 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 77 1. Introduction 79 While telephony encryption devices have been traditionally used for 80 highly sensitive documents, secure fax on the Public Switched 81 Telephone Network (PSTN) was not as widely considered or prioritized 82 because of the challenges involved with physical access to telephony 83 equipment. As real-time communications transition to IP networks, 84 where information might potentially be intercepted or spoofed, an 85 appropriate level of security for fax that offers integrity and 86 confidentiality protection is vital. Some of the security mechanisms 87 for securing fax include: 89 o [ITU.T30.2005] Annex H specifies integrity and confidentiality 90 protection of fax in application layer, independent of protocol 91 for fax transport. 92 o [ITU.T38.2010] specifies fax transport over RTP/SAVP which enables 93 integrity and confidentiality protection of fax in IP network. 95 Despite these mechanisms to secure fax, there is no transport layer 96 security offering integrity and confidentiality protection for UDPTL 98 [ITU.T38.2010], the overwhelmingly predominant fax transport 99 protocol. The protocol stack for fax transport using UDPTL is shown 100 in Table 1. 102 +-----------------------------+ 103 | Protocol | 104 +-----------------------------+ 105 | Internet facsimile protocol | 106 +-----------------------------+ 107 | UDPTL | 108 +-----------------------------+ 109 | UDP | 110 +-----------------------------+ 111 | IP | 112 +-----------------------------+ 114 Table 1: Protocol stack for UDPTL over UDP 116 The 3rd Generation Partnership Project (3GPP) has performed a study 117 on how to provide secure fax in the IP Multimedia Subsystem (IMS) and 118 concluded that secure fax shall be transported using UDPTL over DTLS. 120 This document specifies fax transport using UDPTL over DTLS 121 [RFC6347], which enables integrity and confidentiality protection of 122 fax in IP networks. The protocol stack for integrity and 123 confidentiality protected fax transport using UDPTL over DTLS is 124 shown in Table 2. 126 +-----------------------------+ 127 | Protocol | 128 +-----------------------------+ 129 | Internet facsimile protocol | 130 +-----------------------------+ 131 | UDPTL | 132 +-----------------------------+ 133 | DTLS | 134 +-----------------------------+ 135 | UDP | 136 +-----------------------------+ 137 | IP | 138 +-----------------------------+ 140 Table 2: Protocol stack for UDPTL over UDP 142 The primary motivations for the mechanism in this document are: 144 o The design of DTLS [RFC6347] is clearly defined, well understood 145 and implementations are widely available. 147 o No DTLS extensions are required in order to enable UDPTL transport 148 over DTLS. 149 o Fax transport using UDPTL over DTLS only requires insertion of the 150 DTLS layer between the UDPTL layer and the UDP layer, as shown in 151 Table 2. The UDPTL layer and layers above UDPTL layer require no 152 modification. 153 o UDPTL [ITU.T38.2010] is by far the most widely deployed fax 154 transport protocol in IP networks. 155 o 3GPP needs a mechanism to transport UDPTL over DTLS, in order to 156 provide secure fax in IMS networks. 158 This document specifies the transport of UDPTL over DTLS using the 159 DTLS record layer "application_data" packets [RFC6347]. 161 Since the DTLS record layer "application_data" packet does not 162 indicate whether it carries UDPTL, or some other protocol, the usage 163 of a dedicated DTLS association for transport of UDPTL needs to be 164 negotiated, e.g. using the Session Description Protocol (SDP) 165 [RFC4566] and the SDP offer/answer mechanism [RFC3264]. 167 Therefore, this document specifies a new value [RFC4566] for 168 the SDP media description ("m=" line) [RFC3264], in order to indicate 169 UDPTL over DTLS in SDP messages [RFC4566]. 171 2. Conventions 173 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 174 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 175 document are to be interpreted as described in BCP 14, RFC 2119 176 [RFC2119]. 178 DTLS uses the term "session" to refer to a long-lived set of keying 179 material that spans DTLS associations. In this document, in order to 180 be consistent with SIP/SDP usage of "session" terminology, we use it 181 to refer to a multimedia session and use the term "DTLS session" to 182 refer to the DTLS construct. We use the term "DTLS association" to 183 refer to a particular DTLS cipher suite and keying material set that 184 is associated with a single host/port quartet. The same DTLS session 185 can be used to establish the keying material for multiple DTLS 186 associations. For consistency with other SIP/SDP usage, we use the 187 term "connection" when what's being referred to is a multimedia 188 stream that is not specifically DTLS. 190 3. Secure Channel 192 3.1. Secure Channel Establishment 194 The SDP offer/answer mechanism [RFC3264] is used by other protocols, 195 e.g. the Session Initiation Protocol (SIP) [RFC3261], to negotiate 196 and establish multimedia sessions. 198 In addition to the usual contents of an SDP media description ("m=" 199 line) specified for UDPTL over the UDP, each SDP media description 200 for UDPTL over DTLS over the UDP will also contain several SDP 201 attributes, as specified in [RFC4145] and [RFC4572]. 203 The SDP offer and SDP answer MUST conform to the following 204 requirements: 206 o The endpoint MUST set the "proto" field of the "m=" line to the 207 token specified in Table 3. 208 o The endpoint MUST use the SDP setup attribute [RFC4145]. The 209 offerer MUST assign the SDP setup attribute with setup:actpass 210 value, and MUST be prepared to receive a DTLS client_hello message 211 before it receives the SDP answer. The answerer MUST assign the 212 SDP setup attribute with either setup:active value or 213 setup:passive value. The answerer SHOULD assign the SDP setup 214 attribute with the setup:active value. Whichever party is active 215 MUST initiate a DTLS handshake by sending a ClientHello over each 216 flow (host/port quartet). 217 o The endpoint MUST use the SDP certificate fingerprint attribute 218 [RFC4572]. 219 o The certificate presented during the DTLS handshake MUST match the 220 fingerprint exchanged via the signaling path in the SDP. 221 o If the fingerprint does not match the hashed certificate, then the 222 endpoint MUST tear down the media session immediately. Note that 223 it is permissible to wait until the other side's fingerprint has 224 been received before establishing the connection; however, this 225 may have undesirable latency effects. 227 Editor's note: FFS if connection attribute defined in RFC4145 is 228 needed. 230 3.2. Secure Channel Usage 232 DTLS is used as specified in [RFC6347]. Once the DTLS handshake is 233 completed, the UDPTL packets SHALL be transported in DTLS record 234 layer "application_data" packets. 236 4. Miscellaneous Considerations 237 4.1. Anonymous Calls 239 When making anonymous calls, a new self-signed certificate SHOULD be 240 used for each call and the content of the subjectAltName attribute 241 inside the certificate MUST NOT contain information that either 242 allows correlation or identification of the user making anonymous 243 calls. 245 4.2. Middlebox Interaction 247 The procedures defined for SRTP-DTLS in Section 6.7 of [RFC5763] for 248 interaction with middleboxes also apply to UDPTL over DTLS. 250 The procedures defined for SRTP-DTLS in Section 5.1.2 of [RFC5764] 251 for distinguishing DTLS and STUN packets also apply to UDPTL over 252 DTLS. 254 Editor's note: The complete SRTP-DTLS implementation is not needed. 255 Only the parts for interaction with middleboxes in RFC5763 and for 256 distinguishing DTLS and STUN packets in RFC5764 are needed. Should 257 those be copied into this document? 259 4.3. Rekeying 261 After the DTLS handshake caused by rekeying has completed, because of 262 possible packet reordering on the wire, packets protected by the 263 previous set of keys can arrive. To compensate for this fact, 264 receivers SHOULD maintain both sets of keys for some time in order to 265 be able to decrypt and verify older packets. The duration of 266 maintaining the previous set of keys after the finish of the DTLS 267 handshake is out of scope for this document. 269 5. Security Considerations 271 DTLS media signaled with SIP requires a mechanism to ensure that the 272 communicating peers' certificates are correct. 274 The standard DTLS strategy for authenticating the communicating 275 parties is to give the server (and optionally the client) a PKIX 276 [RFC5280] certificate. The client then verifies the certificate and 277 checks that the name in the certificate matches the server's domain 278 name. This works because there are a relatively small number of 279 servers with well-defined names; a situation that does not usually 280 occur in the VoIP context. 282 The design described in this document is intended to leverage the 283 authenticity of the signaling channel (while not requiring 284 confidentiality). As long as each side of the connection can verify 285 the integrity of the SDP received from the other side, then the DTLS 286 handshake cannot be hijacked via a man-in-the-middle attack. This 287 integrity protection is easily provided by the caller to the callee 288 (see sample message flow in Annex A.2) via the SIP Identity [RFC4474] 289 mechanism. Other mechanisms, such as the S/MIME mechanism [RFC3261], 290 or perhaps future mechanisms yet to be specified could also serve 291 this purpose. 293 While this mechanism can still be used without such integrity 294 mechanisms, the security provided is limited to defense against 295 passive attack by intermediaries. An active attack on the signaling 296 plus an active attack on the media plane can allow an attacker to 297 attack the connection (R-SIG-MEDIA in the notation of [RFC5479]). 299 6. IANA Considerations 301 This document updates the "Session Description Protocol (SDP) 302 Parameters" registry as specified in Section 8.2.2 of [RFC4566]. 303 Specifically, it adds the values in Table 3 to the table for the SDP 304 "proto" field registry. 306 +-------+---------------+------------+ 307 | Type | SDP Name | Reference | 308 +-------+---------------+------------+ 309 | proto | UDP/TLS/UDPTL | [RFC-XXXX] | 310 +-------+---------------+------------+ 312 Table 3: SDP "proto" field values 314 [RFC EDITOR NOTE: Please replace RFC-XXXX with the RFC number of this 315 document.] 317 7. Acknowledgments 319 Special thanks to Peter Dawes, who provided comments on the initial 320 version of the draft, and to Paul E. Jones, James Rafferty and 321 Albrecht Schwarz who provided valuable feedback and input on the 322 MMUSIC mailing list. 324 8. Change Log 326 [RFC EDITOR NOTE: Please remove this section when publishing] 328 Changes from draft-holmberg-mmusic-udptl-dtls-02 330 o Milestone adopted - draft-ietf-mmusic version of the draft 331 submitted. 333 Changes from draft-holmberg-mmusic-udptl-dtls-01 335 o Gonzalo Salgueiro added as co-author. 336 o PSTN comparison text and Introduction text modified. 338 Changes from draft-holmberg-mmusic-udptl-dtls-00 340 o Text about T.30 added. 341 o Latest version of T.38 referenced. 342 o Additional text about the need for secure fax in IP networks. 344 Changes from draft-holmberg-dispatch-udptl-dtls-00 346 o WG changed to MMUSIC. 347 o Added text about 3GPP need for UDPTL/DTLS. 349 9. References 351 9.1. Normative References 353 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 354 Requirement Levels", BCP 14, RFC 2119, March 1997. 356 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 357 A., Peterson, J., Sparks, R., Handley, M., and E. 358 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 359 June 2002. 361 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 362 with Session Description Protocol (SDP)", RFC 3264, June 363 2002. 365 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 366 the Session Description Protocol (SDP)", RFC 4145, 367 September 2005. 369 [RFC4474] Peterson, J. and C. Jennings, "Enhancements for 370 Authenticated Identity Management in the Session 371 Initiation Protocol (SIP)", RFC 4474, August 2006. 373 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 374 Description Protocol", RFC 4566, July 2006. 376 [RFC4572] Lennox, J., "Connection-Oriented Media Transport over the 377 Transport Layer Security (TLS) Protocol in the Session 378 Description Protocol (SDP)", RFC 4572, July 2006. 380 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 381 Housley, R., and W. Polk, "Internet X.509 Public Key 382 Infrastructure Certificate and Certificate Revocation List 383 (CRL) Profile", RFC 5280, May 2008. 385 [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework 386 for Establishing a Secure Real-time Transport Protocol 387 (SRTP) Security Context Using Datagram Transport Layer 388 Security (DTLS)", RFC 5763, May 2010. 390 [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer 391 Security (DTLS) Extension to Establish Keys for the Secure 392 Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. 394 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 395 Security Version 1.2", RFC 6347, January 2012. 397 [ITU.T30.2005] 398 International Telecommunications Union, "Procedures for 399 document facsimile transmission in the general switched 400 telephone network", ITU-T Recommendation T.30, September 401 2005. 403 [ITU.T38.2010] 404 International Telecommunications Union, "Procedures for 405 real-time Group 3 facsimile communication over IP 406 networks", ITU-T Recommendation T.38, September 2010. 408 9.2. Informative References 410 [RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet, 411 "Requirements and Analysis of Media Security Management 412 Protocols", RFC 5479, April 2009. 414 Appendix A. Example 416 A.1. General 418 Prior to establishing the session, both Alice and Bob generate self- 419 signed certificates which are used for a single session or, more 420 likely, reused for multiple sessions. 422 The SIP signaling from Alice to her proxy is transported over TLS to 423 ensure an integrity protected channel between Alice and her identity 424 service. Transport between proxies should also be protected somehow. 426 Only one element is shown for Alice's and Bob's proxies for the 427 purposes of simplification. 429 For the sake of brevity and simplicity, only the mandatory SDP T.38 430 attributes are shown. 432 A.2. Basic Message Flow with Identity 434 Figure 1 shows an example message flow of session establishment for 435 T.38 fax securely transported using UDPTL over DTLS. 437 In this example flow, Alice acts as the passive endpoint of DTLS 438 association and Bob acts as the active endpoint of DTLS association. 440 Alice Proxies Bob 441 | (1) SIP INVITE | | 442 |----------------------->| | 443 | | (2) SIP INVITE | 444 | |----------------------->| 445 | | (3) DTLS ClientHello | 446 |<------------------------------------------------| 447 | (4) remaining messages of DTLS handshake | 448 |<----------------------------------------------->| 449 | | | 450 | | | 451 | | (5) SIP 200 OK | 452 | |<-----------------------| 453 | (6) SIP 200 OK | | 454 |<-----------------------| | 455 | (7) SIP ACK | | 456 |------------------------------------------------>| 457 | (8) T.38 message using UDPTL over DTLS | 458 |<----------------------------------------------->| 459 | | | 461 Figure 1: Basic message flow with Identity 463 Message (1): 465 Figure 2 shows the initial INVITE request sent by Alice to Alice's 466 proxy. The initial INVITE request contains an SDP offer. 468 The "m=" line in the SDP Offer indicates T.38 fax using UDPTL over 469 DTLS. 471 The SDP setup:actpass attribute in the SDP Offer indicates that 472 Alice has requested to be either the active or passive endpoint. 474 The SDP fingerprint attribute in the SDP Offer indicates the 475 certificate fingerprint computed from Alice's self-signed 476 certificate. 478 INVITE sip:bob@example.com SIP/2.0 479 To: 480 From: "Alice";tag=843c7b0b 481 Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj 482 Contact: 483 Call-ID: 6076913b1c39c212@REVMTEpG 484 CSeq: 1 INVITE 485 Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE 486 Max-Forwards: 70 487 Content-Type: application/sdp 488 Content-Length: xxxx 489 Supported: from-change 491 v=0 492 o=- 1181923068 1181923196 IN IP4 ua1.example.com 493 s=example1 494 c=IN IP4 ua1.example.com 495 t=0 0 496 m=image 6056 UDP/TLS/UDPTL t38 497 a=setup:actpass 498 a=fingerprint: SHA-1 \ 499 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 500 a=T38FaxRateManagement:transferredTCF 502 Figure 2: Message (1) 504 Message (2): 506 Figure 3 shows the SIP INVITE request sent by Bob's proxy to Bob. 508 The SIP INVITE request contains an Identity header field and an 509 Identity-Info header fields inserted by Alice's proxy. 511 When received, Bob verifies the identity provided in the SIP 512 INVITE request. 514 INVITE sip:bob@ua2.example.com SIP/2.0 515 To: 516 From: "Alice";tag=843c7b0b 517 Via: SIP/2.0/TLS proxy.example.com;branch=z9hG4bK-0e53sadfkasldk 518 Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj 519 Record-Route: 520 Contact: 521 Call-ID: 6076913b1c39c212@REVMTEpG 522 CSeq: 1 INVITE 523 Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE 524 Max-Forwards: 69 525 Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k 526 3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFC 527 HYWGCl0nB2sNsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI= 528 Identity-Info: https://example.com/cert 529 Content-Type: application/sdp 530 Content-Length: xxxx 531 Supported: from-change 533 v=0 534 o=- 1181923068 1181923196 IN IP4 ua1.example.com 535 s=example1 536 c=IN IP4 ua1.example.com 537 t=0 0 538 m=image 6056 UDP/TLS/UDPTL t38 539 a=setup:actpass 540 a=fingerprint: SHA-1 \ 541 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 542 a=T38FaxRateManagement:transferredTCF 544 Figure 3: Message (2) 546 Message (3): 548 Assuming that Alice's identity is valid, Bob sends a DTLS 549 ClientHello directly to Alice. 551 Message (4): 553 Alice and Bob exchange further messages of DTLS handshake 554 (HelloVerifyRequest, ClientHello, ServerHello, Certificate, 555 ServerKeyExchange, CertificateRequest, ServerHelloDone, 556 Certificate, ClientKeyExchange, CertificateVerify, 557 ChangeCipherSpec, Finished). 559 When Bob receives the certificate of Alice via DTLS, Bob checks 560 whether the certificate fingerprint calculated from the Alice's 561 certificate received via DTLS matches the certificate fingerprint 562 received in the a=fingerprint SDP attribute of Figure 3. In this 563 message flow, the check is successful and thus session setup 564 continues. 566 Message (5): 568 Figure 4 shows a 200 (OK) response to the initial SIP INVITE 569 request, sent by Bob to Bob's proxy. The 200 (OK) response 570 contains an SDP answer. 572 The "m=" line in the SDP Answer indicates T.38 fax using UDPTL 573 over DTLS. 575 The SDP setup:active attribute in the SDP Answer indicates that 576 Bob has requested to be the active endpoint. 578 The SDP fingerprint attribute in the SDP Answer indicates the 579 certificate fingerprint computed from Bob's self-signed 580 certificate. 582 SIP/2.0 200 OK 583 To: ;tag=6418913922105372816 584 From: "Alice" ;tag=843c7b0b 585 Via: SIP/2.0/TLS proxy.example.com:5061;branch=z9hG4bK-0e53sadfkasldk 586 Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj 587 Record-Route: 588 Call-ID: 6076913b1c39c212@REVMTEpG 589 CSeq: 1 INVITE 590 Contact: 591 Content-Type: application/sdp 592 Content-Length: xxxx 593 Supported: from-change 595 v=0 596 o=- 6418913922105372816 2105372818 IN IP4 ua2.example.com 597 s=example2 598 c=IN IP4 ua2.example.com 599 t=0 0 600 m=image 12000 UDP/TLS/UDPTL t38 601 a=setup:active 602 a=fingerprint: SHA-1 \ 603 FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 604 a=T38FaxRateManagement:transferredTCF 606 Figure 4: Message (6) 608 Message (6): 610 Figure 5 shows a 200 (OK) response to the initial SIP INVITE 611 request, sent by Alice's proxy to Alice. Alice checks if the 612 certificate fingerprint calculated from the Bob's certificate 613 received via DTLS is the same as the certificate fingerprint 614 received in the a=fingerprint SDP attribute of Figure 5. In this 615 message flow, the check is successful and thus session setup 616 continues. 618 SIP/2.0 200 OK 619 To: ;tag=6418913922105372816 620 From: "Alice" ;tag=843c7b0b 621 Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj 622 Record-Route: 623 Call-ID: 6076913b1c39c212@REVMTEpG 624 CSeq: 1 INVITE 625 Contact: 626 Content-Type: application/sdp 627 Content-Length: xxxx 628 Supported: from-change 630 v=0 631 o=- 6418913922105372816 2105372818 IN IP4 ua2.example.com 632 s=example2 633 c=IN IP4 ua2.example.com 634 t=0 0 635 m=image 12000 UDP/TLS/UDPTL t38 636 a=setup:active 637 a=fingerprint: SHA-1 \ 638 FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 639 a=T38FaxRateManagement:transferredTCF 641 Figure 5: Message (7) 643 Message (7): 645 Alice sends the SIP ACK request to Bob. 647 Message (8): 649 At this point, Bob and Alice can exchange T.38 fax securely 650 transported using UDPTL over DTLS. 652 Authors' Addresses 654 Christer Holmberg 655 Ericsson 656 Hirsalantie 11 657 Jorvas 02420 658 Finland 660 Email: christer.holmberg@ericsson.com 662 Ivo Sedlacek 663 Ericsson 664 Sokolovska 79 665 Praha 18600 666 Czech Republic 668 Email: ivo.sedlacek@ericsson.com 670 Gonzalo Salgueiro 671 Cisco Systems, Inc. 672 7200-12 Kit Creek Road 673 Research Triangle Park, NC 27709 674 US 676 Email: gsalguei@cisco.com