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Versions: (draft-holmberg-mmusic-udptl-dtls) 00 01 02 03 04 05 06 07 08 09 10 RFC 7345

MMUSIC Working Group                                         C. Holmberg
Internet-Draft                                               I. Sedlacek
Intended status: Standards Track                                Ericsson
Expires: June 8, 2014                                       G. Salgueiro
                                                                   Cisco
                                                        December 5, 2013


   UDP Transport Layer (UDPTL) over Datagram Transport Layer Security
                                 (DTLS)
                    draft-ietf-mmusic-udptl-dtls-02

Abstract

   This document specifies how the UDP Transport Layer (UDPTL) protocol
   can be transported over the Datagram Transport Layer Security (DTLS)
   protocol, how the usage of UDPTL over DTLS is indicated in the
   Session Description Protocol (SDP), and how UDPTL over DTLS is
   negotiated in a session established using the Session Initiation
   Protocol (SIP).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 8, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Secure Channel  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Secure Channel Establishment  . . . . . . . . . . . . . .   5
     3.2.  Secure Channel Usage  . . . . . . . . . . . . . . . . . .   6
   4.  Miscellaneous Considerations  . . . . . . . . . . . . . . . .   6
     4.1.  Anonymous Calls . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Middlebox Interaction . . . . . . . . . . . . . . . . . .   6
       4.2.1.  ICE Interaction . . . . . . . . . . . . . . . . . . .   6
       4.2.2.  Latching Control without ICE  . . . . . . . . . . . .   6
       4.2.3.  STUN Interaction  . . . . . . . . . . . . . . . . . .   7
     4.3.  Rekeying  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  10
     A.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.2.  Basic Message Flow with Identity  . . . . . . . . . . . .  11
     A.3.  Message Flow Of T.38 Fax Replacing Audio Media Stream in
           An Existing Audio-Only Session  . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   While telephony encryption devices have been traditionally used for
   highly sensitive documents, secure fax on the Public Switched
   Telephone Network (PSTN) was not as widely considered or prioritized
   because of the challenges involved with physical access to telephony
   equipment.  As real-time communications transition to IP networks,
   where information might potentially be intercepted or spoofed, an
   appropriate level of security for fax that offers integrity and
   confidentiality protection is vital.  Some of the security mechanisms
   for securing fax include:

   o  [ITU.T30.2005] Annex H specifies integrity and confidentiality
      protection of fax in application layer, independent of protocol
      for fax transport.



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   o  [ITU.T38.2010] specifies fax transport over RTP/SAVP which enables
      integrity and confidentiality protection of fax in IP network.

   Despite these mechanisms to secure fax, there is no transport layer
   security offering integrity and confidentiality protection for UDPTL
   [ITU.T38.2010], the overwhelmingly predominant fax transport
   protocol.  The protocol stack for fax transport using UDPTL is shown
   in Table 1.

                      +-----------------------------+
                      |           Protocol          |
                      +-----------------------------+
                      | Internet facsimile protocol |
                      +-----------------------------+
                      |            UDPTL            |
                      +-----------------------------+
                      |             UDP             |
                      +-----------------------------+
                      |              IP             |
                      +-----------------------------+

                Table 1: Protocol stack for UDPTL over UDP

   The 3rd Generation Partnership Project (3GPP) has performed a study
   on how to provide secure fax in the IP Multimedia Subsystem (IMS) and
   concluded that secure fax shall be transported using UDPTL over DTLS.

   This document specifies fax transport using UDPTL over DTLS
   [RFC6347], which enables integrity and confidentiality protection of
   fax in IP networks.  The protocol stack for integrity and
   confidentiality protected fax transport using UDPTL over DTLS is
   shown in Table 2.

                      +-----------------------------+
                      |           Protocol          |
                      +-----------------------------+
                      | Internet facsimile protocol |
                      +-----------------------------+
                      |            UDPTL            |
                      +-----------------------------+
                      |             DTLS            |
                      +-----------------------------+
                      |             UDP             |
                      +-----------------------------+
                      |              IP             |
                      +-----------------------------+

                Table 2: Protocol stack for UDPTL over UDP



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   The primary motivations for the mechanism in this document are:

   o  The design of DTLS [RFC6347] is clearly defined, well understood
      and implementations are widely available.
   o  No DTLS extensions are required in order to enable UDPTL transport
      over DTLS.
   o  Fax transport using UDPTL over DTLS only requires insertion of the
      DTLS layer between the UDPTL layer and the UDP layer, as shown in
      Table 2.  The UDPTL layer and layers above UDPTL layer require no
      modification.
   o  UDPTL [ITU.T38.2010] is by far the most widely deployed fax
      transport protocol in IP networks.
   o  3GPP needs a mechanism to transport UDPTL over DTLS, in order to
      provide secure fax in IMS networks.

   This document specifies the transport of UDPTL over DTLS using the
   DTLS record layer "application_data" packets [RFC6347].

   Since the DTLS record layer "application_data" packet does not
   indicate whether it carries UDPTL, or some other protocol, the usage
   of a dedicated DTLS association for transport of UDPTL needs to be
   negotiated, e.g. using the Session Description Protocol (SDP)
   [RFC4566] and the SDP offer/answer mechanism [RFC3264].

   Therefore, this document specifies a new <proto> value [RFC4566] for
   the SDP media description ("m=" line) [RFC3264], in order to indicate
   UDPTL over DTLS in SDP messages [RFC4566].

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].

   DTLS uses the term "session" to refer to a long-lived set of keying
   material that spans DTLS associations.  In this document, in order to
   be consistent with SIP/SDP usage of "session" terminology, we use it
   to refer to a multimedia session and use the term "DTLS session" to
   refer to the DTLS construct.  We use the term "DTLS association" to
   refer to a particular DTLS cipher suite and keying material set that
   is associated with a single host/port quartet.  The same DTLS session
   can be used to establish the keying material for multiple DTLS
   associations.  For consistency with other SIP/SDP usage, we use the
   term "connection" when what's being referred to is a multimedia
   stream that is not specifically DTLS.





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3.  Secure Channel

3.1.  Secure Channel Establishment

   The SDP offer/answer mechanism [RFC3264] is used by other protocols,
   e.g. the Session Initiation Protocol (SIP) [RFC3261], to negotiate
   and establish multimedia sessions.

   In addition to the usual contents of an SDP media description ("m="
   line) specified for UDPTL over the UDP, each SDP media description
   for UDPTL over DTLS over the UDP will also contain several SDP
   attributes, as specified in [RFC4145] and [RFC4572].

   The SDP offer and SDP answer MUST conform to the following
   requirements:

   o  The endpoint MUST set the "proto" field of the "m=" line to the
      token specified in Table 3.
   o  The endpoint MUST use the SDP setup attribute [RFC4145].  The
      offerer SHOULD assign the SDP setup attribute with a setup:actpass
      value, and MAY assign the SDP setup attribute with a setup:active
      value or setup:passive value.  The offerer MUST NOT assign the SDP
      setup attribute with a setup:holdconn value.  If the offerer
      assigns the SDP setup attribute with a setup:actpass value or
      setup:passive value, it MUST be prepared to receive a DTLS
      client_hello message before it receives the SDP answer.  If the
      answerer accepts the media stream, then it MUST assign the SDP
      setup attribute with either a setup:active value or setup:passive
      value, according to the procedures in [RFC4145].  The answerer
      MUST NOT assign an SDP setup attribute with a setup:holdconn
      value.  Whichever party is active, it MUST initiate a DTLS
      handshake by sending a ClientHello over each flow (host/port
      quartet).
   o  If the endpoint supports, and is willing to use, a cipher suite
      with an associated certificate, it MUST include an SDP fingerprint
      attribute [RFC4572] in the SDP.
   o  If a cipher suite with an associated certificate is selected
      during the DTLS handshake, the certificate received during the
      DTLS handshake MUST match the fingerprint received in the SDP
      fingerprint attribute.  If the fingerprint does not match the
      hashed certificate, then the endpoint MUST tear down the media
      session immediately.  Note that it is permissible to wait until
      the other side's fingerprint has been received before establishing
      the connection; however, this may have undesirable latency
      effects.
   o  The endpoint MUST NOT use the SDP connection attribute [RFC4145].





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3.2.  Secure Channel Usage

   DTLS is used as specified in [RFC6347].  Once the DTLS handshake is
   completed, the UDPTL packets SHALL be transported in DTLS record
   layer "application_data" packets.

4.  Miscellaneous Considerations

4.1.  Anonymous Calls

   When making anonymous calls, a new self-signed certificate SHOULD be
   used for each call and the content of the subjectAltName attribute
   inside the certificate MUST NOT contain information that either
   allows correlation or identification of the user making anonymous
   calls.

4.2.  Middlebox Interaction

4.2.1.  ICE Interaction

   When ICE [RFC5245] is being used, the ICE connectivity checks are
   performed before the DTLS handshake begins.  Note that if aggressive
   nomination mode is used, multiple candidate pairs may be marked valid
   before ICE finally converges on a single candidate pair.  UAs MUST
   treat all ICE candidate pairs associated with a single component as
   part of the same DTLS association.  Thus, there will be only one DTLS
   handshake even if there are multiple valid candidate pairs.  Note
   that this may mean adjusting the endpoint IP addresses if the
   selected candidate pair shifts, just as if the DTLS packets were an
   ordinary media stream.

4.2.2.  Latching Control without ICE

   When ICE [RFC5245] is not being used and the DTLS handshake has not
   completed upon receiving the other side's SDP, then the passive side
   MUST do a single unauthenticated STUN [RFC5389] connectivity check in
   order to open up the appropriate pinhole.  All UAs MUST be prepared
   to answer this request during the handshake period even if they do
   not otherwise do ICE.  However, the active side MUST proceed with the
   DTLS handshake as appropriate even if no such STUN check is received
   and the passive MUST NOT wait for a STUN answer before sending its
   ServerHello.









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4.2.3.  STUN Interaction

   The UA SHALL send the STUN packets [RFC5389] directly over UDP, not
   over DTLS.

   The UA MUST demultiplex packets arriving on the IP address and port
   associated with the DTLS association as follows:

   o  If the value of the first byte of the packet is 0 or 1, then the
      packet is STUN.
   o  If the value of the first byte of the packet is between 20 and 63
      (inclusive), the packet is DTLS.

4.3.  Rekeying

   After the DTLS handshake caused by rekeying has completed, because of
   possible packet reordering on the wire, packets protected by the
   previous set of keys can arrive.  To compensate for this fact,
   receivers SHOULD maintain both sets of keys for some time in order to
   be able to decrypt and verify older packets.  The duration of
   maintaining the previous set of keys after the finish of the DTLS
   handshake is out of scope for this document.

5.  Security Considerations

   DTLS media signaled with SIP requires a mechanism to ensure that the
   communicating peers' certificates are correct.

   The standard DTLS strategy for authenticating the communicating
   parties is to give the server (and optionally the client) a PKIX
   [RFC5280] certificate.  The client then verifies the certificate and
   checks that the name in the certificate matches the server's domain
   name.  This works because there are a relatively small number of
   servers with well-defined names; a situation that does not usually
   occur in the VoIP context.

   The design described in this document is intended to leverage the
   authenticity of the signaling channel (while not requiring
   confidentiality).  As long as each side of the connection can verify
   the integrity of the SDP received from the other side, then the DTLS
   handshake cannot be hijacked via a man-in-the-middle attack.  This
   integrity protection is easily provided by the caller to the callee
   (see sample message flow in Annex A.2) via the SIP Identity [RFC4474]
   mechanism.  Other mechanisms, such as the S/MIME mechanism [RFC3261],
   or perhaps future mechanisms yet to be specified could also serve
   this purpose.





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   While this mechanism can still be used without such integrity
   mechanisms, the security provided is limited to defense against
   passive attack by intermediaries.  An active attack on the signaling
   plus an active attack on the media plane can allow an attacker to
   attack the connection (R-SIG-MEDIA in the notation of [RFC5479]).

6.  IANA Considerations

   This document updates the "Session Description Protocol (SDP)
   Parameters" registry as specified in Section 8.2.2 of [RFC4566].
   Specifically, it adds the values in Table 3 to the table for the SDP
   "proto" field registry.

                  +-------+---------------+------------+
                  |  Type |    SDP Name   | Reference  |
                  +-------+---------------+------------+
                  | proto | UDP/TLS/UDPTL | [RFC-XXXX] |
                  +-------+---------------+------------+

                     Table 3: SDP "proto" field values

   [RFC EDITOR NOTE: Please replace RFC-XXXX with the RFC number of this
   document.]

7.  Acknowledgments

   Special thanks to Peter Dawes, who provided comments on the initial
   version of the draft, and to Paul E. Jones, James Rafferty, Albrecht
   Schwarz and Oscar Ohlsson who provided valuable feedback and input on
   the MMUSIC mailing list.

8.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing]

   Changes from draft-ietf-mmusic-udptl-dtls-01

   o  Usage of the SDP fingerprint attribute depends on whether a cipher
      suite with an associated certificate is used.
   o  Editor's note in section 4.2 removed.  Procedure text added.

   Changes from draft-ietf-mmusic-udptl-dtls-00

   o  SDP offerer is allowed to assign an a=setup:active or
      a=setup:passive value, in addition to the recommended
      a=setup:actpass (http://www.ietf.org/mail-archive/web/mmusic/
      current/msg12331.html).




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   o  The example for secure fax replacing audio stream in audio-only
      session added (http://www.ietf.org/mail-archive/web/mmusic/current
      /msg12428.html).
   o  Editor's note on the connection attribute resolved by prohibiting
      usage of the SDP connection attribute (http://www.ietf.org/mail-
      archive/web/mmusic/current/msg12772.html).
   o  Editorial corrections.

   Changes from draft-holmberg-mmusic-udptl-dtls-02

   o  Milestone adopted - draft-ietf-mmusic version of the draft
      submitted.

   Changes from draft-holmberg-mmusic-udptl-dtls-01

   o  Gonzalo Salgueiro added as co-author.
   o  PSTN comparison text and Introduction text modified.

   Changes from draft-holmberg-mmusic-udptl-dtls-00

   o  Text about T.30 added.
   o  Latest version of T.38 referenced.
   o  Additional text about the need for secure fax in IP networks.

   Changes from draft-holmberg-dispatch-udptl-dtls-00

   o  WG changed to MMUSIC.
   o  Added text about 3GPP need for UDPTL/DTLS.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June
              2002.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.



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   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572, July 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [ITU.T30.2005]
              International Telecommunications Union, "Procedures for
              document facsimile transmission in the general switched
              telephone network", ITU-T Recommendation T.30, September
              2005.

   [ITU.T38.2010]
              International Telecommunications Union, "Procedures for
              real-time Group 3 facsimile communication over IP
              networks", ITU-T Recommendation T.38, September 2010.

9.2.  Informative References

   [RFC5479]  Wing, D., Fries, S., Tschofenig, H., and F. Audet,
              "Requirements and Analysis of Media Security Management
              Protocols", RFC 5479, April 2009.

Appendix A.  Example





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A.1.  General

   Prior to establishing the session, both Alice and Bob generate self-
   signed certificates which are used for a single session or, more
   likely, reused for multiple sessions.

   The SIP signaling from Alice to her proxy is transported over TLS to
   ensure an integrity protected channel between Alice and her identity
   service.  Transport between proxies should also be protected somehow.

   Only one element is shown for Alice's and Bob's proxies for the
   purposes of simplification.

   For the sake of brevity and simplicity, only the mandatory SDP T.38
   attributes are shown.

A.2.  Basic Message Flow with Identity

   Figure 1 shows an example message flow of session establishment for
   T.38 fax securely transported using UDPTL over DTLS.

   In this example flow, Alice acts as the passive endpoint of DTLS
   association and Bob acts as the active endpoint of DTLS association.

   Alice                    Proxies                   Bob
     | (1) SIP INVITE         |                        |
     |----------------------->|                        |
     |                        | (2) SIP INVITE         |
     |                        |----------------------->|
     |                        |   (3) DTLS ClientHello |
     |<------------------------------------------------|
     |    (4) remaining messages of DTLS handshake     |
     |<----------------------------------------------->|
     |                        |                        |
     |                        |                        |
     |                        |         (5) SIP 200 OK |
     |                        |<-----------------------|
     |         (6) SIP 200 OK |                        |
     |<-----------------------|                        |
     | (7) SIP ACK            |                        |
     |------------------------------------------------>|
     |      (8) T.38 message using UDPTL over DTLS     |
     |<----------------------------------------------->|
     |                        |                        |

                Figure 1: Basic message flow with Identity

   Message (1):



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      Figure 2 shows the initial INVITE request sent by Alice to Alice's
      proxy.  The initial INVITE request contains an SDP offer.


      The "m=" line in the SDP offer indicates T.38 fax using UDPTL over
      DTLS.


      The SDP setup:actpass attribute in the SDP offer indicates that
      Alice has requested to be either the active or passive endpoint.


      The SDP fingerprint attribute in the SDP offer indicates the
      certificate fingerprint computed from Alice's self-signed
      certificate.





   INVITE sip:bob@example.com SIP/2.0
   To: <sip:bob@example.com>
   From: "Alice"<sip:alice@example.com>;tag=843c7b0b
   Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
   Contact: <sip:alice@ua1.example.com>
   Call-ID: 6076913b1c39c212@REVMTEpG
   CSeq: 1 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE
   Max-Forwards: 70
   Content-Type: application/sdp
   Content-Length: xxxx
   Supported: from-change

   v=0
   o=- 1181923068 1181923196 IN IP4 ua1.example.com
   s=-
   c=IN IP4 ua1.example.com
   t=0 0
   m=image 6056 UDP/TLS/UDPTL t38
   a=setup:actpass
   a=fingerprint: SHA-1 \
     4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=T38FaxRateManagement:transferredTCF

                           Figure 2: Message (1)

   Message (2):




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      Figure 3 shows the SIP INVITE request sent by Bob's proxy to Bob.


      The SIP INVITE request contains an Identity header field and an
      Identity-Info header fields inserted by Alice's proxy.


      When received, Bob verifies the identity provided in the SIP
      INVITE request.



   INVITE sip:bob@ua2.example.com SIP/2.0
   To: <sip:bob@example.com>
   From: "Alice"<sip:alice@example.com>;tag=843c7b0b
   Via: SIP/2.0/TLS proxy.example.com;branch=z9hG4bK-0e53sadfkasldk
   Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
   Record-Route: <sip:proxy.example.com;lr>
   Contact: <sip:alice@ua1.example.com>
   Call-ID: 6076913b1c39c212@REVMTEpG
   CSeq: 1 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE
   Max-Forwards: 69
   Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k
             3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFC
             HYWGCl0nB2sNsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
   Identity-Info: https://example.com/cert
   Content-Type: application/sdp
   Content-Length: xxxx
   Supported: from-change

   v=0
   o=- 1181923068 1181923196 IN IP4 ua1.example.com
   s=-
   c=IN IP4 ua1.example.com
   t=0 0
   m=image 6056 UDP/TLS/UDPTL t38
   a=setup:actpass
   a=fingerprint: SHA-1 \
     4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=T38FaxRateManagement:transferredTCF

                           Figure 3: Message (2)

   Message (3):

      Assuming that Alice's identity is valid, Bob sends a DTLS
      ClientHello directly to Alice.



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   Message (4):

      Alice and Bob exchange further messages of DTLS handshake
      (HelloVerifyRequest, ClientHello, ServerHello, Certificate,
      ServerKeyExchange, CertificateRequest, ServerHelloDone,
      Certificate, ClientKeyExchange, CertificateVerify,
      ChangeCipherSpec, Finished).


      When Bob receives the certificate of Alice via DTLS, Bob checks
      whether the certificate fingerprint calculated from the Alice's
      certificate received via DTLS matches the certificate fingerprint
      received in the a=fingerprint SDP attribute of Figure 3.  In this
      message flow, the check is successful and thus session setup
      continues.

   Message (5):

      Figure 4 shows a SIP 200 (OK) response to the initial SIP INVITE
      request, sent by Bob to Bob's proxy.  The SIP 200 (OK) response
      contains an SDP answer.


      The "m=" line in the SDP answer indicates T.38 fax using UDPTL
      over DTLS.


      The SDP setup:active attribute in the SDP answer indicates that
      Bob has requested to be the active endpoint.


      The SDP fingerprint attribute in the SDP answer indicates the
      certificate fingerprint computed from Bob's self-signed
      certificate.





   SIP/2.0 200 OK
   To: <sip:bob@example.com>;tag=6418913922105372816
   From: "Alice" <sip:alice@example.com>;tag=843c7b0b
   Via: SIP/2.0/TLS proxy.example.com:5061;branch=z9hG4bK-0e53sadfkasldk
   Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
   Record-Route: <sip:proxy.example.com;lr>
   Call-ID: 6076913b1c39c212@REVMTEpG
   CSeq: 1 INVITE
   Contact: <sip:bob@ua2.example.com>



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   Content-Type: application/sdp
   Content-Length: xxxx
   Supported: from-change

   v=0
   o=- 8965454521 2105372818 IN IP4 ua2.example.com
   s=-
   c=IN IP4 ua2.example.com
   t=0 0
   m=image 12000 UDP/TLS/UDPTL t38
   a=setup:active
   a=fingerprint: SHA-1 \
     FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=T38FaxRateManagement:transferredTCF

                           Figure 4: Message (5)

   Message (6):

      Figure 5 shows a SIP 200 (OK) response to the initial SIP INVITE
      request, sent by Alice's proxy to Alice.  Alice checks if the
      certificate fingerprint calculated from the Bob's certificate
      received via DTLS is the same as the certificate fingerprint
      received in the a=fingerprint SDP attribute of Figure 5.  In this
      message flow, the check is successful and thus session setup
      continues.



     SIP/2.0 200 OK
     To: <sip:bob@example.com>;tag=6418913922105372816
     From: "Alice" <sip:alice@example.com>;tag=843c7b0b
     Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
     Record-Route: <sip:proxy.example.com;lr>
     Call-ID: 6076913b1c39c212@REVMTEpG
     CSeq: 1 INVITE
     Contact: <sip:bob@ua2.example.com>
     Content-Type: application/sdp
     Content-Length: xxxx
     Supported: from-change

     v=0
     o=- 8965454521 2105372818 IN IP4 ua2.example.com
     s=-
     c=IN IP4 ua2.example.com
     t=0 0
     m=image 12000 UDP/TLS/UDPTL t38
     a=setup:active



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     a=fingerprint: SHA-1 \
       FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
     a=T38FaxRateManagement:transferredTCF

                           Figure 5: Message (6)

   Message (7):

      Alice sends the SIP ACK request to Bob.

   Message (8):

      At this point, Bob and Alice can exchange T.38 fax securely
      transported using UDPTL over DTLS.


A.3.  Message Flow Of T.38 Fax Replacing Audio Media Stream in An
      Existing Audio-Only Session

   Figure 6 focuses on T.38 fax securely transported using UDPTL over
   DTLS replacing audio media stream in an existing audio-only session.

   In this example flow, Alice acts as the passive endpoint of DTLS
   association and Bob acts as the active endpoint of DTLS association.

   Alice                    Proxies                   Bob
     |                        |                        |
     |        (1) Audio-only session initiation        |
     |<-----------------------+----------------------->|
     |                        |                        |
     | (2) SIP re-INVITE      |                        |
     |------------------------------------------------>|
     |                        |   (3) DTLS ClientHello |
     |<------------------------------------------------|
     |    (4) remaining messages of DTLS handshake     |
     |<----------------------------------------------->|
     |                        |                        |
     |                        |                        |
     |                        |         (5) SIP 200 OK |
     |<------------------------------------------------|
     | (6) SIP ACK            |                        |
     |------------------------------------------------>|
     |      (7) T.38 message using UDPTL over DTLS     |
     |<----------------------------------------------->|
     |                        |                        |

   Figure 6: Message Flow Of T.38 Fax Replacing Audio Media Stream in An
                        Existing Audio-Only Session



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   Message (1):

      Session establishment of audio-only session.  The proxies decide
      not to record-route.

   Message (2):

      Alice sends SIP re-INVITE request.  SDP offer included in the SIP
      re-INVITE request shown in Figure 7.


      The first "m=" line in the SDP offer indicates audio media stream
      being removed.  The second "m=" line in the SDP offer indicates
      T.38 fax using UDPTL over DTLS being added.


      The SDP setup:actpass attribute in the SDP offer indicates that
      Alice has requested to be either the active or passive endpoint.


      The SDP fingerprint attribute in the SDP offer indicates the
      certificate fingerprint computed from Alice's self-signed
      certificate.





   v=0
   o=- 2465353433 3524244442 IN IP4 ua1.example.com
   s=-
   c=IN IP4 ua1.example.com
   t=0 0
   m=audio 0 UDP/TLS/RTP/SAVP 0
   m=image 46056 UDP/TLS/UDPTL t38
   a=setup:actpass
   a=fingerprint: SHA-1 \
     4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=T38FaxRateManagement:transferredTCF

                    Figure 7: SDP offer of message (2)

   Message (3):

      Bob sends a DTLS ClientHello directly to Alice.

   Message (4):




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      Alice and Bob exchange further messages of DTLS handshake
      (HelloVerifyRequest, ClientHello, ServerHello, Certificate,
      ServerKeyExchange, CertificateRequest, ServerHelloDone,
      Certificate, ClientKeyExchange, CertificateVerify,
      ChangeCipherSpec, Finished).


      When Bob receives Alice's certificate via DTLS, Bob checks whether
      the certificate fingerprint calculated from Alice's certificate
      received via DTLS matches the certificate fingerprint received in
      the a=fingerprint SDP attribute of Figure 7.  In this message
      flow, the check is successful and thus session setup continues.

   Message (5):

      Bob sends a SIP 200 (OK) response to the SIP re-INVITE request.
      The SIP 200 (OK) response contains an SDP answer shown in Figure
      8.


      The first "m=" line in the SDP offer indicates audio media stream
      being removed.  The second "m=" line in the SDP answer indicates
      T.38 fax using UDPTL over DTLS being added.


      The SDP setup:active attribute in the SDP answer indicates that
      Bob has requested to be the active endpoint.


      The SDP fingerprint attribute in the SDP answer indicates the
      certificate fingerprint computed from Bob's self-signed
      certificate.





   v=0
   o=- 4423478999 5424222292 IN IP4 ua2.example.com
   s=-
   c=IN IP4 ua2.example.com
   t=0 0
   m=audio 0 UDP/TLS/RTP/SAVP 0
   m=image 32000 UDP/TLS/UDPTL t38
   a=setup:active
   a=fingerprint: SHA-1 \
     FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   a=T38FaxRateManagement:transferredTCF



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                    Figure 8: SDP answer of message (5)

   Message (6):

      Alice sends the SIP ACK request to Bob.

   Message (7):

      At this point, Bob and Alice can exchange T.38 fax securely
      transported using UDPTL over DTLS.


Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com


   Ivo Sedlacek
   Ericsson
   Sokolovska 79
   Praha  18600
   Czech Republic

   Email: ivo.sedlacek@ericsson.com


   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   US

   Email: gsalguei@cisco.com












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