One document matched: draft-ietf-mmusic-udptl-dtls-03.xml
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
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]>
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<?rfc compact="yes" ?>
<?rfc subcompact="yes" ?>
<?rfc sortrefs="no" ?>
<?rfc strict="yes" ?>
<rfc ipr="trust200902" category="std" docName="draft-ietf-mmusic-udptl-dtls-03" obsoletes="" updates="" submissionType="IETF" xml:lang="en">
<front>
<title abbrev="UDPTL over DTLS">
UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS)
</title>
<author initials="C.H." surname="Holmberg" fullname="Christer Holmberg">
<organization>Ericsson</organization>
<address>
<postal>
<street>Hirsalantie 11</street>
<code>02420</code>
<city>Jorvas</city>
<country>Finland</country>
</postal>
<email>christer.holmberg@ericsson.com</email>
</address>
</author>
<author initials="I.S." surname="Sedlacek" fullname="Ivo Sedlacek">
<organization>Ericsson</organization>
<address>
<postal>
<street>Sokolovska 79</street>
<code>18600</code>
<city>Praha</city>
<country>Czech Republic</country>
</postal>
<email>ivo.sedlacek@ericsson.com</email>
</address>
</author>
<author initials="G.S" surname="Salgueiro" fullname="Gonzalo Salgueiro">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>7200-12 Kit Creek Road</street>
<city>Research Triangle Park</city>
<region>NC</region>
<code>27709</code>
<country>US</country>
</postal>
<email>gsalguei@cisco.com</email>
</address>
</author>
<date year="2014" />
<area>Transport</area>
<workgroup>MMUSIC Working Group</workgroup>
<keyword>SDP</keyword>
<keyword>SIP</keyword>
<keyword>DTLS</keyword>
<keyword>UDPTL</keyword>
<keyword>fax</keyword>
<keyword>transport</keyword>
<abstract>
<t>
This document specifies how the UDP Transport Layer (UDPTL) protocol,
the predominant transport protocol for T.38 fax, 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).
</t>
</abstract>
</front>
<middle>
<section title="Introduction" toc="default">
<t>
While it is possible to transmit highly sensitive documents using
traditional telephony encryption devices, secure fax on the Public
Switched Telephone Network (PSTN) was never widely considered or
prioritized. This was mainly 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.
</t>
<t>
The overwhelmingly predominant fax transport protocol is UDPTL-based
<xref target="ITU.T38.2010" pageno="false" format="default" />. The
protocol stack for fax transport using UDPTL is shown in <xref
target="table_UDPTL_UDP_stack" pageno="false" format="default"/>.
</t>
<texttable anchor="table_UDPTL_UDP_stack" title="Protocol stack for UDPTL over UDP" style="all">
<ttcol align='center'>Internet facsimile protocol</ttcol>
<c>UDPTL</c>
<c>UDP</c>
<c>IP</c>
</texttable>
<t>
Implementations exist today for securing this fax transport type. Some of these
mechanisms are:
</t>
<t>
<list style="symbols">
<t>
<xref target="ITU.T30.2005" pageno="false" format="default" /> Annex H
specifies integrity and confidentiality protection of fax in the application
layer, independent of protocol for fax transport.
</t>
<t>
<xref target="ITU.T38.2010" pageno="false" format="default" /> specifies
fax transport over RTP/SAVP which enables integrity and confidentiality
protection of fax in IP network.
</t>
</list>
</t>
<t>
Despite these mechanisms to secure fax, there is no transport layer
security offering integrity and confidentiality protection for UDPTL. This issue
was addressed in a study by the 3rd Generation Partnership Project (3GPP)
on how to provide secure fax in the IP Multimedia Subsystem (IMS). They
concluded that secure fax shall be transported using UDPTL over DTLS.
</t>
<t>
This document specifies fax transport using UDPTL over DTLS <xref target="RFC6347"
pageno="false" format="default"/>, which enables integrity and confidentiality
protection of fax in IP networks. The protocol stack which enhances fax transport
to offer integrity and confidentiality using UDPTL over DTLS is shown in
<xref target="table_UDPTL_DTLS_UDP_stack" pageno="false" format="default"/>.
</t>
<texttable anchor="table_UDPTL_DTLS_UDP_stack" title="Protocol stack for UDPTL over DTLS over UDP" style="all">
<ttcol align='center'>Internet facsimile protocol</ttcol>
<c>UDPTL</c>
<c>DTLS</c>
<c>UDP</c>
<c>IP</c>
</texttable>
<t>
The primary motivations for the mechanism in this document are:
<list style="symbols">
<t>
The design of DTLS <xref target="RFC6347" pageno="false" format="default"/> is
clearly defined, well understood and implementations are widely available.
</t>
<t>
No DTLS extensions are required in order to enable UDPTL transport over DTLS.
</t>
<t>
Fax transport using UDPTL over DTLS only requires insertion of the DTLS layer between
the UDPTL layer and the UDP layer, as shown in <xref target="table_UDPTL_DTLS_UDP_stack"
pageno="false" format="default"/>. The UDPTL layer and layers above UDPTL layer require
no modification.
</t>
<t>
UDPTL <xref target="ITU.T38.2010" pageno="false" format="default" /> is by far the
most widely deployed fax transport protocol in IP networks.
</t>
<t>
3GPP and the IP fax community need a mechanism to transport UDPTL over DTLS in order to
provide secure fax in IMS and other SIP-based networks.
</t>
</list>
</t>
<t>
This document specifies the transport of UDPTL over DTLS using the DTLS record
layer "application_data" packets <xref target="RFC6347" pageno="false" format="default"/>.
</t>
<t>
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) <xref target="RFC4566" pageno="false" format="default"/> and the SDP
offer/answer mechanism <xref target="RFC3264" pageno="false" format="default"/>.
</t>
<t>
Therefore, this document specifies a new <proto> value <xref target="RFC4566"
pageno="false" format="default"/> for the SDP media description ("m=" line)
<xref target="RFC3264" pageno="false" format="default"/>, in order to indicate
UDPTL over DTLS in SDP messages <xref target="RFC4566" pageno="false" format="default"/>.
</t>
</section>
<section title="Conventions" toc="default">
<t>
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 <xref target="RFC2119" pageno="false" format="default" />.
</t>
<t>
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 "session" 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.
</t>
</section>
<section title="Secure Channel" toc="default">
<section title="Secure Channel Establishment" toc="default">
<t>
The SDP offer/answer mechanism <xref target="RFC3264" pageno="false" format="default" />
is used by other protocols, e.g. the Session Initiation Protocol (SIP) <xref target="RFC3261"
pageno="false" format="default" />, to negotiate and establish multimedia sessions.
</t>
<t>
In addition to the usual contents of an SDP media description ("m=" line) specified for
UDPTL over UDP, each SDP media description for UDPTL over DTLS over UDP will also
contain several SDP attributes, which were introduced in the context of TCP
<xref target="RFC4145" pageno="false" format="default" /> and TLS
<xref target="RFC4572" pageno="false" format="default" />, and are re-used in this
document.
</t>
<t>
The SDP offer and the SDP answer MUST conform to the following requirements:
<list style="symbols">
<t>
The endpoint MUST set the "proto" field of the "m=" line to the token specified in
<xref target="table_SDP_proto_values" pageno="false" format="default" />.
</t>
<t>
In order to negotiate the TLS roles, the endpoint MUST use the SDP setup attribute <xref target="RFC4145" pageno="false"
format="default" />. 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 <xref
target="RFC4145" pageno="false" format="default" />. 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).
</t>
<t>
If the endpoint supports, and is willing to use, a cipher suite with an associated certificate,
it MUST include an SDP fingerprint attribute <xref target="RFC4572" pageno="false" format="default" />
in the SDP.
</t>
<t>
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.
</t>
<t>
The endpoint MUST NOT use the SDP connection attribute <xref target="RFC4145" pageno="false"
format="default" />.
</t>
</list>
</t>
</section>
<section title="Secure Channel Usage" toc="default">
<t>
DTLS is used as specified in <xref target="RFC6347" pageno="false" format="default" />.
Once the DTLS handshake is successfully completed (in order to prevent facsimile data from being
transmitted insecurely), the UDPTL packets SHALL be transported in DTLS record layer "application_data"
packets.
</t>
</section>
</section>
<section title="Miscellaneous Considerations" toc="default">
<section title="Anonymous Calls" toc="default">
<t>
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.
</t>
</section>
<section title="Middlebox Interaction" toc="default">
<section title="ICE Interaction" toc="default">
<t>
When ICE <xref target="RFC5245" pageno="false" format="default" /> 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.
</t>
</section>
<section title="Latching Control without ICE" toc="default">
<t>
When ICE <xref target="RFC5245" pageno="false" format="default" /> 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 <xref target="RFC5389" pageno="false" format="default" />
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.
</t>
</section>
<section title="STUN Interaction" toc="default">
<t>
The UA SHALL send the STUN packets <xref target="RFC5389" pageno="false" format="default" />
directly over UDP, not over DTLS.
</t>
<t>
The UA MUST demultiplex packets arriving on the IP address and port associated with
the DTLS association as follows:
<list style="symbols">
<t>
If the value of the first byte of the packet is 0 or 1, then the packet is STUN.
</t>
<t>
If the value of the first byte of the packet is between 20 and 63 (inclusive),
the packet is DTLS.
</t>
</list>
</t>
</section>
</section>
<section title="Rekeying" toc="default">
<t>
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.
</t>
</section>
</section>
<section anchor="section.sec" title="Security Considerations">
<t>
Fax may be used to transmit a wide range of sensitive data, including
personal, corporate, and governmental information. It is therefore
critical to be able to protect against threats to the confidentiality
and integrity of the transmitted data.
</t>
<t>
The mechanism in this document provides integrity and confidentiality
protection for fax by specifying fax transport using UDPTL over DTLS
<xref target="RFC6347" pageno="false" format="default" />.
</t>
<t>
DTLS media signaled with SIP requires a mechanism to ensure that the
communicating peers' certificates are correct.
</t>
<t>
The standard DTLS strategy for authenticating the communicating
parties is to give the server (and optionally the client) a PKIX
<xref target="RFC5280" pageno="false" format="default" /> 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.
</t>
<t>
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
via the SIP Identity <xref target="RFC4474"
pageno="false" format="default" /> mechanism. Other mechanisms, such as the
S/MIME mechanism <xref target="RFC3261" pageno="false" format="default" />,
or perhaps future mechanisms yet to be specified could also serve this purpose.
</t>
<t>
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 <xref target="RFC5479"
pageno="false" format="default" />).
</t>
</section>
<section anchor="section.iana" title="IANA Considerations">
<t>
This document updates the "Session Description Protocol (SDP) Parameters" registry as
specified in Section 8.2.2 of <xref target="RFC4566" pageno="false" format="default"/>.
Specifically, it adds the values in <xref target="table_SDP_proto_values"
pageno="false" format="default"/> to the table for the SDP "proto" field
registry.
</t>
<texttable anchor="table_SDP_proto_values" title='SDP "proto" field values'>
<ttcol align='center'>Type</ttcol>
<ttcol align='center'>SDP Name</ttcol>
<ttcol align='center'>Reference</ttcol>
<c>proto</c>
<c>"UDP/TLS/UDPTL"</c>
<c>[RFC-XXXX]</c>
</texttable>
<t>
[RFC EDITOR NOTE: Please replace RFC-XXXX with the RFC number of this document.]
</t>
</section>
<section title="Acknowledgments">
<t>
Special thanks to Peter Dawes, who provided comments on the initial version of the draft, and to
Paul E. Jones, James Rafferty, Albrecht Schwarz, Oscar Ohlsson and David Hanes who provided
valuable feedback and input on the MMUSIC mailing list.
</t>
</section>
<section title="Change Log">
<t>
[RFC EDITOR NOTE: Please remove this section when publishing]
</t>
<t>
Changes from draft-ietf-mmusic-udptl-dtls-02
<list style="symbols">
<t>Editorial comments based on review comments by James Rafferty (http://www.ietf.org/mail-archive/web/mmusic/current/msg12890.html)</t>
<t>Editorial comments based on review comments by David Hanes (http://www.ietf.org/mail-archive/web/mmusic/current/msg12886.html)</t>
<t>Editorial comments based on review comments by Oscar Ohlsson (http://www.ietf.org/mail-archive/web/mmusic/current/msg12882.html)</t>
<t>Editorial comments based on review comments by Albrecht Schwartz (http://www.ietf.org/mail-archive/web/mmusic/current/msg12900.html)</t>
</list>
</t>
<t>
Changes from draft-ietf-mmusic-udptl-dtls-01
<list style="symbols">
<t>Usage of the SDP fingerprint attribute depends on whether a cipher suite with
an associated certificate is used.</t>
<t>Editor's note in section 4.2 removed. Procedure text added.</t>
</list>
</t>
<t>
Changes from draft-ietf-mmusic-udptl-dtls-00
<list style="symbols">
<t>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).</t>
<t>The example for secure fax replacing audio stream in audio-only session
added (http://www.ietf.org/mail-archive/web/mmusic/current/msg12428.html).</t>
<t>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).</t>
<t>Editorial corrections.</t>
</list>
</t>
<t>
Changes from draft-holmberg-mmusic-udptl-dtls-02
<list style="symbols">
<t>Milestone adopted - draft-ietf-mmusic version of the draft submitted.</t>
</list>
</t>
<t>
Changes from draft-holmberg-mmusic-udptl-dtls-01
<list style="symbols">
<t>Gonzalo Salgueiro added as co-author.</t>
<t>PSTN comparison text and Introduction text modified.</t>
</list>
</t>
<t>
Changes from draft-holmberg-mmusic-udptl-dtls-00
<list style="symbols">
<t>Text about T.30 added.</t>
<t>Latest version of T.38 referenced.</t>
<t>Additional text about the need for secure fax in IP networks.</t>
</list>
</t>
<t>
Changes from draft-holmberg-dispatch-udptl-dtls-00
<list style="symbols">
<t>WG changed to MMUSIC.</t>
<t>Added text about 3GPP need for UDPTL/DTLS.</t>
</list>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.3261"?>
<?rfc include="reference.RFC.3264"?>
<?rfc include="reference.RFC.4145"?>
<?rfc include="reference.RFC.4474"?>
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.RFC.4572"?>
<?rfc include="reference.RFC.5245"?>
<?rfc include="reference.RFC.5280"?>
<?rfc include="reference.RFC.5389"?>
<?rfc include="reference.RFC.6347"?>
<reference anchor="ITU.T30.2005">
<front>
<title>Procedures for document facsimile transmission in the general switched telephone network</title>
<author>
<organization>International Telecommunications Union</organization>
</author>
<date month="September" year="2005"/>
</front>
<seriesInfo name="ITU-T" value="Recommendation T.30"/>
</reference>
<reference anchor="ITU.T38.2010">
<front>
<title>Procedures for real-time Group 3 facsimile communication over IP networks</title>
<author>
<organization>International Telecommunications Union</organization>
</author>
<date month="September" year="2010"/>
</front>
<seriesInfo name="ITU-T" value="Recommendation T.38"/>
</reference>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.5479"?>
</references>
<section anchor="section.example" title="Examples">
<section title="General">
<t>
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.
</t>
<t>
The SIP signaling from Alice to her proxy is transported over TLS to ensure an integrity
protected channel between Alice and her identity service. Alice's identity service asserts identity of Alice and protects the SIP message, e.g. using SIP Identity. Transport between proxies should
also be protected somehow.
</t>
<t>
Only one element is shown for Alice's and Bob's proxies for the purposes of simplification.
</t>
<t>
For the sake of brevity and simplicity, only the mandatory SDP T.38
attributes are shown.
</t>
</section>
<section title="Basic Message Flow">
<t>
<xref target="example_1_figure" pageno="false" format="default" /> shows an example message flow of session establishment for T.38 fax securely transported using UDPTL over DTLS.
</t>
<t>
In this example flow, Alice acts as the passive endpoint of the DTLS association and Bob acts as the active endpoint of the DTLS association.
</t>
<t>
<figure anchor="example_1_figure" title="Basic message flow">
<artwork><![CDATA[
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 |
|<----------------------------------------------->|
| | |
]]></artwork>
</figure>
</t>
<t>
<list style="hanging">
<t hangText="Message (1):">
<vspace blankLines="1"/>
<xref target="example_1_message_1" pageno="false" format="default" /> shows the initial INVITE request sent by Alice to Alice's proxy. The initial INVITE request contains an SDP offer.
</t>
<t>
<vspace blankLines="1"/>
The "m=" line in the SDP offer indicates T.38 fax using UDPTL over DTLS.
</t>
<t>
<vspace blankLines="1"/>
The SDP setup:actpass attribute in the SDP offer indicates that Alice has requested to be either the active or passive endpoint.
</t>
<t>
<vspace blankLines="1"/>
The SDP fingerprint attribute in the SDP offer contains the certificate fingerprint computed from Alice's self-signed certificate.
</t>
<t>
<vspace blankLines="2"/>
</t>
<t>
<figure anchor="example_1_message_1" title="Message (1)">
<artwork><![CDATA[
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
]]></artwork>
</figure>
<vspace blankLines="1"/>
</t>
<t hangText="Message (2):">
<vspace blankLines="1"/>
<xref target="example_1_message_2" pageno="false" format="default" /> shows the SIP INVITE request sent by Bob's proxy to Bob.
</t>
<t>
<vspace blankLines="1"/>
When received, Bob verifies the identity provided in the SIP INVITE request.
<vspace blankLines="2"/>
<figure anchor="example_1_message_2" title="Message (2)">
<artwork><![CDATA[
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
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
]]></artwork>
</figure>
<vspace blankLines="1"/>
</t>
<t hangText="Message (3):">
<vspace blankLines="1"/>
Assuming that Alice's identity is valid, Bob sends a DTLS ClientHello directly to Alice.
<vspace blankLines="1"/>
</t>
<t hangText="Message (4):">
<vspace blankLines="1"/>
Alice and Bob exchange further messages of DTLS handshake (HelloVerifyRequest, ClientHello, ServerHello, Certificate, ServerKeyExchange, CertificateRequest, ServerHelloDone, Certificate, ClientKeyExchange, CertificateVerify, ChangeCipherSpec, Finished).
</t>
<t>
<vspace blankLines="1"/>
When Bob receives the certificate of Alice 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 <xref target="example_1_message_2" pageno="false" format="default" />. In this message flow, the check is successful and thus session setup continues.
<vspace blankLines="1"/>
</t>
<t hangText="Message (5):">
<vspace blankLines="1"/>
<xref target="example_1_message_5" pageno="false" format="default" /> 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.
</t>
<t>
<vspace blankLines="1"/>
The "m=" line in the SDP answer indicates T.38 fax using UDPTL over DTLS.
</t>
<t>
<vspace blankLines="1"/>
The SDP setup:active attribute in the SDP answer indicates that Bob has requested to be the active endpoint.
</t>
<t>
<vspace blankLines="1"/>
The SDP fingerprint attribute in the SDP answer contains the certificate fingerprint computed from Bob's self-signed certificate.
</t>
<t>
<vspace blankLines="2"/>
</t>
<t>
<figure anchor="example_1_message_5" title="Message (5)">
<artwork><![CDATA[
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>
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
]]></artwork>
</figure>
<vspace blankLines="1"/>
</t>
<t hangText="Message (6):">
<vspace blankLines="1"/>
<xref target="example_1_message_6" pageno="false" format="default" /> 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 <xref target="example_1_message_6" pageno="false" format="default" />. In this message flow, the check is successful and thus session setup continues.
<vspace blankLines="1"/>
</t>
<t>
<figure anchor="example_1_message_6" align="center" title="Message (6)">
<artwork><![CDATA[
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
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
]]></artwork>
</figure>
<vspace blankLines="1"/>
</t>
<t hangText="Message (7):">
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Alice sends the SIP ACK request to Bob.
<vspace blankLines="1"/>
</t>
<t hangText="Message (8):">
<vspace blankLines="1"/>
At this point, Bob and Alice can exchange T.38 fax securely transported using UDPTL over DTLS.
<vspace blankLines="1"/>
</t>
</list>
</t>
</section>
<section title="Message Flow Of T.38 Fax Replacing Audio Media Stream in An Existing Audio-Only Session">
<t>
Traditionally, most session with non-secure transport of T.38 fax,
transported using UDPTL, are established by modifying an ongoing
audio session into a fax session. <xref target="example_2_figure"
pageno="false" format="default" /> shows an example message flow of
modifying an existing audio session into a session with T.38 fax
securely transported using UDPTL over DTLS.
</t>
<t>
In this example flow, Alice acts as the passive endpoint of the DTLS association
and Bob acts as the active endpoint of the DTLS association.
</t>
<t>
<figure anchor="example_2_figure" title="Message Flow Of T.38 Fax Replacing Audio Media Stream in An Existing Audio-Only Session">
<artwork><![CDATA[
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 |
|<----------------------------------------------->|
| | |
]]></artwork>
</figure>
</t>
<t>
<list style="hanging">
<t hangText="Message (1):">
<vspace blankLines="1"/>
Session establishment of audio-only session. The proxies decide not to record-route.
<vspace blankLines="1"/>
</t>
<t hangText="Message (2):">
<vspace blankLines="1"/>
Alice sends SIP re-INVITE request. The SDP offer included in the SIP re-INVITE request is shown in <xref target="example_2_message_2" pageno="false" format="default" />.
</t>
<t>
<vspace blankLines="1"/>
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.
</t>
<t>
<vspace blankLines="1"/>
The SDP setup:actpass attribute in the SDP offer indicates that Alice has requested to be either the active or passive endpoint.
</t>
<t>
<vspace blankLines="1"/>
The SDP fingerprint attribute in the SDP offer contains the certificate fingerprint computed from Alice's self-signed certificate.
</t>
<t>
<vspace blankLines="2"/>
</t>
<t>
<figure anchor="example_2_message_2" title="SDP offer of message (2)">
<artwork><![CDATA[
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
]]></artwork>
</figure>
<vspace blankLines="1"/>
</t>
<t hangText="Message (3):">
<vspace blankLines="1"/>
Bob sends a DTLS ClientHello directly to Alice.
<vspace blankLines="1"/>
</t>
<t hangText="Message (4):">
<vspace blankLines="1"/>
Alice and Bob exchange further messages of DTLS handshake (HelloVerifyRequest, ClientHello, ServerHello, Certificate, ServerKeyExchange, CertificateRequest, ServerHelloDone, Certificate, ClientKeyExchange, CertificateVerify, ChangeCipherSpec, Finished).
</t>
<t>
<vspace blankLines="1"/>
When Bob receives the certificate of Alice 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 <xref target="example_2_message_2" pageno="false" format="default" />. In this message flow, the check is successful and thus session setup continues.
<vspace blankLines="1"/>
</t>
<t hangText="Message (5):">
<vspace blankLines="1"/>
Bob sends a SIP 200 (OK) response to the SIP re-INVITE request. The SIP 200 (OK) response contains an SDP answer shown in <xref target="example_2_message_5" pageno="false" format="default" />.
</t>
<t>
<vspace blankLines="1"/>
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.
</t>
<t>
<vspace blankLines="1"/>
The SDP setup:active attribute in the SDP answer indicates that Bob has requested to be the active endpoint.
</t>
<t>
<vspace blankLines="1"/>
The SDP fingerprint attribute in the SDP answer contains the certificate fingerprint computed from Bob's self-signed certificate.
</t>
<t>
<vspace blankLines="2"/>
</t>
<t>
<figure anchor="example_2_message_5" title="SDP answer of message (5)">
<artwork><![CDATA[
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
]]></artwork>
</figure>
<vspace blankLines="1"/>
</t>
<t hangText="Message (6):">
<vspace blankLines="1"/>
Alice sends the SIP ACK request to Bob.
<vspace blankLines="1"/>
</t>
<t hangText="Message (7):">
<vspace blankLines="1"/>
At this point, Bob and Alice can exchange T.38 fax securely transported using UDPTL over DTLS.
<vspace blankLines="1"/>
</t>
</list>
</t>
</section>
</section>
</back>
</rfc>
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