draft-ietf-mmusic-udptl-dtls-04.txt   draft-ietf-mmusic-udptl-dtls-05.txt 
MMUSIC Working Group C. Holmberg MMUSIC Working Group C. Holmberg
Internet-Draft I. Sedlacek Internet-Draft I. Sedlacek
Intended status: Standards Track Ericsson Intended status: Standards Track Ericsson
Expires: August 5, 2014 G. Salgueiro Expires: August 18, 2014 G. Salgueiro
Cisco Cisco
February 1, 2014 February 14, 2014
UDP Transport Layer (UDPTL) over Datagram Transport Layer Security UDP Transport Layer (UDPTL) over Datagram Transport Layer Security
(DTLS) (DTLS)
draft-ietf-mmusic-udptl-dtls-04 draft-ietf-mmusic-udptl-dtls-05
Abstract Abstract
This document specifies how the UDP Transport Layer (UDPTL) protocol, This document specifies how the UDP Transport Layer (UDPTL) protocol,
the predominant transport protocol for T.38 fax, can be transported the predominant transport protocol for T.38 fax, can be transported
over the Datagram Transport Layer Security (DTLS) protocol, how the over the Datagram Transport Layer Security (DTLS) protocol, how the
usage of UDPTL over DTLS is indicated in the Session Description usage of UDPTL over DTLS is indicated in the Session Description
Protocol (SDP), and how UDPTL over DTLS is negotiated in a session Protocol (SDP), and how UDPTL over DTLS is negotiated in a session
established using the Session Initiation Protocol (SIP). established using the Session Initiation Protocol (SIP).
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 5, 2014. This Internet-Draft will expire on August 18, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Secure Channel . . . . . . . . . . . . . . . . . . . . . . . 5 3. Secure Channel . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Secure Channel Establishment . . . . . . . . . . . . . . 5 4. SDP Offerer/Answerer Procedures . . . . . . . . . . . . . . . 5
3.2. Secure Channel Usage . . . . . . . . . . . . . . . . . . 6 4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Miscellaneous Considerations . . . . . . . . . . . . . . . . 6 4.2. SDP Offerer Procedures . . . . . . . . . . . . . . . . . 6
4.1. Anonymous Calls . . . . . . . . . . . . . . . . . . . . . 6 4.3. SDP Answerer Procedures . . . . . . . . . . . . . . . . . 6
4.2. Middlebox Interaction . . . . . . . . . . . . . . . . . . 6 5. Miscellaneous Considerations . . . . . . . . . . . . . . . . 7
4.2.1. NAT Traversal With ICE . . . . . . . . . . . . . . . 6 5.1. Anonymous Calls . . . . . . . . . . . . . . . . . . . . . 7
4.2.2. NAT Traversal Without ICE . . . . . . . . . . . . . . 6 5.2. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 7
4.2.3. STUN Interaction . . . . . . . . . . . . . . . . . . 7 5.2.1. ICE Usage . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 7 5.2.2. STUN Interaction . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5.3. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5.4. Compatibility With UDPTL over UDP . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 10 9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 11
A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 12
A.2. Basic Message Flow . . . . . . . . . . . . . . . . . . . 12 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 13
A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 13
A.2. Basic Message Flow . . . . . . . . . . . . . . . . . . . 13
A.3. Message Flow Of T.38 Fax Replacing Audio Media Stream in A.3. Message Flow Of T.38 Fax Replacing Audio Media Stream in
An Existing Audio-Only Session . . . . . . . . . . . . . 17 An Existing Audio-Only Session . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
While it is possible to transmit highly sensitive documents using While it is possible to transmit highly sensitive documents using
traditional telephony encryption devices, secure fax on the Public traditional telephony encryption devices, secure fax on the Public
Switched Telephone Network (PSTN) was never widely considered or Switched Telephone Network (PSTN) was never widely considered or
prioritized. This was mainly because of the challenges involved with prioritized. This was mainly because of the challenges involved with
malevolent physical access to telephony equipment. As real-time malevolent physical access to telephony equipment. As real-time
communications transition to IP networks, where information might communications transition to IP networks, where information might
potentially be intercepted or spoofed, an appropriate level of potentially be intercepted or spoofed, an appropriate level of
security for fax that offers integrity and confidentiality protection security for fax that offers integrity and confidentiality protection
is vital. is vital.
The overwhelmingly predominant fax transport protocol is UDPTL-based The overwhelmingly predominant fax transport protocol is UDPTL-based
[ITU.T38.2010]. The protocol stack for fax transport using UDPTL is [ITU.T38.2010]. The protocol stack for fax transport using UDPTL is
shown in Table 1. shown in Figure 1.
+-----------------------------+ +-----------------------------+
| Internet facsimile protocol | | Internet facsimile protocol |
+-----------------------------+ +-----------------------------+
| UDPTL | | UDPTL |
+-----------------------------+ +-----------------------------+
| UDP | | UDP |
+-----------------------------+ +-----------------------------+
| IP | | IP |
+-----------------------------+ +-----------------------------+
Table 1: Protocol stack for UDPTL over UDP Figure 1: Protocol stack for UDPTL over UDP
Implementations exist today for securing this fax transport type. Implementations exist today for securing this fax transport type.
Some of these mechanisms are: Some of these mechanisms are:
o [ITU.T30.2005] Annex H specifies integrity and confidentiality o [ITU.T30.2005] Annex H specifies integrity and confidentiality
protection of fax in the application layer, independent of protection of fax in the application layer, independent of
protocol for fax transport. protocol for fax transport.
o [ITU.T38.2010] specifies fax transport over RTP/SAVP which enables o [ITU.T38.2010] specifies fax transport over RTP/SAVP which enables
integrity and confidentiality protection of fax in IP network. integrity and confidentiality protection of fax in IP network.
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security offering integrity and confidentiality protection for UDPTL. security offering integrity and confidentiality protection for UDPTL.
This issue was addressed in a study by the 3rd Generation Partnership This issue was addressed in a study by the 3rd Generation Partnership
Project (3GPP) on how to provide secure fax in the IP Multimedia Project (3GPP) on how to provide secure fax in the IP Multimedia
Subsystem (IMS). They concluded that secure fax shall be transported Subsystem (IMS). They concluded that secure fax shall be transported
using UDPTL over DTLS. using UDPTL over DTLS.
This document specifies fax transport using UDPTL over DTLS This document specifies fax transport using UDPTL over DTLS
[RFC6347], which enables integrity and confidentiality protection of [RFC6347], which enables integrity and confidentiality protection of
fax in IP networks. The protocol stack which enhances fax transport fax in IP networks. The protocol stack which enhances fax transport
to offer integrity and confidentiality using UDPTL over DTLS is shown to offer integrity and confidentiality using UDPTL over DTLS is shown
in Table 2. in Figure 2.
+-----------------------------+ +-----------------------------+
| Internet facsimile protocol | | Internet facsimile protocol |
+-----------------------------+ +-----------------------------+
| UDPTL | | UDPTL |
+-----------------------------+ +-----------------------------+
| DTLS | | DTLS |
+-----------------------------+ +-----------------------------+
| UDP | | UDP |
+-----------------------------+ +-----------------------------+
| IP | | IP |
+-----------------------------+ +-----------------------------+
Table 2: Protocol stack for UDPTL over DTLS over UDP Figure 2: Protocol stack for UDPTL over DTLS over UDP
The primary motivations for the mechanism in this document are: The primary motivations for the mechanism in this document are:
o The design of DTLS [RFC6347] is clearly defined, well understood o The design of DTLS [RFC6347] is clearly defined, well understood
and implementations are widely available. and implementations are widely available.
o No DTLS extensions are required in order to enable UDPTL transport o No DTLS extensions are required in order to enable UDPTL transport
over DTLS. over DTLS.
o Fax transport using UDPTL over DTLS only requires insertion of the 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 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 Figure 2. The UDPTL layer and the layers above the UDPTL layer
modification. require no modifications.
o UDPTL [ITU.T38.2010] is by far the most widely deployed fax o UDPTL [ITU.T38.2010] is by far the most widely deployed fax
transport protocol in IP networks. transport protocol in IP networks.
o 3GPP and the IP fax community need a mechanism to transport UDPTL o 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- over DTLS in order to provide secure fax in SIP-based networks
based networks. (including IMS).
This document specifies the transport of UDPTL over DTLS using the This document specifies the transport of UDPTL over DTLS using the
DTLS record layer "application_data" packets [RFC5246] [RFC6347]. DTLS record layer "application_data" packets [RFC5246] [RFC6347].
Since the DTLS record layer "application_data" packet does not Since the DTLS record layer "application_data" packet does not
indicate whether it carries UDPTL, or some other protocol, the usage indicate whether it carries UDPTL, or some other protocol, the usage
of a dedicated DTLS association for transport of UDPTL needs to be of a dedicated DTLS association for transport of UDPTL needs to be
negotiated, e.g. using the Session Description Protocol (SDP) negotiated, e.g. using the Session Description Protocol (SDP)
[RFC4566] and the SDP offer/answer mechanism [RFC3264]. [RFC4566] and the SDP offer/answer mechanism [RFC3264].
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session" to refer to the DTLS construct. We 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 association" to refer to a particular DTLS cipher suite and keying
material set that is associated with a single host/port quartet. The material set that is associated with a single host/port quartet. The
same DTLS session can be used to establish the keying material for same DTLS session can be used to establish the keying material for
multiple DTLS associations. For consistency with other SIP/SDP multiple DTLS associations. For consistency with other SIP/SDP
usage, we use the term "connection" when what's being referred to is usage, we use the term "connection" when what's being referred to is
a multimedia stream that is not specifically DTLS. a multimedia stream that is not specifically DTLS.
3. Secure Channel 3. Secure Channel
3.1. Secure Channel Establishment The UDPTL over DTLS media stream is negotiated using the SDP offer/
answer mechanism [RFC3264]. See Section 4 for more details.
The SDP offer/answer mechanism [RFC3264] is used by other protocols, DTLS is used as specified in [RFC6347]. Once the DTLS handshake is
e.g. the Session Initiation Protocol (SIP) [RFC3261], to negotiate successfully completed (in order to prevent facsimile data from being
and establish multimedia sessions. transmitted insecurely), the UDPTL packets SHALL be transported in
DTLS record layer "application_data" packets.
In addition to the usual contents of an SDP media description ("m=" 4. SDP Offerer/Answerer Procedures
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 [RFC4145] and TLS
[RFC4572], and are re-used in this document.
The SDP offer and the SDP answer MUST conform to the following 4.1. General
requirements:
o The endpoint MUST set the "proto" field of the "m=" line to the An endpoint (i.e. both the offerer and the answerer) MUST create an
token specified in Table 3. SDP media description ("m=" line) for each UDPTL over DTLS media
o In order to negotiate the TLS roles, the endpoint MUST use the SDP stream, and MUST assign a UDP/TLS/UDPTL value (see Table 1) to the
setup attribute [RFC4145]. The offerer SHOULD assign the SDP "proto" field of the "m=" line.
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].
3.2. Secure Channel Usage The procedures in this section apply to an "m=" line associated with
a UDPTL over DTLS media stream.
DTLS is used as specified in [RFC6347]. Once the DTLS handshake is In order to negotiate a UDPTL over DTLS media stream, the following
successfully completed (in order to prevent facsimile data from being SDP attributes are used:
transmitted insecurely), the UDPTL packets SHALL be transported in
DTLS record layer "application_data" packets.
4. Miscellaneous Considerations o The SDP attributes defined for UDPTL over UDP, as described in
[ITU.T38.2010]; and
o The SDP attributes, defined in [RFC4145] and [RFC4572], as
described in this section.
4.1. Anonymous Calls The endpoint MUST NOT use the SDP connection attribute [RFC4145].
In order to negotiate the TLS roles for the UDPTL over DTLS transport
connection, the endpoint MUST use the SDP setup attribute [RFC4145].
If the endpoint supports, and is willing to use, a cipher suite with
an associated certificate, the endpoint MUST include an SDP
fingerprint attribute [RFC4572].
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.
4.2. SDP Offerer Procedures
The offerer SHOULD assign the SDP setup attribute with a
setup:actpass value. Alternatively, the offerer MAY assign the SDP
setup attribute with a setup:active value or setup:passive value.
The offerer MUST NOT assign an 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, the offerer MUST be prepared to receive
a DTLS client_hello message before it receives the SDP answer.
When the offerer receives the associated answer, if the offerer ends
up being active it MUST initiate a DTLS handshake by sending a
ClientHello message on the negotiated media stream, towards the IP
address and port of the answerer.
4.3. SDP Answerer Procedures
If the answerer accepts the offered UDPTL over DTLS transport
connection, in the associated answer the answerer MUST assign an 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.
If the answerer assigns a setup:active value, the answerer MUST
initiate a DTLS handshake by sending a ClientHello message on the
negotiated media stream, towards the IP address and port of the
offerer.
5. Miscellaneous Considerations
5.1. Anonymous Calls
When making anonymous calls, a new self-signed certificate SHOULD be When making anonymous calls, a new self-signed certificate SHOULD be
used for each call and attributes inside the certificate SHALL NOT used for each call and attributes inside the certificate SHALL NOT
contain information that either allows correlation or identification contain information that either allows correlation or identification
of the user making anonymous calls. This is particularly important of the user making anonymous calls. This is particularly important
for the subjectAltName and commonName attributes. for the subjectAltName and commonName attributes.
4.2. Middlebox Interaction 5.2. NAT Traversal
4.2.1. NAT Traversal With ICE 5.2.1. ICE Usage
When ICE [RFC5245] is being used, the ICE connectivity checks are When ICE [RFC5245] is being used, the ICE connectivity checks are
performed before the DTLS handshake begins. Note that if aggressive performed before the DTLS handshake begins. Note that if aggressive
nomination mode is used, multiple candidate pairs may be marked valid nomination mode is used, multiple candidate pairs may be marked valid
before ICE finally converges on a single candidate pair. UAs MUST before ICE finally converges on a single candidate pair. UAs MUST
treat all ICE candidate pairs associated with a single component as treat all ICE candidate pairs associated with a single component as
part of the same DTLS association. Thus, there will be only one DTLS part of the same DTLS association. Thus, there will be only one DTLS
handshake even if there are multiple valid candidate pairs. Note handshake even if there are multiple valid candidate pairs. Note
that this may mean adjusting the endpoint IP addresses if the that this may mean adjusting the endpoint IP addresses if the
selected candidate pair shifts, just as if the DTLS packets were an selected candidate pair shifts, just as if the DTLS packets were an
ordinary media stream. ordinary media stream.
4.2.2. NAT Traversal Without ICE 5.2.2. STUN Interaction
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 side MUST NOT wait for a STUN answer before sending
its ServerHello.
4.2.3. STUN Interaction
The UA SHALL send the STUN packets [RFC5389] directly over UDP, not The UA SHALL send the STUN packets [RFC5389] directly over UDP, not
over DTLS. over DTLS.
The UA MUST demultiplex packets arriving on the IP address and port The UA MUST demultiplex packets arriving on the IP address and port
associated with the DTLS association, e.g. as follows: associated with the DTLS association, e.g. as follows:
o If the value of the first byte of the packet is 0 or 1, then the o If the value of the first byte of the packet is 0 or 1, then the
packet is STUN. packet is STUN.
o If the value of the first byte of the packet is between 20 and 63 o If the value of the first byte of the packet is between 20 and 63
(inclusive), the packet is DTLS. (inclusive), the packet is DTLS.
4.3. Rekeying 5.3. Rekeying
After the DTLS handshake caused by rekeying has completed, because of After the DTLS handshake caused by rekeying has completed, because of
possible packet reordering on the wire, packets protected by the possible packet reordering on the wire, packets protected by the
previous set of keys can arrive. To compensate for this fact, previous set of keys can arrive. To compensate for this fact,
receivers MUST maintain both sets of keys for some time in order to receivers MUST maintain both sets of keys for some time in order to
be able to decrypt and verify older packets. The duration of be able to decrypt and verify older packets. The duration of
maintaining the previous set of keys after the finish of the DTLS maintaining the previous set of keys after the finish of the DTLS
handshake is out of scope for this document. handshake is out of scope for this document.
5. Security Considerations 5.4. Compatibility With UDPTL over UDP
If a user requires fax to be transported securely using UDPTL over
DTLS, and if the remote user does not support UDPTL over DTLS, then a
fax media stream cannot be established.
If a user prefers fax to be transported securely using UDPTL over
DTLS, but is willing to transport the fax insecurely in case the
remote user does not support UDPTL over DTLS, then the SDP Capability
Negotiation mechanism [RFC5939] can be used to offer both UDPTL over
DTLS and UDPTL over UDP. Alternatively, if the remote user rejects
an offer for UDPTL over DTLS, a new offer for a UDPTL over UDP media
stream can be sent.
6. Security Considerations
Fax may be used to transmit a wide range of sensitive data, including Fax may be used to transmit a wide range of sensitive data, including
personal, corporate, and governmental information. It is therefore personal, corporate, and governmental information. It is therefore
critical to be able to protect against threats to the confidentiality critical to be able to protect against threats to the confidentiality
and integrity of the transmitted data. and integrity of the transmitted data.
The mechanism in this document provides integrity and confidentiality The mechanism in this document provides integrity and confidentiality
protection for fax by specifying fax transport using UDPTL over DTLS protection for fax by specifying fax transport using UDPTL over DTLS
[RFC6347]. [RFC6347].
DTLS media signaled with SIP requires a mechanism to ensure that the DTLS media stream negotiated using SIP/SDP requires a mechanism to
communicating peers' certificates are correct. ensure that the certificate received via DTLS was issued by the
remote party of the SIP session.
The standard DTLS strategy for authenticating the communicating The standard DTLS strategy for authenticating the communicating
parties is to give the server (and optionally the client) a PKIX parties is to give the server (and optionally the client) a PKIX
[RFC5280] certificate. The client then verifies the certificate and [RFC5280] certificate. The client then verifies the certificate and
checks that the name in the certificate matches the server's domain checks that the name in the certificate matches the server's domain
name. This works because there are a relatively small number of name. This works because there are a relatively small number of
servers with well-defined names; a situation that does not usually servers with well-defined names; a situation that does not usually
occur in the VoIP context. occur in the VoIP context.
The design described in this document is intended to leverage the The design described in this document is intended to leverage the
authenticity of the signaling channel (while not requiring integrity protection of the SIP signaling, while not requiring
confidentiality). As long as each side of the connection can verify confidentiality. As long as each side of the connection can verify
the integrity of the SDP received from the other side, then the DTLS 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 handshake cannot be hijacked via a man-in-the-middle attack. This
integrity protection is easily provided by the caller to the callee integrity protection is easily provided by the caller to the callee
via the SIP Identity [RFC4474] mechanism. Other mechanisms, such as via the SIP Identity [RFC4474] mechanism. Other mechanisms, such as
the S/MIME mechanism [RFC3261], or perhaps future mechanisms yet to the S/MIME mechanism [RFC3261], or perhaps future mechanisms yet to
be specified could also serve this purpose. be specified could also serve this purpose.
While this mechanism can still be used without such integrity While this mechanism can still be used without such integrity
mechanisms, the security provided is limited to defense against mechanisms, the security provided is limited to defense against
passive attack by intermediaries. An active attack on the signaling passive attack by intermediaries. An active attack on the signaling
plus an active attack on the media plane can allow an attacker to plus an active attack on the media plane can allow an attacker to
attack the connection (R-SIG-MEDIA in the notation of [RFC5479]). attack the connection (R-SIG-MEDIA in the notation of [RFC5479]).
6. IANA Considerations 7. IANA Considerations
This document updates the "Session Description Protocol (SDP) This document updates the "Session Description Protocol (SDP)
Parameters" registry as specified in Section 8.2.2 of [RFC4566]. 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 Specifically, it adds the values in Table 1 to the table for the SDP
"proto" field registry. "proto" field registry.
+-------+-----------------+------------+ +-------+-----------------+------------+
| Type | SDP Name | Reference | | Type | SDP Name | Reference |
+-------+-----------------+------------+ +-------+-----------------+------------+
| proto | "UDP/TLS/UDPTL" | [RFC-XXXX] | | proto | "UDP/TLS/UDPTL" | [RFC-XXXX] |
+-------+-----------------+------------+ +-------+-----------------+------------+
Table 3: SDP "proto" field values Table 1: SDP "proto" field values
[RFC EDITOR NOTE: Please replace RFC-XXXX with the RFC number of this [RFC EDITOR NOTE: Please replace RFC-XXXX with the RFC number of this
document.] document.]
7. Acknowledgments 8. Acknowledgments
Special thanks to Peter Dawes, who provided comments on the initial Special thanks to Peter Dawes, who provided comments on the initial
version of the draft, and to Paul E. Jones, James Rafferty, Albrecht version of the draft, and to Paul E. Jones, James Rafferty, Albrecht
Schwarz, Oscar Ohlsson, David Hanes, Adam Gensler and Ari Keraenen Schwarz, Oscar Ohlsson, David Hanes, Adam Gensler, Ari Keranen and
who provided valuable feedback and input on the MMUSIC mailing list. Flemming Andreasen who provided valuable feedback and input on the
MMUSIC mailing list.
8. Change Log 9. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing] [RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-udptl-dtls-03 Changes from draft-ietf-mmusic-udptl-dtls-04
o Changes based on comments by Flemming Andreasen
o - Addition of SDP Offer/Answer procedure section.
o - Removal of non-ICE NAT traversal procedures.
o - Addition of guidance regarding compatibility with UDPTL over
UDP.
o - Editorial corrections.
o Minor editorial corrections
o -Spelling of Ari's family name.
Changes from draft-ietf-mmusic-udptl-dtls-03
o Changes based on comments by Adam Gensler (http://www.ietf.org/ o Changes based on comments by Adam Gensler (http://www.ietf.org/
mail-archive/web/mmusic/current/msg12945.html) mail-archive/web/mmusic/current/msg12945.html)
o -Indicating that, in case of rekeying, entities MUST maintain both o -Indicating that, in case of rekeying, entities MUST maintain both
set of keys for some time (previously SHOULD). set of keys for some time (previously SHOULD).
o -Explicit mentioning of the commonName attribute in text about o -Explicit mentioning of the commonName attribute in text about
correlation/identification of users. correlation/identification of users.
o Changes based on comments by Ari Keraenen (http://www.ietf.org/ o Changes based on comments by Ari Keranen (http://www.ietf.org/
mail-archive/web/mmusic/current/msg12966.html) mail-archive/web/mmusic/current/msg12966.html)
o -Informative reference to RFC 5246 added. o -Informative reference to RFC 5246 added.
o -Re-naming of sections 4.2.1 and 4.2.2. o -Re-naming of sections 4.2.1 and 4.2.2.
o -Clarifying that documented STUN/DTLS demux mechanism is only one o -Clarifying that documented STUN/DTLS demux mechanism is only one
way of doing the demux. way of doing the demux.
o -Editorial corrections. o -Editorial corrections.
Changes from draft-ietf-mmusic-udptl-dtls-02 Changes from draft-ietf-mmusic-udptl-dtls-02
o Editorial comments based on review comments by James Rafferty o Editorial comments based on review comments by James Rafferty
skipping to change at page 10, line 21 skipping to change at page 11, line 26
o Text about T.30 added. o Text about T.30 added.
o Latest version of T.38 referenced. o Latest version of T.38 referenced.
o Additional text about the need for secure fax in IP networks. o Additional text about the need for secure fax in IP networks.
Changes from draft-holmberg-dispatch-udptl-dtls-00 Changes from draft-holmberg-dispatch-udptl-dtls-00
o WG changed to MMUSIC. o WG changed to MMUSIC.
o Added text about 3GPP need for UDPTL/DTLS. o Added text about 3GPP need for UDPTL/DTLS.
9. References 10. References
9.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
skipping to change at page 11, line 33 skipping to change at page 12, line 37
International Telecommunications Union, "Procedures for International Telecommunications Union, "Procedures for
document facsimile transmission in the general switched document facsimile transmission in the general switched
telephone network", ITU-T Recommendation T.30, September telephone network", ITU-T Recommendation T.30, September
2005. 2005.
[ITU.T38.2010] [ITU.T38.2010]
International Telecommunications Union, "Procedures for International Telecommunications Union, "Procedures for
real-time Group 3 facsimile communication over IP real-time Group 3 facsimile communication over IP
networks", ITU-T Recommendation T.38, September 2010. networks", ITU-T Recommendation T.38, September 2010.
9.2. Informative References 10.2. Informative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet, [RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet,
"Requirements and Analysis of Media Security Management "Requirements and Analysis of Media Security Management
Protocols", RFC 5479, April 2009. Protocols", RFC 5479, April 2009.
[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.
Appendix A. Examples Appendix A. Examples
A.1. General A.1. General
Prior to establishing the session, both Alice and Bob generate self- Prior to establishing the session, both Alice and Bob generate self-
signed certificates which are used for a single session or, more signed certificates which are used for a single session or, more
likely, reused for multiple sessions. likely, reused for multiple sessions.
The SIP signaling from Alice to her proxy is transported over TLS to The SIP signaling from Alice to her proxy is transported over TLS to
ensure an integrity protected channel between Alice and her identity ensure an integrity protected channel between Alice and her identity
skipping to change at page 12, line 15 skipping to change at page 13, line 27
proxies should also be protected somehow. proxies should also be protected somehow.
Only one element is shown for Alice's and Bob's proxies for the Only one element is shown for Alice's and Bob's proxies for the
purposes of simplification. purposes of simplification.
For the sake of brevity and simplicity, only the mandatory SDP T.38 For the sake of brevity and simplicity, only the mandatory SDP T.38
attributes are shown. attributes are shown.
A.2. Basic Message Flow A.2. Basic Message Flow
Figure 1 shows an example message flow of session establishment for Figure 3 shows an example message flow of session establishment for
T.38 fax securely transported using UDPTL over DTLS. T.38 fax securely transported using UDPTL over DTLS.
In this example flow, Alice acts as the passive endpoint of the DTLS 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 and Bob acts as the active endpoint of the DTLS
association. association.
Alice Proxies Bob Alice Proxies Bob
| (1) SIP INVITE | | | (1) SIP INVITE | |
|----------------------->| | |----------------------->| |
| | (2) SIP INVITE | | | (2) SIP INVITE |
skipping to change at page 12, line 43 skipping to change at page 14, line 26
| | (5) SIP 200 OK | | | (5) SIP 200 OK |
| |<-----------------------| | |<-----------------------|
| (6) SIP 200 OK | | | (6) SIP 200 OK | |
|<-----------------------| | |<-----------------------| |
| (7) SIP ACK | | | (7) SIP ACK | |
|------------------------------------------------>| |------------------------------------------------>|
| (8) T.38 message using UDPTL over DTLS | | (8) T.38 message using UDPTL over DTLS |
|<----------------------------------------------->| |<----------------------------------------------->|
| | | | | |
Figure 1: Basic message flow Figure 3: Basic message flow
Message (1): Message (1):
Figure 2 shows the initial INVITE request sent by Alice to Alice's Figure 4 shows the initial INVITE request sent by Alice to Alice's
proxy. The initial INVITE request contains an SDP offer. proxy. The initial INVITE request contains an SDP offer.
The "m=" line in the SDP offer indicates T.38 fax using UDPTL over The "m=" line in the SDP offer indicates T.38 fax using UDPTL over
DTLS. DTLS.
The SDP setup:actpass attribute in the SDP offer indicates that The SDP setup:actpass attribute in the SDP offer indicates that
Alice has requested to be either the active or passive endpoint. Alice has requested to be either the active or passive endpoint.
The SDP fingerprint attribute in the SDP offer contains the The SDP fingerprint attribute in the SDP offer contains the
certificate fingerprint computed from Alice's self-signed certificate fingerprint computed from Alice's self-signed
skipping to change at page 13, line 39 skipping to change at page 15, line 29
o=- 1181923068 1181923196 IN IP4 ua1.example.com o=- 1181923068 1181923196 IN IP4 ua1.example.com
s=- s=-
c=IN IP4 ua1.example.com c=IN IP4 ua1.example.com
t=0 0 t=0 0
m=image 6056 UDP/TLS/UDPTL t38 m=image 6056 UDP/TLS/UDPTL t38
a=setup:actpass a=setup:actpass
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF a=T38FaxRateManagement:transferredTCF
Figure 2: Message (1) Figure 4: Message (1)
Message (2): Message (2):
Figure 3 shows the SIP INVITE request sent by Bob's proxy to Bob. Figure 5 shows the SIP INVITE request sent by Bob's proxy to Bob.
When received, Bob verifies the identity provided in the SIP When received, Bob verifies the identity provided in the SIP
INVITE request. INVITE request.
INVITE sip:bob@ua2.example.com SIP/2.0 INVITE sip:bob@ua2.example.com SIP/2.0
To: <sip:bob@example.com> To: <sip:bob@example.com>
From: "Alice"<sip:alice@example.com>;tag=843c7b0b From: "Alice"<sip:alice@example.com>;tag=843c7b0b
Via: SIP/2.0/TLS proxy.example.com;branch=z9hG4bK-0e53sadfkasldk Via: SIP/2.0/TLS proxy.example.com;branch=z9hG4bK-0e53sadfkasldk
Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
Record-Route: <sip:proxy.example.com;lr> Record-Route: <sip:proxy.example.com;lr>
skipping to change at page 14, line 34 skipping to change at page 16, line 31
o=- 1181923068 1181923196 IN IP4 ua1.example.com o=- 1181923068 1181923196 IN IP4 ua1.example.com
s=- s=-
c=IN IP4 ua1.example.com c=IN IP4 ua1.example.com
t=0 0 t=0 0
m=image 6056 UDP/TLS/UDPTL t38 m=image 6056 UDP/TLS/UDPTL t38
a=setup:actpass a=setup:actpass
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF a=T38FaxRateManagement:transferredTCF
Figure 3: Message (2) Figure 5: Message (2)
Message (3): Message (3):
Assuming that Alice's identity is valid, Bob sends a DTLS Assuming that Alice's identity is valid, Bob sends a DTLS
ClientHello directly to Alice. ClientHello directly to Alice.
Message (4): Message (4):
Alice and Bob exchange further messages of DTLS handshake Alice and Bob exchange further messages of DTLS handshake
(HelloVerifyRequest, ClientHello, ServerHello, Certificate, (HelloVerifyRequest, ClientHello, ServerHello, Certificate,
ServerKeyExchange, CertificateRequest, ServerHelloDone, ServerKeyExchange, CertificateRequest, ServerHelloDone,
Certificate, ClientKeyExchange, CertificateVerify, Certificate, ClientKeyExchange, CertificateVerify,
ChangeCipherSpec, Finished). ChangeCipherSpec, Finished).
When Bob receives the certificate of Alice via DTLS, Bob checks When Bob receives the certificate of Alice via DTLS, Bob checks
whether the certificate fingerprint calculated from Alice's whether the certificate fingerprint calculated from Alice's
certificate received via DTLS matches the certificate fingerprint certificate received via DTLS matches the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 3. In this received in the a=fingerprint SDP attribute of Figure 5. In this
message flow, the check is successful and thus session setup message flow, the check is successful and thus session setup
continues. continues.
Message (5): Message (5):
Figure 4 shows a SIP 200 (OK) response to the initial SIP INVITE Figure 6 shows a SIP 200 (OK) response to the initial SIP INVITE
request, sent by Bob to Bob's proxy. The SIP 200 (OK) response request, sent by Bob to Bob's proxy. The SIP 200 (OK) response
contains an SDP answer. contains an SDP answer.
The "m=" line in the SDP answer indicates T.38 fax using UDPTL The "m=" line in the SDP answer indicates T.38 fax using UDPTL
over DTLS. over DTLS.
The SDP setup:active attribute in the SDP answer indicates that The SDP setup:active attribute in the SDP answer indicates that
Bob has requested to be the active endpoint. Bob has requested to be the active endpoint.
The SDP fingerprint attribute in the SDP answer contains the The SDP fingerprint attribute in the SDP answer contains the
skipping to change at page 16, line 29 skipping to change at page 18, line 29
o=- 8965454521 2105372818 IN IP4 ua2.example.com o=- 8965454521 2105372818 IN IP4 ua2.example.com
s=- s=-
c=IN IP4 ua2.example.com c=IN IP4 ua2.example.com
t=0 0 t=0 0
m=image 12000 UDP/TLS/UDPTL t38 m=image 12000 UDP/TLS/UDPTL t38
a=setup:active a=setup:active
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF a=T38FaxRateManagement:transferredTCF
Figure 4: Message (5) Figure 6: Message (5)
Message (6): Message (6):
Figure 5 shows a SIP 200 (OK) response to the initial SIP INVITE Figure 7 shows a SIP 200 (OK) response to the initial SIP INVITE
request, sent by Alice's proxy to Alice. Alice checks if the request, sent by Alice's proxy to Alice. Alice checks if the
certificate fingerprint calculated from the Bob's certificate certificate fingerprint calculated from the Bob's certificate
received via DTLS is the same as the certificate fingerprint received via DTLS is the same as the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 5. In this received in the a=fingerprint SDP attribute of Figure 7. In this
message flow, the check is successful and thus session setup message flow, the check is successful and thus session setup
continues. continues.
SIP/2.0 200 OK SIP/2.0 200 OK
To: <sip:bob@example.com>;tag=6418913922105372816 To: <sip:bob@example.com>;tag=6418913922105372816
From: "Alice" <sip:alice@example.com>;tag=843c7b0b From: "Alice" <sip:alice@example.com>;tag=843c7b0b
Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
Record-Route: <sip:proxy.example.com;lr> Record-Route: <sip:proxy.example.com;lr>
Call-ID: 6076913b1c39c212@REVMTEpG Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE CSeq: 1 INVITE
skipping to change at page 17, line 28 skipping to change at page 19, line 28
o=- 8965454521 2105372818 IN IP4 ua2.example.com o=- 8965454521 2105372818 IN IP4 ua2.example.com
s=- s=-
c=IN IP4 ua2.example.com c=IN IP4 ua2.example.com
t=0 0 t=0 0
m=image 12000 UDP/TLS/UDPTL t38 m=image 12000 UDP/TLS/UDPTL t38
a=setup:active a=setup:active
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF a=T38FaxRateManagement:transferredTCF
Figure 5: Message (6) Figure 7: Message (6)
Message (7): Message (7):
Alice sends the SIP ACK request to Bob. Alice sends the SIP ACK request to Bob.
Message (8): Message (8):
At this point, Bob and Alice can exchange T.38 fax securely At this point, Bob and Alice can exchange T.38 fax securely
transported using UDPTL over DTLS. transported using UDPTL over DTLS.
A.3. Message Flow Of T.38 Fax Replacing Audio Media Stream in An A.3. Message Flow Of T.38 Fax Replacing Audio Media Stream in An
Existing Audio-Only Session Existing Audio-Only Session
Traditionally, most sessions with non-secure transport of T.38 fax, Traditionally, most sessions with non-secure transport of T.38 fax,
transported using UDPTL, are established by modifying an ongoing transported using UDPTL, are established by modifying an ongoing
audio session into a fax session. Figure 6 shows an example message audio session into a fax session. Figure 8 shows an example message
flow of modifying an existing audio session into a session with T.38 flow of modifying an existing audio session into a session with T.38
fax securely transported using UDPTL over DTLS. fax securely transported using UDPTL over DTLS.
In this example flow, Alice acts as the passive endpoint of the DTLS 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 and Bob acts as the active endpoint of the DTLS
association. association.
Alice Proxies Bob Alice Proxies Bob
| | | | | |
| (1) Audio-only session initiation | | (1) Audio-only session initiation |
skipping to change at page 18, line 30 skipping to change at page 20, line 30
| | | | | |
| | | | | |
| | (5) SIP 200 OK | | | (5) SIP 200 OK |
|<------------------------------------------------| |<------------------------------------------------|
| (6) SIP ACK | | | (6) SIP ACK | |
|------------------------------------------------>| |------------------------------------------------>|
| (7) T.38 message using UDPTL over DTLS | | (7) T.38 message using UDPTL over DTLS |
|<----------------------------------------------->| |<----------------------------------------------->|
| | | | | |
Figure 6: Message Flow Of T.38 Fax Replacing Audio Media Stream in An Figure 8: Message Flow Of T.38 Fax Replacing Audio Media Stream in An
Existing Audio-Only Session Existing Audio-Only Session
Message (1): Message (1):
Session establishment of audio-only session. The proxies decide Session establishment of audio-only session. The proxies decide
not to record-route. not to record-route.
Message (2): Message (2):
Alice sends SIP re-INVITE request. The SDP offer included in the Alice sends SIP re-INVITE request. The SDP offer included in the
SIP re-INVITE request is shown in Figure 7. SIP re-INVITE request is shown in Figure 9.
The first "m=" line in the SDP offer indicates audio media stream The first "m=" line in the SDP offer indicates audio media stream
being removed. The second "m=" line in the SDP offer indicates being removed. The second "m=" line in the SDP offer indicates
T.38 fax using UDPTL over DTLS being added. T.38 fax using UDPTL over DTLS being added.
The SDP setup:actpass attribute in the SDP offer indicates that The SDP setup:actpass attribute in the SDP offer indicates that
Alice has requested to be either the active or passive endpoint. Alice has requested to be either the active or passive endpoint.
The SDP fingerprint attribute in the SDP offer contains the The SDP fingerprint attribute in the SDP offer contains the
certificate fingerprint computed from Alice's self-signed certificate fingerprint computed from Alice's self-signed
skipping to change at page 19, line 21 skipping to change at page 21, line 21
s=- s=-
c=IN IP4 ua1.example.com c=IN IP4 ua1.example.com
t=0 0 t=0 0
m=audio 0 UDP/TLS/RTP/SAVP 0 m=audio 0 UDP/TLS/RTP/SAVP 0
m=image 46056 UDP/TLS/UDPTL t38 m=image 46056 UDP/TLS/UDPTL t38
a=setup:actpass a=setup:actpass
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF a=T38FaxRateManagement:transferredTCF
Figure 7: SDP offer of message (2) Figure 9: SDP offer of message (2)
Message (3): Message (3):
Bob sends a DTLS ClientHello directly to Alice. Bob sends a DTLS ClientHello directly to Alice.
Message (4): Message (4):
Alice and Bob exchange further messages of DTLS handshake Alice and Bob exchange further messages of DTLS handshake
(HelloVerifyRequest, ClientHello, ServerHello, Certificate, (HelloVerifyRequest, ClientHello, ServerHello, Certificate,
ServerKeyExchange, CertificateRequest, ServerHelloDone, ServerKeyExchange, CertificateRequest, ServerHelloDone,
Certificate, ClientKeyExchange, CertificateVerify, Certificate, ClientKeyExchange, CertificateVerify,
ChangeCipherSpec, Finished). ChangeCipherSpec, Finished).
When Bob receives the certificate of Alice via DTLS, Bob checks When Bob receives the certificate of Alice via DTLS, Bob checks
whether the certificate fingerprint calculated from Alice's whether the certificate fingerprint calculated from Alice's
certificate received via DTLS matches the certificate fingerprint certificate received via DTLS matches the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 7. In this received in the a=fingerprint SDP attribute of Figure 9. In this
message flow, the check is successful and thus session setup message flow, the check is successful and thus session setup
continues. continues.
Message (5): Message (5):
Bob sends a SIP 200 (OK) response to the SIP re-INVITE request. Bob sends a SIP 200 (OK) response to the SIP re-INVITE request.
The SIP 200 (OK) response contains an SDP answer shown in The SIP 200 (OK) response contains an SDP answer shown in
Figure 8. Figure 10.
The first "m=" line in the SDP offer indicates audio media stream The first "m=" line in the SDP offer indicates audio media stream
being removed. The second "m=" line in the SDP answer indicates being removed. The second "m=" line in the SDP answer indicates
T.38 fax using UDPTL over DTLS being added. T.38 fax using UDPTL over DTLS being added.
The SDP setup:active attribute in the SDP answer indicates that The SDP setup:active attribute in the SDP answer indicates that
Bob has requested to be the active endpoint. Bob has requested to be the active endpoint.
The SDP fingerprint attribute in the SDP answer contains the The SDP fingerprint attribute in the SDP answer contains the
certificate fingerprint computed from Bob's self-signed certificate fingerprint computed from Bob's self-signed
skipping to change at page 20, line 32 skipping to change at page 22, line 32
s=- s=-
c=IN IP4 ua2.example.com c=IN IP4 ua2.example.com
t=0 0 t=0 0
m=audio 0 UDP/TLS/RTP/SAVP 0 m=audio 0 UDP/TLS/RTP/SAVP 0
m=image 32000 UDP/TLS/UDPTL t38 m=image 32000 UDP/TLS/UDPTL t38
a=setup:active a=setup:active
a=fingerprint: SHA-1 \ a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF a=T38FaxRateManagement:transferredTCF
Figure 8: SDP answer of message (5) Figure 10: SDP answer of message (5)
Message (6): Message (6):
Alice sends the SIP ACK request to Bob. Alice sends the SIP ACK request to Bob.
Message (7): Message (7):
At this point, Bob and Alice can exchange T.38 fax securely At this point, Bob and Alice can exchange T.38 fax securely
transported using UDPTL over DTLS. transported using UDPTL over DTLS.
 End of changes. 63 change blocks. 
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