UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS)
draft-ietf-mmusic-udptl-dtls-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7345.
|
|
|---|---|---|---|
| Authors | Christer Holmberg , Ivo Sedlacek , Gonzalo Salgueiro | ||
| Last updated | 2014-01-17 | ||
| Replaces | draft-holmberg-mmusic-udptl-dtls, draft-holmberg-dispatch-udptl-dtls | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Flemming Andreasen | ||
| IESG | IESG state | Became RFC 7345 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-mmusic-udptl-dtls-03
MMUSIC Working Group C. Holmberg
Internet-Draft I. Sedlacek
Intended status: Standards Track Ericsson
Expires: July 21, 2014 G. Salgueiro
Cisco
January 17, 2014
UDP Transport Layer (UDPTL) over Datagram Transport Layer Security
(DTLS)
draft-ietf-mmusic-udptl-dtls-03
Abstract
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).
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 July 21, 2014.
Copyright Notice
Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11
A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. Basic Message Flow . . . . . . . . . . . . . . . . . . . 11
A.3. Message Flow Of T.38 Fax Replacing Audio Media Stream in
An Existing Audio-Only Session . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
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.
The overwhelmingly predominant fax transport protocol is UDPTL-based
[ITU.T38.2010]. The protocol stack for fax transport using UDPTL is
shown in Table 1.
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+-----------------------------+
| Internet facsimile protocol |
+-----------------------------+
| UDPTL |
+-----------------------------+
| UDP |
+-----------------------------+
| IP |
+-----------------------------+
Table 1: Protocol stack for UDPTL over UDP
Implementations exist today for securing this fax transport type.
Some of these mechanisms are:
o [ITU.T30.2005] Annex H specifies integrity and confidentiality
protection of fax in the application layer, independent of
protocol for fax transport.
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.
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.
This document specifies fax transport using UDPTL over DTLS
[RFC6347], 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 Table 2.
+-----------------------------+
| Internet facsimile protocol |
+-----------------------------+
| UDPTL |
+-----------------------------+
| DTLS |
+-----------------------------+
| UDP |
+-----------------------------+
| IP |
+-----------------------------+
Table 2: Protocol stack for UDPTL over DTLS 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 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.
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
"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.
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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 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
requirements:
o The endpoint MUST set the "proto" field of the "m=" line to the
token specified in Table 3.
o In order to negotiate the TLS roles, 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
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.
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
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.
The mechanism in this document provides integrity and confidentiality
protection for fax by specifying fax transport using UDPTL over DTLS
[RFC6347].
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
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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 [RFC4474] mechanism. Other mechanisms, such as
the S/MIME mechanism [RFC3261], or perhaps future mechanisms yet to
be specified could also serve this purpose.
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, Oscar Ohlsson and David Hanes 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-02
o Editorial comments based on review comments by James Rafferty
(http://www.ietf.org/mail-archive/web/mmusic/current/
msg12890.html)
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o Editorial comments based on review comments by David Hanes (http:/
/www.ietf.org/mail-archive/web/mmusic/current/msg12886.html)
o Editorial comments based on review comments by Oscar Ohlsson
(http://www.ietf.org/mail-archive/web/mmusic/current/
msg12882.html)
o Editorial comments based on review comments by Albrecht Schwartz
(http://www.ietf.org/mail-archive/web/mmusic/current/
msg12900.html)
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).
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.
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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.
[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.
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[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. Examples
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. 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.
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
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 the DTLS
association and Bob acts as the active endpoint of the DTLS
association.
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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
Message (1):
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 contains the
certificate fingerprint computed from Alice's self-signed
certificate.
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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):
Figure 3 shows the SIP INVITE request sent by Bob's proxy to Bob.
When received, Bob verifies the identity provided in the SIP
INVITE request.
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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
Figure 3: Message (2)
Message (3):
Assuming that Alice's identity is valid, Bob sends a DTLS
ClientHello directly to Alice.
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 Alice's
certificate received via DTLS matches the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 3. In this
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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 contains the
certificate fingerprint computed from Bob's self-signed
certificate.
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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
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.
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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
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
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. Figure 6 shows an example message
flow of modifying an existing audio session into a session with T.38
fax securely transported using UDPTL over DTLS.
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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.
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
Message (1):
Session establishment of audio-only session. The proxies decide
not to record-route.
Message (2):
Alice sends SIP re-INVITE request. The SDP offer included in the
SIP re-INVITE request is 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.
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The SDP fingerprint attribute in the SDP offer contains 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):
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 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):
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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 contains 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
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.
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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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