DISPATCH Working Group C. Holmberg
Internet-Draft I. Sedlacek
Intended status: Standards Track Ericsson
Expires: December 22, 2013 June 20, 2013
UDP Transport Layer (UDPTL) over Datagram Transport Layer Security
(DTLS)
draft-holmberg-dispatch-udptl-dtls-00
Abstract
This document specifies how the UDP Transport Layer (UDPTL) protocol
can be transported over the Datagram Transport Layer Security (DTLS)
protocol, how the usage of UDPTL over DTLS is indicated in the
Session Description Protocol (SDP), and how UDPTL over DTLS is
negotiated in a session established using the Session Initiation
Protocol (SIP).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 22, 2013.
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document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Secure Channel Establishment . . . . . . . . . . . . . . 4
3.2. Secure Channel Usage . . . . . . . . . . . . . . . . . . 5
4. Miscellaneous Considerations . . . . . . . . . . . . . . . . 5
4.1. Anonymous Calls . . . . . . . . . . . . . . . . . . . . . 5
4.2. Middlebox Interaction . . . . . . . . . . . . . . . . . . 5
4.3. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 8
A.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 8
A.2. Basic Message Flow with Identity . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
UDPTL [ITU.T38.1998] is the predominant protocol for fax transport in
IP networks. The protocol stack for fax transport using UDPTL is
shown in Table 1.
+-----------------------------+
| Protocol |
+-----------------------------+
| Internet facsimile protocol |
+-----------------------------+
| UDPTL |
+-----------------------------+
| UDP |
+-----------------------------+
| IP |
+-----------------------------+
Table 1: Protocol stack for UDPTL over UDP
UDPTL does not offer integrity and confidentiality protection. To
enable integrity and confidentiality protection, [ITU.T38.2004]
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specifies fax transport over RTP/SAVP. However, fax transport over
RTP/SAVP is not widely supported.
The mechanism in this document provides integrity and confidentiality
protection for fax by specifying fax transport using UDPTL over DTLS
[RFC6347]. The protocol stack for integrity and confidentiality
protected fax transport using UDPTL over DTLS is shown in Table 2.
+-----------------------------+
| Protocol |
+-----------------------------+
| Internet facsimile protocol |
+-----------------------------+
| UDPTL |
+-----------------------------+
| DTLS |
+-----------------------------+
| UDP |
+-----------------------------+
| IP |
+-----------------------------+
Table 2: Protocol stack for UDPTL over UDP
The mechanism in this document is motivated as follows:
o The design of DTLS [RFC6347] is well-known 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.
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 "m=" line [RFC3264], in order to indicate UDPTL over DTLS in
SDP messages [RFC4566].
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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].
The DTLS uses the term "session" to refer to a long-lived set of
keying material that spans DTLS associations. In this document, in
order to be consistent with SIP/SDP usage of "session" terminology,
we use it to refer to a multimedia session and use the term "DTLS
session" to refer to the DTLS construct. We use the term "DTLS
association" to refer to a particular DTLS cipher suite and keying
material set that is associated with a single host/ port quartet.
The same DTLS session can be used to establish the keying material
for multiple DTLS associations. For consistency with other SIP/SDP
usage, we use the term "connection" when what's being referred to is
a multimedia stream that is not specifically DTLS.
3. Secure Channel
3.1. Secure Channel Establishment
The SDP offer/answer mechanism [RFC3264] is used by other protocols,
e.g. the Session Initiation Protocol (SIP) [RFC3261] to negotiate
and establish multimedia sessions.
In addition to the usual contents of an SDP media description ("m="
line) specified for UDPTL over the UDP, each SDP media description
for UDPTL over DTLS over the UDP will also contain several SDP
attributes, as specified in [RFC4145] and [RFC4572].
The SDP offer and SDP answer MUST conform to the following
requirements:
o The endpoint MUST set the "proto" field of the "m=" line to the
token specified in Table 3.
o The endpoint MUST use the SDP setup attribute [RFC4145]. The
offerer MUST assign the SDP setup attribute with setup:actpass
value, and MUST be prepared to receive a DTLS client_hello message
before it receives the SDP answer. The answerer MUST assign the
SDP setup attribute with either setup:active value or
setup:passive value. The answerer SHOULD assign the SDP setup
attribute with the setup:active value. Whichever party is active
MUST initiate a DTLS handshake by sending a ClientHello over each
flow (host/port quartet).
o The endpoint MUST use the SDP certificate fingerprint attribute
[RFC4572].
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o The certificate presented during the DTLS handshake MUST match the
fingerprint exchanged via the signaling path in the SDP.
o 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.
Editor's note: FFS if connection attribute defined in RFC4145 is
needed.
3.2. Secure Channel Usage
The DTLS is used as specified in [RFC6347]. Once the DTLS handshake
is completed, the UDPTL packets SHALL be transported in DTLS record
layer "application_data" packets.
4. Miscellaneous Considerations
4.1. Anonymous Calls
When making anonymous calls, a new self-signed certificate SHOULD be
used for each call and the content of the subjectAltName attribute
inside the certificate MUST NOT contain information that either
allows correlation or identification of the user making anonymous
calls.
4.2. Middlebox Interaction
The procedures defined for SRTP-DTLS in [RFC5763] section 6.7 for
interaction with middleboxes also apply to UPPTL over DTLS.
The procedures defined for SRTP-DTLS in [RFC5764] section 5.1.2 for
distinguishing DTLS and STUN packets also apply to UDPTL over DTLS.
Editor's note: The complete SRTP-DTLS implementation is not needed.
Only the parts for interaction with middleboxes in RFC5763 and for
distinguishing DTLS and STUN packets in RFC5764 are needed. Should
those be copied into this document?
4.3. Rekeying
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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 of this document.
5. Security Considerations
DTLS media signaled with SIP requires a way to ensure that the
communicating peers' certificates are correct.
The standard DTLS strategy for authenticating the communicating
parties is to give the server (and optionally the client) a PKIX
[RFC5280] certificate. The client then verifies the certificate and
checks that the name in the certificate matches the server's domain
name. This works because there are a relatively small number of
servers with well-defined names; a situation that does not usually
occur in the VoIP context.
The design described in this document is intended to leverage the
authenticity of the signaling channel (while not requiring
confidentiality). As long as each side of the connection can verify
the integrity of the SDP received from the other side, then the DTLS
handshake cannot be hijacked via a man-in-the-middle attack. This
integrity protection is easily provided by the caller to the callee
(see Alice to Bob in Section 7) 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 the Table 3 to the table for the
"proto" field.
+-------+---------------+------------+
| Type | SDP Name | Reference |
+-------+---------------+------------+
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| 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 to this draft.
8. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
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.
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[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.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, May 2010.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[ITU.T38.1998]
International Telecommunications Union, "Procedures for
real time Group 3 facsimile communication between
terminals using IP Networks", ITU-T Recommendation T.38,
1998.
[ITU.T38.2004]
International Telecommunications Union, "Procedures for
real-time Group 3 facsimile communication over IP
networks", ITU-T Recommendation T.38, 2004.
9.2. Informative References
[RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet,
"Requirements and Analysis of Media Security Management
Protocols", RFC 5479, April 2009.
Appendix A. Example
A.1. General
Prior to establishing the session, both Alice and Bob generate self-
signed certificates which are used for a single session or, more
likely, reused for multiple sessions.
The SIP signaling from Alice to her proxy is transported over TLS to
ensure an integrity protected channel between Alice and her identity
service. Transport between proxies should also be protected somehow.
Only one element is shown for Alice's and Bob's proxies for the
purposes of simplification.
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Only the mandatory SDP T.38 attributes are shown for simplification.
A.2. Basic Message Flow with Identity
Figure 1 shows an example message flow of session establishment for
T.38 fax securely transported using UDPTL over DTLS.
In this example flow, Alice acts as the passive endpoint of DTLS
association and Bob acts as the active endpoint of DTLS association.
Alice Proxies Bob
| (1) SIP INVITE | |
|----------------------->| |
| | (2) SIP INVITE |
| |----------------------->|
| | (3) DTLS ClientHello |
|<------------------------------------------------|
| (4) remaining messages of DTLS handshake |
|<----------------------------------------------->|
| | |
| | |
| | (5) SIP 200 OK |
| |<-----------------------|
| (6) SIP 200 OK | |
|<-----------------------| |
| (7) SIP ACK | |
|------------------------------------------------>|
| (8) T.38 message using UDPTL over DTLS |
|<----------------------------------------------->|
| | |
Figure 1: Basic message flow with Identity
Message (1):
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.
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The SDP fingerprint attribute in the SDP Offer indicates the
certificate fingerprint computed from Alice's self-signed
certificate.
INVITE sip:bob@example.com SIP/2.0
To: <sip:bob@example.com>
From: "Alice"<sip:alice@example.com>;tag=843c7b0b
Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
Contact: <sip:alice@ua1.example.com>
Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: xxxx
Supported: from-change
v=0
o=- 1181923068 1181923196 IN IP4 ua1.example.com
s=example1
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.
The SIP INVITE request contains an Identity header field and an
Identity-Info header fields inserted by Alice's proxy.
When received, Bob verifies the identity provided in the SIP
INVITE request.
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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
Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFC
HYWGCl0nB2sNsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
Identity-Info: https://example.com/cert
Content-Type: application/sdp
Content-Length: xxxx
Supported: from-change
v=0
o=- 1181923068 1181923196 IN IP4 ua1.example.com
s=example1
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 the Alice's
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certificate received via DTLS matches the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 3. In this
message flow, the check is successful and thus session setup
continues.
Message (5):
Figure 4 shows a 200 (OK) response to the initial SIP INVITE
request, sent by Bob to Bob's proxy. The 200 (OK) response
contains an SDP answer.
The "m=" line in the SDP Answer indicates T.38 fax using UDPTL
over DTLS.
The SDP setup:active attribute in the SDP Answer indicates that
Bob has requested to be the active endpoint.
The SDP fingerprint attribute in the SDP Answer indicates the
certificate fingerprint computed from Bob's self-signed
certificate.
SIP/2.0 200 OK
To: <sip:bob@example.com>;tag=6418913922105372816
From: "Alice" <sip:alice@example.com>;tag=843c7b0b
Via: SIP/2.0/TLS proxy.example.com:5061;branch=z9hG4bK-0e53sadfkasldk
Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
Record-Route: <sip:proxy.example.com;lr>
Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE
Contact: <sip:bob@ua2.example.com>
Content-Type: application/sdp
Content-Length: xxxx
Supported: from-change
v=0
o=- 6418913922105372816 2105372818 IN IP4 ua2.example.com
s=example2
c=IN IP4 ua2.example.com
t=0 0
m=image 12000 UDP/TLS/UDPTL t38
a=setup:active
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a=fingerprint: SHA-1 \
FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=T38FaxRateManagement:transferredTCF
Figure 4: Message (6)
Message (6):
Figure 5 shows a 200 (OK) response to the initial SIP INVITE
request, sent by Alice's proxy to Alice. Alice checks if the
certificate fingerprint calculated from the Bob's certificate
received via DTLS is the same as the certificate fingerprint
received in the a=fingerprint SDP attribute of Figure 5. In this
message flow, the check is successful and thus session setup
continues.
SIP/2.0 200 OK
To: <sip:bob@example.com>;tag=6418913922105372816
From: "Alice" <sip:alice@example.com>;tag=843c7b0b
Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj
Record-Route: <sip:proxy.example.com;lr>
Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE
Contact: <sip:bob@ua2.example.com>
Content-Type: application/sdp
Content-Length: xxxx
Supported: from-change
v=0
o=- 6418913922105372816 2105372818 IN IP4 ua2.example.com
s=example2
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 (7)
Message (7):
Alice sends the SIP ACK request to Bob.
Message (8):
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At this point, Bob and Alice can exchange T.38 fax securely
transported using UDPTL over DTLS.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Ivo Sedlacek
Ericsson
Sokolovska 79
Praha 18600
Czech Republic
Email: ivo.sedlacek@ericsson.com
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