MMUSIC Working Group | C.H. Holmberg |
Internet-Draft | I.S. Sedlacek |
Intended status: Standards Track | Ericsson |
Expires: June 12, 2014 | G.S. Salgueiro |
Cisco | |
December 09, 2013 |
UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS)
draft-ietf-mmusic-udptl-dtls-02
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).
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While telephony encryption devices have been traditionally used for highly sensitive documents, secure fax on the Public Switched Telephone Network (PSTN) was not as widely considered or prioritized because of the challenges involved with physical access to telephony equipment. As real-time communications transition to IP networks, where information might potentially be intercepted or spoofed, an appropriate level of security for fax that offers integrity and confidentiality protection is vital. Some of the security mechanisms for securing fax include:
Despite these mechanisms to secure fax, there is no transport layer security offering integrity and confidentiality protection for UDPTL [ITU.T38.2010], the overwhelmingly predominant fax transport protocol. The protocol stack for fax transport using UDPTL is shown in Table 1.
Protocol |
---|
Internet facsimile protocol |
UDPTL |
UDP |
IP |
The 3rd Generation Partnership Project (3GPP) has performed a study on how to provide secure fax in the IP Multimedia Subsystem (IMS) and concluded that secure fax shall be transported using UDPTL over DTLS.
This document specifies fax transport using UDPTL over DTLS [RFC6347], which enables integrity and confidentiality protection of fax in IP networks. The protocol stack for integrity and confidentiality protected fax transport using UDPTL over DTLS is shown in Table 2.
Protocol |
---|
Internet facsimile protocol |
UDPTL |
DTLS |
UDP |
IP |
The primary motivations for the mechanism in this document are:
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].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119].
DTLS uses the term "session" to refer to a long-lived set of keying material that spans DTLS associations. In this document, in order to be consistent with SIP/SDP usage of "session" terminology, we use it to refer to a multimedia session and use the term "DTLS session" to refer to the DTLS construct. We use the term "DTLS association" to refer to a particular DTLS cipher suite and keying material set that is associated with a single host/port quartet. The same DTLS session can be used to establish the keying material for multiple DTLS associations. For consistency with other SIP/SDP usage, we use the term "connection" when what's being referred to is a multimedia stream that is not specifically DTLS.
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:
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.
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.
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.
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.
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:
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.
DTLS media signaled with SIP requires a mechanism to ensure that the communicating peers' certificates are correct.
The standard DTLS strategy for authenticating the communicating parties is to give the server (and optionally the client) a PKIX [RFC5280] certificate. The client then verifies the certificate and checks that the name in the certificate matches the server's domain name. This works because there are a relatively small number of servers with well-defined names; a situation that does not usually occur in the VoIP context.
The design described in this document is intended to leverage the authenticity of the signaling channel (while not requiring confidentiality). As long as each side of the connection can verify the integrity of the SDP received from the other side, then the DTLS handshake cannot be hijacked via a man-in-the-middle attack. This integrity protection is easily provided by the caller to the callee (see sample message flow in Annex A.2) via the SIP Identity [RFC4474] mechanism. Other mechanisms, such as the S/MIME mechanism [RFC3261], or perhaps future mechanisms yet to be specified could also serve this purpose.
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]).
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] |
[RFC EDITOR NOTE: Please replace RFC-XXXX with the RFC number of this document.]
Special thanks to Peter Dawes, who provided comments on the initial version of the draft, and to Paul E. Jones, James Rafferty, Albrecht Schwarz and Oscar Ohlsson who provided valuable feedback and input on the MMUSIC mailing list.
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from draft-ietf-mmusic-udptl-dtls-01
Changes from draft-ietf-mmusic-udptl-dtls-00
Changes from draft-holmberg-mmusic-udptl-dtls-02
Changes from draft-holmberg-mmusic-udptl-dtls-01
Changes from draft-holmberg-mmusic-udptl-dtls-00
Changes from draft-holmberg-dispatch-udptl-dtls-00
[RFC5479] | Wing, D., Fries, S., Tschofenig, H. and F. Audet, "Requirements and Analysis of Media Security Management Protocols", RFC 5479, April 2009. |
Prior to establishing the session, both Alice and Bob generate self-signed certificates which are used for a single session or, more likely, reused for multiple sessions.
The SIP signaling from Alice to her proxy is transported over TLS to ensure an integrity protected channel between Alice and her identity service. Transport between proxies should also be protected somehow.
Only one element is shown for Alice's and Bob's proxies for the purposes of simplification.
For the sake of brevity and simplicity, only the mandatory SDP T.38 attributes are shown.
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
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)
INVITE sip:bob@ua2.example.com SIP/2.0 To: <sip:bob@example.com> From: "Alice"<sip:alice@example.com>;tag=843c7b0b Via: SIP/2.0/TLS proxy.example.com;branch=z9hG4bK-0e53sadfkasldk Via: SIP/2.0/TLS ua1.example.com;branch=z9hG4bK-0e53sadfkasldkfj Record-Route: <sip:proxy.example.com;lr> Contact: <sip:alice@ua1.example.com> Call-ID: 6076913b1c39c212@REVMTEpG CSeq: 1 INVITE Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, UPDATE Max-Forwards: 69 Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k 3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFC HYWGCl0nB2sNsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI= Identity-Info: https://example.com/cert Content-Type: application/sdp Content-Length: xxxx Supported: from-change v=0 o=- 1181923068 1181923196 IN IP4 ua1.example.com s=- c=IN IP4 ua1.example.com t=0 0 m=image 6056 UDP/TLS/UDPTL t38 a=setup:actpass a=fingerprint: SHA-1 \ 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB a=T38FaxRateManagement:transferredTCF
Figure 3: Message (2)
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)
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)
Figure 6 focuses on T.38 fax securely transported using UDPTL over DTLS replacing audio media stream in an existing audio-only session.
In this example flow, Alice acts as the passive endpoint of DTLS association and Bob acts as the active endpoint of DTLS association.
Alice Proxies Bob | | | | (1) Audio-only session initiation | |<-----------------------+----------------------->| | | | | (2) SIP re-INVITE | | |------------------------------------------------>| | | (3) DTLS ClientHello | |<------------------------------------------------| | (4) remaining messages of DTLS handshake | |<----------------------------------------------->| | | | | | | | | (5) SIP 200 OK | |<------------------------------------------------| | (6) SIP ACK | | |------------------------------------------------>| | (7) T.38 message using UDPTL over DTLS | |<----------------------------------------------->| | | |
Figure 6: Message Flow Of T.38 Fax Replacing Audio Media Stream in An Existing Audio-Only Session
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)
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)