Network Working Group P. Jones
Internet-Draft Cisco Systems
Intended status: Standards Track P. Ellenbogen
Expires: April 18, 2020 Princeton University
N. Ohlmeier
October 16, 2019

DTLS Tunnel between a Media Distributor and Key Distributor to Facilitate Key Exchange


This document defines a DTLS tunneling protocol for use in multimedia conferences that enables a Media Distributor to facilitate key exchange between an endpoint in a conference and the Key Distributor. The protocol is designed to ensure that the keying material used for hop-by-hop encryption and authentication is accessible to the media distributor, while the keying material used for end-to-end encryption and authentication is inaccessible to the media distributor.

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This Internet-Draft will expire on April 18, 2020.

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Table of Contents

1. Introduction

An objective of Privacy-Enhanced RTP Conferencing (PERC) is to ensure that endpoints in a multimedia conference have access to the end-to-end (E2E) and hop-by-hop (HBH) keying material used to encrypt and authenticate Real-time Transport Protocol (RTP) [RFC3550] packets, while the Media Distributor has access only to the hop-by-hop (HBH) keying material for encryption and authentication.

This specification defines a tunneling protocol that enables the media distributor to tunnel DTLS [RFC6347] messages between an endpoint and the key distributor, thus allowing an endpoint to use DTLS-SRTP [RFC5764] for establishing encryption and authentication keys with the key distributor.

The tunnel established between the media distributor and key distributor is a TLS connection that is established before any messages are forwarded by the media distributor on behalf of the endpoint. DTLS packets received from the endpoint are encapsulated by the media distributor inside this tunnel as data to be sent to the key distributor. Likewise, when the media distributor receives data from the key distributor over the tunnel, it extracts the DTLS message inside and forwards the DTLS message to the endpoint. In this way, the DTLS association for the DTLS-SRTP procedures is established between the endpoint and the key distributor, with the media distributor simply forwarding packets between the two entities and having no visibility into the confidential information exchanged.

Following the existing DTLS-SRTP procedures, the endpoint and key distributor will arrive at a selected cipher and keying material, which are used for HBH encryption and authentication by both the endpoint and the media distributor. However, since the media distributor would not have direct access to this information, the key distributor explicitly shares the HBH key information with the media distributor via the tunneling protocol defined in this document. Additionally, the endpoint and key distributor will agree on a cipher for E2E encryption and authentication. The key distributor will transmit keying material to the endpoint for E2E operations, but will not share that information with the media distributor.

By establishing this TLS tunnel between the media distributor and key distributor and implementing the protocol defined in this document, it is possible for the media distributor to facilitate the establishment of a secure DTLS association between an endpoint and the key distributor in order for the endpoint to receive E2E and HBH keying material. At the same time, the key distributor can securely provide the HBH keying material to the media distributor.

2. Conventions Used In This Document

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 [RFC2119] when they appear in ALL CAPS. These words may also appear in this document in lower case as plain English words, absent their normative meanings.

3. Tunneling Concept

A TLS connection (tunnel) is established between the media distributor and the key distributor. This tunnel is used to relay DTLS messages between the endpoint and key distributor, as depicted in Figure 1:

                         |     Key     |
                         | Distributor |
                             # ^ ^ #
                             # | | # <-- TLS Tunnel
                             # | | #
+----------+             +-------------+             +----------+
|          |     DTLS    |             |    DTLS     |          |
| Endpoint |<------------|    Media    |------------>| Endpoint |
|          |    to Key   | Distributor |   to Key    |          |
|          | Distributor |             | Distributor |          |
+----------+             +-------------+             +----------+

Figure 1: TLS Tunnel to Key Distributor

The three entities involved in this communication flow are the endpoint, the media distributor, and the key distributor. The behavior of each entity is described in Section 5.

The key distributor is a logical function that might might be co-resident with a key management server operated by an enterprise, reside in one of the endpoints participating in the conference, or elsewhere that is trusted with E2E keying material.

4. Example Message Flows

This section provides an example message flow to help clarify the procedures described later in this document. It is necessary that the key distributor and media distributor establish a mutually authenticated TLS connection for the purpose of sending tunneled messages, though the complete TLS handshake for the tunnel is not shown in Figure 2 since there is nothing new this document introduces with regard to those procedures.

Once the tunnel is established, it is possible for the media distributor to relay the DTLS messages between the endpoint and the key distributor. Figure 2 shows a message flow wherein the endpoint uses DTLS-SRTP to establish an association with the key distributor. In the process, the media distributor shares its supported SRTP protection profile information (see [RFC5764]) and the key distributor shares HBH keying material and selected cipher with the media distributor.

Endpoint              media distributor          key distributor
    |                         |                         |
    |                         |<=======================>|
    |                         |    TLS Connection Made  |
    |                         |                         |
    |                         |========================>|
    |                         | SupportedProfiles       |
    |                         |                         |
    | DTLS handshake message  | TunneledDtls            |
    |                         |                         |
    |                         |<========================|
    |                         |               MediaKeys |
    |                         |                         |
         .... may be multiple handshake messages ...
    |                         |                         |
    | DTLS handshake message  |            TunneledDtls |
    |                         |                         |

Figure 2: Sample DTLS-SRTP Exchange via the Tunnel

After the initial TLS connection has been established each of the messages on the right-hand side of Figure 2 is a tunneling protocol message as defined in Section Section 6.

SRTP protection profiles supported by the media distributor will be sent in a SupportedProfiles message when the TLS tunnel is initially established. The key distributor will use that information to select a common profile supported by both the endpoint and the media distributor to ensure that hop-by-hop operations can be successfully performed.

As DTLS messages are received from the endpoint by the media distributor, they are forwarded to the key distributor encapsulated inside abbrev TunneledDtls message. Likewise, as TunneledDtls messages are received by the media distributor from the key distributor, the encapsulated DTLS packet is forwarded to the endpoint.

The key distributor will provide the SRTP [RFC3711] keying material to the media distributor for HBH operations via the MediaKeys message. The media distributor will extract this keying material from the MediaKeys message when received and use it for hop-by-hop encryption and authentication.

5. Tunneling Procedures

The following sub-sections explain in detail the expected behavior of the endpoint, the media distributor, and the key distributor.

It is important to note that the tunneling protocol described in this document is not an extension to TLS [RFC5246] or DTLS [RFC6347]. Rather, it is a protocol that transports DTLS messages generated by an endpoint or key distributor as data inside of the TLS connection established between the media distributor and key distributor.

5.1. Endpoint Procedures

The endpoint follows the procedures outlined for DTLS-SRTP [RFC5764] in order to establish the cipher and keys used for encryption and authentication, with the endpoint acting as the client and the key distributor acting as the server. The endpoint does not need to be aware of the fact that DTLS messages it transmits toward the media distributor are being tunneled to the key distributor.

The endpoint MUST include the sdp_tls_id DTLS extension [I-D.thomson-mmusic-sdp-uks] in the ClientHello message when establishing a DTLS association. Likewise, the tls-id SDP [RFC4566] attribute MUST be included in SDP sent by the endpoint in both the offer and answer [RFC3264] messages as per [I-D.ietf-mmusic-dtls-sdp].

When receiving a tls_id value from the key distributor, the client MUST check to ensure that value matches the tls-id value received in SDP. If the values do not match, the endpoint MUST consider any received keying material to be invalid and terminate the DTLS association.

5.2. Tunnel Establishment Procedures

Either the media distributor or key distributor initiates the establishment of a TLS tunnel. Which entity acts as the TLS client when establishing the tunnel and what event triggers the establishment of the tunnel are outside the scope of this document. Further, how the trust relationships are established between the key distributor and media distributor are also outside the scope of this document.

A tunnel MUST be a mutually authenticated TLS connection.

The media distributor or key distributor MUST establish a tunnel prior to forwarding tunneled DTLS messages. Given the time-sensitive nature of DTLS-SRTP procedures, a tunnel SHOULD be established prior to the media distributor receiving a DTLS message from an endpoint.

A single tunnel MAY be used to relay DTLS messages between any number of endpoints and the key distributor.

A media distributor MAY have more than one tunnel established between itself and one or more key distributors. When multiple tunnels are established, which tunnel or tunnels to use to send messages for a given conference is outside the scope of this document.

5.3. Media Distributor Tunneling Procedures

The first message transmitted over the tunnel is the SupportedProfiles (see Section 6). This message informs the key distributor about which DTLS-SRTP profiles the media distributor supports. This message MUST be sent each time a new tunnel connection is established or, in the case of connection loss, when a connection is re-established. The media distributor MUST support the same list of protection profiles for the duration of any endpoint-initiated DTLS association and tunnel connection.

The media distributor MUST assign a unique association identifier for each endpoint-initiated DTLS association and include it in all messages forwarded to the key distributor. The key distributor will subsequently include this identifier in all messages it sends so that the media distributor can map messages received via a tunnel and forward those messages to the correct endpoint. The association identifier MUST be randomly assigned UUID [RFC4122] value.

When a DTLS message is received by the media distributor from an endpoint, it forwards the UDP payload portion of that message to the key distributor encapsulated in a TuneledDtls message. The media distributor is not required to forward all messages received from an endpoint for a given DTLS association through the same tunnel if more than one tunnel has been established between it and a key distributor.

When a MediaKeys message is received, the media distributor MUST extract the cipher and keying material conveyed in order to subsequently perform HBH encryption and authentication operations for RTP and RTCP packets sent between it and an endpoint. Since the HBH keying material will be different for each endpoint, the media distributor uses the association identifier included by the key distributor to ensure that the HBH keying material is used with the correct endpoint.

The media distributor MUST forward all DTLS messages received from either the endpoint or the key distributor (via the TunneledDtls message) to ensure proper communication between those two entities.

When the media distributor detects an endpoint has disconnected or when it receives conference control messages indicating the endpoint is to be disconnected, the media distributors MUST send an EndpointDisconnect message with the association identifier assigned to the endpoint to the key distributor. The media distributor SHOULD take a loss of all RTP and RTCP packets as an indicator that the endpoint has disconnected. The particulars of how RTP and RTCP are to be used to detect an endpoint disconnect, such as timeout period, is not specified. The media distributor MAY use additional indicators to determine when an endpoint has disconnected.

5.4. Key Distributor Tunneling Procedures

Each TLS tunnel established between the media distributor and the key distributor MUST be mutually authenticated.

When the media distributor relays a DTLS message from an endpoint, the media distributor will include an association identifier that is unique per endpoint-originated DTLS association. The association identifier remains constant for the life of the DTLS association. The key distributor identifies each distinct endpoint-originated DTLS association by the association identifier.

When processing an incoming endpoint association, the key distributor MUST extract the tls_id value transmitted in the ClientHello message and match that against tls-id value the endpoint transmitted via SDP. If the values in SDP and the ClientHello do not match, the DTLS association MUST be rejected.

The process through which the tls-id in SDP is conveyed to the key distributor is outside the scope of this document.

The key distributor MUST correlate the certificate fingerprint and tls_id received from endpoint's ClientHello message with the corresponding values received from the SDP transmitted by the endpoint. It is through this correlation that the key distributor can be sure to deliver the correct conference key to the endpoint.

When sending the ServerHello message, the key distributor MUST insert its own tls_id value in the sdp_tls_id extension. This value MUST also be conveyed back to the client via SDP as a tls-id attribute.

The key distributor MUST encapsulate any DTLS message it sends to an endpoint inside a TunneledDtls message (see Section 6). The key distributor is not required to transmit all messages a given DTLS association through the same tunnel if more than one tunnel has been established between it and a media distributor.

The key distributor MUST use the same association identifier in messages sent to an endpoint as was received in messages from that endpoint. This ensures the media distributor can forward the messages to the correct endpoint.

The key distributor extracts tunneled DTLS messages from an endpoint and acts on those messages as if that endpoint had established the DTLS association directly with the key distributor. The key distributor is acting as the DTLS server and the endpoint is acting as the DTLS client. The handling of the messages and certificates is exactly the same as normal DTLS-SRTP procedures between endpoints.

The key distributor MUST send a MediaKeys message to the media distributor as soon as the HBH encryption key is computed and before it sends a DTLS Finished message to the endpoint. The MediaKeys message includes the selected cipher (i.e. protection profile), MKI [RFC3711] value (if any), SRTP master keys, and SRTP master salt values. The key distributor MUST use the same association identifier in the MediaKeys message as is used in the TunneledDtls messages for the given endpoint.

The key distributor uses the certificate fingerprint of the endpoint along with the tls_id value received in the sdp_tls_id extension to determine which conference a given DTLS association is associated.

The key distributor MUST select a cipher that is supported by both the endpoint and the media distributor to ensure proper HBH operations.

When the DTLS association between the endpoint and the key distributor is terminated, regardless of which entity initiated the termination, the key distributor MUST send an EndpointDisconnect message with the association identifier assigned to the endpoint to the media distributor.

5.5. Versioning Considerations

All messages for an established tunnel MUST utilize the same version value.

Since the media distributor sends the first message over the tunnel, it effectively establishes the version of the protocol to be used. If that version is not supported by the key distributor, it MUST discard the message, transmit an UnsupportedVersion message, and close the TLS connection.

The media distributor MUST take note of the version received in an UnsupportedVersion message and use that version when attempting to re-establish a failed tunnel connection. Note that it is not necessary for the media distributor to understand the newer version of the protocol to understand that the first message received is UnsupportedVersion. The media distributor can determine from the first two octets received what the version number is and that the message is UnsupportedVersion. The rest of the data received, if any, would be discarded and the connection closed (if not already closed).

6. Tunneling Protocol

Tunneled messages are transported via the TLS tunnel as application data between the media distributor and the key distributor. Tunnel messages are specified using the format described in [RFC5246] section 4. As in [RFC5246], all values are stored in network byte (big endian) order; the uint32 represented by the hex bytes 01 02 03 04 is equivalent to the decimal value 16909060.

The protocol defines several different messages, each of which containing the the following information:

Each of these messages is a TunnelMessage in the syntax, with a message type indicating the actual content of the message body.

6.1. Tunnel Message Format

The syntax of the protocol is defined below. TunnelMessage defines the structure of all messages sent via the tunnel protocol. That structure includes a field called msg_type that identifies the specific type of message contained within TunnelMessage.

enum {
} MsgType;

opaque uuid[16];

struct {
    MsgType msg_type;
    uint16 length;
    select (MsgType) {
        case supported_profiles:  SupportedProfiles;
        case unsupported_version: UnsupportedVersion;
        case media_keys:          MediaKeys;
        case tunneled_dtls:       TunneledDtls;
        case endpoint_disconnect: EndpointDisconnect;
  } body;
} TunnelMessage;

The elements of TunnelMessage include:

The SupportedProfiles message is defined as:

uint8 SRTPProtectionProfile[2]; /* from RFC5764 */

struct {
  uint8 version;
  SRTPProtectionProfile protection_profiles<0..2^16-1>;
} SupportedProfiles;

This message contains this single element:

The UnsupportedVersion message is defined as follows:

struct {
    uint8 highest_version;
} UnsupportedVersion;

The elements of UnsupportedVersion include:

The MediaKeys message is defined as:

struct {
    uuid association_id;
    SRTPProtectionProfile protection_profile;
    opaque mki<0..255>;
    opaque client_write_SRTP_master_key<1..255>;
    opaque server_write_SRTP_master_key<1..255>;
    opaque client_write_SRTP_master_salt<1..255>;
    opaque server_write_SRTP_master_salt<1..255>;
} MediaKeys;

The fields are described as follows:

The TunneledDtls message is defined as:

struct {
    uuid association_id;
    opaque dtls_message<0..2^16-1>;
} TunneledDtls;

The fields are described as follows:

The EndpointDisconect message is defined as:

struct {
    uuid association_id;
} EndpointDisconnect;

The fields are described as follows:

7. Example Binary Encoding

The TunnelMessage is encoded in binary following the procedures specified in [RFC5246]. This section provides an example of what the bits on the wire would look like for the SupportedProfiles message that advertises support for both DOUBLEAEADAES128GCMAEADAES128GCM and DOUBLEAEADAES256GCMAEADAES256GCM [I-D.ietf-perc-double].

RFC Editor Note: Please replace the values 0009 and 000A in the following two examples with whatever code points IANA assigned for DOUBLEAEADAES128GCMAEADAES128GCM and DOUBLEAEADAES256GCMAEADAES256GCM.

         message_type: 0x01
               length: 0x0007
                   version:  0x00
       protection_profiles:  0x0004 (length)
                             0x0009000A (value)

Thus, the encoding on the wire presented here in network bytes order would be this stream of octets:


8. IANA Considerations

This document establishes a new registry to contain message type values used in the DTLS Tunnel protocol. These data type values are a single octet in length. This document defines the values shown in Table 1 below, leaving the balance of possible values reserved for future specifications:

Data Type Values for the DTLS Tunnel Protocol
MsgType Description
0x01 Supported SRTP Protection Profiles
0x02 Unsupported Version
0x03 Media Keys
0x04 Tunneled DTLS
0x05 Endpoint Disconnect

The value 0x00 and all values in the range 0x06 to 0xFF are reserved.

The name for this registry is "Datagram Transport Layer Security (DTLS) Tunnel Protocol Data Types for Privacy Enhanced Conferencing".

9. Security Considerations

The encapsulated data is protected by the TLS connection from the endpoint to key distributor, and the media distributor is merely an on path entity. The media distributor does not have access to the end-to-end keying material This does not introduce any additional security concerns beyond a normal DTLS-SRTP association.

The HBH keying material is protected by the mutual authenticated TLS connection between the media distributor and key distributor. The key distributor MUST ensure that it only forms associations with authorized media distributors or it could hand HBH keying material to untrusted parties.

The supported profiles information sent from the media distributor to the key distributor is not particularly sensitive as it only provides the cryptographic algorithms supported by the media distributor. Further, it is still protected by the TLS connection between the media distributor and the key distributor.

10. Acknowledgments

The author would like to thank David Benham and Cullen Jennings for reviewing this document and providing constructive comments.

11. References

11.1. Normative References

[I-D.ietf-mmusic-dtls-sdp] Holmberg, C. and R. Shpount, "Session Description Protocol (SDP) Offer/Answer Considerations for Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS)", Internet-Draft draft-ietf-mmusic-dtls-sdp-32, October 2017.
[I-D.thomson-mmusic-sdp-uks] Thomson, M. and E. Rescorla, "Unknown Key Share Attacks on uses of Transport Layer Security with the Session Description Protocol (SDP)", Internet-Draft draft-thomson-mmusic-sdp-uks-00, April 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004.
[RFC4122] Leach, P., Mealling, M. and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, July 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[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, DOI 10.17487/RFC5764, May 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.

11.2. Informative References

[I-D.ietf-perc-double] Jennings, C., Jones, P., Barnes, R. and A. Roach, "SRTP Double Encryption Procedures", Internet-Draft draft-ietf-perc-double-12, August 2019.
[RFC4566] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006.

Authors' Addresses

Paul E. Jones Cisco Systems, Inc. 7025 Kit Creek Rd. Research Triangle Park, North Carolina 27709 USA Phone: +1 919 476 2048 EMail:
Paul M. Ellenbogen Princeton University Phone: +1 206 851 2069 EMail:
Nils H. Ohlmeier Mozilla Phone: +1 408 659 6457 EMail: