Internet DRAFT - draft-ietf-perc-dtls-tunnel
draft-ietf-perc-dtls-tunnel
Network Working Group P. Jones
Internet-Draft Cisco Systems
Intended status: Informational P. Ellenbogen
Expires: 16 May 2022 Princeton University
N. Ohlmeier
8x8, Inc.
12 November 2021
DTLS Tunnel between a Media Distributor and Key Distributor to
Facilitate Key Exchange
draft-ietf-perc-dtls-tunnel-12
Abstract
This document defines a protocol for tunneling DTLS traffic 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.
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 https://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 16 May 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Tunneling Concept . . . . . . . . . . . . . . . . . . . . . . 4
4. Example Message Flows . . . . . . . . . . . . . . . . . . . . 4
5. Tunneling Procedures . . . . . . . . . . . . . . . . . . . . 6
5.1. Endpoint Procedures . . . . . . . . . . . . . . . . . . . 6
5.2. Tunnel Establishment Procedures . . . . . . . . . . . . . 6
5.3. Media Distributor Tunneling Procedures . . . . . . . . . 7
5.4. Key Distributor Tunneling Procedures . . . . . . . . . . 8
5.5. Versioning Considerations . . . . . . . . . . . . . . . . 10
6. Tunneling Protocol . . . . . . . . . . . . . . . . . . . . . 10
6.1. TunnelMessage Structure . . . . . . . . . . . . . . . . . 11
6.2. SupportedProfiles Message . . . . . . . . . . . . . . . . 11
6.3. UnsupportedVersion Message . . . . . . . . . . . . . . . 12
6.4. MediaKeys Message . . . . . . . . . . . . . . . . . . . . 12
6.5. TunneledDtls Message . . . . . . . . . . . . . . . . . . 13
6.6. EndpointDisconnect Message . . . . . . . . . . . . . . . 13
7. Example Binary Encoding . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. Normative References . . . . . . . . . . . . . . . . . . . . 16
12. Informative References . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
An objective of Privacy-Enhanced RTP Conferencing (PERC) [RFC8871] 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 HBH
keying material for encryption and authentication.
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This specification defines a tunneling protocol that enables the
Media Distributor to tunnel DTLS [I-D.ietf-tls-dtls13] messages
between an endpoint and a 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 [RFC8446] connection that is established before
any messages are forwarded by the Media Distributor on behalf of
endpoints. DTLS packets received from an 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 an endpoint and the Key Distributor, with the
Media Distributor forwarding DTLS messages between the two entities
via the established tunnel to the Key Distributor 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 generate 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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This document uses the terms "endpoint", "Media Distributor", and
"Key Distributor" defined in [RFC8871].
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 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
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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 |
| | |
.... may be multiple handshake messages ...
| | |
|<------------------------|<========================|
| DTLS handshake message | TunneledDtls |
| | |
| | |
| |<========================|
| | MediaKeys |
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 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 HBH 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 a "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.
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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 HBH
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 or DTLS. 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 a unique identifier in the "tls-id" SDP
[RFC8866] attribute in all offer and answer messages [RFC3264] that
it generates as per [RFC8842]. Further, the endpoint MUST include
this same unique identifier in the "external_session_id" extension
[RFC8844] in the "ClientHello" message when establishing a DTLS
association.
When receiving a "external_session_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.
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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 value as described
Section 4.4 of [RFC4122].
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
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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 "external_session_id" value transmitted in the
"ClientHello" message and match that against the "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 match the fingerprint of the certificate and
"external_session_id" [RFC8844] received from endpoint via DTLS with
the expected fingerprint [RFC8122] and "tls-id" [RFC8842] values
received via SDP. It is through this process that the Key
Distributor can be sure to deliver the correct conference key to the
endpoint.
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The Key Distributor MUST report its own unique identifier in the
"external_session_id" extension. This extension is sent in the
"EncryptedExtensions" message in DTLS 1.3, and the "ServerHello" in
previous DTLS versions. 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 for a given DTLS
association through the same tunnel if more than one tunnel has been
established between it and the 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 immediately after the DTLS handshake completes. The
"MediaKeys" message includes the selected cipher (i.e. protection
profile), MKI [RFC3711] value (if any), HBH 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.
There are presently two SRTP protection profiles defined for PERC,
namely "DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM" and
"DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM" [RFC8723]. As [RFC8723]
explains in Section 5.2, the Media Distributor is only given the SRTP
master key for HBH operations. As such, the SRTP master key length
advertised in the "MediaKeys" message is half the length of the key
normally associated with selected "double" protection profile.
The Key Distributor uses the certificate fingerprint of the endpoint
along with the unique identifier received in the
"external_session_id" extension to determine which conference a given
DTLS association is associated.
The Key Distributor MUST select a cipher that is supported itself,
the endpoint, and the Media Distributor to ensure proper HBH
operations.
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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
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, the Key
Distributor MUST transmit an "UnsupportedVersion" message containing
the highest version number supported, 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 four 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 [RFC8446]
section 3. As in [RFC8446], 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.
This protocol defines several different messages, each of which
contains the following information:
* Message type identifier
* Message body length
* The message body
Each of the tunnel messages is a "TunnelMessage" structure with the
message type indicating the actual content of the message body.
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6.1. TunnelMessage Structure
The "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 {
supported_profiles(1),
unsupported_version(2),
media_keys(3),
tunneled_dtls(4),
endpoint_disconnect(5),
(255)
} 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:
* "msg_type": the type of message contained within the structure
"body".
* "length": the length in octets of the following "body" of the
message.
* "body": the actual message being conveyed within this
"TunnelMessage" structure.
6.2. SupportedProfiles Message
The "SupportedProfiles" message is defined as:
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uint8 SRTPProtectionProfile[2]; /* from RFC5764 */
struct {
uint8 version;
SRTPProtectionProfile protection_profiles<2..2^16-1>;
} SupportedProfiles;
This message contains this single element:
* "version": This document specifies version 0x00.
* "protection_profiles": The list of two-octet SRTP protection
profile values as per [RFC5764] supported by the Media
Distributor.
6.3. UnsupportedVersion Message
The "UnsupportedVersion" message is defined as follows:
struct {
uint8 highest_version;
} UnsupportedVersion;
The elements of "UnsupportedVersion" include:
* "highest_version": indicates the highest version of the protocol
supported by the Key Distributor.
6.4. MediaKeys Message
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:
* "association_id": A value that identifies a distinct DTLS
association between an endpoint and the Key Distributor.
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* "protection_profiles": The value of the two-octet SRTP protection
profile value as per [RFC5764] used for this DTLS association.
* "mki": Master key identifier [RFC3711]. A zero-length field
indicates that no MKI value is present.
* "client_write_SRTP_master_key": The value of the SRTP master key
used by the client (endpoint).
* "server_write_SRTP_master_key": The value of the SRTP master key
used by the server (Media Distributor).
* "client_write_SRTP_master_salt": The value of the SRTP master salt
used by the client (endpoint).
* "server_write_SRTP_master_salt": The value of the SRTP master salt
used by the server (Media Distributor).
6.5. TunneledDtls Message
The "TunneledDtls" message is defined as:
struct {
uuid association_id;
opaque dtls_message<1..2^16-1>;
} TunneledDtls;
The fields are described as follows:
* "association_id": A value that identifies a distinct DTLS
association between an endpoint and the Key Distributor.
* "dtls_message": the content of the DTLS message received by the
endpoint or to be sent to the endpoint. This includes one or more
complete DTLS records.
6.6. EndpointDisconnect Message
The "EndpointDisconnect" message is defined as:
struct {
uuid association_id;
} EndpointDisconnect;
The fields are described as follows:
* "association_id": An value that identifies a distinct DTLS
association between an endpoint and the Key Distributor.
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7. Example Binary Encoding
The "TunnelMessage" is encoded in binary following the procedures
specified in [RFC8446]. This section provides an example of what the
bits on the wire would look like for the "SupportedProfiles" message
that advertises support for both
"DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM" and
"DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM" [RFC8723].
TunnelMessage:
message_type: 0x01
length: 0x0007
SupportedProfiles:
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:
0x0100070000040009000A
8. IANA Considerations
This document establishes a new registry to contain message type
values used in the DTLS Tunnel protocol. These message 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:
+=========+====================================+
| MsgType | Description |
+=========+====================================+
| 0x01 | Supported SRTP Protection Profiles |
+---------+------------------------------------+
| 0x02 | Unsupported Version |
+---------+------------------------------------+
| 0x03 | Media Keys |
+---------+------------------------------------+
| 0x04 | Tunneled DTLS |
+---------+------------------------------------+
| 0x05 | Endpoint Disconnect |
+---------+------------------------------------+
Table 1: Message Type Values for the DTLS
Tunnel Protocol
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The value 0x00 is reserved and all values in the range 0x06 to 0xFF
are available for allocation. The procedures for updating this table
are those defined as "IETF Review" in section 4.8 of [RFC8126].
The name for this registry is "Datagram Transport Layer Security
(DTLS) Tunnel Protocol Message Types for Privacy Enhanced
Conferencing".
9. Security Considerations
Since the procedures in this document relies on TLS [RFC8446] for
transport security, the security considerations for TLS should be
reviewed when implementing the protocol defined in this document.
While the tunneling protocol defined in this document does not use
DTLS-SRTP [RFC5764] directly, it does convey and negotiate some of
the same information (e.g., protection profile data). As such, a
review of the security considerations found in that document may be
useful.
This document describes a means of securely exchanging keying
material and cryptographic transforms for both E2E and HBH encryption
and authentication of media between an endpoint and a Key Distributor
via a Media Distributor. Additionally, the procedures result in
delivering HBH information to the intermediary Media Distributor.
The Key Distributor and endpoint are the only two entities with
access to both the E2E and HBH keys, while the Media Distributor has
access to only HBH information. Section 8.2 of [RFC8871] enumerates
various attacks against which one must guard when implementing a
Media Distributor and are important to note.
A requirement in this document is that a TLS connection between the
Media Distributor and the Key Distributor be mutually authenticated.
The reason for this requirement is to ensure that only an authorized
Media Distributor receives the HBH keying material. If an
unauthorized Media Distributor gains access to the HBH keying
material, it can easily cause service degradation or denial by
transmitting HBH-valid packets that ultimately fail E2E
authentication or replay protection checks (see Section 3.3.2 of
[RFC3711]). Even if service does not appear degraded in any way,
transmitting and processing bogus packets are a waste of both
computational and network resources.
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The procedures defined in this document assume that the Media
Distributor will properly convey DTLS messages between the endpoint
and Key Distributor. Should it fail in that responsibility by
forwarding DTLS messages from endpoint A advertised as being from
endpoint B, this will result in a failure at the DTLS layer those
DTLS sessions. This could be an additional attack vector that Key
Distributor implementations should consider.
While E2E keying material passes through the Media Distributor via
the protocol defined in this document, the Media Distributor has no
means of gaining access to that information and therefore cannot
affect the E2E media processing function in the endpoint except to
present it with invalid or replayed data. That said, any entity
along the path that interferes with the DTLS exchange between the
endpoint and the Key Distributor, including a malicious Media
Distributor that is not properly authorized, could prevent an
endpoint from properly communicating with the Key Distributor and,
therefore, prevent successful conference participation.
It is worth noting that a compromised Media Distributor can convey
information to an adversary such as participant IP addresses,
negotiates protection profiles, or other metadata. While [RFC8871]
explains that a malicious or compromised Media Distributor can
disrupt communications, an additional attack vector introduced by
this protocol is the potential disruption of DTLS negotiation or
premature removal of a participant from a conference by sending an
"EndpointDisconnect" disconnect message to the Key Distributor.
The Key Distributor should be aware of the possibility that a
malicious Media Distributor might transmit an "EndpointDisconnect"
message to the Key Distributor when the endpoint is, in fact, still
connected.
While the Security Considerations section of [RFC8871] describes
various attacks one needs to consider with respect to the Key
Distributor and denial-of-service, use of this protocol introduces
another possible attack vector. Consider the case where a malicious
endpoint sends unsolicited DTLS-SRTP messages to a Media Distributor.
The Media Distributor will normally forward those messages to the Key
Distributor and, if found invalid, such messages only serve to
consume resources on both the Media Distributor and Key Distributor.
10. Acknowledgments
The author would like to thank David Benham and Cullen Jennings for
reviewing this document and providing constructive comments.
11. Normative References
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[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021,
<https://tools.ietf.org/html/draft-ietf-tls-dtls13-43>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[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,
<https://www.rfc-editor.org/info/rfc5764>.
[RFC8122] Lennox, J. and C. Holmberg, "Connection-Oriented Media
Transport over the Transport Layer Security (TLS) Protocol
in the Session Description Protocol (SDP)", RFC 8122,
DOI 10.17487/RFC8122, March 2017,
<https://www.rfc-editor.org/info/rfc8122>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8723] Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
"Double Encryption Procedures for the Secure Real-Time
Transport Protocol (SRTP)", RFC 8723,
DOI 10.17487/RFC8723, April 2020,
<https://www.rfc-editor.org/info/rfc8723>.
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[RFC8842] Holmberg, C. and R. Shpount, "Session Description Protocol
(SDP) Offer/Answer Considerations for Datagram Transport
Layer Security (DTLS) and Transport Layer Security (TLS)",
RFC 8842, DOI 10.17487/RFC8842, January 2021,
<https://www.rfc-editor.org/info/rfc8842>.
[RFC8844] Thomson, M. and E. Rescorla, "Unknown Key-Share Attacks on
Uses of TLS with the Session Description Protocol (SDP)",
RFC 8844, DOI 10.17487/RFC8844, January 2021,
<https://www.rfc-editor.org/info/rfc8844>.
[RFC8871] Jones, P., Benham, D., and C. Groves, "A Solution
Framework for Private Media in Privacy-Enhanced RTP
Conferencing (PERC)", RFC 8871, DOI 10.17487/RFC8871,
January 2021, <https://www.rfc-editor.org/info/rfc8871>.
12. Informative References
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
[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, <https://www.rfc-editor.org/info/rfc3550>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.
Authors' Addresses
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, North Carolina 27709
United States of America
Phone: +1 919 476 2048
Email: paulej@packetizer.com
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Paul M. Ellenbogen
Princeton University
Phone: +1 206 851 2069
Email: pe5@cs.princeton.edu
Nils H. Ohlmeier
8x8, Inc.
Phone: +1 408 659 6457
Email: nils@ohlmeier.org
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