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Internet Engineering Task Force (IETF) P. Jones
Request for Comments: 9185 Cisco Systems
Category: Informational P. Ellenbogen
ISSN: 2070-1721 Princeton University
N. Ohlmeier
8x8, Inc.
April 2022
DTLS Tunnel between a Media Distributor and Key Distributor to
Facilitate Key Exchange
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 document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9185.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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 and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions Used in This Document
3. Tunneling Concept
4. Example Message Flows
5. Tunneling Procedures
5.1. Endpoint Procedures
5.2. Tunnel Establishment Procedures
5.3. Media Distributor Tunneling Procedures
5.4. Key Distributor Tunneling Procedures
5.5. Versioning Considerations
6. Tunneling Protocol
6.1. TunnelMessage Structure
6.2. SupportedProfiles Message
6.3. UnsupportedVersion Message
6.4. MediaKeys Message
6.5. TunneledDtls Message
6.6. EndpointDisconnect Message
7. Example Binary Encoding
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgements
Authors' Addresses
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) packets
[RFC3550], while the Media Distributor has access only to the HBH
keying material for encryption and authentication.
This specification defines a tunneling protocol that enables the
Media Distributor to tunnel DTLS messages [RFC9147] between an
endpoint and a Key Distributor, thus allowing an endpoint to use DTLS
for the Secure Real-time Transport Protocol (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 [RFC8446] 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.
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, might reside
in one of the endpoints participating in the conference, or might
reside at some other location 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 because 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 the 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.
The Key Distributor will provide the SRTP keying material [RFC3711]
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 subsections 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 Session
Description Protocol (SDP) attribute [RFC8866] 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 an 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.
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 message (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 a version 4 Universally Unique Identifier (UUID),
as described in 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 TunneledDtls 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 RTP Control Protocol (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 Distributor 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,
are 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 message do not match, the DTLS association MUST be
rejected.
The process through which the tls-id value 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 the 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.
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 message
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), Master Key Identifier (MKI) value [RFC3711] (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 explained in
Section 5.2 of [RFC8723], 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 the 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 with which conference a given DTLS association
is associated.
The Key Distributor MUST select a cipher that is supported by itself,
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
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 an
UnsupportedVersion message. The Media Distributor can determine from
the first four octets received what the version number is and that
the message is an UnsupportedVersion message. 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.
6.1. TunnelMessage Structure
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:
uint8 SRTPProtectionProfile[2]; /* from RFC 5764 */
struct {
uint8 version;
SRTPProtectionProfile protection_profiles<2..2^16-1>;
} SupportedProfiles;
The elements of SupportedProfiles include:
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:
struct {
uint8 highest_version;
} UnsupportedVersion;
UnsupportedVersion contains this single element:
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.
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, including one or more
complete DTLS records.
6.6. EndpointDisconnect Message
The EndpointDisconnect message is defined as:
struct {
uuid association_id;
} EndpointDisconnect;
The field is described as follows:
association_id: a value that identifies a distinct DTLS association
between an endpoint and the Key Distributor.
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 byte order,
would be this stream of octets:
0x0100070000040009000A
8. IANA Considerations
This document establishes the "Datagram Transport Layer Security
(DTLS) Tunnel Protocol Message Types for Privacy Enhanced
Conferencing" 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
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].
9. Security Considerations
Since the procedures in this document rely 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; these scenarios 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.
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 of 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,
negotiated 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 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. References
10.1. Normative References
[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>.
[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>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
10.2. 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>.
Acknowledgements
The authors would like to thank David Benham and Cullen Jennings for
reviewing this document and providing constructive comments.
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
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