Network Working Group P. Jones
Internet-Draft Cisco Systems
Intended status: Standards Track July 8, 2016
Expires: January 9, 2017
A DTLS Tunnel between Media Distributor and Key Distributor to
Facilitate Key Exchange
draft-jones-perc-dtls-tunnel-03
Abstract
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.
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
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This Internet-Draft will expire on January 9, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Tunneling Concept . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . 6
5.4. Key Distributor Tunneling Procedures . . . . . . . . . . 8
6. Tunneling Protocol . . . . . . . . . . . . . . . . . . . . . 8
6.1. Tunnel Message . . . . . . . . . . . . . . . . . . . . . 9
6.2. Tunnel Message + Profiles . . . . . . . . . . . . . . . . 9
6.3. Tunnel Message + Key Info . . . . . . . . . . . . . . . . 10
7. PMTU Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. To-Do List . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
12. Normative References . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
An objective of the work in the Privacy-Enhanced RTP Conferencing
(PERC) working group 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 DTLS association 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
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key distributor. Likewise, when the media distributor receives data
from the key distributor over the tunnel, it extracts the DTLS
message inside and forwards that 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 DTLS 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 DTLS association (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:
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+-------------+
| Key |
| Distributor |
+-------------+
# ^ ^ #
# | | # <-- DTLS Tunnel
# | | #
+----------+ +-------------+ +----------+
| | DTLS | | DTLS | |
| Endpoint |<------------| Media |------------>| Endpoint |
| | to Key | Distributor | to Key | |
| | Distributor | | Distributor | |
+----------+ +-------------+ +----------+
Figure 1: DTLS 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. This document
does not preclude any location, only requiring that the key
distributor not allow the media distributor to gain access to the E2E
keying material by following the procedures defined in this document.
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 DTLS association for the purpose of sending tunneled
messages, though the complete DTLS 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. The message used to tunnel the DTLS messages is
named "Tunnel" and can include Profiles or Key Info data.
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Endpoint media distributor key distributor
| | |
| |<========================|
| | DTLS Association Made |
| | |
|------------------------>|========================>|
| DTLS handshake message | Tunnel + Profiles |
| | |
|<------------------------|<========================|
| DTLS handshake message | Tunnel |
| | |
.... may be multiple handshake messages ...
|------------------------>|========================>|
| DTLS handshake message | Tunnel + Profiles |
| | |
|<------------------------|<========================|
| DTLS handshake message | Tunnel + Key Info |
| (including Finished) | |
| | |
Figure 2: Sample DTLS-SRTP Exchange via the Tunnel
Each of these tunneled messages on the right-hand side of Figure 2 is
a message of type "Tunnel" (see Section 6). Each message contains
the following information:
o Protocol version
o Association ID
o DTLS message being tunneled
All messages sent by the media distributor will contain SRTP
protection profiles supported by the media distributor at the end of
the Tunnel message. The key distributor will select a common profile
supported by both the endpoint and the media distributor to ensure
that hop-by-hop operations can be successfully performed.
Further, the key distributor will provide the SRTP [RFC3711] keying
material to the media distributor for HBH operations at the time it
sends a DTLS Finished message to the endpoint via the tunnel. The
media distributor would extract this Key Info when received and use
it for hop-by-hop encryption and authentication. The delivery of the
keying information along with the completion of the DTLS handshake
ensures the delivery of the keying information is fate shared with
completion of the DTLS handshake. This guarantees that the media
distributor will have the HBH keying information before it receives
any media that is encrypted or authenticated with that key.
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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 DTLS association
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.
5.2. Tunnel Establishment Procedures
Either the media distributor or key distributor initiates the
establishment of a DTLS tunnel. Which entity acts as the DTLS 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 DTLS association. It is
used to relay DTLS messages between any number of endpoints and the
key distributor.
The media distributor or key distributor MUST establish a tunnel in
advance of, or no later than the point, when an endpoint attempts to
establish a DTLS association with 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 media distributor MUST 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
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distributor. A media distributor is not precluded from establishing
more than one tunnel to a given key distributor.
Editor's Note: Do we want to have the above requirement or would
we prefer to allow the media distributor to send messages over
more than one tunnel to more than one key distributor? The latter
would provide for higher availability, but at the cost of key
distributor complexity.
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 in 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 SHOULD be randomly assigned and values not re-used for a
period of time sufficient to ensure no late-arriving messages might
be delivered to the wrong endpoint. It is RECOMMENDED that the
association identifier not be re-used for at least 24 hours.
Editor's Note: do we want to recommend a time and is 24 hours
sufficient?
The tunnel protocol enables the key distributor to separately provide
HBH keying material to the media distributor for each of the
individual endpoint DTLS associations, though the media distributor
cannot decrypt messages between the key distributor and endpoints.
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 Tunnel + Profiles message (see
Section 6). The Tunnel + Profiles message allows the media
distributor to signal which SRTP protection profiles it supports for
HBH operations.
The media distributor MUST support the same list of protection
profiles for the life of a given endpoint's DTLS association, which
is represented by the association identifier.
When a message from the key distributor includes "Key Info," 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.
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The media distributor MUST forward all messages received from either
the endpoint or the key distributor to ensure proper communication
between those two entities.
5.4. Key Distributor Tunneling Procedures
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.
The key distributor MUST encapsulate the DTLS message inside a Tunnel
message (see Section 6) when sending a message to an endpoint.
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 server and the endpoint is acting as the
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 DTLS Finished message to the endpoint
at the point the DTLS handshake completes using the Tunnel + Key Info
message. The Key Info 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 select a cipher that is supported by both
the endpoint and the media distributor to ensure proper HBH
operations.
6. Tunneling Protocol
The tunneling protocol is transmitted over the DTLS association
established between the media distributor and key distributor as
application data. The basic message is referred to as the Tunnel
message. The media distributor will append supported SRTP protection
profiles to all Tunnel messages it sends, forming the Tunnel +
Profiles message. The key distributor will append information
necessary for the media distributor to perform HBH encryption and
authentication as it transmits the DTLS Finished message to the
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endpoint, forming the Tunnel + Key Info message. The Tunnel, Tunnel
+ Profiles, and Tunnel + Key Info messages are detailed in the
following sub-sections.
6.1. Tunnel Message
Tunneled DTLS messages are transported via the "Tunnel" message as
application data between the media distributor and the key
distributor. The "Tunnel" Message has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| Association Identifier |
+-------------------------------+-------------------------------+
| DTLS Message Length | :
+-------------------------------+ :
: :
: Tunneled DTLS Message :
: :
+---------------------------------------------------------------+
Figure 3: The "Tunnel" Message
Association Identifier: This is the association identifier used to
uniquely identify each endpoint in a conference (32-bits).
DTLS Message Length: Length in octets of following Tunneled DTLS
Message (16-bits).
Tunneled DTLS Message: This is the DTLS message exchanged between the
endpoint and key distributor. The length varies based on the value
specified in the previous field.
6.2. Tunnel Message + Profiles
Each Tunnel message transmitted by the media distributor contains an
array of SRTP protection profiles at the end of the message. The
format of the message is shown below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| Association Identifier |
+-------------------------------+-------------------------------+
| DTLS Message Length | :
+-------------------------------+ :
: :
: Tunneled DTLS Message :
: :
+---------------+---------------+-------------------------------+
| Data Type | Length | :
+---------------+---------------+ :
: Protection Profiles :
+---------------------------------------------------------------+
Figure 4: The "Tunnel + Profiles" Message
Beyond the fields included in the Tunnel message, this message
introduces the following additional fields.
Data Type: Indicates the type of data that follows. The value is
0x01 for SRTP protection profiles supported by the media distributor.
Length: This is the length in octets of the protection profiles.
This length must be greater than or equal to 2.
Protection Profiles: This is an array of two-octet SRTP protection
profile values as per [RFC5764], with each value represented in
network byte order.
6.3. Tunnel Message + Key Info
When the key distributor has HBH cipher and key information to share
with the media distributor, the key distributor will send a Tunnel
message with the Key Info appended as shown below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| Association Identifier |
+-------------------------------+-------------------------------+
| DTLS Message Length | :
+-------------------------------+ :
: :
: Tunneled DTLS Message :
: :
+---------------+-------------------------------+---------------+
| Data Type | Protection Profile | MKI Length |
+---------------+-------------------------------+---------------+
~ Master Key Identifier (MKI) ~
+---------------+---------------+-------------------------------+
| CWSMK Length | :
+-------------------------------+ :
: Client Write SRTP Master Key :
+-------------------------------+-------------------------------+
| SWSMK Length | :
+-------------------------------+ :
: Server Write SRTP Master Key :
+-------------------------------+-------------------------------+
| CWSMS Length | :
+-------------------------------+ :
: Client Write SRTP Master Salt :
+-------------------------------+-------------------------------+
| SWSMS Length | :
+-------------------------------+ :
: Server Write SRTP Master Salt :
+---------------------------------------------------------------+
Figure 5: The "Tunnel + Key Info" Message
Beyond the fields included in the Tunnel message, this message
introduces the following additional fields.
Data Type: Indicates the type of data that follows. This value is
0x02 for key information.
Protection Profile: This is the SRTP protection profile (see
[RFC5764]) the media distributor MUST use to encrypt and decrypt
packets sent and received between itself and the endpoint.
MKI Length: This is the length in octets of the MKI field. A value
of zero indicates that the MKI field is absent.
CWSMK Length: The length of the "Client Write SRTP Master Key" field.
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Client Write SRTP Master Key: The value of the SRTP master key used
by the client (endpoint).
SWSMK Length: The length of the "Server Write SRTP Master Key" field.
Server Write SRTP Master Key: The value of the SRTP master key used
by the server (media distributor).
CWSMS Length: The length of the "Client Write SRTP Master Salt"
field.
Client Write SRTP Master Salt: The value of the SRTP master salt used
by the client (endpoint).
SWSMS Length: The length of the "Server Write SRTP Master Salt"
field.
Server Write SRTP Master Salt: The value of the SRTP master salt used
by the server (media distributor).
7. PMTU Considerations
Tunneling DTLS messages received by an endpoint inside the DTLS
tunnel between the media distributor and key distributor introduces
only a small risk of message fragmentation, particularly with the
initial handshake messages carrying client and server certificates.
The small risk of fragmentation is considered acceptable given that
DTLS specifies how to recover from loss of handshake messages.
The additional overhead required for the tunnel is calculated to be
approximately 50 octets for messages transmitted from the media
distributor to the key distributor. Messages from the key
distributor would generally have slightly less overhead since they do
not carry a list of protection profiles. The one exception is the
Tunnel + Key Info message, which is slightly larger as it contains
key and salt information for the media distributor. While the Tunnel
+ Key Info message is larger than Tunnel + Profiles, the DTLS
message(s) transmitted in that flight (ChangeCipherSpec and Finished)
are very small and so the overhead does not impose a risk of
introducing packet fragmentation.
8. To-Do List
Given what is presently defined in this draft, it is not possible for
the key distributor to determine which conference to which a given
DTLS-SRTP association belongs, making it impossible for the key
distributor to ensure it is providing the endpoint with the correct
conference key. Observing the client certificate might be
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insufficient if the same client is participating in more than one
conference in parallel. The media distributor and key distributor
may need to coordinate or exchange a "conference identifier" common
to the endpoints a media distributor is bridging together.
Alternatively, information the key distributor needs to know about
conference-to-endpoint correlations might be satisfied by getting
info directly from the endpoints, or some trusted entity on their
behalf, via some other means. Need to revisit this design choice in
the context of all the alternatives.
9. IANA Considerations
This document establishes a new registry to contain "data 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 | Description |
+-----------+------------------------------------+
| 0x01 | Supported SRTP Protection Profiles |
| 0x02 | Key Information |
+-----------+------------------------------------+
Table 1: Data Type Values for the DTLS Tunnel Protocol
The name for this registry is "Datagram Transport Layer Security
(DTLS) Tunnel Protocol Data Types for Privacy Enhanced Conferencing."
10. Security Considerations
TODO - Much more needed.
The encapsulated data is protected by the DTLS session from the
endpoint to key distributor and the media distributor is merely an on
path entity. This does not introduce any additional security
concerns beyond a normal DTLS-SRTP session.
The HBH keying material is protected by the mutual authenticated DTLS
session 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 information
to untrusted parties.
The supported profile information send from the media distributor to
the key distributor is not particularly sensitive as it only provides
the crypt algorithms supported by the media distributor but it is
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still protected by the DTLS session from the media distributor to key
distributor.
11. Acknowledgments
The author would like to thank David Benham and Cullen Jennings for
reviewing this document and providing constructive comments.
12. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[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, <http://www.rfc-editor.org/info/rfc3550>.
[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,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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,
<http://www.rfc-editor.org/info/rfc5764>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
Author's Address
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Paul Jones
Cisco Systems
7025 Kit Creek Rd.
Research Triangle Park, North Carolina 27709
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
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