Internet Engineering Task Force (IETF) D. McGrew

Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 5764 Cisco Systems
Category: Standards Track E. Rescorla
ISSN: 2070-1721 RTFM, Inc.
May 2010

Datagram Transport Layer Security (DTLS) Extension to Establish Keys
for the Secure Real-time Transport Protocol (SRTP)

Abstract

This document describes a Datagram Transport Layer Security (DTLS)
extension to establish keys for Secure RTP (SRTP) and Secure RTP
Control Protocol (SRTCP) flows. DTLS keying happens on the media
path, independent of any out-of-band signalling channel present.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5764.

Copyright Notice

Copyright (c) 2010 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
(http://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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

McGrew & Rescorla Standards Track [Page 1]

RFC 5764 SRTP Extension for DTLS May 2010

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Overview of DTLS-SRTP Operation . . . . . . . . . . . . . . . 4
4. DTLS Extensions for SRTP Key Establishment . . . . . . . . . . 5
4.1. The use_srtp Extension . . . . . . . . . . . . . . . . . . 5
4.1.1. use_srtp Extension Definition . . . . . . . . . . . . 7
4.1.2. SRTP Protection Profiles . . . . . . . . . . . . . . . 8
4.1.3. srtp_mki value . . . . . . . . . . . . . . . . . . . . 9
4.2. Key Derivation . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Key Scope . . . . . . . . . . . . . . . . . . . . . . . . 12
4.4. Key Usage Limitations . . . . . . . . . . . . . . . . . . 12
5. Use of RTP and RTCP over a DTLS-SRTP Channel . . . . . . . . . 13
5.1. Data Protection . . . . . . . . . . . . . . . . . . . . . 13
5.1.1. Transmission . . . . . . . . . . . . . . . . . . . . . 13
5.1.2. Reception . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Rehandshake and Rekey . . . . . . . . . . . . . . . . . . 16
6. Multi-Party RTP Sessions . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Security of Negotiation . . . . . . . . . . . . . . . . . 17
7.2. Framing Confusion . . . . . . . . . . . . . . . . . . . . 17
7.3. Sequence Number Interactions . . . . . . . . . . . . . . . 18
7.3.1. Alerts . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.2. Renegotiation . . . . . . . . . . . . . . . . . . . . 18
7.4. Decryption Cost . . . . . . . . . . . . . . . . . . . . . 19
8. Session Description for RTP/SAVP over DTLS . . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Overview of DTLS . . . . . . . . . . . . . . . . . . 23
Appendix B. Performance of Multiple DTLS Handshakes . . . . . . . 24

McGrew & Rescorla Standards Track [Page 2]

RFC 5764 SRTP Extension for DTLS May 2010

1. Introduction

The Secure RTP (SRTP) profile [RFC3711] can provide confidentiality,
message authentication, and replay protection to RTP data and RTP
Control (RTCP) traffic. SRTP does not provide key management
functionality, but instead depends on external key management to
exchange secret master keys, and to negotiate the algorithms and
parameters for use with those keys.

Datagram Transport Layer Security (DTLS) [RFC4347] is a channel
security protocol that offers integrated key management, parameter
negotiation, and secure data transfer. Because DTLS data transfer
protocol is generic, it is less highly optimized for use with RTP
than is SRTP, which has been specifically tuned for that purpose.

This document describes DTLS-SRTP, a SRTP extension for DTLS that
combines the performance and encryption flexibility benefits of SRTP
with the flexibility and convenience of DTLS-integrated key and
association management. DTLS-SRTP can be viewed in two equivalent
ways: as a new key management method for SRTP, and a new RTP-specific
data format for DTLS.

The key points of DTLS-SRTP are that:

o application data is protected using SRTP,

o the DTLS handshake is used to establish keying material,
algorithms, and parameters for SRTP,

o a DTLS extension is used to negotiate SRTP algorithms, and

o other DTLS record-layer content types are protected using the
ordinary DTLS record format.

The remainder of this memo is structured as follows. Section 2
describes conventions used to indicate normative requirements.
Section 3 provides an overview of DTLS-SRTP operation. Section 4
specifies the DTLS extensions, while Section 5 discusses how RTP and
RTCP are transported over a DTLS-SRTP channel. Section 6 describes
use with multi-party sessions. Section 7 and Section 9 describe
Security and IANA considerations.

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].

McGrew & Rescorla Standards Track [Page 3]

RFC 5764 SRTP Extension for DTLS May 2010

3. Overview of DTLS-SRTP Operation

DTLS-SRTP is defined for point-to-point media sessions, in which
there are exactly two participants. Each DTLS-SRTP session contains
a single DTLS association (called a "connection" in TLS jargon), and
either two SRTP contexts (if media traffic is flowing in both
directions on the same host/port quartet) or one SRTP context (if
media traffic is only flowing in one direction). All SRTP traffic
flowing over that pair in a given direction uses a single SRTP
context. A single DTLS-SRTP session only protects data carried over
a single UDP source and destination port pair.

The general pattern of DTLS-SRTP is as follows. For each RTP or RTCP
flow the peers do a DTLS handshake on the same source and destination
port pair to establish a DTLS association. Which side is the DTLS
client and which side is the DTLS server must be established via some
out-of-band mechanism such as SDP. The keying material from that
handshake is fed into the SRTP stack. Once that association is
established, RTP packets are protected (becoming SRTP) using that
keying material.

RTP and RTCP traffic is usually sent on two separate UDP ports. When
symmetric RTP [RFC4961] is used, two bidirectional DTLS-SRTP sessions
are needed, one for the RTP port, one for the RTCP port. When RTP
flows are not symmetric, four unidirectional DTLS-SRTP sessions are
needed (for inbound and outbound RTP, and inbound and outbound RTCP).

Symmetric RTP [RFC4961] is the case in which there are two RTP
sessions that have their source and destination ports and addresses
reversed, in a manner similar to the way that a TCP connection uses
its ports. Each participant has an inbound RTP session and an
outbound RTP session. When symmetric RTP is used, a single DTLS-SRTP
session can protect both of the RTP sessions. It is RECOMMENDED that
symmetric RTP be used with DTLS-SRTP.

RTP and RTCP traffic MAY be multiplexed on a single UDP port
[RFC5761]. In this case, both RTP and RTCP packets may be sent over
the same DTLS-SRTP session, halving the number of DTLS-SRTP sessions
needed. This improves the cryptographic performance of DTLS, but may
cause problems when RTCP and RTP are subject to different network
treatment (e.g., for bandwidth reservation or scheduling reasons).

Between a single pair of participants, there may be multiple media
sessions. There MUST be a separate DTLS-SRTP session for each
distinct pair of source and destination ports used by a media session
(though the sessions can share a single DTLS session and hence
amortize the initial public key handshake!).

McGrew & Rescorla Standards Track [Page 4]

RFC 5764 SRTP Extension for DTLS May 2010

A DTLS-SRTP session may be indicated by an external signaling
protocol like SIP. When the signaling exchange is integrity-
protected (e.g., when SIP Identity protection via digital signatures
is used), DTLS-SRTP can leverage this integrity guarantee to provide
complete security of the media stream. A description of how to
indicate DTLS-SRTP sessions in SIP and SDP [RFC4566], and how to
authenticate the endpoints using fingerprints can be found in
[RFC5763].

In a naive implementation, when there are multiple media sessions,
there is a new DTLS session establishment (complete with public key
cryptography) for each media channel. For example, a videophone may
be sending both an audio stream and a video stream, each of which
would use a separate DTLS session establishment exchange, which would
proceed in parallel. As an optimization, the DTLS-SRTP
implementation SHOULD use the following strategy: a single DTLS
association is established, and all other DTLS associations wait
until that connection is established before proceeding with their
handshakes. This strategy allows the later sessions to use DTLS
session resumption, which allows the amortization of the expensive
public key cryptography operations over multiple DTLS handshakes.

The SRTP keys used to protect packets originated by the client are
distinct from the SRTP keys used to protect packets originated by the
server. All of the RTP sources originating on the client for the
same channel use the same SRTP keys, and similarly, all of the RTP
sources originating on the server for the same channel use the same
SRTP keys. The SRTP implementation MUST ensure that all of the
synchronization source (SSRC) values for all of the RTP sources
originating from the same device over the same channel are distinct,
in order to avoid the "two-time pad" problem (as described in Section
9.1 of RFC 3711). Note that this is not an issue for separate media
streams (on different host/port quartets) that use independent keying
material even if an SSRC collision occurs.

4. DTLS Extensions for SRTP Key Establishment

4.1. The use_srtp Extension

In order to negotiate the use of SRTP data protection, clients
include an extension of type "use_srtp" in the DTLS extended client
hello. This extension MUST only be used when the data being
transported is RTP or RTCP [RFC3550]. The "extension_data" field of
this extension contains the list of acceptable SRTP protection
profiles, as indicated below.

McGrew & Rescorla Standards Track [Page 5]

RFC 5764 SRTP Extension for DTLS May 2010

Servers that receive an extended hello containing a "use_srtp"
extension can agree to use SRTP by including an extension of type
"use_srtp", with the chosen protection profile in the extended server
hello. This process is shown below.

Client Server

ClientHello + use_srtp -------->
ServerHello + use_srtp
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
SRTP packets <-------> SRTP packets

Note that '*' indicates messages that are not always sent in DTLS.
The CertificateRequest, client and server Certificates, and
CertificateVerify will be sent in DTLS-SRTP.

Once the "use_srtp" extension is negotiated, the RTP or RTCP
application data is protected solely using SRTP. Application data is
never sent in DTLS record-layer "application_data" packets. Rather,
complete RTP or RTCP packets are passed to the DTLS stack, which
passes them to the SRTP stack, which protects them appropriately.
Note that if RTP/RTCP multiplexing [RFC5761] is in use, this means
that RTP and RTCP packets may both be passed to the DTLS stack.
Because the DTLS layer does not process the packets, it does not need
to distinguish them. The SRTP stack can use the procedures of
[RFC5761] to distinguish RTP from RTCP.

When the "use_srtp" extension is in effect, implementations must not
place more than one application data "record" per datagram. (This is
only meaningful from the perspective of DTLS because SRTP is
inherently oriented towards one payload per packet, but this is
stated purely for clarification.)

Data other than RTP/RTCP (i.e., TLS control messages) MUST use
ordinary DTLS framing and MUST be placed in separate datagrams from
SRTP data.

McGrew & Rescorla Standards Track [Page 6]

RFC 5764 SRTP Extension for DTLS May 2010

A DTLS-SRTP handshake establishes one or more SRTP crypto contexts;
however, they all have the same SRTP Protection Profile and Master
Key Identifier (MKI), if any. MKIs are used solely to distinguish
the keying material and protection profiles between distinct
handshakes, for instance, due to rekeying. When an MKI is
established in a DTLS-SRTP session, it MUST apply for all of the
SSRCs within that session -- though a single endpoint may negotiate
multiple DTLS-SRTP sessions due, for instance, to forking. (Note
that RFC 3711 allows packets within the same session but with
different SSRCs to use MKIs differently; in contrast, DTLS-SRTP
requires that MKIs and the keys that they are associated with have
the same meaning and are uniform across the entire SRTP session.)

4.1.1. use_srtp Extension Definition

The client MUST fill the extension_data field of the "use_srtp"
extension with an UseSRTPData value (see Section 9 for the
registration):

uint8 SRTPProtectionProfile[2];

struct {
SRTPProtectionProfiles SRTPProtectionProfiles;
opaque srtp_mki<0..255>;
} UseSRTPData;

SRTPProtectionProfile SRTPProtectionProfiles<2..2^16-1>;

The SRTPProtectionProfiles list indicates the SRTP protection
profiles that the client is willing to support, listed in descending
order of preference. The srtp_mki value contains the SRTP Master Key
Identifier (MKI) value (if any) that the client will use for his SRTP
packets. If this field is of zero length, then no MKI will be used.

Note: for those unfamiliar with TLS syntax, "srtp_mki<0..255>"
indicates a variable-length value with a length between 0 and 255
(inclusive). Thus, the MKI may be up to 255 bytes long.

If the server is willing to accept the use_srtp extension, it MUST
respond with its own "use_srtp" extension in the ExtendedServerHello.
The extension_data field MUST contain a UseSRTPData value with a
single SRTPProtectionProfile value that the server has chosen for use
with this connection. The server MUST NOT select a value that the
client has not offered. If there is no shared profile, the server
SHOULD NOT return the use_srtp extension at which point the
connection falls back to the negotiated DTLS cipher suite. If that
is not acceptable, the server SHOULD return an appropriate DTLS
alert.

McGrew & Rescorla Standards Track [Page 7]

RFC 5764 SRTP Extension for DTLS May 2010

4.1.2. SRTP Protection Profiles

A DTLS-SRTP SRTP Protection Profile defines the parameters and
options that are in effect for the SRTP processing. This document
defines the following SRTP protection profiles.

SRTPProtectionProfile SRTP_AES128_CM_HMAC_SHA1_80 = {0x00, 0x01};
SRTPProtectionProfile SRTP_AES128_CM_HMAC_SHA1_32 = {0x00, 0x02};
SRTPProtectionProfile SRTP_NULL_HMAC_SHA1_80 = {0x00, 0x05};
SRTPProtectionProfile SRTP_NULL_HMAC_SHA1_32 = {0x00, 0x06};

The following list indicates the SRTP transform parameters for each
protection profile. The parameters cipher_key_length,
cipher_salt_length, auth_key_length, and auth_tag_length express the
number of bits in the values to which they refer. The
maximum_lifetime parameter indicates the maximum number of packets
that can be protected with each single set of keys when the parameter
profile is in use. All of these parameters apply to both RTP and
RTCP, unless the RTCP parameters are separately specified.

All of the crypto algorithms in these profiles are from [RFC3711].

SRTP_AES128_CM_HMAC_SHA1_80
cipher: AES_128_CM
cipher_key_length: 128
cipher_salt_length: 112
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 80
SRTP_AES128_CM_HMAC_SHA1_32
cipher: AES_128_CM
cipher_key_length: 128
cipher_salt_length: 112
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 32
RTCP auth_tag_length: 80
SRTP_NULL_HMAC_SHA1_80
cipher: NULL
cipher_key_length: 0
cipher_salt_length: 0
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 80

McGrew & Rescorla Standards Track [Page 8]

RFC 5764 SRTP Extension for DTLS May 2010

SRTP_NULL_HMAC_SHA1_32
cipher: NULL
cipher_key_length: 0
cipher_salt_length: 0
maximum_lifetime: 2^31
auth_function: HMAC-SHA1
auth_key_length: 160
auth_tag_length: 32
RTCP auth_tag_length: 80

With all of these SRTP Parameter profiles, the following SRTP options
are in effect:

o The TLS PseudoRandom Function (PRF) is used to generate keys to
feed into the SRTP Key Derivation Function (KDF). When DTLS 1.2
[DTLS1.2] is in use, the PRF is the one associated with the cipher
suite. Note that this specification is compatible with DTLS 1.0
or DTLS 1.2

o The Key Derivation Rate (KDR) is equal to zero. Thus, keys are
not re-derived based on the SRTP sequence number.

o The key derivation procedures from Section 4.3 with the AES-CM PRF
from RFC 3711 are used.

o For all other parameters (in particular, SRTP replay window size
and FEC order), the default values are used.

If values other than the defaults for these parameters are required,
they can be enabled by writing a separate specification specifying
SDP syntax to signal them.

Applications using DTLS-SRTP SHOULD coordinate the SRTP Protection
Profiles between the DTLS-SRTP session that protects an RTP flow and
the DTLS-SRTP session that protects the associated RTCP flow (in
those cases in which the RTP and RTCP are not multiplexed over a
common port). In particular, identical ciphers SHOULD be used.

New SRTPProtectionProfile values must be defined according to the
"Specification Required" policy as defined by RFC 5226 [RFC5226].
See Section 9 for IANA Considerations.

4.1.3. srtp_mki value

The srtp_mki value MAY be used to indicate the capability and desire
to use the SRTP Master Key Identifier (MKI) field in the SRTP and
SRTCP packets. The MKI field indicates to an SRTP receiver which key
was used to protect the packet that contains that field. The

McGrew & Rescorla Standards Track [Page 9]

RFC 5764 SRTP Extension for DTLS May 2010

srtp_mki field contains the value of the SRTP MKI which is associated
with the SRTP master keys derived from this handshake. Each SRTP
session MUST have exactly one master key that is used to protect
packets at any given time. The client MUST choose the MKI value so
that it is distinct from the last MKI value that was used, and it
SHOULD make these values unique for the duration of the TLS session.

Upon receipt of a "use_srtp" extension containing a "srtp_mki" field,
the server MUST either (assuming it accepts the extension at all):

1. include a matching "srtp_mki" value in its "use_srtp" extension
to indicate that it will make use of the MKI, or
2. return an empty "srtp_mki" value to indicate that it cannot make
use of the MKI.

If the client detects a nonzero-length MKI in the server's response
that is different than the one the client offered, then the client
MUST abort the handshake and SHOULD send an invalid_parameter alert.
If the client and server agree on an MKI, all SRTP packets protected
under the new security parameters MUST contain that MKI.

Note that any given DTLS-SRTP session only has a single active MKI
(if any). Thus, at any given time, a set of endpoints will generally
only be using one MKI (the major exception is during rehandshakes).

4.2. Key Derivation

When SRTP mode is in effect, different keys are used for ordinary
DTLS record protection and SRTP packet protection. These keys are
generated using a TLS exporter [RFC5705] to generate

2 * (SRTPSecurityParams.master_key_len +
SRTPSecurityParams.master_salt_len) bytes of data

which are assigned as shown below. The per-association context value
is empty.

client_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
server_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
client_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
server_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];

The exporter label for this usage is "EXTRACTOR-dtls_srtp". (The
"EXTRACTOR" prefix is for historical compatibility.)

The four keying material values (the master key and master salt for
each direction) are provided as inputs to the SRTP key derivation
mechanism, as shown in Figure 1 and detailed below. By default, the

McGrew & Rescorla Standards Track [Page 10]

RFC 5764 SRTP Extension for DTLS May 2010

mechanism defined in Section 4.3 of [RFC3711] is used, unless another
key derivation mechanism is specified as part of an SRTP Protection
Profile.

The client_write_SRTP_master_key and client_write_SRTP_master_salt
are provided to one invocation of the SRTP key derivation function,
to generate the SRTP keys used to encrypt and authenticate packets
sent by the client. The server MUST only use these keys to decrypt
and to check the authenticity of inbound packets.

The server_write_SRTP_master_key and server_write_SRTP_master_salt
are provided to one invocation of the SRTP key derivation function,
to generate the SRTP keys used to encrypt and authenticate packets
sent by the server. The client MUST only use these keys to decrypt
and to check the authenticity of inbound packets.

TLS master
secret label
| |
v v
+---------------+
| TLS extractor |
+---------------+
| +------+ SRTP
+-> client_write_SRTP_master_key ----+--->| SRTP |-> client
| | +->| KDF | write
| | | +------+ keys
| | |
+-> server_write_SRTP_master_key -- | | +------+ SRTCP
| \ \--->|SRTCP |-> client
| \ +->| KDF | write
| | | +------+ keys
+-> client_write_SRTP_master_salt ---|-+
| |
| | +------+ SRTP
| +--->| SRTP |-> server
+-> server_write_SRTP_master_salt -+-|--->| KDF | write
| | +------+ keys
| |
| | +------+ SRTCP
| +--->|SRTCP |-> server
+----->| KDF | write
+------+ keys

Figure 1: The derivation of the SRTP keys.

McGrew & Rescorla Standards Track [Page 11]

RFC 5764 SRTP Extension for DTLS May 2010

When both RTCP and RTP use the same source and destination ports,
then both the SRTP and SRTCP keys are needed. Otherwise, there are
two DTLS-SRTP sessions, one of which protects the RTP packets and one
of which protects the RTCP packets; each DTLS-SRTP session protects
the part of an SRTP session that passes over a single source/
destination transport address pair, as shown in Figure 2, independent
of which SSRCs are used on that pair. When a DTLS-SRTP session is
protecting RTP, the SRTCP keys derived from the DTLS handshake are
not needed and are discarded. When a DTLS-SRTP session is protecting
RTCP, the SRTP keys derived from the DTLS handshake are not needed
and are discarded.

Client Server
(Sender) (Receiver)
(1) <----- DTLS ------> src/dst = a/b and b/a
------ SRTP ------> src/dst = a/b, uses client write keys

(2) <----- DTLS ------> src/dst = c/d and d/c
------ SRTCP -----> src/dst = c/d, uses client write keys
<----- SRTCP ------ src/dst = d/c, uses server write keys

Figure 2: A DTLS-SRTP session protecting RTP (1) and another one
protecting RTCP (2), showing the transport addresses and keys used.

4.3. Key Scope

Because of the possibility of packet reordering, DTLS-SRTP
implementations SHOULD store multiple SRTP keys sets during a rekey
in order to avoid the need for receivers to drop packets for which
they lack a key.

4.4. Key Usage Limitations

The maximum_lifetime parameter in the SRTP protection profile
indicates the maximum number of packets that can be protected with
each single encryption and authentication key. (Note that, since RTP
and RTCP are protected with independent keys, those protocols are
counted separately for the purposes of determining when a key has
reached the end of its lifetime.) Each profile defines its own
limit. When this limit is reached, a new DTLS session SHOULD be used
to establish replacement keys, and SRTP implementations MUST NOT use
the existing keys for the processing of either outbound or inbound
traffic.

McGrew & Rescorla Standards Track [Page 12]

RFC 5764 SRTP Extension for DTLS May 2010

5. Use of RTP and RTCP over a DTLS-SRTP Channel

5.1. Data Protection

Once the DTLS handshake has completed, the peers can send RTP or RTCP
over the newly created channel. We describe the transmission process
first followed by the reception process.

Within each RTP session, SRTP processing MUST NOT take place before
the DTLS handshake completes.

5.1.1. Transmission

DTLS and TLS define a number of record content types. In ordinary
TLS/DTLS, all data is protected using the same record encoding and
mechanisms. When the mechanism described in this document is in
effect, this is modified so that data written by upper-level protocol
clients of DTLS is assumed to be RTP/RTP and is encrypted using SRTP
rather than the standard TLS record encoding.

When a user of DTLS wishes to send an RTP packet in SRTP mode, it
delivers it to the DTLS implementation as an ordinary application
data write (e.g., SSL_write()). The DTLS implementation then invokes
the processing described in RFC 3711, Sections 3 and 4. The
resulting SRTP packet is then sent directly on the wire as a single
datagram with no DTLS framing. This provides an encapsulation of the
data that conforms to and interoperates with SRTP. Note that the RTP
sequence number rather than the DTLS sequence number is used for
these packets.

5.1.2. Reception

When DTLS-SRTP is used to protect an RTP session, the RTP receiver
needs to demultiplex packets that are arriving on the RTP port.
Arriving packets may be of types RTP, DTLS, or STUN [RFC5389]. If
these are the only types of packets present, the type of a packet can
be determined by looking at its first byte.

The process for demultiplexing a packet is as follows. The receiver
looks at the first byte of the packet. If the value of this byte is
0 or 1, then the packet is STUN. If the value is in between 128 and
191 (inclusive), then the packet is RTP (or RTCP, if both RTCP and
RTP are being multiplexed over the same destination port). If the
value is between 20 and 63 (inclusive), the packet is DTLS. This
process is summarized in Figure 3.

McGrew & Rescorla Standards Track [Page 13]

RFC 5764 SRTP Extension for DTLS May 2010

+----------------+
| 127 < B < 192 -+--> forward to RTP
| |
packet --> | 19 < B < 64 -+--> forward to DTLS
| |
| B < 2 -+--> forward to STUN
+----------------+

Figure 3: The DTLS-SRTP receiver's packet demultiplexing algorithm.
Here the field B denotes the leading byte of the packet.

If other packet types are to be multiplexed as well, implementors
and/or designers SHOULD ensure that they can be demultiplexed from
these three packet types.

In some cases, there will be multiple DTLS-SRTP associations for a
given SRTP endpoint. For instance, if Alice makes a call that is SIP
forked to both Bob and Charlie, she will use the same local host/port
pair for both of them, as shown in Figure 4, where XXX and YYY
represent different DTLS-SRTP associations. (The SSRCs shown are the
ones for data flowing to Alice.)

Bob (192.0.2.1:6666)
/
/
/ SSRC=1
/ DTLS-SRTP=XXX
/
v
Alice (192.0.2.0:5555)
^
\
\ SSRC=2
\ DTLS-SRTP=YYY
\
\
Charlie (192.0.2.2:6666)

Figure 4: RTP sessions with SIP forking.

Because DTLS operates on the host/port quartet, the DTLS association
will still complete correctly, with the foreign host/port pair being
used, to distinguish the associations. However, in RTP the source
host/port is not used and sessions are identified by the destination
host/port and the SSRC. Thus, some mechanism is needed to determine
which SSRCs correspond to which DTLS associations. The following
method SHOULD be used.

McGrew & Rescorla Standards Track [Page 14]

RFC 5764 SRTP Extension for DTLS May 2010

For each local host/port pair, the DTLS-SRTP implementation maintains
a table listing all the SSRCs it knows about and the DTLS-SRTP
associations they correspond to. Initially, this table is empty.
When an SRTP packet is received for a given RTP endpoint (destination
IP/port pair), the following procedure is used:

1. If the SSRC is already known for that endpoint, then the
corresponding DTLS-SRTP association and its keying material is
used to decrypt and verify the packet.
2. If the SSRC is not known, then the receiver tries to decrypt it
with the keying material corresponding to each DTLS-SRTP
association for that endpoint.
3. If the decryption and verification succeeds (the authentication
tag verifies), then an entry is placed in the table mapping the
SSRC to that association.
4. If the decryption and verification fails, then the packet is
silently discarded.
5. When a DTLS-SRTP association is closed (for instance, because the
fork is abandoned), its entries MUST be removed from the mapping
table.

The average cost of this algorithm for a single SSRC is the
decryption and verification time of a single packet times the number
of valid DTLS-SRTP associations corresponding to a single receiving
port on the host. In practice, this means the number of forks; so in
the case shown in Figure 4, that would be two. This cost is only
incurred once for any given SSRC, since afterwards that SSRC is
placed in the map table and looked up immediately. As with normal
RTP, this algorithm allows new SSRCs to be introduced by the source
at any time. They will automatically be mapped to the correct DTLS
association.

Note that this algorithm explicitly allows multiple SSRCs to be sent
from the same address/port pair. One way in which this can happen is
an RTP translator. This algorithm will automatically assign the
SSRCs to the correct associations. Note that because the SRTP
packets are cryptographically protected, such a translator must
either share keying material with one endpoint or refrain from
modifying the packets in a way which would cause the integrity check
to fail. This is a general property of SRTP and is not specific to
DTLS-SRTP.

There are two error cases that should be considered. First, if an
SSRC collision occurs, then only the packets from the first source
will be processed. When the packets from the second source arrive,
the DTLS association with the first source will be used for
decryption and verification, which will fail, and the packet will be
discarded. This is consistent with [RFC3550], which permits the

McGrew & Rescorla Standards Track [Page 15]

RFC 5764 SRTP Extension for DTLS May 2010

receiver to keep the packets from one source and discard those from
the other. Of course the RFC 3550 SSRC collision detection and
handling procedures MUST also be followed.

Second, there may be cases where a malfunctioning source is sending
corrupt packets that cannot be decrypted and verified. In this case,
the SSRC will never be entered into the mapping table because the
decryption and verification always fails. Receivers MAY keep records
of unmapped SSRCs that consistently fail decryption and verification
and abandon attempts to process them once they reach some limit.
That limit MUST be large enough to account for the effects of
transmission errors. Entries MUST be pruned from this table when the
relevant SRTP endpoint is deleted (e.g., the call ends) and SHOULD
time out faster than that (we do not offer a hard recommendation but
10 to 30 seconds seems appropriate) in order to allow for the
possibility that the peer implementation has been corrected.

5.2. Rehandshake and Rekey

Rekeying in DTLS is accomplished by performing a new handshake over
the existing DTLS channel. That is, the handshake messages are
protected by the existing DTLS cipher suite. This handshake can be
performed in parallel with data transport, so no interruption of the
data flow is required. Once the handshake is finished, the newly
derived set of keys is used to protect all outbound packets, both
DTLS and SRTP.

Because of packet reordering, packets protected by the previous set
of keys can appear on the wire after the handshake has completed. To
compensate for this fact, receivers SHOULD maintain both sets of keys
for some time in order to be able to decrypt and verify older
packets. The keys should be maintained for the duration of the
maximum segment lifetime (MSL).

If an MKI is used, then the receiver should use the corresponding set
of keys to process an incoming packet. If no matching MKI is
present, the packet MUST be rejected. Otherwise, when a packet
arrives after the handshake completed, a receiver SHOULD use the
newly derived set of keys to process that packet unless there is an
MKI. (If the packet was protected with the older set of keys, this
fact will become apparent to the receiver as an authentication
failure will occur.) If the authentication check on the packet fails
and no MKI is being used, then the receiver MAY process the packet
with the older set of keys. If that authentication check indicates
that the packet is valid, the packet should be accepted; otherwise,
the packet MUST be discarded and rejected.

McGrew & Rescorla Standards Track [Page 16]

RFC 5764 SRTP Extension for DTLS May 2010

Receivers MAY use the SRTP packet sequence number to aid in the
selection of keys. After a packet has been received and
authenticated with the new key set, any packets with sequence numbers
that are greater will also have been protected with the new key set.

6. Multi-Party RTP Sessions

Since DTLS is a point-to-point protocol, DTLS-SRTP is intended only
to protect unicast RTP sessions. This does not preclude its use with
RTP mixers. For example, a conference bridge may use DTLS-SRTP to
secure the communication to and from each of the participants in a
conference. However, because each flow between an endpoint and a
mixer has its own key, the mixer has to decrypt and then reencrypt
the traffic for each recipient.

A future specification may describe methods for sharing a single key
between multiple DTLS-SRTP associations thus allowing conferencing
systems to avoid the decrypt/reencrypt stage. However, any system in
which the media is modified (e.g., for level balancing or
transcoding) will generally need to be performed on the plaintext and
will certainly break the authentication tag, and therefore will
require a decrypt/reencrypt stage.

7. Security Considerations

The use of multiple data protection framings negotiated in the same
handshake creates some complexities, which are discussed here.

7.1. Security of Negotiation

One concern here is that attackers might be able to implement a bid-
down attack forcing the peers to use ordinary DTLS rather than SRTP.
However, because the negotiation of this extension is performed in
the DTLS handshake, it is protected by the Finished messages.
Therefore, any bid-down attack is automatically detected, which
reduces this to a denial-of-service attack -- which can be mounted by
any attacker who can control the channel.

7.2. Framing Confusion

Because two different framing formats are used, there is concern that
an attacker could convince the receiver to treat an SRTP-framed RTP
packet as a DTLS record (e.g., a handshake message) or vice versa.
This attack is prevented by using different keys for Message
Authentication Code (MAC) verification for each type of data.
Therefore, this type of attack reduces to being able to forge a
packet with a valid MAC, which violates a basic security invariant of
both DTLS and SRTP.

McGrew & Rescorla Standards Track [Page 17]

RFC 5764 SRTP Extension for DTLS May 2010

As an additional defense against injection into the DTLS handshake
channel, the DTLS record type is included in the MAC. Therefore, an
SRTP record would be treated as an unknown type and ignored. (See
Section 6 of [RFC5246].)

7.3. Sequence Number Interactions

As described in Section 5.1.1, the SRTP and DTLS sequence number
spaces are distinct. This means that it is not possible to
unambiguously order a given DTLS control record with respect to an
SRTP packet. In general, this is relevant in two situations: alerts
and rehandshake.

7.3.1. Alerts

Because DTLS handshake and change_cipher_spec messages share the same
sequence number space as alerts, they can be ordered correctly.
Because DTLS alerts are inherently unreliable and SHOULD NOT be
generated as a response to data packets, reliable sequencing between
SRTP packets and DTLS alerts is not an important feature. However,
implementations that wish to use DTLS alerts to signal problems with
the SRTP encoding SHOULD simply act on alerts as soon as they are
received and assume that they refer to the temporally contiguous
stream. Such implementations MUST check for alert retransmission and
discard retransmitted alerts to avoid overreacting to replay attacks.

7.3.2. Renegotiation

Because the rehandshake transition algorithm specified in Section 5.2
requires trying multiple sets of keys if no MKI is used, it slightly
weakens the authentication. For instance, if an n-bit MAC is used
and k different sets of keys are present, then the MAC is weakened by
log_2(k) bits to n - log_2(k). In practice, since the number of keys
used will be very small and the MACs in use are typically strong (the
default for SRTP is 80 bits), the decrease in security involved here
is minimal.

Another concern here is that this algorithm slightly increases the
work factor on the receiver because it needs to attempt multiple
validations. However, again, the number of potential keys will be
very small (and the attacker cannot force it to be larger) and this
technique is already used for rollover counter management, so the
authors do not consider this to be a serious flaw.

McGrew & Rescorla Standards Track [Page 18]

RFC 5764 SRTP Extension for DTLS May 2010

7.4. Decryption Cost

An attacker can impose computational costs on the receiver by sending
superficially valid SRTP packets that do not decrypt correctly. In
general, encryption algorithms are so fast that this cost is
extremely small compared to the bandwidth consumed. The SSRC-DTLS
mapping algorithm described in Section 5.1.2 gives the attacker a
slight advantage here because he can force the receiver to do more
then one decryption per packet. However, this advantage is modest
because the number of decryptions that the receiver does is limited
by the number of associations he has corresponding to a given
destination host/port, which is typically quite small. For
comparison, a single 1024-bit RSA private key operation (the typical
minimum cost to establish a DTLS-SRTP association) is hundreds of
times as expensive as decrypting an SRTP packet.

Implementations can detect this form of attack by keeping track of
the number of SRTP packets that are observed with unknown SSRCs and
that fail the authentication tag check. If under such attack,
implementations SHOULD prioritize decryption and verification of
packets that either have known SSRCs or come from source addresses
that match those of peers with which it has DTLS-SRTP associations.

8. Session Description for RTP/SAVP over DTLS

This specification defines new tokens to describe the protocol used
in SDP media descriptions ("m=" lines and their associated
parameters). The new values defined for the proto field are:

o When a RTP/SAVP or RTP/SAVPF [RFC5124] stream is transported over
DTLS with the Datagram Congestion Control Protocol (DCCP), then
the token SHALL be DCCP/TLS/RTP/SAVP or DCCP/TLS/RTP/SAVPF
respectively.

o When a RTP/SAVP or RTP/SAVPF stream is transported over DTLS with
UDP, the token SHALL be UDP/TLS/RTP/SAVP or UDP/TLS/RTP/SAVPF
respectively.

The "fmt" parameter SHALL be as defined for RTP/SAVP.

See [RFC5763] for how to use offer/answer with DTLS-SRTP.

This document does not specify how to protect RTP data transported
over TCP. Potential approaches include carrying the RTP over TLS
over TCP (see [SRTP-NOT-MAND]) or using a mechanism similar to that
in this document over TCP, either via TLS or DTLS, with DTLS being
used for consistency between reliable and unreliable transports. In

McGrew & Rescorla Standards Track [Page 19]

RFC 5764 SRTP Extension for DTLS May 2010

the latter case, it would be necessary to profile DTLS so that
fragmentation and retransmissions no longer occurred. In either
case, a new document would be required.

9. IANA Considerations

This document adds a new extension for DTLS, in accordance with
[RFC5246]:
enum { use_srtp (14) } ExtensionType;

This extension MUST only be used with DTLS, and not with TLS
[RFC4572], which specifies that TLS can be used over TCP but does not
address TCP for RTP/SAVP.

Section 4.1.2 requires that all SRTPProtectionProfile values be
defined by RFC 5226 "Specification Required". IANA has created a
DTLS SRTPProtectionProfile registry initially populated with values
from Section 4.1.2 of this document. Future values MUST be allocated
via the "Specification Required" profile of [RFC5226].

This specification updates the "Session Description Protocol (SDP)
Parameters" registry as defined in Section 8.2.2 of [RFC4566].
Specifically, it adds the following values to the table for the
"proto" field.

Type SDP Name Reference
---- ------------------ ---------
proto UDP/TLS/RTP/SAVP [RFC5764]
proto DCCP/TLS/RTP/SAVP [RFC5764]

proto UDP/TLS/RTP/SAVPF [RFC5764]
proto DCCP/TLS/RTP/SAVPF [RFC5764]

IANA has registered the "EXTRACTOR-dtls_srtp" value in the TLS
Extractor Label Registry to correspond to this specification.

10. Acknowledgments

Special thanks to Flemming Andreasen, Francois Audet, Pasi Eronen,
Roni Even, Jason Fischl, Cullen Jennings, Colin Perkins, Dan Wing,
and Ben Campbell for input, discussions, and guidance. Pasi Eronen
provided Figure 1.

McGrew & Rescorla Standards Track [Page 20]

RFC 5764 SRTP Extension for DTLS May 2010

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E.,
and K. Norrman, "The Secure Real-time Transport
Protocol (SRTP)", RFC 3711, March 2004.

[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport
Layer Security", RFC 4347, April 2006.

[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol
(RTCP)", BCP 131, RFC 4961, July 2007.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.

[RFC5705] Rescorla, E., "Keying Material Exporters for
Transport Layer Security (TLS)", RFC 5705,
March 2010.

[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP
Data and Control Packets on a Single Port",
RFC 5761, April 2010.

11.2. Informative References

[DTLS1.2] Rescorla, E. and N. Modadugu, "Datagram Transport
Layer Security version 1.2", Work in Progress,
October 2009.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.

[RFC4572] Lennox, J., "Connection-Oriented Media Transport
over the Transport Layer Security (TLS) Protocol in
the Session Description Protocol (SDP)", RFC 4572,
July 2006.

McGrew & Rescorla Standards Track [Page 21]

RFC 5764 SRTP Extension for DTLS May 2010

[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile
for Real-time Transport Control Protocol (RTCP)-
Based Feedback (RTP/SAVPF)", RFC 5124,
February 2008.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)",
RFC 5389, October 2008.

[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla,
"Framework for Establishing a Secure Real-time
Transport Protocol (SRTP) Security Context Using
Datagram Transport Layer Security (DTLS)", RFC 5763,
May 2010.

[SRTP-NOT-MAND] Perkins, C. and M. Westerlund, "Why RTP Does Not
Mandate a Single Security Mechanism", Work in
Progress, January 2010.

McGrew & Rescorla Standards Track [Page 22]

RFC 5764 SRTP Extension for DTLS May 2010

Appendix A. Overview of DTLS

This section provides a brief overview of Datagram TLS (DTLS) for
those who are not familiar with it. DTLS is a channel security
protocol based on the well-known Transport Layer Security (TLS)
[RFC5246] protocol. Where TLS depends on a reliable transport
channel (typically TCP), DTLS has been adapted to support unreliable
transports such as UDP. Otherwise, DTLS is nearly identical to TLS
and generally supports the same cryptographic mechanisms.

Each DTLS association begins with a handshake exchange (shown below)
during which the peers authenticate each other and negotiate
algorithms, modes, and other parameters and establish shared keying
material, as shown below. In order to support unreliable transport,
each side maintains retransmission timers to provide reliable
delivery of these messages. Once the handshake is completed,
encrypted data may be sent.

Client Server

ClientHello -------->
ServerHello
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data

'*' indicates messages that are not always sent.

Figure 5: Basic DTLS Handshake Exchange (after [RFC4347]).

Application data is protected by being sent as a series of DTLS
"records". These records are independent and can be processed
correctly even in the face of loss or reordering. In DTLS-SRTP, this
record protocol is replaced with SRTP [RFC3711]

McGrew & Rescorla Standards Track [Page 23]

RFC 5764 SRTP Extension for DTLS May 2010

Appendix B. Performance of Multiple DTLS Handshakes

Standard practice for security protocols such as TLS, DTLS, and SSH,
which do inline key management, is to create a separate security
association for each underlying network channel (TCP connection, UDP
host/port quartet, etc.). This has dual advantages of simplicity and
independence of the security contexts for each channel.

Three concerns have been raised about the overhead of this strategy
in the context of RTP security. The first concern is the additional
performance overhead of doing a separate public key operation for
each channel. The conventional procedure here (used in TLS and DTLS)
is to establish a master context that can then be used to derive
fresh traffic keys for new associations. In TLS/DTLS, this is called
"session resumption" and can be transparently negotiated between the
peers.

The second concern is network bandwidth overhead for the
establishment of subsequent connections and for rehandshake (for
rekeying) for existing connections. In particular, there is a
concern that the channels will have very narrow capacity requirements
allocated entirely to media that will be overflowed by the
rehandshake. Measurements of the size of the rehandshake (with
resumption) in TLS indicate that it is about 300-400 bytes if a full
selection of cipher suites is offered. (The size of a full handshake
is approximately 1-2 kilobytes larger because of the certificate and
keying material exchange.)

The third concern is the additional round-trips associated with
establishing the second, third, ... channels. In TLS/DTLS, these can
all be done in parallel, but in order to take advantage of session
resumption they should be done after the first channel is
established. For two channels, this provides a ladder diagram
something like this (parenthetical numbers are media channel numbers)

McGrew & Rescorla Standards Track [Page 24]

RFC 5764 SRTP Extension for DTLS May 2010

Alice Bob
-------------------------------------------
<- ClientHello (1)
ServerHello (1) ->
Certificate (1)
ServerHelloDone (1)
<- ClientKeyExchange (1)
ChangeCipherSpec (1)
Finished (1)
ChangeCipherSpec (1)->
Finished (1)->
<--- Channel 1 ready

<- ClientHello (2)
ServerHello (2) ->
ChangeCipherSpec(2)->
Finished(2) ->
<- ChangeCipherSpec (2)
Finished (2)
<--- Channel 2 ready

Figure 6: Parallel DTLS-SRTP negotiations.

So, there is an additional 1 RTT (round-trip time) after Channel 1 is
ready before Channel 2 is ready. If the peers are potentially
willing to forego resumption, they can interlace the handshakes, like
so:

McGrew & Rescorla Standards Track [Page 25]

RFC 5764 SRTP Extension for DTLS May 2010

Alice Bob
-------------------------------------------
<- ClientHello (1)
ServerHello (1) ->
Certificate (1)
ServerHelloDone (1)
<- ClientKeyExchange (1)
ChangeCipherSpec (1)
Finished (1)
<- ClientHello (2)
ChangeCipherSpec (1)->
Finished (1)->
<--- Channel 1 ready
ServerHello (2) ->
ChangeCipherSpec(2)->
Finished(2) ->
<- ChangeCipherSpec (2)
Finished (2)
<--- Channel 2 ready

Figure 7: Interlaced DTLS-SRTP negotiations.

In this case, the channels are ready contemporaneously, but if a
message in handshake (1) is lost, then handshake (2) requires either
a full rehandshake or that Alice be clever and queue the resumption
attempt until the first handshake completes. Note that just dropping
the packet works as well, since Bob will retransmit.

Authors' Addresses

David McGrew
Cisco Systems
510 McCarthy Blvd.
Milpitas, CA 95305
USA

EMail: [email protected]

Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA

EMail: [email protected]

McGrew & Rescorla Standards Track [Page 26]