Network Working Group C. Jennings
Internet-Draft P. Jones
Intended status: Standards Track Cisco Systems
Expires: December 31, 2017 A. Roach
Mozilla
June 29, 2017
SRTP Double Encryption Procedures
draft-ietf-perc-double-05
Abstract
In some conferencing scenarios, it is desirable for an intermediary
to be able to manipulate some RTP parameters, while still providing
strong end-to-end security guarantees. This document defines SRTP
procedures that use two separate but related cryptographic operations
to provide hop-by-hop and end-to-end security guarantees. Both the
end-to-end and hop-by-hop cryptographic algorithms can utilize an
authenticated encryption with associated data scheme or take
advantage of future SRTP transforms with different properties.
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 December 31, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cryptographic Context . . . . . . . . . . . . . . . . . . . . 3
4. Original Header Block . . . . . . . . . . . . . . . . . . . . 4
5. RTP Operations . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Encrypting a Packet . . . . . . . . . . . . . . . . . . . 6
5.2. Relaying a Packet . . . . . . . . . . . . . . . . . . . . 6
5.3. Decrypting a Packet . . . . . . . . . . . . . . . . . . . 8
6. RTCP Operations . . . . . . . . . . . . . . . . . . . . . . . 9
7. Use with Other RTP Mechanisms . . . . . . . . . . . . . . . . 9
7.1. RTX . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. DTMF . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.3. FEC . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Recommended Inner and Outer Cryptographic Algorithms . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10.1. RTP Header Extension . . . . . . . . . . . . . . . . . . 11
10.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Cloud conferencing systems that are based on switched conferencing
have a central Media Distributor device that receives media from
endpoints and distributes it to other endpoints, but does not need to
interpret or change the media content. For these systems, it is
desirable to have one cryptographic key from the sending endpoint to
the receiving endpoint that can encrypt and authenticate the media
end-to-end while still allowing certain RTP header information to be
changed by the Media Distributor. At the same time, a separate
cryptographic key provides integrity and optional confidentiality for
the media flowing between the Media Distributor and the endpoints.
See the framework document that describes this concept in more detail
in more detail in [I-D.ietf-perc-private-media-framework].
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This specification defines an SRTP transform that uses the AES-GCM
algorithm [RFC7714] to provide encryption and integrity for an RTP
packet for the end-to-end cryptographic key as well as a hop-by-hop
cryptographic encryption and integrity between the endpoint and the
Media Distributor. The Media Distributor decrypts and checks
integrity of the hop-by-hop security. The Media Distributor MAY
change some of the RTP header information that would impact the end-
to-end integrity. The original value of any RTP header field that is
changed is included in a new RTP header extension called the Original
Header Block. The new RTP packet is encrypted with the hop-by-hop
cryptographic algorithm before it is sent. The receiving endpoint
decrypts and checks integrity using the hop-by-hop cryptographic
algorithm and then replaces any parameters the Media Distributor
changed using the information in the Original Header Block before
decrypting and checking the end-to-end integrity.
One can think of the double as a normal SRTP transform for encrypting
the RTP in a way where things that only know half of the key, can
decrypt and modify part of the RTP packet but not other parts of if
including the media payload.
2. Terminology
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].
Terms used throughout this document include:
o Media Distributor: media distribution device that routes media
from one endpoint to other endpoints
o end-to-end: meaning the link from one endpoint through one or more
Media Distributors to the endpoint at the other end.
o hop-by-hop: meaning the link from the endpoint to or from the
Media Distributor.
o OHB: Original Header Block is an RTP header extension that
contains the original values from the RTP header that might have
been changed by a Media Distributor.
3. Cryptographic Context
This specification uses a cryptographic context with two parts: an
inner (end-to-end) part that is used by endpoints that originate and
consume media to ensure the integrity of media end-to-end, and an
outer (hop-by-hop) part that is used between endpoints and Media
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Distributors to ensure the integrity of media over a single hop and
to enable a Media Distributor to modify certain RTP header fields.
RTCP is also handled using the hop-by-hop cryptographic part. The
RECOMMENDED cipher for the hop-by-hop and end-to-end algorithm is
AES-GCM. Other combinations of SRTP ciphers that support the
procedures in this document can be added to the IANA registry.
The keys and salt for these algorithms are generated with the
following steps:
o Generate key and salt values of the length required for the
combined inner (end-to-end) and outer (hop-by-hop) algorithms.
o Assign the key and salt values generated for the inner (end-to-
end) algorithm to the first half of the key and salt for the
double algorithm.
o Assign the key and salt values for the outer (hop-by-hop)
algorithm to the second half of the key and salt for the double
algorithm. The first half of the key is revered to as the inner
key while the second out half is referred to as the outer key.
When a key is used by a cryptographic algorithm, the salt used is
the part of the salt generated with that key.
Obviously, if the Media Distributor is to be able to modify header
fields but not decrypt the payload, then it must have cryptographic
key for the outer algorithm, but not the inner (end-to-end)
algorithm. This document does not define how the Media Distributor
should be provisioned with this information. One possible way to
provide keying material for the outer (hop-by-hop) algorithm is to
use [I-D.ietf-perc-dtls-tunnel].
4. Original Header Block
Any SRTP packet processed following these procedures MAY contain an
Original Header Block (OHB) RTP header extension.
The OHB contains the original values of any modified header fields
and MUST be placed after any already-existing RTP header extensions.
Placement of the OHB after any original header extensions is
important so that the receiving endpoint can properly authenticate
the original packet and any originally included RTP header
extensions. The receiving endpoint will authenticate the original
packet by restoring the modified RTP header field values and header
extensions. It does this by copying the original values from the OHB
and then removing the OHB extension and any other RTP header
extensions that appear after the OHB extension.
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The Media Distributor is only permitted to modify the extension (X)
bit, payload type (PT) field, and the RTP sequence number field.
The OHB extension is either one octet in length, two octets in
length, or three octets in length. The length of the OHB indicates
what data is contained in the extension.
If the OHB is one octet in length, it contains the original PT field
value. In this case, the OHB has this form:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+---------------+
| ID | len=0 |R| PT |
+-+-+-+-+-+-+-+-+---------------+
Note that "R" indicates a reserved bit that MUST be set to zero when
sending a packet and ignored upon receipt. ID is the RTP Header
Extension identifier negotiated in the SDP.
If the OHB is two octets in length, it contains the original RTP
packet sequence number. In this case, the OHB has this form:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-------------------------------+
| ID | len=1 | Sequence Number |
+-+-+-+-+-+-+-+-+-------------------------------+
If the OHB is three octets in length, it contains the original PT
field value and RTP packet sequence number. In this case, the OHB
has this form:
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 6 4 5 6 7 8 9 1
+-+-+-+-+-+-+-+-+---------------+-------------------------------+
| ID | len=2 |R| PT | Sequence Number |
+-+-+-+-+-+-+-+-+---------------+-------------------------------+
If a Media Distributor modifies an original RTP header value, the
Media Distributor MUST include the OHB extension to reflect the
changed value, setting the X bit in the RTP header to 1 if no header
extensions were originally present. If another Media Distributor
along the media path makes additional changes to the RTP header and
any original value is already present in the OHB, the Media
Distributor must extend the OHB by adding the changed value to the
OHB. To properly preserve original RTP header values, a Media
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Distributor MUST NOT change a value already present in the OHB
extension.
5. RTP Operations
5.1. Encrypting a Packet
To encrypt a packet, the endpoint encrypts the packet using the inner
(end-to-end) cryptographic key and then encrypts using the outer
(hop-by-hop) cryptographic key. The processes is as follows:
o Form an RTP packet. If there are any header extensions, they MUST
use [RFC5285].
o If the endpoint wishes to insert header extensions that can be
modified by an Media Distributor, it MUST insert an OHB header
extension at the end of any header extensions protected end-to-end
(if any), then add any Media Distributor-modifiable header
extensions. In other cases, the endpoint SHOULD still insert an
OHB header extension. The OHB MUST replicate the information
found in the RTP header following the application of the inner
cryptographic algorithm. If not already set, the endpoint MUST
set the X bit in the RTP header to 1 when introducing the OHB
extension.
o Apply the inner cryptographic algorithm to the RTP packet. If
encrypting RTP header extensions end-to-end, then [RFC6904] MUST
be used when encrypting the RTP packet using the inner
cryptographic key.
o Apply the outer cryptographic algorithm to the RTP packet. If
encrypting RTP header extensions hop-by-hop, then [RFC6904] MUST
be used when encrypting the RTP packet using the outer
cryptographic key.
When using EKT [I-D.ietf-perc-srtp-ekt-diet], the EKT Field comes
after the SRTP packet exactly like using EKT with any other SRTP
transform.
5.2. Relaying a Packet
The Media Distributor does has the part of the key for the outer
(hop-by-hop) but does not have the part of the key for the (end-to-
end) cryptographic algorithm. The cryptographic algorithm and key
used to decrypt a packet and any encrypted RTP header extensions
would be the same as those used in the endpoint's outer algorithm and
key.
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In order to modify a packet, the Media Distributor decrypts the
packet, modifies the packet, updates the OHB with any modifications
not already present in the OHB, and re-encrypts the packet using the
cryptographic using the outer (hop-by-hop) key.
o Apply the outer (bop-by-hop) cryptographic algorithm to decrypt
the packet. If decrypting RTP header extensions hop-by-hop, then
[RFC6904] MUST be used.
o Change any parts of the RTP packet that the relay wishes to change
and are allowed to be changed.
o If a changed RTP header field is not already in the OHB, add it
with its original value to the OHB. A Media Distributor can add
information to the OHB, but MUST NOT change existing information
in the OHB.
o If the Media Distributor resets a parameter to its original value,
it MAY drop it from the OHB as long as there are no other header
extensions following the OHB. Note that this might result in a
decrease in the size of the OHB. It is also possible for the
Media Distributor to remove the OHB entirely if all parameters in
the RTP header are reset to their original values and no other
header extensions follow the OHB. If the OHB is removed and no
other extension is present, the X bit in the RTP header MUST be
set to 0.
o The Media Distributor MUST NOT delete any header extensions before
the OHB, but MAY add, delete, or modify any that follow the OHB.
* If the Media Distributor adds any header extensions, it must
append them and it must maintain the order of the original
header extensions in the [RFC5285] block.
* If the Media Distributor appends header extensions, then it
MUST add the OHB header extension (if not present), even if the
OHB merely replicates the original header field values, and
append the new extensions following the OHB. The OHB serves as
a demarcation point between original RTP header extensions
introduced by the endpoint and those introduced by a Media
Distributor.
o The Media Distributor MAY modify any header extension appearing
after the OHB, but MUST NOT modify header extensions that are
present before the OHB.
o Apply the outer (hop-by-hop) cryptographic algorithm to the
packet. If the RTP Sequence Number has been modified, SRTP
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processing happens as defined in SRTP and will end up using the
new Sequence Number. If encrypting RTP header extensions hop-by-
hop, then [RFC6904] MUST be used.
5.3. Decrypting a Packet
To decrypt a packet, the endpoint first decrypts and verifies using
the outer (hop-by-hop) cryptographic key, then uses the OHB to
reconstruct the original packet, which it decrypts and verifies with
the inner (end-to-end) cryptographic key.
o Apply the outer cryptographic algorithm to the packet. If the
integrity check does not pass, discard the packet. The result of
this is referred to as the outer SRTP packet. If decrypting RTP
header extensions hop-by-hop, then [RFC6904] MUST be used when
decrypting the RTP packet using the outer cryptographic key.
o Form a new synthetic SRTP packet with:
* Header = Received header, with header fields replaced with
values from OHB (if present).
* Insert all header extensions up to the OHB extension, but
exclude the OHB and any header extensions that follow the OHB.
If there are no extensions remaining, then the X bit MUST bet
set to 0. If there are extensions remaining, then the
remaining extensions MUST be padded to the first 32-bit
boundary and the overall length of the header extensions
adjusted accordingly.
* Payload is the encrypted payload from the outer SRTP packet.
o Apply the inner cryptographic algorithm to this synthetic SRTP
packet. Note if the RTP Sequence Number was changed by the Media
Distributor, the synthetic packet has the original Sequence
Number. If the integrity check does not pass, discard the packet.
If decrypting RTP header extensions end-to-end, then [RFC6904]
MUST be used when decrypting the RTP packet using the inner
cryptographic key.
Once the packet has been successfully decrypted, the application
needs to be careful about which information it uses to get the
correct behaviour. The application MUST use only the information
found in the synthetic SRTP packet and MUST NOT use the other data
that was in the outer SRTP packet with the following exceptions:
o The PT from the outer SRTP packet is used for normal matching to
SDP and codec selection.
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o The sequence number from the outer SRTP packet is used for normal
RTP ordering.
The PT and sequence number from the inner SRTP packet can be used for
collection of various statistics.
If any of the following RTP headers extensions are found in the outer
SRTP packet, they MAY be used:
o Mixer-to-client audio level indicators (See [RFC6465])
6. RTCP Operations
Unlike RTP, which is encrypted both hop-by-hop and end-to-end using
two separate cryptographic key, RTCP is encrypted using only the
outer (hop-by-hop) cryptographic key. The procedures for RTCP
encryption are specified in [RFC3711] and this document introduces no
additional steps.
7. Use with Other RTP Mechanisms
There are some RTP related extensions that need special consideration
to be used by a relay when using the double transform due to the end-
to-end protection of the RTP.
7.1. RTX
RTX [RFC4588] is not useable by the relay for hop-by-hop repair.
Some modification or extension would be need to be made to RTX before
it could be used in this way. The problem in using RTX is that the
relay would need to be able to read the first two byes of the payload
of the retransmissions packet which contain the original sequence
number. However, this data is end-to-end encrypted so the relay can
not read it.
7.2. DTMF
When DTMF is sent with [RFC4733], it is end-to-end encrypted and the
relay can not read it so it can not be used to controll the relay.
Other out of band methods to controll the relay can be used instead.
7.3. FEC
The algorithms recommended in [I-D.ietf-rtcweb-fec] for audio work
with no additional considerations.
The algorithm recommend in [I-D.ietf-rtcweb-fec] for video is Flex
FEC [I-D.ietf-payload-flexible-fec-scheme].
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Open Issue: The WG is currently considering how to handle Flex FEC.
The main issue of concern is that the FEC Header, which is needed for
repair, is part of the RTP payload. Flex FEC and be done before or
after the SRTP process with the order controlled by signalling.
[I-D.ietf-rtcweb-fec] recommends not using additional FEC only m-line
in SDP for the repair packets.
8. Recommended Inner and Outer Cryptographic Algorithms
This specification recommends and defines AES-GCM as both the inner
and outer cryptographic algorithms, identified as
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM. These algorithm provide
for authenticated encryption and will consume additional processing
time double-encrypting for hop-by-hop and end-to-end. However, the
approach is secure and simple, and is thus viewed as an acceptable
trade-off in processing efficiency.
Note that names for the cryptographic transforms are of the form
DOUBLE_(inner algorithm)_(outer algorithm).
While this document only defines a profile based on AES-GCM, it is
possible for future documents to define further profiles with
different inner and outer crypto in this same framework. For
example, if a new SRTP transform was defined that encrypts some or
all of the RTP header, it would be reasonable for systems to have the
option of using that for the outer algorithm. Similarly, if a new
transform was defined that provided only integrity, that would also
be reasonable to use for the hop-by-hop as the payload data is
already encrypted by the end-to-end.
The AES-GCM cryptographic algorithm introduces an additional 16
octets to the length of the packet. When using AES-GCM for both the
inner and outer cryptographic algorithms, the total additional length
is 32 octets. If no other header extensions are present in the
packet and the OHB is introduced, that will consume an additional 8
octets. If other extensions are already present, the OHB will
consume up to 4 additional octets.
9. Security Considerations
To summarize what is encrypted and authenticated, we will refer to
all the RTP fields and headers created by the sender and before the
pay load as the initial envelope and the RTP payload information with
the media as the payload. Any additional headers added by the Media
Distributor are referred to as the extra envelope. The sender uses
the end-to-end key to encrypts the payload and authenticate the
payload + initial envelope which using an AEAD cipher results in a
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slight longer new payload. Then the sender uses the hop-by-hop key
to encrypt the new payload and authenticate the initial envelope and
new payload.
The Media Distributor has the hop-by-hop key so it can check the
authentication of the received packet across the initial envelope and
payload data but it can't decrypt the payload as it does not have the
end-to-end key. It can add extra envelope information. It then
authenticates the initial plus extra envelope information plus
payload with a hop-by-hop key. This hop-by-hop for the outgoing
packet is typically different than the hop-by-hop key for the
incoming packet.
The receiver can check the authentication of the initial and extra
envelope information. This, along with the OHB, is used to construct
a synthetic packet that is should be identical to one the sender
created and the receiver can check that it is identical and then
decrypt the original payload.
The end result is that if the authentications succeed, the receiver
knows exactly what the original sender sent, as well as exactly which
modifications were made by the Media Distributor.
It is obviously critical that the intermediary has only the outer
(hop-by-hop) algorithm key and not the half of the key for the the
inner (end-to-end) algorithm. We rely on an external key management
protocol to assure this property.
Modifications by the intermediary result in the recipient getting two
values for changed parameters (original and modified). The recipient
will have to choose which to use; there is risk in using either that
depends on the session setup.
The security properties for both the inner (end-to-end) and outer
(hop-by-hop) key holders are the same as the security properties of
classic SRTP.
10. IANA Considerations
10.1. RTP Header Extension
This document defines a new extension URI in the RTP Compact Header
Extensions part of the Real-Time Transport Protocol (RTP) Parameters
registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:ohb
Description: Original Header Block
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Contact: Cullen Jennings <mailto:fluffy@iii.ca>
Reference: RFCXXXX
Note to RFC Editor: Replace RFCXXXX with the RFC number of this
specification.
10.2. DTLS-SRTP
We request IANA to add the following values to defines a DTLS-SRTP
"SRTP Protection Profile" defined in [RFC5764].
+------------+------------------------------------------+-----------+
| Value | Profile | Reference |
+------------+------------------------------------------+-----------+
| {0x00, | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX |
| 0x09} | | |
| {0x00, | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFCXXXX |
| 0x0A} | | |
+------------+------------------------------------------+-----------+
Note to IANA: Please assign value RFCXXXX and update table to point
at this RFC for these values.
The SRTP transform parameters for each of these protection are:
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM
cipher: AES_128_GCM then AES_128_GCM
cipher_key_length: 256 bits
cipher_salt_length: 192 bits
aead_auth_tag_length: 32 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM
cipher: AES_256_GCM then AES_256_GCM
cipher_key_length: 512 bits
cipher_salt_length: 192 bits
aead_auth_tag_length: 32 octets
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
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The first half of the key and salt is used for the inner (end-to-end)
algorithm and the second half is used for the outer (hop-by-hop)
algorithm.
11. Acknowledgments
Many thanks to Richard Barnes for sending significant text for this
specification. Thank you for reviews and improvements from David
Benham, Paul Jones, Suhas Nandakumar, Nils Ohlmeier, and Magnus
Westerlund.
12. References
12.1. 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,
<http://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,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
[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>.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<http://www.rfc-editor.org/info/rfc6904>.
[RFC7714] McGrew, D. and K. Igoe, "AES-GCM Authenticated Encryption
in the Secure Real-time Transport Protocol (SRTP)",
RFC 7714, DOI 10.17487/RFC7714, December 2015,
<http://www.rfc-editor.org/info/rfc7714>.
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12.2. Informative References
[I-D.ietf-payload-flexible-fec-scheme]
Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP
Payload Format for Flexible Forward Error Correction
(FEC)", draft-ietf-payload-flexible-fec-scheme-04 (work in
progress), March 2017.
[I-D.ietf-perc-dtls-tunnel]
Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
between a Media Distributor and Key Distributor to
Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-01
(work in progress), April 2017.
[I-D.ietf-perc-private-media-framework]
Jones, P., Benham, D., and C. Groves, "A Solution
Framework for Private Media in Privacy Enhanced RTP
Conferencing", draft-ietf-perc-private-media-framework-03
(work in progress), March 2017.
[I-D.ietf-perc-srtp-ekt-diet]
Jennings, C., Mattsson, J., McGrew, D., and D. Wing,
"Encrypted Key Transport for DTLS and Secure RTP", draft-
ietf-perc-srtp-ekt-diet-04 (work in progress), April 2017.
[I-D.ietf-rtcweb-fec]
Uberti, J., "WebRTC Forward Error Correction
Requirements", draft-ietf-rtcweb-fec-05 (work in
progress), May 2017.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
DOI 10.17487/RFC4733, December 2006,
<http://www.rfc-editor.org/info/rfc4733>.
[RFC6465] Ivov, E., Ed., Marocco, E., Ed., and J. Lennox, "A Real-
time Transport Protocol (RTP) Header Extension for Mixer-
to-Client Audio Level Indication", RFC 6465,
DOI 10.17487/RFC6465, December 2011,
<http://www.rfc-editor.org/info/rfc6465>.
Jennings, et al. Expires December 31, 2017 [Page 14]
Internet-Draft Double SRTP June 2017
Authors' Addresses
Cullen Jennings
Cisco Systems
Email: fluffy@iii.ca
Paul E. Jones
Cisco Systems
Email: paulej@packetizer.com
Adam Roach
Mozilla
Email: adam@nostrum.com
Jennings, et al. Expires December 31, 2017 [Page 15]