Network Working Group C. Jennings Internet-Draft P. Jones Intended status: Standards Track Cisco Systems Expires:October 30,December 31, 2017 A. Roach MozillaApril 28,June 29, 2017 SRTP Double Encryption Proceduresdraft-ietf-perc-double-04draft-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 cryptographiccontextsoperations to provide"hop-by-hop"hop-by-hop and"end-to-end"end-to-end security guarantees. Both the end-to-end and hop-by-hop cryptographictransformsalgorithms 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire onOctober 30,December 31, 2017. Copyright Notice Copyright (c) 2017 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. CryptographicContextsContext . . . . . . . . . . . . . . . . . . . . 3 4. Original Header Block . . . . . . . . . . . . . . . . . . . . 4 5. RTP Operations . . . . . . . . . . . . . . . . . . . . . . .56 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 CryptographicTransformsAlgorithms . . . .9 8.10 9. Security Considerations . . . . . . . . . . . . . . . . . . . 109.10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .10 9.1.11 10.1. RTP Header Extension . . . . . . . . . . . . . . . . . . 119.2.10.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . .. 11 10.12 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .12 11.13 12. References . . . . . . . . . . . . . . . . . . . . . . . . .12 11.1.13 12.1. Normative References . . . . . . . . . . . . . . . . . .12 11.2.13 12.2. Informative References . . . . . . . . . . . . . . . . .1314 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .1415 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 cryptographiccontextkey 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 cryptographiccontextkey 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]. This specification defines an SRTP transform that uses the AES-GCMtransformalgorithm [RFC7714] toencryptprovide encryption and integrity for an RTP packet for the end-to-end cryptographiccontext. The output of this is treatedkey asan RTP packet and again encrypted with an SRTP transform used in the hop-by- hopwell as a hop-by-hop cryptographiccontextencryption 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 theend-to-endend- 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 cryptographictransformalgorithm before it is sent. The receiving endpoint decrypts and checks integrity using the hop-by-hop cryptographictransformalgorithm 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 transformasfor 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 oE2E: end-to-end,end-to-end: meaning the link from one endpoint through one or more Media Distributors to the endpoint at the other end. oHBH: hop-by-hop,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. CryptographicContextsContext This specification usestwoa cryptographiccontexts:context with two parts: an inner("end- to-end") context(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") context(hop-by-hop) part that is used between endpoints and Media 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 alsoencryptedhandled using the hop-by-hop cryptographiccontext.part. The RECOMMENDED cipher for the hop-by-hop and end-to-endcontextsalgorithm 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 thesecontextsalgorithms 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)transforms.algorithms. o Assign the key and salt values generated for the inner (end-to- end)transformalgorithm to the first half of the key and salt for the doubletransform.algorithm. o Assign the key and salt values for the outer (hop-by-hop)transformalgorithm to the second half of the key and salt for the doubletransform.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 cryptographiccontextkey for the outertransform,algorithm, but not the innertransform.(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") transform(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. 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 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) cryptographiccontextkey and then encrypts using the outer (hop-by-hop) cryptographiccontext.key. The processes is as follows: o Form an RTP packet. If there are any header extensions, they MUST use [RFC5285]. oApply the inner cryptographic transform 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 context. o IfIf 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 cryptographictransform.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 cryptographictransformalgorithm 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 cryptographiccontext.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 havea notionthe part ofouter or inner cryptographic contexts. Rather,theMedia Distributor has a singlekey for the (end-to- end) cryptographiccontext.algorithm. The cryptographictransformalgorithm and key used to decrypt a packet and any encrypted RTP header extensions would be the same as those used in the endpoint's outercryptographic context.algorithm and key. 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 cryptographiccontext used for next hop.using the outer (hop-by-hop) key. o Apply the outer (bop-by-hop) cryptographictransformalgorithm to decrypt the packet. If decrypting RTP header extensions hop-by-hop, then [RFC6904] MUST be used. o Change anyrequired parametersparts 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) cryptographictransformalgorithm to the packet. If the RTP Sequence Number has been modified, SRTP processing happens as defined in SRTP and will end up using the new Sequence Number. If encrypting RTP header extensionshop-by-hop,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) cryptographiccontext,key, then uses the OHB to reconstruct the original packet, which it decrypts and verifies with the inner (end-to-end) cryptographiccontext.key. o Apply the outer cryptographictransformalgorithm 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 cryptographiccontext.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 cryptographictransformalgorithm 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 cryptographiccontext.key. Once the packet has been successfully decrypted, the application needs to be careful about which information it uses to get the correctbehavior.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. 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 cryptographiccontexts,key, RTCP is encrypted using only the outer(HBH)(hop-by-hop) cryptographiccontext.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]. 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 CryptographicTransformsAlgorithms This specification recommends and defines AES-GCM as both the inner and outer cryptographictransforms,algorithms, identified as DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM. Thesetransformsalgorithm provide for authenticated encryption and will consume additional processing time double-encrypting forHBHhop-by-hop andE2E.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_(innertransform)_(outer transform).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 outertransformscrypto 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 outertransform.algorithm. Similarly, if a new transform was defined that provided only integrity, that would also be reasonable to use for theHBHhop-by-hop as the payload data is already encrypted by theE2E.end-to-end. The AES-GCM cryptographictransformalgorithm introduces an additional 16 octets to the length of the packet. When using AES-GCM for both the inner and outer cryptographictransforms,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.8.9. 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 theE2Eend-to-end key to encrypts the payload and authenticate the payload + initial envelope which using an AEAD cipher results in a slight longer new payload. Then the sender uses theHBHhop-by-hop key to encrypt the new payload and authenticate the initial envelope and new payload. The Media Distributor has theHBHhop-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 theE2Eend-to-end key. It can add extra envelope information. It then authenticates the initial plus extra envelope information plus payload with aHBHhop-by-hop key. ThisHBHhop-by-hop for the outgoing packet is typically different than theHBHhop-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 outertransform parameters(hop-by-hop) algorithm key and not the half of the key for the the innertransform parameters.(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.9.10. IANA Considerations9.1.10.1. 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 Contact: Cullen Jennings <mailto:fluffy@iii.ca> Reference: RFCXXXX Note to RFC Editor: Replace RFCXXXX with the RFC number of this specification.9.2.10.2. DTLS-SRTP We request IANA to add the following values to defines a DTLS-SRTP "SRTP Protection Profile" defined in [RFC5764].+-------+------------------------------------------+-----------++------------+------------------------------------------+-----------+ | Value | Profile | Reference |+-------+------------------------------------------+-----------++------------+------------------------------------------+-----------+ |{TBD}{0x00, | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX | |{TBD}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 The first half of the key and salt is used for the inner(E2E) transform(end-to-end) algorithm and the second half is used for the outer(HBH) transform. 10.(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.11.12. References11.1.12.1. 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>.11.2.12.2. 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-00draft-ietf-perc-dtls-tunnel-01 (work in progress),MarchApril 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-03draft- ietf-perc-srtp-ekt-diet-04 (work in progress),MarchApril 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>. 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