SCIP RTP payload: RTP Payload Format for the Secure Communication Interoperability Protocol (SCIP) Codec

Internet Engineering Task Force (IETF)                         D. Hanson
Request for Comments: 9607                                     M. Faller
Category: Standards Track                                       K. Maver
ISSN: 2070-1721                   General Dynamics Mission Systems, Inc.
                                                               July 2024


    RTP Payload Format for the Secure Communication Interoperability
                         Protocol (SCIP) Codec

Abstract

   This document describes the RTP payload format of the Secure
   Communication Interoperability Protocol (SCIP).  SCIP is an
   application-layer protocol that provides end-to-end session
   establishment, payload encryption, packetization and de-packetization
   of media, and reliable transport.  This document provides a globally
   available reference that can be used for the development of network
   equipment and procurement of services that support SCIP traffic.  The
   intended audience is network security policymakers; network
   administrators, architects, and original equipment manufacturers
   (OEMs); procurement personnel; and government agency and commercial
   industry representatives.

IESG Note

   This IETF specification depends upon a second technical specification
   that is not available publicly, namely [SCIP210].  The IETF was
   therefore unable to conduct a security review of that specification,
   independently or when carried inside Audio/Video Transport (AVT).
   Implementers need to be aware that the IETF hence cannot verify any
   of the security claims contained in this document.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9607.

Copyright Notice

   Copyright (c) 2024 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
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   in the Revised BSD License.

Table of Contents

   1.  Key Points
   2.  Introduction
     2.1.  Conventions
     2.2.  Abbreviations
   3.  Background
   4.  Payload Format
     4.1.  RTP Header Fields
     4.2.  Congestion Control Considerations
     4.3.  Use of Augmented RTPs with SCIP
   5.  Payload Format Parameters
     5.1.  Media Subtype "audio/scip"
     5.2.  Media Subtype "video/scip"
     5.3.  Mapping to SDP
     5.4.  SDP Offer/Answer Considerations
   6.  Security Considerations
   7.  IANA Considerations
   8.  SCIP Contact Information
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

1.  Key Points

   *  SCIP is an application-layer protocol that uses RTP as a
      transport.  This document defines the SCIP media subtypes to be
      listed in the Session Description Protocol (SDP) and only requires
      a basic RTP transport channel for SCIP payloads.  This basic
      transport channel is comparable to Clearmode as defined by
      [RFC4040].

   *  SCIP transmits encrypted traffic and does not require the use of
      Secure RTP (SRTP) for payload protection.  SCIP also provides for
      reliable transport at the application layer, so it is not
      necessary to negotiate RTCP retransmission capabilities.

   *  SCIP includes built-in mechanisms that negotiate protocol message
      versions and capabilities.  To avoid SCIP protocol ossification
      (as described in [RFC9170]), it is important for middleboxes to
      not attempt parsing of the SCIP payload.  As described in this
      document, such parsing serves no useful purpose.

   *  SCIP is designed to be network agnostic.  It can operate over any
      digital link, including non-IP modem-based PSTN and ISDN.  The
      SCIP media subtypes listed in this document were developed for
      SCIP to operate over RTP.

   *  SCIP handles packetization and de-packetization of payloads by
      producing encrypted media packets that are not greater than the
      MTU size.  The SCIP payload is opaque to the network, therefore,
      SCIP functions as a tunneling protocol for the encrypted media,
      without the need for middleboxes to parse SCIP payloads.  Since
      SCIP payloads are integrity protected, modification of the SCIP
      payload is detected as an integrity violation by SCIP endpoints,
      leading to communication failure.

2.  Introduction

   This document details usage of the "audio/scip" and "video/scip"
   pseudo-codecs [MediaTypes] as a secure session establishment protocol
   and media transport protocol over RTP.

   It discusses how:

   1.  encrypted audio and video codec payloads are transported over
       RTP;

   2.  the IP network layer does not implement SCIP as a protocol since
       SCIP operates at the application layer in endpoints;

   3.  the IP network layer enables SCIP traffic to transparently pass
       through the network;

   4.  some network devices do not recognize SCIP and may remove the
       SCIP codecs from the SDP media payload declaration before
       forwarding to the next network node; and finally,

   5.  SCIP endpoint devices do not operate on networks if the SCIP
       media subtype is removed from the SDP media payload declaration.

   The United States, along with its NATO Partners, have implemented
   SCIP in secure voice, video, and data products operating on
   commercial, private, and tactical IP networks worldwide using the
   scip media subtype.  The SCIP data traversing the network is
   encrypted, and network equipment in-line with the session cannot
   interpret the traffic stream in any way.  SCIP-based RTP traffic is
   opaque and can vary significantly in structure and frequency, making
   traffic profiling not possible.  Also, as the SCIP protocol continues
   to evolve independently of this document, any network device that
   attempts to filter traffic (e.g., deep packet inspection) may cause
   unintended consequences in the future when changes to the SCIP
   traffic may not be recognized by the network device.

   The SCIP protocol defined in SCIP-210 [SCIP210] includes built-in
   support for packetization and de-packetization, retransmission,
   capability exchange, version negotiation, and payload encryption.
   Since the traffic is encrypted, neither the RTP transport nor
   middleboxes can usefully parse or modify SCIP payloads; modifications
   are detected as integrity violations resulting in retransmission, and
   eventually, communication failure.

   Because knowledge of the SCIP payload format is not needed to
   transport SCIP signaling or media through middleboxes, SCIP-210
   represents an informative reference.  While older versions of the
   SCIP-210 specification are publicly available, the authors strongly
   encourage network implementers to treat SCIP payloads as opaque
   octets.  When handled correctly, such treatment does not require
   referring to SCIP-210, and any assumptions about the format of SCIP
   messages defined in SCIP-210 are likely to lead to protocol
   ossification and communication failures as the protocol evolves.

      |  Note: The IETF has not conducted a security review of SCIP and
      |  therefore has not verified the claims contained in this
      |  document.

2.1.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The best current practices for writing an RTP payload format
   specification, as per [RFC2736] and [RFC8088], were followed.

   When referring to the Secure Communication Interoperability Protocol,
   the uppercase acronym "SCIP" is used.  When referring to the media
   subtype scip, lowercase "scip" is used.

2.2.  Abbreviations

   The following abbreviations are used in this document.

   AVP:      Audio-Visual Profile

   AVPF:     Audio-Visual Profile with Feedback

   FNBDT:    Future Narrowband Digital Terminal

   ICWG:     Interoperability Control Working Group

   IICWG:    International Interoperability Control Working Group

   MELPe:    Mixed Excitation Linear Prediction Enhanced

   MTU:      Maximum Transmission Unit

   NATO:     North Atlantic Treaty Organization

   OEM:      Original Equipment Manufacturer

   SAVP:     Secure Audio-Visual Profile

   SAVPF:    Secure Audio-Visual Profile with Feedback

   SCIP:     Secure Communication Interoperability Protocol

   SDP:      Session Description Protocol

   SRTP:     Secure Real-time Transport Protocol

   STANAG:   Standardization Agreement

3.  Background

   The Secure Communication Interoperability Protocol (SCIP) allows the
   negotiation of several voice, data, and video applications using
   various cryptographic suites.  SCIP also provides several important
   characteristics that have led to its broad acceptance as a secure
   communications protocol.

   SCIP began in the United States as the Future Narrowband Digital
   Terminal (FNBDT) Protocol in the late 1990s.  A combined U.S.
   Department of Defense and vendor consortium formed a governing
   organization named the Interoperability Control Working Group (ICWG)
   to manage the protocol.  In time, the group expanded to include NATO,
   NATO partners, and European vendors under the name International
   Interoperability Control Working Group (IICWG), which was later
   renamed the SCIP Working Group.

   First generation SCIP devices operated on circuit-switched networks.
   SCIP was then expanded to radio and IP networks.  The scip media
   subtype transports SCIP secure session establishment signaling and
   secure application traffic.  The built-in negotiation and flexibility
   provided by the SCIP protocols make it a natural choice for many
   scenarios that require various secure applications and associated
   encryption suites.  SCIP has been adopted by NATO in STANAG 5068.
   SCIP standards are currently available to participating government
   and military communities and select OEMs of equipment that support
   SCIP.

   However, SCIP must operate over global networks (including private
   and commercial networks).  Without access to necessary information to
   support SCIP, some networks may not support the SCIP media subtypes.
   Issues may occur simply because information is not as readily
   available to OEMs, network administrators, and network architects.

   This document provides essential information about the "audio/scip"
   and "video/scip" media subtypes that enable network equipment
   manufacturers to include settings for "scip" as a known audio and
   video media subtype in their equipment.  This enables network
   administrators to define and implement a compatible security policy
   that includes audio and video media subtypes "audio/scip" and "video/
   scip", respectively, as permitted codecs on the network.

   All current IP-based SCIP endpoints implement "scip" as a media
   subtype.  Registration of scip as a media subtype provides a common
   reference for network equipment manufacturers to recognize SCIP in an
   SDP payload declaration.

4.  Payload Format

   The "scip" media subtype identifies and indicates support for SCIP
   traffic that is being transported over RTP.  Transcoding, lossy
   compression, or other data modifications MUST NOT be performed by the
   network on the SCIP RTP payload.  The "audio/scip" and "video/scip"
   media subtype data streams within the network, including the VoIP
   network, MUST be a transparent relay and be treated as "clear-channel
   data", similar to the Clearmode media subtype defined by [RFC4040].

   [RFC4040] is referenced because Clearmode does not define specific
   RTP payload content, packet size, or packet intervals, but rather
   enables Clearmode devices to signal that they support a compatible
   mode of operation and defines a transparent channel on which devices
   may communicate.  This document takes a similar approach.  Network
   devices that implement support for SCIP need to enable SCIP endpoints
   to signal that they support SCIP and provide a transparent channel on
   which SCIP endpoints may communicate.

   SCIP is an application-layer protocol that is defined in SCIP-210.
   The SCIP traffic consists of encrypted SCIP control messages and
   codec data.  The payload size and interval will vary considerably
   depending on the state of the SCIP protocol within the SCIP device.

   Figure 1 below illustrates the RTP payload format for SCIP.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           RTP Header                          |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   |                          SCIP Payload                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: SCIP RTP Payload Format

   The SCIP codec produces an encrypted bitstream that is transported
   over RTP.  Unlike other codecs, SCIP does not have its own upper
   layer syntax (e.g., no Network Adaptation Layer (NAL) units), but
   rather encrypts the output of the audio and video codecs that it uses
   (e.g., G.729D, H.264 [RFC6184], etc.).  SCIP achieves this by
   encapsulating the encrypted codec output that has been previously
   formatted according to the relevant RTP payload specification for
   that codec.  SCIP endpoints MAY employ mechanisms, such as inter-
   media RTP synchronization as described in [RFC8088], Section 3.3.4,
   to synchronize "audio/scip" and "video/scip" streams.

   Figure 2 below illustrates notionally how codec packets and SCIP
   control messages are packetized for transmission over RTP.

   +-----------+              +-----------------------+
   |   Codec   |              | SCIP control messages |
   +-----------+              +-----------------------+
         |                                |
         |                                |
         V                                V
   +--------------------------------------------------+
   |             Packetizer* (<= MTU size)            |
   +--------------------------------------------------+
             |                        |
             |                        |
             V                        |
     +--------------+                 |
     |  Encryption  |                 |
     +--------------+                 |
             |                        |
             |                        |
             V                        V
   +--------------------------------------------------+
   |                      RTP                         |
   +--------------------------------------------------+

                      Figure 2: SCIP RTP Architecture

   * Packetizer:  The SCIP application layer will ensure that all
      traffic sent to the RTP layer will not exceed the MTU size.  The
      receiving SCIP RTP layer will handle packet identification,
      ordering, and reassembly.  When required, the SCIP application
      layer handles error detection and retransmission.

   As described above, the SCIP RTP payload format is variable and
   cannot be described in specificity in this document.  Details can be
   found in SCIP-210.  SCIP will continue to evolve and, as such, the
   SCIP RTP traffic MUST NOT be filtered by network devices based upon
   what currently is observed or documented.  The focus of this document
   is for network devices to consider the SCIP RTP payload as opaque and
   allow it to traverse the network.  Network devices MUST NOT modify
   SCIP RTP packets.

4.1.  RTP Header Fields

   The SCIP RTP header fields SHALL conform to [RFC3550].

   SCIP traffic may be continuous or discontinuous.  The Timestamp field
   MUST increment based on the sampling clock for discontinuous
   transmission as described in [RFC3550], Section 5.1.  The Timestamp
   field for continuous transmission applications is dependent on the
   sampling rate of the media as specified in the media subtype's
   specification (e.g., Mixed Excitation Linear Prediction Enhanced
   (MELPe)).  Note that during a SCIP session, both discontinuous and
   continuous traffic are highly probable.

   The Marker bit SHALL be set to zero for discontinuous traffic.  The
   Marker bit for continuous traffic is based on the underlying media
   subtype specification.  The underlying media is opaque within SCIP
   RTP packets.

4.2.  Congestion Control Considerations

   The bitrate of SCIP may be adjusted depending on the capability of
   the underlying codec (such as MELPe [RFC8130], G.729D [RFC3551],
   etc.).  The number of encoded audio frames per packet may also be
   adjusted to control congestion.  Discontinuous transmission may also
   be used if supported by the underlying codec.

   Since UDP does not provide congestion control, applications that use
   RTP over UDP SHOULD implement their own congestion control above the
   UDP layer [RFC8085] and MAY also implement a transport circuit
   breaker [RFC8083].  Work in the RTP Media Congestion Avoidance
   Techniques (RMCAT) working group [RMCAT] describes the interactions
   and conceptual interfaces necessary between the application
   components that relate to congestion control, including the RTP
   layer, the higher-level media codec control layer, and the lower-
   level transport interface, as well as components dedicated to
   congestion control functions.

   Use of the packet loss feedback mechanisms in AVPF [RFC4585] and
   SAVPF [RFC5124] are OPTIONAL because SCIP itself manages
   retransmissions of some errored or lost packets.  Specifically, the
   payload-specific feedback messages defined in [RFC4585], Section 6.3
   are OPTIONAL when transporting video data.

4.3.  Use of Augmented RTPs with SCIP

   The SCIP application-layer protocol uses RTP as a basic transport for
   the "audio/scip" and "video/scip" payloads.  Additional RTPs that do
   not modify the SCIP payload are considered OPTIONAL in this document
   and are discretionary for a SCIP device vendor to implement.  Some
   examples include, but are not limited to:

   *  "RTP Payload Format for Generic Forward Error Correction"
      [RFC5109]

   *  "Multiplexing RTP Data and Control Packets on a Single Port"
      [RFC5761]

   *  "Symmetric RTP / RTP Control Protocol (RTCP)" [RFC4961]

   *  "Negotiating Media Multiplexing Using the Session Description
      Protocol (SDP)" a.k.a. BUNDLE [RFC9143]

5.  Payload Format Parameters

   The SCIP RTP payload format is identified using the scip media
   subtype, which is registered in accordance with [RFC4855] and per the
   media type registration template from [RFC6838].  A clock rate of
   8000 Hz SHALL be used for "audio/scip".  A clock rate of 90000 Hz
   SHALL be used for "video/scip".

5.1.  Media Subtype "audio/scip"

   Type name:  audio

   Subtype name:  scip

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  Binary.  This media subtype is only defined
      for transfer via RTP.  There SHALL be no transcoding of the audio
      stream as it traverses the network.

   Security considerations:  See Section 6.

   Interoperability considerations:  N/A

   Published specification:  [SCIP210]

   Applications that use this media type:  N/A

   Fragment identifier considerations:  none

   Additional information:

      Deprecated alias names for this type:  N/A
      Magic number(s):  N/A
      File extension(s):  N/A
      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  Michael
      Faller (michael.faller@gd-ms.com or MichaelFFaller@gmail.com) and
      Daniel Hanson (dan.hanson@gd-ms.com)

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Authors:  Michael Faller (michael.faller@gd-ms.com or
      MichaelFFaller@gmail.com) and Daniel Hanson (dan.hanson@gd-ms.com)

   Change controller:  SCIP Working Group (ncia.cis3@ncia.nato.int)

5.2.  Media Subtype "video/scip"

   Type name:  video

   Subtype name:  scip

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  Binary.  This media subtype is only defined
      for transfer via RTP.  There SHALL be no transcoding of the video
      stream as it traverses the network.

   Security considerations:  See Section 6.

   Interoperability considerations:  N/A

   Published specification:  [SCIP210]

   Applications that use this media type:  N/A

   Fragment identifier considerations:  none

   Additional information:

      Deprecated alias names for this type:  N/A
      Magic number(s):  N/A
      File extension(s):  N/A
      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  Michael
      Faller (michael.faller@gd-ms.com or MichaelFFaller@gmail.com) and
      Daniel Hanson (dan.hanson@gd-ms.com)

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Authors:  Michael Faller (michael.faller@gd-ms.com or
      MichaelFFaller@gmail.com) and Daniel Hanson (dan.hanson@gd-ms.com)

   Change controller:  SCIP Working Group (ncia.cis3@ncia.nato.int)

5.3.  Mapping to SDP

   The mapping of the above-defined payload format media subtype and its
   parameters SHALL be implemented according to Section 3 of [RFC4855].

   Since SCIP includes its own facilities for capabilities exchange, it
   is only necessary to negotiate the use of SCIP within SDP Offer/
   Answer; the specific codecs to be encapsulated within SCIP are then
   negotiated via the exchange of SCIP control messages.

   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [RFC8866], which is commonly used to describe RTP sessions.  When SDP
   is used to specify sessions employing the SCIP codec, the mapping is
   as follows:

   *  The media type ("audio") goes in SDP "m=" as the media name for
      "audio/scip", and the media type ("video") goes in SDP "m=" as the
      media name for "video/scip".

   *  The media subtype ("scip") goes in SDP "a=rtpmap" as the encoding
      name.  The required parameter "rate" also goes in "a=rtpmap" as
      the clock rate.

   *  The optional parameters "ptime" and "maxptime" go in the SDP
      "a=ptime" and "a=maxptime" attributes, respectively.

   An example mapping for "audio/scip" is:

     m=audio 50000 RTP/AVP 96
     a=rtpmap:96 scip/8000

   An example mapping for "video/scip" is:

     m=video 50002 RTP/AVP 97
     a=rtpmap:97 scip/90000

   An example mapping for both "audio/scip" and "video/scip" is:

     m=audio 50000 RTP/AVP 96
     a=rtpmap:96 scip/8000
     m=video 50002 RTP/AVP 97
     a=rtpmap:97 scip/90000

5.4.  SDP Offer/Answer Considerations

   In accordance with the SDP Offer/Answer model [RFC3264], the SCIP
   device SHALL list the SCIP payload type number in order of preference
   in the "m" media line.

   For example, an SDP Offer with scip as the preferred audio media
   subtype:

     m=audio 50000 RTP/AVP 96 0 8
     a=rtpmap:96 scip/8000
     a=rtpmap:0 PCMU/8000
     a=rtpmap:8 PCMA/8000

6.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550], and in any applicable RTP profile such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
   SAVPF [RFC5124].  However, as "Securing the RTP Framework: Why RTP
   Does Not Mandate a Single Media Security Solution" [RFC7202]
   discusses, it is not an RTP payload format's responsibility to
   discuss or mandate what solutions are used to meet the basic security
   goals like confidentiality, integrity, and source authenticity for
   RTP in general.  This responsibility lies on anyone using RTP in an
   application.  They can find guidance on available security mechanisms
   and important considerations in "Options for Securing RTP Sessions"
   [RFC7201].  Applications SHOULD use one or more appropriate strong
   security mechanisms.  The rest of this Security Considerations
   section discusses the security impacting properties of the payload
   format itself.

   This RTP payload format and its media decoder do not exhibit any
   significant non-uniformity in the receiver-side computational
   complexity for packet processing, and thus do not inherently pose a
   denial-of-service threat due to the receipt of pathological data, nor
   does the RTP payload format contain any active content.

   SCIP only encrypts the contents transported in the RTP payload; it
   does not protect the RTP header or RTCP packets.  Applications
   requiring additional RTP headers and/or RTCP security might consider
   mechanisms such as SRTP [RFC3711], however these additional
   mechanisms are considered OPTIONAL in this document.

7.  IANA Considerations

   The "audio/scip" and "video/scip" media subtypes have previously been
   registered in the "Media Types" registry [MediaTypes].  IANA has
   updated these registrations to reference this document.

8.  SCIP Contact Information

   The SCIP protocol is maintained by the SCIP Working Group.  The
   current SCIP-210 specification [SCIP210] may be requested from the
   email address below.

   SCIP Working Group, CIS3 Partnership
   NATO Communications and Information Agency
   Oude Waalsdorperweg 61
   2597 AK The Hague, Netherlands
   Email: ncia.cis3@ncia.nato.int


   An older public version of the SCIP-210 specification can be
   downloaded from https://www.iad.gov/SecurePhone/index.cfm.  A U.S.
   Department of Defense Root Certificate should be installed to access
   this website.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2736]  Handley, M. and C. Perkins, "Guidelines for Writers of RTP
              Payload Format Specifications", BCP 36, RFC 2736,
              DOI 10.17487/RFC2736, December 1999,
              <https://www.rfc-editor.org/info/rfc2736>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <https://www.rfc-editor.org/info/rfc3551>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <https://www.rfc-editor.org/info/rfc5124>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,
              <https://www.rfc-editor.org/info/rfc8866>.

9.2.  Informative References

   [MediaTypes]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [RFC4040]  Kreuter, R., "RTP Payload Format for a 64 kbit/s
              Transparent Call", RFC 4040, DOI 10.17487/RFC4040, April
              2005, <https://www.rfc-editor.org/info/rfc4040>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://www.rfc-editor.org/info/rfc4855>.

   [RFC4961]  Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
              BCP 131, RFC 4961, DOI 10.17487/RFC4961, July 2007,
              <https://www.rfc-editor.org/info/rfc4961>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <https://www.rfc-editor.org/info/rfc5109>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.

   [RFC6184]  Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video", RFC 6184,
              DOI 10.17487/RFC6184, May 2011,
              <https://www.rfc-editor.org/info/rfc6184>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <https://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <https://www.rfc-editor.org/info/rfc7202>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,
              <https://www.rfc-editor.org/info/rfc8083>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8088]  Westerlund, M., "How to Write an RTP Payload Format",
              RFC 8088, DOI 10.17487/RFC8088, May 2017,
              <https://www.rfc-editor.org/info/rfc8088>.

   [RFC8130]  Demjanenko, V. and D. Satterlee, "RTP Payload Format for
              the Mixed Excitation Linear Prediction Enhanced (MELPe)
              Codec", RFC 8130, DOI 10.17487/RFC8130, March 2017,
              <https://www.rfc-editor.org/info/rfc8130>.

   [RFC9143]  Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", RFC 9143,
              DOI 10.17487/RFC9143, February 2022,
              <https://www.rfc-editor.org/info/rfc9143>.

   [RFC9170]  Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
              Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
              December 2021, <https://www.rfc-editor.org/info/rfc9170>.

   [RMCAT]    IETF, "RTP Media Congestion Avoidance Techniques (rmcat)",
              <https://datatracker.ietf.org/wg/rmcat/about>.

   [SCIP210]  SCIP Working Group, "SCIP Signaling Plan, SCIP-210".
              Available by request via email to
              <ncia.cis3@ncia.nato.int>.

Authors' Addresses

   Daniel Hanson
   General Dynamics Mission Systems, Inc.
   150 Rustcraft Road
   Dedham, MA 02026
   United States of America
   Email: dan.hanson@gd-ms.com


   Michael Faller
   General Dynamics Mission Systems, Inc.
   150 Rustcraft Road
   Dedham, MA 02026
   United States of America
   Email: michael.faller@gd-ms.com, MichaelFFaller@gmail.com


   Keith Maver
   General Dynamics Mission Systems, Inc.
   150 Rustcraft Road
   Dedham, MA 02026
   United States of America
   Email: keith.maver@gd-ms.com