Payload Working Group D. Hanson Internet-Draft M. Faller Intended status: Standards Track K. Maver Expires: 30 September 2023 General Dynamics Mission Systems, Inc. 29 March 2023 RTP Payload Format for the Secure Communication Interoperability Protocol (SCIP) Codec draft-ietf-avtcore-rtp-scip-05 Abstract This document describes the RTP payload format of the Secure Communication Interoperability Protocol (SCIP). SCIP is an application layer protocol that defines the establishment of reliable SCIP endpoint to SCIP endpoint secure communications over the RTP channel provided by network equipment. The scope of this document is limited to defining the scip codecs and Session Description Protocol (SDP) and RTP parameters to be supported by network devices with minimal description of the SCIP Application Layer Protocol. Since the SCIP RTP payload is encrypted, it is considered "opaque" to network devices. 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 https://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 on 30 September 2023. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. Hanson, et al. Expires 30 September 2023 [Page 1] Internet-Draft SCIP RTP Payload Format March 2023 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Media Format Description . . . . . . . . . . . . . . . . . . 5 3.1. Congestion Control Considerations . . . . . . . . . . . . 5 4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. RTP Header Fields . . . . . . . . . . . . . . . . . . . . 7 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 7 5.1. Media Subtype "audio/scip" . . . . . . . . . . . . . . . 7 5.2. Media Subtype "video/scip" . . . . . . . . . . . . . . . 9 5.3. Mapping to SDP . . . . . . . . . . . . . . . . . . . . . 10 5.4. SDP Offer/Answer Considerations . . . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. SCIP Contact Information . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction This document details usage of the "audio/scip" and "video/scip" pseudo-codecs [AUDIOSCIP], [VIDEOSCIP] as a secure session establishment protocol and media transport protocol over RTP. It discusses that encrypted audio and video codec payloads are transported over RTP. This document provides a reference for network security policymakers, network equipment OEMs, procurement personnel, and government agency and commercial industry representatives. Note that the IP network layer does not implement SCIP as a protocol since SCIP operates at the application layer in endpoints. However, the IP network layer should enable SCIP traffic to transparently pass through the network. Some network devices do not recognize SCIP, and thus remove the scip codecs from the SDP media payload declaration. When the scip media subtype is removed from the SDP media payload declaration, SCIP endpoint devices will not operate on the network. Hanson, et al. Expires 30 September 2023 [Page 2] Internet-Draft SCIP RTP Payload Format March 2023 The purpose of this document is to provide enough information to enable SCIP payloads to be transported through the network without modification or filtering. SCIP is presently implemented in United States and NATO 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) based on current SCIP traffic profiles may cause unintended consequences in the future when changes to the SCIP traffic may not be recognized by the network device. Network devices do not need to know the details of SCIP protocol as defined in SCIP-210 [SCIP210] to allow it to traverse the network, therefore SCIP-210 is considered an Informative Reference in this document. 1.1. 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. Best current practices for writing an RTP payload format specification were followed [RFC2736] [RFC8088]. 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. 1.2. Abbreviations The following abbreviations are used in this document. AVP: Audio/Video Profile DTX: Discontinuous Transmission ICWG: Interoperability Control Working Group IICWG: International Interoperability Control Working Group NATO: North Atlantic Treaty Organization SCIP: Secure Communication Interoperability Protocol SDP: Session Description Protocol Hanson, et al. Expires 30 September 2023 [Page 3] Internet-Draft SCIP RTP Payload Format March 2023 2. 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 in the United States and within NATO. These capabilities include end-to-end security at the application layer, authentication of user identity, the ability to apply different security levels for each secure session, and secure communication over any end-to-end data connection. 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/ 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 audio/scip and video/scip media subtypes that enables 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 which includes audio and video media subtypes scip/8000 and scip/90000 as permitted codecs on the network. Hanson, et al. Expires 30 September 2023 [Page 4] Internet-Draft SCIP RTP Payload Format March 2023 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 a payload declaration. 3. Media Format Description The "scip" media subtype indicates support for and identifies 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]. RFC 4040 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. 3.1. Congestion Control Considerations The bitrate of SCIP may be adjusted depending on the capability of the underlying codec (MELPe, G.729D, etc.). The number of encoded audio frames per packet may also be adjusted to control congestion. Discontinuous transmission (DTX) 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 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. Hanson, et al. Expires 30 September 2023 [Page 5] Internet-Draft SCIP RTP Payload Format March 2023 4. Payload Format SCIP is an application-layer protocol that is defined in SCIP-210 [SCIP210]. The SCIP traffic consists of both SCIP control messages (some of which may be encrypted) and encrypted codec data. The payload size and interval will vary considerably depending on the state of the SCIP device. 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/video codecs that it uses (e.g., G.729D [RFC3551], 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 1 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 1: SCIP RTP Architecture Hanson, et al. Expires 30 September 2023 [Page 6] Internet-Draft SCIP RTP Payload Format March 2023 | * Packetizer: Media codecs payloads are split into multiple RTP | packets, if they exceed the MTU size, as defined in that media | codec's RFC. SCIP control messages are split into multiple RTP | packets if they exceed the MTU size - the SCIP messages contain | an overall message length field allowing segments to be | reassembled by the receiver. As described above, the SCIP RTP payload format is highly variable and cannot be described in specificity in this document. Details can be found in SCIP-210 [SCIP210]. 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 RFC 3550. 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., MELPe [RFC8130]). 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. 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 form [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" Media type name: audio Media subtype name: scip Hanson, et al. Expires 30 September 2023 [Page 7] Internet-Draft SCIP RTP Payload Format March 2023 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 encoding/decoding (transcoding) of the audio stream as it traverses the network. Security considerations: See Section 6. Interoperability considerations: N/A Published specifications: [SCIP210] Applications which use this media: N/A Fragment Identifier considerations: none Restrictions on usage: N/A Additional information: 1. Deprecated alias names for this type: N/A 2. Magic number(s): N/A 3. File extension(s): N/A 4. Macintosh file type code: N/A 5. Object Identifiers: N/A Person to contact for further information: 1. Name: Michael Faller and Daniel Hanson 2. Email: michael.faller@gd-ms.com and dan.hanson@gd-ms.com Intended usage: Common, Government and Military Authors: Michael Faller - michael.faller@gd-ms.com Daniel Hanson - dan.hanson@gd-ms.com Change controller: Hanson, et al. Expires 30 September 2023 [Page 8] Internet-Draft SCIP RTP Payload Format March 2023 SCIP Working Group - ncia.cis3@ncia.nato.int 5.2. Media Subtype "video/scip" Media type name: video Media 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 encoding/decoding (transcoding) of the video stream as it traverses the network. Security considerations: See Section 6. Interoperability considerations: N/A Published specifications: [SCIP210] Applications which use this media: N/A Fragment Identifier considerations: none Restrictions on usage: N/A Additional information: 1. Deprecated alias names for this type: N/A 2. Magic number(s): N/A 3. File extension(s): N/A 4. Macintosh file type code: N/A 5. Object Identifiers: N/A Person to contact for further information: 1. Name: Michael Faller and Daniel Hanson 2. Email: michael.faller@gd-ms.com and dan.hanson@gd-ms.com Intended usage: Common, Government and Military Hanson, et al. Expires 30 September 2023 [Page 9] Internet-Draft SCIP RTP Payload Format March 2023 Authors: Michael Faller - michael.faller@gd-ms.com 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: Hanson, et al. Expires 30 September 2023 [Page 10] Internet-Draft SCIP RTP Payload Format March 2023 m=audio 50000 RTP/AVP 96 a=rtpmap:96 scip/8000 m=video 50002 RTP/AVP 97 a=rtpmap:97 scip/90000 The application negotiation between endpoints will determine whether the audio and video streams are transported as separate streams over the audio and video payload types or as a single media stream on the video payload type. 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. 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 Protocol 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 lays 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. 7. IANA Considerations The audio/scip and video/scip media subtypes have previously been registered with IANA [AUDIOSCIP] [VIDEOSCIP]. IANA should update [AUDIOSCIP] and [VIDEOSCIP] to reference this document upon publication. Hanson, et al. Expires 30 September 2023 [Page 11] Internet-Draft SCIP RTP Payload Format March 2023 8. SCIP Contact Information The SCIP protocol is maintained by the SCIP Working Group. SCIP Working Group, CIS3 Partnership NATO Communications and Information Agency Oude Waalsdorperweg 61, 2597AK The Hague, The Netherlands Email: ncia.cis3@ncia.nato.int 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, . [RFC2736] Handley, M. and C. Perkins, "Guidelines for Writers of RTP Payload Format Specifications", BCP 36, RFC 2736, DOI 10.17487/RFC2736, December 1999, . [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, . [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, . [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, . [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, . Hanson, et al. Expires 30 September 2023 [Page 12] Internet-Draft SCIP RTP Payload Format March 2023 [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, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: Session Description Protocol", RFC 8866, DOI 10.17487/RFC8866, January 2021, . 9.2. Informative References [AUDIOSCIP] Faller, M. and D. Hanson, "audio/scip: Internet Assigned Numbers Authority (IANA)", 28 January 2021, . [RFC4040] Kreuter, R., "RTP Payload Format for a 64 kbit/s Transparent Call", RFC 4040, DOI 10.17487/RFC4040, April 2005, . [RFC4855] Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, . [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, . [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, . [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, . Hanson, et al. Expires 30 September 2023 [Page 13] Internet-Draft SCIP RTP Payload Format March 2023 [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, . [RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions", RFC 8083, DOI 10.17487/RFC8083, March 2017, . [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017, . [RFC8088] Westerlund, M., "How to Write an RTP Payload Format", RFC 8088, DOI 10.17487/RFC8088, May 2017, . [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, . [RMCAT] IETF, "RTP Media Congestion Avoidance Techniques (rmcat) Working Group", . [SCIP210] SCIP Working Group, "SCIP Signaling Plan", SCIP-210, r3.10, October 2017. [VIDEOSCIP] Faller, M. and D. Hanson, "video/scip: Internet Assigned Numbers Authority (IANA)", 28 January 2021, . 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 Hanson, et al. Expires 30 September 2023 [Page 14] Internet-Draft SCIP RTP Payload Format March 2023 Michael Faller General Dynamics Mission Systems, Inc. 150 Rustcraft Road Dedham, MA 02026 United States of America Email: michael.faller@gd-ms.com Keith Maver General Dynamics Mission Systems, Inc. 150 Rustcraft Road Dedham, MA 02026 United States of America Email: keith.maver@gd-ms.com Hanson, et al. Expires 30 September 2023 [Page 15]