Internet-Draft SCIP RTP Payload Format September 2023
Hanson, et al. Expires 22 March 2024 [Page]
Workgroup:
Payload Working Group
Internet-Draft:
draft-ietf-avtcore-rtp-scip-06
Published:
Intended Status:
Standards Track
Expires:
Authors:
D. Hanson
General Dynamics Mission Systems, Inc.
M. Faller
General Dynamics Mission Systems, Inc.
K. Maver
General Dynamics Mission Systems, Inc.

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 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 the 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. SCIP is considered a tunneling protocol where the contents of the RTP payload will be indeterminant and should not be filtered or altered by the network. SCIP only relies on RTP as a simple transport.

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 22 March 2024.

Table of Contents

1. Key Points

2. 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 before forwarding to the next network node. When the scip media subtype is removed from the SDP media payload declaration, SCIP endpoint devices will not operate on the network. 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) 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.

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.

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.

2.2. Abbreviations

The following abbreviations are used in this document.

AVP:
Audio/Video Profile
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
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 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 "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. 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.

4.1. 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 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.

5. 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, 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

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.

5.1. 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). 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.

6. 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".

6.1. Media Subtype "audio/scip"

Media type name: audio

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 audio stream as it traverses the network.

Security considerations: See Section 7.

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: [email protected] and [email protected]

Intended usage: Common

Authors:

Michael Faller - [email protected]

Daniel Hanson - [email protected]

Change controller:

SCIP Working Group - [email protected]

6.2. 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 7.

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: [email protected] and [email protected]

Intended usage: Common

Authors:

Michael Faller - [email protected]

Daniel Hanson - [email protected]

Change controller:

SCIP Working Group - [email protected]

6.3. 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

6.4. 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

7. 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 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.

8. 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.

9. SCIP Contact Information

The SCIP protocol is maintained by the SCIP Working Group. The current SCIP-210 specification 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: [email protected]

An older public version of the SCIP-210 specification can be downloaded from https://www.iad.gov/SecurePhone/index.cfm.

10. References

10.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8866>.

10.2. Informative References

[AUDIOSCIP]
Faller, M. and D. Hanson, "audio/scip: Internet Assigned Numbers Authority (IANA)", , <https://www.iana.org/assignments/media-types/audio/scip>.
[RFC4040]
Kreuter, R., "RTP Payload Format for a 64 kbit/s Transparent Call", RFC 4040, DOI 10.17487/RFC4040, , <https://www.rfc-editor.org/info/rfc4040>.
[RFC4855]
Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, DOI 10.17487/RFC4855, , <https://www.rfc-editor.org/info/rfc4855>.
[RFC6184]
Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP Payload Format for H.264 Video", RFC 6184, DOI 10.17487/RFC6184, , <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, , <https://www.rfc-editor.org/info/rfc6838>.
[RFC7201]
Westerlund, M. and C. Perkins, "Options for Securing RTP Sessions", RFC 7201, DOI 10.17487/RFC7201, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8085>.
[RFC8088]
Westerlund, M., "How to Write an RTP Payload Format", RFC 8088, DOI 10.17487/RFC8088, , <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, , <https://www.rfc-editor.org/info/rfc8130>.
[RMCAT]
IETF, "RTP Media Congestion Avoidance Techniques (rmcat) Working Group", <https://datatracker.ietf.org/wg/rmcat/about/>.
[SCIP210]
SCIP Working Group, "SCIP Signaling Plan", SCIP-210, r3.10, , <https://www.iad.gov/SecurePhone/index.cfm>.
[VIDEOSCIP]
Faller, M. and D. Hanson, "video/scip: Internet Assigned Numbers Authority (IANA)", , <https://www.iana.org/assignments/media-types/video/scip>.

Authors' Addresses

Daniel Hanson
General Dynamics Mission Systems, Inc.
150 Rustcraft Road
Dedham, MA 02026
United States of America
Michael Faller
General Dynamics Mission Systems, Inc.
150 Rustcraft Road
Dedham, MA 02026
United States of America
Keith Maver
General Dynamics Mission Systems, Inc.
150 Rustcraft Road
Dedham, MA 02026
United States of America