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

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Table of Contents

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 [RFC4040] Clearmode.¶
• 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/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 middle boxes 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.¶
• 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 middle boxes to not attempt parsing of the SCIP payload. As described in this document, such parsing serves no useful purpose.¶

2. Introduction

The purpose of this document is to provide enough information to enable SCIP payloads to be transported through the network without modification or filtering. The document provides a reference for network security policymakers; network equipment OEMs, administrators, and architects; procurement personnel; and government agency and commercial industry representatives.¶

The 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 (1) how encrypted audio and video codec payloads are transported over RTP; (2) the IP network layer not implementing SCIP as a protocol since SCIP operates at the application layer in endpoints; (3) the IP network layer enabling SCIP traffic to transparently pass through the network; (4) network devices not recognizing SCIP, and thus removing the scip codecs from the SDP media payload declaration before forwarding to the next network node; and finally, (5) SCIP endpoint devices not operating on networks due to the scip media subtype removal from the SDP media payload declaration.¶

SCIP is presently implemented in the 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.¶

The SCIP protocol defined in SCIP-210 [SCIP210] includes built-in support for packetization/de-packetization, retransmission, capability exchange, version negotiation and secure key establishment as well as security services such as end-to-end confidentiality and integrity protection. As a result, neither the RTP transport nor middle boxes 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 middle boxes, 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.¶

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. Payload Format

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.¶

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                         |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 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                         |
+--------------------------------------------------+

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.¶

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".¶

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

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

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

  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 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.¶

SCIP only encrypts the contents transported in the RTP payload; it does not protect the RTP header or RTCP packets. Applications requiring additional RTP header 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 with IANA [AUDIOSCIP] [VIDEOSCIP]. IANA should update [AUDIOSCIP] and [VIDEOSCIP] to reference this document upon publication.¶

8. 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: 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.¶

Authors' Addresses