| Internet-Draft | SCIP RTP Payload Format | September 2023 |
| Hanson, et al. | Expires 22 March 2024 | [Page] |
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.¶
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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.¶
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.¶
The following abbreviations are used in this document.¶
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.¶
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.¶
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.¶
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 |
+--------------------------------------------------+
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.¶
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.¶
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".¶
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]¶
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]¶
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:¶
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¶
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¶
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.¶
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.¶
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.¶