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

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 .
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 ), 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.

Introduction This document details usage of the "audio/scip" and "video/scip" pseudo-codecs as a secure session establishment protocol and media transport protocol over RTP. It discusses how:

encrypted audio and video codec payloads are transported over RTP;
the IP network layer does not implement SCIP as a protocol since SCIP operates at the application layer in endpoints;
the IP network layer enables SCIP traffic to transparently pass through the network;
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,
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 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.

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 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 and , 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.

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

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.

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 . 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. below illustrates the RTP payload format for SCIP.

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 , 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 , to synchronize "audio/scip" and "video/scip" streams. below illustrates notionally how codec packets and SCIP control messages are packetized for transmission over RTP.

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.

RTP Header Fields The SCIP RTP header fields SHALL conform to . SCIP traffic may be continuous or discontinuous. The Timestamp field MUST increment based on the sampling clock for discontinuous transmission as described in . 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.

Congestion Control Considerations The bitrate of SCIP may be adjusted depending on the capability of the underlying codec (such as MELPe , G.729D , 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 and MAY also implement a transport circuit breaker . Work in the RTP Media Congestion Avoidance Techniques (RMCAT) working group 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 and SAVPF are OPTIONAL because SCIP itself manages retransmissions of some errored or lost packets. Specifically, the payload-specific feedback messages defined in are OPTIONAL when transporting video data.

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"
"Multiplexing RTP Data and Control Packets on a Single Port"
"Symmetric RTP / RTP Control Protocol (RTCP)"
"Negotiating Media Multiplexing Using the Session Description Protocol (SDP)" a.k.a. BUNDLE

Payload Format Parameters The SCIP RTP payload format is identified using the scip media subtype, which is registered in accordance with and per the media type registration template from . 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 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 .

Interoperability considerations:

N/A

Published specification:

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)

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 .

Interoperability considerations:

N/A

Published specification:

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)

Mapping to SDP The mapping of the above-defined payload format media subtype and its parameters SHALL be implemented according to . 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) , 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

SDP Offer/Answer Considerations In accordance with the SDP Offer/Answer model , 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

Security Considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification , and in any applicable RTP profile such as RTP/AVP , RTP/AVPF , RTP/SAVP , or RTP/SAVPF . However, as "Securing the RTP Framework: Why RTP Does Not Mandate a Single Media Security Solution" 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" . 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 , however these additional mechanisms are considered OPTIONAL in this document.

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

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 ncia.cis3@ncia.nato.int

An older public version of the SCIP-210 specification can be downloaded from . A U.S. Department of Defense Root Certificate should be installed to access this website.