Network Working Group L. Dondeti, Ed. Request for Comments: 5159 QUALCOMM, Inc. Category: Informational A. Jerichow Nokia Siemens Networks March 2008
Session Description Protocol (SDP) Attributes for Open Mobile Alliance (OMA) Broadcast (BCAST) Service and Content Protection
Status of This Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Abstract
This document provides descriptions of Session Description Protocol (SDP) attributes used by the Open Mobile Alliance's Broadcast Service and Content Protection specification.
The Open Mobile Alliance (OMA) Broadcast (BCAST) group is specifying service and content protection mechanisms for broadcast services over wireless networks. As part of that specification, several new SDP attributes are necessary to allow the broadcast server to signal the service and content protection parameters to clients.
Section 8.2.4 of RFC 4566 [1] requires that new SDP attributes are registered through IANA with name, contact information, and description (and other similar parameters). A standards track specification is RECOMMENDED if the new attribute(s) will have widespread use and interoperability considerations.
OMA BCAST specifications are expected to be used by broadcast wireless systems that are based on 3rd Generation Partnership Project (3GPP) Multimedia Broadcast/Multicast Service (MBMS), 3GPP2 Broadcast and Multicast Services (BCMCS), and Digital Video Broadcasting - Handheld (DVB-H). Although this would typically be considered a "widespread" use, in this case IETF chose to use a non-standards- track RFC to register the SDP attributes because OMA maintains change control of the documents that specify the interpretation of the values in the attributes.
This document provides descriptions of the SDP attributes used in the OMA BCAST Service and Content Protection specification [2].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3].
In addition to the notes in Section 7 of RFC 4566 [1], the following considerations may be applicable:
The bcastversion parameter indicates the version of the broadcast system used for distribution of broadcast content. In case future versions indicated by this parameter allow for enhanced or additional security features, the bcastversion parameter, if unprotected, could be utilized for downgrade attacks.
The stkmstream parameter provides references to relevant key management streams so that receivers can map the media streams to key streams and retrieve the necessary keys to decrypt media. As such, this parameter could be utilized, if unprotected, for denial-of- service (DoS) attacks.
IANA has registered a number of OMA BCAST only attributes in the Session Description Protocol Parameters registry [1]. The registration data, according to RFC 4566 [1] follows.
The registration templates below refer to the OMA-TS- BCAST_SvcCntProtection specification [2].
5.1.1. Registration of the Attribute bcastversion:<major>.<minor>
Contact: Anja Jerichow <anja.jerichow@nsn.com>
Phone: +49 89 636-45868
Attribute name: bcastversion
Long-form attribute name: BCAST version
Type of attribute: session level
This attribute is not dependent on charset.
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RFC 5159 OMA BCAST SDP Attributes March 2008
Purpose: This attribute specifies the OMA BCAST version "bcastversion" value in the format x.y.
Specification of attribute values: This attribute has a mandatory value of the form <major>.<minor>, where <major> and <minor> are non-negative decimal numbers. The att-value field is of XML data type decimal. For details, see OMA-TS-BCAST_SvcCntProtection, Section 10.1.1.
5.1.2. Registration of the Attribute stkmstream:<id of the stkm stream>
Contact: Anja Jerichow <anja.jerichow@nsn.com>
Phone: +49 89 636-45868
Attribute name: stkmstream
Long-form attribute name: Short Term Key Message stream identifier
Type of attribute: session level or media level
The attribute can be at session level, in which case it applies to all media streams, or it can be at media level, in which case it only applies to the specified media and would overwrite possible session-level attributes.
This attribute is not dependent on charset.
Purpose: The stkmstream attribute specifies the mapping of Short Term Key Message streams to media streams in the SDP.
Each session or media stream can have multiple stkmstream attributes. By comparing the value of this attribute with the identifier of each STKM stream, the terminal can figure out which one to listen to and process. We note that this attribute is optional and hence would not be there for unencrypted media streams.
Specification of attribute values: This attribute has a mandatory value of the form <id of the stkm stream>, a unique non-zero integer identifying a particular key stream. Numbers are unique within a particular SDP session, i.e., no global numbering is required. The att-value field is of XML data type unsignedShort. For details, see OMA-TS-BCAST_SvcCntProtection, Section 10.1.3.
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RFC 5159 OMA BCAST SDP Attributes March 2008
5.1.3. Registration of the Attribute SRTPAuthentication:<id for SRTP authentication algorithm value>
Contact: Anja Jerichow <anja.jerichow@nsn.com>
Phone: +49 89 636-45868
Attribute name: SRTPAuthentication
Long-form attribute name: SRTP authentication algorithm value identifier
Type of attribute: media level
This attribute is not dependent on charset.
Purpose: When SRTP is used, the attribute SRTPAuthentication states which authentication algorithm to use.
Specification of attribute values: Based on [4], the identifier is a transform-independent parameter of the cryptographic context for SRTP in integer format.
One of the following three integrity transforms registered for the three modes MUST be used: value "2" for RCCm1, "3" for RCCm2, and "4" for RCCm3. For details, see OMA-TS-BCAST_SvcCntProtection, Section 10.4.
5.1.4. Registration of the Attribute SRTPROCTxRate:<ROC transmission rate>
Contact: Anja Jerichow <anja.jerichow@nsn.com>
Phone: +49 89 636-45868
Attribute name: SRTPROCTxRate
Long-form attribute name: ROC transmission rate
Type of attribute: media level
This attribute is not dependent on charset.
Purpose: When SRTP is used, the attribute SRTPROCTxRate defines the ROC transmission rate, R.
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RFC 5159 OMA BCAST SDP Attributes March 2008
Specification of attribute values: The attribute value MUST be a decimal integer R between 1 and 65535 inclusive, as specified in [4]. If the ROC transmission rate is not included in the negotiation, the default value of 1 SHALL be used. For details, see OMA-TS-BCAST_SvcCntProtection, Section 10.4.
Many thanks to Hosame Abu-Amara, Francois Ambrosini, David Castleford, Miguel Garcia, Alfred Hoenes, Charles Lo, and Uwe Rauschenbach for their help and support.
[1] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[2] Open Mobile Alliance, "Service and Content Protection for Mobile Broadcast Services", OMA OMA-TS-BCAST_SvcCntProtection-V1_0- 20071218-D, 2007.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[4] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP)", RFC 4771, January 2007.
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