ITU-T H.248.77 PDF

The secure real-time transport protocol (SRTP) is an RTP profile that provides confidentiality, message authentication and replay protection to RTP and RTCP sessions. The secure RTP package allows a MGC to control the use of SRTP by a MG. This package is defined in detail in clause 6.

By itself, the secure RTP package is incomplete, as it does not provide procedures for key management. Instead, it is designed to rely on existing key-management schemes. Clause 7 provides procedures for the use of one such key-management scheme: SDP security descriptions.

Several reasons exist why this Recommendation is required, in addition to the existing (usually SDP-based) SRTP key-management schemes. The most significant of which are listed below:

– Most existing SDP key-management schemes rely on the SDP offer/answer model (see [b-IETF RFC 3264]). However, the offer/answer model is not used in ITU-T H.248 as it does not fit the nature of the connection between an ITU-T H.248 MGC and a MG.

– Existing SDP key-management schemes do not contain procedures relating to parameter overspecification and wildcarding, which are unique to ITU-T H.248.

– The limited lifetime of SRTP master keys calls for mechanisms for handling master key expiry. The existing mechanisms cannot be used in ITU-T H.248.

– The SRTP package allows explicit control over the key-management scheme employed, allowing easy interoperability with, and migration to future schemes.

– The SRTP package allows an MGC to audit the SRTP capabilities of an MG through the use of the packages descriptor and the properties of the new package.

– The SRTP package allows an MGC to collect statistics regarding the number of security violations encountered by the MG, and the volume of SRTP traffic it processed.

The scope of this Recommendation is limited to use-cases in which a MG applies SRTP procedures, as described in clause 3.3 of [IETF RFC 3711], to the SRTP packets it sends and receives. Usecases in which the MG handles SRTP packets without using those procedures (e.g., transparent forwarding, storage in encrypted form, etc.) are intentionally left out of this Recommendation.

Connection model

All protocol elements and procedures described in this Recommendation are limited to the extent of a single ITU-T H.248 termination. In addition, no assumptions are made regarding either the lower layer protocols beneath the SRTP level or the upper layer protocols/codecs being carried by the SRTP. This allows the use of the Recommendation's procedures in various connection models and use-cases (e.g., a SRTP enabled announcement server, a SRTP to RTP translator, etc.).

Figure 1 details the generic connection-model where a SRTP-enabled termination is connected to a single other termination (either SRTP-enabled or not). The generalization to any number of terminations is trivial.