IP Bearer Control Protocol

Description

The IP Bearer Control Protocol (IPBCP) is a client-server protocol defined by 3GPP for the control of IP bearer resources, primarily in the context of the IP Multimedia Subsystem (IMS) and network-to-network interfaces. It operates as an extension mechanism within the Session Description Protocol (SDP) offer/answer model. Essentially, IPBCP allows two entities involved in a session (like a Media Gateway Control Function - MGCF and a Media Gateway - MGW, or between MGCFs) to negotiate and establish the specific parameters of the IP transport layer that will carry the media stream (e.g., RTP streams for voice), beyond what is possible with standard SDP.

Architecturally, IPBCP is used in conjunction with call control protocols like SIP or BICC. It is carried as an SDP attribute within the body of SIP messages (INVITE, 200 OK, etc.) or within BICC application transport messages. The protocol defines a set of messages and procedures for bearer establishment, modification, and release. A key application is in the context of TrFO (Transcoder Free Operation) and TFO (Tandem Free Operation), where it helps negotiate the use of specific codecs and avoids unnecessary transcoding in the network path. It can also be used to negotiate and control network resources like IP addresses, port numbers, QoS markings, and even to trigger the establishment of dedicated bearers in underlying access networks (though this is often managed by other mechanisms like PCC).

How it works follows a request-response model embedded in SDP. One entity (the client) includes an IPBCP SDP attribute in its SDP offer, proposing certain bearer characteristics. The remote entity (the server) processes this request and includes a corresponding IPBCP attribute in its SDP answer, indicating acceptance, rejection, or an alternative proposal. This negotiation can cover aspects like the connection model (e.g., use of a Media Gateway, or direct endpoint-to-endpoint), the assignment of IP addresses and ports, and the agreement on a single codec to eliminate transcoding. In scenarios involving Media Gateways, the MGCF uses IPBCP to command the MGW to open specific ports and prepare for media reception/transmission with agreed parameters, thereby decoupling call signaling from direct media plane control.

Purpose & Motivation

IPBCP was created to address the need for explicit, dynamic control over the media bearer plane in IP-based telecommunications, which was not sufficiently covered by session description protocols alone. Standard SDP could describe media formats (codecs) but was limited in controlling how the underlying IP transport resources for that media were set up, especially in network-controlled scenarios involving media gateways and inter-operator interfaces. The protocol solves the problem of coordinated media bearer establishment between network nodes that are under the control of different functional entities (e.g., call control in MGCF, media processing in MGW).

Its development was motivated by the desire for efficient media handling and optimal resource utilization. For example, without IPBCP, codec negotiation might result in a suboptimal choice requiring a transcoder in the path, consuming extra processing and potentially degrading quality. IPBCP enables explicit agreement to use a common codec end-to-end (TrFO). Furthermore, in early IMS and CS-IMS interworking, it provided a standardized way for the MGCF to instruct a MGW to allocate specific IP/port resources, replacing vendor-proprietary control protocols. This ensured multi-vendor interoperability and allowed for more intelligent network-based media policy enforcement, contributing to overall network efficiency and service quality.

Key Features

• Bearer negotiation embedded within SDP offer/answer
• Support for TrFO/TFO through explicit codec agreement
• Control of media gateway resources (IP, ports)
• Negotiation of connection models (direct/indirect)
• Dynamic bearer establishment, modification, and release
• Enables network-controlled media resource allocation

Evolution Across Releases

Defining Specifications