DVB-mABR (Multicast Adaptive Bitrate)
Adaptive media streaming over IP multicast
Video delivery has become a dominant class of traffic on public networks. The wider market has embraced unicast streaming with the ability to adapt to network conditions as a means of delivering media on any type of access network. One of the reasons for its widespread adoption is the reuse of existing network technologies used to deliver other Internet services, in particular HTTP and Content Delivery Networks. Dynamic bit rate adaptation allows the streaming session to degrade gracefully as network conditions worsen, and to recover as they improve.
For consumption of the same linear media stream at the same time by a large audience, the number of simultaneous connections to the edge serving infrastructure carrying the same media payloads results in a high degree of redundancy which can be mitigated by the use of multicast packet replication at Layer 3. Unicast streaming is better suited to unsynchronised media consumption, or consumption of linear streams by smaller audiences.
By combining existing media encoding and packaging formats with the efficiency of point-to-multipoint distribution to the edge of IP-based access networks, it is possible to design a system for linear media distribution that is both efficient and scalable to very large audiences, while remaining technically compatible with the largest possible set of already-deployed end user equipment.
Point-to-multipoint topologies also offer opportunities for efficient pre-positioning of assets to devices at the edge of the network. This supports additional non-linear use cases and can help to alleviate peak demand on the access network at synchronisation points in the linear schedule.
This document specifies a reference functional architecture for an end-to-end system that delivers linear content over Internet Protocol (IP) networks in a scalable and standards-compliant manner. Scalability is achieved by means of IP multicast operating in parallel with and alongside conventional unicast delivery. The individual functions required for such a system are depicted in Figure 4.2-1 in clause 4.2, and the interactions between them are shown as named reference points. The functional architecture is intended as an abstract reference: real implementations may, for example, combine multiple functions in a single deployable unit. The architecture is intended to be independent of any particular Internet Protocol address family.
Two mandatory multicast media transport protocols are specified in annex F and annex H.
- Every conformant implementation of the Multicast server shall implement at least one of the mandatory multicast media transport protocols.
- Every conformant implementation of the Multicast gateway shall implement at least one of the mandatory multicast media transport protocols.
- Every conformant implementation of the Unicast repair server implementing reference point M shall implement at least one of the mandatory multicast media transport protocols.
In the following clauses, references to HTTP refer to the Hypertext Transfer Protocol message exchange semantics. Implementations shall, at minimum, conform to HTTP/1.1 protocol syntax as specified in RFC 7230 and may optionally implement subsequent protocol syntaxes, for example HTTP/2.
Similarly, references to HTTPS refer to HTTP/1.1 over TLS 1.3. Implementations may optionally implement equivalent or better transport layer security protocols.
References to HTTP(S) mean “HTTP or HTTPS”.