# An Adaptive Hierarchical Hybrid Multicast Based on Information-Centric Networking
## 2 Related Work  
In the stateless multicast, the root node (such as source, source-side edge router, etc.) needs to track the destinations (such as receiver, receiver-side edge router, etc.) of the group. Some stateless multicast methods directly attach the destination's information (such as the IP addresses of receivers) into the multicast packet header. Therefore, intermediate routers can forward the multicast packet according to the destination's information in the header without the need to maintain the forwarding state. In order to reduce the packet header length, other stateless multicast methods first compress the destinations information and then attach the compressed destinations information to the packet header.  
In Small Group Multicast (SGM) [17], Connectionless Multicast (CLM) [18], Xcast [5], and Multicast for Small Conferences (MSC) [19], the router divides the destinations into different sets according to the next hop of the destination. Then, it copies a packet for each set and forwards the packet with the appropriate header, which contains all the destinations in the set. Due to the limited size of the header, these protocols are only suitable for scenarios with a small number of receivers.  
Xcast+ [20] combines Xcast with the Internet Group Management Protocol (IGMP) [21] to improve scalability. By encoding the Designated Routers (DRs) at receivers' sides into the header instead of receivers, the size of the header can be reduced. Generalized Xcast (GXcast) [22] was proposed to solve the problem that these protocols do not support IP fragmentation.  
As another method similar to Xcast, Sender Initiated Multicast (SIM) [23] can reduce the forwarding cost in Xcast. SIM proposes to use Bitmap technology to compress the receiver address list carried in the packet header. SIM has two forwarding modes: list mode and preset mode. The source always appends the receiver list to the multicast packet in the list mode and periodically appends the receiver list to the packet in the preset mode. In the preset mode, the router that supports SIM protocol maintains a table similar to NFT to forward SIM packets, which severely limits the scalability of SIM.  
In order to reduce the header size, DOM [6] encodes the destinations into a bloom filter. Each router uses unicast IP routing information to determine the necessary copies and forwarding interfaces. However, the bloom filter has the problem of error rate, which leads to loops, packet storms, and other problems.  
BIER [7] is proposed by IETF to improve the scalability of multicast. The ingress router encapsulates a bit string to each multicast packet. Each bit of the bit string is mapped to an egress router. The intermediate router just forwards the BIER packet according to the Bit Index Forwarding Table (BIFT). However, when the number of egress routers is larger than the length of the bit string, BIER needs to divide these egress routers into different Set Identifiers (SIs) and duplicate a packet for each SI, which brings the problem of packet redundancy. Hierarchical Forwarding Bit Index Explicit Replication (HF-BIER) [8] was proposed to solve the packet redundancy problem of BIER in large-scale networks.  
Multicast Design for the MobilityFirst (MMF) [24] manages multicast groups by using the global name resolution service (GNRS) to map the multicast name to the addresses of receivers. MMF appends the names of receivers as additional destinations to the packet header and employs bitmap to compress the destinations. However, in MMF, the source needs to deal with the joining and leaving requests of each receiver, which brings great pressure on the source.