## 3 System Design  
### CCSM Basic Components  
To provide the multicast service under UIN, CCSM introduces the Multicast Service Identifier (MSI) and Multicast Group Identifier (MGI) in access network and core network, respectively. MSI is used to identify the logical multicast group relationship which binds to a set of multicast members (In the following section, we use multicast member to represent multicast source and multicast receiver) identified by AIDs. While MGI is used to present a multicast delivery tree which consists of a set of ASR and CR. The introduction of MSI and MGI separates the multicast member management and multicast packet forwarding. The basic components of CCSM are shown in Figure 2, which consists of control plane, forwarding plane and user plane.  
The control plane mainly consists of AC, IDMS and MMC. AC is responsible for user access authentication for multicast source and multicast receiver. IDMS is used to manage the mapping between AID and RID, and MSI and MGI. MMC is responsible for multicast membership management  
Figure 2: The basic components of CCSM architecture.  
(registration, join and leave) and multicast routing construction. The core component of CCSM is MMC which acquires the core network topology, multicast member and related multicast service requirements, and builds the Shortest Path Tree (SPT) as multicast delivery tree for a given multicast group. As for the multicast tree, the root is selected based on the node centrality and the topology is updated in a fixed period so that SPT could always be efficient. We set a root router for a multicast group so that all sources in one group could reuse the SPT. MMC updates the multicast delivery tree in a central way and distributes the multicast routing table into ASR and CR.  
The forwarding plane consists of access network and core network. Access network is responsible for user attachment, and it uses AID to represent user device and MSI to represent multicast service, respectively. Core network is responsible for data forwarding which adopts RID to represent the data and MGI to represent multicast service, respectively. The ASR connects access network and core network, and it performs the mapping operations between AID and RID, and MSI and MGI with the assist of IDMS. The user plane consists of multicast sources and multicast receivers, which cooperates with AC to provide the multicast source and receiver authentication.  
Based on the CCSM architecture, the multicast membership management is implemented in access network and related group information are stored in MMC for further multicast routing construction. The multicast routing is handled by MMC which computes the multicast delivery tree and installs the multicast routing table in the related ASR and CR. The multicast data delivery between access network and core network performs the mapping and encapsulation procedures to separate the multicast membership management and multicast routing construction. With the help of IDMS, the mapping between MSI and MGI can be one-to-one, one-to-many and many-to-one for different application requirements. Besides, when the access network is multi-hop, the multicast forwarding in access network can base on proxy mechanism like IGMP/MLD proxying [66].