# A Hierarchical Mapping System for Flat Identifier to Locator Resolution Based on Active Degree
## 1 Introduction  
With the popularity of intelligent devices and diversified network services, the Internet has become one of the essential infrastructures of human society. However, it faces many challenges such as scalability, mobility, security, multi-homed, and many other challenges [1]. It is well known that the main cause of these challenges is IP semantic overload [2]. That is, an IP address represents both the ID (identifier) and Locator of an endpoint. Binding the ID of an endpoint with its locator results in conflicts with multi-homing, traffic engineering, mobility, and security. With newly emerging requirements in the industry, an architecture that follows the idea of ID/Locator separation is becoming more and more imperative [3]. Several recent schemes are proposed with the aim to replace the single IP namespace of the Internet with two distinct namespaces (i.e., a locator namespace and an identifier namespace). These schemes are divided into two categories according to their relationship with the existing Internet. One is the evolution of the existing Internet, such as HIP(Host Identity Protocol) [4], LIN6 [5], and LISP (Locator/ID Separation Protocol) [6], while another is a completely new, revolutionary network architecture, such as universal network (UN) [7], as shown in Figure 1.  
The UN consists of an access network (AN) and a core network (CN). Original IP addresses are changed to access identifiers (AIDs) for identifying endpoints and routing identifiers (RIDs) for locating endpoints in the Internet. Access identifiers must be replaced with RIDs when the packets enter a CN from an AN, and RIDs must be changed to AIDs when the packets leave from an CN. That is, a UN uses identifiers _replacing_ the packet's transmission, instead of _encapsulating_ it. Identifier replacing has some merits, as follows. Firstly, it supports mobility strongly like other ID/Locator separation schemes. Secondly, ANs do not know the locators and CNs dont know the IDs, which results in the attackers from ANs not being able to attack the endpoints of a CN. For example, a host attaching to its access router and which wants to attack by DDoS cannot control packets routed to the victim destination, and the attackers from the core part (sometimes an eavesdropper) cannot eavesdrop on the packet of a given user because they do not know the ID of the user. As a result, replacing addresses is more secure as compared to encapsulating packets. The key challenge of ID/Locator separation is how to design a resolution system that can provide a corresponding RID to an AR (Access Router) for a given AID when the packets enter the CN, and provides a corresponding AID to an AR for a given RID when the packets leave the CN. HIP [4] adds the host identity (HI) layer between transport layer and IP layer. The host identity tag (HIT) is used to identify the endpoint host, and IP is used only for routing in the network layer. HIP provides two main solutions for identifier resolving. The first solution uses dynamic DNS for resolving mapping. DNS always keeps the latest IP address and the corresponding HI/HIT for mobile endpoint host. There is no doubt that the host's frequent mobility will cause a sharp performance decline of DNS servers, resulting in poor performance and scalability. Another limitation is that it cannot accurately resolve mapping information when the source and destination hosts move at the same time. The second solution uses a rendezvous server (RS) for resolving mapping. It requires that each home network must be configured by RS, which causes that the HIP is not incrementally deployed. LIN6 [5] divides an IPv6 address into two parts: the identifier (i.e., LIN6 ID) and the exchange routing locator (i.e., LIN6 prefix). LIN6 ID is used in the upper layer for end-to-end communication, and the identifier-to-locator resolving is achieved by the protocol stack of the endpoint host and the mapping agency. However, LIN6 should be distinguished with MIPv6, which causes that the LIN6 namespace is limited too.