## 2 A New Routing Architecture
### Previous Approaches  
There is a multitude of architectures for the future Internet and, in particular, for a new routing and addressing architecture, trying to solve the scalability problem of today's Internet [12]. In the following, we review prominent architectures in this research area, show which design options have been chosen, and address some of their shortcomings.  
The Identifier Locator Network Protocol (ILNP) [13, 14, 15] is similar to GLI-Split in the sense that it splits the IPv6 address into a locator and identifier part, but there are many differences. With ILNP, applications are expected to identify nodes only by fully qualified domain names (FQDNs) and the Domain Name System (DNS) resolves them to possibly several addresses containing the unique identifier of a node and a locator. The lookup is done by the hosts and no gateway interaction is required. Hosts must be upgraded to take advantage of ILNP since gateways cannot take over partial functions as in GLI-Split. GLI-Split and ILNP have evolved from the early ideas of GSE (global, site, and end-system address elements) [16, 17]. GSE essentially codes a global locator, a local locator, and an identifier into an IPv6 address. Addresses are dynamically combined from these parts. It uses only the identifier for TCP checksum calculation and requires host upgrades for deployment.  
The Hierarchical Architecture for Internet Routing (HAIR) [8] is a clean-slate approach and does not need address rewriting by border routers. It implements source routing in the sense that the hosts compose destination addresses containing global locator, local locator, and identifier information. That requires host upgrades since hosts need to perform mapping lookups for that purpose.  
There are several proposals that are intended to be incrementally deployed in the Internet. The locator/identifier separation protocol (LISP) [18, 19] is the most prominent of them. The IP address of a LISP-gateway is a global locator and routable in the Internet backbone. Addresses of LISP-nodes inside LISP-domains are locally routable endpoint identifiers. Interworking with the classic Internet may be done using complex stateful NAT or proxy gateways. LISP-nodes can send packets directly to classic nodes in the general Internet outside LISP-domains. When a classic node sends packets to a node within a LISP-domain, the packets are forwarded by default to a proxy router in the Internet which looks up appropriate global locators and tunnels the packets to the destination LISP-domain using this locator information. Proxy routers have two major disadvantages. First, traffic cannot take the shortest AS-path but takes a detour via the proxy (triangle routing). Second, they attract and forward large data volumes and it is not clear who pays for it. Similar interworking solutions exist also for other map-and-encaps proposals. GLI-Split is intentionally designed to avoid these problems.  
The Host Identity Protocol (HIP) [20] also implements the Loc/ID split. However, its intention is rather enhanced anonymity, security, and mobility instead of improved routing scalability. It could be used on top of other approaches to combine their advantages.  
NPTv6 [21] describes network address translation between IPv6 addresses. GLI-gateways could take advantage of this specification. In this light, GLI-Split resembles large-scale NAT for edge networks, but there are significant differences. The NAT operation performed by GLI-gateways is stateless and nodes in GLI-domains are reachable by global addresses. GLI-Split even improves their reachability beyond provider changes.