The TCP/IP protocol stack uses IP addresses for two distinct roles: identifying hosts and locating their attachment points in the network topology. This dual purpose creates a fundamental tension that has led to routing and forwarding scalability challenges throughout the history of the Internet in unicast packet delivery and, more notably, in multicast delivery. This paper reviews the evolution of routing scalability solutions over the years and makes four observations. First, map-and-encap is a recurring architectural solution shared by all scalable unicast and multicast delivery methods, developed independently across different problem contexts. Second, a new solution tends to succeed when it can bring immediate local gains to early adopters without requiring coordination across administrative domains. Third, network routing and forwarding designs that depend on external factors, such as the number of distinct end sites or even application-specific deliveries, inherently preclude an upper bound on their scalability. Fourth, today's inter-domain routing protocol, BGP, lacks a topological abstraction equivalent to an egress router within a routing domain, thereby inherently preventing a map-and-encap solution for scalability. These observations offer insights into the design of future scalable routing system architectures.

翻译：TCP/IP协议栈使用IP地址承担两个不同的角色：标识主机以及定位其在网络拓扑中的连接点。这种双重作用形成了根本性矛盾，在互联网单播数据包传输乃至更显著的多播传输历史中，持续导致路由与转发可扩展性挑战。本文回顾了多年来路由可扩展性解决方案的演进历程，并提出四点观察。第一，映射封装是一种反复出现的架构性解决方案，被所有可扩展的单播和多播传输方法所共享，且这些方法在不同问题背景下独立发展而来。第二，新解决方案若能无需跨管理域协调即可为早期采用者带来即时本地收益，则更易获得成功。第三，依赖外部因素（如不同终端站点数量甚至特定应用传输）的网络路由与转发设计，本质上无法限定其可扩展性上限。第四，当今域间路由协议BGP缺乏相当于路由域内出口路由器的拓扑抽象，从而从根本上阻碍了采用映射封装方案解决可扩展性。这些观察为未来可扩展路由系统架构的设计提供了启示。