From Map-and-Encap to BIER: Observations on Network Routing Scalability

2026-05-07

AI Summary

This study addresses the scalability bottleneck in unicast and multicast routing caused by the dual role of IP addresses as both identifiers and locators. It systematically traces the evolution of Internet routing scalability solutions, first articulating the map-and-encap architecture as a unifying paradigm and identifying the essential conditions for its successful deployment. Through historical protocol analysis, architectural comparisons, and conceptual abstraction—encompassing approaches such as BIER and tunnel encapsulation—the work reveals that BGP’s lack of intra-domain egress router topology abstraction is a fundamental limitation. The paper proposes core principles to guide future scalable routing designs, emphasizing the critical roles of locally driven incentives and effective topology abstraction in protocol evolution.

Abstract

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.

Problem

• routing scalability
• IP addressing
• multicast delivery
• map-and-encap
• BGP

Innovation

• map-and-encap
• routing scalability
• multicast
• BGP
• topological abstraction