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

TL;DR

This paper analyzes the evolution of network routing scalability solutions, highlighting the recurring use of map-and-encap, the importance of local gains for adoption, and the limitations of current protocols like BGP.

Contribution

It provides four key observations on routing scalability, offering insights into designing future scalable network architectures.

Findings

Map-and-encap is a common solution across scalable delivery methods.

Early local gains drive adoption of new routing solutions.

BGP's lack of topological abstraction limits scalability solutions.

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.