Chapter |Overlay|: Overlay Networks
An overlay is a logical network implemented on top of some underlying network. By this definition, the Internet started out as an overlay network; a packet-switched network implemented on top of the circuits provided by the old telephone network. Eventually those links from the telephone network were replaced with point-to-point links directly connecting the backbone routers. The Internet itself became the foundational network on top of which new overlays could be built.
:numref:`Figure %s <fig-overlay-net>` depicts an overlay implemented on top of an underlying network. Each node in the overlay processes and forwards packets in an application-specific way. Because they run arbitrary code, these overlay nodes are usually servers rather than switches; these server are often co-located with a switch in the underlying network. The links that connect the overlay nodes are implemented as logical links (e.g., tunnels) through the underlying network.
Overlay network layered on top of a physical network.
Overlays can be viewed as forming a network with some topology. The example in :numref:`Figure %s <fig-overlay-net>` is a subgraph of the underlying network, but another common configuration is for the overlay to form a fully-connected graph. This is easy to do when the underlying network is the Internet, since the Internet supports a logical connection between every pair of hosts. We raise this point because, while some overlays do include routing as part of their "application logic", in other cases, the whole purpose of the overlay is to give the application multiple points-of-presence. This improves client-experienced latency (the application is likely to be accessible at a nearby site) and aggregate application throughput (many application sites can be accessed in parallel).
Overlays are an integral part of the software ecosystem running at the edge of the Internet. Like transport protocols that run on individual hosts in support of application programs, overlays run on a distributed collection of edge hosts, also in support of applications. They are a critical part of the Internet's approach to both scaling applications and making them more resilient. This chapter looks at three widely-used examples: Content Distribution Networks (CDNs), the multicast overlays at the heart of video conferencing applications such as Zoom, and peer-to-peer file sharing networks such as BitTorrent. These three examples serve as case studies, helping to illustrate a range of strategies available to applications.