They don't get into it, but the AIM host infrastructure was based on the event-based non-blocking libraries written by Lippke. The host complex was a distributed hash table (routed on relevant data, e.g. Screen name, ICQ number), with the extra step that you can migrate the in-memory state from one host to another while still taking incoming work. Quite a powerful infrastructure to build highly-available services. Only oddity is that you needed to have the clients (AIM clients and internal datacenter callers) have the ability to detach from one end point and re-attach to another end point when the bucket containing doing work for that client migrated to new hardware.

In the layer you were writing code, it was routed by the thing you hashed on, so it was sort of SDN like in its ability to separate the application state from the physical hardware. Amazingly good server technology. AOL had all kinds of cool tech - dynamically assigned (/24) subnets, IP Tunnels from the client, giant compression/caching farms that essentially cached all the static assets on the internet, etc. etc. In the 90s that is; those machines Eric was deploying onto were less powerful than your smart phone.