In computer network protocol design, inter-server communication is an extension of the client–server model in which data are exchanged directly between servers.[1] In some fields server-to-server (S2S) is used as an alternative, and the term inter-domain can in some cases be used interchangeably.
Protocols
Protocols that have inter-server functions as well as the regular client–server communications include the following:
IPsec, secure network protocol that can be used to secure a host-to-host connection
ActivityPub a client/serverAPI for creating, updating and deleting content, as well as a federated server-to-server API for delivering notifications and content.
SMTP which accepts both MUA->MTA traffic, as well as MTA->MTA, but it is usually recommended that different ports are used for these actions
Some of these protocols employ multicast strategies to efficiently deliver information to multiple servers at once.