Inter-server

In computer network protocol design, inter-server communication is an extension of the client–server model in which data are exchanged directly between servers. 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
 * The domain name system (DNS), which uses an inter-server protocol for zone transfers;
 * The Dynamic Host Configuration Protocol (DHCP);
 * FXP, allowing file transfer directly between FTP servers;
 * The Inter-Asterisk eXchange (IAX);
 * InterMUD;
 * The IRC, an Internet chat system with an inter-server protocol allowing clients to be distributed across many servers;
 * The Network News Transfer Protocol (NNTP);
 * The Protocol for SYnchronous Conferencing (PSYC);
 * SIP, a signaling protocol commonly used for Voice over IP;
 * SILC, a secure Internet conferencing protocol;
 * The Extensible Messaging and Presence Protocol (XMPP, formerly named Jabber).
 * ActivityPub a client/server API 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.