Inter-server
In computing, inter-server is a technical term used in network protocol design to refer to the extension of the client-server model by having parts of a protocol which are only exchanged between the servers. In some fields the acronym S2S as in server-to-server is used to mean the same thing. Also the word inter-domain is sometimes used interchangeably.
Protocols that have inter-server functions as well as the regular client-server protocols include: DNS, which uses an inter-server protocol for zone transfers; FXP, allowing file transfer directly between FTP servers; IRC, an Internet chat system with an inter-server protocol allowing clients to be distributed across many servers; SIP and SILC.