The highest server in the hierarchy is sometimes called the origin server—the application server on which a given resource resides or is to be created.
[1] The inverse term, downstream server, is rarely used.
The terms are exclusively used in contexts where requests and responses move in opposite ways.
For example, in the domain name system, a name server in a company's local area network often forwards requests to the internet service provider's (ISP's) name servers, instead of resolving the domain name directly — it can be said that the ISP's name servers are upstream to the local server.
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