Describe an existing example of a standard which allows future extensions, and where client and server can agree on which of a larger set of extensions both support.
The ESMTP (2) extension to SMTP (1 without ESMTP) starts with agreeing to use ESMTP (and not basic SMTP) (1) and then the server sends a list of which extensions it supports (3). The client then selects those services it wants to use, and which the server supports, and can use them in the rest of the session.
To some extent, XML can also be seen as such a standard, since a client can send new types of XML by providing its description through DTD or Schema, CSS and XSLT.
HTTP has a mechanism by which the client can tell the server which media types and languages it prefers, which also can be seen as such an extensible standard.
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