The HTTP-based Memento framework bridges the present and past Web.
It facilitates obtaining representations of prior states of a given
resource by introducing datetime negotiation and TimeMaps. Datetime
negotiation is a variation on content negotiation that leverages the
given resource's URI and a user agent's preferred datetime. TimeMaps
are lists that enumerate URIs of resources that encapsulate prior
states of the given resource. The framework also facilitates
recognizing a resource that encapsulates a frozen prior state of
another resource.
-- RFC 7089