Implementation of rfc7234 caching in Varnish VCL https://tools.ietf.org/html/rfc7230 : Message Syntax and Routing https://tools.ietf.org/html/rfc7231 : Semantics and Content https://tools.ietf.org/html/rfc7234 : Caching https://tools.ietf.org/html/rfc5861 : HTTP Cache-Control Extensions for Stale Content
Old spec: http://tools.ietf.org/html/rfc2616