HTTP Headers

How to use standard or custom HTTP headers

REST Resources
Entities SHOULD be served with an ETag header.
ETag, If-None-Match
Resource lifecycle
either If-Unmodified-Since or If-Match
Clients may specify a desired version as the v parameter of the Accept header
If the version was unspecified, the server should use the latest available version, and specify the Vary header
Internationalisation (i18n)
Localisation is inherently a representation concern, and HTTP mandates such concerns to be addressed using protocol headers.
Single-entity GET endpoints
Responses should include a Last-Modified or ETag header, and may include both
Servers may support the Accept-Encoding header for compression purposes, but this is not mandatory.