gateway: document If-Modified-Since

src/http-gateways/path-gateway.md

@@ -113,7 +113,7 @@ All request headers are optional.
 
 ### `If-None-Match` (request header)
 
-Used for HTTP caching.
+Optional, used for HTTP caching.
 
 Enables advanced cache control based on `Etag`,
 allowing client and server to skip data transfer if previously downloaded
@@ -123,6 +123,22 @@ The Gateway MUST compare Etag values sent in `If-None-Match` with `Etag` that
 would be sent with response. Positive match MUST return HTTP status code 304
 (Not Modified), without any payload.
 
+### `If-Modified-Since` (request header)
+
+Optional, used for HTTP caching of mutable resources.
+
+Enables advanced cache control based on `Last-Modified`,
+allowing client and server to skip data transfer if previously downloaded
+payload did not change.
+
+The Gateway SHOULD compare HTTP-date value sent in `If-Modified-Since` with `Last-Modified` that
+would be sent with response. Positive match MUST return HTTP status code 304
+(Not Modified), without any payload.
+
+While  `Last-Modified` SHOULD be implemented by the gateway itself,
+implementation and handling of this header MAY be delegated to a third-party
+HTTP software such as CDN, reverse proxy, or load-balancer.
+
 ### `Cache-Control` (request header)
 
 Used for HTTP caching.
@@ -272,6 +288,12 @@ Indicates permanent redirection.
 
 The new, canonical URL is returned in the [`Location`](#location-response-header) header.
 
+### `304` Not Modified
+
+Indicates indicates that there is no need to retransmit the requested resources.
+
+See: [`Last-Modified`](#last-modified-response-header).
+
 ### `400` Bad Request
 
 A generic client error returned when it is not possible to return a better one

src/http-gateways/trustless-gateway.md

@@ -65,6 +65,8 @@ Same as in :cite[path-gateway], but with limited number of supported response ty
 
 ## Request Headers
 
+For HTTP Caching, follow `Cache-Control`, `If-Modified-Since`, and `If-None-Match` from :cite[path-gateway].
+
 ### `Accept` (request header)
 
 A Client SHOULD send this HTTP header to leverage content type negotiation
@@ -189,6 +191,8 @@ Below MUST be implemented **in addition** to "HTTP Response" of :cite[path-gatew
 
 ## Response Headers
 
+For HTTP Caching, follow `Cache-Control`, `Last-Modified`, and `Etag` from :cite[path-gateway].
+
 ### `Content-Type` (response header)
 
 MUST be returned and include additional format-specific parameters when possible.