gateway: clarify Last-Modified

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
@@ -397,33 +419,23 @@ Returned directive depends on requested content path and format:
   TXT record.
   - Implementations MAY place an upper bound on any TTL received, as
     noted in Section 8 of :cite[rfc2181].
-  - If TTL value is unknown, implementations SHOULD not send a `Cache-Control`
-  - No matter if TTL value is known or not, implementations SHOULD always
-    send a [`Last-Modified`](#last-modified-response-header) header with the timestamp of the record resolution.
+  - Implementations MAY also place an lower bound of 1 minute to avoid excessive lookups.
+  - If TTL value is unknown, implementations MAY not send a `Cache-Control`
+    but SHOULD send [`Last-Modified`](#last-modified-response-header) header.
 
 ### `Last-Modified` (response header)
 
-Optional, used as additional hint for HTTP caching.
-
-Returning this header depends on the information available:
-
-- The header can be returned with `/ipns/` responses when the gateway
-  implementation knows the exact time a mutable pointer was updated by the
-  publisher.
-
-- When only TTL is known, [`Cache-Control`](#cache-control-response-header)
-  should be used instead.
+Used for HTTP caching, SHOULD be returned with mutable responses.
 
-- Legacy implementations set this header to the current timestamp when reading
-  TTL on  `/ipns/` content paths was not available. This hint was used by web
-  browsers in a process called  "Calculating Heuristic Freshness"
-  (Section 4.2.2 of :cite[rfc9111]). Each browser
-  uses different heuristic, making this an inferior, non-deterministic caching
-  strategy.
+An HTTP-date timestamp ([RFC9110, Section
+5.6.7](https://www.rfc-editor.org/rfc/rfc9110#section-5.6.7)) indicating when
+the resolution of requested content path occured, allowing HTTP proxies and CDNs
+to support inexpensive update checks via `If-Modified-Since`.
 
-- New implementations should not return this header if TTL is not known;
-  providing a static expiration window in `Cache-Control` is easier to reason
-  about than cache expiration based on the fuzzy “heuristic freshness”.
+When `Cache-Control` is not present, `Last-Modified` value also acts as a hint
+for web browsers in a process called  "Calculating Heuristic Freshness"
+(Section 4.2.2 of :cite[rfc9111]). Each browser uses different heuristic,
+making this an inferior, non-deterministic caching strategy.
 
 ### `Content-Type` (response header)
 

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.