And the server sends the new entity-body

Tersely, this header is to If-None-Match and ETag as If-Unmodified-Since is to If-Modified-Since and Last-Modified.

If-Modified-Since

If-None-Match

Type: Request header.

Importance: Low.

This header is used to make a conditional partial GET request. The value of the header comes from the ETag or Last-Modified response header from a previous range request. The server sends the new range only if that part of the entity-body has changed. Oth-erwise the server sends a 304 (“Not Modified”), even if something changed elsewhere in the entity-body.

Importance: Medium.

Normally a client uses the value of the response header Last-Modified as the value of the request header If-Modified-Since to perform a conditional GET request. This header also takes the value of Last-Modified, but it’s usually used for making HTTP actions other than GET into conditional actions.

Type: Response header.

Importance: Very high.

Importance: Very high.

398 | Appendix C:The HTTP Header Top Infinity