Difference between Pragma and Cache-Control headers?

I read about Pragma header on Wikipedia which says:

"The Pragma: no-cache header field is an HTTP/1.0 header intended for use in requests. It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource, not for the server to tell the browser not to cache the resource. Some user agents do pay attention to this header in responses, but the HTTP/1.1 RFC specifically warns against relying on this behavior."

But I haven't understood what it does? What is the difference between the Cache-Control header whose value is no-cache and Pragma whose value is also no-cache?

Pragma is the HTTP/1.0 implementation and cache-control is the HTTP/1.1 implementation of the same concept. They both are meant to prevent the client from caching the response. Older clients may not support HTTP/1.1 which is why that header is still in use.

There is no difference, except that Pragma is only defined as applicable to the requests by the client, whereas Cache-Control may be used by both the requests of the clients and the replies of the servers.

So, as far as standards go, they can only be compared from the perspective of the client making a requests and the server receiving a request from the client. The http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32 defines the scenario as follows:

HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in HTTP.

  Note: because the meaning of "Pragma: no-cache as a response
  header field is not actually specified, it does not provide a
  reliable replacement for "Cache-Control: no-cache" in a response

The way I would read the above:

• if you're writing a client and need no-cache:

  • just use Pragma: no-cache in your requests, since you may not know if Cache-Control is supported by the server;
  • but in replies, to decide on whether to cache, check for Cache-Control

• if you're writing a server:

  • in parsing requests from the clients, check for Cache-Control; if not found, check for Pragma: no-cache, and execute the Cache-Control: no-cache logic;
  • in replies, provide Cache-Control.

Of course, reality might be different from what's written or implied in the RFC!