Can an persistent HTTP client send more than one request at a time?

Question

I am writing a HTTP proxy server and I noticed that many clients use the "Connection: Keep-Alive" header to keep a persistent connection. Is it possible that the client sends another HTTP request before the server processes the first?

For example, the client sends "GET / HTTP/1.1" but before the server has a chance to respond, the client sends "GET /favicon.ico HTTP/1.1". Is that possible? Or will the client pause for the response before sending the second request?

Also, when using a persistent connection, is it safe to assume all requests through that connection will have the same "Host: " header?

Answer 1

"Also, when using a persistent connection, is it safe to assume all requests through that connection will have the same "Host: " header?"

I don't think so, see HTTPbis P1, Section 2.2 :

Recipients MUST consider every message in a connection in isolation; because HTTP is a stateless protocol, it cannot be assumed that two requests on the same connection are from the same client or share any other common attributes. In particular, intermediaries might mix requests from different clients into a single server connection. Note that some existing HTTP extensions (eg, [RFC4559]) violate this requirement, thereby potentially causing interoperability and security problems.

Answer 2

Yes, it is possible for the client to pipeline requests. (See http://en.wikipedia.org/wiki/HTTP_pipelining ).

Turning your last question around... it would not be safe for a client to assume that requests to multiple hosts would be served by a single pipeline. There may be no specs that directly address your question on the Host: header, but it's a safe bet they'll be the same.

Answer 3

Regarding the first question:

Is it possible that the client sends another HTTP request before the server processes the first?

I believe that yes, it can be possible (perhaps I am wrong, I remembered having read that a couple of years ago; the definitive answer is in the HTTP protocol specifications). But I don't understand why you are asking. Also, the client can open several TCP connections at once to the same HTTP server. And of course you have many simultaneous clients.

About the second question

Also, when using a persistent connection, is it safe to assume all requests through that connection will have the same "Host: " header?

I believe it is usually the case, but I won't assume that to be certain. I could imagine that some clever HTTP clients, recognizing that two URL with different Host: headers share the same IP, could re-use the same connection.

But I don't understand why you are asking. Persistent HTTP connections have been invented to minimize the TCP connections which are costly, and the two questions you are asking are an extreme point on that. Perhaps few HTTP clients are doing what you describe today.

And you should be strict on what you send (wrt standard conformance), but flexible on what you accept receiving.