Does empty “Expect:” header mean anything?

Question

Many libraries include Expect: 100-continue on all HTTP 1.1 POST and PUT requests by default.

I intend to reduce perceived latency by removing 100-continue mechanism on the client side on those requests for which I know the expense of sending data right away is less than waiting a roundtrip for 100-continue, namely on short requests.

Of course I still want all the other great features of HTTP 1.1, thus only I want to kill Expect: 100-continue header. I have two options:

• remove expect header entirely, or
• send empty expect header, Expect:\r\n

Is there ever any difference between the two?

Any software that might break for one or the other?

Answer 1

Nothing should break if you remove the Expect header, but I know that Microsoft IIS has had issues with 100 Continue in the past. For example, IIS5 always sends 100 continue responses. So, I wonder if at least some of the uses of it in libraries might be to work around similarly broken behaviour in servers.

Many libraries seem to set this header and then not actually handle 100 Continue properly - e.g. they begin to send the request body immediately without waiting for a 100 Continue and then don't handle the fact that the server might send back any HTTP error code before they've finished sending the request body (the first part's OK, it's the second part which is broken - see later in my answer). This leads me to believe that some authors have just copied it from elsewhere without fully understanding the subtleties.

I can't see any reason to include a blank Expect header - if you're not going to include 100-continue (or some other Expect clause) then omit the header entirely. The only reason to include it would be to work around broken webservers, but I'm not aware of any which behave in this way.

Finally, if you're just looking to reduce roundtrip latencies it seems to me that it wouldn't actually be inconsistent with the RFC to simply begin to transmit the request body immediately. You're not supposed to wait indefinitely to send the request body (as per the RFC), so you're behaving to the spec - it's just your timeout before sending anyway is zero.

You must be aware that servers are at liberty to not send the 100 Continue response if they've already received some of the request body, so you have to handle servers which send 100 Continue, those which send nothing and wait for the full request and those which immediately send any HTTP error code (which may be 417, but more likely a generic 4xx code). In this way, your short requests shouldn't have any overhead (aside from the Expect header) but you won't have to wait for the 100 Continue. Of course, for this approach to work you'll need to be doing things in a way which lets you interrupt the request as soon as the server returns an error code (e.g. non-blocking IO with poll() or select()).

Doing things this way might help keep your code more consistent between small and large requests while reducing the latency. The downside is that it's perhaps not what the RFC authors had in mind, even if it doesn't explicitly violate any of the requirements. Also, it might make your later code more complicated if you're not already doing non-blocking IO or similar.