Techniques are disclosed for a web server to support constraints specified by a client. In one embodiment, the web server receives, from the client, a request for one or more blocks of data. The request includes one or more constraints provided by the client. The web server may perform an action responsive to an increase in network congestion, based on the one or more constraints. Accordingly, the web server may handle the request in a manner that more closely meets the needs of the client.