In various embodiments, data processing apparatus, software, or machine-implemented methods can optimize NFSv3 asynchronous write requests or MSRPC calls that traverse a wide area network, for example, by receiving, from a client, a first request directed to a server across a wide area network; determining whether a related second request has been received previously; when a related second request has been received previously, sending, to the client, a first reply to the second request and forwarding the first request to the server, and otherwise forwarding the first request to the server without sending any reply to the client for the first request. Sending local replies from a WAN optimizer induces the client to send continuous requests, improving throughput, but at least one client request remains unreplied to, and one server error reply is always reported to the client, facilitating correct error processing at the client.