In your example sending the file body will fill the remainder of the packet containing the headers and trigger a packet send. The client waiting for your respon

In your example sending the file body will fill the remainder of the packet containing the headers and trigger a packet send. The client waiting for your response isn't going to lose out. In fact, they'll benefit from having file body data in that first packet. Your example is when you want Nagles algorithm to apply. This exact use case is mentioned on tcp(7) under the TCP_CORK option, which is like a more explicit, application controlled, version of Nagle. Nagles algorithm sucks for HTTP clients who happen to send big HTTP requests. If the HTTP headers being sent are >1400 bytes or so, then the tail end of the request will wait 1 RTT for the ACK of the first part. This will be particularly bad if the HTTP server has delayed-ACKs enabled (TCP_QUICKACK is not on). The server TCP stack has no idea that the HTTP server application hasn't yet got a full request and can't yet respond, so will delay the ACK of the first 1400 request bytes in the hope that it will. The delayed ACK delay is probably small compared to the RTT, so Nagle still dominates