Wednesday, November 21, 2018

Round-trip time (RTT) is the time it takes for a client to send a request and the server to send a response over the network, not including the time required for data transfer.

RTT includes the back-and-forth time on the wire, but excludes the time to fully download the transferred bytes (and is therefore unrelated to bandwidth).

RTTs vary from less than one millisecond on a LAN to over one second in the worst cases, e.g. a modem connection to a service hosted on a different continent from the user. For small download file sizes, such as a search results page, RTT is the major contributing factor to latency on "fast" (broadband) connections. Therefore, an important strategy for speeding up web page performance is to minimize the number of round trips that need to be made. Since the majority of those round trips consist of HTTP requests and responses, it's especially important to minimize the number of requests that the client needs to make and to parallelize them as much as possible.