In the X Window System core protocol, only four kinds of packets are sent, , over the network: requests, replies, events, and errors. "Requests" are sent by a client to the server to ask it to perform some operation (for example, create a new window) and to send back data it holds. "Replies" are sent by the server to provide such data. "Events" are sent by the server to notify clients of user activity or other occurrences they are interested in. "Errors" are packets sent by the server to notify a client of errors occurred during processing of its requests. Requests may generate replies, events, and errors; other than this, the protocol does not mandate a specific order in which packets are sent over the network. Some extensions to the core protocol exist, each one having its own requests, replies, events, and errors.