TBD: The Session Initiation Protocol (SIP) supports multiple transports running both over IPv4 and IPv6 protocols. In more and more cases, a SIP user agent (UA) is connected to network interfaces with multiple address families. In these cases sending a message from a dual stack client to a dual stack server may suffer from the issues described in [RFC6555] (“Happy Eyeballs”): the UA attempts to send the message using IPv6, but IPv6 connectivity is not working to the server. This can cause significant delays in the process of sending the message to the server. This negatively affects the user’s experience. TBD: This document builds on [RFC6555] by modifying the procedures specified in [RFC3263] and related specifications to require that a client ensure that communication targets are accessible before sending messages to them, to allow a client to contact targets out of the order required by other specifications, and to require a client to properly distribute the message load among targets over time.