The Stream Control Transmission Protocol (SCTP) is a message oriented transport protocol supporting arbitrary large user messages. This document adds a new data chunk to SCTP. This allows a sender to interleave different user messages that would otherwise result in head of line blocking at the sender. Whenever an SCTP sender is allowed to send user data, it may choose from multiple outgoing SCTP streams. Multiple ways for performing this selection, called stream schedulers, are defined. A stream scheduler can choose to either implement, or not implement, user message interleaving.