Path MTU discovery and message fragmentation

SCTP is capable of Path Maximum Transmission Unit discovery, as outlined in RFC4821. Two specific alterations have been made to how SCTP handles MTU. First, that endpoints will have separate MTU estimates for each possible multi-homed endpoint. Second, that bundled message fragments (as explained below) will be directed based on MTU calculations, so that retransmissions (if necessary) will be sent without delay to alternate addresses.