When it sends an initial connection request (e.g., a TCP "SYN" segment) or a datagram service request (e.g., a UDP-based query), the transport layer on a multihomed host needs to know which source address to use. If the application does not specify it, the transport layer must ask the IP layer to perform the conceptual mapping:
GET_SRCADDR(remote IP addr, TOS) -> local IP address
Here TOS is the Type-of-Service value (see Section 18.104.22.168), and the result is the desired source address.
The following rules are suggested for implementing this mapping:
In the future, there may be a defined way for a multihomed host to ask the gateways on all connected networks for advice about the best network to use for a given destination.
It will be noted that this process is essentially the same as datagram routing (see Section 3.3.1), and therefore hosts may be able to combine the implementation of the two functions.