clarify "when a handshake is sent, it is sent on all known paths"

[dvanduzer]

Relevant to link and handshake but neither seems to explicitly state this.

[dvanduzer]

The handshake doc refers to a "keepalive handshake" so perhaps there is some additional qualification needed about transport maintenance?

[dvanduzer]

"At any point the transport being used to deliver packets may generate a keepalive handshake request which will start this process."

What are the circumstances a transport can/should do this, and what are the situations where a transport isn't allowed to do this? (e.g. if a Bluetooth radio suddenly reactivates, can it trigger a downgrade to CS1a because it's fresher than a wifi-based CS3a exchange?)

(maybe i'm also suggesting that exchange needs to track information about flapping CS & at values?)

edit: now i'm theorizing a logic bug in underspecified timeout behavior in udp and other transports.