implement tcp.Policy and refactor rto to use it

tcp/policy.go

@@ -1,15 +1,24 @@
 package tcp
 
+// Policy observes segment traffic and steers transmit behaviour: RTO,
+// congestion control and the like (discussion #157). The tcp package holds no
+// clock, so a Policy needing time carries its own (issue #140).
+// Introspection stays off the interface; put it on the concrete type.
 type Policy interface {
+	// Reset returns the Policy to its pre-connection state. Called on every
+	// (re)open and Abort. Must preserve configuration such as a clock.
 	Reset()
 	// PreTx is called before writing to a frame.
 	// The outgoing frame options can be set by the Policy and will be respected if Frame offset >5.
+	// rtxFrom is ignored unless within [snd.UNA, snd.NXT]. Nothing is committed
+	// until PostTx: a transmit attempt may emit no segment at all.
 	PreTx(h *Handler, outgoingOpts Frame) (rtxFrom Value, retransmit, holdNew bool)
 	// PreRx is called by [Handler] on every incoming segment.
 	// PreRx can choose to drop segment if it returns keep=false.
 	PreRx(h *Handler, incoming Frame) (keep bool)
 	// PostRx is called by [Handler] after accepting an incoming segment.
+	// TODO: congestion control will also want the pre-Recv snd.UNA here.
 	PostRx(h *Handler, prevState State, accepted Frame)
-	// PostTx called on leaving the transmit path.
+	// PostTx called on leaving the transmit path with the fully written frame.
 	PostTx(h *Handler, outgoing Frame)
 }