net/netcheck, tailcfg: add DERPHomeParams and use it

This allows providing additional information to the client about how to
select a home DERP region, such as preferring a given DERP region over
all others.

Updates #8603

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I7c4a270f31d8585112fab5408799ffba5b75266f

tailcfg/derpmap.go

@@ -7,6 +7,11 @@ import "sort"
 
 // DERPMap describes the set of DERP packet relay servers that are available.
 type DERPMap struct {
+	// HomeParams, if non-nil, is a change in home parameters.
+	//
+	// The rest of the DEPRMap fields, if zero, means unchanged.
+	HomeParams *DERPHomeParams `json:",omitempty"`
+
 	// Regions is the set of geographic regions running DERP node(s).
 	//
 	// It's keyed by the DERPRegion.RegionID.
@@ -16,6 +21,8 @@ type DERPMap struct {
 
 	// OmitDefaultRegions specifies to not use Tailscale's DERP servers, and only use those
 	// specified in this DERPMap. If there are none set outside of the defaults, this is a noop.
+	//
+	// This field is only meaningful if the Regions map is non-nil (indicating a change).
 	OmitDefaultRegions bool `json:"omitDefaultRegions,omitempty"`
 }
 
@@ -29,6 +36,25 @@ func (m *DERPMap) RegionIDs() []int {
 	return ret
 }
 
+// DERPHomeParams contains parameters from the server related to selecting a
+// DERP home region (sometimes referred to as the "preferred DERP").
+type DERPHomeParams struct {
+	// RegionScore scales latencies of DERP regions by a given scaling
+	// factor when determining which region to use as the home
+	// ("preferred") DERP. Scores in the range (0, 1) will cause this
+	// region to be proportionally more preferred, and scores in the range
+	// (1, ∞) will penalize a region.
+	//
+	// If a region is not present in this map, it is treated as having a
+	// score of 1.0.
+	//
+	// Scores should not be 0 or negative; such scores will be ignored.
+	//
+	// A nil map means no change from the previous value (if any); an empty
+	// non-nil map can be sent to reset all scores back to 1.0.
+	RegionScore map[int]float64 `json:",omitempty"`
+}
+
 // DERPRegion is a geographic region running DERP relay node(s).
 //
 // Client nodes discover which region they're closest to, advertise