Add a flag to filter out high-cost networks.

webrtc/base/network.h

Index: webrtc/base/network.h
diff --git a/webrtc/base/network.h b/webrtc/base/network.h
index a41da4a69a0f01f41fd7344ce89d99b6f0e593d2..17c9c9d7475930ba69bfe13c1d5091d8f0228130 100644
--- a/webrtc/base/network.h
+++ b/webrtc/base/network.h
@@ -37,6 +37,8 @@ class Network;
 class NetworkMonitorInterface;
 class Thread;
 
+static const uint16_t kMaxNetworkCost = 999;
+static const uint16_t kNetworkCostForUnknownType = 10;

[pthatcher1, 2016/05/17 20:44:28]

I think kNetworkCostMax and kNetworkCostUnknown  would be a good pair of names. 

[honghaiz3, 2016/05/18 05:55:02]

Done.

 
 // By default, ignore loopback interfaces on the host.
 const int kDefaultNetworkIgnoreMask = ADAPTER_TYPE_LOOPBACK;
@@ -346,6 +348,17 @@ class Network {
   AdapterType type() const { return type_; }
   void set_type(AdapterType type) { type_ = type; }
 
+  uint16_t GetCost() const {
+    switch (type_) {
+      case rtc::ADAPTER_TYPE_UNKNOWN:
+        return kNetworkCostForUnknownType;
+      case rtc::ADAPTER_TYPE_CELLULAR:
+        return kMaxNetworkCost;

[pthatcher1, 2016/05/17 20:44:28]

Should we use kNetworkCostHigh == 900 or so just in case later we want something
with even higher cost?

[honghaiz3, 2016/05/18 05:55:02]

Done.
I initially chose cellular network cost to be the maximum cost 999 so that one
side can not choose a higher value (say 901) than the other side (choosing 900)
to use the other-side's cellular more often (when both cell-wifi and wifi-cell
connections exist). That may rarely happen because in most cases, we (webrtc)
control both sides. If you think that may still be an issue, let me know. I can
change it back. 

[honghaiz3, 2016/05/18 05:55:02]

Done.

+      default:
+        return 0;

[pthatcher1, 2016/05/17 20:44:28]

Would a kNetworkCostMin be a good constant?

[pthatcher1, 2016/05/17 20:44:28]

Wouldn't it be better to have the default be "unknown", with something like
this?

case rtc::ADAPTER_TYPE_CELLULAR:
   return kNetworkCostHigh;
case rtc::ADAPTER_TYPE_WIFI:
   return kNetworkCostLow;
case rtc::ADAPTER_TYPE_ETHERNET:
     return kNetworkCostMin;
default:
     return kNetworkCostUnknown;

[honghaiz3, 2016/05/18 05:55:02]

Done. I just listed all types we may have now. 

[honghaiz3, 2016/05/18 05:55:02]

Done.

+    }

[pthatcher1, 2016/05/17 20:44:28]

Would it make sense to make WiFi kNetworkCostLow == 1 an Ethernet
kNetworkCostMin so we slightly prefer Ethernet over WiFi?

[honghaiz3, 2016/05/18 05:55:02]

I choose Wifi cost to be 10 to have slightly more difference, although it won't
matter for now.
May be easier in the future if we need to add rtt as part of the cost.

+  }
+
   // A unique id assigned by the network manager, which may be signaled
   // to the remote side in the candidate.
   uint16_t id() const { return id_; }