Add network cost as part of the connection comparison.

webrtc/p2p/base/stun.h

Index: webrtc/p2p/base/stun.h
diff --git a/webrtc/p2p/base/stun.h b/webrtc/p2p/base/stun.h
index 75b89afb8a472b80f7dc6a8d8a2852df8bfb1409..f7225b227d8113858426eac66c5103942a27ce8e 100644
--- a/webrtc/p2p/base/stun.h
+++ b/webrtc/p2p/base/stun.h
@@ -604,6 +604,7 @@ class TurnMessage : public StunMessage {
 enum IceAttributeType {
   STUN_ATTR_PRIORITY                    = 0x0024,  // UInt32
   STUN_ATTR_USE_CANDIDATE               = 0x0025,  // No content, Length = 0
+  STUN_ATTR_NETWORK_COST                = 0x0026,  // UInt32

[pthatcher1, 2016/02/04 23:16:21]

According to RFC 5389, section 18.2, we have to choose a value in the range
0xC000 - 0xFFFF and make sure no one else is using it.

   STUN_ATTR_ICE_CONTROLLED              = 0x8029,  // UInt64
   STUN_ATTR_ICE_CONTROLLING             = 0x802A   // UInt64
 };
@@ -619,7 +620,9 @@ class IceMessage : public StunMessage {
  protected:
   virtual StunAttributeValueType GetAttributeValueType(int type) const {
     switch (type) {
-      case STUN_ATTR_PRIORITY:        return STUN_VALUE_UINT32;
+      case STUN_ATTR_PRIORITY:
+      case STUN_ATTR_NETWORK_COST:

[pthatcher1, 2016/02/04 23:16:21]

32-bits is overkill.  Let's make it uint8.  We'll have to add a little code to
parse a uint8, but that should be trivial.

Actually, I'm tempted to save a byte by making us have no-content and instead
have two attributes: one for HIGH and one for LAST_RESORT, just like
CONTROLLED/CONTROLLING.  Let's ask Justin what he thinks about that. It might be
an over-optimization.

[juberti2, 2016/02/04 23:31:10]

attribs are padded to 32 bits, so no point in going smaller than 32 bits.

[pthatcher1, 2016/02/04 23:51:18]

Yes, I forgot about that.  It's pretty lame when you're trying to save bytes.  
4 bytes seems like a lot to waste.  Should we make the default non-zero so that
cellular networks are the one that can avoid sending it and WiFi networks are
the one that needs to send it?

+        return STUN_VALUE_UINT32;
       case STUN_ATTR_USE_CANDIDATE:   return STUN_VALUE_BYTE_STRING;
       case STUN_ATTR_ICE_CONTROLLED:  return STUN_VALUE_UINT64;
       case STUN_ATTR_ICE_CONTROLLING: return STUN_VALUE_UINT64;