Keep on sending stun binding requests on zero-cost networks.

webrtc/p2p/base/stunport.cc

Index: webrtc/p2p/base/stunport.cc
diff --git a/webrtc/p2p/base/stunport.cc b/webrtc/p2p/base/stunport.cc
index 8f37dd5218bb745b5e06ab6d8b2574faefc9aa44..a2f722b91b730ec6db17b90df5095ceae8731549 100644
--- a/webrtc/p2p/base/stunport.cc
+++ b/webrtc/p2p/base/stunport.cc
@@ -29,6 +29,9 @@ const int RETRY_TIMEOUT = 50 * 1000;    // ICE says 50 secs
 // (in milliseconds) because the connection binding requests should keep
 // the NAT binding alive.
 const int KEEP_ALIVE_TIMEOUT = 2 * 60 * 1000;  // 2 minutes
+// This maximum deadline is used for low cost networks in order to keep the NAT
+// binding alive on a backup connection.
+const uint32_t MAX_KEEP_ALIVE_DEADLINE = 0xFFFFFFFF;
 
 // Handles a binding request sent to the STUN server.
 class StunBindingRequest : public StunRequest {
@@ -397,8 +400,10 @@ void UDPPort::SendStunBindingRequest(const rtc::SocketAddress& stun_addr) {
   } else if (socket_->GetState() == rtc::AsyncPacketSocket::STATE_BOUND) {
     // Check if |server_addr_| is compatible with the port's ip.
     if (IsCompatibleAddress(stun_addr)) {
-      requests_.Send(new StunBindingRequest(this, stun_addr,
-                                            rtc::Time() + KEEP_ALIVE_TIMEOUT));
+      uint32_t deadline = (network_cost() == 0)

[juberti2, 2016/02/26 01:53:41]

I don't like the |deadline| terminology - it's really confusing what the
deadline applies to. Can we call this keepalive_duration (here and above)?

Also, this needs a comment explaining what this code is trying to do.

[honghaiz3, 2016/02/26 18:46:57]

Added comments. 
Renamed it to keepalive_lifetime. I feel that duration may have the confusion
with the duration between two requests.
Also I realize that Time() may wrap around, so use a negative lifetime value to
indicate infinite lifetime.  

+                              ? MAX_KEEP_ALIVE_DEADLINE
+                              : rtc::Time() + KEEP_ALIVE_TIMEOUT;
+      requests_.Send(new StunBindingRequest(this, stun_addr, deadline));
     } else {
       // Since we can't send stun messages to the server, we should mark this
       // port ready.