Add network cost as part of the connection comparison.

talk/app/webrtc/webrtcsdp.cc

Index: talk/app/webrtc/webrtcsdp.cc
diff --git a/talk/app/webrtc/webrtcsdp.cc b/talk/app/webrtc/webrtcsdp.cc
index f5fb7f78e3334edd7f320d6f3e89f49b25132c4f..57e235a01e85372fc21690d3e9f79ff7cd4e6771 100644
--- a/talk/app/webrtc/webrtcsdp.cc
+++ b/talk/app/webrtc/webrtcsdp.cc
@@ -143,6 +143,7 @@ static const char kAttributeCandidateRport[] = "rport";
 static const char kAttributeCandidateUfrag[] = "ufrag";
 static const char kAttributeCandidatePwd[] = "pwd";
 static const char kAttributeCandidateGeneration[] = "generation";
+static const char kAttributeCandidateCost[] = "cost";
 static const char kAttributeFingerprint[] = "fingerprint";
 static const char kAttributeSetup[] = "setup";
 static const char kAttributeFmtp[] = "fmtp";
@@ -1079,6 +1080,7 @@ bool ParseCandidate(const std::string& message, Candidate* candidate,
   std::string username;
   std::string password;
   uint32_t generation = 0;
+  uint32_t cost = 0;
   for (size_t i = current_position; i + 1 < fields.size(); ++i) {
     // RFC 5245
     // *(SP extension-att-name SP extension-att-value)
@@ -1090,6 +1092,10 @@ bool ParseCandidate(const std::string& message, Candidate* candidate,
       username = fields[++i];
     } else if (fields[i] == kAttributeCandidatePwd) {
       password = fields[++i];
+    } else if (fields[i] == kAttributeCandidateCost) {
+      if (!GetValueFromString(first_line, fields[++i], &cost, error)) {
+        return false;
+      }
     } else {
       // Skip the unknown extension.
       ++i;
@@ -1101,6 +1107,7 @@ bool ParseCandidate(const std::string& message, Candidate* candidate,
                          generation, foundation);
   candidate->set_related_address(related_address);
   candidate->set_tcptype(tcptype);
+  candidate->set_network_cost(std::min(cost, cricket::kMaxNetworkCost));

[pthatcher1, 2016/02/04 23:16:20]

I think we should pick "cost" or "network cost" but not both.  I'd say just go
with "cost".  A client might choose to use this for TURN vs. not as well.

[honghaiz3, 2016/02/05 01:36:46]

Discussed in other comments.

[honghaiz3, 2016/02/11 21:12:28]

I think it might be good to put "cost" on the signaling to save a few bytes. For
the rest, I am using NetworkCost. 

   return true;
 }
 
@@ -1775,6 +1782,7 @@ void BuildCandidate(const std::vector<Candidate>& candidates,
     if (include_ufrag && !it->username().empty()) {
       os << " " << kAttributeCandidateUfrag << " " << it->username();
     }
+    os << " " << kAttributeCandidateCost << " " << it->network_cost();
 
     AddLine(os.str(), message);
   }