Mojo UDP socket API definition review.

mojo/services/public/interfaces/network/udp_socket.mojom

Index: mojo/services/public/interfaces/network/udp_socket.mojom
diff --git a/mojo/services/public/interfaces/network/udp_socket.mojom b/mojo/services/public/interfaces/network/udp_socket.mojom
index b5b848d9c764723f1a9cdcf19761178e95bf8872..526a454069ef91dfbedf260dd0d9884a08f2e463 100644
--- a/mojo/services/public/interfaces/network/udp_socket.mojom
+++ b/mojo/services/public/interfaces/network/udp_socket.mojom
@@ -13,8 +13,9 @@ module mojo {
 // - (optional) Set options which are allowed prior to Bind().
 // - Bind the socket.
 // - (optional) Set options which are allowed after Bind().
-// - Send / request to receive packets. Received packets will be delivered to
-//   UDPSocketClient.OnReceived().
+// - Send / request to receive datagrams. Received datagrams will be delivered
+//   to UDPSocketClient.OnReceived().
+
 [Client=UDPSocketClient]
 interface UDPSocket {
   // Allows the socket to share the local address to which it will be bound with
@@ -28,47 +29,80 @@ interface UDPSocket {
   // returned in |bound_addr|.
   Bind(NetAddress addr) => (NetworkError result, NetAddress? bound_addr);
 
-  // Sets the send buffer size (in bytes) for the socket. The socket must be
+  // Sets the OS send buffer size (in bytes) for the socket. The socket must be
   // bound.
-  //
-  // Note: This is only treated as a hint. Even if it succeeds, the service
-  // doesn't guarantee it will conform to the size.
   SetSendBufferSize(uint32 size) => (NetworkError result);
 
-  // Sets the receive buffer size (in bytes) for the socket. The socket must be
-  // bound.
-  //
-  // Note: This is only treated as a hint. Even if it succeeds, the service
-  // doesn't guarantee it will conform to the size.
+  // Sets the OS receive buffer size (in bytes) for the socket. The socket must
+  // be bound.
   SetReceiveBufferSize(uint32 size) => (NetworkError result);
 
-  // Notifies that the client is ready to accept |number| of packets.
+  // Negotiates the maximum number of pending SendTo() requests. If
+  // |requested_size| is set to 0, this method queries the current settings.
+  //
+  // The service stores SendTo() requests in a queue while they are waiting to
+  // be executed (i.e., while they are waiting to be placed in the OS send
+  // buffer and sent out). This method negotiates how many requests (not bytes)
+  // this queue is able to store. If the queue is full, the service fails new
+  // requests directly with error code ERR_INSUFFICIENT_RESOURCES and discards
+  // those datagrams. If the client wants to avoid such failures, it needs to
+  // keep track of how many SendTo() calls are pending and make sure the number
+  // doesn't exceed the result of this method.
+  NegotiateMaxPendingSendRequests(uint32 requested_size)
+      => (uint32 actual_size);
+
+  // Notifies that the client is ready to accept |number| of datagrams.
   // Correspondingly, OnReceived() of the UDPSocketClient interface will be
   // called |number| times (errors also count), unless the connection is closed
   // before that. The socket must be bound.
   //
   // It is allowed to call this method again before the previous request is
   // completely satisfied. For example:
-  //   service->ReceiveMorePackets(3);
+  //   service->ReceiveMore(3);
   //   ...
   //   // OnReceived() is called.
   //   // OnReceived() is called.
   //   ...
-  //   service->ReceiveMorePackets(3);
+  //   service->ReceiveMore(3);
   //   // The client expects 4 more calls to OnReceived().
-  ReceiveMorePackets(uint32 number);
-
-  // Sends data to the specified destination. The socket must be bound.
-  // The method doesn't report the result of the operation.
-  SendToAndForget(NetAddress addr, array<uint8> data);
+  //
+  // Please note that how ReceiveMore() is used will affect performance
+  // significantly. For example:
+  //   // Approach 1:
+  //   service->ReceiveMore(3);
+  //   // OnReceived() is called.
+  //   // OnReceived() is called.
+  //   // OnReceived() is called.
+  //
+  //   // Approach 2:
+  //   service->ReceiveMore(1);
+  //   // OnReceived() is called.
+  //   service->ReceiveMore(1);
+  //   // OnReceived() is called.
+  //   service->ReceiveMore(1);
+  //   // OnReceived() is called.
+  //
+  // It is very likely that approach 1 will perform better than approach 2,
+  // because in approach 2 getting every datagram takes at least the time of a
+  // round trip to the service side.
+  ReceiveMore(uint32 datagram_number);
 
   // Sends data to the specified destination. The socket must be bound.
-  SendTo(NetAddress addr, array<uint8> data) => (NetworkError result);
+  // On success, |result.code| is a non-negative number indicating how many
+  // bytes have been written. Otherwise, it is a network error code, including
+  // (but not limited to):
+  // - ERR_INSUFFICIENT_RESOURCES (-12): The service doesn't have sufficient
+  //   resource to complete the operation. One possible cause is that the client
+  //   tries to send too many datagrams in a short period of time.
+  // TODO(yzshen): Formalize Mojo networking error codes.
+  SendTo(NetAddress dest_addr, array<uint8> data) => (NetworkError result);
 };
 
 interface UDPSocketClient {
-  // |addr| and |data| are non-NULL on success.
-  OnReceived(NetworkError result, NetAddress? addr, array<uint8>? data);
+  // On success, |src_addr| and |data| are non-NULL, and |result.code| is a
+  // non-negative number indicating how many bytes have been received. On
+  // failure, |result.code| is a network error code.
+  OnReceived(NetworkError result, NetAddress? src_addr, array<uint8>? data);
 };
 
 }