Issue resolved by buffering unsent traffic directly in the send queue. Thus if the OS buffers are full, traffic will accumulate in the send queue. If that in turns fills up, then calls to BeginSend will now return a SendQueueFull error, which can be handled appropriately by the user (e.g. by putting the traffic in some external queue). Netcode for GameObjects already handles this error code gracefully. Send queue size is also configurable which provides another way of dealing with the condition.