title: What is the size of CoAP packet?
body: <p>I'm new for this technology, can somebody help me to know about some doubt?</p>
  
  <p><strong>Q-1. What is the size of CoAP packet?</strong><br>
  (I know there is 4 byte fixed header, but what is the maximum size limit including header, option and payload?)</p>
  
  <p><strong>Q-2. Is there any concept for Keep Alive like MQTT?</strong><br>
  (It works on UDP for how much time it keeps open the connection, is there any default time or it keeps open every time when we send packet?)</p>
  
  <p><strong>Q-3. Can we use CoAP with TCP?</strong><br>
  (Main problem with it CoAP is it works on UDP, is there any concept like MQTT QoS? Let's say a sensor publishes some data every one second, if subscriber goes offline, is there any surety in CoAP that subscriber will get all the data when it come online?)</p>
  
  <p><strong>Q-4. What is the duration of connection?</strong><br>
  (CoAP supports publish/subscribe architecture, may be it needs connection open all the time, is it possible with CoAP whether it is based on UDP.)</p>
  
  <p><strong>Q-5. How does it discover the resources?</strong><br>
  (I have one gateway and 5 sensors, how will these sensors connect to the gateway? Will the gateway find these sensors? Or will sensors find the gateway?)</p>
  
  <p><strong>Q-5. How does sensor register with gateway?</strong></p>
  
  <p>Please help me, I really need answer. I'm all new for these kind of things and suggest me something for implementation point of view.</p>
  
  <p>Thanks.</p>