My first notification was dropped

[JavaProgrammerLB]

My first notification was dropped

• the onLoad(CoapResponse response) was invoked when client received the ACK of Observe request from Server
• then CoapClient drop the first NON notification with payload
• then onLoad handle the second notification successful
• the third notification successful...

  I want to know how can I catch the first notification

• In my code I use coapClient.useCons(); to expect load the first notification but the ack

My Observe:

After My code run:

Californium log:

tcpdump packages:

CoapClient coapClient = new CoapClient();
coapClient.setURI("http://*.*.*.*:5683/t/d");
coapClient.useCons();
coapClient.observe(new ObserveHandler());

I use: 2.0.0-M6

tcpdump cap file: devicebus-nbiot-access4.cap.zip

[boaks]

What is your client implementation?

The ACK (first response) contains a OBSERVE 0, response code 2.05 Content, but no payload !!!! The fist NON (MID: 7510) also contains OBSERVE 0, response code 2.05 Content, an payload
The second NON (MID: 7513) also contains OBSERVE 3, response code 2.05 Content, an payload

According https://tools.ietf.org/html/rfc7641#section-3.4, the first NON is not newer than the ACK. Therefore the NON is filtered out!

FMPOV, your client implementation is not proper working (I hope, it's not californium .-) ) It doesn't prepare the observe option of the ACK response proper not does it send the proper first payload in the response.

See also https://tools.ietf.org/html/rfc7641#appendix-A.1

The response to GET has observe 0 and payload, the next notification has observe 9 (and payload), and then 16 (and payload). So that observe option sequence will work. It takes the first payload from the response and then the payload of the following notifies.

[JavaProgrammerLB]

@boaks thanks for your answer，

• I get your point that the observe option in response order the notification.
• my client implementation is Californium 2.0.0-M6.
• Can you give me some suggestion to fix the issue: ignore the ack, handle the notification

[boaks]

What is your client implementation?

Sorry, I was wrong, the coap server implementation is important, it generates the suspicious ACK. If it is also californium 2.0.0-M6, I would appreciate, if you could provide some informations about the examples your using.

[boaks]

Can you give me some suggestion to fix the issue: ignore the ack, handle the notification

Fix the server to sent proper ACK responses, or really just a plain ACK. But this depends on your servers possibilities.

[JavaProgrammerLB]

@boaks thank you very much. The Observe server is run in our customer's IoT device, maybe implement in C.

[JavaProgrammerLB]

@boaks thank you a lot.

• In my case, I can't modify the Observe server's code logic.

• I try to add an if block in ObserveNotificationOrderer.java and modify some code to pass the unit test, which is an informality method to solved my problem.

ObserveNotificationOrderer.java: