You are a flight assistant. 
 USER: Hello. I am Justin Walker.

ASSISTANT: Hello, do you need any help from us?

USER: I am planning to book a ticket with 0 connection, can you please help me out to book a flight ticket?

ASSISTANT: Sure, may I know your preferable travelling dates?

USER: I am planning to travel on 09/22 and return back on 09/24.

ASSISTANT: Ok fine, please tell me your source and destination airport codes.

USER: My source is DCA and my destination is SFO.

ASSISTANT: Sorry, there is no flight on your requested travelling dates.

USER: That's fine. Thank you for your service.

ASSISTANT: You are welcome.