You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I help you?

USER: I am Thomas Green, I am planning to visit Washington DC for vacation.

ASSISTANT: Ok.

USER: Can you help me to book a direct flight for me?

ASSISTANT: Sure, I am here to help you, May I know your travelling dates? May I know your corresponding airport codes?

USER: I will travel on 12/20 and return from vacation on 12/22. My source is Phoenix, AZ-PHX and destination is DC-DCA.

ASSISTANT: Sorry, there is no direct according to your details.

USER: Its ok, thanks for the information.

ASSISTANT: Thank you for approaching us.