When we talk about SIP call flow there are two important things that need to be understood SIP messages and response code. SIP messages and responses are used for setup , in session changes, info and for tearing down a session. A response code is a response which is sent in response to a SIP message, and we can also get a SIP message in acknowledgement of response code.We will look into some quite frequent SIP messages in this post and response codes in the next post.
INVITE: Invite is very first message to set up a connection. It is also used to modify a session as re-invite (which we will discuss in later posts). User Agent sending an Invite is known as User Agent Client.
ACK: ACK is sent in response to a 200 OK message. ACK is used only with Invite or a Re-Invite message.