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.
SIP Messages:
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.
