Here is what the SIP response of user2 will look like.
Via: SIP/2.0/UDP site4.server2.com;branch=z9hG4bKnashds8;received=192.0.2.3
Via: SIP/2.0/UDP site3.server1.com;branch=z9hG4bK77ef4c2312983.1;received=192.0.2.2
Via: SIP/2.0/UDP pc33.server1.com;branch=z9hG4bK776asdhds;received=192.0.2.1
To: user2 <sip:firstname.lastname@example.org>;tag=a6c85cf
From: user1 <sip:email@example.com>;tag=1928301774
CSeq: 314159 INVITE
---- User2 Message Body Not Shown ----
The first line in a response is called Status line.
SIP-Version SP Status-Code SP Reason-Phrase CRLF
[SP = single-space & CRLF=Carriage Return + Line Feed (i.e. the character inserted when you press the "Enter" or "Return" key of your computer)]
Here SIP version is 2, Status-Code is 200 and Reason Phrase is OK.
The header fields that follow the status line are similar to those in a request. I will just mention the differences
Via:There are more than one via field. This is because each element through which the INVITE request has passed has added its identity in the Via field. Three Via fields are added by softphone of user1, server1 the first proxy and server2 the second proxy. The response retraces the path of INVITE using the Via fields. On its way back, each element removes the corresponding Via field before forwarding it back to the caller.
To:Note that the To field now contains a tag. This tag is used to represent the callee in a dialog.
Contact:It contains the exact address of user2. So user1 doesn't need to use the proxy servers to find user2 in the future.
It is a 2xx response. However responses can be differnet depending on particular situations. Learn about the different types of SIP responses.