SIP: Why does proxy server respond with 100 trying message ? What if it does not do this ?

Question

In one of the articles it was mentioned "BYE" SIP message routes end to end and by pass the SIP proxy server. What is it mean ? and when SIP proxy servers release the resources that was reserved during the session establishment ?

Answer 1

When a user agent client sends INVITE request to another user agent, it will go through proxy server. Just to avoid re-transmission of INVITE message, Proxy sever responds back to User agent client with 100 trying.

Answer 2

It is kind of Provisional acknowledgment from proxy to user agent that it has received the INVITE.
The 100 Trying provides you the feedback that your request is being processed by a SIP proxy.
If 100 trying is not sent and if T1 timer gets expired, UAC will send one more invite in order to avoid that 100 trying is being sent back.