100 Trying method is just an indicator saying "I am doing something for you" and it does not do any special task on its own. so the contents is very simple as shown below.

[ Line 1  ] SIP/2.0 100 Trying

[ Line 2  ] Via: SIP/2.0/UDP 192.168.1.15:5060;branch=z9hG4bK3298736468smg;transport=UDP

[ Line 3  ] To: <sip:+10123456789@ims.sharetechnote.com>

[ Line 4  ] From: <sip:+11234567890@ims.sharetechnote.com>;tag=659747293

[ Line 5  ] Call-ID: 131949458@192.168.1.15

[ Line 6  ] CSeq: 1 INVITE

[ Line 7  ] Content-Length: 0

This SIP message clip highlights the following parameters: Via, CSeq, Call-ID, From, To, Content-Length. Use these fields to correlate routing, dialog identity, transaction sequencing, registration/session state, security context, and media negotiation for this part of the procedure.