ACK sip:bob@one.example.com SIP/2.0
Via: SIP/2.0/UDP north.east.isi.edu
From: Alice <sip:alice@wonderland.com>;tag=1
To: Bob <sip:bob@example.com> ;tag=3141593
Call-ID: 602214199@mouse.wonderland.com
CSeq: 1 ACK