When I have chan_sip and chan_pjsip active at the same time (listening on different ports). The channel listening on port 5060 sends SIP "OPTIONS packets with “transport=udp”. The other sends with “rinstance=xxx” where “xxx” is some random alphanumeric (hash?). No “transport=udp” on the second one.
Wondering if maybe this is intentional so that Asterisk can distinguish? The reason I ask is because TCPDUMP does not interpret the second type as SIP packets. So if TCPDump is fooled maybe it could trip up some SIP devices which will then ignore the qualify packets and cause NAT problems.