MOR does not send Contact header
Problem Description
MOR does not include a Contact header in 180/183/200 OK SIP messages sent back to the originating device. The device drops the call immediately after the call is answered.
Possible reasons
This can happen if the ACK message from the client for MOR 401 Unauthorized reaches MOR after second Invite.
As can be seen in the image, packets arrived in MOR out of order; the second Invite reached MOR before the ACK. This can happen if the INVITE packet is too big and is fragmented by the IP stack.
Possible solutions
As mentioned, the INVITE packet can be fragmented by the IP stack if it is too big. An INVITE packet is usually too large when there are many codecs in the SDP, so one way to avoid fragmentation is to ask the client to send only the codecs that are actually needed. For example, if in MOR Device settings only G729 and G711 codecs are enabled, the client should send only these codecs, as MOR will not use any others.
With only the necessary codecs in INVITE, packets will be smaller and not fragmented, which should prevent the IP stack from reordering them.
Note about Wireshark and GUI pcaps
If packets are fragmented, PCAPS in the GUI will not show such packets at all. To see such packets, raw pcap must be downloaded and Wireshark option Reassemble fragmented IPv4 datagrams must be disabled (not checked) in Edit->Preferences->Protocols->IPv4 before doing any filtering on a raw pcap.
