Finding Nocall in the TMSNCC log when carrier is sending 183 Early Media Message.
Scenario: User attempts to make an outgoing call over SIP trunks. The problem could be either
intermittent or constant. When the problem occurs, he hears no ringback.
Characteristics of the problem:
1. TMSNCC log shows a Nocall entry.
12:19:01.416 ( 4348: 4088) C-DE: 4000001E "00050000-1761-56ea-43af-0010490e756c" 83,(NoCall)
sip:TGrp_4,[email protected]:5441
2. The IPBX log and debug (dbg “on ext sip”) show ua_unexpected_msg 6:SIP_S_MSGTYPE_MODIFY_SDP
IPBX log
12:19:01.414 ( 4348: 3128) Switch COR-SG90-1-80 Internal diag:
UA: (++1059++) ua_unexpected_msg - 6:SIP_S_MSGTYPE_MODIFY_SDP, status 100:SIP_STATUS_TRYING
CALL ID = 0050000176156ea43af0010490e756c,
leg_id_str = 010029a7:172.22.10.80:O, leg state = 2:LEG_STATE_OINVITE
Switch debug
UA: (++1071++) ua_unexpected_msg - 6:SIP_S_MSGTYPE_MODIFY_SDP, status 100:SIP_STATUS_TRYING
CALL ID = 0050000176956ea43af0010490e756c,
3. We found the problem is a network issue. Simultaneous Wireshark captures taken at phone switch
and trunk switch shows a key packet was lost. The phone switch was expecting part 2 of the 200 ok
but instead received the “Modify” packet.
Here are the simultaneous Wireshark traces showing the missing packet.
Taken from the trunk switch
Taken from the phone switch
Resolution:
Here is a graphical look at what is supposed to happen when a 183 is sent from the carrier and all works
as expected.
With the SIP profile IgnoreEarlyMedia disabled, we see the carrier sending a 183. The trunk switch
sends a 200 OK to the phone switch. This causes the phone to listen for the RTP stream. Once the 200
OK (both parts 1 and 2) is sent from the carrier, the trunk switch sends a “Modify” packet to the phone
switch normalizing everything.
Problem…
Below shows what happened when the SIP call fails. There is a missing packet in the 200 OK from trunk
switch to phone switch
After the phone switch receives the first of two packets of 200 OK, it expects packet number two of two.
Instead, it received the Modify packet.
Solution…
By enabling IgnoreEarlyMedia in the SIP trunk profile, we are able to simplify the transaction between
trunk switch and phone switch by taking out the Modify.
The phone switch gets all of the packets it expected and SIP calls completed set up 100% of the time.
See case
01414986