CUSP changing transport to TCP from UDP in VIA headers
How does CUSP decide when to use TCP vs UDP when forwarding INVITES to CUCM when both UDP and TCP are configured on the Network Listen Ports? We have two different calls flows coming from our SBC to CUSP. Both INVITES have UDP as the transport. CUSP forwards one call flow to CUCM with TCP as the transport and the other as UDP. Is there something unique to the first INVITE below that would signal CUSP to use TCP?
Invite from SBC that CUSP adds TCP as the transport in the VIA header towards CUCM
40:13.979 On [2:245]10.XXX.XXX.XXX:5060 sent to 10.240.XXX.X:5060
Hi. Did you ever find out the reason and a solution to this problem. I am having the same issue where CUSP decides to change the transport type to TCP (from UDP) when talking to CUCM on certain invites from Acme SBC. On some invites from the same Acme SBC, CUSP doesn't change the transport type and leaves it as UDP.
We never did find out the root cause. In the ACME SBC, we created two SIP-Ports on the SIP Interface. One for UDP and the other for TCP. This allowed the call flow to succeed. Were you able to find a solution.
It is working as designed. Check also your path MTU (Unknown vs 1500) As per the RFCs you need to have the packet within 200 bytes so it does not get changed. Unknown path automatically switches to TCP if the initial INVITE coming from the SBC is 1300bytes or higher CUSP will switch over to TCP as per RFCs 3261 section 18 and 2914
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