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We are using Kamailio 4.2.5 as a registrar and proxy between many dispersed end-users of a soft phone app and our calling platform / switch.

Until now we have used udp exclusively but are trying to introduce tcp between end-users and Kamailio, leaving udp between Kam and our switch...while maintaining the ability for some end-users to use udp to Kam.

With some simple address checks I am able to always send to our switch over udp. If all end-users used tcp I could send everything else tcp, but I need to maintain udp support.

The specific problem I am having is on a reINVITE such as this one from our platform to the a-leg:

INVITE sip:xxxxxx@xxxxxxxxxxxxx:42679;user=phone SIP/2.0
Via: SIP/2.0/UDP xxxxxxxxxxxxx:5060;branch=z9hG4bK218cc8e12ll5035f67INV6a67885312aad
Max-Forwards: 35
Route: <sip:xxxxxxxxxxx;lr;r2=on;ftag=daba971c;did=b57.4872;nat=yes>
Route: <sip:xxxxxxxxxxx:5070;transport=tcp;lr;r2=on;ftag=daba971c;did=b57.4872;nat=yes>
Contact: <sip:xxxxxxxxxx@xxxxxxxxxxxxx:5060>
To: "xxxxxx"<sip:xxxxxx@xxxxxxxxxxxxxxxxxxxxxxxxxxx:5070>;tag=daba971c
From: <sip:xxxxxxxxxx@xxxxxxxxxxxxxxxxxxxxxxxxxxx:5070>;tag=6a678853-co76461-INS002
Call-ID: MDI4ZmFjNmZhN2Y1NWE2ZTViNTkyZGUwNWE2YzUzYmU
CSeq: 7646101 INVITE
Allow: INVITE,ACK,CANCEL,BYE,REFER,OPTIONS,NOTIFY,SUBSCRIBE,PRACK,INFO,UPDATE
Content-Type: application/sdp
Date: Mon, 15 Jun 2015 20:10:18 GMT
User-Agent: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Content-Length: 262

As you might notice, we have rr:enable_double_rr set:

There are some situations when the server needs to insert two Record-Route header fields instead of one. For example when using two disconnected networks or doing cross-protocol forwarding from UDP->TCP. This parameter enables inserting of 2 Record-Routes. The server will later remove both of them.

and I believe it is necessary to keep this way. Without it Kamailio doesn't even see the reINVITE...the switch probably tries tcp and that's not setup between the two.

The invite above is sent to the a-leg over udp but I would expect and need it to be tcp in this case. The reINVITE is part of an existing dialog. We call loose_route() followed by some simple bflag setting and flag checking, t_on_reply(), ... then t_relay().

I do have a functional workaround but would prefer to avoid such manual handling by utilizing built-in functionality properly.

   #                             
   # relay the message           
   #                             
   if(route(TEST_TOGW)) {        
      if (!t_relay_to_udp()) {   
         sl_reply_error();       
      }                          
   }                             
   else {
      if ($(hdr(Route)[-1]) =~ "tcp") {    
         if(!t_relay_to_tcp()) { 
            sl_reply_error();    
         }                       
      }                          
      else if (!t_relay()) {     
         sl_reply_error();       
      }         
   }                             

I'm not 100% sure how reliable or fast this will be, but it does work so far in my simple tests.

Is loose_route supposed to see and use the transport=tcp but isn't for some reason? It seems like the right thing to do to me. If not, is there anything else I can/should be doing in the tm and/or rr modules to make Kamailio realize it needs to send this message over TCP? If not in those two modules is there some recommended way perhaps via registrar or usrloc etc. to make Kamailio remember/store when a user is connected via TCP and be able to do a quick lookup before sending to them? Anything else I'm missing or not thinking of?

Please let me know if I can further explain and rest assured any assistance will be much appreciated!!!