[SR-Users] Kamailio dispatcher -> FreeSWITCH transfers handling.

Тимур Ситдиков sitdikovt at gmail.com
Wed Sep 8 11:27:41 CEST 2021

Hi list! :-)
I'm a total newbie in Kamailio. Trying to study)
Sorry for lots of text, I'm simply don't know how to explain my questions
shorter :-D

I want to configure Kamailio as dispatcher, registrar and NAT traversal in
front of FS farm.
I've managed to set up Kamailio and Fswitches.
Now I need to handle "special cases" with confs, REFER transfers etc

Giovanni Maruzzelli once presented solution for this, with following logic:
1) Kamailio by default distribute calls to various FSs - dispatch or
2) Each time Kamailio sends a call to a FS, it writes down which
destination "number" is going to which FS
server and from which UserID
3) if another call comes for the same (or related) destination, or is
originated from the same UserID
then Kamailio sends the call to that same FS server, bypassing default
distribution algorithm
Sounds good and helpful, but I'm stuck :-(

*Can someone please share an example or point me how and in what form
should I write down call info,and how can I control it (delete on BYE or

Actually there are 2 questions
It seems to me that recordings can be made using HTABLEs but here's a

After the dispatcher sets $du - I write an htable entry
$sht(fsdest=>$fU:$rU) = $du;
Existing entries are removed on catching 'BYE' for normal clearing, and
with failure_route for all failures.
BYE logic is:

  if (has_totag()) {
    if (loose_route()) {
      if (is_method("BYE")) {
          if (sht_match_name("fsdest", "eq", "$fU:$rU")) {
            $sht(fsdest=>$fU:$rU) = $null; //BYE A->B
          } else {
            $sht(fsdest=>$rU:$fU) = $null; //BYE A<-B

Another entry is setting up if blind transfer is made.
If INVITE is coming from one of FSs, and have 'Referred-By' field.
IN DIALOG route:
    if (is_present_hf("Referred-By")) {
      $sht(fsdest=>$fU:$rU) = $si;
For now it's working. Entries are written and deleted properly.

*Problem with Attended transfer.*
1) A --> B = htable entry A:B=xxxx
2) A --> C = htable entry A:C=xxxx(checked and set same FS addr because $fU
is present (If only it's worked as it should))
3) Att transfer (Refer $rU B, referred-by - A. Refer-To - C) B --> C = all
related entries deleted from table over BYE logic.
4) No entry B --> C (no INVITE was sent, bridged by REFER)
So, logic for check active calls in htable is broken now, because we don't
have entry B-->C

*How can I handle writing to htable in Attended transfer case?*
If another Invite will come FROM or TO B or C = there will be no htable
I'm not sure this is even a correct way. Surely missing something.


Anyway, the main problem is with identifying those htables to set proper
If INVITE is coming TO FSs I need to check if caller ID ($fU) or dest ($rU)
is present in htables.

Form I need to write to hash must be like $fU:$rU
Entry must be unique or it will be:
1) rewrited anytime a new invite is going to the same dest.
2) deleted anytime BYE or failure will come

If $sht(fsdest=>$rU) = $dd
1) 1000 --> 3200 = htable entry 3200 = xxxx
2) 1005 --> 3200 = htable entry rewriten to the same 3200 = xxxx
When 1000 or 1005 leaving conf, it sends BYE and removes entry.
So, next call to 3200 may be sent to the wrong FS.

Ok, we have a unique entry ($fU:$rU). Checking for active destination:
if (sht_match_name("fsdest", "re", ":$rU")) {}
If there's an htable with $rU in key - we got a match.
Now we need to assign a value from htable to $du.
But, I don't know how to get the name of this entry.
There's always different $fU: (or $rU if we check for the same caller ID

1000 --> 3200 = htbale 1000 --> 3200
New call 1005 --> 3200
Check sht_match name ":3200" - got match
We need to set: $du = $sht(fsdest=>$fU:$rU)
But it will be $sht(fsdest=>1005:3200) and we don't have this entry.
We need (fsdest=>1003:3200) But i don't see any way to get needed $fU.

*Is there any way to search or identify these tables?*

Can someone help me with this, please? :-)
Or maybe there is a better way to do this?

*My kamailio.cfg routing block in pastebin, if
neededhttps://pastebin.com/1PZ0QSDW <https://pastebin.com/1PZ0QSDW>*

Thank you!
Regards, Tim
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