[SR-Users] P-CSCF

Jason Penton jason.penton at smilecoms.com
Thu Jan 28 20:49:10 CET 2016

Hi Fred,

I can answer the 1st question only as I am not too clued up with mediaproxy
module and server.

The pcscf_is_registered function is used to confirm that the UE you are
sending the request from is actually registered to the IMS. If this is
true, then the P-CSCF will assert the identity and forward the request to
I-CSCF or S-CSCF, depending on state and request type. Normally the
interface between UE and P-CSCF is via ipsec so it's almost a given that
any traffic coming in on the ipsec pipe can be asserted. In the case
without IPSEC however, there are various methods used to make sure the UE
that is sending the request is actually registered to the IMS. There are a
few algorithms that can be configured to check - ie you can check the
contact host and port (this is only works for invites as MESSAGEs don't
have contact headers generally), then you can check the received IP and
port to make sure it's coming from a currently registered contact that used
the same IP:PORT combination, etc, etc.

Alternatively you could remove the check and pass the request onto the
S-CSCF without asserting (not per std though) and then challenge all "cost
incurring" requests with a 407 on the S-CSCF.... One limitatoin of not
getting this working correctly is that you will not be able to use the Rx
interface unless you can match a contact on the P-CSCF (ims_qos module)....
but perhaps you are not interested in Rx interface at the moment?

If you send me a trace (pcap) and logfile I'll take a look as soon as I get
a chance and let you know what the problem is.


On Thu, Jan 28, 2016 at 9:24 PM, Fred Schulz <FSchulz at blackned.de> wrote:

> Hi all,
> we’re just playing around with an IMS setup bases on kamailio. Therefore
> the kamailio is used as P-,I- and S-CSCF.
> We where able to register two clients through all components.
> As we wan to start a call session, the P-CSCF answers with „403 -
> Forbidden. You must register with an S-CSCF“…
> I found this snippet in kamailio.cfg:
> if (!pcscf_is_registered("location")) { send_reply("403","Forbidden - You
> must register first with a S-CSCF“); break; }
> Can one tell me what exactly the kamailio is checking there? As I figured
> out it is looking in database location table.
> The table contains the registered users registered towards the IMS.
> Another „problem“ we faced with is the rtpproxy. As the
> ngcp-mediaproxy-ng is no longer available and replaced with rtpproxy
> we’re trying to use it. But kamailio said that the proxy isn’t answering
> the way expected.
> ERROR: rtpengine [rtpengine.c:1622]: rtpp_test(): proxy responded with
> invalid response
> Any advise would be nice.
> Thank you
> Fred
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*Jason Penton*
*Senior Manager: Applications and Services*
*Smile Communications Pty (Ltd)*
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*Skype:* jason.barry.penton
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