[Kamailio-Users] How should I change Calling Party Number for a call?

[Iñaki Baz Castillo]

El Lunes, 6 de Julio de 2009, Uriel Rozenbaum escribió:
> Hi Guys,
>
> I was wondering how can I implement a proxy with Kamailio that can manage
> privacy and normalization of the calling party.
>
> In an INVITE message I can get the calling party name (or number) in
> several places:
> 1. From Header
> 2. Remote-Party ID
> 3. P-Asserted-Identity
> 4. P-Preferred-Identity
>
> How should I act in order to normalize or block the Calling Party to be
> passed to subsequent hops?
>
> AFAIK I can change the From Header as long as the tag isn't modified. Is
> this really true?
> What about RPID or PAI? Should I Change the aliases there as well?

Forget RPID, it's a expired draft (even if it's widely extended).

If you receive a call from an user (not from a gw or proxy) the only important 
field is the From header (however P-Asserted-Identity could be useful when the 
user desires anonimous call, so the From is "sip:anonymous at XXXX").

P-Asserted-Identity only makes sense when you receive a call from a trusted 
node, this is, a node for which you don't ask authentication, this usually is 
a PSTN gateway, other proxy...

So, if an user sends PAI remove it in the proxy. You can also add it with the 
value YOU want (in the proxy).
If you deliver the call to an user, remove the PAI (and PPI) since this info 
must not be displayed to an untrusted user.


Example A:

- User sip:alice at domain.org sends a call to PSTN number +12345678.

- The call arrives to the proxy which adds the user PSTN CLI:
    P-Asserted-Identity: tel:+3494123434

- The proxy routes the INVITE to the PSTN gw.

- The gw makes use of PAI header (preference over From header) and uses it as 
CLI in the PSTN side.


Example B:

- A PSTN gw sends to the proxy an INVITE with this data:
     INVITE sip:+003494123434 at proxy_ip SIP/2.0
     To: <sip:+003494123434 at proxy_ip>
     From: <sip:anonymous at invalid>;tag=asdasdasd
     P-Asserted-Identity: tel:+4400998877
     Privacy: id

- The caller ask for privace (Privacy: id) so the proxy removes the PAI 
header, locates the local user for that destination (alice) and sends the 
INVITE:
     INVITE sip:alice at IP SIP/2.0
     To: <sip:+003494123434 at proxy_ip>
     From: <sip:anonymous at invalid>;tag=asdasdasd
 
- So Alice receives an anonymous call. Note that the CLI is just hidden to the 
*last* destination. The CLI info is shared between telcos.


Example C:

- Alice wants to call to a PSTN number with hidden CLI, so she sends:
     INVITE sip:+0066444333 at proxy_ip SIP/2.0
     To: <sip:+0066444333 at proxy_ip>
     From: <sip:anonymous at invalid>;tag=asdasdasd
     P-Preferred-Identity: <sip:alice at domain.org>
     Privacy: id

- The proxy must authenticate the user, but cannot do it based on From header 
(anonymous) but based on the PPI header. So the proxy generates a 407 with 
credentials for username=alice and realm=domain.org.

- After authentication, the proxy realizes that Alice is asking for privacy 
(Privacy: id). It adds PAI with Alice's associated PSTN CLI, and sends this 
INVITE to the gw:
     INVITE sip:+0066444333 at proxy_ip SIP/2.0
     To: <sip:+0066444333 at proxy_ip>
     From: <sip:anonymous at invalid>;tag=asdasdasd
     P-Asserted-Identity: tel:+3494123434
     Privacy: id

- The gw uses the PAI value as CLI, but asks for privacy since "Privacy: id" 
is present. It routes the call to the destination telco.

- The destination telco removes the PAI and sends the call to the final user 
(hidden CLI).
    


Hope it helps, but please, read carefully the RFC 3325.



-- 
Iñaki Baz Castillo <ibc at aliax.net>