[SIPForum-discussion] What are the uses of P-Preferred-Identity??

Dwight, Timothy M (Tim) timothy.dwight at verizon.com
Mon Aug 17 19:31:19 UTC 2015

I think the UE is free to use in the P-Preferred-Identity header, any of the identities it gets in the P-Associated-URI header field, in the 200 OK response to its SIP registration.  3GPP doesn’t specify how it populates the From header, since 3GPP basically considers that user-to-user information.

If the UE doesn’t specify a P-Preferred-Identity header, the outbound proxy (3GPP calls this a P-CSCF) will insert a P-Asserted-Identity header whose content is the first URI in the P-Associated-URI header.  The P-CSCF is assumed to have been on the path of the 200 OK, and to have cached the list of URIs from the P-Associated-URI header contained in that response.

For this reason, P-Preferred-Identity doesn’t have to be sent by the UE.  The only reason to send it, as far as I know, is if there were multiple URIs in the P-Associated-URI header and the UE wants to use one that wasn’t the first in the list (the first in the list is the default).  That might happen if the UE is a device that supports multiple “personae” (e.g., a work persona and a personal persona).  Or it might be a server, e.g., a PBX, that hosts many terminals.  In my experience there aren’t a lot of PBX’s that register with the network, but the 3GPP standards allow for such things.


From: pendyala syam [mailto:psyam545 at gmail.com]
Sent: Monday, August 17, 2015 1:15 PM
To: Dwight, Timothy M (Tim)
Cc: discussion at sipforum.org
Subject: RE: [SIPForum-discussion] What are the uses of P-Preferred-Identity??

Dear Tim,

Thanks for providing valuable info on "user asserted" and "network asserted" identities. From 3GPP specs, UE should use the first one of P-Associated-Uris in From header. So when UE has already set From header to the value that is already identified by network, why should UE add P-Preferred-Identity again?  I am doubting this From header can be changed during transmission(Please correct me if I am wrong). If so, Can you please answer me in this perspective, why would network change From header?  Thank you
On Aug 17, 2015 6:40 AM, "Dwight, Timothy M (Tim)" <timothy.dwight at verizon.com<mailto:timothy.dwight at verizon.com>> wrote:

There is in some SIP implementations a distinction made between “user asserted identity” (the caller ID signaled by the originating UE) and “network asserted identity” (the network’s assertion of the caller’s identity).  Usually they are the same, but they can be different.

The From header carries “user asserted caller identity”.

The P-Asserted-Identity header carries “network asserted caller identity”.

P-Preferred-Identity is not intended to be presented to the UAS, it is only meant to communicate between the UA and the network to which it subscribes.  I suppose there could be corner cases where it finds its way to the UAS, but if so I think the UAS would ignore it.

See RFC-3325 and 5876 for additional information.

The purpose of the P-Preferred-Identity header is to allow the UA to influence the network’s assignment of P-Asserted-Identity.  Sometimes the UA has multiple identities.  The UE can when it originates a call, indicate in P-Preferred-Identity header field which of these it wishes the network to assert to the called party as its “network asserted identity”.  The network will typically validate that the identity in the P-Preferred-Identity header field is one that was previously assigned to the UA by the network (in 3GPP SIP networks this assignment happens during SIP registration).  If it matches, the network will use it derive the value of the P-Asserted-Identity header.  That derivation could be to simply copy the contents of P-Preferred-Identity into P-Asserted-Identity, or there might be some additional processing involved.  If the value signaled by the UA in P-Preferred-Identity doesn’t match any of the identities the network considers valid for use by the UA, the network will assign an identity it considers valid for use by the UA and place this in the P-Asserted-Identity header field.

It is possible that a SIP message received by the UAS will have both a From and a P-Asserted-Identity header field.  Either could in theory be rendered by the UAS to its user, as the “caller ID”.  In theory the decision is up to the device.  In practice there may be specifications to which the device is obliged to comply (e.g., if it’s a mobile device operating in a 3GPP network) that will dictate its behavior.  A common implementation is to render the caller ID presented to the user based on P-Asserted-Identity, if present;  and based on From, if P-Asserted-Identity is not present.


From: discussion-bounces at sipforum.org<mailto:discussion-bounces at sipforum.org> [mailto:discussion-bounces at sipforum.org<mailto:discussion-bounces at sipforum.org>] On Behalf Of pendyala syam
Sent: Sunday, August 16, 2015 1:28 AM
To: discussion at sipforum.org<mailto:discussion at sipforum.org>
Subject: [SIPForum-discussion] What are the uses of P-Preferred-Identity??

Hi All,

As far I understand, P-Preferred-Identity will be shown as caller display name in callee UI. When SIP already have From header which carriers the caller identity, why is this P-Preferred-Identity header required? Why can't we use From header to show caller display name at callee UI?. Is there any added advantage of this header. Some body please explain. Thanks in advance.

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