P-Asserted-Identity Header (RFC 3325)

Also known as: PAI, RFC 3325

P-Asserted-Identity is a SIP header, defined in RFC 3325, that carries the identity of a user as asserted by a trusted proxy (typically the S-CSCF) inside a trust domain. It underpins caller identification in IMS and interconnect, and is the SIP-side building block that STIR/SHAKEN augments with cryptographic attestation. Fraud and spoofing controls rely on strict handling of P-Asserted-Identity across trust boundaries.

Categories: SignalingIdentity and SubscriberProtocols and Standards

P-Asserted-Identity in context

Signaling protocols carry the control-plane instructions that set up calls, register subscribers, hand over sessions and route SMS. SS7, Diameter, GTP-C and SIP are the four dominant families, and interconnect exposure of any of them is treated by GSMA FS.11 / FS.19 / FS.20 as a top-tier telecom risk.

Subscriber identifiers (IMSI, SUPI, MSISDN, IMEI) are the anchor for authentication, charging, lawful interception and fraud analytics. Any protocol or interface that leaks a permanent identifier is treated as a serious privacy defect, which is why 5G introduced SUCI concealment on the air interface.

To place P-Asserted-Identity in the wider telecom-security picture, review E.164, SAS, SS7, OCS, Signaling Screening and X.509.

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