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RFC7255 - This specification defines how the Uniform Resource Name (URN) reserved for the Global System for Mobile Communications Association (GSMA) identities and its sub-namespace for the International Mobile station Equipment Identity (IMEI) can be used as an instance-id. Its purpose is to fulfil the requirements for defining how a specific URN needs to be constructed and used in the ’+sip.instance’ Contact header field parameter for outbound behaviour. (https://www.rfc-editor.org/rfc/pdfrfc/rfc7255.txt.pdf).
RFC7254 - This specification defines a Uniform Resource Name (URN) namespace for the Global System for Mobile Communications Association (GSMA) and a Namespace Specific String (NSS) for the International Mobile station Equipment Identity (IMEI), as well as an associated parameter for the International Mobile station Equipment Identity and Software Version number (IMEISV) as per the namespace registration requirement found in RFC 3406 [ 1]. The Namespace Identifier (NID) ’gsma’ is for identities used in GSM, Universal Mobile Telecommunications System (UMTS), and Long Term Evolution (LTE) networks. The IMEI and the IMEISV are managed by the GSMA, so this NID is managed by the GSMA. (https://tools.ietf.org/pdf/rfc7254.pdf).