IETF RFC 7549 PDF

In 3GPP networks, the signaling path between a calling user and a called user can be partitioned into segments, referred to as traffic legs. Each traffic leg may span networks belonging to different operators and will have its own characteristics that can be different from other traffic legs in the same call. A traffic leg might be associated with multiple SIP dialogs, e.g., in case a Back-to-Back User Agent (B2BUA) that modifies the SIP dialog identifier is located within the traffic leg.

This document defines a new SIP URI parameter, 'iotl' (an abbreviation for Inter-Operator Traffic Leg). The parameter can be used in a SIP URI to indicate that the entity associated with the address, or an entity responsible for the host part of the address, represents the end of a specific traffic leg (or multiple traffic legs).

The SIP URI 'iotl' parameter defined in this document has known uses in 3GPP networks. Usage in other networks is also possible.