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This page contains some notes describing the work to fully complete outbound (RFC 5626) support on Kamailio. It is believed that the current Kamailio feature set is sufficient to provide separate Edge Proxy and Registrar servers supporting outbound.
When receiving a REGISTER request, the registrar MUST check from its Via header field if the registrar is the first hop or not. If the registrar is not the first hop, it MUST examine the Path header of the request. If the Path header field is missing or it exists but the first URI does not have an "ob" URI parameter, then outbound processing MUST NOT be applied to the registration. In this case, the following processing applies: if the REGISTER request contains the reg-id and the outbound option tag in a Supported header field, then the registrar MUST respond to the REGISTER request with a 439 (First Hop Lacks Outbound Support) response; otherwise, the registrar MUST ignore the "reg-id" parameter of the Contact header.