ABSTRACT
The use of the Domain Name System (DNS) for resolving of the domain host names (e.g., bob@example.com) to the Internet Protocol (IP) addresses and other attributes related to the Session Initiation Protocol (SIP) functional entities is essential for the client–server SIP protocol. Request for Comment 3263 that is also described here specifies the DNS procedures for locating/discovering the SIP entities. Before we describe the DNS usage in SIP, we describe the DNS architecture itself first because DNS has become almost a general-purpose distributed database for controlling accesses to resources for client–server applications over the IP network. In the beginning, public switched telephone network (PSTN) has been used for telecommunications administered by the International Telecommunication Union-Telephone (ITU-T). ITU-T Recommendation E.164 is used as the number system of the PSTN functional entities, including telephones throughout the world, and the assignment of number prefixes to each country code is also administered by the ITU-T. It implies that interoperability between the IP addresses and E.164 numbers is needed. In this context, the E.164 number (ENUM) technical standard facilitates the mapping using a DNS name parent of e164.arpa. This chapter exclusively discusses both DNS and ENUM address resolution mechanisms, as well as the mapping between the IP addresses and the E.164 telephone numbers in detail.
