e DNS server for a domain, and it is operated by the owner of the domain name. Due to a very efficient caching scheme built into DNS, a DNS request often does not have to route all the way to this server. DNS is also used by an SMTP server to deliver an e-mail message. An SMTP server with an e-mail message to deliver initiates a DNS request for the MX record of the domain name in the destination e-mail address. The response to the request allows the SMTP server to contact the destination SMTP server and transfer the message. A similar process has been proposed for locating a SIP server using SRV, or service, DNS records. 1.7 URLs and URIs Uniform resource locators [15] are names used to represent addresses or locations in the Internet. URLs are designed to encompass a wide range of protocols and resource types in the Internet. The basic form of a URL is scheme specifier-for example, http: /www.artech- house.com/search/search.html.The token http identifies the scheme or protocol to be used, in this case HTTP. The specifier follows the ":" and contains a domain name (www.artechhouse.com), which can be resolved into an IP address and a file name (/search/search.html) URLs can also contain additional parameters or qualifiers relating to trans- port. For example, telnet / host.company.com: 24 indicates that the Telnet Protocol should be used to access host. company com using port 24. New schemes for URLs for new protocols are easily constructed, and dozens have been defined, such as mailto, tel, and https. The sip and sips schemes will be introduced in Section 2.2 and are described in detail in Section 4.2. Most protocols reference URLs, but with SIP we mainly reference URIs. This is due to the mobility aspects of SIP, which means that a particular address (URI) is not tied to a single physical device but instead is a logical entity that may move around and change its location in the Internet. However, the terms URL and URI are often used almost interchangeably in other contexts. 1.8 Multicast In normal Internet packe