escription Protocol (SDP), described in Section 7.1, is shown above SIP in the protocol stack because it is carried in a SIP message body. HTTP, SIP and the Internet SMTP, FTP, and Telnet are all examples of application layer protocols. Because SIP can use any transport protocol, it is shown interacting with both TCP, TLS, and UDP in Figure 1.1. The use of TCP, TLS, SCTP, and UDP transport for SIP will be discussed in the next chapter. 1.5 Utility Applications Two utility applications are also shown in Figure 1.1 as users of UDP. The most common use of the DNS (well-known port number 53) is to resolve a symbolic name (such as domain. com, which is easy to remember) into an IP address (which is required by IP to route the packet). Also shown is the Dynamic Host Configuration Protocol (DHCP). DHCP allows an IP device to download con- figuration information upon initialization. Common fields include a dynami- cally assigned IP address, DNS addresses, subnet masks, maximum transmission unit (MTU), or maximum packet size, and server addresses for mail and Web browsing. Figure 1.4 shows the layer interaction for processing a request. At the top, a URL from the user layer is input to the application layer. URLs, described URL from user Application Request message, URL, and port number Domain Transport Utility Datagram or segment and protocol number IP address Internet Packet and MAC address (if on a LAN) Physical Figure 1.4 Request processing in the Internet Protocol stack. SIP: Understanding the Session Initiation Protocol later in this chapter, are names used to represent resources, hosts, or files on the Internet. The application passes the generated request (for example, a HTTP GET request, which requests a Web page download), the URL, and the port number to the transport layer. The transport layer uses a utility to resolve the domain name extracted from the URL into an IP address. The IP address, data- gram (or segment, depending on the transport layer protocol used), and proto- col number identifying the transport