he Session Initiation Protocol client-server design and the use of URLs and URIs. From SMTP, SIP borrowed a text-encoding scheme and header style. For example, SIP reuses SMTP head- ers such as To, From, Date, and Subject. The interaction of SIP with other Internet protocols such as IP, TCP, UDP, and DNS will be described in the next section. 1.4 Internet Multimedia Protocol Stack Figure 1.1 shows the four-layer Internet Multimedia Protocol stack. The layers shown and protocols identified will be discussed. 1.4.1 Physical Layer The lowest layer is the physical and link layer, which could be an Ethernet local area network (LAN), a telephone line (V.90 or 56k modem) running Point-to- Point Protocol (PPP), or a digital subscriber line (DSL) running asynchronous transport mode (ATM), or even a wireless 802.11 network. This layer performs such functions as symbol exchange, frame synchronization, and physical inter- face specification. 1.4.2 Internet Layer The next layer in Figure 1.1 is the Internet layer. Internet Protocol (IP) [6] is used at this layer to route a packet across the network using the destination IP address. IP is a connectionless, best-effort packet delivery protocol. IP packets can be lost, delayed, or received out of sequence. Each packet is routed on its own, using the IP header appended to the physical packet. Most IP address examples in this book use the older version of IP, version 4 (IPv4). IPv4 Signaling Media Utility Media coding Application layer H.323 Transport layer Internet layer Physical/Link layer Ethernet 802.11 Figure 1.1 The Internet Multimedia Protocol stack. SIP and the Internet addresses are four octets long, usually written in so-called "dotted decimal" notation (for example, 207 134. 3 5). Between each of the dots is a decimal number between 0 and 255. At the IP layer, packets are not acknowledged. A checksum is calculated to detect corruption in the IP header, which could cause a packet to become misrouted. Corruption or errors in the IP payload, however, are not detected; a hi