prehensive in the layers above IP, and it has become extensive in the layers below IP; that is, numerous protocols are supported at the application layer (e.g., FTP and HTTP), and increasingly more options are available at the physical layer for connectivity. Second, IP decouples applications from the transmission or trans- port: IP does not care what the application is, and IP does not care what transport is used. Figure 8.7 identifies the key elements in a TCP/IP network. The TCP/IP trans- mission consists of the application, transport, IP, and L2 layers. The hosts deal with the application and transport layers, and the network acts on the IP and L2 infor- mation. The network is composed of IP edge routers, IP core routers, and the transmission networks between them. The routers examine the IP field to deter- Internet Basics Application Transport Edge Router TCP/IP Edge Router TCP/IP Core Router Routing Protocol Routing Protocol Figure 8.7 The key elements in a TCP/IP network mine the appropriate routing of the message, and the routing protocol determines the specific technique used to determine the proper route. The hosts examine TCP fields to ensure guaranteed service, checking for any errors or lost packets requesting retransmissions when errors occur, and they also work with the applica- tion data. TCP/IP is the darling of the networking world for several important reasons: It provides a common internetworking paradigm-IP enables inter- networking (i.e., creating one big virtual network). It supports many IP routing protocols-The two major routing protocols are BGP and IGP. BGP distributes external network addresses within and between autonomous systems (which are discussed in more detail earlier in this chapter). BGP4 is currently the inter-autonomous system routing pro- tocol of choice. IGP distributes internal prefixes only inside autonomous systems or to border routers leading to external networks. Examples include OSPF, IS-IS, and RIP version 2 (RIPv2). It supports many transport protocols-TCP/IP suppo