Typically a router (or gateway) is loaded with three protocols. Two of these protocols connect to each of the attached networks (e.g., LAN and WAN), and the third protocol is the IP, which provides the network-to-network interface. Hosts typically are equipped with four protocols. To communicate with routers or gateways, a network access protocol and Internet protocol are required. A transport layer protocol assures reliable communication between hosts because end-to-end capability is not provided in either the network access or Internet protocols. Hosts also must have application protocols such as e-mail or file transfer protocols (FTPs). 12.3.2 TCP/IP and Data-Link Layers TCP/IP is transparent to the type of data-link layer involved, and it is also transparent whether it is operating in a LAN or WAN domain or among them. However, there is document support for Ethernet, IEEE 802 series, ARCNET LANs, and frame relay for WANS (Refs. 2, 5). Figure 12.3 shows how upper OSI layers are encapsulated with TCP and IP header information and then incorporated into a data-link layer frame. For the case of IEEE 802 series LAN protocols, advantage is taken of the LLC common to all 802 protocols. The LLC extended header contains the SNAP (sub-network access protocol) such that we have three octets of the LLC header and five octets in the SNAP. The LLC header has its fields fixed as follows (LLC is discussed in Chapter 11): DSAP = 10101010 SSAP = 10101010 Control = 00000011 The five octets in the SNAP have three assigned for protocol ID or organizational code and two octets for "EtherType." EtherType assignments are shown in Table 12.1. EtherType 12.3 TCP/IP AND RELATED PROTOCOLS Data-link Typically Data-link header FTP, SMTP,TELNET header header trailer information Application data TCP segment IP datagram Data-link frame (such as HDLC) Figure 12.3 The incorporation of upper-layer PDUs into a data-link layer frame showing the relationship with TCP and IP. Table 12.1 Ether-Type Assignments Ethernet Decimal Description XEROX PUP 