he official implementation of this layer, and all transmissions are sent in IP messages, referred to as datagrams. The datagrams Applications E-mail, FTP, Layer 4 Web, Peer-to-Peer (P2P) Layer 3 TCP/UDP/SCTP/DCCP Host-to-Host Transport Layer 2 Internetworking Any L1 or L2 Media: Network Interface Layer 1 Ethernet, Frame Relay, ATM, TDM, WDM Figure 5.9 The TCP/IP four-layer reference model Chapter 5 Data Communications Basics are forwarded to their destinations by routers. IP defines an unreliable, best-effort packet-forwarding service. It does not provide error checking, sequencing, or guar- anteed delivery. TCP/IP Layer 3 handles the host-to-host protocols and has the same role as the transport layer of the OSI model (Layer 4). It is used to enable peer entities to talk with one another. These are terminal-to-terminal protocols that run in the end devices, such as the user's PC and the server. There are four possible implementa- tions: TCP, UDP, and two newer transport protocols, Stream Control Transmission Protocol (SCTP) and Datagram Congestion Control Protocol (DCCP) (which are discussed in Chapter 8, "The Internet and IP Infrastructures"). TCP is a connection- oriented protocol that provides reliable, error-free delivery; mind you, sometimes conditions in the network can cause it to fail, and the network connection can be lost. Its role is to split up the message to be transmitted into a form the IP layer can handle. On the receiving side, TCP places packets in order to reconstruct the initial message. TCP is also in charge of the flow control of the connection. In compari- son, UDP is a very simple protocol; it is connectionless and not guaranteed to be reliable, although in practice it works quite well. UDP assumes that there is no need for control of flow of the packets; instead, it relies on some other mechanism to assure quality control. It is used, for example, in support of Voice over IP (VoIP), where the delay associated with retransmissions is not tolerable and instead the end user must ask a person