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Optical Networks: A Practical Perspective - Part 43. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 390 Client Layers of the Optical Layer IP being a network layer protocol does not guarantee reliable in-sequence delivery of data from source to destination. This job is performed by a transport protocol typically the transmission control protocol TCP . Another commonly used transport protocol for simple message transfers over IP is the user datagram protocol UDP . Commonly used applications such as telnet file transfer protocol FTP and rlogin use TCP as their transport protocol whereas certain other applications such as the network file system NFS used to share files across a network and the simple network management protocol SNMP used for management use UDP for transport. We will talk about SNMP in Chapter 9. UDP is also the transport protocol of choice for streaming media. 6.3.1 Routing and Forwarding IP was one of the earliest packet-switching protocols. IP transports information in the form of packets which are of variable length. An IP router is the key network element in an IP network. A router forwards packets from an incoming link onto an outgoing link. Figure 6.15 illustrates how packets are forwarded in an IP network. The nature of this routing is fundamental to IP. Here we describe the classical routing mechanism used by IP. Each router maintains a routing table. The routing table has one or more entries for each destination router in the network. The entry indicates the next node adjacent to this router to which packets need to be forwarded. The forwarding process works as follows. The router looks at the header in a packet arriving on an incoming link. The header contains the identity of the destination router for that packet. The router then does a lookup of its routing table to determine the next adjacent node for that packet and forwards the packet on the link leading to that node. In the example shown in Figure 6.15 consider a packet from node 1 destined for node 4. Node 1 looks at its table and forwards this packet to node 5. Node 5 forwards the
