ed from the source to the destination. Routes can be based on static tables at the network layer that are rarely changed, or they can be dynamic to reflect the current network structure and operational conditions, such as load. The hosts connected to the network are autonomously sending packets when they wish. They usually are not informed about the traffic density of other hosts or network connections. If many hosts happen to be active at the same time, too many packets are transmitted and, hence, have the potential to get in the way of each other and form bottlenecks inside the network. The control of such congestion also belongs to the network layer. In public data networks, an accounting function (if applied) is often built into the network layer. The software in the network layer must count how many packets or bytes are sent by each customer in order to produce the charging information. In isolated small broadcast networks (such as Ethernet), routing is SO simple that the network layer is not needed at all. MAC or hardware addresses identify the hosts inside the LAN. However, if and when these net- works are connected to other networks, network addresses are mandatory. Note that the MAC addresses used in the data link layer have no importance outside one LAN. Here are some examples of network layer protocols: X.121: addressing of digital networks; Introduction to Telecommunications Network Engineering Q.931, I.451: ISDN D-channel, layer 3; Internet Protocol of the Internet: not approved by ISO but it performs basically the same functions as the network layer protocols of OSI. Figure 6.11 shows the relationship between the OSI reference model and the more popular TCP/IP protocol stack, which is explained further in Section 6.6. 6.3.3.5 Transport Layer The transport layer is the first true end-to-end layer. The protocols of hosts from the transport layer upward use the network as an end-to-end connec- tion for communication. The source message may be split into shorter seg- ments or packets, and the destinatio