ore. Others will divert traffic to alternate, less- congested routes, or trickle packets into the network, a process known as "choking." This is shown in Figure 2-34. Frame relay, for example, has the ability to send what is called a choke packet into the network, implicitly telling the receiving device that it should throttle back to reduce conges- tion in the network. Whether it does or not is another story, but the point is that the network has the intelligence to invoke such a command when required. This technique is now used on freeways in large cities: Traffic lights are installed on major onramps that meter the traffic onto the roadway. This results in a dramatic reduction in congestion. Other net- works have the intelligence to diversely route traffic, thus reducing the congestion that occurs in certain areas. Clearly, the Network Layer provides a set of centrally important capa- bilities to the network itself. Through a combination of network proto- cols, routing protocols, and congestion control protocols, routers and switches provide granular control over the integrity of the network. Figure 2-34 "You'll go when | tell you to go! Congestion control using throttling Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols Back to the E-Mail Message Let us return now to our e-mail example. The message has been divided into packets by the Transport Layer and delivered in pieces to the Net- work Layer, which now takes whatever steps are necessary to ensure that the packets are properly addressed for efficient delivery to the des- tination. Each packet now has a header attached to it that contains rout- ing information and a destination address. The next step is to get the packet correctly to the next link in the network chain-in this case, the next switch or router along the way. This is the responsibility of the Data Link 