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20 21 22 23 25 53 69 80 110 119 123 143 443

The Exterior Gateway Protocol, or EGP, has three components, neighbor acquisition, neighbor reachability, and routing information EGP was designed to add a measure of automation to the configuration of routes between different autonomous systems EGP has been superseded by the Border Gateway Protocol, (BGP), so we will not spend too much time on it EGP Neighbor Acquisition We can use Fig 4-21 to discuss how EGP works Suppose router A has been configured to initiate a neighbor relationship with router B This will initiate a neighbor acquisition packet to be sent from router A to router B If router B has been configured to accept a neighbor relationship with A, this packet is accepted The neighbor reachability mechanism is the hello packet, used to verify that a router that is accepted as a neighbor is still functioning The routing information of EGP is similar to distance vector protocols, but it omits the metric for routes advertised EGP was implemented like this because it was designed for the Internet, when it was assumed that there would be a core network with separate routing domains connected to this core by one router The major problem with using EGP in a more generalized network is that, because no use is made of metrics, if there is more than one path to a destination, packets can easily get caught in routing loops (Fig 4-22)

File Transfer Protocol used to transfer data from one machine to another File Transfer Protocol used for control messages of the FTP session Secure Shell Telnet used to create a terminal session Simple Mail Transfer Protocol used to send e-mail across the Internet Domain Name System used to query DNS servers for the IP address of a remote system Trivial File Transfer Protocol Hypertext Transfer Protocol used to deliver web pages from a web server to the web client Post Office Protocol, version 3 a protocol for reading e-mail over the Internet Network News Transfer Protocol used to read news articles from a news server Used by the Network Time Protocol to synchronize the time on systems Internet Message Application Protocol, version 4 another Internet protocol for reading e-mail Secure Hypertext Transfer Protocol used to encrypt web traffic between a client and server

In this exercise, you will look back to the packet capture that you viewed in 2 for the analysis of web traffic and identify the ports that were used by both the web browser (sending system) and the web server (the receiving system) 1 Start Network Monitor by choosing Start | Programs | Administrative Tools | Network Monitor 2 Once Network Monitor has started, open the HTTPTraffic capture file from the LabFiles\PacketCaptures folder 3 Double-click the frame in the packet capture that is posting form data to a web server (frame 16) to expand the details of that packet 4 In the details panel (middle of screen), select the TCP section by clicking it once (as shown in the accompanying illustration)

Figure 7 2: A logical flowchart illustrating how packets are matched against the entries in an access list

5 Notice that the source port and the destination port are shown on this line on the far right side Record the following information in the spaces provided: Source Port: ___________________ Destination Port: ________________

If router R4 receives a packet for AS1, it will have two routes to AS2, one via R3 and one via R6 Because there are no metrics involved, the routes are equal in R4's determination, so it may choose either Let's say it chooses to send the packet to R3 R3 also has two routes to AS1, one via R4 and one to R1 Again, these routes are equal in R3's consideration, so the packet could be returned to R4, where the cycle starts over again Clearly, EGP has problems for a general environment, so I would recommend leaving it alone

4:

6 You should have the following answers recorded in the preceding spaces: Source Port: ____3556__________ Destination Port: ____80_________ 7 Keep in mind that the destination port is port 80, which is the default port of a web server You have found that the source port is 3556, which is a value above 1024 You have learned that a client application usually uses a value above 1024, so this is traffic of a web browser sending data to a web server 8 Close Network Monitor and don t save any information if asked

The access list code contains several points of interest First, notice that only TCP/IP protocols are used in this example, yet numerous protocols other than those in the TCP/IP suite can be specified Table 7 1 shows the types of access lists that can be created for various protocols The number range on the right side of the table specifies the access list number range that is used when creating an access list of the specified type Table 7 1: Access Lists Protocols and Their Number Ranges