EE586 Homework and Laboratory #6

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Please type your homework. We will discuss each problem in class but you must do your own workfor submission.

Homework1. (10/21, 10 points) Consider a datagram network using 32-bit host addresses. Suppose

a router has four links, numbered 0 through 3, and packets are to be forwarded to thelink interfaces as follows:Destination Address Range Link Interface11100000 00000000 00000000 00000000 through11100000 00111111 11111111 11111111

0

11100000 01000000 00000000 00000000 through11100000 01000000 11111111 11111111

1

11100000 01000001 00000000 00000000 through11100001 01111111 11111111 11111111

2

Otherwise 3

a. Provide a forwarding table that has four entries, uses longest prefix matching, andforwards packets to the correct link interfaces.

b. Describe how your forwarding table determines the appropriate link interface fordatagrams with destination addresses:

11001000 10010001 01010001 0101010111100001 01000000 11000011 0011110011100001 10000000 00010001 01110111

2. (10/21, 10 points) Consider a subnet with prefix 128.119.40.128/26. Give an exampleof one IP address (of form xxx.xxx.xxx.xxx) that can be assigned to this network.Suppose an ISPowns the block of addresses of the form 128.119.40.64/26. Suppose itwants to create four subnets from this block, with each block having the same numberof IP addresses. What are the prefixes (of form a.b.c.d/x) for the four subnets?

3. (10/21, 10 points) Suppose you are interested in detecting the number of hosts behinda NAT. You observe that the IP layer stamps an identification number sequentially oneach IP packet. The identification number of the first IP packet generated by a host isa random number, and the identification numbers of the subsequent IP packets aresequentially assigned. Assume all IP packets generated by hosts behind the NAT aresent to the outside world.a. Based on this observation, and assuming you can sniff all packets sent by the

NAT to the outside, can you outline a simple technique that detects the number ofunique hosts behind a NAT? Justify your answer.

b. If the identification numbers are not sequentially assigned but randomly assigned,would your technique work? Justify your answer.

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4. (10/23, 10 points) Consider the following network. With the indicated link costs, useDijkstra’s shortest-path algorithm to compute the shortest path from x to all networknodes. Show how the algorithm works by computing a table similar to that presentedin the lecture.

5. (10/21, 10 pts) Consider the network shown below, and assume that each nodeinitially knows the costs to each of its neighbors. Consider the distance-vectoralgorithm and show the distance table entries at node z.

6. (10/25, 10 pts) Consider the network shown below.

Suppose AS3 and AS2 are running OSPF for their intra-AS routing protocol. SupposeAS1 and AS4 are running RIP for their intra-AS routing protocol. Suppose eBGP andiBGP are used for the inter-AS routing protocol. Initially suppose there is no physicallink between AS2 and AS4.

a. Router 3c learns about prefix x from which routing protocol: OSPF, RIP,eBGP, or iBGP?

b. Router 3a learns about x from which routing protocol?c. Router 1c learns about x from which routing protocol?d. Router 1d learns about x from which routing protocol?

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7. (10/23, 20 pts) Wireshark ProblemIn this lab, we’ll take a quick look at DHCP by examining the DHCP packetscaptured by a host. If you also have administrative access to your DHCP server, youmay want to repeat this lab after making some configuration changes (such as thelease time). If you have a router at home, you most likely can configure your DHCPserver. Because many linux/Unix machines (especially those that serve many users)have a static IP address and because manipulating DHCP on such machines typicallyrequires super-user privileges, we’ll only present a Windows version of this lab below.

In order to observe DHCP in action, we’ll perform several DHCP-related commandsand capture the DHCP messages exchanged as a result of executing these commands.Do the following:

a) Begin by opening the Windows Command Prompt application (which can befound in your Accessories folder). As shown in Figure 1, enter “ipconfig/release”. The executable for ipconfig is in C:\windows\system32. Thiscommand releases your current IP address, so that your host’s IP addressbecomes 0.0.0.0.

b) Start up the Wireshark packet sniffer, as described in the introductoryWireshark lab and begin Wireshark packet capture.

c) Now go back to the Windows Command Prompt and enter “ipconfig /renew”.This instructs your host to obtain a network configuration, including a new IPaddress.

d) Wait until the “ipconfig /renew” has terminated. Then enter the samecommand “ipconfig /renew” again.

e) When the second “ipconfig /renew” terminates, enter the command“ipconfig/release” to release the previously-allocated IP address to yourcomputer.

f) Finally, enter “ipconfig /renew” to again be allocated an IP address for yourcomputer.

g) Stop Wireshark packet capture.h) Now let’s take a look at the resulting Wireshark window. To see only the

DHCP packets, enter into the filter field “bootp”. (DHCP derives from anolder protocol called BOOTP. Both BOOTP and DHCP use the same portnumbers, 67 and 68. To see DHCP packets in the current version of Wireshark,you need to enter “bootp” and not “dhcp” in the filter.)

What to hand in:You should hand in a screen shot of the Command Prompt window. Whenever possible,when answering a question below, you should hand in a printout of the packet(s) withinthe trace that you used to answer the question asked. Annotate the printout to explainyour answer. To print a packet, use File->Print, choose Selected packet only, choosePacket summary line, and select the minimum amount of packet detail that you need toanswer the question.

Questions:1. Are DHCP messages sent over UDP or TCP?2. Draw a timing datagram illustrating the sequence of the first four-packet

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Discover/Offer/Request/ACK DHCP exchange between the client and server. For eachpacket, indicated the source and destination port numbers. Are the port numbers the same asin the example given in this lab assignment?

3. What is the link-layer (e.g., Ethernet) address of your host?4. What values in the DHCP discover message differentiate this message from the DHCP

request message?5. What is the value of the Transaction-ID in each of the first four

(Discover/Offer/Request/ACK) DHCP messages? What are the values of the Transaction-IDin the second set (Request/ACK) set of DHCP messages? What is the purpose of theTransaction-ID field?

6. A host uses DHCP to obtain an IP address, among other things. But a host’s IP address is notconfirmed until the end of the four-message exchange! If the IP address is not set until theend of the four-message exchange, then what values are used in the IP datagrams in the four-message exchange? For each of the four DHCP messages (Discover/Offer/Request/ACKDHCP), indicate the source and destination IP addresses that are carried in the encapsulatingIP datagram.

7. What is the IP address of your DHCP server?8. What IP address is the DHCP server offering to your host in the DHCP Offer message?

Indicate which DHCP message contains the offered DHCP address.9. In the example screenshot in this assignment, there is no relay agent between the host and the

DHCP server. What values in the trace indicate the absence of a relay agent? Is there a relayagent in your experiment? If so what is the IP address of the agent?

10. Explain the purpose of the router and subnet mask lines in the DHCP offer message.11. In the DHCP trace file noted in footnote 2, the DHCP server offers a specific IP address to the

client (see also question 8. above). In the client’s response to the first server OFFERmessage, does the client accept this IP address? Where in the client’s RESPONSE is theclient’s requested address?

12. Explain the purpose of the lease time. How long is the lease time in your experiment?13. What is the purpose of the DHCP release message? Does the DHCP server issue an

acknowledgment of receipt of the client’s DHCP request? What would happen if the client’sDHCP release message is lost?

14. Clear the bootp filter from your Wireshark window. Were any ARP packets sent or receivedduring the DHCP packet-exchange period? If so, explain the purpose of those ARP packets.

8. (10/25, 20 pts) OPNET ProblemSee attached Border Gateway Protocol (BGP) Lab

Border Gateway Protocol (BGP)An Interdomain Routing Protocol

The objective of this lab is to simulate and study the basic features of an interdomainrouting protocol called Border Gateway Protocol (BGP).

The Internet is organized as a set of routing domains. Each routing domain is called anautonomous system (AS). Each AS is controlled by a single administrative entity (e.g., anAS of a single service provider). Each AS has a unique 16-bit identification number. Thisnumber is assigned by a central authority. An AS employs its own intradomain routingprotocol (e.g., RIP or OSPF). Different ASs establish routes among each other throughinterdomain routing protocols. The border gateway protocol (BGP) is one of the majorinterdomain routing protocols.

The main goal of BGP is to find any path to the destination that is loop-free. This isdifferent from the common goal of intradomain routing protocols, which is to find anoptimal route to the destination based on a specific link metric. The routers that connectdifferent ASs are called border gateways. The task of the border gateways is to forwardpackets between ASs. Each AS has also at least one BGP speaker. BGP speakersexchange reachability information among ASs.

BGP advertises the complete path to the destination AS as an enumerated list. This wayrouting loops can be avoided. A BGP speaker can also apply some policies such asbalancing the load over the neighboring ASs. If a BGP speaker has a choice of severaldifferent routes to a destination, it will advertise the best one according to its own localpolicies. BGP is defined to run on top of TCP and hence BGP speakers do not need toworry about acknowledging received information or retransmission of sent information.

In this lab you will set up a network with three different ASs. RIP will be used as theintradomain routing protocol and BGP as the interdomain one. You will analyze the routingtables generated in the routers as well as the effect of applying a simple policy.

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Read section 4.3 from "Computer Networks: A Systems Approach", 4th Edition.

Go to www.net-seal.net/animations.php and play the following animation:- IP Subnets.

Create a New Project

1. Start OPNET IT Guru Academic Edition Choose New from the File menu.

2. Select Project and click OK Name the project <your initials>_BGP, and thescenario No_BGP Click OK.

3. In the Startup Wizard: Initial Topology dialog box, make sure that Create EmptyScenario is selected Click Next Select Campus from the Network Scalelist Click Next three times Click OK.

Create and Configure the Network

Initialize the Network:

1. The Object Palette dialog box should now be on top of your project workspace. If

it is not there, open it by clicking . Make sure that the internet_toolbox isselected from the pull-down menu on the object palette.

2. Add to the project workspace the following objects from the palette: sixethernet4_slip8_gtwy routers and two 100BaseT_LAN objects.

a. To add an object from a palette, click its icon in the object palette Move yourmouse to the workspace Click to place the object Right-click to stopcreating objects of that type.

3. Use bidirectional PPP_DS3 links to connect the routers you just added as in thefollowing figure. Also, rename the network objects as shown (right-click on thenode Set Name).

4. Use a bidirectional 100BaseT link to connect LAN_West to Router1 and another100BaseT link to connect LAN_East to Router6 as in the following figure.

5. Close the Object Palette dialog box.

6. Save your project.

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The ethernet4_slip8_gtwy node modelrepresents an IP-basedgateway supporting fourEthernet hub interfacesand eight serial lineinterfaces. IP packetsarriving on any interfaceare routed to theappropriate outputinterface based on theirdestination IP address.

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Routers Configuration:

1. Highlight or select simultaneously (using shift and left-click) all six routersRight-click on any router Edit Attributes Check the Apply Changes toSelected Objects check box.

2. Expand the BGP Parameters hierarchy and set the following:i. Redistribution Routing Protocols RIP Redistribute w/ Default.

3. Expand the IP Routing Parameters hierarchy and set the following:i. Routing Table Export = Once at End of Simulation. This asks the router to

export its routing table at the end of the simulation to the simulation log.4. Expand the RIP Parameters hierarchy and set the following:

i. Redistribution Routing Protocols Directly ConnectedRedistribute w/ Default.

5. Click OK and then save your project.

Application Configuration:

1. Right-click on LAN_West Edit Attributes Assign All to Application:Supported Services Assign West_Server to the LAN Server Name attributeas shown Click OK.

Notice that two objects for Applications and Profiles will be added automaticallyto the project.

Redistribute w/Default. allows a routerto have a route to adestination that belongsto another AutonomousSystem

2. Right-click on LAN_East Edit Attributes:

i. Expand the Application: Supported Profiles hierarchy Set rows to 1Expand the row 0 hierarchy Set Profile Name to E-commerceCustomer.

ii. Edit the Application: Destination Preferences attribute as follows:

Set rows to 1 Set Symbolic Name to HTTP Server Edit Actual Name Set rows to 1 In the new row, assign West_ Server to the Name column.

3. Click OK three times and then save your project..

Configure the Simulation

Here we need to configure some of the simulation parameters:

1. Click on and the Configure Simulation window should appear.2. Set the duration to be 10.0 minutes.3. Click on the Global Attributes tab and make sure that the following attributes

are assigned as follows:

a. IP Interface Addressing Mode = Auto Addressed/Export.b. IP Routing Table Export/Import = Export.c. RIP Sim Efficiency = Disabled. If this attribute is enabled, RIP will stop

after the "RIP Stop Time." But we need the RIP to keep updating therouting table in case there is any change in the network.

4. Click OK and then save the project.

Choose the Statistics

1. Right-click on LAN_East and select Choose Individual Statistics from the pop-up menu From the Client HTTP hierarchy choose the Traffic Received(bytes/sec) statistic Click OK.

2. Right-click on the link that connects Router2 to Router3 and select ChooseIndividual Statistics from the pop-up menu From the point-to-pointhierarchy choose the “Throughput (bits/sec) -->” statistic Click OK.

Note: If the name of the link is “Router3 <-> Router2” then you will need to choosethe “Throughput (bits/sec) <--” statistic instead.

3. Right-click on the link that connects Router2 to Router4 and select ChooseIndividual Statistics from the pop-up menu From the point-to-pointhierarchy choose the “Throughput (bits/sec) -->” statistic Click OK.

Note: If the name of the link is “Router4 <-> Router2” then you will need to choosethe “Throughput (bits/sec) <--” statistic instead.

4. Save your project.

Auto Addressed meansthat all IP interfaces areassigned IP addressesautomatically duringsimulation. The class ofaddress (e.g., A, B, or C)is determined based onthe number of hosts inthe designed network.Subnet masks assignedto these interfaces arethe default subnet masksfor that class.

Export causes the auto-assigned IP interface tobe exported to a file(name of the file is<net_name>-ip_addresses.gdf andgets saved in the primarymodel directory).

Router Interfaces and IP Addresses

Before setting up the routers to use BGP, we need to get the information of the routers’interfaces along with the IP addresses associated to these interfaces. Recall that these IPaddresses are assigned automatically during simulation, and we set the global attribute IPInterface Addressing Mode to export this information to a file.

1. First we need to run the simulation. Click on and the Configure Simulationwindow should appear Click on Run.

2. After the simulation run completes click Close.3. From the File menu choose Model Files Refresh Model Directories. This

causes OPNET IT Guru to search the model directories and update its list of files.4. From the File menu choose Open From the drop-down menu choose Generic

Data File Select the <<your initials>_BGP-No_BGP -ip_addresses fileClick OK.

The file that contains all the information about router interfaces and their IP addresses willopen. Table 1 shows the interface number and IP addresses between the six outers in ourprojects. For example Router1 is connected to Router2 through interface (IF) 11, which isassigned 192.0.1.1 as its IP address. A router is connected to itself by a Loopbackinterface as shown. Create a similar table for your project but note that your result mayvary due to different nodes placement.

IF: 12IP: 192.0.2.1

IF: 11IP: 192.0.1.1

IF: 10IP: 192.0.1.2

IF: 12IP: 192.0.8.1

IF: 11IP: 192.0.4.2

IF: 4IP: 192.0.7.1

IF: 10IP: 192.0.4.1

IF: 12IP: 192.0.6.1

IF: 11IP: 192.0.5.1

IF: 4IP: 192.0.3.1

IF: 10IP: 192.0.7.2

IF: 11IP: 192.0.5.2

IF: 12IP: 192.0.10.1

IF: 4IP: 192.0.9.1

IF: 10IP: 192.0.3.2

IF: 11IP: 192.0.9.2

IF: 12IP: 192.0.12.1

IF: 4IP: 192.0.11.1

IF: 10IP: 192.0.11.2

IF: 12IP: 192.0.14.1

Table 1: Interfaces that connect the routers and their assigned IP addresses

Creating the BGP Scenario

In the network we just created, all routers belong to the same autonomous system. We willdivide the network into three autonomous systems and utilize BGP to route packetsamong these systems.

1. Select Duplicate Scenario from the Scenarios menu and name it BGP_Simple Click OK.

2. Highlight or select simultaneously (using shift and left-click) Router1 and Router2 Right-click on Router1 Edit Attributes Check the Apply Changes to

Selected Objects check box.3. Expand the IP Routing Parameters hierarchy and set the Autonomous System

Number to 12 Click OK.4. Repeat steps 2 and 3 above for routers Router3 and Router4. Assign their

Autonomous System Number to 34.

5. Repeat steps 2 and 3 above for routers Router5 and Router6. Assign theirAutonomous System Number to 56.

The following figure shows the created autonomous systems. The figure shows also theinterfaces that connect routers across different autonomous systems. There interfaces aretaken from Table 1 above (note: the interface numbers in your project may vary). The nextstep is to disable the RIP protocol on the shown interfaces.

AS 34AS 12

AS 5611

4

10 4

10 4

10

11

6. Right-click on Router2 Edit Attributes Expand the IP RoutingParameters hierarchy Expand the Interface Information hierarchy Expandrow 4 hierarchy Click on the values of the Routing Protocol(s) attributeDisable RIP as shown click OK twice.

7. Repeat step 6 above for all other interfaces that connect routers acrossautonomous systems (i.e., All the eight inter-domain interfaces shown above).

8. Save your project.

Configuring the BGP Neighbor Information

If you try to run the simulation of the BGP_Simple scenario, you will receive hundreds oferrors! This is because there is no routing protocol running between the inter-domainrouters. Therefore, no routing tables are created to deliver packets among autonomoussystems. The solution is to utilize BGP by defining the neighbors of inter-domain routers.Table 2 shows the neighbors of the routers that will run BGP. Neighbors are defined bytheir interface IP address and the AS number. For each router in Table 2 carry out thefollowing step:

1. Right-click on the router Edit Attributes Expand the BGP Parametershierarchy Expand the Neighbor Information hierarchy Assign to the rowsattribute the value 1 for Router1 and Router6. For all other routers, assign thevalue 3 to the rows attribute Utilize Table 2 to assign the corresponding valuesto the IP Address, Remote AS, and Update Source attributes for each of theadded rows.

Note: the values to be assigned to the IP Address attribute have to match thevalues you will collect in your Table 1.

2. Save your project.

BGP Parameters Neighbor InformationRoutersrow 0 row 1 row 2

Router1IP Address: 192.0.8.1Remote AS: 12Update Source: Loopback

Router2IP Address: 192.0.4.1Remote AS: 34Update Source: Not Used

IP Address: 192.0.7.2Remote AS: 34Update Source: Not Used

IP Address: 192.0.2.1Remote AS: 12Update Source: Loopback

Router3IP Address: 192.0.4.2Remote AS: 12Update Source: Not Used

IP Address: 192.0.3.2Remote AS: 56Update Source: Not Used

IP Address: 192.0.10.1Remote AS: 34Update Source: Loopback

Router4IP Address: 192.0.7.1Remote AS: 12Update Source: Not Used

IP Address: 192.0.9.2Remote AS: 56Update Source: Not Used

IP Address: 192.0.6.1Remote AS: 34Update Source: Loopback

Router5IP Address: 192.0.3.1Remote AS: 34Update Source: Not Used

IP Address: 192.0.9.1Remote AS: 34Update Source: Not Used

IP Address: 192.0.14.1Remote AS: 56Update Source: Loopback

Router6IP Address: 192.0.12.1Remote AS: 56Update Source: Loopback

Table 2: Neighbors’ info for inter-domain routers

Creating the BGP with Policy Scenario

BGP allows for routing policies that can be enforced using route maps. We will utilize thisfeature to configure Router2 to redirect its load on the two egress links of its autonomoussystem.

1. First make sure that your project is in the BGP_Simple scenario. SelectDuplicate Scenario from the Scenarios menu and name it BGP_Policy ClickOK.

2. Right-click on Router2 Edit Attributes Expand the IP RoutingParameters hierarchy Expand the Route Map Configuration hierarchySet the attributes as shown in the following figure.

The purpose of the created route map is to reduce the degree of preference ofthe “route to AS 56” to the value 10 (Note: the normal value is "99", which iscalculated as 100 - number of AS that should be crossed to reach thedestination).

The next step is to assign the above route map to the link connecting Router2 to Router3.This way traffic from Router2 to AS 56 will be preferred to go through Router4 instead.

3. Right-click on Router2 Edit Attributes Expand the BGP Parametershierarchy Expand the Neighbor Information hierarchy Expand the row thathas the IP address of Router3 interface (it is row 0 in my project) Expand theRouting Policies hierarchy Set its attribute as shown in the following figure.

4. Click OK and save your project.

Run the Simulation

To run the simulation for the three scenarios simultaneously:

1. Go to the Scenarios menu Select Manage Scenarios.

2. Change the values under the Results column to <collect> (or <recollect>)for the three scenarios. Compare to the following figure.

3. Click OK to run the three simulations. Depending on the speed of yourprocessor, this may take several minutes to complete.

4. After the three simulation runs complete, one for each scenario, click CloseSave your project.

View the Results

Compare the Routing Tables Content:

1. To check the content of the routing tables in Router2 for both scenarios:i. Go to the Results menu Open Simulation Log Expand the hierarchy

on the left as shown below Click on the field COMMON ROUTE TABLE inthe row corresponds to Rouer2.

2. Carry out the previous step for scenario No_BGP and scenario BGP_Simple.The following are partial contents of Router2’s routing table for both scenarios(Note: Your results may vary due to different nodes placement):

Routing table of Router2 for the No_BGP scenario:

Routing table of Router2 for the BGP_Simple scenario:

Compare the load in the network:

1. Select Compare Results from the Results menu.

2. Change the drop-down menu in the right-lower part of the Compare Resultsdialog box from As Is to time_average as shown.

3. Select and show the graphs of the following statistics: Traffic Received inLAN_East, throughput in the Router2-Router3 link, and throughput in theRouter2-Router4 link. The resulting graphs should resemble the graphs below.

A Border Gateway Protocol 4 (BGP-4): IETF RFC number 1771(www.ietf.org/rfc.html).

Application of the Border Gateway Protocol in the Internet: IETF RFC number1772 (www.ietf.org/rfc.html).

BGP-4 Protocol Analysis: IETF RFC number 1774 (www.ietf.org/rfc.html).

1) Obtain and analyze the routing table for Router5 in the project before and afterapplying BGP.

2) Analyze the graphs that show the throughput in both Router2-Router3 link andRouter2-Router4 link. Explain the effect of applying the routing policy on thesethroughputs.

3) Create another scenario as a duplicate of the BGP_Simple scenario. Name thenew scenario BGP_OSPF_RIP. In this new scenario change the intradomainrouting protocol in AS 56 to be OSPF instead of RIP. Run the new scenario andcheck the contents of Router5‘s routing table. Analyze the content of this table.

Prepare a report that follows the guidelines explained in Lab 0. The report should includethe answers to the above exercises as well as the graphs you generated from thesimulation scenarios. Discuss the results you obtained and compare these results withyour expectations. Mention any anomalies or unexplained behaviors.

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