2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 1/7 CSE 123 Home Syllabus Homeworks Exams Announcements Projects Discussion Sections Q & A CSE 123 Projects Packets: Hey Router, Where Should I Go ? :) 1. Building a Simple Router 1.1. Introduction In this assignment you will write a simple router given a static network topology and routing table. Your router will receive raw Ethernet frames. It will process these packets just like a real router and then forward them to the correct outgoing interface. You are responsible for implementing the logic for handling the incoming Ethernet frames (forward them, generate ICMP messages, drop them and more). Your router will route real packets from a emulated host (client) to two emulated application servers (http server 1/2) sitting behind your router. The application servers are each running an HTTP server. When you have finished the forwarding path of your router, you should be able to access these servers using regular client software. In addition, you should be able to ping and traceroute to and through a functioning Internet router. A sample routing topology is shown below: If the router is functioning correctly, you should be able to perform the following operations: Ping the router's interfaces (192.168.2.1, 172.64.3.1 , 10.0.1.1) from the client. Tracerouting from the client to any of the router's interfaces Ping the app servers (192.168.2.2, 172.64.3.10) from the client Tracerouting from the client to any of the app servers Downloading a file using HTTP from one of the app servers Additional requirements are laid out in the 'Required Functionality' section. 1.2. Mininet This assignment runs on top of Mininet which was developed at Stanford. Mininet allows you to emulate a topology on a single machine. It provides the needed isolation between the emulated nodes so that your router node can process and forward real Ethernet frames between the hosts like a real router. You don't have to know how Mininet works to complete this assignment, but more information about Mininet (if you're curious) is available here. 2. Getting Started Note: If you are using the lab machines to run these virtual machine (VM) images, please carry a USB drive (atleast 4GB) with you. The VM image is larger than the total disk quota (~700 MB) assigned to you. So, please perform any downloads and virtual machine operations directly into the USB drive. The Username and Password for your Virtual Machine is: mininet Fetch a pre-installed Virtual Machine image (cse-123-p2.ova) from the Public directory (Shown Below) of the cse123 class folder (Note: Download the cse-123-p2.ova directly to your machine or a USB drive, you will not have enough space in your account to copy the image). Path to the Virtual Machine image: /acsnfs3/software/cse123-vm/vm.ova You can get it using scp with: 2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 2/7 > scp
[email protected]:/acsnfs3/software/cse123-vm/vm.ova /local/directory/path/ Install a Virtual Machine manager to run the virtual machine (cse-123-p2.ova file) on your local machine. Windows : VMWare Player (make sure to get the latest version) Linux: Virtualbox/VMWare Player Mac: VMWare Fusion 11 Log in using the virtual machine image provided as mininet(username)/mininet(password) While using VMware fusion import might fail first time please relax requirements and it should work fine. Also if the VM boot-ups with the GRUB then please select Ubuntu option (post that its as per login instructions) Sidenote 1: If you are using VirtualBox, you'll need to assign a 'Host-only Adapter' to the VM. Do this by opening the Virtual Box preferences menu (the application preferences, not the virtual machine specific settings), selecting 'Network -> Host-only Networks'. If there is one already there (like called 'vboxnet0'), double click on it and select 'Enable Server' under the 'DHCP Server' tab. If one is not there, click the button to add one and then select 'Enable Server'. Then, import the machine by going to 'File -> Import Appliance'. It will give you the option to 'Reinitialize the MAC adresses of all network cards' - please select it. Then, when you are done with your setup but before booting up the VM, go to the 'Network' tab of the VM's settings. There, click on the 'Adapter 2' tab, click to enable the network adapter, and select Host only adapter/vboxnet0. Finally, after you've booted up the VM, run: `sudo dhclient eth1` Sidenote 2: If you have access to it, a commercial hypervisor like VMWare Fusion might be a slightly more reliable option than VirtualBox. You should have received an email on 11/4/19 on how to access VMWare Fusion through the university. If you haven't already, log in using the virtual machine image provided as mininet/mininet From here, you might find it more convenient to ssh into your VM instead of working inside of it directly (you'll be able to copy and paste, among other luxuries). To do this, run: > ifconfig Look for the interface that has an IP address that starts with 192 or 172 and write it down. Then, log out of your vm by typing in: > exit This should take you back to the login screen. You can now minimize the VM window. Then, in a terminal window, type: > ssh -X mininet@
2.1 Configuration Files To get your starter code please follow below steps: You can find the github assignment invitation here > cd ~/cse123-p2/ > git clone https://link/to/your/github/repo.git router > cat ~/cse123-p2/router/rtable 192.168.2.2 192.168.2.2 255.255.255.255 eth1 172.64.3.10 172.64.3.10 255.255.255.255 eth2 10.0.1.100 10.0.1.100 255.255.255.255 eth3 While running "git clone" you will be prompted to enter your github credentials (username and password)! If you run into an error along the lines of 'cannot resolve github.com', try running: 'sudo dhclient eth1' and then reattempt the git clone There are two configuration files. ~/cse123-p2/IP_CONFIG: Listed out the IP addresses assigned to the emulated hosts. It is used by mininet to emulate the topology shown above. ~/cse123-p2/rtable: The static routing table used for the simple router. The default IP_CONFIG and rtable should look like the following: > cat ~/cse123-p2/IP_CONFIG server1 192.168.2.2 server2 172.64.3.10 client 10.0.1.100 sw0-eth1 192.168.2.1 sw0-eth2 172.64.3.1 sw0-eth3 10.0.1.1 > cd ~/cse123-p2/ > cat ~/cse123-p2/rtable 192.168.2.2 192.168.2.2 255.255.255.255 eth1 172.64.3.10 172.64.3.10 255.255.255.255 eth2 10.0.1.100 10.0.1.100 255.255.255.255 eth3 [Note] if nothing appears when you cat these files, you may have missed the 'router' at the end of the git clone command. These files are symlinks to files that exist in the git repository which should be cloned to ~/cse123-p2/router 2.2 Test Connectivity of Your Emulated Topology Mininet requires a controller, which is implemented in POX. To run the controller, use the following command: > cd ~/cse123-p2/ > ./run_pox.sh You should be able to see some output like the following: POX 0.0.0 / Copyright 2011 James McCauley DEBUG:.usr.local.lib.python2.7.dist-packages.cs144-0.0.0-py2.7.egg.cs144.ofhandler:*** ofhandler: Successfully loaded ip settings for hosts {'server1': '192.168.2.2', 'sw0-eth3': '10.0.1.1', 'sw0-eth1': '192.168.2.1', 'sw0-eth2': '172.64.3.1', 'client': '10.0.1.100', 'server2': '172.64.3.10' 2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 3/7 INFO:.usr.local.lib.python2.7.dist-packages.cs144-0.0.0-py2.7.egg.cs144.srhandler:created server DEBUG:.usr.local.lib.python2.7.dist-packages.cs144-0.0.0-py2.7.egg.cs144.srhandler:SRServerListener listening on 8888 DEBUG:core:POX 0.0.0 going up... DEBUG:core:Running on CPython (2.7.4/Apr 19 2013 18:32:33) INFO:core:POX 0.0.0 is up. This program comes with ABSOLUTELY NO WARRANTY. This program is free software, and you are welcome to redistribute it under certain conditions. Type 'help(pox.license)' for details. DEBUG:openflow.of_01:Listening for connections on 0.0.0.0:6633 INFO:openflow.of_01:[Con 1/139234599119694] Connected to 7e-a2-14-d2-2b-4e DEBUG:.usr.local.lib.python2.7.dist-packages.cs144-0.0.0-py2.7.egg.cs144.ofhandler:Connection [Con 1/139234599119694] DEBUG:.usr.local.lib.python2.7.dist-packages.cs144-0.0.0-py2.7.egg.cs144.srhandler:SRServerListener catch RouterInfo even, info={'eth3': ('10.0.1.1', 'c2 Ready. POX> Please note that you have to wait for a few seconds till Mininet is able to connect to the POX controller before you continue to the next step. Keep POX running. Now, open yet another terminal to continue to the next step. (Dont’ do ctrl-z) Start Mininet emulation by using the following command Note: Please use screen to run multiple shell sessions on the same window. For a cheat sheet of keyboard shortcuts available, consult the link or use manpages. > cd ~/cse123-p2/ > ./run_mininet.sh You should be able to see some output like the following: *** Shutting down stale SimpleHTTPServers *** Shutting down stale webservers *** Successfully loaded ip settings for hosts {'server1': '192.168.2.2', 'sw0-eth3': '10.0.1.1', 'sw0-eth1': '192.168.2.1', 'sw0-eth2': '172.64.3.1', 'client': '10.0.1.100', 'server2': '172.64.3.10' *** Creating network *** Creating network *** Adding controller *** Adding hosts: client server1 server2 *** Adding switches: sw0 *** Adding links: (client, sw0) (server1, sw0) (server2, sw0) *** Configuring hosts client server1 server2 *** Starting controller *** Starting 1 switches sw0 *** setting default gateway of host server1 server1 192.168.2.1 *** setting default gateway of host server2 server2 172.64.3.1 *** setting default gateway of host client client 10.0.1.1 *** Starting SimpleHTTPServer on host server1 *** Starting SimpleHTTPServer on host server2 *** Starting CLI: mininet> Keep this terminal open, as you will need the mininet command line for debugging. Now, use another terminal to continue to the next step. (Do not do ctrl-z). If you're using screen (type screen to open new terminal). Use crtl+a+w to list different screens open and use crtl+a+"screen-id-number" to switch to needed screen. Now you are ready to test out the connectivity of the environment setup. To do so, do the following : > cd ~/cse123-p2 > ./sr_solution You should be able to see some output like the following: Using VNS sr stub code revised 2009-10-14 (rev 0.20) Loading routing table from server, clear local routing table. Loading routing table --------------------------------------------- Destination Gateway Mask Iface 192.168.2.2 192.168.2.2 255.255.255.255 eth1 172.64.3.10 172.64.3.10 255.255.255.255 eth2 10.0.1.100 10.0.1.100 255.255.255.255 eth3 --------------------------------------------- Client mininet connecting to Server localhost:8888 Requesting topology 0 successfully authenticated as mininet Loading routing table from server, clear local routing table. Loading routing table --------------------------------------------- Destination Gateway Mask Iface 192.168.2.2 192.168.2.2 255.255.255.255 eth1 172.64.3.10 172.64.3.10 255.255.255.255 eth2 10.0.1.100 10.0.1.100 255.255.255.255 eth3 --------------------------------------------- Router interfaces: eth3 HWaddrc2:e0:f7:04:02:0c inet addr 10.0.1.1 eth2 HWaddr9a:25:38:88:a3:24 inet addr 172.64.3.1 2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 4/7 eth1 HWaddr1a:4c:9c:2b:b2:d9 inet addr 192.168.2.1 <-- Ready to process packets --> In this particular setup, 192.168.2.2 is the IP for server1, and 172.64.3.10 is the IP for server2. You can find the IP addresses in your IP_CONFIG file. Now, back to the terminal where Mininet is running. To issue an command on the emulated host, type the host name followed by the command in the Mininet console. For example, the following command issues 3 pings from the client to the server1. mininet> client ping -c 3 172.64.3.10 PING 172.64.3.10 (172.64.3.10) 56(84) bytes of data. 64 bytes from 172.64.3.10: icmp_req=1 ttl=63 time=184 ms 64 bytes from 172.64.3.10: icmp_req=2 ttl=63 time=60.3 ms 64 bytes from 172.64.3.10: icmp_req=3 ttl=63 time=95.1 ms --- 172.64.3.10 ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2003ms rtt min/avg/max/mdev = 60.317/113.209/184.125/52.127 ms mininet> client traceroute -n 172.64.3.10 You should be able to see the following output. traceroute to 172.64.3.10 (172.64.3.10), 30 hops max, 60 byte packets 1 10.0.1.1 (10.0.1.1) 61.344ms 67.144ms 72.285ms 2 172.64.3.10 (172.64.3.10) 143.898ms 139.766ms 79.988ms mininet> client wget http://172.64.3.10 The above behaviour is what your router must also perform once you have implemented it. Currently if you try running the starter code, > cd ~/cse123-p2/router > make > ./sr You should be able to see some output on running "./sr" like the following: Loading routing table from server, clear local routing table. Loading routing table --------------------------------------------- Destination Gateway Mask Iface 192.168.2.2 192.168.2.2 255.255.255.255 eth1 172.64.3.10 172.64.3.10 255.255.255.255 eth2 10.0.1.100 10.0.1.100 255.255.255.255 eth3 --------------------------------------------- Client mininet connecting to Server localhost:8888 Requesting topology 0 successfully authenticated as mininet Loading routing table from server, clear local routing table. Loading routing table --------------------------------------------- Destination Gateway Mask Iface 192.168.2.2 192.168.2.2 255.255.255.255 eth1 172.64.3.10 172.64.3.10 255.255.255.255 eth2 10.0.1.100 10.0.1.100 255.255.255.255 eth3 --------------------------------------------- Router interfaces: eth3 HWaddrc2:e3:78:bf:71:f9 inet addr 10.0.1.1 eth2 HWaddr26:e4:1f:26:49:ab inet addr 172.64.3.1 eth1 HWaddr56:01:ec:fb:34:09 inet addr 192.168.2.1 <-- Ready to process packets --> Moving back to the mininet terminal, since the skeleton code does not know to handle ping requests. You will get an output for ping test like : mininet> client ping -c 3 172.64.3.10 PING 172.64.3.10 (172.64.3.10) 56(84) bytes of data. From 10.0.1.100 icmp_seq=1 Destination Host Unreachable From 10.0.1.100 icmp_seq=2 Destination Host Unreachable From 10.0.1.100 icmp_seq=3 Destination Host Unreachable --- 172.64.3.10 ping statistics --- 3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2000ms To help you get started, we provide some starter code described in the following section.. 3. Starter Code You should now have all the pieces needed to build and run the router: A routing table file that corresponds to the routing table for the router node in that topology, which we'll call rtable. 3.1 The starter source code You can build and run the starter code (complete section 2.1 which explains how to setup the starter code) as follows: > cd ~/cse123-p2 2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 5/7 > cd router > make >./sr 3.2. Logging Packets Method 1: You can log the packets receives and generated by your SR program by using the "-l" parameter with your SR program. The file will be in pcap format, i.e., you can use wireshark or tcpdump to read it. > ./sr -l logname.pcap Method 2: Besides SR, you can also use mininet to monitor the traffic goes in and out of the emulated nodes, i.e., router, server1 and server2. Mininet provides direct access to each emulated node. Take server1 as an example, to see the packets in and out of it, go to mininet CLI: mininet> server1 sudo tcpdump -n -i server1-eth0 or you can bring up a terminal inside server1 by mininet> xterm server1 then inside the newly popped xterm, > sudo tcpdump -n -i server1-eth0 Method 3 (Recommended): Or, if a GUI is more to your liking, you can use wireshark to inspect packets as they fly by. To get wireshark running, do the following: mininet> xterm sw0 Now, in your new xterm window > sudo wireshark & You can disregard the errors you'll get. In wireshark, click 'Capture Options' and select 'sw0-eth1', 'sw0-eth2', and 'sw0-eth3'. These correspond to the eth1, eth2, and eth3 interfaces on the router whose configurations are described above. Now click 'Start'. Try pinging one of your router's interfaces. You should see some packets show up in this window. Note: To get xterm working, you'll need to have an XServer installed and running. Mac users should install and run XQuartz. Windows users may need to install Xming (try running the above line first and see if it works. If not, install and run Xming). Linux users likely don't need to do anything. 4. IP Forwarding/Handling 4.1. IP Forwarding To get you started, an outline of the forwarding logic for a router follows, although it does not contain all the details. There are two main parts to this assignment: Handling ARP and IP forwarding Given a raw Ethernet frame, if the frame contains an IP packet that is not destined towards one of our interfaces: Sanity-check the packet (meets minimum length and has correct checksum). Decrement the TTL by 1, and recompute the packet checksum over the modified header. Find out which entry in the routing table has the longest prefix match with the destination IP address. Check the ARP cache for the next-hop MAC address corresponding to the next-hop IP. If it's there, send it. Otherwise, send an ARP request for the next-hop IP (if one hasn't been sent within the last second), and add the packet to the queue of packets waiting on this ARP request. Obviously, this is a very simplified version of the forwarding process, and the low-level details follow. For example, if an error occurs in any of the above steps, you will have to send an ICMP message back to the sender notifying them of an error. You may also get an ARP request or reply, which has to interact with the ARP cache correctly. 4.2. IP Packet Destinations An incoming IP packet may be destined for one of your router's IP addresses, or it may be destined elsewhere. If it is sent to one of your router's IP addresses, you should take the following actions, consistent with the section on protocols below: If the packet is an ICMP echo request and its checksum is valid, send an ICMP echo reply to the sending host. If the packet contains a TCP or UDP payload, send an ICMP port unreachable to the sending host. Otherwise, ignore the packet. Packets destined elsewhere should be forwarded using your normal forwarding logic. 5. Protocols to Understand 5.1. Ethernet You are given a raw Ethernet frame and have to send raw Ethernet frames. You should understand source and destination MAC addresses and the idea that we forward a packet one hop by changing the destination MAC address of the forwarded packet to the MAC address of the next hop's incoming interface. 2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 6/7 5.2. Internet Protocol Before operating on an IP packet, you should verify its checksum and make sure it meets the minimum length of an IP packet. You should understand how to find the longest prefix match of a destination IP address in the routing table described in the "Getting Started" section. If you determine that a datagram should be forwarded, you should correctly decrement the TTL field of the header and recompute the checksum over the changed header before forwarding it to the next hop. 5.3. Internet Control Message Protocol ICMP is a simple protocol that can send control information to a host. In this assignment, your router will use ICMP to send messages back to a sending host. You will need to properly generate the following ICMP messages (including the ICMP header checksum) in response to the sending host under the following conditions: Echo reply (type 0) Sent in response to an echo request (ping) to one of the router's interfaces. (This is only for echo requests to any of the router's IPs. An echo request sent elsewhere should be forwarded to the next hop address as usual.) Echo request (type 8) Received as a ping message to either the interfaces of the router or forwarded to any other devices in the network. NOTE: This packet is not generated by our simple router Destination net unreachable (type 3, code 0) ** Sent if there is a non-existent route to the destination IP (no matching entry in routing table when forwarding an IP packet). Destination host unreachable (type 3, code 1) ** Sent if five ARP requests were sent to the next-hop IP without a response. Port unreachable (type 3, code 3) ** Sent if an IP packet containing a UDP or TCP payload is sent to one of the router's interfaces. This is needed for traceroute to work. Time exceeded (type 11, code 0) ** Sent if an IP packet is discarded during processing because the TTL field is 0. This is also needed for traceroute to work. The source address of an ICMP message can be the source address of any of the incoming interfaces, as specified in RFC 792. As it can be seen from above, the only incoming ICMP message destined towards the router's IPs that you have to explicitly process are ICMP echo requests. You may want to create additional structs for ICMP messages for convenience, but make sure to use the packed attribute so that the compiler doesn't try to align the fields in the struct to word boundaries: 5.4. Address Resolution Protocol ARP is needed to determine the next-hop MAC address that corresponds to the next-hop IP address stored in the routing table. Without the ability to generate an ARP request and process ARP replies, your router would not be able to fill out the destination MAC address field of the raw Ethernet frame you are sending over the outgoing interface. Analogously, without the ability to process ARP requests and generate ARP replies, no other router could send your router Ethernet frames. Therefore, your router must generate and process ARP requests and replies. To lessen the number of ARP requests sent out, you are required to cache ARP replies. Cache entries should time out after 15 seconds to minimize staleness. The provided ARP cache class already times the entries out for you. When forwarding a packet to a next-hop IP address, the router should first check the ARP cache for the corresponding MAC address before sending an ARP request. In the case of a cache miss, an ARP request should be sent to a target IP address about once every second until a reply comes in. If the ARP request is sent five times with no reply, an ICMP destination host unreachable is sent back to the source IP as stated above. The provided ARP request queue will help you manage the request queue. In the case of an ARP request, you should only send an ARP reply if the target IP address is one of your router's IP addresses. In the case of an ARP reply, you should only cache the entry if the target IP address is one of your router's IP addresses. Note that ARP requests are sent to the broadcast MAC address (ff-ff-ff-ff-ff-ff). ARP replies are sent directly to the requester's MAC address. 6. Code Overview 6.1. Basic Functions Your router receives a raw Ethernet frame and sends raw Ethernet frames when sending a reply to the sending host or forwarding the frame to the next hop. The basic functions to handle these functions are: void sr_handlepacket(struct sr_instance* sr, uint8_t * packet, unsigned int len, char* interface) This method, located in sr_router.c, is called by the router each time a packet is received. The "packet" argument points to the packet buffer which contains the full packet including the ethernet header. The name of the receiving interface is passed into the method as well. int sr_send_packet(struct sr_instance* sr, uint8_t* buf, unsigned int len, const char* iface) This method, located in sr_vns_comm.c, will send an arbitrary packet of length, len, to the network out of the interface specified by iface. You should not free the buffer given to you in sr_handlepacket (this is why the buffer is labeled as being "lent" to you in the comments). You are responsible for doing correct memory management on the buffers that sr_send_packet borrows from you (that is, sr_send_packet will not call free on the buffers that you pass it). void sr_arpcache_sweepreqs(struct sr_instance *sr) The assignment requires you to send an ARP request about once a second until a reply comes back or we have sent five requests. This function is defined in sr_arpcache.c and called every second, and you should add code that iterates through the ARP request queue and re-sends any outstanding ARP requests that haven't been sent in the past second. If an ARP request has been sent 5 times with no response, a destination host unreachable should go back to all the sender of packets that were waiting on a reply to this ARP request. 6.2. Data Structures The Router (sr_router.h): The full context of the router is housed in the struct sr_instance (sr_router.h). sr_instance contains information about topology the router is routing for as well as the routing table and the list of interfaces. 2019/12/1 CSE 123 Projects https://cseweb.ucsd.edu/classes/fa19/cse123-a/cse123-project2.html 7/7 Interfaces (sr_if.c/h): After connecting, the server will send the client the hardware information for that host. The stub code uses this to create a linked list of interfaces in the router instance at member if_list. Utility methods for handling the interface list can be found at sr_if.c/h. The Routing Table (sr_rt.c/h): The routing table in the stub code is read on from a file (default filename "rtable", can be set with command line option -r ) and stored in a linked list of routing entries in the current routing instance (member routing_table). The ARP Cache and ARP Request Queue (sr_arpcache.c/h): You will need to add ARP requests and packets waiting on responses to those ARP requests to the ARP request queue. When an ARP response arrives, you will have to remove the ARP request from the queue and place it onto the ARP cache, forwarding any packets that were waiting on that ARP request. Pseudocode for these operations is provided in sr_arpcache.h. The base code already creates a thread that times out ARP cache entries 15 seconds after they are added for you. You must fill out the sr_arpcache_sweepreqs function in sr_arpcache.c that gets called every second to iterate through the ARP request queue and re-send ARP requests if necessary. Psuedocode for this is provided in sr_arpcache.h. Protocol Headers (sr_protocol.h) Within the router framework you will be dealing directly with raw Ethernet packets. The stub code itself provides some data structures in sr_protocols.h which you may use to manipulate headers easily. There are a number of resources which describe the protocol headers in detail. Network Sorcery's RFC Sourcebook provides a condensed reference to the packet formats you'll be dealing with: Ethernet IP ICMP ARP For the actual specifications, there are also the RFC's for ARP (RFC826), IP (RFC791), and ICMP (RFC792). Debugging Functions We have provided you with some basic debugging functions in sr_utils.h, sr_utils.c. Feel free to use them to print out network header information from your packets. Below are some functions you may find useful: print_hdrs(uint8_t *buf, uint32_t length) - Prints out all possible headers starting from the Ethernet header in the packet print_addr_ip_int(uint32_t ip) - Prints out a formatted IP address from a uint32_t. Make sure you are passing the IP address in the correct byte ordering. 7. Required Functionality The router must successfully route packets between the Internet and the application servers. The router must correctly handle ARP requests and replies. The router must correctly handle traceroutes through it (where it is not the end host) and to it (where it is the end host). The router must respond correctly to ICMP echo requests. The router must handle TCP/UDP packets sent to one of its interfaces. In this case the router should respond with an ICMP port unreachable. The router must maintain an ARP cache whose entries are invalidated after a timeout period (timeouts should be on the order of 15 seconds). The router must queue all packets waiting for outstanding ARP replies. If a host does not respond to 5 ARP requests, the queued packet is dropped and an ICMP host unreachable message is sent back to the source of the queued packet. The router must not needlessly drop packets (for example when waiting for an ARP reply) The router must enforce guarantees on timeouts--that is, if an ARP request is not responded to within a fixed period of time, the ICMP host unreachable message is generated even if no more packets arrive at the router. (Note: You can guarantee this by implementing the sr_arpcache_sweepreqs function in sr_arpcache.c correctly.) 8. Submission Your final code should be in your git repository that we have assigned for the project. We will take a snapshot of your code at the time of deadline. Don't forget to fill out your README with the description/overview of your implementation! Marks will be reduced if this is absent. The contents of README should start with your name and it must have information on whether you are competing for the George Varghese Espresso prize. In your README file, describe design decisions that you made, and any trade offs that you encountered in the design. 
