CPS 114 Introduction to Computer Networks: Labs

Lab 2: Simple Router


In this lab assignment, you will write a simple router with a static routing table. We will input raw ethernet frames to your code, and you can call existing functions to send out the ethernet frames; however, your task is to implement the forwarding logic between getting a frame and sending it out over another interface.

Figure 1: sample topology

Your router will route real packets from any machine to application servers sitting behind your router. The servers are running HTTP applications. 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 your functioning router. If the router is functioning correctly, all of the following operations should work:

This lab runs on top of Stanford's Virtual Network System. VNS allows you to build virtual network topologies consisting of nodes that operate on actual Ethernet frames. You don't have to know how VNS works to complete this assignment.

Getting started

You should be able to finish your assignment on any machine in the department's linux cluster: linux.cs.duke.edu. If you prefer to finish your assignment on your own machine, you may download and install ubuntu. You may run Ubuntu in a virtual machine such as the free VirtualBox. We do not support MAC or Windows. Sorry.

First of all, ssh into your home directory at the research machine, with assigned username and your own password.

xwy@linux21$ ssh <USER_NAME>@cps114.nicl.cs.duke.edu

You will find the directory router. It contains all the files you need to use for this lab. Try to have a backup of that directory, so that next time, you won't have to copy everything from the course website. You can use scp here.

xwy@linux21$ scp -r <USER_NAME>@cps114.nicl.cs.duke.edu:~/router .

Note that you may run your router code in any current Linux machine. In the following examples, we will use linux21; except for special cases requiring pcap access, we will use cps114.nicl.

  • Your routing table file rtable in your router directory is in the following format:

    Destination     Gateway(next_hop)     Mask     Iface

    Here is one sample rtable file for topology in Figure 1. The base rtable file only has a default route to the firewall.

    You can then build and run the assignment code (TOPOLOGY_ID is in the topology file):

    xwy@linux21$ make
    xwy@linux21$ ./sr -s vns-2.stanford.edu -t <TOPOLOGY_ID> -u <USER_NAME>

    From another terminal or another machine, try pinging one of the router's interfaces. Currently, all the assignment code does is to print that it gets a packet to the command line.

    Understanding the code

    Data Structure

    sr_router.[h|c]: The full context of the router is housed in the struct sr_instance (sr_router.h). sr_instance contains information about the topology the router is routing for as well as the routing table and the list of interfaces.

    sr_if.[h|c]: After connecting, the server will send the client the hardware information for that host. The assignment 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.[h|c].

    sr_rt.[h|c]: 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.

    sr_arpcache.[h|c]: 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.

    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. sr_utils.[c]: It provides useful functions that print out various headers for debugging purpose.

    Important Functions

    void sr_handlepacket(struct sr_instance* sr, uint8_t * packet, unsigned int len, char* interface)

    This function 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. 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.

    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.


    Your simple router must support the following:

    • The router can successfully route packets between the firewall and the application servers.
    • The router correctly handles ARP requests and replies.
    • The router responds correctly to ICMP echo requests.
    • The router maintains an ARP cache whose entries are invalidated after a timeout period (timeouts should be on the order of 15 seconds).
    • The router queues 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 enforces 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.
  • Functions and data structures you need to implement

    There is no specific requirement which function you can/cannot modify. To make the simple router work, you need to handle 4 kinds of packets correctly: Ethernet, IP, ICMP and ARP. For the actual specifications, there are also the RFC's for ARP (RFC826), IP (RFC791), and ICMP (RFC792).

    In our reference implementation, we developed sr_ip.[h|c], sr_icmp.[h|c] (you need to add these source files to the Makefile). We also modified sr_router.[h|c], protocol.h and sr_arpcache.[h|c]. Total work load is around 550 lines of c code.

    Understanding Protocols


    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.

    Internet Protocol

    You should understand how to find the longest prefix match of a destination IP address in the routing table. 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.

    Internet Control Message Protocol

    ICMP is used to send control information back to the 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:

    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.

    Testing and Debugging

    For testing purposes, you may wish to download the test script here. Copy the test script to you assignment directory and modify the values of eth[0|1|2]ip, app[1|2]ip, topology number and username according to your topology information. Run the test script as following:

    xwy@linux21$ python ./test_sr.py

    The above test case does not include Destination Host Unreachable, Destination Unreachable and Protocol Unreachable tests. To do these three tests, you may use test4 at cps114.nicl.cs.duke.edu. Before you start these tests, you need to reconfigure your router/rtable. Let's take Figure 1 as an example.

    Suppose your topology is shown in Figure 1, the allocated IP block is (8 IP addresses). Packets heading to this IP block will be routed to your router. This IP block consists of 4 /31 blocks. They are (connected to eth0),, (connected to eth1) and (connected to eth2). is not connected to any interface.

    For Destination Host Unreachable test, if you add a route entry  eth1

    to your routing table, once your router receives a packet heading to, it will send ARP request for through eth1. There will be no reply since does not exist at all. After 5 ARP requests without any reply, a Destination Host Unreachable ICMP packet will be generated by your router. At this point, your route/rtable should look like              eth0    eth1    eth2    eth1

    For the Destination Unreachable test, We do following changes to the above routing table. First, we remove the default route. Second we remove the routing table entry for Third, we add a entry for any PC in Figure 1. For example, if any PC is cps114.nicl.cs.duke.edu, we add a entry for its IP address ( to the routing table. After these three changes, your routing table should look like.      eth0    eth1    eth2

    Right now, your router/rtable is ready for the Destination Unreachable test. You may simply send a UDP packet heading for from cps114.nicl.cs.duke.edu( Your router should be able to generate a Destination Unreachable ICMP packet and send it to cps114.nicl.cs.duke.edu. At this point, you may think about why do we do the above three modifications. Try not to delete the default route, what do you find?

    For the Protocol Unreachable test, you may simply send one UDP packet to any of your router interfaces. Your router should be able to generate a protocol unreachable packt and send it to you.

    For the Destination Host Unreachable test, Destination Unreachable test and Protocol Unreachable tests, you can log on to cps114.nicl.cs.duke.edu with your cs appartment account and use test4 to send UDP packets and receive ICMP feedbacks. You need to create and modify config.xml. <dst> stands for your UDP packet's destination address, <intf> is the interface IP addresses of your router. Then run the test case as following.

    xwy@cps114:~$ sudo ./test4 config.xml
    This test4 sends 3 UDP packets to your specified IP address, grabs received ICMP packets and check these packets' source addresses, ICMP type and code fields.


    To submit the assignment, do the following:

    In the final submission, you must finish all required functions of this lab. For the checkpoint submission, your code should at least handle ARP requests/replies, and update the ARP cache correspondingly. Please call sr_arpcache_dump function in sr_arpcache_sweepreqs. This will dump your ARP cache periodically. We will use test4 to send UDP packets to one of the http servers. Your router should be able to update ARP cache correctly.

    Collaboration policy

    You can discuss with anyone, but you must not look at each other's code, or copy code from others.


    This lab uses VNS and is one of VNS's assignments. You can read more about how VNS works here.

    Last updated: 02/2010