CSE 123 Projects

Packets: Hey Router, Where Should I Go ? :)

Building a Simple Router


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. Your 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:

alt text

If the router is functioning correctly, you should be able to perform the following operations:

Additional requirements are laid out in the 'Required Functionality' section.


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.

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 (500 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

Configuration Files

There are two configuration files.

The default IP_CONFIG and rtable should look like the following:

> cat ~/cse123-p2/IP_CONFIG
> cat ~/cse123-p2/router/rtable    eth1    eth2    eth3

Test Connectivity of Your Emulated Topology

Start Mininet emulation by using the following command

Note: Please use byobu (pre-installed) to run multiple shell sessions on the same window. For a cheat sheet of keyboard shortcuts available, consult the byobu wiki 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': '', 'sw0-eth3': '', 'sw0-eth1': '', 'sw0-eth2': '', 'client': '', 'server2': ''}
*** Creating network
*** Creating network
*** Adding controller
*** Adding hosts:
client server1 server2
*** Adding switches:
*** Adding links:
(client, sw0) (server1, sw0) (server2, sw0)
*** Configuring hosts
client server1 server2
*** Starting controller
*** Starting 1 switches
*** setting default gateway of host server1
*** setting default gateway of host server2
*** setting default gateway of host client
*** Starting SimpleHTTPServer on host server1
*** Starting SimpleHTTPServer on host server2
*** Starting CLI:
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 byobu (type command byobu-enable to enable), press F2 to open a new terminal. Use F3/F4 keyboard shortcuts to switch the previous/next terminal.

Mininet requires a controller, which we 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': '', 'sw0-eth3': '', 'sw0-eth1': '', 'sw0-eth2': '', 'client': '', 'server2': ''}

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
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': ('', 'c2:e3:78:bf:71:f9', '10Gbps', 3), 'eth2': ('', '26:e4:1f:26:49:ab', '10Gbps', 2), 'eth1': ('', '56:01:ec:fb:34:09', '10Gbps', 1)}, rtable=[('', '', '', 'eth3'), ('', '', '', 'eth1'), ('', '', '', 'eth2')]
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)

Now you are ready to test out the connectivity of the environment setup. To do so, do the following :

> cd ~/cse123-p2/router
> make
> ./sr
You should be able to see some output like the following:

Loading routing table from server, clear local routing table.
Loading routing table
Destination     Gateway         Mask    Iface    eth1    eth2     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    eth1    eth2     eth3
Router interfaces:
eth3    HWaddrc2:e3:78:bf:71:f9
        inet addr
eth2    HWaddr26:e4:1f:26:49:ab
        inet addr
eth1    HWaddr56:01:ec:fb:34:09
        inet addr
 <-- Ready to process packets --> 

In this particular setup, is the IP for server1, and 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.

The skeleton code does not know to handle ping requests. So you will get an output like :
mininet> client ping -c 3
PING ( 56(84) bytes of data.
From icmp_seq=1 Destination Host Unreachable
From icmp_seq=2 Destination Host Unreachable
From icmp_seq=3 Destination Host Unreachable

--- ping statistics ---
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2000ms
After implementing the simple router's functionality, you should be able to see the following output.
mininet> client ping -c 3
PING ( 56(84) bytes of data.
64 bytes from icmp_req=1 ttl=63 time=184 ms
64 bytes from icmp_req=2 ttl=63 time=60.3 ms
64 bytes from icmp_req=3 ttl=63 time=95.1 ms

--- 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

Similarly, after implementing the router's functionality, you can also use traceroute to see the route between client to server1/server2.

mininet> client traceroute -n
You should be able to see the following output.
traceroute to (, 30 hops max, 60 byte packets
 1 ( 61.344ms 67.144ms 72.285ms 
 2 ( 143.898ms 139.766ms 79.988ms 

Finally, to test the web server is properly working at the server1 and server2, issue an HTTP request by using wget or curl.

mininet> client wget
To help you get started, we provide some starter code described in the following section..

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.

The starter source code

You can build and run the starter code as follows:

> cd ~/cse123-p2/router/
> make

Logging Packets

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

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

General Forwarding Logic

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

IP Forwarding

Given a raw Ethernet frame, if the frame contains an IP packet that is not destined towards one of our interfaces:

Protocols to Understand


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

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.

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:

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.

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 above:

Code Overview

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.

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.

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:

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:

Required Functionality

Using GitHub

Use your individual private repository to pull and push your code each time you make an update. The repositories contain the skeleton code (same as the one available in ~/cse123-p2/router directory). In order to clone your git repository onto your VM machine, follow the below steps:

  • Execute the below command in your VM machine.
  • $ssh-keygen -t rsa -b 4096 -C "your_email@example.com"
  • It creates a new public/private rsa key pair, using the provided email as a label. After that, you will be prompted to "Enter a file in which to save the key", just press Enter to continue.
  • Enter file in which to save the key (/Users/you/.ssh/id_rsa): [Press enter]
  • You'll be asked to enter a passphrase. Just press Enter.
  • After you enter a passphrase, you'll be given the fingerprint, or id, of your SSH key, which can be viewed using command, cat /home/mininet/.ssh/id_rsa.pub
  • Add your SSH Keys, by copying and pasting it here, https://github.com/settings/ssh
  • Run the below two commands.
  • eval $(ssh-agent -s)
    ssh-add ~/.ssh/id_rsa
  • Now you should be able to clone your private repository by executing the command 'git clone git@github.com:ucsd-cse123-fa15/CSE123_******.git'
  • A new folder, 'Your_Repo_Name' will be created in your present working directory containing the skeleton code
  • Whenever an update is made to the code in this folder, commit and push the changes to your GitHub Repo using git add, git commit -m and git push origin master commands.

    Also, don't forget to fill out your README with the description/overview of your implementation!


  • Make sure you have a README.md file in your GitHub repository. 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.
  • Make sure to have pushed your final code onto your repository

  • Last updated: 2015-11-04 10:45:46 -0800 [validate xhtml]