Computer Networks

In this assignment you will write a simple router with a static routing table. Your router will receive raw Ethernet frames and forward them to the appropriate outgoing interface.

Your router will route real packets from an emulated host (client) to two emulated HTTP servers sitting behind your router. 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 Internet router. A sample routing topology is shown below:

If the router is functioning correctly, all of the following operations should work:

• Ping from the client to any of the router's interfaces (192.168.2.1, 172.64.3.1, 10.0.1.1).
• Traceroute from the client to any of the router's interfaces
• Ping from the client to any of the HTTP servers (192.168.2.2, 172.64.3.10)
• Traceroute from the client to any of the HTTP servers
• Downloading a file using HTTP from one of the HTTP servers

Network Emulation using Mininet

This assignment runs on top of Mininet which was built 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. We provide scripts to set up the Mininet topology above on which your router will work. In addition your code will use the POX controller on Mininet in order to run your code on the router in the Mininet topology. We also provide the scripts to set up POX and your code on the topology. In the following section we will tell you how to set up your environment to emulate the the topology above using Mininet and POX given the scripts that we have provided.

Environment Setup

In order to start Mininet, first export your Python path: 'export PYTHONPATH=$PYTHONPATH:/usr/lib/python2.7/site-packages/:/usr/lib/python2.6/site-packages/:~/ass2/pox_module/'. Then begin the Mininet set up script that we provide: 'sudo ./run_mininet.sh' Keep this terminal open, as you will need the Mininet command-line for debugging. (Do not do ctrl-z)

Next you will begin the POX controller for Minnet. In a new terminal, begin the POX set up script that we have provided: 'sudo ./run_pox.sh'. Please note that you have to wait until Mininet is able to connect to the POX controller before you continue to the next step.
Once Mininet is connected to POX, keep POX running. (Do not do ctrl-z)

In order to start reference solution proviced, in a third terminal, start the reference solution binary, sr_solution: ' ./sr_solution'. The output from the reference solution will begin by loading the routing table as specified in rtable. IP addresses for the hosts in the topology are in the IP_CONFIG file.

To test the router, you can issue commands in the Mininet console. To run a command on an emulated host, type the host name followed by the command. For example to have the clinet ping HTTP server 1, you would run the following command: 'client ping -c 3 192.168.2.2'. The client can find the route to HTTP server 1, using traceroute in the following command: 'client traceroute -n 192.168.2.2'. The client can retrieve the main page of HTTP server 1 using client 'wget http://192.168.2.2'

To help you get started, we provide some starter code. After following the previous steps in this section, You can build and run the starter code as follows: 'make;./sr'. The starter code initializes the routing table given a file and it gets the router ready to receive packets. You can log the packets received and generated by your program by using the "-l" parameter with your SR program: './sr -l logname.pcap' The file will be in pcap format, so you can use wireshark or tcpdump to read it. Besides SR, you can also use mininet to monitor the traffic that goes in and out of the emulated nodes such as the router, server1 and server2, using tcpdump. (For example to do so for HTTP server one, in the mininet console, use the command: 'server1 sudo tcpdump -n -i server1-eth0'.)

General Forwarding Logic

There are two main parts to this assignment: Handling ARP and IP forwarding.

Your router will receive raw Ethernet frames and have to send raw Ethernet frames. To handle these 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.

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 a received 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.

Given a raw Ethernet frame, if the frame contains an ARP packet, check:

• Is it a request for the router's MAC address? If so, send an ARP reply.
• Is it a reply to a request the router previously sent out? If so, cache the reply and forward the IP packets waiting on this reply.

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). You may simply drop the packet if either of these checks fails.
• 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.

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; ICMP type 8) to one of the router's interfaces. This message is only sent 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.
• 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 mentioned 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:

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:

• 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.

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)

int sr_send_packet(struct sr_instance* sr, uint8_t* buf, unsigned int len, const char* iface)

You should not free the buffer given to you in sr_handlepacket (you'll see the comment "lent" or "borrowed" whenever you should not free a buffer). 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).

The assignment requires you to send an ARP request about once a second until a reply comes back or we have sent five requests. To do so, you must implement the function sr_arpcache_sweepreqs defined in sr_arpcache.c. This function is 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. You must add this functionality to the handle_arpreqs function defined in sr_router.c. Pseudocode for these operations is provided in sr_arpcache.h.

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.

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 how to use the ARP cache and request queue 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.

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 good reference to the packet formats you'll be dealing with:

• Ethernet
• IP
• ICMP
• ARP

Requirements

• 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.)

Also, don't forget to fill out your readme! (It is worth 1 point)

This assignment is considerably harder than the first two assignments, so please get started early. To help you debug your topologies and understand the required behavior, we provide a reference binary and you can find it at ~/ass2/sr_solution in your directory.

Extra Credit

Submission

You should submit your completed router by the due date to CS Dropbox. In your README, describe design decisions that you made, and any trade-offs that you encountered in the design.