For learning purposes, we encourage you to build the following four applications. The setup for these are shown here.

1. Hub

Description: A hub will flood any packet that arrives on a particular port of a switch out of all the other ports.

Packet Logic:

  • On packet_in to a switch,
    • Make list of all ports on the switch
    • Send packet out of all those ports, except the in_port

2. MAC Learning Switch

Description: The learning switch keeps track of where the host with each MAC address is located and accordingly sends packets towards the destination and not FLOOD it like a hub.

Packet logic:

  • Create a dictionary (or HashMap in Java) called mac_to_port
    • For multiple switches in the network, you need a dictionary per switch
  • On packet_in to a switch s1,
    • Parse packet to reveal src and dst MAC addr
    • Store in the dictionary the mapping between src_mac and the in_port
    • Lookup dst_mac in mac_to_port dict of switch s1 to find next hop
    • If next hop is found, create flow_mod and send
    • Else, flood like hub.

3. Stateless Load-balancer

Description: Server load-balancer that maps incoming HTTP requests to a different server in a pre-determined fashion. Main aspect is that the clients only know the IP address of the load-balancer and not the real servers.

Packet logic:

  • Pick static virtual_ip (, virtual_mac (00:00:00:00:00:05) of load-balancer
  • Initialize list of servers and their MAC.
  • On packet_in for virtual_ip from client “X”,
    • Pick server “Y” in round-robin fashion
    • Insert flow:
      • Match: Same as the incoming packet
      • Actions: 1) Rewrite dst_mac, dst_ip of packet to that of “X”, 2) Forward to Mac_to_port[“X”]
    • Proactively insert reverse flow:
      • Match: Src (IP, MAC, TCP_Port) = Y, Dst = X,
      • Action: 1) Rewrite src_mac, src_ip to that of virtual_ip, 2) Forward to port towards “X”
    • All subsequent packets of the request will directly be sent to the chosen server and not be seen by the controller.

4. Content-aware Load-balancer

Description: Based on the exact web page requested, this server load-balancer maps incoming HTTP requests to a different server in a pre-determined fashion.

Packet logic: Compared to the above stateless load-balancer, this load-balancer needs to select the server “Y” and push the bi-directional rules only after the first “GET” request packet arrives on the switch. This means that the first 3 packet_in from the TCP handshake (SYN, SYNC ACK, ACK) will need to be handled using packet_out commands by the controller.

We leave the pseudo-code as your assignment. Contact us if you need help.

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