Computer Networks

Assignment 2: Internet Router

Due 11:59 p.m. Monday, April 6, 2015

Network Address Translation

For the extra credit we will now consider how to make it appear that all traffic to and from the HTTP servers is coming from the same IP address. To do so, the router performs network address translation (NAT) to translate the IP address to a common IP address for all machines within its network. In this case, within its network, the router will only have one external port, eth3. All machines connected to the router at all other ports should be considered part of the internal network. The router will perform NAT on traffic heading in both directions.

Consider the desired behavior for both outbound and inbound traffic, as described below.

Outgoing traffic

If a machine on the internal network sends a packet to a destination that is external to the router's network, the router will perform the following NAT-based actions:

• The router directs forwards the packet via its interface corresponding to the external network. In the topology that we have provided you, this interface is eth3.
• The router updates its Port Allocation Table. The port allocation table has entries as follows: .
• The external IP will always be 10.0.1.11, the IP address of the router's interface that we have designated as "external". The internal IP and internal_port will be in the packet, and the external_port will be a random number between 1024 and 65535. If the internal_IP and internal_port are already in the table, then the router replaces the destination IP and port with the corresponding entries in the port allocation table. Otherwise, the router creates a new entry in the port allocation table.
• The router sends the packet with the updated destination IP address and port.

Incoming traffic

Given a packet the router with dest_IP and dest_port:

• The router will check the port allocation table for a match to external IP and external port.
• If the entry does not exist, the router will drop the packet.
• If the entry exists, the router will update packet's destination IP address and port to the corresponding internal IP address and port from the port allocation table.

We only expect your NAT to handle traffic for connections that hosts on the internal network originate. We do not expect your NAT to handle cases where external hosts attempt to initiate connections to internal hosts.

Setup

We have provided a similar topology to above, but now the two servers would be clients and the client would be a server.

You can download the relevant topology files here. To use the files that we have provided, edit run_mininet.sh to use the script lab3_extra.py. Make sure that IP_CONFIG_EXTRA is in the same directory. Make a copy of the original rtable with the assigment and copy rtable_extra into rtable before running the run_pox.sh script. Also make sure that the included http_server directory is in the same directory as the run_mininet script. The set up instructions should work as in the original assignment.

Note that because Mininet assigns addresses to the hosts in the topology from private address space by default (i.e., 10.0.0.0/8), the internal network hosts need to be numbered from a different internal address space. The IP_CONFIG_EXTRA file makes sure that hosts are numbered appropriately.

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