Question

12. Suppose you have a wireless router connected to a cable modem at your home. And you have four PCs that use 802.11 to wirelessly connect to the router. Finally, suppose your ISP dynamically assigns one IP address to your wireless router. a. How are IP addresses assigned to the four PCs? b. Does your wireless router use NAT? Why or why not? 13. Software-Defined Networking uses a logically-centralized controller to decouple what two main parts of a computer network? 14. How does generalized forwarding (adopted by Software-Defined Networking) differ from destination-based forwarding?

Answer 1

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12. a. Typically the wireless router includes a DHCP server. DHCP is used to assign IP addresses to the 4 PCs and to the router interface.

IP addresses are assigned to 4 PCs the same way that any WiFi connected device in your home gets an IP address from your router: Dynamic Host Configuration Protocol (DHCP).

The new device sends an IP broadcast to UDP port 68 (BootP/DHCP). (Message type DHCPDISCOVER)

Hopefully, there is a DHCP server module somewhere in the network that sees this broadcast and sends back a DHCPOFFER packet containing a proposed IP address. The client may see more than one offer if there are multiple servers; it picks one and sends a DHCPREQUEST asking for a lease on that address. The server sends back DHCPACK agreeing to the lease.

The server may offer the first available address in a range that it is responsible for, or it may have a list of known clients (known by their MAC addresses) and have an assigned IP address for each of those.

b. Yes, the wireless router also uses NAT as it obtains only one IP address from the ISP.

Network Address Translation (NAT) is designed for IP address conservation. It enables private IP networks that use unregistered IP addresses to connect to the Internet. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses before packets are forwarded to another network.

As part of this capability, NAT can be configured to advertise only one address for the entire network to the outside world. This provides additional security by effectively hiding the entire internal network behind that address. NAT offers the dual functions of security and address conservation and is typically implemented in remote-access environments.

13. SDN uses a logically centralized controller to decouple the Data Plane, which contains dummy devices to forward the traffic and the Control Plane, which controls how the traffic will flow through the network.

14. Destination-based routing is the typical, most common type of routing. For this, each message that we send contains the address of the destination and the forwarding decision process makes its forwarding decision solemnly based on this address (and independent of the original sender). So, when constructing routes, one thing we can do is root a spanning tree at the destination. This creates a path from all possible sources to that destination. This is called a "sink tree". Source-based routing is the reverse of destination-based routing. The decision process is based on the source address. So, when sending a message, we only include the address of the sender (and not of the destination). The decision on where to forward the message is based on this source address. Source-based routing is only really useful for doing some kind of broadcast/multicast (e.g. consider a delivery tree when building a multicast group).

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