SDN

Question 1

What does SDN stand for?

Answer 1

Software-defined Networking

Question 2

What are some disadvantages of traditional routing?

Answer 2

• Difficult to make changes
• Constant communication between routers
• Fairly static (long convergence time)
• Dependent on hardware (vendors)

Question 3

How does SDN tackle routing?

Answer 3

Decouple control plane from data plane.

Question 4

Define the two SDN elements (and behaviour)

Answer 4

Controller
– Has global network view
– Centralized decision making
– Programmable
Switches
– Dumb
– Forwarding rules configured by controller

Question 5

What are the advantages of SDN?

Answer 5

They’re programable so allow to be flexible and for finegrained traffic management. They contain a centralized view of the network so it is easier to compute paths, add security or fault tolerance.

Question 6

What are the disadvantages of SDN?

Answer 6

Centralized: Single point of failure.
Scalability: Processing power bottleneck.
Intitial delay: Installing flows on all switches only as they occur will delay the first message.

Question 7

How does data forwarding work in the SDN Data Plane?

Answer 7

Rus dictate the data (forwarding) behaviour: Rule = Match + Action

Example:
* Match: destination IP = 12.3.4.5
* Action: forward packet on port 6

Question 8

Who generates data plane rules in SDN?

Answer 8

the controller

Question 9

Where are forwarding rules stored in SDN?

Answer 9

Switchs store forwarding rules in a flow table.

Question 10

Explain the **Proactive ** Mode of Operation for SDN.

Answer 10

controller
installs rules on
switches before
packets arrive.

Question 11

Explain the **Reactive ** Mode of Operation for SDN.

Answer 11

controller
installs rules on
switches as soon as
packets arrive.

Question 12

What does a switch do in order to install rules from a controller?

Answer 12

It forwards the packet to the controller.

Question 13

What is the SDN Control plane Northbound API?

Answer 13

Interact with network
applications

Question 14

What is the SDN Control plane Southbound API?

Answer 14

Interact with switches

Question 15

What is the SDN Control plane Westbound API?

Answer 15

Interact with other controllers

Question 16

What is the SDN Control plane Eastbound API?

Answer 16

Interact with legacy infrastructure

Question 17

What does ARP stand for?

Answer 17

Adress Resolution protocol

Question 18

Explain the events on a SDN-controller in case of an ARP request by h1 to h2. (asume only 1 switch connecting them)

Answer 18

First h1 will broadcast the ARP Request, in order to find out the adress of h2. The controller will recieve this ARP request (PacketIn), and simple return a flood signal (PacketOut). Upon receival, h2 will send an** ARP reply. Hence, the controller will get the ARP reply from s1, then it will install a flow (FlowMod) that messages from h2 on port 2 to h1 *(Match) must be outpouted to port 1(Action)*. The controller returns the ARP reply to s1 (PacketOut) which gets send to port 1, according to the rule.

Question 19

After ARP request has been completed, explain the events to the controller when h1 pings h2.

Answer 19

First h1 will send the ICMP Request to h2. Hence, the controller will get this request from s1, then it will install a flow (FlowMod) that messages from h1 on port 1 to h2 (Match) must be outpouted to port 2(Action). The controller returns the ICMP request to s1 (PacketOut) which gets send to h2, according to the rule. Then, h2 responds with the ICM Reply. Because a rule for this transmission has already been installed on s1, no events occur with the controller.

Question 20

How can loops in non-SDN networks prevent the ARP reply from arriving. Also explain why ARP Request do not have this problem?

Answer 20

When switches (s1) receive any message from a host (h1) recieved on port x1, they assume messages towards h1 must be forwarded to x1 in the future. the ARP request is a broadcast, meaning all switches/hosts get this message (hence it does not fail to reach the desired destination). However, if your network is cyclic, when h2 sends the ARP Reply it cannot reach h1 anymore, because switch s1 has received the last h1 transmission (the cyclic ARP request) on port x2. So it wants to return the message to h1 via port x2, but it is not allowed to return messages on the same port it received them. When this is the case, a switch will drop that message, and the ARP reply is lost.