Internet Routing II

Question 1

What is routing?

Answer 1

• Routing is the process of populating the forwarding tables
  Routers exchange messages about nets they can reach
  Goal: find optimal route for every destination
  … or maybe good route, or just any route (depending on scale)

Question 2

What is forwarding?

Answer 2

It’s the act of moving packets between ports.
- We look up the destination address in a forwarding table
- Find out-port or (out-port, MAC addr) pair

Question 3

Why are stable routes preferred over rapidly changing ones?

Answer 3

They aid in effective network management.
Higher level optimizations such as TCP RTT estimation, rely on stable routes.

Question 4

What is the difference between global and decentralized routing algorithms?

Answer 4

In global routing algorithms, all routers have the complete topology.
In decentralized, the routers only know their neighbors.

Question 5

What is the difference between intra-domain and inter-domain routing?

Answer 5

Intra-domain routing occurs within a single administrative domain, managing networks under the same control. It scales to around 100 networks, like a campus setup.

In contrast, inter-domain routing is decentralized and scales to the vastness of the Internet.

Question 6

What are the two classes of intra-domain routing algorithms?

Answer 6

Link State & Distance Vector

Question 7

What type of view does the Link State algorithm have? (which nodes can it use?)

Answer 7

Global view, it sees all nodes in the network.

Question 8

What is contained in a Link State Packet?

Answer 8

The Link State Packet (LSP) contains key elements:
- ID of the node that generated the LSP.
- Cost of links to each directly connected neighbor.
- Sequence number (SEQNO) for versioning and avoiding outdated information.
- Time-to-live (TTL) for managing packet lifespan.

Question 9

Is distance vector a distributed or globalized routing algorithm?

Answer 9

Distributed

Question 10

Which equation do you use for calculating the best path in distance-vector algorithms?

Answer 10

Bellman-Ford equation

Question 11

What can distance vector algorithms suffer from? What becomes the implication?

Answer 11

Loops.
infinite converge time.

Question 12

What happens with malfunctioning routers with Link State?

Answer 12

Node can advertise incorrect link cost

Question 13

What happens with malfunctioning routers with Link State?

Answer 13

Node can advertise incorrect path cost
Costs used by others, errors propagate through net

Question 14

What are the two intra-domain routing protocols that has been mentioned in this course?

Answer 14

Routing information protocol (RIP) based on Link-state

Open Shortest Path First (OSPF), based on Distance Vector

Question 15

What algorithm does Routing information protocol (RIP) use?

Answer 15

Distance Vector

Question 16

What’s the difference between a RIPv2 Entry and a RIPv2 Packet?

Answer 16

A “RIPv2 entry” refers to a specific routing record within a route update packet. These entries carry crucial information about available routes within a network.

A packet includes information about routers in a network.

Packet Structure:
Command: Indicates the purpose of the packet, whether it’s a route update, request, or response.

Version: Specifies the RIP version being used (RIPv2 in this case).

Zeroes: Reserved field filled with zeroes for future use.

Question 17

What is the purpose of the Route Tag Field in the context of Routing information protocol (RIP)?

Answer 17

It is used to distinguish between internal and external routes.

Question 18

What the heell is “Next Hop Field”

Answer 18

Indicates the address of the next router or gateway that a data packet should be forwarded to in order to reach its destination.

Question 19

What routing algorithm does Open Shortest Path First (OSPF) use?

Answer 19

Link State

Question 20

What are Open Shortest Path First (OSPF) Areas?

Answer 20

OSPF Areas are subdivisions within an OSPF autonomous system. They provide hierarchy, reduce routing table size, and limit the scope of link-state updates, thereby improving routing efficiency and stability. Area 0 (zero) is the backbone area to which all other areas must connect.

Question 21

What is included in a OSPF Packet header?

Answer 21

Version: This field indicates the OSPF version being used, such as OSPFv2 or OSPFv3.
Type: Specifies the type of OSPF packet (e.g., Hello, Link State Update, Link State Request, Link State Acknowledgment).
Packet Length: Indicates the total length of the OSPF packet, including the header and data.
Router ID: The Router ID of the sender, which is used to identify the source router.
Area ID: Identifies the OSPF area to which the packet belongs.
Checksum: A checksum for error detection in the packet.
Authentication: Contains information related to authentication, such as a password or authentication type.

Question 22

What types of Open Shortest Path First (OSPF) exist?

Answer 22

OSPFv2 and OSPFv3

Question 23

What are pros and cons of Link State and the Distance Vector algorithms

Answer 23

Link State:
Simpler to debug
Require global state (OSPF reduces this through areas)

Distance vector
Harder to debug
Can suffer from loops

Question 24

How do you prevent loops in a distance vector algorithm?

Answer 24

Split horizon

Or Split horizion with poison reverse