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CS3102 Networks > 5: Routing > Flashcards

5: Routing Flashcards

1
Q

What is the network layer? What does it do?

A

The third layer of the reference model that is responsible for packet routing and forwarding in routers. It provides abstracted host-host logical communications.

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2
Q

What is forwarding?

A

The process by which a datagram is moved from a source device to an output device: 1 hop along a path.

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3
Q

What is routing?

A

Routing is the process by which a network as a whole determines the paths deciding how packets go from source to destination: forwarding tables are built.

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4
Q

What plane does forwarding happen on?

A

The data plane

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5
Q

What plane does routing happen on?

A

The control plane

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6
Q

What is the network control plane?

A

The part of the router architecture that is concerned with drawing the network topology, or the information in a (possibly augmented) routing table that defines what to do with incoming packets.

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7
Q

How may the network control plane be structured?

A

With per-router control (traditional) or logically centralised control (software-defined networking).

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8
Q

What is a logically centralised control plane?

A

A distinct, remote controller interacting with control agents in routers, which interact with each other in the control plane to form forwarding tables.

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9
Q

What is a per-router control plane?

A

Routing algorithm components in every router, which interact with each other in the control plane to form forwarding tables.

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10
Q

What is a routing protocol?

A

A protocol specifying how routers in a network communicate, forwarding tables in routers are built, and paths of data between two endpoint nodes are chosen.

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11
Q

What do routing protocols do?

A

Find optimal paths for data transfer between two endpoint hosts in a network.

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12
Q

What is global and decentralised information?

A

In networks with global information, all routers know the complete topology and the link cost information. In networks with decentralised information, a router only knows the link costs to its neighbours.

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13
Q

What are static and dynamic routing algorithms?

A

Static routes change more slowly over time, whereas dynamic routes change more rapidly, often periodically updating in response to link cost changes.

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14
Q

What is Dijkstra’s algorithm?

A

An algorithm to find the shortest paths between nodes in a graph.

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15
Q

What is the time complexity of Dijkstra’s algorithm?

A

O(n ^ 2) but it is possible to implement them within O(n * log(n))

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16
Q

How does Dijkstra’s algorithm work?

A

It finds the costs of going between all adjacent nodes, then the minimum cost of going between all nodes 2 hops apart, then 3, then 4, etc, to build a forwarding table of the minimum cost path between any 2 nodes on the graph.

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17
Q

What is a distance vector?

A

An array (vector) of diatances to other ndoes in the network from a given node.

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18
Q

What is a Distance vector algorithm?

A

A routing algorithm that determines the best route between two nodes based on the distances between nodes.

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19
Q

What is the Bellman-Ford equation?

A

An algorithm to find the shortest path between two nodes on a weighted graph.

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20
Q

How does the Bellman-Ford equation work?

A

Starting from the node x, and aiming to get to the end node y, choose the neighbour node v with the minimum cost of : cost(x to v) + cost(v to y).

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21
Q

What is the Distance vector algorithm?

A

An algoirithmfor estimating the minimum cost of going between two nodes

22
Q

How does the Distance vector algorithm work?

A

Each node stores its distance estimates to other nodes, and other nodes use its distance estimates to other nodes to estimate their distances to their nodes; for instance, the distance of a node x to get to node z via node y = the distance of x to y + the estimated distance of y to z that y informs x of.

In this manner, it is a recursive algorithm. Estimated costs only update when their neighbours do, and starter changes only change due to changes in a local link. It is distributed that changes only occur when neighbours do, so they propagate.

23
Q

What are link cost changes?

A

Changes in the cost for data to be transmitted over a link between two nodes on a network.

24
Q

What is message complexity?

A

??

25
Q

What are link state algorithms?

A

A type of routing algorithm in which all routers independently calculate the best path for data, being aware of all router links and having the entire routing algorithm logic.

This contrasts with distance-vector protocols, in which each router’s routing table is recursively dependent on all others.

26
Q

How do link state and distance vector algorithms compare?

A

In link-state algorithms, all routers independently calculate the best path for data, being aware of all router links and having the entire routing algorithm logic. This contrasts with distance-vector protocols, in which each router’s routing table is recursively dependent on all others.

Link-state algorithms more resistant against router malfunction: nodes only compute its own table rather than tables using each other in DV, where errors propagate.

n ^ 2 complexity LS algorithms converge in O(n * E) messages, where n = no nodes, E = no links, and DV convergence time varies

27
Q

What is administrative autonomy?

A

Network administrators in subnets of the Internet being able to control routing in their own subnet independently.

28
Q

Why do we need to alter routing to make it scalabe?

A

Can’t store all destinations in the Internet in all routing tables in all routers

29
Q

What are autonomous systems? What are they aka?

A

Subdivisions of the Internet akin to subnets which define boundaries in routing; intra-AS routing must use one protocol per AS but inter-AS routing is different, and within each AS, the routing protocol may be different.

30
Q

How do forwarding tables relate to inter and intra-AS routing algorithms?

A

The forwarding table of a node in an AS must contain entries for destinations within the AS from the intra-AS routing algorithm in use, and entries for destinations outside of it using the inter-AS routing algorithm

31
Q

What are inter-AS tasks?

A

??

32
Q

What are interior gateway protocols?

A

Protocols for intra-AS routing

33
Q

What is RIP?

A

Routing Information Protocol is an intra-AS routing protocol (IGP)

34
Q

What is OSPF?

A

Open Shortest Path First is an intra-AS (IGP), link-state routing protocol (IGP) that implements Dijkstra’s algorithm to find the shortest path between two nodes. Various and varying cost metrics are used for assessing links.

35
Q

What is IGRP?

A

Interior Gateway Routing Protocol is an intra-AS (IGP), distance vector routing protocol (IGP) developed by Cisco.

36
Q

How does OSPF work?

A

It is a link-state protocol implementing Dijkstra’s algorithm to find the shortest path between two nodes. Various and varying cost metrics are used for assessing links.

37
Q

What are the key features of OSPF?

A

Broadcast link-state advertisements from all routers to all others in the entire AS to know costs. Secure from authentication, can have multiple same-cost paths, and can have hierarchical OSPF within subnets of a subnet.

38
Q

How does hierarchical OSPF work?

A

AS bound with a boundary router, connecting to backbone routers, in turn that connect to area border routers that define “areas” - subnets in an AS. OSPF used separately within each area. 2-level hierarchy of backbone and local areas.

39
Q

What is BGP?

A

Border Gateway Protocol is the primary inter-AS routing protocol.

40
Q

What are eBGP and iBGP connections?

A

Connections external and internal to an AS that advertise the availability of the nodes in a subnet to the rest of the network and vice versa.

41
Q

How does BGP work?

A

BGP connections have e and i connections to other routers and within an AS and sessions exchange the availability information acquired.

42
Q

What is a BGP session?

A

A semi-permanent connection between two BGP routers that exchange information

43
Q

What is policy-based routing?

A

Routing as per specifications of policy by an AS network admin, e.g. don’t route through another AS and if/when to advertise paths to neighboring ASes.

44
Q

What is path advertisement?

A

When an AS router sends information to others to tell them that a specific network device or region is reachable in the AS.

45
Q

What is import policy in BGP?

A

Criteria set by the network administrator of an AS specifying when to use paths being advertised to the AS router form others.

46
Q

What are the types of BGP message?

A

??

47
Q

How does route selection work in BGP?

A

Depends on configuration; can be policy-defined choice, shortest path, hot potato, etc.

48
Q

What is Hot Potato Routing?

A

When you always pass a packet out of an AS as soon as possible (to another), even if there is another link to another to another AS closer to the goal within the current AS.

49
Q

What are provider networks?

A

??

50
Q

What does it mean for a network to be dual-homed?

A

A device or network interfaced with two others (as opposed to only one)

51
Q

Why are there different protocols for inter and intra AS routing?

A

different weights on policy and performance in the two

52
Q

How does BGP achieve policy via advertisements?

A

??

CS3102 Networks (10 decks)

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