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Computer Networks - Fall 2018 > Midterm 1 > Flashcards

Midterm 1 Flashcards

Concepts from lessons, readings, review guide

1
Q

What does “OSI” stand for?

A

Open Systems Interconnection

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2
Q

What is the 7 Layer OSI Model?

A 
Application = All
Presentation = Pros
Session = Search
Transport = Top
Network = Notch
Data/Link = Donut
Physical = Places
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3
Q

What is the fundamental design goal of the internet?

A

A multiplexed utilization of existing interconnected networks

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4
Q

What is the solution to multiplexed utilization?

A

Packet switching / statistical multiplexing

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5
Q

What is the solution to interconnected networks?

A

IP / the narrow waist

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6
Q

What is Packet Switching?

A

Information for forwarding traffic is contained in destination address of packet.

+ No state in network ahead of time
+ It’s best effort
+ It’s more resilient

  • Delay and dropped packets
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7
Q

What is Circuit Switching?

A

Allows resources between you and recipient to be dedicated.

+ resource control, better accounting

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8
Q

What is the purpose of the DARPA Designs Reading?

A

Discusses one view of the original objectives of the internet architecture and the relation between these goals and the important features of the protocol.

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9
Q

What is the fundamental structure of the internet?

A

A packet switched communications facility in which a number of distinguished networks are connected together using packet communications processors called gateways which implement a store and forward packet forwarding algorithm.

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10
Q

What are the goals of the internet listed in the DARPA Designs Reading?

A 
Sharing
Interconnection
Survivability
Heterogeneity
Distributed Management
Cost
Ease of Attachment
Accountability
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11
Q

What are NOT the goals of the DARPA Designs Reading?

A

Security
Availability
Mobility
Scaling

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12
Q

How was “sharing” addressed?

A

via Packet Switching and sharing the use of a single communications channel

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13
Q

How was “interconnection” addressed?

A

via IP and the narrow waist by having ONE protocol that every device on the network uses

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14
Q

How was “survivability” addressed?

A

Mostly through “fate sharing”, where it’s acceptable to lose the state info for some entity if that entity itself is lost. Another solution would be through replication.

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15
Q

How was “heterogeneity” addressed?

A

via TCP/IP for flow control, reliable delivery, and universal forwarding and offering a “best effort” service

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16
Q

How was “distributed management” addressed?

A

via addressing = ARIN, RIPE, etc.
via naming = DNS
via routing = BGP

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17
Q

How was “east of attachment” addressed?

A

IP is plug and play; the narrow waist

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18
Q

How was “accountability” addressed?

A

Accountability refers to the ability to bill, which datagrams make this difficult to do; still working on this.

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19
Q

What is the End to End Argument and why did the internet grow rapidly because of it?

A

The intelligence required to implement a particular application on the communications system should be placed at the endpoints rather than the middle of the network.

The internet grew rapidly because innovation was at the edge, in the application layer, rather than the middle of the network, which would be harder to change.

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20
Q

Why are Network Address Translators (NATs) a violation of the E2E argument?

A

Machines behind a NAT are not globally addressable or routable

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21
Q

What is STUN?

A

Signaling and Tunneling

A device send an outbound packet to simply create an entry in the NAT table; Once the entry is created, we have a globally routable address and port.

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22
Q

What is the Address Resolution Protocol (ARP)?

A

It’s how a host learns the MAC address of another host.

  1. A broadcast is sent to every host on the LAN
  2. The response is a unicast of a MAC address to host who issued query
  3. The host builds an ARP table when it receives the reply
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23
Q

What does a hub do?

A

LAYER 2 - Ethernet

A hub broadcasts to ALL.

It is prone to flooding, collisions of frames (latency), and misconfigurations.

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24
Q

What does a switch do?

A

LAYER 2 - Ethernet

A switch allows isolation within a LAN segment and does not broadcast to all. It requires a forwarding table that maps a destination MAC to an output port.

If there’s no entry in its table, it still needs to broadcast. If there is an entry, it will send it to the output port in its forwarding table.

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25
Q

What is a Spanning Tree?

A

A spanning tree allows loop free forwarding on a topology that may contain loops. Redundant links in the underlying topology are fine should something happen to them physically, a new spanning tree could be created to forward packets correctly.

26
Q

What does a router do?

A

LAYER 3 - IP

  1. Receive packet
  2. Look at header to determine destination
  3. Look in forwarding table on Line Card to determine output interface
  4. Modify header (e.g. decrease TTL)
  5. Send packet to output interface
27
Q

What is the equation for determining buffer size?

A

B = 2T*C

or

B = 2T*C / sqrt(n), n = flows through router

28
Q

What is routing within a single Autonomous System called?

A

Intradomain Routing

29
Q

What is routing between Autonomous Systems called?

A

Interdomain Routing

30
Q

What are 2 types of Intradomain Routing?

A

Distance Vector Routing Protocol (RIP) - each node sends distance vectors to neighbors

Link State Routing Protocol (OSPF, IS-IS) - each node distributes a network map to every other node and performs a shortest path computation using Dijkstra

31
Q

What is RIP?

A

Routing Information Protocol

Edges have unit cost and infinity = 16; There’s a table refresh every 30 seconds unless an update occurs in which case its table is sent to all neighbors except the one that caused the updates (split horizon).

32
Q

What is OSPF and IS-IS?

A

Open Shortest Paths First and Intermediate System - Intermediate System

33
Q

What is a type of Interdomain Routing?

A

Border Gateway Protocol (BGP)

34
Q

Why is Interdomain Routing needed?

A

There are tens of thousands of ASes and each AS has its own self interest and independent economic and performance objectives, but most cooperate to provide global connectivity.

ASes use route advertisements to provide reachability using an incremental protocol called the Border Gateway Protocol (BGP).

35
Q

What are the attributes on a route advertisement?

A

Destination IP
Next Hop IP
AS Path

36
Q

What are the 2 types of BGP?

A

Both about external destinations.

eBGP = responsible for transporting route advertisements BETWEEN BORDER ROUTERS OF ADJACENT ASes for external destinations

iBGP = responsible for transporting route advertisements BETWEEN ROUTERS WITHIN A SINGLE AS for external destinations

37
Q

What is IGP?

A

Interior Gateway Protocol

Routes inside an AS to internal destinations.

38
Q

What is the BGP Decision Process?

A
  1. Highest LocalPref
  2. Lowest AS Path
  3. Lowest Origin Type
  4. Lowest MED
  5. eBGP learned over iBGP learned
  6. Lowest IGP cost to border router
  7. Lowest Router ID (tie breaker)
39
Q

Which BGP Decision Processes are set locally?

A
  1. Highest LocalPref

6. Lowest IGP cost to border router

40
Q

Which BGP Decision Processes are set by neighbor?

A
  1. Lowest AS Path

4. Lowest MED

41
Q

Which BGP Decision Processes are set by neither itself or the neighbor?

A
  1. Lowest Origin Type
  2. eBGP learned over iBGP learned
  3. Lowest Route ID (tie breaker)
42
Q

What are the 3 types of business relationships in Interdomain Routing?

A

Customer - Provider = one ISP pays another to forward its traffic

Peer - Peer = two ISPs agree connecting would mutually benefit both

Backup = two ISPs set up a link that’s only used in the event a primary route is unavailable

43
Q

How are routes advertised when learned from a customer, provider, and peer?

A

Routes learned from …

customers > everyone else
provider > customers only
peer > customers only

44
Q

What is Classful Addressing (pre-1994)?

A

Class A
1 bit class, 7 bits net id, 24 bits host id

Class B
2 bits class, 14 bits net id, 16 bits host id

Class C
3 bits class, 21 bits net id, 8 bits host id

45
Q

What is CIDR?

A

Classless Interdomain Routing

32 bits: IP Address + Mask e.g. 65.14.248.0/22

46
Q

What is CAM?

A

Content-Addressable Memory

Provides an exact match O(1) for IP address lookup using a tag/value lookup.

47
Q

What are the two ways in which IPv6 Deployment is happening?

A

Dual Stack: host speaks both IPv4 and IPv6 or a translator is used as an intermediary

v6 to v4 Tunneling: wrapping an IPv6 packet in an IPv4 packet; assumes end host uses IPv6

48
Q

What is a crossbar switch?

A

Each input port has a connection to every output port; during each timeslot, each input is connected to zero or one output

49
Q

What is Maximal Matching?

A

It’s an algorithm used for scheduling using a crossbar switch.

In each timeslot, there’s a one-to-one mapping between inputs and outputs.

50
Q

What is Head of Line Blocking?

A

This happens when packets in front that are destined for one output port are blocking other packets that can already be mapped.

The solution is a Virtual Output Queue where there is a queue per output port.

51
Q

What is an A Record?

A

name to IP Address

52
Q

What is an NS Record?

A

name to authoritative nameserver (referrals)

53
Q

What is an MX Record?

A

name to mail server

54
Q

What is a CNAME Record?

A

canonical name

55
Q

What is a PTR Record?

A

IP Address to name (aka reverse lookup)

56
Q

What is an AAAA Record?

A

name to IPv6 Address

57
Q

How are route advertisements processed?

A
  1. Import Policy - applied to determine which routes should be filtered and hence eliminated from consideration, and may append or modify attributes
  2. Decision Process - select most desirable routes
  3. Export Policy - applied to determine which neighbors the chosen route will be exported to
58
Q

ISPs can control how route advertisements are processed by…

A
  1. Modifying the import policy to filter routes it doesn’t want to use
  2. Modifying the route attributes to prefer some routes over others
  3. Modifying the export policy to avoid providing routes for certain neighbors to use
59
Q

What are the “3 knobs” to control import/export policies?

A
  1. Preference - adding/deleting/modifying route attributes in BGP advertisements
  2. Filtering - instructing routers to ignore advertisements with attributes matching a specified value or range
  3. Tagging - associating additional state with a route using the community attribute, a variable length string used to tag routes
60
Q

What is flap damping?

A

This limits the propagation of unstable routes by maintaining a penalty value associated with a route that is incremented. When the penalty > limit, the route is suppressed.

Computer Networks - Fall 2018 (3 decks)

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