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Communication Networks > Network Layer > Flashcards

Network Layer Flashcards

1
Q

What are the network layer services?

A
  • move packets from origin to destination
  • avoid congestion
  • internetworking
  • lowest layer to deal with end to end issues
  • must know topology to choose paths
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2
Q

What is congestion control?

A
  • under heavy traffic performance will collapse
  • all resources involved to make sure network can carry offered traffic
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3
Q

What is flow control?

A

Origin to destination point to point control
- fast senders shouldn’t overwhelm slow receivers

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4
Q

What is a virtual circuit?

A

connection oriented
routing decision only once
nodes know where to forward packets

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5
Q

What is a datagram?

A

connectionless service
no route in advance
datagrams follow different routes
more work but more robust and copes with congestion

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6
Q

What are the virtual circuit signalling protocols?

A

4 way handshake
1. initiate call
2. incoming call
3. accept call
4. call connected

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7
Q

What does a routing algorithm do?

A

determines end to end path through network

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8
Q

What does a forwarding table do?

A

determines local forwarding at particular router

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9
Q

What does the control plane do?

A

how datagram routed in end to end path from sender to destination

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10
Q

What does the data plane do?

A

how datagrams arriving to router’s input link is forwarded to routers output link

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11
Q

What is an example of a forwarding policy and how does it work?

A

weighted fair queueing

  • generalised round robin
  • each traffic class gets weighted amount of service in each cycle
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12
Q

How does datagram fragmentation work?

A

break down datagrams into smaller ones
reassemble at destination

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13
Q

What is the payload?

A

part actually transferred minus the header

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14
Q

What is MTU?

A

maximum transfer unit
- keep chopping datagram into pieces small enough to fit the MTU

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15
Q

What is CIDR?

A

classless interdomain routing
- subnet portion of address of arbitrary length
- format: a.b.c.d/x
- x is no of bits in subnet portion of address

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16
Q

How do subnets work in IP addressing?

A

all the things in the subnet need to know which network they’re connected to

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17
Q

What 3 things does host need to connect to internet in DHCP?

A
  1. IP address
  2. subnet mask
  3. address of nearby router
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18
Q

How does DHCP work?

A

each time user moves or relocated the 3 elements required must be reconfigured

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19
Q

What is required for DHCP?

A
  1. IP address
  2. address of first-hop router for client
  3. name and IP address of DNS server
  4. network mask
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20
Q

What is the difference between IPv4 and IPv6?

A

shorter header to avoid multiple fragmentation

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21
Q

What is tunneling?

A

Can have an IPv4 tunnel connecting IPv6 routers

22
Q

What is a non-adaptive routing algorithm?

A

no consideration to traffic or topology changes

23
Q

What are the 4 characteristics of an adaptive routing algorithm?

A

DRIC
1. Distributed: global and local info
2. Reacts to changes
3. Isolated: local info
4.Centralized: global info

24
Q

What is the objective of a least-cost path algorithm?

A

find minimum length path from one origin to destination

25
Q

What is a distance vector decentralised routing algorithm?

A
  • calculates least cost in iterative, distributed manner
  • each node begins with only knowledge of cost of attached links
  • exchanges info with nieghbours
  • gradually calculates least cost
26
Q

What is the distance vector algorithm?

A

each node:
- wait for change in local link cost c(x,v) or message from neighbour
- update D_x(y) estimates
- if D_x(y) changes then notify neighbours

27
Q

What is a link state global routing algorithm?

A
  • calculate least cost path using complete, global knowledge about network
  • mechanism to obtain info
  • calculations performed in one site or replicated at multiple
28
Q

What is Dijkstra’s algorithm an example of?

A

link-state algorithm

29
Q

How does Dijkstra’s work?

A

initialise:
- N’ = {u}
- for all nodes v adjacent to u
- assign D(v) = c(u,v)
- otherwise D(v) = infinity

Do steps until all w inside N’:
1. find w not in N’ so D(w) is min
2. add w to N’
3. update D(v) for all v adjacent to w and not in N’
4. D(v) = min{D(v), D(w) + c(w,v)}

30
Q

Compare link state and distance vector algorithms for message complexity

A

LS: n nodes, E links, O(nE) messages sent

DV: exchange between neighbours only

31
Q

Compare link state and distance vector algorithms for speed of convergence

A

LS: O(n^2) algorithm required O(nE) messages

DV: convergence time varies

32
Q

Compare link state and distance vector algorithms for robustness

A

LS:
- node advertise incorrect link cost
- node computes only own table

DV:
- node advertise path cost
- table used by others
- error propagates through network

33
Q

What is an autonomous system?

A
  • owned by one administration
  • internal and external routing protocols
34
Q

What are the 2 internet routing protocols?

A

interior gateway protocol
exterior gateway protocol

35
Q

What is interior gateway protocol (IGP)?

A

relies on IP addresses to construct path
- routing info protocol (RIP)
- open shortest path first (OSPF)

36
Q

What is exterior gateway protocol (EGP)?

A

relies on AS numbers to construct AS paths
- border gate protocol (BGPv4)

37
Q

How does RIP work?

A
  • RIP runs on top of UDP
  • distance vector
  • in RIP each router learns from neighbours distance to each dest
  • metric for computation of path is no of hops
  • router sends update message to neighbours every 30 secs
  • mechanisms to reduce routing loops
38
Q

How does OSPF work?

A
  • runs over IP
  • link state
  • each router learns complete network topology
  • routers monitor cost of link to each neighbour
  • floods link state info to other routers on network
  • each router builds identical topology
39
Q

How does BGPv4 work?

A
  • TCP connections
  • path vector
  • de-facto inter-domain routing protocol
  • based on classless address prefixes and policy based routing
  • mechanisms for address aggregation
  • routers keep global view of internet in routing info bases (RIB)
  • speaker exchange network reachability with other adjacent speakers (sequence of ASs that packets traverse to reach dest)
40
Q

What is network management?

A
  • deployment, integration and coordination of hardware, software and human elements
  • monitor, test poll, configure, analyse, evaluate, control network and element resources
  • to meet real time operational performance and QoS requirements
  • at sustainable costs
41
Q

What is the network management framework?

A
  • managing server and operations personnel
  • managed device equipment
  • managed objects: pieces of hardware and configuration parameters
  • management info base
  • network management agent
  • network management protocol needed between MS and MDs
42
Q

What is the simple network management protocol (SNMP)?

A
  • application layer protocol
  • conveys management control and info between managing server and agent
  • request response mode
  • lies on top of UDP
43
Q

What is the internet control message protocol (ICMP)?

A
  • used by hosts and routers to communicate network layer info
  • lies on top of IP
  • ICMP messages carried in IP datagrams
  • host de-multiplexes datagram to ICMP
44
Q

What is Little’s theorem?

A

crowded systems are associates with long customer delays

45
Q

Little’s theorem formula

A

N = lamda T

46
Q

When is mean departure rate equal to mean arrival rate?

A

at equilibrium
if and only if traffic load is less than maximum service rate

47
Q

What is Jackson’s theorem?

A

in a network of queues each node/link behaves as an independent M/M/1 or M/M/n with Poisson flows

48
Q

How do we evaluate routing algorithms?

A

quantify traffic congestion in terms of stats of arrival process on network of queues
- measure congestion by average traffic carried or packet delay

49
Q

What is the quasi-static assumption?

A

origin-destination traffic process is stationary over time

50
Q

What is the fast settling assumption?

A

transients in the flow F_ij due to changes in routing are negligible

51
Q

What is the synchronous update assumption?

A

all link rates F_ij are measured, received and updated simultaneously

52
Q

What 3 assumptions are made for optimal routing problems?

A
  1. quasi-static
  2. fast settling
  3. synchronous update

Communication Networks (8 decks)

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