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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
What is a distance vector decentralised routing algorithm?
- 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
What is the distance vector algorithm?
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
What is a link state global routing algorithm?
- calculate least cost path using complete, global knowledge about network - mechanism to obtain info - calculations performed in one site or replicated at multiple
28
What is Dijkstra's algorithm an example of?
link-state algorithm
29
How does Dijkstra's work?
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
Compare link state and distance vector algorithms for message complexity
LS: n nodes, E links, O(nE) messages sent DV: exchange between neighbours only
31
Compare link state and distance vector algorithms for speed of convergence
LS: O(n^2) algorithm required O(nE) messages DV: convergence time varies
32
Compare link state and distance vector algorithms for robustness
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
What is an autonomous system?
- owned by one administration - internal and external routing protocols
34
What are the 2 internet routing protocols?
interior gateway protocol exterior gateway protocol
35
What is interior gateway protocol (IGP)?
relies on IP addresses to construct path - routing info protocol (RIP) - open shortest path first (OSPF)
36
What is exterior gateway protocol (EGP)?
relies on AS numbers to construct AS paths - border gate protocol (BGPv4)
37
How does RIP work?
- 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
How does OSPF work?
- 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
How does BGPv4 work?
- 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
What is network management?
- 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
What is the network management framework?
- 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
What is the simple network management protocol (SNMP)?
- application layer protocol - conveys management control and info between managing server and agent - request response mode - lies on top of UDP
43
What is the internet control message protocol (ICMP)?
- 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
What is Little's theorem?
crowded systems are associates with long customer delays
45
Little's theorem formula
N = lamda T
46
When is mean departure rate equal to mean arrival rate?
at equilibrium if and only if traffic load is less than maximum service rate
47
What is Jackson's theorem?
in a network of queues each node/link behaves as an independent M/M/1 or M/M/n with Poisson flows
48
How do we evaluate routing algorithms?
quantify traffic congestion in terms of stats of arrival process on network of queues - measure congestion by average traffic carried or packet delay
49
What is the quasi-static assumption?
origin-destination traffic process is stationary over time
50
What is the fast settling assumption?
transients in the flow F_ij due to changes in routing are negligible
51
What is the synchronous update assumption?
all link rates F_ij are measured, received and updated simultaneously
52
What 3 assumptions are made for optimal routing problems?
1. quasi-static 2. fast settling 3. synchronous update

Communication Networks (8 decks)

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