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computer network and systems > network layer > Flashcards

network layer Flashcards

1
Q

role of routers

A

examines datagram headers and decides which port to send it to

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2
Q

forwarding

A

taking the packets at one port of the router and sending it to another based on the header contents

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3
Q

routing

A

determining the path of the packet from end system to end system

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4
Q

what is the role of the sender in the network layer

A

encapsulates segments into datagrams and passes them to the link layer

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5
Q

what is the role of the receiver in the network layer

A

delivers segments to the transport layer protocol

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6
Q

what are the two roles of the network layer

A

forwarding
routing

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7
Q

data plane

A

local per router function that determines how the datagram is forwarded between ports

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8
Q

control plane

A

network wide logic that determines how the datagram is routed between source and destination hosts

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9
Q

what kind of service is the network layer

A

best effort

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10
Q

decentralised switching

A

using header field values to lookup the output port using the forwarding table and input port memory

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11
Q

input port queueing

A

if datagrams arrive faster than the forwarding rate into switch fabric

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12
Q

destination based forwarding

A

forwarding based on the ip address

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13
Q

generalised forwarding

A

forwarding based on any set of header field values

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14
Q

how are ip addresses stored in the lookup table

A

each ip is represented by prefixes instead of the entire thing

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15
Q

longest prefix matching

A

uses the longest address prefix that matches the destination address

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16
Q

what are the 3 types of switch fabric

A

memory
bus
interconnection network

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17
Q

what do the memory and bus switch fabric have in common

A

the interfaces compete with eachother for access
if one is sending data on it already then it cant be used

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18
Q

head of line blocking

A

datagram at the front of the queue prevents others in the queue from moving forward

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19
Q

what do we use to switch through the interconnected network

A

CISCO CRS router

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20
Q

how does the cisco crs router work

A

uses parallel switching planes for faster performance and scalability
8 switching planes per unit
3 stage interconnection network in each plane
supports 100+ tbps switching capacity

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21
Q

why do we use buffering

A

its required when data arrives faster than the link transmission rate

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22
Q

drop policy

A

how we choose which packets to drop when the buffer is full

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23
Q

switching discipline

A

which packets to forward and in which order

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24
Q

priority scheduling

A

drops the packet with the least priority whilst maintaining network neutrality

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25
Q

network neutrality

A

scheduling policy shouldn’t give priority in order to favour some customers as this is against international law

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26
Q

what are the 4 types of packet scheduling

A

first come first served
priority scheduling
round robin scheduling
weighted fair queueing

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27
Q

priority scheduling in packet scheduling

A

the buffer sis split int different partitions based on priority and arriving packets are classified

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28
Q

what is one example of what we can base priority scheduling on

A

how delay tolerant it is
e.g. emails are whilst videos arent

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29
Q

round robin scheduling

A

packets are arranged by priority classification in the buffer and the server sends one packet from each class cyclically

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30
Q

weighted fair queueing

A

round robin plus each class has a weight which signifies how much time to give it once its its turn
higher priority gets more time

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31
Q

what happens when the buffer is too small

A

lots of packet loss and retransmission which isn’t good for the user experience or network utilisation

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32
Q

what happens when the buffer is too large

A

long queueing delays

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33
Q

what is the rule of thumb for buffer size

A

keep the bottle neck link just full enough so its busy but not fuller

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34
Q

how do we calculate the buffer size

A

(rtt * bottleneck link capacity) / sqrt number of concurrent flows in the network

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35
Q

host/router interface

A

connection between the host/router and a physical link

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36
Q

how many interfaces do hosts have

A

1 or 2

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37
Q

how many interfaces do routers have

A

multiple

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38
Q

subnet

A

device interfaces that can physically reach each other without passing through an intervening router

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39
Q

how do we find the subnet

A

removing all devices and creating an isolated network

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40
Q

class a

A

network = 8
host = 24

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41
Q

class b

A

network = 16
host = 16

42
Q

class c

A

network = 24
host = 8

43
Q

loopback

A

sending packets to itself

44
Q

multicast

A

datagrams being sent to a group of devices

45
Q

classless interdomain routing

A

flexible division between the network and host address
no ip classes
creates masks at bit level

46
Q

masks

A

a number that determined the network and host part of the ip

47
Q

fragmentation

A

splitting the datagram into pieces when the mtu is reduced

48
Q

where does reassembly of fragments happen

A

within the network but usually at the receiver side

49
Q

what are the two issues with fragmentation

A

causes inefficient use of recourses as the same packet is being processed twice
loss of any fragment means you have to retransmit the entire datagram

50
Q

dchp

A

dynamic host configuration protocol dynamically assigns ip addresses to hosts

51
Q

how does dchp

A

the host broadcasts a dchp discover message (from 255.255.255.255 to 0.0.0.0)
the server responds with a proposed ip, transaction id and life time
the client broadcasts a message accepting the ip
the server sends an ack and the address

52
Q

what is a life time (dchp)

A

the duration that the ip can be used for as its leased by the client not owned

53
Q

why does the client broadcast the message accepting the proposed ip address

A

other dchp servers may have proposed aswell so they need to know which ip was chosen

54
Q

what happens if the lease is being extended in dchp

A

only the last two dchp messages have to be sent

55
Q

what other information may be given by the dchp server when sending the ip address

A

dns server name and ip
first hop router
network mask

56
Q

how do networks get the subnet part of an ip address

A

the isp owns blocks of ip addresses that it can manage and distribute
it divides the block into smaller subnet blocks and assigns them to different organisations with each one getting a unique subnet

57
Q

what are some benefits of nat

A

secure; hosts aren’t directly visible or accessible by any devices outside the network
can change the address of a host in the network without notifying anyone else
can change the isp without changing host addresses
only one ip needed from the isp for all devices

58
Q

how does nat work

A

the source ip and port number on outgoing datagrams are replaced to their nat ones
incoming datagrams are switched from the nat destination ip and port number to the private one
the nat table stores each translation pair

59
Q

how does Dijkstra’s link state algorithm work

A

starts at one note and initialises the shortest path to itself as 0 and the others as infinity
explore the neighbours and adds their costs to an array then switches to the node with the smallest cost
this is repeated until the shortest path is found creating the shortest path forwarding table

60
Q

optimal substructure property

A

any sub portion of the path starting from the first node is also the shortest path (only positive costs)

61
Q

how does the internet make sure to have scalable routing

A

organising the routes into regions called autonomous systems `

62
Q

intra-as/intra-domain routing

A

handles routing within the same AS
all routers within the AS use the same intradomain routing protocol and in different AS different protocols can be used

63
Q

gateway routers and AS

A

at the edge of the AS and connect routers in other AS and handle both intra and inter AS routing

64
Q

inter-as/ intra-domain routing

A

manages routing between different AS
gateway routers can also do this

65
Q

what are some examples of AS protocols

A

RID; routing information protocol
EIGRD; enhanced interior gateway routing protocol
OSPF; open shortest path first
IS-IS; OSPF with ISO standards

66
Q

how does OSPF work

A

routers share information about the network; link state advertisement
this info is used to build a complete map of the network
using the map users can calculate the best path to send data using Dijkstra’s algorithm

67
Q

how does OSPF handle security

A

each message is checked to make sure it hasn’t been tampered with

68
Q

what are the parts of the two level hierarchy in OSPF and what is their role

A

local area; each router knows the exact details of its own area and each one is connected to the backbone via one or more routers
backbone; connects all the areas together and allows data to travel between them, done by the admin

69
Q

what are the 4 types of routers in OSPF

A

local
area border
backbone
boundary

70
Q

local router

A

work within their area and know the areas map

71
Q

area border router

A

summarize what’s in their area and share it with the backbone

72
Q

backbone router

A

handle communication between areas

73
Q

boundary router

A

connect the network to other external networks

74
Q

tunnelling with ip

A

an ipv6 datagram is carried as a payload in an ipv4 datagram when passing through ipv4 routers
whilst in the tunnel the source and destination changes to the gateway routers being used

75
Q

what does the bellman-ford equation tell us

A

calculates the best path from a router to its destination based on the direct cost to its neighbour and the neighbours cost to reach the destination

76
Q

what happens in the distance vector algorithm for each node

A

waits for a change in the local link cost or a message from a neighbour
recomputes the dv estimate using the dv received from the neighbour
if the dv to any destination has changed then it notifies its neighbours

77
Q

what is the difference between regular dv and iterative dv

A

regular is at intervals whilst iterative changes as soon as it gets an update

78
Q

what happens when a link cost decreases in dv

A

routers quickly add this info as they immediately detect a shorter path

79
Q

count to infinity problem

A

when a link cost increases significantly routers may rely on outdated information from neighbours causing them to repeatedly increase their cost estimates for a destination leading to slow updates

80
Q

what is used to solve the count to infinity problem and how does it work

A

poisoned reverse solution; when a router advertises its distance to a destination and it foes through a neighbour it tells that neighbour that the distance is infinity preventing it from routing back through which breaks the loop

81
Q

what do all AS need

A

unique AS number
needs to know all destinations reachable from neighbouring AS

82
Q

border gateway protocol

A

allows routers to advertise subnets allowing them to be reachable on the internet

83
Q

eBGP

A

passing network reachability info between borders

84
Q

iBGP

A

propagates reachability info to all AS internal routers (intra as)

85
Q

what are the 4 bpg messages

A

OPEN
UPDATE
KEEPALIVE
NOTIFICATION

86
Q

OPEN bpg

A

opens tcp connection to remote bgp peer and authenticates sending peer

87
Q

UPDATE bpg

A

advertises a new path or withdraws an old one

88
Q

KEEPALIVE bpg

A

keeps the connection alive in absence of updates
acknowledges OPEN requests

89
Q

NOTIFICATION bpg

A

reports errors in the previous message
closes the connection

90
Q

hot potato routing

A

the as routes traffic to the nearest exit within its network to hand it off to the nearest as asap minimising internal costs

91
Q

tier 3 as

A

minor isp and customer networks
peer with other t3 providers
has customers of t1 and t2 providers

92
Q

tier 2 as

A

national and regional isp
customers of t1providers
peer with t2 providers

93
Q

tier 1 as

A

international isp (global back bone)
peer with each other (highly connected)
major operators

94
Q

bgp session

A

two bpg routers(peers) exchange bpg messages over a semi permanent tcp connection advertising paths to different destination network prefixes

95
Q

what does a bpg advertised route consist of

A

prefix and attributes

96
Q

bpg prefix

A

destination being advertised
wither a single on or an aggregation of multiple

97
Q

what are two bpg attributes and what do they do

A

AS-PATH; list of as that the prefix advertisement has passed through
NEXT-HOP; the ip address of the next router to reach the prefix

98
Q

how does policy based routing work with bpg

A

when a gateway router on the as receives an ad it applies an import policy to decide whether it should accept it or which one to accept out of multiple
export policies are applied to decide whether or how it advertises routes to neighbouring as

99
Q

customer client relationship

A

any one between a smaller and larger isp
the customer pays the provider for access to the isp

100
Q

peering relationship

A

two domains typically of similar size agree to exchange traffic between their respective customers
the traffic flow volumes are usually similar

101
Q

what happens during failures in ospf

A

when a router detects a failure in its path it stops forwarding packets to it and updates its routing table
however other packets are unaware of this causing transient loops where the same packet keeps getting sent back and forth between two routers
the routing algorithm eventually propagates updates to all affected routers and each one recalculates its routing table

computer network and systems (7 decks)

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