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network layer Flashcards

1
Q

Network layer addresses:

A

How to get from A to B
How to connect to multiple networks
Hot to manage network resources (congestion, quality of service)

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2
Q

the router

A

device that enables new features through new protocols

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3
Q

Who performs store-and-forwarding switching?

A

router

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4
Q

routing

A

finding a path through a network

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5
Q

important properties of routing

A

correctness
simplicity
robustness
stability
fairness
efficiency

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6
Q

routing tables

A

tells us for each packet on which link to forward it

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7
Q

distance vector routing

A

send distance vector to neighbours
update routing table based on incoming distance vectors
good news: propagation
problem: count to infinity when machine fails

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8
Q

link state routing

A

does not suffer from the count to infinity problem, but more is complex
replaces distance vectors in ARPANET 1979
uses a shortest path algorithm

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9
Q

link state routing steps

A
  1. routers only send packets with information about their direct neighbours
  2. these packets are flooded over the network
  3. routers build an overview of the network using these packets and run a shortest path algorithm
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10
Q

What can go wrong with flooding link state packets?

A
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11
Q

hierarchical routing

A

reducing routing table sizes for large newtorks

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12
Q

inter-netwroking

A

getting packets to their destinations across multiple networks

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13
Q

challenges for sending packets end-to-end over multiple networks (inter-networking)

A

technological:
different protocols
different maximum packet sizes
different QoS guarantees

political:
different costs
privacy concern
competition/disputes

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14
Q

inter-networking with autonomous systems

A

routing inside an Autonomous System:
intradomain routing, uses Interior Gateway Protocol, ex: OSPF

routing between Autonomous Systems:
interdomain routing, uses an Exterior Gateway Protocol, ex: BGP

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15
Q

OSPF

A

Open Shortest Path First
routing within an autonomous system
uses a form of link state routing
build a graph representation of the network
uses a hierarchy called areas to manage large networks

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16
Q

areas in OSPF

A

all areas are connected to Area 0 (backbone)
are border router exchanges summarised routing info between areas
boundary routers connect to another AS; uses both interior and exterior gateway protocol
routers only know topology of the area they are in
for destinations not in area: through backbone

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17
Q

BGP

A

Border Gateway Protocol
routing between large independent networks
-supports arbitrary policies put in place by ISPs, companies, organisations, etc. autonomous systems connect at IXPs
stub networks do not need BGP: only one way to send outgoing packets
AS1 provides transit service
uses a path vector protocol (form of distance vector routing)

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18
Q

routing policies of BGP

A

peer above transit: routes learned from peers are preferred over routes learned from transit providers
hot potato routing: nearest exit point from the network as quickly as possible
shortest AS path: the path with the fewest Autonomous System (AS) hops is preferred

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19
Q

connecting networks with different protocols

A

if source and destination networks use different protocols, they cannot communicate

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20
Q

tunneling

A

if an intermediate networks uses different protocols, they can communicate by tunneling
used to route IPv6 packets over IPv4 networks
routers support multiple types of networks
data wrapped in header form multiple networking layers
network layer packet used as payload for other network-layer packet

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21
Q

packet fragmentation

A

packet size can be limited by hardware, software, protocols, law, etc.

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22
Q

transparent fragmentation

A
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23
Q

nontransparent fragmentation

24
Q

avoiding packet fragmentation

A

MTU discovery

25
IP
The Internet Protocol data transfer protocol at network layer
26
challenges of IP
error detection/correction -> header checksum preventing permanently looping packages -> time to live globally identifying computers -> NAT carrying packets over links with different size requirements -> total length, fragment offset
27
IP version 4
frame header-32 bits time to live: counter, prevents infinite loops, the time the packet can spend in the network identification fragment offset: allows for the packet to be sent in fragments, all fragments have an identification number-offset total length = header + data -16 bit length field → packet size can reach 64KiB header checksum - uses checksum - computed by adding all 16-bit half-words in the header
28
802.3 → 802.1Q
- VLAN type > 1500 interpreted as type - maximum frame sizes differs across link-layer protocols
29
IPv4 addresses
uses 32-bit addresses written in dotted decimal notation 32-bit address gives 2^32 > 4 billion addresses
30
How are packets routed to IPv4 addresses with latencies in the order of msec?
by reducing routing table sizes using hierarchical routing
31
routing used in IP
nontransparent fragmentation + MTU (avoiding)
32
NAT
Network Address Translation
33
IP version 6
1. many more addresses 2. simplified header - improves bandwidth/latency 3. easier to add options in the header 4. improved security support
34
IPv6 header
32 bits value 0x06 indicates IPv6 hop limit = time to live next header: specifies transport layer protocol or extension header
35
Addressing the Problem of Too Many Addresses
managing the size of the routing tables
36
Internet Protocol Prefixes and Subnets
routing algorithms can calculate routes to prefixes, instead of to every individual address prefixes handed out by a single organisation: ICANN organisations can further subdivide their prefix to create subnets
37
CIDR
Classless InterDomain Routing ....
38
ICMP
Internet Control Message Protocol if something goes wrong, routers send these messages to seders examples: - destination unreachable - time exceed - used by the program traceroute - “echo” and “echo reply” - used by program ping - router advertisment/solicitation - packet needs fragmentation/packet too big
39
DHCP
Dynamic Host Configuration Protocol - MAC addresses are built into NICs but network addresses are not - used to configure other settings such as: network mask, addresses of default getaways, DNS, time servers, etc. DHCP discover DHCP offer
40
ARP
Address Resolution Protocol problem: we don’t know which value of ad and ed go together request is broadcast over Ethernet ARP packets wrapped in Ethernet headers
41
service types
connectionless connection-oriented
42
connectionless service
- datagrams - routers are routing algorithms to decide where to send each packet individually - used by the IP
43
connection-oriented service
-virtual circuits - decide fixed route during connection setup - all packets part of the connection follow this route - ISPs can use this on top of IP
44
service comparison
datagrams: - no setup required - router failures have low impact - packets contain full addressing information - routers are stateless virtual circuits: - easy congestion control - easy quality of service guarantees - packets contain VC numbers - routers keep track of active VCs
45
Congestion control
combined responsibility of network and transport layers
46
goodput
rate of useful packets arriving at the receiver
47
approaches to congestion control
simplest: resource over-provisioning traffic-aware routing admission control traffic throttling
48
admission control
- if there is congestion, new traffic has to wait - allows new traffic load only if the network has sufficient capacity - Can you find a path that doesn’t result in congestion? Yes → allow traffic No → traffic must wait
49
traffic-aware routing
- if link costs are static, all traffic is routed over lowest-cost links - using dynamic cost calculation can prevent congestion - calculate link cost as a function of current load - need to prevent oscillations 1. small cost updates 2. multiple paths
50
traffic throttling
- send messages in the opposite direction to explicitly indicate network congestion - most common implementation - set special bits in IP packet - inform sender of congestion through TCP - end-to-end - send back a “choke” signal; when the source receives this packet, it slows down transmission - used in TCP/IP via Explicit Congestion Notification (ECN) - link-by-link - send back a “choke” signal; **every router** that receives this packet slows down transmission
51
traffic shaping
regulating network resource usage challenge: limit available data rate, but allow bursty traffic regulates rate and burstiness of data entering the network
52
token bucket
outgoing rate between 0 and B average outgoing rate equal to R sending n bytes requires n tokens bucket can store limited number of tokens (B) incoming token rate: R tokens come at constant rate
53
Load Shedding
choosing partial failure over total system failure
54
RED
Random Early Detection - drop packets randomly if buffer space is almost full - sends an implicit signal to the sender: slow down! - works if transmission errors are unlikely cause of packet loss - wired links are reliable (errors are unlikely) - wireless channels (and other unreliable ones) need to solve transmission errors on the data link layer to hide them from network layer
55
Quality of Service aspects
bandwidth: maximum data rate [bps] delay: time it takes to get from src to dst jitter: variation in packet delay; 0 jitter means delay is constant packet loss: probability of packets being dropped
56
different apps have different requirements
file sharing: bandwidth audio on demand: jitter video on demand: bandwidth, jitter telephony: delay, jitter videoconferencing: bandwidth, delay, jitter

computer networks (6 decks)

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