CS代考 COMP30023 – Computer Systems

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COMP30023 – Computer Systems

Internet (Network) Layer – IP Addresses
and Packet Switching
Dr Lachlan application to transport Layers

• Network / Internet layer
• Addresses and Packet switching
• Subnets and network address translation (NAT)
• Internet layer control protocols

Next four lectures

• Initial look at the network layer
• High level discussion of quality of service
• IP addressing

• Role: get data from the source all the way to the destination
• May not be in a single hop (point-to-point link)

• Traffic must be routed efficiently
• This is performed by network devices called routers

• Nodes must be given names (addresses)

Internet (network) Layer

Connecting multiple networks

Internetworking and Routing

TN 6th 5-46

• Most network layer code runs on routers
• We will refer to the protocol data units as packets
• What types of services does the network layer provide?

• Connectionless
• Packet switching (Internet Protocol – IP)
• Minimum required service: “send packet”
• Called “datagram” network

• Connection-oriented
• (virtual) Circuit Switching

• Asynchronous Transfer Mode – ATM
• MultiProtocol Label Switching MPLS

• Called “virtual circuit” network
• These usually act as a single “link” of an IP network

Network Layer

• The internet is a packet switched network
• Host H1 wants to send a packet to H2

1. Transmits it to the nearest router (A)
2. The packet is buffered while it is arriving, and the checksum is verified
3. If valid, the packet is stored until the outgoing interface is free
4. The router forwards the packet onto the next router in the path
5. Repeat 2-4

Store-and-Forward Packet Switching

TN 6th 5-1

Packet forwarding –
Connectionless

Forwarding tables
(also called
routing tables)

Paths can change
for packets in
the same transport
layer connection

TN 6th 5-2

Packet forwarding –
Connection-oriented

Forwarding table:
“In”: connection ID
“Out”: next hop and
new connection number

Connection number
is local to a hop

TN 6th 5-3

Pros and cons of connections

Issue Datagram Network Virtual Circuit

Type Connectionless Connection-oriented

Addressing (-) Each packet has full
source and destination

(+) Each packet contains a
short VC number

State + Routers do not hold state
information about
connections

– Each VC requires router
table space.
– Router reboots a problem.

Routing Each packet independently Defined at set-up

Quality of Service – Difficult + Easy if enough resources

Congestion control – Difficult + Easy if enough resources

Link failure recover + Simple – Extra work

• Widely deployed Virtual Circuit (connection-oriented) Network
Layer Protocol (below the internet sublayer)
• MPLS network is one IP hop

• Primary purpose is Quality of Service
• Prioritising traffic
• Service Level Agreements for network performance
• Reliable connectivity with known parameters

• Popular with businesses that want to connect multiple sites and
phone companies carrying voice traffic

• Expensive: Price roughly 20-100 times more per Mbps than a
standard internet connection. (Cost is much more similar.)
• https://www.networkworld.com/article/2222196/why-does-mpls-cost-

so-much-more-than-internet-connectivity-.html

MultiProtocol Label Switching

https://www.networkworld.com/article/2222196/why-does-mpls-cost-so-much-more-than-internet-connectivity-.html

• Why is Quality of Service (QoS) important?
• Not all services are equally important or robust to network delay

• VoIP vs file downloads
• VPN connections vs web browsing

• Within your own network, or within a single administered
network (ISP), services can be prioritised
• Own network – typically explicitly
• Shared network – typically implicitly (ISP traffic shaping)

• In the case of explicit prioritisation, the Differentiated Services
header can be used to define classes of traffic

• Useful in an office building with a network that carries both
internet and telephony traffic

Quality of Service

• Be aware that
• connection-oriented services exist, and
• the concept of Quality of Service is important

• Our focus will be on the connectionless Internet Protocol
that forms the backbone of the internet

Network Layer

• Designed with a number of principles in mind, including:
– Something that works OK is better than an ideal standard “in progress”

• Standard of “OK” has gone up as the Internet has become vital
– Keep it simple – Occam’s Razor
– Be strict when sending and tolerant when receiving

• E.g., web browsers handle pages with invalid HTML
– Make clear choices – don’t have different approaches in a standard
– Avoid static options and parameters – negotiate them at runtime
– (New principle: Think about scalability)

• “Best effort”, not guaranteed performance

Internet Protocol

• Responsible for moving the packets through the various
networks from source to destination host

• Multiple paths through the network
– Important for redundancy

• Routing algorithms are used to determine best path
– We will look into these more next week

• Nothing guaranteed – just “best effort”

Internet Protocol

IP Version 4 Protocol (IPv4)

Field Usage

Version Protocol version 4
(This field is also in IPv6)

IHL Header length in 32 bit words;
min 5, max 15

Differentiated

6 bits for service class, 2 bits for
congestion control (ECN)

Total length Including payload, max 65,535

Identification, DF,
MF, Fragment Offset

Used in the handling of
fragmentation

Time to live (TTL) Countdown of hops, at zero packet
is discarded

Protocol Transport layer service
(TCP/UDP/SCTP/DCCP/etc.)

Source and
Destination

IPv4 address

Options Rarely used and poorly supported

TN 6th 5-47

• 32-bit number
• Expressed in decimal notation, each byte is shown as a

decimal, separated by a period, 172.22.44.10 (0xAC162C0A)
• 0.0.0.0 is lowest, 255.255.255.255 is highest
• IP address allocation responsibility of Internet Corporation

for Assigned Names and Numbers (ICANN) by delegation to
IANA and Regional Internet Registries (RIR’s)

• IP addresses are given to interfaces not hosts, i.e. a host
with multiple network cards will have multiple IP addresses

• Supply of IPv4 addresses has basically been exhausted

IPv4 Addresses

• Unicast: One destination (“normal” address)
• Broadcast: Send to everyone
• Multicast: Send to a particular set of nodes

– Used for streaming video of live events
• Anycast: Send to any one of a set of addresses

– Used for database queries, like DNS, NTP
• Geocast: Send to all users in a geographic area

– “Location aware” services
– Send ad to those in store
– Send warning to those near a hazard

2022© University of Melbourne 21

Types of address

Were thought to be
the only types

Not widely used.
To show that
the above aren’t all

• Unicast: One destination (“normal” address)
• Broadcast: Send to everyone
• Multicast: Send to a particular set of nodes

– Used for streaming video of live events
• Anycast: Send to any one of a set of addresses

– Used for database queries, like DNS, NTP
• Geocast: Send to all users in a geographic area

– “Location aware” services
– Send ad to those in store
– Send warning to those near a hazard

2022© University of Melbourne 22

Types of address

Were thought to be
the only types

Not widely used.
To show that
the above aren’t all

• Originally IP addresses were allocated based on classes
• Routing was performed based on the class, which could be

derived from the first part of the address

IPv4 Addresses – Classes
(no longer used, but spoken of)

TN 6th 5-54

• Classes simplify routing
– size of “network” field is implicit in the address

• Wasteful. Networks often much bigger than needed
– Network with 260 nodes must be class B with 65,536 addresses

• Classless InterDomain Routing
– Each interface / route explicitly specifies which bits are the

“network” field
– Network with 260 nodes only needs 9 bits for “host” field
512 addresses
Can have 128 times as many of these networks as class B networks

IP Addresses – CIDR

• Hierarchical – encodes the network and host number
– Network in top bits
– Host in bottom bits

• Assigned to networks in blocks, the network part will be the
same for all hosts on that network
– A network corresponds to a contiguous block of IP address space,

called a prefix
– Prefixes are written as the lowest IP address followed by a slash and

the size of the network portion – 192.0.2.0/24

IP Addresses – CIDR

TN 6th 5-49

• In the case of 192.0.2.0/24
– 24 bits are for the network 192.0.2.0
– Leaving 8 bits for hosts – up to 256 addresses

• In the case of 10.0.0.0/8 (reserved private block)
– 8 bits are for the network 10.0.0.0
– Leaving 24 bits for hosts – up to 16,777,216 addresses

• Can also be written as a subnet mask, a binary mask of 1’s
– In the case of /24: the subnet mask is 255.255.255.0

IP Addresses – Blocks

• Network number = network mask (bitwise-AND) IP address

• This is crucial for efficient routing on the internet
– Since networks are assigned in blocks, intermediary routers need

only maintain routes for the prefixes, not every individual host
– Only when the packet arrives at the destination network does the

host portion need to be read

IP Addresses – Prefixing

IP Addresses – Route aggregation

TN 6th 5-52, 5-53

• Aggregation is performed automatically
• Currently it roughly halves

the size of the routing table

• Prefixes can overlap, in which case the longest matching
prefix is selected

Special IP Addresses

Link local / zero config

169.254.0.0 – 169.254.255.255 169.254.0.0/16 (255.255.0.0) 65,536

Private address range Prefix & Mask Available Addresses

10.0.0.0 – 10.255.255.255 10.0.0.0/8 (255.0.0.0) 16,777,216

172.16.0.0 – 172.31.255.255 172.16.0.0/12 (255.240.0.0) 1,048,576

192.168.0.0 – 192.168.255.255 192.168.0.0/16 (255.255.0.0) 65,536

/Placeholder for “invalid” address

TN 6th 5-55

• Designed over 20 years ago to address the problem of
exhausting the IPv4 address space

• Whilst solving that problem some other changes were made
– Simpler header – allows faster processing
– Improved security – now back-ported to IPv4
– Further Quality of Service support

• IPv6 addresses are 128 bits
– Unlikely ever to run out…
– …unless new wasteful allocation schemes are used.

IPv6 Header

Field Usage

Differentiated

6 bits for service class, 2 bits for
congestion control (ECN)

Flow label Pseudo-Virtual Circuit identifier

Bytes after the 40 byte header

Next header Used to specify additional
headers or Protocol (TCP/UDP)

Hop limit Same as TTL (Time To Live)

Source and
Destination

16 bytes IPv6 addresses

TN 6th 5-57

• Written as 8 groups of (up to) 4 hex digits
– 8000:0000:0000:0000:0123:4567:89AB:CDEF

• Can be optimised by stripping one group of consecutive 0’s
– 8000::123:4567:89AB:CDEF

• Backwards compatibility with IPv4 is achieved with
– ::ffff:192.31.2.46 (note the mix of hex with decimal)

• Still not widely deployed
– Google measures IPv6-capable requesters,

https://www.google.com/intl/en/ipv6/statistics.html
– still around 40% globally, (~28% in Australia)

• Fairly widely supported – likely to see a continued growth

IPv6 Addressing

https://www.google.com/intl/en/ipv6/statistics.html

• One of the disadvantages of a
hierarchical address space is that it
can be quite wasteful if not carefully
assigned, i.e. large parts of the
address space unused if blocks badly

• Combined with the scarcity
(exhaustion) of available IP
addresses, they have become a
valuable commodity

• Not intended to be sold, should
really be returned to the assigning
body for reallocation

• Early adopters of IPv6 were able to
sell their IPv4 address space at a

And finally…

• The slides are based on slides prepared by
based on material developed previously by: ,
, , and .

• Some of the images included in the notes were supplied as
part of the teaching resources accompanying the text books
listed in lecture 1.

– (And also) Computer Networks, 6th Edition, Tanenbaum A., Wetherall. D.
https://ebookcentral.proquest.com/lib/unimelb/detail.action?docID=6481879

• Textbook Reference: 4.1, 4.2.1, 4.3.1, 4.3.3, 4.3.5

Acknowledgement

https://ebookcentral.proquest.com/lib/unimelb/detail.action?docID=6481879

Internet (Network) Layer – IP Addresses and Packet Switching
Next four lectures
Internet (network) Layer
Internetworking and Routing
Network Layer
Store-and-Forward Packet Switching
Packet forwarding –�Connectionless
Packet forwarding –�Connection-oriented
Pros and cons of connections
MultiProtocol Label Switching
Quality of Service
Network Layer
Internet Protocol
Internet Protocol
IP Version 4 Protocol (IPv4)
IPv4 Addresses
Types of address
Types of address
IPv4 Addresses – Classes�(no longer used, but spoken of)
IP Addresses – CIDR
IP Addresses – CIDR
IP Addresses – Blocks
IP Addresses – Prefixing
IP Addresses – Route aggregation
Special IP Addresses
IPv6 Header
IPv6 Addressing
And finally…
Acknowledgement

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