Network Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021
Fragmentation
All networks have a maximum size for packets (Maximum Transmission Unit, MTU)
Hardware and operating system
Protocols and standards compliance Efficiency of transmission
Solution: fragmentation
divides packets into fragments when large packets need to be
routed through a network whose maximum packet size is too small.
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Types of Fragmentation (1)
Solution: Fragmentation and Reassembly.
Transparent: packets fragmented & reassembled in
each network. Route constrained, more work.
G1 fragments G2 reassembles G3 fragments G4 reassembles
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Types of Fragmentation (2)
Solution: Fragmentation and Reassembly.
Non-transparent: fragments are reassembled at destination. Router has less work. IP works this way. Each packet requires packet number, byte offset, end of packet flag
G fragments reassembleat 1 destination
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IP-Style Fragmentation
Original packet: (10 data bytes)
Fragmented: (to 8 data bytes)
Re-fragmented: (to 5 bytes)
Byte offset
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Path MTU Discovery
Alternative to Fragmentation
Advantage: the source knows what length packet to send
If the routes and path MTU change, new error packets will be triggered and the source will adapt to the new path
Try 1200
(error packet) Try 900
(error packet)
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Outline
Network layer in the Internet
Types of services
Internetworking
Tunneling
Fragmentation
Path MTU discovery
Internet Protocol Addressing
Subnetting
Routing algorithms
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IPv4 Datagram Structure (1)
IPv4 (Internet Protocol) datagram consists of a header and payload IPv4 header has fields for the key parts of the protocol
Header format: 20-byte fixed part + variable-length optional part
IPv4 Header TCP, UDP fragmentation Length
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IPv4 Datagram Structure (2)
IHL: Internet Header Length, in 32-bit units, min is 5 and max is 15
Differentiated services: different classes of service
Total Length: header and payload, max length 65535 bytes
Identification:
Allows host to determine which datagram the new fragment belongs to.
All fragments of same datagram have same ID
DF: Don’t Fragment
is used as part of the process to discover the path MTU
MF: More Fragment, is this the last one?
Fragment offset: where in the datagram the current fragment belongs
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IPv4 Datagram Structure (3)
TTL: Time to live, limits packet lifetimes in hops or seconds
Protocol: TCP, UDP
Header Checksum: verifies the header only
Source Address: IP address of the sender
Destination Address: IP address of the receiver
Options: e.g. security, strict vs. loose source routing, record route, timestamp
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IP Addresses (1)
IP address (IPv4) is 32-bit long, written in dotted decimal notation 128.18.3.11
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26
25
24
23
22
21
20
128
64
32
16
8
4
2
1
0
0
0
1
0
0
1
0
range: 0-255
Addresses are hierarchical and can be allocated in blocks e.g. 256 addresses in the block 128.18.3.0 – 128.18.3.255
Overall, IP allocation is managed by Internet Corporation for Assigned Names and Numbers (ICANN)
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IP Addresses (2)
network portion + host portion
Prefix: determined by the network portion, all hosts on a
single network has the same network portion. prefix is written as: lowest address/bit-length
128.18.3.0/24,18.2.0.0/16 Subnet mask: all 1s in the network portion
Extract prefix: ANDed the IP address with the subnet mask
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IP Addressing and Routing Tables
Routing tables are typically built on a triplet: Prefix Address
Subnet Mask
Outgoing Line (physical or virtual)
Example: a row of a routing table
Prefix
Subnet Mask
Interface
128.18.3.0/24
255.255.255.0
Eth 0
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