计算机代考程序代写 assembly algorithm Network Layer COMP90007 Internet Technologies – cscodehelp代写

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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