1. Network Application

1.1 Bidirectional Byte Stream Model

Byte Stream Model

• A/B can write&send the message that B/A can read
• A/B can close the connection

1.2 Word Wide Web

Word Wide Web(HTTP)

• Different kinds of request: GET, POST, DELETE and etc.
• send commands to the SERVER
• HTTP is document-centric, so clients requests name a file
• all in ASCII

1.3 BitTorrent

BitTorrent

• P2P transmission
• Swarms: collections of clients
• Get torrent file: describes the download information and tells who the tracker is
• Tracker keeps track with the name of the clients in the swarm
• when a new client join in, it will connect to other clients and other clients will be notified

1.4 Skype

Skype Skype with NAT

• NAT: client behind NAT can connect to other clients while the reverse is not the same
• personal computers are ways behind the NAT while public servers are not
• reverse connection: A wants to connect to B, but B opens connection with A
• Rendezvous

Skype with more NAT

2.The 4 Layer of Internet Model

Link

• packet: the information that is to be transmitted
• router and link
• eg: WIFI and Ethernet

Network

• packet is the building block of the layer
• provides a service to the Link Layer
• must use Internet Protocol (IP)

Transoport

• helps the information transmitted in order
• in some cases, data don't need to be transmitted in order: video conference

all together

IP: the thins waist of Internet

3. The IP Service

Network Layer Datagram

• IP sends the data based on IP header (IP DA and IP SA)
• hop-by-hop routing

Why IP is so simple?

• To keep the network simple, dumb and minimal
• End-to-end principle: implement the features in the end hosts
• Allows a variety of reliable (or unreliable) services to be built on top
• requires very few assumptions of the link layer below

five features of IP IPv4 Model

4. Life of a packet

three-way handshake

• Sync: C -> S
• Sync/ Ack: S -> C
• Ack: C -> S

TCP Byte Stream Inside the Hop

• see which pattern matches the pattern of the packet
• default: most useful

baidu traceroute

5. Principle: Layering

Layering Example: Postal Service Example: Computer System

Reasons for Layering:

• Modularity
• Well defined Service
• Reuse
• Separation of Concerns
• Continuous Improvement

6. Principle: Encapsulation

• Layer N data is payload to Layer N - 1
• eg: IP encapsulated TCP

Layering and Encapsulation VPN Encapsulation

7. Network Byte Order

Big endian
Little endian

Network Byte Order

网络字节序和主机字节序

8. IPv4 Addresses

8.1 Goal of Internet ProtocolAddresses

1. stitch many different networks together
2. Need network-independent, unique address

网络配置的四大基本要素： IP + Netmask + Gateway + DNS

8.2 Net Mask

如何理解子网掩码？

Net Mask

9. Longest Prefix Matching

Longest Prefix Matching Quiz

10. ARP

Introduction to Computer Networking学习笔记（五）：ARP协议（Address Resolution Protocol）