how an application use the Internet

The structure of the Internet：The 4 layer model

The Internet protocol(IP)

Basic architectural ideas and principles:

Packet switching

Layering

Encapsulation

1.1 A Day in the life of an Application

Dominant model:

bidrectional

reliable byte stream connection

1. World Wide Web (HTTP)

C/S架构

发送commands，接收responses

request："GET / HTTP/1.1"

response: "HTTP/1.1 200 OK"

2. BitTorrent

P2P架构

break files into "pieces"

所有存有pieces file的client被称为swarm，client join and leave swarms

tracker追踪swarm的成员，通过torrent file向一个tracker获取swarm中的client列表

request: "GET list of clients in swarm"

response: "200 OK list of clients"

3. Skype

mixed架构

如果两个client能互联，就在client间建立连接--->P2P

如果有一方存在NAT：Client A-----Internet-----NAT---ClientB

由于NAT，B可以连接A，但A无法连接B

A cannot open connection to B

B在一个Rendezvous上注册，A向Rendezvous发送请求，Rendezvous转发请求给B，B主动和A建立连接

双方均存在NAT

通过一个Relay转发服务器作为中转，转发双方的消息

1.2 What the Internet is: The 4 Layer Internet Model

Application:(http, bit-torrent)

application-specific semantics

Transport:(tcp, udp)

guarantees correct, in-order delivery of data end-to-end

controls congestion

Network:(IP)

must use the Internet Protocol(IP)

best-effort attempt to deliver datagrams, no promises

IP datagrams can get lost, out of order, and can be corrupted

Link:(802.11, 3G, DSL, Ethernet)

one-hop control

delivers data over a single link between an end host and router, or between routers

two extra things need to know:

IP is the "thin waist"

the 7-layer OSI Model

Application------http--------Application----7

Application------ASCII-----Presentation--6

Transport--------TCP-------Session--------5

Transport--------TCP-------Transport------4

Network----------IP----------Network--------3

Link---------------Ethernet----Link------------2

Link---------------Ethernet----Physical------1

1.3 What the Internet is: The IP service model

The Internet Protocol(IP)

Transport Segment

IP Datagram

Link Frame

The IP service Model

Property Behavior
Datagram   Hop by hop routing  
Unreliable   Packets might be dropped because of full wait queue  
Best effort    
Connectionless   No per-flow state. Might be mis-sequenced  

why simple?

faster, more streamlined and lower cost to build and maintain

The end-to-end principle: implement features in the end hosts if possible

Allows a variety of reliable(or not) services to be built on top

Works over any link layer: IP makes very few assumptions about the link layer below

The IP service model

Tries to prevent packets looping forever(TTL, time to live)

Will fragment packets if they are too long

Uses a header checksum to reduce chances of delivering datagram to wrong destination

Allows for new versions of IP

IPv4: 32 bit

IPv6: 128 bit

Allows for new options to be added to header

Quiz:
In an alternative to the Internet Protocol called "ATM" proposed in the 1990s, the source and destination address is replaced by a unique "flow" identifier that is dynamically created for each new end-to-end communication. Before the communication starts, the flow identifier is added to each router along the path, to indicate the next hop, and then removed when the communication is over. Which of the following statements are implications of this design? Check all that apply.

(+)There is state in the network for every communication flow, rather than just for every destination.

(-)If a link fails, there is no need to update any state in the network.

(-)There must be a centralized controller to manage the insertion and removal of identifiers, as well as update them whenever a flow needs to be re-routed.

(-)It means we no longer need a transport layer to reliably deliver correct, in-order data to applications.

cs244a-Introduction to Computer Networking-Unit1