CAN201 W2

This is the note of CAN201(introduction to networking).

This lecture is mainly discussing about Application Layer. 应用程序层

1. Principle of network application
2. Web application
3. Email

The lab of this week is about Introduction of Python.

Lecture

Rich network applications

• Communication: Email, IM, SNS
• Web: countless
• Internet games: countless
• Video based: TikTok
• VoIP / Meeting: Zoom
• P2P file sharing: BitTorrent, Thunder…

---

Architectures for applications

• Client-server
• peer to peer

所有的applications都是这两个架构

---

Client-server architecture

• Server
  • Always-on host 不间断的主机
  • Permanent IP address 永久ip地址
  • High performance 高性能 / Distributed computing
• Clients
  • Link to the server for service 连接服务器进行服务
  • May be intermittently connect to the internet 可能是间歇性连接到互联网
  • Dynamic IP address 动态ip地址
  • Do not communicate directly with each other

P2P architecture

• No always-on server is needed
• Arbitrary end systems directly exchange data
• Peers request service from other peers, provide service in return to other peers
  • Self scalability
• new peers bring new service capacity, as well as new service demands
• Peers are intermittently connected
• Dynamic IP addresses

---

How to communicate over the network?

Host –> Sockets 插座 –> Network
A program with: identifier protocol.
IP地址标记主机，port标记主机里的进程。

Host - Program

• process: a program running within a host
• processes in different / same hosts:
  • socket can be used to exchange messages
• Processes in the same host:
  • Inter-process communication defined by OS
• CS architecture:
  • Client process: initiates communication
  • Server process: waits to be contacted
• P2P architecture:
  • client process / server process on the same host

---

Addressing processes

• Host device has unique 32-bit IPv4 and/or 128-bit IPv6
• Process network identifier:
  • IPv4:port 192.168.1.100:80
  • [IPv6]:port [240e:3a1:4cb1:69d0:f40c:4269:74a2:7ea3]:80

---

App-layer protocol defines

• Types of messages exchanged
  • request, response
• Message syntax
  • what fields in messages & how fields are delineated
• Message semantics:
  • meaning of information in fields
• Message timing: when and how

一共两种协议
Open protocols: 公开协议

• Defined in RFCs
• Allows for interoperability
• e.g., HTTP, SMTP, FTP

Closed protocols: 私有协议

• e.g., Skype, Games, you own protocols…

---

Transport service requirements

There’re 4 requirements in transport service: 必须遵守 (应用协议)

1. data integrity
2. throughput
3. timing
4. security
   

---

Internet transport protocols services (传输层协议)

• TCP 传输控制协议
• UDP 用户数据报协议

---

TCP service

• Reliable transport between sending and receiving process 发送和接收过程之间的可靠传输
• Flow control 流量控制: sender won’t overwhelm receiver 发送方不会淹没接收方
• Congestion control 拥塞控制: throttle sender when network overloaded 当网络过载时限制发送者
• Does not offer 不提供: timing, minimum throughput guarantee, security 记时，最低吞吐量保证，安全性
• Connection-oriented 面向连接: setup required between client and server processes 需要在客户端和服务器进程之间进行设置

---

Internet transport protocols services

The problems with UDP service:

• unreliable data transfer between sending and recieving process
• Does not offer: reliability, flow control, congestion control, timing, throughput guarantee, security, or connection setup. 不提供: 可靠性,流量控制,拥塞控制,时间,吞吐量保证,安全性或连接设置

---

Securing TCP

Secure Sockets Layer - SSL

• TCP & UDP
  • No encryption 无加密
  • Cleartext psws 明文程序状态字 –> Internet
• SSL
  • Provides encrypted TCP connection 提供加密TCP连接
  • Data integrity 数据完整性
  • End-point authentication 端点认证
• SSL is at app layer
  • Apps use SSL libraries, that “talk” to TCP
• SSL socket API
  • Cleartext psw 明文程序状态字 –> encrypted psw –> Internet

---

Web application

Web, HTTP and WWW

• World Wide Web: WWW
• HTTP: Hypertext Transfer Protocol
• Web page consists of base HTML-file which includes several referenced objects, addressable by a URL 网页上的每一个对象都通过url进行寻址

---

HTTP overview

HTTP: hypertext transfer protocol 超文本传输协议

• Application layer protocol 应用层协议
• Client/server model
  • Client: browser that requests, receives, (using HTTP protocol) and show Web objects (Render)
  • Server: Web server sends (using HTTP protocol) objects in response to requests

---

Uses TCP:

1. Client initiates TCP connection (creates socket) to server, port 80 客户端发起TCP连接到服务器，端口80
2. Server accepts TCP connection from client 服务器接受来自客户端的TCP连接
3. HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) 在浏览器和Web服务器之间交换HTTP消息
4. TCP connection closed TCP连接关闭

HTTP is “stateless”: server maintains no information about past client requests. HTTP是“无状态的”:服务器不维护关于过去客户端请求的信息。

Protocols that maintain “state” are complex!

• past history (state) must be maintained.
• if server/client crashed, their view of “state” may be inconsistent, must be reconciled. 如果服务器/客户端崩溃，他们的“状态”视图可能不一致，必须协调

---

Non-persistent HTTP 非持久性HTTP

• At most one object sent over TCP connection 最多一个对象通过TCP连接发送
• Downloading multiple objects required multiple connections 下载多个对象需要多个连接

Persistent HTTP 持久HTTP

• Multiple objects can be sent over single TCP connection between client, server 客户端和服务器之间可以通过单个TCP连接发送多个对象

---

Non-response time

---

Persistent HTTP

Non-persistent HTTP issues

• Requires 2 RTTs per object 每个对象需要2个rtt
• OS overhead for each TCP connection 每个TCP连接的操作系统开销
• Browsers often open parallel TCP connections to fetch referenced objects 浏览器经常打开并行TCP连接来获取引用的对象

Persistent HTTP issues

• Server leaves connection open after sending response 服务器在发送响应后保持连接打开
• Subsequent HTTP messages between same client/server sent over open connection 在同一个客户端/服务器之间通过开放连接发送的后续HTTP消息
• Client sends requests as soon as it encounters a referenced object 客户端一遇到引用对象就发送请求
• As little as one RTT for all the referenced objects 对于所有引用的对象，只有一个RTT

Persistent HTTP is not perfect

---

HTTP request message

• Two types of HTTP messages: request, response
• HTTP 1.x request message:
  • ASCII (human-readable format)

---

general format

---

HTTP 1.1 method

What do we need from a web protocol?
Get, Create, Modify, Delete —- resource / data / information

---

HTTP 1.0 1.1 Additional methods

Get, Post, Put, Delete, Patch —- Safe Methods
Head, Trace, Options, Connect —- Idempotent Methods 幂等方法

---

HTTP response status codes

• Status code appears in 1st line in server-to-client response message.
• Some sample codes:
  • 200 OK
  • 301 Move Permanently
  • 400 Bad Request
  • 404 Not Found
  • 505 HTTP Version Not Supported

---

User-server state: cookies

• many web sites use cookie
• four components:
  • cookie header line of HTTP response message HTTP响应消息的cookie报头行
  • cookie header line in next HTTP request message 下一个HTTP请求消息中的cookie头行
  • cookie file kept on user’s host, managed by user’s browser cookie文件保存在用户的主机上，由用户的浏览器管理
  • back-end database at web site 网站的后端数据库

Example:
Bob always access Internet from PC and visits Taobao site for first time. When initial HTTP requests arrives at site, site creates: unique ID –> cookie, entry in backend database for ID.

---

cookies

What cookies can be used for: cookies可以用来做什么

• authorization 授权
• shopping carts 购物车
• recommendations 建议
• user session state 用户会话状态

How to keep “state”:

• Protocol endpoints: maintain state at sender/receiver over multiple transactions 协议端点:在多个事务中维护发送方/接收方的状态
• Cookies: http messages http消息

Cookies and privacy: cookies permit sites to learn a lot about you; remember to clean your cookies. Cookies允许网站了解更多关于你的信息。

---

Web Caches web缓存(Proxy server代理服务器)

Goal: satisfy client request without involving origin server 满足客户端请求而不涉及原始服务器

• User sets browser: Web accesses via cache 用户设置浏览器:通过缓存访问Web
• Browser sends all HTTP requests to cache 浏览器将所有HTTP请求发送到缓存
  • object in cache: cache returns object 缓存返回对象
  • else cache requests object from origin server, then returns object to client 从源服务器缓存请求对象，然后返回对象给客户端

• Cache acts as both client and server 缓存既是客户端也是服务器端
  • server for original requesting client 服务器为原始请求客户端
  • client to origin server 客户端到源服务器
• Typically cache is installed by ISP (university, company, residential ISP) 缓存通常由ISP安装

Why web caching?

• reduce response time for client request 减少客户的要求的反应时间
• reduce traffic on an institution’s access link 减少机构访问链接的流量
• internet dense with caches: enable “poor” content provides to effectively deliver content (so too does P2P file sharing) 使“糟糕的”内容提供商能够有效地传递内容

---

cache example

Assumptions:

• avg object size: 100K bits
• avg request rate from browsers to origin servers: 15/sec
• avg data rate to browsers: 1.50 Mbps 100k*15
• RTT from institutional router to any origin server: 2 sec
• access link rate: 1.54 Mbps

Consequences:

• LAN utilization: 0.15%
• access link utilization = 99% 1.50/1.54 无限拥塞，以分钟做为单位
• total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs
  
  

---

conditional GET

• Goal: don’t send object if cache has up-to-date cached version
  • no object transmission delay
  • lower link utilization
• cache: specify date of cahced copy in HTTP request
  • If-modified-since:
• server response contains no object if cached copy is up-to-date:
  • HTTP/1.0 304 Not Modified

---

Email

How Email Works:

1. A user (Alice) sends an email message and connects to an SMTP (Simple Mail Transfer Protocol) server as configured in her email client or Mail User Agent (MUA). 用户发送电子邮件消息并连接SMTP服务器配置在她的电子邮件客户端或邮件用户代理。
2. On the SMTP server, a Mail Transfer Agent (MTA) looks at the recipient address and looks up the domain part of the address to determine its destination. 在SMTP服务器上，邮件传输代理查看收件人地址并查找地址的域部分以确定其目的地。
3. After querying a Domain Name System (DNS) server for the name of the Mail eXchanger (MX) for the recipient’s domain name… 在DNS服务器上查询收件人域名的Mail exchange名称后
4. …the SMTP server will send the message to that server via the SMTP protocol. SMTP服务器将通过SMTP协议将消息发送到该服务器。
5. The receiving server will store the message and make it available to the recipient (Bob), who can access it via web, POP, or IMAP. 接收服务器将存储消息并使其对接收方可用，接收方可以通过web、POP或IMAP访问该消息。

---

Email Protocols

Components:

• mail user agent 先从mua发送到ms 然后ms发送到另外一个ms ms再送到收件人的mua
• mail server 邮件服务器
• SMTP (protocol)

Delivery:

• SMTP: simple mail transfer protocol[RFC2821, Apr. 2001]

Receive:

• POP3 - Post Office Protocol - Version 3[RFC1939, May 1996]
• IMAP - Internet Message Access Protocol[RFC3501, Mar. 2003]
• Exchange ActiveSync – Microsoft’s protocol[from 2002]

---

Lab

import re  # 为正则表达式引入的re库
input_str = input(f'input: ')
# 通过正则表达式，提取出两种形式的数字，第一种是整数，第二种是带小数点的数字
numbers = re.findall(r"\d+\.?\d*", input_str)
number_list = []
location_list = []
for number in numbers:
    if '.' in number:
        number_list.append(float(number))
    else:
        number_list.append(int(number))
number_list.sort()
# .sort()方法没有返回值，所以不能直接用print()输出，假如是sorted就可以直接输出
print(number_list)
for number in number_list:
    location_list.append([input_str.index(str(number)), len(str(number))])
print(location_list)

---

References

1. XJTLU MC PowerPoint slides (CAN201 Week2 Lecture2 & Lab2)
2. 计算机网络 自顶向下方法.原书第6版