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Application Layer ​

Jargon(术语) ​

user agent: an interface between user & application

  • browser
  • mail reader
  • media player

Process: a program that is running within a host

IPC: inter-process communication (defined by the OS)

client/server:

  • client
    • initiates contact(speak first)
  • server
    • provide service

API: application programming interface

Interface ​

Socket

This is a interface between application layer & transport layer

enables the application to connect to network

How to identify the other process ​

  • IP address
  • Port number

What service do I need? ​

  • Data loss
  • Timing
  • Bandwidth

Common Protocols ​

ProtocolPortDetails
HTTP80-
FTP21attention:port 21 is for control connection
SMTP25

HTTP ​

stateless (keeping states is complex)

Request ​

HTTP.GET(client to server)

html
GET /xxxx/xxx.html HTTP/1.0
User-agent:Mozilla/4.0
Accept:text/html,image/gif,image/jpeg
Accept-language:fr
\n

HTTP.GET(Response)

html
HTTP/1.0 200 OK
Date:Thu,06 Aug 1998 12:00:15 GMT
Server:Apache/1.3.0(Unix)
Last-Modified:Mon,22 Jun 1998
Content-Length:6821
Content-Type:text/html
\n
data

Framework ​

  • Web Caches(proxy server)
    • this is like CDN
  • Cookie

SMTP ​

simple mail transfer protocol