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第二章-应用层

不同的架构

client-server architecture

  • servers: always-on host; permanent IP address
  • clients: contact with server; maybe offline; may have dynamic address; donot connect with each other directly;

peer-to-peer architecture

  • no always-on server
  • arbitrary host directly communicate
  • peers request and provide service
  • peers are intermittently connected and change IP address(complex management)

进程（Process）间交流

同一主机：由操作系统决定

不同主机：exchanging messages(交换报文)

Sockets（套接字）

进程从套接字接收信息/传递信息给套接字

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Identifier（辨识符）

identifier includes both IP address and port numbers associated with process on host.

为了给特定的进程通信，目的地需要一个地址（IP address + port）

IP 地址用来标识主机，通过端口来区分不同的进程

应用层协议

把报文发送进socket就可以实现网络进程间的通信，但如何构造这些报文？含义？进程、报文如何互动？应用层协议！

HTTP 协议

  • Web’s application-layer protocol
  • client/server model:
  • client: browser that requests, receives, (using HTTP protocol) and “displays” Web objects
  • server: Web server sends (using HTTP protocol) objects in response to requests

HTTP uses TCP and it is “stateless”

  • server maintains no information about past client requests

two types:

  • Non-persistent HTTP
  • persistent HTTP

RTT (round trip time)：time for a small packet to travel from client to server and back

Non-persistent HTTP’s response time:

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Non-persistent HTTP response time for N files = 2N RTT + N file transmission time（每次都要重发请求）

But for Persistent HTTP resonse time = 2 RTT + N file transmission time（带流水线）

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maintaining user/server state: cookies

Web sites and client browser use cookies to maintain some state between transactions

网页缓存：

用户可以配置一个（可以是本地的）网页缓存，这样浏览器会将所有的请求发送给网页缓存，这样网页缓存会在 object in cache 时直接返回，不存在时由 cache 向源服务器发送请求。（代理服务器, proxy server）

Why Web caching?

  • reduce response time for client request
  • cache is closer to client
  • reduce traffic on an institution’s access link
  • Internet is dense with caches
  • enables “poor” content providers to more effectively deliver content

一些乱七八糟的小点：

conditional get(用于检查 cache 是否 up-to-date)

HOL(head of line) blocking: http2 通过按帧分割传输减轻了这个问题

E-mail, SMTP, IMAP

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Three major components:

  • user agents
  • mail servers
  • simple mail transfer protocol: SMTP

User Agent

  • a.k.a. “mail reader”
  • composing, editing, reading mail messages
  • e.g., Outlook, iPhone mail client
  • outgoing, incoming messages stored on server

mail servers:

  • mailbox contains incoming messages for user
  • message queue of outgoing (to be sent) mail messages

SMTP protocol between mail servers to send email messages

  • client: sending mail server
  • “server”: receiving mail server

PPT 第 52 张有一个 smtp(simple mail transfer protocol)+tcp 传输邮件的实例

smtp:push; imap/pop: pull

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The Domain Name System(DNS)

  • hostname-to-IP_address translation
  • canonical, alias names

root dns servers - top-level domain(tld) dns servers - authoritative dns servers - local dns name servers

DNS name resolution:

  • iterated query（迭代查询）

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  • recursive query（递归查询）

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DNS records:

DNS: distributed database storing resource records (RR)

RR format: (name, value, type, ttl)

p2p applications

file distribution: client-server v.s. p2p （可能会有计算题）

video streaming and content distribution networks

上面这个内容没听到，要补（现在看来好像不是很重要）