Chapter 2: Application layer

Application architectures

Client-server arch.

• 伺服器端：永遠在線且有常駐ip位址
• 客戶端：和伺服器連結、可能會間歇性的鏈接、可能為動態ip、客戶端與客戶端彼此不互相連接

P2P(peer-to-peer) arch.

• 無隨時在線的伺服器，點對點傳輸，越多點會使得傳輸速度更快

Process communicating

• process: program running within a host

Socket

• process sends/receives messages to/from its socket
• socket像是現實中的門(出入口)

Addressing process

• to receive messages, process must have "identifier".
• identifer = IP address + port number

Internet transport protocols services

TCP

• reliable transport between sending and receiving process
• flow control: sender won't overwhelm receiver
• congestion control (擁塞控制): throlle (限制) sender when network overloaded
• DOESN'T provide: timing, minimum throughput, guarantee, security
• connection-oriented: setup required between client and server processes

UDP

• DOESN'T provide that TCP service have

Securing TCP

TCP and UDP

• no encryption(加密), cleartext->socket->cleartext

SSL

• provides encrypted TCP connection, data integrity, end-point auth.
• apps use SSL libraries, that "talk" to TCP
• cleartext->socket->encrypt

HTTP overview

use TCP

• client initiates TCP connection(creates socket) to server, port 80
• server accepts TCP connection from client
• HTTP messages exchanged between browser and Web server.
• TCP connection closed

HTTP is "stateless"

• server maintains no information about past client request

HTTP connections

non-persistent HTTP

• at most one object send over TCP connection, connection then closed
• downloading multiple objects required multiple connections
• RTT: time for a small packet to travel from client to server and back
• response time = 2RTT + file transmission time(很重要)

persistent HTTP

• multiple objects can be sent over single TCP connection between client, server
• client sends requests as soon as it encounters a referenced object
• as least one RTT for all referenced objects

HTTP request message

• Two types of HTTP messages: request, response(由可見ASCII字元組成)
• 分為request line(GET, POST與其他指令), header lines與body
• status code: 200 OK, 301 Moved Permanently, 400 Bad Request, 404 Not found, 505 Http Version Not Supported
• 詳情請看Http貓

Method types

• HTTP/1.0: GET, POST, HEAD
• HTTP/1.1: 1.0 + PUT + DELETE(ppt裡現在沒有了)

User-server state: cookies

• Four components:

1. cookie header line of HTTP response message
2. cookie header line of HTTP request message
3. cookie file kept on user's host, managed by user's browser
4. back-end DB at Web site

Web caches(proxy server)

• satisfy client request without involving origin server
• user sets browser: Web accesses via cache
• browser sends all HTTP requests to cache
• object in cache: cache returns object
• else cache requests object from origin server, then returns object to client

Electronic mail

• Three major components: user agents, mail servers, Simple Mail Transfer Protocol(SMTP)

user agent

• function: composing, editing, reading mail messages.
• e.g., Outlook, Thunderbird

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

SMTP(RFC 2821)

• uses TCP to reliably transfer email message from client to server, port 25
• three phase of transfer: handshaking, transfer of messages, closure

SMTP: final words

• SMTP uses persistent connections, and requires message(header & body) to be in ASCII
• SMTP server used CRLF.CRLF(\r\n\r\n) determine end of message

comparison with HTTP

• protocol type: HTTP: pull, SMTP: push
• (HTTP：TCP由想要「接收」檔案的主機建立，SMTP則是由想要「送出」的檔案建立)
• both have ASCII command/response interaction, status codes
• HTTP: each object encapsulated(封裝) in its own response message
• SMTP: multiple objects sent in multipart message

Mail message format

• SMTP: protocol for exchanging email messages
• RFC822: standard for text message format
• header(To:, From:, Subject:), blank line and body(the message, ASCII char only)

Mail access protocols

• SMTP: delivery/storage to receiver's server
• mail access protocol: retrieval from server
• POP: Post Office protocol(RFC 1939): authorization, download
• IMAP: Internet Mail Access protocol(RFC 1730): more feature, including manipulation of stored messages on server
• HTTP: gmail, Hotmail, etc.

POP3(POP version 3)

• auth. phase:
• client commands: user, pass
• server responses: +OK, -ERR
• transaction phase: list, retr, dele, quit

IMAP

• keeps all messages in server
• allows user to organize messages in folders
• keeps user state across sessions

DNS(Domain name system)

[IMPORTANT]Peer to peer(P2P)

• no always-on server
• End systems connect with each other
• peers can connected\disconnect and change IP addresses during operation

File distribution time

Client server(CS)

• Server sends.
  • $u_s$ is the upload speed of the server.
• Client downloads.
  • $d_{min}$ is the minimum client download rate

time to distribution F to n Clients using CS model.

$D_{c-s}\ge max(NF/u_s,F/d_{min})$

• $F/u_s$ is the distribution time when
  $NF/u_s<=d_{min}$
• $F/d_{min}$ is the distribution time when
  $NF/u_s>d_{min}$

• File distribution time

P2P

• client
  • is the sum of client upload rate

time to distribution F to n Clients using CS model.

• $F/u_s$ is the distribution time when
  $Us<=(u_s+\sum _{}^{}{u}_i)/N$
• $F/d_{min}$ is the distribution time for the slowest client to receive file
• $NF/(u_s+\sum _{}^{}{u}_i)$ is the distribution time when
  $Us>=(u_s+\sum _{}^{}{u}_i)/N$

tags: Computer Network CSnote