[컴퓨터네트워크] 2. Application Layer (3)

HTTP

▶ Non-persistent

- Single TCP

- Parallel TCP

• Problem 1) 하나를 위한 HTTP (낭비)

 

▶ Persistent

• Solution 1) Persistent HTTP: 한 번의 connection으로 multiple object를 전송한다.
• Problem 2) HOL(Head of Line blocking) 큰 object --> 다른 object delay 

 

HTTP/2

• Solution 2) object를 frame으로 나눈다. --> HOL 방지
  + Server: push unrequested object to client.
  + Priority 설정 --> FCFS (x), 다른 순서 가능
• Problem 3) 하나의 object packet이 drop되면, 오류 복구 전까지 진행이 멈춘다.
  <-> Non-persistent - parallel: 서로 다른 object들은 서로 다른 TCP connection을 이용하기 때문에, 방해 x
• Problem 4) Security 문제

 

HTTP/3

• Solution 3, 4) per object error and congestion control over UDP. (unreliable transfer)

 

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2.3 e-mail

• SMTP
• POP3, IMAP, HTTP

 

E-mail

▶ Three major components:

E-mail

• MUA (Mail User Agent)
• MTA (Mail Transfer Agent) (= mail server)
• SMTP (Simple Mail Transfer Protocol)

 

Mail servers

Mail servers

• mailbox: incoming message 보관
• message queue: outgoing message 보관

▶ SMTP: Protocol between mail servers to send e-mail messages.

• SMTP client: sending mail server
• SMTP server: receiving mail server

 

Cf) P2P vs. SMTP

• P2P(Peer-to-Peer): Client-Server 구조 x
• SMTP: server들끼리의 통신, sending server/receiving server

 

SMTP

• TCP(reliable transfer), Port 25
• Direct transfer: sending server --> receiving server
• Three phases of transfer: handshaking --> transfer of messages --> closure
• Command/Response interaction
  - Commands: ASCII text
  - Response: status code and phrase
• Message must be 7-bit ASCII

 

Scenario: Alice sends message to Bob

Scenario: Alice sends message to Bob

1. Alice's UA sends message to Alice's mail server --> message: Alice's message queue
2. Client side of SMTP opens TCP connection with Bob's mail server.
3. SMTP client sends message over the TCP connection to Bob's mail server --> message: Bob's mailbox
4. Bob's mail server invokes Bob's UA to read message.

▶ SMTP 사용:

- SMTP: push protocol

1. Alice's UA --> Alice's mail server
2. Alice's mail server --> Bob's mail server

Cf) Bob's mail server --> Bob's UA: message access protocol (POP, IMAP)

 

▶ Why does Alice's UA not directly sends her e-mail to Bob's mail server?

: Alice's UA는 sending을 계속 시도하지 않는다. 따라서 Alice's mail server message queue에서 maximum time trial.

 

▶ Who does DNS resolution for Bob's mail server?

: Alice's mail server

 

Sample SMTP interaction

1. Handshaking
2. Message transfer
3. Closure

Sample SMTP interaction

 

SMTP vs. HTTP

▶ Push protocol vs. Pull protocol

• SMTP: push protocol --> sending mail server pushes the data onto the receiving mail server by initiating a TCP connection.
• HTTP: pull protocol --> client pulls the information available on a server by initiating a TCP connection.

▶ Persistent connection vs. Non-persistent connection

• SMTP: persistent connection (message queue에 받은 message를 저장하고, 하나의 handshake로 모든 정보를 보내고 받는다.)
• HTTP: persistent connection, non-persistent connection 
  - HTTP/1: non-persistent connection
  - HTTP/2, HTTP/2: persistent connection

▶ Port number

• SMTP: Port 25
• HTTP: Port 80

▶ Encoding format

• SMTP: 7-bit ASCII
• HTTP: Does not require binary multimedia data to be encoded in 7-bit ASCII.

▶ Object 저장 방식

• SMTP: Places all the object into a single message.
• HTTP: Places each object in its own HTTP message.

 

Mail access protocols

Mail access protocols

▶ SMTP: push protocol --> mail server에게 보낼 때

 

▶ mail access protocol --> mail server에서 보낼 때

• POP(Post Office Protocol): authorization, download
• IMAP(Internet Mail Access Protocol): 더 많은 기능 포함 (+ manipulation of stored messages; UA가 mail server의 mailbox를 organize)
• HTTP: 웹 메일

 

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POP3 protocol

POP3 protocol

 

POP3 & IMAP

POP3

▶ POP3 mode

• download and delete mode: mailbox를 비운다. --> Bob cannot re-read e-mail if he changes client.
• download and keep mode: copies messages on different clients. (server의 mailbox가 비워지지 않는다.)

▶ stateless across sessions.

 

IMAP

• Keep all messages in one place: mail server.
• Allows UA to organize mailbox in folders.
• Keeps UA's state accross sessions.

 

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2.4 DNS (Domain Name System)

: Domain name --> IP address

DNS sevices, structure

▶ Domai Name System:

• Distributed database 
• Application-layer protocol: 모든 application에게 서비스를 제공하는 application-layer protocol. (network-layer x)