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Character 2 - Application Layer

Application architectures​

Client-server​

Server

  • Always-on host, permanent IP address
  • Data centers for scaling

Client

  • Do not communicate directly with each other client

Peer-to-peer (P2P)​

  • No always-on server
  • Higher self scalability
  • Complex management

Processes communicating​

Process mean a program running within a host

Sockets​

Software interface, a process sends/receives messages to/from network

Addressing Processes​

To identify receiving process, we need to specify two info:

  1. Address of host: unique 32-bit IP address
  2. Identifier specifies receiving process in destination host: port number

IP address and port number

One IP is not enough to identifying process, because many processes can be running on same host

App-layer protocol defines​

Two main types of app-layer protocols:

Open protocols​

Defined in RFCs, e.g. HTTP, SMTP

Proprietary protocols​

Not defined in RFCs or elsewhere, e.g. Skype

What transport service does an app need?​

  1. Reliable data transfer
  2. Throughput
  3. Timing: low delay
  4. Security

Internet transport protocols services​

TCP​

  • Reliable
  • Flow control
  • Congestion control
  • e.g. Email, file transfer

UDP​

  • Higher throughput
  • e.g. Audio streaming

Securing TCP​

TCP and UDP no encryption

Use Secure Sockets Layer (SSL)

HTTP​

筆記一下學習網路五層#2.2.1 - 2.2.3 - iT 邦幫忙::一起幫忙解決難題，拯救 IT 人的一天

Hypertext transfer protocol, port 80

HTTP is "stateless"

  • Server maintains no information about past client requests

Question: should each request/response pair be sen over a separate or the same TCP connection?​

開發者可以決定當送出 request/response pair 時，是透過 separet TCP connections 或者是 同一個 TCP connection 實行，若為前者則稱為 Non-Persistent 後者則為 Persistent

Non-persistent HTTP 非持久性​

Round-trip time (RTT): time of packet travel from client to server and back

假使有一個 html 裡面有十張圖片

One RTT to initiate TCP connection

One RTT request message + receives response

Then request 10 image

Total 11 TCP connection, 22 RTT

Persistent HTTP​

One RTT to initiate TCP connection

One RTT request message + receives response

Then 10 RTT for 10 image

Total 1 TCP connection, 12 RTT

Concurrent HTTP​

Not part of the HTTP specification, but implemented by almost all web browsers

After initial HTTP connection which retrieves the HTML body, initiate multiple (4~6) HTTP sessions (per domain) to retrieve multiple objects (e.g. images) in parallel

HTTP request message general format​

HTTP Messages - HTTP | MDN

  • Request line
  • Header lines
  • Body

| method | sp | URL | sp | version | cr | lf | | header field name | | value | cr | lf | | header field name | | value | cr | lf | ... | cr | lf | | entity body |

Cookies​

使用 HTTP Cookie - HTTP | MDN

HTTP is stateless, how can identify users? Cookies

Defined in RFC 6265

Cookies允許網站用於追蹤使用者紀錄，主要構成有四部分

  • cookie header line in request message
  • cookie header line in response message
  • cookie file 存放於各個使用者的browser
  • back-end database in the Web site

Cookies can be used for:

  • Authorization
  • Shopping carts
  • Recommendations

Web caches (proxy server)​

Reduce response time for client request

Reduce traffic

Electronic mail​

SMTP, POP3, IMAP

SMTP​

RFC 2821, port 25

SMTP uses persistent connections

Comparison with HTTP:​

HTTP: pull protocol (pull the information from the server)

SMTP: push protocol (sending mail server pushes the file to the receiving mail server)

Eail access protocol​

POP: Post Office Protocol

IMAP: Internet Mail Access Protocol

or use HTTP

POP3​

Simple

TCP connection to the mail server on port 110

Three phases:

  1. Authorization
  • Declare username, password
  • +OK, -ERR
  1. Transaction
  • list, retr, dele, quit
  1. Update
  • Two modes of operations:
    1. download and delete
    2. download and keep

POP3 is stateless across sessions 跨會話是無狀態的

IMAP​

Keeps user state across sessions

Domain Name System (DNS)​

Application-layer protocol, distributed database

Services:

  • Hostname to IP address
  • Host aliasing
  • Mail server aliasing
  • Load distribution
    • Many IP addresses correspond to one name

Why not centralize DNS?​

  • Single point of failure

  • Traffic

  • Maintenance

Top-level domain (TLD) servers​

Responsible for top-level country domains, e.g.: uk, fr, ca, jp

Further delegates query to authoritative DNS servers

Authoritative DNS servers​

Organization’s own DNS server

Providing authoritative hostname to IP mappings for organization’ s named hosts (e.g., *.cityu.edu.hk), known as a DNS zone

Can further sub-divide into smaller DNS zones

DNS caching​

Once (any) name server learns mapping, it caches it

Improve delay performance and to reduce the number of DNS messages ricocheting around

Cached entries may be out-of-date

cache entries removed after time to live (TTL)

DNS records​

What are DNS records? | Cloudflare

DNS records (aka zone files)

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

There are many type of DNS record, more than 20 kind

Common type of DNS record​

A / AAAA​

What is a DNS A record? | Cloudflare

Hostname to IP address

A for IPv4, AAAA for IPv6

NS​

DNS NS record | Cloudflare

Stands for "nameserver"

Indicates which DNS server is authoritative for that domain

CNAME​

What is a DNS CNAME record? | Cloudflare

Align name to A record

CNAME record can point to another CNAME record, but it slower

MX and NS records cannot point to a CNAME record

MX​

What is a DNS MX record? | Cloudflare

"mail exchange" (MX)

MX records cannot point to a CNAME, MX must directly point to A or AAAA record

DNS protocol​

DNS Message Format - GeeksforGeeks

Query and reply messages, both with same message format

Don't need quiz, I think

Attacking DNS​

  • DDoS attacks
  • Redirect attacks
    • Man-in-middle
    • DNS poisoning
  • Exploit DNS for DDoS

P2P applications​

Examples:

  • file distribution (BitTorrent)
  • Streaming (KanKan)
  • VoIP (Skype)

File distribution - Client-server​

Server send N copies with F file size

usu_s: server upload capacity

Time to send one copy: Fus\frac{F}{u_s}

Time to send N copies: NFus\frac{NF}{u_s}

dmind_{min}: min client download rate

Min client download time: Fdmin\frac{F}{d_{min}}

Time to distribute F to N clients using client-server approach:

Dc−s≥max{NFus,Fdmin}D_{c-s} ≥ max\{\frac{NF}{u_s}, \frac{F}{d_{min}}\}

File distribution - P2P​

Client can download and upload to other client at the same time

Streaming multimedia: DASH​

What is MPEG-DASH? | HLS vs. DASH | Cloudflare

Wiki

DASH: Dynamic, Adaptive Streaming over HTTP

Content distribution networks (CDN)​

What is a content delivery network (CDN)? | How do CDNs work? | Cloudflare

Close to users