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7.3 Protocols

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By So Hee Park 5 min read

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✅ 7.4 Protocols

  • Protocols = books or rules for different applications.

☑️ Application Layer Protocol Categories

  • 1 book for naming things.
  • 2 books for IP configuration.
  • 3 books for email.
  • 2 books for sending attachments.
  • 2 books for browsing/navigating the internet.

✅ DNS (Domain Name System)

  • Set of rules for transforming IP addresses into human-readable names.
  • Purpose: understand what an IP belongs to.

☑️ Example

  • www.google.es
  • www = public webpage.
  • google = company name.
  • .es = Spanish server/domain.

☑️ Important Concepts

  • Human-readable name is commonly called a URL.
  • More accurately: FQDN (Fully Qualified Domain Name).
  • DNS transforms names into technical IPs.

  • Example:

    • https://mail.ef.com
    • becomes https://192.2.14.2:25

☑️ DNS Database

  • Huge distributed database.

  • Different servers manage different domains:

    • .com
    • .org
    • .net
  • Solving DNS = converting FQDN into IP address.

☑️ FQDN Levels

  • Information at the end = more general.
  • Information at the beginning = more specific.

✔️ Example

  • mail.ef.com

    • .com = level 1 (general)
    • ef = level 2
    • mail = level 3 (specific)

✅ DHCP (Dynamic Host Configuration Protocol)

  • Protocol that gives devices an IP address dynamically.
  • Used when devices move between networks.

☑️ DHCP Server

  • Machine that provides IP addresses.

  • Connected to database of:

    • free IPs
    • shareable IPs

☑️ Server vs Client

  • Server = gives something.
  • Client = requests/needs something.

☑️ DHCP Process (DORA)

✔️ 1. Discover

  • Client requests an IP.

✔️ 2. Offer

  • Server offers an IP.
  • Multiple servers can offer IPs.

✔️ 3. Request

  • Client requests one offered IP.
  • Usually chooses highest range.

✔️ 4. ACK (Acknowledgement)

  • Server confirms assigned IP.

☑️ Extra Note

  • Another Discover message is sent when signal becomes weak.

✅ BOOTP (Bootstrap Protocol)

  • Gives a temporary IP address.
  • Used for remote Operating System installation.

✅ HTTP (Hypertext Transfer Protocol)

  • Protocol used for navigating the internet.

☑️ Communication

  • Client = computer requesting webpage.
  • Server = machine sending webpage.

☑️ How HTTP Works

  • Client sends request with GET code.

  • Server replies with:

    • HTML code
    • HTML version
    • status code

☑️ Status Codes

✔️ 1XX

  • Not finished, wait.

✔️ 2XX

  • OK.

✔️ 3XX

  • Request forwarded.

✔️ 4XX

  • Bad request/webpage missing.

✔️ 5XX

  • Server error.

☑️ Important Characteristic

  • HTTP has no session memory.
  • If error happens, process restarts from beginning.

  • Faster browsing usually due to:

    • temporary internet files
    • cookies

✅ HTTPS (HTTP Secure)

  • Secure version of HTTP.

☑️ Security

  • Uses:

    • public keys
    • private keys
    • PGP (Pretty Good Privacy)

☑️ Encryption Idea

  • Public key = template/container.
  • Private key = password to decrypt content.

☑️ Important Notes

  • HTTPS prevents interception.
  • HTTPS does NOT mean malware-free.

  • Many systems besides HTTP use PGP:

    • emails
    • encrypted messaging
    • WhatsApp end-to-end encryption

✅ FTP (File Transfer Protocol)

  • Protocol for sending attachments/files.

☑️ TFTP (Trivial FTP)

  • Sends file pieces randomly.
  • No control mechanism.
  • Chaotic and less reliable.

✔️ Example

  • Decorative image transfer.

☑️ Normal FTP

  • Sends file pieces with control.
  • Failed transfers restart.
  • More reliable than TFTP.

✔️ Example

  • Sending school/work files.

☑️ FTP Actors

✔️ FTP Client

  • Host sending file.

✔️ FTP Server

  • Host receiving file.

☑️ Ports

  • Control ports:

    • 3000
    • 21

  • Data ports:

    • client uses ports >1024
    • server uses port 20

☑️ FTP Process

✔️ Step 1

  • Client port 3000 contacts server port 21.
  • Includes client data port >1024.

✔️ Step 2

  • Server replies ACK.
  • Requests confirmation.

✔️ Step 3

  • Client replies ACK.
  • Double check completed.

✔️ Step 4

  • File pieces sent:

    • client >1024
    • server 20

  • Every piece needs ACK.

✔️ Step 5

  • EOF (End Of File) sent.
  • Final confirmation received.

✅ SFTP (Secure FTP)

  • FTP with encryption.
  • Uses PGP keys.
  • All communication encrypted.

✅ Emails

☑️ Structure

  • Header + body.

☑️ Header Contains

  • From
  • To
  • Subject
  • CC
  • CCO

☑️ Communication Type

  • Asynchronous.
  • Receiver may read later.

☑️ Email Flow

  • Sender sends email to server.
  • Server later sends to destination.

☑️ Email Protocols

✔️ SMTP

  • Sends email from origin to server.

✔️ POP3

  • Email removed from server after download.
  • Uses port 110.

✔️ IMAP

  • Keeps original email on server.
  • Destination receives copies.
  • Allows multiplatform access.
  • Uses port 143.

☑️ POP3 vs IMAP

  • POP3:

    • cheaper
    • less storage
    • common in governmental systems

  • IMAP:

    • modern
    • flexible
    • common in Gmail/Educamadrid

✅ SMTP Process

☑️ Ports

  • Uses port 25 on both machines.

☑️ Start of Communication

✔️ Message 1

  • “hello” → ACK

✔️ Message 2

  • “From:” → ACK

✔️ Message 3

  • “To:” → ACK

☑️ Sending

  • Email sent in pieces.

☑️ Ending

✔️ Step 1

  • Sender says email finished.

✔️ Step 2

  • Receiver ACK.

✔️ Step 3

  • Sender leaves.

✔️ Step 4

  • Receiver ACK.

☑️ Important Notes

  • Each email generates many messages.

  • Attachments use:

    • SMTP for email
    • FTP for file transfer
  • Emails consume many resources.
  • Avoid unnecessary “thank you” replies in companies.

✅ Email Actors

☑️ MUA (Mail User Agent)

  • Final sender and receiver.
  • Sending uses port 25.

  • Receiving uses:

    • 110
    • 143

☑️ MTA (Mail Transfer Agent)

  • Transfers email between servers.
  • Like a truck carrying packages.

☑️ MDA (Mail Delivery Agent)

  • Delivers email close to destination.
  • Like a mailman.

☑️ MAA (Mail Access Agent)

  • Rebuilds email from pieces.
  • Gives completed email to client.

✅ SSH (Secure Shell)

  • Adds security to insecure communications.
  • Provides PGP encryption.

☑️ Example

  • Emails normally unencrypted.
  • SSH adds encryption first.

☑️ SSH Process

✔️ Step 1

  • Machine 1 requests public key.

✔️ Step 2

  • Machine 2 sends public key and requests one.

✔️ Step 3

  • Machine 1 sends public key.

✔️ Step 4

  • Communication occurs encrypted.

✅ Proxy-Cache Service

☑️ Proxy

  • Hides user IP.
  • Communications appear from proxy IP.

☑️ Cache

  • Stores webpage copies locally.
  • Avoids repeated internet downloads.
  • Saves pages for future access.

☑️ If Proxy-Cache Disconnects

  • Real IP becomes visible.
  • Device always reconnects to internet.
  • Temporary web files help future downloads.

✅ Firewall Service

  • Service that opens/closes ports.
  • Controls traffic access.

☑️ Port Behavior

✔️ Open Port

  • Allows data through.

✔️ Closed Port

  • Blocks and deletes data.

☑️ Examples

  • Close port 25:

    • cannot send emails.

  • Close port 80:

    • cannot browse internet.

  • Close ports 20/21:

    • cannot send attachments.

☑️ Company Security

  • First machine in company is usually firewall.

✅ Firewall Zones

☑️ MZ (Militarized Zone)

  • Protected internal company network.
  • Requires login/access permissions.

☑️ DMZ (Demilitarized Zone)

  • Publicly accessible systems.
  • Reachable from internet.

☑️ Why Both Are Needed

✔️ DMZ

  • Public webpages.
  • Marketing services.

✔️ MZ

  • Customer/private processes.
  • Restricted access.

☑️ Working Principle

  • Try using DMZ first.
  • Move to MZ only when necessary.
DAW bilingual, Computer System
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