Three major components of the Internet email system:

  1. User Agent (UA)
     - Reading, composing, and sending email
     - Examples: Outlook, Gmail web, Thunderbird

  2. Mail Server
     - Mailbox: Stores incoming messages
     - Message queue: Outgoing message queue

  3. SMTP (Simple Mail Transfer Protocol)
     - Protocol for transferring messages between mail servers

Email transmission process:

  Alice's UA -> Alice's mail server -> Bob's mail server -> Bob's UA
                    SMTP transfer          SMTP transfer

  1. Alice composes a message -> UA sends to Alice's mail server
  2. Alice's mail server stores message in the message queue
  3. SMTP client opens TCP connection to Bob's mail server SMTP server (port 25)
  4. SMTP handshaking followed by message transfer
  5. Bob's mail server stores message in Bob's mailbox
  6. Bob reads the message via UA

S: 220 mail.example.com SMTP ready
C: HELO mail.alice.com
S: 250 Hello mail.alice.com
C: MAIL FROM: alice@alice.com
S: 250 OK
C: RCPT TO: bob@example.com
S: 250 OK
C: DATA
S: 354 Start mail input
C: From: alice@alice.com
C: To: bob@example.com
C: Subject: Hello
C:
C: Hi Bob, how are you?
C: .
S: 250 OK
C: QUIT
S: 221 Bye

Sending                               Receiving
Alice UA --SMTP--> Mail server --SMTP--> Mail server --POP3/IMAP/HTTP--> Bob UA
          (Push)                    (Push)                            (Pull)

User wants to access: www.example.com
  |
DNS query: www.example.com -> ?
  |
DNS response: 93.184.216.34
  |
HTTP connection: 93.184.216.34:80

1. Host Aliasing
   Assign a simple alias to a complex canonical hostname
   relay1.east.example.com -> www.example.com

2. Mail Server Aliasing
   Specify mail servers via MX records
   Mail for example.com -> mail.example.com

3. Load Distribution
   Map a single name to multiple IP addresses
   www.example.com -> 93.184.216.34, 93.184.216.35, ...
   DNS rotates IP order in responses

Problems with centralized DNS:
  +-- Single point of failure: Server crash brings down entire Internet
  +-- Traffic volume: Cannot handle worldwide DNS queries
  +-- Distant database: Increased latency
  +-- Maintenance: Impossible to update a single database

-> DNS is designed as a distributed hierarchical database!

                    Root DNS server (.)
                    /      |      \
               .com      .org     .kr
               /    \      |       |
         example   google  wiki   naver

1. Root DNS Server
   - 13 root server clusters worldwide (A through M)
   - Provides IP addresses of TLD servers

2. TLD Server (Top-Level Domain Server)
   - Handles .com, .org, .net, .kr, .jp, etc.
   - Provides IP addresses of authoritative DNS servers

3. Authoritative DNS Server
   - Maintains DNS records for an organization's public hosts
   - Provides the final hostname-to-IP mapping

Each ISP has a local DNS server (default name server)
  - When a host sends a DNS query, the local DNS server queries on its behalf
  - Located nearby for fast responses

Host -> Local DNS -> Root DNS -> TLD DNS -> Authoritative DNS
                                              |
Host <- Local DNS <- Root DNS <- TLD DNS <- Authoritative DNS

Each server queries the next server on behalf of the requester
and returns the result back up the chain

Host -> Local DNS --query--> Root DNS
                  <--reply-- "Ask the TLD server"

         Local DNS --query--> TLD DNS
                  <--reply-- "Ask the authoritative server"

         Local DNS --query--> Authoritative DNS
                  <--reply-- "IP address: 93.184.216.34"

Host <- Local DNS (final answer)

DNS caching behavior:
  1. Local DNS server caches responses it receives
  2. Responds directly from cache for the same name queries
  3. Cache expires after TTL (Time To Live)

Effect: Most queries can be resolved without contacting root/TLD servers

+----------------------+
|   Header (12 bytes)  |
|  ID, flags, counts   |
+----------------------+
|   Question section   |
|  Query name, type    |
+----------------------+
|   Answer section     |
|  Resource records    |
+----------------------+
|   Authority section  |
|  Authority server RRs|
+----------------------+
|   Additional section |
|  Additional RRs      |
+----------------------+

nslookup www.example.com

Server must send N copies:
  D_cs >= max(NF/u_s, F/d_min)

  u_s: Server upload speed
  d_min: Slowest client's download speed

  As N increases -> distribution time increases linearly!

All peers contribute to uploading:
  D_p2p >= max(F/u_s, F/d_min, NF/(u_s + sum(u_i)))

  sum(u_i): Total upload capacity of all peers

  As N increases -> total upload capacity also increases!
  -> Distribution time increases only logarithmically

Distribution time comparison (as N increases):

Time
  ^
  |     / Client-server (linear increase)
  |    /
  |   /        ___
  |  /    ____/    P2P (sub-linear)
  | / ___/
  |//
  +---------------------> N (number of peers)

BitTorrent terminology:
  - Torrent: The set of peers participating in file distribution
  - Tracker: A server that tracks peers in the torrent
  - Chunk: A piece of the file (typically 256KB)

1. Rarest First:
   A peer requests the rarest chunks from its neighbors first
   -> Increases copies of rare chunks -> Improves availability

2. Tit-for-Tat:
   Prioritizes sending data to the 4 peers providing the fastest data
   -> Prevents free-riding

3. Optimistic Unchoking:
   Every 30 seconds, randomly unchokes one additional peer
   -> Gives new peers a chance to participate

Regular hash table:
  key -> hash(key) -> bucket -> value

Distributed hash table:
  key -> hash(key) -> responsible peer -> value

  Each peer is responsible for a portion of the key space

Peers arranged in a circle (0 to 2^n - 1):

         0
       /   \
     15      1
    /          \
  14            2
  |              |
  13            3
  |              |
  12            4
    \          /
     11      5
       \   /
    10--9--8--7--6

Key k is assigned to the closest peer >= k
Example: Key 11 -> Peer 12 is responsible

If Peer 3 wants to find the value for key 11:

  Peer 3 -> Peer 4 -> Peer 5 -> ... -> Peer 12

  Queries forwarded sequentially to successor peers
  -> O(N) messages needed (inefficient)

Each peer maintains a few shortcuts:

  Peer 3's shortcuts: Peer 4, Peer 8, Peer 14

  Peer 3 -> Peer 8 -> Peer 12 (3 steps)

  -> Reduced to O(log N) messages

Email system:
  +-- SMTP: Push transfer between mail servers (port 25)
  +-- POP3/IMAP/HTTP: Users read mail (Pull)
  +-- 7-bit ASCII, extended via MIME

DNS:
  +-- Hostname -> IP address translation
  +-- Hierarchical distributed database
  +-- Root -> TLD -> Authoritative DNS servers
  +-- Iterative/recursive queries
  +-- Caching for performance improvement

P2P:
  +-- Self-scalability (more peers -> more capacity)
  +-- BitTorrent: Chunk-based, Tit-for-Tat
  +-- DHT: Distributed key-value store