Network samajhna matlab ek packet ki poori journey samajhna.

Computer Networks ka syllabus bada lagta hai, lekin asal mein bas ek hi kahani hai: tumhare browser se ek request nikli, aur wo Google ke server tak pahunchi aur wapas aayi. Bas isi journey ko 5 layers mein tod diya gaya hai. Har layer ka ek kaam hai, aur har layer ke apne protocols hain.

Padhne ka order

1. Section 01–03 must hai. Basics + layering + delay formulas. Inke bina baaki sab hawa mein hai.
2. Section 04, 05, 09, 12, 20 — ye 5 sections interview mein sabse zyada poochhe jaate hain (HTTP, DNS, TCP, subnetting, ARP). Agar time kam hai to yahin se start karo.
3. Baaki sections depth ke liye — college exam / on-campus test / SDE-1 deep rounds mein kaam aate hain.
4. Last mein Section 22 (google.com walk-through) padho — wo saare topics ko ek dhaage mein baandh deta hai. Wo hi sabse common interview question hai.

Badges ka matlab

1.1 Sabse pehle: network ki definition

Internet ko do tareeke se dekha jaata hai — ye distinction interview mein "What is the Internet?" ka perfect answer banata hai:

Basic vocabulary — inko confuse mat karna

1.2 Network ka structure: edge, core, access

   NETWORK EDGE              ACCESS NETWORK            NETWORK CORE
   (hosts: clients,      →      (last mile: DSL,      →      (mesh of routers,
    servers, IoT)                cable, fiber, LTE,           ISP backbones,
                                 WiFi, Ethernet)              IXPs, peering)
    

• Network edge — applications yahin chalti hain (tumhara laptop, data center servers).
• Access network — edge ko core se jodne wali "last mile" link. Yahan par bandwidth shared hai ya dedicated ye important hai: cable Internet mein neighbourhood share karta hai (isliye shaam ko slow), DSL/fiber dedicated hota hai.
• Network core — interconnected routers ka mesh jo packets ko source se destination tak pahunchata hai. Core ke do main kaam: forwarding (local: input port se sahi output port pe bhejo) aur routing (global: pura raasta decide karo).

1.3 Physical media (short but asked)

1.4 Internet structure: network of networks ★★ common

Sawaal: crores ke access ISPs ko aapas mein kaise jodein? Har ISP ko har doosre se jodo to O(N²) links chahiye — impossible. Isliye hierarchy bani:

        Tier-1 ISPs (global: AT&T, Level3/Lumen, Tata Comm.)  ← ye ek doosre se
             │        peer karte hain, kisi ko paise nahi dete
        Regional ISPs (country/state level)
             │
        Access ISPs (Jio, Airtel, local broadband)  → tum yahan ho
             │
        You

  Side-by-side: IXP (Internet Exchange Point) — jahan bahut saare ISPs
  ek jagah milke traffic exchange karte hain (sasta, kam latency).
  Content Provider Networks (Google, Netflix, Akamai) — apna private
  network + CDN banate hain, aur seedha access ISPs se peer karte hain
  taaki Tier-1 ko paise na dena pade aur latency kam ho.
    

1.5 Circuit switching vs Packet switching ★★★ asked a lot

FDM (Frequency Division Multiplexing): har user ko alag frequency band, poore time ke liye. TDM (Time Division Multiplexing): time ko frames mein baanta, har user ko har frame mein ek fixed slot.

2.1 Layering kyun? ★★★ asked a lot

• Modularity — har layer independently change/upgrade ho sakti hai (jaise IPv4 → IPv6 ke liye HTTP badalna nahi pada).
• Abstraction — upar wali layer ko neeche ka implementation nahi pata, sirf service pata hai.
• Debugging aasan — problem kis layer par hai, isolate kar sakte ho.
• Nuksaan: thoda overhead (har layer apna header lagati hai), aur kabhi kabhi ek hi kaam do layers karti hain (error check L2 pe bhi, L4 pe bhi).

2.2 OSI 7 layers

Mnemonic (top→bottom): All People Seem To Need Data Processing.

2.3 TCP/IP model (jo asli mein chalta hai)

  OSI (7)                       TCP/IP (5-layer, Kurose)      TCP/IP (4-layer, RFC)
  ┌─────────────┐
  │ Application │ ┐
  ├─────────────┤ │
  │Presentation │ ├──────────►  Application            Application
  ├─────────────┤ │
  │  Session    │ ┘
  ├─────────────┤
  │  Transport  │ ──────────►  Transport              Transport
  ├─────────────┤
  │  Network    │ ──────────►  Network                Internet
  ├─────────────┤
  │  Data Link  │ ──────────►  Link          ┐
  ├─────────────┤                               ├──►  Network Access
  │  Physical   │ ──────────►  Physical      ┘
  └─────────────┘
    

2.4 Encapsulation — packet ke upar layers ke header ★★★ asked a lot

 Sender pe (neeche jaate hue header lagta hai — "encapsulation"):

 App:    [                    Message (e.g. HTTP GET)                  ]
 Trans:  [ TCP hdr |               Message                            ]  → segment
 Net:    [ IP hdr  | TCP hdr |     Message                            ]  → datagram
 Link:   [ Eth hdr | IP hdr  | TCP hdr | Message | Eth trailer(CRC) ]  → frame
 Phy:    101101000111010101110100…                                          → bits

 Receiver pe ulta: har layer apna header hata ke upar bhejti hai — "decapsulation".
    

3.1 Chaar tarah ke delay ★★★ asked a lot

        A ──────────────► [ROUTER] ──────────────► B
                          ↑  ↑  ↑  ↑
                          │  │  │  └─ 4. propagation (link pe travel)
                          │  │  └──── 3. transmission (bits ko link pe push)
                          │  └─────── 2. queuing (buffer mein wait)
                          └────────── 1. processing (header check, output link decide)
    

3.2 Queuing delay aur traffic intensity

• La/R ≈ 0 → average queuing delay bahut kam.
• La/R → 1 → delay tezi se badhta hai (exponentially).
• La/R > 1 → jitna aa raha hai utna nikal nahi sakta → queue infinite badhegi → packet drop. Design rule: traffic intensity ko 1 ke paas mat jaane do.

 avg queuing
   delay │           ╱
         │          ╱
         │        ╱
         │     ╱
         │ ╱
         └────────────── La/R
         0            1
    

3.3 Packet loss

Router ke buffer ki capacity finite hai. Jab queue full ho jaye aur naya packet aaye, to packet drop ho jaata hai — yehi packet loss hai. Lost packet ya to previous node se ya source se retransmit hota hai (TCP karta hai), ya kabhi retransmit hi nahi hota (UDP).

3.4 Throughput ★★ common

Multiple connections sharing a link: agar 10 connections ek common link R (bottleneck) share kar rahe hain, to har ek ko R/10 milta hai (fair share). Aur agar access links R_s aur R_c hain to per-connection throughput = min(Rs, Rc, R/10).

3.5 Store-and-forward: N hops ka total delay ★★★ asked a lot

Packet switch poora packet receive karne ke baad hi forward karta hai. To agar source aur destination ke beech N links hain (yaani N−1 routers), aur sab links same rate R hain, propagation ignore karke:

Intuition: pehla packet ko N hops paar karne mein N×(L/R) lagta hai; uske baad har agla packet ek-ek L/R ke gap pe aata rehta hai (pipeline), to (P−1) aur.

4.1 Application architectures

4.2 Process communication: socket aur addressing

App ko transport se kya chahiye?

Internet ka TCP ye deta hai: reliable, in-order, flow control, congestion control — par timing ya minimum bandwidth guarantee nahi deta. Ye line interview mein bolna.

4.3 HTTP basics ★★★ asked a lot

• HyperText Transfer Protocol — web ka application layer protocol, client-server model.
• TCP use karta hai, port 80 (HTTPS: 443). Pehle TCP connection banega, phir HTTP messages.
• Stateless — server client ke pichle requests ke baare mein kuch yaad nahi rakhta. (Isliye cookies ki zaroorat padi.)
• Web page = ek base HTML file + N referenced objects (images, CSS, JS). Har object ka apna URL.

4.4 Non-persistent vs Persistent HTTP ★★★ asked a lot

4.5 HTTP message format

REQUEST
GET /somedir/page.html HTTP/1.1        ← request line: METHOD  URL  VERSION
Host: www.someschool.edu              ← header lines
Connection: close
User-Agent: Mozilla/5.0
Accept-language: en
(blank line = CR LF)
(entity body — POST/PUT mein data yahan)

RESPONSE
HTTP/1.1 200 OK                       ← status line
Date: Tue, 09 Aug 2026 15:44:04 GMT
Server: Apache/2.4.41
Last-Modified: Tue, 09 Aug 2026 15:11:03 GMT
Content-Length: 6821
Content-Type: text/html
(blank line)
<html> … data … </html>              ← entity body
    

HTTP methods

GET vs POST ★★★ asked a lot

Status codes — ye poochhe jaate hain

4.6 Cookies — stateless protocol mein state ★★ common

1. Pehli baar tum site pe jaate ho. Server ek unique ID generate karta hai aur backend database mein entry banata hai.
2. Response mein header aata hai: Set-Cookie: 1678
3. Browser use apni cookie file mein save kar leta hai (domain ke against).
4. Aage har request mein browser bhejta hai: Cookie: 1678
5. Server us ID se tumhe pehchaan leta hai → shopping cart, login session, recommendations.

Cookies ke 4 kaam: authorization, shopping carts, recommendations, user session state.

4.7 Web caching (proxy server) ★★ common

Cache = client ke paas wala server jo popular objects ki copy rakhta hai. Browser saari requests cache ko bhejta hai. Agar object cache mein hai → turant de deta hai. Nahi hai → cache origin server se laata hai, apne paas rakhta hai, aur client ko deta hai.

• Faayde: response time kam, institutional access link ka traffic kam (paisa bachta hai), origin server pe load kam.
• Cache client bhi hai aur server bhi — origin ke liye client, browser ke liye server.
• CDN (Akamai, Cloudflare) yehi cache ko global scale pe karte hain.

Conditional GET — cache stale na ho jaye

 Cache → Server:   GET /fruit/kiwi.gif HTTP/1.1
                   If-Modified-Since: Wed, 7 Sep 2026 09:23:24

 Agar object badla nahi:        Agar object badal gaya:
 HTTP/1.1 304 Not Modified       HTTP/1.1 200 OK
 (body khaali — bandwidth bacha)   (naya object body mein)
    

Modern version: ETag + If-None-Match (hash-based, timestamp se better). Cache-Control: max-age=3600 se to network trip hi nahi hoti.

4.8 HTTP versions: 1.1 → 2 → 3 ★★ common

HTTP/2 vocabulary: Stream = ek bidirectional flow of bytes ek connection ke andar (har request-response ka apna stream ID). Frame = sabse chhoti unit (HEADERS, DATA, SETTINGS…). Objects ko frames mein toda jaata hai aur frames interleave karke bheje jaate hain — isliye chhota object bade object ke peeche atakta nahi.

4.9 HTTPS aur TLS (bonus but poochha jaata hai) ★★★ asked a lot

HTTPS = HTTP + TLS (Transport Layer Security, purana naam SSL). Port 443. TLS TCP ke upar aur HTTP ke neeche baithta hai.

TLS kya deta hai: (1) Confidentiality — encryption, (2) Integrity — MAC se tampering pakadna, (3) Authentication — certificate se server ki pehchaan.

1. ClientHello — client supported cipher suites aur random number bhejta hai.
2. ServerHello + Certificate — server cipher choose karta hai, apna X.509 certificate (public key + CA ka signature) bhejta hai.
3. Client CA chain verify karta hai (browser mein pre-installed root CAs se).
4. Key exchange — client pre-master secret banata hai (RSA se encrypt karke bhejta hai, ya modern mein ECDHE se dono milke derive karte hain — forward secrecy ke liye).
5. Dono taraf symmetric session key derive hoti hai. Actual data symmetric encryption (AES) se hota hai kyunki wo fast hai.
6. Finished messages — handshake verify, phir encrypted application data start.

5.1 DNS kya karta hai

DNS ki services:

• Hostname → IP address translation (main kaam)
• Host aliasing — canonical name vs alias (relay1.west.enterprise.com ↔ www.enterprise.com)
• Mail server aliasing — MX records
• Load distribution — ek hostname ke liye multiple IPs, rotate karke return karta hai (poor man's load balancer)

5.2 DNS hierarchy ★★★ asked a lot

                        Root DNS servers  (13 logical: a.root-servers.net … m,
                             │           actually 1000+ physical via anycast)
        ┌────────────────────┼────────────────────┐
     .com TLD            .org TLD            .in TLD       ← Top Level Domain servers
        │                    │                    │
   google.com          wikipedia.org      bits-pilani.ac.in  ← Authoritative servers
   authoritative                                              (organization ke apne records)

  + Local DNS server / Resolver (ISP ya 8.8.8.8) — hierarchy ka hissa nahi,
    par har query pehle yahin jaati hai; ye cache rakhta hai aur pura kaam karwata hai.
    

• Root server — TLD server ka pata deta hai. (ICANN manage karta hai Verisign ke saath.)
• TLD server — .com, .org, .edu, country domains (.in, .uk). Authoritative server ka pata deta hai.
• Authoritative server — organization ka apna DNS server, jisme uske hosts ke actual records hain. Final answer yahin se aata hai.
• Local DNS server (resolver) — ISP ka server (ya Google 8.8.8.8, Cloudflare 1.1.1.1). Cache aur proxy dono ka kaam karta hai.

5.3 Iterative vs Recursive query ★★★ asked a lot

Host → Local DNS: "IP bata do"
Local → Root:    "IP bata do"
Root  → TLD:     "IP bata do"
TLD   → Auth:    "IP bata do"
       ← ← ← ← answer wapas chain se

Host  → Local DNS: "IP bata do" (recursive)
Local → Root: → "mujhe nahi pata,
                 TLD se poochho"
Local → TLD:  → "auth server se poochho"
Local → Auth: → "ye rahi IP"
Local → Host: "ye rahi IP"

5.4 DNS caching

• Koi bhi DNS server jo mapping seekhta hai, use cache kar leta hai.
• Har record ke saath TTL (Time To Live) hoti hai — utni der baad entry delete ho jaati hai.
• Isi wajah se root servers pe traffic bahut kam hai — TLD mappings local servers mein cached rehti hain.
• Downside: agar koi host apni IP badal le, to purani cached entry TTL expire hone tak galat rahegi (isliye migration se pehle TTL kam kar dete hain).
• Caching layers: browser cache → OS cache → local resolver cache.

5.5 DNS records (RR = Resource Record)

Format: (Name, Value, Type, TTL)

5.6 DNS message + transport

• Query aur reply ka same format hota hai: Header (identification, flags) + Question + Answer + Authority + Additional sections.
• UDP port 53 — kyun UDP? Kam overhead, no handshake (fast), query chhoti hoti hai, aur retry app khud kar leti hai. Speed > reliability yahan.
• TCP kab? Jab response 512 bytes se bada ho (ab EDNS0 se 4096 tak UDP chal jaata hai), aur zone transfers ke liye (secondary server master se pura zone copy karta hai).

5.7 Domain register karna

Registrar (GoDaddy, Namecheap) ke through domain lo → usko apne authoritative server ke NS records aur unke A records (glue records) dene padte hain → registrar unhe .com TLD server mein daal deta hai. Ab duniya tumhare server tak pahunch sakti hai.

5.8 DNS attacks (bonus, security round mein)

• DNS spoofing / cache poisoning — attacker fake response bhej deta hai jo resolver cache kar leta hai, phir users galat IP pe redirect ho jaate hain. Fix: DNSSEC (records ko digitally sign karna).
• DDoS on root/TLD — hua hai, par caching aur anycast ki wajah se zyada asar nahi hua.
• DNS amplification — chhoti spoofed query se badi response victim pe bhejwana.
• DoH / DoT — DNS over HTTPS/TLS, privacy ke liye query ko encrypt karna.

6.1 Email ke 3 components

User agents (Outlook, Gmail UI) + Mail servers (mailbox + outgoing message queue) + SMTP (servers ke beech ka protocol).

 Alice ────SMTP───► Alice ka mail ────SMTP───► Bob ka mail ───POP3/IMAP──► Bob
 (user agent)        server                     server                    (user agent)
                  PUSH protocol                              PULL protocol
    

SMTP (Simple Mail Transfer Protocol) ★★ common

• TCP port 25 (submission ke liye 587, SMTPS 465), reliable transfer.
• Push protocol — sender apna message dhakelta hai (HTTP pull hai — client maangta hai).
• 3 phases: handshaking → transfer of messages → closure.
• Commands: HELO/EHLO, MAIL FROM, RCPT TO, DATA, QUIT. Response codes numeric (250 OK, 354 start mail input, 221 bye).
• Historically 7-bit ASCII body; binary attachments ke liye MIME encoding (base64) chahiye.
• Message ka end: single line with just .
• Ek hi TCP connection pe multiple messages bhej sakta hai (persistent).

Mail access protocols: POP3 vs IMAP ★★ common

Note: Sending hamesha SMTP se hi hota hai, chahe tum Gmail web UI use karo (browser→Gmail server HTTP, Gmail server→receiver SMTP).

6.2 FTP — File Transfer Protocol

• TCP, do connections: control connection port 21 (commands, "out-of-band") aur data connection port 20 (actual file).
• Stateful — server current directory aur authentication state yaad rakhta hai (HTTP stateless hai — ye difference poochha jaata hai).
• Control connection puri session tak khuli rehti hai; har file transfer ke liye nayi data connection banti hai.
• Active mode: server client se data connection banata hai (client ke firewall/NAT ke liye problem). Passive mode: client hi data connection banata hai — isliye aaj passive default hai.
• Commands: USER, PASS, LIST, RETR, STOR. Replies: 331 username OK, 425 can't open data connection, 452 error writing file.
• Plaintext hai → aaj SFTP (SSH-based, port 22) ya FTPS use karna chahiye.

6.3 P2P file distribution ★★ common

Sawaal: ek file (size F) ko 1 server se N peers tak pahunchane mein kitna time?

BitTorrent

• File chunks (256 KB typical) mein toot jaati hai. Peers ka group = torrent; tracker peers ka register rakhta hai.
• Naya peer tracker se peer list leta hai, kuch se TCP connection banata hai (neighbours).
• Rarest first — jo chunk swarm mein sabse kam hai, wo pehle maango (taaki file swarm mein zinda rahe aur distribution balanced ho).
• Tit-for-tat (choking): jo top-4 peers tumhe sabse tez de rahe hain, unhi ko tum bhejo ("unchoked"). Har 10 sec re-evaluate.
• Optimistic unchoking: har 30 sec ek random naye peer ko bhi mauka do — isse naye peers shuru kar paate hain aur better partners mil sakte hain.
• Free-riding (sirf download karna) is tarah discourage hota hai.
• Seeder = poori file rakhne wala, Leecher = abhi download kar raha.

DHT aur Chord (short)

Problem: kis peer ke paas kaunsi file hai, ye kaise dhoondein bina central server ke? Solution: Distributed Hash Table — (key, value) pairs peers ke beech baant do. Har peer aur har key ko hash karke ek m-bit ID do; key ko us peer pe rakho jiska ID key ke successor (closest ID ≥ key) hai.

• Circular DHT: har peer sirf successor jaanta hai → lookup O(N) messages. Simple par slow.
• Chord with finger table: har node m entries rakhta hai (2⁰, 2¹, … 2^m−1 aage wale successors) → lookup O(log N).
• Peer churn handle karne ke liye har peer apne successor ke saath-saath doosre successor ko bhi track karta hai.

7.1 Transport layer ka kaam

7.2 Multiplexing aur Demultiplexing ★★★ asked a lot

Multiplexing (sender pe): alag-alag sockets se data lo, har ek pe header (port numbers) lagao, aur network layer ko pass karo.
Demultiplexing (receiver pe): aaye hue segment ko dekh ke sahi socket tak pahunchao.

(A:26145, S:80)   ← client A, connection 1
(A:7532,  S:80)   ← client A, connection 2   (source port alag)
(B:26145, S:80)   ← client B, connection 1   (source IP alag — port same ho to bhi chalega)
(B:9157,  S:80)   ← client B, connection 2

7.3 UDP — User Datagram Protocol ★★★ asked a lot

"No frills, bare bones" transport. Best-effort service — segment lost ho sakta hai, out-of-order aa sakta hai.

 UDP header — sirf 8 bytes:
 ┌───────────────────────┬───────────────────────┐
 │  Source Port (16)     │  Dest Port (16)       │
 ├───────────────────────┼───────────────────────┤
 │  Length (16)          │  Checksum (16)        │   Length = header + data, bytes mein
 ├───────────────────────┴───────────────────────┤
 │                Application data                │
 └───────────────────────────────────────────────┘
    

UDP kahan use hota hai: DNS, DHCP, SNMP, RIP, TFTP, VoIP/video streaming, online gaming, aur QUIC (HTTP/3).

UDP checksum ★★ common

1. Segment ke contents ko 16-bit integers ki sequence maano.
2. Sab ko jodo — 1's complement addition (carry ko wapas LSB pe add karo, "wrap around").
3. Result ka 1's complement (sab bits invert) le lo — wahi checksum hai.
4. Receiver saare 16-bit words + checksum ko jodta hai. Agar answer 1111111111111111 aaye → no error detected. Warna error.

  1110011001100110
+ 1101010101010101
──────────────────
 11011101110111011      ← 17 bits, carry nikal gaya

 wraparound: 1011101110111011 + 1 = 1011101110111100   ← sum

 checksum = 1's complement of sum = 0100010001000011

7.4 TCP vs UDP — table ratt lo ★★★ asked a lot

Follow-up jo aata hai: "Video streaming TCP use karta hai ya UDP?" → Live/real-time (video call, gaming) UDP; on-demand streaming (YouTube, Netflix) actually TCP/HTTP (DASH) use karta hai kyunki buffering se latency chal jaati hai aur firewalls TCP ko allow karte hain.

7.5 Socket programming ka essence

Server:
socket() → bind() → listen()
        → accept()  ← blocks
        → recv()/send() → close()

Client:
socket() → connect()
        → send()/recv() → close()

Server:
socket() → bind()
        → recvfrom() / sendto()
        → close()

Client:
socket()
        → sendto() / recvfrom()
        → close()

• accept() ek naya connection socket return karta hai — listening socket alag rehta hai. Isliye server multiple clients handle kar sakta hai (thread/process per connection ya epoll).
• UDP mein har datagram ke saath destination address dena padta hai (sendto), kyunki connection hi nahi hai.
• Byte order: network big-endian (Network Byte Order) use karta hai. Isliye htons(), htonl(), ntohs(), ntohl() se convert karna padta hai — warna x86 (little-endian) pe port number ulta chala jayega.

8.1 Problem statement

Neeche wala channel unreliable hai (bits flip ho sakte hain, packets lost ho sakte hain). Upar wali app ko reliable channel chahiye. Beech mein reliable data transfer (rdt) protocol banate hain — step by step.

8.2 Stop-and-Wait ki performance problem ★★★ asked a lot

8.3 Pipelining: Go-Back-N aur Selective Repeat ★★★ asked a lot

Idea: ACK ka wait kiye bina ek saath N packets "in-flight" bhejo. Iske liye chahiye: (1) bada sequence number space, (2) sender/receiver pe buffering, (3) window management.

9.1 TCP ke characteristics (ek saans mein bolna aana chahiye)

"Byte stream" ka matlab: TCP message boundaries preserve nahi karta. Agar tum 3 baar send() karo 100-100 bytes, receiver ko ek hi recv() mein 300 bytes mil sakte hain, ya 5 tukdon mein. Isliye application ko khud message framing karni padti hai (length prefix ya delimiter). Ye interview mein bahut poochha jaata hai.

9.2 TCP segment structure ★★★ asked a lot

  0                   8                  16                  24                31
 ┌───────────────────────────────────────┬───────────────────────────────────────┐
 │         Source port (16)              │        Destination port (16)          │
 ├───────────────────────────────────────┴───────────────────────────────────────┤
 │                        Sequence number (32)                                  │
 ├───────────────────────────────────────────────────────────────────────────────┤
 │                     Acknowledgement number (32)                              │
 ├──────┬──────────┬─────────────────────┬───────────────────────────────────────┤
 │Hdr   │ unused   │ U A P R S F         │       Receive window (16)              │
 │len(4)│          │ R C S S Y I         │       (flow control ke liye)          │
 │      │          │ G K H T N N         │                                       │
 ├──────┴──────────┴─────────────────────┼───────────────────────────────────────┤
 │        Checksum (16)                   │      Urgent data pointer (16)         │
 ├───────────────────────────────────────┴───────────────────────────────────────┤
 │                  Options (variable: MSS, window scale, SACK, timestamp)       │
 ├───────────────────────────────────────────────────────────────────────────────┤
 │                          Application data                                    │
 └───────────────────────────────────────────────────────────────────────────────┘
    

9.3 Sequence aur ACK numbers ★★★ asked a lot

 Telnet example — A "C" character type karta hai (echo back hota hai)

 Host A                                        Host B
   │   Seq=42, ACK=79, data='C'                  │
   │ ───────────────────────────────────────────►│   B ko 1 byte mila (42)
   │                                             │   ab B ko 43 chahiye
   │   Seq=79, ACK=43, data='C' (echo)          │
   │◄─────────────────────────────────────────── │
   │   Seq=43, ACK=80                           │
   │ ───────────────────────────────────────────►│
    

9.4 Three-way handshake ★★★ asked a lot

 Client                                                Server
   │                                        (LISTEN)     │
   │  1. SYN=1, Seq=x                                    │
   │ ───────────────────────────────────────────────────►│
   │                                    (SYN_RCVD)       │
   │  2. SYN=1, ACK=1, Seq=y, Ack=x+1                  │
   │◄─────────────────────────────────────────────────── │
   │ (ESTABLISHED)                                       │
   │  3. ACK=1, Seq=x+1, Ack=y+1   (data ja sakta hai)  │
   │ ───────────────────────────────────────────────────►│
   │                                    (ESTABLISHED)    │
    

9.5 Connection termination — 4-way handshake

 Client                                                Server
   │  1. FIN, Seq=x                                     │
   │ ───────────────────────────────────────────────────►│  (CLOSE_WAIT)
   │  2. ACK, Ack=x+1                                    │
   │◄─────────────────────────────────────────────────── │  server abhi bhi
   │  (FIN_WAIT_2)          [server bacha data bhej sakta hai — half close]
   │  3. FIN, Seq=y                                     │  (LAST_ACK)
   │◄─────────────────────────────────────────────────── │
   │  4. ACK, Ack=y+1                                    │
   │ ───────────────────────────────────────────────────►│  (CLOSED)
   │  (TIME_WAIT — 2×MSL wait, phir CLOSED)              │
    

9.6 TCP timeout aur RTT estimation ★★ common

Timeout kitna rakhein? RTT se bada hona chahiye — par RTT to badalta rehta hai. Bahut chhota → faltu retransmission. Bahut bada → loss recover karne mein der.

• Ye EWMA (exponentially weighted moving average) hai — purane samples ka weight exponentially girta hai.
• Karn's algorithm: retransmitted segment ka SampleRTT mat lo (pata nahi chalega ki ACK original ka tha ya retransmission ka). Aur har timeout pe timeout value double kar do (exponential backoff).
• DevRTT "safety margin" hai — RTT jitna zyada fluctuate karega, margin utna bada.

9.7 Fast retransmit

Timeout ka wait lamba hota hai. Lekin agar sender ko 3 duplicate ACKs mil jaayein (yaani 4 baar same ACK number), to iska matlab uske baad ke segments to pahunch rahe hain, sirf ek beech wala gaayab hai. To timeout ka intezaar mat karo — turant retransmit karo.

 Sender bhejta hai: seg1(Seq=100) seg2(200) seg3(300) seg4(400) seg5(500)
 seg2 LOST.
 Receiver: seg1 aaya → ACK 200
           seg3 aaya (out of order) → ACK 200   ← dup 1
           seg4 aaya → ACK 200                  ← dup 2
           seg5 aaya → ACK 200                  ← dup 3
 Sender: 3 dup ACKs mile → turant seg2 retransmit (timeout se pehle)
    

9.8 TCP: GBN hai ya Selective Repeat? ★★ common

9.9 Flow control ★★★ asked a lot

• Receiver har ACK ke saath apna current rwnd header mein bhejta hai.
• Zero window problem: agar rwnd = 0 ho jaye to sender ruk jaata hai — par phir receiver ko naya rwnd batane ka mauka kaise mile (kyunki ACK data ke saath jaata hai)? Solution: sender chhote window probe segments (1 byte) bhejta rehta hai jab tak window khule nahi.

9.10 Useful TCP options

• MSS (Maximum Segment Size) — ek segment mein max application data. Typically 1460 bytes (1500 MTU − 20 IP − 20 TCP). SYN mein negotiate hota hai.
• Window scaling — 16-bit window field max 65,535 bytes deta hai, jo high-BDP links ke liye kam hai. Scale factor se window 1 GB tak ja sakta hai.
• SACK — receiver exactly bata deta hai kaunse byte-ranges mile.
• Timestamps — better RTT measurement + PAWS (wrapped sequence numbers se bachav).
• Nagle's algorithm — chhote-chhote packets ko jama karke bhejta hai (network efficiency), par latency badhati hai → interactive apps TCP_NODELAY set karti hain.
• Delayed ACK — receiver 500 ms tak (ya 2nd segment tak) ACK rokta hai taaki piggyback ho sake.

9.11 SYN flood attack (security round mein)

Attacker bahut saare SYN bhejta hai par kabhi teesra ACK nahi bhejta. Server har SYN pe resources allocate karke half-open connection table bhar leta hai → legit users ko jagah nahi milti. Fix: SYN cookies — server state allocate hi nahi karta; wo ISN mein ek cryptographic hash encode kar deta hai, aur jab client ka ACK aata hai tab hash verify karke connection banata hai.

10.1 Congestion kya hai

Congestion ki "costs" (Kurose ke 3 scenarios ka nichod)

1. Scenario 1 (infinite buffer, 1 router): jaise-jaise sending rate capacity R/2 ke paas jaata hai, delay infinite hone lagta hai. Throughput to R/2 tak hi ja sakta hai.
2. Scenario 2 (finite buffer): loss hone lagta hai → sender ko retransmit karna padta hai → "goodput" (useful throughput) sending rate se kam ho jaata hai. Premature timeout se unneeded duplicates bhi bante hain, jo aur capacity khaate hain.
3. Scenario 3 (multi-hop): jab ek packet upstream links par bandwidth kha ke aage jaake drop ho jaata hai, to us packet pe kharch ki gayi saari upstream capacity waste ho gayi. Isliye congestion collapse itna bura hota hai.

10.2 Congestion control ke do approach

10.3 TCP congestion control ka core ★★★ asked a lot

cwnd (congestion window) sender khud maintain karta hai. Do phases hain:

10.4 Loss detect hone pe kya hota hai — Tahoe vs Reno ★★★ asked a lot

TCP NewReno partial ACKs ko better handle karta hai (ek window mein multiple losses). TCP SACK selective ACK use karta hai. Aaj Linux ka default CUBIC hai.

 cwnd
  │        Tahoe: dup ACK pe bhi 1 tak gir jaata hai
 24│      ╱|
 20│    ╱  |         Reno: dup ACK pe aadha (fast recovery)
 16│  ╱    |       ╱‾‾‾╲          ╱‾‾╲
 12│╱      |     ╱      ╲___    ╱     ╲___
  8│       |   ╱                         ← "sawtooth" pattern
  4│       | ╱
  1│_______|╱________________________________
   └───────────────────────────────────────── time (RTT)
    slow start   CA    loss   CA     loss
    

10.5 AIMD — Additive Increase, Multiplicative Decrease

10.6 TCP throughput

10.7 TCP fairness

• Goal: agar K TCP connections ek bottleneck link R share kar rahi hain, to sabko ~R/K milna chahiye. AIMD isko approximately achieve karta hai.
• RTT unfairness: chhote RTT wale connections zyada le jaate hain.
• Parallel connections se cheating: agar ek app 9 parallel TCP connections khole aur doosri 1, to 9 wali ko 9/10 bandwidth mil jayegi. Browsers yahi karte the!
• UDP fair nahi hai — wo congestion control follow hi nahi karta, isliye TCP flows ko dabaa sakta hai.

10.8 Modern congestion control (bonus)

10.9 QUIC — HTTP/3 ka transport ★★ common

• UDP ke upar banaya gaya application-layer protocol (kernel change nahi karna padta, browser update se deploy ho jaata hai).
• Reliability, congestion control, flow control — sab QUIC khud implement karta hai (TCP jaisa hi logic).
• TLS 1.3 built-in — connection setup + security ek hi handshake mein → 1-RTT, aur repeat connection pe 0-RTT. TCP+TLS mein ye 3 RTT tak lagta tha.
• Per-stream reliability — ek stream ka packet loss doosri streams ko block nahi karta → TCP HOL blocking solved.
• Connection ID — IP badal jaye (WiFi → mobile data) to bhi connection zinda rehti hai (connection migration).

11.1 Network layer ke do plane ★★ common

Internet ka service model = "best effort": no guarantee on delivery, ordering, timing, ya bandwidth. Simplicity ki wajah se hi Internet itna scale kar paya — saari complexity edge (hosts) pe daali gayi.

11.2 IPv4 datagram format ★★★ asked a lot

  0        4         8              16                            31
 ┌────────┬─────────┬──────────────┬──────────────────────────────┐
 │Version │ IHL     │ Type of Svc  │      Total Length (16 bits)     │  ← bytes mein, max 65535
 │ (4)    │ (4)     │ (8) / DSCP   │                              │
 ├────────┴─────────┴──────────────┼──────┬───────────────────────┤
 │      Identification (16)          │Flags │  Fragment Offset (13) │  ← fragmentation
 │                                 │ (3)  │  (8-byte units mein)  │
 ├──────────────────┬──────────────┼──────┴───────────────────────┤
 │   TTL (8)        │ Protocol (8) │    Header Checksum (16)      │
 ├──────────────────┴──────────────┴──────────────────────────────┤
 │                    Source IP address (32)                      │
 ├────────────────────────────────────────────────────────────────┤
 │                  Destination IP address (32)                   │
 ├────────────────────────────────────────────────────────────────┤
 │                  Options (variable, 0–40 bytes)                │
 ├────────────────────────────────────────────────────────────────┤
 │                          Data (payload)                        │
 └────────────────────────────────────────────────────────────────┘
    

11.3 IP fragmentation ★★★ asked a lot

12.1 IP address ka basic

• 32-bit identifier, dotted-decimal mein likha jaata hai: 223.1.1.1 = 11011111 00000001 00000001 00000001
• IP address host ka nahi, interface ka hota hai. Router ke har interface ka apna IP hota hai (isliye router ke paas multiple IPs hote hain).
• Address ke do hisse: Network (prefix) + Host part.

12.2 Classful addressing (purana, par poochha jaata hai)

Classful kyun mar gaya: Class B (65k hosts) chhoti company ke liye bahut bada, Class C (254) bahut chhota. Beech ka koi option nahi tha → address space ki barbaadi. Isliye CIDR aaya.

12.3 CIDR aur subnet mask ★★★ asked a lot

Yaad rakhne ka trick: mask ke octet values hamesha inhi mein se hoti hain — 128, 192, 224, 240, 248, 252, 254, 255 (har baar aadha).

12.4 Special addresses

12.5 Subnetting — step by step

12.6 Longest Prefix Matching ★★★ asked a lot

Router ki forwarding table mein prefixes hote hain. Agar destination IP ek se zyada entries se match kare, to router sabse lamba (most specific) prefix chunta hai.

12.7 Route aggregation (supernetting)

ISP apne saare customers ke prefixes ko ek bade prefix mein "jodh" ke advertise karta hai — isse global routing tables chhoti rehti hain.

4 = 00000100
5 = 00000101
6 = 00000110
7 = 00000111
     ↑ common 6 bits

13.1 DHCP — IP address milta kaise hai ★★★ asked a lot

Do sawaal alag hain: (1) network ko block kaise milta hai → ISP se, jo aage ICANN/RIR se leta hai. (2) host ko address kaise milta hai → manually (static) ya DHCP se ("plug-and-play protocol").

DHCP sirf IP address nahi deta — ye bhi deta hai: subnet mask, default gateway (first-hop router) ka IP, aur local DNS server ka IP. Ye 4 cheezein interview mein poochhi jaati hain.

• UDP pe chalta hai, port 67 (server) / 68 (client).
• Broadcast kyun? Kyunki client ke paas abhi apna IP hi nahi hai aur server ka pata bhi nahi.
• Lease hoti hai — time khatam hone se pehle renew karna padta hai, warna address wapas pool mein.
• Agar DHCP server doosre subnet mein ho to DHCP relay agent router pe chahiye.

13.2 NAT — Network Address Translation ★★★ asked a lot

 Local network (private)                    NAT router                 Internet
 ┌─────────────────────┐            ┌────────────────────┐
 │ 10.0.0.1:3345 ──────┼───────────►│ WAN IP 138.76.29.7 │──────► 128.119.40.186:80
 │                     │            │ port 5001           │
 │ 10.0.0.2:3345 ──────┼───────────►│ port 5002           │──────►
 └─────────────────────┘            └────────────────────┘

 NAT translation table
 ┌──────────────────────────┬──────────────────────────┐
 │ WAN side (bahar)         │ LAN side (andar)         │
 ├──────────────────────────┼──────────────────────────┤
 │ 138.76.29.7 : 5001       │ 10.0.0.1 : 3345          │
 │ 138.76.29.7 : 5002       │ 10.0.0.2 : 3345          │
 └──────────────────────────┴──────────────────────────┘
    

1. Outgoing packet: source (10.0.0.1, 3345) → router use (138.76.29.7, 5001) se replace karta hai aur table mein entry banata hai.
2. Reply aata hai destination (138.76.29.7, 5001) pe.
3. Router table dekhta hai → destination ko (10.0.0.1, 3345) mein badal ke andar bhej deta hai.

NAT traversal ke solutions: (1) Static port forwarding — router pe manually configure. (2) UPnP / IGD — app khud router se mapping maang leti hai. (3) Relaying (TURN) — beech mein ek public server jisse dono connect karte hain (Skype yahi karta tha). (4) STUN + hole punching — dono side apna mapping seekh ke direct connect karte hain.

13.3 ICMP — network ka "error report" protocol ★★ common

• Hosts aur routers network-level info exchange karne ke liye use karte hain — error reporting aur diagnostics.
• ICMP messages IP datagram ke payload mein jaate hain (Protocol number = 1). Isliye ye "IP ke upar" hai, par network layer ka hi hissa maana jaata hai (kyunki ye IP ki service ka part hai, application ka nahi).
• Har message mein Type + Code hote hain + jis packet se error hua uske header ke pehle 8 bytes.

13.4 IPv6 ★★ common

Motivation: 32-bit address space khatam ho raha tha. Saath hi header ko simplify karke routers ko fast banana tha.

IPv4 → IPv6 transition

• Dual stack — devices dono protocols chalate hain, jahan jo mile use karte hain.
• Tunneling — IPv6 datagram ko IPv4 datagram ke payload mein daal ke IPv4-only region ke paar bhejna. Doosri taraf tunnel endpoint use nikal leta hai.

 IPv6 host ─── IPv6 net ───[tunnel entry]═══ IPv4 network ═══[tunnel exit]─── IPv6 net ─── IPv6 host
                            IPv6 packet ko            IPv4 header ke andar        IPv6 packet
                            IPv4 me wrap kiya          IPv6 packet safar karta hai  wapas nikala
    

14.1 Router architecture

        ┌──────────────────────────────────────────────┐
        │  Routing processor (control plane, software)   │  ← routing protocols
        │  routing table banata hai                     │     chalata hai
        └──────────────────────┬───────────────────────┘
                 forwarding table copy ↓
   ┌──────────┐         ┌───────────────────┐         ┌──────────┐
   │ Input    │────────►│  Switching        │────────►│ Output   │
   │ ports    │         │  fabric            │         │ ports    │
   │ L1,L2,    │         │  (memory/bus/     │         │ queue +  │
   │ lookup +  │         │   crossbar)       │         │ L2,L1    │
   │ queue     │         └───────────────────┘         └──────────┘
   └──────────┘
       ← data plane — hardware mein, nanoseconds mein →
    

Input port ka kaam: physical layer termination → link layer processing (decapsulation) → lookup (destination IP se forwarding table mein longest prefix match, aksar TCAM hardware se) → queue → fabric.

14.2 Switching fabric ke 3 tareeke

14.3 Queuing kahan hoti hai ★★ common

14.4 Packet scheduling

14.5 AQM aur RED

Problem with drop-tail: buffer full hone tak wait karte ho, phir aane wale saare packets drop — isse (a) queue hamesha full rehti hai (delay zyada), (b) global synchronization — sab TCP flows ek saath backoff karte hain aur ek saath badhte hain, link utilization ghatti-badhti rehti hai.

Faayde: average queue length chhoti (kam delay), global synchronization se bachav, aur bursty traffic ko accommodate karta hai. Modern versions: CoDel, FQ-CoDel (bufferbloat ke liye).

15.1 Graph abstraction

Network = graph G = (N, E). Nodes = routers, edges = links, edge cost c(x,y) = link ka cost (delay, 1/bandwidth, ya monetary cost). Goal: har source-destination pair ke liye least cost path.

15.2 Link State routing — Dijkstra ★★★ asked a lot

• Har router apne links ka status poore network mein broadcast karta hai (link state broadcast/flooding).
• Sabke paas identical, complete topology map aa jaata hai.
• Har router khud Dijkstra chalake apna shortest path tree aur forwarding table banata hai.

          5
     u ────────── v
     │ \        / │ \
   1 │  \2    2/  │3  \1
     │   \    /   │    \
     w ─── x ──── y ──── z
        3      1      2

 Edges: u-v=5, u-w=1, u-x=2, v-x=2, v-y=3, v-z=1,
        w-x=3, x-y=1, y-z=2

15.3 Distance Vector — Bellman-Ford ★★★ asked a lot

Algorithm (har node par):

1. Apna distance vector D_x (har destination ke liye estimate) rakho.
2. Padosiyon ko apna DV bhejo.
3. Jab kisi padosi ka DV aaye, Bellman-Ford se apna DV update karo.
4. Agar apna DV badla → padosiyon ko phir bhejo. Warna chup raho.

Properties: iterative, asynchronous (sab ko step-lock mein chalne ki zaroorat nahi), self-terminating (jab kuch nahi badalta, messages ruk jaate hain), aur distributed.

D_x = [x:0, y:2, z:7]
D_y = [x:2, y:0, z:1]
D_z = [x:7, y:1, z:0]

15.4 Count-to-infinity problem ★★★ asked a lot

  X ──1── Y ──1── Z

 Normal: D_Y(X)=1, D_Z(X)=2 (via Y)

 Ab X–Y link toot gaya:
 Y sochta hai: "X ko direct nahi ja sakta. Par Z bolta hai uska X tak cost 2 hai!"
   → D_Y(X) = 1 (Y→Z ka cost) + 2 = 3   ← Y ko nahi pata ki Z ka raasta khud Y se hoke jaata hai
 Z ko Y ka naya DV milta hai: D_Z(X) = 1 + 3 = 4
 Y phir: 1 + 4 = 5 … Z: 6 … Y: 7 …
 
 Ye tab tak chalta hai jab tak cost "infinity" (RIP mein 16) na pahunch jaye.

15.5 LS vs DV — comparison table ★★ common

16.1 Autonomous System (AS) — routing ko do tier mein kyun toda ★★ common

Internet ke crores routers pe ek hi flat routing algorithm nahi chal sakta. Do reason:

• Scale — 600 million+ destinations ki table aur uske liye link-state broadcast — impossible.
• Administrative autonomy — har organization apna network apne hisaab se chalana chahti hai, aur apni policy chhupana chahti hai.

Isliye routers ko Autonomous Systems (AS) mein baanta gaya (har AS ka apna ASN number). Do level:

16.2 RIP (Routing Information Protocol)

• Distance vector, metric = hop count (har link cost 1), max 15 hops (16 = infinity).
• Har 30 sec mein padosiyon ko poori routing table bhejta hai (RIP advertisement, max 25 destination entries).
• 180 sec tak koi advertisement na aaye → neighbour dead maano, routes invalidate karo, padosiyon ko batao.
• UDP port 520 pe chalta hai, application-level process (routed daemon) ke roop mein.
• Weaknesses: chhote networks tak hi (15 hops), hop count bandwidth ignore karta hai (1 Gbps aur 1 Mbps dono cost 1!), slow convergence, count-to-infinity.

16.3 OSPF (Open Shortest Path First) ★★ common

• Link state + Dijkstra. "Open" = publicly available spec.
• Har router poore AS mein flooding se link state advertisements (LSA) bhejta hai — seedha IP ke upar (protocol 89), TCP/UDP nahi.
• Advanced features: authentication (fake LSA se bachav), multiple same-cost paths (ECMP load balancing), ToS-based routing (alag traffic ke liye alag cost), integrated uni/multicast.
• Hierarchical OSPF — bade AS ke liye do level: local areas + backbone (area 0). Area ke andar detail flood hoti hai, bahar sirf summary. Area border routers areas ko jodte hain; boundary routers doosre AS se.
• Messages: HELLO (neighbour discovery/keepalive), Database Description, Link State Request/Update/Ack.

16.4 BGP — Border Gateway Protocol ★★★ asked a lot

Do flavours:

• eBGP — doosre AS ke gateway router se prefix reachability info lena/dena.
• iBGP — apne AS ke andar baaki routers tak wo info propagate karna.

BGP kya karta hai (3 kaam): (1) padosi AS se prefix reachability seekhna, (2) apne AS ke andar propagate karna, (3) policy ke hisaab se best route chunna.

Path attributes

BGP route selection order ★★ common

1. Highest LOCAL PREF (policy decision — sabse important)
2. Shortest AS-PATH
3. Closest NEXT-HOP router — yehi hot potato routing hai
4. Additional criteria (lowest MED, eBGP over iBGP, lowest router ID…)

BGP messages: OPEN (session start), UPDATE (naya route ya withdrawal), KEEPALIVE, NOTIFICATION (error/close).

BGP ki weakness: trust-based hai — koi galat prefix advertise kar de to traffic hijack ho sakta hai (BGP hijacking, jaise "Pakistan Telecom ne YouTube hijack kiya tha" wala famous incident). Fix attempts: RPKI, BGPsec.

16.5 SDN — Software Defined Networking

Faayde: centralized programmability, easier traffic engineering, vendor-neutral, network apps (load balancer, firewall) software mein likhi ja sakti hain. Google apne data centers ke beech (B4) SDN use karta hai.

Match-plus-action: OpenFlow generalized forwarding — sirf destination IP nahi, header ke kisi bhi field (MAC, IP, TCP port, VLAN) pe match karke action (forward, drop, modify, send to controller) le sakte ho. Isse ek hi device router, switch, firewall, NAT — sab ban sakta hai.

17.1 Link layer kya karta hai

Datagram ko ek node se physically judne wale agle node tak pahunchana (ek hop). Har hop pe alag link protocol ho sakta hai (WiFi pe pehla hop, Ethernet pe doosra, fiber pe teesra).

Implementation: link layer NIC (Network Interface Card) mein hoti hai — hardware + firmware. Sending side: datagram encapsulate, error bits add. Receiving side: error check, datagram nikaal ke upar do.

17.2 Error detection ka basic idea

 Sender:   [ Data D ] + [ EDC ] ────────► noisy channel ────────► Receiver
                                                     ↓
                                          D' aur EDC' se check karo:
                                          error hai ya nahi?
    

17.3 Parity checking

• Single bit parity: d data bits ke saath 1 parity bit. Even parity mein total 1s ki sankhya even honi chahiye. Sirf odd number of bit errors pakad sakta hai — 2 bits flip ho jayein to fail.
• Two-dimensional (2D) parity: data ko rows aur columns mein arrange karo, har row aur har column ka parity bit. Ye single-bit error ko detect AND correct kar sakta hai (row aur column dono se position pata chal jaata hai). 2-bit errors detect kar leta hai.

       c1 c2 c3 c4 | row parity
 r1     1  0  1  0 |  0   (do 1s → even → 0)
 r2     1  1  0  1 |  1   (teen 1s → odd → 1 lagao)
 r3     0  1  1  0 |  0
 r4     1  0  0  1 |  0
 ─────────────────────
 col    1  0  0  0 |  1
 par

17.4 Hamming code (error correction) ★★ common

Position:  1    2    3    4    5    6    7
Bit:      P1   P2   d1   P4   d2   d3   d4
Data 1011 → d1=1, d2=0, d3=1, d4=1
Position:  P1   P2    1   P4    0    1    1

17.5 CRC — Cyclic Redundancy Check ★★★ asked a lot

Parity se kahin zyada powerful. Ethernet, WiFi, ATM — sab CRC use karte hain (hardware mein bahut fast).

          1 0 1 0 1 1        ← quotient (isse matlab nahi)
      ┌──────────────
1001  │ 1 0 1 1 1 0 0 0 0
        1 0 0 1                 ← XOR
        ───────
        0 0 1 0 1
            1 0 1 1 0
            1 0 0 1
            ───────
            0 0 1 0 0
                1 0 0 0
                1 0 0 1
                ───────
                0 0 0 1 0
                    1 0 0
                    ← 3 bits bache: 011

18.1 Problem: ek shared channel, bahut saare nodes

Do ya zyada nodes ek saath bhejein to collision ho jaati hai — signals mix, dono frames barbaad. Chahiye ek Multiple Access Protocol jo decide kare ki kaun kab bolega — aur ye decision usi shared channel pe hi karna hai (koi alag coordination channel nahi).

18.2 Teen categories

TDMA: har node ko har round mein fixed time slot. FDMA: har node ko alag frequency band. CDMA: sab ek saath bolte hain par alag orthogonal codes se — receiver code se apna signal nikaal leta hai (cellular mein).

Polling: master node baari-baari poochhta hai "kuch bhejna hai?" — polling delay + master single point of failure. Token passing: ek token frame ghoomta rehta hai, jiske paas token wo bhej sakta hai — token lost ho jaye to problem (Token Ring, FDDI).

18.3 Pure ALOHA vs Slotted ALOHA ★★ common

S = throughput (successful frames per frame-time), G = offered load (attempted transmissions per frame-time). Slotted, pure se exactly double efficient hai — kyunki vulnerable period aadha ho gaya.

18.4 CSMA — Carrier Sense Multiple Access

Basic idea: "bolne se pehle suno" — channel busy hai to mat bhejo. Insaan bhi yahi karte hain.

Persistence methods

18.5 CSMA/CD — Collision Detection ★★★ asked a lot

Ethernet ka protocol: "listen before talk" + "listen while talking". Collision detect hote hi transmission abort kar do — kyunki aage bhejna waste hai. Isse channel waste kam hota hai.

Ethernet CSMA/CD algorithm:

1. NIC datagram le ke frame banata hai.
2. Channel idle hai → bhejna shuru. Busy hai → idle hone tak wait, phir bhejo (1-persistent).
3. Poora frame bina collision ke chala gaya → done.
4. Collision detect hui → abort karo aur 48-bit jam signal bhejo (taaki sabko pata chal jaye).
5. Binary exponential backoff: m-th collision ke baad {0, 1, 2, …, 2^m−1} mein se random K chuno aur K × 512 bit-times wait karo. 10 collisions ke baad max 1023 pe cap, 16 baar fail hone par frame drop.

18.6 CSMA/CA (WiFi) — collision avoidance

CSMA/CA ka solution: collision avoid karo — sense karo, DIFS ka wait, random backoff counter (busy hone pe freeze), phir bhejo. Receiver SIFS ke baad ACK bhejta hai (kyunki sender ko khud pata nahi chalega ki frame gaya ya nahi). Optionally RTS/CTS handshake — chhote RTS/CTS frames se channel reserve karo, isse hidden terminal problem kam hoti hai.

19.1 Ethernet — LAN ka raja

Sabse zyada use hone wali LAN technology. Sasta, simple, aur 10 Mbps se 400 Gbps tak evolve kar gaya. Bob Metcalfe ne 1970s mein banaya.

 Ethernet frame structure

 ┌──────────┬──────┬────────┬────────┬──────┬─────────────────┬──────┐
 │ Preamble │ Dest │ Source │  Type  │ Data (payload)        │ CRC  │
 │ 8 bytes  │ MAC  │  MAC   │2 bytes │ 46 – 1500 bytes       │4 byte│
 │          │6 byte│ 6 byte │        │                       │      │
 └──────────┴──────┴────────┴────────┴──────┴─────────────────┴──────┘

 Preamble: 7 bytes of 10101010 + 1 byte 10101011 — receiver clock sync ke liye
 Type: upar wala protocol (0x0800 = IPv4, 0x0806 = ARP, 0x86DD = IPv6)
 Min frame = 64 bytes (14 header + 46 data + 4 CRC), Max = 1518 bytes
    

Ethernet standards: 10BASE-T, 100BASE-TX (Fast Ethernet), 1000BASE-T (Gigabit), 10GBASE-… — naming: speed BASE media. Sabka frame format same hai, sirf physical layer aur speed alag.

19.2 MAC address ★★ common

• 48-bit (6 bytes), hex mein likha jaata hai: 1A-2F-BB-76-09-AD.
• NIC ke ROM mein burned hota hai (software se badla ja sakta hai). IEEE allocate karta hai — pehle 3 bytes = manufacturer (OUI).
• Flat address space — portable hai. LAN badal do, MAC wahi rahega. IP hierarchical hai — network badla to IP badalna padega.
• Broadcast MAC = FF-FF-FF-FF-FF-FF (sab NICs receive karti hain).
• First byte ka LSB: 0 = unicast, 1 = multicast.

19.3 Hub vs Bridge vs Switch vs Router ★★★ asked a lot

19.4 Switch self-learning ★★★ asked a lot

19.5 Switch vs Router — ek line ka jawab

19.6 VLAN — Virtual LAN ★★ common

Problem: ek badi switched LAN mein (a) sab ek hi broadcast domain hain — ek ARP broadcast poore network mein jaata hai, (b) security/isolation nahi (HR aur Engineering ek hi LAN pe), (c) koi banda floor badle to physically cable badalni padti.

Solution: ek hi physical switch ko multiple logical LANs mein baant do. Har VLAN apna alag broadcast domain hai.

• Port-based VLAN: switch ke ports ko VLAN ID assign karte hain (ports 1–8 = VLAN 10 "EE", ports 9–16 = VLAN 20 "CS").
• Inter-VLAN routing: VLAN 10 se VLAN 20 baat karne ke liye router (ya L3 switch) chahiye — kyunki alag broadcast domain matlab alag subnet. ("Router on a stick" configuration.)
• Trunk port + 802.1Q: ek hi physical link se multiple VLANs ka traffic bhejne ke liye frame mein 4-byte VLAN tag lagta hai (VLAN ID 12 bits → 4096 VLANs). Isse Ethernet frame ka max size 1518 → 1522 bytes ho jaata hai.

19.7 MPLS aur virtual circuits (bonus)

Virtual Circuit (VC): datagram network ka ulta — connection setup hota hai, har packet mein VC number hota hai (poora destination address nahi), aur har router mein per-VC state hoti hai. Har hop pe VC number badal sakta hai (label swapping). ATM, Frame Relay, X.25 ne use kiya.

MPLS (Multiprotocol Label Switching): IP aur link layer ke beech ek "2.5 layer". Packet ke aage 32-bit MPLS header lagta hai jisme 20-bit label hota hai. Router (LSR) sirf label dekhta hai — poora IP longest-prefix-match nahi karta → fast forwarding.

• Faayda: traffic engineering — tum path choose kar sakte ho (IP mein sirf shortest path milta hai). QoS, VPNs, aur fast reroute.
• MPLS routers IP-only routers ke saath interoperate kar sakte hain (backward compatible).
• Signaling: OSPF/IS-IS ko extend karke labels distribute kiye jaate hain (RSVP-TE, LDP).

20.1 ARP kya karta hai ★★★ asked a lot

ARP table har host/router ke paas hoti hai: <IP address, MAC address, TTL>. TTL typically 20 minutes.

1. A ko B ka MAC nahi pata → A ek ARP query broadcast karta hai (dest MAC = FF-FF-FF-FF-FF-FF): "Kiske paas IP 192.168.1.5 hai?"
2. LAN ke saare nodes wo frame receive karte hain.
3. Jiska IP match karta hai (B), wo unicast mein ARP reply bhejta hai apne MAC ke saath.
4. A apni ARP table mein entry cache kar leta hai (TTL tak).

20.2 Doosre subnet mein bhejna — poora walkthrough ★★★ asked a lot

Ye sequence samajh gaye to CN ka aadha syllabus samajh gaye. A (subnet 1) se B (subnet 2) ko datagram bhejna hai, beech mein router R hai.

   Subnet 1                    R                     Subnet 2
  ┌────────┐          ┌──────────────────┐          ┌────────┐
  │   A    │──────────│ R1        R2     │──────────│   B    │
  │IP: A   │          │IP: R1   IP: R2   │          │IP: B   │
  │MAC: A  │          │MAC: R1  MAC: R2  │          │MAC: B  │
  └────────┘          └──────────────────┘          └────────┘
    

1. A apna IP datagram banata hai: source IP = A, destination IP = B. (Ye poore safar mein nahi badlega.)
2. A dekhta hai (apne subnet mask se) ki B doosre subnet mein hai → to frame default gateway (R1) ko bhejna hoga.
3. A ko R1 ka MAC chahiye → ARP (agar cache mein nahi hai). R1 apna MAC bhej deta hai.
4. A frame banata hai: src MAC = A, dest MAC = R1, payload mein IP datagram (A → B).
5. R frame receive karta hai, IP datagram nikalta hai, routing table dekh ke pata karta hai ki B interface R2 se jayega. TTL−1, checksum recompute.
6. R ko B ka MAC chahiye → ARP subnet 2 pe. B apna MAC deta hai.
7. R naya frame banata hai: src MAC = R2, dest MAC = B, wahi IP datagram andar.
8. B frame receive karta hai, datagram nikaal ke upar transport layer ko de deta hai.

20.3 ARP spoofing (security)

ARP mein koi authentication nahi hai — jo bhi reply bhej de, host maan leta hai. Attacker fake ARP reply bhej ke keh sakta hai "gateway ka MAC main hoon" → poora traffic uske through jayega (Man-in-the-Middle). Isse ARP cache poisoning kehte hain.

Bachav: static ARP entries (critical hosts ke liye), Dynamic ARP Inspection (DAI) switches pe, port security, aur upar TLS use karna (taaki MITM data padh na sake).

21.1 WiFi (802.11) — jitna chahiye

• Infrastructure mode: devices ek Access Point (AP) se jude, AP wired network se juda. Ad-hoc mode: devices seedha aapas mein.
• Association: host beacon frames sunta hai (SSID + AP MAC), ek AP chunta hai, association request bhejta hai, phir typically DHCP se IP leta hai.
• Passive scanning = AP ke beacons ka wait; Active scanning = probe request broadcast karna.
• Bands: 2.4 GHz (zyada range, kam speed, crowded — 11 channels jinme sirf 1, 6, 11 non-overlapping) aur 5 GHz (zyada speed, kam range). 6 GHz WiFi 6E mein.
• MAC: CSMA/CA (Section 18 dekho) + link-layer ACK, kyunki wireless mein error rate zyada hai.
• Security evolution: WEP (toota hua) → WPA → WPA2 (AES-CCMP) → WPA3 (SAE handshake).

21.2 Security ke 4 goals ★★ common

Symmetric vs Asymmetric encryption

Isiliye TLS hybrid hai: asymmetric se authenticate karke ek symmetric session key establish karo, phir saara data symmetric se. (Section 4.9 dekho.)

Digital signature: sender message ka hash apni private key se encrypt karta hai. Koi bhi uski public key se decrypt karke hash verify kar sakta hai → isse authentication + integrity + non-repudiation milta hai.

Certificate (X.509): ek CA (Certificate Authority) gawah banti hai ki "ye public key sach mein google.com ki hai", aur us binding pe apna digital signature laga deti hai. Browser mein root CAs pehle se installed hote hain — wahi trust chain ka anchor hai.

21.3 Common attacks (rapid list)

Firewall types: (1) Stateless packet filter — har packet ko header rules se allow/deny. (2) Stateful filter — connection state track karta hai (sirf established connections ke reply allow). (3) Application gateway/proxy — application data tak dekhta hai (jaise sirf kuch users ko Telnet allow).

VPN: public Internet ke upar ek encrypted tunnel (IPsec/WireGuard/OpenVPN) — remote employees ko aisa lagta hai jaise wo office LAN pe hon.

22.1 Chhota version (jo pehle bolna hai)

1. Browser URL parse karta hai; cache check karta hai.
2. DNS se google.com ka IP nikalta hai.
3. Us IP ke saath TCP 3-way handshake (port 443).
4. TLS handshake — certificate verify, session key.
5. HTTP GET request jaati hai, server response deta hai.
6. Browser HTML parse karta hai, aur objects (CSS/JS/images) ke liye aur requests bhejta hai.
7. Render — DOM + CSSOM → render tree → layout → paint.

22.2 Poora version (jab detail maangein)

Phase 0 — Network se judna (agar naya connect hua ho)

• DHCP DORA — laptop ko IP, subnet mask, default gateway, DNS server milta hai. Ye messages UDP (68→67) mein, IP broadcast (255.255.255.255) mein, aur Ethernet broadcast (FF:FF:FF:FF:FF:FF) frame mein jaate hain.
• ARP — default gateway ka MAC pata karna, kyunki har outgoing frame usi ko jayega.

Phase 1 — URL resolution (DNS)

• Browser check karta hai: browser cache → OS cache (/etc/hosts bhi) → router cache → ISP resolver cache.
• Miss hone par local DNS resolver iterative queries karta hai: root → .com TLD → google.com ka authoritative server.
• Query UDP port 53 pe jaati hai. Answer TTL ke saath cache ho jaata hai.
• Aksar CDN involved hota hai — tumhari location ke hisaab se nearest edge server ka IP milta hai.

Phase 2 — Transport connection

• OS ek ephemeral source port choose karta hai; socket = (src IP, src port, dst IP, 443).
• 3-way handshake: SYN → SYN-ACK → ACK. Yahin MSS, window scaling, SACK jaise options negotiate hote hain.
• Beech mein packet har hop pe: L3 routing (longest prefix match, TTL−1), L2 pe naya frame har link ke liye.

Phase 3 — TLS

• ClientHello (cipher suites, SNI) → ServerHello + certificate → certificate chain verify (CA signature, validity, hostname match, revocation) → key exchange (ECDHE) → session keys → Finished.
• TLS 1.3 mein ye 1 RTT mein ho jaata hai (aur resumption pe 0-RTT).

Phase 4 — HTTP

• GET / HTTP/1.1 + Host header + cookies + Accept-Encoding (gzip/br).
• Server response: status line + headers (Content-Type, Content-Length/Transfer-Encoding, Cache-Control, Set-Cookie) + body.
• Redirect (301/302) hua to browser naye URL pe dubara sab kuch karta hai (aksar http → https redirect, ya HSTS se seedha https).

Phase 5 — Rendering

• HTML parse → DOM; CSS → CSSOM; JS execute (parser block kar sakta hai).
• Har referenced object ke liye phir se HTTP request (HTTP/2 mein same connection pe multiplexed streams).
• Layout → paint → composite.

Phase 6 — Connection close

• Keep-alive timeout ya explicit close → 4-way FIN handshake, phir TIME_WAIT (2×MSL).

Basics & layering

Application layer

Transport layer

Network layer

Link layer

25.1 Formula sheet

25.2 Port numbers

OSPF aur ICMP transport layer use hi nahi karte — wo seedha IP ke upar hain (protocol number 89 aur 1).

25.3 Header sizes (yaad rakho)

25.4 Protocol → layer map

* Nuance: RIP actually UDP pe chalta hai aur BGP TCP pe — to technically wo application-layer processes hain jo network layer ka kaam (routing) karte hain. Interview mein ye nuance bolna achha lagta hai. OSPF seedha IP pe (protocol 89).

25.5 Sabse common comparisons — one-liners

26.1 Agar 3 din hain

26.2 Agar sirf 1 din hai (emergency mode)

1. Section 25 (cheat sheet) — 20 min, sab formulas aur comparisons.
2. Section 23 (50 rapid-fire) — 60 min, har question loud bolke answer karo.
3. Section 22 (google.com walkthrough) — 20 min, ye zaroor aayega.
4. Sections 09, 10, 12, 20 — TCP, congestion control, subnetting, ARP. 2 ghante.
5. Section 24 se 5–6 numericals solve karo. 1 ghanta.

26.3 Final checklist — tick karke jao

26.4 Interview mein bolne ka tareeka

• Structure do. "Iske teen parts hain…" bolke start karo. Interviewer ko follow karna aasan hota hai.
• Definition → why → example. Har concept isi order mein bolo.
• Trade-off zaroor mention karo. "TCP reliable hai par overhead aur latency zyada hai" — ye maturity dikhata hai.
• Real-world se jodo. "Isliye Netflix DASH over TCP use karta hai" jaise references bonus dete hain.
• Nahi pata to bolo. "Mujhe exact detail yaad nahi, par mera samajhna ye hai ki…" — bakwaas banane se accha hai.