🌐 Computer Networks — Dari Physical Layer ke Application
Jaringan komputer adalah fondasi dari setiap sistem digital modern . Catatan ini memetakan 7 lapisan OSI model + 4 lapisan TCP/IP — bukan dari lensa keamanan (sudah ada hierarchy-network-security), tapi sebagai fondasi yang menjelaskan bagaimana bit bergerak dari satu mesin ke mesin lain. Layer 1-4 adalah domain yang sama sekali belum terpetakan vault.
Daftar Isi
1. Premise — Networking Adalah Sistem Transportasi Digital 2. OSI Model vs TCP/IP Stack 3. Layer 1 — Physical Layer 4. Layer 2 — Data Link Layer 5. Layer 3 — Network Layer (IP) 6. Layer 4 — Transport Layer (TCP/UDP) 7. Layer 5 — Session Layer 8. Layer 6 — Presentation Layer 9. Layer 7 — Application Layer 10. Routing & Switching
1. Premise — Networking Adalah Sistem Transportasi Digital
Setiap byte yang dikirim melalui jaringan melewati transformasi 7 lapisan sebelum mencapai tujuannya:
[Application] → HTTP request → ...
[Presentation] → Encryption, encoding
[Session] → Connection management
[Transport] → TCP segments
[Network] → IP packets
[Data Link] → Ethernet frames
[Physical] → Electrical/optical signals
Kenapa ini BOLEV besar di vault:
Ada hierarchy-network-security (firewall, IDS, WAF) — tapi tidak ada hierarchy-computer-networks (bagaimana network BEKERJA)
Semua attack vector (ARP spoof, MitM, DDoS) membutuhkan pemahaman OSI layers
68 hits di vault merujuk ke networking — tertinggi domain fundamental
2. OSI Model vs TCP/IP Stack
2.1 Tujuh Lapisan OSI
Layer Nama Fungsi PDU Contoh Hardware/Protocol L7 Application Antarmuka ke aplikasi Data HTTP, SMTP, FTP, DNS, SSH L6 Presentation Encoding, encryption, compression Data TLS, SSL, JPEG, ASCII, MIME L5 Session Sesi komunikasi, checkpoint Data NetBIOS, RPC, SIP, SOCKS L4 Transport End-to-end reliability Segment TCP, UDP, QUIC, SCTP L3 Network Routing, logical addressing Packet IP, ICMP, OSPF, BGP, IPsec L2 Data Link Framing, MAC addressing Frame Ethernet, Wi-Fi, PPP, ARP L1 Physical Bit transmission Bit Coaxial, Fiber, RS-232, 10BASE-T
2.2 TCP/IP Model (4 Lapisan)
+----------------------+
| Application (L5-L7) | ← HTTP, SMTP, DNS, SSH, TLS
+----------------------+
| Transport (L4) | ← TCP, UDP, QUIC
+----------------------+
| Internet (L3) | ← IP, ICMP, ARP
+----------------------+
| Network Access (L1-2) | ← Ethernet, Wi-Fi, PPP
+----------------------+
2.3 OSI vs TCP/IP
Aspek OSI Model TCP/IP Model Layers 7 4 Status Referensi (teoretis) Implementasi (aktual) Kelebihan Detail, konseptual Praktis, real-world Penggunaan Teaching, troubleshooting Engineering, deployment
3. Layer 1 — Physical Layer
Media Bandwidth Max Jarak Interference Twisted Pair (Cat 5e/6/6a/8) 40 Gbps 30-100 m Moderate Coaxial 10 Gbps 500 m Low Multi-mode Fiber (MMF) 100 Gbps 550 m None Single-mode Fiber (SMF) 800 Gbps+ 40 km+ None Wireless (5 GHz) 1-9.6 Gbps 30-100 m High (walls, noise) Wireless (60 GHz) 20 Gbps < 10 m Very high
3.2 Signaling
Teknik Mekanisme Digunakan Oleh NRZ (Non-Return-to-Zero) Voltage high = 1, low = 0 Ethernet (10BASE-T) Manchester Transition mid-bit = clock Legacy Ethernet PAM-4 4 amplitude levels = 2 bit/symbol 200G/400G Ethernet QAM-64/256 Amplitude + phase modulation Wi-Fi 6/7, DOCSIS OFDM Multiple subcarriers Wi-Fi, 4G/5G, DSL
Key insight: Semakin tinggi frekuensi → semakin pendek jarak → semakin rentan terhadap noise.
4. Layer 2 — Data Link Layer
4.1 Ethernet Frame Structure
Preamble (7B) | SFD (1B) | Dest MAC (6B) | Src MAC (6B) | EtherType (2B) | Payload (46-1500B) | FCS (4B)
Keterangan:
Preamble — 7 byte sinkronisasiSFD (Start Frame Delimiter) — 1 byte tanda mulai frameMAC Address — 6 byte per alamat (48-bit)EtherType — 0x0800 = IPv4, 0x86DD = IPv6, 0x0806 = ARPPayload — 46-1500 byte (MTU, MTU = Maximum Transmission Unit)FCS (Frame Check Sequence) — CRC32
4.2 MAC Address dan ARP
MAC Address Format: 00:1A:2B:3C:4D:5E
OUI (24-bit) = vendor (Cisco, Intel, …)
NIC-specific (24-bit) = perangkat
ARP (Address Resolution Protocol): L3 address (IP) → L2 address (MAC)
Host A: "Who has 192.168.1.2? Tell 192.168.1.1" (broadcast)
Host B: "192.168.1.2 is at 00:1A:2B:3C:4D:5E" (unicast)
ARP Spoofing: Host jahat bisa menjawab untuk IP orang lain → membuat traffic dialihkan ke dirinya.
4.3 Switching
Teknik Switching Latency Error Check Store-and-forward Penuh (seluruh frame diterima) ✅ Sebelum forward Cut-through Minimal (dest MAC saja) ❌ Fragment-free Menunggu 64 byte pertama Partial
Switch internals:
MAC Address Table — mapping MAC → Port
CAM Table — ternary content-addressable memory (TCAM)
Spanning Tree Protocol (STP) — mencegah loop
5. Layer 3 — Network Layer (IP)
5.1 IPv4 Packet Structure
Version (4) | IHL (4) | DSCP (6) | ECN (2) | Total Length (16) | ID (16) | Flags (3) | Fragment Offset (13) | TTL (8) | Protocol (8) | Header Checksum (16) | Src IP (32) | Dest IP (32) | Options | Payload
Field penting:
TTL (Time To Live) — maksimal hop sebelum packet dibuangProtocol — 1=ICMP, 6=TCP, 17=UDPFragment Offset — fragmentasi di level IP (ketika MTU < paket)Checksum — hanya header, bukan payload
5.2 IPv6 — Alasan dan Perbedaan
Aspek IPv4 IPv6 Alamat 32-bit (4.3 milyar) 128-bit (340 undecillion) Format 192.168.1.1 2001:db8::1 Header 20-60 byte (variable) 40 byte (fixed) Checksum Ada ❌ Tidak ada Fragmentasi Router bisa fragment Hanya host asal Broadcast ✅ ARP broadcast ✅ No ARP (multicast instead) NAT Wajib (IP shortage) Tidak perlu
5.3 Subnetting
IPv4: 192.168.1.0/24
├─ Network: 192.168.1.0
├─ Broadcast: 192.168.1.255
├─ Hosts: 192.168.1.1 - 192.168.1.254 (254 hosts)
└─ Subnet mask: 255.255.255.0 (/24)
Subnet prefix lengths:
CIDR Mask Hosts Use Case /16 255.255.0.0 65,534 Large org /24 255.255.255.0 254 Standard LAN /27 255.255.255.224 30 Small team /30 255.255.255.252 2 Point-to-point link /128 — 1 Loopback (IPv6)
6. Layer 4 — Transport Layer (TCP/UDP)
6.1 TCP Segment
Src Port (16) | Dst Port (16) | Seq Num (32) | Ack Num (32) | Offset (4) | Flags (12) | Window (16) | Checksum (16) | Urgent Pointer (16) | Options | Payload
Flags:
SYN — Mulai koneksi (three-way handshake)ACK — AcknowledgmentFIN — Akhiri koneksiRST — Reset koneksiPSH — Push data ke aplikasiURG — Urgent data
6.2 TCP 3-Way Handshake
CLIENT SERVER
│ │
│ SYN (seq=x) │
│ ───────────────────────> │
│ │
│ SYN-ACK (seq=y, ack=x+1)│
│ <─────────────────────── │
│ │
│ ACK (seq=x+1, ack=y+1)│
│ ───────────────────────> │
│ │
6.3 TCP vs UDP
Aspek TCP UDP Connection Connection-oriented Connectionless Reliability ✅ Retransmission + ACK ❌ No ACK Ordering ✅ Sequence numbers ❌ No ordering Flow control ✅ Sliding window ❌ Congestion control ✅ AIMD, CUBIC ❌ Header size 20-60 bytes 8 bytes Use cases HTTP, SMTP, SSH, FTP DNS, streaming, VoIP, gaming
6.4 TCP Congestion Control
Algoritma Approach Throughput Fairness CUBIC (default Linux)Cubic function, RTT-independent Tinggi untuk high-BDP 🟡 BBR (Google)Model-based (bandwidth + RTT) Sangat tinggi ✅ NewReno AIMD: +1 per ACK, /2 di loss Moderate ✅ DCTCP ECN-based Tinggi (datacenter) ✅
7. Layer 5 — Session Layer
Tanggung jawab: mengelola sesi komunikasi (establish, maintain, terminate).
Protokol Session
Protocol Fungsi Lapisan OSI SOCKS5 Proxy session establishment L5 RPC Remote procedure call session L5/L7 NetBIOS Session service (legacy Windows) L5 TLS Handshake Session key exchange + crypto parameters L5/L6
8. Layer 6 — Presentation Layer
8.1 Encoding
Format Type Contoh Character ASCII, UTF-8, UTF-16 Text encoding Image JPEG, PNG, WebP, AVIF Visual data Video H.264, VP9, AV1 Moving visual Audio AAC, MP3, Opus, FLAC Sound Serialization JSON, XML, Protobuf, Avro Structured data
8.2 Compression
Algorithm Type Ratio Speed gzip (deflate)Lossless 2-5× Fast brotli Lossless 3-8× Medium zstd Lossless 2-10× Fast JPEG Lossy 10-50× Fast H.264 Lossy video 100-1000× Complex
8.3 Encryption (Presentation Layer Role)
TLS menyediakan session encryption — namun berada di L5/L6
X.509 certificates — autentikasi server
Perfect Forward Secrecy (PFS) — setiap sesi punya kunci berbeda
9. Layer 7 — Application Layer
Protokol aplikasi paling penting:
Protocol Port Transport Fungsi HTTP/1.1 80 TCP Web HTTPS (HTTP/2) 443 TCP Web aman HTTP/3 443 QUIC (UDP) Web low-latency DNS 53 UDP (query) / TCP (zone) Name resolution DHCP 67/68 UDP IP assignment SMTP 25/587 TCP Email delivery SSH 22 TCP Remote shell FTP 20/21 TCP File transfer
10. Routing & Switching
10.1 Routing Protocols
Protocol Type Algorithm Metric Convergence OSPF Link-state SPF (Dijkstra) Cost/bw Fast IS-IS Link-state SPF Cost/bw Fast EIGRP Hybrid DUAL Composite Very fast RIP Distance-vector Bellman-Ford Hops Slow BGP Path-vector Path selection AS path + policies Slow (intentional)
10.2 Routing vs Switching
Aspek Switching (L2) Routing (L3) Unit Frame Packet Address MAC IP Decision MAC table → port Routing table → next hop Domain Single broadcast domain Multiple subnets Loop prevention STP/RSTP TTL + routing protocol Hardware ASIC (hardware forwarding) ASIC + CPU
11. Cross-Reference ke Vault
References
Kurose, J. & Ross, K. “Computer Networking: A Top-Down Approach.” 8th ed., Pearson, 2021.
Stevens, W. R. “TCP/IP Illustrated, Vol. 1: The Protocols.” 2nd ed., Addison-Wesley, 2011.
Fall, K. & Stevens, W. R. “TCP/IP Illustrated, Vol. 2.” 2012.
Tanenbaum, A. & Wetherall, D. “Computer Networks.” 6th ed., Pearson, 2021.
Postel, J. “RFC 791: Internet Protocol.” IETF, 1981.
Postel, J. “RFC 793: Transmission Control Protocol.” IETF, 1981.
Deering, S. & Hinden, R. “RFC 2460: Internet Protocol, Version 6.” IETF, 1998.
Perlman, R. “Interconnections: Bridges, Routers, Switches.” 2nd ed., Addison-Wesley, 2000.
Jacobson, V. “Congestion Avoidance and Control.” SIGCOMM 1988.
Cardwell, N. et al. “BBR: Congestion-Based Congestion Control.” CACM 2017.
Varghese, G. “Network Algorithmics.” Morgan Kaufmann, 2005.