Networking

For SDE-3 Interview Preparation Deep dive into protocols, communication patterns, and network optimizations.

Table of Contents

1. OSI Model vs TCP/IP

2. TCP vs UDP

3. HTTP Evolution (1.1, 2, 3)

4. Communication Protocols

   • REST

   • GraphQL

   • gRPC

   • WebSockets

5. Load Balancing (L4 vs L7)

6. API Gateway Pattern

---

OSI Model vs TCP/IP

OSI Layer	TCP/IP Layer	Protocols	Responsibility
7. Application	Application	HTTP, DNS, SMTP	User-level data
6. Presentation	Application	SSL/TLS, JPEG	Formatting/Encryption
5. Session	Application	Sockets	Session management
4. Transport	Transport	TCP, UDP	End-to-end delivery
3. Network	Internet	IP, ICMP, BGP	Routing packets
2. Data Link	Link	Ethernet, MAC	Node-to-node transfer
1. Physical	Link	Fiber, WiFi	Bits over wire

---

TCP vs UDP

TCP (Transmission Control Protocol)

• Features: Connection-oriented (3-way handshake), Reliable (ACKs), Ordered, Flow Control, Congestion Control.

• Overhead: High (Handshake, headers).

• Use Cases: Web (HTTP), Email (SMTP), File Transfer (FTP).

UDP (User Datagram Protocol)

• Features: Connectionless, Unreliable (fire and forget), Unordered, Fast.

• Overhead: Low.

• Use Cases: Video Streaming/VoIP (WebRTC), Gaming, DNS, DHCP.

QUIC (UDP + Reliability)

• Built on top of UDP to provide reliability and security (TLS 1.3 baked in).

• Basis for HTTP/3.

• Solves "Head-of-Line Blocking" present in TCP.

---

HTTP Evolution

HTTP/1.1

• Plain Text: Human readable.

• One Request per Connection (Keep-Alive added later).

• Head-of-Line Blocking: One slow request blocks queue.

HTTP/2

• Binary Protocols: More efficient parsing.

• Multiplexing: Multiple requests over single TCP connection.

• Header Compression (HPACK).

• Server Push.

• Limitation: Still suffers from TCP Head-of-Line blocking (packet loss blocks all streams).

HTTP/3

• Runs on QUIC (UDP).

• True Multiplexing: Packet loss in one stream doesn't block others.

• Built-in TLS 1.3.

• Faster Handshake: 0-RTT or 1-RTT.

---

Communication Protocols

1. REST (Representational State Transfer)

• Resource-based: GET /users/123.

• Stateless: Server stores no session.

• Pros: Cacheable, Simple, Universal support.

• Cons: Over-fetching/Under-fetching data.

2. GraphQL

• Query Language: Client requests exactly what it needs.

• Schema-Driven: Strongly typed APIs.

• Pros: No over-fetching, Single round-trip.

• Cons: Complex caching (POST based), N+1 query problem.

3. gRPC (Google Remote Procedure Call)

• Based on Protobuf: Binary serialization (smaller/faster than JSON).

• HTTP/2: Supports streaming (Client/Server/Bi-directional).

• Strongly Typed: .proto files define contract.

• Use Cases: Microservices internal communication, Mobile-to-Server.

REST vs gRPC:

Feature	REST	gRPC
Format	JSON (Text)	Protobuf (Binary)
Protocol	HTTP/1.1	HTTP/2
Interface	Resource	Method (RPC)
Latency	Medium	Low
Browser Support	Excellent	Requires Proxy (gRPC-Web)

4. WebSockets

• Full Duplex: Persistent bi-directional connection.

• Protocol: Starts as HTTP Upgrade connection -> switches to WS.

• Use Cases: Chat apps, Live Dashboards, Multiplayer games.

---

Load Balancing

L4 Load Balancer (Transport Layer)

• Routes based on IP + Port.

• Simple packet forwarding (NAT).

• Fast, low CPU.

• No data inspection (can't route based on URL).

• Examples: LVS, HAProxy (TCP mode).

L7 Load Balancer (Application Layer)

• Routes based on Content (URL, Headers, Cookies).

• Terminates SSL/TLS.

• Smarter: Can do A/B testing routing, Auth checking.

• Slower: Needs to decrypt/inspect.

• Examples: Nginx, AWS ALB, HAProxy (HTTP mode).

Algorithm Choices:

• Round Robin (Simple)

• Least Connections (Adaptive)

• Consistent Hashing (Sticky cache affinity)

---

API Gateway Pattern

Central entry point for all client requests.

Functions:

1. Routing: Route /api/user to User Service.

2. Auth: Verify JWT tokens.

3. Rate Limiting: Protect backend.

4. Protocol Translation: REST (Public) -> gRPC (Internal).

5. Caching: Response caching.

6. Circuit Breaking: Fail fast if service is down.

Technologies: Kong, Amazon API Gateway, Nginx, Zuul.