Networking Essentials for System Design

This article provides a deep dive into the core networking concepts essential for system design interviews. It begins by demystifying the OSI model, specifically focusing on the layers most relevant to software engineers: the Network Layer (Layer 3), Transport Layer (Layer 4), and Application Layer (Layer 7). It uses a practical example of a simple web request to illustrate how these layers interact, from DNS resolution and the TCP handshake to the HTTP request cycle and connection teardown.

The guide then contrasts the two primary Transport Layer protocols: TCP and UDP. It highlights TCP's reliability and order guarantees, making it suitable for web browsing and file transfers, versus UDP's speed and lack of overhead, which is ideal for real-time applications like video streaming and gaming. It further explores various Application Layer protocols, including the ubiquity of HTTP/REST, the flexibility of GraphQL, the efficiency of gRPC for microservices, and real-time solutions like WebSockets, Server-Sent Events (SSE), and WebRTC.

Finally, the article covers critical infrastructure components and patterns. It discusses Load Balancing strategies, differentiating between Layer 4 (performance-focused) and Layer 7 (content-aware) load balancers, and explains algorithms like Round Robin and Least Connections. It also addresses challenges like latency, which can be mitigated via CDNs and regionalization, and system reliability, advocating for patterns like Timeouts, Retries with Backoff, and Circuit Breakers to handle failures gracefully.

Key Concepts

• OSI Layers: Focus on Layer 3 (IP), Layer 4 (Transport), and Layer 7 (Application) as they are most relevant for system design.
• TCP vs. UDP: TCP ensures reliable, ordered delivery but with overhead; UDP is connectionless and faster but unreliable.
• Protocol Selection: Use HTTP/REST for general APIs, gRPC for internal high-performance services, and WebSockets/SSE for real-time capabilities.
• Load Balancing: Distribute traffic using L4 (connection-level) or L7 (application-level) load balancers to ensure scalability and availability.
• Circuit Breakers: A stability pattern that temporarily disables calls to a failing service to prevent cascading failures and allow recovery.
• Content Delivery Networks (CDNs): Use geographically distributed servers to cache content closer to users, reducing latency and backend load.