Transport Layer Protocols Questions

Comprehensive understanding of transport layer protocols, primarily Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), and related protocols used for diagnostics such as Internet Control Message Protocol (ICMP). Candidates should be able to explain TCP as a connection oriented, reliable, ordered, and flow controlled protocol including the three way handshake for connection establishment, the four step connection teardown, retransmission and timeout behavior, and high level congestion control and flow control mechanisms. Describe TCP header structure and key fields used for reliability and ordering. Explain UDP as a connectionless, best effort, lower latency protocol, its datagram model, simple header structure, and trade offs for reliability and ordering. Give real world use cases and justify protocol choice, for example reliable file transfer and web traffic versus low latency streaming, real time voice, and many DNS queries. Discuss port numbers and common service ports such as HTTP port 80, HTTPS port 443, DNS port 53, SSH port 22, and SMTP port 25, and how sockets and ports map to endpoints. Cover practical topics such as when UDP may fall back to TCP, how fragmentation and packet loss affect each protocol, and the role of ICMP for network diagnostics and error reporting.

MediumTechnical

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An SRE observes that large HTTP responses fail when routed through a particular path but succeed via other routes. Explain fragmentation and Path MTU Discovery (PMTU). Describe the role of the DF (Don't Fragment) bit and ICMP 'fragmentation needed' messages, the problems that occur when ICMP is filtered, and step-by-step debugging and mitigation actions in cloud networks.

HardSystem Design

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Discuss the trade-offs of terminating TLS at the edge load balancer versus upstream application servers for both TCP-based TLS and QUIC (which encrypts much of its transport metadata). Consider observability, DDoS protection, key management, middlebox compatibility, latency, and operational complexity in a multi-tenant cloud environment.

EasyTechnical

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List common service ports (HTTP 80, HTTPS 443, DNS 53, SSH 22, SMTP 25) and describe how blocking or NAT rules on these ports can impact service availability. As an SRE, explain practical steps and tools you would use to verify port reachability and diagnose whether a reachable port issue is caused by firewall, ACLs, load balancer, or service binding.

EasyTechnical

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Describe how sockets and ports map to endpoints on a host. Explain ephemeral ports, well-known ports, how a server binds to an address:port, and the impact of options like SO_REUSEADDR and SO_REUSEPORT. From an SRE viewpoint, how do port collisions and process restarts manifest and how would you debug them?

HardSystem Design

0 practiced

Design a globally distributed, low-latency UDP-based architecture for a competitive real-time game server supporting thousands of matches. Address transport considerations: compensating for packet loss (FEC, application-level retransmits), choosing tick rate, NAT traversal and UDP hole punching for clients, server-authoritative vs client-prediction models, and metrics and alerts to detect transport-layer degradation across regions.

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