Latency
Latency is the time delay between a request being sent and the first byte of a response being received. Measured in milliseconds. Network latency is dominated by the speed of light over physical distance (1ms per 100km), plus queuing and processing delays.
Definition
Network latency is the round-trip time (RTT) from sender to receiver and back. It has a hard physical lower bound: light travels approximately 200km/ms in fiber (accounting for refraction). A request from New York to London (~5,500km) has a theoretical minimum of ~27ms RTT. Real-world latency adds: TCP handshake overhead, TLS negotiation, DNS resolution, server processing, and network queuing. Latency is distinct from bandwidth – a satellite link may have high bandwidth but very high latency (~600ms). Latency is the primary performance constraint for interactive applications, while bandwidth matters more for bulk transfers.
Examples
- –CDN reduces latency by serving from edge nodes geographically near users
- –TCP has 1 RTT handshake overhead before data can flow
- –TLS 1.3 adds 1 RTT for the TLS handshake
- –0-RTT resumption in TLS 1.3 / QUIC eliminates handshake latency for returning clients
Related Protocols
Related Terms
Bandwidth
Bandwidth is the maximum rate of data transfer across a network path, measured in bits per second (Mbps, Gbps). High bandwidth allows large files to transfer quickly. Bandwidth is often confused with latency – a high-bandwidth connection can still have high latency.
Round-Trip Time (RTT)
Round-Trip Time (RTT) is the time for a packet to travel from source to destination and a response to return. RTT determines how many round trips fit in a given time window. TCP handshakes, TLS negotiations, and DNS lookups each consume RTTs before data can flow.
Time to First Byte (TTFB)
Time to First Byte (TTFB) is the duration from when a client sends an HTTP request to when it receives the first byte of the response. TTFB includes DNS lookup, TCP connection, TLS handshake, server processing, and network transit. Google's Core Web Vitals threshold is TTFB under 800ms for a 'good' score.
Jitter
Jitter is the variation in packet arrival times. If packets arrive at inconsistent intervals (5ms, 12ms, 3ms, 20ms), jitter is high. VoIP and video streaming are sensitive to jitter – inconsistent delivery causes choppy audio and video stuttering. Jitter buffers smooth out variation at the cost of added latency.