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.
Definition
RTT is the fundamental unit of network latency accounting. TCP connection setup takes 1 RTT. TLS 1.2 adds 2 RTT. TLS 1.3 adds 1 RTT (0-RTT for resumption). HTTP/1.1 without pipelining: 1 RTT per request. HTTP/2 over TLS 1.3: 2 RTT total (1 TCP + 1 TLS) before first request. DNS adds 1+ RTT. QUIC (HTTP/3) reduces connection setup to 1 RTT or 0-RTT. Protocol design focuses heavily on minimizing RTT count because every RTT adds the full network latency.
Examples
- –TCP handshake: SYN → SYN-ACK → ACK = 1 RTT
- –TLS 1.3: ClientHello → ServerHello+Finished = 1 RTT
- –QUIC: Initial → Handshake Complete = 1 RTT
Related Protocols
Related Terms
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.
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.
Handshake
A handshake is the initial exchange of messages between two parties to establish a connection, agree on parameters, and authenticate before data transfer begins. TCP uses a 3-way handshake (SYN/SYN-ACK/ACK). TLS uses its own handshake on top of TCP. QUIC integrates both into a single handshake.