Pairs with Lesson 4: Connection Reuse.
| Term | Meaning |
|---|---|
| RTT (round-trip time) | Time for a message to travel to the other side and its acknowledgment to travel back. The basic unit of network setup cost. |
| TCP handshake | SYN, SYN-ACK, ACK exchange that establishes a TCP connection. Roughly one RTT before either side can send application data. |
| TLS handshake | Negotiates encryption for HTTPS on top of an already-open TCP connection. Adds its own round trip(s) before the first encrypted application byte can be sent. |
| TLS 1.3 1-RTT / 0-RTT resumption | TLS 1.3 typically completes a full handshake in one round trip; resuming a session with a host seen recently can skip the full handshake, though exact behavior depends on client/server support and is worth verifying rather than assuming. |
| Keep-alive | Keeping an HTTP connection open after a response so it can carry later requests to the same host, skipping handshake setup on each one. |
| Connection pooling | A client (browser or backend service) keeps a set of already-established connections ready to hand out, instead of opening a new one per call. |
| HTTP/2 multiplexing | Multiple requests share a single connection concurrently instead of needing one connection per in-flight request — makes connection reuse matter even more, since one warm connection now covers many parallel calls. |
If you're making frequent calls to the same host — browser to API, or backend service to a downstream API or database — connection reuse turns a multi-RTT setup tax into a one-time cost. Undersized pools or disabled/short keep-alive push that tax back onto every request under load, which shows up as a latency spike that looks like the dependency got slower even though its own processing time didn't change.