Transport Layer Security (TLS) is the protocol that encrypts traffic between a client and a server and proves the server's identity using a certificate. SSL (Secure Sockets Layer) was its predecessor; every SSL version is obsolete, but "SSL certificate" remains the common name for what is technically a TLS certificate.
Why it matters
TLS prevents eavesdropping and tampering, and browsers flag sites without it as "Not secure". It is also a frequent cause of outages: an expired certificate, an incomplete chain or a protocol mismatch makes your site unreachable even though the server is running.
Example
A TLS 1.3 connection (RFC 8446) completes its handshake in one round trip. You can inspect what a server negotiates:
openssl s_client -connect example.com:443 -servername example.com </dev/null 2>/dev/null | grep -E "Protocol|Cipher"TLS 1.0 and 1.1 were formally deprecated by RFC 8996. Most sites today should allow TLS 1.2 and 1.3 only.
Common mistakes
- Forgetting the
-servernameflag (SNI) when testing, which returns the wrong certificate on shared IPs. - Installing only the leaf certificate and omitting the intermediate, so some clients fail to build the chain.
- Letting a certificate expire because renewal was manual or the automated job silently failed.
- Leaving old protocol versions enabled for legacy clients that no longer exist in your traffic.
Related
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