Facebook and its sister sites faced a mass outage on October 4th. The primary reason being the misconfiguration of facebook's BGP (Border Gateway protocol) records. BGP is a mechanism by which Internet service providers of the world share information about which providers are responsible for routing Internet traffic to which specific groups of Internet addresses. There is a detailed blog post by Cloudfare regarding this protocol and another one on outage.
PII stands for Personally Identifiable Information.
Found an informative blog that explains SSH Tunnelling.
Envelope Encryption: The process of encrypting a key with another key is known as envelope encryption.
Tink is a multi-language, cross-platform, open source library that provides cryptographic APIs that are secure, easy to use correctly, and hard(er) to misuse.