18.6 Mandatory-to-implement cipher suites

For compatibility purposes, every TLS endpoint must fulfill a minimum set of requirements. To ensure this, Chapter 9 in RFC 8446 defines three types of compliance requirements:

1. Mandatory-to-implement cipher suites

2. Mandatory-to-implement TLS extensions

3. Protocol invariants that every TLS endpoint and middlebox must follow

If there is no dedicated application profile standard that prescribes different algorithms, TLS 1.3 specification requires a TLS endpoint to implement cipher suites given in Table 18.2. Note, however, the difference in the requirement level for the specific cipher suites. The exact meaning of the capital words MUST and SHOULD is defined in IETF RFC 2119, Key words for use in RFCs to Indicate Requirement Levels.

The term MUST (or its equivalents, REQUIRED or SHALL) means that the requirement is absolutely mandatory – there is no room for exceptions. In contrast, the term SHOULD (or its equivalent, RECOMMENDED...