requirement
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... backwards compatibility is achieved based on the TLS
requirement that servers that are not "extensions-aware" ignore data
added to client hellos that they do not recognize; for example, see
...
... client MUST ensure that labels are separated only by the byte 0x2E,
representing the dot character U+002E (requirement 1 in Section 3.1
of [IDNA] notwithstanding). If the server needs to match the
...
... OCSP client
will need to be checked for conformance to this requirement).
Servers that receive a client hello ...
... Certificates MUST conform to [PKIX]. (This should be
interpreted as a requirement to encode only PKIX-conformant
certificates ...
... Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. ...