Judging from the IETF mailing list it was forgotten inside the RFC document.
Relevant mail is the second in the thread:
Good point.
DTLS is intended to support extensions--and OpenSSL, at least, supports them in the same way as it does for TLS.
There probably should be a definition of ExtendedClientHello in 4346 and 6347, but it's exactly the PDU you would expect. I.e., the extensions come after the CompressionMethod.
-Ekr
Thread search results here. Contents here:
[TLS] DTLS lacking TLS extensions ?, Martin Rex (Archived here.)
Re: [TLS] DTLS lacking TLS extensions ?, Eric Rescorla (Archived here.)
Re: [TLS] DTLS lacking TLS extensions ?, Martin Rex (Archived here.)
Re: [TLS] DTLS lacking TLS extensions ?, Marsh Ray (Archived here.)