"1.2 Normative References reads in part:

• • • • • 
          COAP Shelby, Z., Hartke, K., Bormann, C., ""Constrained Application Protocol (CoAP)"", IETF Internet Draft, Version 18, 28 June 2013.

Which expired on December 30, 2013 by its own terms. http://tools.ietf.org/search/draft-ietf-core-coap-18

Further, an Internet-Draft, expired or not, is not eligible to function as a normative reference.

As the draft bears on its face:

• • • • • Internet-Drafts are draft documents valid for a maximum of six months
          and may be updated, replaced, or obsoleted by other documents at any
          time. It is inappropriate to use Internet-Drafts as reference
          material or to cite them other than as ""work in progress.""

There is no way to require confomance to (the function of a normative reference) a document that may go away at any moment. Moreover, that is so stated by its issuing agency."