RFC 7252, "The Constrained Application Protocol (CoAP)", June 2014

Errata ID: 5078
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Jim Schaad
Date Reported: 2017-08-07

Section 7.2.1 says:

The Content-Format code
attribute MAY include a space-separated sequence of Content-Format
codes, indicating that multiple content-formats are available.  The
syntax of the attribute value is summarized in the production "ct-
value" in Figure 12, where "cardinal", "SP", and "DQUOTE" are defined
as in [RFC6690].

It should say:

The Content-Format code
attribute MAY include a space-separated sequence of Content-Format
codes, indicating that multiple content-formats are available.
The Content-Format code attribute MUST NOT appear more than once in a 
link.  The syntax of the attribute value is summarized in the 
production "ct-value" in Figure 12, where "cardinal", "SP", and 
"DQUOTE" are defined as in [RFC6690].

Notes:

Insert a sentence that says that the code MUST NOT appear more than once. This appears to be what was intended, but not stated, by the authors since it supports the space separated values to appear in a single attribute value.

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