can't get MDC values in my logs

[Stefano Zanmarchi]

Hi,
following the wiki
https://wiki.shibboleth.net/confluence/display/SHIB2/IdPLogging#IdPLogging-MappedDiagnosticContext
I've tried to add %mdc{KEY} information to my idp-audit.log file, but
I get empty values
for all KEY values (clientIP, idpSessionId, JSESSIONID).
Here's is my  idp-audit.log section, am I doing something wrong or
might there be a bug
in my IdP version (2.2.1 on Tomcat 6.0.29)?
Thank you very much,
Stefano

<appender name="IDP_AUDIT"
class="ch.qos.logback.core.rolling.RollingFileAppender">
 <File>/home/idpadmin/shibboleth-idp/logs/idp-audit.log</File>
 <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
 <FileNamePattern>/home/idpadmin/shibboleth-idp/logs/idp-audit-%d{yyyy-MM-dd}.log</FileNamePattern>
   </rollingPolicy>
     <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
        <charset>UTF-8</charset>
         <Pattern>[%mdc{clientIP}] - %msg%n</Pattern>
     </encoder>
    </appender>