code: public void setContentLength(int len) {
  		checkMessageState();
  		checkCommitted();
  		try {
  			ContentLengthHeader h = SipFactoryImpl.headerFactory.createContentLengthHeader(len);
  			this.message.setHeader(h);
  		} catch (InvalidArgumentException e) {
  			throw new IllegalStateException("Impossible to set a content length lower than 0", e);
  		}
  	}
docs: /*
  (non-Javadoc)
  @see javax.servlet.sip.SipServletMessage#setContentLength(int)