docs: public long size() {
          long retval=(long)Global.BYTE_SIZE   // leading byte
                  + Global.SHORT_SIZE;   // flags
          if(dest != null)
              retval+=Util.size(dest);
          if(sender != null)
              retval+=Util.size(sender);
  
          retval+=Global.SHORT_SIZE;  // number of headers
          retval+=Headers.marshalledSize(this.headers);
  
          if(buf != null)
              retval+=Global.INT_SIZE // length (integer)
                + length;       // number of bytes in the buffer
          return retval;
      }