static inline
    unsigned char *silc_buffer_put(SilcBuffer sb,
                                     const unsigned char *data,
                                     SilcUInt32 len);

    Puts data at the start of the valid data area. Returns a pointer
    to the copied data area.  Returns NULL on error.

    ---------------------------------
    | head  | data       | tail     |
    ---------------------------------
            ^
            Puts data to the data section.

    silc_buffer_put(sb, data, data_len);