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2                                                 Transport aspects


            S.6.2   Software image download

            The software image download operation is a file transfer from the DPU to the NT.
            The  atomic  unit  of  file  transfer  is  the  section,  the  amount  of  data  that  can  be  transferred  in  a  single
            download section message. The maximum size of the section is limited by the maximum payload size of the
            datagram eoc command (see clause 11.2.2.4.2). The DPU may send smaller sections at will, including the
            final  section  of  a  file  transfer.  Because  the  message  format  allows  for  variable  length,  software  image
            sections are never padded.
            A number of sections comprise a so-called window. A window shall not exceed 256 sections. Figure S.4
            illustrtes the relationship between a software image and its decomposition into windows and sections.


























                               Figure S.4 – Relationship between image, windows and sections


            During the initial software download message exchange, the DPU proposes a maximum window size, but a
            lower value can be stipulated by the NT, which shall be accepted by the DPU. The DPU may send windows
            with fewer sections than this negotiated maximum, but shall not exceed the maximum. Though it is not a
            preferred  choice,  the  DPU  may  send  all  windows  at  the  full  negotiated  maximum  size,  with  the  final
            window of the download operation with download section messages containing only null bytes.
            Each download section message contains a sequence number, which begins anew at 0 with each window.
            By  tracking  the  incrementing  sequence  numbers,  the  NT  can  confirm  that  it  has  in  fact  received  each
            section of code.

            In the message type field of the last download section message of each window, the DPU indicates the end
            of the window by setting the AR (acknowledgement request) bit – prior download section messages are
            unacknowledged.  If  the  NT  has  not  received  the  entire  window  correctly,  i.e.,  if  it  misses  a  sequence
            number, it acknowledges with a command processing error result, whereupon the DPU falls back to the
            beginning of the window and tries again. To improve the chance of successful transmission, the DPU may
            choose to reduce the size of the window on its next attempt.

            When  the  final  window  has  been  successfully  downloaded,  the  DPU  sends  an  end  software  download
            message whose contents include the size of the downloaded image in bytes, along with a CRC-32 computed
            according to [ITU-T I.363.5], across the entire image. If the NT agrees with both of these values, it updates
            the  software  image  validity  attribute  to  indicate  that  the  newly  downloaded  image  is  valid.  Figure  S.5
            illustrates this process.










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