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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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