On 24/04/2020 15:51, Jürgen Groß wrote:
On 24.04.20 16:38, Julien Grall wrote:
Hi,
On 24/04/2020 15:26, Jürgen Groß wrote:
+```
+ 0 1 2 3 4 5 6 7 octet
++-------+-------+-------+-------+-------+-------+-------+-------+
+| type | len |
++-------------------------------+-------------------------------+
+| body
+...
+| | padding (0 to 7 octets) |
++-------+-------------------------------------------------------+
+```
+
+NOTE: padding octets here and in all subsequent format
specifications must be
+ zero, unless stated otherwise.
What about: "... are written as zero and should be ignored on read."
I would rather not say "ignored" because it doesn't allow us to extend
the record if needed in a safe way. The read side should abort if it
sees an other value than 0 in the padding.
I'd rather ignore unknown fields. This allows to add optional data
without having to special case it.
You will need a special case for 0 in any case.
Writing zeroes is the important part
here, of course.
Ignoring those fields would e.g. enable a downgrade more easily, while
aborting could make that impossible.
You are assuming the fields may contain optional data. Now imagine, we
realize in a few months we missed some important fields. How would you
describe the required fields?
I can see two ways:
1) Re-using the padding fields if possible
2) Extending the record
If you re-use the padding fields, then when you downgrade you may lose
information. If you extend the size of the record, then you can't downgrade.
Cheers,
--
Julien Grall
