Must signed.version be incremented every time a role is re-signed?

Description of issue or feature request:

After reading the TUF specification and studying the basic_repo.py example, one thing remains unclear to me:

When exactly do we need to increment signed.version?

It is clear that we need to increment the signed.version after e.g. adding a new target, as detailed in the basic_repo.py example.

However, what if we only modify signed.expires, after a role has expired, without changing anything else? Do we also need to increment signed.version in that case? That would imply e.g. the version of timestamp is incremented every time it is re-signed.

In general, do we need to increment a role’s signed.version every time we re-sign that role, without exception?

Current behavior:

It is not explicitly clear from the documentation when signed.version needs to be incremented.

Expected behavior:

It would be very helpful if the documentation/specification could clarify this point explicitly.

Perhaps the basic_repo.py example could also show a snippet where an expired timestamp is re-signed (without any changes to the root, targets, or snapshot metadata).

[lukpueh]

bump the version whenever someone signs a role

… and publishes it for someone to consume.

I would say.

[jku]

Good point, stuff like this would make the docs much better. I believe any change to published signed metadata should lead to version bump: otherwise clients can’t know when they need to download new metadata (in the case that they already have the previous version downloaded).

The only modification to metadata that doesn’t need this that I could imagine is adding signatures… but even that seems safe only before the metadata is made public to clients (otherwise some clients could have fewer signatures and then signing keys might get changed and who knows how that would end).