Timestamp renewals

["Carl Wallace" <cwallace@xxxxxxxxxxxx>]

Hash tree renewal is described in section 4.2 and includes the following
statement:

   3.  atsc(i) is the encoded ArchiveTimeStampSequence, the
       concatenation of all previous Archive Time-Stamp Chains (in
       chronological order) related to data object d(i).
       Generate hash value ha(i) = H(atsc(i)).
       Note: The ArchiveTimeStampChains used are DER encoded, i.e. they
       contain sequence and length tags.

Is it desirable for the reducedHashtree field to be a factor in this
renewal operation?  Including it requires the generator to prepare the
sequence of chains for each reduced hash tree.  Why not factor in the
last timestamp from the previous chain plus a new set of data object
hashes instead?

For timestamp renewal, the new timestamp is generated for a hash of the
timestamp field of its predecessor, and thus does not factor in the
reducedHashtree field.  For timestamp renewal, it seems like the
reducedHashtree field should be absent (i.e., the reducedHashtree field
should only be populated in the first element of an
ArchiveTimeStampChain).  However, section 4.1 states that "within an
ArchiveTimeStampChain all reducedHashtrees of the contained
ArchiveTimeStamps MUST use the same Hash-Algorithm."  In an
ArchiveTimeStampChain, why would any elements other than the first
element include a reducedHashtree?  Is it really necessary to support
aggregation at each renewal point?  Aggregation for the first timestamp
in each chain seems good enough.