[Ndn-interest] Packet Format v0.3: semantics of FreshnessPeriod=0

[Junxiao Shi]

Dear folks

I have a question regarding the semantics of FreshnessPeriod=0 in Packet
Format v0.3.

The protocol says
<https://named-data.net/doc/NDN-packet-spec/current/data.html#freshnessperiod>
:

If the Data does not have a FreshnessPeriod or if it has a FreshnessPeriod
equal to zero, it MUST be immediately marked “non-fresh”.
If an Interest contains MustBeFresh element, a node MUST NOT return
“non-fresh” Data in response to this Interest. The effect is the same as if
that “non-fresh” Data did not exist (i.e., the Interest might be matched by
some other Data in the store, or, failing that, get forwarded to other
nodes).


My question is:

   1. A node receives an Interest containing MustBeFresh element from
   downstream, and forwards the Interest to upstream.
   2. The upstream returns a Data with FreshnessPeriod=0.
   3. At this moment, can the node return this Data to downstream?


>From my understanding of the protocol, upon receiving the Data, it is
already "non-fresh". Since "non-fresh" Data cannot satisfy an Interest
containing MustBeFresh element, this Data cannot go to downstream.
Is this understanding correct?

Yours, Junxiao
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