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Data Packet

The NDN Data packet is a TLV defined as follows:

Data = DATA-TYPE TLV-LENGTH
         Name
         [MetaInfo]
         [Content]
         DataSignature

The Data packet represents some arbitrary binary data (held in the optional :ref:`Content` element) together with its Name, some additional bits of optional information (:ref:`MetaInfo`), and a digital signature (:ref:`DataSignature <DataSignature>`).

As recommended by the :ref:`TLV evolvability guidelines <evolvability>`, unrecognized non-critical TLV elements may appear in a Data packet. However, they must not appear before the Name element.

Name

See :ref:`Name`.

MetaInfo

MetaInfo = META-INFO-TYPE TLV-LENGTH
             [ContentType]
             [FreshnessPeriod]
             [FinalBlockId]

ContentType

ContentType = CONTENT-TYPE-TYPE TLV-LENGTH NonNegativeInteger

The following ContentTypes are currently defined:

ContentType Assigned number Description of the content
BLOB 0 payload identified by the data name; this is the default ContentType
LINK 1 list of delegations (see :ref:`Link`)
KEY 2 public key (see :ref:`Certificate`)
NACK 3 application-level NACK

Other ContentType numbers are assigned and maintained in the NDN Packet Specification Wiki.

FreshnessPeriod

FreshnessPeriod = FRESHNESS-PERIOD-TYPE TLV-LENGTH NonNegativeInteger

The optional FreshnessPeriod indicates how long a node should wait after the arrival of this data before marking it "non-fresh". The encoded value is number of milliseconds. Note that the "non-fresh" data is still valid data; the expiration of FreshnessPeriod only means that the producer may have produced newer data.

If the Data packet carries a FreshnessPeriod greater than zero, a node should initially consider it "fresh". After the Data has resided in the node for FreshnessPeriod milliseconds, it will be marked as "non-fresh". If the Data does not have a FreshnessPeriod or if it has a FreshnessPeriod equal to zero, it MUST be marked "non-fresh" immediately after processing any pending Interests.

If an Interest contains MustBeFresh element, a content store 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 content store, or, failing that, get forwarded to other nodes). When an exact duplicate of the "non-fresh" Data packet with a positive FreshnessPeriod value arrives at the node, the node SHOULD re-mark it as "fresh" for the specified duration.

FinalBlockId

FinalBlockId = FINAL-BLOCK-ID-TYPE TLV-LENGTH NameComponent

The optional FinalBlockId identifies the final block in a sequence of fragments. It should be present in the final block itself, and may also be present in other fragments to provide advanced warning of the end to consumers. The value here should be equal to the last explicit name component of the final block.

Content

Content = CONTENT-TYPE TLV-LENGTH *OCTET

The Content element can carry any arbitrary sequence of bytes.

DataSignature

See :ref:`DataSignature`.