Interest Packet

The NDN Interest packet is a TLV defined as follows:

Interest = INTEREST-TYPE TLV-LENGTH
             Name
             [CanBePrefix]
             [MustBeFresh]
             [ForwardingHint]
             [Nonce]
             [InterestLifetime]
             [HopLimit]
             [ApplicationParameters [InterestSignature]]

Name is the only required element in an Interest packet. Nonce is required when an Interest is transmitted over the network links, i.e., a compliant forwarder must augment the Interest with the Nonce element if it is missing. CanBePrefix, MustBeFresh, InterestLifetime, and ForwardingHint are optional elements to guide Interest matching or forwarding. Interest can also include an optional ApplicationParameters element.

If an Interest contains InterestSignature, it is considered a Signed Interest. See Signed Interest section for details.

As recommended by TLV evolvability guidelines, unrecognized non-critical TLV elements may appear in the Interest packet. However, they must not appear before the Name element.

Name

See Name for details.

The Name element that can be put in the Interest is further restricted to have at least one name component. Interests that include Name TLV that has zero name components MUST be discarded.

CanBePrefix

CanBePrefix = CAN-BE-PREFIX-TYPE
              TLV-LENGTH ; == 0

When present, Name element in the Interest is a prefix, exact, or full name of the requested Data packet.

When not present, the Name element is either exact or full name of the Data packet:

  • if the last component of the Name has type ImplicitSha256DigestComponent, Interest can be matched only to a Data packet with full name that includes the implicit digest component;

  • if the last component has any other type, Interest is matched to Data if all name components in Interest’s Name element equal to components in Data’s Name element, without consideration of the implicit digest component.

MustBeFresh

MustBeFresh = MUST-BE-FRESH-TYPE
              TLV-LENGTH ; == 0

The presence or absence of the MustBeFresh element indicates whether a content store may satisfy the Interest with stale Data. See FreshnessPeriod for more information.

ForwardingHint

ForwardingHint = FORWARDING-HINT-TYPE TLV-LENGTH 1*Name

The ForwardingHint element contains a list of Names (“delegations”). Presence of the forwarding hint implies that Data can be retrieved by forwarding the Interest over path(s) pointed by the listed Names. Specifics of the forwarding logic for Interests with ForwardingHint will be defined in a separated document.

Nonce

Nonce = NONCE-TYPE
        TLV-LENGTH ; == 4
        4OCTET

The Nonce carries a randomly-generated 4-octet long byte-string. The combination of Name and Nonce should uniquely identify an Interest packet. This is used to detect looping Interests.

InterestLifetime

InterestLifetime = INTEREST-LIFETIME-TYPE TLV-LENGTH NonNegativeInteger

InterestLifetime indicates the (approximate) time remaining before the Interest times out. The value is the number of milliseconds. The timeout is relative to the arrival time of the Interest at the current node.

Nodes that forward Interests may decrease the lifetime to account for the time spent in the node before forwarding, but are not required to do so. It is recommended that these adjustments be done only for relatively large delays (measured in seconds).

It is the application that sets the value for InterestLifetime. If the InterestLifetime element is omitted, a default value of 4 seconds is used (4000). The missing element may be added before forwarding.

HopLimit

HopLimit = HOP-LIMIT-TYPE
           TLV-LENGTH ; == 1
           OCTET

The optional HopLimit element indicates the number of hops the Interest is allowed to be forwarded. The value is encoded as a 1-byte unsigned integer value in the range [0, 255].

If element is present:

  • if the HopLimit value is larger than or equal to 1, a node should accept the packet and decrease the encoded value by 1.

    If the HopLimit value becomes 0, a node can satisfy this Interest locally (cache or applications bound to local faces), but must not forward the Interests to any non-local faces.

  • if HopLimit is 0, a node must drop the packet

If omitted:

  • a node should accept the packet;

  • when desired, a node can augment the Interest with the HopLimit element.

ApplicationParameters

ApplicationParameters = APPLICATION-PARAMETERS-TYPE TLV-LENGTH *OCTET

The ApplicationParameters element can carry any arbitrary data that parameterizes the request for Data. The Interest’s name MUST include a Interest parameters digest component to ensure uniqueness and integrity of the parameterized Interest (see Parameters Digest Component for additional details).

InterestSignature

See Interest Signature.