Interest Packet

NDN Interest packet is TLV defined as follows:

Interest ::= INTEREST-TYPE TLV-LENGTH
               Name
               Selectors?
               Nonce
               InterestLifetime?
               Link?
               SelectedDelegation?

Name and Nonce are the only two required elements in an Interest packet. Selectors are optional elements that further qualify Data that may match the Interest. They are used for discovering and selecting the Data that matches best to what the application wants. Selectors are placed right after the Name to facilitate implementations that may use continuous memory block of Name and Selectors TLVs together as the index for PIT lookup. By using a TLV to group all the Selectors, an implementation can easily skip them to find Nonce, which is used together with Name to identify looping Interests. If Selectors TLV is present in the Interest, it MUST contain at least one selector.

InterestLifetime is optional and is referred to as a Guider. It affects Interest forwarding behavior, i.e., how long an Interest may be kept in the PIT.

Name

The Name element in an Interest is synonymous with the term prefix. See Name section for details.

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

Selectors

Selectors ::= SELECTORS-TYPE TLV-LENGTH
                MinSuffixComponents?
                MaxSuffixComponents?
                PublisherPublicKeyLocator?
                Exclude?
                ChildSelector?
                MustBeFresh?

MinSuffixComponents, MaxSuffixComponents

MinSuffixComponents ::= MIN-SUFFIX-COMPONENTS-TYPE TLV-LENGTH
                          nonNegativeInteger

MaxSuffixComponents ::= MAX-SUFFIX-COMPONENTS-TYPE TLV-LENGTH
                          nonNegativeInteger

When needed, MinSuffixComponents and MaxSuffixComponents allow a data consumer to indicate whether the Name in the Interest is the full name including the digest, or the full name except for the digest, or the content it is seeking has a known range of legitimate component counts. These two parameters refer to the number of name components beyond those in the prefix, and counting the implicit digest, that may occur in the matching Data. The default for MinSuffixComponents is 0 and for MaxSuffixComponents is effectively infinite, meaning that any Data whose name starts with the prefix is a match. Often only one of these will be needed to get the desired effect.

PublisherPublicKeyLocator

PublisherPublicKeyLocator ::= KeyLocator

This element specifies the name of the key which is used to sign the Data packet that the consumer is requesting. This is a way for the Interest to select answers from a particular publisher.

See KeyLocator section for more detail.

Exclude

Exclude ::= EXCLUDE-TYPE TLV-LENGTH AnyLeading? (NameComponent | AnyBetween)* AnyTrailing?
AnyLeading ::= Any NameComponent
AnyBetween ::= NameComponent Any NameComponent
AnyTrailing ::= NameComponent Any
Any ::= ANY-TYPE TLV-LENGTH(=0)

The Exclude selectors allow requesters to specify list and/or ranges of name components that MUST NOT appear as a continuation of the Name prefix in the responding Data packet to the Interest. For example, if Interest is expressed for /ndn/edu and Exclude specifies one name component ucla, then neither data producer nor conforming NDN routers are allowed to return any Data packet that has prefix /ndn/edu/ucla.

Exclude filter applies only to a name component of the Data packet name that is located at a position that numerically equals to the number of name components in the Interest packet, assuming 0 is the first name component.

The Components in the exclusion list MUST occur in strictly increasing order according to the canonical NDN name component ordering (Name Section), with optional leading, trailing, and interleaved Any components. The following defines processing of Any components:

  • If none of the Any components are specified, the filter excludes only the names specified in the Exclude list.
  • If a leading Any component is specified, then the filter excludes all names that are smaller or equal (in NDN name component canonical ordering) to the first NameComponent in the Exclude list.
  • If a trailing Any component is specified, then the filter excludes all names that are larger or equal (in NDN name component canonical ordering) to the last NameComponent in the Exclude list.
  • If Any component is specified between two NameComponents in the list, then the filter excludes all names from the range from the right NameComponent to the left NameComponent, including both ends.

Exclude filter MUST not consist of a single Any component or one NameComponent with leading and trailing Any components.

ChildSelector

ChildSelector ::= CHILD-SELECTOR-TYPE TLV-LENGTH
                    nonNegativeInteger

Often a given Interest can match more than one Data within a given content store. The ChildSelector provides a way of expressing a preference for which of these should be returned. If the value is 0, the leftmost child is preferred. If 1, the rightmost child is preferred. Here leftmost and rightmost refer to the least and greatest components according to the canonical NDN name component ordering (Name Section). This ordering is only done at the level of the name hierarchy one past the name prefix.

For example, assuming in the name hierarchy the component immediately after the name prefix is the version number, whose next level is the segment number, then setting ChildSelector to be 1 will retrieve the rightmost version number (i.e., the latest version) and the leftmost segment number (i.e., the first segment). However, this selection is only done with respect to a single content store, not globally. Additional rounds that exclude the earlier versions may be used to explore other content stores for newer versions. In this case, the use of ChildSelector does not change the multi-round outcome, but it decreases the number of rounds needed to converge to an answer.

MustBeFresh

MustBeFresh ::= MUST-BE-FRESH-TYPE TLV-LENGTH(=0)

This selector is encoded with Type and Length but no Value part. When it is absent from an Interest packet, the router can respond with a Data packet from its content store whose FreshnessPeriod is either still valid or expired. When it is present in an Interest packet, the router should not return Data packet from its content store whose FreshnessPeriod has expired.

The FreshnessPeriod carried in each Data packet (Data Section) is set by the original producer. It starts counting down when the Data packet arrives at a node. Consequently if a node is N hops away from the original producer, it may not consider the Data stale until N X FreshnessPeriod after the Data is produced.

Nonce

Nonce defined as follows:

Nonce ::= NONCE-TYPE TLV-LENGTH(=4) BYTE{4}

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.

Guiders

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.

Selected Delegation

The SelectedDelegation field indicates the index of the delegation in the attached Link that was chosen by a downstream forwarder(s).

The format is the following:

SelectedDelegation ::= SELECTED-DELEGATION-TYPE TLV-LENGTH
                         nonNegativeInteger

If Link field is not present, the SelectedDelegation field MUST NOT be present.

The index value of the SelectedDelegation field MUST be less than the number of delegations within the Link.