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Forwarding Plane

NDN uses a stateful forwarding plane1 that maintains several tables at forwarders. This is in contrast to the stateless forwarding plane of IP, which does not maintain any per-packet state. The stateful forwarding plane of NDN enables Data multicast and caching, along with other features such as privacy, loop detection, network-layer congestion control2 and DoS protection3.

Each instance of NFD in an NDN network maintains several tables. Some of these tables store per-packet state, thus making the forwarding plane stateful. The most important of these tables are described briefly in the following sections.

Further Reading

If you are interested in the implementation details of NFD, please refer to the NFD Developer Guide.

Forwarding Information Base

The FIB is a forwarding table similar to the one found in IP routers, which maps name prefixes to outgoing faces. It is used by NFD to forward Interest packets. Each entry in the FIB contains the following information.

  • A name prefix.
  • A set of next hops (outgoing faces).
  • The cost of each next hop face.

When NFD receives an Interest which must be forwarded, it looks up the FIB for the longest matching prefix. If a matching entry is found, the Interest may be forwarded to one or more faces specified in the entry. The exact decision of which face(s) to forward the Interest on is made by the strategy matching the prefix, as described in the next section.

Strategy Table

The strategy table is used by NFD to decide which strategy to use for forwarding a given Interest. Each entry in the strategy table contains the following information.

  • A name prefix.
  • The name of the strategy to use for forwarding Interests matching the prefix.

When NFD receives an Interest which must be forwarded, it looks up the strategy table for the longest matching prefix. The matching strategy is then used to decide which face(s) to forward the Interest on.

Some examples of strategies are,

  • Best route: Use the next hop with the lowest cost.
  • Multicast: Forward the Interest to all next hops.
  • ASF: Use the Adaptive Smoothed RTT-based Forwarding (ASF) algorithm4.

Pending Interest Table

The PIT is the most important table in the forwarding plane, and it provides the basic support for the pull model of communication in NDN.

When NFD receives an Interest, it stores a PIT entry in this table. Each PIT entry contains the following information.

  • The name of the Interest along with any selectors.
  • The incoming face on which the Interest was received (in-record).
  • The outgoing face(s) on which the Interest was forwarded (out-record).
  • The time at which the Interest will expire (lifetime).

On receiving a Data packet, NFD performs the following steps.

  • Looks up the PIT for an entry matching the Data packet.
  • If such an entry is found, the Data packet is forwarded on the incoming face of the PIT entry, and the PIT entry is removed from the table.
  • If no matching PIT entry is found, the Data packet is dropped.

The PIT entry is also dropped if the Interest times out before a matching Data packet is received.

One-Interest One-Data

A fundamental principle of NDN is that one outgoing Interest can bring back at most one Data packet. This is enforced by the PIT, since the PIT entry is removed from the table as soon as a matching Data packet is received and forwarded to the incoming face.

Breadcrumb Trail

In an NDN network containing multiple forwarders, PIT entries for an Interest serve as a breadcrumb trail that the Data packet follows back to the consumer. As a result, no source address is required in the Interest packet for sending the Data packet back to the consumer.

Implementation of PIT

For efficiency, forwarders may implement the PIT using tree-based data structures or hash tables. The entry expiration is typically implemented using priority queues.

Data Multicast

The per-packet statefulness of the PIT directly enables Data multicast by allowing a single Data packet to be forwarded to multiple consumers that have expressed an Interest in the same data.

When NFD receives an Interest, it first looks up the PIT for an entry matching the Interest. If such an entry is found, the Interest is aggregated with the existing entry, by appending an in-record to the entry for the incoming face of the new Interest. The Interest is then not forwarded further.

When a matching Data packet is received, it is forwarded on all the incoming faces of the PIT entry. As a result, multiple Interests are satisfied with a single Data packet, thus realizing Data multicast.

Multicast and Interest Selectors

Interests can generally be aggregated only if they have the same name and selectors. For instance, two Interests having the same name but different values for the MustBeFresh selector cannot be aggregated, since they are potentially requesting different Data packets.

Consumer Privacy

Since the PIT stores the incoming face of each Interest, the outgoing Interest does not need to contain any information about the incoming face. This enables consumer privacy in NDN, by ensuring that Interest packets cannot be used to identify the original sender.

Loop Detection

The PIT also lets NFD detect and prevent looping Interests. Each Interest in NDN carries a randomly generated nonce, which is stored in the PIT entry along with the Interest name. When NFD receives an Interest, it first looks up the PIT for an entry matching the Interest. If such an entry is found including a matching nonce, then the incoming Interest is dropped.

Longer Loops

While the PIT prevents some types of looping Interests, it fails to detect longer loops, since an Interest may be satisfied by a Data packet before it loops back to the same forwarder. To detect such looping Interests, NFD uses a separate table called the Dead Nonce List, which stores nonces of recently satisfied Interests.

Content Store

The Content Store is a cache of Data packets, which is used by NFD to satisfy Interests. When NFD receives an Interest, it first looks up the Content Store for a matching Data packet before looking up the FIB and Strategy Tables. If a matching Data packet is found, it is directly used to satisfy the Interest, and the Interest is not forwarded further.

The Content Store is a key component of NDN, since it enables Data caching at the network layer. As a result, NDN supports both synchronous and asynchronous multicast of Data packets.

Data Freshness

NDN Data packets carry a FreshnessPeriod, which specifies the time in milliseconds for which the data is fresh. A Data packet cached earlier than its freshness period is considered "non-fresh", and cannot be used to satisfy Interests with the MustBeFresh selector.

  1. Yi, C. et al. 2013. A case for stateful forwarding plane. Computer Communications. 36, 7 (2013), 779–791. DOI:https://doi.org/https://doi.org/10.1016/j.comcom.2013.01.005. ↩

  2. Song, S. and Zhang, L. 2022. Effective NDN congestion control based on queue size feedback. Proceedings of the 9th ACM conference on information-centric networking (New York, NY, USA, 2022), 11–21. ↩

  3. Gasti, P. et al. 2013. DoS and DDoS in named data networking. 2013 22nd international conference on computer communication and networks (ICCCN) (2013), 1–7. ↩

  4. Lehman, V. et al. 2016. An experimental investigation of hyperbolic routing with a smart forwarding plane in NDN. 2016 IEEE/ACM 24th international symposium on quality of service (IWQoS) (2016), 1–10. ↩