Several ICN architectures have been proposed thus far that share the same principles at the network layer but differ in the implementation details [3
]. Among them, NDN [10
] has been mainly considered as the reference architecture for vehicular environments, thanks to its effective data delivery mechanism and robust security support [11
]. Research works have been dealing with NDN-based V2X networks, mainly focused on name-based forwarding strategies. In the following, after summarizing the basics of the NDN architecture, we give an overview of the current state of the art about NDN for V2X networking.
3.1. NDN Basics
In NDN, the original content is segmented into chunks, which are individually named and secured. The resulting unit may be transmitted in a single Data packet or further fragmented and transferred over the channel in more than one packet of smaller size. NDN follows a receiver-driven approach: an Interest is used to request a given named Data packet.
NDN names are application-specific and hierarchically structured. For instance, a video produced in a classroom during the maths lecture at the university of Reggio Calabria (UNIRC) may have the name /unirc/videos/classrooms/maths.mpg, where “/” delineates name components in text representations, similar to Uniform Resource Locators (URLs).
An NDN consumer, i.e., an end device interested in a given content, includes the content name in an Interest packet and sends it to the network without specifying a destination address. The network nodes apply a forwarding-by-name mechanism to move the request toward the source(s) generating or owning that content. The node(s) will return a Data packet that contains the content with the addressed name. Data packets are signed by the content source with its key so that the consumer and any network node can check the packets authentication and integrity. As a result, NDN natively overcomes many conventional security attacks [11
] and represents a valid alternative to other security frameworks (e.g., [12
The Interest processing at each NDN node involves three tables: (i) the Content Store
(CS) that is used to cache incoming Data packets; (ii) the Pending Interest Table
(PIT) that keeps track of all incoming unsatisfied Interests with the related incoming node’s interfaces (named faces
in NDN terminology); and (iii) the Forwarding Information Base
(FIB) that records, per each name prefix, one or more outgoing faces for the Interest packets forwarding. The FIB can be dynamically populated by a routing protocol [13
Specifically, at the Interest reception, a node N first looks up in its CS for a matching content. If it is found, then the cached Data packet is returned to the consumer. Conversely, N looks up in the PIT for the same pending request. If a PIT entry for the same content already exists, then it is updated with the new incoming face and the Interest is discarded; otherwise, a FIB look-up is performed. If a matching is found in the FIB, then the Interest is forwarded towards the next hop(s) through the face(s) stored in the FIB, otherwise it is discarded. As soon as a node owning the content is reached, the Data packets follow the chain of PIT entries back to the consumer(s). Data packets can be cached by en-route nodes and made available for future requests.
Two main decision engines are implemented at the NDN forwarding plane that may affect the data retrieval performance:
The forwarding strategy permits each NDN node to take actions related to the Interest forwarding, specifically to decide if, when and where forwarding the packet. Depending on the application requirements, the strategy may prioritize the delivery of certain packets or even transmit them in parallel over multiple interfaces. In the case of transmission over a wireless medium with access procedures managed in a distributed manner, the node may defer the packets in order to limit the collision probability with other potentially transmitting nodes. In the case that no route is available in the FIB, it may issue a negative acknowledgement (NACK) message indicating the error type toward the previous node.
The content caching/replacement strategy
permits each NDN node to decide whether to cache the incoming Data packet(s) and which buffered packets to replace in the case the CS is full. The simplest and most intuitive cache-and-replacement strategy implemented in NDN is cache-everything-everywhere with Least Frequently Used (LFU) replacement. This means that all
nodes cache all
Data packets and replace the least requested ones. More advanced policies have been proposed for data caching and replacement [14
], with decisions performed according to a variety of parameters such as content popularity, freshness, and geographical validity of information.
3.2. NDN-Based Content Delivery for V2X: An Overview
Related work on vehicular NDN mainly focused on effective and low-overhead forwarding strategies for the IEEE 802.11 access technology, with only a few works [15
] analyzing hybrid cellular-IEEE 802.11 V2X deployments.
When considering Interest/Data forwarding, the following main aspects have been explored thus far: (i) the design of content retrieval schemes over the shared IEEE 802.11 wireless medium; (ii) the design of reliable transport services; (iii) the design of prioritized transmission schemes that distinguish contents with different requirements, e.g., in terms of latency; and (iv) the definition of transmission schemes over hybrid network access (i.e., cellular/802.11).
Content retrieval schemes. After receiving an Interest and performing CS and PIT lookup without finding a matching, vehicles have to apply a forwarding logic to decide if they can further forward the packet or not, based on the FIB content.
The forwarding schemes available in the literature can be distinguished into (i) pure broadcast-based
schemes, where only broadcast transmissions are enforced with a controlled-flooding mechanism used during Interest forwarding; and (ii) unicast-based
schemes, where after an initial content discovery phase based on broadcasting, Interests are forwarded in unicast mode from node to node (this type of forwarding is possible in NDN provided that the identifier of the next-hop, e.g., the IP address [17
] or the MAC address [18
], is tracked in the FIB).
Unicast-based schemes reduce the adverse broadcast storm effect due to multi-hop Interest broadcasting (in multi-hop wireless networks, broadcast storm is defined as a condition in which, due to a high number of broadcasted packets, nodes may experiment a high level of contention and collisions at the link layer [19
]), as demonstrated in [17
]. However, these schemes may suffer from connectivity breakages due to node mobility and harsh propagation conditions, and may limit the data sharing capability of the wireless medium, enabled by packets overhearing and distributed in-network caching. This is why, thus far, the majority of the works on vehicular NDN have preferred broadcast-based schemes enhanced with proper broadcast storm mitigation mechanisms and/or self-eligibility forwarding decisions that nodes can implement during the data retrieval. In particular, in [20
], a vehicle is considered an eligible forwarder only if it is in the path towards the data source, which has been discovered during a preliminary flooding stage. In [21
], the authors presented the Distributed Interest Forwarder Selection (DIFS) algorithm, where eligible forwarders are only the vehicles that have maximum connectivity time and good link quality with the consumer. In [22
], instead, eligible forwarding vehicles are selected based on the neighbor distance, node density, and the closeness to the consumer.
Forwarding metrics can be carried in the Interest and/or Data packets and can be considered to further improve the forwarding decision. For instance, in [23
], to maximize the packet dissemination scope, Interests carry the geographical position of the sender, and the best forwarder is identified as the farthest away node from the sender. In [24
], such position-based forwarding strategy is further extended with information about the vehicle trajectory, while, in [25
], vehicles consider both their distance from the previous sender and from the target data source.
The forwarding decision is usually coupled with a channel overhearing mechanism to further limit packet collisions and redundancy. A defer time is calculated before each Interest transmission: if the same packet or the related Data is overheard during the waiting time, the Interest packet is dropped. To further control the Interests propagation, time-to-live (TTL) information, such as the hop count, can be included in the packet [26
Although the above-mentioned schemes can be very effective in limiting packet collisions, the percentage of packet losses in vehicular environments can still be huge, due to adverse propagation effects and mobility. It is therefore crucial to design robust packet retransmission schemes to quickly recover from losses. Being a content usually composed of multiple Data packets, the authors of [27
] proposed to extend the Interest header with a bitmap
field indicating the missing packets of a content chunk. A retransmission time-out (RTO), estimated as a moving average of round-trip-time samples, is maintained by the consumer. When the RTO expires, the Interest is retransmitted with the bitmap
set to request selected (missing) Data packets.
Bouk et al. [28
] observed that frequent Interest/Data loss over the wireless channel may cause adverse effects on the PIT size. Indeed, each Interest is maintained in the PIT until it is consumed by the Data or the PIT Entry Lifetime (PEL) expires. A PEL of 4 s is usually considered in the vanilla NDN implementation, but such a static value can be too high in the vehicular scenario. The authors of [28
], instead, proposed a hop limit based adaptive PIT Entry Lifetime (LAPEL) scheme, which adaptively estimates the PEL at each Interest forwarding vehicle. By doing so, the PIT scalability and a more reliable transport service are achieved.
Vehicular applications may have different requirements in terms of latency, throughput and reliability. These requirements can be codified into well-defined name prefixes and translated into differentiated treatments at the NDN layer. For instance, the work in [29
] uses two main name prefixes, /high and /low, to identify the content priority and to set accordingly the logic for the defer time calculation before (re-)broadcasting the packet. Similarly, in [30
], content priorities are used by vehicles to compute the defer time, but they are codified with integer values and included as an additional header field in the Interest packet. For instance, value 1 is assigned to road conditions information that have the highest priority, value 2 is assigned to vehicle status information, value 10 to social media information and so on.
Hybrid cellular-IEEE 802.11 forwarding
. To improve the delivery performance and reduce the network congestion, the work in [15
] proposes to forward Interest and Data packets over different network interfaces. Instead of using IEEE 802.11 to convey all messages, signaling information (i.e., Interest packets) are transmitted over the cellular network, while Data are transmitted over ad hoc contacts. This solution offloads the cellular network of Data exchange, and guarantees that content requests reach the interested nodes with a high probability. In [16
], instead, vehicles can use the cellular and the IEEE 802.11 interfaces in parallel, if the application requires high reliability and low latency.