「2013-5-27」Introduction to Named Data Networking

In this blog post, I will briefly go over a few concepts of Named Data Networking (NDN), and explain why some mechanisms of NDN may be beneficial to Connected Vehicle communication environment.

The entities enrolled in NDN communication can be categorized into data consumer and data producer. The communication process is consumer driven, i.e. the consumer sends out request (which is named Interest packet in NDN terminology) and the producer returns the requested content (named the Data packet). The concept of the consumer-producer pair of NDN is much like the subscriber-publisher pair which is common in real-world application protocols based on TCP/IP, such as web feed which eases the process of keeping track of web content.

After taking a glimpse of the big picture of NDN communication, a more detailed description is as below.

Suppose a data consumer want to obtain some data. Firstly, it sends out an Interest packet which carries a name prefix that uniquely identifies the desired Data packet. When the Interest packet reaches a router, the router remembers the interface the Interest comes in, then forwards it according to Forwarding Information Base (FIB). FIB is much like the routing table in nowadays IP-based routers; the main difference is FIB is based on name prefix which specifies the desired content, while routing table is based on IP address which specifies the desired communication host. That’s why NDN is called content-centric (instead of host-centric) communication.

It is noteworthy that not all the Interest packets need to be forwarded. Each received Interest is stored in Pending Interest Table (PIT), an embedded database indexed by name prefix. When an Interest with the same name prefix (i.e. request for the exactly the same Data with some pending Interest packet) arrives, the router simply records the arrival interface of the Interest in the matching PIT entry, then discards the Interest packet.

Once the Interest packet reaches a node with the requested Data packet (a node can either be a communication entity or a NDN-based router), the Data is sent back according to the incoming interface from which the Interest packet comes in. Each Data packet consists of a) a name prefix which uniquely identifies itself and b) the actual data, together with c) other data segments which are designed to ensure the security and robustness (integrity) of communication, such as the per-packet digital signature. When a router sees a Data packet, it will try to find the matching entry in PIT, and forwards the Data packet to interfaces according to the PIT entry, then removes the entry from PIT. In others words, a Data packet responded by some node will traverse along the reverse path with which the corresponding Interest comes to the node.

One of the most interesting mechanisms of NDN is caching. When the Data packet is sent back to data consumer, the intermediate nodes along the path will cache the Data packet in their Content Store (CS). The caching mechanism of NDN is much like the Content Distribution Network (CDN), both aim to reduce the average time to access the desired data. The difference is, while CDN is a system built upon IP networking; the caching mechanism of NDN is simply intrinsic. When an Interest packet is received by a node, the node will first check its CS; if a copy of the desired Data (with the matching name prefix) is found, the copy is immediately sent back and the request is immediately satisfied; thanks to this wonderful mechanism, the Interest does not need to travel a long distance to the original data producer.

The naming prefix and caching mechanism described above, along with other mechanisms that not covered here (such as strategy layer within each node), enables many desirable properties we need in Connected Vehicle communication environment.

First, the caching can bring lower delay (think about to what extent CDN has impoved our Internet experience in nowadays IP world). Second, mobility environment demands highly dynamic connection between mobile stations (end users) and infrastructures. To IP-based host-to-host communication, quick and effective IP address assignment under mobility environment is always a problem. However, the content-centric and name prefix based NDN communication does not require layered IP addresses, layer-2 MAC addresses (which is universally unique to each device) is enough. Besides name prefix, the strategy layer just mentioned above also benefits the mobility communication when multiple connectivities (either wired or wireless) coexist; because it can adaptively choose the best connectivity for forwarding.