Network coding has been shown to help achieve optimal throughput in directed networks with known link capacities. However, as real-world networks in the Internet are bi-directed in nature, it is important to investigate theoretical and practical advantages of network coding in more realistic bi-directed and peer-to-peer (P2P) network settings. In this chapter, we begin with a discussion of the fundamental limitations of network coding in improving routing throughput and cost in the classic undirected network model. A finite bound of 2 is proved for a single communication session. We then extend the discussions to bi-directed Internet-like networks and to the case of multiple communication sessions. Finally, we investigate advantages of network coding in a practical peer-to-peer network setting, and present both theoretical and experimental results on the use of network coding in P2P content distribution and media streaming.

Network coding background

Network coding is a fairly recent paradigm of research in information theory and data networking. It allows essentially every node in a network to perform information coding, besides normal forwarding and replication operations. Information flows can therefore be “mixed” during the course of routing. In contrast to source coding, the encoding and decoding operations are not restricted to the terminal nodes (sources and destinations) only, and may happen at all nodes across the network. In contrast to channel coding, network coding works beyond a single communication channel, it contains an integrated coding scheme that dictates the transmission at every link towards a common network-wise goal. The power of network coding can be appreciated with two classic examples in the literature, one for the wireline setting and one for the wireless setting, as shown in Figure 17.1.