1. Theoretical Basis
This project aims to address the inefficiency of the traditional TCP protocol in high-loss network environments by introducing a transformation based on network coding. The source code is modified by integrating network coding logic into both the TCP layer and the IP layer. A new network coding layer is introduced, which enables encoding at the sender, decoding at the receiver, and dynamically adjusting the transmission redundancy of encoded segments based on real-time network loss rates. This helps mitigate packet loss at the TCP level and improves overall reliability. Additionally, we incorporate processor sharing technology to replace the traditional TCP slow start mechanism with a more efficient initial transmission rate. This approach ultimately enhances network throughput, increases data delivery reliability, and reduces the time required for data flows to complete.
2. Network Coding
Network coding was first proposed in 2000 by researchers from the Department of Information Engineering at the Chinese University of Hong Kong. It addresses the limitations of traditional routing by allowing intermediate nodes to encode and combine data packets before forwarding them. At the receiving end, these packets are decoded to recover the original information. This method significantly improves efficiency, especially in multicast scenarios.
The concept of network coding can be clearly illustrated using the butterfly topology shown in Figure 1-1:
Figure 1-1: Basic Principles of Network Coding
In this diagram, each link can transmit only one bit per unit time. If traditional routing is used, the UV link becomes a bottleneck, limiting the transmission of either 'a' or 'b'. As a result, each sink node receives an average of 1.5 bits. However, with network coding, the two messages 'a' and 'b' are combined and transmitted over the UV link. Sink P can then decode 'b' from the received code, while Q can decode 'a', resulting in an average throughput of 2 bits per sink node. This demonstrates that network coding can achieve the maximum capacity of a multicast network, even when traditional routing cannot.
Since its introduction, network coding has evolved rapidly, leading to extensive research across various fields such as random coding, linear and nonlinear coding, static and dynamic codes, convolutional codes, group codes, error detection and correction, multi-user channels, wireless networks, and more. Many top universities and research institutions, including Princeton University, MIT, EPFL, Microsoft Research, Bell Labs, and AT&T’s Shannon Information Lab, have actively contributed to both theoretical and applied studies in this area. In recent years, domestic research groups in China, such as those at Tsinghua University, Xi’an Electronic Science and Technology University, and Beijing University of Posts and Telecommunications, have also started exploring network coding technologies.
3. TCP Protocol
The Transmission Control Protocol (TCP) is a connection-oriented, reliable, byte-stream transport layer protocol. It operates at the fourth layer of the OSI model and provides process-to-process communication through port numbers. TCP is stream-oriented, meaning it treats data as a continuous sequence of bytes rather than discrete packets.
Each byte sent over a TCP connection is assigned a sequence number, starting with a randomly generated initial value. When data is received, the receiver acknowledges the bytes it has successfully received using an acknowledgment number, which is cumulative and indicates the next expected byte. This mechanism ensures reliable delivery and flow control.
To avoid congestion, TCP uses a sliding window mechanism. However, in long-distance transmissions, waiting for acknowledgments can lead to underutilization of the channel. By implementing network coding, we aim to reduce this inefficiency and improve performance in lossy environments.
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