Network Coding Meets TCP: Theory and Implementation

Abstract

This paper explains how network coding can be applied in a manner compatible with the transmission control protocol (TCP).By Jay Kumar Sundararajan, Devavrat Shah, Muriel Médard, Fellow IEEE, Szymon Jakubczak, Michael Mitzenmacher, and João Barros ABSTRACT | The theory of network coding promises significant benefits in network performance, especially in lossy networks and in multicast and multipath scenarios. To realize these benefits in practice, we need to understand how coding across packets interacts with the acknowledgment (ACK)-based flow control mechanism that forms a central part of today's Internet protocols such as transmission control protocol (TCP). Current approaches such as rateless codes and batch-based coding are not compatible with TCP's retransmission and sliding-window mechanisms. In this paper, we propose a new mechanism called TCP/NC that incorporates network coding into TCP with only minor changes to the protocol stack, thereby allowing incremental deployment. In our scheme, the source transmits random linear combinations of packets currently in the congestion window. At the heart of our scheme is a new interpretation of ACKsVthe sink acknowledges every degree of freedom (i.e., a linear combination that reveals one unit of new information) even if it does not reveal an original packet immediately. Thus, our new TCP ACK rule takes into account the network coding operations in the lower layer and enables a TCP-compatible sliding-window approach to network coding. Coding essentially masks losses from the congestion control algorithm and allows TCP/NC to react smoothly to losses, resulting in a novel and effective approach for congestion control over lossy networks such as wireless networks. An important feature of our solution is that it allows intermediate nodes to perform re-encoding of packets, which is known to provide significant throughput gains in lossy networks and multicast scenarios. Simulations show that our scheme, with or without re-encoding inside the network, achieves much higher throughput compared to TCP over lossy wireless links. We present a real-world implementation of this protocol that addresses the practical aspects of incorporating network coding and decoding with TCP's window management mechanism. We work with TCP-Reno, which is a widespread and practical variant of TCP. Our implementation significantly advances the goal of designing a deployable, general, TCP-compatible protocol that provides the benefits of network coding.

An Implementation of Network Coding with Association Policies in Heterogeneous Networks

“…Typically, TCP segments have a length of around 1500 bytes. With maximum value of n = W = 12, the NWC header overhead per TCP segment will be 2.2% in contrast to 4.467% for the packet format proposed in [2] which adds (7 + 5n) bytes of overhead. For a more detailed description, refer to [10].…”

“…2b shows the packet format used by the network coding (NWC) module. This packet format is a modified version of the packet format used in [2]. This header structure adds (9 + 2n) bytes of overhead, where n is the number of source packets involved in the random linear combination.…”

“…Note that for TCP control packets, only the first 8 bytes are prepended as these packets are not involved in coding operations. The coding vector information in the header is packet-based as opposed to byte-based in [2] that helps reducing the header overhead. Typically, TCP segments have a length of around 1500 bytes.…”

“…It has proven its merits over traditional routing and forwarding approaches by considering data as algebraic entities that can be modified inside the elements of both wireline and wireless networks [6], [7], [8]. We use the solution proposed in [1], [2] and implement it in the widely used OPNET Modeler [9]. Following [1] the model introduces a network coding layer between the transport and network layers of the TCP/IP protocol stack.…”

“…Following [1] the model introduces a network coding layer between the transport and network layers of the TCP/IP protocol stack. This solution does not require any changes to the behavior of the Transmission Control Protocol (TCP) or the Internet Protocol (IP) as packets are seamlessly transmitted across the known interfaces [2].…”