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TCP NewReno
TCP NewReno is an extension of the Transmission Control Protocol (TCP) that enhances its fast recovery and retransmission algorithms. Originally introduced in RFC 2582 and later refined in RFC 3782 and RFC 6582, TCP NewReno was designed to handle packet loss more efficiently, especially in scenarios where multiple packets are lost within a single transmission window. This improvement significantly boosts the performance of TCP in environments with higher packet loss rates, such as wireless networks or congested links.
The key contribution of TCP NewReno is its handling of partial acknowledgments (ACKs) during the fast recovery phase. In traditional TCP Reno, the fast recovery process ends when an acknowledgment for new data is received, even if not all lost packets have been recovered. This could cause unnecessary timeouts and retransmissions. TCP NewReno, however, introduces a method where the sender can identify and respond to partial ACKs, allowing it to retransmit the next lost packet without exiting the fast recovery phase. This improvement prevents premature exits from fast recovery and improves throughput when multiple packets are lost.
Another core aspect of TCP NewReno is its interaction with the fast retransmit algorithm. When three duplicate ACKs are received, the sender assumes that a packet was lost and immediately retransmits it. The congestion window is reduced, but instead of fully resetting, TCP NewReno allows the sender to continue transmitting new data while waiting for further ACKs. This adaptive approach prevents drastic reductions in transmission rates, making the protocol more resilient in high-loss scenarios.
The recovery process in TCP NewReno continues until all outstanding packets (those that were sent but not acknowledged) are successfully acknowledged. Full acknowledgments allow the sender to exit fast recovery, while partial acknowledgments trigger retransmissions of any remaining lost packets. This ensures that data loss is addressed more dynamically, without relying solely on timeouts.
A limitation of TCP NewReno is its reliance on duplicate ACKs for loss detection, which can delay recovery when fewer than three duplicate ACKs are generated. This limitation is addressed in protocols like TCP SACK (Selective Acknowledgment), which provides more detailed feedback about which packets have been lost or received. Nevertheless, TCP NewReno remains widely used due to its simplicity and significant improvements over earlier congestion control methods like TCP Reno.
TCP NewReno was designed with backward compatibility in mind, meaning it can be deployed in mixed environments without requiring significant changes to existing network infrastructure. This compatibility allows for gradual adoption of the protocol, making it a preferred choice for many network administrators looking to improve performance without overhauling their systems.
For technical specifications and additional details, refer to the following official resources: - RFC 6582: https://www.rfc-editor.org/info/rfc6582 - RFC 3782: https://www.rfc-editor.org/info/rfc3782 - Wikipedia on TCP congestion control: https://en.wikipedia.org/wiki/TCP_congestion_control
Conclusion
TCP NewReno improves upon earlier versions of TCP by enhancing the fast recovery process and addressing packet loss more effectively, especially in networks with higher loss rates. Its ability to handle partial ACKs allows for a smoother and more efficient recovery process, making it a vital protocol for maintaining throughput in environments prone to congestion and loss. While newer protocols like TCP SACK offer more advanced features, TCP NewReno continues to play a crucial role in the evolution of TCP congestion control.