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Optimization of TCP SACK

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  1. Introduction
  2. The Transport Control Protocol - TCP
  3. TCP implementations
  4. TCP sack performance issues
  5. Conclusions
  6. Bibliography

Internet is expanding rapidly over entire glob with heterogeneous networks. TCP/IP protocol suite is an inseparable part of Internet. Hence, the efficiency of the protocols is plays a vital role in performance of Internet. The increasing use of wireless links in expanding Internet has brought out some serious performance issues of TCP, which was originally designed and deployed over a wired network. Wired links are less prone to channel errors. Due to inherent problem of TCP, it fails to differentiate losses by actual reason, so they are not treated fairly. Therefore, performance is often compromised by unnecessary congestion window, cwnd reductions and RTOs. Basic TCP has gone through a number of revisions in order to improve performance. Among all of them, SACK TCP is considered to be the most efficient scheme because of its ability to avoid unnecessary retransmissions based on SACK information. SACK TCP also fails to discriminate the cause of the loss: congestion or corruption. If random losses can be categorized apart from losses due to severe congestion, then irrational decision of 'cwnd' reduction can be avoided. This will surely lead to performance improvement due to effective bandwidth utilization and consistent flow in random packet losses. In this paper, SACK TCP is modified to delay fast recovery after one packet loss, until subsequent loss in the same windows is encountered. Keywords: TCP SACK, cwnd, Congestion, Corruption, Fast Recovery

[...] TCP Reno's Fast Recovery algorithm is optimized for the case when a single packet is dropped during a window of data. The TCP Reno retransmits at most one dropped packet per round-trip time. TCP Reno significantly improves upon the behavior of TCP Tahoe when a single packet is dropped from a window of data, but can suffer from performance problems when multiple packets are dropped during a window of data TCP New-Reno TCP New-Reno is the enhanced TCP Reno using a modified version of Fast Recovery Janey Hoe proposed a modification to TCP Reno usually called New-Reno, which addressed two problems in TCP Reno, these ideas are gradually finding acceptance within the IETF. [...]


[...] SIMULATION TOPOLOGY Erroneous Environment with no Congestion Fig Simulation Topology 1 Congested Network with an Erroneous Link Table Number of Packets Delivered (Errors & Congestion 100 Seconds) Error Rate Original SACK Modified TCP SACK TCP CONCLUSIONS Fig Simulation Topology 2 Simulations were carried out on ns-2 with two types of topologies: erroneous but congestion free environment & congested network along with random errors. Following observations were made from the results of simulations attempted on baseline and modified SACK TCP schemes. [...]

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