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Technical Report


TCP Libra: Exploring RTT-Fairness for TCP


The majority of Internet traffic relies on the Transmission Control Protocol (TCP1) devised in the early 1970s to provide a reliable data transfer across the ARPANET. Today’s users download large multimedia files from remote servers using TCP. If these TCP sessions share the same bottleneck, they are expected to receive the same share of bandwidth, thus achieving the same transfer rate. Unfortunately, this is not the case when the round trip delay RTT is very different among sessions. This may have negative practical implications in downloads of delay-sensitive (though non-real-time) information from servers. For instance, suppose in a popular internet caf`e in New York City several users are simultaneously downloading multimedia files from various servers. Most of the servers are local. However, one customer is downloading a large file from a remote server in Australia. All customers share the same 11 MBPS WiFi bottleneck. The customer connected to Australia will make no visible progress until all the customers downloading from local servers are done! One solution would be to set up a time day when only remote customers download. But this is not quite application transparent. Here we propose a new version of TCP, namely TCP Libra, that guarantees fair sharing regardless of RTT. Libra in Latin means scale, thus indicating a balance between the sessions. In this paper we describe the design of TCP Libra and prove that it is indeed RTT fair. The key element of TCP Libra is the window adjustment algorithm. The algorithm is derived by modeling TCP performance as a utility function and by optimizing this function such that the result is independent of RTT, yet maintaining the function stability. Non-linear optimization is used, leading to a solution that provides fairness among TCP flows that share the same bottleneck link regardless of RTT. The remarkable property of TCP Libra is that it achieves fairness while still maintaining throughput efficiency and friendliness with respect to TCP New Reno. Another important property from the implementation standpoint is the “sender-side-only modification introduced by TCP Libra. The fairness and stability properties are proved analytically and are extensively tested via simulation. A comparison is also carried out with other TCP versions that have been reported as RTT fair in the literature.

Paper: PDF file of paper

Information & Date

UCLA CSD Technical Report, , September. 2005


Gustavo Marfia
Claudio Enrico Palazzi
Giovanni Pau
Mario Gerla
M. Y. Sanadidi
Marco Roccetti