Although many research efforts have been devoted to network congestion in the face of an increase in the Internet traffic, there is little discussion on performance improvements for endhosts. We propose a new architecture, called scalable socket buffer tuning (SSBT), to provide high-performance and fair service for many TCP connections at Internet endhosts. SSBT has two major features. One is to reduce the number of memory accesses at the sender host by using some new system calls, called simple memory-copy reduction (SMR) scheme. The other is equation-based automatic TCP buffer tuning (E-ATBT), where the sender host estimates 'expected' throughput of the TCP connections through a simple mathematical equation, and assigns a send socket buffer to them according to the estimated throughput. If the socket buffer is short, the max-min fairness policy is used. We confirm the effectiveness of our proposed algorithm through both a simulation technique and an experimental system. From the experimental results, we have found that our SSBT can achieve up to a 30% gain for Web server throughput, and a fair and effective usage of the sender socket buffer can be achieved.