Issues in the Implementation of Selective Acknowledgements for Tcp

Researchers are investigating on the deenition and the eeectiveness of selective acknowledgement (SACK) options for TCP, as it is expected that a SACK mechanism, together with a suitable congestion control algorithm, can improve TCP performance over lossy networks. Due to the widely variable traac and congestion conditions on the Internet, we believe that work on the above subjects should be supported by extensive experiments in the eld, in addition to theoretical analysis and trace-based simulations. We also believe that the design of new mechanisms, such as SACKs, should be done with special attention to their eeectiveness when interoperating with older implementations. In this paper we give two contributions in this direction. First, we show how the sender node can process incoming SACKs with a small, constant overhead and memory usage with respect to non-SACK implementations. Second, we show in detail how to transport SACK information in modiied RFC1323 timestamps, which we call TSACKs. TSACKs are of trivial implementation, but allow the exploitation of the advantages of SACKs when sending data to RFC1323-compliant nodes. The patterns of traac on the Internet (where relatively few, large FTP/WWW servers probably account for the majority of bytes sent), make this a very important feature. An implementation of the two algorithms above is available from the author. Developed on FreeBSD, a 4.4BSD derivative, the code is compatible with the current IETF draft on the encoding of SACK options.