Performance and Setup Guide for the NOS TCP/IP Protocol Used on HF Near-Vertical-Incidence-Skywave (NVIS) Radio Paths

This note describes two years of testing of the Transmission Control Protocol and Internet Protocol (TCP/IP) protocol stack over near-vertical-incidence-skywave (NVIS) propagation paths in Massachusetts and New Hampshire. Results of the testing bear on the evaluation of a number of COTS and developmental systems that transfer data over high frequency (HF) radio. Among these are systems being considered in "dual-use" HF radio configurations on board military aircraft being modified under the US Air Force Global Air Traffic Management (GATM) upgrade program. One use of such radio systems is communication of civilian air traffic control and "airline operational control" data over commercial networks. A second use is communication of encrypted military command and control data over purely military networks. Our results provide throughput and probability of correct message delivery baselines for both modes of use. Our study also allows baseline performance comparison for systems that exchange data between tactical ("short-haul") military users over beyond-line-of-sight radio links, and for systems that can communicate IP-addressable data among platforms being integrated into the worldwide US military Global Grid. In the last case, the TCP/IP protocol stack we have evaluated can itself be used in preliminary studies of direct transmission of TCP/IP datagrams over half-duplex radio. To aid the use by others of the shareware stack we evaluated, we have expanded the scope of our report to include not only performance results but also selected information on how to set up and effectively operate the stack over error-prone radio links. The report is not, however, a comprehensive manual on configuration and operation of TCP/IP over radio. NVIS paths are relatively short radio paths (ground distances from about 20 to 400 miles) used for tactical military communications and various civilian purposes. Difficult conditions are often encountered during NVIS communications. Among these conditions are multipath interference at sunrise and sundown, high D-layer signal absorption at midday and strong interference from long-distance broadcast stations at night. Protocol performance evaluations over NVIS links therefore provide conservative (i.e., lower-bound) assessments of the long-distance HF skywave communications normally associated with the concept of "shortwave" radio. The tests are part of an on-going, multi-year program to assess and compare reliable, modern systems for communicating data over HF radio. Such systems comprise, at each station, an HF radio and antenna, a controller (usually a computer) that runs a user interface, and an HF modem that does waveform generation, signal processing and forward error correction. The software that implements a particular data transmission protocol runs either on the controller or in the modem or both. "Reliable" in the sense of this report means that the protocol includes an automatic repeat request (ARQ) sub-protocol that causes a station receiving erroneous and uncorrectable data to ask the transmitting station to repeat it. Although the standard TCP/IP stack was not developed for use on radio channels, its performance (in half-duplex modes) is still good enough for fielded military use and to serve as a baseline for the performance of (non-TCP) ARQ protocols that are tailored to HF channels. The TCP/IP stack used for these tests is the Net Operating System (NOS), a 16-bit, DOS-based stack developed by amateur radio operators and subsequently used commercially and by the military. By installing special software "shims" we have interfaced IP-based 32-bit Windows 95 applications such as E-mail clients and Worldwide Web (WWW) browsers with NOS and have sent E-mail and other TCP data over NVIS links using standard Windows 95 applications. This process is described in the note. Because of their inability to distinguish between contention and channel errors (to be discussed below), standard (RFC-1122-compliant) TCP/IP stacks like NOS are ill suited to highly efficient use over radio channels. In spite of this, performance over HF links that is acceptable in many data-communication applications can be achieved by adapting the stack's window- and segment-size parameters, and the HF modem's data rate and interleaver settings, to noise and propagation conditions on the channel. Our experience with this approach, which is meant to defeat standard TCP's unsuitable contention-control protocols, is also discussed in the note.