Prior measurement studies of the Internet have explored traffic and topology, but have largely ignored edge hosts. While the number of Internet hosts is very large, and many are hidden behind firewalls or in private address space, there is much to be learned from examining the population of visible hosts, those with public unicast addresses that respond to messages. In this paper we introduce two new approaches to explore the visible Internet. Applying statistical population sampling, we use censuses to walk the entire Internet address space, and surveys to probe frequently a fraction of that space. We then use these tools to evaluate address usage, where we find that only 3.6% of allocated addresses are actually occupied by visible hosts, and that occupancy is unevenly distributed, with a quarter of responsive /24 address blocks (subnets) less than 5% full, and only 9% of blocks more than half full. We show about 34 million addresses are very stable and visible to our probes (about 16% of responsive addresses), and we project from this up to 60 million stable Internet-accessible computers. The remainder of allocated addresses are used intermittently, with a median occupancy of 81 minutes. Finally, we show that many firewalls are visible, measuring significant diversity in the distribution of firewalled block size. To our knowledge, we are the first to take a census of edge hosts in the visible Internet since 1982, to evaluate the accuracy of active probing for address census and survey, and to quantify these aspects of the Internet.

TCP with Van Jacobson congestion control (VJCC) is known to have poor performance over large bandwidth-delay product paths. Long delay paths, in particular, can display very poor behavior with VJCC slowly probing to acquire available capacity. TCP performance enhancing proxies (PEPs) constitute one mechanism for ameliorating poor VJCC end-to-end performance by splitting TCP connections around a long-delay link or network, and using alternate congestion control dynamics across the troublesome portion. Previous congestion control techniques for use over satellite links have typically constituted either tweaks to the Van Jacobson algorithms, Vegas-style congestion control or disabling congestion control altogether in favor of a manual send-rate. This paper analyzes results from measurements of a new congestion control mechanism, the explicit Control protocol or XCP, between PEPs with a simulated geosynchronous satellite link in the path. We show that connections using an XCP PEP acquire their share of expensive satellite bandwidth up to 70 times faster than end-to-end TCP with VJCC.