Van Jacobson is among the most influential figures in the practical engineering of the Internet’s transport layer. Working at Lawrence Berkeley Laboratory in the 1980s, he tackled the congestion collapses that were degrading the young network. With Michael Karels of the University of California at Berkeley, he produced the 1988 SIGCOMM paper “Congestion Avoidance and Control,” which introduced slow start, congestion avoidance, and the additive-increase, multiplicative-decrease behavior that still governs how TCP shares network capacity. The paper is one of the most cited in all of networking.
Jacobson’s contributions were not only theoretical. He wrote and contributed to a suite of diagnostic tools that became standard equipment for anyone operating an IP network. The LBNL Network Research Group’s own page lists “traceroute for printing the route packets take to a network host” and “tcpdump, the protocol packet capture and dumper program” among the software it distributed. Traceroute, which exploits the IP time-to-live field to map the hops between two hosts, and tcpdump, which captures and decodes packets off the wire, remain in daily use by network engineers worldwide.
He also worked on tools for measuring the Internet itself, including pathchar, listed by the LBNL group as a program “for inferring the characteristics of Internet paths” such as per-hop bandwidth and latency. This line of work reflected a consistent theme in his career: building instruments that let engineers see what packets were actually doing, rather than guessing from theory. Much of the BSD networking code’s robust real-world behavior traces back to the careful measurement and tuning that tools like these enabled.
After his years at Berkeley, Jacobson worked at Cisco Systems and at PARC, where his attention turned to a more radical idea. The LBNL group page later notes that he moved on to PARC, where he became a central figure in Content-Centric Networking, also known as Named Data Networking. That work proposed addressing and routing data by name rather than by host location, a rethinking of the Internet’s basic addressing model intended to better fit how the network is actually used for distributing content.
Jacobson’s recognition includes election to the National Academy of Engineering and a place in the Internet Hall of Fame. His enduring importance, though, is best measured by how invisible his work has become: every TCP connection that quietly finds its fair share of a congested link, and every network operator who reaches for traceroute or tcpdump to diagnose a problem, is using tools and ideas that he built.