Radia, which by the way is pronounced to sound like "radio" but with an "a" at the end, is brilliant, quirky, funny, and a terrific teacher. She radiates intelligence, humor, mother-earth friendliness, and strength. I first met her in 2002 when I took her USENIX course on network security protocols. I had admired her for many years before that, though. Like most networking people of a certain age, I cut my teeth on her terrific book, Interconnections: Bridges, Routers, Switches, and Internetworking Protocols.
Radia invented mechanisms that make modern link-state routing protocols efficient and robust. Her thesis on routing in the presence of malicious failures remains the most important work in routing security. She has also made contributions in such areas as strong password protocols, analysis and redesign of IPsec's IKE protocols, PKI models, efficient certificate revocation, and distributed authorization. She recently was granted a patent for her work on ephemeral decryption which can make files reliably disappear. (Oh, I wish I could use that to make some emails I've sent disappear! :-)
Radia is the inventor of the spanning tree algorithm used by network bridges (switches). The photo above shows her posing as a spanning tree. Many computer scientists have worked on spanning tree algorithms, but Radia's work is noted for having a long life outside the academic lab. Her spanning-tree algorithm is at the heart of most enterprise networks.
Bridges (and the spanning tree protocol) were invented to deal with endnodes that run applications directly above Ethernet, without the benefit of a "Layer 3" protocol (e.g., IP). Bridges were a method of moving Ethernet frames around without a Layer 3 header. According to a recent email discussion I had with Radia, she assumed once everyone implemented Layer 3, bridges would go away. This probably would have happened if the world had adopted ISO's Layer 3, the Connectionless Network Layer Protocol (CLNP), Radia said. But the world adopted IP.
CLNP supported a prefix that an entire corporate network shared. Nodes could move around within the corporate network and keep their Layer 3 address. In contrast, with IP, every link requires its own prefix, which means routers have to be configured, addresses have to change if a node moves, and addresses get wasted. Once the world adopted IP, network administrators tended to avoid the extra configuration required with IP by using bridges (switches). The spanning tree algorithm lets administrators create large Ethernet bridged networks with redundant links that take over upon failure of the links chosen to be part of the spanning tree.
The spanning-tree algorithm has many shortcomings. It doesn't use optimal paths, doesn't allow splitting network traffic across multiple paths, and is lacking many other features of Layer 3 routing protocols. It is also intrinsically fragile because the Ethernet header doesn't contain a Time to Live (TTL) or hop count field, so any loop that forms can be devastating. Frames loops forever and bridges flood broadcasts over and over again. Something as simple as missed spanning-tree protocol messages can result in these deadly loops.
Radia is currently working on a new type of device that has the ease of management of bridging, but the robustness and features of routers. This is being standardized in the Transparent Interconnection of Lots of Links (TRILL) working group of the IETF. The TRILL working group is designing a solution for shortest-path frame forwarding in multihop Ethernet networks with arbitrary topologies, using Radia's work on link-state routing protocols. Radia isn't afraid to say it's time to retire her famous spanning-tree protocol.
One of the reasons I admire Radia is because she has a knack for remaining friendly while speaking controversial or unwelcome truths. In 2005, the Anita Borg Institute for Women and Technology honored Radia with a Women of Vision award. I highly recommend listening to her keynote speech which is available here. She recently told me that some of the things she said in this speech make her cringe, but I think her thoughts on the topic of women in technology are awesome, just like I think she's awesome. Well, now I've probably made her cringe again. Oh well!