Open-source network emulators and simulators
The most important and most useful thing is the hardest to obtain: latency simulators.<p>I just want an IPv4/IPv6 proxy that does <i>nothing</i> other than delay, rate-limit, and/or drop packets.<p>This is a critical aspect of application performance that all developers just ignore because it's such a PITA to simulate.<p>Your loopback adapter has a latency of 30 microseconds, but that same web-to-database connection might traverse multiple switches, routers, firewalls, and load balancers and have a latency of over 1,000 microseconds (1ms) in production. That's <i>thirty times worse</i>, and I'm not even talking about zone-redundant deployments, regional-to-region failover, or any such thing!<p>Ditto with web apps, where every developer mistakenly believes that their Angular or React monstrosity is "performant"... because it is... on their own machine. With loopback networking. Try that over a 3G connection... in rural India. Not Indiana. India.