Well, it is a core part of TCP. It's also part of http in a lot of ways. Browsers can make N simultaneous connections to a server. If those requests get queued

Well, it is a core part of TCP. It's also part of http in a lot of ways. Browsers can make N simultaneous connections to a server. If those requests get queued up on the server side, then the client won't proceed with a new request until one finishes. The pattern I see all to frequently is asynchronous worker queues. Those can very easily undermine backpressure. The rise of Ruby/Python and their limited concurrency models has placed a taboo on synchronous operations. However, synchronicity natural lends itself to backpressure.