On 1/3/21 4:22 PM, Mark Delany wrote:
Creating quiescent sockets has certainly been discussed in the context of RSS where you might want to server-notify a large number of long-held client connections very infrequently.
While a kernel could quiesce a TCP socket down to maybe 100 bytes or so (endpoint tuples, sequence numbers, window sizes and a few other odds and sods), a big residual cost is application state - in particular TLS state.
Even with a participating application, quiescing in-memory state to something less than, say, 1KB is probably hard but might be doable with a participating TLS library. If so, a million quiescent connections could conceivably be stashed in a coupla GB of memory. And of course if you're prepared to wear a disk read to recover quiescent state, your in-memory cost could be less than 100 bytes allowing many millions of quiescent connections per server.
Having said all that, as far as I understand it, none of the DNS-over-TCP systems imply centralization, that's just how a few applications have chosen to deploy. We deploy DOH to a private self-managed server pool which consume a measly 10-20 concurrent TCP sessions.
I was thinking more in the original context of this thread w.r.t. potential distribution of emergency alerts. That could, if semi-centralized, easily result in 100s of million connections to juggle across a single service just for the USA. While it presumably wouldn't be quite that centralized, it's a sizable problem to manage. Obviously you could distribute it out ala the CDN model that the content providers use, but then you're potentially devoting a sizable chunk of hardware resources at something that really doesn't otherwise require it. The nice thing is that such emergency alerts don't require confidentiality and can relatively easily bear in-band, application-level authentication (in fact, that seems preferable to only using session-level authentication). That means you could easily carry them over plain HTTP or similar which removes the TLS overhead you mention. Several GB of RAM is nothing for a modern server, of course. It sounds like you'd probably run into other scaling issues before you hit memory limitations needed to juggle legitimate TCP connection state. -- Brandon Martin