diego's weblog

there and back again

too bad phone number records don’t have a TTL

It’s been almost a day since I ported my phone number from AT&T to Google Voice, and I still can’t receive SMS or international calls. Mind you, the porting process explicitly stated that SMS could take up to three days to transfer over, but while I was doing it I didn’t pay much attention to that.

Yesterday, when someone was trying to call me from abroad and got “this number has been disconnected” as a response, the obvious struck me: phone numbers, like DNS, clearly has a set of local caches at your local endpoint that know who is responsible for that phone number and lets it figure out the rounting. When the owner of the record changes the changes must propagate world-wide, and this takes time. Caching policies must vary between providers, and given how rarely phone numbers change, it’s reasonable that the cache would be fairly long-lived. SMS and international calls must both use the same name-resolution (or number-resolution) routing system.

As I said, fairly obvious, but it’s one of those things that we rarely think about. I’ve been searching online for a bit on information on these systems but can’t seem to find any. I’m curious about the protocol they use, and how much like (or unlike) DNS it may be. My bet is that with the benefit of hindsight it’s an arcane, bizarre system, evolved painfully and slowly over a long time that everyone would like to change, but can’t. Oh, wait, that is DNS.

Either that, or there’s a giant building somewhere with rows of filing cabinets disappearing into the distance, with grim-looking people pushing carts full of punchcards that have to be moved from one filing cabinet to another. I’ll keep looking and see which one it is.

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