For what it's worth, there was consideration of replacing protocol buffers when modifying BIP70 to function with the altcoin I work on (changes were required anyway in eliminate any risk that payment requests could not be accidentally applied to the wrong blockchain). The eventual conclusion was that while we might have used JSON or XML if we were starting from scratch, there's no choice that's clearly better. While deployed infrastructure for payment protocol is still quite limited, it seems that the cost to replace at this point is higher than not.

If there's ever a major reworking of the standard, for example to handle recurring payments, it's probably worth thinking about then, but protocol buffers result in a compact data format which is supported by most major languages (and size is a concern if dealing with Bluetooth or NFC), and has no major drawbacks I am aware of.

Ross

On 19/01/2015 20:40, Mike Hearn wrote:

I'm a bit confused.  It's been a long time since I looked at protobuf (and
will have to dig into it soon), but I seem to recall it doesn't have any of
the determinism properties you guys just said.

It's not guaranteed no, which is why we store signed sub-messages as byte
arrays instead of typed submessages. In practice though, most
implementations do seem to serialise things the same way. I recall Python
used to be an odd one out, unsure if it still is.

OK, I guess we can boil this down more simply. BIP 70 uses protocol buffers
because I designed it and implemented the original prototype (with lots of
input from Gavin and an earlier proposal by sipa). I used protocol buffers
because, beyond all their nice properties, I used to work at Google and so
was very familiar with them.

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