[Bitcoin-development] Payment Protocol Proposal: Invoices/Payments/Receipts

[Mike Hearn]

> So, if a bitcoin client is getting Invoice messages via email or from
> a web server, the version will be specified as part of the MIME type;
> for example:
>    Content-Type: application/x-bitcoin-invoice; version=1
> The version= syntax is part of the MIME standard.

I think that's OK. However, you should only be getting the version you
expect because when you request an invoice, your client should be
telling the merchant what protocol version you implement.

Does it make sense to have this spec not include the details of
bootstrapping? It's not complicated - we extend the URI spec in a
backwards compatible way:

   bitcoin:1AbCdEfG?value=10.0&label=Pay%20for%20Foo&invoice=https://merchant.com/inv/aB425az

When a compatible client sees the invoice param, it ignores the rest
of the URI and downloads the URL
https://merchant.com/inv/aB425az?ver=1.0

A server on merchant.com sees that the client expects a version 1.0
invoice and vends it. If ver=2.0 or whatever, it knows it can use 2.0
features. If extensions are supported, add new query params.

We should define a simple mechanism for extending the protocol now, so
people who want to make proprietary extensions don't conflict. The
simplest is to just say, if you want to add new fields to an Invoice
message, please update a wiki page with the tag numbers you're going
to use, and start from number X. Protobufs have a simple way to
formalize this in the language:

   https://developers.google.com/protocol-buffers/docs/proto#extensions

message Invoice {
  extensions 1000 to max;
}

The point of this is to allow you to define new parts of the messages
in separate .proto files. It's only a minor convenience but it means
if you want to use, say, two extensions that weren't yet folded into
the main spec, you can more easily do so without having to do a manual
merge of the message definitions together.

For instance, if you wanted to extend the protocol to support
specification of recurring billing, you could make a file called
recurring-invoices.proto containing:

message Recurrences {
  required uint32 every_seconds = 1;
  optional uint32 start_time = 2;
}

extend Invoice {
  optional Recurrences recurrences = 1005;
}

then you update the wiki page to claim tag number 1005 and apps can
easily use your new features. If/when the feature gets standardized
via a BIP, the core .proto definition can be extended to include these
messages and the extensions can go away.