enum PaymentFrequencyType {

        WEEKLY = 1;

        MONTHLY = 2;

        QUARTERLY = 3;

        ANNUAL = 4;

}
message RecurringPaymentDetails {
        // Namespace for the merchant such as org.foo.bar

        required string merchant_id = 1;                 
        // Id for the recurring subscription
        required bytes subscription_id = 2;              
        // Contracts associated with a given subscription

        repeated RecurringPaymentContract contracts = 3; 
}
message RecurringPaymentContract {
        // Unique id for a given contract

        required bytes contract_id = 1;                           
        // URL to poll to get the next PaymentRequest

        required string polling_url = 2;                          
        // Timestamp; when this contract starts
        required uint64 starts = 3;                               
        // Timestamp; when this contract should be considered invalid 

        optional uint64 ends = 4;                                 
        // Expected payment frequency
        optional PaymentFrequencyType payment_frequency_type = 5; 
        // Max payment amount within that frequency (e.g. no more than 5 BTC per month)

        optional uint64 max_payment_per_period  = 6;              
        // Max payment amount (e.g. no more than 3 BTC per payment)

        optional uint64 max_payment_amount = 7;                   
}

1. There's no need to serialize RecurringPaymentDetails as bytes here. It's done that way outside of PaymentDetails in order to support digital signatures over protobufs that may have extensions the wallet app isn't aware of, but it's a pain and inside PaymentDetails (and therefore for most extensions) it shouldn't be necessary. So you can just use "optional RecurringPamentDetails recurring_payments = 8;"
   
   
2. There's only 4 possibilities here for recurrences. That seems rather restrictive. Is the cost of being more expressive really so high? Why not allow more flexible specification of periods?
   
   
3. If there's no payment_frequency_type field then what happens? A quirk of protobufs to be aware of is that making an enum field "required" can hurt backwards compatibility. Because it will be expressed using a languages underlying enum type, if there's a new enum member added later old software that attempts to deserialize this will throw exceptions because the new "unknown" member would be unrepresentable in the old model. Making the field optional avoids this problem (it will be treated as missing instead) but means software needs to be written to know what to do when it can't read the enum value / sees enum values from the future.
   
   
4. I assume the amounts are specified in terms of satoshi, and timestamps are UNIX time, but better to make that explicit.
   
   
5. Seems there's an implicit value constraint that max_payment_amount <= max_payment_per_period. What happens if that constraint is violated? Best to document that.
   
   
6. What's the "merchant ID" namespace thing about? What's it for? What happens if I set my competitors merchant ID there?
   
   
7. What's the "subscription ID"? Is this stuff not duplicative/redundant with the existing merchant_data field?
   
   
8. In what situations would you have >1 contract per payment request? I'm not sure I understand why it's repeated. Presumably if there are zero contracts included the data should be ignored, or an error thrown and the entire payment request rejected? Which should it be?
   
   
9. It's unclear to me given such a contract when the payment should actually occur. For instance if it's "monthly" then what day in the month would the payment occur?
   
   
10. You'll notice I moved the comments to be above the field definitions. I know the current proto isn't done that way, but let's change it - long comments are good and putting them above the field definitions encourages people to write enough detail without being put off by line length constraints