CallbackAddress information missing - help wanted

[IgorBalog-Eng]

UseCase provider posts offer right arrow:

• provider posts ContractOfferMessage to the consumer (https://consumer.com/negotiations/offers); message contains provider callback address https://provider.com/return_path/ for example)
• consumer accepts request, process it and returns ContractNegotiation.OFFERED
• consumer review offer, and for simplicity, accepts it
• consumer sends ContractNegotiationEventMessage.ACCEPTED to the provider (https://provider.com/return_path//negotiations/consumerPid_id/events), using provider callback address from offer request
• provider recieve event message, process it, and return 200 OK; contract negotiation status on both sides is ACCEPTED
• provider creates ContractAgreementMessage and sends it to the consumer https://consumer.com/:callback/negotiations/:consumerPid/agreement

Problem! how can provider know which is consumer callback address since this information is not send by the consumer in any of the previous steps?
Callback address can have that :callback part, which is unknow for the provider to be able to proceed with contract negotiation. Event message does not have callbackAddress so it can be used in following calls.
Provider cannot know what is :callback part so it can somehow construct agreement URL from first (ContractOfferMessage) request.

[arnoweiss]

I think it's the ContractRequestMessage (Consumer --> Provider) that the Provider must then append with the path elements for each endpoint.

ids-specification/negotiation/contract.negotiation.binding.https.md

Line 302 in 50f2a64

All callback paths are relative to the `callbackAddress` base URL specified in the [Contract Request Message](./contract.negotiation.protocol.md#21-contract-request-message) that initiated a CN. For example, if the `callbackAddress` is specified as `https://consumer.com/callback` and a callback path binding is `negotiations/:consumerPid/offers`, the resolved URL will be `https://consumer.com/callback/negotiations/:consumerPid/offers`.

The definition of callbackAddress fits the requirement pretty well, I suppose.

ids-specification/negotiation/contract.negotiation.protocol.md

Line 66 in 50f2a64

- The `callbackAddress` is a URL indicating where messages to the [Consumer](../model/terminology.md#consumer) should be sent in asynchronous settings. If the address is not understood, the [Provider](../model/terminology.md#provider) MUST return an UNRECOVERABLE error.

[IgorBalog-Eng]

Hello Arno,
here I'm considering second use-case, Provider initiating contract negotiation with posting offer to consumer, consumer accepts offer and sends event message. Next step is that Provider sends agreement message, but Provider does not have Consumer callback address, to append proper endpoint. Callback address was not sent by the Consumer, since event message does not have it.
Assuming that there is not "negotiation" to find better "deal", involving sending request message from Consumer to Provider (having Consumer callback address)

Problem that I have is that Provider does not have Consumer callback address to append valid context, so that Provider can continue with negotiation.