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High Level Design decisions for Business Network (#369)

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* ENT-1331: Placeholder for design decisions previously discussed.

* ENT-1331: Describe TLS approach.

* ENT-1331: Describe Membership approach.

* ENT-1331: Describe Node vs. CordApp membership approaches.

* ENT-1331: Including remarks/corrections from @josecoll

* ENT-1331: Including remarks/corrections from @josecoll, part 2
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![Corda](https://www.corda.net/wp-content/uploads/2016/11/fg005_corda_b.png)
--------------------------------------------
Design Decision: Business Network Membership control: Node level or CorDapp level?
============================================
## Background / Context
During discussion of [Business Networks](../design.md) document multiple people voiced concerns
about Business Network membership for different CorDapps deployed on the Corda Node.
## Options Analysis
### 1. One set of Business Networks for the whole Node
The idea is that a Node has knowledge of what Business Networks it is a member of. E.g. a Node will be notified by one or
many BN operator node(s) of it's membership. Configurability and management of Membership is the responsibility of the
BN Operator node with updates are pushed to member Nodes.
In other words, Business Network membership is enforced on the Node level
and **all** the CorDapps installed on a node can communicate with **all** the Business Networks node been included into.
#### Advantages
1. Business Network remote communication API will become node level API and it will be a single API to use;
2. The change in Business Network composition will be quickly propagated to all the peer nodes via push mechanism.
#### Disadvantages
1. A set of CorDapps may need to be split and hosted by multiple Corda Nodes. A member will need to run a separate
Corda Node for every Business Network it wants to participate in;
Deployment of a node may be a big deal as it requires new X.500 name, Node registration through
Doorman, separate production process monitoring, etc.
2. BNO node will have to know about Corda member nodes to push Business Network information to them. Not only this
requires a uniform remote API that every node will have to support, but also member nodes' IP addresses
(which may change from time to time) should be known to BNO node. This might be problematic as with maximum privacy
enforced member node may not be listed on the NetworkMap.
### 2. Allow CorDapps to specify Business Network name
The idea is that every CorDapp will be able to specify which Business Network(s) it can work with.
Upon Corda Node start-up (or CorDapp re-load) CorDapps will be inspected to establish super-set of Business Networks
for the current Node run.
After that a call will be made to each of the BNO nodes to inform about Node's IP address such that the node can be
communicated with.
#### Advantages
1. Flexibility for different CorDapps to work with multiple Business Network(s) which do not have to be aligned;
2. No need for multiple Nodes - a single Node may host many CorDapps which belong to many Business Networks.
#### Disadvantages
1. Difficult to know upfront which set of Business Networks a Corda Node is going to connect to.
It is entirely dependant on which CorDapps are installed on the Node.
This can be mitigated by explicitly white-listing Business Networks in Node configuration, such that only intersection
of Business Network name set obtained from CorDapps and Node configuration will be the resulting set of Business Networks
a Node can connect to.
## Recommendation and justification
This is a key decision for Technical Design and will be discussed during next Business Network working group meeting.

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![Corda](https://www.corda.net/wp-content/uploads/2016/11/fg005_corda_b.png)
--------------------------------------------
Design Decision: Using TLS signing vs. Membership lists for Business Network composition.
============================================
## Background / Context
As per High Level Design document for [Business Networks](../design.md), there has to be a mechanism established
for composition of the Business Network.
## Options Analysis
### 1. Use Transport Layer Security (TLS) signing
The idea is to employ Public/Private keys mechanism and certification path to be able to prove that certain
member belongs to a Business Network.
Simplified approach can be as follows:
1. NodeA wants to perform communication with NodeB on the assumption that they both belong to the same
Business Network (BN1);
2. During initial handshake each node presents a certificate signed by BNO node confirming that given
node indeed belongs to the said BN1;
3. Nodes cross-check the certificates to ensure that signature is indeed valid.
#### Advantages
1. Complete de-centralization.
Even if BNO node is down, as long as it is public key known - signature can be verified.
2. Approach can scale to a great majority of the nodes.
#### Disadvantages
1. Revocation of membership becomes problematic;
This could be mitigated by introducing some form of a "blacklist" or by issuing certificates with expiration. But this will
add pressure on BNO node to be more available in order to be able to re-new certificates.
2. Privacy requirement will not be achieved.
Both NodeA and NodeB will have to advertise themselves on the global Network Map, which might be undesired.
3. Cannot produce a list of BN participants;
Since BN participation is established after certificate is checked, it is not quite possible to establish
composition of the Business Network without talking to **each** node in the whole universe of the Compatibility Zone (CZ).
This has been discussed with Mike Hearn in great details on [this PR](https://github.com/corda/enterprise/pull/101#pullrequestreview-77476717).
### 2. Make BNO node maintain membership list for Business Network
The idea is that BNO node will hold a "golden" copy of Business Network membership list and will vend
content to the parties who are entitled to know BN composition.
That said, if an outsider makes an enquiry about composition of Business Network, such request is likely
to be rejected.
#### Advantages
1. Satisfies all the requirements know so far for Business Network functionality, including:
* Joining/leaving Business Network;
* Privacy;
Privacy is enforced by BNO node that is only going to vend membership information to the parties that need to know.
Also member node no longer has to register with global NetworkMap and may register with BNO instead.
* Ability to discover Business Network peers;
* BNO owner has ultimate control as for how membership information is stored (e.g. DB or CSV file).
#### Disadvantages
1. BNO node gains a critical role and must be highly available for flows to work within Business Network.
2. When the Business Network will be expanding BNO node need to be reasonably performant to cope with the load.
This can be mitigated by holding local caches of Business Network membership on the Node side to make requests
to BNO node less frequent.
3. There is no pub-sub facility which would allow a member node to learn about new nodes joining Business Network.
But at all times it is possible to approach BNO node and download complete list of current Business Network members.
## Recommendation and justification
Proceed with Option 2 after discussion with Mike Hearn and Richard Brown on [this PR](https://github.com/corda/enterprise/pull/101).
The PR was about having Proof of Concept implementation for Business Networks to demonstrate how it might work.

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## Design Decisions
| Description | Recommendation | Approval |
| ---------------------------------------- | --------------- | ----------------------- |
| [TLS vs. Membership](decisions/tlsVsMembership.md) | Proceed with Membership | Mike Hearn |
| [BN membership: Node vs. CorDapp](decisions/nodeVsCorDapp.md) | TBD | TBD |
* Per [New Network Map](https://r3-cev.atlassian.net/wiki/spaces/AWG/pages/127710793/New+Network+Map):
* R3 will serve a file containing NetworkParameters to all members of the Corda Connect Compatibility Zone, made publicly available via CDN.
*NetworkParameters are only served by the network map and cannot be overwritten by BNOs*.