Ripple Inter Server Protocol

Allows interaction between nodes distributed on many servers.

Ripple Protocol Design Technical Documents

Discussions and designs for a protocol by which independent Ripple servers could communicate to route payments between them.

JSON messages over HTTP.

Protocol 0.6 - Draft of Ripple core transaction protocol v0.6.
Exchange Publishing Routing Method - Extension for routing by exchange publishing.
Registry Commit Method - Extension for transaction atomicity.
Bare Commit Method - Extension for more decentralized, but not completely atomic commit.
BlockChain Commit Method - Extension for transaction atomicity using a proof of work blockchain as the registry.

Custom binary message transport with messages encoded as Protocol Buffers.

Definitions - What's a node?
Identity - How do people and hosts address each other?
Knowledge Routing - Give each peer enough information to compute routes itself.
Messages - Description of protocol messages
Commit Block Chain - Improve transaction atomicity by creating a block chain of commit messages.
Building Cells By Block Chain - Improve privacy and scalability by letting well-connected groups of peer endpoints self-organize into cells whose internal connections are then abstracted away from the greater network.
Tunneling - Improve anonymity and privacy with secure communication tunnels to other endpoints.
Proxies - Proxy transactions through other endpoints, obscuring the true source or destination of transactions.
Solution Stack - All the other bits and pieces that form an overall distributed payments and marketplace product from a user perspective
Distributed Architectures - Different ways of organizing a Ripple network.
Commit scripts - A proposal for substituting registries and enable new features.

You can find an animation of a basic transaction here ( .odp open office presentation)

These are meant to be refined during the process of building a reference implementation.