LSPS0: Transport layer protocol description.

LSPS0/README.md

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+# LSPS0 Transport Layer
+
+| Name    | `base_api`       |
+| ------- | ---------------- |
+| Version | 1                |
+
+## Motivation
+
+The transport layer describes how a client is able to request
+services from the LSP.
+
+### Actors
+
+'LSP' is the API provider, and acts as the server.
+'Client' is the API consumer.
+
+## Protocol
+
+### Lightning Peer-To-Peer Transport
+
+The Lightning Network [BOLT8][] specification describes the transport
+layer for Lightning Network peer-to-peer messages.
+
+[BOLT8]: https://github.com/lightning/bolts/blob/f7dcc32694b8cd4f3a1768b904f58cb177168f29/08-transport.md
+
+Lightning Network BOLT8 describes messages as having two components:
+
+* A 16-bit message ID (encoded in big-endian).
+* A variable-length message payload (the remainder of the message).
+
+LSPS messages and access to LSP endpoints is via BOLT8 messages.
+
+> **Rationale** All clients and LSPs are expected to be Lightning
+> Network participants, and need to somehow talk using the BOLT8
+> protocol.
+> Thus, this is not an additional dependency for any client and
+> any LSP, the way an HTTP(S) or gRPC protocol would pull in
+> additional dependencies.
+
+For LSPS0, the message ID SHALL be 37913 (`lsps0_message_id`).
+All LSPS messages use only this message ID.
+
+> **Rationale** We indicate a single message ID as this reduces
+> the "footprint" of all LSPS specifications to only that single
+> message ID, increasing the probability that other protocols
+> using the Lightning BOLT8 peer-to-peer transport will not
+> conflict with the LSPS specifications.
+> The message ID 37913 is odd in order to comply with the "it's
+> OK to be odd" rule, and is in the 32768 to 65535 range reserved
+> for extensions of the BOLT protocol.
+> Some implementations, such as Core Lightning, only expose
+> odd-numbered messages to custom message handlers.
+> The message ID was otherwise selected by random and has no
+> special meaning.
+
+The payload contains the UTF-8 encoding of a complete JSON
+object.
+
+If a client or LSP receives a BOLT8 message with message ID
+37913:
+
+* If the payload does not parse as a complete JSON object
+  (i.e. a JSON key-value store or dictionary), MUST disconnect
+  from the peer and ignore the message.
+* If the payload parses as more than a single complete JSON
+  object, MUST disconnect from the peer and ignore the message.
+* If any byte in the payload is 0, MUST disconnect from the
+  peer and ignore the message.
+* MUST NOT expect the payload to be terminated by a NUL
+  character (0 byte).
+
+> **Rationale** Requiring a single complete JSON object
+> simplifies handling of messages, so that a single message
+> maps to a single request or response.
+> C uses the NUL character as a string terminator and thus
+> embedded 0 bytes may cause problems in implementations
+> that pass the JSON string to C code.
+> Conversely, we do not require the payload to be terminated
+> by a 0 byte / NUL character as it is unnecessary in many
+> modern non-C-based languages; C code can copy the buffer
+> and append the NUL character if necessary, as the payload
+> size is known.
+
+The JSON object embedded in the message payload is defined
+by the [JSON-RPC 2.0][] protocol.
+
+[JSON-RPC 2.0]: https://www.jsonrpc.org/specification
+
+The LSP acts as a JSON-RPC 2.0 server, while the client acts
+as a JSON-RPC 2.0 client.
+
+> **Rationale** JSON is a simple format that is easy to
+> extend and is extensively supported.
+> The Lightning Network peer-to-peer transport protocol in
+> BOLT8 is not inherently a request-response protocol like
+> Lightning or gRPC, and JSON-RPC 2.0 describes a
+> JSON-based protocol that builds a request-response
+> protocol from a simple tunnel, with message ID 37913
+> acting as that tunnel.
+> JSON-RPC 2.0 is simple to implement.
+> Although BOLT8 messages are limited to payloads of 65533
+> bytes, it is expected that both requests and responses
+> would be far below that limit, and thus the ability to
+> "cut" a large object across multiple messages, the
+> use of a compression algorithm, and the use of a binary
+> format instead of JSON, are all considered unnecessary.
+
+The client:
+
+* MUST send single complete JSON-RPC 2.0 request objects,
+  UTF-8-encoded, as payloads for message ID 37913.
+* MUST NOT send JSON-RPC 2.0 notification objects
+  (i.e. every object it sends must have an `id` field).
+* MUST NOT use by-position parameter structures, and MUST
+  use by-name parameter structures.
+* MUST NOT batch multiple JSON-RPC 2.0 request objects in
+  an array as described in the "Batch" section of the
+  JSON-RPC 2.0 specification.
+
+> **Rationale** By disallowing by-position parameter
+> structures, other LSPS specifications need only to define
+> parameter names and not some canonical order of parameters
+> for by-position use.
+> Having to handle only by-name parameter structures also
+> simplifies the LSP code, as it does not have to check
+> whether the `params` value is an array or a dictionary,
+> and to separately map array elements to `params` keys.
+> Batched requests require batched responses according to
+> JSON-RPC 2.0, and it may be simpler for LSPs to handle
+> unrelated LSPS methods separately without requiring
+> re-batching of the responses; it gives a simple "one
+> message is one request / response" rule.
+
+The LSP:
+
+* MUST send single complete JSON-RPC 2.0 response objects,
+  UTF-8-encoded, as payloads for message ID 37913.
+* MUST NOT send JSON-RPC 2.0 notification objects
+  (i.e. every object it sends must have an `id` field).
+* MUST respond with an error -32602 if a request `params` field
+  is not recognized for the given `method`.
+* MUST NOT send message ID 37913 unless the peer first sent a
+  message ID 37913.
+
+> **Rationale** The peer sending message ID 37913 is an
+> indicator that it understands the LSPS0 protocol and wishes
+> to act as a client.
+> If the peer does not send it, then it is not an LSPS client
+> and the LSP has no reason to send message ID 37913.
+
+JSON-RPC 2.0 is an asynchronous protocol, and the order
+in which the LSP sends responses MAY be different from the
+order in which the client sent the requests.
+
+Other LSPS specifications MUST indicate `method` names for
+each API endpoint, as well as the key names of the `params`
+for each `method`, and the meanings of each parameter.
+Other LSPS specifications MUST also describe the possible
+error codes for each API endpoint.
+
+Other LSPS specifications SHOULD define `response` values
+for its API endpoints that are objects (dictionaries), and
+MUST define each key.
+
+LSPs MAY return additional keys in the `response` values
+that are not defined in the relevant LSPS specification.
+Clients conversely MUST ignore unrecognized keys.
+
+#### Error Handling
+
+JSON-RPC 2.0 `error`s include a `message` field which is a
+human-readable error message.
+
+Clients MUST carefully filter error messages of possibly
+problematic characters, such as NUL characters, `<`, newlines,
+or ASCII control characters, which may cause problems when
+displaying in a dynamic context such as an HTML webpage, a
+TTY, or C string processing functions.
+
+Clients SHOULD NOT directly expose error `message`s to users
+unless users have enabled some kind of "advanced" mode or
+"developer" mode.
+Clients MAY write them in a log that is accessible to "advanced"
+or "developer" mode users.
+
+### Lightning Feature Bit
+
+The [BOLT7][] specification describes the `node_announcement`
+gossip message, which includes a `features` bit field.
+The [BOLT1][] specification describes the `init` message,
+which also includes a `features` bit field.
+
+[BOLT7]: https://github.com/lightning/bolts/blob/f7dcc32694b8cd4f3a1768b904f58cb177168f29/07-routing-gossip.md
+[BOLT1]: https://github.com/lightning/bolts/blob/f7dcc32694b8cd4f3a1768b904f58cb177168f29/01-messaging.md
+
+LSPs MUST set the `features` bit numbered 729
+(`option_lsps0_lsp`) in both the `init` message on connection
+establishment, and in their own advertised `node_announcement`.
+Clients MUST NOT set `features` bit numbered 729 in either
+context.
+
+> **Rationale** As clients are the one who must initiate any
+> message ID 37913 messages before LSPs respond, they need to
+> separately discover whether a peer they connect to is
+> indeed an LSPS LSP that recognizes that message.
+> Broadcasting this as part of gossip allows a client to
+> discover new LSPs by simply downloading gossip.
+> The bit 729 was chosen by random and has no special meaning.
+
+### LSPS Specification Support Query
+
+The client can determine if an LSP supports a particular LSPS
+specification other than LSPS0 via the `method` named
+`listprotocols`, which has the following parameters:
+
+```JSON
+{
+  "comment": "This comment is ignored"
+}
+```
+
+`comment` is an optional string, and is ignored by the LSP.
+It defaults to the empty string.
+
+> **Rationale** In addition to allowing clients to determine what
+> protocols an LSP supports, this query is also an example of what
+> other LSPS specifications need to describe, such as the parameters
+> of an API endpoint, the type of each parameter, whether each
+> parameter is optional, the default value if the parameter is
+> optional but not specified by the client, and what each parameter
+> means for the LSP.
+
+The response datum is an object like the below:
+
+```JSON
+{
+  "protocols": [1, 3]
+}
+```
+
+`protocols` is an array of numbers, indicating the LSPS specification
+number for the LSPS specification the LSP supports.
+LSPs do not advertise LSPS0 support and 0 MUST NOT appear in the
+`protocols` array.
+
+> **Rationale** LSPS0 support is advertised via `features` bit 729
+> already, so specifying `0` here is redundant.
+
+> **Non-normative** The example below would not be necessary for other
+> LSPS specifications, but gives an idea of how the JSON-RPC 2.0
+> protocol would look like, when using this API endpoint.
+
+As a concrete example, a client might send the JSON object below
+inside a message ID 37913 in order to query what LSPS protocols
+the LSP supports:
+
+```JSON
+{
+  "method": "listprotocols",
+  "jsonrpc": "2.0",
+  "id": "example00",
+  "params": {}
+}
+```
+
+The LSP could then respond with a message ID 37913 with the following
+payload, indicating it supports LSPS1 and LSPS3 (in addition to LSPS0):
+
+```JSON
+{
+  "jsonrpc": "2.0",
+  "id": "example00",
+  "result": {
+    "protocols": [1, 3],
+    "example-undefined-key-that-clients-should-ignore": true
+  }
+}
+```