User:Justmoon/BIP Draft: Custom Services
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This page describes a BIP (Bitcoin Improvement Proposal). |
BIP: Draft Title: Custom Services Author: Stefan Thomas <justmoon@members.fsf.org> Status: Draft Type: Standards Track Created: 03-08-2012
Abstract
This BIP adds new fields to the version message which clients can use to announce custom services without polluting the limited 64-bit services field. It also makes some non-binding recommendations regarding the implementation of custom services.
Motivation
We would like to encourage experimentation with custom services that extend the Bitcoin protocol with useful functionality. Examples include Distributed Hash Tables (DHT), distributed pools, lightweight client support protocols, directed message routing and support for custom transports. However, without a general, extensible framework for protocol extensions, these custom services are likely to collide in various ways. This BIP provides such a framework.
Specification
Two new fields are added to the version command, after extra_height:
| Field Size | Description | Data type | Comments |
|---|---|---|---|
| 1+ | service_count | var_int | Number of extra services |
| ? | service_list | service[] | List of service definitions |
The service definitions are given in the following format:
| Field Size | Description | Data type | Comments |
|---|---|---|---|
| ? | service_name | var_str | Unique service identifier |
| 4 | service_version | uint32_t | Identifies service version being used by the node |
| ? | service_data | var_str | Additional service-specific data |
A node MUST NOT announce two services with the same service_name. If a remote node sends such a version message the client MAY disconnect.
The service_version is service-specific and can be any integer. Higher versions SHOULD be higher integers. When a service is standardized, it is assigned a NODE_* constant for use with the services field and future iterations of the protocol depend on the Bitcoin protocol version. Both the NODE_* flag and the custom service entry MAY be provided for the duration of a transitional period.
Services SHOULD pass an empty string (0x00) as service_data and use a custom handshake to initialize their protocol, exchange information about capabilities etc. Note that to become a standardized service, a service MUST NOT rely on service_data since there is no corresponding mechanism for the standard services defined in the services field.
However, services MAY use service_data if they do not intend to become standard services and need a simple way to transmit a small amount of initialization data. For example, a node offering a custom transport like UDP or WebSocket, may choose to announce this as a service and include the port number in service_data. The format for service_data is service-specific and may be any binary or ASCII data.
Service identifier
Each service SHOULD choose a new identifier that is not used by any other service.
Service identifiers that are reserved or used by an accepted BIP MUST NOT be used except in the way specified by that BIP.
Service identifiers MUST be between five (5) and eleven (11) characters long. Service identifiers MUST use only ASCII characters, excluding: / * _ :
Valid examples:
MySampleSvcsmartservP-Pool
Valid, but discouraged examples:
MySVC 1.0(useservice_versionto differentiate versions)@@---.(identifiers should be pronounceable)lightweight(avoid too generic names)
Invalid examples:
Pppc(too short)SuperService(too long)Cool_Svc(invalid character)
Optional: Service messages
All messages of a custom service SHOULD be represented by a single "command" on the Bitcoin network, consisting of an underscore, followed by the service identifier. For example: _MySampleSvc
To distinguish different messages within the service, the following format SHOULD be used:
| Field Size | Description | Data type | Comments |
|---|---|---|---|
| 12 | subcommand | char[12] | ASCII string identifying the service command, NULL padded (non-NULL padding results in packet rejected) |
| ? | subpayload | uchar[] | The actual data |
The length of subpayload is derived from the length of the total payload minus twelve (12) bytes for the subcommand. Implementations MUST NOT rely on this format to be used by unknown services. Clients SHOULD ignore any services or subcommands they don't explicitly understand.
The recommended way to refer to messages following this format in documentation is by the service identifier, followed by a colon, followed by the subcommand. For example, the subcommand search for the MySampleSvc service would be referred to as: MySampleSvc:search
Full hexdump of an example MySampleSvc:search message:
0000 F9 BE B4 D9 5F 4D 79 53 61 6D 70 6C 65 53 76 63 ...._MySampleSvc
0010 14 00 00 00 73 D5 56 77 73 65 61 72 63 68 00 00 ....s.Vwsearch..
0020 00 00 00 00 12 34 56 78 9A BC DE F0 .....4Vx....
Message header:
F9 BE B4 D9 - Main network magic bytes
5F 4D 79 53 61 6D 70 6C 65 53 76 63 - "_MySampleSvc" command
14 00 00 00 - Payload is 20 bytes long
(includes 12 bytes for subcommand)
73 D5 56 77 - Checksum
Service header:
73 65 61 72 63 68 00 00 00 00 00 00 - "search" subcommand
Search message:
12 34 56 78 9A BC DE F0 - Payload
Optional: Announcing known nodes
Bitcoin includes the services for each node in the addr message. In order to provide the same functionality, a custom service X MAY define a message X:addr with the following format:
- If
X:addris empty or starts with TYPE_ALL (0x00), then all addresses in the nextaddrmessage support the service. - If
X:addrstarts with TYPE_BITS (0x01), then the rest of the data is a bit array marking which nodes in the next addr message support the service. Any nodes not covered by the bitarray don't support the service. - If
X:addrstarts with TYPE_RLE (0x02), then the next byte determines whether the first address supports the service and the remaining data is a series of var_ints determining the lengths of runs of nodes that support or don't support the service, not counting the first node in the run. Any nodes not covered by the RLE have the opposite value of the last node covered. - If there was no
X:addrmessage since the lastaddrmessage or the beginning of the connection, then none of the nodes in the nextaddrmessage support the service.
Examples:
00 - TYPE_ALL - all nodes in the next addr message support the service
01 - TYPE_BITS
FC - 11111100 - First six nodes support the service, next two don't
1F - 00011111 - Next three don't, next five do
- Remaining nodes don't
02 - TYPE_RLE
01 - First set of nodes are nodes that support the service
05 - First six nodes support the service
FD 42 03 - Next 835 nodes don't
00 - Next node does
08 - Next nine nodes don't
- Remaining nodes do
Multiple X:addr messages for different services MUST all be applied to the next addr message. If there are multiple X:addr messages for the same service, only the most recent one is used.
If a node knows about many different services, the recommendation is to store a bitmask of all the recognized services for each node, order by that field and then use X:addr messages using TYPE_RLE.
Rationale
This BIP aims to fulfill the following goals:
- Minimize the risk of namespace collisions, ambiguities or other issues arising from conflicting custom services
- Provide an easy upgrade path for custom services to become standardized services with their own
NODE_*flag - Place minimum restrictions on custom service authors
- Allow custom services to be created with minimum effort
- Allow clients to support multiple/many custom services at once
To achieve these goals this BIP adds two new fields to the version message. It would have been possible to avoid changes to version by adding a new message instead. However, it makes sense to keep both types of service announcements in the same message so that the life cycle of standardized services and custom services remains exactly the same. This also simplifies detecting a service which is in the transition from a custom to a standardized service (and being announced using both methods.)
While writing this BIP it became apparent that twelve characters were not sufficient to contain both the service identifier and the subcommand. This lead to the definition of a recommended encoding for service-specific messages. Services where this encoding is not appropriate are able to supercede it.
For announcing other nodes supporting the service, it would have been possible to extend the addr message, however some services may not require this type of announcement whereas other services may wish to include additional information per node. Therefore we provide a recommended scheme that implements efficiently the same functionality provided by the services bitmask. Services for which this scheme is not appropriate can supercede these recommendations as needed.
Finally, this BIP defines both explicitly and implicitly some useful common nomenclature that can be used when discussing custom services, e.g. "subcommand", "subpayload", "service identifier" and the colon format for referring to subcommands.
Copyright
This document is placed in the public domain.