11 KiB
BOLT #1: Base Protocol
Overview
This protocol assumes an underlying authenticated and ordered transport mechanism that takes care of framing individual messages. BOLT #8 specifies the canonical transport layer used in Lightning, though it can be replaced by any transport that fulfills the above guarantees.
The default TCP port is 9735. This corresponds to hexadecimal 0x2607, the unicode code point for LIGHTNING.1
All data fields are big-endian unless otherwise specified.
Table of Contents
- Connection handling and multiplexing
- Lightning Message Format
- Setup Messages
- Control Messages
- Acknowledgements
- References
- Authors
Connection handling and multiplexing
Implementations MUST use one connection per peer, channel messages (which include a channel id) being multiplexed over this single connection.
Lightning Message Format
After decryption, all lightning messages are of the form:
type: 2 byte big-endian field indicating the type of the message.payload: variable length payload. It comprises the remainder of the message and conforms to the format matching thetype.
The type field indicates how to interpret the payload field.
The format for each individual type is specified in a specification in this repository.
The type follows the it's ok to be odd rule, so nodes MAY send odd-numbered types without ascertaining that the recipient understands it.
A node MUST NOT send an evenly-typed message not listed here without prior negotiation.
A node MUST ignore a received message of unknown type, if that type is odd.
A node MUST fail the channels if it receives a message of unknown type, if that type is even.
The messages are grouped logically into 4 groups by their most significant set bit:
- Setup & Control (types
0-31): messages related to connection setup, control, supported features, and error reporting. These are described below. - Channel (types
32-127): comprises messages used to setup and tear down micropayment channels. These are described in BOLT #2. - Commitment (types
128-255: comprises messages related to updating the current commitment transaction, which includes adding, revoking, and settling HTLCs, as well as updating fees and exchanging signatures. These are described in BOLT #2. - Routing (types
256-511): node and channel announcements, as well as any active route exploration. These are described in BOLT #7.
The size of the message is required to fit into a 2 byte unsigned int by the transport layer, therefore the maximum possible size is 65535 bytes. A node MUST ignore any additional data within a message, beyond the length it expects for that type. A node MUST fail the channels if it receives a known message with insufficient length for the contents.
Rationale
The standard endian of SHA2 and the encoding of Bitcoin public keys
are big endian, thus it would be unusual to use a different endian for
other fields.
Length is limited to 65535 bytes by the cryptographic wrapping, and messages in the protocol are never more than that length anyway.
The "it's OK to be odd" rule allows for future optional extensions without negotiation or special coding in clients. The "ignore additional data" rule similarly allows for future expansion.
Implementations may prefer to have message data aligned on an 8 byte boundary (the largest natural alignment requirement of any type here), but adding a 6 byte padding after the type field was considered wasteful: alignment may be achieved by decrypting the message into a buffer with 6 bytes of pre-padding.
Setup Messages
The init message
Once authentication is complete, the first message reveals the features supported or required by this node, even if this is a reconnection. Odd features are optional, even features are compulsory (it's OK to be odd). The meaning of these bits will be defined in the future.
- type: 16 (
init) - data:
- [2:gflen]
- [gflen:globalfeatures]
- [2:lflen]
- [lflen:localfeatures]
The 2 byte gflen and lflen fields indicate the number of bytes in the immediately following field.
Requirements
The sending node MUST send init as the first lightning message for any
connection.
The sending node SHOULD use the minimum lengths required to represent
the feature fields. The sending node MUST set feature bits
corresponding to features it requires the peer to support, and SHOULD
set feature bits corresponding to features it optionally supports.
The receiving node MUST fail the channels if it receives a
globalfeatures or localfeatures with an even bit set which it does
not understand.
Each node MUST wait to receive init before sending any other messages.
Rationale
The even/odd semantic allows future incompatible changes, or backward compatible changes. Bits should generally be assigned in pairs, so that optional features can later become compulsory.
Nodes wait for receipt of the other's features to simplify error diagnosis where features are incompatible.
The feature masks are split into local features which only affect the protocol between these two nodes, and global features which can affect HTLCs and thus are also advertised to other nodes.
The error message
For simplicity of diagnosis, it is often useful to tell the peer that something is incorrect.
- type: 17 (
error) - data:
- [32:channel-id]
- [2:len]
- [len:data]
The 2-byte len field indicates the number of bytes in the immediately following field.
Requirements
The channel is referred to by channel-id unless channel-id is zero (ie. all bytes zero), in which case it refers to all channels.
A node SHOULD send error for protocol violations or internal
errors which make channels unusable or further communication unusable.
A node MAY send an empty [data] field. A node sending error MUST
fail the channel referred to by the error message, or if channel-id is zero, it MUST
fail all channels and MUST close the connection.
A node MUST set len equal to the length of data. A node SHOULD include the raw, hex-encoded transaction in reply to a funding_created, funding_signed, closing_signed or commitment_signed message when failure was caused by an invalid signature check.
A node receiving error MUST fail the channel referred to by the message,
or if channel-id is zero, it MUST fail all channels and MUST close the connection. If no existing channel is referred to by the message, the receiver MUST ignore the message. A receiving node MUST truncate
len to the remainder of the packet if it is larger.
A receiving node SHOULD only print out data verbatim if the string is composed solely of printable ASCII characters.
For referece, the printable character set includes byte values 32 through 127 inclusive.
Rationale
There are unrecoverable errors which require an abort of conversations; if the connection is simply dropped then the peer may retry the connection. It's also useful to describe protocol violations for diagnosis, as it indicates that one peer has a bug.
It may be wise not to distinguish errors in production settings, lest it leak information, thus the optional data field.
Control Messages
The ping and pong messages
In order to allow for the existence of very long-lived TCP connections, at times it may be required that both ends keep alive the TCP connection at the application level. Such messages also allow obsfusation of traffic patterns.
- type: 18 (
ping) - data:
- [2:num_pong_bytes]
- [2:byteslen]
- [byteslen:ignored]
The pong message is to be sent whenever a ping message is received. It
serves as a reply, and also serves to keep the connection alive while
explicitly notifying the other end that the receiver is still active. Within
the received ping message, the sender will specify the number of bytes to be
included within the data payload of the pong message
- type: 19 (
pong) - data:
- [2:byteslen]
- [byteslen:ignored]
Requirements
A node sending pong or ping SHOULD set ignored to zeroes, but MUST NOT
set ignored to sensitive data such as secrets, or portions of initialized
memory.
A node SHOULD NOT send ping messages more often than once every 30 seconds,
and MAY terminate the network connection if it does not receive a corresponding
pong: it MUST NOT fail the channels in this case.
A node receiving a ping message SHOULD fail the channels if it has received
significantly in excess of one ping per 30 seconds, otherwise if
num_pong_bytes is less than 65534 it MUST respond by sending a pong message
with byteslen equal to num_pong_bytes, otherwise it MUST ignore the ping.
A node receiving a pong message MAY fail the channels if byteslen does not
correspond to any ping num_pong_bytes value it has sent.
Rationale
Connections between nodes within the network may be very long lived as payment channels have an indefinite lifetime. However, it's likely that for a significant portion of the life-time of a connection, no new data will be exchanged. Additionally, on several platforms it's possible that Lightning clients will be put to sleep without prior warning. As a result, we use a distinct ping message in order to probe for the liveness of the connection on the other side, and also to keep the established connection active.
Additionally, the ability for a sender to request that the receiver send a response with a particular number of bytes enables nodes on the network to create synthetic traffic. Such traffic can be used to partially defend against packet and timing analysis as nodes can fake the traffic patterns of typical exchanges, without applying any true updates to their respective channels.
When combined with the onion routing protocol defined in BOLT #4, careful statistically driven synthetic traffic can serve to further bolster the privacy of participants within the network.
Limited precautions are recommended against ping flooding, however some
latitude is given because of network delays. Note that there are other methods
of incoming traffic flooding (eg. sending odd unknown message types, or padding
every message maximally).
Finally, the usage of periodic ping messages serves to promote frequent key
rotations as specified within BOLT #8.
Acknowledgements
TODO(roasbeef); fin
References
Authors
FIXME
This work is licensed under a Creative Commons Attribution 4.0 International License.