lightning-bolts/01-messaging.md

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
  • The init message
  • The error message
• 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:

1. type: 2 byte big-endian field indicating the type of the message.
2. payload: variable length payload. It comprises the remainder of the message and conforms to the format matching the type.

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 & signalling (types 0-31): messages related to 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.

1. type: 16 (init)
2. 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.

1. type: 17 (error)
2. 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.

Acknowledgements

TODO(roasbeef); fin

References

1. http://www.unicode.org/charts/PDF/U2600.pdf

Authors

FIXME

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