This appendix lists all the currently defined message types used in the Lightning P2P protocol. Additionally, we show the structure of each message, grouping the messages into logical groupings based on the protocol flows.
|
Note
|
Lightning Protocol messages are extensible and their structure may change during network-wide upgrades. For the authoritative information, consult the latest version of the BOLTs found in the Github - Lightning-RFC repository. |
Currently defined message types are:
| Type Integer | Message Name | Category |
|---|---|---|
16 |
|
Connection Establishment |
17 |
|
Error Communication |
18 |
|
Connection Liveness |
19 |
|
Connection Liveness |
32 |
|
Channel Funding |
33 |
|
Channel Funding |
34 |
|
Channel Funding |
35 |
|
Channel Funding |
36 |
|
Channel Funding + Channel Operation |
38 |
|
Channel Closing |
39 |
|
Channel Closing |
128 |
|
Channel Operation |
130 |
|
Channel Operation |
131 |
|
Channel Operation |
132 |
|
Channel Operation |
133 |
|
Channel Operation |
134 |
|
Channel Operation |
135 |
|
Channel Operation |
136 |
|
Channel Operation |
256 |
|
Channel Announcement |
257 |
|
Channel Announcement |
258 |
|
Channel Announcement |
259 |
|
Channel Announcement |
261 |
|
Channel Graph Syncing |
262 |
|
Channel Graph Syncing |
263 |
|
Channel Graph Syncing |
264 |
|
Channel Graph Syncing |
265 |
|
Channel Graph Syncing |
In [message_types], the Category field allows us to quickly categorize a
message based on its functionality within the protocol itself. At a high level,
we place a message into one of 8 (non exhaustive) buckets including:
-
Connection Establishment: Sent when a peer to peer connection is first established. Also used in order to negotiate the set of feature supported by a new connection.
-
Error Communication: Used by peer to communicate the occurrence of protocol level errors to each other.
-
Connection Liveness: Used by peers to check that a given transport connection is still live.
-
Channel Funding: Used by peers to create a new payment channel. This process is also known as the channel funding process.
-
Channel Operation: The act of updating a given channel off-chain. This includes sending and receiving payments, as well as forwarding payments within the network.
-
Channel Announcement: The process of announcing a new public channel to the wider network so it can be used for routing purposes.
-
Channel Graph Syncing: The process of downloading & verifying the channel graph.
Notice how messages that belong to the same category typically share an adjacent message type as well. This is done on purpose in order to group semantically similar messages together within the specification itself.
We now detail each message category in order to define the precise structure and semantics of all defined messages within the LN protocol.
Messages in this category are the very first message sent between peers once
they establish a transport connection. At the time of writing of this chapter,
there exists only a single messages within this category, the init message.
The init message is sent by both sides of the connection once it has been
first established. No other messages are to be sent before the init message
has been sent by both parties.
The structure of the init message is defined as follows:
-
type: 16
-
fields:
-
uint16:global_features_len -
global_features_len*byte:global_features -
uint16:features_len -
features_len*byte:features -
tlv_stream_tlvs
-
Structurally, the init message is composed of two variable size bytes slices
that each store a set of feature bits. As we see in [feature_bits], feature bits are a
primitive used within the protocol in order to advertise the set of protocol
features a node either understands (optional features), or demands (required
features).
Note that modern node implementations will only use the features field, with
items residing within the global_features vector for primarily historical
purposes (backwards compatibility).
What follows after the core message is a series of T.L.V, or Type Length Value records which can be used to extend the message in a forwards+backwards compatible manner in the future. We’ll cover what TLV records are and how they’re used later in the chapter.
An init message is then examined by a peer in order to determine if the
connection is well defined based on the set of optional and required feature
bits advertised by both sides.
An optional feature means that a peer knows about a feature, but they don’t consider it critical to the operation of a new connection. An example of one would be something like the ability to understand the semantics of a newly added field to an existing message.
On the other hand, required feature indicate that if the other peer doesn’t know about the feature, then the connection isn’t well defined. An example of such a feature would be a theoretical new channel type within the protocol: if your peer doesn’t know of this feature, they you don’t want to keep the connection as they’re unable to open your new preferred channel type.
Messages in this category are used to send connection level errors between two peers. Another type of error exists in the protocol: an HTLC forwarding level error. Connection level errors may signal things like feature bit incompatibility, or the intent to force close (unilaterally broadcast the latest signed commitment)
The sole message in this category is the error message:
-
type: 17
-
fields:
-
channel_id:chan_id -
uint16:data_len -
data_len*byte:data
-
An error message can be sent within the scope of a particular channel by
setting the channel_id, to the channel_id of the channel under going this
new error state. Alternatively, if the error applies to the connection in
general, then the channel_id field should be set to all zeroes. This all zero
channel_id is also known as the connection level identifier for an error.
Depending on the nature of the error, sending an error message to a peer you
have a channel with may indicate that the channel cannot continue without
manual intervention, so the only option at that point is to force close the
channel by broadcasting the latest commitment state of the channel.
Messages in this section are used to probe to determine if a connection is
still live or not. As the LN protocol somewhat abstracts over the underlying
transport being used to transmit the messages, a set of protocol level ping
and pong messages are defined.
-
type: 18
-
fields:
-
uint16:num_pong_bytes -
uint16:ping_body_len -
ping_body_len*bytes:ping_body
-
Next it’s companion, the pong message.
-
type: 19
-
fields:
-
uint16:pong_body_len -
ping_body_len*bytes:pong_body
-
A ping message can be sent by either party at any time.
The ping message includes a num_pong_bytes field that is used to instruct
the receiving node with respect to how large the payload it sends in its pong
message is. The ping message also includes a ping_body opaque set of bytes
which can be safely ignored. It only serves to allow a sender to pad out ping
messages they send, which can be useful in attempting to thwart certain
de-anonymization techniques based on packet sizes on the wire.
A pong message should be sent in response to a received ping message. The
receiver should read a set of num_pong_bytes random bytes to send back as the
pong_body field. Clever use of these fields/messages may allow a privacy
concious routing node to attempt to thwart certain classes of network
de-anonymization attempts, as they can create a "fake" transcript that
resembles other messages based on the packet sizes set across. Remember that by
default the LN uses an encrypted transport, so a passive network monitor
cannot read the plaintext bytes, thus only has timing and packet sizes to go
off of.
As we go on, we enter into the territory of the core messages that govern the functionality and semantics of the Lightning Protocol. In this section, we explore the messages sent during the process of creating a new channel. We’ll only describe the fields used as we leave a in in-depth analysis of the funding process to [payment_channels].
Messages that are sent during the channel funding flow belong to the following
set of 5 messages: open_channel, accept_channel, funding_created,
funding_signed, funding_locked.
The detailed protocol flow using these messages is described in [payment_channels].
-
type: 32
-
fields:
-
chain_hash:chain_hash -
32*byte:temp_chan_id -
uint64:funding_satoshis -
uint64:push_msat -
uint64:dust_limit_satoshis -
uint64:max_htlc_value_in_flight_msat -
uint64:channel_reserve_satoshis -
uint64:htlc_minimum_msat -
uint32:feerate_per_kw -
uint16:to_self_delay -
uint16:max_accepted_htlcs -
pubkey:funding_pubkey -
pubkey:revocation_basepoint -
pubkey:payment_basepoint -
pubkey:delayed_payment_basepoint -
pubkey:htlc_basepoint -
pubkey:first_per_commitment_point -
byte:channel_flags -
tlv_stream:tlvs
-
This is the first message sent when a node wishes to execute a new funding flow with another node. This message contains all the necessary information required for both peers to constructs both the funding transaction as well as the commitment transaction.
At the time of writing of this chapter, a single TLV record is defined within the set of optional TLV records that may be appended to the end of a defined message:
-
type: 0
-
data:
upfront_shutdown_script
The upfront_shutdown_script is a variable sized byte slice that MUST be a
valid public key script as accepted by the Bitcoin networks' consensus
algorithm. By providing such an address, the sending party is able to
effectively create a "closed loop" for their channel, as neither side will sign
off an cooperative closure transaction that pays to any other address. In
practice, this address is usually one derived from a cold storage wallet.
The channel_flags field is a bitfield of which at the time of writing, only
the first bit has any sort of significance. If this bit is set, then this
denotes that this channel is to be advertised to the public network as a route
bal channel. Otherwise, the channel is considered to be unadvertised, also
commonly referred to as a "private" channel.
The accept_channel message is the response to the open_channel message:
-
type: 33
-
fields:
-
32*byte:temp_chan_id -
uint64:dust_limit_satoshis -
uint64:max_htlc_value_in_flight_msat -
uint64:channel_reserve_satoshis -
uint64:htlc_minimum_msat -
uint32:minimum_depth -
uint16:to_self_delay -
uint16:max_accepted_htlcs -
pubkey:funding_pubkey -
pubkey:revocation_basepoint -
pubkey:payment_basepoint -
pubkey:delayed_payment_basepoint -
pubkey:htlc_basepoint -
pubkey:first_per_commitment_point -
tlv_stream:tlvs
-
The accept_channel message is the second message sent during the funding flow
process. It serves to acknowledge an intent to open a channel with a new remote
peer. The message mostly echoes the set of parameters that the responder wishes
to apply to their version of the commitment transaction. In [payment_channels],
when we go into the funding process in details, we do a deep dive to explore
the implications of the various parameters that can be set when opening a new
channel.
In response, the initiator will send the funding_created message.
-
type: 34
-
fields:
-
32*byte:temp_chan_id -
32*byte:funding_txid -
uint16:funding_output_index -
sig:commit_sig
-
Once the initiator of a channel receives the accept_channel message from the
responder, they they have all the materials they need in order to construct the
commitment transaction, as well as the funding transaction. As channels by
default are single funder (only one side commits funds), only the initiator
needs to construct the funding transaction. As a result, in order to allow the
responder to sign a version of a commitment transaction for the initiator, the
initiator, only needs to send the funding outpoint of the channel.
To conclude the responder sends the funding_signed message.
-
type: 34
-
fields:
-
channel_id:channel_id -
sig:signature
-
To conclude after the responder receivers the funding_created message, they
now own a valid signature of the commitment transaction by the initiator. With
this signature they’re able to exit the channel at any time by signing their
half of the multi-sig funding output, and broadcasting the transaction. This is
referred to as a force close. In order to give the initiator the ability to do
so was well, before the channel can be used, the responder then signs the
initiator’s commitment transaction as well.
Once this message has been received by the initiator, it’s safe for them to broadcast the funding transaction, as they’re now able to exit the channel agreement unilaterally.
Once the funding transaction has received enough confirmations, the
funding_locked is sent.
-
type: 36
-
fields:
-
channel_id:channel_id -
pubkey:next_per_commitment_point
-
Once the funding transaction obtains a minimum_depth number of confirmations,
then the funding_locked message is to be sent by both sides. Only after this
message has been received, and sent can the channel being to be used.
Channel closing is a multi-step process. One node initiates by sending the shutdown message. The two channel partners then exchange a series of channel_closing messages to negotiate mutually acceptable fees for the closing transaction. The channel funder sends the first closing_signed message and the other side can accept by sending a closing_signed message with the same fee values.
In this section, we briefly describe the set of messages used to allow nodes to operate a channel. By operation, we mean being able to send receive, and forward payments for a given channel.
In order to send, receive or forward a payment over a channel, an HTLC must first be added to both commitment transactions that comprise of a channel link.
The update_add_htlc message allows either side to add a new HTLC to the
opposite commitment transaction.
-
type: 128
-
fields:
-
channel_id:channel_id -
uint64:id -
uint64:amount_msat -
sha256:payment_hash -
uint32:cltv_expiry -
1366*byte:onion_routing_packet
-
Sending this message allows one party to initiate either sending a new payment,
or forwarding an existing payment that arrived via in incoming channel. The
message specifies the amount (amount_msat) along with the payment hash that
unlocks the payment itself. The set of forwarding instructions of the next hop
are onion encrypted within the onion_routing_packet field. In [onion_routing], on
multi-hop HTLC forwarding, we detail the onion routing protocol used in the
Lighting Network in detail.
Note that each HTLC sent uses an auto incrementing ID which is used by any message which modifies an HTLC (settle or cancel) to reference the HTLC in a unique manner scoped to the channel.
The update_fulfill_hltc allow redemption (receipt) of an active HTLC.
-
type: 130
-
fields:
-
channel_id:channel_id -
uint64:id -
32*byte:payment_preimage
-
This message is sent by the HTLC receiver to the proposer in order to redeem an
active HTLC. The message references the id of the HTLC in question, and also
provides the pre-image (which unlocks the HLTC) as well.
The update_fail_htlc is sent to remove an HTLC from a commitment transaction.
-
type: 131
-
fields:
-
channel_id:channel_id -
uint64:id -
uint16:len -
len*byte:reason
-
The update_fail_htlc is the opposite of the update_fulfill_hltc message as
it allows the receiver of an HTLC to remove the very same HTLC. This message is
typically sent when an HTLC cannot be properly routed upstream, and needs to be
sent back to the sender in order to unravel the HTLC chain. As we explore in
Chapter XX, the message contains an encrypted failure reason (reason) which
may allow the sender to either adjust their payment route, or terminate if the
failure itself is a terminal one.
The commitment_signed message is used to stamp the creation of a new commitment transaction
-
type: 132
-
fields:
-
channel_id:channel_id -
sig:signature -
uint16:num_htlcs -
num_htlcs*sig:htlc_signature
-
In addition to sending a signature for the next commitment transaction, the sender of this message also needs to send a signature for each HTLC that’s present on the commitment transaction. This is due to the existence of the
The revoke_and_ack is sent to revoke a dated commitment:
-
type: 133
-
fields:
-
channel_id:channel_id -
32*byte:per_commitment_secret -
pubkey:next_per_commitment_point
-
As the Lightning Network uses a replace-by-revoke commitment transaction, after
receiving a new commitment transaction via the commit_sig message, a party
must revoke their past commitment before they’re able to receive another one.
While revoking a commitment transaction, the revoker then also provides the
next commitment point that’s required to allow the other party to send them a
new commitment state.
The update_fee is sent to update the fee on the current commitment
transactions.
-
type: 134
-
fields:
-
channel_id:channel_id -
uint32:feerate_per_kw
-
This message can only be sent by the initiator of the channel they’re the ones that will pay for the commitment fee of the channel as along as it’s open.
The update_fail_malformed_htlc is sent to remove a corrupted HTLC:
-
type: 135
-
fields:
-
channel_id:channel_id -
uint64:id -
sha256:sha256_of_onion -
uint16:failure_code
-
This message is similar to the update_fail_htlc but it’s rarely used in
practice. As mentioned above, each HTLC carries an onion encrypted routing
packet that also covers the integrity of portions of the HTLC itself. If a
party receives an onion packet that has somehow been corrupted along the way,
then it won’t be able to decrypt the packet. As a result it also can’t properly
forward the HTLC, therefore it’ll send this message to signify that the HTLC
has been corrupted somewhere along the route back to the sender.
Messages in this category are used to announce components of the Channel Graph authenticated data structure to the wider network. The Channel Graph has a series of unique properties due to the condition that all data added to the channel graph MUST also be anchored in the base Bitcoin blockchain. As a result, in order to add a new entry to the channel graph, an agent must be an on chain transaction fee. This serves as a natural spam de tenace for the Lightning Network.
The channel_announcement is used to announce a new channel to the wider
network.
-
type: 256
-
fields:
-
sig:node_signature_1 -
sig:node_signature_2 -
sig:bitcoin_signature_1 -
sig:bitcoin_signature_2 -
uint16:len -
len*byte:features -
chain_hash:chain_hash -
short_channel_id:short_channel_id -
pubkey:node_id_1 -
pubkey:node_id_2 -
pubkey:bitcoin_key_1 -
pubkey:bitcoin_key_2
-
The series of signatures and public keys in the message serves to create a
proof that the channel actually exists within the base Bitcoin blockchain. As
we detail in [sid], each channel is uniquely identified by a locator
that encodes it’s location within the blockchain. This locator is called this
short_channel_id and can fit into a 64-bit integer.
The node_announcement allows a node to announce/update it’s vertex within the
greater Channel Graph.
-
type: 257
-
fields:
-
sig:signature -
uint64:flen -
flen*byte:features -
uint32:timestamp -
pubkey:node_id -
3*byte:rgb_color -
32*byte:alias -
uint16:addrlen -
addrlen*byte:addresses
-
Note that if a node doesn’t have any advertised channel within the Channel Graph, then this message is ignored in order to ensure that adding an item to the Channel Graph bares an on-chain cost. In this case, the on-chain cost will the cost of creating the channel which this node is connected to.
In addition to advertising its feature set, this message also allows a node to
announce/update the set of network addresses that it can be reached at.
The channel_update message is sent to update the properties and policies of
an active channel edge within the Channel graph.
-
type: 258
-
fields:
-
signature:signature -
chain_hash:chain_hash -
short_channel_id:short_channel_id -
uint32:timestamp -
byte:message_flags -
byte:channel_flags -
uint16:cltv_expiry_delta -
uint64:htlc_minimum_msat -
uint32:fee_base_msat -
uint32:fee_proportional_millionths -
uint16:htlc_maximum_msat
-
In addition to being able to enable/disable a channel this message allows a node to update it’s routing fees as well as other fields that shape the type of payment that is permitted to flow through this channel.
The announce_signatures message is exchange by channel peers in order to
assemble the set of signatures required to produce a channel_announcement
message.
-
type: 259
-
fields:
-
channel_id:channel_id -
short_channel_id:short_channel_id -
sig:node_signature -
sig:bitcoin_signature
-
After the funding_locked message has been sent, if both sides wish to
advertise their channel to the network, then they’ll each send the
announce_signatures message which allows both sides to emplace the 4
signatures required to generate a announce_signatures message.
The query_short_chan_ids allows a peer to obtain the channel information
related to a series of short channel IDs.
-
type: 261
-
fields:
-
chain_hash:chain_hash -
u16:len -
len*byte:encoded_short_ids -
query_short_channel_ids_tlvs:tlvs
-
As we learn in [gossip], these channel IDs may be a series of channels that were new to the sender, or were out of date which allows the sender to obtain the latest set of information for a set of channels.
The reply_short_chan_ids_end message is sent after a peer finishes responding
to a prior query_short_chan_ids message.
-
type: 262
-
fields:
-
chain_hash:chain_hash -
byte:full_information
-
This message signals to the receiving party that if they wish to send another query message, they can now do so.
The query_channel_range message allows a node to query for the set of channel
opened within a block range.
-
type: 263
-
fields:
-
chain_hash:chain_hash -
u32:first_blocknum -
u32:number_of_blocks -
query_channel_range_tlvs:tlvs
-
As channels are represented using a short channel ID that encodes the location of a channel in the chain, a node on the network can use a block height as a sort of cursor to seek through the chain in order to discover a set of newly opened channels.
The reply_channel_range message is the response to query_channel_range and
includes the set of short channel IDs for known channels within that range.
-
type: 264
-
fields:
-
chain_hash:chain_hash -
u32:first_blocknum -
u32:number_of_blocks -
byte:sync_complete -
u16:len -
len*byte:encoded_short_ids -
reply_channel_range_tlvs:tlvs
-
As a response to query_channel_range, this message sends back the set of
channels that were opened within that range. This process can be repeated with
the requester advancing their cursor further down the chain in order to
continue syncing the Channel Graph.
The gossip_timestamp_range message allows a peer to start receiving new
incoming gossip messages on the network.
-
type: 265
-
fields:
-
chain_hash:chain_hash -
u32:first_timestamp -
u32:timestamp_range
-
Once a peer has synced the channel graph, they can send this message if they
wish to receive real-time updates on changes in the Channel Graph. They can
also set the first_timestamp and timestamp_range fields if they wish to
receive a backlog of updates they may have missed while they were down.