lnbook/appendix_protocol_messages.asciidoc

[appendix]
[[wire_protocol_enumeration]]
[[protocol_messages]]
[[messages]]
== Wire Protocol Messages

((("wire protocol messages", id="ix_appendix_protocol_messages-asciidoc0", range="startofrange")))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 https://github.com/lightningnetwork/lightning-rfc[GitHub Lightning-RFC repository].
====

=== Message Types

((("wire protocol messages","message types", id="ix_appendix_protocol_messages-asciidoc1", range="startofrange")))Currently defined message types are listed in <<apdx_message_types>>.

[[apdx_message_types]]
.Message types
[options="header"]
|===
| Type integer | Message name | Category
| 16  | `init`             |  Connection Establishment
| 17  | `error`             | Error Communication
| 18  | `ping`             | Connection Liveness
| 19  | `pong`             | Connection Liveness
| 32  | `open_channel`             |          Channel Funding
| 33  | `accept_channel`             |          Channel Funding
| 34  | `funding_created`             |          Channel Funding
| 35  | `funding_signed`             |          Channel Funding
| 36  | `funding_locked`             |          Channel Funding + Channel Operation
| 38  | `shutdown`             | Channel Closing
| 39  | `closing_signed`             |         Channel Closing
| 128 | `update_add_htlc`             |          Channel Operation
| 130 | `update_fulfill_hltc`             |          Channel Operation
| 131 | `update_fail_htlc`             |          Channel Operation
| 132 | `commit_sig`             |          Channel Operation
| 133 | `revoke_and_ack`             |          Channel Operation
| 134 | `update_fee`             |          Channel Operation
| 135 | `update_fail_malformed_htlc`             |          Channel Operation
| 136 | `channel_reestablish`             |         Channel Operation
| 256 | `channel_announcement`             |          Channel Announcement
| 257 | `node_announcement`             |          Channel Announcement
| 258 | `channel_update`             |          Channel Announcement
| 259 | `announce_signatures`             |          Channel Announcement
| 261 | `query_short_chan_ids`             |          Channel Graph Syncing
| 262 | `reply_short_chan_ids_end`             |          Channel Graph Syncing
| 263 | `query_channel_range`             |          Channel Graph Syncing
| 264 | `reply_channel_range`             |          Channel Graph Syncing
| 265 | `gossip_timestamp_range`             |          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 eight (nonexhaustive) buckets including:

Connection Establishment:: Sent when a peer-to-peer connection is first
   established. Also used to negotiate the set of features supported
   by a new connection.

Error Communication:: Used by peers 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 and 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 to group
semantically similar messages together within the specification itself.(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc1")))

=== Message Structure

((("wire protocol messages","message structure", id="ix_appendix_protocol_messages-asciidoc2", range="startofrange")))We now detail each message category in order to define
the precise structure and semantics of all defined messages within the LN
protocol.

==== Connection Establishment Messages

((("wire protocol messages","connection establishment messages")))Messages in this category are the very first message sent between peers once
they establish a transport connection. At the time of writing this chapter,
there exists only a single message 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.


[[apdx_init_message]]
===== The init message

((("wire protocol messages","init message")))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_. ((("feature bits","defined")))As we see in <<feature_bits>>, feature bits are a
primitive used within the protocol 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 (backward compatibility).

What follows after the core message is a series of Type-Length-Value (TLV) records that can be used to extend the message in a forward- and backward-compatible manner in the future. We'll cover what TLV records are and how
they're used later in this appendix.

An `init` message is then examined by a peer 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 features 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, then you don't want to keep the
connection because they're unable to open your new preferred channel type.

==== Error Communication Messages

((("wire protocol messages","error communication messages")))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).

[[apdx_error_message]]
===== The error message

((("wire protocol messages","error message")))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 undergoing 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.

==== Connection Liveness

((("wire protocol messages","connection liveness messages")))Messages in this section are used to probe to determine if a connection is
still live or not. Because the LN protocol somewhat abstracts over the underlying
transport being used to transmit the messages, a set of protocol level ((("wire protocol messages","ping message")))((("wire protocol messages","pong message")))`ping`
and `pong` messages are defined.

[[apdx_ping_message]]
===== The ping message

The `ping` message is used to check whether the other party in a connection is "live." It contains the following fields:

 * Type: 18
 * Fields:
  ** `uint16` : `num_pong_bytes`
  ** `uint16` : `ping_body_len`
  ** `ping_body_len*bytes` : `ping_body`

Next its companion, the `pong` message.

[[apdx_pong_message]]
===== The pong message

The +pong+ message is sent in response to the +ping+ message and contains the following fields:

 * 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
conscious routing node to attempt to thwart certain classes of network
de-anonymization attempts because they can create a "fake" transcript that
resembles other messages based on the packet sizes sent across. Remember that by
default the Lightning Network uses an _encrypted_ transport, so a passive network monitor
cannot read the plain-text bytes and thus only has timing and packet sizes to go
off of.

==== Channel Funding

((("wire protocol messages","channel funding", id="ix_appendix_protocol_messages-asciidoc3", range="startofrange")))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 an 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 five messages: `open_channel`, `accept_channel`, `funding_created`,
`funding_signed`, and `funding_locked`.

The detailed protocol flow using these messages is described in <<payment_channels>>.

[[apdx_open_channel_message]]
===== The open_channel message

The +open_channel+ message starts the channel funding process and contains the following fields:

 * 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`

((("open_channel message")))((("wire protocol messages","open_channel message")))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 construct both the funding transaction as well as the
commitment transaction.

At the time of writing 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 network's 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 channel is to be advertised to the public network as a routable channel. Otherwise, the channel is considered to be unadvertised, also
commonly referred to as a private channel.

[[apdx_accept_channel_message]]
===== The accept_channel message

((("accept_channel message")))((("wire protocol messages","accept_channel message")))The `accept_channel` message is the response to the `open_channel` message.

[role="pagebreak-before"]
 * 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 detail, we explore
the implications of the various parameters that can be set when opening a new
channel.

[[apdx_funding_created_message]]
===== The funding_created message

((("funding_created message")))((("wire protocol messages","funding_created message")))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 have all the materials they need 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, 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.

[[apdx_funding_signed_message]]
===== The funding_signed message

((("funding_signed message")))((("wire protocol messages","funding_signed message")))To conclude, the responder sends the `funding_signed` message.

 * Type: 34
 * Fields:
  ** `channel_id` : `channel_id`
  ** `sig` : `signature`

To conclude after the responder receives 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 multisig funding output and broadcasting the transaction. This is
referred to as a force close. Conversely, to give the initiator the ability to close the channel, the responder also signs the initiator's commitment transaction.

Once this message has been received by the initiator, it's safe for them to
broadcast the funding transaction because they're now able to exit the channel
agreement unilaterally.

[[apdx_funding_locked_message]]
===== The funding_locked message

((("funding_locked message")))((("wire protocol messages","funding_locked message")))Once the funding transaction has received enough confirmations, the
`funding_locked` message 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 begin to be used.(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc3")))

==== Channel Closing

((("wire protocol messages","channel closing")))Channel closing is a multistep process. ((("wire protocol messages","shutdown message")))One node initiates by sending the `shutdown` message. The two channel partners then exchange a series of `closing_signed` messages to negotiate mutually acceptable fees for the closing transaction. ((("closing_signed message")))((("wire protocol messages","closing_signed message")))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.

[[apdx_shutdown_message]]
===== The shutdown message

The +shutdown+ message initiates the process of closing a channel and contains the following fields:

 * Type: 38
 * Fields:
  ** `channel_id` : `channel_id`
  ** `u16` : `len`
  ** `len*byte` : `scriptpubkey`

[[apdx_closing_signed_message]]
===== The closing_signed message

The +closing_signed+ message is sent by each channel partner until they agree on fees. It contains the following fields:

 * Type: 39
 * Fields:
  ** `channel_id` : `channel_id`
  ** `u64` : `fee_satoshis`
  ** `signature` : `signature`

==== Channel Operation

((("wire protocol messages","channel operation", id="ix_appendix_protocol_messages-asciidoc4", range="startofrange")))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.

To send, receive, or forward a payment over a channel, an HTLC must
first be added to both commitment transactions that comprise a channel link.

[role="pagebreak-before less_space"]
[[apdx_update_add_htlc_message]]
===== The update_add_htlc message

((("channel operation","update_add_htlc message")))((("update_add_htlc message")))((("wire protocol messages","update_add_htlc message")))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 an 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
multihop HTLC forwarding, we cover the onion routing protocol used in the
Lightning Network in detail.

Note that each HTLC sent uses an automatically 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.

[[apdx_update_fulfill_hltc_message]]
===== The update_fulfill_hltc message

((("channel operation","update_fulfill_hltc message")))((("update_fulfill_hltc message")))The `update_fulfill_hltc` message allows 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 to redeem an
active HTLC. The message references the `id` of the HTLC in question, and also
provides the preimage (which unlocks the HLTC).

[[apdx_update_fail_htlc_message]]
===== The update_fail_htlc message

((("channel operation","update_fail_htlc message")))((("update_fail_htlc message")))The `update_fail_htlc` message 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` message is the opposite of the `update_fulfill_hltc` message in that
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 to unravel the HTLC chain. As we explore in
<<failure_messages>>, 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.

[[apdx_commitment_signed_message]]
===== The commitment_signed message

((("channel operation","commitment_signed message")))((("commitment_signed message")))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.

[role="pagebreak-before less_space"]
[[apdx_revoke_and_ack_message]]
===== The revoke_and_ack message

((("channel operation","revoke_and_ack message")))((("revoke_and_ack message")))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`

Because 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.

[[apdx_update_fee_message]]
===== The update_fee message

((("channel operation","update_fee message")))((("update_fee message")))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.

[[apdx_update_fail_malformed_htlc_message]]
===== The update_fail_malformed_htlc message

((("channel operation","update_fail_malformed_htlc message")))((("update_fail_malformed_htlc message")))The `update_fail_malformed_htlc` message 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` message, but it's rarely used in
practice. As mentioned previously, 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.(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc4")))

==== Channel Announcement

((("channel_announcement message", id="ix_appendix_protocol_messages-asciidoc5", range="startofrange")))((("wire protocol messages","channel announcement", id="ix_appendix_protocol_messages-asciidoc6", range="startofrange")))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, to add a new entry to the channel graph, an agent must be an
on-chain transaction fee. This serves as a natural spam deterrent for the
Lightning Network.


[[apdx_channel_announcement_message]]
===== The channel_announcement message

The `channel_announcement` message 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 <<scid>>, each channel is uniquely identified by a locator
that encodes its _location_ within the blockchain. This locator is called this
`short_channel_id` and can fit into a 64-bit integer.

[[apdx_node_announcement_message]]
===== The node_announcement message

((("channel_announcement message","node_announcement message")))((("node_announcement message")))The `node_announcement` message allows a node to announce/update its 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 to ensure that adding an item to
the channel graph bears an on-chain cost. In this case, the on-chain cost will be
the cost of creating the channel to which this node is connected.

In addition to advertising its feature set, this message also allows a node to
announce/update the set of network `addresses` where it can be reached.

[[apdx_channel_update_message]]
===== The channel_update message

((("channel_announcement message","channel_update message")))((("channel_update message")))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 its routing fees as well as other fields that shape the type of
payment that is permitted to flow through this channel.

[[apdx_announce_signatures_message]]
===== The announce_signatures message

((("announce_signatures message")))((("channel_announcement message","announce_signatures message")))The `announce_signatures` message is exchanged by channel peers 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 four
signatures required to generate an `announce_signatures` message.(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc6")))(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc5")))

==== Channel Graph Syncing

Nodes create a local perspective of the channel graph using five messages: +query_short_chan_ids+, +reply_short_chan_ids_end+, +query_channel_range+, +reply_channel_range+, and +gossip_timestamp_range+.

[[apdx_query_short_chan_ids_message]]
===== The query_short_chan_ids message

((("channel graph syncing messages", id="ix_appendix_protocol_messages-asciidoc7", range="startofrange")))((("wire protocol messages","channel graph syncing", id="ix_appendix_protocol_messages-asciidoc8", range="startofrange")))The ((("channel graph syncing messages","query_short_chan_ids message")))((("query_short_chan_ids message")))`query_short_chan_ids` message 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.

[[apdx_reply_short_chan_ids_end_message]]
===== The reply_short_chan_ids_end message

((("channel graph syncing messages","reply_short_chan_ids_end message")))((("reply_short_chan_ids_end message")))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.

[[apdx_query_channel_range_message]]
===== The query_channel_range message

((("channel graph syncing messages","query_channel_range message")))((("query_channel_range message")))The `query_channel_range` message allows a node to query for the set of channels
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.

[[apdx_reply_channel_range_message]]
===== The reply_channel_range message

((("channel graph syncing messages","reply_channel_range message")))((("reply_channel_range message")))The `reply_channel_range` message is the response to the `query_channel_range` message 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 to
continue syncing the channel graph.

[[apdx_gossip_timestamp_range_message]]
===== The gossip_timestamp_range message

((("channel graph syncing messages","gossip_timestamp_range message")))((("gossip_timestamp_range message")))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(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc8")))(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc7"))) down(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc2"))).(((range="endofrange", startref="ix_appendix_protocol_messages-asciidoc0")))