[appendix]
[[wire_protocol_enumeration]]
==== Wire Messages

In this section, well outline the precise structure of each of the wire
messages within the protocol. We'll do so in two parts: first we'll enumerate
all the currently defined wire message types along with the message name
corresponding to that type, we'll then double back and define the structure of
each of the wire messages (partitioned into logical groupings).

First, we'll lead with an enumeration of all the currently defined 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 the above table, the `Category` field allows us to quickly categonize 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. With
this roadmap laid out, we'll now visit each message category in order to define
the precise structure and semantics of all defined messages within the LN
protocol.

===== 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 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:

`init` message:

 * 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'll see later, 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.

===== Error Communication Messages

Messages in this category are used to send connection level errors between two
peers. As we'll see later, 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.

===== Connection Liveness

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.

First, the `ping` message:

  * 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.

===== Channel Funding

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'll
explore the messages sent during the process of creating a new channel. We'll
only describe the fields used as we'll leave a in in-depth analysis of the
funding process to chapter XXX.

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`. We'll leave a description of the precise
protocol flow involving these messages for a chapter XXX. In this section,
we'll simply enumerate the set of fields and briefly describe each one.

The `open_channel` message:

  * 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 echos the set of parameters that the responder wishes
to apply to their version of the commitment transaction. Later in Chapter XXX,
when we go into the funding process in details, we'll do a deep dive to explore
the implications of the various par maters 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

* type: `38`
* fields:
  [channel_id:channel_id]
[u16:len]
[len*byte:scriptpubkey]

* type: `39`
* fields:
  [channel_id:channel_id]
[u64:fee_satoshis]
[signature:signature]

#### Channel Operation

In this section, we'll briefly describe the set of messages used to allow
anodes 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 Chapter XXX on
multi-hop HTLC forwarding, we details 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 na 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'll 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 `commit_sig` 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.

===== Channel Announcement

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'll detail in Chapter XXX, 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` messages 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.

===== Channel Graph Syncing

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'll learn in Chapter XXX, 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. In Chapter XXX, we'll go through the protocol peers use to
sync the channel graph in more detail.


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.
