[[wire_protocol]]
== Wire Protocol: Framing and Extensibility

In this chapter, we dive into the wire protocol of the Lightning Network
and also cover all the various extensibility levers that have been built into
the protocol. By the end of this chapter, an ambitious reader should be able to
write their very own wire protocol parser for the Lightning Network. In addition
to being able to write a custom wire protocol parser, a reader of this chapter
will gain a deep understanding of the various upgrade mechanisms
that have been built into the protocol.

=== Messaging Layer in the Lightning Protocol Suite

The messaging layer, which is detailed in this chapter, consists of "Message Framing and Format," "Type Length Value (TLV)" encoding, and "Feature Bits." These components are highlighted by a double outline in the protocol suite, shown in <<LN_protocol_wire_message_highlight>>.

[[LN_protocol_wire_message_highlight]]
.The Lightning Network Protocol Suite
image::images/mtln_1301.png["The Lightning Network Protocol Suite"]

=== Wire Framing

We begin by describing the high-level structure of the wire _framing_
within the protocol. When we say framing, we mean the way that the bytes are
packed on the wire to _encode_ a particular protocol message. Without knowledge
of the framing system used in the protocol, a string of bytes on the wire would
resemble a series of random bytes because no structure has been imposed. By applying
proper framing to decode these bytes on the wire, we'll be able to extract
structure and finally parse this structure into protocol messages within our
higher-level language.

It's important to note that the Lightning Network is an _end-to-end
encrypted_ protocol, and the wire framing is itself encapsulated within an
_encrypted_ message transport layer. As we see in <<encrypted_message_transport>>, the Lightning
Network uses a custom variant of the Noise Protocol to handle
transport encryption. Within this chapter, whenever we give an example of wire
framing, we assume the encryption layer has already been stripped away (when
decoding), or that we haven't yet encrypted the set of bytes before we send
them on the wire (encoding).

==== High-Level Wire Framing

With that said, we're ready to describe the high-level schema used to
encode messages on the wire:

  * Messages on the wire begin with a _2-byte_ type field, followed by a
    message payload.
  * The message payload itself can be up to 65 KB in size.
  * All integers are encoded in big-endian (network order).
  * Any bytes that follow after a defined message can be safely ignored.

Yep, that's it. As the protocol relies on an _encapsulating_ transport protocol
encryption layer, we don't need an explicit length for each message type. This
is due to the fact that transport encryption works at the _message_ level, so
by the time we're ready to decode the next message, we already know the total
number of bytes of the message itself. Using 2 bytes for the message type
(encoded in big-endian) means that the protocol can have up to `2^16 – 1` or
`65,535` distinct messages. Continuing, because we know all messages must be below
65 KB, this simplifies our parsing as we can use a _fixed_ size buffer and
maintain strong bounds on the total amount of memory required to parse an
incoming wire message.

The final bullet point allows for a degree of _backward_ compatibility  because new nodes are able to provide information in the wire messages that older nodes
(which may not understand them) can safely ignore. As we see subsequently, this
feature combined with a very flexible wire message extensibility format also
allows the protocol to achieve _forward_ compatibility as well.

==== Type Encoding

With this high-level background provided, we now start at the most primitive
layer: parsing primitive types. In addition to encoding integers, the Lightning
Protocol also allows for encoding of a vast array of types including variable
length byte slices, elliptic curve public keys, Bitcoin addresses, and
signatures. When we describe the _structure_ of wire messages later in this
chapter, we refer to the high-level type (the abstract type) rather than the
lower-level representation of said type. In this section, we peel back this
abstraction layer to ensure our future wire parser is able to properly
encoding/decode any of the higher-level types.

In <<message_types>>, we map the name of a given message type to the
high-level routine used to encode/decode the type.

[[message_types]]
.High-level message types
[options="header"]
|===
| High-level type | Framing | Comment
| `node_alias` | A 32-byte fixed-length byte slice      | When decoding, reject if contents are not a valid UTF-8 string
| `channel_id` | A 32-byte fixed-length byte slice that maps an outpoint to a 32-byte value     | Given an outpoint, one can convert it to a `channel_id` by taking the TxID of the outpoint and XORing it with the index (interpreted as the lower 2 bytes)
| `short_chan_id` | An unsigned 64-bit integer (`uint64`) | Composed of the block height (24 bits), transaction index (24 bits), and output index (16 bits) packed into 8 bytes
| `milli_satoshi` | An unsigned 64-bit integer (`uint64`) | Represents 1000th of a satoshi
| `satoshi` | An unsigned 64-bit integer (`uint64`) | The base unit of bitcoin
| `pubkey`  | An secp256k1 public key encoded in _compressed_ format, occupying 33 bytes | Occupies a fixed 33-byte length on the wire
| `sig`     | An ECDSA signature of the secp256k1 Elliptic Curve. | Encoded as a _fixed_ 64-byte byte slice, packed as `R \|\| S`
| `uint8`   | An 8-bit integer  |
| `uint16`  | A 16-bit integer  |
| `uint64`  | A 64-bit integer  |
| `[]byte`  | A variable length byte slice | Prefixed with a 16-bit integer denoting the length of the bytes
| `color_rgb` | RGB color encoding | Encoded as a series if 8-bit integers
| `net_addr` | The encoding of a network address | Encoded with a 1-byte prefix that denotes the type of address, followed by the address body
|===

In the next section, we describe the structure of each of the wire messages
including the prefix type of the message along with the contents of its message
body.

[[tlv_message_extensions]]
=== Type Length Value (TLV) Message Extensions

Earlier in this chapter we mentioned that messages can be up to 65 KB in size,
and if while parsing a message, extra bytes are left over, then those bytes
are to be ignored. At an initial glance, this requirement may appear to be
somewhat arbitrary; however, this requirement allows for decoupled desynchronized evolution of the Lightning
Protocol itself. We discuss this more toward the end of the chapter. But first, we turn our attention to exactly what those "extra bytes" at
the end of a message can be used for.

==== The Protocol Buffers Message Format

The Protocol Buffers (Protobuf) message serialization format started out as an
internal format used at Google and has blossomed into one of the most popular
message serialization formats used by developers globally. The Protobuf format
describes how a message (usually some sort of data structure related to an API)
is encoded on the wire and decoded on the other end. Several "Protobuf
compilers" exists in dozens of languages which act as a bridge that allows any
language to encode a Protobuf that will be able to decode by a compliant decode
in another language. Such cross-language data structure compatibility allows
for a wide range of innovation because it's possible to transmit structure and even
typed data structures across language and abstraction boundaries.

Protobufs are also known for their flexibility with respect to how they
handle changes in the underlying messages structure. As long as the field
numbering schema is adhered to, then it's possible for a newer write of
Protobufs to include information within a Protobuf that may be unknown to any
older readers. When the old reader encounters the new serialized format, if
there are types/fields that it doesn't understand, then it simply _ignores_
them. This allows old clients and new clients to coexist because all clients can
parse some portion of the newer message format.

==== Forward and Backward Compatibility

Protobufs are extremely popular amongst developers because they have built in
support for both forward and backward compatibility. Most developers are
likely familiar with the concept of backward compatibility. In simple terms,
the principle states that any changes to a message format or API should be
done in a manner that doesn't break support for older clients. Within our preceding Protobuf extensibility examples, backward compatibility is achieved by
ensuring that new additions to the Protobuf format don't break the known portions
of older readers. Forward compatibility, on the other hand, is just as important
for desynchronized updates; however, it's less commonly known. For a change to
be forward compatible, clients are to simply ignore any information
they don't understand. The soft fork mechanism of upgrading the Bitcoin
consensus system can be said to be both forward and backward compatible: any
clients that don't update can still use Bitcoin, and if they encounter any
transactions they don't understand, then they simply ignore them as their funds
aren't using those new features.

[[tlv]]
=== Type-Length-Value (TLV) Format

To be able to upgrade messages in a manner that is both forward and backward
compatible, in addition to feature bits (more on that later), the Lightning Network utilizes a custom message serialization format plainly called Type-Length-Value, or TLV for short. The format was inspired by the widely used Protobuf
format and borrows many concepts by significantly simplifying the
implementation as well as the software that interacts with message parsing. A
curious reader might ask "why not just use Protobufs?" In response, the
Lightning developers would respond that we're able to have the best of the
extensibility of Protobufs while also having the benefit of a smaller
implementation and thus smaller attack. As of version 3.15.6, the Protobuf
compiler weighs in at over 656,671 lines of code.  In comparison LND's
implementation of the TLV message format weighs in at only 2.3k lines of code
(including tests).

With the necessary background presented, we're now ready to describe the TLV
format in detail. A TLV message extension is said to be a stream of
individual TLV records. A single TLV record has three components: the type of
the record, the length of the record, and finally the opaque value of the
record:

`type`:: An integer representing the name of the record being encoded.
`length`:: The length of the record.
`value`:: The opaque value of the record.

Both the `type` and `length` are encoded using a variable sized integer that's inspired by the variable sized integer (varint) used in Bitcoin's P2P protocol, called `BigSize` for short.

==== BigSize Integer Encoding

In its fullest form, a `BigSize`
integer can represent value up to 64 bits. In contrast to Bitcoin's varint
format, the `BigSize` format instead encodes integers using a big-endian byte
ordering.

The `BigSize` varint has two components: the discriminant and the body. In the
context of the `BigSize` integer, the discriminant communicates to the decoder
the size of the variable sized integer that follows. Remember that the unique thing about
variable sized integers is that they allow a parser to use fewer bytes to encode
smaller integers than larger ones, saving space. Encoding of a `BigSize`
integer follows one of the four following options:

1. If the value is less than `0xfd` (`253`): Then the discriminant isn't really used, and the encoding is simply the integer itself. This allows us to encode very small integers with no additional overhead.

2. If the value is less than or equal to `0xffff` (`65535`):The discriminant is encoded as `0xfd`, which indicates that the value that follows is larger than `0xfd`, but smaller than `0xffff`). The number is then encoded as a 16-bit integer. Including the discriminant, then we can encode a value that is greater than 253, but less than 65,535 using 3 bytes.

3. If the value is less than `0xffffffff` (`4294967295`): The discriminant is encoded as `0xfe`. The body is encoded using 32-bit integer, including the discriminant, then we can encode a value that's less than `4,294,967,295` using 5 bytes.

4. Otherwise, we just encode the value as a full-size 64-bit integer.


====  TLV Encoding Constraints

Within the context of a TLV message, record types below `2^16` are said to be _reserved_ for future use. Types beyond this
range are to be used for "custom" message extensions used by higher-level application protocols.

The `value` of a record depends on the `type`. In other words, it can take any form because parsers will attempt to interpret it depending on the context of the type itself.

==== TLV Canonical Encoding

One issue with the Protobuf format is that encodings of the same message may
output an entirely different set of bytes when encoded by two different
versions of the compiler. Such instances of a noncanonical encoding are not
acceptable within the context of Lightning, as many messages contain a
signature of the message digest. If it's possible for a message to be encoded
in two different ways, then it would be possible to break the authentication of
a signature inadvertently by re-encoding a message using a slightly different
set of bytes on the wire.

To ensure that all encoded messages are canonical, the following
constraints are defined when encoding:

  * All records within a TLV stream must be encoded in order of strictly
    increasing type.

  * All records must minimally encode the `type` and `length` fields. In other words, the smallest `BigSize` representation for an integer must be used at all times.

  * Each `type` may only appear once within a given TLV stream.

In addition to these encoding constraints, a series of higher-level
interpretation requirements is also defined based on the _arity_ of a given `type` integer. We dive further into these details toward the end of the
chapter once we describe how the Lightning Protocol is upgraded in practice and
in theory.

[[feature_bits]]
=== Feature Bits & Protocol Extensibility

Because the Lightning Network is a decentralized system, no single entity can enforce a
protocol change or modification upon all the users of the system. This
characteristic is also seen in other decentralized networks such as Bitcoin.
However, unlike Bitcoin, overwhelming consensus _is not_ required to change a
subset of the Lightning Network. Lightning is able to evolve at will without a
strong requirement of coordination because, unlike Bitcoin, there is no global consensus required in the Lightning Network. Due to this fact and the several
upgrade mechanisms embedded in the Lightning Network, only the
participants that wish to use these new Lightning Network features need to
upgrade, and then they are able to interact with each other.

In this section, we explore the various ways that developers and users are
able to design and deploy new features to the Lightning Network. The
designers of the original Lightning Network knew that there were many possible future directions for the network and the underlying protocol. As a result, they made sure to implement several
extensibility mechanisms within the system, which can be used to upgrade it partially or fully in a decoupled, desynchronized, and decentralized
manner.

==== Feature Bits As an Upgrade Discoverability Mechanism

An astute reader may have noticed the various locations that feature bits are
included within the Lightning Protocol. A _feature bit_ is a bitfield that can
be used to advertise understanding or adherence to a possible network protocol
update. Feature bits are commonly assigned in pairs, meaning that each
potential new feature/upgrade always defines two bits within the bitfield.
One bit signals that the advertised feature is _optional_, meaning that the
node knows about the feature and can use it, but doesn't
consider it required for normal operation. The other bit signals that the
feature is instead _required_, meaning that the node will not continue
operation if a prospective peer doesn't understand that feature.

Using these two bits (optional and required), we can construct a simple
compatibility matrix that nodes/users can consult to determine if a peer is compatible with a desired feature, as shown in <<table1302>>.

[[table1302]]
.Feature bit compatibility matrix
[options="header"]
|===
|Bit type|Remote optional|Remote required|Remote unknown
|Local optional|✅|✅|✅
|Local required|✅|✅|❌
|Local unknown|✅|❌|❌
|===

From this simplified compatibility matrix, we can see that as long as the other
party knows about our feature bit, then we can interact with them using the
protocol. If the party doesn't even know about what bit we're referring to
_and_ they require the feature, then we are incompatible with them. Within the
network, optional features are signaled using an _odd bit number_ while
required features are signaled using an _even bit number_. As an example, if a peer signals that they known of a feature that uses bit +15+, then we know that
this is an optional feature, and we can interact with them or respond to
their messages even if we don't know about the feature. If
they instead signaled the feature using bit +16+, then we know this is a
required feature, and we can't interact with them unless our node also
understands that feature.

The Lightning developers have come up with an easy to remember phrase that
encodes this matrix: "it's OK to be odd." This simple rule  allows for a
rich set of interactions within the protocol, as a simple bitmask operation
between two feature bit vectors allows peers to determine if certain
interactions are compatible with each other or not. In other words, feature
bits are used as an upgrade discoverability mechanism: they easily allow to
peers to understand if they are compatible or not based on the concepts of
optional, required, and unknown feature bits.

Feature bits are found in the `node_announcement`, `channel_announcement`, and
`init` messages within the protocol. As a result, these three messages can be
used to signal the knowledge and/or understanding of in-flight protocol
updates within the network. The feature bits found in the `node_announcement`
message can allow a peer to determine if their _connections_ are compatible or
not. The feature bits within the `channel_announcement` messages allow a peer
to determine if a given payment type or HTLC can transit through a given peer or
not. The feature bits within the `init` message allow peers to understand if
they can maintain a connection, and also which features are negotiated for the
lifetime of a given connection.

==== TLV for Forward and Backward Compatibility

As we learned earlier in the chapter, TLV records can be
used to extend messages in a forward and backward compatible manner.
Over time, these records have been used to extend existing messages without
breaking the protocol by utilizing the "undefined" area within a message beyond
that set of known bytes.

As an example, the original Lightning Protocol didn't have a concept of the
"largest amount HTLC" that could traverse through a channel as dictated by a routing
policy. Later on, the `max_htlc` field was added to the `channel_update`
message to phase in this concept over time. Peers that receive a
`channel_update` that sets such a field but don't even know the upgrade existed
are unaffected by the change, but have their HTLCs rejected if they are
beyond the limit. Newer peers, on the other hand, are able to parse, verify,
and utilize the new field.

Those familiar with the concept of soft forks in Bitcoin may now see some
similarities between the two mechanisms.  Unlike Bitcoin consensus-level
soft forks, upgrades to the Lightning Network don't require overwhelming
consensus to be adopted. Instead, at minimum, only two peers within the
network need to understand a new upgrade to start using it. Commonly these two peers may be the recipient and sender of a
payment, or may be the channel partners of a new payment channel.

==== A Taxonomy of Upgrade Mechanisms

Rather than there being a single widely utilized upgrade mechanism within the
network (such as soft forks for Bitcoin), there exist several possible upgrade mechanisms within the Lightning Network. In this
section, we enumerate these upgrade mechanisms and
provide a real-world example of their use in the past.

===== Internal network upgrades

We start with the upgrade type that requires the most protocol-level
coordination: internal network upgrades. An internal network upgrade is
characterized by one that requires _every single node_ within a prospective payment path to understand the new feature. Such an upgrade is similar to any
upgrade within the internet that requires hardware-level upgrades within
the core-relay portion of the upgrade. In the context of the Lightning Network, however, we deal
with pure software, so such upgrades are easier to deploy, yet they still
require much more coordination than any other upgrade mechanism in the
network.

One example of such an upgrade within the network was the introduction of a TLV
encoding for the routing information encoded within the onion
packets. The prior format used a hardcoded fixed-length message
format to communicate information such as the next hop.
Because this format was fixed, it meant that new protocol-level upgrades  weren't possible. The move to the more flexible TLV
format meant that after this upgrade, any sort of feature that
modified the type of information communicated at each hop could be rolled out at will.

It's worth mentioning that the TLV onion upgrade was a sort of "soft" internal
network upgrade, in that if a payment wasn't using any new feature beyond
that new routing information encoding, then a payment could be transmitted
using a mixed set of nodes.

===== End-to-end upgrades

To contrast the internal network upgrade, in this section we describe the
_end-to-end_ network upgrade. This upgrade mechanism differs from the internal
network upgrade in that it only requires the "ends" of the payment, the sender
and recipient, to upgrade.

This type of upgrade allows
for a wide array of unrestricted innovation within the network. Because of the
onion encrypted nature of payments within the network, those forwarding HTLCs
within the center of the network may not even know that new features are being
utilized.

One example of an end-to-end upgrade within the network was the rollout of multipart payments (MPP). MPP is a protocol-level feature that enables a
single payment to be split into multiple parts or paths, to be assembled at the
recipient for settlement. The rollout of MPP was coupled with a new
`node_announcement` level feature bit that indicates that the recipient knows
how to handle partial payments. Assuming a sender and recipient know about each
other (possibly via a BOLT 11 invoice), then they're able to use the new
feature without any further negotiation.

Another example of an end-to-end upgrade are the various types of
_spontaneous_ payments deployed within the network. One early type of
spontaneous payments called _keysend_ worked by simply placing the preimage of a payment within the encrypted onion. Upon receipt, the destination would decrypt the
preimage, then use that to settle the payment. Because the entire packet is end-to-end encrypted, this payment type was safe, since none of the intermediate nodes
are able to fully unwrap the onion to uncover the payment preimage.

==== Channel Construction Level Updates

The final broad category of updates are those that happen at
the channel construction level, but which don't modify the structure of the HTLC used widely within the network. When we say channel construction, we mean
how the channel is funded or created. As an example, the eltoo channel type
can be rolled out within the network using a new `node_announcement` level
feature bit as well as a `channel_announcement` level feature bit. Only the two
peers on the sides of the channels need to understand and advertise these new
features. This channel pair can then be used to forward any payment type
granted the channel supports it.

Another is the _anchor outputs_ channel format which allows the commitment fee to be
bumped via Bitcoin's Child-Pays-For-Parent (CPFP) fee management mechanism.

=== Conclusion

Lightning's wire protocol is incredibly flexible and allows for rapid innovation and interoperability without strict consensus. It is one of the reasons that the Lightning Network is experiencing much faster development and is attractive to many developers, who might otherwise find Bitcoin's development style too conservative and slow.
