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.
The messaging layer, which is detailed in this chapter, consists of "Framing and message format," "Type-Length-Value (TLV)" encoding, and "Feature bits." These components are highlighted by an outline in the protocol suite, shown in Messaging layer in the Lightning protocol suite.
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).
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 less than 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, allows the protocol to achieve forward compatibility as well.
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 that our future wire parser is able to properly encode/decode any of the higher-level types.
In High-level message types, we map the name of a given message type to the high-level routine used to encode/decode the type.
| High-level type | Framing | Comment |
|---|---|---|
|
A 32-byte fixed-length byte slice |
When decoding, reject if contents are not a valid UTF-8 string |
|
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 |
|
An unsigned 64-bit integer ( |
Composed of the block height (24 bits), transaction index (24 bits), and output index (16 bits) packed into 8 bytes |
|
An unsigned 64-bit integer ( |
Represents 1000th of a satoshi |
|
An unsigned 64-bit integer ( |
The base unit of bitcoin |
|
An secp256k1 public key encoded in compressed format, occupying 33 bytes |
Occupies a fixed 33-byte length on the wire |
|
An ECDSA signature of the secp256k1 elliptic curve |
Encoded as a fixed 64-byte byte slice, packed as |
|
An 8-bit integer |
|
|
A 16-bit integer |
|
|
A 64-bit integer |
|
|
A variable-length byte slice |
Prefixed with a 16-bit integer denoting the length of the bytes |
|
RGB color encoding |
Encoded as a series of 8-bit integers |
|
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 wire message, including the prefix type of the message along with the contents of its message body.
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 (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.
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.
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.
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:
-
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. -
If the value is less than or equal to
0xffff(65535): The discriminant is encoded as0xfd, which indicates that the value that follows is larger than0xfd, but smaller than0xffff. The number is then encoded as a 16-bit integer. Including the discriminant, we can encode a value that is greater than 253, but less than 65,535 using 3 bytes. -
If the value is less than
0xffffffff(4294967295): The discriminant is encoded as0xfe. The body is encoded using a 32-bit integer, including the discriminant, and we can encode a value that’s less than4,294,967,295using 5 bytes. -
Otherwise, we just encode the value as a full-size 64-bit integer.
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.
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
typeandlengthfields. In other words, the smallestBigSizerepresentation for an integer must be used at all times. -
Each
typemay 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.
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.
An astute reader may have noticed the various locations where 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 Feature bit compatibility matrix.
| 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 know 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.
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.
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.
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.
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.
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.
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.