07-routing-gossip.md

BOLT #7: P2P Node and Channel Discovery

This specification describes simple node discovery, channel discovery, and channel update mechanisms that do not rely on a third-party to disseminate the information.

Node and channel discovery serve two different purposes:

• Node discovery allows nodes to broadcast their ID, host, and port, so that other nodes can open connections and establish payment channels with them.
• Channel discovery allows the creation and maintenance of a local view of the network's topology, so that a node can discover routes to desired destinations.

To support channel and node discovery, three gossip messages are supported:

• For node discovery, peers exchange node_announcement messages, which supply additional information about the nodes. There may be multiple node_announcement messages, in order to update the node information.

• For channel discovery, peers in the network exchange channel_announcement messages containing information regarding new channels between the two nodes. They can also exchange channel_update messages, which update information about a channel. There can only be one valid channel_announcement for any channel, but at least two channel_update messages are expected.

Table of Contents

• Definition of short_channel_id
• The announcement_signatures Message
• The channel_announcement Message
• The node_announcement Message
• The channel_update Message
• Query Messages
• Rebroadcasting
• HTLC Fees
• Pruning the Network View
• Recommendations for Routing
• References

Definition of short_channel_id

The short_channel_id is the unique description of the funding transaction. It is constructed as follows:

1. the most significant 3 bytes: indicating the block height
2. the next 3 bytes: indicating the transaction index within the block
3. the least significant 2 bytes: indicating the output index that pays to the channel.

The standard human readable format for short_channel_id is created by printing the above components, in the order: block height, transaction index, and output index. Each component is printed as a decimal number, and separated from each other by the small letter x. For example, a short_channel_id might be written as 539268x845x1, indicating a channel on the output 1 of the transaction at index 845 of the block at height 539268.

Rationale

The short_channel_id human readable format is designed so that double-clicking or double-tapping it will select the entire ID on most systems. Humans prefer decimal when reading numbers, so the ID components are written in decimal. The small letter x is used since on most fonts, the x is visibly smaller than decimal digits, making it easy to visibly group each component of the ID.

The announcement_signatures Message

This is a direct message between the two endpoints of a channel and serves as an opt-in mechanism to allow the announcement of the channel to the rest of the network. It contains the necessary signatures, by the sender, to construct the channel_announcement message.

1. type: 259 (announcement_signatures)
2. data:
   • [channel_id:channel_id]
   • [short_channel_id:short_channel_id]
   • [signature:node_signature]
   • [signature:bitcoin_signature]

The willingness of the initiating node to announce the channel is signaled during channel opening by setting the announce_channel bit in channel_flags (see BOLT #2).

Requirements

The announcement_signatures message is created by constructing a channel_announcement message, corresponding to the newly confirmed channel funding transaction, and signing it with the secrets matching an endpoint's node_id and bitcoin_key.

A node:

• If the open_channel message has the announce_channel bit set AND a shutdown message has not been sent:
  • After channel_ready has been sent and received AND the funding transaction has enough confirmations to ensure that it won't be reorganized:
    • MUST send announcement_signatures for the funding transaction.
• Otherwise:
  • MUST NOT send the announcement_signatures message.
• Upon reconnection (once the above timing requirements have been met):
  • If it has NOT previously received announcement_signatures for the funding transaction:
    • MUST send its own announcement_signatures message.
  • If it receives announcement_signatures for the funding transaction:
    • MUST respond with its own announcement_signatures message.

A recipient node:

• If the short_channel_id is NOT correct:
  • SHOULD send a warning and close the connection, or send an error and fail the channel.
• If the node_signature OR the bitcoin_signature is NOT correct:
  • MAY send a warning and close the connection, or send an error and fail the channel.
• If it has sent AND received a valid announcement_signatures message:
  • If the funding transaction has at least 6 confirmations:
    • SHOULD queue the channel_announcement message for its peers.
• If it has not sent channel_ready:
  • SHOULD defer handling the announcement_signatures until after it has sent channel_ready.

Rationale

Channels must not be announced before the funding transaction has enough confirmations, because a blockchain reorganization would otherwise invalidate the short_channel_id.

The channel_announcement Message

This gossip message contains ownership information regarding a channel. It ties each on-chain Bitcoin key to the associated Lightning node key, and vice-versa. The channel is not practically usable until at least one side has announced its fee levels and expiry, using channel_update.

Proving the existence of a channel between node_1 and node_2 requires:

1. proving that the funding transaction pays to bitcoin_key_1 and bitcoin_key_2
2. proving that node_1 owns bitcoin_key_1
3. proving that node_2 owns bitcoin_key_2

Assuming that all nodes know the unspent transaction outputs, the first proof is accomplished by a node finding the output given by the short_channel_id and verifying that it is indeed a P2WSH funding transaction output for those keys specified in BOLT #3.

The last two proofs are accomplished through explicit signatures: bitcoin_signature_1 and bitcoin_signature_2 are generated for each bitcoin_key and each of the corresponding node_ids are signed.

It's also necessary to prove that node_1 and node_2 both agree on the announcement message: this is accomplished by having a signature from each node_id (node_signature_1 and node_signature_2) signing the message.

1. type: 256 (channel_announcement)
2. data:
   • [signature:node_signature_1]
   • [signature:node_signature_2]
   • [signature:bitcoin_signature_1]
   • [signature:bitcoin_signature_2]
   • [u16:len]
   • [len*byte:features]
   • [chain_hash:chain_hash]
   • [short_channel_id:short_channel_id]
   • [point:node_id_1]
   • [point:node_id_2]
   • [point:bitcoin_key_1]
   • [point:bitcoin_key_2]

Requirements

The origin node:

• MUST set chain_hash to the 32-byte hash that uniquely identifies the chain that the channel was opened within:
  • for the Bitcoin blockchain:
    • MUST set chain_hash value (encoded in hex) equal to 6fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000.
• MUST set short_channel_id to refer to the confirmed funding transaction, as specified in BOLT #2.
  • Note: the corresponding output MUST be a P2WSH, as described in BOLT #3.
• MUST set node_id_1 and node_id_2 to the public keys of the two nodes operating the channel, such that node_id_1 is the lexicographically-lesser of the two compressed keys sorted in ascending lexicographic order.
• MUST set bitcoin_key_1 and bitcoin_key_2 to node_id_1 and node_id_2's respective funding_pubkeys.
• MUST compute the double-SHA256 hash h of the message, beginning at offset 256, up to the end of the message.
  • Note: the hash skips the 4 signatures but hashes the rest of the message, including any future fields appended to the end.
• MUST set node_signature_1 and node_signature_2 to valid signatures of the hash h (using node_id_1 and node_id_2's respective secrets).
• MUST set bitcoin_signature_1 and bitcoin_signature_2 to valid signatures of the hash h (using bitcoin_key_1 and bitcoin_key_2's respective secrets).
• MUST set features based on what features were negotiated for this channel, according to BOLT #9
• MUST set len to the minimum length required to hold the features bits it sets.
• If the funding transaction has less than 6 confirmations:
  • MUST NOT send channel_announcement.

The receiving node:

• MUST verify the integrity AND authenticity of the message by verifying the signatures.
• if there is an unknown even bit in the features field:
  • MUST NOT attempt to route messages through the channel.
• if the short_channel_id's output does NOT correspond to a P2WSH (using bitcoin_key_1 and bitcoin_key_2, as specified in BOLT #3) OR the output is spent:
  • MUST ignore the message.
• if the specified chain_hash is unknown to the receiver:
  • MUST ignore the message.
• if the short_channel_id's output does NOT have at least 6 confirmations:
  • MAY accept the message if the output is close to 6 confirmations, in case the receiving node hasn't received the latest block(s) yet.
  • otherwise:
    • SHOULD ignore the message.
• otherwise:
  • if bitcoin_signature_1, bitcoin_signature_2, node_signature_1 OR node_signature_2 are invalid OR NOT correct:
    • SHOULD send a warning.
    • MAY close the connection.
    • MUST ignore the message.
  • otherwise:
    • if node_id_1 OR node_id_2 are blacklisted:
      • SHOULD ignore the message.
    • otherwise:
      • if the transaction referred to was NOT previously announced as a channel:
        • SHOULD queue the message for rebroadcasting.
        • MAY choose NOT to for messages longer than the minimum expected length.
    • if it has previously received a valid channel_announcement, for the same transaction, in the same block, but for a different node_id_1 or node_id_2:
      • SHOULD blacklist the previous message's node_id_1 and node_id_2, as well as this node_id_1 and node_id_2 AND forget any channels connected to them.
    • otherwise:
      • SHOULD store this channel_announcement.
• once its funding output has been spent OR reorganized out:
  • SHOULD forget a channel after a 12-block delay.

Rationale

Both nodes are required to sign to indicate they are willing to route other payments via this channel (i.e. be part of the public network); requiring their Bitcoin signatures proves that they control the channel.

The blacklisting of conflicting nodes disallows multiple different announcements. Such conflicting announcements should never be broadcast by any node, as this implies that keys have leaked.

While channels should not be advertised before they are sufficiently deep, the requirement against rebroadcasting only applies if the transaction has not moved to a different block.

In order to avoid storing excessively large messages, yet still allow for reasonable future expansion, nodes are permitted to restrict rebroadcasting (perhaps statistically).

New channel features are possible in the future: backwards compatible (or optional) features will have odd feature bits, while incompatible features will have even feature bits ("It's OK to be odd!").

A delay of 12 blocks is used when forgetting a channel on funding output spend as to permit a new channel_announcement to propagate which indicates this channel was spliced.

The node_announcement Message

This gossip message allows a node to indicate extra data associated with it, in addition to its public key. To avoid trivial denial of service attacks, nodes not associated with an already known channel are ignored.

1. type: 257 (node_announcement)
2. data:
   • [signature:signature]
   • [u16:flen]
   • [flen*byte:features]
   • [u32:timestamp]
   • [point:node_id]
   • [3*byte:rgb_color]
   • [32*byte:alias]
   • [u16:addrlen]
   • [addrlen*byte:addresses]

timestamp allows for the ordering of messages, in the case of multiple announcements. rgb_color and alias allow intelligence services to assign nodes colors like black and cool monikers like 'IRATEMONK' and 'WISTFULTOLL'.

addresses allows a node to announce its willingness to accept incoming network connections: it contains a series of address descriptors for connecting to the node. The first byte describes the address type and is followed by the appropriate number of bytes for that type.

The following address descriptor types are defined:

• 1: ipv4; data = [4:ipv4_addr][2:port] (length 6)
• 2: ipv6; data = [16:ipv6_addr][2:port] (length 18)
• 3: Deprecated (length 12). Used to contain Tor v2 onion services.
• 4: Tor v3 onion service; data = [35:onion_addr][2:port] (length 37)
  • version 3 (prop224) onion service addresses; Encodes: [32:32_byte_ed25519_pubkey] || [2:checksum] || [1:version], where checksum = sha3(".onion checksum" || pubkey || version)[:2].
• 5: DNS hostname; data = [1:hostname_len][hostname_len:hostname][2:port] (length up to 258)
  • hostname bytes MUST be ASCII characters.
  • Non-ASCII characters MUST be encoded using Punycode: https://en.wikipedia.org/wiki/Punycode

Requirements

The origin node:

• MUST set timestamp to be greater than that of any previous node_announcement it has previously created.
  • MAY base it on a UNIX timestamp.
• MUST set signature to the signature of the double-SHA256 of the entire remaining packet after signature (using the key given by node_id).
• MAY set alias AND rgb_color to customize its appearance in maps and graphs.
  • Note: the first byte of rgb_color is the red value, the second byte is the green value, and the last byte is the blue value.
• MUST set alias to a valid UTF-8 string, with any alias trailing-bytes equal to 0.
• SHOULD fill addresses with an address descriptor for each public network address that expects incoming connections.
• MUST set addrlen to the number of bytes in addresses.
• MUST place address descriptors in ascending order.
• SHOULD NOT place any zero-typed address descriptors anywhere.
• SHOULD use placement only for aligning fields that follow addresses.
• MUST NOT create a type 1, type 2 or type 5 address descriptor with port equal to 0.
• SHOULD ensure ipv4_addr AND ipv6_addr are routable addresses.
• MUST set features according to BOLT #9
• SHOULD set flen to the minimum length required to hold the features bits it sets.
• SHOULD not announce a Tor v2 onion service.
• MUST NOT announce more than one type 5 DNS hostname.

The receiving node:

• if node_id is NOT a valid compressed public key:
  • SHOULD send a warning.
  • MAY close the connection.
  • MUST NOT process the message further.
• if signature is NOT a valid signature (using node_id of the double-SHA256 of the entire message following the signature field, including any future fields appended to the end):
  • SHOULD send a warning.
  • MAY close the connection.
  • MUST NOT process the message further.
• if features field contains unknown even bits:
  • SHOULD NOT connect to the node.
  • Unless paying a BOLT #11 invoice which does not have the same bit(s) set, MUST NOT attempt to send payments to the node.
  • MUST NOT route a payment through the node.
• SHOULD ignore the first address descriptor that does NOT match the types defined above.
• if addrlen is insufficient to hold the address descriptors of the known types:
  • SHOULD send a warning.
  • MAY close the connection.
• if port is equal to 0:
  • SHOULD ignore ipv6_addr OR ipv4_addr OR hostname.
• if node_id is NOT previously known from a channel_announcement message, OR if timestamp is NOT greater than the last-received node_announcement from this node_id:
  • SHOULD ignore the message.
• otherwise:
  • if timestamp is greater than the last-received node_announcement from this node_id:
    • SHOULD queue the message for rebroadcasting.
    • MAY choose NOT to queue messages longer than the minimum expected length.
• MAY use rgb_color AND alias to reference nodes in interfaces.
  • SHOULD insinuate their self-signed origins.
• SHOULD ignore Tor v2 onion services.
• if more than one type 5 address is announced:
  • SHOULD ignore the additional data.
  • MUST not forward the node_announcement.

Rationale

New node features are possible in the future: backwards compatible (or optional) ones will have odd feature bits, incompatible ones will have even feature bits. These will be propagated normally; incompatible feature bits here refer to the nodes, not the node_announcement message itself.

New address types may be added in the future; as address descriptors have to be ordered in ascending order, unknown ones can be safely ignored. Additional fields beyond addresses may also be added in the future—with optional padding within addresses, if they require certain alignment.

Security Considerations for Node Aliases

Node aliases are user-defined and provide a potential avenue for injection attacks, both during the process of rendering and during persistence.

Node aliases should always be sanitized before being displayed in HTML/Javascript contexts or any other dynamically interpreted rendering frameworks. Similarly, consider using prepared statements, input validation, and escaping to protect against injection vulnerabilities and persistence engines that support SQL or other dynamically interpreted querying languages.

• Stored and Reflected XSS Prevention
• DOM-based XSS Prevention
• SQL Injection Prevention

Don't be like the school of Little Bobby Tables.

The channel_update Message

After a channel has been initially announced, each side independently announces the fees and minimum expiry delta it requires to relay HTLCs through this channel. Each uses the 8-byte channel shortid that matches the channel_announcement and the 1-bit channel_flags field to indicate which end of the channel it's on (origin or final). A node can do this multiple times, in order to change fees.

Note that the channel_update gossip message is only useful in the context of relaying payments, not sending payments. When making a payment A -> B -> C -> D, only the channel_updates related to channels B -> C (announced by B) and C -> D (announced by C) will come into play. When building the route, amounts and expiries for HTLCs need to be calculated backward from the destination to the source. The exact initial value for amount_msat and the minimal value for cltv_expiry, to be used for the last HTLC in the route, are provided in the payment request (see BOLT #11).

1. type: 258 (channel_update)
2. data:
   • [signature:signature]
   • [chain_hash:chain_hash]
   • [short_channel_id:short_channel_id]
   • [u32:timestamp]
   • [byte:message_flags]
   • [byte:channel_flags]
   • [u16:cltv_expiry_delta]
   • [u64:htlc_minimum_msat]
   • [u32:fee_base_msat]
   • [u32:fee_proportional_millionths]
   • [u64:htlc_maximum_msat]

The channel_flags bitfield is used to indicate the direction of the channel: it identifies the node that this update originated from and signals various options concerning the channel. The following table specifies the meaning of its individual bits:

Bit Position	Name	Meaning
0	direction	Direction this update refers to.
1	disable	Disable the channel.

The message_flags bitfield is used to provide additional details about the message:

Bit Position	Name
0	must_be_one
1	dont_forward

The node_id for the signature verification is taken from the corresponding channel_announcement: node_id_1 if the least-significant bit of flags is 0 or node_id_2 otherwise.

Requirements

The origin node:

• MUST NOT send a created channel_update before channel_ready has been received.
• MAY create a channel_update to communicate the channel parameters to the channel peer, even though the channel has not yet been announced (i.e. the announce_channel bit was not set or the channel_update is sent before the peers exchanged announcement signatures).
  • MUST set the short_channel_id to either an alias it has received from the peer, or the real channel short_channel_id.
  • MUST set dont_forward to 1 in message_flags
  • MUST NOT forward such a channel_update to other peers, for privacy reasons.
  • Note: such a channel_update, one not preceded by a channel_announcement, is invalid to any other peer and would be discarded.
• MUST set signature to the signature of the double-SHA256 of the entire remaining packet after signature, using its own node_id.
• MUST set chain_hash AND short_channel_id to match the 32-byte hash AND 8-byte channel ID that uniquely identifies the channel specified in the channel_announcement message.
• if the origin node is node_id_1 in the message:
  • MUST set the direction bit of channel_flags to 0.
• otherwise:
  • MUST set the direction bit of channel_flags to 1.
• MUST set htlc_maximum_msat to the maximum value it will send through this channel for a single HTLC.
  • MUST set this to less than or equal to the channel capacity.
  • MUST set this to less than or equal to max_htlc_value_in_flight_msat it received from the peer.
  • MUST set this to greater than or equal to htlc_minimum_msat.
• MUST set must_be_one in message_flags to 1.
• MUST set bits in channel_flags and message_flags that are not assigned a meaning to 0.
• MAY create and send a channel_update with the disable bit set to 1, to signal a channel's temporary unavailability (e.g. due to a loss of connectivity) OR permanent unavailability (e.g. prior to an on-chain settlement).
  • MAY sent a subsequent channel_update with the disable bit set to 0 to re-enable the channel.
• MUST set timestamp to greater than 0, AND to greater than any previously-sent channel_update for this short_channel_id.
  • SHOULD base timestamp on a UNIX timestamp.
• MUST set cltv_expiry_delta to the number of blocks it will subtract from an incoming HTLC's cltv_expiry.
• MUST set htlc_minimum_msat to the minimum HTLC value (in millisatoshi) that the channel peer will accept.
  • MUST set htlc_minimum_msat to less than or equal to htlc_maximum_msat.
• MUST set fee_base_msat to the base fee (in millisatoshi) it will charge for any HTLC.
• MUST set fee_proportional_millionths to the amount (in millionths of a satoshi) it will charge per transferred satoshi.
• SHOULD NOT create redundant channel_updates
• If it creates a new channel_update with updated channel parameters:
  • SHOULD keep accepting the previous channel parameters for 10 minutes

The receiving node:

• if the short_channel_id does NOT match a previous channel_announcement, OR if the channel has been closed in the meantime:
  • MUST ignore channel_updates that do NOT correspond to one of its own channels.
• SHOULD accept channel_updates for its own channels (even if non-public), in order to learn the associated origin nodes' forwarding parameters.
• if signature is not a valid signature, using node_id of the double-SHA256 of the entire message following the signature field (including unknown fields following fee_proportional_millionths):
  • SHOULD send a warning and close the connection.
  • MUST NOT process the message further.
• if the specified chain_hash value is unknown (meaning it isn't active on the specified chain):
  • MUST ignore the channel update.
• if the timestamp is equal to the last-received channel_update for this short_channel_id AND node_id:
  • if the fields below timestamp differ:
    • MAY blacklist this node_id.
    • MAY forget all channels associated with it.
  • if the fields below timestamp are equal:
    • SHOULD ignore this message
• if timestamp is lower than that of the last-received channel_update for this short_channel_id AND for node_id:
  • SHOULD ignore the message.
• otherwise:
  • if the timestamp is unreasonably far in the future:
    • MAY discard the channel_update.
  • otherwise:
    • SHOULD queue the message for rebroadcasting.
    • MAY choose NOT to for messages longer than the minimum expected length.
• if htlc_maximum_msat < htlc_minimum_msat:
  • SHOULD ignore this channel during route considerations.
• if htlc_maximum_msat is greater than channel capacity:
  • MAY blacklist this node_id
  • SHOULD ignore this channel during route considerations.
• otherwise:
  • SHOULD consider the htlc_maximum_msat when routing.

Rationale

The timestamp field is used by nodes for pruning channel_updates that are either too far in the future or have not been updated in two weeks; so it makes sense to have it be a UNIX timestamp (i.e. seconds since UTC 1970-01-01). This cannot be a hard requirement, however, given the possible case of two channel_updates within a single second.

It is assumed that more than one channel_update message changing the channel parameters in the same second may be a DoS attempt, and therefore, the node responsible for signing such messages may be blacklisted. However, a node may send a same channel_update message with a different signature (changing the nonce in signature signing), and hence fields apart from signature are checked to see if the channel parameters have changed for the same timestamp. It is also important to note that ECDSA signatures are malleable. So, an intermediate node who received the channel_update message can rebroadcast it just by changing the s component of signature with -s. This should however not result in the blacklist of the node_id from where the message originated.

The recommendation against redundant channel_updates minimizes spamming the network, however it is sometimes inevitable. For example, a channel with a peer which is unreachable will eventually cause a channel_update to indicate that the channel is disabled, with another update re-enabling the channel when the peer reestablishes contact. Because gossip messages are batched and replace previous ones, the result may be a single seemingly-redundant update.

When a node creates a new channel_update to change its channel parameters, it will take some time to propagate through the network and payers may use older parameters. It is recommended to keep accepting older parameters for at least 10 minutes to improve payment latency and reliability.

The must_be_one field in message_flags was previously used to indicate the presence of the htlc_maximum_msat field. This field must now always be present, so must_be_one is a constant value, and ignored by receivers.

Query Messages

Understanding of messages used to be indicated with the gossip_queries feature bit; now these messages are universally supported, that feature has now been slightly repurposed. Not offering this feature means a node is not worth querying for gossip: either they do not store the entire gossip map, or they are only connected to a single peer (this one).

There are several messages which contain a long array of short_channel_ids (called encoded_short_ids) so we include an encoding byte which allows for different encoding schemes to be defined in the future, if they provide benefit.

Encoding types:

• 0: uncompressed array of short_channel_id types, in ascending order.
• 1: Previously used for zlib compression, this encoding MUST NOT be used.

This encoding is also used for arrays of other types (timestamps, flags, ...), and specified with an encoded_ prefix. For example, encoded_timestamps is an array of timestamps with a 0 prefix.

Query messages can be extended with optional fields that can help reduce the number of messages needed to synchronize routing tables by enabling:

• timestamp-based filtering of channel_update messages: only ask for channel_update messages that are newer than the ones you already have.
• checksum-based filtering of channel_update messages: only ask for channel_update messages that carry different information from the ones you already have.

Nodes can signal that they support extended gossip queries with the gossip_queries_ex feature bit.

The query_short_channel_ids/reply_short_channel_ids_end Messages

1. type: 261 (query_short_channel_ids)

2. data:

   • [chain_hash:chain_hash]
   • [u16:len]
   • [len*byte:encoded_short_ids]
   • [query_short_channel_ids_tlvs:tlvs]

3. tlv_stream: query_short_channel_ids_tlvs

4. types:

   1. type: 1 (query_flags)
   2. data:
      • [byte:encoding_type]
      • [...*byte:encoded_query_flags]

encoded_query_flags is an array of bitfields, one bigsize per bitfield, one bitfield for each short_channel_id. Bits have the following meaning:

Bit Position	Meaning
0	Sender wants channel_announcement
1	Sender wants channel_update for node 1
2	Sender wants channel_update for node 2
3	Sender wants node_announcement for node 1
4	Sender wants node_announcement for node 2

Query flags must be minimally encoded, which means that one flag will be encoded with a single byte.

1. type: 262 (reply_short_channel_ids_end)
2. data:
   • [chain_hash:chain_hash]
   • [byte:full_information]

This is a general mechanism which lets a node query for the channel_announcement and channel_update messages for specific channels (identified via short_channel_ids). This is usually used either because a node sees a channel_update for which it has no channel_announcement or because it has obtained previously unknown short_channel_ids from reply_channel_range.

Requirements

The sender:

• SHOULD NOT send this to a peer which does not offer gossip_queries.
• MUST NOT send query_short_channel_ids if it has sent a previous query_short_channel_ids to this peer and not received reply_short_channel_ids_end.
• MUST set chain_hash to the 32-byte hash that uniquely identifies the chain that the short_channel_ids refer to.
• MUST set the first byte of encoded_short_ids to the encoding type.
• MUST encode a whole number of short_channel_ids to encoded_short_ids
• MAY send this if it receives a channel_update for a short_channel_id for which it has no channel_announcement.
• SHOULD NOT send this if the channel referred to is not an unspent output.
• MAY include an optional query_flags. If so:
  • MUST set encoding_type, as for encoded_short_ids.
  • Each query flag is a minimally-encoded bigsize.
  • MUST encode one query flag per short_channel_id.

The receiver:

• if the first byte of encoded_short_ids is not a known encoding type:
  • MAY send a warning.
  • MAY close the connection.
• if encoded_short_ids does not decode into a whole number of short_channel_id:
  • MAY send a warning.
  • MAY close the connection.
• if it has not sent reply_short_channel_ids_end to a previously received query_short_channel_ids from this sender:
  • MAY send a warning.
  • MAY close the connection.
• if the incoming message includes query_short_channel_ids_tlvs:
  • if encoding_type is not a known encoding type:
    • MAY send a warning.
    • MAY close the connection.
  • if encoded_query_flags does not decode to exactly one flag per short_channel_id:
    • MAY send a warning.
    • MAY close the connection.
• MUST respond to each known short_channel_id:
  • if the incoming message does not include encoded_query_flags:
    • with a channel_announcement and the latest channel_update for each end
    • MUST follow with any node_announcements for each channel_announcement
  • otherwise:
    • We define query_flag for the Nth short_channel_id in encoded_short_ids to be the Nth bigsize of the decoded encoded_query_flags.
    • if bit 0 of query_flag is set:
      • MUST reply with a channel_announcement
    • if bit 1 of query_flag is set and it has received a channel_update from node_id_1:
      • MUST reply with the latest channel_update for node_id_1
    • if bit 2 of query_flag is set and it has received a channel_update from node_id_2:
      • MUST reply with the latest channel_update for node_id_2
    • if bit 3 of query_flag is set and it has received a node_announcement from node_id_1:
      • MUST reply with the latest node_announcement for node_id_1
    • if bit 4 of query_flag is set and it has received a node_announcement from node_id_2:
      • MUST reply with the latest node_announcement for node_id_2
  • SHOULD NOT wait for the next outgoing gossip flush to send these.
• SHOULD avoid sending duplicate node_announcements in response to a single query_short_channel_ids.
• MUST follow these responses with reply_short_channel_ids_end.
• if does not maintain up-to-date channel information for chain_hash:
  • MUST set full_information to 0.
• otherwise:
  • SHOULD set full_information to 1.

Rationale

Future nodes may not have complete information; they certainly won't have complete information on unknown chain_hash chains. While this full_information field (previously and confusingly called complete) cannot be trusted, a 0 does indicate that the sender should search elsewhere for additional data.

The explicit reply_short_channel_ids_end message means that the receiver can indicate it doesn't know anything, and the sender doesn't need to rely on timeouts. It also causes a natural ratelimiting of queries.

The query_channel_range and reply_channel_range Messages

1. type: 263 (query_channel_range)

2. data:

   • [chain_hash:chain_hash]
   • [u32:first_blocknum]
   • [u32:number_of_blocks]
   • [query_channel_range_tlvs:tlvs]

3. tlv_stream: query_channel_range_tlvs

4. types:

   1. type: 1 (query_option)
   2. data:
      • [bigsize:query_option_flags]

query_option_flags is a bitfield represented as a minimally-encoded bigsize. Bits have the following meaning:

Bit Position	Meaning
0	Sender wants timestamps
1	Sender wants checksums

Though it is possible, it would not be very useful to ask for checksums without asking for timestamps too: the receiving node may have an older channel_update with a different checksum, asking for it would be useless. And if a channel_update checksum is actually 0 (which is quite unlikely) it will not be queried.

1. type: 264 (reply_channel_range)

2. data:

   • [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]

3. tlv_stream: reply_channel_range_tlvs

4. types:

   1. type: 1 (timestamps_tlv)
   2. data:
      • [byte:encoding_type]
      • [...*byte:encoded_timestamps]
   3. type: 3 (checksums_tlv)
   4. data:
      • [...*channel_update_checksums:checksums]

For a single channel_update, timestamps are encoded as:

1. subtype: channel_update_timestamps
2. data:
   • [u32:timestamp_node_id_1]
   • [u32:timestamp_node_id_2]

Where:

• timestamp_node_id_1 is the timestamp of the channel_update for node_id_1, or 0 if there was no channel_update from that node.
• timestamp_node_id_2 is the timestamp of the channel_update for node_id_2, or 0 if there was no channel_update from that node.

For a single channel_update, checksums are encoded as:

1. subtype: channel_update_checksums
2. data:
   • [u32:checksum_node_id_1]
   • [u32:checksum_node_id_2]

Where:

• checksum_node_id_1 is the checksum of the channel_update for node_id_1, or 0 if there was no channel_update from that node.
• checksum_node_id_2 is the checksum of the channel_update for node_id_2, or 0 if there was no channel_update from that node.

The checksum of a channel_update is the CRC32C checksum as specified in RFC3720 of this channel_update without its signature and timestamp fields.

This allows querying for channels within specific blocks.

Requirements

The sender of query_channel_range:

• SHOULD NOT send this to a peer which does not offer gossip_queries.
• MUST NOT send this if it has sent a previous query_channel_range to this peer and not received all reply_channel_range replies.
• MUST set chain_hash to the 32-byte hash that uniquely identifies the chain that it wants the reply_channel_range to refer to
• MUST set first_blocknum to the first block it wants to know channels for
• MUST set number_of_blocks to 1 or greater.
• MAY append an additional query_channel_range_tlv, which specifies the type of extended information it would like to receive.

The receiver of query_channel_range:

• if it has not sent all reply_channel_range to a previously received query_channel_range from this sender:
  • MAY send a warning.
  • MAY close the connection.
• MUST respond with one or more reply_channel_range:
  • MUST set with chain_hash equal to that of query_channel_range,
  • MUST limit number_of_blocks to the maximum number of blocks whose results could fit in encoded_short_ids
  • MAY split block contents across multiple reply_channel_range.
  • the first reply_channel_range message:
    • MUST set first_blocknum less than or equal to the first_blocknum in query_channel_range
    • MUST set first_blocknum plus number_of_blocks greater than first_blocknum in query_channel_range.
  • successive reply_channel_range message:
    • MUST have first_blocknum equal or greater than the previous first_blocknum.
  • MUST set sync_complete to false if this is not the final reply_channel_range.
  • the final reply_channel_range message:
    • MUST have first_blocknum plus number_of_blocks equal or greater than the query_channel_range first_blocknum plus number_of_blocks.
  • MUST set sync_complete to true.

If the incoming message includes query_option, the receiver MAY append additional information to its reply:

• if bit 0 in query_option_flags is set, the receiver MAY append a timestamps_tlv that contains channel_update timestamps for all short_channel_ids in encoded_short_ids
• if bit 1 in query_option_flags is set, the receiver MAY append a checksums_tlv that contains channel_update checksums for all short_channel_ids in encoded_short_ids

Rationale

A single response might be too large for a single packet, so multiple replies may be required. We want to allow a peer to store canned results for (say) 1000-block ranges, so replies can exceed the requested range. However, we require that each reply be relevant (overlapping the requested range).

By insisting that replies be in increasing order, the receiver can easily determine if replies are done: simply check if first_blocknum plus number_of_blocks equals or exceeds the first_blocknum plus number_of_blocks it asked for.

The addition of timestamp and checksum fields allow a peer to omit querying for redundant updates.

The gossip_timestamp_filter Message

1. type: 265 (gossip_timestamp_filter)
2. data:
   • [chain_hash:chain_hash]
   • [u32:first_timestamp]
   • [u32:timestamp_range]

This message allows a node to constrain future gossip messages to a specific range. A node which wants any gossip messages has to send this, otherwise no gossip messages would be received.

Note that this filter replaces any previous one, so it can be used multiple times to change the gossip from a peer.

Requirements

The sender:

• MUST set chain_hash to the 32-byte hash that uniquely identifies the chain that it wants the gossip to refer to.
• If the receiver does not offer gossip_queries:
  • SHOULD set first_timestamp to 0xFFFFFFFF and timestamp_range to 0.

The receiver:

• SHOULD send all gossip messages whose timestamp is greater or equal to first_timestamp, and less than first_timestamp plus timestamp_range.
  • MAY wait for the next outgoing gossip flush to send these.
• SHOULD send gossip messages as it generates them regardless of timestamp.
• Otherwise (relayed gossip):
  • SHOULD restrict future gossip messages to those whose timestamp is greater or equal to first_timestamp, and less than first_timestamp plus timestamp_range.
• If a channel_announcement has no corresponding channel_updates:
  • MUST NOT send the channel_announcement.
• Otherwise:
  • MUST consider the timestamp of the channel_announcement to be the timestamp of a corresponding channel_update.
  • MUST consider whether to send the channel_announcement after receiving the first corresponding channel_update.
• If a channel_announcement is sent:
  • MUST send the channel_announcement prior to any corresponding channel_updates and node_announcements.

Rationale

Since channel_announcement doesn't have a timestamp, we generate a likely one. If there's no channel_update then it is not sent at all, which is most likely in the case of pruned channels.

Otherwise the channel_announcement is usually followed immediately by a channel_update. Ideally we would specify that the first (oldest) channel_update's timestamp is to be used as the time of the channel_announcement, but new nodes on the network will not have this, and further would require the first channel_update timestamp to be stored. Instead, we allow any update to be used, which is simple to implement.

In the case where the channel_announcement is nonetheless missed, query_short_channel_ids can be used to retrieve it.

Nodes can use timestamp_filter to reduce their gossip load when they have many peers (eg. setting first_timestamp to 0xFFFFFFFF after the first few peers, in the assumption that propagation is adequate). This assumption of adequate propagation does not apply for gossip messages generated directly by the node itself, so they should ignore filters.

Requirements

A node:

• MUST NOT relay any gossip messages it did not generate itself, unless explicitly requested.

Rebroadcasting

Requirements

A receiving node:

• upon receiving a new channel_announcement or a channel_update or node_announcement with an updated timestamp:
  • SHOULD update its local view of the network's topology accordingly.
• after applying the changes from the announcement:
  • if there are no channels associated with the corresponding origin node:
    • MAY purge the origin node from its set of known nodes.
  • otherwise:
    • SHOULD update the appropriate metadata AND store the signature associated with the announcement.
      • Note: this will later allow the node to rebuild the announcement for its peers.

A node:

• MUST not send gossip it did not generate itself, until it receives gossip_timestamp_filter.
• SHOULD flush outgoing gossip messages once every 60 seconds, independently of the arrival times of the messages.
  • Note: this results in staggered announcements that are unique (not duplicated).
  • SHOULD NOT forward gossip messages to peers who sent networks in init and did not specify the chain_hash of this gossip message.
• MAY re-announce its channels regularly.
  • Note: this is discouraged, in order to keep the resource requirements low.

Rationale

Once the gossip message has been processed, it's added to a list of outgoing messages, destined for the processing node's peers, replacing any older updates from the origin node. This list of gossip messages will be flushed at regular intervals; such a store-and-delayed-forward broadcast is called a staggered broadcast. Also, such batching forms a natural rate limit with low overhead.

HTLC Fees

Requirements

The origin node:

• SHOULD accept HTLCs that pay a fee equal to or greater than:
  • fee_base_msat + ( amount_to_forward * fee_proportional_millionths / 1000000 )
• SHOULD accept HTLCs that pay an older fee, for some reasonable time after sending channel_update.
  • Note: this allows for any propagation delay.

Pruning the Network View

Requirements

A node:

• SHOULD monitor the funding transactions in the blockchain, to identify channels that are being closed.
• if the funding output of a channel is spent and received 12 block confirmations:
  • SHOULD be removed from the local network view AND be considered closed.
• if the announced node no longer has any associated open channels:
  • MAY prune nodes added through node_announcement messages from their local view.
    • Note: this is a direct result of the dependency of a node_announcement being preceded by a channel_announcement.

Recommendation on Pruning Stale Entries

Requirements

A node:

• if the timestamp of the latest channel_update in either direction is older than two weeks (1209600 seconds):
  • MAY prune the channel.
  • MAY ignore the channel.
  • Note: this is an individual node policy and MUST NOT be enforced by forwarding peers, e.g. by closing channels when receiving outdated gossip messages.

Rationale

Several scenarios may result in channels becoming unusable and its endpoints becoming unable to send updates for these channels. For example, this occurs if both endpoints lose access to their private keys and can neither sign channel_updates nor close the channel on-chain. In this case, the channels are unlikely to be part of a computed route, since they would be partitioned off from the rest of the network; however, they would remain in the local network view would be forwarded to other peers indefinitely.

The oldest channel_update is used to prune the channel since both sides need to be active in order for the channel to be usable. Doing so prunes channels even if one side continues to send fresh channel_updates but the other node has disappeared.

Recommendations for Routing

When calculating a route for an HTLC, both the cltv_expiry_delta and the fee need to be considered: the cltv_expiry_delta contributes to the time that funds will be unavailable in the event of a worst-case failure. The relationship between these two attributes is unclear, as it depends on the reliability of the nodes involved.

If a route is computed by simply routing to the intended recipient and summing the cltv_expiry_deltas, then it's possible for intermediate nodes to guess their position in the route. Knowing the CLTV of the HTLC, the surrounding network topology, and the cltv_expiry_deltas gives an attacker a way to guess the intended recipient. Therefore, it's highly desirable to add a random offset to the CLTV that the intended recipient will receive, which bumps all CLTVs along the route.

In order to create a plausible offset, the origin node MAY start a limited random walk on the graph, starting from the intended recipient and summing the cltv_expiry_deltas, and use the resulting sum as the offset. This effectively creates a shadow route extension to the actual route and provides better protection against this attack vector than simply picking a random offset would.

Other more advanced considerations involve diversification of route selection, to avoid single points of failure and detection, and balancing of local channels.

Routing Example

Consider four nodes:

   B
  / \
 /   \
A     C
 \   /
  \ /
   D

Each advertises the following cltv_expiry_delta on its end of every channel:

1. A: 10 blocks
2. B: 20 blocks
3. C: 30 blocks
4. D: 40 blocks

C also uses a min_final_cltv_expiry_delta of 18 (the default) when requesting payments.

Also, each node has a set fee scheme that it uses for each of its channels:

1. A: 100 base + 1000 millionths
2. B: 200 base + 2000 millionths
3. C: 300 base + 3000 millionths
4. D: 400 base + 4000 millionths

The network will see eight channel_update messages:

1. A->B: cltv_expiry_delta = 10, fee_base_msat = 100, fee_proportional_millionths = 1000
2. A->D: cltv_expiry_delta = 10, fee_base_msat = 100, fee_proportional_millionths = 1000
3. B->A: cltv_expiry_delta = 20, fee_base_msat = 200, fee_proportional_millionths = 2000
4. D->A: cltv_expiry_delta = 40, fee_base_msat = 400, fee_proportional_millionths = 4000
5. B->C: cltv_expiry_delta = 20, fee_base_msat = 200, fee_proportional_millionths = 2000
6. D->C: cltv_expiry_delta = 40, fee_base_msat = 400, fee_proportional_millionths = 4000
7. C->B: cltv_expiry_delta = 30, fee_base_msat = 300, fee_proportional_millionths = 3000
8. C->D: cltv_expiry_delta = 30, fee_base_msat = 300, fee_proportional_millionths = 3000

B->C. If B were to send 4,999,999 millisatoshi directly to C, it would neither charge itself a fee nor add its own cltv_expiry_delta, so it would use C's requested min_final_cltv_expiry_delta of 18. Presumably it would also add a shadow route to give an extra CLTV of 42. Additionally, it could add extra CLTV deltas at other hops, as these values represent a minimum, but chooses not to do so here, for the sake of simplicity:

• amount_msat: 4999999
• cltv_expiry: current-block-height + 18 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 18 + 42

A->B->C. If A were to send 4,999,999 millisatoshi to C via B, it needs to pay B the fee it specified in the B->C channel_update, calculated as per HTLC Fees:

    fee_base_msat + ( amount_to_forward * fee_proportional_millionths / 1000000 )

    200 + ( 4999999 * 2000 / 1000000 ) = 10199

Similarly, it would need to add B->C's channel_update cltv_expiry_delta (20), C's requested min_final_cltv_expiry_delta (18), and the cost for the shadow route (42). Thus, A->B's update_add_htlc message would be:

• amount_msat: 5010198
• cltv_expiry: current-block-height + 20 + 18 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 18 + 42

B->C's update_add_htlc would be the same as B->C's direct payment above.

A->D->C. Finally, if for some reason A chose the more expensive route via D, A->D's update_add_htlc message would be:

• amount_msat: 5020398
• cltv_expiry: current-block-height + 40 + 18 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 18 + 42

And D->C's update_add_htlc would again be the same as B->C's direct payment above.

This work is licensed under a Creative Commons Attribution 4.0 International License.