lightning-bolts/07-routing-gossip.md

BOLT #7: P2P Node and Channel Discovery

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

Node and channel discovery serve two different purposes:

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

Peers in the network exchange channel_announcement messages that contain information about new channels between two nodes. They can also exchange node_announcement messages which supply additional information about nodes, and channel_update messages which update information about a channel.

There can only be one valid channel_announcement for any channel, but multiple node_announcement messages are possible (to update node information), and at least two channel_update messages are expected.

The announcement_signatures message

This is a direct message between 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:
   • [32:channel_id]
   • [8:short_channel_id]
   • [64:node_signature]
   • [64: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

If the open_channel message had the announce_channel bit set, then both nodes must send the announcement_signatures message, otherwise they MUST NOT.

If sent, announcement_signatures messages MUST NOT be sent until funding_locked has been sent and the funding transaction has at least 6 confirmations.

The short_channel_id is the unique description of the funding transaction. It is constructed with the most significant 3 bytes as the block height, the next 3 bytes indicating the transaction index within the block, and the least significant two bytes indicating the output index which pays to the channel.

The announcement_signatures message is created by constructing a channel_announcement message corresponding to the newly established channel, and sign it with the secrets matching their node_id and bitcoin_key, and send them using an announcement_signatures. The recipient MAY fail the channel if the node_signature or bitcoin_signature is incorrect. The recipient SHOULD queue the channel_announcement message for its peers if it has sent and received a valid announcement_signatures message.

On reconnection, a node SHOULD retransmit the announcement_signatures message if it has not received an announcement_signatures message, and MUST respond to the first announcement_signatures message after reconnection with its own announcement_signatures message.

The channel_announcement message

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

In order to prove the existence of channel between node_1 and node_2 we need to:

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

The first one is done by assuming that all nodes know the unspent transaction outputs, and thus can find the output given by short_channel_id and validate that it is indeed a P2WSH funding transaction output as to those keys specified in BOLT #3.

The second two are done by explicit signatures (bitcoin_signature_1 and bitcoin_signature_2, generated by each bitcoin_key and signing the corresponding node_id.

We also need to prove that node_1 and node_2 both agree on this announcement message; that is done by having a signature from each node_id signing the message (node_signature_1 and node_signature_2).

1. type: 256 (channel_announcement)
2. data:
   • [64:node_signature_1]
   • [64:node_signature_2]
   • [64:bitcoin_signature_1]
   • [64:bitcoin_signature_2]
   • [2:len]
   • [len:features]
   • [32:chain_hash]
   • [8:short_channel_id]
   • [33:node_id_1]
   • [33:node_id_2]
   • [33:bitcoin_key_1]
   • [33:bitcoin_key_2]

Requirements

The creating node MUST set chain_hash to the 32-byte hash that uniquely identifies the chain the channel was opened within. For the Bitcoin blockchain, the chain_hash value MUST be (encoded in hex): 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f.

The creating node MUST set short_channel_id to refer to the confirmed funding transaction as specified in BOLT #2. The corresponding output MUST be a P2WSH as described in BOLT #3.

The creating node MUST set node_id_1 and node_id_2 to the public keys of the two nodes who are operating the channel, such that node_id_1 is the numerically-lesser of the two DER encoded keys sorted in ascending numerical order, and MUST set bitcoin_key_1 and bitcoin_key_2 to funding_pubkeys of node_id_1 and node_id_2 respectively.

The creating node MUST compute the double-SHA256 hash h of the message, starting at offset 256, up to the end of the message. Thus the hash skips the 4 signatures, but hashes the rest of the message, including any future fields appended to the end. node_signature_1 and node_signature_2 MUST be valid signatures of the hash h using the secret associated with node_id_1 and node_id_2 respectively. bitcoin_signature_1 and bitcoin_signature_2 MUST be valid signatures of the hash h using the secret associated with bitcoin_key_1 and bitcoin_key_2 respectively.

The creating node SHOULD set len to the minimum length required to hold the features bits it sets.

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 the receiving node MUST NOT parse the remainder of the message and MUST NOT add the channel to its local network view, and SHOULD NOT forward the announcement.

The receiving node MUST ignore the message if the output specified by short_channel_id does not correspond to a P2WSH using bitcoin_key_1 and bitcoin_key_2 as specified in BOLT #3. The receiving node MUST ignore the message if this output is spent.

The receiving node MUST ignore the message if the specified chain_hash is unknown to the receiver.

Otherwise, the receiving node SHOULD fail the connection if bitcoin_signature_1, bitcoin_signature_2, node_signature_1 or node_signature_2 are invalid or not correct.

Otherwise, if node_id_1 or node_id_2 are blacklisted, it SHOULD ignore the message.

Otherwise, if the transaction referred to was not previously announced as a channel, the receiving node SHOULD queue the message for rebroadcasting, but 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 different node_id_1 or node_id_2, it 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 channels connected to them, otherwise it SHOULD store this channel_announcement.

The receiving node SHOULD forget a channel once its funding output has been spent or reorganized out.

Rationale

Requiring both nodes to sign indicates they are both willing to route other payments via this node (ie. take part of the public network). Requiring the Bitcoin signatures proves they control the channel.

The blacklisting of conflicting nodes means that we disallow multiple different announcements: no node should ever do this, as it implies that keys have leaked.

While channels shouldn't be advertised before they are sufficiently deep, the requirement against rebroadcasting only applies if the transaction hasn't moved to a different block.

To avoid having to store excessive-sized messages, yet allow reasonable expansion in future, nodes are allowed to restrict rebroadcasting (perhaps statistically).

New channel features are possible in future; backwards compatible (or optional) ones will have odd feature bits, incompatible ones will have even feature bits ("It's OK to be odd!"). Incompatible features will result in the announcement not being forwarded by nodes that don't understand them.

The node_announcement message

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

1. type: 257 (node_announcement)
2. data:
   • [64:signature]
   • [2:flen]
   • [flen:features]
   • [4:timestamp]
   • [33:node_id]
   • [3:rgb_color]
   • [32:alias]
   • [2:addrlen]
   • [addrlen:addresses]

The timestamp allows ordering in the case of multiple announcements; the rgb_color and alias allow intelligence services to give their nodes cool monikers like IRATEMONK and WISTFULTOLL and use the color black.

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

The following address descriptor types are defined:

• 0: padding. data = none (length 0).
• 1: ipv4. data = [4:ipv4_addr][2:port] (length 6)
• 2: ipv6. data = [16:ipv6_addr][2:port] (length 18)
• 3: tor v2 onion service. data = [10:onion_addr][2:port] (length 12)
  • Version 2 onion service addresses. Encodes an 80-bit truncated SHA-1 hash of a 1024-bit RSA public key for the onion service.
• 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]

Requirements

The creating node MUST set timestamp to be greater than any previous node_announcement it has created. It MAY base it on a UNIX timestamp. It MUST set signature to the signature of the double-SHA256 of the entire remaining packet after signature using the key given by node_id. It MAY set alias and rgb_color to customize their node's appearance in maps and graphs, where the first byte of rgb is the red value, the second byte is the green value and the last byte is the blue value. It MUST set alias to a valid UTF-8 string, with any alias bytes following equal to zero.

The creating node SHOULD fill addresses with an address descriptor for each public network address which expects incoming connections, and MUST set addrlen to the number of bytes in addresses. Non-zero typed address descriptors MUST be placed in ascending order; any number of zero-typed address descriptors MAY be placed anywhere, but SHOULD only be used for aligning fields following addresses.

The creating node MUST NOT create a type 1 or type 2 address descriptor with port equal to zero, and SHOULD ensure ipv4_addr and ipv6_addr are routable addresses. The creating node MUST NOT include more than one address descriptor of the same type.

The creating node SHOULD set flen to the minimum length required to hold the features bits it sets.

The receiving node SHOULD fail the connection if node_id is not a valid compressed public key, and MUST NOT further process the message.

The receiving node SHOULD fail the connection 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 alias), and MUST NOT further process the message.

If the features field contains unknown even bits the receiving node MUST NOT parse the remainder of the message and MAY discard the message altogether. The node MAY forward node_announcements that contain unknown features bit set, even though it hasn't parsed the announcement.

The receiving node SHOULD ignore the first address descriptor which does not match the types defined above. The receiving node SHOULD fail the connection if addrlen is insufficient to hold the address descriptors of the known types.

The receiving node SHOULD ignore ipv6_addr or ipv4_addr if port is zero.

The receiving node SHOULD ignore the message 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. Otherwise, if the timestamp is greater than the last-received node_announcement from this node_id the receiving node SHOULD queue the message for rebroadcasting, but MAY choose not to for messages longer than the minimum expected length.

The receiving node SHOULD NOT connect to a node which has an unknown features bit set in the node_announcement which is even.

The receiving node MAY use rgb_color and alias to reference nodes in interfaces, but SHOULD insinuate their self-signed origin.

Rationale

New node features are possible in future; backwards compatible (or optional) ones will have odd feature bits, incompatible ones will have even feature bits. These may be propagated by nodes even if they can't use the announcements themselves.

New address types can be added in future; as address descriptors have to be ordered in ascending order so they will be safely ignored. Future fields beyond addresses can still be added, optionally with padding within addresses if they require certain alignment.

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. It uses the 8-byte channel shortid which matches the channel_announcement and one bit in the flags field to indicate which end this is. It can do this multiple times, if it wants to change fees.

Note that the channel_update 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 backwards from the destination to the source. The initial exact value for amount_msat and minimal value for htlc_expiry, which are to be used for the last HTLC in the route, are provided in the payment request (see BOLT #11).

A node MAY still create a channel_update to communicate the channel parameters to the other endpoint, even though the channel has not been announced, e.g., because the announce_channel bit was not set. For further privacy such a channel_update MUST NOT be forwarded to other peers. Note that such a channel_update that is not preceded by a channel_announcement is invalid to any other peer and would be discarded.

1. type: 258 (channel_update)
2. data:
   • [64:signature]
   • [32:chain_hash]
   • [8:short_channel_id]
   • [4:timestamp]
   • [2:flags]
   • [2:cltv_expiry_delta]
   • [8:htlc_minimum_msat]
   • [4:fee_base_msat]
   • [4:fee_proportional_millionths]

The flags bitfield is used to indicate the direction of the channel this update concerns, i.e., it identifies the node that this update originated from, and signal various options concerning the channel. The following table specifies the meaning of the individual bits:

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

Requirements

The creating node MUST set signature to the signature of the double-SHA256 of the entire remaining packet after signature using its own node_id.

The creating node MUST set chain_hash and short_channel_id to match the 32-byte hash and 8-byte channel ID that uniquely identifies the channel within the channel_announcement message.

The creating node MUST set short_channel_id to match those in the already-sent channel_announcement message, and MUST set the direction bit of flags to 0 if the creating node is node_id_1 in that message, otherwise 1. Bits which are not assigned a meaning must be set to 0.

A node MAY create and send a channel_update with the disable bit set to signal the temporary unavailability of a channel, e.g., due to loss of connectivity, or the permanent unavailability, e.g., ahead of an on-chain settlement. A subsequent channel_update with the disable bit unset MAY re-enable the channel.

The creating node MUST set timestamp to greater than zero, and MUST set it to greater than any previously-sent channel_update for this short_channel_id.

It MUST set cltv_expiry_delta to the number of blocks it will subtract from an incoming HTLCs cltv_expiry. It MUST set htlc_minimum_msat to the minimum HTLC value it will accept, in millisatoshi. It MUST set fee_base_msat to the base fee it will charge for any HTLC, in millisatoshi, and fee_proportional_millionths to the amount it will charge per transferred satoshi in millionths of a satoshi.

The receiving nodes MUST ignore the channel_update if it does not correspond to one of its own channels, if the short_channel_id does not match a previous channel_announcement, or the channel has been closed in the meantime. It SHOULD accept channel_updates for its own channels in order to learn the other end's forwarding parameters, even for non-public channels.

The node_id for the signature verification is taken from the corresponding channel_announcement: node_id_1 if least-significant bit of flags is 0 or node_id_2 otherwise. The receiving node SHOULD fail the connection 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), and MUST NOT further process the message.

The receiving node MUST ignore the channel update if the specified chain_hash value is unknown, meaning it isn't active on the specified chain.

The receiving node SHOULD ignore the message if timestamp is not greater than than the last-received channel_announcement for this short_channel_id and node_id. Otherwise, if the timestamp is equal to the last-received channel_announcement and the fields other than signature differ, the node MAY blacklist this node_id and forget all channels associated with it. Otherwise the receiving node SHOULD queue the message for rebroadcasting, but MAY choose not to for messages longer than the minimum expected length.

Initial Sync

Upon establishing a connection, the two endpoints negotiate whether to perform an initial sync by setting the initial_routing_sync flags in the init message. The endpoint SHOULD set the initial_routing_sync flag if it requires a full copy of the other endpoint's routing state. Upon receiving an init message with the initial_routing_sync flag set the node sends channel_announcements, channel_updates and node_announcements for all known channels and nodes as if they were just received.

If the initial_routing_sync flag is not set, or initial sync was completed, then the node resumes normal operation, see the Rebroadcasting section for details.

Rebroadcasting

Nodes receiving a new channel_announcement or a channel_update or node_announcement with an updated timestamp update their local view of the network's topology accordingly.

Once the announcement has been processed it is added to a list of outgoing announcements (perhaps replacing older updates) to the processing node's peers, which will be flushed at regular intervals. This store and delayed forward broadcast is called a staggered broadcast

If, after applying the changes from the announcement, there are no channels associated with the announcing node, then the receiving node MAY purge the announcing node from the set of known nodes. Otherwise the receiving node updates the metadata and stores the signature associated with the announcement. This will later allow the receiving node to rebuild the announcement for its peers.

After processing the announcement the receiving node adds the announcement to a list of outgoing announcements.

Requirements

Each node SHOULD flush outgoing announcements once every 60 seconds, independently of the arrival times of announcements, resulting in a staggered announcement and deduplication of announcements.

Nodes MAY re-announce their channels regularly, however this is discouraged in order to keep the resource requirements low.

Nodes SHOULD send all channel_announcement messages followed by the latest node_announcement and channel_update messages upon connection establishment.

Rationale

Batching announcements form a natural ratelimit with low overhead.

The sending of all announcements on reconnection is naive, but simple, and allows bootstrap for new nodes as well as updating for nodes which have been offline for some time.

HTLC Fees

The node creating channel_update SHOULD accept HTLCs which pay a fee equal or greater than:

fee_base_msat + amount_msat * fee_proportional_millionths / 1000000

The node creating channel_update SHOULD accept HTLCs which pay an older fee for some time after sending channel_update to allow for propagation delay.

Pruning the Network View

Nodes SHOULD monitor the funding transactions in the blockchain to identify channels that are being closed. If the funding output of a channel is being spent, then the channel is to be considered closed and SHOULD be removed from the local network view.

Nodes MAY prune nodes added through node_announcement messages from their local view if the announced node no longer has any open channels associated. This is a direct result from the dependency of a node_announcement being preceded by a channel_announcement.

Recommendation on pruning stale entries

Several scenarios may result in channels becoming unusable and the endpoints unable to send updates for these channels. This happens for example in the case that both endpoints lose access to their private keys and cannot sign a channel_update nor close the channel on-chain. These 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 and be forwarded to other nodes forever. For this reason nodes MAY prune channels should the timestamp of the latest channel_update be older than 2 weeks (1209600 seconds). In addition nodes MAY ignore channels with a timestamp older than 2 weeks. Notice that this is a node policy and MUST NOT be enforced by peers, e.g., by closing channels when receiving outdated gossip messages.

Recommendations for Routing

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

If a route is computed by simply routing to the intended recipient, summing up the cltv_expiry_deltas, then nodes along the route may guess their position in the route. Knowing the CLTV of the HTLC and the surrounding topology with the cltv_expiry_deltas gives an attacker a way to guess the intended recipient. Therefore it is highly suggested to add a random offset to the CLTV that the intended recipient will receive, bumping all CLTVs along the route. In order to create a plausible offset the sender MAY start a limited random walk on the graph, starting from the intended recipient, sum the cltv_expiry_deltas, and then use the sum as the offset. This effectively creates a shadow route extension to the actual route, providing better protection against this kind of attack than simply picking a random offset.

Other more advanced considerations involve diversity of routes to avoid single points of failure and detection, and channel balance 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 amin_final_cltv_expiry of 9 (the default) when requesting payments.

Also, each node has the same fee scheme which 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

If B were to send 4,999,999 millisatoshi directly to C, it wouldn't charge itself a fee nor add its own cltv_expiry_delta, so it would use C's requested min_final_cltv_expiry of 9. We also assume it adds a "shadow route" to give an extra CLTV of 42. It could also add extra cltv deltas at other hops, as these values are a minimum, but we don't here for simplicity:

• amount_msat: 4999999
• cltv_expiry: current-block-height + 9 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 9 + 42

If A were to send an 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:

200 + 4999999 * 2000 / 1000000 = 10199

Similarly, it would need to add the cltv_expiry from B->C's channel_update (20), plus C's requested min_final_cltv_expiry (9), plus 42 for the "shadow route". Thus the update_add_htlc message from A to B would be:

• amount_msat: 5010198
• cltv_expiry: current-block-height + 20 + 9 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 9 + 42

The update_add_htlc from B to C would be the same as the B->C direct payment above.

Finally, if for some reason A chose the more expensive route via D, it would send the following update_add_htlc to D:

• amount_msat: 5020398
• cltv_expiry: current-block-height + 40 + 9 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 9 + 42

And the update_add_htlc from D to C would be the same as the B->C direct payment again.

References

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