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:

• 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.
• Node discovery allows nodes to broadcast their ID, host, and port, so that other nodes can open connections and establish payment channels with them.

To support 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.

To support 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.

Table of Contents

• The announcement_signatures Message
• The channel_announcement Message
• The node_announcement Message
• The channel_update Message
• Initial Sync
• Rebroadcasting
• HTLC Fees
• Pruning the Network View
• Recommendations for Routing
• References

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:
   • [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

The announcement_signatures message is created by constructing a channel_announcement message, corresponding to the newly established channel, and signing it with the secrets matching an endpoint's node_id and bitcoin_key. After it's signed, the announcement_signatures message may be sent.

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.

A node:

• if the open_channel message has the announce_channel bit set AND a shutdown message has not been sent:
  • MUST send the announcement_signatures message.
    • MUST NOT send announcement_signatures messages until funding_locked has been sent AND the funding transaction has at least six confirmations.
• otherwise:
  • MUST NOT send the announcement_signatures message.
• upon reconnection:
  • MUST respond to the first announcement_signatures message with its own announcement_signatures message.
  • if it has NOT received an announcement_signatures message:
    • SHOULD retransmit the announcement_signatures message.

A recipient node:

• if the node_signature OR the bitcoin_signature is NOT correct:
  • MAY fail the channel.
• if it has sent AND received a valid announcement_signatures message:
  • SHOULD queue the channel_announcement message for its peers.

The channel_announcement Message

This 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:
   • [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 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 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f.
• 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 numerically-lesser of the two DER-encoded keys sorted in ascending numerical 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).
• SHOULD set len to the minimum length required to hold the features bits it sets.

The final 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 parse the remainder of the message.
  • MUST NOT add the channel to its local network view.
  • SHOULD NOT forward the announcement.
• 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.
• otherwise:
  • if bitcoin_signature_1, bitcoin_signature_2, node_signature_1 OR node_signature_2 are invalid OR NOT correct:
    • SHOULD fail the connection.
  • 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.

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!"). Incompatible features will result in the announcement not being forwarded by nodes that do not understand them.

The node_announcement Message

This 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:
   • [64:signature]
   • [2:flen]
   • [flen:features]
   • [4:timestamp]
   • [33:node_id]
   • [3:rgb_color]
   • [32:alias]
   • [2:addrlen]
   • [addrlen: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:

• 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 (a.k.a. Tor hidden 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 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 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 non-zero typed address descriptors in ascending order.
• MAY place any number of zero-typed address descriptors anywhere.
• SHOULD use placement only for aligning fields that follow addresses.
• MUST NOT create a type 1 OR type 2 address descriptor with port equal to 0.
• SHOULD ensure ipv4_addr AND ipv6_addr are routable addresses.
• MUST NOT include more than one address descriptor of the same type.
• SHOULD set flen to the minimum length required to hold the features bits it sets.

The final node:

• if node_id is NOT a valid compressed public key:
  • SHOULD fail 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 unknown fields following alias):
  • SHOULD fail the connection.
  • MUST NOT process the message further.
• if features field contains unknown even bits:
  • MUST NOT parse the remainder of the message.
  • MAY discard the message altogether.
  • SHOULD NOT connect to the node.
• MAY forward node_announcements that contain an unknown features bit, regardless of if it has parsed the announcement or not.
• 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 fail the connection.
• if port is equal to 0:
  • SHOULD ignore ipv6_addr OR ipv4_addr.
• 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.

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 may be propagated by nodes even if they cannot process the announcements themselves.

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 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 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:
   • [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: 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 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:

• MAY create a channel_update to communicate the channel parameters to the final node, even though the channel has not yet been announced (i.e. the announce_channel bit was not set).
  • 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 flags to 0.
• otherwise:
  • MUST set the direction bit of flags to 1.
• MUST set bits 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 final node will accept.
• 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.

The final 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):
  • MUST NOT process the message further.
  • SHOULD fail the connection.
• if the specified chain_hash value is unknown (meaning it isn't active on the specified chain):
  • MUST ignore the channel update.
• if timestamp is NOT greater than that of the last-received channel_announcement for this short_channel_id AND for node_id:
  • SHOULD ignore the message.
• otherwise:
  • if the timestamp is equal to the last-received channel_announcement AND the fields (other than signature) differ:
    • MAY blacklist this node_id.
    • MAY forget all channels associated with it.
• if the timestamp is unreasonably far in the future:
  • MAY discard the channel_announcement.
• otherwise:
  • SHOULD queue the message for rebroadcasting.
  • MAY choose NOT to for messages longer than the minimum expected length.

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.

Initial Sync

Requirements

An endpoint node:

• upon establishing a connection:
  • SHOULD set the init message's initial_routing_sync flag to 1, to negotiate an initial sync.
• if it requires a full copy of the other endpoint's routing state:
  • SHOULD set the initial_routing_sync flag to 1.
• upon receiving an init message with the initial_routing_sync flag set to 1:
  • SHOULD send 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 set to 0, OR if the initial sync was completed:
  • SHOULD resume normal operation, as specified in the following Rebroadcasting section.

Rebroadcasting

Requirements

The final 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 final node to rebuild the announcement for its peers.

An endpoint node:

• SHOULD flush outgoing announcements once every 60 seconds, independently of the arrival times of announcements.
  • Note: this results in staggered announcements that are unique (not duplicated).
• MAY re-announce its channels regularly.
  • Note: this is discouraged, in order to keep the resource requirements low.
• upon connection establishment:
  • SHOULD send all channel_announcement messages, followed by the latest node_announcement AND channel_update messages.

Rationale

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

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

HTLC Fees

Requirements

The origin node:

• SHOULD accept HTLCs that pay a fee equal to or greater than:
  • fee_base_msat + ( amount_msat * 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 being spent:
  • 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

An endpoint node:

• if a channel's latest channel_updates timestamp is older than two weeks (1209600 seconds):
  • MAY prune the channel.
  • MAY ignore the channel.
  • Note: this is an endpoint node policy and MUST NOT be enforced by forwarding peers, e.g. by closing channels when receiving outdated gossip messages. [ FIXME: is this intended meaning? ]

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.

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 of 9 (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 of 9. 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 + 9 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 9 + 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_msat * fee_proportional_millionths / 1000000 )

200 + ( 4999999 * 2000 / 1000000 ) = 10199

Similarly, it would need to add B->C's channel_update cltv_expiry (20), C's requested min_final_cltv_expiry (9), 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 + 9 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 9 + 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 + 9 + 42
• onion_routing_packet:
  • amt_to_forward = 4999999
  • outgoing_cltv_value = current-block-height + 9 + 42

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

References

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