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BOLT 4: rationale for the last node's payload

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Specify the payload for the last node in the route and how it is used to return
errors. The idea is to prevent the next to last node to guess if the next node is
the final one.
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sstone 2017-05-17 17:57:35 +02:00 • committed by Rusty Russell
parent c60e5e05ec
commit f7277cea43
1 changed files with 14 additions and 1 deletions
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15
04-onion-routing.md
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@ -166,6 +166,19 @@ Nodes forwarding HTLCs MUST construct the outgoing HTLC as specified within
`per_hop`. Otherwise, deviation from the specified HTLC parameters
may lead to extraneous routing failure.
### Payload for the last node
The last node in the route could just discard its payload since it will not forward payments. However, when building the route, the original
sender must use a payload for the last node with the following values:
* `outgoing_cltv_value` is set to the final expiry specified by the recipient
* `amt_to_forward` is set to the final amount specified by the recipient
This way, the final node can check these values and return errors if needed, which will defeat probing attacks by the next to last node which could
try to find out if the next node is the last one (by re-sending HTLCs with different amounts/expiries):
The last node will extract its onion payload from the HTLC it has received and compare its values to the HTLC values.
See the [Returning Errors](#returning-errors) section below for more details.
## Packet Construction
Assuming a _sender node_ `n_0` wants to route a packet to a _final recipient_ `n_r`.
@ -584,7 +597,7 @@ HTLC at the final hop:
2. data:
* [`4`:`cltv_expiry`]
If the `amt_to_forward` does not match the `incoming_htlc_amt` of
If the `amt_to_forward` is higher than `incoming_htlc_amt` of
the HTLC at the final hop:
1. type: 19 (`final_incorrect_htlc_amount`)