Consider hop_payloads length and hmac fields.

Author: tnull

lightning/src/routing/router.rs

@@ -68,7 +68,7 @@ pub struct Route {
 	/// last RouteHop in each path must be the same.
 	/// Each entry represents a list of hops, NOT INCLUDING our own, where the last hop is the
 	/// destination. Thus, this must always be at least length one. While the maximum length of any
-	/// given path is variable, keeping the length of any path to less than 40 should currently
+	/// given path is variable, keeping the length of any path to less than 20 should currently
 	/// ensure it is viable.
 	pub paths: Vec<Vec<RouteHop>>,
 	/// The `payment_params` parameter passed to [`find_route`].
@@ -1476,21 +1476,21 @@ where L::Target: Logger {
 	}
 
 	// Step (5).
-	// We only consider paths shorter than our maximum length estimate. 
+	// We only consider paths shorter than our maximum length estimate.
 	// In the legacy onion format, the maximum number of hops used to be a fixed value of 20.
 	// However, in the TLV onion format, there is no fixed maximum length, but the `hop_payloads`
 	// field is always 1300 bytes. As the `tlv_payload` for each hop may vary in length, we have to
 	// estimate how many hops the route may have so that it actually fits the `hop_payloads` field.
 	//
-	// We estimate 2+8 (amt_to_forward) + 2+4 (outgoing_cltv_value) + 2+8 (short_channel_id) = 26
-	// bytes for each intermediate hop and 2+8 (amt_to_forward) + 2+4 (outgoing_cltv_value) +
-	// 2+32+8 (payment_secret and total_msat) = 58 bytes for the final hop.
-	// Since the length of the potentially included `payment_metadata` is unkown to us, we
-	// generously round down from (1300-58) / 26 = 47 to arrive at a conservative estimate of a
-	// feasible maximum path length of 40 hops.
-	const MAX_PATH_LENGTH_ESTIMATE: usize = 40;
+	// We estimate 3+32 (payload length and HMAC) + 1+8 (amt_to_forward) + 1+4 (outgoing_cltv_value) +
+	// 1+8 (short_channel_id) = 58 bytes for each intermediate hop and 3+32
+	// (payload length and HMAC) + 1+8 (amt_to_forward) + 1+4 (outgoing_cltv_value) + 1+32+8
+	// (payment_secret and total_msat) = 90 bytes for the final hop.
+	// Since the length of the potentially included `payment_metadata` is unkown to us, we round
+	// down from (1300-90) / 58 = 21 just to arrive again at a conservative estimate of 20.
+	const MAX_PATH_LENGTH_ESTIMATE: usize = 20;
 	payment_paths.retain(|path| path.hops.len() <= MAX_PATH_LENGTH_ESTIMATE);
-	
+
 	if payment_paths.len() == 0 {
 		return Err(LightningError{err: "Failed to find a path to the given destination".to_owned(), action: ErrorAction::IgnoreError});
 	}