Specify max HTLC nLocktime for expiry_too_far

Author: ariard

04-onion-routing.md

@@ -62,6 +62,7 @@ A node:
   * [Returning Errors](#returning-errors)
     * [Failure Messages](#failure-messages)
     * [Receiving Failure Codes](#receiving-failure-codes)
+  * [`max_htlc_cltv` Selection](#max-htlc-cltv-selection)
   * [Test Vector](#test-vector)
     * [Returning Errors](#returning-errors)
   * [References](#references)
@@ -1325,7 +1326,7 @@ A _forwarding node_ MAY, but a _final node_ MUST NOT:
     - report the `cltv_expiry` of the outgoing HTLC and the current channel setting for the outgoing
     channel.
     - return an `incorrect_cltv_expiry` error.
-  - if the `cltv_expiry` is unreasonably near the present:
+  - if the `cltv_expiry` is more than `max_htlc_cltv` near the present:
     - report the current channel setting for the outgoing channel.
     - return an `expiry_too_soon` error.
   - if the `cltv_expiry` is unreasonably far in the future:
@@ -1407,6 +1408,13 @@ The _origin node_:
   - MAY use the data specified in the various failure types for debugging
   purposes.
 
+## `max_htlc_cltv` Selection
+
+If the HTLC expires too far in the future, a forwarding node can return an `expiry_too_far` error.
+
+This `max_htlc_ctlv` value is defined by default to be 2016 blocks, based on historical default
+deployed by Lightning implementations.
+
 # Test Vector
 
 ## Returning Errors