bolt-02: add endorsement byte to update_add_hltc

Add an endorsement field to allow nodes to signal whether a HTLC is
expected to resolve quickly or is unknown to the forwarder. The
addition of this field allows for the introduction of local reputation
tracking that still allows new and unknown entrants access to resources.

Author: carlaKC

02-peer-protocol.md

@@ -1978,7 +1978,10 @@ is destined, is described in [BOLT #4](04-onion-routing.md).
     1. type: 0 (`blinded_path`)
     2. data:
         * [`point`:`path_key`]
-
+    1. type: 1 (`endorsed`)
+    2. data:
+        * [`byte`:`endorsed`]
+ 
 #### Requirements
 
 A sending node:
@@ -2016,6 +2019,20 @@ A sending node:
   - MUST increase the value of `id` by 1 for each successive offer.
   - if it is relaying a payment inside a blinded route:
     - MUST set `path_key` (see [Route Blinding](04-onion-routing.md#route-blinding))
+  - if it is the original source of the HTLC:
+    - if it does not expect immediate fulfillment upon receipt by the 
+      final destination: 
+      - SHOULD set `endorsed` to `0`.
+    - otherwise:  
+      - SHOULD set `endorsed` to `1`. 
+      - MAY choose to set `endorsed` to `0` to mimic endorsement patterns of 
+        HTLCs it has forwarded.
+  - otherwise: 
+    - if `endorsed` is present and non-zero for the corresponding incoming HTLC
+      AND the incoming peer is considered to have sufficient local reputation: 
+      - SHOULD set `endorsed` to `1`
+    - otherwise: 
+      - SHOULD set `endorsed` to `0`.
 
 `id` MUST NOT be reset to 0 after the update is complete (i.e. after `revoke_and_ack` has
 been received). It MUST continue incrementing instead.
@@ -2047,6 +2064,9 @@ A receiving node:
     - MUST respond with an error as detailed in [Failure Messages](04-onion-routing.md#failure-messages)
   - Otherwise:
     - MUST follow the requirements for the reader of `payload` in [Payload Format](04-onion-routing.md#payload-format)
+  - if `endorsed` is not provided OR `endorsed` is zero:
+    - MAY choose to limit the liquidity and slots available to forward the 
+      corresponding outgoing HTLC in `onion_routing_packet`, if any.
 
 The `onion_routing_packet` contains an obfuscated list of hops and instructions for each hop along the path.
 It commits to the HTLC by setting the `payment_hash` as associated data, i.e. includes the `payment_hash` in the computation of HMACs.