There's a kinda-neat intersection between the "use TLV" proposal and the
"multi-cell-onion" idea, so I want to make a concrete proposal (wording
needs formalization):
Multi-cell structure:
1. `realm` (or `per_hop_type` if you prefer) meaning expanded.
2. Lower 4 bits is `num_extra_cells` to use (ie. total 1-17 cells).
3. Upper 4 bits reserved: if set, drop.
4. HMAC on end covers that many per-hops.
5. Padding is thus 12 bytes + 64 * `num_extra_cells`.
Structure of padding changes:
1. We make the onion `padding` contain TLV, rename to `tlv`.
2. TLVs (as always!) are in lexicographical order (with shortest-wins on
tiebreak).
3. TLVs follow unknown-odd-is-ok rule.
4. No 0-type; that terminates (backwards compat with 0-filled padding).
New onion error value:
1. type: PERM|22 (`tlv_element_invalid`)
2. `2`:`offset`
The writer MUST set `offset` to a byte offset within the `tlv` field
of the tlv element it rejected. It SHOULD use the offset of the `type` byte of
the TLV
element if it didn't understand it, the offset the `len` byte of the TLV
element if it was an incorrect length, or otherwise within the `value`
if the value was somehow invalid.
TLVs defined for initial onion:
* type 2: `switch_chain`
length: 32
value: chain_id of new chain.
Used to switch chains during transit or at final hop.
---
Use with multi-part payment ("base AMP"):
* type 4: `total_payment`
length: variable, <= 8
value: amount of total payment, in msat (big-endian of course).
Writer must only use for final hop, and only if bolt11 flagged it as
available (bolt11-multi_part_available). May use even if this payment
meets the total_payment requirement.
Reader MUST reject if not final hop, MAY reject if invoice was not
`bolt11-multi_part_available`. Reader SHOULD wait until total parts
meet or exceed `total_payment` (exceed may be due to fuzzing) [rest
as per previous posts].
Thoughts?
Rusty.
PS. I prefer 'multi-part-payment' to 'base AMP' in the spec. It's more
explicit, and leaves the namespace clear for more atomicy AMPs.
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