BOLT 2: move closing tx definition to BOLT #3, and insist recipient c…

…heck fee

The recipient should check the fee is valid, otherwise we might get
fun overflow games.  So might as well check that it's better than the
previous offer (as the sender is already required to do).

Changing BOLT #3 to define the mutual close tx is next patch.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

02-peer-protocol.md

@@ -408,44 +408,36 @@ Nodes SHOULD send a `closing_signed` message after `shutdown` has
 been received and no HTLCs remain in either commitment transaction.
 
 A sending node MUST set `fee_satoshis` lower than or equal to the
-fee of the final commitment transaction.
+base fee of the final commitment transaction as calculated in [BOLT #3](03-transactions.md#fee-calculation).
 
 The sender SHOULD set the initial `fee_satoshis` according to its
 estimate of cost of inclusion in a block.
 
 The sender MUST set `signature` to the Bitcoin signature of the close
-transaction with the node responsible for paying the bitcoin fee
-paying `fee_satoshis`, without populating any output which is below
-its own `dust_limit_satoshis`. The sender MAY also eliminate its own
-output from the mutual close transaction.
+transaction as specified in [BOLT #3](03-transactions.md#closing-transaction).
 
-The receiver MUST check `signature` is valid for either the close
-transaction with the given `fee_satoshis` as detailed above and its
-own `dust_limit_satoshis` OR that same transaction with the sender's
-output eliminated, and MUST fail the connection if it is not.
+The receiver MUST check `signature` is valid for either variant of close
+transaction specified in [BOLT #3](03-transactions.md#closing-transaction),
+and MUST fail the connection if it is not.
+
+If `fee_satoshis` is equal to its previously sent `fee_satoshis`, the receiver
+SHOULD close the connection and SHOULD sign and broadcast the final closing
+transaction.  Otherwise, if this is the first `closing_signed` received, the
+recipient MUST fail the connection if `fee_satoshis` is greater than the base
+fee of the final commitment transaction as calculated in
+[BOLT #3](03-transactions.md#fee-calculation), otherwise the recipient SHOULD
+fail the connection if `fee_satoshis` is not strictly between its last-sent
+`fee_satoshis` and its previously-received `fee_satoshis`.
 
 If the receiver agrees with the fee, it SHOULD reply with a
 `closing_signed` with the same `fee_satoshis` value, otherwise it
-SHOULD propose a value strictly between the received `fee_satoshis`
+MUST propose a value strictly between the received `fee_satoshis`
 and its previously-sent `fee_satoshis`.
 
-Once a node has sent or received a `closing_signed` with matching
-`fee_satoshis` it SHOULD close the connection and SHOULD sign and
-broadcast the final closing transaction.
-
 #### Rationale
 
-There is a possibility of irreparable differences on closing if one
-node considers the other's output too small to allow propagation on
-the bitcoin network (aka "dust"), and that other node instead
-considers that output to be too valuable to discard.  This is why each
-side uses its own `dust_limit_satoshis`, and the result can be a
-signature validation failure, if they disagree on what the closing
-transaction should look like.
-
-However, if one side chooses to eliminate its own output, there's no
-reason for the other side to fail the closing protocol, so this is
-explicitly allowed.
+The "strictly between" requirements ensure that we make forward
+progress, even if only by a single satoshi at a time.
 
 Note that there is limited risk if the closing transaction is
 delayed, and it will be broadcast very soon, so there is usually no