ZIP 401: allow implementations to use a different approximation of the

memory size of a transaction than its serialized size.

fixes zcash#673

Signed-off-by: Daira Emma Hopwood <daira@jacaranda.org>

zip-0401.rst

@@ -79,7 +79,11 @@ potentially other implementations that have adopted this specification in full).
 The mempool of a node holds a set of transactions. Each transaction has a *cost*,
 which is an integer defined as:
 
-  max(serialized transaction size in bytes, 4000)
+  max(memory size in bytes, 4000)
+
+The memory size is an estimate of the size that a transaction occupies in the
+memory of a node. It MAY be approximated as the serialized transaction size in
+bytes.
 
 Each transaction also has an *eviction weight*, which is *cost* + *low_fee_penalty*,
 where *low_fee_penalty* is 16000 if the transaction pays a fee less than the
@@ -135,6 +139,13 @@ be that the complexity of accounting for this overhead is unwarranted given that
 the format of a transaction already imposes a minimum size. However, the proposed
 cost function is almost as simple as using transaction size directly.
 
+There is some ambiguity in the specification of "memory size", allowing
+implementations to use different approximations. Currently, `zcashd` uses a size
+computed by the `RecursiveDynamicUsage` function, and `zebra` uses the serialized
+size. This has been changed from a previous version of this ZIP that specified
+use of the serialized size, to reflect how the implementation in `zcashd` has
+worked since it was first deployed [#size-ambiguity]_.
+
 The threshold 4000 for the cost function is chosen so that the size in bytes of a
 typical fully shielded Sapling transaction (with, say, 2 shielded outputs and up
 to 5 shielded inputs) will fall below the threshold. This has the effect of
@@ -212,4 +223,5 @@ References
 .. [#zip-0239] `ZIP 239: Relay of Version 5 Transactions <zip-0239.rst>`_
 .. [#zip-0252] `ZIP 252: Deployment of the NU5 Network Upgrade <zip-0252.rst>`_
 .. [#zip-0317] `ZIP 317: Proportional Transfer Fee Mechanism <zip-0317.rst>`_
+.. [#size-ambiguity] `zcash/zips issue #673 - ZIP 401 uses serialized size to calculate cost but the zcashd implementation uses RecursiveDynamicUsage <https://github.com/zcash/zips/issues/673>`_
 .. [#BitcoinCore-PR6722] `Bitcoin Core PR 6722: Limit mempool by throwing away the cheapest txn and setting min relay fee to it <https://github.com/bitcoin/bitcoin/pull/6722>`_