<pre>
  BIP: 339
  Layer: Peer Services
  Title: WTXID-based transaction relay
  Authors: Suhas Daftuar <sdaftuar@chaincode.com>
  Status: Deployed
  Type: Specification
  Assigned: 2020-02-03
  License: BSD-2-Clause
</pre>

==Abstract==

This BIP describes two changes to the p2p protocol to support transaction relay
based on the BIP 141 wtxid of a transaction, rather than its txid.

==Motivation==

Historically, the inv messages sent on the Bitcoin peer-to-peer network to
announce transactions refer to transactions by their txid, which is a hash of
the transaction that does not include the witness (see BIP 141). This has been
the case even since Segregated Witness (BIP 141/143/144) has been adopted by
the network.

Not committing to the witness in transaction announcements creates
inefficiencies: because a transaction's witness can be malleated without
altering the txid, a node in receipt of a witness transaction that the node
does not accept will generally still download that same transaction when
announced by other peers. This is because the alternative -- of not downloading
a given txid after rejecting a transaction with that txid -- would allow a
third party to interfere with transaction relay by malleating a transaction's
witness and announcing the resulting invalid transaction to nodes, preventing
relay of the valid version of the transaction as well.

We can eliminate this concern by using the wtxid in place of the txid when
announcing and fetching transactions.

==Specification==

# A new wtxidrelay message is added, which is defined as an empty message where pchCommand == "wtxidrelay".
# The protocol version of nodes implementing this BIP must be set to 70016 or higher.
# The wtxidrelay message MUST be sent in response to a version message from a peer whose protocol version is >= 70016 and prior to sending a verack. A wtxidrelay message received after a verack message MUST be ignored or treated as invalid.
# A new inv type MSG_WTX (0x00000005) is added, for use in both inv messages and getdata requests, indicating that the hash being referenced is a transaction's wtxid. In the case of getdata requests, MSG_WTX implies that the transaction being requested should be serialized with witness as well, as described in BIP 144.
# After a node has received a wtxidrelay message from a peer, the node MUST use the MSG_WTX inv type when announcing transactions to that peer.
# After a node has received a wtxidrelay message from a peer, the node SHOULD use a MSG_WTX getdata message to request any announced transactions. A node MAY still request transactions from that peer using MSG_TX getdata messages, such as for transactions not recently announced by that peer (like the parents of recently announced transactions).

==Backward compatibility==

As wtxid-based transaction relay is only enabled between peers that both support it, older clients remain fully compatible and interoperable after this change.

==Implementation==

https://github.com/bitcoin/bitcoin/pull/18044

==Copyright==

This BIP is licensed under the 2-clause BSD license.
