Assign BIP 339: WTXID-based transaction relay

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+<pre>
+  BIP: 339
+  Layer: Peer Services
+  Title: WTXID-based transaction relay
+  Author: Suhas Daftuar <sdaftuar@chaincode.com>
+  Comments-Summary: No comments yet.
+  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0339
+  Status: Draft
+  Type: Standards Track
+  Created: 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 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 sent and received a "wtxidrelay" message to/from a given peer, the node is required to use the MSG_WTX inv-type when announcing transactions to that peer, or requesting announced transactions from that peer.
+
+==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.