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Draft of Overwinter network handshake
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ZIP: ???
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Title: Network Upgrade Zero ("OverWinter") Network Handshake
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Author: Simon Liu <simon@z.cash>
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Comments-Summary: No comments yet.
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Category: Process
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Created: 2018-01-15
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License: MIT
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Terminology
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===========
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The key words "MUST", "MUST NOT", "SHOULD", and "MAY" in this document are to be interpreted as described in RFC 2119.
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"Legacy" - pre-NU0
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"NU0" - Network upgade zero
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"Overwinter" - Code-name for NU0
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Abstract
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========
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This proposal defines an upgrade to network handshake format required for Network Upgrade Activation Mechanism [#zip-0???]_.
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Related to [#zip-0143]_
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Motivation
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==========
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With scheduled network upgrades, at the activation height, nodes on each branch should disconnect from nodes on other branches and only accept new incoming connections from nodes on the same branch (??? or accepted set of branches ???)
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Specification
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=============
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When a new inbound connection is received or an outbound connection
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created, a CNode object is instantiated with the version field set to
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INIT_PROTO_VERSION (209). This value is not transmitted across the network, but for legacy reasons and technical debt beyond the scope of this ZIP, this value will not be changed.
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Once the two nodes have connected and perform a handshake to negotiate the protocol version, version field of CNode will be updated. As documented here https://en.bitcoin.it/wiki/Version_Handshake
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on connection, "version" and "verack" messages are exchanged.::
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L -> R: Send version message with the local peer's version
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R -> L: Send version message back
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R -> L: Send verack message
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R: Sets version to the minimum of the 2 versions
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L -> R: Send verack message after receiving version message from R
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L: Sets version to the minimum of the 2 versions
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To send a version message, the node will invoke PushVersion()::
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void CNode::PushVersion()
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PushMessage("version", PROTOCOL_VERSION, nLocalServices, nTime, addrYou, addrMe,
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Where:
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- Legacy PROTOCOL_VERSION is 170002
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- Overwinter PROTOCOL_VERSION is 170003
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Rejecting Legacy Connections
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----------------------------
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Currently, nodes will reject connections from nodes with a protocol version lower than the other node's minimum supported protocol version. This value is defined as::
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//! disconnect from peers older than this proto version
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static const int MIN_PEER_PROTO_VERSION = 170002;
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...
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if (pfrom->nVersion < MIN_PEER_PROTO_VERSION)
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{
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// disconnect from old peers running protocol version we don't support.
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Prior to activation, Overwinter nodes will contain the following constants::
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static const int PROTOCOL_VERSION = 170003;
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static const int MIN_PEER_PROTO_VERSION = 170002;
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This allows pre-Overwinter nodes and Overwinter nodes to remain connected prior to activation.
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However, once activation occurs, Overwinter nodes must reject connections from legacy 170002 nodes.
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To achieve this, the version message will be upgraded for Overwinter to include one extra field, a 32-bit BRANCH_ID. Since legacy nodes do not send this field as part of their version message during the handshake, the legacy node's connection will be rejected.
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We can do this by implementing code such as::
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bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv, int64_t nTimeReceived)
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...
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if (strCommand == "version")
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...
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vRecv >> addrFrom >> nNonce;
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vRecv >> LIMITED_STRING(pfrom->strSubVer, 256);
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pfrom->cleanSubVer = SanitizeString(pfrom->strSubVer);
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vRecv >> pfrom->nStartingHeight;
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vRecv >> pfrom->fRelayTxes; // set to true after we get the first filter* message
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// Overwinter node
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if (isOverwinterHandshakeActivated()) {
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if (!vRecv.empty()) {
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vRecv >> pfrom->branchID
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}
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else {
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// disconnect as branch ID is missing
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LogPrintf("peer=%d using obsolete version %i; disconnecting\n", pfrom->id, pfrom->nVersion);
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pfrom->PushMessage("reject", strCommand, REJECT_OBSOLETE,
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strprintf("Version must be %d or greater", OVERWINTER_PROTO_VERSION));
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pfrom->fDisconnect = true;
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return false;
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}
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}
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Network Coalescence
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-------------------
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Prior to the activation of Overwinter, nodes running pre-Overwinter protocol version 170002 and the Overwinter protocol version 170003 remain connected with the same consensus rules, but it would be preferable for nodes supporting Overwinter to connect to other nodes supporting Overwinter.
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This will help the network partition smoothly, since nodes should already be connected to (a majority of) peers running the same protocol version. Otherwise am Overwinter node may find their connections to legacy nodes dropped at the activation height, leaving them isolated and potentially susceptible to eclipse attacks. [link]
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To assist network coalescence before the activation height, we update the eviction process to place a higher priority on evicting legacy nodes.
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This can be activated at n blocks before the activation block height, where n could be defined by a constant such as::
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static const int NETWORK_COALESCE_BLOCK_PERIOD = 1000.
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The eviction code can be updated as follows::
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static bool AttemptToEvictConnection(bool fPreferNewConnection) {
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...
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// Protect connections with certain characteristics
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...
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// Check version of eviction candidates
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if (current_block_height >= (activationheight - NETWORK_COALESCE_BLOCK_PERIOD)) {
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// if there exist any legacy nodes, keep them in the eviction set
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// and at the same time remove overwinter nodes from eviction set.
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// if there do not exist any legacy nodes,
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// continue with existing behaviour.
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Disconnecting Existing Connections
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----------------------------------
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It is likely that at the activation block height, an Overwinter node will still be connected to some Legacy nodes.
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Currently, when connecting, a node must perform the networking handshake, and send the version message, before any other messages are processed.
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To disconnect existing connections, we can modify ProcessMessage so that the protocol version is always checked after Overwinter activates.
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Example code::
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bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv, int64_t nTimeReceived)
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...
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else if (pfrom->nVersion == 0)
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{
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// Must have a version message before anything else
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Misbehaving(pfrom->GetId(), 1);
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return false;
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}
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else if (strCommand == "verack")
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{
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...
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}
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// Disconnect existing connection if:
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// 1. The version message has been received
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// 2. Overwinter is active
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// 3. Version is legacy
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else if (
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pfrom->nVersion != 0 &&
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isOverwinterActivated() &&
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pfrom->nVersion < OVERWINTER_PROTO_VERSION )
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{
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LogPrintf("peer=%d using obsolete version %i; disconnecting\n", pfrom->id, pfrom->nVersion);
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pfrom->PushMessage("reject", strCommand, REJECT_OBSOLETE,
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strprintf("Version must be %d or greater", OVERWINTER_PROTO_VERSION));
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pfrom->fDisconnect = true;
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return false;
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}
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Deployment
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==========
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This proposal will be deployed with the Overwinter network upgrade.
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Testnet:
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Mainnet:
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Backward compatibility
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======================
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This proposal intentionally creates what is known as a "bilateral hard fork" between Legacy software and Overwinter compatible software. Use of this new handshake requires that all network participants upgrade their software to a compatible version within the upgrade window
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Legacy software will accept the numerically larger Overwinter protocol version as valid, but Overwinter compatible software will reject the legacy nodes as they will not send the BRANCH_ID as part of the version message.
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Reference Implementation
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========================
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TBC
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References
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==========
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Partition nodes with old protocol version from network in advance of hard fork https://github.com/zcash/zcash/issues/2775
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https://en.bitcoin.it/wiki/Protocol_documentation#version
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.. [#zip-0???] Network Upgrade Activation Mechanism
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