InstantSend P2P messages / Guide

- Add diagram of message exchange
 - Add section to guide

_includes/devdoc/guide_p2p_network.md

@@ -607,6 +607,40 @@ features in a decentralized, deterministic way.
 {% endautocrossref %}
 
 
+### InstantSend
+
+{% include helpers/subhead-links.md %}
+
+{% autocrossref %}
+
+Dash Core's InstantSend feature provides a way to lock transaction inputs and
+enable secure, instantaneous transactions.
+
+*InstantSend Data Flow*
+
+| **InstantSend Client** | **Direction**  | **Peers**   | **Description** |
+| `inv` message (ix)          | → |                         | Client sends inventory for transaction lock request
+|                             | ← | `getdata` message (ix)  | Peer responds with request for transaction lock request
+| `ix` message                | → |                         | Client sends InstantSend transaction lock request
+|                             | ← | `inv` message (txlvote) | Masternodes in the [quorum](#quorum-selection) respond with votes
+| `getdata` message (txlvote) | → |                         | Client requests vote
+|                             | ← | `txlvote` message       | Peer responds with vote
+
+Once a sufficient number of votes approve the transaction lock, the InstantSend
+transaction is approved and shows 5 confirmations (`DEFAULT_INSTANTSEND_DEPTH`).
+If an InstantSend transaction is a valid transaction but does not receive a
+transaction lock, it reverts to being a standard transaction.
+
+There are a number of limitations on InstantSend transactions:
+
+* To be used in an InstantSend transaction, an input must have 6+ confirmations (`INSTANTSEND_CONFIRMATIONS_REQUIRED`)
+* The lock request will timeout 15 seconds after the first vote is seen (`INSTANTSEND_LOCK_TIMEOUT_SECONDS`)
+* The lock request will fail if it has not been locked after 60 seconds (`INSTANTSEND_FAILED_TIMEOUT_SECONDS`)
+* A minimum fee (0.001 Dash) is required since the transaction involves the masternodes in addition to miners. Activation of [DIP-0001](https://github.com/dashpay/dips/blob/master/dip-0001.md) will reduce the fee by an order of magnitude (to 0.0001 Dash).
+
+{% endautocrossref %}
+
+
 ### PrivateSend
 
 {% include helpers/subhead-links.md %}

_includes/devdoc/ref_p2p_networking.md

@@ -1444,7 +1444,11 @@ bc8f5e5400000000 ............................ Epoch time: 1415483324
 
 The following network messages all help control the InstantSend feature of Dash.
 InstantSend uses the masternode network to lock transaction inputs and enable
-secure, instantaneous transactions.
+secure, instantaneous transactions. For additional details, refer to
+the Developer Guide [InstantSend section](developer-guide#instantsend).
+
+![Overview Of P2P Protocol InstantSend Request And Reply Messages](/img/dev/en-p2p-instantsend-messages.svg)
+
 
 {% endautocrossref %}
 

img/dev/en-p2p-instantsend-messages.dot

@@ -0,0 +1,28 @@
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+
+size="6.25";
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+splines="false"
+
+edge [ penwidth = 1.75, fontname="Sans", dir="none" ]
+node [ penwidth = 1.75, shape = "box", fontname="Sans", ]
+graph [ penwidth = 1.75, fontname="Sans" ]
+
+ix -> "inv (txlvote)";
+"inv (txlvote)" -> getdata;
+getdata -> txlvote;
+
+{
+    node [ shape = "none" ];
+    label1 [ label = "Client Requests\nTransaction Lock" ];
+    label2 [ label = "Masternodes Reply\nWith Inventory" ];
+    label3 [ label = "Request For\nMasternode Vote" ];
+    label4 [ label = "Reply With\nRequested Vote" ];
+
+    label1 -> label2 -> label3 -> label4 [style = "invis" ];
+}
+
+label = "Overview Of P2P Protocol InstantSend Request And Reply Messages"
+}

img/dev/en-p2p-instantsend-messages.svg

@@ -0,0 +1,69 @@
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+<!-- Title: %3 Pages: 1 -->
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+<!-- label1 -->
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+</g>
+<!-- label2 -->
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