http: provide keep-alive timeout response header

lib/_http_outgoing.js

@@ -28,6 +28,7 @@ const {
   ObjectKeys,
   ObjectPrototypeHasOwnProperty,
   ObjectSetPrototypeOf,
+  MathFloor,
   Symbol,
 } = primordials;
 
@@ -123,6 +124,8 @@ function OutgoingMessage() {
   this._header = null;
   this[kOutHeaders] = null;
 
+  this._keepAliveTimeout = 0;
+
   this._onPendingData = noopPendingOutput;
 }
 ObjectSetPrototypeOf(OutgoingMessage.prototype, Stream.prototype);
@@ -424,6 +427,10 @@ function _storeHeader(firstLine, headers) {
         (state.contLen || this.useChunkedEncodingByDefault || this.agent);
     if (shouldSendKeepAlive) {
       header += 'Connection: keep-alive\r\n';
+      if (this._keepAliveTimeout) {
+        const timeoutSeconds = MathFloor(this._keepAliveTimeout) / 1000;
+        header += `Keep-Alive: timeout=${timeoutSeconds}\r\n`;
+      }
     } else {
       this._last = true;
       header += 'Connection: close\r\n';

lib/_http_server.js

@@ -766,6 +766,7 @@ function parserOnIncoming(server, socket, state, req, keepAlive) {
   }
 
   const res = new server[kServerResponse](req);
+  res._keepAliveTimeout = server.keepAliveTimeout;
   res._onPendingData = updateOutgoingData.bind(undefined, socket, state);
 
   res.shouldKeepAlive = keepAlive;

test/parallel/test-http-keep-alive-timeout.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const common = require('../common');
+const http = require('http');
+const assert = require('assert');
+
+const server = http.createServer(common.mustCall((req, res) => {
+  const body = 'hello world\n';
+
+  res.writeHead(200, { 'Content-Length': body.length });
+  res.write(body);
+  res.end();
+}));
+server.keepAliveTimeout = 12000;
+
+const agent = new http.Agent({ maxSockets: 1, keepAlive: true });
+
+server.listen(0, common.mustCall(function() {
+  http.get({
+    path: '/', port: this.address().port, agent: agent
+  }, common.mustCall((response) => {
+    response.resume();
+    assert.strictEqual(
+      response.headers['keep-alive'], 'timeout=12');
+    server.close();
+    agent.destroy();
+  }));
+}));