diff --git a/docs/Server/Monitoring.md b/docs/Server/Monitoring.md
index 30f3aeaa..dc5b3638 100644
--- a/docs/Server/Monitoring.md
+++ b/docs/Server/Monitoring.md
@@ -54,6 +54,7 @@ off rather than returned empty.
 | `connectionsRejected` | Connections turned away at the connection limit. |
 | `connectionsWriteTimeouts`, `connectionsIdleTimeouts` | Connections closed by the write or idle timeout. |
 | `connectionsRequestSizeExceeded` | Connections dropped because a request exceeded `setMaxRequestSize`. |
+| `connectionsProtocolErrors` | Connections dropped by a malformed/undecodable request PDU (Notice of Disconnection). |
 | `connectionsMax` | The configured connection limit (`0` is unlimited). |
 | `operationsCompleted`, `operationsFailed` | Total operations and the failed subset. |
 | `operationsByType` | Per-type counts, e.g. `search=1402, bind=210, add=8`. |
diff --git a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler.php b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler.php
index 1c783ced..cb565fb7 100644
--- a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler.php
+++ b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler.php
@@ -101,6 +101,7 @@ public function handle(): ?ConnectionObservation
             # Per RFC 4511 §4.1.1, a PDU that cannot be processed (malformed) warrants a disconnect with a protocol
             # error. The NoticeOfDisconnectSent event records the specific reason.
             $this->sendNoticeOfDisconnect('The message could not be processed.');
+            $closeReason = ConnectionObservation::ProtocolError;
         } catch (Throwable $e) {
             if ($this->queue->isConnected()) {
                 $this->sendNoticeOfDisconnect(cause: $e);
diff --git a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerMonitorHandler.php b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerMonitorHandler.php
index 4d43f807..fd7c84e7 100644
--- a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerMonitorHandler.php
+++ b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerMonitorHandler.php
@@ -100,6 +100,7 @@ private function attributes(MetricsSnapshot $snapshot): array
             'connectionsWriteTimeouts' => [(string) $connections->writeTimeouts],
             'connectionsIdleTimeouts' => [(string) $connections->idleTimeouts],
             'connectionsRequestSizeExceeded' => [(string) $connections->requestSizeExceeded],
+            'connectionsProtocolErrors' => [(string) $connections->protocolErrors],
             'connectionsMax' => [(string) $this->options->getMaxConnections()],
             'operationsCompleted' => [(string) $operations->total()],
             'operationsFailed' => [(string) $operations->totalErrors()],
diff --git a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerPagingHandler.php b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerPagingHandler.php
index 90d88dff..78e45f3a 100644
--- a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerPagingHandler.php
+++ b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerPagingHandler.php
@@ -14,7 +14,6 @@
 namespace FreeDSx\Ldap\Protocol\ServerProtocolHandler;
 
 use FreeDSx\Ldap\Control\PagingControl;
-use FreeDSx\Ldap\Control\Sorting\SortingResponseControl;
 use FreeDSx\Ldap\Entry\Entries;
 use FreeDSx\Ldap\Entry\Entry;
 use FreeDSx\Ldap\Exception\OperationException;
@@ -119,9 +118,12 @@ public function handleRequest(
             $controls[] = new PagingControl(0, '');
         }
 
-        $sortControl = $this->sortingControl($message);
-        if ($sortControl !== null) {
-            $controls[] = new SortingResponseControl(0);
+        $sortResponse = $this->sortingResponseControl(
+            $this->sortingControl($message),
+            $this->schema,
+        );
+        if ($sortResponse !== null) {
+            $controls[] = $sortResponse;
         }
 
         $pagingRequest->markProcessed();
diff --git a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchHandler.php b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchHandler.php
index a9923dc5..a1336b38 100644
--- a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchHandler.php
+++ b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchHandler.php
@@ -13,7 +13,6 @@
 
 namespace FreeDSx\Ldap\Protocol\ServerProtocolHandler;
 
-use FreeDSx\Ldap\Control\Sorting\SortingResponseControl;
 use FreeDSx\Ldap\Entry\Entry;
 use FreeDSx\Ldap\Operation\Request\AbandonRequest;
 use FreeDSx\Ldap\Operation\Request\CancelRequest;
@@ -88,9 +87,12 @@ public function handleRequest(
             $state,
         );
 
-        $sortControl = $this->sortingControl($message);
-        $responseControls = $sortControl !== null
-            ? [new SortingResponseControl(0)]
+        $sortResponse = $this->sortingResponseControl(
+            $this->sortingControl($message),
+            $this->schema,
+        );
+        $responseControls = $sortResponse !== null
+            ? [$sortResponse]
             : [];
 
         $this->sendEntriesToClient(
diff --git a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchTrait.php b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchTrait.php
index e6f1932a..f5dd1eec 100644
--- a/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchTrait.php
+++ b/src/FreeDSx/Ldap/Protocol/ServerProtocolHandler/ServerSearchTrait.php
@@ -17,6 +17,7 @@
 use FreeDSx\Ldap\Control\ControlBag;
 use FreeDSx\Ldap\Control\PagingControl;
 use FreeDSx\Ldap\Control\Sorting\SortingControl;
+use FreeDSx\Ldap\Control\Sorting\SortingResponseControl;
 use FreeDSx\Ldap\Entry\Dn;
 use FreeDSx\Ldap\Exception\OperationException;
 use FreeDSx\Ldap\Exception\RuntimeException;
@@ -30,6 +31,7 @@
 use FreeDSx\Ldap\Protocol\LdapMessageRequest;
 use FreeDSx\Ldap\Protocol\LdapMessageResponse;
 use FreeDSx\Ldap\Protocol\Queue\ServerQueue;
+use FreeDSx\Ldap\Schema\Schema;
 use Generator;
 
 trait ServerSearchTrait
@@ -191,4 +193,41 @@ private function sortingControl(LdapMessageRequest $message): ?SortingControl
             ? $control
             : null;
     }
+
+    /**
+     * The RFC 2891 sort response control, with the result code per §1.2 (first unsortable key wins).
+     */
+    private function sortingResponseControl(
+        ?SortingControl $sortControl,
+        Schema $schema,
+    ): ?SortingResponseControl {
+        if ($sortControl === null) {
+            return null;
+        }
+
+        foreach ($sortControl->getSortKeys() as $sortKey) {
+            $attribute = $sortKey->getAttribute();
+            $attributeType = $schema->getAttributeType($attribute);
+
+            if ($attributeType === null) {
+                return new SortingResponseControl(
+                    ResultCode::NO_SUCH_ATTRIBUTE,
+                    $attribute,
+                );
+            }
+
+            $orderingRule = $sortKey->getOrderingRule();
+            $unknownRule = $orderingRule !== null
+                && $schema->getMatchingRule($orderingRule) === null;
+
+            if ($unknownRule || ($orderingRule === null && $attributeType->orderingOid === null)) {
+                return new SortingResponseControl(
+                    ResultCode::INAPPROPRIATE_MATCHING,
+                    $attribute,
+                );
+            }
+        }
+
+        return new SortingResponseControl(ResultCode::SUCCESS);
+    }
 }
diff --git a/src/FreeDSx/Ldap/Server/Metrics/Observation/ConnectionObservation.php b/src/FreeDSx/Ldap/Server/Metrics/Observation/ConnectionObservation.php
index 74c7d4c7..9e990f2b 100644
--- a/src/FreeDSx/Ldap/Server/Metrics/Observation/ConnectionObservation.php
+++ b/src/FreeDSx/Ldap/Server/Metrics/Observation/ConnectionObservation.php
@@ -31,4 +31,6 @@ enum ConnectionObservation: string
     case IdleTimeout = 'idle_timeout';
 
     case RequestSizeExceeded = 'request_size_exceeded';
+
+    case ProtocolError = 'protocol_error';
 }
diff --git a/src/FreeDSx/Ldap/Server/Metrics/Recorder/InMemoryMetricsRecorder.php b/src/FreeDSx/Ldap/Server/Metrics/Recorder/InMemoryMetricsRecorder.php
index 0b8a6c05..a60e5999 100644
--- a/src/FreeDSx/Ldap/Server/Metrics/Recorder/InMemoryMetricsRecorder.php
+++ b/src/FreeDSx/Ldap/Server/Metrics/Recorder/InMemoryMetricsRecorder.php
@@ -75,6 +75,11 @@ final class InMemoryMetricsRecorder implements MetricsRecorderInterface, Metrics
      */
     private int $requestSizeExceeded = 0;
 
+    /**
+     * @var int<0, max>
+     */
+    private int $protocolErrors = 0;
+
     /**
      * @var array<string, int<0, max>>
      */
@@ -177,6 +182,7 @@ public function connectionObserved(ConnectionObservation $observation): void
             ConnectionObservation::WriteTimeout => $this->writeTimeouts++,
             ConnectionObservation::IdleTimeout => $this->idleTimeouts++,
             ConnectionObservation::RequestSizeExceeded => $this->requestSizeExceeded++,
+            ConnectionObservation::ProtocolError => $this->protocolErrors++,
         };
     }
 
@@ -206,6 +212,7 @@ public function snapshot(): MetricsSnapshot
                 $this->writeTimeouts,
                 $this->idleTimeouts,
                 $this->requestSizeExceeded,
+                $this->protocolErrors,
             ),
             $this->operationMetrics(),
             $this->operationsInProgress,
diff --git a/src/FreeDSx/Ldap/Server/Metrics/Snapshot/ConnectionMetrics.php b/src/FreeDSx/Ldap/Server/Metrics/Snapshot/ConnectionMetrics.php
index ee7c73d2..33bc57b0 100644
--- a/src/FreeDSx/Ldap/Server/Metrics/Snapshot/ConnectionMetrics.php
+++ b/src/FreeDSx/Ldap/Server/Metrics/Snapshot/ConnectionMetrics.php
@@ -27,6 +27,7 @@ public function __construct(
         public int $writeTimeouts = 0,
         public int $idleTimeouts = 0,
         public int $requestSizeExceeded = 0,
+        public int $protocolErrors = 0,
     ) {}
 
     /**
@@ -41,6 +42,7 @@ public function toArray(): array
             'write_timeouts' => $this->writeTimeouts,
             'idle_timeouts' => $this->idleTimeouts,
             'request_size_exceeded' => $this->requestSizeExceeded,
+            'protocol_errors' => $this->protocolErrors,
         ];
     }
 
@@ -56,6 +58,7 @@ public static function fromArray(array $data): self
             writeTimeouts: SnapshotValue::toInt($data['write_timeouts'] ?? null),
             idleTimeouts: SnapshotValue::toInt($data['idle_timeouts'] ?? null),
             requestSizeExceeded: SnapshotValue::toInt($data['request_size_exceeded'] ?? null),
+            protocolErrors: SnapshotValue::toInt($data['protocol_errors'] ?? null),
         );
     }
 }
diff --git a/src/FreeDSx/Ldap/Server/ServerRunner/PcntlServerRunner.php b/src/FreeDSx/Ldap/Server/ServerRunner/PcntlServerRunner.php
index 618c0fe9..2d211b7f 100644
--- a/src/FreeDSx/Ldap/Server/ServerRunner/PcntlServerRunner.php
+++ b/src/FreeDSx/Ldap/Server/ServerRunner/PcntlServerRunner.php
@@ -54,6 +54,8 @@ class PcntlServerRunner implements ServerRunnerInterface
 
     private const EXIT_CODE_REQUEST_SIZE_EXCEEDED = 12;
 
+    private const EXIT_CODE_PROTOCOL_ERROR = 13;
+
     private SocketServer $server;
 
     /**
@@ -214,6 +216,7 @@ private function childExitCodeFor(?ConnectionObservation $closeReason): int
             ConnectionObservation::WriteTimeout => self::EXIT_CODE_WRITE_TIMEOUT,
             ConnectionObservation::IdleTimeout => self::EXIT_CODE_IDLE_TIMEOUT,
             ConnectionObservation::RequestSizeExceeded => self::EXIT_CODE_REQUEST_SIZE_EXCEEDED,
+            ConnectionObservation::ProtocolError => self::EXIT_CODE_PROTOCOL_ERROR,
             default => 0,
         };
     }
@@ -233,6 +236,7 @@ private function recordChildCloseReason(
             self::EXIT_CODE_WRITE_TIMEOUT => ConnectionObservation::WriteTimeout,
             self::EXIT_CODE_IDLE_TIMEOUT => ConnectionObservation::IdleTimeout,
             self::EXIT_CODE_REQUEST_SIZE_EXCEEDED => ConnectionObservation::RequestSizeExceeded,
+            self::EXIT_CODE_PROTOCOL_ERROR => ConnectionObservation::ProtocolError,
             default => null,
         };
 
diff --git a/tests/unit/Protocol/ServerProtocolHandler/ServerMonitorHandlerTest.php b/tests/unit/Protocol/ServerProtocolHandler/ServerMonitorHandlerTest.php
index 28adf04c..9f3c5e58 100644
--- a/tests/unit/Protocol/ServerProtocolHandler/ServerMonitorHandlerTest.php
+++ b/tests/unit/Protocol/ServerProtocolHandler/ServerMonitorHandlerTest.php
@@ -258,6 +258,19 @@ public function test_it_reports_connections_closed_by_an_oversized_request(): vo
         );
     }
 
+    public function test_it_reports_connections_closed_by_a_protocol_error(): void
+    {
+        $this->metrics->connectionObserved(ConnectionObservation::ProtocolError);
+        $this->metrics->connectionObserved(ConnectionObservation::ProtocolError);
+
+        $entry = $this->handleAndCaptureEntry();
+
+        self::assertSame(
+            ['2'],
+            $entry->get('connectionsProtocolErrors')?->getValues(),
+        );
+    }
+
     public function test_it_reports_traffic_totals(): void
     {
         $this->metrics->trafficObserved(new TrafficObservation(
diff --git a/tests/unit/Protocol/ServerProtocolHandler/ServerPagingHandlerTest.php b/tests/unit/Protocol/ServerProtocolHandler/ServerPagingHandlerTest.php
index 927e7ded..ddfb96f9 100644
--- a/tests/unit/Protocol/ServerProtocolHandler/ServerPagingHandlerTest.php
+++ b/tests/unit/Protocol/ServerProtocolHandler/ServerPagingHandlerTest.php
@@ -31,6 +31,7 @@
 use FreeDSx\Ldap\Protocol\Queue\ServerQueue;
 use FreeDSx\Ldap\Protocol\ServerProtocolHandler\ServerPagingHandler;
 use FreeDSx\Ldap\Schema\Schema;
+use FreeDSx\Ldap\Schema\StandardSchemaProvider;
 use FreeDSx\Ldap\Search\Filters;
 use FreeDSx\Ldap\Server\AccessControl\AccessControlInterface;
 use FreeDSx\Ldap\Server\Backend\LdapBackendInterface;
@@ -77,7 +78,7 @@ protected function setUp(): void
         $this->mockFilterEvaluator = $this->createMock(FilterEvaluatorInterface::class);
         $this->mockAccessControl = $this->createMock(AccessControlInterface::class);
         $this->requestHistory = new RequestHistory();
-        $this->schema = new Schema();
+        $this->schema = StandardSchemaProvider::buildCore();
         $this->sentMessages = [];
 
         $this->mockFilterEvaluator
@@ -623,6 +624,39 @@ public function test_sort_control_appends_sorting_response_control_to_done_messa
         );
     }
 
+    public function test_sort_by_unknown_attribute_reports_no_such_attribute(): void
+    {
+        $message = new LdapMessageRequest(
+            2,
+            $this->makeSearchRequest(),
+            new PagingControl(10, ''),
+            new SortingControl(SortKey::ascending('bogusAttr')),
+        );
+
+        $this->mockBackend
+            ->method('search')
+            ->willReturn(new EntryStream($this->makeGenerator()));
+
+        $this->subject->handleRequest(
+            $message,
+            $this->mockToken,
+        );
+
+        $sortControl = $this->doneMessage()->controls()->get(Control::OID_SORTING_RESPONSE);
+        self::assertInstanceOf(
+            SortingResponseControl::class,
+            $sortControl,
+        );
+        self::assertSame(
+            ResultCode::NO_SUCH_ATTRIBUTE,
+            $sortControl->getResult(),
+        );
+        self::assertSame(
+            'bogusAttr',
+            $sortControl->getAttribute(),
+        );
+    }
+
     public function test_no_sort_control_does_not_append_sorting_response_control(): void
     {
         $message = $this->makeSearchMessage(size: 10);
diff --git a/tests/unit/Protocol/ServerProtocolHandler/ServerSearchHandlerTest.php b/tests/unit/Protocol/ServerProtocolHandler/ServerSearchHandlerTest.php
index dcbc7377..1d61cb46 100644
--- a/tests/unit/Protocol/ServerProtocolHandler/ServerSearchHandlerTest.php
+++ b/tests/unit/Protocol/ServerProtocolHandler/ServerSearchHandlerTest.php
@@ -32,6 +32,7 @@
 use FreeDSx\Ldap\Protocol\Queue\ServerQueue;
 use FreeDSx\Ldap\Protocol\ServerProtocolHandler\ServerSearchHandler;
 use FreeDSx\Ldap\Schema\Schema;
+use FreeDSx\Ldap\Schema\StandardSchemaProvider;
 use FreeDSx\Ldap\Search\Filters;
 use FreeDSx\Ldap\Server\AccessControl\AccessControlInterface;
 use FreeDSx\Ldap\Server\Backend\LdapBackendInterface;
@@ -73,7 +74,7 @@ protected function setUp(): void
         $this->mockBackend = $this->createMock(LdapBackendInterface::class);
         $this->mockFilterEvaluator = $this->createMock(FilterEvaluatorInterface::class);
         $this->mockAccessControl = $this->createMock(AccessControlInterface::class);
-        $this->schema = new Schema();
+        $this->schema = StandardSchemaProvider::buildCore();
         $this->sentMessages = [];
 
         $this->mockAccessControl
@@ -646,6 +647,70 @@ public function test_sort_control_appends_sorting_response_control_to_done_messa
         );
     }
 
+    public function test_sort_by_unknown_attribute_reports_no_such_attribute(): void
+    {
+        $search = new LdapMessageRequest(
+            2,
+            (new SearchRequest(Filters::present('cn')))->base('dc=foo,dc=bar'),
+            new SortingControl(SortKey::ascending('bogusAttr')),
+        );
+
+        $this->mockBackend
+            ->method('search')
+            ->willReturn(new EntryStream($this->makeGenerator()));
+
+        $this->subject->handleRequest(
+            $search,
+            $this->mockToken,
+        );
+
+        $done = end($this->sentMessages);
+        self::assertInstanceOf(LdapMessageResponse::class, $done);
+        $sortControl = $done->controls()->get(Control::OID_SORTING_RESPONSE);
+        self::assertInstanceOf(
+            SortingResponseControl::class,
+            $sortControl,
+        );
+        self::assertSame(
+            ResultCode::NO_SUCH_ATTRIBUTE,
+            $sortControl->getResult(),
+        );
+        self::assertSame(
+            'bogusAttr',
+            $sortControl->getAttribute(),
+        );
+    }
+
+    public function test_sort_by_attribute_without_ordering_rule_reports_inappropriate_matching(): void
+    {
+        $search = new LdapMessageRequest(
+            2,
+            (new SearchRequest(Filters::present('cn')))->base('dc=foo,dc=bar'),
+            new SortingControl(SortKey::ascending('userPassword')),
+        );
+
+        $this->mockBackend
+            ->method('search')
+            ->willReturn(new EntryStream($this->makeGenerator()));
+
+        $this->subject->handleRequest(
+            $search,
+            $this->mockToken,
+        );
+
+        $done = end($this->sentMessages);
+        self::assertInstanceOf(LdapMessageResponse::class, $done);
+        $sortControl = $done->controls()->get(Control::OID_SORTING_RESPONSE);
+        self::assertInstanceOf(
+            SortingResponseControl::class,
+            $sortControl,
+        );
+        self::assertSame(
+            ResultCode::INAPPROPRIATE_MATCHING,
+            $sortControl->getResult(),
+        );
+    }
+
     public function test_no_sort_control_does_not_append_sorting_response_control(): void
     {
         $search = new LdapMessageRequest(
diff --git a/tests/unit/Protocol/ServerProtocolHandlerTest.php b/tests/unit/Protocol/ServerProtocolHandlerTest.php
index 4ef3a9e7..ebad250a 100644
--- a/tests/unit/Protocol/ServerProtocolHandlerTest.php
+++ b/tests/unit/Protocol/ServerProtocolHandlerTest.php
@@ -159,8 +159,13 @@ public function test_it_sends_a_notice_of_disconnect_on_a_protocol_exception_fro
 
         $this->expectNoticeOfDisconnect('The message could not be processed.');
 
-        $this->handlerWith(new StubMiddlewareHandler(OperationOutcomeResult::succeeded()))
+        $closeReason = $this->handlerWith(new StubMiddlewareHandler(OperationOutcomeResult::succeeded()))
             ->handle();
+
+        self::assertSame(
+            ConnectionObservation::ProtocolError,
+            $closeReason,
+        );
     }
 
     public function test_a_request_size_exceeded_is_recorded_and_sends_a_notice_of_disconnect(): void
@@ -196,8 +201,13 @@ public function test_it_sends_a_notice_of_disconnect_on_an_encoder_exception_fro
 
         $this->expectNoticeOfDisconnect('The message could not be processed.');
 
-        $this->handlerWith(new StubMiddlewareHandler(OperationOutcomeResult::succeeded()))
+        $closeReason = $this->handlerWith(new StubMiddlewareHandler(OperationOutcomeResult::succeeded()))
             ->handle();
+
+        self::assertSame(
+            ConnectionObservation::ProtocolError,
+            $closeReason,
+        );
     }
 
     public function test_it_ends_normally_on_a_socket_exception_from_the_message_queue(): void
diff --git a/tests/unit/Server/Metrics/Recorder/InMemoryMetricsRecorderTest.php b/tests/unit/Server/Metrics/Recorder/InMemoryMetricsRecorderTest.php
index 8bad9511..07046cc5 100644
--- a/tests/unit/Server/Metrics/Recorder/InMemoryMetricsRecorderTest.php
+++ b/tests/unit/Server/Metrics/Recorder/InMemoryMetricsRecorderTest.php
@@ -187,6 +187,7 @@ public function test_it_counts_rejected_and_timed_out_connections(): void
         $this->subject->connectionObserved(ConnectionObservation::WriteTimeout);
         $this->subject->connectionObserved(ConnectionObservation::IdleTimeout);
         $this->subject->connectionObserved(ConnectionObservation::RequestSizeExceeded);
+        $this->subject->connectionObserved(ConnectionObservation::ProtocolError);
 
         $connections = $this->subject->snapshot()->connections;
 
@@ -206,6 +207,10 @@ public function test_it_counts_rejected_and_timed_out_connections(): void
             1,
             $connections->requestSizeExceeded,
         );
+        self::assertSame(
+            1,
+            $connections->protocolErrors,
+        );
     }
 
     public function test_taking_the_delta_returns_the_metrics_and_resets_them(): void