[srp-client] message MTU check and single service mode (#7776)

This commit adds a new mechanism in `Srp::Client`: When preparing an
update message, client will check whether the prepared message fits
in an IPv6 MTU (1280 bytes). If it does not, the SRP client would
temporarily enable "single service mode" which ensures only a single
service (along with its sub-types) to be appended in the SRP update
message. After the message is sent SRP client would switch back to
normal behavior and disables the single service mode.

This change addresses situation where user may register many services
(with multiple sub-types and long TXT data) which then may not fit
in a single UDP message payload.

This commit also adds `test_srp_many_services_mtu_check` test which
verifies the newly added behavior.

src/core/net/srp_client.cpp

@@ -335,6 +335,8 @@ void Client::Stop(Requester aRequester, StopMode aMode)
 
     VerifyOrExit(GetState() != kStateStopped);
 
+    mSingleServiceMode.Disable();
+
     // State changes:
     //   kAdding     -> kToRefresh
     //   kRefreshing -> kToRefresh
@@ -403,6 +405,8 @@ void Client::Pause(void)
         /* (7) kRemoved    -> */ kRemoved,
     };
 
+    mSingleServiceMode.Disable();
+
     // State changes:
     //   kAdding     -> kToRefresh
     //   kRefreshing -> kToRefresh
@@ -710,6 +714,11 @@ void Client::ChangeHostAndServiceStates(const ItemState *aNewStates)
 
     for (Service &service : mServices)
     {
+        if (mSingleServiceMode.IsEnabled() && mSingleServiceMode.GetService() != &service)
+        {
+            continue;
+        }
+
         service.SetState(aNewStates[service.GetState()]);
     }
 
@@ -768,9 +777,21 @@ void Client::SendUpdate(void)
 
     Error    error   = kErrorNone;
     Message *message = mSocket.NewMessage(0);
+    uint32_t length;
 
     VerifyOrExit(message != nullptr, error = kErrorNoBufs);
     SuccessOrExit(error = PrepareUpdateMessage(*message));
+
+    length = message->GetLength() + sizeof(Ip6::Udp::Header) + sizeof(Ip6::Header);
+
+    if (length >= Ip6::kMaxDatagramLength)
+    {
+        LogInfo("Msg len %u is larger than MTU, enabling single service mode", length);
+        mSingleServiceMode.Enable();
+        IgnoreError(message->SetLength(0));
+        SuccessOrExit(error = PrepareUpdateMessage(*message));
+    }
+
     SuccessOrExit(error = mSocket.SendTo(*message, Ip6::MessageInfo()));
 
     LogInfo("Send update");
@@ -808,6 +829,7 @@ exit:
 
         LogInfo("Failed to send update: %s", ErrorToString(error));
 
+        mSingleServiceMode.Disable();
         FreeMessage(message);
 
         SetState(kStateToRetry);
@@ -882,6 +904,11 @@ Error Client::PrepareUpdateMessage(Message &aMessage)
         for (Service &service : mServices)
         {
             SuccessOrExit(error = AppendServiceInstructions(service, aMessage, info));
+
+            if (mSingleServiceMode.IsEnabled() && (mSingleServiceMode.GetService() != nullptr))
+            {
+                break;
+            }
         }
     }
 
@@ -1054,6 +1081,11 @@ Error Client::AppendServiceInstructions(Service &aService, Message &aMessage, In
     }
 #endif
 
+    if (mSingleServiceMode.IsEnabled())
+    {
+        mSingleServiceMode.SetService(aService);
+    }
+
 exit:
     return error;
 }
@@ -1473,6 +1505,7 @@ void Client::ProcessResponse(Message &aMessage)
     //   kRemoving   -> kRemoved
 
     ChangeHostAndServiceStates(kNewStateOnUpdateDone);
+    mSingleServiceMode.Disable();
 
     HandleUpdateDone();
     UpdateState();
@@ -1742,6 +1775,7 @@ void Client::HandleTimer(void)
         break;
 
     case kStateUpdating:
+        mSingleServiceMode.Disable();
         LogRetryWaitInterval();
         LogInfo("Timed out, no response");
         GrowRetryWaitInterval();

src/core/net/srp_client.hpp

@@ -854,6 +854,26 @@ private:
         kKeepRetryInterval,
     };
 
+    class SingleServiceMode
+    {
+    public:
+        SingleServiceMode(void)
+            : mEnabled(false)
+            , mService(nullptr)
+        {
+        }
+
+        void     Enable(void) { mEnabled = true, mService = nullptr; }
+        void     Disable(void) { mEnabled = false; }
+        bool     IsEnabled(void) const { return mEnabled; }
+        Service *GetService(void) { return mService; }
+        void     SetService(Service &aService) { mService = &aService; }
+
+    private:
+        bool     mEnabled;
+        Service *mService;
+    };
+
 #if OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_API_ENABLE
     class AutoStart : Clearable<AutoStart>
     {
@@ -995,6 +1015,7 @@ private:
     const char *        mDomainName;
     HostInfo            mHostInfo;
     LinkedList<Service> mServices;
+    SingleServiceMode   mSingleServiceMode;
     TimerMilli          mTimer;
 #if OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_API_ENABLE
     AutoStart mAutoStart;

src/posix/platform/openthread-core-posix-config.h

@@ -287,4 +287,16 @@
 #define OPENTHREAD_CONFIG_LOG_PREPEND_UPTIME 1
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_SRP_CLIENT_BUFFERS_MAX_SERVICES
+ *
+ * Specifies number of service entries in the SRP client service pool.
+ *
+ * This config is applicable only when `OPENTHREAD_CONFIG_SRP_CLIENT_BUFFERS_ENABLE` is enabled.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_SRP_CLIENT_BUFFERS_MAX_SERVICES
+#define OPENTHREAD_CONFIG_SRP_CLIENT_BUFFERS_MAX_SERVICES 20
+#endif
+
 #endif // OPENTHREAD_CORE_POSIX_CONFIG_H_

tests/scripts/thread-cert/Makefile.am

@@ -193,6 +193,7 @@ EXTRA_DIST                                                         = \
     test_srp_client_remove_host.py                                   \
     test_srp_client_save_server_info.py                              \
     test_srp_lease.py                                                \
+    test_srp_many_services_mtu_check.py                              \
     test_srp_name_conflicts.py                                       \
     test_srp_register_single_service.py                              \
     test_srp_server_anycast_mode.py                                  \
@@ -270,6 +271,7 @@ check_SCRIPTS                                                      = \
     test_srp_client_remove_host.py                                   \
     test_srp_client_save_server_info.py                              \
     test_srp_lease.py                                                \
+    test_srp_many_services_mtu_check.py                              \
     test_srp_name_conflicts.py                                       \
     test_srp_register_single_service.py                              \
     test_srp_server_anycast_mode.py                                  \

tests/scripts/thread-cert/test_srp_many_services_mtu_check.py

@@ -0,0 +1,124 @@
+#!/usr/bin/env python3
+#
+#  Copyright (c) 2022, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+#
+
+import ipaddress
+import unittest
+
+import command
+import config
+import thread_cert
+
+# Test description:
+#
+#   This test verifies SRP client behavior when there are many services
+#   causing SRP Update message going over the IPv6 MTU size. In such
+#   a case the SRP client attempts to register a single service at
+#   a time.
+#
+# Topology:
+#
+#     LEADER (SRP server)
+#       |
+#       |
+#     ROUTER (SRP client)
+#
+
+SERVER = 1
+CLIENT = 2
+
+
+class SrpManyServicesMtuCheck(thread_cert.TestCase):
+    USE_MESSAGE_FACTORY = False
+    SUPPORT_NCP = False
+
+    TOPOLOGY = {
+        SERVER: {
+            'name': 'SRP_SERVER',
+            'mode': 'rdn',
+        },
+        CLIENT: {
+            'name': 'SRP_CLIENT',
+            'mode': 'rdn',
+        },
+    }
+
+    def test(self):
+        server = self.nodes[SERVER]
+        client = self.nodes[CLIENT]
+
+        # Start the server & client devices.
+
+        server.start()
+        self.simulator.go(5)
+        self.assertEqual(server.get_state(), 'leader')
+
+        client.start()
+        self.simulator.go(config.ROUTER_STARTUP_DELAY)
+        self.assertEqual(client.get_state(), 'router')
+
+        server.srp_server_set_enabled(True)
+        client.srp_client_enable_auto_start_mode()
+        self.simulator.go(5)
+
+        # Register 8 services with long name, 6 sub-types and long txt record.
+        # The 8 services won't be fit in a single MTU (1280 bytes) UDP message
+        # SRP Client would then register services one by one.
+
+        num_services = 8
+
+        client.srp_client_set_host_name('hosthosthosthosthosthosthosthosthosthosthosthosthosthosthosthos')
+        client.srp_client_set_host_address('2001::1')
+
+        txt_info = ["xyz=987654321", "uvw=abcdefghijklmnopqrstu", "qwp=564321abcdefghij"]
+
+        for num in range(num_services):
+            name = chr(ord('a') + num) * 63
+            client.srp_client_add_service(
+                name,
+                '_longsrvname._udp,_subtype1,_subtype2,_subtype3,_subtype4,_subtype5,_subtype6',
+                1977,
+                txt_entries=txt_info)
+
+        self.simulator.go(10)
+
+        # Make sure all services are successfully registered
+        # (from both client and server perspectives).
+
+        client_services = client.srp_client_get_services()
+        self.assertEqual(len(client_services), num_services)
+
+        for service in client_services:
+            self.assertEqual(service['state'], 'Registered')
+
+        server_services = server.srp_server_get_services()
+        self.assertEqual(len(server_services), num_services)
+
+
+if __name__ == '__main__':
+    unittest.main()