[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.

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()