From 0715bbf1d6b8b2b948b133df2b8e8ffd017c91da Mon Sep 17 00:00:00 2001
From: Sameer Puri <sameerpuri@google.com>
Date: Tue, 27 Oct 2026 15:18:53 -0700
Subject: [PATCH] Fragment outbound UCI control packets in Python host

Host.send_control previously serialized UCI control packets into a
single frame without fragmentation. Control payloads exceeding the
255-byte UCI payload limit, such as multicast list updates containing
multiple extended subsession keys, overflowed the single-byte length
field and failed transmission.

Segment serialized control payloads into 255-byte chunks, set the
Packet Boundary Flag on intermediate frames, and add test coverage
exercising multicast list updates across packet boundaries.
---
 py/pica/pica/__init__.py |  9 ++---
 tests/ranging.py         | 72 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+), 4 deletions(-)

diff --git a/py/pica/pica/__init__.py b/py/pica/pica/__init__.py
index 423824f..7c277c3 100644
--- a/py/pica/pica/__init__.py
+++ b/py/pica/pica/__init__.py
@@ -97,10 +97,11 @@ async def _recv_data(self) -> bytes:
         return await self.data_queue.get()
 
     def send_control(self, packet: uci.ControlPacket):
-        # TODO packet fragmentation.
-        packet = bytearray(packet.serialize())
-        packet[3] = len(packet) - 4
-        self.writer.write(packet)
+        raw = packet.serialize()
+        chunks = [raw[i : i + 255] for i in range(4, len(raw), 255)] or [b""]
+        for i, chunk in enumerate(chunks):
+            pbf = 0x10 if i < len(chunks) - 1 else 0
+            self.writer.write(bytes([raw[0] | pbf, raw[1], raw[2], len(chunk)]) + chunk)
 
     def send_data(self, packet: uci.DataPacket):
         packet = bytearray(packet.serialize())
diff --git a/tests/ranging.py b/tests/ranging.py
index eb96185..4db736e 100755
--- a/tests/ranging.py
+++ b/tests/ranging.py
@@ -531,6 +531,78 @@ async def multicast_list_ntf(host: Host, peer: Host):
         )
     )
 
+    host.send_control(
+        uci.SessionUpdateControllerMulticastListCmd(
+            session_token=0,
+            action=uci.UpdateMulticastListAction.REMOVE_CONTROLEE,
+            payload=uci.SessionUpdateControllerMulticastListCmdPayload(
+                controlees=[
+                    uci.Controlee(
+                        short_address=list(peer.mac_address),
+                        subsession_id=0,
+                    )
+                ]
+            ).serialize(),
+        )
+    )
+    await host.expect_control(
+        uci.SessionUpdateControllerMulticastListRsp(
+            status=uci.Status.OK,
+            controlee_status=[
+                uci.ControleeStatus(
+                    mac_address=list(peer.mac_address),
+                    status=uci.MulticastUpdateStatus.OK_MULTICAST_LIST_UPDATE,
+                )
+            ],
+        )
+    )
+
+    host.send_control(
+        uci.SessionSetAppConfigCmd(
+            session_token=0,
+            tlvs=[
+                uci.AppConfigTlv(
+                    cfg_id=uci.AppConfigTlvType.STS_CONFIG,
+                    v=bytes([uci.StsConfig.PROVISIONED_FOR_RESPONDER_SUB_SESSION_KEY]),
+                )
+            ],
+        )
+    )
+    await host.expect_control(
+        uci.SessionSetAppConfigRsp(status=uci.Status.OK, cfg_status=[])
+    )
+
+    # 8 * 38-byte controlees -> 310-byte payload, placing the 8th MAC address in the 2nd PBF fragment.
+    controlees = [
+        uci.Controlee_V2_0_32_Byte_Version(
+            short_address=bytearray([0, i + 3]),
+            subsession_id=i + 1,
+            subsession_key=bytearray(32),
+        )
+        for i in range(8)
+    ]
+    host.send_control(
+        uci.SessionUpdateControllerMulticastListCmd(
+            session_token=0,
+            action=uci.UpdateMulticastListAction.ADD_CONTROLEE_WITH_EXTENDED_SUB_SESSION_KEY,
+            payload=uci.SessionUpdateControllerMulticastListCmd_2_0_32_Byte_Payload(
+                controlees=controlees
+            ).serialize(),
+        )
+    )
+    await host.expect_control(
+        uci.SessionUpdateControllerMulticastListRsp(
+            status=uci.Status.OK,
+            controlee_status=[
+                uci.ControleeStatus(
+                    mac_address=[0, i + 3],
+                    status=uci.MulticastUpdateStatus.OK_MULTICAST_LIST_UPDATE,
+                )
+                for i in range(8)
+            ],
+        )
+    )
+
     host.send_control(uci.SessionDeinitCmd(session_token=0))
 
     await host.expect_control(uci.SessionDeinitRsp(status=uci.Status.OK))