From db5a2310a4f8c881a066cf838b15b8350837fa0d Mon Sep 17 00:00:00 2001
From: Bryan Biedenkapp <gatekeep@gmail.com>
Date: Sat, 15 Mar 2025 23:44:08 -0400
Subject: [PATCH] send a return ACK on reception of a Start of Stream;

---
 src/common/p25/dfsi/frames/FrameDefines.h |  2 ++
 src/host/modem/ModemV24.cpp               | 30 +++++++++++++++++++++++
 src/host/modem/ModemV24.h                 |  4 +++
 3 files changed, 36 insertions(+)

diff --git a/src/common/p25/dfsi/frames/FrameDefines.h b/src/common/p25/dfsi/frames/FrameDefines.h
index 890c2671..a2db0b9e 100644
--- a/src/common/p25/dfsi/frames/FrameDefines.h
+++ b/src/common/p25/dfsi/frames/FrameDefines.h
@@ -85,6 +85,8 @@ namespace p25
                     START_OF_STREAM = 9,                //! Start of Stream
                     END_OF_STREAM = 10,                 //! End of Stream
 
+                    START_OF_STREAM_ACK = 14,           //! Start of Stream Ack
+
                     UNDEFINED = 127                     //! Undefined
                 };
             }
diff --git a/src/host/modem/ModemV24.cpp b/src/host/modem/ModemV24.cpp
index c2e41e89..d3b8527b 100644
--- a/src/host/modem/ModemV24.cpp
+++ b/src/host/modem/ModemV24.cpp
@@ -1140,6 +1140,10 @@ void ModemV24::convertToAirTIA(const uint8_t *data, uint32_t length)
             // bryanb: maybe compare the NACs?
 
             dataOffs += StartOfStream::LENGTH;
+
+            // ack start of stream
+            // bryanb: is this really the right place to be doing this...
+            ackStartOfStreamTIA(); 
         }
         break;
         case BlockType::END_OF_STREAM:
@@ -1895,6 +1899,32 @@ void ModemV24::endOfStreamTIA()
     m_txCallInProgress = false;
 }
 
+/* Send a start of stream ACK. */
+
+void ModemV24::ackStartOfStreamTIA()
+{
+    uint16_t length = 0U;
+    uint8_t buffer[P25_HDU_LENGTH_BYTES];
+    ::memset(buffer, 0x00U, P25_HDU_LENGTH_BYTES);
+
+    // generate control octet
+    ControlOctet ctrl = ControlOctet();
+    ctrl.setBlockHeaderCnt(1U);
+    ctrl.encode(buffer);
+    length += ControlOctet::LENGTH;
+
+    // generate block header
+    BlockHeader hdr = BlockHeader();
+    hdr.setBlockType(BlockType::START_OF_STREAM_ACK);
+    hdr.encode(buffer + 1U);
+    length += BlockHeader::LENGTH;
+
+    if (m_trace)
+        Utils::dump(1U, "ModemV24::ackStartOfStreamTIA() Ack StartOfStream", buffer, length);
+
+    queueP25Frame(buffer, length, STT_NON_IMBE);
+}
+
 /* Internal helper to convert from TIA-102 air interface to V.24/DFSI. */
 
 void ModemV24::convertFromAir(uint8_t* data, uint32_t length)
diff --git a/src/host/modem/ModemV24.h b/src/host/modem/ModemV24.h
index cb48e31a..f4069f91 100644
--- a/src/host/modem/ModemV24.h
+++ b/src/host/modem/ModemV24.h
@@ -391,6 +391,10 @@ namespace modem
          * @brief Send an end of stream sequence (TDU, etc) to the connected UDP TIA-102 device.
          */
         void endOfStreamTIA();
+        /**
+         * @brief Send a start of stream ACK.
+         */
+        void ackStartOfStreamTIA();
 
         /**
          * @brief Internal helper to convert from TIA-102 air interface to V.24/DFSI.