diff --git a/common/JfjochTCP.h b/common/JfjochTCP.h
index 3ee50967..268adc66 100644
--- a/common/JfjochTCP.h
+++ b/common/JfjochTCP.h
@@ -47,6 +47,6 @@ struct alignas(64) TcpFrameHeader {
     uint32_t ack_processed_images = 0;
     uint16_t ack_code = 0;
     uint16_t ack_for = 0;
-
-    uint64_t reserved[2] = {0, 0};
+    uint64_t ack_fifo_occupancy = 0;
+    uint64_t reserved = 0;
 };
\ No newline at end of file
diff --git a/docs/IMAGE_STREAM.md b/docs/IMAGE_STREAM.md
index ac7c1161..fc2b369f 100644
--- a/docs/IMAGE_STREAM.md
+++ b/docs/IMAGE_STREAM.md
@@ -125,20 +125,21 @@ On Linux, large payload transmission (`DATA` and `CALIBRATION` frames) can use k
 
 #### TCP frame header (`TcpFrameHeader`)
 
-| Field | Type | Description |
-|---|---|---|
-| `magic` | `uint32_t` | Protocol magic (`0x4A464A54`, `"JFJT"`) |
-| `version` | `uint16_t` | Protocol version (`2`) |
-| `type` | `uint16_t` | Frame type (see table above) |
-| `image_number` | `uint64_t` | Image index for `DATA` frames |
-| `payload_size` | `uint64_t` | Number of payload bytes after header |
-| `socket_number` | `uint32_t` | Connection index in split-stream mode |
-| `flags` | `uint32_t` | ACK flags (`OK`, `FATAL`, `HAS_ERROR_TEXT`) |
-| `run_number` | `uint64_t` | Run identifier |
-| `ack_processed_images` | `uint32_t` | In `ACK`: number of images processed by receiver |
-| `ack_code` | `uint16_t` | In `ACK`: error/status code |
-| `ack_for` | `uint16_t` | In `ACK`: frame type being acknowledged |
-| `reserved` | `uint64_t[2]` | Reserved, set to `0` |
+| Field                  | Type | Description                                              |
+|------------------------|---|----------------------------------------------------------|
+| `magic`                | `uint32_t` | Protocol magic (`0x4A464A54`, `"JFJT"`)                  |
+| `version`              | `uint16_t` | Protocol version (`2`)                                   |
+| `type`                 | `uint16_t` | Frame type (see table above)                             |
+| `image_number`         | `uint64_t` | Image index for `DATA` frames                            |
+| `payload_size`         | `uint64_t` | Number of payload bytes after header                     |
+| `socket_number`        | `uint32_t` | Connection index in split-stream mode                    |
+| `flags`                | `uint32_t` | ACK flags (`OK`, `FATAL`, `HAS_ERROR_TEXT`)              |
+| `run_number`           | `uint64_t` | Run identifier                                           |
+| `ack_processed_images` | `uint32_t` | In `ACK`: number of images processed by receiver         |
+| `ack_code`             | `uint16_t` | In `ACK`: error/status code                              |
+| `ack_for`              | `uint16_t` | In `ACK`: frame type being acknowledged                  |
+| `ack_fifo_occupancy`   | `uint16_t` | In `ACK`: occupancy of input FIFO in the `jfjoch_writer` |
+| `reserved`             | `uint64_t` | Reserved, set to `0`                                     |
 
 The header is 64-byte aligned (`alignas(64)`).
 
diff --git a/image_puller/TCPImagePuller.cpp b/image_puller/TCPImagePuller.cpp
index ecaa2162..0077b051 100644
--- a/image_puller/TCPImagePuller.cpp
+++ b/image_puller/TCPImagePuller.cpp
@@ -108,6 +108,7 @@ bool TCPImagePuller::SendAck(const PullerAckMessage &ack) {
     h.ack_processed_images = ack.processed_images;
     h.ack_code = static_cast<uint32_t>(ack.error_code);
     h.payload_size = ack.error_text.size();
+    h.ack_fifo_occupancy = cbor_fifo.GetCurrentUtilization();
 
     if (!SendAll(&h, sizeof(h)))
         return false;