Default message chunk size is now 300, not 900.

--message-chunk-size is now treated as a requested maximum.
If the requested value is too large for the Reticulum link budget, the program caps it and logs that it did so.
The cap accounts for file metadata and send_epoch.

examples/ble_dual_node_echo.py

@@ -38,6 +38,8 @@ import time
 
 APP_NAME = "ble_reticulum_poc"
 APP_ASPECT = "echo"
+DEFAULT_MESSAGE_CHUNK_SIZE = 300
+LINK_PAYLOAD_BUDGET = 420
 
 RNS = None
 running = True
@@ -46,7 +48,7 @@ active_links_lock = threading.Lock()
 temporary_config_dir = None
 message_file_text = None
 message_file_path = None
-message_chunk_size = 900
+message_chunk_size = DEFAULT_MESSAGE_CHUNK_SIZE
 
 
 
@@ -299,24 +301,46 @@ def utf8_chunks(text, max_bytes):
         yield chunk
 
 
+def file_chunk_prefix(index, total, chunk_bytes):
+    return (
+        f"file_chunk {index}/{total} from {NODE_NAME} "
+        f"bytes={chunk_bytes} file={os.path.basename(message_file_path)} data="
+    )
+
+
+def file_data_chunk_size(total, total_bytes):
+    prefix = file_chunk_prefix(total, total, total_bytes)
+    send_epoch_suffix = " send_epoch=1778951576.861234"
+    metadata_bytes = len(prefix.encode("utf-8")) + len(send_epoch_suffix.encode("utf-8"))
+    safe_size = max(1, LINK_PAYLOAD_BUDGET - metadata_bytes)
+    return min(message_chunk_size, safe_size)
+
+
 def send_message_file(link):
     if message_file_text is None:
         return
 
-    chunks = list(utf8_chunks(message_file_text, message_chunk_size))
-    total = max(1, len(chunks))
     total_bytes = len(message_file_text.encode("utf-8"))
+    estimated_total = max(1, (total_bytes + message_chunk_size - 1) // message_chunk_size)
+    data_chunk_size = file_data_chunk_size(estimated_total, total_bytes)
+    chunks = list(utf8_chunks(message_file_text, data_chunk_size))
+    total = max(1, len(chunks))
+    data_chunk_size = file_data_chunk_size(total, total_bytes)
+    chunks = list(utf8_chunks(message_file_text, data_chunk_size))
+    total = max(1, len(chunks))
+    if data_chunk_size < message_chunk_size:
+        log(
+            f"Requested message chunk size {message_chunk_size} exceeds Reticulum link budget; "
+            f"using {data_chunk_size} data bytes per chunk"
+        )
     log(f"Sending file {message_file_path} as {total} chunk(s), {total_bytes} bytes")
 
     for index, chunk_text in enumerate(chunks):
         if not running or link.status != RNS.Link.ACTIVE:
             return
 
-        send_link_packet(
-            link,
-            f"file_chunk {index + 1}/{total} from {NODE_NAME} "
-            f"bytes={len(chunk_text.encode('utf-8'))} file={os.path.basename(message_file_path)} data={chunk_text}",
-        )
+        chunk_bytes = len(chunk_text.encode("utf-8"))
+        send_link_packet(link, f"{file_chunk_prefix(index + 1, total, chunk_bytes)}{chunk_text}")
         time.sleep(0.1)
 
 
@@ -465,8 +489,8 @@ def parse_args():
     parser.add_argument(
         "--message-chunk-size",
         type=int,
-        default=900,
-        help="Maximum UTF-8 bytes per file chunk packet",
+        default=DEFAULT_MESSAGE_CHUNK_SIZE,
+        help="Requested maximum UTF-8 data bytes per file chunk packet",
     )
     parser.add_argument(
         "--announce-only-when-disconnected",