Link test: download series (bulk sampled, warm bursts, paced) and burst-vs-idle; car results

Orin -> tablet over NCM: paced streams up to 4 MB/s are clean; unpaced
bursts and bulk hit multi-second holds that inflate the RTO to 10-16 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

tool/linktest_server.py

@@ -4,7 +4,12 @@
 The tablet's own sockets are the only ones bound to the USB link (`adb shell`
 traffic goes over WiFi), so the app drives the test and this end only serves:
 
-  7791/tcp  down: client sends "<id> <secs>\\n", gets zeros for <secs>
+  7791/tcp  down: client sends one line, then reads until the server closes:
+              "<id> <secs>"               zeros for <secs>
+              "<id> burst"                then per line "<bytes>": exactly <bytes> zeros,
+                                          on the same connection until the client closes
+              "<id> paced <B/s> <secs>"   30 frames/s, each [u32 length][f64 send time][zeros],
+                                          length counting the whole frame
   7792/tcp  up:   client sends "<id>\\n" then data until EOF; gets "<bytes>\\n" back
   7793/udp  echo
   7794/tcp  results: one JSON line per connection, appended to the results file
@@ -51,15 +56,37 @@ def readline(conn):
 
 
 def down(conn, peer):
-    test_id, secs = readline(conn).split()
-    end = time.monotonic() + float(secs)
+    test_id, *args = readline(conn).split()
+    mode = args[0] if args[0] in ("burst", "paced") else "bulk"
     sent = 0
     try:
-        while time.monotonic() < end:
-            sent += conn.send(CHUNK)
+        if mode == "burst":
+            # One warm connection, as an utterance would arrive on the open WebSocket.
+            while line := readline(conn):
+                left = int(line)
+                while left > 0:
+                    n = conn.send(CHUNK[:min(left, len(CHUNK))])
+                    left -= n
+                    sent += n
+        elif mode == "paced":
+            rate, secs = float(args[1]), float(args[2])
+            size = max(12, int(rate / 30))
+            t0 = time.monotonic()
+            for i in range(int(secs * 30)):
+                # Pace on the schedule, not on the last send: a stall must not lower the offered rate.
+                delay = t0 + i / 30 - time.monotonic()
+                if delay > 0:
+                    time.sleep(delay)
+                conn.sendall(struct.pack("!Id", size, time.time()) + bytes(size - 12))
+                sent += size
+        else:
+            end = time.monotonic() + float(args[0])
+            while time.monotonic() < end:
+                sent += conn.send(CHUNK)
     except OSError as e:
-        log({"id": test_id, "kind": "tcp_down_orin", "error": str(e)})
-    log({"id": test_id, "kind": "tcp_down_orin", "peer": peer, "bytes": sent, **tcp_info(conn)})
+        log({"id": test_id, "kind": "tcp_down_orin", "mode": mode, "error": str(e)})
+    log({"id": test_id, "kind": "tcp_down_orin", "mode": mode, "args": args, "peer": peer, "bytes": sent,
+         **tcp_info(conn)})
 
 
 def up(conn, peer):