Link test: per-socket pacing (SO_MAX_PACING_RATE) at 5 MB/s removes the NCM holds

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

tool/linktest_server.py

@@ -16,6 +16,9 @@ traffic goes over WiFi), so the app drives the test and this end only serves:
 
 Every TCP test also logs this end's TCP_INFO (retransmits, RTT), which the app
 cannot read. Run: python3 linktest_server.py [results.jsonl]
+
+LINKTEST_PACE=<bytes/s> caps every download socket with SO_MAX_PACING_RATE, TCP's
+own pacing (no qdisc needed; the Tegra kernel ships without fq/tbf/htb).
 """
 
 import json
@@ -26,6 +29,8 @@ import sys
 import threading
 import time
 
+PACE = int(os.environ.get("LINKTEST_PACE", "0"))
+SO_MAX_PACING_RATE = 47
 RESULTS = os.path.expanduser(sys.argv[1] if len(sys.argv) > 1 else "~/linktest/results.jsonl")
 CHUNK = b"\0" * 65536
 lock = threading.Lock()
@@ -58,6 +63,8 @@ def readline(conn):
 def down(conn, peer):
     test_id, *args = readline(conn).split()
     mode = args[0] if args[0] in ("burst", "paced") else "bulk"
+    if PACE:
+        conn.setsockopt(socket.SOL_SOCKET, SO_MAX_PACING_RATE, PACE)
     sent = 0
     try:
         if mode == "burst":
@@ -85,7 +92,7 @@ def down(conn, peer):
                 sent += conn.send(CHUNK)
     except OSError as e:
         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,
+    log({"id": test_id, "kind": "tcp_down_orin", "mode": mode, "args": args, "pace": PACE, "peer": peer, "bytes": sent,
          **tcp_info(conn)})