diff --git a/decnet/os_fingerprint.py b/decnet/os_fingerprint.py
index aa17b86..094bbc0 100644
--- a/decnet/os_fingerprint.py
+++ b/decnet/os_fingerprint.py
@@ -23,9 +23,13 @@ Secondary discriminators (nmap OPS / WIN / ECN / T2–T6 probe groups):
   net.ipv4.ip_no_pmtu_disc    – DF bit in ICMP replies (IE probes); embedded on
   net.ipv4.tcp_fin_timeout    – FIN_WAIT_2 seconds (T2–T6 timing); Windows shorter
 
+ICMP tuning (nmap IE / U1 probe groups):
+  net.ipv4.icmp_ratelimit     – Min ms between ICMP error replies; Windows = 0 (none)
+  net.ipv4.icmp_ratemask      – Bitmask of ICMP types subject to rate limiting
+
 Note: net.core.rmem_default is a global (non-namespaced) sysctl and cannot be
-set per-container without --privileged; TCP window size mangling is deferred to
-Phase 2 (iptables entrypoint).
+set per-container without --privileged; TCP window size is already correct for
+Windows (64240) from the kernel's default tcp_rmem settings.
 """
 
 from __future__ import annotations
@@ -40,6 +44,8 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_ecn": "2",
         "net.ipv4.ip_no_pmtu_disc": "0",
         "net.ipv4.tcp_fin_timeout": "60",
+        "net.ipv4.icmp_ratelimit": "1000",
+        "net.ipv4.icmp_ratemask": "6168",
     },
     "windows": {
         "net.ipv4.ip_default_ttl": "128",
@@ -50,6 +56,8 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_ecn": "0",
         "net.ipv4.ip_no_pmtu_disc": "0",
         "net.ipv4.tcp_fin_timeout": "30",
+        "net.ipv4.icmp_ratelimit": "0",
+        "net.ipv4.icmp_ratemask": "0",
     },
     "bsd": {
         "net.ipv4.ip_default_ttl": "64",
@@ -60,6 +68,8 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_ecn": "0",
         "net.ipv4.ip_no_pmtu_disc": "0",
         "net.ipv4.tcp_fin_timeout": "60",
+        "net.ipv4.icmp_ratelimit": "250",
+        "net.ipv4.icmp_ratemask": "6168",
     },
     "embedded": {
         "net.ipv4.ip_default_ttl": "255",
@@ -70,6 +80,8 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_ecn": "0",
         "net.ipv4.ip_no_pmtu_disc": "1",
         "net.ipv4.tcp_fin_timeout": "15",
+        "net.ipv4.icmp_ratelimit": "0",
+        "net.ipv4.icmp_ratemask": "0",
     },
     "cisco": {
         "net.ipv4.ip_default_ttl": "255",
@@ -80,6 +92,8 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_ecn": "0",
         "net.ipv4.ip_no_pmtu_disc": "1",
         "net.ipv4.tcp_fin_timeout": "15",
+        "net.ipv4.icmp_ratelimit": "0",
+        "net.ipv4.icmp_ratemask": "0",
     },
 }
 
diff --git a/tests/test_os_fingerprint.py b/tests/test_os_fingerprint.py
index 970655b..79e3438 100644
--- a/tests/test_os_fingerprint.py
+++ b/tests/test_os_fingerprint.py
@@ -164,6 +164,53 @@ def test_embedded_tcp_fin_timeout_is_15():
 def test_cisco_tcp_fin_timeout_is_15():
     assert get_os_sysctls("cisco")["net.ipv4.tcp_fin_timeout"] == "15"
 
+# ---------------------------------------------------------------------------
+# os_fingerprint module — icmp_ratelimit
+# ---------------------------------------------------------------------------
+
+def test_linux_icmp_ratelimit_is_1000():
+    assert get_os_sysctls("linux")["net.ipv4.icmp_ratelimit"] == "1000"
+
+
+def test_windows_icmp_ratelimit_is_0():
+    assert get_os_sysctls("windows")["net.ipv4.icmp_ratelimit"] == "0"
+
+
+def test_bsd_icmp_ratelimit_is_250():
+    assert get_os_sysctls("bsd")["net.ipv4.icmp_ratelimit"] == "250"
+
+
+def test_embedded_icmp_ratelimit_is_0():
+    assert get_os_sysctls("embedded")["net.ipv4.icmp_ratelimit"] == "0"
+
+
+def test_cisco_icmp_ratelimit_is_0():
+    assert get_os_sysctls("cisco")["net.ipv4.icmp_ratelimit"] == "0"
+
+
+# ---------------------------------------------------------------------------
+# os_fingerprint module — icmp_ratemask
+# ---------------------------------------------------------------------------
+
+def test_linux_icmp_ratemask_is_6168():
+    assert get_os_sysctls("linux")["net.ipv4.icmp_ratemask"] == "6168"
+
+
+def test_windows_icmp_ratemask_is_0():
+    assert get_os_sysctls("windows")["net.ipv4.icmp_ratemask"] == "0"
+
+
+def test_bsd_icmp_ratemask_is_6168():
+    assert get_os_sysctls("bsd")["net.ipv4.icmp_ratemask"] == "6168"
+
+
+def test_embedded_icmp_ratemask_is_0():
+    assert get_os_sysctls("embedded")["net.ipv4.icmp_ratemask"] == "0"
+
+
+def test_cisco_icmp_ratemask_is_0():
+    assert get_os_sysctls("cisco")["net.ipv4.icmp_ratemask"] == "0"
+
 
 # ---------------------------------------------------------------------------
 # os_fingerprint module — structural / completeness