updated comments for MTU probing

references: https://blog.cloudflare.com/ip-fragmentation-is-broken/

sysctl.conf

@@ -215,21 +215,22 @@ net.ipv4.ip_local_port_range = 1024 65535
 # - 2: enable timestamps without random offsets
 net.ipv4.tcp_timestamps = 0
 
-# SSR could impact TCP's performance on a fixed-speed network (e.g., wired)
-#   but it could be helpful on a variable-speed network (e.g., LTE)
-# uncomment this if you are on a fixed-speed network
-#net.ipv4.tcp_slow_start_after_idle = 0
-
-# useful if there are ICMP blackholes between you and your clients (most likely there are).
-net.ipv4.tcp_mtu_probing=1
-
 # enabling SACK can increase the throughput
 # but SACK is commonly exploited and rarely used
 net.ipv4.tcp_sack = 0
 
 # divide socket buffer evenly between TCP window size and application
 net.ipv4.tcp_adv_win_scale = 1
 
+# SSR could impact TCP's performance on a fixed-speed network (e.g., wired)
+#   but it could be helpful on a variable-speed network (e.g., LTE)
+# uncomment this if you are on a fixed-speed network
+#net.ipv4.tcp_slow_start_after_idle = 0
+
+# enabling MTU probing helps mitigating PMTU blackhole issues
+# this may not be desirable on congested networks
+#net.ipv4.tcp_mtu_probing = 1
+
 # increase memory thresholds to prevent packet dropping
 #net.ipv4.tcp_rmem = 4096 87380 8388608
 #net.ipv4.tcp_wmem = 4096 87380 8388608