Enable tcp mss clamp for Tethering

With this change, IPv4 tethering clients (such as PC) with default
MTU size 1500 will be configured again in the phone to use MTU
matching the carrier network, then no IP fragemention in the phone
and have higher data throughput for tethering.

bug:17552732
Change-Id: Ia306b1b8454da1b0e2d9c320e770aeff172d5fc4

diff --git a/server/NatController.cpp b/server/NatController.cpp
index 902756b..d9a779c 100644 (file)
--- a/server/NatController.cpp
+++ b/server/NatController.cpp
@@ -83,12 +83,25 @@ int NatController::setupIptablesHooks() {
 
     struct CommandsAndArgs defaultCommands[] = {
         /*
-         * Chain for tethering counters.
+         * First chain is for tethering counters.
          * This chain is reached via --goto, and then RETURNS.
+         *
+         * Second chain is used to limit downstream mss to the upstream pmtu
+         * so we don't end up fragmenting every large packet tethered devices
+         * send.  Note this feature requires kernel support with flag
+         * CONFIG_NETFILTER_XT_TARGET_TCPMSS=y, which not all builds will have,
+         * so the final rule is allowed to fail.
+         * Bug 17629786 asks to make the failure more obvious, or even fatal
+         * so that all builds eventually gain the performance improvement.
          */
         {{IPTABLES_PATH, "-F", LOCAL_TETHER_COUNTERS_CHAIN,}, 0},
         {{IPTABLES_PATH, "-X", LOCAL_TETHER_COUNTERS_CHAIN,}, 0},
         {{IPTABLES_PATH, "-N", LOCAL_TETHER_COUNTERS_CHAIN,}, 1},
+        {{IPTABLES_PATH, "-t", "mangle", "-F", LOCAL_FORWARD,}, 0},
+        {{IPTABLES_PATH, "-t", "mangle", "-X", LOCAL_FORWARD,}, 0},
+        {{IPTABLES_PATH, "-t", "mangle", "-N", LOCAL_FORWARD,}, 1},
+        {{IPTABLES_PATH, "-t", "mangle", "-A", LOCAL_FORWARD, "-p", "tcp", "--tcp-flags",
+                "SYN", "SYN", "-j", "TCPMSS", "--clamp-mss-to-pmtu"}, 0},
     };
     for (unsigned int cmdNum = 0; cmdNum < ARRAY_SIZE(defaultCommands); cmdNum++) {
         if (runCmd(ARRAY_SIZE(defaultCommands[cmdNum].cmd), defaultCommands[cmdNum].cmd) &&