Use a real network MTU when throttling localhost

README.md

@@ -111,6 +111,8 @@ This is useful if you test a local web server or run [WebPageReplay](https://git
 throttle --rtt 200 --localhost
 ```
 
+On Linux, throttle also sets the MTU of the loopback interface to 1500, the packet size of real networks. With the loopback default (65536), the receive window of the browser limits the transfer, and the number of round trips changes from run to run. Stop sets the MTU back to 65536. If you do this on a host and not in a container, all local traffic uses the smaller packets until you stop.
+
 ## Stop adding delay on localhost
 
 ```

lib/localHostTc.js

@@ -1,5 +1,11 @@
 import sudo from './sudo.js';
 
+// Real networks use 1500-byte packets. With the loopback default (65536),
+// the receive window of the browser limits the transfer, and the number of
+// round trips changes from run to run.
+const NETWORK_MTU = '1500';
+const LOOPBACK_DEFAULT_MTU = '65536';
+
 export async function start(delay) {
   const halfWayDelay = delay / 2;
 
@@ -22,7 +28,9 @@ export async function start(delay) {
     'delay',
     `${halfWayDelay}ms`
   );
+  await sudo('ip', 'link', 'set', 'dev', 'lo', 'mtu', NETWORK_MTU);
 }
 export async function stop() {
+  await sudo('ip', 'link', 'set', 'dev', 'lo', 'mtu', LOOPBACK_DEFAULT_MTU);
   await sudo('tc', 'qdisc', 'del', 'dev', 'lo', 'root');
 }

test/start.sh

@@ -26,4 +26,10 @@ tc qdisc del dev "$INTERFACE" clsact
 ## Same thing when a plain ingress qdisc already exists
 tc qdisc add dev "$INTERFACE" ingress
 bin/index.js --profile cable
-bin/index.js --stop
+bin/index.js --stop
+
+## Localhost sets a real network MTU and stop restores the loopback default
+bin/index.js --rtt 200 --localhost
+ip link show lo | grep -q 'mtu 1500'
+bin/index.js --stop --localhost
+ip link show lo | grep -q 'mtu 65536'