TCPMSS clamp to PMTU

[ml70]

I'm seeing some weird behavior from android clients regarding MTU and MSS. My connection is PPPoE, but the MTU resolves down to 1480, not 1492 (ISP setting probably). The wifi networks (eth1, eth2, br0) are on default settings, MTU 1500.

Regardless, the Android clients (phones and tablets) seem to be insisting on a MSS of 1460 (=MTU 1500) all of the time for any outgoing traffic. I tried lowering eth1, eth2 and br0 MTU to 1480 to fit with the PPPoE, but the android clients are still pushing for MSS of 1460.

In iptables, the rule for TCPMSS "clamp to PMTU" has some 126k packet hits in last 24 hours alone...

Any ideas what's going on? I also noticed that using "service restart_wireless" also returns MTU to default setting 1500. What's the proper way of forcing the new MTU to wifi clients?

If it would help in this case, would it be possible to have a MTU setting in Wireless>Professional in order to force a certain MTU for wifi?

 

[Nullity]

I think you can use dnsmasq to share your preferred MSS/MTU via DHCP. I used a modified dnsmasq config through the /jffs folder... I think.

For me, it was successful but I gained nothing in stability, speed, or anything else I could notice, so I gave up on the whole idea and let mss clamping do it's dirty work.

 

[Chrysalis]

I'm seeing some weird behavior from android clients regarding MTU and MSS. My connection is PPPoE, but the MTU resolves down to 1480, not 1492 (ISP setting probably). The wifi networks (eth1, eth2, br0) are on default settings, MTU 1500.

Regardless, the Android clients (phones and tablets) seem to be insisting on a MSS of 1460 (=MTU 1500) all of the time for any outgoing traffic. I tried lowering eth1, eth2 and br0 MTU to 1480 to fit with the PPPoE, but the android clients are still pushing for MSS of 1460.

In iptables, the rule for TCPMSS "clamp to PMTU" has some 126k packet hits in last 24 hours alone...

Any ideas what's going on? I also noticed that using "service restart_wireless" also returns MTU to default setting 1500. What's the proper way of forcing the new MTU to wifi clients?

If it would help in this case, would it be possible to have a MTU setting in Wireless>Professional in order to force a certain MTU for wifi?

Can I ask how you finding out what MSS they using?

 

[ml70]

Can I ask how you finding out what MSS they using?

ping -f -l 1500 host.domain

, it will show an error when unfragmented packet can't get through anymore. Asus' basic ping defaults to Busybox and doesn't have -f option, but you can install hping (ipkg install hping), use it like

hping -V -1 -c 1 --dontfrag --mtu 1500 -d 1472 host.domain -I eth0

 

[sinshiva]

an alternative to clamp-to-pmtu is;

iptables -I FORWARD -o br0 -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1453:65535 -j TCPMSS --set-mss 1452
iptables -I FORWARD -o ppp0 -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1453:65535 -j TCPMSS --set-mss 1452

i also use it for openvpn server at 1300ish. this is for pppoe mtu 1492.

for 1480;

iptables -I FORWARD -o br0 -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1441:65535 -j TCPMSS --set-mss 1440
iptables -I FORWARD -o ppp0 -p tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1441:65535 -j TCPMSS --set-mss 1440