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optio
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MTU over LTE and PMTUD

Mon Mar 20, 2023 7:54 pm

Hi,

When LTE connection is up there is a log: lte1 mbim: network advertises lower mtu: 1358
But MTU is still set to 1500 on lte1 and I can ping from MT device with option do-not-fragment and 1500 size, so I guess that's ok even it is reported from network to have lower MTU.
From computer (MacOS) connected over wifi I can do with max 1480, wifi interface is on 1500 MTU (automatic over PMTUD).
Does ROS need additional setup for clients PMTUD to get correct MTU?
 
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Amm0
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Re: MTU over LTE and PMTUD

Mon Mar 20, 2023 9:37 pm

LOL, I had a support request on this one too. They just log the value they get, nothing is done based on it, according to support.

On the "network advertises [higher/lower] mtu: XXXX" logs, they did suggest:
Changing TCP MSS is only half solution and it does work for other protocols, so anyway there should not be a blackhole router in the packet path which does not send or drop other router ICMP notifications about to big packets.
We are not aware of providers which do not actually supports MTU of 1500, but they recommend lowering MTU because:
each subscriber connection from the tower to the packet core is tunneled using GTP tunnels using the transport network
optionally GTP tunnels may be encrypted using IPsec
transport networks may be built using same MTU as 1500
so when clients sent packets bigger than the transport network MTU minus GTP and encryption headers then this packet must be split into two packets in the transport network which slightly increases the data amount but doubles the processing amount on the transport network/devices.

So does " doubles the processing amount on the transport network/device" make a difference in real world is still an open question. But MT suggest PMTUD detection should work through an LTE network, so higher MTU may not be a problem. I historically just set the MTU match these logs message (which also match the MTU the carrier documents, AT&T in US in my case ). But also seen larger MTU's accepted than the value reported in the log messages.

So I suspect the value reported MAY be the lowest supported by the carrier's overall network, but maybe higher on some paths. But dunno.
 
optio
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Re: MTU over LTE and PMTUD

Mon Mar 20, 2023 10:58 pm

Thx, what confuses me, how local computer (any on my network, checked multiple devices, all have mtu 1500) gets interface MTU over PMTUD, is it pinging path to default gateway (router ip) which is my case at 0 distance or someting else?
My router is not black hole on lan and if OS is pinging default gw it should get 1480, which is max in my case:
# /sbin/ping -g 1470 -G 1500 -D 192.168.100.1
PING 192.168.100.1 (192.168.100.1): (1470 ... 1500) data bytes
1478 bytes from 192.168.100.1: icmp_seq=0 ttl=64 time=14.163 ms
1479 bytes from 192.168.100.1: icmp_seq=1 ttl=64 time=1.914 ms
1480 bytes from 192.168.100.1: icmp_seq=2 ttl=64 time=1.420 ms
ping: sendto: Message too long
 
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Amm0
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Re: MTU over LTE and PMTUD

Mon Mar 20, 2023 11:13 pm

I think the gist is the that LTE is accepting the packets, and inside the LTE network is may be getting fragmented, but the fragmentation is hidden inside the LTE network, since packets come out on the other end at 1500. But fragmentation is "more work" inside the network, so it can effect speeds (and if fragments get separated even more work).

But with LTE... it's hard to attribute a speed difference based on fragmentation, vs just congestion/band changes/etc.

Mikrotik used to use lower defaults for LTE, but recently the default has been MTU 1500 for LTE. So leaving at 1500 may be okay, since the next-hop in LTE is accepting it. If the link happened to be lower upstream in LTE netowrk, PMTUD should kick-in.

As to what the "right thing" to do is, I'm not sure. If default of 1500 is working for you...and ping DF test is working...stick with that. Especially if the device isn't moving to another a location – where at a different tower/location/etc MTU behavior might be different.
 
optio
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Re: MTU over LTE and PMTUD

Mon Mar 20, 2023 11:21 pm

But his is ping between local computer interface and router interface, in this case LTE is not involved. When I tested directly from router terminal to external ip ping with 1500 passed, but this local MTU worries me.

And another mystery on Mac, it does't allow me to set manual MTU over 1436, but when it is on automatic it is set to 1500
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Last edited by optio on Mon Mar 20, 2023 11:26 pm, edited 1 time in total.
 
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Amm0
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Re: MTU over LTE and PMTUD

Mon Mar 20, 2023 11:26 pm

On the PC you're seeing the overhead of ping, why it's 1480. I think Mikrotik take the icmp overhead into account when you input a desired packet size ("Packet size to be used in bytes (includes payload and IP header)").
 
optio
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Re: MTU over LTE and PMTUD

Mon Mar 20, 2023 11:28 pm

On the PC you're seeing the overhead of ping, why it's 1480. I think Mikrotik take the icmp overhead into account when you input a desired packet size ("Packet size to be used in bytes (includes payload and IP header)").
Ah, didn't know that, ok so 1500 is actually correct MTU. Thx

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