MTU network settings - what are they?

[liquidtmd]

I appreciate this is a niche gaming thread and Mods feel free to close if inappropriate or move to a tech thread, but yesterday and for around the last 12 hours I've had a weird problem that I've resolved and I felt compelled to write to ask. It's a story thread, born of relief, beer (well, hangover) and it having been a crappy week

- 'why the hell did it do this'
- 'what is this MTU setting broadly'
- 'how the hell was anyone supposed to know how to fix this easily'

The situation:

- Bought a PS5 on launch day
- Hooked it up to my internet. Setup is BT Hub downstairs (I'm in UK), extender Wi-disc upstairs
- Mainly play it for single player. Connects to Wi-fi flawlessly, can download stuff / updates / access PSN. Bought COD Cold War on launch, online worked great
- A year goes by. I'd long stopped playing COD. I think the first time I'd noticed the problem I'm going to describe, I was trying to use Capcom Arcade Collection - i could download it fine but it insisted I had no Network features because I had no connection.

- Then I noticed it with all games that required a game specific network functionality or online play - all games insisted I couldn't play online, despite my internet connection tests on the PS5 coming back fine and I could download updates at my full download speed (70MB)
- Since I mainly play it for single player, I shrug and carry on thinking it a short glitch. No game gave me a specific error code
- I kept running into the issue across a few PS+ titles so I started to do the standard troubleshooting stuff (power cycling / router cycling)

- At around that point I download the COD Vanguard weekend trial and lo and behold, same problem but it threw an 'DHUK-GUERNSEY' error (which lead me to an article which lead to the solution but I'll come back to this). At this point I was knee deep in port forwarding on my BT-Hub trying to fix it, and playing with DNS settings / all network PS5 settings trying to resolve it following various online guides (if I see another advice piece that just says 'turn off your router and on again, you'll be fine I'll cry)

- It finally fixed and resolved itself by my various tinkering that day, but I'm an idiot and didn't mark which step actually fixed it

- Skip to the present. At the moment I'm in love with the Evil Dead game. It's great, online has been great. Now on Thursday my WiFi extender disc upstairs had a few connection issues and I had to do a full repair process. At the same time, the Evil Dead downloaded an update (which I came to realise quickly wasn't the problem). Post both of these things, Evil Dead wasn't working in pretty much the same way as my problem last year - PSN fine, downloads fine at great speed, but

- For Evil Dead, it doesnt usually let you get to the above screen...unless it already knows youve got internet! Argh
- Since I don't really play much online, I didn't have much to test but Guilty Gear Strive, network features - yep, all borked.

- Now, if you Google 'My PS5 internet works but network features dont' you get absolutely zero helpful threads. Genuinely, sod all. Recycle your router! So away I go again looking at port settings, my hub, recycles and....WAIT BRAIN, WHAT WAS THAT ERROR CODE VANGUARD GAVE YOU THAT LEAD YOU TO THAT ONE NEW PIECE OF ADVICE.

- I couldn't remember it was DHUK-GUERSNEY, so I redownloaded the whole of Vanguard again to get it. And behold, it clicked immediately back thats what it was and lo-behold, this time I only did this variable change and it FIXED MY PROBLEM

***I changed the PS5 network MTU variable from 1500 to 1473***

Seriously, how was anyone supposed to know how to do that? Why would my net setup make my PS5 have to do that? And I guess to anyone out there who has the same issue, try that.

TL:DR: PS5 had no issue connecting to my internet, could connect to PSN, could download updates but no network features worked. Changing MTU setting from 1500 to 1473 fixed it. I made a thread because I read 50 odd Google threads about fixing network issues and this fix solution featured in barely any of them. Also Id like someone technical to explain the working here?

 

[Edward850]

MTU stands for Maximum Transmission Unit, it's what controls the maximum size a packet payload can be. Packets larger than it have to be fragmented by the hardware or dropped entirely if the hardware cannot handle fragmentation.

Usually you never have to change it unless you are on a specialist network such as a VPN (as they have much lower MTU sizes, they can get as low as 512). For whatever reason, the network hardware you have doesn't seem to be complying under normal rules, so the playstation under default settings wasn't able to successfully communicate with remote servers as its packets were larger than what it's configured for (that's not normal) and you have something between your PS5 and your ISP that's just dropping data entirely instead of fragmenting it (extremely not normal, the DONTFRAGMENT flag is almost never used).

And yes, this does imply MTUs can't be automatically detected. It's an annoying oddity. You can sort of test them using progressively larger packets, though this is very not common to do as it's an unreliable metric and complicated to design around.

 

[neoak]

Maximum transmission unit - Wikipedia

en.wikipedia.org

MTU being the issue is rare. Really rare.

And it's more a thing with PSN. Sony even published this as an official solution back in 2014, so... it's PSN.

PS4's MTU 1473 Temporary PlayStation Network Fix Made Official by Sony (UPDATED)

We've been talking about it for a couple days, and looks like Sony decided that in sight of the continuing problems some are suffering with the PSN, it

www.dualshockers.com

 

[bobmonkey]

From an old Reddit thread on this topic:

Reddit - Dive into anything

www.reddit.com

An MTU size of 1473 is almost a magic number. Reducing the size of the MTU has one significant use, and that is to reduce/eliminate/avoid datagram ("packet") fragmentation. Every active device between your PS4 and PSN's servers has an MTU, and most will be 1500. In ideal circumstances, each of those devices will pass on the datagram untouched to the next. if one device has a lower MTU, then it will slice the packets to size creating more packets that have to be reassembled. Reducing the MTU allows the origin (your PS4) to take responsibility for slicing the data. This might avoid certain network problems.

I say that 1473 is almost a magic number because the header size for a datagram is 28 bytes. So, if your MTU is 1500 (normal) then the maximum payload is 1472 (a datagram is payload plus header, so 1472 + 28 == 1500). Why does this matter ? Because IP tunneling stuffs datagrams inside datagrams. In other words, if you have a datagram that originates with an MTU of 1472 (payload of 1444 + 28 == 1472), then you can stuff that datagram inside another datagram ( +28 bytes) and still hit the 1500 byte datagram size.

1472 is absolutely a magic number for MTU size. However, I certainly can't explain why 1473 is a relevant number for avoiding PSN's current problems. I could hazard a guess:

The DDOS is almost certainly UDP based. UDP is stateless and easier to disguise the origin. Also UDP has no guarantee of delivery. In other words legitimate data from a PS should be sent with an algorithm that can cope with non-delivery. I'd guess that the DDOS UDP packets are a specific size...perhaps small. SONY has configured their systems to ignore, or drop packets of this specific size, maybe with additional characteristics. SONY may have reasoned that this was OK, since legitimate UDP data from a PS would be resent or at least handled gracefully. However, some of us have a router somewhere in our path to SONY that is fragmenting packets resulting in a large packet, plus the leftover data in a small packet. that small packet looks like the DDOS packet and is being rejected by SONY, which will cause the "large" packet to be discarded also since is missing the data in its companion packet. Reducing the MTU is preventing that small packet from being created and our data no longer looks like the DDOS data.

Click to shrink...

 

[liquidtmd]

Maximum transmission unit - Wikipedia

en.wikipedia.org

MTU being the issue is rare. Really rare.

And it's more a thing with PSN. Sony even published this as an official solution back in 2014, so... it's PSN.

PS4's MTU 1473 Temporary PlayStation Network Fix Made Official by Sony (UPDATED)

We've been talking about it for a couple days, and looks like Sony decided that in sight of the continuing problems some are suffering with the PSN, it

www.dualshockers.com

Click to shrink...

Interesting. Very interesting

I would class myself as 'vaguely' technical in that my troubleshooting is usually fairly rigid.

I know for sure I changed the MTU setting to 1473 when I fixed it last time, and when I came to change it this week it was back to 1500)

I 'do' know for sure changing the MTU setting yesterday was the key to resolution, as it was the only variable I changed during that phase of my testing

It took a fair bit of patience on my part and as I tried to get across in my OP, googling 'my PS5 can browse PSN, download stuff and seems fine but cannot use network features' seems to be a niche and obscure problem to fix

 

[neoak]

1500 is the standard Ethernet packet. Pretty much everything now runs on that.

There is some connections that require less, some forms of VDSL (1492) and PPPoE iirc, but never have personally needed to change it. But those are like super specific cases that you set it once and never touch it again.

 

[liquidtmd]

MTU stands for Maximum Transmission Unit, it's what controls the maximum size a packet payload can be. Packets larger than it have to be fragmented by the hardware or dropped entirely if the hardware cannot handle fragmentation.

Usually you never have to change it unless you are on a specialist network such as a VPN (as they have much lower MTU sizes, they can get as low as 512). For whatever reason, the network hardware you have doesn't seem to be complying under normal rules, so the playstation under default settings wasn't able to successfully communicate with remote servers as its packets were larger than what it's configured for (that's not normal) and you have something between your PS5 and your ISP that's just dropping data entirely instead of fragmenting it (extremely not normal, the DONTFRAGMENT flag is almost never used).

And yes, this does imply MTUs can't be automatically detected. It's an annoying oddity. You can sort of test them using progressively larger packets, though this is very not common to do as it's an unreliable metric and complicated to design around.

Click to shrink...

Thank you for this - something does indeed seem odd with my netsetup here.

It's one of those where 'everything else connects fine in my house, my kids can use their tablets, my wife can use her phone so who cares enough to deep dive....EXCEPT IT CAME FOR MY EVIL DEAD' 😂

 

[neoak]

It's one of those where 'everything else connects fine in my house, my kids can use their tablets, my wife can use her phone so who cares enough to deep dive....EXCEPT IT CAME FOR MY EVIL DEAD' 😂

Click to shrink...

Hence why it wasn't on your network or internet.

It was PSN. Some duct tape peeled off and they needed time to fix it.

 

[kurahador]

Damn this brings me back. I remember having this problem back when I was using an old modem decades ago ---- as in really old, like 2mbps max modem. Discovering the MTU thing feels huge.

 

[neoak]

Damn this brings me back. I remember having this problem back when I was using an old modem ---- as in really old, like 2mbps max modem.

Click to shrink...

was it DSL? MTU was 1492 and depending on the quality of the line, you could need to lower it because 30+ years old copper without any maintenance (because big telcos ofc) couldn't handle big packets.

 

[liquidtmd]

Damn this brings me back. I remember having this problem back when I was using an old modem ---- as in really old, like 2mbps max modem.

Click to shrink...

Heh

FWIW I've got a BT Homehub 3, BT extender disc and typically 70 MB / 10MB (down/up). I live out in the sticks but it's not bad - I used to be nearer a town and bizarrely only got freaking 8MB

 

[liquidtmd]

Hence why it wasn't on your network or internet.

It was PSN. Some duct tape peeled off and they needed time to fix it.

Click to shrink...

This is good to know also and makes sense. Just so weird to me that I could download stuff fine and browse the PSN store fine, but all games network wise were broke

 

[kurahador]

was it DSL? MTU was 1492 and depending on the quality of the line, you could need to lower it because 30+ years old copper without any maintenance (because big telcos ofc) couldn't handle big packets.

Click to shrink...

Oh yeah, that's it --- 1492, and it was DSL. Forgot what game that cause me problem at the time.

 

[SCUMMbag]

When are we going to start talking about Jumbo Frames?

 

[neoak]

When are we going to start talking about Jumbo Frames?

Click to shrink...

Ain't happening with all the copper getting wrecked by the elements and lack of maintenance.

 

[jroc74]

Hell, I expected this to be about getting in a public solo lobby in GTA Online, lol.

Good to know, I had no idea about MTU issues and PSN.

 

[Dave.]

Heh

FWIW I've got a BT Homehub 3, BT extender disc and typically 70 MB / 10MB (down/up). I live out in the sticks but it's not bad - I used to be nearer a town and bizarrely only got freaking 8MB

Click to shrink...

FWIW neoak , this is a DSL service. FTTC "Fiber to the cabinet" then VDSL for the final run over (probably ancient) copper.