rounder Posted March 18 Share Posted March 18 There is. a specific reason why i would need lower MTU on router level For example, if I lower MTU to 1492 on R3 wan advanced inteface, restart etc... Clients still get full 1500 MTU ping -D -s 1472 www.google.com PING www.google.com (142.251.41.68): 1472 data bytes Request timeout for icmp_seq 0 Request timeout for icmp_seq 1 vs ping -D -s 1473 www.google.com PING www.google.com (142.251.41.68): 1473 data bytes ping: sendto: Message too long Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 18 Share Posted March 18 I don't quite understand what you are trying to say. 1492 - 20 (Ethernet header) are the 1472 mentioned, which is why a ping with packet size 1472 goes through and 1473 does not. If, as you say, the MTU restriction did not apply, packets up to 1480 would theoretically still go through. EDIT: sorry, I forgot the 8 bit for the ICMP header like @DanologyUK has mentioned below.. So yes it’s looks strange. What type of connection you use? PPPoE or DHCP / Static IP? Link to comment Share on other sites More sharing options...
DanologyUK Posted March 18 Share Posted March 18 1 hour ago, ChrisG82 said: I don't quite understand what you are trying to say. 1492 - 20 (Ethernet header) are the 1472 mentioned, which is why a ping with packet size 1472 goes through and 1473 does not. If, as you say, the MTU restriction did not apply, packets up to 1480 would theoretically still go through. Isnt it 1492 - 28 (20 are reserved for the IP header and 8 bytes needed for the ICMP Echo Request header)? Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 18 Share Posted March 18 I have assumed that the 8 bytes are included in the 1492 as he sends ICMP packets. But yes I think you are right. Link to comment Share on other sites More sharing options...
Proud R3 Owner Posted March 18 Share Posted March 18 I've always used 1452 for my mtu. Always just felt snappier to me. Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 18 Share Posted March 18 I have tested it in my network. My MTU is set to 1492. The maximum packet size for me is then 1464. From a packet size of 1465, the ICMP packet no longer goes out. So the limitation actually seems to work. PS: my R3 establishes a PPPoE connection. Netduma Fraser 1 Link to comment Share on other sites More sharing options...
rounder Posted March 18 Author Share Posted March 18 For me it doesnt work, if i set R3 to 1492 and clients are still able to send 1500 or 1472 + headers.... Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 18 Share Posted March 18 Ok let’s ask me again: what type of internet connection you use? Link to comment Share on other sites More sharing options...
Administrators Netduma Fraser Posted March 19 Administrators Share Posted March 19 Also check your ISP modem/router, if that has a conflicting MTU you should set it as the same Link to comment Share on other sites More sharing options...
rounder Posted March 19 Author Share Posted March 19 its a dhcp, full dmz via fibre but that really doesnt matter, I am asking why netduma is not limiting MTU to clients if i set it up in advanced WAN settings? Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 19 Share Posted March 19 11 hours ago, rounder said: its a dhcp, full dmz via fibre but that really doesnt matter, I am asking why netduma is not limiting MTU to clients if i set it up in advanced WAN settings? So you're saying that giving technical details about a problem that apparently only you have doesn't matter? Okay, then it looks to me like you're not seriously interested in troubleshooting. It works completely problem-free for me. So it doesn't seem to be a general problem and only occurs in certain cases. But I'm sure you'll find out eventually. Link to comment Share on other sites More sharing options...
Fuzy Posted March 19 Share Posted March 19 It doesn't work like the XR series (netgear) The modification of the MTU is only at the WAN level, the LAN part is not affected! Devices downstream of the R3 will not take it into account. (Like for example a test on an xbox...) Link to comment Share on other sites More sharing options...
Administrators Netduma Fraser Posted March 19 Administrators Share Posted March 19 As above, if your PC for example has an MTU set at 1500 it's still going to use that. You can use netsh interface ipv4 show subinterface in command prompt to check what MTU your PC is using (likely 1500 by default). What is your use case out of interest? Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 19 Share Posted March 19 @Fuzy @Netduma Fraser I think he wants to avoid packet fragmentation. And yes, of course a PC on which an MTU of 1500 is set uses this for the packets. However, if the packets are to be sent to the Internet via the WAN interface of the R3, packets that are larger than the set MTU should NOT be processed if there is a DF flag on the packet. But this is apparently not the case with him. And in his example he has actually shown that he is sending an ICMP packet where he specifies the size manually. If he has specified an MTU of 1492 in the advanced settings in R3, no packets with a DF flag that are larger than 1464 should go out. For me it works as it should. The question is whether it does not work for him because his connection runs via DHCP and mine via PPPoE So I can only imagine exactly two things that could be the cause. Either the R3 ignores the maximum MTU set for a connection via DHCP or he has not connected his ISP router to the WAN port of the R3. Link to comment Share on other sites More sharing options...
rounder Posted March 19 Author Share Posted March 19 4 hours ago, Fuzy said: It doesn't work like the XR series (netgear) The modification of the MTU is only at the WAN level, the LAN part is not affected! Devices downstream of the R3 will not take it into account. (Like for example a test on an xbox...) OK, thx, thats what i wanted to know coz it had no effect on connected devices. I assumed changing mtu would affect clients too... Link to comment Share on other sites More sharing options...
rounder Posted March 19 Author Share Posted March 19 4 hours ago, ChrisG82 said: So you're saying that giving technical details about a problem that apparently only you have doesn't matter? Okay, then it looks to me like you're not seriously interested in troubleshooting. It works completely problem-free for me. So it doesn't seem to be a general problem and only occurs in certain cases. But I'm sure you'll find out eventually. sry, i didnt mean to be rude, its just i think we misunderstand each other what i was trying to do btw, R3 is in advanced DMZ mode via telco fiber modem so it gets public WAN IP, no pppoe, 1500 mtu, basically changing MTU on R3 wan advanced interface doesn't affect any clients connected to R3, Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 19 Share Posted March 19 Yeah, no problem. But I still don't understand exactly why it works for me and not for you. The only difference is that my R3 establishes a PPPoE connection and you use DHCP. If I reduce the MTU in my advanced settings, all clients are affected. They then can only send packets with the DF flag that comply with this maximum MTU and without the DF flag the packets are fragmented accordingly... And I think this behaviour is completely correct. Because with this setting I tell the R3 that it should only send packets up to a certain size to the outside via the WAN interface. So here is my request to @Netduma Fraser Please clarify with the devs what behaviour is really desired when the MTU is changed in the advanced settings. Link to comment Share on other sites More sharing options...
rounder Posted March 21 Author Share Posted March 21 Thank you, that is what i exactly need to know, is it a bug or as intended but then how are you able to limit MTU for clients ?... On 3/19/2024 at 2:46 PM, ChrisG82 said: Yeah, no problem. But I still don't understand exactly why it works for me and not for you. The only difference is that my R3 establishes a PPPoE connection and you use DHCP. If I reduce the MTU in my advanced settings, all clients are affected. They then can only send packets with the DF flag that comply with this maximum MTU and without the DF flag the packets are fragmented accordingly... And I think this behaviour is completely correct. Because with this setting I tell the R3 that it should only send packets up to a certain size to the outside via the WAN interface. So here is my request to @Netduma Fraser Please clarify with the devs what behaviour is really desired when the MTU is changed in the advanced settings. Link to comment Share on other sites More sharing options...
ChrisG82 Posted March 21 Share Posted March 21 (edited) I have only reduced the MTU in the advanced settings. If I now (like you) send an ICMP to Google, for example, and set the packet size manually, I just don't get above 1464. Of course, only if I also say that the packets should not be fragmented. EDIT: So here are two screenshots, one with a registered MTU of 1492 and one with an MTU of 1450. The first ping per screenshot is directly against the R3. Here you can see that the LAN side is not affected and accepts packets with 1472 bytes without any problems. However, as soon as the packets go out via the WAN interface of the R3, you can see that the packets are limited in size. Depending on the MTU in the R3 settings (And no: I have not changed any settings on the client itself) Edited March 21 by ChrisG82 Link to comment Share on other sites More sharing options...
killi_34 Posted March 21 Share Posted March 21 Ich habe die MTU mal auf 1 gestellt beim Netduma R3. Theroetisch sollte ich so nicht zum Beispiel Call of Duty zocken können , oder ? Meine Verbindungen sind PS5 --- Netduma --- Glasfaserkonverter . Beim Glasfaserkonverter kann ich keine MTU einstellen. Link to comment Share on other sites More sharing options...
Fairypusher Posted March 21 Share Posted March 21 The Deutsche Telekom use 1492 MTU. To find the right MTU for Router and Windows you can use SG TCP Optimizer and then go to MTU / Latancy and klick on Largest MTU. Link to comment Share on other sites More sharing options...
White_Mamba Posted March 22 Share Posted March 22 Different MTU numbers are for different kinds of connections. 1492 is specifically for those who are using PPPOE connections. That is what I use, I forgot what other numbers may be used for but you need to check with your internet provider to see what connection you use and then just google the answer from there. Link to comment Share on other sites More sharing options...
Fuzy Posted March 23 Share Posted March 23 For information : https://en.wikipedia.org/wiki/Maximum_transmission_unit Link to comment Share on other sites More sharing options...
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