Tweaking your MTU settings.

[Cosmic_Nomad]

I have been having loads of disconnections on my ADSL line @ 384 kbps / 128 kbps


I checked on the router and that it was a default 1472. I must also mention that I'm about 7 Km from the exchange, so every bit counts....

After doing the exercise I have now changed it to 1398.

Go to Start/ Programs/ Accessories/ Command Prompt and type the following:

ping -f -l 1472 www.google.com

(That is a dash lower case "L," not a dash "1." Also note the spaces in between the sections.)

Press Enter.

If you get the "packet needs to be fragmented" error message. Then reduce 1472 by 10 until you no longer get the "packet needs to be fragmented" error message.

C:\Users\Timo>ping www.google.com -f -l 1420

Pinging www.l.google.com [64.233.179.104] with 1420 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 64.233.179.104:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Timo>ping www.google.com -f -l 1400

Pinging www.l.google.com [64.233.179.104] with 1400 bytes of data:
Reply from 64.233.179.104: bytes=64 (sent 1400) time=495ms TTL=50
Reply from 64.233.179.104: bytes=64 (sent 1400) time=496ms TTL=50
Reply from 64.233.179.104: bytes=64 (sent 1400) time=496ms TTL=50
Reply from 64.233.179.104: bytes=64 (sent 1400) time=496ms TTL=50

Ping statistics for 64.233.179.104:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 495ms, Maximum = 496ms, Average = 495ms

C:\Users\Timo>ping www.google.com -f -l 1408

Pinging www.l.google.com [64.233.179.104] with 1408 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 64.233.179.104:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Timo>ping www.google.com -f -l 1370

Pinging www.l.google.com [64.233.179.104] with 1370 bytes of data:
Reply from 64.233.179.104: bytes=64 (sent 1370) time=491ms TTL=50
Reply from 64.233.179.104: bytes=64 (sent 1370) time=491ms TTL=50
Reply from 64.233.179.104: bytes=64 (sent 1370) time=492ms TTL=50
Reply from 64.233.179.104: bytes=64 (sent 1370) time=492ms TTL=50

You will have to add 28 more to this (this is just the ping packet size, not including IP/ICMP header of 28 bytes), and this is your MaxMTU.

I have no longer got the problem with ADSL going bang at two in the morning.( Holding thumbs )

Did any one else have a major change from what was default or have I just got to sell the house and move closer?


All info from .dslreports.com/faq/tweaks/4._MTU and .kitz.co.uk/adsl/tweak2.htm

 

[Tinuva]

Usually you don't need to do this, which is what makes it strange. Do you use your adsl modem to connect to the internet perhaps? What make/model is it.

On linux, if you let it dial the pppoe connection and you have this problem, you set up a iptables rule to clamp the packets, meaning it resizes it for you where needed then you also don't have this problem.

 

[Cosmic_Nomad]

Hi Tinuva,

It's a Netgear DG834Gv5
Firmware Version V6.00.25

It have also changed the DSL mode from Auto ----> ADSL (G.DMT)

I have another problem with the following:

Line Attenuation 60.0 db <------ 31.5 db
Noise Margin 19.0 db 15 db

I have spoken to the ISP who handles both my account and ADSL circuit, and they have mentioned that a Telkom " Technician" will have a look at the high line attenuation but blame it on how far I am away from the exchange.

When and "if" I plug the phone in it sounds like a bicycle chain running through a jockey......( Should a digital line not be crystal clear )

I hope the technician will find an error some place at the exchange.

Any input would be welcome !

 

[Dimpie (COMPUTEK)]

You say the default is 1472 ..... should it not be 1492 :erm:

 

[Cosmic_Nomad]

Well that's what it was before I changed it ..(It was obtained second hand )

Any higher than what it is and the line falls over.

From the Netgear forum: http://kb.netgear.com/app/answers/detail/a_id/1153

Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size:

* 1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections. The default value for NETGEAR routers, adapters and switches.
* 1492. The size PPPoE prefers.
* 1472. Maximum size to use for pinging. (Bigger packets are fragmented.)
* 1468. The size DHCP prefers.
* 1460. Usable by AOL if you don't have large email attachments, etc.
* 1430. The size VPN and PPTP prefer.
* 1400. Maximum size for AOL DSL.
* 576. Typical value to connect to dial-up ISPs.


Details About MTU

A packet sent to a device larger than its MTU is broken into pieces. Ideally, MTU would be set to the same — large — value on all your computers, routers and switches, as well as on all the parts of the Internet that you access. But you cannot control the MTU on the Internet, and in practice the optimum MTU size on your LAN is related to your hardware, software, wireless interference, etc.

* Tweaking MTU size may work well in one situation, but cause performance and connection problems in others................ ( you just can't win )

 

[Tinuva]

Hi Tinuva,

It's a Netgear DG834Gv5
Firmware Version V6.00.25

It have also changed the DSL mode from Auto ----> ADSL (G.DMT)

I have another problem with the following:

Line Attenuation 60.0 db <------ 31.5 db
Noise Margin 19.0 db 15 db

I have spoken to the ISP who handles both my account and ADSL circuit, and they have mentioned that a Telkom " Technician" will have a look at the high line attenuation but blame it on how far I am away from the exchange.

When and "if" I plug the phone in it sounds like a bicycle chain running through a jockey......( Should a digital line not be crystal clear )

I hope the technician will find an error some place at the exchange.

Any input would be welcome !

ADSL is not digital, it is in fact very analog. It shares the same line, different frequencies. You need a POTS filter for every phone you want to connect to the line to ensure it doesn't interfere with the ADSL signal, else that will cause problems browsing/using the net whilst using a phone.

Strange though, those Netgear modems never used to give me any problems. But I have heard those having problems on the new Exchanges that got upgraded to be compatible for the new 10mbit lines.

As for my MTU, the Mikrotik automatically set it at 1480 for me, no problems at all with it. Generally speaking that is about right, Ethernet runs at 1500, thus PPPoE can't run at 1500, so usually runs at something a bit lower, up to 1492 sounds about right. Generally speaking, if you can't connect to MSN ect, but browsing works your MTU is too high, at least that is how most people will notice it.

I must add, I don't think using ping is a good way to test what you should set your MTU to. For example, if I try it with 1472 it doesn't work yet my MTU is at 1480, but it works to a destination on my LAN, which uses a MTU of 1500 because its the default for ethernet. Again pinging with a length of 1500 on the Ethernet doesn't work.

Remember, ping needs the lower layers in the OSI model to add data to the packet as well, which uses up that 28 from 1472 to 1500.

 

[Nickste]

My dlink says my MTU is at 1500?

I have very good attenuation and SNR.

 

[ponder]

ADSL is not digital, it is in fact very analog. It shares the same line, different frequencies. You need a POTS filter for every phone you want to connect to the line to ensure it doesn't interfere with the ADSL signal, else that will cause problems browsing/using the net whilst using a phone.

Erm, with that mindset ISDN, Dstv etc would also be analog, so is most other digital transmission technology by your reasoning.

 

[Tinuva]

Erm, with that mindset ISDN, Dstv etc would also be analog, so is most other digital transmission technology by your reasoning.

No it would not. You actually need to know what you are talking about.

ISDN = digital
DSTV = digital
ADSL over ISDN is digital, but we don't have this in SA.

 

[ponder]

You actually need to know what you are talking about.

+1

Think of things along the general lines of baseband and carrier signals. ISDN was a bad call on my part.

 

[Tinuva]

Well tbh, I was sort of wrong with DSTV, it is both digital and analog.

Digital from the Sending networking via Satellite up to the decoder. Depending on the decoder and the TV, from the Decoder to the TV could be analog or digital. The regular models are mostly used analog, where as the newer HDTV models will use digital via HDMI.