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Author Topic: MTU  (Read 2403 times)

noddy

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MTU
« on: March 04, 2017, 10:16:18 AM »

Hi while I wait for the exchange to be upgraded  :fingers: which may or may not improve things ( when it's windy :) ) I started looking at MTU settings and after a few ping test it was showing 1430 as the highest with no packet loss ( almost sounds as if I know what I'm talking about - I don't really  ;) ) so that leads me to think my MTU should be set at 1458 ? now should I set the router to this as well as the computers ? or does it not make a deal off difference ? cheers
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ejs

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Re: MTU
« Reply #1 on: March 04, 2017, 10:52:52 AM »

What is the current setting on your router?
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noddy

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Re: MTU
« Reply #2 on: March 04, 2017, 11:20:17 AM »

1492
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ejs

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Re: MTU
« Reply #3 on: March 04, 2017, 02:27:02 PM »

How were you testing it? What were you pinging? Some servers might have lower MTU values than what your connection can generally support.

1458 was an old default value in some Netgear routers. With a PPPoA connection, 1500 is a reasonable choice, I think 1478 gives a very small throughput increase due to optimizing ATM cell packing.
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noddy

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Re: MTU
« Reply #4 on: March 04, 2017, 03:02:53 PM »

I was pinging plusnet.net thinking my provider was a good a choice as any , using cmd , could be anything to do with our exchange being 20cn ?
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ejs

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Re: MTU
« Reply #5 on: March 04, 2017, 03:21:36 PM »

I've just spent a few minutes pinging plusnet.net before eventually realising that it should of course be plus.net.

I wouldn't have thought it's a 20cn issue, an MTU of 1500 isn't a new thing.
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nallar

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Re: MTU
« Reply #6 on: March 04, 2017, 03:36:03 PM »

1500 when possible. 1492 if you're stuck with hardware which doesn't support RFC 4638.
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Virgin Media cable, A&A and Sky DSL. pfSense router.

noddy

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Re: MTU
« Reply #7 on: March 04, 2017, 03:58:44 PM »

I've just spent a few minutes pinging plusnet.net before eventually realising that it should of course be plus.net.

I wouldn't have thought it's a 20cn issue, an MTU of 1500 isn't a new thing.

sorry for that  :-[
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aesmith

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Re: MTU
« Reply #8 on: March 05, 2017, 09:14:47 PM »

On 20cn I'm using 1500 on a combined modem/router.

Sent from my Nexus 7 using Tapatalk

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Weaver

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Re: MTU
« Reply #9 on: March 05, 2017, 10:01:20 PM »

ejs is exactly correct. All depends on what protocols you are using ADSL, VDSL, FTTC? PPPoA or PPPoE. Single-box modem-router combined or separate modem and router.

Tuning will only give you at most 3% more speed, or 0% depending on your setup. Let us know _all the details about kit, setting, services, protocols, line type_ and we will be able to advise.

Here is the rule in a nutshell:

About what works / doesn't work: If you have a separate modem, 1492 is the safe choice. Otherwise always use 1500 for maximum compatibility with the world at large. Beyond that all else is tuning/tweaking and we would have to advise you on that with a knowledge of all your details.

Earlier posters’ tips are valid or not dependent on your setup.

Tuning/tweaking: the possible gains from tweaking are not large (3% or 0% max) and we don't know how to do it without your details. ejs' excellent advice is protocol-specific, as he says, and 1478 is an optimum for efficiency with PPPoA _only_. But we don't know if you are using PPPoA or not.

(Further reading: The wikipedia article on PPPoE has a section on PPPoEoA over ADSL written by your truly, which goes through all the arithmetic in gruesome boring detail. It might be worth a read even if you are not using PPPoE. see other MTU thread)
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Weaver

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Re: MTU
« Reply #10 on: March 05, 2017, 10:15:51 PM »

There is a website that will test the combination of your own kit, ISP and internet connection and check out your MRU so that you can see whether your chosen setting are actually working for incoming stuff. I've used it several times but I foolishly didn't save the url, so I would have to google it again, but it wasn't that hard to find.

I used it to test that my three separate DLink DSL-320B-Z1 modems are actually working properly with 1500-byte IP packets inbound (using separate router, ADSL2, BTW 21CN, with PPPoE on the link from router to the modems and PPPoEoA over DSL on the copper line). In my case, luckily (i) modems, and (ii) router, and (iii) the particular link from my modem to the BRAS in BT Land can all handle 1500+8=1508-byte long PPP PDUs.

You can of course try sending outbound packets of different lengths, but you need help from such a test server to test inbound stuff.
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noddy

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Re: MTU
« Reply #11 on: March 05, 2017, 10:22:21 PM »

well thanks for all the use full advise , as I thought I have a lot to learn  :-[ I'll get back with more info , so thanks yet again
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Weaver

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Re: MTU
« Reply #12 on: March 06, 2017, 12:19:29 AM »

Noddy, it's like Russian Dolls - there are all these layers upon layers of comms protocols and packets within packets within packets, each with some wrapping surrounding the payload within,  then an outer wrapping that encloses that one and so on. Each one adding more overhead, and contributing more inefficiency. So there are all these terms for protocols and different standards' documents that contain pictures, and we have to say what layer off wrapping it is that we're talking about when we mention the length of something or talk about length limits, "it's the length of exactly what?" - including/excluding which wrappers. That's the reason for all the jargon that gets dragged in. There are concise pictures of all these levels of wrapping, I would have to dig for a min.

[Moderator edited to correct a typo.]
« Last Edit: March 06, 2017, 06:01:35 PM by burakkucat »
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noddy

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Re: MTU
« Reply #13 on: March 06, 2017, 07:09:50 AM »

I'm beginning to see what you mean , and thanks for your patience . 
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aesmith

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Re: MTU
« Reply #14 on: March 06, 2017, 10:18:04 AM »

On PPPoA, as you are, you should be able to use 1500.  That's what I used when I was with Plusnet, and what I use now with A&A.  All on 20CN.
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