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coold8
Premium
join:2005-05-15
Arlington, VA

[Speed] Correct MTU Settings?

Hi,

I have 105/10 and when I run a manual MTU test I can't do anything between than 1472 through my router through a regular ping test. ping -c 15 -D -s 1470 news.giganews.com (on mac).

However when I run the DSLReports Tweak Test at 1470 it always tells me to up it to 1500.

I have noticed inconclusive speed results between the two. What are you opinons on what I should set my routers MTU to? I also have an AUTO option. I am running a Linksys E4500 with the new Cisco Cloud Connect firmware.

Thanks.

-Dave


NetFixer
Freedom is NOT Free
Premium
join:2004-06-24
The Boro
Reviews:
·Cingular Wireless
·Comcast Business..
·Vonage

3 edits
said by coold8:

Hi,

I have 105/10 and when I run a manual MTU test I can't do anything between than 1472 through my router through a regular ping test. ping -c 15 -D -s 1470 news.giganews.com (on mac).

However when I run the DSLReports Tweak Test at 1470 it always tells me to up it to 1500.

I have noticed inconclusive speed results between the two. What are you opinons on what I should set my routers MTU to? I also have an AUTO option. I am running a Linksys E4500 with the new Cisco Cloud Connect firmware.

Thanks.

-Dave

With the ping test you are running, you must add 28 to the packet size in order to get the real MTU because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header (the result should be 1500 for a Comcast connection...and amazingly that is the default value for every router and TCPIP stack I know of).

If you want to find out what your automatic MTU is, I suggest running the ICSI Netalyzer test.

My Netalyzer results are (not surprisingly) 1500 (both througn my Netgear router connected to Comcast, and also through a tethered cell phone connection to AT&T):

Path MTU (?): OK –
The path between your network and our system supports an MTU of at least 1500 bytes, and the path between our system and your network has an MTU of 1500 bytes.

--
History does not long entrust the care of freedom to the weak or the timid.
-- Dwight D. Eisenhower

coold8
Premium
join:2005-05-15
Arlington, VA
Just to clarify:

1472 should be able to ping but 1500 would not due to overhead. In addition, from your explanation 1472 is essentially 1500. Therefore the correct mac command would be:

ping -c 15 -D -s 1472 news.giganews.com

Thanks for your help.

-Dave


Mike Wolf

join:2009-05-24
Beachwood, NJ
kudos:4

1 edit
reply to NetFixer
thanks for your time.


NetFixer
Freedom is NOT Free
Premium
join:2004-06-24
The Boro
Reviews:
·Cingular Wireless
·Comcast Business..
·Vonage
said by Mike Wolf:

so whats the answer without having to install anything.

The OP's problem was just a misunderstanding of what the ping test was doing, so the only thing that needed to be installed was knowledge.

What is your problem and what is your question?
--
History does not long entrust the care of freedom to the weak or the timid.
-- Dwight D. Eisenhower


Mike Wolf

join:2009-05-24
Beachwood, NJ
kudos:4
i dont have either, just was curious as to what you think the MTU should be set for
Expand your moderator at work


plencnerb
Premium
join:2000-09-25
Carpentersville, IL
kudos:3

1 recommendation

reply to coold8

Re: [Speed] Correct MTU Settings?

Many years ago (like 2001), with the help of this site I believe, there was a whole "formula" to compute your ideal RWIN and MTU values. I have the document archived somewhere, but from what I remember, it involved running a ping test to some far away server multiple times. Once you did that, you took the average TTL (or was it ping time..again, its been a while), and you did some math and came up with a value. That value was your "ideal" RWIN. There was a similar set of tests and math to compute your "ideal" MTU as well.

Once you got that, there was a program written to help you modify the registry without actually going into the registry to change these values. The program was called Dr. TCP (screen shot attached). And yes, I still have that program, but I don't use it anymore.

However, my understanding was that all this work was done because the older OS's (Windows 95/98, 2000, and Windows XP) did not have (by default) the proper values in the registry to support a broadband connection. Again, if you think back, we were really happy to be getting 1 or 2 MB on the download side, and 512 KB on the upload back in the early 2000's. When Vista came out (and of course Windows 7), the "need" to modify the default values for these was moot, as Microsoft had corrected the problem.

--Brian
--
============================
--Brian Plencner

E-Mail: CoasterBrian72Cancer@gmail.com
Note: Kill Cancer to Reply via e-mail


NetFixer
Freedom is NOT Free
Premium
join:2004-06-24
The Boro
Reviews:
·Cingular Wireless
·Comcast Business..
·Vonage

1 edit
reply to Mike Wolf
said by Mike Wolf:

i dont have either, just was curious as to what you think the MTU should be set for

That was already answered in this post: »Re: [Speed] Correct MTU Settings?

--
History does not long entrust the care of freedom to the weak or the timid.
-- Dwight D. Eisenhower


Mike Wolf

join:2009-05-24
Beachwood, NJ
kudos:4
Thank you again