# Solved: MTU is it really 1492



## D-Rock (Oct 31, 2007)

Okay guys, I have a question but first lets go over a few things. I know that an MTU of 1492 is standard for a connection over PPPoE. I have an ADSL connection over PPPoE and my bandwidth is 6Mbps/512Kbps. I have been playing with the command console in XP lately and its has mostly involved the ping parameters. Now correct me if I am wrong but isn't the MTU the largest possible size a packet can be without being fragmented? If I set the ping parameters in the command console to acknowledge that the packet size should be 1464 and it should not be fragmented it transfers just fine. If I set a value any higher that 1464 then it says that that the packet needs to be fragmented. I believe that means that my MTU is 1464 and not 1492. All that means to me is that the 1492 was an acquired number through speculation.

What I'm trying to figure out is if I am right and 1464 is my MTU would I see any benefit configuring my router and other software apps to acknowledge this MTU instead of 1492.


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## JohnWill (Oct 19, 2002)

If it starts fragmenting at sizes larger than 1464, that's the correct place to configure it.  The 1492 was not idle speculation, and for most PPPoE connections it's the correct value. I suspect there's something else going on here that results in a larger header in the packets.


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## D-Rock (Oct 31, 2007)

Someone told me a few minutes ago that its because there are 28 ICMP and Ip headers and that the correct MTU is still 1492. What are ICMP and IP headers. I know that ICMP is essentially ping but what are the headers?


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## JohnWill (Oct 19, 2002)

D-Rock said:


> Someone told me a few minutes ago that its because there are 28 ICMP and Ip headers and that the correct MTU is still 1492. What are ICMP and IP headers. I know that ICMP is essentially ping but what are the headers?


http://www.freesoft.org/CIE/Course/Section3/7.htm

http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol


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## D-Rock (Oct 31, 2007)

Well thank you.


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## fatalchaoz (Feb 23, 2008)

After finding the max transmission unit which is 1424 and after adding 28 is 1452. But the whole problem is I'm under PPPoE connection and my MTU value must be 1492 and no other value for PPPoE user. My question is 1492 value appropiate since it uses high bandwidth and the packet needs to be fragmented. What if I uses 1452 instead and does it effect my connection?


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## D-Rock (Oct 31, 2007)

how sure are you that your mtu is 1452?

If your using windows, open a command prompt and type without the parenthasis

"ping -n 10 -f -l 1464 www.yahoo.com"

If you get a message back that says " paket needs to be fragmented but....." then decrease the 1464 until you dont get those errors anymore. Once you find the highest value, add 28, then you will have your real MTU.


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## fatalchaoz (Feb 23, 2008)

Pinging www.yahoo-ht3.akadns.net [209.131.36.158] with 1464 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.
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.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 209.131.36.158:
Packets: Sent = 10, Received = 0, Lost = 10 (100% loss),

I decrease from 1464 till 1424 and I get 10% loss packets. Adding with 28 is 1452. So as a PPPoE user, should I mark it or otherwise? MTU value = 1492 or 1452?


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## JohnWill (Oct 19, 2002)

What do you get with a ping of 1424 then of 1426?

*ping -n 10 -f -l 1424 www.yahoo.com*

*ping -n 10 -f -l 1426 www.yahoo.com*


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## fatalchaoz (Feb 23, 2008)

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\user>ping -n 10 -f -l 1424 www.yahoo.com

Pinging www.yahoo-ht3.akadns.net [209.131.36.158] with 1424 bytes of data:

Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=280ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=282ms TTL=50
Reply from 209.131.36.158: bytes=1424 time=284ms TTL=50

Ping statistics for 209.131.36.158:
Packets: Sent = 10, Received = 10, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 280ms, Maximum = 284ms, Average = 280ms

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\user>ping -n 10 -f -l 1426 www.yahoo.com

Pinging www.yahoo-ht3.akadns.net [209.131.36.158] with 1426 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.
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.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 209.131.36.158:
Packets: Sent = 10, Received = 0, Lost = 10 (100% loss),

1424 is the max.


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## JohnWill (Oct 19, 2002)

OK, just curious.


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## D-Rock (Oct 31, 2007)

Someone correct me if im wrong because im not sure, but on a high bandwidth connection it wouldnt really matter if your mtu was just a little bit off. If it were say isdn or dialup it would make a difference but on any type of dsl connection i dont think it would.

However, I would wait for someone to confirm this.


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## JohnWill (Oct 19, 2002)

If your MTU is high, it will make a significant difference on any speed connection. However, if it's a little low, it will not be significant.


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## fatalchaoz (Feb 23, 2008)

Below picture shows my default gateways settings. As you can see in red box shown, that is my MTU value. Am I supposed to change to 1492 or leave it.? And another thing, I have fixed ip but I'm using PPPoE connection. So should I use PPPoE connection or static ip for WAN access type? Btw, what difference between the two?


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## fatalchaoz (Feb 23, 2008)

OK, I had done research at the time when I was waiting for answers from you two. I had solved my 1st case by changing from 1452 to 1492 in the connection setup and edit my MTU value to 1492 in registry. You can refer below. Btw what does TcpMaxDupAck do? My value is 2

C:\Documents and Settings\user>ping -n 10 -f -l 1464 www.yahoo.com

Pinging www.yahoo-ht3.akadns.net [209.131.36.158] with 1464 bytes of data:

Reply from 209.131.36.158: bytes=1464 time=349ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=350ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=349ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=350ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=350ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=350ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=350ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=349ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=349ms TTL=48
Reply from 209.131.36.158: bytes=1464 time=350ms TTL=48

Ping statistics for 209.131.36.158:
Packets: Sent = 10, Received = 10, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 349ms, Maximum = 350ms, Average = 349ms


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## D-Rock (Oct 31, 2007)

This is from cisco.com and heres a direct link to the paper.



> Duplicate ACK Threshold for Windows
> The TCP > TCP Parameters > Duplicate ACK Threshold default value has been
> modified for "Windows 2000" and "Windows XP" symbol maps. The default
> value was modified from 3 to 2, in reference to Microsoft Windows 2000 TCP/IP
> ...


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## fatalchaoz (Feb 23, 2008)

Still don't really get it. So TcpMaxDubAck set to 2 is better than under any circumstances?


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## D-Rock (Oct 31, 2007)

i couldt really tell you because i cant find enough info about it, what i can tell you is that a setting of 2 or 3 is normal. sorry.


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## fatalchaoz (Feb 23, 2008)

There's no need to be sorry. Thanks for the reply and the hard work btw.


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