Analyzer and Optimizer show different MTU

**bege** · 04-02-10, 02:37 PM

Hi everybody,

I am quite confused about the results of the Analyzer and Optimizer.

My ISP (T-Online Business) requests a special MTU of 1456. So are the settings in my computer and the router.
This is what the Optimizer finds out in the Largest MTU tab:

Pinging [66.230.207.58] with 40 bytes ->bytes=40 time=218ms TTL=45
Pinging [66.230.207.58] with 750 bytes ->bytes=750 time=240ms TTL=45
Pinging [66.230.207.58] with 1125 bytes ->bytes=1125 time=254ms TTL=45
Pinging [66.230.207.58] with 1312 bytes ->bytes=1312 time=260ms TTL=45
Pinging [66.230.207.58] with 1406 bytes ->bytes=1406 time=273ms TTL=45
Pinging [66.230.207.58] with 1453 bytes -> ..fragmented
Pinging [66.230.207.58] with 1430 bytes -> ..fragmented
Pinging [66.230.207.58] with 1418 bytes ->bytes=1418 time=262ms TTL=45
Pinging [66.230.207.58] with 1424 bytes ->bytes=1424 time=262ms TTL=45
Pinging [66.230.207.58] with 1427 bytes ->bytes=1427 time=276ms TTL=45
Pinging [66.230.207.58] with 1428 bytes ->bytes=1428 time=269ms TTL=45
Pinging [66.230.207.58] with 1429 bytes -> ..fragmented
The largest possible non-fragmented packet is 1428 (1456 - 28 ICMP & IP headers).
You can set your MTU to 1456

But the Analyzer shows an MTU of 1416:

« SpeedGuide.net TCP Analyzer Results »
Tested on: 04.02.2010 14:57
IP address: 80.129.xxx.xxx
Client OS: Windows XP

TCP options string: 020405600103030101010402
MSS: 1376
MTU: 1416
TCP Window: 124608 (NOT multiple of MSS)
RWIN Scaling: 1 bits (2^1=2)
Unscaled RWIN : 62304
Recommended RWINs: 63296, 126592, 253184, 506368, 1012736
BDP limit (200ms): 4984kbps (623KBytes/s)
BDP limit (500ms): 1994kbps (249KBytes/s)
MTU Discovery: ON
TTL: 45
Timestamps: OFF
SACKs: ON
IP ToS: 00000000 (0)

How come?

Does anybody have an idea?
Regards

**Rollingstone** · 04-02-10, 08:04 PM

Try the following with TCP Optimizer & see if MTU will change:

General Settings tab:
Custom settings - check
Modify All Network Adapters - check
network adapter selection - your NIC
MTU - 1500
TTL - 64
Windows Scaling - uncheck
TCP Receive Window - leave blank
MTU Discovery - Yes
Black Hole Detect - No
Selective Acks - Yes
Max Duplicate ACKs - 2
TCP 1323 Options:
Timestamps - uncheck

Advanced Settings tab:
Max Connections per Server - 10
Max Connections per 1.0 Server - 10
LocalPriority - 5
Host Priority - 6
DNSPriority - 7
NetbtPriority - 8
Lan Browsing speedup - optimized
QoS: NonBestEffortLimit - 0
ToS: DisableUserTOSSetting - 0
ToS: DefaultTOSValue - 0
MaxNegativeCacheTtl - 0
NetFailureCacheTime - 0
NegativeSOACache Time - 0
LAN Request Buffer Size - 32768

**bege** · 04-15-10, 09:17 AM

Rollingstone, thank you for your answer.
(Sorry for the long delay. I didn't get an e-mail notification of your answer and forgot to come back to the forum until today.)

Unfortunately the strange MTU is still shown by the Analyzer, plus a weird RWIN
By the way, I just notice that the TTL shown by Optimizer and Analyzer also differ. The Analyzer shows 45, the Optimizer 64.

« SpeedGuide.net TCP Analyzer Results »
Tested on: 04.15.2010 10:04
IP address: 80.129.xxx.xxx
Client OS: Windows XP

TCP options string: 020405600103030e01010402
MSS: 1376
MTU: 1416
TCP Window: 1073725440 (NOT multiple of MSS)
RWIN Scaling: 14 bits (2^14=16384)
Unscaled RWIN : 65535
Recommended RWINs: 63296, 126592, 253184, 506368, 1012736
BDP limit (200ms): 42949018kbps (5368627KBytes/s)
BDP limit (500ms): 17179607kbps (2147451KBytes/s)
MTU Discovery: ON
TTL: 45
Timestamps: OFF
SACKs: ON
IP ToS: 00000000 (0)

**bege** · 04-15-10, 09:34 AM

... and this is what the Analyzer shows with the standard windows settings:

« SpeedGuide.net TCP Analyzer Results »
Tested on: 04.15.2010 10:30
IP address: 80.129.xxx.xxx
Client OS: Windows XP

TCP options string: 02040560
MSS: 1376
MTU: 1416
TCP Window: 16512 (multiple of MSS)
RWIN Scaling: 0 bits
Unscaled RWIN : 16512
Recommended RWINs: 63296, 126592, 253184, 506368, 1012736
BDP limit (200ms): 660kbps (83KBytes/s)
BDP limit (500ms): 264kbps (33KBytes/s)
MTU Discovery: ON
TTL: 109
Timestamps: OFF
SACKs: OFF
IP ToS: 00000000 (0)

**bege** · 04-15-10, 11:36 AM

... and after having set the MTU in the router to 1492 (the maximum the router allows) Analyzer shows MTU 1452 (computer settings still are windows default = no MTU setting).

So Analyzer always shows 40 less than the router and/or the computer are set to.

**Rollingstone** · 04-15-10, 08:09 PM

What's brand & model of your router ? What speed are you paying for ?

**bege** · 04-17-10, 10:36 PM

Originally Posted by Rollingstone

What's brand & model of your router ? What speed are you paying for ?

The router is Netgear WGR614 v7

The speed is 3000kBit/s

**Rollingstone** · 04-18-10, 01:00 AM

Try the following with TCP Optimizer:

General Settings tab:
Custom settings - check
Modify All Network Adapters - check
network adapter selection - your NIC
MTU - 1376
TTL - 64
Windows Scaling - uncheck
TCP Receive Window - 63296
MTU Discovery - Yes
Black Hole Detect - No
Selective Acks - Yes
Max Duplicate ACKs - 2
TCP 1323 Options:
Timestamps - uncheck

Advanced Settings tab:
Max Connections per Server - 10
Max Connections per 1.0 Server - 10
LocalPriority - 5
Host Priority - 6
DNSPriority - 7
NetbtPriority - 8
Lan Browsing speedup - optimized
QoS: NonBestEffortLimit - 0
ToS: DisableUserTOSSetting - 0
ToS: DefaultTOSValue - 0
MaxNegativeCacheTtl - 0
NetFailureCacheTime - 0
NegativeSOACache Time - 0
LAN Request Buffer Size - 32768

**bege** · 04-20-10, 09:24 AM

Now Analyzer and Optimizer show the same MTU. Looks a bit like wizardry to a "layperson" like me.

Please, can you explain to me why this was not the case with different MTU values?

Why did you recommend this MTU value? It is now the same in Optimizer and Analyzer, but not the one the ISP requires.

And what does the low RWIN value mean? It's below the windows standard which is for slower connection, I think, and far less than what the Optimizer recommends.

Why is the LAN Request Buffer Size double the size as in the optimal settings of the Optimizer?

**Rollingstone** · 04-20-10, 07:23 PM

Now Analyzer and Optimizer show the same MTU. Looks a bit like wizardry to a "layperson" like me.

Please, can you explain to me why this was not the case with different MTU values?

Why did you recommend this MTU value? It is now the same in Optimizer and Analyzer, but not the one the ISP requires.

And what does the low RWIN value mean? It's below the windows standard which is for slower connection, I think, and far less than what the Optimizer recommends.

Why is the LAN Request Buffer Size double the size as in the optimal settings of the Optimizer?

MTU : In some cases, your modem/router is MTU Limitation !

RWIN Forumula : Bandwidth * Max.latency / 8

Your case : 3000 * 150 /8 = 56250 then compare with Recommened RWINS (in ANalyzer) ! The Closest Number is 63296

LAN Request Buffer Size : Why dont you try max number possible ?

**bege** · 04-21-10, 05:37 AM

Originally Posted by Rollingstone

MTU : In some cases, your modem/router is MTU Limitation !

Thank you!
Still one thing not clear:
When the router was set to MTU 1456 (ISP), the max. MTU tab in Optimizer showed the same value, but the Analyzer showed 1416.
I still don't understand, what the real MTU is and why they are different in Analyzer and Optimizer. (Ingoing and outgoing different?)
When the router was set to 1492 (maximum of the router), the Analyzer showed 1452. What do you mean is the limitation of the router? The router settings tell it's 1492, so 1456 shouldn't be a problem.