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Huge gain in download speeds

Essential MTU Protocol setting for Win95
Probable speed gain for Win98, SE, ME

Correcting a fault in Windows 98/95 can give most home users a very dramatic increase in Internet performance (dialup PPP connections).

Just changing the MTU protocol setting can be like getting a major modem upgrade.

This is a VITAL ALTERATION for most Home users.

QUICK FIX     All Users

MTU Quick Fix (home modems)

Windows 95, and to some extent Windows 98, comes optimized for office use and uses an incorrect setting (MaxMTU) for home use with PPP . For a quick fix :-

95 Windows 95 users should just:
• Download the MTU Speed utility (freeware, 152KB)
• Run the utility, and click on Optimum Settings (that's for a MTU of 576)
• Click on Update Registry, and reboot.
Home users can expect a very substantial improvement in download speeds - if not, then rerun the utility, undo the changes, reboot, and then read ALL of Modems at this Site!

98+ Windows 98 users should just:
• Look in Control Panel • Network • Dialup Adapter
• Set the Packet Size to small (a IPMTU of 576), or experiment with Automatic

Most users benefit by setting the (IP)MTU to a permanently lower level - to the MTU used by your own ISP (usually 576).

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Why change the MaxMTU setting ?

Microsoft's Dial-up Networking (DUN) still has a setting which is not suitable for most Home users. It is designed for Ethernet networks in offices, and not for the home users who have a connection over a phone line (a PPP connection). This means that your downloading is probably dramatically slower than it should be, but the fault can be remedied.

When you request a file download (even a Web page), you also request that it be sent in many specifically sized segments (called 'Packets' or 'Frames') for transmission. This packet size can be controlled by making a new Registry setting, the MaxMTU.

Small packets get through more quickly on inherently noisy phone lines.

The Internet standard size for MaxMTU is 576. Windows 95/98 sets it to 1500 (office LAN standard) which is the wrong size for the majority of home users and drags down the modem's performance. This is a major flaw in Windows 95/98 for home modem user.

To ensure efficient data transfer rates the users at both end of the line must strictly match the same protocols i.e. a MaxMTU (or IPMTU) of 576. Failure to do so drags down the effective data transfer rate even if 'appears' to be adequate.
• MTU is referred to as IPMTU in Windows 98+.

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Use the Correct MTU

The above figures presume your ISP uses 576 for his own MaxMTU. His modem is vital as your entry point to the Internet. If you find he uses other than 576, then you should use his (usually) and adjust the other settings accordingly (and complain to the ISP).

Use Ping from DOS to find your ISP's MaxMTU. Better still, get the utility FindMTU (free, 168KB, Zip file) which is very easy to use in Windows 95/98.
Those Protocol numbers
If you deviate from the optional settings of a MTU-fixing utility then remember:
MTU is most important, and most commonly set to 576 for PPP connections
MSS will be set automatically by Windows (at MTU minus 40)
RWIN is set to a multiple (4,6,8,or 10) of MSS (MSS=MTU minus 40)
TTL (usually less important) is set to 32, 64, or 128
Blackhole and others can usually be left unaltered

A MTU lower than the ISP's 576 (568, 560, 552, 548, 536, 528, 520) might improve modem throughput, thus increase transfer rate, especially when accessing huge graphics or large compressed files (i.e. ZIP files) from WWW/FTP sites. Try experimenting.

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Tuning your MTU (home modems)

MSS : (Maximum Segment Size) Windows 98/95 automatically sets MSS to a value of MaxMTU minus 40, so you do not need to change MSS when altering MaxMTU (that's IPMTU in Windows 98). You can safely ignore any recommendation that you alter MSS yourself as Win98/95 will ignore you.

RWIN : (Receive Window) When MaxMTU is set, it is natural to also set its associated Receive Window which is a whole-number multiple of MSS (=MTU-40). Basically, RWIN is the maximum number of packets that can be received before an ACKnowledgement must be sent back to the sender. Then he can send you more. If he does not receive the ACK within a time limit, he will assume they are 'lost' and will resend them (time wasting, and slower download).

TTL (Time To Live). Every Packet of data is sent via a large variety of possible Routers who, if not too busy at that moment, will forward the Packet. TTL now means the number of hops (routers) that can be tried before giving up.

Black Hole (PMTUBlackHoleDetect) Black Holes are so rare that it is not worth while checking for them, so disable this option. Should you encounter Black Holes you need to change ISPs immediately - yours is living in the stone age!
NDI Ignore it! You should not be using this IPX/SPX Protocol in a TCP/IP Internet connection.

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FIX THE MTU FAULT [2]     All Users

Use a Utility (home modems)

The importance of the MaxMTU problem is so widely recognized by now that there are numerous utilities available to simplify its correction. Without such a utility it would be necessary for all users to manually alter the Registry settings.

You can use ONE of these reliable utilities:
MTU-Speed v4.03 free, 152KB, easy to use
NetLightening friendlyware; also caches IP addresses
PPP Boost v1.2 free, 200KB, little help functions
The utility allows you to alter the MTU, RWIN, and TTL settings with a few mouse clicks.

Look for these changes:

MaxMTU to 576 (MSS to 536) - or, preferably, use the MTU used by your ISP

RecWindow to 2,144 (MSS x 4. A few of users prefer not to set it, initially)

TTL to 64 (or perhaps 128 - not as important) (Do not change if using Win95 & DUN)

The vast majority of users do not alter the Blackhole or other associated settings.

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MTU & WINDOWS 98     98+     All Users

IPMTU is used in Windows 98

Microsoft quote "uses a more efficient MTU value of 576" if it detects a slow Internet connection; and that "As line errors increase on an Internet connection, a smaller MTU such as 576 makes re-transmissions faster than a larger MTU such as 1500"

IPMTU is used where MTU is used in Windows 95.
The installation IPMTU default is 1500 (the office network setting). When a slower connection (phone line) is recognised it is reduced dynamically to 1000 and then 576 (the setting for a phone line connection) - it CAN NOT be set any lower!

Control Panel • Network • Dial-Up Adapter • Advanced • IP Packet Size
Change the IP Packet Size from Automatic to Small (MTU fixed at 576)
Make sure the "Client for Microsoft Networks" item is present on your Network applet list (install it if necessary), to enable the saving of your ISP logon password!

Many Home users claim that download speeds are still improved by setting the MTU to the permanently lower (576) level. This Site remains undecided. It may be wiser to continue with the utility for the time being.

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Receive Window

RecWindow (RWIN or Receive Window) sets the amount of data the computer is prepared to receive before ACKnowledging its safe receipt. Its size should really be determined by the speed of your data transfer (type of modem, quality of phone line, ISP response, Routers, Host Site).

If set too high for your own situation, then your ACKnowledgement of safe receipt of data will be too slow and the same data will be resent. Your transfer rate will look normal but your effective transfer rate will deteriorate.

It is normal to alter the RecWindow in line with MaxMTU - to keep them in balance. RecWindow is set to 4 (6,8,10) times the MSS (and MSS is the MaxMTU minus 40) .

The main benefit comes from the MaxMTU alteration. However the results can vary and it is possible the RecWindow setting causes this variability. Therefore, in the first place, you should allow the utility to select the RecWindow setting, and alter it yourself only if you do not have a substantial increase in download rates.

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Sometimes better results occur only after making the RecWindow settings. Occasionally the MaxMTU change produces good results which are then negated by the RecWindow setting. Most often results are good only after both are changed.

Try for the best transfer rate performance by calling for a number of data packets (RWIN) corresponding as closely as possible to your modem's maximum transfer rate capabilities. These are usually: [3.5kcps for a 28,800 modem] [4.1kcps for 33,600] [6Kcps for 56K, theoretically, but about 4.8K in practice}- all under excellent phone line conditions!

If your data transfer is constantly on the slow side, then you may benefit from a lower RecWindow. This would give your modem time to receive the data and ACKnowledge its safe receipt before more data arrives. Some experimentation may be necessary.

Framing Errors may occur if RWIN is set larger than your ISP's.

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• A possible sign of having the wrong MaxMTU. You can use System Monitor (sysmon.exe in \Windows) to look at the Bytes received/sec. and compare it to the Characters/sec reported by your web browser. Bytes received/sec should be no more than 10%-12% greater than Characters/sec. If Characters/sec is substantially lower then you are getting a lot of unneeded retransmissions that are quite possibly caused by a Windows 95/98 MaxMTU problem for Home users.

• If you are a very regular visitor to one specific site, you can 'Tracert' the route taken, establish the bottlenecks, and set your MaxMTU accordingly.

Using the MTU Speed utility with 95 or 98

When using MTU-Speed most users will find the Optimum Settings option is all that need be enabled to achieve best performance. A few will need to experiment.
Leaving MTUBlackHoleDetect at Disable is the norm, but a few have found Enabling it made all the difference - it may depend on the type of server used by your ISP.

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Please remember that you alone are responsible for the consequences of any changes you make to your computer hardware or software.

Copyright © LarryM 1998-2015