What exactly is this mystical MTU anyway? The Maximum Transmission Unit of a network is the greatest amount of data or "packet" size that can be transferred in one physical frame on that network. This packet also contains the header and trailer information, which are kind of like addresses for each packet that are required by the routers on the network. Fragmentation will occur if a packet is sent across a network that has an MTU smaller than the packet's frame length. This will lead to reduced performance levels as fragments need to be reassembled. When a connection is opened between two computers, they must agree on an MTU.Windows uses this default packet size to negotiate with. This is done by comparing MTUs and selecting the smaller of the two. If this value is set too high for the routers to handle between the two computers, then this info is fragmented into a packet size that the router can handle. This negotiation takes some time and can double the time it takes to send a single packet. Therefore; if the ISP uses an MTU then every time a connection is started, Windows must adjust to an acceptable MTU. By setting this value manually, you can greatly reduce the amount of work that Windows must do to negotiate and reach an acceptable MTU.
 

With Windows 95, all versions, including OSR2, have a "hidden" default for MTU set in the registry at 1500. This is appropriate for LAN's and all Ethernet connections, but can be very inefficient when using a PPP dialup connection. This has now been recognized by Microsoft and corrected in Windows 98 to some extent. It's really quite simple. Any intermediary IP routers encountered on the Internet which are set to an MTU less than 1500, when they handle your TCP/IP request, will often fragment an MTU, breaking it up into smaller units, before sending it out on the network.
 

MTU is normally set in conjunction with MSS, the Maximum Segment Size, and RWIN, the TCP Receive WINdow. MSS is the largest segment of TCP data that the winsock is prepared to receive on that connection. MSS must be smaller than MTU by at least 40 bytes, the size of the header and trailer information. RWIN determines how much data the receiving computer is prepared to receive. If RWIN is set too large it will result in greater loss of data if a packet is lost or damaged. If it is set too small (e.g. 1x MSS), transmission will be very slow. Normally RWIN is set to either 4x, 6x or 8x MSS. Here's a simple analogy. TCP/IP takes an e-mail letter (or web page) and cuts it into nice neat squares (packets). It puts each piece in an envelope with the address (header) of where itÆs going for the routers to send it along its way. Then all the letters are delivered to your mailbox. So...

MSS = the size of each cut square of paper
MTU = the size of the envelope (which must be slightly bigger than the square of paper)
RWIN = the size of the mailbox which "receives" the letters
 

The maximum speed of TCP/IP transfers through a SLIP (Serial Line Protocol) or PPP (Point to Point Protocol) connection is, first and foremost, limited by the speed of the modem. In an ideal world, for an already compressed file, you should be able to get 3.2 Kbytes/sec transfers with a 28.8 Kbps (kilobits per second) connection; 2.7 Kbytes/sec with a 24 Kbps connection; 1.6 Kbytes/sec with a 14.4 Kbps connection; etc. Roughly speaking, each one Byte/sec of delivered TCP data rate requires 9 bps of modem connect rate. Note that newer modems allow for much greater compression of certain types of files. Text files and html files can often be compressed by a factor of 2x or 3x, effectively doubling or tripling the speeds mentioned above.
 

The data transfer speeds mentioned above are the ideal speeds one can achieve, but this is often thwarted by fragmentation which may occur in the transmission of packets of data. As you visit various web sites, you may encounter routers on the download path with an IP default MTU of only 576 bytes. Thus, if you use the Windows default MTU settings of 1500, this can often result in packet fragmentation slowdown (since you are trying to use TCP segments (MSS) larger than 536 bytes.) Avoiding fragmentation has a major effect on file download speed, whether we are talking about html and image files from the web or straight text data bits in your e-mail program.
 

An MTU of 576 used to be the most common MTU setting in use and was generally referred to as the "Internet Standard". However modern routers are more often than not set to 1500 as an MTU. So why use a smaller MTU? Well in an ideal world, you would not do that any longer. You'd use 1500 and be done with it. However, there is still a good deal of Internet congestion out there, more than ever, really. And with this congestion, packets of data are "dropped" and as a result need to be retransmitted. Simply put, it takes just over 1/3 as long to retransmit a smaller packet, say 576 bytes in size, than it does to retransmit one 1500 bytes in size.
 

So in summary, whether youÆre ISP, the site you are going to. Or the intermediary routers involved have any 576 MTU settings or not, setting your own computer's MTU to a setting smaller than 1500 (like 576) will often result in faster download speeds for those using Dialup connections.
Download speeds for Broadband connections like cable DSL, and Satellite. Broadband connections work best with an MTU setting of 1500; however they work better with significantly higher RWIN settings than Dialup .So how can you avoid fragmentation during TCP data transfers? Very simple... you can limit the MTU of your TCP/IP request to a smaller or optimum size. Most people that are using Windows 95 and the Win95 Dialup Adapter, which employs the winsock.dll in C:\Windows and wsock32.dll file in C:\Windows\System (wsock32n.dll belongs to MSIE). You are probably using a PPP dialup connection which most ISP's use these days. All versions of Windows 95 are set up by default to request a maximum MTU of 1500. You can, however, change that default setting. And, it turns out, depending on how your ISP and other routers encountered on the Internet handle your TCP/IP requests, that a MaxMTU setting of 576, often referred to as the "Internet Standard", will in many cases avoid the fragmentation of packets of data and the slow transfer speeds which result. Note that if your ISP is using an MTU lower than 576, (typical with a less efficient SLIP connection, for example), then you will want to lower your MaxMTU setting to match your ISP's. So, check with your ISP to see what he is using for MTU, if you find that the recommended settings are not producing good results for you.