frame所能夠接受的範圍,導致PPPoE傳送「don't fragment」的ICMP而不 
是「must fragment」,最終導致網站沒有回應。 
這種情況稱為Black Hole Router,以下是解決辦法: 


Interfaces\<ID for Adapter> 



Q: Why do MacOS and Windows 98 connections freeze when running PPPoE on the gateway 

A: Thanks to Michael Wozniak <> for figuring this out and 
Dan Flemming <> for the Mac solution: 

This is due to what's called a "Black Hole" router. MacOS and Windows 98 
(and maybe other Microsoft OSs) send TCP packets with a requested segment size 
too big to fit into a PPPoE frame (MTU is 1500 by default for ethernet) and 
have the "don't fragment" bit set (default of TCP) and the Telco router is 
not sending ICMP "must fragment" back to the www site you are trying to load. 
When the www server is sending you frames that don't fit into the PPPoE pipe 
the Telco router drops them on the floor and your page doesn't load 
(some pages/graphics do as they are smaller than a MSS.) This seems to be the 
default of most Telco PPPoE configurations (if only they knew how to program 
a router... sigh...) 

One fix is to use regedit on your 95/98 boxes to add the following registry entry... 


It should be a string with a value ``1450'' (more accurately it should be ``1464'' 
to fit TCP packets into a PPPoE frame perfectly but the ``1450'' gives you a 
margin of error for other IP protocols you may encounter). 

Refer to MS KB # ``Q158474 - Windows TCPIP Registry Entries'' and 
``Q120642 - TCPIP & NBT Configuration Parameters for Windows NT '' for more 
information on changing Windoze MTU to work with a NAT/PPPoE router. 

Unfortunately, MacOS does not provide an interface for changing TCP/IP settings. 
However, there is commercial software available, such as OTAdvancedTuner 
(OT for OpenTransport, the MacOS TCP/IP stack) by Sustainable Softworks, 
that will allow users to customize TCP/IP settings. MacOS NAT users should 
select ip_interface_MTU from the drop-down menu, enter 1450 instead of 1500 
in the box, click the box next to Save as Auto Configure, 
and click Make Active. 


1.MS KB # ``Q158474 - Windows TCPIP Registry Entries'' 

The entries in this section must be added to the following registry key, 
where n represents the particular TCP/IP-to-network adapter binding. 
MaxMTU = 16-bit integer 

Data Type: String 

Specifies the maximum size datagram IP that can pass to a media driver. 
SNAP and source routing headers (if used on the media) are not included 
in this value. For example, on an Ethernet network, MaxMTU will default 
to 1500. The actual value used will be the minimum of the value specified 
with this parameter and the size reported by the media driver. The default 
is the size reported by the media driver. 

2.MS KB #``Q120642 - TCPIP & NBT Configuration Parameters for Windows NT '' 
The information in this article applies to: 

Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0 
Microsoft Windows NT Server versions 3.5, 3.51, 4.0 
Microsoft Windows 2000 Professional 
Microsoft Windows 2000 Server 
Microsoft Windows 2000 Advanced Server 


WARNING: Using Registry Editor incorrectly can cause serious, 
system-wide problems that may require you to reinstall Windows NT 
to correct them. Microsoft cannot guarantee that any problems 
resulting from the use of Registry Editor can be solved. 
Use this tool at your own risk. 

To change these parameters, use the following procedure: 

Run Registry Editor (REGEDT32.EXE). 

From the HKEY_LOCAL_MACHINE subtree, go to the following key: 


Add a value to the key described in the appropriate entry below, 
by selecting "Edit -> Add Value", typing in the value, and using 
the "Data Type" checkbox to set the value type. 

Select OK. 

Exit the Registry Editor. 

Reboot the system to make the change take effect. 

All of the TCP/IP parameters are registry values located under one of 
two different subkeys of 

<Adapter Name>\Parameters\Tcpip 

where <Adapter Name> refers to the subkey for a network adapter that 
TCP/IP is bound to, such as Lance01. Values under the latter key(s) 
are specific to each adapter. Parameters for which there may be both 
a DHCP and statically configured value may or may not exist depending 
on whether the system/adapter is DHCP configured and/or static 
override values have been specified. A reboot of the system is required 
for a change in any of these parameters to take effect. 

IMPORTANT NOTE: The Windows NT 3.5 Resource Kit documentation was not 
updated properly from version 3.1, and lists a number of incorrect TCP/IP 
registry parameters. The parameters listed in this document should be 
used in their place. The Windows NT 3.5 TCP/IP stack was a complete 
re-write, so many of the old parameters are no longer valid. 

Key: <Adapter Name>\Parameters\Tcpip 

NOTE: In Windows 2000 this value is under the following key: 
Key: Tcpip\Parameters\Interfaces\<ID for Adapter> 
Value Type: REG_DWORD Number 
Valid Range: 68 - <the MTU of the underlying network> 
Default: 0xFFFFFFFF 
Description: This parameter overrides the default Maximum Transmission 
Unit (MTU) for a network interface. The MTU is the maximum packet size 
in bytes that the transport will transmit over the underlying network. 
The size includes the transport header. Note that an IP datagram may 
span multiple packets. Values larger than the default for the underlying 
network will result in the transport using the network default MTU. 
Values smaller than 68 will result in the tranport using an MTU of 68.