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About MTU

2008/02/08
by
Something basic, something overlooked, but important to know the following case.
SYMPTOMS
When a Microsoft Internet Security and Acceleration Server (ISA) 2004-based computer is operating under heavy load conditions, you may experience high CPU use. For example, CPU use on the ISA Server computer may be more than 50 percent.
CAUSE
This behavior may occur because of the TCP/IP maximum transmission unit (MTU) setting that is applied during ISA Server installation.

To prevent an attacker from changing the MTU value, ISA Server 2004 disables path MTU (PMTU) discovery. This setting is documented in Microsoft security bulletin MS05-019. To see this bulletin, visit the following Microsoft Web site:
http://www.microsoft.com/technet/security/Bulletin/MS05-019.mspx
Notes

  • By default, Windows uses an MTU setting of 1,480 bytes and accepts Internet Control Message Protocol (ICMP) messages that request smaller packet sizes.
  • If MTU discovery is disabled on a Windows-based server, the server uses an MTU setting of 576 bytes.
MS/KB#902347 – CPU use may be more than 50 percent when an ISA Server 2004 computer is operating under heavy load conditions
 
 
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