Microsoft Knowledge Base Article 900926 contains the following summary and introduction:
Summatry The MS05-019 security update modifies how the operating system validates Internet Control Message Protocol (ICMP) requests. This security update prevents an ICMP-based attack. However, under special circumstances, this security update may cause the computer to lose network connectivity. This article describes three methods that you can use to help prevent the computer from losing network connectivity when the MS05-019 security update is installed.
Introduction This article describes the recommended TCP/IP settings for wide area network (WAN) links with a Maximum Transmission Unit (MTU) size of less than 576.