Improved Reliability

The Windows Vista and Windows 7 TCP/IP networking features also offer improvements designed to increase reliability when network conditions are less than optimal:

• Neighbor Unreachability Detection for IPv4 Neighbor Unreachability Detection is a feature of IPv6 in which a node tracks whether a neighboring node is reachable, providing better error detection and recovery when nodes suddenly become unavailable. Windows also supports Neighbor Unreachability Detection for IPv4 traffic by tracking the reachable state of IPv4 neighbors in the IPv4 route cache. IPv4 Neighbor Unreachability Detection determines reachability through an exchange of unicast ARP Request and ARP Reply messages or by relying on upper-layer protocols such as TCP. With IPv4 Neighbor Unreachability Detection, IPv4-based communications benefit by determining when neighboring nodes, including routers, are no longer reachable and reporting the condition.
• Changes in dead gateway detection Dead gateway detection in TCP/IP for Windows Server 2003 and Windows XP provides a failover function, but it does not provide a failback function in which a dead gateway is tried again to determine whether it has become available. Windows Vista and Windows 7, however, also provide failback for dead gateways by periodically attempting to send TCP traffic through the previously detected dead gateway. If the TCP traffic sent through the dead gateway is successful, Windows switches the default gateway to the previously detected dead gateway. Support for failback to primary default gateways can provide faster throughput by sending traffic through the primary default gateway on the subnet.
• Changes in PMTU black-hole router detection Path maximum transmission unit (PMTU) discovery, defined in RFC 1191, relies on the receipt of ICMP Destination Unreachable-Fragmentation Needed and Don't Fragment (DF) Set messages from routers containing the MTU of the next link. However, in some cases, intermediate routers silently discard packets that cannot be fragmented. These types of routers are known as black-hole PMTU routers. In addition, intermediate routers might drop ICMP messages because of configured firewall rules. As a result, TCP connections can time out and terminate because intermediate routers silently discard large TCP segments, their retransmissions, and the ICMP error messages for PMTU discovery. PTMU black-hole router detection senses when large TCP segments are being retransmitted and automatically adjusts the PMTU for the connection rather than relying on the receipt of the ICMP Destination Unreachable-Fragmentation Needed and DF Set messages. With TCP/IP in Windows Server 2003 and Windows XP, PMTU black-hole router detection is disabled by default because enabling it increases the maximum number of retransmissions that are performed for a given segment. However, with increasing use of firewall rules on routers to drop ICMP traffic, Windows Vista and Windows 7 enable PMTU black-hole router detection by default to prevent TCP connections from terminating. PMTU black-hole router detection is triggered on a TCP connection when it begins retransmitting full-sized segments with the DF flag set. TCP resets the PMTU for the connection to 536 bytes and retransmits its segments with the DF flag cleared. This maintains the TCP connection, although possibly at a lower PMTU size than actually exists for the connection.