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New features for TCP/IP

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

New features for TCP/IP

TCP/IP includes the following new features for Microsoft® Windows Server 2003 family:

  • IGMP version 3

  • Alternate configuration

  • Automatic determination of the interface metric

  • IP version 6

IGMP version 3

Internet Group Management Protocol (IGMP) version 3, described in the Internet draft entitled "Internet Group Management Protocol, version 3," provides source-based multicast group membership reporting. Hosts can request to receive multicast traffic from specified sources or from all but a specific set of sources. Source-specific reporting prevents multicast-enabled routers from delivering multicast traffic to a subnet where there are no listening hosts. For more information, see Internet Group Management Protocol (IGMP).

Alternate configuration

Alternate configuration enables a computer to use an alternate, manually configured IP address configuration in the absence of a Dynamic Host Configuration Protocol (DHCP) server. You can use an alternate configuration when a computer is used on more than one network, at least one of the networks does not have a DHCP server, and automatic configuration is not wanted.

For example, if you have a laptop computer that you use both at the office and at home, it is useful to configure TCP/IP for an alternate configuration. At the office, the laptop uses a DHCP-allocated TCP/IP configuration. At home, where there is no DHCP server present, the laptop automatically uses the alternate configuration, which provides easy access to home network devices and the Internet and allow seamless operation on both networks, without the manual reconfiguration of TCP/IP settings.

Without an alternate configuration, TCP/IP uses Automatic Private IP Addressing (APIPA).

For more information, see Configure TCP/IP for an alternate configuration.

Automatic determination of the routing metric

By default, TCP/IP automatically determines the routing metric for the default gateway of each interface, based on the speed of its associated interface. If interfaces of different speeds are configured to use the same default gateway, the fastest interface has the lowest routing metric for its default route and is used to forward traffic to its default gateway. If there are multiple interfaces of the fastest speed, the interface that is listed first in the binding order is used to forward traffic to its default gateway. For more information, see The IP routing table.

Automatic determination of the interface metric is enabled by default through the Automatic metric check box on the IP Settings tab and when you manually configure default gateways in Advanced TCP/IP Settings. For more information, see Default gateways.

IP version 6

The Windows Server 2003 family includes the latest version of Internet Protocol (IP), known as IP version 6. For more information, see IP Version 6.

Configuration Features

TCP/IP includes several features that simplify configuration on a single subnet and optimize TCP performance in high-bandwidth network environments.

These features include support for:

  • Automatic and alternate private address configuration

  • Large TCP windows

  • Better roundtrip time (RTT) estimation

  • Selective acknowledgments

  • ICMP router discovery

  • DNS caching

  • Disabling NetBIOS over TCP/IP

Automatic and alternate private address configuration

Use APIPA to automate TCP/IP address configuration for single-subnet networks that do not contain a DHCP server.

By default, computers running Windows Server 2003 operating systems first try to contact a DHCP server on the network to dynamically obtain configuration for each installed network connection.

  • If a DHCP server is reached and the leased configuration is successful, TCP/IP configuration is completed.

  • If a DHCP server is not reached, the computer checks for an alternate configuration. If an alternate configuration exists, then its settings are used to configure TCP/IP.

  • If an alternate configuration does not exist, TCP/IP uses APIPA to automatically configure TCP/IP. When you use APIPA, a server running Windows Server 2003 determines an address in the reserved IP address range from 169.254.0.1 through 169.254.255.254. This address is used as a temporary IP address configuration until a DHCP server is located. The subnet mask is set to 255.255.0.0.

The APIPA range of IP addresses is reserved by the Internet Assigned Numbers Authority (IANA). Any IP addresses within this range are not used on the Internet.

APIPA eliminates IP address configuration for single-network small office or home office networks that are not connected to the Internet. For information about disabling APIPA, see Disable automatic address configuration.

Large TCP windows

Window size reflects the maximum number of packets that can be sent without waiting for positive acknowledgment. Large TCP windows improve TCP/IP performance when large amounts of data are in transit between the sender and receiver. In typical TCP-based communication, the maximum window size is usually fixed at the onset of connection and limited to 64 kilobytes.

With large window support, you can dynamically recalculate and scale the actual window size by using a TCP option as needed during longer sessions. With this option, more data packets are in transit on the network at one time, which increases throughput.

By default, computers running Windows Server 2003 operating systems only accept client requests for the large TCP windows option from TCP1323Opts-enabled computers that they are connecting with. TCP1323Opts-enabled computers make requests for the large TCP windows option during the initial three-way handshake. If you want your computer to be able to make requests for the large TCP windows option, you must enable TCP1323Opts in the registry. For more information about large TCP windows, see RFC 1323, "TCP Extensions for High Performance."

Better RTT estimation

TCP uses roundtrip time (RTT) to estimate the amount of time that is needed for roundtrip communication between a sender and a receiver. Servers running Windows Server 2003 support the use of the RFC 1323 TCP timestamp option to improve the way that RTT is estimated. By calculating more accurate RTT information more often, TCP uses better estimates for setting retransmission timers, which helps improve overall TCP speed and performance.

Improvements in estimating RTT help significantly over longer roundtrip network links, such as WANs that span continents or that use either wireless or satellite communication links.

By default, computers running Windows Server 2003 operating systems only accept client requests for the TCP timestamp option from TCP1323Opts-enabled computers that they are connecting with. TCP1323Opts-enabled computers make requests for the TCP timestamp option during the initial three-way handshake. If you want your computer to be able to make requests for the TCP timestamp option, you must enable TCP1323Opts in the registry. For more information about TCP timestamp, see RFC 1323, "TCP Extensions for High Performance."

Selective acknowledgments

In typical TCP-based communication, acknowledgments are cumulative. TCP only acknowledges segments received that are contiguous with previously acknowledged segments. Noncontiguous segments--segments received out of sequence--are not explicitly acknowledged. TCP requires that segments are received and acknowledged within a brief time period, or the missing segment, and all subsequent segments that follow it, must be retransmitted.

Selective acknowledgments are a recent TCP option that allow the receiver to selectively notify and request that a sender resend only data that is actually missing. This results in smaller amounts of data that require retransmission and better use of network bandwidth.

For more information about selective acknowledgments, see RFC 2018, "TCP Selective Acknowledgment Options."

ICMP router discovery

Internet Control Message Protocol (ICMP) router discovery is the use of ICMP messages to discover the default gateway on a network segment when a default gateway is not manually configured or assigned by using DHCP. ICMP router discovery consists of two ICMP messages: the router solicitation and the router advertisement. A router solicitation is sent by a host to discover the routers on the network. A router advertisement is sent by a router in response to a router solicitation and periodically to notify hosts on the network that the router is still available. ICMP router discovery is disabled by default on TCP/IP for host computers running Windows XP or Windows Server 2003 operating systems, unless the host receives the perform router discovery option from a DHCP server.

You can configure a server running Windows Server 2003 and the Routing and Remote Access service to support ICMP router discovery as a router. For more information, see Routing Overview.

ICMP router discovery is described in RFC 1256, "ICMP Router Discovery Messages".

DNS caching

The Domain Name System (DNS) resolver in TCP/IP caches DNS name queries. You can view and flush the contents of the DNS cache by using the Ipconfig utility. For more information, see Client features and Flush and reset a client resolver cache using the ipconfig command.

Disabling NetBIOS over TCP/IP

With computers running Windows Server 2003 operating systems, you can disable NetBIOS over TCP/IP (NetBT) for each network connection. This feature is intended for computers that only use DNS name registration and resolution techniques and communicate by using the Client for Microsoft Networks and the File and Print Sharing for Microsoft Networks components with other computers where NetBT is disabled. Examples of disabling NetBT include computers in specialized or secured roles for your network, such as an edge proxy server or bastion host in a firewall environment, where NetBT support is not required or desired.

The following are considerations for disabling NetBT on computers running Windows Server 2003 operating systems:

  • The computer no longer listens for traffic to the NetBIOS datagram service at User Datagram Protocol (UDP) port 138, the NetBIOS name service at UDP port 137, and the NetBIOS session service at Transmission Control Protocol (TCP) port 139.

  • TCP/IP-based connections that use the Client for Microsoft Networks and the File and Print Sharing for Microsoft Networks components are only possible to other computers that have NetBT disabled. This affects the ability to browse the network to see network computers and to connect to file shares and network printers.

  • NetBIOS name resolution techniques such as WINS, local subnet broadcasts, and the Lmhosts file are no longer used. All name resolution occurs through DNS queries and the Hosts file.

  • If the computer needs to participate in WINS as a client, it must have NetBT enabled on at least one network connection.

  • If a server running Windows Server 2003 needs to run the WINS service, it must have NetBT enabled on at least one private network connection.

For example, consider disabling NetBT if you have a server computer that has a connection to a private network and a connection to an external network, such as the Internet. In this case, NetBT is not required for the Internet connection. By disabling NetBT on only the Internet connection, the dual-homed computer continues to function as either a WINS server or client for the internal network, and WINS clients are still serviced for connections made by using other physical network adapters installed on the computer.

You can disable NetBT on the WINS tab in the properties of the TCP/IP protocol. For more information, see Configure TCP/IP to use WINS. You can also disable NetBT through DHCP by using a Microsoft vendor-specific DHCP option. For more information, see "Dynamic Host Configuration Protocol" at the Microsoft Resource Kits Web site.

Additional feature information

For information about obtaining RFC documents, see TCP/IP RFCs.

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