Operating System Microsoft Windows 2000 tcp/ip implementation Details


Critical Client Services and Stack Components



Download 0.63 Mb.
Page11/21
Date31.07.2017
Size0.63 Mb.
#25712
1   ...   7   8   9   10   11   12   13   14   ...   21

Critical Client Services and Stack Components


The focus of this paper is on core TCP/IP stack components, not on the many available services that use it. However, the stack itself relies upon a few services for configuration information and name and address resolution. A few of these critical client services are discussed here.

Automatic Client Configuration and Media Sense


One of the most important client services is the Dynamic Host Configuration Protocol (DHCP) client. The DHCP client has an expanded role in Windows 2000. Its primary new feature is the ability to automatically configure an IP address and subnet mask when the client is started on a small private network without a DHCP server available to assign addresses (such as a home network). Another new feature is support for Media Sense, which can improve the roaming experience for portable device users.

  1. If a Microsoft TCP/IP client is installed and set to dynamically obtain TCP/IP protocol configuration information from a DHCP server (instead of being manually configured with an IP address and other parameters), the DHCP client service is engaged each time the computer is restarted. The DHCP client service now uses a two-step process to configure the client with an IP address and other configuration information.

  2. When the client is installed, it attempts to locate a DHCP server and obtain a configuration from it. Many TCP/IP networks use DHCP servers that are administratively configured to hand out information to clients on the network. If this attempt to locate a DHCP server fails, the Windows 2000 DHCP client autoconfigures its stack with a selected IP address from the IANA-reserved class B network 169.254.0.0 with the subnet mask 255.255.0.09. The DHCP client tests (using a gratuitous ARP) to make sure that the IP address that it has chosen is not already in use. If it is in use, it selects another IP address (it does this for up to 10 addresses). Once the DHCP client has selected an address that is verifiably not in use, it configures the interface with this address. It continues to check for a DHCP server in the background every 5 minutes. If a DHCP server is found, the autoconfiguration information is abandoned, and the configuration offered by the DHCP server is used instead. This autoconfiguration feature is known as Automatic Private IP Addressing (APIPA) and allows single subnet home office or small office networks to use TCP/IP without static configuration or the administration of a DHCP server.

If the DHCP client has previously obtained a lease from a DHCP server, the following modified sequence of events occurs:

  1. If the client’s lease is still valid (not expired) at boot time, the client tries to renew its lease with the DHCP server. If the client fails to locate a DHCP server during the renewal attempt, it tries to ping the default gateway that is listed in the lease. If pinging the default gateway succeeds, the DHCP client assumes that it is still located on the same network where it obtained its current lease and continues to use the lease. By default, the client attempts to renew its lease in the background when half of its assigned lease time has expired.

  2. If the attempt to ping the default gateway fails, the client assumes that it has been moved to a network that has no DHCP services currently available (such as a home network), and autoconfigures itself as described above. Once autoconfigured, it continues to try to locate a DHCP server every 5 minutes, in the background.

Media Sense support was added in NDIS 5.0. It provides a mechanism for the Network Interface Card (NIC) to notify the protocol stack of media connect and media disconnect events. Windows 2000 TCP/IP utilizes these notifications to assist in automatic configuration. For instance, in Windows NT 4.0, when a portable computer was located and DHCP was configured on an Ethernet subnet, and then moved to another subnet without rebooting, the protocol stack received no indication of the move. This meant that the configuration parameters became stale, and not relevant to the new network. Additionally, if the computer was shut off, carried home and rebooted, the protocol stack was not aware that the NIC was no longer connected to a network, and again stale configuration parameters remained. This could be problematic, as subnet routes, default gateways, and so on, could conflict with dial-up parameters.

Media Sense support allows the protocol stack to react to events and invalidate stale parameters. For instance, if a computer running Windows 2000 is unplugged from the network (assuming the NIC supports Media Sense), after a damping period implemented in the stack (currently 3 seconds), TCP/IP will invalidate the parameters associated with the network which has been disconnected. The IP address(es) will no longer allow sends, and any routes associated with the interface are invalidated. You can make the network connection status visible on the taskbar by selecting a connection, right-clicking it, clicking Properties, and then selecting the Show icon in taskbar when connected check box. The network connection icon will also appear automatically with a red “X” when the adapter is having a connectivity problem.

If an application is bound to a socket that is using an invalidated address, it should handle the event and recover in a graceful way, such as attempting to use another IP address on the system or notifying the user of the disconnect.

Dynamic Update DNS Client


Windows 2000 includes support for dynamic updates to DNS as described in RFC 2136. Every time there is an address event (new address or renewal), the DHCP client sends option 81 and its fully qualified name to the DHCP server, and requests the DHCP server to register a DNS pointer resource record PTR RR on its behalf. The dynamic update client handles the A RR registration on its own. This is done because only the client knows which IP addresses on the host map to that name. The DHCP server may not be able to properly do the A RR registration because it has incomplete knowledge. However, the DHCP server can be configured to instruct the client to allow the server to register both records with the DNS. Registry parameters associated with the dynamic update DNS client are documented in Appendix C.

The Windows 2000 DHCP server handles option 81 requests as specified in the draft RFC10. If a Windows 2000 DHCP client talks to a down-level DHCP server that does not handle option 81, it registers a PTR RR on its own. The Windows 2000 DNS server is capable of handling dynamic updates.

Statically configured (non-DHCP) clients register both the A RR and the PTR RR with the DNS server themselves.



Download 0.63 Mb.

Share with your friends:
1   ...   7   8   9   10   11   12   13   14   ...   21



The database is protected by copyright ©ininet.org 2024
send message
    Main page
information contained
current view
issues discussed as