REG: TCP/IP Transport Entries, PART 1

• Windows NT operating system version 3.1
• Windows NT Advanced Server version 3.1

• TCP/IP Parameters Subkey Entries
• Adapter Card Parameters for TCP/IP

This is the first of two articles on these entries; for the other entries, see "TCP/IP Transport Entries, Part 2."

TCP/IP TRANSPORT ENTRIES

The various TCP/IP keys do not appear in the Registry unless TCP/IP is installed using the Network icon in Control Panel.

With Registry Editor, you can modify the following parameters for the TCP/IP transport. This section does not include all the TCP/IP parameters that can be set using the Networks application in Control Panel.

The startup parameters defined in this section are found in these subkeys of HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services:

   TCPIP\Parameters
   adapter_name#\Parameters\TCPIP, (where adapter_name# indicates a
                      Services subkey for a network adapter
                      card)
   Ftpsvc\Parameters
   NBT\Parameters
   Streams\Parameters

TCP/IP PARAMETERS SUBKEY ENTRIES

The entries for TCP/IP parameters appear under the following Registry path:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

ArpCacheLife   REG_DWORD   Number of Seconds

Determines the default lifetime for entries in the ARP cache table. Once an entry is placed in the ARP cache, it is allowed to remain there until its lifetime expires or until its table entry is reused because it is the oldest entry.

ArpCacheSize   REG_DWORD   Number

Determines the maximum number of entries that the ARP cache table can hold. The ARP cache is allowed to grow dynamically until this size is reached. After the table reaches this size, new entries can only be added by replacing the oldest entries that exist.

DatabasePath   REG_EXPAND_SZ   Pathname

Determines where TCP/IP and NBT look for the hosts, services, networks, protocols, NETRC, and LMHOSTS files.

IpEnableRouter   REG_DWORD   0 or 1

Determines whether IP routing is enabled between local interfaces. IP routing is always enabled for packets generated by the local host. This parameter determines whether, for packets received on an interface and not destined for this host, IP will attempt to actively forward the packets to hosts that can be reached via its other interfaces.

IpReassemblyTimeout   REG_DWORD   Number of seconds

Determines how long IP accepts fragments when attempting to reassemble a previously fragmented packet. That is, if a packet is fragmented, all of the fragments must make it to the destination within this time limit; otherwise, the fragments will be discarded and the packet will be lost.

TcpDisableReceiveChecksum   REG_DWORD   0 or 1

Specifies whether Checksums is disabled on receive.

TcpDisableSendChecksum   REG_DWORD   0 or 1

Specifies whether Checksums is disabled on send.

TcpKeepCnt   REG_DWORD   Number in Minutes

Specifies how often TCP/IP will generate keep-alive traffic. When TCP/IP determines that no activity has occurred on the connection within the specified time, it generates keep-alive traffic to probe the connection. After trying TcpKeepTries number of times to deliver the keep-alive traffic without success, it marks the connection as down.

TcpKeepTries   REG_DWORD   Number

Specifies the maximum number of times that TCP/IP will attempt to deliver keep-alive traffic before marking a connection as down.

TcpLogLevel   REG_DWORD   Number

Specifies how verbose TCP/IP should be about logging events in the event log. The highest level of verbosity is 16, and 1 is the lowest level. The following shows general information about these levels.

   Level   Events to be included
   ---------------------------------------------
   1     Only the most critical errors
   4     Serious protocol violations
   8     Nonserious protocol violations
   12    Information about unusual events
   16    Information about unusual events that
       some networks normally allow

TcpMaxConnectAttempts   REG_DWORD   Number

Specifies the maximum number of times TCP/IP attempts to establish a connection before reporting failure. The initial delay between connection attempts is 3 seconds. This delay is doubled after each attempt.

TcpMaxRetransmissionAttempts   REG_DWORD   Number

Specifies the maximum number of times that TCP/IP attempts to retransmit a piece of data on an established connection before ending the connection. The initial delay before retransmitting is based on the current estimate TCP/IP makes of the round-trip time on the connection. This delay is doubled after each retransmission. Acknowledgment of the data results in a recalculation of the estimate for the round-trip time.

TcpNumConnections   REG_DWORD   Number

Specifies the maximum number of TCP endpoints.

TcpRecvSegmentSize   REG_DWORD   Bytes

Specifies the maximum receive segment size.

TcpSendDownMax   REG_DWORD   Number

Specifies the maximum number of bytes queued by TCP/IP.

TcpSendSegmentSize   REG_DWORD   Bytes

Specifies the maximum send segment size.

TcpWindowSize   REG_DWORD   Number

Sets the size of the TCP send and receive windows, which is the amount of data that can be accepted in a single transaction. This parameter is important in transferring files between a client and a server and is critical for performance for one-way traffic, such as for FTP.

UdpDisableReceiveChecksum   REG_DWORD   0 or 1

Specifies whether Checksums is disabled on receive.

UdpDisableSendChecksum   REG_DWORD   0 or 1

Specifies whether Checksums is disabled on send.

UdpNumConnections   REG_DWORD   Number

Specifies the maximum number of UDP endpoints.

ADAPTER CARD PARAMETERS FOR TCP/IP

These parameters for TCP/IP are specific to individual network adapter cards. These appear under the following Registry path, where adapter name# refers to the Services subkey for the specific adapter card:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
      \adapter name#\Parameters\Tcpip

BroadcastType   REG_DWORD    0  or 1

Determines whether broadcast packets contain all 0’s or all 1’s as the broadcast address. The most common broadcast type is all 1’s. The all-0’s setting is provided for compatibility with BSD 4.2 systems.

ForwardBroadcasts   REG_DWORD    0 or 1

Specifies whether broadcasts should be forwarded between adapters. If enabled, broadcasts seen by this interface are forwarded to other IP interfaces.

KeepAlive   REG_DWORD   0 or 1

Determines whether TCP connections that request keep-alive packets result in keep-alive packets being sent. This feature is used to determine when inactive connections can be disconnected. When a connection becomes inactive, keep-alive packets are periodically exchanged. When 20 (the default value for TCPKeepTries) consecutive keep-alive packets go unanswered, the connection is broken. This disconnect is initiated by the endpoint that is sending keep-alive packets.

MTU   REG_DWORD   Number in octets

Specifies the maximum transmission unit size of an interface. Each interface used by TCP/IP may have a different MTU value specified. The MTU is usually determined through negotiation with the lower driver, using that lower driver’s value. However, that value may be overridden.

Ideally, the MTU should be large enough to hold any datagram in one frame. The limiting factor is usually the technology making the transfer. Some technologies limit the maximum size to as little as 128; Ethernet limits transfers to 1500; and proNet-10 allows as many as 2044 octets per frame.

Datagrams larger than the MTU value are automatically divided into smaller pieces called fragments; size is a multiple of eight octets. Fragmentation usually occurs somewhere through which the traffic must pass whose MTU is smaller than the encapsulated datagram. If fragmentation occurs, the fragments travel separately to the destination computer, where they are automatically reassembled before the datagram is processed.

RouterMTU   REG_DWORD   Number in octets

Specifies the maximum transmission unit size that should be used when the destination IP address is on a different subnet. Each interface used by TCP/IP may have a different RouterMTU value specified. In many implementations, the value of RouterMTU is set to 576 octets. This is the minimum size that must be supported by any IP node. Because modern routers can usually handle MTUs larger than 576 octets, the default value for this parameter is the same value as that used by MTU.

Trailers   REG_DWORD   0 or 1

Specifies whether the trailer format is used. This feature provides compatibility with BSD 4.2 systems. When this feature is enabled, TCP/IP header information follows the data area of IP packets.

Reference: "The Windows NT Resource Kit for Operating System Version 3.1"