TCP/IP and Data Communications Administration Guide

TCP Large Window Parameters

The configuration parameters are associated with the TCP device, /dev/tcp, and may be inspected or modified using ndd(5). Normally, these parameters would be set in one of the shell scripts executed by init(1M) when the system is booted (see init.d(4) for information on how to add a new script).

A list of the available parameters and their meaning are.

tcp_xmit_hiwat

Specifies the default value for a connection's send buffer space. The default is 8K.

tcp_recv_hiwat

Specifies the default value for a connection's receive buffer space; that is, the amount of buffer space allocated for received data (and thus the maximum possible advertised receive window). The default is 8K.

tcp_wscale_always

If this parameter is nonzero, a window scale option is always sent when connecting to a remote system. Otherwise, the option will be sent if-and-only-if the user has requested a receive window larger than 64K. The default is zero.

Regardless of the value of this parameter, a window scale option is always included in a connect acknowledgment if the connecting system has used the option.

tcp_tstamp_always

If this parameter is nonzero, a timestamp option is always sent when connecting to a remote system. The default is zero.

Regardless of the value of this parameter, a timestamp option is always included in a connect acknowledgment (and all succeeding packets) if the connecting system has used the option.

tcp_tstamp_if_wscale

If this parameter is nonzero, the timestamp option is sent when connecting to a remote system if the user has requested a receive window larger than 64K (that is, if a window scale option with a nonzero scale is being used). The default is zero.

tcp_max_buf

Specifies the maximum buffer size a user is allowed to specify with the SO_SNDBUF or SO_RCVBUF options. Attempts to use larger buffers fail with EINVAL. The default is 256K. It is unwise to make this parameter much larger than the maximum buffer size your applications require, since that could allow malfunctioning or malicious applications to consume unreasonable amounts of kernel memory.

tcp_host_param

This parameter is a table of IP addresses, networks, and subnetworks, along with default values for certain TCP parameters to be used on connections with the specified hosts. The table can be displayed with the ndd command as follows:

example# ndd /dev/tcp tcp_host_param
Hash HSP     Address         Subnet Mask     Send       Receive    TStamp
027 fc31eea4 129.154.000.000 255.255.255.000 0000008192 0000008192      0
131 fc308244 129.154.152.000 000.000.000.000 0000032000 0000032000      0
133 fc30bd64 129.154.152.006 000.000.000.000 0000128000 0000128000      1

Each element in the table specifies either a host, a network (with optional subnet mask), or a subnet, along with the default send buffer space and receive buffer space, and a flag indicating whether timestamps are to be used.

The default values specified in the table are used for both active and passive connections (that is, both connect() and listen()). The most applicable match found is used; first the full host address, then the subnet, and finally the network. For subnet recognition to work properly, there must be an entry for that subnet's network which specifies the subnet mask.

The example table above specifies that:

Connections with host 129.154.152.6 uses send and receive buffer sizes of 128000 bytes, and will use timestamps.
Connections with other hosts on the 129.154.152 subnet uses send and receive buffer sizes of 32000 bytes.
Connections with other hosts on the 129.154 network uses send and receive buffer sizes of 8192 bytes.

Elements are added to or removed from the table with ndd as follows:

ndd -set /dev/tcp tcp_host_param '<command>'

where <command> is either:

<ipaddr>	[ mask <ipmask>] [ sendspace <integer> ]
				[ recvspace <integer> ] [ timestamp { 0 | 1 } ]

<ipaddr> delete

For example, the table above was created by:

example# ndd -set /dev/tcp tcp_host_param '129.154.0.0
mask 255.255.255.0 sendspace 8192 recvspace 8192'
            

example# ndd -set /dev/tcp tcp_host_param
'129.154.152.0 sendspace 32000 recvspace 32000'
           

example# ndd -set /dev/tcp tcp_host_param
'129.154.152.6 sendspace 128000 recvspace 128000 
timestamp 1'

It could be removed by:

example# ndd -set /dev/tcp tcp_host_param '129.154.152.6 delete'
	 

example# ndd -set /dev/tcp tcp_host_param '129.154.152.0 delete'


example# ndd -set /dev/tcp tcp_host_param '129.154.0.0	 delete'

Networks and subnets are specified by leaving the host bits zero. The same syntax used to add entries can also be used to modify existing entries.

The send and receive space values from the tcp_host_param table will only be used if they are larger than the values set by the user (or obtained from tcp_xmit_hiwat and tcp_recv_hiwat). This is so that the user can specify larger values for improved throughput and not have them erroneously reduced.

If timestamp value in the tcp_host_param table is 1, the timestamp option will be sent to the selected host or hosts when a connection is initiated. However, if the value is 0, the timestamp option may still be sent, depending on the settings of the tcp_tstamp_always and tcp_tstamp_if_wscale options.