Appendix E: Configurable Options

# Configurable Options

We've seen many features of TCP/IP that we've had to describe with the qualifier "it depends on the configuration." Typical examples are whether or not UDP checksums are enabled (Section 11.3), whether destination IP addresses with the same network ID but a different subnet ID are local or nonlocal (Section 18.4), and whether directed broadcasts are forwarded or not (Section 12.3). Indeed, many operating characteristics of a given TCP/IP implementation can be modified by the system administrator.

This appendix lists some of the configurable options for the various TCP/IP implementations that have been used throughout the text. As you might expect, every vendor does things differently from all others. Nevertheless, this appendix gives an idea of the types of parameters different implementations allow one to modify. A few options that are highly implementation specific, such as the low-water mark for the memory buffer pool, are not described.

These variables are described for informational purposes only. Their names, default values, or interpretation can change from one release to the next. Always check your vendor's documentation (or bug them for adequate documentation) for the final word on these variables.

This appendix does not cover the initialization that takes place every time the system is bootstrapped: the initialization of each network interface using `ifconfig` (setting the IP address, the subnet mask, etc.), entering static routes into the routing table, and the like. Instead, this appendix focuses on the configuration options that affect how TCP/IP operates.

### E.1 BSD/386 Version 1.0

This system is an example of the "classical" BSD configuration that has been used since 4.2BSD. Since the source code is distributed with the system, configuration options are specified by the administrator, and the kernel is recompiled. There are two types of options: constants that are defined in the kernel configuration file (see the `config`(8) manual page), and variable initializations in various C source files. Brave and knowledgeable administrators can also change the values of these C variables in either the running kernel or the kernel's disk image, using a debugger, to avoid rebuilding the kernel. Here are the constants that can be changed in the kernel's configuration file.

`IPFORWARDING
` The value of this constant initializes the kernel variable `ipforwarding`. If 0 (default), IP datagrams are not forwarded. If 1, forwarding is always enabled.

`GATEWAY
` If defined, causes `IPFORWARDING` to be set to 1. Additionally, defining this constant causes certain system tables (the ARP cache and the routing table) to be larger.

`SUBNETSARELOCAL
` The value of this constant initializes the kernel variable `subnetsarelocal`. If 1 (default), a destination IP address with the same network ID as the sending host but a different subnet ID is considered local. If 0, only destination IP addresses on an attached subnet are considered local. This is summarized in Figure E.1.

Network IDs
Subnet IDs
`subnetsarelocal`
Comment
1
0
same
same
different
same
different
-
local
local
nonlocal
local
nonlocal
nonlocal
always local
depends on configuration
always nonlocal

Figure E.1 Interpretation of subnetsarelocal kernel variable.

This affects the MSS selected by TCP. When sending to local destinations, TCP chooses the MSS based on the MTU of the outgoing interface. When sending to nonlocal destinations, TCP uses the variable `tcp_mssdflt` as the MSS.

`IPSENDREDIRECTS
` The value of this constant initializes the kernel variable `ipsendredirects`. If 1 (default), the host will send ICMP redirects when forwarding IP datagrams. If 0, ICMP redirects are not sent.

`DIRECTED_BROADCAST
` If 1 (default), received datagrams whose destination address is the directed broadcast address of an attached interface are forwarded as a link-layer broadcast. If 0, these datagrams are silently discarded.

The following variables can also be modified. These variables are spread throughout different files in the `/usr/src/sys/netinet` directory.

`tcprexmtthresh` | The number of consecutive ACKs that triggers the fast retransmit and fast recovery algorithm. The default value is 3.
`tcp_ttl` | The default value for the TTL field for TCP segments. Default value is 60.
`tcp_mssdflt` | The default TCP MSS for nonlocal destinations. Default value is 512.
`tcp_keepidle` | Number of 500-ms clock ticks before sending a keepalive probe. Default value is 14400 (2 hours).
`tcp_keepintvl` | Number of 500-ms clock ticks between successive keepalive probes, when no response is received. Default value is 150 (75 seconds).
`tcp_sendspace` | The default size of the TCP send buffer. Default value is 4096.
`tcp_recvspace` | The default size of the TCP receive buffer. This affects the window size that is offered. Default value is 4096.
`udpcksum` | If nonzero, UDP checksums are calculated for outgoing UDP datagrams, and incoming UDP datagrams containing nonzero checksums have their checksum verified. If 0, outgoing UDP datagrams do not contain a checksum, and no checksum verification is performed on incoming UDP datagrams, even if the sender calculated a checksum. Default is 1.
`udp_ttl` | The default value for the TTL field in UDP datagrams. Default value is 30.
`udp_sendspace` | The default size of the UDP send buffer. Defines the maximum UDP datagram that can be sent. Default is 9216.
`udp_recvspace` | The default size of the UDP receive buffer. The default is 41600, allowing for 40 1024-byte datagrams.

### E.2 SunOS 4.1.3

The method used with SunOS 4.1.3 is similar to what we saw with BSD/386. Since most of the kernel sources are not distributed, all the C variable initializations are contained in a single C source file that is provided.

The administrator's kernel configuration file (see the `config`(8) manual page) can define the following variables. After modifying your configuration file, a new kernel must be made and rebooted.

`IPFORWARDING
` The value of this constant initializes the kernel variable `ip_forwarding`. If -1, IP datagrams are never forwarded. Furthermore, the variable is never changed. If 0 (default), IP datagrams are not forwarded, but the variable's value is changed to 1 if multiple interfaces are up. If 1, forwarding is always enabled.

`SUBNETSARELOCAL
` The value of the kernel variable `ip_subnetsarelocal` is initialized from this constant. If 1 (default), a destination IP address with the same network ID as the sending host but a different subnet ID is considered local. If 0, only destination IP addresses on an attached subnet are considered local. This is summarized in Figure E.1. When sending to local destinations, TCP chooses the MSS based on the MTU of the outgoing interface. When sending to nonlocal destinations, TCP uses the variable `tcp_default_mss`.

`IPSENDREDIRECTS
` The value of this constant initializes the kernel variable `ip_sendredirects`. If 1 (default), the host will send ICMP redirects when forwarding IP datagrams. If 0, ICMP redirects are not sent.

`DIRECTED_BROADCAST
` The value of this constant initializes the kernel variable `ip_dirbroadcast`. If 1 (default), received datagrams whose destination .address is the directed broadcast address of an attached interface are forwarded as a link-layer broadcast. If 0, these datagrams are silently discarded.

The file `/usr/kvm/sys/netinet/in_proto.c` defines the following variables that can be changed. Once these variables are changed, a new kernel must be made and rebooted.

`tcp_default_mss` | The default TCP MSS for nonlocal destinations. Default value is 512.
`tcp_sendspace` | The default size of the TCP send buffer. Default value is 4096.
`tcp_recvspace` | The default size of the TCP receive buffer. This affects the window size that is offered. Default value is 4096.
`tcp_keeplen` | A keepalive probe to a 4.2BSD host must contain a single byte of data to get a response. Set the variable to 1 for compatibility with these older implementations. Default value is 1.
`tcp_ttl` | The default value for the TTL field for TCP segments. Default value is 60.
`tcp_nodelack` | If nonzero, ACKs are not delayed. Default value is 0.
`tcp_keepidle` | Number of 500-ms clock ticks before sending a keepalive probe. Default value is 14400 (2 hours).
`tcp_keepintvl` | Number of 500-ms clock ticks between successive keepalive probes, when no response is received. Default value is 150 (75 seconds).
`udp_cksum` | If nonzero, UDP checksums are calculated for outgoing UDP datagrams, and incoming UDP datagrams containing nonzero checksums have their checksum verified. If 0, outgoing UDP datagrams do not contain a checksum, and no checksum verification is performed on incoming UDP datagrams, even if the sender calculated a checksum. Default is 0.
`udp_ttl` | The default value for the TTL field in UDP datagrams. Default value is 60.
`udp_sendspace` | The default size of the UDP send buffer. Defines the maximum UDP datagram that can be sent. Default is 9000.
`udp recvspace` | The default size of the UDP receive buffer. The default is 18000, allowing for two 9000-byte datagrams.

### E.3 System V Release 4

The TCP/IP configuration of SVR4 is similar to the previous two systems, but fewer options are available. In the file `/etc/conf/pack.d/ip/space.c` two constants can be defined, and the kernel must then be rebuilt and rebooted.

`IPFORWARDING
` The value of this constant initializes the kernel variable `ipforwarding`. If 0 (default), IP datagrams are not forwarded. If 1, forwarding is always enabled.

`IPSENDREDIRECTS
` The value of this constant initializes the kernel variable `ipsendredirects`. If 1 (default), the host will send ICMP redirects when forwarding IP datagrams. If 0, ICMP redirects are not sent.

Many of the variables that we've described in the previous two sections are defined in the kernel, but one must patch the kernel to modify them. For example, there is a variable named `tcp_keepidle` with a value of 14400.

### E.4 Solaris 2.2

Solaris 2.2 is typical of the newer Unix systems that provide a program for the administrator to run to change the configuration options of the TCP/IP system. This allows reconfiguration without having to modify source files and rebuild a kernel.

The configuration program is `ndd`(l). We can run the program to see what parameters we can examine or modify in the UDP module:

`solaris % ndd /dev/udp \?`
`udp_wroff_extra` | `(read and write)`
`udp_def_ttl` | `(read and write)`
`udp_first_anon_port` | `(read and write)`
`udp_trust_optlen` | `(read and write)`
`udp_do_checksum` | `(read and write)`
`udp_status` | `(read only)`

There are five modules we can specify: `/dev/ip`, `/dev/icmp`, `/dev/arp`, `/dev/udp`, and `/dev/tcp`. The question mark argument (which we have to prevent the shell from interpreting by preceding it with a backslash) tells the program to list all the parameters for that module. An example that queries the value of a variable is:

`solaris % ndd /dev/tcp tcp_mss_def
536`

To change the value of a variable we need superuser privilege and type:

`solaris # ndd -set /dev/ip ip_forwarding 0`

These variables can be divided into three categories:

1. Configuration variables that a system administrator can change (e.g., `ip_forwarding`).
2. Status variables that can only be displayed (e.g., the ARP cache). Normally this information is provided in an easier to understand format by the commands `ifconfig`, `netstat`, and `arp`.
3. Debugging variables intended for those with kernel source code. Enabling some of these generates kernel debug output at runtime, which can degrade performance.

We now describe the parameters in each module. All parameters are read-write, unless marked "(Read only)." The read-only parameters are the status variables from case 2 above. We also mark the "(Debug)" variables from case 3. Unless otherwise noted, all the timing variables are specified in milliseconds, which differs from the other systems that normally specify times as some number of 500-ms clock ticks.

#### /dev/ip

`ip_cksum_choice
` (Debug) Selects between two independent implementations of the IP checksum algorithm.

`ip_debug
` (Debug) Enables printing of debug output by the kernel, if greater than 0. Larger values generate more output. Default is 0.

`ip_def_ttl
` Default TTL for outgoing IP datagrams, if not specified by transport layer. Default is 255.

`ip_forward_directed_broadcasts
` If 1 (default), received datagrams whose destination address is the directed broadcast address of an attached interface are forwarded as a link-layer broadcast. If 0, these datagrams are silently discarded.

`ip_forward_src_routed
` If 1 (default), received datagrams containing a source route option are forwarded. If 0, these datagrams are discarded.

`ip_forwarding
` Specifies whether the system forwards incoming IP datagrams: 0 means never forward, 1 means always forward, and 2 (default) means only forward when two or more interfaces are up.

`ip_icmp_return_data_bytes
` The number of bytes of data beyond the IP header that are returned in an ICMP error. Default is 64.

`ip_ignore_delete_time
` (Debug) Minimum lifetime of an IP routing table entry (IRE). Default is 30 seconds. (This parameter is in seconds, not milliseconds.)

`ip_ill_status
` (Read only) Displays the status of each IP lower layer data structure. There is one lower layer structure for each interface.

`ip_ipif_status
` (Read only) Displays the status of each IP interface data structure (IP address, subnet mask, etc.). There is one of these structures for each interface.

`ip_ire_cleanup_interval
` (Debug) The interval at which the IP routing table entries are scanned for possible deletions. Default is 30000 ms (30 seconds).

`ip_ire_flush_interval
` The interval at which ARP information in unconditionally flushed from the IP routing table. Default is 1200000 ms (20 minutes).

`ip_irepathmtu_interval
` The interval at which the path MTU discovery algorithm tries to increase the MTU. Default is 30000 ms (30 seconds).

`ip_ire_redirect_interval
` The interval at which IP routing table entries that are from ICMP redirects are deleted. Default is 60000 ms (60 seconds).

`ip_ire_status
` (Read only) Displays all the IP routing table entries.

`ip_local_cksum
` If 0 (default), IP does not calculate the IP checksum or the higher layer protocol checksum (i.e., TCP, UDP, ICMP, or IGMP) for datagrams sent or received through the loopback interface. If 1, these checksums are calculated.

`ip_mrtdebug
` (Debug) Enables printing of debug output concerning multicast routing by the kernel, if 1. Default is 0.

`ip_path_mtu_discovery
` If 1 (default), path MTU discovery is performed by IP. If 0, IP never sets the "don't fragment" bit in outgoing datagrams.

`ip_respond_to_address_mask
` If 0 (default), the host does not respond to ICMP address mask requests. If 1, it does respond.

`ip_respond_to_echo_broadcast
` If 1 (default), the host responds to ICMP echo requests that are sent to a broadcast address. If 0, it does not respond.

`ip_respond_to_timestamp
` If 0 (default), the host does not respond to ICMP timestamp requests. If 1, the host responds.

`ip_respond_to_timestamp_broadcast
` If 0 (default), the host does not respond to ICMP timestamp requests that are sent to a broadcast address. If 1, it does respond.

`ip_rput_pullups
` (Debug) Count of number of buffers from the network interface driver that needed to be pulled up to access the full IP header. Initialized to 0 at bootstrap time, and can be reset to 0.

`ip_send_redirects
` If 1 (default), the host sends ICMP redirects when acting as a router. If 0, these are not sent.

`ip_send_source_quench
` If 1 (default), the host generates ICMP source quench errors when incoming datagrams are discarded. If 0, these are not generated.

`ip_wroff_extra
` (Debug) Number of bytes of extra space to allocate in buffers for IP headers. Default is 32.

#### /dev/icmp

`icmp_bsd_compat
` (Debug) If 1 (default), the length field in the IP header of received datagrams is adjusted to exclude the length of the IP header. This is compatible with Berkeley-derived implementations and is for applications reading raw IP or raw ICMP packets. If 0, the length field is not changed.

`icmp_def_ttl
` The default TTL for outgoing ICMP messages. Default is 255.

`icmp_wroff_extra
` (Debug) Number of bytes of extra space to allocate in buffers for IP options and data-link headers. Default is 32.

#### /dev/arp

`arp_cache_report
` (Readonly) The ARP cache.

`arp_cleanup_interval
` The interval after which ARP entries are discarded from ARP's cache. Default is 300000 ms (5 minutes). (IP maintains its own cache of completed ARP translations; see ip_ire flush_interval.)

`arp_debug
` (Debug) If 1, enables printing of debug output by the ARP driver. Default is 0.

#### /dev/udp

`udp_def_ttl
` The default TTL for outgoing UDP datagrams. Default value is 255.

`udp_do_checksum
` If 1 (default), UDP checksums are calculated for outgoing UDP datagrams. If 0, outgoing UDP datagrams do not contain a checksum. (Unlike most other implementations, this UDP checksum flag does not affect incoming datagrams. If a received datagram has a nonzero checksum, it is always verified.)

`udp_largest_anon_port
` Largest port number to allocate for UDP ephemeral ports. Default is 65535.

`udp_smallest_anon_port
` Starting port number to allocate for UDP ephemeral ports. Default is 32768.

`udp_smallest_nonpriv_port
` A process requires superuser privilege to assign itself a port number less than this. Default is 1024.

`udp_status
` (Read only) The status of all local UDP end points: local IP address and port, foreign IP address and port.

`udp_trust_optlen
` (Debug) No longer used.

`udp_wroff_extra
` (Debug) Number of bytes of extra space to allocate in buffers for IP options and data-link headers. Default is 32.

#### /dev/tcp

`tcp_close_wait_interval
` The 2MSL value: the time spent in the TIME_WAIT state. Default is 240000 ms (4 minutes).

`tcp_conn_grace_period
` (Debug) Additional time added to the timer interval when sending a SYN. Default is 500 ms.

`tcp_conn_req_max
` The maximum number of pending connection requests queued for any listening end point. Default is 5.

`tcp_cwnd_max
` The maximum value of the congestion window. Default is 32768.

`tcp_debug
` (Debug) If 1, enables printing of debug output by TCP. Default is 0.

`tcp_deferred_ack_interval
` The time to wait before sending a delayed ACK. Default is 50 ms.

`tcp_dupack_fast_retransmit
` The number of consecutive duplicate ACKs that triggers the fast retransmit, fast recovery algorithm. Default is 3.

`tcp_eager_listeners
` (Debug) If 1 (default), TCP completes the three-way handshake before returning a new connection to an application with a pending passive open. "This is the way most TCP implementations operate. If 0, TCP passes an incoming connection request (received SYN) to the application, and does not complete the three-way handshake until the application accepts the connection. (Setting this to 0 might break many existing applications.)

`tcp_ignore_path_mtu
` (Debug) If 1, path MTU discovery ignores received ICMP fragmentation needed messages. If 0 (default), path MTU discovery is enabled for TCP.

`tcp_ip abort_cinterval
` The total retransmit timeout value when TCP is performing an active open. Default is 240000 ms (4 minutes).

`tcp_ip_abort_interval
` The total retransmit timeout value for a TCP connection after it is established. Default is 120000 ms (2 minutes).

`tcp_ip_notify_cinterval
` The timeout value when TCP is performing an active open after which TCP notifies IP to find a new route. Default is 10000 ms (10 seconds).

`tcp_ip_notify_interval
` The timeout value for an established connection after which TCP notifies IP to find a new route. Default is 10000 ms (10 seconds).

`tcp_ip_ttl
` The TTL to use for outgoing TCP segments. Default is 255.

`tcp_keepalive_interval
` The time that a connection must be idle before a keepalive probe is sent. Default is 7200000 ms (2 hours).

`tcp_largest_anon_port
` Largest port number to allocate for TCP ephemeral ports. Default is 65535.

`tcp_maxpsz_multiplier
` (Debug) Specifies the multiple of the MSS into which the stream head packetizes the application's write data. Default is 1.

`tcp_mss_def
` Default MSS for nonlocal destinations. Default is 536.

`tcp_mss_max
` The maximum MSS. Default is 65495.

`tcp_mss_min
` The minimum MSS. Default is 1.

`tcp_naglim_def
` (Debug) Maximum value of the per-connection Nagle algorithm threshold. Default is 65535. The per-connection value starts out as the minimum of the MSS or this value. The per-connection value is set to 1 by the TCP_NODELAY socket option, which disables the Nagle algorithm.

`tcp_old_urp_interpretation
` (Debug) If 1 (default), the older (but more common) BSD interpretation of the urgent pointer is used: it points 1 byte beyond the last byte of urgent data. If 0, the Host Requirements RFC interpretation is used; it points to the last byte of urgent data.

`tcp_rcv_push_wait
` (Debug) Maximum number of bytes received without the PUSH flag set before the data is passed to the application. Default is 16384.

`tcp_rexmit_interval_initial
` (Debug) Initial retransmit timeout interval. Default is 500 ms.

`tcp_rexmit_interval_max
` (Debug) Maximum retransmit timeout interval. Default is 60000 ms (60 seconds).

`tcp_rexmit_interval_min
` (Debug) Minimum retransmit timeout interval. Default is 200 ms.

`tcp_rwin_credit_pct
` (Debug) Percentage of receive window that must be buffered before flow control is checked on every received segment. Default is 50%.

`tcp_smallest_anon_port
` Starting port number to allocate for TCP ephemeral ports. Default is 32768.

`tcp_smallest_nonpriv_port
` A process requires superuser privilege to assign itself a port number less than this. Default is 1024.

`tcp_snd_lowat_fraction
` (Debug) If nonzero, the send buffer low-water mark is the send buffer size divided by this value. Default is 0 (disabled).

`tcp_status
` (Read only) Information on all TCP connections.

`tcp_sth_rcv_hiwat
` (Debug) If nonzero, the value to set the stream head high-water mark to. Default is 0.

`tcp_sth_rcv_lowat
` (Debug) If nonzero, the value to set the stream head low-water mark to. Default is 0.

`tcp_wroff_xtra
` (Debug) Number of bytes of extra space to allocate in buffers for IP options and data-link headers. Default is 32.

### E.5 AIX 3.2.2

AIX 3.2.2 allows network options to be set at runtime using the no command. It can display the value of an option, set the value of an option, or set an option value back to its default. For example, to display an option we type:

`aix % no -o udp_ttl
udp_ttl = 30`

The following options can be modified.

`arpt_killc
` The time (in minutes) before an inactive completed ARP entry is removed. Default is 20.

`ipforwarding
` If 1 (default), IP datagrams are always forwarded. If 0, forwarding is disabled.

`ipfragttl
` The time to live (in seconds) for IP fragments awaiting reassembly. Default is 60.

`ipsendredirects
` If 1 (default), the host will send ICMP redirects when forwarding IP datagrams. If 0, ICMP redirects are not sent.

`loop_check_sum
` If 1 (default), the IP checksum is calculated for datagrams sent through the loop-back interface. If 0, this checksum is not calculated.

`nonlocsrcroute
` If 1 (default), received datagrams containing a source route option are forwarded. If 0, these datagrams are discarded.

`subnetsarelocal
` If 1 (default), a destination IP address with the same network ID as the sending host but a different subnet ID is considered local. If 0, only destination IP addresses on an attached subnet are considered local. This is summarized in Figure E.1. When sending to local destinations, TCP chooses the MSS based on the MTU of the outgoing interface. When sending to nonlocal destinations, TCP uses the default (536) as the MSS.

`tcp_keepidle
` Number of 500-ms clock ticks before sending a keepalive probe. Default value is 14400 (2 hours).

`tcp_keepintvl
` Number of 500-ms clock ticks between successive keepalive probes, when no response is received. Default value is 150 (75 seconds).

`tcp_recvspace
` The default size of the TCP receive buffer. This affects the window size that is offered. Default value is 16384.

`tcp_sendspace
` The default size of the TCP send buffer. Default value is 16384.

`tcp_ttl
` The default value for the TTL field for TCP segments. Default value is 60.

`udp_recvspace
` The default size of the UDP receive buffer. The default is 41600, allowing for 40 1024-byte datagrams.

`udp_sendspace
` The default size of the UDP send buffer. Defines the maximum UDP datagram that can be sent. Default is 9216.

`udp_ttl
` The default value for the TTL field in UDP datagrams. Default value is 30.

### E.6 4.4BSD

4.4BSD is the first of the Berkeley releases to provide dynamic configuration for numerous kernel parameters. The `sysctl`(8) command is used. The names for the parameters were chosen to look like MIB names from SNMP. To examine a parameter we type:

`vangogh % sysctl net.inet.ip.forwarding
net.inet.ip.forwarding = 1`

To change a parameter we need superuser privilege and then type:

`vangogh # sysctl -w nat.inet.ip.ttl=128`

The following parameters can be changed.

`net.inet.ip.forwarding
` If 0 (default), IP datagrams are not forwarded. If 1, forwarding is enabled.

`net.inet.ip.redirect
` If 1 (default), the host will send ICMP redirects when forwarding IP datagrams. If 0, ICMP redirects are not sent.

`net.inet.ip.tti
` The default TTL for both TCP and UDP. The default is 64.

`net.inet.icmp.maskrepi
` If 0 (default), the host does not respond to ICMP address mask requests. If 1, it does respond.

`net.inet.udp.checksum
` If 1 (default), UDP checksums are calculated for outgoing UDP datagrams, and incoming UDP datagrams containing nonzero checksums have their checksum verified. If 0, outgoing UDP datagrams do not contain a checksum, and no checksum verification is performed on incoming UDP datagrams, even if the sender calculated a checksum.

Additionally, numerous variables that we've described earlier in this appendix are scattered among various source files (`tcp_keepidle`, `subnetsarelocal`, etc.) and can be modified.