Run time configurable parameters

The kernel allows you to change many parameters at run time. If you take a careful look at the /proc/sys/net/ directory structure you will see many files with useful names that describe various parameters for the network configuration. The files in the /proc/sys/net/ax25/ directory each represent one configured AX.25 port. The name of the file relates to the name of the port.

AX.25

The structure of the files in /proc/sys/net/ax25/portname/ is as follows:

Filename	Meaning	Values	Default
ip_default_mode	IP Default Mode	0=DG 1=VC	0
ax25_default_mode	AX.25 Default Mode	0=Normal 1=Extended	0
backoff_type	Backoff	0=Linear 1=Exponential	1
connect_mode	Connected Mode	0=No 1=Yes	1
standard_window_size	Standard Window	1 .. 7	2
extended_window_size	Extended Window	1 .. 63	32
t1_timeout	T1 Timeout	1s .. 30s	10s
t2_timeout	T2 Timeout	1s .. 20s	3s
t3_timeout	T3 Timeout	0s .. 3600s	300s
idle_timeout	Idle Timeout	0m or greater	20m
maximum_retry_count	N2	1 .. 31	10
maximum_packet_length	AX.25 Frame Length	1 .. 512	256

The range for the T1, T3 and T3 timers in above table is given in seconds and the idle timeout in minutes. However note that the procfs and sysctl interfaces actually used miliseconds rsp. seconds starting Linux 2.6.17. Before that the values for T1, T2 and T3 were measured in the unit which the kernel was running internal interrupt timer. For Linux 2.4 and older running on i386 MIPS and a few other architectures that was 100Hz (so time was given in units of 100th of a seconds that is multiple of 10ms), some Linux 2.4 Suse kernels however were running at 1000Hz (so AX.25 timings were in units of 1ms), any Linux version on Alpha was always running at 1000Hz except on Rawhide which were running at 1200Hz (thus making the timing values multiple of 0.833ms). Too complicated? Upgrade to Linux 2.6.17 or newer.

NET/ROM

The structure of the files in /proc/sys/net/netrom/ is as follows:

Filename	Meaning	Values	Default
default_path_quality			10
link_fails_count			2
network_ttl_initialiser			16
obsolescence_count_initialiser			6
routing_control			1
transport_acknowledge_delay			50
transport_busy_delay			1800
transport_maximum_tries			3
transport_requested_window_size			4
transport_timeout			1200

The same details which were explained above for the AX.25 timers also apply the NETROM timers. This affects the T1, T2, T4 and idle timers.

ROSE

The structure of the files in /proc/sys/net/rose/ is as follows:

Filename	Meaning	Values	Default
acknowledge_hold_back_timeout			50
call_request_timeout			2000
clear_request_timeout			1800
link_fail_timeout			1200
maximum_virtual_circuits			50
reset_request_timeout			1800
restart_request_timeout			1800
routing_control			1
window_size			3

Again the units of time for the T0, T1, T2, T3, Holdback and idle timers were changed to miliseconds in Linux 2.6.17

To set a parameter all you need to do is write the desired value to the file itself, for example to check and set the ROSE window size you'd use something like:

# cat /proc/sys/net/rose/window_size
3
# echo 4 >/proc/sys/net/rose/window_size
# cat /proc/sys/net/rose/window_size
4