PERMLINKS(9) XR32 REFERENCE MANUAL 31/1/2013
NAME
PERMLINKS -- Permanent NetRom Neighbour Links.
SYNOPSIS
This file explains why XR32 tries to keep neighbour node
links permanently open, why that is desirable, and why you
shouldn't worry about it.
DESCRIPTION
Old-fashioned nodes close their AX25 level 2 links with
neighbour nodes after a period of inactivity, which can be
less than ideal...
Firstly, it takes a finite time to establish an AX25 L2
interlink from "cold", which adds an unnecessary extra
delay for NetRom link setup, and for L3 frames which arrive
infrequently. If it takes 5 seconds to establish a link,
and a frame has to pass through 10 nodes, that's an extra
50 seconds of delay!
Secondly there is the very common and highly disruptive
problem known as the "ONE WAY LINK". These can occur when
an RF path fades, or when there is local interference, or
when a transmitter, receiver, antenna, TNC or AX*P link
malfunctions, leaving one node hearing the other but not
vice versa.
When a one-way link occurs, one peer can hear the other's
nodes broadcasts, but there is no reverse path. The node
which received the broadcast mistakenly thinks it has a
route to the node which made the broadcast, but since the
two-way path required by an AX25 connection is not
available, no connection can be made. No traffic can be
transferred, so the neigbour node and all nodes advertised
by him are unreachable.
To make matters worse, the node which received the
broadcast advertises all the nodes learned via that
broadcast to its other neighbours. So in turn they think
they have routes to distant nodes, when in fact they don't.
The network is seriously disrupted by one broken link,
causing a black hole where all packets are LOST. This
situation can not recover until the one way link is fixed.
The Better Way
~~~~~~~~~~~~~~
In contrast, XR32 attempts to maintain permanent L2 links
between neighbour nodes. You may find this disconcerting,
but it has the following advantages:
Firstly, it removes the link establishment time, so NetRom
traffic can be routed without delay, no matter how
infrequently it arrives.
Secondly, it solves the "one way link" problem by promptly
detecting when the link cannot accept traffic, and marking
all nodes advertised by the peer as unusable, so that
alternative routes can be used.
Lastly, it enables continuous measurement of route quality
and round trip time, used in making routing decisions. RF
links, especially long ones, can be remarkably variable
throughout the day and night, and these problems can be
discovered through the process of continuous link audit.
There is no need for concern. The only cost associated with
keeping a link open is a tiny link check packet every 3
minutes. Hardly a great waste of bandwidth! Remember that
the old-fashioned way has a bandwidth cost too - at least
two packets to open a link and two to close it, in addition
to the link check packets. The benefits of permanent links
far outweigh the disadvantages.
How it Works
~~~~~~~~~~~~
If port QUALITY and NODESINTERVAL are both non-zero, as soon
as XR32 hears a nodes broadcast from a previously unknown
neighbour, it will attempt to connect to it. If the connect
is successful, the round trip time is measured, and the
route is considered viable. The link is checked every 3
minutes, and will close if non-viable.
If the connect fails, the route is marked as unusable,
therefore no nodes will be routed via it. The connection is
retried at regular intervals.
If the path is marginal, connection attempts may sometimes
succeed and sometimes fail. It is not worth using a route
like this, so you can prevent it from re-trying by locking
in the neighbour with a zero quality.
The "R" commands will show a ">" in the left-most column if
the interlink is fully open, a "~" if it is opening or
closing, or a blank if the link can't be opened. This
provides a handy way of detecting link problems which might
otherwise go un-noticed.
SEE ALSO
ROUTES(1) -- NetRom Routes Commands.
XROUTER.CFG(8) -- Main Configuration File.
PERMLINKS(9) END OF DOCUMENT
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