INP3(9) XR32 REFERENCE MANUAL 8/2/2013
NAME
INP3 -- Inter-Node Protocol 3.
DESCRIPTION
INP3 is version 3 of the so-called "Internode Protocol". This
is a protocol for exchanging time-based routing information
between neighbour nodes. Despite the misleading name, it is
not concerned with general inter-node traffic, just the
routing information.
INP3 is analogous to NetRom nodes broadcasts, except that the
information is "unicast" to each neighbour via the normal
AX25L2 inter-node links, instead of being broadcast via UI
frames.
NetRom nodes broadcasts and INP3 can happily co-exist on the
same network, as they are are concerned with different
metrics. NetRom broadcasts propogate route QUALITY, whilst
INP3 unicasts propogate TRIP TIME. These are two completely
different metrics, and there is NO valid way to convert one to
the other, especially quality to time, since route quality is
such a nebulous and subjective quantity.
However, there is one caveat: Some types of node software,
including Linux and (X)net (no relation to XRouter!) convert
time to quality and vice versa. This causes distorted routing
when qualities are converted to bogus trip times. Assigning
slightly lower route qualities to these neighbours helps to
prevent traffic being diverted through them.
The protocol is summarised below:
Information Exchange
~~~~~~~~~~~~~~~~~~~~
All INP3 routing information traffic between two neighbour
nodes is carried alongside regular inter-node traffic, by
AX25L2 numbered information frames with PID 0xCF (NetRom).
Link Quality Estimation
~~~~~~~~~~~~~~~~~~~~~~~
The "quality" of an inter-node link is determined by its mean
one-way frame transport time, usually measured by the exchange
of L3RTT frames. This value is called the Smoothed Neighbour
Transport Time (SNTT).
Trip Time
~~~~~~~~~
This is the all-important metric. For a given target node, it
is simply the sum of all the intermediate nodes' SNTTs. It is
measured in 10ms units, with a maximum (horizon) value of
60000, i.e. 600 seconds. A trip time of 60000 is used to mark
a route as unusable.
The sysop may set a local limit which is less than 60000 using
MAXTT. Routes which exceed the local limit are propogated
with the horizon value.
In this context "route" means a path through the network,
involving one or more intermediate nodes. The optimum route
to a target node is generally considered to be the one with
the lowest trip time.
Hop Count
~~~~~~~~~
Each inter-node link is counted as one HOP. This is a second
metric which is propogated alongside trip time. Routes with
lower hop counts are preferred. A hop count of 30 is
considered to be the horizon, and marks the route as unusable.
The sysop may set a local limit which is less than 30 using
MAXHOPS. Routes which exceed the local limit are propogated
with the horizon value.
Propogation of Routing Information
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A route via a neighbour is valid as long as it is regularly
updated in unicasts from that neighbor, with a trip time less
than the horizon. If the route is not regularly updated, or
the trip time reaches the horizon, that route is marked as
unusable. If the link with the neighbour is broken, all
routes via that neighbour are marked as unusable.
Routes with trip time below the horizon are called positive
information and represent available nodes. Positive
information is not time critical and, like NetRom nodes
broadcasts, is sent at scheduled intervals only.
Any update of positive information with a slower target time
(or route loss) is called negative information. This is
always propogated immediately.
Information about nodes which are routed via a neighbour is
never returned to that neighbour. Instead it is returned with
horizon values. This is called "poisoned reverse".
Packet Structure
~~~~~~~~~~~~~~~~
Routing information is exchanged using "Routing Information
Frames" (RIF), containing one or more Routing Information
Packet" (RIP). Each RIP contains information about one node,
such as callsign, trip time and hop count, plus optional
fields such as alias and IP address.
A RIF starts with a single identification byte of 0xFF. This
value is guaranteed not to appear in normal L3 frame headers
as the first byte. The end of the RIF is marked by a zero
byte, 0x00. The following diagram shows a RIF containing
one RIP:
Byt: 1 7 1 2 n 1
.------.----------.--------.-------------.-----------.------.
| 0xFF | <axcall> | <hops> | <trip time> | <options> | 0x00 |
'------'----------'--------'-------------'-----------'------'
<------ Routing Information Packet --------->
<-------------- Routing Information Frame ---------------->
<axcall> Callsign in AX25 format
<hops> No. of hops to target (1-30)
<trip time> Trip time to target in 10ms units (1-60000)
<options> Variable length options field as below:
Bytes: 1 1 n = <length>
.----------.--------.--------------.
| <length> | <type> | <data> |
'----------'--------'--------------'
<length> Length of data field in bytes (0-255)
<type> Type of data field (0=alias, 1=IP addr)
<data> Data field.
SEE ALSO
L3RTT(9) -- Layer 3 Round Trip Time
MAXHOPS(7) -- Maximum Hop Count.
MAXTT(7) -- Maximum Trip Time.
QUALITY(1) -- NetRom Route Quality.
ROUTES(1) -- Add, Drop & List Neighbour Routes.
INP3(9) END OF DOCUMENT
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