INP3

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. The name is misleading, as it is not concerned with general inter-node traffic, only 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(5) -- Layer 3 Round Trip Time
MAXHOPS(7) -- Maximum Hop Count.
MAXTT(7) -- Maximum Trip Time.
QUALITY(2) -- NetRom Route Quality.
ROUTES(1) -- Add, Drop & List Neighbour Routes.