[Xastir] path question

[Tom Henderson]

Thank you very much, and especially for relating it to IP, since I'm a 
lot more comfortable with that.

So how does the path work when you use it in the Igate -> RF Path 
section of the TNC interface config in Xastir?

Mine is set up as WIDE2-1. What effect would changing that to WIDE2-2 have?

Tom Henderson

On 08/30/2018 09:42 AM, Jason KG4WSV wrote:
> On Thu, Aug 30, 2018 at 7:36 AM Tom Henderson <Tom at henderson4.us> wrote:
>> Is there a good explanation of paths and how they work in APRS someone
>> could refer me to? I see bits and pieces out there, but nothing
>> comprehensive tat explains it from the ground up.
> I don't know of one off the top of my head.
>
>
> An AX.25 path is called "source routing" in general networking
> terminology.  This is because the path that the packet will travel
> through the network is determined by the source of the packet.  E.g.
> If I know routers Bob and Sally get me to Jim, my destination address
> would be Jim with a path of Bob,Sally.  In more typical routing
> scenarios (e.g. Internet Protocol aka IP) the source node only knows
> how to get to the first router in the path (almost always your
> "default gateway"), nd that node knows how to get the next step toward
> the destination.
>
> IP includes a field called TTL (time to live) which is really a hop
> count, not a time.  The source of the packet sets the maximum number
> of routers to be traversed ("hops") and each router decrements the TTL
> by 1.  If the TTL drops to 0, the packet is discarded.  AX.25 has no
> such provision - it isn't needed, because the route length is fixed by
> the path.
>
> Another element of AX.25 addressing is the "SSID" secondary station
> identifier.  An AX.25 address is a callsign plus a 4 bit number (0 to
> 15) so that a single ham can have 16 stations on the network at the
> same time.  For a bit of added confusion, the SSID 0 is often left
> off, so you'll see KG4WSV-3, but rarely KG4WSV-0; that'll be
> abbreviated to KG4WSV. (On a side note "CALL-N" is just a convention
> for representing what is actually a fixed width binary address; every
> AX.25 address takes up exactly the same number of bits when it's going
> over the air.)
>
> In an AX.25 packet, the source address, destination address, and 0 to
> 8 path addresses are all standard AX.25 addresses.
>
> In order to accomplish a broadcast address (which doesn't exist in
> AX.25 AFAIK), the address "WIDE" was used. AX.25 repeaters were
> configured to answer to an alias of WIDE (in addition to their
> callsign).  This allows a packet to travel in multiple directions
> (network-wise), and it allows a generic PATH to work everywhere, which
> is critical to making the network useful in a mobile environment.
>
> The WIDEn-N concept used by APRS is a mashup.  Instead of a fixed
> address in the source path (e.g. "Bob" or Sally" or KG4WSV) it uses a
> generic address of WIDEn-N, where "n" is the requested number of hops
> (like the initial TTL in IP) and "N" is the number of hops remaining
> (analogous to the current TTL in IP). note the "n" is purely
> documentation; the functional part of the count is the "N", the part
> of the address that's in those 4 bits representing the SSID.  In
> addition to WIDE, there are special uses, e.g. STn-N where ST is the 2
> letter state abbreviation. Effectively the n-N method uses the 4 bits
> of the SSID part of the address as a hop count.
>
> An added twist is that there's a hop count for each of these generic
> path elements and Ax.25 allows for multiple addresses in the path, so
> a path of WIDE2-2.WIDE2-2 would give me a total of 4 hops.
>
> There are additional historical artifacts, like the hack to allow
> ancient crude hardware incapable of the WIDEn-N SSID decrement trick
> to participate in the network as an edge repeater, where a packet
> could enter the network if the transmitting station couldn't be heard
> by a more capable n-N repeater.  A path the looks like "WIDE,WIDE2-1"
> is a result.
>
> Many of the confusing aspects of APRS networking, IMO, are due to
> hacks to support hardware that should be retired, e.g. the whole
> "WIDE,WIDE2-1" thing.
>
>
> Hope this helps a bit.
>
> -Jason
> kg4wsv
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