[aprssig] what is RELAY, WIDE, TRACE, etc?

[Jason Winningham]

I'm trying to figure out how the various addresses work with APRS.  I 
understand (and correct me if I'm wrong) that my station's call-ssid is 
the AX.25 source address, the destination address is one of the generic 
APRS destination addresses (typically identifying my rig), and the 
digipeater addresses field is typically "RELAY,WIDE2-2" (or something 
along those lines).  These generic digipeater addresses may get 
substituted or modified as they pass through digis.

Assuming I've got all that right, first question: what is the 
difference between RELAY and WIDE? As far as I can tell from the 
standpoint of an end user who is transmitting or receiving there is no 
difference: RELAY,RELAY; RELAY,WIDE; WIDE,RELAY; WIDE,WIDE; and WIDE2-2 
would all get me digipeated twice.  I assume what various digipeaters 
do with these may depend on what kind of digi it is and how it's 
configured?

What's wrong with some sort of simple hop count/TTL? (Ignoring the fact 
that it probably breaks the current system.)  Wouldn't this allow digi 
operators to easily reign in users who try to use 8 hops, as well as 
shorten the overall packet length and reduce bandwidth?

Would it be better for the digipeater operator and not the user to 
decide how many times a packet should get repeated, (e.g., default 
"digipeater addresses" field is blank)?

How does TRACE fit into the picture, and how does it compare to WIDE?

Is there some documentation (book, URL, etc) that describes APRS 
digipeating?  Google gets me bits and pieces, but I can't find the 
whole picture anywhere.

thanks,
Jason
kg4wsv