ua1zbe
/
aprsc
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

IGATE-HINTS: iGates dropping duplicate packets unnecessarily

This commit is contained in:
Heikki Hannikainen 2017-04-17 00:22:09 +03:00
parent 89c6dd5d59
commit dc9fcd397e
1 changed files with 36 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

36
doc/IGATE-HINTS.md
View File

@ -85,6 +85,42 @@ for a network socket, do not set UTF-8 encoding for the APRS-IS socket.
Treat it as a binary stream.
iGates dropping duplicate packets unnecessarily
-------------------------------------------------
Since the APRS-IS servers normally drop all duplicate packets seen within 30
seconds, many igates have adopted the same function, probably hoping to
reduce server load. Several implementations have been quite simple
(calculating a 16-bit CRC, storing it, and checking for later packets having
the same checksum).
16-bit CRC has a fairly high chance of collisions - another completely
different packet may have the same checksum and may get dropped. With 16
bits, there are 65536 different checksum values. Given two different random
packets, there is a 1-in-65536 chance that they have the same checksum.
Given 100 random packets, there is a whopping 7.3% chance that two packets
will have the same checksum (for proof, see the
[Birthday paradox](https://en.wikipedia.org/wiki/Birthday_problem)).
The implementation on the APRS-IS servers is more complicated. It takes
into account various common packet corruptions, and is not as likely to do
unnecessary drops. On aprsc for example, a checksum/hash is used as a key
to hash data structure, and the actual data contents of stored packets are
compared before dropping as a duplicate.
The servers can, in fact, benefit from having the duplicate packets
available. Duplicates can be extracted from the servers for improved
network analysis ("who heard who") and in the future, servers could possibly
do improved APRS message routing based on this information.
***Solution:***
Do not implement duplicate packet filtering in an RX igate (RF > APRS-IS).
Duplicate filtering only needs to be done when digipeating. When doing TX
igating (APRS-IS > RF) you might prefer to have a rate limit instead to
prevent too many packets being sent overall, even if they are different.
iGates not supporting DNS
----------------------------