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Date: Sat, 6 Aug 2022 15:10:00 +0200
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To: ~pukkamustard/eris@lists.sr.ht
From: Katharina Fey <kookie@spacekookie.de>
Subject: Introduction of smaller block sizes
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Hello everyone,

as part of the Irdest project [1] we are working on a LoRa driver for 
long-range, low-power, low-bandwidth uses.

Lora was implemented with the goals of providing an ultra low power, 
long range, radio link for IoT devices in free to access radio bands. 
Lora is particularly suitable for use in the Irdest project as it will 
enable our users to set up peer to peer radio communications without the 
need for ground station infrastructure or the requirement for users to 
be licensed radio operators.

LoRa, along with its accompanying de-facto link layer protocol, LoraWAN, 
is governed by the LoRa Alliance, with further restrictions enforced by 
regional radio regulators. The unlicensed nature of the Lora band, and 
its low power design goals unfortunately means that very little 
bandwidth is available to peers operating in the LoRa band. To permit 
fair and reliable access to the band limitations on Lora packet lengths 
and radio transmit duty cycles. These restrictions are implemented at 
the regulatory level.

Due to the variable bit-rate of the LoRa physical layer protocol  MTU's 
for Lora messages defined per region and bit-rate, to prevent the 
broadcast of messages more than a few milliseconds to prevent a handful 
of stations from tying up the entire band. In the best case Lora's MTU 
is 242 bytes. In the worst case it is only 19 bytes. Lora operates at no 
more than 27Kbit/s and most regions enforce station broadcast limits of 
<1% duty cycle.

In the best case Lora radio environment in the case of attempting to 
transmit a standard 1KB ERIS frame over Lora using reasonable framing 
and the maximum permissible 242byte MTU, frame transmission would 
require being split over 5 Lora Messages, which must be separated by (at 
minimum) 2 seconds each. This would require at least 10 seconds to 
transmit any Ratman message regardless of actual size.

These restrictions are built into the physical and legal layers of the 
protocol, as such Irdest cannot bypass them. Because of this we must 
request a modification to the ERIS spec.

Currently ERIS frames are padded to the nearest 1K block size before 
encrypting. We wish for smaller power of 2 padding options of 512, 256, 
128 and 64 bytes be made available at the point of packet construction 
or later slicing, to allow for us to make efficient use of the limited 
radio spectrum available to us. With this change it will be possible for 
Irdest to offer medium to long range fully encrypted mesh networking for 
simple communication payloads to anyone without the requirement of using 
licensed radio bands.


Cheers,

- Katharina Fey
- Lauren Brown <SEGFAULT>

[1]: https://irde.st