rns.recipes

Hi together! I'm currently working with Reticulum and would like to set up a local LoRa network with standalone transport nodes running microReticulum. I've noticed that, unfortunately, many peer-to-peer connections are unidirectional – Node A can hear Node B, but not the other way around.

The announce system in Reticulum works fine in one way but lets imagine the following situation:

Node A <--> Node B <-- Node C
^ ^
| |
∨ ∨
Node D <--> Node E <--> Node F

Node C sends an announce. The announce makes its way through the mesh using two parallel paths. Node B receives the repeated announce from Node C and from Node E. Node B only saves the direct path to Node C because it is the shortest path to Node C AND NOT the path via Node E. Finally, the announce arrives Node A.
Now, Node A wants to send a packet or establish a link to Node C. The first packet is sent to Node B. Node B acts as a transport node and repeats the packet that is now "addressed" to Node C. Since the connection between Node B and Node C is unidirectional, Node B is unable to send any packets to Node C. The link establishment fails or the packet is dropped.
Node A maybe tries to make a path request but this would just lead to establishing the very same (broken) path again. I think the path_states table might allow for a longer path temporarily, but I think this is originally meant for another purpose. Since the shorter path is still valid (in the receiving direction), the "broken"/unidirectional path would be restored in the long run.

Do you have any thought about that and do you have ideas for solving this issue? I already thought about two mechanisms:

• score different paths: increase the score for transported proof packets, decrease the score for proof timeouts (maybe also link timeouts)
• keeping track of reachable neighbors by listening to recently sent announces with hops=1 and preferring routes with next_hop in reachable_neighbors

Cheers!

Somehow the format got messed up. The right vertical arrow should connect Node C and Node F in both directions :)

I'd like to note that this issue also applies to a malicious transport nodes which only sends out announcements and never lets any other data types through. If the malicious transport node is the only connection between two disconnected networks then obviously not much can be done but even if there is alternative path for data to travel, nodes may see an announcement coming from the malicious transport node first and try to pass data through it, made worse by the fact that this transport node doesn't have the overhead of transmitting other traffic and thus may be faster than most other transport nodes, so a few malicious nodes could damage the network's ability to get a valid two way path between destinations.

Reticulum tends assumes an interface is either bi-directional for all traffic, or dead. So there's a lot of odd edgecases for stuff like this. The simplest protocol would be for the lora nodes one hop away from eachother to periodically (or on first hearing an announcement) to form a link destination between each other and test the packet loss between each other, and if it is unacceptable then to not use each other as possible paths unless there is no better option, but it's possible that a lora node may be just barely audible and then later on the conditions change and now the interface is one-directional again. This is basically the two generals problem on steroids made harder through the limited bit rate of lora. Maybe there should be way of saying to reticulum that "This interface exists but is only 85% reliable on uploading data, only use this if there's nothing better".

For the more abstract and general case of a malicious node as everything a node could do on accident it can also do maliciously and worse. The only kinda effective way I have come up with to solve to this in all cases would be to have Three nodes, Node A, Node B, and Node C. Assuming that they are all directly connected to each other, If node A and C suspect that node B is malicious, they can generate completely new identities on the network, fake that they are further away in hops and then they can attempt to pass data/link/any other type of traffic through that node B to check if it is actually routing traffic as it should. But this Requires node A and C to trust each other fully, though it might be way of doing this without trust. I don't think there's one size fits all solution to this issue either

In all these cases, if a node were to be found to one-directional then they would just be completely removed from pathing and the link would just not exist. It would be nice if reticulum could take advantage of these one-way interfaces instead of just completely ignoring them, specially since for some interfaces, the bitrate in one direction is higher than another (most regular network connections have more download speed than upload). But having a link destination using one path for upload or download or having to somehow deal with path tables for both ways makes everything quadratically and is probably be incompatible with the current system.

This can be easily implemented i think.

Keep track of how many times a peer does not respond to requests while still sending data to us.
If it reach a limit in a specific timeframe then this means the connection is unidirectional(at least temporarily, i.e., LoRa interference or noise etc.).
Mark it as such, and use alternative paths, maybe forgive it after a while and test it again for bidirectional communication.

The mechanism is similar to announce limitation, as you check how many announces are sent in a timeframe and take an action.

The disadvantage is that maybe you use a neighbour node which rely on the "malicious" node as well, but didn't marked it as unidirectional yet, so this process propagate a little bit and may take a while until the "malicious" node is not optimal anymore.

This can be done with probably 10-15 bytes of data and few checks for each request.

Oh I LOVE this question because I'm working on a path request video and this is right up this alley.

Good news: there are a few simple mechanisms that help recover from this, already!

When A tries to establish a Link to C, it'll ultimately fail. At that point, A calls Transport.expire_path(C). That zeroes out the timestamp, then the cull pass that runs later inthe same jobs() tick removes that entry.

Then, A will fire a path request openly. B and D both hear it (and B, which would have marked the path to C it has as UNRESPONSIVE, would not have cleared its cached path entry yet, so it will reply with that path despite it being unresponsive). Both B and D then respond. But, because A has dropped out its path entirely, it will accept whichever shows up first. Let's assume D does. Because they're identical (just the saved copy it got from C originally), despite the shorter hops that B will claim, it will not override the now-establishe dpath through D. Then, with a path via D, the LInk will presumably work and all moves on from there.

Also, if A was a transport node itself, remember that it will forward announces too, meaning B will hear a path to C from A. If that happens to trigger after B has marked the C path UNRESPONSIVE, then in that case even B will also take the -> A -> D -> E -> F -> C route too.

Now if you're thinking "okay but race condition!", more good news. That's exaclty why PATHFINDER_RW exists. It's a half-second jitter in response to path requests, so D has a great shot at winning this.

LXMF (which I assume you're using alongside all this) retries up to 5 times. Very good chance for D to win

Thanks for sharing your thoughts! @welo that’s a good point, malicious nodes even complicate the situation!

I did not know the details about the path request procedure! That’s very interesting. But even tho there is chance to find a bidirectional path, it might be better to have a deterministic way for reliable paths. As I understood it correctly, the functioning bidirectional path also will be dropped again, as soon as Node C sends a regular announce, right?

nilu96 wrote:

it might be better to have a deterministic way for reliable paths.

if only we could haha! that's the thing, this is messy RF. Node D suddenly goes offline, you're out of luck anyway, right? Node B moves and now A can't reach it, then waht? And so on.

I think the very important question here is why is C -> B unidirectional? Is it just at the fade edge? then sometimes it might still get lossy bidirectional traffic. Is node B intentionally a directional link in C's direction? Is node B just doing Rx-side techniques for better sensitivity, creating that imbalance (kinda implies C is near the fade edge too). One really nice thing about Reticulum is adding a node is cheap (in terms of compute, and overall $ cost all things considered). Plenty of hops are possible seamlessly.

It sounds a bit heavy-handed but ironically the best approach may simply be "add another node or two". Or if you're coordinating (which is what it sounds like from your original post; let's ignore malicious actors for the time being), get B and C on a true point to point link or something.

Not to say some tweaks or application-level choices couldn't help smooth those over! But I think the fundamental problem still might lie somewhere else

KenAKAFrosty wrote:

It sounds a bit heavy-handed but ironically the best approach may simply be "add another node or two".

Even if you add a node in between C and B, let's say node Z, wouldn't node B hear an announcement from C first instead of Z anyways, thus then just prioritizing and trying to resolve paths through that unidirectional link ignoring Z and trying to go straight from B to C. If you get to organise the nodes yourself you could maybe control this, but if not in a randomly distributed sea of more and more transport lora nodes in one area, the chance that any one of the receivers is just in the perfect range for the the transmission to be unidirectional increases , and because those links would be the most efficient route for traffic to pass if they were good connections, a lot of traffic will try to go through these links and fail one way or another.

I hear ya, and I promise I'm trying to steelman this side too! Where it gets weirder though is when we lift out of the diagram which is basically modeled as a bunch of point-to-point links, right? In reality we're probably talking the typical dipole donut broadcast from each node.

It's already WAY harder to even create this configuration once you deal with that (and ignoring other 3d geometry or interference; even just assuming "a wide open plain" where each node lives, this is the kind of arrangement it would have to look like to fit that diagram

Notice how even here, C has to have a notably broader reach than the others. and E is nearly hearing it too. IT's just really really hard to create that arrangement in actual all-lora-only-broadcast scenarios.

If C has similar reach as the rest, then it quickly would end up in a spot where it would be bidirectional

Since this is LoRa and on boards we're probably talking about, there isn't really much "rx-sensitivity tricks" going on there, the main variable will just be antenna placement, interfernce, and power (like a heltec v4 having 28 vs 22 for example)

And it's pretty load-bearing that B itself is just reliably, losslessly just always faster to get there in the first place, which seems unlikely for all sorts of reasons from CSMA/CA behavior to moving interference to announce queueing and so on.

THis is just an extreme edge type of case, and I think the final rub is this:
Even if you accounted for all of this, and made explicit adjustments for this exact kind of scenario, that all becomes moot when node A can still end up with a bad link via that path for whatever other reason haha. the joy of distributed systems, right?!

"try, observe, adjust" from A's PoV really is the only way to appropriately handle lossy links, for whatever reason it's lossy. And after first failure you have a working system.

To nilu's point, yes after C announces this could reset. But the announce rate by default is 6 hours.

The one, simplest change I could think of (and note I have not traced any other ramifications of this yet), is that paths that are currently marked UNRESPONSIVE don't get sent back as a response to a path request (which would just allow D to cleanly win on the first re-attempt instead of the jitter dance)

I like your idea to not answer path requests if a path is marked UNRESPONSIVE and instead repeating the path request to heal a path. Even tho this would only heal the path temporarily, this would still be a win – and as you said, in a lora only mesh there is always change and the network has to handle broken paths anyway.

I am still curious what do you think of alternative solutions, like I mentioned in my original post or the idea from Nomad1n0:

Keep track of how many times a peer does not respond to requests while still sending data to us.
If it reach a limit in a specific timeframe then this means the connection is unidirectional(at least temporarily, i.e., LoRa interference or noise etc.). Mark it as such, and use alternative paths, maybe forgive it after a while and test it again for bidirectional communication.

nilu96 wrote:

I like your idea to not answer path requests if a path is marked UNRESPONSIVE and instead repeating the path request to heal a path. Even tho this would only heal the path temporarily, this would still be a win – and as you said, in a lora only mesh there is always change and the network has to handle broken paths anyway.

I am still curious what do you think of alternative solutions, like I mentioned in my original post or the idea from Nomad1n0:

Keep track of how many times a peer does not respond to requests while still sending data to us.
If it reach a limit in a specific timeframe then this means the connection is unidirectional(at least temporarily, i.e., LoRa interference or noise etc.). Mark it as such, and use alternative paths, maybe forgive it after a while and test it again for bidirectional communication.

Oh sorry don't get me wrong! I'm ALL FOR continually improved orchestration calls like this (read some outputs/metrics, make decisions, adjust). Many or most of which should probably be optional, just because some devices will be more constrained with the size they can manage. So treating these as "optional upgrades or hints" could have a really nice path, definitely

welo wrote:

It would be nice if reticulum could take advantage of these one-way interfaces

There are many cases where there are one-way connections, and there can even be networks where everyone can reach everyone, but connections are only one way (think e.g. of three nodes in a triangle).

Having some laser connections for high bandwidth over a long line of sight, it's half the price to have it only unidirectional. If the main data flow is only in one direction, the other direction could be done by more "conventional" connections. Or if we have a few nodes, build a unidirectional ring to save ressources but still have high bandwidth connection.

With radio, unidirectional connections are also common -- I can think of different scenarios (unintentional and intentional); with MeshCore we have it often enough in practice.

Good for me to now have confirmation that Reticulum does not really work when there are only unidirectional physical connections.

Regards!

dreieck wrote:

@welo wrote:

It would be nice if reticulum could take advantage of these one-way interfaces

There are many cases where there are one-way connections, and there can even be networks where everyone can reach everyone, but connections are only one way (think e.g. of three nodes in a triangle).

Having some laser connections for high bandwidth over a long line of sight, it's half the price to have it only unidirectional. If the main data flow is only in one direction, the other direction could be done by more "conventional" connections. Or if we have a few nodes, build a unidirectional ring to save ressources but still have high bandwidth connection.

With radio, unidirectional connections are also common -- I can think of different scenarios (unintentional and intentional); with MeshCore we have it often enough in practice.

Good for me to now have confirmation that Reticulum does not really work when there are only unidirectional physical connections.

Regards!

Actually this scenario is pretty tractable! What you're describing is quite a bit different (espeically when you expect it, like the free space optical you mentioned; side note, QR stream as optical data is actually a fully live thing! see https://rns.recipes/forum/showcase/retiqr-an-alternative-reticulum-qrhid-interface)

The main answer to this (I think) we can do even without any core code changes is just a custom interface! It just needs to know to define the inbound/outbound as those different physical mediums.

In your intentional triangle setup, I'll use a laser chain cuz that's cool as hell: it'd just mean that your outgoing bytes are wired up to your laser, and your incoming bytes would come from your photodetector. Would love to make a demo of that some day! (i have to resist the urge now, too many things on plate already haha).

Great question!!

edit: forgot to mention! if you just mean "absorb data from one direction, but never have return data flow", that's completely doable already too! there's an outgoing field on the interface; set that to False and you're set!

I'd like to chime in here because I've taken a deep dive into this asymmetry issue recently and I'm convinced that it's a much more common and serious problem than some of the posts here imply.

Take just the simple case of an involuntary asymmetry as an example to avoid confounding the problem with intentional unidirectional lnks or malicious nodes. As I think we can all agree, this sort of asymmetry is all too common in a LoRa mesh network, especially a largely uncoordinated one where transport nodes of all (dis)ability are strewn haphazardly about.

I've reviewed some of the code that @KenAKAFrosty is referring to and while I haven't studied it in detail I trust that maybe a legitimate Link establishment attempt could detect and temporarily route around an asymmetry.
But if instead node C was just sending basic Packets to node A, as would be the case if A were a sensor node just sending readings to a hub or a messaging app sending an adhoc message for example, then there would be no way to detect the asymmetry between node C and node B, and those Packets would just silently fall into a blackhole with no way for any node in the path to detect the failure.

In essence, what I've concluded is that RNS naively assumes through its core announce mechanism that if an interface is configured as bidirectional, then communication with another node over that interface is always bidirectional.

Is this a valid current limitation of RNS that we can all agree on, or am I missing something?

gdane wrote:

I'd like to chime in here because I've taken a deep dive into this asymmetry issue recently and I'm convinced that it's a much more common and serious problem than some of the posts here imply.

Take just the simple case of an involuntary asymmetry as an example to avoid confounding the problem with intentional unidirectional lnks or malicious nodes. As I think we can all agree, this sort of asymmetry is all too common in a LoRa mesh network, especially a largely uncoordinated one where transport nodes of all (dis)ability are strewn haphazardly about.

I've reviewed some of the code that @KenAKAFrosty is referring to and while I haven't studied it in detail I trust that maybe a legitimate Link establishment attempt could detect and temporarily route around an asymmetry.
But if instead node C was just sending basic Packets to node A, as would be the case if A were a sensor node just sending readings to a hub or a messaging app sending an adhoc message for example, then there would be no way to detect the asymmetry between node C and node B, and those Packets would just silently fall into a blackhole with no way for any node in the path to detect the failure.

In essence, what I've concluded is that RNS naively assumes through its core announce mechanism that if an interface is configured as bidirectional, then communication with another node over that interface is always bidirectional.

Is this a valid current limitation of RNS that we can all agree on, or am I missing something?

You've put this very well, and I'm actually mostly with you on this, just with one caveat.

First I just want to say: I hope in this thread I didn't come off as adversarial or blindly dismissive! I really didn't mean for that. I too am in the arena with y'all, and I also want to help make RNS the best it can be (i think this will be obvious especially in a few weeks from now).

So, "Is this a valid current limitation of RNS that we can all agree on?" Yes, with the caveat: I think it's very sensible where it already is today, but agree that improvements are still possible. The main distinction I wrestle with is how much of this should truly be on RNS,vs how much is the app's responsibility. Distributed systems are tough, and changes to the core should be very careful.

Where it gets messier is also the fact that LXMF itself is an app on top, even though it's easy for us to blend it all together into a monolith in our heads.

I'd love for us to get some real data on this too, because all of us (myself included) are going on limited n=1 experiences and thought experiments.

What can we do to actually collect and share that kind of data? That seems like the healthiest question for us to answer first

KenAKAFrosty wrote:

gdane wrote:

...
You've put this very well, and I'm actually mostly with you on this, just with one caveat.

First I just want to say: I hope in this thread I didn't come off as adversarial or blindly dismissive! I really didn't mean for that. I too am in the arena with y'all, and I also want to help make RNS the best it can be (i think this will be obvious especially in a few weeks from now).

No I didn't feel you were being adversarial at all, and the point that you made was valid. I just wanted to highlight alternate uses of RNS that don't involve Links which don't benefit from the stacks ability to mark a path as UNRESPONSIVE.

So, "Is this a valid current limitation of RNS that we can all agree on?" Yes, with the caveat: I think it's very sensible where it already is today, but agree that improvements are still possible. The main distinction I wrestle with is how much of this should truly be on RNS,vs how much is the app's responsibility. Distributed systems are tough, and changes to the core should be very careful.

I completely agree with you, and I've had the same inner dialog. If there were a way for the app to solve this problem without a change to the core, and without having to resort to using a Link for something as trivial as being able to (quasi) reliably send a simple Packet, then I would agree that the onus should be on the app. I just don't see a way with the current design.

Maybe the solution doesn't even belong in the core-core, but in a higher level of the stack like where Links and Resources live. Something like a ReliablePacket?

Where it gets messier is also the fact that LXMF itself is an app on top, even though it's easy for us to blend it all together into a monolith in our heads.

I'd love for us to get some real data on this too, because all of us (myself included) are going on limited n=1 experiences and thought experiments.

What can we do to actually collect and share that kind of data? That seems like the healthiest question for us to answer first

I can definitely help here. Most of my focus recently with microReticulum has been on getting to a place where I can release some LoRa transport nodes into the wild with enough remote management and telemetry exposed that we can start to understand how they really behave instead of just being a black box doing who knows what. My goal is to have the first off-grid node deployed by the end of the month, and I'd be happy to make all of the accumulated stats available online for anyone interested.

BTW, your remark "especially in a few weeks from now" has got me really curious about what you're working on. Sounds like something great.

I believe it has to solved at the RNS layer rather than individual apps or implementation because most of the solutions for detecting asymmetric links would require both nodes to cooperate and send some kind of data; if only 50% of the implementations are doing this then only 25% of nodes could verify that they are bidirectional and the remaining 75% affects everyone on the network (might be possible able to use rnprobe and avoid that specific problem of getting adoption but I'm not certain about that). The chance of one interface being unintentionally assymetric grows quadratically with the amount of rnodes in a chain and this completely breaks pathing for every usecase so this is a pretty important thing to fix.

On that note I think adding proper support for an asymmetric connection at the implementation layer would be possible to do without a breaking change just by sending single/announcement/plain packets through interfaces which are either upload or bidirectional and then put link through bidirectional ones only (there's a weird edgecase of a node with only upload and download interfaces with no bidirectional interfaces because then normal packets work fine but links cannot function or take advantage of it and to fix that is a breaking change probably, if minor). The difficult part of that would be then figuring out pathing because an interface that is upload only will not send you any announcements so the regular reticulum pathing rules don't apply, but there's nothing stopping a more advanced fancy reticulum interface in the future from having some way of querying for that information using packets or whatever. Though it would be better if we could find better solution that would be built into directly into the protocol, I'm just saying that if we can't find a better solution then someone in the future can just make a decent solution without breaking everything worst case scenario.

I absolutely think it can be solved without a breaking change, but it will require opt-in by all transport nodes and possibly end nodes in the network, or at least along the paths that will be used.
Since mostly only RF transports suffer from involuntary asymmetry, that does narrow the scope of the issue a lot so whatever solution is implemented will really only be crucial on LoRa and similar meshes.
I’ve thought about solutions ranging from a full blown neighbor discovery protocol (too heavy) to a low-level ACK request mechanism (too core). Where I’m leaning at the moment is a simple 2-way handshake between a transport node and the next-hop that essentially involves a “can you hear me” request and an expected “yeah I hear you” response before a path is marked as bidirectional. Both would be very small packets, and the handshake would only occur when a path is actually used (as opposed to when it’s learned) to keep airtime requirements low.
I’m sure there other ways of going about this and would love to hear more ideas.

All the discussion here has been very good, and I've been thinking about and researching this over the past few days.

It's top of mind for me because I'm deep in the weeds of my own RNS implementation (which also supports self-contained RNS on embedded devices; @gdane, the bz2 compression situation is biting me too but I have some graceful workaround things in mind. if you'd like I'm more than happy to share those in your microreticulum thread?)

Anyway, I have some good news, some bad news, and a solution proposal to the bad news.

I double-checked and LXMF also does the drop_path+request_path combo even on OPPORTUNISTIC messages. LXMF proves every incoming message by default, so the recipient is expected to provide a proof; LXMF uses that lack of proof as the failure signal, and goes through its retry cycle. So for clarity, this isn't a Link-only affordance. Today, right now, LXMF clients give you some recovery for this. That's the good news!

The bad news is that would only happen once during the 5-part retry cycle, leaving us subject to that jitter race with only one attempt to get it clean.

Also, other transport nodes (in RNS, not LXMF) only react during the Link case. Transport nodes 1 hop away to the initiator or recipient mark the path unresponsive when the Link fails.

This same thing keeps coming up now, and it seems like the thread here agrees: a node really shouldn't reply to a path request if that path is currently marked unresponsive. I think this is very high priority:

• It's a very small change in terms of code
• It's semantically accurate and sensible. "Someone else doesn't know of a clean path, and is asking its neighbors if they happen to know" is the point of the path request. An unresponsive path is not helpful, and as we can see here, sometimes actively unhelpful.
• It integrates cleanly from day 1. Nothing about this would break other nodes that don't have this change yet
• It's extremely high value: it takes us from hoping the jitter race wins, to reliably getting the better path every time.

For the Link case, this would basically solve the entire problem on its own, since transport nodes in RNS already play nicely by marking their paths unresponsive when Links fail.

In fact, that's not even really my solution proposal, because that seems like a shoe-in at this point. Happy to hear thoughts from everyone else if they see clear issues with, or objections to, doing this!

I'm going to break the proposal into a separate message beacuse it includes a wire change, and that warrants an isolated review.

This proposal would extend the previous benefits to SINGLE DATA packets (and by extension, LXMF's OPPORTUNISTIC message type), not just Links.

This following fix is also quite simple & integrates cleanly, but due to it including a wire modification, is also a call for scrutiny from you, the reader.

A SINGLE DATA packet should have a way to say "the initiator is expecting a proof". It doesn't change recipient behavior (and it shouldn't; this is the trap to be careful to avoid). Recipients still decide to prove or not.

Note that LXMF already proves every packet itself, so most nodes out there already choose to prove. That's one reason this integrates cleanly

Thankfully, the context flag is still fair game for a SINGLE DATA packet. Currently it just gets ignored and then ultimately zeroed out by every transport node when the packet is forwarded. That happens because each transport node constructs a new flags byte, and only carries parts of the old one across. Context isn't actively preserved, so by omission it gets effectively set back to 0.

So, the first proposed change is for the initiator to set the context flag on a SINGLE DATA packet when expecting a proof to come back.

The next change is transport nodes marking that expects-proof flag in their reverse tables, and forwarding the context flag instead of zoeroing it by omission.

Then, the node should also mark the path unresponsive if the expected-proof (context flag) was set but the proof timed out. Setting this up can just use the same proof_timeout configurations we have for Links, since it's already proven and useful there.

Assuming the previous shoe-in change is also live (unresponsive paths don't get sent in response to path requests), this proposal improves that change by also applying it to single-data-with-proof-expectation scenarios.

Also a nice relief: the context flag doesn't change the proof hash. Proofs would still match with zero special handling, even for current nodes out there. The wire change is actually poised for a clean integration with no extra work; not anywhere as scary as something that requires careful migration.

And one final change is needed too: RNS finally uses its mark_path_responsive (which is currently declares but is not called anywhere), to heal its tables when it does get a proof back, for both Links & singles.

Thoughts, objections, additions, concerns?