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[liberationtech] [serval-project-dev] Implementing a different routing protocol

Eugen Leitl eugen at leitl.org
Tue Nov 27 23:24:55 PST 2012


----- Forwarded message from Paul Gardner-Stephen <paul at servalproject.org> -----

From: Paul Gardner-Stephen <paul at servalproject.org>
Date: Wed, 28 Nov 2012 09:28:33 +1030
To: serval-project-developers at googlegroups.com
Subject: Re: [serval-project-dev] Implementing a different routing protocol
Reply-To: serval-project-developers at googlegroups.com

Hello Fraser,

Sounds like an interesting project.

Jeremy has been doing the most work on the mesh routing parts of
Serval, so I expect that he will chime in with where things are in the
current state of the code.  Note that routing is currently under
active development, so things are liable to change.

Back to your actual goal, which is to stream multimedia content for
disaster recovery scenarios, this is something that we have been
thinking about from the earliest, and it is nice to hear that someone
is looking to work on it.

Thinking about the general approach you are considering around greedy
routing, it may make more sense to use the Serval Rhizome
store-and-forward scheme as the basis, rather than the MDP/overlay
real-time routing.  Rhizome understands the idea of a "journal" which
is really just a file that grows in successive versions.  Nodes
receiving a journal can, in principle at least, pull just the new part
of the file.  If the file has grown further in the meantime, then
another pull will occur.    There would still be some tweaking
required using this approach, such as making Rhizome be more selective
about who it exchanges with so that it can be directed towards the
destination, but I think it would give you more resilient routing. The
tradeoff is likely to be increased latency, although I think that the
actual useful throughput would increase, because packet loss and
retransmission would be dealt with each hop.  You would also be able
to use WiFi unicast packets, and thus the full WiFi bandwidth.

You should also take a look at Serval Maps that provides functionality
for nodes to share their geographic location (via Rhizome), and that
could be used in place of adding geo tags to each packet.

I guess overall I am envisaging a solution where Serval Maps provides
the geolocation information, and also possibly the user interface for
choosing which phone you want to see the video from.  Then the video
or other content is pulled down via the improved Rhizome that you
would create.  By using Rhizome, it doesn't matter if a link drops for
a short while, as the content will be cached on intermediate nodes,
and so it will deliver as soon as it is able.

Anyway, happy to keep thinking through the options with you, and
looking forward to seeing what you create.

We would prefer that you contribute any modifications you make back to
our repo so that everyone can benefit.  We have a standard Harmony
Project issued contributor agreement that can facilitate that fairly
painlessly.

Paul.

On Wed, Nov 28, 2012 at 6:30 AM, Fraser Cadger <cadge01 at googlemail.com> wrote:
> Hi all,
>
>
> First of all, let me start by stating that I am very impressed with the work
> of the Serval Project and the Serval app. I appreciate that it is still
> under development, but having experimented on several Android phones I have
> found it really easy to use and effective.
>
>
> My name is Fraser Cadger and I am a third year PhD student at the University
> of Ulster in Northern Ireland. My project is concerned with developing a
> framework to allow the streaming of multimedia content both live (i.e. video
> call) and on-demand (recorded videos) in disaster recovery scenarios using a
> mesh network of WiFi-enabled devices (currently this entails a testbed of
> six Android phones). As I am working with Android devices this obviously
> adds a layer of difficult when trying to implement ad-hoc networking. After
> doing some searching I came across several different implementations of
> ad-hoc routing on Android, and after some experimentation the two I was most
> interested in were Serval and Commotion (who I believe the Serval Project
> collaborates with). In the end I decided to work with the Serval app because
> I felt that was the closest to what I was doing, and I also liked how it
> worked on the phones.
>
>
> Currently what I am interested in doing is implementing my own routing
> protocol (which is still under development) on the phones using Serval as a
> base. That is to say, that I want to replace the modified BATMAN code Serval
> uses for routing with the current version of my routing algorithm
> (originally written in C++ for ns-2 but re-writing in C should not be a huge
> problem). Obviously I realise this will not be a simple as copying and
> pasting my code in and that is why I am sending this message. From reading
> various comments in the code I understand that one of the main modifications
> to Serval is to restrict broadcasting to link-local nodes (i.e. not
> network-wide broadcasting), if I have understand the code correctly that is.
> The protocol I am developing is a variation of the greedy routing protocol
> GPSR http://www0.cs.ucl.ac.uk/staff/b.karp/gpsr/gpsr.html . Both the
> original GPSR and my own protocol use limited broadcasting as well; beacon
> (regular hello messages) are broadcast as far as one hop and nodes maintain
> tables of neighbours who can be reached directly only. There is no
> conventional collection of routes; instead each node forwards a packet to
> their neighbour who best meets the criterion/criteria (generally geographic
> location, i.e. located closest to the destination) one hop at a time. So
> packets are effectively passed from node to node without a formal route
> existing. This version of geographic routing is not perfect, and that is why
> we are working on several modifications, but for now I am content to have
> some form of working geographic routing up and running.
>
>
> I have been reading through the code and trying to determine what parts I
> need to change and where to add my code. What I am looking for is the point
> at which a node determines where to send a packet. I realise that this will
> vary depending on the packet's origin, that is to say that when a node
> generates a new packet it will usually be treated differently from when an
> intermediate node receives a packet from another node. Now, if I understand
> correctly Serval's version of BATMAN does not use an explicit routing table
> structure. I have came across a struct called subscriber defined in
> overlay_address.h, and from what I have read this seems to act as a record
> of different nodes (destinations). Within the subscriber struct there is an
> integer variable called reachable and this determines whether a node is
> reachable directly via unicast, broadcast, or must be reached indirectly. If
> a node must be reached indirectly then there is a field called next_hop
> which if I understand correctly is a pointer to another struct (the
> intermediate node between ourselves and the destination). Is this correct?
> Now, what I have noticed in the code is that sometimes next_hop contains a
> pointer to another next_hop (i.e. next_hop->next_hop). What I'm guessing
> this means is that if there are multiple intermediate nodes (i.e. to send a
> packet to node D node A needs to send it via B and C), then this is a way of
> linking them as a route. So in essence, the subscriber struct contains the
> route to a destination (by way of the next_hop attribute).
>
>
> For the actual routing, from reading the code I'm guessing that the
> 'overlay_route_recalc_node_metrics' function is used to determine whether a
> destination can be reached directly or indirectly, and if indirectly it will
> then assign the appropriate intermediate nodes as next_hop's. Therefore, to
> create or change a route this function is called. Is this correct?
>
>
> In my case, I would like to do things slightly differently. As I am not
> doing end-to-end routing I do not need a list of destinations, instead all I
> want is a list of 1-hop neighbours who can be accessed directly. Then from
> that list I would determine which of these is the most suitable as the next
> hop (obviously in my case this will require other stuff, for instance adding
> GPS coordinates to the packet header and storing this in the subscriber
> field) and forward the packet to that node, and so on until the packet has
> been delivered (or has to be dropped).
>
>
> The main questions I have are:
>
>
> Exactly where is a packet received and the node to which it should be sent
> decided?
>
> i.e. if I want to decide which node to forward a packet to where should I
> decide this?
>
> I came across a method called 'overlay_mdp_receive' in mdp_client.c, is this
> maybe what I'm looking for?
>
> Concerning the subscriber entity, is there an actual table/list/array of
> these – as I can't seem to find one?
>
> i.e. a list of neighbours/known nodes/destinations?
>
>
> I apologise if my questions and this email aren't very well-worded.
> Essentially what I'm looking for is some advice/guidance on exactly how
> routing (determining intermediate nodes for nodes which cannot be reached
> directly) and forwarding (looking at a received/originated packet and
> determining which node to send it to) is done. As I indicated earlier in
> this message, there are a few functions/structs I have stumbled across that
> I think are relevant and I have made some guesses at what they are doing, so
> I would appreciate if someone could correct/expand on my guesses.
>
>
> Any help/guidance I have would be greatly appreciated. It goes without
> saying that any code I develop myself I will happily share, and any
> issues/bugs I come across with Serval will be reported.
>
>
> Thank you for taking the time to read this message, I'm sorry it's a bit on
> the long side but hopefully I've made myself clear.
>
>
> Regards,
>
>
> Fraser
>
>
> Ps. I realise this topic has been covered before, but I think some of the
> questions I am asking in this message are new.
>
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----- End forwarded message -----
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Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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