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

Eugen Leitl eugen at leitl.org
Thu Dec 13 11:44:38 PST 2012


----- Forwarded message from Fraser Cadger <cadge01 at googlemail.com> -----

From: Fraser Cadger <cadge01 at googlemail.com>
Date: Thu, 13 Dec 2012 19:27:35 +0000
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

Jeremy,

Thanks again for your help. I appreciate that as you said, it can be
difficult trying to help people understand your own software. I decided to
send my initial message because I had tried reading through the code
several times, picking up bits and pieces, but being unsure about exactly
how things fitted together. I genuinely found your previous comments very
illuminating and useful, and when I compared your comments with the code a
great deal of it made sense and I then started thinking about ways of
implementing my own routing. In fact, I produced a short report for my
supervisor based on this and detailing the steps I intended to take to
implement our routing protocol on Serval. I was about to dive in with this
when I realised that I did not really know how the update process worked
and that this was a very important factor! So I looked at the code again
and tried to work my way through, making notes, those notes were then used
in my last post. As you said in your comment it is better to work with
people who are making independent progress, so that was why I tried to
explain my understanding instead of jumping straight in with questions.
Actually, I have been meaning to check back with the current version of the
software in repository, as I noticed a few functions were in different
locations and you mentioned that the self announce had been redesigned, so
this is something I'll look at.

Ok, so I'll now try to explain what I intend to do with routing.

Our overall aim is to develop a protocol called Geographic QoS Predictive
Routing (GQPR) which uses geographic/location/mobility information as well
as other context to make QoS predictions for neighbours, these predictions
are then used as the basis for forwarding decisions. GQPR is intended to
act as the routing element in a framework called Geographic QoS
Peer-to-peer Streaming (GQP2PS) which will run on WiFi devices (at the
moment a testbed of six Android phones + my own Android tablet)
and facilitate the streaming of both live (i.e. video call) and on-demand
(i.e. recorded video) between the devices. The intended use case is for
disaster recovery scenarios, for instance someone with basic medical/first
aid training comes across an injured person but is not exactly sure what
course of action to take, so they can initiate a video call with a doctor
located at base station (or anywhere else covered by the network) who is
able to view the patient and perform a diagnosis and if necessary supervise
the treatment. This is just on scenario, it could also be used for
repairing infrastructure where an engineer supervises and guides the repair
process. Indeed, the technology could be used for many purposes unrelated
to disaster recovery, but we are focussing on this to give us a specific
aim, although the ideas for the technology actually came before the idea
for application.

What we have at present is the building blocks of GQPR which we have called
GPR - Geographic Predictive Routing - GPR contains some of the main
elements of what will become GQPR but not all of the functionality. At
present GPR uses location predictions (provided by an Artificial Neural
Network) as well as some other factors (congestion level, radio range, a
metric we'd developed called neighbour range (this relates to the positions
of a neighbour's neighbours), amongst others to make forwarding decisions.
I think explaining GPR would make the most sense if I first explained how
basic geographic routing worked and then discussed our modifications to
it. As the name implies, geographic routing makes forwarding decisions
based on node's geographic locations instead of logical addresses. The most
basic form of geographic routing is greedy geographic routing.

Greedy geographic routing works as follows:

   - A packet is received, the node inspects the packet and records the
   location of the destination
   - The node then calculates the physical distance between itself and the
   destination
   - All of the node's neighbours are then compared against this distance
   - The one with the shortest distance to the destination is selected as
   the next hop

Greedy geographic routing does not create end-to-end routes (which is why
some people prefer the term forwarding, although the terms are
used interchangeably in literature), instead nodes are forwarded on a
per-hop basis using the greedy criterion. This does seem a bit similar to
BATMAN, where packets are also forwarded on a per-hop basis, however the
big difference is that in greedy geographic routing nodes only have
knowledge about their directly connected neighbours (i.e. those within
their radio range) and they do not know about destinations. So while BATMAN
nodes know of the destination node's existence (but only know the next hop
to reach it and not a full route), greedy geographic routing nodes know
nothing about the destination (aside from its address and location) and
simply make the forwarding decision based on which neighbour is closest to
that destination. The idea being that by shortening the geographic distance
at each hop we are finding the quickest way to reach a destination.

Obviously this does not always hold true in practice, and their is a
problem known as the local maximum where nodes are forced to drop packets
if they cannot find a neighbour closer to the destination than themselves
(to prevent loops, the packet cannot travel physically backwards). However,
greedy geographic routing is relatively lightweight, and highly localised
as nodes are only required to know their directly connected neighbours and
so are not affected by topology changes at the other side of the network
(although this does mean nodes will sometimes forward packets to
unreachable destinations). There have been quite a few modifications and
variants to standard greedy geographic routing, some of which are more
similar to conventional routing protocols by establishing end-to-end routes
(but using geographic information to do so), while others focus on
particular problems (QoS, energy consumption, security, etc.) I spent a
large period of my first year surveying different geographic routing
protocols and published this as a paper if you're interested in finding
more about geographic routing (shameless plug for my own paper!!!);
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6238283&tag=1 .

Getting back to basic greedy routing, obviously nodes need a way of
discovering their immediate neighbours as well as finding the locations of
other nodes who they are not directly connected to. The first is achieved
through the sending of beacon messages at set periods of time, beacons are
just your bog standard hello-type messages except they contain the
coordinates of the sending node. Neighbours are stored in a dedicated
neighbour table along with their coordinates, this neighbour table is the
only table nodes store. The second is more difficult, but typically
involves the use of something called a location service which usually
involves certain nodes being designated as location servers. Regular nodes
periodically send location updates to these servers and when a node needs
to send a packet to a destination it is not connected to it will query the
relevant location server for that node's destination. Exactly how the
location service works depends on the particular scheme being used.

So now to talk about GPR.

Structurally GPR is very similar to classical greedy geographic routing, it
was originally implemented on top of the code developed by Karp and Kung
for their protocol known as GPSR (Geographic Perimeter Stateless Routing).
What we did was first implement a neural network algorithm which predicts
the future location of a device based on two previous coordinates, and the
times they were recorded at. Tests found that the location prediction
algorithm was able to accurately predict the future locations of
neighbouring devices with an error of less than 1m in most instances. At
present the use of the location prediction algorithm is relatively simple,
all we are doing is looking at our neighbours two previous locations (and
timestamps) and using these to determine what position the neighbour will
be in when we send the packet. This allows us to use greedy geographic
routing, but with the ability to cope with changes in location. In order to
avoid sending packets to nodes that are too far away, we always check their
predicted location against our transmission range to ensure they are within
it. So in addition to using the location predictions we developed a
modified congestion control algorithm which weighs the neighbour's
congestion level against the distance between the neighbour and other
factors such as the reliability of the node's information. The calculation
is stored as a value t, and then we evaluate the neighbour's neighbour
range (i.e. the physical range covered by its own neighbours), and if that
neighbour is the best it becomes the candidate node and the other node's
values for t are compare against it, and so until we finish the loop and
end up with the best neighbour (or none in which case we'll drop the
packet).

Another factor is that because we are using location predictions we do not
need node updates as frequently as geographic routing normally dictates. So
while ordinary geographic routing protocols usually send these messages
over 0.5s we send them every 13.6s which reduces the amount of control
traffic. This is a number based on ns-2 experimentation and it is likely
that in real wireless network it will need to be more frequent, but we
still expect it to be a lot less frequent than the conventional 0.5s.

It might not seem like a huge modification, but it performs a lot better
than unmodified greedy geographic routing in simulations of video calling
and streaming traffic, and compares well with AODV, DSR, and DSDV. If you
look at the bulletin points I typed above to describe greedy geographic
routing, it is very similar except our means of calculating the best next
hop is slightly different.

As I said earlier, I had thought about and discussed with my supervisor how
we could go about implementing GPR in Serval. We both decided that it was
best to implement basic greedy geographic routing first and then work from
there. There is a possibility that the NN algorithm we used in simulations
will be too resource-intensive for the phones we are using and will either
need to be modified or replaced with a different approach (we have some
backups), so our plan is to implement basic geographic routing first,
conduct separate experiments of the NN algorithm on the phones, and if
everything goes well combine them, build GPR, and then continue working on
the phones and simulations to create GQPR.

In terms of implementing basic greedy geographic routing, I think making
the forwarding decisions could be quite simple. When we are in
overlay_stuff_packet(), instead of looking at the destination's next_hop
field we would call a function which would loop through our list of
neighbours and find the neighbour closest to the destination and select
that as the next hop for the packet (or drop it if we can't find a suitable
next hop). So that part seems quite simple.

However, what seems a bit more complicated is implementing the beaconing
messages. I can see two ways of doing this; keep the current Serval system
of self announcements, self announcement acks, and node announcements and
simply add locations to these messages. The other would be to get rid of
node announcements, and just use self announcement and self announcement
acks to transmit location information between directly connected nodes.
This actually brings me to a question I forgot to ask; if self announce
messages are regularly sent why do we need to ack these? If we are sending
our own acks and receiving acks from other neighbours do we need an
explicit reply? I.e. if we just send self announce messages, but after a
period of time we don't get any self announce messages from neighour x, we
remove them from our list of neighbours. Slight digression.

Both of the two approaches I described have their advantages; the first
is ostensibly simpler as we don't need to stop anything being sent, while
the second means that we are getting something closer to the 'traditional'
beacon approach used in geographic routing as well as avoiding the
transmission of node announcements and the recording of indirect nodes
which we will just ignore. On the other hand, if we keep node announcements
this could actually act as a 'surrogate' for the location service if we
include their locations. So if for instance we have three nodes; a, b, and
with b being connected to a and c, a only being connected to b, and c only
being connected to c. If b advertises c to a and a to c, then a is able to
know the location of c (and vice versa) despite not being connected and
without the need for a location service (obviously this is a trivial
example as in this instance a can only reach c via b, so b is not really
compared against anyone else).

I realise that Serval's implementation of BATMAN is intended to limit the
number of nodes a particular device knows about to avoid bandwidth going
out of control, but as we only have a testbed of six this shouldn't be a
problem for our experiments. Although we would obviously need to rethink
for future uses which involve larger networks. So I imagine this would
really be a temporary way of avoiding a location service, and in the long
run we would still try to implement an explicit location service (possibly
based on Serval Maps which Paul pointed me in the direction of). However, I
did discuss the idea of retaining some BATMAN elements in our
implementation (where we have some knowledge of nodes we are not directly
connected to), instead of going for a pure greedy geographic routing
approach (where we only know about our one-hop neighbours) as we are by no
means obliged to use a particular approach and our ultimate aim is
performance. Therefore it is possible that this will lead to us developing
a hybrid approach between GPR/GQPR and BATMAN.

However, I think for now the best approach might be for us to try both
messages of messaging and see how they perform. I'm meeting my supervisor
tomorrow, and I'm going to discuss this with him. I've read through your
comments fully, so I think I should be able to start planning how I will
modify Serval announcement/advertisement messages and then try to get
something running next week. Unfortunately I am going back home for the
Christmas holidays, and I will only have access to an Android tablet and a
Windows laptop (which belongs to someone else!), so I'm not sure how much
work I'll get done until January, but if I can get Android NDK up and
running on the laptop I might be able to get some routing work done.

Hopefully, the explanation of my work makes sense (or enough sense), and as
I indicated before I am happy to share any code I develop myself (I have
spoken about this with Paul). So in addition to developing my routing
protocol I intend to work on video calling, and Paul stated this was
something the Serval team wanted to include, so if I get that working I am
more than happy to put the code in the repo.

Regards,

Fraser

On 13 December 2012 07:12, Jeremy Lakeman <jeremy at servalproject.org> wrote:

> > I'd much rather help someone who can demonstrate that they've tried to
> > understand the problem on their own.
>
> I suppose I should clarify, just in case you take that sentence the
> wrong way. It's very encouraging that you are showing an interest in
> understanding our software for yourself. It can be very frustrating
> working with people who fail to demonstrate any independent progress.
>
> Feel free to shoot off an email with any difficult unanswered
> questions you have at the end of each day. With timezone differences
> it's likely we can answer them before you start in the morning. I
> don't mind taking half an hour here or there to solve a problem that
> might take you a couple of days to solve otherwise.
>
> Also note that our software has been undergoing quite rapid changes
> lately. Some of the research and understanding you have gained may
> already be obsolete.
>
> If you can let me know some more details about your previous routing
> experiments, I'll probably be able to refactor the interfaces to our
> existing routing layer to make it easier for you to make the changes
> you need. I'm intending to replace the routing layer eventually
> anyway, so this will certainly be a productive use of my time.
>
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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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