[OLSR-users] Dot topology plugin

[Ignacio García Pérez]

Hi,

Today I needed to represent my network topology and voila!, there is the dot
plugin!. However, after closer examination, I've found several shortcomings.
I may be wrong, so I'll share my thoughts here before starting to code:

1- Admits only *one* connection.

2- The DOT description is issued only when network topology changes.

The second point makes this plugin pretty useless for anything but
debugging. I guess the right way of operation would be to send current
topology when a client connects to the socket, and then send new topology
only on changes. In addition, some terminator should be sent to indicate the
end of the DOT description.

Implementing this should be pretty easy. Just write the current topology
after the accept(). Accepting several connections should be not much harder:
just write to all client sockets the new DOT when topology changes.

I see also another more subtle shortcoming: what does happen to a socket
whose connection is lost when you repeatedly write() or send() ?. Suppose
the client is disconnected from the network. Though the TCP connection will
eventually time out, I guess there is a chance that, if you write a lot of
data, the write() or send() call will block, whenever the underlying buffers
are full. Since writes are done from the plugin event callback, if write()
or send() blocks then OLSRD will block.

Yeah, I know the plugin currently only accepts connections from the loopback
interface and in this situation a blocking is not very likely to happen, but
I also want to modify the plugin to allow remote connections.

I've addressed this problem before: make the plugin multithreaded, one
thread per client. The event callback creates the DOT description in a
memory buffer protected by a mutex and signals one semaphore per thread.
Threads wake up, make a local copy of the DOT (using the mutex to avoid race
conditions) and then rushlessly write from the local copy to the socket.
Note that this can use quite a lot of memory if topology is big and there
are many clients connected.

Any suggestions of which path to take ?

Thanks.


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