Why Multicast over Ham Radio.

Have you ever watched the LAN send the same chat message, or
the same PBBS message over and over again to multiple recipients
at the same time?  This is such a waste, it seems like someone
should be working on a solution.  The answer is that yes, people
have been working on this, and the result is IP Multicasting. 

IP Multicast is a technology designed to allow broadcast packets to be sent
across many lans on the internet.  Unlike normal broadcast packets, 
which are limited to the local networks, multicast packets can travel 
through routers and over the internet to everyone interested in 
receiving that data.  

Applications where data needs to be sent from a single source to
many destinations are candidates for using IP multicast rather
than a unicast IP program.  For example, with
a traditional chat room system, typically a set of computers
act as chat servers.  Users connect to the chat servers, and
then the chat servers negotiate among themselves, how data is
transmitted among them.  For each user connection to a chat
server, the server needs to send out a packet for each user --
even if they are on the same LAN.  The connections between
chat servers result in difficult code in the servers themselves
to arrange for routing of packets.  

In comparison, with with an ip multicast chat room, users would broadcast
chat packets, to all users with a single 'sendblock' command.
The routers would be responsible for delivering the chat messages
to all users.  The routers should be smart enough to eliminate
duplicate packets sent over the same data pipes.  IP Multicast
is a unreliable, UDP, style mechanism, so some system needs to
be added to ensure the reliable, or even likely, delivery of
packets. With an IP Multicast chat system, there would be no
chat servers.  All of the logic for distributing the chat packets
across the network are moved down to the OS level.   As multicast
routers are added to the system, chat access then is automatically
available to each new area -- since, no new chat server is required.

That is the job of scaling the size of the application is moved to
the network -- and the application itself can remain relatively
simple.  The ease of programming multicast applications may then
result in more experimtation since programmers can create
the applications, without having to build a network of servers
across the planet -- provided there's a multicast network in 
place.   

I believe IP Multicast is a good idea for ham radio networks
-- as the data that travels over ham radio is often of a
broadcast nature.  Whether IP Multicast in its current form 
is suitable for ham radio networks remains to be seen -- 
however, the programming API, and socket level interfaces 
to IP Multicast are well-designed, and should provide a base for 
applications.  Both in Windows and Linux.


Here are some basic multicasting concepts that I've picked up.


mrouted

The program mrouted provides for routing of multicast packets.
It's typically used to connect multiple nets, say an ax25 net
and an ethernet, or an ethernet to another ethernet via a
tunnel.  Mrouted doesn't actually route the packets, but instead
directs the Linux kernel to forward the packets accordingly. 


Membership

IP Multicast works according to a concept called 'Membership' 
A setsockopt command called IP_ADD_MEMBERSHIP specifies which
of the multicast IP addresses that machine receives packets
from.  An application just specifies that it wishes membership
in one of the multicast IP's in the range 224.0.0.0/4, and
then the routers take care of the rest, and deliver all packets
for that multicast IP address that are sent anywhere on the
network.  


IGMP

IGMP packets are used by applications to indicate to the
routers how multicast packets should be routed.  


What are the advantages?

1.  IP Multicast is optimizized so that packets are sent only
to the routers that need them.  The lastest mrouted 3.9 implements
a mechanims to reduce this traffic.  IP Multicast will only send
a packet once over a tunnel, even if there are multiple recipients
on the other end.  Compare this to traditional IP programs, which
will broadcast a duplicate copy of the same data to everyone on the
other end of the pipe.

2.  IP Mulitcast is the result of lots of research, and testing.
And, should be stable enough for ham radio networks.  IP Multicast
has in it a level of maturity that would be extremely
difficult to duplicate with a ham radio network built from scratch.

3.  IP Multicast has a very simple programming interface for the
applications programmer.  Most of the complexity for distributing
of packets over a complicated network is placed in the operating 
system.

4.  By building applications using the existing IP Multicast programming
interface, these applications can then take advantage of future
advances in IP Multicast routers.
 
5.  IP Multicast, though not universally supported by Internet
service providers, is widely supported by OS and software venders.
So, Win98 and Linux both have native support for IP Multicast.


What are the disadvantages?

1.  mrouted, the multicast router is only available for UNIX style
machines.  Win98 machines can run multicast applications, but as
far as I'm aware, they cannot act as routers.

2.  Multicast is untested over low speed ham radio links.  There are 
certainly tuning adjustments which will need to be made.

3.  There aren't many multicast applications at this time. 

4.  Multicast routing isn't supported by most ISP's.  Anyone interested
in exerimenting in multicast routing, will need to build their own
network.    

5.  IP Multicast is an IP based technology, and inevitably involves
the overhead associated with IP -- as compared to native AX25 systems.

6.  IP Multicast does not guarantee delivery, so applications, developed
for TCP/IP or reliable packet delivery will need substantial reworking
to use IP multicast efficiently.


How do you get started?

First, I believe you want to be running a Linux version with multicast
enabled, multicast routing enabled and ax25 also enabled.  If you haven't
done ham radio networking before, get ax25 working first, and then get
your ax25 devices running first.  Go to linux.org, and look in the
support section for the HOWTO's.  There you'll find the ax25 howto,
and the multicast howto.  Make sure to include IP Multicasting and
IP Multicast routing when you build your kernel.  (These options
seem relatively benign, even if you don't use IP Multicasting.) 
Also, I turn on IP Tunneling, though I'm not absolutely sure this
is necessary.  My testing is on Linux 2.2.x based kernels.  


Once, your AX25 network is running, you'll want to setup for
IP Multicast. First, you need to set MULTICAST flag for all devices
that use multicast.  This is done with the following command: 

'/sbin/ifconfig ax0 multicast'

Do a command '/sbin/ifconfig' and you should see the word 
MULTICAST following each interace that you intend to use for
multicast.  My eth0 devices comes up with multicast on by
default.  


Then, you need to change your rouring.  Make sure to 

'add -net 224.0.0.0 netmask 240.0.0.0 eth0'

and

'add -net 224.0.0.0 netmask 240.0.0.0 ax0'

where eth0 is your local ethernet, and ax0 is your ham radio network.
The net of 224.0.0.0/4 is a special set of ip addresses set aside for
ip multicast.


Then, you'll need to get mrouted up and running.  I'll try to make
versions available here, though mrouted is also available on the net.
For mrouted to be useful, you'll need at least 2 networks or a 
tunnel.  That is for example an ax25 network and an ethernet network.
Or, an ethernet network and a tunnel to another ethernet network.

I have a beacon test program which will send out multicast packets.
With a beacon monitoring program, which I'm working on now, you should
be able to see your beacons travel from one net to another.  If you're
connected to another user via a tunnel, you should see his beacons also.
 
Then send me an email, and either I'll connect your multicast network
to my system, or hopefully find someone near you who can connect you.  
(I'm at cathryn@junglevision.com)


Here's how I start IP multicast.

/sbin/route add -net 224.0.0.0 netmask 240.0.0.0 ax2
/sbin/route add -net 224.0.0.0 netmask 240.0.0.0 eth0
/sbin/ifconfig ax2 multicast
/usr/sbin/mrouted


You also might want to try

/usr/sbin/mrouted -d

To watch mrouted perform its functions.


If /usr/sbin/mrouted looks functional, it's time to setup the tunnels.  
The mrouted.conf file should be at /etc/mrouted.conf.  Follow the
instructions in mrouted.conf.  (I'm currently working this out myself
-- that is how to properly setup tunnels.)


Finally, watch out for firewall issues.  

This one bit me for awhile. I run IP Multicast over 
an inteface with masqueraded IP addresses.  The second 
line sets up the IP Masquerading, and the third line 
is necessary, in this example, to allow the interface 
to forward IP Multicast packets.  


/sbin/ipchains -P forward DENY
/sbin/ipchains -A forward -s 192.168.1.0/24 -j MASQ
/sbin/ipchains -A forward -d 224.0.0.0/4 -j ACCEPT


IP Multicast on Win98

I have been testing small IP Multicast applications on Win98 and 
AX25 using the SV2AGW software.  This combination works
pretty good, and can talk to Linux IP multicast with no problems.

Win98 works well for running Multicast applications, but does
not work as a multicast router. 

The MSDN website has some sample code for IP Multicast if 
you're interested in creating multicast applications for 
Win98.

 
IP Multicast with NOS

I don't run currently run any NOS code, so I'm not sure if
these work or not.  If you're a NOS programmer, give me the
scoop, and I'll put that information here. 


What is the Future?

1.  As a short term goal, I'd say it's important to get a few tunnels
up and running, and verify the basic functionality of mrouted over
ax25.  Let's learn the basics of running IP Multicast.

2.  Develop beacons and beacon monitoring as an application.  Sending
and monitoring beacons is trivial from the programming side, but will
give us some information on how well the tunnels are working.  Also,
beacons can possibly serve some use for distributing information widely
across the ham radio networks around the world.  A beacon system, might
be used to find people to meet on 20meters -- or to advertise a ham
web page or something.  Using ip multicast for beacons will also set
up the right expectations about what the network is capable of doing.

3.  Develop a chat system.  IP Multicast will allow us to make more
scalable chat systems.  Especially chat systems where many users 
are in the same chat room.  Chat is slightly more advanced than
beaconing, since there's an expectation that users will see all the
messages that people type -- and this requires some kind of information
to be stored by the users or in the network. 

4.  Adapt existing IP Multicast multimedia applications to the ham
radio network.  These applications will probably run natively, but
may require more bandwidth than we can give them.  However, assuming
some kind of bandwidth improvement in ham radio networking, (well, 
we can hope!) IP Multicast is a natural mechanism for delivering,
for example, Newsline audio broadcasts to a wide variety of sites. 

Cathryn Mataga
KE6I

http://ke6i.com
cathryn@junglevision.com