So I think that thinking about whiteboard and WebWisdom in Tango is
quite fruitful as it reveals some useful general points about
robustness and archiving of Tango Sessions. 
Firstly note one cannot assume the client applet is there. It takes an
unknown time to load and unload. Further user can destroy it or its
connection to webwisdom with an innocent click of their mouse. Thus one
needs strategy used in shared browser and WebWisdom where one resends
URL every 30 seconds or so. One checks URL of client and changes if
the new value is different from old one. This covers case where slide
changes and also case where user or system has destroyed connection.
For
applet webwisdom client will create it if necessary and if it exists
just load a new image without reloading applet. 

Now in whiteboard version of WebWisdom we assume 4 layers
1: background Image defined by Image URL
2: Master annotations maintained on master client but sent to all
clients in session
3: Separately for each client, user annotations maintained on each
machine and NOT sent anywhere
4: Telecursor/marker maintained by master and sent to all clients.
Distinguished from layer 2 as not preserved in save operation

Layer 1 is created by WebWisdom by calling applet either in create or
update mode.
Layer 2 and 4 are updated by whiteboard separately from WebWisdom;
however for late comers and clients that screw-up, we need an interface
to request update of these layers on a particular client. This is done
by WebWisdom calling at its or users request an appropriate method in
client applet. This applet communicates with master through "bytes"
channels.

For robustness I need an interface to be able to extract "state" of
layers 1 2 and 4. I suggest you label state by a 5-vector (Overall Info
gives a fifth label and lets include
layer 3 for completeness) which I can determine by calling a suitable
method. See below for
discussion of state label

Database is created for near future by automatic or user requested save
of applet to disk. The disk used is on machine where annotations
create. We must label the object saved for two reasons. Firstly it will
help robustness as I can check from state if all clients do indeed see
same state. Secondly we must be able to label each file uniquely so we
can properly store into a real database and also check that if reread a
file after a crash, we have read back correct version as while reading
back master might have changed URL. So what is a good database record
label?
First note we do have a vector or tuple. Label is

Global label: Some Session Name created by Tango; datetime started and
              perhaps a special user optional field
Layer 1 label: URL and datetime fetched
Layer 2 label: datetime last updated
Layer 3 label: datetime last updated
Layer 4 label: datetime last updated

Note this type of structure holds for other applications and says Tango
needs to generate unique ID's for sessions to label data they produce.
These labels need to be distributed to clients so they can be used to
label consistently related information from each client. In whiteboard,
the applet generates datetime labels but Tango/WebWisdom specifies rest
of label.

Note that WebWisdom can generate naturally save directory and file name
and it may be good to have a method call to set this.

Finally can I tell what type of data that is being sent from channel
or is this always 41?