documentation/specification/protocols.txt

i-scream Protocols
==================

Contents
--------
 - Conventions

 - Host Connection Protocol
 - Host Heartbeat Protocol
 - Host Data Protocol

 - Client Control Protocol
 - Client Data Protocol

Conventions
-----------
All protocols in this document assume a valid connection of
the appropriate type has been made, and that data streams
are already available.  All strings should, and will be,
terminated with a \n (newline character) to indicate the end
of the message.  All messages are sent as ASCII Strings.

Fixed protocol messages will appear in [] brackets, if there
is a variety of options they will be separated with the |
character.  For example:

    [OK|ERROR]
  
Indicates that the message "OK" OR the message "ERROR" will
be sent depending on the result of the last action.

Data messages will appear in {} brackets, where the name in
the brackets indicates the type (as in kind, not data type)
of data that will be returned.  For example:

    {lastmodified}

Indicates that data for use as 'last modified' will be
returned.  

If [ERROR] is sent back at any time, this indicates that all
communication is over. EXCEPT where otherwise specified!

Host Connection Protocol
------------------------
The initial connection of a host to the i-scream server is
through the FilterManager.  A Host gets its configuration
and then gets assigned a Filter to connect to and start
sending data.

The port number of the FilterManager is fully configurable,
however the default at this time is 4567.

Host         (direction) Server         Comment
------------------------------------------------------------
[STARTCONFIG]---------->                Requests to start
                                        receiving config
                                        information
           
             <---------- [OK|ERROR]     If the server ok's
                                        the request or not

[LASTMODIFIED]---------->               Asks when the config
                                        was last modified
                                        (used when checking
                                        if the config has
                                        changed)
           
             <---------- [{lastmodified}Returns a long int
                         |ERROR]        time since epoch
                                        eg, 123456789
                                        To indicate what
                                        format this is in,
                                        I quote from the
                                        Java 1.3 JDK API
                                        "measured in milli
                                        -seconds since the
                                        epoch"
                                       
[FILELIST]   ---------->                Asks the server for
                                        the list of files
                                        that were used to
                                        build the config
                                        (used when checking
                                        if the config has
                                        changed)
      
             <---------- [{filelist}    Returns a semi-colon
                         |ERROR]        separated list of
                                        filenames
                                        eg,
                                        a.conf;b.conf;c.conf

[FQDN]       ---------->                Asks the server to
                                        send the FQDN (fully
                                        qualified domain
                                        name) of the
                                        connecting host.
                                        
             <---------- [{fqdn}        Returns the FQDN of
                         |ERROR]        the connected host,
                                        or error if it can't
                                        be looked up.

             ********** LOOP START **********
             This loop reads configuration
             properties from the config

[{property}] ---------->                Sends the name of a
                                        property to be
                                        retrieved from the
                                        config file
                                        eg, UDPUpdateTime
           
             <---------- [{value}|ERROR]Returns the value of
                                        the requested config
                                        property
                                        eg, 120
                                        If it is unable to
                                        find the requested
                                        property it returns
                                        ERROR to indicate
                                        that fact, but
                                        communication still
                                        proceeds
                                      
             ********** LOOP UNTIL **********
             The loop continues until the host
             sends the following message
           
[ENDCONFIG]  ---------->                Indicates that the
                                        host requires no
                                        more config
           
             ********** LOOP  END **********
             Communication continues
           
             <---------- [OK]           Indicates that the
                                        server is ready to
                                        proceed
                                      
[FILTER]     ---------->                Asks the server to
                                        send it the host
                                        information of a
                                        filter that it
                                        should connect to
                                      
             <---------- {filter}       Returns a semi-colon
                                        separated list of
                                        FQDN hostname, UDP
                                        port number and TCP
                                        port number that a
                                        configured Filter is
                                        running on and
                                        assigned to the
                                        calling host
                                        eg,
                                        raptor.ukc.ac.uk;
                                        1234;5678

[END]        ---------->                Indicates to the
                                        server that the host
                                        has finished an will
                                        disconnect
           
             <---------- [OK|ERROR]     Indicates that the
                                        server is either ok
                                        or that it thought
                                        there was an error
------------------------------------------------------------
      
Host Heartbeat Protocol
-----------------------
When a host is configured after it has connected it should
obtain a property TCPUpdateTime.  This indicates how often
a host should send a "Heart Beat", which is a pro-active
connection to the server by the host to indicate that it is
still alive.  This "Heart Beat" also allows a host to see
its configuration has changed and act accordingly.  In a
well written host this should just be a case of dropping out
of a loop and heading back to the start (connecting to the
filter manager).

Host            (direction) Server      Comment
------------------------------------------------------------
[HEARTBEAT]     ---------->             Starts the
                                        heartbeat protocol
                                        off
                                      
                <---------- [OK|ERROR]  Indicates that the
                                        server is ok or not

[CONFIG]        ---------->             Indicates that the
                                        host wants to check
                                        its config

                <---------- [OK|ERROR]  Indicates that the
                                        server is ok or not

[{filelist}]    ---------->             Send a semi-colon
                                        separated list of
                                        filenames
                                        eg,
                                        a.conf;b.conf;c.conf
                                        This should be
                                        identical to the one
                                        received in the
                                        connection protocol

                <---------- [OK]        Indicates that the
                                        server is ok

[{lastmodified}]---------->             Returns a long int
                                        time since epoch
                                        eg, 123456789
                                        This should be
                                        identical to the one
                                        received in the
                                        connection protocol
                                        To indicate what
                                        format this is in,
                                        I quote from the
                                        Java 1.3 JDK API
                                        "measured in milli
                                        -seconds since the
                                        epoch"

                <---------- [OK|ERROR]  The server then
                                        checks all the files
                                        in the file list to
                                        see if they have
                                        been modified more
                                        recently than the
                                        lastmodified value.
                                        If they HAVE that
                                        indicates that the
                                        configuration has
                                        changed and the host
                                        should re-configure,
                                        thus it sends ERROR.
                                        If the files remain
                                        unchanged the server
                                        will return OK to
                                        indicate the host
                                        should continue as
                                        before

[ENDHEARTBEAT]  ---------->             Indicates to the
                                        server that the host
                                        has finished an will
                                        disconnect
              
                <---------- [OK|ERROR]  Indicates that the
                                        server is either ok
                                        or that it thought
                                        there was an error
------------------------------------------------------------

Host Data Protocol
------------------
The UDP data packets that are sent by the host contain
simply XML data.  For information on the format of this
data see the XML via UDP specification document at:

http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt

Client Control Protocol
-----------------------

The client control protocol channel is opened by the client
and allows a variety of actions to be carried out by the
client at anytime.  Unlike previous protocols, this is NOT
sequential, all of the requests can be carried out in
any order.

All client protocols are backwards compatible, and the
version is shown by the protocol identifier.

There are three sections to this protocol.

1) Initialise (sent only at start)
2) Send command(s) - unlimited number in any order
3) Disconnect (sent only at end)

If at anytime the client sends something the server does
not understand, an [ERROR] will be sent

All are indicated with <> brackets.

v1.0 - Protocol identifier: "PROTOCOL 1.0" (without quotes)
       Supported commands:
            STARTCONFIG
            STARTDATA
            STOPDATA

Host         (direction) Server         Comment
------------------------------------------------------------
<initialise>
             <---------- [{protocol}]   The server sends the
                                        protocol identifier
                                       
[{name}]     ---------->                The client sends its
                                        "name".  This name
                                        is used to identify
                                        the type of client
                                        to the system and
                                        obtain its config
                                        eg, Conient
            
             <---------- [OK]           Indicates the server
                                        is ok to proceed
                       

<command #1> (allows client to obtain its configuration)
[STARTCONFIG]---------->                Tell the server we
                                        want to start this
                                        command
            
             <---------- [OK]           Indicates the server
                                        is ok to proceed
            
             ********** LOOP START **********
             This loop reads configuration
             properties from the config

[{property}] ---------->                Sends the name of a
                                        property to be
                                        retrieved from the
                                        config file
                                        Clients can obtain
                                        host information
                                        eg,
                                        Host.UDPUpdateTime
                                        Otherwise it must
                                        prefix requests
                                        with "Client."
                                        All other requests
                                        will be ignored as
                                        if it was unable to
                                        retrieve the
                                        property
                                       
             <---------- [{value}|ERROR]Returns the value of
                                        the requested config
                                        property
                                        eg, 120
                                        If it is unable to
                                        find the requested
                                        property it returns
                                        ERROR to indicate
                                        that fact
                                       
             ********** LOOP UNTIL **********
             The loop continues until the client
             sends the following message
            
[ENDCONFIG]  ---------->                Indicates that the
                                        client requires no
                                        more config
            
             ********** LOOP  END **********
             Communication continues
            
             <---------- [OK]           Indicates that the
                                        server is ready to
                                        proceed

<command #2> (tells the server to start the data link)
[STARTDATA]  ---------->                Tell the server we
                                        want to start this
                                        command
           
             <---------- [{portnumber}  The server then sets
                         |ERROR]        itself listening for
                                        a connection on its
                                        data link socket for
                                        this client.  It
                                        returns the port no.
                                        that it is listening
                                        on eg, 12367
                                        If the data link is
                                        already started the
                                        server will return
                                        ERROR
            
             <---------- [OK]           Indicates the client 
                                        has successfully 
                                        connected to the 
                                        data socket.

<command #3> (tells the server to stop the data link)
[STOPDATA]   ---------->                Tell the server we
                                        want to start this
                                        command
                                        The server then
                                        shuts down the
                                        data link to the
                                        client

             <---------- [OK|ERROR]     Returns OK is the
                                        server is ready to
                                        proceed, or ERROR
                                        if the data link
                                        was not open in the
                                        first place

<disconnect>
[DISCONNECT] ---------->                Tells the server the
                                        client wants to
                                        close the control
                                        link
                                       
             <---------- [OK]           The last word from
                                        the server, it will
                                        disappear after 
                                        this, and close the 
                                        data link if 
                                        required
------------------------------------------------------------

v1.1 - Protocol identifier: "PROTOCOL 1.1" (without quotes)
       Supported commands:
            SETHOSTLIST

Host         (direction) Server         Comment
------------------------------------------------------------
<command #4> (indicates to the server which hosts the client
              wants data from)
[SETHOSTLIST]---------->                Tell the server we
                                        want to start this
                                        command
            
             <---------- [OK|ERROR]     The server will
                                        return an ERROR if
                                        the data link is
                                        open, as a host list
                                        must ONLY be set
                                        if the data link is
                                        closed
                                        If the server is
                                        ok to proceed with
                                        the command it says
                                        [OK]

[{hostlist}] ---------->                This is a semi-colon
                                        seperated list of
                                        FQDN hostnames that
                                        the client wishes to
                                        recieve data from
                                        eg,
        raptor.ukc.ac.uk;killigrew.ukc.ac.uk;chalk.ukc.ac.uk
                                       
                                        If the list is sent
                                        blank (or if no list
                                        is set at all) then
                                        data from ALL hosts
                                        will be sent to the
                                        client (this is the 
                                        default if no 
                                        SETHOSTLIST is not 
                                        run by the client
                                       
             <---------- [OK]           Indicates the server
                                        is ok to proceed and
                                        the host list has
                                        been accepted
------------------------------------------------------------

Client Data Protocol
--------------------
Once the data link has been opened by the control link, the
server will send XML formatted data packets separated by the
\n (newline) character.  For information on the format of
this data see the XML via UDP specification document at:

http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt

About
-----
This document was written by AJ Moore [ajm4@ukc.ac.uk] for
use by the team working on a 3rd year Computer Science
project called "i-scream". More details can be found on the
project website;

http://www.i-scream.org.uk
Revision:	1.8
Committed:	Tue Feb 27 18:45:15 2001 UTC (23 years, 8 months ago) by tdb
Content type:	text/plain
Branch:	MAIN
Changes since 1.7:	+11 -0 lines
Log Message:	Added a feature to send the FQDN of the connected host.
#	User	Rev	Content
1	ajm	1.1	i-scream Protocols
2			==================
3
4			Contents
5			--------
6			- Conventions
7
8			- Host Connection Protocol
9			- Host Heartbeat Protocol
10			- Host Data Protocol
11
12			- Client Control Protocol
13			- Client Data Protocol
14
15			Conventions
16			-----------
17			All protocols in this document assume a valid connection of
18			the appropriate type has been made, and that data streams
19	tdb	1.2	are already available. All strings should, and will be,
20	ajm	1.1	terminated with a \n (newline character) to indicate the end
21			of the message. All messages are sent as ASCII Strings.
22
23			Fixed protocol messages will appear in [] brackets, if there
24	tdb	1.2	is a variety of options they will be separated with the \|
25	ajm	1.1	character. For example:
26
27			[OK\|ERROR]
28	tdb	1.6
29	ajm	1.1	Indicates that the message "OK" OR the message "ERROR" will
30			be sent depending on the result of the last action.
31
32			Data messages will appear in {} brackets, where the name in
33			the brackets indicates the type (as in kind, not data type)
34			of data that will be returned. For example:
35
36			{lastmodified}
37
38	tdb	1.6	Indicates that data for use as 'last modified' will be
39			returned.
40	tdb	1.2
41	tdb	1.6	If [ERROR] is sent back at any time, this indicates that all
42	tdb	1.2	communication is over. EXCEPT where otherwise specified!
43	ajm	1.1
44			Host Connection Protocol
45			------------------------
46			The initial connection of a host to the i-scream server is
47			through the FilterManager. A Host gets its configuration
48	tdb	1.2	and then gets assigned a Filter to connect to and start
49	ajm	1.1	sending data.
50
51	tdb	1.2	The port number of the FilterManager is fully configurable,
52	ajm	1.1	however the default at this time is 4567.
53
54			Host (direction) Server Comment
55			------------------------------------------------------------
56			[STARTCONFIG]----------> Requests to start
57			receiving config
58			information
59	tdb	1.6
60	ajm	1.1	<---------- [OK\|ERROR] If the server ok's
61			the request or not
62
63			[LASTMODIFIED]----------> Asks when the config
64			was last modified
65			(used when checking
66	tdb	1.2	if the config has
67	ajm	1.1	changed)
68	tdb	1.6
69	ajm	1.1	<---------- [{lastmodified}Returns a long int
70	tdb	1.2	\|ERROR] time since epoch
71	ajm	1.1	eg, 123456789
72	ajm	1.5	To indicate what
73			format this is in,
74			I quote from the
75			Java 1.3 JDK API
76			"measured in milli
77			-seconds since the
78			epoch"
79	tdb	1.6
80	ajm	1.1	[FILELIST] ----------> Asks the server for
81			the list of files
82			that were used to
83			build the config
84			(used when checking
85			if the config has
86			changed)
87	tdb	1.6
88	ajm	1.1	<---------- [{filelist} Returns a semi-colon
89	tdb	1.2	\|ERROR] separated list of
90	ajm	1.1	filenames
91			eg,
92			a.conf;b.conf;c.conf
93
94	tdb	1.8	[FQDN] ----------> Asks the server to
95			send the FQDN (fully
96			qualified domain
97			name) of the
98			connecting host.
99
100			<---------- [{fqdn} Returns the FQDN of
101			\|ERROR] the connected host,
102			or error if it can't
103			be looked up.
104
105	ajm	1.1	******** LOOP START ********
106			This loop reads configuration
107			properties from the config
108
109			[{property}] ----------> Sends the name of a
110			property to be
111			retrieved from the
112			config file
113			eg, UDPUpdateTime
114	tdb	1.6
115	ajm	1.1	<---------- [{value}\|ERROR]Returns the value of
116			the requested config
117			property
118			eg, 120
119			If it is unable to
120	tdb	1.2	find the requested
121	ajm	1.1	property it returns
122			ERROR to indicate
123	tdb	1.6	that fact, but
124			communication still
125	tdb	1.2	proceeds
126	tdb	1.6
127	ajm	1.1	******** LOOP UNTIL ********
128			The loop continues until the host
129			sends the following message
130	tdb	1.6
131	tdb	1.2	[ENDCONFIG] ----------> Indicates that the
132	ajm	1.1	host requires no
133			more config
134	tdb	1.6
135	ajm	1.1	******** LOOP END ********
136			Communication continues
137	tdb	1.6
138	ajm	1.1	<---------- [OK] Indicates that the
139			server is ready to
140			proceed
141	tdb	1.6
142	ajm	1.1	[FILTER] ----------> Asks the server to
143			send it the host
144	tdb	1.2	information of a
145	ajm	1.1	filter that it
146			should connect to
147	tdb	1.6
148	ajm	1.1	<---------- {filter} Returns a semi-colon
149	tdb	1.2	separated list of
150	ajm	1.1	FQDN hostname, UDP
151			port number and TCP
152	tdb	1.2	port number that a
153	ajm	1.1	configured Filter is
154			running on and
155	tdb	1.2	assigned to the
156	ajm	1.1	calling host
157			eg,
158			raptor.ukc.ac.uk;
159			1234;5678
160
161			[END] ----------> Indicates to the
162			server that the host
163			has finished an will
164			disconnect
165	tdb	1.6
166	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
167			server is either ok
168			or that it thought
169			there was an error
170	ajm	1.3	------------------------------------------------------------
171	tdb	1.6
172	ajm	1.1	Host Heartbeat Protocol
173			-----------------------
174			When a host is configured after it has connected it should
175			obtain a property TCPUpdateTime. This indicates how often
176	tdb	1.2	a host should send a "Heart Beat", which is a pro-active
177	ajm	1.1	connection to the server by the host to indicate that it is
178			still alive. This "Heart Beat" also allows a host to see
179	tdb	1.2	its configuration has changed and act accordingly. In a
180	ajm	1.1	well written host this should just be a case of dropping out
181			of a loop and heading back to the start (connecting to the
182			filter manager).
183
184			Host (direction) Server Comment
185			------------------------------------------------------------
186			[HEARTBEAT] ----------> Starts the
187			heartbeat protocol
188			off
189	tdb	1.6
190	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
191			server is ok or not
192
193			[CONFIG] ----------> Indicates that the
194			host wants to check
195			its config
196
197			<---------- [OK\|ERROR] Indicates that the
198			server is ok or not
199
200			[{filelist}] ----------> Send a semi-colon
201	tdb	1.2	separated list of
202	ajm	1.1	filenames
203			eg,
204			a.conf;b.conf;c.conf
205			This should be
206			identical to the one
207	tdb	1.2	received in the
208	ajm	1.1	connection protocol
209
210			<---------- [OK] Indicates that the
211			server is ok
212
213			[{lastmodified}]----------> Returns a long int
214	tdb	1.2	time since epoch
215	ajm	1.1	eg, 123456789
216			This should be
217			identical to the one
218	tdb	1.2	received in the
219	ajm	1.1	connection protocol
220	ajm	1.5	To indicate what
221			format this is in,
222			I quote from the
223			Java 1.3 JDK API
224			"measured in milli
225			-seconds since the
226			epoch"
227	ajm	1.1
228			<---------- [OK\|ERROR] The server then
229			checks all the files
230			in the file list to
231			see if they have
232			been modified more
233			recently than the
234	tdb	1.2	lastmodified value.
235			If they HAVE that
236	ajm	1.1	indicates that the
237			configuration has
238			changed and the host
239			should re-configure,
240			thus it sends ERROR.
241			If the files remain
242			unchanged the server
243			will return OK to
244			indicate the host
245			should continue as
246			before
247
248			[ENDHEARTBEAT] ----------> Indicates to the
249			server that the host
250			has finished an will
251			disconnect
252	tdb	1.6
253	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
254			server is either ok
255			or that it thought
256			there was an error
257	ajm	1.3	------------------------------------------------------------
258	ajm	1.1
259			Host Data Protocol
260			------------------
261			The UDP data packets that are sent by the host contain
262			simply XML data. For information on the format of this
263			data see the XML via UDP specification document at:
264
265	ajm	1.4	http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt
266	ajm	1.1
267			Client Control Protocol
268			-----------------------
269
270	ajm	1.3	The client control protocol channel is opened by the client
271			and allows a variety of actions to be carried out by the
272			client at anytime. Unlike previous protocols, this is NOT
273	tdb	1.6	sequential, all of the requests can be carried out in
274	ajm	1.3	any order.
275
276			All client protocols are backwards compatible, and the
277			version is shown by the protocol identifier.
278
279			There are three sections to this protocol.
280
281			1) Initialise (sent only at start)
282			2) Send command(s) - unlimited number in any order
283			3) Disconnect (sent only at end)
284
285			If at anytime the client sends something the server does
286			not understand, an [ERROR] will be sent
287
288			All are indicated with <> brackets.
289
290			v1.0 - Protocol identifier: "PROTOCOL 1.0" (without quotes)
291			Supported commands:
292			STARTCONFIG
293			STARTDATA
294			STOPDATA
295
296			Host (direction) Server Comment
297			------------------------------------------------------------
298			<initialise>
299			<---------- [{protocol}] The server sends the
300			protocol identifier
301	tdb	1.6
302	ajm	1.3	[{name}] ----------> The client sends its
303			"name". This name
304			is used to identify
305			the type of client
306			to the system and
307			obtain its config
308			eg, Conient
309	tdb	1.6
310	ajm	1.3	<---------- [OK] Indicates the server
311			is ok to proceed
312	tdb	1.6
313	ajm	1.3
314			<command #1> (allows client to obtain its configuration)
315			[STARTCONFIG]----------> Tell the server we
316			want to start this
317			command
318	tdb	1.6
319	ajm	1.3	<---------- [OK] Indicates the server
320			is ok to proceed
321	tdb	1.6
322	ajm	1.3	******** LOOP START ********
323			This loop reads configuration
324			properties from the config
325
326			[{property}] ----------> Sends the name of a
327			property to be
328			retrieved from the
329			config file
330			Clients can obtain
331			host information
332	tdb	1.6	eg,
333	ajm	1.3	Host.UDPUpdateTime
334			Otherwise it must
335			prefix requests
336			with "Client."
337			All other requests
338			will be ignored as
339			if it was unable to
340	tdb	1.6	retrieve the
341			property
342
343	ajm	1.3	<---------- [{value}\|ERROR]Returns the value of
344			the requested config
345			property
346			eg, 120
347			If it is unable to
348	tdb	1.6	find the requested
349	ajm	1.3	property it returns
350			ERROR to indicate
351			that fact
352	tdb	1.6
353	ajm	1.3	******** LOOP UNTIL ********
354			The loop continues until the client
355			sends the following message
356	tdb	1.6
357			[ENDCONFIG] ----------> Indicates that the
358	ajm	1.3	client requires no
359			more config
360	tdb	1.6
361	ajm	1.3	******** LOOP END ********
362			Communication continues
363	tdb	1.6
364	ajm	1.3	<---------- [OK] Indicates that the
365			server is ready to
366			proceed
367
368			<command #2> (tells the server to start the data link)
369			[STARTDATA] ----------> Tell the server we
370			want to start this
371			command
372	tdb	1.6
373	ajm	1.3	<---------- [{portnumber} The server then sets
374			\|ERROR] itself listening for
375			a connection on its
376			data link socket for
377			this client. It
378			returns the port no.
379			that it is listening
380			on eg, 12367
381			If the data link is
382	tdb	1.6	already started the
383			server will return
384			ERROR
385
386			<---------- [OK] Indicates the client
387			has successfully
388			connected to the
389			data socket.
390	ajm	1.3
391			<command #3> (tells the server to stop the data link)
392			[STOPDATA] ----------> Tell the server we
393			want to start this
394			command
395			The server then
396	tdb	1.6	shuts down the
397	ajm	1.3	data link to the
398			client
399
400			<---------- [OK\|ERROR] Returns OK is the
401			server is ready to
402			proceed, or ERROR
403			if the data link
404			was not open in the
405			first place
406
407			<disconnect>
408			[DISCONNECT] ----------> Tells the server the
409			client wants to
410			close the control
411			link
412	tdb	1.6
413	ajm	1.3	<---------- [OK] The last word from
414			the server, it will
415	tdb	1.6	disappear after
416			this, and close the
417			data link if
418			required
419	ajm	1.3	------------------------------------------------------------
420
421			v1.1 - Protocol identifier: "PROTOCOL 1.1" (without quotes)
422			Supported commands:
423			SETHOSTLIST
424
425			Host (direction) Server Comment
426			------------------------------------------------------------
427			<command #4> (indicates to the server which hosts the client
428			wants data from)
429			[SETHOSTLIST]----------> Tell the server we
430			want to start this
431			command
432	tdb	1.6
433	ajm	1.3	<---------- [OK\|ERROR] The server will
434			return an ERROR if
435			the data link is
436			open, as a host list
437	tdb	1.6	must ONLY be set
438	ajm	1.3	if the data link is
439			closed
440			If the server is
441			ok to proceed with
442			the command it says
443			[OK]
444
445			[{hostlist}] ----------> This is a semi-colon
446			seperated list of
447			FQDN hostnames that
448			the client wishes to
449			recieve data from
450			eg,
451			raptor.ukc.ac.uk;killigrew.ukc.ac.uk;chalk.ukc.ac.uk
452	tdb	1.6
453	ajm	1.3	If the list is sent
454			blank (or if no list
455			is set at all) then
456			data from ALL hosts
457			will be sent to the
458	tdb	1.6	client (this is the
459			default if no
460			SETHOSTLIST is not
461			run by the client
462
463	ajm	1.3	<---------- [OK] Indicates the server
464			is ok to proceed and
465	tdb	1.6	the host list has
466	ajm	1.3	been accepted
467			------------------------------------------------------------
468	ajm	1.1
469			Client Data Protocol
470			--------------------
471	ajm	1.3	Once the data link has been opened by the control link, the
472	tdb	1.6	server will send XML formatted data packets separated by the
473			\n (newline) character. For information on the format of
474	ajm	1.3	this data see the XML via UDP specification document at:
475	ajm	1.1
476	ajm	1.4	http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt
477	ajm	1.1
478			About
479			-----
480			This document was written by AJ Moore [ajm4@ukc.ac.uk] for
481			use by the team working on a 3rd year Computer Science
482			project called "i-scream". More details can be found on the
483			project website;
484
485			http://www.i-scream.org.uk