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

             ********** 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>
[CONNECT]    ---------->                Starts off the
                                        client protocol
            
             <---------- [{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:	1.6
Committed:	Sun Jan 28 22:01:14 2001 UTC (23 years, 9 months ago) by tdb
Content type:	text/plain
Branch:	MAIN
Changes since 1.5:	+65 -61 lines
Log Message:	A few changes in the client-server protocol.
#	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			******** LOOP START ********
95			This loop reads configuration
96			properties from the config
97
98			[{property}] ----------> Sends the name of a
99			property to be
100			retrieved from the
101			config file
102			eg, UDPUpdateTime
103	tdb	1.6
104	ajm	1.1	<---------- [{value}\|ERROR]Returns the value of
105			the requested config
106			property
107			eg, 120
108			If it is unable to
109	tdb	1.2	find the requested
110	ajm	1.1	property it returns
111			ERROR to indicate
112	tdb	1.6	that fact, but
113			communication still
114	tdb	1.2	proceeds
115	tdb	1.6
116	ajm	1.1	******** LOOP UNTIL ********
117			The loop continues until the host
118			sends the following message
119	tdb	1.6
120	tdb	1.2	[ENDCONFIG] ----------> Indicates that the
121	ajm	1.1	host requires no
122			more config
123	tdb	1.6
124	ajm	1.1	******** LOOP END ********
125			Communication continues
126	tdb	1.6
127	ajm	1.1	<---------- [OK] Indicates that the
128			server is ready to
129			proceed
130	tdb	1.6
131	ajm	1.1	[FILTER] ----------> Asks the server to
132			send it the host
133	tdb	1.2	information of a
134	ajm	1.1	filter that it
135			should connect to
136	tdb	1.6
137	ajm	1.1	<---------- {filter} Returns a semi-colon
138	tdb	1.2	separated list of
139	ajm	1.1	FQDN hostname, UDP
140			port number and TCP
141	tdb	1.2	port number that a
142	ajm	1.1	configured Filter is
143			running on and
144	tdb	1.2	assigned to the
145	ajm	1.1	calling host
146			eg,
147			raptor.ukc.ac.uk;
148			1234;5678
149
150			[END] ----------> Indicates to the
151			server that the host
152			has finished an will
153			disconnect
154	tdb	1.6
155	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
156			server is either ok
157			or that it thought
158			there was an error
159	ajm	1.3	------------------------------------------------------------
160	tdb	1.6
161	ajm	1.1	Host Heartbeat Protocol
162			-----------------------
163			When a host is configured after it has connected it should
164			obtain a property TCPUpdateTime. This indicates how often
165	tdb	1.2	a host should send a "Heart Beat", which is a pro-active
166	ajm	1.1	connection to the server by the host to indicate that it is
167			still alive. This "Heart Beat" also allows a host to see
168	tdb	1.2	its configuration has changed and act accordingly. In a
169	ajm	1.1	well written host this should just be a case of dropping out
170			of a loop and heading back to the start (connecting to the
171			filter manager).
172
173			Host (direction) Server Comment
174			------------------------------------------------------------
175			[HEARTBEAT] ----------> Starts the
176			heartbeat protocol
177			off
178	tdb	1.6
179	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
180			server is ok or not
181
182			[CONFIG] ----------> Indicates that the
183			host wants to check
184			its config
185
186			<---------- [OK\|ERROR] Indicates that the
187			server is ok or not
188
189			[{filelist}] ----------> Send a semi-colon
190	tdb	1.2	separated list of
191	ajm	1.1	filenames
192			eg,
193			a.conf;b.conf;c.conf
194			This should be
195			identical to the one
196	tdb	1.2	received in the
197	ajm	1.1	connection protocol
198
199			<---------- [OK] Indicates that the
200			server is ok
201
202			[{lastmodified}]----------> Returns a long int
203	tdb	1.2	time since epoch
204	ajm	1.1	eg, 123456789
205			This should be
206			identical to the one
207	tdb	1.2	received in the
208	ajm	1.1	connection protocol
209	ajm	1.5	To indicate what
210			format this is in,
211			I quote from the
212			Java 1.3 JDK API
213			"measured in milli
214			-seconds since the
215			epoch"
216	ajm	1.1
217			<---------- [OK\|ERROR] The server then
218			checks all the files
219			in the file list to
220			see if they have
221			been modified more
222			recently than the
223	tdb	1.2	lastmodified value.
224			If they HAVE that
225	ajm	1.1	indicates that the
226			configuration has
227			changed and the host
228			should re-configure,
229			thus it sends ERROR.
230			If the files remain
231			unchanged the server
232			will return OK to
233			indicate the host
234			should continue as
235			before
236
237			[ENDHEARTBEAT] ----------> Indicates to the
238			server that the host
239			has finished an will
240			disconnect
241	tdb	1.6
242	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
243			server is either ok
244			or that it thought
245			there was an error
246	ajm	1.3	------------------------------------------------------------
247	ajm	1.1
248			Host Data Protocol
249			------------------
250			The UDP data packets that are sent by the host contain
251			simply XML data. For information on the format of this
252			data see the XML via UDP specification document at:
253
254	ajm	1.4	http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt
255	ajm	1.1
256			Client Control Protocol
257			-----------------------
258
259	ajm	1.3	The client control protocol channel is opened by the client
260			and allows a variety of actions to be carried out by the
261			client at anytime. Unlike previous protocols, this is NOT
262	tdb	1.6	sequential, all of the requests can be carried out in
263	ajm	1.3	any order.
264
265			All client protocols are backwards compatible, and the
266			version is shown by the protocol identifier.
267
268			There are three sections to this protocol.
269
270			1) Initialise (sent only at start)
271			2) Send command(s) - unlimited number in any order
272			3) Disconnect (sent only at end)
273
274			If at anytime the client sends something the server does
275			not understand, an [ERROR] will be sent
276
277			All are indicated with <> brackets.
278
279			v1.0 - Protocol identifier: "PROTOCOL 1.0" (without quotes)
280			Supported commands:
281			STARTCONFIG
282			STARTDATA
283			STOPDATA
284
285			Host (direction) Server Comment
286			------------------------------------------------------------
287			<initialise>
288	tdb	1.6	[CONNECT] ----------> Starts off the
289	ajm	1.3	client protocol
290	tdb	1.6
291	ajm	1.3	<---------- [{protocol}] The server sends the
292			protocol identifier
293	tdb	1.6
294	ajm	1.3	[{name}] ----------> The client sends its
295			"name". This name
296			is used to identify
297			the type of client
298			to the system and
299			obtain its config
300			eg, Conient
301	tdb	1.6
302	ajm	1.3	<---------- [OK] Indicates the server
303			is ok to proceed
304	tdb	1.6
305	ajm	1.3
306			<command #1> (allows client to obtain its configuration)
307			[STARTCONFIG]----------> Tell the server we
308			want to start this
309			command
310	tdb	1.6
311	ajm	1.3	<---------- [OK] Indicates the server
312			is ok to proceed
313	tdb	1.6
314	ajm	1.3	******** LOOP START ********
315			This loop reads configuration
316			properties from the config
317
318			[{property}] ----------> Sends the name of a
319			property to be
320			retrieved from the
321			config file
322			Clients can obtain
323			host information
324	tdb	1.6	eg,
325	ajm	1.3	Host.UDPUpdateTime
326			Otherwise it must
327			prefix requests
328			with "Client."
329			All other requests
330			will be ignored as
331			if it was unable to
332	tdb	1.6	retrieve the
333			property
334
335	ajm	1.3	<---------- [{value}\|ERROR]Returns the value of
336			the requested config
337			property
338			eg, 120
339			If it is unable to
340	tdb	1.6	find the requested
341	ajm	1.3	property it returns
342			ERROR to indicate
343			that fact
344	tdb	1.6
345	ajm	1.3	******** LOOP UNTIL ********
346			The loop continues until the client
347			sends the following message
348	tdb	1.6
349			[ENDCONFIG] ----------> Indicates that the
350	ajm	1.3	client requires no
351			more config
352	tdb	1.6
353	ajm	1.3	******** LOOP END ********
354			Communication continues
355	tdb	1.6
356	ajm	1.3	<---------- [OK] Indicates that the
357			server is ready to
358			proceed
359
360			<command #2> (tells the server to start the data link)
361			[STARTDATA] ----------> Tell the server we
362			want to start this
363			command
364	tdb	1.6
365	ajm	1.3	<---------- [{portnumber} The server then sets
366			\|ERROR] itself listening for
367			a connection on its
368			data link socket for
369			this client. It
370			returns the port no.
371			that it is listening
372			on eg, 12367
373			If the data link is
374	tdb	1.6	already started the
375			server will return
376			ERROR
377
378			<---------- [OK] Indicates the client
379			has successfully
380			connected to the
381			data socket.
382	ajm	1.3
383			<command #3> (tells the server to stop the data link)
384			[STOPDATA] ----------> Tell the server we
385			want to start this
386			command
387			The server then
388	tdb	1.6	shuts down the
389	ajm	1.3	data link to the
390			client
391
392			<---------- [OK\|ERROR] Returns OK is the
393			server is ready to
394			proceed, or ERROR
395			if the data link
396			was not open in the
397			first place
398
399			<disconnect>
400			[DISCONNECT] ----------> Tells the server the
401			client wants to
402			close the control
403			link
404	tdb	1.6
405	ajm	1.3	<---------- [OK] The last word from
406			the server, it will
407	tdb	1.6	disappear after
408			this, and close the
409			data link if
410			required
411	ajm	1.3	------------------------------------------------------------
412
413			v1.1 - Protocol identifier: "PROTOCOL 1.1" (without quotes)
414			Supported commands:
415			SETHOSTLIST
416
417			Host (direction) Server Comment
418			------------------------------------------------------------
419			<command #4> (indicates to the server which hosts the client
420			wants data from)
421			[SETHOSTLIST]----------> Tell the server we
422			want to start this
423			command
424	tdb	1.6
425	ajm	1.3	<---------- [OK\|ERROR] The server will
426			return an ERROR if
427			the data link is
428			open, as a host list
429	tdb	1.6	must ONLY be set
430	ajm	1.3	if the data link is
431			closed
432			If the server is
433			ok to proceed with
434			the command it says
435			[OK]
436
437			[{hostlist}] ----------> This is a semi-colon
438			seperated list of
439			FQDN hostnames that
440			the client wishes to
441			recieve data from
442			eg,
443			raptor.ukc.ac.uk;killigrew.ukc.ac.uk;chalk.ukc.ac.uk
444	tdb	1.6
445	ajm	1.3	If the list is sent
446			blank (or if no list
447			is set at all) then
448			data from ALL hosts
449			will be sent to the
450	tdb	1.6	client (this is the
451			default if no
452			SETHOSTLIST is not
453			run by the client
454
455	ajm	1.3	<---------- [OK] Indicates the server
456			is ok to proceed and
457	tdb	1.6	the host list has
458	ajm	1.3	been accepted
459			------------------------------------------------------------
460	ajm	1.1
461			Client Data Protocol
462			--------------------
463	ajm	1.3	Once the data link has been opened by the control link, the
464	tdb	1.6	server will send XML formatted data packets separated by the
465			\n (newline) character. For information on the format of
466	ajm	1.3	this data see the XML via UDP specification document at:
467	ajm	1.1
468	ajm	1.4	http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt
469	ajm	1.1
470			About
471			-----
472			This document was written by AJ Moore [ajm4@ukc.ac.uk] for
473			use by the team working on a 3rd year Computer Science
474			project called "i-scream". More details can be found on the
475			project website;
476
477			http://www.i-scream.org.uk