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}|ERROR 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
                                        or ERROR if none of
                                        configured Filter's
                                        could be found.

[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:	1.9
Committed:	Wed Mar 14 13:14:57 2001 UTC (24 years, 7 months ago) by tdb
Content type:	text/plain
Branch:	MAIN
Changes since 1.8:	+4 -1 lines
Log Message:	Clarified the protocol if a Filter cannot be found.
#	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	tdb	1.9	<---------- {filter}\|ERROR 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	tdb	1.9	or ERROR if none of
161			configured Filter's
162			could be found.
163	ajm	1.1
164			[END] ----------> Indicates to the
165			server that the host
166			has finished an will
167			disconnect
168	tdb	1.6
169	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
170			server is either ok
171			or that it thought
172			there was an error
173	ajm	1.3	------------------------------------------------------------
174	tdb	1.6
175	ajm	1.1	Host Heartbeat Protocol
176			-----------------------
177			When a host is configured after it has connected it should
178			obtain a property TCPUpdateTime. This indicates how often
179	tdb	1.2	a host should send a "Heart Beat", which is a pro-active
180	ajm	1.1	connection to the server by the host to indicate that it is
181			still alive. This "Heart Beat" also allows a host to see
182	tdb	1.2	its configuration has changed and act accordingly. In a
183	ajm	1.1	well written host this should just be a case of dropping out
184			of a loop and heading back to the start (connecting to the
185			filter manager).
186
187			Host (direction) Server Comment
188			------------------------------------------------------------
189			[HEARTBEAT] ----------> Starts the
190			heartbeat protocol
191			off
192	tdb	1.6
193	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
194			server is ok or not
195
196			[CONFIG] ----------> Indicates that the
197			host wants to check
198			its config
199
200			<---------- [OK\|ERROR] Indicates that the
201			server is ok or not
202
203			[{filelist}] ----------> Send a semi-colon
204	tdb	1.2	separated list of
205	ajm	1.1	filenames
206			eg,
207			a.conf;b.conf;c.conf
208			This should be
209			identical to the one
210	tdb	1.2	received in the
211	ajm	1.1	connection protocol
212
213			<---------- [OK] Indicates that the
214			server is ok
215
216			[{lastmodified}]----------> Returns a long int
217	tdb	1.2	time since epoch
218	ajm	1.1	eg, 123456789
219			This should be
220			identical to the one
221	tdb	1.2	received in the
222	ajm	1.1	connection protocol
223	ajm	1.5	To indicate what
224			format this is in,
225			I quote from the
226			Java 1.3 JDK API
227			"measured in milli
228			-seconds since the
229			epoch"
230	ajm	1.1
231			<---------- [OK\|ERROR] The server then
232			checks all the files
233			in the file list to
234			see if they have
235			been modified more
236			recently than the
237	tdb	1.2	lastmodified value.
238			If they HAVE that
239	ajm	1.1	indicates that the
240			configuration has
241			changed and the host
242			should re-configure,
243			thus it sends ERROR.
244			If the files remain
245			unchanged the server
246			will return OK to
247			indicate the host
248			should continue as
249			before
250
251			[ENDHEARTBEAT] ----------> Indicates to the
252			server that the host
253			has finished an will
254			disconnect
255	tdb	1.6
256	ajm	1.1	<---------- [OK\|ERROR] Indicates that the
257			server is either ok
258			or that it thought
259			there was an error
260	ajm	1.3	------------------------------------------------------------
261	ajm	1.1
262			Host Data Protocol
263			------------------
264			The UDP data packets that are sent by the host contain
265			simply XML data. For information on the format of this
266			data see the XML via UDP specification document at:
267
268	ajm	1.4	http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt
269	ajm	1.1
270			Client Control Protocol
271			-----------------------
272
273	ajm	1.3	The client control protocol channel is opened by the client
274			and allows a variety of actions to be carried out by the
275			client at anytime. Unlike previous protocols, this is NOT
276	tdb	1.6	sequential, all of the requests can be carried out in
277	ajm	1.3	any order.
278
279			All client protocols are backwards compatible, and the
280			version is shown by the protocol identifier.
281
282			There are three sections to this protocol.
283
284			1) Initialise (sent only at start)
285			2) Send command(s) - unlimited number in any order
286			3) Disconnect (sent only at end)
287
288			If at anytime the client sends something the server does
289			not understand, an [ERROR] will be sent
290
291			All are indicated with <> brackets.
292
293			v1.0 - Protocol identifier: "PROTOCOL 1.0" (without quotes)
294			Supported commands:
295			STARTCONFIG
296			STARTDATA
297			STOPDATA
298
299			Host (direction) Server Comment
300			------------------------------------------------------------
301			<initialise>
302			<---------- [{protocol}] The server sends the
303			protocol identifier
304	tdb	1.6
305	ajm	1.3	[{name}] ----------> The client sends its
306			"name". This name
307			is used to identify
308			the type of client
309			to the system and
310			obtain its config
311			eg, Conient
312	tdb	1.6
313	ajm	1.3	<---------- [OK] Indicates the server
314			is ok to proceed
315	tdb	1.6
316	ajm	1.3
317			<command #1> (allows client to obtain its configuration)
318			[STARTCONFIG]----------> Tell the server we
319			want to start this
320			command
321	tdb	1.6
322	ajm	1.3	<---------- [OK] Indicates the server
323			is ok to proceed
324	tdb	1.6
325	ajm	1.3	******** LOOP START ********
326			This loop reads configuration
327			properties from the config
328
329			[{property}] ----------> Sends the name of a
330			property to be
331			retrieved from the
332			config file
333			Clients can obtain
334			host information
335	tdb	1.6	eg,
336	ajm	1.3	Host.UDPUpdateTime
337			Otherwise it must
338			prefix requests
339			with "Client."
340			All other requests
341			will be ignored as
342			if it was unable to
343	tdb	1.6	retrieve the
344			property
345
346	ajm	1.3	<---------- [{value}\|ERROR]Returns the value of
347			the requested config
348			property
349			eg, 120
350			If it is unable to
351	tdb	1.6	find the requested
352	ajm	1.3	property it returns
353			ERROR to indicate
354			that fact
355	tdb	1.6
356	ajm	1.3	******** LOOP UNTIL ********
357			The loop continues until the client
358			sends the following message
359	tdb	1.6
360			[ENDCONFIG] ----------> Indicates that the
361	ajm	1.3	client requires no
362			more config
363	tdb	1.6
364	ajm	1.3	******** LOOP END ********
365			Communication continues
366	tdb	1.6
367	ajm	1.3	<---------- [OK] Indicates that the
368			server is ready to
369			proceed
370
371			<command #2> (tells the server to start the data link)
372			[STARTDATA] ----------> Tell the server we
373			want to start this
374			command
375	tdb	1.6
376	ajm	1.3	<---------- [{portnumber} The server then sets
377			\|ERROR] itself listening for
378			a connection on its
379			data link socket for
380			this client. It
381			returns the port no.
382			that it is listening
383			on eg, 12367
384			If the data link is
385	tdb	1.6	already started the
386			server will return
387			ERROR
388
389			<---------- [OK] Indicates the client
390			has successfully
391			connected to the
392			data socket.
393	ajm	1.3
394			<command #3> (tells the server to stop the data link)
395			[STOPDATA] ----------> Tell the server we
396			want to start this
397			command
398			The server then
399	tdb	1.6	shuts down the
400	ajm	1.3	data link to the
401			client
402
403			<---------- [OK\|ERROR] Returns OK is the
404			server is ready to
405			proceed, or ERROR
406			if the data link
407			was not open in the
408			first place
409
410			<disconnect>
411			[DISCONNECT] ----------> Tells the server the
412			client wants to
413			close the control
414			link
415	tdb	1.6
416	ajm	1.3	<---------- [OK] The last word from
417			the server, it will
418	tdb	1.6	disappear after
419			this, and close the
420			data link if
421			required
422	ajm	1.3	------------------------------------------------------------
423
424			v1.1 - Protocol identifier: "PROTOCOL 1.1" (without quotes)
425			Supported commands:
426			SETHOSTLIST
427
428			Host (direction) Server Comment
429			------------------------------------------------------------
430			<command #4> (indicates to the server which hosts the client
431			wants data from)
432			[SETHOSTLIST]----------> Tell the server we
433			want to start this
434			command
435	tdb	1.6
436	ajm	1.3	<---------- [OK\|ERROR] The server will
437			return an ERROR if
438			the data link is
439			open, as a host list
440	tdb	1.6	must ONLY be set
441	ajm	1.3	if the data link is
442			closed
443			If the server is
444			ok to proceed with
445			the command it says
446			[OK]
447
448			[{hostlist}] ----------> This is a semi-colon
449			seperated list of
450			FQDN hostnames that
451			the client wishes to
452			recieve data from
453			eg,
454			raptor.ukc.ac.uk;killigrew.ukc.ac.uk;chalk.ukc.ac.uk
455	tdb	1.6
456	ajm	1.3	If the list is sent
457			blank (or if no list
458			is set at all) then
459			data from ALL hosts
460			will be sent to the
461	tdb	1.6	client (this is the
462			default if no
463			SETHOSTLIST is not
464			run by the client
465
466	ajm	1.3	<---------- [OK] Indicates the server
467			is ok to proceed and
468	tdb	1.6	the host list has
469	ajm	1.3	been accepted
470			------------------------------------------------------------
471	ajm	1.1
472			Client Data Protocol
473			--------------------
474	ajm	1.3	Once the data link has been opened by the control link, the
475	tdb	1.6	server will send XML formatted data packets separated by the
476			\n (newline) character. For information on the format of
477	ajm	1.3	this data see the XML via UDP specification document at:
478	ajm	1.1
479	ajm	1.4	http://www.i-scream.org.uk/cgi-bin/docs.cgi?doc=specification/xml_via_udp.txt
480	ajm	1.1
481			About
482			-----
483			This document was written by AJ Moore [ajm4@ukc.ac.uk] for
484			use by the team working on a 3rd year Computer Science
485			project called "i-scream". More details can be found on the
486			project website;
487
488			http://www.i-scream.org.uk