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[paraslash.git] / web / manual.m4
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2 dnl m4 web/manual.m4 | grutatxt --toc
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6 define(`REMOVE_NEWLINE', `translit(`$1',`
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8
9 define(`REFERENCE', ./``#''`LOCAL_LINK_NAME($1)' (`REMOVE_NEWLINE($2)'))
10 define(`XREFERENCE', `$1' (`REMOVE_NEWLINE($2)'))
11 define(`EMPH', ``_''`REMOVE_NEWLINE($1)'``_'')
12
13 Paraslash user manual
14 =====================
15
16 This document describes how to install, configure and use the paraslash
17 network audio streaming system. Most chapters start with a chapter
18 overview and conclude with an example section. We try to focus on
19 general concepts and on the interaction of the various pieces of the
20 paraslash package. Hence this user manual is not meant as a replacement
21 for the manual pages that describe all command line options of each
22 paraslash executable.
23
24 ------------
25 Introduction
26 ------------
27
28 In this chapter we give an REFERENCE(Overview, overview) of the
29 interactions of the two main programs contained in the paraslash
30 package, followed by REFERENCE(The paraslash executables, brief
31 descriptions) of all executables.
32
33 Overview
34 ~~~~~~~~
35
36 The core functionality of the para suite is provided by two main
37 executables, para_server and para_audiod. The former maintains a
38 database of audio files and streams these files to para_audiod which
39 receives and plays the stream.
40
41 In a typical setting, both para_server and para_audiod act as
42 background daemons whose functionality is controlled by client
43 programs: the para_audioc client controls para_audiod over a local
44 socket while the para_client program connects to para_server over a
45 local or remote networking connection.
46
47 Typically, these two daemons run on different hosts but a local setup
48 is also possible.
49
50 A simplified picture of a typical setup is as follows
51 <<
52 <pre>
53 server_host client_host
54 ~~~~~~~~~~~ ~~~~~~~~~~~
55
56 +-----------+ audio stream +-----------+
57 |para_server| -----------------------------> |para_audiod|
58 +-----------+ +-----------+
59 ^ ^
60 | |
61 | | connect
62 | |
63 | |
64 | +-----------+
65 | |para_audioc|
66 | +-----------+
67 |
68 |
69 | connect +-----------+
70 +-------------------------------------- |para_client|
71 +-----------+
72 </pre>
73 >>
74
75 The paraslash executables
76 ~~~~~~~~~~~~~~~~~~~~~~~~~
77
78 *para_server*
79
80 para_server streams binary audio data (MP3, OGG/Vorbis, OGG/Speex,
81 M4A, WMA files) over local and/or remote networks. It listens on a
82 TCP port and accepts commands such as play, stop, pause, next from
83 authenticated clients. There are many more commands though, see the
84 man page of para_server for a description of all commands.
85
86 It supports three built-in network streaming protocols
87 (senders/receivers): HTTP, DCCP, or UDP. This is explained in more
88 detail in the section on REFERENCE(Networking, networking).
89
90 The built-in audio file selector of paraslash is used to manage your
91 audio files. It maintains statistics on the usage of all available
92 audio files such as last-played time, and the number of times each
93 file was selected.
94
95 Additional information may be added to the database to allow
96 fine-grained selection based on various properties of the audio file,
97 including information found in (ID3) tags. However, old-fashioned
98 playlists are also supported.
99
100 It is also possible to store images (album covers) and lyrics in the
101 database and associate these to the corresponding audio files.
102
103 The section on the REFERENCE(The audio file selector, audio file
104 selector) discusses this topic.
105
106
107 *para_client*
108
109 The client program to connect to para_server. paraslash commands
110 are sent to para_server and the response is dumped to STDOUT. This
111 can be used by any scripting language to produce user interfaces with
112 little programming effort.
113
114 All connections between para_server and para_client are encrypted
115 with a symmetric RC4 session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
117
118
119 *para_audiod*
120
121 The local daemon that collects information from para_server.
122
123 It runs on the client side and connects to para_server. As soon as
124 para_server announces the availability of an audio stream, para_audiod
125 starts an appropriate receiver, any number of filters and a paraslash
126 writer to play the stream.
127
128 Moreover, para_audiod listens on a local socket and sends status
129 information about para_server and para_audiod to local clients on
130 request. Access via this local socket may be restricted by using Unix
131 socket credentials, if available.
132
133
134 *para_audioc*
135
136 The client program which talks to para_audiod. Used to control
137 para_audiod, to receive status info, or to grab the stream at any
138 point of the decoding process.
139
140 *para_recv*
141
142 A command line HTTP/DCCP/UDP stream grabber. The http mode is
143 compatible with arbitrary HTTP streaming sources (e.g. icecast).
144
145 *para_filter*
146
147 A filter program that reads from STDIN and writes to STDOUT.
148 Like para_recv, this is an atomic building block which can be used to
149 assemble higher-level audio receiving facilities. It combines several
150 different functionalities in one tool: decoders for multiple audio
151 formats and a number of processing filters, among these a normalizer
152 for audio volume.
153
154 *para_afh*
155
156 A small stand-alone program that prints tech info about the given
157 audio file to STDOUT. It can be instructed to print a "chunk table",
158 an array of offsets within the audio file or to write the content of
159 the audio file in complete chunks 'just in time'.
160
161 This allows third-party streaming software that is unaware of the
162 particular audio format to send complete frames in real time.
163
164 *para_write*
165
166 A modular audio stream writer. It supports a simple file writer
167 output plug-in and optional WAV/raw players for ALSA (Linux) and for
168 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
169 or raw audio player.
170
171
172 *para_gui*
173
174 Curses-based gui that presents status information obtained in a curses
175 window. Appearance can be customized via themes. para_gui provides
176 key-bindings for the most common server commands and new key-bindings
177 can be added easily.
178
179
180 *para_fade*
181
182 An (OSS-only) alarm clock and volume-fader.
183
184 -----------
185 Quick start
186 -----------
187
188 This chapter lists the REFERENCE(Requirements, necessary software)
189 that must be installed to compile the paraslash package, describes
190 how to REFERENCE(Installation, compile and install) the paraslash
191 source code and the steps that have to be performed in order to
192 REFERENCE(Quick start, set up) a typical server and client.
193
194 Requirements
195 ~~~~~~~~~~~~
196
197 In any case you'll need
198
199 - XREFERENCE(http://systemlinux.org/~maan/osl/, libosl).
200 The _object storage layer_ library is used by para_server. To
201 clone the source code repository, execute
202
203 git clone git://git.tuebingen.mpg.de/osl
204
205 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc) or
206 XREFERENCE(http://clang.llvm.org, clang). All gcc versions
207 >= 3.3 are currently supported. Clang version 1.1 or newer
208 should work as well.
209
210 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
211 also shipped with the disto. On BSD systems the gnu make
212 executable is often called gmake.
213
214 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
215 scripts which run during compilation require the EMPH(Bourne
216 again shell). It is most likely already installed.
217
218 - XREFERENCE(http://www.openssl.org/, openssl) or
219 XREFERENCE(ftp://ftp.gnupg.org/gcrypt/libgcrypt/, libgcrypt).
220 At least one of these two libraries is needed as the backend
221 for cryptographic routines on both the server and the client
222 side. Both openssl and libgcrypt are usually shipped with the
223 distro, but you might have to install the development package
224 (libssl-dev or libgcrypt-dev on debian systems) as well.
225
226 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/help2man, help2man)
227 is used to create the man pages.
228
229 Optional:
230
231 - XREFERENCE(http://www.underbit.com/products/mad/, libmad).
232 To compile in MP3 support for paraslash, the development
233 package must be installed. It is called libmad0-dev on
234 debian-based systems. Note that libmad is not necessary on
235 the server side, i.e. for sending MP3 files.
236
237 - XREFERENCE(http://www.underbit.com/products/mad/,
238 libid3tag). For version-2 ID3 tag support, you'll need
239 the libid3tag development package libid3tag0-dev. Without
240 libid3tag, only version one tags are recognized.
241
242 - XREFERENCE(http://www.xiph.org/downloads/, ogg vorbis).
243 For ogg vorbis streams you'll need libogg, libvorbis,
244 libvorbisfile. The corresponding Debian packages are called
245 libogg-dev and libvorbis-dev.
246
247 - XREFERENCE(http://www.audiocoding.com/, libfaad). For aac
248 files (m4a) you'll need libfaad (libfaad-dev).
249
250 - XREFERENCE(http://www.speex.org/, speex). In order to stream
251 or decode speex files, libspeex (libspeex-dev) is required.
252
253 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
254 Linux, you'll need to have ALSA's development package
255 libasound2-dev installed.
256
257 - XREFERENCE(http://downloads.xiph.org/releases/ao/,
258 libao). Needed to build the ao writer (ESD, PulseAudio,...).
259 Debian package: libao-dev.
260
261 Installation
262 ~~~~~~~~~~~~
263
264 First make sure all non-optional packages listed in the section on
265 REFERENCE(Requirements, required software) are installed on your
266 system.
267
268 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
269 OGG/Speex and AAC support are all optional. The configure script will
270 detect what is installed on your system and will only try to build
271 those executables that can be built with your setup.
272
273 Note that no special decoder library (not even the MP3 decoding library
274 libmad) is needed for para_server if you only want to stream MP3 or WMA
275 files. Also, it's fine to use para_server on a box without sound card.
276
277 Next, install the paraslash package on all machines, you'd like this
278 software to run on:
279
280 (./configure && make) > /dev/null
281
282 There should be no errors but probably some warnings about missing
283 packages which usually implies that not all audio formats will be
284 supported. If headers or libs are installed at unusual locations you
285 might need to tell the configure script where to find them. Try
286
287 ./configure --help
288
289 to see a list of options. If the paraslash package was compiled
290 successfully, execute (optionally)
291
292 make test
293
294 to run the paraslash test suite. If all tests pass, execute as root
295
296 make install
297
298 to install executables under /usr/local/bin and the man pages under
299 /usr/local/man.
300
301 Configuration
302 ~~~~~~~~~~~~~
303
304 *Step 1*: Create a paraslash user
305
306 In order to control para_server at runtime you must create a paraslash
307 user. As authentication is based on the RSA crypto system you'll have
308 to create an RSA key pair. If you already have a user and an RSA key
309 pair, you may skip this step.
310
311 In this section we'll assume a typical setup: You would like to run
312 para_server on some host called server_host as user foo, and you want
313 to connect to para_server from another machine called client_host as
314 user bar.
315
316 As foo@server_host, create ~/.paraslash/server.users by typing the
317 following commands:
318
319 user=bar
320 target=~/.paraslash/server.users
321 key=~/.paraslash/id_rsa.pub.$user
322 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
323 mkdir -p ~/.paraslash
324 echo "user $user $key $perms" >> $target
325
326 Next, change to the "bar" account on client_host and generate the
327 key pair with the commands
328
329 ssh-keygen -t rsa -b 2048
330 # hit enter twice to create a key with no passphrase
331
332 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
333 that paraslash can also read keys generated by the "openssl genrsa"
334 command. However, since keys created with ssh-keygen can also be used
335 for ssh, this method is recommended.
336
337 Note that para_server refuses to use a key if it is shorter than 2048
338 bits. In particular, the RSA keys of paraslash 0.3.x will not work
339 with version 0.4.x. Moreover, para_client refuses to use a (private)
340 key which is world-readable.
341
342 para_server only needs to know the public key of the key pair just
343 created. Copy this public key to server_host:
344
345 src=~/.ssh/id_rsa.pub
346 dest=.paraslash/id_rsa.pub.$LOGNAME
347 scp $src foo@server_host:$dest
348
349 Finally, tell para_client to connect to server_host:
350
351 conf=~/.paraslash/client.conf
352 echo 'hostname server_host' > $conf
353
354
355 *Step 2*: Start para_server
356
357 Before starting the server make sure you have write permissions to
358 the directory /var/paraslash that has been created during installation:
359
360 sudo chown $LOGNAME /var/paraslash
361
362 Alternatively, use the --afs_socket Option to specify a different
363 location for the AFS command socket.
364
365 For this first try, we'll use the info loglevel to make the output
366 of para_server more verbose.
367
368 para_server -l info
369
370 Now you can use para_client to connect to the server and issue
371 commands. Open a new shell as bar@client_host and try
372
373 para_client help
374 para_client si
375
376 to retrieve the list of available commands and some server info.
377 Don't proceed if this doesn't work.
378
379 *Step 3*: Create and populate the database
380
381 An empty database is created with
382
383 para_client init
384
385 This initializes a couple of empty tables under
386 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
387 tables, but it's good to know that you can start from scratch with
388
389 rm -rf ~/.paraslash/afs_database-0.4
390
391 in case something went wrong.
392
393 Next, you need to add some audio files to that database so that
394 para_server knows about them. Choose an absolute path to a directory
395 containing some audio files and add them to the audio file table:
396
397 para_client add /my/mp3/dir
398
399 This might take a while, so it is a good idea to start with a directory
400 containing not too many files. Note that the table only contains data
401 about the audio files found, not the files themselves.
402
403 You may print the list of all known audio files with
404
405 para_client ls
406
407 *Step 4*: Configure para_audiod
408
409 para_audiod needs to create a "well-known" socket for the clients to
410 connect to. The default path for this socket is
411
412 /var/paraslash/audiod_socket.$HOSTNAME
413
414 In order to make this directory writable for para_audiod, execute
415 as bar@client_host
416
417 sudo chown $LOGNAME /var/paraslash
418
419
420 We will also have to tell para_audiod that it should receive the
421 audio stream from server_host via http:
422
423 para_audiod -l info -r '.:http -i server_host'
424
425 You should now be able to listen to the audio stream once para_server
426 starts streaming. To activate streaming, execute
427
428 para_client play
429
430 Since no playlist has been specified yet, the "dummy" mode which
431 selects all known audio files is activated automatically. See the
432 section on the REFERENCE(The audio file selector, audio file selector)
433 for how to use playlists and moods to specify which files should be
434 streamed in which order.
435
436 *Troubleshooting*
437
438 It did not work? To find out why, try to receive, decode and play the
439 stream manually using para_recv, para_filter and para_write as follows.
440
441 For simplicity we assume that you're running Linux/ALSA and that only
442 MP3 files have been added to the database.
443
444 para_recv -r 'http -i server_host' > file.mp3
445 # (interrupt with CTRL+C after a few seconds)
446 ls -l file.mp3 # should not be empty
447 para_filter -f mp3dec -f wav < file.mp3 > file.wav
448 ls -l file.wav # should be much bigger than file.mp3
449 para_write -w alsa < file.wav
450
451 Double check what is logged by para_server and use the --loglevel
452 option of para_recv, para_filter and para_write to increase verbosity.
453
454 ---------------
455 User management
456 ---------------
457
458 para_server uses a challenge-response mechanism to authenticate
459 requests from incoming connections, similar to ssh's public key
460 authentication method. Authenticated connections are encrypted using
461 the RC4 stream cipher.
462
463 In this chapter we briefly describe RSA and RC4 and sketch the
464 REFERENCE(Client-server authentication, authentication handshake)
465 between para_client and para_server. User management is discussed
466 in the section on REFERENCE(The user_list file, the user_list file).
467 These sections are all about communication between the client and the
468 server. Connecting para_audiod is a different matter and is described
469 in a REFERENCE(Connecting para_audiod, separate section).
470
471
472
473 RSA and RC4
474 ~~~~~~~~~~~
475
476 RSA is an asymmetric block cipher which is used in many applications,
477 including ssh and gpg. An RSA key consists in fact of two keys,
478 called the public key and the private key. A message can be encrypted
479 with either key and only the counterpart of that key can decrypt
480 the message. While RSA can be used for both signing and encrypting
481 a message, paraslash only uses RSA only for the latter purpose. The
482 RSA public key encryption and signatures algorithms are defined in
483 detail in RFC 2437.
484
485 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
486 key stream to produce the output. Decryption uses the same function
487 calls as encryption. While RC4 supports variable key lengths,
488 paraslash uses a fixed length of 256 bits, which is considered a
489 strong encryption by today's standards. Since the same key must never
490 be used twice, a different, randomly-generated key is used for every
491 new connection.
492
493 Client-server authentication
494 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
495
496 The authentication handshake between para_client and para_server goes
497 as follows:
498
499 - para_client connects to para_server and sends an
500 authentication request for a user. It does so by connecting
501 to TCP port 2990 of the server host. This port is called the
502 para_server _control port_.
503
504 - para_server accepts the connection and forks a child process
505 which handles the incoming request. The parent process keeps
506 listening on the control port while the child process (also
507 called para_server below) continues as follows.
508
509 - para_server loads the RSA public key of that user, fills a
510 fixed-length buffer with random bytes, encrypts that buffer
511 using the public key and sends the encrypted buffer to the
512 client. The first part of the buffer is the challenge which
513 is used for authentication while the second part is the RC4
514 session key.
515
516 - para_client receives the encrypted buffer and decrypts it
517 with the user's private key, thereby obtaining the challenge
518 buffer and the session key. It sends the SHA1 hash value of
519 the challenge back to para_server and stores the session key
520 for further use.
521
522 - para_server also computes the SHA1 hash of the challenge
523 and compares it against what was sent back by the client.
524
525 - If the two hashes do not match, the authentication has
526 failed and para_server closes the connection.
527
528 - Otherwise the user is considered authenticated and the client
529 is allowed to proceed by sending a command to be executed. From
530 this point on the communication is encrypted using the RC4
531 stream cipher with the session key known to both peers.
532
533 paraslash relies on the quality of the pseudo-random bytes provided
534 by the crypto library (openssl or libgcrypt), on the security of
535 the implementation of the RSA and RC4 crypto routines and on the
536 infeasibility to invert the SHA1 function.
537
538 Neither para_server or para_client create RSA keys on their own. This
539 has to be done once for each user as sketched in REFERENCE(Quick start,
540 Quick start) and discussed in more detail REFERENCE(The user_list
541 file, below).
542
543 The user_list file
544 ~~~~~~~~~~~~~~~~~~
545
546 At startup para_server reads the user list file which contains one
547 line per user. The default location of the user list file may be
548 changed with the --user_list option.
549
550 There should be at least one user in this file. Each user must have
551 an RSA key pair. The public part of the key is needed by para_server
552 while the private key is needed by para_client. Each line of the
553 user list file must be of the form
554
555 user <username> <key> <perms>
556
557 where _username_ is an arbitrary string (usually the user's login
558 name), _key_ is the full path to that user's public RSA key, and
559 _perms_ is a comma-separated list of zero or more of the following
560 permission bits:
561
562 +---------------------------------------------------------+
563 | AFS_READ | read the contents of the databases |
564 +-----------+---------------------------------------------+
565 | AFS_WRITE | change database contents |
566 +-----------+---------------------------------------------+
567 | VSS_READ | obtain information about the current stream |
568 +-----------+---------------------------------------------+
569 | VSS_WRITE | change the current stream |
570 +---------------------------------------------------------+
571
572 The permission bits specify which commands the user is allowed to
573 execute. The output of
574
575 para_client help
576
577 contains in the third column the permissions needed to execute the
578 command.
579
580 It is possible to make para_server reread the user_list file by
581 executing the paraslash "hup" command or by sending SIGHUP to the
582 PID of para_server.
583
584
585 Connecting para_audiod
586 ~~~~~~~~~~~~~~~~~~~~~~
587
588 para_audiod listens on a Unix domain socket. Those sockets are
589 for local communication only, so only local users can connect to
590 para_audiod. The default is to let any user connect but this can be
591 restricted on platforms that support UNIX socket credentials which
592 allow para_audiod to obtain the Unix credentials of the connecting
593 process.
594
595 Use para_audiod's --user_allow option to allow connections only for
596 a limited set of users.
597
598 -----------------------
599 The audio file selector
600 -----------------------
601
602 paraslash comes with a sophisticated audio file selector (AFS),
603 whose main task is to determine which file to stream next, based on
604 information on the audio files stored in a database. It communicates
605 also with para_client whenever an AFS command is executed, for example
606 to answer a database query.
607
608 Besides the traditional playlists, AFS supports audio file selection
609 based on _moods_ which act as a filter that limits the set of all
610 known audio files to those which satisfy certain criteria. It also
611 maintains tables containing images (e.g. album cover art) and lyrics
612 that can be associated with one or more audio files.
613
614 AFS uses XREFERENCE(http://systemlinux.org/~maan/osl/, libosl), the
615 object storage layer library, as the backend library for storing
616 information on audio files, playlists, etc. This library offers
617 functionality similar to a relational database, but is much more
618 lightweight than a full database backend.
619
620 In this chapter we sketch the setup of the REFERENCE(The AFS process,
621 AFS process) during server startup and proceed with the description
622 of the REFERENCE(Database layout, layout) of the various database
623 tables. The section on REFERENCE(Playlists and moods, playlists
624 and moods) explains these two audio file selection mechanisms
625 in detail and contains pratical examples. The way REFERENCE(File
626 renames and content changes, file renames and content changes) are
627 detected is discussed briefly before the REFERENCE(Troubleshooting,
628 Troubleshooting) section concludes the chapter.
629
630 The AFS process
631 ~~~~~~~~~~~~~~~
632
633 On startup, para_server forks to create the AFS process which opens
634 the OSL database tables. The server process communicates with the
635 AFS process via pipes and shared memory. Usually, the AFS process
636 awakes only briefly whenever the current audio file changes. The AFS
637 process determines the next audio file, opens it, verifies it has
638 not been changed since it was added to the database and passes the
639 open file descriptor to the server process, along with audio file
640 meta-data such as file name, duration, audio format and so on. The
641 server process then starts to stream the audio file.
642
643 The AFS process also accepts connections from local clients via
644 a well-known socket. However, only child processes of para_server
645 may connect through this socket. All server commands that have the
646 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
647 change the database.
648
649 Database layout
650 ~~~~~~~~~~~~~~~
651
652 *The audio file table*
653
654 This is the most important and usually also the largest table of the
655 AFS database. It contains the information needed to stream each audio
656 file. In particular the following data is stored for each audio file.
657
658 - SHA1 hash value of the audio file contents. This is computed
659 once when the file is added to the database. Whenever AFS
660 selects this audio file for streaming the hash value is
661 recomputed and checked against the value stored in the
662 database to detect content changes.
663
664 - The time when this audio file was last played.
665
666 - The number of times the file has been played so far.
667
668 - The attribute bitmask.
669
670 - The image id which describes the image associated with this
671 audio file.
672
673 - The lyrics id which describes the lyrics associated with
674 this audio file.
675
676 - The audio format id (MP3, OGG, ...).
677
678 - An amplification value that can be used by the amplification
679 filter to pre-amplify the decoded audio stream.
680
681 - The chunk table. It describes the location and the timing
682 of the building blocks of the audio file. This is used by
683 para_server to send chunks of the file at appropriate times.
684
685 - The duration of the audio file.
686
687 - Tag information contained in the audio file (ID3 tags,
688 Vorbis comments, ...).
689
690 - The number of channels
691
692 - The encoding bitrate.
693
694 - The sampling frequency.
695
696 To add or refresh the data contained in the audio file table, the _add_
697 command is used. It takes the full path of either an audio file or a
698 directory. In the latter case, the directory is traversed recursively
699 and all files which are recognized as valid audio files are added to
700 the database.
701
702 *The attribute table*
703
704 The attribute table contains two columns, _name_ and _bitnum_. An
705 attribute is simply a name for a certain bit number in the attribute
706 bitmask of the audio file table.
707
708 Each of the 64 bits of the attribute bitmask can be set for each
709 audio file individually. Hence up to 64 different attributes may be
710 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
711 "german_lyrics", "speech", whatever. You are free to choose as
712 many attributes as you like and there are no naming restrictions
713 for attributes.
714
715 A new attribute "test" is created by
716
717 para_client addatt test
718 and
719 para_client lsatt
720
721 lists all available attributes. You can set the "test" attribute for
722 an audio file by executing
723
724 para_client setatt test+ /path/to/the/audio/file
725
726 Similarly, the "test" bit can be removed from an audio file with
727
728 para_client setatt test- /path/to/the/audio/file
729
730 Instead of a path you may use a shell wildcard pattern. The attribute
731 is applied to all audio files matching this pattern:
732
733 para_client setatt test+ '/test/directory/*'
734
735 The command
736
737 para_client -- ls -lv
738
739 gives you a verbose listing of your audio files also showing which
740 attributes are set.
741
742 In case you wonder why the double-dash in the above command is needed:
743 It tells para_client to not interpret the options after the dashes. If
744 you find this annoying, just say
745
746 alias para='para_client --'
747
748 and be happy. In what follows we shall use this alias.
749
750 The "test" attribute can be dropped from the database with
751
752 para rmatt test
753
754 Read the output of
755
756 para help ls
757 para help setatt
758
759 for more information and a complete list of command line options to
760 these commands.
761
762 *Blob tables*
763
764 The image, lyrics, moods and playlists tables are all blob tables.
765 Blob tables consist of three columns each: The identifier which is
766 a positive non-negative number that is auto-incremented, the name
767 (an arbitrary string) and the content (the blob).
768
769 All blob tables support the same set of actions: cat, ls, mv, rm
770 and add. Of course, _add_ is used for adding new blobs to the table
771 while the other actions have the same meaning as the corresponding
772 Unix commands. The paraslash commands to perform these actions are
773 constructed as the concatenation of the table name and the action. For
774 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
775 manipulate or query the image table.
776
777 The add variant of these commands is special as these commands read
778 the blob contents from stdin. To add an image to the image table the
779 command
780
781 para addimg image_name < file.jpg
782
783 can be used.
784
785 Note that the images and lyrics are not interpreted at all, and also
786 the playlist and the mood blobs are only investigated when the mood
787 or playlist is activated with the select command.
788
789 *The score table*
790
791 Unlike all other tables the contents of the score table remain in
792 memory and are never stored on disk. The score table contains two
793 columns: The SHA1 hash value (of an audio file) and its current
794 score.
795
796 However, only those files which are admissible for the current mood
797 or playlist are contained in the score table. The audio file selector
798 always chooses the row with the highest score as the file to stream
799 next. While doing so, it computes the new score and updates the
800 last_played and the num_played fields in the audio file table.
801
802 The score table is recomputed by the select command which loads a
803 mood or playlist. Audio files are chosen for streaming from the rows
804 of the score table on a highest-score-first basis.
805
806
807 Playlists and moods
808 ~~~~~~~~~~~~~~~~~~~
809
810 Playlists and moods offer two different ways of specifying the set of
811 admissible files. A playlist in itself describes a set of admissible
812 files. A mood, in contrast, describes the set of admissible files in
813 terms of attributes and other type of information available in the
814 audio file table. As an example, a mood can define a filename pattern,
815 which is then matched against the names of audio files in the table.
816
817 *Playlists*
818
819 Playlists are accommodated in the playlist table of the afs database,
820 using the aforementioned blob format for tables. A new playlist is
821 created with the addpl command by specifying the full (absolute)
822 paths of all desired audio files, separated by newlines. Example:
823
824 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
825
826 If _my_playlist_ already exists it is overwritten. To activate the
827 new playlist, execute
828
829 para select p/my_playlist
830
831 The audio file selector will assign scores to each entry of the list,
832 in descending order so that files will be selected in order. If a
833 file could not be opened for streaming, its entry is removed from
834 the score table (but not from the playlist).
835
836 *Moods*
837
838 A mood consists of a unique name and its *mood definition*, which is
839 a set of *mood lines* containing expressions in terms of attributes
840 and other data contained in the database.
841
842 At any time at most one mood can be *active* which means that
843 para_server is going to select only files from that subset of
844 admissible files.
845
846 So in order to create a mood definition one has to write a set of
847 mood lines. Mood lines come in three flavours: Accept lines, deny
848 lines and score lines.
849
850 The general syntax of the three types of mood lines is
851
852
853 accept [with score <score>] [if] [not] <mood_method> [options]
854 deny [with score <score>] [if] [not] <mood_method> [options]
855 score <score> [if] [not] <mood_method> [options]
856
857
858 Here <score> is either an integer or the string "random" which assigns
859 a random score to all matching files. The score value changes the
860 order in which admissible files are going to be selected, but is of
861 minor importance for this introduction.
862
863 So we concentrate on the first two forms, i.e. accept and deny
864 lines. As usual, everything in square brackets is optional, i.e.
865 accept/deny lines take the following form when ignoring scores:
866
867 accept [if] [not] <mood_method> [options]
868
869 and analogously for the deny case. The "if" keyword is only syntactic
870 sugar and has no function. The "not" keyword just inverts the result,
871 so the essence of a mood line is the mood method part and the options
872 following thereafter.
873
874 A *mood method* is realized as a function which takes an audio file
875 and computes a number from the data contained in the database.
876 If this number is non-negative, we say the file *matches* the mood
877 method. The file matches the full mood line if it either
878
879 - matches the mood method and the "not" keyword is not given,
880 or
881 - does not match the mood method, but the "not" keyword is given.
882
883 The set of admissible files for the whole mood is now defined as those
884 files which match at least one accept mood line, but no deny mood line.
885 More formally, an audio file F is admissible if and only if
886
887 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
888
889 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
890 lines and "~" means "matches".
891
892 The cases where no mood lines of accept/deny type are defined need
893 special treatment:
894
895 - Neither accept nor deny lines: This treats all files as
896 admissible (in fact, that is the definition of the dummy mood
897 which is activated automatically if no moods are available).
898
899 - Only accept lines: A file is admissible iff it matches at
900 least one accept line:
901
902 F ~ AL1 or F ~ AL2 or ...
903
904 - Only deny lines: A file is admissible iff it matches no
905 deny line:
906
907 not (F ~ DL1 or F ~ DN2 ...)
908
909
910
911 *List of mood_methods*
912
913 no_attributes_set
914
915 Takes no arguments and matches an audio file if and only if no
916 attributes are set.
917
918 is_set <attribute_name>
919
920 Takes the name of an attribute and matches iff that attribute is set.
921
922 path_matches <pattern>
923
924 Takes a filename pattern and matches iff the path of the audio file
925 matches the pattern.
926
927 artist_matches <pattern>
928 album_matches <pattern>
929 title_matches <pattern>
930 comment_matches <pattern>
931
932 Takes an extended regular expression and matches iff the text of the
933 corresponding tag of the audio file matches the pattern. If the tag
934 is not set, the empty string is matched against the pattern.
935
936 year ~ <num>
937 bitrate ~ <num>
938 frequency ~ <num>
939 channels ~ <num>
940 num_played ~ <num>
941
942 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
943 <num>. Matches an audio file iff the condition <val> ~ <num> is
944 satisfied where val is the corresponding value of the audio file
945 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
946 count, play count).
947
948 The year tag is special as its value is undefined if the audio file
949 has no year tag or the content of the year tag is not a number. Such
950 audio files never match. Another difference is the special treatment
951 if the year tag is a two-digit number. In this case either 1900 or
952 2000 is added to the tag value, depending on whether the number is
953 greater than 2000 plus the current year.
954
955
956 *Mood usage*
957
958 To create a new mood called "my_mood", write its definition into
959 some temporary file, say "tmpfile", and add it to the mood table
960 by executing
961
962 para addmood my_mood < tmpfile
963
964 If the mood definition is really short, you may just pipe it to the
965 client instead of using temporary files. Like this:
966
967 echo "$MOOD_DEFINITION" | para addmood my_mood
968
969 There is no need to keep the temporary file since you can always use
970 the catmood command to get it back:
971
972 para catmood my_mood
973
974 A mood can be activated by executing
975
976 para select m/my_mood
977
978 Once active, the list of admissible files is shown by the ls command
979 if the "-a" switch is given:
980
981 para ls -a
982
983
984 *Example mood definition*
985
986 Suppose you have defined attributes "punk" and "rock" and want to define
987 a mood containing only Punk-Rock songs. That is, an audio file should be
988 admissible if and only if both attributes are set. Since
989
990 punk and rock
991
992 is obviously the same as
993
994 not (not punk or not rock)
995
996 (de Morgan's rule), a mood definition that selects only Punk-Rock
997 songs is
998
999 deny if not is_set punk
1000 deny if not is_set rock
1001
1002
1003
1004 File renames and content changes
1005 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1006
1007 Since the audio file selector knows the SHA1 of each audio file that
1008 has been added to the afs database, it recognizes if the content of
1009 a file has changed, e.g. because an ID3 tag was added or modified.
1010 Also, if a file has been renamed or moved to a different location,
1011 afs will detect that an entry with the same hash value already exists
1012 in the audio file table.
1013
1014 In both cases it is enough to just re-add the new file. In the
1015 first case (file content changed), the audio table is updated, while
1016 metadata such as the num_played and last_played fields, as well as
1017 the attributes, remain unchanged. In the other case, when the file
1018 is moved or renamed, only the path information is updated, all other
1019 data remains as before.
1020
1021 It is possible to change the behaviour of the add command by using the
1022 "-l" (lazy add) or the "-f" (force add) option.
1023
1024 Troubleshooting
1025 ~~~~~~~~~~~~~~~
1026
1027 Use the debug loglevel (option -l debug for most commands) to show
1028 debugging info. Almost all paraslash executables have a brief online
1029 help which is displayed by using the -h switch. The --detailed-help
1030 option prints the full help text.
1031
1032 If para_server crashed or was killed by SIGKILL (signal 9), it
1033 may refuse to start again because of "dirty osl tables". In this
1034 case you'll have to run the oslfsck program of libosl to fix your
1035 database. It might be necessary to use --force (even if your name
1036 isn't Luke). However, make sure para_server isn't running before
1037 executing oslfsck --force.
1038
1039 If you don't mind to recreate your database you can start
1040 from scratch by removing the entire database directory, i.e.
1041
1042 rm -rf ~/.paraslash/afs_database-0.4
1043
1044 Be aware that this removes all attribute definitions, all playlists
1045 and all mood definitions and requires to re-initialize the tables.
1046
1047 Although oslfsck fixes inconsistencies in database tables it doesn't
1048 care about the table contents. To check for invalid table contents, use
1049
1050 para_client check
1051
1052 This prints out references to missing audio files as well as invalid
1053 playlists and mood definitions.
1054
1055 ---------------------------------------
1056 Audio formats and audio format handlers
1057 ---------------------------------------
1058
1059 Audio formats
1060 ~~~~~~~~~~~~~
1061
1062 The following audio formats are supported by paraslash:
1063
1064 *MP3*
1065
1066 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1067 designed as part of its MPEG-1 standard. An MP3 file is made up of
1068 multiple MP3 frames, which consist of a header and a data block. The
1069 size of an MP3 frame depends on the bit rate and on the number
1070 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1071 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1072 400 bytes large.
1073
1074 *OGG/Vorbis*
1075
1076 OGG is a standardized audio container format, while Vorbis is an
1077 open source codec for lossy audio compression. Since Vorbis is most
1078 commonly made available via the OGG container format, it is often
1079 referred to as OGG/Vorbis. The OGG container format divides data into
1080 chunks called OGG pages. A typical OGG page is about 4KB large. The
1081 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1082 may vary considerably.
1083
1084 *OGG/Speex*
1085
1086 Speex is an open-source speech codec that is based on CELP (Code
1087 Excited Linear Prediction) coding. It is designed for voice
1088 over IP applications, has modest complexity and a small memory
1089 footprint. Wideband and narrowband (telephone quality) speech are
1090 supported. As for Vorbis audio, Speex bit-streams are often stored
1091 in OGG files.
1092
1093 *AAC*
1094
1095 Advanced Audio Coding (AAC) is a standardized, lossy compression
1096 and encoding scheme for digital audio which is the default audio
1097 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1098 the container format and audio files encoded with AAC have the .m4a
1099 extension. A typical AAC frame is about 700 bytes large.
1100
1101 *WMA*
1102
1103 Windows Media Audio (WMA) is an audio data compression technology
1104 developed by Microsoft. A WMA file is usually encapsulated in the
1105 Advanced Systems Format (ASF) container format, which also specifies
1106 how meta data about the file is to be encoded. The bit stream of WMA
1107 is composed of superframes, each containing one or more frames of
1108 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1109
1110 Meta data
1111 ~~~~~~~~~
1112
1113 Unfortunately, each audio format has its own conventions how meta
1114 data is added as tags to the audio file.
1115
1116 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1117 is rather simple but also very limited as it supports only artist,
1118 title, album, year and comment tags. Each of these can only be at most
1119 32 characters long. ID3, version 2 is much more flexible but requires
1120 a separate library being installed for paraslash to support it.
1121
1122 Ogg vorbis files contain meta data as Vorbis comments, which are
1123 typically implemented as strings of the form "[TAG]=[VALUE]". Unlike
1124 ID3 version 1 tags, one may use whichever tags are appropriate for
1125 the content.
1126
1127 AAC files usually use the MPEG-4 container format for storing meta
1128 data while WMA files wrap meta data as special objects within the
1129 ASF container format.
1130
1131 paraslash only tracks the most common tags that are supported by
1132 all tag variants: artist, title, year, album, comment. When a file
1133 is added to the AFS database, the meta data of the file is extracted
1134 and stored in the audio file table.
1135
1136 Chunks and chunk tables
1137 ~~~~~~~~~~~~~~~~~~~~~~~
1138
1139 paraslash uses the word "chunk" as common term for the building blocks
1140 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1141 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1142 size varies considerably between audio formats, from a few hundred
1143 bytes (MP3) up to 8K (WMA).
1144
1145 The chunk table contains the offsets within the audio file that
1146 correspond to the chunk boundaries of the file. Like the meta data,
1147 the chunk table is computed and stored in the database whenever an
1148 audio file is added.
1149
1150 The paraslash senders (see below) always send complete chunks. The
1151 granularity for seeking is therefore determined by the chunk size.
1152
1153 Audio format handlers
1154 ~~~~~~~~~~~~~~~~~~~~~
1155
1156 For each audio format paraslash contains an audio format handler whose
1157 first task is to tell whether a given file is a valid audio file of
1158 this type. If so, the audio file handler extracts some technical data
1159 (duration, sampling rate, number of channels etc.), computes the
1160 chunk table and reads the meta data.
1161
1162 The audio format handler code is linked into para_server and executed
1163 via the _add_ command. The same code is also available as a stand-alone
1164 tool, para_afh, which can be used to print the technical data, the
1165 chunk table and the meta data of a file. Furthermore, one can use
1166 para_afh to cut an audio file, i.e. to select some of its chunks to
1167 produce a new file containing only these chunks.
1168
1169 ----------
1170 Networking
1171 ----------
1172
1173 Paraslash uses different network connections for control and data.
1174 para_client communicates with para_server over a dedicated TCP control
1175 connection. To transport audio data, separate data connections are
1176 used. For these data connections, a variety of transports (UDP, DCCP,
1177 HTTP) can be chosen.
1178
1179 The chapter starts with the REFERENCE(The paraslash control
1180 service, control service), followed by a section on the various
1181 REFERENCE(Streaming protocols, streaming protocols) in which the data
1182 connections are described. The way audio file headers are embedded into
1183 the stream is discussed REFERENCE(Streams with headers and headerless
1184 streams, briefly) before the REFERENCE(Networking examples, example
1185 section) which illustrates typical commands for real-life scenarios.
1186
1187 Both IPv4 and IPv6 are supported.
1188
1189 The paraslash control service
1190 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1191
1192 para_server is controlled at runtime via the paraslash control
1193 connection. This connection is used for server commands (play, stop,
1194 ...) as well as for afs commands (ls, select, ...).
1195
1196 The server listens on a TCP port and accepts connections from clients
1197 that connect the open port. Each connection causes the server to fork
1198 off a client process which inherits the connection and deals with that
1199 client only. In this classical accept/fork approach the server process
1200 is unaffected if the child dies or goes crazy for whatever reason. In
1201 fact, the child process can not change address space of server process.
1202
1203 The section on REFERENCE(Client-server authentication, client-server
1204 authentication) above described the early connection establishment
1205 from the crypto point of view. Here it is described what happens
1206 after the connection (including crypto setup) has been established.
1207 There are four processes involved during command dispatch as sketched
1208 in the following diagram.
1209
1210 <<
1211 <pre>
1212 server_host client_host
1213 ~~~~~~~~~~~ ~~~~~~~~~~~
1214
1215 +-----------+ connect +-----------+
1216 |para_server|<------------------------------ |para_client|
1217 +-----------+ +-----------+
1218 | ^
1219 | fork +---+ |
1220 +----------> |AFS| |
1221 | +---+ |
1222 | ^ |
1223 | | |
1224 | | connect (cookie) |
1225 | | |
1226 | | |
1227 | fork +-----+ inherited connection |
1228 +---------->|child|<--------------------------+
1229 +-----+
1230 </pre>
1231 >>
1232
1233 Note that the child process is not a child of the afs process,
1234 so communication of these two processes has to happen via local
1235 sockets. In order to avoid abuse of the local socket by unrelated
1236 processes, a magic cookie is created once at server startup time just
1237 before the server process forks off the AFS process. This cookie is
1238 known to the server, AFS and the child, but not to unrelated processes.
1239
1240 There are two different kinds of commands: First there are commands
1241 that cause the server to respond with some answer such as the list
1242 of all audio files. All but the addblob commands (addimg, addlyr,
1243 addpl, addmood) are of this kind. The addblob commands add contents
1244 to the database, so they need to transfer data the other way round,
1245 from the client to the server.
1246
1247 There is no knowledge about the server commands built into para_client,
1248 so it does not know about addblob commands. Instead, it inspects the
1249 first data package sent by the server for a magic string. If this
1250 string was found, it sends STDIN to the server, otherwise it dumps
1251 data from the server to STDOUT.
1252
1253 Streaming protocols
1254 ~~~~~~~~~~~~~~~~~~~
1255
1256 A network (audio) stream usually consists of one streaming source,
1257 the _sender_, and one or more _receivers_ which read data over the
1258 network from the streaming source.
1259
1260 Senders are thus part of para_server while receivers are part of
1261 para_audiod. Moreover, there is the stand-alone tool para_recv which
1262 can be used to manually download a stream, either from para_server
1263 or from a web-based audio streaming service.
1264
1265 The following three streaming protocols are supported by paraslash:
1266
1267 - HTTP. Recommended for public streams that can be played by
1268 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1269 sender is supported on all operating systems and all platforms.
1270
1271 - DCCP. Recommended for LAN streaming. DCCP is currently
1272 available only for Linux.
1273
1274 - UDP. Recommended for multicast LAN streaming.
1275
1276 See the Appendix on REFERENCE(Network protocols, network protocols)
1277 for brief descriptions of the various protocols relevant for network
1278 audio streaming with paraslash.
1279
1280 It is possible to activate more than one sender simultaneously.
1281 Senders can be controlled at run time and via config file and command
1282 line options.
1283
1284 Note that audio connections are _not_ encrypted. Transport or Internet
1285 layer encryption should be used if encrypted data connections are
1286 needed.
1287
1288 Since DCCP and TCP are both connection-oriented protocols, connection
1289 establishment/teardown and access control are very similar between
1290 these two streaming protocols. UDP is the most lightweight option,
1291 since in contrast to TCP/DCCP it is connectionless. It is also the
1292 only protocol supporting IP multicast.
1293
1294 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1295 clients to connect and establish a connection via some protocol-defined
1296 handshake mechanism. Both senders maintain two linked lists each:
1297 The list of all clients which are currently connected, and the list
1298 of access control entries which determines who is allowed to connect.
1299 IP-based access control may be configured through config file and
1300 command line options and via the "allow" and "deny" sender subcommands.
1301
1302 Upon receiving a GET request from the client, the HTTP sender sends
1303 back a status line and a message. The body of this message is the
1304 audio stream. This is common practice and is supported by many popular
1305 clients which can thus be used to play a stream offered by para_server.
1306 For DCCP things are a bit simpler: No messages are exchanged between
1307 the receiver and sender. The client simply connects and the sender
1308 starts to stream.
1309
1310 DCCP is an experimental protocol which offers a number of new features
1311 not available for TCP. Both ends can negotiate these features using
1312 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1313 datagram-based (no retransmissions) and thus should not be used over
1314 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1315 is access to a variety of different congestion-control mechanisms
1316 called CCIDs. Two different CCIDs are available per default on Linux:
1317
1318
1319 - _CCID 2_. A Congestion Control mechanism similar to that
1320 of TCP. The sender maintains a congestion window and halves
1321 this window in response to congestion.
1322
1323
1324 - _CCID-3_. Designed to be fair when competing for bandwidth.
1325 It has lower variation of throughput over time compared with
1326 TCP, which makes it suitable for streaming media.
1327
1328 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1329 single list, the _target list_. This list describes the set of clients
1330 to which the stream is sent. There is no list for access control and
1331 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1332 and "delete" commands can be used to modify the target list.
1333
1334 Since both UDP and DCCP offer an unreliable datagram-based transport,
1335 additional measures are necessary to guard against disruptions over
1336 networks that are lossy or which may be subject to interference (as
1337 is for instance the case with WiFi). Paraslash uses FEC (Forward
1338 Error Correction) to guard against packet losses and reordering. The
1339 stream is FEC-encoded before it is sent through the UDP socket and
1340 must be decoded accordingly on the receiver side.
1341
1342 The packet size and the amount of redundancy introduced by FEC can
1343 be configured via the FEC parameters which are dictated by server
1344 and may also be configured through the "sender" command. The FEC
1345 parameters are encoded in the header of each network packet, so no
1346 configuration is necessary on the receiver side. See the section on
1347 REFERENCE(Forward error correction, FEC) below.
1348
1349 Streams with headers and headerless streams
1350 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1351
1352 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1353 needed to decode the stream is only contained in the audio file
1354 header of the container format but not in each data chunk. Clients
1355 must be able to obtain this information in case streaming starts in
1356 the middle of the file or if para_audiod is started while para_server
1357 is already sending a stream.
1358
1359 This is accomplished in different ways, depending on the streaming
1360 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1361 header is sent prior to audio file data. This technique however does
1362 not work for the connectionless UDP transport. Hence the audio file
1363 header is periodically being embedded into the UDP audio data stream.
1364 By default, the header is resent after five seconds. The receiver has
1365 to wait until the next header arrives before it can start decoding
1366 the stream.
1367
1368 Examples
1369 ~~~~~~~~
1370
1371 The sender command of para_server allows to (de-)activate senders
1372 and to change the access permissions senders at runtime. The "si"
1373 (server info) command is used to list the streaming options of the
1374 currently running server as well as the various sender access lists.
1375
1376 -> Show client/target/access lists:
1377
1378 para_client si
1379
1380 -> Obtain general help for the sender command:
1381
1382 para_client help sender
1383
1384 -> Get help for a specific sender (contains further examples):
1385
1386 s=http # or dccp or udp
1387 para_client sender $s help
1388
1389 By default para_server activates both the HTTP and th DCCP sender on
1390 startup. This can be changed via command line options or para_server's
1391 config file.
1392
1393 -> List config file options for senders:
1394
1395 para_server -h
1396
1397 All senders share the "on" and "off" commands, so senders may be
1398 activated and deactivated independently of each other.
1399
1400 -> Switch off the http sender:
1401
1402 para_client sender http off
1403
1404 -> Receive a DCCP stream using CCID2 and write the output into a file:
1405
1406 host=foo.org; ccid=2; filename=bar
1407 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1408
1409 Note the quotes around the arguments for the dccp receiver. Each
1410 receiver has its own set of command line options and its own command
1411 line parser, so arguments for the dccp receiver must be protected
1412 from being interpreted by para_recv.
1413
1414 -> Start UDP multicast, using the default multicast address:
1415
1416 para_client sender udp add 224.0.1.38
1417
1418 -> Receive FEC-encoded multicast stream and write the output into a file:
1419
1420 filename=foo
1421 para_recv -r udp > $filename
1422
1423 -> Add an UDP unicast for a client to the target list of the UDP sender:
1424
1425 t=client.foo.org
1426 para_client sender udp add $t
1427
1428 -> Receive this (FEC-encoded) unicast stream:
1429
1430 filename=foo
1431 para_recv -r 'udp -i 0.0.0.0' > $filename
1432
1433 -> Create a minimal config for para_audiod for HTTP streams:
1434
1435 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1436 echo receiver \".:http -i $s\" > $c
1437 para_audiod --config $c
1438
1439 -------
1440 Filters
1441 -------
1442
1443 A paraslash filter is a module which transforms an input stream into
1444 an output stream. Filters are included in the para_audiod executable
1445 and in the stand-alone tool para_filter which usually contains the
1446 same modules.
1447
1448 While para_filter reads its input stream from STDIN and writes
1449 the output to STDOUT, the filter modules of para_audiod are always
1450 connected to a receiver which produces the input stream and a writer
1451 which absorbs the output stream.
1452
1453 Some filters depend on a specific library being installed and are
1454 not compiled in if this library was not found at compile time. To
1455 see the list of supported filters, run para_filter and para_audiod
1456 with the --help option. The output looks similar to the following:
1457
1458 Available filters:
1459 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1460
1461 Out of these filter modules, a chain of filters can be constructed,
1462 much in the way Unix pipes can be chained, and analogous to the use
1463 of modules in gstreamer: The output of the first filter becomes the
1464 input of the second filter. There is no limitation on the number of
1465 filters and the same filter may occur more than once.
1466
1467 Like receivers, each filter has its own command line options which
1468 must be quoted to protect them from the command line options of
1469 the driving application (para_audiod or para_filter). Example:
1470
1471 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1472
1473 For para_audiod, each audio format has its own set of filters. The
1474 name of the audio format for which the filter should be applied can
1475 be used as the prefix for the filter option. Example:
1476
1477 para_audiod -f 'mp3:prebuffer --duration 300'
1478
1479 The "mp3" prefix above is actually interpreted as a POSIX extended
1480 regular expression. Therefore
1481
1482 para_audiod -f '.:prebuffer --duration 300'
1483
1484 activates the prebuffer filter for all supported audio formats (because
1485 "." matches all audio formats) while
1486
1487 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1488
1489 activates it only for wma and ogg streams.
1490
1491 Decoders
1492 ~~~~~~~~
1493
1494 For each supported audio format there is a corresponding filter
1495 which decodes audio data in this format to 16 bit PCM data which
1496 can be directly sent to the sound device or any other software that
1497 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1498 filters are called _decoders_ in general, and xxxdec is the name of
1499 the paraslash decoder for the audio format xxx. For example, the mp3
1500 decoder filter is called mp3dec.
1501
1502 Note that the output of the decoder is about 10 times larger than
1503 its input. This means that filters that operate on the decoded audio
1504 stream have to deal with much more data than filters that transform
1505 the audio stream before it is fed to the decoder.
1506
1507 Paraslash relies on external libraries for most decoders, so these
1508 libraries must be installed for the decoder to be included in the
1509 para_filter and para_audiod executables. The oggdec filter depends
1510 on the libogg and libvorbis libraries for example.
1511
1512 Forward error correction
1513 ~~~~~~~~~~~~~~~~~~~~~~~~
1514
1515 As already mentioned REFERENCE(Streaming protocols, earlier),
1516 paraslash uses forward error correction (FEC) for the unreliable UDP
1517 and DCCP transports. FEC is a technique which was invented already
1518 in 1960 by Reed and Solomon and which is widely used for the parity
1519 calculations of storage devices (RAID arrays). It is based on the
1520 algebraic concept of finite fields, today called Galois fields, in
1521 honour of the mathematician Galois (1811-1832). The FEC implementation
1522 of paraslash is based on code by Luigi Rizzo.
1523
1524 Although the details require a sound knowledge of the underlying
1525 mathematics, the basic idea is not hard to understand: For positive
1526 integers k and n with k < n it is possible to compute for any k given
1527 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1528 ..., p_r such that all data bytes can be reconstructed from *any*
1529 k bytes of the set
1530
1531 {d_1, ..., d_k, p_1, ..., p_r}.
1532
1533 FEC-encoding for unreliable network transports boils down to slicing
1534 the audio stream into groups of k suitably sized pieces called _slices_
1535 and computing the r corresponding parity slices. This step is performed
1536 in para_server which then sends both the data and the parity slices
1537 over the unreliable network connection. If the client was able
1538 to receive at least k of the n = k + r slices, it can reconstruct
1539 (FEC-decode) the original audio stream.
1540
1541 From these observations it is clear that there are three different
1542 FEC parameters: The slice size, the number of data slices k, and the
1543 total number of slices n. It is crucial to choose the slice size
1544 such that no fragmentation of network packets takes place because
1545 FEC only guards against losses and reordering but fails if slices are
1546 received partially.
1547
1548 FEC decoding in paralash is performed through the fecdec filter which
1549 usually is the first filter (there can be other filters before fecdec
1550 if these do not alter the audio stream).
1551
1552
1553 Volume adjustment (amp and compress)
1554 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1555
1556 The amp and the compress filter both adjust the volume of the audio
1557 stream. These filters operate on uncompressed audio samples. Hence
1558 they are usually placed directly after the decoding filter. Each
1559 sample is multiplied with a scaling factor (>= 1) which makes amp
1560 and compress quite expensive in terms of computing power.
1561
1562 *amp*
1563
1564 The amp filter amplifies the audio stream by a fixed scaling factor
1565 that must be known in advance. For para_audiod this factor is derived
1566 from the amplification field of the audio file's entry in the audio
1567 file table while para_filter uses the value given at the command line.
1568
1569 The optimal scaling factor F for an audio file is the largest real
1570 number F >= 1 such that after multiplication with F all samples still
1571 fit into the sample interval [-32768, 32767]. One can use para_filter
1572 in combination with the sox utility to compute F:
1573
1574 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1575
1576 The amplification value V which is stored in the audio file table,
1577 however, is an integer between 0 and 255 which is connected to F
1578 through the formula
1579
1580 V = (F - 1) * 64.
1581
1582 To store V in the audio file table, the command
1583
1584 para_client -- touch -a=V file.mp3
1585
1586 is used. The reader is encouraged to write a script that performs
1587 these computations :)
1588
1589 *compress*
1590
1591 Unlike the amplification filter, the compress filter adjusts the volume
1592 of the audio stream dynamically without prior knowledge about the peak
1593 value. It maintains the maximal volume of the last n samples of the
1594 audio stream and computes a suitable amplification factor based on that
1595 value and the various configuration options. It tries to chose this
1596 factor such that the adjusted volume meets the desired target level.
1597
1598 Note that it makes sense to combine amp and compress.
1599
1600 Misc filters (wav and prebuffer)
1601 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1602
1603 These filters are rather simple and do not modify the audio stream at
1604 all. The wav filter is only useful with para_filter and in connection
1605 with a decoder. It asks the decoder for the number of channels and the
1606 sample rate of the stream and adds a Microsoft wave header containing
1607 this information at the beginning. This allows to write wav files
1608 rather than raw PCM files (which do not contain any information about
1609 the number of channels and the sample rate).
1610
1611 The prebuffer filter simply delays the output until the given time has
1612 passed (starting from the time the first byte was available in its
1613 input queue) or until the given amount of data has accumulated. It
1614 is mainly useful for para_audiod if the standard parameters result
1615 in buffer underruns.
1616
1617 Both filters require almost no additional computing time, even when
1618 operating on uncompressed audio streams, since data buffers are simply
1619 "pushed down" rather than copied.
1620
1621 Examples
1622 ~~~~~~~~
1623
1624 -> Decode an mp3 file to wav format:
1625
1626 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1627
1628 -> Amplify a raw audio file by a factor of 1.5:
1629
1630 para_filter -f amp --amp 32 < foo.raw > bar.raw
1631
1632 ------
1633 Output
1634 ------
1635
1636 Once an audio stream has been received and decoded to PCM format,
1637 it can be sent to a sound device for playback. This part is performed
1638 by paraslash _writers_ which are described in this chapter.
1639
1640 Writers
1641 ~~~~~~~
1642
1643 A paraslash writer acts as a data sink that consumes but does not
1644 produce audio data. Paraslash writers operate on the client side and
1645 are contained in para_audiod and in the stand-alone tool para_write.
1646
1647 The para_write program reads uncompressed audio data from STDIN. If
1648 this data starts with a wav header, sample rate, sample format and
1649 channel count are read from the header. Otherwise CD audio (44.1KHz
1650 16 bit little endian, stereo) is assumed but this can be overridden
1651 by command line options. para_audiod, on the other hand, obtains
1652 the sample rate and the number of channels from the decoder.
1653
1654 Like receivers and filters, each writer has an individual set of
1655 command line options, and for para_audiod writers can be configured
1656 per audio format separately. It is possible to activate more than
1657 one writer for the same stream simultaneously.
1658
1659 OS-dependent APIs
1660 ~~~~~~~~~~~~~~~~~
1661
1662 Unfortunately, the various flavours of Unix on which paraslash
1663 runs on have different APIs for opening a sound device and starting
1664 playback. Hence for each such API there is a paraslash writer that
1665 can play the audio stream via this API.
1666
1667 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1668 Linux systems. Although there are several mid-layer APIs in use by
1669 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1670 currently supports only the low-level ALSA API which is not supposed
1671 to be change. ALSA is very feature-rich, in particular it supports
1672 software mixing via its DMIX plugin. ALSA is the default writer on
1673 Linux systems.
1674
1675 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1676 is also available on Linux systems, usually provided by ALSA as an
1677 emulation for backwards compatibility. This API is rather simple but
1678 also limited. For example only one application can open the device
1679 at any time. The OSS writer is activated by default on BSD Systems.
1680
1681 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1682 for this API is only compiled in on such systems and is of course
1683 the default there.
1684
1685 *FILE*. The file writer allows to capture the audio stream and
1686 write the PCM data to a file on the file system rather than playing
1687 it through a sound device. It is supported on all platforms and is
1688 always compiled in.
1689
1690 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1691 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1692 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1693 through an output plugin of libao.
1694
1695 Examples
1696 ~~~~~~~~
1697
1698 -> Use the OSS writer to play a wav file:
1699
1700 para_write --writer oss < file.wav
1701
1702 -> Enable ALSA software mixing for mp3 streams
1703
1704 para_audiod --writer 'mp3:alsa -d plug:swmix'
1705
1706
1707 ---
1708 Gui
1709 ---
1710
1711 para_gui executes an arbitrary command which is supposed to print
1712 status information to STDOUT. It then displays this information in
1713 a curses window. By default the command
1714
1715 para_audioc -- stat -p
1716
1717 is executed, but this can be customized via the --stat_cmd option. In
1718 particular it possible to use
1719
1720 para_client -- stat -p
1721
1722 to make para_gui work on systems on which para_audiod is not running.
1723
1724 Key bindings
1725 ~~~~~~~~~~~~
1726
1727 It is possible to bind keys to arbitrary commands via custom
1728 key-bindings. Besides the internal keys which can not be changed (help,
1729 quit, loglevel, version...), the following flavours of key-bindings
1730 are supported:
1731
1732 - external: Shutdown curses before launching the given command.
1733 Useful for starting other ncurses programs from within
1734 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1735 output format to write a mailbox containing one mail for each
1736 (admissible) file the audio file selector knows about. Then
1737 start mutt from within para_gui to browse your collection!
1738
1739 - display: Launch the command and display its stdout in
1740 para_gui's bottom window.
1741
1742 - para: Like display, but start "para_client <specified
1743 command>" instead of "<specified command>".
1744
1745 The general form of a key binding is
1746
1747 key_map k:m:c
1748
1749 which maps key k to command c using mode m. Mode may be x, d or p
1750 for external, display and paraslash commands, respectively.
1751
1752 Themes
1753 ~~~~~~
1754
1755 Currently there are only two themes for para_gui. It is easy, however,
1756 to add more themes. To create a new theme one has to define the
1757 position, color and geometry for for each status item that should be
1758 shown by this theme. See gui_theme.c for examples.
1759
1760 The "." and "," keys are used to switch between themes.
1761
1762 Examples
1763 ~~~~~~~~
1764
1765 -> Show server info:
1766
1767 key_map "i:p:si"
1768
1769 -> Jump to the middle of the current audio file by pressing F5:
1770
1771 key_map "<F5>:p:jmp 50"
1772
1773 -> vi-like bindings for jumping around:
1774
1775 key_map "l:p:ff 10"
1776 key_map "h:p:ff 10-"
1777 key_map "w:p:ff 60"
1778 key_map "b:p:ff 60-"
1779
1780 -> Print the current date and time:
1781
1782 key_map "D:d:date"
1783
1784 -> Call other curses programs:
1785
1786 key_map "U:x:aumix"
1787 key_map "!:x:/bin/bash"
1788 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1789
1790 -----------
1791 Development
1792 -----------
1793
1794 Tools
1795 ~~~~~
1796
1797 In order to compile the sources from the git repository (rather than
1798 from tar balls) and for contributing non-trivial changes to the
1799 paraslash project, some additional tools should be installed on a
1800 developer machine.
1801
1802 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1803 branches, below), the git source code management tool is used for
1804 paraslash development. It is necessary for cloning the git repository
1805 and for getting updates.
1806
1807 ftp://ftp.gnu.org/pub/gnu/gengetopt/ (gengetopt). The C code for
1808 the command line parsers of all paraslash executables is generated
1809 by gengetopt. The generated C files are shipped in the tarballs but
1810 are not contained in the git repository.
1811
1812 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1813 are generated from templates by the m4 macro processor.
1814
1815 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1816 the configure file which is shipped in the tarballs but has to be
1817 generated when compiling from git.
1818
1819 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1820 HTML version of this manual and some of the paraslash web pages are
1821 generated by the grutatxt plain text to HTML converter. If changes
1822 are made to these text files the grutatxt package must be installed
1823 to regenerate the HTML files.
1824
1825 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1826 of paraslash's C sources uses the doxygen documentation system. The
1827 conventions for documenting the source code is described in the
1828 REFERENCE(Doxygen, Doxygen section).
1829
1830 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1831 browsable HTML from the C sources. It is needed by doxygen.
1832
1833 Git branches
1834 ~~~~~~~~~~~~
1835
1836 Paraslash has been developed using the git source code management
1837 tool since 2006. Development is organized roughly in the same spirit
1838 as the git development itself, as described below.
1839
1840 The following text passage is based on "A note from the maintainer",
1841 written by Junio C Hamano, the maintainer of git.
1842
1843 There are four branches in the paraslash repository that track the
1844 source tree: "master", "maint", "next", and "pu".
1845
1846 The "master" branch is meant to contain what is well tested and
1847 ready to be used in a production setting. There could occasionally be
1848 minor breakages or brown paper bag bugs but they are not expected to
1849 be anything major, and more importantly quickly and easily fixable.
1850 Every now and then, a "feature release" is cut from the tip of this
1851 branch, named with three dotted decimal digits, like 0.4.2.
1852
1853 Whenever changes are about to be included that will eventually lead to
1854 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1855 "master" at that point. Obvious, safe and urgent fixes after the major
1856 release are applied to this branch and maintenance releases are cut
1857 from it. New features never go to this branch. This branch is also
1858 merged into "master" to propagate the fixes forward.
1859
1860 A trivial and safe enhancement goes directly on top of "master".
1861 New development does not usually happen on "master", however.
1862 Instead, a separate topic branch is forked from the tip of "master",
1863 and it first is tested in isolation; Usually there are a handful such
1864 topic branches that are running ahead of "master". The tip of these
1865 branches is not published in the public repository to keep the number
1866 of branches that downstream developers need to worry about low.
1867
1868 The quality of topic branches varies widely. Some of them start out as
1869 "good idea but obviously is broken in some areas" and then with some
1870 more work become "more or less done and can now be tested by wider
1871 audience". Luckily, most of them start out in the latter, better shape.
1872
1873 The "next" branch is to merge and test topic branches in the latter
1874 category. In general, this branch always contains the tip of "master".
1875 It might not be quite rock-solid production ready, but is expected to
1876 work more or less without major breakage. The maintainer usually uses
1877 the "next" version of paraslash for his own pleasure, so it cannot
1878 be _that_ broken. The "next" branch is where new and exciting things
1879 take place.
1880
1881 The two branches "master" and "maint" are never rewound, and "next"
1882 usually will not be either (this automatically means the topics that
1883 have been merged into "next" are usually not rebased, and you can find
1884 the tip of topic branches you are interested in from the output of
1885 "git log next"). You should be able to safely build on top of them.
1886
1887 However, at times "next" will be rebuilt from the tip of "master" to
1888 get rid of merge commits that will never be in "master". The commit
1889 that replaces "next" will usually have the identical tree, but it
1890 will have different ancestry from the tip of "master".
1891
1892 The "pu" (proposed updates) branch bundles the remainder of the
1893 topic branches. The "pu" branch, and topic branches that are only in
1894 "pu", are subject to rebasing in general. By the above definition
1895 of how "next" works, you can tell that this branch will contain quite
1896 experimental and obviously broken stuff.
1897
1898 When a topic that was in "pu" proves to be in testable shape, it
1899 graduates to "next". This is done with
1900
1901 git checkout next
1902 git merge that-topic-branch
1903
1904 Sometimes, an idea that looked promising turns out to be not so good
1905 and the topic can be dropped from "pu" in such a case.
1906
1907 A topic that is in "next" is expected to be polished to perfection
1908 before it is merged to "master". Similar to the above, this is
1909 done with
1910
1911 git checkout master
1912 git merge that-topic-branch
1913 git branch -d that-topic-branch
1914
1915 Note that being in "next" is not a guarantee to appear in the next
1916 release (being in "master" is such a guarantee, unless it is later
1917 found seriously broken and reverted), nor even in any future release.
1918
1919 Coding Style
1920 ~~~~~~~~~~~~
1921
1922 The preferred coding style for paraslash coincides more or less
1923 with the style of the Linux kernel. So rather than repeating what is
1924 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
1925 there), here are the most important points.
1926
1927 - Burn the GNU coding standards.
1928 - Never use spaces for indentation.
1929 - Tabs are 8 characters, and thus indentations are also 8 characters.
1930 - Don't put multiple assignments on a single line.
1931 - Avoid tricky expressions.
1932 - Don't leave whitespace at the end of lines.
1933 - The limit on the length of lines is 80 columns.
1934 - Use K&R style for placing braces and spaces:
1935
1936 if (x is true) {
1937 we do y
1938 }
1939
1940 - Use a space after (most) keywords.
1941 - Do not add spaces around (inside) parenthesized expressions.
1942 - Use one space around (on each side of) most binary and ternary operators.
1943 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1944 - Mixed-case names are frowned upon.
1945 - Descriptive names for global variables are a must.
1946 - Avoid typedefs.
1947 - Functions should be short and sweet, and do just one thing.
1948 - The number of local variables shouldn't exceed 10.
1949 - Gotos are fine if they improve readability and reduce nesting.
1950 - Don't use C99-style "// ..." comments.
1951 - Names of macros defining constants and labels in enums are capitalized.
1952 - Enums are preferred when defining several related constants.
1953 - Always use the paraslash wrappers for allocating memory.
1954 - If the name of a function is an action or an imperative.
1955 command, the function should return an error-code integer
1956 (<0 means error, >=0 means success). If the name is a
1957 predicate, the function should return a "succeeded" boolean.
1958
1959
1960 Doxygen
1961 ~~~~~~~
1962
1963 Doxygen is a documentation system for various programming
1964 languages. The paraslash project uses Doxygen for generating the API
1965 reference on the web pages, but good source code documentation is
1966 also beneficial to people trying to understand the code structure
1967 and the interactions between the various source files.
1968
1969 It is more illustrative to look at the source code for examples than
1970 to describe the conventions for documenting the source in this manual,
1971 so we only describe which parts of the code need doxygen comments,
1972 but leave out details on documentation conventions.
1973
1974 As a rule, only the public part of the C source is documented with
1975 Doxygen. This includes structures, defines and enumerations in header
1976 files as well as public (non-static) C functions. These should be
1977 documented completely. For example each parameter and the return
1978 value of a public function should get a descriptive comment.
1979
1980 No doxygen comments are necessary for static functions and for
1981 structures and enumerations in C files (which are used only within
1982 this file). This does not mean, however, that those entities need
1983 no documentation at all. Instead, common sense should be applied to
1984 document what is not obvious from reading the code.
1985
1986 --------
1987 Appendix
1988 --------
1989
1990 Network protocols
1991 ~~~~~~~~~~~~~~~~~
1992
1993 *IP*. The _Internet Protocol_ is the primary networking protocol
1994 used for the Internet. All protocols described below use IP as the
1995 underlying layer. Both the prevalent IPv4 and the next-generation
1996 IPv6 variant are being deployed actively worldwide.
1997
1998 *Connection-oriented and connectionless protocols*. Connectionless
1999 protocols differ from connection-oriented ones in that state
2000 associated with the sending/receiving endpoints is treated
2001 implicitly. Connectionless protocols maintain no internal knowledge
2002 about the state of the connection. Hence they are not capable of
2003 reacting to state changes, such as sudden loss or congestion on the
2004 connection medium. Connection-oriented protocols, in contrast, make
2005 this knowledge explicit. The connection is established only after
2006 a bidirectional handshake which requires both endpoints to agree
2007 on the state of the connection, and may also involve negotiating
2008 specific parameters for the particular connection. Maintaining an
2009 up-to-date internal state of the connection also in general means
2010 that the sending endpoints perform congestion control, adapting to
2011 qualitative changes of the connection medium.
2012
2013 *Reliability*. In IP networking, packets can be lost, duplicated,
2014 or delivered out of order, and different network protocols handle
2015 these problems in different ways. We call a transport-layer protocol
2016 _reliable_, if it turns the unreliable IP delivery into an ordered,
2017 duplicate- and loss-free delivery of packets. Sequence numbers
2018 are used to discard duplicates and re-arrange packets delivered
2019 out-of-order. Retransmission is used to guarantee loss-free
2020 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2021 or data integrity.
2022
2023 *Classification*. With these definitions the protocols which are used
2024 by paraslash for steaming audio data may be classified as follows.
2025
2026 - HTTP/TCP: connection-oriented, reliable,
2027 - UDP: connectionless, unreliable,
2028 - DCCP: connection-oriented, unreliable.
2029
2030 Below we give a short descriptions of these protocols.
2031
2032 *TCP*. The _Transmission Control Protocol_ provides reliable,
2033 ordered delivery of a stream and a classic window-based congestion
2034 control. In contrast to UDP and DCCP (see below), TCP does not have
2035 record-oriented or datagram-based syntax, i.e. it provides a stream
2036 which is unaware and independent of any record (packet) boundaries.
2037 TCP is used extensively by many application layers. Besides HTTP (the
2038 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2039 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2040 top of TCP.
2041
2042 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2043 protocol, built as a thin layer directly on top of IP. For this reason,
2044 it offers the same best-effort service as IP itself, i.e. there is no
2045 detection of duplicate or reordered packets. Being a connectionless
2046 protocol, only minimal internal state about the connection is
2047 maintained, which means that there is no protection against packet
2048 loss or network congestion. Error checking and correction (if at all)
2049 are performed in the application.
2050
2051 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2052 connection-oriented state maintenance known from TCP with the
2053 unreliable, datagram-based transport of UDP. This means that it
2054 is capable of reacting to changes in the connection by performing
2055 congestion control, offering multiple alternative approaches. But it
2056 is bound to datagram boundaries (the maximum packet size supported
2057 by a medium), and like UDP it lacks retransmission to protect
2058 against loss. Due to the use of sequence numbers, it is however
2059 able to react to loss (interpreted as a congestion indication) and
2060 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2061 to negotiate specific, binding features for a connection, such as
2062 the choice of congestion control: classic, window-based congestion
2063 control known from TCP is available as CCID-2, rate-based, "smooth"
2064 congestion control is offered as CCID-3.
2065
2066 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2067 protocol on top of TCP. It is spoken by web servers and is most often
2068 used for web services. However, as can be seen by the many Internet
2069 radio stations and YouTube/Flash videos, http is by far not limited to
2070 the delivery of web pages only. Being a simple request/response based
2071 protocol, the semantics of the protocol also allow the delivery of
2072 multimedia content, such as audio over http.
2073
2074 *Multicast*. IP multicast is not really a protocol but a technique
2075 for one-to-many communication over an IP network. The challenge is to
2076 deliver information to a group of destinations simultaneously using
2077 the most efficient strategy to send the messages over each link of
2078 the network only once. This has benefits for streaming multimedia:
2079 the standard one-to-one unicast offered by TCP/DCCP means that
2080 n clients listening to the same stream also consume n-times the
2081 resources, whereas multicast requires to send the stream just once,
2082 irrespective of the number of receivers. Since it would be costly to
2083 maintain state for each listening receiver, multicast often implies
2084 connectionless transport, which is the reason that it is currently
2085 only available via UDP.
2086
2087 License
2088 ~~~~~~~
2089
2090 Paraslash is licensed under the GPL, version 2. Most of the code
2091 base has been written from scratch, and those parts are GPL V2
2092 throughout. Notable exceptions are FEC and the WMA decoder. See the
2093 corresponding source files for licencing details for these parts. Some
2094 code sniplets of several other third party software packages have
2095 been incorporated into the paraslash sources, for example log message
2096 coloring was taken from the git sources. These third party software
2097 packages are all published under the GPL or some other license
2098 compatible to the GPL.
2099
2100 Acknowledgements
2101 ~~~~~~~~~~~~~~~~
2102
2103 Many thanks to Gerrit Renker who read an early draft of this manual
2104 and contributed significant improvements.
2105
2106 ----------
2107 References
2108 ----------
2109
2110 Articles
2111 ~~~~~~~~
2112 - Reed, Irving S.; Solomon, Gustave (1960),
2113 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2114 Polynomial Codes over Certain Finite Fields), Journal of the
2115 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2116 300-304, doi:10.1137/0108018)
2117
2118 RFCs
2119 ~~~~
2120
2121 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2122 User Datagram Protocol
2123 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2124 Internet Protocol
2125 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2126 RSA Cryptography Specifications
2127 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2128 (2006): Datagram Congestion Control Protocol (DCCP)
2129 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2130 Congestion Control ID 2: TCP-like Congestion Control
2131 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2132 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2133
2134 Application web pages
2135 ~~~~~~~~~~~~~~~~~~~~~
2136
2137 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash)
2138 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2139 - XREFERENCE(http://www.mpg123.de/, mpg123)
2140 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2141 - XREFERENCE(http://www.icecast.org/, icecast)
2142 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2143
2144 External documentation
2145 ~~~~~~~~~~~~~~~~~~~~~~
2146
2147 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2148 H. Peter Anvin: The mathematics of Raid6)
2149 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2150 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2151 Communication Protocols)
2152
2153 Code
2154 ~~~~
2155 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2156 Original FEC implementation by Luigi Rizzo)
2157