Merge branch 't/utf8'
[paraslash.git] / web / manual.m4
1 dnl To generate the html version, execute
2 dnl m4 web/manual.m4 | grutatxt --toc
3
4 define(`LOCAL_LINK_NAME', `translit(`$1', `A-Z
5 ', `a-z__')')
6 define(`REMOVE_NEWLINE', `translit(`$1',`
7 ', ` ')')
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, ...) over local and/or
81 remote networks. It listens on a TCP port and accepts commands such
82 as play, stop, pause, next from authenticated clients. There are
83 many more commands though, see the man page of para_server for a
84 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 If para_client is started without non-option arguments, an interactive
119 session (shell) is started. Command history and command completion are
120 supported through libreadline.
121
122 *para_audiod*
123
124 The local daemon that collects information from para_server.
125
126 It runs on the client side and connects to para_server. As soon as
127 para_server announces the availability of an audio stream, para_audiod
128 starts an appropriate receiver, any number of filters and a paraslash
129 writer to play the stream.
130
131 Moreover, para_audiod listens on a local socket and sends status
132 information about para_server and para_audiod to local clients on
133 request. Access via this local socket may be restricted by using Unix
134 socket credentials, if available.
135
136
137 *para_audioc*
138
139 The client program which talks to para_audiod. Used to control
140 para_audiod, to receive status info, or to grab the stream at any
141 point of the decoding process. Like para_client, para_audioc supports
142 interactive sessions on systems with libreadline.
143
144 *para_recv*
145
146 A command line HTTP/DCCP/UDP stream grabber. The http mode is
147 compatible with arbitrary HTTP streaming sources (e.g. icecast).
148 In addition to the three network streaming modes, para_recv can also
149 operate in local (afh) mode. In this mode it writes the content of
150 an audio file on the local file system in complete chunks to stdout,
151 optionally 'just in time'. This allows to cut an audio file without
152 first decoding it, and it enables third-party software which is unaware
153 of the particular audio format to send complete frames in real time.
154
155 *para_filter*
156
157 A filter program that reads from STDIN and writes to STDOUT.
158 Like para_recv, this is an atomic building block which can be used to
159 assemble higher-level audio receiving facilities. It combines several
160 different functionalities in one tool: decoders for multiple audio
161 formats and a number of processing filters, among these a normalizer
162 for audio volume.
163
164 *para_afh*
165
166 A small stand-alone program that prints tech info about the given
167 audio file to STDOUT. It can be instructed to print a "chunk table",
168 an array of offsets within the audio file.
169
170 *para_write*
171
172 A modular audio stream writer. It supports a simple file writer
173 output plug-in and optional WAV/raw players for ALSA (Linux) and for
174 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
175 or raw audio player.
176
177 *para_play*
178
179 A command line audio player.
180
181 *para_gui*
182
183 Curses-based gui that presents status information obtained in a curses
184 window. Appearance can be customized via themes. para_gui provides
185 key-bindings for the most common server commands and new key-bindings
186 can be added easily.
187
188
189 *para_fade*
190
191 An alarm clock and volume-fader for OSS and ALSA.
192
193 -----------
194 Quick start
195 -----------
196
197 This chapter lists the REFERENCE(Requirements, necessary software)
198 that must be installed to compile the paraslash package, describes
199 how to REFERENCE(Installation, compile and install) the paraslash
200 source code and the steps that have to be performed in order to
201 REFERENCE(Quick start, set up) a typical server and client.
202
203 Requirements
204 ~~~~~~~~~~~~
205
206 In any case you'll need
207
208 - XREFERENCE(http://systemlinux.org/~maan/osl/, libosl).
209 The _object storage layer_ library is used by para_server. To
210 clone the source code repository, execute
211
212 git clone git://git.tuebingen.mpg.de/osl
213
214 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc) or
215 XREFERENCE(http://clang.llvm.org, clang). All gcc versions
216 >= 3.3 are currently supported. Clang version 1.1 or newer
217 should work as well.
218
219 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
220 also shipped with the disto. On BSD systems the gnu make
221 executable is often called gmake.
222
223 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
224 scripts which run during compilation require the EMPH(Bourne
225 again shell). It is most likely already installed.
226
227 - XREFERENCE(http://www.openssl.org/, openssl) or
228 XREFERENCE(ftp://ftp.gnupg.org/gcrypt/libgcrypt/, libgcrypt).
229 At least one of these two libraries is needed as the backend
230 for cryptographic routines on both the server and the client
231 side. Both openssl and libgcrypt are usually shipped with the
232 distro, but you might have to install the development package
233 (libssl-dev or libgcrypt-dev on debian systems) as well.
234
235 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gengetopt/, gengetopt)
236 is needed to generate the C code for the command line parsers
237 of all paraslash executables.
238
239 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/help2man, help2man)
240 is used to create the man pages.
241
242 Optional:
243
244 - XREFERENCE(http://www.underbit.com/products/mad/, libmad).
245 To compile in MP3 support for paraslash, the development
246 package must be installed. It is called libmad0-dev on
247 debian-based systems. Note that libmad is not necessary on
248 the server side, i.e. for sending MP3 files.
249
250 - XREFERENCE(http://www.underbit.com/products/mad/,
251 libid3tag). For version-2 ID3 tag support, you'll need
252 the libid3tag development package libid3tag0-dev. Without
253 libid3tag, only version one tags are recognized.
254
255 - XREFERENCE(http://www.xiph.org/downloads/, ogg vorbis).
256 For ogg vorbis streams you'll need libogg, libvorbis,
257 libvorbisfile. The corresponding Debian packages are called
258 libogg-dev and libvorbis-dev.
259
260 - XREFERENCE(http://www.audiocoding.com/, libfaad). For aac
261 files (m4a) you'll need libfaad (libfaad-dev).
262
263 - XREFERENCE(http://www.speex.org/, speex). In order to stream
264 or decode speex files, libspeex (libspeex-dev) is required.
265
266 - XREFERENCE(http://flac.sourceforge.net/, flac). To stream
267 or decode files encoded with the _Free Lossless Audio Codec_,
268 libFLAC (libFLAC-dev) must be installed.
269
270 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
271 Linux, you'll need to have ALSA's development package
272 libasound2-dev installed.
273
274 - XREFERENCE(http://downloads.xiph.org/releases/ao/,
275 libao). Needed to build the ao writer (ESD, PulseAudio,...).
276 Debian package: libao-dev.
277
278 - XREFERENCE(http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html,
279 GNU Readline). If this library (libreadline-dev) is installed,
280 para_client, para_audioc and para_play support interactive
281 sessions.
282
283 Installation
284 ~~~~~~~~~~~~
285
286 First make sure all non-optional packages listed in the section on
287 REFERENCE(Requirements, required software) are installed on your
288 system.
289
290 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
291 OGG/Speex and AAC support are all optional. The configure script will
292 detect what is installed on your system and will only try to build
293 those executables that can be built with your setup.
294
295 Note that no special decoder library (not even the MP3 decoding library
296 libmad) is needed for para_server if you only want to stream MP3 or WMA
297 files. Also, it's fine to use para_server on a box without sound card.
298
299 Next, install the paraslash package on all machines, you'd like this
300 software to run on. If you compile from a released tarball, execute
301
302 (./configure && make) > /dev/null
303
304 When compiling from git or from snapshots downloaded via gitweb,
305 the above command will not work because the configure script is not
306 included in the git repository. In this case the following command
307 should be used instead:
308
309 ./autogen.sh
310
311 This runs autoconf to generate the configure script, then runs it as
312 above. Therefore you'll need autoconf for this to work.
313
314 There should be no errors but probably some warnings about missing
315 packages which usually implies that not all audio formats will be
316 supported. If headers or libs are installed at unusual locations you
317 might need to tell the configure script where to find them. Try
318
319 ./configure --help
320
321 to see a list of options. If the paraslash package was compiled
322 successfully, execute (optionally)
323
324 make test
325
326 to run the paraslash test suite. If all tests pass, execute as root
327
328 make install
329
330 to install executables under /usr/local/bin and the man pages under
331 /usr/local/man.
332
333 Configuration
334 ~~~~~~~~~~~~~
335
336 *Step 1*: Create a paraslash user
337
338 In order to control para_server at runtime you must create a paraslash
339 user. As authentication is based on the RSA crypto system you'll have
340 to create an RSA key pair. If you already have a user and an RSA key
341 pair, you may skip this step.
342
343 In this section we'll assume a typical setup: You would like to run
344 para_server on some host called server_host as user foo, and you want
345 to connect to para_server from another machine called client_host as
346 user bar.
347
348 As foo@server_host, create ~/.paraslash/server.users by typing the
349 following commands:
350
351 user=bar
352 target=~/.paraslash/server.users
353 key=~/.paraslash/id_rsa.pub.$user
354 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
355 mkdir -p ~/.paraslash
356 echo "user $user $key $perms" >> $target
357
358 Next, change to the "bar" account on client_host and generate the
359 key pair with the commands
360
361 ssh-keygen -t rsa -b 2048
362 # hit enter twice to create a key with no passphrase
363
364 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
365 that paraslash can also read keys generated by the "openssl genrsa"
366 command. However, since keys created with ssh-keygen can also be used
367 for ssh, this method is recommended.
368
369 Note that para_server refuses to use a key if it is shorter than 2048
370 bits. In particular, the RSA keys of paraslash 0.3.x will not work
371 with version 0.4.x. Moreover, para_client refuses to use a (private)
372 key which is world-readable.
373
374 para_server only needs to know the public key of the key pair just
375 created. Copy this public key to server_host:
376
377 src=~/.ssh/id_rsa.pub
378 dest=.paraslash/id_rsa.pub.$LOGNAME
379 scp $src foo@server_host:$dest
380
381 Finally, tell para_client to connect to server_host:
382
383 conf=~/.paraslash/client.conf
384 echo 'hostname server_host' > $conf
385
386
387 *Step 2*: Start para_server
388
389 Before starting the server make sure you have write permissions to
390 the directory /var/paraslash that has been created during installation:
391
392 sudo chown $LOGNAME /var/paraslash
393
394 Alternatively, use the --afs_socket Option to specify a different
395 location for the AFS command socket.
396
397 For this first try, we'll use the info loglevel to make the output
398 of para_server more verbose.
399
400 para_server -l info
401
402 Now you can use para_client to connect to the server and issue
403 commands. Open a new shell as bar@client_host and try
404
405 para_client help
406 para_client si
407
408 to retrieve the list of available commands and some server info.
409 Don't proceed if this doesn't work.
410
411 *Step 3*: Create and populate the database
412
413 An empty database is created with
414
415 para_client init
416
417 This initializes a couple of empty tables under
418 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
419 tables, but it's good to know that you can start from scratch with
420
421 rm -rf ~/.paraslash/afs_database-0.4
422
423 in case something went wrong.
424
425 Next, you need to add some audio files to that database so that
426 para_server knows about them. Choose an absolute path to a directory
427 containing some audio files and add them to the audio file table:
428
429 para_client add /my/mp3/dir
430
431 This might take a while, so it is a good idea to start with a directory
432 containing not too many files. Note that the table only contains data
433 about the audio files found, not the files themselves.
434
435 You may print the list of all known audio files with
436
437 para_client ls
438
439 *Step 4*: Configure para_audiod
440
441 para_audiod needs to create a "well-known" socket for the clients to
442 connect to. The default path for this socket is
443
444 /var/paraslash/audiod_socket.$HOSTNAME
445
446 In order to make this directory writable for para_audiod, execute
447 as bar@client_host
448
449 sudo chown $LOGNAME /var/paraslash
450
451
452 We will also have to tell para_audiod that it should receive the
453 audio stream from server_host via http:
454
455 para_audiod -l info -r '.:http -i server_host'
456
457 You should now be able to listen to the audio stream once para_server
458 starts streaming. To activate streaming, execute
459
460 para_client play
461
462 Since no playlist has been specified yet, the "dummy" mode which
463 selects all known audio files is activated automatically. See the
464 section on the REFERENCE(The audio file selector, audio file selector)
465 for how to use playlists and moods to specify which files should be
466 streamed in which order.
467
468 *Troubleshooting*
469
470 It did not work? To find out why, try to receive, decode and play the
471 stream manually using para_recv, para_filter and para_write as follows.
472
473 For simplicity we assume that you're running Linux/ALSA and that only
474 MP3 files have been added to the database.
475
476 para_recv -r 'http -i server_host' > file.mp3
477 # (interrupt with CTRL+C after a few seconds)
478 ls -l file.mp3 # should not be empty
479 para_filter -f mp3dec -f wav < file.mp3 > file.wav
480 ls -l file.wav # should be much bigger than file.mp3
481 para_write -w alsa < file.wav
482
483 Double check what is logged by para_server and use the --loglevel
484 option of para_recv, para_filter and para_write to increase verbosity.
485
486 ---------------
487 User management
488 ---------------
489
490 para_server uses a challenge-response mechanism to authenticate
491 requests from incoming connections, similar to ssh's public key
492 authentication method. Authenticated connections are encrypted using
493 the RC4 stream cipher.
494
495 In this chapter we briefly describe RSA and RC4 and sketch the
496 REFERENCE(Client-server authentication, authentication handshake)
497 between para_client and para_server. User management is discussed
498 in the section on REFERENCE(The user_list file, the user_list file).
499 These sections are all about communication between the client and the
500 server. Connecting para_audiod is a different matter and is described
501 in a REFERENCE(Connecting para_audiod, separate section).
502
503
504
505 RSA and RC4
506 ~~~~~~~~~~~
507
508 RSA is an asymmetric block cipher which is used in many applications,
509 including ssh and gpg. An RSA key consists in fact of two keys,
510 called the public key and the private key. A message can be encrypted
511 with either key and only the counterpart of that key can decrypt
512 the message. While RSA can be used for both signing and encrypting
513 a message, paraslash uses RSA only for the latter purpose. The
514 RSA public key encryption and signatures algorithms are defined in
515 detail in RFC 2437.
516
517 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
518 key stream to produce the output. Decryption uses the same function
519 calls as encryption. While RC4 supports variable key lengths,
520 paraslash uses a fixed length of 256 bits, which is considered a
521 strong encryption by today's standards. Since the same key must never
522 be used twice, a different, randomly-generated key is used for every
523 new connection.
524
525 Client-server authentication
526 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
527
528 The authentication handshake between para_client and para_server goes
529 as follows:
530
531 - para_client connects to para_server and sends an
532 authentication request for a user. It does so by connecting
533 to TCP port 2990 of the server host. This port is called the
534 para_server _control port_.
535
536 - para_server accepts the connection and forks a child process
537 which handles the incoming request. The parent process keeps
538 listening on the control port while the child process (also
539 called para_server below) continues as follows.
540
541 - para_server loads the RSA public key of that user, fills a
542 fixed-length buffer with random bytes, encrypts that buffer
543 using the public key and sends the encrypted buffer to the
544 client. The first part of the buffer is the challenge which
545 is used for authentication while the second part is the RC4
546 session key.
547
548 - para_client receives the encrypted buffer and decrypts it
549 with the user's private key, thereby obtaining the challenge
550 buffer and the session key. It sends the SHA1 hash value of
551 the challenge back to para_server and stores the session key
552 for further use.
553
554 - para_server also computes the SHA1 hash of the challenge
555 and compares it against what was sent back by the client.
556
557 - If the two hashes do not match, the authentication has
558 failed and para_server closes the connection.
559
560 - Otherwise the user is considered authenticated and the client
561 is allowed to proceed by sending a command to be executed. From
562 this point on the communication is encrypted using the RC4
563 stream cipher with the session key known to both peers.
564
565 paraslash relies on the quality of the pseudo-random bytes provided
566 by the crypto library (openssl or libgcrypt), on the security of
567 the implementation of the RSA and RC4 crypto routines and on the
568 infeasibility to invert the SHA1 function.
569
570 Neither para_server or para_client create RSA keys on their own. This
571 has to be done once for each user as sketched in REFERENCE(Quick start,
572 Quick start) and discussed in more detail REFERENCE(The user_list
573 file, below).
574
575 The user_list file
576 ~~~~~~~~~~~~~~~~~~
577
578 At startup para_server reads the user list file which contains one
579 line per user. The default location of the user list file may be
580 changed with the --user_list option.
581
582 There should be at least one user in this file. Each user must have
583 an RSA key pair. The public part of the key is needed by para_server
584 while the private key is needed by para_client. Each line of the
585 user list file must be of the form
586
587 user <username> <key> <perms>
588
589 where _username_ is an arbitrary string (usually the user's login
590 name), _key_ is the full path to that user's public RSA key, and
591 _perms_ is a comma-separated list of zero or more of the following
592 permission bits:
593
594 +---------------------------------------------------------+
595 | AFS_READ | read the contents of the databases |
596 +-----------+---------------------------------------------+
597 | AFS_WRITE | change database contents |
598 +-----------+---------------------------------------------+
599 | VSS_READ | obtain information about the current stream |
600 +-----------+---------------------------------------------+
601 | VSS_WRITE | change the current stream |
602 +---------------------------------------------------------+
603
604 The permission bits specify which commands the user is allowed to
605 execute. The output of
606
607 para_client help
608
609 contains in the third column the permissions needed to execute the
610 command.
611
612 It is possible to make para_server reread the user_list file by
613 executing the paraslash "hup" command or by sending SIGHUP to the
614 PID of para_server.
615
616
617 Connecting para_audiod
618 ~~~~~~~~~~~~~~~~~~~~~~
619
620 para_audiod listens on a Unix domain socket. Those sockets are
621 for local communication only, so only local users can connect to
622 para_audiod. The default is to let any user connect but this can be
623 restricted on platforms that support UNIX socket credentials which
624 allow para_audiod to obtain the Unix credentials of the connecting
625 process.
626
627 Use para_audiod's --user_allow option to allow connections only for
628 a limited set of users.
629
630 -----------------------
631 The audio file selector
632 -----------------------
633
634 paraslash comes with a sophisticated audio file selector (AFS),
635 whose main task is to determine which file to stream next, based on
636 information on the audio files stored in a database. It communicates
637 also with para_client whenever an AFS command is executed, for example
638 to answer a database query.
639
640 Besides the traditional playlists, AFS supports audio file selection
641 based on _moods_ which act as a filter that limits the set of all
642 known audio files to those which satisfy certain criteria. It also
643 maintains tables containing images (e.g. album cover art) and lyrics
644 that can be associated with one or more audio files.
645
646 AFS uses XREFERENCE(http://systemlinux.org/~maan/osl/, libosl), the
647 object storage layer library, as the backend library for storing
648 information on audio files, playlists, etc. This library offers
649 functionality similar to a relational database, but is much more
650 lightweight than a full database backend.
651
652 In this chapter we sketch the setup of the REFERENCE(The AFS process,
653 AFS process) during server startup and proceed with the description
654 of the REFERENCE(Database layout, layout) of the various database
655 tables. The section on REFERENCE(Playlists and moods, playlists
656 and moods) explains these two audio file selection mechanisms
657 in detail and contains pratical examples. The way REFERENCE(File
658 renames and content changes, file renames and content changes) are
659 detected is discussed briefly before the REFERENCE(Troubleshooting,
660 Troubleshooting) section concludes the chapter.
661
662 The AFS process
663 ~~~~~~~~~~~~~~~
664
665 On startup, para_server forks to create the AFS process which opens
666 the OSL database tables. The server process communicates with the
667 AFS process via pipes and shared memory. Usually, the AFS process
668 awakes only briefly whenever the current audio file changes. The AFS
669 process determines the next audio file, opens it, verifies it has
670 not been changed since it was added to the database and passes the
671 open file descriptor to the server process, along with audio file
672 meta-data such as file name, duration, audio format and so on. The
673 server process then starts to stream the audio file.
674
675 The AFS process also accepts connections from local clients via
676 a well-known socket. However, only child processes of para_server
677 may connect through this socket. All server commands that have the
678 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
679 change the database.
680
681 Database layout
682 ~~~~~~~~~~~~~~~
683
684 *The audio file table*
685
686 This is the most important and usually also the largest table of the
687 AFS database. It contains the information needed to stream each audio
688 file. In particular the following data is stored for each audio file.
689
690 - SHA1 hash value of the audio file contents. This is computed
691 once when the file is added to the database. Whenever AFS
692 selects this audio file for streaming the hash value is
693 recomputed and checked against the value stored in the
694 database to detect content changes.
695
696 - The time when this audio file was last played.
697
698 - The number of times the file has been played so far.
699
700 - The attribute bitmask.
701
702 - The image id which describes the image associated with this
703 audio file.
704
705 - The lyrics id which describes the lyrics associated with
706 this audio file.
707
708 - The audio format id (MP3, OGG, ...).
709
710 - An amplification value that can be used by the amplification
711 filter to pre-amplify the decoded audio stream.
712
713 - The chunk table. It describes the location and the timing
714 of the building blocks of the audio file. This is used by
715 para_server to send chunks of the file at appropriate times.
716
717 - The duration of the audio file.
718
719 - Tag information contained in the audio file (ID3 tags,
720 Vorbis comments, ...).
721
722 - The number of channels
723
724 - The encoding bitrate.
725
726 - The sampling frequency.
727
728 To add or refresh the data contained in the audio file table, the _add_
729 command is used. It takes the full path of either an audio file or a
730 directory. In the latter case, the directory is traversed recursively
731 and all files which are recognized as valid audio files are added to
732 the database.
733
734 *The attribute table*
735
736 The attribute table contains two columns, _name_ and _bitnum_. An
737 attribute is simply a name for a certain bit number in the attribute
738 bitmask of the audio file table.
739
740 Each of the 64 bits of the attribute bitmask can be set for each
741 audio file individually. Hence up to 64 different attributes may be
742 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
743 "german_lyrics", "speech", whatever. You are free to choose as
744 many attributes as you like and there are no naming restrictions
745 for attributes.
746
747 A new attribute "test" is created by
748
749 para_client addatt test
750 and
751 para_client lsatt
752
753 lists all available attributes. You can set the "test" attribute for
754 an audio file by executing
755
756 para_client setatt test+ /path/to/the/audio/file
757
758 Similarly, the "test" bit can be removed from an audio file with
759
760 para_client setatt test- /path/to/the/audio/file
761
762 Instead of a path you may use a shell wildcard pattern. The attribute
763 is applied to all audio files matching this pattern:
764
765 para_client setatt test+ '/test/directory/*'
766
767 The command
768
769 para_client -- ls -lv
770
771 gives you a verbose listing of your audio files also showing which
772 attributes are set.
773
774 In case you wonder why the double-dash in the above command is needed:
775 It tells para_client to not interpret the options after the dashes. If
776 you find this annoying, just say
777
778 alias para='para_client --'
779
780 and be happy. In what follows we shall use this alias.
781
782 The "test" attribute can be dropped from the database with
783
784 para rmatt test
785
786 Read the output of
787
788 para help ls
789 para help setatt
790
791 for more information and a complete list of command line options to
792 these commands.
793
794 *Blob tables*
795
796 The image, lyrics, moods and playlists tables are all blob tables.
797 Blob tables consist of three columns each: The identifier which is
798 a positive non-negative number that is auto-incremented, the name
799 (an arbitrary string) and the content (the blob).
800
801 All blob tables support the same set of actions: cat, ls, mv, rm
802 and add. Of course, _add_ is used for adding new blobs to the table
803 while the other actions have the same meaning as the corresponding
804 Unix commands. The paraslash commands to perform these actions are
805 constructed as the concatenation of the table name and the action. For
806 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
807 manipulate or query the image table.
808
809 The add variant of these commands is special as these commands read
810 the blob contents from stdin. To add an image to the image table the
811 command
812
813 para addimg image_name < file.jpg
814
815 can be used.
816
817 Note that the images and lyrics are not interpreted at all, and also
818 the playlist and the mood blobs are only investigated when the mood
819 or playlist is activated with the select command.
820
821 *The score table*
822
823 Unlike all other tables the contents of the score table remain in
824 memory and are never stored on disk. The score table contains two
825 columns: The SHA1 hash value (of an audio file) and its current
826 score.
827
828 However, only those files which are admissible for the current mood
829 or playlist are contained in the score table. The audio file selector
830 always chooses the row with the highest score as the file to stream
831 next. While doing so, it computes the new score and updates the
832 last_played and the num_played fields in the audio file table.
833
834 The score table is recomputed by the select command which loads a
835 mood or playlist. Audio files are chosen for streaming from the rows
836 of the score table on a highest-score-first basis.
837
838
839 Playlists and moods
840 ~~~~~~~~~~~~~~~~~~~
841
842 Playlists and moods offer two different ways of specifying the set of
843 admissible files. A playlist in itself describes a set of admissible
844 files. A mood, in contrast, describes the set of admissible files in
845 terms of attributes and other type of information available in the
846 audio file table. As an example, a mood can define a filename pattern,
847 which is then matched against the names of audio files in the table.
848
849 *Playlists*
850
851 Playlists are accommodated in the playlist table of the afs database,
852 using the aforementioned blob format for tables. A new playlist is
853 created with the addpl command by specifying the full (absolute)
854 paths of all desired audio files, separated by newlines. Example:
855
856 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
857
858 If _my_playlist_ already exists it is overwritten. To activate the
859 new playlist, execute
860
861 para select p/my_playlist
862
863 The audio file selector will assign scores to each entry of the list,
864 in descending order so that files will be selected in order. If a
865 file could not be opened for streaming, its entry is removed from
866 the score table (but not from the playlist).
867
868 *Moods*
869
870 A mood consists of a unique name and its *mood definition*, which is
871 a set of *mood lines* containing expressions in terms of attributes
872 and other data contained in the database.
873
874 At any time at most one mood can be *active* which means that
875 para_server is going to select only files from that subset of
876 admissible files.
877
878 So in order to create a mood definition one has to write a set of
879 mood lines. Mood lines come in three flavours: Accept lines, deny
880 lines and score lines.
881
882 The general syntax of the three types of mood lines is
883
884
885 accept [with score <score>] [if] [not] <mood_method> [options]
886 deny [with score <score>] [if] [not] <mood_method> [options]
887 score <score> [if] [not] <mood_method> [options]
888
889
890 Here <score> is either an integer or the string "random" which assigns
891 a random score to all matching files. The score value changes the
892 order in which admissible files are going to be selected, but is of
893 minor importance for this introduction.
894
895 So we concentrate on the first two forms, i.e. accept and deny
896 lines. As usual, everything in square brackets is optional, i.e.
897 accept/deny lines take the following form when ignoring scores:
898
899 accept [if] [not] <mood_method> [options]
900
901 and analogously for the deny case. The "if" keyword is only syntactic
902 sugar and has no function. The "not" keyword just inverts the result,
903 so the essence of a mood line is the mood method part and the options
904 following thereafter.
905
906 A *mood method* is realized as a function which takes an audio file
907 and computes a number from the data contained in the database.
908 If this number is non-negative, we say the file *matches* the mood
909 method. The file matches the full mood line if it either
910
911 - matches the mood method and the "not" keyword is not given,
912 or
913 - does not match the mood method, but the "not" keyword is given.
914
915 The set of admissible files for the whole mood is now defined as those
916 files which match at least one accept mood line, but no deny mood line.
917 More formally, an audio file F is admissible if and only if
918
919 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
920
921 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
922 lines and "~" means "matches".
923
924 The cases where no mood lines of accept/deny type are defined need
925 special treatment:
926
927 - Neither accept nor deny lines: This treats all files as
928 admissible (in fact, that is the definition of the dummy mood
929 which is activated automatically if no moods are available).
930
931 - Only accept lines: A file is admissible iff it matches at
932 least one accept line:
933
934 F ~ AL1 or F ~ AL2 or ...
935
936 - Only deny lines: A file is admissible iff it matches no
937 deny line:
938
939 not (F ~ DL1 or F ~ DN2 ...)
940
941
942
943 *List of mood_methods*
944
945 no_attributes_set
946
947 Takes no arguments and matches an audio file if and only if no
948 attributes are set.
949
950 is_set <attribute_name>
951
952 Takes the name of an attribute and matches iff that attribute is set.
953
954 path_matches <pattern>
955
956 Takes a filename pattern and matches iff the path of the audio file
957 matches the pattern.
958
959 artist_matches <pattern>
960 album_matches <pattern>
961 title_matches <pattern>
962 comment_matches <pattern>
963
964 Takes an extended regular expression and matches iff the text of the
965 corresponding tag of the audio file matches the pattern. If the tag
966 is not set, the empty string is matched against the pattern.
967
968 year ~ <num>
969 bitrate ~ <num>
970 frequency ~ <num>
971 channels ~ <num>
972 num_played ~ <num>
973
974 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
975 <num>. Matches an audio file iff the condition <val> ~ <num> is
976 satisfied where val is the corresponding value of the audio file
977 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
978 count, play count).
979
980 The year tag is special as its value is undefined if the audio file
981 has no year tag or the content of the year tag is not a number. Such
982 audio files never match. Another difference is the special treatment
983 if the year tag is a two-digit number. In this case either 1900 or
984 2000 is added to the tag value, depending on whether the number is
985 greater than 2000 plus the current year.
986
987
988 *Mood usage*
989
990 To create a new mood called "my_mood", write its definition into
991 some temporary file, say "tmpfile", and add it to the mood table
992 by executing
993
994 para addmood my_mood < tmpfile
995
996 If the mood definition is really short, you may just pipe it to the
997 client instead of using temporary files. Like this:
998
999 echo "$MOOD_DEFINITION" | para addmood my_mood
1000
1001 There is no need to keep the temporary file since you can always use
1002 the catmood command to get it back:
1003
1004 para catmood my_mood
1005
1006 A mood can be activated by executing
1007
1008 para select m/my_mood
1009
1010 Once active, the list of admissible files is shown by the ls command
1011 if the "-a" switch is given:
1012
1013 para ls -a
1014
1015
1016 *Example mood definition*
1017
1018 Suppose you have defined attributes "punk" and "rock" and want to define
1019 a mood containing only Punk-Rock songs. That is, an audio file should be
1020 admissible if and only if both attributes are set. Since
1021
1022 punk and rock
1023
1024 is obviously the same as
1025
1026 not (not punk or not rock)
1027
1028 (de Morgan's rule), a mood definition that selects only Punk-Rock
1029 songs is
1030
1031 deny if not is_set punk
1032 deny if not is_set rock
1033
1034
1035
1036 File renames and content changes
1037 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1038
1039 Since the audio file selector knows the SHA1 of each audio file that
1040 has been added to the afs database, it recognizes if the content of
1041 a file has changed, e.g. because an ID3 tag was added or modified.
1042 Also, if a file has been renamed or moved to a different location,
1043 afs will detect that an entry with the same hash value already exists
1044 in the audio file table.
1045
1046 In both cases it is enough to just re-add the new file. In the
1047 first case (file content changed), the audio table is updated, while
1048 metadata such as the num_played and last_played fields, as well as
1049 the attributes, remain unchanged. In the other case, when the file
1050 is moved or renamed, only the path information is updated, all other
1051 data remains as before.
1052
1053 It is possible to change the behaviour of the add command by using the
1054 "-l" (lazy add) or the "-f" (force add) option.
1055
1056 Troubleshooting
1057 ~~~~~~~~~~~~~~~
1058
1059 Use the debug loglevel (option -l debug for most commands) to show
1060 debugging info. Almost all paraslash executables have a brief online
1061 help which is displayed by using the -h switch. The --detailed-help
1062 option prints the full help text.
1063
1064 If para_server crashed or was killed by SIGKILL (signal 9), it
1065 may refuse to start again because of "dirty osl tables". In this
1066 case you'll have to run the oslfsck program of libosl to fix your
1067 database. It might be necessary to use --force (even if your name
1068 isn't Luke). However, make sure para_server isn't running before
1069 executing oslfsck --force.
1070
1071 If you don't mind to recreate your database you can start
1072 from scratch by removing the entire database directory, i.e.
1073
1074 rm -rf ~/.paraslash/afs_database-0.4
1075
1076 Be aware that this removes all attribute definitions, all playlists
1077 and all mood definitions and requires to re-initialize the tables.
1078
1079 Although oslfsck fixes inconsistencies in database tables it doesn't
1080 care about the table contents. To check for invalid table contents, use
1081
1082 para_client check
1083
1084 This prints out references to missing audio files as well as invalid
1085 playlists and mood definitions.
1086
1087 ---------------------------------------
1088 Audio formats and audio format handlers
1089 ---------------------------------------
1090
1091 Audio formats
1092 ~~~~~~~~~~~~~
1093
1094 The following audio formats are supported by paraslash:
1095
1096 *MP3*
1097
1098 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1099 designed as part of its MPEG-1 standard. An MP3 file is made up of
1100 multiple MP3 frames, which consist of a header and a data block. The
1101 size of an MP3 frame depends on the bit rate and on the number
1102 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1103 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1104 400 bytes large.
1105
1106 *OGG/Vorbis*
1107
1108 OGG is a standardized audio container format, while Vorbis is an
1109 open source codec for lossy audio compression. Since Vorbis is most
1110 commonly made available via the OGG container format, it is often
1111 referred to as OGG/Vorbis. The OGG container format divides data into
1112 chunks called OGG pages. A typical OGG page is about 4KB large. The
1113 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1114 may vary considerably.
1115
1116 *OGG/Speex*
1117
1118 Speex is an open-source speech codec that is based on CELP (Code
1119 Excited Linear Prediction) coding. It is designed for voice
1120 over IP applications, has modest complexity and a small memory
1121 footprint. Wideband and narrowband (telephone quality) speech are
1122 supported. As for Vorbis audio, Speex bit-streams are often stored
1123 in OGG files.
1124
1125 *AAC*
1126
1127 Advanced Audio Coding (AAC) is a standardized, lossy compression
1128 and encoding scheme for digital audio which is the default audio
1129 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1130 the container format and audio files encoded with AAC have the .m4a
1131 extension. A typical AAC frame is about 700 bytes large.
1132
1133 *WMA*
1134
1135 Windows Media Audio (WMA) is an audio data compression technology
1136 developed by Microsoft. A WMA file is usually encapsulated in the
1137 Advanced Systems Format (ASF) container format, which also specifies
1138 how meta data about the file is to be encoded. The bit stream of WMA
1139 is composed of superframes, each containing one or more frames of
1140 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1141
1142 *FLAC*
1143
1144 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1145 loss. It gives better compression ratios than a general purpose
1146 compressor like zip or bzip2 because FLAC is designed specifically
1147 for audio. A FLAC-encoded file consits of frames of varying size, up
1148 to 16K. Each frame starts with a header that contains all information
1149 necessary to decode the frame.
1150
1151 Meta data
1152 ~~~~~~~~~
1153
1154 Unfortunately, each audio format has its own conventions how meta
1155 data is added as tags to the audio file.
1156
1157 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1158 is rather simple but also very limited as it supports only artist,
1159 title, album, year and comment tags. Each of these can only be at most
1160 32 characters long. ID3, version 2 is much more flexible but requires
1161 a separate library being installed for paraslash to support it.
1162
1163 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1164 comments, which are typically implemented as strings of the form
1165 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1166 tags are appropriate for the content.
1167
1168 AAC files usually use the MPEG-4 container format for storing meta
1169 data while WMA files wrap meta data as special objects within the
1170 ASF container format.
1171
1172 paraslash only tracks the most common tags that are supported by
1173 all tag variants: artist, title, year, album, comment. When a file
1174 is added to the AFS database, the meta data of the file is extracted
1175 and stored in the audio file table.
1176
1177 Chunks and chunk tables
1178 ~~~~~~~~~~~~~~~~~~~~~~~
1179
1180 paraslash uses the word "chunk" as common term for the building blocks
1181 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1182 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1183 size varies considerably between audio formats, from a few hundred
1184 bytes (MP3) up to 16K (FLAC).
1185
1186 The chunk table contains the offsets within the audio file that
1187 correspond to the chunk boundaries of the file. Like the meta data,
1188 the chunk table is computed and stored in the database whenever an
1189 audio file is added.
1190
1191 The paraslash senders (see below) always send complete chunks. The
1192 granularity for seeking is therefore determined by the chunk size.
1193
1194 Audio format handlers
1195 ~~~~~~~~~~~~~~~~~~~~~
1196
1197 For each audio format paraslash contains an audio format handler whose
1198 first task is to tell whether a given file is a valid audio file of
1199 this type. If so, the audio file handler extracts some technical data
1200 (duration, sampling rate, number of channels etc.), computes the
1201 chunk table and reads the meta data.
1202
1203 The audio format handler code is linked into para_server and executed
1204 via the _add_ command. The same code is also available as a stand-alone
1205 tool, para_afh, which can be used to print the technical data, the
1206 chunk table and the meta data of a file. Furthermore, one can use
1207 para_afh to cut an audio file, i.e. to select some of its chunks to
1208 produce a new file containing only these chunks.
1209
1210 ----------
1211 Networking
1212 ----------
1213
1214 Paraslash uses different network connections for control and data.
1215 para_client communicates with para_server over a dedicated TCP control
1216 connection. To transport audio data, separate data connections are
1217 used. For these data connections, a variety of transports (UDP, DCCP,
1218 HTTP) can be chosen.
1219
1220 The chapter starts with the REFERENCE(The paraslash control
1221 service, control service), followed by a section on the various
1222 REFERENCE(Streaming protocols, streaming protocols) in which the data
1223 connections are described. The way audio file headers are embedded into
1224 the stream is discussed REFERENCE(Streams with headers and headerless
1225 streams, briefly) before the REFERENCE(Networking examples, example
1226 section) which illustrates typical commands for real-life scenarios.
1227
1228 Both IPv4 and IPv6 are supported.
1229
1230 The paraslash control service
1231 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1232
1233 para_server is controlled at runtime via the paraslash control
1234 connection. This connection is used for server commands (play, stop,
1235 ...) as well as for afs commands (ls, select, ...).
1236
1237 The server listens on a TCP port and accepts connections from clients
1238 that connect the open port. Each connection causes the server to fork
1239 off a client process which inherits the connection and deals with that
1240 client only. In this classical accept/fork approach the server process
1241 is unaffected if the child dies or goes crazy for whatever reason. In
1242 fact, the child process can not change address space of server process.
1243
1244 The section on REFERENCE(Client-server authentication, client-server
1245 authentication) above described the early connection establishment
1246 from the crypto point of view. Here it is described what happens
1247 after the connection (including crypto setup) has been established.
1248 There are four processes involved during command dispatch as sketched
1249 in the following diagram.
1250
1251 <<
1252 <pre>
1253 server_host client_host
1254 ~~~~~~~~~~~ ~~~~~~~~~~~
1255
1256 +-----------+ connect +-----------+
1257 |para_server|<------------------------------ |para_client|
1258 +-----------+ +-----------+
1259 | ^
1260 | fork +---+ |
1261 +----------> |AFS| |
1262 | +---+ |
1263 | ^ |
1264 | | |
1265 | | connect (cookie) |
1266 | | |
1267 | | |
1268 | fork +-----+ inherited connection |
1269 +---------->|child|<--------------------------+
1270 +-----+
1271 </pre>
1272 >>
1273
1274 Note that the child process is not a child of the afs process,
1275 so communication of these two processes has to happen via local
1276 sockets. In order to avoid abuse of the local socket by unrelated
1277 processes, a magic cookie is created once at server startup time just
1278 before the server process forks off the AFS process. This cookie is
1279 known to the server, AFS and the child, but not to unrelated processes.
1280
1281 There are two different kinds of commands: First there are commands
1282 that cause the server to respond with some answer such as the list
1283 of all audio files. All but the addblob commands (addimg, addlyr,
1284 addpl, addmood) are of this kind. The addblob commands add contents
1285 to the database, so they need to transfer data the other way round,
1286 from the client to the server.
1287
1288 There is no knowledge about the server commands built into para_client,
1289 so it does not know about addblob commands. Instead, it inspects the
1290 first data package sent by the server for a magic string. If this
1291 string was found, it sends STDIN to the server, otherwise it dumps
1292 data from the server to STDOUT.
1293
1294 Streaming protocols
1295 ~~~~~~~~~~~~~~~~~~~
1296
1297 A network (audio) stream usually consists of one streaming source,
1298 the _sender_, and one or more _receivers_ which read data over the
1299 network from the streaming source.
1300
1301 Senders are thus part of para_server while receivers are part of
1302 para_audiod. Moreover, there is the stand-alone tool para_recv which
1303 can be used to manually download a stream, either from para_server
1304 or from a web-based audio streaming service.
1305
1306 The following three streaming protocols are supported by paraslash:
1307
1308 - HTTP. Recommended for public streams that can be played by
1309 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1310 sender is supported on all operating systems and all platforms.
1311
1312 - DCCP. Recommended for LAN streaming. DCCP is currently
1313 available only for Linux.
1314
1315 - UDP. Recommended for multicast LAN streaming.
1316
1317 See the Appendix on REFERENCE(Network protocols, network protocols)
1318 for brief descriptions of the various protocols relevant for network
1319 audio streaming with paraslash.
1320
1321 It is possible to activate more than one sender simultaneously.
1322 Senders can be controlled at run time and via config file and command
1323 line options.
1324
1325 Note that audio connections are _not_ encrypted. Transport or Internet
1326 layer encryption should be used if encrypted data connections are
1327 needed.
1328
1329 Since DCCP and TCP are both connection-oriented protocols, connection
1330 establishment/teardown and access control are very similar between
1331 these two streaming protocols. UDP is the most lightweight option,
1332 since in contrast to TCP/DCCP it is connectionless. It is also the
1333 only protocol supporting IP multicast.
1334
1335 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1336 clients to connect and establish a connection via some protocol-defined
1337 handshake mechanism. Both senders maintain two linked lists each:
1338 The list of all clients which are currently connected, and the list
1339 of access control entries which determines who is allowed to connect.
1340 IP-based access control may be configured through config file and
1341 command line options and via the "allow" and "deny" sender subcommands.
1342
1343 Upon receiving a GET request from the client, the HTTP sender sends
1344 back a status line and a message. The body of this message is the
1345 audio stream. This is common practice and is supported by many popular
1346 clients which can thus be used to play a stream offered by para_server.
1347 For DCCP things are a bit simpler: No messages are exchanged between
1348 the receiver and sender. The client simply connects and the sender
1349 starts to stream.
1350
1351 DCCP is an experimental protocol which offers a number of new features
1352 not available for TCP. Both ends can negotiate these features using
1353 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1354 datagram-based (no retransmissions) and thus should not be used over
1355 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1356 is access to a variety of different congestion-control mechanisms
1357 called CCIDs. Two different CCIDs are available per default on Linux:
1358
1359
1360 - _CCID 2_. A Congestion Control mechanism similar to that
1361 of TCP. The sender maintains a congestion window and halves
1362 this window in response to congestion.
1363
1364
1365 - _CCID-3_. Designed to be fair when competing for bandwidth.
1366 It has lower variation of throughput over time compared with
1367 TCP, which makes it suitable for streaming media.
1368
1369 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1370 single list, the _target list_. This list describes the set of clients
1371 to which the stream is sent. There is no list for access control and
1372 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1373 and "delete" commands can be used to modify the target list.
1374
1375 Since both UDP and DCCP offer an unreliable datagram-based transport,
1376 additional measures are necessary to guard against disruptions over
1377 networks that are lossy or which may be subject to interference (as
1378 is for instance the case with WiFi). Paraslash uses FEC (Forward
1379 Error Correction) to guard against packet losses and reordering. The
1380 stream is FEC-encoded before it is sent through the UDP socket and
1381 must be decoded accordingly on the receiver side.
1382
1383 The packet size and the amount of redundancy introduced by FEC can
1384 be configured via the FEC parameters which are dictated by server
1385 and may also be configured through the "sender" command. The FEC
1386 parameters are encoded in the header of each network packet, so no
1387 configuration is necessary on the receiver side. See the section on
1388 REFERENCE(Forward error correction, FEC) below.
1389
1390 Streams with headers and headerless streams
1391 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1392
1393 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1394 needed to decode the stream is only contained in the audio file
1395 header of the container format but not in each data chunk. Clients
1396 must be able to obtain this information in case streaming starts in
1397 the middle of the file or if para_audiod is started while para_server
1398 is already sending a stream.
1399
1400 This is accomplished in different ways, depending on the streaming
1401 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1402 header is sent prior to audio file data. This technique however does
1403 not work for the connectionless UDP transport. Hence the audio file
1404 header is periodically being embedded into the UDP audio data stream.
1405 By default, the header is resent after five seconds. The receiver has
1406 to wait until the next header arrives before it can start decoding
1407 the stream.
1408
1409 Examples
1410 ~~~~~~~~
1411
1412 The sender command of para_server allows to (de-)activate senders
1413 and to change the access permissions senders at runtime. The "si"
1414 (server info) command is used to list the streaming options of the
1415 currently running server as well as the various sender access lists.
1416
1417 -> Show client/target/access lists:
1418
1419 para_client si
1420
1421 -> Obtain general help for the sender command:
1422
1423 para_client help sender
1424
1425 -> Get help for a specific sender (contains further examples):
1426
1427 s=http # or dccp or udp
1428 para_client sender $s help
1429
1430 By default para_server activates both the HTTP and th DCCP sender on
1431 startup. This can be changed via command line options or para_server's
1432 config file.
1433
1434 -> List config file options for senders:
1435
1436 para_server -h
1437
1438 All senders share the "on" and "off" commands, so senders may be
1439 activated and deactivated independently of each other.
1440
1441 -> Switch off the http sender:
1442
1443 para_client sender http off
1444
1445 -> Receive a DCCP stream using CCID2 and write the output into a file:
1446
1447 host=foo.org; ccid=2; filename=bar
1448 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1449
1450 Note the quotes around the arguments for the dccp receiver. Each
1451 receiver has its own set of command line options and its own command
1452 line parser, so arguments for the dccp receiver must be protected
1453 from being interpreted by para_recv.
1454
1455 -> Start UDP multicast, using the default multicast address:
1456
1457 para_client sender udp add 224.0.1.38
1458
1459 -> Receive FEC-encoded multicast stream and write the output into a file:
1460
1461 filename=foo
1462 para_recv -r udp > $filename
1463
1464 -> Add an UDP unicast for a client to the target list of the UDP sender:
1465
1466 t=client.foo.org
1467 para_client sender udp add $t
1468
1469 -> Receive this (FEC-encoded) unicast stream:
1470
1471 filename=foo
1472 para_recv -r 'udp -i 0.0.0.0' > $filename
1473
1474 -> Create a minimal config for para_audiod for HTTP streams:
1475
1476 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1477 echo receiver \".:http -i $s\" > $c
1478 para_audiod --config $c
1479
1480 -------
1481 Filters
1482 -------
1483
1484 A paraslash filter is a module which transforms an input stream into
1485 an output stream. Filters are included in the para_audiod executable
1486 and in the stand-alone tool para_filter which usually contains the
1487 same modules.
1488
1489 While para_filter reads its input stream from STDIN and writes
1490 the output to STDOUT, the filter modules of para_audiod are always
1491 connected to a receiver which produces the input stream and a writer
1492 which absorbs the output stream.
1493
1494 Some filters depend on a specific library being installed and are
1495 not compiled in if this library was not found at compile time. To
1496 see the list of supported filters, run para_filter and para_audiod
1497 with the --help option. The output looks similar to the following:
1498
1499 Available filters:
1500 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1501
1502 Out of these filter modules, a chain of filters can be constructed,
1503 much in the way Unix pipes can be chained, and analogous to the use
1504 of modules in gstreamer: The output of the first filter becomes the
1505 input of the second filter. There is no limitation on the number of
1506 filters and the same filter may occur more than once.
1507
1508 Like receivers, each filter has its own command line options which
1509 must be quoted to protect them from the command line options of
1510 the driving application (para_audiod or para_filter). Example:
1511
1512 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1513
1514 For para_audiod, each audio format has its own set of filters. The
1515 name of the audio format for which the filter should be applied can
1516 be used as the prefix for the filter option. Example:
1517
1518 para_audiod -f 'mp3:prebuffer --duration 300'
1519
1520 The "mp3" prefix above is actually interpreted as a POSIX extended
1521 regular expression. Therefore
1522
1523 para_audiod -f '.:prebuffer --duration 300'
1524
1525 activates the prebuffer filter for all supported audio formats (because
1526 "." matches all audio formats) while
1527
1528 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1529
1530 activates it only for wma and ogg streams.
1531
1532 Decoders
1533 ~~~~~~~~
1534
1535 For each supported audio format there is a corresponding filter
1536 which decodes audio data in this format to 16 bit PCM data which
1537 can be directly sent to the sound device or any other software that
1538 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1539 filters are called _decoders_ in general, and xxxdec is the name of
1540 the paraslash decoder for the audio format xxx. For example, the mp3
1541 decoder filter is called mp3dec.
1542
1543 Note that the output of the decoder is about 10 times larger than
1544 its input. This means that filters that operate on the decoded audio
1545 stream have to deal with much more data than filters that transform
1546 the audio stream before it is fed to the decoder.
1547
1548 Paraslash relies on external libraries for most decoders, so these
1549 libraries must be installed for the decoder to be included in the
1550 para_filter and para_audiod executables. The oggdec filter depends
1551 on the libogg and libvorbis libraries for example.
1552
1553 Forward error correction
1554 ~~~~~~~~~~~~~~~~~~~~~~~~
1555
1556 As already mentioned REFERENCE(Streaming protocols, earlier),
1557 paraslash uses forward error correction (FEC) for the unreliable UDP
1558 and DCCP transports. FEC is a technique which was invented already
1559 in 1960 by Reed and Solomon and which is widely used for the parity
1560 calculations of storage devices (RAID arrays). It is based on the
1561 algebraic concept of finite fields, today called Galois fields, in
1562 honour of the mathematician Galois (1811-1832). The FEC implementation
1563 of paraslash is based on code by Luigi Rizzo.
1564
1565 Although the details require a sound knowledge of the underlying
1566 mathematics, the basic idea is not hard to understand: For positive
1567 integers k and n with k < n it is possible to compute for any k given
1568 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1569 ..., p_r such that all data bytes can be reconstructed from *any*
1570 k bytes of the set
1571
1572 {d_1, ..., d_k, p_1, ..., p_r}.
1573
1574 FEC-encoding for unreliable network transports boils down to slicing
1575 the audio stream into groups of k suitably sized pieces called _slices_
1576 and computing the r corresponding parity slices. This step is performed
1577 in para_server which then sends both the data and the parity slices
1578 over the unreliable network connection. If the client was able
1579 to receive at least k of the n = k + r slices, it can reconstruct
1580 (FEC-decode) the original audio stream.
1581
1582 From these observations it is clear that there are three different
1583 FEC parameters: The slice size, the number of data slices k, and the
1584 total number of slices n. It is crucial to choose the slice size
1585 such that no fragmentation of network packets takes place because
1586 FEC only guards against losses and reordering but fails if slices are
1587 received partially.
1588
1589 FEC decoding in paralash is performed through the fecdec filter which
1590 usually is the first filter (there can be other filters before fecdec
1591 if these do not alter the audio stream).
1592
1593
1594 Volume adjustment (amp and compress)
1595 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1596
1597 The amp and the compress filter both adjust the volume of the audio
1598 stream. These filters operate on uncompressed audio samples. Hence
1599 they are usually placed directly after the decoding filter. Each
1600 sample is multiplied with a scaling factor (>= 1) which makes amp
1601 and compress quite expensive in terms of computing power.
1602
1603 *amp*
1604
1605 The amp filter amplifies the audio stream by a fixed scaling factor
1606 that must be known in advance. For para_audiod this factor is derived
1607 from the amplification field of the audio file's entry in the audio
1608 file table while para_filter uses the value given at the command line.
1609
1610 The optimal scaling factor F for an audio file is the largest real
1611 number F >= 1 such that after multiplication with F all samples still
1612 fit into the sample interval [-32768, 32767]. One can use para_filter
1613 in combination with the sox utility to compute F:
1614
1615 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1616
1617 The amplification value V which is stored in the audio file table,
1618 however, is an integer between 0 and 255 which is connected to F
1619 through the formula
1620
1621 V = (F - 1) * 64.
1622
1623 To store V in the audio file table, the command
1624
1625 para_client -- touch -a=V file.mp3
1626
1627 is used. The reader is encouraged to write a script that performs
1628 these computations :)
1629
1630 *compress*
1631
1632 Unlike the amplification filter, the compress filter adjusts the volume
1633 of the audio stream dynamically without prior knowledge about the peak
1634 value. It maintains the maximal volume of the last n samples of the
1635 audio stream and computes a suitable amplification factor based on that
1636 value and the various configuration options. It tries to chose this
1637 factor such that the adjusted volume meets the desired target level.
1638
1639 Note that it makes sense to combine amp and compress.
1640
1641 Misc filters (wav and prebuffer)
1642 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1643
1644 These filters are rather simple and do not modify the audio stream at
1645 all. The wav filter is only useful with para_filter and in connection
1646 with a decoder. It asks the decoder for the number of channels and the
1647 sample rate of the stream and adds a Microsoft wave header containing
1648 this information at the beginning. This allows to write wav files
1649 rather than raw PCM files (which do not contain any information about
1650 the number of channels and the sample rate).
1651
1652 The prebuffer filter simply delays the output until the given time has
1653 passed (starting from the time the first byte was available in its
1654 input queue) or until the given amount of data has accumulated. It
1655 is mainly useful for para_audiod if the standard parameters result
1656 in buffer underruns.
1657
1658 Both filters require almost no additional computing time, even when
1659 operating on uncompressed audio streams, since data buffers are simply
1660 "pushed down" rather than copied.
1661
1662 Examples
1663 ~~~~~~~~
1664
1665 -> Decode an mp3 file to wav format:
1666
1667 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1668
1669 -> Amplify a raw audio file by a factor of 1.5:
1670
1671 para_filter -f amp --amp 32 < foo.raw > bar.raw
1672
1673 ------
1674 Output
1675 ------
1676
1677 Once an audio stream has been received and decoded to PCM format,
1678 it can be sent to a sound device for playback. This part is performed
1679 by paraslash _writers_ which are described in this chapter.
1680
1681 Writers
1682 ~~~~~~~
1683
1684 A paraslash writer acts as a data sink that consumes but does not
1685 produce audio data. Paraslash writers operate on the client side and
1686 are contained in para_audiod and in the stand-alone tool para_write.
1687
1688 The para_write program reads uncompressed audio data from STDIN. If
1689 this data starts with a wav header, sample rate, sample format and
1690 channel count are read from the header. Otherwise CD audio (44.1KHz
1691 16 bit little endian, stereo) is assumed but this can be overridden
1692 by command line options. para_audiod, on the other hand, obtains
1693 the sample rate and the number of channels from the decoder.
1694
1695 Like receivers and filters, each writer has an individual set of
1696 command line options, and for para_audiod writers can be configured
1697 per audio format separately. It is possible to activate more than
1698 one writer for the same stream simultaneously.
1699
1700 OS-dependent APIs
1701 ~~~~~~~~~~~~~~~~~
1702
1703 Unfortunately, the various flavours of Unix on which paraslash
1704 runs on have different APIs for opening a sound device and starting
1705 playback. Hence for each such API there is a paraslash writer that
1706 can play the audio stream via this API.
1707
1708 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1709 Linux systems. Although there are several mid-layer APIs in use by
1710 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1711 currently supports only the low-level ALSA API which is not supposed
1712 to be change. ALSA is very feature-rich, in particular it supports
1713 software mixing via its DMIX plugin. ALSA is the default writer on
1714 Linux systems.
1715
1716 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1717 is also available on Linux systems, usually provided by ALSA as an
1718 emulation for backwards compatibility. This API is rather simple but
1719 also limited. For example only one application can open the device
1720 at any time. The OSS writer is activated by default on BSD Systems.
1721
1722 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1723 for this API is only compiled in on such systems and is of course
1724 the default there.
1725
1726 *FILE*. The file writer allows to capture the audio stream and
1727 write the PCM data to a file on the file system rather than playing
1728 it through a sound device. It is supported on all platforms and is
1729 always compiled in.
1730
1731 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1732 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1733 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1734 through an output plugin of libao.
1735
1736 Examples
1737 ~~~~~~~~
1738
1739 -> Use the OSS writer to play a wav file:
1740
1741 para_write --writer oss < file.wav
1742
1743 -> Enable ALSA software mixing for mp3 streams
1744
1745 para_audiod --writer 'mp3:alsa -d plug:swmix'
1746
1747
1748 ---
1749 Gui
1750 ---
1751
1752 para_gui executes an arbitrary command which is supposed to print
1753 status information to STDOUT. It then displays this information in
1754 a curses window. By default the command
1755
1756 para_audioc -- stat -p
1757
1758 is executed, but this can be customized via the --stat_cmd option. In
1759 particular it possible to use
1760
1761 para_client -- stat -p
1762
1763 to make para_gui work on systems on which para_audiod is not running.
1764
1765 Key bindings
1766 ~~~~~~~~~~~~
1767
1768 It is possible to bind keys to arbitrary commands via custom
1769 key-bindings. Besides the internal keys which can not be changed (help,
1770 quit, loglevel, version...), the following flavours of key-bindings
1771 are supported:
1772
1773 - external: Shutdown curses before launching the given command.
1774 Useful for starting other ncurses programs from within
1775 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1776 output format to write a mailbox containing one mail for each
1777 (admissible) file the audio file selector knows about. Then
1778 start mutt from within para_gui to browse your collection!
1779
1780 - display: Launch the command and display its stdout in
1781 para_gui's bottom window.
1782
1783 - para: Like display, but start "para_client <specified
1784 command>" instead of "<specified command>".
1785
1786 The general form of a key binding is
1787
1788 key_map k:m:c
1789
1790 which maps key k to command c using mode m. Mode may be x, d or p
1791 for external, display and paraslash commands, respectively.
1792
1793 Themes
1794 ~~~~~~
1795
1796 Currently there are only two themes for para_gui. It is easy, however,
1797 to add more themes. To create a new theme one has to define the
1798 position, color and geometry for for each status item that should be
1799 shown by this theme. See gui_theme.c for examples.
1800
1801 The "." and "," keys are used to switch between themes.
1802
1803 Examples
1804 ~~~~~~~~
1805
1806 -> Show server info:
1807
1808 key_map "i:p:si"
1809
1810 -> Jump to the middle of the current audio file by pressing F5:
1811
1812 key_map "<F5>:p:jmp 50"
1813
1814 -> vi-like bindings for jumping around:
1815
1816 key_map "l:p:ff 10"
1817 key_map "h:p:ff 10-"
1818 key_map "w:p:ff 60"
1819 key_map "b:p:ff 60-"
1820
1821 -> Print the current date and time:
1822
1823 key_map "D:d:date"
1824
1825 -> Call other curses programs:
1826
1827 key_map "U:x:aumix"
1828 key_map "!:x:/bin/bash"
1829 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1830
1831 -----------
1832 Development
1833 -----------
1834
1835 Tools
1836 ~~~~~
1837
1838 In order to compile the sources from the git repository (rather than
1839 from tar balls) and for contributing non-trivial changes to the
1840 paraslash project, some additional tools should be installed on a
1841 developer machine.
1842
1843 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1844 branches, below), the git source code management tool is used for
1845 paraslash development. It is necessary for cloning the git repository
1846 and for getting updates.
1847
1848 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1849 are generated from templates by the m4 macro processor.
1850
1851 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1852 the configure file which is shipped in the tarballs but has to be
1853 generated when compiling from git.
1854
1855 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1856 HTML version of this manual and some of the paraslash web pages are
1857 generated by the grutatxt plain text to HTML converter. If changes
1858 are made to these text files the grutatxt package must be installed
1859 to regenerate the HTML files.
1860
1861 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1862 of paraslash's C sources uses the doxygen documentation system. The
1863 conventions for documenting the source code is described in the
1864 REFERENCE(Doxygen, Doxygen section).
1865
1866 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1867 browsable HTML from the C sources. It is needed by doxygen.
1868
1869 Git branches
1870 ~~~~~~~~~~~~
1871
1872 Paraslash has been developed using the git source code management
1873 tool since 2006. Development is organized roughly in the same spirit
1874 as the git development itself, as described below.
1875
1876 The following text passage is based on "A note from the maintainer",
1877 written by Junio C Hamano, the maintainer of git.
1878
1879 There are four branches in the paraslash repository that track the
1880 source tree: "master", "maint", "next", and "pu".
1881
1882 The "master" branch is meant to contain what is well tested and
1883 ready to be used in a production setting. There could occasionally be
1884 minor breakages or brown paper bag bugs but they are not expected to
1885 be anything major, and more importantly quickly and easily fixable.
1886 Every now and then, a "feature release" is cut from the tip of this
1887 branch, named with three dotted decimal digits, like 0.4.2.
1888
1889 Whenever changes are about to be included that will eventually lead to
1890 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1891 "master" at that point. Obvious, safe and urgent fixes after the major
1892 release are applied to this branch and maintenance releases are cut
1893 from it. New features never go to this branch. This branch is also
1894 merged into "master" to propagate the fixes forward.
1895
1896 A trivial and safe enhancement goes directly on top of "master".
1897 New development does not usually happen on "master", however.
1898 Instead, a separate topic branch is forked from the tip of "master",
1899 and it first is tested in isolation; Usually there are a handful such
1900 topic branches that are running ahead of "master". The tip of these
1901 branches is not published in the public repository to keep the number
1902 of branches that downstream developers need to worry about low.
1903
1904 The quality of topic branches varies widely. Some of them start out as
1905 "good idea but obviously is broken in some areas" and then with some
1906 more work become "more or less done and can now be tested by wider
1907 audience". Luckily, most of them start out in the latter, better shape.
1908
1909 The "next" branch is to merge and test topic branches in the latter
1910 category. In general, this branch always contains the tip of "master".
1911 It might not be quite rock-solid production ready, but is expected to
1912 work more or less without major breakage. The maintainer usually uses
1913 the "next" version of paraslash for his own pleasure, so it cannot
1914 be _that_ broken. The "next" branch is where new and exciting things
1915 take place.
1916
1917 The two branches "master" and "maint" are never rewound, and "next"
1918 usually will not be either (this automatically means the topics that
1919 have been merged into "next" are usually not rebased, and you can find
1920 the tip of topic branches you are interested in from the output of
1921 "git log next"). You should be able to safely build on top of them.
1922
1923 However, at times "next" will be rebuilt from the tip of "master" to
1924 get rid of merge commits that will never be in "master". The commit
1925 that replaces "next" will usually have the identical tree, but it
1926 will have different ancestry from the tip of "master".
1927
1928 The "pu" (proposed updates) branch bundles the remainder of the
1929 topic branches. The "pu" branch, and topic branches that are only in
1930 "pu", are subject to rebasing in general. By the above definition
1931 of how "next" works, you can tell that this branch will contain quite
1932 experimental and obviously broken stuff.
1933
1934 When a topic that was in "pu" proves to be in testable shape, it
1935 graduates to "next". This is done with
1936
1937 git checkout next
1938 git merge that-topic-branch
1939
1940 Sometimes, an idea that looked promising turns out to be not so good
1941 and the topic can be dropped from "pu" in such a case.
1942
1943 A topic that is in "next" is expected to be polished to perfection
1944 before it is merged to "master". Similar to the above, this is
1945 done with
1946
1947 git checkout master
1948 git merge that-topic-branch
1949 git branch -d that-topic-branch
1950
1951 Note that being in "next" is not a guarantee to appear in the next
1952 release (being in "master" is such a guarantee, unless it is later
1953 found seriously broken and reverted), nor even in any future release.
1954
1955 Coding Style
1956 ~~~~~~~~~~~~
1957
1958 The preferred coding style for paraslash coincides more or less
1959 with the style of the Linux kernel. So rather than repeating what is
1960 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
1961 there), here are the most important points.
1962
1963 - Burn the GNU coding standards.
1964 - Never use spaces for indentation.
1965 - Tabs are 8 characters, and thus indentations are also 8 characters.
1966 - Don't put multiple assignments on a single line.
1967 - Avoid tricky expressions.
1968 - Don't leave whitespace at the end of lines.
1969 - The limit on the length of lines is 80 columns.
1970 - Use K&R style for placing braces and spaces:
1971
1972 if (x is true) {
1973 we do y
1974 }
1975
1976 - Use a space after (most) keywords.
1977 - Do not add spaces around (inside) parenthesized expressions.
1978 - Use one space around (on each side of) most binary and ternary operators.
1979 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1980 - Mixed-case names are frowned upon.
1981 - Descriptive names for global variables are a must.
1982 - Avoid typedefs.
1983 - Functions should be short and sweet, and do just one thing.
1984 - The number of local variables shouldn't exceed 10.
1985 - Gotos are fine if they improve readability and reduce nesting.
1986 - Don't use C99-style "// ..." comments.
1987 - Names of macros defining constants and labels in enums are capitalized.
1988 - Enums are preferred when defining several related constants.
1989 - Always use the paraslash wrappers for allocating memory.
1990 - If the name of a function is an action or an imperative.
1991 command, the function should return an error-code integer
1992 (<0 means error, >=0 means success). If the name is a
1993 predicate, the function should return a "succeeded" boolean.
1994
1995
1996 Doxygen
1997 ~~~~~~~
1998
1999 Doxygen is a documentation system for various programming
2000 languages. The paraslash project uses Doxygen for generating the API
2001 reference on the web pages, but good source code documentation is
2002 also beneficial to people trying to understand the code structure
2003 and the interactions between the various source files.
2004
2005 It is more illustrative to look at the source code for examples than
2006 to describe the conventions for documenting the source in this manual,
2007 so we only describe which parts of the code need doxygen comments,
2008 but leave out details on documentation conventions.
2009
2010 As a rule, only the public part of the C source is documented with
2011 Doxygen. This includes structures, defines and enumerations in header
2012 files as well as public (non-static) C functions. These should be
2013 documented completely. For example each parameter and the return
2014 value of a public function should get a descriptive comment.
2015
2016 No doxygen comments are necessary for static functions and for
2017 structures and enumerations in C files (which are used only within
2018 this file). This does not mean, however, that those entities need
2019 no documentation at all. Instead, common sense should be applied to
2020 document what is not obvious from reading the code.
2021
2022 --------
2023 Appendix
2024 --------
2025
2026 Network protocols
2027 ~~~~~~~~~~~~~~~~~
2028
2029 *IP*. The _Internet Protocol_ is the primary networking protocol
2030 used for the Internet. All protocols described below use IP as the
2031 underlying layer. Both the prevalent IPv4 and the next-generation
2032 IPv6 variant are being deployed actively worldwide.
2033
2034 *Connection-oriented and connectionless protocols*. Connectionless
2035 protocols differ from connection-oriented ones in that state
2036 associated with the sending/receiving endpoints is treated
2037 implicitly. Connectionless protocols maintain no internal knowledge
2038 about the state of the connection. Hence they are not capable of
2039 reacting to state changes, such as sudden loss or congestion on the
2040 connection medium. Connection-oriented protocols, in contrast, make
2041 this knowledge explicit. The connection is established only after
2042 a bidirectional handshake which requires both endpoints to agree
2043 on the state of the connection, and may also involve negotiating
2044 specific parameters for the particular connection. Maintaining an
2045 up-to-date internal state of the connection also in general means
2046 that the sending endpoints perform congestion control, adapting to
2047 qualitative changes of the connection medium.
2048
2049 *Reliability*. In IP networking, packets can be lost, duplicated,
2050 or delivered out of order, and different network protocols handle
2051 these problems in different ways. We call a transport-layer protocol
2052 _reliable_, if it turns the unreliable IP delivery into an ordered,
2053 duplicate- and loss-free delivery of packets. Sequence numbers
2054 are used to discard duplicates and re-arrange packets delivered
2055 out-of-order. Retransmission is used to guarantee loss-free
2056 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2057 or data integrity.
2058
2059 *Classification*. With these definitions the protocols which are used
2060 by paraslash for steaming audio data may be classified as follows.
2061
2062 - HTTP/TCP: connection-oriented, reliable,
2063 - UDP: connectionless, unreliable,
2064 - DCCP: connection-oriented, unreliable.
2065
2066 Below we give a short descriptions of these protocols.
2067
2068 *TCP*. The _Transmission Control Protocol_ provides reliable,
2069 ordered delivery of a stream and a classic window-based congestion
2070 control. In contrast to UDP and DCCP (see below), TCP does not have
2071 record-oriented or datagram-based syntax, i.e. it provides a stream
2072 which is unaware and independent of any record (packet) boundaries.
2073 TCP is used extensively by many application layers. Besides HTTP (the
2074 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2075 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2076 top of TCP.
2077
2078 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2079 protocol, built as a thin layer directly on top of IP. For this reason,
2080 it offers the same best-effort service as IP itself, i.e. there is no
2081 detection of duplicate or reordered packets. Being a connectionless
2082 protocol, only minimal internal state about the connection is
2083 maintained, which means that there is no protection against packet
2084 loss or network congestion. Error checking and correction (if at all)
2085 are performed in the application.
2086
2087 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2088 connection-oriented state maintenance known from TCP with the
2089 unreliable, datagram-based transport of UDP. This means that it
2090 is capable of reacting to changes in the connection by performing
2091 congestion control, offering multiple alternative approaches. But it
2092 is bound to datagram boundaries (the maximum packet size supported
2093 by a medium), and like UDP it lacks retransmission to protect
2094 against loss. Due to the use of sequence numbers, it is however
2095 able to react to loss (interpreted as a congestion indication) and
2096 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2097 to negotiate specific, binding features for a connection, such as
2098 the choice of congestion control: classic, window-based congestion
2099 control known from TCP is available as CCID-2, rate-based, "smooth"
2100 congestion control is offered as CCID-3.
2101
2102 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2103 protocol on top of TCP. It is spoken by web servers and is most often
2104 used for web services. However, as can be seen by the many Internet
2105 radio stations and YouTube/Flash videos, http is by far not limited to
2106 the delivery of web pages only. Being a simple request/response based
2107 protocol, the semantics of the protocol also allow the delivery of
2108 multimedia content, such as audio over http.
2109
2110 *Multicast*. IP multicast is not really a protocol but a technique
2111 for one-to-many communication over an IP network. The challenge is to
2112 deliver information to a group of destinations simultaneously using
2113 the most efficient strategy to send the messages over each link of
2114 the network only once. This has benefits for streaming multimedia:
2115 the standard one-to-one unicast offered by TCP/DCCP means that
2116 n clients listening to the same stream also consume n-times the
2117 resources, whereas multicast requires to send the stream just once,
2118 irrespective of the number of receivers. Since it would be costly to
2119 maintain state for each listening receiver, multicast often implies
2120 connectionless transport, which is the reason that it is currently
2121 only available via UDP.
2122
2123 License
2124 ~~~~~~~
2125
2126 Paraslash is licensed under the GPL, version 2. Most of the code
2127 base has been written from scratch, and those parts are GPL V2
2128 throughout. Notable exceptions are FEC and the WMA decoder. See the
2129 corresponding source files for licencing details for these parts. Some
2130 code sniplets of several other third party software packages have
2131 been incorporated into the paraslash sources, for example log message
2132 coloring was taken from the git sources. These third party software
2133 packages are all published under the GPL or some other license
2134 compatible to the GPL.
2135
2136 Acknowledgements
2137 ~~~~~~~~~~~~~~~~
2138
2139 Many thanks to Gerrit Renker who read an early draft of this manual
2140 and contributed significant improvements.
2141
2142 ----------
2143 References
2144 ----------
2145
2146 Articles
2147 ~~~~~~~~
2148 - Reed, Irving S.; Solomon, Gustave (1960),
2149 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2150 Polynomial Codes over Certain Finite Fields), Journal of the
2151 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2152 300-304, doi:10.1137/0108018)
2153
2154 RFCs
2155 ~~~~
2156
2157 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2158 User Datagram Protocol
2159 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2160 Internet Protocol
2161 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2162 RSA Cryptography Specifications
2163 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2164 (2006): Datagram Congestion Control Protocol (DCCP)
2165 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2166 Congestion Control ID 2: TCP-like Congestion Control
2167 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2168 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2169
2170 Application web pages
2171 ~~~~~~~~~~~~~~~~~~~~~
2172
2173 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash)
2174 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2175 - XREFERENCE(http://www.mpg123.de/, mpg123)
2176 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2177 - XREFERENCE(http://www.icecast.org/, icecast)
2178 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2179
2180 External documentation
2181 ~~~~~~~~~~~~~~~~~~~~~~
2182
2183 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2184 H. Peter Anvin: The mathematics of Raid6)
2185 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2186 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2187 Communication Protocols)
2188
2189 Code
2190 ~~~~
2191 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2192 Original FEC implementation by Luigi Rizzo)
2193