manual: Improve troubleshooting section.
[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 For the impatient:
206
207 git clone git://git.tuebingen.mpg.de/osl
208 cd osl && make && sudo make install && sudo ldconfig
209 sudo apt-get install autoconf libssl-dev help2man gengetopt \
210 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
211 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev \
212 libasound2-dev libao-dev libreadline-dev libncurses-dev
213
214 Detailed description: In any case you'll need
215
216 - XREFERENCE(http://systemlinux.org/~maan/osl/, libosl).
217 The _object storage layer_ library is used by para_server. To
218 clone the source code repository, execute
219
220 git clone git://git.tuebingen.mpg.de/osl
221
222 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc) or
223 XREFERENCE(http://clang.llvm.org, clang). All gcc versions
224 >= 3.3 are currently supported. Clang version 1.1 or newer
225 should work as well.
226
227 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
228 also shipped with the disto. On BSD systems the gnu make
229 executable is often called gmake.
230
231 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
232 scripts which run during compilation require the EMPH(Bourne
233 again shell). It is most likely already installed.
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.openssl.org/, openssl) or
245 XREFERENCE(ftp://ftp.gnupg.org/gcrypt/libgcrypt/, libgcrypt).
246 At least one of these two libraries is needed as the backend
247 for cryptographic routines on both the server and the client
248 side. Both openssl and libgcrypt are usually shipped with the
249 distro, but you might have to install the development package
250 (libssl-dev or libgcrypt-dev on debian systems) as well.
251
252 - XREFERENCE(http://www.underbit.com/products/mad/, libmad).
253 To compile in MP3 support for paraslash, the development
254 package must be installed. It is called libmad0-dev on
255 debian-based systems. Note that libmad is not necessary on
256 the server side, i.e. for sending MP3 files.
257
258 - XREFERENCE(http://www.underbit.com/products/mad/,
259 libid3tag). For version-2 ID3 tag support, you'll need
260 the libid3tag development package libid3tag0-dev. Without
261 libid3tag, only version one tags are recognized.
262
263 - XREFERENCE(http://www.xiph.org/downloads/, ogg vorbis).
264 For ogg vorbis streams you'll need libogg, libvorbis,
265 libvorbisfile. The corresponding Debian packages are called
266 libogg-dev and libvorbis-dev.
267
268 - XREFERENCE(http://www.audiocoding.com/, libfaad). For aac
269 files (m4a) you'll need libfaad (libfaad-dev).
270
271 - XREFERENCE(http://www.speex.org/, speex). In order to stream
272 or decode speex files, libspeex (libspeex-dev) is required.
273
274 - XREFERENCE(http://flac.sourceforge.net/, flac). To stream
275 or decode files encoded with the _Free Lossless Audio Codec_,
276 libFLAC (libFLAC-dev) must be installed.
277
278 - XREFERENCE(http://www.mega-nerd.com/SRC/index.html,
279 libsamplerate). The resample filter will only be compiled if
280 this library is installed. Debian package: libsamplerate-dev.
281
282 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
283 Linux, you'll need to have ALSA's development package
284 libasound2-dev installed.
285
286 - XREFERENCE(http://downloads.xiph.org/releases/ao/,
287 libao). Needed to build the ao writer (ESD, PulseAudio,...).
288 Debian package: libao-dev.
289
290 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/ncurses, curses). Needed
291 for para_gui. Debian package: libncurses-dev.
292
293 - XREFERENCE(http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html,
294 GNU Readline). If this library (libreadline-dev) is installed,
295 para_client, para_audioc and para_play support interactive
296 sessions.
297
298 Installation
299 ~~~~~~~~~~~~
300
301 First make sure all non-optional packages listed in the section on
302 REFERENCE(Requirements, required software) are installed on your
303 system.
304
305 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
306 OGG/Speex and AAC support are all optional. The configure script will
307 detect what is installed on your system and will only try to build
308 those executables that can be built with your setup.
309
310 Note that no special decoder library (not even the MP3 decoding library
311 libmad) is needed for para_server if you only want to stream MP3 or WMA
312 files. Also, it's fine to use para_server on a box without sound card.
313
314 Next, install the paraslash package on all machines, you'd like this
315 software to run on. If you compile from a released tarball, execute
316
317 (./configure && make) > /dev/null
318
319 When compiling from git or from snapshots downloaded via gitweb,
320 the above command will not work because the configure script is not
321 included in the git repository. In this case the following command
322 should be used instead:
323
324 ./autogen.sh
325
326 This runs autoconf to generate the configure script, then runs it as
327 above. Therefore you'll need autoconf for this to work.
328
329 There should be no errors but probably some warnings about missing
330 packages which usually implies that not all audio formats will be
331 supported. If headers or libs are installed at unusual locations you
332 might need to tell the configure script where to find them. Try
333
334 ./configure --help
335
336 to see a list of options. If the paraslash package was compiled
337 successfully, execute (optionally)
338
339 make test
340
341 to run the paraslash test suite. If all tests pass, execute as root
342
343 make install
344
345 to install executables under /usr/local/bin and the man pages under
346 /usr/local/man.
347
348 Configuration
349 ~~~~~~~~~~~~~
350
351 *Step 1*: Create a paraslash user
352
353 In order to control para_server at runtime you must create a paraslash
354 user. As authentication is based on the RSA crypto system you'll have
355 to create an RSA key pair. If you already have a user and an RSA key
356 pair, you may skip this step.
357
358 In this section we'll assume a typical setup: You would like to run
359 para_server on some host called server_host as user foo, and you want
360 to connect to para_server from another machine called client_host as
361 user bar.
362
363 As foo@server_host, create ~/.paraslash/server.users by typing the
364 following commands:
365
366 user=bar
367 target=~/.paraslash/server.users
368 key=~/.paraslash/id_rsa.pub.$user
369 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
370 mkdir -p ~/.paraslash
371 echo "user $user $key $perms" >> $target
372
373 Next, change to the "bar" account on client_host and generate the
374 key pair with the commands
375
376 ssh-keygen -t rsa -b 2048
377 # hit enter twice to create a key with no passphrase
378
379 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
380 that paraslash can also read keys generated by the "openssl genrsa"
381 command. However, since keys created with ssh-keygen can also be used
382 for ssh, this method is recommended.
383
384 Note that para_server refuses to use a key if it is shorter than 2048
385 bits. In particular, the RSA keys of paraslash 0.3.x will not work
386 with version 0.4.x. Moreover, para_client refuses to use a (private)
387 key which is world-readable.
388
389 para_server only needs to know the public key of the key pair just
390 created. Copy this public key to server_host:
391
392 src=~/.ssh/id_rsa.pub
393 dest=.paraslash/id_rsa.pub.$LOGNAME
394 scp $src foo@server_host:$dest
395
396 Finally, tell para_client to connect to server_host:
397
398 conf=~/.paraslash/client.conf
399 echo 'hostname server_host' > $conf
400
401
402 *Step 2*: Start para_server
403
404 Before starting the server make sure you have write permissions to
405 the directory /var/paraslash that has been created during installation:
406
407 sudo chown $LOGNAME /var/paraslash
408
409 Alternatively, use the --afs-socket Option to specify a different
410 location for the AFS command socket.
411
412 For this first try, we'll use the info loglevel to make the output
413 of para_server more verbose.
414
415 para_server -l info
416
417 Now you can use para_client to connect to the server and issue
418 commands. Open a new shell as bar@client_host and try
419
420 para_client help
421 para_client si
422
423 to retrieve the list of available commands and some server info.
424 Don't proceed if this doesn't work.
425
426 *Step 3*: Create and populate the database
427
428 An empty database is created with
429
430 para_client init
431
432 This initializes a couple of empty tables under
433 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
434 tables, but it's good to know that you can start from scratch with
435
436 rm -rf ~/.paraslash/afs_database-0.4
437
438 in case something went wrong.
439
440 Next, you need to add some audio files to that database so that
441 para_server knows about them. Choose an absolute path to a directory
442 containing some audio files and add them to the audio file table:
443
444 para_client add /my/mp3/dir
445
446 This might take a while, so it is a good idea to start with a directory
447 containing not too many files. Note that the table only contains data
448 about the audio files found, not the files themselves.
449
450 You may print the list of all known audio files with
451
452 para_client ls
453
454 *Step 4*: Configure para_audiod
455
456 para_audiod needs to create a "well-known" socket for the clients to
457 connect to. The default path for this socket is
458
459 /var/paraslash/audiod_socket.$HOSTNAME
460
461 In order to make this directory writable for para_audiod, execute
462 as bar@client_host
463
464 sudo chown $LOGNAME /var/paraslash
465
466
467 We will also have to tell para_audiod that it should receive the
468 audio stream from server_host via http:
469
470 para_audiod -l info -r '.:http -i server_host'
471
472 You should now be able to listen to the audio stream once para_server
473 starts streaming. To activate streaming, execute
474
475 para_client play
476
477 Since no playlist has been specified yet, the "dummy" mode which
478 selects all known audio files is activated automatically. See the
479 section on the REFERENCE(The audio file selector, audio file selector)
480 for how to use playlists and moods to specify which files should be
481 streamed in which order.
482
483 *Troubleshooting*
484
485 It did not work? To find out why, try to receive, decode and play the
486 stream manually using para_recv, para_filter and para_write as follows.
487
488 For simplicity we assume that you're running Linux/ALSA and that only
489 MP3 files have been added to the database.
490
491 para_recv -r 'http -i server_host' > file.mp3
492 # (interrupt with CTRL+C after a few seconds)
493 ls -l file.mp3 # should not be empty
494 para_filter -f mp3dec -f wav < file.mp3 > file.wav
495 ls -l file.wav # should be much bigger than file.mp3
496 para_write -w alsa < file.wav
497
498 Double check what is logged by para_server and use the --loglevel
499 option of para_recv, para_filter and para_write to increase verbosity.
500
501 ---------------
502 User management
503 ---------------
504
505 para_server uses a challenge-response mechanism to authenticate
506 requests from incoming connections, similar to ssh's public key
507 authentication method. Authenticated connections are encrypted using
508 the RC4 stream cipher.
509
510 In this chapter we briefly describe RSA and RC4 and sketch the
511 REFERENCE(Client-server authentication, authentication handshake)
512 between para_client and para_server. User management is discussed
513 in the section on REFERENCE(The user_list file, the user_list file).
514 These sections are all about communication between the client and the
515 server. Connecting para_audiod is a different matter and is described
516 in a REFERENCE(Connecting para_audiod, separate section).
517
518
519
520 RSA and RC4
521 ~~~~~~~~~~~
522
523 RSA is an asymmetric block cipher which is used in many applications,
524 including ssh and gpg. An RSA key consists in fact of two keys,
525 called the public key and the private key. A message can be encrypted
526 with either key and only the counterpart of that key can decrypt
527 the message. While RSA can be used for both signing and encrypting
528 a message, paraslash uses RSA only for the latter purpose. The
529 RSA public key encryption and signatures algorithms are defined in
530 detail in RFC 2437.
531
532 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
533 key stream to produce the output. Decryption uses the same function
534 calls as encryption. While RC4 supports variable key lengths,
535 paraslash uses a fixed length of 256 bits, which is considered a
536 strong encryption by today's standards. Since the same key must never
537 be used twice, a different, randomly-generated key is used for every
538 new connection.
539
540 Client-server authentication
541 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
542
543 The authentication handshake between para_client and para_server goes
544 as follows:
545
546 - para_client connects to para_server and sends an
547 authentication request for a user. It does so by connecting
548 to TCP port 2990 of the server host. This port is called the
549 para_server _control port_.
550
551 - para_server accepts the connection and forks a child process
552 which handles the incoming request. The parent process keeps
553 listening on the control port while the child process (also
554 called para_server below) continues as follows.
555
556 - para_server loads the RSA public key of that user, fills a
557 fixed-length buffer with random bytes, encrypts that buffer
558 using the public key and sends the encrypted buffer to the
559 client. The first part of the buffer is the challenge which
560 is used for authentication while the second part is the RC4
561 session key.
562
563 - para_client receives the encrypted buffer and decrypts it
564 with the user's private key, thereby obtaining the challenge
565 buffer and the session key. It sends the SHA1 hash value of
566 the challenge back to para_server and stores the session key
567 for further use.
568
569 - para_server also computes the SHA1 hash of the challenge
570 and compares it against what was sent back by the client.
571
572 - If the two hashes do not match, the authentication has
573 failed and para_server closes the connection.
574
575 - Otherwise the user is considered authenticated and the client
576 is allowed to proceed by sending a command to be executed. From
577 this point on the communication is encrypted using the RC4
578 stream cipher with the session key known to both peers.
579
580 paraslash relies on the quality of the pseudo-random bytes provided
581 by the crypto library (openssl or libgcrypt), on the security of
582 the implementation of the RSA and RC4 crypto routines and on the
583 infeasibility to invert the SHA1 function.
584
585 Neither para_server or para_client create RSA keys on their own. This
586 has to be done once for each user as sketched in REFERENCE(Quick start,
587 Quick start) and discussed in more detail REFERENCE(The user_list
588 file, below).
589
590 The user_list file
591 ~~~~~~~~~~~~~~~~~~
592
593 At startup para_server reads the user list file which contains one
594 line per user. The default location of the user list file may be
595 changed with the --user-list option.
596
597 There should be at least one user in this file. Each user must have
598 an RSA key pair. The public part of the key is needed by para_server
599 while the private key is needed by para_client. Each line of the
600 user list file must be of the form
601
602 user <username> <key> <perms>
603
604 where _username_ is an arbitrary string (usually the user's login
605 name), _key_ is the full path to that user's public RSA key, and
606 _perms_ is a comma-separated list of zero or more of the following
607 permission bits:
608
609 +---------------------------------------------------------+
610 | AFS_READ | read the contents of the databases |
611 +-----------+---------------------------------------------+
612 | AFS_WRITE | change database contents |
613 +-----------+---------------------------------------------+
614 | VSS_READ | obtain information about the current stream |
615 +-----------+---------------------------------------------+
616 | VSS_WRITE | change the current stream |
617 +---------------------------------------------------------+
618
619 The permission bits specify which commands the user is allowed to
620 execute. The output of
621
622 para_client help
623
624 contains in the third column the permissions needed to execute the
625 command.
626
627 It is possible to make para_server reread the user_list file by
628 executing the paraslash "hup" command or by sending SIGHUP to the
629 PID of para_server.
630
631
632 Connecting para_audiod
633 ~~~~~~~~~~~~~~~~~~~~~~
634
635 para_audiod listens on a Unix domain socket. Those sockets are
636 for local communication only, so only local users can connect to
637 para_audiod. The default is to let any user connect but this can be
638 restricted on platforms that support UNIX socket credentials which
639 allow para_audiod to obtain the Unix credentials of the connecting
640 process.
641
642 Use para_audiod's --user-allow option to allow connections only for
643 a limited set of users.
644
645 -----------------------
646 The audio file selector
647 -----------------------
648
649 paraslash comes with a sophisticated audio file selector (AFS),
650 whose main task is to determine which file to stream next, based on
651 information on the audio files stored in a database. It communicates
652 also with para_client whenever an AFS command is executed, for example
653 to answer a database query.
654
655 Besides the traditional playlists, AFS supports audio file selection
656 based on _moods_ which act as a filter that limits the set of all
657 known audio files to those which satisfy certain criteria. It also
658 maintains tables containing images (e.g. album cover art) and lyrics
659 that can be associated with one or more audio files.
660
661 AFS uses XREFERENCE(http://systemlinux.org/~maan/osl/, libosl), the
662 object storage layer library, as the backend library for storing
663 information on audio files, playlists, etc. This library offers
664 functionality similar to a relational database, but is much more
665 lightweight than a full database backend.
666
667 In this chapter we sketch the setup of the REFERENCE(The AFS process,
668 AFS process) during server startup and proceed with the description
669 of the REFERENCE(Database layout, layout) of the various database
670 tables. The section on REFERENCE(Playlists and moods, playlists
671 and moods) explains these two audio file selection mechanisms
672 in detail and contains pratical examples. The way REFERENCE(File
673 renames and content changes, file renames and content changes) are
674 detected is discussed briefly before the REFERENCE(Troubleshooting,
675 Troubleshooting) section concludes the chapter.
676
677 The AFS process
678 ~~~~~~~~~~~~~~~
679
680 On startup, para_server forks to create the AFS process which opens
681 the OSL database tables. The server process communicates with the
682 AFS process via pipes and shared memory. Usually, the AFS process
683 awakes only briefly whenever the current audio file changes. The AFS
684 process determines the next audio file, opens it, verifies it has
685 not been changed since it was added to the database and passes the
686 open file descriptor to the server process, along with audio file
687 meta-data such as file name, duration, audio format and so on. The
688 server process then starts to stream the audio file.
689
690 The AFS process also accepts connections from local clients via
691 a well-known socket. However, only child processes of para_server
692 may connect through this socket. All server commands that have the
693 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
694 change the database.
695
696 Database layout
697 ~~~~~~~~~~~~~~~
698
699 *The audio file table*
700
701 This is the most important and usually also the largest table of the
702 AFS database. It contains the information needed to stream each audio
703 file. In particular the following data is stored for each audio file.
704
705 - SHA1 hash value of the audio file contents. This is computed
706 once when the file is added to the database. Whenever AFS
707 selects this audio file for streaming the hash value is
708 recomputed and checked against the value stored in the
709 database to detect content changes.
710
711 - The time when this audio file was last played.
712
713 - The number of times the file has been played so far.
714
715 - The attribute bitmask.
716
717 - The image id which describes the image associated with this
718 audio file.
719
720 - The lyrics id which describes the lyrics associated with
721 this audio file.
722
723 - The audio format id (MP3, OGG, ...).
724
725 - An amplification value that can be used by the amplification
726 filter to pre-amplify the decoded audio stream.
727
728 - The chunk table. It describes the location and the timing
729 of the building blocks of the audio file. This is used by
730 para_server to send chunks of the file at appropriate times.
731
732 - The duration of the audio file.
733
734 - Tag information contained in the audio file (ID3 tags,
735 Vorbis comments, ...).
736
737 - The number of channels
738
739 - The encoding bitrate.
740
741 - The sampling frequency.
742
743 To add or refresh the data contained in the audio file table, the _add_
744 command is used. It takes the full path of either an audio file or a
745 directory. In the latter case, the directory is traversed recursively
746 and all files which are recognized as valid audio files are added to
747 the database.
748
749 *The attribute table*
750
751 The attribute table contains two columns, _name_ and _bitnum_. An
752 attribute is simply a name for a certain bit number in the attribute
753 bitmask of the audio file table.
754
755 Each of the 64 bits of the attribute bitmask can be set for each
756 audio file individually. Hence up to 64 different attributes may be
757 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
758 "german_lyrics", "speech", whatever. You are free to choose as
759 many attributes as you like and there are no naming restrictions
760 for attributes.
761
762 A new attribute "test" is created by
763
764 para_client addatt test
765 and
766 para_client lsatt
767
768 lists all available attributes. You can set the "test" attribute for
769 an audio file by executing
770
771 para_client setatt test+ /path/to/the/audio/file
772
773 Similarly, the "test" bit can be removed from an audio file with
774
775 para_client setatt test- /path/to/the/audio/file
776
777 Instead of a path you may use a shell wildcard pattern. The attribute
778 is applied to all audio files matching this pattern:
779
780 para_client setatt test+ '/test/directory/*'
781
782 The command
783
784 para_client -- ls -lv
785
786 gives you a verbose listing of your audio files also showing which
787 attributes are set.
788
789 In case you wonder why the double-dash in the above command is needed:
790 It tells para_client to not interpret the options after the dashes. If
791 you find this annoying, just say
792
793 alias para='para_client --'
794
795 and be happy. In what follows we shall use this alias.
796
797 The "test" attribute can be dropped from the database with
798
799 para rmatt test
800
801 Read the output of
802
803 para help ls
804 para help setatt
805
806 for more information and a complete list of command line options to
807 these commands.
808
809 *Blob tables*
810
811 The image, lyrics, moods and playlists tables are all blob tables.
812 Blob tables consist of three columns each: The identifier which is
813 a positive non-negative number that is auto-incremented, the name
814 (an arbitrary string) and the content (the blob).
815
816 All blob tables support the same set of actions: cat, ls, mv, rm
817 and add. Of course, _add_ is used for adding new blobs to the table
818 while the other actions have the same meaning as the corresponding
819 Unix commands. The paraslash commands to perform these actions are
820 constructed as the concatenation of the table name and the action. For
821 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
822 manipulate or query the image table.
823
824 The add variant of these commands is special as these commands read
825 the blob contents from stdin. To add an image to the image table the
826 command
827
828 para addimg image_name < file.jpg
829
830 can be used.
831
832 Note that the images and lyrics are not interpreted at all, and also
833 the playlist and the mood blobs are only investigated when the mood
834 or playlist is activated with the select command.
835
836 *The score table*
837
838 Unlike all other tables the contents of the score table remain in
839 memory and are never stored on disk. The score table contains two
840 columns: The SHA1 hash value (of an audio file) and its current
841 score.
842
843 However, only those files which are admissible for the current mood
844 or playlist are contained in the score table. The audio file selector
845 always chooses the row with the highest score as the file to stream
846 next. While doing so, it computes the new score and updates the
847 last_played and the num_played fields in the audio file table.
848
849 The score table is recomputed by the select command which loads a
850 mood or playlist. Audio files are chosen for streaming from the rows
851 of the score table on a highest-score-first basis.
852
853
854 Playlists and moods
855 ~~~~~~~~~~~~~~~~~~~
856
857 Playlists and moods offer two different ways of specifying the set of
858 admissible files. A playlist in itself describes a set of admissible
859 files. A mood, in contrast, describes the set of admissible files in
860 terms of attributes and other type of information available in the
861 audio file table. As an example, a mood can define a filename pattern,
862 which is then matched against the names of audio files in the table.
863
864 *Playlists*
865
866 Playlists are accommodated in the playlist table of the afs database,
867 using the aforementioned blob format for tables. A new playlist is
868 created with the addpl command by specifying the full (absolute)
869 paths of all desired audio files, separated by newlines. Example:
870
871 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
872
873 If _my_playlist_ already exists it is overwritten. To activate the
874 new playlist, execute
875
876 para select p/my_playlist
877
878 The audio file selector will assign scores to each entry of the list,
879 in descending order so that files will be selected in order. If a
880 file could not be opened for streaming, its entry is removed from
881 the score table (but not from the playlist).
882
883 *Moods*
884
885 A mood consists of a unique name and its *mood definition*, which is
886 a set of *mood lines* containing expressions in terms of attributes
887 and other data contained in the database.
888
889 At any time at most one mood can be *active* which means that
890 para_server is going to select only files from that subset of
891 admissible files.
892
893 So in order to create a mood definition one has to write a set of
894 mood lines. Mood lines come in three flavours: Accept lines, deny
895 lines and score lines.
896
897 The general syntax of the three types of mood lines is
898
899
900 accept [with score <score>] [if] [not] <mood_method> [options]
901 deny [with score <score>] [if] [not] <mood_method> [options]
902 score <score> [if] [not] <mood_method> [options]
903
904
905 Here <score> is either an integer or the string "random" which assigns
906 a random score to all matching files. The score value changes the
907 order in which admissible files are going to be selected, but is of
908 minor importance for this introduction.
909
910 So we concentrate on the first two forms, i.e. accept and deny
911 lines. As usual, everything in square brackets is optional, i.e.
912 accept/deny lines take the following form when ignoring scores:
913
914 accept [if] [not] <mood_method> [options]
915
916 and analogously for the deny case. The "if" keyword is only syntactic
917 sugar and has no function. The "not" keyword just inverts the result,
918 so the essence of a mood line is the mood method part and the options
919 following thereafter.
920
921 A *mood method* is realized as a function which takes an audio file
922 and computes a number from the data contained in the database.
923 If this number is non-negative, we say the file *matches* the mood
924 method. The file matches the full mood line if it either
925
926 - matches the mood method and the "not" keyword is not given,
927 or
928 - does not match the mood method, but the "not" keyword is given.
929
930 The set of admissible files for the whole mood is now defined as those
931 files which match at least one accept mood line, but no deny mood line.
932 More formally, an audio file F is admissible if and only if
933
934 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
935
936 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
937 lines and "~" means "matches".
938
939 The cases where no mood lines of accept/deny type are defined need
940 special treatment:
941
942 - Neither accept nor deny lines: This treats all files as
943 admissible (in fact, that is the definition of the dummy mood
944 which is activated automatically if no moods are available).
945
946 - Only accept lines: A file is admissible iff it matches at
947 least one accept line:
948
949 F ~ AL1 or F ~ AL2 or ...
950
951 - Only deny lines: A file is admissible iff it matches no
952 deny line:
953
954 not (F ~ DL1 or F ~ DN2 ...)
955
956
957
958 *List of mood_methods*
959
960 no_attributes_set
961
962 Takes no arguments and matches an audio file if and only if no
963 attributes are set.
964
965 is_set <attribute_name>
966
967 Takes the name of an attribute and matches iff that attribute is set.
968
969 path_matches <pattern>
970
971 Takes a filename pattern and matches iff the path of the audio file
972 matches the pattern.
973
974 artist_matches <pattern>
975 album_matches <pattern>
976 title_matches <pattern>
977 comment_matches <pattern>
978
979 Takes an extended regular expression and matches iff the text of the
980 corresponding tag of the audio file matches the pattern. If the tag
981 is not set, the empty string is matched against the pattern.
982
983 year ~ <num>
984 bitrate ~ <num>
985 frequency ~ <num>
986 channels ~ <num>
987 num_played ~ <num>
988
989 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
990 <num>. Matches an audio file iff the condition <val> ~ <num> is
991 satisfied where val is the corresponding value of the audio file
992 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
993 count, play count).
994
995 The year tag is special as its value is undefined if the audio file
996 has no year tag or the content of the year tag is not a number. Such
997 audio files never match. Another difference is the special treatment
998 if the year tag is a two-digit number. In this case either 1900 or
999 2000 is added to the tag value, depending on whether the number is
1000 greater than 2000 plus the current year.
1001
1002
1003 *Mood usage*
1004
1005 To create a new mood called "my_mood", write its definition into
1006 some temporary file, say "tmpfile", and add it to the mood table
1007 by executing
1008
1009 para addmood my_mood < tmpfile
1010
1011 If the mood definition is really short, you may just pipe it to the
1012 client instead of using temporary files. Like this:
1013
1014 echo "$MOOD_DEFINITION" | para addmood my_mood
1015
1016 There is no need to keep the temporary file since you can always use
1017 the catmood command to get it back:
1018
1019 para catmood my_mood
1020
1021 A mood can be activated by executing
1022
1023 para select m/my_mood
1024
1025 Once active, the list of admissible files is shown by the ls command
1026 if the "-a" switch is given:
1027
1028 para ls -a
1029
1030
1031 *Example mood definition*
1032
1033 Suppose you have defined attributes "punk" and "rock" and want to define
1034 a mood containing only Punk-Rock songs. That is, an audio file should be
1035 admissible if and only if both attributes are set. Since
1036
1037 punk and rock
1038
1039 is obviously the same as
1040
1041 not (not punk or not rock)
1042
1043 (de Morgan's rule), a mood definition that selects only Punk-Rock
1044 songs is
1045
1046 deny if not is_set punk
1047 deny if not is_set rock
1048
1049
1050
1051 File renames and content changes
1052 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1053
1054 Since the audio file selector knows the SHA1 of each audio file that
1055 has been added to the afs database, it recognizes if the content of
1056 a file has changed, e.g. because an ID3 tag was added or modified.
1057 Also, if a file has been renamed or moved to a different location,
1058 afs will detect that an entry with the same hash value already exists
1059 in the audio file table.
1060
1061 In both cases it is enough to just re-add the new file. In the
1062 first case (file content changed), the audio table is updated, while
1063 metadata such as the num_played and last_played fields, as well as
1064 the attributes, remain unchanged. In the other case, when the file
1065 is moved or renamed, only the path information is updated, all other
1066 data remains as before.
1067
1068 It is possible to change the behaviour of the add command by using the
1069 "-l" (lazy add) or the "-f" (force add) option.
1070
1071 Troubleshooting
1072 ~~~~~~~~~~~~~~~
1073
1074 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1075 executables have a brief online help which is displayed when -h is
1076 given. The --detailed-help option prints the full help text.
1077
1078 If para_server crashed or was killed by SIGKILL (signal 9), it
1079 may refuse to start again because of "dirty osl tables". In this
1080 case you'll have to run the oslfsck program of libosl to fix your
1081 database:
1082
1083 oslfsck -fd ~/.paraslash/afs_database-0.4
1084
1085 However, make sure para_server isn't running before executing oslfsck.
1086
1087 If you don't mind to recreate your database you can start
1088 from scratch by removing the entire database directory, i.e.
1089
1090 rm -rf ~/.paraslash/afs_database-0.4
1091
1092 Be aware that this removes all attribute definitions, all playlists
1093 and all mood definitions and requires to re-initialize the tables.
1094
1095 Although oslfsck fixes inconsistencies in database tables it doesn't
1096 care about the table contents. To check for invalid table contents, use
1097
1098 para_client check
1099
1100 This prints out references to missing audio files as well as invalid
1101 playlists and mood definitions.
1102
1103 Similarly, para_audiod refuses to start if its socket file exists, since
1104 this indicates that another instance of para_audiod is running. After
1105 a crash a stale socket file might remain and you must run
1106
1107 para_audiod --force
1108
1109 once to fix it up.
1110
1111 ---------------------------------------
1112 Audio formats and audio format handlers
1113 ---------------------------------------
1114
1115 Audio formats
1116 ~~~~~~~~~~~~~
1117
1118 The following audio formats are supported by paraslash:
1119
1120 *MP3*
1121
1122 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1123 designed as part of its MPEG-1 standard. An MP3 file is made up of
1124 multiple MP3 frames, which consist of a header and a data block. The
1125 size of an MP3 frame depends on the bit rate and on the number
1126 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1127 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1128 400 bytes large.
1129
1130 *OGG/Vorbis*
1131
1132 OGG is a standardized audio container format, while Vorbis is an
1133 open source codec for lossy audio compression. Since Vorbis is most
1134 commonly made available via the OGG container format, it is often
1135 referred to as OGG/Vorbis. The OGG container format divides data into
1136 chunks called OGG pages. A typical OGG page is about 4KB large. The
1137 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1138 may vary considerably.
1139
1140 *OGG/Speex*
1141
1142 Speex is an open-source speech codec that is based on CELP (Code
1143 Excited Linear Prediction) coding. It is designed for voice
1144 over IP applications, has modest complexity and a small memory
1145 footprint. Wideband and narrowband (telephone quality) speech are
1146 supported. As for Vorbis audio, Speex bit-streams are often stored
1147 in OGG files.
1148
1149 *AAC*
1150
1151 Advanced Audio Coding (AAC) is a standardized, lossy compression
1152 and encoding scheme for digital audio which is the default audio
1153 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1154 the container format and audio files encoded with AAC have the .m4a
1155 extension. A typical AAC frame is about 700 bytes large.
1156
1157 *WMA*
1158
1159 Windows Media Audio (WMA) is an audio data compression technology
1160 developed by Microsoft. A WMA file is usually encapsulated in the
1161 Advanced Systems Format (ASF) container format, which also specifies
1162 how meta data about the file is to be encoded. The bit stream of WMA
1163 is composed of superframes, each containing one or more frames of
1164 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1165
1166 *FLAC*
1167
1168 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1169 loss. It gives better compression ratios than a general purpose
1170 compressor like zip or bzip2 because FLAC is designed specifically
1171 for audio. A FLAC-encoded file consits of frames of varying size, up
1172 to 16K. Each frame starts with a header that contains all information
1173 necessary to decode the frame.
1174
1175 Meta data
1176 ~~~~~~~~~
1177
1178 Unfortunately, each audio format has its own conventions how meta
1179 data is added as tags to the audio file.
1180
1181 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1182 is rather simple but also very limited as it supports only artist,
1183 title, album, year and comment tags. Each of these can only be at most
1184 32 characters long. ID3, version 2 is much more flexible but requires
1185 a separate library being installed for paraslash to support it.
1186
1187 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1188 comments, which are typically implemented as strings of the form
1189 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1190 tags are appropriate for the content.
1191
1192 AAC files usually use the MPEG-4 container format for storing meta
1193 data while WMA files wrap meta data as special objects within the
1194 ASF container format.
1195
1196 paraslash only tracks the most common tags that are supported by
1197 all tag variants: artist, title, year, album, comment. When a file
1198 is added to the AFS database, the meta data of the file is extracted
1199 and stored in the audio file table.
1200
1201 Chunks and chunk tables
1202 ~~~~~~~~~~~~~~~~~~~~~~~
1203
1204 paraslash uses the word "chunk" as common term for the building blocks
1205 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1206 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1207 size varies considerably between audio formats, from a few hundred
1208 bytes (MP3) up to 16K (FLAC).
1209
1210 The chunk table contains the offsets within the audio file that
1211 correspond to the chunk boundaries of the file. Like the meta data,
1212 the chunk table is computed and stored in the database whenever an
1213 audio file is added.
1214
1215 The paraslash senders (see below) always send complete chunks. The
1216 granularity for seeking is therefore determined by the chunk size.
1217
1218 Audio format handlers
1219 ~~~~~~~~~~~~~~~~~~~~~
1220
1221 For each audio format paraslash contains an audio format handler whose
1222 first task is to tell whether a given file is a valid audio file of
1223 this type. If so, the audio file handler extracts some technical data
1224 (duration, sampling rate, number of channels etc.), computes the
1225 chunk table and reads the meta data.
1226
1227 The audio format handler code is linked into para_server and executed
1228 via the _add_ command. The same code is also available as a stand-alone
1229 tool, para_afh, which can be used to print the technical data, the
1230 chunk table and the meta data of a file. Furthermore, one can use
1231 para_afh to cut an audio file, i.e. to select some of its chunks to
1232 produce a new file containing only these chunks.
1233
1234 ----------
1235 Networking
1236 ----------
1237
1238 Paraslash uses different network connections for control and data.
1239 para_client communicates with para_server over a dedicated TCP control
1240 connection. To transport audio data, separate data connections are
1241 used. For these data connections, a variety of transports (UDP, DCCP,
1242 HTTP) can be chosen.
1243
1244 The chapter starts with the REFERENCE(The paraslash control
1245 service, control service), followed by a section on the various
1246 REFERENCE(Streaming protocols, streaming protocols) in which the data
1247 connections are described. The way audio file headers are embedded into
1248 the stream is discussed REFERENCE(Streams with headers and headerless
1249 streams, briefly) before the REFERENCE(Networking examples, example
1250 section) which illustrates typical commands for real-life scenarios.
1251
1252 Both IPv4 and IPv6 are supported.
1253
1254 The paraslash control service
1255 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1256
1257 para_server is controlled at runtime via the paraslash control
1258 connection. This connection is used for server commands (play, stop,
1259 ...) as well as for afs commands (ls, select, ...).
1260
1261 The server listens on a TCP port and accepts connections from clients
1262 that connect the open port. Each connection causes the server to fork
1263 off a client process which inherits the connection and deals with that
1264 client only. In this classical accept/fork approach the server process
1265 is unaffected if the child dies or goes crazy for whatever reason. In
1266 fact, the child process can not change address space of server process.
1267
1268 The section on REFERENCE(Client-server authentication, client-server
1269 authentication) above described the early connection establishment
1270 from the crypto point of view. Here it is described what happens
1271 after the connection (including crypto setup) has been established.
1272 There are four processes involved during command dispatch as sketched
1273 in the following diagram.
1274
1275 <<
1276 <pre>
1277 server_host client_host
1278 ~~~~~~~~~~~ ~~~~~~~~~~~
1279
1280 +-----------+ connect +-----------+
1281 |para_server|<------------------------------ |para_client|
1282 +-----------+ +-----------+
1283 | ^
1284 | fork +---+ |
1285 +----------> |AFS| |
1286 | +---+ |
1287 | ^ |
1288 | | |
1289 | | connect (cookie) |
1290 | | |
1291 | | |
1292 | fork +-----+ inherited connection |
1293 +---------->|child|<--------------------------+
1294 +-----+
1295 </pre>
1296 >>
1297
1298 Note that the child process is not a child of the afs process,
1299 so communication of these two processes has to happen via local
1300 sockets. In order to avoid abuse of the local socket by unrelated
1301 processes, a magic cookie is created once at server startup time just
1302 before the server process forks off the AFS process. This cookie is
1303 known to the server, AFS and the child, but not to unrelated processes.
1304
1305 There are two different kinds of commands: First there are commands
1306 that cause the server to respond with some answer such as the list
1307 of all audio files. All but the addblob commands (addimg, addlyr,
1308 addpl, addmood) are of this kind. The addblob commands add contents
1309 to the database, so they need to transfer data the other way round,
1310 from the client to the server.
1311
1312 There is no knowledge about the server commands built into para_client,
1313 so it does not know about addblob commands. Instead, it inspects the
1314 first data package sent by the server for a magic string. If this
1315 string was found, it sends STDIN to the server, otherwise it dumps
1316 data from the server to STDOUT.
1317
1318 Streaming protocols
1319 ~~~~~~~~~~~~~~~~~~~
1320
1321 A network (audio) stream usually consists of one streaming source,
1322 the _sender_, and one or more _receivers_ which read data over the
1323 network from the streaming source.
1324
1325 Senders are thus part of para_server while receivers are part of
1326 para_audiod. Moreover, there is the stand-alone tool para_recv which
1327 can be used to manually download a stream, either from para_server
1328 or from a web-based audio streaming service.
1329
1330 The following three streaming protocols are supported by paraslash:
1331
1332 - HTTP. Recommended for public streams that can be played by
1333 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1334 sender is supported on all operating systems and all platforms.
1335
1336 - DCCP. Recommended for LAN streaming. DCCP is currently
1337 available only for Linux.
1338
1339 - UDP. Recommended for multicast LAN streaming.
1340
1341 See the Appendix on REFERENCE(Network protocols, network protocols)
1342 for brief descriptions of the various protocols relevant for network
1343 audio streaming with paraslash.
1344
1345 It is possible to activate more than one sender simultaneously.
1346 Senders can be controlled at run time and via config file and command
1347 line options.
1348
1349 Note that audio connections are _not_ encrypted. Transport or Internet
1350 layer encryption should be used if encrypted data connections are
1351 needed.
1352
1353 Since DCCP and TCP are both connection-oriented protocols, connection
1354 establishment/teardown and access control are very similar between
1355 these two streaming protocols. UDP is the most lightweight option,
1356 since in contrast to TCP/DCCP it is connectionless. It is also the
1357 only protocol supporting IP multicast.
1358
1359 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1360 clients to connect and establish a connection via some protocol-defined
1361 handshake mechanism. Both senders maintain two linked lists each:
1362 The list of all clients which are currently connected, and the list
1363 of access control entries which determines who is allowed to connect.
1364 IP-based access control may be configured through config file and
1365 command line options and via the "allow" and "deny" sender subcommands.
1366
1367 Upon receiving a GET request from the client, the HTTP sender sends
1368 back a status line and a message. The body of this message is the
1369 audio stream. This is common practice and is supported by many popular
1370 clients which can thus be used to play a stream offered by para_server.
1371 For DCCP things are a bit simpler: No messages are exchanged between
1372 the receiver and sender. The client simply connects and the sender
1373 starts to stream.
1374
1375 DCCP is an experimental protocol which offers a number of new features
1376 not available for TCP. Both ends can negotiate these features using
1377 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1378 datagram-based (no retransmissions) and thus should not be used over
1379 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1380 is access to a variety of different congestion-control mechanisms
1381 called CCIDs. Two different CCIDs are available per default on Linux:
1382
1383
1384 - _CCID 2_. A Congestion Control mechanism similar to that
1385 of TCP. The sender maintains a congestion window and halves
1386 this window in response to congestion.
1387
1388
1389 - _CCID-3_. Designed to be fair when competing for bandwidth.
1390 It has lower variation of throughput over time compared with
1391 TCP, which makes it suitable for streaming media.
1392
1393 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1394 single list, the _target list_. This list describes the set of clients
1395 to which the stream is sent. There is no list for access control and
1396 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1397 and "delete" commands can be used to modify the target list.
1398
1399 Since both UDP and DCCP offer an unreliable datagram-based transport,
1400 additional measures are necessary to guard against disruptions over
1401 networks that are lossy or which may be subject to interference (as
1402 is for instance the case with WiFi). Paraslash uses FEC (Forward
1403 Error Correction) to guard against packet losses and reordering. The
1404 stream is FEC-encoded before it is sent through the UDP socket and
1405 must be decoded accordingly on the receiver side.
1406
1407 The packet size and the amount of redundancy introduced by FEC can
1408 be configured via the FEC parameters which are dictated by server
1409 and may also be configured through the "sender" command. The FEC
1410 parameters are encoded in the header of each network packet, so no
1411 configuration is necessary on the receiver side. See the section on
1412 REFERENCE(Forward error correction, FEC) below.
1413
1414 Streams with headers and headerless streams
1415 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1416
1417 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1418 needed to decode the stream is only contained in the audio file
1419 header of the container format but not in each data chunk. Clients
1420 must be able to obtain this information in case streaming starts in
1421 the middle of the file or if para_audiod is started while para_server
1422 is already sending a stream.
1423
1424 This is accomplished in different ways, depending on the streaming
1425 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1426 header is sent prior to audio file data. This technique however does
1427 not work for the connectionless UDP transport. Hence the audio file
1428 header is periodically being embedded into the UDP audio data stream.
1429 By default, the header is resent after five seconds. The receiver has
1430 to wait until the next header arrives before it can start decoding
1431 the stream.
1432
1433 Examples
1434 ~~~~~~~~
1435
1436 The sender command of para_server allows to (de-)activate senders
1437 and to change the access permissions senders at runtime. The "si"
1438 (server info) command is used to list the streaming options of the
1439 currently running server as well as the various sender access lists.
1440
1441 -> Show client/target/access lists:
1442
1443 para_client si
1444
1445 -> Obtain general help for the sender command:
1446
1447 para_client help sender
1448
1449 -> Get help for a specific sender (contains further examples):
1450
1451 s=http # or dccp or udp
1452 para_client sender $s help
1453
1454 By default para_server activates both the HTTP and th DCCP sender on
1455 startup. This can be changed via command line options or para_server's
1456 config file.
1457
1458 -> List config file options for senders:
1459
1460 para_server -h
1461
1462 All senders share the "on" and "off" commands, so senders may be
1463 activated and deactivated independently of each other.
1464
1465 -> Switch off the http sender:
1466
1467 para_client sender http off
1468
1469 -> Receive a DCCP stream using CCID2 and write the output into a file:
1470
1471 host=foo.org; ccid=2; filename=bar
1472 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1473
1474 Note the quotes around the arguments for the dccp receiver. Each
1475 receiver has its own set of command line options and its own command
1476 line parser, so arguments for the dccp receiver must be protected
1477 from being interpreted by para_recv.
1478
1479 -> Start UDP multicast, using the default multicast address:
1480
1481 para_client sender udp add 224.0.1.38
1482
1483 -> Receive FEC-encoded multicast stream and write the output into a file:
1484
1485 filename=foo
1486 para_recv -r udp > $filename
1487
1488 -> Add an UDP unicast for a client to the target list of the UDP sender:
1489
1490 t=client.foo.org
1491 para_client sender udp add $t
1492
1493 -> Receive this (FEC-encoded) unicast stream:
1494
1495 filename=foo
1496 para_recv -r 'udp -i 0.0.0.0' > $filename
1497
1498 -> Create a minimal config for para_audiod for HTTP streams:
1499
1500 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1501 echo receiver \".:http -i $s\" > $c
1502 para_audiod --config $c
1503
1504 -------
1505 Filters
1506 -------
1507
1508 A paraslash filter is a module which transforms an input stream into
1509 an output stream. Filters are included in the para_audiod executable
1510 and in the stand-alone tool para_filter which usually contains the
1511 same modules.
1512
1513 While para_filter reads its input stream from STDIN and writes
1514 the output to STDOUT, the filter modules of para_audiod are always
1515 connected to a receiver which produces the input stream and a writer
1516 which absorbs the output stream.
1517
1518 Some filters depend on a specific library and are not compiled in
1519 if this library was not found at compile time. To see the list of
1520 supported filters, run para_filter and para_audiod with the --help
1521 option. The output looks similar to the following:
1522
1523 Available filters:
1524 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1525
1526 Out of these filter modules, a chain of filters can be constructed,
1527 much in the way Unix pipes can be chained, and analogous to the use
1528 of modules in gstreamer: The output of the first filter becomes the
1529 input of the second filter. There is no limitation on the number of
1530 filters and the same filter may occur more than once.
1531
1532 Like receivers, each filter has its own command line options which
1533 must be quoted to protect them from the command line options of
1534 the driving application (para_audiod or para_filter). Example:
1535
1536 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1537
1538 For para_audiod, each audio format has its own set of filters. The
1539 name of the audio format for which the filter should be applied can
1540 be used as the prefix for the filter option. Example:
1541
1542 para_audiod -f 'mp3:prebuffer --duration 300'
1543
1544 The "mp3" prefix above is actually interpreted as a POSIX extended
1545 regular expression. Therefore
1546
1547 para_audiod -f '.:prebuffer --duration 300'
1548
1549 activates the prebuffer filter for all supported audio formats (because
1550 "." matches all audio formats) while
1551
1552 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1553
1554 activates it only for wma and ogg streams.
1555
1556 Decoders
1557 ~~~~~~~~
1558
1559 For each supported audio format there is a corresponding filter
1560 which decodes audio data in this format to 16 bit PCM data which
1561 can be directly sent to the sound device or any other software that
1562 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1563 filters are called _decoders_ in general, and xxxdec is the name of
1564 the paraslash decoder for the audio format xxx. For example, the mp3
1565 decoder filter is called mp3dec.
1566
1567 Note that the output of the decoder is about 10 times larger than
1568 its input. This means that filters that operate on the decoded audio
1569 stream have to deal with much more data than filters that transform
1570 the audio stream before it is fed to the decoder.
1571
1572 Paraslash relies on external libraries for most decoders, so these
1573 libraries must be installed for the decoder to be included in the
1574 para_filter and para_audiod executables. The oggdec filter depends
1575 on the libogg and libvorbis libraries for example.
1576
1577 Forward error correction
1578 ~~~~~~~~~~~~~~~~~~~~~~~~
1579
1580 As already mentioned REFERENCE(Streaming protocols, earlier),
1581 paraslash uses forward error correction (FEC) for the unreliable UDP
1582 and DCCP transports. FEC is a technique which was invented already
1583 in 1960 by Reed and Solomon and which is widely used for the parity
1584 calculations of storage devices (RAID arrays). It is based on the
1585 algebraic concept of finite fields, today called Galois fields, in
1586 honour of the mathematician Galois (1811-1832). The FEC implementation
1587 of paraslash is based on code by Luigi Rizzo.
1588
1589 Although the details require a sound knowledge of the underlying
1590 mathematics, the basic idea is not hard to understand: For positive
1591 integers k and n with k < n it is possible to compute for any k given
1592 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1593 ..., p_r such that all data bytes can be reconstructed from *any*
1594 k bytes of the set
1595
1596 {d_1, ..., d_k, p_1, ..., p_r}.
1597
1598 FEC-encoding for unreliable network transports boils down to slicing
1599 the audio stream into groups of k suitably sized pieces called _slices_
1600 and computing the r corresponding parity slices. This step is performed
1601 in para_server which then sends both the data and the parity slices
1602 over the unreliable network connection. If the client was able
1603 to receive at least k of the n = k + r slices, it can reconstruct
1604 (FEC-decode) the original audio stream.
1605
1606 From these observations it is clear that there are three different
1607 FEC parameters: The slice size, the number of data slices k, and the
1608 total number of slices n. It is crucial to choose the slice size
1609 such that no fragmentation of network packets takes place because
1610 FEC only guards against losses and reordering but fails if slices are
1611 received partially.
1612
1613 FEC decoding in paralash is performed through the fecdec filter which
1614 usually is the first filter (there can be other filters before fecdec
1615 if these do not alter the audio stream).
1616
1617
1618 Volume adjustment (amp and compress)
1619 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1620
1621 The amp and the compress filter both adjust the volume of the audio
1622 stream. These filters operate on uncompressed audio samples. Hence
1623 they are usually placed directly after the decoding filter. Each
1624 sample is multiplied with a scaling factor (>= 1) which makes amp
1625 and compress quite expensive in terms of computing power.
1626
1627 *amp*
1628
1629 The amp filter amplifies the audio stream by a fixed scaling factor
1630 that must be known in advance. For para_audiod this factor is derived
1631 from the amplification field of the audio file's entry in the audio
1632 file table while para_filter uses the value given at the command line.
1633
1634 The optimal scaling factor F for an audio file is the largest real
1635 number F >= 1 such that after multiplication with F all samples still
1636 fit into the sample interval [-32768, 32767]. One can use para_filter
1637 in combination with the sox utility to compute F:
1638
1639 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1640
1641 The amplification value V which is stored in the audio file table,
1642 however, is an integer between 0 and 255 which is connected to F
1643 through the formula
1644
1645 V = (F - 1) * 64.
1646
1647 To store V in the audio file table, the command
1648
1649 para_client -- touch -a=V file.mp3
1650
1651 is used. The reader is encouraged to write a script that performs
1652 these computations :)
1653
1654 *compress*
1655
1656 Unlike the amplification filter, the compress filter adjusts the volume
1657 of the audio stream dynamically without prior knowledge about the peak
1658 value. It maintains the maximal volume of the last n samples of the
1659 audio stream and computes a suitable amplification factor based on that
1660 value and the various configuration options. It tries to chose this
1661 factor such that the adjusted volume meets the desired target level.
1662
1663 Note that it makes sense to combine amp and compress.
1664
1665 Misc filters (wav and prebuffer)
1666 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1667
1668 These filters are rather simple and do not modify the audio stream at
1669 all. The wav filter is only useful with para_filter and in connection
1670 with a decoder. It asks the decoder for the number of channels and the
1671 sample rate of the stream and adds a Microsoft wave header containing
1672 this information at the beginning. This allows to write wav files
1673 rather than raw PCM files (which do not contain any information about
1674 the number of channels and the sample rate).
1675
1676 The prebuffer filter simply delays the output until the given time has
1677 passed (starting from the time the first byte was available in its
1678 input queue) or until the given amount of data has accumulated. It
1679 is mainly useful for para_audiod if the standard parameters result
1680 in buffer underruns.
1681
1682 Both filters require almost no additional computing time, even when
1683 operating on uncompressed audio streams, since data buffers are simply
1684 "pushed down" rather than copied.
1685
1686 Examples
1687 ~~~~~~~~
1688
1689 -> Decode an mp3 file to wav format:
1690
1691 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1692
1693 -> Amplify a raw audio file by a factor of 1.5:
1694
1695 para_filter -f amp --amp 32 < foo.raw > bar.raw
1696
1697 ------
1698 Output
1699 ------
1700
1701 Once an audio stream has been received and decoded to PCM format,
1702 it can be sent to a sound device for playback. This part is performed
1703 by paraslash _writers_ which are described in this chapter.
1704
1705 Writers
1706 ~~~~~~~
1707
1708 A paraslash writer acts as a data sink that consumes but does not
1709 produce audio data. Paraslash writers operate on the client side and
1710 are contained in para_audiod and in the stand-alone tool para_write.
1711
1712 The para_write program reads uncompressed audio data from STDIN. If
1713 this data starts with a wav header, sample rate, sample format and
1714 channel count are read from the header. Otherwise CD audio (44.1KHz
1715 16 bit little endian, stereo) is assumed but this can be overridden
1716 by command line options. para_audiod, on the other hand, obtains
1717 the sample rate and the number of channels from the decoder.
1718
1719 Like receivers and filters, each writer has an individual set of
1720 command line options, and for para_audiod writers can be configured
1721 per audio format separately. It is possible to activate more than
1722 one writer for the same stream simultaneously.
1723
1724 OS-dependent APIs
1725 ~~~~~~~~~~~~~~~~~
1726
1727 Unfortunately, the various flavours of Unix on which paraslash
1728 runs on have different APIs for opening a sound device and starting
1729 playback. Hence for each such API there is a paraslash writer that
1730 can play the audio stream via this API.
1731
1732 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1733 Linux systems. Although there are several mid-layer APIs in use by
1734 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1735 currently supports only the low-level ALSA API which is not supposed
1736 to be change. ALSA is very feature-rich, in particular it supports
1737 software mixing via its DMIX plugin. ALSA is the default writer on
1738 Linux systems.
1739
1740 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1741 is also available on Linux systems, usually provided by ALSA as an
1742 emulation for backwards compatibility. This API is rather simple but
1743 also limited. For example only one application can open the device
1744 at any time. The OSS writer is activated by default on BSD Systems.
1745
1746 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1747 for this API is only compiled in on such systems and is of course
1748 the default there.
1749
1750 *FILE*. The file writer allows to capture the audio stream and
1751 write the PCM data to a file on the file system rather than playing
1752 it through a sound device. It is supported on all platforms and is
1753 always compiled in.
1754
1755 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1756 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1757 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1758 through an output plugin of libao.
1759
1760 Examples
1761 ~~~~~~~~
1762
1763 -> Use the OSS writer to play a wav file:
1764
1765 para_write --writer oss < file.wav
1766
1767 -> Enable ALSA software mixing for mp3 streams
1768
1769 para_audiod --writer 'mp3:alsa -d plug:swmix'
1770
1771
1772 ---
1773 Gui
1774 ---
1775
1776 para_gui executes an arbitrary command which is supposed to print
1777 status information to STDOUT. It then displays this information in
1778 a curses window. By default the command
1779
1780 para_audioc -- stat -p
1781
1782 is executed, but this can be customized via the --stat-cmd option. In
1783 particular it possible to use
1784
1785 para_client -- stat -p
1786
1787 to make para_gui work on systems on which para_audiod is not running.
1788
1789 Key bindings
1790 ~~~~~~~~~~~~
1791
1792 It is possible to bind keys to arbitrary commands via custom
1793 key-bindings. Besides the internal keys which can not be changed (help,
1794 quit, loglevel, version...), the following flavours of key-bindings
1795 are supported:
1796
1797 - external: Shutdown curses before launching the given command.
1798 Useful for starting other ncurses programs from within
1799 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1800 output format to write a mailbox containing one mail for each
1801 (admissible) file the audio file selector knows about. Then
1802 start mutt from within para_gui to browse your collection!
1803
1804 - display: Launch the command and display its stdout in
1805 para_gui's bottom window.
1806
1807 - para: Like display, but start "para_client <specified
1808 command>" instead of "<specified command>".
1809
1810 The general form of a key binding is
1811
1812 key_map k:m:c
1813
1814 which maps key k to command c using mode m. Mode may be x, d or p
1815 for external, display and paraslash commands, respectively.
1816
1817 Themes
1818 ~~~~~~
1819
1820 Currently there are only two themes for para_gui. It is easy, however,
1821 to add more themes. To create a new theme one has to define the
1822 position, color and geometry for for each status item that should be
1823 shown by this theme. See gui_theme.c for examples.
1824
1825 The "." and "," keys are used to switch between themes.
1826
1827 Examples
1828 ~~~~~~~~
1829
1830 -> Show server info:
1831
1832 key_map "i:p:si"
1833
1834 -> Jump to the middle of the current audio file by pressing F5:
1835
1836 key_map "<F5>:p:jmp 50"
1837
1838 -> vi-like bindings for jumping around:
1839
1840 key_map "l:p:ff 10"
1841 key_map "h:p:ff 10-"
1842 key_map "w:p:ff 60"
1843 key_map "b:p:ff 60-"
1844
1845 -> Print the current date and time:
1846
1847 key_map "D:d:date"
1848
1849 -> Call other curses programs:
1850
1851 key_map "U:x:aumix"
1852 key_map "!:x:/bin/bash"
1853 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1854
1855 -----------
1856 Development
1857 -----------
1858
1859 Tools
1860 ~~~~~
1861
1862 In order to compile the sources from the git repository (rather than
1863 from tar balls) and for contributing non-trivial changes to the
1864 paraslash project, some additional tools should be installed on a
1865 developer machine.
1866
1867 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1868 branches, below), the git source code management tool is used for
1869 paraslash development. It is necessary for cloning the git repository
1870 and for getting updates.
1871
1872 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1873 are generated from templates by the m4 macro processor.
1874
1875 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1876 the configure file which is shipped in the tarballs but has to be
1877 generated when compiling from git.
1878
1879 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1880 HTML version of this manual and some of the paraslash web pages are
1881 generated by the grutatxt plain text to HTML converter. If changes
1882 are made to these text files the grutatxt package must be installed
1883 to regenerate the HTML files.
1884
1885 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1886 of paraslash's C sources uses the doxygen documentation system. The
1887 conventions for documenting the source code is described in the
1888 REFERENCE(Doxygen, Doxygen section).
1889
1890 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1891 browsable HTML from the C sources. It is needed by doxygen.
1892
1893 Git branches
1894 ~~~~~~~~~~~~
1895
1896 Paraslash has been developed using the git source code management
1897 tool since 2006. Development is organized roughly in the same spirit
1898 as the git development itself, as described below.
1899
1900 The following text passage is based on "A note from the maintainer",
1901 written by Junio C Hamano, the maintainer of git.
1902
1903 There are four branches in the paraslash repository that track the
1904 source tree: "master", "maint", "next", and "pu".
1905
1906 The "master" branch is meant to contain what is well tested and
1907 ready to be used in a production setting. There could occasionally be
1908 minor breakages or brown paper bag bugs but they are not expected to
1909 be anything major, and more importantly quickly and easily fixable.
1910 Every now and then, a "feature release" is cut from the tip of this
1911 branch, named with three dotted decimal digits, like 0.4.2.
1912
1913 Whenever changes are about to be included that will eventually lead to
1914 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1915 "master" at that point. Obvious, safe and urgent fixes after the major
1916 release are applied to this branch and maintenance releases are cut
1917 from it. New features never go to this branch. This branch is also
1918 merged into "master" to propagate the fixes forward.
1919
1920 A trivial and safe enhancement goes directly on top of "master".
1921 New development does not usually happen on "master", however.
1922 Instead, a separate topic branch is forked from the tip of "master",
1923 and it first is tested in isolation; Usually there are a handful such
1924 topic branches that are running ahead of "master". The tip of these
1925 branches is not published in the public repository to keep the number
1926 of branches that downstream developers need to worry about low.
1927
1928 The quality of topic branches varies widely. Some of them start out as
1929 "good idea but obviously is broken in some areas" and then with some
1930 more work become "more or less done and can now be tested by wider
1931 audience". Luckily, most of them start out in the latter, better shape.
1932
1933 The "next" branch is to merge and test topic branches in the latter
1934 category. In general, this branch always contains the tip of "master".
1935 It might not be quite rock-solid production ready, but is expected to
1936 work more or less without major breakage. The maintainer usually uses
1937 the "next" version of paraslash for his own pleasure, so it cannot
1938 be _that_ broken. The "next" branch is where new and exciting things
1939 take place.
1940
1941 The two branches "master" and "maint" are never rewound, and "next"
1942 usually will not be either (this automatically means the topics that
1943 have been merged into "next" are usually not rebased, and you can find
1944 the tip of topic branches you are interested in from the output of
1945 "git log next"). You should be able to safely build on top of them.
1946
1947 However, at times "next" will be rebuilt from the tip of "master" to
1948 get rid of merge commits that will never be in "master". The commit
1949 that replaces "next" will usually have the identical tree, but it
1950 will have different ancestry from the tip of "master".
1951
1952 The "pu" (proposed updates) branch bundles the remainder of the
1953 topic branches. The "pu" branch, and topic branches that are only in
1954 "pu", are subject to rebasing in general. By the above definition
1955 of how "next" works, you can tell that this branch will contain quite
1956 experimental and obviously broken stuff.
1957
1958 When a topic that was in "pu" proves to be in testable shape, it
1959 graduates to "next". This is done with
1960
1961 git checkout next
1962 git merge that-topic-branch
1963
1964 Sometimes, an idea that looked promising turns out to be not so good
1965 and the topic can be dropped from "pu" in such a case.
1966
1967 A topic that is in "next" is expected to be polished to perfection
1968 before it is merged to "master". Similar to the above, this is
1969 done with
1970
1971 git checkout master
1972 git merge that-topic-branch
1973 git branch -d that-topic-branch
1974
1975 Note that being in "next" is not a guarantee to appear in the next
1976 release (being in "master" is such a guarantee, unless it is later
1977 found seriously broken and reverted), nor even in any future release.
1978
1979 Coding Style
1980 ~~~~~~~~~~~~
1981
1982 The preferred coding style for paraslash coincides more or less
1983 with the style of the Linux kernel. So rather than repeating what is
1984 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
1985 there), here are the most important points.
1986
1987 - Burn the GNU coding standards.
1988 - Never use spaces for indentation.
1989 - Tabs are 8 characters, and thus indentations are also 8 characters.
1990 - Don't put multiple assignments on a single line.
1991 - Avoid tricky expressions.
1992 - Don't leave whitespace at the end of lines.
1993 - The limit on the length of lines is 80 columns.
1994 - Use K&R style for placing braces and spaces:
1995
1996 if (x is true) {
1997 we do y
1998 }
1999
2000 - Use a space after (most) keywords.
2001 - Do not add spaces around (inside) parenthesized expressions.
2002 - Use one space around (on each side of) most binary and ternary operators.
2003 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2004 - Mixed-case names are frowned upon.
2005 - Descriptive names for global variables are a must.
2006 - Avoid typedefs.
2007 - Functions should be short and sweet, and do just one thing.
2008 - The number of local variables shouldn't exceed 10.
2009 - Gotos are fine if they improve readability and reduce nesting.
2010 - Don't use C99-style "// ..." comments.
2011 - Names of macros defining constants and labels in enums are capitalized.
2012 - Enums are preferred when defining several related constants.
2013 - Always use the paraslash wrappers for allocating memory.
2014 - If the name of a function is an action or an imperative.
2015 command, the function should return an error-code integer
2016 (<0 means error, >=0 means success). If the name is a
2017 predicate, the function should return a "succeeded" boolean.
2018
2019
2020 Doxygen
2021 ~~~~~~~
2022
2023 Doxygen is a documentation system for various programming
2024 languages. The paraslash project uses Doxygen for generating the API
2025 reference on the web pages, but good source code documentation is
2026 also beneficial to people trying to understand the code structure
2027 and the interactions between the various source files.
2028
2029 It is more illustrative to look at the source code for examples than
2030 to describe the conventions for documenting the source in this manual,
2031 so we only describe which parts of the code need doxygen comments,
2032 but leave out details on documentation conventions.
2033
2034 As a rule, only the public part of the C source is documented with
2035 Doxygen. This includes structures, defines and enumerations in header
2036 files as well as public (non-static) C functions. These should be
2037 documented completely. For example each parameter and the return
2038 value of a public function should get a descriptive comment.
2039
2040 No doxygen comments are necessary for static functions and for
2041 structures and enumerations in C files (which are used only within
2042 this file). This does not mean, however, that those entities need
2043 no documentation at all. Instead, common sense should be applied to
2044 document what is not obvious from reading the code.
2045
2046 --------
2047 Appendix
2048 --------
2049
2050 Network protocols
2051 ~~~~~~~~~~~~~~~~~
2052
2053 *IP*. The _Internet Protocol_ is the primary networking protocol
2054 used for the Internet. All protocols described below use IP as the
2055 underlying layer. Both the prevalent IPv4 and the next-generation
2056 IPv6 variant are being deployed actively worldwide.
2057
2058 *Connection-oriented and connectionless protocols*. Connectionless
2059 protocols differ from connection-oriented ones in that state
2060 associated with the sending/receiving endpoints is treated
2061 implicitly. Connectionless protocols maintain no internal knowledge
2062 about the state of the connection. Hence they are not capable of
2063 reacting to state changes, such as sudden loss or congestion on the
2064 connection medium. Connection-oriented protocols, in contrast, make
2065 this knowledge explicit. The connection is established only after
2066 a bidirectional handshake which requires both endpoints to agree
2067 on the state of the connection, and may also involve negotiating
2068 specific parameters for the particular connection. Maintaining an
2069 up-to-date internal state of the connection also in general means
2070 that the sending endpoints perform congestion control, adapting to
2071 qualitative changes of the connection medium.
2072
2073 *Reliability*. In IP networking, packets can be lost, duplicated,
2074 or delivered out of order, and different network protocols handle
2075 these problems in different ways. We call a transport-layer protocol
2076 _reliable_, if it turns the unreliable IP delivery into an ordered,
2077 duplicate- and loss-free delivery of packets. Sequence numbers
2078 are used to discard duplicates and re-arrange packets delivered
2079 out-of-order. Retransmission is used to guarantee loss-free
2080 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2081 or data integrity.
2082
2083 *Classification*. With these definitions the protocols which are used
2084 by paraslash for steaming audio data may be classified as follows.
2085
2086 - HTTP/TCP: connection-oriented, reliable,
2087 - UDP: connectionless, unreliable,
2088 - DCCP: connection-oriented, unreliable.
2089
2090 Below we give a short descriptions of these protocols.
2091
2092 *TCP*. The _Transmission Control Protocol_ provides reliable,
2093 ordered delivery of a stream and a classic window-based congestion
2094 control. In contrast to UDP and DCCP (see below), TCP does not have
2095 record-oriented or datagram-based syntax, i.e. it provides a stream
2096 which is unaware and independent of any record (packet) boundaries.
2097 TCP is used extensively by many application layers. Besides HTTP (the
2098 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2099 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2100 top of TCP.
2101
2102 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2103 protocol, built as a thin layer directly on top of IP. For this reason,
2104 it offers the same best-effort service as IP itself, i.e. there is no
2105 detection of duplicate or reordered packets. Being a connectionless
2106 protocol, only minimal internal state about the connection is
2107 maintained, which means that there is no protection against packet
2108 loss or network congestion. Error checking and correction (if at all)
2109 are performed in the application.
2110
2111 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2112 connection-oriented state maintenance known from TCP with the
2113 unreliable, datagram-based transport of UDP. This means that it
2114 is capable of reacting to changes in the connection by performing
2115 congestion control, offering multiple alternative approaches. But it
2116 is bound to datagram boundaries (the maximum packet size supported
2117 by a medium), and like UDP it lacks retransmission to protect
2118 against loss. Due to the use of sequence numbers, it is however
2119 able to react to loss (interpreted as a congestion indication) and
2120 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2121 to negotiate specific, binding features for a connection, such as
2122 the choice of congestion control: classic, window-based congestion
2123 control known from TCP is available as CCID-2, rate-based, "smooth"
2124 congestion control is offered as CCID-3.
2125
2126 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2127 protocol on top of TCP. It is spoken by web servers and is most often
2128 used for web services. However, as can be seen by the many Internet
2129 radio stations and YouTube/Flash videos, http is by far not limited to
2130 the delivery of web pages only. Being a simple request/response based
2131 protocol, the semantics of the protocol also allow the delivery of
2132 multimedia content, such as audio over http.
2133
2134 *Multicast*. IP multicast is not really a protocol but a technique
2135 for one-to-many communication over an IP network. The challenge is to
2136 deliver information to a group of destinations simultaneously using
2137 the most efficient strategy to send the messages over each link of
2138 the network only once. This has benefits for streaming multimedia:
2139 the standard one-to-one unicast offered by TCP/DCCP means that
2140 n clients listening to the same stream also consume n-times the
2141 resources, whereas multicast requires to send the stream just once,
2142 irrespective of the number of receivers. Since it would be costly to
2143 maintain state for each listening receiver, multicast often implies
2144 connectionless transport, which is the reason that it is currently
2145 only available via UDP.
2146
2147 License
2148 ~~~~~~~
2149
2150 Paraslash is licensed under the GPL, version 2. Most of the code
2151 base has been written from scratch, and those parts are GPL V2
2152 throughout. Notable exceptions are FEC and the WMA decoder. See the
2153 corresponding source files for licencing details for these parts. Some
2154 code sniplets of several other third party software packages have
2155 been incorporated into the paraslash sources, for example log message
2156 coloring was taken from the git sources. These third party software
2157 packages are all published under the GPL or some other license
2158 compatible to the GPL.
2159
2160 Acknowledgements
2161 ~~~~~~~~~~~~~~~~
2162
2163 Many thanks to Gerrit Renker who read an early draft of this manual
2164 and contributed significant improvements.
2165
2166 ----------
2167 References
2168 ----------
2169
2170 Articles
2171 ~~~~~~~~
2172 - Reed, Irving S.; Solomon, Gustave (1960),
2173 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2174 Polynomial Codes over Certain Finite Fields), Journal of the
2175 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2176 300-304, doi:10.1137/0108018)
2177
2178 RFCs
2179 ~~~~
2180
2181 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2182 User Datagram Protocol
2183 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2184 Internet Protocol
2185 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2186 RSA Cryptography Specifications
2187 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2188 (2006): Datagram Congestion Control Protocol (DCCP)
2189 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2190 Congestion Control ID 2: TCP-like Congestion Control
2191 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2192 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2193
2194 Application web pages
2195 ~~~~~~~~~~~~~~~~~~~~~
2196
2197 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash)
2198 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2199 - XREFERENCE(http://www.mpg123.de/, mpg123)
2200 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2201 - XREFERENCE(http://www.icecast.org/, icecast)
2202 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2203
2204 External documentation
2205 ~~~~~~~~~~~~~~~~~~~~~~
2206
2207 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2208 H. Peter Anvin: The mathematics of Raid6)
2209 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2210 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2211 Communication Protocols)
2212
2213 Code
2214 ~~~~
2215 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2216 Original FEC implementation by Luigi Rizzo)
2217