Merge branch 'maint'
[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 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 a stream cipher, either RC4 or AES in integer counter mode.
509
510 In this chapter we briefly describe RSA, RC4 and AES, 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, RC4, AES
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 AES, the advanced encryption standard, is a well-known symmetric block
541 cipher, i.e. a transformation operating on fixed-length blocks which
542 is determined by a single key for both encryption and decryption. Any
543 block cipher can be turned into a stream cipher by generating
544 a pseudo-random key stream by encrypting successive values of a
545 counter. The AES_CTR128 stream cipher used in paraslash is obtained
546 in this way from the AES block cipher with a 128 bit block size.
547
548
549 Client-server authentication
550 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
551
552 The authentication handshake between para_client and para_server goes
553 as follows:
554
555 - para_client connects to para_server and sends an
556 authentication request for a user. It does so by connecting
557 to TCP port 2990 of the server host. This port is called the
558 para_server _control port_.
559
560 - para_server accepts the connection and forks a child process
561 which handles the incoming request. The parent process keeps
562 listening on the control port while the child process (also
563 called para_server below) continues as follows.
564
565 - para_server loads the RSA public key of that user, fills a
566 fixed-length buffer with random bytes, encrypts that buffer
567 using the public key and sends the encrypted buffer to the
568 client. The first part of the buffer is the challenge which
569 is used for authentication while the second part is the
570 session key.
571
572 - para_client receives the encrypted buffer and decrypts it
573 with the user's private key, thereby obtaining the challenge
574 buffer and the session key. It sends the SHA1 hash value of
575 the challenge back to para_server and stores the session key
576 for further use.
577
578 - para_server also computes the SHA1 hash of the challenge
579 and compares it against what was sent back by the client.
580
581 - If the two hashes do not match, the authentication has
582 failed and para_server closes the connection.
583
584 - Otherwise the user is considered authenticated and the client
585 is allowed to proceed by sending a command to be executed. From
586 this point on the communication is encrypted using the stream
587 cipher with the session key known to both peers.
588
589 paraslash relies on the quality of the pseudo-random bytes provided
590 by the crypto library (openssl or libgcrypt), on the security of
591 the implementation of the RSA, RC4 and AES crypto routines and on the
592 infeasibility to invert the SHA1 function.
593
594 Neither para_server or para_client create RSA keys on their own. This
595 has to be done once for each user as sketched in REFERENCE(Quick start,
596 Quick start) and discussed in more detail REFERENCE(The user_list
597 file, below).
598
599 The user_list file
600 ~~~~~~~~~~~~~~~~~~
601
602 At startup para_server reads the user list file which contains one
603 line per user. The default location of the user list file may be
604 changed with the --user-list option.
605
606 There should be at least one user in this file. Each user must have
607 an RSA key pair. The public part of the key is needed by para_server
608 while the private key is needed by para_client. Each line of the
609 user list file must be of the form
610
611 user <username> <key> <perms>
612
613 where _username_ is an arbitrary string (usually the user's login
614 name), _key_ is the full path to that user's public RSA key, and
615 _perms_ is a comma-separated list of zero or more of the following
616 permission bits:
617
618 +---------------------------------------------------------+
619 | AFS_READ | read the contents of the databases |
620 +-----------+---------------------------------------------+
621 | AFS_WRITE | change database contents |
622 +-----------+---------------------------------------------+
623 | VSS_READ | obtain information about the current stream |
624 +-----------+---------------------------------------------+
625 | VSS_WRITE | change the current stream |
626 +---------------------------------------------------------+
627
628 The permission bits specify which commands the user is allowed to
629 execute. The output of
630
631 para_client help
632
633 contains in the third column the permissions needed to execute the
634 command.
635
636 It is possible to make para_server reread the user_list file by
637 executing the paraslash "hup" command or by sending SIGHUP to the
638 PID of para_server.
639
640
641 Connecting para_audiod
642 ~~~~~~~~~~~~~~~~~~~~~~
643
644 para_audiod listens on a Unix domain socket. Those sockets are
645 for local communication only, so only local users can connect to
646 para_audiod. The default is to let any user connect but this can be
647 restricted on platforms that support UNIX socket credentials which
648 allow para_audiod to obtain the Unix credentials of the connecting
649 process.
650
651 Use para_audiod's --user-allow option to allow connections only for
652 a limited set of users.
653
654 -----------------------
655 The audio file selector
656 -----------------------
657
658 paraslash comes with a sophisticated audio file selector (AFS),
659 whose main task is to determine which file to stream next, based on
660 information on the audio files stored in a database. It communicates
661 also with para_client whenever an AFS command is executed, for example
662 to answer a database query.
663
664 Besides the traditional playlists, AFS supports audio file selection
665 based on _moods_ which act as a filter that limits the set of all
666 known audio files to those which satisfy certain criteria. It also
667 maintains tables containing images (e.g. album cover art) and lyrics
668 that can be associated with one or more audio files.
669
670 AFS uses XREFERENCE(http://systemlinux.org/~maan/osl/, libosl), the
671 object storage layer library, as the backend library for storing
672 information on audio files, playlists, etc. This library offers
673 functionality similar to a relational database, but is much more
674 lightweight than a full database backend.
675
676 In this chapter we sketch the setup of the REFERENCE(The AFS process,
677 AFS process) during server startup and proceed with the description
678 of the REFERENCE(Database layout, layout) of the various database
679 tables. The section on REFERENCE(Playlists and moods, playlists
680 and moods) explains these two audio file selection mechanisms
681 in detail and contains pratical examples. The way REFERENCE(File
682 renames and content changes, file renames and content changes) are
683 detected is discussed briefly before the REFERENCE(Troubleshooting,
684 Troubleshooting) section concludes the chapter.
685
686 The AFS process
687 ~~~~~~~~~~~~~~~
688
689 On startup, para_server forks to create the AFS process which opens
690 the OSL database tables. The server process communicates with the
691 AFS process via pipes and shared memory. Usually, the AFS process
692 awakes only briefly whenever the current audio file changes. The AFS
693 process determines the next audio file, opens it, verifies it has
694 not been changed since it was added to the database and passes the
695 open file descriptor to the server process, along with audio file
696 meta-data such as file name, duration, audio format and so on. The
697 server process then starts to stream the audio file.
698
699 The AFS process also accepts connections from local clients via
700 a well-known socket. However, only child processes of para_server
701 may connect through this socket. All server commands that have the
702 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
703 change the database.
704
705 Database layout
706 ~~~~~~~~~~~~~~~
707
708 *The audio file table*
709
710 This is the most important and usually also the largest table of the
711 AFS database. It contains the information needed to stream each audio
712 file. In particular the following data is stored for each audio file.
713
714 - SHA1 hash value of the audio file contents. This is computed
715 once when the file is added to the database. Whenever AFS
716 selects this audio file for streaming the hash value is
717 recomputed and checked against the value stored in the
718 database to detect content changes.
719
720 - The time when this audio file was last played.
721
722 - The number of times the file has been played so far.
723
724 - The attribute bitmask.
725
726 - The image id which describes the image associated with this
727 audio file.
728
729 - The lyrics id which describes the lyrics associated with
730 this audio file.
731
732 - The audio format id (MP3, OGG, ...).
733
734 - An amplification value that can be used by the amplification
735 filter to pre-amplify the decoded audio stream.
736
737 - The chunk table. It describes the location and the timing
738 of the building blocks of the audio file. This is used by
739 para_server to send chunks of the file at appropriate times.
740
741 - The duration of the audio file.
742
743 - Tag information contained in the audio file (ID3 tags,
744 Vorbis comments, ...).
745
746 - The number of channels
747
748 - The encoding bitrate.
749
750 - The sampling frequency.
751
752 To add or refresh the data contained in the audio file table, the _add_
753 command is used. It takes the full path of either an audio file or a
754 directory. In the latter case, the directory is traversed recursively
755 and all files which are recognized as valid audio files are added to
756 the database.
757
758 *The attribute table*
759
760 The attribute table contains two columns, _name_ and _bitnum_. An
761 attribute is simply a name for a certain bit number in the attribute
762 bitmask of the audio file table.
763
764 Each of the 64 bits of the attribute bitmask can be set for each
765 audio file individually. Hence up to 64 different attributes may be
766 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
767 "german_lyrics", "speech", whatever. You are free to choose as
768 many attributes as you like and there are no naming restrictions
769 for attributes.
770
771 A new attribute "test" is created by
772
773 para_client addatt test
774 and
775 para_client lsatt
776
777 lists all available attributes. You can set the "test" attribute for
778 an audio file by executing
779
780 para_client setatt test+ /path/to/the/audio/file
781
782 Similarly, the "test" bit can be removed from an audio file with
783
784 para_client setatt test- /path/to/the/audio/file
785
786 Instead of a path you may use a shell wildcard pattern. The attribute
787 is applied to all audio files matching this pattern:
788
789 para_client setatt test+ '/test/directory/*'
790
791 The command
792
793 para_client -- ls -lv
794
795 gives you a verbose listing of your audio files also showing which
796 attributes are set.
797
798 In case you wonder why the double-dash in the above command is needed:
799 It tells para_client to not interpret the options after the dashes. If
800 you find this annoying, just say
801
802 alias para='para_client --'
803
804 and be happy. In what follows we shall use this alias.
805
806 The "test" attribute can be dropped from the database with
807
808 para rmatt test
809
810 Read the output of
811
812 para help ls
813 para help setatt
814
815 for more information and a complete list of command line options to
816 these commands.
817
818 *Blob tables*
819
820 The image, lyrics, moods and playlists tables are all blob tables.
821 Blob tables consist of three columns each: The identifier which is
822 a positive non-negative number that is auto-incremented, the name
823 (an arbitrary string) and the content (the blob).
824
825 All blob tables support the same set of actions: cat, ls, mv, rm
826 and add. Of course, _add_ is used for adding new blobs to the table
827 while the other actions have the same meaning as the corresponding
828 Unix commands. The paraslash commands to perform these actions are
829 constructed as the concatenation of the table name and the action. For
830 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
831 manipulate or query the image table.
832
833 The add variant of these commands is special as these commands read
834 the blob contents from stdin. To add an image to the image table the
835 command
836
837 para addimg image_name < file.jpg
838
839 can be used.
840
841 Note that the images and lyrics are not interpreted at all, and also
842 the playlist and the mood blobs are only investigated when the mood
843 or playlist is activated with the select command.
844
845 *The score table*
846
847 Unlike all other tables the contents of the score table remain in
848 memory and are never stored on disk. The score table contains two
849 columns: The SHA1 hash value (of an audio file) and its current
850 score.
851
852 However, only those files which are admissible for the current mood
853 or playlist are contained in the score table. The audio file selector
854 always chooses the row with the highest score as the file to stream
855 next. While doing so, it computes the new score and updates the
856 last_played and the num_played fields in the audio file table.
857
858 The score table is recomputed by the select command which loads a
859 mood or playlist. Audio files are chosen for streaming from the rows
860 of the score table on a highest-score-first basis.
861
862
863 Playlists and moods
864 ~~~~~~~~~~~~~~~~~~~
865
866 Playlists and moods offer two different ways of specifying the set of
867 admissible files. A playlist in itself describes a set of admissible
868 files. A mood, in contrast, describes the set of admissible files in
869 terms of attributes and other type of information available in the
870 audio file table. As an example, a mood can define a filename pattern,
871 which is then matched against the names of audio files in the table.
872
873 *Playlists*
874
875 Playlists are accommodated in the playlist table of the afs database,
876 using the aforementioned blob format for tables. A new playlist is
877 created with the addpl command by specifying the full (absolute)
878 paths of all desired audio files, separated by newlines. Example:
879
880 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
881
882 If _my_playlist_ already exists it is overwritten. To activate the
883 new playlist, execute
884
885 para select p/my_playlist
886
887 The audio file selector will assign scores to each entry of the list,
888 in descending order so that files will be selected in order. If a
889 file could not be opened for streaming, its entry is removed from
890 the score table (but not from the playlist).
891
892 *Moods*
893
894 A mood consists of a unique name and its *mood definition*, which is
895 a set of *mood lines* containing expressions in terms of attributes
896 and other data contained in the database.
897
898 At any time at most one mood can be *active* which means that
899 para_server is going to select only files from that subset of
900 admissible files.
901
902 So in order to create a mood definition one has to write a set of
903 mood lines. Mood lines come in three flavours: Accept lines, deny
904 lines and score lines.
905
906 The general syntax of the three types of mood lines is
907
908
909 accept [with score <score>] [if] [not] <mood_method> [options]
910 deny [with score <score>] [if] [not] <mood_method> [options]
911 score <score> [if] [not] <mood_method> [options]
912
913
914 Here <score> is either an integer or the string "random" which assigns
915 a random score to all matching files. The score value changes the
916 order in which admissible files are going to be selected, but is of
917 minor importance for this introduction.
918
919 So we concentrate on the first two forms, i.e. accept and deny
920 lines. As usual, everything in square brackets is optional, i.e.
921 accept/deny lines take the following form when ignoring scores:
922
923 accept [if] [not] <mood_method> [options]
924
925 and analogously for the deny case. The "if" keyword is only syntactic
926 sugar and has no function. The "not" keyword just inverts the result,
927 so the essence of a mood line is the mood method part and the options
928 following thereafter.
929
930 A *mood method* is realized as a function which takes an audio file
931 and computes a number from the data contained in the database.
932 If this number is non-negative, we say the file *matches* the mood
933 method. The file matches the full mood line if it either
934
935 - matches the mood method and the "not" keyword is not given,
936 or
937 - does not match the mood method, but the "not" keyword is given.
938
939 The set of admissible files for the whole mood is now defined as those
940 files which match at least one accept mood line, but no deny mood line.
941 More formally, an audio file F is admissible if and only if
942
943 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
944
945 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
946 lines and "~" means "matches".
947
948 The cases where no mood lines of accept/deny type are defined need
949 special treatment:
950
951 - Neither accept nor deny lines: This treats all files as
952 admissible (in fact, that is the definition of the dummy mood
953 which is activated automatically if no moods are available).
954
955 - Only accept lines: A file is admissible iff it matches at
956 least one accept line:
957
958 F ~ AL1 or F ~ AL2 or ...
959
960 - Only deny lines: A file is admissible iff it matches no
961 deny line:
962
963 not (F ~ DL1 or F ~ DN2 ...)
964
965
966
967 *List of mood_methods*
968
969 no_attributes_set
970
971 Takes no arguments and matches an audio file if and only if no
972 attributes are set.
973
974 is_set <attribute_name>
975
976 Takes the name of an attribute and matches iff that attribute is set.
977
978 path_matches <pattern>
979
980 Takes a filename pattern and matches iff the path of the audio file
981 matches the pattern.
982
983 artist_matches <pattern>
984 album_matches <pattern>
985 title_matches <pattern>
986 comment_matches <pattern>
987
988 Takes an extended regular expression and matches iff the text of the
989 corresponding tag of the audio file matches the pattern. If the tag
990 is not set, the empty string is matched against the pattern.
991
992 year ~ <num>
993 bitrate ~ <num>
994 frequency ~ <num>
995 channels ~ <num>
996 num_played ~ <num>
997
998 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
999 <num>. Matches an audio file iff the condition <val> ~ <num> is
1000 satisfied where val is the corresponding value of the audio file
1001 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
1002 count, play count).
1003
1004 The year tag is special as its value is undefined if the audio file
1005 has no year tag or the content of the year tag is not a number. Such
1006 audio files never match. Another difference is the special treatment
1007 if the year tag is a two-digit number. In this case either 1900 or
1008 2000 is added to the tag value, depending on whether the number is
1009 greater than 2000 plus the current year.
1010
1011
1012 *Mood usage*
1013
1014 To create a new mood called "my_mood", write its definition into
1015 some temporary file, say "tmpfile", and add it to the mood table
1016 by executing
1017
1018 para addmood my_mood < tmpfile
1019
1020 If the mood definition is really short, you may just pipe it to the
1021 client instead of using temporary files. Like this:
1022
1023 echo "$MOOD_DEFINITION" | para addmood my_mood
1024
1025 There is no need to keep the temporary file since you can always use
1026 the catmood command to get it back:
1027
1028 para catmood my_mood
1029
1030 A mood can be activated by executing
1031
1032 para select m/my_mood
1033
1034 Once active, the list of admissible files is shown by the ls command
1035 if the "-a" switch is given:
1036
1037 para ls -a
1038
1039
1040 *Example mood definition*
1041
1042 Suppose you have defined attributes "punk" and "rock" and want to define
1043 a mood containing only Punk-Rock songs. That is, an audio file should be
1044 admissible if and only if both attributes are set. Since
1045
1046 punk and rock
1047
1048 is obviously the same as
1049
1050 not (not punk or not rock)
1051
1052 (de Morgan's rule), a mood definition that selects only Punk-Rock
1053 songs is
1054
1055 deny if not is_set punk
1056 deny if not is_set rock
1057
1058
1059
1060 File renames and content changes
1061 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1062
1063 Since the audio file selector knows the SHA1 of each audio file that
1064 has been added to the afs database, it recognizes if the content of
1065 a file has changed, e.g. because an ID3 tag was added or modified.
1066 Also, if a file has been renamed or moved to a different location,
1067 afs will detect that an entry with the same hash value already exists
1068 in the audio file table.
1069
1070 In both cases it is enough to just re-add the new file. In the
1071 first case (file content changed), the audio table is updated, while
1072 metadata such as the num_played and last_played fields, as well as
1073 the attributes, remain unchanged. In the other case, when the file
1074 is moved or renamed, only the path information is updated, all other
1075 data remains as before.
1076
1077 It is possible to change the behaviour of the add command by using the
1078 "-l" (lazy add) or the "-f" (force add) option.
1079
1080 Troubleshooting
1081 ~~~~~~~~~~~~~~~
1082
1083 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1084 executables have a brief online help which is displayed when -h is
1085 given. The --detailed-help option prints the full help text.
1086
1087 If para_server crashed or was killed by SIGKILL (signal 9), it
1088 may refuse to start again because of "dirty osl tables". In this
1089 case you'll have to run the oslfsck program of libosl to fix your
1090 database:
1091
1092 oslfsck -fd ~/.paraslash/afs_database-0.4
1093
1094 However, make sure para_server isn't running before executing oslfsck.
1095
1096 If you don't mind to recreate your database you can start
1097 from scratch by removing the entire database directory, i.e.
1098
1099 rm -rf ~/.paraslash/afs_database-0.4
1100
1101 Be aware that this removes all attribute definitions, all playlists
1102 and all mood definitions and requires to re-initialize the tables.
1103
1104 Although oslfsck fixes inconsistencies in database tables it doesn't
1105 care about the table contents. To check for invalid table contents, use
1106
1107 para_client check
1108
1109 This prints out references to missing audio files as well as invalid
1110 playlists and mood definitions.
1111
1112 Similarly, para_audiod refuses to start if its socket file exists, since
1113 this indicates that another instance of para_audiod is running. After
1114 a crash a stale socket file might remain and you must run
1115
1116 para_audiod --force
1117
1118 once to fix it up.
1119
1120 ---------------------------------------
1121 Audio formats and audio format handlers
1122 ---------------------------------------
1123
1124 Audio formats
1125 ~~~~~~~~~~~~~
1126
1127 The following audio formats are supported by paraslash:
1128
1129 *MP3*
1130
1131 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1132 designed as part of its MPEG-1 standard. An MP3 file is made up of
1133 multiple MP3 frames, which consist of a header and a data block. The
1134 size of an MP3 frame depends on the bit rate and on the number
1135 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1136 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1137 400 bytes large.
1138
1139 *OGG/Vorbis*
1140
1141 OGG is a standardized audio container format, while Vorbis is an
1142 open source codec for lossy audio compression. Since Vorbis is most
1143 commonly made available via the OGG container format, it is often
1144 referred to as OGG/Vorbis. The OGG container format divides data into
1145 chunks called OGG pages. A typical OGG page is about 4KB large. The
1146 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1147 may vary considerably.
1148
1149 *OGG/Speex*
1150
1151 Speex is an open-source speech codec that is based on CELP (Code
1152 Excited Linear Prediction) coding. It is designed for voice
1153 over IP applications, has modest complexity and a small memory
1154 footprint. Wideband and narrowband (telephone quality) speech are
1155 supported. As for Vorbis audio, Speex bit-streams are often stored
1156 in OGG files. As of 2012 this codec is considered obsolete since the
1157 Oppus codec, described below, surpasses its performance in all areas.
1158
1159 *OGG/Opus*
1160
1161 Opus is a lossy audio compression format standardized through RFC
1162 6716 in 2012. It combines the speech-oriented SILK codec and the
1163 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1164 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1165 containers. All known software patents which cover Opus are licensed
1166 under royalty-free terms.
1167
1168 *AAC*
1169
1170 Advanced Audio Coding (AAC) is a standardized, lossy compression
1171 and encoding scheme for digital audio which is the default audio
1172 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1173 the container format and audio files encoded with AAC have the .m4a
1174 extension. A typical AAC frame is about 700 bytes large.
1175
1176 *WMA*
1177
1178 Windows Media Audio (WMA) is an audio data compression technology
1179 developed by Microsoft. A WMA file is usually encapsulated in the
1180 Advanced Systems Format (ASF) container format, which also specifies
1181 how meta data about the file is to be encoded. The bit stream of WMA
1182 is composed of superframes, each containing one or more frames of
1183 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1184
1185 *FLAC*
1186
1187 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1188 loss. It gives better compression ratios than a general purpose
1189 compressor like zip or bzip2 because FLAC is designed specifically
1190 for audio. A FLAC-encoded file consits of frames of varying size, up
1191 to 16K. Each frame starts with a header that contains all information
1192 necessary to decode the frame.
1193
1194 Meta data
1195 ~~~~~~~~~
1196
1197 Unfortunately, each audio format has its own conventions how meta
1198 data is added as tags to the audio file.
1199
1200 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1201 is rather simple but also very limited as it supports only artist,
1202 title, album, year and comment tags. Each of these can only be at most
1203 32 characters long. ID3, version 2 is much more flexible but requires
1204 a separate library being installed for paraslash to support it.
1205
1206 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1207 comments, which are typically implemented as strings of the form
1208 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1209 tags are appropriate for the content.
1210
1211 AAC files usually use the MPEG-4 container format for storing meta
1212 data while WMA files wrap meta data as special objects within the
1213 ASF container format.
1214
1215 paraslash only tracks the most common tags that are supported by
1216 all tag variants: artist, title, year, album, comment. When a file
1217 is added to the AFS database, the meta data of the file is extracted
1218 and stored in the audio file table.
1219
1220 Chunks and chunk tables
1221 ~~~~~~~~~~~~~~~~~~~~~~~
1222
1223 paraslash uses the word "chunk" as common term for the building blocks
1224 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1225 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1226 size varies considerably between audio formats, from a few hundred
1227 bytes (MP3) up to 16K (FLAC).
1228
1229 The chunk table contains the offsets within the audio file that
1230 correspond to the chunk boundaries of the file. Like the meta data,
1231 the chunk table is computed and stored in the database whenever an
1232 audio file is added.
1233
1234 The paraslash senders (see below) always send complete chunks. The
1235 granularity for seeking is therefore determined by the chunk size.
1236
1237 Audio format handlers
1238 ~~~~~~~~~~~~~~~~~~~~~
1239
1240 For each audio format paraslash contains an audio format handler whose
1241 first task is to tell whether a given file is a valid audio file of
1242 this type. If so, the audio file handler extracts some technical data
1243 (duration, sampling rate, number of channels etc.), computes the
1244 chunk table and reads the meta data.
1245
1246 The audio format handler code is linked into para_server and executed
1247 via the _add_ command. The same code is also available as a stand-alone
1248 tool, para_afh, which can be used to print the technical data, the
1249 chunk table and the meta data of a file.
1250
1251 ----------
1252 Networking
1253 ----------
1254
1255 Paraslash uses different network connections for control and data.
1256 para_client communicates with para_server over a dedicated TCP control
1257 connection. To transport audio data, separate data connections are
1258 used. For these data connections, a variety of transports (UDP, DCCP,
1259 HTTP) can be chosen.
1260
1261 The chapter starts with the REFERENCE(The paraslash control
1262 service, control service), followed by a section on the various
1263 REFERENCE(Streaming protocols, streaming protocols) in which the data
1264 connections are described. The way audio file headers are embedded into
1265 the stream is discussed REFERENCE(Streams with headers and headerless
1266 streams, briefly) before the REFERENCE(Networking examples, example
1267 section) which illustrates typical commands for real-life scenarios.
1268
1269 Both IPv4 and IPv6 are supported.
1270
1271 The paraslash control service
1272 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1273
1274 para_server is controlled at runtime via the paraslash control
1275 connection. This connection is used for server commands (play, stop,
1276 ...) as well as for afs commands (ls, select, ...).
1277
1278 The server listens on a TCP port and accepts connections from clients
1279 that connect the open port. Each connection causes the server to fork
1280 off a client process which inherits the connection and deals with that
1281 client only. In this classical accept/fork approach the server process
1282 is unaffected if the child dies or goes crazy for whatever reason. In
1283 fact, the child process can not change address space of server process.
1284
1285 The section on REFERENCE(Client-server authentication, client-server
1286 authentication) above described the early connection establishment
1287 from the crypto point of view. Here it is described what happens
1288 after the connection (including crypto setup) has been established.
1289 There are four processes involved during command dispatch as sketched
1290 in the following diagram.
1291
1292 <<
1293 <pre>
1294 server_host client_host
1295 ~~~~~~~~~~~ ~~~~~~~~~~~
1296
1297 +-----------+ connect +-----------+
1298 |para_server|<------------------------------ |para_client|
1299 +-----------+ +-----------+
1300 | ^
1301 | fork +---+ |
1302 +----------> |AFS| |
1303 | +---+ |
1304 | ^ |
1305 | | |
1306 | | connect (cookie) |
1307 | | |
1308 | | |
1309 | fork +-----+ inherited connection |
1310 +---------->|child|<--------------------------+
1311 +-----+
1312 </pre>
1313 >>
1314
1315 Note that the child process is not a child of the afs process,
1316 so communication of these two processes has to happen via local
1317 sockets. In order to avoid abuse of the local socket by unrelated
1318 processes, a magic cookie is created once at server startup time just
1319 before the server process forks off the AFS process. This cookie is
1320 known to the server, AFS and the child, but not to unrelated processes.
1321
1322 There are two different kinds of commands: First there are commands
1323 that cause the server to respond with some answer such as the list
1324 of all audio files. All but the addblob commands (addimg, addlyr,
1325 addpl, addmood) are of this kind. The addblob commands add contents
1326 to the database, so they need to transfer data the other way round,
1327 from the client to the server.
1328
1329 There is no knowledge about the server commands built into para_client,
1330 so it does not know about addblob commands. Instead, it inspects the
1331 first data package sent by the server for a magic string. If this
1332 string was found, it sends STDIN to the server, otherwise it dumps
1333 data from the server to STDOUT.
1334
1335 Streaming protocols
1336 ~~~~~~~~~~~~~~~~~~~
1337
1338 A network (audio) stream usually consists of one streaming source,
1339 the _sender_, and one or more _receivers_ which read data over the
1340 network from the streaming source.
1341
1342 Senders are thus part of para_server while receivers are part of
1343 para_audiod. Moreover, there is the stand-alone tool para_recv which
1344 can be used to manually download a stream, either from para_server
1345 or from a web-based audio streaming service.
1346
1347 The following three streaming protocols are supported by paraslash:
1348
1349 - HTTP. Recommended for public streams that can be played by
1350 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1351 sender is supported on all operating systems and all platforms.
1352
1353 - DCCP. Recommended for LAN streaming. DCCP is currently
1354 available only for Linux.
1355
1356 - UDP. Recommended for multicast LAN streaming.
1357
1358 See the Appendix on REFERENCE(Network protocols, network protocols)
1359 for brief descriptions of the various protocols relevant for network
1360 audio streaming with paraslash.
1361
1362 It is possible to activate more than one sender simultaneously.
1363 Senders can be controlled at run time and via config file and command
1364 line options.
1365
1366 Note that audio connections are _not_ encrypted. Transport or Internet
1367 layer encryption should be used if encrypted data connections are
1368 needed.
1369
1370 Since DCCP and TCP are both connection-oriented protocols, connection
1371 establishment/teardown and access control are very similar between
1372 these two streaming protocols. UDP is the most lightweight option,
1373 since in contrast to TCP/DCCP it is connectionless. It is also the
1374 only protocol supporting IP multicast.
1375
1376 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1377 clients to connect and establish a connection via some protocol-defined
1378 handshake mechanism. Both senders maintain two linked lists each:
1379 The list of all clients which are currently connected, and the list
1380 of access control entries which determines who is allowed to connect.
1381 IP-based access control may be configured through config file and
1382 command line options and via the "allow" and "deny" sender subcommands.
1383
1384 Upon receiving a GET request from the client, the HTTP sender sends
1385 back a status line and a message. The body of this message is the
1386 audio stream. This is common practice and is supported by many popular
1387 clients which can thus be used to play a stream offered by para_server.
1388 For DCCP things are a bit simpler: No messages are exchanged between
1389 the receiver and sender. The client simply connects and the sender
1390 starts to stream.
1391
1392 DCCP is an experimental protocol which offers a number of new features
1393 not available for TCP. Both ends can negotiate these features using
1394 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1395 datagram-based (no retransmissions) and thus should not be used over
1396 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1397 is access to a variety of different congestion-control mechanisms
1398 called CCIDs. Two different CCIDs are available per default on Linux:
1399
1400
1401 - _CCID 2_. A Congestion Control mechanism similar to that
1402 of TCP. The sender maintains a congestion window and halves
1403 this window in response to congestion.
1404
1405
1406 - _CCID-3_. Designed to be fair when competing for bandwidth.
1407 It has lower variation of throughput over time compared with
1408 TCP, which makes it suitable for streaming media.
1409
1410 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1411 single list, the _target list_. This list describes the set of clients
1412 to which the stream is sent. There is no list for access control and
1413 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1414 and "delete" commands can be used to modify the target list.
1415
1416 Since both UDP and DCCP offer an unreliable datagram-based transport,
1417 additional measures are necessary to guard against disruptions over
1418 networks that are lossy or which may be subject to interference (as
1419 is for instance the case with WiFi). Paraslash uses FEC (Forward
1420 Error Correction) to guard against packet losses and reordering. The
1421 stream is FEC-encoded before it is sent through the UDP socket and
1422 must be decoded accordingly on the receiver side.
1423
1424 The packet size and the amount of redundancy introduced by FEC can
1425 be configured via the FEC parameters which are dictated by server
1426 and may also be configured through the "sender" command. The FEC
1427 parameters are encoded in the header of each network packet, so no
1428 configuration is necessary on the receiver side. See the section on
1429 REFERENCE(Forward error correction, FEC) below.
1430
1431 Streams with headers and headerless streams
1432 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1433
1434 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1435 needed to decode the stream is only contained in the audio file
1436 header of the container format but not in each data chunk. Clients
1437 must be able to obtain this information in case streaming starts in
1438 the middle of the file or if para_audiod is started while para_server
1439 is already sending a stream.
1440
1441 This is accomplished in different ways, depending on the streaming
1442 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1443 header is sent prior to audio file data. This technique however does
1444 not work for the connectionless UDP transport. Hence the audio file
1445 header is periodically being embedded into the UDP audio data stream.
1446 By default, the header is resent after five seconds. The receiver has
1447 to wait until the next header arrives before it can start decoding
1448 the stream.
1449
1450 Examples
1451 ~~~~~~~~
1452
1453 The sender command of para_server allows to (de-)activate senders
1454 and to change the access permissions senders at runtime. The "si"
1455 (server info) command is used to list the streaming options of the
1456 currently running server as well as the various sender access lists.
1457
1458 -> Show client/target/access lists:
1459
1460 para_client si
1461
1462 -> Obtain general help for the sender command:
1463
1464 para_client help sender
1465
1466 -> Get help for a specific sender (contains further examples):
1467
1468 s=http # or dccp or udp
1469 para_client sender $s help
1470
1471 By default para_server activates both the HTTP and th DCCP sender on
1472 startup. This can be changed via command line options or para_server's
1473 config file.
1474
1475 -> List config file options for senders:
1476
1477 para_server -h
1478
1479 All senders share the "on" and "off" commands, so senders may be
1480 activated and deactivated independently of each other.
1481
1482 -> Switch off the http sender:
1483
1484 para_client sender http off
1485
1486 -> Receive a DCCP stream using CCID2 and write the output into a file:
1487
1488 host=foo.org; ccid=2; filename=bar
1489 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1490
1491 Note the quotes around the arguments for the dccp receiver. Each
1492 receiver has its own set of command line options and its own command
1493 line parser, so arguments for the dccp receiver must be protected
1494 from being interpreted by para_recv.
1495
1496 -> Start UDP multicast, using the default multicast address:
1497
1498 para_client sender udp add 224.0.1.38
1499
1500 -> Receive FEC-encoded multicast stream and write the output into a file:
1501
1502 filename=foo
1503 para_recv -r udp > $filename
1504
1505 -> Add an UDP unicast for a client to the target list of the UDP sender:
1506
1507 t=client.foo.org
1508 para_client sender udp add $t
1509
1510 -> Receive this (FEC-encoded) unicast stream:
1511
1512 filename=foo
1513 para_recv -r 'udp -i 0.0.0.0' > $filename
1514
1515 -> Create a minimal config for para_audiod for HTTP streams:
1516
1517 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1518 echo receiver \".:http -i $s\" > $c
1519 para_audiod --config $c
1520
1521 -------
1522 Filters
1523 -------
1524
1525 A paraslash filter is a module which transforms an input stream into
1526 an output stream. Filters are included in the para_audiod executable
1527 and in the stand-alone tool para_filter which usually contains the
1528 same modules.
1529
1530 While para_filter reads its input stream from STDIN and writes
1531 the output to STDOUT, the filter modules of para_audiod are always
1532 connected to a receiver which produces the input stream and a writer
1533 which absorbs the output stream.
1534
1535 Some filters depend on a specific library and are not compiled in
1536 if this library was not found at compile time. To see the list of
1537 supported filters, run para_filter and para_audiod with the --help
1538 option. The output looks similar to the following:
1539
1540 Available filters:
1541 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1542
1543 Out of these filter modules, a chain of filters can be constructed,
1544 much in the way Unix pipes can be chained, and analogous to the use
1545 of modules in gstreamer: The output of the first filter becomes the
1546 input of the second filter. There is no limitation on the number of
1547 filters and the same filter may occur more than once.
1548
1549 Like receivers, each filter has its own command line options which
1550 must be quoted to protect them from the command line options of
1551 the driving application (para_audiod or para_filter). Example:
1552
1553 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1554
1555 For para_audiod, each audio format has its own set of filters. The
1556 name of the audio format for which the filter should be applied can
1557 be used as the prefix for the filter option. Example:
1558
1559 para_audiod -f 'mp3:prebuffer --duration 300'
1560
1561 The "mp3" prefix above is actually interpreted as a POSIX extended
1562 regular expression. Therefore
1563
1564 para_audiod -f '.:prebuffer --duration 300'
1565
1566 activates the prebuffer filter for all supported audio formats (because
1567 "." matches all audio formats) while
1568
1569 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1570
1571 activates it only for wma and ogg streams.
1572
1573 Decoders
1574 ~~~~~~~~
1575
1576 For each supported audio format there is a corresponding filter
1577 which decodes audio data in this format to 16 bit PCM data which
1578 can be directly sent to the sound device or any other software that
1579 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1580 filters are called _decoders_ in general, and xxxdec is the name of
1581 the paraslash decoder for the audio format xxx. For example, the mp3
1582 decoder filter is called mp3dec.
1583
1584 Note that the output of the decoder is about 10 times larger than
1585 its input. This means that filters that operate on the decoded audio
1586 stream have to deal with much more data than filters that transform
1587 the audio stream before it is fed to the decoder.
1588
1589 Paraslash relies on external libraries for most decoders, so these
1590 libraries must be installed for the decoder to be included in the
1591 para_filter and para_audiod executables. The oggdec filter depends
1592 on the libogg and libvorbis libraries for example.
1593
1594 Forward error correction
1595 ~~~~~~~~~~~~~~~~~~~~~~~~
1596
1597 As already mentioned REFERENCE(Streaming protocols, earlier),
1598 paraslash uses forward error correction (FEC) for the unreliable UDP
1599 and DCCP transports. FEC is a technique which was invented already
1600 in 1960 by Reed and Solomon and which is widely used for the parity
1601 calculations of storage devices (RAID arrays). It is based on the
1602 algebraic concept of finite fields, today called Galois fields, in
1603 honour of the mathematician Galois (1811-1832). The FEC implementation
1604 of paraslash is based on code by Luigi Rizzo.
1605
1606 Although the details require a sound knowledge of the underlying
1607 mathematics, the basic idea is not hard to understand: For positive
1608 integers k and n with k < n it is possible to compute for any k given
1609 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1610 ..., p_r such that all data bytes can be reconstructed from *any*
1611 k bytes of the set
1612
1613 {d_1, ..., d_k, p_1, ..., p_r}.
1614
1615 FEC-encoding for unreliable network transports boils down to slicing
1616 the audio stream into groups of k suitably sized pieces called _slices_
1617 and computing the r corresponding parity slices. This step is performed
1618 in para_server which then sends both the data and the parity slices
1619 over the unreliable network connection. If the client was able
1620 to receive at least k of the n = k + r slices, it can reconstruct
1621 (FEC-decode) the original audio stream.
1622
1623 From these observations it is clear that there are three different
1624 FEC parameters: The slice size, the number of data slices k, and the
1625 total number of slices n. It is crucial to choose the slice size
1626 such that no fragmentation of network packets takes place because
1627 FEC only guards against losses and reordering but fails if slices are
1628 received partially.
1629
1630 FEC decoding in paralash is performed through the fecdec filter which
1631 usually is the first filter (there can be other filters before fecdec
1632 if these do not alter the audio stream).
1633
1634
1635 Volume adjustment (amp and compress)
1636 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1637
1638 The amp and the compress filter both adjust the volume of the audio
1639 stream. These filters operate on uncompressed audio samples. Hence
1640 they are usually placed directly after the decoding filter. Each
1641 sample is multiplied with a scaling factor (>= 1) which makes amp
1642 and compress quite expensive in terms of computing power.
1643
1644 *amp*
1645
1646 The amp filter amplifies the audio stream by a fixed scaling factor
1647 that must be known in advance. For para_audiod this factor is derived
1648 from the amplification field of the audio file's entry in the audio
1649 file table while para_filter uses the value given at the command line.
1650
1651 The optimal scaling factor F for an audio file is the largest real
1652 number F >= 1 such that after multiplication with F all samples still
1653 fit into the sample interval [-32768, 32767]. One can use para_filter
1654 in combination with the sox utility to compute F:
1655
1656 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1657
1658 The amplification value V which is stored in the audio file table,
1659 however, is an integer between 0 and 255 which is connected to F
1660 through the formula
1661
1662 V = (F - 1) * 64.
1663
1664 To store V in the audio file table, the command
1665
1666 para_client -- touch -a=V file.mp3
1667
1668 is used. The reader is encouraged to write a script that performs
1669 these computations :)
1670
1671 *compress*
1672
1673 Unlike the amplification filter, the compress filter adjusts the volume
1674 of the audio stream dynamically without prior knowledge about the peak
1675 value. It maintains the maximal volume of the last n samples of the
1676 audio stream and computes a suitable amplification factor based on that
1677 value and the various configuration options. It tries to chose this
1678 factor such that the adjusted volume meets the desired target level.
1679
1680 Note that it makes sense to combine amp and compress.
1681
1682 Misc filters (wav and prebuffer)
1683 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1684
1685 These filters are rather simple and do not modify the audio stream at
1686 all. The wav filter is only useful with para_filter and in connection
1687 with a decoder. It asks the decoder for the number of channels and the
1688 sample rate of the stream and adds a Microsoft wave header containing
1689 this information at the beginning. This allows to write wav files
1690 rather than raw PCM files (which do not contain any information about
1691 the number of channels and the sample rate).
1692
1693 The prebuffer filter simply delays the output until the given time has
1694 passed (starting from the time the first byte was available in its
1695 input queue) or until the given amount of data has accumulated. It
1696 is mainly useful for para_audiod if the standard parameters result
1697 in buffer underruns.
1698
1699 Both filters require almost no additional computing time, even when
1700 operating on uncompressed audio streams, since data buffers are simply
1701 "pushed down" rather than copied.
1702
1703 Examples
1704 ~~~~~~~~
1705
1706 -> Decode an mp3 file to wav format:
1707
1708 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1709
1710 -> Amplify a raw audio file by a factor of 1.5:
1711
1712 para_filter -f amp --amp 32 < foo.raw > bar.raw
1713
1714 ------
1715 Output
1716 ------
1717
1718 Once an audio stream has been received and decoded to PCM format,
1719 it can be sent to a sound device for playback. This part is performed
1720 by paraslash _writers_ which are described in this chapter.
1721
1722 Writers
1723 ~~~~~~~
1724
1725 A paraslash writer acts as a data sink that consumes but does not
1726 produce audio data. Paraslash writers operate on the client side and
1727 are contained in para_audiod and in the stand-alone tool para_write.
1728
1729 The para_write program reads uncompressed audio data from STDIN. If
1730 this data starts with a wav header, sample rate, sample format and
1731 channel count are read from the header. Otherwise CD audio (44.1KHz
1732 16 bit little endian, stereo) is assumed but this can be overridden
1733 by command line options. para_audiod, on the other hand, obtains
1734 the sample rate and the number of channels from the decoder.
1735
1736 Like receivers and filters, each writer has an individual set of
1737 command line options, and for para_audiod writers can be configured
1738 per audio format separately. It is possible to activate more than
1739 one writer for the same stream simultaneously.
1740
1741 OS-dependent APIs
1742 ~~~~~~~~~~~~~~~~~
1743
1744 Unfortunately, the various flavours of Unix on which paraslash
1745 runs on have different APIs for opening a sound device and starting
1746 playback. Hence for each such API there is a paraslash writer that
1747 can play the audio stream via this API.
1748
1749 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1750 Linux systems. Although there are several mid-layer APIs in use by
1751 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1752 currently supports only the low-level ALSA API which is not supposed
1753 to be change. ALSA is very feature-rich, in particular it supports
1754 software mixing via its DMIX plugin. ALSA is the default writer on
1755 Linux systems.
1756
1757 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1758 is also available on Linux systems, usually provided by ALSA as an
1759 emulation for backwards compatibility. This API is rather simple but
1760 also limited. For example only one application can open the device
1761 at any time. The OSS writer is activated by default on BSD Systems.
1762
1763 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1764 for this API is only compiled in on such systems and is of course
1765 the default there.
1766
1767 *FILE*. The file writer allows to capture the audio stream and
1768 write the PCM data to a file on the file system rather than playing
1769 it through a sound device. It is supported on all platforms and is
1770 always compiled in.
1771
1772 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1773 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1774 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1775 through an output plugin of libao.
1776
1777 Examples
1778 ~~~~~~~~
1779
1780 -> Use the OSS writer to play a wav file:
1781
1782 para_write --writer oss < file.wav
1783
1784 -> Enable ALSA software mixing for mp3 streams
1785
1786 para_audiod --writer 'mp3:alsa -d plug:swmix'
1787
1788
1789 ---
1790 Gui
1791 ---
1792
1793 para_gui executes an arbitrary command which is supposed to print
1794 status information to STDOUT. It then displays this information in
1795 a curses window. By default the command
1796
1797 para_audioc -- stat -p
1798
1799 is executed, but this can be customized via the --stat-cmd option. In
1800 particular it possible to use
1801
1802 para_client -- stat -p
1803
1804 to make para_gui work on systems on which para_audiod is not running.
1805
1806 Key bindings
1807 ~~~~~~~~~~~~
1808
1809 It is possible to bind keys to arbitrary commands via custom
1810 key-bindings. Besides the internal keys which can not be changed (help,
1811 quit, loglevel, version...), the following flavours of key-bindings
1812 are supported:
1813
1814 - external: Shutdown curses before launching the given command.
1815 Useful for starting other ncurses programs from within
1816 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1817 output format to write a mailbox containing one mail for each
1818 (admissible) file the audio file selector knows about. Then
1819 start mutt from within para_gui to browse your collection!
1820
1821 - display: Launch the command and display its stdout in
1822 para_gui's bottom window.
1823
1824 - para: Like display, but start "para_client <specified
1825 command>" instead of "<specified command>".
1826
1827 The general form of a key binding is
1828
1829 key_map k:m:c
1830
1831 which maps key k to command c using mode m. Mode may be x, d or p
1832 for external, display and paraslash commands, respectively.
1833
1834 Themes
1835 ~~~~~~
1836
1837 Currently there are only two themes for para_gui. It is easy, however,
1838 to add more themes. To create a new theme one has to define the
1839 position, color and geometry for for each status item that should be
1840 shown by this theme. See gui_theme.c for examples.
1841
1842 The "." and "," keys are used to switch between themes.
1843
1844 Examples
1845 ~~~~~~~~
1846
1847 -> Show server info:
1848
1849 key_map "i:p:si"
1850
1851 -> Jump to the middle of the current audio file by pressing F5:
1852
1853 key_map "<F5>:p:jmp 50"
1854
1855 -> vi-like bindings for jumping around:
1856
1857 key_map "l:p:ff 10"
1858 key_map "h:p:ff 10-"
1859 key_map "w:p:ff 60"
1860 key_map "b:p:ff 60-"
1861
1862 -> Print the current date and time:
1863
1864 key_map "D:d:date"
1865
1866 -> Call other curses programs:
1867
1868 key_map "U:x:aumix"
1869 key_map "!:x:/bin/bash"
1870 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1871
1872 -----------
1873 Development
1874 -----------
1875
1876 Tools
1877 ~~~~~
1878
1879 In order to compile the sources from the git repository (rather than
1880 from tar balls) and for contributing non-trivial changes to the
1881 paraslash project, some additional tools should be installed on a
1882 developer machine.
1883
1884 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1885 branches, below), the git source code management tool is used for
1886 paraslash development. It is necessary for cloning the git repository
1887 and for getting updates.
1888
1889 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1890 are generated from templates by the m4 macro processor.
1891
1892 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1893 the configure file which is shipped in the tarballs but has to be
1894 generated when compiling from git.
1895
1896 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1897 HTML version of this manual and some of the paraslash web pages are
1898 generated by the grutatxt plain text to HTML converter. If changes
1899 are made to these text files the grutatxt package must be installed
1900 to regenerate the HTML files.
1901
1902 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1903 of paraslash's C sources uses the doxygen documentation system. The
1904 conventions for documenting the source code is described in the
1905 REFERENCE(Doxygen, Doxygen section).
1906
1907 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1908 browsable HTML from the C sources. It is needed by doxygen.
1909
1910 Git branches
1911 ~~~~~~~~~~~~
1912
1913 Paraslash has been developed using the git source code management
1914 tool since 2006. Development is organized roughly in the same spirit
1915 as the git development itself, as described below.
1916
1917 The following text passage is based on "A note from the maintainer",
1918 written by Junio C Hamano, the maintainer of git.
1919
1920 There are four branches in the paraslash repository that track the
1921 source tree: "master", "maint", "next", and "pu".
1922
1923 The "master" branch is meant to contain what is well tested and
1924 ready to be used in a production setting. There could occasionally be
1925 minor breakages or brown paper bag bugs but they are not expected to
1926 be anything major, and more importantly quickly and easily fixable.
1927 Every now and then, a "feature release" is cut from the tip of this
1928 branch, named with three dotted decimal digits, like 0.4.2.
1929
1930 Whenever changes are about to be included that will eventually lead to
1931 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1932 "master" at that point. Obvious, safe and urgent fixes after the major
1933 release are applied to this branch and maintenance releases are cut
1934 from it. New features never go to this branch. This branch is also
1935 merged into "master" to propagate the fixes forward.
1936
1937 A trivial and safe enhancement goes directly on top of "master".
1938 New development does not usually happen on "master", however.
1939 Instead, a separate topic branch is forked from the tip of "master",
1940 and it first is tested in isolation; Usually there are a handful such
1941 topic branches that are running ahead of "master". The tip of these
1942 branches is not published in the public repository to keep the number
1943 of branches that downstream developers need to worry about low.
1944
1945 The quality of topic branches varies widely. Some of them start out as
1946 "good idea but obviously is broken in some areas" and then with some
1947 more work become "more or less done and can now be tested by wider
1948 audience". Luckily, most of them start out in the latter, better shape.
1949
1950 The "next" branch is to merge and test topic branches in the latter
1951 category. In general, this branch always contains the tip of "master".
1952 It might not be quite rock-solid production ready, but is expected to
1953 work more or less without major breakage. The maintainer usually uses
1954 the "next" version of paraslash for his own pleasure, so it cannot
1955 be _that_ broken. The "next" branch is where new and exciting things
1956 take place.
1957
1958 The two branches "master" and "maint" are never rewound, and "next"
1959 usually will not be either (this automatically means the topics that
1960 have been merged into "next" are usually not rebased, and you can find
1961 the tip of topic branches you are interested in from the output of
1962 "git log next"). You should be able to safely build on top of them.
1963
1964 However, at times "next" will be rebuilt from the tip of "master" to
1965 get rid of merge commits that will never be in "master". The commit
1966 that replaces "next" will usually have the identical tree, but it
1967 will have different ancestry from the tip of "master".
1968
1969 The "pu" (proposed updates) branch bundles the remainder of the
1970 topic branches. The "pu" branch, and topic branches that are only in
1971 "pu", are subject to rebasing in general. By the above definition
1972 of how "next" works, you can tell that this branch will contain quite
1973 experimental and obviously broken stuff.
1974
1975 When a topic that was in "pu" proves to be in testable shape, it
1976 graduates to "next". This is done with
1977
1978 git checkout next
1979 git merge that-topic-branch
1980
1981 Sometimes, an idea that looked promising turns out to be not so good
1982 and the topic can be dropped from "pu" in such a case.
1983
1984 A topic that is in "next" is expected to be polished to perfection
1985 before it is merged to "master". Similar to the above, this is
1986 done with
1987
1988 git checkout master
1989 git merge that-topic-branch
1990 git branch -d that-topic-branch
1991
1992 Note that being in "next" is not a guarantee to appear in the next
1993 release (being in "master" is such a guarantee, unless it is later
1994 found seriously broken and reverted), nor even in any future release.
1995
1996 Coding Style
1997 ~~~~~~~~~~~~
1998
1999 The preferred coding style for paraslash coincides more or less
2000 with the style of the Linux kernel. So rather than repeating what is
2001 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
2002 there), here are the most important points.
2003
2004 - Burn the GNU coding standards.
2005 - Never use spaces for indentation.
2006 - Tabs are 8 characters, and thus indentations are also 8 characters.
2007 - Don't put multiple assignments on a single line.
2008 - Avoid tricky expressions.
2009 - Don't leave whitespace at the end of lines.
2010 - The limit on the length of lines is 80 columns.
2011 - Use K&R style for placing braces and spaces:
2012
2013 if (x is true) {
2014 we do y
2015 }
2016
2017 - Use a space after (most) keywords.
2018 - Do not add spaces around (inside) parenthesized expressions.
2019 - Use one space around (on each side of) most binary and ternary operators.
2020 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2021 - Mixed-case names are frowned upon.
2022 - Descriptive names for global variables are a must.
2023 - Avoid typedefs.
2024 - Functions should be short and sweet, and do just one thing.
2025 - The number of local variables shouldn't exceed 10.
2026 - Gotos are fine if they improve readability and reduce nesting.
2027 - Don't use C99-style "// ..." comments.
2028 - Names of macros defining constants and labels in enums are capitalized.
2029 - Enums are preferred when defining several related constants.
2030 - Always use the paraslash wrappers for allocating memory.
2031 - If the name of a function is an action or an imperative.
2032 command, the function should return an error-code integer
2033 (<0 means error, >=0 means success). If the name is a
2034 predicate, the function should return a "succeeded" boolean.
2035
2036
2037 Doxygen
2038 ~~~~~~~
2039
2040 Doxygen is a documentation system for various programming
2041 languages. The paraslash project uses Doxygen for generating the API
2042 reference on the web pages, but good source code documentation is
2043 also beneficial to people trying to understand the code structure
2044 and the interactions between the various source files.
2045
2046 It is more illustrative to look at the source code for examples than
2047 to describe the conventions for documenting the source in this manual,
2048 so we only describe which parts of the code need doxygen comments,
2049 but leave out details on documentation conventions.
2050
2051 As a rule, only the public part of the C source is documented with
2052 Doxygen. This includes structures, defines and enumerations in header
2053 files as well as public (non-static) C functions. These should be
2054 documented completely. For example each parameter and the return
2055 value of a public function should get a descriptive comment.
2056
2057 No doxygen comments are necessary for static functions and for
2058 structures and enumerations in C files (which are used only within
2059 this file). This does not mean, however, that those entities need
2060 no documentation at all. Instead, common sense should be applied to
2061 document what is not obvious from reading the code.
2062
2063 --------
2064 Appendix
2065 --------
2066
2067 Network protocols
2068 ~~~~~~~~~~~~~~~~~
2069
2070 *IP*. The _Internet Protocol_ is the primary networking protocol
2071 used for the Internet. All protocols described below use IP as the
2072 underlying layer. Both the prevalent IPv4 and the next-generation
2073 IPv6 variant are being deployed actively worldwide.
2074
2075 *Connection-oriented and connectionless protocols*. Connectionless
2076 protocols differ from connection-oriented ones in that state
2077 associated with the sending/receiving endpoints is treated
2078 implicitly. Connectionless protocols maintain no internal knowledge
2079 about the state of the connection. Hence they are not capable of
2080 reacting to state changes, such as sudden loss or congestion on the
2081 connection medium. Connection-oriented protocols, in contrast, make
2082 this knowledge explicit. The connection is established only after
2083 a bidirectional handshake which requires both endpoints to agree
2084 on the state of the connection, and may also involve negotiating
2085 specific parameters for the particular connection. Maintaining an
2086 up-to-date internal state of the connection also in general means
2087 that the sending endpoints perform congestion control, adapting to
2088 qualitative changes of the connection medium.
2089
2090 *Reliability*. In IP networking, packets can be lost, duplicated,
2091 or delivered out of order, and different network protocols handle
2092 these problems in different ways. We call a transport-layer protocol
2093 _reliable_, if it turns the unreliable IP delivery into an ordered,
2094 duplicate- and loss-free delivery of packets. Sequence numbers
2095 are used to discard duplicates and re-arrange packets delivered
2096 out-of-order. Retransmission is used to guarantee loss-free
2097 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2098 or data integrity.
2099
2100 *Classification*. With these definitions the protocols which are used
2101 by paraslash for steaming audio data may be classified as follows.
2102
2103 - HTTP/TCP: connection-oriented, reliable,
2104 - UDP: connectionless, unreliable,
2105 - DCCP: connection-oriented, unreliable.
2106
2107 Below we give a short descriptions of these protocols.
2108
2109 *TCP*. The _Transmission Control Protocol_ provides reliable,
2110 ordered delivery of a stream and a classic window-based congestion
2111 control. In contrast to UDP and DCCP (see below), TCP does not have
2112 record-oriented or datagram-based syntax, i.e. it provides a stream
2113 which is unaware and independent of any record (packet) boundaries.
2114 TCP is used extensively by many application layers. Besides HTTP (the
2115 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2116 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2117 top of TCP.
2118
2119 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2120 protocol, built as a thin layer directly on top of IP. For this reason,
2121 it offers the same best-effort service as IP itself, i.e. there is no
2122 detection of duplicate or reordered packets. Being a connectionless
2123 protocol, only minimal internal state about the connection is
2124 maintained, which means that there is no protection against packet
2125 loss or network congestion. Error checking and correction (if at all)
2126 are performed in the application.
2127
2128 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2129 connection-oriented state maintenance known from TCP with the
2130 unreliable, datagram-based transport of UDP. This means that it
2131 is capable of reacting to changes in the connection by performing
2132 congestion control, offering multiple alternative approaches. But it
2133 is bound to datagram boundaries (the maximum packet size supported
2134 by a medium), and like UDP it lacks retransmission to protect
2135 against loss. Due to the use of sequence numbers, it is however
2136 able to react to loss (interpreted as a congestion indication) and
2137 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2138 to negotiate specific, binding features for a connection, such as
2139 the choice of congestion control: classic, window-based congestion
2140 control known from TCP is available as CCID-2, rate-based, "smooth"
2141 congestion control is offered as CCID-3.
2142
2143 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2144 protocol on top of TCP. It is spoken by web servers and is most often
2145 used for web services. However, as can be seen by the many Internet
2146 radio stations and YouTube/Flash videos, http is by far not limited to
2147 the delivery of web pages only. Being a simple request/response based
2148 protocol, the semantics of the protocol also allow the delivery of
2149 multimedia content, such as audio over http.
2150
2151 *Multicast*. IP multicast is not really a protocol but a technique
2152 for one-to-many communication over an IP network. The challenge is to
2153 deliver information to a group of destinations simultaneously using
2154 the most efficient strategy to send the messages over each link of
2155 the network only once. This has benefits for streaming multimedia:
2156 the standard one-to-one unicast offered by TCP/DCCP means that
2157 n clients listening to the same stream also consume n-times the
2158 resources, whereas multicast requires to send the stream just once,
2159 irrespective of the number of receivers. Since it would be costly to
2160 maintain state for each listening receiver, multicast often implies
2161 connectionless transport, which is the reason that it is currently
2162 only available via UDP.
2163
2164 License
2165 ~~~~~~~
2166
2167 Paraslash is licensed under the GPL, version 2. Most of the code
2168 base has been written from scratch, and those parts are GPL V2
2169 throughout. Notable exceptions are FEC and the WMA decoder. See the
2170 corresponding source files for licencing details for these parts. Some
2171 code sniplets of several other third party software packages have
2172 been incorporated into the paraslash sources, for example log message
2173 coloring was taken from the git sources. These third party software
2174 packages are all published under the GPL or some other license
2175 compatible to the GPL.
2176
2177 Acknowledgements
2178 ~~~~~~~~~~~~~~~~
2179
2180 Many thanks to Gerrit Renker who read an early draft of this manual
2181 and contributed significant improvements.
2182
2183 ----------
2184 References
2185 ----------
2186
2187 Articles
2188 ~~~~~~~~
2189 - Reed, Irving S.; Solomon, Gustave (1960),
2190 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2191 Polynomial Codes over Certain Finite Fields), Journal of the
2192 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2193 300-304, doi:10.1137/0108018)
2194
2195 RFCs
2196 ~~~~
2197
2198 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2199 User Datagram Protocol
2200 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2201 Internet Protocol
2202 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2203 RSA Cryptography Specifications
2204 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2205 (2006): Datagram Congestion Control Protocol (DCCP)
2206 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2207 Congestion Control ID 2: TCP-like Congestion Control
2208 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2209 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2210 - XREFERENCE(http://www.ietf.org/rfc/rfc6716.txt, RFC 6716) (2012):
2211 Definition of the Opus Audio Codec
2212
2213 Application web pages
2214 ~~~~~~~~~~~~~~~~~~~~~
2215
2216 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash)
2217 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2218 - XREFERENCE(http://www.mpg123.de/, mpg123)
2219 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2220 - XREFERENCE(http://www.icecast.org/, icecast)
2221 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2222
2223 External documentation
2224 ~~~~~~~~~~~~~~~~~~~~~~
2225
2226 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2227 H. Peter Anvin: The mathematics of Raid6)
2228 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2229 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2230 Communication Protocols)
2231
2232 Code
2233 ~~~~
2234 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2235 Original FEC implementation by Luigi Rizzo)
2236