Merge branch 'maint'
[paraslash.git] / web / manual.md
1 **Paraslash user manual**
2
3 This document describes how to install, configure and use the paraslash
4 network audio streaming system. Most chapters start with a chapter
5 overview and conclude with an example section. We try to focus on
6 general concepts and on the interaction of the various pieces of the
7 paraslash package. Hence this user manual is not meant as a replacement
8 for the manual pages that describe all command line options of each
9 paraslash executable.
10
11 ============
12 Introduction
13 ============
14
15 In this chapter we give an [overview](#Overview) of the interactions
16 of the programs contained in the paraslash package, followed by
17 [brief descriptions](#The.paraslash.executables) of all executables.
18
19 Overview
20 --------
21
22 The core functionality of the para suite is provided by two main
23 applications, para_server and para_audiod. para_server maintains
24 the audio file database and acts as the streaming source, while
25 para_audiod is the streaming client. Usually, both run in the
26 background on different hosts but a local setup is also possible.
27
28 A simplified picture of a typical setup is as follows
29
30
31 .____________________.
32 | ______ |
33 .-----------------------. | .d########b. |
34 |.---------------------.| | .d############b |
35 || || | .d######""####//b. |
36 || || | 9######( )######P |
37 || || | 'b######++######d' |
38 || Screen || | "9############P" |
39 || || | "9a########P" |
40 || || | `""""'' |
41 |`---------------------'| | ________________ |
42 `-----------------------' | |________________| |
43 ___) (___ |____________________|
44 `-._______.-' loudspeaker
45 | |
46 | |
47 | |
48 .____/ \___. ._____________. ._____/ \_____.
49 | | | | | |
50 | para_gui |-----| para_audioc |-----| para_audiod |
51 |____ ___| |_____________| |_____ _____|
52 \ / \ /
53 | |
54 | |
55 | |
56 ._____/ \_____. ._____/ \_____.
57 | | | |
58 | para_client |-----------------------| para_server |
59 |_____________| |_____ _____|
60 \ /
61 |
62 |
63 .-'"""`-.
64 ( )
65 |`-.___.-'|
66 | |
67 |. ' " ` .|
68 | |
69 `-.___.-'
70 Database
71
72 The two client programs, para_client and para_audioc communicate with
73 para_server and para_audiod, respectively.
74
75 para_gui controls para_server and para_audiod by executing para_client
76 and para_audioc. In particular, it runs a command to obtain the state
77 of para_audiod and para_server, and the metadata of the current audio
78 file. This information is pretty-printed in a curses window.
79
80 The paraslash executables
81 -------------------------
82
83 <h3> para_server </h3>
84
85 para_server streams binary audio data (MP3, ...) over local and/or
86 remote networks. It listens on a TCP port and accepts commands such
87 as play, stop, pause, next from authenticated clients. The components
88 of para_server are illustrated in the following diagram:
89
90 ______________________________________________________________________ network
91 | | | | |
92 | .-'""""`-. | | | |
93 | ( ) | | | |
94 .____/ \_____. |`-.____.-'| .____/ \____/ \____/ \____. |
95 | | | | | | |
96 | dispatcher | | database | | senders (http/udp/dccp) | |
97 |____ _____| | | |___________ ___________| |
98 \ / |. ' "" ` .| \ / |
99 | | | | |
100 | `-.____.-' | |
101 | | | |
102 | | | |
103 | | | |
104 | ._____/ \_____. .________/ \________. |
105 | | | | | |
106 | | audio file |________| virtual streaming | |
107 | | selector | | system | |
108 | |_____ _____| |________ ________| |
109 | \ / \ / |
110 | | | |
111 | | | |
112 | | ._________________. | |
113 | | | | | |
114 | `---| command handler |---' |
115 | |____ ___ ____| |
116 | \ / \ / |
117 | | | |
118 | | | |
119 | | | |
120 `-------------------------' `--------------------------'
121
122
123 Incoming connections arrive at the dispatcher which creates a process
124 dedicated to the connection. Its task is to authenticate the client
125 and to run the command handler which forwards the client request to
126 either the audio file selector or the virtual streaming system. Results
127 (if any) are sent back to the client.
128
129 The audio file selector manages audio files using various database
130 tables. It maintains statistics on the usage of all audio files such as
131 last-played time and the number of times each file was selected. It
132 is also responsible for selecting and loading audio files for
133 streaming. Additional information may be added to the database to allow
134 fine-grained selection based on various properties of the audio file,
135 including information found in (ID3) tags. Simple playlists are also
136 supported. It is possible to store images (album covers) and lyrics
137 in the database and associate these to the corresponding audio files.
138 The section on the [audio file selector](#The.audio.file.selector)
139 discusses this topic in more detail.
140
141 Another component of para_server is the virtual streaming system,
142 which controls the paraslash senders. During streaming it requests
143 small chunks of data (e.g., mp3 frames) from the audio file selector
144 and feeds them to the senders which forward the chunks to connected
145 clients.
146
147 The three senders of para_server correspond to network streaming
148 protocols based on HTTP, DCCP, or UDP. This is explained in the
149 section on [networking](#Networking).
150
151 <h3> para_client </h3>
152
153 The client program to connect to para_server. paraslash commands
154 are sent to para_server and the response is dumped to STDOUT. This
155 can be used by any scripting language to produce user interfaces with
156 little programming effort.
157
158 All connections between para_server and para_client are encrypted
159 with a symmetric session key. For each user of paraslash you must
160 create a public/secret RSA key pair for authentication.
161
162 If para_client is started without non-option arguments, an interactive
163 session (shell) is started. Command history and command completion are
164 supported through libreadline.
165
166 <h3> para_audiod </h3>
167
168 The purpose of para_audiod is to download, decode and play an audio
169 stream received from para_server. A typical setup looks as follows.
170
171
172 .----------------------------.
173 | |
174 | |
175 ._____/ \_____. .___/ \____.
176 | | .----------| |
177 | para_server | | .______| receiver |
178 |_____ ____| | | |___ ____|
179 \ / | | \ /
180 | | | |
181 | | | |
182 | | | |
183 ._____/ \_____. | | .___/ \____.
184 | | | | | |
185 | status task |-----+ | | filter 1 |
186 |_____________| | |___ ____|
187 | \ /
188 | | .____________________.
189 | | | ______ |
190 .____________. | .___/ \____. | .d########b. |
191 | | | | | | .d############b |
192 | dispatcher |----------' | filter 2 | | .d######""####//b. |
193 |_____ ____| |___ ____| | 9######( )######P |
194 \ / \ / | 'b######++######d' |
195 | | | "9############P" |
196 | | | "9a########P" |
197 ._____/ \_____. .___/ \____. | `""""'' |
198 | | | | | ________________ |
199 | para_audioc | | writer |------| |________________| |
200 |_____________| |__________| |____________________|
201
202
203 The status task of para_audiod connects to para_server and runs the
204 "stat" command to retrieve the current server status. If an audio
205 stream is available, para_audiod starts a so-called buffer tree to
206 play the stream.
207
208 The buffer tree consists of a receiver, any number of filters and a
209 writer. The receiver downloads the audio stream from para_server and
210 the filters decode or modify the received data. The writer plays the
211 decoded stream.
212
213 The dispatcher of para_audiod listens on a local socket and runs
214 audiod commands on behalf of para_audioc. For example, para_gui runs
215 para_audioc to obtain status information about para_audiod and the
216 current audio file. Access to the local socket may be restricted by
217 means of Unix socket credentials.
218
219 <h3> para_audioc </h3>
220
221 The client program which talks to para_audiod. Used to control
222 para_audiod, to receive status info, or to grab the stream at any
223 point of the decoding process. Like para_client, para_audioc supports
224 interactive sessions on systems with libreadline.
225
226 <h3> para_recv </h3>
227
228 A command line HTTP/DCCP/UDP stream grabber. The http mode is
229 compatible with arbitrary HTTP streaming sources (e.g. icecast).
230 In addition to the three network streaming modes, para_recv can also
231 operate in local (afh) mode. In this mode it writes the content of
232 an audio file on the local file system in complete chunks to stdout,
233 optionally 'just in time'. This allows to cut an audio file without
234 first decoding it, and it enables third-party software which is unaware
235 of the particular audio format to send complete frames in real time.
236
237 <h3> para_filter </h3>
238
239 A filter program that reads from STDIN and writes to STDOUT.
240 Like para_recv, this is an atomic building block which can be used to
241 assemble higher-level audio receiving facilities. It combines several
242 different functionalities in one tool: decoders for multiple audio
243 formats and a number of processing filters, among these a normalizer
244 for audio volume.
245
246 <h3> para_afh </h3>
247
248 A small stand-alone program that prints tech info about the given
249 audio file to STDOUT. It can be instructed to print a "chunk table",
250 an array of offsets within the audio file.
251
252 <h3> para_write </h3>
253
254 A modular audio stream writer. It supports a simple file writer
255 output plug-in and optional WAV/raw players for ALSA (Linux) and OSS.
256 para_write can also be used as a stand-alone WAV or raw audio player.
257
258 <h3> para_play </h3>
259
260 A command line audio player which supports the same audio formats as
261 para_server. It differs from other players in that it has an insert
262 and a command mode, like the vi editor. Line editing is based on
263 libreadline, and tab completion and command history are supported.
264
265 <h3> para_gui </h3>
266
267 Curses-based gui that presents status information obtained in a curses
268 window. Appearance can be customized via themes. para_gui provides
269 key-bindings for the most common server commands and new key-bindings
270 can be added easily.
271
272 <h3> para_mixer </h3>
273
274 An alarm clock and volume-fader for OSS and ALSA.
275
276 ===========
277 Quick start
278 ===========
279
280 This chapter lists the [necessary software](#Requirements)
281 that must be installed to compile the paraslash package, describes
282 how to [compile and install](#Installation) the paraslash
283 source code and the steps that have to be performed in order to
284 [set up](#Configuration) a typical server and client.
285
286 Requirements
287 ------------
288 <h3> For the impatient </h3>
289
290 git clone git://git.tuebingen.mpg.de/lopsub
291 cd lopsub && make && sudo make install
292 git clone git://git.tuebingen.mpg.de/osl
293 cd osl && make && sudo make install && sudo ldconfig
294 sudo apt-get install autoconf libssl-dev m4 \
295 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
296 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev realpath \
297 libasound2-dev libao-dev libreadline-dev libncurses-dev \
298 libopus-dev
299
300 <h3> Detailed description </h3>
301
302 In any case you will need
303
304 - [lopsub](http://people.tuebingen.mpg.de/maan/lopsub/). The long
305 option parser for subcommands generates the command line and config
306 file parsers for all paraslash executables. Clone the source code
307 repository with
308
309 git clone git://git.tuebingen.mpg.de/lopsub
310
311 - [gcc](ftp://ftp.gnu.org/pub/gnu/gcc) or
312 [clang](http://clang.llvm.org). All gcc versions >= 4.2 are currently
313 supported. Clang version 1.1 or newer should work as well.
314
315 - [gnu make](ftp://ftp.gnu.org/pub/gnu/make) is also shipped with the
316 disto. On BSD systems the gnu make executable is often called gmake.
317
318 - [bash](ftp://ftp.gnu.org/pub/gnu/bash). Some scripts which run
319 during compilation require the _Bourne again shell_. It is most
320 likely already installed.
321
322 - [m4](ftp://ftp.gnu.org/pub/gnu/m4/). Some source files are generated
323 from templates by the m4 macro processor.
324
325 Optional:
326
327 - [libosl](http://people.tuebingen.mpg.de/maan/osl/). The _object
328 storage layer_ library is used by para_server. To clone the source
329 code repository, execute
330
331 git clone git://git.tuebingen.mpg.de/osl
332
333 - [openssl](http://www.openssl.org/) or
334 [libgcrypt](ftp://ftp.gnupg.org/gcrypt/libgcrypt/). At least one
335 of these two libraries is needed as the backend for cryptographic
336 routines on both the server and the client side. Both openssl and
337 libgcrypt are usually shipped with the distro, but you might have
338 to install the development package (`libssl-dev` or `libgcrypt-dev`
339 on debian systems) as well.
340
341 - [libmad](http://www.underbit.com/products/mad/). To compile in MP3
342 support for paraslash, the development package must be installed. It
343 is called `libmad0-dev` on debian-based systems. Note that libmad is
344 not necessary on the server side, i.e., for sending MP3 files.
345
346 - [libid3tag](http://www.underbit.com/products/mad/). For version-2
347 ID3 tag support, you willl need the libid3tag development package
348 `libid3tag0-dev`. Without libid3tag, only version-1 tags are
349 recognized. The mp3 tagger also needs this library for modifying
350 (id3v1 and id3v2) tags.
351
352 - [ogg vorbis](http://www.xiph.org/downloads/). For ogg vorbis streams
353 you need libogg, libvorbis, libvorbisfile. The corresponding Debian
354 packages are called `libogg-dev` and `libvorbis-dev`.
355
356 - [libfaad and mp4ff](http://www.audiocoding.com/). For aac files
357 (m4a) you need libfaad and libmp4ff (package: `libfaad-dev`). Note
358 that for some distributions, e.g. Ubuntu, mp4ff is not part of the
359 libfaad package. Install the faad library from sources (available
360 through the above link) to get the mp4ff library and header files.
361
362 - [speex](http://www.speex.org/). In order to stream or decode speex
363 files, libspeex (`libspeex-dev`) is required.
364
365 - [flac](http://flac.sourceforge.net/). To stream or decode files
366 encoded with the _Free Lossless Audio Codec_, libFLAC (`libFLAC-dev`)
367 must be installed.
368
369 - [libsamplerate](http://www.mega-nerd.com/SRC/index.html). The
370 resample filter will only be compiled if this library is
371 installed. Debian package: `libsamplerate-dev`.
372
373 - [alsa-lib](ftp://ftp.alsa-project.org/pub/lib/). On Linux, you will
374 need to have the ALSA development package `libasound2-dev` installed.
375
376 - [libao](http://downloads.xiph.org/releases/ao/). Needed to build
377 the ao writer (ESD, PulseAudio,...). Debian package: `libao-dev`.
378
379 - [curses](ftp://ftp.gnu.org/pub/gnu/ncurses). Needed for
380 para_gui. Debian package: `libncurses-dev`.
381
382 - [GNU
383 Readline](http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html). If
384 this library (`libreadline-dev`) is installed, para_client, para_audioc
385 and para_play support interactive sessions.
386
387 Installation
388 ------------
389 To build the sources from a tarball, execute
390
391 ./configure && make
392
393 To build from git or a gitweb snapshot, run this command instead:
394
395 ./autogen.sh
396
397 There should be no errors but probably some warnings about missing
398 packages which usually implies that not all audio formats will be
399 supported. If headers or libs are installed at unusual locations you
400 might need to tell the configure script where to find them. Try
401
402 ./configure --help
403
404 to see a list of options. If the paraslash package was compiled
405 successfully, execute (optionally)
406
407 make test
408
409 to run the paraslash test suite. If all tests pass, execute as root
410
411 make install
412
413 to install executables under /usr/local/bin and the man pages under
414 /usr/local/man.
415
416 Configuration
417 -------------
418
419 <h3> Create a paraslash user </h3>
420
421 In order to control para_server at runtime you must create a paraslash
422 user. As authentication is based on the RSA crypto system you'll have
423 to create an RSA key pair. If you already have a user and an RSA key
424 pair, you may skip this step.
425
426 In this section we'll assume a typical setup: You would like to run
427 para_server on some host called server_host as user foo, and you want
428 to connect to para_server from another machine called client_host as
429 user bar.
430
431 As foo@server_host, create ~/.paraslash/server.users by typing the
432 following commands:
433
434 user=bar
435 target=~/.paraslash/server.users
436 key=~/.paraslash/id_rsa.pub.$user
437 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
438 mkdir -p ~/.paraslash
439 echo "user $user $key $perms" >> $target
440
441 Next, change to the "bar" account on client_host and generate the
442 key pair with the commands
443
444 ssh-keygen -q -t rsa -b 2048 -N '' -f $key
445
446 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
447 that para_server won't accept keys shorter than 2048 bits. Moreover,
448 para_client rejects private keys which are world-readable.
449
450 para_server only needs to know the public key of the key pair just
451 created. Copy this public key to server_host:
452
453 src=~/.ssh/id_rsa.pub
454 dest=.paraslash/id_rsa.pub.$LOGNAME
455 scp $src foo@server_host:$dest
456
457 Finally, tell para_client to connect to server_host:
458
459 conf=~/.paraslash/client.conf
460 echo 'hostname server_host' > $conf
461
462
463 <h3> Start para_server </h3>
464
465 For this first try, we'll use the info loglevel to make the output
466 of para_server more verbose.
467
468 para_server -l info
469
470 Now you can use para_client to connect to the server and issue
471 commands. Open a new shell as bar@client_host and try
472
473 para_client help
474 para_client si
475
476 to retrieve the list of available commands and some server info.
477 Don't proceed if this doesn't work.
478
479 <h3> Create and populate the database </h3>
480
481 An empty database is created with
482
483 para_client init
484
485 This initializes a couple of empty tables under
486 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
487 tables, but it's good to know that you can start from scratch with
488
489 rm -rf ~/.paraslash/afs_database-0.4
490
491 in case something went wrong.
492
493 Next, you need to add some audio files to that database so that
494 para_server knows about them. Choose an absolute path to a directory
495 containing some audio files and add them to the audio file table:
496
497 para_client add /my/mp3/dir
498
499 This might take a while, so it is a good idea to start with a directory
500 containing not too many files. Note that the table only contains data
501 about the audio files found, not the files themselves.
502
503 You may print the list of all known audio files with
504
505 para_client ls
506
507 <h3> Configure para_audiod </h3>
508
509 We will have to tell para_audiod that it should receive the audio
510 stream from server_host via http:
511
512 para_audiod -l info -r '.:http -i server_host'
513
514 You should now be able to listen to the audio stream once para_server
515 starts streaming. To activate streaming, execute
516
517 para_client play
518
519 Since no playlist has been specified yet, the "dummy" mode which
520 selects all known audio files is activated automatically. See the
521 section on the [audio file selector](#The.audio.file.selector) for how
522 to use playlists and moods to specify which files should be streamed
523 in which order.
524
525 Troubleshooting
526 ---------------
527
528 To identify streaming problems try to receive, decode and play the
529 stream manually using para_recv, para_filter and para_write as follows.
530 For simplicity we assume that you're running Linux/ALSA and that only
531 MP3 files have been added to the database.
532
533 para_recv -r 'http -i server_host' > file.mp3
534 # (interrupt with CTRL+C after a few seconds)
535 ls -l file.mp3 # should not be empty
536 para_filter -f mp3dec -f wav < file.mp3 > file.wav
537 ls -l file.wav # should be much bigger than file.mp3
538 para_write -w alsa < file.wav
539
540 Double check what is logged by para_server and use the --loglevel
541 option of para_recv, para_filter and para_write to increase verbosity.
542
543 ===============
544 User management
545 ===============
546
547 para_server uses a challenge-response mechanism to authenticate
548 requests from incoming connections, similar to ssh's public key
549 authentication method. Authenticated connections are encrypted using
550 the AES stream cipher in integer counter mode.
551
552 In this chapter we briefly describe RSA and AES, and sketch the
553 [authentication handshake](#Client-server.authentication)
554 between para_client and para_server. User management is discussed
555 in the section on [the user_list file](#The.user_list.file).
556 These sections are all about communication between the client and the
557 server. Connecting para_audiod is a different matter and is described
558 in a [separate section](#Connecting.para_audiod).
559
560 RSA and AES
561 -----------
562
563 A block cipher is a transformation which operates on fixed-length
564 blocks. For symmetric block ciphers the transformation is determined
565 by a single key for both encryption and decryption. For asymmetric
566 block ciphers, on the other hand, the key consists of two parts,
567 called the public key and the private key. A message can be encrypted
568 with either key and only the counterpart of that key can decrypt the
569 message. Asymmetric block ciphers can be used for both signing and
570 encrypting a message.
571
572 RSA is an asymmetric block cipher which is used in many applications,
573 including ssh and gpg. The RSA public key encryption and signatures
574 algorithms are defined in detail in RFC 2437. Paraslash relies on
575 RSA for authentication.
576
577 Stream ciphers XOR the input with a pseudo-random key stream to produce
578 the output. Decryption uses the same function calls as encryption.
579 Any block cipher can be turned into a stream cipher by generating the
580 pseudo-random key stream by encrypting successive values of a counter
581 (counter mode).
582
583 AES, the advanced encryption standard, is a well-known symmetric block
584 cipher. Paraslash employs AES in counter mode as described above to
585 encrypt communications. Since a stream cipher key must not be used
586 twice, a random key is generated for every new connection.
587
588 Client-server authentication
589 ----------------------------
590
591 The authentication handshake between para_client and para_server goes
592 as follows:
593
594 - para_client connects to para_server and sends an authentication
595 request for a user. It does so by connecting to TCP port 2990 of the
596 server host. This port is called the para_server _control port_.
597
598 - para_server accepts the connection and forks a child process which
599 handles the incoming request. The parent process keeps listening on the
600 control port while the child process (also called para_server below)
601 continues as follows.
602
603 - para_server loads the RSA public key of that user, fills a
604 fixed-length buffer with random bytes, encrypts that buffer using the
605 public key and sends the encrypted buffer to the client. The first
606 part of the buffer is the challenge which is used for authentication
607 while the second part is the session key.
608
609 - para_client receives the encrypted buffer and decrypts it with the
610 user's private key, thereby obtaining the challenge buffer and the
611 session key. It sends the SHA1 hash value of the challenge back to
612 para_server and stores the session key for further use.
613
614 - para_server also computes the SHA1 hash of the challenge and compares
615 it against what was sent back by the client.
616
617 - If the two hashes do not match, the authentication has failed and
618 para_server closes the connection.
619
620 - Otherwise the user is considered authenticated and the client is
621 allowed to proceed by sending a command to be executed. From this
622 point on the communication is encrypted using the stream cipher with
623 the session key known to both peers.
624
625 paraslash relies on the quality of the pseudo-random bytes provided
626 by the crypto library (openssl or libgcrypt), on the security of
627 the implementation of the RSA and AES crypto routines and on the
628 infeasibility to invert the SHA1 function.
629
630 Neither para_server or para_client create RSA keys on their
631 own. This has to be done once for each user as sketched in
632 [Quick start](#Quick.start) and discussed in more detail
633 [below](#The.user_list.file).
634
635 The user_list file
636 ------------------
637
638 At startup para_server reads the user list file which contains one
639 line per user. The default location of the user list file may be
640 changed with the --user-list option.
641
642 There should be at least one user in this file. Each user must have
643 an RSA key pair. The public part of the key is needed by para_server
644 while the private key is needed by para_client. Each line of the
645 user list file must be of the form
646
647 user <username> <key> <perms>
648
649 where _username_ is an arbitrary string (usually the user's login
650 name), _key_ is the full path to that user's public RSA key, and
651 _perms_ is a comma-separated list of zero or more of the following
652 permission bits:
653
654 +---------------------------------------------------------+
655 | AFS_READ | read the contents of the databases |
656 +-----------+---------------------------------------------+
657 | AFS_WRITE | change database contents |
658 +-----------+---------------------------------------------+
659 | VSS_READ | obtain information about the current stream |
660 +-----------+---------------------------------------------+
661 | VSS_WRITE | change the current stream |
662 +---------------------------------------------------------+
663
664 The permission bits specify which commands the user is allowed to
665 execute. The output of
666
667 para_client help
668
669 contains the permissions needed to execute the command.
670
671 It is possible to make para_server reread the user_list file by
672 executing the paraslash "hup" command or by sending SIGHUP to the
673 PID of para_server.
674
675 Connecting para_audiod
676 ----------------------
677
678 para_audiod listens on a Unix domain socket. Those sockets are
679 for local communication only, so only local users can connect to
680 para_audiod. The default is to let any user connect but this can be
681 restricted on platforms that support UNIX socket credentials which
682 allow para_audiod to obtain the Unix credentials of the connecting
683 process.
684
685 Use para_audiod's --user-allow option to allow connections only for
686 a limited set of users.
687
688 =======================
689 The audio file selector
690 =======================
691
692 paraslash comes with a sophisticated audio file selector (AFS),
693 whose main task is to determine which file to stream next, based on
694 information on the audio files stored in a database. It communicates
695 also with para_client via the command handler whenever an AFS command
696 is executed, for example to answer a database query.
697
698 Besides the simple playlists, AFS supports audio file selection
699 based on _moods_ which act as a filter that limits the set of all
700 known audio files to those which satisfy certain criteria. It also
701 maintains tables containing images (e.g. album cover art) and lyrics
702 that can be associated with one or more audio files.
703
704 In this chapter we sketch the setup of the [AFS
705 process](#The.AFS.process) during server startup and proceed with the
706 description of the [layout](#Database.layout) of the various database
707 tables. The section on [playlists and moods](#Playlists.and.moods)
708 explains these two audio file selection mechanisms in detail
709 and contains practical examples. The way [file renames and content
710 changes](#File.renames.and.content.changes) are detected is discussed
711 briefly before the [Troubleshooting](#Troubleshooting) section
712 concludes the chapter.
713
714 The AFS process
715 ---------------
716
717 On startup, para_server forks to create the AFS process which opens
718 the database tables. The AFS process accepts incoming connections
719 which arrive either on a pipe which is shared with para_server,
720 or on the local socket it is listening on. The setup is as follows.
721
722 .___________________. .______________.
723 | | | |
724 | virtual streaming | | audio format |
725 | system | | handler |
726 |_________ _______| |_____ ______|
727 \ / \ /
728 | |
729 .-'""""`-. | | .-'""""`-.
730 ( ) | | ( )
731 |`-.____.-'| .__/ \________________/ \___. |`-.____.-'|
732 | | | | | |
733 | file |----| AFS (audio file selector) |----| OSL |
734 | system | | process | | database |
735 | | |___________________________| | |
736 |. ' "" ` .| | |. ' "" ` .|
737 | | | | |
738 `-.____.-' | `-.____.-'
739 ._______/ \_______.
740 | |
741 | command handler |
742 |_______ _______|
743 \ /
744 |
745 |
746 |
747 ._____/ \_____.
748 | |
749 | para_client |
750 |_____________|
751
752 The virtual streaming system, which is part of the server process,
753 communicates with the AFS process via pipes and shared memory. When
754 the current audio file changes, it sends a notification through the
755 shared pipe. The AFS process queries the database to determine the
756 next audio file, opens it, verifies that it has not been changed since
757 it was added to the database and passes the open file descriptor back
758 to the virtual streaming system, along with audio file meta-data such
759 as file name, duration, audio format and so on. The virtual streaming
760 system then starts to stream the file.
761
762 The command handlers of all AFS server commands use the local socket
763 to query or update the database. For example, the command handler of
764 the add command sends the path of an audio file to the local socket.
765 The AFS process opens the file and tries to find an audio format
766 handler which recognizes the file. If all goes well, a new database
767 entry with metadata obtained from the audio format handler is added
768 to the database.
769
770 Note that AFS employs
771 [libosl](http://people.tuebingen.mpg.de/maan/osl/), the object
772 storage layer library, as the database backend. This library offers
773 functionality similar to a relational database, but is much more
774 lightweight than a full featured database management system.
775
776 Database layout
777 ---------------
778
779 Metadata about the known audio files is stored in an OSL database. This
780 database consists of the following tables:
781
782 - The audio file table contains path, hash and metadata of each
783 known file.
784
785 - The "attributes" table maps each of the 64 possible attributes to a
786 string.
787
788 - The "blob" tables store images, lyrics, moods, playlists. All of
789 these are optional.
790
791 - The "score" table describes the subset of admissible files for the
792 current playlist or mood.
793
794 All tables are described in more detail below.
795
796 <h3> The audio file table </h3>
797
798 This is the most important and usually also the largest table of the
799 AFS database. It contains the information needed to stream each audio
800 file. In particular the following data is stored for each audio file.
801
802 - SHA1 hash value of the audio file contents. This is computed once
803 when the file is added to the database. Whenever AFS selects this
804 audio file for streaming the hash value is recomputed and checked
805 against the value stored in the database to detect content changes.
806
807 - The time when this audio file was last played.
808
809 - The number of times the file has been played so far.
810
811 - The attribute bitmask.
812
813 - The image id which describes the image associated with this audio
814 file.
815
816 - The lyrics id which describes the lyrics associated with this
817 audio file.
818
819 - The audio format id (MP3, OGG, ...).
820
821 - An amplification value that can be used by the amplification filter
822 to pre-amplify the decoded audio stream.
823
824 - The chunk table. It describes the location and the timing of the
825 building blocks of the audio file. This is used by para_server to
826 send chunks of the file at appropriate times.
827
828 - The duration of the audio file.
829
830 - Tag information contained in the audio file (ID3 tags, Vorbis
831 comments, ...).
832
833 - The number of channels
834
835 - The encoding bitrate.
836
837 - The sampling frequency.
838
839 To add or refresh the data contained in the audio file table, the _add_
840 command is used. It takes the full path of either an audio file or a
841 directory. In the latter case, the directory is traversed recursively
842 and all files which are recognized as valid audio files are added to
843 the database.
844
845 <h3> The attribute table </h3>
846
847 The attribute table contains two columns, _name_ and _bitnum_. An
848 attribute is simply a name for a certain bit number in the attribute
849 bitmask of the audio file table.
850
851 Each of the 64 bits of the attribute bitmask can be set for each
852 audio file individually. Hence up to 64 different attributes may be
853 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
854 "german_lyrics", "speech", whatever. You are free to choose as
855 many attributes as you like and there are no naming restrictions
856 for attributes.
857
858 A new attribute "test" is created by
859
860 para_client addatt test
861 and
862 para_client lsatt
863
864 lists all available attributes. You can set the "test" attribute for
865 an audio file by executing
866
867 para_client setatt test+ /path/to/the/audio/file
868
869 Similarly, the "test" bit can be removed from an audio file with
870
871 para_client setatt test- /path/to/the/audio/file
872
873 Instead of a path you may use a shell wildcard pattern. The attribute
874 is applied to all audio files matching this pattern:
875
876 para_client setatt test+ '/test/directory/*'
877
878 The command
879
880 para_client -- ls -l=v
881
882 gives you a verbose listing of your audio files also showing which
883 attributes are set.
884
885 In case you wonder why the double-dash in the above command is needed:
886 It tells para_client to not interpret the options after the dashes. If
887 you find this annoying, just say
888
889 alias para='para_client --'
890
891 and be happy. In what follows we shall use this alias.
892
893 The "test" attribute can be dropped from the database with
894
895 para rmatt test
896
897 Read the output of
898
899 para help ls
900 para help setatt
901
902 for more information and a complete list of command line options to
903 these commands.
904
905 <h3> Blob tables </h3>
906
907 The image, lyrics, moods and playlists tables are all blob tables.
908 Blob tables consist of three columns each: The identifier which is
909 a positive number that is auto-incremented, the name (an arbitrary
910 string) and the content (the blob).
911
912 All blob tables support the same set of actions: cat, ls, mv, rm
913 and add. Of course, _add_ is used for adding new blobs to the table
914 while the other actions have the same meaning as the corresponding
915 Unix commands. The paraslash commands to perform these actions are
916 constructed as the concatenation of the table name and the action. For
917 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
918 manipulate or query the image table.
919
920 The add variant of these commands is special as these commands read
921 the blob contents from stdin. To add an image to the image table the
922 command
923
924 para addimg image_name < file.jpg
925
926 can be used.
927
928 Note that the images and lyrics are not interpreted at all, and also
929 the playlist and the mood blobs are only investigated when the mood
930 or playlist is activated with the select command.
931
932 <h3> The score table </h3>
933
934 The score table describes those audio files which are admissible for
935 the current mood or playlist (see below). The table has two columns:
936 a pointer to a row of the audio file table and a score value.
937
938 Unlike all other tables of the database, the score table remains in
939 memory and is never stored on disk. It is initialized at startup and
940 recomputed when the select command loads a new mood or playlist.
941
942 When the audio file selector is asked to open the next audio file,
943 it picks the row with the highest score, opens the corresponding
944 file and passes the file descriptor to the virtual streaming system.
945 At this point the last_played and the num_played fields of the selected
946 file are updated and the score is recomputed.
947
948 Playlists and moods
949 -------------------
950
951 Playlists and moods offer two different ways of specifying the set of
952 admissible files. A playlist in itself describes a set of admissible
953 files. A mood, in contrast, describes the set of admissible files in
954 terms of attributes and other type of information available in the
955 audio file table. As an example, a mood can define a filename pattern,
956 which is then matched against the names of audio files in the table.
957
958 <h3> Playlists </h3>
959
960 Playlists are accommodated in the playlist table of the afs database,
961 using the aforementioned blob format for tables. A new playlist is
962 created with the addpl command by specifying the full (absolute)
963 paths of all desired audio files, separated by newlines. Example:
964
965 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
966
967 If _my_playlist_ already exists it is overwritten. To activate the
968 new playlist, execute
969
970 para select p/my_playlist
971
972 The audio file selector will assign scores to each entry of the list,
973 in descending order so that files will be selected in order. If a
974 file could not be opened for streaming, its entry is removed from
975 the score table (but not from the playlist).
976
977 <h3> Moods </h3>
978
979 A mood consists of a unique name and its *mood definition*, which is
980 a set of *mood lines* containing expressions in terms of attributes
981 and other data contained in the database.
982
983 At any time at most one mood can be *active* which means that
984 para_server is going to select only files from that subset of
985 admissible files.
986
987 So in order to create a mood definition one has to write a set of
988 mood lines. Mood lines come in three flavours: Accept lines, deny
989 lines and score lines.
990
991 The general syntax of the three types of mood lines is
992
993
994 accept [with score <score>] [if] [not] <mood_method> [options]
995 deny [with score <score>] [if] [not] <mood_method> [options]
996 score <score> [if] [not] <mood_method> [options]
997
998
999 Here <score> is either an integer or the string "random" which assigns
1000 a random score to all matching files. The score value changes the
1001 order in which admissible files are going to be selected, but is of
1002 minor importance for this introduction.
1003
1004 So we concentrate on the first two forms, i.e. accept and deny
1005 lines. As usual, everything in square brackets is optional, i.e.
1006 accept/deny lines take the following form when ignoring scores:
1007
1008 accept [if] [not] <mood_method> [options]
1009
1010 and analogously for the deny case. The "if" keyword is only syntactic
1011 sugar and has no function. The "not" keyword just inverts the result,
1012 so the essence of a mood line is the mood method part and the options
1013 following thereafter.
1014
1015 A *mood method* is realized as a function which takes an audio file
1016 and computes a number from the data contained in the database.
1017 If this number is non-negative, we say the file *matches* the mood
1018 method. The file matches the full mood line if it either
1019
1020 - matches the mood method and the "not" keyword is not given,
1021 or
1022 - does not match the mood method, but the "not" keyword is given.
1023
1024 The set of admissible files for the whole mood is now defined as those
1025 files which match at least one accept mood line, but no deny mood line.
1026 More formally, an audio file F is admissible if and only if
1027
1028 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
1029
1030 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
1031 lines and "~" means "matches".
1032
1033 The cases where no mood lines of accept/deny type are defined need
1034 special treatment:
1035
1036 - Neither accept nor deny lines: This treats all files as
1037 admissible (in fact, that is the definition of the dummy mood
1038 which is activated automatically if no moods are available).
1039
1040 - Only accept lines: A file is admissible iff it matches at
1041 least one accept line:
1042
1043 F ~ AL1 or F ~ AL2 or ...
1044
1045 - Only deny lines: A file is admissible iff it matches no
1046 deny line:
1047
1048 not (F ~ DL1 or F ~ DN2 ...)
1049
1050
1051
1052 <h3> List of mood_methods </h3>
1053
1054 no_attributes_set
1055
1056 Takes no arguments and matches an audio file if and only if no
1057 attributes are set.
1058
1059 is_set <attribute_name>
1060
1061 Takes the name of an attribute and matches iff that attribute is set.
1062
1063 path_matches <pattern>
1064
1065 Takes a filename pattern and matches iff the path of the audio file
1066 matches the pattern.
1067
1068 artist_matches <pattern>
1069 album_matches <pattern>
1070 title_matches <pattern>
1071 comment_matches <pattern>
1072
1073 Takes an extended regular expression and matches iff the text of the
1074 corresponding tag of the audio file matches the pattern. If the tag
1075 is not set, the empty string is matched against the pattern.
1076
1077 year ~ <num>
1078 bitrate ~ <num>
1079 frequency ~ <num>
1080 channels ~ <num>
1081 num_played ~ <num>
1082 image_id ~ <num>
1083 lyrics_id ~ <num>
1084
1085 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
1086 <num>. Matches an audio file iff the condition <val> ~ <num> is
1087 satisfied where val is the corresponding value of the audio file
1088 (value of the year tag, bitrate in kbit/s, etc.).
1089
1090 The year tag is special as its value is undefined if the audio file
1091 has no year tag or the content of the year tag is not a number. Such
1092 audio files never match. Another difference is the special treatment
1093 if the year tag is a two-digit number. In this case either 1900 or
1094 2000 is added to the tag value, depending on whether the number is
1095 greater than 2000 plus the current year.
1096
1097
1098 <h3> Mood usage </h3>
1099
1100 To create a new mood called "my_mood", write its definition into
1101 some temporary file, say "tmpfile", and add it to the mood table
1102 by executing
1103
1104 para addmood my_mood < tmpfile
1105
1106 If the mood definition is really short, you may just pipe it to the
1107 client instead of using temporary files. Like this:
1108
1109 echo "$MOOD_DEFINITION" | para addmood my_mood
1110
1111 There is no need to keep the temporary file since you can always use
1112 the catmood command to get it back:
1113
1114 para catmood my_mood
1115
1116 A mood can be activated by executing
1117
1118 para select m/my_mood
1119
1120 Once active, the list of admissible files is shown by the ls command
1121 if the "-a" switch is given:
1122
1123 para ls -a
1124
1125
1126 <h3> Example mood definition </h3>
1127
1128 Suppose you have defined attributes "punk" and "rock" and want to define
1129 a mood containing only Punk-Rock songs. That is, an audio file should be
1130 admissible if and only if both attributes are set. Since
1131
1132 punk and rock
1133
1134 is obviously the same as
1135
1136 not (not punk or not rock)
1137
1138 (de Morgan's rule), a mood definition that selects only Punk-Rock
1139 songs is
1140
1141 deny if not is_set punk
1142 deny if not is_set rock
1143
1144
1145
1146 File renames and content changes
1147 --------------------------------
1148
1149 Since the audio file selector knows the SHA1 of each audio file that
1150 has been added to the afs database, it recognizes if the content of
1151 a file has changed, e.g. because an ID3 tag was added or modified.
1152 Also, if a file has been renamed or moved to a different location,
1153 afs will detect that an entry with the same hash value already exists
1154 in the audio file table.
1155
1156 In both cases it is enough to just re-add the new file. In the
1157 first case (file content changed), the audio table is updated, while
1158 metadata such as the num_played and last_played fields, as well as
1159 the attributes, remain unchanged. In the other case, when the file
1160 is moved or renamed, only the path information is updated, all other
1161 data remains as before.
1162
1163 It is possible to change the behaviour of the add command by using the
1164 "-l" (lazy add) or the "-f" (force add) option.
1165
1166 Troubleshooting
1167 ---------------
1168
1169 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1170 executables have a brief online help which is displayed when -h is
1171 given. The --detailed-help option prints the full help text.
1172
1173 If para_server crashed or was killed by SIGKILL (signal 9), it
1174 may refuse to start again because of "dirty osl tables". In this
1175 case you'll have to run the oslfsck program of libosl to fix your
1176 database:
1177
1178 oslfsck -fd ~/.paraslash/afs_database-0.4
1179
1180 However, make sure para_server isn't running before executing oslfsck.
1181
1182 If you don't mind to recreate your database you can start
1183 from scratch by removing the entire database directory, i.e.
1184
1185 rm -rf ~/.paraslash/afs_database-0.4
1186
1187 Be aware that this removes all attribute definitions, all playlists
1188 and all mood definitions and requires to re-initialize the tables.
1189
1190 Although oslfsck fixes inconsistencies in database tables it doesn't
1191 care about the table contents. To check for invalid table contents, use
1192
1193 para_client check
1194
1195 This prints out references to missing audio files as well as invalid
1196 playlists and mood definitions.
1197
1198 Similarly, para_audiod refuses to start if its socket file exists, since
1199 this indicates that another instance of para_audiod is running. After
1200 a crash a stale socket file might remain and you must run
1201
1202 para_audiod --force
1203
1204 once to fix it up.
1205
1206 =======================================
1207 Audio formats and audio format handlers
1208 =======================================
1209
1210 Audio formats
1211 -------------
1212
1213 The following audio formats are supported by paraslash:
1214
1215 <h3> MP3 </h3>
1216
1217 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1218 designed as part of its MPEG-1 standard. An MP3 file is made up of
1219 multiple MP3 frames, which consist of a header and a data block. The
1220 size of an MP3 frame depends on the bit rate and on the number
1221 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1222 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1223 400 bytes large.
1224
1225 <h3> OGG/Vorbis </h3>
1226
1227 OGG is a standardized audio container format, while Vorbis is an
1228 open source codec for lossy audio compression. Since Vorbis is most
1229 commonly made available via the OGG container format, it is often
1230 referred to as OGG/Vorbis. The OGG container format divides data into
1231 chunks called OGG pages. A typical OGG page is about 4KB large. The
1232 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1233 may vary considerably.
1234
1235 <h3> OGG/Speex </h3>
1236
1237 Speex is an open-source speech codec that is based on CELP (Code
1238 Excited Linear Prediction) coding. It is designed for voice
1239 over IP applications, has modest complexity and a small memory
1240 footprint. Wideband and narrowband (telephone quality) speech are
1241 supported. As for Vorbis audio, Speex bit-streams are often stored
1242 in OGG files. As of 2012 this codec is considered obsolete since the
1243 Oppus codec, described below, surpasses its performance in all areas.
1244
1245 <h3> OGG/Opus </h3>
1246
1247 Opus is a lossy audio compression format standardized through RFC
1248 6716 in 2012. It combines the speech-oriented SILK codec and the
1249 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1250 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1251 containers. All known software patents which cover Opus are licensed
1252 under royalty-free terms.
1253
1254 <h3> AAC </h3>
1255
1256 Advanced Audio Coding (AAC) is a standardized, lossy compression
1257 and encoding scheme for digital audio which is the default audio
1258 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1259 the container format and audio files encoded with AAC have the .m4a
1260 extension. A typical AAC frame is about 700 bytes large.
1261
1262 <h3> WMA </h3>
1263
1264 Windows Media Audio (WMA) is an audio data compression technology
1265 developed by Microsoft. A WMA file is usually encapsulated in the
1266 Advanced Systems Format (ASF) container format, which also specifies
1267 how meta data about the file is to be encoded. The bit stream of WMA
1268 is composed of superframes, each containing one or more frames of
1269 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1270
1271 <h3> FLAC </h3>
1272
1273 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1274 loss. It gives better compression ratios than a general purpose
1275 compressor like zip or bzip2 because FLAC is designed specifically
1276 for audio. A FLAC-encoded file consists of frames of varying size, up
1277 to 16K. Each frame starts with a header that contains all information
1278 necessary to decode the frame.
1279
1280 Meta data
1281 ---------
1282
1283 Unfortunately, each audio format has its own conventions how meta
1284 data is added as tags to the audio file.
1285
1286 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1287 is rather simple but also very limited as it supports only artist,
1288 title, album, year and comment tags. Each of these can only be at most
1289 32 characters long. ID3, version 2 is much more flexible but requires
1290 a separate library being installed for paraslash to support it.
1291
1292 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1293 comments, which are typically implemented as strings of the form
1294 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1295 tags are appropriate for the content.
1296
1297 AAC files usually use the MPEG-4 container format for storing meta
1298 data while WMA files wrap meta data as special objects within the
1299 ASF container format.
1300
1301 paraslash only tracks the most common tags that are supported by
1302 all tag variants: artist, title, year, album, comment. When a file
1303 is added to the AFS database, the meta data of the file is extracted
1304 and stored in the audio file table.
1305
1306 Chunks and chunk tables
1307 -----------------------
1308
1309 paraslash uses the word "chunk" as common term for the building blocks
1310 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1311 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1312 size varies considerably between audio formats, from a few hundred
1313 bytes (MP3) up to 16K (FLAC).
1314
1315 The chunk table contains the offsets within the audio file that
1316 correspond to the chunk boundaries of the file. Like the meta data,
1317 the chunk table is computed and stored in the database whenever an
1318 audio file is added.
1319
1320 The paraslash senders (see below) always send complete chunks. The
1321 granularity for seeking is therefore determined by the chunk size.
1322
1323 Audio format handlers
1324 ---------------------
1325
1326 For each audio format paraslash contains an audio format handler whose
1327 first task is to tell whether a given file is a valid audio file of
1328 this type. If so, the audio file handler extracts some technical data
1329 (duration, sampling rate, number of channels etc.), computes the
1330 chunk table and reads the meta data.
1331
1332 The audio format handler code is linked into para_server and executed
1333 via the _add_ command. The same code is also available as a stand-alone
1334 tool, para_afh, which prints the technical data, the chunk table
1335 and the meta data of a file. Moreover, all audio format handlers are
1336 combined in the afh receiver which is part of para_recv and para_play.
1337
1338 ==========
1339 Networking
1340 ==========
1341
1342 Paraslash uses different network connections for control and data.
1343 para_client communicates with para_server over a dedicated TCP control
1344 connection. To transport audio data, separate data connections are
1345 used. For these data connections, a variety of transports (UDP, DCCP,
1346 HTTP) can be chosen.
1347
1348 The chapter starts with the [control
1349 service](#The.paraslash.control.service), followed by a section
1350 on the various [streaming protocols](#Streaming.protocols)
1351 in which the data connections are described. The way
1352 audio file headers are embedded into the stream is discussed
1353 [briefly](#Streams.with.headers.and.headerless.streams) before the
1354 [example section](#Networking.examples) which illustrates typical
1355 commands for real-life scenarios.
1356
1357 Both IPv4 and IPv6 are supported.
1358
1359 The paraslash control service
1360 -----------------------------
1361
1362 para_server is controlled at runtime via the paraslash control
1363 connection. This connection is used for server commands (play, stop,
1364 ...) as well as for afs commands (ls, select, ...).
1365
1366 The server listens on a TCP port and accepts connections from clients
1367 that connect the open port. Each connection causes the server to fork
1368 off a client process which inherits the connection and deals with that
1369 client only. In this classical accept/fork approach the server process
1370 is unaffected if the child dies or goes crazy for whatever reason. In
1371 fact, the child process can not change address space of server process.
1372
1373 The section on [client-server
1374 authentication](#Client-server.authentication) above described the
1375 early connection establishment from the crypto point of view. Here
1376 it is described what happens after the connection (including crypto
1377 setup) has been established. There are four processes involved during
1378 command dispatch as sketched in the following diagram.
1379
1380 server_host client_host
1381 ~~~~~~~~~~~ ~~~~~~~~~~~
1382
1383 +-----------+ connect +-----------+
1384 |para_server|<------------------------------ |para_client|
1385 +-----------+ +-----------+
1386 | ^
1387 | fork +---+ |
1388 +----------> |AFS| |
1389 | +---+ |
1390 | ^ |
1391 | | |
1392 | | connect (cookie) |
1393 | | |
1394 | | |
1395 | fork +-----+ inherited connection |
1396 +---------->|child|<--------------------------+
1397 +-----+
1398
1399 Note that the child process is not a child of the afs process,
1400 so communication of these two processes has to happen via local
1401 sockets. In order to avoid abuse of the local socket by unrelated
1402 processes, a magic cookie is created once at server startup time just
1403 before the server process forks off the AFS process. This cookie is
1404 known to the server, AFS and the child, but not to unrelated processes.
1405
1406 There are two different kinds of commands: First there are commands
1407 that cause the server to respond with some answer such as the list
1408 of all audio files. All but the addblob commands (addimg, addlyr,
1409 addpl, addmood) are of this kind. The addblob commands add contents
1410 to the database, so they need to transfer data the other way round,
1411 from the client to the server.
1412
1413 There is no knowledge about the server commands built into para_client,
1414 so it does not know about addblob commands. Instead, the server sends
1415 a special "awaiting data" packet for these commands. If the client
1416 receives this packet, it sends STDIN to the server, otherwise it
1417 dumps data from the server to STDOUT.
1418
1419 Streaming protocols
1420 -------------------
1421
1422 A network (audio) stream usually consists of one streaming source,
1423 the _sender_, and one or more _receivers_ which read data over the
1424 network from the streaming source.
1425
1426 Senders are thus part of para_server while receivers are part of
1427 para_audiod. Moreover, there is the stand-alone tool para_recv which
1428 can be used to manually download a stream, either from para_server
1429 or from a web-based audio streaming service.
1430
1431 The following three streaming protocols are supported by paraslash:
1432
1433 - HTTP. Recommended for public streams that can be played by any
1434 player like mpg123, xmms, itunes, winamp, etc. The HTTP sender is
1435 supported on all operating systems and all platforms.
1436
1437 - DCCP. Recommended for LAN streaming. DCCP is currently available
1438 only for Linux.
1439
1440 - UDP. Recommended for multicast LAN streaming.
1441
1442 See the Appendix on [network protocols](/#Network.protocols)
1443 for brief descriptions of the various protocols relevant for network
1444 audio streaming with paraslash.
1445
1446 It is possible to activate more than one sender simultaneously.
1447 Senders can be controlled at run time and via config file and command
1448 line options.
1449
1450 Note that audio connections are _not_ encrypted. Transport or Internet
1451 layer encryption should be used if encrypted data connections are
1452 needed.
1453
1454 Since DCCP and TCP are both connection-oriented protocols, connection
1455 establishment/teardown and access control are very similar between
1456 these two streaming protocols. UDP is the most lightweight option,
1457 since in contrast to TCP/DCCP it is connectionless. It is also the
1458 only protocol supporting IP multicast.
1459
1460 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1461 clients to connect and establish a connection via some protocol-defined
1462 handshake mechanism. Both senders maintain two linked lists each:
1463 The list of all clients which are currently connected, and the list
1464 of access control entries which determines who is allowed to connect.
1465 IP-based access control may be configured through config file and
1466 command line options and via the "allow" and "deny" sender subcommands.
1467
1468 Upon receiving a GET request from the client, the HTTP sender sends
1469 back a status line and a message. The body of this message is the
1470 audio stream. This is common practice and is supported by many popular
1471 clients which can thus be used to play a stream offered by para_server.
1472 For DCCP things are a bit simpler: No messages are exchanged between
1473 the receiver and sender. The client simply connects and the sender
1474 starts to stream.
1475
1476 DCCP is an experimental protocol which offers a number of new features
1477 not available for TCP. Both ends can negotiate these features using
1478 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1479 datagram-based (no retransmissions) and thus should not be used over
1480 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1481 is access to a variety of different congestion-control mechanisms
1482 called CCIDs. Two different CCIDs are available per default on Linux:
1483
1484
1485 - _CCID 2_. A Congestion Control mechanism similar to that of TCP. The
1486 sender maintains a congestion window and halves this window in response
1487 to congestion.
1488
1489
1490 - _CCID-3_. Designed to be fair when competing for bandwidth.
1491 It has lower variation of throughput over time compared with TCP,
1492 which makes it suitable for streaming media.
1493
1494 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1495 single list, the _target list_. This list describes the set of clients
1496 to which the stream is sent. There is no list for access control and
1497 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1498 and "delete" commands can be used to modify the target list.
1499
1500 Since both UDP and DCCP offer an unreliable datagram-based transport,
1501 additional measures are necessary to guard against disruptions over
1502 networks that are lossy or which may be subject to interference (as
1503 is for instance the case with WiFi). Paraslash uses FEC (Forward
1504 Error Correction) to guard against packet losses and reordering. The
1505 stream is FEC-encoded before it is sent through the UDP socket and
1506 must be decoded accordingly on the receiver side.
1507
1508 The packet size and the amount of redundancy introduced by FEC can
1509 be configured via the FEC parameters which are dictated by server
1510 and may also be configured through the "sender" command. The FEC
1511 parameters are encoded in the header of each network packet, so no
1512 configuration is necessary on the receiver side. See the section on
1513 [FEC](#Forward.error.correction) below.
1514
1515 Streams with headers and headerless streams
1516 -------------------------------------------
1517
1518 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1519 needed to decode the stream is only contained in the audio file
1520 header of the container format but not in each data chunk. Clients
1521 must be able to obtain this information in case streaming starts in
1522 the middle of the file or if para_audiod is started while para_server
1523 is already sending a stream.
1524
1525 This is accomplished in different ways, depending on the streaming
1526 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1527 header is sent prior to audio file data. This technique however does
1528 not work for the connectionless UDP transport. Hence the audio file
1529 header is periodically being embedded into the UDP audio data stream.
1530 By default, the header is resent after five seconds. The receiver has
1531 to wait until the next header arrives before it can start decoding
1532 the stream.
1533
1534 Networking examples
1535 -------------------
1536
1537 The "si" (server info) command lists some information about the
1538 currently running server process.
1539
1540 -> Show PIDs, number of connected clients, uptime, and more:
1541
1542 para_client si
1543
1544 The sender command of para_server prints information about senders,
1545 like the various access control lists, and it allows to (de-)activate
1546 senders and to change the access permissions at runtime.
1547
1548 -> List all senders
1549
1550 para_client sender
1551
1552 -> Obtain general help for the sender command:
1553
1554 para_client help sender
1555
1556 -> Get help for a specific sender (contains further examples):
1557
1558 s=http # or dccp or udp
1559 para_client sender $s help
1560
1561 -> Show status of the http sender
1562
1563 para_client sender http status
1564
1565 By default para_server activates both the HTTP and th DCCP sender on
1566 startup. This can be changed via command line options or para_server's
1567 config file.
1568
1569 -> List config file options for senders:
1570
1571 para_server -h
1572
1573 All senders share the "on" and "off" commands, so senders may be
1574 activated and deactivated independently of each other.
1575
1576 -> Switch off the http sender:
1577
1578 para_client sender http off
1579
1580 -> Receive a DCCP stream using CCID2 and write the output into a file:
1581
1582 host=foo.org; ccid=2; filename=bar
1583 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1584
1585 Note the quotes around the arguments for the dccp receiver. Each
1586 receiver has its own set of command line options and its own command
1587 line parser, so arguments for the dccp receiver must be protected
1588 from being interpreted by para_recv.
1589
1590 -> Start UDP multicast, using the default multicast address:
1591
1592 para_client sender udp add 224.0.1.38
1593
1594 -> Receive FEC-encoded multicast stream and write the output into a file:
1595
1596 filename=foo
1597 para_recv -r udp > $filename
1598
1599 -> Add an UDP unicast for a client to the target list of the UDP sender:
1600
1601 t=client.foo.org
1602 para_client sender udp add $t
1603
1604 -> Receive this (FEC-encoded) unicast stream:
1605
1606 filename=foo
1607 para_recv -r 'udp -i 0.0.0.0' > $filename
1608
1609 -> Create a minimal config for para_audiod for HTTP streams:
1610
1611 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1612 echo receiver \".:http -i $s\" > $c
1613 para_audiod --config $c
1614
1615 =======
1616 Filters
1617 =======
1618
1619 A paraslash filter is a module which transforms an input stream into
1620 an output stream. Filters are included in the para_audiod executable
1621 and in the stand-alone tool para_filter which usually contains the
1622 same modules.
1623
1624 While para_filter reads its input stream from STDIN and writes
1625 the output to STDOUT, the filter modules of para_audiod are always
1626 connected to a receiver which produces the input stream and a writer
1627 which absorbs the output stream.
1628
1629 Some filters depend on a specific library and are not compiled in
1630 if this library was not found at compile time. To see the list of
1631 supported filters, run para_filter and para_audiod with the --help
1632 option. The output looks similar to the following:
1633
1634 Available filters:
1635 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1636
1637 Out of these filter modules, a chain of filters can be constructed,
1638 much in the way Unix pipes can be chained, and analogous to the use
1639 of modules in gstreamer: The output of the first filter becomes the
1640 input of the second filter. There is no limitation on the number of
1641 filters and the same filter may occur more than once.
1642
1643 Like receivers, each filter has its own command line options which
1644 must be quoted to protect them from the command line options of
1645 the driving application (para_audiod or para_filter). Example:
1646
1647 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1648
1649 For para_audiod, each audio format has its own set of filters. The
1650 name of the audio format for which the filter should be applied can
1651 be used as the prefix for the filter option. Example:
1652
1653 para_audiod -f 'mp3:prebuffer --duration 300'
1654
1655 The "mp3" prefix above is actually interpreted as a POSIX extended
1656 regular expression. Therefore
1657
1658 para_audiod -f '.:prebuffer --duration 300'
1659
1660 activates the prebuffer filter for all supported audio formats (because
1661 "." matches all audio formats) while
1662
1663 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1664
1665 activates it only for wma and ogg streams.
1666
1667 Decoders
1668 --------
1669
1670 For each supported audio format there is a corresponding filter
1671 which decodes audio data in this format to 16 bit PCM data which
1672 can be directly sent to the sound device or any other software that
1673 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1674 filters are called _decoders_ in general, and xxxdec is the name of
1675 the paraslash decoder for the audio format xxx. For example, the mp3
1676 decoder is called mp3dec.
1677
1678 Note that the output of the decoder is about 10 times larger than
1679 its input. This means that filters that operate on the decoded audio
1680 stream have to deal with much more data than filters that transform
1681 the audio stream before it is fed to the decoder.
1682
1683 Paraslash relies on external libraries for most decoders, so these
1684 libraries must be installed for the decoder to be included in the
1685 executables. For example, the mp3dec filter depends on the mad library.
1686
1687 Forward error correction
1688 ------------------------
1689
1690 As already mentioned [earlier](#Streaming.protocols), paraslash
1691 uses forward error correction (FEC) for the unreliable UDP and
1692 DCCP transports. FEC is a technique which was invented already in
1693 1960 by Reed and Solomon and which is widely used for the parity
1694 calculations of storage devices (RAID arrays). It is based on the
1695 algebraic concept of finite fields, today called Galois fields, in
1696 honour of the mathematician Galois (1811-1832). The FEC implementation
1697 of paraslash is based on code by Luigi Rizzo.
1698
1699 Although the details require a sound knowledge of the underlying
1700 mathematics, the basic idea is not hard to understand: For positive
1701 integers k and n with k < n it is possible to compute for any k given
1702 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1703 ..., p_r such that all data bytes can be reconstructed from *any*
1704 k bytes of the set
1705
1706 {d_1, ..., d_k, p_1, ..., p_r}.
1707
1708 FEC-encoding for unreliable network transports boils down to slicing
1709 the audio stream into groups of k suitably sized pieces called _slices_
1710 and computing the r corresponding parity slices. This step is performed
1711 in para_server which then sends both the data and the parity slices
1712 over the unreliable network connection. If the client was able
1713 to receive at least k of the n = k + r slices, it can reconstruct
1714 (FEC-decode) the original audio stream.
1715
1716 From these observations it is clear that there are three different
1717 FEC parameters: The slice size, the number of data slices k, and the
1718 total number of slices n. It is crucial to choose the slice size
1719 such that no fragmentation of network packets takes place because
1720 FEC only guards against losses and reordering but fails if slices are
1721 received partially.
1722
1723 FEC decoding in paralash is performed through the fecdec filter which
1724 usually is the first filter (there can be other filters before fecdec
1725 if these do not alter the audio stream).
1726
1727 Volume adjustment (amp and compress)
1728 ------------------------------------
1729
1730 The amp and the compress filter both adjust the volume of the audio
1731 stream. These filters operate on uncompressed audio samples. Hence
1732 they are usually placed directly after the decoding filter. Each
1733 sample is multiplied with a scaling factor (>= 1) which makes amp
1734 and compress quite expensive in terms of computing power.
1735
1736 <h3> amp </h3>
1737
1738 The amp filter amplifies the audio stream by a fixed scaling factor
1739 that must be known in advance. For para_audiod this factor is derived
1740 from the amplification field of the audio file's entry in the audio
1741 file table while para_filter uses the value given at the command line.
1742
1743 The optimal scaling factor F for an audio file is the largest real
1744 number F >= 1 such that after multiplication with F all samples still
1745 fit into the sample interval [-32768, 32767]. One can use para_filter
1746 in combination with the sox utility to compute F:
1747
1748 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1749
1750 The amplification value V which is stored in the audio file table,
1751 however, is an integer between 0 and 255 which is connected to F
1752 through the formula
1753
1754 V = (F - 1) * 64.
1755
1756 To store V in the audio file table, the command
1757
1758 para_client -- touch -a=V file.mp3
1759
1760 is used. The reader is encouraged to write a script that performs
1761 these computations :)
1762
1763 <h3> compress </h3>
1764
1765 Unlike the amplification filter, the compress filter adjusts the volume
1766 of the audio stream dynamically without prior knowledge about the peak
1767 value. It maintains the maximal volume of the last n samples of the
1768 audio stream and computes a suitable amplification factor based on that
1769 value and the various configuration options. It tries to chose this
1770 factor such that the adjusted volume meets the desired target level.
1771
1772 Note that it makes sense to combine amp and compress.
1773
1774 Misc filters (wav and prebuffer)
1775 --------------------------------
1776
1777 These filters are rather simple and do not modify the audio stream at
1778 all. The wav filter is only useful with para_filter and in connection
1779 with a decoder. It asks the decoder for the number of channels and the
1780 sample rate of the stream and adds a Microsoft wave header containing
1781 this information at the beginning. This allows to write wav files
1782 rather than raw PCM files (which do not contain any information about
1783 the number of channels and the sample rate).
1784
1785 The prebuffer filter simply delays the output until the given time has
1786 passed (starting from the time the first byte was available in its
1787 input queue) or until the given amount of data has accumulated. It
1788 is mainly useful for para_audiod if the standard parameters result
1789 in buffer underruns.
1790
1791 Both filters require almost no additional computing time, even when
1792 operating on uncompressed audio streams, since data buffers are simply
1793 "pushed down" rather than copied.
1794
1795 Examples
1796 --------
1797
1798 -> Decode an mp3 file to wav format:
1799
1800 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1801
1802 -> Amplify a raw audio file by a factor of 1.5:
1803
1804 para_filter -f amp --amp 32 < foo.raw > bar.raw
1805
1806 ======
1807 Output
1808 ======
1809
1810 Once an audio stream has been received and decoded to PCM format,
1811 it can be sent to a sound device for playback. This part is performed
1812 by paraslash _writers_ which are described in this chapter.
1813
1814 Writers
1815 -------
1816
1817 A paraslash writer acts as a data sink that consumes but does not
1818 produce audio data. Paraslash writers operate on the client side and
1819 are contained in para_audiod and in the stand-alone tool para_write.
1820
1821 The para_write program reads uncompressed audio data from STDIN. If
1822 this data starts with a wav header, sample rate, sample format and
1823 channel count are read from the header. Otherwise CD audio (44.1KHz
1824 16 bit little endian, stereo) is assumed but this can be overridden
1825 by command line options. para_audiod, on the other hand, obtains
1826 the sample rate and the number of channels from the decoder.
1827
1828 Like receivers and filters, each writer has an individual set of
1829 command line options, and for para_audiod writers can be configured
1830 per audio format separately. It is possible to activate more than
1831 one writer for the same stream simultaneously.
1832
1833 OS-dependent APIs
1834 -----------------
1835
1836 Unfortunately, the various flavours of Unix on which paraslash
1837 runs on have different APIs for opening a sound device and starting
1838 playback. Hence for each such API there is a paraslash writer that
1839 can play the audio stream via this API.
1840
1841 - *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1842 Linux systems. Although there are several mid-layer APIs in use by
1843 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1844 currently supports only the low-level ALSA API which is not supposed
1845 to be change. ALSA is very feature-rich, in particular it supports
1846 software mixing via its DMIX plugin. ALSA is the default writer on
1847 Linux systems.
1848
1849 - *OSS*. The _Open Sound System_ is the only API on \*BSD Unixes and
1850 is also available on Linux systems, usually provided by ALSA as an
1851 emulation for backwards compatibility. This API is rather simple but
1852 also limited. For example only one application can open the device
1853 at any time. The OSS writer is activated by default on BSD Systems.
1854
1855 - *FILE*. The file writer allows to capture the audio stream and
1856 write the PCM data to a file on the file system rather than playing
1857 it through a sound device. It is supported on all platforms and is
1858 always compiled in.
1859
1860 - *AO*. _Libao_ is a cross-platform audio library which supports a wide
1861 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1862 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1863 through an output plugin of libao.
1864
1865 Examples
1866 --------
1867
1868 -> Use the OSS writer to play a wav file:
1869
1870 para_write --writer oss < file.wav
1871
1872 -> Enable ALSA software mixing for mp3 streams:
1873
1874 para_audiod --writer 'mp3:alsa -d plug:swmix'
1875
1876
1877 ===
1878 Gui
1879 ===
1880
1881 para_gui executes an arbitrary command which is supposed to print
1882 status information to STDOUT. It then displays this information in
1883 a curses window. By default the command
1884
1885 para_audioc -- stat -p
1886
1887 is executed, but this can be customized via the --stat-cmd option. In
1888 particular it possible to use
1889
1890 para_client -- stat -p
1891
1892 to make para_gui work on systems on which para_audiod is not running.
1893
1894 Key bindings
1895 ------------
1896
1897 It is possible to bind keys to arbitrary commands via custom
1898 key-bindings. Besides the internal keys which can not be changed (help,
1899 quit, loglevel, version...), the following flavours of key-bindings
1900 are supported:
1901
1902 - external: Shutdown curses before launching the given command.
1903 Useful for starting other ncurses programs from within para_gui,
1904 e.g. aumix or dialog scripts. Or, use the mbox output format to write
1905 a mailbox containing one mail for each (admissible) file the audio
1906 file selector knows about. Then start mutt from within para_gui to
1907 browse your collection!
1908
1909 - display: Launch the command and display its stdout in para_gui's
1910 bottom window.
1911
1912 - para: Like display, but start "para_client <specified command>"
1913 instead of "<specified command>".
1914
1915 The general form of a key binding is
1916
1917 key_map k:m:c
1918
1919 which maps key k to command c using mode m. Mode may be x, d or p
1920 for external, display and paraslash commands, respectively.
1921
1922 Themes
1923 ------
1924
1925 Currently there are only two themes for para_gui. It is easy, however,
1926 to add more themes. To create a new theme one has to define the
1927 position, color and geometry for for each status item that should be
1928 shown by this theme. See gui_theme.c for examples.
1929
1930 The "." and "," keys are used to switch between themes.
1931
1932 Examples
1933 --------
1934
1935 -> Show server info:
1936
1937 key_map "i:p:si"
1938
1939 -> Jump to the middle of the current audio file by pressing F5:
1940
1941 key_map "<F5>:p:jmp 50"
1942
1943 -> vi-like bindings for jumping around:
1944
1945 key_map "l:p:ff 10"
1946 key_map "h:p:ff 10-"
1947 key_map "w:p:ff 60"
1948 key_map "b:p:ff 60-"
1949
1950 -> Print the current date and time:
1951
1952 key_map "D:d:date"
1953
1954 -> Call other curses programs:
1955
1956 key_map "U:x:aumix"
1957 key_map "!:x:/bin/bash"
1958 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1959
1960 ===========
1961 Development
1962 ===========
1963
1964 Contributing
1965 ------------
1966
1967 Paraslash is an open source project and contributions are
1968 welcome. Here's a list of things you can do to help the project:
1969
1970 - Report problems with building, installing or running the software.
1971 In particular, test the experimental git branches ("next" and "pu").
1972 This helps to identify and fix problems before the code gets merged
1973 and thus keeps the master branch as stable as possible.
1974 - Proofread the documentation (manual, web pages, man pages, source
1975 code documentation) and point out unclear or poorly written parts. If
1976 you are a native English speaker you will easily find a lot of text
1977 that could be improved.
1978 - Run analysis tools (coverity, afl, sparse, etc.) and report issues
1979 found by those tools.
1980 - Suggest new features you would like to see implemented.
1981 - Compile and test on your favorite architecture or operating
1982 system. The code is tested only on a limited set of systems, so you
1983 will probably encounter problems when building on different systems.
1984 - Post about paraslash on your blog or on social networks.
1985 - Build and maintain Debian/RPM packages for your favorite distribution.
1986
1987 Note that there is no mailing list, no bug tracker and no discussion
1988 forum for paraslash. If you'd like to contribute, or have questions
1989 about contributing, send email to Andre Noll <maan@tuebingen.mpg.de>.
1990 New releases are announced by email. If you would like to receive
1991 these announcements, contact the author through the above address.
1992
1993 Tools
1994 -----
1995
1996 In order to compile the sources from the git repository (rather than
1997 from tar balls) and for contributing non-trivial changes to the
1998 paraslash project, some additional tools should be installed on a
1999 developer machine.
2000
2001 - [git](http://git.or.cz/). As described in more detail
2002 [below](#Git.branches), the git source code management tool is used for
2003 paraslash development. It is necessary for cloning the git repository
2004 and for getting updates.
2005
2006 - [autoconf](ftp://ftp.gnu.org/pub/gnu/autoconf/) GNU autoconf creates
2007 the configure file which is shipped in the tarballs but has to be
2008 generated when compiling from git.
2009
2010 - [discount](http://www.pell.portland.or.us/~orc/Code/discount). The
2011 HTML version of this manual and some of the paraslash web pages are
2012 written in the Markdown markup language and are translated into html
2013 with the converter of the *Discount* package.
2014
2015 - [doxygen](http://www.stack.nl/~dimitri/doxygen/). The documentation
2016 of paraslash's C sources uses the doxygen documentation system. The
2017 conventions for documenting the source code is described in the
2018 [Doxygen section](#Doxygen).
2019
2020 - [global](ftp://ftp.gnu.org/pub/gnu/global). This is used to generate
2021 browsable HTML from the C sources. It is needed by doxygen.
2022
2023 Git branches
2024 ------------
2025
2026 Paraslash has been developed using the git source code management
2027 tool since 2006. Development is organized roughly in the same spirit
2028 as the git development itself, as described below.
2029
2030 The following text passage is based on "A note from the maintainer",
2031 written by Junio C Hamano, the maintainer of git.
2032
2033 There are four branches in the paraslash repository that track the
2034 source tree: "master", "maint", "next", and "pu".
2035
2036 The "master" branch is meant to contain what is well tested and
2037 ready to be used in a production setting. There could occasionally be
2038 minor breakages or brown paper bag bugs but they are not expected to
2039 be anything major, and more importantly quickly and easily fixable.
2040 Every now and then, a "feature release" is cut from the tip of this
2041 branch, named with three dotted decimal digits, like 0.4.2.
2042
2043 Whenever changes are about to be included that will eventually lead to
2044 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
2045 "master" at that point. Obvious, safe and urgent fixes after the major
2046 release are applied to this branch and maintenance releases are cut
2047 from it. New features never go to this branch. This branch is also
2048 merged into "master" to propagate the fixes forward.
2049
2050 A trivial and safe enhancement goes directly on top of "master".
2051 New development does not usually happen on "master", however.
2052 Instead, a separate topic branch is forked from the tip of "master",
2053 and it first is tested in isolation; Usually there are a handful such
2054 topic branches that are running ahead of "master". The tip of these
2055 branches is not published in the public repository to keep the number
2056 of branches that downstream developers need to worry about low.
2057
2058 The quality of topic branches varies widely. Some of them start out as
2059 "good idea but obviously is broken in some areas" and then with some
2060 more work become "more or less done and can now be tested by wider
2061 audience". Luckily, most of them start out in the latter, better shape.
2062
2063 The "next" branch is to merge and test topic branches in the latter
2064 category. In general, this branch always contains the tip of "master".
2065 It might not be quite rock-solid production ready, but is expected to
2066 work more or less without major breakage. The maintainer usually uses
2067 the "next" version of paraslash for his own pleasure, so it cannot
2068 be _that_ broken. The "next" branch is where new and exciting things
2069 take place.
2070
2071 The two branches "master" and "maint" are never rewound, and "next"
2072 usually will not be either (this automatically means the topics that
2073 have been merged into "next" are usually not rebased, and you can find
2074 the tip of topic branches you are interested in from the output of
2075 "git log next"). You should be able to safely build on top of them.
2076
2077 However, at times "next" will be rebuilt from the tip of "master" to
2078 get rid of merge commits that will never be in "master". The commit
2079 that replaces "next" will usually have the identical tree, but it
2080 will have different ancestry from the tip of "master".
2081
2082 The "pu" (proposed updates) branch bundles the remainder of the
2083 topic branches. The "pu" branch, and topic branches that are only in
2084 "pu", are subject to rebasing in general. By the above definition
2085 of how "next" works, you can tell that this branch will contain quite
2086 experimental and obviously broken stuff.
2087
2088 When a topic that was in "pu" proves to be in testable shape, it
2089 graduates to "next". This is done with
2090
2091 git checkout next
2092 git merge that-topic-branch
2093
2094 Sometimes, an idea that looked promising turns out to be not so good
2095 and the topic can be dropped from "pu" in such a case.
2096
2097 A topic that is in "next" is expected to be polished to perfection
2098 before it is merged to "master". Similar to the above, this is
2099 done with
2100
2101 git checkout master
2102 git merge that-topic-branch
2103 git branch -d that-topic-branch
2104
2105 Note that being in "next" is not a guarantee to appear in the next
2106 release (being in "master" is such a guarantee, unless it is later
2107 found seriously broken and reverted), nor even in any future release.
2108
2109 Coding Style
2110 ------------
2111
2112 The preferred coding style for paraslash coincides more or less
2113 with the style of the Linux kernel. So rather than repeating what is
2114 written [there](http://www.kernel.org/doc/Documentation/process/coding-style.rst),
2115 here are the most important points.
2116
2117 - Burn the GNU coding standards.
2118 - Never use spaces for indentation.
2119 - Tabs are 8 characters, and thus indentations are also 8 characters.
2120 - Don't put multiple assignments on a single line.
2121 - Avoid tricky expressions.
2122 - Don't leave whitespace at the end of lines.
2123 - The limit on the length of lines is 80 columns.
2124 - Use K&R style for placing braces and spaces:
2125
2126 if (x is true) {
2127 we do y
2128 }
2129
2130 - Use a space after (most) keywords.
2131 - Do not add spaces around (inside) parenthesized expressions.
2132 - Use one space around (on each side of) most binary and ternary operators.
2133 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2134 - Mixed-case names are frowned upon.
2135 - Descriptive names for global variables are a must.
2136 - Avoid typedefs.
2137 - Functions should be short and sweet, and do just one thing.
2138 - The number of local variables shouldn't exceed 10.
2139 - Gotos are fine if they improve readability and reduce nesting.
2140 - Don't use C99-style "// ..." comments.
2141 - Names of macros defining constants and labels in enums are capitalized.
2142 - Enums are preferred when defining several related constants.
2143 - Always use the paraslash wrappers for allocating memory.
2144 - If the name of a function is an action or an imperative.
2145 command, the function should return an error-code integer
2146 (<0 means error, >=0 means success). If the name is a
2147 predicate, the function should return a "succeeded" boolean.
2148
2149 Doxygen
2150 -------
2151
2152 Doxygen is a documentation system for various programming
2153 languages. The API reference on the paraslash web page is generated
2154 by doxygen.
2155
2156 It is more illustrative to look at the source code for examples than
2157 to describe the conventions in this manual, so we only describe which
2158 parts of the code need doxygen comments, but leave out details on
2159 documentation conventions.
2160
2161 As a rule, only the public part of the C source is documented with
2162 Doxygen. This includes structures, defines and enumerations in header
2163 files as well as public (non-static) C functions. These should be
2164 documented completely. For example, each parameter and the return
2165 value of a public function should get a descriptive doxygen comment.
2166
2167 No doxygen comments are necessary for static functions and for
2168 structures and enumerations in C files (which are used only within
2169 this file). This does not mean, however, that those entities need
2170 no documentation at all. Instead, common sense should be applied to
2171 document what is not obvious from reading the code.
2172
2173 ========
2174 Appendix
2175 ========
2176
2177 Network protocols
2178 -----------------
2179
2180 <h3> IP </h3>
2181
2182 The _Internet Protocol_ is the primary networking protocol used for
2183 the Internet. All protocols described below use IP as the underlying
2184 layer. Both the prevalent IPv4 and the next-generation IPv6 variant
2185 are being deployed actively worldwide.
2186
2187 <h3> Connection-oriented and connectionless protocols </h3>
2188
2189 Connectionless protocols differ from connection-oriented ones in
2190 that state associated with the sending/receiving endpoints is treated
2191 implicitly. Connectionless protocols maintain no internal knowledge
2192 about the state of the connection. Hence they are not capable of
2193 reacting to state changes, such as sudden loss or congestion on the
2194 connection medium. Connection-oriented protocols, in contrast, make
2195 this knowledge explicit. The connection is established only after
2196 a bidirectional handshake which requires both endpoints to agree
2197 on the state of the connection, and may also involve negotiating
2198 specific parameters for the particular connection. Maintaining an
2199 up-to-date internal state of the connection also in general means
2200 that the sending endpoints perform congestion control, adapting to
2201 qualitative changes of the connection medium.
2202
2203 <h3> Reliability </h3>
2204
2205 In IP networking, packets can be lost, duplicated, or delivered
2206 out of order, and different network protocols handle these
2207 problems in different ways. We call a transport-layer protocol
2208 _reliable_, if it turns the unreliable IP delivery into an ordered,
2209 duplicate- and loss-free delivery of packets. Sequence numbers
2210 are used to discard duplicates and re-arrange packets delivered
2211 out-of-order. Retransmission is used to guarantee loss-free
2212 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2213 or data integrity.
2214
2215 <h3> Classification </h3>
2216
2217 With these definitions the protocols which are used by paraslash for
2218 steaming audio data may be classified as follows.
2219
2220 - HTTP/TCP: connection-oriented, reliable,
2221 - UDP: connectionless, unreliable,
2222 - DCCP: connection-oriented, unreliable.
2223
2224 Below we give a short descriptions of these protocols.
2225
2226 <h3> TCP </h3>
2227
2228 The _Transmission Control Protocol_ provides reliable, ordered delivery
2229 of a stream and a classic window-based congestion control. In contrast
2230 to UDP and DCCP (see below), TCP does not have record-oriented or
2231 datagram-based syntax, i.e. it provides a stream which is unaware
2232 and independent of any record (packet) boundaries. TCP is used
2233 extensively by many application layers. Besides HTTP (the Hypertext
2234 Transfer Protocol), also FTP (the File Transfer protocol), SMTP (Simple
2235 Mail Transfer Protocol), SSH (Secure Shell) all sit on top of TCP.
2236
2237 <h3> UDP </h3>
2238
2239 The _User Datagram Protocol_ is the simplest transport-layer protocol,
2240 built as a thin layer directly on top of IP. For this reason, it offers
2241 the same best-effort service as IP itself, i.e. there is no detection
2242 of duplicate or reordered packets. Being a connectionless protocol,
2243 only minimal internal state about the connection is maintained, which
2244 means that there is no protection against packet loss or network
2245 congestion. Error checking and correction (if at all) are performed
2246 in the application.
2247
2248 <h3> DCCP </h3>
2249
2250 The _Datagram Congestion Control Protocol_ combines the
2251 connection-oriented state maintenance known from TCP with the
2252 unreliable, datagram-based transport of UDP. This means that it
2253 is capable of reacting to changes in the connection by performing
2254 congestion control, offering multiple alternative approaches. But it
2255 is bound to datagram boundaries (the maximum packet size supported
2256 by a medium), and like UDP it lacks retransmission to protect
2257 against loss. Due to the use of sequence numbers, it is however
2258 able to react to loss (interpreted as a congestion indication) and
2259 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2260 to negotiate specific, binding features for a connection, such as
2261 the choice of congestion control: classic, window-based congestion
2262 control known from TCP is available as CCID-2, rate-based, "smooth"
2263 congestion control is offered as CCID-3.
2264
2265 <h3> HTTP </h3>
2266
2267 The _Hypertext Transfer Protocol_ is an application layer protocol
2268 on top of TCP. It is spoken by web servers and is most often used
2269 for web services. However, as can be seen by the many Internet radio
2270 stations and YouTube/Flash videos, http is by far not limited to the
2271 delivery of web pages only. Being a simple request/response based
2272 protocol, the semantics of the protocol also allow the delivery of
2273 multimedia content, such as audio over http.
2274
2275 <h3> Multicast </h3>
2276
2277 IP multicast is not really a protocol but a technique for one-to-many
2278 communication over an IP network. The challenge is to deliver
2279 information to a group of destinations simultaneously using the
2280 most efficient strategy to send the messages over each link of the
2281 network only once. This has benefits for streaming multimedia: the
2282 standard one-to-one unicast offered by TCP/DCCP means that n clients
2283 listening to the same stream also consume n-times the resources,
2284 whereas multicast requires to send the stream just once, irrespective
2285 of the number of receivers. Since it would be costly to maintain state
2286 for each listening receiver, multicast often implies connectionless
2287 transport, which is the reason that it is currently only available
2288 via UDP.
2289
2290 Abstract socket namespace
2291 -------------------------
2292 UNIX domain sockets are a traditional way to communicate between
2293 processes on the same machine. They are always reliable (see above)
2294 and don't reorder datagrams. Unlike TCP and UDP, UNIX domain sockets
2295 support passing open file descriptors or process credentials to
2296 other processes.
2297
2298 The usual way to set up a UNIX domain socket (as obtained from
2299 socket(2)) for listening is to first bind the socket to a file system
2300 pathname and then call listen(2), then accept(2). Such sockets are
2301 called _pathname sockets_ because bind(2) creates a special socket
2302 file at the specified path. Pathname sockets allow unrelated processes
2303 to communicate with the listening process by binding to the same path
2304 and calling connect(2).
2305
2306 There are two problems with pathname sockets:
2307
2308 * The listing process must be able to (safely) create the
2309 socket special in a directory which is also accessible to
2310 the connecting process.
2311
2312 * After an unclean shutdown of the listening process, a stale
2313 socket special may reside on the file system.
2314
2315 The abstract socket namespace is a non-portable Linux feature which
2316 avoids these problems. Abstract sockets are still bound to a name,
2317 but the name has no connection with file system pathnames.
2318
2319 License
2320 -------
2321
2322 Paraslash is licensed under the GPL, version 2. Most of the code
2323 base has been written from scratch, and those parts are GPL V2
2324 throughout. Notable exceptions are FEC and the WMA decoder. See the
2325 corresponding source files for licencing details for these parts. Some
2326 code sniplets of several other third party software packages have
2327 been incorporated into the paraslash sources, for example log message
2328 coloring was taken from the git sources. These third party software
2329 packages are all published under the GPL or some other license
2330 compatible to the GPL.
2331
2332 Acknowledgements
2333 ----------------
2334
2335 Many thanks to Gerrit Renker who read an early draft of this manual
2336 and contributed significant improvements.
2337
2338 ==========
2339 References
2340 ==========
2341
2342 Articles
2343 --------
2344 - [Polynomial Codes over Certain Finite
2345 Fields](http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF) by Reed, Irving
2346 S.; Solomon, Gustave (1960), Journal of the Society for Industrial
2347 and Applied Mathematics (SIAM) 8 (2): 300-304, doi:10.1137/0108018)
2348
2349 RFCs
2350 ----
2351
2352 - [RFC 768](http://www.ietf.org/rfc/rfc768.txt) (1980): User Datagram
2353 Protocol
2354
2355 - [RFC 791](http://www.ietf.org/rfc/rfc791.txt) (1981): Internet
2356 Protocol
2357
2358 - [RFC 2437](http://www.ietf.org/rfc/rfc2437.txt) (1998): RSA
2359 Cryptography Specifications
2360
2361 - [RFC 4340](http://www.ietf.org/rfc/rfc4340.txt) (2006): Datagram
2362 Congestion Control Protocol (DCCP)
2363
2364 - [RFC 4341](http://www.ietf.org/rfc/rfc4341.txt) (2006): Congestion
2365 Control ID 2: TCP-like Congestion Control
2366
2367 - [RFC 4342](http://www.ietf.org/rfc/rfc4342.txt) (2006): Congestion
2368 Control ID 3: TCP-Friendly Rate Control (TFRC)
2369
2370 - [RFC 6716](http://www.ietf.org/rfc/rfc6716.txt) (2012): Definition
2371 of the Opus Audio Codec
2372
2373 Application web pages
2374 ---------------------
2375
2376 - [paraslash](http://people.tuebingen.mpg.de/maan/paraslash/)
2377 - [xmms](http://xmms2.org/wiki/Main_Page)
2378 - [mpg123](http://www.mpg123.de/)
2379 - [gstreamer](http://gstreamer.freedesktop.org/)
2380 - [icecast](http://www.icecast.org/)
2381 - [Audio Compress](http://beesbuzz.biz/code/audiocompress.php)
2382
2383 External documentation
2384 ----------------------
2385
2386 - [The mathematics of
2387 Raid6](http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf)
2388 by H. Peter Anvin
2389
2390 - [Effective Erasure Codes for reliable Computer Communication
2391 Protocols](http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz) by Luigi
2392 Rizzo
2393
2394 Code
2395 ----
2396 - [Original FEC
2397 implementation](http://info.iet.unipi.it/~luigi/vdm.tar.gz) by
2398 Luigi Rizzo)