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[paraslash.git] / web / manual.m4
1 dnl To generate the html version, execute
2 dnl m4 web/manual.m4 | grutatxt --toc
3
4 define(`LOCAL_LINK_NAME', `translit(`$1', `A-Z
5 ', `a-z__')')
6 define(`REMOVE_NEWLINE', `translit(`$1',`
7 ', ` ')')
8
9 define(`REFERENCE', ./``#''`LOCAL_LINK_NAME($1)' (`REMOVE_NEWLINE($2)'))
10 define(`XREFERENCE', `$1' (`REMOVE_NEWLINE($2)'))
11 define(`EMPH', ``_''`REMOVE_NEWLINE($1)'``_'')
12
13 Paraslash user manual
14 =====================
15
16 This document describes how to install, configure and use the paraslash
17 network audio streaming system. Most chapters start with a chapter
18 overview and conclude with an example section. We try to focus on
19 general concepts and on the interaction of the various pieces of the
20 paraslash package. Hence this user manual is not meant as a replacement
21 for the manual pages that describe all command line options of each
22 paraslash executable.
23
24 ------------
25 Introduction
26 ------------
27
28 In this chapter we give an REFERENCE(Overview, overview) of the
29 interactions of the two main programs contained in the paraslash
30 package, followed by REFERENCE(The paraslash executables, brief
31 descriptions) of all executables.
32
33 Overview
34 ~~~~~~~~
35
36 The core functionality of the para suite is provided by two main
37 executables, para_server and para_audiod. The former maintains a
38 database of audio files and streams these files to para_audiod which
39 receives and plays the stream.
40
41 In a typical setting, both para_server and para_audiod act as
42 background daemons whose functionality is controlled by client
43 programs: the para_audioc client controls para_audiod over a local
44 socket while the para_client program connects to para_server over a
45 local or remote networking connection.
46
47 Typically, these two daemons run on different hosts but a local setup
48 is also possible.
49
50 A simplified picture of a typical setup is as follows
51 <<
52 <pre>
53 server_host client_host
54 ~~~~~~~~~~~ ~~~~~~~~~~~
55
56 +-----------+ audio stream +-----------+
57 |para_server| -----------------------------> |para_audiod|
58 +-----------+ +-----------+
59 ^ ^
60 | |
61 | | connect
62 | |
63 | |
64 | +-----------+
65 | |para_audioc|
66 | +-----------+
67 |
68 |
69 | connect +-----------+
70 +-------------------------------------- |para_client|
71 +-----------+
72 </pre>
73 >>
74
75 The paraslash executables
76 ~~~~~~~~~~~~~~~~~~~~~~~~~
77
78 *para_server*
79
80 para_server streams binary audio data (MP3, ...) over local and/or
81 remote networks. It listens on a TCP port and accepts commands such
82 as play, stop, pause, next from authenticated clients. There are
83 many more commands though, see the man page of para_server for a
84 description of all commands.
85
86 It supports three built-in network streaming protocols
87 (senders/receivers): HTTP, DCCP, or UDP. This is explained in more
88 detail in the section on REFERENCE(Networking, networking).
89
90 The built-in audio file selector of paraslash is used to manage your
91 audio files. It maintains statistics on the usage of all available
92 audio files such as last-played time, and the number of times each
93 file was selected.
94
95 Additional information may be added to the database to allow
96 fine-grained selection based on various properties of the audio file,
97 including information found in (ID3) tags. However, old-fashioned
98 playlists are also supported.
99
100 It is also possible to store images (album covers) and lyrics in the
101 database and associate these to the corresponding audio files.
102
103 The section on the REFERENCE(The audio file selector, audio file
104 selector) discusses this topic.
105
106
107 *para_client*
108
109 The client program to connect to para_server. paraslash commands
110 are sent to para_server and the response is dumped to STDOUT. This
111 can be used by any scripting language to produce user interfaces with
112 little programming effort.
113
114 All connections between para_server and para_client are encrypted
115 with a symmetric session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
117
118 If para_client is started without non-option arguments, an interactive
119 session (shell) is started. Command history and command completion are
120 supported through libreadline.
121
122 *para_audiod*
123
124 The local daemon that collects information from para_server.
125
126 It runs on the client side and connects to para_server. As soon as
127 para_server announces the availability of an audio stream, para_audiod
128 starts an appropriate receiver, any number of filters and a paraslash
129 writer to play the stream.
130
131 Moreover, para_audiod listens on a local socket and sends status
132 information about para_server and para_audiod to local clients on
133 request. Access via this local socket may be restricted by using Unix
134 socket credentials, if available.
135
136
137 *para_audioc*
138
139 The client program which talks to para_audiod. Used to control
140 para_audiod, to receive status info, or to grab the stream at any
141 point of the decoding process. Like para_client, para_audioc supports
142 interactive sessions on systems with libreadline.
143
144 *para_recv*
145
146 A command line HTTP/DCCP/UDP stream grabber. The http mode is
147 compatible with arbitrary HTTP streaming sources (e.g. icecast).
148 In addition to the three network streaming modes, para_recv can also
149 operate in local (afh) mode. In this mode it writes the content of
150 an audio file on the local file system in complete chunks to stdout,
151 optionally 'just in time'. This allows to cut an audio file without
152 first decoding it, and it enables third-party software which is unaware
153 of the particular audio format to send complete frames in real time.
154
155 *para_filter*
156
157 A filter program that reads from STDIN and writes to STDOUT.
158 Like para_recv, this is an atomic building block which can be used to
159 assemble higher-level audio receiving facilities. It combines several
160 different functionalities in one tool: decoders for multiple audio
161 formats and a number of processing filters, among these a normalizer
162 for audio volume.
163
164 *para_afh*
165
166 A small stand-alone program that prints tech info about the given
167 audio file to STDOUT. It can be instructed to print a "chunk table",
168 an array of offsets within the audio file.
169
170 *para_write*
171
172 A modular audio stream writer. It supports a simple file writer
173 output plug-in and optional WAV/raw players for ALSA (Linux) and for
174 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
175 or raw audio player.
176
177 *para_play*
178
179 A command line audio player.
180
181 *para_gui*
182
183 Curses-based gui that presents status information obtained in a curses
184 window. Appearance can be customized via themes. para_gui provides
185 key-bindings for the most common server commands and new key-bindings
186 can be added easily.
187
188
189 *para_fade*
190
191 An alarm clock and volume-fader for OSS and ALSA.
192
193 -----------
194 Quick start
195 -----------
196
197 This chapter lists the REFERENCE(Requirements, necessary software)
198 that must be installed to compile the paraslash package, describes
199 how to REFERENCE(Installation, compile and install) the paraslash
200 source code and the steps that have to be performed in order to
201 REFERENCE(Quick start, set up) a typical server and client.
202
203 Requirements
204 ~~~~~~~~~~~~
205 For the impatient:
206
207 git clone git://git.tuebingen.mpg.de/osl
208 cd osl && make && sudo make install && sudo ldconfig
209 sudo apt-get install autoconf libssl-dev help2man gengetopt \
210 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
211 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev \
212 libasound2-dev libao-dev libreadline-dev libncurses-dev \
213 libopus-dev
214
215 Detailed description: In any case you'll need
216
217 - XREFERENCE(http://people.tuebingen.mpg.de/maan/osl/, libosl).
218 The _object storage layer_ library is used by para_server. To
219 clone the source code repository, execute
220
221 git clone git://git.tuebingen.mpg.de/osl
222
223 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc) or
224 XREFERENCE(http://clang.llvm.org, clang). All gcc versions
225 >= 3.3 are currently supported. Clang version 1.1 or newer
226 should work as well.
227
228 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
229 also shipped with the disto. On BSD systems the gnu make
230 executable is often called gmake.
231
232 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
233 scripts which run during compilation require the EMPH(Bourne
234 again shell). It is most likely already installed.
235
236 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gengetopt/, gengetopt)
237 is needed to generate the C code for the command line parsers
238 of all paraslash executables.
239
240 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/help2man, help2man)
241 is used to create the man pages.
242
243 Optional:
244
245 - XREFERENCE(http://www.openssl.org/, openssl) or
246 XREFERENCE(ftp://ftp.gnupg.org/gcrypt/libgcrypt/, libgcrypt).
247 At least one of these two libraries is needed as the backend
248 for cryptographic routines on both the server and the client
249 side. Both openssl and libgcrypt are usually shipped with the
250 distro, but you might have to install the development package
251 (libssl-dev or libgcrypt-dev on debian systems) as well.
252
253 - XREFERENCE(http://www.underbit.com/products/mad/, libmad).
254 To compile in MP3 support for paraslash, the development
255 package must be installed. It is called libmad0-dev on
256 debian-based systems. Note that libmad is not necessary on
257 the server side, i.e. for sending MP3 files.
258
259 - XREFERENCE(http://www.underbit.com/products/mad/,
260 libid3tag). For version-2 ID3 tag support, you'll need
261 the libid3tag development package libid3tag0-dev. Without
262 libid3tag, only version one tags are recognized.
263
264 - XREFERENCE(http://www.xiph.org/downloads/, ogg vorbis).
265 For ogg vorbis streams you'll need libogg, libvorbis,
266 libvorbisfile. The corresponding Debian packages are called
267 libogg-dev and libvorbis-dev.
268
269 - XREFERENCE(http://www.audiocoding.com/, libfaad). For aac
270 files (m4a) you'll need libfaad (libfaad-dev).
271
272 - XREFERENCE(http://www.speex.org/, speex). In order to stream
273 or decode speex files, libspeex (libspeex-dev) is required.
274
275 - XREFERENCE(http://flac.sourceforge.net/, flac). To stream
276 or decode files encoded with the _Free Lossless Audio Codec_,
277 libFLAC (libFLAC-dev) must be installed.
278
279 - XREFERENCE(http://www.mega-nerd.com/SRC/index.html,
280 libsamplerate). The resample filter will only be compiled if
281 this library is installed. Debian package: libsamplerate-dev.
282
283 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
284 Linux, you'll need to have ALSA's development package
285 libasound2-dev installed.
286
287 - XREFERENCE(http://downloads.xiph.org/releases/ao/,
288 libao). Needed to build the ao writer (ESD, PulseAudio,...).
289 Debian package: libao-dev.
290
291 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/ncurses, curses). Needed
292 for para_gui. Debian package: libncurses-dev.
293
294 - XREFERENCE(http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html,
295 GNU Readline). If this library (libreadline-dev) is installed,
296 para_client, para_audioc and para_play support interactive
297 sessions.
298
299 Installation
300 ~~~~~~~~~~~~
301 To build the sources from a tarball, execute
302
303 ./configure && make
304
305 To build from git or a gitweb snapshot, run this command instead:
306
307 ./autogen.sh
308
309 There should be no errors but probably some warnings about missing
310 packages which usually implies that not all audio formats will be
311 supported. If headers or libs are installed at unusual locations you
312 might need to tell the configure script where to find them. Try
313
314 ./configure --help
315
316 to see a list of options. If the paraslash package was compiled
317 successfully, execute (optionally)
318
319 make test
320
321 to run the paraslash test suite. If all tests pass, execute as root
322
323 make install
324
325 to install executables under /usr/local/bin and the man pages under
326 /usr/local/man.
327
328 Configuration
329 ~~~~~~~~~~~~~
330
331 *Step 1*: Create a paraslash user
332
333 In order to control para_server at runtime you must create a paraslash
334 user. As authentication is based on the RSA crypto system you'll have
335 to create an RSA key pair. If you already have a user and an RSA key
336 pair, you may skip this step.
337
338 In this section we'll assume a typical setup: You would like to run
339 para_server on some host called server_host as user foo, and you want
340 to connect to para_server from another machine called client_host as
341 user bar.
342
343 As foo@server_host, create ~/.paraslash/server.users by typing the
344 following commands:
345
346 user=bar
347 target=~/.paraslash/server.users
348 key=~/.paraslash/id_rsa.pub.$user
349 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
350 mkdir -p ~/.paraslash
351 echo "user $user $key $perms" >> $target
352
353 Next, change to the "bar" account on client_host and generate the
354 key pair with the commands
355
356 ssh-keygen -q -t rsa -b 2048 -N '' -f $key
357
358 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
359 that para_server won't accept keys shorter than 2048 bits. Moreover,
360 para_client rejects private keys which are world-readable.
361
362 para_server only needs to know the public key of the key pair just
363 created. Copy this public key to server_host:
364
365 src=~/.ssh/id_rsa.pub
366 dest=.paraslash/id_rsa.pub.$LOGNAME
367 scp $src foo@server_host:$dest
368
369 Finally, tell para_client to connect to server_host:
370
371 conf=~/.paraslash/client.conf
372 echo 'hostname server_host' > $conf
373
374
375 *Step 2*: Start para_server
376
377 Before starting the server make sure you have write permissions to
378 the directory /var/paraslash that has been created during installation:
379
380 sudo chown $LOGNAME /var/paraslash
381
382 Alternatively, use the --afs-socket Option to specify a different
383 location for the AFS command socket.
384
385 For this first try, we'll use the info loglevel to make the output
386 of para_server more verbose.
387
388 para_server -l info
389
390 Now you can use para_client to connect to the server and issue
391 commands. Open a new shell as bar@client_host and try
392
393 para_client help
394 para_client si
395
396 to retrieve the list of available commands and some server info.
397 Don't proceed if this doesn't work.
398
399 *Step 3*: Create and populate the database
400
401 An empty database is created with
402
403 para_client init
404
405 This initializes a couple of empty tables under
406 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
407 tables, but it's good to know that you can start from scratch with
408
409 rm -rf ~/.paraslash/afs_database-0.4
410
411 in case something went wrong.
412
413 Next, you need to add some audio files to that database so that
414 para_server knows about them. Choose an absolute path to a directory
415 containing some audio files and add them to the audio file table:
416
417 para_client add /my/mp3/dir
418
419 This might take a while, so it is a good idea to start with a directory
420 containing not too many files. Note that the table only contains data
421 about the audio files found, not the files themselves.
422
423 You may print the list of all known audio files with
424
425 para_client ls
426
427 *Step 4*: Configure para_audiod
428
429 para_audiod needs to create a "well-known" socket for the clients to
430 connect to. The default path for this socket is
431
432 /var/paraslash/audiod_socket.$HOSTNAME
433
434 In order to make this directory writable for para_audiod, execute
435 as bar@client_host
436
437 sudo chown $LOGNAME /var/paraslash
438
439
440 We will also have to tell para_audiod that it should receive the
441 audio stream from server_host via http:
442
443 para_audiod -l info -r '.:http -i server_host'
444
445 You should now be able to listen to the audio stream once para_server
446 starts streaming. To activate streaming, execute
447
448 para_client play
449
450 Since no playlist has been specified yet, the "dummy" mode which
451 selects all known audio files is activated automatically. See the
452 section on the REFERENCE(The audio file selector, audio file selector)
453 for how to use playlists and moods to specify which files should be
454 streamed in which order.
455
456 *Troubleshooting*
457
458 It did not work? To find out why, try to receive, decode and play the
459 stream manually using para_recv, para_filter and para_write as follows.
460
461 For simplicity we assume that you're running Linux/ALSA and that only
462 MP3 files have been added to the database.
463
464 para_recv -r 'http -i server_host' > file.mp3
465 # (interrupt with CTRL+C after a few seconds)
466 ls -l file.mp3 # should not be empty
467 para_filter -f mp3dec -f wav < file.mp3 > file.wav
468 ls -l file.wav # should be much bigger than file.mp3
469 para_write -w alsa < file.wav
470
471 Double check what is logged by para_server and use the --loglevel
472 option of para_recv, para_filter and para_write to increase verbosity.
473
474 ---------------
475 User management
476 ---------------
477
478 para_server uses a challenge-response mechanism to authenticate
479 requests from incoming connections, similar to ssh's public key
480 authentication method. Authenticated connections are encrypted using
481 a stream cipher, either RC4 or AES in integer counter mode.
482
483 In this chapter we briefly describe RSA, RC4 and AES, and sketch the
484 REFERENCE(Client-server authentication, authentication handshake)
485 between para_client and para_server. User management is discussed
486 in the section on REFERENCE(The user_list file, the user_list file).
487 These sections are all about communication between the client and the
488 server. Connecting para_audiod is a different matter and is described
489 in a REFERENCE(Connecting para_audiod, separate section).
490
491
492
493 RSA, RC4, AES
494 ~~~~~~~~~~~~~
495
496 RSA is an asymmetric block cipher which is used in many applications,
497 including ssh and gpg. An RSA key consists in fact of two keys,
498 called the public key and the private key. A message can be encrypted
499 with either key and only the counterpart of that key can decrypt
500 the message. While RSA can be used for both signing and encrypting
501 a message, paraslash uses RSA only for the latter purpose. The
502 RSA public key encryption and signatures algorithms are defined in
503 detail in RFC 2437.
504
505 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
506 key stream to produce the output. Decryption uses the same function
507 calls as encryption. While RC4 supports variable key lengths,
508 paraslash uses a fixed length of 256 bits, which is considered a
509 strong encryption by today's standards. Since the same key must never
510 be used twice, a different, randomly-generated key is used for every
511 new connection.
512
513 AES, the advanced encryption standard, is a well-known symmetric block
514 cipher, i.e. a transformation operating on fixed-length blocks which
515 is determined by a single key for both encryption and decryption. Any
516 block cipher can be turned into a stream cipher by generating
517 a pseudo-random key stream by encrypting successive values of a
518 counter. The AES_CTR128 stream cipher used in paraslash is obtained
519 in this way from the AES block cipher with a 128 bit block size.
520
521
522 Client-server authentication
523 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
524
525 The authentication handshake between para_client and para_server goes
526 as follows:
527
528 - para_client connects to para_server and sends an
529 authentication request for a user. It does so by connecting
530 to TCP port 2990 of the server host. This port is called the
531 para_server _control port_.
532
533 - para_server accepts the connection and forks a child process
534 which handles the incoming request. The parent process keeps
535 listening on the control port while the child process (also
536 called para_server below) continues as follows.
537
538 - para_server loads the RSA public key of that user, fills a
539 fixed-length buffer with random bytes, encrypts that buffer
540 using the public key and sends the encrypted buffer to the
541 client. The first part of the buffer is the challenge which
542 is used for authentication while the second part is the
543 session key.
544
545 - para_client receives the encrypted buffer and decrypts it
546 with the user's private key, thereby obtaining the challenge
547 buffer and the session key. It sends the SHA1 hash value of
548 the challenge back to para_server and stores the session key
549 for further use.
550
551 - para_server also computes the SHA1 hash of the challenge
552 and compares it against what was sent back by the client.
553
554 - If the two hashes do not match, the authentication has
555 failed and para_server closes the connection.
556
557 - Otherwise the user is considered authenticated and the client
558 is allowed to proceed by sending a command to be executed. From
559 this point on the communication is encrypted using the stream
560 cipher with the session key known to both peers.
561
562 paraslash relies on the quality of the pseudo-random bytes provided
563 by the crypto library (openssl or libgcrypt), on the security of
564 the implementation of the RSA, RC4 and AES crypto routines and on the
565 infeasibility to invert the SHA1 function.
566
567 Neither para_server or para_client create RSA keys on their own. This
568 has to be done once for each user as sketched in REFERENCE(Quick start,
569 Quick start) and discussed in more detail REFERENCE(The user_list
570 file, below).
571
572 The user_list file
573 ~~~~~~~~~~~~~~~~~~
574
575 At startup para_server reads the user list file which contains one
576 line per user. The default location of the user list file may be
577 changed with the --user-list option.
578
579 There should be at least one user in this file. Each user must have
580 an RSA key pair. The public part of the key is needed by para_server
581 while the private key is needed by para_client. Each line of the
582 user list file must be of the form
583
584 user <username> <key> <perms>
585
586 where _username_ is an arbitrary string (usually the user's login
587 name), _key_ is the full path to that user's public RSA key, and
588 _perms_ is a comma-separated list of zero or more of the following
589 permission bits:
590
591 +---------------------------------------------------------+
592 | AFS_READ | read the contents of the databases |
593 +-----------+---------------------------------------------+
594 | AFS_WRITE | change database contents |
595 +-----------+---------------------------------------------+
596 | VSS_READ | obtain information about the current stream |
597 +-----------+---------------------------------------------+
598 | VSS_WRITE | change the current stream |
599 +---------------------------------------------------------+
600
601 The permission bits specify which commands the user is allowed to
602 execute. The output of
603
604 para_client help
605
606 contains in the third column the permissions needed to execute the
607 command.
608
609 It is possible to make para_server reread the user_list file by
610 executing the paraslash "hup" command or by sending SIGHUP to the
611 PID of para_server.
612
613
614 Connecting para_audiod
615 ~~~~~~~~~~~~~~~~~~~~~~
616
617 para_audiod listens on a Unix domain socket. Those sockets are
618 for local communication only, so only local users can connect to
619 para_audiod. The default is to let any user connect but this can be
620 restricted on platforms that support UNIX socket credentials which
621 allow para_audiod to obtain the Unix credentials of the connecting
622 process.
623
624 Use para_audiod's --user-allow option to allow connections only for
625 a limited set of users.
626
627 -----------------------
628 The audio file selector
629 -----------------------
630
631 paraslash comes with a sophisticated audio file selector (AFS),
632 whose main task is to determine which file to stream next, based on
633 information on the audio files stored in a database. It communicates
634 also with para_client whenever an AFS command is executed, for example
635 to answer a database query.
636
637 Besides the traditional playlists, AFS supports audio file selection
638 based on _moods_ which act as a filter that limits the set of all
639 known audio files to those which satisfy certain criteria. It also
640 maintains tables containing images (e.g. album cover art) and lyrics
641 that can be associated with one or more audio files.
642
643 AFS uses XREFERENCE(http://people.tuebingen.mpg.de/maan/osl/, libosl), the
644 object storage layer library, as the backend library for storing
645 information on audio files, playlists, etc. This library offers
646 functionality similar to a relational database, but is much more
647 lightweight than a full database backend.
648
649 In this chapter we sketch the setup of the REFERENCE(The AFS process,
650 AFS process) during server startup and proceed with the description
651 of the REFERENCE(Database layout, layout) of the various database
652 tables. The section on REFERENCE(Playlists and moods, playlists
653 and moods) explains these two audio file selection mechanisms
654 in detail and contains pratical examples. The way REFERENCE(File
655 renames and content changes, file renames and content changes) are
656 detected is discussed briefly before the REFERENCE(Troubleshooting,
657 Troubleshooting) section concludes the chapter.
658
659 The AFS process
660 ~~~~~~~~~~~~~~~
661
662 On startup, para_server forks to create the AFS process which opens
663 the OSL database tables. The server process communicates with the
664 AFS process via pipes and shared memory. Usually, the AFS process
665 awakes only briefly whenever the current audio file changes. The AFS
666 process determines the next audio file, opens it, verifies it has
667 not been changed since it was added to the database and passes the
668 open file descriptor to the server process, along with audio file
669 meta-data such as file name, duration, audio format and so on. The
670 server process then starts to stream the audio file.
671
672 The AFS process also accepts connections from local clients via
673 a well-known socket. However, only child processes of para_server
674 may connect through this socket. All server commands that have the
675 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
676 change the database.
677
678 Database layout
679 ~~~~~~~~~~~~~~~
680
681 *The audio file table*
682
683 This is the most important and usually also the largest table of the
684 AFS database. It contains the information needed to stream each audio
685 file. In particular the following data is stored for each audio file.
686
687 - SHA1 hash value of the audio file contents. This is computed
688 once when the file is added to the database. Whenever AFS
689 selects this audio file for streaming the hash value is
690 recomputed and checked against the value stored in the
691 database to detect content changes.
692
693 - The time when this audio file was last played.
694
695 - The number of times the file has been played so far.
696
697 - The attribute bitmask.
698
699 - The image id which describes the image associated with this
700 audio file.
701
702 - The lyrics id which describes the lyrics associated with
703 this audio file.
704
705 - The audio format id (MP3, OGG, ...).
706
707 - An amplification value that can be used by the amplification
708 filter to pre-amplify the decoded audio stream.
709
710 - The chunk table. It describes the location and the timing
711 of the building blocks of the audio file. This is used by
712 para_server to send chunks of the file at appropriate times.
713
714 - The duration of the audio file.
715
716 - Tag information contained in the audio file (ID3 tags,
717 Vorbis comments, ...).
718
719 - The number of channels
720
721 - The encoding bitrate.
722
723 - The sampling frequency.
724
725 To add or refresh the data contained in the audio file table, the _add_
726 command is used. It takes the full path of either an audio file or a
727 directory. In the latter case, the directory is traversed recursively
728 and all files which are recognized as valid audio files are added to
729 the database.
730
731 *The attribute table*
732
733 The attribute table contains two columns, _name_ and _bitnum_. An
734 attribute is simply a name for a certain bit number in the attribute
735 bitmask of the audio file table.
736
737 Each of the 64 bits of the attribute bitmask can be set for each
738 audio file individually. Hence up to 64 different attributes may be
739 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
740 "german_lyrics", "speech", whatever. You are free to choose as
741 many attributes as you like and there are no naming restrictions
742 for attributes.
743
744 A new attribute "test" is created by
745
746 para_client addatt test
747 and
748 para_client lsatt
749
750 lists all available attributes. You can set the "test" attribute for
751 an audio file by executing
752
753 para_client setatt test+ /path/to/the/audio/file
754
755 Similarly, the "test" bit can be removed from an audio file with
756
757 para_client setatt test- /path/to/the/audio/file
758
759 Instead of a path you may use a shell wildcard pattern. The attribute
760 is applied to all audio files matching this pattern:
761
762 para_client setatt test+ '/test/directory/*'
763
764 The command
765
766 para_client -- ls -lv
767
768 gives you a verbose listing of your audio files also showing which
769 attributes are set.
770
771 In case you wonder why the double-dash in the above command is needed:
772 It tells para_client to not interpret the options after the dashes. If
773 you find this annoying, just say
774
775 alias para='para_client --'
776
777 and be happy. In what follows we shall use this alias.
778
779 The "test" attribute can be dropped from the database with
780
781 para rmatt test
782
783 Read the output of
784
785 para help ls
786 para help setatt
787
788 for more information and a complete list of command line options to
789 these commands.
790
791 *Blob tables*
792
793 The image, lyrics, moods and playlists tables are all blob tables.
794 Blob tables consist of three columns each: The identifier which is
795 a positive non-negative number that is auto-incremented, the name
796 (an arbitrary string) and the content (the blob).
797
798 All blob tables support the same set of actions: cat, ls, mv, rm
799 and add. Of course, _add_ is used for adding new blobs to the table
800 while the other actions have the same meaning as the corresponding
801 Unix commands. The paraslash commands to perform these actions are
802 constructed as the concatenation of the table name and the action. For
803 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
804 manipulate or query the image table.
805
806 The add variant of these commands is special as these commands read
807 the blob contents from stdin. To add an image to the image table the
808 command
809
810 para addimg image_name < file.jpg
811
812 can be used.
813
814 Note that the images and lyrics are not interpreted at all, and also
815 the playlist and the mood blobs are only investigated when the mood
816 or playlist is activated with the select command.
817
818 *The score table*
819
820 Unlike all other tables the contents of the score table remain in
821 memory and are never stored on disk. The score table contains two
822 columns: The SHA1 hash value (of an audio file) and its current
823 score.
824
825 However, only those files which are admissible for the current mood
826 or playlist are contained in the score table. The audio file selector
827 always chooses the row with the highest score as the file to stream
828 next. While doing so, it computes the new score and updates the
829 last_played and the num_played fields in the audio file table.
830
831 The score table is recomputed by the select command which loads a
832 mood or playlist. Audio files are chosen for streaming from the rows
833 of the score table on a highest-score-first basis.
834
835
836 Playlists and moods
837 ~~~~~~~~~~~~~~~~~~~
838
839 Playlists and moods offer two different ways of specifying the set of
840 admissible files. A playlist in itself describes a set of admissible
841 files. A mood, in contrast, describes the set of admissible files in
842 terms of attributes and other type of information available in the
843 audio file table. As an example, a mood can define a filename pattern,
844 which is then matched against the names of audio files in the table.
845
846 *Playlists*
847
848 Playlists are accommodated in the playlist table of the afs database,
849 using the aforementioned blob format for tables. A new playlist is
850 created with the addpl command by specifying the full (absolute)
851 paths of all desired audio files, separated by newlines. Example:
852
853 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
854
855 If _my_playlist_ already exists it is overwritten. To activate the
856 new playlist, execute
857
858 para select p/my_playlist
859
860 The audio file selector will assign scores to each entry of the list,
861 in descending order so that files will be selected in order. If a
862 file could not be opened for streaming, its entry is removed from
863 the score table (but not from the playlist).
864
865 *Moods*
866
867 A mood consists of a unique name and its *mood definition*, which is
868 a set of *mood lines* containing expressions in terms of attributes
869 and other data contained in the database.
870
871 At any time at most one mood can be *active* which means that
872 para_server is going to select only files from that subset of
873 admissible files.
874
875 So in order to create a mood definition one has to write a set of
876 mood lines. Mood lines come in three flavours: Accept lines, deny
877 lines and score lines.
878
879 The general syntax of the three types of mood lines is
880
881
882 accept [with score <score>] [if] [not] <mood_method> [options]
883 deny [with score <score>] [if] [not] <mood_method> [options]
884 score <score> [if] [not] <mood_method> [options]
885
886
887 Here <score> is either an integer or the string "random" which assigns
888 a random score to all matching files. The score value changes the
889 order in which admissible files are going to be selected, but is of
890 minor importance for this introduction.
891
892 So we concentrate on the first two forms, i.e. accept and deny
893 lines. As usual, everything in square brackets is optional, i.e.
894 accept/deny lines take the following form when ignoring scores:
895
896 accept [if] [not] <mood_method> [options]
897
898 and analogously for the deny case. The "if" keyword is only syntactic
899 sugar and has no function. The "not" keyword just inverts the result,
900 so the essence of a mood line is the mood method part and the options
901 following thereafter.
902
903 A *mood method* is realized as a function which takes an audio file
904 and computes a number from the data contained in the database.
905 If this number is non-negative, we say the file *matches* the mood
906 method. The file matches the full mood line if it either
907
908 - matches the mood method and the "not" keyword is not given,
909 or
910 - does not match the mood method, but the "not" keyword is given.
911
912 The set of admissible files for the whole mood is now defined as those
913 files which match at least one accept mood line, but no deny mood line.
914 More formally, an audio file F is admissible if and only if
915
916 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
917
918 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
919 lines and "~" means "matches".
920
921 The cases where no mood lines of accept/deny type are defined need
922 special treatment:
923
924 - Neither accept nor deny lines: This treats all files as
925 admissible (in fact, that is the definition of the dummy mood
926 which is activated automatically if no moods are available).
927
928 - Only accept lines: A file is admissible iff it matches at
929 least one accept line:
930
931 F ~ AL1 or F ~ AL2 or ...
932
933 - Only deny lines: A file is admissible iff it matches no
934 deny line:
935
936 not (F ~ DL1 or F ~ DN2 ...)
937
938
939
940 *List of mood_methods*
941
942 no_attributes_set
943
944 Takes no arguments and matches an audio file if and only if no
945 attributes are set.
946
947 is_set <attribute_name>
948
949 Takes the name of an attribute and matches iff that attribute is set.
950
951 path_matches <pattern>
952
953 Takes a filename pattern and matches iff the path of the audio file
954 matches the pattern.
955
956 artist_matches <pattern>
957 album_matches <pattern>
958 title_matches <pattern>
959 comment_matches <pattern>
960
961 Takes an extended regular expression and matches iff the text of the
962 corresponding tag of the audio file matches the pattern. If the tag
963 is not set, the empty string is matched against the pattern.
964
965 year ~ <num>
966 bitrate ~ <num>
967 frequency ~ <num>
968 channels ~ <num>
969 num_played ~ <num>
970
971 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
972 <num>. Matches an audio file iff the condition <val> ~ <num> is
973 satisfied where val is the corresponding value of the audio file
974 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
975 count, play count).
976
977 The year tag is special as its value is undefined if the audio file
978 has no year tag or the content of the year tag is not a number. Such
979 audio files never match. Another difference is the special treatment
980 if the year tag is a two-digit number. In this case either 1900 or
981 2000 is added to the tag value, depending on whether the number is
982 greater than 2000 plus the current year.
983
984
985 *Mood usage*
986
987 To create a new mood called "my_mood", write its definition into
988 some temporary file, say "tmpfile", and add it to the mood table
989 by executing
990
991 para addmood my_mood < tmpfile
992
993 If the mood definition is really short, you may just pipe it to the
994 client instead of using temporary files. Like this:
995
996 echo "$MOOD_DEFINITION" | para addmood my_mood
997
998 There is no need to keep the temporary file since you can always use
999 the catmood command to get it back:
1000
1001 para catmood my_mood
1002
1003 A mood can be activated by executing
1004
1005 para select m/my_mood
1006
1007 Once active, the list of admissible files is shown by the ls command
1008 if the "-a" switch is given:
1009
1010 para ls -a
1011
1012
1013 *Example mood definition*
1014
1015 Suppose you have defined attributes "punk" and "rock" and want to define
1016 a mood containing only Punk-Rock songs. That is, an audio file should be
1017 admissible if and only if both attributes are set. Since
1018
1019 punk and rock
1020
1021 is obviously the same as
1022
1023 not (not punk or not rock)
1024
1025 (de Morgan's rule), a mood definition that selects only Punk-Rock
1026 songs is
1027
1028 deny if not is_set punk
1029 deny if not is_set rock
1030
1031
1032
1033 File renames and content changes
1034 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1035
1036 Since the audio file selector knows the SHA1 of each audio file that
1037 has been added to the afs database, it recognizes if the content of
1038 a file has changed, e.g. because an ID3 tag was added or modified.
1039 Also, if a file has been renamed or moved to a different location,
1040 afs will detect that an entry with the same hash value already exists
1041 in the audio file table.
1042
1043 In both cases it is enough to just re-add the new file. In the
1044 first case (file content changed), the audio table is updated, while
1045 metadata such as the num_played and last_played fields, as well as
1046 the attributes, remain unchanged. In the other case, when the file
1047 is moved or renamed, only the path information is updated, all other
1048 data remains as before.
1049
1050 It is possible to change the behaviour of the add command by using the
1051 "-l" (lazy add) or the "-f" (force add) option.
1052
1053 Troubleshooting
1054 ~~~~~~~~~~~~~~~
1055
1056 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1057 executables have a brief online help which is displayed when -h is
1058 given. The --detailed-help option prints the full help text.
1059
1060 If para_server crashed or was killed by SIGKILL (signal 9), it
1061 may refuse to start again because of "dirty osl tables". In this
1062 case you'll have to run the oslfsck program of libosl to fix your
1063 database:
1064
1065 oslfsck -fd ~/.paraslash/afs_database-0.4
1066
1067 However, make sure para_server isn't running before executing oslfsck.
1068
1069 If you don't mind to recreate your database you can start
1070 from scratch by removing the entire database directory, i.e.
1071
1072 rm -rf ~/.paraslash/afs_database-0.4
1073
1074 Be aware that this removes all attribute definitions, all playlists
1075 and all mood definitions and requires to re-initialize the tables.
1076
1077 Although oslfsck fixes inconsistencies in database tables it doesn't
1078 care about the table contents. To check for invalid table contents, use
1079
1080 para_client check
1081
1082 This prints out references to missing audio files as well as invalid
1083 playlists and mood definitions.
1084
1085 Similarly, para_audiod refuses to start if its socket file exists, since
1086 this indicates that another instance of para_audiod is running. After
1087 a crash a stale socket file might remain and you must run
1088
1089 para_audiod --force
1090
1091 once to fix it up.
1092
1093 ---------------------------------------
1094 Audio formats and audio format handlers
1095 ---------------------------------------
1096
1097 Audio formats
1098 ~~~~~~~~~~~~~
1099
1100 The following audio formats are supported by paraslash:
1101
1102 *MP3*
1103
1104 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1105 designed as part of its MPEG-1 standard. An MP3 file is made up of
1106 multiple MP3 frames, which consist of a header and a data block. The
1107 size of an MP3 frame depends on the bit rate and on the number
1108 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1109 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1110 400 bytes large.
1111
1112 *OGG/Vorbis*
1113
1114 OGG is a standardized audio container format, while Vorbis is an
1115 open source codec for lossy audio compression. Since Vorbis is most
1116 commonly made available via the OGG container format, it is often
1117 referred to as OGG/Vorbis. The OGG container format divides data into
1118 chunks called OGG pages. A typical OGG page is about 4KB large. The
1119 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1120 may vary considerably.
1121
1122 *OGG/Speex*
1123
1124 Speex is an open-source speech codec that is based on CELP (Code
1125 Excited Linear Prediction) coding. It is designed for voice
1126 over IP applications, has modest complexity and a small memory
1127 footprint. Wideband and narrowband (telephone quality) speech are
1128 supported. As for Vorbis audio, Speex bit-streams are often stored
1129 in OGG files. As of 2012 this codec is considered obsolete since the
1130 Oppus codec, described below, surpasses its performance in all areas.
1131
1132 *OGG/Opus*
1133
1134 Opus is a lossy audio compression format standardized through RFC
1135 6716 in 2012. It combines the speech-oriented SILK codec and the
1136 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1137 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1138 containers. All known software patents which cover Opus are licensed
1139 under royalty-free terms.
1140
1141 *AAC*
1142
1143 Advanced Audio Coding (AAC) is a standardized, lossy compression
1144 and encoding scheme for digital audio which is the default audio
1145 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1146 the container format and audio files encoded with AAC have the .m4a
1147 extension. A typical AAC frame is about 700 bytes large.
1148
1149 *WMA*
1150
1151 Windows Media Audio (WMA) is an audio data compression technology
1152 developed by Microsoft. A WMA file is usually encapsulated in the
1153 Advanced Systems Format (ASF) container format, which also specifies
1154 how meta data about the file is to be encoded. The bit stream of WMA
1155 is composed of superframes, each containing one or more frames of
1156 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1157
1158 *FLAC*
1159
1160 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1161 loss. It gives better compression ratios than a general purpose
1162 compressor like zip or bzip2 because FLAC is designed specifically
1163 for audio. A FLAC-encoded file consists of frames of varying size, up
1164 to 16K. Each frame starts with a header that contains all information
1165 necessary to decode the frame.
1166
1167 Meta data
1168 ~~~~~~~~~
1169
1170 Unfortunately, each audio format has its own conventions how meta
1171 data is added as tags to the audio file.
1172
1173 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1174 is rather simple but also very limited as it supports only artist,
1175 title, album, year and comment tags. Each of these can only be at most
1176 32 characters long. ID3, version 2 is much more flexible but requires
1177 a separate library being installed for paraslash to support it.
1178
1179 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1180 comments, which are typically implemented as strings of the form
1181 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1182 tags are appropriate for the content.
1183
1184 AAC files usually use the MPEG-4 container format for storing meta
1185 data while WMA files wrap meta data as special objects within the
1186 ASF container format.
1187
1188 paraslash only tracks the most common tags that are supported by
1189 all tag variants: artist, title, year, album, comment. When a file
1190 is added to the AFS database, the meta data of the file is extracted
1191 and stored in the audio file table.
1192
1193 Chunks and chunk tables
1194 ~~~~~~~~~~~~~~~~~~~~~~~
1195
1196 paraslash uses the word "chunk" as common term for the building blocks
1197 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1198 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1199 size varies considerably between audio formats, from a few hundred
1200 bytes (MP3) up to 16K (FLAC).
1201
1202 The chunk table contains the offsets within the audio file that
1203 correspond to the chunk boundaries of the file. Like the meta data,
1204 the chunk table is computed and stored in the database whenever an
1205 audio file is added.
1206
1207 The paraslash senders (see below) always send complete chunks. The
1208 granularity for seeking is therefore determined by the chunk size.
1209
1210 Audio format handlers
1211 ~~~~~~~~~~~~~~~~~~~~~
1212
1213 For each audio format paraslash contains an audio format handler whose
1214 first task is to tell whether a given file is a valid audio file of
1215 this type. If so, the audio file handler extracts some technical data
1216 (duration, sampling rate, number of channels etc.), computes the
1217 chunk table and reads the meta data.
1218
1219 The audio format handler code is linked into para_server and executed
1220 via the _add_ command. The same code is also available as a stand-alone
1221 tool, para_afh, which prints the technical data, the chunk table
1222 and the meta data of a file. Moreover, all audio format handlers are
1223 combined in the afh receiver which is part of para_recv and para_play.
1224
1225 ----------
1226 Networking
1227 ----------
1228
1229 Paraslash uses different network connections for control and data.
1230 para_client communicates with para_server over a dedicated TCP control
1231 connection. To transport audio data, separate data connections are
1232 used. For these data connections, a variety of transports (UDP, DCCP,
1233 HTTP) can be chosen.
1234
1235 The chapter starts with the REFERENCE(The paraslash control
1236 service, control service), followed by a section on the various
1237 REFERENCE(Streaming protocols, streaming protocols) in which the data
1238 connections are described. The way audio file headers are embedded into
1239 the stream is discussed REFERENCE(Streams with headers and headerless
1240 streams, briefly) before the REFERENCE(Networking examples, example
1241 section) which illustrates typical commands for real-life scenarios.
1242
1243 Both IPv4 and IPv6 are supported.
1244
1245 The paraslash control service
1246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1247
1248 para_server is controlled at runtime via the paraslash control
1249 connection. This connection is used for server commands (play, stop,
1250 ...) as well as for afs commands (ls, select, ...).
1251
1252 The server listens on a TCP port and accepts connections from clients
1253 that connect the open port. Each connection causes the server to fork
1254 off a client process which inherits the connection and deals with that
1255 client only. In this classical accept/fork approach the server process
1256 is unaffected if the child dies or goes crazy for whatever reason. In
1257 fact, the child process can not change address space of server process.
1258
1259 The section on REFERENCE(Client-server authentication, client-server
1260 authentication) above described the early connection establishment
1261 from the crypto point of view. Here it is described what happens
1262 after the connection (including crypto setup) has been established.
1263 There are four processes involved during command dispatch as sketched
1264 in the following diagram.
1265
1266 <<
1267 <pre>
1268 server_host client_host
1269 ~~~~~~~~~~~ ~~~~~~~~~~~
1270
1271 +-----------+ connect +-----------+
1272 |para_server|<------------------------------ |para_client|
1273 +-----------+ +-----------+
1274 | ^
1275 | fork +---+ |
1276 +----------> |AFS| |
1277 | +---+ |
1278 | ^ |
1279 | | |
1280 | | connect (cookie) |
1281 | | |
1282 | | |
1283 | fork +-----+ inherited connection |
1284 +---------->|child|<--------------------------+
1285 +-----+
1286 </pre>
1287 >>
1288
1289 Note that the child process is not a child of the afs process,
1290 so communication of these two processes has to happen via local
1291 sockets. In order to avoid abuse of the local socket by unrelated
1292 processes, a magic cookie is created once at server startup time just
1293 before the server process forks off the AFS process. This cookie is
1294 known to the server, AFS and the child, but not to unrelated processes.
1295
1296 There are two different kinds of commands: First there are commands
1297 that cause the server to respond with some answer such as the list
1298 of all audio files. All but the addblob commands (addimg, addlyr,
1299 addpl, addmood) are of this kind. The addblob commands add contents
1300 to the database, so they need to transfer data the other way round,
1301 from the client to the server.
1302
1303 There is no knowledge about the server commands built into para_client,
1304 so it does not know about addblob commands. Instead, it inspects the
1305 first data package sent by the server for a magic string. If this
1306 string was found, it sends STDIN to the server, otherwise it dumps
1307 data from the server to STDOUT.
1308
1309 Streaming protocols
1310 ~~~~~~~~~~~~~~~~~~~
1311
1312 A network (audio) stream usually consists of one streaming source,
1313 the _sender_, and one or more _receivers_ which read data over the
1314 network from the streaming source.
1315
1316 Senders are thus part of para_server while receivers are part of
1317 para_audiod. Moreover, there is the stand-alone tool para_recv which
1318 can be used to manually download a stream, either from para_server
1319 or from a web-based audio streaming service.
1320
1321 The following three streaming protocols are supported by paraslash:
1322
1323 - HTTP. Recommended for public streams that can be played by
1324 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1325 sender is supported on all operating systems and all platforms.
1326
1327 - DCCP. Recommended for LAN streaming. DCCP is currently
1328 available only for Linux.
1329
1330 - UDP. Recommended for multicast LAN streaming.
1331
1332 See the Appendix on REFERENCE(Network protocols, network protocols)
1333 for brief descriptions of the various protocols relevant for network
1334 audio streaming with paraslash.
1335
1336 It is possible to activate more than one sender simultaneously.
1337 Senders can be controlled at run time and via config file and command
1338 line options.
1339
1340 Note that audio connections are _not_ encrypted. Transport or Internet
1341 layer encryption should be used if encrypted data connections are
1342 needed.
1343
1344 Since DCCP and TCP are both connection-oriented protocols, connection
1345 establishment/teardown and access control are very similar between
1346 these two streaming protocols. UDP is the most lightweight option,
1347 since in contrast to TCP/DCCP it is connectionless. It is also the
1348 only protocol supporting IP multicast.
1349
1350 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1351 clients to connect and establish a connection via some protocol-defined
1352 handshake mechanism. Both senders maintain two linked lists each:
1353 The list of all clients which are currently connected, and the list
1354 of access control entries which determines who is allowed to connect.
1355 IP-based access control may be configured through config file and
1356 command line options and via the "allow" and "deny" sender subcommands.
1357
1358 Upon receiving a GET request from the client, the HTTP sender sends
1359 back a status line and a message. The body of this message is the
1360 audio stream. This is common practice and is supported by many popular
1361 clients which can thus be used to play a stream offered by para_server.
1362 For DCCP things are a bit simpler: No messages are exchanged between
1363 the receiver and sender. The client simply connects and the sender
1364 starts to stream.
1365
1366 DCCP is an experimental protocol which offers a number of new features
1367 not available for TCP. Both ends can negotiate these features using
1368 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1369 datagram-based (no retransmissions) and thus should not be used over
1370 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1371 is access to a variety of different congestion-control mechanisms
1372 called CCIDs. Two different CCIDs are available per default on Linux:
1373
1374
1375 - _CCID 2_. A Congestion Control mechanism similar to that
1376 of TCP. The sender maintains a congestion window and halves
1377 this window in response to congestion.
1378
1379
1380 - _CCID-3_. Designed to be fair when competing for bandwidth.
1381 It has lower variation of throughput over time compared with
1382 TCP, which makes it suitable for streaming media.
1383
1384 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1385 single list, the _target list_. This list describes the set of clients
1386 to which the stream is sent. There is no list for access control and
1387 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1388 and "delete" commands can be used to modify the target list.
1389
1390 Since both UDP and DCCP offer an unreliable datagram-based transport,
1391 additional measures are necessary to guard against disruptions over
1392 networks that are lossy or which may be subject to interference (as
1393 is for instance the case with WiFi). Paraslash uses FEC (Forward
1394 Error Correction) to guard against packet losses and reordering. The
1395 stream is FEC-encoded before it is sent through the UDP socket and
1396 must be decoded accordingly on the receiver side.
1397
1398 The packet size and the amount of redundancy introduced by FEC can
1399 be configured via the FEC parameters which are dictated by server
1400 and may also be configured through the "sender" command. The FEC
1401 parameters are encoded in the header of each network packet, so no
1402 configuration is necessary on the receiver side. See the section on
1403 REFERENCE(Forward error correction, FEC) below.
1404
1405 Streams with headers and headerless streams
1406 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1407
1408 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1409 needed to decode the stream is only contained in the audio file
1410 header of the container format but not in each data chunk. Clients
1411 must be able to obtain this information in case streaming starts in
1412 the middle of the file or if para_audiod is started while para_server
1413 is already sending a stream.
1414
1415 This is accomplished in different ways, depending on the streaming
1416 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1417 header is sent prior to audio file data. This technique however does
1418 not work for the connectionless UDP transport. Hence the audio file
1419 header is periodically being embedded into the UDP audio data stream.
1420 By default, the header is resent after five seconds. The receiver has
1421 to wait until the next header arrives before it can start decoding
1422 the stream.
1423
1424 Examples
1425 ~~~~~~~~
1426
1427 The "si" (server info) command lists some information about the
1428 currently running server process.
1429
1430 -> Show PIDs, number of connected clients, uptime, and more:
1431
1432 para_client si
1433
1434 The sender command of para_server prints information about senders,
1435 like the various access control lists, and it allows to (de-)activate
1436 senders and to change the access permissions at runtime.
1437
1438 -> List all senders
1439
1440 para_client sender
1441
1442 -> Obtain general help for the sender command:
1443
1444 para_client help sender
1445
1446 -> Get help for a specific sender (contains further examples):
1447
1448 s=http # or dccp or udp
1449 para_client sender $s help
1450
1451 -> Show status of the http sender
1452
1453 para_client sender http status
1454
1455 By default para_server activates both the HTTP and th DCCP sender on
1456 startup. This can be changed via command line options or para_server's
1457 config file.
1458
1459 -> List config file options for senders:
1460
1461 para_server -h
1462
1463 All senders share the "on" and "off" commands, so senders may be
1464 activated and deactivated independently of each other.
1465
1466 -> Switch off the http sender:
1467
1468 para_client sender http off
1469
1470 -> Receive a DCCP stream using CCID2 and write the output into a file:
1471
1472 host=foo.org; ccid=2; filename=bar
1473 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1474
1475 Note the quotes around the arguments for the dccp receiver. Each
1476 receiver has its own set of command line options and its own command
1477 line parser, so arguments for the dccp receiver must be protected
1478 from being interpreted by para_recv.
1479
1480 -> Start UDP multicast, using the default multicast address:
1481
1482 para_client sender udp add 224.0.1.38
1483
1484 -> Receive FEC-encoded multicast stream and write the output into a file:
1485
1486 filename=foo
1487 para_recv -r udp > $filename
1488
1489 -> Add an UDP unicast for a client to the target list of the UDP sender:
1490
1491 t=client.foo.org
1492 para_client sender udp add $t
1493
1494 -> Receive this (FEC-encoded) unicast stream:
1495
1496 filename=foo
1497 para_recv -r 'udp -i 0.0.0.0' > $filename
1498
1499 -> Create a minimal config for para_audiod for HTTP streams:
1500
1501 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1502 echo receiver \".:http -i $s\" > $c
1503 para_audiod --config $c
1504
1505 -------
1506 Filters
1507 -------
1508
1509 A paraslash filter is a module which transforms an input stream into
1510 an output stream. Filters are included in the para_audiod executable
1511 and in the stand-alone tool para_filter which usually contains the
1512 same modules.
1513
1514 While para_filter reads its input stream from STDIN and writes
1515 the output to STDOUT, the filter modules of para_audiod are always
1516 connected to a receiver which produces the input stream and a writer
1517 which absorbs the output stream.
1518
1519 Some filters depend on a specific library and are not compiled in
1520 if this library was not found at compile time. To see the list of
1521 supported filters, run para_filter and para_audiod with the --help
1522 option. The output looks similar to the following:
1523
1524 Available filters:
1525 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1526
1527 Out of these filter modules, a chain of filters can be constructed,
1528 much in the way Unix pipes can be chained, and analogous to the use
1529 of modules in gstreamer: The output of the first filter becomes the
1530 input of the second filter. There is no limitation on the number of
1531 filters and the same filter may occur more than once.
1532
1533 Like receivers, each filter has its own command line options which
1534 must be quoted to protect them from the command line options of
1535 the driving application (para_audiod or para_filter). Example:
1536
1537 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1538
1539 For para_audiod, each audio format has its own set of filters. The
1540 name of the audio format for which the filter should be applied can
1541 be used as the prefix for the filter option. Example:
1542
1543 para_audiod -f 'mp3:prebuffer --duration 300'
1544
1545 The "mp3" prefix above is actually interpreted as a POSIX extended
1546 regular expression. Therefore
1547
1548 para_audiod -f '.:prebuffer --duration 300'
1549
1550 activates the prebuffer filter for all supported audio formats (because
1551 "." matches all audio formats) while
1552
1553 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1554
1555 activates it only for wma and ogg streams.
1556
1557 Decoders
1558 ~~~~~~~~
1559
1560 For each supported audio format there is a corresponding filter
1561 which decodes audio data in this format to 16 bit PCM data which
1562 can be directly sent to the sound device or any other software that
1563 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1564 filters are called _decoders_ in general, and xxxdec is the name of
1565 the paraslash decoder for the audio format xxx. For example, the mp3
1566 decoder filter is called mp3dec.
1567
1568 Note that the output of the decoder is about 10 times larger than
1569 its input. This means that filters that operate on the decoded audio
1570 stream have to deal with much more data than filters that transform
1571 the audio stream before it is fed to the decoder.
1572
1573 Paraslash relies on external libraries for most decoders, so these
1574 libraries must be installed for the decoder to be included in the
1575 para_filter and para_audiod executables. The oggdec filter depends
1576 on the libogg and libvorbis libraries for example.
1577
1578 Forward error correction
1579 ~~~~~~~~~~~~~~~~~~~~~~~~
1580
1581 As already mentioned REFERENCE(Streaming protocols, earlier),
1582 paraslash uses forward error correction (FEC) for the unreliable UDP
1583 and DCCP transports. FEC is a technique which was invented already
1584 in 1960 by Reed and Solomon and which is widely used for the parity
1585 calculations of storage devices (RAID arrays). It is based on the
1586 algebraic concept of finite fields, today called Galois fields, in
1587 honour of the mathematician Galois (1811-1832). The FEC implementation
1588 of paraslash is based on code by Luigi Rizzo.
1589
1590 Although the details require a sound knowledge of the underlying
1591 mathematics, the basic idea is not hard to understand: For positive
1592 integers k and n with k < n it is possible to compute for any k given
1593 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1594 ..., p_r such that all data bytes can be reconstructed from *any*
1595 k bytes of the set
1596
1597 {d_1, ..., d_k, p_1, ..., p_r}.
1598
1599 FEC-encoding for unreliable network transports boils down to slicing
1600 the audio stream into groups of k suitably sized pieces called _slices_
1601 and computing the r corresponding parity slices. This step is performed
1602 in para_server which then sends both the data and the parity slices
1603 over the unreliable network connection. If the client was able
1604 to receive at least k of the n = k + r slices, it can reconstruct
1605 (FEC-decode) the original audio stream.
1606
1607 From these observations it is clear that there are three different
1608 FEC parameters: The slice size, the number of data slices k, and the
1609 total number of slices n. It is crucial to choose the slice size
1610 such that no fragmentation of network packets takes place because
1611 FEC only guards against losses and reordering but fails if slices are
1612 received partially.
1613
1614 FEC decoding in paralash is performed through the fecdec filter which
1615 usually is the first filter (there can be other filters before fecdec
1616 if these do not alter the audio stream).
1617
1618
1619 Volume adjustment (amp and compress)
1620 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1621
1622 The amp and the compress filter both adjust the volume of the audio
1623 stream. These filters operate on uncompressed audio samples. Hence
1624 they are usually placed directly after the decoding filter. Each
1625 sample is multiplied with a scaling factor (>= 1) which makes amp
1626 and compress quite expensive in terms of computing power.
1627
1628 *amp*
1629
1630 The amp filter amplifies the audio stream by a fixed scaling factor
1631 that must be known in advance. For para_audiod this factor is derived
1632 from the amplification field of the audio file's entry in the audio
1633 file table while para_filter uses the value given at the command line.
1634
1635 The optimal scaling factor F for an audio file is the largest real
1636 number F >= 1 such that after multiplication with F all samples still
1637 fit into the sample interval [-32768, 32767]. One can use para_filter
1638 in combination with the sox utility to compute F:
1639
1640 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1641
1642 The amplification value V which is stored in the audio file table,
1643 however, is an integer between 0 and 255 which is connected to F
1644 through the formula
1645
1646 V = (F - 1) * 64.
1647
1648 To store V in the audio file table, the command
1649
1650 para_client -- touch -a=V file.mp3
1651
1652 is used. The reader is encouraged to write a script that performs
1653 these computations :)
1654
1655 *compress*
1656
1657 Unlike the amplification filter, the compress filter adjusts the volume
1658 of the audio stream dynamically without prior knowledge about the peak
1659 value. It maintains the maximal volume of the last n samples of the
1660 audio stream and computes a suitable amplification factor based on that
1661 value and the various configuration options. It tries to chose this
1662 factor such that the adjusted volume meets the desired target level.
1663
1664 Note that it makes sense to combine amp and compress.
1665
1666 Misc filters (wav and prebuffer)
1667 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1668
1669 These filters are rather simple and do not modify the audio stream at
1670 all. The wav filter is only useful with para_filter and in connection
1671 with a decoder. It asks the decoder for the number of channels and the
1672 sample rate of the stream and adds a Microsoft wave header containing
1673 this information at the beginning. This allows to write wav files
1674 rather than raw PCM files (which do not contain any information about
1675 the number of channels and the sample rate).
1676
1677 The prebuffer filter simply delays the output until the given time has
1678 passed (starting from the time the first byte was available in its
1679 input queue) or until the given amount of data has accumulated. It
1680 is mainly useful for para_audiod if the standard parameters result
1681 in buffer underruns.
1682
1683 Both filters require almost no additional computing time, even when
1684 operating on uncompressed audio streams, since data buffers are simply
1685 "pushed down" rather than copied.
1686
1687 Examples
1688 ~~~~~~~~
1689
1690 -> Decode an mp3 file to wav format:
1691
1692 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1693
1694 -> Amplify a raw audio file by a factor of 1.5:
1695
1696 para_filter -f amp --amp 32 < foo.raw > bar.raw
1697
1698 ------
1699 Output
1700 ------
1701
1702 Once an audio stream has been received and decoded to PCM format,
1703 it can be sent to a sound device for playback. This part is performed
1704 by paraslash _writers_ which are described in this chapter.
1705
1706 Writers
1707 ~~~~~~~
1708
1709 A paraslash writer acts as a data sink that consumes but does not
1710 produce audio data. Paraslash writers operate on the client side and
1711 are contained in para_audiod and in the stand-alone tool para_write.
1712
1713 The para_write program reads uncompressed audio data from STDIN. If
1714 this data starts with a wav header, sample rate, sample format and
1715 channel count are read from the header. Otherwise CD audio (44.1KHz
1716 16 bit little endian, stereo) is assumed but this can be overridden
1717 by command line options. para_audiod, on the other hand, obtains
1718 the sample rate and the number of channels from the decoder.
1719
1720 Like receivers and filters, each writer has an individual set of
1721 command line options, and for para_audiod writers can be configured
1722 per audio format separately. It is possible to activate more than
1723 one writer for the same stream simultaneously.
1724
1725 OS-dependent APIs
1726 ~~~~~~~~~~~~~~~~~
1727
1728 Unfortunately, the various flavours of Unix on which paraslash
1729 runs on have different APIs for opening a sound device and starting
1730 playback. Hence for each such API there is a paraslash writer that
1731 can play the audio stream via this API.
1732
1733 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1734 Linux systems. Although there are several mid-layer APIs in use by
1735 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1736 currently supports only the low-level ALSA API which is not supposed
1737 to be change. ALSA is very feature-rich, in particular it supports
1738 software mixing via its DMIX plugin. ALSA is the default writer on
1739 Linux systems.
1740
1741 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1742 is also available on Linux systems, usually provided by ALSA as an
1743 emulation for backwards compatibility. This API is rather simple but
1744 also limited. For example only one application can open the device
1745 at any time. The OSS writer is activated by default on BSD Systems.
1746
1747 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1748 for this API is only compiled in on such systems and is of course
1749 the default there.
1750
1751 *FILE*. The file writer allows to capture the audio stream and
1752 write the PCM data to a file on the file system rather than playing
1753 it through a sound device. It is supported on all platforms and is
1754 always compiled in.
1755
1756 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1757 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1758 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1759 through an output plugin of libao.
1760
1761 Examples
1762 ~~~~~~~~
1763
1764 -> Use the OSS writer to play a wav file:
1765
1766 para_write --writer oss < file.wav
1767
1768 -> Enable ALSA software mixing for mp3 streams
1769
1770 para_audiod --writer 'mp3:alsa -d plug:swmix'
1771
1772
1773 ---
1774 Gui
1775 ---
1776
1777 para_gui executes an arbitrary command which is supposed to print
1778 status information to STDOUT. It then displays this information in
1779 a curses window. By default the command
1780
1781 para_audioc -- stat -p
1782
1783 is executed, but this can be customized via the --stat-cmd option. In
1784 particular it possible to use
1785
1786 para_client -- stat -p
1787
1788 to make para_gui work on systems on which para_audiod is not running.
1789
1790 Key bindings
1791 ~~~~~~~~~~~~
1792
1793 It is possible to bind keys to arbitrary commands via custom
1794 key-bindings. Besides the internal keys which can not be changed (help,
1795 quit, loglevel, version...), the following flavours of key-bindings
1796 are supported:
1797
1798 - external: Shutdown curses before launching the given command.
1799 Useful for starting other ncurses programs from within
1800 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1801 output format to write a mailbox containing one mail for each
1802 (admissible) file the audio file selector knows about. Then
1803 start mutt from within para_gui to browse your collection!
1804
1805 - display: Launch the command and display its stdout in
1806 para_gui's bottom window.
1807
1808 - para: Like display, but start "para_client <specified
1809 command>" instead of "<specified command>".
1810
1811 The general form of a key binding is
1812
1813 key_map k:m:c
1814
1815 which maps key k to command c using mode m. Mode may be x, d or p
1816 for external, display and paraslash commands, respectively.
1817
1818 Themes
1819 ~~~~~~
1820
1821 Currently there are only two themes for para_gui. It is easy, however,
1822 to add more themes. To create a new theme one has to define the
1823 position, color and geometry for for each status item that should be
1824 shown by this theme. See gui_theme.c for examples.
1825
1826 The "." and "," keys are used to switch between themes.
1827
1828 Examples
1829 ~~~~~~~~
1830
1831 -> Show server info:
1832
1833 key_map "i:p:si"
1834
1835 -> Jump to the middle of the current audio file by pressing F5:
1836
1837 key_map "<F5>:p:jmp 50"
1838
1839 -> vi-like bindings for jumping around:
1840
1841 key_map "l:p:ff 10"
1842 key_map "h:p:ff 10-"
1843 key_map "w:p:ff 60"
1844 key_map "b:p:ff 60-"
1845
1846 -> Print the current date and time:
1847
1848 key_map "D:d:date"
1849
1850 -> Call other curses programs:
1851
1852 key_map "U:x:aumix"
1853 key_map "!:x:/bin/bash"
1854 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1855
1856 -----------
1857 Development
1858 -----------
1859
1860 Tools
1861 ~~~~~
1862
1863 In order to compile the sources from the git repository (rather than
1864 from tar balls) and for contributing non-trivial changes to the
1865 paraslash project, some additional tools should be installed on a
1866 developer machine.
1867
1868 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1869 branches, below), the git source code management tool is used for
1870 paraslash development. It is necessary for cloning the git repository
1871 and for getting updates.
1872
1873 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1874 are generated from templates by the m4 macro processor.
1875
1876 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1877 the configure file which is shipped in the tarballs but has to be
1878 generated when compiling from git.
1879
1880 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1881 HTML version of this manual and some of the paraslash web pages are
1882 generated by the grutatxt plain text to HTML converter. If changes
1883 are made to these text files the grutatxt package must be installed
1884 to regenerate the HTML files.
1885
1886 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1887 of paraslash's C sources uses the doxygen documentation system. The
1888 conventions for documenting the source code is described in the
1889 REFERENCE(Doxygen, Doxygen section).
1890
1891 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1892 browsable HTML from the C sources. It is needed by doxygen.
1893
1894 Git branches
1895 ~~~~~~~~~~~~
1896
1897 Paraslash has been developed using the git source code management
1898 tool since 2006. Development is organized roughly in the same spirit
1899 as the git development itself, as described below.
1900
1901 The following text passage is based on "A note from the maintainer",
1902 written by Junio C Hamano, the maintainer of git.
1903
1904 There are four branches in the paraslash repository that track the
1905 source tree: "master", "maint", "next", and "pu".
1906
1907 The "master" branch is meant to contain what is well tested and
1908 ready to be used in a production setting. There could occasionally be
1909 minor breakages or brown paper bag bugs but they are not expected to
1910 be anything major, and more importantly quickly and easily fixable.
1911 Every now and then, a "feature release" is cut from the tip of this
1912 branch, named with three dotted decimal digits, like 0.4.2.
1913
1914 Whenever changes are about to be included that will eventually lead to
1915 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1916 "master" at that point. Obvious, safe and urgent fixes after the major
1917 release are applied to this branch and maintenance releases are cut
1918 from it. New features never go to this branch. This branch is also
1919 merged into "master" to propagate the fixes forward.
1920
1921 A trivial and safe enhancement goes directly on top of "master".
1922 New development does not usually happen on "master", however.
1923 Instead, a separate topic branch is forked from the tip of "master",
1924 and it first is tested in isolation; Usually there are a handful such
1925 topic branches that are running ahead of "master". The tip of these
1926 branches is not published in the public repository to keep the number
1927 of branches that downstream developers need to worry about low.
1928
1929 The quality of topic branches varies widely. Some of them start out as
1930 "good idea but obviously is broken in some areas" and then with some
1931 more work become "more or less done and can now be tested by wider
1932 audience". Luckily, most of them start out in the latter, better shape.
1933
1934 The "next" branch is to merge and test topic branches in the latter
1935 category. In general, this branch always contains the tip of "master".
1936 It might not be quite rock-solid production ready, but is expected to
1937 work more or less without major breakage. The maintainer usually uses
1938 the "next" version of paraslash for his own pleasure, so it cannot
1939 be _that_ broken. The "next" branch is where new and exciting things
1940 take place.
1941
1942 The two branches "master" and "maint" are never rewound, and "next"
1943 usually will not be either (this automatically means the topics that
1944 have been merged into "next" are usually not rebased, and you can find
1945 the tip of topic branches you are interested in from the output of
1946 "git log next"). You should be able to safely build on top of them.
1947
1948 However, at times "next" will be rebuilt from the tip of "master" to
1949 get rid of merge commits that will never be in "master". The commit
1950 that replaces "next" will usually have the identical tree, but it
1951 will have different ancestry from the tip of "master".
1952
1953 The "pu" (proposed updates) branch bundles the remainder of the
1954 topic branches. The "pu" branch, and topic branches that are only in
1955 "pu", are subject to rebasing in general. By the above definition
1956 of how "next" works, you can tell that this branch will contain quite
1957 experimental and obviously broken stuff.
1958
1959 When a topic that was in "pu" proves to be in testable shape, it
1960 graduates to "next". This is done with
1961
1962 git checkout next
1963 git merge that-topic-branch
1964
1965 Sometimes, an idea that looked promising turns out to be not so good
1966 and the topic can be dropped from "pu" in such a case.
1967
1968 A topic that is in "next" is expected to be polished to perfection
1969 before it is merged to "master". Similar to the above, this is
1970 done with
1971
1972 git checkout master
1973 git merge that-topic-branch
1974 git branch -d that-topic-branch
1975
1976 Note that being in "next" is not a guarantee to appear in the next
1977 release (being in "master" is such a guarantee, unless it is later
1978 found seriously broken and reverted), nor even in any future release.
1979
1980 Coding Style
1981 ~~~~~~~~~~~~
1982
1983 The preferred coding style for paraslash coincides more or less
1984 with the style of the Linux kernel. So rather than repeating what is
1985 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
1986 there), here are the most important points.
1987
1988 - Burn the GNU coding standards.
1989 - Never use spaces for indentation.
1990 - Tabs are 8 characters, and thus indentations are also 8 characters.
1991 - Don't put multiple assignments on a single line.
1992 - Avoid tricky expressions.
1993 - Don't leave whitespace at the end of lines.
1994 - The limit on the length of lines is 80 columns.
1995 - Use K&R style for placing braces and spaces:
1996
1997 if (x is true) {
1998 we do y
1999 }
2000
2001 - Use a space after (most) keywords.
2002 - Do not add spaces around (inside) parenthesized expressions.
2003 - Use one space around (on each side of) most binary and ternary operators.
2004 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2005 - Mixed-case names are frowned upon.
2006 - Descriptive names for global variables are a must.
2007 - Avoid typedefs.
2008 - Functions should be short and sweet, and do just one thing.
2009 - The number of local variables shouldn't exceed 10.
2010 - Gotos are fine if they improve readability and reduce nesting.
2011 - Don't use C99-style "// ..." comments.
2012 - Names of macros defining constants and labels in enums are capitalized.
2013 - Enums are preferred when defining several related constants.
2014 - Always use the paraslash wrappers for allocating memory.
2015 - If the name of a function is an action or an imperative.
2016 command, the function should return an error-code integer
2017 (<0 means error, >=0 means success). If the name is a
2018 predicate, the function should return a "succeeded" boolean.
2019
2020
2021 Doxygen
2022 ~~~~~~~
2023
2024 Doxygen is a documentation system for various programming
2025 languages. The paraslash project uses Doxygen for generating the API
2026 reference on the web pages, but good source code documentation is
2027 also beneficial to people trying to understand the code structure
2028 and the interactions between the various source files.
2029
2030 It is more illustrative to look at the source code for examples than
2031 to describe the conventions for documenting the source in this manual,
2032 so we only describe which parts of the code need doxygen comments,
2033 but leave out details on documentation conventions.
2034
2035 As a rule, only the public part of the C source is documented with
2036 Doxygen. This includes structures, defines and enumerations in header
2037 files as well as public (non-static) C functions. These should be
2038 documented completely. For example each parameter and the return
2039 value of a public function should get a descriptive comment.
2040
2041 No doxygen comments are necessary for static functions and for
2042 structures and enumerations in C files (which are used only within
2043 this file). This does not mean, however, that those entities need
2044 no documentation at all. Instead, common sense should be applied to
2045 document what is not obvious from reading the code.
2046
2047 --------
2048 Appendix
2049 --------
2050
2051 Network protocols
2052 ~~~~~~~~~~~~~~~~~
2053
2054 *IP*. The _Internet Protocol_ is the primary networking protocol
2055 used for the Internet. All protocols described below use IP as the
2056 underlying layer. Both the prevalent IPv4 and the next-generation
2057 IPv6 variant are being deployed actively worldwide.
2058
2059 *Connection-oriented and connectionless protocols*. Connectionless
2060 protocols differ from connection-oriented ones in that state
2061 associated with the sending/receiving endpoints is treated
2062 implicitly. Connectionless protocols maintain no internal knowledge
2063 about the state of the connection. Hence they are not capable of
2064 reacting to state changes, such as sudden loss or congestion on the
2065 connection medium. Connection-oriented protocols, in contrast, make
2066 this knowledge explicit. The connection is established only after
2067 a bidirectional handshake which requires both endpoints to agree
2068 on the state of the connection, and may also involve negotiating
2069 specific parameters for the particular connection. Maintaining an
2070 up-to-date internal state of the connection also in general means
2071 that the sending endpoints perform congestion control, adapting to
2072 qualitative changes of the connection medium.
2073
2074 *Reliability*. In IP networking, packets can be lost, duplicated,
2075 or delivered out of order, and different network protocols handle
2076 these problems in different ways. We call a transport-layer protocol
2077 _reliable_, if it turns the unreliable IP delivery into an ordered,
2078 duplicate- and loss-free delivery of packets. Sequence numbers
2079 are used to discard duplicates and re-arrange packets delivered
2080 out-of-order. Retransmission is used to guarantee loss-free
2081 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2082 or data integrity.
2083
2084 *Classification*. With these definitions the protocols which are used
2085 by paraslash for steaming audio data may be classified as follows.
2086
2087 - HTTP/TCP: connection-oriented, reliable,
2088 - UDP: connectionless, unreliable,
2089 - DCCP: connection-oriented, unreliable.
2090
2091 Below we give a short descriptions of these protocols.
2092
2093 *TCP*. The _Transmission Control Protocol_ provides reliable,
2094 ordered delivery of a stream and a classic window-based congestion
2095 control. In contrast to UDP and DCCP (see below), TCP does not have
2096 record-oriented or datagram-based syntax, i.e. it provides a stream
2097 which is unaware and independent of any record (packet) boundaries.
2098 TCP is used extensively by many application layers. Besides HTTP (the
2099 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2100 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2101 top of TCP.
2102
2103 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2104 protocol, built as a thin layer directly on top of IP. For this reason,
2105 it offers the same best-effort service as IP itself, i.e. there is no
2106 detection of duplicate or reordered packets. Being a connectionless
2107 protocol, only minimal internal state about the connection is
2108 maintained, which means that there is no protection against packet
2109 loss or network congestion. Error checking and correction (if at all)
2110 are performed in the application.
2111
2112 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2113 connection-oriented state maintenance known from TCP with the
2114 unreliable, datagram-based transport of UDP. This means that it
2115 is capable of reacting to changes in the connection by performing
2116 congestion control, offering multiple alternative approaches. But it
2117 is bound to datagram boundaries (the maximum packet size supported
2118 by a medium), and like UDP it lacks retransmission to protect
2119 against loss. Due to the use of sequence numbers, it is however
2120 able to react to loss (interpreted as a congestion indication) and
2121 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2122 to negotiate specific, binding features for a connection, such as
2123 the choice of congestion control: classic, window-based congestion
2124 control known from TCP is available as CCID-2, rate-based, "smooth"
2125 congestion control is offered as CCID-3.
2126
2127 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2128 protocol on top of TCP. It is spoken by web servers and is most often
2129 used for web services. However, as can be seen by the many Internet
2130 radio stations and YouTube/Flash videos, http is by far not limited to
2131 the delivery of web pages only. Being a simple request/response based
2132 protocol, the semantics of the protocol also allow the delivery of
2133 multimedia content, such as audio over http.
2134
2135 *Multicast*. IP multicast is not really a protocol but a technique
2136 for one-to-many communication over an IP network. The challenge is to
2137 deliver information to a group of destinations simultaneously using
2138 the most efficient strategy to send the messages over each link of
2139 the network only once. This has benefits for streaming multimedia:
2140 the standard one-to-one unicast offered by TCP/DCCP means that
2141 n clients listening to the same stream also consume n-times the
2142 resources, whereas multicast requires to send the stream just once,
2143 irrespective of the number of receivers. Since it would be costly to
2144 maintain state for each listening receiver, multicast often implies
2145 connectionless transport, which is the reason that it is currently
2146 only available via UDP.
2147
2148 Abstract socket namespace
2149 ~~~~~~~~~~~~~~~~~~~~~~~~~
2150 UNIX domain sockets are a traditional way to communicate between
2151 processes on the same machine. They are always reliable (see above)
2152 and don't reorder datagrams. Unlike TCP and UDP, UNIX domain sockets
2153 support passing open file descriptors or process credentials to
2154 other processes.
2155
2156 The usual way to set up a UNIX domain socket (as obtained from
2157 socket(2)) for listening is to first bind the socket to a file system
2158 pathname and then call listen(2), then accept(2). Such sockets are
2159 called _pathname sockets_ because bind(2) creates a special socket
2160 file at the specified path. Pathname sockets allow unrelated processes
2161 to communicate with the listening process by binding to the same path
2162 and calling connect(2).
2163
2164 There are two problems with pathname sockets:
2165
2166 * The listing process must be able to (safely) create the
2167 socket special in a directory which is also accessible to
2168 the connecting process.
2169
2170 * After an unclean shutdown of the listening process, a stale
2171 socket special may reside on the file system.
2172
2173 The abstract socket namespace is a non-portable Linux feature which
2174 avoids these problems. Abstract sockets are still bound to a name,
2175 but the name has no connection with file system pathnames.
2176
2177 License
2178 ~~~~~~~
2179
2180 Paraslash is licensed under the GPL, version 2. Most of the code
2181 base has been written from scratch, and those parts are GPL V2
2182 throughout. Notable exceptions are FEC and the WMA decoder. See the
2183 corresponding source files for licencing details for these parts. Some
2184 code sniplets of several other third party software packages have
2185 been incorporated into the paraslash sources, for example log message
2186 coloring was taken from the git sources. These third party software
2187 packages are all published under the GPL or some other license
2188 compatible to the GPL.
2189
2190 Acknowledgements
2191 ~~~~~~~~~~~~~~~~
2192
2193 Many thanks to Gerrit Renker who read an early draft of this manual
2194 and contributed significant improvements.
2195
2196 ----------
2197 References
2198 ----------
2199
2200 Articles
2201 ~~~~~~~~
2202 - Reed, Irving S.; Solomon, Gustave (1960),
2203 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2204 Polynomial Codes over Certain Finite Fields), Journal of the
2205 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2206 300-304, doi:10.1137/0108018)
2207
2208 RFCs
2209 ~~~~
2210
2211 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2212 User Datagram Protocol
2213 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2214 Internet Protocol
2215 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2216 RSA Cryptography Specifications
2217 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2218 (2006): Datagram Congestion Control Protocol (DCCP)
2219 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2220 Congestion Control ID 2: TCP-like Congestion Control
2221 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2222 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2223 - XREFERENCE(http://www.ietf.org/rfc/rfc6716.txt, RFC 6716) (2012):
2224 Definition of the Opus Audio Codec
2225
2226 Application web pages
2227 ~~~~~~~~~~~~~~~~~~~~~
2228
2229 - XREFERENCE(http://people.tuebingen.mpg.de/maan/paraslash/, paraslash)
2230 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash (alternative page))
2231 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2232 - XREFERENCE(http://www.mpg123.de/, mpg123)
2233 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2234 - XREFERENCE(http://www.icecast.org/, icecast)
2235 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2236
2237 External documentation
2238 ~~~~~~~~~~~~~~~~~~~~~~
2239
2240 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2241 H. Peter Anvin: The mathematics of Raid6)
2242 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2243 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2244 Communication Protocols)
2245
2246 Code
2247 ~~~~
2248 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2249 Original FEC implementation by Luigi Rizzo)
2250