Convert audiod commands to lopsub.
[paraslash.git] / web / manual.md
1 **Paraslash user manual**
2
3 This document describes how to install, configure and use the paraslash
4 network audio streaming system. Most chapters start with a chapter
5 overview and conclude with an example section. We try to focus on
6 general concepts and on the interaction of the various pieces of the
7 paraslash package. Hence this user manual is not meant as a replacement
8 for the manual pages that describe all command line options of each
9 paraslash executable.
10
11 ============
12 Introduction
13 ============
14
15 In this chapter we give an [overview](#Overview) of the interactions of
16 the two main programs contained in the paraslash package, followed by
17 [brief descriptions](#The.paraslash.executables) of all executables.
18
19 Overview
20 --------
21
22 The core functionality of the para suite is provided by two main
23 executables, para_server and para_audiod. The former maintains a
24 database of audio files and streams these files to para_audiod which
25 receives and plays the stream.
26
27 In a typical setting, both para_server and para_audiod act as
28 background daemons whose functionality is controlled by client
29 programs: the para_audioc client controls para_audiod over a local
30 socket while the para_client program connects to para_server over a
31 local or remote networking connection.
32
33 Typically, these two daemons run on different hosts but a local setup
34 is also possible.
35
36 A simplified picture of a typical setup is as follows
37
38 server_host client_host
39 ~~~~~~~~~~~ ~~~~~~~~~~~
40
41 +-----------+ audio stream +-----------+
42 |para_server| -----------------------------> |para_audiod|
43 +-----------+ +-----------+
44 ^ ^
45 | |
46 | | connect
47 | |
48 | |
49 | +-----------+
50 | |para_audioc|
51 | +-----------+
52 |
53 |
54 | connect +-----------+
55 +-------------------------------------- |para_client|
56 +-----------+
57 The paraslash executables
58 -------------------------
59
60 ### para_server ###
61
62 para_server streams binary audio data (MP3, ...) over local and/or
63 remote networks. It listens on a TCP port and accepts commands such
64 as play, stop, pause, next from authenticated clients. There are
65 many more commands though, see the man page of para_server for a
66 description of all commands.
67
68 It supports three built-in network streaming protocols
69 (senders/receivers): HTTP, DCCP, or UDP. This is explained in more
70 detail in the section on [networking](#Networking).
71
72 The built-in audio file selector of paraslash is used to manage your
73 audio files. It maintains statistics on the usage of all available
74 audio files such as last-played time, and the number of times each
75 file was selected.
76
77 Additional information may be added to the database to allow
78 fine-grained selection based on various properties of the audio file,
79 including information found in (ID3) tags. However, old-fashioned
80 playlists are also supported.
81
82 It is also possible to store images (album covers) and lyrics in the
83 database and associate these to the corresponding audio files.
84
85 The section on the [audio file selector](#The.audio.file.selector)
86 discusses this topic.
87
88
89 ### para_client ###
90
91 The client program to connect to para_server. paraslash commands
92 are sent to para_server and the response is dumped to STDOUT. This
93 can be used by any scripting language to produce user interfaces with
94 little programming effort.
95
96 All connections between para_server and para_client are encrypted
97 with a symmetric session key. For each user of paraslash you must
98 create a public/secret RSA key pair for authentication.
99
100 If para_client is started without non-option arguments, an interactive
101 session (shell) is started. Command history and command completion are
102 supported through libreadline.
103
104 ### para_audiod ###
105
106 The local daemon that collects information from para_server.
107
108 It runs on the client side and connects to para_server. As soon as
109 para_server announces the availability of an audio stream, para_audiod
110 starts an appropriate receiver, any number of filters and a paraslash
111 writer to play the stream.
112
113 Moreover, para_audiod listens on a local socket and sends status
114 information about para_server and para_audiod to local clients on
115 request. Access via this local socket may be restricted by using Unix
116 socket credentials, if available.
117
118
119 ### para_audioc ###
120
121 The client program which talks to para_audiod. Used to control
122 para_audiod, to receive status info, or to grab the stream at any
123 point of the decoding process. Like para_client, para_audioc supports
124 interactive sessions on systems with libreadline.
125
126 ### para_recv ###
127
128 A command line HTTP/DCCP/UDP stream grabber. The http mode is
129 compatible with arbitrary HTTP streaming sources (e.g. icecast).
130 In addition to the three network streaming modes, para_recv can also
131 operate in local (afh) mode. In this mode it writes the content of
132 an audio file on the local file system in complete chunks to stdout,
133 optionally 'just in time'. This allows to cut an audio file without
134 first decoding it, and it enables third-party software which is unaware
135 of the particular audio format to send complete frames in real time.
136
137 ### para_filter ###
138
139 A filter program that reads from STDIN and writes to STDOUT.
140 Like para_recv, this is an atomic building block which can be used to
141 assemble higher-level audio receiving facilities. It combines several
142 different functionalities in one tool: decoders for multiple audio
143 formats and a number of processing filters, among these a normalizer
144 for audio volume.
145
146 ### para_afh ###
147
148 A small stand-alone program that prints tech info about the given
149 audio file to STDOUT. It can be instructed to print a "chunk table",
150 an array of offsets within the audio file.
151
152 ### para_write ###
153
154 A modular audio stream writer. It supports a simple file writer
155 output plug-in and optional WAV/raw players for ALSA (Linux) and for
156 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
157 or raw audio player.
158
159 ### para_play ###
160
161 A command line audio player.
162
163 ### para_gui ###
164
165 Curses-based gui that presents status information obtained in a curses
166 window. Appearance can be customized via themes. para_gui provides
167 key-bindings for the most common server commands and new key-bindings
168 can be added easily.
169
170 ### para_fade ###
171
172 An alarm clock and volume-fader for OSS and ALSA.
173
174 ===========
175 Quick start
176 ===========
177
178 This chapter lists the [necessary software](#Requirements)
179 that must be installed to compile the paraslash package, describes
180 how to [compile and install](#Installation) the paraslash
181 source code and the steps that have to be performed in order to
182 [set up](#Configuration) a typical server and client.
183
184 Requirements
185 ------------
186 ### For the impatient ###
187
188 git clone git://git.tuebingen.mpg.de/osl
189 cd osl && make && sudo make install && sudo ldconfig
190 sudo apt-get install autoconf libssl-dev help2man gengetopt m4 \
191 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
192 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev realpath \
193 libasound2-dev libao-dev libreadline-dev libncurses-dev \
194 libopus-dev
195
196 ### Detailed description ###
197
198 In any case you will need
199
200 - [libosl](http://people.tuebingen.mpg.de/maan/osl/). The _object
201 storage layer_ library is used by para_server. To clone the source
202 code repository, execute
203
204 git clone git://git.tuebingen.mpg.de/osl
205
206 - [lopsub](http://people.tuebingen.mpg.de/maan/lopsub/). The long
207 option parser for subcommands generates the command line and config
208 file parsers for all paraslash executables. Clone the source code
209 repository with
210
211 git clone git://git.tuebingen.mpg.de/lopsub
212
213 - [gcc](ftp://ftp.gnu.org/pub/gnu/gcc) or
214 [clang](http://clang.llvm.org). All gcc versions >= 4.2 are currently
215 supported. Clang version 1.1 or newer should work as well.
216
217 - [gnu make](ftp://ftp.gnu.org/pub/gnu/make) is also shipped with the
218 disto. On BSD systems the gnu make executable is often called gmake.
219
220 - [bash](ftp://ftp.gnu.org/pub/gnu/bash). Some scripts which run
221 during compilation require the _Bourne again shell_. It is most
222 likely already installed.
223
224 - [gengetopt](ftp://ftp.gnu.org/pub/gnu/gengetopt/) is needed to
225 generate the C code for the command line parsers of all paraslash
226 executables.
227
228 - [help2man](ftp://ftp.gnu.org/pub/gnu/help2man) is used to create
229 the man pages.
230
231 - [m4](ftp://ftp.gnu.org/pub/gnu/m4/). Some source files are generated
232 from templates by the m4 macro processor.
233
234 Optional:
235
236 - [openssl](http://www.openssl.org/) or
237 [libgcrypt](ftp://ftp.gnupg.org/gcrypt/libgcrypt/). At least one
238 of these two libraries is needed as the backend for cryptographic
239 routines on both the server and the client side. Both openssl and
240 libgcrypt are usually shipped with the distro, but you might have
241 to install the development package (`libssl-dev` or `libgcrypt-dev`
242 on debian systems) as well.
243
244 - [libmad](http://www.underbit.com/products/mad/). To compile in MP3
245 support for paraslash, the development package must be installed. It
246 is called `libmad0-dev` on debian-based systems. Note that libmad is
247 not necessary on the server side, i.e., for sending MP3 files.
248
249 - [libid3tag](http://www.underbit.com/products/mad/). For version-2
250 ID3 tag support, you willl need the libid3tag development package
251 `libid3tag0-dev`. Without libid3tag, only version-1 tags are
252 recognized. The mp3 tagger also needs this library for modifying
253 (id3v1 and id3v2) tags.
254
255 - [ogg vorbis](http://www.xiph.org/downloads/). For ogg vorbis streams
256 you need libogg, libvorbis, libvorbisfile. The corresponding Debian
257 packages are called `libogg-dev` and `libvorbis-dev`.
258
259 - [libfaad](http://www.audiocoding.com/). For aac files (m4a) you
260 need libfaad (`libfaad-dev`).
261
262 - [speex](http://www.speex.org/). In order to stream or decode speex
263 files, libspeex (`libspeex-dev`) is required.
264
265 - [flac](http://flac.sourceforge.net/). To stream or decode files
266 encoded with the _Free Lossless Audio Codec_, libFLAC (`libFLAC-dev`)
267 must be installed.
268
269 - [libsamplerate](http://www.mega-nerd.com/SRC/index.html). The
270 resample filter will only be compiled if this library is
271 installed. Debian package: `libsamplerate-dev`.
272
273 - [alsa-lib](ftp://ftp.alsa-project.org/pub/lib/). On Linux, you will
274 need to have the ALSA development package `libasound2-dev` installed.
275
276 - [libao](http://downloads.xiph.org/releases/ao/). Needed to build
277 the ao writer (ESD, PulseAudio,...). Debian package: `libao-dev`.
278
279 - [curses](ftp://ftp.gnu.org/pub/gnu/ncurses). Needed for
280 para_gui. Debian package: `libncurses-dev`.
281
282 - [GNU
283 Readline](http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html). If
284 this library (`libreadline-dev`) is installed, para_client, para_audioc
285 and para_play support interactive sessions.
286
287 Installation
288 ------------
289 To build the sources from a tarball, execute
290
291 ./configure && make
292
293 To build from git or a gitweb snapshot, run this command instead:
294
295 ./autogen.sh
296
297 There should be no errors but probably some warnings about missing
298 packages which usually implies that not all audio formats will be
299 supported. If headers or libs are installed at unusual locations you
300 might need to tell the configure script where to find them. Try
301
302 ./configure --help
303
304 to see a list of options. If the paraslash package was compiled
305 successfully, execute (optionally)
306
307 make test
308
309 to run the paraslash test suite. If all tests pass, execute as root
310
311 make install
312
313 to install executables under /usr/local/bin and the man pages under
314 /usr/local/man.
315
316 Configuration
317 -------------
318
319 ### Create a paraslash user ###
320
321 In order to control para_server at runtime you must create a paraslash
322 user. As authentication is based on the RSA crypto system you'll have
323 to create an RSA key pair. If you already have a user and an RSA key
324 pair, you may skip this step.
325
326 In this section we'll assume a typical setup: You would like to run
327 para_server on some host called server_host as user foo, and you want
328 to connect to para_server from another machine called client_host as
329 user bar.
330
331 As foo@server_host, create ~/.paraslash/server.users by typing the
332 following commands:
333
334 user=bar
335 target=~/.paraslash/server.users
336 key=~/.paraslash/id_rsa.pub.$user
337 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
338 mkdir -p ~/.paraslash
339 echo "user $user $key $perms" >> $target
340
341 Next, change to the "bar" account on client_host and generate the
342 key pair with the commands
343
344 ssh-keygen -q -t rsa -b 2048 -N '' -f $key
345
346 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
347 that para_server won't accept keys shorter than 2048 bits. Moreover,
348 para_client rejects private keys which are world-readable.
349
350 para_server only needs to know the public key of the key pair just
351 created. Copy this public key to server_host:
352
353 src=~/.ssh/id_rsa.pub
354 dest=.paraslash/id_rsa.pub.$LOGNAME
355 scp $src foo@server_host:$dest
356
357 Finally, tell para_client to connect to server_host:
358
359 conf=~/.paraslash/client.conf
360 echo 'hostname server_host' > $conf
361
362
363 ### Start para_server ###
364
365 For this first try, we'll use the info loglevel to make the output
366 of para_server more verbose.
367
368 para_server -l info
369
370 Now you can use para_client to connect to the server and issue
371 commands. Open a new shell as bar@client_host and try
372
373 para_client help
374 para_client si
375
376 to retrieve the list of available commands and some server info.
377 Don't proceed if this doesn't work.
378
379 ### Create and populate the database ###
380
381 An empty database is created with
382
383 para_client init
384
385 This initializes a couple of empty tables under
386 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
387 tables, but it's good to know that you can start from scratch with
388
389 rm -rf ~/.paraslash/afs_database-0.4
390
391 in case something went wrong.
392
393 Next, you need to add some audio files to that database so that
394 para_server knows about them. Choose an absolute path to a directory
395 containing some audio files and add them to the audio file table:
396
397 para_client add /my/mp3/dir
398
399 This might take a while, so it is a good idea to start with a directory
400 containing not too many files. Note that the table only contains data
401 about the audio files found, not the files themselves.
402
403 You may print the list of all known audio files with
404
405 para_client ls
406
407 ### Configure para_audiod ###
408
409 We will have to tell para_audiod that it should receive the audio
410 stream from server_host via http:
411
412 para_audiod -l info -r '.:http -i server_host'
413
414 You should now be able to listen to the audio stream once para_server
415 starts streaming. To activate streaming, execute
416
417 para_client play
418
419 Since no playlist has been specified yet, the "dummy" mode which
420 selects all known audio files is activated automatically. See the
421 section on the [audio file selector](#The.audio.file.selector) for how
422 to use playlists and moods to specify which files should be streamed
423 in which order.
424
425 Troubleshooting
426 ---------------
427
428 If you receive a socket related error on server or audiod startup,
429 make sure you have write permissions to the /var/paraslash directory:
430
431 sudo chown $LOGNAME /var/paraslash
432
433 Alternatively, use the --afs-socket (para_server) or --socket
434 (para_audiod) option to specify a different socket pathname.
435
436 To identify streaming problems try to receive, decode and play the
437 stream manually using para_recv, para_filter and para_write as follows.
438 For simplicity we assume that you're running Linux/ALSA and that only
439 MP3 files have been added to the database.
440
441 para_recv -r 'http -i server_host' > file.mp3
442 # (interrupt with CTRL+C after a few seconds)
443 ls -l file.mp3 # should not be empty
444 para_filter -f mp3dec -f wav < file.mp3 > file.wav
445 ls -l file.wav # should be much bigger than file.mp3
446 para_write -w alsa < file.wav
447
448 Double check what is logged by para_server and use the --loglevel
449 option of para_recv, para_filter and para_write to increase verbosity.
450
451 ===============
452 User management
453 ===============
454
455 para_server uses a challenge-response mechanism to authenticate
456 requests from incoming connections, similar to ssh's public key
457 authentication method. Authenticated connections are encrypted using
458 a stream cipher, either RC4 or AES in integer counter mode.
459
460 In this chapter we briefly describe RSA, RC4 and AES, and sketch the
461 [authentication handshake](#Client-server.authentication)
462 between para_client and para_server. User management is discussed
463 in the section on [the user_list file](#The.user_list.file).
464 These sections are all about communication between the client and the
465 server. Connecting para_audiod is a different matter and is described
466 in a [separate section](#Connecting.para_audiod).
467
468 RSA, RC4, AES
469 -------------
470
471 RSA is an asymmetric block cipher which is used in many applications,
472 including ssh and gpg. An RSA key consists in fact of two keys,
473 called the public key and the private key. A message can be encrypted
474 with either key and only the counterpart of that key can decrypt
475 the message. While RSA can be used for both signing and encrypting
476 a message, paraslash uses RSA only for the latter purpose. The
477 RSA public key encryption and signatures algorithms are defined in
478 detail in RFC 2437.
479
480 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
481 key stream to produce the output. Decryption uses the same function
482 calls as encryption. While RC4 supports variable key lengths,
483 paraslash uses a fixed length of 256 bits, which is considered a
484 strong encryption by today's standards. Since the same key must never
485 be used twice, a different, randomly-generated key is used for every
486 new connection.
487
488 AES, the advanced encryption standard, is a well-known symmetric block
489 cipher, i.e. a transformation operating on fixed-length blocks which
490 is determined by a single key for both encryption and decryption. Any
491 block cipher can be turned into a stream cipher by generating
492 a pseudo-random key stream by encrypting successive values of a
493 counter. The AES_CTR128 stream cipher used in paraslash is obtained
494 in this way from the AES block cipher with a 128 bit block size.
495
496 Client-server authentication
497 ----------------------------
498
499 The authentication handshake between para_client and para_server goes
500 as follows:
501
502 - para_client connects to para_server and sends an authentication
503 request for a user. It does so by connecting to TCP port 2990 of the
504 server host. This port is called the para_server _control port_.
505
506 - para_server accepts the connection and forks a child process which
507 handles the incoming request. The parent process keeps listening on the
508 control port while the child process (also called para_server below)
509 continues as follows.
510
511 - para_server loads the RSA public key of that user, fills a
512 fixed-length buffer with random bytes, encrypts that buffer using the
513 public key and sends the encrypted buffer to the client. The first
514 part of the buffer is the challenge which is used for authentication
515 while the second part is the session key.
516
517 - para_client receives the encrypted buffer and decrypts it with the
518 user's private key, thereby obtaining the challenge buffer and the
519 session key. It sends the SHA1 hash value of the challenge back to
520 para_server and stores the session key for further use.
521
522 - para_server also computes the SHA1 hash of the challenge and compares
523 it against what was sent back by the client.
524
525 - If the two hashes do not match, the authentication has failed and
526 para_server closes the connection.
527
528 - Otherwise the user is considered authenticated and the client is
529 allowed to proceed by sending a command to be executed. From this
530 point on the communication is encrypted using the stream cipher with
531 the session key known to both peers.
532
533 paraslash relies on the quality of the pseudo-random bytes provided
534 by the crypto library (openssl or libgcrypt), on the security of the
535 implementation of the RSA, RC4 and AES crypto routines and on the
536 infeasibility to invert the SHA1 function.
537
538 Neither para_server or para_client create RSA keys on their
539 own. This has to be done once for each user as sketched in
540 [Quick start](#Quick.start) and discussed in more detail
541 [below](#The.user_list.file).
542
543 The user_list file
544 ------------------
545
546 At startup para_server reads the user list file which contains one
547 line per user. The default location of the user list file may be
548 changed with the --user-list option.
549
550 There should be at least one user in this file. Each user must have
551 an RSA key pair. The public part of the key is needed by para_server
552 while the private key is needed by para_client. Each line of the
553 user list file must be of the form
554
555 user <username> <key> <perms>
556
557 where _username_ is an arbitrary string (usually the user's login
558 name), _key_ is the full path to that user's public RSA key, and
559 _perms_ is a comma-separated list of zero or more of the following
560 permission bits:
561
562 +---------------------------------------------------------+
563 | AFS_READ | read the contents of the databases |
564 +-----------+---------------------------------------------+
565 | AFS_WRITE | change database contents |
566 +-----------+---------------------------------------------+
567 | VSS_READ | obtain information about the current stream |
568 +-----------+---------------------------------------------+
569 | VSS_WRITE | change the current stream |
570 +---------------------------------------------------------+
571
572 The permission bits specify which commands the user is allowed to
573 execute. The output of
574
575 para_client help
576
577 contains in the third column the permissions needed to execute the
578 command.
579
580 It is possible to make para_server reread the user_list file by
581 executing the paraslash "hup" command or by sending SIGHUP to the
582 PID of para_server.
583
584 Connecting para_audiod
585 ----------------------
586
587 para_audiod listens on a Unix domain socket. Those sockets are
588 for local communication only, so only local users can connect to
589 para_audiod. The default is to let any user connect but this can be
590 restricted on platforms that support UNIX socket credentials which
591 allow para_audiod to obtain the Unix credentials of the connecting
592 process.
593
594 Use para_audiod's --user-allow option to allow connections only for
595 a limited set of users.
596
597 =======================
598 The audio file selector
599 =======================
600
601 paraslash comes with a sophisticated audio file selector (AFS),
602 whose main task is to determine which file to stream next, based on
603 information on the audio files stored in a database. It communicates
604 also with para_client whenever an AFS command is executed, for example
605 to answer a database query.
606
607 Besides the traditional playlists, AFS supports audio file selection
608 based on _moods_ which act as a filter that limits the set of all
609 known audio files to those which satisfy certain criteria. It also
610 maintains tables containing images (e.g. album cover art) and lyrics
611 that can be associated with one or more audio files.
612
613 AFS employs [libosl](http://people.tuebingen.mpg.de/maan/osl/), the
614 object storage layer library, as the backend library for storing
615 information on audio files, playlists, etc. This library offers
616 functionality similar to a relational database, but is much more
617 lightweight than a full database backend.
618
619 In this chapter we sketch the setup of the [AFS
620 process](#The.AFS.process) during server startup and proceed with the
621 description of the [layout](#Database.layout) of the various database
622 tables. The section on [playlists and moods](#Playlists.and.moods)
623 explains these two audio file selection mechanisms in detail
624 and contains pratical examples. The way [file renames and content
625 changes](#File.renames.and.content.changes) are detected is discussed
626 briefly before the [Troubleshooting](#Troubleshooting) section
627 concludes the chapter.
628
629 The AFS process
630 ---------------
631
632 On startup, para_server forks to create the AFS process which opens
633 the OSL database tables. The server process communicates with the
634 AFS process via pipes and shared memory. Usually, the AFS process
635 awakes only briefly whenever the current audio file changes. The AFS
636 process determines the next audio file, opens it, verifies it has
637 not been changed since it was added to the database and passes the
638 open file descriptor to the server process, along with audio file
639 meta-data such as file name, duration, audio format and so on. The
640 server process then starts to stream the audio file.
641
642 The AFS process also accepts connections from local clients via
643 a well-known socket. However, only child processes of para_server
644 may connect through this socket. All server commands that have the
645 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
646 change the database.
647
648 Database layout
649 ---------------
650
651 ### The audio file table ###
652
653 This is the most important and usually also the largest table of the
654 AFS database. It contains the information needed to stream each audio
655 file. In particular the following data is stored for each audio file.
656
657 - SHA1 hash value of the audio file contents. This is computed once
658 when the file is added to the database. Whenever AFS selects this
659 audio file for streaming the hash value is recomputed and checked
660 against the value stored in the database to detect content changes.
661
662 - The time when this audio file was last played.
663
664 - The number of times the file has been played so far.
665
666 - The attribute bitmask.
667
668 - The image id which describes the image associated with this audio
669 file.
670
671 - The lyrics id which describes the lyrics associated with this
672 audio file.
673
674 - The audio format id (MP3, OGG, ...).
675
676 - An amplification value that can be used by the amplification filter
677 to pre-amplify the decoded audio stream.
678
679 - The chunk table. It describes the location and the timing of the
680 building blocks of the audio file. This is used by para_server to
681 send chunks of the file at appropriate times.
682
683 - The duration of the audio file.
684
685 - Tag information contained in the audio file (ID3 tags, Vorbis
686 comments, ...).
687
688 - The number of channels
689
690 - The encoding bitrate.
691
692 - The sampling frequency.
693
694 To add or refresh the data contained in the audio file table, the _add_
695 command is used. It takes the full path of either an audio file or a
696 directory. In the latter case, the directory is traversed recursively
697 and all files which are recognized as valid audio files are added to
698 the database.
699
700 ### The attribute table ###
701
702 The attribute table contains two columns, _name_ and _bitnum_. An
703 attribute is simply a name for a certain bit number in the attribute
704 bitmask of the audio file table.
705
706 Each of the 64 bits of the attribute bitmask can be set for each
707 audio file individually. Hence up to 64 different attributes may be
708 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
709 "german_lyrics", "speech", whatever. You are free to choose as
710 many attributes as you like and there are no naming restrictions
711 for attributes.
712
713 A new attribute "test" is created by
714
715 para_client addatt test
716 and
717 para_client lsatt
718
719 lists all available attributes. You can set the "test" attribute for
720 an audio file by executing
721
722 para_client setatt test+ /path/to/the/audio/file
723
724 Similarly, the "test" bit can be removed from an audio file with
725
726 para_client setatt test- /path/to/the/audio/file
727
728 Instead of a path you may use a shell wildcard pattern. The attribute
729 is applied to all audio files matching this pattern:
730
731 para_client setatt test+ '/test/directory/*'
732
733 The command
734
735 para_client -- ls -l=v
736
737 gives you a verbose listing of your audio files also showing which
738 attributes are set.
739
740 In case you wonder why the double-dash in the above command is needed:
741 It tells para_client to not interpret the options after the dashes. If
742 you find this annoying, just say
743
744 alias para='para_client --'
745
746 and be happy. In what follows we shall use this alias.
747
748 The "test" attribute can be dropped from the database with
749
750 para rmatt test
751
752 Read the output of
753
754 para help ls
755 para help setatt
756
757 for more information and a complete list of command line options to
758 these commands.
759
760 ### Blob tables ###
761
762 The image, lyrics, moods and playlists tables are all blob tables.
763 Blob tables consist of three columns each: The identifier which is
764 a positive number that is auto-incremented, the name (an arbitrary
765 string) and the content (the blob).
766
767 All blob tables support the same set of actions: cat, ls, mv, rm
768 and add. Of course, _add_ is used for adding new blobs to the table
769 while the other actions have the same meaning as the corresponding
770 Unix commands. The paraslash commands to perform these actions are
771 constructed as the concatenation of the table name and the action. For
772 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
773 manipulate or query the image table.
774
775 The add variant of these commands is special as these commands read
776 the blob contents from stdin. To add an image to the image table the
777 command
778
779 para addimg image_name < file.jpg
780
781 can be used.
782
783 Note that the images and lyrics are not interpreted at all, and also
784 the playlist and the mood blobs are only investigated when the mood
785 or playlist is activated with the select command.
786
787 ### The score table ###
788
789 The score table describes those audio files which are admissible for
790 the current mood or playlist (see below). The table has two columns:
791 a pointer to a row of the audio file table and a score value.
792
793 Unlike all other tables of the database, the score table remains in
794 memory and is never stored on disk. It is initialized at startup and
795 recomputed when the select command loads a new mood or playlist.
796
797 When the audio file selector is asked to open the next audio file,
798 it picks the row with the highest score, opens the corresponding
799 file and passes the file descriptor to the virtual streaming system.
800 At this point the last_played and the num_played fields of the selected
801 file are updated and the score is recomputed.
802
803 Playlists and moods
804 -------------------
805
806 Playlists and moods offer two different ways of specifying the set of
807 admissible files. A playlist in itself describes a set of admissible
808 files. A mood, in contrast, describes the set of admissible files in
809 terms of attributes and other type of information available in the
810 audio file table. As an example, a mood can define a filename pattern,
811 which is then matched against the names of audio files in the table.
812
813 ### Playlists ###
814
815 Playlists are accommodated in the playlist table of the afs database,
816 using the aforementioned blob format for tables. A new playlist is
817 created with the addpl command by specifying the full (absolute)
818 paths of all desired audio files, separated by newlines. Example:
819
820 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
821
822 If _my_playlist_ already exists it is overwritten. To activate the
823 new playlist, execute
824
825 para select p/my_playlist
826
827 The audio file selector will assign scores to each entry of the list,
828 in descending order so that files will be selected in order. If a
829 file could not be opened for streaming, its entry is removed from
830 the score table (but not from the playlist).
831
832 ### Moods ###
833
834 A mood consists of a unique name and its *mood definition*, which is
835 a set of *mood lines* containing expressions in terms of attributes
836 and other data contained in the database.
837
838 At any time at most one mood can be *active* which means that
839 para_server is going to select only files from that subset of
840 admissible files.
841
842 So in order to create a mood definition one has to write a set of
843 mood lines. Mood lines come in three flavours: Accept lines, deny
844 lines and score lines.
845
846 The general syntax of the three types of mood lines is
847
848
849 accept [with score <score>] [if] [not] <mood_method> [options]
850 deny [with score <score>] [if] [not] <mood_method> [options]
851 score <score> [if] [not] <mood_method> [options]
852
853
854 Here <score> is either an integer or the string "random" which assigns
855 a random score to all matching files. The score value changes the
856 order in which admissible files are going to be selected, but is of
857 minor importance for this introduction.
858
859 So we concentrate on the first two forms, i.e. accept and deny
860 lines. As usual, everything in square brackets is optional, i.e.
861 accept/deny lines take the following form when ignoring scores:
862
863 accept [if] [not] <mood_method> [options]
864
865 and analogously for the deny case. The "if" keyword is only syntactic
866 sugar and has no function. The "not" keyword just inverts the result,
867 so the essence of a mood line is the mood method part and the options
868 following thereafter.
869
870 A *mood method* is realized as a function which takes an audio file
871 and computes a number from the data contained in the database.
872 If this number is non-negative, we say the file *matches* the mood
873 method. The file matches the full mood line if it either
874
875 - matches the mood method and the "not" keyword is not given,
876 or
877 - does not match the mood method, but the "not" keyword is given.
878
879 The set of admissible files for the whole mood is now defined as those
880 files which match at least one accept mood line, but no deny mood line.
881 More formally, an audio file F is admissible if and only if
882
883 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
884
885 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
886 lines and "~" means "matches".
887
888 The cases where no mood lines of accept/deny type are defined need
889 special treatment:
890
891 - Neither accept nor deny lines: This treats all files as
892 admissible (in fact, that is the definition of the dummy mood
893 which is activated automatically if no moods are available).
894
895 - Only accept lines: A file is admissible iff it matches at
896 least one accept line:
897
898 F ~ AL1 or F ~ AL2 or ...
899
900 - Only deny lines: A file is admissible iff it matches no
901 deny line:
902
903 not (F ~ DL1 or F ~ DN2 ...)
904
905
906
907 ### List of mood_methods ###
908
909 no_attributes_set
910
911 Takes no arguments and matches an audio file if and only if no
912 attributes are set.
913
914 is_set <attribute_name>
915
916 Takes the name of an attribute and matches iff that attribute is set.
917
918 path_matches <pattern>
919
920 Takes a filename pattern and matches iff the path of the audio file
921 matches the pattern.
922
923 artist_matches <pattern>
924 album_matches <pattern>
925 title_matches <pattern>
926 comment_matches <pattern>
927
928 Takes an extended regular expression and matches iff the text of the
929 corresponding tag of the audio file matches the pattern. If the tag
930 is not set, the empty string is matched against the pattern.
931
932 year ~ <num>
933 bitrate ~ <num>
934 frequency ~ <num>
935 channels ~ <num>
936 num_played ~ <num>
937 image_id ~ <num>
938 lyrics_id ~ <num>
939
940 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
941 <num>. Matches an audio file iff the condition <val> ~ <num> is
942 satisfied where val is the corresponding value of the audio file
943 (value of the year tag, bitrate in kbit/s, etc.).
944
945 The year tag is special as its value is undefined if the audio file
946 has no year tag or the content of the year tag is not a number. Such
947 audio files never match. Another difference is the special treatment
948 if the year tag is a two-digit number. In this case either 1900 or
949 2000 is added to the tag value, depending on whether the number is
950 greater than 2000 plus the current year.
951
952
953 ### Mood usage ###
954
955 To create a new mood called "my_mood", write its definition into
956 some temporary file, say "tmpfile", and add it to the mood table
957 by executing
958
959 para addmood my_mood < tmpfile
960
961 If the mood definition is really short, you may just pipe it to the
962 client instead of using temporary files. Like this:
963
964 echo "$MOOD_DEFINITION" | para addmood my_mood
965
966 There is no need to keep the temporary file since you can always use
967 the catmood command to get it back:
968
969 para catmood my_mood
970
971 A mood can be activated by executing
972
973 para select m/my_mood
974
975 Once active, the list of admissible files is shown by the ls command
976 if the "-a" switch is given:
977
978 para ls -a
979
980
981 ### Example mood definition ###
982
983 Suppose you have defined attributes "punk" and "rock" and want to define
984 a mood containing only Punk-Rock songs. That is, an audio file should be
985 admissible if and only if both attributes are set. Since
986
987 punk and rock
988
989 is obviously the same as
990
991 not (not punk or not rock)
992
993 (de Morgan's rule), a mood definition that selects only Punk-Rock
994 songs is
995
996 deny if not is_set punk
997 deny if not is_set rock
998
999
1000
1001 File renames and content changes
1002 --------------------------------
1003
1004 Since the audio file selector knows the SHA1 of each audio file that
1005 has been added to the afs database, it recognizes if the content of
1006 a file has changed, e.g. because an ID3 tag was added or modified.
1007 Also, if a file has been renamed or moved to a different location,
1008 afs will detect that an entry with the same hash value already exists
1009 in the audio file table.
1010
1011 In both cases it is enough to just re-add the new file. In the
1012 first case (file content changed), the audio table is updated, while
1013 metadata such as the num_played and last_played fields, as well as
1014 the attributes, remain unchanged. In the other case, when the file
1015 is moved or renamed, only the path information is updated, all other
1016 data remains as before.
1017
1018 It is possible to change the behaviour of the add command by using the
1019 "-l" (lazy add) or the "-f" (force add) option.
1020
1021 Troubleshooting
1022 ---------------
1023
1024 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1025 executables have a brief online help which is displayed when -h is
1026 given. The --detailed-help option prints the full help text.
1027
1028 If para_server crashed or was killed by SIGKILL (signal 9), it
1029 may refuse to start again because of "dirty osl tables". In this
1030 case you'll have to run the oslfsck program of libosl to fix your
1031 database:
1032
1033 oslfsck -fd ~/.paraslash/afs_database-0.4
1034
1035 However, make sure para_server isn't running before executing oslfsck.
1036
1037 If you don't mind to recreate your database you can start
1038 from scratch by removing the entire database directory, i.e.
1039
1040 rm -rf ~/.paraslash/afs_database-0.4
1041
1042 Be aware that this removes all attribute definitions, all playlists
1043 and all mood definitions and requires to re-initialize the tables.
1044
1045 Although oslfsck fixes inconsistencies in database tables it doesn't
1046 care about the table contents. To check for invalid table contents, use
1047
1048 para_client check
1049
1050 This prints out references to missing audio files as well as invalid
1051 playlists and mood definitions.
1052
1053 Similarly, para_audiod refuses to start if its socket file exists, since
1054 this indicates that another instance of para_audiod is running. After
1055 a crash a stale socket file might remain and you must run
1056
1057 para_audiod --force
1058
1059 once to fix it up.
1060
1061 =======================================
1062 Audio formats and audio format handlers
1063 =======================================
1064
1065 Audio formats
1066 -------------
1067
1068 The following audio formats are supported by paraslash:
1069
1070 ### MP3 ###
1071
1072 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1073 designed as part of its MPEG-1 standard. An MP3 file is made up of
1074 multiple MP3 frames, which consist of a header and a data block. The
1075 size of an MP3 frame depends on the bit rate and on the number
1076 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1077 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1078 400 bytes large.
1079
1080 ### OGG/Vorbis ###
1081
1082 OGG is a standardized audio container format, while Vorbis is an
1083 open source codec for lossy audio compression. Since Vorbis is most
1084 commonly made available via the OGG container format, it is often
1085 referred to as OGG/Vorbis. The OGG container format divides data into
1086 chunks called OGG pages. A typical OGG page is about 4KB large. The
1087 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1088 may vary considerably.
1089
1090 ### OGG/Speex ###
1091
1092 Speex is an open-source speech codec that is based on CELP (Code
1093 Excited Linear Prediction) coding. It is designed for voice
1094 over IP applications, has modest complexity and a small memory
1095 footprint. Wideband and narrowband (telephone quality) speech are
1096 supported. As for Vorbis audio, Speex bit-streams are often stored
1097 in OGG files. As of 2012 this codec is considered obsolete since the
1098 Oppus codec, described below, surpasses its performance in all areas.
1099
1100 ### OGG/Opus ###
1101
1102 Opus is a lossy audio compression format standardized through RFC
1103 6716 in 2012. It combines the speech-oriented SILK codec and the
1104 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1105 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1106 containers. All known software patents which cover Opus are licensed
1107 under royalty-free terms.
1108
1109 ### AAC ###
1110
1111 Advanced Audio Coding (AAC) is a standardized, lossy compression
1112 and encoding scheme for digital audio which is the default audio
1113 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1114 the container format and audio files encoded with AAC have the .m4a
1115 extension. A typical AAC frame is about 700 bytes large.
1116
1117 ### WMA ###
1118
1119 Windows Media Audio (WMA) is an audio data compression technology
1120 developed by Microsoft. A WMA file is usually encapsulated in the
1121 Advanced Systems Format (ASF) container format, which also specifies
1122 how meta data about the file is to be encoded. The bit stream of WMA
1123 is composed of superframes, each containing one or more frames of
1124 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1125
1126 ### FLAC ###
1127
1128 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1129 loss. It gives better compression ratios than a general purpose
1130 compressor like zip or bzip2 because FLAC is designed specifically
1131 for audio. A FLAC-encoded file consists of frames of varying size, up
1132 to 16K. Each frame starts with a header that contains all information
1133 necessary to decode the frame.
1134
1135 Meta data
1136 ---------
1137
1138 Unfortunately, each audio format has its own conventions how meta
1139 data is added as tags to the audio file.
1140
1141 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1142 is rather simple but also very limited as it supports only artist,
1143 title, album, year and comment tags. Each of these can only be at most
1144 32 characters long. ID3, version 2 is much more flexible but requires
1145 a separate library being installed for paraslash to support it.
1146
1147 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1148 comments, which are typically implemented as strings of the form
1149 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1150 tags are appropriate for the content.
1151
1152 AAC files usually use the MPEG-4 container format for storing meta
1153 data while WMA files wrap meta data as special objects within the
1154 ASF container format.
1155
1156 paraslash only tracks the most common tags that are supported by
1157 all tag variants: artist, title, year, album, comment. When a file
1158 is added to the AFS database, the meta data of the file is extracted
1159 and stored in the audio file table.
1160
1161 Chunks and chunk tables
1162 -----------------------
1163
1164 paraslash uses the word "chunk" as common term for the building blocks
1165 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1166 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1167 size varies considerably between audio formats, from a few hundred
1168 bytes (MP3) up to 16K (FLAC).
1169
1170 The chunk table contains the offsets within the audio file that
1171 correspond to the chunk boundaries of the file. Like the meta data,
1172 the chunk table is computed and stored in the database whenever an
1173 audio file is added.
1174
1175 The paraslash senders (see below) always send complete chunks. The
1176 granularity for seeking is therefore determined by the chunk size.
1177
1178 Audio format handlers
1179 ---------------------
1180
1181 For each audio format paraslash contains an audio format handler whose
1182 first task is to tell whether a given file is a valid audio file of
1183 this type. If so, the audio file handler extracts some technical data
1184 (duration, sampling rate, number of channels etc.), computes the
1185 chunk table and reads the meta data.
1186
1187 The audio format handler code is linked into para_server and executed
1188 via the _add_ command. The same code is also available as a stand-alone
1189 tool, para_afh, which prints the technical data, the chunk table
1190 and the meta data of a file. Moreover, all audio format handlers are
1191 combined in the afh receiver which is part of para_recv and para_play.
1192
1193 ==========
1194 Networking
1195 ==========
1196
1197 Paraslash uses different network connections for control and data.
1198 para_client communicates with para_server over a dedicated TCP control
1199 connection. To transport audio data, separate data connections are
1200 used. For these data connections, a variety of transports (UDP, DCCP,
1201 HTTP) can be chosen.
1202
1203 The chapter starts with the [control
1204 service](#The.paraslash.control.service), followed by a section
1205 on the various [streaming protocols](#Streaming.protocols)
1206 in which the data connections are described. The way
1207 audio file headers are embedded into the stream is discussed
1208 [briefly](#Streams.with.headers.and.headerless.streams) before the
1209 [example section](#Networking.examples) which illustrates typical
1210 commands for real-life scenarios.
1211
1212 Both IPv4 and IPv6 are supported.
1213
1214 The paraslash control service
1215 -----------------------------
1216
1217 para_server is controlled at runtime via the paraslash control
1218 connection. This connection is used for server commands (play, stop,
1219 ...) as well as for afs commands (ls, select, ...).
1220
1221 The server listens on a TCP port and accepts connections from clients
1222 that connect the open port. Each connection causes the server to fork
1223 off a client process which inherits the connection and deals with that
1224 client only. In this classical accept/fork approach the server process
1225 is unaffected if the child dies or goes crazy for whatever reason. In
1226 fact, the child process can not change address space of server process.
1227
1228 The section on [client-server
1229 authentication](#Client-server.authentication) above described the
1230 early connection establishment from the crypto point of view. Here
1231 it is described what happens after the connection (including crypto
1232 setup) has been established. There are four processes involved during
1233 command dispatch as sketched in the following diagram.
1234
1235 server_host client_host
1236 ~~~~~~~~~~~ ~~~~~~~~~~~
1237
1238 +-----------+ connect +-----------+
1239 |para_server|<------------------------------ |para_client|
1240 +-----------+ +-----------+
1241 | ^
1242 | fork +---+ |
1243 +----------> |AFS| |
1244 | +---+ |
1245 | ^ |
1246 | | |
1247 | | connect (cookie) |
1248 | | |
1249 | | |
1250 | fork +-----+ inherited connection |
1251 +---------->|child|<--------------------------+
1252 +-----+
1253
1254 Note that the child process is not a child of the afs process,
1255 so communication of these two processes has to happen via local
1256 sockets. In order to avoid abuse of the local socket by unrelated
1257 processes, a magic cookie is created once at server startup time just
1258 before the server process forks off the AFS process. This cookie is
1259 known to the server, AFS and the child, but not to unrelated processes.
1260
1261 There are two different kinds of commands: First there are commands
1262 that cause the server to respond with some answer such as the list
1263 of all audio files. All but the addblob commands (addimg, addlyr,
1264 addpl, addmood) are of this kind. The addblob commands add contents
1265 to the database, so they need to transfer data the other way round,
1266 from the client to the server.
1267
1268 There is no knowledge about the server commands built into para_client,
1269 so it does not know about addblob commands. Instead, the server sends
1270 a special "awaiting data" packet for these commands. If the client
1271 receives this packet, it sends STDIN to the server, otherwise it
1272 dumps data from the server to STDOUT.
1273
1274 Streaming protocols
1275 -------------------
1276
1277 A network (audio) stream usually consists of one streaming source,
1278 the _sender_, and one or more _receivers_ which read data over the
1279 network from the streaming source.
1280
1281 Senders are thus part of para_server while receivers are part of
1282 para_audiod. Moreover, there is the stand-alone tool para_recv which
1283 can be used to manually download a stream, either from para_server
1284 or from a web-based audio streaming service.
1285
1286 The following three streaming protocols are supported by paraslash:
1287
1288 - HTTP. Recommended for public streams that can be played by any
1289 player like mpg123, xmms, itunes, winamp, etc. The HTTP sender is
1290 supported on all operating systems and all platforms.
1291
1292 - DCCP. Recommended for LAN streaming. DCCP is currently available
1293 only for Linux.
1294
1295 - UDP. Recommended for multicast LAN streaming.
1296
1297 See the Appendix on [network protocols](/#Network.protocols)
1298 for brief descriptions of the various protocols relevant for network
1299 audio streaming with paraslash.
1300
1301 It is possible to activate more than one sender simultaneously.
1302 Senders can be controlled at run time and via config file and command
1303 line options.
1304
1305 Note that audio connections are _not_ encrypted. Transport or Internet
1306 layer encryption should be used if encrypted data connections are
1307 needed.
1308
1309 Since DCCP and TCP are both connection-oriented protocols, connection
1310 establishment/teardown and access control are very similar between
1311 these two streaming protocols. UDP is the most lightweight option,
1312 since in contrast to TCP/DCCP it is connectionless. It is also the
1313 only protocol supporting IP multicast.
1314
1315 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1316 clients to connect and establish a connection via some protocol-defined
1317 handshake mechanism. Both senders maintain two linked lists each:
1318 The list of all clients which are currently connected, and the list
1319 of access control entries which determines who is allowed to connect.
1320 IP-based access control may be configured through config file and
1321 command line options and via the "allow" and "deny" sender subcommands.
1322
1323 Upon receiving a GET request from the client, the HTTP sender sends
1324 back a status line and a message. The body of this message is the
1325 audio stream. This is common practice and is supported by many popular
1326 clients which can thus be used to play a stream offered by para_server.
1327 For DCCP things are a bit simpler: No messages are exchanged between
1328 the receiver and sender. The client simply connects and the sender
1329 starts to stream.
1330
1331 DCCP is an experimental protocol which offers a number of new features
1332 not available for TCP. Both ends can negotiate these features using
1333 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1334 datagram-based (no retransmissions) and thus should not be used over
1335 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1336 is access to a variety of different congestion-control mechanisms
1337 called CCIDs. Two different CCIDs are available per default on Linux:
1338
1339
1340 - _CCID 2_. A Congestion Control mechanism similar to that of TCP. The
1341 sender maintains a congestion window and halves this window in response
1342 to congestion.
1343
1344
1345 - _CCID-3_. Designed to be fair when competing for bandwidth.
1346 It has lower variation of throughput over time compared with TCP,
1347 which makes it suitable for streaming media.
1348
1349 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1350 single list, the _target list_. This list describes the set of clients
1351 to which the stream is sent. There is no list for access control and
1352 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1353 and "delete" commands can be used to modify the target list.
1354
1355 Since both UDP and DCCP offer an unreliable datagram-based transport,
1356 additional measures are necessary to guard against disruptions over
1357 networks that are lossy or which may be subject to interference (as
1358 is for instance the case with WiFi). Paraslash uses FEC (Forward
1359 Error Correction) to guard against packet losses and reordering. The
1360 stream is FEC-encoded before it is sent through the UDP socket and
1361 must be decoded accordingly on the receiver side.
1362
1363 The packet size and the amount of redundancy introduced by FEC can
1364 be configured via the FEC parameters which are dictated by server
1365 and may also be configured through the "sender" command. The FEC
1366 parameters are encoded in the header of each network packet, so no
1367 configuration is necessary on the receiver side. See the section on
1368 [FEC](#Forward.error.correction) below.
1369
1370 Streams with headers and headerless streams
1371 -------------------------------------------
1372
1373 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1374 needed to decode the stream is only contained in the audio file
1375 header of the container format but not in each data chunk. Clients
1376 must be able to obtain this information in case streaming starts in
1377 the middle of the file or if para_audiod is started while para_server
1378 is already sending a stream.
1379
1380 This is accomplished in different ways, depending on the streaming
1381 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1382 header is sent prior to audio file data. This technique however does
1383 not work for the connectionless UDP transport. Hence the audio file
1384 header is periodically being embedded into the UDP audio data stream.
1385 By default, the header is resent after five seconds. The receiver has
1386 to wait until the next header arrives before it can start decoding
1387 the stream.
1388
1389 Networking examples
1390 -------------------
1391
1392 The "si" (server info) command lists some information about the
1393 currently running server process.
1394
1395 -> Show PIDs, number of connected clients, uptime, and more:
1396
1397 para_client si
1398
1399 The sender command of para_server prints information about senders,
1400 like the various access control lists, and it allows to (de-)activate
1401 senders and to change the access permissions at runtime.
1402
1403 -> List all senders
1404
1405 para_client sender
1406
1407 -> Obtain general help for the sender command:
1408
1409 para_client help sender
1410
1411 -> Get help for a specific sender (contains further examples):
1412
1413 s=http # or dccp or udp
1414 para_client sender $s help
1415
1416 -> Show status of the http sender
1417
1418 para_client sender http status
1419
1420 By default para_server activates both the HTTP and th DCCP sender on
1421 startup. This can be changed via command line options or para_server's
1422 config file.
1423
1424 -> List config file options for senders:
1425
1426 para_server -h
1427
1428 All senders share the "on" and "off" commands, so senders may be
1429 activated and deactivated independently of each other.
1430
1431 -> Switch off the http sender:
1432
1433 para_client sender http off
1434
1435 -> Receive a DCCP stream using CCID2 and write the output into a file:
1436
1437 host=foo.org; ccid=2; filename=bar
1438 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1439
1440 Note the quotes around the arguments for the dccp receiver. Each
1441 receiver has its own set of command line options and its own command
1442 line parser, so arguments for the dccp receiver must be protected
1443 from being interpreted by para_recv.
1444
1445 -> Start UDP multicast, using the default multicast address:
1446
1447 para_client sender udp add 224.0.1.38
1448
1449 -> Receive FEC-encoded multicast stream and write the output into a file:
1450
1451 filename=foo
1452 para_recv -r udp > $filename
1453
1454 -> Add an UDP unicast for a client to the target list of the UDP sender:
1455
1456 t=client.foo.org
1457 para_client sender udp add $t
1458
1459 -> Receive this (FEC-encoded) unicast stream:
1460
1461 filename=foo
1462 para_recv -r 'udp -i 0.0.0.0' > $filename
1463
1464 -> Create a minimal config for para_audiod for HTTP streams:
1465
1466 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1467 echo receiver \".:http -i $s\" > $c
1468 para_audiod --config $c
1469
1470 =======
1471 Filters
1472 =======
1473
1474 A paraslash filter is a module which transforms an input stream into
1475 an output stream. Filters are included in the para_audiod executable
1476 and in the stand-alone tool para_filter which usually contains the
1477 same modules.
1478
1479 While para_filter reads its input stream from STDIN and writes
1480 the output to STDOUT, the filter modules of para_audiod are always
1481 connected to a receiver which produces the input stream and a writer
1482 which absorbs the output stream.
1483
1484 Some filters depend on a specific library and are not compiled in
1485 if this library was not found at compile time. To see the list of
1486 supported filters, run para_filter and para_audiod with the --help
1487 option. The output looks similar to the following:
1488
1489 Available filters:
1490 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1491
1492 Out of these filter modules, a chain of filters can be constructed,
1493 much in the way Unix pipes can be chained, and analogous to the use
1494 of modules in gstreamer: The output of the first filter becomes the
1495 input of the second filter. There is no limitation on the number of
1496 filters and the same filter may occur more than once.
1497
1498 Like receivers, each filter has its own command line options which
1499 must be quoted to protect them from the command line options of
1500 the driving application (para_audiod or para_filter). Example:
1501
1502 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1503
1504 For para_audiod, each audio format has its own set of filters. The
1505 name of the audio format for which the filter should be applied can
1506 be used as the prefix for the filter option. Example:
1507
1508 para_audiod -f 'mp3:prebuffer --duration 300'
1509
1510 The "mp3" prefix above is actually interpreted as a POSIX extended
1511 regular expression. Therefore
1512
1513 para_audiod -f '.:prebuffer --duration 300'
1514
1515 activates the prebuffer filter for all supported audio formats (because
1516 "." matches all audio formats) while
1517
1518 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1519
1520 activates it only for wma and ogg streams.
1521
1522 Decoders
1523 --------
1524
1525 For each supported audio format there is a corresponding filter
1526 which decodes audio data in this format to 16 bit PCM data which
1527 can be directly sent to the sound device or any other software that
1528 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1529 filters are called _decoders_ in general, and xxxdec is the name of
1530 the paraslash decoder for the audio format xxx. For example, the mp3
1531 decoder is called mp3dec.
1532
1533 Note that the output of the decoder is about 10 times larger than
1534 its input. This means that filters that operate on the decoded audio
1535 stream have to deal with much more data than filters that transform
1536 the audio stream before it is fed to the decoder.
1537
1538 Paraslash relies on external libraries for most decoders, so these
1539 libraries must be installed for the decoder to be included in the
1540 executables. For example, the mp3dec filter depends on the mad library.
1541
1542 Forward error correction
1543 ------------------------
1544
1545 As already mentioned [earlier](#Streaming.protocols), paraslash
1546 uses forward error correction (FEC) for the unreliable UDP and
1547 DCCP transports. FEC is a technique which was invented already in
1548 1960 by Reed and Solomon and which is widely used for the parity
1549 calculations of storage devices (RAID arrays). It is based on the
1550 algebraic concept of finite fields, today called Galois fields, in
1551 honour of the mathematician Galois (1811-1832). The FEC implementation
1552 of paraslash is based on code by Luigi Rizzo.
1553
1554 Although the details require a sound knowledge of the underlying
1555 mathematics, the basic idea is not hard to understand: For positive
1556 integers k and n with k < n it is possible to compute for any k given
1557 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1558 ..., p_r such that all data bytes can be reconstructed from *any*
1559 k bytes of the set
1560
1561 {d_1, ..., d_k, p_1, ..., p_r}.
1562
1563 FEC-encoding for unreliable network transports boils down to slicing
1564 the audio stream into groups of k suitably sized pieces called _slices_
1565 and computing the r corresponding parity slices. This step is performed
1566 in para_server which then sends both the data and the parity slices
1567 over the unreliable network connection. If the client was able
1568 to receive at least k of the n = k + r slices, it can reconstruct
1569 (FEC-decode) the original audio stream.
1570
1571 From these observations it is clear that there are three different
1572 FEC parameters: The slice size, the number of data slices k, and the
1573 total number of slices n. It is crucial to choose the slice size
1574 such that no fragmentation of network packets takes place because
1575 FEC only guards against losses and reordering but fails if slices are
1576 received partially.
1577
1578 FEC decoding in paralash is performed through the fecdec filter which
1579 usually is the first filter (there can be other filters before fecdec
1580 if these do not alter the audio stream).
1581
1582 Volume adjustment (amp and compress)
1583 ------------------------------------
1584
1585 The amp and the compress filter both adjust the volume of the audio
1586 stream. These filters operate on uncompressed audio samples. Hence
1587 they are usually placed directly after the decoding filter. Each
1588 sample is multiplied with a scaling factor (>= 1) which makes amp
1589 and compress quite expensive in terms of computing power.
1590
1591 ### amp ###
1592
1593 The amp filter amplifies the audio stream by a fixed scaling factor
1594 that must be known in advance. For para_audiod this factor is derived
1595 from the amplification field of the audio file's entry in the audio
1596 file table while para_filter uses the value given at the command line.
1597
1598 The optimal scaling factor F for an audio file is the largest real
1599 number F >= 1 such that after multiplication with F all samples still
1600 fit into the sample interval [-32768, 32767]. One can use para_filter
1601 in combination with the sox utility to compute F:
1602
1603 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1604
1605 The amplification value V which is stored in the audio file table,
1606 however, is an integer between 0 and 255 which is connected to F
1607 through the formula
1608
1609 V = (F - 1) * 64.
1610
1611 To store V in the audio file table, the command
1612
1613 para_client -- touch -a=V file.mp3
1614
1615 is used. The reader is encouraged to write a script that performs
1616 these computations :)
1617
1618 ### compress ###
1619
1620 Unlike the amplification filter, the compress filter adjusts the volume
1621 of the audio stream dynamically without prior knowledge about the peak
1622 value. It maintains the maximal volume of the last n samples of the
1623 audio stream and computes a suitable amplification factor based on that
1624 value and the various configuration options. It tries to chose this
1625 factor such that the adjusted volume meets the desired target level.
1626
1627 Note that it makes sense to combine amp and compress.
1628
1629 Misc filters (wav and prebuffer)
1630 --------------------------------
1631
1632 These filters are rather simple and do not modify the audio stream at
1633 all. The wav filter is only useful with para_filter and in connection
1634 with a decoder. It asks the decoder for the number of channels and the
1635 sample rate of the stream and adds a Microsoft wave header containing
1636 this information at the beginning. This allows to write wav files
1637 rather than raw PCM files (which do not contain any information about
1638 the number of channels and the sample rate).
1639
1640 The prebuffer filter simply delays the output until the given time has
1641 passed (starting from the time the first byte was available in its
1642 input queue) or until the given amount of data has accumulated. It
1643 is mainly useful for para_audiod if the standard parameters result
1644 in buffer underruns.
1645
1646 Both filters require almost no additional computing time, even when
1647 operating on uncompressed audio streams, since data buffers are simply
1648 "pushed down" rather than copied.
1649
1650 Examples
1651 --------
1652
1653 -> Decode an mp3 file to wav format:
1654
1655 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1656
1657 -> Amplify a raw audio file by a factor of 1.5:
1658
1659 para_filter -f amp --amp 32 < foo.raw > bar.raw
1660
1661 ======
1662 Output
1663 ======
1664
1665 Once an audio stream has been received and decoded to PCM format,
1666 it can be sent to a sound device for playback. This part is performed
1667 by paraslash _writers_ which are described in this chapter.
1668
1669 Writers
1670 -------
1671
1672 A paraslash writer acts as a data sink that consumes but does not
1673 produce audio data. Paraslash writers operate on the client side and
1674 are contained in para_audiod and in the stand-alone tool para_write.
1675
1676 The para_write program reads uncompressed audio data from STDIN. If
1677 this data starts with a wav header, sample rate, sample format and
1678 channel count are read from the header. Otherwise CD audio (44.1KHz
1679 16 bit little endian, stereo) is assumed but this can be overridden
1680 by command line options. para_audiod, on the other hand, obtains
1681 the sample rate and the number of channels from the decoder.
1682
1683 Like receivers and filters, each writer has an individual set of
1684 command line options, and for para_audiod writers can be configured
1685 per audio format separately. It is possible to activate more than
1686 one writer for the same stream simultaneously.
1687
1688 OS-dependent APIs
1689 -----------------
1690
1691 Unfortunately, the various flavours of Unix on which paraslash
1692 runs on have different APIs for opening a sound device and starting
1693 playback. Hence for each such API there is a paraslash writer that
1694 can play the audio stream via this API.
1695
1696 - *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1697 Linux systems. Although there are several mid-layer APIs in use by
1698 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1699 currently supports only the low-level ALSA API which is not supposed
1700 to be change. ALSA is very feature-rich, in particular it supports
1701 software mixing via its DMIX plugin. ALSA is the default writer on
1702 Linux systems.
1703
1704 - *OSS*. The _Open Sound System_ is the only API on \*BSD Unixes and
1705 is also available on Linux systems, usually provided by ALSA as an
1706 emulation for backwards compatibility. This API is rather simple but
1707 also limited. For example only one application can open the device
1708 at any time. The OSS writer is activated by default on BSD Systems.
1709
1710 - *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1711 for this API is only compiled in on such systems and is of course
1712 the default there.
1713
1714 - *FILE*. The file writer allows to capture the audio stream and
1715 write the PCM data to a file on the file system rather than playing
1716 it through a sound device. It is supported on all platforms and is
1717 always compiled in.
1718
1719 - *AO*. _Libao_ is a cross-platform audio library which supports a wide
1720 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1721 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1722 through an output plugin of libao.
1723
1724 Examples
1725 --------
1726
1727 -> Use the OSS writer to play a wav file:
1728
1729 para_write --writer oss < file.wav
1730
1731 -> Enable ALSA software mixing for mp3 streams:
1732
1733 para_audiod --writer 'mp3:alsa -d plug:swmix'
1734
1735
1736 ===
1737 Gui
1738 ===
1739
1740 para_gui executes an arbitrary command which is supposed to print
1741 status information to STDOUT. It then displays this information in
1742 a curses window. By default the command
1743
1744 para_audioc -- stat -p
1745
1746 is executed, but this can be customized via the --stat-cmd option. In
1747 particular it possible to use
1748
1749 para_client -- stat -p
1750
1751 to make para_gui work on systems on which para_audiod is not running.
1752
1753 Key bindings
1754 ------------
1755
1756 It is possible to bind keys to arbitrary commands via custom
1757 key-bindings. Besides the internal keys which can not be changed (help,
1758 quit, loglevel, version...), the following flavours of key-bindings
1759 are supported:
1760
1761 - external: Shutdown curses before launching the given command.
1762 Useful for starting other ncurses programs from within para_gui,
1763 e.g. aumix or dialog scripts. Or, use the mbox output format to write
1764 a mailbox containing one mail for each (admissible) file the audio
1765 file selector knows about. Then start mutt from within para_gui to
1766 browse your collection!
1767
1768 - display: Launch the command and display its stdout in para_gui's
1769 bottom window.
1770
1771 - para: Like display, but start "para_client <specified command>"
1772 instead of "<specified command>".
1773
1774 The general form of a key binding is
1775
1776 key_map k:m:c
1777
1778 which maps key k to command c using mode m. Mode may be x, d or p
1779 for external, display and paraslash commands, respectively.
1780
1781 Themes
1782 ------
1783
1784 Currently there are only two themes for para_gui. It is easy, however,
1785 to add more themes. To create a new theme one has to define the
1786 position, color and geometry for for each status item that should be
1787 shown by this theme. See gui_theme.c for examples.
1788
1789 The "." and "," keys are used to switch between themes.
1790
1791 Examples
1792 --------
1793
1794 -> Show server info:
1795
1796 key_map "i:p:si"
1797
1798 -> Jump to the middle of the current audio file by pressing F5:
1799
1800 key_map "<F5>:p:jmp 50"
1801
1802 -> vi-like bindings for jumping around:
1803
1804 key_map "l:p:ff 10"
1805 key_map "h:p:ff 10-"
1806 key_map "w:p:ff 60"
1807 key_map "b:p:ff 60-"
1808
1809 -> Print the current date and time:
1810
1811 key_map "D:d:date"
1812
1813 -> Call other curses programs:
1814
1815 key_map "U:x:aumix"
1816 key_map "!:x:/bin/bash"
1817 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1818
1819 ===========
1820 Development
1821 ===========
1822
1823 Contributing
1824 ------------
1825
1826 Paraslash is an open source project and contributions are
1827 welcome. Here's a list of things you can do to help the project:
1828
1829 - Report problems with building, installing or running the software.
1830 In particular, test the experimental git branches ("next" and "pu").
1831 This helps to identify and fix problems before the code gets merged
1832 and thus keeps the master branch as stable as possible.
1833 - Proofread the documentation (manual, web pages, man pages, source
1834 code documentation) and point out unclear or poorly written parts. If
1835 you are a native English speaker you will easily find a lot of text
1836 that could be improved.
1837 - Run analysis tools (coverity, afl, sparse, etc.) and report issues
1838 found by those tools.
1839 - Suggest new features you would like to see implemented.
1840 - Compile and test on your favorite architecture or operating
1841 system. The code is tested only on a limited set of systems, so you
1842 will probably encounter problems when building on different systems.
1843 - Post about about paraslash on your blog or on social networks.
1844 - Build and maintain Debian/RPM packages for your favorite distribution.
1845
1846 Note that there is no mailing list, no bug tracker and no discussion
1847 forum for paraslash. If you'd like to contribute, or have questions
1848 about contributing, send email to Andre Noll <maan@tuebingen.mpg.de>.
1849
1850 Tools
1851 -----
1852
1853 In order to compile the sources from the git repository (rather than
1854 from tar balls) and for contributing non-trivial changes to the
1855 paraslash project, some additional tools should be installed on a
1856 developer machine.
1857
1858 - [git](http://git.or.cz/). As described in more detail
1859 [below](#Git.branches), the git source code management tool is used for
1860 paraslash development. It is necessary for cloning the git repository
1861 and for getting updates.
1862
1863 - [autoconf](ftp://ftp.gnu.org/pub/gnu/autoconf/) GNU autoconf creates
1864 the configure file which is shipped in the tarballs but has to be
1865 generated when compiling from git.
1866
1867 - [discount](http://www.pell.portland.or.us/~orc/Code/discount). The
1868 HTML version of this manual and some of the paraslash web pages are
1869 written in the Markdown markup language and are translated into html
1870 with the converter of the *Discount* package.
1871
1872 - [doxygen](http://www.stack.nl/~dimitri/doxygen/). The documentation
1873 of paraslash's C sources uses the doxygen documentation system. The
1874 conventions for documenting the source code is described in the
1875 [Doxygen section](#Doxygen).
1876
1877 - [global](ftp://ftp.gnu.org/pub/gnu/global). This is used to generate
1878 browsable HTML from the C sources. It is needed by doxygen.
1879
1880 Git branches
1881 ------------
1882
1883 Paraslash has been developed using the git source code management
1884 tool since 2006. Development is organized roughly in the same spirit
1885 as the git development itself, as described below.
1886
1887 The following text passage is based on "A note from the maintainer",
1888 written by Junio C Hamano, the maintainer of git.
1889
1890 There are four branches in the paraslash repository that track the
1891 source tree: "master", "maint", "next", and "pu".
1892
1893 The "master" branch is meant to contain what is well tested and
1894 ready to be used in a production setting. There could occasionally be
1895 minor breakages or brown paper bag bugs but they are not expected to
1896 be anything major, and more importantly quickly and easily fixable.
1897 Every now and then, a "feature release" is cut from the tip of this
1898 branch, named with three dotted decimal digits, like 0.4.2.
1899
1900 Whenever changes are about to be included that will eventually lead to
1901 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1902 "master" at that point. Obvious, safe and urgent fixes after the major
1903 release are applied to this branch and maintenance releases are cut
1904 from it. New features never go to this branch. This branch is also
1905 merged into "master" to propagate the fixes forward.
1906
1907 A trivial and safe enhancement goes directly on top of "master".
1908 New development does not usually happen on "master", however.
1909 Instead, a separate topic branch is forked from the tip of "master",
1910 and it first is tested in isolation; Usually there are a handful such
1911 topic branches that are running ahead of "master". The tip of these
1912 branches is not published in the public repository to keep the number
1913 of branches that downstream developers need to worry about low.
1914
1915 The quality of topic branches varies widely. Some of them start out as
1916 "good idea but obviously is broken in some areas" and then with some
1917 more work become "more or less done and can now be tested by wider
1918 audience". Luckily, most of them start out in the latter, better shape.
1919
1920 The "next" branch is to merge and test topic branches in the latter
1921 category. In general, this branch always contains the tip of "master".
1922 It might not be quite rock-solid production ready, but is expected to
1923 work more or less without major breakage. The maintainer usually uses
1924 the "next" version of paraslash for his own pleasure, so it cannot
1925 be _that_ broken. The "next" branch is where new and exciting things
1926 take place.
1927
1928 The two branches "master" and "maint" are never rewound, and "next"
1929 usually will not be either (this automatically means the topics that
1930 have been merged into "next" are usually not rebased, and you can find
1931 the tip of topic branches you are interested in from the output of
1932 "git log next"). You should be able to safely build on top of them.
1933
1934 However, at times "next" will be rebuilt from the tip of "master" to
1935 get rid of merge commits that will never be in "master". The commit
1936 that replaces "next" will usually have the identical tree, but it
1937 will have different ancestry from the tip of "master".
1938
1939 The "pu" (proposed updates) branch bundles the remainder of the
1940 topic branches. The "pu" branch, and topic branches that are only in
1941 "pu", are subject to rebasing in general. By the above definition
1942 of how "next" works, you can tell that this branch will contain quite
1943 experimental and obviously broken stuff.
1944
1945 When a topic that was in "pu" proves to be in testable shape, it
1946 graduates to "next". This is done with
1947
1948 git checkout next
1949 git merge that-topic-branch
1950
1951 Sometimes, an idea that looked promising turns out to be not so good
1952 and the topic can be dropped from "pu" in such a case.
1953
1954 A topic that is in "next" is expected to be polished to perfection
1955 before it is merged to "master". Similar to the above, this is
1956 done with
1957
1958 git checkout master
1959 git merge that-topic-branch
1960 git branch -d that-topic-branch
1961
1962 Note that being in "next" is not a guarantee to appear in the next
1963 release (being in "master" is such a guarantee, unless it is later
1964 found seriously broken and reverted), nor even in any future release.
1965
1966 Coding Style
1967 ------------
1968
1969 The preferred coding style for paraslash coincides more or less
1970 with the style of the Linux kernel. So rather than repeating what is
1971 written [there](http://www.kernel.org/doc/Documentation/process/coding-style.rst),
1972 here are the most important points.
1973
1974 - Burn the GNU coding standards.
1975 - Never use spaces for indentation.
1976 - Tabs are 8 characters, and thus indentations are also 8 characters.
1977 - Don't put multiple assignments on a single line.
1978 - Avoid tricky expressions.
1979 - Don't leave whitespace at the end of lines.
1980 - The limit on the length of lines is 80 columns.
1981 - Use K&R style for placing braces and spaces:
1982
1983 if (x is true) {
1984 we do y
1985 }
1986
1987 - Use a space after (most) keywords.
1988 - Do not add spaces around (inside) parenthesized expressions.
1989 - Use one space around (on each side of) most binary and ternary operators.
1990 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1991 - Mixed-case names are frowned upon.
1992 - Descriptive names for global variables are a must.
1993 - Avoid typedefs.
1994 - Functions should be short and sweet, and do just one thing.
1995 - The number of local variables shouldn't exceed 10.
1996 - Gotos are fine if they improve readability and reduce nesting.
1997 - Don't use C99-style "// ..." comments.
1998 - Names of macros defining constants and labels in enums are capitalized.
1999 - Enums are preferred when defining several related constants.
2000 - Always use the paraslash wrappers for allocating memory.
2001 - If the name of a function is an action or an imperative.
2002 command, the function should return an error-code integer
2003 (<0 means error, >=0 means success). If the name is a
2004 predicate, the function should return a "succeeded" boolean.
2005
2006 Doxygen
2007 -------
2008
2009 Doxygen is a documentation system for various programming
2010 languages. The API reference on the paraslash web page is generated
2011 by doxygen.
2012
2013 It is more illustrative to look at the source code for examples than
2014 to describe the conventions in this manual, so we only describe which
2015 parts of the code need doxygen comments, but leave out details on
2016 documentation conventions.
2017
2018 As a rule, only the public part of the C source is documented with
2019 Doxygen. This includes structures, defines and enumerations in header
2020 files as well as public (non-static) C functions. These should be
2021 documented completely. For example, each parameter and the return
2022 value of a public function should get a descriptive doxygen comment.
2023
2024 No doxygen comments are necessary for static functions and for
2025 structures and enumerations in C files (which are used only within
2026 this file). This does not mean, however, that those entities need
2027 no documentation at all. Instead, common sense should be applied to
2028 document what is not obvious from reading the code.
2029
2030 ========
2031 Appendix
2032 ========
2033
2034 Network protocols
2035 -----------------
2036
2037 ### IP ###
2038
2039 The _Internet Protocol_ is the primary networking protocol used for
2040 the Internet. All protocols described below use IP as the underlying
2041 layer. Both the prevalent IPv4 and the next-generation IPv6 variant
2042 are being deployed actively worldwide.
2043
2044 ### Connection-oriented and connectionless protocols ###
2045
2046 Connectionless protocols differ from connection-oriented ones in
2047 that state associated with the sending/receiving endpoints is treated
2048 implicitly. Connectionless protocols maintain no internal knowledge
2049 about the state of the connection. Hence they are not capable of
2050 reacting to state changes, such as sudden loss or congestion on the
2051 connection medium. Connection-oriented protocols, in contrast, make
2052 this knowledge explicit. The connection is established only after
2053 a bidirectional handshake which requires both endpoints to agree
2054 on the state of the connection, and may also involve negotiating
2055 specific parameters for the particular connection. Maintaining an
2056 up-to-date internal state of the connection also in general means
2057 that the sending endpoints perform congestion control, adapting to
2058 qualitative changes of the connection medium.
2059
2060 ### Reliability ###
2061
2062 In IP networking, packets can be lost, duplicated, or delivered
2063 out of order, and different network protocols handle these
2064 problems in different ways. We call a transport-layer protocol
2065 _reliable_, if it turns the unreliable IP delivery into an ordered,
2066 duplicate- and loss-free delivery of packets. Sequence numbers
2067 are used to discard duplicates and re-arrange packets delivered
2068 out-of-order. Retransmission is used to guarantee loss-free
2069 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2070 or data integrity.
2071
2072 ### Classification ###
2073
2074 With these definitions the protocols which are used by paraslash for
2075 steaming audio data may be classified as follows.
2076
2077 - HTTP/TCP: connection-oriented, reliable,
2078 - UDP: connectionless, unreliable,
2079 - DCCP: connection-oriented, unreliable.
2080
2081 Below we give a short descriptions of these protocols.
2082
2083 ### TCP ###
2084
2085 The _Transmission Control Protocol_ provides reliable, ordered delivery
2086 of a stream and a classic window-based congestion control. In contrast
2087 to UDP and DCCP (see below), TCP does not have record-oriented or
2088 datagram-based syntax, i.e. it provides a stream which is unaware
2089 and independent of any record (packet) boundaries. TCP is used
2090 extensively by many application layers. Besides HTTP (the Hypertext
2091 Transfer Protocol), also FTP (the File Transfer protocol), SMTP (Simple
2092 Mail Transfer Protocol), SSH (Secure Shell) all sit on top of TCP.
2093
2094 ### UDP ###
2095
2096 The _User Datagram Protocol_ is the simplest transport-layer protocol,
2097 built as a thin layer directly on top of IP. For this reason, it offers
2098 the same best-effort service as IP itself, i.e. there is no detection
2099 of duplicate or reordered packets. Being a connectionless protocol,
2100 only minimal internal state about the connection is maintained, which
2101 means that there is no protection against packet loss or network
2102 congestion. Error checking and correction (if at all) are performed
2103 in the application.
2104
2105 ### DCCP ###
2106
2107 The _Datagram Congestion Control Protocol_ combines the
2108 connection-oriented state maintenance known from TCP with the
2109 unreliable, datagram-based transport of UDP. This means that it
2110 is capable of reacting to changes in the connection by performing
2111 congestion control, offering multiple alternative approaches. But it
2112 is bound to datagram boundaries (the maximum packet size supported
2113 by a medium), and like UDP it lacks retransmission to protect
2114 against loss. Due to the use of sequence numbers, it is however
2115 able to react to loss (interpreted as a congestion indication) and
2116 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2117 to negotiate specific, binding features for a connection, such as
2118 the choice of congestion control: classic, window-based congestion
2119 control known from TCP is available as CCID-2, rate-based, "smooth"
2120 congestion control is offered as CCID-3.
2121
2122 ### HTTP ###
2123
2124 The _Hypertext Transfer Protocol_ is an application layer protocol
2125 on top of TCP. It is spoken by web servers and is most often used
2126 for web services. However, as can be seen by the many Internet radio
2127 stations and YouTube/Flash videos, http is by far not limited to the
2128 delivery of web pages only. Being a simple request/response based
2129 protocol, the semantics of the protocol also allow the delivery of
2130 multimedia content, such as audio over http.
2131
2132 ### Multicast ###
2133
2134 IP multicast is not really a protocol but a technique for one-to-many
2135 communication over an IP network. The challenge is to deliver
2136 information to a group of destinations simultaneously using the
2137 most efficient strategy to send the messages over each link of the
2138 network only once. This has benefits for streaming multimedia: the
2139 standard one-to-one unicast offered by TCP/DCCP means that n clients
2140 listening to the same stream also consume n-times the resources,
2141 whereas multicast requires to send the stream just once, irrespective
2142 of the number of receivers. Since it would be costly to maintain state
2143 for each listening receiver, multicast often implies connectionless
2144 transport, which is the reason that it is currently only available
2145 via UDP.
2146
2147 Abstract socket namespace
2148 -------------------------
2149 UNIX domain sockets are a traditional way to communicate between
2150 processes on the same machine. They are always reliable (see above)
2151 and don't reorder datagrams. Unlike TCP and UDP, UNIX domain sockets
2152 support passing open file descriptors or process credentials to
2153 other processes.
2154
2155 The usual way to set up a UNIX domain socket (as obtained from
2156 socket(2)) for listening is to first bind the socket to a file system
2157 pathname and then call listen(2), then accept(2). Such sockets are
2158 called _pathname sockets_ because bind(2) creates a special socket
2159 file at the specified path. Pathname sockets allow unrelated processes
2160 to communicate with the listening process by binding to the same path
2161 and calling connect(2).
2162
2163 There are two problems with pathname sockets:
2164
2165 * The listing process must be able to (safely) create the
2166 socket special in a directory which is also accessible to
2167 the connecting process.
2168
2169 * After an unclean shutdown of the listening process, a stale
2170 socket special may reside on the file system.
2171
2172 The abstract socket namespace is a non-portable Linux feature which
2173 avoids these problems. Abstract sockets are still bound to a name,
2174 but the name has no connection with file system pathnames.
2175
2176 License
2177 -------
2178
2179 Paraslash is licensed under the GPL, version 2. Most of the code
2180 base has been written from scratch, and those parts are GPL V2
2181 throughout. Notable exceptions are FEC and the WMA decoder. See the
2182 corresponding source files for licencing details for these parts. Some
2183 code sniplets of several other third party software packages have
2184 been incorporated into the paraslash sources, for example log message
2185 coloring was taken from the git sources. These third party software
2186 packages are all published under the GPL or some other license
2187 compatible to the GPL.
2188
2189 Acknowledgements
2190 ----------------
2191
2192 Many thanks to Gerrit Renker who read an early draft of this manual
2193 and contributed significant improvements.
2194
2195 ==========
2196 References
2197 ==========
2198
2199 Articles
2200 --------
2201 - [Polynomial Codes over Certain Finite
2202 Fields](http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF) by Reed, Irving
2203 S.; Solomon, Gustave (1960), Journal of the Society for Industrial
2204 and Applied Mathematics (SIAM) 8 (2): 300-304, doi:10.1137/0108018)
2205
2206 RFCs
2207 ----
2208
2209 - [RFC 768](http://www.ietf.org/rfc/rfc768.txt) (1980): User Datagram
2210 Protocol
2211
2212 - [RFC 791](http://www.ietf.org/rfc/rfc791.txt) (1981): Internet
2213 Protocol
2214
2215 - [RFC 2437](http://www.ietf.org/rfc/rfc2437.txt) (1998): RSA
2216 Cryptography Specifications
2217
2218 - [RFC 4340](http://www.ietf.org/rfc/rfc4340.txt) (2006): Datagram
2219 Congestion Control Protocol (DCCP)
2220
2221 - [RFC 4341](http://www.ietf.org/rfc/rfc4341.txt) (2006): Congestion
2222 Control ID 2: TCP-like Congestion Control
2223
2224 - [RFC 4342](http://www.ietf.org/rfc/rfc4342.txt) (2006): Congestion
2225 Control ID 3: TCP-Friendly Rate Control (TFRC)
2226
2227 - [RFC 6716](http://www.ietf.org/rfc/rfc6716.txt) (2012): Definition
2228 of the Opus Audio Codec
2229
2230 Application web pages
2231 ---------------------
2232
2233 - [paraslash](http://people.tuebingen.mpg.de/maan/paraslash/)
2234 - [xmms](http://xmms2.org/wiki/Main_Page)
2235 - [mpg123](http://www.mpg123.de/)
2236 - [gstreamer](http://gstreamer.freedesktop.org/)
2237 - [icecast](http://www.icecast.org/)
2238 - [Audio Compress](http://beesbuzz.biz/code/audiocompress.php)
2239
2240 External documentation
2241 ----------------------
2242
2243 - [The mathematics of
2244 Raid6](http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf)
2245 by H. Peter Anvin
2246
2247 - [Effective Erasure Codes for reliable Computer Communication
2248 Protocols](http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz) by Luigi
2249 Rizzo
2250
2251 Code
2252 ----
2253 - [Original FEC
2254 implementation](http://info.iet.unipi.it/~luigi/vdm.tar.gz) by
2255 Luigi Rizzo)