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