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