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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, OGG/Vorbis, OGG/Speex,
81 M4A, WMA files) over local and/or remote networks. It listens on a
82 TCP port and accepts commands such as play, stop, pause, next from
83 authenticated clients. There are many more commands though, see the
84 man page of para_server for a 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 RC4 session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
119 *para_audiod*
121 The local daemon that collects information from para_server.
123 It runs on the client side and connects to para_server. As soon as
124 para_server announces the availability of an audio stream, para_audiod
125 starts an appropriate receiver, any number of filters and a paraslash
126 writer to play the stream.
128 Moreover, para_audiod listens on a local socket and sends status
129 information about para_server and para_audiod to local clients on
130 request. Access via this local socket may be restricted by using Unix
131 socket credentials, if available.
134 *para_audioc*
136 The client program which talks to para_audiod. Used to control
137 para_audiod, to receive status info, or to grab the stream at any
138 point of the decoding process.
140 *para_recv*
142 A command line HTTP/DCCP/UDP stream grabber. The http mode is
143 compatible with arbitrary HTTP streaming sources (e.g. icecast).
145 *para_filter*
147 A filter program that reads from STDIN and writes to STDOUT.
148 Like para_recv, this is an atomic building block which can be used to
149 assemble higher-level audio receiving facilities. It combines several
150 different functionalities in one tool: decoders for multiple audio
151 formats and a number of processing filters, among these a normalizer
152 for audio volume.
154 *para_afh*
156 A small stand-alone program that prints tech info about the given
157 audio file to STDOUT. It can be instructed to print a "chunk table",
158 an array of offsets within the audio file or to write the content of
159 the audio file in complete chunks 'just in time'.
161 This allows third-party streaming software that is unaware of the
162 particular audio format to send complete frames in real time.
164 *para_write*
166 A modular audio stream writer. It supports a simple file writer
167 output plug-in and optional WAV/raw players for ALSA (Linux) and for
168 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
169 or raw audio player.
172 *para_gui*
174 Curses-based gui that presents status information obtained in a curses
175 window. Appearance can be customized via themes. para_gui provides
176 key-bindings for the most common server commands and new key-bindings
177 can be added easily.
180 *para_fade*
182 An (OSS-only) alarm clock and volume-fader.
184 -----------
185 Quick start
186 -----------
188 This chapter lists the REFERENCE(Requirements, necessary software)
189 that must be installed to compile the paraslash package, describes
190 how to REFERENCE(Installation, compile and install) the paraslash
191 source code and the steps that have to be performed in order to
192 REFERENCE(Quick start, set up) a typical server and client.
194 Requirements
195 ~~~~~~~~~~~~
197 In any case you'll need
199 - XREFERENCE(http://systemlinux.org/~maan/osl/, libosl).
200 The _object storage layer_ library is used by para_server. To
201 clone the source code repository, execute
203 git clone git://git.tuebingen.mpg.de/osl
205 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc). The
206 EMPH(gnu compiler collection) is usually shipped with the
207 distro. gcc-3.3 or newer is required.
209 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
210 also shipped with the disto. On BSD systems the gnu make
211 executable is often called gmake.
213 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
214 scripts which run during compilation require the EMPH(Bourne
215 again shell). It is most likely already installed.
217 - XREFERENCE(http://www.openssl.org/, openssl). The EMPH(Secure
218 Sockets Layer) library is needed for cryptographic routines
219 on both the server and the client side. It is usually shipped
220 with the distro, but you might have to install the "development
221 package" (called libssl-dev on debian systems) as well.
223 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/help2man, help2man)
224 is used to create the man pages.
226 Optional:
228 - XREFERENCE(http://www.underbit.com/products/mad/, libmad).
229 To compile in MP3 support for paraslash, the development
230 package must be installed. It is called libmad0-dev on
231 debian-based systems. Note that libmad is not necessary on
232 the server side, i.e. for sending MP3 files.
234 - XREFERENCE(http://www.underbit.com/products/mad/,
235 libid3tag). For version-2 ID3 tag support, you'll need
236 the libid3tag development package libid3tag0-dev. Without
237 libid3tag, only version one tags are recognized.
239 - XREFERENCE(http://www.xiph.org/downloads/, ogg vorbis).
240 For ogg vorbis streams you'll need libogg, libvorbis,
241 libvorbisfile. The corresponding Debian packages are called
242 libogg-dev and libvorbis-dev.
244 - XREFERENCE(http://www.audiocoding.com/, libfaad). For aac
245 files (m4a) you'll need libfaad (libfaad-dev).
247 - XREFERENCE(http://www.speex.org/, speex). In order to stream
248 or decode speex files, libspeex (libspeex-dev) is required.
250 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
251 Linux, you'll need to have ALSA's development package
252 libasound2-dev installed.
254 Installation
255 ~~~~~~~~~~~~
257 First make sure all non-optional packages listed in the section on
258 REFERENCE(Requirements, required software) are installed on your
259 system.
261 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
262 OGG/Speex and AAC support are all optional. The configure script will
263 detect what is installed on your system and will only try to build
264 those executables that can be built with your setup.
266 Note that no special decoder library (not even the MP3 decoding library
267 libmad) is needed for para_server if you only want to stream MP3 or WMA
268 files. Also, it's fine to use para_server on a box without sound card.
270 Next, install the paraslash package on all machines, you'd like this
271 software to run on:
273 (./configure && make) > /dev/null
275 There should be no errors but probably some warnings about missing
276 packages which usually implies that not all audio formats will be
277 supported. If headers or libs are installed at unusual locations you
278 might need to tell the configure script where to find them. Try
280 ./configure --help
282 to see a list of options. If the paraslash package was compiled
283 successfully, execute (optionally)
285 make test
287 to run the paraslash test suite. If all tests pass, execute as root
289 make install
291 to install executables under /usr/local/bin and the man pages under
292 /usr/local/man.
294 Configuration
295 ~~~~~~~~~~~~~
297 *Step 1*: Create a paraslash user
299 In order to control para_server at runtime you must create a paraslash
300 user. As authentication is based on the RSA crypto system you'll have
301 to create an RSA key pair. If you already have a user and an RSA key
302 pair, you may skip this step.
304 In this section we'll assume a typical setup: You would like to run
305 para_server on some host called server_host as user foo, and you want
306 to connect to para_server from another machine called client_host as
307 user bar.
309 As foo@server_host, create ~/.paraslash/server.users by typing the
310 following commands:
312 user=bar
313 target=~/.paraslash/server.users
314 key=~/.paraslash/id_rsa.pub.$user
316 mkdir -p ~/.paraslash
317 echo "user $user $key $perms" >> $target
319 Next, change to the "bar" account on client_host and generate the
320 key pair with the commands
322 ssh-keygen -t rsa -b 2048
323 # hit enter twice to create a key with no passphrase
325 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
326 that paraslash can also read keys generated by the "openssl genrsa"
327 command. However, since keys created with ssh-keygen can also be used
328 for ssh, this method is recommended.
330 Note that para_server refuses to use a key if it is shorter than 2048
331 bits. In particular, the RSA keys of paraslash 0.3.x will not work
332 with version 0.4.x. Moreover, para_client refuses to use a (private)
333 key which is world-readable.
335 para_server only needs to know the public key of the key pair just
336 created. Copy this public key to server_host:
338 src=~/.ssh/id_rsa.pub
339 dest=.paraslash/id_rsa.pub.$LOGNAME
340 scp $src foo@server_host:$dest
342 Finally, tell para_client to connect to server_host:
344 conf=~/.paraslash/client.conf
345 echo 'hostname server_host' > $conf
348 *Step 2*: Start para_server
350 Before starting the server make sure you have write permissions to
351 the directory /var/paraslash that has been created during installation:
353 sudo chown $LOGNAME /var/paraslash
355 Alternatively, use the --afs_socket Option to specify a different
356 location for the AFS command socket.
358 For this first try, we'll use the info loglevel to make the output
359 of para_server more verbose.
361 para_server -l info
363 Now you can use para_client to connect to the server and issue
364 commands. Open a new shell as bar@client_host and try
366 para_client help
367 para_client si
369 to retrieve the list of available commands and some server info.
370 Don't proceed if this doesn't work.
372 *Step 3*: Create and populate the database
374 An empty database is created with
376 para_client init
378 This initializes a couple of empty tables under
379 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
380 tables, but it's good to know that you can start from scratch with
382 rm -rf ~/.paraslash/afs_database-0.4
384 in case something went wrong.
386 Next, you need to add some audio files to that database so that
387 para_server knows about them. Choose an absolute path to a directory
388 containing some audio files and add them to the audio file table:
390 para_client add /my/mp3/dir
392 This might take a while, so it is a good idea to start with a directory
393 containing not too many files. Note that the table only contains data
394 about the audio files found, not the files themselves.
396 You may print the list of all known audio files with
398 para_client ls
400 *Step 4*: Configure para_audiod
402 para_audiod needs to create a "well-known" socket for the clients to
403 connect to. The default path for this socket is
405 /var/paraslash/audiod_socket.$HOSTNAME
407 In order to make this directory writable for para_audiod, execute
408 as bar@client_host
410 sudo chown $LOGNAME /var/paraslash
413 We will also have to tell para_audiod that it should receive the
414 audio stream from server_host:
416 para_audiod -l info -r 'mp3:http -i server_host'
418 You should now be able to listen to the audio stream once para_server
419 starts streaming. To activate streaming, execute
421 para_client play
423 Since no playlist has been specified yet, the "dummy" mode which
424 selects all known audio files is activated automatically. See the
425 section on the REFERENCE(The audio file selector, audio file selector)
426 for how to use playlists and moods to specify which files should be
427 streamed in which order.
429 *Troubleshooting*
431 It did not work? To find out why, try to receive, decode and play the
432 stream manually using para_recv, para_filter and para_write as follows.
434 For simplicity we assume that you're running Linux/ALSA and that only
435 MP3 files have been added to the database.
437 para_recv -r 'http -i server_host' > file.mp3
438 # (interrupt with CTRL+C after a few seconds)
439 ls -l file.mp3 # should not be empty
440 para_filter -f mp3dec -f wav < file.mp3 > file.wav
441 ls -l file.wav # should be much bigger than file.mp3
442 para_write -w alsa < file.wav
444 Double check what is logged by para_server and use the --loglevel
445 option of para_recv, para_filter and para_write to increase verbosity.
447 ---------------
448 User management
449 ---------------
451 para_server uses a challenge-response mechanism to authenticate
452 requests from incoming connections, similar to ssh's public key
453 authentication method. Authenticated connections are encrypted using
454 the RC4 stream cipher.
456 In this chapter we briefly describe RSA and RC4 and sketch the
457 REFERENCE(Client-server authentication, authentication handshake)
458 between para_client and para_server. User management is discussed
459 in the section on REFERENCE(The user_list file, the user_list file).
460 These sections are all about communication between the client and the
461 server. Connecting para_audiod is a different matter and is described
462 in a REFERENCE(Connecting para_audiod, separate section).
466 RSA and RC4
467 ~~~~~~~~~~~
469 RSA is an asymmetric block cipher which is used in many applications,
470 including ssh and gpg. An RSA key consists in fact of two keys,
471 called the public key and the private key. A message can be encrypted
472 with either key and only the counterpart of that key can decrypt
473 the message. While RSA can be used for both signing and encrypting
474 a message, paraslash only uses RSA only for the latter purpose. The
475 RSA public key encryption and signatures algorithms are defined in
476 detail in RFC 2437.
478 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
479 key stream to produce the output. Decryption uses the same function
480 calls as encryption. While RC4 supports variable key lengths,
481 paraslash uses a fixed length of 256 bits, which is considered a
482 strong encryption by today's standards. Since the same key must never
483 be used twice, a different, randomly-generated key is used for every
484 new connection.
486 Client-server authentication
487 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
489 The authentication handshake between para_client and para_server goes
490 as follows:
492 - para_client connects to para_server and sends an
493 authentication request for a user. It does so by connecting
494 to para_server, TCP 2990, the control port of para_server.
496 - para_server accepts the connection and forks a child process
497 which is supposed to handle the connection. The parent process
498 keeps listening on the control port while the child process
499 (also called para_server below) continues as follows.
501 - para_server loads the RSA public key of that user, fills a
502 fixed-length buffer with random bytes, encrypts that buffer
503 using the public key and sends the encrypted buffer to the
504 client. The first part of the buffer is the challenge which
505 is used for authentication while the second part is the RC4
506 session key.
508 - para_client receives the encrypted buffer and decrypts it
509 using the user's private key, thereby obtaining the challenge
510 buffer and the session key. It sends the SHA1 hash value of
511 the challenge back to para_server and stores the session key
512 for further use.
514 - para_server also computes the SHA1 hash of the challenge
515 and compares it against what was sent back by the client.
517 - If the two hashes do not match, the authentication has
518 failed and para_server closes the connection.
520 - Otherwise the user is considered authenticated and the client
521 is allowed to proceed by sending a command to be executed. From
522 this point on the communication is encrypted using the RC4
523 stream cipher with the session key known to both peers.
525 paraslash relies on the quality of openssl's cryptographically strong
526 pseudo-random bytes, on the security of the implementation of the
527 openssl RSA and RC4 crypto routines and on the infeasibility to invert
528 the SHA1 function.
530 Neither para_server or para_client create RSA keys on their own. This
531 has to be done once for each user as sketched in REFERENCE(Quick start,
532 Quick start) and discussed in more detail REFERENCE(The user_list
533 file, below).
535 The user_list file
536 ~~~~~~~~~~~~~~~~~~
538 At startup para_server reads the user list file which must contain
539 one line per user. The default location of the user list file may be
540 changed with the --user_list option.
542 There should be at least one user in this file. Each user must have
543 an RSA key pair. The public part of the key is needed by para_server
544 while the private key is needed by para_client. Each line of the
545 user list file must be of the form
547 user <username> <key> <perms>
549 where _username_ is an arbitrary string (usually the user's login
550 name), _key_ is the full path to that user's public RSA key, and
551 _perms_ is a comma-separated list of zero or more of the following
552 permission bits:
554 +---------------------------------------------------------+
555 | AFS_READ | read the contents of the databases |
556 +-----------+---------------------------------------------+
557 | AFS_WRITE | change database contents |
558 +-----------+---------------------------------------------+
559 | VSS_READ | obtain information about the current stream |
560 +-----------+---------------------------------------------+
561 | VSS_WRITE | change the current stream |
562 +---------------------------------------------------------+
564 The permission bits specify which commands the user is allowed to
565 execute. The output of
567 para_client help
569 contains in the third column the permissions needed to execute the
570 command.
572 It is possible to make para_server reread the user_list file by
573 executing the paraslash "hup" command or by sending SIGHUP to the
574 PID of para_server.
577 Connecting para_audiod
578 ~~~~~~~~~~~~~~~~~~~~~~
580 para_audiod listens on a Unix domain socket. Those sockets are
581 for local communication only, so only local users can connect to
582 para_audiod. The default is to let any user connect but this can be
583 restricted on platforms that support UNIX socket credentials which
584 allow para_audiod to obtain the Unix credentials of the connecting
585 process.
587 Use para_audiod's --user_allow option to allow connections only for
588 a limited set of users.
590 -----------------------
591 The audio file selector
592 -----------------------
594 paraslash comes with a sophisticated audio file selector (AFS),
595 whose main task is to determine which file to stream next, based on
596 information on the audio files stored in a database. It communicates
597 also with para_client whenever an AFS command is executed, for example
598 to answer a database query.
600 Besides the traditional playlists, AFS supports audio file selection
601 based on _moods_ which act as a filter that limits the set of all
602 known audio files to those which satisfy certain criteria. It also
603 maintains tables containing images (e.g. album cover art) and lyrics
604 that can be associated with one or more audio files.
606 AFS uses libosl, the object storage layer, as the backend library
607 for storing information on audio files, playlists, etc. This library
608 offers functionality similar to a relational database, but is much
609 more lightweight than a full database backend.
611 In this chapter we sketch the setup of the REFERENCE(The AFS process,
612 AFS process) during server startup and proceed with the description
613 of the REFERENCE(Database layout, layout) of the various database
614 tables. The section on REFERENCE(Playlists and moods, playlists
615 and moods) explains these two audio file selection mechanisms
616 in detail and contains pratical examples. The way REFERENCE(File
617 renames and content changes, file renames and content changes) are
618 detected is discussed briefly before the REFERENCE(Troubleshooting,
619 Troubleshooting) section which concludes the chapter.
621 The AFS process
622 ~~~~~~~~~~~~~~~
624 On startup, para_server forks to create the AFS process which opens
625 the OSL database tables. The server process communicates with the
626 AFS process via pipes and shared memory. Usually, the AFS process
627 awakes only briefly whenever the current audio file changes. The AFS
628 process determines the next audio file, opens it, verifies it has
629 not been changed since it was added to the database and passes the
630 open file descriptor to the server process, along with audio file
631 meta-data such as file name, duration, audio format and so on. The
632 server process then starts to stream the audio file.
634 The AFS process also accepts connections from local clients via
635 a well-known socket. However, only child processes of para_server
636 may connect through this socket. All server commands that have the
637 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
638 change the database.
640 Database layout
641 ~~~~~~~~~~~~~~~
643 *The audio file table*
645 This is the most important and usually also the largest table of the
646 AFS database. It contains the information needed to stream each audio
647 file. In particular the following data is stored for each audio file.
649 - SHA1 hash value of the audio file contents. This is computed
650 once when the file is added to the database. Whenever AFS
651 selects this audio file for streaming the hash value is
652 recomputed and checked against the value stored in the
653 database to detect content changes.
655 - The time when this audio file was last played.
657 - The number of times the file has been played so far.
659 - The attribute bitmask.
661 - The image id which describes the image associated with this
662 audio file.
664 - The lyrics id which describes the lyrics associated with
665 this audio file.
667 - The audio format id (MP3, OGG, ...).
669 - An amplification value that can be used by the amplification
670 filter to pre-amplify the decoded audio stream.
672 - The chunk table. It describes the location and the timing
673 of the building blocks of the audio file. This is used by
674 para_server to send chunks of the file at appropriate times.
676 - The duration of the audio file.
678 - Tag information contained in the audio file (ID3 tags,
679 Vorbis comments, ...).
681 - The number of channels
683 - The encoding bitrate.
685 - The sampling frequency.
687 To add or refresh the data contained in the audio file table, the _add_
688 command is used. It takes the full path of either an audio file or a
689 directory. In the latter case, the directory is traversed recursively
690 and all files which are recognized as valid audio files are added to
691 the database.
693 *The attribute table*
695 The attribute table contains two columns, _name_ and _bitnum_. An
696 attribute is simply a name for a certain bit number in the attribute
697 bitmask of the audio file table.
699 Each of the 64 bits of the attribute bitmask can be set for each
700 audio file individually. Hence up to 64 different attributes may be
701 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
702 "german_lyrics", "speech", whatever. You are free to choose as
703 many attributes as you like and there are no naming restrictions
704 for attributes.
706 A new attribute "test" is created by
708 para_client addatt test
709 and
710 para_client lsatt
712 lists all available attributes. You can set the "test" attribute for
713 an audio file by executing
715 para_client setatt test+ /path/to/the/audio/file
717 Similarly, the "test" bit can be removed from an audio file with
719 para_client setatt test- /path/to/the/audio/file
721 Instead of a path you may use a shell wildcard pattern. The attribute
722 is applied to all audio files matching that pattern:
724 para_client setatt test+ '/test/directory/*'
726 The command
728 para_client -- ls -lv
730 gives you a verbose listing of your audio files also showing which
731 attributes are set.
733 In case you wonder why the double-dash in the above command is needed:
734 It tells para_client to not interpret the options after the dashes. If
735 you find this annoying, just say
737 alias para='para_client --'
739 and be happy. In what follows we shall use this alias.
741 The "test" attribute can be dropped from the database with
743 para rmatt test
745 Read the output of
747 para help ls
748 para help setatt
750 for more information and a complete list of command line options to
751 these commands.
753 *Blob tables*
755 The image, lyrics, moods and playlists tables are all blob tables.
756 Blob tables consist of three columns each: The identifier which is
757 a positive non-negative number that is auto-incremented, the name
758 (an arbitrary string) and the content (the blob).
760 All blob tables support the same set of actions: cat, ls, mv, rm
761 and add. Of course, _add_ is used for adding new blobs to the table
762 while the other actions have the same meaning as the corresponding
763 Unix commands. The paraslash commands to perform these actions are
764 constructed as the concatenation of the table name and the action. For
765 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
766 manipulate or query the image table.
768 The add variant of these commands is special as these commands read
769 the blob contents from stdin. To add an image to the image table the
770 command
772 para addimg image_name < file.jpg
774 can be used.
776 Note that the images and lyrics are not interpreted at all, and also
777 the playlist and the mood blobs are only investigated when the mood
778 or playlist is activated by using the select command.
780 *The score table*
782 Unlike all other tables the contents of the score table remain in
783 memory and are never stored on disk. The score table contains two
784 columns: The SHA1 hash value (of an audio file) and its current
785 score.
787 However, only those files which are admissible for the current mood
788 or playlist are contained in the score table. The audio file selector
789 always chooses the row with the highest score as the file to stream
790 next. While doing so, it computes the new score and updates the
791 last_played and the num_played fields in the audio file table.
793 The score table is recomputed by the select command which loads a
794 new mood or playlist.
796 Playlists and moods
797 ~~~~~~~~~~~~~~~~~~~
799 Playlists and moods offer two different ways of specifying the set of
800 admissible files. A playlist in itself describes a set of admissible
801 files. A mood, in contrast, describes the set of admissible files in
802 terms of attributes and other type of information available in the
803 audio file table. As an example, a mood can define a filename pattern,
804 which is then matched against the names of audio files in the table.
806 Selecting a mood or playlist means the generation of a ranking
807 (a score table) for the set of admissible files. Audio files are
808 then selected on a highest-score-first basis. The score table is
809 recomputed at the moment the mood or playlist is selected.
811 *Playlists*
813 Playlists are accommodated in the playlist table of the afs database,
814 using the aforementioned blob format for tables. A new filelist is
815 created using the addpl command, by specifying the full (absolute)
816 paths of all desired audio files, separated by newlines. For example
818 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
820 If _my_playlist_ already exists it is overwritten. To activate the
821 new playlist, execute
823 para select p/my_playlist
825 The audio file selector will assign scores to each entry of the list,
826 in descending order so that files will be selected in order. If a
827 file could not be opened for streaming, its entry is removed from
828 the score table (but not from the playlist).
830 *Moods*
832 A mood consists of a unique name and its *mood definition*, which is
833 a set of *mood lines* containing expressions in terms of attributes
834 and other data contained in the database.
836 At any time, at most one mood can be *active* which means that
837 para_server is going to select only files from that subset of
838 admissible files.
840 So in order to create a mood definition one has to write a set of
841 mood lines. Mood lines come in three flavours: Accept lines, deny
842 lines and score lines.
844 The general syntax of the three types of mood lines is
847 accept [with score <score>] [if] [not] <mood_method> [options]
848 deny [with score <score>] [if] [not] <mood_method> [options]
849 score <score> [if] [not] <mood_method> [options]
852 Here <score> is either an integer or the string "random" which assigns
853 a random score to all matching files. The score value changes the
854 order in which admissible files are going to be selected, but is of
855 minor importance for this introduction.
857 So we concentrate on the first two forms, i.e. accept and deny
858 lines. As usual, everything in square brackets is optional, i.e.
859 accept/deny lines take the following form when ignoring scores:
861 accept [if] [not] <mood_method> [options]
863 and analogously for the deny case. The "if" keyword is only syntactic
864 sugar and has no function. The "not" keyword just inverts the result,
865 so the essence of a mood line is the mood method part and the options
866 following thereafter.
868 A *mood method* is realized as a function which takes an audio file
869 and computes a number from the data contained in the database.
870 If this number is non-negative, we say the file *matches* the mood
871 method. The file matches the full mood line if it either
873 - matches the mood method and the "not" keyword is not given,
874 or
875 - does not match the mood method, but the "not" keyword is given.
877 The set of admissible files for the whole mood is now defined as those
878 files which match at least one accept mood line, but no deny mood line.
879 More formally, an audio file F is admissible if and only if
881 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
883 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
884 lines and "~" means "matches".
886 The cases where no mood lines of accept/deny type are defined need
887 special treatment:
889 - Neither accept nor deny lines: This treats all files as
890 admissible (in fact, that is the definition of the dummy mood
891 which is activated automatically if no moods are available).
893 - Only accept lines: A file is admissible iff it matches at
894 least one accept line:
896 F ~ AL1 or F ~ AL2 or ...
898 - Only deny lines: A file is admissible iff it matches no
899 deny line:
901 not (F ~ DL1 or F ~ DN2 ...)
905 *List of mood_methods*
907 no_attributes_set
909 Takes no arguments and matches an audio file if and only if no
910 attributes are set.
912 is_set <attribute_name>
914 Takes the name of an attribute and matches iff that attribute is set.
916 path_matches <pattern>
918 Takes a filename pattern and matches iff the path of the audio file
919 matches the pattern.
921 artist_matches <pattern>
922 album_matches <pattern>
923 title_matches <pattern>
924 comment_matches <pattern>
926 Takes an extended regular expression and matches iff the text of the
927 corresponding tag of the audio file matches the pattern. If the tag
928 is not set, the empty string is matched against the pattern.
930 year ~ <num>
931 bitrate ~ <num>
932 frequency ~ <num>
933 channels ~ <num>
934 num_played ~ <num>
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, frequency in Hz, channel
940 count, play count).
942 The year tag is special as its value is undefined if the audio file
943 has no year tag or the content of the year tag is not a number. Such
944 audio files never match. Another difference is the special treatment
945 if the year tag is a two-digit number. In this case either 1900 or
946 2000 are added to the tag value depending on whether the number is
947 greater than 2000 plus the current year.
950 *Mood usage*
952 To create a new mood called "my_mood", write its definition into
953 some temporary file, say "tmpfile", and add it to the mood table
954 by executing
956 para addmood my_mood < tmpfile
958 If the mood definition is really short, you may just pipe it to the
959 client instead of using temporary files. Like this:
961 echo "$MOOD_DEFINITION" | para addmood my_mood
963 There is no need to keep the temporary file since you can always use
964 the catmood command to get it back:
966 para catmood my_mood
968 A mood can be activated by executing
970 para select m/my_mood
972 Once active, the list of admissible files is shown by the ls command
973 if the "-a" switch is given:
975 para ls -a
978 *Example mood definition*
980 Suppose you have defined attributes "punk" and "rock" and want to define
981 a mood containing only Punk-Rock songs. That is, an audio file should be
982 admissible if and only if both attributes are set. Since
984 punk and rock
986 is obviously the same as
988 not (not punk or not rock)
990 (de Morgan's rule), a mood definition that selects only Punk-Rock
991 songs is
993 deny if not is_set punk
994 deny if not is_set rock
998 File renames and content changes
999 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1001 Since the audio file selector knows the SHA1 of each audio file that
1002 has been added to the afs database, it recognizes if the content of
1003 a file has changed, e.g. because an ID3 tag was added or modified.
1004 Also, if a file has been renamed or moved to a different location,
1005 afs will detect that an entry with the same hash value already exists
1006 in the audio file table.
1008 In both cases it is enough to just re-add the new file. In the
1009 first case (file content changed), the audio table is updated, while
1010 metadata such as the num_played and last_played fields, as well as
1011 the attributes, remain unchanged. In the other case, when the file
1012 is moved or renamed, only the path information is updated, all other
1013 data remains as before.
1015 It is possible to change the behaviour of the add command by using the
1016 "-l" (lazy add) or the "-f" (force add) option.
1018 Troubleshooting
1019 ~~~~~~~~~~~~~~~
1021 Use the debug loglevel (option -l debug for most commands) to show
1022 debugging info. Almost all paraslash executables have a brief online
1023 help which is displayed by using the -h switch. The --detailed-help
1024 option prints the full help text.
1026 If para_server crashed or was killed by SIGKILL (signal 9), it
1027 may refuse to start again because of "dirty osl tables". In this
1028 case you'll have to run the oslfsck program of libosl to fix your
1029 database. It might be necessary to use --force (even if your name
1030 isn't Luke). However, make sure para_server isn't running before
1031 executing oslfsck --force.
1033 If you don't mind to recreate your database you can start
1034 from scratch by removing the entire database directory, i.e.
1036 rm -rf ~/.paraslash/afs_database-0.4
1038 Be aware that this removes all attribute definitions, all playlists
1039 and all mood definitions and requires to re-initialize the tables.
1041 Although oslfsck fixes inconsistencies in database tables it doesn't
1042 care about the table contents. To check for invalid table contents, use
1044 para_client check
1046 This prints out references to missing audio files as well as invalid
1047 playlists and mood definitions.
1049 ---------------------------------------
1050 Audio formats and audio format handlers
1051 ---------------------------------------
1053 Audio formats
1054 ~~~~~~~~~~~~~
1056 The following audio formats are supported by paraslash:
1058 *MP3*
1060 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1061 designed as part of its MPEG-1 standard. An MP3 file is made up of
1062 multiple MP3 frames, which consist of a header and a data block. The
1063 size of an MP3 frame depends on the bit rate and on the number
1064 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1065 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1066 400 bytes large.
1068 *OGG/Vorbis*
1070 OGG is a standardized audio container format, while Vorbis is an
1071 open source codec for lossy audio compression. Since Vorbis is most
1072 commonly made available via the OGG container format, it is often
1073 referred to as OGG/Vorbis. The OGG container format divides data into
1074 chunks called OGG pages. A typical OGG page is about 4KB large. The
1075 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1076 may vary considerably.
1078 *OGG/Speex*
1080 Speex is an open-source speech codec that is based on CELP (Code
1081 Excited Linear Prediction) coding. It is designed for voice
1082 over IP applications, has modest complexity and a small memory
1083 footprint. Wideband and narrowband (telephone quality) speech are
1084 supported. As for Vorbis audio, Speex bit-streams are often stored
1085 in OGG files.
1087 *AAC*
1089 Advanced Audio Coding (AAC) is a standardized, lossy compression
1090 and encoding scheme for digital audio which is the default audio
1091 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1092 the container format and audio files encoded with AAC have the .m4a
1093 extension. A typical AAC frame is about 700 bytes large.
1095 *WMA*
1097 Windows Media Audio (WMA) is an audio data compression technology
1098 developed by Microsoft. A WMA file is usually encapsulated in the
1099 Advanced Systems Format (ASF) container format, which also specifies
1100 how meta data about the file is to be encoded. The bit stream of WMA
1101 is composed of superframes, each containing one or more frames of
1102 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1104 Meta data
1105 ~~~~~~~~~
1107 Unfortunately, each audio format has its own conventions how meta
1108 data is added as tags to the audio file.
1110 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1111 is rather simple but also very limited as it supports only artist,
1112 title, album, year and comment tags. Each of these can only be at most
1113 32 characters long. ID3, version 2 is much more flexible but requires
1114 a separate library being installed for paraslash to support it.
1116 Ogg vorbis files contain meta data as Vorbis comments, which are
1117 typically implemented as strings of the form "[TAG]=[VALUE]". Unlike
1118 ID3 version 1 tags, one may use whichever tags are appropriate for
1119 the content.
1121 AAC files usually use the MPEG-4 container format for storing meta
1122 data while WMA files wrap meta data as special objects within the
1123 ASF container format.
1125 paraslash only tracks the most common tags that are supported by
1126 all tag variants: artist, title, year, album, comment. When a file
1127 is added to the AFS database, the meta data of the file is extracted
1128 and stored in the audio file table.
1130 Chunks and chunk tables
1131 ~~~~~~~~~~~~~~~~~~~~~~~
1133 paraslash uses the word "chunk" as common term for the building blocks
1134 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1135 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1136 size varies considerably between audio formats, from a few hundred
1137 bytes (MP3) up to 8K (WMA).
1139 The chunk table contains the offsets within the audio file that
1140 correspond to the chunk boundaries of the file. Like the meta data,
1141 the chunk table is computed and stored in the database whenever an
1142 audio file is added.
1144 The paraslash senders (see below) always send complete chunks. The
1145 granularity for seeking is therefore determined by the chunk size.
1147 Audio format handlers
1148 ~~~~~~~~~~~~~~~~~~~~~
1150 For each audio format paraslash contains an audio format handler whose
1151 first task is to tell whether a given file is a valid audio file of
1152 this type. If so, the audio file handler extracts some technical data
1153 (duration, sampling rate, number of channels etc.), computes the
1154 chunk table and reads the meta data.
1156 The audio format handler code is linked into para_server and executed
1157 via the _add_ command. The same code is also available as a stand-alone
1158 tool, para_afh, which can be used to print the technical data, the
1159 chunk table and the meta data of a file. Furthermore, one can use
1160 para_afh to cut an audio file, i.e. to select some of its chunks to
1161 produce a new file containing only these chunks.
1163 ----------
1164 Networking
1165 ----------
1167 Paraslash uses different network connections for control and data.
1168 para_client communicates with para_server over a dedicated TCP control
1169 connection. To transport audio data, separate data connections are
1170 used. For these data connections, a variety of transports (UDP, DCCP,
1171 HTTP) can be chosen.
1173 The chapter starts with the REFERENCE(The paraslash control
1174 service, control service), followed by a section on the various
1175 REFERENCE(Streaming protocols, streaming protocols) in which the data
1176 connections are described. The way audio file headers are embedded into
1177 the stream is discussed REFERENCE(Streams with headers and headerless
1178 streams, briefly) before the REFERENCE(Networking examples, example
1179 section) which illustrates typical commands for real-life scenarios.
1181 Both IPv4 and IPv6 are supported.
1183 The paraslash control service
1184 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1186 para_server is controlled at runtime via the paraslash control
1187 connection. This connection is used for server commands (play, stop,
1188 ...) as well as for afs commands (ls, select, ...).
1190 The server listens on a TCP port and accepts connections from clients
1191 that connect the open port. Each connection causes the server to fork
1192 off a client process which inherits the connection and deals with that
1193 client only. In this classical accept/fork approach the server process
1194 is unaffected if the child dies or goes crazy for whatever reason. In
1195 fact, the child process can not change address space of server process.
1197 The section on REFERENCE(Client-server authentication, client-server
1198 authentication) above described the early connection establishment
1199 from the crypto point of view. Here it is described what happens
1200 after the connection (including crypto setup) has been established.
1201 There are four processes involved during command dispatch as sketched
1202 in the following diagram.
1204 <<
1205 <pre>
1206 server_host client_host
1207 ~~~~~~~~~~~ ~~~~~~~~~~~
1209 +-----------+ connect +-----------+
1210 |para_server|<------------------------------ |para_client|
1211 +-----------+ +-----------+
1212 | ^
1213 | fork +---+ |
1214 +----------> |AFS| |
1215 | +---+ |
1216 | ^ |
1217 | | |
1218 | | connect (cookie) |
1219 | | |
1220 | | |
1221 | fork +-----+ inherited connection |
1222 +---------->|child|<--------------------------+
1223 +-----+
1224 </pre>
1225 >>
1227 Note that the child process is not a child of the afs process,
1228 so communication of these two processes has to happen via local
1229 sockets. In order to avoid abuse of the local socket by unrelated
1230 processes, a magic cookie is created once at server startup time just
1231 before the server process forks off the AFS process. This cookie is
1232 known to the server, AFS and the child, but not to unrelated processes.
1234 There are two different kinds of commands: First there are commands
1235 that cause the server to respond with some answer such as the list
1236 of all audio files. All but the addblob commands (addimg, addlyr,
1237 addpl, addmood) are of this kind. The addblob commands add contents
1238 to the database, so they need to transfer data the other way round,
1239 from the client to the server.
1241 There is no knowledge about the server commands built into para_client,
1242 so it does not know about addblob commands. Instead, it inspects the
1243 first data package sent by the server for a magic string. If this
1244 string was found, it sends STDIN to the server, otherwise it dumps
1245 data from the server to STDOUT.
1247 Streaming protocols
1248 ~~~~~~~~~~~~~~~~~~~
1250 A network (audio) stream usually consists of one streaming source,
1251 the _sender_, and one or more _receivers_ which read data over the
1252 network from the streaming source.
1254 Senders are thus part of para_server while receivers are part of
1255 para_audiod. Moreover, there is the stand-alone tool para_recv which
1256 can be used to manually download a stream, either from para_server
1257 or from a web-based audio streaming service.
1259 The following three streaming protocols are supported by paraslash:
1261 - HTTP. Recommended for public streams that can be played by
1262 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1263 sender is supported on all operating systems and all platforms.
1265 - DCCP. Recommended for LAN streaming. DCCP is currently
1266 available only for Linux.
1268 - UDP. Recommended for multicast LAN streaming.
1270 See the Appendix on REFERENCE(Network protocols, network protocols)
1271 for brief descriptions of the various protocols relevant for network
1272 audio streaming with paraslash.
1274 It is possible to activate more than one sender simultaneously.
1275 Senders can be controlled at run time and via config file and command
1276 line options.
1278 Note that audio connections are _not_ encrypted. Transport or Internet
1279 layer encryption should be used if encrypted data connections are
1280 needed.
1282 Since DCCP and TCP are both connection-oriented protocols, connection
1283 establishment/teardown and access control are very similar between
1284 these two streaming protocols. UDP is the most lightweight option,
1285 since in contrast to TCP/DCCP it is connectionless. It is also the
1286 only protocol supporting IP multicast.
1288 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1289 clients to connect and establish a connection via some protocol-defined
1290 handshake mechanism. Both senders maintain two linked lists each:
1291 The list of all clients which are currently connected, and the list
1292 of access control entries which determines who is allowed to connect.
1293 IP-based access control may be configured through config file and
1294 command line options and via the "allow" and "deny" sender subcommands.
1296 Upon receiving a GET request from the client, the HTTP sender sends
1297 back a status line and a message. The body of this message is the
1298 audio stream. This is common practice and is supported by many popular
1299 clients which can thus be used to play a stream offered by para_server.
1300 For DCCP things are a bit simpler: No messages are exchanged between
1301 the receiver and sender. The client simply connects and the sender
1302 starts to stream.
1304 DCCP is an experimental protocol which offers a number of new features
1305 not available for TCP. Both ends can negotiate these features using
1306 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1307 datagram-based (no retransmissions) and thus should not be used over
1308 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1309 is access to a variety of different congestion-control mechanisms
1310 called CCIDs. Two different CCIDs are available per default on Linux:
1313 - _CCID 2_. A Congestion Control mechanism similar to that
1314 of TCP. The sender maintains a congestion window and halves
1315 this window in response to congestion.
1318 - _CCID-3_. Designed to be fair when competing for bandwidth.
1319 It has lower variation of throughput over time compared with
1320 TCP, which makes it suitable for streaming media.
1322 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1323 single list, the _target list_. This list describes the set of clients
1324 to which the stream is sent. There is no list for access control and
1325 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1326 and "delete" commands can be used to modify the target list.
1328 Since both UDP and DCCP offer an unreliable datagram-based transport,
1329 additional measures are necessary to guard against disruptions over
1330 networks that are lossy or which may be subject to interference (as
1331 is for instance the case with WiFi). Paraslash uses FEC (Forward
1332 Error Correction) to guard against packet losses and reordering. The
1333 stream is FEC-encoded before it is sent through the UDP socket and
1334 must be decoded accordingly on the receiver side.
1336 The packet size and the amount of redundancy introduced by FEC can
1337 be configured via the FEC parameters which are dictated by server
1338 and may also be configured through the "sender" command. The FEC
1339 parameters are encoded in the header of each network packet, so no
1340 configuration is necessary on the receiver side. See the section on
1341 REFERENCE(Forward error correction, FEC) below.
1343 Streams with headers and headerless streams
1344 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1346 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1347 needed to decode the stream is only contained in the audio file
1348 header of the container format but not in each data chunk. Clients
1349 must be able to obtain this information in case streaming starts in
1350 the middle of the file or if para_audiod is started while para_server
1351 is already sending a stream.
1353 This is accomplished in different ways, depending on the streaming
1354 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1355 header is sent prior to audio file data. This technique however does
1356 not work for the connectionless UDP transport. Hence the audio file
1357 header is periodically being embedded into the UDP audio data stream.
1358 By default, the header is resent after five seconds. The receiver has
1359 to wait until the next header arrives before it can start decoding
1360 the stream.
1362 Examples
1363 ~~~~~~~~
1365 The sender command of para_server allows to (de-)activate senders
1366 and to change the access permissions senders at runtime. The "si"
1367 (server info) command is used to list the streaming options of the
1368 currently running server as well as the various sender access lists.
1370 -> Show client/target/access lists:
1372 para_client si
1374 -> Obtain general help for the sender command:
1376 para_client help sender
1378 -> Get help for a specific sender (contains further examples):
1380 s=http # or dccp or udp
1381 para_client sender $s help
1383 By default para_server activates both the HTTP and th DCCP sender on
1384 startup. This can be changed via command line options or para_server's
1385 config file.
1387 -> List config file options for senders:
1389 para_server -h
1391 All senders share the "on" and "off" commands, so senders may be
1392 activated and deactivated independently of each other.
1394 -> Switch off the http sender:
1396 para_client sender http off
1398 -> Receive a DCCP stream using CCID2 and write the output into a file:
1400 host=foo.org; ccid=2; filename=bar
1401 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1403 Note the quotes around the arguments for the dccp receiver. Each
1404 receiver has its own set of command line options and its own command
1405 line parser, so arguments for the dccp receiver must be protected
1406 from being interpreted by para_recv.
1408 -> Start UDP multicast, using the default multicast address:
1410 para_client sender udp add
1412 -> Receive FEC-encoded multicast stream and write the output into a file:
1414 filename=foo
1415 para_recv -r udp > $filename
1417 -> Add an UDP unicast for a client to the target list of the UDP sender:
1419 t=client.foo.org
1420 para_client sender udp add $t
1422 -> Receive this (FEC-encoded) unicast stream:
1424 filename=foo
1425 para_recv -r 'udp -i' > $filename
1427 -> Create a minimal config for para_audiod for HTTP streams:
1429 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1430 formats="mp3 ogg aac wma" # remove what you do not have
1431 for f in $formats; do echo receiver \"$f:http -i $s\"; done > $c
1432 para_audiod --config $c
1434 -------
1435 Filters
1436 -------
1438 A paraslash filter is a module which transforms an input stream into
1439 an output stream. Filters are included in the para_audiod executable
1440 and in the stand-alone tool para_filter which usually contains the
1441 same modules.
1443 While para_filter reads its input stream from STDIN and writes
1444 the output to STDOUT, the filter modules of para_audiod are always
1445 connected to a receiver which produces the input stream and a writer
1446 which absorbs the output stream.
1448 Some filters depend on a specific library being installed and are
1449 not compiled in if this library was not found at compile time. To
1450 see the list of supported filters, run para_filter and para_audiod
1451 with the --help option. The output looks similar to the following:
1453 Available filters:
1454 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1456 Out of these filter modules, a chain of filters can be constructed,
1457 much in the way Unix pipes can be chained, and analogous to the use
1458 of modules in gstreamer: The output of the first filter becomes the
1459 input of the second filter. There is no limitation on the number of
1460 filters and the same filter may occur more than once.
1462 Like receivers, each filter has its own command line options which
1463 must be quoted to protect them from the command line options of
1464 the driving application (para_audiod or para_filter). Example:
1466 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1468 For para_audiod, each audio format has its own set of filters. The
1469 name of the audio format for which the filter should be applied is
1470 used as the prefix for the filter option. Example:
1472 para_audiod -f 'mp3:prebuffer --duration 300'
1474 Decoders
1475 ~~~~~~~~
1477 For each supported audio format there is a corresponding filter
1478 which decodes audio data in this format to 16 bit PCM data which
1479 can be directly sent to the sound device or any other software that
1480 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1481 filters are called _decoders_ in general, and xxxdec is the name of
1482 the paraslash decoder for the audio format xxx. For example, the mp3
1483 decoder filter is called mp3dec.
1485 Note that the output of the decoder is about 10 times larger than
1486 its input. This means that filters that operate on the decoded audio
1487 stream have to deal with much more data than filters that transform
1488 the audio stream before it is fed to the decoder.
1490 Paraslash relies on external libraries for most decoders, so these
1491 libraries must be installed for the decoder to be included in the
1492 para_filter and para_audiod executables. The oggdec filter depends
1493 on the libogg and libvorbis libraries for example.
1495 Forward error correction
1496 ~~~~~~~~~~~~~~~~~~~~~~~~
1498 As already mentioned REFERENCE(Streaming protocols, earlier),
1499 paraslash uses forward error correction (FEC) for the unreliable UDP
1500 and DCCP transports. FEC is a technique which was invented already
1501 in 1960 by Reed and Solomon and which is widely used for the parity
1502 calculations of storage devices (RAID arrays). It is based on the
1503 algebraic concept of finite fields, today called Galois fields, in
1504 honour of the mathematician Galois (1811-1832). The FEC implementation
1505 of paraslash is based on code by Luigi Rizzo.
1507 Although the details require a sound knowledge of the underlying
1508 mathematics, the basic idea is not hard to understand: For positive
1509 integers k and n with k < n it is possible to compute for any k given
1510 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1511 ..., p_r such that all data bytes can be reconstructed from *any*
1512 k bytes of the set
1514 {d_1, ..., d_k, p_1, ..., p_r}.
1516 FEC-encoding for unreliable network transports boils down to slicing
1517 the audio stream into groups of k suitably sized pieces called _slices_
1518 and computing the r corresponding parity slices. This step is performed
1519 in para_server which then sends both the data and the parity slices
1520 over the unreliable network connection. If the client was able
1521 to receive at least k of the n = k + r slices, it can reconstruct
1522 (FEC-decode) the original audio stream.
1524 From these observations it is clear that there are three different
1525 FEC parameters: The slice size, the number of data slices k, and the
1526 total number of slices n. It is crucial to choose the slice size
1527 such that no fragmentation of network packets takes place because
1528 FEC only guards against losses and reodering but fails if slices are
1529 received partially.
1531 FEC decoding in paralash is performed through the fecdec filter which
1532 usually is the first filter (there can be other filters before fecdec
1533 if these do not alter the audio stream).
1536 Volume adjustment (amp and compress)
1537 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1539 The amp and the compress filter both adjust the volume of the audio
1540 stream. These filters operate on uncompressed audio samples. Hence
1541 they are usually placed directly after the decoding filter. Each
1542 sample is multiplied with a scaling factor (>= 1) which makes amp
1543 and compress quite expensive in terms of computing power.
1545 *amp*
1547 The amp filter amplifies the audio stream by a fixed scaling factor
1548 that must be known in advance. For para_audiod this factor is derived
1549 from the amplification field of the audio file's entry in the audio
1550 file table while para_filter uses the value given at the command line.
1552 The optimal scaling factor F for an audio file is the largest real
1553 number F >= 1 such that after multiplication with F all samples still
1554 fit into the sample interval [-32768, 32767]. One can use para_filter
1555 in combination with the sox utility to compute F:
1557 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1559 The amplification value V which is stored in the audio file table,
1560 however, is an integer between 0 and 255 which is connected to F
1561 through the formula
1563 V = (F - 1) * 64.
1565 To store V in the audio file table, the command
1567 para_client -- touch -a=V file.mp3
1569 is used. The reader is encouraged to write a script that performs
1570 these computations :)
1572 *compress*
1574 Unlike the amplification filter, the compress filter adjusts the volume
1575 of the audio stream dynamically without prior knowledge about the peak
1576 value. It maintains the maximal volume of the last n samples of the
1577 audio stream and computes a suitable amplification factor based on that
1578 value and the various configuration options. It tries to chose this
1579 factor such that the adjusted volume meets the desired target level.
1581 Note that it makes sense to combine amp and compress.
1583 Misc filters (wav and prebuffer)
1584 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1586 These filters are rather simple and do not modify the audio stream at
1587 all. The wav filter is only useful with para_filter and in connection
1588 with a decoder. It asks the decoder for the number of channels and the
1589 sample rate of the stream and adds a Microsoft wave header containing
1590 this information at the beginning. This allows to write wav files
1591 rather than raw PCM files (which do not contain any information about
1592 the number of channels and the sample rate).
1594 The prebuffer filter simply delays the output until the given time has
1595 passed (starting from the time the first byte was available in its
1596 input queue) or until the given amount of data has accumulated. It
1597 is mainly useful for para_audiod if the standard parameters result
1598 in buffer underruns.
1600 Both filters require almost no additional computing time, even when
1601 operating on uncompressed audio streams, since data buffers are simply
1602 "pushed down" rather than copied.
1604 Examples
1605 ~~~~~~~~
1607 -> Decode an mp3 file to wav format:
1609 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1611 -> Amplify a raw audio file by a factor of 1.5:
1613 para_filter -f amp --amp 32 < foo.raw > bar.raw
1615 ------
1616 Output
1617 ------
1619 Once an audio stream has been received and decoded to PCM format,
1620 it can be sent to a sound device for playback. This part is performed
1621 by paraslash _writers_ which are described in this chapter.
1623 Writers
1624 ~~~~~~~
1626 A paraslash writer acts as a data sink that consumes but does not
1627 produce audio data. Paraslash writers operate on the client side and
1628 are contained in para_audiod and in the stand-alone tool para_write.
1630 The para_write program reads uncompressed audio data from STDIN. If
1631 this data starts with a wav header, sample rate, sample format and
1632 channel count are read from the header. Otherwise CD audio (44.1KHz
1633 16 bit little endian, stereo) is assumed but this can be overridden
1634 by command line options. para_audiod, on the other hand, obtains
1635 the sample rate and the number of channels from the decoder.
1637 Like receivers and filters, each writer has an individual set of
1638 command line options, and for para_audiod writers can be configured
1639 per audio format separately. It is possible to activate more than
1640 one writer for the same stream simultaneously.
1642 OS-dependent APIs
1643 ~~~~~~~~~~~~~~~~~
1645 Unfortunately, the various flavours of Unix on which paraslash
1646 runs on have different APIs for opening a sound device and starting
1647 playback. Hence for each such API there is a paraslash writer that
1648 can play the audio stream via this API.
1650 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1651 Linux systems. Although there are several mid-layer APIs in use by
1652 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1653 currently supports only the low-level ALSA API which is not supposed
1654 to be change. ALSA is very feature-rich, in particular it supports
1655 software mixing via its DMIX plugin. ALSA is the default writer on
1656 Linux systems.
1658 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1659 is also available on Linux systems, usually provided by ALSA as an
1660 emulation for backwards compatibility. This API is rather simple but
1661 also limited. For example only one application can open the device
1662 at any time. The OSS writer is activated by default on BSD Systems.
1664 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1665 for this API is only compiled in on such systems and is of course
1666 the default there.
1668 *FILE*. The file writer allows to capture the audio stream and
1669 write the PCM data to a file on the file system rather than playing
1670 it through a sound device. It is supported on all platforms and is
1671 always compiled in.
1673 Examples
1674 ~~~~~~~~
1676 -> Use the OSS writer to play a wav file:
1678 para_write --writer oss < file.wav
1680 -> Enable ALSA software mixing for mp3 streams
1682 para_audiod --writer 'mp3:alsa -d plug:swmix'
1685 ---
1686 Gui
1687 ---
1689 para_gui executes an arbitrary command which is supposed to print
1690 status information to STDOUT. It then displays this information in
1691 a curses window. By default the command
1693 para_audioc -- stat -p
1695 is executed, but this can be customized via the --stat_cmd option. In
1696 particular it possible to use
1698 para_client -- stat -p
1700 to make para_gui work on systems on which para_audiod is not running.
1702 Key bindings
1703 ~~~~~~~~~~~~
1705 It is possible to bind keys to arbitrary commands via custom
1706 key-bindings. Besides the internal keys which can not be changed (help,
1707 quit, loglevel, version...), the following flavours of key-bindings
1708 are supported:
1710 - external: Shutdown curses before launching the given command.
1711 Useful for starting other ncurses programs from within
1712 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1713 output format to write a mailbox containing one mail for each
1714 (admissible) file the audio file selector knows about. Then
1715 start mutt from within para_gui to browse your collection!
1717 - display: Launch the command and display its stdout in
1718 para_gui's bottom window.
1720 - para: Like display, but start "para_client <specified
1721 command>" instead of "<specified command>".
1723 The general form of a key binding is
1725 key_map k:m:c
1727 which maps key k to command c using mode m. Mode may be x, d or p
1728 for external, display and paraslash commands, respectively.
1730 Themes
1731 ~~~~~~
1733 Currently there are only two themes for para_gui. It is easy, however,
1734 to add more themes. To create a new theme one has to define the
1735 position, color and geometry for for each status item that should be
1736 shown by this theme. See gui_theme.c for examples.
1738 The "." and "," keys are used to switch between themes.
1740 Examples
1741 ~~~~~~~~
1743 -> Show server info:
1745 key_map "i:p:si"
1747 -> Jump to the middle of the current audio file by pressing F5:
1749 key_map "<F5>:p:jmp 50"
1751 -> vi-like bindings for jumping around:
1753 key_map "l:p:ff 10"
1754 key_map "h:p:ff 10-"
1755 key_map "w:p:ff 60"
1756 key_map "b:p:ff 60-"
1758 -> Print the current date and time:
1760 key_map "D:d:date"
1762 -> Call other curses programs:
1764 key_map "U:x:aumix"
1765 key_map "!:x:/bin/bash"
1766 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1768 -----------
1769 Development
1770 -----------
1772 Tools
1773 ~~~~~
1775 In order to compile the sources from the git repository (rather than
1776 from tar balls) and for contributing non-trivial changes to the
1777 paraslash project, some additional tools should be installed on a
1778 developer machine.
1780 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1781 branches, below), the git source code management tool is used for
1782 paraslash development. It is necessary for cloning the git repository
1783 and for getting updates.
1785 ftp://ftp.gnu.org/pub/gnu/gengetopt/ (gengetopt). The C code for
1786 the command line parsers of all paraslash executables is generated
1787 by gengetopt. The generated C files are shipped in the tarballs but
1788 are not contained in the git repository.
1790 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1791 are generated from templates by the m4 macro processor.
1793 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1794 the configure file which is shipped in the tarballs but has to be
1795 generated when compiling from git.
1797 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1798 HTML version of this manual and some of the paraslash web pages are
1799 generated by the grutatxt plain text to HTML converter. If changes
1800 are made to these text files the grutatxt package must be installed
1801 to regenerate the HTML files.
1803 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1804 of paraslash's C sources uses the doxygen documentation system. The
1805 conventions for documenting the source code is described in the
1806 REFERENCE(Doxygen, Doxygen section).
1808 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1809 browsable HTML from the C sources. It is needed by doxygen.
1811 Git branches
1812 ~~~~~~~~~~~~
1814 Paraslash has been developed using the git source code management
1815 tool since 2006. Development is organized roughly in the same spirit
1816 as the git development itself, as described below.
1818 The following text passage is based on "A note from the maintainer",
1819 written by Junio C Hamano, the maintainer of git.
1821 There are four branches in the paraslash repository that track the
1822 source tree: "master", "maint", "next", and "pu".
1824 The "master" branch is meant to contain what is well tested and
1825 ready to be used in a production setting. There could occasionally be
1826 minor breakages or brown paper bag bugs but they are not expected to
1827 be anything major, and more importantly quickly and easily fixable.
1828 Every now and then, a "feature release" is cut from the tip of this
1829 branch, named with three dotted decimal digits, like 0.4.2.
1831 Whenever changes are about to be included that will eventually lead to
1832 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1833 "master" at that point. Obvious, safe and urgent fixes after the major
1834 release are applied to this branch and maintenance releases are cut
1835 from it. New features never go to this branch. This branch is also
1836 merged into "master" to propagate the fixes forward.
1838 A trivial and safe enhancement goes directly on top of "master".
1839 New development does not usually happen on "master", however.
1840 Instead, a separate topic branch is forked from the tip of "master",
1841 and it first is tested in isolation; Usually there are a handful such
1842 topic branches that are running ahead of "master". The tip of these
1843 branches is not published in the public repository to keep the number
1844 of branches that downstream developers need to worry about low.
1846 The quality of topic branches varies widely. Some of them start out as
1847 "good idea but obviously is broken in some areas" and then with some
1848 more work become "more or less done and can now be tested by wider
1849 audience". Luckily, most of them start out in the latter, better shape.
1851 The "next" branch is to merge and test topic branches in the latter
1852 category. In general, this branch always contains the tip of "master".
1853 It might not be quite rock-solid production ready, but is expected to
1854 work more or less without major breakage. The maintainer usually uses
1855 the "next" version of paraslash for his own pleasure, so it cannot
1856 be _that_ broken. The "next" branch is where new and exciting things
1857 take place.
1859 The two branches "master" and "maint" are never rewound, and "next"
1860 usually will not be either (this automatically means the topics that
1861 have been merged into "next" are usually not rebased, and you can find
1862 the tip of topic branches you are interested in from the output of
1863 "git log next"). You should be able to safely build on top of them.
1865 However, at times "next" will be rebuilt from the tip of "master" to
1866 get rid of merge commits that will never be in "master. The commit
1867 that replaces "next" will usually have the identical tree, but it
1868 will have different ancestry from the tip of "master".
1870 The "pu" (proposed updates) branch bundles the remainder of the
1871 topic branches. The "pu" branch, and topic branches that are only in
1872 "pu", are subject to rebasing in general. By the above definition
1873 of how "next" works, you can tell that this branch will contain quite
1874 experimental and obviously broken stuff.
1876 When a topic that was in "pu" proves to be in testable shape, it
1877 graduates to "next". This is done with
1879 git checkout next
1880 git merge that-topic-branch
1882 Sometimes, an idea that looked promising turns out to be not so good
1883 and the topic can be dropped from "pu" in such a case.
1885 A topic that is in "next" is expected to be polished to perfection
1886 before it is merged to "master". Similar to the above, this is
1887 done with
1889 git checkout master
1890 git merge that-topic-branch
1891 git branch -d that-topic-branch
1893 Note that being in "next" is not a guarantee to appear in the next
1894 release (being in "master" is such a guarantee, unless it is later
1895 found seriously broken and reverted), nor even in any future release.
1897 Coding Style
1898 ~~~~~~~~~~~~
1900 The preferred coding style for paraslash coincides more or less
1901 with the style of the Linux kernel. So rather than repeating what is
1902 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
1903 there), here are the most important points.
1905 - Burn the GNU coding standards.
1906 - Never use spaces for indentation.
1907 - Tabs are 8 characters, and thus indentations are also 8 characters.
1908 - Don't put multiple assignments on a single line.
1909 - Avoid tricky expressions.
1910 - Don't leave whitespace at the end of lines.
1911 - The limit on the length of lines is 80 columns.
1912 - Use K&R style for placing braces and spaces:
1914 if (x is true) {
1915 we do y
1916 }
1918 - Use a space after (most) keywords.
1919 - Do not add spaces around (inside) parenthesized expressions.
1920 - Use one space around (on each side of) most binary and ternary operators.
1921 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1922 - Mixed-case names are frowned upon.
1923 - Descriptive names for global variables are a must.
1924 - Avoid typedefs.
1925 - Functions should be short and sweet, and do just one thing.
1926 - The number of local variables shouldn't exceed 10.
1927 - Gotos are fine if they improve readability and reduce nesting.
1928 - Don't use C99-style "// ..." comments.
1929 - Names of macros defining constants and labels in enums are capitalized.
1930 - Enums are preferred when defining several related constants.
1931 - Always use the paraslash wrappers for allocating memory.
1932 - If the name of a function is an action or an imperative.
1933 command, the function should return an error-code integer
1934 (<0 means error, >=0 means success). If the name is a
1935 predicate, the function should return a "succeeded" boolean.
1938 Doxygen
1939 ~~~~~~~
1941 Doxygen is a documentation system for various programming
1942 languages. The paraslash project uses Doxygen for generating the API
1943 reference on the web pages, but good source code documentation is
1944 also beneficial to people trying to understand the code structure
1945 and the interactions between the various source files.
1947 It is more illustrative to look at the source code for examples than
1948 to describe the conventions for documenting the source in this manual,
1949 so we only describe which parts of the code need doxygen comments,
1950 but leave out details on documentation conventions.
1952 As a rule, only the public part of the C source is documented with
1953 Doxygen. This includes structures, defines and enumerations in header
1954 files as well as public (non-static) C functions. These should be
1955 documented completely. For example each parameter and the return
1956 value of a public function should get a descriptive comment.
1958 No doxygen comments are necessary for static functions and for
1959 structures and enumerations in C files (which are used only within
1960 this file). This does not mean, however, that those entities need
1961 no documentation at all. Instead, common sense should be applied to
1962 document what is not obvious from reading the code.
1964 --------
1965 Appendix
1966 --------
1968 Network protocols
1969 ~~~~~~~~~~~~~~~~~
1971 *IP*. The _Internet Protocol_ is the primary networking protocol
1972 used for the Internet. All protocols described below use IP as the
1973 underlying layer. Both the prevalent IPv4 and the next-generation
1974 IPv6 variant are being deployed actively worldwide.
1976 *Connection-oriented and connectionless protocols*. Connectionless
1977 protocols differ from connection-oriented ones in that state
1978 associated with the sending/receiving endpoints is treated
1979 implicitly. Connectionless protocols maintain no internal knowledge
1980 about the state of the connection. Hence they are not capable of
1981 reacting to state changes, such as sudden loss or congestion on the
1982 connection medium. Connection-oriented protocols, in contrast, make
1983 this knowledge explicit. The connection is established only after
1984 a bidirectional handshake which requires both endpoints to agree
1985 on the state of the connection, and may also involve negotiating
1986 specific parameters for the particular connection. Maintaining an
1987 up-to-date internal state of the connection also in general means
1988 that the sending endpoints perform congestion control, adapting to
1989 qualitative changes of the connection medium.
1991 *Reliability*. In IP networking, packets can be lost, duplicated,
1992 or delivered out of order, and different network protocols handle
1993 these problems in different ways. We call a transport-layer protocol
1994 _reliable_, if it turns the unreliable IP delivery into an ordered,
1995 duplicate- and loss-free delivery of packets. Sequence numbers
1996 are used to discard duplicates and re-arrange packets delivered
1997 out-of-order. Retransmission is used to guarantee loss-free
1998 delivery. Unreliable protocols, in contrast, do not guarantee ordering
1999 or data integrity.
2001 *Classification*. With these definitions the protocols which are used
2002 by paraslash for steaming audio data may be classified as follows.
2004 - HTTP/TCP: connection-oriented, reliable,
2005 - UDP: connectionless, unreliable,
2006 - DCCP: connection-oriented, unreliable.
2008 Below we give a short descriptions of these protocols.
2010 *TCP*. The _Transmission Control Protocol_ provides reliable,
2011 ordered delivery of a stream and a classic window-based congestion
2012 control. In contrast to UDP and DCCP (see below), TCP does not have
2013 record-oriented or datagram-based syntax, i.e. it provides a stream
2014 which is unaware and independent of any record (packet) boundaries.
2015 TCP is used extensively by many application layers. Besides HTTP (the
2016 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2017 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2018 top of TCP.
2020 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2021 protocol, built as a thin layer directly on top of IP. For this reason,
2022 it offers the same best-effort service as IP itself, i.e. there is no
2023 detection of duplicate or reordered packets. Being a connectionless
2024 protocol, only minimal internal state about the connection is
2025 maintained, which means that there is no protection against packet
2026 loss or network congestion. Error checking and correction (if at all)
2027 are performed in the application.'
2029 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2030 connection-oriented state maintenance known from TCP with the
2031 unreliable, datagram-based transport of UDP. This means that it
2032 is capable of reacting to changes in the connection by performing
2033 congestion control, offering multiple alternative approaches. But it
2034 is bound to datagram boundaries (the maximum packet size supported
2035 by a medium), and like UDP it lacks retransmission to protect
2036 against loss. Due to the use of sequence numbers, it is however
2037 able to react to loss (interpreted as a congestion indication) and
2038 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2039 to negotiate specific, binding features for a connection, such as
2040 the choice of congestion control: classic, window-based congestion
2041 control known from TCP is available as CCID-2, rate-based, "smooth"
2042 congestion control is offered as CCID-3.
2044 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2045 protocol on top of TCP. It is spoken by web servers and is most often
2046 used for web services. However, as can be seen by the many Internet
2047 radio stations and YouTube/Flash videos, http is by far not limited to
2048 the delivery of web pages only. Being a simple request/response based
2049 protocol, the semantics of the protocol also allow the delivery of
2050 multimedia content, such as audio over http.
2052 *Multicast*. IP multicast is not really a protocol but a technique
2053 for one-to-many communication over an IP network. The challenge is to
2054 deliver information to a group of destinations simultaneously using
2055 the most efficient strategy to send the messages over each link of
2056 the network only once. This has benefits for streaming multimedia:
2057 the standard one-to-one unicast offered by TCP/DCCP means that
2058 n clients listening to the same stream also consume n-times the
2059 resources, whereas multicast requires to send the stream just once,
2060 irrespective of the number of receivers. Since it would be costly to
2061 maintain state for each listening receiver, multicast often implies
2062 connectionless transport, which is the reason that it is currently
2063 only available via UDP.
2065 License
2066 ~~~~~~~
2068 Paraslash is licensed under the GPL, version 2. Most of the code
2069 base has been written from scratch, and those parts are GPL V2
2070 throughout. Notable exceptions are FEC and the WMA decoder. See the
2071 corresponding source files for licencing details for these parts. Some
2072 code sniplets of several other third party software packages have
2073 been incorporated into the paraslash sources, for example log message
2074 coloring was taken from the git sources. These third party software
2075 packages are all published under the GPL or some other license
2076 compatible to the GPL.
2078 Acknowledgements
2079 ~~~~~~~~~~~~~~~~
2081 Many thanks to Gerrit Renker who read an early draft of this manual
2082 and contributed significant improvements.
2084 ----------
2085 References
2086 ----------
2088 Articles
2089 ~~~~~~~~
2090 - Reed, Irving S.; Solomon, Gustave (1960),
2091 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2092 Polynomial Codes over Certain Finite Fields), Journal of the
2093 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2094 300-304, doi:10.1137/0108018)
2096 RFCs
2097 ~~~~
2099 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2100 User Datagram Protocol
2101 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2102 Internet Protocol
2103 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2104 RSA Cryptography Specifications
2105 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2106 (2006): Datagram Congestion Control Protocol (DCCP)
2107 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2108 Congestion Control ID 2: TCP-like Congestion Control
2109 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2110 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2112 Application web pages
2113 ~~~~~~~~~~~~~~~~~~~~~
2115 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash)
2116 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2117 - XREFERENCE(http://www.mpg123.de/, mpg123)
2118 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2119 - XREFERENCE(http://www.icecast.org/, icecast)
2120 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2122 External documentation
2123 ~~~~~~~~~~~~~~~~~~~~~~
2125 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2126 H. Peter Anvin: The mathematics of Raid6)
2127 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2128 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2129 Communication Protocols)
2131 Code
2132 ~~~~
2133 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2134 Original FEC implementation by Luigi Rizzo)