doc: Add flac documentation to manual and FEATURES.
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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, 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(, libosl).
200 The _object storage layer_ library is used by para_server. To
201 clone the source code repository, execute
203 git clone git://
205 - XREFERENCE(, gcc) or
206 XREFERENCE(, clang). All gcc versions
207 >= 3.3 are currently supported. Clang version 1.1 or newer
208 should work as well.
210 - XREFERENCE(, gnu make) is
211 also shipped with the disto. On BSD systems the gnu make
212 executable is often called gmake.
214 - XREFERENCE(, bash). Some
215 scripts which run during compilation require the EMPH(Bourne
216 again shell). It is most likely already installed.
218 - XREFERENCE(, openssl) or
219 XREFERENCE(, libgcrypt).
220 At least one of these two libraries is needed as the backend
221 for cryptographic routines on both the server and the client
222 side. Both openssl and libgcrypt are usually shipped with the
223 distro, but you might have to install the development package
224 (libssl-dev or libgcrypt-dev on debian systems) as well.
226 - XREFERENCE(, gengetopt)
227 is needed to generate the C code for the command line parsers
228 of all paraslash executables.
230 - XREFERENCE(, help2man)
231 is used to create the man pages.
233 Optional:
235 - XREFERENCE(, libmad).
236 To compile in MP3 support for paraslash, the development
237 package must be installed. It is called libmad0-dev on
238 debian-based systems. Note that libmad is not necessary on
239 the server side, i.e. for sending MP3 files.
242 libid3tag). For version-2 ID3 tag support, you'll need
243 the libid3tag development package libid3tag0-dev. Without
244 libid3tag, only version one tags are recognized.
246 - XREFERENCE(, ogg vorbis).
247 For ogg vorbis streams you'll need libogg, libvorbis,
248 libvorbisfile. The corresponding Debian packages are called
249 libogg-dev and libvorbis-dev.
251 - XREFERENCE(, libfaad). For aac
252 files (m4a) you'll need libfaad (libfaad-dev).
254 - XREFERENCE(, speex). In order to stream
255 or decode speex files, libspeex (libspeex-dev) is required.
257 - XREFERENCE(, flac). To stream
258 or decode files encoded with the _Free Lossless Audio Codec_,
259 libFLAC (libFLAC-dev) must be installed.
261 - XREFERENCE(, alsa-lib). On
262 Linux, you'll need to have ALSA's development package
263 libasound2-dev installed.
266 libao). Needed to build the ao writer (ESD, PulseAudio,...).
267 Debian package: libao-dev.
269 Installation
270 ~~~~~~~~~~~~
272 First make sure all non-optional packages listed in the section on
273 REFERENCE(Requirements, required software) are installed on your
274 system.
276 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
277 OGG/Speex and AAC support are all optional. The configure script will
278 detect what is installed on your system and will only try to build
279 those executables that can be built with your setup.
281 Note that no special decoder library (not even the MP3 decoding library
282 libmad) is needed for para_server if you only want to stream MP3 or WMA
283 files. Also, it's fine to use para_server on a box without sound card.
285 Next, install the paraslash package on all machines, you'd like this
286 software to run on:
288 (./configure && make) > /dev/null
290 There should be no errors but probably some warnings about missing
291 packages which usually implies that not all audio formats will be
292 supported. If headers or libs are installed at unusual locations you
293 might need to tell the configure script where to find them. Try
295 ./configure --help
297 to see a list of options. If the paraslash package was compiled
298 successfully, execute (optionally)
300 make test
302 to run the paraslash test suite. If all tests pass, execute as root
304 make install
306 to install executables under /usr/local/bin and the man pages under
307 /usr/local/man.
309 Configuration
310 ~~~~~~~~~~~~~
312 *Step 1*: Create a paraslash user
314 In order to control para_server at runtime you must create a paraslash
315 user. As authentication is based on the RSA crypto system you'll have
316 to create an RSA key pair. If you already have a user and an RSA key
317 pair, you may skip this step.
319 In this section we'll assume a typical setup: You would like to run
320 para_server on some host called server_host as user foo, and you want
321 to connect to para_server from another machine called client_host as
322 user bar.
324 As foo@server_host, create ~/.paraslash/server.users by typing the
325 following commands:
327 user=bar
328 target=~/.paraslash/server.users
329 key=~/.paraslash/$user
331 mkdir -p ~/.paraslash
332 echo "user $user $key $perms" >> $target
334 Next, change to the "bar" account on client_host and generate the
335 key pair with the commands
337 ssh-keygen -t rsa -b 2048
338 # hit enter twice to create a key with no passphrase
340 This generates the two files id_rsa and in ~/.ssh. Note
341 that paraslash can also read keys generated by the "openssl genrsa"
342 command. However, since keys created with ssh-keygen can also be used
343 for ssh, this method is recommended.
345 Note that para_server refuses to use a key if it is shorter than 2048
346 bits. In particular, the RSA keys of paraslash 0.3.x will not work
347 with version 0.4.x. Moreover, para_client refuses to use a (private)
348 key which is world-readable.
350 para_server only needs to know the public key of the key pair just
351 created. Copy this public key to server_host:
353 src=~/.ssh/
354 dest=.paraslash/$LOGNAME
355 scp $src foo@server_host:$dest
357 Finally, tell para_client to connect to server_host:
359 conf=~/.paraslash/client.conf
360 echo 'hostname server_host' > $conf
363 *Step 2*: Start para_server
365 Before starting the server make sure you have write permissions to
366 the directory /var/paraslash that has been created during installation:
368 sudo chown $LOGNAME /var/paraslash
370 Alternatively, use the --afs_socket Option to specify a different
371 location for the AFS command socket.
373 For this first try, we'll use the info loglevel to make the output
374 of para_server more verbose.
376 para_server -l info
378 Now you can use para_client to connect to the server and issue
379 commands. Open a new shell as bar@client_host and try
381 para_client help
382 para_client si
384 to retrieve the list of available commands and some server info.
385 Don't proceed if this doesn't work.
387 *Step 3*: Create and populate the database
389 An empty database is created with
391 para_client init
393 This initializes a couple of empty tables under
394 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
395 tables, but it's good to know that you can start from scratch with
397 rm -rf ~/.paraslash/afs_database-0.4
399 in case something went wrong.
401 Next, you need to add some audio files to that database so that
402 para_server knows about them. Choose an absolute path to a directory
403 containing some audio files and add them to the audio file table:
405 para_client add /my/mp3/dir
407 This might take a while, so it is a good idea to start with a directory
408 containing not too many files. Note that the table only contains data
409 about the audio files found, not the files themselves.
411 You may print the list of all known audio files with
413 para_client ls
415 *Step 4*: Configure para_audiod
417 para_audiod needs to create a "well-known" socket for the clients to
418 connect to. The default path for this socket is
420 /var/paraslash/audiod_socket.$HOSTNAME
422 In order to make this directory writable for para_audiod, execute
423 as bar@client_host
425 sudo chown $LOGNAME /var/paraslash
428 We will also have to tell para_audiod that it should receive the
429 audio stream from server_host via http:
431 para_audiod -l info -r '.:http -i server_host'
433 You should now be able to listen to the audio stream once para_server
434 starts streaming. To activate streaming, execute
436 para_client play
438 Since no playlist has been specified yet, the "dummy" mode which
439 selects all known audio files is activated automatically. See the
440 section on the REFERENCE(The audio file selector, audio file selector)
441 for how to use playlists and moods to specify which files should be
442 streamed in which order.
444 *Troubleshooting*
446 It did not work? To find out why, try to receive, decode and play the
447 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 the RC4 stream cipher.
471 In this chapter we briefly describe RSA and RC4 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 and RC4
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 only 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 Client-server authentication
502 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
504 The authentication handshake between para_client and para_server goes
505 as follows:
507 - para_client connects to para_server and sends an
508 authentication request for a user. It does so by connecting
509 to TCP port 2990 of the server host. This port is called the
510 para_server _control port_.
512 - para_server accepts the connection and forks a child process
513 which handles the incoming request. The parent process keeps
514 listening on the control port while the child process (also
515 called para_server below) continues as follows.
517 - para_server loads the RSA public key of that user, fills a
518 fixed-length buffer with random bytes, encrypts that buffer
519 using the public key and sends the encrypted buffer to the
520 client. The first part of the buffer is the challenge which
521 is used for authentication while the second part is the RC4
522 session key.
524 - para_client receives the encrypted buffer and decrypts it
525 with the user's private key, thereby obtaining the challenge
526 buffer and the session key. It sends the SHA1 hash value of
527 the challenge back to para_server and stores the session key
528 for further use.
530 - para_server also computes the SHA1 hash of the challenge
531 and compares it against what was sent back by the client.
533 - If the two hashes do not match, the authentication has
534 failed and para_server closes the connection.
536 - Otherwise the user is considered authenticated and the client
537 is allowed to proceed by sending a command to be executed. From
538 this point on the communication is encrypted using the RC4
539 stream cipher with the session key known to both peers.
541 paraslash relies on the quality of the pseudo-random bytes provided
542 by the crypto library (openssl or libgcrypt), on the security of
543 the implementation of the RSA and RC4 crypto routines and on the
544 infeasibility to invert the SHA1 function.
546 Neither para_server or para_client create RSA keys on their own. This
547 has to be done once for each user as sketched in REFERENCE(Quick start,
548 Quick start) and discussed in more detail REFERENCE(The user_list
549 file, below).
551 The user_list file
552 ~~~~~~~~~~~~~~~~~~
554 At startup para_server reads the user list file which contains one
555 line per user. The default location of the user list file may be
556 changed with the --user_list option.
558 There should be at least one user in this file. Each user must have
559 an RSA key pair. The public part of the key is needed by para_server
560 while the private key is needed by para_client. Each line of the
561 user list file must be of the form
563 user <username> <key> <perms>
565 where _username_ is an arbitrary string (usually the user's login
566 name), _key_ is the full path to that user's public RSA key, and
567 _perms_ is a comma-separated list of zero or more of the following
568 permission bits:
570 +---------------------------------------------------------+
571 | AFS_READ | read the contents of the databases |
572 +-----------+---------------------------------------------+
573 | AFS_WRITE | change database contents |
574 +-----------+---------------------------------------------+
575 | VSS_READ | obtain information about the current stream |
576 +-----------+---------------------------------------------+
577 | VSS_WRITE | change the current stream |
578 +---------------------------------------------------------+
580 The permission bits specify which commands the user is allowed to
581 execute. The output of
583 para_client help
585 contains in the third column the permissions needed to execute the
586 command.
588 It is possible to make para_server reread the user_list file by
589 executing the paraslash "hup" command or by sending SIGHUP to the
590 PID of para_server.
593 Connecting para_audiod
594 ~~~~~~~~~~~~~~~~~~~~~~
596 para_audiod listens on a Unix domain socket. Those sockets are
597 for local communication only, so only local users can connect to
598 para_audiod. The default is to let any user connect but this can be
599 restricted on platforms that support UNIX socket credentials which
600 allow para_audiod to obtain the Unix credentials of the connecting
601 process.
603 Use para_audiod's --user_allow option to allow connections only for
604 a limited set of users.
606 -----------------------
607 The audio file selector
608 -----------------------
610 paraslash comes with a sophisticated audio file selector (AFS),
611 whose main task is to determine which file to stream next, based on
612 information on the audio files stored in a database. It communicates
613 also with para_client whenever an AFS command is executed, for example
614 to answer a database query.
616 Besides the traditional playlists, AFS supports audio file selection
617 based on _moods_ which act as a filter that limits the set of all
618 known audio files to those which satisfy certain criteria. It also
619 maintains tables containing images (e.g. album cover art) and lyrics
620 that can be associated with one or more audio files.
622 AFS uses XREFERENCE(, libosl), the
623 object storage layer library, as the backend library for storing
624 information on audio files, playlists, etc. This library offers
625 functionality similar to a relational database, but is much more
626 lightweight than a full database backend.
628 In this chapter we sketch the setup of the REFERENCE(The AFS process,
629 AFS process) during server startup and proceed with the description
630 of the REFERENCE(Database layout, layout) of the various database
631 tables. The section on REFERENCE(Playlists and moods, playlists
632 and moods) explains these two audio file selection mechanisms
633 in detail and contains pratical examples. The way REFERENCE(File
634 renames and content changes, file renames and content changes) are
635 detected is discussed briefly before the REFERENCE(Troubleshooting,
636 Troubleshooting) section concludes the chapter.
638 The AFS process
639 ~~~~~~~~~~~~~~~
641 On startup, para_server forks to create the AFS process which opens
642 the OSL database tables. The server process communicates with the
643 AFS process via pipes and shared memory. Usually, the AFS process
644 awakes only briefly whenever the current audio file changes. The AFS
645 process determines the next audio file, opens it, verifies it has
646 not been changed since it was added to the database and passes the
647 open file descriptor to the server process, along with audio file
648 meta-data such as file name, duration, audio format and so on. The
649 server process then starts to stream the audio file.
651 The AFS process also accepts connections from local clients via
652 a well-known socket. However, only child processes of para_server
653 may connect through this socket. All server commands that have the
654 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
655 change the database.
657 Database layout
658 ~~~~~~~~~~~~~~~
660 *The audio file table*
662 This is the most important and usually also the largest table of the
663 AFS database. It contains the information needed to stream each audio
664 file. In particular the following data is stored for each audio file.
666 - SHA1 hash value of the audio file contents. This is computed
667 once when the file is added to the database. Whenever AFS
668 selects this audio file for streaming the hash value is
669 recomputed and checked against the value stored in the
670 database to detect content changes.
672 - The time when this audio file was last played.
674 - The number of times the file has been played so far.
676 - The attribute bitmask.
678 - The image id which describes the image associated with this
679 audio file.
681 - The lyrics id which describes the lyrics associated with
682 this audio file.
684 - The audio format id (MP3, OGG, ...).
686 - An amplification value that can be used by the amplification
687 filter to pre-amplify the decoded audio stream.
689 - The chunk table. It describes the location and the timing
690 of the building blocks of the audio file. This is used by
691 para_server to send chunks of the file at appropriate times.
693 - The duration of the audio file.
695 - Tag information contained in the audio file (ID3 tags,
696 Vorbis comments, ...).
698 - The number of channels
700 - The encoding bitrate.
702 - The sampling frequency.
704 To add or refresh the data contained in the audio file table, the _add_
705 command is used. It takes the full path of either an audio file or a
706 directory. In the latter case, the directory is traversed recursively
707 and all files which are recognized as valid audio files are added to
708 the database.
710 *The attribute table*
712 The attribute table contains two columns, _name_ and _bitnum_. An
713 attribute is simply a name for a certain bit number in the attribute
714 bitmask of the audio file table.
716 Each of the 64 bits of the attribute bitmask can be set for each
717 audio file individually. Hence up to 64 different attributes may be
718 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
719 "german_lyrics", "speech", whatever. You are free to choose as
720 many attributes as you like and there are no naming restrictions
721 for attributes.
723 A new attribute "test" is created by
725 para_client addatt test
726 and
727 para_client lsatt
729 lists all available attributes. You can set the "test" attribute for
730 an audio file by executing
732 para_client setatt test+ /path/to/the/audio/file
734 Similarly, the "test" bit can be removed from an audio file with
736 para_client setatt test- /path/to/the/audio/file
738 Instead of a path you may use a shell wildcard pattern. The attribute
739 is applied to all audio files matching this pattern:
741 para_client setatt test+ '/test/directory/*'
743 The command
745 para_client -- ls -lv
747 gives you a verbose listing of your audio files also showing which
748 attributes are set.
750 In case you wonder why the double-dash in the above command is needed:
751 It tells para_client to not interpret the options after the dashes. If
752 you find this annoying, just say
754 alias para='para_client --'
756 and be happy. In what follows we shall use this alias.
758 The "test" attribute can be dropped from the database with
760 para rmatt test
762 Read the output of
764 para help ls
765 para help setatt
767 for more information and a complete list of command line options to
768 these commands.
770 *Blob tables*
772 The image, lyrics, moods and playlists tables are all blob tables.
773 Blob tables consist of three columns each: The identifier which is
774 a positive non-negative number that is auto-incremented, the name
775 (an arbitrary string) and the content (the blob).
777 All blob tables support the same set of actions: cat, ls, mv, rm
778 and add. Of course, _add_ is used for adding new blobs to the table
779 while the other actions have the same meaning as the corresponding
780 Unix commands. The paraslash commands to perform these actions are
781 constructed as the concatenation of the table name and the action. For
782 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
783 manipulate or query the image table.
785 The add variant of these commands is special as these commands read
786 the blob contents from stdin. To add an image to the image table the
787 command
789 para addimg image_name < file.jpg
791 can be used.
793 Note that the images and lyrics are not interpreted at all, and also
794 the playlist and the mood blobs are only investigated when the mood
795 or playlist is activated with the select command.
797 *The score table*
799 Unlike all other tables the contents of the score table remain in
800 memory and are never stored on disk. The score table contains two
801 columns: The SHA1 hash value (of an audio file) and its current
802 score.
804 However, only those files which are admissible for the current mood
805 or playlist are contained in the score table. The audio file selector
806 always chooses the row with the highest score as the file to stream
807 next. While doing so, it computes the new score and updates the
808 last_played and the num_played fields in the audio file table.
810 The score table is recomputed by the select command which loads a
811 mood or playlist. Audio files are chosen for streaming from the rows
812 of the score table on a highest-score-first basis.
815 Playlists and moods
816 ~~~~~~~~~~~~~~~~~~~
818 Playlists and moods offer two different ways of specifying the set of
819 admissible files. A playlist in itself describes a set of admissible
820 files. A mood, in contrast, describes the set of admissible files in
821 terms of attributes and other type of information available in the
822 audio file table. As an example, a mood can define a filename pattern,
823 which is then matched against the names of audio files in the table.
825 *Playlists*
827 Playlists are accommodated in the playlist table of the afs database,
828 using the aforementioned blob format for tables. A new playlist is
829 created with the addpl command by specifying the full (absolute)
830 paths of all desired audio files, separated by newlines. Example:
832 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
834 If _my_playlist_ already exists it is overwritten. To activate the
835 new playlist, execute
837 para select p/my_playlist
839 The audio file selector will assign scores to each entry of the list,
840 in descending order so that files will be selected in order. If a
841 file could not be opened for streaming, its entry is removed from
842 the score table (but not from the playlist).
844 *Moods*
846 A mood consists of a unique name and its *mood definition*, which is
847 a set of *mood lines* containing expressions in terms of attributes
848 and other data contained in the database.
850 At any time at most one mood can be *active* which means that
851 para_server is going to select only files from that subset of
852 admissible files.
854 So in order to create a mood definition one has to write a set of
855 mood lines. Mood lines come in three flavours: Accept lines, deny
856 lines and score lines.
858 The general syntax of the three types of mood lines is
861 accept [with score <score>] [if] [not] <mood_method> [options]
862 deny [with score <score>] [if] [not] <mood_method> [options]
863 score <score> [if] [not] <mood_method> [options]
866 Here <score> is either an integer or the string "random" which assigns
867 a random score to all matching files. The score value changes the
868 order in which admissible files are going to be selected, but is of
869 minor importance for this introduction.
871 So we concentrate on the first two forms, i.e. accept and deny
872 lines. As usual, everything in square brackets is optional, i.e.
873 accept/deny lines take the following form when ignoring scores:
875 accept [if] [not] <mood_method> [options]
877 and analogously for the deny case. The "if" keyword is only syntactic
878 sugar and has no function. The "not" keyword just inverts the result,
879 so the essence of a mood line is the mood method part and the options
880 following thereafter.
882 A *mood method* is realized as a function which takes an audio file
883 and computes a number from the data contained in the database.
884 If this number is non-negative, we say the file *matches* the mood
885 method. The file matches the full mood line if it either
887 - matches the mood method and the "not" keyword is not given,
888 or
889 - does not match the mood method, but the "not" keyword is given.
891 The set of admissible files for the whole mood is now defined as those
892 files which match at least one accept mood line, but no deny mood line.
893 More formally, an audio file F is admissible if and only if
895 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
897 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
898 lines and "~" means "matches".
900 The cases where no mood lines of accept/deny type are defined need
901 special treatment:
903 - Neither accept nor deny lines: This treats all files as
904 admissible (in fact, that is the definition of the dummy mood
905 which is activated automatically if no moods are available).
907 - Only accept lines: A file is admissible iff it matches at
908 least one accept line:
910 F ~ AL1 or F ~ AL2 or ...
912 - Only deny lines: A file is admissible iff it matches no
913 deny line:
915 not (F ~ DL1 or F ~ DN2 ...)
919 *List of mood_methods*
921 no_attributes_set
923 Takes no arguments and matches an audio file if and only if no
924 attributes are set.
926 is_set <attribute_name>
928 Takes the name of an attribute and matches iff that attribute is set.
930 path_matches <pattern>
932 Takes a filename pattern and matches iff the path of the audio file
933 matches the pattern.
935 artist_matches <pattern>
936 album_matches <pattern>
937 title_matches <pattern>
938 comment_matches <pattern>
940 Takes an extended regular expression and matches iff the text of the
941 corresponding tag of the audio file matches the pattern. If the tag
942 is not set, the empty string is matched against the pattern.
944 year ~ <num>
945 bitrate ~ <num>
946 frequency ~ <num>
947 channels ~ <num>
948 num_played ~ <num>
950 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
951 <num>. Matches an audio file iff the condition <val> ~ <num> is
952 satisfied where val is the corresponding value of the audio file
953 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
954 count, play count).
956 The year tag is special as its value is undefined if the audio file
957 has no year tag or the content of the year tag is not a number. Such
958 audio files never match. Another difference is the special treatment
959 if the year tag is a two-digit number. In this case either 1900 or
960 2000 is added to the tag value, depending on whether the number is
961 greater than 2000 plus the current year.
964 *Mood usage*
966 To create a new mood called "my_mood", write its definition into
967 some temporary file, say "tmpfile", and add it to the mood table
968 by executing
970 para addmood my_mood < tmpfile
972 If the mood definition is really short, you may just pipe it to the
973 client instead of using temporary files. Like this:
975 echo "$MOOD_DEFINITION" | para addmood my_mood
977 There is no need to keep the temporary file since you can always use
978 the catmood command to get it back:
980 para catmood my_mood
982 A mood can be activated by executing
984 para select m/my_mood
986 Once active, the list of admissible files is shown by the ls command
987 if the "-a" switch is given:
989 para ls -a
992 *Example mood definition*
994 Suppose you have defined attributes "punk" and "rock" and want to define
995 a mood containing only Punk-Rock songs. That is, an audio file should be
996 admissible if and only if both attributes are set. Since
998 punk and rock
1000 is obviously the same as
1002 not (not punk or not rock)
1004 (de Morgan's rule), a mood definition that selects only Punk-Rock
1005 songs is
1007 deny if not is_set punk
1008 deny if not is_set rock
1012 File renames and content changes
1013 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1015 Since the audio file selector knows the SHA1 of each audio file that
1016 has been added to the afs database, it recognizes if the content of
1017 a file has changed, e.g. because an ID3 tag was added or modified.
1018 Also, if a file has been renamed or moved to a different location,
1019 afs will detect that an entry with the same hash value already exists
1020 in the audio file table.
1022 In both cases it is enough to just re-add the new file. In the
1023 first case (file content changed), the audio table is updated, while
1024 metadata such as the num_played and last_played fields, as well as
1025 the attributes, remain unchanged. In the other case, when the file
1026 is moved or renamed, only the path information is updated, all other
1027 data remains as before.
1029 It is possible to change the behaviour of the add command by using the
1030 "-l" (lazy add) or the "-f" (force add) option.
1032 Troubleshooting
1033 ~~~~~~~~~~~~~~~
1035 Use the debug loglevel (option -l debug for most commands) to show
1036 debugging info. Almost all paraslash executables have a brief online
1037 help which is displayed by using the -h switch. The --detailed-help
1038 option prints the full help text.
1040 If para_server crashed or was killed by SIGKILL (signal 9), it
1041 may refuse to start again because of "dirty osl tables". In this
1042 case you'll have to run the oslfsck program of libosl to fix your
1043 database. It might be necessary to use --force (even if your name
1044 isn't Luke). However, make sure para_server isn't running before
1045 executing oslfsck --force.
1047 If you don't mind to recreate your database you can start
1048 from scratch by removing the entire database directory, i.e.
1050 rm -rf ~/.paraslash/afs_database-0.4
1052 Be aware that this removes all attribute definitions, all playlists
1053 and all mood definitions and requires to re-initialize the tables.
1055 Although oslfsck fixes inconsistencies in database tables it doesn't
1056 care about the table contents. To check for invalid table contents, use
1058 para_client check
1060 This prints out references to missing audio files as well as invalid
1061 playlists and mood definitions.
1063 ---------------------------------------
1064 Audio formats and audio format handlers
1065 ---------------------------------------
1067 Audio formats
1068 ~~~~~~~~~~~~~
1070 The following audio formats are supported by paraslash:
1072 *MP3*
1074 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1075 designed as part of its MPEG-1 standard. An MP3 file is made up of
1076 multiple MP3 frames, which consist of a header and a data block. The
1077 size of an MP3 frame depends on the bit rate and on the number
1078 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1079 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1080 400 bytes large.
1082 *OGG/Vorbis*
1084 OGG is a standardized audio container format, while Vorbis is an
1085 open source codec for lossy audio compression. Since Vorbis is most
1086 commonly made available via the OGG container format, it is often
1087 referred to as OGG/Vorbis. The OGG container format divides data into
1088 chunks called OGG pages. A typical OGG page is about 4KB large. The
1089 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1090 may vary considerably.
1092 *OGG/Speex*
1094 Speex is an open-source speech codec that is based on CELP (Code
1095 Excited Linear Prediction) coding. It is designed for voice
1096 over IP applications, has modest complexity and a small memory
1097 footprint. Wideband and narrowband (telephone quality) speech are
1098 supported. As for Vorbis audio, Speex bit-streams are often stored
1099 in OGG files.
1101 *AAC*
1103 Advanced Audio Coding (AAC) is a standardized, lossy compression
1104 and encoding scheme for digital audio which is the default audio
1105 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1106 the container format and audio files encoded with AAC have the .m4a
1107 extension. A typical AAC frame is about 700 bytes large.
1109 *WMA*
1111 Windows Media Audio (WMA) is an audio data compression technology
1112 developed by Microsoft. A WMA file is usually encapsulated in the
1113 Advanced Systems Format (ASF) container format, which also specifies
1114 how meta data about the file is to be encoded. The bit stream of WMA
1115 is composed of superframes, each containing one or more frames of
1116 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1118 *FLAC*
1120 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1121 loss. It gives better compression ratios than a general purpose
1122 compressor like zip or bzip2 because FLAC is designed specifically
1123 for audio. A FLAC-encoded file consits of frames of varying size, up
1124 to 16K. Each frame starts with a header that contains all information
1125 necessary to decode the frame.
1127 Meta data
1128 ~~~~~~~~~
1130 Unfortunately, each audio format has its own conventions how meta
1131 data is added as tags to the audio file.
1133 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1134 is rather simple but also very limited as it supports only artist,
1135 title, album, year and comment tags. Each of these can only be at most
1136 32 characters long. ID3, version 2 is much more flexible but requires
1137 a separate library being installed for paraslash to support it.
1139 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1140 comments, which are typically implemented as strings of the form
1141 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1142 tags are appropriate for the content.
1144 AAC files usually use the MPEG-4 container format for storing meta
1145 data while WMA files wrap meta data as special objects within the
1146 ASF container format.
1148 paraslash only tracks the most common tags that are supported by
1149 all tag variants: artist, title, year, album, comment. When a file
1150 is added to the AFS database, the meta data of the file is extracted
1151 and stored in the audio file table.
1153 Chunks and chunk tables
1154 ~~~~~~~~~~~~~~~~~~~~~~~
1156 paraslash uses the word "chunk" as common term for the building blocks
1157 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1158 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1159 size varies considerably between audio formats, from a few hundred
1160 bytes (MP3) up to 16K (FLAC).
1162 The chunk table contains the offsets within the audio file that
1163 correspond to the chunk boundaries of the file. Like the meta data,
1164 the chunk table is computed and stored in the database whenever an
1165 audio file is added.
1167 The paraslash senders (see below) always send complete chunks. The
1168 granularity for seeking is therefore determined by the chunk size.
1170 Audio format handlers
1171 ~~~~~~~~~~~~~~~~~~~~~
1173 For each audio format paraslash contains an audio format handler whose
1174 first task is to tell whether a given file is a valid audio file of
1175 this type. If so, the audio file handler extracts some technical data
1176 (duration, sampling rate, number of channels etc.), computes the
1177 chunk table and reads the meta data.
1179 The audio format handler code is linked into para_server and executed
1180 via the _add_ command. The same code is also available as a stand-alone
1181 tool, para_afh, which can be used to print the technical data, the
1182 chunk table and the meta data of a file. Furthermore, one can use
1183 para_afh to cut an audio file, i.e. to select some of its chunks to
1184 produce a new file containing only these chunks.
1186 ----------
1187 Networking
1188 ----------
1190 Paraslash uses different network connections for control and data.
1191 para_client communicates with para_server over a dedicated TCP control
1192 connection. To transport audio data, separate data connections are
1193 used. For these data connections, a variety of transports (UDP, DCCP,
1194 HTTP) can be chosen.
1196 The chapter starts with the REFERENCE(The paraslash control
1197 service, control service), followed by a section on the various
1198 REFERENCE(Streaming protocols, streaming protocols) in which the data
1199 connections are described. The way audio file headers are embedded into
1200 the stream is discussed REFERENCE(Streams with headers and headerless
1201 streams, briefly) before the REFERENCE(Networking examples, example
1202 section) which illustrates typical commands for real-life scenarios.
1204 Both IPv4 and IPv6 are supported.
1206 The paraslash control service
1207 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1209 para_server is controlled at runtime via the paraslash control
1210 connection. This connection is used for server commands (play, stop,
1211 ...) as well as for afs commands (ls, select, ...).
1213 The server listens on a TCP port and accepts connections from clients
1214 that connect the open port. Each connection causes the server to fork
1215 off a client process which inherits the connection and deals with that
1216 client only. In this classical accept/fork approach the server process
1217 is unaffected if the child dies or goes crazy for whatever reason. In
1218 fact, the child process can not change address space of server process.
1220 The section on REFERENCE(Client-server authentication, client-server
1221 authentication) above described the early connection establishment
1222 from the crypto point of view. Here it is described what happens
1223 after the connection (including crypto setup) has been established.
1224 There are four processes involved during command dispatch as sketched
1225 in the following diagram.
1227 <<
1228 <pre>
1229 server_host client_host
1230 ~~~~~~~~~~~ ~~~~~~~~~~~
1232 +-----------+ connect +-----------+
1233 |para_server|<------------------------------ |para_client|
1234 +-----------+ +-----------+
1235 | ^
1236 | fork +---+ |
1237 +----------> |AFS| |
1238 | +---+ |
1239 | ^ |
1240 | | |
1241 | | connect (cookie) |
1242 | | |
1243 | | |
1244 | fork +-----+ inherited connection |
1245 +---------->|child|<--------------------------+
1246 +-----+
1247 </pre>
1248 >>
1250 Note that the child process is not a child of the afs process,
1251 so communication of these two processes has to happen via local
1252 sockets. In order to avoid abuse of the local socket by unrelated
1253 processes, a magic cookie is created once at server startup time just
1254 before the server process forks off the AFS process. This cookie is
1255 known to the server, AFS and the child, but not to unrelated processes.
1257 There are two different kinds of commands: First there are commands
1258 that cause the server to respond with some answer such as the list
1259 of all audio files. All but the addblob commands (addimg, addlyr,
1260 addpl, addmood) are of this kind. The addblob commands add contents
1261 to the database, so they need to transfer data the other way round,
1262 from the client to the server.
1264 There is no knowledge about the server commands built into para_client,
1265 so it does not know about addblob commands. Instead, it inspects the
1266 first data package sent by the server for a magic string. If this
1267 string was found, it sends STDIN to the server, otherwise it dumps
1268 data from the server to STDOUT.
1270 Streaming protocols
1271 ~~~~~~~~~~~~~~~~~~~
1273 A network (audio) stream usually consists of one streaming source,
1274 the _sender_, and one or more _receivers_ which read data over the
1275 network from the streaming source.
1277 Senders are thus part of para_server while receivers are part of
1278 para_audiod. Moreover, there is the stand-alone tool para_recv which
1279 can be used to manually download a stream, either from para_server
1280 or from a web-based audio streaming service.
1282 The following three streaming protocols are supported by paraslash:
1284 - HTTP. Recommended for public streams that can be played by
1285 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1286 sender is supported on all operating systems and all platforms.
1288 - DCCP. Recommended for LAN streaming. DCCP is currently
1289 available only for Linux.
1291 - UDP. Recommended for multicast LAN streaming.
1293 See the Appendix on REFERENCE(Network protocols, network protocols)
1294 for brief descriptions of the various protocols relevant for network
1295 audio streaming with paraslash.
1297 It is possible to activate more than one sender simultaneously.
1298 Senders can be controlled at run time and via config file and command
1299 line options.
1301 Note that audio connections are _not_ encrypted. Transport or Internet
1302 layer encryption should be used if encrypted data connections are
1303 needed.
1305 Since DCCP and TCP are both connection-oriented protocols, connection
1306 establishment/teardown and access control are very similar between
1307 these two streaming protocols. UDP is the most lightweight option,
1308 since in contrast to TCP/DCCP it is connectionless. It is also the
1309 only protocol supporting IP multicast.
1311 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1312 clients to connect and establish a connection via some protocol-defined
1313 handshake mechanism. Both senders maintain two linked lists each:
1314 The list of all clients which are currently connected, and the list
1315 of access control entries which determines who is allowed to connect.
1316 IP-based access control may be configured through config file and
1317 command line options and via the "allow" and "deny" sender subcommands.
1319 Upon receiving a GET request from the client, the HTTP sender sends
1320 back a status line and a message. The body of this message is the
1321 audio stream. This is common practice and is supported by many popular
1322 clients which can thus be used to play a stream offered by para_server.
1323 For DCCP things are a bit simpler: No messages are exchanged between
1324 the receiver and sender. The client simply connects and the sender
1325 starts to stream.
1327 DCCP is an experimental protocol which offers a number of new features
1328 not available for TCP. Both ends can negotiate these features using
1329 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1330 datagram-based (no retransmissions) and thus should not be used over
1331 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1332 is access to a variety of different congestion-control mechanisms
1333 called CCIDs. Two different CCIDs are available per default on Linux:
1336 - _CCID 2_. A Congestion Control mechanism similar to that
1337 of TCP. The sender maintains a congestion window and halves
1338 this window in response to congestion.
1341 - _CCID-3_. Designed to be fair when competing for bandwidth.
1342 It has lower variation of throughput over time compared with
1343 TCP, which makes it suitable for streaming media.
1345 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1346 single list, the _target list_. This list describes the set of clients
1347 to which the stream is sent. There is no list for access control and
1348 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1349 and "delete" commands can be used to modify the target list.
1351 Since both UDP and DCCP offer an unreliable datagram-based transport,
1352 additional measures are necessary to guard against disruptions over
1353 networks that are lossy or which may be subject to interference (as
1354 is for instance the case with WiFi). Paraslash uses FEC (Forward
1355 Error Correction) to guard against packet losses and reordering. The
1356 stream is FEC-encoded before it is sent through the UDP socket and
1357 must be decoded accordingly on the receiver side.
1359 The packet size and the amount of redundancy introduced by FEC can
1360 be configured via the FEC parameters which are dictated by server
1361 and may also be configured through the "sender" command. The FEC
1362 parameters are encoded in the header of each network packet, so no
1363 configuration is necessary on the receiver side. See the section on
1364 REFERENCE(Forward error correction, FEC) below.
1366 Streams with headers and headerless streams
1367 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1369 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1370 needed to decode the stream is only contained in the audio file
1371 header of the container format but not in each data chunk. Clients
1372 must be able to obtain this information in case streaming starts in
1373 the middle of the file or if para_audiod is started while para_server
1374 is already sending a stream.
1376 This is accomplished in different ways, depending on the streaming
1377 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1378 header is sent prior to audio file data. This technique however does
1379 not work for the connectionless UDP transport. Hence the audio file
1380 header is periodically being embedded into the UDP audio data stream.
1381 By default, the header is resent after five seconds. The receiver has
1382 to wait until the next header arrives before it can start decoding
1383 the stream.
1385 Examples
1386 ~~~~~~~~
1388 The sender command of para_server allows to (de-)activate senders
1389 and to change the access permissions senders at runtime. The "si"
1390 (server info) command is used to list the streaming options of the
1391 currently running server as well as the various sender access lists.
1393 -> Show client/target/access lists:
1395 para_client si
1397 -> Obtain general help for the sender command:
1399 para_client help sender
1401 -> Get help for a specific sender (contains further examples):
1403 s=http # or dccp or udp
1404 para_client sender $s help
1406 By default para_server activates both the HTTP and th DCCP sender on
1407 startup. This can be changed via command line options or para_server's
1408 config file.
1410 -> List config file options for senders:
1412 para_server -h
1414 All senders share the "on" and "off" commands, so senders may be
1415 activated and deactivated independently of each other.
1417 -> Switch off the http sender:
1419 para_client sender http off
1421 -> Receive a DCCP stream using CCID2 and write the output into a file:
1423; ccid=2; filename=bar
1424 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1426 Note the quotes around the arguments for the dccp receiver. Each
1427 receiver has its own set of command line options and its own command
1428 line parser, so arguments for the dccp receiver must be protected
1429 from being interpreted by para_recv.
1431 -> Start UDP multicast, using the default multicast address:
1433 para_client sender udp add
1435 -> Receive FEC-encoded multicast stream and write the output into a file:
1437 filename=foo
1438 para_recv -r udp > $filename
1440 -> Add an UDP unicast for a client to the target list of the UDP sender:
1443 para_client sender udp add $t
1445 -> Receive this (FEC-encoded) unicast stream:
1447 filename=foo
1448 para_recv -r 'udp -i' > $filename
1450 -> Create a minimal config for para_audiod for HTTP streams:
1452 c=$HOME/.paraslash/audiod.conf.min;
1453 echo receiver \".:http -i $s\" > $c
1454 para_audiod --config $c
1456 -------
1457 Filters
1458 -------
1460 A paraslash filter is a module which transforms an input stream into
1461 an output stream. Filters are included in the para_audiod executable
1462 and in the stand-alone tool para_filter which usually contains the
1463 same modules.
1465 While para_filter reads its input stream from STDIN and writes
1466 the output to STDOUT, the filter modules of para_audiod are always
1467 connected to a receiver which produces the input stream and a writer
1468 which absorbs the output stream.
1470 Some filters depend on a specific library being installed and are
1471 not compiled in if this library was not found at compile time. To
1472 see the list of supported filters, run para_filter and para_audiod
1473 with the --help option. The output looks similar to the following:
1475 Available filters:
1476 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1478 Out of these filter modules, a chain of filters can be constructed,
1479 much in the way Unix pipes can be chained, and analogous to the use
1480 of modules in gstreamer: The output of the first filter becomes the
1481 input of the second filter. There is no limitation on the number of
1482 filters and the same filter may occur more than once.
1484 Like receivers, each filter has its own command line options which
1485 must be quoted to protect them from the command line options of
1486 the driving application (para_audiod or para_filter). Example:
1488 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1490 For para_audiod, each audio format has its own set of filters. The
1491 name of the audio format for which the filter should be applied can
1492 be used as the prefix for the filter option. Example:
1494 para_audiod -f 'mp3:prebuffer --duration 300'
1496 The "mp3" prefix above is actually interpreted as a POSIX extended
1497 regular expression. Therefore
1499 para_audiod -f '.:prebuffer --duration 300'
1501 activates the prebuffer filter for all supported audio formats (because
1502 "." matches all audio formats) while
1504 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1506 activates it only for wma and ogg streams.
1508 Decoders
1509 ~~~~~~~~
1511 For each supported audio format there is a corresponding filter
1512 which decodes audio data in this format to 16 bit PCM data which
1513 can be directly sent to the sound device or any other software that
1514 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1515 filters are called _decoders_ in general, and xxxdec is the name of
1516 the paraslash decoder for the audio format xxx. For example, the mp3
1517 decoder filter is called mp3dec.
1519 Note that the output of the decoder is about 10 times larger than
1520 its input. This means that filters that operate on the decoded audio
1521 stream have to deal with much more data than filters that transform
1522 the audio stream before it is fed to the decoder.
1524 Paraslash relies on external libraries for most decoders, so these
1525 libraries must be installed for the decoder to be included in the
1526 para_filter and para_audiod executables. The oggdec filter depends
1527 on the libogg and libvorbis libraries for example.
1529 Forward error correction
1530 ~~~~~~~~~~~~~~~~~~~~~~~~
1532 As already mentioned REFERENCE(Streaming protocols, earlier),
1533 paraslash uses forward error correction (FEC) for the unreliable UDP
1534 and DCCP transports. FEC is a technique which was invented already
1535 in 1960 by Reed and Solomon and which is widely used for the parity
1536 calculations of storage devices (RAID arrays). It is based on the
1537 algebraic concept of finite fields, today called Galois fields, in
1538 honour of the mathematician Galois (1811-1832). The FEC implementation
1539 of paraslash is based on code by Luigi Rizzo.
1541 Although the details require a sound knowledge of the underlying
1542 mathematics, the basic idea is not hard to understand: For positive
1543 integers k and n with k < n it is possible to compute for any k given
1544 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1545 ..., p_r such that all data bytes can be reconstructed from *any*
1546 k bytes of the set
1548 {d_1, ..., d_k, p_1, ..., p_r}.
1550 FEC-encoding for unreliable network transports boils down to slicing
1551 the audio stream into groups of k suitably sized pieces called _slices_
1552 and computing the r corresponding parity slices. This step is performed
1553 in para_server which then sends both the data and the parity slices
1554 over the unreliable network connection. If the client was able
1555 to receive at least k of the n = k + r slices, it can reconstruct
1556 (FEC-decode) the original audio stream.
1558 From these observations it is clear that there are three different
1559 FEC parameters: The slice size, the number of data slices k, and the
1560 total number of slices n. It is crucial to choose the slice size
1561 such that no fragmentation of network packets takes place because
1562 FEC only guards against losses and reordering but fails if slices are
1563 received partially.
1565 FEC decoding in paralash is performed through the fecdec filter which
1566 usually is the first filter (there can be other filters before fecdec
1567 if these do not alter the audio stream).
1570 Volume adjustment (amp and compress)
1571 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1573 The amp and the compress filter both adjust the volume of the audio
1574 stream. These filters operate on uncompressed audio samples. Hence
1575 they are usually placed directly after the decoding filter. Each
1576 sample is multiplied with a scaling factor (>= 1) which makes amp
1577 and compress quite expensive in terms of computing power.
1579 *amp*
1581 The amp filter amplifies the audio stream by a fixed scaling factor
1582 that must be known in advance. For para_audiod this factor is derived
1583 from the amplification field of the audio file's entry in the audio
1584 file table while para_filter uses the value given at the command line.
1586 The optimal scaling factor F for an audio file is the largest real
1587 number F >= 1 such that after multiplication with F all samples still
1588 fit into the sample interval [-32768, 32767]. One can use para_filter
1589 in combination with the sox utility to compute F:
1591 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1593 The amplification value V which is stored in the audio file table,
1594 however, is an integer between 0 and 255 which is connected to F
1595 through the formula
1597 V = (F - 1) * 64.
1599 To store V in the audio file table, the command
1601 para_client -- touch -a=V file.mp3
1603 is used. The reader is encouraged to write a script that performs
1604 these computations :)
1606 *compress*
1608 Unlike the amplification filter, the compress filter adjusts the volume
1609 of the audio stream dynamically without prior knowledge about the peak
1610 value. It maintains the maximal volume of the last n samples of the
1611 audio stream and computes a suitable amplification factor based on that
1612 value and the various configuration options. It tries to chose this
1613 factor such that the adjusted volume meets the desired target level.
1615 Note that it makes sense to combine amp and compress.
1617 Misc filters (wav and prebuffer)
1618 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1620 These filters are rather simple and do not modify the audio stream at
1621 all. The wav filter is only useful with para_filter and in connection
1622 with a decoder. It asks the decoder for the number of channels and the
1623 sample rate of the stream and adds a Microsoft wave header containing
1624 this information at the beginning. This allows to write wav files
1625 rather than raw PCM files (which do not contain any information about
1626 the number of channels and the sample rate).
1628 The prebuffer filter simply delays the output until the given time has
1629 passed (starting from the time the first byte was available in its
1630 input queue) or until the given amount of data has accumulated. It
1631 is mainly useful for para_audiod if the standard parameters result
1632 in buffer underruns.
1634 Both filters require almost no additional computing time, even when
1635 operating on uncompressed audio streams, since data buffers are simply
1636 "pushed down" rather than copied.
1638 Examples
1639 ~~~~~~~~
1641 -> Decode an mp3 file to wav format:
1643 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1645 -> Amplify a raw audio file by a factor of 1.5:
1647 para_filter -f amp --amp 32 < foo.raw > bar.raw
1649 ------
1650 Output
1651 ------
1653 Once an audio stream has been received and decoded to PCM format,
1654 it can be sent to a sound device for playback. This part is performed
1655 by paraslash _writers_ which are described in this chapter.
1657 Writers
1658 ~~~~~~~
1660 A paraslash writer acts as a data sink that consumes but does not
1661 produce audio data. Paraslash writers operate on the client side and
1662 are contained in para_audiod and in the stand-alone tool para_write.
1664 The para_write program reads uncompressed audio data from STDIN. If
1665 this data starts with a wav header, sample rate, sample format and
1666 channel count are read from the header. Otherwise CD audio (44.1KHz
1667 16 bit little endian, stereo) is assumed but this can be overridden
1668 by command line options. para_audiod, on the other hand, obtains
1669 the sample rate and the number of channels from the decoder.
1671 Like receivers and filters, each writer has an individual set of
1672 command line options, and for para_audiod writers can be configured
1673 per audio format separately. It is possible to activate more than
1674 one writer for the same stream simultaneously.
1676 OS-dependent APIs
1677 ~~~~~~~~~~~~~~~~~
1679 Unfortunately, the various flavours of Unix on which paraslash
1680 runs on have different APIs for opening a sound device and starting
1681 playback. Hence for each such API there is a paraslash writer that
1682 can play the audio stream via this API.
1684 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1685 Linux systems. Although there are several mid-layer APIs in use by
1686 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1687 currently supports only the low-level ALSA API which is not supposed
1688 to be change. ALSA is very feature-rich, in particular it supports
1689 software mixing via its DMIX plugin. ALSA is the default writer on
1690 Linux systems.
1692 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1693 is also available on Linux systems, usually provided by ALSA as an
1694 emulation for backwards compatibility. This API is rather simple but
1695 also limited. For example only one application can open the device
1696 at any time. The OSS writer is activated by default on BSD Systems.
1698 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1699 for this API is only compiled in on such systems and is of course
1700 the default there.
1702 *FILE*. The file writer allows to capture the audio stream and
1703 write the PCM data to a file on the file system rather than playing
1704 it through a sound device. It is supported on all platforms and is
1705 always compiled in.
1707 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1708 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1709 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1710 through an output plugin of libao.
1712 Examples
1713 ~~~~~~~~
1715 -> Use the OSS writer to play a wav file:
1717 para_write --writer oss < file.wav
1719 -> Enable ALSA software mixing for mp3 streams
1721 para_audiod --writer 'mp3:alsa -d plug:swmix'
1724 ---
1725 Gui
1726 ---
1728 para_gui executes an arbitrary command which is supposed to print
1729 status information to STDOUT. It then displays this information in
1730 a curses window. By default the command
1732 para_audioc -- stat -p
1734 is executed, but this can be customized via the --stat_cmd option. In
1735 particular it possible to use
1737 para_client -- stat -p
1739 to make para_gui work on systems on which para_audiod is not running.
1741 Key bindings
1742 ~~~~~~~~~~~~
1744 It is possible to bind keys to arbitrary commands via custom
1745 key-bindings. Besides the internal keys which can not be changed (help,
1746 quit, loglevel, version...), the following flavours of key-bindings
1747 are supported:
1749 - external: Shutdown curses before launching the given command.
1750 Useful for starting other ncurses programs from within
1751 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1752 output format to write a mailbox containing one mail for each
1753 (admissible) file the audio file selector knows about. Then
1754 start mutt from within para_gui to browse your collection!
1756 - display: Launch the command and display its stdout in
1757 para_gui's bottom window.
1759 - para: Like display, but start "para_client <specified
1760 command>" instead of "<specified command>".
1762 The general form of a key binding is
1764 key_map k:m:c
1766 which maps key k to command c using mode m. Mode may be x, d or p
1767 for external, display and paraslash commands, respectively.
1769 Themes
1770 ~~~~~~
1772 Currently there are only two themes for para_gui. It is easy, however,
1773 to add more themes. To create a new theme one has to define the
1774 position, color and geometry for for each status item that should be
1775 shown by this theme. See gui_theme.c for examples.
1777 The "." and "," keys are used to switch between themes.
1779 Examples
1780 ~~~~~~~~
1782 -> Show server info:
1784 key_map "i:p:si"
1786 -> Jump to the middle of the current audio file by pressing F5:
1788 key_map "<F5>:p:jmp 50"
1790 -> vi-like bindings for jumping around:
1792 key_map "l:p:ff 10"
1793 key_map "h:p:ff 10-"
1794 key_map "w:p:ff 60"
1795 key_map "b:p:ff 60-"
1797 -> Print the current date and time:
1799 key_map "D:d:date"
1801 -> Call other curses programs:
1803 key_map "U:x:aumix"
1804 key_map "!:x:/bin/bash"
1805 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1807 -----------
1808 Development
1809 -----------
1811 Tools
1812 ~~~~~
1814 In order to compile the sources from the git repository (rather than
1815 from tar balls) and for contributing non-trivial changes to the
1816 paraslash project, some additional tools should be installed on a
1817 developer machine.
1819 (git). As described in more detail REFERENCE(Git
1820 branches, below), the git source code management tool is used for
1821 paraslash development. It is necessary for cloning the git repository
1822 and for getting updates.
1824 (m4). Some input files for gengetopt
1825 are generated from templates by the m4 macro processor.
1827 (autoconf) GNU autoconf creates
1828 the configure file which is shipped in the tarballs but has to be
1829 generated when compiling from git.
1831 (grutatxt). The
1832 HTML version of this manual and some of the paraslash web pages are
1833 generated by the grutatxt plain text to HTML converter. If changes
1834 are made to these text files the grutatxt package must be installed
1835 to regenerate the HTML files.
1837 (doxygen). The documentation
1838 of paraslash's C sources uses the doxygen documentation system. The
1839 conventions for documenting the source code is described in the
1840 REFERENCE(Doxygen, Doxygen section).
1842 (global). This is used to generate
1843 browsable HTML from the C sources. It is needed by doxygen.
1845 Git branches
1846 ~~~~~~~~~~~~
1848 Paraslash has been developed using the git source code management
1849 tool since 2006. Development is organized roughly in the same spirit
1850 as the git development itself, as described below.
1852 The following text passage is based on "A note from the maintainer",
1853 written by Junio C Hamano, the maintainer of git.
1855 There are four branches in the paraslash repository that track the
1856 source tree: "master", "maint", "next", and "pu".
1858 The "master" branch is meant to contain what is well tested and
1859 ready to be used in a production setting. There could occasionally be
1860 minor breakages or brown paper bag bugs but they are not expected to
1861 be anything major, and more importantly quickly and easily fixable.
1862 Every now and then, a "feature release" is cut from the tip of this
1863 branch, named with three dotted decimal digits, like 0.4.2.
1865 Whenever changes are about to be included that will eventually lead to
1866 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1867 "master" at that point. Obvious, safe and urgent fixes after the major
1868 release are applied to this branch and maintenance releases are cut
1869 from it. New features never go to this branch. This branch is also
1870 merged into "master" to propagate the fixes forward.
1872 A trivial and safe enhancement goes directly on top of "master".
1873 New development does not usually happen on "master", however.
1874 Instead, a separate topic branch is forked from the tip of "master",
1875 and it first is tested in isolation; Usually there are a handful such
1876 topic branches that are running ahead of "master". The tip of these
1877 branches is not published in the public repository to keep the number
1878 of branches that downstream developers need to worry about low.
1880 The quality of topic branches varies widely. Some of them start out as
1881 "good idea but obviously is broken in some areas" and then with some
1882 more work become "more or less done and can now be tested by wider
1883 audience". Luckily, most of them start out in the latter, better shape.
1885 The "next" branch is to merge and test topic branches in the latter
1886 category. In general, this branch always contains the tip of "master".
1887 It might not be quite rock-solid production ready, but is expected to
1888 work more or less without major breakage. The maintainer usually uses
1889 the "next" version of paraslash for his own pleasure, so it cannot
1890 be _that_ broken. The "next" branch is where new and exciting things
1891 take place.
1893 The two branches "master" and "maint" are never rewound, and "next"
1894 usually will not be either (this automatically means the topics that
1895 have been merged into "next" are usually not rebased, and you can find
1896 the tip of topic branches you are interested in from the output of
1897 "git log next"). You should be able to safely build on top of them.
1899 However, at times "next" will be rebuilt from the tip of "master" to
1900 get rid of merge commits that will never be in "master". The commit
1901 that replaces "next" will usually have the identical tree, but it
1902 will have different ancestry from the tip of "master".
1904 The "pu" (proposed updates) branch bundles the remainder of the
1905 topic branches. The "pu" branch, and topic branches that are only in
1906 "pu", are subject to rebasing in general. By the above definition
1907 of how "next" works, you can tell that this branch will contain quite
1908 experimental and obviously broken stuff.
1910 When a topic that was in "pu" proves to be in testable shape, it
1911 graduates to "next". This is done with
1913 git checkout next
1914 git merge that-topic-branch
1916 Sometimes, an idea that looked promising turns out to be not so good
1917 and the topic can be dropped from "pu" in such a case.
1919 A topic that is in "next" is expected to be polished to perfection
1920 before it is merged to "master". Similar to the above, this is
1921 done with
1923 git checkout master
1924 git merge that-topic-branch
1925 git branch -d that-topic-branch
1927 Note that being in "next" is not a guarantee to appear in the next
1928 release (being in "master" is such a guarantee, unless it is later
1929 found seriously broken and reverted), nor even in any future release.
1931 Coding Style
1932 ~~~~~~~~~~~~
1934 The preferred coding style for paraslash coincides more or less
1935 with the style of the Linux kernel. So rather than repeating what is
1936 written XREFERENCE(,
1937 there), here are the most important points.
1939 - Burn the GNU coding standards.
1940 - Never use spaces for indentation.
1941 - Tabs are 8 characters, and thus indentations are also 8 characters.
1942 - Don't put multiple assignments on a single line.
1943 - Avoid tricky expressions.
1944 - Don't leave whitespace at the end of lines.
1945 - The limit on the length of lines is 80 columns.
1946 - Use K&R style for placing braces and spaces:
1948 if (x is true) {
1949 we do y
1950 }
1952 - Use a space after (most) keywords.
1953 - Do not add spaces around (inside) parenthesized expressions.
1954 - Use one space around (on each side of) most binary and ternary operators.
1955 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1956 - Mixed-case names are frowned upon.
1957 - Descriptive names for global variables are a must.
1958 - Avoid typedefs.
1959 - Functions should be short and sweet, and do just one thing.
1960 - The number of local variables shouldn't exceed 10.
1961 - Gotos are fine if they improve readability and reduce nesting.
1962 - Don't use C99-style "// ..." comments.
1963 - Names of macros defining constants and labels in enums are capitalized.
1964 - Enums are preferred when defining several related constants.
1965 - Always use the paraslash wrappers for allocating memory.
1966 - If the name of a function is an action or an imperative.
1967 command, the function should return an error-code integer
1968 (<0 means error, >=0 means success). If the name is a
1969 predicate, the function should return a "succeeded" boolean.
1972 Doxygen
1973 ~~~~~~~
1975 Doxygen is a documentation system for various programming
1976 languages. The paraslash project uses Doxygen for generating the API
1977 reference on the web pages, but good source code documentation is
1978 also beneficial to people trying to understand the code structure
1979 and the interactions between the various source files.
1981 It is more illustrative to look at the source code for examples than
1982 to describe the conventions for documenting the source in this manual,
1983 so we only describe which parts of the code need doxygen comments,
1984 but leave out details on documentation conventions.
1986 As a rule, only the public part of the C source is documented with
1987 Doxygen. This includes structures, defines and enumerations in header
1988 files as well as public (non-static) C functions. These should be
1989 documented completely. For example each parameter and the return
1990 value of a public function should get a descriptive comment.
1992 No doxygen comments are necessary for static functions and for
1993 structures and enumerations in C files (which are used only within
1994 this file). This does not mean, however, that those entities need
1995 no documentation at all. Instead, common sense should be applied to
1996 document what is not obvious from reading the code.
1998 --------
1999 Appendix
2000 --------
2002 Network protocols
2003 ~~~~~~~~~~~~~~~~~
2005 *IP*. The _Internet Protocol_ is the primary networking protocol
2006 used for the Internet. All protocols described below use IP as the
2007 underlying layer. Both the prevalent IPv4 and the next-generation
2008 IPv6 variant are being deployed actively worldwide.
2010 *Connection-oriented and connectionless protocols*. Connectionless
2011 protocols differ from connection-oriented ones in that state
2012 associated with the sending/receiving endpoints is treated
2013 implicitly. Connectionless protocols maintain no internal knowledge
2014 about the state of the connection. Hence they are not capable of
2015 reacting to state changes, such as sudden loss or congestion on the
2016 connection medium. Connection-oriented protocols, in contrast, make
2017 this knowledge explicit. The connection is established only after
2018 a bidirectional handshake which requires both endpoints to agree
2019 on the state of the connection, and may also involve negotiating
2020 specific parameters for the particular connection. Maintaining an
2021 up-to-date internal state of the connection also in general means
2022 that the sending endpoints perform congestion control, adapting to
2023 qualitative changes of the connection medium.
2025 *Reliability*. In IP networking, packets can be lost, duplicated,
2026 or delivered out of order, and different network protocols handle
2027 these problems in different ways. We call a transport-layer protocol
2028 _reliable_, if it turns the unreliable IP delivery into an ordered,
2029 duplicate- and loss-free delivery of packets. Sequence numbers
2030 are used to discard duplicates and re-arrange packets delivered
2031 out-of-order. Retransmission is used to guarantee loss-free
2032 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2033 or data integrity.
2035 *Classification*. With these definitions the protocols which are used
2036 by paraslash for steaming audio data may be classified as follows.
2038 - HTTP/TCP: connection-oriented, reliable,
2039 - UDP: connectionless, unreliable,
2040 - DCCP: connection-oriented, unreliable.
2042 Below we give a short descriptions of these protocols.
2044 *TCP*. The _Transmission Control Protocol_ provides reliable,
2045 ordered delivery of a stream and a classic window-based congestion
2046 control. In contrast to UDP and DCCP (see below), TCP does not have
2047 record-oriented or datagram-based syntax, i.e. it provides a stream
2048 which is unaware and independent of any record (packet) boundaries.
2049 TCP is used extensively by many application layers. Besides HTTP (the
2050 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2051 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2052 top of TCP.
2054 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2055 protocol, built as a thin layer directly on top of IP. For this reason,
2056 it offers the same best-effort service as IP itself, i.e. there is no
2057 detection of duplicate or reordered packets. Being a connectionless
2058 protocol, only minimal internal state about the connection is
2059 maintained, which means that there is no protection against packet
2060 loss or network congestion. Error checking and correction (if at all)
2061 are performed in the application.
2063 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2064 connection-oriented state maintenance known from TCP with the
2065 unreliable, datagram-based transport of UDP. This means that it
2066 is capable of reacting to changes in the connection by performing
2067 congestion control, offering multiple alternative approaches. But it
2068 is bound to datagram boundaries (the maximum packet size supported
2069 by a medium), and like UDP it lacks retransmission to protect
2070 against loss. Due to the use of sequence numbers, it is however
2071 able to react to loss (interpreted as a congestion indication) and
2072 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2073 to negotiate specific, binding features for a connection, such as
2074 the choice of congestion control: classic, window-based congestion
2075 control known from TCP is available as CCID-2, rate-based, "smooth"
2076 congestion control is offered as CCID-3.
2078 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2079 protocol on top of TCP. It is spoken by web servers and is most often
2080 used for web services. However, as can be seen by the many Internet
2081 radio stations and YouTube/Flash videos, http is by far not limited to
2082 the delivery of web pages only. Being a simple request/response based
2083 protocol, the semantics of the protocol also allow the delivery of
2084 multimedia content, such as audio over http.
2086 *Multicast*. IP multicast is not really a protocol but a technique
2087 for one-to-many communication over an IP network. The challenge is to
2088 deliver information to a group of destinations simultaneously using
2089 the most efficient strategy to send the messages over each link of
2090 the network only once. This has benefits for streaming multimedia:
2091 the standard one-to-one unicast offered by TCP/DCCP means that
2092 n clients listening to the same stream also consume n-times the
2093 resources, whereas multicast requires to send the stream just once,
2094 irrespective of the number of receivers. Since it would be costly to
2095 maintain state for each listening receiver, multicast often implies
2096 connectionless transport, which is the reason that it is currently
2097 only available via UDP.
2099 License
2100 ~~~~~~~
2102 Paraslash is licensed under the GPL, version 2. Most of the code
2103 base has been written from scratch, and those parts are GPL V2
2104 throughout. Notable exceptions are FEC and the WMA decoder. See the
2105 corresponding source files for licencing details for these parts. Some
2106 code sniplets of several other third party software packages have
2107 been incorporated into the paraslash sources, for example log message
2108 coloring was taken from the git sources. These third party software
2109 packages are all published under the GPL or some other license
2110 compatible to the GPL.
2112 Acknowledgements
2113 ~~~~~~~~~~~~~~~~
2115 Many thanks to Gerrit Renker who read an early draft of this manual
2116 and contributed significant improvements.
2118 ----------
2119 References
2120 ----------
2122 Articles
2123 ~~~~~~~~
2124 - Reed, Irving S.; Solomon, Gustave (1960),
2126 Polynomial Codes over Certain Finite Fields), Journal of the
2127 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2128 300-304, doi:10.1137/0108018)
2130 RFCs
2131 ~~~~
2133 - XREFERENCE(, RFC 768) (1980):
2134 User Datagram Protocol
2135 - XREFERENCE(, RFC 791) (1981):
2136 Internet Protocol
2137 - XREFERENCE(, RFC 2437) (1998):
2138 RSA Cryptography Specifications
2139 - XREFERENCE(, RFC 4340)
2140 (2006): Datagram Congestion Control Protocol (DCCP)
2141 - XREFERENCE(, RFC 4341) (2006):
2142 Congestion Control ID 2: TCP-like Congestion Control
2143 - XREFERENCE(, RFC 4342) (2006):
2144 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2146 Application web pages
2147 ~~~~~~~~~~~~~~~~~~~~~
2149 - XREFERENCE(, paraslash)
2150 - XREFERENCE(, xmms)
2151 - XREFERENCE(, mpg123)
2152 - XREFERENCE(, gstreamer)
2153 - XREFERENCE(, icecast)
2154 - XREFERENCE(, Audio Compress)
2156 External documentation
2157 ~~~~~~~~~~~~~~~~~~~~~~
2160 H. Peter Anvin: The mathematics of Raid6)
2162 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2163 Communication Protocols)
2165 Code
2166 ~~~~
2168 Original FEC implementation by Luigi Rizzo)