Implement the flac decoding filter.
[paraslash.git] / web / manual.m4
1 dnl To generate the html version, execute
2 dnl m4 web/manual.m4 | grutatxt --toc
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10 define(`XREFERENCE', `$1' (`REMOVE_NEWLINE($2)'))
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(, alsa-lib). On
258 Linux, you'll need to have ALSA's development package
259 libasound2-dev installed.
262 libao). Needed to build the ao writer (ESD, PulseAudio,...).
263 Debian package: libao-dev.
265 Installation
266 ~~~~~~~~~~~~
268 First make sure all non-optional packages listed in the section on
269 REFERENCE(Requirements, required software) are installed on your
270 system.
272 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
273 OGG/Speex and AAC support are all optional. The configure script will
274 detect what is installed on your system and will only try to build
275 those executables that can be built with your setup.
277 Note that no special decoder library (not even the MP3 decoding library
278 libmad) is needed for para_server if you only want to stream MP3 or WMA
279 files. Also, it's fine to use para_server on a box without sound card.
281 Next, install the paraslash package on all machines, you'd like this
282 software to run on:
284 (./configure && make) > /dev/null
286 There should be no errors but probably some warnings about missing
287 packages which usually implies that not all audio formats will be
288 supported. If headers or libs are installed at unusual locations you
289 might need to tell the configure script where to find them. Try
291 ./configure --help
293 to see a list of options. If the paraslash package was compiled
294 successfully, execute (optionally)
296 make test
298 to run the paraslash test suite. If all tests pass, execute as root
300 make install
302 to install executables under /usr/local/bin and the man pages under
303 /usr/local/man.
305 Configuration
306 ~~~~~~~~~~~~~
308 *Step 1*: Create a paraslash user
310 In order to control para_server at runtime you must create a paraslash
311 user. As authentication is based on the RSA crypto system you'll have
312 to create an RSA key pair. If you already have a user and an RSA key
313 pair, you may skip this step.
315 In this section we'll assume a typical setup: You would like to run
316 para_server on some host called server_host as user foo, and you want
317 to connect to para_server from another machine called client_host as
318 user bar.
320 As foo@server_host, create ~/.paraslash/server.users by typing the
321 following commands:
323 user=bar
324 target=~/.paraslash/server.users
325 key=~/.paraslash/$user
327 mkdir -p ~/.paraslash
328 echo "user $user $key $perms" >> $target
330 Next, change to the "bar" account on client_host and generate the
331 key pair with the commands
333 ssh-keygen -t rsa -b 2048
334 # hit enter twice to create a key with no passphrase
336 This generates the two files id_rsa and in ~/.ssh. Note
337 that paraslash can also read keys generated by the "openssl genrsa"
338 command. However, since keys created with ssh-keygen can also be used
339 for ssh, this method is recommended.
341 Note that para_server refuses to use a key if it is shorter than 2048
342 bits. In particular, the RSA keys of paraslash 0.3.x will not work
343 with version 0.4.x. Moreover, para_client refuses to use a (private)
344 key which is world-readable.
346 para_server only needs to know the public key of the key pair just
347 created. Copy this public key to server_host:
349 src=~/.ssh/
350 dest=.paraslash/$LOGNAME
351 scp $src foo@server_host:$dest
353 Finally, tell para_client to connect to server_host:
355 conf=~/.paraslash/client.conf
356 echo 'hostname server_host' > $conf
359 *Step 2*: Start para_server
361 Before starting the server make sure you have write permissions to
362 the directory /var/paraslash that has been created during installation:
364 sudo chown $LOGNAME /var/paraslash
366 Alternatively, use the --afs_socket Option to specify a different
367 location for the AFS command socket.
369 For this first try, we'll use the info loglevel to make the output
370 of para_server more verbose.
372 para_server -l info
374 Now you can use para_client to connect to the server and issue
375 commands. Open a new shell as bar@client_host and try
377 para_client help
378 para_client si
380 to retrieve the list of available commands and some server info.
381 Don't proceed if this doesn't work.
383 *Step 3*: Create and populate the database
385 An empty database is created with
387 para_client init
389 This initializes a couple of empty tables under
390 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
391 tables, but it's good to know that you can start from scratch with
393 rm -rf ~/.paraslash/afs_database-0.4
395 in case something went wrong.
397 Next, you need to add some audio files to that database so that
398 para_server knows about them. Choose an absolute path to a directory
399 containing some audio files and add them to the audio file table:
401 para_client add /my/mp3/dir
403 This might take a while, so it is a good idea to start with a directory
404 containing not too many files. Note that the table only contains data
405 about the audio files found, not the files themselves.
407 You may print the list of all known audio files with
409 para_client ls
411 *Step 4*: Configure para_audiod
413 para_audiod needs to create a "well-known" socket for the clients to
414 connect to. The default path for this socket is
416 /var/paraslash/audiod_socket.$HOSTNAME
418 In order to make this directory writable for para_audiod, execute
419 as bar@client_host
421 sudo chown $LOGNAME /var/paraslash
424 We will also have to tell para_audiod that it should receive the
425 audio stream from server_host via http:
427 para_audiod -l info -r '.:http -i server_host'
429 You should now be able to listen to the audio stream once para_server
430 starts streaming. To activate streaming, execute
432 para_client play
434 Since no playlist has been specified yet, the "dummy" mode which
435 selects all known audio files is activated automatically. See the
436 section on the REFERENCE(The audio file selector, audio file selector)
437 for how to use playlists and moods to specify which files should be
438 streamed in which order.
440 *Troubleshooting*
442 It did not work? To find out why, try to receive, decode and play the
443 stream manually using para_recv, para_filter and para_write as follows.
445 For simplicity we assume that you're running Linux/ALSA and that only
446 MP3 files have been added to the database.
448 para_recv -r 'http -i server_host' > file.mp3
449 # (interrupt with CTRL+C after a few seconds)
450 ls -l file.mp3 # should not be empty
451 para_filter -f mp3dec -f wav < file.mp3 > file.wav
452 ls -l file.wav # should be much bigger than file.mp3
453 para_write -w alsa < file.wav
455 Double check what is logged by para_server and use the --loglevel
456 option of para_recv, para_filter and para_write to increase verbosity.
458 ---------------
459 User management
460 ---------------
462 para_server uses a challenge-response mechanism to authenticate
463 requests from incoming connections, similar to ssh's public key
464 authentication method. Authenticated connections are encrypted using
465 the RC4 stream cipher.
467 In this chapter we briefly describe RSA and RC4 and sketch the
468 REFERENCE(Client-server authentication, authentication handshake)
469 between para_client and para_server. User management is discussed
470 in the section on REFERENCE(The user_list file, the user_list file).
471 These sections are all about communication between the client and the
472 server. Connecting para_audiod is a different matter and is described
473 in a REFERENCE(Connecting para_audiod, separate section).
477 RSA and RC4
478 ~~~~~~~~~~~
480 RSA is an asymmetric block cipher which is used in many applications,
481 including ssh and gpg. An RSA key consists in fact of two keys,
482 called the public key and the private key. A message can be encrypted
483 with either key and only the counterpart of that key can decrypt
484 the message. While RSA can be used for both signing and encrypting
485 a message, paraslash only uses RSA only for the latter purpose. The
486 RSA public key encryption and signatures algorithms are defined in
487 detail in RFC 2437.
489 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
490 key stream to produce the output. Decryption uses the same function
491 calls as encryption. While RC4 supports variable key lengths,
492 paraslash uses a fixed length of 256 bits, which is considered a
493 strong encryption by today's standards. Since the same key must never
494 be used twice, a different, randomly-generated key is used for every
495 new connection.
497 Client-server authentication
498 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
500 The authentication handshake between para_client and para_server goes
501 as follows:
503 - para_client connects to para_server and sends an
504 authentication request for a user. It does so by connecting
505 to TCP port 2990 of the server host. This port is called the
506 para_server _control port_.
508 - para_server accepts the connection and forks a child process
509 which handles the incoming request. The parent process keeps
510 listening on the control port while the child process (also
511 called para_server below) continues as follows.
513 - para_server loads the RSA public key of that user, fills a
514 fixed-length buffer with random bytes, encrypts that buffer
515 using the public key and sends the encrypted buffer to the
516 client. The first part of the buffer is the challenge which
517 is used for authentication while the second part is the RC4
518 session key.
520 - para_client receives the encrypted buffer and decrypts it
521 with the user's private key, thereby obtaining the challenge
522 buffer and the session key. It sends the SHA1 hash value of
523 the challenge back to para_server and stores the session key
524 for further use.
526 - para_server also computes the SHA1 hash of the challenge
527 and compares it against what was sent back by the client.
529 - If the two hashes do not match, the authentication has
530 failed and para_server closes the connection.
532 - Otherwise the user is considered authenticated and the client
533 is allowed to proceed by sending a command to be executed. From
534 this point on the communication is encrypted using the RC4
535 stream cipher with the session key known to both peers.
537 paraslash relies on the quality of the pseudo-random bytes provided
538 by the crypto library (openssl or libgcrypt), on the security of
539 the implementation of the RSA and RC4 crypto routines and on the
540 infeasibility to invert the SHA1 function.
542 Neither para_server or para_client create RSA keys on their own. This
543 has to be done once for each user as sketched in REFERENCE(Quick start,
544 Quick start) and discussed in more detail REFERENCE(The user_list
545 file, below).
547 The user_list file
548 ~~~~~~~~~~~~~~~~~~
550 At startup para_server reads the user list file which contains one
551 line per user. The default location of the user list file may be
552 changed with the --user_list option.
554 There should be at least one user in this file. Each user must have
555 an RSA key pair. The public part of the key is needed by para_server
556 while the private key is needed by para_client. Each line of the
557 user list file must be of the form
559 user <username> <key> <perms>
561 where _username_ is an arbitrary string (usually the user's login
562 name), _key_ is the full path to that user's public RSA key, and
563 _perms_ is a comma-separated list of zero or more of the following
564 permission bits:
566 +---------------------------------------------------------+
567 | AFS_READ | read the contents of the databases |
568 +-----------+---------------------------------------------+
569 | AFS_WRITE | change database contents |
570 +-----------+---------------------------------------------+
571 | VSS_READ | obtain information about the current stream |
572 +-----------+---------------------------------------------+
573 | VSS_WRITE | change the current stream |
574 +---------------------------------------------------------+
576 The permission bits specify which commands the user is allowed to
577 execute. The output of
579 para_client help
581 contains in the third column the permissions needed to execute the
582 command.
584 It is possible to make para_server reread the user_list file by
585 executing the paraslash "hup" command or by sending SIGHUP to the
586 PID of para_server.
589 Connecting para_audiod
590 ~~~~~~~~~~~~~~~~~~~~~~
592 para_audiod listens on a Unix domain socket. Those sockets are
593 for local communication only, so only local users can connect to
594 para_audiod. The default is to let any user connect but this can be
595 restricted on platforms that support UNIX socket credentials which
596 allow para_audiod to obtain the Unix credentials of the connecting
597 process.
599 Use para_audiod's --user_allow option to allow connections only for
600 a limited set of users.
602 -----------------------
603 The audio file selector
604 -----------------------
606 paraslash comes with a sophisticated audio file selector (AFS),
607 whose main task is to determine which file to stream next, based on
608 information on the audio files stored in a database. It communicates
609 also with para_client whenever an AFS command is executed, for example
610 to answer a database query.
612 Besides the traditional playlists, AFS supports audio file selection
613 based on _moods_ which act as a filter that limits the set of all
614 known audio files to those which satisfy certain criteria. It also
615 maintains tables containing images (e.g. album cover art) and lyrics
616 that can be associated with one or more audio files.
618 AFS uses XREFERENCE(, libosl), the
619 object storage layer library, as the backend library for storing
620 information on audio files, playlists, etc. This library offers
621 functionality similar to a relational database, but is much more
622 lightweight than a full database backend.
624 In this chapter we sketch the setup of the REFERENCE(The AFS process,
625 AFS process) during server startup and proceed with the description
626 of the REFERENCE(Database layout, layout) of the various database
627 tables. The section on REFERENCE(Playlists and moods, playlists
628 and moods) explains these two audio file selection mechanisms
629 in detail and contains pratical examples. The way REFERENCE(File
630 renames and content changes, file renames and content changes) are
631 detected is discussed briefly before the REFERENCE(Troubleshooting,
632 Troubleshooting) section concludes the chapter.
634 The AFS process
635 ~~~~~~~~~~~~~~~
637 On startup, para_server forks to create the AFS process which opens
638 the OSL database tables. The server process communicates with the
639 AFS process via pipes and shared memory. Usually, the AFS process
640 awakes only briefly whenever the current audio file changes. The AFS
641 process determines the next audio file, opens it, verifies it has
642 not been changed since it was added to the database and passes the
643 open file descriptor to the server process, along with audio file
644 meta-data such as file name, duration, audio format and so on. The
645 server process then starts to stream the audio file.
647 The AFS process also accepts connections from local clients via
648 a well-known socket. However, only child processes of para_server
649 may connect through this socket. All server commands that have the
650 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
651 change the database.
653 Database layout
654 ~~~~~~~~~~~~~~~
656 *The audio file table*
658 This is the most important and usually also the largest table of the
659 AFS database. It contains the information needed to stream each audio
660 file. In particular the following data is stored for each audio file.
662 - SHA1 hash value of the audio file contents. This is computed
663 once when the file is added to the database. Whenever AFS
664 selects this audio file for streaming the hash value is
665 recomputed and checked against the value stored in the
666 database to detect content changes.
668 - The time when this audio file was last played.
670 - The number of times the file has been played so far.
672 - The attribute bitmask.
674 - The image id which describes the image associated with this
675 audio file.
677 - The lyrics id which describes the lyrics associated with
678 this audio file.
680 - The audio format id (MP3, OGG, ...).
682 - An amplification value that can be used by the amplification
683 filter to pre-amplify the decoded audio stream.
685 - The chunk table. It describes the location and the timing
686 of the building blocks of the audio file. This is used by
687 para_server to send chunks of the file at appropriate times.
689 - The duration of the audio file.
691 - Tag information contained in the audio file (ID3 tags,
692 Vorbis comments, ...).
694 - The number of channels
696 - The encoding bitrate.
698 - The sampling frequency.
700 To add or refresh the data contained in the audio file table, the _add_
701 command is used. It takes the full path of either an audio file or a
702 directory. In the latter case, the directory is traversed recursively
703 and all files which are recognized as valid audio files are added to
704 the database.
706 *The attribute table*
708 The attribute table contains two columns, _name_ and _bitnum_. An
709 attribute is simply a name for a certain bit number in the attribute
710 bitmask of the audio file table.
712 Each of the 64 bits of the attribute bitmask can be set for each
713 audio file individually. Hence up to 64 different attributes may be
714 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
715 "german_lyrics", "speech", whatever. You are free to choose as
716 many attributes as you like and there are no naming restrictions
717 for attributes.
719 A new attribute "test" is created by
721 para_client addatt test
722 and
723 para_client lsatt
725 lists all available attributes. You can set the "test" attribute for
726 an audio file by executing
728 para_client setatt test+ /path/to/the/audio/file
730 Similarly, the "test" bit can be removed from an audio file with
732 para_client setatt test- /path/to/the/audio/file
734 Instead of a path you may use a shell wildcard pattern. The attribute
735 is applied to all audio files matching this pattern:
737 para_client setatt test+ '/test/directory/*'
739 The command
741 para_client -- ls -lv
743 gives you a verbose listing of your audio files also showing which
744 attributes are set.
746 In case you wonder why the double-dash in the above command is needed:
747 It tells para_client to not interpret the options after the dashes. If
748 you find this annoying, just say
750 alias para='para_client --'
752 and be happy. In what follows we shall use this alias.
754 The "test" attribute can be dropped from the database with
756 para rmatt test
758 Read the output of
760 para help ls
761 para help setatt
763 for more information and a complete list of command line options to
764 these commands.
766 *Blob tables*
768 The image, lyrics, moods and playlists tables are all blob tables.
769 Blob tables consist of three columns each: The identifier which is
770 a positive non-negative number that is auto-incremented, the name
771 (an arbitrary string) and the content (the blob).
773 All blob tables support the same set of actions: cat, ls, mv, rm
774 and add. Of course, _add_ is used for adding new blobs to the table
775 while the other actions have the same meaning as the corresponding
776 Unix commands. The paraslash commands to perform these actions are
777 constructed as the concatenation of the table name and the action. For
778 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
779 manipulate or query the image table.
781 The add variant of these commands is special as these commands read
782 the blob contents from stdin. To add an image to the image table the
783 command
785 para addimg image_name < file.jpg
787 can be used.
789 Note that the images and lyrics are not interpreted at all, and also
790 the playlist and the mood blobs are only investigated when the mood
791 or playlist is activated with the select command.
793 *The score table*
795 Unlike all other tables the contents of the score table remain in
796 memory and are never stored on disk. The score table contains two
797 columns: The SHA1 hash value (of an audio file) and its current
798 score.
800 However, only those files which are admissible for the current mood
801 or playlist are contained in the score table. The audio file selector
802 always chooses the row with the highest score as the file to stream
803 next. While doing so, it computes the new score and updates the
804 last_played and the num_played fields in the audio file table.
806 The score table is recomputed by the select command which loads a
807 mood or playlist. Audio files are chosen for streaming from the rows
808 of the score table on a highest-score-first basis.
811 Playlists and moods
812 ~~~~~~~~~~~~~~~~~~~
814 Playlists and moods offer two different ways of specifying the set of
815 admissible files. A playlist in itself describes a set of admissible
816 files. A mood, in contrast, describes the set of admissible files in
817 terms of attributes and other type of information available in the
818 audio file table. As an example, a mood can define a filename pattern,
819 which is then matched against the names of audio files in the table.
821 *Playlists*
823 Playlists are accommodated in the playlist table of the afs database,
824 using the aforementioned blob format for tables. A new playlist is
825 created with the addpl command by specifying the full (absolute)
826 paths of all desired audio files, separated by newlines. Example:
828 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
830 If _my_playlist_ already exists it is overwritten. To activate the
831 new playlist, execute
833 para select p/my_playlist
835 The audio file selector will assign scores to each entry of the list,
836 in descending order so that files will be selected in order. If a
837 file could not be opened for streaming, its entry is removed from
838 the score table (but not from the playlist).
840 *Moods*
842 A mood consists of a unique name and its *mood definition*, which is
843 a set of *mood lines* containing expressions in terms of attributes
844 and other data contained in the database.
846 At any time at most one mood can be *active* which means that
847 para_server is going to select only files from that subset of
848 admissible files.
850 So in order to create a mood definition one has to write a set of
851 mood lines. Mood lines come in three flavours: Accept lines, deny
852 lines and score lines.
854 The general syntax of the three types of mood lines is
857 accept [with score <score>] [if] [not] <mood_method> [options]
858 deny [with score <score>] [if] [not] <mood_method> [options]
859 score <score> [if] [not] <mood_method> [options]
862 Here <score> is either an integer or the string "random" which assigns
863 a random score to all matching files. The score value changes the
864 order in which admissible files are going to be selected, but is of
865 minor importance for this introduction.
867 So we concentrate on the first two forms, i.e. accept and deny
868 lines. As usual, everything in square brackets is optional, i.e.
869 accept/deny lines take the following form when ignoring scores:
871 accept [if] [not] <mood_method> [options]
873 and analogously for the deny case. The "if" keyword is only syntactic
874 sugar and has no function. The "not" keyword just inverts the result,
875 so the essence of a mood line is the mood method part and the options
876 following thereafter.
878 A *mood method* is realized as a function which takes an audio file
879 and computes a number from the data contained in the database.
880 If this number is non-negative, we say the file *matches* the mood
881 method. The file matches the full mood line if it either
883 - matches the mood method and the "not" keyword is not given,
884 or
885 - does not match the mood method, but the "not" keyword is given.
887 The set of admissible files for the whole mood is now defined as those
888 files which match at least one accept mood line, but no deny mood line.
889 More formally, an audio file F is admissible if and only if
891 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
893 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
894 lines and "~" means "matches".
896 The cases where no mood lines of accept/deny type are defined need
897 special treatment:
899 - Neither accept nor deny lines: This treats all files as
900 admissible (in fact, that is the definition of the dummy mood
901 which is activated automatically if no moods are available).
903 - Only accept lines: A file is admissible iff it matches at
904 least one accept line:
906 F ~ AL1 or F ~ AL2 or ...
908 - Only deny lines: A file is admissible iff it matches no
909 deny line:
911 not (F ~ DL1 or F ~ DN2 ...)
915 *List of mood_methods*
917 no_attributes_set
919 Takes no arguments and matches an audio file if and only if no
920 attributes are set.
922 is_set <attribute_name>
924 Takes the name of an attribute and matches iff that attribute is set.
926 path_matches <pattern>
928 Takes a filename pattern and matches iff the path of the audio file
929 matches the pattern.
931 artist_matches <pattern>
932 album_matches <pattern>
933 title_matches <pattern>
934 comment_matches <pattern>
936 Takes an extended regular expression and matches iff the text of the
937 corresponding tag of the audio file matches the pattern. If the tag
938 is not set, the empty string is matched against the pattern.
940 year ~ <num>
941 bitrate ~ <num>
942 frequency ~ <num>
943 channels ~ <num>
944 num_played ~ <num>
946 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
947 <num>. Matches an audio file iff the condition <val> ~ <num> is
948 satisfied where val is the corresponding value of the audio file
949 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
950 count, play count).
952 The year tag is special as its value is undefined if the audio file
953 has no year tag or the content of the year tag is not a number. Such
954 audio files never match. Another difference is the special treatment
955 if the year tag is a two-digit number. In this case either 1900 or
956 2000 is added to the tag value, depending on whether the number is
957 greater than 2000 plus the current year.
960 *Mood usage*
962 To create a new mood called "my_mood", write its definition into
963 some temporary file, say "tmpfile", and add it to the mood table
964 by executing
966 para addmood my_mood < tmpfile
968 If the mood definition is really short, you may just pipe it to the
969 client instead of using temporary files. Like this:
971 echo "$MOOD_DEFINITION" | para addmood my_mood
973 There is no need to keep the temporary file since you can always use
974 the catmood command to get it back:
976 para catmood my_mood
978 A mood can be activated by executing
980 para select m/my_mood
982 Once active, the list of admissible files is shown by the ls command
983 if the "-a" switch is given:
985 para ls -a
988 *Example mood definition*
990 Suppose you have defined attributes "punk" and "rock" and want to define
991 a mood containing only Punk-Rock songs. That is, an audio file should be
992 admissible if and only if both attributes are set. Since
994 punk and rock
996 is obviously the same as
998 not (not punk or not rock)
1000 (de Morgan's rule), a mood definition that selects only Punk-Rock
1001 songs is
1003 deny if not is_set punk
1004 deny if not is_set rock
1008 File renames and content changes
1009 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1011 Since the audio file selector knows the SHA1 of each audio file that
1012 has been added to the afs database, it recognizes if the content of
1013 a file has changed, e.g. because an ID3 tag was added or modified.
1014 Also, if a file has been renamed or moved to a different location,
1015 afs will detect that an entry with the same hash value already exists
1016 in the audio file table.
1018 In both cases it is enough to just re-add the new file. In the
1019 first case (file content changed), the audio table is updated, while
1020 metadata such as the num_played and last_played fields, as well as
1021 the attributes, remain unchanged. In the other case, when the file
1022 is moved or renamed, only the path information is updated, all other
1023 data remains as before.
1025 It is possible to change the behaviour of the add command by using the
1026 "-l" (lazy add) or the "-f" (force add) option.
1028 Troubleshooting
1029 ~~~~~~~~~~~~~~~
1031 Use the debug loglevel (option -l debug for most commands) to show
1032 debugging info. Almost all paraslash executables have a brief online
1033 help which is displayed by using the -h switch. The --detailed-help
1034 option prints the full help text.
1036 If para_server crashed or was killed by SIGKILL (signal 9), it
1037 may refuse to start again because of "dirty osl tables". In this
1038 case you'll have to run the oslfsck program of libosl to fix your
1039 database. It might be necessary to use --force (even if your name
1040 isn't Luke). However, make sure para_server isn't running before
1041 executing oslfsck --force.
1043 If you don't mind to recreate your database you can start
1044 from scratch by removing the entire database directory, i.e.
1046 rm -rf ~/.paraslash/afs_database-0.4
1048 Be aware that this removes all attribute definitions, all playlists
1049 and all mood definitions and requires to re-initialize the tables.
1051 Although oslfsck fixes inconsistencies in database tables it doesn't
1052 care about the table contents. To check for invalid table contents, use
1054 para_client check
1056 This prints out references to missing audio files as well as invalid
1057 playlists and mood definitions.
1059 ---------------------------------------
1060 Audio formats and audio format handlers
1061 ---------------------------------------
1063 Audio formats
1064 ~~~~~~~~~~~~~
1066 The following audio formats are supported by paraslash:
1068 *MP3*
1070 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1071 designed as part of its MPEG-1 standard. An MP3 file is made up of
1072 multiple MP3 frames, which consist of a header and a data block. The
1073 size of an MP3 frame depends on the bit rate and on the number
1074 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1075 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1076 400 bytes large.
1078 *OGG/Vorbis*
1080 OGG is a standardized audio container format, while Vorbis is an
1081 open source codec for lossy audio compression. Since Vorbis is most
1082 commonly made available via the OGG container format, it is often
1083 referred to as OGG/Vorbis. The OGG container format divides data into
1084 chunks called OGG pages. A typical OGG page is about 4KB large. The
1085 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1086 may vary considerably.
1088 *OGG/Speex*
1090 Speex is an open-source speech codec that is based on CELP (Code
1091 Excited Linear Prediction) coding. It is designed for voice
1092 over IP applications, has modest complexity and a small memory
1093 footprint. Wideband and narrowband (telephone quality) speech are
1094 supported. As for Vorbis audio, Speex bit-streams are often stored
1095 in OGG files.
1097 *AAC*
1099 Advanced Audio Coding (AAC) is a standardized, lossy compression
1100 and encoding scheme for digital audio which is the default audio
1101 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1102 the container format and audio files encoded with AAC have the .m4a
1103 extension. A typical AAC frame is about 700 bytes large.
1105 *WMA*
1107 Windows Media Audio (WMA) is an audio data compression technology
1108 developed by Microsoft. A WMA file is usually encapsulated in the
1109 Advanced Systems Format (ASF) container format, which also specifies
1110 how meta data about the file is to be encoded. The bit stream of WMA
1111 is composed of superframes, each containing one or more frames of
1112 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1114 Meta data
1115 ~~~~~~~~~
1117 Unfortunately, each audio format has its own conventions how meta
1118 data is added as tags to the audio file.
1120 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1121 is rather simple but also very limited as it supports only artist,
1122 title, album, year and comment tags. Each of these can only be at most
1123 32 characters long. ID3, version 2 is much more flexible but requires
1124 a separate library being installed for paraslash to support it.
1126 Ogg vorbis files contain meta data as Vorbis comments, which are
1127 typically implemented as strings of the form "[TAG]=[VALUE]". Unlike
1128 ID3 version 1 tags, one may use whichever tags are appropriate for
1129 the content.
1131 AAC files usually use the MPEG-4 container format for storing meta
1132 data while WMA files wrap meta data as special objects within the
1133 ASF container format.
1135 paraslash only tracks the most common tags that are supported by
1136 all tag variants: artist, title, year, album, comment. When a file
1137 is added to the AFS database, the meta data of the file is extracted
1138 and stored in the audio file table.
1140 Chunks and chunk tables
1141 ~~~~~~~~~~~~~~~~~~~~~~~
1143 paraslash uses the word "chunk" as common term for the building blocks
1144 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1145 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1146 size varies considerably between audio formats, from a few hundred
1147 bytes (MP3) up to 8K (WMA).
1149 The chunk table contains the offsets within the audio file that
1150 correspond to the chunk boundaries of the file. Like the meta data,
1151 the chunk table is computed and stored in the database whenever an
1152 audio file is added.
1154 The paraslash senders (see below) always send complete chunks. The
1155 granularity for seeking is therefore determined by the chunk size.
1157 Audio format handlers
1158 ~~~~~~~~~~~~~~~~~~~~~
1160 For each audio format paraslash contains an audio format handler whose
1161 first task is to tell whether a given file is a valid audio file of
1162 this type. If so, the audio file handler extracts some technical data
1163 (duration, sampling rate, number of channels etc.), computes the
1164 chunk table and reads the meta data.
1166 The audio format handler code is linked into para_server and executed
1167 via the _add_ command. The same code is also available as a stand-alone
1168 tool, para_afh, which can be used to print the technical data, the
1169 chunk table and the meta data of a file. Furthermore, one can use
1170 para_afh to cut an audio file, i.e. to select some of its chunks to
1171 produce a new file containing only these chunks.
1173 ----------
1174 Networking
1175 ----------
1177 Paraslash uses different network connections for control and data.
1178 para_client communicates with para_server over a dedicated TCP control
1179 connection. To transport audio data, separate data connections are
1180 used. For these data connections, a variety of transports (UDP, DCCP,
1181 HTTP) can be chosen.
1183 The chapter starts with the REFERENCE(The paraslash control
1184 service, control service), followed by a section on the various
1185 REFERENCE(Streaming protocols, streaming protocols) in which the data
1186 connections are described. The way audio file headers are embedded into
1187 the stream is discussed REFERENCE(Streams with headers and headerless
1188 streams, briefly) before the REFERENCE(Networking examples, example
1189 section) which illustrates typical commands for real-life scenarios.
1191 Both IPv4 and IPv6 are supported.
1193 The paraslash control service
1194 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1196 para_server is controlled at runtime via the paraslash control
1197 connection. This connection is used for server commands (play, stop,
1198 ...) as well as for afs commands (ls, select, ...).
1200 The server listens on a TCP port and accepts connections from clients
1201 that connect the open port. Each connection causes the server to fork
1202 off a client process which inherits the connection and deals with that
1203 client only. In this classical accept/fork approach the server process
1204 is unaffected if the child dies or goes crazy for whatever reason. In
1205 fact, the child process can not change address space of server process.
1207 The section on REFERENCE(Client-server authentication, client-server
1208 authentication) above described the early connection establishment
1209 from the crypto point of view. Here it is described what happens
1210 after the connection (including crypto setup) has been established.
1211 There are four processes involved during command dispatch as sketched
1212 in the following diagram.
1214 <<
1215 <pre>
1216 server_host client_host
1217 ~~~~~~~~~~~ ~~~~~~~~~~~
1219 +-----------+ connect +-----------+
1220 |para_server|<------------------------------ |para_client|
1221 +-----------+ +-----------+
1222 | ^
1223 | fork +---+ |
1224 +----------> |AFS| |
1225 | +---+ |
1226 | ^ |
1227 | | |
1228 | | connect (cookie) |
1229 | | |
1230 | | |
1231 | fork +-----+ inherited connection |
1232 +---------->|child|<--------------------------+
1233 +-----+
1234 </pre>
1235 >>
1237 Note that the child process is not a child of the afs process,
1238 so communication of these two processes has to happen via local
1239 sockets. In order to avoid abuse of the local socket by unrelated
1240 processes, a magic cookie is created once at server startup time just
1241 before the server process forks off the AFS process. This cookie is
1242 known to the server, AFS and the child, but not to unrelated processes.
1244 There are two different kinds of commands: First there are commands
1245 that cause the server to respond with some answer such as the list
1246 of all audio files. All but the addblob commands (addimg, addlyr,
1247 addpl, addmood) are of this kind. The addblob commands add contents
1248 to the database, so they need to transfer data the other way round,
1249 from the client to the server.
1251 There is no knowledge about the server commands built into para_client,
1252 so it does not know about addblob commands. Instead, it inspects the
1253 first data package sent by the server for a magic string. If this
1254 string was found, it sends STDIN to the server, otherwise it dumps
1255 data from the server to STDOUT.
1257 Streaming protocols
1258 ~~~~~~~~~~~~~~~~~~~
1260 A network (audio) stream usually consists of one streaming source,
1261 the _sender_, and one or more _receivers_ which read data over the
1262 network from the streaming source.
1264 Senders are thus part of para_server while receivers are part of
1265 para_audiod. Moreover, there is the stand-alone tool para_recv which
1266 can be used to manually download a stream, either from para_server
1267 or from a web-based audio streaming service.
1269 The following three streaming protocols are supported by paraslash:
1271 - HTTP. Recommended for public streams that can be played by
1272 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1273 sender is supported on all operating systems and all platforms.
1275 - DCCP. Recommended for LAN streaming. DCCP is currently
1276 available only for Linux.
1278 - UDP. Recommended for multicast LAN streaming.
1280 See the Appendix on REFERENCE(Network protocols, network protocols)
1281 for brief descriptions of the various protocols relevant for network
1282 audio streaming with paraslash.
1284 It is possible to activate more than one sender simultaneously.
1285 Senders can be controlled at run time and via config file and command
1286 line options.
1288 Note that audio connections are _not_ encrypted. Transport or Internet
1289 layer encryption should be used if encrypted data connections are
1290 needed.
1292 Since DCCP and TCP are both connection-oriented protocols, connection
1293 establishment/teardown and access control are very similar between
1294 these two streaming protocols. UDP is the most lightweight option,
1295 since in contrast to TCP/DCCP it is connectionless. It is also the
1296 only protocol supporting IP multicast.
1298 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1299 clients to connect and establish a connection via some protocol-defined
1300 handshake mechanism. Both senders maintain two linked lists each:
1301 The list of all clients which are currently connected, and the list
1302 of access control entries which determines who is allowed to connect.
1303 IP-based access control may be configured through config file and
1304 command line options and via the "allow" and "deny" sender subcommands.
1306 Upon receiving a GET request from the client, the HTTP sender sends
1307 back a status line and a message. The body of this message is the
1308 audio stream. This is common practice and is supported by many popular
1309 clients which can thus be used to play a stream offered by para_server.
1310 For DCCP things are a bit simpler: No messages are exchanged between
1311 the receiver and sender. The client simply connects and the sender
1312 starts to stream.
1314 DCCP is an experimental protocol which offers a number of new features
1315 not available for TCP. Both ends can negotiate these features using
1316 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1317 datagram-based (no retransmissions) and thus should not be used over
1318 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1319 is access to a variety of different congestion-control mechanisms
1320 called CCIDs. Two different CCIDs are available per default on Linux:
1323 - _CCID 2_. A Congestion Control mechanism similar to that
1324 of TCP. The sender maintains a congestion window and halves
1325 this window in response to congestion.
1328 - _CCID-3_. Designed to be fair when competing for bandwidth.
1329 It has lower variation of throughput over time compared with
1330 TCP, which makes it suitable for streaming media.
1332 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1333 single list, the _target list_. This list describes the set of clients
1334 to which the stream is sent. There is no list for access control and
1335 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1336 and "delete" commands can be used to modify the target list.
1338 Since both UDP and DCCP offer an unreliable datagram-based transport,
1339 additional measures are necessary to guard against disruptions over
1340 networks that are lossy or which may be subject to interference (as
1341 is for instance the case with WiFi). Paraslash uses FEC (Forward
1342 Error Correction) to guard against packet losses and reordering. The
1343 stream is FEC-encoded before it is sent through the UDP socket and
1344 must be decoded accordingly on the receiver side.
1346 The packet size and the amount of redundancy introduced by FEC can
1347 be configured via the FEC parameters which are dictated by server
1348 and may also be configured through the "sender" command. The FEC
1349 parameters are encoded in the header of each network packet, so no
1350 configuration is necessary on the receiver side. See the section on
1351 REFERENCE(Forward error correction, FEC) below.
1353 Streams with headers and headerless streams
1354 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1356 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1357 needed to decode the stream is only contained in the audio file
1358 header of the container format but not in each data chunk. Clients
1359 must be able to obtain this information in case streaming starts in
1360 the middle of the file or if para_audiod is started while para_server
1361 is already sending a stream.
1363 This is accomplished in different ways, depending on the streaming
1364 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1365 header is sent prior to audio file data. This technique however does
1366 not work for the connectionless UDP transport. Hence the audio file
1367 header is periodically being embedded into the UDP audio data stream.
1368 By default, the header is resent after five seconds. The receiver has
1369 to wait until the next header arrives before it can start decoding
1370 the stream.
1372 Examples
1373 ~~~~~~~~
1375 The sender command of para_server allows to (de-)activate senders
1376 and to change the access permissions senders at runtime. The "si"
1377 (server info) command is used to list the streaming options of the
1378 currently running server as well as the various sender access lists.
1380 -> Show client/target/access lists:
1382 para_client si
1384 -> Obtain general help for the sender command:
1386 para_client help sender
1388 -> Get help for a specific sender (contains further examples):
1390 s=http # or dccp or udp
1391 para_client sender $s help
1393 By default para_server activates both the HTTP and th DCCP sender on
1394 startup. This can be changed via command line options or para_server's
1395 config file.
1397 -> List config file options for senders:
1399 para_server -h
1401 All senders share the "on" and "off" commands, so senders may be
1402 activated and deactivated independently of each other.
1404 -> Switch off the http sender:
1406 para_client sender http off
1408 -> Receive a DCCP stream using CCID2 and write the output into a file:
1410; ccid=2; filename=bar
1411 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1413 Note the quotes around the arguments for the dccp receiver. Each
1414 receiver has its own set of command line options and its own command
1415 line parser, so arguments for the dccp receiver must be protected
1416 from being interpreted by para_recv.
1418 -> Start UDP multicast, using the default multicast address:
1420 para_client sender udp add
1422 -> Receive FEC-encoded multicast stream and write the output into a file:
1424 filename=foo
1425 para_recv -r udp > $filename
1427 -> Add an UDP unicast for a client to the target list of the UDP sender:
1430 para_client sender udp add $t
1432 -> Receive this (FEC-encoded) unicast stream:
1434 filename=foo
1435 para_recv -r 'udp -i' > $filename
1437 -> Create a minimal config for para_audiod for HTTP streams:
1439 c=$HOME/.paraslash/audiod.conf.min;
1440 echo receiver \".:http -i $s\" > $c
1441 para_audiod --config $c
1443 -------
1444 Filters
1445 -------
1447 A paraslash filter is a module which transforms an input stream into
1448 an output stream. Filters are included in the para_audiod executable
1449 and in the stand-alone tool para_filter which usually contains the
1450 same modules.
1452 While para_filter reads its input stream from STDIN and writes
1453 the output to STDOUT, the filter modules of para_audiod are always
1454 connected to a receiver which produces the input stream and a writer
1455 which absorbs the output stream.
1457 Some filters depend on a specific library being installed and are
1458 not compiled in if this library was not found at compile time. To
1459 see the list of supported filters, run para_filter and para_audiod
1460 with the --help option. The output looks similar to the following:
1462 Available filters:
1463 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1465 Out of these filter modules, a chain of filters can be constructed,
1466 much in the way Unix pipes can be chained, and analogous to the use
1467 of modules in gstreamer: The output of the first filter becomes the
1468 input of the second filter. There is no limitation on the number of
1469 filters and the same filter may occur more than once.
1471 Like receivers, each filter has its own command line options which
1472 must be quoted to protect them from the command line options of
1473 the driving application (para_audiod or para_filter). Example:
1475 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1477 For para_audiod, each audio format has its own set of filters. The
1478 name of the audio format for which the filter should be applied can
1479 be used as the prefix for the filter option. Example:
1481 para_audiod -f 'mp3:prebuffer --duration 300'
1483 The "mp3" prefix above is actually interpreted as a POSIX extended
1484 regular expression. Therefore
1486 para_audiod -f '.:prebuffer --duration 300'
1488 activates the prebuffer filter for all supported audio formats (because
1489 "." matches all audio formats) while
1491 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1493 activates it only for wma and ogg streams.
1495 Decoders
1496 ~~~~~~~~
1498 For each supported audio format there is a corresponding filter
1499 which decodes audio data in this format to 16 bit PCM data which
1500 can be directly sent to the sound device or any other software that
1501 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1502 filters are called _decoders_ in general, and xxxdec is the name of
1503 the paraslash decoder for the audio format xxx. For example, the mp3
1504 decoder filter is called mp3dec.
1506 Note that the output of the decoder is about 10 times larger than
1507 its input. This means that filters that operate on the decoded audio
1508 stream have to deal with much more data than filters that transform
1509 the audio stream before it is fed to the decoder.
1511 Paraslash relies on external libraries for most decoders, so these
1512 libraries must be installed for the decoder to be included in the
1513 para_filter and para_audiod executables. The oggdec filter depends
1514 on the libogg and libvorbis libraries for example.
1516 Forward error correction
1517 ~~~~~~~~~~~~~~~~~~~~~~~~
1519 As already mentioned REFERENCE(Streaming protocols, earlier),
1520 paraslash uses forward error correction (FEC) for the unreliable UDP
1521 and DCCP transports. FEC is a technique which was invented already
1522 in 1960 by Reed and Solomon and which is widely used for the parity
1523 calculations of storage devices (RAID arrays). It is based on the
1524 algebraic concept of finite fields, today called Galois fields, in
1525 honour of the mathematician Galois (1811-1832). The FEC implementation
1526 of paraslash is based on code by Luigi Rizzo.
1528 Although the details require a sound knowledge of the underlying
1529 mathematics, the basic idea is not hard to understand: For positive
1530 integers k and n with k < n it is possible to compute for any k given
1531 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1532 ..., p_r such that all data bytes can be reconstructed from *any*
1533 k bytes of the set
1535 {d_1, ..., d_k, p_1, ..., p_r}.
1537 FEC-encoding for unreliable network transports boils down to slicing
1538 the audio stream into groups of k suitably sized pieces called _slices_
1539 and computing the r corresponding parity slices. This step is performed
1540 in para_server which then sends both the data and the parity slices
1541 over the unreliable network connection. If the client was able
1542 to receive at least k of the n = k + r slices, it can reconstruct
1543 (FEC-decode) the original audio stream.
1545 From these observations it is clear that there are three different
1546 FEC parameters: The slice size, the number of data slices k, and the
1547 total number of slices n. It is crucial to choose the slice size
1548 such that no fragmentation of network packets takes place because
1549 FEC only guards against losses and reordering but fails if slices are
1550 received partially.
1552 FEC decoding in paralash is performed through the fecdec filter which
1553 usually is the first filter (there can be other filters before fecdec
1554 if these do not alter the audio stream).
1557 Volume adjustment (amp and compress)
1558 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1560 The amp and the compress filter both adjust the volume of the audio
1561 stream. These filters operate on uncompressed audio samples. Hence
1562 they are usually placed directly after the decoding filter. Each
1563 sample is multiplied with a scaling factor (>= 1) which makes amp
1564 and compress quite expensive in terms of computing power.
1566 *amp*
1568 The amp filter amplifies the audio stream by a fixed scaling factor
1569 that must be known in advance. For para_audiod this factor is derived
1570 from the amplification field of the audio file's entry in the audio
1571 file table while para_filter uses the value given at the command line.
1573 The optimal scaling factor F for an audio file is the largest real
1574 number F >= 1 such that after multiplication with F all samples still
1575 fit into the sample interval [-32768, 32767]. One can use para_filter
1576 in combination with the sox utility to compute F:
1578 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1580 The amplification value V which is stored in the audio file table,
1581 however, is an integer between 0 and 255 which is connected to F
1582 through the formula
1584 V = (F - 1) * 64.
1586 To store V in the audio file table, the command
1588 para_client -- touch -a=V file.mp3
1590 is used. The reader is encouraged to write a script that performs
1591 these computations :)
1593 *compress*
1595 Unlike the amplification filter, the compress filter adjusts the volume
1596 of the audio stream dynamically without prior knowledge about the peak
1597 value. It maintains the maximal volume of the last n samples of the
1598 audio stream and computes a suitable amplification factor based on that
1599 value and the various configuration options. It tries to chose this
1600 factor such that the adjusted volume meets the desired target level.
1602 Note that it makes sense to combine amp and compress.
1604 Misc filters (wav and prebuffer)
1605 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1607 These filters are rather simple and do not modify the audio stream at
1608 all. The wav filter is only useful with para_filter and in connection
1609 with a decoder. It asks the decoder for the number of channels and the
1610 sample rate of the stream and adds a Microsoft wave header containing
1611 this information at the beginning. This allows to write wav files
1612 rather than raw PCM files (which do not contain any information about
1613 the number of channels and the sample rate).
1615 The prebuffer filter simply delays the output until the given time has
1616 passed (starting from the time the first byte was available in its
1617 input queue) or until the given amount of data has accumulated. It
1618 is mainly useful for para_audiod if the standard parameters result
1619 in buffer underruns.
1621 Both filters require almost no additional computing time, even when
1622 operating on uncompressed audio streams, since data buffers are simply
1623 "pushed down" rather than copied.
1625 Examples
1626 ~~~~~~~~
1628 -> Decode an mp3 file to wav format:
1630 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1632 -> Amplify a raw audio file by a factor of 1.5:
1634 para_filter -f amp --amp 32 < foo.raw > bar.raw
1636 ------
1637 Output
1638 ------
1640 Once an audio stream has been received and decoded to PCM format,
1641 it can be sent to a sound device for playback. This part is performed
1642 by paraslash _writers_ which are described in this chapter.
1644 Writers
1645 ~~~~~~~
1647 A paraslash writer acts as a data sink that consumes but does not
1648 produce audio data. Paraslash writers operate on the client side and
1649 are contained in para_audiod and in the stand-alone tool para_write.
1651 The para_write program reads uncompressed audio data from STDIN. If
1652 this data starts with a wav header, sample rate, sample format and
1653 channel count are read from the header. Otherwise CD audio (44.1KHz
1654 16 bit little endian, stereo) is assumed but this can be overridden
1655 by command line options. para_audiod, on the other hand, obtains
1656 the sample rate and the number of channels from the decoder.
1658 Like receivers and filters, each writer has an individual set of
1659 command line options, and for para_audiod writers can be configured
1660 per audio format separately. It is possible to activate more than
1661 one writer for the same stream simultaneously.
1663 OS-dependent APIs
1664 ~~~~~~~~~~~~~~~~~
1666 Unfortunately, the various flavours of Unix on which paraslash
1667 runs on have different APIs for opening a sound device and starting
1668 playback. Hence for each such API there is a paraslash writer that
1669 can play the audio stream via this API.
1671 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1672 Linux systems. Although there are several mid-layer APIs in use by
1673 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1674 currently supports only the low-level ALSA API which is not supposed
1675 to be change. ALSA is very feature-rich, in particular it supports
1676 software mixing via its DMIX plugin. ALSA is the default writer on
1677 Linux systems.
1679 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1680 is also available on Linux systems, usually provided by ALSA as an
1681 emulation for backwards compatibility. This API is rather simple but
1682 also limited. For example only one application can open the device
1683 at any time. The OSS writer is activated by default on BSD Systems.
1685 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1686 for this API is only compiled in on such systems and is of course
1687 the default there.
1689 *FILE*. The file writer allows to capture the audio stream and
1690 write the PCM data to a file on the file system rather than playing
1691 it through a sound device. It is supported on all platforms and is
1692 always compiled in.
1694 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1695 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1696 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1697 through an output plugin of libao.
1699 Examples
1700 ~~~~~~~~
1702 -> Use the OSS writer to play a wav file:
1704 para_write --writer oss < file.wav
1706 -> Enable ALSA software mixing for mp3 streams
1708 para_audiod --writer 'mp3:alsa -d plug:swmix'
1711 ---
1712 Gui
1713 ---
1715 para_gui executes an arbitrary command which is supposed to print
1716 status information to STDOUT. It then displays this information in
1717 a curses window. By default the command
1719 para_audioc -- stat -p
1721 is executed, but this can be customized via the --stat_cmd option. In
1722 particular it possible to use
1724 para_client -- stat -p
1726 to make para_gui work on systems on which para_audiod is not running.
1728 Key bindings
1729 ~~~~~~~~~~~~
1731 It is possible to bind keys to arbitrary commands via custom
1732 key-bindings. Besides the internal keys which can not be changed (help,
1733 quit, loglevel, version...), the following flavours of key-bindings
1734 are supported:
1736 - external: Shutdown curses before launching the given command.
1737 Useful for starting other ncurses programs from within
1738 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1739 output format to write a mailbox containing one mail for each
1740 (admissible) file the audio file selector knows about. Then
1741 start mutt from within para_gui to browse your collection!
1743 - display: Launch the command and display its stdout in
1744 para_gui's bottom window.
1746 - para: Like display, but start "para_client <specified
1747 command>" instead of "<specified command>".
1749 The general form of a key binding is
1751 key_map k:m:c
1753 which maps key k to command c using mode m. Mode may be x, d or p
1754 for external, display and paraslash commands, respectively.
1756 Themes
1757 ~~~~~~
1759 Currently there are only two themes for para_gui. It is easy, however,
1760 to add more themes. To create a new theme one has to define the
1761 position, color and geometry for for each status item that should be
1762 shown by this theme. See gui_theme.c for examples.
1764 The "." and "," keys are used to switch between themes.
1766 Examples
1767 ~~~~~~~~
1769 -> Show server info:
1771 key_map "i:p:si"
1773 -> Jump to the middle of the current audio file by pressing F5:
1775 key_map "<F5>:p:jmp 50"
1777 -> vi-like bindings for jumping around:
1779 key_map "l:p:ff 10"
1780 key_map "h:p:ff 10-"
1781 key_map "w:p:ff 60"
1782 key_map "b:p:ff 60-"
1784 -> Print the current date and time:
1786 key_map "D:d:date"
1788 -> Call other curses programs:
1790 key_map "U:x:aumix"
1791 key_map "!:x:/bin/bash"
1792 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1794 -----------
1795 Development
1796 -----------
1798 Tools
1799 ~~~~~
1801 In order to compile the sources from the git repository (rather than
1802 from tar balls) and for contributing non-trivial changes to the
1803 paraslash project, some additional tools should be installed on a
1804 developer machine.
1806 (git). As described in more detail REFERENCE(Git
1807 branches, below), the git source code management tool is used for
1808 paraslash development. It is necessary for cloning the git repository
1809 and for getting updates.
1811 (m4). Some input files for gengetopt
1812 are generated from templates by the m4 macro processor.
1814 (autoconf) GNU autoconf creates
1815 the configure file which is shipped in the tarballs but has to be
1816 generated when compiling from git.
1818 (grutatxt). The
1819 HTML version of this manual and some of the paraslash web pages are
1820 generated by the grutatxt plain text to HTML converter. If changes
1821 are made to these text files the grutatxt package must be installed
1822 to regenerate the HTML files.
1824 (doxygen). The documentation
1825 of paraslash's C sources uses the doxygen documentation system. The
1826 conventions for documenting the source code is described in the
1827 REFERENCE(Doxygen, Doxygen section).
1829 (global). This is used to generate
1830 browsable HTML from the C sources. It is needed by doxygen.
1832 Git branches
1833 ~~~~~~~~~~~~
1835 Paraslash has been developed using the git source code management
1836 tool since 2006. Development is organized roughly in the same spirit
1837 as the git development itself, as described below.
1839 The following text passage is based on "A note from the maintainer",
1840 written by Junio C Hamano, the maintainer of git.
1842 There are four branches in the paraslash repository that track the
1843 source tree: "master", "maint", "next", and "pu".
1845 The "master" branch is meant to contain what is well tested and
1846 ready to be used in a production setting. There could occasionally be
1847 minor breakages or brown paper bag bugs but they are not expected to
1848 be anything major, and more importantly quickly and easily fixable.
1849 Every now and then, a "feature release" is cut from the tip of this
1850 branch, named with three dotted decimal digits, like 0.4.2.
1852 Whenever changes are about to be included that will eventually lead to
1853 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1854 "master" at that point. Obvious, safe and urgent fixes after the major
1855 release are applied to this branch and maintenance releases are cut
1856 from it. New features never go to this branch. This branch is also
1857 merged into "master" to propagate the fixes forward.
1859 A trivial and safe enhancement goes directly on top of "master".
1860 New development does not usually happen on "master", however.
1861 Instead, a separate topic branch is forked from the tip of "master",
1862 and it first is tested in isolation; Usually there are a handful such
1863 topic branches that are running ahead of "master". The tip of these
1864 branches is not published in the public repository to keep the number
1865 of branches that downstream developers need to worry about low.
1867 The quality of topic branches varies widely. Some of them start out as
1868 "good idea but obviously is broken in some areas" and then with some
1869 more work become "more or less done and can now be tested by wider
1870 audience". Luckily, most of them start out in the latter, better shape.
1872 The "next" branch is to merge and test topic branches in the latter
1873 category. In general, this branch always contains the tip of "master".
1874 It might not be quite rock-solid production ready, but is expected to
1875 work more or less without major breakage. The maintainer usually uses
1876 the "next" version of paraslash for his own pleasure, so it cannot
1877 be _that_ broken. The "next" branch is where new and exciting things
1878 take place.
1880 The two branches "master" and "maint" are never rewound, and "next"
1881 usually will not be either (this automatically means the topics that
1882 have been merged into "next" are usually not rebased, and you can find
1883 the tip of topic branches you are interested in from the output of
1884 "git log next"). You should be able to safely build on top of them.
1886 However, at times "next" will be rebuilt from the tip of "master" to
1887 get rid of merge commits that will never be in "master". The commit
1888 that replaces "next" will usually have the identical tree, but it
1889 will have different ancestry from the tip of "master".
1891 The "pu" (proposed updates) branch bundles the remainder of the
1892 topic branches. The "pu" branch, and topic branches that are only in
1893 "pu", are subject to rebasing in general. By the above definition
1894 of how "next" works, you can tell that this branch will contain quite
1895 experimental and obviously broken stuff.
1897 When a topic that was in "pu" proves to be in testable shape, it
1898 graduates to "next". This is done with
1900 git checkout next
1901 git merge that-topic-branch
1903 Sometimes, an idea that looked promising turns out to be not so good
1904 and the topic can be dropped from "pu" in such a case.
1906 A topic that is in "next" is expected to be polished to perfection
1907 before it is merged to "master". Similar to the above, this is
1908 done with
1910 git checkout master
1911 git merge that-topic-branch
1912 git branch -d that-topic-branch
1914 Note that being in "next" is not a guarantee to appear in the next
1915 release (being in "master" is such a guarantee, unless it is later
1916 found seriously broken and reverted), nor even in any future release.
1918 Coding Style
1919 ~~~~~~~~~~~~
1921 The preferred coding style for paraslash coincides more or less
1922 with the style of the Linux kernel. So rather than repeating what is
1923 written XREFERENCE(,
1924 there), here are the most important points.
1926 - Burn the GNU coding standards.
1927 - Never use spaces for indentation.
1928 - Tabs are 8 characters, and thus indentations are also 8 characters.
1929 - Don't put multiple assignments on a single line.
1930 - Avoid tricky expressions.
1931 - Don't leave whitespace at the end of lines.
1932 - The limit on the length of lines is 80 columns.
1933 - Use K&R style for placing braces and spaces:
1935 if (x is true) {
1936 we do y
1937 }
1939 - Use a space after (most) keywords.
1940 - Do not add spaces around (inside) parenthesized expressions.
1941 - Use one space around (on each side of) most binary and ternary operators.
1942 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1943 - Mixed-case names are frowned upon.
1944 - Descriptive names for global variables are a must.
1945 - Avoid typedefs.
1946 - Functions should be short and sweet, and do just one thing.
1947 - The number of local variables shouldn't exceed 10.
1948 - Gotos are fine if they improve readability and reduce nesting.
1949 - Don't use C99-style "// ..." comments.
1950 - Names of macros defining constants and labels in enums are capitalized.
1951 - Enums are preferred when defining several related constants.
1952 - Always use the paraslash wrappers for allocating memory.
1953 - If the name of a function is an action or an imperative.
1954 command, the function should return an error-code integer
1955 (<0 means error, >=0 means success). If the name is a
1956 predicate, the function should return a "succeeded" boolean.
1959 Doxygen
1960 ~~~~~~~
1962 Doxygen is a documentation system for various programming
1963 languages. The paraslash project uses Doxygen for generating the API
1964 reference on the web pages, but good source code documentation is
1965 also beneficial to people trying to understand the code structure
1966 and the interactions between the various source files.
1968 It is more illustrative to look at the source code for examples than
1969 to describe the conventions for documenting the source in this manual,
1970 so we only describe which parts of the code need doxygen comments,
1971 but leave out details on documentation conventions.
1973 As a rule, only the public part of the C source is documented with
1974 Doxygen. This includes structures, defines and enumerations in header
1975 files as well as public (non-static) C functions. These should be
1976 documented completely. For example each parameter and the return
1977 value of a public function should get a descriptive comment.
1979 No doxygen comments are necessary for static functions and for
1980 structures and enumerations in C files (which are used only within
1981 this file). This does not mean, however, that those entities need
1982 no documentation at all. Instead, common sense should be applied to
1983 document what is not obvious from reading the code.
1985 --------
1986 Appendix
1987 --------
1989 Network protocols
1990 ~~~~~~~~~~~~~~~~~
1992 *IP*. The _Internet Protocol_ is the primary networking protocol
1993 used for the Internet. All protocols described below use IP as the
1994 underlying layer. Both the prevalent IPv4 and the next-generation
1995 IPv6 variant are being deployed actively worldwide.
1997 *Connection-oriented and connectionless protocols*. Connectionless
1998 protocols differ from connection-oriented ones in that state
1999 associated with the sending/receiving endpoints is treated
2000 implicitly. Connectionless protocols maintain no internal knowledge
2001 about the state of the connection. Hence they are not capable of
2002 reacting to state changes, such as sudden loss or congestion on the
2003 connection medium. Connection-oriented protocols, in contrast, make
2004 this knowledge explicit. The connection is established only after
2005 a bidirectional handshake which requires both endpoints to agree
2006 on the state of the connection, and may also involve negotiating
2007 specific parameters for the particular connection. Maintaining an
2008 up-to-date internal state of the connection also in general means
2009 that the sending endpoints perform congestion control, adapting to
2010 qualitative changes of the connection medium.
2012 *Reliability*. In IP networking, packets can be lost, duplicated,
2013 or delivered out of order, and different network protocols handle
2014 these problems in different ways. We call a transport-layer protocol
2015 _reliable_, if it turns the unreliable IP delivery into an ordered,
2016 duplicate- and loss-free delivery of packets. Sequence numbers
2017 are used to discard duplicates and re-arrange packets delivered
2018 out-of-order. Retransmission is used to guarantee loss-free
2019 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2020 or data integrity.
2022 *Classification*. With these definitions the protocols which are used
2023 by paraslash for steaming audio data may be classified as follows.
2025 - HTTP/TCP: connection-oriented, reliable,
2026 - UDP: connectionless, unreliable,
2027 - DCCP: connection-oriented, unreliable.
2029 Below we give a short descriptions of these protocols.
2031 *TCP*. The _Transmission Control Protocol_ provides reliable,
2032 ordered delivery of a stream and a classic window-based congestion
2033 control. In contrast to UDP and DCCP (see below), TCP does not have
2034 record-oriented or datagram-based syntax, i.e. it provides a stream
2035 which is unaware and independent of any record (packet) boundaries.
2036 TCP is used extensively by many application layers. Besides HTTP (the
2037 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2038 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2039 top of TCP.
2041 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2042 protocol, built as a thin layer directly on top of IP. For this reason,
2043 it offers the same best-effort service as IP itself, i.e. there is no
2044 detection of duplicate or reordered packets. Being a connectionless
2045 protocol, only minimal internal state about the connection is
2046 maintained, which means that there is no protection against packet
2047 loss or network congestion. Error checking and correction (if at all)
2048 are performed in the application.
2050 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2051 connection-oriented state maintenance known from TCP with the
2052 unreliable, datagram-based transport of UDP. This means that it
2053 is capable of reacting to changes in the connection by performing
2054 congestion control, offering multiple alternative approaches. But it
2055 is bound to datagram boundaries (the maximum packet size supported
2056 by a medium), and like UDP it lacks retransmission to protect
2057 against loss. Due to the use of sequence numbers, it is however
2058 able to react to loss (interpreted as a congestion indication) and
2059 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2060 to negotiate specific, binding features for a connection, such as
2061 the choice of congestion control: classic, window-based congestion
2062 control known from TCP is available as CCID-2, rate-based, "smooth"
2063 congestion control is offered as CCID-3.
2065 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2066 protocol on top of TCP. It is spoken by web servers and is most often
2067 used for web services. However, as can be seen by the many Internet
2068 radio stations and YouTube/Flash videos, http is by far not limited to
2069 the delivery of web pages only. Being a simple request/response based
2070 protocol, the semantics of the protocol also allow the delivery of
2071 multimedia content, such as audio over http.
2073 *Multicast*. IP multicast is not really a protocol but a technique
2074 for one-to-many communication over an IP network. The challenge is to
2075 deliver information to a group of destinations simultaneously using
2076 the most efficient strategy to send the messages over each link of
2077 the network only once. This has benefits for streaming multimedia:
2078 the standard one-to-one unicast offered by TCP/DCCP means that
2079 n clients listening to the same stream also consume n-times the
2080 resources, whereas multicast requires to send the stream just once,
2081 irrespective of the number of receivers. Since it would be costly to
2082 maintain state for each listening receiver, multicast often implies
2083 connectionless transport, which is the reason that it is currently
2084 only available via UDP.
2086 License
2087 ~~~~~~~
2089 Paraslash is licensed under the GPL, version 2. Most of the code
2090 base has been written from scratch, and those parts are GPL V2
2091 throughout. Notable exceptions are FEC and the WMA decoder. See the
2092 corresponding source files for licencing details for these parts. Some
2093 code sniplets of several other third party software packages have
2094 been incorporated into the paraslash sources, for example log message
2095 coloring was taken from the git sources. These third party software
2096 packages are all published under the GPL or some other license
2097 compatible to the GPL.
2099 Acknowledgements
2100 ~~~~~~~~~~~~~~~~
2102 Many thanks to Gerrit Renker who read an early draft of this manual
2103 and contributed significant improvements.
2105 ----------
2106 References
2107 ----------
2109 Articles
2110 ~~~~~~~~
2111 - Reed, Irving S.; Solomon, Gustave (1960),
2113 Polynomial Codes over Certain Finite Fields), Journal of the
2114 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2115 300-304, doi:10.1137/0108018)
2117 RFCs
2118 ~~~~
2120 - XREFERENCE(, RFC 768) (1980):
2121 User Datagram Protocol
2122 - XREFERENCE(, RFC 791) (1981):
2123 Internet Protocol
2124 - XREFERENCE(, RFC 2437) (1998):
2125 RSA Cryptography Specifications
2126 - XREFERENCE(, RFC 4340)
2127 (2006): Datagram Congestion Control Protocol (DCCP)
2128 - XREFERENCE(, RFC 4341) (2006):
2129 Congestion Control ID 2: TCP-like Congestion Control
2130 - XREFERENCE(, RFC 4342) (2006):
2131 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2133 Application web pages
2134 ~~~~~~~~~~~~~~~~~~~~~
2136 - XREFERENCE(, paraslash)
2137 - XREFERENCE(, xmms)
2138 - XREFERENCE(, mpg123)
2139 - XREFERENCE(, gstreamer)
2140 - XREFERENCE(, icecast)
2141 - XREFERENCE(, Audio Compress)
2143 External documentation
2144 ~~~~~~~~~~~~~~~~~~~~~~
2147 H. Peter Anvin: The mathematics of Raid6)
2149 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2150 Communication Protocols)
2152 Code
2153 ~~~~
2155 Original FEC implementation by Luigi Rizzo)