server: Add mood methods image_id and lyrics_id.
[paraslash.git] / web / manual.m4
1 dnl To generate the html version, execute
2 dnl m4 web/manual.m4 | grutatxt --toc
4 define(`LOCAL_LINK_NAME', `translit(`$1', `A-Z
5 ', `a-z__')')
6 define(`REMOVE_NEWLINE', `translit(`$1',`
7 ', ` ')')
9 define(`REFERENCE', ./``#''`LOCAL_LINK_NAME($1)' (`REMOVE_NEWLINE($2)'))
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, ...) over local and/or
81 remote networks. It listens on a TCP port and accepts commands such
82 as play, stop, pause, next from authenticated clients. There are
83 many more commands though, see the man page of para_server for a
84 description of all commands.
86 It supports three built-in network streaming protocols
87 (senders/receivers): HTTP, DCCP, or UDP. This is explained in more
88 detail in the section on REFERENCE(Networking, networking).
90 The built-in audio file selector of paraslash is used to manage your
91 audio files. It maintains statistics on the usage of all available
92 audio files such as last-played time, and the number of times each
93 file was selected.
95 Additional information may be added to the database to allow
96 fine-grained selection based on various properties of the audio file,
97 including information found in (ID3) tags. However, old-fashioned
98 playlists are also supported.
100 It is also possible to store images (album covers) and lyrics in the
101 database and associate these to the corresponding audio files.
103 The section on the REFERENCE(The audio file selector, audio file
104 selector) discusses this topic.
107 *para_client*
109 The client program to connect to para_server. paraslash commands
110 are sent to para_server and the response is dumped to STDOUT. This
111 can be used by any scripting language to produce user interfaces with
112 little programming effort.
114 All connections between para_server and para_client are encrypted
115 with a symmetric session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
118 If para_client is started without non-option arguments, an interactive
119 session (shell) is started. Command history and command completion are
120 supported through libreadline.
122 *para_audiod*
124 The local daemon that collects information from para_server.
126 It runs on the client side and connects to para_server. As soon as
127 para_server announces the availability of an audio stream, para_audiod
128 starts an appropriate receiver, any number of filters and a paraslash
129 writer to play the stream.
131 Moreover, para_audiod listens on a local socket and sends status
132 information about para_server and para_audiod to local clients on
133 request. Access via this local socket may be restricted by using Unix
134 socket credentials, if available.
137 *para_audioc*
139 The client program which talks to para_audiod. Used to control
140 para_audiod, to receive status info, or to grab the stream at any
141 point of the decoding process. Like para_client, para_audioc supports
142 interactive sessions on systems with libreadline.
144 *para_recv*
146 A command line HTTP/DCCP/UDP stream grabber. The http mode is
147 compatible with arbitrary HTTP streaming sources (e.g. icecast).
148 In addition to the three network streaming modes, para_recv can also
149 operate in local (afh) mode. In this mode it writes the content of
150 an audio file on the local file system in complete chunks to stdout,
151 optionally 'just in time'. This allows to cut an audio file without
152 first decoding it, and it enables third-party software which is unaware
153 of the particular audio format to send complete frames in real time.
155 *para_filter*
157 A filter program that reads from STDIN and writes to STDOUT.
158 Like para_recv, this is an atomic building block which can be used to
159 assemble higher-level audio receiving facilities. It combines several
160 different functionalities in one tool: decoders for multiple audio
161 formats and a number of processing filters, among these a normalizer
162 for audio volume.
164 *para_afh*
166 A small stand-alone program that prints tech info about the given
167 audio file to STDOUT. It can be instructed to print a "chunk table",
168 an array of offsets within the audio file.
170 *para_write*
172 A modular audio stream writer. It supports a simple file writer
173 output plug-in and optional WAV/raw players for ALSA (Linux) and for
174 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
175 or raw audio player.
177 *para_play*
179 A command line audio player.
181 *para_gui*
183 Curses-based gui that presents status information obtained in a curses
184 window. Appearance can be customized via themes. para_gui provides
185 key-bindings for the most common server commands and new key-bindings
186 can be added easily.
189 *para_fade*
191 An alarm clock and volume-fader for OSS and ALSA.
193 -----------
194 Quick start
195 -----------
197 This chapter lists the REFERENCE(Requirements, necessary software)
198 that must be installed to compile the paraslash package, describes
199 how to REFERENCE(Installation, compile and install) the paraslash
200 source code and the steps that have to be performed in order to
201 REFERENCE(Quick start, set up) a typical server and client.
203 Requirements
204 ~~~~~~~~~~~~
205 For the impatient:
207 git clone git://
208 cd osl && make && sudo make install && sudo ldconfig
209 sudo apt-get install autoconf libssl-dev help2man gengetopt \
210 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
211 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev \
212 libasound2-dev libao-dev libreadline-dev libncurses-dev \
213 libopus-dev
215 Detailed description: In any case you'll need
217 - XREFERENCE(, libosl).
218 The _object storage layer_ library is used by para_server. To
219 clone the source code repository, execute
221 git clone git://
223 - XREFERENCE(, gcc) or
224 XREFERENCE(, clang). All gcc versions
225 >= 3.3 are currently supported. Clang version 1.1 or newer
226 should work as well.
228 - XREFERENCE(, gnu make) is
229 also shipped with the disto. On BSD systems the gnu make
230 executable is often called gmake.
232 - XREFERENCE(, bash). Some
233 scripts which run during compilation require the EMPH(Bourne
234 again shell). It is most likely already installed.
236 - XREFERENCE(, gengetopt)
237 is needed to generate the C code for the command line parsers
238 of all paraslash executables.
240 - XREFERENCE(, help2man)
241 is used to create the man pages.
243 Optional:
245 - XREFERENCE(, openssl) or
246 XREFERENCE(, libgcrypt).
247 At least one of these two libraries is needed as the backend
248 for cryptographic routines on both the server and the client
249 side. Both openssl and libgcrypt are usually shipped with the
250 distro, but you might have to install the development package
251 (libssl-dev or libgcrypt-dev on debian systems) as well.
253 - XREFERENCE(, libmad).
254 To compile in MP3 support for paraslash, the development
255 package must be installed. It is called libmad0-dev on
256 debian-based systems. Note that libmad is not necessary on
257 the server side, i.e. for sending MP3 files.
260 libid3tag). For version-2 ID3 tag support, you'll need
261 the libid3tag development package libid3tag0-dev. Without
262 libid3tag, only version-1 tags are recognized. The mp3 tagger
263 also needs this library for modifying (id3v1 and id3v2) tags.
265 - XREFERENCE(, ogg vorbis).
266 For ogg vorbis streams you'll need libogg, libvorbis,
267 libvorbisfile. The corresponding Debian packages are called
268 libogg-dev and libvorbis-dev.
270 - XREFERENCE(, libfaad). For aac
271 files (m4a) you'll need libfaad (libfaad-dev).
273 - XREFERENCE(, speex). In order to stream
274 or decode speex files, libspeex (libspeex-dev) is required.
276 - XREFERENCE(, flac). To stream
277 or decode files encoded with the _Free Lossless Audio Codec_,
278 libFLAC (libFLAC-dev) must be installed.
281 libsamplerate). The resample filter will only be compiled if
282 this library is installed. Debian package: libsamplerate-dev.
284 - XREFERENCE(, alsa-lib). On
285 Linux, you'll need to have ALSA's development package
286 libasound2-dev installed.
289 libao). Needed to build the ao writer (ESD, PulseAudio,...).
290 Debian package: libao-dev.
292 - XREFERENCE(, curses). Needed
293 for para_gui. Debian package: libncurses-dev.
296 GNU Readline). If this library (libreadline-dev) is installed,
297 para_client, para_audioc and para_play support interactive
298 sessions.
300 Installation
301 ~~~~~~~~~~~~
302 To build the sources from a tarball, execute
304 ./configure && make
306 To build from git or a gitweb snapshot, run this command instead:
308 ./
310 There should be no errors but probably some warnings about missing
311 packages which usually implies that not all audio formats will be
312 supported. If headers or libs are installed at unusual locations you
313 might need to tell the configure script where to find them. Try
315 ./configure --help
317 to see a list of options. If the paraslash package was compiled
318 successfully, execute (optionally)
320 make test
322 to run the paraslash test suite. If all tests pass, execute as root
324 make install
326 to install executables under /usr/local/bin and the man pages under
327 /usr/local/man.
329 Configuration
330 ~~~~~~~~~~~~~
332 *Step 1*: Create a paraslash user
334 In order to control para_server at runtime you must create a paraslash
335 user. As authentication is based on the RSA crypto system you'll have
336 to create an RSA key pair. If you already have a user and an RSA key
337 pair, you may skip this step.
339 In this section we'll assume a typical setup: You would like to run
340 para_server on some host called server_host as user foo, and you want
341 to connect to para_server from another machine called client_host as
342 user bar.
344 As foo@server_host, create ~/.paraslash/server.users by typing the
345 following commands:
347 user=bar
348 target=~/.paraslash/server.users
349 key=~/.paraslash/$user
351 mkdir -p ~/.paraslash
352 echo "user $user $key $perms" >> $target
354 Next, change to the "bar" account on client_host and generate the
355 key pair with the commands
357 ssh-keygen -q -t rsa -b 2048 -N '' -f $key
359 This generates the two files id_rsa and in ~/.ssh. Note
360 that para_server won't accept keys shorter than 2048 bits. Moreover,
361 para_client rejects private keys which are world-readable.
363 para_server only needs to know the public key of the key pair just
364 created. Copy this public key to server_host:
366 src=~/.ssh/
367 dest=.paraslash/$LOGNAME
368 scp $src foo@server_host:$dest
370 Finally, tell para_client to connect to server_host:
372 conf=~/.paraslash/client.conf
373 echo 'hostname server_host' > $conf
376 *Step 2*: Start para_server
378 For this first try, we'll use the info loglevel to make the output
379 of para_server more verbose.
381 para_server -l info
383 Now you can use para_client to connect to the server and issue
384 commands. Open a new shell as bar@client_host and try
386 para_client help
387 para_client si
389 to retrieve the list of available commands and some server info.
390 Don't proceed if this doesn't work.
392 *Step 3*: Create and populate the database
394 An empty database is created with
396 para_client init
398 This initializes a couple of empty tables under
399 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
400 tables, but it's good to know that you can start from scratch with
402 rm -rf ~/.paraslash/afs_database-0.4
404 in case something went wrong.
406 Next, you need to add some audio files to that database so that
407 para_server knows about them. Choose an absolute path to a directory
408 containing some audio files and add them to the audio file table:
410 para_client add /my/mp3/dir
412 This might take a while, so it is a good idea to start with a directory
413 containing not too many files. Note that the table only contains data
414 about the audio files found, not the files themselves.
416 You may print the list of all known audio files with
418 para_client ls
420 *Step 4*: Configure para_audiod
422 We will have to tell para_audiod that it should receive the audio
423 stream from server_host via http:
425 para_audiod -l info -r '.:http -i server_host'
427 You should now be able to listen to the audio stream once para_server
428 starts streaming. To activate streaming, execute
430 para_client play
432 Since no playlist has been specified yet, the "dummy" mode which
433 selects all known audio files is activated automatically. See the
434 section on the REFERENCE(The audio file selector, audio file selector)
435 for how to use playlists and moods to specify which files should be
436 streamed in which order.
438 *Troubleshooting*
440 If you receive a socket related error on server or audiod startup,
441 make sure you have write permissions to the /var/paraslash directory:
443 sudo chown $LOGNAME /var/paraslash
445 Alternatively, use the --afs-socket (para_server) or --socket
446 (para_audiod) option to specify a different socket pathname.
448 To identify streaming problems try to receive, decode and play the
449 stream manually using para_recv, para_filter and para_write as follows.
450 For simplicity we assume that you're running Linux/ALSA and that only
451 MP3 files have been added to the database.
453 para_recv -r 'http -i server_host' > file.mp3
454 # (interrupt with CTRL+C after a few seconds)
455 ls -l file.mp3 # should not be empty
456 para_filter -f mp3dec -f wav < file.mp3 > file.wav
457 ls -l file.wav # should be much bigger than file.mp3
458 para_write -w alsa < file.wav
460 Double check what is logged by para_server and use the --loglevel
461 option of para_recv, para_filter and para_write to increase verbosity.
463 ---------------
464 User management
465 ---------------
467 para_server uses a challenge-response mechanism to authenticate
468 requests from incoming connections, similar to ssh's public key
469 authentication method. Authenticated connections are encrypted using
470 a stream cipher, either RC4 or AES in integer counter mode.
472 In this chapter we briefly describe RSA, RC4 and AES, and sketch the
473 REFERENCE(Client-server authentication, authentication handshake)
474 between para_client and para_server. User management is discussed
475 in the section on REFERENCE(The user_list file, the user_list file).
476 These sections are all about communication between the client and the
477 server. Connecting para_audiod is a different matter and is described
478 in a REFERENCE(Connecting para_audiod, separate section).
482 RSA, RC4, AES
483 ~~~~~~~~~~~~~
485 RSA is an asymmetric block cipher which is used in many applications,
486 including ssh and gpg. An RSA key consists in fact of two keys,
487 called the public key and the private key. A message can be encrypted
488 with either key and only the counterpart of that key can decrypt
489 the message. While RSA can be used for both signing and encrypting
490 a message, paraslash uses RSA only for the latter purpose. The
491 RSA public key encryption and signatures algorithms are defined in
492 detail in RFC 2437.
494 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
495 key stream to produce the output. Decryption uses the same function
496 calls as encryption. While RC4 supports variable key lengths,
497 paraslash uses a fixed length of 256 bits, which is considered a
498 strong encryption by today's standards. Since the same key must never
499 be used twice, a different, randomly-generated key is used for every
500 new connection.
502 AES, the advanced encryption standard, is a well-known symmetric block
503 cipher, i.e. a transformation operating on fixed-length blocks which
504 is determined by a single key for both encryption and decryption. Any
505 block cipher can be turned into a stream cipher by generating
506 a pseudo-random key stream by encrypting successive values of a
507 counter. The AES_CTR128 stream cipher used in paraslash is obtained
508 in this way from the AES block cipher with a 128 bit block size.
511 Client-server authentication
512 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
514 The authentication handshake between para_client and para_server goes
515 as follows:
517 - para_client connects to para_server and sends an
518 authentication request for a user. It does so by connecting
519 to TCP port 2990 of the server host. This port is called the
520 para_server _control port_.
522 - para_server accepts the connection and forks a child process
523 which handles the incoming request. The parent process keeps
524 listening on the control port while the child process (also
525 called para_server below) continues as follows.
527 - para_server loads the RSA public key of that user, fills a
528 fixed-length buffer with random bytes, encrypts that buffer
529 using the public key and sends the encrypted buffer to the
530 client. The first part of the buffer is the challenge which
531 is used for authentication while the second part is the
532 session key.
534 - para_client receives the encrypted buffer and decrypts it
535 with the user's private key, thereby obtaining the challenge
536 buffer and the session key. It sends the SHA1 hash value of
537 the challenge back to para_server and stores the session key
538 for further use.
540 - para_server also computes the SHA1 hash of the challenge
541 and compares it against what was sent back by the client.
543 - If the two hashes do not match, the authentication has
544 failed and para_server closes the connection.
546 - Otherwise the user is considered authenticated and the client
547 is allowed to proceed by sending a command to be executed. From
548 this point on the communication is encrypted using the stream
549 cipher with the session key known to both peers.
551 paraslash relies on the quality of the pseudo-random bytes provided
552 by the crypto library (openssl or libgcrypt), on the security of
553 the implementation of the RSA, RC4 and AES crypto routines and on the
554 infeasibility to invert the SHA1 function.
556 Neither para_server or para_client create RSA keys on their own. This
557 has to be done once for each user as sketched in REFERENCE(Quick start,
558 Quick start) and discussed in more detail REFERENCE(The user_list
559 file, below).
561 The user_list file
562 ~~~~~~~~~~~~~~~~~~
564 At startup para_server reads the user list file which contains one
565 line per user. The default location of the user list file may be
566 changed with the --user-list option.
568 There should be at least one user in this file. Each user must have
569 an RSA key pair. The public part of the key is needed by para_server
570 while the private key is needed by para_client. Each line of the
571 user list file must be of the form
573 user <username> <key> <perms>
575 where _username_ is an arbitrary string (usually the user's login
576 name), _key_ is the full path to that user's public RSA key, and
577 _perms_ is a comma-separated list of zero or more of the following
578 permission bits:
580 +---------------------------------------------------------+
581 | AFS_READ | read the contents of the databases |
582 +-----------+---------------------------------------------+
583 | AFS_WRITE | change database contents |
584 +-----------+---------------------------------------------+
585 | VSS_READ | obtain information about the current stream |
586 +-----------+---------------------------------------------+
587 | VSS_WRITE | change the current stream |
588 +---------------------------------------------------------+
590 The permission bits specify which commands the user is allowed to
591 execute. The output of
593 para_client help
595 contains in the third column the permissions needed to execute the
596 command.
598 It is possible to make para_server reread the user_list file by
599 executing the paraslash "hup" command or by sending SIGHUP to the
600 PID of para_server.
603 Connecting para_audiod
604 ~~~~~~~~~~~~~~~~~~~~~~
606 para_audiod listens on a Unix domain socket. Those sockets are
607 for local communication only, so only local users can connect to
608 para_audiod. The default is to let any user connect but this can be
609 restricted on platforms that support UNIX socket credentials which
610 allow para_audiod to obtain the Unix credentials of the connecting
611 process.
613 Use para_audiod's --user-allow option to allow connections only for
614 a limited set of users.
616 -----------------------
617 The audio file selector
618 -----------------------
620 paraslash comes with a sophisticated audio file selector (AFS),
621 whose main task is to determine which file to stream next, based on
622 information on the audio files stored in a database. It communicates
623 also with para_client whenever an AFS command is executed, for example
624 to answer a database query.
626 Besides the traditional playlists, AFS supports audio file selection
627 based on _moods_ which act as a filter that limits the set of all
628 known audio files to those which satisfy certain criteria. It also
629 maintains tables containing images (e.g. album cover art) and lyrics
630 that can be associated with one or more audio files.
632 AFS uses XREFERENCE(, libosl), the
633 object storage layer library, as the backend library for storing
634 information on audio files, playlists, etc. This library offers
635 functionality similar to a relational database, but is much more
636 lightweight than a full database backend.
638 In this chapter we sketch the setup of the REFERENCE(The AFS process,
639 AFS process) during server startup and proceed with the description
640 of the REFERENCE(Database layout, layout) of the various database
641 tables. The section on REFERENCE(Playlists and moods, playlists
642 and moods) explains these two audio file selection mechanisms
643 in detail and contains pratical examples. The way REFERENCE(File
644 renames and content changes, file renames and content changes) are
645 detected is discussed briefly before the REFERENCE(Troubleshooting,
646 Troubleshooting) section concludes the chapter.
648 The AFS process
649 ~~~~~~~~~~~~~~~
651 On startup, para_server forks to create the AFS process which opens
652 the OSL database tables. The server process communicates with the
653 AFS process via pipes and shared memory. Usually, the AFS process
654 awakes only briefly whenever the current audio file changes. The AFS
655 process determines the next audio file, opens it, verifies it has
656 not been changed since it was added to the database and passes the
657 open file descriptor to the server process, along with audio file
658 meta-data such as file name, duration, audio format and so on. The
659 server process then starts to stream the audio file.
661 The AFS process also accepts connections from local clients via
662 a well-known socket. However, only child processes of para_server
663 may connect through this socket. All server commands that have the
664 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
665 change the database.
667 Database layout
668 ~~~~~~~~~~~~~~~
670 *The audio file table*
672 This is the most important and usually also the largest table of the
673 AFS database. It contains the information needed to stream each audio
674 file. In particular the following data is stored for each audio file.
676 - SHA1 hash value of the audio file contents. This is computed
677 once when the file is added to the database. Whenever AFS
678 selects this audio file for streaming the hash value is
679 recomputed and checked against the value stored in the
680 database to detect content changes.
682 - The time when this audio file was last played.
684 - The number of times the file has been played so far.
686 - The attribute bitmask.
688 - The image id which describes the image associated with this
689 audio file.
691 - The lyrics id which describes the lyrics associated with
692 this audio file.
694 - The audio format id (MP3, OGG, ...).
696 - An amplification value that can be used by the amplification
697 filter to pre-amplify the decoded audio stream.
699 - The chunk table. It describes the location and the timing
700 of the building blocks of the audio file. This is used by
701 para_server to send chunks of the file at appropriate times.
703 - The duration of the audio file.
705 - Tag information contained in the audio file (ID3 tags,
706 Vorbis comments, ...).
708 - The number of channels
710 - The encoding bitrate.
712 - The sampling frequency.
714 To add or refresh the data contained in the audio file table, the _add_
715 command is used. It takes the full path of either an audio file or a
716 directory. In the latter case, the directory is traversed recursively
717 and all files which are recognized as valid audio files are added to
718 the database.
720 *The attribute table*
722 The attribute table contains two columns, _name_ and _bitnum_. An
723 attribute is simply a name for a certain bit number in the attribute
724 bitmask of the audio file table.
726 Each of the 64 bits of the attribute bitmask can be set for each
727 audio file individually. Hence up to 64 different attributes may be
728 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
729 "german_lyrics", "speech", whatever. You are free to choose as
730 many attributes as you like and there are no naming restrictions
731 for attributes.
733 A new attribute "test" is created by
735 para_client addatt test
736 and
737 para_client lsatt
739 lists all available attributes. You can set the "test" attribute for
740 an audio file by executing
742 para_client setatt test+ /path/to/the/audio/file
744 Similarly, the "test" bit can be removed from an audio file with
746 para_client setatt test- /path/to/the/audio/file
748 Instead of a path you may use a shell wildcard pattern. The attribute
749 is applied to all audio files matching this pattern:
751 para_client setatt test+ '/test/directory/*'
753 The command
755 para_client -- ls -l=v
757 gives you a verbose listing of your audio files also showing which
758 attributes are set.
760 In case you wonder why the double-dash in the above command is needed:
761 It tells para_client to not interpret the options after the dashes. If
762 you find this annoying, just say
764 alias para='para_client --'
766 and be happy. In what follows we shall use this alias.
768 The "test" attribute can be dropped from the database with
770 para rmatt test
772 Read the output of
774 para help ls
775 para help setatt
777 for more information and a complete list of command line options to
778 these commands.
780 *Blob tables*
782 The image, lyrics, moods and playlists tables are all blob tables.
783 Blob tables consist of three columns each: The identifier which is
784 a positive non-negative number that is auto-incremented, the name
785 (an arbitrary string) and the content (the blob).
787 All blob tables support the same set of actions: cat, ls, mv, rm
788 and add. Of course, _add_ is used for adding new blobs to the table
789 while the other actions have the same meaning as the corresponding
790 Unix commands. The paraslash commands to perform these actions are
791 constructed as the concatenation of the table name and the action. For
792 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
793 manipulate or query the image table.
795 The add variant of these commands is special as these commands read
796 the blob contents from stdin. To add an image to the image table the
797 command
799 para addimg image_name < file.jpg
801 can be used.
803 Note that the images and lyrics are not interpreted at all, and also
804 the playlist and the mood blobs are only investigated when the mood
805 or playlist is activated with the select command.
807 *The score table*
809 Unlike all other tables the contents of the score table remain in
810 memory and are never stored on disk. The score table contains two
811 columns: The SHA1 hash value (of an audio file) and its current
812 score.
814 However, only those files which are admissible for the current mood
815 or playlist are contained in the score table. The audio file selector
816 always chooses the row with the highest score as the file to stream
817 next. While doing so, it computes the new score and updates the
818 last_played and the num_played fields in the audio file table.
820 The score table is recomputed by the select command which loads a
821 mood or playlist. Audio files are chosen for streaming from the rows
822 of the score table on a highest-score-first basis.
825 Playlists and moods
826 ~~~~~~~~~~~~~~~~~~~
828 Playlists and moods offer two different ways of specifying the set of
829 admissible files. A playlist in itself describes a set of admissible
830 files. A mood, in contrast, describes the set of admissible files in
831 terms of attributes and other type of information available in the
832 audio file table. As an example, a mood can define a filename pattern,
833 which is then matched against the names of audio files in the table.
835 *Playlists*
837 Playlists are accommodated in the playlist table of the afs database,
838 using the aforementioned blob format for tables. A new playlist is
839 created with the addpl command by specifying the full (absolute)
840 paths of all desired audio files, separated by newlines. Example:
842 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
844 If _my_playlist_ already exists it is overwritten. To activate the
845 new playlist, execute
847 para select p/my_playlist
849 The audio file selector will assign scores to each entry of the list,
850 in descending order so that files will be selected in order. If a
851 file could not be opened for streaming, its entry is removed from
852 the score table (but not from the playlist).
854 *Moods*
856 A mood consists of a unique name and its *mood definition*, which is
857 a set of *mood lines* containing expressions in terms of attributes
858 and other data contained in the database.
860 At any time at most one mood can be *active* which means that
861 para_server is going to select only files from that subset of
862 admissible files.
864 So in order to create a mood definition one has to write a set of
865 mood lines. Mood lines come in three flavours: Accept lines, deny
866 lines and score lines.
868 The general syntax of the three types of mood lines is
871 accept [with score <score>] [if] [not] <mood_method> [options]
872 deny [with score <score>] [if] [not] <mood_method> [options]
873 score <score> [if] [not] <mood_method> [options]
876 Here <score> is either an integer or the string "random" which assigns
877 a random score to all matching files. The score value changes the
878 order in which admissible files are going to be selected, but is of
879 minor importance for this introduction.
881 So we concentrate on the first two forms, i.e. accept and deny
882 lines. As usual, everything in square brackets is optional, i.e.
883 accept/deny lines take the following form when ignoring scores:
885 accept [if] [not] <mood_method> [options]
887 and analogously for the deny case. The "if" keyword is only syntactic
888 sugar and has no function. The "not" keyword just inverts the result,
889 so the essence of a mood line is the mood method part and the options
890 following thereafter.
892 A *mood method* is realized as a function which takes an audio file
893 and computes a number from the data contained in the database.
894 If this number is non-negative, we say the file *matches* the mood
895 method. The file matches the full mood line if it either
897 - matches the mood method and the "not" keyword is not given,
898 or
899 - does not match the mood method, but the "not" keyword is given.
901 The set of admissible files for the whole mood is now defined as those
902 files which match at least one accept mood line, but no deny mood line.
903 More formally, an audio file F is admissible if and only if
905 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
907 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
908 lines and "~" means "matches".
910 The cases where no mood lines of accept/deny type are defined need
911 special treatment:
913 - Neither accept nor deny lines: This treats all files as
914 admissible (in fact, that is the definition of the dummy mood
915 which is activated automatically if no moods are available).
917 - Only accept lines: A file is admissible iff it matches at
918 least one accept line:
920 F ~ AL1 or F ~ AL2 or ...
922 - Only deny lines: A file is admissible iff it matches no
923 deny line:
925 not (F ~ DL1 or F ~ DN2 ...)
929 *List of mood_methods*
931 no_attributes_set
933 Takes no arguments and matches an audio file if and only if no
934 attributes are set.
936 is_set <attribute_name>
938 Takes the name of an attribute and matches iff that attribute is set.
940 path_matches <pattern>
942 Takes a filename pattern and matches iff the path of the audio file
943 matches the pattern.
945 artist_matches <pattern>
946 album_matches <pattern>
947 title_matches <pattern>
948 comment_matches <pattern>
950 Takes an extended regular expression and matches iff the text of the
951 corresponding tag of the audio file matches the pattern. If the tag
952 is not set, the empty string is matched against the pattern.
954 year ~ <num>
955 bitrate ~ <num>
956 frequency ~ <num>
957 channels ~ <num>
958 num_played ~ <num>
959 image_id ~ <num>
960 lyrics_id ~ <num>
962 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
963 <num>. Matches an audio file iff the condition <val> ~ <num> is
964 satisfied where val is the corresponding value of the audio file
965 (value of the year tag, bitrate in kbit/s, etc.).
967 The year tag is special as its value is undefined if the audio file
968 has no year tag or the content of the year tag is not a number. Such
969 audio files never match. Another difference is the special treatment
970 if the year tag is a two-digit number. In this case either 1900 or
971 2000 is added to the tag value, depending on whether the number is
972 greater than 2000 plus the current year.
975 *Mood usage*
977 To create a new mood called "my_mood", write its definition into
978 some temporary file, say "tmpfile", and add it to the mood table
979 by executing
981 para addmood my_mood < tmpfile
983 If the mood definition is really short, you may just pipe it to the
984 client instead of using temporary files. Like this:
986 echo "$MOOD_DEFINITION" | para addmood my_mood
988 There is no need to keep the temporary file since you can always use
989 the catmood command to get it back:
991 para catmood my_mood
993 A mood can be activated by executing
995 para select m/my_mood
997 Once active, the list of admissible files is shown by the ls command
998 if the "-a" switch is given:
1000 para ls -a
1003 *Example mood definition*
1005 Suppose you have defined attributes "punk" and "rock" and want to define
1006 a mood containing only Punk-Rock songs. That is, an audio file should be
1007 admissible if and only if both attributes are set. Since
1009 punk and rock
1011 is obviously the same as
1013 not (not punk or not rock)
1015 (de Morgan's rule), a mood definition that selects only Punk-Rock
1016 songs is
1018 deny if not is_set punk
1019 deny if not is_set rock
1023 File renames and content changes
1024 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1026 Since the audio file selector knows the SHA1 of each audio file that
1027 has been added to the afs database, it recognizes if the content of
1028 a file has changed, e.g. because an ID3 tag was added or modified.
1029 Also, if a file has been renamed or moved to a different location,
1030 afs will detect that an entry with the same hash value already exists
1031 in the audio file table.
1033 In both cases it is enough to just re-add the new file. In the
1034 first case (file content changed), the audio table is updated, while
1035 metadata such as the num_played and last_played fields, as well as
1036 the attributes, remain unchanged. In the other case, when the file
1037 is moved or renamed, only the path information is updated, all other
1038 data remains as before.
1040 It is possible to change the behaviour of the add command by using the
1041 "-l" (lazy add) or the "-f" (force add) option.
1043 Troubleshooting
1044 ~~~~~~~~~~~~~~~
1046 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1047 executables have a brief online help which is displayed when -h is
1048 given. The --detailed-help option prints the full help text.
1050 If para_server crashed or was killed by SIGKILL (signal 9), it
1051 may refuse to start again because of "dirty osl tables". In this
1052 case you'll have to run the oslfsck program of libosl to fix your
1053 database:
1055 oslfsck -fd ~/.paraslash/afs_database-0.4
1057 However, make sure para_server isn't running before executing oslfsck.
1059 If you don't mind to recreate your database you can start
1060 from scratch by removing the entire database directory, i.e.
1062 rm -rf ~/.paraslash/afs_database-0.4
1064 Be aware that this removes all attribute definitions, all playlists
1065 and all mood definitions and requires to re-initialize the tables.
1067 Although oslfsck fixes inconsistencies in database tables it doesn't
1068 care about the table contents. To check for invalid table contents, use
1070 para_client check
1072 This prints out references to missing audio files as well as invalid
1073 playlists and mood definitions.
1075 Similarly, para_audiod refuses to start if its socket file exists, since
1076 this indicates that another instance of para_audiod is running. After
1077 a crash a stale socket file might remain and you must run
1079 para_audiod --force
1081 once to fix it up.
1083 ---------------------------------------
1084 Audio formats and audio format handlers
1085 ---------------------------------------
1087 Audio formats
1088 ~~~~~~~~~~~~~
1090 The following audio formats are supported by paraslash:
1092 *MP3*
1094 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1095 designed as part of its MPEG-1 standard. An MP3 file is made up of
1096 multiple MP3 frames, which consist of a header and a data block. The
1097 size of an MP3 frame depends on the bit rate and on the number
1098 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1099 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1100 400 bytes large.
1102 *OGG/Vorbis*
1104 OGG is a standardized audio container format, while Vorbis is an
1105 open source codec for lossy audio compression. Since Vorbis is most
1106 commonly made available via the OGG container format, it is often
1107 referred to as OGG/Vorbis. The OGG container format divides data into
1108 chunks called OGG pages. A typical OGG page is about 4KB large. The
1109 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1110 may vary considerably.
1112 *OGG/Speex*
1114 Speex is an open-source speech codec that is based on CELP (Code
1115 Excited Linear Prediction) coding. It is designed for voice
1116 over IP applications, has modest complexity and a small memory
1117 footprint. Wideband and narrowband (telephone quality) speech are
1118 supported. As for Vorbis audio, Speex bit-streams are often stored
1119 in OGG files. As of 2012 this codec is considered obsolete since the
1120 Oppus codec, described below, surpasses its performance in all areas.
1122 *OGG/Opus*
1124 Opus is a lossy audio compression format standardized through RFC
1125 6716 in 2012. It combines the speech-oriented SILK codec and the
1126 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1127 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1128 containers. All known software patents which cover Opus are licensed
1129 under royalty-free terms.
1131 *AAC*
1133 Advanced Audio Coding (AAC) is a standardized, lossy compression
1134 and encoding scheme for digital audio which is the default audio
1135 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1136 the container format and audio files encoded with AAC have the .m4a
1137 extension. A typical AAC frame is about 700 bytes large.
1139 *WMA*
1141 Windows Media Audio (WMA) is an audio data compression technology
1142 developed by Microsoft. A WMA file is usually encapsulated in the
1143 Advanced Systems Format (ASF) container format, which also specifies
1144 how meta data about the file is to be encoded. The bit stream of WMA
1145 is composed of superframes, each containing one or more frames of
1146 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1148 *FLAC*
1150 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1151 loss. It gives better compression ratios than a general purpose
1152 compressor like zip or bzip2 because FLAC is designed specifically
1153 for audio. A FLAC-encoded file consists of frames of varying size, up
1154 to 16K. Each frame starts with a header that contains all information
1155 necessary to decode the frame.
1157 Meta data
1158 ~~~~~~~~~
1160 Unfortunately, each audio format has its own conventions how meta
1161 data is added as tags to the audio file.
1163 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1164 is rather simple but also very limited as it supports only artist,
1165 title, album, year and comment tags. Each of these can only be at most
1166 32 characters long. ID3, version 2 is much more flexible but requires
1167 a separate library being installed for paraslash to support it.
1169 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1170 comments, which are typically implemented as strings of the form
1171 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1172 tags are appropriate for the content.
1174 AAC files usually use the MPEG-4 container format for storing meta
1175 data while WMA files wrap meta data as special objects within the
1176 ASF container format.
1178 paraslash only tracks the most common tags that are supported by
1179 all tag variants: artist, title, year, album, comment. When a file
1180 is added to the AFS database, the meta data of the file is extracted
1181 and stored in the audio file table.
1183 Chunks and chunk tables
1184 ~~~~~~~~~~~~~~~~~~~~~~~
1186 paraslash uses the word "chunk" as common term for the building blocks
1187 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1188 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1189 size varies considerably between audio formats, from a few hundred
1190 bytes (MP3) up to 16K (FLAC).
1192 The chunk table contains the offsets within the audio file that
1193 correspond to the chunk boundaries of the file. Like the meta data,
1194 the chunk table is computed and stored in the database whenever an
1195 audio file is added.
1197 The paraslash senders (see below) always send complete chunks. The
1198 granularity for seeking is therefore determined by the chunk size.
1200 Audio format handlers
1201 ~~~~~~~~~~~~~~~~~~~~~
1203 For each audio format paraslash contains an audio format handler whose
1204 first task is to tell whether a given file is a valid audio file of
1205 this type. If so, the audio file handler extracts some technical data
1206 (duration, sampling rate, number of channels etc.), computes the
1207 chunk table and reads the meta data.
1209 The audio format handler code is linked into para_server and executed
1210 via the _add_ command. The same code is also available as a stand-alone
1211 tool, para_afh, which prints the technical data, the chunk table
1212 and the meta data of a file. Moreover, all audio format handlers are
1213 combined in the afh receiver which is part of para_recv and para_play.
1215 ----------
1216 Networking
1217 ----------
1219 Paraslash uses different network connections for control and data.
1220 para_client communicates with para_server over a dedicated TCP control
1221 connection. To transport audio data, separate data connections are
1222 used. For these data connections, a variety of transports (UDP, DCCP,
1223 HTTP) can be chosen.
1225 The chapter starts with the REFERENCE(The paraslash control
1226 service, control service), followed by a section on the various
1227 REFERENCE(Streaming protocols, streaming protocols) in which the data
1228 connections are described. The way audio file headers are embedded into
1229 the stream is discussed REFERENCE(Streams with headers and headerless
1230 streams, briefly) before the REFERENCE(Networking examples, example
1231 section) which illustrates typical commands for real-life scenarios.
1233 Both IPv4 and IPv6 are supported.
1235 The paraslash control service
1236 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1238 para_server is controlled at runtime via the paraslash control
1239 connection. This connection is used for server commands (play, stop,
1240 ...) as well as for afs commands (ls, select, ...).
1242 The server listens on a TCP port and accepts connections from clients
1243 that connect the open port. Each connection causes the server to fork
1244 off a client process which inherits the connection and deals with that
1245 client only. In this classical accept/fork approach the server process
1246 is unaffected if the child dies or goes crazy for whatever reason. In
1247 fact, the child process can not change address space of server process.
1249 The section on REFERENCE(Client-server authentication, client-server
1250 authentication) above described the early connection establishment
1251 from the crypto point of view. Here it is described what happens
1252 after the connection (including crypto setup) has been established.
1253 There are four processes involved during command dispatch as sketched
1254 in the following diagram.
1256 <<
1257 <pre>
1258 server_host client_host
1259 ~~~~~~~~~~~ ~~~~~~~~~~~
1261 +-----------+ connect +-----------+
1262 |para_server|<------------------------------ |para_client|
1263 +-----------+ +-----------+
1264 | ^
1265 | fork +---+ |
1266 +----------> |AFS| |
1267 | +---+ |
1268 | ^ |
1269 | | |
1270 | | connect (cookie) |
1271 | | |
1272 | | |
1273 | fork +-----+ inherited connection |
1274 +---------->|child|<--------------------------+
1275 +-----+
1276 </pre>
1277 >>
1279 Note that the child process is not a child of the afs process,
1280 so communication of these two processes has to happen via local
1281 sockets. In order to avoid abuse of the local socket by unrelated
1282 processes, a magic cookie is created once at server startup time just
1283 before the server process forks off the AFS process. This cookie is
1284 known to the server, AFS and the child, but not to unrelated processes.
1286 There are two different kinds of commands: First there are commands
1287 that cause the server to respond with some answer such as the list
1288 of all audio files. All but the addblob commands (addimg, addlyr,
1289 addpl, addmood) are of this kind. The addblob commands add contents
1290 to the database, so they need to transfer data the other way round,
1291 from the client to the server.
1293 There is no knowledge about the server commands built into para_client,
1294 so it does not know about addblob commands. Instead, the server sends
1295 a special "awaiting data" packet for these commands. If the client
1296 receives this packet, it sends STDIN to the server, otherwise it
1297 dumps data from the server to STDOUT.
1299 Streaming protocols
1300 ~~~~~~~~~~~~~~~~~~~
1302 A network (audio) stream usually consists of one streaming source,
1303 the _sender_, and one or more _receivers_ which read data over the
1304 network from the streaming source.
1306 Senders are thus part of para_server while receivers are part of
1307 para_audiod. Moreover, there is the stand-alone tool para_recv which
1308 can be used to manually download a stream, either from para_server
1309 or from a web-based audio streaming service.
1311 The following three streaming protocols are supported by paraslash:
1313 - HTTP. Recommended for public streams that can be played by
1314 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1315 sender is supported on all operating systems and all platforms.
1317 - DCCP. Recommended for LAN streaming. DCCP is currently
1318 available only for Linux.
1320 - UDP. Recommended for multicast LAN streaming.
1322 See the Appendix on REFERENCE(Network protocols, network protocols)
1323 for brief descriptions of the various protocols relevant for network
1324 audio streaming with paraslash.
1326 It is possible to activate more than one sender simultaneously.
1327 Senders can be controlled at run time and via config file and command
1328 line options.
1330 Note that audio connections are _not_ encrypted. Transport or Internet
1331 layer encryption should be used if encrypted data connections are
1332 needed.
1334 Since DCCP and TCP are both connection-oriented protocols, connection
1335 establishment/teardown and access control are very similar between
1336 these two streaming protocols. UDP is the most lightweight option,
1337 since in contrast to TCP/DCCP it is connectionless. It is also the
1338 only protocol supporting IP multicast.
1340 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1341 clients to connect and establish a connection via some protocol-defined
1342 handshake mechanism. Both senders maintain two linked lists each:
1343 The list of all clients which are currently connected, and the list
1344 of access control entries which determines who is allowed to connect.
1345 IP-based access control may be configured through config file and
1346 command line options and via the "allow" and "deny" sender subcommands.
1348 Upon receiving a GET request from the client, the HTTP sender sends
1349 back a status line and a message. The body of this message is the
1350 audio stream. This is common practice and is supported by many popular
1351 clients which can thus be used to play a stream offered by para_server.
1352 For DCCP things are a bit simpler: No messages are exchanged between
1353 the receiver and sender. The client simply connects and the sender
1354 starts to stream.
1356 DCCP is an experimental protocol which offers a number of new features
1357 not available for TCP. Both ends can negotiate these features using
1358 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1359 datagram-based (no retransmissions) and thus should not be used over
1360 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1361 is access to a variety of different congestion-control mechanisms
1362 called CCIDs. Two different CCIDs are available per default on Linux:
1365 - _CCID 2_. A Congestion Control mechanism similar to that
1366 of TCP. The sender maintains a congestion window and halves
1367 this window in response to congestion.
1370 - _CCID-3_. Designed to be fair when competing for bandwidth.
1371 It has lower variation of throughput over time compared with
1372 TCP, which makes it suitable for streaming media.
1374 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1375 single list, the _target list_. This list describes the set of clients
1376 to which the stream is sent. There is no list for access control and
1377 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1378 and "delete" commands can be used to modify the target list.
1380 Since both UDP and DCCP offer an unreliable datagram-based transport,
1381 additional measures are necessary to guard against disruptions over
1382 networks that are lossy or which may be subject to interference (as
1383 is for instance the case with WiFi). Paraslash uses FEC (Forward
1384 Error Correction) to guard against packet losses and reordering. The
1385 stream is FEC-encoded before it is sent through the UDP socket and
1386 must be decoded accordingly on the receiver side.
1388 The packet size and the amount of redundancy introduced by FEC can
1389 be configured via the FEC parameters which are dictated by server
1390 and may also be configured through the "sender" command. The FEC
1391 parameters are encoded in the header of each network packet, so no
1392 configuration is necessary on the receiver side. See the section on
1393 REFERENCE(Forward error correction, FEC) below.
1395 Streams with headers and headerless streams
1396 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1398 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1399 needed to decode the stream is only contained in the audio file
1400 header of the container format but not in each data chunk. Clients
1401 must be able to obtain this information in case streaming starts in
1402 the middle of the file or if para_audiod is started while para_server
1403 is already sending a stream.
1405 This is accomplished in different ways, depending on the streaming
1406 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1407 header is sent prior to audio file data. This technique however does
1408 not work for the connectionless UDP transport. Hence the audio file
1409 header is periodically being embedded into the UDP audio data stream.
1410 By default, the header is resent after five seconds. The receiver has
1411 to wait until the next header arrives before it can start decoding
1412 the stream.
1414 Examples
1415 ~~~~~~~~
1417 The "si" (server info) command lists some information about the
1418 currently running server process.
1420 -> Show PIDs, number of connected clients, uptime, and more:
1422 para_client si
1424 The sender command of para_server prints information about senders,
1425 like the various access control lists, and it allows to (de-)activate
1426 senders and to change the access permissions at runtime.
1428 -> List all senders
1430 para_client sender
1432 -> Obtain general help for the sender command:
1434 para_client help sender
1436 -> Get help for a specific sender (contains further examples):
1438 s=http # or dccp or udp
1439 para_client sender $s help
1441 -> Show status of the http sender
1443 para_client sender http status
1445 By default para_server activates both the HTTP and th DCCP sender on
1446 startup. This can be changed via command line options or para_server's
1447 config file.
1449 -> List config file options for senders:
1451 para_server -h
1453 All senders share the "on" and "off" commands, so senders may be
1454 activated and deactivated independently of each other.
1456 -> Switch off the http sender:
1458 para_client sender http off
1460 -> Receive a DCCP stream using CCID2 and write the output into a file:
1462; ccid=2; filename=bar
1463 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1465 Note the quotes around the arguments for the dccp receiver. Each
1466 receiver has its own set of command line options and its own command
1467 line parser, so arguments for the dccp receiver must be protected
1468 from being interpreted by para_recv.
1470 -> Start UDP multicast, using the default multicast address:
1472 para_client sender udp add
1474 -> Receive FEC-encoded multicast stream and write the output into a file:
1476 filename=foo
1477 para_recv -r udp > $filename
1479 -> Add an UDP unicast for a client to the target list of the UDP sender:
1482 para_client sender udp add $t
1484 -> Receive this (FEC-encoded) unicast stream:
1486 filename=foo
1487 para_recv -r 'udp -i' > $filename
1489 -> Create a minimal config for para_audiod for HTTP streams:
1491 c=$HOME/.paraslash/audiod.conf.min;
1492 echo receiver \".:http -i $s\" > $c
1493 para_audiod --config $c
1495 -------
1496 Filters
1497 -------
1499 A paraslash filter is a module which transforms an input stream into
1500 an output stream. Filters are included in the para_audiod executable
1501 and in the stand-alone tool para_filter which usually contains the
1502 same modules.
1504 While para_filter reads its input stream from STDIN and writes
1505 the output to STDOUT, the filter modules of para_audiod are always
1506 connected to a receiver which produces the input stream and a writer
1507 which absorbs the output stream.
1509 Some filters depend on a specific library and are not compiled in
1510 if this library was not found at compile time. To see the list of
1511 supported filters, run para_filter and para_audiod with the --help
1512 option. The output looks similar to the following:
1514 Available filters:
1515 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1517 Out of these filter modules, a chain of filters can be constructed,
1518 much in the way Unix pipes can be chained, and analogous to the use
1519 of modules in gstreamer: The output of the first filter becomes the
1520 input of the second filter. There is no limitation on the number of
1521 filters and the same filter may occur more than once.
1523 Like receivers, each filter has its own command line options which
1524 must be quoted to protect them from the command line options of
1525 the driving application (para_audiod or para_filter). Example:
1527 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1529 For para_audiod, each audio format has its own set of filters. The
1530 name of the audio format for which the filter should be applied can
1531 be used as the prefix for the filter option. Example:
1533 para_audiod -f 'mp3:prebuffer --duration 300'
1535 The "mp3" prefix above is actually interpreted as a POSIX extended
1536 regular expression. Therefore
1538 para_audiod -f '.:prebuffer --duration 300'
1540 activates the prebuffer filter for all supported audio formats (because
1541 "." matches all audio formats) while
1543 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1545 activates it only for wma and ogg streams.
1547 Decoders
1548 ~~~~~~~~
1550 For each supported audio format there is a corresponding filter
1551 which decodes audio data in this format to 16 bit PCM data which
1552 can be directly sent to the sound device or any other software that
1553 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1554 filters are called _decoders_ in general, and xxxdec is the name of
1555 the paraslash decoder for the audio format xxx. For example, the mp3
1556 decoder is called mp3dec.
1558 Note that the output of the decoder is about 10 times larger than
1559 its input. This means that filters that operate on the decoded audio
1560 stream have to deal with much more data than filters that transform
1561 the audio stream before it is fed to the decoder.
1563 Paraslash relies on external libraries for most decoders, so these
1564 libraries must be installed for the decoder to be included in the
1565 executables. For example, the mp3dec filter depends on the mad library.
1567 Forward error correction
1568 ~~~~~~~~~~~~~~~~~~~~~~~~
1570 As already mentioned REFERENCE(Streaming protocols, earlier),
1571 paraslash uses forward error correction (FEC) for the unreliable UDP
1572 and DCCP transports. FEC is a technique which was invented already
1573 in 1960 by Reed and Solomon and which is widely used for the parity
1574 calculations of storage devices (RAID arrays). It is based on the
1575 algebraic concept of finite fields, today called Galois fields, in
1576 honour of the mathematician Galois (1811-1832). The FEC implementation
1577 of paraslash is based on code by Luigi Rizzo.
1579 Although the details require a sound knowledge of the underlying
1580 mathematics, the basic idea is not hard to understand: For positive
1581 integers k and n with k < n it is possible to compute for any k given
1582 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1583 ..., p_r such that all data bytes can be reconstructed from *any*
1584 k bytes of the set
1586 {d_1, ..., d_k, p_1, ..., p_r}.
1588 FEC-encoding for unreliable network transports boils down to slicing
1589 the audio stream into groups of k suitably sized pieces called _slices_
1590 and computing the r corresponding parity slices. This step is performed
1591 in para_server which then sends both the data and the parity slices
1592 over the unreliable network connection. If the client was able
1593 to receive at least k of the n = k + r slices, it can reconstruct
1594 (FEC-decode) the original audio stream.
1596 From these observations it is clear that there are three different
1597 FEC parameters: The slice size, the number of data slices k, and the
1598 total number of slices n. It is crucial to choose the slice size
1599 such that no fragmentation of network packets takes place because
1600 FEC only guards against losses and reordering but fails if slices are
1601 received partially.
1603 FEC decoding in paralash is performed through the fecdec filter which
1604 usually is the first filter (there can be other filters before fecdec
1605 if these do not alter the audio stream).
1608 Volume adjustment (amp and compress)
1609 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1611 The amp and the compress filter both adjust the volume of the audio
1612 stream. These filters operate on uncompressed audio samples. Hence
1613 they are usually placed directly after the decoding filter. Each
1614 sample is multiplied with a scaling factor (>= 1) which makes amp
1615 and compress quite expensive in terms of computing power.
1617 *amp*
1619 The amp filter amplifies the audio stream by a fixed scaling factor
1620 that must be known in advance. For para_audiod this factor is derived
1621 from the amplification field of the audio file's entry in the audio
1622 file table while para_filter uses the value given at the command line.
1624 The optimal scaling factor F for an audio file is the largest real
1625 number F >= 1 such that after multiplication with F all samples still
1626 fit into the sample interval [-32768, 32767]. One can use para_filter
1627 in combination with the sox utility to compute F:
1629 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1631 The amplification value V which is stored in the audio file table,
1632 however, is an integer between 0 and 255 which is connected to F
1633 through the formula
1635 V = (F - 1) * 64.
1637 To store V in the audio file table, the command
1639 para_client -- touch -a=V file.mp3
1641 is used. The reader is encouraged to write a script that performs
1642 these computations :)
1644 *compress*
1646 Unlike the amplification filter, the compress filter adjusts the volume
1647 of the audio stream dynamically without prior knowledge about the peak
1648 value. It maintains the maximal volume of the last n samples of the
1649 audio stream and computes a suitable amplification factor based on that
1650 value and the various configuration options. It tries to chose this
1651 factor such that the adjusted volume meets the desired target level.
1653 Note that it makes sense to combine amp and compress.
1655 Misc filters (wav and prebuffer)
1656 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1658 These filters are rather simple and do not modify the audio stream at
1659 all. The wav filter is only useful with para_filter and in connection
1660 with a decoder. It asks the decoder for the number of channels and the
1661 sample rate of the stream and adds a Microsoft wave header containing
1662 this information at the beginning. This allows to write wav files
1663 rather than raw PCM files (which do not contain any information about
1664 the number of channels and the sample rate).
1666 The prebuffer filter simply delays the output until the given time has
1667 passed (starting from the time the first byte was available in its
1668 input queue) or until the given amount of data has accumulated. It
1669 is mainly useful for para_audiod if the standard parameters result
1670 in buffer underruns.
1672 Both filters require almost no additional computing time, even when
1673 operating on uncompressed audio streams, since data buffers are simply
1674 "pushed down" rather than copied.
1676 Examples
1677 ~~~~~~~~
1679 -> Decode an mp3 file to wav format:
1681 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1683 -> Amplify a raw audio file by a factor of 1.5:
1685 para_filter -f amp --amp 32 < foo.raw > bar.raw
1687 ------
1688 Output
1689 ------
1691 Once an audio stream has been received and decoded to PCM format,
1692 it can be sent to a sound device for playback. This part is performed
1693 by paraslash _writers_ which are described in this chapter.
1695 Writers
1696 ~~~~~~~
1698 A paraslash writer acts as a data sink that consumes but does not
1699 produce audio data. Paraslash writers operate on the client side and
1700 are contained in para_audiod and in the stand-alone tool para_write.
1702 The para_write program reads uncompressed audio data from STDIN. If
1703 this data starts with a wav header, sample rate, sample format and
1704 channel count are read from the header. Otherwise CD audio (44.1KHz
1705 16 bit little endian, stereo) is assumed but this can be overridden
1706 by command line options. para_audiod, on the other hand, obtains
1707 the sample rate and the number of channels from the decoder.
1709 Like receivers and filters, each writer has an individual set of
1710 command line options, and for para_audiod writers can be configured
1711 per audio format separately. It is possible to activate more than
1712 one writer for the same stream simultaneously.
1714 OS-dependent APIs
1715 ~~~~~~~~~~~~~~~~~
1717 Unfortunately, the various flavours of Unix on which paraslash
1718 runs on have different APIs for opening a sound device and starting
1719 playback. Hence for each such API there is a paraslash writer that
1720 can play the audio stream via this API.
1722 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1723 Linux systems. Although there are several mid-layer APIs in use by
1724 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1725 currently supports only the low-level ALSA API which is not supposed
1726 to be change. ALSA is very feature-rich, in particular it supports
1727 software mixing via its DMIX plugin. ALSA is the default writer on
1728 Linux systems.
1730 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1731 is also available on Linux systems, usually provided by ALSA as an
1732 emulation for backwards compatibility. This API is rather simple but
1733 also limited. For example only one application can open the device
1734 at any time. The OSS writer is activated by default on BSD Systems.
1736 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1737 for this API is only compiled in on such systems and is of course
1738 the default there.
1740 *FILE*. The file writer allows to capture the audio stream and
1741 write the PCM data to a file on the file system rather than playing
1742 it through a sound device. It is supported on all platforms and is
1743 always compiled in.
1745 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1746 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1747 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1748 through an output plugin of libao.
1750 Examples
1751 ~~~~~~~~
1753 -> Use the OSS writer to play a wav file:
1755 para_write --writer oss < file.wav
1757 -> Enable ALSA software mixing for mp3 streams
1759 para_audiod --writer 'mp3:alsa -d plug:swmix'
1762 ---
1763 Gui
1764 ---
1766 para_gui executes an arbitrary command which is supposed to print
1767 status information to STDOUT. It then displays this information in
1768 a curses window. By default the command
1770 para_audioc -- stat -p
1772 is executed, but this can be customized via the --stat-cmd option. In
1773 particular it possible to use
1775 para_client -- stat -p
1777 to make para_gui work on systems on which para_audiod is not running.
1779 Key bindings
1780 ~~~~~~~~~~~~
1782 It is possible to bind keys to arbitrary commands via custom
1783 key-bindings. Besides the internal keys which can not be changed (help,
1784 quit, loglevel, version...), the following flavours of key-bindings
1785 are supported:
1787 - external: Shutdown curses before launching the given command.
1788 Useful for starting other ncurses programs from within
1789 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1790 output format to write a mailbox containing one mail for each
1791 (admissible) file the audio file selector knows about. Then
1792 start mutt from within para_gui to browse your collection!
1794 - display: Launch the command and display its stdout in
1795 para_gui's bottom window.
1797 - para: Like display, but start "para_client <specified
1798 command>" instead of "<specified command>".
1800 The general form of a key binding is
1802 key_map k:m:c
1804 which maps key k to command c using mode m. Mode may be x, d or p
1805 for external, display and paraslash commands, respectively.
1807 Themes
1808 ~~~~~~
1810 Currently there are only two themes for para_gui. It is easy, however,
1811 to add more themes. To create a new theme one has to define the
1812 position, color and geometry for for each status item that should be
1813 shown by this theme. See gui_theme.c for examples.
1815 The "." and "," keys are used to switch between themes.
1817 Examples
1818 ~~~~~~~~
1820 -> Show server info:
1822 key_map "i:p:si"
1824 -> Jump to the middle of the current audio file by pressing F5:
1826 key_map "<F5>:p:jmp 50"
1828 -> vi-like bindings for jumping around:
1830 key_map "l:p:ff 10"
1831 key_map "h:p:ff 10-"
1832 key_map "w:p:ff 60"
1833 key_map "b:p:ff 60-"
1835 -> Print the current date and time:
1837 key_map "D:d:date"
1839 -> Call other curses programs:
1841 key_map "U:x:aumix"
1842 key_map "!:x:/bin/bash"
1843 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1845 -----------
1846 Development
1847 -----------
1849 Tools
1850 ~~~~~
1852 In order to compile the sources from the git repository (rather than
1853 from tar balls) and for contributing non-trivial changes to the
1854 paraslash project, some additional tools should be installed on a
1855 developer machine.
1857 (git). As described in more detail REFERENCE(Git
1858 branches, below), the git source code management tool is used for
1859 paraslash development. It is necessary for cloning the git repository
1860 and for getting updates.
1862 (m4). Some input files for gengetopt
1863 are generated from templates by the m4 macro processor.
1865 (autoconf) GNU autoconf creates
1866 the configure file which is shipped in the tarballs but has to be
1867 generated when compiling from git.
1869 (grutatxt). The
1870 HTML version of this manual and some of the paraslash web pages are
1871 generated by the grutatxt plain text to HTML converter. If changes
1872 are made to these text files the grutatxt package must be installed
1873 to regenerate the HTML files.
1875 (doxygen). The documentation
1876 of paraslash's C sources uses the doxygen documentation system. The
1877 conventions for documenting the source code is described in the
1878 REFERENCE(Doxygen, Doxygen section).
1880 (global). This is used to generate
1881 browsable HTML from the C sources. It is needed by doxygen.
1883 Git branches
1884 ~~~~~~~~~~~~
1886 Paraslash has been developed using the git source code management
1887 tool since 2006. Development is organized roughly in the same spirit
1888 as the git development itself, as described below.
1890 The following text passage is based on "A note from the maintainer",
1891 written by Junio C Hamano, the maintainer of git.
1893 There are four branches in the paraslash repository that track the
1894 source tree: "master", "maint", "next", and "pu".
1896 The "master" branch is meant to contain what is well tested and
1897 ready to be used in a production setting. There could occasionally be
1898 minor breakages or brown paper bag bugs but they are not expected to
1899 be anything major, and more importantly quickly and easily fixable.
1900 Every now and then, a "feature release" is cut from the tip of this
1901 branch, named with three dotted decimal digits, like 0.4.2.
1903 Whenever changes are about to be included that will eventually lead to
1904 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1905 "master" at that point. Obvious, safe and urgent fixes after the major
1906 release are applied to this branch and maintenance releases are cut
1907 from it. New features never go to this branch. This branch is also
1908 merged into "master" to propagate the fixes forward.
1910 A trivial and safe enhancement goes directly on top of "master".
1911 New development does not usually happen on "master", however.
1912 Instead, a separate topic branch is forked from the tip of "master",
1913 and it first is tested in isolation; Usually there are a handful such
1914 topic branches that are running ahead of "master". The tip of these
1915 branches is not published in the public repository to keep the number
1916 of branches that downstream developers need to worry about low.
1918 The quality of topic branches varies widely. Some of them start out as
1919 "good idea but obviously is broken in some areas" and then with some
1920 more work become "more or less done and can now be tested by wider
1921 audience". Luckily, most of them start out in the latter, better shape.
1923 The "next" branch is to merge and test topic branches in the latter
1924 category. In general, this branch always contains the tip of "master".
1925 It might not be quite rock-solid production ready, but is expected to
1926 work more or less without major breakage. The maintainer usually uses
1927 the "next" version of paraslash for his own pleasure, so it cannot
1928 be _that_ broken. The "next" branch is where new and exciting things
1929 take place.
1931 The two branches "master" and "maint" are never rewound, and "next"
1932 usually will not be either (this automatically means the topics that
1933 have been merged into "next" are usually not rebased, and you can find
1934 the tip of topic branches you are interested in from the output of
1935 "git log next"). You should be able to safely build on top of them.
1937 However, at times "next" will be rebuilt from the tip of "master" to
1938 get rid of merge commits that will never be in "master". The commit
1939 that replaces "next" will usually have the identical tree, but it
1940 will have different ancestry from the tip of "master".
1942 The "pu" (proposed updates) branch bundles the remainder of the
1943 topic branches. The "pu" branch, and topic branches that are only in
1944 "pu", are subject to rebasing in general. By the above definition
1945 of how "next" works, you can tell that this branch will contain quite
1946 experimental and obviously broken stuff.
1948 When a topic that was in "pu" proves to be in testable shape, it
1949 graduates to "next". This is done with
1951 git checkout next
1952 git merge that-topic-branch
1954 Sometimes, an idea that looked promising turns out to be not so good
1955 and the topic can be dropped from "pu" in such a case.
1957 A topic that is in "next" is expected to be polished to perfection
1958 before it is merged to "master". Similar to the above, this is
1959 done with
1961 git checkout master
1962 git merge that-topic-branch
1963 git branch -d that-topic-branch
1965 Note that being in "next" is not a guarantee to appear in the next
1966 release (being in "master" is such a guarantee, unless it is later
1967 found seriously broken and reverted), nor even in any future release.
1969 Coding Style
1970 ~~~~~~~~~~~~
1972 The preferred coding style for paraslash coincides more or less
1973 with the style of the Linux kernel. So rather than repeating what is
1974 written XREFERENCE(,
1975 there), here are the most important points.
1977 - Burn the GNU coding standards.
1978 - Never use spaces for indentation.
1979 - Tabs are 8 characters, and thus indentations are also 8 characters.
1980 - Don't put multiple assignments on a single line.
1981 - Avoid tricky expressions.
1982 - Don't leave whitespace at the end of lines.
1983 - The limit on the length of lines is 80 columns.
1984 - Use K&R style for placing braces and spaces:
1986 if (x is true) {
1987 we do y
1988 }
1990 - Use a space after (most) keywords.
1991 - Do not add spaces around (inside) parenthesized expressions.
1992 - Use one space around (on each side of) most binary and ternary operators.
1993 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1994 - Mixed-case names are frowned upon.
1995 - Descriptive names for global variables are a must.
1996 - Avoid typedefs.
1997 - Functions should be short and sweet, and do just one thing.
1998 - The number of local variables shouldn't exceed 10.
1999 - Gotos are fine if they improve readability and reduce nesting.
2000 - Don't use C99-style "// ..." comments.
2001 - Names of macros defining constants and labels in enums are capitalized.
2002 - Enums are preferred when defining several related constants.
2003 - Always use the paraslash wrappers for allocating memory.
2004 - If the name of a function is an action or an imperative.
2005 command, the function should return an error-code integer
2006 (<0 means error, >=0 means success). If the name is a
2007 predicate, the function should return a "succeeded" boolean.
2010 Doxygen
2011 ~~~~~~~
2013 Doxygen is a documentation system for various programming
2014 languages. The paraslash project uses Doxygen for generating the API
2015 reference on the web pages, but good source code documentation is
2016 also beneficial to people trying to understand the code structure
2017 and the interactions between the various source files.
2019 It is more illustrative to look at the source code for examples than
2020 to describe the conventions for documenting the source in this manual,
2021 so we only describe which parts of the code need doxygen comments,
2022 but leave out details on documentation conventions.
2024 As a rule, only the public part of the C source is documented with
2025 Doxygen. This includes structures, defines and enumerations in header
2026 files as well as public (non-static) C functions. These should be
2027 documented completely. For example each parameter and the return
2028 value of a public function should get a descriptive comment.
2030 No doxygen comments are necessary for static functions and for
2031 structures and enumerations in C files (which are used only within
2032 this file). This does not mean, however, that those entities need
2033 no documentation at all. Instead, common sense should be applied to
2034 document what is not obvious from reading the code.
2036 --------
2037 Appendix
2038 --------
2040 Network protocols
2041 ~~~~~~~~~~~~~~~~~
2043 *IP*. The _Internet Protocol_ is the primary networking protocol
2044 used for the Internet. All protocols described below use IP as the
2045 underlying layer. Both the prevalent IPv4 and the next-generation
2046 IPv6 variant are being deployed actively worldwide.
2048 *Connection-oriented and connectionless protocols*. Connectionless
2049 protocols differ from connection-oriented ones in that state
2050 associated with the sending/receiving endpoints is treated
2051 implicitly. Connectionless protocols maintain no internal knowledge
2052 about the state of the connection. Hence they are not capable of
2053 reacting to state changes, such as sudden loss or congestion on the
2054 connection medium. Connection-oriented protocols, in contrast, make
2055 this knowledge explicit. The connection is established only after
2056 a bidirectional handshake which requires both endpoints to agree
2057 on the state of the connection, and may also involve negotiating
2058 specific parameters for the particular connection. Maintaining an
2059 up-to-date internal state of the connection also in general means
2060 that the sending endpoints perform congestion control, adapting to
2061 qualitative changes of the connection medium.
2063 *Reliability*. In IP networking, packets can be lost, duplicated,
2064 or delivered out of order, and different network protocols handle
2065 these problems in different ways. We call a transport-layer protocol
2066 _reliable_, if it turns the unreliable IP delivery into an ordered,
2067 duplicate- and loss-free delivery of packets. Sequence numbers
2068 are used to discard duplicates and re-arrange packets delivered
2069 out-of-order. Retransmission is used to guarantee loss-free
2070 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2071 or data integrity.
2073 *Classification*. With these definitions the protocols which are used
2074 by paraslash for steaming audio data may be classified as follows.
2076 - HTTP/TCP: connection-oriented, reliable,
2077 - UDP: connectionless, unreliable,
2078 - DCCP: connection-oriented, unreliable.
2080 Below we give a short descriptions of these protocols.
2082 *TCP*. The _Transmission Control Protocol_ provides reliable,
2083 ordered delivery of a stream and a classic window-based congestion
2084 control. In contrast to UDP and DCCP (see below), TCP does not have
2085 record-oriented or datagram-based syntax, i.e. it provides a stream
2086 which is unaware and independent of any record (packet) boundaries.
2087 TCP is used extensively by many application layers. Besides HTTP (the
2088 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2089 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2090 top of TCP.
2092 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2093 protocol, built as a thin layer directly on top of IP. For this reason,
2094 it offers the same best-effort service as IP itself, i.e. there is no
2095 detection of duplicate or reordered packets. Being a connectionless
2096 protocol, only minimal internal state about the connection is
2097 maintained, which means that there is no protection against packet
2098 loss or network congestion. Error checking and correction (if at all)
2099 are performed in the application.
2101 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2102 connection-oriented state maintenance known from TCP with the
2103 unreliable, datagram-based transport of UDP. This means that it
2104 is capable of reacting to changes in the connection by performing
2105 congestion control, offering multiple alternative approaches. But it
2106 is bound to datagram boundaries (the maximum packet size supported
2107 by a medium), and like UDP it lacks retransmission to protect
2108 against loss. Due to the use of sequence numbers, it is however
2109 able to react to loss (interpreted as a congestion indication) and
2110 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2111 to negotiate specific, binding features for a connection, such as
2112 the choice of congestion control: classic, window-based congestion
2113 control known from TCP is available as CCID-2, rate-based, "smooth"
2114 congestion control is offered as CCID-3.
2116 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2117 protocol on top of TCP. It is spoken by web servers and is most often
2118 used for web services. However, as can be seen by the many Internet
2119 radio stations and YouTube/Flash videos, http is by far not limited to
2120 the delivery of web pages only. Being a simple request/response based
2121 protocol, the semantics of the protocol also allow the delivery of
2122 multimedia content, such as audio over http.
2124 *Multicast*. IP multicast is not really a protocol but a technique
2125 for one-to-many communication over an IP network. The challenge is to
2126 deliver information to a group of destinations simultaneously using
2127 the most efficient strategy to send the messages over each link of
2128 the network only once. This has benefits for streaming multimedia:
2129 the standard one-to-one unicast offered by TCP/DCCP means that
2130 n clients listening to the same stream also consume n-times the
2131 resources, whereas multicast requires to send the stream just once,
2132 irrespective of the number of receivers. Since it would be costly to
2133 maintain state for each listening receiver, multicast often implies
2134 connectionless transport, which is the reason that it is currently
2135 only available via UDP.
2137 Abstract socket namespace
2138 ~~~~~~~~~~~~~~~~~~~~~~~~~
2139 UNIX domain sockets are a traditional way to communicate between
2140 processes on the same machine. They are always reliable (see above)
2141 and don't reorder datagrams. Unlike TCP and UDP, UNIX domain sockets
2142 support passing open file descriptors or process credentials to
2143 other processes.
2145 The usual way to set up a UNIX domain socket (as obtained from
2146 socket(2)) for listening is to first bind the socket to a file system
2147 pathname and then call listen(2), then accept(2). Such sockets are
2148 called _pathname sockets_ because bind(2) creates a special socket
2149 file at the specified path. Pathname sockets allow unrelated processes
2150 to communicate with the listening process by binding to the same path
2151 and calling connect(2).
2153 There are two problems with pathname sockets:
2155 * The listing process must be able to (safely) create the
2156 socket special in a directory which is also accessible to
2157 the connecting process.
2159 * After an unclean shutdown of the listening process, a stale
2160 socket special may reside on the file system.
2162 The abstract socket namespace is a non-portable Linux feature which
2163 avoids these problems. Abstract sockets are still bound to a name,
2164 but the name has no connection with file system pathnames.
2166 License
2167 ~~~~~~~
2169 Paraslash is licensed under the GPL, version 2. Most of the code
2170 base has been written from scratch, and those parts are GPL V2
2171 throughout. Notable exceptions are FEC and the WMA decoder. See the
2172 corresponding source files for licencing details for these parts. Some
2173 code sniplets of several other third party software packages have
2174 been incorporated into the paraslash sources, for example log message
2175 coloring was taken from the git sources. These third party software
2176 packages are all published under the GPL or some other license
2177 compatible to the GPL.
2179 Acknowledgements
2180 ~~~~~~~~~~~~~~~~
2182 Many thanks to Gerrit Renker who read an early draft of this manual
2183 and contributed significant improvements.
2185 ----------
2186 References
2187 ----------
2189 Articles
2190 ~~~~~~~~
2191 - Reed, Irving S.; Solomon, Gustave (1960),
2193 Polynomial Codes over Certain Finite Fields), Journal of the
2194 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2195 300-304, doi:10.1137/0108018)
2197 RFCs
2198 ~~~~
2200 - XREFERENCE(, RFC 768) (1980):
2201 User Datagram Protocol
2202 - XREFERENCE(, RFC 791) (1981):
2203 Internet Protocol
2204 - XREFERENCE(, RFC 2437) (1998):
2205 RSA Cryptography Specifications
2206 - XREFERENCE(, RFC 4340)
2207 (2006): Datagram Congestion Control Protocol (DCCP)
2208 - XREFERENCE(, RFC 4341) (2006):
2209 Congestion Control ID 2: TCP-like Congestion Control
2210 - XREFERENCE(, RFC 4342) (2006):
2211 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2212 - XREFERENCE(, RFC 6716) (2012):
2213 Definition of the Opus Audio Codec
2215 Application web pages
2216 ~~~~~~~~~~~~~~~~~~~~~
2218 - XREFERENCE(, paraslash)
2219 - XREFERENCE(, paraslash (alternative page))
2220 - XREFERENCE(, xmms)
2221 - XREFERENCE(, mpg123)
2222 - XREFERENCE(, gstreamer)
2223 - XREFERENCE(, icecast)
2224 - XREFERENCE(, Audio Compress)
2226 External documentation
2227 ~~~~~~~~~~~~~~~~~~~~~~
2230 H. Peter Anvin: The mathematics of Raid6)
2232 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2233 Communication Protocols)
2235 Code
2236 ~~~~
2238 Original FEC implementation by Luigi Rizzo)