manual: Simplify keygen command.
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11 define(`EMPH', ``_''`REMOVE_NEWLINE($1)'``_'')
13 Paraslash user manual
14 =====================
16 This document describes how to install, configure and use the paraslash
17 network audio streaming system. Most chapters start with a chapter
18 overview and conclude with an example section. We try to focus on
19 general concepts and on the interaction of the various pieces of the
20 paraslash package. Hence this user manual is not meant as a replacement
21 for the manual pages that describe all command line options of each
22 paraslash executable.
24 ------------
25 Introduction
26 ------------
28 In this chapter we give an REFERENCE(Overview, overview) of the
29 interactions of the two main programs contained in the paraslash
30 package, followed by REFERENCE(The paraslash executables, brief
31 descriptions) of all executables.
33 Overview
34 ~~~~~~~~
36 The core functionality of the para suite is provided by two main
37 executables, para_server and para_audiod. The former maintains a
38 database of audio files and streams these files to para_audiod which
39 receives and plays the stream.
41 In a typical setting, both para_server and para_audiod act as
42 background daemons whose functionality is controlled by client
43 programs: the para_audioc client controls para_audiod over a local
44 socket while the para_client program connects to para_server over a
45 local or remote networking connection.
47 Typically, these two daemons run on different hosts but a local setup
48 is also possible.
50 A simplified picture of a typical setup is as follows
51 <<
52 <pre>
53 server_host client_host
54 ~~~~~~~~~~~ ~~~~~~~~~~~
56 +-----------+ audio stream +-----------+
57 |para_server| -----------------------------> |para_audiod|
58 +-----------+ +-----------+
59 ^ ^
60 | |
61 | | connect
62 | |
63 | |
64 | +-----------+
65 | |para_audioc|
66 | +-----------+
67 |
68 |
69 | connect +-----------+
70 +-------------------------------------- |para_client|
71 +-----------+
72 </pre>
73 >>
75 The paraslash executables
76 ~~~~~~~~~~~~~~~~~~~~~~~~~
78 *para_server*
80 para_server streams binary audio data (MP3, ...) over local and/or
81 remote networks. It listens on a TCP port and accepts commands such
82 as play, stop, pause, next from authenticated clients. There are
83 many more commands though, see the man page of para_server for a
84 description of all commands.
86 It supports three built-in network streaming protocols
87 (senders/receivers): HTTP, DCCP, or UDP. This is explained in more
88 detail in the section on REFERENCE(Networking, networking).
90 The built-in audio file selector of paraslash is used to manage your
91 audio files. It maintains statistics on the usage of all available
92 audio files such as last-played time, and the number of times each
93 file was selected.
95 Additional information may be added to the database to allow
96 fine-grained selection based on various properties of the audio file,
97 including information found in (ID3) tags. However, old-fashioned
98 playlists are also supported.
100 It is also possible to store images (album covers) and lyrics in the
101 database and associate these to the corresponding audio files.
103 The section on the REFERENCE(The audio file selector, audio file
104 selector) discusses this topic.
107 *para_client*
109 The client program to connect to para_server. paraslash commands
110 are sent to para_server and the response is dumped to STDOUT. This
111 can be used by any scripting language to produce user interfaces with
112 little programming effort.
114 All connections between para_server and para_client are encrypted
115 with a symmetric session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
118 If para_client is started without non-option arguments, an interactive
119 session (shell) is started. Command history and command completion are
120 supported through libreadline.
122 *para_audiod*
124 The local daemon that collects information from para_server.
126 It runs on the client side and connects to para_server. As soon as
127 para_server announces the availability of an audio stream, para_audiod
128 starts an appropriate receiver, any number of filters and a paraslash
129 writer to play the stream.
131 Moreover, para_audiod listens on a local socket and sends status
132 information about para_server and para_audiod to local clients on
133 request. Access via this local socket may be restricted by using Unix
134 socket credentials, if available.
137 *para_audioc*
139 The client program which talks to para_audiod. Used to control
140 para_audiod, to receive status info, or to grab the stream at any
141 point of the decoding process. Like para_client, para_audioc supports
142 interactive sessions on systems with libreadline.
144 *para_recv*
146 A command line HTTP/DCCP/UDP stream grabber. The http mode is
147 compatible with arbitrary HTTP streaming sources (e.g. icecast).
148 In addition to the three network streaming modes, para_recv can also
149 operate in local (afh) mode. In this mode it writes the content of
150 an audio file on the local file system in complete chunks to stdout,
151 optionally 'just in time'. This allows to cut an audio file without
152 first decoding it, and it enables third-party software which is unaware
153 of the particular audio format to send complete frames in real time.
155 *para_filter*
157 A filter program that reads from STDIN and writes to STDOUT.
158 Like para_recv, this is an atomic building block which can be used to
159 assemble higher-level audio receiving facilities. It combines several
160 different functionalities in one tool: decoders for multiple audio
161 formats and a number of processing filters, among these a normalizer
162 for audio volume.
164 *para_afh*
166 A small stand-alone program that prints tech info about the given
167 audio file to STDOUT. It can be instructed to print a "chunk table",
168 an array of offsets within the audio file.
170 *para_write*
172 A modular audio stream writer. It supports a simple file writer
173 output plug-in and optional WAV/raw players for ALSA (Linux) and for
174 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
175 or raw audio player.
177 *para_play*
179 A command line audio player.
181 *para_gui*
183 Curses-based gui that presents status information obtained in a curses
184 window. Appearance can be customized via themes. para_gui provides
185 key-bindings for the most common server commands and new key-bindings
186 can be added easily.
189 *para_fade*
191 An alarm clock and volume-fader for OSS and ALSA.
193 -----------
194 Quick start
195 -----------
197 This chapter lists the REFERENCE(Requirements, necessary software)
198 that must be installed to compile the paraslash package, describes
199 how to REFERENCE(Installation, compile and install) the paraslash
200 source code and the steps that have to be performed in order to
201 REFERENCE(Quick start, set up) a typical server and client.
203 Requirements
204 ~~~~~~~~~~~~
205 For the impatient:
207 git clone git://
208 cd osl && make && sudo make install && sudo ldconfig
209 sudo apt-get install autoconf libssl-dev help2man gengetopt \
210 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
211 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev \
212 libasound2-dev libao-dev libreadline-dev libncurses-dev \
213 libopus-dev
215 Detailed description: In any case you'll need
217 - XREFERENCE(, libosl).
218 The _object storage layer_ library is used by para_server. To
219 clone the source code repository, execute
221 git clone git://
223 - XREFERENCE(, gcc) or
224 XREFERENCE(, clang). All gcc versions
225 >= 3.3 are currently supported. Clang version 1.1 or newer
226 should work as well.
228 - XREFERENCE(, gnu make) is
229 also shipped with the disto. On BSD systems the gnu make
230 executable is often called gmake.
232 - XREFERENCE(, bash). Some
233 scripts which run during compilation require the EMPH(Bourne
234 again shell). It is most likely already installed.
236 - XREFERENCE(, gengetopt)
237 is needed to generate the C code for the command line parsers
238 of all paraslash executables.
240 - XREFERENCE(, help2man)
241 is used to create the man pages.
243 Optional:
245 - XREFERENCE(, openssl) or
246 XREFERENCE(, libgcrypt).
247 At least one of these two libraries is needed as the backend
248 for cryptographic routines on both the server and the client
249 side. Both openssl and libgcrypt are usually shipped with the
250 distro, but you might have to install the development package
251 (libssl-dev or libgcrypt-dev on debian systems) as well.
253 - XREFERENCE(, libmad).
254 To compile in MP3 support for paraslash, the development
255 package must be installed. It is called libmad0-dev on
256 debian-based systems. Note that libmad is not necessary on
257 the server side, i.e. for sending MP3 files.
260 libid3tag). For version-2 ID3 tag support, you'll need
261 the libid3tag development package libid3tag0-dev. Without
262 libid3tag, only version one tags are recognized.
264 - XREFERENCE(, ogg vorbis).
265 For ogg vorbis streams you'll need libogg, libvorbis,
266 libvorbisfile. The corresponding Debian packages are called
267 libogg-dev and libvorbis-dev.
269 - XREFERENCE(, libfaad). For aac
270 files (m4a) you'll need libfaad (libfaad-dev).
272 - XREFERENCE(, speex). In order to stream
273 or decode speex files, libspeex (libspeex-dev) is required.
275 - XREFERENCE(, flac). To stream
276 or decode files encoded with the _Free Lossless Audio Codec_,
277 libFLAC (libFLAC-dev) must be installed.
280 libsamplerate). The resample filter will only be compiled if
281 this library is installed. Debian package: libsamplerate-dev.
283 - XREFERENCE(, alsa-lib). On
284 Linux, you'll need to have ALSA's development package
285 libasound2-dev installed.
288 libao). Needed to build the ao writer (ESD, PulseAudio,...).
289 Debian package: libao-dev.
291 - XREFERENCE(, curses). Needed
292 for para_gui. Debian package: libncurses-dev.
295 GNU Readline). If this library (libreadline-dev) is installed,
296 para_client, para_audioc and para_play support interactive
297 sessions.
299 Installation
300 ~~~~~~~~~~~~
302 First make sure all non-optional packages listed in the section on
303 REFERENCE(Requirements, required software) are installed on your
304 system.
306 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
307 OGG/Speex and AAC support are all optional. The configure script will
308 detect what is installed on your system and will only try to build
309 those executables that can be built with your setup.
311 Note that no special decoder library (not even the MP3 decoding library
312 libmad) is needed for para_server if you only want to stream MP3 or WMA
313 files. Also, it's fine to use para_server on a box without sound card.
315 Next, install the paraslash package on all machines, you'd like this
316 software to run on. If you compile from a released tarball, execute
318 (./configure && make) > /dev/null
320 When compiling from git or from snapshots downloaded via gitweb,
321 the above command will not work because the configure script is not
322 included in the git repository. In this case the following command
323 should be used instead:
325 ./
327 This runs autoconf to generate the configure script, then runs it as
328 above. Therefore you'll need autoconf for this to work.
330 There should be no errors but probably some warnings about missing
331 packages which usually implies that not all audio formats will be
332 supported. If headers or libs are installed at unusual locations you
333 might need to tell the configure script where to find them. Try
335 ./configure --help
337 to see a list of options. If the paraslash package was compiled
338 successfully, execute (optionally)
340 make test
342 to run the paraslash test suite. If all tests pass, execute as root
344 make install
346 to install executables under /usr/local/bin and the man pages under
347 /usr/local/man.
349 Configuration
350 ~~~~~~~~~~~~~
352 *Step 1*: Create a paraslash user
354 In order to control para_server at runtime you must create a paraslash
355 user. As authentication is based on the RSA crypto system you'll have
356 to create an RSA key pair. If you already have a user and an RSA key
357 pair, you may skip this step.
359 In this section we'll assume a typical setup: You would like to run
360 para_server on some host called server_host as user foo, and you want
361 to connect to para_server from another machine called client_host as
362 user bar.
364 As foo@server_host, create ~/.paraslash/server.users by typing the
365 following commands:
367 user=bar
368 target=~/.paraslash/server.users
369 key=~/.paraslash/$user
371 mkdir -p ~/.paraslash
372 echo "user $user $key $perms" >> $target
374 Next, change to the "bar" account on client_host and generate the
375 key pair with the commands
377 ssh-keygen -q -t rsa -b 2048 -N '' -f $key
379 This generates the two files id_rsa and in ~/.ssh. Note
380 that para_server won't accept keys shorter than 2048 bits. Moreover,
381 para_client rejects private keys which are world-readable.
383 para_server only needs to know the public key of the key pair just
384 created. Copy this public key to server_host:
386 src=~/.ssh/
387 dest=.paraslash/$LOGNAME
388 scp $src foo@server_host:$dest
390 Finally, tell para_client to connect to server_host:
392 conf=~/.paraslash/client.conf
393 echo 'hostname server_host' > $conf
396 *Step 2*: Start para_server
398 Before starting the server make sure you have write permissions to
399 the directory /var/paraslash that has been created during installation:
401 sudo chown $LOGNAME /var/paraslash
403 Alternatively, use the --afs-socket Option to specify a different
404 location for the AFS command socket.
406 For this first try, we'll use the info loglevel to make the output
407 of para_server more verbose.
409 para_server -l info
411 Now you can use para_client to connect to the server and issue
412 commands. Open a new shell as bar@client_host and try
414 para_client help
415 para_client si
417 to retrieve the list of available commands and some server info.
418 Don't proceed if this doesn't work.
420 *Step 3*: Create and populate the database
422 An empty database is created with
424 para_client init
426 This initializes a couple of empty tables under
427 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
428 tables, but it's good to know that you can start from scratch with
430 rm -rf ~/.paraslash/afs_database-0.4
432 in case something went wrong.
434 Next, you need to add some audio files to that database so that
435 para_server knows about them. Choose an absolute path to a directory
436 containing some audio files and add them to the audio file table:
438 para_client add /my/mp3/dir
440 This might take a while, so it is a good idea to start with a directory
441 containing not too many files. Note that the table only contains data
442 about the audio files found, not the files themselves.
444 You may print the list of all known audio files with
446 para_client ls
448 *Step 4*: Configure para_audiod
450 para_audiod needs to create a "well-known" socket for the clients to
451 connect to. The default path for this socket is
453 /var/paraslash/audiod_socket.$HOSTNAME
455 In order to make this directory writable for para_audiod, execute
456 as bar@client_host
458 sudo chown $LOGNAME /var/paraslash
461 We will also have to tell para_audiod that it should receive the
462 audio stream from server_host via http:
464 para_audiod -l info -r '.:http -i server_host'
466 You should now be able to listen to the audio stream once para_server
467 starts streaming. To activate streaming, execute
469 para_client play
471 Since no playlist has been specified yet, the "dummy" mode which
472 selects all known audio files is activated automatically. See the
473 section on the REFERENCE(The audio file selector, audio file selector)
474 for how to use playlists and moods to specify which files should be
475 streamed in which order.
477 *Troubleshooting*
479 It did not work? To find out why, try to receive, decode and play the
480 stream manually using para_recv, para_filter and para_write as follows.
482 For simplicity we assume that you're running Linux/ALSA and that only
483 MP3 files have been added to the database.
485 para_recv -r 'http -i server_host' > file.mp3
486 # (interrupt with CTRL+C after a few seconds)
487 ls -l file.mp3 # should not be empty
488 para_filter -f mp3dec -f wav < file.mp3 > file.wav
489 ls -l file.wav # should be much bigger than file.mp3
490 para_write -w alsa < file.wav
492 Double check what is logged by para_server and use the --loglevel
493 option of para_recv, para_filter and para_write to increase verbosity.
495 ---------------
496 User management
497 ---------------
499 para_server uses a challenge-response mechanism to authenticate
500 requests from incoming connections, similar to ssh's public key
501 authentication method. Authenticated connections are encrypted using
502 a stream cipher, either RC4 or AES in integer counter mode.
504 In this chapter we briefly describe RSA, RC4 and AES, and sketch the
505 REFERENCE(Client-server authentication, authentication handshake)
506 between para_client and para_server. User management is discussed
507 in the section on REFERENCE(The user_list file, the user_list file).
508 These sections are all about communication between the client and the
509 server. Connecting para_audiod is a different matter and is described
510 in a REFERENCE(Connecting para_audiod, separate section).
514 RSA, RC4, AES
515 ~~~~~~~~~~~~~
517 RSA is an asymmetric block cipher which is used in many applications,
518 including ssh and gpg. An RSA key consists in fact of two keys,
519 called the public key and the private key. A message can be encrypted
520 with either key and only the counterpart of that key can decrypt
521 the message. While RSA can be used for both signing and encrypting
522 a message, paraslash uses RSA only for the latter purpose. The
523 RSA public key encryption and signatures algorithms are defined in
524 detail in RFC 2437.
526 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
527 key stream to produce the output. Decryption uses the same function
528 calls as encryption. While RC4 supports variable key lengths,
529 paraslash uses a fixed length of 256 bits, which is considered a
530 strong encryption by today's standards. Since the same key must never
531 be used twice, a different, randomly-generated key is used for every
532 new connection.
534 AES, the advanced encryption standard, is a well-known symmetric block
535 cipher, i.e. a transformation operating on fixed-length blocks which
536 is determined by a single key for both encryption and decryption. Any
537 block cipher can be turned into a stream cipher by generating
538 a pseudo-random key stream by encrypting successive values of a
539 counter. The AES_CTR128 stream cipher used in paraslash is obtained
540 in this way from the AES block cipher with a 128 bit block size.
543 Client-server authentication
544 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
546 The authentication handshake between para_client and para_server goes
547 as follows:
549 - para_client connects to para_server and sends an
550 authentication request for a user. It does so by connecting
551 to TCP port 2990 of the server host. This port is called the
552 para_server _control port_.
554 - para_server accepts the connection and forks a child process
555 which handles the incoming request. The parent process keeps
556 listening on the control port while the child process (also
557 called para_server below) continues as follows.
559 - para_server loads the RSA public key of that user, fills a
560 fixed-length buffer with random bytes, encrypts that buffer
561 using the public key and sends the encrypted buffer to the
562 client. The first part of the buffer is the challenge which
563 is used for authentication while the second part is the
564 session key.
566 - para_client receives the encrypted buffer and decrypts it
567 with the user's private key, thereby obtaining the challenge
568 buffer and the session key. It sends the SHA1 hash value of
569 the challenge back to para_server and stores the session key
570 for further use.
572 - para_server also computes the SHA1 hash of the challenge
573 and compares it against what was sent back by the client.
575 - If the two hashes do not match, the authentication has
576 failed and para_server closes the connection.
578 - Otherwise the user is considered authenticated and the client
579 is allowed to proceed by sending a command to be executed. From
580 this point on the communication is encrypted using the stream
581 cipher with the session key known to both peers.
583 paraslash relies on the quality of the pseudo-random bytes provided
584 by the crypto library (openssl or libgcrypt), on the security of
585 the implementation of the RSA, RC4 and AES crypto routines and on the
586 infeasibility to invert the SHA1 function.
588 Neither para_server or para_client create RSA keys on their own. This
589 has to be done once for each user as sketched in REFERENCE(Quick start,
590 Quick start) and discussed in more detail REFERENCE(The user_list
591 file, below).
593 The user_list file
594 ~~~~~~~~~~~~~~~~~~
596 At startup para_server reads the user list file which contains one
597 line per user. The default location of the user list file may be
598 changed with the --user-list option.
600 There should be at least one user in this file. Each user must have
601 an RSA key pair. The public part of the key is needed by para_server
602 while the private key is needed by para_client. Each line of the
603 user list file must be of the form
605 user <username> <key> <perms>
607 where _username_ is an arbitrary string (usually the user's login
608 name), _key_ is the full path to that user's public RSA key, and
609 _perms_ is a comma-separated list of zero or more of the following
610 permission bits:
612 +---------------------------------------------------------+
613 | AFS_READ | read the contents of the databases |
614 +-----------+---------------------------------------------+
615 | AFS_WRITE | change database contents |
616 +-----------+---------------------------------------------+
617 | VSS_READ | obtain information about the current stream |
618 +-----------+---------------------------------------------+
619 | VSS_WRITE | change the current stream |
620 +---------------------------------------------------------+
622 The permission bits specify which commands the user is allowed to
623 execute. The output of
625 para_client help
627 contains in the third column the permissions needed to execute the
628 command.
630 It is possible to make para_server reread the user_list file by
631 executing the paraslash "hup" command or by sending SIGHUP to the
632 PID of para_server.
635 Connecting para_audiod
636 ~~~~~~~~~~~~~~~~~~~~~~
638 para_audiod listens on a Unix domain socket. Those sockets are
639 for local communication only, so only local users can connect to
640 para_audiod. The default is to let any user connect but this can be
641 restricted on platforms that support UNIX socket credentials which
642 allow para_audiod to obtain the Unix credentials of the connecting
643 process.
645 Use para_audiod's --user-allow option to allow connections only for
646 a limited set of users.
648 -----------------------
649 The audio file selector
650 -----------------------
652 paraslash comes with a sophisticated audio file selector (AFS),
653 whose main task is to determine which file to stream next, based on
654 information on the audio files stored in a database. It communicates
655 also with para_client whenever an AFS command is executed, for example
656 to answer a database query.
658 Besides the traditional playlists, AFS supports audio file selection
659 based on _moods_ which act as a filter that limits the set of all
660 known audio files to those which satisfy certain criteria. It also
661 maintains tables containing images (e.g. album cover art) and lyrics
662 that can be associated with one or more audio files.
664 AFS uses XREFERENCE(, libosl), the
665 object storage layer library, as the backend library for storing
666 information on audio files, playlists, etc. This library offers
667 functionality similar to a relational database, but is much more
668 lightweight than a full database backend.
670 In this chapter we sketch the setup of the REFERENCE(The AFS process,
671 AFS process) during server startup and proceed with the description
672 of the REFERENCE(Database layout, layout) of the various database
673 tables. The section on REFERENCE(Playlists and moods, playlists
674 and moods) explains these two audio file selection mechanisms
675 in detail and contains pratical examples. The way REFERENCE(File
676 renames and content changes, file renames and content changes) are
677 detected is discussed briefly before the REFERENCE(Troubleshooting,
678 Troubleshooting) section concludes the chapter.
680 The AFS process
681 ~~~~~~~~~~~~~~~
683 On startup, para_server forks to create the AFS process which opens
684 the OSL database tables. The server process communicates with the
685 AFS process via pipes and shared memory. Usually, the AFS process
686 awakes only briefly whenever the current audio file changes. The AFS
687 process determines the next audio file, opens it, verifies it has
688 not been changed since it was added to the database and passes the
689 open file descriptor to the server process, along with audio file
690 meta-data such as file name, duration, audio format and so on. The
691 server process then starts to stream the audio file.
693 The AFS process also accepts connections from local clients via
694 a well-known socket. However, only child processes of para_server
695 may connect through this socket. All server commands that have the
696 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
697 change the database.
699 Database layout
700 ~~~~~~~~~~~~~~~
702 *The audio file table*
704 This is the most important and usually also the largest table of the
705 AFS database. It contains the information needed to stream each audio
706 file. In particular the following data is stored for each audio file.
708 - SHA1 hash value of the audio file contents. This is computed
709 once when the file is added to the database. Whenever AFS
710 selects this audio file for streaming the hash value is
711 recomputed and checked against the value stored in the
712 database to detect content changes.
714 - The time when this audio file was last played.
716 - The number of times the file has been played so far.
718 - The attribute bitmask.
720 - The image id which describes the image associated with this
721 audio file.
723 - The lyrics id which describes the lyrics associated with
724 this audio file.
726 - The audio format id (MP3, OGG, ...).
728 - An amplification value that can be used by the amplification
729 filter to pre-amplify the decoded audio stream.
731 - The chunk table. It describes the location and the timing
732 of the building blocks of the audio file. This is used by
733 para_server to send chunks of the file at appropriate times.
735 - The duration of the audio file.
737 - Tag information contained in the audio file (ID3 tags,
738 Vorbis comments, ...).
740 - The number of channels
742 - The encoding bitrate.
744 - The sampling frequency.
746 To add or refresh the data contained in the audio file table, the _add_
747 command is used. It takes the full path of either an audio file or a
748 directory. In the latter case, the directory is traversed recursively
749 and all files which are recognized as valid audio files are added to
750 the database.
752 *The attribute table*
754 The attribute table contains two columns, _name_ and _bitnum_. An
755 attribute is simply a name for a certain bit number in the attribute
756 bitmask of the audio file table.
758 Each of the 64 bits of the attribute bitmask can be set for each
759 audio file individually. Hence up to 64 different attributes may be
760 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
761 "german_lyrics", "speech", whatever. You are free to choose as
762 many attributes as you like and there are no naming restrictions
763 for attributes.
765 A new attribute "test" is created by
767 para_client addatt test
768 and
769 para_client lsatt
771 lists all available attributes. You can set the "test" attribute for
772 an audio file by executing
774 para_client setatt test+ /path/to/the/audio/file
776 Similarly, the "test" bit can be removed from an audio file with
778 para_client setatt test- /path/to/the/audio/file
780 Instead of a path you may use a shell wildcard pattern. The attribute
781 is applied to all audio files matching this pattern:
783 para_client setatt test+ '/test/directory/*'
785 The command
787 para_client -- ls -lv
789 gives you a verbose listing of your audio files also showing which
790 attributes are set.
792 In case you wonder why the double-dash in the above command is needed:
793 It tells para_client to not interpret the options after the dashes. If
794 you find this annoying, just say
796 alias para='para_client --'
798 and be happy. In what follows we shall use this alias.
800 The "test" attribute can be dropped from the database with
802 para rmatt test
804 Read the output of
806 para help ls
807 para help setatt
809 for more information and a complete list of command line options to
810 these commands.
812 *Blob tables*
814 The image, lyrics, moods and playlists tables are all blob tables.
815 Blob tables consist of three columns each: The identifier which is
816 a positive non-negative number that is auto-incremented, the name
817 (an arbitrary string) and the content (the blob).
819 All blob tables support the same set of actions: cat, ls, mv, rm
820 and add. Of course, _add_ is used for adding new blobs to the table
821 while the other actions have the same meaning as the corresponding
822 Unix commands. The paraslash commands to perform these actions are
823 constructed as the concatenation of the table name and the action. For
824 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
825 manipulate or query the image table.
827 The add variant of these commands is special as these commands read
828 the blob contents from stdin. To add an image to the image table the
829 command
831 para addimg image_name < file.jpg
833 can be used.
835 Note that the images and lyrics are not interpreted at all, and also
836 the playlist and the mood blobs are only investigated when the mood
837 or playlist is activated with the select command.
839 *The score table*
841 Unlike all other tables the contents of the score table remain in
842 memory and are never stored on disk. The score table contains two
843 columns: The SHA1 hash value (of an audio file) and its current
844 score.
846 However, only those files which are admissible for the current mood
847 or playlist are contained in the score table. The audio file selector
848 always chooses the row with the highest score as the file to stream
849 next. While doing so, it computes the new score and updates the
850 last_played and the num_played fields in the audio file table.
852 The score table is recomputed by the select command which loads a
853 mood or playlist. Audio files are chosen for streaming from the rows
854 of the score table on a highest-score-first basis.
857 Playlists and moods
858 ~~~~~~~~~~~~~~~~~~~
860 Playlists and moods offer two different ways of specifying the set of
861 admissible files. A playlist in itself describes a set of admissible
862 files. A mood, in contrast, describes the set of admissible files in
863 terms of attributes and other type of information available in the
864 audio file table. As an example, a mood can define a filename pattern,
865 which is then matched against the names of audio files in the table.
867 *Playlists*
869 Playlists are accommodated in the playlist table of the afs database,
870 using the aforementioned blob format for tables. A new playlist is
871 created with the addpl command by specifying the full (absolute)
872 paths of all desired audio files, separated by newlines. Example:
874 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
876 If _my_playlist_ already exists it is overwritten. To activate the
877 new playlist, execute
879 para select p/my_playlist
881 The audio file selector will assign scores to each entry of the list,
882 in descending order so that files will be selected in order. If a
883 file could not be opened for streaming, its entry is removed from
884 the score table (but not from the playlist).
886 *Moods*
888 A mood consists of a unique name and its *mood definition*, which is
889 a set of *mood lines* containing expressions in terms of attributes
890 and other data contained in the database.
892 At any time at most one mood can be *active* which means that
893 para_server is going to select only files from that subset of
894 admissible files.
896 So in order to create a mood definition one has to write a set of
897 mood lines. Mood lines come in three flavours: Accept lines, deny
898 lines and score lines.
900 The general syntax of the three types of mood lines is
903 accept [with score <score>] [if] [not] <mood_method> [options]
904 deny [with score <score>] [if] [not] <mood_method> [options]
905 score <score> [if] [not] <mood_method> [options]
908 Here <score> is either an integer or the string "random" which assigns
909 a random score to all matching files. The score value changes the
910 order in which admissible files are going to be selected, but is of
911 minor importance for this introduction.
913 So we concentrate on the first two forms, i.e. accept and deny
914 lines. As usual, everything in square brackets is optional, i.e.
915 accept/deny lines take the following form when ignoring scores:
917 accept [if] [not] <mood_method> [options]
919 and analogously for the deny case. The "if" keyword is only syntactic
920 sugar and has no function. The "not" keyword just inverts the result,
921 so the essence of a mood line is the mood method part and the options
922 following thereafter.
924 A *mood method* is realized as a function which takes an audio file
925 and computes a number from the data contained in the database.
926 If this number is non-negative, we say the file *matches* the mood
927 method. The file matches the full mood line if it either
929 - matches the mood method and the "not" keyword is not given,
930 or
931 - does not match the mood method, but the "not" keyword is given.
933 The set of admissible files for the whole mood is now defined as those
934 files which match at least one accept mood line, but no deny mood line.
935 More formally, an audio file F is admissible if and only if
937 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
939 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
940 lines and "~" means "matches".
942 The cases where no mood lines of accept/deny type are defined need
943 special treatment:
945 - Neither accept nor deny lines: This treats all files as
946 admissible (in fact, that is the definition of the dummy mood
947 which is activated automatically if no moods are available).
949 - Only accept lines: A file is admissible iff it matches at
950 least one accept line:
952 F ~ AL1 or F ~ AL2 or ...
954 - Only deny lines: A file is admissible iff it matches no
955 deny line:
957 not (F ~ DL1 or F ~ DN2 ...)
961 *List of mood_methods*
963 no_attributes_set
965 Takes no arguments and matches an audio file if and only if no
966 attributes are set.
968 is_set <attribute_name>
970 Takes the name of an attribute and matches iff that attribute is set.
972 path_matches <pattern>
974 Takes a filename pattern and matches iff the path of the audio file
975 matches the pattern.
977 artist_matches <pattern>
978 album_matches <pattern>
979 title_matches <pattern>
980 comment_matches <pattern>
982 Takes an extended regular expression and matches iff the text of the
983 corresponding tag of the audio file matches the pattern. If the tag
984 is not set, the empty string is matched against the pattern.
986 year ~ <num>
987 bitrate ~ <num>
988 frequency ~ <num>
989 channels ~ <num>
990 num_played ~ <num>
992 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
993 <num>. Matches an audio file iff the condition <val> ~ <num> is
994 satisfied where val is the corresponding value of the audio file
995 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
996 count, play count).
998 The year tag is special as its value is undefined if the audio file
999 has no year tag or the content of the year tag is not a number. Such
1000 audio files never match. Another difference is the special treatment
1001 if the year tag is a two-digit number. In this case either 1900 or
1002 2000 is added to the tag value, depending on whether the number is
1003 greater than 2000 plus the current year.
1006 *Mood usage*
1008 To create a new mood called "my_mood", write its definition into
1009 some temporary file, say "tmpfile", and add it to the mood table
1010 by executing
1012 para addmood my_mood < tmpfile
1014 If the mood definition is really short, you may just pipe it to the
1015 client instead of using temporary files. Like this:
1017 echo "$MOOD_DEFINITION" | para addmood my_mood
1019 There is no need to keep the temporary file since you can always use
1020 the catmood command to get it back:
1022 para catmood my_mood
1024 A mood can be activated by executing
1026 para select m/my_mood
1028 Once active, the list of admissible files is shown by the ls command
1029 if the "-a" switch is given:
1031 para ls -a
1034 *Example mood definition*
1036 Suppose you have defined attributes "punk" and "rock" and want to define
1037 a mood containing only Punk-Rock songs. That is, an audio file should be
1038 admissible if and only if both attributes are set. Since
1040 punk and rock
1042 is obviously the same as
1044 not (not punk or not rock)
1046 (de Morgan's rule), a mood definition that selects only Punk-Rock
1047 songs is
1049 deny if not is_set punk
1050 deny if not is_set rock
1054 File renames and content changes
1055 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1057 Since the audio file selector knows the SHA1 of each audio file that
1058 has been added to the afs database, it recognizes if the content of
1059 a file has changed, e.g. because an ID3 tag was added or modified.
1060 Also, if a file has been renamed or moved to a different location,
1061 afs will detect that an entry with the same hash value already exists
1062 in the audio file table.
1064 In both cases it is enough to just re-add the new file. In the
1065 first case (file content changed), the audio table is updated, while
1066 metadata such as the num_played and last_played fields, as well as
1067 the attributes, remain unchanged. In the other case, when the file
1068 is moved or renamed, only the path information is updated, all other
1069 data remains as before.
1071 It is possible to change the behaviour of the add command by using the
1072 "-l" (lazy add) or the "-f" (force add) option.
1074 Troubleshooting
1075 ~~~~~~~~~~~~~~~
1077 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1078 executables have a brief online help which is displayed when -h is
1079 given. The --detailed-help option prints the full help text.
1081 If para_server crashed or was killed by SIGKILL (signal 9), it
1082 may refuse to start again because of "dirty osl tables". In this
1083 case you'll have to run the oslfsck program of libosl to fix your
1084 database:
1086 oslfsck -fd ~/.paraslash/afs_database-0.4
1088 However, make sure para_server isn't running before executing oslfsck.
1090 If you don't mind to recreate your database you can start
1091 from scratch by removing the entire database directory, i.e.
1093 rm -rf ~/.paraslash/afs_database-0.4
1095 Be aware that this removes all attribute definitions, all playlists
1096 and all mood definitions and requires to re-initialize the tables.
1098 Although oslfsck fixes inconsistencies in database tables it doesn't
1099 care about the table contents. To check for invalid table contents, use
1101 para_client check
1103 This prints out references to missing audio files as well as invalid
1104 playlists and mood definitions.
1106 Similarly, para_audiod refuses to start if its socket file exists, since
1107 this indicates that another instance of para_audiod is running. After
1108 a crash a stale socket file might remain and you must run
1110 para_audiod --force
1112 once to fix it up.
1114 ---------------------------------------
1115 Audio formats and audio format handlers
1116 ---------------------------------------
1118 Audio formats
1119 ~~~~~~~~~~~~~
1121 The following audio formats are supported by paraslash:
1123 *MP3*
1125 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1126 designed as part of its MPEG-1 standard. An MP3 file is made up of
1127 multiple MP3 frames, which consist of a header and a data block. The
1128 size of an MP3 frame depends on the bit rate and on the number
1129 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1130 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1131 400 bytes large.
1133 *OGG/Vorbis*
1135 OGG is a standardized audio container format, while Vorbis is an
1136 open source codec for lossy audio compression. Since Vorbis is most
1137 commonly made available via the OGG container format, it is often
1138 referred to as OGG/Vorbis. The OGG container format divides data into
1139 chunks called OGG pages. A typical OGG page is about 4KB large. The
1140 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1141 may vary considerably.
1143 *OGG/Speex*
1145 Speex is an open-source speech codec that is based on CELP (Code
1146 Excited Linear Prediction) coding. It is designed for voice
1147 over IP applications, has modest complexity and a small memory
1148 footprint. Wideband and narrowband (telephone quality) speech are
1149 supported. As for Vorbis audio, Speex bit-streams are often stored
1150 in OGG files. As of 2012 this codec is considered obsolete since the
1151 Oppus codec, described below, surpasses its performance in all areas.
1153 *OGG/Opus*
1155 Opus is a lossy audio compression format standardized through RFC
1156 6716 in 2012. It combines the speech-oriented SILK codec and the
1157 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1158 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1159 containers. All known software patents which cover Opus are licensed
1160 under royalty-free terms.
1162 *AAC*
1164 Advanced Audio Coding (AAC) is a standardized, lossy compression
1165 and encoding scheme for digital audio which is the default audio
1166 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1167 the container format and audio files encoded with AAC have the .m4a
1168 extension. A typical AAC frame is about 700 bytes large.
1170 *WMA*
1172 Windows Media Audio (WMA) is an audio data compression technology
1173 developed by Microsoft. A WMA file is usually encapsulated in the
1174 Advanced Systems Format (ASF) container format, which also specifies
1175 how meta data about the file is to be encoded. The bit stream of WMA
1176 is composed of superframes, each containing one or more frames of
1177 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1179 *FLAC*
1181 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1182 loss. It gives better compression ratios than a general purpose
1183 compressor like zip or bzip2 because FLAC is designed specifically
1184 for audio. A FLAC-encoded file consists of frames of varying size, up
1185 to 16K. Each frame starts with a header that contains all information
1186 necessary to decode the frame.
1188 Meta data
1189 ~~~~~~~~~
1191 Unfortunately, each audio format has its own conventions how meta
1192 data is added as tags to the audio file.
1194 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1195 is rather simple but also very limited as it supports only artist,
1196 title, album, year and comment tags. Each of these can only be at most
1197 32 characters long. ID3, version 2 is much more flexible but requires
1198 a separate library being installed for paraslash to support it.
1200 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1201 comments, which are typically implemented as strings of the form
1202 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1203 tags are appropriate for the content.
1205 AAC files usually use the MPEG-4 container format for storing meta
1206 data while WMA files wrap meta data as special objects within the
1207 ASF container format.
1209 paraslash only tracks the most common tags that are supported by
1210 all tag variants: artist, title, year, album, comment. When a file
1211 is added to the AFS database, the meta data of the file is extracted
1212 and stored in the audio file table.
1214 Chunks and chunk tables
1215 ~~~~~~~~~~~~~~~~~~~~~~~
1217 paraslash uses the word "chunk" as common term for the building blocks
1218 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1219 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1220 size varies considerably between audio formats, from a few hundred
1221 bytes (MP3) up to 16K (FLAC).
1223 The chunk table contains the offsets within the audio file that
1224 correspond to the chunk boundaries of the file. Like the meta data,
1225 the chunk table is computed and stored in the database whenever an
1226 audio file is added.
1228 The paraslash senders (see below) always send complete chunks. The
1229 granularity for seeking is therefore determined by the chunk size.
1231 Audio format handlers
1232 ~~~~~~~~~~~~~~~~~~~~~
1234 For each audio format paraslash contains an audio format handler whose
1235 first task is to tell whether a given file is a valid audio file of
1236 this type. If so, the audio file handler extracts some technical data
1237 (duration, sampling rate, number of channels etc.), computes the
1238 chunk table and reads the meta data.
1240 The audio format handler code is linked into para_server and executed
1241 via the _add_ command. The same code is also available as a stand-alone
1242 tool, para_afh, which prints the technical data, the chunk table
1243 and the meta data of a file. Moreover, all audio format handlers are
1244 combined in the afh receiver which is part of para_recv and para_play.
1246 ----------
1247 Networking
1248 ----------
1250 Paraslash uses different network connections for control and data.
1251 para_client communicates with para_server over a dedicated TCP control
1252 connection. To transport audio data, separate data connections are
1253 used. For these data connections, a variety of transports (UDP, DCCP,
1254 HTTP) can be chosen.
1256 The chapter starts with the REFERENCE(The paraslash control
1257 service, control service), followed by a section on the various
1258 REFERENCE(Streaming protocols, streaming protocols) in which the data
1259 connections are described. The way audio file headers are embedded into
1260 the stream is discussed REFERENCE(Streams with headers and headerless
1261 streams, briefly) before the REFERENCE(Networking examples, example
1262 section) which illustrates typical commands for real-life scenarios.
1264 Both IPv4 and IPv6 are supported.
1266 The paraslash control service
1267 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1269 para_server is controlled at runtime via the paraslash control
1270 connection. This connection is used for server commands (play, stop,
1271 ...) as well as for afs commands (ls, select, ...).
1273 The server listens on a TCP port and accepts connections from clients
1274 that connect the open port. Each connection causes the server to fork
1275 off a client process which inherits the connection and deals with that
1276 client only. In this classical accept/fork approach the server process
1277 is unaffected if the child dies or goes crazy for whatever reason. In
1278 fact, the child process can not change address space of server process.
1280 The section on REFERENCE(Client-server authentication, client-server
1281 authentication) above described the early connection establishment
1282 from the crypto point of view. Here it is described what happens
1283 after the connection (including crypto setup) has been established.
1284 There are four processes involved during command dispatch as sketched
1285 in the following diagram.
1287 <<
1288 <pre>
1289 server_host client_host
1290 ~~~~~~~~~~~ ~~~~~~~~~~~
1292 +-----------+ connect +-----------+
1293 |para_server|<------------------------------ |para_client|
1294 +-----------+ +-----------+
1295 | ^
1296 | fork +---+ |
1297 +----------> |AFS| |
1298 | +---+ |
1299 | ^ |
1300 | | |
1301 | | connect (cookie) |
1302 | | |
1303 | | |
1304 | fork +-----+ inherited connection |
1305 +---------->|child|<--------------------------+
1306 +-----+
1307 </pre>
1308 >>
1310 Note that the child process is not a child of the afs process,
1311 so communication of these two processes has to happen via local
1312 sockets. In order to avoid abuse of the local socket by unrelated
1313 processes, a magic cookie is created once at server startup time just
1314 before the server process forks off the AFS process. This cookie is
1315 known to the server, AFS and the child, but not to unrelated processes.
1317 There are two different kinds of commands: First there are commands
1318 that cause the server to respond with some answer such as the list
1319 of all audio files. All but the addblob commands (addimg, addlyr,
1320 addpl, addmood) are of this kind. The addblob commands add contents
1321 to the database, so they need to transfer data the other way round,
1322 from the client to the server.
1324 There is no knowledge about the server commands built into para_client,
1325 so it does not know about addblob commands. Instead, it inspects the
1326 first data package sent by the server for a magic string. If this
1327 string was found, it sends STDIN to the server, otherwise it dumps
1328 data from the server to STDOUT.
1330 Streaming protocols
1331 ~~~~~~~~~~~~~~~~~~~
1333 A network (audio) stream usually consists of one streaming source,
1334 the _sender_, and one or more _receivers_ which read data over the
1335 network from the streaming source.
1337 Senders are thus part of para_server while receivers are part of
1338 para_audiod. Moreover, there is the stand-alone tool para_recv which
1339 can be used to manually download a stream, either from para_server
1340 or from a web-based audio streaming service.
1342 The following three streaming protocols are supported by paraslash:
1344 - HTTP. Recommended for public streams that can be played by
1345 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1346 sender is supported on all operating systems and all platforms.
1348 - DCCP. Recommended for LAN streaming. DCCP is currently
1349 available only for Linux.
1351 - UDP. Recommended for multicast LAN streaming.
1353 See the Appendix on REFERENCE(Network protocols, network protocols)
1354 for brief descriptions of the various protocols relevant for network
1355 audio streaming with paraslash.
1357 It is possible to activate more than one sender simultaneously.
1358 Senders can be controlled at run time and via config file and command
1359 line options.
1361 Note that audio connections are _not_ encrypted. Transport or Internet
1362 layer encryption should be used if encrypted data connections are
1363 needed.
1365 Since DCCP and TCP are both connection-oriented protocols, connection
1366 establishment/teardown and access control are very similar between
1367 these two streaming protocols. UDP is the most lightweight option,
1368 since in contrast to TCP/DCCP it is connectionless. It is also the
1369 only protocol supporting IP multicast.
1371 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1372 clients to connect and establish a connection via some protocol-defined
1373 handshake mechanism. Both senders maintain two linked lists each:
1374 The list of all clients which are currently connected, and the list
1375 of access control entries which determines who is allowed to connect.
1376 IP-based access control may be configured through config file and
1377 command line options and via the "allow" and "deny" sender subcommands.
1379 Upon receiving a GET request from the client, the HTTP sender sends
1380 back a status line and a message. The body of this message is the
1381 audio stream. This is common practice and is supported by many popular
1382 clients which can thus be used to play a stream offered by para_server.
1383 For DCCP things are a bit simpler: No messages are exchanged between
1384 the receiver and sender. The client simply connects and the sender
1385 starts to stream.
1387 DCCP is an experimental protocol which offers a number of new features
1388 not available for TCP. Both ends can negotiate these features using
1389 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1390 datagram-based (no retransmissions) and thus should not be used over
1391 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1392 is access to a variety of different congestion-control mechanisms
1393 called CCIDs. Two different CCIDs are available per default on Linux:
1396 - _CCID 2_. A Congestion Control mechanism similar to that
1397 of TCP. The sender maintains a congestion window and halves
1398 this window in response to congestion.
1401 - _CCID-3_. Designed to be fair when competing for bandwidth.
1402 It has lower variation of throughput over time compared with
1403 TCP, which makes it suitable for streaming media.
1405 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1406 single list, the _target list_. This list describes the set of clients
1407 to which the stream is sent. There is no list for access control and
1408 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1409 and "delete" commands can be used to modify the target list.
1411 Since both UDP and DCCP offer an unreliable datagram-based transport,
1412 additional measures are necessary to guard against disruptions over
1413 networks that are lossy or which may be subject to interference (as
1414 is for instance the case with WiFi). Paraslash uses FEC (Forward
1415 Error Correction) to guard against packet losses and reordering. The
1416 stream is FEC-encoded before it is sent through the UDP socket and
1417 must be decoded accordingly on the receiver side.
1419 The packet size and the amount of redundancy introduced by FEC can
1420 be configured via the FEC parameters which are dictated by server
1421 and may also be configured through the "sender" command. The FEC
1422 parameters are encoded in the header of each network packet, so no
1423 configuration is necessary on the receiver side. See the section on
1424 REFERENCE(Forward error correction, FEC) below.
1426 Streams with headers and headerless streams
1427 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1429 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1430 needed to decode the stream is only contained in the audio file
1431 header of the container format but not in each data chunk. Clients
1432 must be able to obtain this information in case streaming starts in
1433 the middle of the file or if para_audiod is started while para_server
1434 is already sending a stream.
1436 This is accomplished in different ways, depending on the streaming
1437 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1438 header is sent prior to audio file data. This technique however does
1439 not work for the connectionless UDP transport. Hence the audio file
1440 header is periodically being embedded into the UDP audio data stream.
1441 By default, the header is resent after five seconds. The receiver has
1442 to wait until the next header arrives before it can start decoding
1443 the stream.
1445 Examples
1446 ~~~~~~~~
1448 The "si" (server info) command lists some information about the
1449 currently running server process.
1451 -> Show PIDs, number of connected clients, uptime, and more:
1453 para_client si
1455 The sender command of para_server prints information about senders,
1456 like the various access control lists, and it allows to (de-)activate
1457 senders and to change the access permissions at runtime.
1459 -> List all senders
1461 para_client sender
1463 -> Obtain general help for the sender command:
1465 para_client help sender
1467 -> Get help for a specific sender (contains further examples):
1469 s=http # or dccp or udp
1470 para_client sender $s help
1472 -> Show status of the http sender
1474 para_client sender http status
1476 By default para_server activates both the HTTP and th DCCP sender on
1477 startup. This can be changed via command line options or para_server's
1478 config file.
1480 -> List config file options for senders:
1482 para_server -h
1484 All senders share the "on" and "off" commands, so senders may be
1485 activated and deactivated independently of each other.
1487 -> Switch off the http sender:
1489 para_client sender http off
1491 -> Receive a DCCP stream using CCID2 and write the output into a file:
1493; ccid=2; filename=bar
1494 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1496 Note the quotes around the arguments for the dccp receiver. Each
1497 receiver has its own set of command line options and its own command
1498 line parser, so arguments for the dccp receiver must be protected
1499 from being interpreted by para_recv.
1501 -> Start UDP multicast, using the default multicast address:
1503 para_client sender udp add
1505 -> Receive FEC-encoded multicast stream and write the output into a file:
1507 filename=foo
1508 para_recv -r udp > $filename
1510 -> Add an UDP unicast for a client to the target list of the UDP sender:
1513 para_client sender udp add $t
1515 -> Receive this (FEC-encoded) unicast stream:
1517 filename=foo
1518 para_recv -r 'udp -i' > $filename
1520 -> Create a minimal config for para_audiod for HTTP streams:
1522 c=$HOME/.paraslash/audiod.conf.min;
1523 echo receiver \".:http -i $s\" > $c
1524 para_audiod --config $c
1526 -------
1527 Filters
1528 -------
1530 A paraslash filter is a module which transforms an input stream into
1531 an output stream. Filters are included in the para_audiod executable
1532 and in the stand-alone tool para_filter which usually contains the
1533 same modules.
1535 While para_filter reads its input stream from STDIN and writes
1536 the output to STDOUT, the filter modules of para_audiod are always
1537 connected to a receiver which produces the input stream and a writer
1538 which absorbs the output stream.
1540 Some filters depend on a specific library and are not compiled in
1541 if this library was not found at compile time. To see the list of
1542 supported filters, run para_filter and para_audiod with the --help
1543 option. The output looks similar to the following:
1545 Available filters:
1546 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1548 Out of these filter modules, a chain of filters can be constructed,
1549 much in the way Unix pipes can be chained, and analogous to the use
1550 of modules in gstreamer: The output of the first filter becomes the
1551 input of the second filter. There is no limitation on the number of
1552 filters and the same filter may occur more than once.
1554 Like receivers, each filter has its own command line options which
1555 must be quoted to protect them from the command line options of
1556 the driving application (para_audiod or para_filter). Example:
1558 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1560 For para_audiod, each audio format has its own set of filters. The
1561 name of the audio format for which the filter should be applied can
1562 be used as the prefix for the filter option. Example:
1564 para_audiod -f 'mp3:prebuffer --duration 300'
1566 The "mp3" prefix above is actually interpreted as a POSIX extended
1567 regular expression. Therefore
1569 para_audiod -f '.:prebuffer --duration 300'
1571 activates the prebuffer filter for all supported audio formats (because
1572 "." matches all audio formats) while
1574 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1576 activates it only for wma and ogg streams.
1578 Decoders
1579 ~~~~~~~~
1581 For each supported audio format there is a corresponding filter
1582 which decodes audio data in this format to 16 bit PCM data which
1583 can be directly sent to the sound device or any other software that
1584 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1585 filters are called _decoders_ in general, and xxxdec is the name of
1586 the paraslash decoder for the audio format xxx. For example, the mp3
1587 decoder filter is called mp3dec.
1589 Note that the output of the decoder is about 10 times larger than
1590 its input. This means that filters that operate on the decoded audio
1591 stream have to deal with much more data than filters that transform
1592 the audio stream before it is fed to the decoder.
1594 Paraslash relies on external libraries for most decoders, so these
1595 libraries must be installed for the decoder to be included in the
1596 para_filter and para_audiod executables. The oggdec filter depends
1597 on the libogg and libvorbis libraries for example.
1599 Forward error correction
1600 ~~~~~~~~~~~~~~~~~~~~~~~~
1602 As already mentioned REFERENCE(Streaming protocols, earlier),
1603 paraslash uses forward error correction (FEC) for the unreliable UDP
1604 and DCCP transports. FEC is a technique which was invented already
1605 in 1960 by Reed and Solomon and which is widely used for the parity
1606 calculations of storage devices (RAID arrays). It is based on the
1607 algebraic concept of finite fields, today called Galois fields, in
1608 honour of the mathematician Galois (1811-1832). The FEC implementation
1609 of paraslash is based on code by Luigi Rizzo.
1611 Although the details require a sound knowledge of the underlying
1612 mathematics, the basic idea is not hard to understand: For positive
1613 integers k and n with k < n it is possible to compute for any k given
1614 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1615 ..., p_r such that all data bytes can be reconstructed from *any*
1616 k bytes of the set
1618 {d_1, ..., d_k, p_1, ..., p_r}.
1620 FEC-encoding for unreliable network transports boils down to slicing
1621 the audio stream into groups of k suitably sized pieces called _slices_
1622 and computing the r corresponding parity slices. This step is performed
1623 in para_server which then sends both the data and the parity slices
1624 over the unreliable network connection. If the client was able
1625 to receive at least k of the n = k + r slices, it can reconstruct
1626 (FEC-decode) the original audio stream.
1628 From these observations it is clear that there are three different
1629 FEC parameters: The slice size, the number of data slices k, and the
1630 total number of slices n. It is crucial to choose the slice size
1631 such that no fragmentation of network packets takes place because
1632 FEC only guards against losses and reordering but fails if slices are
1633 received partially.
1635 FEC decoding in paralash is performed through the fecdec filter which
1636 usually is the first filter (there can be other filters before fecdec
1637 if these do not alter the audio stream).
1640 Volume adjustment (amp and compress)
1641 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1643 The amp and the compress filter both adjust the volume of the audio
1644 stream. These filters operate on uncompressed audio samples. Hence
1645 they are usually placed directly after the decoding filter. Each
1646 sample is multiplied with a scaling factor (>= 1) which makes amp
1647 and compress quite expensive in terms of computing power.
1649 *amp*
1651 The amp filter amplifies the audio stream by a fixed scaling factor
1652 that must be known in advance. For para_audiod this factor is derived
1653 from the amplification field of the audio file's entry in the audio
1654 file table while para_filter uses the value given at the command line.
1656 The optimal scaling factor F for an audio file is the largest real
1657 number F >= 1 such that after multiplication with F all samples still
1658 fit into the sample interval [-32768, 32767]. One can use para_filter
1659 in combination with the sox utility to compute F:
1661 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1663 The amplification value V which is stored in the audio file table,
1664 however, is an integer between 0 and 255 which is connected to F
1665 through the formula
1667 V = (F - 1) * 64.
1669 To store V in the audio file table, the command
1671 para_client -- touch -a=V file.mp3
1673 is used. The reader is encouraged to write a script that performs
1674 these computations :)
1676 *compress*
1678 Unlike the amplification filter, the compress filter adjusts the volume
1679 of the audio stream dynamically without prior knowledge about the peak
1680 value. It maintains the maximal volume of the last n samples of the
1681 audio stream and computes a suitable amplification factor based on that
1682 value and the various configuration options. It tries to chose this
1683 factor such that the adjusted volume meets the desired target level.
1685 Note that it makes sense to combine amp and compress.
1687 Misc filters (wav and prebuffer)
1688 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1690 These filters are rather simple and do not modify the audio stream at
1691 all. The wav filter is only useful with para_filter and in connection
1692 with a decoder. It asks the decoder for the number of channels and the
1693 sample rate of the stream and adds a Microsoft wave header containing
1694 this information at the beginning. This allows to write wav files
1695 rather than raw PCM files (which do not contain any information about
1696 the number of channels and the sample rate).
1698 The prebuffer filter simply delays the output until the given time has
1699 passed (starting from the time the first byte was available in its
1700 input queue) or until the given amount of data has accumulated. It
1701 is mainly useful for para_audiod if the standard parameters result
1702 in buffer underruns.
1704 Both filters require almost no additional computing time, even when
1705 operating on uncompressed audio streams, since data buffers are simply
1706 "pushed down" rather than copied.
1708 Examples
1709 ~~~~~~~~
1711 -> Decode an mp3 file to wav format:
1713 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1715 -> Amplify a raw audio file by a factor of 1.5:
1717 para_filter -f amp --amp 32 < foo.raw > bar.raw
1719 ------
1720 Output
1721 ------
1723 Once an audio stream has been received and decoded to PCM format,
1724 it can be sent to a sound device for playback. This part is performed
1725 by paraslash _writers_ which are described in this chapter.
1727 Writers
1728 ~~~~~~~
1730 A paraslash writer acts as a data sink that consumes but does not
1731 produce audio data. Paraslash writers operate on the client side and
1732 are contained in para_audiod and in the stand-alone tool para_write.
1734 The para_write program reads uncompressed audio data from STDIN. If
1735 this data starts with a wav header, sample rate, sample format and
1736 channel count are read from the header. Otherwise CD audio (44.1KHz
1737 16 bit little endian, stereo) is assumed but this can be overridden
1738 by command line options. para_audiod, on the other hand, obtains
1739 the sample rate and the number of channels from the decoder.
1741 Like receivers and filters, each writer has an individual set of
1742 command line options, and for para_audiod writers can be configured
1743 per audio format separately. It is possible to activate more than
1744 one writer for the same stream simultaneously.
1746 OS-dependent APIs
1747 ~~~~~~~~~~~~~~~~~
1749 Unfortunately, the various flavours of Unix on which paraslash
1750 runs on have different APIs for opening a sound device and starting
1751 playback. Hence for each such API there is a paraslash writer that
1752 can play the audio stream via this API.
1754 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1755 Linux systems. Although there are several mid-layer APIs in use by
1756 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1757 currently supports only the low-level ALSA API which is not supposed
1758 to be change. ALSA is very feature-rich, in particular it supports
1759 software mixing via its DMIX plugin. ALSA is the default writer on
1760 Linux systems.
1762 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1763 is also available on Linux systems, usually provided by ALSA as an
1764 emulation for backwards compatibility. This API is rather simple but
1765 also limited. For example only one application can open the device
1766 at any time. The OSS writer is activated by default on BSD Systems.
1768 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1769 for this API is only compiled in on such systems and is of course
1770 the default there.
1772 *FILE*. The file writer allows to capture the audio stream and
1773 write the PCM data to a file on the file system rather than playing
1774 it through a sound device. It is supported on all platforms and is
1775 always compiled in.
1777 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1778 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1779 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1780 through an output plugin of libao.
1782 Examples
1783 ~~~~~~~~
1785 -> Use the OSS writer to play a wav file:
1787 para_write --writer oss < file.wav
1789 -> Enable ALSA software mixing for mp3 streams
1791 para_audiod --writer 'mp3:alsa -d plug:swmix'
1794 ---
1795 Gui
1796 ---
1798 para_gui executes an arbitrary command which is supposed to print
1799 status information to STDOUT. It then displays this information in
1800 a curses window. By default the command
1802 para_audioc -- stat -p
1804 is executed, but this can be customized via the --stat-cmd option. In
1805 particular it possible to use
1807 para_client -- stat -p
1809 to make para_gui work on systems on which para_audiod is not running.
1811 Key bindings
1812 ~~~~~~~~~~~~
1814 It is possible to bind keys to arbitrary commands via custom
1815 key-bindings. Besides the internal keys which can not be changed (help,
1816 quit, loglevel, version...), the following flavours of key-bindings
1817 are supported:
1819 - external: Shutdown curses before launching the given command.
1820 Useful for starting other ncurses programs from within
1821 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1822 output format to write a mailbox containing one mail for each
1823 (admissible) file the audio file selector knows about. Then
1824 start mutt from within para_gui to browse your collection!
1826 - display: Launch the command and display its stdout in
1827 para_gui's bottom window.
1829 - para: Like display, but start "para_client <specified
1830 command>" instead of "<specified command>".
1832 The general form of a key binding is
1834 key_map k:m:c
1836 which maps key k to command c using mode m. Mode may be x, d or p
1837 for external, display and paraslash commands, respectively.
1839 Themes
1840 ~~~~~~
1842 Currently there are only two themes for para_gui. It is easy, however,
1843 to add more themes. To create a new theme one has to define the
1844 position, color and geometry for for each status item that should be
1845 shown by this theme. See gui_theme.c for examples.
1847 The "." and "," keys are used to switch between themes.
1849 Examples
1850 ~~~~~~~~
1852 -> Show server info:
1854 key_map "i:p:si"
1856 -> Jump to the middle of the current audio file by pressing F5:
1858 key_map "<F5>:p:jmp 50"
1860 -> vi-like bindings for jumping around:
1862 key_map "l:p:ff 10"
1863 key_map "h:p:ff 10-"
1864 key_map "w:p:ff 60"
1865 key_map "b:p:ff 60-"
1867 -> Print the current date and time:
1869 key_map "D:d:date"
1871 -> Call other curses programs:
1873 key_map "U:x:aumix"
1874 key_map "!:x:/bin/bash"
1875 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1877 -----------
1878 Development
1879 -----------
1881 Tools
1882 ~~~~~
1884 In order to compile the sources from the git repository (rather than
1885 from tar balls) and for contributing non-trivial changes to the
1886 paraslash project, some additional tools should be installed on a
1887 developer machine.
1889 (git). As described in more detail REFERENCE(Git
1890 branches, below), the git source code management tool is used for
1891 paraslash development. It is necessary for cloning the git repository
1892 and for getting updates.
1894 (m4). Some input files for gengetopt
1895 are generated from templates by the m4 macro processor.
1897 (autoconf) GNU autoconf creates
1898 the configure file which is shipped in the tarballs but has to be
1899 generated when compiling from git.
1901 (grutatxt). The
1902 HTML version of this manual and some of the paraslash web pages are
1903 generated by the grutatxt plain text to HTML converter. If changes
1904 are made to these text files the grutatxt package must be installed
1905 to regenerate the HTML files.
1907 (doxygen). The documentation
1908 of paraslash's C sources uses the doxygen documentation system. The
1909 conventions for documenting the source code is described in the
1910 REFERENCE(Doxygen, Doxygen section).
1912 (global). This is used to generate
1913 browsable HTML from the C sources. It is needed by doxygen.
1915 Git branches
1916 ~~~~~~~~~~~~
1918 Paraslash has been developed using the git source code management
1919 tool since 2006. Development is organized roughly in the same spirit
1920 as the git development itself, as described below.
1922 The following text passage is based on "A note from the maintainer",
1923 written by Junio C Hamano, the maintainer of git.
1925 There are four branches in the paraslash repository that track the
1926 source tree: "master", "maint", "next", and "pu".
1928 The "master" branch is meant to contain what is well tested and
1929 ready to be used in a production setting. There could occasionally be
1930 minor breakages or brown paper bag bugs but they are not expected to
1931 be anything major, and more importantly quickly and easily fixable.
1932 Every now and then, a "feature release" is cut from the tip of this
1933 branch, named with three dotted decimal digits, like 0.4.2.
1935 Whenever changes are about to be included that will eventually lead to
1936 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1937 "master" at that point. Obvious, safe and urgent fixes after the major
1938 release are applied to this branch and maintenance releases are cut
1939 from it. New features never go to this branch. This branch is also
1940 merged into "master" to propagate the fixes forward.
1942 A trivial and safe enhancement goes directly on top of "master".
1943 New development does not usually happen on "master", however.
1944 Instead, a separate topic branch is forked from the tip of "master",
1945 and it first is tested in isolation; Usually there are a handful such
1946 topic branches that are running ahead of "master". The tip of these
1947 branches is not published in the public repository to keep the number
1948 of branches that downstream developers need to worry about low.
1950 The quality of topic branches varies widely. Some of them start out as
1951 "good idea but obviously is broken in some areas" and then with some
1952 more work become "more or less done and can now be tested by wider
1953 audience". Luckily, most of them start out in the latter, better shape.
1955 The "next" branch is to merge and test topic branches in the latter
1956 category. In general, this branch always contains the tip of "master".
1957 It might not be quite rock-solid production ready, but is expected to
1958 work more or less without major breakage. The maintainer usually uses
1959 the "next" version of paraslash for his own pleasure, so it cannot
1960 be _that_ broken. The "next" branch is where new and exciting things
1961 take place.
1963 The two branches "master" and "maint" are never rewound, and "next"
1964 usually will not be either (this automatically means the topics that
1965 have been merged into "next" are usually not rebased, and you can find
1966 the tip of topic branches you are interested in from the output of
1967 "git log next"). You should be able to safely build on top of them.
1969 However, at times "next" will be rebuilt from the tip of "master" to
1970 get rid of merge commits that will never be in "master". The commit
1971 that replaces "next" will usually have the identical tree, but it
1972 will have different ancestry from the tip of "master".
1974 The "pu" (proposed updates) branch bundles the remainder of the
1975 topic branches. The "pu" branch, and topic branches that are only in
1976 "pu", are subject to rebasing in general. By the above definition
1977 of how "next" works, you can tell that this branch will contain quite
1978 experimental and obviously broken stuff.
1980 When a topic that was in "pu" proves to be in testable shape, it
1981 graduates to "next". This is done with
1983 git checkout next
1984 git merge that-topic-branch
1986 Sometimes, an idea that looked promising turns out to be not so good
1987 and the topic can be dropped from "pu" in such a case.
1989 A topic that is in "next" is expected to be polished to perfection
1990 before it is merged to "master". Similar to the above, this is
1991 done with
1993 git checkout master
1994 git merge that-topic-branch
1995 git branch -d that-topic-branch
1997 Note that being in "next" is not a guarantee to appear in the next
1998 release (being in "master" is such a guarantee, unless it is later
1999 found seriously broken and reverted), nor even in any future release.
2001 Coding Style
2002 ~~~~~~~~~~~~
2004 The preferred coding style for paraslash coincides more or less
2005 with the style of the Linux kernel. So rather than repeating what is
2006 written XREFERENCE(,
2007 there), here are the most important points.
2009 - Burn the GNU coding standards.
2010 - Never use spaces for indentation.
2011 - Tabs are 8 characters, and thus indentations are also 8 characters.
2012 - Don't put multiple assignments on a single line.
2013 - Avoid tricky expressions.
2014 - Don't leave whitespace at the end of lines.
2015 - The limit on the length of lines is 80 columns.
2016 - Use K&R style for placing braces and spaces:
2018 if (x is true) {
2019 we do y
2020 }
2022 - Use a space after (most) keywords.
2023 - Do not add spaces around (inside) parenthesized expressions.
2024 - Use one space around (on each side of) most binary and ternary operators.
2025 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2026 - Mixed-case names are frowned upon.
2027 - Descriptive names for global variables are a must.
2028 - Avoid typedefs.
2029 - Functions should be short and sweet, and do just one thing.
2030 - The number of local variables shouldn't exceed 10.
2031 - Gotos are fine if they improve readability and reduce nesting.
2032 - Don't use C99-style "// ..." comments.
2033 - Names of macros defining constants and labels in enums are capitalized.
2034 - Enums are preferred when defining several related constants.
2035 - Always use the paraslash wrappers for allocating memory.
2036 - If the name of a function is an action or an imperative.
2037 command, the function should return an error-code integer
2038 (<0 means error, >=0 means success). If the name is a
2039 predicate, the function should return a "succeeded" boolean.
2042 Doxygen
2043 ~~~~~~~
2045 Doxygen is a documentation system for various programming
2046 languages. The paraslash project uses Doxygen for generating the API
2047 reference on the web pages, but good source code documentation is
2048 also beneficial to people trying to understand the code structure
2049 and the interactions between the various source files.
2051 It is more illustrative to look at the source code for examples than
2052 to describe the conventions for documenting the source in this manual,
2053 so we only describe which parts of the code need doxygen comments,
2054 but leave out details on documentation conventions.
2056 As a rule, only the public part of the C source is documented with
2057 Doxygen. This includes structures, defines and enumerations in header
2058 files as well as public (non-static) C functions. These should be
2059 documented completely. For example each parameter and the return
2060 value of a public function should get a descriptive comment.
2062 No doxygen comments are necessary for static functions and for
2063 structures and enumerations in C files (which are used only within
2064 this file). This does not mean, however, that those entities need
2065 no documentation at all. Instead, common sense should be applied to
2066 document what is not obvious from reading the code.
2068 --------
2069 Appendix
2070 --------
2072 Network protocols
2073 ~~~~~~~~~~~~~~~~~
2075 *IP*. The _Internet Protocol_ is the primary networking protocol
2076 used for the Internet. All protocols described below use IP as the
2077 underlying layer. Both the prevalent IPv4 and the next-generation
2078 IPv6 variant are being deployed actively worldwide.
2080 *Connection-oriented and connectionless protocols*. Connectionless
2081 protocols differ from connection-oriented ones in that state
2082 associated with the sending/receiving endpoints is treated
2083 implicitly. Connectionless protocols maintain no internal knowledge
2084 about the state of the connection. Hence they are not capable of
2085 reacting to state changes, such as sudden loss or congestion on the
2086 connection medium. Connection-oriented protocols, in contrast, make
2087 this knowledge explicit. The connection is established only after
2088 a bidirectional handshake which requires both endpoints to agree
2089 on the state of the connection, and may also involve negotiating
2090 specific parameters for the particular connection. Maintaining an
2091 up-to-date internal state of the connection also in general means
2092 that the sending endpoints perform congestion control, adapting to
2093 qualitative changes of the connection medium.
2095 *Reliability*. In IP networking, packets can be lost, duplicated,
2096 or delivered out of order, and different network protocols handle
2097 these problems in different ways. We call a transport-layer protocol
2098 _reliable_, if it turns the unreliable IP delivery into an ordered,
2099 duplicate- and loss-free delivery of packets. Sequence numbers
2100 are used to discard duplicates and re-arrange packets delivered
2101 out-of-order. Retransmission is used to guarantee loss-free
2102 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2103 or data integrity.
2105 *Classification*. With these definitions the protocols which are used
2106 by paraslash for steaming audio data may be classified as follows.
2108 - HTTP/TCP: connection-oriented, reliable,
2109 - UDP: connectionless, unreliable,
2110 - DCCP: connection-oriented, unreliable.
2112 Below we give a short descriptions of these protocols.
2114 *TCP*. The _Transmission Control Protocol_ provides reliable,
2115 ordered delivery of a stream and a classic window-based congestion
2116 control. In contrast to UDP and DCCP (see below), TCP does not have
2117 record-oriented or datagram-based syntax, i.e. it provides a stream
2118 which is unaware and independent of any record (packet) boundaries.
2119 TCP is used extensively by many application layers. Besides HTTP (the
2120 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2121 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2122 top of TCP.
2124 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2125 protocol, built as a thin layer directly on top of IP. For this reason,
2126 it offers the same best-effort service as IP itself, i.e. there is no
2127 detection of duplicate or reordered packets. Being a connectionless
2128 protocol, only minimal internal state about the connection is
2129 maintained, which means that there is no protection against packet
2130 loss or network congestion. Error checking and correction (if at all)
2131 are performed in the application.
2133 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2134 connection-oriented state maintenance known from TCP with the
2135 unreliable, datagram-based transport of UDP. This means that it
2136 is capable of reacting to changes in the connection by performing
2137 congestion control, offering multiple alternative approaches. But it
2138 is bound to datagram boundaries (the maximum packet size supported
2139 by a medium), and like UDP it lacks retransmission to protect
2140 against loss. Due to the use of sequence numbers, it is however
2141 able to react to loss (interpreted as a congestion indication) and
2142 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2143 to negotiate specific, binding features for a connection, such as
2144 the choice of congestion control: classic, window-based congestion
2145 control known from TCP is available as CCID-2, rate-based, "smooth"
2146 congestion control is offered as CCID-3.
2148 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2149 protocol on top of TCP. It is spoken by web servers and is most often
2150 used for web services. However, as can be seen by the many Internet
2151 radio stations and YouTube/Flash videos, http is by far not limited to
2152 the delivery of web pages only. Being a simple request/response based
2153 protocol, the semantics of the protocol also allow the delivery of
2154 multimedia content, such as audio over http.
2156 *Multicast*. IP multicast is not really a protocol but a technique
2157 for one-to-many communication over an IP network. The challenge is to
2158 deliver information to a group of destinations simultaneously using
2159 the most efficient strategy to send the messages over each link of
2160 the network only once. This has benefits for streaming multimedia:
2161 the standard one-to-one unicast offered by TCP/DCCP means that
2162 n clients listening to the same stream also consume n-times the
2163 resources, whereas multicast requires to send the stream just once,
2164 irrespective of the number of receivers. Since it would be costly to
2165 maintain state for each listening receiver, multicast often implies
2166 connectionless transport, which is the reason that it is currently
2167 only available via UDP.
2169 License
2170 ~~~~~~~
2172 Paraslash is licensed under the GPL, version 2. Most of the code
2173 base has been written from scratch, and those parts are GPL V2
2174 throughout. Notable exceptions are FEC and the WMA decoder. See the
2175 corresponding source files for licencing details for these parts. Some
2176 code sniplets of several other third party software packages have
2177 been incorporated into the paraslash sources, for example log message
2178 coloring was taken from the git sources. These third party software
2179 packages are all published under the GPL or some other license
2180 compatible to the GPL.
2182 Acknowledgements
2183 ~~~~~~~~~~~~~~~~
2185 Many thanks to Gerrit Renker who read an early draft of this manual
2186 and contributed significant improvements.
2188 ----------
2189 References
2190 ----------
2192 Articles
2193 ~~~~~~~~
2194 - Reed, Irving S.; Solomon, Gustave (1960),
2196 Polynomial Codes over Certain Finite Fields), Journal of the
2197 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2198 300-304, doi:10.1137/0108018)
2200 RFCs
2201 ~~~~
2203 - XREFERENCE(, RFC 768) (1980):
2204 User Datagram Protocol
2205 - XREFERENCE(, RFC 791) (1981):
2206 Internet Protocol
2207 - XREFERENCE(, RFC 2437) (1998):
2208 RSA Cryptography Specifications
2209 - XREFERENCE(, RFC 4340)
2210 (2006): Datagram Congestion Control Protocol (DCCP)
2211 - XREFERENCE(, RFC 4341) (2006):
2212 Congestion Control ID 2: TCP-like Congestion Control
2213 - XREFERENCE(, RFC 4342) (2006):
2214 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2215 - XREFERENCE(, RFC 6716) (2012):
2216 Definition of the Opus Audio Codec
2218 Application web pages
2219 ~~~~~~~~~~~~~~~~~~~~~
2221 - XREFERENCE(, paraslash)
2222 - XREFERENCE(, paraslash (alternative page))
2223 - XREFERENCE(, xmms)
2224 - XREFERENCE(, mpg123)
2225 - XREFERENCE(, gstreamer)
2226 - XREFERENCE(, icecast)
2227 - XREFERENCE(, Audio Compress)
2229 External documentation
2230 ~~~~~~~~~~~~~~~~~~~~~~
2233 H. Peter Anvin: The mathematics of Raid6)
2235 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2236 Communication Protocols)
2238 Code
2239 ~~~~
2241 Original FEC implementation by Luigi Rizzo)