oss: Avoid sound artefacts on some setups.
[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://git.tuebingen.mpg.de/osl
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(http://people.tuebingen.mpg.de/maan/osl/, libosl).
218 The _object storage layer_ library is used by para_server. To
219 clone the source code repository, execute
221 git clone git://git.tuebingen.mpg.de/osl
223 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc) or
224 XREFERENCE(http://clang.llvm.org, clang). All gcc versions
225 >= 3.3 are currently supported. Clang version 1.1 or newer
226 should work as well.
228 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
229 also shipped with the disto. On BSD systems the gnu make
230 executable is often called gmake.
232 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
233 scripts which run during compilation require the EMPH(Bourne
234 again shell). It is most likely already installed.
236 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gengetopt/, gengetopt)
237 is needed to generate the C code for the command line parsers
238 of all paraslash executables.
240 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/help2man, help2man)
241 is used to create the man pages.
243 Optional:
245 - XREFERENCE(http://www.openssl.org/, openssl) or
246 XREFERENCE(ftp://ftp.gnupg.org/gcrypt/libgcrypt/, 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(http://www.underbit.com/products/mad/, 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.
259 - XREFERENCE(http://www.underbit.com/products/mad/,
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(http://www.xiph.org/downloads/, 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(http://www.audiocoding.com/, libfaad). For aac
270 files (m4a) you'll need libfaad (libfaad-dev).
272 - XREFERENCE(http://www.speex.org/, speex). In order to stream
273 or decode speex files, libspeex (libspeex-dev) is required.
275 - XREFERENCE(http://flac.sourceforge.net/, flac). To stream
276 or decode files encoded with the _Free Lossless Audio Codec_,
277 libFLAC (libFLAC-dev) must be installed.
279 - XREFERENCE(http://www.mega-nerd.com/SRC/index.html,
280 libsamplerate). The resample filter will only be compiled if
281 this library is installed. Debian package: libsamplerate-dev.
283 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
284 Linux, you'll need to have ALSA's development package
285 libasound2-dev installed.
287 - XREFERENCE(http://downloads.xiph.org/releases/ao/,
288 libao). Needed to build the ao writer (ESD, PulseAudio,...).
289 Debian package: libao-dev.
291 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/ncurses, curses). Needed
292 for para_gui. Debian package: libncurses-dev.
294 - XREFERENCE(http://cnswww.cns.cwru.edu/php/chet/readline/rltop.html,
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 ./autogen.sh
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/id_rsa.pub.$user
371 mkdir -p ~/.paraslash
372 echo "user $user $key $perms" >> $target
374 Next, change to the "bar" account on client_host and generate the
375 key pair with the commands
377 ssh-keygen -t rsa -b 2048
378 # hit enter twice to create a key with no passphrase
380 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
381 that paraslash can also read keys generated by the "openssl genrsa"
382 command. However, since keys created with ssh-keygen can also be used
383 for ssh, this method is recommended.
385 Note that para_server refuses to use a key if it is shorter than 2048
386 bits. In particular, the RSA keys of paraslash 0.3.x will not work
387 with version 0.4.x. Moreover, para_client refuses to use a (private)
388 key which is world-readable.
390 para_server only needs to know the public key of the key pair just
391 created. Copy this public key to server_host:
393 src=~/.ssh/id_rsa.pub
394 dest=.paraslash/id_rsa.pub.$LOGNAME
395 scp $src foo@server_host:$dest
397 Finally, tell para_client to connect to server_host:
399 conf=~/.paraslash/client.conf
400 echo 'hostname server_host' > $conf
403 *Step 2*: Start para_server
405 Before starting the server make sure you have write permissions to
406 the directory /var/paraslash that has been created during installation:
408 sudo chown $LOGNAME /var/paraslash
410 Alternatively, use the --afs-socket Option to specify a different
411 location for the AFS command socket.
413 For this first try, we'll use the info loglevel to make the output
414 of para_server more verbose.
416 para_server -l info
418 Now you can use para_client to connect to the server and issue
419 commands. Open a new shell as bar@client_host and try
421 para_client help
422 para_client si
424 to retrieve the list of available commands and some server info.
425 Don't proceed if this doesn't work.
427 *Step 3*: Create and populate the database
429 An empty database is created with
431 para_client init
433 This initializes a couple of empty tables under
434 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
435 tables, but it's good to know that you can start from scratch with
437 rm -rf ~/.paraslash/afs_database-0.4
439 in case something went wrong.
441 Next, you need to add some audio files to that database so that
442 para_server knows about them. Choose an absolute path to a directory
443 containing some audio files and add them to the audio file table:
445 para_client add /my/mp3/dir
447 This might take a while, so it is a good idea to start with a directory
448 containing not too many files. Note that the table only contains data
449 about the audio files found, not the files themselves.
451 You may print the list of all known audio files with
453 para_client ls
455 *Step 4*: Configure para_audiod
457 para_audiod needs to create a "well-known" socket for the clients to
458 connect to. The default path for this socket is
460 /var/paraslash/audiod_socket.$HOSTNAME
462 In order to make this directory writable for para_audiod, execute
463 as bar@client_host
465 sudo chown $LOGNAME /var/paraslash
468 We will also have to tell para_audiod that it should receive the
469 audio stream from server_host via http:
471 para_audiod -l info -r '.:http -i server_host'
473 You should now be able to listen to the audio stream once para_server
474 starts streaming. To activate streaming, execute
476 para_client play
478 Since no playlist has been specified yet, the "dummy" mode which
479 selects all known audio files is activated automatically. See the
480 section on the REFERENCE(The audio file selector, audio file selector)
481 for how to use playlists and moods to specify which files should be
482 streamed in which order.
484 *Troubleshooting*
486 It did not work? To find out why, try to receive, decode and play the
487 stream manually using para_recv, para_filter and para_write as follows.
489 For simplicity we assume that you're running Linux/ALSA and that only
490 MP3 files have been added to the database.
492 para_recv -r 'http -i server_host' > file.mp3
493 # (interrupt with CTRL+C after a few seconds)
494 ls -l file.mp3 # should not be empty
495 para_filter -f mp3dec -f wav < file.mp3 > file.wav
496 ls -l file.wav # should be much bigger than file.mp3
497 para_write -w alsa < file.wav
499 Double check what is logged by para_server and use the --loglevel
500 option of para_recv, para_filter and para_write to increase verbosity.
502 ---------------
503 User management
504 ---------------
506 para_server uses a challenge-response mechanism to authenticate
507 requests from incoming connections, similar to ssh's public key
508 authentication method. Authenticated connections are encrypted using
509 a stream cipher, either RC4 or AES in integer counter mode.
511 In this chapter we briefly describe RSA, RC4 and AES, and sketch the
512 REFERENCE(Client-server authentication, authentication handshake)
513 between para_client and para_server. User management is discussed
514 in the section on REFERENCE(The user_list file, the user_list file).
515 These sections are all about communication between the client and the
516 server. Connecting para_audiod is a different matter and is described
517 in a REFERENCE(Connecting para_audiod, separate section).
521 RSA, RC4, AES
522 ~~~~~~~~~~~~~
524 RSA is an asymmetric block cipher which is used in many applications,
525 including ssh and gpg. An RSA key consists in fact of two keys,
526 called the public key and the private key. A message can be encrypted
527 with either key and only the counterpart of that key can decrypt
528 the message. While RSA can be used for both signing and encrypting
529 a message, paraslash uses RSA only for the latter purpose. The
530 RSA public key encryption and signatures algorithms are defined in
531 detail in RFC 2437.
533 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
534 key stream to produce the output. Decryption uses the same function
535 calls as encryption. While RC4 supports variable key lengths,
536 paraslash uses a fixed length of 256 bits, which is considered a
537 strong encryption by today's standards. Since the same key must never
538 be used twice, a different, randomly-generated key is used for every
539 new connection.
541 AES, the advanced encryption standard, is a well-known symmetric block
542 cipher, i.e. a transformation operating on fixed-length blocks which
543 is determined by a single key for both encryption and decryption. Any
544 block cipher can be turned into a stream cipher by generating
545 a pseudo-random key stream by encrypting successive values of a
546 counter. The AES_CTR128 stream cipher used in paraslash is obtained
547 in this way from the AES block cipher with a 128 bit block size.
550 Client-server authentication
551 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
553 The authentication handshake between para_client and para_server goes
554 as follows:
556 - para_client connects to para_server and sends an
557 authentication request for a user. It does so by connecting
558 to TCP port 2990 of the server host. This port is called the
559 para_server _control port_.
561 - para_server accepts the connection and forks a child process
562 which handles the incoming request. The parent process keeps
563 listening on the control port while the child process (also
564 called para_server below) continues as follows.
566 - para_server loads the RSA public key of that user, fills a
567 fixed-length buffer with random bytes, encrypts that buffer
568 using the public key and sends the encrypted buffer to the
569 client. The first part of the buffer is the challenge which
570 is used for authentication while the second part is the
571 session key.
573 - para_client receives the encrypted buffer and decrypts it
574 with the user's private key, thereby obtaining the challenge
575 buffer and the session key. It sends the SHA1 hash value of
576 the challenge back to para_server and stores the session key
577 for further use.
579 - para_server also computes the SHA1 hash of the challenge
580 and compares it against what was sent back by the client.
582 - If the two hashes do not match, the authentication has
583 failed and para_server closes the connection.
585 - Otherwise the user is considered authenticated and the client
586 is allowed to proceed by sending a command to be executed. From
587 this point on the communication is encrypted using the stream
588 cipher with the session key known to both peers.
590 paraslash relies on the quality of the pseudo-random bytes provided
591 by the crypto library (openssl or libgcrypt), on the security of
592 the implementation of the RSA, RC4 and AES crypto routines and on the
593 infeasibility to invert the SHA1 function.
595 Neither para_server or para_client create RSA keys on their own. This
596 has to be done once for each user as sketched in REFERENCE(Quick start,
597 Quick start) and discussed in more detail REFERENCE(The user_list
598 file, below).
600 The user_list file
601 ~~~~~~~~~~~~~~~~~~
603 At startup para_server reads the user list file which contains one
604 line per user. The default location of the user list file may be
605 changed with the --user-list option.
607 There should be at least one user in this file. Each user must have
608 an RSA key pair. The public part of the key is needed by para_server
609 while the private key is needed by para_client. Each line of the
610 user list file must be of the form
612 user <username> <key> <perms>
614 where _username_ is an arbitrary string (usually the user's login
615 name), _key_ is the full path to that user's public RSA key, and
616 _perms_ is a comma-separated list of zero or more of the following
617 permission bits:
619 +---------------------------------------------------------+
620 | AFS_READ | read the contents of the databases |
621 +-----------+---------------------------------------------+
622 | AFS_WRITE | change database contents |
623 +-----------+---------------------------------------------+
624 | VSS_READ | obtain information about the current stream |
625 +-----------+---------------------------------------------+
626 | VSS_WRITE | change the current stream |
627 +---------------------------------------------------------+
629 The permission bits specify which commands the user is allowed to
630 execute. The output of
632 para_client help
634 contains in the third column the permissions needed to execute the
635 command.
637 It is possible to make para_server reread the user_list file by
638 executing the paraslash "hup" command or by sending SIGHUP to the
639 PID of para_server.
642 Connecting para_audiod
643 ~~~~~~~~~~~~~~~~~~~~~~
645 para_audiod listens on a Unix domain socket. Those sockets are
646 for local communication only, so only local users can connect to
647 para_audiod. The default is to let any user connect but this can be
648 restricted on platforms that support UNIX socket credentials which
649 allow para_audiod to obtain the Unix credentials of the connecting
650 process.
652 Use para_audiod's --user-allow option to allow connections only for
653 a limited set of users.
655 -----------------------
656 The audio file selector
657 -----------------------
659 paraslash comes with a sophisticated audio file selector (AFS),
660 whose main task is to determine which file to stream next, based on
661 information on the audio files stored in a database. It communicates
662 also with para_client whenever an AFS command is executed, for example
663 to answer a database query.
665 Besides the traditional playlists, AFS supports audio file selection
666 based on _moods_ which act as a filter that limits the set of all
667 known audio files to those which satisfy certain criteria. It also
668 maintains tables containing images (e.g. album cover art) and lyrics
669 that can be associated with one or more audio files.
671 AFS uses XREFERENCE(http://people.tuebingen.mpg.de/maan/osl/, libosl), the
672 object storage layer library, as the backend library for storing
673 information on audio files, playlists, etc. This library offers
674 functionality similar to a relational database, but is much more
675 lightweight than a full database backend.
677 In this chapter we sketch the setup of the REFERENCE(The AFS process,
678 AFS process) during server startup and proceed with the description
679 of the REFERENCE(Database layout, layout) of the various database
680 tables. The section on REFERENCE(Playlists and moods, playlists
681 and moods) explains these two audio file selection mechanisms
682 in detail and contains pratical examples. The way REFERENCE(File
683 renames and content changes, file renames and content changes) are
684 detected is discussed briefly before the REFERENCE(Troubleshooting,
685 Troubleshooting) section concludes the chapter.
687 The AFS process
688 ~~~~~~~~~~~~~~~
690 On startup, para_server forks to create the AFS process which opens
691 the OSL database tables. The server process communicates with the
692 AFS process via pipes and shared memory. Usually, the AFS process
693 awakes only briefly whenever the current audio file changes. The AFS
694 process determines the next audio file, opens it, verifies it has
695 not been changed since it was added to the database and passes the
696 open file descriptor to the server process, along with audio file
697 meta-data such as file name, duration, audio format and so on. The
698 server process then starts to stream the audio file.
700 The AFS process also accepts connections from local clients via
701 a well-known socket. However, only child processes of para_server
702 may connect through this socket. All server commands that have the
703 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
704 change the database.
706 Database layout
707 ~~~~~~~~~~~~~~~
709 *The audio file table*
711 This is the most important and usually also the largest table of the
712 AFS database. It contains the information needed to stream each audio
713 file. In particular the following data is stored for each audio file.
715 - SHA1 hash value of the audio file contents. This is computed
716 once when the file is added to the database. Whenever AFS
717 selects this audio file for streaming the hash value is
718 recomputed and checked against the value stored in the
719 database to detect content changes.
721 - The time when this audio file was last played.
723 - The number of times the file has been played so far.
725 - The attribute bitmask.
727 - The image id which describes the image associated with this
728 audio file.
730 - The lyrics id which describes the lyrics associated with
731 this audio file.
733 - The audio format id (MP3, OGG, ...).
735 - An amplification value that can be used by the amplification
736 filter to pre-amplify the decoded audio stream.
738 - The chunk table. It describes the location and the timing
739 of the building blocks of the audio file. This is used by
740 para_server to send chunks of the file at appropriate times.
742 - The duration of the audio file.
744 - Tag information contained in the audio file (ID3 tags,
745 Vorbis comments, ...).
747 - The number of channels
749 - The encoding bitrate.
751 - The sampling frequency.
753 To add or refresh the data contained in the audio file table, the _add_
754 command is used. It takes the full path of either an audio file or a
755 directory. In the latter case, the directory is traversed recursively
756 and all files which are recognized as valid audio files are added to
757 the database.
759 *The attribute table*
761 The attribute table contains two columns, _name_ and _bitnum_. An
762 attribute is simply a name for a certain bit number in the attribute
763 bitmask of the audio file table.
765 Each of the 64 bits of the attribute bitmask can be set for each
766 audio file individually. Hence up to 64 different attributes may be
767 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
768 "german_lyrics", "speech", whatever. You are free to choose as
769 many attributes as you like and there are no naming restrictions
770 for attributes.
772 A new attribute "test" is created by
774 para_client addatt test
775 and
776 para_client lsatt
778 lists all available attributes. You can set the "test" attribute for
779 an audio file by executing
781 para_client setatt test+ /path/to/the/audio/file
783 Similarly, the "test" bit can be removed from an audio file with
785 para_client setatt test- /path/to/the/audio/file
787 Instead of a path you may use a shell wildcard pattern. The attribute
788 is applied to all audio files matching this pattern:
790 para_client setatt test+ '/test/directory/*'
792 The command
794 para_client -- ls -lv
796 gives you a verbose listing of your audio files also showing which
797 attributes are set.
799 In case you wonder why the double-dash in the above command is needed:
800 It tells para_client to not interpret the options after the dashes. If
801 you find this annoying, just say
803 alias para='para_client --'
805 and be happy. In what follows we shall use this alias.
807 The "test" attribute can be dropped from the database with
809 para rmatt test
811 Read the output of
813 para help ls
814 para help setatt
816 for more information and a complete list of command line options to
817 these commands.
819 *Blob tables*
821 The image, lyrics, moods and playlists tables are all blob tables.
822 Blob tables consist of three columns each: The identifier which is
823 a positive non-negative number that is auto-incremented, the name
824 (an arbitrary string) and the content (the blob).
826 All blob tables support the same set of actions: cat, ls, mv, rm
827 and add. Of course, _add_ is used for adding new blobs to the table
828 while the other actions have the same meaning as the corresponding
829 Unix commands. The paraslash commands to perform these actions are
830 constructed as the concatenation of the table name and the action. For
831 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
832 manipulate or query the image table.
834 The add variant of these commands is special as these commands read
835 the blob contents from stdin. To add an image to the image table the
836 command
838 para addimg image_name < file.jpg
840 can be used.
842 Note that the images and lyrics are not interpreted at all, and also
843 the playlist and the mood blobs are only investigated when the mood
844 or playlist is activated with the select command.
846 *The score table*
848 Unlike all other tables the contents of the score table remain in
849 memory and are never stored on disk. The score table contains two
850 columns: The SHA1 hash value (of an audio file) and its current
851 score.
853 However, only those files which are admissible for the current mood
854 or playlist are contained in the score table. The audio file selector
855 always chooses the row with the highest score as the file to stream
856 next. While doing so, it computes the new score and updates the
857 last_played and the num_played fields in the audio file table.
859 The score table is recomputed by the select command which loads a
860 mood or playlist. Audio files are chosen for streaming from the rows
861 of the score table on a highest-score-first basis.
864 Playlists and moods
865 ~~~~~~~~~~~~~~~~~~~
867 Playlists and moods offer two different ways of specifying the set of
868 admissible files. A playlist in itself describes a set of admissible
869 files. A mood, in contrast, describes the set of admissible files in
870 terms of attributes and other type of information available in the
871 audio file table. As an example, a mood can define a filename pattern,
872 which is then matched against the names of audio files in the table.
874 *Playlists*
876 Playlists are accommodated in the playlist table of the afs database,
877 using the aforementioned blob format for tables. A new playlist is
878 created with the addpl command by specifying the full (absolute)
879 paths of all desired audio files, separated by newlines. Example:
881 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
883 If _my_playlist_ already exists it is overwritten. To activate the
884 new playlist, execute
886 para select p/my_playlist
888 The audio file selector will assign scores to each entry of the list,
889 in descending order so that files will be selected in order. If a
890 file could not be opened for streaming, its entry is removed from
891 the score table (but not from the playlist).
893 *Moods*
895 A mood consists of a unique name and its *mood definition*, which is
896 a set of *mood lines* containing expressions in terms of attributes
897 and other data contained in the database.
899 At any time at most one mood can be *active* which means that
900 para_server is going to select only files from that subset of
901 admissible files.
903 So in order to create a mood definition one has to write a set of
904 mood lines. Mood lines come in three flavours: Accept lines, deny
905 lines and score lines.
907 The general syntax of the three types of mood lines is
910 accept [with score <score>] [if] [not] <mood_method> [options]
911 deny [with score <score>] [if] [not] <mood_method> [options]
912 score <score> [if] [not] <mood_method> [options]
915 Here <score> is either an integer or the string "random" which assigns
916 a random score to all matching files. The score value changes the
917 order in which admissible files are going to be selected, but is of
918 minor importance for this introduction.
920 So we concentrate on the first two forms, i.e. accept and deny
921 lines. As usual, everything in square brackets is optional, i.e.
922 accept/deny lines take the following form when ignoring scores:
924 accept [if] [not] <mood_method> [options]
926 and analogously for the deny case. The "if" keyword is only syntactic
927 sugar and has no function. The "not" keyword just inverts the result,
928 so the essence of a mood line is the mood method part and the options
929 following thereafter.
931 A *mood method* is realized as a function which takes an audio file
932 and computes a number from the data contained in the database.
933 If this number is non-negative, we say the file *matches* the mood
934 method. The file matches the full mood line if it either
936 - matches the mood method and the "not" keyword is not given,
937 or
938 - does not match the mood method, but the "not" keyword is given.
940 The set of admissible files for the whole mood is now defined as those
941 files which match at least one accept mood line, but no deny mood line.
942 More formally, an audio file F is admissible if and only if
944 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
946 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
947 lines and "~" means "matches".
949 The cases where no mood lines of accept/deny type are defined need
950 special treatment:
952 - Neither accept nor deny lines: This treats all files as
953 admissible (in fact, that is the definition of the dummy mood
954 which is activated automatically if no moods are available).
956 - Only accept lines: A file is admissible iff it matches at
957 least one accept line:
959 F ~ AL1 or F ~ AL2 or ...
961 - Only deny lines: A file is admissible iff it matches no
962 deny line:
964 not (F ~ DL1 or F ~ DN2 ...)
968 *List of mood_methods*
970 no_attributes_set
972 Takes no arguments and matches an audio file if and only if no
973 attributes are set.
975 is_set <attribute_name>
977 Takes the name of an attribute and matches iff that attribute is set.
979 path_matches <pattern>
981 Takes a filename pattern and matches iff the path of the audio file
982 matches the pattern.
984 artist_matches <pattern>
985 album_matches <pattern>
986 title_matches <pattern>
987 comment_matches <pattern>
989 Takes an extended regular expression and matches iff the text of the
990 corresponding tag of the audio file matches the pattern. If the tag
991 is not set, the empty string is matched against the pattern.
993 year ~ <num>
994 bitrate ~ <num>
995 frequency ~ <num>
996 channels ~ <num>
997 num_played ~ <num>
999 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
1000 <num>. Matches an audio file iff the condition <val> ~ <num> is
1001 satisfied where val is the corresponding value of the audio file
1002 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
1003 count, play count).
1005 The year tag is special as its value is undefined if the audio file
1006 has no year tag or the content of the year tag is not a number. Such
1007 audio files never match. Another difference is the special treatment
1008 if the year tag is a two-digit number. In this case either 1900 or
1009 2000 is added to the tag value, depending on whether the number is
1010 greater than 2000 plus the current year.
1013 *Mood usage*
1015 To create a new mood called "my_mood", write its definition into
1016 some temporary file, say "tmpfile", and add it to the mood table
1017 by executing
1019 para addmood my_mood < tmpfile
1021 If the mood definition is really short, you may just pipe it to the
1022 client instead of using temporary files. Like this:
1024 echo "$MOOD_DEFINITION" | para addmood my_mood
1026 There is no need to keep the temporary file since you can always use
1027 the catmood command to get it back:
1029 para catmood my_mood
1031 A mood can be activated by executing
1033 para select m/my_mood
1035 Once active, the list of admissible files is shown by the ls command
1036 if the "-a" switch is given:
1038 para ls -a
1041 *Example mood definition*
1043 Suppose you have defined attributes "punk" and "rock" and want to define
1044 a mood containing only Punk-Rock songs. That is, an audio file should be
1045 admissible if and only if both attributes are set. Since
1047 punk and rock
1049 is obviously the same as
1051 not (not punk or not rock)
1053 (de Morgan's rule), a mood definition that selects only Punk-Rock
1054 songs is
1056 deny if not is_set punk
1057 deny if not is_set rock
1061 File renames and content changes
1062 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1064 Since the audio file selector knows the SHA1 of each audio file that
1065 has been added to the afs database, it recognizes if the content of
1066 a file has changed, e.g. because an ID3 tag was added or modified.
1067 Also, if a file has been renamed or moved to a different location,
1068 afs will detect that an entry with the same hash value already exists
1069 in the audio file table.
1071 In both cases it is enough to just re-add the new file. In the
1072 first case (file content changed), the audio table is updated, while
1073 metadata such as the num_played and last_played fields, as well as
1074 the attributes, remain unchanged. In the other case, when the file
1075 is moved or renamed, only the path information is updated, all other
1076 data remains as before.
1078 It is possible to change the behaviour of the add command by using the
1079 "-l" (lazy add) or the "-f" (force add) option.
1081 Troubleshooting
1082 ~~~~~~~~~~~~~~~
1084 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1085 executables have a brief online help which is displayed when -h is
1086 given. The --detailed-help option prints the full help text.
1088 If para_server crashed or was killed by SIGKILL (signal 9), it
1089 may refuse to start again because of "dirty osl tables". In this
1090 case you'll have to run the oslfsck program of libosl to fix your
1091 database:
1093 oslfsck -fd ~/.paraslash/afs_database-0.4
1095 However, make sure para_server isn't running before executing oslfsck.
1097 If you don't mind to recreate your database you can start
1098 from scratch by removing the entire database directory, i.e.
1100 rm -rf ~/.paraslash/afs_database-0.4
1102 Be aware that this removes all attribute definitions, all playlists
1103 and all mood definitions and requires to re-initialize the tables.
1105 Although oslfsck fixes inconsistencies in database tables it doesn't
1106 care about the table contents. To check for invalid table contents, use
1108 para_client check
1110 This prints out references to missing audio files as well as invalid
1111 playlists and mood definitions.
1113 Similarly, para_audiod refuses to start if its socket file exists, since
1114 this indicates that another instance of para_audiod is running. After
1115 a crash a stale socket file might remain and you must run
1117 para_audiod --force
1119 once to fix it up.
1121 ---------------------------------------
1122 Audio formats and audio format handlers
1123 ---------------------------------------
1125 Audio formats
1126 ~~~~~~~~~~~~~
1128 The following audio formats are supported by paraslash:
1130 *MP3*
1132 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1133 designed as part of its MPEG-1 standard. An MP3 file is made up of
1134 multiple MP3 frames, which consist of a header and a data block. The
1135 size of an MP3 frame depends on the bit rate and on the number
1136 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1137 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1138 400 bytes large.
1140 *OGG/Vorbis*
1142 OGG is a standardized audio container format, while Vorbis is an
1143 open source codec for lossy audio compression. Since Vorbis is most
1144 commonly made available via the OGG container format, it is often
1145 referred to as OGG/Vorbis. The OGG container format divides data into
1146 chunks called OGG pages. A typical OGG page is about 4KB large. The
1147 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1148 may vary considerably.
1150 *OGG/Speex*
1152 Speex is an open-source speech codec that is based on CELP (Code
1153 Excited Linear Prediction) coding. It is designed for voice
1154 over IP applications, has modest complexity and a small memory
1155 footprint. Wideband and narrowband (telephone quality) speech are
1156 supported. As for Vorbis audio, Speex bit-streams are often stored
1157 in OGG files. As of 2012 this codec is considered obsolete since the
1158 Oppus codec, described below, surpasses its performance in all areas.
1160 *OGG/Opus*
1162 Opus is a lossy audio compression format standardized through RFC
1163 6716 in 2012. It combines the speech-oriented SILK codec and the
1164 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1165 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1166 containers. All known software patents which cover Opus are licensed
1167 under royalty-free terms.
1169 *AAC*
1171 Advanced Audio Coding (AAC) is a standardized, lossy compression
1172 and encoding scheme for digital audio which is the default audio
1173 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1174 the container format and audio files encoded with AAC have the .m4a
1175 extension. A typical AAC frame is about 700 bytes large.
1177 *WMA*
1179 Windows Media Audio (WMA) is an audio data compression technology
1180 developed by Microsoft. A WMA file is usually encapsulated in the
1181 Advanced Systems Format (ASF) container format, which also specifies
1182 how meta data about the file is to be encoded. The bit stream of WMA
1183 is composed of superframes, each containing one or more frames of
1184 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1186 *FLAC*
1188 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1189 loss. It gives better compression ratios than a general purpose
1190 compressor like zip or bzip2 because FLAC is designed specifically
1191 for audio. A FLAC-encoded file consits of frames of varying size, up
1192 to 16K. Each frame starts with a header that contains all information
1193 necessary to decode the frame.
1195 Meta data
1196 ~~~~~~~~~
1198 Unfortunately, each audio format has its own conventions how meta
1199 data is added as tags to the audio file.
1201 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1202 is rather simple but also very limited as it supports only artist,
1203 title, album, year and comment tags. Each of these can only be at most
1204 32 characters long. ID3, version 2 is much more flexible but requires
1205 a separate library being installed for paraslash to support it.
1207 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1208 comments, which are typically implemented as strings of the form
1209 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1210 tags are appropriate for the content.
1212 AAC files usually use the MPEG-4 container format for storing meta
1213 data while WMA files wrap meta data as special objects within the
1214 ASF container format.
1216 paraslash only tracks the most common tags that are supported by
1217 all tag variants: artist, title, year, album, comment. When a file
1218 is added to the AFS database, the meta data of the file is extracted
1219 and stored in the audio file table.
1221 Chunks and chunk tables
1222 ~~~~~~~~~~~~~~~~~~~~~~~
1224 paraslash uses the word "chunk" as common term for the building blocks
1225 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1226 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1227 size varies considerably between audio formats, from a few hundred
1228 bytes (MP3) up to 16K (FLAC).
1230 The chunk table contains the offsets within the audio file that
1231 correspond to the chunk boundaries of the file. Like the meta data,
1232 the chunk table is computed and stored in the database whenever an
1233 audio file is added.
1235 The paraslash senders (see below) always send complete chunks. The
1236 granularity for seeking is therefore determined by the chunk size.
1238 Audio format handlers
1239 ~~~~~~~~~~~~~~~~~~~~~
1241 For each audio format paraslash contains an audio format handler whose
1242 first task is to tell whether a given file is a valid audio file of
1243 this type. If so, the audio file handler extracts some technical data
1244 (duration, sampling rate, number of channels etc.), computes the
1245 chunk table and reads the meta data.
1247 The audio format handler code is linked into para_server and executed
1248 via the _add_ command. The same code is also available as a stand-alone
1249 tool, para_afh, which prints the technical data, the chunk table
1250 and the meta data of a file. Moreover, all audio format handlers are
1251 combined in the afh receiver which is part of para_recv and para_play.
1253 ----------
1254 Networking
1255 ----------
1257 Paraslash uses different network connections for control and data.
1258 para_client communicates with para_server over a dedicated TCP control
1259 connection. To transport audio data, separate data connections are
1260 used. For these data connections, a variety of transports (UDP, DCCP,
1261 HTTP) can be chosen.
1263 The chapter starts with the REFERENCE(The paraslash control
1264 service, control service), followed by a section on the various
1265 REFERENCE(Streaming protocols, streaming protocols) in which the data
1266 connections are described. The way audio file headers are embedded into
1267 the stream is discussed REFERENCE(Streams with headers and headerless
1268 streams, briefly) before the REFERENCE(Networking examples, example
1269 section) which illustrates typical commands for real-life scenarios.
1271 Both IPv4 and IPv6 are supported.
1273 The paraslash control service
1274 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1276 para_server is controlled at runtime via the paraslash control
1277 connection. This connection is used for server commands (play, stop,
1278 ...) as well as for afs commands (ls, select, ...).
1280 The server listens on a TCP port and accepts connections from clients
1281 that connect the open port. Each connection causes the server to fork
1282 off a client process which inherits the connection and deals with that
1283 client only. In this classical accept/fork approach the server process
1284 is unaffected if the child dies or goes crazy for whatever reason. In
1285 fact, the child process can not change address space of server process.
1287 The section on REFERENCE(Client-server authentication, client-server
1288 authentication) above described the early connection establishment
1289 from the crypto point of view. Here it is described what happens
1290 after the connection (including crypto setup) has been established.
1291 There are four processes involved during command dispatch as sketched
1292 in the following diagram.
1294 <<
1295 <pre>
1296 server_host client_host
1297 ~~~~~~~~~~~ ~~~~~~~~~~~
1299 +-----------+ connect +-----------+
1300 |para_server|<------------------------------ |para_client|
1301 +-----------+ +-----------+
1302 | ^
1303 | fork +---+ |
1304 +----------> |AFS| |
1305 | +---+ |
1306 | ^ |
1307 | | |
1308 | | connect (cookie) |
1309 | | |
1310 | | |
1311 | fork +-----+ inherited connection |
1312 +---------->|child|<--------------------------+
1313 +-----+
1314 </pre>
1315 >>
1317 Note that the child process is not a child of the afs process,
1318 so communication of these two processes has to happen via local
1319 sockets. In order to avoid abuse of the local socket by unrelated
1320 processes, a magic cookie is created once at server startup time just
1321 before the server process forks off the AFS process. This cookie is
1322 known to the server, AFS and the child, but not to unrelated processes.
1324 There are two different kinds of commands: First there are commands
1325 that cause the server to respond with some answer such as the list
1326 of all audio files. All but the addblob commands (addimg, addlyr,
1327 addpl, addmood) are of this kind. The addblob commands add contents
1328 to the database, so they need to transfer data the other way round,
1329 from the client to the server.
1331 There is no knowledge about the server commands built into para_client,
1332 so it does not know about addblob commands. Instead, it inspects the
1333 first data package sent by the server for a magic string. If this
1334 string was found, it sends STDIN to the server, otherwise it dumps
1335 data from the server to STDOUT.
1337 Streaming protocols
1338 ~~~~~~~~~~~~~~~~~~~
1340 A network (audio) stream usually consists of one streaming source,
1341 the _sender_, and one or more _receivers_ which read data over the
1342 network from the streaming source.
1344 Senders are thus part of para_server while receivers are part of
1345 para_audiod. Moreover, there is the stand-alone tool para_recv which
1346 can be used to manually download a stream, either from para_server
1347 or from a web-based audio streaming service.
1349 The following three streaming protocols are supported by paraslash:
1351 - HTTP. Recommended for public streams that can be played by
1352 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1353 sender is supported on all operating systems and all platforms.
1355 - DCCP. Recommended for LAN streaming. DCCP is currently
1356 available only for Linux.
1358 - UDP. Recommended for multicast LAN streaming.
1360 See the Appendix on REFERENCE(Network protocols, network protocols)
1361 for brief descriptions of the various protocols relevant for network
1362 audio streaming with paraslash.
1364 It is possible to activate more than one sender simultaneously.
1365 Senders can be controlled at run time and via config file and command
1366 line options.
1368 Note that audio connections are _not_ encrypted. Transport or Internet
1369 layer encryption should be used if encrypted data connections are
1370 needed.
1372 Since DCCP and TCP are both connection-oriented protocols, connection
1373 establishment/teardown and access control are very similar between
1374 these two streaming protocols. UDP is the most lightweight option,
1375 since in contrast to TCP/DCCP it is connectionless. It is also the
1376 only protocol supporting IP multicast.
1378 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1379 clients to connect and establish a connection via some protocol-defined
1380 handshake mechanism. Both senders maintain two linked lists each:
1381 The list of all clients which are currently connected, and the list
1382 of access control entries which determines who is allowed to connect.
1383 IP-based access control may be configured through config file and
1384 command line options and via the "allow" and "deny" sender subcommands.
1386 Upon receiving a GET request from the client, the HTTP sender sends
1387 back a status line and a message. The body of this message is the
1388 audio stream. This is common practice and is supported by many popular
1389 clients which can thus be used to play a stream offered by para_server.
1390 For DCCP things are a bit simpler: No messages are exchanged between
1391 the receiver and sender. The client simply connects and the sender
1392 starts to stream.
1394 DCCP is an experimental protocol which offers a number of new features
1395 not available for TCP. Both ends can negotiate these features using
1396 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1397 datagram-based (no retransmissions) and thus should not be used over
1398 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1399 is access to a variety of different congestion-control mechanisms
1400 called CCIDs. Two different CCIDs are available per default on Linux:
1403 - _CCID 2_. A Congestion Control mechanism similar to that
1404 of TCP. The sender maintains a congestion window and halves
1405 this window in response to congestion.
1408 - _CCID-3_. Designed to be fair when competing for bandwidth.
1409 It has lower variation of throughput over time compared with
1410 TCP, which makes it suitable for streaming media.
1412 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1413 single list, the _target list_. This list describes the set of clients
1414 to which the stream is sent. There is no list for access control and
1415 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1416 and "delete" commands can be used to modify the target list.
1418 Since both UDP and DCCP offer an unreliable datagram-based transport,
1419 additional measures are necessary to guard against disruptions over
1420 networks that are lossy or which may be subject to interference (as
1421 is for instance the case with WiFi). Paraslash uses FEC (Forward
1422 Error Correction) to guard against packet losses and reordering. The
1423 stream is FEC-encoded before it is sent through the UDP socket and
1424 must be decoded accordingly on the receiver side.
1426 The packet size and the amount of redundancy introduced by FEC can
1427 be configured via the FEC parameters which are dictated by server
1428 and may also be configured through the "sender" command. The FEC
1429 parameters are encoded in the header of each network packet, so no
1430 configuration is necessary on the receiver side. See the section on
1431 REFERENCE(Forward error correction, FEC) below.
1433 Streams with headers and headerless streams
1434 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1436 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1437 needed to decode the stream is only contained in the audio file
1438 header of the container format but not in each data chunk. Clients
1439 must be able to obtain this information in case streaming starts in
1440 the middle of the file or if para_audiod is started while para_server
1441 is already sending a stream.
1443 This is accomplished in different ways, depending on the streaming
1444 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1445 header is sent prior to audio file data. This technique however does
1446 not work for the connectionless UDP transport. Hence the audio file
1447 header is periodically being embedded into the UDP audio data stream.
1448 By default, the header is resent after five seconds. The receiver has
1449 to wait until the next header arrives before it can start decoding
1450 the stream.
1452 Examples
1453 ~~~~~~~~
1455 The sender command of para_server allows to (de-)activate senders
1456 and to change the access permissions senders at runtime. The "si"
1457 (server info) command is used to list the streaming options of the
1458 currently running server as well as the various sender access lists.
1460 -> Show client/target/access lists:
1462 para_client si
1464 -> Obtain general help for the sender command:
1466 para_client help sender
1468 -> Get help for a specific sender (contains further examples):
1470 s=http # or dccp or udp
1471 para_client sender $s help
1473 By default para_server activates both the HTTP and th DCCP sender on
1474 startup. This can be changed via command line options or para_server's
1475 config file.
1477 -> List config file options for senders:
1479 para_server -h
1481 All senders share the "on" and "off" commands, so senders may be
1482 activated and deactivated independently of each other.
1484 -> Switch off the http sender:
1486 para_client sender http off
1488 -> Receive a DCCP stream using CCID2 and write the output into a file:
1490 host=foo.org; ccid=2; filename=bar
1491 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1493 Note the quotes around the arguments for the dccp receiver. Each
1494 receiver has its own set of command line options and its own command
1495 line parser, so arguments for the dccp receiver must be protected
1496 from being interpreted by para_recv.
1498 -> Start UDP multicast, using the default multicast address:
1500 para_client sender udp add
1502 -> Receive FEC-encoded multicast stream and write the output into a file:
1504 filename=foo
1505 para_recv -r udp > $filename
1507 -> Add an UDP unicast for a client to the target list of the UDP sender:
1509 t=client.foo.org
1510 para_client sender udp add $t
1512 -> Receive this (FEC-encoded) unicast stream:
1514 filename=foo
1515 para_recv -r 'udp -i' > $filename
1517 -> Create a minimal config for para_audiod for HTTP streams:
1519 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1520 echo receiver \".:http -i $s\" > $c
1521 para_audiod --config $c
1523 -------
1524 Filters
1525 -------
1527 A paraslash filter is a module which transforms an input stream into
1528 an output stream. Filters are included in the para_audiod executable
1529 and in the stand-alone tool para_filter which usually contains the
1530 same modules.
1532 While para_filter reads its input stream from STDIN and writes
1533 the output to STDOUT, the filter modules of para_audiod are always
1534 connected to a receiver which produces the input stream and a writer
1535 which absorbs the output stream.
1537 Some filters depend on a specific library and are not compiled in
1538 if this library was not found at compile time. To see the list of
1539 supported filters, run para_filter and para_audiod with the --help
1540 option. The output looks similar to the following:
1542 Available filters:
1543 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1545 Out of these filter modules, a chain of filters can be constructed,
1546 much in the way Unix pipes can be chained, and analogous to the use
1547 of modules in gstreamer: The output of the first filter becomes the
1548 input of the second filter. There is no limitation on the number of
1549 filters and the same filter may occur more than once.
1551 Like receivers, each filter has its own command line options which
1552 must be quoted to protect them from the command line options of
1553 the driving application (para_audiod or para_filter). Example:
1555 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1557 For para_audiod, each audio format has its own set of filters. The
1558 name of the audio format for which the filter should be applied can
1559 be used as the prefix for the filter option. Example:
1561 para_audiod -f 'mp3:prebuffer --duration 300'
1563 The "mp3" prefix above is actually interpreted as a POSIX extended
1564 regular expression. Therefore
1566 para_audiod -f '.:prebuffer --duration 300'
1568 activates the prebuffer filter for all supported audio formats (because
1569 "." matches all audio formats) while
1571 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1573 activates it only for wma and ogg streams.
1575 Decoders
1576 ~~~~~~~~
1578 For each supported audio format there is a corresponding filter
1579 which decodes audio data in this format to 16 bit PCM data which
1580 can be directly sent to the sound device or any other software that
1581 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1582 filters are called _decoders_ in general, and xxxdec is the name of
1583 the paraslash decoder for the audio format xxx. For example, the mp3
1584 decoder filter is called mp3dec.
1586 Note that the output of the decoder is about 10 times larger than
1587 its input. This means that filters that operate on the decoded audio
1588 stream have to deal with much more data than filters that transform
1589 the audio stream before it is fed to the decoder.
1591 Paraslash relies on external libraries for most decoders, so these
1592 libraries must be installed for the decoder to be included in the
1593 para_filter and para_audiod executables. The oggdec filter depends
1594 on the libogg and libvorbis libraries for example.
1596 Forward error correction
1597 ~~~~~~~~~~~~~~~~~~~~~~~~
1599 As already mentioned REFERENCE(Streaming protocols, earlier),
1600 paraslash uses forward error correction (FEC) for the unreliable UDP
1601 and DCCP transports. FEC is a technique which was invented already
1602 in 1960 by Reed and Solomon and which is widely used for the parity
1603 calculations of storage devices (RAID arrays). It is based on the
1604 algebraic concept of finite fields, today called Galois fields, in
1605 honour of the mathematician Galois (1811-1832). The FEC implementation
1606 of paraslash is based on code by Luigi Rizzo.
1608 Although the details require a sound knowledge of the underlying
1609 mathematics, the basic idea is not hard to understand: For positive
1610 integers k and n with k < n it is possible to compute for any k given
1611 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1612 ..., p_r such that all data bytes can be reconstructed from *any*
1613 k bytes of the set
1615 {d_1, ..., d_k, p_1, ..., p_r}.
1617 FEC-encoding for unreliable network transports boils down to slicing
1618 the audio stream into groups of k suitably sized pieces called _slices_
1619 and computing the r corresponding parity slices. This step is performed
1620 in para_server which then sends both the data and the parity slices
1621 over the unreliable network connection. If the client was able
1622 to receive at least k of the n = k + r slices, it can reconstruct
1623 (FEC-decode) the original audio stream.
1625 From these observations it is clear that there are three different
1626 FEC parameters: The slice size, the number of data slices k, and the
1627 total number of slices n. It is crucial to choose the slice size
1628 such that no fragmentation of network packets takes place because
1629 FEC only guards against losses and reordering but fails if slices are
1630 received partially.
1632 FEC decoding in paralash is performed through the fecdec filter which
1633 usually is the first filter (there can be other filters before fecdec
1634 if these do not alter the audio stream).
1637 Volume adjustment (amp and compress)
1638 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1640 The amp and the compress filter both adjust the volume of the audio
1641 stream. These filters operate on uncompressed audio samples. Hence
1642 they are usually placed directly after the decoding filter. Each
1643 sample is multiplied with a scaling factor (>= 1) which makes amp
1644 and compress quite expensive in terms of computing power.
1646 *amp*
1648 The amp filter amplifies the audio stream by a fixed scaling factor
1649 that must be known in advance. For para_audiod this factor is derived
1650 from the amplification field of the audio file's entry in the audio
1651 file table while para_filter uses the value given at the command line.
1653 The optimal scaling factor F for an audio file is the largest real
1654 number F >= 1 such that after multiplication with F all samples still
1655 fit into the sample interval [-32768, 32767]. One can use para_filter
1656 in combination with the sox utility to compute F:
1658 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1660 The amplification value V which is stored in the audio file table,
1661 however, is an integer between 0 and 255 which is connected to F
1662 through the formula
1664 V = (F - 1) * 64.
1666 To store V in the audio file table, the command
1668 para_client -- touch -a=V file.mp3
1670 is used. The reader is encouraged to write a script that performs
1671 these computations :)
1673 *compress*
1675 Unlike the amplification filter, the compress filter adjusts the volume
1676 of the audio stream dynamically without prior knowledge about the peak
1677 value. It maintains the maximal volume of the last n samples of the
1678 audio stream and computes a suitable amplification factor based on that
1679 value and the various configuration options. It tries to chose this
1680 factor such that the adjusted volume meets the desired target level.
1682 Note that it makes sense to combine amp and compress.
1684 Misc filters (wav and prebuffer)
1685 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1687 These filters are rather simple and do not modify the audio stream at
1688 all. The wav filter is only useful with para_filter and in connection
1689 with a decoder. It asks the decoder for the number of channels and the
1690 sample rate of the stream and adds a Microsoft wave header containing
1691 this information at the beginning. This allows to write wav files
1692 rather than raw PCM files (which do not contain any information about
1693 the number of channels and the sample rate).
1695 The prebuffer filter simply delays the output until the given time has
1696 passed (starting from the time the first byte was available in its
1697 input queue) or until the given amount of data has accumulated. It
1698 is mainly useful for para_audiod if the standard parameters result
1699 in buffer underruns.
1701 Both filters require almost no additional computing time, even when
1702 operating on uncompressed audio streams, since data buffers are simply
1703 "pushed down" rather than copied.
1705 Examples
1706 ~~~~~~~~
1708 -> Decode an mp3 file to wav format:
1710 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1712 -> Amplify a raw audio file by a factor of 1.5:
1714 para_filter -f amp --amp 32 < foo.raw > bar.raw
1716 ------
1717 Output
1718 ------
1720 Once an audio stream has been received and decoded to PCM format,
1721 it can be sent to a sound device for playback. This part is performed
1722 by paraslash _writers_ which are described in this chapter.
1724 Writers
1725 ~~~~~~~
1727 A paraslash writer acts as a data sink that consumes but does not
1728 produce audio data. Paraslash writers operate on the client side and
1729 are contained in para_audiod and in the stand-alone tool para_write.
1731 The para_write program reads uncompressed audio data from STDIN. If
1732 this data starts with a wav header, sample rate, sample format and
1733 channel count are read from the header. Otherwise CD audio (44.1KHz
1734 16 bit little endian, stereo) is assumed but this can be overridden
1735 by command line options. para_audiod, on the other hand, obtains
1736 the sample rate and the number of channels from the decoder.
1738 Like receivers and filters, each writer has an individual set of
1739 command line options, and for para_audiod writers can be configured
1740 per audio format separately. It is possible to activate more than
1741 one writer for the same stream simultaneously.
1743 OS-dependent APIs
1744 ~~~~~~~~~~~~~~~~~
1746 Unfortunately, the various flavours of Unix on which paraslash
1747 runs on have different APIs for opening a sound device and starting
1748 playback. Hence for each such API there is a paraslash writer that
1749 can play the audio stream via this API.
1751 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1752 Linux systems. Although there are several mid-layer APIs in use by
1753 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1754 currently supports only the low-level ALSA API which is not supposed
1755 to be change. ALSA is very feature-rich, in particular it supports
1756 software mixing via its DMIX plugin. ALSA is the default writer on
1757 Linux systems.
1759 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1760 is also available on Linux systems, usually provided by ALSA as an
1761 emulation for backwards compatibility. This API is rather simple but
1762 also limited. For example only one application can open the device
1763 at any time. The OSS writer is activated by default on BSD Systems.
1765 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1766 for this API is only compiled in on such systems and is of course
1767 the default there.
1769 *FILE*. The file writer allows to capture the audio stream and
1770 write the PCM data to a file on the file system rather than playing
1771 it through a sound device. It is supported on all platforms and is
1772 always compiled in.
1774 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1775 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1776 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1777 through an output plugin of libao.
1779 Examples
1780 ~~~~~~~~
1782 -> Use the OSS writer to play a wav file:
1784 para_write --writer oss < file.wav
1786 -> Enable ALSA software mixing for mp3 streams
1788 para_audiod --writer 'mp3:alsa -d plug:swmix'
1791 ---
1792 Gui
1793 ---
1795 para_gui executes an arbitrary command which is supposed to print
1796 status information to STDOUT. It then displays this information in
1797 a curses window. By default the command
1799 para_audioc -- stat -p
1801 is executed, but this can be customized via the --stat-cmd option. In
1802 particular it possible to use
1804 para_client -- stat -p
1806 to make para_gui work on systems on which para_audiod is not running.
1808 Key bindings
1809 ~~~~~~~~~~~~
1811 It is possible to bind keys to arbitrary commands via custom
1812 key-bindings. Besides the internal keys which can not be changed (help,
1813 quit, loglevel, version...), the following flavours of key-bindings
1814 are supported:
1816 - external: Shutdown curses before launching the given command.
1817 Useful for starting other ncurses programs from within
1818 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1819 output format to write a mailbox containing one mail for each
1820 (admissible) file the audio file selector knows about. Then
1821 start mutt from within para_gui to browse your collection!
1823 - display: Launch the command and display its stdout in
1824 para_gui's bottom window.
1826 - para: Like display, but start "para_client <specified
1827 command>" instead of "<specified command>".
1829 The general form of a key binding is
1831 key_map k:m:c
1833 which maps key k to command c using mode m. Mode may be x, d or p
1834 for external, display and paraslash commands, respectively.
1836 Themes
1837 ~~~~~~
1839 Currently there are only two themes for para_gui. It is easy, however,
1840 to add more themes. To create a new theme one has to define the
1841 position, color and geometry for for each status item that should be
1842 shown by this theme. See gui_theme.c for examples.
1844 The "." and "," keys are used to switch between themes.
1846 Examples
1847 ~~~~~~~~
1849 -> Show server info:
1851 key_map "i:p:si"
1853 -> Jump to the middle of the current audio file by pressing F5:
1855 key_map "<F5>:p:jmp 50"
1857 -> vi-like bindings for jumping around:
1859 key_map "l:p:ff 10"
1860 key_map "h:p:ff 10-"
1861 key_map "w:p:ff 60"
1862 key_map "b:p:ff 60-"
1864 -> Print the current date and time:
1866 key_map "D:d:date"
1868 -> Call other curses programs:
1870 key_map "U:x:aumix"
1871 key_map "!:x:/bin/bash"
1872 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1874 -----------
1875 Development
1876 -----------
1878 Tools
1879 ~~~~~
1881 In order to compile the sources from the git repository (rather than
1882 from tar balls) and for contributing non-trivial changes to the
1883 paraslash project, some additional tools should be installed on a
1884 developer machine.
1886 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1887 branches, below), the git source code management tool is used for
1888 paraslash development. It is necessary for cloning the git repository
1889 and for getting updates.
1891 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1892 are generated from templates by the m4 macro processor.
1894 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1895 the configure file which is shipped in the tarballs but has to be
1896 generated when compiling from git.
1898 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1899 HTML version of this manual and some of the paraslash web pages are
1900 generated by the grutatxt plain text to HTML converter. If changes
1901 are made to these text files the grutatxt package must be installed
1902 to regenerate the HTML files.
1904 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1905 of paraslash's C sources uses the doxygen documentation system. The
1906 conventions for documenting the source code is described in the
1907 REFERENCE(Doxygen, Doxygen section).
1909 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1910 browsable HTML from the C sources. It is needed by doxygen.
1912 Git branches
1913 ~~~~~~~~~~~~
1915 Paraslash has been developed using the git source code management
1916 tool since 2006. Development is organized roughly in the same spirit
1917 as the git development itself, as described below.
1919 The following text passage is based on "A note from the maintainer",
1920 written by Junio C Hamano, the maintainer of git.
1922 There are four branches in the paraslash repository that track the
1923 source tree: "master", "maint", "next", and "pu".
1925 The "master" branch is meant to contain what is well tested and
1926 ready to be used in a production setting. There could occasionally be
1927 minor breakages or brown paper bag bugs but they are not expected to
1928 be anything major, and more importantly quickly and easily fixable.
1929 Every now and then, a "feature release" is cut from the tip of this
1930 branch, named with three dotted decimal digits, like 0.4.2.
1932 Whenever changes are about to be included that will eventually lead to
1933 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1934 "master" at that point. Obvious, safe and urgent fixes after the major
1935 release are applied to this branch and maintenance releases are cut
1936 from it. New features never go to this branch. This branch is also
1937 merged into "master" to propagate the fixes forward.
1939 A trivial and safe enhancement goes directly on top of "master".
1940 New development does not usually happen on "master", however.
1941 Instead, a separate topic branch is forked from the tip of "master",
1942 and it first is tested in isolation; Usually there are a handful such
1943 topic branches that are running ahead of "master". The tip of these
1944 branches is not published in the public repository to keep the number
1945 of branches that downstream developers need to worry about low.
1947 The quality of topic branches varies widely. Some of them start out as
1948 "good idea but obviously is broken in some areas" and then with some
1949 more work become "more or less done and can now be tested by wider
1950 audience". Luckily, most of them start out in the latter, better shape.
1952 The "next" branch is to merge and test topic branches in the latter
1953 category. In general, this branch always contains the tip of "master".
1954 It might not be quite rock-solid production ready, but is expected to
1955 work more or less without major breakage. The maintainer usually uses
1956 the "next" version of paraslash for his own pleasure, so it cannot
1957 be _that_ broken. The "next" branch is where new and exciting things
1958 take place.
1960 The two branches "master" and "maint" are never rewound, and "next"
1961 usually will not be either (this automatically means the topics that
1962 have been merged into "next" are usually not rebased, and you can find
1963 the tip of topic branches you are interested in from the output of
1964 "git log next"). You should be able to safely build on top of them.
1966 However, at times "next" will be rebuilt from the tip of "master" to
1967 get rid of merge commits that will never be in "master". The commit
1968 that replaces "next" will usually have the identical tree, but it
1969 will have different ancestry from the tip of "master".
1971 The "pu" (proposed updates) branch bundles the remainder of the
1972 topic branches. The "pu" branch, and topic branches that are only in
1973 "pu", are subject to rebasing in general. By the above definition
1974 of how "next" works, you can tell that this branch will contain quite
1975 experimental and obviously broken stuff.
1977 When a topic that was in "pu" proves to be in testable shape, it
1978 graduates to "next". This is done with
1980 git checkout next
1981 git merge that-topic-branch
1983 Sometimes, an idea that looked promising turns out to be not so good
1984 and the topic can be dropped from "pu" in such a case.
1986 A topic that is in "next" is expected to be polished to perfection
1987 before it is merged to "master". Similar to the above, this is
1988 done with
1990 git checkout master
1991 git merge that-topic-branch
1992 git branch -d that-topic-branch
1994 Note that being in "next" is not a guarantee to appear in the next
1995 release (being in "master" is such a guarantee, unless it is later
1996 found seriously broken and reverted), nor even in any future release.
1998 Coding Style
1999 ~~~~~~~~~~~~
2001 The preferred coding style for paraslash coincides more or less
2002 with the style of the Linux kernel. So rather than repeating what is
2003 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
2004 there), here are the most important points.
2006 - Burn the GNU coding standards.
2007 - Never use spaces for indentation.
2008 - Tabs are 8 characters, and thus indentations are also 8 characters.
2009 - Don't put multiple assignments on a single line.
2010 - Avoid tricky expressions.
2011 - Don't leave whitespace at the end of lines.
2012 - The limit on the length of lines is 80 columns.
2013 - Use K&R style for placing braces and spaces:
2015 if (x is true) {
2016 we do y
2017 }
2019 - Use a space after (most) keywords.
2020 - Do not add spaces around (inside) parenthesized expressions.
2021 - Use one space around (on each side of) most binary and ternary operators.
2022 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2023 - Mixed-case names are frowned upon.
2024 - Descriptive names for global variables are a must.
2025 - Avoid typedefs.
2026 - Functions should be short and sweet, and do just one thing.
2027 - The number of local variables shouldn't exceed 10.
2028 - Gotos are fine if they improve readability and reduce nesting.
2029 - Don't use C99-style "// ..." comments.
2030 - Names of macros defining constants and labels in enums are capitalized.
2031 - Enums are preferred when defining several related constants.
2032 - Always use the paraslash wrappers for allocating memory.
2033 - If the name of a function is an action or an imperative.
2034 command, the function should return an error-code integer
2035 (<0 means error, >=0 means success). If the name is a
2036 predicate, the function should return a "succeeded" boolean.
2039 Doxygen
2040 ~~~~~~~
2042 Doxygen is a documentation system for various programming
2043 languages. The paraslash project uses Doxygen for generating the API
2044 reference on the web pages, but good source code documentation is
2045 also beneficial to people trying to understand the code structure
2046 and the interactions between the various source files.
2048 It is more illustrative to look at the source code for examples than
2049 to describe the conventions for documenting the source in this manual,
2050 so we only describe which parts of the code need doxygen comments,
2051 but leave out details on documentation conventions.
2053 As a rule, only the public part of the C source is documented with
2054 Doxygen. This includes structures, defines and enumerations in header
2055 files as well as public (non-static) C functions. These should be
2056 documented completely. For example each parameter and the return
2057 value of a public function should get a descriptive comment.
2059 No doxygen comments are necessary for static functions and for
2060 structures and enumerations in C files (which are used only within
2061 this file). This does not mean, however, that those entities need
2062 no documentation at all. Instead, common sense should be applied to
2063 document what is not obvious from reading the code.
2065 --------
2066 Appendix
2067 --------
2069 Network protocols
2070 ~~~~~~~~~~~~~~~~~
2072 *IP*. The _Internet Protocol_ is the primary networking protocol
2073 used for the Internet. All protocols described below use IP as the
2074 underlying layer. Both the prevalent IPv4 and the next-generation
2075 IPv6 variant are being deployed actively worldwide.
2077 *Connection-oriented and connectionless protocols*. Connectionless
2078 protocols differ from connection-oriented ones in that state
2079 associated with the sending/receiving endpoints is treated
2080 implicitly. Connectionless protocols maintain no internal knowledge
2081 about the state of the connection. Hence they are not capable of
2082 reacting to state changes, such as sudden loss or congestion on the
2083 connection medium. Connection-oriented protocols, in contrast, make
2084 this knowledge explicit. The connection is established only after
2085 a bidirectional handshake which requires both endpoints to agree
2086 on the state of the connection, and may also involve negotiating
2087 specific parameters for the particular connection. Maintaining an
2088 up-to-date internal state of the connection also in general means
2089 that the sending endpoints perform congestion control, adapting to
2090 qualitative changes of the connection medium.
2092 *Reliability*. In IP networking, packets can be lost, duplicated,
2093 or delivered out of order, and different network protocols handle
2094 these problems in different ways. We call a transport-layer protocol
2095 _reliable_, if it turns the unreliable IP delivery into an ordered,
2096 duplicate- and loss-free delivery of packets. Sequence numbers
2097 are used to discard duplicates and re-arrange packets delivered
2098 out-of-order. Retransmission is used to guarantee loss-free
2099 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2100 or data integrity.
2102 *Classification*. With these definitions the protocols which are used
2103 by paraslash for steaming audio data may be classified as follows.
2105 - HTTP/TCP: connection-oriented, reliable,
2106 - UDP: connectionless, unreliable,
2107 - DCCP: connection-oriented, unreliable.
2109 Below we give a short descriptions of these protocols.
2111 *TCP*. The _Transmission Control Protocol_ provides reliable,
2112 ordered delivery of a stream and a classic window-based congestion
2113 control. In contrast to UDP and DCCP (see below), TCP does not have
2114 record-oriented or datagram-based syntax, i.e. it provides a stream
2115 which is unaware and independent of any record (packet) boundaries.
2116 TCP is used extensively by many application layers. Besides HTTP (the
2117 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2118 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2119 top of TCP.
2121 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2122 protocol, built as a thin layer directly on top of IP. For this reason,
2123 it offers the same best-effort service as IP itself, i.e. there is no
2124 detection of duplicate or reordered packets. Being a connectionless
2125 protocol, only minimal internal state about the connection is
2126 maintained, which means that there is no protection against packet
2127 loss or network congestion. Error checking and correction (if at all)
2128 are performed in the application.
2130 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2131 connection-oriented state maintenance known from TCP with the
2132 unreliable, datagram-based transport of UDP. This means that it
2133 is capable of reacting to changes in the connection by performing
2134 congestion control, offering multiple alternative approaches. But it
2135 is bound to datagram boundaries (the maximum packet size supported
2136 by a medium), and like UDP it lacks retransmission to protect
2137 against loss. Due to the use of sequence numbers, it is however
2138 able to react to loss (interpreted as a congestion indication) and
2139 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2140 to negotiate specific, binding features for a connection, such as
2141 the choice of congestion control: classic, window-based congestion
2142 control known from TCP is available as CCID-2, rate-based, "smooth"
2143 congestion control is offered as CCID-3.
2145 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2146 protocol on top of TCP. It is spoken by web servers and is most often
2147 used for web services. However, as can be seen by the many Internet
2148 radio stations and YouTube/Flash videos, http is by far not limited to
2149 the delivery of web pages only. Being a simple request/response based
2150 protocol, the semantics of the protocol also allow the delivery of
2151 multimedia content, such as audio over http.
2153 *Multicast*. IP multicast is not really a protocol but a technique
2154 for one-to-many communication over an IP network. The challenge is to
2155 deliver information to a group of destinations simultaneously using
2156 the most efficient strategy to send the messages over each link of
2157 the network only once. This has benefits for streaming multimedia:
2158 the standard one-to-one unicast offered by TCP/DCCP means that
2159 n clients listening to the same stream also consume n-times the
2160 resources, whereas multicast requires to send the stream just once,
2161 irrespective of the number of receivers. Since it would be costly to
2162 maintain state for each listening receiver, multicast often implies
2163 connectionless transport, which is the reason that it is currently
2164 only available via UDP.
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),
2192 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
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(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2201 User Datagram Protocol
2202 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2203 Internet Protocol
2204 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2205 RSA Cryptography Specifications
2206 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2207 (2006): Datagram Congestion Control Protocol (DCCP)
2208 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2209 Congestion Control ID 2: TCP-like Congestion Control
2210 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2211 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2212 - XREFERENCE(http://www.ietf.org/rfc/rfc6716.txt, RFC 6716) (2012):
2213 Definition of the Opus Audio Codec
2215 Application web pages
2216 ~~~~~~~~~~~~~~~~~~~~~
2218 - XREFERENCE(http://people.tuebingen.mpg.de/maan/paraslash/, paraslash)
2219 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash (alternative page))
2220 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2221 - XREFERENCE(http://www.mpg123.de/, mpg123)
2222 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2223 - XREFERENCE(http://www.icecast.org/, icecast)
2224 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2226 External documentation
2227 ~~~~~~~~~~~~~~~~~~~~~~
2229 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2230 H. Peter Anvin: The mathematics of Raid6)
2231 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2232 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2233 Communication Protocols)
2235 Code
2236 ~~~~
2237 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2238 Original FEC implementation by Luigi Rizzo)