Merge branch 't/cpsi_improvement'
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6 define(`REMOVE_NEWLINE', `translit(`$1',`
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9 define(`REFERENCE', ./``#''`LOCAL_LINK_NAME($1)' (`REMOVE_NEWLINE($2)'))
10 define(`XREFERENCE', `$1' (`REMOVE_NEWLINE($2)'))
11 define(`EMPH', ``_''`REMOVE_NEWLINE($1)'``_'')
13 Paraslash user manual
14 =====================
16 This document describes how to install, configure and use the paraslash
17 network audio streaming system. Most chapters start with a chapter
18 overview and conclude with an example section. We try to focus on
19 general concepts and on the interaction of the various pieces of the
20 paraslash package. Hence this user manual is not meant as a replacement
21 for the manual pages that describe all command line options of each
22 paraslash executable.
24 ------------
25 Introduction
26 ------------
28 In this chapter we give an REFERENCE(Overview, overview) of the
29 interactions of the two main programs contained in the paraslash
30 package, followed by REFERENCE(The paraslash executables, brief
31 descriptions) of all executables.
33 Overview
34 ~~~~~~~~
36 The core functionality of the para suite is provided by two main
37 executables, para_server and para_audiod. The former maintains a
38 database of audio files and streams these files to para_audiod which
39 receives and plays the stream.
41 In a typical setting, both para_server and para_audiod act as
42 background daemons whose functionality is controlled by client
43 programs: the para_audioc client controls para_audiod over a local
44 socket while the para_client program connects to para_server over a
45 local or remote networking connection.
47 Typically, these two daemons run on different hosts but a local setup
48 is also possible.
50 A simplified picture of a typical setup is as follows
51 <<
52 <pre>
53 server_host client_host
54 ~~~~~~~~~~~ ~~~~~~~~~~~
56 +-----------+ audio stream +-----------+
57 |para_server| -----------------------------> |para_audiod|
58 +-----------+ +-----------+
59 ^ ^
60 | |
61 | | connect
62 | |
63 | |
64 | +-----------+
65 | |para_audioc|
66 | +-----------+
67 |
68 |
69 | connect +-----------+
70 +-------------------------------------- |para_client|
71 +-----------+
72 </pre>
73 >>
75 The paraslash executables
76 ~~~~~~~~~~~~~~~~~~~~~~~~~
78 *para_server*
80 para_server streams binary audio data (MP3, ...) over local and/or
81 remote networks. It listens on a TCP port and accepts commands such
82 as play, stop, pause, next from authenticated clients. There are
83 many more commands though, see the man page of para_server for a
84 description of all commands.
86 It supports three built-in network streaming protocols
87 (senders/receivers): HTTP, DCCP, or UDP. This is explained in more
88 detail in the section on REFERENCE(Networking, networking).
90 The built-in audio file selector of paraslash is used to manage your
91 audio files. It maintains statistics on the usage of all available
92 audio files such as last-played time, and the number of times each
93 file was selected.
95 Additional information may be added to the database to allow
96 fine-grained selection based on various properties of the audio file,
97 including information found in (ID3) tags. However, old-fashioned
98 playlists are also supported.
100 It is also possible to store images (album covers) and lyrics in the
101 database and associate these to the corresponding audio files.
103 The section on the REFERENCE(The audio file selector, audio file
104 selector) discusses this topic.
107 *para_client*
109 The client program to connect to para_server. paraslash commands
110 are sent to para_server and the response is dumped to STDOUT. This
111 can be used by any scripting language to produce user interfaces with
112 little programming effort.
114 All connections between para_server and para_client are encrypted
115 with a symmetric session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
118 If para_client is started without non-option arguments, an interactive
119 session (shell) is started. Command history and command completion are
120 supported through libreadline.
122 *para_audiod*
124 The local daemon that collects information from para_server.
126 It runs on the client side and connects to para_server. As soon as
127 para_server announces the availability of an audio stream, para_audiod
128 starts an appropriate receiver, any number of filters and a paraslash
129 writer to play the stream.
131 Moreover, para_audiod listens on a local socket and sends status
132 information about para_server and para_audiod to local clients on
133 request. Access via this local socket may be restricted by using Unix
134 socket credentials, if available.
137 *para_audioc*
139 The client program which talks to para_audiod. Used to control
140 para_audiod, to receive status info, or to grab the stream at any
141 point of the decoding process. Like para_client, para_audioc supports
142 interactive sessions on systems with libreadline.
144 *para_recv*
146 A command line HTTP/DCCP/UDP stream grabber. The http mode is
147 compatible with arbitrary HTTP streaming sources (e.g. icecast).
148 In addition to the three network streaming modes, para_recv can also
149 operate in local (afh) mode. In this mode it writes the content of
150 an audio file on the local file system in complete chunks to stdout,
151 optionally 'just in time'. This allows to cut an audio file without
152 first decoding it, and it enables third-party software which is unaware
153 of the particular audio format to send complete frames in real time.
155 *para_filter*
157 A filter program that reads from STDIN and writes to STDOUT.
158 Like para_recv, this is an atomic building block which can be used to
159 assemble higher-level audio receiving facilities. It combines several
160 different functionalities in one tool: decoders for multiple audio
161 formats and a number of processing filters, among these a normalizer
162 for audio volume.
164 *para_afh*
166 A small stand-alone program that prints tech info about the given
167 audio file to STDOUT. It can be instructed to print a "chunk table",
168 an array of offsets within the audio file.
170 *para_write*
172 A modular audio stream writer. It supports a simple file writer
173 output plug-in and optional WAV/raw players for ALSA (Linux) and for
174 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
175 or raw audio player.
177 *para_play*
179 A command line audio player.
181 *para_gui*
183 Curses-based gui that presents status information obtained in a curses
184 window. Appearance can be customized via themes. para_gui provides
185 key-bindings for the most common server commands and new key-bindings
186 can be added easily.
189 *para_fade*
191 An alarm clock and volume-fader for OSS and ALSA.
193 -----------
194 Quick start
195 -----------
197 This chapter lists the REFERENCE(Requirements, necessary software)
198 that must be installed to compile the paraslash package, describes
199 how to REFERENCE(Installation, compile and install) the paraslash
200 source code and the steps that have to be performed in order to
201 REFERENCE(Quick start, set up) a typical server and client.
203 Requirements
204 ~~~~~~~~~~~~
205 For the impatient:
207 git clone git://
208 cd osl && make && sudo make install && sudo ldconfig
209 sudo apt-get install autoconf libssl-dev help2man gengetopt \
210 libmad0-dev libid3tag0-dev libasound2-dev libvorbis-dev \
211 libfaad-dev libspeex-dev libFLAC-dev libsamplerate-dev \
212 libasound2-dev libao-dev libreadline-dev libncurses-dev
214 Detailed description: In any case you'll need
216 - XREFERENCE(, libosl).
217 The _object storage layer_ library is used by para_server. To
218 clone the source code repository, execute
220 git clone git://
222 - XREFERENCE(, gcc) or
223 XREFERENCE(, clang). All gcc versions
224 >= 3.3 are currently supported. Clang version 1.1 or newer
225 should work as well.
227 - XREFERENCE(, gnu make) is
228 also shipped with the disto. On BSD systems the gnu make
229 executable is often called gmake.
231 - XREFERENCE(, bash). Some
232 scripts which run during compilation require the EMPH(Bourne
233 again shell). It is most likely already installed.
235 - XREFERENCE(, gengetopt)
236 is needed to generate the C code for the command line parsers
237 of all paraslash executables.
239 - XREFERENCE(, help2man)
240 is used to create the man pages.
242 Optional:
244 - XREFERENCE(, openssl) or
245 XREFERENCE(, libgcrypt).
246 At least one of these two libraries is needed as the backend
247 for cryptographic routines on both the server and the client
248 side. Both openssl and libgcrypt are usually shipped with the
249 distro, but you might have to install the development package
250 (libssl-dev or libgcrypt-dev on debian systems) as well.
252 - XREFERENCE(, libmad).
253 To compile in MP3 support for paraslash, the development
254 package must be installed. It is called libmad0-dev on
255 debian-based systems. Note that libmad is not necessary on
256 the server side, i.e. for sending MP3 files.
259 libid3tag). For version-2 ID3 tag support, you'll need
260 the libid3tag development package libid3tag0-dev. Without
261 libid3tag, only version one tags are recognized.
263 - XREFERENCE(, ogg vorbis).
264 For ogg vorbis streams you'll need libogg, libvorbis,
265 libvorbisfile. The corresponding Debian packages are called
266 libogg-dev and libvorbis-dev.
268 - XREFERENCE(, libfaad). For aac
269 files (m4a) you'll need libfaad (libfaad-dev).
271 - XREFERENCE(, speex). In order to stream
272 or decode speex files, libspeex (libspeex-dev) is required.
274 - XREFERENCE(, flac). To stream
275 or decode files encoded with the _Free Lossless Audio Codec_,
276 libFLAC (libFLAC-dev) must be installed.
279 libsamplerate). The resample filter will only be compiled if
280 this library is installed. Debian package: libsamplerate-dev.
282 - XREFERENCE(, alsa-lib). On
283 Linux, you'll need to have ALSA's development package
284 libasound2-dev installed.
287 libao). Needed to build the ao writer (ESD, PulseAudio,...).
288 Debian package: libao-dev.
290 - XREFERENCE(, curses). Needed
291 for para_gui. Debian package: libncurses-dev.
294 GNU Readline). If this library (libreadline-dev) is installed,
295 para_client, para_audioc and para_play support interactive
296 sessions.
298 Installation
299 ~~~~~~~~~~~~
301 First make sure all non-optional packages listed in the section on
302 REFERENCE(Requirements, required software) are installed on your
303 system.
305 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
306 OGG/Speex and AAC support are all optional. The configure script will
307 detect what is installed on your system and will only try to build
308 those executables that can be built with your setup.
310 Note that no special decoder library (not even the MP3 decoding library
311 libmad) is needed for para_server if you only want to stream MP3 or WMA
312 files. Also, it's fine to use para_server on a box without sound card.
314 Next, install the paraslash package on all machines, you'd like this
315 software to run on. If you compile from a released tarball, execute
317 (./configure && make) > /dev/null
319 When compiling from git or from snapshots downloaded via gitweb,
320 the above command will not work because the configure script is not
321 included in the git repository. In this case the following command
322 should be used instead:
324 ./
326 This runs autoconf to generate the configure script, then runs it as
327 above. Therefore you'll need autoconf for this to work.
329 There should be no errors but probably some warnings about missing
330 packages which usually implies that not all audio formats will be
331 supported. If headers or libs are installed at unusual locations you
332 might need to tell the configure script where to find them. Try
334 ./configure --help
336 to see a list of options. If the paraslash package was compiled
337 successfully, execute (optionally)
339 make test
341 to run the paraslash test suite. If all tests pass, execute as root
343 make install
345 to install executables under /usr/local/bin and the man pages under
346 /usr/local/man.
348 Configuration
349 ~~~~~~~~~~~~~
351 *Step 1*: Create a paraslash user
353 In order to control para_server at runtime you must create a paraslash
354 user. As authentication is based on the RSA crypto system you'll have
355 to create an RSA key pair. If you already have a user and an RSA key
356 pair, you may skip this step.
358 In this section we'll assume a typical setup: You would like to run
359 para_server on some host called server_host as user foo, and you want
360 to connect to para_server from another machine called client_host as
361 user bar.
363 As foo@server_host, create ~/.paraslash/server.users by typing the
364 following commands:
366 user=bar
367 target=~/.paraslash/server.users
368 key=~/.paraslash/$user
370 mkdir -p ~/.paraslash
371 echo "user $user $key $perms" >> $target
373 Next, change to the "bar" account on client_host and generate the
374 key pair with the commands
376 ssh-keygen -t rsa -b 2048
377 # hit enter twice to create a key with no passphrase
379 This generates the two files id_rsa and in ~/.ssh. Note
380 that paraslash can also read keys generated by the "openssl genrsa"
381 command. However, since keys created with ssh-keygen can also be used
382 for ssh, this method is recommended.
384 Note that para_server refuses to use a key if it is shorter than 2048
385 bits. In particular, the RSA keys of paraslash 0.3.x will not work
386 with version 0.4.x. Moreover, para_client refuses to use a (private)
387 key which is world-readable.
389 para_server only needs to know the public key of the key pair just
390 created. Copy this public key to server_host:
392 src=~/.ssh/
393 dest=.paraslash/$LOGNAME
394 scp $src foo@server_host:$dest
396 Finally, tell para_client to connect to server_host:
398 conf=~/.paraslash/client.conf
399 echo 'hostname server_host' > $conf
402 *Step 2*: Start para_server
404 Before starting the server make sure you have write permissions to
405 the directory /var/paraslash that has been created during installation:
407 sudo chown $LOGNAME /var/paraslash
409 Alternatively, use the --afs-socket Option to specify a different
410 location for the AFS command socket.
412 For this first try, we'll use the info loglevel to make the output
413 of para_server more verbose.
415 para_server -l info
417 Now you can use para_client to connect to the server and issue
418 commands. Open a new shell as bar@client_host and try
420 para_client help
421 para_client si
423 to retrieve the list of available commands and some server info.
424 Don't proceed if this doesn't work.
426 *Step 3*: Create and populate the database
428 An empty database is created with
430 para_client init
432 This initializes a couple of empty tables under
433 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
434 tables, but it's good to know that you can start from scratch with
436 rm -rf ~/.paraslash/afs_database-0.4
438 in case something went wrong.
440 Next, you need to add some audio files to that database so that
441 para_server knows about them. Choose an absolute path to a directory
442 containing some audio files and add them to the audio file table:
444 para_client add /my/mp3/dir
446 This might take a while, so it is a good idea to start with a directory
447 containing not too many files. Note that the table only contains data
448 about the audio files found, not the files themselves.
450 You may print the list of all known audio files with
452 para_client ls
454 *Step 4*: Configure para_audiod
456 para_audiod needs to create a "well-known" socket for the clients to
457 connect to. The default path for this socket is
459 /var/paraslash/audiod_socket.$HOSTNAME
461 In order to make this directory writable for para_audiod, execute
462 as bar@client_host
464 sudo chown $LOGNAME /var/paraslash
467 We will also have to tell para_audiod that it should receive the
468 audio stream from server_host via http:
470 para_audiod -l info -r '.:http -i server_host'
472 You should now be able to listen to the audio stream once para_server
473 starts streaming. To activate streaming, execute
475 para_client play
477 Since no playlist has been specified yet, the "dummy" mode which
478 selects all known audio files is activated automatically. See the
479 section on the REFERENCE(The audio file selector, audio file selector)
480 for how to use playlists and moods to specify which files should be
481 streamed in which order.
483 *Troubleshooting*
485 It did not work? To find out why, try to receive, decode and play the
486 stream manually using para_recv, para_filter and para_write as follows.
488 For simplicity we assume that you're running Linux/ALSA and that only
489 MP3 files have been added to the database.
491 para_recv -r 'http -i server_host' > file.mp3
492 # (interrupt with CTRL+C after a few seconds)
493 ls -l file.mp3 # should not be empty
494 para_filter -f mp3dec -f wav < file.mp3 > file.wav
495 ls -l file.wav # should be much bigger than file.mp3
496 para_write -w alsa < file.wav
498 Double check what is logged by para_server and use the --loglevel
499 option of para_recv, para_filter and para_write to increase verbosity.
501 ---------------
502 User management
503 ---------------
505 para_server uses a challenge-response mechanism to authenticate
506 requests from incoming connections, similar to ssh's public key
507 authentication method. Authenticated connections are encrypted using
508 a stream cipher, either RC4 or AES in integer counter mode.
510 In this chapter we briefly describe RSA, RC4 and AES, and sketch the
511 REFERENCE(Client-server authentication, authentication handshake)
512 between para_client and para_server. User management is discussed
513 in the section on REFERENCE(The user_list file, the user_list file).
514 These sections are all about communication between the client and the
515 server. Connecting para_audiod is a different matter and is described
516 in a REFERENCE(Connecting para_audiod, separate section).
520 RSA, RC4, AES
521 ~~~~~~~~~~~~~
523 RSA is an asymmetric block cipher which is used in many applications,
524 including ssh and gpg. An RSA key consists in fact of two keys,
525 called the public key and the private key. A message can be encrypted
526 with either key and only the counterpart of that key can decrypt
527 the message. While RSA can be used for both signing and encrypting
528 a message, paraslash uses RSA only for the latter purpose. The
529 RSA public key encryption and signatures algorithms are defined in
530 detail in RFC 2437.
532 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
533 key stream to produce the output. Decryption uses the same function
534 calls as encryption. While RC4 supports variable key lengths,
535 paraslash uses a fixed length of 256 bits, which is considered a
536 strong encryption by today's standards. Since the same key must never
537 be used twice, a different, randomly-generated key is used for every
538 new connection.
540 AES, the advanced encryption standard, is a well-known symmetric block
541 cipher, i.e. a transformation operating on fixed-length blocks which
542 is determined by a single key for both encryption and decryption. Any
543 block cipher can be turned into a stream cipher by generating
544 a pseudo-random key stream by encrypting successive values of a
545 counter. The AES_CTR128 stream cipher used in paraslash is obtained
546 in this way from the AES block cipher with a 128 bit block size.
549 Client-server authentication
550 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
552 The authentication handshake between para_client and para_server goes
553 as follows:
555 - para_client connects to para_server and sends an
556 authentication request for a user. It does so by connecting
557 to TCP port 2990 of the server host. This port is called the
558 para_server _control port_.
560 - para_server accepts the connection and forks a child process
561 which handles the incoming request. The parent process keeps
562 listening on the control port while the child process (also
563 called para_server below) continues as follows.
565 - para_server loads the RSA public key of that user, fills a
566 fixed-length buffer with random bytes, encrypts that buffer
567 using the public key and sends the encrypted buffer to the
568 client. The first part of the buffer is the challenge which
569 is used for authentication while the second part is the
570 session key.
572 - para_client receives the encrypted buffer and decrypts it
573 with the user's private key, thereby obtaining the challenge
574 buffer and the session key. It sends the SHA1 hash value of
575 the challenge back to para_server and stores the session key
576 for further use.
578 - para_server also computes the SHA1 hash of the challenge
579 and compares it against what was sent back by the client.
581 - If the two hashes do not match, the authentication has
582 failed and para_server closes the connection.
584 - Otherwise the user is considered authenticated and the client
585 is allowed to proceed by sending a command to be executed. From
586 this point on the communication is encrypted using the stream
587 cipher with the session key known to both peers.
589 paraslash relies on the quality of the pseudo-random bytes provided
590 by the crypto library (openssl or libgcrypt), on the security of
591 the implementation of the RSA, RC4 and AES crypto routines and on the
592 infeasibility to invert the SHA1 function.
594 Neither para_server or para_client create RSA keys on their own. This
595 has to be done once for each user as sketched in REFERENCE(Quick start,
596 Quick start) and discussed in more detail REFERENCE(The user_list
597 file, below).
599 The user_list file
600 ~~~~~~~~~~~~~~~~~~
602 At startup para_server reads the user list file which contains one
603 line per user. The default location of the user list file may be
604 changed with the --user-list option.
606 There should be at least one user in this file. Each user must have
607 an RSA key pair. The public part of the key is needed by para_server
608 while the private key is needed by para_client. Each line of the
609 user list file must be of the form
611 user <username> <key> <perms>
613 where _username_ is an arbitrary string (usually the user's login
614 name), _key_ is the full path to that user's public RSA key, and
615 _perms_ is a comma-separated list of zero or more of the following
616 permission bits:
618 +---------------------------------------------------------+
619 | AFS_READ | read the contents of the databases |
620 +-----------+---------------------------------------------+
621 | AFS_WRITE | change database contents |
622 +-----------+---------------------------------------------+
623 | VSS_READ | obtain information about the current stream |
624 +-----------+---------------------------------------------+
625 | VSS_WRITE | change the current stream |
626 +---------------------------------------------------------+
628 The permission bits specify which commands the user is allowed to
629 execute. The output of
631 para_client help
633 contains in the third column the permissions needed to execute the
634 command.
636 It is possible to make para_server reread the user_list file by
637 executing the paraslash "hup" command or by sending SIGHUP to the
638 PID of para_server.
641 Connecting para_audiod
642 ~~~~~~~~~~~~~~~~~~~~~~
644 para_audiod listens on a Unix domain socket. Those sockets are
645 for local communication only, so only local users can connect to
646 para_audiod. The default is to let any user connect but this can be
647 restricted on platforms that support UNIX socket credentials which
648 allow para_audiod to obtain the Unix credentials of the connecting
649 process.
651 Use para_audiod's --user-allow option to allow connections only for
652 a limited set of users.
654 -----------------------
655 The audio file selector
656 -----------------------
658 paraslash comes with a sophisticated audio file selector (AFS),
659 whose main task is to determine which file to stream next, based on
660 information on the audio files stored in a database. It communicates
661 also with para_client whenever an AFS command is executed, for example
662 to answer a database query.
664 Besides the traditional playlists, AFS supports audio file selection
665 based on _moods_ which act as a filter that limits the set of all
666 known audio files to those which satisfy certain criteria. It also
667 maintains tables containing images (e.g. album cover art) and lyrics
668 that can be associated with one or more audio files.
670 AFS uses XREFERENCE(, libosl), the
671 object storage layer library, as the backend library for storing
672 information on audio files, playlists, etc. This library offers
673 functionality similar to a relational database, but is much more
674 lightweight than a full database backend.
676 In this chapter we sketch the setup of the REFERENCE(The AFS process,
677 AFS process) during server startup and proceed with the description
678 of the REFERENCE(Database layout, layout) of the various database
679 tables. The section on REFERENCE(Playlists and moods, playlists
680 and moods) explains these two audio file selection mechanisms
681 in detail and contains pratical examples. The way REFERENCE(File
682 renames and content changes, file renames and content changes) are
683 detected is discussed briefly before the REFERENCE(Troubleshooting,
684 Troubleshooting) section concludes the chapter.
686 The AFS process
687 ~~~~~~~~~~~~~~~
689 On startup, para_server forks to create the AFS process which opens
690 the OSL database tables. The server process communicates with the
691 AFS process via pipes and shared memory. Usually, the AFS process
692 awakes only briefly whenever the current audio file changes. The AFS
693 process determines the next audio file, opens it, verifies it has
694 not been changed since it was added to the database and passes the
695 open file descriptor to the server process, along with audio file
696 meta-data such as file name, duration, audio format and so on. The
697 server process then starts to stream the audio file.
699 The AFS process also accepts connections from local clients via
700 a well-known socket. However, only child processes of para_server
701 may connect through this socket. All server commands that have the
702 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
703 change the database.
705 Database layout
706 ~~~~~~~~~~~~~~~
708 *The audio file table*
710 This is the most important and usually also the largest table of the
711 AFS database. It contains the information needed to stream each audio
712 file. In particular the following data is stored for each audio file.
714 - SHA1 hash value of the audio file contents. This is computed
715 once when the file is added to the database. Whenever AFS
716 selects this audio file for streaming the hash value is
717 recomputed and checked against the value stored in the
718 database to detect content changes.
720 - The time when this audio file was last played.
722 - The number of times the file has been played so far.
724 - The attribute bitmask.
726 - The image id which describes the image associated with this
727 audio file.
729 - The lyrics id which describes the lyrics associated with
730 this audio file.
732 - The audio format id (MP3, OGG, ...).
734 - An amplification value that can be used by the amplification
735 filter to pre-amplify the decoded audio stream.
737 - The chunk table. It describes the location and the timing
738 of the building blocks of the audio file. This is used by
739 para_server to send chunks of the file at appropriate times.
741 - The duration of the audio file.
743 - Tag information contained in the audio file (ID3 tags,
744 Vorbis comments, ...).
746 - The number of channels
748 - The encoding bitrate.
750 - The sampling frequency.
752 To add or refresh the data contained in the audio file table, the _add_
753 command is used. It takes the full path of either an audio file or a
754 directory. In the latter case, the directory is traversed recursively
755 and all files which are recognized as valid audio files are added to
756 the database.
758 *The attribute table*
760 The attribute table contains two columns, _name_ and _bitnum_. An
761 attribute is simply a name for a certain bit number in the attribute
762 bitmask of the audio file table.
764 Each of the 64 bits of the attribute bitmask can be set for each
765 audio file individually. Hence up to 64 different attributes may be
766 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
767 "german_lyrics", "speech", whatever. You are free to choose as
768 many attributes as you like and there are no naming restrictions
769 for attributes.
771 A new attribute "test" is created by
773 para_client addatt test
774 and
775 para_client lsatt
777 lists all available attributes. You can set the "test" attribute for
778 an audio file by executing
780 para_client setatt test+ /path/to/the/audio/file
782 Similarly, the "test" bit can be removed from an audio file with
784 para_client setatt test- /path/to/the/audio/file
786 Instead of a path you may use a shell wildcard pattern. The attribute
787 is applied to all audio files matching this pattern:
789 para_client setatt test+ '/test/directory/*'
791 The command
793 para_client -- ls -lv
795 gives you a verbose listing of your audio files also showing which
796 attributes are set.
798 In case you wonder why the double-dash in the above command is needed:
799 It tells para_client to not interpret the options after the dashes. If
800 you find this annoying, just say
802 alias para='para_client --'
804 and be happy. In what follows we shall use this alias.
806 The "test" attribute can be dropped from the database with
808 para rmatt test
810 Read the output of
812 para help ls
813 para help setatt
815 for more information and a complete list of command line options to
816 these commands.
818 *Blob tables*
820 The image, lyrics, moods and playlists tables are all blob tables.
821 Blob tables consist of three columns each: The identifier which is
822 a positive non-negative number that is auto-incremented, the name
823 (an arbitrary string) and the content (the blob).
825 All blob tables support the same set of actions: cat, ls, mv, rm
826 and add. Of course, _add_ is used for adding new blobs to the table
827 while the other actions have the same meaning as the corresponding
828 Unix commands. The paraslash commands to perform these actions are
829 constructed as the concatenation of the table name and the action. For
830 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
831 manipulate or query the image table.
833 The add variant of these commands is special as these commands read
834 the blob contents from stdin. To add an image to the image table the
835 command
837 para addimg image_name < file.jpg
839 can be used.
841 Note that the images and lyrics are not interpreted at all, and also
842 the playlist and the mood blobs are only investigated when the mood
843 or playlist is activated with the select command.
845 *The score table*
847 Unlike all other tables the contents of the score table remain in
848 memory and are never stored on disk. The score table contains two
849 columns: The SHA1 hash value (of an audio file) and its current
850 score.
852 However, only those files which are admissible for the current mood
853 or playlist are contained in the score table. The audio file selector
854 always chooses the row with the highest score as the file to stream
855 next. While doing so, it computes the new score and updates the
856 last_played and the num_played fields in the audio file table.
858 The score table is recomputed by the select command which loads a
859 mood or playlist. Audio files are chosen for streaming from the rows
860 of the score table on a highest-score-first basis.
863 Playlists and moods
864 ~~~~~~~~~~~~~~~~~~~
866 Playlists and moods offer two different ways of specifying the set of
867 admissible files. A playlist in itself describes a set of admissible
868 files. A mood, in contrast, describes the set of admissible files in
869 terms of attributes and other type of information available in the
870 audio file table. As an example, a mood can define a filename pattern,
871 which is then matched against the names of audio files in the table.
873 *Playlists*
875 Playlists are accommodated in the playlist table of the afs database,
876 using the aforementioned blob format for tables. A new playlist is
877 created with the addpl command by specifying the full (absolute)
878 paths of all desired audio files, separated by newlines. Example:
880 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
882 If _my_playlist_ already exists it is overwritten. To activate the
883 new playlist, execute
885 para select p/my_playlist
887 The audio file selector will assign scores to each entry of the list,
888 in descending order so that files will be selected in order. If a
889 file could not be opened for streaming, its entry is removed from
890 the score table (but not from the playlist).
892 *Moods*
894 A mood consists of a unique name and its *mood definition*, which is
895 a set of *mood lines* containing expressions in terms of attributes
896 and other data contained in the database.
898 At any time at most one mood can be *active* which means that
899 para_server is going to select only files from that subset of
900 admissible files.
902 So in order to create a mood definition one has to write a set of
903 mood lines. Mood lines come in three flavours: Accept lines, deny
904 lines and score lines.
906 The general syntax of the three types of mood lines is
909 accept [with score <score>] [if] [not] <mood_method> [options]
910 deny [with score <score>] [if] [not] <mood_method> [options]
911 score <score> [if] [not] <mood_method> [options]
914 Here <score> is either an integer or the string "random" which assigns
915 a random score to all matching files. The score value changes the
916 order in which admissible files are going to be selected, but is of
917 minor importance for this introduction.
919 So we concentrate on the first two forms, i.e. accept and deny
920 lines. As usual, everything in square brackets is optional, i.e.
921 accept/deny lines take the following form when ignoring scores:
923 accept [if] [not] <mood_method> [options]
925 and analogously for the deny case. The "if" keyword is only syntactic
926 sugar and has no function. The "not" keyword just inverts the result,
927 so the essence of a mood line is the mood method part and the options
928 following thereafter.
930 A *mood method* is realized as a function which takes an audio file
931 and computes a number from the data contained in the database.
932 If this number is non-negative, we say the file *matches* the mood
933 method. The file matches the full mood line if it either
935 - matches the mood method and the "not" keyword is not given,
936 or
937 - does not match the mood method, but the "not" keyword is given.
939 The set of admissible files for the whole mood is now defined as those
940 files which match at least one accept mood line, but no deny mood line.
941 More formally, an audio file F is admissible if and only if
943 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
945 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
946 lines and "~" means "matches".
948 The cases where no mood lines of accept/deny type are defined need
949 special treatment:
951 - Neither accept nor deny lines: This treats all files as
952 admissible (in fact, that is the definition of the dummy mood
953 which is activated automatically if no moods are available).
955 - Only accept lines: A file is admissible iff it matches at
956 least one accept line:
958 F ~ AL1 or F ~ AL2 or ...
960 - Only deny lines: A file is admissible iff it matches no
961 deny line:
963 not (F ~ DL1 or F ~ DN2 ...)
967 *List of mood_methods*
969 no_attributes_set
971 Takes no arguments and matches an audio file if and only if no
972 attributes are set.
974 is_set <attribute_name>
976 Takes the name of an attribute and matches iff that attribute is set.
978 path_matches <pattern>
980 Takes a filename pattern and matches iff the path of the audio file
981 matches the pattern.
983 artist_matches <pattern>
984 album_matches <pattern>
985 title_matches <pattern>
986 comment_matches <pattern>
988 Takes an extended regular expression and matches iff the text of the
989 corresponding tag of the audio file matches the pattern. If the tag
990 is not set, the empty string is matched against the pattern.
992 year ~ <num>
993 bitrate ~ <num>
994 frequency ~ <num>
995 channels ~ <num>
996 num_played ~ <num>
998 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
999 <num>. Matches an audio file iff the condition <val> ~ <num> is
1000 satisfied where val is the corresponding value of the audio file
1001 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
1002 count, play count).
1004 The year tag is special as its value is undefined if the audio file
1005 has no year tag or the content of the year tag is not a number. Such
1006 audio files never match. Another difference is the special treatment
1007 if the year tag is a two-digit number. In this case either 1900 or
1008 2000 is added to the tag value, depending on whether the number is
1009 greater than 2000 plus the current year.
1012 *Mood usage*
1014 To create a new mood called "my_mood", write its definition into
1015 some temporary file, say "tmpfile", and add it to the mood table
1016 by executing
1018 para addmood my_mood < tmpfile
1020 If the mood definition is really short, you may just pipe it to the
1021 client instead of using temporary files. Like this:
1023 echo "$MOOD_DEFINITION" | para addmood my_mood
1025 There is no need to keep the temporary file since you can always use
1026 the catmood command to get it back:
1028 para catmood my_mood
1030 A mood can be activated by executing
1032 para select m/my_mood
1034 Once active, the list of admissible files is shown by the ls command
1035 if the "-a" switch is given:
1037 para ls -a
1040 *Example mood definition*
1042 Suppose you have defined attributes "punk" and "rock" and want to define
1043 a mood containing only Punk-Rock songs. That is, an audio file should be
1044 admissible if and only if both attributes are set. Since
1046 punk and rock
1048 is obviously the same as
1050 not (not punk or not rock)
1052 (de Morgan's rule), a mood definition that selects only Punk-Rock
1053 songs is
1055 deny if not is_set punk
1056 deny if not is_set rock
1060 File renames and content changes
1061 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1063 Since the audio file selector knows the SHA1 of each audio file that
1064 has been added to the afs database, it recognizes if the content of
1065 a file has changed, e.g. because an ID3 tag was added or modified.
1066 Also, if a file has been renamed or moved to a different location,
1067 afs will detect that an entry with the same hash value already exists
1068 in the audio file table.
1070 In both cases it is enough to just re-add the new file. In the
1071 first case (file content changed), the audio table is updated, while
1072 metadata such as the num_played and last_played fields, as well as
1073 the attributes, remain unchanged. In the other case, when the file
1074 is moved or renamed, only the path information is updated, all other
1075 data remains as before.
1077 It is possible to change the behaviour of the add command by using the
1078 "-l" (lazy add) or the "-f" (force add) option.
1080 Troubleshooting
1081 ~~~~~~~~~~~~~~~
1083 Use the debug loglevel (-l debug) to show debugging info. All paraslash
1084 executables have a brief online help which is displayed when -h is
1085 given. The --detailed-help option prints the full help text.
1087 If para_server crashed or was killed by SIGKILL (signal 9), it
1088 may refuse to start again because of "dirty osl tables". In this
1089 case you'll have to run the oslfsck program of libosl to fix your
1090 database:
1092 oslfsck -fd ~/.paraslash/afs_database-0.4
1094 However, make sure para_server isn't running before executing oslfsck.
1096 If you don't mind to recreate your database you can start
1097 from scratch by removing the entire database directory, i.e.
1099 rm -rf ~/.paraslash/afs_database-0.4
1101 Be aware that this removes all attribute definitions, all playlists
1102 and all mood definitions and requires to re-initialize the tables.
1104 Although oslfsck fixes inconsistencies in database tables it doesn't
1105 care about the table contents. To check for invalid table contents, use
1107 para_client check
1109 This prints out references to missing audio files as well as invalid
1110 playlists and mood definitions.
1112 Similarly, para_audiod refuses to start if its socket file exists, since
1113 this indicates that another instance of para_audiod is running. After
1114 a crash a stale socket file might remain and you must run
1116 para_audiod --force
1118 once to fix it up.
1120 ---------------------------------------
1121 Audio formats and audio format handlers
1122 ---------------------------------------
1124 Audio formats
1125 ~~~~~~~~~~~~~
1127 The following audio formats are supported by paraslash:
1129 *MP3*
1131 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1132 designed as part of its MPEG-1 standard. An MP3 file is made up of
1133 multiple MP3 frames, which consist of a header and a data block. The
1134 size of an MP3 frame depends on the bit rate and on the number
1135 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1136 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1137 400 bytes large.
1139 *OGG/Vorbis*
1141 OGG is a standardized audio container format, while Vorbis is an
1142 open source codec for lossy audio compression. Since Vorbis is most
1143 commonly made available via the OGG container format, it is often
1144 referred to as OGG/Vorbis. The OGG container format divides data into
1145 chunks called OGG pages. A typical OGG page is about 4KB large. The
1146 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1147 may vary considerably.
1149 *OGG/Speex*
1151 Speex is an open-source speech codec that is based on CELP (Code
1152 Excited Linear Prediction) coding. It is designed for voice
1153 over IP applications, has modest complexity and a small memory
1154 footprint. Wideband and narrowband (telephone quality) speech are
1155 supported. As for Vorbis audio, Speex bit-streams are often stored
1156 in OGG files. As of 2012 this codec is considered obsolete since the
1157 Oppus codec, described below, surpasses its performance in all areas.
1159 *OGG/Opus*
1161 Opus is a lossy audio compression format standardized through RFC
1162 6716 in 2012. It combines the speech-oriented SILK codec and the
1163 low-latency CELT (Constrained Energy Lapped Transform) codec. Like
1164 OGG/Vorbis and OGG/Speex, Opus data is usually encapsulated in OGG
1165 containers. All known software patents which cover Opus are licensed
1166 under royalty-free terms.
1168 *AAC*
1170 Advanced Audio Coding (AAC) is a standardized, lossy compression
1171 and encoding scheme for digital audio which is the default audio
1172 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1173 the container format and audio files encoded with AAC have the .m4a
1174 extension. A typical AAC frame is about 700 bytes large.
1176 *WMA*
1178 Windows Media Audio (WMA) is an audio data compression technology
1179 developed by Microsoft. A WMA file is usually encapsulated in the
1180 Advanced Systems Format (ASF) container format, which also specifies
1181 how meta data about the file is to be encoded. The bit stream of WMA
1182 is composed of superframes, each containing one or more frames of
1183 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1185 *FLAC*
1187 The Free Lossless Audio Codec (FLAC) compresses audio without quality
1188 loss. It gives better compression ratios than a general purpose
1189 compressor like zip or bzip2 because FLAC is designed specifically
1190 for audio. A FLAC-encoded file consits of frames of varying size, up
1191 to 16K. Each frame starts with a header that contains all information
1192 necessary to decode the frame.
1194 Meta data
1195 ~~~~~~~~~
1197 Unfortunately, each audio format has its own conventions how meta
1198 data is added as tags to the audio file.
1200 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1201 is rather simple but also very limited as it supports only artist,
1202 title, album, year and comment tags. Each of these can only be at most
1203 32 characters long. ID3, version 2 is much more flexible but requires
1204 a separate library being installed for paraslash to support it.
1206 Ogg vorbis, ogg speex and flac files contain meta data as Vorbis
1207 comments, which are typically implemented as strings of the form
1208 "[TAG]=[VALUE]". Unlike ID3 version 1 tags, one may use whichever
1209 tags are appropriate for the content.
1211 AAC files usually use the MPEG-4 container format for storing meta
1212 data while WMA files wrap meta data as special objects within the
1213 ASF container format.
1215 paraslash only tracks the most common tags that are supported by
1216 all tag variants: artist, title, year, album, comment. When a file
1217 is added to the AFS database, the meta data of the file is extracted
1218 and stored in the audio file table.
1220 Chunks and chunk tables
1221 ~~~~~~~~~~~~~~~~~~~~~~~
1223 paraslash uses the word "chunk" as common term for the building blocks
1224 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1225 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1226 size varies considerably between audio formats, from a few hundred
1227 bytes (MP3) up to 16K (FLAC).
1229 The chunk table contains the offsets within the audio file that
1230 correspond to the chunk boundaries of the file. Like the meta data,
1231 the chunk table is computed and stored in the database whenever an
1232 audio file is added.
1234 The paraslash senders (see below) always send complete chunks. The
1235 granularity for seeking is therefore determined by the chunk size.
1237 Audio format handlers
1238 ~~~~~~~~~~~~~~~~~~~~~
1240 For each audio format paraslash contains an audio format handler whose
1241 first task is to tell whether a given file is a valid audio file of
1242 this type. If so, the audio file handler extracts some technical data
1243 (duration, sampling rate, number of channels etc.), computes the
1244 chunk table and reads the meta data.
1246 The audio format handler code is linked into para_server and executed
1247 via the _add_ command. The same code is also available as a stand-alone
1248 tool, para_afh, which prints the technical data, the chunk table
1249 and the meta data of a file. Moreover, all audio format handlers are
1250 combined in the afh receiver which is part of para_recv and para_play.
1252 ----------
1253 Networking
1254 ----------
1256 Paraslash uses different network connections for control and data.
1257 para_client communicates with para_server over a dedicated TCP control
1258 connection. To transport audio data, separate data connections are
1259 used. For these data connections, a variety of transports (UDP, DCCP,
1260 HTTP) can be chosen.
1262 The chapter starts with the REFERENCE(The paraslash control
1263 service, control service), followed by a section on the various
1264 REFERENCE(Streaming protocols, streaming protocols) in which the data
1265 connections are described. The way audio file headers are embedded into
1266 the stream is discussed REFERENCE(Streams with headers and headerless
1267 streams, briefly) before the REFERENCE(Networking examples, example
1268 section) which illustrates typical commands for real-life scenarios.
1270 Both IPv4 and IPv6 are supported.
1272 The paraslash control service
1273 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1275 para_server is controlled at runtime via the paraslash control
1276 connection. This connection is used for server commands (play, stop,
1277 ...) as well as for afs commands (ls, select, ...).
1279 The server listens on a TCP port and accepts connections from clients
1280 that connect the open port. Each connection causes the server to fork
1281 off a client process which inherits the connection and deals with that
1282 client only. In this classical accept/fork approach the server process
1283 is unaffected if the child dies or goes crazy for whatever reason. In
1284 fact, the child process can not change address space of server process.
1286 The section on REFERENCE(Client-server authentication, client-server
1287 authentication) above described the early connection establishment
1288 from the crypto point of view. Here it is described what happens
1289 after the connection (including crypto setup) has been established.
1290 There are four processes involved during command dispatch as sketched
1291 in the following diagram.
1293 <<
1294 <pre>
1295 server_host client_host
1296 ~~~~~~~~~~~ ~~~~~~~~~~~
1298 +-----------+ connect +-----------+
1299 |para_server|<------------------------------ |para_client|
1300 +-----------+ +-----------+
1301 | ^
1302 | fork +---+ |
1303 +----------> |AFS| |
1304 | +---+ |
1305 | ^ |
1306 | | |
1307 | | connect (cookie) |
1308 | | |
1309 | | |
1310 | fork +-----+ inherited connection |
1311 +---------->|child|<--------------------------+
1312 +-----+
1313 </pre>
1314 >>
1316 Note that the child process is not a child of the afs process,
1317 so communication of these two processes has to happen via local
1318 sockets. In order to avoid abuse of the local socket by unrelated
1319 processes, a magic cookie is created once at server startup time just
1320 before the server process forks off the AFS process. This cookie is
1321 known to the server, AFS and the child, but not to unrelated processes.
1323 There are two different kinds of commands: First there are commands
1324 that cause the server to respond with some answer such as the list
1325 of all audio files. All but the addblob commands (addimg, addlyr,
1326 addpl, addmood) are of this kind. The addblob commands add contents
1327 to the database, so they need to transfer data the other way round,
1328 from the client to the server.
1330 There is no knowledge about the server commands built into para_client,
1331 so it does not know about addblob commands. Instead, it inspects the
1332 first data package sent by the server for a magic string. If this
1333 string was found, it sends STDIN to the server, otherwise it dumps
1334 data from the server to STDOUT.
1336 Streaming protocols
1337 ~~~~~~~~~~~~~~~~~~~
1339 A network (audio) stream usually consists of one streaming source,
1340 the _sender_, and one or more _receivers_ which read data over the
1341 network from the streaming source.
1343 Senders are thus part of para_server while receivers are part of
1344 para_audiod. Moreover, there is the stand-alone tool para_recv which
1345 can be used to manually download a stream, either from para_server
1346 or from a web-based audio streaming service.
1348 The following three streaming protocols are supported by paraslash:
1350 - HTTP. Recommended for public streams that can be played by
1351 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1352 sender is supported on all operating systems and all platforms.
1354 - DCCP. Recommended for LAN streaming. DCCP is currently
1355 available only for Linux.
1357 - UDP. Recommended for multicast LAN streaming.
1359 See the Appendix on REFERENCE(Network protocols, network protocols)
1360 for brief descriptions of the various protocols relevant for network
1361 audio streaming with paraslash.
1363 It is possible to activate more than one sender simultaneously.
1364 Senders can be controlled at run time and via config file and command
1365 line options.
1367 Note that audio connections are _not_ encrypted. Transport or Internet
1368 layer encryption should be used if encrypted data connections are
1369 needed.
1371 Since DCCP and TCP are both connection-oriented protocols, connection
1372 establishment/teardown and access control are very similar between
1373 these two streaming protocols. UDP is the most lightweight option,
1374 since in contrast to TCP/DCCP it is connectionless. It is also the
1375 only protocol supporting IP multicast.
1377 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1378 clients to connect and establish a connection via some protocol-defined
1379 handshake mechanism. Both senders maintain two linked lists each:
1380 The list of all clients which are currently connected, and the list
1381 of access control entries which determines who is allowed to connect.
1382 IP-based access control may be configured through config file and
1383 command line options and via the "allow" and "deny" sender subcommands.
1385 Upon receiving a GET request from the client, the HTTP sender sends
1386 back a status line and a message. The body of this message is the
1387 audio stream. This is common practice and is supported by many popular
1388 clients which can thus be used to play a stream offered by para_server.
1389 For DCCP things are a bit simpler: No messages are exchanged between
1390 the receiver and sender. The client simply connects and the sender
1391 starts to stream.
1393 DCCP is an experimental protocol which offers a number of new features
1394 not available for TCP. Both ends can negotiate these features using
1395 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1396 datagram-based (no retransmissions) and thus should not be used over
1397 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1398 is access to a variety of different congestion-control mechanisms
1399 called CCIDs. Two different CCIDs are available per default on Linux:
1402 - _CCID 2_. A Congestion Control mechanism similar to that
1403 of TCP. The sender maintains a congestion window and halves
1404 this window in response to congestion.
1407 - _CCID-3_. Designed to be fair when competing for bandwidth.
1408 It has lower variation of throughput over time compared with
1409 TCP, which makes it suitable for streaming media.
1411 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1412 single list, the _target list_. This list describes the set of clients
1413 to which the stream is sent. There is no list for access control and
1414 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1415 and "delete" commands can be used to modify the target list.
1417 Since both UDP and DCCP offer an unreliable datagram-based transport,
1418 additional measures are necessary to guard against disruptions over
1419 networks that are lossy or which may be subject to interference (as
1420 is for instance the case with WiFi). Paraslash uses FEC (Forward
1421 Error Correction) to guard against packet losses and reordering. The
1422 stream is FEC-encoded before it is sent through the UDP socket and
1423 must be decoded accordingly on the receiver side.
1425 The packet size and the amount of redundancy introduced by FEC can
1426 be configured via the FEC parameters which are dictated by server
1427 and may also be configured through the "sender" command. The FEC
1428 parameters are encoded in the header of each network packet, so no
1429 configuration is necessary on the receiver side. See the section on
1430 REFERENCE(Forward error correction, FEC) below.
1432 Streams with headers and headerless streams
1433 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1435 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1436 needed to decode the stream is only contained in the audio file
1437 header of the container format but not in each data chunk. Clients
1438 must be able to obtain this information in case streaming starts in
1439 the middle of the file or if para_audiod is started while para_server
1440 is already sending a stream.
1442 This is accomplished in different ways, depending on the streaming
1443 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1444 header is sent prior to audio file data. This technique however does
1445 not work for the connectionless UDP transport. Hence the audio file
1446 header is periodically being embedded into the UDP audio data stream.
1447 By default, the header is resent after five seconds. The receiver has
1448 to wait until the next header arrives before it can start decoding
1449 the stream.
1451 Examples
1452 ~~~~~~~~
1454 The sender command of para_server allows to (de-)activate senders
1455 and to change the access permissions senders at runtime. The "si"
1456 (server info) command is used to list the streaming options of the
1457 currently running server as well as the various sender access lists.
1459 -> Show client/target/access lists:
1461 para_client si
1463 -> Obtain general help for the sender command:
1465 para_client help sender
1467 -> Get help for a specific sender (contains further examples):
1469 s=http # or dccp or udp
1470 para_client sender $s help
1472 By default para_server activates both the HTTP and th DCCP sender on
1473 startup. This can be changed via command line options or para_server's
1474 config file.
1476 -> List config file options for senders:
1478 para_server -h
1480 All senders share the "on" and "off" commands, so senders may be
1481 activated and deactivated independently of each other.
1483 -> Switch off the http sender:
1485 para_client sender http off
1487 -> Receive a DCCP stream using CCID2 and write the output into a file:
1489; ccid=2; filename=bar
1490 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1492 Note the quotes around the arguments for the dccp receiver. Each
1493 receiver has its own set of command line options and its own command
1494 line parser, so arguments for the dccp receiver must be protected
1495 from being interpreted by para_recv.
1497 -> Start UDP multicast, using the default multicast address:
1499 para_client sender udp add
1501 -> Receive FEC-encoded multicast stream and write the output into a file:
1503 filename=foo
1504 para_recv -r udp > $filename
1506 -> Add an UDP unicast for a client to the target list of the UDP sender:
1509 para_client sender udp add $t
1511 -> Receive this (FEC-encoded) unicast stream:
1513 filename=foo
1514 para_recv -r 'udp -i' > $filename
1516 -> Create a minimal config for para_audiod for HTTP streams:
1518 c=$HOME/.paraslash/audiod.conf.min;
1519 echo receiver \".:http -i $s\" > $c
1520 para_audiod --config $c
1522 -------
1523 Filters
1524 -------
1526 A paraslash filter is a module which transforms an input stream into
1527 an output stream. Filters are included in the para_audiod executable
1528 and in the stand-alone tool para_filter which usually contains the
1529 same modules.
1531 While para_filter reads its input stream from STDIN and writes
1532 the output to STDOUT, the filter modules of para_audiod are always
1533 connected to a receiver which produces the input stream and a writer
1534 which absorbs the output stream.
1536 Some filters depend on a specific library and are not compiled in
1537 if this library was not found at compile time. To see the list of
1538 supported filters, run para_filter and para_audiod with the --help
1539 option. The output looks similar to the following:
1541 Available filters:
1542 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1544 Out of these filter modules, a chain of filters can be constructed,
1545 much in the way Unix pipes can be chained, and analogous to the use
1546 of modules in gstreamer: The output of the first filter becomes the
1547 input of the second filter. There is no limitation on the number of
1548 filters and the same filter may occur more than once.
1550 Like receivers, each filter has its own command line options which
1551 must be quoted to protect them from the command line options of
1552 the driving application (para_audiod or para_filter). Example:
1554 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1556 For para_audiod, each audio format has its own set of filters. The
1557 name of the audio format for which the filter should be applied can
1558 be used as the prefix for the filter option. Example:
1560 para_audiod -f 'mp3:prebuffer --duration 300'
1562 The "mp3" prefix above is actually interpreted as a POSIX extended
1563 regular expression. Therefore
1565 para_audiod -f '.:prebuffer --duration 300'
1567 activates the prebuffer filter for all supported audio formats (because
1568 "." matches all audio formats) while
1570 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1572 activates it only for wma and ogg streams.
1574 Decoders
1575 ~~~~~~~~
1577 For each supported audio format there is a corresponding filter
1578 which decodes audio data in this format to 16 bit PCM data which
1579 can be directly sent to the sound device or any other software that
1580 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1581 filters are called _decoders_ in general, and xxxdec is the name of
1582 the paraslash decoder for the audio format xxx. For example, the mp3
1583 decoder filter is called mp3dec.
1585 Note that the output of the decoder is about 10 times larger than
1586 its input. This means that filters that operate on the decoded audio
1587 stream have to deal with much more data than filters that transform
1588 the audio stream before it is fed to the decoder.
1590 Paraslash relies on external libraries for most decoders, so these
1591 libraries must be installed for the decoder to be included in the
1592 para_filter and para_audiod executables. The oggdec filter depends
1593 on the libogg and libvorbis libraries for example.
1595 Forward error correction
1596 ~~~~~~~~~~~~~~~~~~~~~~~~
1598 As already mentioned REFERENCE(Streaming protocols, earlier),
1599 paraslash uses forward error correction (FEC) for the unreliable UDP
1600 and DCCP transports. FEC is a technique which was invented already
1601 in 1960 by Reed and Solomon and which is widely used for the parity
1602 calculations of storage devices (RAID arrays). It is based on the
1603 algebraic concept of finite fields, today called Galois fields, in
1604 honour of the mathematician Galois (1811-1832). The FEC implementation
1605 of paraslash is based on code by Luigi Rizzo.
1607 Although the details require a sound knowledge of the underlying
1608 mathematics, the basic idea is not hard to understand: For positive
1609 integers k and n with k < n it is possible to compute for any k given
1610 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1611 ..., p_r such that all data bytes can be reconstructed from *any*
1612 k bytes of the set
1614 {d_1, ..., d_k, p_1, ..., p_r}.
1616 FEC-encoding for unreliable network transports boils down to slicing
1617 the audio stream into groups of k suitably sized pieces called _slices_
1618 and computing the r corresponding parity slices. This step is performed
1619 in para_server which then sends both the data and the parity slices
1620 over the unreliable network connection. If the client was able
1621 to receive at least k of the n = k + r slices, it can reconstruct
1622 (FEC-decode) the original audio stream.
1624 From these observations it is clear that there are three different
1625 FEC parameters: The slice size, the number of data slices k, and the
1626 total number of slices n. It is crucial to choose the slice size
1627 such that no fragmentation of network packets takes place because
1628 FEC only guards against losses and reordering but fails if slices are
1629 received partially.
1631 FEC decoding in paralash is performed through the fecdec filter which
1632 usually is the first filter (there can be other filters before fecdec
1633 if these do not alter the audio stream).
1636 Volume adjustment (amp and compress)
1637 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1639 The amp and the compress filter both adjust the volume of the audio
1640 stream. These filters operate on uncompressed audio samples. Hence
1641 they are usually placed directly after the decoding filter. Each
1642 sample is multiplied with a scaling factor (>= 1) which makes amp
1643 and compress quite expensive in terms of computing power.
1645 *amp*
1647 The amp filter amplifies the audio stream by a fixed scaling factor
1648 that must be known in advance. For para_audiod this factor is derived
1649 from the amplification field of the audio file's entry in the audio
1650 file table while para_filter uses the value given at the command line.
1652 The optimal scaling factor F for an audio file is the largest real
1653 number F >= 1 such that after multiplication with F all samples still
1654 fit into the sample interval [-32768, 32767]. One can use para_filter
1655 in combination with the sox utility to compute F:
1657 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1659 The amplification value V which is stored in the audio file table,
1660 however, is an integer between 0 and 255 which is connected to F
1661 through the formula
1663 V = (F - 1) * 64.
1665 To store V in the audio file table, the command
1667 para_client -- touch -a=V file.mp3
1669 is used. The reader is encouraged to write a script that performs
1670 these computations :)
1672 *compress*
1674 Unlike the amplification filter, the compress filter adjusts the volume
1675 of the audio stream dynamically without prior knowledge about the peak
1676 value. It maintains the maximal volume of the last n samples of the
1677 audio stream and computes a suitable amplification factor based on that
1678 value and the various configuration options. It tries to chose this
1679 factor such that the adjusted volume meets the desired target level.
1681 Note that it makes sense to combine amp and compress.
1683 Misc filters (wav and prebuffer)
1684 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1686 These filters are rather simple and do not modify the audio stream at
1687 all. The wav filter is only useful with para_filter and in connection
1688 with a decoder. It asks the decoder for the number of channels and the
1689 sample rate of the stream and adds a Microsoft wave header containing
1690 this information at the beginning. This allows to write wav files
1691 rather than raw PCM files (which do not contain any information about
1692 the number of channels and the sample rate).
1694 The prebuffer filter simply delays the output until the given time has
1695 passed (starting from the time the first byte was available in its
1696 input queue) or until the given amount of data has accumulated. It
1697 is mainly useful for para_audiod if the standard parameters result
1698 in buffer underruns.
1700 Both filters require almost no additional computing time, even when
1701 operating on uncompressed audio streams, since data buffers are simply
1702 "pushed down" rather than copied.
1704 Examples
1705 ~~~~~~~~
1707 -> Decode an mp3 file to wav format:
1709 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1711 -> Amplify a raw audio file by a factor of 1.5:
1713 para_filter -f amp --amp 32 < foo.raw > bar.raw
1715 ------
1716 Output
1717 ------
1719 Once an audio stream has been received and decoded to PCM format,
1720 it can be sent to a sound device for playback. This part is performed
1721 by paraslash _writers_ which are described in this chapter.
1723 Writers
1724 ~~~~~~~
1726 A paraslash writer acts as a data sink that consumes but does not
1727 produce audio data. Paraslash writers operate on the client side and
1728 are contained in para_audiod and in the stand-alone tool para_write.
1730 The para_write program reads uncompressed audio data from STDIN. If
1731 this data starts with a wav header, sample rate, sample format and
1732 channel count are read from the header. Otherwise CD audio (44.1KHz
1733 16 bit little endian, stereo) is assumed but this can be overridden
1734 by command line options. para_audiod, on the other hand, obtains
1735 the sample rate and the number of channels from the decoder.
1737 Like receivers and filters, each writer has an individual set of
1738 command line options, and for para_audiod writers can be configured
1739 per audio format separately. It is possible to activate more than
1740 one writer for the same stream simultaneously.
1742 OS-dependent APIs
1743 ~~~~~~~~~~~~~~~~~
1745 Unfortunately, the various flavours of Unix on which paraslash
1746 runs on have different APIs for opening a sound device and starting
1747 playback. Hence for each such API there is a paraslash writer that
1748 can play the audio stream via this API.
1750 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1751 Linux systems. Although there are several mid-layer APIs in use by
1752 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1753 currently supports only the low-level ALSA API which is not supposed
1754 to be change. ALSA is very feature-rich, in particular it supports
1755 software mixing via its DMIX plugin. ALSA is the default writer on
1756 Linux systems.
1758 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1759 is also available on Linux systems, usually provided by ALSA as an
1760 emulation for backwards compatibility. This API is rather simple but
1761 also limited. For example only one application can open the device
1762 at any time. The OSS writer is activated by default on BSD Systems.
1764 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1765 for this API is only compiled in on such systems and is of course
1766 the default there.
1768 *FILE*. The file writer allows to capture the audio stream and
1769 write the PCM data to a file on the file system rather than playing
1770 it through a sound device. It is supported on all platforms and is
1771 always compiled in.
1773 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1774 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1775 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1776 through an output plugin of libao.
1778 Examples
1779 ~~~~~~~~
1781 -> Use the OSS writer to play a wav file:
1783 para_write --writer oss < file.wav
1785 -> Enable ALSA software mixing for mp3 streams
1787 para_audiod --writer 'mp3:alsa -d plug:swmix'
1790 ---
1791 Gui
1792 ---
1794 para_gui executes an arbitrary command which is supposed to print
1795 status information to STDOUT. It then displays this information in
1796 a curses window. By default the command
1798 para_audioc -- stat -p
1800 is executed, but this can be customized via the --stat-cmd option. In
1801 particular it possible to use
1803 para_client -- stat -p
1805 to make para_gui work on systems on which para_audiod is not running.
1807 Key bindings
1808 ~~~~~~~~~~~~
1810 It is possible to bind keys to arbitrary commands via custom
1811 key-bindings. Besides the internal keys which can not be changed (help,
1812 quit, loglevel, version...), the following flavours of key-bindings
1813 are supported:
1815 - external: Shutdown curses before launching the given command.
1816 Useful for starting other ncurses programs from within
1817 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1818 output format to write a mailbox containing one mail for each
1819 (admissible) file the audio file selector knows about. Then
1820 start mutt from within para_gui to browse your collection!
1822 - display: Launch the command and display its stdout in
1823 para_gui's bottom window.
1825 - para: Like display, but start "para_client <specified
1826 command>" instead of "<specified command>".
1828 The general form of a key binding is
1830 key_map k:m:c
1832 which maps key k to command c using mode m. Mode may be x, d or p
1833 for external, display and paraslash commands, respectively.
1835 Themes
1836 ~~~~~~
1838 Currently there are only two themes for para_gui. It is easy, however,
1839 to add more themes. To create a new theme one has to define the
1840 position, color and geometry for for each status item that should be
1841 shown by this theme. See gui_theme.c for examples.
1843 The "." and "," keys are used to switch between themes.
1845 Examples
1846 ~~~~~~~~
1848 -> Show server info:
1850 key_map "i:p:si"
1852 -> Jump to the middle of the current audio file by pressing F5:
1854 key_map "<F5>:p:jmp 50"
1856 -> vi-like bindings for jumping around:
1858 key_map "l:p:ff 10"
1859 key_map "h:p:ff 10-"
1860 key_map "w:p:ff 60"
1861 key_map "b:p:ff 60-"
1863 -> Print the current date and time:
1865 key_map "D:d:date"
1867 -> Call other curses programs:
1869 key_map "U:x:aumix"
1870 key_map "!:x:/bin/bash"
1871 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1873 -----------
1874 Development
1875 -----------
1877 Tools
1878 ~~~~~
1880 In order to compile the sources from the git repository (rather than
1881 from tar balls) and for contributing non-trivial changes to the
1882 paraslash project, some additional tools should be installed on a
1883 developer machine.
1885 (git). As described in more detail REFERENCE(Git
1886 branches, below), the git source code management tool is used for
1887 paraslash development. It is necessary for cloning the git repository
1888 and for getting updates.
1890 (m4). Some input files for gengetopt
1891 are generated from templates by the m4 macro processor.
1893 (autoconf) GNU autoconf creates
1894 the configure file which is shipped in the tarballs but has to be
1895 generated when compiling from git.
1897 (grutatxt). The
1898 HTML version of this manual and some of the paraslash web pages are
1899 generated by the grutatxt plain text to HTML converter. If changes
1900 are made to these text files the grutatxt package must be installed
1901 to regenerate the HTML files.
1903 (doxygen). The documentation
1904 of paraslash's C sources uses the doxygen documentation system. The
1905 conventions for documenting the source code is described in the
1906 REFERENCE(Doxygen, Doxygen section).
1908 (global). This is used to generate
1909 browsable HTML from the C sources. It is needed by doxygen.
1911 Git branches
1912 ~~~~~~~~~~~~
1914 Paraslash has been developed using the git source code management
1915 tool since 2006. Development is organized roughly in the same spirit
1916 as the git development itself, as described below.
1918 The following text passage is based on "A note from the maintainer",
1919 written by Junio C Hamano, the maintainer of git.
1921 There are four branches in the paraslash repository that track the
1922 source tree: "master", "maint", "next", and "pu".
1924 The "master" branch is meant to contain what is well tested and
1925 ready to be used in a production setting. There could occasionally be
1926 minor breakages or brown paper bag bugs but they are not expected to
1927 be anything major, and more importantly quickly and easily fixable.
1928 Every now and then, a "feature release" is cut from the tip of this
1929 branch, named with three dotted decimal digits, like 0.4.2.
1931 Whenever changes are about to be included that will eventually lead to
1932 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1933 "master" at that point. Obvious, safe and urgent fixes after the major
1934 release are applied to this branch and maintenance releases are cut
1935 from it. New features never go to this branch. This branch is also
1936 merged into "master" to propagate the fixes forward.
1938 A trivial and safe enhancement goes directly on top of "master".
1939 New development does not usually happen on "master", however.
1940 Instead, a separate topic branch is forked from the tip of "master",
1941 and it first is tested in isolation; Usually there are a handful such
1942 topic branches that are running ahead of "master". The tip of these
1943 branches is not published in the public repository to keep the number
1944 of branches that downstream developers need to worry about low.
1946 The quality of topic branches varies widely. Some of them start out as
1947 "good idea but obviously is broken in some areas" and then with some
1948 more work become "more or less done and can now be tested by wider
1949 audience". Luckily, most of them start out in the latter, better shape.
1951 The "next" branch is to merge and test topic branches in the latter
1952 category. In general, this branch always contains the tip of "master".
1953 It might not be quite rock-solid production ready, but is expected to
1954 work more or less without major breakage. The maintainer usually uses
1955 the "next" version of paraslash for his own pleasure, so it cannot
1956 be _that_ broken. The "next" branch is where new and exciting things
1957 take place.
1959 The two branches "master" and "maint" are never rewound, and "next"
1960 usually will not be either (this automatically means the topics that
1961 have been merged into "next" are usually not rebased, and you can find
1962 the tip of topic branches you are interested in from the output of
1963 "git log next"). You should be able to safely build on top of them.
1965 However, at times "next" will be rebuilt from the tip of "master" to
1966 get rid of merge commits that will never be in "master". The commit
1967 that replaces "next" will usually have the identical tree, but it
1968 will have different ancestry from the tip of "master".
1970 The "pu" (proposed updates) branch bundles the remainder of the
1971 topic branches. The "pu" branch, and topic branches that are only in
1972 "pu", are subject to rebasing in general. By the above definition
1973 of how "next" works, you can tell that this branch will contain quite
1974 experimental and obviously broken stuff.
1976 When a topic that was in "pu" proves to be in testable shape, it
1977 graduates to "next". This is done with
1979 git checkout next
1980 git merge that-topic-branch
1982 Sometimes, an idea that looked promising turns out to be not so good
1983 and the topic can be dropped from "pu" in such a case.
1985 A topic that is in "next" is expected to be polished to perfection
1986 before it is merged to "master". Similar to the above, this is
1987 done with
1989 git checkout master
1990 git merge that-topic-branch
1991 git branch -d that-topic-branch
1993 Note that being in "next" is not a guarantee to appear in the next
1994 release (being in "master" is such a guarantee, unless it is later
1995 found seriously broken and reverted), nor even in any future release.
1997 Coding Style
1998 ~~~~~~~~~~~~
2000 The preferred coding style for paraslash coincides more or less
2001 with the style of the Linux kernel. So rather than repeating what is
2002 written XREFERENCE(,
2003 there), here are the most important points.
2005 - Burn the GNU coding standards.
2006 - Never use spaces for indentation.
2007 - Tabs are 8 characters, and thus indentations are also 8 characters.
2008 - Don't put multiple assignments on a single line.
2009 - Avoid tricky expressions.
2010 - Don't leave whitespace at the end of lines.
2011 - The limit on the length of lines is 80 columns.
2012 - Use K&R style for placing braces and spaces:
2014 if (x is true) {
2015 we do y
2016 }
2018 - Use a space after (most) keywords.
2019 - Do not add spaces around (inside) parenthesized expressions.
2020 - Use one space around (on each side of) most binary and ternary operators.
2021 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
2022 - Mixed-case names are frowned upon.
2023 - Descriptive names for global variables are a must.
2024 - Avoid typedefs.
2025 - Functions should be short and sweet, and do just one thing.
2026 - The number of local variables shouldn't exceed 10.
2027 - Gotos are fine if they improve readability and reduce nesting.
2028 - Don't use C99-style "// ..." comments.
2029 - Names of macros defining constants and labels in enums are capitalized.
2030 - Enums are preferred when defining several related constants.
2031 - Always use the paraslash wrappers for allocating memory.
2032 - If the name of a function is an action or an imperative.
2033 command, the function should return an error-code integer
2034 (<0 means error, >=0 means success). If the name is a
2035 predicate, the function should return a "succeeded" boolean.
2038 Doxygen
2039 ~~~~~~~
2041 Doxygen is a documentation system for various programming
2042 languages. The paraslash project uses Doxygen for generating the API
2043 reference on the web pages, but good source code documentation is
2044 also beneficial to people trying to understand the code structure
2045 and the interactions between the various source files.
2047 It is more illustrative to look at the source code for examples than
2048 to describe the conventions for documenting the source in this manual,
2049 so we only describe which parts of the code need doxygen comments,
2050 but leave out details on documentation conventions.
2052 As a rule, only the public part of the C source is documented with
2053 Doxygen. This includes structures, defines and enumerations in header
2054 files as well as public (non-static) C functions. These should be
2055 documented completely. For example each parameter and the return
2056 value of a public function should get a descriptive comment.
2058 No doxygen comments are necessary for static functions and for
2059 structures and enumerations in C files (which are used only within
2060 this file). This does not mean, however, that those entities need
2061 no documentation at all. Instead, common sense should be applied to
2062 document what is not obvious from reading the code.
2064 --------
2065 Appendix
2066 --------
2068 Network protocols
2069 ~~~~~~~~~~~~~~~~~
2071 *IP*. The _Internet Protocol_ is the primary networking protocol
2072 used for the Internet. All protocols described below use IP as the
2073 underlying layer. Both the prevalent IPv4 and the next-generation
2074 IPv6 variant are being deployed actively worldwide.
2076 *Connection-oriented and connectionless protocols*. Connectionless
2077 protocols differ from connection-oriented ones in that state
2078 associated with the sending/receiving endpoints is treated
2079 implicitly. Connectionless protocols maintain no internal knowledge
2080 about the state of the connection. Hence they are not capable of
2081 reacting to state changes, such as sudden loss or congestion on the
2082 connection medium. Connection-oriented protocols, in contrast, make
2083 this knowledge explicit. The connection is established only after
2084 a bidirectional handshake which requires both endpoints to agree
2085 on the state of the connection, and may also involve negotiating
2086 specific parameters for the particular connection. Maintaining an
2087 up-to-date internal state of the connection also in general means
2088 that the sending endpoints perform congestion control, adapting to
2089 qualitative changes of the connection medium.
2091 *Reliability*. In IP networking, packets can be lost, duplicated,
2092 or delivered out of order, and different network protocols handle
2093 these problems in different ways. We call a transport-layer protocol
2094 _reliable_, if it turns the unreliable IP delivery into an ordered,
2095 duplicate- and loss-free delivery of packets. Sequence numbers
2096 are used to discard duplicates and re-arrange packets delivered
2097 out-of-order. Retransmission is used to guarantee loss-free
2098 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2099 or data integrity.
2101 *Classification*. With these definitions the protocols which are used
2102 by paraslash for steaming audio data may be classified as follows.
2104 - HTTP/TCP: connection-oriented, reliable,
2105 - UDP: connectionless, unreliable,
2106 - DCCP: connection-oriented, unreliable.
2108 Below we give a short descriptions of these protocols.
2110 *TCP*. The _Transmission Control Protocol_ provides reliable,
2111 ordered delivery of a stream and a classic window-based congestion
2112 control. In contrast to UDP and DCCP (see below), TCP does not have
2113 record-oriented or datagram-based syntax, i.e. it provides a stream
2114 which is unaware and independent of any record (packet) boundaries.
2115 TCP is used extensively by many application layers. Besides HTTP (the
2116 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2117 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2118 top of TCP.
2120 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2121 protocol, built as a thin layer directly on top of IP. For this reason,
2122 it offers the same best-effort service as IP itself, i.e. there is no
2123 detection of duplicate or reordered packets. Being a connectionless
2124 protocol, only minimal internal state about the connection is
2125 maintained, which means that there is no protection against packet
2126 loss or network congestion. Error checking and correction (if at all)
2127 are performed in the application.
2129 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2130 connection-oriented state maintenance known from TCP with the
2131 unreliable, datagram-based transport of UDP. This means that it
2132 is capable of reacting to changes in the connection by performing
2133 congestion control, offering multiple alternative approaches. But it
2134 is bound to datagram boundaries (the maximum packet size supported
2135 by a medium), and like UDP it lacks retransmission to protect
2136 against loss. Due to the use of sequence numbers, it is however
2137 able to react to loss (interpreted as a congestion indication) and
2138 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2139 to negotiate specific, binding features for a connection, such as
2140 the choice of congestion control: classic, window-based congestion
2141 control known from TCP is available as CCID-2, rate-based, "smooth"
2142 congestion control is offered as CCID-3.
2144 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2145 protocol on top of TCP. It is spoken by web servers and is most often
2146 used for web services. However, as can be seen by the many Internet
2147 radio stations and YouTube/Flash videos, http is by far not limited to
2148 the delivery of web pages only. Being a simple request/response based
2149 protocol, the semantics of the protocol also allow the delivery of
2150 multimedia content, such as audio over http.
2152 *Multicast*. IP multicast is not really a protocol but a technique
2153 for one-to-many communication over an IP network. The challenge is to
2154 deliver information to a group of destinations simultaneously using
2155 the most efficient strategy to send the messages over each link of
2156 the network only once. This has benefits for streaming multimedia:
2157 the standard one-to-one unicast offered by TCP/DCCP means that
2158 n clients listening to the same stream also consume n-times the
2159 resources, whereas multicast requires to send the stream just once,
2160 irrespective of the number of receivers. Since it would be costly to
2161 maintain state for each listening receiver, multicast often implies
2162 connectionless transport, which is the reason that it is currently
2163 only available via UDP.
2165 License
2166 ~~~~~~~
2168 Paraslash is licensed under the GPL, version 2. Most of the code
2169 base has been written from scratch, and those parts are GPL V2
2170 throughout. Notable exceptions are FEC and the WMA decoder. See the
2171 corresponding source files for licencing details for these parts. Some
2172 code sniplets of several other third party software packages have
2173 been incorporated into the paraslash sources, for example log message
2174 coloring was taken from the git sources. These third party software
2175 packages are all published under the GPL or some other license
2176 compatible to the GPL.
2178 Acknowledgements
2179 ~~~~~~~~~~~~~~~~
2181 Many thanks to Gerrit Renker who read an early draft of this manual
2182 and contributed significant improvements.
2184 ----------
2185 References
2186 ----------
2188 Articles
2189 ~~~~~~~~
2190 - Reed, Irving S.; Solomon, Gustave (1960),
2192 Polynomial Codes over Certain Finite Fields), Journal of the
2193 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2194 300-304, doi:10.1137/0108018)
2196 RFCs
2197 ~~~~
2199 - XREFERENCE(, RFC 768) (1980):
2200 User Datagram Protocol
2201 - XREFERENCE(, RFC 791) (1981):
2202 Internet Protocol
2203 - XREFERENCE(, RFC 2437) (1998):
2204 RSA Cryptography Specifications
2205 - XREFERENCE(, RFC 4340)
2206 (2006): Datagram Congestion Control Protocol (DCCP)
2207 - XREFERENCE(, RFC 4341) (2006):
2208 Congestion Control ID 2: TCP-like Congestion Control
2209 - XREFERENCE(, RFC 4342) (2006):
2210 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2211 - XREFERENCE(, RFC 6716) (2012):
2212 Definition of the Opus Audio Codec
2214 Application web pages
2215 ~~~~~~~~~~~~~~~~~~~~~
2217 - XREFERENCE(, paraslash)
2218 - XREFERENCE(, xmms)
2219 - XREFERENCE(, mpg123)
2220 - XREFERENCE(, gstreamer)
2221 - XREFERENCE(, icecast)
2222 - XREFERENCE(, Audio Compress)
2224 External documentation
2225 ~~~~~~~~~~~~~~~~~~~~~~
2228 H. Peter Anvin: The mathematics of Raid6)
2230 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2231 Communication Protocols)
2233 Code
2234 ~~~~
2236 Original FEC implementation by Luigi Rizzo)