Merge branch 't/crypto'
[paraslash.git] / web / manual.m4
1 dnl To generate the html version, execute
2 dnl m4 web/manual.m4 | grutatxt --toc
3
4 define(`LOCAL_LINK_NAME', `translit(`$1', `A-Z
5 ', `a-z__')')
6 define(`REMOVE_NEWLINE', `translit(`$1',`
7 ', ` ')')
8
9 define(`REFERENCE', ./``#''`LOCAL_LINK_NAME($1)' (`REMOVE_NEWLINE($2)'))
10 define(`XREFERENCE', `$1' (`REMOVE_NEWLINE($2)'))
11 define(`EMPH', ``_''`REMOVE_NEWLINE($1)'``_'')
12
13 Paraslash user manual
14 =====================
15
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.
23
24 ------------
25 Introduction
26 ------------
27
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.
32
33 Overview
34 ~~~~~~~~
35
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.
40
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.
46
47 Typically, these two daemons run on different hosts but a local setup
48 is also possible.
49
50 A simplified picture of a typical setup is as follows
51 <<
52 <pre>
53 server_host client_host
54 ~~~~~~~~~~~ ~~~~~~~~~~~
55
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 >>
74
75 The paraslash executables
76 ~~~~~~~~~~~~~~~~~~~~~~~~~
77
78 *para_server*
79
80 para_server streams binary audio data (MP3, OGG/Vorbis, OGG/Speex,
81 M4A, WMA files) over local and/or remote networks. It listens on a
82 TCP port and accepts commands such as play, stop, pause, next from
83 authenticated clients. There are many more commands though, see the
84 man page of para_server for a description of all commands.
85
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).
89
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.
94
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.
99
100 It is also possible to store images (album covers) and lyrics in the
101 database and associate these to the corresponding audio files.
102
103 The section on the REFERENCE(The audio file selector, audio file
104 selector) discusses this topic.
105
106
107 *para_client*
108
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.
113
114 All connections between para_server and para_client are encrypted
115 with a symmetric RC4 session key. For each user of paraslash you must
116 create a public/secret RSA key pair for authentication.
117
118
119 *para_audiod*
120
121 The local daemon that collects information from para_server.
122
123 It runs on the client side and connects to para_server. As soon as
124 para_server announces the availability of an audio stream, para_audiod
125 starts an appropriate receiver, any number of filters and a paraslash
126 writer to play the stream.
127
128 Moreover, para_audiod listens on a local socket and sends status
129 information about para_server and para_audiod to local clients on
130 request. Access via this local socket may be restricted by using Unix
131 socket credentials, if available.
132
133
134 *para_audioc*
135
136 The client program which talks to para_audiod. Used to control
137 para_audiod, to receive status info, or to grab the stream at any
138 point of the decoding process.
139
140 *para_recv*
141
142 A command line HTTP/DCCP/UDP stream grabber. The http mode is
143 compatible with arbitrary HTTP streaming sources (e.g. icecast).
144
145 *para_filter*
146
147 A filter program that reads from STDIN and writes to STDOUT.
148 Like para_recv, this is an atomic building block which can be used to
149 assemble higher-level audio receiving facilities. It combines several
150 different functionalities in one tool: decoders for multiple audio
151 formats and a number of processing filters, among these a normalizer
152 for audio volume.
153
154 *para_afh*
155
156 A small stand-alone program that prints tech info about the given
157 audio file to STDOUT. It can be instructed to print a "chunk table",
158 an array of offsets within the audio file or to write the content of
159 the audio file in complete chunks 'just in time'.
160
161 This allows third-party streaming software that is unaware of the
162 particular audio format to send complete frames in real time.
163
164 *para_write*
165
166 A modular audio stream writer. It supports a simple file writer
167 output plug-in and optional WAV/raw players for ALSA (Linux) and for
168 coreaudio (Mac OS). para_write can also be used as a stand-alone WAV
169 or raw audio player.
170
171
172 *para_gui*
173
174 Curses-based gui that presents status information obtained in a curses
175 window. Appearance can be customized via themes. para_gui provides
176 key-bindings for the most common server commands and new key-bindings
177 can be added easily.
178
179
180 *para_fade*
181
182 An (OSS-only) alarm clock and volume-fader.
183
184 -----------
185 Quick start
186 -----------
187
188 This chapter lists the REFERENCE(Requirements, necessary software)
189 that must be installed to compile the paraslash package, describes
190 how to REFERENCE(Installation, compile and install) the paraslash
191 source code and the steps that have to be performed in order to
192 REFERENCE(Quick start, set up) a typical server and client.
193
194 Requirements
195 ~~~~~~~~~~~~
196
197 In any case you'll need
198
199 - XREFERENCE(http://systemlinux.org/~maan/osl/, libosl).
200 The _object storage layer_ library is used by para_server. To
201 clone the source code repository, execute
202
203 git clone git://git.tuebingen.mpg.de/osl
204
205 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/gcc, gcc) or
206 XREFERENCE(http://clang.llvm.org, clang). All gcc versions
207 >= 3.3 are currently supported. Clang version 1.1 or newer
208 should work as well.
209
210 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/make, gnu make) is
211 also shipped with the disto. On BSD systems the gnu make
212 executable is often called gmake.
213
214 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/bash, bash). Some
215 scripts which run during compilation require the EMPH(Bourne
216 again shell). It is most likely already installed.
217
218 - XREFERENCE(http://www.openssl.org/, openssl) or
219 XREFERENCE(ftp://ftp.gnupg.org/gcrypt/libgcrypt/, libgcrypt).
220 At least one of these two libraries is needed as the backend
221 for cryptographic routines on both the server and the client
222 side. Both openssl and libgcrypt are usually shipped with the
223 distro, but you might have to install the development package
224 (libssl-dev or libgcrypt-dev on debian systems) as well.
225
226 - XREFERENCE(ftp://ftp.gnu.org/pub/gnu/help2man, help2man)
227 is used to create the man pages.
228
229 Optional:
230
231 - XREFERENCE(http://www.underbit.com/products/mad/, libmad).
232 To compile in MP3 support for paraslash, the development
233 package must be installed. It is called libmad0-dev on
234 debian-based systems. Note that libmad is not necessary on
235 the server side, i.e. for sending MP3 files.
236
237 - XREFERENCE(http://www.underbit.com/products/mad/,
238 libid3tag). For version-2 ID3 tag support, you'll need
239 the libid3tag development package libid3tag0-dev. Without
240 libid3tag, only version one tags are recognized.
241
242 - XREFERENCE(http://www.xiph.org/downloads/, ogg vorbis).
243 For ogg vorbis streams you'll need libogg, libvorbis,
244 libvorbisfile. The corresponding Debian packages are called
245 libogg-dev and libvorbis-dev.
246
247 - XREFERENCE(http://www.audiocoding.com/, libfaad). For aac
248 files (m4a) you'll need libfaad (libfaad-dev).
249
250 - XREFERENCE(http://www.speex.org/, speex). In order to stream
251 or decode speex files, libspeex (libspeex-dev) is required.
252
253 - XREFERENCE(ftp://ftp.alsa-project.org/pub/lib/, alsa-lib). On
254 Linux, you'll need to have ALSA's development package
255 libasound2-dev installed.
256
257 - XREFERENCE(http://downloads.xiph.org/releases/ao/,
258 libao). Needed to build the ao writer (ESD, PulseAudio,...).
259 Debian package: libao-dev.
260
261 Installation
262 ~~~~~~~~~~~~
263
264 First make sure all non-optional packages listed in the section on
265 REFERENCE(Requirements, required software) are installed on your
266 system.
267
268 You don't need everything listed there. In particular, MP3, OGG/Vorbis,
269 OGG/Speex and AAC support are all optional. The configure script will
270 detect what is installed on your system and will only try to build
271 those executables that can be built with your setup.
272
273 Note that no special decoder library (not even the MP3 decoding library
274 libmad) is needed for para_server if you only want to stream MP3 or WMA
275 files. Also, it's fine to use para_server on a box without sound card.
276
277 Next, install the paraslash package on all machines, you'd like this
278 software to run on:
279
280 (./configure && make) > /dev/null
281
282 There should be no errors but probably some warnings about missing
283 packages which usually implies that not all audio formats will be
284 supported. If headers or libs are installed at unusual locations you
285 might need to tell the configure script where to find them. Try
286
287 ./configure --help
288
289 to see a list of options. If the paraslash package was compiled
290 successfully, execute (optionally)
291
292 make test
293
294 to run the paraslash test suite. If all tests pass, execute as root
295
296 make install
297
298 to install executables under /usr/local/bin and the man pages under
299 /usr/local/man.
300
301 Configuration
302 ~~~~~~~~~~~~~
303
304 *Step 1*: Create a paraslash user
305
306 In order to control para_server at runtime you must create a paraslash
307 user. As authentication is based on the RSA crypto system you'll have
308 to create an RSA key pair. If you already have a user and an RSA key
309 pair, you may skip this step.
310
311 In this section we'll assume a typical setup: You would like to run
312 para_server on some host called server_host as user foo, and you want
313 to connect to para_server from another machine called client_host as
314 user bar.
315
316 As foo@server_host, create ~/.paraslash/server.users by typing the
317 following commands:
318
319 user=bar
320 target=~/.paraslash/server.users
321 key=~/.paraslash/id_rsa.pub.$user
322 perms=AFS_READ,AFS_WRITE,VSS_READ,VSS_WRITE
323 mkdir -p ~/.paraslash
324 echo "user $user $key $perms" >> $target
325
326 Next, change to the "bar" account on client_host and generate the
327 key pair with the commands
328
329 ssh-keygen -t rsa -b 2048
330 # hit enter twice to create a key with no passphrase
331
332 This generates the two files id_rsa and id_rsa.pub in ~/.ssh. Note
333 that paraslash can also read keys generated by the "openssl genrsa"
334 command. However, since keys created with ssh-keygen can also be used
335 for ssh, this method is recommended.
336
337 Note that para_server refuses to use a key if it is shorter than 2048
338 bits. In particular, the RSA keys of paraslash 0.3.x will not work
339 with version 0.4.x. Moreover, para_client refuses to use a (private)
340 key which is world-readable.
341
342 para_server only needs to know the public key of the key pair just
343 created. Copy this public key to server_host:
344
345 src=~/.ssh/id_rsa.pub
346 dest=.paraslash/id_rsa.pub.$LOGNAME
347 scp $src foo@server_host:$dest
348
349 Finally, tell para_client to connect to server_host:
350
351 conf=~/.paraslash/client.conf
352 echo 'hostname server_host' > $conf
353
354
355 *Step 2*: Start para_server
356
357 Before starting the server make sure you have write permissions to
358 the directory /var/paraslash that has been created during installation:
359
360 sudo chown $LOGNAME /var/paraslash
361
362 Alternatively, use the --afs_socket Option to specify a different
363 location for the AFS command socket.
364
365 For this first try, we'll use the info loglevel to make the output
366 of para_server more verbose.
367
368 para_server -l info
369
370 Now you can use para_client to connect to the server and issue
371 commands. Open a new shell as bar@client_host and try
372
373 para_client help
374 para_client si
375
376 to retrieve the list of available commands and some server info.
377 Don't proceed if this doesn't work.
378
379 *Step 3*: Create and populate the database
380
381 An empty database is created with
382
383 para_client init
384
385 This initializes a couple of empty tables under
386 ~/.paraslash/afs_database-0.4. You normally don't need to look at these
387 tables, but it's good to know that you can start from scratch with
388
389 rm -rf ~/.paraslash/afs_database-0.4
390
391 in case something went wrong.
392
393 Next, you need to add some audio files to that database so that
394 para_server knows about them. Choose an absolute path to a directory
395 containing some audio files and add them to the audio file table:
396
397 para_client add /my/mp3/dir
398
399 This might take a while, so it is a good idea to start with a directory
400 containing not too many files. Note that the table only contains data
401 about the audio files found, not the files themselves.
402
403 You may print the list of all known audio files with
404
405 para_client ls
406
407 *Step 4*: Configure para_audiod
408
409 para_audiod needs to create a "well-known" socket for the clients to
410 connect to. The default path for this socket is
411
412 /var/paraslash/audiod_socket.$HOSTNAME
413
414 In order to make this directory writable for para_audiod, execute
415 as bar@client_host
416
417 sudo chown $LOGNAME /var/paraslash
418
419
420 We will also have to tell para_audiod that it should receive the
421 audio stream from server_host via http:
422
423 para_audiod -l info -r '.:http -i server_host'
424
425 You should now be able to listen to the audio stream once para_server
426 starts streaming. To activate streaming, execute
427
428 para_client play
429
430 Since no playlist has been specified yet, the "dummy" mode which
431 selects all known audio files is activated automatically. See the
432 section on the REFERENCE(The audio file selector, audio file selector)
433 for how to use playlists and moods to specify which files should be
434 streamed in which order.
435
436 *Troubleshooting*
437
438 It did not work? To find out why, try to receive, decode and play the
439 stream manually using para_recv, para_filter and para_write as follows.
440
441 For simplicity we assume that you're running Linux/ALSA and that only
442 MP3 files have been added to the database.
443
444 para_recv -r 'http -i server_host' > file.mp3
445 # (interrupt with CTRL+C after a few seconds)
446 ls -l file.mp3 # should not be empty
447 para_filter -f mp3dec -f wav < file.mp3 > file.wav
448 ls -l file.wav # should be much bigger than file.mp3
449 para_write -w alsa < file.wav
450
451 Double check what is logged by para_server and use the --loglevel
452 option of para_recv, para_filter and para_write to increase verbosity.
453
454 ---------------
455 User management
456 ---------------
457
458 para_server uses a challenge-response mechanism to authenticate
459 requests from incoming connections, similar to ssh's public key
460 authentication method. Authenticated connections are encrypted using
461 the RC4 stream cipher.
462
463 In this chapter we briefly describe RSA and RC4 and sketch the
464 REFERENCE(Client-server authentication, authentication handshake)
465 between para_client and para_server. User management is discussed
466 in the section on REFERENCE(The user_list file, the user_list file).
467 These sections are all about communication between the client and the
468 server. Connecting para_audiod is a different matter and is described
469 in a REFERENCE(Connecting para_audiod, separate section).
470
471
472
473 RSA and RC4
474 ~~~~~~~~~~~
475
476 RSA is an asymmetric block cipher which is used in many applications,
477 including ssh and gpg. An RSA key consists in fact of two keys,
478 called the public key and the private key. A message can be encrypted
479 with either key and only the counterpart of that key can decrypt
480 the message. While RSA can be used for both signing and encrypting
481 a message, paraslash only uses RSA only for the latter purpose. The
482 RSA public key encryption and signatures algorithms are defined in
483 detail in RFC 2437.
484
485 RC4 is a stream cipher, i.e. the input is XORed with a pseudo-random
486 key stream to produce the output. Decryption uses the same function
487 calls as encryption. While RC4 supports variable key lengths,
488 paraslash uses a fixed length of 256 bits, which is considered a
489 strong encryption by today's standards. Since the same key must never
490 be used twice, a different, randomly-generated key is used for every
491 new connection.
492
493 Client-server authentication
494 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
495
496 The authentication handshake between para_client and para_server goes
497 as follows:
498
499 - para_client connects to para_server and sends an
500 authentication request for a user. It does so by connecting
501 to para_server, TCP 2990, the control port of para_server.
502
503 - para_server accepts the connection and forks a child process
504 which is supposed to handle the connection. The parent process
505 keeps listening on the control port while the child process
506 (also called para_server below) continues as follows.
507
508 - para_server loads the RSA public key of that user, fills a
509 fixed-length buffer with random bytes, encrypts that buffer
510 using the public key and sends the encrypted buffer to the
511 client. The first part of the buffer is the challenge which
512 is used for authentication while the second part is the RC4
513 session key.
514
515 - para_client receives the encrypted buffer and decrypts it
516 using the user's private key, thereby obtaining the challenge
517 buffer and the session key. It sends the SHA1 hash value of
518 the challenge back to para_server and stores the session key
519 for further use.
520
521 - para_server also computes the SHA1 hash of the challenge
522 and compares it against what was sent back by the client.
523
524 - If the two hashes do not match, the authentication has
525 failed and para_server closes the connection.
526
527 - Otherwise the user is considered authenticated and the client
528 is allowed to proceed by sending a command to be executed. From
529 this point on the communication is encrypted using the RC4
530 stream cipher with the session key known to both peers.
531
532 paraslash relies on the quality of the pseudo-random bytes provided
533 by the crypto library (openssl or libgcrypt), on the security of
534 the implementation of the RSA and RC4 crypto routines and on the
535 infeasibility to invert the SHA1 function.
536
537 Neither para_server or para_client create RSA keys on their own. This
538 has to be done once for each user as sketched in REFERENCE(Quick start,
539 Quick start) and discussed in more detail REFERENCE(The user_list
540 file, below).
541
542 The user_list file
543 ~~~~~~~~~~~~~~~~~~
544
545 At startup para_server reads the user list file which must contain
546 one line per user. The default location of the user list file may be
547 changed with the --user_list option.
548
549 There should be at least one user in this file. Each user must have
550 an RSA key pair. The public part of the key is needed by para_server
551 while the private key is needed by para_client. Each line of the
552 user list file must be of the form
553
554 user <username> <key> <perms>
555
556 where _username_ is an arbitrary string (usually the user's login
557 name), _key_ is the full path to that user's public RSA key, and
558 _perms_ is a comma-separated list of zero or more of the following
559 permission bits:
560
561 +---------------------------------------------------------+
562 | AFS_READ | read the contents of the databases |
563 +-----------+---------------------------------------------+
564 | AFS_WRITE | change database contents |
565 +-----------+---------------------------------------------+
566 | VSS_READ | obtain information about the current stream |
567 +-----------+---------------------------------------------+
568 | VSS_WRITE | change the current stream |
569 +---------------------------------------------------------+
570
571 The permission bits specify which commands the user is allowed to
572 execute. The output of
573
574 para_client help
575
576 contains in the third column the permissions needed to execute the
577 command.
578
579 It is possible to make para_server reread the user_list file by
580 executing the paraslash "hup" command or by sending SIGHUP to the
581 PID of para_server.
582
583
584 Connecting para_audiod
585 ~~~~~~~~~~~~~~~~~~~~~~
586
587 para_audiod listens on a Unix domain socket. Those sockets are
588 for local communication only, so only local users can connect to
589 para_audiod. The default is to let any user connect but this can be
590 restricted on platforms that support UNIX socket credentials which
591 allow para_audiod to obtain the Unix credentials of the connecting
592 process.
593
594 Use para_audiod's --user_allow option to allow connections only for
595 a limited set of users.
596
597 -----------------------
598 The audio file selector
599 -----------------------
600
601 paraslash comes with a sophisticated audio file selector (AFS),
602 whose main task is to determine which file to stream next, based on
603 information on the audio files stored in a database. It communicates
604 also with para_client whenever an AFS command is executed, for example
605 to answer a database query.
606
607 Besides the traditional playlists, AFS supports audio file selection
608 based on _moods_ which act as a filter that limits the set of all
609 known audio files to those which satisfy certain criteria. It also
610 maintains tables containing images (e.g. album cover art) and lyrics
611 that can be associated with one or more audio files.
612
613 AFS uses libosl, the object storage layer, as the backend library
614 for storing information on audio files, playlists, etc. This library
615 offers functionality similar to a relational database, but is much
616 more lightweight than a full database backend.
617
618 In this chapter we sketch the setup of the REFERENCE(The AFS process,
619 AFS process) during server startup and proceed with the description
620 of the REFERENCE(Database layout, layout) of the various database
621 tables. The section on REFERENCE(Playlists and moods, playlists
622 and moods) explains these two audio file selection mechanisms
623 in detail and contains pratical examples. The way REFERENCE(File
624 renames and content changes, file renames and content changes) are
625 detected is discussed briefly before the REFERENCE(Troubleshooting,
626 Troubleshooting) section which concludes the chapter.
627
628 The AFS process
629 ~~~~~~~~~~~~~~~
630
631 On startup, para_server forks to create the AFS process which opens
632 the OSL database tables. The server process communicates with the
633 AFS process via pipes and shared memory. Usually, the AFS process
634 awakes only briefly whenever the current audio file changes. The AFS
635 process determines the next audio file, opens it, verifies it has
636 not been changed since it was added to the database and passes the
637 open file descriptor to the server process, along with audio file
638 meta-data such as file name, duration, audio format and so on. The
639 server process then starts to stream the audio file.
640
641 The AFS process also accepts connections from local clients via
642 a well-known socket. However, only child processes of para_server
643 may connect through this socket. All server commands that have the
644 AFS_READ or AFS_WRITE permission bits use this mechanism to query or
645 change the database.
646
647 Database layout
648 ~~~~~~~~~~~~~~~
649
650 *The audio file table*
651
652 This is the most important and usually also the largest table of the
653 AFS database. It contains the information needed to stream each audio
654 file. In particular the following data is stored for each audio file.
655
656 - SHA1 hash value of the audio file contents. This is computed
657 once when the file is added to the database. Whenever AFS
658 selects this audio file for streaming the hash value is
659 recomputed and checked against the value stored in the
660 database to detect content changes.
661
662 - The time when this audio file was last played.
663
664 - The number of times the file has been played so far.
665
666 - The attribute bitmask.
667
668 - The image id which describes the image associated with this
669 audio file.
670
671 - The lyrics id which describes the lyrics associated with
672 this audio file.
673
674 - The audio format id (MP3, OGG, ...).
675
676 - An amplification value that can be used by the amplification
677 filter to pre-amplify the decoded audio stream.
678
679 - The chunk table. It describes the location and the timing
680 of the building blocks of the audio file. This is used by
681 para_server to send chunks of the file at appropriate times.
682
683 - The duration of the audio file.
684
685 - Tag information contained in the audio file (ID3 tags,
686 Vorbis comments, ...).
687
688 - The number of channels
689
690 - The encoding bitrate.
691
692 - The sampling frequency.
693
694 To add or refresh the data contained in the audio file table, the _add_
695 command is used. It takes the full path of either an audio file or a
696 directory. In the latter case, the directory is traversed recursively
697 and all files which are recognized as valid audio files are added to
698 the database.
699
700 *The attribute table*
701
702 The attribute table contains two columns, _name_ and _bitnum_. An
703 attribute is simply a name for a certain bit number in the attribute
704 bitmask of the audio file table.
705
706 Each of the 64 bits of the attribute bitmask can be set for each
707 audio file individually. Hence up to 64 different attributes may be
708 defined. For example, "pop", "rock", "blues", "jazz", "instrumental",
709 "german_lyrics", "speech", whatever. You are free to choose as
710 many attributes as you like and there are no naming restrictions
711 for attributes.
712
713 A new attribute "test" is created by
714
715 para_client addatt test
716 and
717 para_client lsatt
718
719 lists all available attributes. You can set the "test" attribute for
720 an audio file by executing
721
722 para_client setatt test+ /path/to/the/audio/file
723
724 Similarly, the "test" bit can be removed from an audio file with
725
726 para_client setatt test- /path/to/the/audio/file
727
728 Instead of a path you may use a shell wildcard pattern. The attribute
729 is applied to all audio files matching that pattern:
730
731 para_client setatt test+ '/test/directory/*'
732
733 The command
734
735 para_client -- ls -lv
736
737 gives you a verbose listing of your audio files also showing which
738 attributes are set.
739
740 In case you wonder why the double-dash in the above command is needed:
741 It tells para_client to not interpret the options after the dashes. If
742 you find this annoying, just say
743
744 alias para='para_client --'
745
746 and be happy. In what follows we shall use this alias.
747
748 The "test" attribute can be dropped from the database with
749
750 para rmatt test
751
752 Read the output of
753
754 para help ls
755 para help setatt
756
757 for more information and a complete list of command line options to
758 these commands.
759
760 *Blob tables*
761
762 The image, lyrics, moods and playlists tables are all blob tables.
763 Blob tables consist of three columns each: The identifier which is
764 a positive non-negative number that is auto-incremented, the name
765 (an arbitrary string) and the content (the blob).
766
767 All blob tables support the same set of actions: cat, ls, mv, rm
768 and add. Of course, _add_ is used for adding new blobs to the table
769 while the other actions have the same meaning as the corresponding
770 Unix commands. The paraslash commands to perform these actions are
771 constructed as the concatenation of the table name and the action. For
772 example addimg, catimg, lsimg, mvimg, rmimg are the commands that
773 manipulate or query the image table.
774
775 The add variant of these commands is special as these commands read
776 the blob contents from stdin. To add an image to the image table the
777 command
778
779 para addimg image_name < file.jpg
780
781 can be used.
782
783 Note that the images and lyrics are not interpreted at all, and also
784 the playlist and the mood blobs are only investigated when the mood
785 or playlist is activated by using the select command.
786
787 *The score table*
788
789 Unlike all other tables the contents of the score table remain in
790 memory and are never stored on disk. The score table contains two
791 columns: The SHA1 hash value (of an audio file) and its current
792 score.
793
794 However, only those files which are admissible for the current mood
795 or playlist are contained in the score table. The audio file selector
796 always chooses the row with the highest score as the file to stream
797 next. While doing so, it computes the new score and updates the
798 last_played and the num_played fields in the audio file table.
799
800 The score table is recomputed by the select command which loads a
801 new mood or playlist.
802
803 Playlists and moods
804 ~~~~~~~~~~~~~~~~~~~
805
806 Playlists and moods offer two different ways of specifying the set of
807 admissible files. A playlist in itself describes a set of admissible
808 files. A mood, in contrast, describes the set of admissible files in
809 terms of attributes and other type of information available in the
810 audio file table. As an example, a mood can define a filename pattern,
811 which is then matched against the names of audio files in the table.
812
813 Selecting a mood or playlist means the generation of a ranking
814 (a score table) for the set of admissible files. Audio files are
815 then selected on a highest-score-first basis. The score table is
816 recomputed at the moment the mood or playlist is selected.
817
818 *Playlists*
819
820 Playlists are accommodated in the playlist table of the afs database,
821 using the aforementioned blob format for tables. A new filelist is
822 created using the addpl command, by specifying the full (absolute)
823 paths of all desired audio files, separated by newlines. For example
824
825 find /my/mp3/dir -name "*.mp3" | para addpl my_playlist
826
827 If _my_playlist_ already exists it is overwritten. To activate the
828 new playlist, execute
829
830 para select p/my_playlist
831
832 The audio file selector will assign scores to each entry of the list,
833 in descending order so that files will be selected in order. If a
834 file could not be opened for streaming, its entry is removed from
835 the score table (but not from the playlist).
836
837 *Moods*
838
839 A mood consists of a unique name and its *mood definition*, which is
840 a set of *mood lines* containing expressions in terms of attributes
841 and other data contained in the database.
842
843 At any time, at most one mood can be *active* which means that
844 para_server is going to select only files from that subset of
845 admissible files.
846
847 So in order to create a mood definition one has to write a set of
848 mood lines. Mood lines come in three flavours: Accept lines, deny
849 lines and score lines.
850
851 The general syntax of the three types of mood lines is
852
853
854 accept [with score <score>] [if] [not] <mood_method> [options]
855 deny [with score <score>] [if] [not] <mood_method> [options]
856 score <score> [if] [not] <mood_method> [options]
857
858
859 Here <score> is either an integer or the string "random" which assigns
860 a random score to all matching files. The score value changes the
861 order in which admissible files are going to be selected, but is of
862 minor importance for this introduction.
863
864 So we concentrate on the first two forms, i.e. accept and deny
865 lines. As usual, everything in square brackets is optional, i.e.
866 accept/deny lines take the following form when ignoring scores:
867
868 accept [if] [not] <mood_method> [options]
869
870 and analogously for the deny case. The "if" keyword is only syntactic
871 sugar and has no function. The "not" keyword just inverts the result,
872 so the essence of a mood line is the mood method part and the options
873 following thereafter.
874
875 A *mood method* is realized as a function which takes an audio file
876 and computes a number from the data contained in the database.
877 If this number is non-negative, we say the file *matches* the mood
878 method. The file matches the full mood line if it either
879
880 - matches the mood method and the "not" keyword is not given,
881 or
882 - does not match the mood method, but the "not" keyword is given.
883
884 The set of admissible files for the whole mood is now defined as those
885 files which match at least one accept mood line, but no deny mood line.
886 More formally, an audio file F is admissible if and only if
887
888 (F ~ AL1 or F ~ AL2...) and not (F ~ DL1 or F ~ DN2 ...)
889
890 where AL1, AL2... are the accept lines, DL1, DL2... are the deny
891 lines and "~" means "matches".
892
893 The cases where no mood lines of accept/deny type are defined need
894 special treatment:
895
896 - Neither accept nor deny lines: This treats all files as
897 admissible (in fact, that is the definition of the dummy mood
898 which is activated automatically if no moods are available).
899
900 - Only accept lines: A file is admissible iff it matches at
901 least one accept line:
902
903 F ~ AL1 or F ~ AL2 or ...
904
905 - Only deny lines: A file is admissible iff it matches no
906 deny line:
907
908 not (F ~ DL1 or F ~ DN2 ...)
909
910
911
912 *List of mood_methods*
913
914 no_attributes_set
915
916 Takes no arguments and matches an audio file if and only if no
917 attributes are set.
918
919 is_set <attribute_name>
920
921 Takes the name of an attribute and matches iff that attribute is set.
922
923 path_matches <pattern>
924
925 Takes a filename pattern and matches iff the path of the audio file
926 matches the pattern.
927
928 artist_matches <pattern>
929 album_matches <pattern>
930 title_matches <pattern>
931 comment_matches <pattern>
932
933 Takes an extended regular expression and matches iff the text of the
934 corresponding tag of the audio file matches the pattern. If the tag
935 is not set, the empty string is matched against the pattern.
936
937 year ~ <num>
938 bitrate ~ <num>
939 frequency ~ <num>
940 channels ~ <num>
941 num_played ~ <num>
942
943 Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
944 <num>. Matches an audio file iff the condition <val> ~ <num> is
945 satisfied where val is the corresponding value of the audio file
946 (value of the year tag, bitrate in kbit/s, frequency in Hz, channel
947 count, play count).
948
949 The year tag is special as its value is undefined if the audio file
950 has no year tag or the content of the year tag is not a number. Such
951 audio files never match. Another difference is the special treatment
952 if the year tag is a two-digit number. In this case either 1900 or
953 2000 are added to the tag value depending on whether the number is
954 greater than 2000 plus the current year.
955
956
957 *Mood usage*
958
959 To create a new mood called "my_mood", write its definition into
960 some temporary file, say "tmpfile", and add it to the mood table
961 by executing
962
963 para addmood my_mood < tmpfile
964
965 If the mood definition is really short, you may just pipe it to the
966 client instead of using temporary files. Like this:
967
968 echo "$MOOD_DEFINITION" | para addmood my_mood
969
970 There is no need to keep the temporary file since you can always use
971 the catmood command to get it back:
972
973 para catmood my_mood
974
975 A mood can be activated by executing
976
977 para select m/my_mood
978
979 Once active, the list of admissible files is shown by the ls command
980 if the "-a" switch is given:
981
982 para ls -a
983
984
985 *Example mood definition*
986
987 Suppose you have defined attributes "punk" and "rock" and want to define
988 a mood containing only Punk-Rock songs. That is, an audio file should be
989 admissible if and only if both attributes are set. Since
990
991 punk and rock
992
993 is obviously the same as
994
995 not (not punk or not rock)
996
997 (de Morgan's rule), a mood definition that selects only Punk-Rock
998 songs is
999
1000 deny if not is_set punk
1001 deny if not is_set rock
1002
1003
1004
1005 File renames and content changes
1006 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1007
1008 Since the audio file selector knows the SHA1 of each audio file that
1009 has been added to the afs database, it recognizes if the content of
1010 a file has changed, e.g. because an ID3 tag was added or modified.
1011 Also, if a file has been renamed or moved to a different location,
1012 afs will detect that an entry with the same hash value already exists
1013 in the audio file table.
1014
1015 In both cases it is enough to just re-add the new file. In the
1016 first case (file content changed), the audio table is updated, while
1017 metadata such as the num_played and last_played fields, as well as
1018 the attributes, remain unchanged. In the other case, when the file
1019 is moved or renamed, only the path information is updated, all other
1020 data remains as before.
1021
1022 It is possible to change the behaviour of the add command by using the
1023 "-l" (lazy add) or the "-f" (force add) option.
1024
1025 Troubleshooting
1026 ~~~~~~~~~~~~~~~
1027
1028 Use the debug loglevel (option -l debug for most commands) to show
1029 debugging info. Almost all paraslash executables have a brief online
1030 help which is displayed by using the -h switch. The --detailed-help
1031 option prints the full help text.
1032
1033 If para_server crashed or was killed by SIGKILL (signal 9), it
1034 may refuse to start again because of "dirty osl tables". In this
1035 case you'll have to run the oslfsck program of libosl to fix your
1036 database. It might be necessary to use --force (even if your name
1037 isn't Luke). However, make sure para_server isn't running before
1038 executing oslfsck --force.
1039
1040 If you don't mind to recreate your database you can start
1041 from scratch by removing the entire database directory, i.e.
1042
1043 rm -rf ~/.paraslash/afs_database-0.4
1044
1045 Be aware that this removes all attribute definitions, all playlists
1046 and all mood definitions and requires to re-initialize the tables.
1047
1048 Although oslfsck fixes inconsistencies in database tables it doesn't
1049 care about the table contents. To check for invalid table contents, use
1050
1051 para_client check
1052
1053 This prints out references to missing audio files as well as invalid
1054 playlists and mood definitions.
1055
1056 ---------------------------------------
1057 Audio formats and audio format handlers
1058 ---------------------------------------
1059
1060 Audio formats
1061 ~~~~~~~~~~~~~
1062
1063 The following audio formats are supported by paraslash:
1064
1065 *MP3*
1066
1067 Mp3, MPEG-1 Audio Layer 3, is a common audio format for audio storage,
1068 designed as part of its MPEG-1 standard. An MP3 file is made up of
1069 multiple MP3 frames, which consist of a header and a data block. The
1070 size of an MP3 frame depends on the bit rate and on the number
1071 of channels. For a typical CD-audio file (sample rate of 44.1 kHz
1072 stereo), encoded with a bit rate of 128 kbit, an MP3 frame is about
1073 400 bytes large.
1074
1075 *OGG/Vorbis*
1076
1077 OGG is a standardized audio container format, while Vorbis is an
1078 open source codec for lossy audio compression. Since Vorbis is most
1079 commonly made available via the OGG container format, it is often
1080 referred to as OGG/Vorbis. The OGG container format divides data into
1081 chunks called OGG pages. A typical OGG page is about 4KB large. The
1082 Vorbis codec creates variable-bitrate (VBR) data, where the bitrate
1083 may vary considerably.
1084
1085 *OGG/Speex*
1086
1087 Speex is an open-source speech codec that is based on CELP (Code
1088 Excited Linear Prediction) coding. It is designed for voice
1089 over IP applications, has modest complexity and a small memory
1090 footprint. Wideband and narrowband (telephone quality) speech are
1091 supported. As for Vorbis audio, Speex bit-streams are often stored
1092 in OGG files.
1093
1094 *AAC*
1095
1096 Advanced Audio Coding (AAC) is a standardized, lossy compression
1097 and encoding scheme for digital audio which is the default audio
1098 format for Apple's iPhone, iPod, iTunes. Usually MPEG-4 is used as
1099 the container format and audio files encoded with AAC have the .m4a
1100 extension. A typical AAC frame is about 700 bytes large.
1101
1102 *WMA*
1103
1104 Windows Media Audio (WMA) is an audio data compression technology
1105 developed by Microsoft. A WMA file is usually encapsulated in the
1106 Advanced Systems Format (ASF) container format, which also specifies
1107 how meta data about the file is to be encoded. The bit stream of WMA
1108 is composed of superframes, each containing one or more frames of
1109 2048 samples. For 16 bit stereo a WMA superframe is about 8K large.
1110
1111 Meta data
1112 ~~~~~~~~~
1113
1114 Unfortunately, each audio format has its own conventions how meta
1115 data is added as tags to the audio file.
1116
1117 For MP3 files, ID3, version 1 and 2 are widely used. ID3 version 1
1118 is rather simple but also very limited as it supports only artist,
1119 title, album, year and comment tags. Each of these can only be at most
1120 32 characters long. ID3, version 2 is much more flexible but requires
1121 a separate library being installed for paraslash to support it.
1122
1123 Ogg vorbis files contain meta data as Vorbis comments, which are
1124 typically implemented as strings of the form "[TAG]=[VALUE]". Unlike
1125 ID3 version 1 tags, one may use whichever tags are appropriate for
1126 the content.
1127
1128 AAC files usually use the MPEG-4 container format for storing meta
1129 data while WMA files wrap meta data as special objects within the
1130 ASF container format.
1131
1132 paraslash only tracks the most common tags that are supported by
1133 all tag variants: artist, title, year, album, comment. When a file
1134 is added to the AFS database, the meta data of the file is extracted
1135 and stored in the audio file table.
1136
1137 Chunks and chunk tables
1138 ~~~~~~~~~~~~~~~~~~~~~~~
1139
1140 paraslash uses the word "chunk" as common term for the building blocks
1141 of an audio file. For MP3 files, a chunk is the same as an MP3 frame,
1142 while for OGG files a chunk is an OGG page, etc. Therefore the chunk
1143 size varies considerably between audio formats, from a few hundred
1144 bytes (MP3) up to 8K (WMA).
1145
1146 The chunk table contains the offsets within the audio file that
1147 correspond to the chunk boundaries of the file. Like the meta data,
1148 the chunk table is computed and stored in the database whenever an
1149 audio file is added.
1150
1151 The paraslash senders (see below) always send complete chunks. The
1152 granularity for seeking is therefore determined by the chunk size.
1153
1154 Audio format handlers
1155 ~~~~~~~~~~~~~~~~~~~~~
1156
1157 For each audio format paraslash contains an audio format handler whose
1158 first task is to tell whether a given file is a valid audio file of
1159 this type. If so, the audio file handler extracts some technical data
1160 (duration, sampling rate, number of channels etc.), computes the
1161 chunk table and reads the meta data.
1162
1163 The audio format handler code is linked into para_server and executed
1164 via the _add_ command. The same code is also available as a stand-alone
1165 tool, para_afh, which can be used to print the technical data, the
1166 chunk table and the meta data of a file. Furthermore, one can use
1167 para_afh to cut an audio file, i.e. to select some of its chunks to
1168 produce a new file containing only these chunks.
1169
1170 ----------
1171 Networking
1172 ----------
1173
1174 Paraslash uses different network connections for control and data.
1175 para_client communicates with para_server over a dedicated TCP control
1176 connection. To transport audio data, separate data connections are
1177 used. For these data connections, a variety of transports (UDP, DCCP,
1178 HTTP) can be chosen.
1179
1180 The chapter starts with the REFERENCE(The paraslash control
1181 service, control service), followed by a section on the various
1182 REFERENCE(Streaming protocols, streaming protocols) in which the data
1183 connections are described. The way audio file headers are embedded into
1184 the stream is discussed REFERENCE(Streams with headers and headerless
1185 streams, briefly) before the REFERENCE(Networking examples, example
1186 section) which illustrates typical commands for real-life scenarios.
1187
1188 Both IPv4 and IPv6 are supported.
1189
1190 The paraslash control service
1191 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1192
1193 para_server is controlled at runtime via the paraslash control
1194 connection. This connection is used for server commands (play, stop,
1195 ...) as well as for afs commands (ls, select, ...).
1196
1197 The server listens on a TCP port and accepts connections from clients
1198 that connect the open port. Each connection causes the server to fork
1199 off a client process which inherits the connection and deals with that
1200 client only. In this classical accept/fork approach the server process
1201 is unaffected if the child dies or goes crazy for whatever reason. In
1202 fact, the child process can not change address space of server process.
1203
1204 The section on REFERENCE(Client-server authentication, client-server
1205 authentication) above described the early connection establishment
1206 from the crypto point of view. Here it is described what happens
1207 after the connection (including crypto setup) has been established.
1208 There are four processes involved during command dispatch as sketched
1209 in the following diagram.
1210
1211 <<
1212 <pre>
1213 server_host client_host
1214 ~~~~~~~~~~~ ~~~~~~~~~~~
1215
1216 +-----------+ connect +-----------+
1217 |para_server|<------------------------------ |para_client|
1218 +-----------+ +-----------+
1219 | ^
1220 | fork +---+ |
1221 +----------> |AFS| |
1222 | +---+ |
1223 | ^ |
1224 | | |
1225 | | connect (cookie) |
1226 | | |
1227 | | |
1228 | fork +-----+ inherited connection |
1229 +---------->|child|<--------------------------+
1230 +-----+
1231 </pre>
1232 >>
1233
1234 Note that the child process is not a child of the afs process,
1235 so communication of these two processes has to happen via local
1236 sockets. In order to avoid abuse of the local socket by unrelated
1237 processes, a magic cookie is created once at server startup time just
1238 before the server process forks off the AFS process. This cookie is
1239 known to the server, AFS and the child, but not to unrelated processes.
1240
1241 There are two different kinds of commands: First there are commands
1242 that cause the server to respond with some answer such as the list
1243 of all audio files. All but the addblob commands (addimg, addlyr,
1244 addpl, addmood) are of this kind. The addblob commands add contents
1245 to the database, so they need to transfer data the other way round,
1246 from the client to the server.
1247
1248 There is no knowledge about the server commands built into para_client,
1249 so it does not know about addblob commands. Instead, it inspects the
1250 first data package sent by the server for a magic string. If this
1251 string was found, it sends STDIN to the server, otherwise it dumps
1252 data from the server to STDOUT.
1253
1254 Streaming protocols
1255 ~~~~~~~~~~~~~~~~~~~
1256
1257 A network (audio) stream usually consists of one streaming source,
1258 the _sender_, and one or more _receivers_ which read data over the
1259 network from the streaming source.
1260
1261 Senders are thus part of para_server while receivers are part of
1262 para_audiod. Moreover, there is the stand-alone tool para_recv which
1263 can be used to manually download a stream, either from para_server
1264 or from a web-based audio streaming service.
1265
1266 The following three streaming protocols are supported by paraslash:
1267
1268 - HTTP. Recommended for public streams that can be played by
1269 any player like mpg123, xmms, itunes, winamp, etc. The HTTP
1270 sender is supported on all operating systems and all platforms.
1271
1272 - DCCP. Recommended for LAN streaming. DCCP is currently
1273 available only for Linux.
1274
1275 - UDP. Recommended for multicast LAN streaming.
1276
1277 See the Appendix on REFERENCE(Network protocols, network protocols)
1278 for brief descriptions of the various protocols relevant for network
1279 audio streaming with paraslash.
1280
1281 It is possible to activate more than one sender simultaneously.
1282 Senders can be controlled at run time and via config file and command
1283 line options.
1284
1285 Note that audio connections are _not_ encrypted. Transport or Internet
1286 layer encryption should be used if encrypted data connections are
1287 needed.
1288
1289 Since DCCP and TCP are both connection-oriented protocols, connection
1290 establishment/teardown and access control are very similar between
1291 these two streaming protocols. UDP is the most lightweight option,
1292 since in contrast to TCP/DCCP it is connectionless. It is also the
1293 only protocol supporting IP multicast.
1294
1295 The HTTP and the DCCP sender listen on a (TCP/DCCP) port waiting for
1296 clients to connect and establish a connection via some protocol-defined
1297 handshake mechanism. Both senders maintain two linked lists each:
1298 The list of all clients which are currently connected, and the list
1299 of access control entries which determines who is allowed to connect.
1300 IP-based access control may be configured through config file and
1301 command line options and via the "allow" and "deny" sender subcommands.
1302
1303 Upon receiving a GET request from the client, the HTTP sender sends
1304 back a status line and a message. The body of this message is the
1305 audio stream. This is common practice and is supported by many popular
1306 clients which can thus be used to play a stream offered by para_server.
1307 For DCCP things are a bit simpler: No messages are exchanged between
1308 the receiver and sender. The client simply connects and the sender
1309 starts to stream.
1310
1311 DCCP is an experimental protocol which offers a number of new features
1312 not available for TCP. Both ends can negotiate these features using
1313 a built-in negotiation mechanism. In contrast to TCP/HTTP, DCCP is
1314 datagram-based (no retransmissions) and thus should not be used over
1315 lossy media (e.g. WiFi networks). One useful feature offered by DCCP
1316 is access to a variety of different congestion-control mechanisms
1317 called CCIDs. Two different CCIDs are available per default on Linux:
1318
1319
1320 - _CCID 2_. A Congestion Control mechanism similar to that
1321 of TCP. The sender maintains a congestion window and halves
1322 this window in response to congestion.
1323
1324
1325 - _CCID-3_. Designed to be fair when competing for bandwidth.
1326 It has lower variation of throughput over time compared with
1327 TCP, which makes it suitable for streaming media.
1328
1329 Unlike the HTTP and DCCP senders, the UDP sender maintains only a
1330 single list, the _target list_. This list describes the set of clients
1331 to which the stream is sent. There is no list for access control and
1332 no "allow" and "deny" commands for the UDP sender. Instead, the "add"
1333 and "delete" commands can be used to modify the target list.
1334
1335 Since both UDP and DCCP offer an unreliable datagram-based transport,
1336 additional measures are necessary to guard against disruptions over
1337 networks that are lossy or which may be subject to interference (as
1338 is for instance the case with WiFi). Paraslash uses FEC (Forward
1339 Error Correction) to guard against packet losses and reordering. The
1340 stream is FEC-encoded before it is sent through the UDP socket and
1341 must be decoded accordingly on the receiver side.
1342
1343 The packet size and the amount of redundancy introduced by FEC can
1344 be configured via the FEC parameters which are dictated by server
1345 and may also be configured through the "sender" command. The FEC
1346 parameters are encoded in the header of each network packet, so no
1347 configuration is necessary on the receiver side. See the section on
1348 REFERENCE(Forward error correction, FEC) below.
1349
1350 Streams with headers and headerless streams
1351 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1352
1353 For OGG/Vorbis, OGG/Speex and wma streams, some of the information
1354 needed to decode the stream is only contained in the audio file
1355 header of the container format but not in each data chunk. Clients
1356 must be able to obtain this information in case streaming starts in
1357 the middle of the file or if para_audiod is started while para_server
1358 is already sending a stream.
1359
1360 This is accomplished in different ways, depending on the streaming
1361 protocol. For connection-oriented streams (HTTP, DCCP) the audio file
1362 header is sent prior to audio file data. This technique however does
1363 not work for the connectionless UDP transport. Hence the audio file
1364 header is periodically being embedded into the UDP audio data stream.
1365 By default, the header is resent after five seconds. The receiver has
1366 to wait until the next header arrives before it can start decoding
1367 the stream.
1368
1369 Examples
1370 ~~~~~~~~
1371
1372 The sender command of para_server allows to (de-)activate senders
1373 and to change the access permissions senders at runtime. The "si"
1374 (server info) command is used to list the streaming options of the
1375 currently running server as well as the various sender access lists.
1376
1377 -> Show client/target/access lists:
1378
1379 para_client si
1380
1381 -> Obtain general help for the sender command:
1382
1383 para_client help sender
1384
1385 -> Get help for a specific sender (contains further examples):
1386
1387 s=http # or dccp or udp
1388 para_client sender $s help
1389
1390 By default para_server activates both the HTTP and th DCCP sender on
1391 startup. This can be changed via command line options or para_server's
1392 config file.
1393
1394 -> List config file options for senders:
1395
1396 para_server -h
1397
1398 All senders share the "on" and "off" commands, so senders may be
1399 activated and deactivated independently of each other.
1400
1401 -> Switch off the http sender:
1402
1403 para_client sender http off
1404
1405 -> Receive a DCCP stream using CCID2 and write the output into a file:
1406
1407 host=foo.org; ccid=2; filename=bar
1408 para_recv --receiver "dccp --host $host --ccid $ccid" > $filename
1409
1410 Note the quotes around the arguments for the dccp receiver. Each
1411 receiver has its own set of command line options and its own command
1412 line parser, so arguments for the dccp receiver must be protected
1413 from being interpreted by para_recv.
1414
1415 -> Start UDP multicast, using the default multicast address:
1416
1417 para_client sender udp add 224.0.1.38
1418
1419 -> Receive FEC-encoded multicast stream and write the output into a file:
1420
1421 filename=foo
1422 para_recv -r udp > $filename
1423
1424 -> Add an UDP unicast for a client to the target list of the UDP sender:
1425
1426 t=client.foo.org
1427 para_client sender udp add $t
1428
1429 -> Receive this (FEC-encoded) unicast stream:
1430
1431 filename=foo
1432 para_recv -r 'udp -i 0.0.0.0' > $filename
1433
1434 -> Create a minimal config for para_audiod for HTTP streams:
1435
1436 c=$HOME/.paraslash/audiod.conf.min; s=server.foo.com
1437 echo receiver \".:http -i $s\" > $c
1438 para_audiod --config $c
1439
1440 -------
1441 Filters
1442 -------
1443
1444 A paraslash filter is a module which transforms an input stream into
1445 an output stream. Filters are included in the para_audiod executable
1446 and in the stand-alone tool para_filter which usually contains the
1447 same modules.
1448
1449 While para_filter reads its input stream from STDIN and writes
1450 the output to STDOUT, the filter modules of para_audiod are always
1451 connected to a receiver which produces the input stream and a writer
1452 which absorbs the output stream.
1453
1454 Some filters depend on a specific library being installed and are
1455 not compiled in if this library was not found at compile time. To
1456 see the list of supported filters, run para_filter and para_audiod
1457 with the --help option. The output looks similar to the following:
1458
1459 Available filters:
1460 compress wav amp fecdec wmadec prebuffer oggdec aacdec mp3dec
1461
1462 Out of these filter modules, a chain of filters can be constructed,
1463 much in the way Unix pipes can be chained, and analogous to the use
1464 of modules in gstreamer: The output of the first filter becomes the
1465 input of the second filter. There is no limitation on the number of
1466 filters and the same filter may occur more than once.
1467
1468 Like receivers, each filter has its own command line options which
1469 must be quoted to protect them from the command line options of
1470 the driving application (para_audiod or para_filter). Example:
1471
1472 para_filter -f 'mp3dec --ignore-crc' -f 'compress --damp 1'
1473
1474 For para_audiod, each audio format has its own set of filters. The
1475 name of the audio format for which the filter should be applied can
1476 be used as the prefix for the filter option. Example:
1477
1478 para_audiod -f 'mp3:prebuffer --duration 300'
1479
1480 The "mp3" prefix above is actually interpreted as a POSIX extended
1481 regular expression. Therefore
1482
1483 para_audiod -f '.:prebuffer --duration 300'
1484
1485 activates the prebuffer filter for all supported audio formats (because
1486 "." matches all audio formats) while
1487
1488 para_audiod -f 'wma|ogg:prebuffer --duration 300'
1489
1490 activates it only for wma and ogg streams.
1491
1492 Decoders
1493 ~~~~~~~~
1494
1495 For each supported audio format there is a corresponding filter
1496 which decodes audio data in this format to 16 bit PCM data which
1497 can be directly sent to the sound device or any other software that
1498 operates on undecoded PCM data (visualizers, equalizers etc.). Such
1499 filters are called _decoders_ in general, and xxxdec is the name of
1500 the paraslash decoder for the audio format xxx. For example, the mp3
1501 decoder filter is called mp3dec.
1502
1503 Note that the output of the decoder is about 10 times larger than
1504 its input. This means that filters that operate on the decoded audio
1505 stream have to deal with much more data than filters that transform
1506 the audio stream before it is fed to the decoder.
1507
1508 Paraslash relies on external libraries for most decoders, so these
1509 libraries must be installed for the decoder to be included in the
1510 para_filter and para_audiod executables. The oggdec filter depends
1511 on the libogg and libvorbis libraries for example.
1512
1513 Forward error correction
1514 ~~~~~~~~~~~~~~~~~~~~~~~~
1515
1516 As already mentioned REFERENCE(Streaming protocols, earlier),
1517 paraslash uses forward error correction (FEC) for the unreliable UDP
1518 and DCCP transports. FEC is a technique which was invented already
1519 in 1960 by Reed and Solomon and which is widely used for the parity
1520 calculations of storage devices (RAID arrays). It is based on the
1521 algebraic concept of finite fields, today called Galois fields, in
1522 honour of the mathematician Galois (1811-1832). The FEC implementation
1523 of paraslash is based on code by Luigi Rizzo.
1524
1525 Although the details require a sound knowledge of the underlying
1526 mathematics, the basic idea is not hard to understand: For positive
1527 integers k and n with k < n it is possible to compute for any k given
1528 data bytes d_1, ..., d_k the corresponding r := n -k parity bytes p_1,
1529 ..., p_r such that all data bytes can be reconstructed from *any*
1530 k bytes of the set
1531
1532 {d_1, ..., d_k, p_1, ..., p_r}.
1533
1534 FEC-encoding for unreliable network transports boils down to slicing
1535 the audio stream into groups of k suitably sized pieces called _slices_
1536 and computing the r corresponding parity slices. This step is performed
1537 in para_server which then sends both the data and the parity slices
1538 over the unreliable network connection. If the client was able
1539 to receive at least k of the n = k + r slices, it can reconstruct
1540 (FEC-decode) the original audio stream.
1541
1542 From these observations it is clear that there are three different
1543 FEC parameters: The slice size, the number of data slices k, and the
1544 total number of slices n. It is crucial to choose the slice size
1545 such that no fragmentation of network packets takes place because
1546 FEC only guards against losses and reordering but fails if slices are
1547 received partially.
1548
1549 FEC decoding in paralash is performed through the fecdec filter which
1550 usually is the first filter (there can be other filters before fecdec
1551 if these do not alter the audio stream).
1552
1553
1554 Volume adjustment (amp and compress)
1555 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1556
1557 The amp and the compress filter both adjust the volume of the audio
1558 stream. These filters operate on uncompressed audio samples. Hence
1559 they are usually placed directly after the decoding filter. Each
1560 sample is multiplied with a scaling factor (>= 1) which makes amp
1561 and compress quite expensive in terms of computing power.
1562
1563 *amp*
1564
1565 The amp filter amplifies the audio stream by a fixed scaling factor
1566 that must be known in advance. For para_audiod this factor is derived
1567 from the amplification field of the audio file's entry in the audio
1568 file table while para_filter uses the value given at the command line.
1569
1570 The optimal scaling factor F for an audio file is the largest real
1571 number F >= 1 such that after multiplication with F all samples still
1572 fit into the sample interval [-32768, 32767]. One can use para_filter
1573 in combination with the sox utility to compute F:
1574
1575 para_filter -f mp3dec -f wav < file.mp3 | sox -t wav - -e stat -v
1576
1577 The amplification value V which is stored in the audio file table,
1578 however, is an integer between 0 and 255 which is connected to F
1579 through the formula
1580
1581 V = (F - 1) * 64.
1582
1583 To store V in the audio file table, the command
1584
1585 para_client -- touch -a=V file.mp3
1586
1587 is used. The reader is encouraged to write a script that performs
1588 these computations :)
1589
1590 *compress*
1591
1592 Unlike the amplification filter, the compress filter adjusts the volume
1593 of the audio stream dynamically without prior knowledge about the peak
1594 value. It maintains the maximal volume of the last n samples of the
1595 audio stream and computes a suitable amplification factor based on that
1596 value and the various configuration options. It tries to chose this
1597 factor such that the adjusted volume meets the desired target level.
1598
1599 Note that it makes sense to combine amp and compress.
1600
1601 Misc filters (wav and prebuffer)
1602 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1603
1604 These filters are rather simple and do not modify the audio stream at
1605 all. The wav filter is only useful with para_filter and in connection
1606 with a decoder. It asks the decoder for the number of channels and the
1607 sample rate of the stream and adds a Microsoft wave header containing
1608 this information at the beginning. This allows to write wav files
1609 rather than raw PCM files (which do not contain any information about
1610 the number of channels and the sample rate).
1611
1612 The prebuffer filter simply delays the output until the given time has
1613 passed (starting from the time the first byte was available in its
1614 input queue) or until the given amount of data has accumulated. It
1615 is mainly useful for para_audiod if the standard parameters result
1616 in buffer underruns.
1617
1618 Both filters require almost no additional computing time, even when
1619 operating on uncompressed audio streams, since data buffers are simply
1620 "pushed down" rather than copied.
1621
1622 Examples
1623 ~~~~~~~~
1624
1625 -> Decode an mp3 file to wav format:
1626
1627 para_filter -f mp3dec -f wav < file.mp3 > file.wav
1628
1629 -> Amplify a raw audio file by a factor of 1.5:
1630
1631 para_filter -f amp --amp 32 < foo.raw > bar.raw
1632
1633 ------
1634 Output
1635 ------
1636
1637 Once an audio stream has been received and decoded to PCM format,
1638 it can be sent to a sound device for playback. This part is performed
1639 by paraslash _writers_ which are described in this chapter.
1640
1641 Writers
1642 ~~~~~~~
1643
1644 A paraslash writer acts as a data sink that consumes but does not
1645 produce audio data. Paraslash writers operate on the client side and
1646 are contained in para_audiod and in the stand-alone tool para_write.
1647
1648 The para_write program reads uncompressed audio data from STDIN. If
1649 this data starts with a wav header, sample rate, sample format and
1650 channel count are read from the header. Otherwise CD audio (44.1KHz
1651 16 bit little endian, stereo) is assumed but this can be overridden
1652 by command line options. para_audiod, on the other hand, obtains
1653 the sample rate and the number of channels from the decoder.
1654
1655 Like receivers and filters, each writer has an individual set of
1656 command line options, and for para_audiod writers can be configured
1657 per audio format separately. It is possible to activate more than
1658 one writer for the same stream simultaneously.
1659
1660 OS-dependent APIs
1661 ~~~~~~~~~~~~~~~~~
1662
1663 Unfortunately, the various flavours of Unix on which paraslash
1664 runs on have different APIs for opening a sound device and starting
1665 playback. Hence for each such API there is a paraslash writer that
1666 can play the audio stream via this API.
1667
1668 *ALSA*. The _Advanced Linux Sound Architecture_ is only available on
1669 Linux systems. Although there are several mid-layer APIs in use by
1670 the various Linux distributions (ESD, Jack, PulseAudio), paraslash
1671 currently supports only the low-level ALSA API which is not supposed
1672 to be change. ALSA is very feature-rich, in particular it supports
1673 software mixing via its DMIX plugin. ALSA is the default writer on
1674 Linux systems.
1675
1676 *OSS*. The _Open Sound System_ is the only API on *BSD Unixes and
1677 is also available on Linux systems, usually provided by ALSA as an
1678 emulation for backwards compatibility. This API is rather simple but
1679 also limited. For example only one application can open the device
1680 at any time. The OSS writer is activated by default on BSD Systems.
1681
1682 *OSX*. Mac OS X has yet another API called CoreAudio. The OSX writer
1683 for this API is only compiled in on such systems and is of course
1684 the default there.
1685
1686 *FILE*. The file writer allows to capture the audio stream and
1687 write the PCM data to a file on the file system rather than playing
1688 it through a sound device. It is supported on all platforms and is
1689 always compiled in.
1690
1691 *AO*. _Libao_ is a cross-platform audio library which supports a wide
1692 variety of platforms including PulseAudio (gnome), ESD (Enlightened
1693 Sound Daemon), AIX, Solaris and IRIX. The ao writer plays audio
1694 through an output plugin of libao.
1695
1696 Examples
1697 ~~~~~~~~
1698
1699 -> Use the OSS writer to play a wav file:
1700
1701 para_write --writer oss < file.wav
1702
1703 -> Enable ALSA software mixing for mp3 streams
1704
1705 para_audiod --writer 'mp3:alsa -d plug:swmix'
1706
1707
1708 ---
1709 Gui
1710 ---
1711
1712 para_gui executes an arbitrary command which is supposed to print
1713 status information to STDOUT. It then displays this information in
1714 a curses window. By default the command
1715
1716 para_audioc -- stat -p
1717
1718 is executed, but this can be customized via the --stat_cmd option. In
1719 particular it possible to use
1720
1721 para_client -- stat -p
1722
1723 to make para_gui work on systems on which para_audiod is not running.
1724
1725 Key bindings
1726 ~~~~~~~~~~~~
1727
1728 It is possible to bind keys to arbitrary commands via custom
1729 key-bindings. Besides the internal keys which can not be changed (help,
1730 quit, loglevel, version...), the following flavours of key-bindings
1731 are supported:
1732
1733 - external: Shutdown curses before launching the given command.
1734 Useful for starting other ncurses programs from within
1735 para_gui, e.g. aumix or dialog scripts. Or, use the mbox
1736 output format to write a mailbox containing one mail for each
1737 (admissible) file the audio file selector knows about. Then
1738 start mutt from within para_gui to browse your collection!
1739
1740 - display: Launch the command and display its stdout in
1741 para_gui's bottom window.
1742
1743 - para: Like display, but start "para_client <specified
1744 command>" instead of "<specified command>".
1745
1746 The general form of a key binding is
1747
1748 key_map k:m:c
1749
1750 which maps key k to command c using mode m. Mode may be x, d or p
1751 for external, display and paraslash commands, respectively.
1752
1753 Themes
1754 ~~~~~~
1755
1756 Currently there are only two themes for para_gui. It is easy, however,
1757 to add more themes. To create a new theme one has to define the
1758 position, color and geometry for for each status item that should be
1759 shown by this theme. See gui_theme.c for examples.
1760
1761 The "." and "," keys are used to switch between themes.
1762
1763 Examples
1764 ~~~~~~~~
1765
1766 -> Show server info:
1767
1768 key_map "i:p:si"
1769
1770 -> Jump to the middle of the current audio file by pressing F5:
1771
1772 key_map "<F5>:p:jmp 50"
1773
1774 -> vi-like bindings for jumping around:
1775
1776 key_map "l:p:ff 10"
1777 key_map "h:p:ff 10-"
1778 key_map "w:p:ff 60"
1779 key_map "b:p:ff 60-"
1780
1781 -> Print the current date and time:
1782
1783 key_map "D:d:date"
1784
1785 -> Call other curses programs:
1786
1787 key_map "U:x:aumix"
1788 key_map "!:x:/bin/bash"
1789 key_map "^E:x:/bin/sh -c 'vi ~/.paraslash/gui.conf'"
1790
1791 -----------
1792 Development
1793 -----------
1794
1795 Tools
1796 ~~~~~
1797
1798 In order to compile the sources from the git repository (rather than
1799 from tar balls) and for contributing non-trivial changes to the
1800 paraslash project, some additional tools should be installed on a
1801 developer machine.
1802
1803 http://git.or.cz/ (git). As described in more detail REFERENCE(Git
1804 branches, below), the git source code management tool is used for
1805 paraslash development. It is necessary for cloning the git repository
1806 and for getting updates.
1807
1808 ftp://ftp.gnu.org/pub/gnu/gengetopt/ (gengetopt). The C code for
1809 the command line parsers of all paraslash executables is generated
1810 by gengetopt. The generated C files are shipped in the tarballs but
1811 are not contained in the git repository.
1812
1813 ftp://ftp.gnu.org/pub/gnu/m4/ (m4). Some input files for gengetopt
1814 are generated from templates by the m4 macro processor.
1815
1816 ftp://ftp.gnu.org/pub/gnu/autoconf/ (autoconf) GNU autoconf creates
1817 the configure file which is shipped in the tarballs but has to be
1818 generated when compiling from git.
1819
1820 http://www.triptico.com/software/grutatxt.html (grutatxt). The
1821 HTML version of this manual and some of the paraslash web pages are
1822 generated by the grutatxt plain text to HTML converter. If changes
1823 are made to these text files the grutatxt package must be installed
1824 to regenerate the HTML files.
1825
1826 http://www.stack.nl/~dimitri/doxygen/ (doxygen). The documentation
1827 of paraslash's C sources uses the doxygen documentation system. The
1828 conventions for documenting the source code is described in the
1829 REFERENCE(Doxygen, Doxygen section).
1830
1831 ftp://ftp.gnu.org/pub/gnu/global (global). This is used to generate
1832 browsable HTML from the C sources. It is needed by doxygen.
1833
1834 Git branches
1835 ~~~~~~~~~~~~
1836
1837 Paraslash has been developed using the git source code management
1838 tool since 2006. Development is organized roughly in the same spirit
1839 as the git development itself, as described below.
1840
1841 The following text passage is based on "A note from the maintainer",
1842 written by Junio C Hamano, the maintainer of git.
1843
1844 There are four branches in the paraslash repository that track the
1845 source tree: "master", "maint", "next", and "pu".
1846
1847 The "master" branch is meant to contain what is well tested and
1848 ready to be used in a production setting. There could occasionally be
1849 minor breakages or brown paper bag bugs but they are not expected to
1850 be anything major, and more importantly quickly and easily fixable.
1851 Every now and then, a "feature release" is cut from the tip of this
1852 branch, named with three dotted decimal digits, like 0.4.2.
1853
1854 Whenever changes are about to be included that will eventually lead to
1855 a new major release (e.g. 0.5.0), a "maint" branch is forked off from
1856 "master" at that point. Obvious, safe and urgent fixes after the major
1857 release are applied to this branch and maintenance releases are cut
1858 from it. New features never go to this branch. This branch is also
1859 merged into "master" to propagate the fixes forward.
1860
1861 A trivial and safe enhancement goes directly on top of "master".
1862 New development does not usually happen on "master", however.
1863 Instead, a separate topic branch is forked from the tip of "master",
1864 and it first is tested in isolation; Usually there are a handful such
1865 topic branches that are running ahead of "master". The tip of these
1866 branches is not published in the public repository to keep the number
1867 of branches that downstream developers need to worry about low.
1868
1869 The quality of topic branches varies widely. Some of them start out as
1870 "good idea but obviously is broken in some areas" and then with some
1871 more work become "more or less done and can now be tested by wider
1872 audience". Luckily, most of them start out in the latter, better shape.
1873
1874 The "next" branch is to merge and test topic branches in the latter
1875 category. In general, this branch always contains the tip of "master".
1876 It might not be quite rock-solid production ready, but is expected to
1877 work more or less without major breakage. The maintainer usually uses
1878 the "next" version of paraslash for his own pleasure, so it cannot
1879 be _that_ broken. The "next" branch is where new and exciting things
1880 take place.
1881
1882 The two branches "master" and "maint" are never rewound, and "next"
1883 usually will not be either (this automatically means the topics that
1884 have been merged into "next" are usually not rebased, and you can find
1885 the tip of topic branches you are interested in from the output of
1886 "git log next"). You should be able to safely build on top of them.
1887
1888 However, at times "next" will be rebuilt from the tip of "master" to
1889 get rid of merge commits that will never be in "master. The commit
1890 that replaces "next" will usually have the identical tree, but it
1891 will have different ancestry from the tip of "master".
1892
1893 The "pu" (proposed updates) branch bundles the remainder of the
1894 topic branches. The "pu" branch, and topic branches that are only in
1895 "pu", are subject to rebasing in general. By the above definition
1896 of how "next" works, you can tell that this branch will contain quite
1897 experimental and obviously broken stuff.
1898
1899 When a topic that was in "pu" proves to be in testable shape, it
1900 graduates to "next". This is done with
1901
1902 git checkout next
1903 git merge that-topic-branch
1904
1905 Sometimes, an idea that looked promising turns out to be not so good
1906 and the topic can be dropped from "pu" in such a case.
1907
1908 A topic that is in "next" is expected to be polished to perfection
1909 before it is merged to "master". Similar to the above, this is
1910 done with
1911
1912 git checkout master
1913 git merge that-topic-branch
1914 git branch -d that-topic-branch
1915
1916 Note that being in "next" is not a guarantee to appear in the next
1917 release (being in "master" is such a guarantee, unless it is later
1918 found seriously broken and reverted), nor even in any future release.
1919
1920 Coding Style
1921 ~~~~~~~~~~~~
1922
1923 The preferred coding style for paraslash coincides more or less
1924 with the style of the Linux kernel. So rather than repeating what is
1925 written XREFERENCE(http://www.kernel.org/doc/Documentation/CodingStyle,
1926 there), here are the most important points.
1927
1928 - Burn the GNU coding standards.
1929 - Never use spaces for indentation.
1930 - Tabs are 8 characters, and thus indentations are also 8 characters.
1931 - Don't put multiple assignments on a single line.
1932 - Avoid tricky expressions.
1933 - Don't leave whitespace at the end of lines.
1934 - The limit on the length of lines is 80 columns.
1935 - Use K&R style for placing braces and spaces:
1936
1937 if (x is true) {
1938 we do y
1939 }
1940
1941 - Use a space after (most) keywords.
1942 - Do not add spaces around (inside) parenthesized expressions.
1943 - Use one space around (on each side of) most binary and ternary operators.
1944 - Do not use cute names like ThisVariableIsATemporaryCounter, call it tmp.
1945 - Mixed-case names are frowned upon.
1946 - Descriptive names for global variables are a must.
1947 - Avoid typedefs.
1948 - Functions should be short and sweet, and do just one thing.
1949 - The number of local variables shouldn't exceed 10.
1950 - Gotos are fine if they improve readability and reduce nesting.
1951 - Don't use C99-style "// ..." comments.
1952 - Names of macros defining constants and labels in enums are capitalized.
1953 - Enums are preferred when defining several related constants.
1954 - Always use the paraslash wrappers for allocating memory.
1955 - If the name of a function is an action or an imperative.
1956 command, the function should return an error-code integer
1957 (<0 means error, >=0 means success). If the name is a
1958 predicate, the function should return a "succeeded" boolean.
1959
1960
1961 Doxygen
1962 ~~~~~~~
1963
1964 Doxygen is a documentation system for various programming
1965 languages. The paraslash project uses Doxygen for generating the API
1966 reference on the web pages, but good source code documentation is
1967 also beneficial to people trying to understand the code structure
1968 and the interactions between the various source files.
1969
1970 It is more illustrative to look at the source code for examples than
1971 to describe the conventions for documenting the source in this manual,
1972 so we only describe which parts of the code need doxygen comments,
1973 but leave out details on documentation conventions.
1974
1975 As a rule, only the public part of the C source is documented with
1976 Doxygen. This includes structures, defines and enumerations in header
1977 files as well as public (non-static) C functions. These should be
1978 documented completely. For example each parameter and the return
1979 value of a public function should get a descriptive comment.
1980
1981 No doxygen comments are necessary for static functions and for
1982 structures and enumerations in C files (which are used only within
1983 this file). This does not mean, however, that those entities need
1984 no documentation at all. Instead, common sense should be applied to
1985 document what is not obvious from reading the code.
1986
1987 --------
1988 Appendix
1989 --------
1990
1991 Network protocols
1992 ~~~~~~~~~~~~~~~~~
1993
1994 *IP*. The _Internet Protocol_ is the primary networking protocol
1995 used for the Internet. All protocols described below use IP as the
1996 underlying layer. Both the prevalent IPv4 and the next-generation
1997 IPv6 variant are being deployed actively worldwide.
1998
1999 *Connection-oriented and connectionless protocols*. Connectionless
2000 protocols differ from connection-oriented ones in that state
2001 associated with the sending/receiving endpoints is treated
2002 implicitly. Connectionless protocols maintain no internal knowledge
2003 about the state of the connection. Hence they are not capable of
2004 reacting to state changes, such as sudden loss or congestion on the
2005 connection medium. Connection-oriented protocols, in contrast, make
2006 this knowledge explicit. The connection is established only after
2007 a bidirectional handshake which requires both endpoints to agree
2008 on the state of the connection, and may also involve negotiating
2009 specific parameters for the particular connection. Maintaining an
2010 up-to-date internal state of the connection also in general means
2011 that the sending endpoints perform congestion control, adapting to
2012 qualitative changes of the connection medium.
2013
2014 *Reliability*. In IP networking, packets can be lost, duplicated,
2015 or delivered out of order, and different network protocols handle
2016 these problems in different ways. We call a transport-layer protocol
2017 _reliable_, if it turns the unreliable IP delivery into an ordered,
2018 duplicate- and loss-free delivery of packets. Sequence numbers
2019 are used to discard duplicates and re-arrange packets delivered
2020 out-of-order. Retransmission is used to guarantee loss-free
2021 delivery. Unreliable protocols, in contrast, do not guarantee ordering
2022 or data integrity.
2023
2024 *Classification*. With these definitions the protocols which are used
2025 by paraslash for steaming audio data may be classified as follows.
2026
2027 - HTTP/TCP: connection-oriented, reliable,
2028 - UDP: connectionless, unreliable,
2029 - DCCP: connection-oriented, unreliable.
2030
2031 Below we give a short descriptions of these protocols.
2032
2033 *TCP*. The _Transmission Control Protocol_ provides reliable,
2034 ordered delivery of a stream and a classic window-based congestion
2035 control. In contrast to UDP and DCCP (see below), TCP does not have
2036 record-oriented or datagram-based syntax, i.e. it provides a stream
2037 which is unaware and independent of any record (packet) boundaries.
2038 TCP is used extensively by many application layers. Besides HTTP (the
2039 Hypertext Transfer Protocol), also FTP (the File Transfer protocol),
2040 SMTP (Simple Mail Transfer Protocol), SSH (Secure Shell) all sit on
2041 top of TCP.
2042
2043 *UDP*. The _User Datagram Protocol_ is the simplest transport-layer
2044 protocol, built as a thin layer directly on top of IP. For this reason,
2045 it offers the same best-effort service as IP itself, i.e. there is no
2046 detection of duplicate or reordered packets. Being a connectionless
2047 protocol, only minimal internal state about the connection is
2048 maintained, which means that there is no protection against packet
2049 loss or network congestion. Error checking and correction (if at all)
2050 are performed in the application.
2051
2052 *DCCP*. The _Datagram Congestion Control Protocol_ combines the
2053 connection-oriented state maintenance known from TCP with the
2054 unreliable, datagram-based transport of UDP. This means that it
2055 is capable of reacting to changes in the connection by performing
2056 congestion control, offering multiple alternative approaches. But it
2057 is bound to datagram boundaries (the maximum packet size supported
2058 by a medium), and like UDP it lacks retransmission to protect
2059 against loss. Due to the use of sequence numbers, it is however
2060 able to react to loss (interpreted as a congestion indication) and
2061 to ignore out-of-order and duplicate packets. Unlike TCP it allows
2062 to negotiate specific, binding features for a connection, such as
2063 the choice of congestion control: classic, window-based congestion
2064 control known from TCP is available as CCID-2, rate-based, "smooth"
2065 congestion control is offered as CCID-3.
2066
2067 *HTTP*. _The Hypertext Transfer Protocol_ is an application layer
2068 protocol on top of TCP. It is spoken by web servers and is most often
2069 used for web services. However, as can be seen by the many Internet
2070 radio stations and YouTube/Flash videos, http is by far not limited to
2071 the delivery of web pages only. Being a simple request/response based
2072 protocol, the semantics of the protocol also allow the delivery of
2073 multimedia content, such as audio over http.
2074
2075 *Multicast*. IP multicast is not really a protocol but a technique
2076 for one-to-many communication over an IP network. The challenge is to
2077 deliver information to a group of destinations simultaneously using
2078 the most efficient strategy to send the messages over each link of
2079 the network only once. This has benefits for streaming multimedia:
2080 the standard one-to-one unicast offered by TCP/DCCP means that
2081 n clients listening to the same stream also consume n-times the
2082 resources, whereas multicast requires to send the stream just once,
2083 irrespective of the number of receivers. Since it would be costly to
2084 maintain state for each listening receiver, multicast often implies
2085 connectionless transport, which is the reason that it is currently
2086 only available via UDP.
2087
2088 License
2089 ~~~~~~~
2090
2091 Paraslash is licensed under the GPL, version 2. Most of the code
2092 base has been written from scratch, and those parts are GPL V2
2093 throughout. Notable exceptions are FEC and the WMA decoder. See the
2094 corresponding source files for licencing details for these parts. Some
2095 code sniplets of several other third party software packages have
2096 been incorporated into the paraslash sources, for example log message
2097 coloring was taken from the git sources. These third party software
2098 packages are all published under the GPL or some other license
2099 compatible to the GPL.
2100
2101 Acknowledgements
2102 ~~~~~~~~~~~~~~~~
2103
2104 Many thanks to Gerrit Renker who read an early draft of this manual
2105 and contributed significant improvements.
2106
2107 ----------
2108 References
2109 ----------
2110
2111 Articles
2112 ~~~~~~~~
2113 - Reed, Irving S.; Solomon, Gustave (1960),
2114 XREFERENCE(http://kom.aau.dk/~heb/kurser/NOTER/KOFA01.PDF,
2115 Polynomial Codes over Certain Finite Fields), Journal of the
2116 Society for Industrial and Applied Mathematics (SIAM) 8 (2):
2117 300-304, doi:10.1137/0108018)
2118
2119 RFCs
2120 ~~~~
2121
2122 - XREFERENCE(http://www.ietf.org/rfc/rfc768.txt, RFC 768) (1980):
2123 User Datagram Protocol
2124 - XREFERENCE(http://www.ietf.org/rfc/rfc791.txt, RFC 791) (1981):
2125 Internet Protocol
2126 - XREFERENCE(http://www.ietf.org/rfc/rfc2437.txt, RFC 2437) (1998):
2127 RSA Cryptography Specifications
2128 - XREFERENCE(http://www.ietf.org/rfc/rfc4340.txt, RFC 4340)
2129 (2006): Datagram Congestion Control Protocol (DCCP)
2130 - XREFERENCE(http://www.ietf.org/rfc/rfc4341.txt, RFC 4341) (2006):
2131 Congestion Control ID 2: TCP-like Congestion Control
2132 - XREFERENCE(http://www.ietf.org/rfc/rfc4342.txt, RFC 4342) (2006):
2133 Congestion Control ID 3: TCP-Friendly Rate Control (TFRC)
2134
2135 Application web pages
2136 ~~~~~~~~~~~~~~~~~~~~~
2137
2138 - XREFERENCE(http://paraslash.systemlinux.org/, paraslash)
2139 - XREFERENCE(http://xmms2.org/wiki/Main_Page, xmms)
2140 - XREFERENCE(http://www.mpg123.de/, mpg123)
2141 - XREFERENCE(http://gstreamer.freedesktop.org/, gstreamer)
2142 - XREFERENCE(http://www.icecast.org/, icecast)
2143 - XREFERENCE(http://beesbuzz.biz/code/audiocompress.php, Audio Compress)
2144
2145 External documentation
2146 ~~~~~~~~~~~~~~~~~~~~~~
2147
2148 - XREFERENCE(http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf,
2149 H. Peter Anvin: The mathematics of Raid6)
2150 - XREFERENCE(http://info.iet.unipi.it/~luigi/fec_ccr.ps.gz,
2151 Luigi Rizzo: Effective Erasure Codes for reliable Computer
2152 Communication Protocols)
2153
2154 Code
2155 ~~~~
2156 - XREFERENCE(http://info.iet.unipi.it/~luigi/vdm.tar.gz,
2157 Original FEC implementation by Luigi Rizzo)
2158