OSC Encoding tentative draft

The Open Sound Control Specification

Version 1.0, March 26 2002, Matt Wright

April 19 2012, Adrian Freed


Open Sound Control (OSC) is a open, transport-independent, message-based encoding developed for communication among agents, process, threads, computers, sound synthesizers, and other multimedia devices.

OSC Syntax

This section defines the syntax of OSC data.

Atomic Data Types

All OSC data is composed of the following fundamental data types:

32-bit big-endian two's complement integer
64-bit big-endian fixed-point time tag, semantics defined below
32-bit big-endian IEEE 754 floating point number
A sequence of non-null ASCII UTF-8 characters followed by a null, followed by 0-3 additional null characters to make the total number of bits a multiple of 32. (OSC-string examples) In this document, example OSC-strings will be written without the null characters, surrounded by double quotes.
An int32 size count, followed by that many 8-bit bytes of arbitrary binary data, followed by 0-3 additional zero bytes to make the total number of bits a multiple of 32.

The size of every atomic data type in OSC is a multiple of 32 bits. This guarantees that if the beginning of a block of OSC data is 32-bit aligned, every number in the OSC data will be 32-bit aligned.

OSC Packets

The unit of transmission of OSC is an OSC Packet. Any application that sends OSC Packet sources are called OSC Clients; any application that receives OSC Packet sinks are called OSC Servers.

An OSC packet consists of its contents, a contiguous block of binary data, and its size, the number of 8-bit bytes that comprise the contents. The size of an OSC packet is always a multiple of 4.

The underlying network that delivers an OSC packet is responsible for delivering both the contents and the size to the OSC application.
An OSC packet can be represented directly as a datagram in a transport such as UDP. In a reliable, 8-bit stream-based protocol such as TCP, OSC packets should
be framed by preceding each one with an int32 specifying their size in bytes. In an unreliable, 8-bit stream-based protocol such as USB Serial, OSC packets should be framed by wrapping them using SLIP with END preceding and concluding each payload as described in RFC1055.

The contents of an OSC packet must be either an OSC Message or an OSC Bundle. The first byte of the packet's contents unambiguously distinguishes between these two alternatives.

OSC Messages

An OSC message consists of an OSC Address Pattern followed by an OSC Type Tag String followed by zero or more OSC Arguments.

Note: some older implementations of OSC may omit the OSC Type Tag string. Until all such implementations are updated, OSC implementations should be robust in the case of a missing OSC Type Tag String.

OSC Address Patterns

An OSC Address Pattern is an OSC-string beginning with the character '/' (forward slash).

OSC Type Tag String

An OSC Type Tag String is an OSC-string beginning with the character ',' (comma) followed by a sequence of characters corresponding exactly to the sequence of OSC Arguments in the given message. Each character after the comma is called an OSC Type Tag and represents the type of the corresponding OSC Argument. (The requirement for OSC Type Tag Strings to start with a comma makes it easier for the recipient of an OSC Message to determine whether that OSC Message is lacking an OSC Type Tag String.)

This table lists the correspondance between each OSC Type Tag and the type of its corresponding OSC Argument:

The meaning of each OSC Type Tag
OSC Type Tag Type of corresponding argument
i int32
f float32
s OSC-string
b OSC-blob

Some OSC applications communicate among instances of themselves with additional, nonstandard argument types beyond those specified above.
OSC applications are not required to recognize these types; an OSC application should discard any message whose OSC Type Tag String contains any unrecognized OSC Type Tags. An application that does use any additional argument types must encode them with the OSC Type Tags in this table:

OSC Type Tags that must be used for certain nonstandard argument types
OSC Type Tag Type of corresponding argument
h 64 bit big-endian two's complement integer
t OSC-timetag
d 64 bit ("double") IEEE 754 floating point number
S Alternate type represented as an OSC-string (for example, for systems that differentiate "symbols" from "strings")
c an ascii character, sent as 32 bits
r 32 bit RGBA color
m 4 byte MIDI message. Bytes from MSB to LSB are: port id, status byte, data1, data2
T True. No bytes are allocated in the argument data.
F False. No bytes are allocated in the argument data.
N Nil. No bytes are allocated in the argument data.
I Infinitum. No bytes are allocated in the argument data.
[ Indicates the beginning of an array. The tags following are for data in the Array until a close brace tag is reached.
] Indicates the end of an array.
#Named nested bundle

OSC Type Tag String examples.

OSC Arguments

A sequence of OSC Arguments is represented by a contiguous sequence of the binary representations of each argument.

OSC Bundles

An OSC Bundle consists of the OSC-string "#bundle" followed by an OSC Time Tag, followed by zero or more OSC Bundle Elements.
The OSC-timetag is a 64-bit fixed point time tag whose semantics interpretation are described below.

An OSC Bundle Element consists of its size and its contents. The size is an int32 representing the number of 8-bit bytes in the contents, and will always be a multiple of 4. The contents are either an OSC Message or an OSC Bundle. Note this recursive definition: bundles may contain bundles.

This table shows the parts of a two-or-more-element OSC Bundle and the size (in 8-bit bytes) of each part.

Parts of an OSC Bundle
Data Size Purpose
OSC-string "#bundle" 8 bytes How to know that this data is a bundle
OSC-timetag 8 bytes Time tag that applies to the entire bundle
Size of first bundle element int32 = 4 bytes First bundle element
First bundle element's contents As many bytes as given by "size of first bundle element"
Size of second bundle element int32 = 4 bytes Second bundle element
Second bundle element's contents As many bytes as given by "size of second bundle element"
etc.   Addtional bundle elements

OSC Interpretation

This section defines the semantics interpretation of OSC data.

OSC Addresses include Regular Expressions

The syntax of OSC addresses was chosen to resemble that of URL paths. (OSC Address Examples)

The "/" character is the recommended character for interpretations of OSC Addresses as a hierarchical tree structure called OSC Address Spaces.

 OSC adddresses contain characters with special functions most to express regular expressions.

Special Characters in OSC Addresses
character name ASCII code (decimal)Function
' ' space 32Reserved
# number sign 35Reserved
* asterisk 42Regular Expression
, within { }  pairs comma 44Regular Expression
/ forward slash 47Hierarchy
? question mark 63Regular Expression
[ open bracket 91Regular Expression
] close bracket 93Regular Expression
{ open curly brace 123Regular Expression
} close curly brace 125Regular Expression
- within [ ] pairsdash??Regular Expression
! within [ ] pairsexclamation??Regular Expression

These are the matching rules for characters in the OSC Address Pattern:

  1. '?' in the OSC Address Pattern matches any single character
  2. '*' in the OSC Address Pattern matches any sequence of zero or more characters
  3. A string of characters in square brackets (e.g., "[string]") in the OSC Address Pattern matches any character in the string. Inside square brackets, the minus sign (-) and exclamation point (!) have special meanings:
    • two characters separated by a minus sign indicate the range of characters between the given two in ASCII collating sequence. 
      (A minus sign at the end of the string has no special meaning.)
    • An exclamation point at the beginning of a bracketed string negates the sense of the list, meaning that the list matches any character not in the list. 
      (An exclamation point anywhere besides the first character after the open bracket has no special meaning.)
  4. A comma-separated list of strings enclosed in curly braces (e.g., "{foo,bar}") in the OSC Address Pattern matches any of the strings in the list.
  5. Any other character in an OSC Address Pattern can match only the same character.

How can we clarify the use of regular expressions without using method invokation language? How do we talk about what is matched against?

How can we clarify atomicity without method invokation? Parallel Assignment?

Every OSC server has a set of OSC Methods.
OSC Methods are the potential destinations of OSC messages received by the OSC server and correspond to each of the points of control that the application makes available.
"Invoking" an OSC method is analogous to a procedure call; it means supplying the method with arguments and causing the method's effect to take place.
The "/" character is used to arrange an OSC Server's OSC Methods in a tree structure called an OSC Address Space.
The leaves of this tree are the OSC Methods and the branch nodes are called OSC Containers
An OSC Server's OSC Address Space can be dynamic; that is, its contents and shape can change over time. 
Each OSC Method and each OSC Container other than the root of the tree has a symbolic name, an ASCII string consisting of printable characters other than the following:


The OSC Address of an OSC Method is a symbolic name giving the full path to the OSC Method in the OSC Address Space, starting from the root of the tree. An OSC Method's OSC Address begins with the character '/' (forward slash), followed by the names of all the containers, in order, along the path from the root of the tree to the OSC Method, separated by forward slash characters, followed by the name of the OSC Method. 

OSC Message Dispatching and Pattern Matching

When an OSC server receives an OSC Message, it must invoke the appropriate OSC Methods in its OSC Address Space based on the OSC Message's OSC Address Pattern. This process is called dispatching the OSC Message to the OSC Methods that match its OSC Address Pattern.
All the matching OSC Methods are invoked with the same argument data, namely, the OSC Arguments in the OSC Message.

The parts of an OSC Address or an OSC Address Pattern are the substrings between adjacent pairs of forward slash characters and the substring after the last forward slash character. (examples)

A received OSC Message must be disptched to every OSC method in the current OSC Address Space whose OSC Address matches the OSC Message's OSC Address Pattern. An OSC Address Pattern matches an OSC Address if

  1. The OSC Address and the OSC Address Pattern contain the same number of parts; and
  2. Each part of the OSC Address Pattern matches the corresponding part of the OSC Address.

A part of an OSC Address Pattern matches a part of an OSC Address if every consecutive character in the OSC Address Pattern matches the next consecutive substring of the OSC Address and every character in the OSC Address is matched by something in the OSC Address Pattern.


 Temporal Interpretation and OSC Time Tags

An OSC server must have access to a representation of the correct current absolute time. OSC does not provide any mechanism for clock synchronization.

When a received OSC Packet contains only a single OSC Message, the OSC Server should invoke the correponding OSC Methods immediately, i.e., as soon as possible after receipt of the packet.

Otherwise a received OSC Packet contains an OSC Bundle, in which case the OSC Bundle's OSC Time Tag is interpreted.

Time tags are represented by a 64 bit fixed point number. The first 32 bits specify the number of seconds since midnight on January 1, 1900, and the last 32 bits specify fractional parts of a second to a precision of about 200 picoseconds. This is the UTC representation used by Internet NTP timestamps.

if the LSB of a Time Tag is 0  the rest of the bits  represent a time derived from the OSC client's local clock.

If the LSB of a Time Tag is 1 the rest of the Time Tag represents a desired "presentation time", i.e. the time when an OSC server should invoke the correponding OSC Methods. OSC client and Server have to be synchronized to a common clock for this "presentation time" to be correctly interpreted.

If the presentation time represented by the OSC Time Tag is before or equal to the current time, the OSC Server should invoke the methods immediately (unless the user has configured the OSC Server to discard messages that arrive too late). Otherwise the OSC Time Tag represents a time in the future, and the OSC server must store the OSC Bundle until the specified time and then invoke the appropriate OSC Methods.

The time tag value consisting of 63 zero bits followed by a one in the least significant bit is a special case meaning the OSC server should invoke the correponding OSC Methods "immediately."

OSC Messages in the same OSC Bundle are atomic; their corresponding OSC Methods should change state in the OSC Server as if no other processing took place between OSC Method invocations.

When an OSC Address Pattern is dispatched to multiple OSC Methods, the order in which the matching OSC Methods are invoked is unspecified. When an OSC Bundle contains multiple OSC Messages, the sets of OSC Methods corresponding to the OSC Messages must be invoked in the same order as the OSC Messages appear in the packet. (example)

When bundles contain other bundles, the OSC Time Tag of the enclosed bundle must be greater than or equal to the OSC Time Tag of the enclosing bundle. The atomicity requirement for OSC Messages in the same OSC Bundle does not apply to OSC Bundles within an OSC Bundle.