Octet Stream
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In computer and network technology, an octet represents an 8-bit quantity. Octets range in mathematical value from 0 to 255.
The term octet is also used in other contexts, such as musical performance, to refer to a group of eight people or parts.
Octets vs. Bytes
All modern computer systems implement a byte as an eight-bit quantity. Octets and bytes are the same from this perspective. For this reason, some people use the two terms interchangeably. Historically, however, computers have supported bytes having different numbers of bits; octets and bytes mean different things in this context. Network professionals began using the term octet many years ago to maintain this distinction.
Computer systems engineers often use the term nibble when referring to a four-bit quantity (half of one octet or byte) rather than calling it a half octet (or quartet, as is common in music).
Octet Strings in IP Addresses and Network Protocols
The term octet string refers to a collection of any number of related octets. Octet strings are most commonly found in Internet protocol (IP) addressing, in which the four bytes of an IPv4 address consists of four octets. In dotted-decimal notation, an IP address appears as [ octet ] . [ octet ] . [ octet ] . [ octet ], as in 192.168.0.1.
An IPv6 address contains 16 octets rather than four. Whereas IPv4 notation separates every single octet with a dot (.), IPv6 notation separates pairs of octets with a colon, like this: [octet][octet]:[octet][octet]::::::[octet][octet].
Octets also can refer to individual byte units within network protocol headers or footers. Network engineers sometimes classify protocols as octet stuffing or octet counting. An octet-stuffing protocol supports message units with special (hard-coded) sequences of bits (one or more octets) inserted to signify the end of the message. An octet counting protocol supports message units with their sizes (number of octets) encoded within the protocol header. Both approaches allow message recipients to determine when they are finished with processing the incoming data, although each has its advantages depending on the intended usage of the protocol. (A third method, called connection blasting, has the message sender terminate its end of the connection to signify that no more data is being sent.)
Octet Stream
In web browsers, the MIME type application/octet-stream refers to a binary file that is delivered by the server over an HTTP connection. Web clients typically use octet streams when working with multiple types of binary files and when they're unable to recognize the type by its file name or to assume any one particular format.
Browsers often prompt the user to identify the file type of an octet stream by saving the file with a specific filename extension.
A bitstream (or bit stream), also known as binary sequence, is a sequence of bits.
A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity (octets), and so the term octet stream is sometimes used interchangeably. An octet may be encoded as a sequence of 8 bits in multiple different ways (see endianness) so there is no unique and direct translation between bytestreams and bitstreams.
Bitstreams and bytestreams are used extensively in telecommunications and computing. For example, synchronous bitstreams are carried by SONET, and Transmission Control Protocol transports an asynchronous bytestream.
Relationship between bitstreams and bytestreams[edit]
In practice, bitstreams are not used directly to encode bytestreams; a communication channel may use a signalling method that does not directly translate to bits (for instance, by transmitting signals of multiple frequencies) and typically also encodes other information such as framing and error correction together with its data.[citation needed]
Examples[edit]
The term bitstream is frequently used to describe the configuration data to be loaded into a field-programmable gate array (FPGA). Although most FPGAs also support a byte-parallel loading method as well, this usage may have originated based on the common method of configuring the FPGA from a serial bit stream, typically from a serial PROM or flash memory chip. The detailed format of the bitstream for a particular FPGA is typically proprietary to the FPGA vendor.
In mathematics, several specific infinite sequences of bits have been studied for their mathematical properties; these include the Baum–Sweet sequence, Ehrenfeucht–Mycielski sequence, Fibonacci word, Kolakoski sequence, regular paperfolding sequence, Rudin–Shapiro sequence, and Thue–Morse sequence.
On most operating systems, including Unix-like and Windows, standard I/O libraries convert lower-level paged or buffered file access to a bytestream paradigm. In particular in Unix-like operating systems, each process has three standard streams, that are examples of unidirectional bytestreams. The Unix pipe mechanism provides bytestream communications between different processes.
Compression algorithms often code in bitstreams, as the 8 bits offered by a byte (the smallest addressable unit of memory) may be wasteful. Although typically implemented in low-level languages, some high-level languages such as Python[1] and Java[2] offer native interfaces for bitstream I/O.
One well-known example of a communication protocol which provides a byte-stream service to its clients is the Transmission Control Protocol (TCP) of the Internet protocol suite, which provides a bidirectional bytestream.
The Internet media type for an arbitrary bytestream is application/octet-stream. Other media types are defined for bytestreams in well-known formats.
Flow control[edit]
Often the contents of a bytestream are dynamically created, such as the data from the keyboard and other peripherals (/dev/tty), data from the pseudorandom number generator (/dev/urandom), etc.
In those cases, when the destination of a bytestream (the consumer) uses bytes faster than they can be generated, the system uses process synchronization to make the destination wait until the next byte is available.
When bytes are generated faster than the destination can use them, there are several techniques to deal with the situation:
- When the producer is a software algorithm, the system pauses the producer with the same process synchronization techniques
- When the producer supports flow control, the system only sends the ready signal when the consumer is ready for the next byte
- When the producer can't be paused—it is a keyboard or some hardware that doesn't support flow control—the system typically attempts to temporarily store the data until the consumer is ready for it, typically using a queue. Often the receiver can empty the buffer before it gets completely full. A producer that continues to produce data faster than it can be consumed, even after the buffer is full, leads to unwanted buffer overflow, packet loss, and network congestion.
See also[edit]
Octet Stream Error
References[edit]
Octet Stream Base64
- ^'Bitstream'. Archived from the original on 2016-09-08.
- ^'Class BitSet'. Archived from the original on 2016-11-30.