Out-of-band data

Out of band data is a logically independent transmission channel between a pair of connected stream sockets. In computer networking, out-of-band data is the data transferred through a stream that is independent from the main in-band data stream. An out-of-band data mechanism provides a conceptually independent channel, which allows any data sent via that mechanism to be kept separate from in-band data. The out-of-band data mechanism should be provided as an inherent characteristic of the data channel and transmission protocol, rather than requiring a separate channel and endpoints to be established. The term "out-of-band data" probably derives from out-of-band signaling, as used in the telecommunications industry.

Consider a networking application that data from a remote data source to a remote destination. The data being tunneled may consist of any bit patterns. The sending end of the tunnel may at times have conditions that it needs to notify the receiving end about. However, it cannot simply insert a message to the receiving end, because that end will not be able to distinguish the message from data sent by the data source. But using an out-of-band mechanism, the sending end can send the message to the receiving end out of band. The receiving end will be notified in some fashion of the arrival of out-of-band data, and it can read the out of band data and know that this is a message intended for it from the sending end, independent of the data from the data source.

It is possible to implement out-of-band data transmission using a physically separate channel, but most commonly out-of-band data is a feature provided by a transmission protocol; typically such a protocol will divide transmitted data into chunks, which each chunk flagged as to whether it is in-band or out-of-band. A typical protocol might divide the data to be transmitted into blocks, which each block having a header word that identifies the type of data being sent, and a count of the data bytes or words to be sent in the block. The header will identify the data as being in-band or out-of-band, along with other identification and routing information. At the receiving end, the protocol looks at the header and routes the data to the normal reception endpoint if it is in-band, and to a separate mechanism if it is out-of-band. Depending on the implementation, there may be some mechanism for notifying or interrupting the receiving application when out-of-band data has arrived.

The most commonly used protocol containing an out-of-band data mechanism is the Internet's . It implements out-of-band data using an "urgent pointer", which marks certain data in the transmitted data stream as out-of-band. Unfortunately, a long-existing discrepancy between RFC 793 and RFC 1122 limits the usability of this feature of TCP; nonetheless, it is heavily used by certain standard application protocols, notably the Telnet protocol.

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