RFCs (Request for Comments) are documents, in which technical components - e. g. architecture of the internet, the design of protocols, the design of email headers etc. - are described in detail. These documents are available for free in the internet and can be downloaded, e. g. from the addresses http://www.rfc-editor.org/ or ftp://ftp.denic.de/pub/rfc/.
RFC 1006 with the title "ISO Transport Service on top of the TCP" (ISO transport service through TCP) is a protocol extension for the TCP protocol. This means information in addition to the TCP data is transmitted between partners, in order to provide certain services for the user (ISO services as expansion to TCP).
Behaviour of the TCP protocol
When data is transmitted using TCP protocol, then transmission is focused on the data stream. Thereby no information on length nor on begining and end of a message are transmitted. This is no problem when sending, as the sender knows how many bytes of data he wants to send. The addressee, however, is not able to recognize, where one message ends and where the next message begins within the data stream.
Behaviour of the RFC 1006 protocol expansion
In most applications of automation technology however, it is indispensable to work message focused. Completed message blocks, which are recognized as such by the recipient, are sent through a connection. In order to ensure recognition, RFC 1006 now specifies the information that must be added as a header to the data to be transmitted.
RFC 1006 therefore provides applications, which are built on the data stream focused TCP protocol, with a message oriented transmission.
This functionality is available through the communication processors CP 443-1 and CP 343-1 for SIMATIC S7, through CP 1430 TCP for SIMATIC S5 and through CP 1613 or Softnet Industrial Ethernet for PC. Furthermore, the function is supported by the S7-300 CPUs (from V2.5) and S7-400 CPUs (from V5.0) with integrated PROFINET interface.
When using TCP without RFC 1006, e. g. because a communication partner does not support the protocoll, ensure - regarding the connection - you work with a definite, arranged length of data.
Further sources of information on RFC 1006:
On the NCM 1430 TCP floppy disk, you will find a document "Information zur Arbeitsweise des RFC 1006" with information on the use of RFC 1006 within CP 1430 TCP in German.
Alternatively you can also obtain the document at http://tools.ietf.org/html/rfc1006.