RFC 1576 (rfc1576) - Page 2 of 12
TN3270 Current Practices
Alternative Format: Original Text Document
RFC 1576 TN3270 Current Practices January 1994 5. Testing for session presence . . . . . . . . . . 7 6. Handling 3270 data . . . . . . . . . . . . . . . 7 7. 3270 Structured Fields . . . . . . . . . . . . . 8 8. The 3270 ATTN (Attention) Key . . . . . . . . . . 8 9. The 3270 SYSREQ Key . . . . . . . . . . . . . . . 9 10. Items not addressed by TN3270 . . . . . . . . . . 10 11. References . . . . . . . . . . . . . . . . . . . 10 12. Security Considerations . . . . . . . . . . . . . 11 13. Author's Note . . . . . . . . . . . . . . . . . . 11 14. Author's Address . . . . . . . . . . . . . . . . 12 1. Motivation 3270 display terminal data differs from traditional display terminal data in that it is block mode and uses EBCDIC instead of ASCII character representation. These two differences are the primary reason for the differentiation of TN3270 from standard Telnet in this document. 2. Background Existing complex IBM 3270 display terminal networks are not easily integrated with the increasing number of multi-platform networking environments, specifically TCP/IP. These complex networks include terminals attached to a 3270 host using SNA (Systems Network Architecture) and non-SNA connections. To address the issue of easily connecting display terminals to 3270 hosts using IP networks, several vendors have introduced telnet servers that provide TCP/IP users a connection to existing IBM mainframes by supporting display terminal emulation using a subset of the existing telnet protocol. Telnet servers may exist on the host itself, or be connected to the host using SNA or non-SNA methods. IBM terminals are generically referred to as 3270's which includes a broad range of terminals and devices, not all of which actually begin with the numbers 327x. 3270 terminals in the IBM SNA network environment have two sessions with the host computer application. One is used for communicating with the host application, the other is used for communicating with the SSCP (System Services Control Point) that links the terminal with the appropriate host computer. For the purposes of TN3270, this distinction is not apparent or relevant since there is actually only a single telnet session with the host computer or server. On an IBM SNA network, the 3270 terminal has a special key that toggles between the two sessions (SYSREQ). A brief discussion on how some telnet servers deal with this is included. TN3270 Enhancements Working Group
