RFC 1576 (rfc1576) - Page 2 of 12

TN3270 Current Practices

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