HP X.25/9000 User's Guide HP-UX 11i v3 (5900-1523, August 2011)

Figure 1 X.25 Link architecture
Application (L7), Presentation (L6), and Session (L5) levels
The X.25 link does not provide any component for the general support of the application and
presentation levels (levels 7 and 6, respectively), although X.25/9000 PAD Services do provide
some of the functionality of these levels.
User-written application programs, Internet Services/Berkeley Services (via BSD Sockets) and NS
(via NetIPC Sockets), are accessed by means of TCP or UDP Transport Level (level 4) protocols.
For application level services, you can install OSI Services to run over the X.25 network (for more
information, refer to OSI Services documentation). The OTS/9000 product provides access to
X.25 for OSI Services. You can install application level services such as Internet/Berkeley Services
and Network Services (for more information, refer to the documentation for these products).
BSD IPC (Berkeley Software Distribution InterProcess Communication) allows direct programmatic
access to the X.25 packet level (level 3), or TCP/UDP at the transport level (level 4), for user-written
application programs.
NetIPC sockets also provide a programmatic interface to TCP/UDP at the transport level.
Transport level (L4)
At the transport level (level 4), the X.25 link provides TCP (based on the DARPA standard) and
UDP. These Transport level protocols are used by Internet/Berkeley Services, by NS, and by user
application programs that access the TCP/UDP and IP protocols.
The TCP protocol is a connection-based protocol. TCP verifies that all data is delivered without
duplication to its destination. The UDP protocol, unlike TCP, has no concept of a connection.
Messages are sent as a unit with source and destination information in the header.
8 About the X.25 product