Information Technology Reference
In-Depth Information
Figure11-1
DNA and the OSI Reference Model
OSI reference model
DNA
7
Application
DNA applications
OSI applications
6
Presentation
OSI presentation
DNA
name
service
DNA
session
control
5
Session
OSI session
NSP, TP0, TP2, TP4
4
Transport
ES-IS
IS-IS
Connectionless
(CLNP, CLNS)
Connection-oriented
(X.25, CMNP)
3
Network
2
Data link
Various link-access protocols
1
Physical
As Figure 11-1 shows, DNA supports a variety of media and link implementations. Among these are
well-known standards such as Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), IEEE
802.2, and X.25. DNA also offers a traditional point-to-point link-layer protocol called Digital Data
Communications Message Protocol (DDCMP) and a 70-Mbps bus used in the VAX cluster called the
computer-room interconnect bus (CI bus).
The Network Layer
DECnet supports both connectionless and connection-oriented network layers. Both network layers are
implemented by OSI protocols. The connectionless implementation uses the Connectionless Network Protocol
(CLNP) and the Connectionless Network Service (CLNS). The connection-oriented network layer uses the
X.25 Packet-Level Protocol (PLP), which is also known as X.25 Level 3, and the Connection-Mode
Network Protocol (CMNP).
Although most of DNA was brought into OSI conformance with DECnet Phase V, DECnet Phase IV
routing was already very similar to OSI routing. Phase V DNA routing consists of OSI routing (ES-IS
and IS-IS), plus continued support for the DECnet Phase IV routing protocol.
DECnet Phase IV Routing Frame Format
The DECnet Phase IV routing protocol differs from IS-IS in several ways. One difference is in the
protocol header. The DNA Phase IV routing layer header is shown in Figure 11-2; IS-IS packet formats
are shown in Chapter 12, “Troubleshooting ISO CLNS.”
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