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Chapter 12: AI2524 X.25 Configuration Steps
August 1997 Page 12-17
2524UM
Understand Protocol Identification
The various methods and protocols used in X.25 SVC encapsulation
are identified in a specific field of the call packet; this field is defined
by X.25 to carry CUD. Since PVCs do not use the X.25 call setup pro-
cedures, only PVCs do not use CUD to identify their encapsulation.
The primary difference between the available AI2524 and IETF en-
capsulation methods is the specific value used to identify a protocol.
When any of the methods establishes a virtual circuit for carrying
single protocol, the protocol is identified in the call packet by the
CUD. When a virtual circuit is established to carry more than one pro-
tocol (only available using the RFC 1356 methodology), a protocol
identification field precedes the datagram encapsulated in the X.25
data packet; every datagram exchanged over that virtual circuit has its
protocol identified.
This table summarizes the values used in the CUD field to identify
protocols.
Protocol
AI2524
Protocol
Identifier
IETF RFC 1356
Protocol Identifier
Apollo Domain 0xD4 0x80 (5-byte SNAP
encoding
1
)
Bridging 0xD5 (Not implemented)
ISO CLNS 0x81
0x81
2
Compressed TCP 0xD8 0x00 (5-byte SNAP
encoding)
3
IP 0xCC
0xCC
4
or 0x80 (5-byte
SNAP encoding)
Novell IPX 0xD3 0x80 (5-byte SNAP
encoding)
PAD
0x01
5
0x01
5
QLLC 0xC3 (Not available)
Multiprotocol (Not available) 0x00