Chapter 12: AI2524 X.25 Configuration Steps

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.

 

AI2524

IETF RFC 1356

Protocol

Protocol

Protocol Identifier

 

Identifier

 

 

 

 

 

Apollo Domain

0xD4

0x80 (5-byte SNAP

 

 

encoding1)

Bridging

0xD5

(Not implemented)

 

 

 

ISO CLNS

0x81

0x812

Compressed TCP

0xD8

0x00 (5-byte SNAP

 

 

encoding)3

IP

0xCC

0xCC4 or 0x80 (5-byte

 

 

SNAP encoding)

 

 

 

Novell IPX

0xD3

0x80 (5-byte SNAP

 

 

encoding)

 

 

 

PAD

0x015

0x015

QLLC

0xC3

(Not available)

 

 

 

Multiprotocol

(Not available)

0x00

 

 

 

August 1997

Page 12-17

2524UM

 

Page 275
Image 275
AIS AI2524 Understand Protocol Identification, Protocol Protocol Identifier, 0x01, 0xC3 Not available Multiprotocol 0x00