Using PPP

Protocol-Reject

LCP packet code ®eld is set to 8. The transmission of this packet type indicates that a PPP frame has been received that contains an unsupported or unknown protocol (the PPP ªprotocolº ®eld was unrecognized for some packet). This usually occurs if you try to negotiate some NCP for a protocol that the other end doesn't support. For example, if DECnet CP (DNCP) sends a Con®g-Request and the other end does not know about DECnet, the other end replies with an LCP Protocol-Reject on DNCP. Upon receiving a Protocol-Reject packet, the link stops transmitting the incorrect protocol.

Note: NCP packet types and structure are the same as LCP, although there are a few additional ªcodeº ®elds associated with some NCPs.

Link Termination Packets

Link Termination Packets terminate a link and include the following packet types:

Terminate-Request

LCP packet code ®eld is set to 5. LCP transmits this packet type when a point-to-point link needs to be closed. These packets are sent until a Terminate-Ack packet is sent back, or until a retry counter is exceeded while waiting for an Ack.

Terminate-Ack

LCP packet code ®eld is set to 6. Upon receiving a Terminate-Request packet, this packet type must be transmitted with the code ®eld set to 6. Reception of an Terminate-Ack packet that was not expected indicates that the link has been closed.

Link Maintenance Packets

Link Maintenance Packets manage and debug a link, and include the following packet types:

Echo-Request and Echo-Reply

LCP packet code ®elds are set to 9 and 10 respectively. LCP transmits these packet types in order to provide a Data Link Layer loopback mechanism for both directions on the link. This feature is useful, for example, in debugging a faulty link to determine link quality. These packets are sent only when the link is in the Open state.

Discard-Request

LCP packet code ®eld is set to 11. LCP transmits this packet type to provide a data sink for Data link Layer testing. A peer that receives a Discard-Request must throw away the packet. This is useful in debugging a link. These packets are sent only when the link is in the Open state.

PPPAuthentication Protocols

PPPauthentication protocols provide a form of security between two nodes connected via a PPP link. If authentication is required on a box, then immediately after the two boxes successfully negotiate the use of the link at the LCP layer (LCP packets are exchanged until LCP goes into an ªopenº state), they go into an ªauthenticationº phase where they exchange authentication packets. A box is neither able to carry network data packets nor negotiate the use of a network protocol (NCP traffic) until authentication negotiation completes successfully.

Chapter 32. Using Point-to-Point Protocol Interfaces 455

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IBM SC30-3681-08 manual Link Termination Packets, Link Maintenance Packets, PPP Authentication Protocols