Protocol-Reject
LCP packet code field 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 “protocol” field 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 Config-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 “code” fields associated with some NCPs.
Link Termination Packets
Link TerminationPackets terminate a link and include the following packet types:
Terminate-Request
LCP packet code field 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-Ackpacket is sent back, or until a retry counter is exceeded
while waiting for an Ack.
Terminate-Ack
LCP packet code field is set to 6. Upon receiving a Terminate-Request
packet, this packet type must be transmitted with the code field set to 6.
Reception of an Terminate-Ackpacket 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 fields 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 field is set to 11.LCP transmits this packet type to
provide a data sink for Data link Layer testing. Apeer 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.
PPP Authentication Protocols
PPP authentication 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 “open” state), they go into an
“authentication” phase where they exchange authentication packets. Abox is neither
able to carry network data packets nor negotiate the use of a network protocol
(NCP traffic) until authentication negotiation completes successfully.
Using PPP
Chapter32. Using Point-to-Point Protocol Interfaces 455