establishment type packets that LCP defines. They may implement some of the
additional LCP packet types, and they also may define additional packet types
beyond what LCP uses. Unlike LCP packets, the structure of an NCP frame may be
modified according to options negotiated by LCP during the link establishment
phase.
Link Establishment Packets
Link Establishment Packets establish and configure a point-to-point link including
the following packet types:
Configure-Request
LCP packet code field is set to 1. LCP transmits this packet type when it
wants to open a point-to-point link. Upon receiving a Configure-Request, a
peer station’s LCP entity sends an appropriate reply,depending on whether
it is ready to process packets.
Configure-Ack
LCP packet code field is set to 2. The peer transmits this packet type when
every configuration option in a Configure-Request packet is acceptable.
Upon receiving the Configure-Ack (ack = acknowledgment), the originating
station checks the Identifier field. This field must match the one from the
last-transmitted Configure-Request or the packet is invalid.
Both ends send Configure-Request and both ends must receive a
Configure-Ack before the link opens. Options negotiated for one direction
may differ from that negotiated for the other direction. There is no
“master-slave” relationship. Rather, each end works symmetrically.
Configure-Nak
LCP packet code field is set to 3. The peer transmits this packet type when
some part of the configuration option in a Configure-Request packet is
unacceptable. The Identifier field is copied from the received
Configure-Request and the Data (option) field is filled with the received
unacceptable configuration options. The Identifier field must match the one
from the last-transmitted Configure-Request or the packet is invalid and is
discarded.
When the originator receives a Configure-Nak packet, a new
Configure-Request packet is sent that includes modified, acceptable
configuration options.
Configure-Reject
LCP packet code field is set to 4. The peer transmits this packet type when
some part of the configuration options in a Configure-Request packet is
unacceptable. The Identifier field is copied from the received
Configure-Request and the Data (option) field is filled with the received
unacceptable configuration options. The Identifier field must match the one
from the last-transmitted Configure-Request or the packet is invalid and is
discarded.
When the originator receives a Configure-Reject packet, a new
Configure-Request packet is sent that does not include any of the
configuration options received in the Configure-Reject packet.
Code-Reject
LCP packet code field is set to 7. The transmission of this packet type
indicates that the LCP “code” field on a received packet is not recognized
as a valid value. While this can indicate an error, it also can indicate that
the peer does not implement some feature that you are trying to use.
Using PPP
454 MRS V3.2 Software User’s Guide