The Point-to-Point Protocol (PPP)
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3
2. PPP Encapsulation ..................................... 4
Simpson [Page i]
3. PPP Link Operation .................................... 6
3.1 Overview ........................................ 6
3.2 Phase Diagram ................................... 6
3.3 Link Dead (physical-layer not ready) ............ 7
3.4 Link Establishment Phase ........................ 7
3.5 Authentication Phase ............................ 8
3.6 Network-Layer Protocol Phase .................... 8
3.7 Link Termination Phase .......................... 9
4. The Option Negotiation Automaton ...................... 11
4.1 State Transition Table .......................... 12
4.2 States .......................................... 14
4.3 Events .......................................... 16
4.4 Actions ......................................... 21
4.5 Loop Avoidance .................................. 23
4.6 Counters and Timers ............................. 24
5. LCP Packet Formats .................................... 26
5.1 Configure-Request ............................... 28
5.2 Configure-Ack ................................... 29
5.3 Configure-Nak ................................... 30
5.4 Configure-Reject ................................ 31
5.5 Terminate-Request and Terminate-Ack ............. 33
5.6 Code-Reject ..................................... 34
5.7 Protocol-Reject ................................. 35
5.8 Echo-Request and Echo-Reply ..................... 36
5.9 Discard-Request ................................. 37
6. LCP Configuration Options ............................. 39
6.1 Maximum-Receive-Unit (MRU) ...................... 41
6.2 Authentication-Protocol ......................... 42
6.3 Quality-Protocol ................................ 43
6.4 Magic-Number .................................... 45
6.5 Protocol-Field-Compression (PFC) ................ 48
6.6 Address-and-Control-Field-Compression (ACFC)
SECURITY CONSIDERATIONS ...................................... 51
REFERENCES ................................................... 51
ACKNOWLEDGEMENTS ............................................. 51
CHAIR'S ADDRESS .............................................. 52
EDITOR'S ADDRESS ............................................. 52
Simpson [Page ii]
The Point-to-Point Protocol is designed for simple links which
transport packets between two peers. These links provide full-duplex
simultaneous bi-directional operation, and are assumed to deliver
packets in order. It is intended that PPP provide a common solution
for easy connection of a wide variety of hosts, bridges and routers
The PPP encapsulation provides for multiplexing of different
network-layer protocols simultaneously over the same link. The
PPP encapsulation has been carefully designed to retain
compatibility with most commonly used supporting hardware.
Only 8 additional octets are necessary to form the encapsulation
when used within the default HDLC-like framing. In environments
where bandwidth is at a premium, the encapsulation and framing may
be shortened to 2 or 4 octets.
To support high speed implementations, the default encapsulation
uses only simple fields, only one of which needs to be examined
for demultiplexing. The default header and information fields
fall on 32-bit boundaries, and the trailer may be padded to an
Link Control Protocol
In order to be sufficiently versatile to be portable to a wide
variety of environments, PPP provides a Link Control Protocol
(LCP). The LCP is used to automatically agree upon the
encapsulation format options, handle varying limits on sizes of
packets, detect a looped-back link and other common
misconfiguration errors, and terminate the link. Other optional
facilities provided are authentication of the identity of its peer
on the link, and determination when a link is functioning properly
and when it is failing.
Network Control Protocols
Point-to-Point links tend to exacerbate many problems with the
current family of network protocols. For instance, assignment and
management of IP addresses, which is a problem even in LAN
environments, is especially difficult over circuit-switched
point-to-point links (such as dial-up modem servers). These
problems are handled by a family of Network Control Protocols
(NCPs), which each manage the specific needs required by their
Simpson [Page 1]
respective network-layer protocols. These NCPs are defined in
It is intended that PPP links be easy to configure. By design,
the standard defaults handle all common configurations. The
implementor can specify improvements to the default configuration,
which are automatically communicated to the peer without operator
intervention. Finally, the operator may explicitly configure
options for the link which enable the link to operate in
environments where it would otherwise be impossible.
This self-configuration is implemented through an extensible
option negotiation mechanism, wherein each end of the link
describes to the other its capabilities and requirements.
Although the option negotiation mechanism described in this
document is specified in terms of the Link Control Protocol (LCP),
the same facilities are designed to be used by other control
protocols, especially the family of NCPs.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
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This document frequently uses the following terms:
datagram The unit of transmission in the network layer (such as IP).
A datagram may be encapsulated in one or more packets
passed to the data link layer.
frame The unit of transmission at the data link layer. A frame
may include a header and/or a trailer, along with some
number of units of data.
packet The basic unit of encapsulation, which is passed across the
interface between the network layer and the data link
layer. A packet is usually mapped to a frame; the
exceptions are when data link layer fragmentation is being
performed, or when multiple packets are incorporated into a
peer The other end of the point-to-point link.
The implementation discards the packet without further
processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
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2. PPP Encapsulation
The PPP encapsulation is used to disambiguate multiprotocol
datagrams. This encapsulation requires framing to indicate the
beginning and end of the encapsulation. Methods of providing framing
are specified in companion documents.
A summary of the PPP encapsulation is shown below. The fields are
transmitted from left to right.
| Protocol | Information | Padding |
| 8/16 bits| * | * |
The Protocol field is one or two octets, and its value identifies
the datagram encapsulated in the Information field of the packet.
The field is transmitted and received most significant octet
The structure of this field is consistent with the ISO 3309
extension mechanism for address fields. All Protocols MUST be
odd; the least significant bit of the least significant octet MUST
equal "1". Also, all Protocols MUST be assigned such that the
least significant bit of the most significant octet equals "0".
Frames received which don't comply with these rules MUST be
treated as having an unrecognized Protocol.
Protocol field values in the "0***" to "3***" range identify the
network-layer protocol of specific packets, and values in the
"8***" to "b***" range identify packets belonging to the
associated Network Control Protocols (NCPs), if any.
Protocol field values in the "4***" to "7***" range are used for
protocols with low volume traffic which have no associated NCP.
Protocol field values in the "c***" to "f***" range identify
packets as link-layer Control Protocols (such as LCP).
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Up-to-date values of the Protocol field are specified in the most
recent "Assigned Numbers" RFC . This specification reserves
the following values:
Value (in hex) Protocol Name
0001 Padding Protocol
0003 to 001f reserved (transparency inefficient)
007d reserved (Control Escape)
00cf reserved (PPP NLPID)
00ff reserved (compression inefficient)
8001 to 801f unused
c021 Link Control Protocol
c023 Password Authentication Protocol
c025 Link Quality Report
c223 Challenge Handshake Authentication Protocol
Developers of new protocols MUST obtain a number from the Internet
Assigned Numbers Authority (IANA), at IANA@isi.edu.
The Information field is zero or more octets. The Information
field contains the datagram for the protocol specified in the
The maximum length for the Information field, including Padding,
but not including the Protocol field, is termed the Maximum
Receive Unit (MRU), which defaults to 1500 octets. By
negotiation, consenting PPP implementations may use other values
for the MRU.
On transmission, the Information field MAY be padded with an
arbitrary number of octets up to the MRU. It is the
responsibility of each protocol to distinguish padding octets from
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3. PPP Link Operation
In order to establish communications over a point-to-point link, each
end of the PPP link MUST first send LCP packets to configure and test
the data link. After the link has been established, the peer MAY be
Then, PPP MUST send NCP packets to choose and configure one or more
network-layer protocols. Once each of the chosen network-layer
protocols has been configured, datagrams from each network-layer
protocol can be sent over the link.
The link will remain configured for communications until explicit LCP
or NCP packets close the link down, or until some external event
occurs (an inactivity timer expires or network administrator
3.2. Phase Diagram
In the process of configuring, maintaining and terminating the
point-to-point link, the PPP link goes through several distinct
phases which are specified in the following simplified state diagram:
+------+ +-----------+ +--------------+
| | UP | | OPENED | | SUCCESS/NONE
| Dead |------->| Establish |---------->| Authenticate |--+
| | | | | | |
+------+ +-----------+ +--------------+ |
^ | | |
| FAIL | FAIL | |
+<--------------+ +----------+ |
| | |
| +-----------+ | +---------+ |
| DOWN | | | CLOSING | | |
+------------| Terminate |<---+<----------| Network |<-+
| | | |
Not all transitions are specified in this diagram. The following
semantics MUST be followed.
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3.3. Link Dead (physical-layer not ready)
The link necessarily begins and ends with this phase. When an
external event (such as carrier detection or network administrator
configuration) indicates that the physical-layer is ready to be used,
PPP will proceed to the Link Establishment phase.
During this phase, the LCP automaton (described later) will be in the
Initial or Starting states. The transition to the Link Establishment
phase will signal an Up event to the LCP automaton.
Typically, a link will return to this phase automatically after
the disconnection of a modem. In the case of a hard-wired link,
this phase may be extremely short -- merely long enough to detect
the presence of the device.
3.4. Link Establishment Phase
The Link Control Protocol (LCP) is used to establish the connection
through an exchange of Configure packets. This exchange is complete,
and the LCP Opened state entered, once a Configure-Ack packet
(described later) has been both sent and received.
All Configuration Options are assumed to be at default values unless
altered by the configuration exchange. See the chapter on LCP
Configuration Options for further discussion.
It is important to note that only Configuration Options which are
independent of particular network-layer protocols are configured by
LCP. Configuration of individual network-layer protocols is handled
by separate Network Control Protocols (NCPs) during the Network-Layer
Any non-LCP packets received during this phase MUST be silently
The receipt of the LCP Configure-Request causes a return to the Link
Establishment phase from the Network-Layer Protocol phase or
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3.5. Authentication Phase
On some links it may be desirable to require a peer to authenticate
itself before allowing network-layer protocol packets to be
By default, authentication is not mandatory. If an implementation
desires that the peer authenticate with some specific authentication
protocol, then it MUST request the use of that authentication
protocol during Link Establishment phase.
Authentication SHOULD take place as soon as possible after link
establishment. However, link quality determination MAY occur
concurrently. An implementation MUST NOT allow the exchange of link
quality determination packets to delay authentication indefinitely.
Advancement from the Authentication phase to the Network-Layer
Protocol phase MUST NOT occur until authentication has completed. If
authentication fails, the authenticator SHOULD proceed instead to the
Link Termination phase.
Only Link Control Protocol, authentication protocol, and link quality
monitoring packets are allowed during this phase. All other packets
received during this phase MUST be silently discarded.
An implementation SHOULD NOT fail authentication simply due to
timeout or lack of response. The authentication SHOULD allow some
method of retransmission, and proceed to the Link Termination
phase only after a number of authentication attempts has been
The implementation responsible for commencing Link Termination
phase is the implementation which has refused authentication to
3.6. Network-Layer Protocol Phase
Once PPP has finished the previous phases, each network-layer
protocol (such as IP, IPX, or AppleTalk) MUST be separately
configured by the appropriate Network Control Protocol (NCP).
Each NCP MAY be Opened and Closed at any time.
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Because an implementation may initially use a significant amount
of time for link quality determination, implementations SHOULD
avoid fixed timeouts when waiting for their peers to configure a
After a NCP has reached the Opened state, PPP will carry the
corresponding network-layer protocol packets. Any supported
network-layer protocol packets received when the corresponding NCP is
not in the Opened state MUST be silently discarded.
While LCP is in the Opened state, any protocol packet which is
unsupported by the implementation MUST be returned in a Protocol-
Reject (described later). Only protocols which are supported are
During this phase, link traffic consists of any possible combination
of LCP, NCP, and network-layer protocol packets.
3.7. Link Termination Phase
PPP can terminate the link at any time. This might happen because of
the loss of carrier, authentication failure, link quality failure,
the expiration of an idle-period timer, or the administrative closing
of the link.
LCP is used to close the link through an exchange of Terminate
packets. When the link is closing, PPP informs the network-layer
protocols so that they may take appropriate action.
After the exchange of Terminate packets, the implementation SHOULD
signal the physical-layer to disconnect in order to enforce the
termination of the link, particularly in the case of an
authentication failure. The sender of the Terminate-Request SHOULD
disconnect after receiving a Terminate-Ack, or after the Restart
counter expires. The receiver of a Terminate-Request SHOULD wait for
the peer to disconnect, and MUST NOT disconnect until at least one
Restart time has passed after sending a Terminate-Ack. PPP SHOULD
proceed to the Link Dead phase.
Any non-LCP packets received during this phase MUST be silently
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The closing of the link by LCP is sufficient. There is no need
for each NCP to send a flurry of Terminate packets. Conversely,
the fact that one NCP has Closed is not sufficient reason to cause
the termination of the PPP link, even if that NCP was the only NCP
currently in the Opened state.
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4. The Option Negotiation Automaton
The finite-state automaton is defined by events, actions and state
transitions. Events include reception of external commands such as
Open and Close, expiration of the Restart timer, and reception of
packets from a peer. Actions include the starting of the Restart
timer and transmission of packets to the peer.
Some types of packets -- Configure-Naks and Configure-Rejects, or
Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
Discard-Requests -- are not differentiated in the automaton
descriptions. As will be described later, these packets do indeed
serve different functions. However, they always cause the same
Up = lower layer is Up tlu = This-Layer-Up
Down = lower layer is Down tld = This-Layer-Down
Open = administrative Open tls = This-Layer-Started
Close= administrative Close tlf = This-Layer-Finished
TO+ = Timeout with counter > 0 irc = Initialize-Restart-Count
TO- = Timeout with counter expired zrc = Zero-Restart-Count
RCR+ = Receive-Configure-Request (Good) scr = Send-Configure-Request
RCR- = Receive-Configure-Request (Bad)
RCA = Receive-Configure-Ack sca = Send-Configure-Ack
RCN = Receive-Configure-Nak/Rej scn = Send-Configure-Nak/Rej
RTR = Receive-Terminate-Request str = Send-Terminate-Request
RTA = Receive-Terminate-Ack sta = Send-Terminate-Ack
RUC = Receive-Unknown-Code scj = Send-Code-Reject
RXJ+ = Receive-Code-Reject (permitted)
RXJ- = Receive-Code-Reject (catastrophic)
RXR = Receive-Echo-Request ser = Send-Echo-Reply
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4.1. State Transition Table
The complete state transition table follows. States are indicated
horizontally, and events are read vertically. State transitions and
actions are represented in the form action/new-state. Multiple
actions are separated by commas, and may continue on succeeding lines
as space requires; multiple actions may be implemented in any
convenient order. The state may be followed by a letter, which
indicates an explanatory footnote. The dash ('-') indicates an
| 0 1 2 3 4 5
Events| Initial Starting Closed Stopped Closing Stopping
Up | 2 irc,scr/6 - - - -
Down | - - 0 tls/1 0 1
Open | tls/1 1 irc,scr/6 3r 5r 5r
Close| 0 tlf/0 2 2 4 4
TO+ | - - - - str/4 str/5
TO- | - - - - tlf/2 tlf/3
RCR+ | - - sta/2 irc,scr,sca/8 4 5
RCR- | - - sta/2 irc,scr,scn/6 4 5
RCA | - - sta/2 sta/3 4 5
RCN | - - sta/2 sta/3 4 5
RTR | - - sta/2 sta/3 sta/4 sta/5
RTA | - - 2 3 tlf/2 tlf/3
RUC | - - scj/2 scj/3 scj/4 scj/5
RXJ+ | - - 2 3 4 5
RXJ- | - - tlf/2 tlf/3 tlf/2 tlf/3
RXR | - - 2 3 4 5
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| 6 7 8 9
Events| Req-Sent Ack-Rcvd Ack-Sent Opened
Up | - - - -
Down | 1 1 1 tld/1
Open | 6 7 8 9r
Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
TO+ | scr/6 scr/6 scr/8 -
TO- | tlf/3p tlf/3p tlf/3p -
RCR+ | sca/8 sca,tlu/9 sca/8 tld,scr,sca/8
RCR- | scn/6 scn/7 scn/6 tld,scr,scn/6
RCA | irc/7 scr/6x irc,tlu/9 tld,scr/6x
RCN |irc,scr/6 scr/6x irc,scr/8 tld,scr/6x
RTR | sta/6 sta/6 sta/6 tld,zrc,sta/5
RTA | 6 6 8 tld,scr/6
RUC | scj/6 scj/7 scj/8 scj/9
RXJ+ | 6 6 8 9
RXJ- | tlf/3 tlf/3 tlf/3 tld,irc,str/5
RXR | 6 7 8 ser/9
The states in which the Restart timer is running are identifiable by
the presence of TO events. Only the Send-Configure-Request, Send-
Terminate-Request and Zero-Restart-Count actions start or re-start
the Restart timer. The Restart timer is stopped when transitioning
from any state where the timer is running to a state where the timer
is not running.
The events and actions are defined according to a message passing
architecture, rather than a signalling architecture. If an action is
desired to control specific signals (such as DTR), additional actions
are likely to be required.
[p] Passive option; see Stopped state discussion.
[r] Restart option; see Open event discussion.
[x] Crossed connection; see RCA event discussion.
Simpson [Page 13]
Following is a more detailed description of each automaton state.
In the Initial state, the lower layer is unavailable (Down), and
no Open has occurred. The Restart timer is not running in the
The Starting state is the Open counterpart to the Initial state.
An administrative Open has been initiated, but the lower layer is
still unavailable (Down). The Restart timer is not running in the
When the lower layer becomes available (Up), a Configure-Request
In the Closed state, the link is available (Up), but no Open has
occurred. The Restart timer is not running in the Closed state.
Upon reception of Configure-Request packets, a Terminate-Ack is
sent. Terminate-Acks are silently discarded to avoid creating a
The Stopped state is the Open counterpart to the Closed state. It
is entered when the automaton is waiting for a Down event after
the This-Layer-Finished action, or after sending a Terminate-Ack.
The Restart timer is not running in the Stopped state.
Upon reception of Configure-Request packets, an appropriate
response is sent. Upon reception of other packets, a Terminate-
Ack is sent. Terminate-Acks are silently discarded to avoid
creating a loop.
The Stopped state is a junction state for link termination,
link configuration failure, and other automaton failure modes.
These potentially separate states have been combined.
There is a race condition between the Down event response (from
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the This-Layer-Finished action) and the Receive-Configure-
Request event. When a Configure-Request arrives before the
Down event, the Down event will supercede by returning the
automaton to the Starting state. This prevents attack by
After the peer fails to respond to Configure-Requests, an
implementation MAY wait passively for the peer to send
Configure-Requests. In this case, the This-Layer-Finished
action is not used for the TO- event in states Req-Sent, Ack-
Rcvd and Ack-Sent.
This option is useful for dedicated circuits, or circuits which
have no status signals available, but SHOULD NOT be used for
In the Closing state, an attempt is made to terminate the
connection. A Terminate-Request has been sent and the Restart
timer is running, but a Terminate-Ack has not yet been received.
Upon reception of a Terminate-Ack, the Closed state is entered.
Upon the expiration of the Restart timer, a new Terminate-Request
is transmitted, and the Restart timer is restarted. After the
Restart timer has expired Max-Terminate times, the Closed state is
The Stopping state is the Open counterpart to the Closing state.
A Terminate-Request has been sent and the Restart timer is
running, but a Terminate-Ack has not yet been received.
The Stopping state provides a well defined opportunity to
terminate a link before allowing new traffic. After the link
has terminated, a new configuration may occur via the Stopped
or Starting states.
In the Request-Sent state an attempt is made to configure the
connection. A Configure-Request has been sent and the Restart
timer is running, but a Configure-Ack has not yet been received
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nor has one been sent.
In the Ack-Received state, a Configure-Request has been sent and a
Configure-Ack has been received. The Restart timer is still
running, since a Configure-Ack has not yet been sent.
In the Ack-Sent state, a Configure-Request and a Configure-Ack
have both been sent, but a Configure-Ack has not yet been
received. The Restart timer is running, since a Configure-Ack has
not yet been received.
In the Opened state, a Configure-Ack has been both sent and
received. The Restart timer is not running.
When entering the Opened state, the implementation SHOULD signal
the upper layers that it is now Up. Conversely, when leaving the
Opened state, the implementation SHOULD signal the upper layers
that it is now Down.
Transitions and actions in the automaton are caused by events.
This event occurs when a lower layer indicates that it is ready to
Typically, this event is used by a modem handling or calling
process, or by some other coupling of the PPP link to the physical
media, to signal LCP that the link is entering Link Establishment
It also can be used by LCP to signal each NCP that the link is
entering Network-Layer Protocol phase. That is, the This-Layer-Up
action from LCP triggers the Up event in the NCP.
This event occurs when a lower layer indicates that it is no
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longer ready to carry packets.
Typically, this event is used by a modem handling or calling
process, or by some other coupling of the PPP link to the physical
media, to signal LCP that the link is entering Link Dead phase.
It also can be used by LCP to signal each NCP that the link is
leaving Network-Layer Protocol phase. That is, the This-Layer-
Down action from LCP triggers the Down event in the NCP.
This event indicates that the link is administratively available
for traffic; that is, the network administrator (human or program)
has indicated that the link is allowed to be Opened. When this
event occurs, and the link is not in the Opened state, the
automaton attempts to send configuration packets to the peer.
If the automaton is not able to begin configuration (the lower
layer is Down, or a previous Close event has not completed), the
establishment of the link is automatically delayed.
When a Terminate-Request is received, or other events occur which
cause the link to become unavailable, the automaton will progress
to a state where the link is ready to re-open. No additional
administrative intervention is necessary.
Experience has shown that users will execute an additional Open
command when they want to renegotiate the link. This might
indicate that new values are to be negotiated.
Since this is not the meaning of the Open event, it is
suggested that when an Open user command is executed in the
Opened, Closing, Stopping, or Stopped states, the
implementation issue a Down event, immediately followed by an
Up event. Care must be taken that an intervening Down event
cannot occur from another source.
The Down followed by an Up will cause an orderly renegotiation
of the link, by progressing through the Starting to the
Request-Sent state. This will cause the renegotiation of the
link, without any harmful side effects.
This event indicates that the link is not available for traffic;
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that is, the network administrator (human or program) has
indicated that the link is not allowed to be Opened. When this
event occurs, and the link is not in the Closed state, the
automaton attempts to terminate the connection. Futher attempts
to re-configure the link are denied until a new Open event occurs.
When authentication fails, the link SHOULD be terminated, to
prevent attack by repetition and denial of service to other
users. Since the link is administratively available (by
definition), this can be accomplished by simulating a Close
event to the LCP, immediately followed by an Open event. Care
must be taken that an intervening Close event cannot occur from
The Close followed by an Open will cause an orderly termination
of the link, by progressing through the Closing to the Stopping
state, and the This-Layer-Finished action can disconnect the
link. The automaton waits in the Stopped or Starting states
for the next connection attempt.
This event indicates the expiration of the Restart timer. The
Restart timer is used to time responses to Configure-Request and
The TO+ event indicates that the Restart counter continues to be
greater than zero, which triggers the corresponding Configure-
Request or Terminate-Request packet to be retransmitted.
The TO- event indicates that the Restart counter is not greater
than zero, and no more packets need to be retransmitted.
This event occurs when a Configure-Request packet is received from
the peer. The Configure-Request packet indicates the desire to
open a connection and may specify Configuration Options. The
Configure-Request packet is more fully described in a later
The RCR+ event indicates that the Configure-Request was
acceptable, and triggers the transmission of a corresponding
The RCR- event indicates that the Configure-Request was
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unacceptable, and triggers the transmission of a corresponding
Configure-Nak or Configure-Reject.
These events may occur on a connection which is already in the
Opened state. The implementation MUST be prepared to
immediately renegotiate the Configuration Options.
This event occurs when a valid Configure-Ack packet is received
from the peer. The Configure-Ack packet is a positive response to
a Configure-Request packet. An out of sequence or otherwise
invalid packet is silently discarded.
Since the correct packet has already been received before
reaching the Ack-Rcvd or Opened states, it is extremely
unlikely that another such packet will arrive. As specified,
all invalid Ack/Nak/Rej packets are silently discarded, and do
not affect the transitions of the automaton.
However, it is not impossible that a correctly formed packet
will arrive through a coincidentally-timed cross-connection.
It is more likely to be the result of an implementation error.
At the very least, this occurance SHOULD be logged.
This event occurs when a valid Configure-Nak or Configure-Reject
packet is received from the peer. The Configure-Nak and
Configure-Reject packets are negative responses to a Configure-
Request packet. An out of sequence or otherwise invalid packet is
Although the Configure-Nak and Configure-Reject cause the same
state transition in the automaton, these packets have
significantly different effects on the Configuration Options
sent in the resulting Configure-Request packet.
This event occurs when a Terminate-Request packet is received.
The Terminate-Request packet indicates the desire of the peer to
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close the connection.
This event is not identical to the Close event (see above), and
does not override the Open commands of the local network
administrator. The implementation MUST be prepared to receive
a new Configure-Request without network administrator
This event occurs when a Terminate-Ack packet is received from the
peer. The Terminate-Ack packet is usually a response to a
Terminate-Request packet. The Terminate-Ack packet may also
indicate that the peer is in Closed or Stopped states, and serves
to re-synchronize the link configuration.
This event occurs when an un-interpretable packet is received from
the peer. A Code-Reject packet is sent in response.
Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
This event occurs when a Code-Reject or a Protocol-Reject packet
is received from the peer.
The RXJ+ event arises when the rejected value is acceptable, such
as a Code-Reject of an extended code, or a Protocol-Reject of a
NCP. These are within the scope of normal operation. The
implementation MUST stop sending the offending packet type.
The RXJ- event arises when the rejected value is catastrophic,
such as a Code-Reject of Configure-Request, or a Protocol-Reject
of LCP! This event communicates an unrecoverable error that
terminates the connection.
Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
This event occurs when an Echo-Request, Echo-Reply or Discard-
Request packet is received from the peer. The Echo-Reply packet
is a response to an Echo-Request packet. There is no reply to an
Echo-Reply or Discard-Request packet.
Simpson [Page 20]
Nak, receive Configure-Nak) will repeat over and over again. If
the link is not looped-back, this sequence might occur a few
times, but it is extremely unlikely to occur repeatedly. More
likely, the Magic-Numbers chosen at either end will quickly
diverge, terminating the sequence. The following table shows the
probability of collisions assuming that both ends of the link
select Magic-Numbers with a perfectly uniform distribution:
Number of Collisions Probability
1 1/2**32 = 2.3 E-10
2 1/2**32**2 = 5.4 E-20
3 1/2**32**3 = 1.3 E-29
Good sources of uniqueness or randomness are required for this
divergence to occur. If a good source of uniqueness cannot be
found, it is recommended that this Configuration Option not be
enabled; Configure-Requests with the option SHOULD NOT be
transmitted and any Magic-Number Configuration Options which the
peer sends SHOULD be either acknowledged or rejected. In this
case, looped-back links cannot be reliably detected by the
implementation, although they may still be detectable by the peer.
If an implementation does transmit a Configure-Request with a
Magic-Number Configuration Option, then it MUST NOT respond with a
Configure-Reject when it receives a Configure-Request with a
Magic-Number Configuration Option. That is, if an implementation
desires to use Magic Numbers, then it MUST also allow its peer to
do so. If an implementation does receive a Configure-Reject in
response to a Configure-Request, it can only mean that the link is
not looped-back, and that its peer will not be using Magic-
Numbers. In this case, an implementation SHOULD act as if the
negotiation had been successful (as if it had instead received a
The Magic-Number also may be used to detect looped-back links
during normal operation, as well as during Configuration Option
negotiation. All LCP Echo-Request, Echo-Reply, and Discard-
Request packets have a Magic-Number field. If Magic-Number has
been successfully negotiated, an implementation MUST transmit
these packets with the Magic-Number field set to its negotiated
The Magic-Number field of these packets SHOULD be inspected on
reception. All received Magic-Number fields MUST be equal to
either zero or the peer's unique Magic-Number, depending on
whether or not the peer negotiated a Magic-Number.
Simpson [Page 46]
Reception of a Magic-Number field equal to the negotiated local
Magic-Number indicates a looped-back link. Reception of a Magic-
Number other than the negotiated local Magic-Number, the peer's
negotiated Magic-Number, or zero if the peer didn't negotiate one,
indicates a link which has been (mis)configured for communications
with a different peer.
Procedures for recovery from either case are unspecified, and may
vary from implementation to implementation. A somewhat
pessimistic procedure is to assume a LCP Down event. A further
Open event will begin the process of re-establishing the link,
which can't complete until the looped-back condition is
terminated, and Magic-Numbers are successfully negotiated. A more
optimistic procedure (in the case of a looped-back link) is to
begin transmitting LCP Echo-Request packets until an appropriate
Echo-Reply is received, indicating a termination of the looped-
A summary of the Magic-Number Configuration Option format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length | Magic-Number
Magic-Number (cont) |
The Magic-Number field is four octets, and indicates a number
which is very likely to be unique to one end of the link. A
Magic-Number of zero is illegal and MUST always be Nak'd, if it is
not Rejected outright.
Simpson [Page 47]
6.5. Protocol-Field-Compression (PFC)
This Configuration Option provides a method to negotiate the
compression of the PPP Protocol field. By default, all
implementations MUST transmit packets with two octet PPP Protocol
PPP Protocol field numbers are chosen such that some values may be
compressed into a single octet form which is clearly
distinguishable from the two octet form. This Configuration
Option is sent to inform the peer that the implementation can
receive such single octet Protocol fields.
As previously mentioned, the Protocol field uses an extension
mechanism consistent with the ISO 3309 extension mechanism for the
Address field; the Least Significant Bit (LSB) of each octet is
used to indicate extension of the Protocol field. A binary "0" as
the LSB indicates that the Protocol field continues with the
following octet. The presence of a binary "1" as the LSB marks
the last octet of the Protocol field. Notice that any number of
"0" octets may be prepended to the field, and will still indicate
the same value (consider the two binary representations for 3,
00000011 and 00000000 00000011).
When using low speed links, it is desirable to conserve bandwidth
by sending as little redundant data as possible. The Protocol-
Field-Compression Configuration Option allows a trade-off between
implementation simplicity and bandwidth efficiency. If
successfully negotiated, the ISO 3309 extension mechanism may be
used to compress the Protocol field to one octet instead of two.
The large majority of packets are compressible since data
protocols are typically assigned with Protocol field values less
Compressed Protocol fields MUST NOT be transmitted unless this
Configuration Option has been negotiated. When negotiated, PPP
implementations MUST accept PPP packets with either double-octet
or single-octet Protocol fields, and MUST NOT distinguish between
The Protocol field is never compressed when sending any LCP
packet. This rule guarantees unambiguous recognition of LCP
When a Protocol field is compressed, the Data Link Layer FCS field
is calculated on the compressed frame, not the original
Simpson [Page 48]
A summary of the Protocol-Field-Compression Configuration Option
format is shown below. The fields are transmitted from left to
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
| Type | Length |
Simpson [Page 49]
6.6. Address-and-Control-Field-Compression (ACFC)
This Configuration Option provides a method to negotiate the
compression of the Data Link Layer Address and Control fields. By
default, all implementations MUST transmit frames with Address and
Control fields appropriate to the link framing.
Since these fields usually have constant values for point-to-point
links, they are easily compressed. This Configuration Option is
sent to inform the peer that the implementation can receive
compressed Address and Control fields.
If a compressed frame is received when Address-and-Control-Field-
Compression has not been negotiated, the implementation MAY
silently discard the frame.
The Address and Control fields MUST NOT be compressed when sending
any LCP packet. This rule guarantees unambiguous recognition of
When the Address and Control fields are compressed, the Data Link
Layer FCS field is calculated on the compressed frame, not the
original uncompressed frame.
A summary of the Address-and-Control-Field-Compression configuration
option format is shown below. The fields are transmitted from left
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
| Type | Length |
Simpson [Page 50]
Security issues are briefly discussed in sections concerning the
Authentication Phase, the Close event, and the Authentication-
Protocol Configuration Option.
 Perkins, D., "Requirements for an Internet Standard Point-to-
Point Protocol", RFC 1547, Carnegie Mellon University,
 Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.
This document is the product of the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). Comments should
be submitted to the email@example.com mailing list.
Much of the text in this document is taken from the working group
requirements ; and RFCs 1171 & 1172, by Drew Perkins while at
Carnegie Mellon University, and by Russ Hobby of the University of
California at Davis.
William Simpson was principally responsible for introducing
consistent terminology and philosophy, and the re-design of the phase
and negotiation state machines.
Many people spent significant time helping to develop the Point-to-
Point Protocol. The complete list of people is too numerous to list,
but the following people deserve special thanks: Rick Adams, Ken
Adelman, Fred Baker, Mike Ballard, Craig Fox, Karl Fox, Phill Gross,
Kory Hamzeh, former WG chair Russ Hobby, David Kaufman, former WG
chair Steve Knowles, Mark Lewis, former WG chair Brian Lloyd, John
LoVerso, Bill Melohn, Mike Patton, former WG chair Drew Perkins, Greg
Satz, John Shriver, Vernon Schryver, and Asher Waldfogel.
Special thanks to Morning Star Technologies for providing computing
resources and network access support for writing this specification.
Simpson [Page 51]
The working group can be contacted via the current chair:
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California 93117
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