Network Working Group W. Simpson, Editor
Request for Comments: 1661 Daydreamer
STD: 51 July 1994
Obsoletes: 1548
Category: Standards Track

The Point-to-Point Protocol (PPP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

The Point-to-Point Protocol (PPP) provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
is comprised of three main components:

1. A method for encapsulating multi-protocol datagrams.

2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.

3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network-layer protocols.

This document defines the PPP organization and methodology, and the
PPP encapsulation, together with an extensible option negotiation
mechanism which is able to negotiate a rich assortment of
configuration parameters and provides additional management
functions. The PPP Link Control Protocol (LCP) is described in terms
of this mechanism.

Table of Contents

1. IntrodUCtion .......................................... 1
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3

2. PPP Encapsulation ..................................... 4

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

1. Introduction

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
[1].

Encapsulation

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
arbitrary boundary.

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

respective network-layer protocols. These NCPs are defined in
companion documents.

Configuration

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
different course.

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.

1.2. Terminology

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
single frame.

peer The other end of the point-to-point link.

silently discard
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.

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 * *
+----------+-------------+---------+

Protocol Field

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
first.

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).

Up-to-date values of the Protocol field are specified in the most
recent "Assigned Numbers" RFC[2]. 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
807d unused
80cf unused
80ff 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.

Information Field

The Information field is zero or more octets. The Information
field contains the datagram for the protocol specified in the
Protocol field.

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.

Padding

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
real information.

3. PPP Link Operation

3.1. Overview

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
authenticated.

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
intervention).

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.

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.

Implementation Note:

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
Protocol phase.

Any non-LCP packets received during this phase MUST be silently
discarded.

The receipt of the LCP Configure-Request causes a return to the Link
Establishment phase from the Network-Layer Protocol phase or
Authentication phase.

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
exchanged.

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.

Implementation Notes:

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
exceeded.

The implementation responsible for commencing Link Termination
phase is the implementation which has refused authentication to
its peer.

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.

Implementation Note:

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
NCP.

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.

Implementation Note:

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
silently discarded.

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
discarded.

Implementation Note:

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.

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
transitions.

Events Actions

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)
or Receive-Protocol-Reject
RXJ- = Receive-Code-Reject (catastrophic)
or Receive-Protocol-Reject
RXR = Receive-Echo-Request ser = Send-Echo-Reply
or Receive-Echo-Reply
or Receive-Discard-Request

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
illegal transition.

State
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

State
6 7 8 9
Events Req-Sent Ack-Rcvd Ack-Sent Opened
------+-----------------------------------------
Up - - - -
Down 1 1 1 tld/1
Open 6 7 8 9r
Closeirc,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.

4.2. States

Following is a more detailed description of each automaton state.

Initial

In the Initial state, the lower layer is unavailable (Down), and
no Open has occurred. The Restart timer is not running in the
Initial state.

Starting

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
Starting state.

When the lower layer becomes available (Up), a Configure-Request
is sent.

Closed

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
loop.

Stopped

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.

Rationale:

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

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
repetition.

Implementation Option:

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
switched circuits.

Closing

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
entered.

Stopping

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.

Rationale:

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.

Request-Sent

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

nor has one been sent.

Ack-Received

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.

Ack-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.

Opened

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.

4.3. Events

Transitions and actions in the automaton are caused by events.

Up

This event occurs when a lower layer indicates that it is 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 Establishment
phase.

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.

Down

This event occurs when a lower layer indicates that it is no

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.

Open

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.

Implementation Option:

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.

Close

This event indicates that the link is not available for traffic;

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.

Implementation Note:

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
another source.

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.

Timeout (TO+,TO-)

This event indicates the expiration of the Restart timer. The
Restart timer is used to time responses to Configure-Request and
Terminate-Request packets.

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.

Receive-Configure-Request (RCR+,RCR-)

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
section.

The RCR+ event indicates that the Configure-Request was
acceptable, and triggers the transmission of a corresponding
Configure-Ack.

The RCR- event indicates that the Configure-Request was

unacceptable, and triggers the transmission of a corresponding
Configure-Nak or Configure-Reject.

Implementation Note:

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.

Receive-Configure-Ack (RCA)

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.

Implementation Note:

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.

Receive-Configure-Nak/Rej (RCN)

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
silently discarded.

Implementation Note:

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.

Receive-Terminate-Request (RTR)

This event occurs when a Terminate-Request packet is received.
The Terminate-Request packet indicates the desire of the peer to

close the connection.

Implementation Note:

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
intervention.

Receive-Terminate-Ack (RTA)

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.

Receive-Unknown-Code (RUC)

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
(RXR)

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.

4.4. Actions

Actions in the automaton are caused by events and typically indicate
the transmission of packets and/or the starting or stopping of the
Restart timer.

Illegal-Event (-)

This indicates an event that cannot occur in a properly
implemented automaton. The implementation has an internal error,
which should be reported and logged. No transition is taken, and
the implementation SHOULD NOT reset or freeze.

This-Layer-Up (tlu)

This action indicates to the upper layers that the automaton is
entering the Opened state.

Typically, this action is used by the LCP to signal the Up event
to a NCP, Authentication Protocol, or Link Quality Protocol, or
MAY be used by a NCP to indicate that the link is available for
its network layer traffic.

This-Layer-Down (tld)

This action indicates to the upper layers that the automaton is
leaving the Opened state.

Typically, this action is used by the LCP to signal the Down event
to a NCP, Authentication Protocol, or Link Quality Protocol, or
MAY be used by a NCP to indicate that the link is no longer
available for its network layer traffic.

This-Layer-Started (tls)

This action indicates to the lower layers that the automaton is
entering the Starting state, and the lower layer is needed for the
link. The lower layer SHOULD respond with an Up event when the
lower layer is available.

This results of this action are highly implementation dependent.

This-Layer-Finished (tlf)

This action indicates to the lower layers that the automaton is
entering the Initial, Closed or Stopped states, and the lower
layer is no longer needed for the link. The lower layer SHOULD
respond with a Down event when the lower layer has terminated.

Typically, this action MAY be used by the LCP to advance to the
Link Dead phase, or MAY be used by a NCP to indicate to the LCP
that the link may terminate when there are no other NCPs open.

This results of this action are highly implementation dependent.

Initialize-Restart-Count (irc)

This action sets the Restart counter to the appropriate value
(Max-Terminate or Max-Configure). The counter is decremented for
each transmission, including the first.

Implementation Note:

In addition to setting the Restart counter, the implementation
MUST set the timeout period to the initial value when Restart
timer bacKOFf is used.

Zero-Restart-Count (zrc)

This action sets the Restart counter to zero.

Implementation Note:

This action enables the FSA to pause before proceeding to the
desired final state, allowing traffic to be processed by the
peer. In addition to zeroing the Restart counter, the
implementation MUST set the timeout period to an appropriate
value.

Send-Configure-Request (scr)

A Configure-Request packet is transmitted. This indicates the
desire to open a connection with a specified set of Configuration
Options. The Restart timer is started when the Configure-Request
packet is transmitted, to guard against packet loss. The Restart
counter is decremented each time a Configure-Request is sent.

Send-Configure-Ack (sca)

A Configure-Ack packet is transmitted. This acknowledges the
reception of a Configure-Request packet with an acceptable set of
Configuration Options.

Send-Configure-Nak (scn)

A Configure-Nak or Configure-Reject packet is transmitted, as
appropriate. This negative response reports the reception of a

Configure-Request packet with an unacceptable set of Configuration
Options.

Configure-Nak packets are used to refuse a Configuration Option
value, and to suggest a new, acceptable value. Configure-Reject
packets are used to refuse all negotiation about a Configuration
Option, typically because it is not recognized or implemented.
The use of Configure-Nak versus Configure-Reject is more fully
described in the chapter on LCP Packet Formats.

Send-Terminate-Request (str)

A Terminate-Request packet is transmitted. This indicates the
desire to close a connection. The Restart timer is started when
the Terminate-Request packet is transmitted, to guard against
packet loss. The Restart counter is decremented each time a
Terminate-Request is sent.

Send-Terminate-Ack (sta)

A Terminate-Ack packet is transmitted. This acknowledges the
reception of a Terminate-Request packet or otherwise serves to
synchronize the automatons.

Send-Code-Reject (scj)

A Code-Reject packet is transmitted. This indicates the reception
of an unknown type of packet.

Send-Echo-Reply (ser)

An Echo-Reply packet is transmitted. This acknowledges the
reception of an Echo-Request packet.

4.5. Loop Avoidance

The protocol makes a reasonable attempt at avoiding Configuration
Option negotiation loops. However, the protocol does NOT guarantee
that loops will not happen. As with any negotiation, it is possible
to configure two PPP implementations with conflicting policies that
will never converge. It is also possible to configure policies which
do converge, but which take significant time to do so. Implementors
should keep this in mind and SHOULD implement loop detection
mechanisms or higher level timeouts.

4.6. Counters and Timers

Restart Timer

There is one special timer used by the automaton. The Restart
timer is used to time transmissions of Configure-Request and
Terminate-Request packets. Expiration of the Restart timer causes
a Timeout event, and retransmission of the corresponding
Configure-Request or Terminate-Request packet. The Restart timer
MUST be configurable, but SHOULD default to three (3) seconds.

Implementation Note:

The Restart timer SHOULD be based on the speed of the link.
The default value is designed for low speed (2,400 to 9,600
bps), high switching latency links (typical telephone lines).
Higher speed links, or links with low switching latency, SHOULD
have correspondingly faster retransmission times.

Instead of a constant value, the Restart timer MAY begin at an
initial small value and increase to the configured final value.
Each successive value less than the final value SHOULD be at
least twice the previous value. The initial value SHOULD be
large enough to account for the size of the packets, twice the
round trip time for transmission at the link speed, and at
least an additional 100 milliseconds to allow the peer to
process the packets before responding. Some circuits add
another 200 milliseconds of satellite delay. Round trip times
for modems operating at 14,400 bps have been measured in the
range of 160 to more than 600 milliseconds.

Max-Terminate

There is one required restart counter for Terminate-Requests.
Max-Terminate indicates the number of Terminate-Request packets
sent without receiving a Terminate-Ack before assuming that the
peer is unable to respond. Max-Terminate MUST be configurable,
but SHOULD default to two (2) transmissions.

Max-Configure

A similar counter is recommended for Configure-Requests. Max-
Configure indicates the number of Configure-Request packets sent
without receiving a valid Configure-Ack, Configure-Nak or
Configure-Reject before assuming that the peer is unable to
respond. Max-Configure MUST be configurable, but SHOULD default
to ten (10) transmissions.

Max-Failure

A related counter is recommended for Configure-Nak. Max-Failure
indicates the number of Configure-Nak packets sent without sending
a Configure-Ack before assuming that configuration is not
converging. Any further Configure-Nak packets for peer requested
options are converted to Configure-Reject packets, and locally
desired options are no longer appended. Max-Failure MUST be
configurable, but SHOULD default to five (5) transmissions.

5. LCP Packet Formats

There are three classes of LCP packets:

1. Link Configuration packets used to establish and configure a
link (Configure-Request, Configure-Ack, Configure-Nak and
Configure-Reject).

2. Link Termination packets used to terminate a link (Terminate-
Request and Terminate-Ack).

3. Link Maintenance packets used to manage and debug a link
(Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
Discard-Request).

In the interest of simplicity, there is no version field in the LCP
packet. A correctly functioning LCP implementation will always
respond to unknown Protocols and Codes with an easily recognizable
LCP packet, thus providing a deterministic fallback mechanism for
implementations of other versions.

Regardless of which Configuration Options are enabled, all LCP Link
Configuration, Link Termination, and Code-Reject packets (codes 1
through 7) are always sent as if no Configuration Options were
negotiated. In particular, each Configuration Option specifies a
default value. This ensures that such LCP packets are always
recognizable, even when one end of the link mistakenly believes the
link to be open.

Exactly one LCP packet is encapsulated in the PPP Information field,
where the PPP Protocol field indicates type hex c021 (Link Control
Protocol).

A summary of the Link Control Protocol packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data ...
+-+-+-+-+

Code

The Code field is one octet, and identifies the kind of LCP

packet. When a packet is received with an unknown Code field, a
Code-Reject packet is transmitted.

Up-to-date values of the LCP Code field are specified in the most
recent "Assigned Numbers" RFC[2]. This document concerns the
following values:

1 Configure-Request
2 Configure-Ack
3 Configure-Nak
4 Configure-Reject
5 Terminate-Request
6 Terminate-Ack
7 Code-Reject
8 Protocol-Reject
9 Echo-Request
10 Echo-Reply
11 Discard-Request

Identifier

The Identifier field is one octet, and aids in matching requests
and replies. When a packet is received with an invalid Identifier
field, the packet is silently discarded without affecting the
automaton.

Length

The Length field is two octets, and indicates the length of the
LCP packet, including the Code, Identifier, Length and Data
fields. The Length MUST NOT exceed the MRU of the link.

Octets outside the range of the Length field are treated as
padding and are ignored on reception. When a packet is received
with an invalid Length field, the packet is silently discarded
without affecting the automaton.

Data

The Data field is zero or more octets, as indicated by the Length
field. The format of the Data field is determined by the Code
field.

5.1. Configure-Request

Description

An implementation wishing to open a connection MUST transmit a
Configure-Request. The Options field is filled with any desired
changes to the link defaults. Configuration Options SHOULD NOT be
included with default values.

Upon reception of a Configure-Request, an appropriate reply MUST
be transmitted.

A summary of the Configure-Request packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Options ...
+-+-+-+-+

Code

1 for Configure-Request.

Identifier

The Identifier field MUST be changed whenever the contents of the
Options field changes, and whenever a valid reply has been
received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.

Options

The options field is variable in length, and contains the list of
zero or more Configuration Options that the sender desires to
negotiate. All Configuration Options are always negotiated
simultaneously. The format of Configuration Options is further
described in a later chapter.

5.2. Configure-Ack

Description

If every Configuration Option received in a Configure-Request is
recognizable and all values are acceptable, then the
implementation MUST transmit a Configure-Ack. The acknowledged
Configuration Options MUST NOT be reordered or modified in any
way.

On reception of a Configure-Ack, the Identifier field MUST match
that of the last transmitted Configure-Request. Additionally, the
Configuration Options in a Configure-Ack MUST exactly match those
of the last transmitted Configure-Request. Invalid packets are
silently discarded.

A summary of the Configure-Ack packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Options ...
+-+-+-+-+

Code

2 for Configure-Ack.

Identifier

The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Ack.

Options

The Options field is variable in length, and contains the list of
zero or more Configuration Options that the sender is
acknowledging. All Configuration Options are always acknowledged
simultaneously.

5.3. Configure-Nak

Description

If every instance of the received Configuration Options is
recognizable, but some values are not acceptable, then the
implementation MUST transmit a Configure-Nak. The Options field
is filled with only the unacceptable Configuration Options from
the Configure-Request. All acceptable Configuration Options are
filtered out of the Configure-Nak, but otherwise the Configuration
Options from the Configure-Request MUST NOT be reordered.

Options which have no value fields (boolean options) MUST use the
Configure-Reject reply instead.

Each Configuration Option which is allowed only a single instance
MUST be modified to a value acceptable to the Configure-Nak
sender. The default value MAY be used, when this differs from the
requested value.

When a particular type of Configuration Option can be listed more
than once with different values, the Configure-Nak MUST include a
list of all values for that option which are acceptable to the
Configure-Nak sender. This includes acceptable values that were
present in the Configure-Request.

Finally, an implementation may be configured to request the
negotiation of a specific Configuration Option. If that option is
not listed, then that option MAY be appended to the list of Nak'd
Configuration Options, in order to prompt the peer to include that
option in its next Configure-Request packet. Any value fields for
the option MUST indicate values acceptable to the Configure-Nak
sender.

On reception of a Configure-Nak, the Identifier field MUST match
that of the last transmitted Configure-Request. Invalid packets
are silently discarded.

Reception of a valid Configure-Nak indicates that when a new
Configure-Request is sent, the Configuration Options MAY be
modified as specified in the Configure-Nak. When multiple
instances of a Configuration Option are present, the peer SHOULD
select a single value to include in its next Configure-Request
packet.

Some Configuration Options have a variable length. Since the
Nak'd Option has been modified by the peer, the implementation
MUST be able to handle an Option length which is different from

the original Configure-Request.

A summary of the Configure-Nak packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Options ...
+-+-+-+-+

Code

3 for Configure-Nak.

Identifier

The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Nak.

Options

The Options field is variable in length, and contains the list of
zero or more Configuration Options that the sender is Nak'ing.
All Configuration Options are always Nak'd simultaneously.

5.4. Configure-Reject

Description

If some Configuration Options received in a Configure-Request are
not recognizable or are not acceptable for negotiation (as
configured by a network administrator), then the implementation
MUST transmit a Configure-Reject. The Options field is filled
with only the unacceptable Configuration Options from the
Configure-Request. All recognizable and negotiable Configuration
Options are filtered out of the Configure-Reject, but otherwise
the Configuration Options MUST NOT be reordered or modified in any
way.

On reception of a Configure-Reject, the Identifier field MUST
match that of the last transmitted Configure-Request.
Additionally, the Configuration Options in a Configure-Reject MUST

be a proper subset of those in the last transmitted Configure-
Request. Invalid packets are silently discarded.

Reception of a valid Configure-Reject indicates that when a new
Configure-Request is sent, it MUST NOT include any of the
Configuration Options listed in the Configure-Reject.

A summary of the Configure-Reject packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Options ...
+-+-+-+-+

Code

4 for Configure-Reject.

Identifier

The Identifier field is a copy of the Identifier field of the
Configure-Request which caused this Configure-Reject.

Options

The Options field is variable in length, and contains the list of
zero or more Configuration Options that the sender is rejecting.
All Configuration Options are always rejected simultaneously.

5.5. Terminate-Request and Terminate-Ack

Description

LCP includes Terminate-Request and Terminate-Ack Codes in order to
provide a mechanism for closing a connection.

An implementation wishing to close a connection SHOULD transmit a
Terminate-Request. Terminate-Request packets SHOULD continue to
be sent until Terminate-Ack is received, the lower layer indicates
that it has gone down, or a sufficiently large number have been
transmitted such that the peer is down with reasonable certainty.

Upon reception of a Terminate-Request, a Terminate-Ack MUST be
transmitted.

Reception of an unelicited Terminate-Ack indicates that the peer
is in the Closed or Stopped states, or is otherwise in need of
re-negotiation.

A summary of the Terminate-Request and Terminate-Ack packet formats
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data ...
+-+-+-+-+

Code

5 for Terminate-Request;

6 for Terminate-Ack.

Identifier

On transmission, the Identifier field MUST be changed whenever the
content of the Data field changes, and whenever a valid reply has
been received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.

On reception, the Identifier field of the Terminate-Request is
copied into the Identifier field of the Terminate-Ack packet.

Data

The Data field is zero or more octets, and contains uninterpreted
data for use by the sender. The data may consist of any binary
value. The end of the field is indicated by the Length.

5.6. Code-Reject

Description

Reception of a LCP packet with an unknown Code indicates that the
peer is operating with a different version. This MUST be reported
back to the sender of the unknown Code by transmitting a Code-
Reject.

Upon reception of the Code-Reject of a code which is fundamental
to this version of the protocol, the implementation SHOULD report
the problem and drop the connection, since it is unlikely that the
situation can be rectified automatically.

A summary of the Code-Reject packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Rejected-Packet ...
+-+-+-+-+-+-+-+-+

Code

7 for Code-Reject.

Identifier

The Identifier field MUST be changed for each Code-Reject sent.

Rejected-Packet

The Rejected-Packet field contains a copy of the LCP packet which
is being rejected. It begins with the Information field, and does
not include any Data Link Layer headers nor an FCS. The
Rejected-Packet MUST be truncated to comply with the peer's

established MRU.

5.7. Protocol-Reject

Description

Reception of a PPP packet with an unknown Protocol field indicates
that the peer is attempting to use a protocol which is
unsupported. This usually occurs when the peer attempts to
configure a new protocol. If the LCP automaton is in the Opened
state, then this MUST be reported back to the peer by transmitting
a Protocol-Reject.

Upon reception of a Protocol-Reject, the implementation MUST stop
sending packets of the indicated protocol at the earliest
opportunity.

Protocol-Reject packets can only be sent in the LCP Opened state.
Protocol-Reject packets received in any state other than the LCP
Opened state SHOULD be silently discarded.

A summary of the Protocol-Reject packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Rejected-Protocol Rejected-Information ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code

8 for Protocol-Reject.

Identifier

The Identifier field MUST be changed for each Protocol-Reject
sent.

Rejected-Protocol

The Rejected-Protocol field is two octets, and contains the PPP
Protocol field of the packet which is being rejected.

Rejected-Information

The Rejected-Information field contains a copy of the packet which
is being rejected. It begins with the Information field, and does
not include any Data Link Layer headers nor an FCS. The
Rejected-Information MUST be truncated to comply with the peer's
established MRU.

5.8. Echo-Request and Echo-Reply

Description

LCP includes Echo-Request and Echo-Reply Codes in order to provide
a Data Link Layer loopback mechanism for use in exercising both
directions of the link. This is useful as an aid in debugging,
link quality determination, performance testing, and for numerous
other functions.

Upon reception of an Echo-Request in the LCP Opened state, an
Echo-Reply MUST be transmitted.

Echo-Request and Echo-Reply packets MUST only be sent in the LCP
Opened state. Echo-Request and Echo-Reply packets received in any
state other than the LCP Opened state SHOULD be silently
discarded.

A summary of the Echo-Request and Echo-Reply packet formats 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Magic-Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data ...
+-+-+-+-+

Code

9 for Echo-Request;

10 for Echo-Reply.

Identifier

On transmission, the Identifier field MUST be changed whenever the
content of the Data field changes, and whenever a valid reply has
been received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.

On reception, the Identifier field of the Echo-Request is copied
into the Identifier field of the Echo-Reply packet.

Magic-Number

The Magic-Number field is four octets, and aids in detecting links
which are in the looped-back condition. Until the Magic-Number
Configuration Option has been successfully negotiated, the Magic-
Number MUST be transmitted as zero. See the Magic-Number
Configuration Option for further explanation.

Data

The Data field is zero or more octets, and contains uninterpreted
data for use by the sender. The data may consist of any binary
value. The end of the field is indicated by the Length.

5.9. Discard-Request

Description

LCP includes a Discard-Request Code in order to provide a Data
Link Layer sink mechanism for use in exercising the local to
remote direction of the link. This is useful as an aid in
debugging, performance testing, and for numerous other functions.

Discard-Request packets MUST only be sent in the LCP Opened state.
On reception, the receiver MUST silently discard any Discard-
Request that it receives.

A summary of the Discard-Request packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Magic-Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data ...
+-+-+-+-+

Code

11 for Discard-Request.

Identifier

The Identifier field MUST be changed for each Discard-Request
sent.

Magic-Number

The Magic-Number field is four octets, and aids in detecting links
which are in the looped-back condition. Until the Magic-Number
Configuration Option has been successfully negotiated, the Magic-
Number MUST be transmitted as zero. See the Magic-Number
Configuration Option for further explanation.

Data

The Data field is zero or more octets, and contains uninterpreted
data for use by the sender. The data may consist of any binary
value. The end of the field is indicated by the Length.

6. LCP Configuration Options

LCP Configuration Options allow negotiation of modifications to the
default characteristics of a point-to-point link. If a Configuration
Option is not included in a Configure-Request packet, the default
value for that Configuration Option is assumed.

Some Configuration Options MAY be listed more than once. The effect
of this is Configuration Option specific, and is specified by each
such Configuration Option description. (None of the Configuration
Options in this specification can be listed more than once.)

The end of the list of Configuration Options is indicated by the
Length field of the LCP packet.

Unless otherwise specified, all Configuration Options apply in a
half-duplex fashion; typically, in the receive direction of the link
from the point of view of the Configure-Request sender.

Design Philosophy

The options indicate additional capabilities or requirements of
the implementation that is requesting the option. An
implementation which does not understand any option SHOULD
interoperate with one which implements every option.

A default is specified for each option which allows the link to
correctly function without negotiation of the option, although
perhaps with less than optimal performance.

Except where explicitly specified, acknowledgement of an option
does not require the peer to take any additional action other than
the default.

It is not necessary to send the default values for the options in
a Configure-Request.

A summary of the Configuration Option format is shown below. The
fields are transmitted from left to right.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

The Type field is one octet, and indicates the type of
Configuration Option. Up-to-date values of the LCP Option Type
field are specified in the most recent "Assigned Numbers" RFC[2].
This document concerns the following values:

0 RESERVED
1 Maximum-Receive-Unit
3 Authentication-Protocol
4 Quality-Protocol
5 Magic-Number
7 Protocol-Field-Compression
8 Address-and-Control-Field-Compression

Length

The Length field is one octet, and indicates the length of this
Configuration Option including the Type, Length and Data fields.

If a negotiable Configuration Option is received in a Configure-
Request, but with an invalid or unrecognized Length, a Configure-
Nak SHOULD be transmitted which includes the desired Configuration
Option with an appropriate Length and Data.

Data

The Data field is zero or more octets, and contains information
specific to the Configuration Option. The format and length of
the Data field is determined by the Type and Length fields.

When the Data field is indicated by the Length to extend beyond
the end of the Information field, the entire packet is silently
discarded without affecting the automaton.

6.1. Maximum-Receive-Unit (MRU)

Description

This Configuration Option may be sent to inform the peer that the
implementation can receive larger packets, or to request that the
peer send smaller packets.

The default value is 1500 octets. If smaller packets are
requested, an implementation MUST still be able to receive the
full 1500 octet information field in case link synchronization is
lost.

Implementation Note:

This option is used to indicate an implementation capability.
The peer is not required to maximize the use of the capacity.
For example, when a MRU is indicated which is 2048 octets, the
peer is not required to send any packet with 2048 octets. The
peer need not Configure-Nak to indicate that it will only send
smaller packets, since the implementation will always require
support for at least 1500 octets.

A summary of the Maximum-Receive-Unit 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 Maximum-Receive-Unit
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

1

Length

4

Maximum-Receive-Unit

The Maximum-Receive-Unit field is two octets, and specifies the
maximum number of octets in the Information and Padding fields.
It does not include the framing, Protocol field, FCS, nor any
transparency bits or bytes.

6.2. Authentication-Protocol

Description

On some links it may be desirable to require a peer to
authenticate itself before allowing network-layer protocol packets
to be exchanged.

This Configuration Option provides a method to negotiate the use
of a specific protocol for authentication. By default,
authentication is not required.

An implementation MUST NOT include multiple Authentication-
Protocol Configuration Options in its Configure-Request packets.
Instead, it SHOULD attempt to configure the most desirable
protocol first. If that protocol is Configure-Nak'd, then the
implementation SHOULD attempt the next most desirable protocol in
the next Configure-Request.

The implementation sending the Configure-Request is indicating
that it expects authentication from its peer. If an
implementation sends a Configure-Ack, then it is agreeing to
authenticate with the specified protocol. An implementation
receiving a Configure-Ack SHOULD expect the peer to authenticate
with the acknowledged protocol.

There is no requirement that authentication be full-duplex or that
the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.

A summary of the Authentication-Protocol 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 Authentication-Protocol
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data ...
+-+-+-+-+

Type

3

Length

>= 4

Authentication-Protocol

The Authentication-Protocol field is two octets, and indicates the
authentication protocol desired. Values for this field are always
the same as the PPP Protocol field values for that same
authentication protocol.

Up-to-date values of the Authentication-Protocol field are
specified in the most recent "Assigned Numbers" RFC[2]. Current
values are assigned as follows:

Value (in hex) Protocol

c023 Password Authentication Protocol
c223 Challenge Handshake Authentication Protocol

Data

The Data field is zero or more octets, and contains additional
data as determined by the particular protocol.

6.3. Quality-Protocol

Description

On some links it may be desirable to determine when, and how
often, the link is dropping data. This process is called link
quality monitoring.

This Configuration Option provides a method to negotiate the use
of a specific protocol for link quality monitoring. By default,
link quality monitoring is disabled.

The implementation sending the Configure-Request is indicating
that it expects to receive monitoring information from its peer.
If an implementation sends a Configure-Ack, then it is agreeing to
send the specified protocol. An implementation receiving a
Configure-Ack SHOULD expect the peer to send the acknowledged
protocol.

There is no requirement that quality monitoring be full-duplex or

that the same protocol be used in both directions. It is
perfectly acceptable for different protocols to be used in each
direction. This will, of course, depend on the specific protocols
negotiated.

A summary of the Quality-Protocol 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 Quality-Protocol
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data ...
+-+-+-+-+

Type

4

Length

>= 4

Quality-Protocol

The Quality-Protocol field is two octets, and indicates the link
quality monitoring protocol desired. Values for this field are
always the same as the PPP Protocol field values for that same
monitoring protocol.

Up-to-date values of the Quality-Protocol field are specified in
the most recent "Assigned Numbers" RFC[2]. Current values are
assigned as follows:

Value (in hex) Protocol

c025 Link Quality Report

Data

The Data field is zero or more octets, and contains additional
data as determined by the particular protocol.

6.4. Magic-Number

Description

This Configuration Option provides a method to detect looped-back
links and other Data Link Layer anomalies. This Configuration
Option MAY be required by some other Configuration Options such as
the Quality-Protocol Configuration Option. By default, the
Magic-Number is not negotiated, and zero is inserted where a
Magic-Number might otherwise be used.

Before this Configuration Option is requested, an implementation
MUST choose its Magic-Number. It is recommended that the Magic-
Number be chosen in the most random manner possible in order to
guarantee with very high probability that an implementation will
arrive at a unique number. A good way to choose a unique random
number is to start with a unique seed. Suggested sources of
uniqueness include machine serial numbers, other network hardware
addresses, time-of-day clocks, etc. Particularly good random
number seeds are precise measurements of the inter-arrival time of
physical events such as packet reception on other connected
networks, server response time, or the typing rate of a human
user. It is also suggested that as many sources as possible be
used simultaneously.

When a Configure-Request is received with a Magic-Number
Configuration Option, the received Magic-Number is compared with
the Magic-Number of the last Configure-Request sent to the peer.
If the two Magic-Numbers are different, then the link is not
looped-back, and the Magic-Number SHOULD be acknowledged. If the
two Magic-Numbers are equal, then it is possible, but not certain,
that the link is looped-back and that this Configure-Request is
actually the one last sent. To determine this, a Configure-Nak
MUST be sent specifying a different Magic-Number value. A new
Configure-Request SHOULD NOT be sent to the peer until normal
processing would cause it to be sent (that is, until a Configure-
Nak is received or the Restart timer runs out).

Reception of a Configure-Nak with a Magic-Number different from
that of the last Configure-Nak sent to the peer proves that a link
is not looped-back, and indicates a unique Magic-Number. If the
Magic-Number is equal to the one sent in the last Configure-Nak,
the possibility of a looped-back link is increased, and a new
Magic-Number MUST be chosen. In either case, a new Configure-
Request SHOULD be sent with the new Magic-Number.

If the link is indeed looped-back, this sequence (transmit
Configure-Request, receive Configure-Request, transmit Configure-

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
Configure-Ack).

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
Magic-Number.

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.

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-
back condition.

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)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

5

Length

6

Magic-Number

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.

6.5. Protocol-Field-Compression (PFC)

Description

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
fields.

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
than 256.

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
them.

The Protocol field is never compressed when sending any LCP
packet. This rule guarantees unambiguous recognition of LCP
packets.

When a Protocol field is compressed, the Data Link Layer FCS field
is calculated on the compressed frame, not the original

uncompressed frame.

A summary of the Protocol-Field-Compression Configuration Option
format is shown below. The fields are transmitted from left to
right.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

7

Length

2

6.6. Address-and-Control-Field-Compression (ACFC)

Description

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
LCP packets.

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
to right.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

8

Length

2

Security Considerations

Security issues are briefly discussed in sections concerning the
Authentication Phase, the Close event, and the Authentication-
Protocol Configuration Option.

References

[1] Perkins, D., "Requirements for an Internet Standard Point-to-
Point Protocol", RFC1547, Carnegie Mellon University,
December 1993.

[2] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.

Acknowledgements

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 ietf-ppp@merit.edu mailing list.

Much of the text in this document is taken from the working group
requirements [1]; 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.

Chair's Address

The working group can be contacted via the current chair:

Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California 93117

fbaker@acc.com

Editor's Address

Questions about this memo can also be directed to:

William Allen Simpson
Daydreamer
Computer Systems Consulting Services
1384 Fontaine
Madison Heights, Michigan 48071