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Network Working Group W. Simpson,


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

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RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

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



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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.

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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.

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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| * | * |

+----------+-------------+---------+

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).

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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.

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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.

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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.

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.

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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

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.

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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.

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RFC 1661 Point-to-Point Protocol July 1994

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.

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RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

| 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

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.

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RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

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;

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RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 18]

RFC 1661 Point-to-Point Protocol July 1994

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

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RFC 1661 Point-to-Point Protocol July 1994

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.

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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.

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RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 22]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 23]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 24]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 25]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 26]

RFC 1661 Point-to-Point Protocol July 1994



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.

Simpson [Page 27]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 28]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 29]

RFC 1661 Point-to-Point Protocol July 1994



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

Simpson [Page 30]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 31]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 32]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 33]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 34]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 35]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 36]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 37]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 38]

RFC 1661 Point-to-Point Protocol July 1994

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 ...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Simpson [Page 39]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 40]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 41]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 42]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 43]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 44]

RFC 1661 Point-to-Point Protocol July 1994

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-

Simpson [Page 45]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 46]

RFC 1661 Point-to-Point Protocol July 1994

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.

Simpson [Page 47]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 48]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 49]

RFC 1661 Point-to-Point Protocol July 1994

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

Simpson [Page 50]

RFC 1661 Point-to-Point Protocol July 1994

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", RFC 1547, 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.

Simpson [Page 51]

RFC 1661 Point-to-Point Protocol July 1994

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

Bill.Simpson@um.cc.umich.edu

bsimpson@MorningStar.com

Simpson [Page 52]


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