Network Working Group W. Simpson
Request for Comments: 1548 Daydreamer
Obsoletes: RFC 1331 December 1993
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.
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@ucdavis.edu mailing list.
Simpson [Page 1]RFC 1548 The Point-to-Point Protocol December 1993
Table of Contents
1. Introduction ................................................3
1.1 Specification of Requirements ...............................4
1.2 Terminology .................................................5
2. PPP Encapsulation ...........................................5
3. PPP Link Operation ..........................................8
3.1 Overview ....................................................8
3.2 Phase Diagram ...............................................8
3.3 Link Dead (physical-layer not ready) ........................9
3.4 Link Establishment Phase ....................................9
3.5 Authentication Phase ........................................9
3.6 Network-Layer Protocol Phase ................................10
3.7 Link Termination Phase ......................................10
4. The Option Negotiation Automaton ............................11
4.1 State Diagram ...............................................12
4.2 State Transition Table ......................................14
4.3 A Day in the Life ...........................................15
4.4 States ......................................................16
4.5 Events ......................................................19
4.6 Actions .....................................................23
4.7 Loop Avoidance ..............................................26
4.8 Counters and Timers .........................................26
5. LCP Packet Formats ..........................................27
5.1 Configure-Request ...........................................29
5.2 Configure-Ack ...............................................30
5.3 Configure-Nak ...............................................31
5.4 Configure-Reject ............................................33
5.5 Terminate-Request and Terminate-Ack .........................34
5.6 Code-Reject .................................................35
5.7 Protocol-Reject .............................................36
5.8 Echo-Request and Echo-Reply .................................37
5.9 Discard-Request .............................................39
6. LCP Configuration Options ...................................40
6.1 Maximum-Receive-Unit ........................................41
6.2 Async-Control-Character-Map .................................42
6.3 Authentication-Protocol .....................................43
6.4 Quality-Protocol ............................................45
6.5 Magic-Number ................................................46
6.6 Protocol-Field-Compression ..................................49
6.7 Address-and-Control-Field-Compression .......................50
APPENDIX A. LCP Recommended Options ..............................51
SECURITY CONSIDERATIONS ..........................................51
REFERENCES .......................................................52
ACKNOWLEDGEMENTS .................................................52
CHAIR'S ADDRESS ..................................................52
EDITOR'S ADDRESS .................................................53
Simpson [Page 2]
RFC 1548 The Point-to-Point Protocol December 1993
1. Introduction
Encapsulation
The PPP encapsulation provides for multiplexing of different
network-layer protocols simultaneously over the same link. It is
intended that PPP provide a common solution for easy connection of
a wide variety of hosts, bridges and routers [1].
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 with the default HDLC 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, authenticate the identity of its peer on the link,
determine when a link is functioning properly and when it is
defunct, detect a looped-back link and other common
misconfiguration errors, and terminate the link.
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.
Simpson [Page 3]RFC 1548 The Point-to-Point Protocol December 1993
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.
Simpson [Page 4]RFC 1548 The Point-to-Point Protocol December 1993
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
This means 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.
Simpson [Page 5]RFC 1548 The Point-to-Point Protocol December 1993
A summary of the PPP encapsulation is shown below. The fields are
transmitted from left to right.
+----------+-------------+---------+
| Protocol | Information | Padding |
| 16 bits | * | * |
+----------+-------------+---------+
Protocol Field
The Protocol field is 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]. Current values are assigned as
follows:
Value (in hex) Protocol Name
0001 Padding Protocol
0003 to 001f reserved (transparency inefficient)
0021 Internet Protocol
0023 OSI Network Layer
0025 Xerox NS IDP
0027 DECnet Phase IV
0029 Appletalk
002b Novell IPX
002d Van Jacobson Compressed TCP/IP
002f Van Jacobson Uncompressed TCP/IP
Simpson [Page 6]RFC 1548 The Point-to-Point Protocol December 1993
0031 Bridging PDU
0033 Stream Protocol (ST-II)
0035 Banyan Vines
0037 unused
0039 AppleTalk EDDP
003b AppleTalk SmartBuffered
003d Multi-Link
005d reserved (compression inefficient)
00cf reserved (PPP NLPID)
00fd 1st choice compression
00ff reserved (compression inefficient)
0201 802.1d Hello Packets
0203 IBM Source Routing BPDU
0231 Luxcom
0233 Sigma Network Systems
8021 Internet Protocol Control Protocol
8023 OSI Network Layer Control Protocol
8025 Xerox NS IDP Control Protocol
8027 DECnet Phase IV Control Protocol
8029 Appletalk Control Protocol
802b Novell IPX Control Protocol
802d Reserved
802f Reserved
8031 Bridging NCP
8033 Stream Protocol Control Protocol
8035 Banyan Vines Control Protocol
8037 unused
8039 Reserved
803b Reserved
803d Multi-Link Control Protocol
80fd Compression Control Protocol
80ff Reserved
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.
Simpson [Page 7]RFC 1548 The Point-to-Point Protocol December 1993
The maximum length for the Information field, including Padding,
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:
+------+ +-----------+ +--------------+
| | UP | | OPENED | | SUCCESS/NONE
| Dead |------->| Establish |---------->| Authenticate |--+
| | | | | | |
+------+ +-----------+ +--------------+ |
^ FAIL | FAIL | |
+<--------------+ +----------+ |
| | |
| +-----------+ | +---------+ |
| DOWN | | | CLOSING | | |
+------------| Terminate |<---+<----------| Network |<-+
| | | |
+-----------+ +---------+
Simpson [Page 8]RFC 1548 The Point-to-Point Protocol December 1993
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 below) will be in the
Initial or Starting states. The transition to the Link Establishment
phase will signal an Up event to the 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 line,
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 below) has been both sent and received.
All Configuration Options are assumed to be at default values unless
altered by the configuration exchange. See the section 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.
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 negotiate the use of that authentication
protocol during Link Establishment phase.
Simpson [Page 9]RFC 1548 The Point-to-Point Protocol December 1993
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,
using the negotiated authentication protocol. If authentication
fails, PPP SHOULD proceed instead to the Link Termination phase.
Any Network Control Protocol or network-layer protocol packets
received during this phase MUST be silently discarded.
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 network-layer
protocol packets received when the corresponding NCP is not in the
Opened state MUST be silently discarded.
Implementation Note:
There is an exception to the preceding paragraphs, due to the
availability of the LCP Protocol-Reject (described below). While
LCP is in the Opened state, any protocol packet which is
unsupported by the implementation MUST be returned in a Protocol-
Reject. 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
Simpson [Page 10]RFC 1548 The Point-to-Point Protocol December 1993
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
Simpson [Page 11]RFC 1548 The Point-to-Point Protocol December 1993
TO+ = Timeout with counter > 0 irc = Initialize-Restart-Counter
TO- = Timeout with counter expired zrc = Zero-Restart-Counter
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 Diagram
The simplified state diagram which follows describes the sequence of
events for reaching agreement on Configuration Options (opening the
PPP link) and for later termination of the link.
This diagram is not a complete representation of the automaton.
Implementation MUST be done by consulting the actual state transition
table.
Events are in upper case. Actions are in lower case. For these
purposes, the state machine is initially in the Closed state. Once
the Opened state has been reached, both ends of the link have met the
requirement of having both sent and received a Configure-Ack packet.
Simpson [Page 12]RFC 1548 The Point-to-Point Protocol December 1993
RCR TO+
+--sta-->+ +------->+
| | | |
+-------+ | RTA +-------+ | Close +-------+
| |<-----+<------| |<-str-+<------| |
|Closed | |Closing| |Opened |
| | Open | | | |
| |------+ | | | |
+-------+ | +-------+ +-------+
| ^
| |
| +-sca----------------->+
| | ^
RCN,TO+ V RCR+ | RCR- RCA | RCN,TO+
+------->+ | +------->+ | +--scr-->+
| | | | | | | |
+-------+ | TO+ +-------+ | +-------+ |
| |<-scr-+<------| |<-scn-+ | |<-----+
| Req- | | Ack- | | Ack- |
| Sent | RCA | Rcvd | | Sent |
+-scn->| |------------->| | +-sca->| |
| +-------+ +-------+ | +-------+
| RCR- | | RCR+ | RCR+ | | RCR-
| | +------------------------------->+<-------+ |
| | |
+<-------+<------------------------------------------------+
Simpson [Page 13]RFC 1548 The Point-to-Point Protocol December 1993
4.2 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 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
Simpson [Page 14]RFC 1548 The Point-to-Point Protocol December 1993
| 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-Counter 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.
[p] Passive option; see Stopped state discussion.
[r] Restart option; see Open event discussion.
[x] Crossed connection; see RCA event discussion.
4.3 A Day in the Life
Here is an example of how a typical implementation might use the
automaton to implement LCP in a dial-up environment:
- The Network Access Server is powered on (Initial state, Link Dead
phase).
- A configuration file indicates that a particular link is to be
Simpson [Page 15]RFC 1548 The Point-to-Point Protocol December 1993
used for PPP access (Open: tls/Starting). The This-Layer-Started
event turns on DTR to a modem, readying it for accepting calls.
- An incoming call is answered. The modem CD triggers configuration
negotiation (Up: irc,scr/Req-Sent, Link Establishment phase).
- A Configure-Request is received, which is acknowleged (RCR+:
sca/Ack-Sent).
- The Request is acknowleged (RCA: irc,tlu/Opened). The This-
Layer-Up event starts authentication and quality monitoring
protocols (Authentication phase).
- When authentication and quality monitoring are satisfied, they
send an Up event to start the available NCPs (Network-Layer
Protocol phase).
- Later, the peer is finished, and closes the link. A Terminate-
Request arrives (RTR: tld,zrc,sta/Stopping, Termination phase).
The This-Layer-Down action sends the Down event to any NCPs, while
the Terminate-Ack is sent. The Zero-Restart-Counter action causes
the link to wait for the peer to process the Terminate-Ack, with
no retries.
- When the Restart Timer times out (TO-: tlf/Stopped), the This-
Layer-Finished action signals the modem to hang up by dropping
DTR.
- When the CD from the modem drops (Down: tls/Starting), the This-
Layer-Started action raises DTR again, readying it for the next
call (returning to the Link Dead phase).
4.4 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.
Simpson [Page 16]RFC 1548 The Point-to-Point Protocol December 1993
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.
Simpson [Page 17]RFC 1548 The Point-to-Point Protocol December 1993
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, this action may be
skipped, and the Closed state may be 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 always running in the Ack-Sent state.
Opened
In the Opened state, a Configure-Ack has been both sent and
received. The Restart timer is not running in the Opened state.
Simpson [Page 18]RFC 1548 The Point-to-Point Protocol December 1993
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.
4.5 Events
Transitions and actions in the automaton are caused by events.
Up
The Up 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
The Down 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
The Open 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.
Simpson [Page 19]RFC 1548 The Point-to-Point Protocol December 1993
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. This will cause the
renegotiation of the link, without any harmful side effects.
Close
The Close 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.
The Close followed by an Open will cause an orderly termination of
the link, by progressing from 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-
Simpson [Page 20]RFC 1548 The Point-to-Point Protocol December 1993
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)
The Receive-Configure-Ack 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.
Simpson [Page 21]RFC 1548 The Point-to-Point Protocol December 1993
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)
The Receive-Terminate-Request 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)
The Receive-Terminate-Ack 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)
The Receive-Unknown-Code 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
Simpson [Page 22]RFC 1548 The Point-to-Point Protocol December 1993
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 a Echo-Request packet. There is no reply to an Echo-
Reply or Discard-Request packet.
4.6 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
Simpson [Page 23]RFC 1548 The Point-to-Point Protocol December 1993
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.
Implementation Note:
This results of this action are highly implementation dependent.
The transitions where this event is indicated are defined
according to a message passing architecture, rather than a
signalling architecture. If the action is desired to control
specific signals (such as DTR), other transitions for the action
are likely to be required (Open in Closed, RCR in Stopped).
This-Layer-Finished (tlf)
This action indicates to the lower layers that the automaton is
entering the Stopped or Closed 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.
Implementation Note:
This results of this action are highly implementation dependent.
The transitions where this event is indicated are defined
according to a message passing architecture, rather than a
signalling architecture. If the action is desired to control
specific signals (such as DTR), other transitions for the action
are likely to be required (Close in Starting, Down in Closing).
Initialize-Restart-Counter (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.
Simpson [Page 24]RFC 1548 The Point-to-Point Protocol December 1993
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-Counter (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)
The Send-Configure-Request action transmits a Configure-Request
packet. 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)
The Send-Configure-Ack action transmits a Configure-Ack packet.
This acknowledges the reception of a Configure-Request packet with
an acceptable set of Configuration Options.
Send-Configure-Nak (scn)
The Send-Configure-Nak action transmits a Configure-Nak or
Configure-Reject packet, 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 section on LCP Packet Formats.
Send-Terminate-Request (str)
The Send-Terminate-Request action transmits a Terminate-Request
Simpson [Page 25]RFC 1548 The Point-to-Point Protocol December 1993
packet. 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)
The Send-Terminate-Ack action transmits a Terminate-Ack packet.
This acknowledges the reception of a Terminate-Request packet or
otherwise serves to synchronize the state machines.
Send-Code-Reject (scj)
The Send-Code-Reject action transmits a Code-Reject packet. This
indicates the reception of an unknown type of packet.
Send-Echo-Reply (ser)
The Send-Echo-Reply action transmits an Echo-Reply packet. This
acknowledges the reception of an Echo-Request packet.
4.7 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.
4.8 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
Simpson [Page 26]RFC 1548 The Point-to-Point Protocol December 1993
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 are converted to
Configure-Reject packets. Max-Failure MUST be configurable, but
SHOULD default to ten (10) 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
Simpson [Page 27]RFC 1548 The Point-to-Point Protocol December 1993
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).
This document describes Version 1 of the Link Control Protocol. In
the interest of simplicity, there is no version field in the LCP
packet. If a new version of LCP is necessary in the future, the
intention is that a new PPP Protocol field value will be used to
differentiate Version 1 LCP from all other versions. A correctly
functioning Version 1 LCP implementation will always respond to
unknown Protocols (including other versions) with an easily
recognizable Version 1 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
enabled. This ensures that such LCP packets are always recognizable
even when one end of the link mistakenly believes the link to be
open.
Implementation Note:
In particular, the Async-Control-Character-Map (ACCM) default for
the type of link is used, and no address, control, or protocol
field compression is allowed.
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 ...
+-+-+-+-+
Simpson [Page 28]RFC 1548 The Point-to-Point Protocol December 1993
Code
The Code field is one octet and identifies the kind of LCP packet.
When a packet is received with an invalid 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 specification 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.
Length
The Length field is two octets and indicates the length of the LCP
packet including the Code, Identifier, Length and Data fields.
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.
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 LCP
packet with the Code field set to 1 (Configure-Request), and the
Simpson [Page 29]RFC 1548 The Point-to-Point Protocol December 1993
Options field 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 content 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 section.
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 LCP packet with the Code field set
to 2 (Configure-Ack), the Identifier field copied from the
received Configure-Request, and the Options field copied from the
received Configure-Request. The acknowledged Configuration
Options MUST NOT be reordered or modified in any way.
Simpson [Page 30]RFC 1548 The Point-to-Point Protocol December 1993
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 element of the received Configuration Options is
recognizable but some values are not acceptable, then the
implementation MUST transmit a LCP packet with the Code field set
to 3 (Configure-Nak), the Identifier field copied from the
received Configure-Request, and the Options field 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
Simpson [Page 31]RFC 1548 The Point-to-Point Protocol December 1993
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 a new
Configure-Request MAY be sent with the Configuration Options
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 ...
+-+-+-+-+
Simpson [Page 32]RFC 1548 The Point-to-Point Protocol December 1993
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 LCP packet with the Code field set to 4
(Configure-Reject), the Identifier field copied from the received
Configure-Request, and the Options field 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 a new
Configure-Request SHOULD be sent which does 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.
Simpson [Page 33]RFC 1548 The Point-to-Point Protocol December 1993
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.
A LCP implementation wishing to close a connection SHOULD transmit
a LCP packet with the Code field set to 5 (Terminate-Request), and
the Data field filled with any desired data. 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 LCP packet MUST be
transmitted with the Code field set to 6 (Terminate-Ack), the
Identifier field copied from the Terminate-Request packet, and the
Data field filled with any desired data.
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
Simpson [Page 34]RFC 1548 The Point-to-Point Protocol December 1993
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 and may be of any length from zero to the peer's established
MRU minus four.
5.6 Code-Reject
Description
Reception of a LCP packet with an unknown Code indicates that one
of the communicating LCP implementations is faulty or incomplete.
This error MUST be reported back to the sender of the unknown Code
by transmitting a LCP packet with the Code field set to 7 (Code-
Reject), and the inducing packet copied to the Rejected-
Information field.
Upon reception of a Code-Reject, the implementation SHOULD report
the error, since it is unlikely that the situation can be
rectified automatically.
Simpson [Page 35]RFC 1548 The Point-to-Point Protocol December 1993
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-Information
The Rejected-Information 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-Information 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 state machine is in the
Opened state, then this error MUST be reported back to the peer by
transmitting a LCP packet with the Code field set to 8 (Protocol-
Reject), the Rejected-Protocol field set to the received Protocol,
and the inducing packet copied to the Rejected-Information field.
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.
Simpson [Page 36]RFC 1548 The Point-to-Point Protocol December 1993
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.
An Echo-Request sender transmits a LCP packet with the Code field
set to 9 (Echo-Request), the Identifier field set, the local
Magic-Number (if any) inserted, and the Data field filled with any
desired data, but not exceeding the peer's established MRU minus
eight.
Simpson [Page 37]RFC 1548 The Point-to-Point Protocol December 1993
Upon reception of an Echo-Request, a LCP packet MUST be
transmitted with the Code field set to 10 (Echo-Reply), the
Identifier field copied from the received Echo-Request, the local
Magic-Number (if any) inserted, and the Data field copied from the
Echo-Request, truncating as necessary to avoid exceeding the
peer's established MRU.
Echo-Request and Echo-Reply packets may 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.
Simpson [Page 38]RFC 1548 The Point-to-Point Protocol December 1993
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 and may be of any length from zero to the peer's established
MRU minus eight.
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.
The sender transmits a LCP packet with the Code field set to 11
(Discard-Request), the Identifier field set, the local Magic-
Number (if any) inserted, and the Data field filled with any
desired data, but not exceeding the peer's established MRU minus
eight.
Discard-Request packets may only be sent in the LCP Opened state.
On reception, the receiver MUST simply throw away 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.
Simpson [Page 39]RFC 1548 The Point-to-Point Protocol December 1993
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 and may be of any length from zero to the peer's established
MRU minus four.
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 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.
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].
Simpson [Page 40]RFC 1548 The Point-to-Point Protocol December 1993
This specification concerns the following values:
1 Maximum-Receive-Unit
2 Async-Control-Character-Map
3 Authentication-Protocol
4 Quality-Protocol
5 Magic-Number
6 RESERVED
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 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 information specific to
the Configuration Option. The format and length of the Data field
is determined by the Type and Length fields.
6.1 Maximum-Receive-Unit
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.
Simpson [Page 41]RFC 1548 The Point-to-Point Protocol December 1993
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 Async-Control-Character-Map
Description
This Configuration Option provides a method to negotiate the use
of control character transparency on asynchronous links.
For asynchronous links, the default value is 0xffffffff, which
causes all octets less than 0x20 to be mapped into an appropriate
two octet sequence. For most other links, the default value is 0,
since there is no need for mapping.
However, it is rarely necessary to map all control characters, and
often it is unnecessary to map any control characters. The
Configuration Option is used to inform the peer which control
characters MUST remain mapped when the peer sends them.
The peer MAY still send any other octets in mapped format, if it
is necessary because of constraints known to the peer. The peer
SHOULD Configure-Nak with the logical union of the sets of mapped
octets, so that when such octets are spuriously introduced they
can be ignored on receipt.
Simpson [Page 42]RFC 1548 The Point-to-Point Protocol December 1993
A summary of the Async-Control-Character-Map 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 | Async-Control-Character-Map
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACCM (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
6
Async-Control-Character-Map
The Async-Control-Character-Map field is four octets and indicates
the set of control characters to be mapped. The map is sent most
significant octet first.
Each numbered bit corresponds to the octet of the same value. If
the bit is cleared to zero, then that octet need not be mapped.
If the bit is set to one, then that octet MUST remain mapped. For
example, if bit 19 is set to zero, then the ASCII control
character 19 (DC3, Control-S) MAY be sent in the clear.
Note: The least significant bit of the least significant octet
(the final octet transmitted) is numbered bit 0, and would map
to the ASCII control character NUL.
6.3 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 authentication protocol. By default, authentication
is not required.
Simpson [Page 43]RFC 1548 The Point-to-Point Protocol December 1993
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.
If an implementation sends a Configure-Ack with this Configuration
Option, then it is agreeing to authenticate with the specified
protocol. An implementation receiving a Configure-Ack with this
Configuration Option 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:
Simpson [Page 44]RFC 1548 The Point-to-Point Protocol December 1993
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.
6.4 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.
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
Simpson [Page 45]RFC 1548 The Point-to-Point Protocol December 1993
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.
6.5 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 an 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.
Simpson [Page 46]RFC 1548 The Point-to-Point Protocol December 1993
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, loop-backs 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 if 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
Simpson [Page 47]RFC 1548 The Point-to-Point Protocol December 1993
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 or 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 loop-back condition is terminated
and Magic-Numbers are successfully negotiated. A more optimistic
procedure (in the case of a loop-back) is to begin transmitting
LCP Echo-Request packets until an appropriate Echo-Reply is
received, indicating a termination of the loop-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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Simpson [Page 48]RFC 1548 The Point-to-Point Protocol December 1993
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.
6.6 Protocol-Field-Compression
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
Simpson [Page 49]RFC 1548 The Point-to-Point Protocol December 1993
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
6.7 Address-and-Control-Field-Compression
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.
Simpson [Page 50]RFC 1548 The Point-to-Point Protocol December 1993
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
A. LCP Recommended Options
The following Configurations Options are recommended:
SYNC LINES
Magic Number Link Quality Monitoring No Address and Control Field
Compression No Protocol Field Compression
ASYNC LINES
Async Control Character Map Magic Number Address and Control Field
Compression Protocol Field Compression
Security Considerations
Security issues are briefly discussed in sections concerning the
Authentication Phase, the Close event, and the Authentication-
Simpson [Page 51]RFC 1548 The Point-to-Point Protocol December 1993
Protocol Configuration Option. Further discussion is in a companion
document entitled PPP Authentication Protocols.
References
[1] Perkins, D., "Requirements for an Internet Standard
Point-to-Point Protocol", RFC 1547, December 1993.
[2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
Acknowledgments
Much of the text in this document is taken from the WG Requirements,
and RFCs 1171 & 1172, by Drew Perkins of Carnegie Mellon University,
and by Russ Hobby of the University of California at Davis.
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 (UUNET),
Ken Adelman (TGV), Fred Baker (ACC), Mike Ballard (Telebit), Craig
Fox (Network Systems), Karl Fox (Morning Star Technologies), Phill
Gross (AN&S), former WG chair Russ Hobby (UC Davis), David Kaufman
(Proteon), former WG chair Steve Knowles (FTP Software), former WG
chair Brian Lloyd (L&A), John LoVerso (Xylogics), Bill Melohn (Sun
Microsystems), Mike Patton (MIT), former WG chair Drew Perkins
(Fore), Greg Satz (cisco systems), John Shriver (Proteon), Vernon
Schryver (Silicon Graphics), and Asher Waldfogel (Wellfleet).
The "Day in the Life" example was instigated by Kory Hamzeh (Avatar).
In this version, improvements in wording were also provided by Scott
Ginsburg, Mark Moraes, and Timon Sloan, as they worked on
implementations.
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, 93111
EMail: fbaker@acc.com
Simpson [Page 52]RFC 1548 The Point-to-Point Protocol December 1993
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
EMail: Bill.Simpson@um.cc.umich.edu
Simpson [Page 53]
