Network Working Group W. Simpson
Request for Comments: 1331 Daydreamer
Obsoletes: RFCs 1171, 1172 May 1992 The Point-to-Point Protocol (PPP)
Transmission of Multi-protocol Datagrams
over Point-to-Point Links
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
The Point-to-Point Protocol (PPP) provides a method for transmitting
datagrams over serial point-to-point links. PPP is comprised of
three main components:
1. A method for encapsulating datagrams over serial links.
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 encapsulation scheme, together with the
PPP Link Control Protocol (LCP), an extensible option negotiation
protocol which is able to negotiate a rich assortment of
configuration parameters and provides additional management
This RFC is a product of the Point-to-Point Protocol Working Group of
the Internet Engineering Task Force (IETF). Comments on this memo
should be submitted to the firstname.lastname@example.org mailing list.
In the last few years, the Internet has seen explosive growth in
the number of hosts supporting TCP/IP. The vast majority of these
hosts are connected to Local Area Networks (LANs) of various
types, Ethernet being the most common. Most of the other hosts
are connected through Wide Area Networks (WANs) such as X.25 style
Public Data Networks (PDNs). Relatively few of these hosts are
connected with simple point-to-point (i.e., serial) links. Yet,
point-to-point links are among the oldest methods of data
communications and almost every host supports point-to-point
connections. For example, asynchronous RS-232-C  interfaces
are essentially ubiquitous.
One reason for the small number of point-to-point IP links is the
lack of a standard encapsulation protocol. There are plenty of
non-standard (and at least one de facto standard) encapsulation
protocols available, but there is not one which has been agreed
upon as an Internet Standard. By contrast, standard encapsulation
schemes do exist for the transmission of datagrams over most
PPP provides an encapsulation protocol over both bit-oriented
synchronous links and asynchronous links with 8 bits of data and
no parity. These links MUST be full-duplex, but MAY be either
dedicated or circuit-switched. PPP uses HDLC as a basis for the
PPP has been carefully designed to retain compatibility with most
commonly used supporting hardware. In addition, an escape
mechanism is specified to allow control data such as XON/XOFF to
be transmitted transparently over the link, and to remove spurious
control data which may be injected into the link by intervening
hardware and software.
The PPP encapsulation also 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.
Some protocols expect error free transmission, and either provide
error detection only on a conditional basis, or do not provide it
at all. PPP uses the HDLC Frame Check Sequence for error
detection. This is commonly available in hardware
implementations, and a software implementation is provided.
By default, only 8 additional octets are necessary to form the
encapsulation. In environments where bandwidth is at a premium,
the encapsulation may be shortened to as few as 2 octets. To
support high speed hardware implementations, PPP provides that the
default encapsulation header and information fields fall on 32-bit
boundaries, and allows the trailer to be padded to an arbitrary
Link Control Protocol
More importantly, the Point-to-Point Protocol defines more than
just an encapsulation scheme. 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
It is intended that PPP be easy to configure. By design, the
standard defaults should handle all common configurations. The
implementor may 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 may be used by the Internet Protocol Control
Protocol (IPCP) and others in 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.
This word, or the adjective "required", means that the definition
is an absolute requirement of the specification.
This phrase means that the definition is an absolute prohibition
of the specification.
This word, or the adjective "recommended", means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and carefully
weighed before choosing a different course.
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.
This document frequently uses the following terms:
The other end of the point-to-point link.
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
2. Physical Layer Requirements
The Point-to-Point Protocol is capable of operating across any
DTE/DCE interface (e.g., EIA RS-232-C, EIA RS-422, EIA RS-423 and
CCITT V.35). The only absolute requirement imposed by PPP is the
provision of a full-duplex circuit, either dedicated or circuit-
switched, which can operate in either an asynchronous (start/stop) or
synchronous bit-serial mode, transparent to PPP Data Link Layer
frames. PPP does not impose any restrictions regarding transmission
rate, other than those imposed by the particular DTE/DCE interface in
PPP does not require any particular synchronous encoding, such as FM,
NRZ, or NRZI.
NRZ is currently most widely available, and on that basis is
recommended as a default. When configuration of the encoding is
allowed, NRZI is recommended as an alternative, because of its
relative immunity to signal inversion configuration errors.
PPP does not require the use of modem control signals, such as
Request To Send (RTS), Clear To Send (CTS), Data Carrier Detect
(DCD), and Data Terminal Ready (DTR).
When available, using such signals can allow greater functionality
and performance. In particular, such signals SHOULD be used to
signal the Up and Down events in the Option Negotiation Automaton
3. The Data Link Layer
The Point-to-Point Protocol uses the principles, terminology, and
frame structure of the International Organization For
Standardization's (ISO) High-level Data Link Control (HDLC)
procedures (ISO 3309-1979 ), as modified by ISO 3309:1984/PDAD1
"Addendum 1: Start/stop transmission" . ISO 3309-1979 specifies
the HDLC frame structure for use in synchronous environments. ISO
3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to
allow its use in asynchronous environments.
The PPP control procedures use the definitions and Control field
encodings standardized in ISO 4335-1979  and ISO 4335-
1979/Addendum 1-1979 . The PPP frame structure is also consistent
with CCITT Recommendation X.25 LAPB , since that too is based on
The purpose of this memo is not to document what is already
standardized in ISO 3309. We assume that the reader is already
familiar with HDLC, or has access to a copy of  or . Instead,
this paper attempts to give a concise summary and point out specific
options and features used by PPP. Since "Addendum 1: Start/stop
transmission", is not yet standardized and widely available, it is
summarized in Appendix A.
To remain consistent with standard Internet practice, and avoid
confusion for people used to reading RFCs, all binary numbers in the
following descriptions are in Most Significant Bit to Least
Significant Bit order, reading from left to right, unless otherwise
indicated. Note that this is contrary to standard ISO and CCITT
practice which orders bits as transmitted (i.e., network bit order).
Keep this in mind when comparing this document with the international
3.1. Frame Format
A summary of the standard PPP frame structure is shown below. This
figure does not include start/stop bits (for asynchronous links), nor
any bits or octets inserted for transparency. The fields are
transmitted from left to right.
| Flag | Address | Control | Protocol | Information
| 01111110 | 11111111 | 00000011 | 16 bits | *
| FCS | Flag | Inter-frame Fill
| 16 bits | 01111110 | or next Address
Inter-frame Time Fill
For asynchronous links, inter-frame time fill SHOULD be accomplished
in the same manner as inter-octet time fill, by transmitting
continuous "1" bits (mark-hold state).
For synchronous links, the Flag Sequence SHOULD be transmitted during
inter-frame time fill. There is no provision for inter-octet time
Mark idle (continuous ones) SHOULD NOT be used for idle
synchronous inter-frame time fill. However, certain types of
circuit-switched links require the use of mark idle, particularly
those that calculate accounting based on bit activity. When mark
idle is used on a synchronous link, the implementation MUST ensure
at least 15 consecutive "1" bits between Flags, and that the Flag
Sequence is generated at the beginning and end of a frame.
The Flag Sequence is a single octet and indicates the beginning or
end of a frame. The Flag Sequence consists of the binary sequence
01111110 (hexadecimal 0x7e).
The Flag is a frame separator. Only one Flag is required between two
frames. Two consecutive Flags constitute an empty frame, which is
The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT
be used. When not avoidable, such an implementation MUST ensure
that the first Flag Sequence detected (the end of the frame) is
promptly communicated to the link layer.
The Address field is a single octet and contains the binary sequence
11111111 (hexadecimal 0xff), the All-Stations address. PPP does not
assign individual station addresses. The All-Stations address MUST
always be recognized and received. The use of other address lengths
and values may be defined at a later time, or by prior agreement.
Frames with unrecognized Addresses SHOULD be silently discarded, and
reported through the normal network management facility.
The Control field is a single octet and contains the binary sequence
00000011 (hexadecimal 0x03), the Unnumbered Information (UI) command
with the P/F bit set to zero. Frames with other Control field values
SHOULD be silently discarded.
The Protocol field is two octets and its value identifies the
protocol encapsulated in the Information field of the frame.
This Protocol field is defined by PPP and is not a field defined by
HDLC. However, the Protocol 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 considered as
having an unrecognized Protocol, and handled as specified by the LCP.
The Protocol field is transmitted and received most significant octet
Protocol field values in the "0---" to "3---" range identify the
network-layer protocol of specific datagrams, and values in the "8--
-" to "b---" range identify datagrams 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
datagrams as link-layer Control Protocols (such as LCP).
The most up-to-date values of the Protocol field are specified in the
most recent "Assigned Numbers" RFC . Current values are assigned
Value (in hex) Protocol Name
0001 to 001f reserved (transparency inefficient)
0021 Internet Protocol
0023 OSI Network Layer
0025 Xerox NS IDP
0027 DECnet Phase IV
002b Novell IPX
002d Van Jacobson Compressed TCP/IP
002f Van Jacobson Uncompressed TCP/IP
0031 Bridging PDU
0033 Stream Protocol (ST-II)
0035 Banyan Vines
0037 reserved (until 1993)
00ff reserved (compression inefficient)
0201 802.1d Hello Packets
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
8031 Bridging NCP
8033 Stream Protocol Control Protocol
8035 Banyan Vines Control Protocol
c021 Link Control Protocol
c023 Password Authentication Protocol
c025 Link Quality Report
c223 Challenge Handshake Authentication Protocol
Developers of new protocols MUST obtain a number from the Internet
Assigned Numbers Authority (IANA), at IANA@isi.edu.
The Information field is zero or more octets. The Information field
contains the datagram for the protocol specified in the Protocol
field. The end of the Information field is found by locating the
closing Flag Sequence and allowing two octets for the Frame Check
Sequence field. The default maximum length of the Information field
is 1500 octets. By negotiation, consenting PPP implementations may
use other values for the maximum Information field length.
On transmission, the Information field may be padded with an
arbitrary number of octets up to the maximum length. It is the
responsibility of each protocol to disambiguate padding octets from
Frame Check Sequence (FCS) Field
The Frame Check Sequence field is normally 16 bits (two octets). The
use of other FCS lengths may be defined at a later time, or by prior
The FCS field is calculated over all bits of the Address, Control,
Protocol and Information fields not including any start and stop bits
(asynchronous) and any bits (synchronous) or octets (asynchronous)
inserted for transparency. This does not include the Flag Sequences
or the FCS field itself. The FCS is transmitted with the coefficient
of the highest term first.
Note: When octets are received which are flagged in the Async-
Control-Character-Map, they are discarded before calculating the
FCS. See the description in Appendix A.
For more information on the specification of the FCS, see ISO 3309
 or CCITT X.25 .
Note: A fast, table-driven implementation of the 16-bit FCS
algorithm is shown in Appendix B. This implementation is based on
, , and .
Modifications to the Basic Frame Format
The Link Control Protocol can negotiate modifications to the standard
PPP frame structure. However, modified frames will always be clearly
distinguishable from standard frames.
4. PPP Link Operation
In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure and test
the data link. After the link has been established, the peer may be
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
4.2. Phase Diagram
In the process of configuring, maintaining and terminating the
point-to-point link, the PPP link goes through several distinct
+------+ +-----------+ +--------------+
| | UP | | OPENED | | SUCCESS/NONE
| Dead |------->| Establish |---------->| Authenticate |--+
| | | | | | |
+------+ +-----------+ +--------------+ |
^ FAIL | FAIL | |
+<--------------+ +----------+ |
| | |
| +-----------+ | +---------+ |
| DOWN | | | CLOSING | | |
+------------| Terminate |<---+<----------| Network |<-+
| | | |
4.3. 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.
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.
4.4. 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. Any non-LCP
packets received during this phase MUST be silently discarded.
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
4.5. Authentication Phase
On some links it may be desirable to require a peer to authenticate
itself before allowing network-layer protocol packets to be
By default, authentication is not necessary. If an implementation
requires that the peer authenticate with some specific authentication
protocol, then it MUST negotiate 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 the peer is successfully
authenticated using the negotiated authentication protocol. In the
event of failure to authenticate, PPP SHOULD proceed instead to the
Link Termination phase.
4.6. Network-Layer Protocol Phase
Once PPP has finished the previous phases, each network-layer
protocol (such as IP) MUST be separately configured by the
appropriate Network Control Protocol (NCP).
Each NCP may be Opened and Closed at any time.
Because an implementation may initially use a significant amount
of time for link quality determination, implementations SHOULD
avoid fixed timeouts when waiting for their peers to configure a
After a NCP has reached the Opened state, PPP will carry the
corresponding network-layer protocol packets. Any network-layer
protocol packets received when the corresponding NCP is not in the
Opened state SHOULD be silently discarded.
During this phase, link traffic consists of any possible combinations
of LCP, NCP, and network-layer protocol packets. Any NCP or
network-layer protocol packets received during any other phase SHOULD
be silently discarded.
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 supported protocols are silently discarded.
4.7. Link Termination Phase
PPP may terminate the link at any time. This will usually be done at
the request of a human user, but might happen because of a physical
event such as the loss of carrier, authentication failure, link
quality failure, or the expiration of an idle-period timer.
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.
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 a NCP has Closed is not sufficient reason to cause
the termination of the PPP link, even if that NCP was the only
currently NCP in the Opened state.
5. The Option Negotiation Automaton
The finite-state automaton is defined by events, actions and state
transitions. Events include reception of external commands such as
Open and Close, expiration of the Restart timer, and reception of
packets from a peer. Actions include the starting of the Restart
timer and transmission of packets to the peer.
Some types of packets -- Configure-Naks and Configure-Rejects, or
Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
Discard-Requests -- are not differentiated in the automaton
descriptions. As will be described later, these packets do indeed
serve different functions. However, they always cause the same
Up = lower layer is Up tlu = This-Layer-Up
Down = lower layer is Down tld = This-Layer-Down
Open = administrative Open tls = This-Layer-Start
Close= administrative Close tlf = This-Layer-Finished
TO+ = Timeout with counter > 0 irc = initialize restart
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)
RXJ- = Receive-Code-Reject (catastrophic)
RXR = Receive-Echo-Request ser = Send-Echo-Reply
- = illegal action
5.1. 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
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.
| | | |
+-------+ | RTA +-------+ | Close +-------+
| |<-----+<------| |<-str-+<------| |
|Closed | |Closing| |Opened |
| | Open | | | |
| |------+ | | | |
+-------+ | +-------+ +-------+
| | ^
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-
| | +------------------------------->+<-------+ |
| | |
5.2. 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. The state may be followed by a letter, which
indicates an explanatory footnote.
In previous versions of this table, a simplified non-deterministic
finite-state automaton was used, with considerable detailed
information specified in the semantics. This lead to
interoperability problems from differing interpretations.
This table functions similarly to the previous versions, with the
up/down flags expanded to explicit states, and the active/passive
paradigm eliminated. It is believed that this table interoperates
with previous versions better than those versions themselves.
| 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
| 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 tld,scj,scr/6
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 SHOULD be 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.
Following is a more detailed description of each automaton state.
In the Initial state, the lower layer is unavailable (Down), and
no Open has occurred. The Restart timer is not running in the
The Starting state is the Open counterpart to the Initial state.
An administrative Open has been initiated, but the lower layer is
still unavailable (Down). The Restart timer is not running in the
When the lower layer becomes available (Up), a Configure-Request
In the Closed state, the link is available (Up), but no Open has
occurred. The Restart timer is not running in the Closed state.
Upon reception of Configure-Request packets, a Terminate-Ack is
sent. Terminate-Acks are silently discarded to avoid creating a
The Stopped state is the Open counterpart to the Closed state. It
is entered when the automaton is waiting for a Down event after
the This-Layer-Finished action, or after sending a Terminate-Ack.
The Restart timer is not running in the Stopped state.
Upon reception of Configure-Request packets, an appropriate
response is sent. Upon reception of other packets, a Terminate-
Ack is sent. Terminate-Acks are silently discarded to avoid
creating a loop.
The Stopped state is a junction state for link termination,
link configuration failure, and other automaton failure modes.
These potentially separate states have been combined.
There is a race condition between the Down event response (from
the This-Layer-Finished action) and the Receive-Configure-
Request event. When a Configure-Request arrives before the
Down event, the Down event will supercede by returning the
automaton to the Starting state. This prevents attack by
After the peer fails to respond to Configure-Requests, an
implementation MAY wait passively for the peer to send
Configure-Requests. In this case, the This-Layer-Finished
action is not used for the TO- event in states Req-Sent, Ack-
Rcvd and Ack-Sent.
This option is useful for dedicated circuits, or circuits which
have no status signals available, but SHOULD NOT be used for
In the Closing state, an attempt is made to terminate the
connection. A Terminate-Request has been sent and the Restart
timer is running, but a Terminate-Ack has not yet been received.
Upon reception of a Terminate-Ack, the Closed state is entered.
Upon the expiration of the Restart timer, a new Terminate-Request
is transmitted and the Restart timer is restarted. After the
Restart timer has expired Max-Terminate times, this action may be
skipped, and the Closed state may be entered.
The Stopping state is the Open counterpart to the Closing state.
A Terminate-Request has been sent and the Restart timer is
running, but a Terminate-Ack has not yet been received.
The Stopping state provides a well defined opportunity to
terminate a link before allowing new traffic. After the link
has terminated, a new configuration may occur via the Stopped
or Starting states.
In the Request-Sent state an attempt is made to configure the
connection. A Configure-Request has been sent and the Restart
timer is running, but a Configure-Ack has not yet been received
nor has one been sent.
In the Ack-Received state, a Configure-Request has been sent and a
Configure-Ack has been received. The Restart timer is still
running since a Configure-Ack has not yet been sent.
In the Ack-Sent state, a Configure-Request and a Configure-Ack
have both been sent but a Configure-Ack has not yet been received.
The Restart timer is always running in the Ack-Sent state.
In the Opened state, a Configure-Ack has been both sent and
received. The Restart timer is not running in the Opened state.
When entering the Opened state, the implementation SHOULD signal
the upper layers that it is now Up. Conversely, when leaving the
Opened state, the implementation SHOULD signal the upper layers
that it is now Down.
Transitions and actions in the automaton are caused by events.
The Up event occurs when a lower layer indicates that it is ready
to carry packets. Typically, this event is used to signal LCP
that the link is entering Link Establishment phase, or used to
signal a NCP that the link is entering Network-Layer Protocol
The Down event occurs when a lower layer indicates that it is no
longer ready to carry packets. Typically, this event is used to
signal LCP that the link is entering Link Dead phase, or used to
signal a NCP that the link is leaving Network-Layer Protocol
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.
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 should be necessary.
Experience has shown that users will execute an additional Open
command when they want to renegotiate the link. 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.
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.
This event indicates the expiration of the Restart timer. The
Restart timer is used to time responses to Configure-Request and
The TO+ event indicates that the Restart counter continues to be
greater than zero, which triggers the corresponding Configure-
Request or Terminate-Request packet to be retransmitted.
The TO- event indicates that the Restart counter is not greater
than zero, and no more packets need to be retransmitted.
This event occurs when a Configure-Request packet is received from
the peer. The Configure-Request packet indicates the desire to
open a connection and may specify Configuration Options. The
Configure-Request packet is more fully described in a later
The RCR+ event indicates that the Configure-Request was
acceptable, and triggers the transmission of a corresponding
The RCR- event indicates that the Configure-Request was
unacceptable, and triggers the transmission of a corresponding
Configure-Nak or Configure-Reject.
These events may occur on a connection which is already in the
Opened state. The implementation MUST be prepared to
immediately renegotiate the Configuration Options.
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.
Since the correct packet has already been received before
reaching the Ack-Rcvd or Opened states, it is extremely
unlikely that another such packet will arrive. As specified,
all invalid Ack/Nak/Rej packets are silently discarded, and do
not affect the transitions of the automaton.
However, it is not impossible that a correctly formed packet
will arrive through a coincidentally-timed cross-connection.
It is more likely to be the result of an implementation error.
At the very least, this occurance should be logged.
This event occurs when a valid Configure-Nak or Configure-Reject
packet is received from the peer. The Configure-Nak and
Configure-Reject packets are negative responses to a Configure-
Request packet. An out of sequence or otherwise invalid packet is
Although the Configure-Nak and Configure-Reject cause the same
state transition in the automaton, these packets have
significantly different effects on the Configuration Options
sent in the resulting Configure-Request packet.
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.
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
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.
The Receive-Unknown-Code event occurs when an un-interpretable
packet is received from the peer. A Code-Reject packet is sent in
Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
This event occurs when a Code-Reject or a Protocol-Reject packet
is received from the peer.
The RXJ+ event arises when the rejected value is acceptable, such
as a Code-Reject of an extended code, or a Protocol-Reject of a
NCP. These are within the scope of normal operation. The
implementation MUST stop sending the offending packet type.
The RXJ- event arises when the rejected value is catastrophic,
such as a Code-Reject of Configure-Request, or a Protocol-Reject
of LCP! This event communicates an unrecoverable error that
terminates the connection.
Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
This event occurs when an Echo-Request, Echo-Reply or Discard-
Request packet is received from the peer. The Echo-Reply packet
is a response to a Echo-Request packet. There is no reply to an
Echo-Reply or Discard-Request packet.
Actions in the automaton are caused by events and typically indicate
the transmission of packets and/or the starting or stopping of the
This indicates an event that SHOULD NOT occur. The implementation
probably has an internal error.
This action indicates to the upper layers that the automaton is
entering the Opened state.
Typically, this action MAY be 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
This action indicates to the upper layers that the automaton is
leaving the Opened state.
Typically, this action MAY be 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 traffic.
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 action is highly implementation dependent.
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.
This action is highly implementation dependent.
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.
This action sets the Restart counter to zero.
This action enables the FSA to pause before proceeding to the
desired final state. In addition to zeroing the Restart
counter, the implementation MUST set the timeout period to an
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.
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.
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.
The Send-Terminate-Request action transmits a Terminate-Request
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.
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.
The Send-Code-Reject action transmits a Code-Reject packet. This
indicates the reception of an unknown type of packet.
The Send-Echo-Reply action transmits an Echo-Reply packet. This
acknowledges the reception of an Echo-Request packet.
5.6. 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.
5.7. Counters and Timers
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 MAY default to three (3) seconds.
The Restart timer SHOULD be based on the speed of the link. The
default value is designed for low speed (19,200 bps or less), high
switching latency links (typical telephone lines). Higher speed
links, or links with low switching latency, SHOULD have
correspondingly faster retransmission times.
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.
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)
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