Network Working Group D. Perkins
Request for Comments: 1171 CMU
Obsoletes: RFC 1134 July 1990
The Point-to-Point Protocol
for the
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.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
This proposal is the 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 IETF Point-to-Point Protocol Working
Group chair.
Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) provides a method for transmitting
datagrams over serial point-to-point links. PPP is composed of three
parts:
1. A method for encapsulating datagrams over serial links.
2. An extensible Link Control Protocol (LCP).
3. A family of Network Control Protocols (NCP) for establishing
and configuring different network-layer protocols.
This document defines the encapsulation scheme, the basic LCP, and an
NCP for establishing and configuring the Internet Protocol (IP)
(called the IP Control Protocol, IPCP).
The options and facilities used by the LCP and the IPCP are defined
in separate documents. Control protocols for configuring and
utilizing other network-layer protocols besides IP (e.g., DECNET,
OSI) are expected to be developed as needed.
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Table of Contents
1. Introduction .......................................... 1
1.1 Motivation ...................................... 1
1.2 Overview of PPP ................................. 1
1.3 Organization of the document .................... 2
2. Physical Layer Requirements ........................... 3
3. The Data Link Layer ................................... 4
3.1 Frame Format .................................... 5
4. The PPP Link Control Protocol (LCP) ................... 9
4.1 The LCP Automaton ............................... 11
4.1.1 Overview ........................................ 11
4.1.2 State Diagram ................................... 11
4.1.3 State Transition Table .......................... 13
4.1.4 Events .......................................... 13
4.1.5 Actions ......................................... 16
4.1.6 States .......................................... 17
4.2 Loop Avoidance .................................. 20
4.3 Timers and Counters ............................. 20
4.4 Packet Format ................................... 21
4.4.1 Configure-Request ............................... 23
4.4.2 Configure-Ack ................................... 24
4.4.3 Configure-Nak ................................... 25
4.4.4 Configure-Reject ................................ 27
4.4.5 Terminate-Request and Terminate-Ack ............. 29
4.4.6 Code-Reject ..................................... 31
4.4.7 Protocol-Reject ................................. 32
4.4.8 Echo-Request and Echo-Reply ..................... 34
4.4.9 Discard-Request ................................. 36
4.5 Configuration Options ........................... 38
4.5.1 Format .......................................... 39
5. A PPP Network Control Protocol (NCP) for IP ........... 40
5.1 Sending IP Datagrams ............................ 40
APPENDICES ................................................... 42
A. Asynchronous HDLC ..................................... 42
B. Fast Frame Check Sequence (FCS) Implementation ........ 44
B.1 FCS Computation Method .......................... 44
B.2 Fast FCS table generator ........................ 46
REFERENCES ................................................... 47
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SECURITY CONSIDERATIONS ...................................... 48
CHAIRMAN'S ADDRESS ........................................... 48
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1. Introduction
1.1. Motivation
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 [1] 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 defacto 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 popular LANs.
One purpose of this memo is to remedy this problem. But even more
importantly, the Point-to-Point Protocol proposes more than just an
encapsulation scheme. 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 circuits (e.g., dialups).
Some additional issues addressed by this specification of PPP include
asynchronous (start/stop) and bit-oriented synchronous encapsulation,
network protocol multiplexing, link configuration, link quality
testing, error detection, and option negotiation for such
capabilities as network-layer address negotiation and data
compression negotiation.
PPP addresses these issues by providing an extensible Link Control
Protocol (LCP) and a family of Network Control Protocols (NCP) to
negotiate optional configuration parameters and facilities.
1.2. Overview of PPP
PPP has three main components:
1. A method for encapsulating datagrams over serial links. PPP
uses HDLC as a basis for encapsulating datagrams over point-
to-point links. At this time, PPP specifies the use of
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asynchronous or synchronous duplex circuits, either dedicated
or circuit switched.
2. An extensible Link Control Protocol (LCP) to establish,
configure, and test the data-link connection.
3. A family of Network Control Protocols (NCP) for establishing
and configuring different network-layer protocols. PPP is
designed to allow the simultaneous use of multiple network-
layer protocols.
In order to establish communications over a point-to-point link, the
originating PPP would first send LCP packets to configure and test
the data link. After the link has been establish and optional
facilities have been negotiated as needed by the LCP, the originating
PPP would 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 (e.g., inactivity timer expires or user intervention).
1.3. Organization of the document
This memo is divided into several sections. Section 2 discusses the
physical-layer requirements of PPP. Section 3 describes the Data
Link Layer including the PPP frame format and data link encapsulation
scheme. Section 4 specifies the LCP including the connection
establishment and option negotiation procedures. Section 5 specifies
the IP Control Protocol (IPCP), which is the NCP for the Internet
Protocol, and describes the encapsulation of IP datagrams within PPP
packets. Appendix A summarizes important features of asynchronous
HDLC, and Appendix B describes an efficient table-lookup algorithm
for fast Frame Check Sequence (FCS) computation.
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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 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
use.
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). However, using such signals
when available can allow greater functionality and performance.
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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 [2]), as modified by ISO 3309:1984/PDAD1
"Addendum 1: Start/stop transmission" [5]. 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 [3] and ISO 4335-
1979/Addendum 1-1979 [4]. The PPP frame structure is also consistent
with CCITT Recommendation X.25 LAPB [6], since that too is based on
HDLC.
Note: ISO 3309:1984/PDAD1 is a Proposed Draft standard. At
present, it seems that ISO 3309:1984/PDAD1 is stable and likely to
become an International Standard. Therefore, we feel comfortable
about using it before it becomes an International Standard. The
progress of this proposal should be tracked and encouraged by the
Internet community.
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 [2] or [6]. 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.
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3.1. Frame Format
A summary of the standard PPP frame structure is shown below. The
fields are transmitted from left to right.
+----------+----------+----------+----------+------------
| Flag | Address | Control | Protocol | Information
| 01111110 | 11111111 | 00000011 | 16 bits | *
+----------+----------+----------+----------+------------
---+---------+----------+
| FCS | Flag |
| 16 bits | 01111110 |
---+---------+----------+
This figure does not include start/stop bits (for asynchronous links)
or any bits or octets inserted for transparency. When asynchronous
links are used, all octets are transmitted with one start bit, eight
bits of data, and one stop bit. There is no provision in either PPP
or ISO 3309:1984/PDAD1 for seven bit asynchronous links.
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
standards documents.
Flag Sequence
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).
Address Field
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 should 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 reported through the normal network management facility.
Control Field
The Control field is a single octet and contains the binary
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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.
Protocol Field
The Protocol field is two octets and its value identifies the
protocol encapsulated in the Information field of the frame. The
most up-to-date values of the Protocol field are specified in the
most recent "Assigned Numbers" RFC [12]. Initial values are also
listed below.
Protocol field values in the "cxxx" range identify datagrams as
belonging to the Link Control Protocol (LCP) or associated
protocols. Values in the "8xxx" range identify datagrams belonging
to the family of Network Control Protocols (NCP). Values in the
"0xxx" range identify the network protocol of specific datagrams.
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 should be
considered as having an unrecognized Protocol, and should be
handled as specified by the LCP. The Protocol field is
transmitted and received most significant octet first.
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The Protocol field is initially assigned as follows:
Value (in hex) Protocol
0001 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 1
002f * Van Jacobson Compressed TCP/IP 2
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
c021 Link Control Protocol
c023 * User/Password Authentication Protocol
* Reserved for future use; not described in this document.
Information Field
The Information field is zero or more octets. The Information
field contains the datagram for the protocol specified in the
Protocol field. The 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 prior agreement, 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 real information.
Frame Check Sequence (FCS) Field
The Frame Check Sequence field is normally 16 bits (two octets).
By prior agreement, consenting PPP implementations may use a 32-
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bit (four-octet) FCS for improved error detection.
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 FCS field. The FCS is transmitted with the
coefficient of the highest term first.
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 [7], [8], and [9].
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.
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4. The PPP Link Control Protocol (LCP)
The Link Control Protocol (LCP) provides a method of establishing,
configuring, maintaining and terminating the point-to-point
connection. LCP goes through four distinct phases:
Phase 1: Link Establishment and Configuration Negotiation
Before any network-layer datagrams (e.g., IP) may be exchanged,
LCP must first open the connection through an exchange of
Configure packets. This exchange is complete, and the Open state
entered, once a Configure-Ack packet (described below) has been
both sent and received. Any non-LCP packets received before this
exchange is complete are silently discarded.
It is important to note that LCP handles configuration only of the
link; LCP does not handle configuration of individual network-
layer protocols. In particular, all Configuration Parameters
which are independent of particular network-layer protocols are
configured by LCP. All Configuration Options are assumed to be at
default values unless altered by the configuration exchange.
Phase 2: Link Quality Determination
LCP allows an optional Link Quality Determination phase following
transition to the LCP Open state. In this phase, the link is
tested to determine if the link quality is sufficient to bring up
network-layer protocols. This phase is completely optional. LCP
may delay transmission of network-layer protocol information until
this phase is completed.
The procedure for Link Quality Determination is unspecified and
may vary from implementation to implementation, or because of
user-configured parameters, but only so long as the procedure
doesn't violate other aspects of LCP. One suggested method is to
use LCP Echo-Request and Echo-Reply packets.
What is important is that this phase may persist for any length of
time. Therefore, implementations should avoid fixed timeouts when
waiting for their peers to advance to phase 3.
Phase 3: Network-Layer Protocol Configuration Negotiation
Once LCP has finished the Link Quality Determination phase,
network-layer protocols may be separately configured by the
appropriate Network Control Protocols (NCP), and may be brought up
and taken down at any time. If LCP closes the link, it informs
the network-layer protocols so that they may take appropriate
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action.
Phase 4: Link Termination
LCP may terminate the link at any time. This will usually be done
at the request of a human user, but may happen because of a
physical event such as the loss of carrier, or the expiration of
an idle-period timer.
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4.1. The LCP Automaton
4.1.1. Overview
LCP is specified by a number of packet formats and a finite-state
automaton. This section presents an overview of the LCP automaton,
followed by a representation of it as both a state diagram and a
state transition table.
There are three classes of LCP packets:
1. Link Establishment packets used to establish and configure a
link, (e.g., Configure-Request, Configure-Ack, Configure-Nak
and Configure-Reject)
2. Link Termination packets used to terminate a link, (e.g.,
Terminate-Request and Terminate-Ack)
3. Link Maintenance packets used to manage and debug a link,
(e.g., Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply
and Discard-Request)
The finite-state automaton is defined by events, state transitions
and actions. Events include receipt of external commands such as
Open and Close, expiration of the Restart timer, and receipt of
packets from a LCP peer. Actions include the starting of the Restart
timer and transmission of packets.
4.1.2. State Diagram
The state diagram which follows describes the sequence of events for
reaching agreement on Configuration Options (opening the PPP
connection) and for later closing of the connection. The state
machine is initially in the Closed state (1). Once the Open state
(6) has been reached, both ends of the link have met the requirement
of having both sent and received a Configure-Ack packet.
In the state diagram, events are shown above horizontal lines.
Actions are shown below horizontal lines. Two types of LCP packets -
Configure-Naks and Configure-Rejects - are not differentiated in the
state diagram. As will be described later, these packets do indeed
serve different, though similar, functions. However, at the level of
detail of this state diagram, they always cause the same transition.
Since a more detailed specification of the LCP automaton is given in
a state transition table in the following section, implementation
should be done by consulting it rather than this state diagram.
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+------------------------------+
| |
V |
+---2---+ PO +---1---+ RTA +---7---+ |
| |<-------------| |<-----------| | |
|Listen | |Closed | |Closing| |
RCR | | C | | PLD | | |
+----| |----->+------>| |<---Any | |<--+ |
|scr +-------+ ^ +-------+ State +-------+ | |
| | AO | ^ | TO | |
| +-----------+ --- | | +---->+ |
| | SCR | | str ^ |
| C | RCN/TO | | C | |
| --- | +-------->+<--------+ | --- | |
| | | scr | | | |
| +---3---+ V TO +---4---+ +-------+ | |
| | |<-----+<------| |<-----------| | | |
| | Req- | scr | Ack- | scn | Good | | |
| | Sent | RCA | Rcvd | RCR | Req? | | |
| | |------------->| |----------->| | | |
| +-------+ +-------+ +-------+ | |
| | ^ | | |
| RCR | +<--------+ | | |
| --- | | | TO RCN --- | | |
| | | --- +---------+ +-----+ sca | | |
| V | scn scr | | scr | V | |
| +-------+ +---5---+ | +---6---+ C | |
+--->| |------------->| |<--+ | |---+ |
| Good | sca | Ack- | | Open | str |
| Req? | RCR | Sent | RCA | | |
| |<-------------| |----------->| | |
+-------+ +-------+ +-------+ |
^ | | |
| RCR | | RTR |
+---------------------------------------+ +--------+
scr sta
Events Actions
RCR - Receive-Configure-Request scr - Send Configure-Request
RCA - Receive-Configure-Ack sca - Send Configure-Ack
RCN - Receive-Configure-Nak or Reject scn - Send Configure-Nak
RTR - Receive-Terminate-Req or Reject
RTA - Receive-Terminate-Ack str - Send Terminate-Req
AO - Active-Open sta - Sent Terminate-Ack
PO - Passive-Open
C - Close
TO - Timeout
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PLD - Physical-Layer-Down
4.1.3. 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. Two actions
caused by the same event are represented as action1&action2.
| State
| 1 2 3 4 5 6 7
Events| Closed Listen Req-Sent Ack-Rcvd Ack-Sent Open Closing
------+-------------------------------------------------------------
AO | scr/3 scr/3 3 4 5 6 scr/3
PO | 2 2 2* 4 5 6 sta/3*
C | 1 1 1* 1 str/7 str/7 7
TO | 1 2 scr/3 scr/3 scr/3 6 str/7*
PLD | 1 1 1 1 1 1 1
RCR+ | sta/1 scr&sca/5 sca/5 sca/6 sca/5 scr&sca/5 7
RCR- | sta/1 scr&scn/3 scn/3 scn/4 scn/3 scr&scn/3 7
RCA | sta/1 sta/2 4 scr/3 6 scr/3 7
RCN | sta/1 sta/2 scr/3 scr/3 scr/5 scr/3 7
RTR | sta/1 sta/2 sta/3 sta/3 sta/3 sta/1 sta/7
RTA | 1 2 3 3 3 1 1
RCJ | 1 2 1 1 1 1 1
RUC | scj/1 scj/2 scj/1 scj/1 scj/1 scj/1 1 scj/7
RER | sta/1 sta/2 3 4 5 ser/6 7
Notes:
RCR+ - Receive-Configure-Request (Good)
RCR- - Receive-Configure-Request (Bad)
RCJ - Receive-Code-Reject
RUC - Receive-Unknown-Code
RER - Receive-Echo-Request
scj - Send-Code-Reject
ser - Send-Echo-Reply
* - Special attention necessary, see detailed text
4.1.4. Events
Transitions and actions in the LCP state machine are caused by
events. Some events are caused by commands executed at the local end
(e.g., Active-Open, Passive-Open, and Close), others are caused by
the receipt of packets from the remote end (e.g., Receive-
Configure-Request, Receive-Configure-Ack, Receive-Configure-Nak,
Receive- Terminate-Request and Receive-Terminate-Ack), and still
others are caused by the expiration of the Restart timer started as
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the result of other events (e.g., Timeout).
Following is a list of LCP events.
Active-Open (AO)
The Active-Open event indicates the local execution of an Active-
Open command by the network administrator (human or program).
When this event occurs, LCP should immediately attempt to open the
connection by exchanging configuration packets with the LCP peer.
Passive-Open (PO)
The Passive-Open event is similar to the Active-Open event.
However, instead of immediately exchanging configuration packets,
LCP should wait for the peer to send the first packet. This will
only happen after an Active-Open event in the LCP peer.
Close (C)
The Close event indicates the local execution of a Close command.
When this event occurs, LCP should immediately attempt to close
the connection.
Timeout (TO)
The Timeout event indicates the expiration of the LCP Restart
timer. The LCP Restart timer is started as the result of other
LCP events.
The Restart timer is used to time out transmissions of Configure-
Request and Terminate-Request packets. Expiration of the Restart
timer causes a Timeout event, which triggers the corresponding
Configure-Request or Terminate-Request packet to be retransmitted.
The Restart timer MUST be configurable, but should default to
three (3) seconds.
Receive-Configure-Request (RCR)
The Receive-Configure-Request event occurs when a Configure-
Request packet is received from the LCP peer. The Configure-
Request packet indicates the desire to open a LCP connection and
may specify Configuration Options. The Configure-Request packet
is more fully described in a later section.
Receive-Configure-Ack (RCA)
The Receive-Configure-Ack event occurs when a valid Configure-Ack
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packet is received from the LCP peer. The Configure-Ack packet is
a positive response to a Configure-Request packet.
Receive-Configure-Nak (RCN)
The Receive-Configure-Nak event occurs when a valid Configure-Nak
or Configure-Reject packet is received from the LCP peer. The
Configure-Nak and Configure-Reject packets are negative responses
to a Configure-Request packet.
Receive-Terminate-Request (RTR)
The Receive-Terminate-Request event occurs when a Terminate-
Request packet is received from the LCP peer. The Terminate-
Request packet indicates the desire to close the LCP connection.
Receive-Terminate-Ack (RTA)
The Receive-Terminate-Ack event occurs when a Terminate-Ack packet
is received from the LCP peer. The Terminate-Ack packet is a
response to a Terminate-Request packet.
Receive-Code-Reject (RCJ)
The Receive-Code-Reject event occurs when a Code-Reject packet is
received from the LCP peer. The Code-Reject packet communicates
an error that immediately closes the connection.
Receive-Unknown-Code (RUC)
The Receive-Unknown-Code event occurs when an un-interpretable
packet is received from the LCP peer. The Code-Reject packet is a
response to an unknown packet.
Receive-Echo-Request (RER)
The Receive-Echo-Request event occurs when a Echo-Request, Echo-
Reply, or Discard-Request packet is received from the LCP peer.
The Echo-Reply packet is a response to a Echo-Request packet.
There is no reply to a Discard-Request.
Physical-Layer-Down (PLD)
The Physical-Layer-Down event occurs when the Physical Layer
indicates that it is down.
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4.1.5. Actions
Actions in the LCP state machine are caused by events and typically
indicate the transmission of packets and/or the starting or stopping
of the Restart timer. Following is a list of LCP actions.
Send-Configure-Request (scr)
The Send-Configure-Request action transmits a Configure-Request
packet. This indicates the desire to open a LCP connection with a
specified set of Configuration Options. The Restart timer is
started after the Configure-Request packet is transmitted, to
guard against packet loss.
Send-Configure-Ack (sca)
The Send-Configure-Ack action transmits a Configure-Ack packet.
This acknowledges the receipt 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 receipt 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 vs. Configure-Reject is more fully described in the
section on LCP Packet Formats.
Send-Terminate-Req (str)
The Send-Terminate-Request action transmits a Terminate-Request
packet. This indicates the desire to close a LCP connection. The
Restart timer is started after the Terminate-Request packet is
transmitted, to guard against packet loss.
Send-Terminate-Ack (sta)
The Send-Terminate-Request action transmits a Terminate-Ack
packet. This acknowledges the receipt of a Terminate-Request
packet or otherwise confirms the belief that a LCP connection is
Closed.
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Send-Code-Reject (scj)
The Send-Code-Reject action transmits a Code-Reject packet. This
indicates the receipt of an unknown type of packet. This is an
unrecoverable error which causes immediate transitions to the
Closed state on both ends of the link.
Send-Echo-Reply (ser)
The Send-Echo-Reply action transmits an Echo-Reply packet. This
acknowledges the receipt of an Echo-Request packet.
4.1.6. States
Following is a more detailed description of each LCP state.
Closed (1)
The initial and final state is the Closed state. In the Closed
state the connection is down and there is no attempt to open it;
all connection requests from peers are rejected. Physical-Layer-
Down events always cause an immediate transition to the Closed
state.
There are two events which cause a transition out of the Closed
state, Active-Open and Passive-Open. Upon an Active-Open event, a
Configure-Request is transmitted, the Restart timer is started,
and the Request-Sent state is entered. Upon a Passive-Open event,
the Listen state is entered immediately. Upon receipt of any
packet, with the exception of a Terminate-Ack, a Terminate-Ack is
sent. Terminate-Acks are silently discarded to avoid creating a
loop.
The Restart timer is not running in the Closed state.
The Physical Layer connection may be disconnected at any time when
in the LCP Closed state.
Listen (2)
The Listen state is similar to the Closed state in that the
connection is down and there is no attempt to open it. However,
peer connection requests are no longer rejected.
Upon receipt of a Configure-Request, a Configure-Request is
immediately transmitted and the Restart timer is started. The
received Configuration Options are examined and the proper
response is sent. If a Configure-Ack is sent, the Ack-Sent state
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is entered. Otherwise, if a Configure-Nak or Configure-Reject is
sent, the Request-Sent state is entered. In either case, LCP
exits its passive state, and begins to actively open the
connection. Terminate-Ack packets are sent in response to either
Configure-Ack or Configure-Nak packets,
The Restart timer is not running in the Listen state.
Request-Sent (3)
In the Request-Sent state an active attempt is made to open 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.
Upon receipt of a Configure-Ack, the Ack-Received state is
immediately entered. Upon receipt of a Configure-Nak or
Configure-Reject, the Configure-Request Configuration Options are
adjusted appropriately, a new Configure-Request is transmitted,
and the Restart timer is restarted. Similarly, upon the
expiration of the Restart timer, a new Configure-Request is
transmitted and the Restart timer is restarted. Upon receipt of a
Configure-Request, the Configuration Options are examined and if
acceptable, a Configure-Ack is sent and the Ack-Sent state is
entered. If the Configuration Options are unacceptable, a
Configure-Nak or Configure-Reject is sent as appropriate.
Since there is an outstanding Configure-Request in the Request-
Sent state, special care must be taken to implement the Passive-
Open and Close events; otherwise, it is possible for the LCP peer
to think the connection is open. Processing of either event
should be postponed until there is reasonable assurance that the
peer is not open. In particular, the Restart timer should be
allowed to expire.
Ack-Received (4)
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 transmitted.
Upon receipt of a Configure-Request with acceptable Configuration
Options, a Configure-Ack is transmitted, the Restart timer is
stopped and the Open state is entered. If the Configuration
Options are unacceptable, a Configure-Nak or Configure-Reject is
sent as appropriate. Upon the expiration of the Restart timer, a
new Configure-Request is transmitted, the Restart timer is
restarted, and the state machine returns to the Request-Sent
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state.
Ack-Sent (5)
In the Ack-Sent state, a Configure-Ack and a Configure-Request
have been sent but a Configure-Ack has not yet been received. The
Restart timer is always running in the Ack-Sent state.
Upon receipt of a Configure-Ack, the Restart timer is stopped and
the Open state is entered. Upon receipt of a Configure-Nak or
Configure-Reject, the Configure-Request Configuration Options are
adjusted appropriately, a new Configure-Request is transmitted,
and the Restart timer is restarted. Upon the expiration of the
Restart timer, a new Configure-Request is transmitted, the Restart
timer is restarted, and the state machine returns to the Request-
Sent state.
Open (6)
In the Open state, a connection exists and data may be
communicated over the link. The Restart timer is not running in
the Open state.
In normal operation, only two events cause transitions out of the
Open state. Upon receipt of a Close command, a Terminate-Request
is transmitted, the Restart timer is started, and the Closing
state is entered. Upon receipt of a Terminate-Request, a
Terminate-Ack is transmitted and the Closed state is entered.
Upon receipt of an Echo-Request, an Echo-Reply is transmitted.
Similarly, Echo-Reply and Discard-Request packets are silently
discarded or processed as expected. All other events cause
immediate transitions out of the Open state and should be handled
as if the state machine were in the Listen state.
Closing (7)
In the Closing state, an active attempt is made to close the
connection. A Terminate-Request has been sent and the Restart
timer is running, but a Terminate-Ack has not yet been received.
Upon receipt of a Terminate-Ack, the Closed state is immediately
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-Restart times,
this action may be skipped, and the Closed state may be entered.
Max-Restart MUST be a configurable parameter.
Since there is an outstanding Terminate-Request in the Closing
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state, special care must be taken to implement the Passive-Open
event; otherwise, it is possible for the LCP peer to think the
connection is open. Processing of the Passive-Open event should
be postponed until there is reasonable assurance that the peer is
not open. In particular, the implementation should wait until the
state machine would normally transition to the Closed state
because of a Receive-Terminate-Ack event or Max-Restart Timeout
events.
4.2. Loop Avoidance
Note that 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. If a timeout is
implemented, it MUST be configurable.
4.3. Timers and Counters
There is one special timer used by LCP, the Restart timer. The
Restart timer is used to time out transmissions of Configure-Request
and Terminate-Request packets. Expiration of the Restart timer
causes a Timeout event, and the corresponding Configure-Request or
Terminate-Request packet retransmission. The Restart timer MUST be
configurable, but should default to three (3) seconds.
There is one additional restart parameter, Max-Restarts. Max-
Restarts indicates the number of packet retransmissions that are
required before there is reasonable assurance that the link closed.
Max-Restarts MUST also be configurable, but should default to ten
(10) retransmissions.
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4.4. Packet Format
Exactly one Link Control Protocol packet is encapsulated in the
Information field of PPP Data Link Layer frames where the Protocol
field indicates type hex c021 (Link Control Protocol).
A summary of the Link Control Protocol packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
The Code field is one octet and identifies the kind of LCP packet.
LCP Codes are assigned as follows:
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.
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 should be treated as
Data Link Layer padding and should be ignored on reception.
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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.
Regardless of which Configuration Options are enabled, all LCP
packets are always sent in the full, standard form, as if no
Configuration Options were enabled. This ensures that LCP
Configure-Request packets are always recognizable even when one end
of the link mistakenly believes the link to be Open.
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 Data Link Layer Protocol field value should
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.
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4.4.1. Configure-Request
Description
A LCP implementation wishing to open a connection MUST transmit a
LCP packet with the Code field set to 1 (Configure-Request) and
the Options field filled with any desired changes to the default
link Configuration Options.
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 should be changed on each transmission. On
reception, the Identifier field should be copied into the
Identifier field of the appropriate reply packet.
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.
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4.4.2. Configure-Ack
Description
If every Configuration Option received in a Configure-Request is
both recognizable and acceptable, then a LCP implementation should
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.
On reception of a Configure-Ack, the Identifier field must match
that of the last transmitted Configure-Request, or the packet is
invalid. Additionally, the Configuration Options in a Configure-
Ack must match those of the last transmitted Configure-Request, or
the packet is invalid. Invalid packets should be silently
discarded.
Reception of a valid Configure-Ack indicates that all
Configuration Options sent in the last Configure-Request are
acceptable.
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
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acknowledging. All Configuration Options are always acknowledged
simultaneously.
4.4.3. Configure-Nak
Description
If every element of the received Configuration Options is
recognizable but some are not acceptable, then a LCP
implementation should 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 should be filtered out of the
Configure-Nak, but otherwise the Configuration Options from the
Configure-Request MUST NOT be reordered. Each of the nak'd
Configuration Options MUST be modified to a value acceptable to
the Configure-Nak sender. Finally, an implementation may be
configured to require the negotiation of a specific option. If
that option is not listed, then that option may be appended to the
list of nak'd Configuration Options in order to request the remote
end to list that option in its next Configure-Request packet. The
appended option must include a value acceptable to the Configure-
Nak sender.
On reception of a Configure-Nak, the Identifier field must match
that of the last transmitted Configure-Request, or the packet is
invalid and should be silently discarded.
Reception of a valid Configure-Nak indicates that a new
Configure-Request should be sent with the Configuration Options
modified as specified in the Configure-Nak.
A summary of the Configure-Nak packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
Code
3 for Configure-Nak.
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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.
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4.4.4. 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 manager), then a LCP implementation should
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 unrecognized
Configuration Options from the Configure-Request. All
recognizable and negotiable Configuration Options must be filtered
out of the Configure-Reject, but otherwise the Configuration
Options MUST not be reordered.
On reception of a Configure-Reject, the Identifier field must
match that of the last transmitted Configure-Request, or the
packet is invalid. Additionally, the Configuration Options in a
Configure-Reject must be a proper subset of those in the last
transmitted Configure-Request, or the packet is invalid. Invalid
packets should be silently discarded.
Reception of a 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.
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.
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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.
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4.4.5. 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 Physical Layer indicates that it has gone down, or a
sufficiently large number have been transmitted such that the
remote end 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
connection has been closed.
A summary of the Terminate-Request and Terminate-Ack packet formats
is shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
5 for Terminate-Request;
6 for Terminate-Ack.
Identifier
The Identifier field is one octet and aids in matching requests
and replies.
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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 established
maximum Information field length minus four.
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4.4.6. 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-Packet
field.
Upon reception of a Code-Reject, a LCP implementation should make
an immediate transition to the Closed state, and should report the
error, since it is unlikely that the situation can be rectified
automatically.
A summary of the Code-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Packet ...
+-+-+-+-+-+-+-+-+
Code
7 for Code-Reject.
Identifier
The Identifier field is one octet and is for use by the
transmitter.
Rejected-Packet
The Rejected-Packet field contains a copy of the LCP packet which
is being rejected. It begins with the rejected Code field; it
does not include any PPP Data Link Layer headers. The Rejected-
Packet should be truncated to comply with the established maximum
Information field length.
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4.4.7. Protocol-Reject
Description
Reception of a PPP frame with an unknown Data Link Layer Protocol
indicates that the remote end is attempting to use a protocol
which is unsupported at the local end. This typically occurs when
the remote end attempts to configure a new, but unsupported
protocol. If the LCP state machine is in the Open state, then
this error MUST be reported back to the sender of the unknown
protocol 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 Data field filled with any desired data.
Upon reception of a Protocol-Reject, a LCP implementation should
stop transmitting frames of the indicated protocol.
Protocol-Reject packets may only be sent in the LCP Open state.
Protocol-Reject packets received in any state other than the LCP
Open state should be discarded and no further action should be
taken.
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 is one octet and is for use by the
transmitter.
Rejected-Protocol
The Rejected-Protocol field is two octets and contains the
Protocol of the Data Link Layer frame which is being rejected.
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Rejected-Information
The Rejected-Information field contains a copy from the frame
which is being rejected. It begins with the Information field,
and does not include any PPP Data Link Layer headers or the FCS.
The Rejected-Information field should be truncated to comply with
the established maximum Information field length.
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4.4.8. 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) and the Data field filled with any desired
data, up to but not exceeding the receiver's established maximum
Information field length minus eight.
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, and the
Data field copied from the Echo-Request, truncating as necessary
to avoid exceeding the peer's established maximum Information
field length.
Echo-Request and Echo-Reply packets may only be sent in the LCP
Open state. Echo-Request and Echo-Reply packets received in any
state other than the LCP Open state should be discarded and no
further action should be taken.
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.
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Identifier
The Identifier field is one octet and aids in matching Echo-
Requests and Echo-Replies.
Magic-Number
The Magic-Number field is four octets and aids in detecting
loopbacked links. Unless modified by a Configuration Option, the
Magic-Number MUST always be transmitted as zero and MUST always be
ignored on reception. Further use of the Magic-Number is beyond
the scope of this discussion.
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 established
maximum Information field length minus eight.
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4.4.9. Discard-Request
Description
LCP includes a Discard-Request Code in order to provide a Data
Link Layer data 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 and for numerous other
functions.
A discard sender transmits a LCP packet with the Code field set to
11 (Discard-Request) and the Data field filled with any desired
data, up to but not exceeding the receiver's established maximum
Information field length minus eight.
A discard receiver MUST simply throw away an Discard-Request that
it receives.
Discard-Request packets may only be sent in the LCP Open state.
A summary of the Discard-Request 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
11 for Discard-Request.
Identifier
The Identifier field is one octet and is for use by the Discard-
Request transmitter.
Magic-Number
The Magic-Number field is four octets and aids in detecting
loopbacked links. Unless modified by a configuration option, the
Magic-Number MUST always be transmitted as zero and MUST always be
ignored on reception. Further use of the Magic-Number is beyond
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the scope of this discussion.
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 established
maximum Information field length minus four.
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4.5. Configuration Options
LCP Configuration Options allow modifications to the standard
characteristics of a point-to-point link to be negotiated.
Negotiable modifications include such things as the maximum receive
unit, async control character mapping, the link authentication
method, etc. The Configuration Options themselves are described in
separate documents. If a Configuration Option is not included in a
Configure-Request packet, the default value for that Configuration
Option is assumed.
The end of the list of Configuration Options is indicated by the end
of the LCP packet.
Unless otherwise specified, a specific Configuration Option should be
listed no more than once in a Configuration Options list. Specific
Configuration Options may override this general rule and may be
listed more than once. The effect of this is Configuration Option
specific and is specified by each such Configuration Option.
Also unless otherwise specified, all Configuration Options apply in a
half-duplex fashion. When negotiated, they apply to only one
direction of the link, typically in the receive direction when
interpreted from the point of view of the Configure-Request sender.
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4.5.1. Format
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. The most up-to-date values of the Type
field are specified in the most recent "Assigned Numbers" RFC
[12].
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 indicates the value or
other information for this Configuration Option. The format and
length of the Data field is determined by the Type and Length
fields.
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5. A PPP Network Control Protocol (NCP) for IP
The IP Control Protocol (IPCP) is responsible for configuring,
enabling, and disabling the IP protocol modules on both ends of the
point-to-point link. As with the Link Control Protocol, this is
accomplished through an exchange of packets. IPCP packets may not be
exchanged until LCP has reached the Network-Layer Protocol
Configuration Negotiation phase. IPCP packets received before this
phase is reached should be silently discarded. Likewise, IP
datagrams may not be exchanged until IPCP has first opened the
connection (reached the Open state).
The IP Control Protocol is exactly the same as the Link Control
Protocol with the following exceptions:
Data Link Layer Protocol Field
Exactly one IP Control Protocol packet is encapsulated in the
Information field of PPP Data Link Layer frames where the Protocol
field indicates type hex 8021 (IP Control Protocol).
Code field
Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack
and Code-Reject) are used. Other Codes should be treated as
unrecognized and should result in Code-Rejects.
Timeouts
IPCP packets may not be exchanged until the Link Control Protocol
has reached the network-layer Protocol Configuration Negotiation
phase. An implementation should be prepared to wait for Link
Quality testing to finish before timing out waiting for a
Configure-Ack or other response. It is suggested that an
implementation give up only after user intervention or a
configurable amount of time.
Configuration Option Types
The IPCP has a separate set of Configuration Options. The most
up-to-date values of the type field are specified in the most
recent "Assigned Numbers" RFC [12].
5.1. Sending IP Datagrams
Before any IP packets may be communicated, both the Link Control
Protocol and the IP Control Protocol must reach the Open state.
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RFC 1171 Point-to-Point Protocol July 1990
Exactly one IP packet is encapsulated in the Information field of PPP
Data Link Layer frames where the Protocol field indicates type hex
0021 (Internet Protocol).
The maximum length of an IP packet transmitted over a PPP link is the
same as the maximum length of the Information field of a PPP data
link layer frame. Larger IP datagrams must be fragmented as
necessary. If a system wishes to avoid fragmentation and reassembly,
it should use the TCP Maximum Segment Size option [13], or a similar
mechanism, to discourage others from sending large datagrams.
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RFC 1171 Point-to-Point Protocol July 1990
A. Asynchronous HDLC
This appendix summarizes the modifications to ISO 3309-1979 proposed
in ISO 3309:1984/PDAD1. These modifications allow HDLC to be used
with 8-bit asynchronous links.
Transmission Considerations
Each octet is delimited by a start and a stop element.
Flag Sequence
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).
Transparency
On asynchronous links, a character stuffing procedure is used.
The Control Escape octet is defined as binary 01111101
(hexadecimal 0x7d) where the bit positions are numbered 87654321
(not 76543210, BEWARE).
After FCS computation, the transmitter examines the entire frame
between the two Flag Sequences. Each Flag Sequence, Control
Escape octet and octet with value less than hexadecimal 0x20 is
replaced by a two octet sequence consisting of the Control Escape
octet and the original octet with bit 6 complemented (i.e.,
exclusive-or'd with hexadecimal 0x20).
Prior to FCS computation, the receiver examines the entire frame
between the two Flag Sequences. Each octet with value less than
hexadecimal 0x20 is simply removed (it may have been inserted by
intervening data communications equipment). For each Control
Escape octet, that octet is also removed, but bit 6 of the
following octet is complemented. A Control Escape octet
immediately preceding the closing Flag Sequence indicates an
invalid frame.
Note: The inclusion of all octets less than hexadecimal 0x20
allows all ASCII control characters [10] excluding DEL (Delete)
to be transparently communicated through almost all known data
communications equipment.
A few examples may make this more clear. Packet data is
transmitted on the link as follows:
0x7e is encoded as 0x7d, 0x5e.
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RFC 1171 Point-to-Point Protocol July 1990
0x7d is encoded as 0x7d, 0x5d.
0x01 is encoded as 0x7d, 0x21.
Aborting a Transmission
On asynchronous links, frames may be aborted by transmitting a "0"
stop bit where a "1" bit is expected (framing error) or by
transmitting a Control Escape octet followed immediately by a
closing Flag Sequence.
Inter-frame Time Fill
On asynchronous links, inter-octet and inter-frame time fill
should be accomplished by transmitting continuous "1" bits (mark-
hold state).
Note: On asynchronous links, inter-frame time fill can be
viewed as extended inter-octet time fill. Doing so can save
one octet for every frame, decreasing delay and increasing
bandwidth. This is possible since a Flag Sequence may serve as
both a frame close and a frame begin. After having received
any frame, an idle receiver will always be in a frame begin
state.
Robust transmitters should avoid using this trick over-
zealously since the price for decreased delay is decreased
reliability. Noisy links may cause the receiver to receive
garbage characters and interpret them as part of an incoming
frame. If the transmitter does not transmit a new opening Flag
Sequence before sending the next frame, then that frame will be
appended to the noise characters causing an invalid frame (with
high reliability). Transmitters should avoid this by
transmitting an open Flag Sequence whenever "appreciable time"
has elapsed since the prior closing Flag Sequence. It is
suggested that implementations will achieve the best results by
always sending an opening Flag Sequence if the new frame is not
back-to-back with the last. The maximum value for "appreciable
time" is likely to be no greater than the typing rate of a slow
to average typist, say 1 second.
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RFC 1171 Point-to-Point Protocol July 1990
B. Fast Frame Check Sequence (FCS) Implementation
B.1. FCS Computation Method
The following code provides a table lookup computation for
calculating the Frame Check Sequence as data arrives at the
interface. The table is created by the code in section 2.
/*
* u16 represents an unsigned 16-bit number. Adjust the typedef for
* your hardware.
*/
typedef unsigned short u16;
/*
* FCS lookup table as calculated by the table generator in section 2.
*/
static u16 fcstab[256] = {
0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
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RFC 1171 Point-to-Point Protocol July 1990
0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};
#define PPPINITFCS 0xffff /* Initial FCS value */
#define PPPGOODFCS 0xf0b8 /* Good final FCS value */
/*
* Calculate a new fcs given the current fcs and the new data.
*/
u16 pppfcs(fcs, cp, len)
register u16 fcs;
register unsigned char *cp;
register int len;
{
ASSERT(sizeof (u16) == 2);
ASSERT(((u16) -1) > 0);
while (len--)
fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];
return (fcs);
}
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RFC 1171 Point-to-Point Protocol July 1990
B.2. Fast FCS table generator
The following code creates the lookup table used to calculate the
FCS.
/*
* Generate a FCS table for the HDLC FCS.
*
* Drew D. Perkins at Carnegie Mellon University.
*
* Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier.
*/
/*
* The HDLC polynomial: x**0 + x**5 + x**12 + x**16 (0x8408).
*/
#define P 0x8408
main()
{
register unsigned int b, v;
register int i;
printf("typedef unsigned short u16;\n");
printf("static u16 fcstab[256] = {");
for (b = 0; ; ) {
if (b % 8 == 0)
printf("\n");
v = b;
for (i = 8; i--; )
v = v & 1 ? (v >> 1) ^ P : v >> 1;
printf("0x%04x", v & 0xFFFF);
if (++b == 256)
break;
printf(",");
}
printf("\n};\n");
}
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RFC 1171 Point-to-Point Protocol July 1990
References
[1] Electronic Industries Association, EIA Standard RS-232-C,
"Interface Between Data Terminal Equipment and Data
Communications Equipment Employing Serial Binary Data
Interchange", August 1969.
[2] International Organization For Standardization, ISO Standard
3309-1979, "Data communication - High-level data link control
procedures - Frame structure", 1979.
[3] International Organization For Standardization, ISO Standard
4335-1979, "Data communication - High-level data link control
procedures - Elements of procedures", 1979.
[4] International Organization For Standardization, ISO Standard
4335-1979/Addendum 1, "Data communication - High-level data
link control procedures - Elements of procedures - Addendum 1",
1979.
[5] International Organization For Standardization, Proposed Draft
International Standard ISO 3309:1983/PDAD1, "Information
processing systems - Data communication - High-level data link
control procedures - Frame structure - Addendum 1: Start/stop
transmission", 1984.
[6] International Telecommunication Union, CCITT Recommendation
X.25, "Interface Between Data Terminal Equipment (DTE) and Data
Circuit Terminating Equipment (DCE) for Terminals Operating in
the Packet Mode on Public Data Networks", CCITT Red Book,
Volume VIII, Fascicle VIII.3, Rec. X.25., October 1984.
[7] Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983.
[8] Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
September 1986.
[9] LeVan, J., "A Fast CRC", Byte, November 1987.
[10] American National Standards Institute, ANSI X3.4-1977,
"American National Standard Code for Information Interchange",
1977.
[11] Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
[12] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
USC/Information Sciences Institute, March 1990.
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RFC 1171 Point-to-Point Protocol July 1990
[13] Postel, J., "The TCP Maximum Segment Size Option and Related
Topics", RFC 879, USC/Information Sciences Institute, November
1983.
Security Considerations
Security issues are not discussed in this memo.
Chairman's Address
This proposal is the product of the Point-to-Point Protocol Working
Group of the Internet Engineering Task Force (IETF). The working group
can be contacted via the chair:
Russ Hobby
UC Davis
Computing Services
Davis, CA 95616
Phone: (916) 752-0236
EMail: rdhobby@ucdavis.edu
Author's Address
Questions about this memo can also be directed to the author:
Drew D. Perkins
Carnegie Mellon University
Networking and Communications
Pittsburgh, PA 15213
Phone: (412) 268-8576
EMail: ddp@andrew.cmu.edu
Acknowledgments
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: Ken Adelman (TGV),
Craig Fox (NSC), Phill Gross (NRI), Russ Hobby (UC Davis), David
Kaufman (Proteon), John LoVerso (Xylogics), Bill Melohn (Sun
Microsystems), Mike Patton (MIT), Drew Perkins (CMU), Greg Satz
(cisco systems) and Asher Waldfogel (Wellfleet).
Perkins [Page 48]
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