RFC 1331 (rfc1331) - Page 1 of 66
The Point-to-Point Protocol (PPP) for the Transmission of Multi-protocol Datagrams over Point-to-Point Links
Alternative Format: Original Text Document
Network Working Group W. Simpson
Request for Comments: 1331 Daydreamer
Obsoletes: RFCs 1171, 1172 May 1992
The Point-to-Point Protocol (PPP)
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, 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.
Abstract
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
functions.
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 mailing list.
Simpson [Page i]
RFC 1331 Point-to-Point Protocol May 1992
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 3
1.2 Terminology ..................................... 3
2. Physical Layer Requirements ........................... 4
3. The Data Link Layer ................................... 5
3.1 Frame Format .................................... 6
4. PPP Link Operation .................................... 10
4.1 Overview ........................................ 10
4.2 Phase Diagram ................................... 10
4.3 Link Dead (physical-layer not ready) ............ 10
4.4 Link Establishment Phase ........................ 11
4.5 Authentication Phase ............................ 11
4.6 Network-Layer Protocol Phase .................... 12
4.7 Link Termination Phase .......................... 12
5. The Option Negotiation Automaton ...................... 14
5.1 State Diagram ................................... 15
5.2 State Transition Table .......................... 16
5.3 States .......................................... 18
5.4 Events .......................................... 20
5.5 Actions ......................................... 24
5.6 Loop Avoidance .................................. 26
5.7 Counters and Timers ............................. 27
6. LCP Packet Formats .................................... 28
6.1 Configure-Request ............................... 30
6.2 Configure-Ack ................................... 31
6.3 Configure-Nak ................................... 32
6.4 Configure-Reject ................................ 33
6.5 Terminate-Request and Terminate-Ack ............. 35
6.6 Code-Reject ..................................... 36
6.7 Protocol-Reject ................................. 38
6.8 Echo-Request and Echo-Reply ..................... 39
6.9 Discard-Request ................................. 40
7. LCP Configuration Options ............................. 42
7.1 Format .......................................... 43
7.2 Maximum-Receive-Unit ............................ 44
7.3 Async-Control-Character-Map ..................... 45
7.4 Authentication-Protocol ......................... 47
7.5 Quality-Protocol ................................ 49
7.6 Magic-Number .................................... 51
Simpson [Page ii]
RFC 1331 Point-to-Point Protocol May 1992
7.7 Protocol-Field-Compression ...................... 54
7.8 Address-and-Control-Field-Compression ........... 56
APPENDICES ................................................... 58
A. Asynchronous HDLC ..................................... 58
B. Fast Frame Check Sequence (FCS) Implementation ........ 61
B.1 FCS Computation Method .......................... 61
B.2 Fast FCS table generator ........................ 63
C. LCP Recommended Options ............................... 64
SECURITY CONSIDERATIONS ...................................... 65
REFERENCES ................................................... 65
ACKNOWLEDGEMENTS ............................................. 66
CHAIR'S ADDRESS .............................................. 66
AUTHOR'S ADDRESS ............................................. 66
Simpson [Page iii]
RFC 1331 Point-to-Point Protocol May 1992
1. Introduction
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.
Encapsulation
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
popular LANs.
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
encapsulation.
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
Simpson
