RFC 1661 (rfc1661) – Page 1 of 52
The Point-to-Point Protocol (PPP)
Alternative Format: Original Text Document
Network Working Group W. Simpson, Editor
Request for Comments: 1661 Daydreamer
STD: 51 July 1994
Obsoletes: 1548
Category: Standards Track
The Point-to-Point Protocol (PPP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Point-to-Point Protocol (PPP) provides a standard method for
transporting multi-protocol datagrams over point-to-point links. PPP
is comprised of three main components:
1. A method for encapsulating multi-protocol datagrams.
2. A Link Control Protocol (LCP) for establishing, configuring,
and testing the data-link connection.
3. A family of Network Control Protocols (NCPs) for establishing
and configuring different network-layer protocols.
This document defines the PPP organization and methodology, and the
PPP encapsulation, together with an extensible option negotiation
mechanism which is able to negotiate a rich assortment of
configuration parameters and provides additional management
functions. The PPP Link Control Protocol (LCP) is described in terms
of this mechanism.
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 2
1.2 Terminology ..................................... 3
2. PPP Encapsulation ..................................... 4
Simpson [Page i]
RFC 1661 Point-to-Point Protocol July 1994
3. PPP Link Operation .................................... 6
3.1 Overview ........................................ 6
3.2 Phase Diagram ................................... 6
3.3 Link Dead (physical-layer not ready) ............ 7
3.4 Link Establishment Phase ........................ 7
3.5 Authentication Phase ............................ 8
3.6 Network-Layer Protocol Phase .................... 8
3.7 Link Termination Phase .......................... 9
4. The Option Negotiation Automaton ...................... 11
4.1 State Transition Table .......................... 12
4.2 States .......................................... 14
4.3 Events .......................................... 16
4.4 Actions ......................................... 21
4.5 Loop Avoidance .................................. 23
4.6 Counters and Timers ............................. 24
5. LCP Packet Formats .................................... 26
5.1 Configure-Request ............................... 28
5.2 Configure-Ack ................................... 29
5.3 Configure-Nak ................................... 30
5.4 Configure-Reject ................................ 31
5.5 Terminate-Request and Terminate-Ack ............. 33
5.6 Code-Reject ..................................... 34
5.7 Protocol-Reject ................................. 35
5.8 Echo-Request and Echo-Reply ..................... 36
5.9 Discard-Request ................................. 37
6. LCP Configuration Options ............................. 39
6.1 Maximum-Receive-Unit (MRU) ...................... 41
6.2 Authentication-Protocol ......................... 42
6.3 Quality-Protocol ................................ 43
6.4 Magic-Number .................................... 45
6.5 Protocol-Field-Compression (PFC) ................ 48
6.6 Address-and-Control-Field-Compression (ACFC)
SECURITY CONSIDERATIONS ...................................... 51
REFERENCES ................................................... 51
ACKNOWLEDGEMENTS ............................................. 51
CHAIR'S ADDRESS .............................................. 52
EDITOR'S ADDRESS ............................................. 52
Simpson [Page ii]
RFC 1661 Point-to-Point Protocol July 1994
1. Introduction
The Point-to-Point Protocol is designed for simple links which
transport packets between two peers. These links provide full-duplex
simultaneous bi-directional operation, and are assumed to deliver
packets in order. It is intended that PPP provide a common solution
for easy connection of a wide variety of hosts, bridges and routers
[1].
Encapsulation
The PPP encapsulation provides for multiplexing of different
network-layer protocols simultaneously over the same link. The
PPP encapsulation has been carefully designed to retain
compatibility with most commonly used supporting hardware.
Only 8 additional octets are necessary to form the encapsulation
when used within the default HDLC-like framing. In environments
where bandwidth is at a premium, the encapsulation and framing may
be shortened to 2 or 4 octets.
To support high speed implementations, the default encapsulation
uses only simple fields, only one of which needs to be examined
for demultiplexing. The default header and information fields
fall on 32-bit boundaries, and the trailer may be padded to an
arbitrary boundary.
Link Control Protocol
In order to be sufficiently versatile to be portable to a wide
variety of environments, PPP provides a Link Control Protocol
(LCP). The LCP is used to automatically agree upon the
encapsulation format options, handle varying limits on sizes of
packets, detect a looped-back link and other common
misconfiguration errors, and terminate the link. Other optional
facilities provided are authentication of the identity of its peer
on the link, and determination when a link is functioning properly
and when it is failing.
Network Control Protocols
Point-to-Point links tend to exacerbate many problems with the
current family of network protocols. For instance, assignment and
management of IP addresses, which is a problem even in LAN
environments, is especially difficult over circuit-switched
point-to-point links (such as dial-up modem servers). These
problems are handled by a family of Network Control Protocols
(NCPs), which each manage the specific needs required by their
Simpson
