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+
+Network Working Group W. Simpson
+Request for Comments: 1548 Daydreamer
+Obsoletes: RFC 1331 December 1993
+Category: Standards Track
+
+
+ The Point-to-Point Protocol (PPP)
+
+Status of this Memo
+
+ This document specifies an Internet standards track protocol for the
+ Internet community, and requests discussion and suggestions for
+ improvements. Please refer to the current edition of the "Internet
+ Official Protocol Standards" (STD 1) for the standardization state
+ and status of this protocol. Distribution of this memo is unlimited.
+
+Abstract
+
+ The Point-to-Point Protocol (PPP) provides a standard method for
+ transporting multi-protocol datagrams over point-to-point links. PPP
+ is comprised of three main components:
+
+ 1. A method for encapsulating multi-protocol datagrams.
+
+ 2. A Link Control Protocol (LCP) for establishing, configuring,
+ and testing the data-link connection.
+
+ 3. A family of Network Control Protocols (NCPs) for establishing
+ and configuring different network-layer protocols.
+
+ This document defines the PPP organization and methodology, and the
+ PPP encapsulation, together with an extensible option negotiation
+ mechanism which is able to negotiate a rich assortment of
+ configuration parameters and provides additional management
+ functions. The PPP Link Control Protocol (LCP) is described in terms
+ of this mechanism.
+
+ This document is the product of the Point-to-Point Protocol Working
+ Group of the Internet Engineering Task Force (IETF). Comments should
+ be submitted to the ietf-ppp@ucdavis.edu mailing list.
+
+
+
+
+
+
+
+
+
+
+
+Simpson [Page 1]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+Table of Contents
+
+ 1. Introduction ................................................3
+ 1.1 Specification of Requirements ...............................4
+ 1.2 Terminology .................................................5
+ 2. PPP Encapsulation ...........................................5
+ 3. PPP Link Operation ..........................................8
+ 3.1 Overview ....................................................8
+ 3.2 Phase Diagram ...............................................8
+ 3.3 Link Dead (physical-layer not ready) ........................9
+ 3.4 Link Establishment Phase ....................................9
+ 3.5 Authentication Phase ........................................9
+ 3.6 Network-Layer Protocol Phase ................................10
+ 3.7 Link Termination Phase ......................................10
+ 4. The Option Negotiation Automaton ............................11
+ 4.1 State Diagram ...............................................12
+ 4.2 State Transition Table ......................................14
+ 4.3 A Day in the Life ...........................................15
+ 4.4 States ......................................................16
+ 4.5 Events ......................................................19
+ 4.6 Actions .....................................................23
+ 4.7 Loop Avoidance ..............................................26
+ 4.8 Counters and Timers .........................................26
+ 5. LCP Packet Formats ..........................................27
+ 5.1 Configure-Request ...........................................29
+ 5.2 Configure-Ack ...............................................30
+ 5.3 Configure-Nak ...............................................31
+ 5.4 Configure-Reject ............................................33
+ 5.5 Terminate-Request and Terminate-Ack .........................34
+ 5.6 Code-Reject .................................................35
+ 5.7 Protocol-Reject .............................................36
+ 5.8 Echo-Request and Echo-Reply .................................37
+ 5.9 Discard-Request .............................................39
+ 6. LCP Configuration Options ...................................40
+ 6.1 Maximum-Receive-Unit ........................................41
+ 6.2 Async-Control-Character-Map .................................42
+ 6.3 Authentication-Protocol .....................................43
+ 6.4 Quality-Protocol ............................................45
+ 6.5 Magic-Number ................................................46
+ 6.6 Protocol-Field-Compression ..................................49
+ 6.7 Address-and-Control-Field-Compression .......................50
+ APPENDIX A. LCP Recommended Options ..............................51
+ SECURITY CONSIDERATIONS ..........................................51
+ REFERENCES .......................................................52
+ ACKNOWLEDGEMENTS .................................................52
+ CHAIR'S ADDRESS ..................................................52
+ EDITOR'S ADDRESS .................................................53
+
+
+
+
+Simpson [Page 2]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+1. Introduction
+
+ Encapsulation
+
+ The PPP encapsulation provides for multiplexing of different
+ network-layer protocols simultaneously over the same link. It is
+ intended that PPP provide a common solution for easy connection of
+ a wide variety of hosts, bridges and routers [1].
+
+ The PPP encapsulation has been carefully designed to retain
+ compatibility with most commonly used supporting hardware.
+
+ Only 8 additional octets are necessary to form the encapsulation
+ when used with the default HDLC framing. In environments where
+ bandwidth is at a premium, the encapsulation and framing may be
+ shortened to 2 or 4 octets.
+
+ To support high speed implementations, the default encapsulation
+ uses only simple fields, only one of which needs to be examined
+ for demultiplexing. The default header and information fields
+ fall on 32-bit boundaries, and the trailer may be padded to an
+ arbitrary boundary.
+
+ Link Control Protocol
+
+ In order to be sufficiently versatile to be portable to a wide
+ variety of environments, PPP provides a Link Control Protocol
+ (LCP). The LCP is used to automatically agree upon the
+ encapsulation format options, handle varying limits on sizes of
+ packets, authenticate the identity of its peer on the link,
+ determine when a link is functioning properly and when it is
+ defunct, detect a looped-back link and other common
+ misconfiguration errors, and terminate the link.
+
+ Network Control Protocols
+
+ Point-to-Point links tend to exacerbate many problems with the
+ current family of network protocols. For instance, assignment and
+ management of IP addresses, which is a problem even in LAN
+ environments, is especially difficult over circuit-switched
+ point-to-point links (such as dial-up modem servers). These
+ problems are handled by a family of Network Control Protocols
+ (NCPs), which each manage the specific needs required by their
+ respective network-layer protocols. These NCPs are defined in
+ companion documents.
+
+
+
+
+
+
+Simpson [Page 3]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Configuration
+
+ It is intended that PPP links be easy to configure. By design,
+ the standard defaults handle all common configurations. The
+ implementor can specify improvements to the default configuration,
+ which are automatically communicated to the peer without operator
+ intervention. Finally, the operator may explicitly configure
+ options for the link which enable the link to operate in
+ environments where it would otherwise be impossible.
+
+ This self-configuration is implemented through an extensible
+ option negotiation mechanism, wherein each end of the link
+ describes to the other its capabilities and requirements.
+ Although the option negotiation mechanism described in this
+ document is specified in terms of the Link Control Protocol (LCP),
+ the same facilities are designed to be used by other control
+ protocols, especially the family of NCPs.
+
+1.1 Specification of Requirements
+
+ In this document, several words are used to signify the
+ requirements of the specification. These words are often
+ capitalized.
+
+ MUST
+
+ This word, or the adjective "required", means that the definition
+ is an absolute requirement of the specification.
+
+ MUST NOT
+
+ This phrase means that the definition is an absolute prohibition
+ of the specification.
+
+ SHOULD
+
+ This word, or the adjective "recommended", means that there may
+ exist valid reasons in particular circumstances to ignore this
+ item, but the full implications must be understood and carefully
+ weighed before choosing a different course.
+
+ MAY
+
+ This word, or the adjective "optional", means that this item is
+ one of an allowed set of alternatives. An implementation which
+ does not include this option MUST be prepared to interoperate with
+ another implementation which does include the option.
+
+
+
+
+Simpson [Page 4]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+1.2 Terminology
+
+ This document frequently uses the following terms:
+
+ datagram
+
+ The unit of transmission in the network layer (such as IP). A
+ datagram may be encapsulated in one or more packets passed to the
+ data link layer.
+
+ frame
+
+ The unit of transmission at the data link layer. A frame may
+ include a header and/or a trailer, along with some number of units
+ of data.
+
+ packet
+
+ The basic unit of encapsulation, which is passed across the
+ interface between the network layer and the data link layer. A
+ packet is usually mapped to a frame; the exceptions are when data
+ link layer fragmentation is being performed, or when multiple
+ packets are incorporated into a single frame.
+
+ peer
+
+ The other end of the point-to-point link.
+
+ silently discard
+
+ This means the implementation discards the packet without further
+ processing. The implementation SHOULD provide the capability of
+ logging the error, including the contents of the silently
+ discarded packet, and SHOULD record the event in a statistics
+ counter.
+
+2. PPP Encapsulation
+
+ The PPP encapsulation is used to disambiguate multiprotocol
+ datagrams. This encapsulation requires framing to indicate the
+ beginning and end of the encapsulation. Methods of providing framing
+ are specified in companion documents.
+
+
+
+
+
+
+
+
+
+Simpson [Page 5]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ A summary of the PPP encapsulation is shown below. The fields are
+ transmitted from left to right.
+
+ +----------+-------------+---------+
+ | Protocol | Information | Padding |
+ | 16 bits | * | * |
+ +----------+-------------+---------+
+
+ Protocol Field
+
+ The Protocol field is two octets and its value identifies the
+ datagram encapsulated in the Information field of the packet. The
+ field is transmitted and received most significant octet first.
+
+ The structure of this field is consistent with the ISO 3309
+ extension mechanism for address fields. All Protocols MUST be
+ odd; the least significant bit of the least significant octet MUST
+ equal "1". Also, all Protocols MUST be assigned such that the
+ least significant bit of the most significant octet equals "0".
+ Frames received which don't comply with these rules MUST be
+ treated as having an unrecognized Protocol.
+
+ Protocol field values in the "0***" to "3***" range identify the
+ network-layer protocol of specific packets, and values in the
+ "8***" to "b***" range identify packets belonging to the
+ associated Network Control Protocols (NCPs), if any.
+
+ Protocol field values in the "4***" to "7***" range are used for
+ protocols with low volume traffic which have no associated NCP.
+ Protocol field values in the "c***" to "f***" range identify
+ packets as link-layer Control Protocols (such as LCP).
+
+ Up-to-date values of the Protocol field are specified in the most
+ recent "Assigned Numbers" RFC [2]. Current values are assigned as
+ follows:
+
+ Value (in hex) Protocol Name
+
+ 0001 Padding Protocol
+ 0003 to 001f reserved (transparency inefficient)
+ 0021 Internet Protocol
+ 0023 OSI Network Layer
+ 0025 Xerox NS IDP
+ 0027 DECnet Phase IV
+ 0029 Appletalk
+ 002b Novell IPX
+ 002d Van Jacobson Compressed TCP/IP
+ 002f Van Jacobson Uncompressed TCP/IP
+
+
+
+Simpson [Page 6]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ 0031 Bridging PDU
+ 0033 Stream Protocol (ST-II)
+ 0035 Banyan Vines
+ 0037 unused
+ 0039 AppleTalk EDDP
+ 003b AppleTalk SmartBuffered
+ 003d Multi-Link
+ 005d reserved (compression inefficient)
+ 00cf reserved (PPP NLPID)
+ 00fd 1st choice compression
+ 00ff reserved (compression inefficient)
+
+ 0201 802.1d Hello Packets
+ 0203 IBM Source Routing BPDU
+ 0231 Luxcom
+ 0233 Sigma Network Systems
+
+ 8021 Internet Protocol Control Protocol
+ 8023 OSI Network Layer Control Protocol
+ 8025 Xerox NS IDP Control Protocol
+ 8027 DECnet Phase IV Control Protocol
+ 8029 Appletalk Control Protocol
+ 802b Novell IPX Control Protocol
+ 802d Reserved
+ 802f Reserved
+ 8031 Bridging NCP
+ 8033 Stream Protocol Control Protocol
+ 8035 Banyan Vines Control Protocol
+ 8037 unused
+ 8039 Reserved
+ 803b Reserved
+ 803d Multi-Link Control Protocol
+ 80fd Compression Control Protocol
+ 80ff Reserved
+
+ c021 Link Control Protocol
+ c023 Password Authentication Protocol
+ c025 Link Quality Report
+ c223 Challenge Handshake Authentication Protocol
+
+ Developers of new protocols MUST obtain a number from the Internet
+ Assigned Numbers Authority (IANA), at IANA@isi.edu.
+
+ Information Field
+
+ The Information field is zero or more octets. The Information
+ field contains the datagram for the protocol specified in the
+ Protocol field.
+
+
+
+Simpson [Page 7]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ The maximum length for the Information field, including Padding,
+ is termed the Maximum Receive Unit (MRU), which defaults to 1500
+ octets. By negotiation, consenting PPP implementations may use
+ other values for the MRU.
+
+ Padding
+
+ On transmission, the Information field MAY be padded with an
+ arbitrary number of octets up to the MRU. It is the
+ responsibility of each protocol to distinguish padding octets from
+ real information.
+
+3. PPP Link Operation
+
+3.1 Overview
+
+ In order to establish communications over a point-to-point link, each
+ end of the PPP link MUST first send LCP packets to configure and test
+ the data link. After the link has been established, the peer MAY be
+ authenticated. Then, PPP MUST send NCP packets to choose and
+ configure one or more network-layer protocols. Once each of the
+ chosen network-layer protocols has been configured, datagrams from
+ each network-layer protocol can be sent over the link.
+
+ The link will remain configured for communications until explicit LCP
+ or NCP packets close the link down, or until some external event
+ occurs (an inactivity timer expires or network administrator
+ intervention).
+
+3.2 Phase Diagram
+
+ In the process of configuring, maintaining and terminating the
+ point-to-point link, the PPP link goes through several distinct
+ phases:
+
+ +------+ +-----------+ +--------------+
+ | | UP | | OPENED | | SUCCESS/NONE
+ | Dead |------->| Establish |---------->| Authenticate |--+
+ | | | | | | |
+ +------+ +-----------+ +--------------+ |
+ ^ FAIL | FAIL | |
+ +<--------------+ +----------+ |
+ | | |
+ | +-----------+ | +---------+ |
+ | DOWN | | | CLOSING | | |
+ +------------| Terminate |<---+<----------| Network |<-+
+ | | | |
+ +-----------+ +---------+
+
+
+
+Simpson [Page 8]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+3.3 Link Dead (physical-layer not ready)
+
+ The link necessarily begins and ends with this phase. When an
+ external event (such as carrier detection or network administrator
+ configuration) indicates that the physical-layer is ready to be used,
+ PPP will proceed to the Link Establishment phase.
+
+ During this phase, the LCP automaton (described below) will be in the
+ Initial or Starting states. The transition to the Link Establishment
+ phase will signal an Up event to the automaton.
+
+ Implementation Note:
+
+ Typically, a link will return to this phase automatically after
+ the disconnection of a modem. In the case of a hard-wired line,
+ this phase may be extremely short -- merely long enough to detect
+ the presence of the device.
+
+3.4 Link Establishment Phase
+
+ The Link Control Protocol (LCP) is used to establish the connection
+ through an exchange of Configure packets. This exchange is complete,
+ and the LCP Opened state entered, once a Configure-Ack packet
+ (described below) has been both sent and received.
+
+ All Configuration Options are assumed to be at default values unless
+ altered by the configuration exchange. See the section on LCP
+ Configuration Options for further discussion.
+
+ It is important to note that only Configuration Options which are
+ independent of particular network-layer protocols are configured by
+ LCP. Configuration of individual network-layer protocols is handled
+ by separate Network Control Protocols (NCPs) during the Network-Layer
+ Protocol phase.
+
+ Any non-LCP packets received during this phase MUST be silently
+ discarded.
+
+3.5 Authentication Phase
+
+ On some links it may be desirable to require a peer to authenticate
+ itself before allowing network-layer protocol packets to be
+ exchanged.
+
+ By default, authentication is not mandatory. If an implementation
+ desires that the peer authenticate with some specific authentication
+ protocol, then it MUST negotiate the use of that authentication
+ protocol during Link Establishment phase.
+
+
+
+Simpson [Page 9]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Authentication SHOULD take place as soon as possible after link
+ establishment. However, link quality determination MAY occur
+ concurrently. An implementation MUST NOT allow the exchange of link
+ quality determination packets to delay authentication indefinitely.
+
+ Advancement from the Authentication phase to the Network-Layer
+ Protocol phase MUST NOT occur until authentication has completed,
+ using the negotiated authentication protocol. If authentication
+ fails, PPP SHOULD proceed instead to the Link Termination phase.
+
+ Any Network Control Protocol or network-layer protocol packets
+ received during this phase MUST be silently discarded.
+
+3.6 Network-Layer Protocol Phase
+
+ Once PPP has finished the previous phases, each network-layer
+ protocol (such as IP, IPX, or AppleTalk) MUST be separately
+ configured by the appropriate Network Control Protocol (NCP).
+
+ Each NCP MAY be Opened and Closed at any time.
+
+ Implementation Note:
+
+ Because an implementation may initially use a significant amount
+ of time for link quality determination, implementations SHOULD
+ avoid fixed timeouts when waiting for their peers to configure a
+ NCP.
+
+ After a NCP has reached the Opened state, PPP will carry the
+ corresponding network-layer protocol packets. Any network-layer
+ protocol packets received when the corresponding NCP is not in the
+ Opened state MUST be silently discarded.
+
+ Implementation Note:
+
+ There is an exception to the preceding paragraphs, due to the
+ availability of the LCP Protocol-Reject (described below). While
+ LCP is in the Opened state, any protocol packet which is
+ unsupported by the implementation MUST be returned in a Protocol-
+ Reject. Only protocols which are supported are silently
+ discarded.
+
+ During this phase, link traffic consists of any possible
+ combination of LCP, NCP, and network-layer protocol packets.
+
+3.7 Link Termination Phase
+
+ PPP can terminate the link at any time. This might happen because of
+
+
+
+Simpson [Page 10]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ the loss of carrier, authentication failure, link quality failure,
+ the expiration of an idle-period timer, or the administrative closing
+ of the link. LCP is used to close the link through an exchange of
+ Terminate packets. When the link is closing, PPP informs the
+ network-layer protocols so that they may take appropriate action.
+
+ After the exchange of Terminate packets, the implementation SHOULD
+ signal the physical-layer to disconnect in order to enforce the
+ termination of the link, particularly in the case of an
+ authentication failure. The sender of the Terminate-Request SHOULD
+ disconnect after receiving a Terminate-Ack, or after the Restart
+ counter expires. The receiver of a Terminate-Request SHOULD wait for
+ the peer to disconnect, and MUST NOT disconnect until at least one
+ Restart time has passed after sending a Terminate-Ack. PPP SHOULD
+ proceed to the Link Dead phase.
+
+ Any non-LCP packets received during this phase MUST be silently
+ discarded.
+
+ Implementation Note:
+
+ The closing of the link by LCP is sufficient. There is no need
+ for each NCP to send a flurry of Terminate packets. Conversely,
+ the fact that one NCP has Closed is not sufficient reason to cause
+ the termination of the PPP link, even if that NCP was the only NCP
+ currently in the Opened state.
+
+4. The Option Negotiation Automaton
+
+ The finite-state automaton is defined by events, actions and state
+ transitions. Events include reception of external commands such as
+ Open and Close, expiration of the Restart timer, and reception of
+ packets from a peer. Actions include the starting of the Restart
+ timer and transmission of packets to the peer.
+
+ Some types of packets -- Configure-Naks and Configure-Rejects, or
+ Code-Rejects and Protocol-Rejects, or Echo-Requests, Echo-Replies and
+ Discard-Requests -- are not differentiated in the automaton
+ descriptions. As will be described later, these packets do indeed
+ serve different functions. However, they always cause the same
+ transitions.
+
+Events Actions
+
+Up = lower layer is Up tlu = This-Layer-Up
+Down = lower layer is Down tld = This-Layer-Down
+Open = administrative Open tls = This-Layer-Started
+Close= administrative Close tlf = This-Layer-Finished
+
+
+
+Simpson [Page 11]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+TO+ = Timeout with counter > 0 irc = Initialize-Restart-Counter
+TO- = Timeout with counter expired zrc = Zero-Restart-Counter
+
+RCR+ = Receive-Configure-Request (Good) scr = Send-Configure-Request
+RCR- = Receive-Configure-Request (Bad)
+RCA = Receive-Configure-Ack sca = Send-Configure-Ack
+RCN = Receive-Configure-Nak/Rej scn = Send-Configure-Nak/Rej
+
+RTR = Receive-Terminate-Request str = Send-Terminate-Request
+RTA = Receive-Terminate-Ack sta = Send-Terminate-Ack
+
+RUC = Receive-Unknown-Code scj = Send-Code-Reject
+RXJ+ = Receive-Code-Reject (permitted)
+ or Receive-Protocol-Reject
+RXJ- = Receive-Code-Reject (catastrophic)
+ or Receive-Protocol-Reject
+RXR = Receive-Echo-Request ser = Send-Echo-Reply
+ or Receive-Echo-Reply
+ or Receive-Discard-Request
+
+4.1 State Diagram
+
+ The simplified state diagram which follows describes the sequence of
+ events for reaching agreement on Configuration Options (opening the
+ PPP link) and for later termination of the link.
+
+ This diagram is not a complete representation of the automaton.
+ Implementation MUST be done by consulting the actual state transition
+ table.
+
+ Events are in upper case. Actions are in lower case. For these
+ purposes, the state machine is initially in the Closed state. Once
+ the Opened state has been reached, both ends of the link have met the
+ requirement of having both sent and received a Configure-Ack packet.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Simpson [Page 12]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ RCR TO+
+ +--sta-->+ +------->+
+ | | | |
+ +-------+ | RTA +-------+ | Close +-------+
+ | |<-----+<------| |<-str-+<------| |
+ |Closed | |Closing| |Opened |
+ | | Open | | | |
+ | |------+ | | | |
+ +-------+ | +-------+ +-------+
+ | ^
+ | |
+ | +-sca----------------->+
+ | | ^
+ RCN,TO+ V RCR+ | RCR- RCA | RCN,TO+
+ +------->+ | +------->+ | +--scr-->+
+ | | | | | | | |
+ +-------+ | TO+ +-------+ | +-------+ |
+ | |<-scr-+<------| |<-scn-+ | |<-----+
+ | Req- | | Ack- | | Ack- |
+ | Sent | RCA | Rcvd | | Sent |
+ +-scn->| |------------->| | +-sca->| |
+ | +-------+ +-------+ | +-------+
+ | RCR- | | RCR+ | RCR+ | | RCR-
+ | | +------------------------------->+<-------+ |
+ | | |
+ +<-------+<------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Simpson [Page 13]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+4.2 State Transition Table
+
+ The complete state transition table follows. States are indicated
+ horizontally, and events are read vertically. State transitions and
+ actions are represented in the form action/new-state. Multiple
+ actions are separated by commas, and may continue on succeeding lines
+ as space requires; multiple actions may be implemented in any
+ convenient order. The state may be followed by a letter, which
+ indicates an explanatory footnote. The dash ('-') indicates an
+ illegal transition.
+
+
+ | State
+ | 0 1 2 3 4 5
+ Events| Initial Starting Closed Stopped Closing Stopping
+ ------+-----------------------------------------------------------
+ Up | 2 irc,scr/6 - - - -
+ Down | - - 0 tls/1 0 1
+ Open | tls/1 1 irc,scr/6 3r 5r 5r
+ Close| 0 0 2 2 4 4
+ |
+ TO+ | - - - - str/4 str/5
+ TO- | - - - - tlf/2 tlf/3
+ |
+ RCR+ | - - sta/2 irc,scr,sca/8 4 5
+ RCR- | - - sta/2 irc,scr,scn/6 4 5
+ RCA | - - sta/2 sta/3 4 5
+ RCN | - - sta/2 sta/3 4 5
+ |
+ RTR | - - sta/2 sta/3 sta/4 sta/5
+ RTA | - - 2 3 tlf/2 tlf/3
+ |
+ RUC | - - scj/2 scj/3 scj/4 scj/5
+ RXJ+ | - - 2 3 4 5
+ RXJ- | - - tlf/2 tlf/3 tlf/2 tlf/3
+ |
+ RXR | - - 2 3 4 5
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Simpson [Page 14]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ | State
+ | 6 7 8 9
+ Events| Req-Sent Ack-Rcvd Ack-Sent Opened
+ ------+-----------------------------------------
+ Up | - - - -
+ Down | 1 1 1 tld/1
+ Open | 6 7 8 9r
+ Close|irc,str/4 irc,str/4 irc,str/4 tld,irc,str/4
+ |
+ TO+ | scr/6 scr/6 scr/8 -
+ TO- | tlf/3p tlf/3p tlf/3p -
+ |
+ RCR+ | sca/8 sca,tlu/9 sca/8 tld,scr,sca/8
+ RCR- | scn/6 scn/7 scn/6 tld,scr,scn/6
+ RCA | irc/7 scr/6x irc,tlu/9 tld,scr/6x
+ RCN |irc,scr/6 scr/6x irc,scr/8 tld,scr/6x
+ |
+ RTR | sta/6 sta/6 sta/6 tld,zrc,sta/5
+ RTA | 6 6 8 tld,scr/6
+ |
+ RUC | scj/6 scj/7 scj/8 scj/9
+ RXJ+ | 6 6 8 9
+ RXJ- | tlf/3 tlf/3 tlf/3 tld,irc,str/5
+ |
+ RXR | 6 7 8 ser/9
+
+ The states in which the Restart timer is running are identifiable by
+ the presence of TO events. Only the Send-Configure-Request, Send-
+ Terminate-Request and Zero-Restart-Counter actions start or re-start
+ the Restart timer. The Restart timer is stopped when transitioning
+ from any state where the timer is running to a state where the timer
+ is not running.
+
+ [p] Passive option; see Stopped state discussion.
+
+ [r] Restart option; see Open event discussion.
+
+ [x] Crossed connection; see RCA event discussion.
+
+4.3 A Day in the Life
+
+ Here is an example of how a typical implementation might use the
+ automaton to implement LCP in a dial-up environment:
+
+ - The Network Access Server is powered on (Initial state, Link Dead
+ phase).
+
+ - A configuration file indicates that a particular link is to be
+
+
+
+Simpson [Page 15]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ used for PPP access (Open: tls/Starting). The This-Layer-Started
+ event turns on DTR to a modem, readying it for accepting calls.
+
+ - An incoming call is answered. The modem CD triggers configuration
+ negotiation (Up: irc,scr/Req-Sent, Link Establishment phase).
+
+ - A Configure-Request is received, which is acknowleged (RCR+:
+ sca/Ack-Sent).
+
+ - The Request is acknowleged (RCA: irc,tlu/Opened). The This-
+ Layer-Up event starts authentication and quality monitoring
+ protocols (Authentication phase).
+
+ - When authentication and quality monitoring are satisfied, they
+ send an Up event to start the available NCPs (Network-Layer
+ Protocol phase).
+
+ - Later, the peer is finished, and closes the link. A Terminate-
+ Request arrives (RTR: tld,zrc,sta/Stopping, Termination phase).
+ The This-Layer-Down action sends the Down event to any NCPs, while
+ the Terminate-Ack is sent. The Zero-Restart-Counter action causes
+ the link to wait for the peer to process the Terminate-Ack, with
+ no retries.
+
+ - When the Restart Timer times out (TO-: tlf/Stopped), the This-
+ Layer-Finished action signals the modem to hang up by dropping
+ DTR.
+
+ - When the CD from the modem drops (Down: tls/Starting), the This-
+ Layer-Started action raises DTR again, readying it for the next
+ call (returning to the Link Dead phase).
+
+4.4 States
+
+ Following is a more detailed description of each automaton state.
+
+ Initial
+
+ In the Initial state, the lower layer is unavailable (Down), and
+ no Open has occurred. The Restart timer is not running in the
+ Initial state.
+
+ Starting
+
+ The Starting state is the Open counterpart to the Initial state.
+ An administrative Open has been initiated, but the lower layer is
+ still unavailable (Down). The Restart timer is not running in the
+ Starting state.
+
+
+
+Simpson [Page 16]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ When the lower layer becomes available (Up), a Configure-Request
+ is sent.
+
+ Closed
+
+ In the Closed state, the link is available (Up), but no Open has
+ occurred. The Restart timer is not running in the Closed state.
+
+ Upon reception of Configure-Request packets, a Terminate-Ack is
+ sent. Terminate-Acks are silently discarded to avoid creating a
+ loop.
+
+ Stopped
+
+ The Stopped state is the Open counterpart to the Closed state. It
+ is entered when the automaton is waiting for a Down event after
+ the This-Layer-Finished action, or after sending a Terminate-Ack.
+ The Restart timer is not running in the Stopped state.
+
+ Upon reception of Configure-Request packets, an appropriate
+ response is sent. Upon reception of other packets, a Terminate-
+ Ack is sent. Terminate-Acks are silently discarded to avoid
+ creating a loop.
+
+ Rationale:
+
+ The Stopped state is a junction state for link termination, link
+ configuration failure, and other automaton failure modes. These
+ potentially separate states have been combined.
+
+ There is a race condition between the Down event response (from
+ the This-Layer-Finished action) and the Receive-Configure- Request
+ event. When a Configure-Request arrives before the Down event,
+ the Down event will supercede by returning the automaton to the
+ Starting state. This prevents attack by repetition.
+
+ Implementation Option:
+
+ After the peer fails to respond to Configure-Requests, an
+ implementation MAY wait passively for the peer to send Configure-
+ Requests. In this case, the This-Layer-Finished action is not
+ used for the TO- event in states Req-Sent, Ack- Rcvd and Ack-Sent.
+
+ This option is useful for dedicated circuits, or circuits which
+ have no status signals available, but SHOULD NOT be used for
+ switched circuits.
+
+
+
+
+
+Simpson [Page 17]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Closing
+
+ In the Closing state, an attempt is made to terminate the
+ connection. A Terminate-Request has been sent and the Restart
+ timer is running, but a Terminate-Ack has not yet been received.
+
+ Upon reception of a Terminate-Ack, the Closed state is entered.
+ Upon the expiration of the Restart timer, a new Terminate-Request
+ is transmitted and the Restart timer is restarted. After the
+ Restart timer has expired Max-Terminate times, this action may be
+ skipped, and the Closed state may be entered.
+
+ Stopping
+
+ The Stopping state is the Open counterpart to the Closing state.
+ A Terminate-Request has been sent and the Restart timer is
+ running, but a Terminate-Ack has not yet been received.
+
+ Rationale:
+
+ The Stopping state provides a well defined opportunity to
+ terminate a link before allowing new traffic. After the link has
+ terminated, a new configuration may occur via the Stopped or
+ Starting states.
+
+ Request-Sent
+
+ In the Request-Sent state an attempt is made to configure the
+ connection. A Configure-Request has been sent and the Restart
+ timer is running, but a Configure-Ack has not yet been received
+ nor has one been sent.
+
+ Ack-Received
+
+ In the Ack-Received state, a Configure-Request has been sent and a
+ Configure-Ack has been received. The Restart timer is still
+ running since a Configure-Ack has not yet been sent.
+
+ Ack-Sent
+
+ In the Ack-Sent state, a Configure-Request and a Configure-Ack
+ have both been sent but a Configure-Ack has not yet been received.
+ The Restart timer is always running in the Ack-Sent state.
+
+ Opened
+
+ In the Opened state, a Configure-Ack has been both sent and
+ received. The Restart timer is not running in the Opened state.
+
+
+
+Simpson [Page 18]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ When entering the Opened state, the implementation SHOULD signal
+ the upper layers that it is now Up. Conversely, when leaving the
+ Opened state, the implementation SHOULD signal the upper layers
+ that it is now Down.
+
+4.5 Events
+
+ Transitions and actions in the automaton are caused by events.
+
+ Up
+
+ The Up event occurs when a lower layer indicates that it is ready
+ to carry packets.
+
+ Typically, this event is used by a modem handling or calling
+ process, or by some other coupling of the PPP link to the physical
+ media, to signal LCP that the link is entering Link Establishment
+ phase.
+
+ It also can be used by LCP to signal each NCP that the link is
+ entering Network-Layer Protocol phase. That is, the This-Layer-Up
+ action from LCP triggers the Up event in the NCP.
+
+ Down
+
+ The Down event occurs when a lower layer indicates that it is no
+ longer ready to carry packets.
+
+ Typically, this event is used by a modem handling or calling
+ process, or by some other coupling of the PPP link to the physical
+ media, to signal LCP that the link is entering Link Dead phase.
+
+ It also can be used by LCP to signal each NCP that the link is
+ leaving Network-Layer Protocol phase. That is, the This-Layer-
+ Down action from LCP triggers the Down event in the NCP.
+
+ Open
+
+ The Open event indicates that the link is administratively
+ available for traffic; that is, the network administrator (human
+ or program) has indicated that the link is allowed to be Opened.
+ When this event occurs, and the link is not in the Opened state,
+ the automaton attempts to send configuration packets to the peer.
+
+ If the automaton is not able to begin configuration (the lower
+ layer is Down, or a previous Close event has not completed), the
+ establishment of the link is automatically delayed.
+
+
+
+
+Simpson [Page 19]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ When a Terminate-Request is received, or other events occur which
+ cause the link to become unavailable, the automaton will progress
+ to a state where the link is ready to re-open. No additional
+ administrative intervention is necessary.
+
+ Implementation Option:
+
+ Experience has shown that users will execute an additional Open
+ command when they want to renegotiate the link. This might
+ indicate that new values are to be negotiated.
+
+ Since this is not the meaning of the Open event, it is suggested
+ that when an Open user command is executed in the Opened, Closing,
+ Stopping, or Stopped states, the implementation issue a Down
+ event, immediately followed by an Up event. This will cause the
+ renegotiation of the link, without any harmful side effects.
+
+ Close
+
+ The Close event indicates that the link is not available for
+ traffic; that is, the network administrator (human or program) has
+ indicated that the link is not allowed to be Opened. When this
+ event occurs, and the link is not in the Closed state, the
+ automaton attempts to terminate the connection. Futher attempts
+ to re-configure the link are denied until a new Open event occurs.
+
+ Implementation Note:
+
+ When authentication fails, the link SHOULD be terminated, to
+ prevent attack by repetition and denial of service to other users.
+ Since the link is administratively available (by definition), this
+ can be accomplished by simulating a Close event to the LCP,
+ immediately followed by an Open event.
+
+ The Close followed by an Open will cause an orderly termination of
+ the link, by progressing from the Closing to the Stopping state,
+ and the This-Layer-Finished action can disconnect the link. The
+ automaton waits in the Stopped or Starting states for the next
+ connection attempt.
+
+ Timeout (TO+,TO-)
+
+ This event indicates the expiration of the Restart timer. The
+ Restart timer is used to time responses to Configure-Request and
+ Terminate-Request packets.
+
+ The TO+ event indicates that the Restart counter continues to be
+ greater than zero, which triggers the corresponding Configure-
+
+
+
+Simpson [Page 20]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Request or Terminate-Request packet to be retransmitted.
+
+ The TO- event indicates that the Restart counter is not greater
+ than zero, and no more packets need to be retransmitted.
+
+ Receive-Configure-Request (RCR+,RCR-)
+
+ This event occurs when a Configure-Request packet is received from
+ the peer. The Configure-Request packet indicates the desire to
+ open a connection and may specify Configuration Options. The
+ Configure-Request packet is more fully described in a later
+ section.
+
+ The RCR+ event indicates that the Configure-Request was
+ acceptable, and triggers the transmission of a corresponding
+ Configure-Ack.
+
+ The RCR- event indicates that the Configure-Request was
+ unacceptable, and triggers the transmission of a corresponding
+ Configure-Nak or Configure-Reject.
+
+ Implementation Note:
+
+ These events may occur on a connection which is already in the
+ Opened state. The implementation MUST be prepared to immediately
+ renegotiate the Configuration Options.
+
+ Receive-Configure-Ack (RCA)
+
+ The Receive-Configure-Ack event occurs when a valid Configure-Ack
+ packet is received from the peer. The Configure-Ack packet is a
+ positive response to a Configure-Request packet. An out of
+ sequence or otherwise invalid packet is silently discarded.
+
+ Implementation Note:
+
+ Since the correct packet has already been received before reaching
+ the Ack-Rcvd or Opened states, it is extremely unlikely that
+ another such packet will arrive. As specified, all invalid
+ Ack/Nak/Rej packets are silently discarded, and do not affect the
+ transitions of the automaton.
+
+ However, it is not impossible that a correctly formed packet will
+ arrive through a coincidentally-timed cross-connection. It is
+ more likely to be the result of an implementation error. At the
+ very least, this occurance SHOULD be logged.
+
+
+
+
+
+Simpson [Page 21]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Receive-Configure-Nak/Rej (RCN)
+
+ This event occurs when a valid Configure-Nak or Configure-Reject
+ packet is received from the peer. The Configure-Nak and
+ Configure-Reject packets are negative responses to a Configure-
+ Request packet. An out of sequence or otherwise invalid packet is
+ silently discarded.
+
+ Implementation Note:
+
+ Although the Configure-Nak and Configure-Reject cause the same
+ state transition in the automaton, these packets have
+ significantly different effects on the Configuration Options sent
+ in the resulting Configure-Request packet.
+
+ Receive-Terminate-Request (RTR)
+
+ The Receive-Terminate-Request event occurs when a Terminate-
+ Request packet is received. The Terminate-Request packet
+ indicates the desire of the peer to close the connection.
+
+ Implementation Note:
+
+ This event is not identical to the Close event (see above), and
+ does not override the Open commands of the local network
+ administrator. The implementation MUST be prepared to receive a
+ new Configure-Request without network administrator intervention.
+
+ Receive-Terminate-Ack (RTA)
+
+ The Receive-Terminate-Ack event occurs when a Terminate-Ack packet
+ is received from the peer. The Terminate-Ack packet is usually a
+ response to a Terminate-Request packet. The Terminate-Ack packet
+ may also indicate that the peer is in Closed or Stopped states,
+ and serves to re-synchronize the link configuration.
+
+ Receive-Unknown-Code (RUC)
+
+ The Receive-Unknown-Code event occurs when an un-interpretable
+ packet is received from the peer. A Code-Reject packet is sent in
+ response.
+
+ Receive-Code-Reject, Receive-Protocol-Reject (RXJ+,RXJ-)
+
+ This event occurs when a Code-Reject or a Protocol-Reject packet
+ is received from the peer.
+
+ The RXJ+ event arises when the rejected value is acceptable, such
+
+
+
+Simpson [Page 22]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ as a Code-Reject of an extended code, or a Protocol-Reject of a
+ NCP. These are within the scope of normal operation. The
+ implementation MUST stop sending the offending packet type.
+
+ The RXJ- event arises when the rejected value is catastrophic,
+ such as a Code-Reject of Configure-Request, or a Protocol-Reject
+ of LCP! This event communicates an unrecoverable error that
+ terminates the connection.
+
+ Receive-Echo-Request, Receive-Echo-Reply, Receive-Discard-Request
+ (RXR)
+
+ This event occurs when an Echo-Request, Echo-Reply or Discard-
+ Request packet is received from the peer. The Echo-Reply packet is
+ a response to a Echo-Request packet. There is no reply to an Echo-
+ Reply or Discard-Request packet.
+
+4.6 Actions
+
+ Actions in the automaton are caused by events and typically indicate
+ the transmission of packets and/or the starting or stopping of the
+ Restart timer.
+
+ Illegal-Event (-)
+
+ This indicates an event that cannot occur in a properly
+ implemented automaton. The implementation has an internal error,
+ which should be reported and logged. No transition is taken, and
+ the implementation SHOULD NOT reset or freeze.
+
+ This-Layer-Up (tlu)
+
+ This action indicates to the upper layers that the automaton is
+ entering the Opened state.
+
+ Typically, this action is used by the LCP to signal the Up event
+ to a NCP, Authentication Protocol, or Link Quality Protocol, or
+ MAY be used by a NCP to indicate that the link is available for
+ its network layer traffic.
+
+ This-Layer-Down (tld)
+
+ This action indicates to the upper layers that the automaton is
+ leaving the Opened state.
+
+ Typically, this action is used by the LCP to signal the Down event
+ to a NCP, Authentication Protocol, or Link Quality Protocol, or
+ MAY be used by a NCP to indicate that the link is no longer
+
+
+
+Simpson [Page 23]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ available for its network layer traffic.
+
+ This-Layer-Started (tls)
+
+ This action indicates to the lower layers that the automaton is
+ entering the Starting state, and the lower layer is needed for the
+ link. The lower layer SHOULD respond with an Up event when the
+ lower layer is available.
+
+ Implementation Note:
+
+ This results of this action are highly implementation dependent.
+
+ The transitions where this event is indicated are defined
+ according to a message passing architecture, rather than a
+ signalling architecture. If the action is desired to control
+ specific signals (such as DTR), other transitions for the action
+ are likely to be required (Open in Closed, RCR in Stopped).
+
+ This-Layer-Finished (tlf)
+
+ This action indicates to the lower layers that the automaton is
+ entering the Stopped or Closed states, and the lower layer is no
+ longer needed for the link. The lower layer SHOULD respond with a
+ Down event when the lower layer has terminated.
+
+ Typically, this action MAY be used by the LCP to advance to the
+ Link Dead phase, or MAY be used by a NCP to indicate to the LCP
+ that the link may terminate when there are no other NCPs open.
+
+ Implementation Note:
+
+ This results of this action are highly implementation dependent.
+
+ The transitions where this event is indicated are defined
+ according to a message passing architecture, rather than a
+ signalling architecture. If the action is desired to control
+ specific signals (such as DTR), other transitions for the action
+ are likely to be required (Close in Starting, Down in Closing).
+
+ Initialize-Restart-Counter (irc)
+
+ This action sets the Restart counter to the appropriate value
+ (Max-Terminate or Max-Configure). The counter is decremented for
+ each transmission, including the first.
+
+
+
+
+
+
+Simpson [Page 24]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Implementation Note:
+
+ In addition to setting the Restart counter, the implementation
+ MUST set the timeout period to the initial value when Restart
+ timer backoff is used.
+
+ Zero-Restart-Counter (zrc)
+
+ This action sets the Restart counter to zero.
+
+ Implementation Note:
+
+ This action enables the FSA to pause before proceeding to the
+ desired final state, allowing traffic to be processed by the peer.
+ In addition to zeroing the Restart counter, the implementation
+ MUST set the timeout period to an appropriate value.
+
+ Send-Configure-Request (scr)
+
+ The Send-Configure-Request action transmits a Configure-Request
+ packet. This indicates the desire to open a connection with a
+ specified set of Configuration Options. The Restart timer is
+ started when the Configure-Request packet is transmitted, to guard
+ against packet loss. The Restart counter is decremented each time
+ a Configure-Request is sent.
+
+ Send-Configure-Ack (sca)
+
+ The Send-Configure-Ack action transmits a Configure-Ack packet.
+ This acknowledges the reception of a Configure-Request packet with
+ an acceptable set of Configuration Options.
+
+ Send-Configure-Nak (scn)
+
+ The Send-Configure-Nak action transmits a Configure-Nak or
+ Configure-Reject packet, as appropriate. This negative response
+ reports the reception of a Configure-Request packet with an
+ unacceptable set of Configuration Options. Configure-Nak packets
+ are used to refuse a Configuration Option value, and to suggest a
+ new, acceptable value. Configure-Reject packets are used to
+ refuse all negotiation about a Configuration Option, typically
+ because it is not recognized or implemented. The use of
+ Configure-Nak versus Configure-Reject is more fully described in
+ the section on LCP Packet Formats.
+
+ Send-Terminate-Request (str)
+
+ The Send-Terminate-Request action transmits a Terminate-Request
+
+
+
+Simpson [Page 25]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ packet. This indicates the desire to close a connection. The
+ Restart timer is started when the Terminate-Request packet is
+ transmitted, to guard against packet loss. The Restart counter is
+ decremented each time a Terminate-Request is sent.
+
+ Send-Terminate-Ack (sta)
+
+ The Send-Terminate-Ack action transmits a Terminate-Ack packet.
+ This acknowledges the reception of a Terminate-Request packet or
+ otherwise serves to synchronize the state machines.
+
+ Send-Code-Reject (scj)
+
+ The Send-Code-Reject action transmits a Code-Reject packet. This
+ indicates the reception of an unknown type of packet.
+
+ Send-Echo-Reply (ser)
+
+ The Send-Echo-Reply action transmits an Echo-Reply packet. This
+ acknowledges the reception of an Echo-Request packet.
+
+4.7 Loop Avoidance
+
+ The protocol makes a reasonable attempt at avoiding Configuration
+ Option negotiation loops. However, the protocol does NOT guarantee
+ that loops will not happen. As with any negotiation, it is possible
+ to configure two PPP implementations with conflicting policies that
+ will never converge. It is also possible to configure policies which
+ do converge, but which take significant time to do so. Implementors
+ should keep this in mind and SHOULD implement loop detection
+ mechanisms or higher level timeouts.
+
+
+4.8 Counters and Timers
+
+ Restart Timer
+
+ There is one special timer used by the automaton. The Restart
+ timer is used to time transmissions of Configure-Request and
+ Terminate- Request packets. Expiration of the Restart timer
+ causes a Timeout event, and retransmission of the corresponding
+ Configure-Request or Terminate-Request packet. The Restart timer
+ MUST be configurable, but SHOULD default to three (3) seconds.
+
+ Implementation Note:
+
+ The Restart timer SHOULD be based on the speed of the link. The
+ default value is designed for low speed (2,400 to 9,600 bps), high
+
+
+
+Simpson [Page 26]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ switching latency links (typical telephone lines). Higher speed
+ links, or links with low switching latency, SHOULD have
+ correspondingly faster retransmission times.
+
+ Instead of a constant value, the Restart timer MAY begin at an
+ initial small value and increase to the configured final value.
+ Each successive value less than the final value SHOULD be at least
+ twice the previous value. The initial value SHOULD be large
+ enough to account for the size of the packets, twice the round
+ trip time for transmission at the link speed, and at least an
+ additional 100 milliseconds to allow the peer to process the
+ packets before responding. Some circuits add another 200
+ milliseconds of satellite delay. Round trip times for modems
+ operating at 14,400 bps have been measured in the range of 160 to
+ more than 600 milliseconds.
+
+ Max-Terminate
+
+ There is one required restart counter for Terminate-Requests.
+ Max- Terminate indicates the number of Terminate-Request packets
+ sent without receiving a Terminate-Ack before assuming that the
+ peer is unable to respond. Max-Terminate MUST be configurable,
+ but SHOULD default to two (2) transmissions.
+
+ Max-Configure
+
+ A similar counter is recommended for Configure-Requests. Max-
+ Configure indicates the number of Configure-Request packets sent
+ without receiving a valid Configure-Ack, Configure-Nak or
+ Configure- Reject before assuming that the peer is unable to
+ respond. Max- Configure MUST be configurable, but SHOULD default
+ to ten (10) transmissions.
+
+ Max-Failure
+
+ A related counter is recommended for Configure-Nak. Max-Failure
+ indicates the number of Configure-Nak packets sent without sending
+ a Configure-Ack before assuming that configuration is not
+ converging. Any further Configure-Nak packets are converted to
+ Configure-Reject packets. Max-Failure MUST be configurable, but
+ SHOULD default to ten (10) transmissions.
+
+5. LCP Packet Formats
+
+ There are three classes of LCP packets:
+
+ 1. Link Configuration packets used to establish and configure a
+ link (Configure-Request, Configure-Ack, Configure-Nak and
+
+
+
+Simpson [Page 27]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Configure-Reject).
+
+ 2. Link Termination packets used to terminate a link (Terminate-
+ Request and Terminate-Ack).
+
+ 3. Link Maintenance packets used to manage and debug a link
+ (Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
+ Discard-Request).
+
+ This document describes Version 1 of the Link Control Protocol. In
+ the interest of simplicity, there is no version field in the LCP
+ packet. If a new version of LCP is necessary in the future, the
+ intention is that a new PPP Protocol field value will be used to
+ differentiate Version 1 LCP from all other versions. A correctly
+ functioning Version 1 LCP implementation will always respond to
+ unknown Protocols (including other versions) with an easily
+ recognizable Version 1 packet, thus providing a deterministic
+ fallback mechanism for implementations of other versions.
+
+ Regardless of which Configuration Options are enabled, all LCP Link
+ Configuration, Link Termination, and Code-Reject packets (codes 1
+ through 7) are always sent as if no Configuration Options were
+ enabled. This ensures that such LCP packets are always recognizable
+ even when one end of the link mistakenly believes the link to be
+ open.
+
+ Implementation Note:
+
+ In particular, the Async-Control-Character-Map (ACCM) default for
+ the type of link is used, and no address, control, or protocol
+ field compression is allowed.
+
+ Exactly one LCP packet is encapsulated in the PPP Information
+ field, where the PPP Protocol field indicates type hex c021 (Link
+ Control Protocol).
+
+ A summary of the Link Control Protocol packet format is shown below.
+ The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data ...
+ +-+-+-+-+
+
+
+
+
+
+Simpson [Page 28]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Code
+
+ The Code field is one octet and identifies the kind of LCP packet.
+ When a packet is received with an invalid Code field, a Code-
+ Reject packet is transmitted.
+
+ Up-to-date values of the LCP Code field are specified in the most
+ recent "Assigned Numbers" RFC [2]. This specification concerns
+ the following values:
+
+ 1 Configure-Request
+ 2 Configure-Ack
+ 3 Configure-Nak
+ 4 Configure-Reject
+ 5 Terminate-Request
+ 6 Terminate-Ack
+ 7 Code-Reject
+ 8 Protocol-Reject
+ 9 Echo-Request
+ 10 Echo-Reply
+ 11 Discard-Request
+
+ Identifier
+
+ The Identifier field is one octet and aids in matching requests
+ and replies. When a packet is received with an invalid Identifier
+ field, the packet is silently discarded.
+
+ Length
+
+ The Length field is two octets and indicates the length of the LCP
+ packet including the Code, Identifier, Length and Data fields.
+ Octets outside the range of the Length field are treated as
+ padding and are ignored on reception. When a packet is received
+ with an invalid Length field, the packet is silently discarded.
+
+ Data
+
+ The Data field is zero or more octets as indicated by the Length
+ field. The format of the Data field is determined by the Code
+ field.
+
+5.1 Configure-Request
+
+ Description
+
+ An implementation wishing to open a connection MUST transmit a LCP
+ packet with the Code field set to 1 (Configure-Request), and the
+
+
+
+Simpson [Page 29]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Options field filled with any desired changes to the link
+ defaults. Configuration Options SHOULD NOT be included with
+ default values.
+
+ Upon reception of a Configure-Request, an appropriate reply MUST
+ be transmitted.
+
+ A summary of the Configure-Request packet format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Options ...
+ +-+-+-+-+
+
+ Code
+
+ 1 for Configure-Request.
+
+ Identifier
+
+ The Identifier field MUST be changed whenever the content of the
+ Options field changes, and whenever a valid reply has been
+ received for a previous request. For retransmissions, the
+ Identifier MAY remain unchanged.
+
+ Options
+
+ The options field is variable in length and contains the list of
+ zero or more Configuration Options that the sender desires to
+ negotiate. All Configuration Options are always negotiated
+ simultaneously. The format of Configuration Options is further
+ described in a later section.
+
+5.2 Configure-Ack
+
+ Description
+
+ If every Configuration Option received in a Configure-Request is
+ recognizable and all values are acceptable, then the
+ implementation MUST transmit a LCP packet with the Code field set
+ to 2 (Configure-Ack), the Identifier field copied from the
+ received Configure-Request, and the Options field copied from the
+ received Configure-Request. The acknowledged Configuration
+ Options MUST NOT be reordered or modified in any way.
+
+
+
+Simpson [Page 30]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ On reception of a Configure-Ack, the Identifier field MUST match
+ that of the last transmitted Configure-Request. Additionally, the
+ Configuration Options in a Configure-Ack MUST exactly match those
+ of the last transmitted Configure-Request. Invalid packets are
+ silently discarded.
+
+ A summary of the Configure-Ack packet format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Options ...
+ +-+-+-+-+
+
+ Code
+
+ 2 for Configure-Ack.
+
+ Identifier
+
+ The Identifier field is a copy of the Identifier field of the
+ Configure-Request which caused this Configure-Ack.
+
+ Options
+
+ The Options field is variable in length and contains the list of
+ zero or more Configuration Options that the sender is
+ acknowledging. All Configuration Options are always acknowledged
+ simultaneously.
+
+5.3 Configure-Nak
+
+ Description
+
+ If every element of the received Configuration Options is
+ recognizable but some values are not acceptable, then the
+ implementation MUST transmit a LCP packet with the Code field set
+ to 3 (Configure-Nak), the Identifier field copied from the
+ received Configure-Request, and the Options field filled with only
+ the unacceptable Configuration Options from the Configure-Request.
+ All acceptable Configuration Options are filtered out of the
+ Configure-Nak, but otherwise the Configuration Options from the
+ Configure-Request MUST NOT be reordered.
+
+ Options which have no value fields (boolean options) MUST use the
+
+
+
+Simpson [Page 31]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Configure-Reject reply instead.
+
+ Each Configuration Option which is allowed only a single instance
+ MUST be modified to a value acceptable to the Configure-Nak
+ sender. The default value MAY be used, when this differs from the
+ requested value.
+
+ When a particular type of Configuration Option can be listed more
+ than once with different values, the Configure-Nak MUST include a
+ list of all values for that option which are acceptable to the
+ Configure-Nak sender. This includes acceptable values that were
+ present in the Configure-Request.
+
+ Finally, an implementation may be configured to request the
+ negotiation of a specific Configuration Option. If that option is
+ not listed, then that option MAY be appended to the list of Nak'd
+ Configuration Options in order to prompt the peer to include that
+ option in its next Configure-Request packet. Any value fields for
+ the option MUST indicate values acceptable to the Configure-Nak
+ sender.
+
+ On reception of a Configure-Nak, the Identifier field MUST match
+ that of the last transmitted Configure-Request. Invalid packets
+ are silently discarded.
+
+ Reception of a valid Configure-Nak indicates that a new
+ Configure-Request MAY be sent with the Configuration Options
+ modified as specified in the Configure-Nak. When multiple
+ instances of a Configuration Option are present, the peer SHOULD
+ select a single value to include in its next Configure-Request
+ packet.
+
+ Some Configuration Options have a variable length. Since the
+ Nak'd Option has been modified by the peer, the implementation
+ MUST be able to handle an Option length which is different from
+ the original Configure-Request.
+
+ A summary of the Configure-Nak packet format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Options ...
+ +-+-+-+-+
+
+
+
+
+Simpson [Page 32]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Code
+
+ 3 for Configure-Nak.
+
+ Identifier
+
+ The Identifier field is a copy of the Identifier field of the
+ Configure-Request which caused this Configure-Nak.
+
+ Options
+
+ The Options field is variable in length and contains the list of
+ zero or more Configuration Options that the sender is Nak'ing.
+ All Configuration Options are always Nak'd simultaneously.
+
+5.4 Configure-Reject
+
+ Description
+
+ If some Configuration Options received in a Configure-Request are
+ not recognizable or are not acceptable for negotiation (as
+ configured by a network administrator), then the implementation
+ MUST transmit a LCP packet with the Code field set to 4
+ (Configure-Reject), the Identifier field copied from the received
+ Configure-Request, and the Options field filled with only the
+ unacceptable Configuration Options from the Configure-Request.
+ All recognizable and negotiable Configuration Options are filtered
+ out of the Configure-Reject, but otherwise the Configuration
+ Options MUST NOT be reordered or modified in any way.
+
+ On reception of a Configure-Reject, the Identifier field MUST
+ match that of the last transmitted Configure-Request.
+ Additionally, the Configuration Options in a Configure-Reject MUST
+ be a proper subset of those in the last transmitted Configure-
+ Request. Invalid packets are silently discarded.
+
+ Reception of a valid Configure-Reject indicates that a new
+ Configure-Request SHOULD be sent which does not include any of the
+ Configuration Options listed in the Configure-Reject.
+
+ A summary of the Configure-Reject packet format is shown below. The
+ fields are transmitted from left to right.
+
+
+
+
+
+
+
+
+
+Simpson [Page 33]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Options ...
+ +-+-+-+-+
+
+ Code
+
+ 4 for Configure-Reject.
+
+ Identifier
+
+ The Identifier field is a copy of the Identifier field of the
+ Configure-Request which caused this Configure-Reject.
+
+ Options
+
+ The Options field is variable in length and contains the list of
+ zero or more Configuration Options that the sender is rejecting.
+ All Configuration Options are always rejected simultaneously.
+
+5.5 Terminate-Request and Terminate-Ack
+
+ Description
+
+ LCP includes Terminate-Request and Terminate-Ack Codes in order to
+ provide a mechanism for closing a connection.
+
+ A LCP implementation wishing to close a connection SHOULD transmit
+ a LCP packet with the Code field set to 5 (Terminate-Request), and
+ the Data field filled with any desired data. Terminate-Request
+ packets SHOULD continue to be sent until Terminate-Ack is
+ received, the lower layer indicates that it has gone down, or a
+ sufficiently large number have been transmitted such that the peer
+ is down with reasonable certainty.
+
+ Upon reception of a Terminate-Request, a LCP packet MUST be
+ transmitted with the Code field set to 6 (Terminate-Ack), the
+ Identifier field copied from the Terminate-Request packet, and the
+ Data field filled with any desired data.
+
+ Reception of an unelicited Terminate-Ack indicates that the peer
+ is in the Closed or Stopped states, or is otherwise in need of
+ re-negotiation.
+
+ A summary of the Terminate-Request and Terminate-Ack packet formats
+
+
+
+Simpson [Page 34]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ is shown below. The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data ...
+ +-+-+-+-+
+
+ Code
+
+ 5 for Terminate-Request;
+
+ 6 for Terminate-Ack.
+
+ Identifier
+
+ On transmission, the Identifier field MUST be changed whenever the
+ content of the Data field changes, and whenever a valid reply has
+ been received for a previous request. For retransmissions, the
+ Identifier MAY remain unchanged. On reception, the Identifier
+ field of the Terminate-Request is copied into the Identifier field
+ of the Terminate-Ack packet.
+
+ Data
+
+ The Data field is zero or more octets and contains uninterpreted
+ data for use by the sender. The data may consist of any binary
+ value and may be of any length from zero to the peer's established
+ MRU minus four.
+
+
+5.6 Code-Reject
+
+ Description
+
+ Reception of a LCP packet with an unknown Code indicates that one
+ of the communicating LCP implementations is faulty or incomplete.
+ This error MUST be reported back to the sender of the unknown Code
+ by transmitting a LCP packet with the Code field set to 7 (Code-
+ Reject), and the inducing packet copied to the Rejected-
+ Information field.
+
+ Upon reception of a Code-Reject, the implementation SHOULD report
+ the error, since it is unlikely that the situation can be
+ rectified automatically.
+
+
+
+
+Simpson [Page 35]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ A summary of the Code-Reject packet format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Rejected-Packet ...
+ +-+-+-+-+-+-+-+-+
+
+ Code
+
+ 7 for Code-Reject.
+
+ Identifier
+
+ The Identifier field MUST be changed for each Code-Reject sent.
+
+ Rejected-Information
+
+ The Rejected-Information field contains a copy of the LCP packet
+ which is being rejected. It begins with the Information field,
+ and does not include any Data Link Layer headers nor an FCS. The
+ Rejected-Information MUST be truncated to comply with the peer's
+ established MRU.
+
+
+5.7 Protocol-Reject
+
+ Description
+
+ Reception of a PPP packet with an unknown Protocol field indicates
+ that the peer is attempting to use a protocol which is
+ unsupported. This usually occurs when the peer attempts to
+ configure a new protocol. If the LCP state machine is in the
+ Opened state, then this error MUST be reported back to the peer by
+ transmitting a LCP packet with the Code field set to 8 (Protocol-
+ Reject), the Rejected-Protocol field set to the received Protocol,
+ and the inducing packet copied to the Rejected-Information field.
+
+ Upon reception of a Protocol-Reject, the implementation MUST stop
+ sending packets of the indicated protocol at the earliest
+ opportunity.
+
+ Protocol-Reject packets can only be sent in the LCP Opened state.
+ Protocol-Reject packets received in any state other than the LCP
+ Opened state SHOULD be silently discarded.
+
+
+
+Simpson [Page 36]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ A summary of the Protocol-Reject packet format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Rejected-Protocol | Rejected-Information ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Code
+
+ 8 for Protocol-Reject.
+
+ Identifier
+
+ The Identifier field MUST be changed for each Protocol-Reject
+ sent.
+
+ Rejected-Protocol
+
+ The Rejected-Protocol field is two octets and contains the PPP
+ Protocol field of the packet which is being rejected.
+
+ Rejected-Information
+
+ The Rejected-Information field contains a copy of the packet which
+ is being rejected. It begins with the Information field, and does
+ not include any Data Link Layer headers nor an FCS. The
+ Rejected-Information MUST be truncated to comply with the peer's
+ established MRU.
+
+5.8 Echo-Request and Echo-Reply
+
+ Description
+
+ LCP includes Echo-Request and Echo-Reply Codes in order to provide
+ a Data Link Layer loopback mechanism for use in exercising both
+ directions of the link. This is useful as an aid in debugging,
+ link quality determination, performance testing, and for numerous
+ other functions.
+
+ An Echo-Request sender transmits a LCP packet with the Code field
+ set to 9 (Echo-Request), the Identifier field set, the local
+ Magic-Number (if any) inserted, and the Data field filled with any
+ desired data, but not exceeding the peer's established MRU minus
+ eight.
+
+
+
+Simpson [Page 37]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Upon reception of an Echo-Request, a LCP packet MUST be
+ transmitted with the Code field set to 10 (Echo-Reply), the
+ Identifier field copied from the received Echo-Request, the local
+ Magic-Number (if any) inserted, and the Data field copied from the
+ Echo-Request, truncating as necessary to avoid exceeding the
+ peer's established MRU.
+
+ Echo-Request and Echo-Reply packets may only be sent in the LCP
+ Opened state. Echo-Request and Echo-Reply packets received in any
+ state other than the LCP Opened state SHOULD be silently
+ discarded.
+
+ A summary of the Echo-Request and Echo-Reply packet formats is shown
+ below. The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Magic-Number |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data ...
+ +-+-+-+-+
+
+ Code
+
+ 9 for Echo-Request;
+
+ 10 for Echo-Reply.
+
+ Identifier
+
+ On transmission, the Identifier field MUST be changed whenever the
+ content of the Data field changes, and whenever a valid reply has
+ been received for a previous request. For retransmissions, the
+ Identifier MAY remain unchanged.
+
+ On reception, the Identifier field of the Echo-Request is copied
+ into the Identifier field of the Echo-Reply packet.
+
+ Magic-Number
+
+ The Magic-Number field is four octets and aids in detecting links
+ which are in the looped-back condition. Until the Magic-Number
+ Configuration Option has been successfully negotiated, the Magic-
+ Number MUST be transmitted as zero. See the Magic-Number
+ Configuration Option for further explanation.
+
+
+
+Simpson [Page 38]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Data
+
+ The Data field is zero or more octets and contains uninterpreted
+ data for use by the sender. The data may consist of any binary
+ value and may be of any length from zero to the peer's established
+ MRU minus eight.
+
+5.9 Discard-Request
+
+ Description
+
+ LCP includes a Discard-Request Code in order to provide a Data
+ Link Layer sink mechanism for use in exercising the local to
+ remote direction of the link. This is useful as an aid in
+ debugging, performance testing, and for numerous other functions.
+
+ The sender transmits a LCP packet with the Code field set to 11
+ (Discard-Request), the Identifier field set, the local Magic-
+ Number (if any) inserted, and the Data field filled with any
+ desired data, but not exceeding the peer's established MRU minus
+ eight.
+
+ Discard-Request packets may only be sent in the LCP Opened state.
+ On reception, the receiver MUST simply throw away any Discard-
+ Request that it receives.
+
+ A summary of the Discard-Request packet format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Code | Identifier | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Magic-Number |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data ...
+ +-+-+-+-+
+
+
+ Code
+
+ 11 for Discard-Request.
+
+ Identifier
+
+ The Identifier field MUST be changed for each Discard-Request
+ sent.
+
+
+
+Simpson [Page 39]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Magic-Number
+
+ The Magic-Number field is four octets and aids in detecting links
+ which are in the looped-back condition. Until the Magic-Number
+ Configuration Option has been successfully negotiated, the Magic-
+ Number MUST be transmitted as zero. See the Magic-Number
+ Configuration Option for further explanation.
+
+ Data
+
+ The Data field is zero or more octets and contains uninterpreted
+ data for use by the sender. The data may consist of any binary
+ value and may be of any length from zero to the peer's established
+ MRU minus four.
+
+6. LCP Configuration Options
+
+ LCP Configuration Options allow negotiation of modifications to the
+ default characteristics of a point-to-point link. If a Configuration
+ Option is not included in a Configure-Request packet, the default
+ value for that Configuration Option is assumed.
+
+ Some Configuration Options MAY be listed more than once. The effect
+ of this is Configuration Option specific, and is specified by each
+ such Configuration Option description. (None of the Configuration
+ Options in this specification can be listed more than once.)
+
+ The end of the list of Configuration Options is indicated by the
+ length of the LCP packet.
+
+ Unless otherwise specified, all Configuration Options apply in a
+ half-duplex fashion; typically, in the receive direction of the link
+ from the point of view of the Configure-Request sender.
+
+ A summary of the Configuration Option format is shown below. The
+ fields are transmitted from left to right.
+
+ 0 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length | Data ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Type
+
+ The Type field is one octet and indicates the type of
+ Configuration Option. Up-to-date values of the LCP Option Type
+ field are specified in the most recent "Assigned Numbers" RFC [2].
+
+
+
+Simpson [Page 40]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ This specification concerns the following values:
+
+ 1 Maximum-Receive-Unit
+ 2 Async-Control-Character-Map
+ 3 Authentication-Protocol
+ 4 Quality-Protocol
+ 5 Magic-Number
+ 6 RESERVED
+ 7 Protocol-Field-Compression
+ 8 Address-and-Control-Field-Compression
+
+ Length
+
+ The Length field is one octet and indicates the length of this
+ Configuration Option including the Type, Length and Data fields.
+ If a negotiable Configuration Option is received in a Configure-
+ Request but with an invalid Length, a Configure-Nak SHOULD be
+ transmitted which includes the desired Configuration Option with
+ an appropriate Length and Data.
+
+ Data
+
+ The Data field is zero or more octets and information specific to
+ the Configuration Option. The format and length of the Data field
+ is determined by the Type and Length fields.
+
+6.1 Maximum-Receive-Unit
+
+ Description
+
+ This Configuration Option may be sent to inform the peer that the
+ implementation can receive larger packets, or to request that the
+ peer send smaller packets.
+
+ The default value is 1500 octets. If smaller packets are
+ requested, an implementation MUST still be able to receive the
+ full 1500 octet information field in case link synchronization is
+ lost.
+
+ Implementation Note:
+
+ This option is used to indicate an implementation capability. The
+ peer is not required to maximize the use of the capacity. For
+ example, when a MRU is indicated which is 2048 octets, the peer is
+ not required to send any packet with 2048 octets. The peer need
+ not Configure-Nak to indicate that it will only send smaller
+ packets, since the implementation will always require support for
+ at least 1500 octets.
+
+
+
+Simpson [Page 41]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ A summary of the Maximum-Receive-Unit Configuration Option format is
+ shown below. The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length | Maximum-Receive-Unit |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+ Type
+
+ 1
+
+ Length
+
+ 4
+
+ Maximum-Receive-Unit
+
+ The Maximum-Receive-Unit field is two octets, and specifies the
+ maximum number of octets in the Information and Padding fields.
+ It does not include the framing, Protocol field, FCS, nor any
+ transparency bits or bytes.
+
+6.2 Async-Control-Character-Map
+
+ Description
+
+ This Configuration Option provides a method to negotiate the use
+ of control character transparency on asynchronous links.
+
+ For asynchronous links, the default value is 0xffffffff, which
+ causes all octets less than 0x20 to be mapped into an appropriate
+ two octet sequence. For most other links, the default value is 0,
+ since there is no need for mapping.
+
+ However, it is rarely necessary to map all control characters, and
+ often it is unnecessary to map any control characters. The
+ Configuration Option is used to inform the peer which control
+ characters MUST remain mapped when the peer sends them.
+
+ The peer MAY still send any other octets in mapped format, if it
+ is necessary because of constraints known to the peer. The peer
+ SHOULD Configure-Nak with the logical union of the sets of mapped
+ octets, so that when such octets are spuriously introduced they
+ can be ignored on receipt.
+
+
+
+
+Simpson [Page 42]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ A summary of the Async-Control-Character-Map Configuration Option
+ format is shown below. The fields are transmitted from left to
+ right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length | Async-Control-Character-Map
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | ACCM (cont) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Type
+
+ 2
+
+ Length
+
+ 6
+
+ Async-Control-Character-Map
+
+ The Async-Control-Character-Map field is four octets and indicates
+ the set of control characters to be mapped. The map is sent most
+ significant octet first.
+
+ Each numbered bit corresponds to the octet of the same value. If
+ the bit is cleared to zero, then that octet need not be mapped.
+ If the bit is set to one, then that octet MUST remain mapped. For
+ example, if bit 19 is set to zero, then the ASCII control
+ character 19 (DC3, Control-S) MAY be sent in the clear.
+
+ Note: The least significant bit of the least significant octet
+ (the final octet transmitted) is numbered bit 0, and would map
+ to the ASCII control character NUL.
+
+6.3 Authentication-Protocol
+
+ Description
+
+ On some links it may be desirable to require a peer to
+ authenticate itself before allowing network-layer protocol packets
+ to be exchanged.
+
+ This Configuration Option provides a method to negotiate the use
+ of a specific authentication protocol. By default, authentication
+ is not required.
+
+
+
+
+Simpson [Page 43]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ An implementation MUST NOT include multiple Authentication-
+ Protocol Configuration Options in its Configure-Request packets.
+ Instead, it SHOULD attempt to configure the most desirable
+ protocol first. If that protocol is Configure-Nak'd, then the
+ implementation SHOULD attempt the next most desirable protocol in
+ the next Configure-Request.
+
+ If an implementation sends a Configure-Ack with this Configuration
+ Option, then it is agreeing to authenticate with the specified
+ protocol. An implementation receiving a Configure-Ack with this
+ Configuration Option SHOULD expect the peer to authenticate with
+ the acknowledged protocol.
+
+ There is no requirement that authentication be full duplex or that
+ the same protocol be used in both directions. It is perfectly
+ acceptable for different protocols to be used in each direction.
+ This will, of course, depend on the specific protocols negotiated.
+
+ A summary of the Authentication-Protocol Configuration Option format
+ is shown below. The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length | Authentication-Protocol |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data ...
+ +-+-+-+-+
+
+ Type
+
+ 3
+
+ Length
+
+ >= 4
+
+ Authentication-Protocol
+
+ The Authentication-Protocol field is two octets and indicates the
+ authentication protocol desired. Values for this field are always
+ the same as the PPP Protocol field values for that same
+ authentication protocol.
+
+ Up-to-date values of the Authentication-Protocol field are
+ specified in the most recent "Assigned Numbers" RFC [2]. Current
+ values are assigned as follows:
+
+
+
+
+Simpson [Page 44]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Value (in hex) Protocol
+
+ c023 Password Authentication Protocol
+ c223 Challenge Handshake Authentication Protocol
+
+ Data
+
+ The Data field is zero or more octets and contains additional data
+ as determined by the particular protocol.
+
+6.4 Quality-Protocol
+
+ Description
+
+ On some links it may be desirable to determine when, and how
+ often, the link is dropping data. This process is called link
+ quality monitoring.
+
+ This Configuration Option provides a method to negotiate the use
+ of a specific protocol for link quality monitoring. By default,
+ link quality monitoring is disabled.
+
+ There is no requirement that quality monitoring be full duplex or
+ that the same protocol be used in both directions. It is
+ perfectly acceptable for different protocols to be used in each
+ direction. This will, of course, depend on the specific protocols
+ negotiated.
+
+ A summary of the Quality-Protocol Configuration Option format is
+ shown below. The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length | Quality-Protocol |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Data ...
+ +-+-+-+-+
+
+ Type
+
+ 4
+
+ Length
+
+ >= 4
+
+
+
+
+
+Simpson [Page 45]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Quality-Protocol
+
+ The Quality-Protocol field is two octets and indicates the link
+ quality monitoring protocol desired. Values for this field are
+ always the same as the PPP Protocol field values for that same
+ monitoring protocol.
+
+ Up-to-date values of the Quality-Protocol field are specified in
+ the most recent "Assigned Numbers" RFC [2]. Current values are
+ assigned as follows:
+
+ Value (in hex) Protocol
+
+ c025 Link Quality Report
+
+ Data
+
+ The Data field is zero or more octets and contains additional data
+ as determined by the particular protocol.
+
+6.5 Magic-Number
+
+ Description
+
+ This Configuration Option provides a method to detect looped-back
+ links and other Data Link Layer anomalies. This Configuration
+ Option MAY be required by some other Configuration Options such as
+ the Quality-Protocol Configuration Option. By default, the
+ Magic-Number is not negotiated, and zero is inserted where a
+ Magic-Number might otherwise be used.
+
+ Before this Configuration Option is requested, an implementation
+ MUST choose its Magic-Number. It is recommended that the Magic-
+ Number be chosen in the most random manner possible in order to
+ guarantee with very high probability that an implementation will
+ arrive at a unique number. A good way to choose a unique random
+ number is to start with an unique seed. Suggested sources of
+ uniqueness include machine serial numbers, other network hardware
+ addresses, time-of-day clocks, etc. Particularly good random
+ number seeds are precise measurements of the inter-arrival time of
+ physical events such as packet reception on other connected
+ networks, server response time, or the typing rate of a human
+ user. It is also suggested that as many sources as possible be
+ used simultaneously.
+
+ When a Configure-Request is received with a Magic-Number
+ Configuration Option, the received Magic-Number is compared with
+ the Magic-Number of the last Configure-Request sent to the peer.
+
+
+
+Simpson [Page 46]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ If the two Magic-Numbers are different, then the link is not
+ looped-back, and the Magic-Number SHOULD be acknowledged. If the
+ two Magic-Numbers are equal, then it is possible, but not certain,
+ that the link is looped-back and that this Configure-Request is
+ actually the one last sent. To determine this, a Configure-Nak
+ MUST be sent specifying a different Magic-Number value. A new
+ Configure-Request SHOULD NOT be sent to the peer until normal
+ processing would cause it to be sent (that is, until a Configure-
+ Nak is received or the Restart timer runs out).
+
+ Reception of a Configure-Nak with a Magic-Number different from
+ that of the last Configure-Nak sent to the peer proves that a link
+ is not looped-back, and indicates a unique Magic-Number. If the
+ Magic-Number is equal to the one sent in the last Configure-Nak,
+ the possibility of a looped-back link is increased, and a new
+ Magic-Number MUST be chosen. In either case, a new Configure-
+ Request SHOULD be sent with the new Magic-Number.
+
+ If the link is indeed looped-back, this sequence (transmit
+ Configure-Request, receive Configure-Request, transmit Configure-
+ Nak, receive Configure-Nak) will repeat over and over again. If
+ the link is not looped-back, this sequence might occur a few
+ times, but it is extremely unlikely to occur repeatedly. More
+ likely, the Magic-Numbers chosen at either end will quickly
+ diverge, terminating the sequence. The following table shows the
+ probability of collisions assuming that both ends of the link
+ select Magic-Numbers with a perfectly uniform distribution:
+
+ Number of Collisions Probability
+ -------------------- ---------------------
+ 1 1/2**32 = 2.3 E-10
+ 2 1/2**32**2 = 5.4 E-20
+ 3 1/2**32**3 = 1.3 E-29
+
+ Good sources of uniqueness or randomness are required for this
+ divergence to occur. If a good source of uniqueness cannot be
+ found, it is recommended that this Configuration Option not be
+ enabled; Configure-Requests with the option SHOULD NOT be
+ transmitted and any Magic-Number Configuration Options which the
+ peer sends SHOULD be either acknowledged or rejected. In this
+ case, loop-backs cannot be reliably detected by the
+ implementation, although they may still be detectable by the peer.
+
+ If an implementation does transmit a Configure-Request with a
+ Magic-Number Configuration Option, then it MUST NOT respond with a
+ Configure-Reject if it receives a Configure-Request with a Magic-
+ Number Configuration Option. That is, if an implementation
+ desires to use Magic Numbers, then it MUST also allow its peer to
+
+
+
+Simpson [Page 47]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ do so. If an implementation does receive a Configure-Reject in
+ response to a Configure-Request, it can only mean that the link is
+ not looped-back, and that its peer will not be using Magic-
+ Numbers. In this case, an implementation SHOULD act as if the
+ negotiation had been successful (as if it had instead received a
+ Configure-Ack).
+
+ The Magic-Number also may be used to detect looped-back links
+ during normal operation as well as during Configuration Option
+ negotiation. All LCP Echo-Request, Echo-Reply, and Discard-
+ Request packets have a Magic-Number field. If Magic-Number has
+ been successfully negotiated, an implementation MUST transmit
+ these packets with the Magic-Number field set to its negotiated
+ Magic-Number.
+
+ The Magic-Number field of these packets SHOULD be inspected on
+ reception. All received Magic-Number fields MUST be equal to
+ either zero or the peer's unique Magic-Number, depending on
+ whether or not the peer negotiated a Magic-Number. Reception of a
+ Magic-Number field equal to the negotiated local Magic-Number
+ indicates a looped-back link. Reception of a Magic- Number other
+ than the negotiated local Magic-Number or the peer's negotiated
+ Magic-Number, or zero if the peer didn't negotiate one, indicates
+ a link which has been (mis)configured for communications with a
+ different peer.
+
+ Procedures for recovery from either case are unspecified and may
+ vary from implementation to implementation. A somewhat
+ pessimistic procedure is to assume a LCP Down event. A further
+ Open event will begin the process of re-establishing the link,
+ which can't complete until the loop-back condition is terminated
+ and Magic-Numbers are successfully negotiated. A more optimistic
+ procedure (in the case of a loop-back) is to begin transmitting
+ LCP Echo-Request packets until an appropriate Echo-Reply is
+ received, indicating a termination of the loop-back condition.
+
+ A summary of the Magic-Number Configuration Option format is shown
+ below. The fields are transmitted from left to right.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length | Magic-Number
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Magic-Number (cont) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+
+
+
+Simpson [Page 48]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Type
+
+ 5
+
+ Length
+
+ 6
+
+ Magic-Number
+
+ The Magic-Number field is four octets and indicates a number which
+ is very likely to be unique to one end of the link. A Magic-
+ Number of zero is illegal and MUST always be Nak'd, if it is not
+ Rejected outright.
+
+6.6 Protocol-Field-Compression
+
+ Description
+
+ This Configuration Option provides a method to negotiate the
+ compression of the PPP Protocol field. By default, all
+ implementations MUST transmit packets with two octet PPP Protocol
+ fields.
+
+ PPP Protocol field numbers are chosen such that some values may be
+ compressed into a single octet form which is clearly
+ distinguishable from the two octet form. This Configuration
+ Option is sent to inform the peer that the implementation can
+ receive such single octet Protocol fields.
+
+ As previously mentioned, the Protocol field uses an extension
+ mechanism consistent with the ISO 3309 extension mechanism for the
+ Address field; the Least Significant Bit (LSB) of each octet is
+ used to indicate extension of the Protocol field. A binary "0" as
+ the LSB indicates that the Protocol field continues with the
+ following octet. The presence of a binary "1" as the LSB marks
+ the last octet of the Protocol field. Notice that any number of
+ "0" octets may be prepended to the field, and will still indicate
+ the same value (consider the two binary representations for 3,
+ 00000011 and 00000000 00000011).
+
+ When using low speed links, it is desirable to conserve bandwidth
+ by sending as little redundant data as possible. The Protocol-
+ Field-Compression Configuration Option allows a trade-off between
+ implementation simplicity and bandwidth efficiency. If
+ successfully negotiated, the ISO 3309 extension mechanism may be
+ used to compress the Protocol field to one octet instead of two.
+ The large majority of packets are compressible since data
+
+
+
+Simpson [Page 49]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ protocols are typically assigned with Protocol field values less
+ than 256.
+
+ Compressed Protocol fields MUST NOT be transmitted unless this
+ Configuration Option has been negotiated. When negotiated, PPP
+ implementations MUST accept PPP packets with either double-octet
+ or single-octet Protocol fields, and MUST NOT distinguish between
+ them.
+
+ The Protocol field is never compressed when sending any LCP
+ packet. This rule guarantees unambiguous recognition of LCP
+ packets.
+
+ When a Protocol field is compressed, the Data Link Layer FCS field
+ is calculated on the compressed frame, not the original
+ uncompressed frame.
+
+ A summary of the Protocol-Field-Compression Configuration Option
+ format is shown below. The fields are transmitted from left to
+ right.
+
+ 0 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Type
+
+ 7
+
+ Length
+
+ 2
+
+6.7 Address-and-Control-Field-Compression
+
+ Description
+
+ This Configuration Option provides a method to negotiate the
+ compression of the Data Link Layer Address and Control fields. By
+ default, all implementations MUST transmit frames with Address and
+ Control fields appropriate to the link framing.
+
+ Since these fields usually have constant values for point-to-point
+ links, they are easily compressed. This Configuration Option is
+ sent to inform the peer that the implementation can receive
+ compressed Address and Control fields.
+
+
+
+Simpson [Page 50]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ If a compressed frame is received when Address-and-Control-Field-
+ Compression has not been negotiated, the implementation MAY
+ silently discard the frame.
+
+ The Address and Control fields MUST NOT be compressed when sending
+ any LCP packet. This rule guarantees unambiguous recognition of
+ LCP packets.
+
+ When the Address and Control fields are compressed, the Data Link
+ Layer FCS field is calculated on the compressed frame, not the
+ original uncompressed frame.
+
+ A summary of the Address-and-Control-Field-Compression configuration
+ option format is shown below. The fields are transmitted from left
+ to right.
+
+ 0 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Type
+
+ 8
+
+ Length
+
+ 2
+
+A. LCP Recommended Options
+
+ The following Configurations Options are recommended:
+
+ SYNC LINES
+
+ Magic Number Link Quality Monitoring No Address and Control Field
+ Compression No Protocol Field Compression
+
+ ASYNC LINES
+
+ Async Control Character Map Magic Number Address and Control Field
+ Compression Protocol Field Compression
+
+Security Considerations
+
+ Security issues are briefly discussed in sections concerning the
+ Authentication Phase, the Close event, and the Authentication-
+
+
+
+Simpson [Page 51]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+ Protocol Configuration Option. Further discussion is in a companion
+ document entitled PPP Authentication Protocols.
+
+
+References
+
+ [1] Perkins, D., "Requirements for an Internet Standard
+ Point-to-Point Protocol", RFC 1547, December 1993.
+
+ [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
+ USC/Information Sciences Institute, July 1992.
+
+Acknowledgments
+
+ Much of the text in this document is taken from the WG Requirements,
+ and RFCs 1171 & 1172, by Drew Perkins of Carnegie Mellon University,
+ and by Russ Hobby of the University of California at Davis.
+
+ Many people spent significant time helping to develop the Point-to-
+ Point Protocol. The complete list of people is too numerous to list,
+ but the following people deserve special thanks: Rick Adams (UUNET),
+ Ken Adelman (TGV), Fred Baker (ACC), Mike Ballard (Telebit), Craig
+ Fox (Network Systems), Karl Fox (Morning Star Technologies), Phill
+ Gross (AN&S), former WG chair Russ Hobby (UC Davis), David Kaufman
+ (Proteon), former WG chair Steve Knowles (FTP Software), former WG
+ chair Brian Lloyd (L&A), John LoVerso (Xylogics), Bill Melohn (Sun
+ Microsystems), Mike Patton (MIT), former WG chair Drew Perkins
+ (Fore), Greg Satz (cisco systems), John Shriver (Proteon), Vernon
+ Schryver (Silicon Graphics), and Asher Waldfogel (Wellfleet).
+
+ The "Day in the Life" example was instigated by Kory Hamzeh (Avatar).
+ In this version, improvements in wording were also provided by Scott
+ Ginsburg, Mark Moraes, and Timon Sloan, as they worked on
+ implementations.
+
+ Special thanks to Morning Star Technologies for providing computing
+ resources and network access support for writing this specification.
+
+Chair's Address
+
+ The working group can be contacted via the current chair:
+
+ Fred Baker
+ Advanced Computer Communications
+ 315 Bollay Drive
+ Santa Barbara, California, 93111
+
+ EMail: fbaker@acc.com
+
+
+
+Simpson [Page 52]
+
+RFC 1548 The Point-to-Point Protocol December 1993
+
+
+Editor's Address
+
+ Questions about this memo can also be directed to:
+
+ William Allen Simpson
+ Daydreamer
+ Computer Systems Consulting Services
+ 1384 Fontaine
+ Madison Heights, Michigan 48071
+
+ EMail: Bill.Simpson@um.cc.umich.edu
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Simpson [Page 53]
+
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