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