RFCs in HTML Format


RFC 1661

                   The Point-to-Point Protocol (PPP)


Table of Contents


     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    2
        1.2       Terminology .....................................    3

     2.     PPP Encapsulation .....................................    4


Simpson                                                         [Page i]

3. PPP Link Operation ....................................
6 3.1 Overview ........................................ 6 3.2 Phase Diagram ................................... 6 3.3 Link Dead (physical-layer not ready) ............ 7 3.4 Link Establishment Phase ........................ 7 3.5 Authentication Phase ............................ 8 3.6 Network-Layer Protocol Phase .................... 8 3.7 Link Termination Phase .......................... 9 4. The Option Negotiation Automaton ...................... 11 4.1 State Transition Table .......................... 12 4.2 States .......................................... 14 4.3 Events .......................................... 16 4.4 Actions ......................................... 21 4.5 Loop Avoidance .................................. 23 4.6 Counters and Timers ............................. 24 5. LCP Packet Formats .................................... 26 5.1 Configure-Request ............................... 28 5.2 Configure-Ack ................................... 29 5.3 Configure-Nak ................................... 30 5.4 Configure-Reject ................................ 31 5.5 Terminate-Request and Terminate-Ack ............. 33 5.6 Code-Reject ..................................... 34 5.7 Protocol-Reject ................................. 35 5.8 Echo-Request and Echo-Reply ..................... 36 5.9 Discard-Request ................................. 37 6. LCP Configuration Options ............................. 39 6.1 Maximum-Receive-Unit (MRU) ...................... 41 6.2 Authentication-Protocol ......................... 42 6.3 Quality-Protocol ................................ 43 6.4 Magic-Number .................................... 45 6.5 Protocol-Field-Compression (PFC) ................ 48 6.6 Address-and-Control-Field-Compression (ACFC) SECURITY CONSIDERATIONS ...................................... 51 REFERENCES ................................................... 51 ACKNOWLEDGEMENTS ............................................. 51 CHAIR'S ADDRESS .............................................. 52 EDITOR'S ADDRESS ............................................. 52 Simpson [Page ii]
1. Introduction The Point-to-Point Protocol is designed for simple links which transport packets between two peers. These links provide full-duplex simultaneous bi-directional operation, and are assumed to deliver packets in order. It is intended that PPP provide a common solution for easy connection of a wide variety of hosts, bridges and routers [1]. Encapsulation The PPP encapsulation provides for multiplexing of different network-layer protocols simultaneously over the same link. The PPP encapsulation has been carefully designed to retain compatibility with most commonly used supporting hardware. Only 8 additional octets are necessary to form the encapsulation when used within the default HDLC-like framing. In environments where bandwidth is at a premium, the encapsulation and framing may be shortened to 2 or 4 octets. To support high speed implementations, the default encapsulation uses only simple fields, only one of which needs to be examined for demultiplexing. The default header and information fields fall on 32-bit boundaries, and the trailer may be padded to an arbitrary boundary. Link Control Protocol In order to be sufficiently versatile to be portable to a wide variety of environments, PPP provides a Link Control Protocol (LCP). The LCP is used to automatically agree upon the encapsulation format options, handle varying limits on sizes of packets, detect a looped-back link and other common misconfiguration errors, and terminate the link. Other optional facilities provided are authentication of the identity of its peer on the link, and determination when a link is functioning properly and when it is failing. Network Control Protocols Point-to-Point links tend to exacerbate many problems with the current family of network protocols. For instance, assignment and management of IP addresses, which is a problem even in LAN environments, is especially difficult over circuit-switched point-to-point links (such as dial-up modem servers). These problems are handled by a family of Network Control Protocols (NCPs), which each manage the specific needs required by their Simpson [Page 1]

respective network-layer protocols. These NCPs are defined in companion documents. 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 2]

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 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. Simpson [Page 3]

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. A summary of the PPP encapsulation is shown below. The fields are transmitted from left to right. +----------+-------------+---------+ | Protocol | Information | Padding | | 8/16 bits| * | * | +----------+-------------+---------+ Protocol Field The Protocol field is one or 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). Simpson [Page 4]

Up-to-date values of the Protocol field are specified in the most recent "Assigned Numbers" RFC [2]. This specification reserves the following values: Value (in hex) Protocol Name 0001 Padding Protocol 0003 to 001f reserved (transparency inefficient) 007d reserved (Control Escape) 00cf reserved (PPP NLPID) 00ff reserved (compression inefficient) 8001 to 801f unused 807d unused 80cf unused 80ff unused 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. The maximum length for the Information field, including Padding, but not including the Protocol field, 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. Simpson [Page 5]

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 which are specified in the following simplified state diagram: +------+ +-----------+ +--------------+ | | UP | | OPENED | | SUCCESS/NONE | Dead |------->| Establish |---------->| Authenticate |--+ | | | | | | | +------+ +-----------+ +--------------+ | ^ | | | | FAIL | FAIL | | +<--------------+ +----------+ | | | | | +-----------+ | +---------+ | | DOWN | | | CLOSING | | | +------------| Terminate |<---+<----------| Network |<-+ | | | | +-----------+ +---------+ Not all transitions are specified in this diagram. The following semantics MUST be followed. Simpson [Page 6]

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 later) will be in the Initial or Starting states. The transition to the Link Establishment phase will signal an Up event to the LCP 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 link, 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 later) has been both sent and received. All Configuration Options are assumed to be at default values unless altered by the configuration exchange. See the chapter 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. The receipt of the LCP Configure-Request causes a return to the Link Establishment phase from the Network-Layer Protocol phase or Authentication phase. Simpson [Page 7]

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 request the use of that authentication protocol during Link Establishment phase. 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. If authentication fails, the authenticator SHOULD proceed instead to the Link Termination phase. Only Link Control Protocol, authentication protocol, and link quality monitoring packets are allowed during this phase. All other packets received during this phase MUST be silently discarded. Implementation Notes: An implementation SHOULD NOT fail authentication simply due to timeout or lack of response. The authentication SHOULD allow some method of retransmission, and proceed to the Link Termination phase only after a number of authentication attempts has been exceeded. The implementation responsible for commencing Link Termination phase is the implementation which has refused authentication to its peer. 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. Simpson [Page 8]

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 supported network-layer protocol packets received when the corresponding NCP is not in the Opened state MUST be silently discarded. Implementation Note: While LCP is in the Opened state, any protocol packet which is unsupported by the implementation MUST be returned in a Protocol- Reject (described later). 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 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. Simpson [Page 9]

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. Simpson [Page 10]

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 TO+ = Timeout with counter > 0 irc = Initialize-Restart-Count TO- = Timeout with counter expired zrc = Zero-Restart-Count 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 Simpson [Page 11]

4.1. 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 tlf/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 12]

| 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-Count 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. The events and actions are defined according to a message passing architecture, rather than a signalling architecture. If an action is desired to control specific signals (such as DTR), additional actions are likely to be required. [p] Passive option; see Stopped state discussion. [r] Restart option; see Open event discussion. [x] Crossed connection; see RCA event discussion. Simpson [Page 13]

4.2. 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. 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 Simpson [Page 14]

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. 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, the Closed state is 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 Simpson [Page 15]

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 running, since a Configure-Ack has not yet been received. Opened In the Opened state, a Configure-Ack has been both sent and received. The Restart timer is not running. 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. 4.3. Events Transitions and actions in the automaton are caused by events. Up This 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 This event occurs when a lower layer indicates that it is no Simpson [Page 16]

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 This 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. 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. Care must be taken that an intervening Down event cannot occur from another source. The Down followed by an Up will cause an orderly renegotiation of the link, by progressing through the Starting to the Request-Sent state. This will cause the renegotiation of the link, without any harmful side effects. Close This event indicates that the link is not available for traffic; Simpson [Page 17]

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. Care must be taken that an intervening Close event cannot occur from another source. The Close followed by an Open will cause an orderly termination of the link, by progressing through 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- 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 Simpson [Page 18]

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) This 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. 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) This event occurs when a Terminate-Request packet is received. The Terminate-Request packet indicates the desire of the peer to Simpson [Page 19]

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) This 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) This 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 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 an Echo-Request packet. There is no reply to an Echo-Reply or Discard-Request packet. Simpson [Page 20]


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, looped-back links 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 when 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 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. Simpson [Page 46]

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, 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 looped-back condition is terminated, and Magic-Numbers are successfully negotiated. A more optimistic procedure (in the case of a looped-back link) is to begin transmitting LCP Echo-Request packets until an appropriate Echo-Reply is received, indicating a termination of the looped- 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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. Simpson [Page 47]

6.5. Protocol-Field-Compression (PFC) 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 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 Simpson [Page 48]

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 Simpson [Page 49]

6.6. Address-and-Control-Field-Compression (ACFC) 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. 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 Simpson [Page 50]

Security Considerations Security issues are briefly discussed in sections concerning the Authentication Phase, the Close event, and the Authentication- Protocol Configuration Option. References [1] Perkins, D., "Requirements for an Internet Standard Point-to- Point Protocol",
RFC 1547, Carnegie Mellon University, December 1993. [2] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC 1340, USC/Information Sciences Institute, July 1992. Acknowledgements 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@merit.edu mailing list. Much of the text in this document is taken from the working group requirements [1]; and RFCs 1171 & 1172, by Drew Perkins while at Carnegie Mellon University, and by Russ Hobby of the University of California at Davis. William Simpson was principally responsible for introducing consistent terminology and philosophy, and the re-design of the phase and negotiation state machines. 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, Ken Adelman, Fred Baker, Mike Ballard, Craig Fox, Karl Fox, Phill Gross, Kory Hamzeh, former WG chair Russ Hobby, David Kaufman, former WG chair Steve Knowles, Mark Lewis, former WG chair Brian Lloyd, John LoVerso, Bill Melohn, Mike Patton, former WG chair Drew Perkins, Greg Satz, John Shriver, Vernon Schryver, and Asher Waldfogel. Special thanks to Morning Star Technologies for providing computing resources and network access support for writing this specification. Simpson [Page 51]
Chair's Address The working group can be contacted via the current chair: Fred Baker Advanced Computer Communications 315 Bollay Drive Santa Barbara, California 93117 fbaker@acc.com

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