RFCs in HTML Format


RFC 1548

                   The Point-to-Point Protocol (PPP)






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
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



Back to RFC index

 

Associates:

 



Sponsered-Sites:

Register domain name and transfer | Cheap webhosting service | Domain name registration

 

 

""