Internet Draft PPP Working Group Kory Hamzeh INTERNET DRAFT Ascend Communications Category: Internet Draft Tim Kolar Title: draft-ietf-pppext-l2tp-04.txt Cisco Systems Date: June 1997 Morgan Littlewood Cisco Systems Gurdeep Singh Pall Microsoft Corporation Jeff Taarud Copper Mountain Networks Andrew J. Valencia Cisco Systems William Verthein U.S. Robotics Layer Two Tunneling Protocol "L2TP" Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au. Abstract Virtual dial-up allows many separate and autonomous protocol domains to share common access infrastructure including modems, Access Servers, and ISDN routers. RFC1661 specifies multiprotocol dial-up via PPP [1]. This document describes the Layer Two Tunneling Protocol (L2TP) which permits the tunneling of the link layer (i.e., HDLC, async HDLC) of PPP. Using such tunnels, it is possible to divorce the location of the initial dial-up server from the location at which the dial-up protocol connection is terminated and access to the network provided. Table of Contents 1.0 Introduction 1.1 Conventions 1.2 Terminology Valencia expires December 1997 [Page 1] INTERNET DRAFT June 1997 2.0 Problem Space Overview 2.1 Initial Assumptions 2.2 Topology 2.3 Providing Virtual Dial-up Services--a walk-through 3.0 Service Model Issues 3.1 Security 3.2 Address Allocation 3.3 Authentication 3.4 Accounting 4.0 Protocol Overview 4.1 Control Message Overview 4.2 Payload Packet Overview 5.0 Message Format and Protocol Extensibility 5.1 AVP 5.2 Control Message Format 5.3 Payload Message Format 5.4 Control Message Types 5.5 AVP Summary 5.6 Result and Error Code Summary 5.7 Hiding of AVP values 6.0 Control Connection Protocol Specification 6.1 Start-Control-Connection-Request 6.2 Start-Control-Connection-Reply 6.3 Start-Control-Connection-Connected 6.4 Stop-Control-Connection-Request 6.5 Stop-Control-Connection-Reply 6.6 Hello 6.7 Outgoing-Call-Request 6.8 Outgoing-Call-Reply 6.9 Outgoing-Call-Connected 6.10 Incoming-Call-Request 6.11 Incoming-Call-Reply 6.12 Incoming-Call-Connected 6.13 Call-Clear-Request 6.14 Call-Disconnect-Notify 6.15 WAN-Error-Notify 6.16 Set-Link-Info 7.0 Control Connection State Machines 7.1 Control Connection Protocol Operation 7.2 Control Connection States 7.2.1 Control Connection Originator 7.2.2 Control connection Receiver 7.3 Timing considerations 7.4 Incoming Calls 7.4.1 LAC Incoming Call States 7.4.2 LNS Incoming Call States 7.5 Outgoing calls 7.5.1 LAC Outgoing Call States 7.5.2 LNS Outgoing Call States 8.0 L2TP Over Specific Media 8.1 IP/UDP 8.2 IP 9.0 Security Considerations 9.1 Tunnel Endpoint Security Valencia expires December 1997 [Page 2] INTERNET DRAFT June 1997 9.2 Client Security 10.0 Acknowledgments 11.0 Contacts 12.0 References Appendix A: Acknowledgment Time-Outs Appendix B: Acknowledgment Time-Out and Window Adjustment Appendix C: Handling of out-of-order packets Appendix D: Transport Layer Adaptive Time-Outs and Window Adjustment Appendix E: Examples of L2TP sequence numbering E.1 Lock-step tunnel establishment E.2 Multiple packets acknowledged E.3 Lost packet with retransmission 1.0 Introduction The traditional dial-up network service on the Internet is for registered IP addresses only. A new class of virtual dial-up application which allows multiple protocols and unregistered IP addresses is also desired on the Internet. Examples of this class of network application are support for privately addressed IP, IPX, and AppleTalk dial-up via PPP across existing Internet infrastructure. The support of these multiprotocol virtual dial-up applications is of significant benefit to end users, enterprises, and Internet Service providers as it allows the sharing of very large investments in access and core infrastructure and allows local calls to be used. It also allows existing investments in non-IP protocol applications to be supported in a secure manner while still leveraging the access infrastructure of the Internet. It is the purpose of this draft to identify the issues encountered in integrating multiprotocol dial-up services into an existing Internet Service Provider's Point of Presence (hereafter referred to as ISP and POP, respectively), and to describe the L2TP protocol which permits the leveraging of existing access protocols. This protocol may also be used to solve the "multilink hunt-group splitting" problem. Multilink PPP, often used to aggregate ISDN B channels, requires that all channels composing a multilink bundle be grouped at a single NAS. Because L2TP makes a PPP session appear at a location other than the physical point at which the session was physically received, it can be used to make all channels appear at a single NAS, allowing multilink operation even when the physical calls are spread across distinct physical NAS's. 1.1 Conventions The following language conventions are used in the items of specification in this document: o MUST, SHALL, or MANDATORY -- This item is an absolute requirement of the specification. o SHOULD or RECOMMEND -- This item should generally be followed Valencia expires December 1997 [Page 3] INTERNET DRAFT June 1997 for all but exceptional circumstances. o MAY or OPTIONAL -- This item is truly optional and may be followed or ignored according to the needs of the implementor. 1.2 Terminology Analog Channel A circuit-switched communication path which is intended to carry 3.1 Khz audio in each direction. Digital Channel A circuit-switched communication path which is intended to carry digital information in each direction. Call A connection or attempted connection between two terminal endpoints on a PSTN or ISDN--for example, a telephone call between two modems. CHAP Challenge Authentication Protocol, a PPP cryptographic challenge/response authentication protocol in which the cleartext password is not passed in the clear over the line. CLID Calling Line ID, an indication to the receiver of a call as to the phone number of the caller. Control Messages Control messages are exchanged between LAC, LNS pairs, and operate in-band within the tunnel protocol. Control messages govern aspects of the tunnel and sessions within the tunnel. Dial User An end-system or router attached to an on-demand PSTN or ISDN which is either the initiator or recipient of a call. DNIS Dialed Number Information String, an indication to the receiver of a call as to what phone number the caller used to reach it. EAP Extensible Authentication Protocol, a framework for a family of PPP authentication protocols, including cleartext, challenge/response, and arbitrary dialog sequences. Valencia expires December 1997 [Page 4] INTERNET DRAFT June 1997 L2TP Access Concentrator (LAC) A device attached to one or more PSTN or ISDN lines capable of PPP operation and of handling the L2TP protocol. The LAC needs only implement the media over which L2TP is to operate to pass traffic to one or more LNS's. It may tunnel any protocol carried within PPP. L2TP Network Server (LNS) An LNS operates on any platform capable of PPP termination. The LNS handles the server side of the L2TP protocol. Since L2TP relies only on the single media over which L2TP tunnels arrive, the LNS may have only a single LAN or WAN interface, yet still be able to terminate calls arriving at any LAC's full range of PPP interfaces (async, synchronous ISDN, V.120, etc.). Network Access Server (NAS) A device providing temporary, on-demand network access to users. This access is point-to-point using PSTN or ISDN lines. PAP Password Authentication Protocol, a simple PPP authentication mechanism in which a cleartext username and password are transmitted to prove identity. Session L2TP is connection-oriented. The LNS and LAC maintain state for each user that is attached to an LAC. A session is created when an end-to-end PPP connection is attempted between a dial user and the LNS, or when a outbound call is initiated. The datagrams related to a session are sent over the tunnel between the LAC and LNS. Quality of Service (QOS) A given Quality of Service level is sometimes required for a given user being tunneled between an LNS-LAC pair. For this scenario, a unique L2TP tunnel is created (generally on top of a new SVC) and encapsulated directly on top of the media providing the indicated QOS. Switched Virtual Circuit (SVC) An L2TP-compatible media on top of which L2TP is directly encapsulated. SVC's are dynamically created, permitting tunnel media to be created dynamically in response to desired LNS-LAC connectivity requirements. Tunnel Valencia expires December 1997 [Page 5] INTERNET DRAFT June 1997 A tunnel is defined by an LNS-LAC pair. The tunnel carries PPP datagrams between the LAC and the LNS; many sessions can be multiplexed over a single tunnel. A control connection operating in-band over the same tunnel controls the establishment, release, and maintenance of sessions and of the tunnel itself. 2.0 Problem Space Overview In this section we describe in high level terms the scope of the problem that will be explored in more detail in later sections. 2.1 Initial Assumptions We begin by assuming that Internet access is provided by an ISP and that the ISP wishes to offer services other than traditional registered IP address based services to dial-up users of the network. We also assume that the user of such a service wants all of the security facilities that are available to him in a dedicated dial-up configuration. In particular, the end user requires: + End System transparency: Neither the remote end system nor his home site hosts should require any special software to use this service in a secure manner. + Authentication as provided via dial-up PPP CHAP, PAP, EAP, or through other dialogs, for instance, a textual exchange on V.120 before starting PPP. This will include TACACS+ [7] and RADIUS [8] solutions as well as support for smart cards and one-time passwords. The authentication should be manageable by the user independently of the ISP. + Addressing should be as manageable as dedicated dial-up solutions. The address should be assigned by the home site and not the ISP. + Authorization should be managed by the home site as it would in a direct dial-up solution. + Accounting should be performed both by the ISP (for billing purposes) and by the user (for charge-back and auditing). 2.2 Topology Shown below is a generic Internet with Public switched Telephone Network (PSTN) access (i.e., async PPP via modems) and Integrated Services Digital Network (ISDN) access (i.e., synchronous PPP access). Remote users (either async or ISDN PPP) will access the Home LAN as if they were dialed into the L2TP Network Server (LNS), although their physical dial-up is via the ISP Network Access Server. Valencia expires December 1997 [Page 6] INTERNET DRAFT June 1997 ...----[L]----+---[L]-----... | | [H] | ________|________________________ | | ________|__ ______|________ | | | | | PSTN [R] [R] ISDN | | Cloud | | Cloud [N]__[U] | | Internet | | | | [R] | [N]______[R] |_____________| | | | | | | [U] |________________________________| [H] = LNS [L] = Home LAN(s) [R] = Router [U] = Remote User [N] = ISP Network Access Server 2.3 Providing Virtual dial-up Services--a walk-through To motivate the following discussion, this section walks through an example of what might happen when a Virtual dial-up client initiates access. The remote user initiates a PPP connection to an ISP via either the PSTN or ISDN. The Network Access Server (NAS) accepts the connection and the PPP link is established (L2TP also permits the NAS to check with an LNS after call indication prior to accepting the call--this is useful where DNIS or CLID information is available in the incoming call notification). The ISP may now undertake a partial authentication of the end system/user. Only the username field would be interpreted to determine whether the user requires a Virtual dial-up service. It is expected--but not required--that usernames will be structured (e.g. username@company.com). Alternatively, the ISP may maintain a database mapping users to services. In the case of Virtual dial-up, the mapping will name a specific endpoint, the LNS. Alternatively, the ISP may have already determined the target LNS from DNIS. If the LNS is willing to accept tunnel creation without any authentication of the caller, the NAS may tunnel the PPP connection without ever having communicated with the remote user. If a virtual dial-up service is not required, standard access to the Internet may be provided. Valencia expires December 1997 [Page 7] INTERNET DRAFT June 1997 If no tunnel connection currently exists to the desired LNS, one is initiated. L2TP is designed to be largely insulated from the details of the media over which the tunnel is established; L2TP requires only that the tunnel media provide packet oriented point-to-point connectivity. Obvious examples of such media are UDP, Frame Relay PVC's, or X.25 VC's. Once the tunnel exists, an unused slot within the tunnel, a "Call ID", is allocated, and a connect indication is sent to notify the LNS of this new dial-up session. The LNS either accepts the connection, or rejects it. Rejection may include a reason indication, which may be displayed to the dial-up user, after which the call should be disconnected. The initial setup notification may include the authentication information required to allow the LNS to authenticate the user and decide to accept or decline the connection. In the case of CHAP, the set-up packet includes the challenge, username and raw response. For PAP or text dialog, it includes username and clear text password. The LNS may choose to use this information to complete its authentication, avoiding an additional cycle of authentication. If the LAC negotiated PPP LCP before initiating the tunnel, the initial setup notification may also include a copy of the LCP CONFREQ's sent in each direction which completed LCP negotiation. The LNS may use this information to initialize its own PPP state (thus avoiding an additional LCP negotiation), or it may choose to initiate a new LCP CONFREQ exchange. If the LNS accepts the connection, it creates a "virtual interface" for PPP in a manner analogous to what it would use for a direct- dialed connection. With this "virtual interface" in place, link layer frames may now pass over this tunnel in both directions. Frames from the remote user are received at the POP, stripped of CRC, link framing, and transparency bytes, encapsulated in L2TP, and forwarded over the appropriate tunnel. The LNS accepts these frames, strips L2TP, and processes them as normal incoming frames for the appropriate interface and protocol. The "virtual interface" behaves very much like a hardware interface, with the exception that the hardware in this case is physically located at the ISP POP. The other direction behaves analogously, with the LNS encapsulating the packet in L2TP, and the NAS stripping L2TP before transmitting it out the physical interface to the remote user. For the remainder of this document, a NAS operating as a peer to an LNS will be referred to as an L2TP Access Concentrator, or "LAC". At this point, the connectivity is a point-to-point PPP session whose endpoints are the remote user's networking application on one end and the termination of this connectivity into the LNS's PPP support on the other. Because the remote user has become simply another dial-up client of the LNS, client connectivity can now be managed using traditional mechanisms with respect to further authorization, Valencia expires December 1997 [Page 8] INTERNET DRAFT June 1997 protocol access, and packet filtering. Accounting can be performed at both the LAC as well as the LNS. This document illustrates some Accounting techniques which are possible using L2TP, but the policies surrounding such Accounting are outside the scope of this specification. L2TP offers optional facilities which maximize compatibility with legacy client requirements; L2TP connect notifications for PPP clients can contain sufficient information for an LNS to authenticate and initialize its LCP state machine. With these facilities, the remote user need not be queried a second time for PPP authentication, nor undergo multiple rounds of LCP negotiation and convergence. These techniques are intended to optimize connection setup, and are not intended to deprecate any functions required by the PPP specification. 3.0 Service Model Issues There are several significant differences between the standard Internet access service and the Virtual dial-up service with respect to authentication, address allocation, authorization and accounting. The details of the differences between these services and the problems presented by these differences are described below. The mechanisms used for Virtual Dial-up service are intended to coexist with more traditional mechanisms; it is intended that an ISP's POP can simultaneously service ISP clients as well as Virtual dial-up clients. 3.1 Security For the Virtual dial-up service, the ISP pursues authentication only to the extent required to discover the user's apparent identity (and by implication, their desired LNS). This may involve no more than detecting DNIS information when a call arrives, or may involve full LCP negotiation and initiation of PPP authentication. As soon as the apparent identity is determined, a connection to the LNS is initiated with any authentication information gathered by the ISP. The LNS completes the authentication by either accepting the connection, or rejecting it. The LNS may need to protect against attempts by third parties to establish tunnels to the LNS. Tunnel establishment can include authentication to protect against such attacks. 3.2 Address Allocation For an Internet service, the user accepts that the IP address may be allocated dynamically from a pool of ISP addresses. This model often means that the remote user has little or no access to their home network's resources, due to firewalls and other security policies applied by the home network to accesses from external IP addresses. For the Virtual dial-up service, the LNS can exist behind the home Valencia expires December 1997 [Page 9] INTERNET DRAFT June 1997 firewall, allocating addresses which are internal (and, in fact, can be RFC1918 addresses, or non-IP addresses). Because L2TP tunnels exclusively at the frame layer, the actual policies of such address management are irrelevant to correct Virtual dial-up service; for all purposes of PPP protocol handling, the dial-in user appears to have connected at the LNS. 3.3 Authentication The authentication of the user occurs in three phases; the first at the ISP, and the second and optional third at the LNS. The ISP uses DNIS, CLID, or username to determine that a Virtual dial-up service is required and initiates the tunnel connection to the appropriate LNS. Once a tunnel is established, The ISP NAS allocates a new Call ID and initiates a session by forwarding the gathered authentication information. The LNS undertakes the second phase by deciding whether or not to accept the connection. The connection indication may include CHAP, PAP, EAP, or textual authentication information. Based on this information, the LNS may accept the connection, or may reject it (for instance, it was a PAP request and the username/password are found to be incorrect). Once the connection is accepted, the LNS is free to pursue a third phase of authentication at the PPP layer. These activities are outside the scope of this specification, but might include an additional cycle of LCP authentication, proprietary PPP extensions, or textual challenges carried via a TCP/IP telnet session. 3.4 Accounting It is a requirement that both the LAC and the LNS be capable of providing accounting data and hence both may count packets, octets and connection start and stop times. Since Virtual dial-up is an access service, accounting of connection attempts (in particular, failed connection attempts) is of significant interest. The LNS can reject new connections based on the authentication information gathered by the LAC, with corresponding logging. For cases where the LNS accepts the connection and then continues with further authentication, the LNS might subsequently disconnect the client. For such scenarios, the disconnection indication back to the LAC may also include a reason. Because the LNS can decline a connection based on the authentication information collected by the LAC, accounting can easily draw a distinction between a series of failed connection attempts and a series of brief successful connections. Lacking this facility, the LNS must always accept connection requests, and would need to exchange a number of PPP packets with the remote system. Note that the LNS could use this information to decide to accept the connection (which protects against most invalid connection attempts) while still Valencia expires December 1997 [Page 10] INTERNET DRAFT June 1997 insisting on running its own CHAP authentication (for instance, to protect against CHAP replay attacks). 4.0 Protocol Overview There are two parallel components of L2TP operating over a given tunnel: control messages between each LAC-LNS pair, and payload packets between the same LAC-LNS pair. The latter are used to transport L2TP encapsulated PPP packets for user sessions between the pair. The Nr (Next Received) and Ns (Next Sent) fields are always present in control messages, and are optionally present in payload messages. A single sequence number state is maintained for all control messages, and a distinct state is maintained for the payload of each user session within the tunnel. Each state is initialized so the first packet is sent with an Ns of 0. Nr is sent reflecting one more than the last in-order received packet; if sent before any packet is received it would be 0, indicating that it expects the next new Ns value received to be 0. The sequence number state is maintained and updated as packets are sent. A message (control or payload) with a zero-length body indicates that the packet is only used to communicate Nr and Ns fields. The Nr and Ns fields are filled in as above, but the sequence number state remains the same. For non-zero-length body messages, the sequence number state is incremented (modulo 2**16) after it is copied to the packet's Ns field. Thus a zero-length message's Ns field will reflect one more than the Ns of the last non-zero-length message sent. If a new Ns value has been received by the peer, and no packet has been sent with an Nr value to indicate this reception within 1/4 of the timeout interval, such a zero-length message is sent. See Appendix E for some examples of how sequence numbers progress. 4.1 Control Message Overview Before PPP tunneling can occur between an LAC and LNS, control messages must be exchanged between them. Control messages are exchanged over the same tunnel which will be used to forward payload data once L2TP call control and management information have been passed. The control messages are responsible for establishment, management, and release of sessions carried through the tunnel, as well as status on the tunnel itself. It is the means by which an LNS is notified of an incoming call at an associated LAC, as well as the means by which an LAC is instructed to place an outgoing dial call. A tunnel may be established by either an LAC (for incoming calls) or an LNS (for outgoing calls). Following the establishment of the tunnel, the LNS and LAC configure the tunnel by exchanging Start-Control-Connection-Request and -Reply messages. These Valencia expires December 1997 [Page 11] INTERNET DRAFT June 1997 messages are also used to exchange information about basic operating capabilities of the LAC and LNS. Once the control message exchange is complete, the LAC may initiate sessions by indicating inbound requests, or the LNS by requesting outbound calls. Control messages may indicate changes in operating characteristics of an individual user session with a Set-Link-Info message. Individual sessions may be released by either the LAC or LNS, also through control messages. Independent Call ID values are established for each end of a user session. The sender of a packet associated with a particular session places the Call ID established by its peer in the Call ID header field of all outgoing packets. For the cases where a Call ID has not yet been assigned from the peer (i.e., during call establishment of a new session), the Call ID field is sent as 0, and further fields within the message are used to identify the session. The Call ID value of 0 is thus special and MUST NOT be used as an Assigned Call ID. Two mechanisms provide for detection of tunnel connectivity problems, one by the reliable transport layer of L2TP and another by the higher layer. The transport layer of L2TP performs control message retransmission. If the number of retransmission attempts for a given control message exceeds a configured maximum value, the tunnel is reset. This retransmission mechanism exists in both the LNS and LAC ends of the tunnel. In addition, keepalive control echo messages are injected into the control stream by the higher L2TP layer after a certain duration of inactivity on a given tunnel is detected. A response to the sent keepalive is expected within a configured time interval. If not received within the allowed time interval, the tunnel is reset. These two mechanisms ensure that a connectivity failure between the LNS and the LAC can be detected at either end of a tunnel in a timely manner. It is intended that control messages will also carry management related information in the future, such as a message allowing the LNS to request the status of a given LAC; these message types are not defined in this document. 4.2 Payload Packet Overview Once a tunnel is established and control messages have completed tunnel setup, the tunnel can be used to carry user session PPP packets for sessions involving a given LNS-LAC pair. The "Call ID" field in the L2TP header indicates to which session a particular PPP packet belongs. In this manner, PPP packets are multiplexed and demultiplexed over a single tunnel between a given LNS-LAC pair. The "Call ID" field value is established during the exchange of call setup control messages. It is legal for multiple tunnels to exist between a given LNS-LAC pair. This is useful where each tunnel is used for a single user session, and the tunnel media (an SVC, for instance) has specific Valencia expires December 1997 [Page 12] INTERNET DRAFT June 1997 QOS attributes dedicated to a given user. L2TP provides a tunnel identifier so that individual tunnels can be identified, even when arriving from a single source LAC or LNS. The L2TP header also contains optional acknowledgment and sequencing information that can be used to perform congestion control over the tunnel. Control messages are used to determine rate and buffering parameters that are used to regulate the flow of PPP packets for a particular session over the tunnel. All implementations MUST implement flow control, but may indicate that flow control is not desired by omitting the Packet Window Size and Packet Processing Delay AVP's during call setup. L2TP does not specify the particular algorithms to use for congestion and flow control. Suggested algorithms for the determination of adaptive time-outs to recover from dropped data or acknowledgments on the tunnel are included in Appendix A of this document. L2TP does not include a "Receive-Not-Ready" function. It is expected that the flow-control mechanism used will provide an adequate "pacing" mechanism so that the sender does not overflow the receiver's allotted receive window and receive buffers. It is permissible for the receiving peer to withhold Acks if it is unable to accept more data for a connection. Thus, unlike for the Control Message session, the sending peer MUST NOT clear a session (or the whole tunnel) as a result of not receiving timely acknowledgments for transmitted packets. The job of detecting a non-functioning tunnel lies solely with the Control Message functions of L2TP. 5. Message Format and Protocol Extensibility L2TP defines a set of control messages sent in packets over the tunnel between an LNS and a given LAC. The exact technique for initiating a tunnel between an LNS-LAC pair is specific to the tunnel media, and specific media are described in section 8.0. Once media-level connectivity is reached, L2TP message formats define the protocol for an LAC and LNS to manage the tunnel and its associated sessions. In each case where a field is optional, if the field is not present, its space does not exist in the packet. Existing fields are placed back-to-back to form the packet. 5.1 AVP To maximize extensibility while still permitting interoperability, a uniform method for encoding message types and bodies is used throughout L2TP. This encoding will be termed an AVP (Attribute- Value Pair) in the remainder of this document. Each AVP is encoded as: Valencia expires December 1997 [Page 13] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H|0|0|0|0| Overall Length | Vendor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute | Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | [until Overall Length is reached]... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The first six bits are a bit mask, describing the general attributes of the AVP. The M bit, known as the "mandatory" bit, controls the behavior required of an implementation which receives an AVP which it does not recognize. If M is set, any session associated with this AVP MUST be terminated. If the AVP is associated with the overall tunnel, the entire tunnel (and all sessions within) MUST be terminated. If M is not set, an unrecognized AVP should be ignored. The H bit, known as the "hidden" bit, controls the hiding of the data in the value field of an AVP. This capability can be used to avoid the passing of sensitive data, such as user passwords, as cleartext in an AVP. Section 5.7 describes the procedure for performing AVP value hiding. Overall Length encodes the number of octets (including the Overall Length field itself) contained in this AVP. It is 10 bits, permitting a maximum of 1024 bytes of data in a single AVP. Vendor ID is the IANA assigned "SMI Network Management Private Enterprise Codes" value, encoded in network byte order. The value 0, reserved in this table, corresponds to IETF adopted Attribute values, defined within this document. Any vendor wishing to implement L2TP extensions can use their own Vendor ID along with private Attribute values, guaranteeing that they will not collide with any other vendor's extensions, nor with future IETF extensions. Attribute is the actual attribute, a 16-bit value with a unique interpretation across all AVP's defined under a given Vendor ID. Value follows immediately after the Attribute field, and runs for the remaining octets indicated in the Overall Length (i.e., Overall Length minus six octets of header). AVP's should be kept compact; the combined AVP's within a control message MUST NOT ever cause a control message's total length to exceed 1500 bytes in length. 5.2 Control Message Format Each L2TP control message begins with a 16 octet header portion followed by zero or more AVP's. This header is formatted: Valencia expires December 1997 [Page 14] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |T|1|1|1|1|0|0|0| | Ver | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tunnel ID | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ns | Nr | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Type AVP... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... (8 bytes) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The T bit MUST be 1, indicating a control message. The next four bits MUST be set to 1, making the header more compatible in encoding with the payload message (defined in the next section). The K bit is optional, documented below. The bit following the K bit MUST be 0. Ver MUST be the value 002, indicating a version 1 L2TP message (values 000 and 001 are reserved to permit detection of L2F [2] and PPTP [3] packets if they arrive intermixed). Length is the overall length of the message, including header, message type AVP, plus any additional AVP's associated with a given control message type. Tunnel ID and Call ID identify the tunnel and user session within the tunnel to which a control message applies. If a control message does not apply to a single user session within the tunnel (for instance, a Stop-Control-Connection-Request message), Call ID MUST be set to 0. If an Assigned Tunnel ID has not yet been received from the peer, Tunnel ID MUST be set to 0. Once an Assigned Tunnel ID is received, all further packets MUST be sent with Tunnel ID set to the indicated value. Nr and Ns reflect the currently transmitted packet and latest received packet respectively. See section 4.0. 5.3 Payload Message Format PPP payload packets tunneled within L2TP have a smaller encapsulation than the L2TP control message header, reducing overhead of L2TP during the life of a tunneled PPP session. The MTU for the user data packets encapsulated in L2TP is expected to be 1500 octets, not including L2TP and media encapsulation. The smallest L2TP encapsulation is 2 octets; the largest is 14 octets (plus padding bytes, if present). See section 8.1 for further MTU considerations. Valencia expires December 1997 [Page 15] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |T|L|I|C|F|K|O|P| | Ver | Length (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tunnel ID (opt) | Call ID (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ns (opt) | Nr (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Offset Size (opt) | Offset pad... (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The T bit MUST be 0, indicating payload. Ver MUST be 002, indicating version 1 of the L2TP protocol. If the L bit is set, the Length field is present, indicating the total length of the received packet. The I and C bits indicate the presence of Tunnel ID and Call ID, respectively. The interpretation of these fields, if present, is described in section 5.2. If the F bit is set, both the Nr and Ns fields are present. Ns indicates the sequence number of the packet being sent. The current packet will be this sequence number if the payload size is non-zero, otherwise this packet is only an acknowledgment (sequence number does not advance). Nr indicates the next packet sequence number to be received (if the last data packet had Ns set to 1, the Nr sent back would be 2). Together, these fields can be used to handle out-of-order packets, and to provide flow control for the connection. An L2TP peer setting the F bit, and placing Nr and Ns fields in its messages, MUST have previously received or sent a Receive Window Size AVP from its peer during establishment of the session. The Nr and Ns fields are present and updated as described in section 4.0 if either side has specified an intention to do payload flow control. The K bit MUST be set to 0. This bit position represents a function no longer present in L2TP. The Offset Size field is present if the O bit is set in the header flags. This field specifies the number of bytes past the L2TP header at which the payload data is expected to start. It is recommended that data thus skipped be initialized to 0's. If Offset Size is 0, or the O bit is not set, the first byte following the last byte of L2TP header is the first byte of payload data. If the P bit is set, this packet should receive preferential treatment in its queueing for transmission. LCP echo requests used as a keepalive for the link, for instance, should generally Valencia expires December 1997 [Page 16] INTERNET DRAFT June 1997 be sent with this bit set. Without it, a temporary interval of congestion of the transmission queues could result in the interference with keepalive messages and unnecessary loss of the link. 5.4 Control Message Types Control message and AVP types defined in this specification exist under Vendor ID 0, indicating IETF defined behavior. The actual message and AVP semantics are defined in the next section. This section includes tables that summarize all currently defined message and AVP types. Each message type entry in the table below consists of 3 columns. The "Num." column indicates the integer value assigned to this message type. i The "(Abbrev)" column lists the abbreviation for each message type used in the AVP table that follows. The number in the "Value" column is placed in the Value field of the Message Type AVP. This AVP MUST be the first AVP in a message. The currently defined control message types, grouped by function, are: Control Connection Management 1 (SCCRQ) Start-Control-Connection-Request 2 (SCCRP) Start-Control-Connection-Reply 3 (SCCCN) Start-Control-Connection-Connected 4 (StopCCRQ) Stop-Control-Connection-Request 5 (StopCCRP) Stop-Control-Connection-Reply 6 Hello Call Management 7 (OCRQ) Outgoing-Call-Request 8 (OCRP) Outgoing-Call-Reply 9 (OCCN) Outgoing-Call-Connected 10 (ICRQ) Incoming-Call-Request 11 (ICRP) Incoming-Call-Reply 12 (ICCN) Incoming-Call-Connected 13 (CCRQ) Call-Clear-Request 14 (CDN) Call-Disconnect-Notify Error Reporting 15 (WEN) WAN-Error-Notify PPP Session Control 16 (SLI) Set-Link-Info 5.5 AVP Summary The following table lists all standard L2TP attributes currently Valencia expires December 1997 [Page 17] INTERNET DRAFT June 1997 defined. The "Attr" column indicates the integer value assigned to this attribute. The "M" column indicates the setting of the "Mandatory" bit of the AVP header for each attribute. The "Len" field indicates the size of the AVP including the AVP header. A "+" in this column indicates that the length varies depending upon the length of the actual contents of the value field. Under the Name column, the parenthesized lists of message type abbreviations indicate the message types that utilize each AVP (See command table above). An abbreviation shown in mixed or uppercase letters indicates that the corresponding AVP MUST be present in this message type; All lowercase indicates that the AVP may optionally appear in this message type. A "+" appended to a message type abbreviation indicates that the AVP is only mandatory in a "positive" (non-error) condition -- The AVP is optional in a message indicating an error condition. A brief summary of the type and contents of the value field for each attribute is also given for each entry. Refer to the individual message type descriptions that appear in Section 6 for further details about the use of a particular AVP in a particular message type. Attr M Len Attribute Name (usage) 0 1 8 Message Type (ALL MESSAGES) 16 bit integer value indicating the message type, as defined in table above. MUST be the first AVP in each message 1 1 10+ Result Code (CDN, ICRP, OCCN, OCRP, SCCRP, StopCCRP, StopCCRQ) 16 bit Integer value indicating result of corresponding request or reason for issuing a request, 16 bit integer General Error code and an optional ASCII string error message. See Result and General Error code tables below. 2 1 8 Protocol Version (SCCRP, SCCRQ) 8 bit L2TP Protocol and Revision numbers 3 1 10 Framing Capabilities (SCCRP, SCCRQ) 32 bit bitmask indicating supported framing types (e.g., synchronous and asynchronous) 4 1 10 Bearer Capabilities (SCCRP, SCCRQ) 32 bit bitmask indicating supported bearer types (e.g., analog and digital) 5 0 14 Tie Breaker (sccrq) 8 byte value used to break control connection establishment collisions 6 0 8 Firmware Revision (sccrp, sccrq) 16 bit integer representing vendor's firmware revision Valencia expires December 1997 [Page 18] INTERNET DRAFT June 1997 7 0 6+ Host Name (sccrp, sccrq) ASCII string name (e.g., DNS name) of issuer 8 0 6+ Vendor Name (sccrp, sccrq) ASCII string describing issuing device 9 1 8 Assigned Tunnel ID (SCCRP+, SCCRQ, StopCCRQ) 16 bit integer tunnel ID assigned by sender 10 1 8 Receive Window Size (iccn, icrp, occn, ocrq, sccrp, sccrq) 16 bit integer receive window size offered by sender for a given call or control session 11 1 6+ Challenge (sccrp, sccrq) 1 or more octet value issued by sender wishing to authenticate control session peer 12 0 9+ Q.931 Cause Code (cdn, occn) 16 bit cause code, 1 octet cause message, and optional ASCII advisory message 13 1 22 Challenge Response (scccn, sccrp) 16 octet CHAP type response to peer's Challenge 14 1 8 Assigned Call ID (CCRQ, CDN, ICRP+, ICRQ, OCRP+, OCRQ) 16 bit integer ID assigned to a call by sender 15 1 6+ Call Serial Number (ICRQ, OCRQ) 1 or more octet identifier assigned to a call 16 1 10 Minimum BPS (OCRQ) 32 bit integer indicating lowest acceptable line speed for call 17 1 10 Maximum BPS (OCRQ) 32 bit integer indicating highest acceptable line speed for call 18 1 10 Bearer Type (ICRQ, OCRQ) Indicates bearer type (i.e., analog or digital) for call 19 1 10 Framing Type (ICCN, OCCN+, OCRQ) Indicates framing type (i.e., synchronous or asynchronous) for call 20 1 8 Packet Processing Delay (iccn, icrp, occn, ocrq) 16 bit integer estimate of processing time of full window of received packets by sender 21 1 6+ Dialed Number (icrq, OCRQ) ASCII string phone number called or to be called 22 1 6+ Dialing Number (icrq) ASCII string phone number of caller Valencia expires December 1997 [Page 19] INTERNET DRAFT June 1997 23 1 6+ Sub-Address (icrq, ocrq) ASCII string containing additional dialing information 24 1 10 Connect Speed (ICCN, OCCN+, OCRP+) 16 bit integer actual line speed of connection 25 1 10 Physical Channel ID (icrq, ocrp) 16 bit vendor specific physical device identifier used for call 26 0 6+ Initial LCP Confreq (iccn) Octet string containing initial CONFREQ received from client 27 0 6+ Last Sent LCP Confreq (iccn) Octet string containing final CONFREQ sent to client 28 0 6+ Last Received LCP Confreq (iccn) Octet string containing final CONFREQ received from client 29 1 8 Proxy Authen Type (ICCN) 16 bit integer code indicating client authentication type negotiated (e.g., PAP, CHAP) 30 0 6+ Proxy Authen Name (iccn) ASCII string containing name returned by client in authentication response 31 0 6+ Proxy Authen Challenge (iccn) Octet string Challenge presented by LAC to client 32 0 8 Proxy Authen ID (iccn) 16 bit integer of which low order octet is ID presented to client with Challenge. High order octet must be 0. 33 1 6+ Proxy Authen Response (iccn) Octet string CHAP response or ASCII string password depending on authentication type used 34 1 32 Call Errors (WEN) A reserved 16 bit word set to 0 followed by 6 32 bit integer connection error counters 35 1 16 ACCM (SLI) A reserved 16 bit word set to 0 followed by 2 32 bit bitmasks containing Send and Receive ACCM values respectively 36 1 6+ Random Vector (all messages) Variable length octet string containing a random sequence of values used to accomplish the optional "hiding" of other AVP values (See "H" bit description) 5.6 Result and Error Code Summary In general, all Reply Message types contain a Result Code AVP which indicates the result of the requested operation. The Result Code can Valencia expires December 1997 [Page 20] INTERNET DRAFT June 1997 indicate that additional information pertaining to an error situation can be found in the Error Code field of the Result Code AVP. The meaning of the result code is tabulated under the specific type of message containing the result. Each 16-bit Result Code is immediately followed (in the same AVP) by a 16-bit General Error code value. General error codes pertain to types of errors which are not specific to any particular L2TP request, but rather to protocol or message format errors. If an L2TP reply indicates in its Result Code that a general error occurred, the General Error value should be examined to determine what the error was. The currently defined General Error codes and their meanings are: 0 - No general error 1 - No control connection exists yet for this LAC-LNS pair 2 - Length is wrong 3 - One of the field values was out of range or reserved field was non-zero 4 - Insufficient resources to handle this operation now 5 - The Call ID is invalid in this context 6 - A generic vendor-specific error occurred in the LAC 7 - Try another. If LAC is aware of other possible LNS destinations, it should try one of them. This can be used to guide an LAC based on LNS policy, for instance, the existence of multilink PPP bundles. If the length of the Result Code AVP specifies that the Value field is more than four octets in length, the remaining bytes after the General Error Code field are an arbitrary string providing further (possibly human readable) text associated with the condition. Generally, when a General Error Code of 6 is used, additional information about the error will be included in the Result Code AVP in the Optional Message field that follows the Error Code field. 5.7 Hiding of AVP values The H ("Hidden") bit in the header of each AVP in a control message provides a mechanism to indicate to the receiving peer whether the contents of the AVP are hidden or present in cleartext. This feature can be used to hide sensitive control message data such as user passwords or user ID's. The H bit MUST NOT be set in the Random Vector AVP. The H bit MUST only be set if tunnel authentication was used and, therefore, a shared secret exists between the peers on either end of the tunnel. Therefore, the H bit MUST NOT be set in AVP's contained within the Start-Control-Connection-Request, -Reply, and -Connected messages. If the H bit is set in any AVP(s) in a given command message, a Random Vector AVP must also be present in the message and MUST preceed the first AVP having an H-bit of 1. The following mechanism is applied to the contents of the value field Valencia expires December 1997 [Page 21] INTERNET DRAFT June 1997 of each AVP to which hiding is to be applied. An MD5 hash is performed on the concatenation of: - the 2 octet Attribute number of the AVP - the shared authentication secret - and an arbitrary length random vector The value of the random vector used in this hash is passed in the value field of a Random Vector AVP. This Random Vector AVP must be placed in the message by the sender before any hidden AVPs. The same random vector can be used for more than one hidden AVP in the same message. If a different random vector is used for the hiding of subsequent AVPs then a new Random Vector AVP must be placed in the command message before the first AVP to which it applies. The MD5 hash value is then XORed with the first 16 octet or less segment of the Value and placed in the Value field of the AVP. If the Value is less than 16 octets, the Value is transformed as if the Value field had been padded to 16 octets before the XOR, but only the actual bytes present in the Value are modified, and the length of the AVP is not altered. If the value is longer than 16 octets, a second one-way MD5 hash is calculated over a stream of octets consisting of the shared secret followed by the result of the first XOR. That hash is XORed with the second 16 octet or less segment of the value and placed in the corresponding octets of the Value field of the AVP. If necessary, this operation is repeated, with each XOR result being used along with the shared secret to generate the next hash to XOR the next segment of the value with. This technique results in the content of the AVP being obscured, although the length of the AVP is still known. On receipt, the random vector is taken from the last Random Vector AVP encountered in the message prior to the AVP to be unhidden. The above process is then reversed to yield the original value. For more details on this hiding method, consult the RADIUS [8] RFC. The Random Vector AVP has the following format: Random Vector 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + String length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 36 | Random Octet String ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Random Vector AVP may be used in any message type. The Attribute value is 36 and it is marked mandatory. It is used to enable the hiding of the values of arbitrary AVPs. It MUST preceed any AVP Valencia expires December 1997 [Page 22] INTERNET DRAFT June 1997 containing an AVP with the H-bit set but it MUST NOT itself have the H-bit set. More than one Random Vector AVP may appear in a message, in which case the one most closely preceeding an AVP with the H-bit set pertains to that AVP. The Random Octet String is the random vector value to use in computing the MD5 hash to retrieve the original value of a hidden AVP. This string can be of arbitrary length, although a random vector of at least 16 octets is recommended. 6.0 Control Connection Protocol Specification Control Connection messages are used to establish and clear user sessions. The first set of Control Connection messages are used to maintain the control connection itself. The control connection is initiated by an LAC or LNS after establishing the underlying tunnel- over-media connection. 6.0.1 Control Connection Collision For the case where an LAC and LNS both initiate tunnels to each other concurrently, and where the LAC and LNS both determine that a single tunnel suffices (generally because of media characteristic considerations, for instance, whether individual tunnels are needed to gain QOS guarantees for each tunnel), a "tie breaker" may be undertaken. The details of breaking a tie are documented with the tunnel establishment messages. 6.0.2 Reliable Delivery of Control Messages Since L2TP may run across media where packets may be lost, an L2TP peer sending a control message will retransmit the control message after deciding that its remote peer has not received it. The reliable transport mechanism built into L2TP is essentially a lower layer transport service; the Nr and Ns fields of the control message header belong to this transport layer. The higher layer functions of L2TP are not concerned with retransmission or ordering of control messages. Each tunnel maintains a queue of control messages to be transmitted to the peer. The message at the front of the queue is sent with a given Ns value, and is held until a control message arrives from the peer in which the Nr field indicates receipt of this message. After a fixed (recommended default is 1 second) or adaptive (see Appendix D) timeout interval expires without receiving such an acknowledgment, the control message packet is retransmitted. The retransmitted packet contains the same Ns value, but the Nr value MUST be updated to reflect any packets received in the interim. If no peer response is detected after several retransmissions (a recommended default is 5, but may be altered due to media considerations), the tunnel and all sessions within MUST be cleared. Valencia expires December 1997 [Page 23] INTERNET DRAFT June 1997 When a tunnel is being shut down for reasons other than loss of connectivity, the state and reliable delivery mechanisms MUST be maintained and operated for the full retransmission interval after the final message exchange has occurred. This permits reliable delivery of closing messages in environments where these closing messages might be dropped. Unlike payload traffic, a peer MUST NOT withhold acknowledgment of packets as a technique for flow controlling control messages. An L2TP implementation is expected to be able to keep up with incoming control messages, possible responding to some with errors reflecting an inability to honor the requested action. A sliding window mechanism is used, by default, for control message transmission. The default is to permit four control message to be outstanding on a given tunnel. If a peer specifies a control message window in the Start-Control-Connection-Request and -Reply packets, up to the indicated number of control messages may be sent and held outstanding. An implementation may only support a receive window of 1, but MUST accept at least a window of 4 from its peer. The transport layer at a receiving peer is responsible for making sure that control messages are delivered in order to the higher layer and that duplicate messages are not delivered to the higher layer. Messages arriving out of order may be queued for in-order delivery when the missing messages are received or they may be discarded, requiring a retransmission. 6.0.3 Control Message Format The following Control Connection messages are all sent as packets on the established tunnel connection between a given LNS-LAC pair. All data is sent in network order (high order octets first). Any "reserved" or "empty" fields MUST be sent as 0 values to allow for protocol extensibility. Each control message has a header, specified in section 5.2, including an AVP indicating the type of control message, followed by zero or more AVP's appropriate for the given type of control message. Each control message is described first at a block level, and then with details of each AVP. 6.1 Start-Control-Connection-Request The Start-Control-Connection-Request is an L2TP control message used to initialize the tunnel between an LNS and an LAC. The tunnel must be initialized through the exchange of these control messages before any other L2TP messages can be issued. The establishment of the control connection is started by the initiator of the underlying tunnel. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | Valencia expires December 1997 [Page 24] INTERNET DRAFT June 1997 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Start-Control-Connection-Request | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol Version | | Framing Capabilities | | Bearer Capabilities | | Tie Breaker | | Firmware Revision | | Host Name | | Vendor Name | | Assigned Tunnel ID | | Receive Window Size | | Challenge | +-+-+-+-+-+-+-+-+-+-+-+-+ Start-Control-Connection-Request 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 1, indicating Start- Control-Connection-Request. The Flags indicate a mandatory option. Details associated with this tunneled session follow in additional AVP's within the packet. Protocol Version 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0x01 | 0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Protocol Version AVP within a Start-Control-Connection-Request packet indicates the L2TP protocol version available. The Attribute value is 2, indicating Protocol Version, and is marked mandatory. This AVP MUST be present. The Value field is a 16-bit hexadecimal value 0x100, indicating L2TP protocol version 1, revision 0. Framing Capabilities 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 25] INTERNET DRAFT June 1997 | 3 | 0x00 | 0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 |0|0|0|0|0|0|A|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Framing Capabilities AVP within a Start-Control-Connection- Request indicates the type of framing that the sender of this message can provide. The Attribute value is 3, indicating Framing Capabilities, and is marked mandatory. This AVP MUST be present. The Value field is a 32-bit quantity, with two bits defined. If bit A is set, asynchronous framing is supported. If bit S is set, synchronous framing is supported. Bearer Capabilities 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4 | 0x00 | 0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 |0|0|0|0|0|0|A|D| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Bearer Capabilities AVP within a Start-Control-Connection- Request indicates the bearer capabilities that the sender of this message can provide. The Attribute value is 4, indicating Bearer Capabilities, and is marked mandatory. This AVP MUST be present. The Value field is a 32-bit quantity with two bits defined. If bit A is set, analog access is supported. If bit D is set, digital access is supported. Tie Breaker 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0| 14 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 5 | Tie Break Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...(64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Tie Breaker AVP within a Start-Control-Connection-Request contains a 64-bit Value used to break ties in tunnel establishment between an LAC-LNS pair. The Attribute value is 5, indicating Tie Breaker, and is marked optional. This AVP itself is optional. The 8 byte Value is used as a 64-bit tie breaker value. If present, it indicates the sender wishes a single tunnel to Valencia expires December 1997 [Page 26] INTERNET DRAFT June 1997 exist between the given LAC-LNS pair, and this value will be used to choose a single tunnel where both LAC and LNS initiate a tunnel concurrently. The recipient of a Start-Control-Connection-Request must check to see if a Start-Control-Connection-Request has been sent to the peer, and if so, must compare its Tie Breaker value with the received one. The lower value "wins", and the "loser" MUST initiate a tunnel disconnect for their outstanding tunnel. In the case where a tie breaker is present on both sides, and the value is equal, both sides MUST initiate tunnel disconnects. If a tie breaker is received, and the outstanding Start-Control- Connection-Request had no tie breaker value, the initiator which included the Tie Breaker AVP "wins". It is recommended that the Value be set to the MAC address of a LAN interface on the sender. If no MAC address is available, a 64-bit random number should be used instead. Firmware Revision 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 | Firmware Revision | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Firmware Revision AVP within a Start-Control-Connection- Request indicates the firmware revision of the issuing device. The Attribute value is 6, indicating Firmware Revision, and is marked optional. This AVP itself is optional. The Value field is a 16-bit integer encoded in a vendor specific format. For devices which do not have a firmware revision (general purpose computers running L2TP software modules, for instance), the revision of the L2TP software module may be reported instead. Host Name 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Host name length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 7 | Host name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Host Name AVP within a Start-Control-Connection-Request indicates the name of the issuing LAC or LNS. The Attribute value is 7, indicating Host Name, and is marked mandatory. This AVP itself MUST be present. This name should be as broadly unique as possible; for hosts participating in DNS [4], a hostname with fully qualified domain would be appropriate. Valencia expires December 1997 [Page 27] INTERNET DRAFT June 1997 Vendor Name 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|6 + vendor name length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | Vendor name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Vendor Name AVP within a Start-Control-Connection-Request contains a vendor specific string describing the type of LAC or LNS being used. The Attribute value is 8, indicating Vendor Name, and is marked optional. This AVP itself is optional. The Value is the indicated number of bytes representing the vendor string. Assigned Tunnel ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 9 | Tunnel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Tunnel ID AVP within a Start-Control-Connection- Request specifies the Tunnel ID which the receiving peer MUST use in the Tunnel ID field of all subsequent packets. The Attribute value is 9, indicating Assigned Tunnel ID, and is marked mandatory. This AVP MUST be present. Before the Assigned Tunnel ID AVP is received, packets MUST be sent with a Tunnel ID value of 0. The Value is a 16-bit non-zero Tunnel ID value. Receive Window Size 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 10 | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Receive Window Size AVP within a Start-Control-Connection- Request specifies the receive window size being offered to the remote peer. The Attribute value is 10, indicating Receive Window Size, and is mandatory. This AVP itself is optional. Value is a 16-bit word indicating the offered window size. If absent, the peer must assume a value of 4 for its transmit window. The remote peer may send the specified number of control messages before it must wait for an acknowledgment. Challenge Valencia expires December 1997 [Page 28] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Challenge length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 11 | Challenge... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Challenge AVP within a Start-Control-Connection-Request indicates that the issuing peer wishes to authenticate the tunnel endpoints. The Attribute value is 11, indicating Challenge, and is marked mandatory. This AVP is optional. The Value is one or more octets of challenge value. 6.2 Start-Control-Connection-Reply The Start-Control-Connection-Reply is an L2TP control message sent in reply to a received Start-Control-Connection-Request message. This message contains a result code indicating the result of the control connection establishment attempt. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Start-Control-Connection-Reply | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol Version | | Result Code | | Framing Capabilities | | Bearer Capabilities | | Firmware Revision | | Host Name | | Vendor Name | | Assigned Tunnel ID | | Receive Window Size | | Challenge | | Challenge Response | +-+-+-+-+-+-+-+-+-+-+-+-+ Start-Control-Connection-Reply 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 2, indicating Start- Control-Connection-Reply. The Flags indicate a mandatory option. Protocol Version Valencia expires December 1997 [Page 29] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0x01 | 0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Protocol Version AVP within a Start-Control-Connection-Reply packet indicates the L2TP protocol version available. The Attribute value is 2, indicating Protocol Version, and the Value field is a 16-bit hexadecimal value 0x100, indicating L2TP protocol version 1, revision 0. This AVP MUST be present. Framing Capabilities 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3 | 0x00 | 0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 |0|0|0|0|0|0|A|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Framing Capabilities AVP within a Start-Control-Connection- Reply indicates the type of framing that the sender of this message can provide. The Attribute is 3, it is a mandatory AVP, the Value field is a 32-bit quantity, with two bits defined. If bit A is set, asynchronous framing is supported. If bit S is set, synchronous framing is supported. This AVP MUST be present if the Result AVP indicates success. Bearer Capabilities 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4 | 0x00 | 0x00 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 |0|0|0|0|0|0|A|D| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Bearer Capabilities AVP within a Start-Control-Connection- Reply indicates the bearer capabilities that the sender of this message can provide. The Attribute is 4, it is a mandatory AVP, the Value field is a 32-bit quantity with two bits defined. If bit A is set, analog access is supported. If bit D is set, digital access is supported. This AVP MUST be present if the Result AVP indicates success. Valencia expires December 1997 [Page 30] INTERNET DRAFT June 1997 Firmware Revision 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 | Firmware Revision | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Firmware Revision AVP within a Start-Control-Connection-Reply indicates the firmware revision of the issuing device. The Attribute is 6, it is not a mandatory AVP, the Value field is a 16-bit integer encoded in a vendor specific format. For devices which do not have a firmware revision (general purposes computers running L2TP software modules, for instance), the revision of the L2TP software module may be reported instead. This AVP is optional. Host Name 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Host name length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 7 | Host name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Host Name AVP within a Start-Control-Connection-Reply indicates the name of the issuing LAC or LNS. See the notes in section 6.1 concerning Host Name contents. It is encoded as the Attribute 7, mandatory, with the indicated number of bytes representing the host name string. This AVP MUST be present. Vendor Name 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|6 + Vendor name length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 |Vendor name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Vendor Name AVP within a Start-Control-Connection-Reply contains a vendor specific string describing the type of LAC or LNS being used. It is encoded as the Attribute 8, not mandatory, with the indicated number of bytes representing the vendor string. This AVP is optional. Assigned Tunnel ID 0 1 2 3 Valencia expires December 1997 [Page 31] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 9 | Tunnel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Tunnel ID AVP within a Start-Control-Connection-Reply specifies the Tunnel ID which the receiving peer MUST use in all subsequent packets. It is encoded as the Attribute 9, mandatory, with a 16-bit non-zero Tunnel ID value. This AVP MUST be present if the Result Code indicates "Successful channel establishment". Receive Window Size 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 10 | size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Receive Window Size AVP within a Start-Control-Connection- Reply specifies the receive window size being offered to the remote peer. The Attribute value is 10, indicating Receive Window Size, and is mandatory. This AVP itself is optional. Value is a 16-bit word indicating the offered window size. If absent, the peer must assume a value of 4 for its transmit window. The remote peer may send the specified number of control messages before it must wait for an acknowledgment. Result Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within a Start-Control-Connection-Reply packet indicates the result of the control channel establishment attempt. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16-bit word following the Result Code field contains the Error Code value. The Result Code value indicates whether the Error Code value is meaningful or not. If it is not meaningful it MUST be set to a value of 0. An optional error message can follow the Error Code field. Its presence and length is indicated by the value of the AVP length field. Valencia expires December 1997 [Page 32] INTERNET DRAFT June 1997 Result code values are: 1 - Successful channel establishment 2 - General error--Error Code indicates the problem 3 - Control channel already exists 4 - Requester is not authorized to establish a control channel 5 - The protocol version of the requester is not supported Challenge 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Challenge length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 11 | Challenge... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Challenge AVP within a Start-Control-Connection-Reply indicates that the peer wishes to authenticate the tunnel initiator. It is encoded as the Attribute 11, mandatory, with at least one byte of challenge value embedded. If this AVP is not present, it indicates to the receiving peer that the sender does not wish to authenticate that peer. Challenge Response 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 22 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 13 | Response... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Response... (128 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Response AVP within a Start-Control-Connection-Reply packet provides a response to a challenge received. The Attribute value is 13, indicating Response, and the Value field is a 128-bit value reflecting the CHAP-style response to the challenge. This AVP marked mandatory, and MUST be present if a challenge was received and this Start-Control-Connection-Reply indicates success. For purposes of the ID value in the CHAP response calculation, the fixed value 0 MUST be used. 6.3 Start-Control-Connection-Connected The Start-Control-Connection-Connected message is an L2TP control message sent in reply to a received Start-Control-Connection-Reply message. This message provides closure to the tunnel establishment process, and includes a challenge response if the peer sent a challenge in the Start-Control-Connection-Reply message. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 33] INTERNET DRAFT June 1997 | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Start-Control-Connection-Connected | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Challenge Response | +-+-+-+-+-+-+-+-+-+-+-+-+ Start-Control-Connection-Connected 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 3, indicating Start- Control-Connection-Connected. The Flags indicate a mandatory option. Challenge Response 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 22 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 13 | Response... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Response... (128 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Challenge Response AVP within a Start-Control-Connection- Connected packet provides a response to a challenge received. The Attribute value is 13, indicating Response, and the Value field is a 128-bit value reflecting the CHAP-style response to the challenge. This AVP is marked mandatory, and MUST be present if a challenge was received, otherwise MUST be omitted. For purposes of the ID value in the CHAP response calculation, the fixed value 0 MUST be used. 6.4 Stop-Control-Connection-Request The Stop-Control-Connection-Request is an L2TP control message sent by one peer of an LAC-LNS control connection to inform the other peer that the control connection should be closed. In addition to closing the control connection, all active user calls are implicitly cleared. The reason for issuing this request is indicated in the Result Code AVP. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 34] INTERNET DRAFT June 1997 | Stop-Control-Connection-Request | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Tunnel ID | | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+ Stop-Control-Connection-Request AVP 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 4, indicating Stop- Control-Connection-Request. The Flags indicate a mandatory option. Assigned Tunnel ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 9 | Tunnel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Attribute value is 9, indicating Assigned Tunnel ID, and is marked mandatory. This AVP MUST be present. The Value MUST be the same Assigned Tunnel ID first sent to the receiving peer. This AVP permits the peer to identify the appropriate tunnel even if Stop-Control-Connection-Request must be sent before an Assigned Tunnel ID is received. Result Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within a Stop-Control-Connection-Request packet indicates the reason for terminating the control channel. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16-bit word following the Result Code field contains Valencia expires December 1997 [Page 35] INTERNET DRAFT June 1997 the Error Code value, which for a Stop-Control-Connection-Request is always 0. An optional message can follow the Error Code field. Its presence and length is indicated by the value of the AVP length field. Defined Result Code values are: 1 - General request to clear control connection 2 - Can't support peer's version of the protocol 3 - Requester is being shut down 6.5 Stop-Control-Connection-Reply The Stop-Control-Connection-Reply is an L2TP control message sent by one peer of an LAC-LNS control connection upon receipt of a Stop- Control-Connection-Request from the other peer. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Stop-Control-Connection-Reply | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+ Stop-Control-Connection-Reply 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 5, indicating Stop- Control-Connection-Reply. The Flags indicate a mandatory option. Result Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within a Stop-Control-Connection-Reply packet indicates the result of the control channel close attempt. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16-bit word following the Result Code field contains the Error Code value. The Result Code value indicates whether the Error Valencia expires December 1997 [Page 36] INTERNET DRAFT June 1997 Code value is meaningful or not. If it is not meaningful it should be set to a value of 0. An optional error message can follow the Error Code field. Defined Values are: 1 - Control connection closed 2 - Control connection not closed for reason indicated in Error Code 6.6 Hello The Hello message is an L2TP control message sent by either peer of a LAC-LNS control connection. This control message is used as a "keepalive" for the control connection. Keepalives should be implemented by sending a Hello once every 60 seconds if 60 seconds have passed without sending a message to the peer. When a Hello is received, it MUST be silently discarded (after updating any effects of the indicated Nr/Ns values). Because a Hello is a control message, and control messages are reliably sent by the lower level transport, this keepalive function operates by causing the transport level to reliably deliver a message. If a media interruption has occurred, the reliable transport will be unable to deliver the Hello across, and will clean up the tunnel. Hello messages are global to the tunnel; the Call ID field of these control messages MUST be 0. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hello | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Hello 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 6 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 6, indicating Hello The Flags indicate a mandatory option. 6.7 Outgoing-Call-Request The Outgoing-Call-Request is an L2TP control message sent by the LNS to the LAC to indicate that an outbound call from the LNS is to be established. This request provides the LAC with information required to make the call. It also provides information to the LAC that is used to regulate the transmission of data to the LNS for this session Valencia expires December 1997 [Page 37] INTERNET DRAFT June 1997 once it is established. This message is the first in the "three-way handshake" used by L2TP for establishing outgoing calls. The first message requests the call; the second indicates that the call was successfully initiated. The third and final message indicates that the call was established. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Outgoing-Call-Request | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Call ID | | Call Serial Number | | Minimum BPS | | Maximum BPS | | Bearer Type | | Framing Type | | Receive Window Size | | Packet Processing Delay | | Dialed Number | | Sub-Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+ Outgoing-Call-Request 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 7 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 7, indicating Outgoing- Call-Request. The Outgoing-Call-Request encodes a request to an LAC to establish an outgoing call. The flags indicate a mandatory option. Assigned Call ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Call ID AVP encodes the ID being assigned to this call by the LNS. The Attribute value is 14, indicating Assigned Call ID, and is marked mandatory. This AVP MUST be present. The LAC places this value in the Call ID header field of all command and payload packets that it subsequently transmits over the tunnel Valencia expires December 1997 [Page 38] INTERNET DRAFT June 1997 that belong to this call. The Call ID value is an identifier assigned by the LNS to this session. It is used to multiplex and demultiplex data sent over that tunnel between the LNS and LAC. Call Serial Number 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Number length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 15 | Number... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Call Serial Number AVP encodes an identifier assigned by the LNS to this call. Attribute is 15, indicating Call Serial Number, and is marked mandatory. This AVP MUST be present. The Call Serial Number is intended to be an easy reference for administrators on both ends of a tunnel to use when investigating call failure problems. Call Serial Numbers should be set to progressively increasing values, which are likely to be unique for a significant period of time across all interconnected LNS and LACs. Other identification information may also be prepended. Minimum BPS 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 16 | BPS (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Minimum BPS AVP encodes the lowest acceptable line speed for this call. Attribute is 16, Minimum BPS, and is marked mandatory. This AVP MUST be present. The BPS value indicates the speed in bits/second. Maximum BPS 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 17 | BPS (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 39] INTERNET DRAFT June 1997 Maximum BPS AVP encodes the highest acceptable line speed for this call. Attribute is 17, indicating Maximum BPS, and is marked mandatory. This AVP MUST be present. The BPS value indicates the speed in bits/second. Bearer Type 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 18 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|D| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Bearer Type AVP encodes the bearer type for the requested call. The value bit field Attribute is 18, indicating Bearer Type, and is marked mandatory. This AVP MUST be present. The Value is a 32-bit quantity indicating the bearer capability required for this outgoing call. If set, bit A indicates that the call should be on an analog channel. If set, bit D indicates that the call should be on a digital channel. Both may be set, indicating that the call can be of either type. Framing Type 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 19 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Framing Type AVP encodes the framing type for the requested call. Attribute is 19, indicating Framing Type, and is marked mandatory. This AVP MUST be present. The 32-bit field indicates the type of PPP framing to be used for the outgoing call. Bit A if set indicates that asynchronous framing should be used. Bit S is set indicates that synchronous framing should be used. Receive Window Size 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 10 | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 40] INTERNET DRAFT June 1997 Receive Window Size AVP encodes the window size being advertised by the LNS for this call. Attribute is 10, indicating Receive Window Size, and is marked mandatory. This AVP is optional. The Size value indicates the number of received data packets the LNS will buffer for this call, which is also the maximum number of data packets the LAC should send before waiting for an acknowledgment. The absence of this AVP indicates that Sequence and Acknowledgment Numbers are not to be used in the payload session for this call. Packet Processing Delay 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 20 | Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Packet Processing Delay AVP encodes the delay LNS has for processing a window full of packets sent by the LAC. Attribute is 20, indicating Packet Processing Delay, and is marked mandatory. This AVP is optional. The Delay value is specified in units of 1/10 seconds. Refer to Appendix A for a description of how this value is determined and used. Dialed Number 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0|6 + Phone Number length| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 21 | Phone Number.. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Phone Number AVP encodes the phone number to be called. Attribute is 21, indicating Phone Number, and is marked mandatory. This AVP MUST be present. The Phone Number value is an ASCII string. Sub-Address 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0|6 + Sub-Address length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 23 |Sub-Address ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Sub-Address AVP encodes additional dialing information. Attribute is 23, indicating Sub-Address, and is marked mandatory. This AVP is optional. The Sub-Address value is an ASCII string. Valencia expires December 1997 [Page 41] INTERNET DRAFT June 1997 6.8 Outgoing-Call-Reply The Outgoing-Call-Reply is an L2TP control message sent by the LAC to the LNS in response to a received Outgoing-Call-Request message. The reply indicates whether or not the LAC is able to attempt the outbound call and also returns certain parameters regarding the call attempt. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Outgoing-Call-Reply | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Call ID | | Result Code | | Connect Speed | | Physical Channel Id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Outgoing-Call-Reply 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 8, indicating Outgoing- Call-Reply. The Outgoing-Call-Reply message encodes the immediate result of attempting an outgoing call request. The flags indicate a mandatory option. Assigned Call ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Call ID AVP encodes the ID being assigned to this call by the LAC. Attribute is 14, indicating Assigned Call ID, and is marked mandatory. This AVP MUST be present if the Result Code indicates a call is in progress. Call ID value is an identifier, unique within the tunnel, assigned by the sender to this session. The remote peer MUST place this Call ID in the Call ID portion of all future packets it sends associated with this session. It is used to multiplex and demultiplex data sent over that tunnel between the LNS and LAC. Valencia expires December 1997 [Page 42] INTERNET DRAFT June 1997 Result Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within an Outgoing-Call-Request indicates the result of the outgoing call establishment attempt. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16-bit word following the Result Code field contains the Error Code value. The Result Code value indicates whether the Error Code value is meaningful or not. If it is not meaningful it should be set to a value of 0. An optional error message can follow the Error Code field. Its presence and length is indicated by the value of the AVP length field. Defined Result Code values are: 1 - Call attempt in progress 2 - Outgoing Call not attempted for the reason indicated in Error Code 3 - No appropriate facilities are available (temporary condition) 4 - No appropriate facilities are available (permanent condition) 5 - Invalid destination 6 - Outgoing Call administratively prohibited Connect Speed 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 24 | BPS (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Connect Speed BPS AVP encodes the speed of the facility chosen for the connection attempt. The Attribute value is 24, indicating Connect Speed, and is marked mandatory. This AVP MUST be present if the Result indicates a call is in progress. The BPS is a 32- bit value indicating the speed in bits/second. Physical Channel ID 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 Valencia expires December 1997 [Page 43] INTERNET DRAFT June 1997 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 25 | ID (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Physical Channel ID AVP encodes the vendor specific physical channel number used for the call. The Attribute value is 25, indicating Physical Channel ID, and is marked optional. This AVP itself is optional. ID is a 32-bit value in network byte order. The value is used for logging purposes only. 6.9 Outgoing-Call-Connected Outgoing-Call-Connected is an L2TP control message sent by the LAC to the LNS to indicate the result of a requested outgoing call. The LAC MUST send the corresponding Outgoing-Call-Reply to the LNS before sending this message. This message provides information to the LNS about the particular parameters used for the call. It provides information to allow the LNS to regulate the transmission of data to the LAC for this session. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Outgoing-Call-Connected | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Result Code | | Q.931 Cause Code | | Connect Speed | | Framing Type | | Receive Window Size | | Packet Processing Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Outgoing-Call-Connected 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 9 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 16, indicating Outgoing- Call-Connected. The Outgoing-Call-Connected message encodes the final result of an outgoing call request. The flags indicate a mandatory option. Result Code Valencia expires December 1997 [Page 44] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within an Outgoing-Call-Connected message indicates the final result of the outgoing call establishment attempt. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16-bit word following the Result Code field contains the Error Code value. The Result Code value indicates whether the Error Code value is meaningful or not. If it is not meaningful it should be set to a value of 0. An optional error message can follow the Error Code field. Its presence and length is indicated by the value of the AVP length field. Defined Result Code values are: 1 - Call established with no errors 2 - Outgoing Call not established for the reason indicated in Error Code 3 - Outgoing Call failed due to no carrier detected 4 - Outgoing Call failed due to detection of a busy signal 5 - Outgoing Call failed due to lack of a dial tone 6 - Outgoing Call was not established within time allotted by LAC 7 - Outgoing Call was connected but no appropriate framing was detected Q.931 Cause Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|9 + Advisory Msg length| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 12 | Cause Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Msg |Advisory Msg... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Q.931 Cause Code AVP is used to give additional information in cases of call failure. The Attribute value is 12, indicating Cause Code, and is marked mandatory. This AVP is optional. It is only relevant when the LAC uses Q.931/DSS1 for the outbound call attempt. Cause Code is the returned Q.931 Cause code and Cause Msg is the returned Q.931 message code (e.g., DISCONNECT) associated with the Cause Code. Both values are returned in their native ITU encodings. An additional ASCII text Advisory Message may also be included (presence indicated by the AVP length) to further explain the call failure. Connect Speed Valencia expires December 1997 [Page 45] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 24 | BPS (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Connect Speed BPS AVP encodes the final negotiated speed for the connection. The Attribute value is 24, indicating Connect Speed, and is marked mandatory. This AVP MUST be present if the call attempt is successful. The BPS value indicates the speed in bits/second. Framing Type 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 19 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Framing Type AVP encodes the framing type for the call. The Attribute value is 19, indicating Framing Type, and is marked mandatory. This AVP MUST be present if the call attempt is successful. The value bit field indicates the type of PPP framing is used for the call. If set, bit A indicates that asynchronous framing is being used. If set, bit S indicates that synchronous framing is being used. Receive Window Size 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 10 | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Receive Window Size AVP encodes the window size being offered by the LNS for this call. The Attribute value is 10, indicating Receive Window Size, and is marked mandatory. The Size is a 16- bit value indicating the number of received data packets the LAC will buffer for this call, which is also the maximum number of data packets the LNS should send before waiting for an acknowledgment. This AVP MUST be present if and only if Sequence and Acknowledgment Numbers are to be used in the payload session Valencia expires December 1997 [Page 46] INTERNET DRAFT June 1997 for this call. Packet Processing Delay 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 20 | Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Packet Processing Delay AVP encodes the delay the LAC expects for processing a window full of packets sent by the LNS. The Attribute value is 20, indicating Packet Processing Delay, and is marked mandatory. This AVP is optional. The Delay value is specified in units of 1/10 seconds. Refer to Appendix A to see a description of how this value is determined and used. 6.10 Incoming-Call-Request Incoming-Call-Request is an L2TP control message sent by the LAC to the LNS to indicate that an inbound call is to be established from the LAC. This request provides the LNS with parameter information for the incoming call. This message is the first in the "three-way handshake" used by L2TP for establishing incoming calls. The LAC may defer answering the call until it has received an Incoming-Call-Reply from the LNS indicating that the call should be established. This mechanism allows the LNS to obtain sufficient information about the call before it is answered to determine whether the call should be answered or not. Alternatively, the LAC may answer the call, negotiate LCP and PPP authentication, and use the information gained to choose the LNS. In this case, the call has already been answered by the time the Incoming-Call-Reply message is received; the LAC simply spoofs the "call indication/answer call" phase in this case. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Incoming-Call-Request | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Call ID | | Call Serial Number | | Bearer Type | | Physical Channel ID | | Dialed Number | | Dialing Number | | Sub-Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Incoming-Call-Request Valencia expires December 1997 [Page 47] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 10 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 10, indicating Incoming- Call-Request. The Incoming-Call-Request message encodes an incoming call being indicated by the LAC. The flags indicate a mandatory option. Assigned Call ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Call ID AVP encodes the Call ID being assigned to call by the LAC. The Attribute value is 14, indicating Call ID, and is marked mandatory. This AVP MUST be present. The LNS places this value in the Call ID header field of all command and payload packets that it subsequently transmits over the tunnel that belong to this call. The Call ID value is an identifier assigned by the LAC to this session. It is used to multiplex and demultiplex data sent over that tunnel between the LNS and LAC. Call Serial Number 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Number length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 15 | Number... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Call Serial Number AVP encodes an identifier assigned by the LAC to this call. The Attribute value is 15, Call Serial Number, and is marked mandatory. This AVP MUST be present. Please refer to the description of this field from section 6.8. Bearer Type 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 48] INTERNET DRAFT June 1997 | 18 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|D| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Bearer Type AVP encodes the bearer type for the incoming call. The Attribute value is 18, Bearer Type, and is marked mandatory. This AVP MUST be present. The Value is a 32-bit field indicating the bearer capability being used by the incoming call. If set, bit A indicates that the call is on an analog channel. If set, bit D indicates that the call is on a digital channel. Physical Channel ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 25 | ID (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Physical Channel ID AVP encodes the vendor specific physical channel number used for the call. The Attribute value is 25, Physical Channel ID, and is marked mandatory. The presence of this AVP is optional. ID is a 32-bit value in network byte order. The value is used for logging purposes only. Dialed Number 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0|6 + Phone Number length| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 21 | Phone Number.. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Dialed Number AVP encodes the dialed number for the incoming call, that is, DNIS. The Attribute value is 21, Dialed Number, and is marked mandatory. The presence of this AVP is optional. The value is an ASCII string. Dialing Number 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0|6 + Phone Number length| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 22 |Phone Number... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 49] INTERNET DRAFT June 1997 Dialing Number AVP encodes the originating number for the incoming call, that is, CLID. The Attribute value is 22, Dialing Number, and is marked mandatory. The presence of this AVP is optional. The value is an ASCII string. Sub-Address 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0|6 + Sub-Address length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 23 |Sub-Address ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Sub-Address AVP encodes additional dialing information. The Attribute value is 23, Sub-Address, and is marked mandatory. The presence of this AVP is optional. The Sub-Address value is an ASCII string. 6.11 Incoming-Call-Reply The Incoming-Call-Reply is an L2TP control message sent by the LNS to the LAC in response to a received Incoming-Call-Request message. The reply indicates the result of the incoming call attempt. It also provides information to allow the LAC to regulate the transmission of data to the LNS for this session. This message is the second in the three-way handshake used by L2TP for establishing incoming calls. It indicates to the LAC whether the call should be answered or not. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Incoming-Call-Reply | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Call ID | | Result Code | | Receive Window Size | | Packet Processing Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Incoming-Call-Reply 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 11 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 10, indicating Incoming- Valencia expires December 1997 [Page 50] INTERNET DRAFT June 1997 Call-Reply. The Incoming-Call-Reply message encodes a response by the LNS to the incoming call indication given by the LAC. The flags indicate a mandatory option. Assigned Call ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Call ID AVP encodes the ID being assigned to call by the LNS. The Attribute value is 14, Assigned Call ID, and is marked mandatory. This AVP MUST be present if the Result Code indicates the call was successful. The LAC places this value in the Call ID header field of all command and payload packets that it subsequently transmits over the tunnel that belong to this call. The Call ID value is an identifier assigned by the LNS to this session. It is used to multiplex and demultiplex data sent over that tunnel between the LNS and LAC. Result Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within an Incoming-Call-Reply message indicates the result of the incoming call establishment attempt. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16-bit word following the Result Code field contains the Error Code value. The Result Code value indicates whether the Error Code value is meaningful or not. If it is not meaningful it should be set to a value of 0. An optional error message can follow the Error Code field. Its presence and length is indicated by the value of the AVP length field. Defined Result Code values are: 1 - The LAC should answer the incoming call 2 - The Incoming Call should not be established due to the reason indicated in Error Code 3 - The LAC should not accept the incoming call. It should hang up or issue a busy indication Valencia expires December 1997 [Page 51] INTERNET DRAFT June 1997 Receive Window Size 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 10 | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Optional Message... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Receive Window Size AVP encodes the receive window size being offered by the LNS for this call. The Attribute value is 10, Receive Window Size, and is marked mandatory. The Size value indicates the number of received data packets the LNS will buffer for this call, which is also the maximum number of data packets the LAC should send before waiting for an acknowledgment. This AVP is optional if Sequence and Acknowledgment Numbers are not to be used in the payload session for this call, or if the Result AVP indicates failure. Packet Processing Delay 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 20 | Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Packet Processing Delay AVP encodes the delay the LNS expects for processing a window full of packets sent by the LAC. The Attribute value is 20, Packet Processing Delay AVP, and is marked mandatory. The presence of this AVP is optional. The Delay value is specified in units of 1/10 seconds. Refer to Appendix A to see a description of how this value is determined and used. 6.12 Incoming-Call-Connected The Incoming-Call-Connected message is an L2TP control message sent by the LAC to the LNS in response to a received Incoming-Call-Reply. It provides information to the LNS about particular parameters used for the call. It also provides information to allow the LNS to regulate the transmission of data to the LAC for this session. This message is the third in the three-way handshake used by L2TP for establishing incoming calls. It provides a mechanism for providing the LNS with additional information about the call that cannot, in general, be obtained at the time the Incoming-Call-Request is issued by the LAC. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Valencia expires December 1997 [Page 52] INTERNET DRAFT June 1997 | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Incoming-Call-Connected | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Connect Speed | | Framing Type | | Receive Window Size | | Packet Processing Delay | | Initial LCP Confreq | | Last Sent LCP Confreq | | Last Received LCP Confreq | | Proxy authen type | | Proxy authen name | | Proxy authen challenge | | Proxy authen ID | | Proxy authen response | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Incoming-Call-Connected 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 12 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 11, indicating Incoming- Call-Connected. The Incoming-Call-Connected message encodes a response by the LAC to the Incoming-Call-Reply message sent by the LAC. The flags indicate a mandatory option. Connect Speed 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 24 | BPS (H) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Connect Speed BPS AVP encodes the speed for the connection. The Attribute value is 24, Connect Speed, and is marked mandatory. This AVP MUST be present. The value is a 32-bit quantity indicating the speed in bits/second. Framing Type 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 Valencia expires December 1997 [Page 53] INTERNET DRAFT June 1997 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 19 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|S| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Framing Type AVP encodes the framing type for the call. The Attribute value is 19, Framing Type, and is marked mandatory. This AVP MUST be present. The value is a 32-bit bit field indicating the type of PPP framing used for the call. If set, bit A indicates that asynchronous framing is being used. If set, bit S indicates that synchronous framing is being used. Receive Window Size 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 10 | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Receive Window Size AVP encodes the window size being offered by the LAC for this call. The Attribute value is 10, Receive Window Size, and is marked mandatory. This AVP is optional if Sequence and Acknowledgment Numbers are not to be used in the payload session for this call. The 16-bit Size value indicates the number of received data packets the LAC will buffer for this call, which is also the maximum number of data packets the LNS should send before waiting for an acknowledgment. Packet Processing Delay 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 20 | Delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Packet Processing Delay AVP encodes the delay the LAC expects for processing a window full of packets sent by the LNS. The Attribute value is 20, Packet Processing Delay, and is marked mandatory. The presence of this AVP is optional. The 16-bit Delay value is specified in units of 1/10 seconds. Refer to Appendix A to see a description of how this value is determined and used. Initial LCP Confreq 0 1 2 3 Valencia expires December 1997 [Page 54] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|6 + LCP confreq length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 26 | LCP confreq... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The LAC may have answered the phone call and negotiated LCP with the dial-in client in order to establish the client's apparent identity. In this case, this option may be included to indicate the link properties the client requested in its initial LCP CONFREQ request. The Attribute value is 26, Initial LCP Confreq, and is marked optional. The presence of this AVP is optional. The Value field is a copy of the body of the initial CONFREQ received, starting at the first option within this packet's body. Last Sent LCP Confreq 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|6 + LCP confreq length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 27 | LCP confreq... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ See Initial LCP Confreq above for rationale. The Attribute value is 27, Last Sent LCP Confreq, and is marked optional. The presence of this AVP is optional. The Value field is a copy of the body of the final CONFREQ sent to the client to complete LCP negotiation, starting at the first option within this packet's body. Last Received LCP Confreq 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|6 + LCP confreq length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 28 | LCP confreq... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ See Initial LCP Confreq above for rationale. The Attribute value is 28, Last Received LCP Confreq, and is marked optional. The presence of this AVP is optional. The Value field is a copy of the body of the final CONFREQ received from the client to complete LCP negotiation, starting at the first option within this packet's body. Proxy Authen Type 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | Valencia expires December 1997 [Page 55] INTERNET DRAFT June 1997 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 29 | Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Attribute value is 29, Proxy Authen Type, and is marked mandatory. This AVP MUST be present. The value Type is a 16-bit word, holding a value: 1 - Textual username/password exchange 2 - PPP CHAP 3 - PPP PAP 4 - None Associated AVP's for each type of authentication follow. Proxy Authen Name 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0| 6 + Name length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 30 | Name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Attribute value is 30, Proxy Authen Name, and is marked mandatory. This AVP MUST be present for Proxy Authen Type values 1, 2, and 3. The Name field contains the name specified in the client's authentication response. Note that the AVP H bit may be desirable for obscuring the cleartext name. Proxy Authen Challenge 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0| 6 + Challenge length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 31 | Challenge... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Attribute value is 31, Proxy Authen Challenge, and is marked mandatory. The AVP itself MUST be present for Proxy authen type 2. The Challenge field contains the value presented to the client by the LAC. Proxy Authen ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 32 | ID | Valencia expires December 1997 [Page 56] INTERNET DRAFT June 1997 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Attribute value is 32, Proxy Authen ID, and is marked mandatory. The AVP itself MUST be present for Proxy authen types 2 and 3. For CHAP, the ID field contains the byte ID value presented to the client by the LAC in its Challenge. For PAP, it is the Identifier value of the Authenticate-Request. The most significant 8 bits of ID MUST be 0, and are reserved. Proxy Authen Response 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 6 + Response length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 33 | Response... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Attribute value is 33, Proxy Authen Response, and is marked mandatory. The AVP itself MUST be present for Proxy authen types 1, 2, and 3. The Response field contains the client's response to the challenge. For Proxy authen type 2, this field contains the response value received by the LAC. For types 1 or 3, it contains the clear text password received from the client by the LAC. In the case of cleartext passwords, use of the AVP H bit is recommended. 6.13 Call-Clear-Request The Call-Clear-Request is an L2TP control message sent by the LNS to the LAC indicating that a particular call is to be disconnected. The call being cleared can be either an incoming or outgoing call, in any state. The LAC responds to this message with a Call-Disconnect- Notify message. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Call-Clear-Request | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+ Call-Clear-Request 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 13 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 12, indicating Call-Clear- Valencia expires December 1997 [Page 57] INTERNET DRAFT June 1997 Request. The Call-Clear-Request message encodes a request by the LNS to the LAC to disconnect the call. The flags indicate a mandatory option. Assigned Call ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This attribute is used to provide the LAC with the Call ID assigned by the LNS for the call to be cleared in the case where the LNS has not yet learned the LAC's Call ID for the call. The Attribute value is 14, Assigned Call ID, and is marked mandatory. This AVP MUST be present. The value Call ID MUST be the same value sent from the LNS to the LAC in the initial call setup exchange. 6.14 Call-Disconnect-Notify The Call-Disconnect-Notify message is an L2TP control message sent by the LAC to the LNS. It is issued whenever a call is disconnected, due to the receipt by the LAC of a Call-Clear-Request or for any other reason. Its purpose is to inform the LNS of the disconnection and the reason why the disconnection occurred. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Call-Disconnect-Notify | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Result Code | | Q.931 Cause Code | | Assigned Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+ Call-Disconnect-Notify 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 14 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 13, indicating Call- Disconnect-Notify. The Call-Disconnect-Notify message encodes a disconnect indication from the LAC to the LNS. The flags indicate a mandatory option. Valencia expires December 1997 [Page 58] INTERNET DRAFT June 1997 Result Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 10 + Message length | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Code | Optional Message ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code AVP within a Call-Disconnect-Notify message indicates the reason for the call disconnect. It is encoded as Attribute 1, indicating a Result Code AVP. This AVP is mandatory and MUST be present. The Result Code is a 16-bit word. The 16- bit word following the Result Code field contains the Error Code value. The Result Code value indicates whether the Error Code value is meaningful or not. If it is not meaningful it should be set to a value of 0. An optional error message can follow the Error Code field. Its presence and length is indicated by the value of the AVP length field. Defined Result Code values are: 1 (Lost Carrier) - Call disconnected due to loss of carrier 2 (General Error) - Call disconnected for the reason indicated in Error Code. 3 (Admin Shutdown) - Call disconnected for administrative reasons 4 (Request) - Call disconnected due to received Call-Clear- Request Q.931 Cause Code 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0|0|9 + Advisory Msg length| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 12 | Cause Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cause Msg |Advisory Msg... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Q.931 Cause Code AVP is used to give additional information in case of unsolicited call disconnection. The Attribute value is 12, Cause Code, and is marked mandatory. The presence of this AVP is optional. The Cause Code AVP is used to give additional information about the reason for disconnecting. It is only relevant when the LAC is using Q.931/DSS1 for the call. This AVP is optional. Cause Code is the returned Q.931 Cause code and Cause Msg is the returned Q.931 message code (e.g., DISCONNECT) associated with the Cause Code. Both values are returned in their native ITU encodings. An additional Ascii text Advisory Message may also be included (presence indicated by the AVP length) to Valencia expires December 1997 [Page 59] INTERNET DRAFT June 1997 further explain the reason for disconnecting. Assigned Call ID 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Call ID which was provided to the LNS from this LAC is included in the Call-Disconnect-Notify message. This permits a connection to be terminated even before the LNS has provided its own Assigned Call ID to this LAC (the Call ID field in the control message header is 0). The Attribute value is 14, Assigned Call ID, and is marked mandatory. This AVP MUST be present. 6.15 WAN-Error-Notify The WAN-Error-Notify message is an L2TP control message sent by the LAC to the LNS to indicate WAN error conditions (conditions that occur on the interface supporting PPP). The counters in this message are cumulative. This message should only be sent when an error occurs, and not more than once every 60 seconds. The counters are reset when a new call is established. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | WAN-Error-Notify | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Call Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ WAN-Error-Notify 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 15 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 14, indicating WAN-Error- Notify. The WAN-Error-Notify message encodes information about line and other errors detected on the LAC's physical interface to the client. This message is sent by the LAC to the LNS. The flags indicate a mandatory option. Call Errors Valencia expires December 1997 [Page 60] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 32 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 34 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CRC Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Framing Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hardware Overruns | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Buffer Overruns | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time-out Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Alignment Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Call Errors AVP is used by the LAC to send error information to the LNS. The Attribute value is 34, WAN-Error-Notify, and is marked mandatory. This AVP MUST be present. The value contains the following fields: Reserved - Not used, MUST be 0 CRC Errors - Number of PPP frames received with CRC errors since session was established Framing Errors - Number of improperly framed PPP packets received Hardware Overruns - Number of receive buffer over-runs since session was established Buffer Overruns - Number of buffer over-runs detected since session was established Time-out Errors - Number of time-outs since call was established Alignment Errors - Number of alignment errors since call was established 6.16 Set-Link-Info The Set-Link-Info message is an L2TP control message sent by the LNS to the LAC to set PPP-negotiated options. Because these options can change at any time during the life of the call, the LAC MUST be able to update its internal call information dynamically and update its behavior on an active PPP session. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Control Message Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Set-Link-Info | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ACCM | +-+-+-+-+-+-+-+-+-+-+-+-+-+ Set-Link-Info Valencia expires December 1997 [Page 61] INTERNET DRAFT June 1997 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 8 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type AVP contains a Value of 15, indicating Set-Link- Info. The Set-Link-Info message encodes ACCM information sent from the LNS to the LAC after it is negotiated in LCP. The flags indicate a mandatory option. ACCM 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0|0| 32 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 35 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Send ACCM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receive ACCM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The ACCM AVP is used by the LNS to inform LAC of the ACCM to the LNS. The Attribute value is 35, ACCM, and is marked mandatory. This attribute MUST be present. The value contains Send ACCM and Receive ACCM fields. The send ACCM value should be used by the LAC to process packets it is sends on the connection. The receive ACCM value should be used by the LAC to process incoming packets on the connection. The default values used by the LAC for both these fields are 0xFFFFFFFF. The LAC should honor these fields unless it has specific configuration information to indicate that the requested mask must be modified to permit operation. 7.0 Control Connection State Machines The control messages defined in section 6 are exchanged by way of state tables defined in this section. Tables are defined for incoming call placement, outgoing call placement, as well as for initiation of the tunnel itself. The state tables do not encode timeout and retransmission behavior, as this is handled in the underlying semantics defined in 6.0.2. 7.1 Control Connection Protocol Operation This section describes the operation of various L2TP control connection functions and the Control Connection messages which are used to support them. Receipt of an invalid or malformed Control Connection message should Valencia expires December 1997 [Page 62] INTERNET DRAFT June 1997 be logged appropriately, and the control connection should be closed and restarted to ensure recovery into a known state. 7.2 Control Connection States Control messages are carried over the same media as the payload messages which are carried following successful connection establishment. The L2TP control connection protocol is not distinguishable between the LNS and LAC, but is distinguishable between the originator and receiver. The originating peer is the one which first establishes the tunnel. Since either LAC or LNS can be the originator, a collision can occur. See Section 6.0.1 for a description of this and its resolution situation. 7.2.1 Control Connection Originator State Event Action New State ----- ----- ------ --------- idle Open request Send wait-ctl-reply Start-Control- Connection-Request wait-ctl-reply Collision If terminating, idle clean-up. wait-ctl-reply Collision If not terminating, wait-stop-reply Send Stop-Control- Connection-Request wait-ctl-reply Receive If version OK established Start-Control- Send Start-Control- Connection-Reply Connection-Connected wait-ctl-reply Receive If version not OK wait-stop-reply Start-Control- or bad auth, Send Connection-Reply Stop-Control- Connection-Request established Local terminate Send wait-stop-reply Stop-Control- Connection-Request established Receive Acknowledge via idle Stop-Control- reliable transport Connection- Request wait-stop-reply Receive Clean-up idle Acknowledgement via reliable transport idle Valencia expires December 1997 [Page 63] INTERNET DRAFT June 1997 The control connection originator attempts to open a connection to the peer during idle state. When the connection is open, the originator transmits a send Start-Control-Connection-Request and then enters the wait-ctl-reply state. wait-ctl-reply The originator checks to see if another connection has been requested from the same peer, and if so, handles the collision situation described in Section 6.0.1. When a Start-Control-Connection-Reply is received, it is examined for a compatible version. If the version of the reply is lower than the version sent in the request, the older (lower) version should be used provided it is supported. If the version in the reply is earlier and supported, the originator moves to the established state. If the version is earlier and not supported, a Stop-Control-Connection-Request SHOULD be sent to the peer and the originator moves into the wait-stop-reply state. established An established connection may be terminated by either a local condition or the receipt of a Stop-Control-Connection-Request. In the event of a local termination, the originator MUST send a Stop-Control-Connection-Request and enter the wait-stop-reply state. If the originator receives a Stop-Control-Connection-Request it MUST run its reliable delivery mechanism to acknowledge the stop request (see 6.0.2) before destroying the tunnel. wait-stop-reply Once the reliable transport has verified that the peer has received the stop request, the tunnel may be cleared. 7.2.2 Control connection Receiver State Event Action New State ----- ----- ------ --------- idle Receive If version not OK idle Start-Control- send Connection-Request Start-Control- Connection-Reply with error idle Receive Version OK, send wait-ctl-reply Start-Control- Start-Control- Connection-Request Connection-Reply wait-ctl-reply Receive Clean-up, send idle Stop-Control- Stop-Control- Connection-Request Connection-Reply wait-ctl-reply Receive If auth OK established Start-Control- Valencia expires December 1997 [Page 64] INTERNET DRAFT June 1997 Connection-Connected wait-ctl-reply Receive If auth not OK wait-stop-reply Start-Control- Send Stop-Control- Connection- Connection-Request Connected established Receive Acknowledge via idle Stop-Control- reliable transport Connection- Request established Local terminate Send wait-stop-reply Stop-Control- Connection-Request wait-stop-reply Receive Clean-up idle Acknowledgement via reliable transport idle The control connection receiver waits for an incoming connection attempt. When notified of a new connection, it should prepare to receive L2TP messages. When a Start-Control-Connection-Request is received its version field MUST be examined. If the version is earlier than the receiver's version and the earlier version can be supported by the receiver, the receiver SHOULD send a Start- Control-Connection-Reply. If the version is earlier than the receiver's version and the version cannot be supported, the receiver SHOULD send a Start- Connection-Reply message indicating this error and remain in the idle state. If the receiver's version is the same as or earlier than the peer's, the receiver SHOULD send a Start-Control- Connection-Reply with the receiver's version and enter the wait- ctl-reply state. wait-ctl-reply The peer waits in this state after sending a Start-Control- Connection-Reply. If it receives a Start-Control-Connection- Reply, it checks to see if the message is properly authenticated and, if so, it enters the established state. If authentication fails, a Stop-Control-Connection-Request with the reason code set appropriately is sent and wait-stop-reply state is entered. if a Stop-Control-Connection-Request is received, a Stop-Control- Connection-Reply is issued and idle state is entered. established An established connection may be terminated by either a local condition or the reception of a Stop-Control-Connection-Request. In the event of a local termination, the originator MUST send a Stop-Control-Connection-Request and enter the wait-stop-reply Valencia expires December 1997 [Page 65] INTERNET DRAFT June 1997 state. If the originator receives a Stop-Control-Connection-Request it MUST run its reliable delivery mechanism to acknowledge the stop request (see 6.0.2) before destroying the tunnel. wait-stop-reply Once the reliable transport has verified that the peer has received the stop request, the tunnel may be cleared. 7.3 Timing considerations Because of the real-time nature of telephone signaling, both the LNS and LAC should be implemented with multi-threaded architectures such that messages related to multiple calls are not serialized and blocked. The call and connection state figures do not specify exceptions caused by timers. These are addressed in Section 6.0.2. 7.4 Incoming calls An Incoming-Call-Request message is generated by the LAC when an associated telephone line rings. The LAC selects a Call ID and serial number and indicates the call bearer type. Modems should always indicate analog call type. ISDN calls should indicate digital when unrestricted digital service or rate adaption is used and analog if digital modems are involved. CLID, DNIS, and subaddress may be included in the message if they are available from the telephone network. Once the LAC sends the Incoming-Call-Request, it waits for a response from the LNS but it does not necessarily answer the call from the telephone network yet. The LNS may choose not to accept the call if: - No resources are available to handle more sessions - The dialed, dialing, or subaddress fields are not indicative of an authorized user - The bearer service is not authorized or supported If the LNS chooses to accept the call, it responds with an Incoming- Call-Reply which also indicates window sizes (see Appendix B). When the LAC receives the Incoming-Call-Reply, it attempts to connect the call, assuming the calling party has not hung up. A final call connected message from the LAC to the LNS indicates that the call states for both the LAC and the LNS should enter the established state. When the dialed-in client hangs up, the call is cleared normally and the LAC sends a Call-Disconnect-Notify message. If the LNS wishes to clear a call, it sends a Call-Clear-Request message and then waits for a Call-Disconnect-Notify. 7.4.1 LAC Incoming Call States State Event Action New State ----- ----- ------ --------- idle Ring OR Send wait-reply Valencia expires December 1997 [Page 66] INTERNET DRAFT June 1997 Ready to indicate Incoming-Call-Request incoming conn. wait-reply Receive Clean-up idle Incoming-Call-Reply Not Accepting wait-reply Receive Answer call established Incoming-Call-Reply Send Accepting Incoming-Call-Connected wait-reply Abort Clean-up idle Send Call-Disconnect- Notify established Receive Hang-up and send idle Call-Clear-Request Call-Disconnect-Notify established telco line drop Send idle Call-Disconnect-Notify established local disconnect Send idle Call-Disconnect-Notify The states associated with the LAC for incoming calls are: idle The LAC detects an incoming call on one of its telco interfaces. Typically this means an analog line is ringing or an ISDN TE has detected an incoming Q.931 SETUP message. The LAC sends an Incoming-Call-Request message and moves to the wait-reply state. wait-reply The LAC receives an Incoming-Call-Reply message indicating non- willingness to accept the call (general error or don't accept) and moves back into the idle state. If the reply message indicates that the call is accepted, the LAC sends an Incoming-Call-Connected message and enters the established state. established Data is exchanged over the tunnel. The call may be cleared following: An event on the telco connection. The LAC sends a Call- Disconnect-Notify message Receipt of a Call-Clear-Request. The LAC sends a Call- Disconnect-Notify message A local reason. The LAC sends a Call-Disconnect-Notify message. 7.4.2 LNS Incoming Call States Valencia expires December 1997 [Page 67] INTERNET DRAFT June 1997 State Event Action New State ----- ----- ------ --------- idle Receive If not accepting idle Incoming-Call-Request Send Incoming-Call-Reply with Error idle Receive If accepting wait-connect Incoming-Call-Request Send Incoming-Call-Reply wait-connect Receive Clean-up idle Call-Disconnect-Notify wait-connect Receive Get ready for data established Incoming-Call-Connect established Receive Clean-up idle Call-Disconnect-Notify established Local terminate Send wait- Call-Clear-Request disconnect wait- Receive Clean-up idle disconnect Call-Disconnect-Notify The states associated with the LNS for incoming calls are: idle An Incoming-Call-Request message is received. If the request is not acceptable, an Incoming-Call-Reply is sent back to the LAC and the LNS remains in the idle state. If the Incoming-Call-Request message is acceptable, an Incoming-Call-Reply is sent indicating accept in the result code. The session moves to the wait-connect state. wait-connect If the session is still connected on the LAC, the LAC sends an incoming call connect message to the LNS which then moves into established state. The LAC may send a Call-Disconnect-Notify to indicate that the incoming caller could not be connected. This could happen, for example, if a telephone user accidentally places a standard voice call to an LAC resulting in a handshake failure on the called modem. established The session is terminated either by receipt of a Call-Disconnect- Notify message from the LAC or by sending a Call-Clear-Request. Once a Call-Clear-Request has been sent, the session enters the wait-disconnect state. Valencia expires December 1997 [Page 68] INTERNET DRAFT June 1997 wait-disconnect Once a Call-Disconnect-Notify is received the session moves back to the idle state. 7.5 Outgoing calls Outgoing calls are initiated by an LNS and instruct an LAC to place a call on a telco interface. There are three messages for outgoing calls: Outgoing-Call-Request, Outgoing-Call-Reply, and Outgoing-Call-Connected. The LNS sends an Outgoing-Call-Request specifying the dialed party phone number and subaddress as well as speed and window parameters. The LAC MUST respond to the Outgoing-Call-Request message with an Outgoing- Call-Reply message once the LAC determines that the proper facilities exist to place the call and the call is administratively authorized. For example, is this LNS allowed to dial an international call? Once the outbound call is connected the LAC sends an Outgoing-Call-Connected message to the LNS indicating the final result of the call attempt: 7.5.1 LAC Outgoing Call States State Event Action New State ----- ----- ------ --------- idle Receive If cannot service, idle Outgoing-Call- send Outgoing-Call-Reply Request with Error idle Receive If can service, wait-cs-ans Outgoing-Call- send Request Outgoing-Call-Reply wait-cs-ans Telco answer Send established and framing Outgoing-Call-Connected detected wait-cs-ans Call failure Send Outgoing-Call-Connected idle with Error wait-cs-ans Receive Hang-up, send idle Call-Clear-Request Call-Disconnect-Notify established Receive Hang-up, send idle Call-Clear-Request Call-Disconnect-Notify or detect call disconnected The states associated with the LAC for outgoing calls are: idle Received Outgoing-Call-Request. If this is received in error, respond with an Outgoing-Call-Reply with error condition set. Otherwise, allocate physical channel to dial on and send an Valencia expires December 1997 [Page 69] INTERNET DRAFT June 1997 Outgoing-Call-Reply. Place the outbound call and move to the wait-cs-ans state. wait-cs-ans If the call is not completed or a timer expires waiting for the call to complete, send an Outgoing-Call-Connected with the appropriate error condition set and go to idle state. If a circuit switched connection is established and framing is detected, send an Outgoing-Call-Connected indicating success and go to established state. established If a Call-Clear-Request is received, the telco call SHOULD be released via appropriate mechanisms and a Call-Disconnect-Notify message SHOULD BE sent to the LNS. If the call is disconnected by the client or by the telco interface, a Call-Disconnect-Notify message MUST be sent to the LNS. Return to idle state after sending the Call-Disconnect-Notify. 7.5.2 LNS Outgoing Call States State Event Action New State ----- ----- ------ --------- idle Open request Send wait-reply Outgoing-Call-Request wait-reply Receive Clean-up idle Outgoing-Call-Reply with Error wait-reply Receive Null wait-connect Outgoing-Call-Reply wait-reply Abort request Send wait-disconnect Call-Clear-Request wait-connect Abort request Send Call-Clear-Request wait-disconnect wait-connect Receive Get ready for data established Outgoing-Call-Connected no Error wait-connect Receive Clean-up idle Outgoing-Call-Connected with Error established Receive Clean-up idle Call-Disconnect-Notify established Local terminate Send wait-disconnect Call-Clear-Request Valencia expires December 1997 [Page 70] INTERNET DRAFT June 1997 wait-disconnect Receive Clean-up idle Call-Disconnect- Notify The states associated with the LNS for outgoing calls are: idle An Outgoing-Call-Request message is sent to the LAC and the session moves into the wait-reply state. wait-reply An Outgoing-Call-Reply is received which indicates an error. The session returns to idle state. If the Outgoing-Call-Reply does not indicate an error, the telco call is in progress and the session moves to the wait-connect state. wait-connect An Outgoing-Call-Connected is received which indicates an error. The session returns to idle state. No telco call is active. If the Outgoing-Call-Connected does not indicate an error, the telco call is established If a Call-Disconnect-Notify is received, the telco call has been terminated for the reason indicated in the Result and Cause Codes. The session moves back to the idle state. If the LNS chooses to terminate the session, it sends a Call-Clear-Request to the LAC and then enters the wait-disconnect state. wait-disconnect A session disconnection is waiting to be confirmed by the LAC. Once the LNS receives the Call-Disconnect-Notify message, the session enters idle state. 8.0 L2TP Over Specific Media L2TP tries to be self-describing, operating at a level above the particular media over which it is carried. However, some details of its connection to media are required to permit interoperable implementations. The following sections describe details needed to permit interoperability over specific media. 8.1 IP/UDP L2TP uses the well-known UDP port 1701 [3]. The entire L2TP packet, including payload and L2TP header, is sent within a UDP datagram. The initiator of an L2TP tunnel picks an available source UDP port, and sends to the desired destination at port 1701. The recipient picks a free port on its own system, and sends its reply to the initiator's UDP port, setting its own UDP source port set to the free port it found. All subsequent packets exchanged will use these UDP ports. It is legal for a peer's IP address used for a given tunnel to change over the life of a connection; this may correspond to a peer with multiple IP interfaces responding to a network topology Valencia expires December 1997 [Page 71] INTERNET DRAFT June 1997 change. Responses should reflect the last source IP address for that Tunnel ID. IP fragmentation may occur as the L2TP packet travels over the IP substrate. L2TP makes no special efforts to optimize this. A LAC implementation MAY cause its LCP to negotiate for a specific MRU, which could optimize for LAC environments in which the MTUs of the path over which the L2TP packets are likely to travel have a consistent value. When operating over UDP, both the I and C bits MUST be present, and are used to permit correct demultiplexing and tunnel identification. The default for any L2TP implementation is that UDP checksums MUST be enabled for both control and payload messages. An L2TP implementation MAY provide an option to disable UDP checksums for payload packets. It is recommended that UDP checksums always be enabled on control packets. Port 1701 is used for both L2F [5] and L2TP packets. The two types of packets may be detected by their headers; L2TP has a Vers field of 2, L2F has a 1 in this field instead. An L2TP implementation running on a system which does not support L2F MUST silently discard all packets whose Vers field is set to 1. 8.2 IP When operating in IP environments, L2TP MUST use the UDP encapsulation described in 8.1. 9.0 Security Considerations L2TP encounters several security issues in its operation. The general approach of L2TP to these issues is documented here. 9.1 Tunnel Endpoint Security The tunnel endpoints may authenticate each other during tunnel establishment. This authentication has the same security attributes as CHAP, and has reasonable protection against reply and snooping. For L2TP tunnels over IP, IP-level packet security provides very strong protection of the tunnel. This requires no modification to the L2TP protocol, and leverages extensive IETF work in this area. For L2TP tunnels over Frame Relay or other switched networks, current practice indicates that these media are much less likely to experience attacks of in-transit data. If these attacks became prevalent, either the media or the L2TP packets would have to be encrypted. 9.2 Client Security Valencia expires December 1997 [Page 72] INTERNET DRAFT June 1997 A more systematic method of protection in tunneled PPP environments may be achieved through client security. PPP layer encryption would provide end-to-end security for both direct dial-in clients as well as PPP clients carried within a tunnel. With this level of client security, sessions are protected against attacks against the carrying tunnel, as well as attacks on the LAC itself. Because both encryption and compression can occur at the PPP layer, the two can be coordinated, permitting compression to precede encryption. 9.3 Proxy Authentication The optional proxy CHAP function of L2TP can permit an entirely transparent PPP tunnel, with a single LCP and CHAP sequence being seen by the client. For cases where the LAC and the entire path to the LNS are operated by a single entity, this function may provide acceptable security. For cases where LNS-initiated authentication is required, proxy CHAP still permits an initial access decision to be made before accepting the tunnel, permitting the LNS in most cases to reject tunnel initiations rather than accept them and later disconnect. The optional proxy PAP may result in the cleartext password traversing the tunnel. Where PAP is being used in conjunction with static passwords, this may pose a significant security issue. Where PAP is only used to transport one-time passwords, such issues may be greatly mitigated. The H bit of the carrying AVP may be used to protect against this. 10.0 Acknowledgments The AVP construct comes from Glen Zorn, who thought up the framework for permitting multiple vendors to contribute to a common attribute space in a relatively orderly fashion. Dory Leifer and Allan Rubens of Ascend Communications made valuable refinements to the protocol definition of L2TP and contributed to the editing of this document. 11.0 Contacts Kory Hamzeh Ascend Communications 1275 Harbor Bay Parkway Alameda, CA 94502 kory@ascend.com Tim Kolar, Morgan Littlewood, Andrew J. Valencia Cisco Systems 170 West Tasman Drive San Jose CA 95134-1706 tkolar@cisco.com littlewo@cisco.com vandys@cisco.com Valencia expires December 1997 [Page 73] INTERNET DRAFT June 1997 Gurdeep Singh Pall Microsoft Corporation Redmond, WA gurdeep@microsoft.com Jeff Taarud Copper Mountain Networks jtaarud@coppermountain.com William Verthein U.S. Robotics 12.0 References [1] W. Simpson, "The Point-to-Point Protocol (PPP)", RFC 1661, 7/21/1994 [2] A. Valencia, M. Littlewood, T. Kolar, "Layer 2 Forwarding", Internet draft, April 1996 [3] K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, "Point-to-Point Tunneling Protocol", Internet draft, June 1996 [4] P. Mockapetris, "Domain Names - Concepts and Facilities", RFC1034, November 1987 [5] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340, USC/Information Sciences Institute, July 1992. [6] Sally Floyd, Van Jacobson, "Random Early Detection Gateways for Congestion Avoidance", IEEE/ACM Transactions on Networking, August 1993 [7] D. Carrel, L. Grant, "The TACACS+ Protocol", draft-grant-tacacs-00.txt, October 1996 [8] C. Rigney, A. Rubens, W. A. Simpson, S. Willens. "Remote Authentication Dial In User Service (RADIUS)." draft-ietf-radius-radius-05.txt, Livingston, Merit, Daydreamer, July 1996. Appendix A: Acknowledgment Time-Outs L2TP uses sliding windows and time-outs to provide both user session flow-control across the underlying medium (which may be an internetwork) and to perform efficient data buffering to keep the LAC-LNS data channels full without causing receive buffer overflow. L2TP requires that a time-out be used to recover from dropped data or acknowledgment packets. The exact implementation of the time-out is vendor-specific. It is suggested that an adaptive time-out be implemented with backoff for congestion control. The time-out mechanism proposed here has the following properties: Independent time-outs for each session. A device (LAC or LNS) Valencia expires December 1997 [Page 74] INTERNET DRAFT June 1997 will have to maintain and calculate time-outs for every active session. An administrator-adjustable maximum time-out, MaxTimeOut, unique to each device. An adaptive time-out mechanism that compensates for changing throughput. To reduce packet processing overhead, vendors may choose not to recompute the adaptive time-out for every received acknowledgment. The result of this overhead reduction is that the time-out will not respond as quickly to rapid network changes. Timer backoff on time-out to reduce congestion. The backed-off timer value is limited by the configurable maximum time-out value. Timer backoff is done every time an acknowledgment time-out occurs. In general, this mechanism has the desirable behavior of quickly backing off upon a time-out and of slowly decreasing the time-out value as packets are delivered without time-outs. Some definitions: Packet Processing Delay, "PPD", is the amount of time required for each peer to process the maximum amount of data buffered in their offered receive packet window. The PPD is the value exchanged between the LAC and LNS when a call is established. For the LNS, this number should be small. For an LAC supporting modem connections, this number could be significant. "Sample" is the actual amount of time incurred receiving an acknowledgment for a packet. The Sample is measured, not calculated. Round-Trip Time, "RTT", is the estimated round-trip time for an Acknowledgment to be received for a given transmitted packet. When the network link is a local network, this delay will be minimal (if not zero). When the network link is the Internet, this delay could be substantial and vary widely. RTT is adaptive: it will adjust to include the PPD and whatever shifting network delays contribute to the time between a packet being transmitted and receiving its acknowledgment. Adaptive Time-Out, "ATO", is the time that must elapse before an acknowledgment is considered lost. After a time-out, the sliding window is partially closed and the ATO is backed off. Packet Processing Delay (PPD) The PPD parameter is a 16-bit time value exchanged during the Call Control phase expressed in units of tenths of a second (64 means 6.4 seconds). The protocol only specifies that the parameter is exchanged, it does not specify how it is calculated. The way values for ATO are calculated is implementation-dependent and need not be Valencia expires December 1997 [Page 75] INTERNET DRAFT June 1997 variable (static time-outs are allowed). IF adaptive time-outs are to be used then the PPD should be exchanged in the call connect sequences. A possible way to calculate the PPD is: PPD = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / ConnectRate + LACFudge (for an LAC) or PPD = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / AvePathRate + LNSFudge (for an LNS) Header is the total size of the L2TP and media dependent headers. MTU is the overall MTU for the link between the LAC and LNS. WindowSize represents the number of packets in the sliding window, and is implementation-dependent. The latency of the underlying connection path between the LAC and LNS could be used to pick a window size sufficient to keep the current session's pipe full. The constant 8 converts octets to bits (assuming ConnectRate is in bits per second). If ConnectRate is in bytes per second, omit the 8. LACFudge and LNSFudge are not required but can be used to take overall processing overhead of the LAC or LNS into account. In the case of the computed PPD for an LNS, AvePathRate is the average bit rate of the path between the LNS and LAC. Given that this number is probably very large and WindowSize is relatively small, LNSFudge will be the dominant factor in the computation of PPD. It is recommended that the minimum value of PPD be on the order of 0.5 second. The value of PPD is used to seed the adaptive algorithm with the initial RTT[n-1] value. A.1 Calculating Adaptive Acknowledgment Time-Out We still must decide how much time to allow for acknowledgments to return. If the time-out is set too high, we may wait an unnecessarily long time for dropped packets. If the time-out is too short, we may time out just before the acknowledgment arrives. The acknowledgment time-out should also be reasonable and responsive to changing network conditions. The suggested adaptive algorithm detailed below is based on the TCP 1989 implementation and is explained in Richard Steven's book TCP/IP Illustrated, Volume 1 (page 300). 'n' means this iteration of the calculation, and 'n-1' refers to values from the last calculation. DIFF[n] = SAMPLE[n] - RTT[n-1] DEV[n] = DEV[n-1] + (beta * (|DIFF[n]| - DEV[n-1])) RTT[n] = RTT[n-1] + (alpha * DIFF[n]) ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut) DIFF represents the error between the last estimated round-trip time and the measured time. DIFF is calculated on each iteration. DEV is the estimated mean deviation. This approximates the standard deviation. DEV is calculated on each iteration and Valencia expires December 1997 [Page 76] INTERNET DRAFT June 1997 stored for use in the next iteration. Initially, it is set to 0. RTT is the estimated round-trip time of an average packet. RTT is calculated on each iteration and stored for use in the next iteration. Initially, it is set to PPD. ATO is the adaptive time-out for the next transmitted packet. ATO is calculated on each iteration. Its value is limited, by the MIN function, to be a maximum of the configured MaxTimeOut value. Alpha is the gain for the average and is typically 1/8 (0.125). Beta is the gain for the deviation and is typically 1/4 (0.250). Chi is the gain for the time-out and is typically set to 4. To eliminate division operations for fractional gain elements, the entire set of equations can be scaled. With the suggested gain constants, they should be scaled by 8 to eliminate all division. To simplify calculations, all gain values are kept to powers of two so that shift operations can be used in place of multiplication or division. The above calculations are carried out each time an acknowledgment is received for a packet that was not retransmitted (no time-out occurs). A.2 Congestion Control: Adjusting for Time-Out This section describes how the calculation of ATO is modified in the case where a time-out does occur. When a time-out occurs, the time- out value should be adjusted rapidly upward. Although L2TP payload packets are not retransmitted when a time-out occurs, the time-out should be adjusted up toward a maximum limit. To compensate for shifting internetwork time delays, a strategy must be employed to increase the time-out when it expires. A simple formula called Karn's Algorithm is used in TCP implementations and may be used in implementing the backoff timers for the LNS or the LAC. Notice that in addition to increasing the time-out, we also shrink the size of the window as described in the next section. Karn's timer backoff algorithm, as used in TCP, is: NewTIMEOUT = delta * TIMEOUT Adapted to our time-out calculations, for an interval in which a time-out occurs, the new time-out interval ATO is calculated as: RTT[n] = delta * RTT[n-1] DEV[n] = DEV[n-1] ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut) In this modified calculation of ATO, only the two values that contribute to ATO and that are stored for the next iteration are calculated. RTT is scaled by delta, and DEV is unmodified. DIFF is not carried forward and is not used in this scenario. A value of 2 for Delta, the time-out gain factor for RTT, is suggested. Valencia expires December 1997 [Page 77] INTERNET DRAFT June 1997 Appendix B: Acknowledgment Time-Out and Window Adjustment B.1 Initial Window Size Although each side has indicated the maximum size of its receive window, it is recommended that a slow start method be used to begin transmitting data. The initial window size on the transmitter is set to half the maximum size the receiver requested, with a minimum size of one packet. The transmitter stops sending packets when the number of packets awaiting acknowledgment is equal to the current window size. As the receiver successfully digests each window, the window size on the transmitter is bumped up by one packet until the maximum is reached. This method prevents a system from flooding an already congested network because no history has been established. When for any reason an LAC or LNS receives more data than it can queue for the tunnel, a packet must be discarded. In this case, it is recommended that a "random early discard" algorithm [6] be used rather than the obvious "drop last" algorithm. B.2 Closing the Window When a time-out does occur on a packet, the sender adjusts the size of the transmit window down to one half its value when it failed. Fractions are rounded up, and the minimum window size is one. B.3 Opening the Window With every successful transmission of a window's worth of packets without a time-out, the transmit window size is increased by one packet until it reaches the maximum window size that was sent by the other side when the call was connected. As stated earlier, no retransmission is done on a time-out. After a time-out, the transmission resumes with the window starting at one half the size of the transmit window when the time-out occurred and adjusting upward by one each time the transmit window is filled with packets that are all acknowledged without time-outs. B.4 Window Overflow When a receiver's window overflows with too many incoming packets, excess packets are thrown away. This situation should not arise if the sliding window procedures are being properly followed by the transmitter and receiver. It is assumed that, on the transmit side, packets are buffered for transmission and are no longer accepted from the packet source when the transmit buffer fills. Appendix C: Handling of out-of-order packets When the Sequence Number and Acknowledgment Number fields are present in payload packets, they are used to manage packet rate. In addition, they may be used to handle out-of-order arrival of packets. A simple L2TP client would simply discard any packet other than a packet with a sequence greater than that last received; if packets 1, Valencia expires December 1997 [Page 78] INTERNET DRAFT June 1997 2, 3 arrived as 1, 3, 2, this would result in packet 2 being discarded. Such behavior does not affect the L2TP protocol itself, but significantly improved throughput in such environments may be attained by queueing and reordering packets when they arrive out of order. The number of packets to be queued is a function of memory resources on the L2TP implementation, but should never be more than 1/4 of the total sequence number space (i.e., 16384 packets), to avoid aliasing. An implementation which queues packets in this way must also employ an algorithm for deciding that a given sequence number corresponds to a packet which is lost, rather than one which is out of order but still in transit. Such a decision would likely be based upon timing, buffering conditions, and packet arrival characteristics. The details of such a tradeoff are outside the scope of this specification, but it is recommended a packet should be afforded an interval at least twice the estimated RTT from the L2TP peer. Appendix D: Transport Layer Adaptive Time-outs and Window Adjustment Appendixes A, B, and C dealt with operation of the payload packet sessions of L2TP. This Appendix explains how these three algorithms pertain to the transport layer for L2TP control sessions. Appendix B, Time-out Window Management, directly applies to the Transport Layer except in the case where a window size of 1 is being used. Appendix C, does not really apply because, for the Control Session, control messages MUST always be processed by the receiver in order. Also, there are no lost control packets because they are retransmitted by the L2TP Transport Layer. Thus, the main topic of this Appendix is how to adapt the example adaptive time-out algorithm of Appendix A to the Control Session Transport Layer. There are two main differences between the Control Session and payload sessions: 1) Unlike lost payload packets, lost control messages are retransmitted and 2) There is no Packet Processing Delay value provided in the control session setup messages. The latter affects the manner in which the initial value of the RTT estimate is determined. The former really doesn't affect the algorithm at all, except that upon a time-out, retransmission of unacknowledged control messages should be attempted, up to the number that fit in the sliding window with size computed as in Appendix B. Using the symbol definitions of Appendix A, the calculation of the value for the PPD of the remote peer can be estimated as: PPD = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / AvePathRate + Fudge This is simply the number of bits in a full control session window divided by the average speed of the path between the LAC and LNS with a fudge factor added on to make it work. In cases where the average rate of the connection between LAC and LNS isn't known, it is Valencia expires December 1997 [Page 79] INTERNET DRAFT June 1997 suggested that some value be configured that is associated with each possible peer. Because Control Session windows will most likely be small and the connection speed will be quite high, fudge will be the dominant factor in this calculation. For this reason, just configuring a single fixed initial PPD estimate to be used for all possible peers will probably provide adequate results. This fudge factor should probably be at least 0.5 second. Appendix E: Examples of L2TP sequence numbering Although sequence numbers serve distinct purposes for control and data messages, both types of messages use identical techniques for assigning sequence numbers. This appendix shows several common scenarios, and illustrates how sequence number state progresses and is intepreted. E.1: Lock-step tunnel establishment In this example, an LAC establishes a tunnel, with the exchange involving each side alternating in sending messages. This example is contrived, in that the final acknowledgement in the example is explicitly sent within a zero-length message, although most typically the acknowledgement would have been included in the processing of the Incoming-Call-Request which had prompted the LAC to initiate the tunnel in the first place. LAC -> LNS: Start-Control-Connection-Request Nr: 0, Ns: 0 LNS -> LAC: Start-Control-Connection-Reply Nr: 1, Ns: 0 LAC -> LNS: Start-Control-Connection-Connected Nr: 1, Ns: 1 (delay) LNS -> LAC: (zero-length) Nr: 2, Ns: 1 E.2: Multiple packets acknowledged This example shows a flow of payload packets from the LNS to the LAC, with the LAC having no traffic of its own. The LAC is waiting 1/4 of its timeout interval, and then acknowledging all packets seen since the last interval. (previous packet flow precedes this) LAC -> LNS: (zero-length) Nr: 7000, Ns: 1000 LNS -> LAC: (payload) Nr: 1000, Ns: 7000 Valencia expires December 1997 [Page 80] INTERNET DRAFT June 1997 LNS -> LAC: (payload) Nr: 1000, Ns: 7001 LNS -> LAC: (payload) Nr: 1000, Ns: 7002 (LAC's timer indicates it should acknowledge pending traffic) LAC -> LNS: (zero-length) Nr: 7003, Ns: 1000 E.3: Lost packet with retransmission As a final example, an existing tunnel has a new session requested by the LAC. The Incoming-Call-Reply is lost and must be retransmitted by the LNS. This example continues from the sequence state reached at the end of example E.1. Note that loss of the -Reply has two impacts: not only does it keep the upper level state machine from progressive, but it also keeps the LAC from seeing a timely lower level acknowledgement of its -Request packet. LAC -> LNS: Incoming-Call-Request Nr: 1, Ns: 2 LNS -> LAC: Incoming-Call-Reply Nr: 3, Ns: 1 (pause; LAC's timer started first, so fires first) LAC -> LNS: Incoming-Call-Request Nr: 1, Ns: 2 (LNS realizes he has already seen this packet) (LNS might use this as a cue to retransmit, as in this example) LNS -> LAC: Incoming-Call-Reply Nr: 3, Ns: 1 LAC -> LNS: Incoming-Call-Connected Nr: 2, Ns: 3 (delay) LNS -> LAC: (zero-length) Nr: 4, Ns: 2 Valencia expires December 1997 [Page 81]