Network Working Group J. Lau Internet-Draft M. Townsley Category: Standards Track A. Valencia <draft-ietf-l2tpext-l2tp-base-01.txt> G. Zorn cisco Systems I. Goyret Lucent Technologies G. Pall Microsoft Corporation A. Rubens Nexthop B. Palter Redback Networks October 2001 Layer Two Tunneling Protocol "L2TP" Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress''. The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). The distribution of this memo is unlimited. It is filed as <draft- ietf-l2tpext-l2tp-base-01.txt> and expires April 2002. Please send comments to the L2TP mailing list (l2tp@l2tp.net). Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Townsley, et al. Standards Track [Page 1]
INTERNET DRAFT L2TP October 2001 Abstract This document describes the Layer Two Tunneling Protocol (L2TP). L2TP tunnels Layer 2 packets across an intervening network in a way that is as transparent as possible to both end-users and applications. Acknowledgments The basic concept for L2TP and many of its protocol constructs were adopted from L2F [RFC2341] and PPTP [RFC2637]. Authors of these are A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, and G. Zorn. The L2TP rewrite team for splitting RFC 2661 into the base and companion PPP specifications consisted of Ignacio Goyret, Jed Lau, Bill Palter, Mark Townsley, and Madhvi Verma. Skip Booth also provided helpful review and comment. Stewart Bryant and Simon Barber provided helpful input for the new L2TP over IP header. This document was based upon RFC 2661, for which a number of people provided valuable input and effort. John Bray, Greg Burns, Rich Garrett, Don Grosser, Matt Holdrege, Terry Johnson, Dory Leifer, and Rich Shea provided valuable input and review at the 43rd IETF in Orlando, FL, which led to improvement of the overall readability and clarity of RFC 2661. Thomas Narten provided a great deal of critical review, formatting, and wrote the IANA Considerations section. Dory Leifer made valuable refinements to the protocol definition of L2TP and contributed to the editing of early drafts leading to RFC2661. Steve Cobb and Evan Caves redesigned the state machine tables. Barney Wolff provided a great deal of design input on the endpoint authentication mechanism. Townsley, et al. Standards Track [Page 2]
INTERNET DRAFT L2TP October 2001 Contents Status of this Memo.......................................... 1 1. Introduction............................................. 5 1.1 Changes from RFC 2661................................ 5 1.2 Specification of Requirements........................ 6 1.3 Terminology.......................................... 6 2. Topology................................................. 9 3. Protocol Overview........................................ 10 3.1 Control Message Types................................ 11 3.2 L2TP Header Formats.................................. 12 3.2.1 L2TP Control Message Header..................... 12 3.2.2 L2TP Data Message............................... 14 3.3 Control Connection Management........................ 15 3.3.1 Control Connection Establishment................ 15 3.3.2 Control Connection Teardown..................... 15 3.4 Call Management...................................... 16 3.4.1 Incoming Call Establishment..................... 16 3.4.2 Outgoing Call Establishment..................... 16 3.4.3 Session Teardown................................ 16 4. Protocol Operation....................................... 17 4.1 L2TP Over Specific Packet Switched Networks (PSNs)... 17 4.1.1 L2TP over IP.................................... 18 4.1.2 L2TP over UDP................................... 19 4.1.3 IP Fragmentation Issues......................... 21 4.2 Reliable Delivery of Control Messages................ 21 4.3 Tunnel Endpoint Authentication....................... 24 4.4 Keepalive (Hello).................................... 24 4.5 Forwarding Session Frames............................ 25 4.6 L2TPv2/v3 Interoperability and Migration............. 25 4.6.1 L2TPv3 over IP.................................. 25 4.6.2 L2TPv3 over UDP................................. 26 4.6.3 Automatic L2TPv2 Fallback....................... 26 5. Control Message Attribute Value Pairs.................... 26 5.1 AVP Format........................................... 27 5.2 Mandatory AVPs....................................... 28 5.3 Hiding of AVP Attribute Values....................... 29 5.4 AVP Summary.......................................... 31 5.4.1 AVPs Applicable to All Control Messages......... 31 5.4.2 Result and Error Codes.......................... 33 5.4.3 Control Connection Management AVPs.............. 35 5.4.4 Call Management AVPs............................ 41 5.4.5 Call Status AVPs................................ 48 Townsley, et al. Standards Track [Page 3]
INTERNET DRAFT L2TP October 2001 6. Control Connection Protocol Specification................ 48 6.1 Start-Control-Connection-Request (SCCRQ)............. 49 6.2 Start-Control-Connection-Reply (SCCRP)............... 49 6.3 Start-Control-Connection-Connected (SCCCN)........... 50 6.4 Stop-Control-Connection-Notification (StopCCN)....... 50 6.5 Hello (HELLO)........................................ 50 6.6 Incoming-Call-Request (ICRQ)......................... 51 6.7 Incoming-Call-Reply (ICRP)........................... 51 6.8 Incoming-Call-Connected (ICCN)....................... 52 6.9 Outgoing-Call-Request (OCRQ)......................... 52 6.10 Outgoing-Call-Reply (OCRP).......................... 53 6.11 Outgoing-Call-Connected (OCCN)...................... 53 6.12 Call-Disconnect-Notify (CDN)........................ 54 6.13 WAN-Error-Notify (WEN).............................. 54 7. Control Connection State Machines........................ 54 7.1 Malformed Control Messages........................... 55 7.2 Timing Considerations................................ 56 7.3 Control Connection States............................ 56 7.4 Incoming Calls....................................... 58 7.4.1 ICRQ Sender States.............................. 58 7.4.2 ICRQ Recipient States........................... 60 7.5 Outgoing Calls....................................... 61 7.5.1 OCRQ Sender States.............................. 61 7.5.2 OCRQ Recipient (LAC) States..................... 62 7.6 Termination of a Control Connection.................. 64 8. Security Considerations.................................. 64 8.1 Control Connection Endpoint Security................. 64 8.2 Packet Level Security................................ 65 8.3 End-to-End Security.................................. 65 8.4 L2TP and IPsec....................................... 65 9. IANA Considerations...................................... 66 9.1 AVP Attributes....................................... 66 9.2 Message Type AVP Values.............................. 66 9.3 Result Code AVP Values............................... 66 9.3.1 Result Code Field Values........................ 66 9.3.2 Error Code Field Values......................... 67 9.4 AVP Header Bits...................................... 67 10. References.............................................. 67 11. Editors' Addresses...................................... 69 Appendix A: Control Slow Start and Congestion Avoidance...... 69 Appendix B: Control Message Examples......................... 70 Townsley, et al. Standards Track [Page 4]
INTERNET DRAFT L2TP October 2001 Appendix C: Intellectual Property Notice..................... 72 1. Introduction The Layer Two Tunneling Protocol (L2TP) provides a dynamic tunneling mechanism for multiple Layer 2 (L2) circuits across a packet-oriented data network. L2TP, as originally defined in RFC 2661, describes a standard method for tunneling PPP sessions. L2TP has since been adopted for tunneling a number of other L2 protocols. In order to provide greater modularity, this document describes the base L2TP protocol, independent of the L2 payload that is being tunneled. The base L2TP protocol consists of (1) the control protocol for dynamic creation, maintenance, and teardown of L2TP sessions, and (2) the L2TP data encapsulation to multiplex and demultiplex L2 data streams between IP-connected L2TP nodes. 1.1 Changes from RFC 2661 Most of the protocol constructs described in this document are carried over from RFC 2661. Changes include clarifications based on years of interoperability and deployment experience as well as modifications to either improve protocol operation or provide a clearer separation from PPP. The intent of these modifications is to achieve a healthy balance between code, interoperability experience with RFC 2661, and a thoughtful and directed evolution of the protocol as it is applied to new tasks. When the designation between L2TPv2 and L2TPv3 is necessary, L2TP as defined in RFC 2661 will be referred to as "L2TPv2", corresponding to the value in the Version field of an L2TP control message header. (L2F is defined as "version 1".) At times, L2TP as defined in this document will be referred to as "L2TPv3". Otherwise, the acronym "L2TP" will refer to L2TPv3 or L2TP in general. Notable differences between L2TPv2 and L2TPv3 include: - Separation of all PPP-related AVPs, references, etc., including a portion of the L2TP data header that was specific to the needs of PPP. The PPP-specific contructs are described in a companion document. - Transition from a 16-bit Session ID and Tunnel ID to a 32-bit Session ID and Control Connection ID. Details of these changes and a recommendation for transitioning to L2TPv3 may be found in Section 4.6. Townsley, et al. Standards Track [Page 5]
INTERNET DRAFT L2TP October 2001 1.2 Specification of Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.3 Terminology Attribute Value Pair (AVP) The variable-length concatenation of a unique Attribute (represented by an integer) and a Value containing the actual value identified by the attribute. Multiple AVPs make up control messages, which are used in the establishment, maintenance, and teardown of control connections. This construct is known as the Type-Length-Value (TLV) in some specifications. (See also: Control Connection, Control Message.) Call The action of transitioning a circuit on an LAC to an "up" or "established" state. A call may be dynamically established through signaling properties (e.g. an incoming or outgoing call through the PSTN) or statically established (e.g. provisioning a VC on an interface). A call is defined by its properties (e.g. type of call, called number, etc.) and its data traffic. (See also: Circuit, Session, Incoming Call, Outgoing Call, Outgoing Call Request.) CHAP Challenge Handshake Authentication Protocol [RFC1994], a point-to- point cryptographic challenge/response authentication protocol in which the cleartext password is not passed over the line. Circuit A general term identifying any one of a wide range of L2 connections. A circuit may be virtual in nature (e.g. an ATM PVC or an L2TP session), or it may have direct correlation to a physical layer (e.g. an RS-232 serial line). Circuits may be statically configured with a relatively long-lived uptime, or dynamically established with some type of control channel governing the establishment, maintenance, and teardown of the circuit. For the purposes of this document, a statically configured circuit is considered to be largely equivalent to a simple dynamic circuit. (See also: Call, Remote System.) Townsley, et al. Standards Track [Page 6]
INTERNET DRAFT L2TP October 2001 Client (See Remote System.) Control Connection An L2TP control connection is a reliable control channel that is used to establish, maintain, and release individual L2TP sessions as well as the control channel itself. (See also: Control Message, Data Channel.) Control Message An L2TP message used by the control connection. (See also: Control Connection.) Data Message Message used by the data channel. (See also: Data Channel.) Data Channel The channel of L2TP-encapsulated L2 traffic that passes between two LCCEs, utilizing a specific data encapsulation method. L2TP defines one base encapsulation method for L2 traffic, although others may be used as well. (See also: Control Connection, Data Message.) Dominant LCCE The LCCE that either solely initiated establishment of a control connection or won the tie breaker during control connection establishment. (See also: LCCE, Section 5.4.3.) Incoming Call The action of receiving a call on an LAC. The call may have been placed by a remote system (e.g. a phone call over a PSTN), or it may have been triggered by a local event (e.g. interesting traffic routed to a virtual interface). An incoming call that needs to be tunneled (as determined by the LAC) results in the generation of an L2TP ICRQ message. (See also: Call, Outgoing Call, Outgoing Call Request.) L2TP Access Concentrator (LAC) An LCCE that tunnels a circuit (either physically connected or logically connected, as via another L2TP session) to another Townsley, et al. Standards Track [Page 7]
INTERNET DRAFT L2TP October 2001 location using L2TP, without performing any native L2 packet processing on the circuit. The LAC may tunnel to either an LNS or another LAC. (See also: LCCE, LNS.) L2TP Control Connection Endpoint (LCCE) One end of an L2TP control connection, either an LAC or an LNS. (See also: LAC, LNS.) L2TP Network Server (LNS) An LCCE that logically terminates a tunneled circuit locally and that processes the tunneled traffic as though the circuit were physically connected to the device. The LNS may tunnel to either an LAC or another LNS. (See also: LCCE, LAC.) Outgoing Call The action of placing a call on an LAC, typically in response to policy directed by the peer in an Outgoing Call Request message. (See also: Call, Incoming Call, Outgoing Call Request.) Outgoing Call Request A request sent to an LAC to place an outgoing call. The request contains specific information for the LAC in placing the call, information that is typically not known a priori by the LAC. (See also: Call, Incoming Call, Outgoing Call.) Packet-Switched Network (PSN) A network layer that uses packet-switching technology for data delivery (e.g. an IP network). Peer When used in context with L2TP, Peer refers to the far end of an L2TP control connection (i.e. the far LCCE). An LAC's peer may be either an LNS or another LAC. Similarly, an LNS's peer may be either an LAC or another LNS. (See also: LAC, LCCE, LNS.) Remote System An end-system or router connected by a circuit to an LAC. Session An L2TP session is created by a particular L2TP control connection Townsley, et al. Standards Track [Page 8]
INTERNET DRAFT L2TP October 2001 between two LCCEs when a circuit is successfully established. The circuit may either pass through (LAC) or terminate locally (LNS) on the LCCEs, which maintain state for the circuit. There is a one-to-one relationship between established L2TP sessions and their associated circuits. (See also: Circuit, LAC, LCCE, LNS.) Zero-Length Body (ZLB) Message A control packet with only an L2TP header. ZLB messages are used for explicitly acknowledging packets on the reliable control channel. 2. Topology L2TP operates between two L2TP Control Connection Endpoints (LCCEs), tunneling circuit traffic across a packet network. An L2TP Network Server (LNS) is an LCCE that decapsulates tunneled L2 traffic and directs it as incoming data towards a virtual L2 interface. In contrast, an L2TP Access Concentrator (LAC) is an LCCE that merely forwards tunneled traffic directly to a circuit (which may even be another L2TP session). There are three predominant tunneling models in which L2TP operates: LAC-LNS (or vice versa), LAC-LAC, and LNS-LNS. These models are diagrammed below. (Dotted lines designate network connections. Solid lines designate circuit connections.) Figure 2.0: L2TP Reference Models (a) LAC-LNS Reference Model: On one side, the LAC receives traffic from an L2 circuit, which it forwards via L2TP across an IP or other packet-based network. On the other side, an LNS logically terminates the L2 circuit locally and routes traffic (at Layer 3) to the home network. The action of session establishment may be driven by the LAC (perhaps as an incoming call) or the LNS (perhaps as an outgoing call). This model typically has, but does not require, a clear initiator and responder. +-----+ L2 +-----+ +-----+ | |------| LAC |....[packet network]....| LNS |...[home network] +-----+ +-----+ +-----+ remote system |<-- emulated service -->| |<----------- L2 service ------------>| (b) LAC-LAC Reference Model: In this model, both LCCEs are LACs. Each LAC forwards circuit traffic from the remote system to the peer Townsley, et al. Standards Track [Page 9]
INTERNET DRAFT L2TP October 2001 LAC using L2TP, and vice versa. A LAC does not perform any native handling of the tunneled L2 frame, and thus, does not utilize a virtual L2 interface. Rather, a LAC acts as a simple cross-connect between a circuit and an L2TP session. This model typically involves symmetric establishment; that is, either side of the connection may initiate a session at any time (or perhaps simultaneously). +-----+ L2 +-----+ +-----+ L2 +-----+ | |------| LAC |...[packet network]...| LAC |------| | +-----+ +-----+ +-----+ +-----+ remote remote system system |<- emulated service ->| |<----------------- L2 service ----------------->| (c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs. Each LNS logically terminates the L2TP session locally, requiring virtual L2 interfaces for each L2TP session on each side of the L2TP session. A user-level or traffic-generated event typically drives session establishment from one side of the control connection. Also known as "voluntary tunneling" [RFC2809]. +-----+ +-----+ [home network]...| LNS |...[packet network]...| LNS |...[home network] +-----+ +-----+ |<- emulated service ->| |<---- L2 service ---->| Note: If an LNS initiates session establishment due to an event (generally user-driven), the LNS is sometimes referred to as a "LAC Client" as defined in [RFC2661]. 3. Protocol Overview L2TP utilizes two types of messages: control messages and data messages. Control messages are used in the establishment, maintenance, and clearing of control connections and calls. These messages utilize a reliable control channel within L2TP to guarantee delivery (see Section 4.2 for details). Data messages are used to encapsulate the L2 traffic being carried over the L2TP session. Unlike control messages, data messages are not retransmitted when packet loss occurs. While both the L2TP control channel and the L2TP data channel are defined strictly in this document, the L2TP data channel MAY be substituted with a different L2 tunneling encapsulation whose format can negotiated by the L2TP control connection. Furthermore, the L2TP data channel MAY be used without the control channel, if so desired. Townsley, et al. Standards Track [Page 10]
INTERNET DRAFT L2TP October 2001 However, it is strongly recommended that such practice be limited to relatively small-scale deployments, or deployments in which some other form of automatic control information distribution is employed. Figure 3.0: L2TPv3 Structure +-------------------+ | L2 Frames | +-------------------+ +-----------------------+ | L2TP Data Messages| | L2TP Control Messages | +-------------------+ +-----------------------+ | L2TP Data Channel | | L2TP Control Channel | | (unreliable) | | (reliable) | +-------------------+----+-----------------------+ | Packet Switched Network (IP, FR, ATM, etc.) | +------------------------------------------------+ Figure 3.0 depicts the relationship of control messages and data messages over the L2TP control and data channels, respectively. Data messages are passed over an unreliable data channel, encapsulated first by an L2TP header and sent over a Packet Switched Network (PSN) such as IP, UDP, Frame Relay, ATM, etc. Control messages are sent over a reliable L2TP control channel, which operates in-band over the same PSN. The necessary setup for tunneling a session with L2TP consists of two steps: (1) Establishing the control connection, and (2) establishing a session as triggered by an incoming call or outgoing call. The control connection MUST be established before an incoming or outgoing call is initiated. An L2TP session MUST be established before L2TP can begin to forward session frames. Multiple sessions may be bound to a single control connection, and multiple control connections may exist between the same two LCCEs. 3.1 Control Message Types The Message Type AVP (see Section 5.4.1) defines the specific type of control message being sent. This document defines the following control message types (see Sections 6.1 through 6.13 for details on the construction and use of each message): Control Connection Management 0 (reserved) 1 (SCCRQ) Start-Control-Connection-Request 2 (SCCRP) Start-Control-Connection-Reply Townsley, et al. Standards Track [Page 11]
INTERNET DRAFT L2TP October 2001 3 (SCCCN) Start-Control-Connection-Connected 4 (StopCCN) Stop-Control-Connection-Notification 5 (reserved) 6 (HELLO) 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 (reserved) 14 (CDN) Call-Disconnect-Notify Error Reporting 15 (WEN) WAN-Error-Notify 3.2 L2TP Header Formats This section defines header formats for L2TP control messages and L2TP data messages. All values are placed into their respective fields and sent in network order (high order octets first). 3.2.1 L2TP Control Message Header The L2TP control message header provides information for the reliable transport of messages that govern the establishment, maintenance, and teardown of L2TP sessions. By default, control messages are sent over the underlying media in-band with L2TP data messages. As such, L2TP also includes a default method (borrowing from RFC 2661 by utilizing the high bit of the first octet in the L2TP header) that may be used to distinguish L2TP control messages from L2TP data messages. Other methods for distinguishing between control and data MAY be utilized for specific media (an example is L2TP over IP as defined in 4.1). The L2TP control message header is formatted as follows: Townsley, et al. Standards Track [Page 12]
INTERNET DRAFT L2TP October 2001 Figure 3.2.1: L2TP Control Message Header 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|x|x|S|x|x|x|x|x|x|x| Ver | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Control Connection ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ns | Nr | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The T bit MUST be set to 1, indicating that this is a control message. The L and S bits MUST be set to 1, indicating that the Length field and sequence numbers are present. The x bits are reserved for future extensions. All reserved bits MUST be set to 0 on outgoing messages and ignored on incoming messages. The Ver field indicates the version of the L2TP control message header described in this document. On sending, this field MUST be set to 3 for all messages (unless operating in an environment with L2TPv2 [RFC2661] and/or L2F [RFC2341], see Section 4.1 for details). The Length field indicates the total length of the message in octets, always calculated from the start of the control message header itself (beginning with the T bit). The Control Connection ID field contains the identifier for the control connection. L2TP control connections are named by identifiers that have local significance only. That is, the same control connection will be given unique Control Connection IDs by each LCCE from within each endpoint's own Control Connection ID number space. As such, the Control Connection ID in each message is that of the intended recipient, not the sender. Non-zero Control Connection IDs are selected and exchanged as Assigned Control Connection ID AVPs during the creation of a control connection. Ns indicates the sequence number for this control message, beginning at zero and incrementing by one (modulo 2**16) for each message sent. See Section 4.2 for more information on using this field. Nr indicates the sequence number expected in the next control message to be received. Thus, Nr is set to the Ns of the last in-order message received plus one (modulo 2**16). See Section 4.2 for more Townsley, et al. Standards Track [Page 13]
INTERNET DRAFT L2TP October 2001 information on using this field. 3.2.2 L2TP Data Message In general, an L2TP data message consists of a (1) Tunnel Header, (2) an L2-Specific Sublayer (when needed), and (3) the Tunneled L2 Frame, as depicted below. Figure 3.2.2: L2TP Data Message +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Packet Switched Network (PSN) Delivery Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2TP Tunnel Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | L2-Specific Sublayer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tunneled L2 Frame | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The L2TP Tunnel Header is specific to the PSN over which the L2TP traffic is delivered. The tunnel header MUST, at a minimum, provide (1) a 32-bit longword-aligned Session ID field to uniquely identify a tunneled stream of data, and (2) a method of distinguishing data messages from control messages. Each type of PSN MUST define its own tunnel header, clearly identifying the format of the header and parameters necessary to setup the session. Section 4.1 defines two tunnel headers, one for transport over UDP and one for transport over IP. Either of these formats MAY be used for other PSNs, but the actual definition of such remains outside the scope of this document. The L2-specific sublayer is an intermediary layer between the fixed L2TP tunnel header and the start of the inner L2 frame. It may contain control fields that the are used to facilitate the tunneling of the L2 frames (e.g. offset bytes or sequence numbers). Since the sublayer is specific to each L2 payload that may be tunneled using the L2TP data encapsulation, the format of the sublayer is determined by the Pseudo Wire Type AVP (see Section 5.4.4), which identifies the L2 payload. Specific sublayer formats are defined in the appropriate L2 payload-specific companion documents. The tunneled L2 frame consists of the encapsulated L2 traffic, including any necessary L2 framing as defined in the payload-specific companion documents. Townsley, et al. Standards Track [Page 14]
INTERNET DRAFT L2TP October 2001 3.3 Control Connection Management The L2TP Control Connection handles dynamic establishment, teardown, and maintenance of the L2TP sessions and of the control connection itself. The reliable delivery of control messages is described in Section 4.2. This section describes the typical control connection establishment and teardown exchanges. It is important to note that, in the diagrams that follow, the reliable control message delivery mechanism exists independently of the L2TP state machine. For instance, ZLB ACKs may be sent after any of the control messages indicated in the exchanges below if an acknowledgement is not piggybacked on a later control message. 3.3.1 Control Connection Establishment Establishment of the control connection involves an exchange of AVPs that identifies the peer and its capabilities. A three-message exchange is used to establish the control connection. The following is a typical message exchange: LCCE A LCCE B ------ ------ SCCRQ -> <- SCCRP SCCCN -> 3.3.2 Control Connection Teardown Control connection teardown may be initiated by either LCCE and is accomplished by sending a single StopCCN control message. As part of the reliable control message delivery mechanism, the recipient of a StopCCN MUST send a ZLB ACK to acknowledge receipt of the message and maintain enough control connection state to properly accept StopCCN retransmissions over at least a full retransmission cycle (in case the ZLB ACK is lost). The recommended time for a full retransmission cycle is at least 31 seconds (see Section 4.2). The following is an example of a typical control message exchange: LCCE A LCCE B ------ ------ StopCCN -> (Clean up) (Wait) (Clean up) Townsley, et al. Standards Track [Page 15]
INTERNET DRAFT L2TP October 2001 An implementation may shut down an entire control connection and all sessions associated with the control connection by sending the StopCCN. Thus, it is not necessary to clear each session individually when tearing down the whole control connection. 3.4 Call Management After successful control connection establishment, individual sessions may be created. Each session corresponds to a single data stream between the two LCCEs. This section describes the typical call establishment and teardown exchanges. 3.4.1 Incoming Call Establishment A three-message exchange is used to establish the session. The following is a typical sequence of events: LCCE A LCCE B ------ ------ (Call Detected) ICRQ -> <- ICRP ICCN -> 3.4.2 Outgoing Call Establishment A three-message exchange is used to set up the session. The following is a typical sequence of events: LCCE A LCCE B ------ ------ <- OCRQ OCRP -> (Perform Call Operation) OCCN -> 3.4.3 Session Teardown Session teardown may be initiated by either the LAC or LNS and is accomplished by sending a CDN control message. After the last session is cleared, the control connection MAY be torn down as well (and typically is). The following is an example of a typical control Townsley, et al. Standards Track [Page 16]
INTERNET DRAFT L2TP October 2001 message exchange: LCCE A LCCE B ------ ------ CDN -> (Clean up) (Clean up) 4. Protocol Operation This section addresses various operational issues in both the control connection and data channel of L2TP. 4.1 L2TP Over Specific Packet Switched Networks (PSNs) L2TP is designed to allow operation over a variety of Packet Switched Networks. In consideration of any specific characteristics of an underlying PSN, the actual L2TP Tunnel Header encapsulation may vary. For instance, a payload length field for data packets traversing a UDP or IP network is unnecessary as this is readily available from the underlying layer. This document describes the standard method for operation of L2TP over IPv4 networks. There are two modes described, L2TP over IP (section 4.1.1) and L2TP over UDP (section 4.1.2). L2TPv3 implementations MUST support L2TP over IP, and SHOULD support L2TP over UDP for better NAT and FW traversal, integration with IPsec [L2TPIPSEC], and easier migration from L2TPv2. L2TP over other PSNs may be defined, but the specifics are outside the scope of this document. Whenever possible, the field definitions in this section should be used as they are described here. Examples of L2TPv2 over other PSNs include [RFC3070], and [L2TPAAL5]. The following field definitions are defined for use in all L2TP Tunnel Header encapsulations. Session ID A 32-bit field containing a non-zero identifier for a session. L2TP sessions are named by identifiers that have local significance only. That is, the same logical session will be given different Session IDs by each end of the tunnel for the life of the session. When the L2TP control connection is used for session establishment, Session IDs are selected and exchanged as Local Session ID AVPs during the creation of a session. Townsley, et al. Standards Track [Page 17]
INTERNET DRAFT L2TP October 2001 Cookie The optional Cookie field contains a variable length (maximum 64 bits), longword-aligned value used to check the association of a received data message with the session identified by the Session ID. The Cookie MUST be configured with a random value utilizing all bits in the field. The Cookie provides an additional level of guarantee, beyond the Session ID lookup, that a data message has been directed to the proper session. A well-chosen Cookie may prevent inadvertent misdirection of stray packets with recently reused Session IDs, Session IDs subject to packet corruption, etc. When the L2TP control connection is used for session establishment, random Cookie values are selected and exchanged as Assigned Cookie AVPs during the creation of a session. The maximum size of the Cookie field is 64 bits. 4.1.1 L2TP over IP L2TP over IP utilizes the IANA assigned IP protocol ID 115. 4.1.1.1 L2TP over IP Tunnel Header Unlike L2TP over UDP, the L2TPv3 tunnel header over IP is free of any restrictions imposed by coexistence with L2TPv2 and L2F. As such, the header format has been redesigned to optimize packet processing. The following tunnel header format is utilized when operating L2TPv3 over IP: Figure 4.1.1.1: L2TPv3 over IP Tunnel Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Session ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cookie (optional, maximum 64 bits)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Session ID and Cookie fields are as defined in Section 4.1. The Session ID of zero is reserved for use by L2TP control messages (see Section 4.1.1.2). It should be noted that the absence of the Version and Flags fields, which are present in L2TP over UDP, prevents straightforward version extension flexibility for data messages. However, given the freedom Townsley, et al. Standards Track [Page 18]
INTERNET DRAFT L2TP October 2001 of setting the first 32 bits in the data message header here, this limitation can be alleviated with an acceptable workaround if an extension to the demultiplexing capabilities of L2TP is ever in need of further revision. In contrast, the control message header still retains all version checking ability. 4.1.1.2 L2TP Control and Data Traffic over IP As shown in Section 4.1.1.1, there are no Version and Flags fields in the L2TP Tunnel Header over IP. Specifically, the T bit does not exist to distinguish control packets and data packets. Instead, all control packets are sent over the reserved session ID of 0. It is presumed that this method is more efficient -- both in header size for data packets and in processing speed for distinguishing control messages -- than checking for the presence of certain bits. Thus, the entire control message header over IP, including the zero session ID, appears as follows: Figure 4.1.1.2: L2TPv3 over IP Control Message Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (32 bits of zeros) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |T|L|x|x|S|x|x|x|x|x|x|x| Ver | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Control Connection ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ns | Nr | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Named fields are as defined in Section 3.2.1. Note that the Length field is still calculated from the beginning of the control message header, beginning with the the T bit. The length calculation does NOT include the "(32 bits of zeros)" depicted above. 4.1.2 L2TP over UDP L2TPv3 over UDP must take into careful consideration other L2 tunneling protocols that may be operating in the same environment, including L2TPv2 [RFC2661] and L2F [RFC2341]. While there are efficiencies gained by running L2TP directly over IP, there are possible side effects as well. For instance, in some circumstances, L2TP over IP may not be as NAT-friendly as L2TP over UDP. Also, control messages transmitted over IP are not protected by Townsley, et al. Standards Track [Page 19]
INTERNET DRAFT L2TP October 2001 a network-layer checksum as they are with UDP. As such, and for easier migration from L2TPv2, this mode over operation is provided. 4.1.2.1 L2TP over UDP Tunnel Header The following tunnel header format is utilized when operating L2TPv3 over UDP: Figure 4.1.2.1: L2TPv3 over UDP Tunnel Header 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|x|x|x|x|x|x|x|x|x|x|x| Ver | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Session ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cookie (optional, maximum 64 bits)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The T bit MUST be set to 0, indicating that this is a data message. The x bits and Reserved field are reserved for future extensions. All reserved values MUST be set to 0 on outgoing messages and ignored on incoming messages. The Ver field MUST be set to 3, indicating an L2TPv3 message. The Session ID and Cookie fields are as defined in Section 4.1. 4.1.2.2 L2TP over UDP Port Selection L2TPv3 utilizes the same UDP port selection method as defined in L2TPv2 [RFC2661]. When negotiating a control connection over UDP, control messages first must be sent as UDP datagrams using the registered UDP port 1701 [RFC1700]. The initiator of an L2TP control connection picks an available source UDP port (which may or may not be 1701), and sends to the desired destination address at port 1701. The recipient picks a free port on its own system (which may or may not be 1701), and sends its reply to the initiator's UDP port and address, setting its own source port to the free port it found. Any subsequent traffic associated with this control connection (either control traffic or data traffic from a session established Townsley, et al. Standards Track [Page 20]
INTERNET DRAFT L2TP October 2001 through this control connection) must use these same UDP ports. This method has some inefficiencies with regard to packet processing. However, it is the most NAT-friendly method since there is only one entry in the NAT table to be kept valid, and the control connection can provide a keepalive to ensure that the NAT entry remains valid. Also, firewalls can be configured to pass all control and data traffic with a single entry rather than separate entries for control and for data. It has been suggested that having the recipient choose an arbitrary source port (as opposed to using the destination port in the packet initiating the control connection, i.e., 1701) may make it more difficult for L2TP to traverse some NAT devices. Implementations should consider the potential implication of this before choosing an arbitrary source port. Any NAT device that can pass TFTP traffic should be able to pass L2TP UDP traffic as they employ similar policies with regard to UDP port selection. 4.1.2.3 UDP Checksum UDP checksums MUST be enabled for control messages and MAY be enabled for data messages. It should be noted, however, that enabling checksums on data packets may significantly increase packet processing burden. 4.1.3 IP Fragmentation Issues IP fragmentation may occur as the L2TP packet travels over the IP substrate. L2TP makes no special efforts defined in this document to optimize this. 4.2 Reliable Delivery of Control Messages L2TP provides a lower level reliable delivery service for all control messages. The Nr and Ns fields of the control message header (see Section 3.2.1) belong to this delivery mechanism. The upper level functions of L2TP are not concerned with retransmission or ordering of control messages. The reliable control messaging mechanism is a sliding window mechanism that provides control message retransmission and congestion control. Each peer maintains separate sequence number state for each control connection. The message sequence number, Ns, begins at 0. Each subsequent message is sent with the next increment of the sequence number. The sequence number is thus a free-running counter represented modulo 65536. The sequence number in the header of a received message is considered less than or equal to the last received number if its value lies in the range of the last received number and the preceding Townsley, et al. Standards Track [Page 21]
INTERNET DRAFT L2TP October 2001 32767 values, inclusive. For example, if the last received sequence number was 15, then messages with sequence numbers 0 through 15, as well as 32784 through 65535, would be considered less than or equal. Such a message would be considered a duplicate of a message already received and ignored from processing. However, in order to ensure that all messages are acknowledged properly (particularly in the case of a lost ZLB ACK message), receipt of duplicate messages MUST be acknowledged by the reliable delivery mechanism. This acknowledgement may either piggybacked on a message in queue or sent explicitly via a ZLB ACK. All control messages take up one slot in the control message sequence number space, except the ZLB acknowledgement. Thus, Ns is not incremented after a ZLB message is sent. The last received message number, Nr, is used to acknowledge messages received by an L2TP peer. It contains the sequence number of the message the peer expects to receive next (e.g. the last Ns of a non- ZLB message received plus 1, modulo 65536). While the Nr in a received ZLB is used to flush messages from the local retransmit queue (see below), the Nr of the next message sent is not updated by the Ns of the ZLB. As a precaution, Nr should be sanity-checked before flushing the retransmit queue. For instance, if the Nr received in a control message is greater than the last Ns sent plus 1 modulo 65536, it is clearly invalid. The reliable delivery mechanism at a receiving peer is responsible for making sure that control messages are delivered in order and without duplication to the upper level. Messages arriving out of order may be queued for in-order delivery when the missing messages are received. Alternatively, they may be discarded, thus requiring a retransmission by the peer. When dropping out of order control packets, Nr MAY be updated before the packet is discarded. Each control connection maintains a queue of control messages to be transmitted to its 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 period of time (a recommended default is 1 second) passes without acknowledgement, the message is retransmitted. The retransmitted message contains the same Ns value, but the Nr value MUST be updated with the sequence number of the next expected message. Each subsequent retransmission of a message MUST employ an exponential backoff interval. Thus, if the first retransmission occurred after 1 second, the next retransmission should occur after 2 seconds has elapsed, then 4 seconds, etc. An implementation MAY Townsley, et al. Standards Track [Page 22]
INTERNET DRAFT L2TP October 2001 place a cap upon the maximum interval between retransmissions. This cap MUST be no less than 8 seconds per retransmission. If no peer response is detected after several retransmissions (a recommended default is 5, but SHOULD be configurable), the control connection and all associated sessions MUST be cleared. When a control connection 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. A sliding window mechanism is used for control message transmission. Consider two peers, A and B. Suppose A specifies a Receive Window Size AVP with a value of N in the SCCRQ or SCCRP message. B is now allowed to have up to N outstanding control messages. Once N messages have been sent, B must wait for an acknowledgment from A that advances the window before sending new control messages. An implementation may support a receive window of only 1 (e.g. by sending out a Receive Window Size AVP with a value of 1), but MUST accept a window of up to 4 from its peer (i.e. have the ability to send 4 messages before backing off). A value of 0 for the Receive Window Size AVP is invalid. When retransmitting control messages, a slow start and congestion avoidance window adjustment procedure SHOULD be utilized. A recommended procedure is described in Appendix A. A peer MUST NOT withhold acknowledgment of messages as a technique for flow controlling control messages. An L2TP implementation is expected to be able to keep up with incoming control messages, possibly responding to some with errors reflecting an inability to honor the requested action. In addition, a peer MUST NOT withhold acknowledgement of messages in order to maintain state in the L2TP state machine. Conversely, the L2TP state machine MUST be capable of maintaining state if a ZLB ACK is received in response to a control message. However, determining when a state should no longer be maintained (e.g. how long to wait in wait-reply state for an ICRP from the peer) before destroying a session or control connection is an issue that is left to each implementation. Appendix B contains examples of control message transmission, acknowledgement, and retransmission. Townsley, et al. Standards Track [Page 23]
INTERNET DRAFT L2TP October 2001 4.3 Tunnel Endpoint Authentication L2TP incorporates a simple, optional, CHAP-like [RFC1994] authentication system for each LCCE during control connection establishment. If an LAC or LNS wishes to authenticate the identity of its peer, a Challenge AVP is included in the SCCRQ or SCCRP message. If a Challenge AVP is received in an SCCRQ or SCCRP, a Challenge Response AVP MUST be sent in the following SCCRP or SCCCN, respectively. If the expected response received from a peer does not match, establishment of the control connection MUST be disallowed. To participate in LCCE authentication, a single shared secret MUST exist between the two LCCEs. This is the same shared secret used for AVP hiding (see Section 5.3). See Section 5.4.3 for details on construction of the Challenge and Response AVPs. 4.4 Keepalive (Hello) A keepalive mechanism is employed by L2TP in order to differentiate control connection outages from extended periods of no control or data activity on a control connection. This is accomplished by injecting Hello control messages (see Section 6.5) after a specified period of time has elapsed since the last data message or control message was received on an L2TP session or control connection, respectively. As for any other control message, if the Hello message is not reliably delivered, the control connection is declared down and is reset. The delivery reset mechanism along with the injection of Hello messages ensures that a connectivity failure between the LCCEs will be detected at both ends of a control connection. The sending of Hello messages and the policy for sending them are left up to the implementation. A peer MUST NOT expect Hello messages at any time or interval. As with all messages sent on the control connection, the receiver will return either a ZLB ACK or an (unrelated) message piggybacking the necessary acknowledgement information. Since a Hello is a control message, and since control messages are reliably sent by the lower level delivery mechanism, this keepalive function operates by causing the reliable delivery of a message. If a media interruption has occurred, the delivery mechanism will be unable to deliver the Hello across and will clean up the control connection. Keepalives for the control connection MAY be implemented by sending a Hello if a period of time (a recommended default is 60 seconds, but SHOULD be configurable) has passed without receiving any message (data or control) from the peer. Townsley, et al. Standards Track [Page 24]
INTERNET DRAFT L2TP October 2001 An LCCE sending Hello messages across multiple control connections between the same LCCE endpoints SHOULD employ a jittered timer mechanism. 4.5 Forwarding Session Frames Once session establishment is complete, L2 frames are received at an LCCE, encapsulated in L2TP (with appropriate attention to framing and L2 dependencies as described in documents for the particular Pseudo Wire Type), and forwarded over the appropriate session. For every outgoing data message, the sender places the identifier specified in the Local Session ID AVP (received from peer during session establishment) in the Session ID field of the L2TP data header. In this manner, session frames are multiplexed and demultiplexed between a given pair of LCCEs. Multiple control connections may exist between a given pair of LCCEs, and multiple sessions may be associated with the same control connection. The peer LCCE receiving the L2TP data packet identifies the session with which the packet is associated by the Session ID in the data packet's header. The LCCE then checks the Cookie field in the data packet against the Cookie value received in the Assigned Cookie AVP during session establishment. Any received data packets that contain invalid Session IDs or associated Cookie values MUST be dropped. Finally, the LCCE either processes the encapsulated session frame locally (i.e. as an LNS) or forwards the frame to a circuit (i.e. as an LAC). 4.6 L2TPv2/v3 Interoperability and Migration L2TPv2 and L2TPv3 environments should be able to coexist while a migration to L2TPv3 is made. Migration issues are discussed for each media type in this section. Most issues apply only to implementations that require both L2TPv2 and L2TPv3 operation. However, even L2TPv3-only implementations must be mindful of these issues in order to interoperate with implementations that support both versions. 4.6.1 L2TPv3 over IP L2TPv3 implementations running strictly over IP with no desire to interoperate with L2TPv2 implementations may safely disregard most migration issues from L2TPv2. All control messages and data messages are sent as described in this document. An L2TP implementation may first attempt to operate in L2TPv3 over IP mode, then fall back to L2TPv2 (over UDP) if L2TPv3 over IP is unavailable. It does so by first sending an L2TPv3-formatted SCCRQ Townsley, et al. Standards Track [Page 25]
INTERNET DRAFT L2TP October 2001 over IP to try to initiate an L2TPv3 control connection. If the SCCRQ elicits no response, the implementation may fall back to L2TPv2 operation, as defined in [RFC2661]. Fallback to L2TPv2 should be seamless and occur automatically. (See Section 4.6.3 for further details.) 4.6.2 L2TPv3 over UDP In order to allow simultaneous operation with L2TPv2, L2TPv3 uses the same UDP port (port 1701) as L2TPv2 and shares the first two octets of header format (via the tunnel header) with L2TPv2. Furthermore, though the control message and data message headers have changed, an LCCE sends an SCCRQ that looks enough like an L2TPv2 SCCRQ to be accepted by both L2TPv2 and L2TPv3 implementations. If the response to the SCCRQ is a properly formatted L2TPv3 message, then operation can continue as described in this document for an L2TPv3 implementation. If the response is a properly formatted L2TPv2 message, then an L2TPv2 mode of operation must be adopted. 4.6.3 Automatic L2TPv2 Fallback When running over UDP, an implementation may detect whether a peer is L2TPv3-capable by sending an L2TPv3-formatted SCCRQ. The SCCRQ is sent with the Ver field set to 2, and any L2TPv3-specific AVPs within the message are sent without setting the M bit on each AVP (so that they may be ignored by an L2TPv2 implementation). Note that, in both L2TPv2 and L2TPv3, the value contained in the space of the control message header utilized by the 32-bit Control Connection ID (16-bit Tunnel ID and 16-bit Session ID in L2TPv2) is always 0 for an SCCRQ, a key feature for this capability. If the peer implementation is an L2TPv3-capable implementation, a control message with Ver set to 3 and corresponding header and message format will be sent in response to the SCCRQ. Operation may then continue as L2TPv3. If a message is received with Ver set to 2, one may assume that the peer implementation is L2TPv2-only and fall back to L2TPv2 mode if local policy and capability permits. The auto-detection mode requires that an L2TPv3-only implementation be liberal in its acceptance of SCCRQ control messages with the Ver field set to 2. Thus, an L2TPv3 over UDP implementation MUST allow receipt of an SCCRQ with Ver field of 2 or Ver field of 3. 5. Control Message Attribute Value Pairs 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 AVP (Attribute Value Townsley, et al. Standards Track [Page 26]
INTERNET DRAFT L2TP October 2001 Pair) in the remainder of this document. 5.1 AVP Format Each AVP is encoded as: 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| rsvd | Length | Vendor ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute Type | Attribute Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ (until Length is reached)... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The first six bits comprise a bit mask that describe the general attributes of the AVP. Two bits are defined in this document; the remaining bits are reserved for future extensions. Reserved bits MUST be set to 0. An AVP received with a reserved bit set to 1 MUST be treated as an unrecognized AVP. Mandatory (M) bit: Controls the behavior required of an implementation that receives an AVP that is unrecognized or malformed. The M bit of a given AVP should only be checked if the AVP is unrecognized or malformed. If the M bit is set on an unrecognized or malformed AVP in a control message associated with a particular session, the session MUST be terminated. If the M bit is set on an unrecognized or malformed AVP within a control message associated with a control connection, the control connection (and all sessions bound to the control connection) MUST be terminated. If the M bit is not set, an unrecognized AVP MUST be ignored. The control message must then continue to be processed as if the AVP had not been present. Hidden (H) bit: Identifies the hiding of data in the Attribute 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 5.3 describes the procedure for performing AVP hiding. Length: Encodes the number of octets (including the Overall Length and bit mask fields) contained in this AVP. The Length may be calculated as 6 + the length of the Attribute Value field in octets. The field itself is 10 bits, permitting a maximum of 1023 octets of data in a single AVP. The minimum Length of an AVP is 6. If the Length is 6, then the Attribute Value field is absent. Vendor ID: The IANA assigned "SMI Network Management Private Townsley, et al. Standards Track [Page 27]
INTERNET DRAFT L2TP October 2001 Enterprise Codes" [RFC1700] value. The value 0, corresponding to IETF adopted attribute values, is used for all AVPs defined within this document. Any vendor wishing to implement its own L2TP extensions can use its own Vendor ID along with private Attribute values, guaranteeing that they will not collide with any other vendor's extensions or future IETF extensions. Note that there are 16 bits allocated for the Vendor ID, thus limiting this feature to the first 65,535 enterprises. Attribute Type: A 2-octet value with a unique interpretation across all AVPs defined under a given Vendor ID. Attribute Value: This is the actual value as indicated by the Vendor ID and Attribute Type. It follows immediately after the Attribute Type field and runs for the remaining octets indicated in the Length (i.e., Length minus 6 octets of header). This field is absent if the Length is 6. 5.2 Mandatory AVPs Receipt of an unrecognized or malformed AVP that has the M bit set is catastrophic to the session or control connection with which it is associated. Thus, the M bit should only be defined for AVPs that are absolutely crucial to proper operation of the session or control connection. Furthermore, in the case in which the LAC or LNS receives an unknown AVP with the M bit set and shuts down the session or control connection accordingly, it is the full responsibility of the peer sending the Mandatory AVP to accept fault for causing a non- interoperable situation. Before defining an AVP with the M bit set, particularly a vendor-specific AVP, be sure that this is the intended consequence. When an adequate alternative exists to use of the M bit, it should be utilized. For example, rather than simply sending an AVP with the M bit set to determine if a specific extension exists, availability may be identified by sending an AVP in a request message and expecting a corresponding AVP in a reply message. Use of the M bit with new AVPs (i.e. those not defined in this document) MUST provide the ability to configure the associated feature off, such that the AVP either is not sent or is sent with the M bit not set. On the other side, the recipient of a control message should only check the M bit of an AVP when the AVP is determined to be unrecognized or malformed. The M bit should not be checked for a recognized and well-formatted AVP. This rule prevents the possibility of a valid AVP resulting in a session or control Townsley, et al. Standards Track [Page 28]
INTERNET DRAFT L2TP October 2001 connection teardown, simply because its M bit was set to a value that was unexpected by the receiving LCCE. 5.3 Hiding of AVP Attribute Values The H bit in the header of each AVP 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 IDs. The H bit MUST only be set if (1) a shared secret exists between the LCCEs and (2) LCCE authentication has completed. The shared secret is the same secret that is used for LCCE authentication (see Section 4.3). Hidden values MUST NOT be unhidden until after LCCE authentication has completed successfully (perhaps requiring the hidden value to be stored until after receipt of additional setup messages). To do otherwise runs the risk of AVP data being utilized without verifying the integrity of the shared secret. If the H bit is set in any AVP(s) in a given control message, a Random Vector AVP must also be present in the message and MUST precede the first AVP having an H bit of 1. Hiding an AVP value is done in several steps. The first step is to take the length and value fields of the original (cleartext) AVP and encode them into a Hidden AVP Subformat as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length of Original Value | Original Attribute Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Padding... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Length of Original Attribute Value: This is length of the Original Attribute Value to be obscured in octets. This is necessary to determine the original length of the Attribute Value that is lost when the additional Padding is added. Original Attribute Value: Attribute Value that is to be obscured. Padding: Random additional octets used to obscure length of the Attribute Value that is being hidden. To mask the size of the data being hidden, the resulting subformat MAY be padded as shown above. Padding does NOT alter the value placed in the Length of Original Attribute Value field, but does alter the length of the resultant AVP that is being created. For Townsley, et al. Standards Track [Page 29]
INTERNET DRAFT L2TP October 2001 example, if an Attribute Value to be hidden is 4 octets in length, the unhidden AVP length would be 10 octets (6 + Attribute Value length). After hiding, the length of the AVP will become 6 + Attribute Value length + size of the Length of Original Attribute Value field + Padding. Thus, if Padding is 12 octets, the AVP length will be 6 + 4 + 2 + 12 = 24 octets. Next, an MD5 hash is performed (in network byte order) on the concatenation of the following: + the 2-octet Attribute number of the AVP + the shared secret + 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 may 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 Hidden AVP Subformat and placed in the Attribute Value field of the Hidden AVP. If the Hidden AVP Subformat is less than 16 octets, the Subformat is transformed as if the Attribute Value field had been padded to 16 octets before the XOR. Only the actual octets present in the Subformat are modified, and the length of the AVP is not altered. If the Subformat 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 Subformat and placed in the corresponding octets of the Value field of the Hidden AVP. If necessary, this operation is repeated, with the shared secret used along with each XOR result to generate the next hash to XOR the next segment of the value with. The hiding method was adapted from [RFC2138], which was taken from the "Mixing in the Plaintext" section in the book "Network Security" by Kaufman, Perlman and Speciner [KPS]. A detailed explanation of the method follows: Call the shared secret S, the Random Vector RV, and the Attribute Value AV. Break the value field into 16-octet chunks p1, p2, etc., with the last one padded at the end with random data to a 16-octet Townsley, et al. Standards Track [Page 30]
INTERNET DRAFT L2TP October 2001 boundary. Call the ciphertext blocks c(1), c(2), etc. We will also define intermediate values b1, b2, etc. b1 = MD5(AV + S + RV) c(1) = p1 xor b1 b2 = MD5(S + c(1)) c(2) = p2 xor b2 . . . . . . bi = MD5(S + c(i-1)) c(i) = pi xor bi The String will contain c(1)+c(2)+...+c(i), where + denotes concatenation. 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. 5.4 AVP Summary The following sections contain a list of all L2TP AVPs defined in this document. Following the name of the AVP is a list indicating the message types that utilize each AVP. After each AVP title follows a short description of the purpose of the AVP, a detail (including a graphic) of the format for the Attribute Value, and any additional information needed for proper use of the AVP. 5.4.1 AVPs Applicable to All Control Messages Message Type (All Messages) The Message Type AVP, Attribute Type 0, identifies the control message herein and defines the context in which the exact meaning of the following AVPs will be determined. The Attribute Value field for this AVP has the following format: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Message Type is a 2-octet unsigned integer. The Message Type AVP MUST be the first AVP in a message, immediately following the control message header (defined in Section 3.2.1). See Townsley, et al. Standards Track [Page 31]
INTERNET DRAFT L2TP October 2001 Section 3.1 for the list of defined control message types and their identifiers. The Mandatory (M) bit within the Message Type AVP has special meaning. Rather than an indication as to whether the AVP itself should be ignored if not recognized or malformed, it is an indication as to whether the control message itself should be ignored. If the M bit is set within the Message Type AVP and the Message Type is unknown to the implementation, the control connection MUST be cleared. If the M bit is not set, then the implementation may ignore an unknown message type. The M bit MUST be set to 1 for all message types defined in this document. This AVP may not be hidden (the H bit MUST be 0). The Length of this AVP is 8. A vendor-specific control message may be defined by setting the Vendor ID of the Message Type AVP to a value other than the IETF Vendor ID of 0 (see Section 5.1). The Message Type AVP MUST still be the first AVP in the control message. Random Vector (All Messages) The Random Vector AVP, Attribute Type 36, is used to enable the hiding of the Attribute Value of arbitrary AVPs. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Random Octet String... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Random Octet String may be of arbitrary length, although a random vector of at least 16 octets is recommended. The string contains the random vector for use in computing the MD5 hash to retrieve or hide the Attribute Value of a hidden AVP (see Section 5.3). More than one Random Vector AVP may appear in a message, in which case a hidden AVP uses the Random Vector AVP most closely preceding it. This AVP MUST precede the first AVP with the H bit set. The M bit for this AVP SHOULD be set to 1. This AVP MUST NOT be hidden (the H bit MUST be 0). The Length of this AVP is 6 plus the length of the Random Octet String. Townsley, et al. Standards Track [Page 32]
INTERNET DRAFT L2TP October 2001 5.4.2 Result and Error Codes Result Code (CDN, StopCCN) The Result Code AVP, Attribute Type 1, indicates the reason for terminating the control channel or session. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Result Code | Error Code (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Message (optional)... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Result Code is a 2-octet unsigned integer. The optional Error Code is a 2-octet unsigned integer. An optional Error Message can follow the Error Code field. Presence of the Error Code and Message is indicated by the AVP Length field. The Error Message contains an arbitrary string providing further (human-readable) text associated with the condition. Human-readable text in all error messages MUST be provided in the UTF-8 charset using the Default Language [RFC2277]. This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this AVP SHOULD be set to 1. The Length is 8 if there is no Error Code or Message, 10 if there is an Error Code and no Error Message, or 10 + the length of the Error Message if there is an Error Code and Message. Defined Result Code values for the StopCCN message are as follows: 0 - Reserved 1 - General request to clear control connection 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, Error Code indicates highest version supported 6 - Requester is being shut down 7 - Finite State Machine error Defined Result Code values for the CDN message are as follows: 0 - Reserved 1 - Call disconnected due to loss of carrier or circuit disconnect Townsley, et al. Standards Track [Page 33]
INTERNET DRAFT L2TP October 2001 2 - Call disconnected for the reason indicated in Error Code 3 - Call disconnected for administrative reasons 4 - Call failed due to lack of appropriate facilities being available (temporary condition) 5 - Call failed due to lack of appropriate facilities being available (permanent condition) 6 - Invalid destination 7 - Call failed due to no carrier detected 8 - Call failed due to detection of a busy signal 9 - Call failed due to lack of a dial tone 10 - Call was not established within time allotted 11 - Call was connected but no appropriate framing was detected TBA - Call was not established due to losing tie breaker The Error Codes defined below pertain to types of errors that 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 as follows: 0 - No general error 1 - No control connection exists yet for this pair of LCCEs 2 - Length is wrong 3 - One of the field values was out of range 4 - Insufficient resources to handle this operation now 5 - Invalid Session ID 6 - A generic vendor-specific error occurred 7 - Try another. If initiator is aware of other possible responder destinations, it should try one of them. This can be used to guide an LAC or LNS based on policy. 8 - The session or control connection was shutdown due to receipt of an unknown AVP with the M bit set (see Section 5.2). The Error Message SHOULD contain the attribute of the offending AVP in (human-readable) text form. 9 - Try another directed. If an LAC or LNS is aware of other possible destinations, it should inform the initiator of the control connection or session. The Error Message MUST contain a comma-separated list of addresses from which the initiator may choose. If the L2TP data channel runs over IPv4, then this would be a comma-separated list of IP addresses in the canonical dotted-decimal format (i.e. "10.0.0.1, 10.0.0.2, 10.0.0.3") in the UTF-8 charset using the Default Language [RFC2277]. If there are no servers for the LAC or LNS to suggest, then Error Code 7 should be used. The delimiter between addresses MUST be precisely a single comma and a single space. Townsley, et al. Standards Track [Page 34]
INTERNET DRAFT L2TP October 2001 When a General Error Code of 6 is used, additional information about the error SHOULD be included in the Error Message field. Furthermore, a vendor-specific AVP MAY be sent to indicate the problem more precisely. 5.4.3 Control Connection Management AVPs Protocol Version (SCCRP, SCCRQ) The Protocol Version AVP, Attribute Type 2, indicates the L2TP protocol version of the sender. The Attribute Value field for this AVP has the following format: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ver | Rev | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Ver field is a 1-octet unsigned integer containing the value 1. Rev field is a 1-octet unsigned integer containing 0. This pertains to L2TP version 1, revision 0. Note this is not the same version number that is included in the header of each message. This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this AVP SHOULD be set to 1. The Length of this AVP is 8. Tie Breaker (SCCRQ) The Tie Breaker AVP, Attribute Type 5, indicates that the sender desires a single control connection to exist between the given LCCE pair. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tie Breaker Value... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...(64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Tie Breaker Value is an 8-octet value that is used to choose a single control connection when two LCCEs request a control connection concurrently. The recipient of a SCCRQ must check to see if a SCCRQ has been sent to the peer, and if so, must compare its Tie Breaker Townsley, et al. Standards Track [Page 35]
INTERNET DRAFT L2TP October 2001 value with the received one. The lower value "wins", and the "loser" MUST silently discard its control connection. In the case in which a tie breaker is present on both sides and the value is equal, both sides MUST discard their control connections and restart control connection negotiation. If a tie breaker is received and an outstanding SCCRQ has no tie breaker value, the initiator that included the Tie Breaker AVP "wins". If neither side issues a tie breaker, then two separate control connections are opened. In the case of a tie, the "winner" of the tie is declared the "dominant LCCE". Session-level ties, as detected by End Identifier AVP, are always won by the dominant LCCE. If there is no tie, the dominant LCCE is always the initiator of the control connection (the sender of the SCCRQ). Tie breaker values MUST be random values. This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this AVP SHOULD be set to 0. The Length of this AVP is 14. Firmware Revision (SCCRP, SCCRQ) The Firmware Revision AVP, Attribute Type 6, indicates the firmware revision of the issuing device. The Attribute Value field for this AVP has the following format: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Firmware Revision | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Firmware Revision is a 2-octet unsigned integer encoded in a vendor-specific format. For devices that do not have a firmware revision (e.g. general purpose computers running L2TP software modules), the revision of the L2TP software module may be reported instead. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 0. The Length (before hiding) is 8. Host Name (SCCRP, SCCRQ) The Host Name AVP, Attribute Type 7, indicates the name of the Townsley, et al. Standards Track [Page 36]
INTERNET DRAFT L2TP October 2001 issuing LAC or LNS. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Host Name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Host Name is of arbitrary length, but MUST be at least 1 octet. This name should be as broadly unique as possible; for hosts participating in DNS [RFC1034], a hostname with fully qualified domain would be appropriate. The Host Name MAY be used to identify LCCE configuration, including the shared secret for LCCE authentication (if enabled) and any other options defined for the control connection. This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this AVP SHOULD be set to 1. The Length of this AVP is 6 plus the length of the Host Name. Vendor Name (SCCRP, SCCRQ) The Vendor Name AVP, Attribute Type 8, contains a vendor-specific (possibly human-readable) string describing the type of LAC or LNS being used. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor Name... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Vendor Name is the indicated number of octets representing the vendor string. Human-readable text for this AVP MUST be provided in the UTF-8 charset using the Default Language [RFC2277]. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 6 plus the length of the Vendor Name. Assigned Control Connection ID (SCCRP, SCCRQ, StopCCN) The Assigned Control Connection ID AVP, Attribute Type TBA, encodes Townsley, et al. Standards Track [Page 37]
INTERNET DRAFT L2TP October 2001 the ID being assigned to this control connection by the sender. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Control Connection ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Control Connection ID is a 4-octet non-zero unsigned integer. The Assigned Control Connection ID AVP establishes the identifier used to multiplex and demultiplex multiple control connections between a pair of LCCEs. Once the Assigned Control Connection ID AVP has been received by an LCCE, the Control Connection ID specified in the AVP MUST be included in the Control Connection ID field of all control packets sent to the peer for the lifetime of the control connection. Before the Assigned Control Connection ID AVP is received from a peer, all control messages MUST be sent to that peer with a Control Connection ID value of 0 in the header. Because a Control Connection ID value of 0 is used in this special manner, the zero value MUST NOT be sent as an Assigned Control Connection ID value. Under certain circumstances, an LCCE may need to send a StopCCN to a peer without having yet received an Assigned Control Connection ID AVP from the peer (i.e. SCCRQ sent, no SCCRP received yet). In this case, the Assigned Control Connection ID AVP that had been sent to the peer (i.e. in the SCCRQ) MUST be sent as the Assigned Control Connection ID AVP in the StopCCN. This policy allows the peer to try to identify the appropriate control connection via a reverse lookup. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1 (see Section 4.6.3). The Length (before hiding) of this AVP is 10. Receive Window Size (SCCRP, SCCRQ) The Receive Window Size AVP, Attribute Type 10, specifies the receive window size being offered to the remote peer. The Attribute Value field for this AVP has the following format: Townsley, et al. Standards Track [Page 38]
INTERNET DRAFT L2TP October 2001 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Window Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Window Size is a 2-octet unsigned integer. If absent, the peer must assume a Window Size of 4 for its transmit window. The remote peer may send the specified number of control messages before it must wait for an acknowledgment. This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this AVP SHOULD be set to 1. The Length of this AVP is 8. Challenge (SCCRP, SCCRQ) The Challenge AVP, Attribute Type 11, indicates that the issuing peer wishes to authenticate the LCCE using a CHAP-style authentication mechanism. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Challenge... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Challenge is one or more octets of random data. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 6 plus the length of the Challenge. Challenge Response (SCCCN, SCCRP) The Response AVP, Attribute Type 13, provides a response to a challenge received. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Response... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Townsley, et al. Standards Track [Page 39]
INTERNET DRAFT L2TP October 2001 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...(16 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Response is a 16-octet value reflecting the CHAP-style [RFC1994] response to the challenge. This AVP MUST be present in an SCCRP or SCCCN if a challenge was received in the preceding SCCRQ or SCCRP, respectively. For purposes of the ID value in the CHAP response calculation, the value of the Message Type AVP for this message is used (e.g. 2 for an SCCRP, 3 for an SCCCN). This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 22. Pseudo Wire Capabilities List (SCCRP, SCCRQ) The Pseudo Wire Capabilities List AVP, Attribute Type TBA, indicates the L2 payload types that will be accepted by the sender. The specific payload type of a given session is identified by the Pseudo Wire Type AVP. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pseudo Wire Type 0 | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Pseudo Wire Type N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Defined Pseudo Wire Types that may be included in the Pseudo Wire Capabilities List are as follows: 0 - PPP This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1 (see Section 4.6.3). The Length (before hiding) of this AVP is 8 octets with one Pseudo Wire Type specified, plus 2 octets for each additional Pseudo Wire Type. Townsley, et al. Standards Track [Page 40]
INTERNET DRAFT L2TP October 2001 5.4.4 Call Management AVPs Local Session ID (CDN, ICRP, ICRQ, OCRP, OCRQ) The Local Session ID AVP (analogous to the Assigned Session ID in L2TPv2), Attribute Type TBA, encodes the ID being assigned to this session by the sender. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Local Session ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Local Session ID is a 4-octet non-zero unsigned integer. The Local Session ID AVP establishes the identifier used to multiplex and demultiplex both data and control connection traffic for a given session between two LCCEs. The local session lookup mechanism chooses a free value that it expects to see in all received data messages for this session. This value is advertised to the peer via this AVP. The receiving LCCE MUST use this value as the Session ID for all data messages sent to this peer. In addition, the receiving LCCE MUST echo this value back as the Remote Session ID AVP in all session-related control messages, allowing efficient session context lookup when processing these control messages. See Section 4.1 for additional information about the Session ID. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP MUST be 1 for implementations that support only L2TPv3 (see Section 4.6 for L2TPv2 migration issues). The Length (before hiding) of this AVP is 10. Remote Session ID (CDN, ICRP, ICRQ, ICCN, OCRP, OCRQ, OCCN, WEN) The Remote Session ID AVP, Attribute Type TBA, encodes the ID that was assigned to this session by the peer. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Session ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Townsley, et al. Standards Track [Page 41]
INTERNET DRAFT L2TP October 2001 The Remote Session ID is a 4-octet non-zero unsigned integer. The Remote Session ID AVP echoes the session identifier advertised by our peer via the Local Session ID AVP. Upon receipt, it is used to determine the session context for a session-specific control message. For non-session-oriented messages, the Remote Session ID AVP MUST NOT be present. For all session-oriented messages, it MUST be present. If a Local Session ID AVP has not yet been received, this AVP MUST be sent with a value of zero to indicate that the Session ID is currently unknown. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP MUST be 1 for implementations that support only L2TPv3 (see Section 4.6 for L2TPv2 migration issues). The Length (before hiding) of this AVP is 10. Assigned Cookie (ICRP, ICRQ, OCRP, OCRQ) The Assigned Cookie AVP, Attribute Type TBA, encodes the Cookie value being assigned to this session by the sender. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Assigned Cookie (32 or 64 bits)... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Assigned Cookie is a 4-octet or 8-octet random value. The Assigned Cookie AVP contains the value used to check the association of a received data message with the session identified by the Session ID. All data messages sent to a peer MUST use the Assigned Cookie sent by the peer in this AVP. The value's length (0, 32, or 64 bits) is obtained by the Length of the AVP. A Cookie value of zero length serves as positive acknowledgement that the Cookie field should not be present in any data packets sent to this LCCE. The Assigned Cookie AVP MAY not be sent, which has the same effect as sending the AVP to designate a Cookie value of zero length. See Section 4.1 for additional information about the Assigned Cookie. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP MUST be 1 for implementations that support only L2TPv3 (see Section 4.6 for L2TPv2 migration issues). The Length (before hiding) of this AVP may be 6, 10, or 14 octets. Townsley, et al. Standards Track [Page 42]
INTERNET DRAFT L2TP October 2001 Call Serial Number (ICRQ, OCRQ) The Call Serial Number AVP, Attribute Type 15, encodes an identifier assigned by the LAC or LNS to this call. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Call Serial Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Call Serial Number is a 32-bit value. The Call Serial Number is intended to be an easy reference for administrators on both ends of a control connection 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 LNSs and LACs. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 10. End Identifier AVP (ICRQ, OCRQ) The End Identifier AVP, Attribute Type TBA, encodes an identifier assigned by the LAC or LNS to this call, used to detect ties in session establishment for the same circuit. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | End Identifier ... (arbitrary number of octets) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The End Identifier contains interface, circuit, and other information, depending on the circuit that is being tunneled. The field may be a simple ASCII string. For example, a source interface serial 1/1 and DLCI 100, and a destination interface serial 1/1 with DLCI 200, could be represented as "serial 1/1 DLCI 100, serial 1/1 DLCI 200". The format of the information contained in this AVP should be agreed Townsley, et al. Standards Track [Page 43]
INTERNET DRAFT L2TP October 2001 on by the administrators at the two LCCEs. Specification of this format is outside the scope of this document. A session-level tie is detected if an LCCE receives an ICRQ or OCRQ with an End Identifier AVP whose value matches the End Identifier AVP that was just sent in an outgoing ICRQ or OCRQ to the same peer. If the two End Identifier values match, an LCCE recognizes that a tie exists (i.e. both LCCEs are attempting to establish sessions for the same circuit). The tie is broken by the dominant LCCE. The "losing" LCCE must send a CDN to its peer to cancel the ICRQ or OCRQ that it had sent to the peer. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 6 plus the length of the End Identifier value. Minimum BPS (OCRQ) The Minimum BPS AVP, Attribute Type 16, encodes the lowest acceptable line speed for this call. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Minimum BPS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Minimum BPS is a 32-bit value indicates the speed in bits per second. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 10. Maximum BPS (OCRQ) The Maximum BPS AVP, Attribute Type 17, encodes the highest acceptable line speed for this call. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum BPS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Townsley, et al. Standards Track [Page 44]
INTERNET DRAFT L2TP October 2001 The Maximum BPS is a 32-bit value indicates the speed in bits per second. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 10. Pseudo Wire Type (ICRQ, OCRQ) The Pseudo Wire Type AVP, Attribute Type TBA, indicates the L2 payload type for the requested call. The Attribute Value field for this AVP has the following format: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Pseudo Wire Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ See the definition for the Pseudo Wire Capabilities List AVP for a list of defined Pseudo Wire Type values. A peer MUST NOT request an incoming or outgoing call with a Pseudo Wire Type AVP specifying a value not advertised in the Pseudo Wire Capabilities List AVP it received during control connection establishment. Attempts to do so will result in the call being rejected. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP MUST be 1 for implementations that support only L2TPv3 (see Section 4.6 for L2TPv2 migration issues). The Length (before hiding) of this AVP is 8. Tx Connect Speed (ICCN, OCCN) The Tx Connect Speed BPS AVP, Attribute Type 24, encodes the speed of the facility chosen for the connection attempt. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (H) | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Tx Connect Speed BPS is a 4-octet value indicating the speed in Townsley, et al. Standards Track [Page 45]
INTERNET DRAFT L2TP October 2001 bits per second. A value of zero indicates that the speed is indeterminable or that there is no physical point-to-point link. When the optional Rx Connect Speed AVP is present, the value in this AVP represents the transmit connect speed from the perspective of the LAC (e.g. data flowing from the LAC to the remote system). When the optional Rx Connect Speed AVP is NOT present, the connection speed between the remote system and LAC is assumed to be symmetric and is represented by the single value in this AVP. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 10. Rx Connect Speed (ICCN, OCCN) The Rx Connect Speed AVP, Attribute Type 38, represents the speed of the connection from the perspective of the LAC (e.g. data flowing from the remote system to the LAC). The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BPS (H) | BPS (L) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ BPS is a 4-octet value indicating the speed in bits per second. A value of zero indicates that the speed is indeterminable or that there is no physical point-to-point link. Presence of this AVP implies that the connection speed may be asymmetric with respect to the transmit connect speed given in the Tx Connect Speed AVP. This AVP may be hidden (the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 10. Physical Channel ID (ICRQ, OCRP) The Physical Channel ID AVP, Attribute Type 25, encodes the vendor- specific physical channel number used for a call. The Attribute Value field for this AVP has the following format: Townsley, et al. Standards Track [Page 46]
INTERNET DRAFT L2TP October 2001 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Physical Channel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Physical Channel ID is a 4-octet value intended to be used for logging purposes only. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 10. Private Group ID (ICCN) The Private Group ID AVP, Attribute Type 37, is used by the LAC to indicate that this call is to be associated with a particular customer group. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Private Group ID ... (arbitrary number of octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Private Group ID is a string of octets of arbitrary length. The LNS MAY treat the session as well as network traffic through this session in a special manner determined by the peer. For example, if the LNS is individually connected to several private networks using unregistered addresses, this AVP may be included by the LAC to indicate that a given call should be associated with one of the private networks. The Private Group ID is a string corresponding to a table in the LNS that defines the particular characteristics of the selected group. A LAC MAY determine the Private Group ID from a RADIUS response, local configuration, or some other source. This AVP may be hidden (the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to 0. The Length (before hiding) of this AVP is 6 plus the length of the Private Group ID. Townsley, et al. Standards Track [Page 47]
INTERNET DRAFT L2TP October 2001 5.4.5 Call Status AVPs Circuit Errors (WEN) The Circuit Errors AVP, Attribute Type 34, conveys circuit error information to the peer. The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hardware Overruns | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Buffer Overruns | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timeout Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Alignment Errors | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The following fields are defined: Reserved: 2 octets of Reserved data is present (providing longword alignment within the AVP of the following values). Reserved data MUST be zero on sending and ignored upon receipt. Hardware Overruns: Number of receive buffer overruns since call was established. Buffer Overruns: Number of buffer overruns detected since call was established. Timeout Errors: Number of timeouts since call was established. Alignment Errors: Number of alignment errors since call was established. This AVP may be hidden (the H bit may be 0 or 1). The M bit for this AVP SHOULD be set to 1. The Length (before hiding) of this AVP is 32. 6. Control Connection Protocol Specification The following control messages are used to establish, maintain, and tear down L2TP control connections. All data are sent in network order (high order octets first). Any "reserved" or "empty" fields MUST be sent as 0 values to allow for protocol extensibility. The exchanges in which these messages are involved are outlined in Townsley, et al. Standards Track [Page 48]
INTERNET DRAFT L2TP October 2001 Section 3.3. 6.1 Start-Control-Connection-Request (SCCRQ) Start-Control-Connection-Request (SCCRQ) is a control message used to initiate a control connection between two LCCEs. It is sent by either the LAC or the LNS to begin the control connection establishment process. The following AVPs MUST be present in the SCCRQ: Message Type AVP Protocol Version Host Name Assigned Control Connection ID Pseudo Wire Capabilities List The following AVPs MAY be present in the SCCRQ: Receive Window Size Challenge Tie Breaker Firmware Revision Vendor Name 6.2 Start-Control-Connection-Reply (SCCRP) Start-Control-Connection-Reply (SCCRP) is a control message sent in reply to a received SCCRQ message. The SCCRP is used to indicate that the SCCRQ was accepted and establishment of the control connection should continue. The following AVPs MUST be present in the SCCRP: Message Type Protocol Version Host Name Assigned Control Connection ID Pseudo Wire Capabilities List The following AVPs MAY be present in the SCCRP: Firmware Revision Vendor Name Receive Window Size Challenge Challenge Response Townsley, et al. Standards Track [Page 49]
INTERNET DRAFT L2TP October 2001 6.3 Start-Control-Connection-Connected (SCCCN) Start-Control-Connection-Connected (SCCCN) is a control message sent in reply to an SCCRP. The SCCCN completes the control connection establishment process. The following AVP MUST be present in the SCCCN: Message Type The following AVP MAY be present in the SCCCN: Challenge Response 6.4 Stop-Control-Connection-Notification (StopCCN) Stop-Control-Connection-Notification (StopCCN) is a control message sent by either LCCE to inform its peer that the control connection is being shut down and that the control connection should be closed. In addition, all active sessions are implicitly cleared (without sending any explicit session control messages). The reason for issuing this request is indicated in the Result Code AVP. There is no explicit reply to the message, only the implicit ACK that is received by the reliable control message delivery layer. The following AVPs MUST be present in the StopCCN: Message Type Result Code Additionally, the Assigned Control Connection ID AVP MUST be present in the StopCCN if it has been sent in a previous message (see Section 5.4.3). 6.5 Hello (HELLO) The Hello (HELLO) message is an L2TP control message sent by either peer of a control connection. This control message is used as a "keepalive" for the control connection. See Section 4.2 for a description of the keepalive mechanism. HELLO messages are global to the control connection. The Session ID in a HELLO message MUST be 0. The following AVP MUST be present in the HELLO: Message Type Townsley, et al. Standards Track [Page 50]
INTERNET DRAFT L2TP October 2001 6.6 Incoming-Call-Request (ICRQ) Incoming-Call-Request (ICRQ) is a control message sent by an LCCE to a peer when an incoming call is detected (although the ICRQ may also be sent as a result of a local event). It is the first in a three- message exchange used for establishing a session via an L2TP control connection. The ICRQ is used to indicate that a session is to be established between an LCCE and a peer. The sender of an ICRQ provides the peer with parameter information for the session. However, the sender makes no demands about how the session is terminated at the peer (i.e. whether the L2 traffic is processed locally, forwarded, etc.). The following AVPs MUST be present in the ICRQ: Message Type Local Session ID Remote Session ID Call Serial Number Pseudo Wire Type The following AVP MAY be present in the ICRQ: Assigned Cookie End Identifier Physical Channel ID 6.7 Incoming-Call-Reply (ICRP) Incoming-Call-Reply (ICRP) is a control message sent by an LCCE in response to an ICRQ. It is the second in the three-message exchange used for establishing sessions within an L2TP control connection. The ICRP is used to indicate that the ICRQ was successful and that the peer should establish (e.g. answer) the incoming call if it has not already done so. It also allows the sender to indicate specific parameters about the L2TP session. The following AVPs MUST be present in the ICRP: Message Type Local Session ID Remote Session ID The following AVP MAY be present in the ICRP: Assigned Cookie Townsley, et al. Standards Track [Page 51]
INTERNET DRAFT L2TP October 2001 End Identifier 6.8 Incoming-Call-Connected (ICCN) Incoming-Call-Connected (ICCN) is a control message sent by the LCCE who originally sent an ICRQ, upon receiving an ICRP from its peer. It is the final message in the three-message exchange used for establishing sessions within an L2TP control connection. The ICCN is used to indicate that the ICRP was accepted, that the call has been established, and that the L2TP session should move to the established state. It also allows the sender to indicate specific parameters about the established call (parameters that may not have been available at the time the ICRQ is issued). The following AVPs MUST be present in the ICCN: Message Type Remote Session ID Tx Connect Speed The following AVPs MAY be present in the ICCN: Private Group ID Rx Connect Speed 6.9 Outgoing-Call-Request (OCRQ) Outgoing-Call-Request (OCRQ) is a control message sent by an LCCE to an LAC to indicate that an outbound call at the LAC is to be established based on specific destination information sent in this message. It is the first in a three-message exchange used for establishing a session and placing a call on behalf of the initiating LCCE. Note that a call may be any L2 connection requiring well-known destination information to be sent from an LCCE to an LAC. This could be a dialup connection to the PSTN, an SVC connection, the IP address of another LCCE, or any other destination dictated by the sender of this message. The following AVPs MUST be present in the OCRQ: Message Type Local Session ID Remote Session ID Call Serial Number Minimum BPS Townsley, et al. Standards Track [Page 52]
INTERNET DRAFT L2TP October 2001 Maximum BPS Pseudo Wire The following AVPs MAY be present in the OCRQ: Assigned Cookie End Identifier 6.10 Outgoing-Call-Reply (OCRP) Outgoing-Call-Reply (OCRP) is a control message sent by an LAC to an LCCE in response to an OCRQ. It is the second in a three-message exchange used for establishing a session within an L2TP control connection. OCRP is used to indicate that the LAC has been able to attempt the outbound call. The message returns any relevant parameters regarding the call attempt. Data MUST not be forwarded until the OCCN is received indicating that the call has been placed. The following AVPs MUST be present in the OCRP: Message Type Local Session ID Remote Session ID The following AVPs MAY be present in the OCRP: Assigned Cookie End Identifier Physical Channel ID 6.11 Outgoing-Call-Connected (OCCN) Outgoing-Call-Connected (OCCN) is a control message sent by an LAC to the an LCCE following the OCRP and after the outgoing call has been completed. It is the final message in a three-message exchange used for establishing a session within an L2TP control connection. OCCN is used to indicate that the result of a requested outgoing call was successful. It also provides information to the LCCE who requested the call about the particular parameters obtained after the call was established. The following AVPs MUST be present in the OCCN: Message Type Remote Session ID Townsley, et al. Standards Track [Page 53]
INTERNET DRAFT L2TP October 2001 Tx Connect Speed The following AVPs MAY be present in the OCCN: Rx Connect Speed Sequencing Required 6.12 Call-Disconnect-Notify (CDN) The Call-Disconnect-Notify (CDN) is a control message sent by an LCCE to request disconnection of a specific session. Its purpose is to inform the peer of the disconnection and the reason for the disconnection. The peer MUST clean up any resources, and does not send back any indication of success or failure for such cleanup. The following AVPs MUST be present in the CDN: Message Type Remote Session ID Result Code Additionally, the Local Session ID AVP MUST be present in the CDN if it has been sent in a previous message (see Section 5.4.4). The following AVPs MAY be present in the CDN: Q.931 Cause Code 6.13 WAN-Error-Notify (WEN) The WAN-Error-Notify (WEN) is a control message sent from an LAC to an LNS to indicate WAN error conditions. 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. The following AVPs MUST be present in the WEN: Message Type Circuit Errors Remote Session ID 7. Control Connection State Machines The state tables defined in this section govern the exchange of control messages defined in Section 6. Tables are defined for incoming call placement and outgoing call placement, as well as for initiation of the control connection itself. The state tables do not Townsley, et al. Standards Track [Page 54]
INTERNET DRAFT L2TP October 2001 encode timeout and retransmission behavior, as this is handled in the underlying reliable control message delivery mechanism (see Section 4.2). 7.1 Malformed Control Messages Receipt of an invalid or unrecoverable malformed control message SHOULD be logged appropriately and the control connection cleared to ensure recovery to a known state. The control connection may then be restarted by the initiator. An invalid control message is defined as (1) a message that contains a Message Type marked as mandatory (see Section 5.4.1) but that is unknown to the implementation, or (2) a control message that is received in the wrong state. Examples of malformed control messages include (1) a message that has an invalid value in its header, (2) a message that contains an AVP that is formatted incorrectly or whose value is out of range, and (3) a message that is missing a required AVP. A control message with a malformed header MUST be discarded. If a malformed AVP is received with the M bit set, the session or control connection MUST be terminated with a proper Result or Error Code sent. A malformed yet non-mandatory (M bit is not set) AVP within a control message should be handled like an unrecognized non-mandatory AVP. That is, the AVP MUST be ignored (with the exception of logging a local error message), and the message MUST be accepted. This policy MUST NOT be considered a license to send malformed AVPs, but rather, a guide towards how to handle an improperly formatted message if one is received. It is impossible to list all potential malformations of a given message and give advice for each. That said, one example of a recoverable, malformed AVP might be if the Rx Connect Speed AVP, attribute 38, is received with a length of 8 rather than 10, and the BPS given in 2 octets rather than 4. Since the Rx Connect Speed is non-mandatory, this condition should not be considered catastrophic. As such, the control message should be accepted as if the AVP had not been received (with the exception of a local error message being logged). In several cases in the following tables, a protocol message is sent, and then a "clean up" occurs. Note that, regardless of the initiator of the control connection destruction, the reliable delivery mechanism must be allowed to run (see Section 4.2) before destroying the control connection. This permits the control connection management messages to be reliably delivered to the peer. Townsley, et al. Standards Track [Page 55]
INTERNET DRAFT L2TP October 2001 Appendix B.1 contains an example of lock-step control connection establishment. 7.2 Timing Considerations Due to the real-time nature of L2 circuit signaling, an LCCE should be implemented using a multi-threaded architecture 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. 7.3 Control Connection States The L2TP control connection protocol is not distinguishable between the two LCCEs but is distinguishable between the originator and receiver. The originating peer is the one that first initiates establishment of the control connection. (In a tie breaker situation, this is the winner of the tie.) Since either the LAC or the LNS can be the originator, a collision can occur. See the Tie Breaker AVP in Section 5.4.3 for a description of this and its resolution. State Event Action New State ----- ----- ------ --------- idle Local open Send SCCRQ wait-ctl-reply request idle Receive SCCRQ, Send SCCRP wait-ctl-conn acceptable idle Receive SCCRQ, Send StopCCN, idle not acceptable clean up idle Receive SCCRP Send StopCCN, idle clean up idle Receive SCCCN Clean up idle wait-ctl-reply Receive SCCRP, Send SCCCN, established acceptable send control-conn open event to waiting sessions wait-ctl-reply Receive SCCRP, Send StopCCN, idle not acceptable clean up wait-ctl-reply Receive SCCRQ, Clean up, idle lose tie breaker re-queue SCCRQ for idle state Townsley, et al. Standards Track [Page 56]
INTERNET DRAFT L2TP October 2001 wait-ctl-reply Receive SCCCN Send StopCCN, idle clean up wait-ctl-conn Receive SCCCN, Send control-conn established acceptable open event to waiting sessions wait-ctl-conn Receive SCCCN, Send StopCCN, idle not acceptable clean up wait-ctl-conn Receive SCCRP, Send StopCCN, idle SCCRQ clean up established Local open Send control-conn established request open event to (new call) waiting sessions established Administrative Send StopCCN, idle control-conn clean up close event established Receive SCCRQ, Send StopCCN, idle SCCRP, SCCCN clean up idle, Receive StopCCN Clean up idle wait-ctl-reply, wait-ctl-conn, established The states associated with an LCCE for control connection establishment are as follows: idle Both initiator and recipient start from this state. An initiator transmits an SCCRQ, while a recipient remains in the idle state until receiving an SCCRQ. 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 5.4.3. wait-ctl-conn Awaiting an SCCCN. Upon receipt, the challenge response contained in the message is checked. The control connection is established if authentication succeeds; otherwise, it is torn down. Townsley, et al. Standards Track [Page 57]
INTERNET DRAFT L2TP October 2001 established An established connection may be terminated by either a local condition or the receipt of a StopCCN. In the event of a local termination, the originator MUST send a StopCCN and clean up the control connection. If the originator receives a StopCCN, it MUST also clean up the control connection. 7.4 Incoming Calls An ICRQ is generated by an LCCE, typically in response to an incoming call or a local event. Once the LCCE sends the ICRQ, it waits for a response from the peer. However, it may choose to postpone establishment of the call (e.g. answering the call, bringing up the circuit) until the peer has indicated with an ICRP that it will accept the call. The peer may choose not to accept the call if, for instance, there are insufficient resources to handle an additional session. If the peer chooses to accept the call, it responds with an ICRP. When the local LCCE receives the ICRP, it attempts to establish the call. A final call connected message, the ICCN, is sent from the local LCCE to the peer to indicate that the call states for both LCCEs should enter the established state. If the call is terminated before the peer can accept it, a CDN is sent by the local LCCE to indicate this condition. When a call transitions to a "disconnected" or "down" state, the call is cleared normally, and the local LCCE sends a CDN. Similarly, if the peer wishes to clear a call, it sends a CDN and cleans up its session. 7.4.1 ICRQ Sender States State Event Action New State ----- ----- ------ --------- idle Call signal or Initiate local wait-control-conn ready to receive control-conn incoming conn open idle Receive ICCN, Clean up idle ICRP, CDN wait-control- Bearer line drop Clean up idle conn or local close request wait-control- control-conn-open Send ICRQ wait-reply conn Townsley, et al. Standards Track [Page 58]
INTERNET DRAFT L2TP October 2001 wait-reply Receive ICRP, Send ICCN established acceptable wait-reply Receive ICRP, Send CDN, idle Not acceptable clean up wait-reply Receive ICRQ Send CDN, idle clean up wait-reply Receive CDN, Clean up idle ICCN wait-reply Local close Send CDN, idle request clean up established Receive CDN Clean up idle established Receive ICRQ, Send CDN, idle ICRP, ICCN clean up established Local close Send CDN, idle request clean up The states associated with the ICRQ sender are as follows: idle The LCCE detects an incoming call on one of its interfaces (e.g. an analog PSTN line rings, or an ATM PVC is provisioned), or a local event occurs. The LCCE initiates its control connection establishment state machine and moves to a state waiting for confirmation of the existence of a control connection. wait-control-connection In this state, the session is waiting for either the control connection to be opened or for verification that the control connection is already open. Once an indication that the control connection has been opened is received, session control messages may be exchanged. The first of these is the ICRQ. wait-reply The ICRQ sender receives either (1) a CDN indicating the peer is not willing to accept the call (general error or do not accept) and moves back into the idle state, or (2) an ICRP indicating the call is accepted. In the latter case, the LCCE sends an ICCN and enters the established state. established Data is exchanged over the session. The call may be cleared by Townsley, et al. Standards Track [Page 59]
INTERNET DRAFT L2TP October 2001 any of the following: + An event on the connected interface: The LCCE sends a CDN. + Receipt of a CDN: The LCCE cleans up, disconnecting the call. + A local reason: The LCCE sends a CDN. 7.4.2 ICRQ Recipient States State Event Action New State ----- ----- ------ --------- idle Receive ICRQ, Send ICRP wait-connect acceptable idle Receive ICRQ, Send CDN, idle not acceptable clean up idle Receive ICRP Send CDN idle clean up idle Receive ICCN Clean up idle wait-connect Receive ICCN Prepare for established acceptable data wait-connect Receive ICCN Send CDN, idle not acceptable clean up wait-connect Receive ICRQ, Send CDN, idle ICRP clean up idle, Receive CDN Clean up idle wait-connect, established wait-connect Local close Send CDN, idle established request clean up established Receive ICRQ, Send CDN, idle ICRP, ICCN clean up The states associated with the ICRQ recipient are as follows: idle An ICRQ is received. If the request is not acceptable, a CDN is sent back to the peer LCCE, and the local LCCE remains in the idle state. If the ICRQ is acceptable, an ICRP is sent. The session moves to the wait-connect state. Townsley, et al. Standards Track [Page 60]
INTERNET DRAFT L2TP October 2001 wait-connect The local LCCE is waiting for an ICCN from the peer. Upon receipt of the ICCN, the local LCCE moves to established state. established The session is terminated either by sending a CDN or by receiving a CDN from the peer. Clean up follows on both sides regardless of the initiator. 7.5 Outgoing Calls Outgoing calls instruct an LAC to place a call. There are three messages for outgoing calls: OCRQ, OCRP, and OCCN. An LCCE first sends an OCRQ to an LAC to request an outgoing call. The LAC MUST respond to the OCRQ with an OCRP once it determines that the proper facilities exist to place the call and that the call is administratively authorized. Once the outbound call is connected, the LAC sends an OCCN to the peer indicating the final result of the call attempt. 7.5.1 OCRQ Sender States State Event Action New State ----- ----- ------ --------- idle Local open Initiate local wait-control-conn request control-conn-open idle Receive OCCN, Clean up idle OCRP wait-control- control-conn-open Send OCRQ wait-reply conn wait-reply Receive OCRP, none wait-connect acceptable wait-reply Receive OCRP, Send CDN, idle not acceptable clean up wait-reply Receive OCCN, Send CDN, idle OCRQ clean up wait-connect Receive OCCN none established wait-connect Receive OCRQ, Send CDN, idle OCRP clean up Townsley, et al. Standards Track [Page 61]
INTERNET DRAFT L2TP October 2001 idle, Receive CDN Clean up idle wait-reply, wait-connect, established established Receive OCRQ, Send CDN, idle OCRP, OCCN clean up wait-reply, Local close Send CDN, idle wait-connect, request clean up established wait-control- Local close Clean up idle conn request The states associated with the OCRQ sender are as follows: idle, wait-control-conn When an outgoing call request is initiated, a control connection is created as described above, if not already present. Once the control connection is established, an OCRQ is sent to the LAC, and the session moves into the wait-reply state. wait-reply If a CDN is received, the session is cleaned up and returns to idle state. If an OCRP is received, the call is in progress, and the session moves to the wait-connect state. wait-connect If a CDN is received, the session is cleaned up and returns to idle state. If an OCCN is received, the call has succeeded, and the session may now exchange data. established If a CDN is received, the session is cleaned up and returns to idle state. Alternatively, if the LCCE chooses to terminate the session, it sends a CDN to the LAC, cleans up the session, and moves the session to idle state. 7.5.2 OCRQ Recipient (LAC) States State Event Action New State ----- ----- ------ --------- idle Receive OCRQ, Send OCRP, wait-cs-answer acceptable Place call idle Receive OCRQ, Send CDN, idle not acceptable clean up Townsley, et al. Standards Track [Page 62]
INTERNET DRAFT L2TP October 2001 idle Receive OCRP Send CDN, idle clean up idle Receive OCCN, Clean up idle CDN wait-cs-answer Call placement Send OCCN established successful wait-cs-answer Call placement Send CDN, idle failed clean up wait-cs-answer Receive OCRQ, Send CDN, idle OCRP, OCCN clean up established Receive OCRQ, Send CDN, idle OCRP, OCCN clean up wait-cs-answer, Receive CDN Clean up idle established established Local close Send CDN, idle request clean up The states associated with the LAC for outgoing calls are as follows: idle If the OCRQ is received in error, respond with a CDN. Otherwise, place the call, send an OCRP, and move to the wait-cs-answer state. wait-cs-answer If the call is not completed or a timer expires while waiting for the call to complete, send a CDN with the appropriate error condition set, and go to idle state. If a circuit-switched connection is established, send an OCCN indicating success, and go to established state. established If the LAC receives a CDN from the peer, the call MUST be released via appropriate mechanisms, and the session cleaned up. If the call is disconnected because the circuit transitions to a "disconnected" or "down" state, the LAC MUST send a CDN to the peer and return to idle state. Townsley, et al. Standards Track [Page 63]
INTERNET DRAFT L2TP October 2001 7.6 Termination of a Control Connection The termination of a control connection consists of either peer issuing a StopCCN. The sender of this message SHOULD wait a finite period of time for the acknowledgment of this message before releasing the control information associated with the control connection. The recipient of this message should send an acknowledgment of the message to the peer, then release the associated control information. When to release a control connection is an implementation issue and is not specified in this document. A particular implementation may use whatever policy is appropriate for determining when to release a control connection. Some implementations may leave a control connection open for a period of time or perhaps indefinitely after the last session for that control connection is cleared. Others may choose to disconnect the control connection immediately after the last call on the control connection disconnects. 8. Security Considerations L2TP encounters several security issues in its operation. The general approach of L2TP to these issues is documented here. 8.1 Control Connection Endpoint Security The LCCEs may optionally perform an authentication procedure of one another during control connection establishment. This authentication has the same security attributes as CHAP and has reasonable protection against replay and snooping during the control connection establishment process. This mechanism is not designed to provide any authentication beyond control connection establishment; it is fairly simple for a malicious user who can snoop the control connection stream to inject packets once an authenticated control connection establishment has been completed successfully. For authentication to occur, the LCCE pair MUST share a single secret. Each side uses this same secret when acting as authenticatee as well as authenticator. Since a single secret is used, the control connection authentication AVPs include differentiating values in the CHAP ID fields for each message digest calculation to guard against replay attacks. The Assigned Control Connection ID and Assigned Session ID (see Section 5.4) SHOULD be selected in an unpredictable manner rather than sequentially or otherwise. Doing so will help deter hijacking of a session by a malicious user who does not have access to packet traces between the LCCEs. Townsley, et al. Standards Track [Page 64]
INTERNET DRAFT L2TP October 2001 The Assigned Cookie value MUST be selected in an unpredictable manner. However, the Cookie MUST not be regarded as packet-level authentication or security of any kind. It should be used for nothing more than simple configuration error detection and identification of misrouted packets. Since the Cookie is sent and advertised in the clear, it is by no means a true packet-level security measure, such as that offered by IPsec. 8.2 Packet Level Security Securing L2TP requires that the underlying transport make available encryption, integrity, and authentication services for all L2TP traffic. This secure transport operates on the entire L2TP packet and is functionally independent of the data being carried on an L2TP data session. As such, L2TP is only concerned with confidentiality, authenticity, and integrity of the L2TP packets between two LCCEs, not unlike link-layer encryption being concerned only about protecting the confidentiality of traffic between its physical endpoints. 8.3 End-to-End Security Protecting the L2TP packet stream via a secure transport does, in turn, also protect the data within the tunneled session packets while transported from one LCCE to the other. Such protection should not be considered a substitution for end-to-end security between communicating hosts or applications. 8.4 L2TP and IPsec When running over IP, IPsec provides packet-level security via ESP and/or AH. All L2TP control and data packets for a particular control connection appear as homogeneous UDP/IP data packets to the IPsec system. In addition to IP transport security, IPsec defines a mode of operation that allows tunneling of IP packets. The packet-level encryption and authentication provided by IPsec tunnel mode and that provided by L2TP secured with IPsec provide an equivalent level of security for these requirements. IPsec also defines access control features that are required of a compliant IPsec implementation. These features allow filtering of packets based upon network and transport layer characteristics such as IP address, ports, etc. In the L2TP tunneling model, analogous filtering is logically performed at the network layer above L2TP. These network layer access control features may be handled at an LCCE via vendor-specific authorization features based upon the Townsley, et al. Standards Track [Page 65]
INTERNET DRAFT L2TP October 2001 authenticated user, or at the network layer itself by using IPsec transport mode end-to-end between the communicating hosts. The requirements for access control mechanisms are not a part of the L2TP specification and as such are outside the scope of this document. 9. IANA Considerations This document defines a number of "magic" numbers to be maintained by the IANA. This section explains the criteria to be used by the IANA to assign additional numbers in each of these lists. The following subsections describe the assignment policy for the namespaces defined elsewhere in this document. 9.1 AVP Attributes As defined in Section 5.1, AVPs contain Vendor ID, Attribute, and Value fields. For a Vendor ID value of 0, IANA will maintain a registry of assigned Attributes and, in some cases, Values. Attributes 0-39 are assigned as defined in Section 5.4. The remaining values are available for assignment upon Expert Review [RFC2434]. 9.2 Message Type AVP Values As defined in Section 5.4.1, Message Type AVPs (Attribute Type 0) have an associated value maintained by IANA. Values 0-16 are defined in Section 3.1. The remaining values are available for assignment upon Expert Review [RFC2434]. 9.3 Result Code AVP Values As defined in Section 5.4.2, Result Code AVPs (Attribute Type 1) contain three fields. Two of these fields (the Result Code and Error Code fields) have associated values maintained by IANA. 9.3.1 Result Code Field Values The Result Code AVP may be included in CDN and StopCCN messages. The allowable values for the Result Code field of the AVP differ depending upon the value of the Message Type AVP. For the StopCCN message, values 0-7 are defined in Section 5.4.2; for the CDN message, values 0-11 are defined in the same section. The remaining values of the Result Code field for both messages are available for assignment upon Expert Review [RFC2434]. Townsley, et al. Standards Track [Page 66]
INTERNET DRAFT L2TP October 2001 9.3.2 Error Code Field Values Values 0-9 are defined in Section 5.4.2. The remaining values are available for assignment upon Expert Review [RFC2434]. 9.4 AVP Header Bits There are four remaining reserved bits in the AVP header. Additional bits should only be assigned via a Standards Action [RFC2434]. 10. References [DSS1] ITU-T Recommendation, "Digital subscriber Signaling System No. 1 (DSS 1) - ISDN user-network interface layer 3 specification for basic call control", Rec. Q.931(I.451), May 1998 [KPS] Kaufman, C., Perlman, R., and Speciner, M., "Network Security: Private Communications in a Public World", Prentice Hall, March 1995, ISBN 0-13-061466-1 [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13, RFC 1034, November 1987. [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, July 1994. [RFC1663] Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994. [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. See also: http://www.iana.org/numbers.html [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996. [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. Townsley, et al. Standards Track [Page 67]
INTERNET DRAFT L2TP October 2001 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2138] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote Authentication Dial In User Service (RADIUS)", RFC 2138, April 1997. [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [RFC2341] Valencia, A., Littlewood, M. and T. Kolar, "Cisco Layer Two Forwarding (Protocol) L2F", RFC 2341, May 1998. [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations section in RFCs", BCP 26, RFC 2434, October 1998. [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W. and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637, July 1999. [RFC2661] Townsley W., et al., "Layer Two Tunneling Layer Two Tunneling Protocol (L2TP)", RFC 2661, August 1999. [RFC3070] V. Rawat, R. Tio, S. Nanji, R. Verma, "Layer Two Tunneling Protocol (L2TP) over Frame Relay," RFC 3070, February 2001. [STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I The Protocols", Addison-Wesley Publishing Company, Inc., March 1996, ISBN 0-201-63346-9 [L2TPIPSEC] B. Patel, B. Aboba, W. Dixon, G. Zorn, S. Booth, "Securing L2TP using IPsec," Internet Draft, October 2001, draft-ietf-l2tpext-security-08.txt. [L2TPAAL5] M. Davison, A. Lin, A. Singh, J. Stephens, R. Turner, R. Tio, S. Nanji, "L2TP Over AAL5," Internet Draft, August 2001, draft-ietf-l2tpext-l2tp-atm-02.txt. Townsley, et al. Standards Track [Page 68]
INTERNET DRAFT L2TP October 2001 11. Editors' Addresses Jed Lau cisco Systems 170 W. Tasman Drive San Jose, CA 95134 jedlau@cisco.com Gurdeep Singh Pall Microsoft Corporation Redmond, WA gurdeep@microsoft.com Bill Palter RedBack Networks, Inc 1389 Moffett Park Drive Sunnyvale, CA 94089 palter@zev.net Allan Rubens acr@del.com W. Mark Townsley cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 mark@townsley.net Andrew J. Valencia P.O. Box 2928 Vashon, WA 98070 vandys@zendo.com Glen Zorn cisco Systems 500 108th Avenue N.E., Suite 500 Bellevue, WA 98004 gwz@cisco.com Appendix A: Control Slow Start and Congestion Avoidance Although each side has indicated the maximum size of its receive window, it is recommended that a slow start and congestion avoidance method be used to transmit control packets. The methods described here are based upon the TCP congestion avoidance algorithm as described in section 21.6 of TCP/IP Illustrated, Volume I, by W. Richard Stevens [STEVENS]. Townsley, et al. Standards Track [Page 69]
INTERNET DRAFT L2TP October 2001 Slow start and congestion avoidance make use of several variables. The congestion window (CWND) defines the number of packets a sender may send before waiting for an acknowledgment. The size of CWND expands and contracts as described below. Note however, that CWND is never allowed to exceed the size of the advertised window obtained from the Receive Window AVP (in the text below, it is assumed any increase will be limited by the Receive Window Size). The variable SSTHRESH determines when the sender switches from slow start to congestion avoidance. Slow start is used while CWND is less than SSHTRESH. A sender starts out in the slow start phase. CWND is initialized to one packet, and SSHTRESH is initialized to the advertised window (obtained from the Receive Window AVP). The sender then transmits one packet and waits for its acknowledgement (either explicit or piggybacked). When the acknowledgement is received, the congestion window is incremented from one to two. During slow start, CWND is increased by one packet each time an ACK (explicit ZLB or piggybacked) is received. Increasing CWND by one on each ACK has the effect of doubling CWND with each round trip, resulting in an exponential increase. When the value of CWND reaches SSHTRESH, the slow start phase ends and the congestion avoidance phase begins. During congestion avoidance, CWND expands more slowly. Specifically, it increases by 1/CWND for every new ACK received. That is, CWND is increased by one packet after CWND new ACKs have been received. Window expansion during the congestion avoidance phase is effectively linear, with CWND increasing by one packet each round trip. When congestion occurs (indicated by the triggering of a retransmission) one half of the CWND is saved in SSTHRESH, and CWND is set to one. The sender then reenters the slow start phase. Appendix B: Control Message Examples B.1: Lock-Step Control Connection Establishment In this example, an LCCE establishes a control connection, with the exchange involving each side alternating in sending messages. This example shows the final acknowledgment explicitly sent within a ZLB ACK message. An alternative would be to piggyback the acknowledgement within a message sent as a reply to the ICRQ or OCRQ that will likely follow from the side that initiated the control connection. Townsley, et al. Standards Track [Page 70]
INTERNET DRAFT L2TP October 2001 LCCE A LCCE B ------ ------ SCCRQ -> Nr: 0, Ns: 0 <- SCCRP Nr: 1, Ns: 0 SCCCN -> Nr: 1, Ns: 1 <- ZLB Nr: 2, Ns: 1 B.2: Lost Packet with Retransmission An existing control connection has a new session requested by LCCE A. The ICRP is lost and must be retransmitted by LCCE B. Note that loss of the ICRP has two impacts: It not only keeps the upper level state machine from progressing, but also keeps LCCE A from seeing a timely lower level acknowledgment of its ICRQ. LCCE A LCCE B ------ ------ ICRQ -> Nr: 1, Ns: 2 (packet lost) <- ICRP Nr: 3, Ns: 1 (pause; LCCE A's timer started first, so fires first) ICRQ -> Nr: 1, Ns: 2 (Realizing that it has already seen this packet, LCCE B discards the packet and sends a ZLB) <- ZLB Nr: 3, Ns: 2 (LCCE B's retransmit timer fires) <- ICRP Nr: 3, Ns: 1 ICCN -> Nr: 2, Ns: 3 <- ZLB Nr: 4, Ns: 2 Townsley, et al. Standards Track [Page 71]
INTERNET DRAFT L2TP October 2001 Appendix C: Intellectual Property Notice The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF Secretariat." The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights. Townsley, et al. Standards Track [Page 72]
