Network Working Group J. Lau
Internet-Draft M. Townsley
Category: Standards Track cisco Systems
<draft-ietf-l2tpext-l2tp-base-08.txt> I. Goyret
Lucent Technologies
Editors
June 2003
Layer Two Tunneling Protocol (Version 3)
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes "version 3" of the Layer Two Tunneling
Protocol (L2TPv3). L2TPv3 defines the base control protocol and
encapsulation for tunneling multiple layer 2 connections between two
IP connected nodes. Additional documents detail the specifics for
each link-type being emulated.
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Contents
Status of this Memo.......................................... 1
1. Introduction............................................. 4
1.1 Changes from RFC 2661................................ 5
1.2 Specification of Requirements........................ 5
1.3 Terminology.......................................... 5
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............................... 13
3.3 Control Connection Management........................ 14
3.3.1 Control Connection Establishment................ 15
3.3.2 Control Connection Teardown..................... 15
3.4 Session Management................................... 16
3.4.1 Session Establishment for an Incoming Call...... 16
3.4.2 Session Establishment for an Outgoing Call...... 16
3.4.3 Session Teardown................................ 16
4. Protocol Operation....................................... 17
4.1 L2TP Over Specific Packet-Switched Networks (PSN).... 17
4.1.1 L2TPv3 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 Control Connection and Control Message Authentication 23
4.4 Keepalive (Hello).................................... 24
4.5 Forwarding Session Data Frames....................... 25
4.6 Default L2-Specific Sublayer......................... 25
4.6.1 Sequencing Data Packets......................... 26
4.7 L2TPv2/v3 Interoperability and Migration............. 27
4.7.1 L2TPv3 over IP.................................. 27
4.7.2 L2TPv3 over UDP................................. 28
4.7.3 Automatic L2TPv2 Fallback....................... 28
5. Control Message Attribute Value Pairs.................... 28
5.1 AVP Format........................................... 29
5.2 Mandatory AVPs....................................... 30
5.3 Hiding of AVP Attribute Values....................... 31
5.4 AVP Summary.......................................... 33
5.4.1 General Control Message AVPs.................... 33
5.4.2 Result and Error Codes.......................... 37
5.4.3 Control Connection Management AVPs.............. 39
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5.4.4 Session Management AVPs......................... 44
5.4.5 Circuit Status AVPs............................. 53
6. Control Connection Protocol Specification................ 55
6.1 Start-Control-Connection-Request (SCCRQ)............. 55
6.2 Start-Control-Connection-Reply (SCCRP)............... 56
6.3 Start-Control-Connection-Connected (SCCCN)........... 56
6.4 Stop-Control-Connection-Notification (StopCCN)....... 57
6.5 Hello (HELLO)........................................ 57
6.6 Incoming-Call-Request (ICRQ)......................... 58
6.7 Incoming-Call-Reply (ICRP)........................... 58
6.8 Incoming-Call-Connected (ICCN)....................... 59
6.9 Outgoing-Call-Request (OCRQ)......................... 60
6.10 Outgoing-Call-Reply (OCRP).......................... 60
6.11 Outgoing-Call-Connected (OCCN)...................... 61
6.12 Call-Disconnect-Notify (CDN)........................ 61
6.13 WAN-Error-Notify (WEN).............................. 62
6.14 Set-Link-Info (SLI)................................. 62
7. Control Connection State Machines........................ 63
7.1 Malformed Control Messages........................... 63
7.2 Timing Considerations................................ 64
7.3 Control Connection States............................ 65
7.4 Incoming Calls....................................... 67
7.4.1 ICRQ Sender States.............................. 67
7.4.2 ICRQ Recipient States........................... 69
7.5 Outgoing Calls....................................... 70
7.5.1 OCRQ Sender States.............................. 70
7.5.2 OCRQ Recipient (LAC) States..................... 71
7.6 Termination of a Control Connection.................. 72
8. Security Considerations.................................. 73
8.1 Control Connection Endpoint and Message Security..... 73
8.2 Data Channel Security................................ 73
8.3 End-to-End Security.................................. 74
8.4 L2TP and IPsec....................................... 74
8.5 Impact of L2TPv3 Features on RFC 3193................ 74
9. Internationalization Considerations...................... 75
10. IANA Considerations..................................... 75
10.1 Control Message Attribute Value Pairs (AVPs)........ 75
10.2 Message Type AVP Values............................. 76
10.3 Result Code AVP Values.............................. 76
10.3.2 Error Code Field Values........................ 76
10.4 AVP Header Bits..................................... 76
10.5 L2TP Control Message Header Bits.................... 76
10.6 Pseudowire Types..................................... 77
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10.7 Application Code..................................... 77
10.8 Circuit Status Bits.................................. 77
10.9 Default L2-Specific Sublayer bits.................... 77
10.10 L2-Specific Sublayer Type........................... 77
13. Acknowledgments......................................... 78
11. References.............................................. 79
11.1 Normative References................................ 79
11.2 Informative References.............................. 80
12. Editors' Addresses...................................... 81
Appendix A: Control Slow Start and Congestion Avoidance...... 81
Appendix B: Control Message Examples......................... 82
Appendix C: Processing Sequence Numbers...................... 83
Appendix D: Intellectual Property Notice..................... 85
Appendix E: Full Copyright Statement......................... 86
1. Introduction
The Layer Two Tunneling Protocol (L2TP) provides a dynamic mechanism
for tunneling Layer 2 (L2) "circuits" across a packet-oriented data
network (e.g., over IP). L2TP, as originally defined in RFC 2661, is
a standard method for tunneling Point to Point Protocol PPP [RFC1661]
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 defined here 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 two L2TP peers across an IP
network. Additional documents are expected to be published for each
layer 2 data link emulation type (a.k.a. pseudowire-type) supported
by L2TP (i.e., PPP, Ethernet, Frame Relay, etc.). These documents
will contain any individual details that are outside the scope of
this base specification.
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1.1 Changes from RFC 2661
Many 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 reuse, interoperability experience, and a
directed evolution of L2TP 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 header. (Layer 2
Forwarding, L2F, [RFC2341] was 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 the following:
- 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 constructs 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, respectively.
Details of these changes and a recommendation for transitioning to
L2TPv3 may be found in Section 4.7.
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), a length field, and a Value
containing the actual value identified by the attribute. Zero or
more AVPs make up the body of control messages, which are used in
the establishment, maintenance, and teardown of control
connections. This basic construct is sometimes referred to as a
Type-Length-Value (TLV) in some specifications. (See also:
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Control Connection, Control Message.)
Call (Circuit Up)
The action of transitioning a circuit on an L2TP Access
Concentrator (LAC) to an "up" or "active" state. A call may be
dynamically established through signaling properties (e.g., an
incoming or outgoing call through the Public Switched Telephone
Network (PSTN)) or statically configured (e.g., provisioning a
Virtual Circuit 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.)
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, point-to-point ethernet, 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 signaling to
govern the establishment, maintenance, and teardown of the
circuit. For the purposes of this document, a statically
configured circuit is considered to be essentially the same as a
very simple, long-lived, dynamic circuit. (See also: Call, Remote
System.)
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. (a.k.a. Data Packet, See also:
Data Channel.)
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Data Channel
The channel for L2TP-encapsulated data traffic that passes between
two LCCEs over a Packet Switched Network (i.e. IP). (See also:
Control Connection, Data Message.)
Incoming Call
The action of receiving a call (circuit up event) 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)
If a given circuit or L2TP session is cross-connected directly
from one data link (L2) connection to another, we refer to this
L2TP node as an L2TP Access Concentrator (LAC). A given L2TP
Control Connection Endpoint (LCCE) may act as both an L2TP Network
Server (LNS) for some sessions and an LAC for others, so these
terms must only be used within the context of a given set of
sessions unless the LCCE is in fact single purpose for a given
topology. (See also: LCCE, LNS.)
L2TP Control Connection Endpoint (LCCE)
An L2TP node which exists at either end of an L2TP control
connection. May also be referred to as an LAC or LNS, depending on
whether tunneled session data packets are processed at the data
link (L2) or network layer (L3). (See also: LAC, LNS.)
L2TP Network Server (LNS)
If a given L2TP session is terminated at the L2TP node and the
encapsulated network layer (L3) packet processed on a virtual
interface, we refer to this L2TP node as an L2TP Network Server
(LNS). A given LCCE may act as both an LNS for some sessions and
an LAC for others, so these terms must only be used within the
context of a given set of sessions unless the LCCE is in fact
single purpose for a given topology. (See also: LCCE, LAC.)
Outgoing Call
The action of placing a call by an LAC, typically in response to
policy directed by the peer in an Outgoing Call Request message.
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(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 (i.e., a number to dial) 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. For L2TPv3, this layer is principally IP. Other
examples include MPLS, FR, and ATM.
Peer
When used in context with L2TP, Peer refers to the far end of an
L2TP control connection (i.e., the remote 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.)
Pseudowire (PW)
An emulated circuit as it traverses a PSN. There is one
Pseudowire per L2TP Session. (See also: Packet-Switched Network,
Session.)
Pseudowire Type
The payload type being carried within an L2TP session. Examples
include PPP, Ethernet, and Frame Relay. (See also: Session.)
Remote System
An end-system or router connected by a circuit to an LAC.
Session
An L2TP session is the entity which is created between two LCCEs
in order to exchange parameters for and maintain an emulated L2
connection. Multiple sessions may be associated with a single
Control Connection.
Zero-Length Body (ZLB) Message
A control message with only an L2TP header. ZLB messages are used
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only to acknowledge messages on the L2TP reliable control channel.
(See also: Control Message.)
2. Topology
L2TP operates between two L2TP Control Connection Endpoints (LCCEs),
tunneling traffic across a packet network. For semantic convenience,
if a given circuit or L2TP session is cross-connected directly from
one layer-2 connection to another, we can refer to this L2TP node as
an L2TP Access Concentrator (LAC). If the L2TP session is terminated
at the L2TP node and the encapsulated network layer packet processed
on a virtual interface, we refer to the L2TP node as an L2TP Network
Server (LNS). A given LCCE may act as both an LNS for some sessions
and an LAC for others, so these terms must only be used within the
context of a given set of sessions unless the LCCE is in fact single
purpose for a given topology.
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 network traffic to the home
network. The action of session establishment is driven by the LAC
(as an incoming call) or the LNS (as an outgoing call).
+-----+ L2 +-----+ +-----+
| |------| LAC |.........[ IP ].........| 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
LAC using L2TP, and vice versa. In its simplest form, an LAC acts as
a simple cross-connect between a circuit to a remote system 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 simultaneously, in which a tie-breaking mechanism is
utilized).
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+-----+ L2 +-----+ +-----+ L2 +-----+
| |------| LAC |........[ IP ]........| LAC |------| |
+-----+ +-----+ +-----+ +-----+
remote remote
system system
|<- emulated service ->|
|<----------------- L2 service ----------------->|
(c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs. A
user-level, traffic-generated, or signaled event typically drives
session establishment from one side of the tunnel (though it is
permitted to be symmetric). For example, a tunnel generated from a PC
or other customer premises equipment.
+-----+ +-----+
[home network]...| LNS |........[ IP ]........| LNS |...[home network]
+-----+ +-----+
|<- emulated service ->|
|<---- L2 service ---->|
Note: In L2TPv2, user-driven tunneling of this type is often referred
to as "voluntary tunneling" [RFC2809]. Further, an LNS acting as part
of a software package on a host is sometimes referred to as an "LAC
Client" [RFC2661].
3. Protocol Overview
L2TP utilizes two types of messages, control messages and data
messages (sometimes referred to as "control packets" and "data
packets", respectively). Control messages are used in the
establishment, maintenance, and clearing of control connections and
sessions. 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.
The L2TPv3 control message format defined in this document borrows
largely from L2TPv2. These control messages are used in conjunction
with the associated protocol state machines that govern the dynamic
setup, maintenance, and teardown for L2TP sessions. The data message
format for tunneling data packets may be utilized with or without the
L2TP control channel, either via manual configuration or other
signaling methods to pre-configure or distribute L2TP session
information. Utilization of the L2TP data message format with other
signaling methods is outside the scope of this document.
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Figure 3.0: L2TPv3 Structure
+-------------------+ +-----------------------+
| Tunneled Frame | | L2TP Control Message |
+-------------------+ +-----------------------+
| L2TP Data Header | | L2TP Control Header |
+-------------------+ +-----------------------+
| L2TP Data Channel | | L2TP Control Channel |
| (unreliable) | | (reliable) |
+-------------------+----+-----------------------+
| Packet-Switched Network (IP, FR, MPLS, 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 by
an L2TP header, and sent over a Packet-Switched Network (PSN) such as
IP, UDP, Frame Relay, ATM, MPLS, etc. Control messages are sent over
a reliable L2TP control channel, which operates 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. 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
3 (SCCCN) Start-Control-Connection-Connected
4 (StopCCN) Stop-Control-Connection-Notification
5 (reserved)
6 (HELLO) Hello
TBA-M1 (ACK) Explicit Acknowledgement
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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
Link Status Change Reporting
16 (SLI) Set-Link-Info
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.
The L2TP control message header is formatted as follows:
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.
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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 which
includes L2TPv2 [RFC2661] and/or L2F [RFC2341] as well, 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
information on using this field.
3.2.2 L2TP Data Message
In general, an L2TP data message consists of a (1) Session Header,
(2) an optional L2-Specific Sublayer, and (3) the Tunnel Payload, as
depicted below.
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Figure 3.2.2: L2TP Data Message Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Session Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-Specific Sublayer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Payload ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The L2TP Session Header is specific to the encapsulating PSN over
which the L2TP traffic is delivered. The Session Header MUST provide
(1) a method of distinguishing traffic among multiple L2TP data
sessions and (2) a method of distinguishing data messages from
control messages.
Each type of encapsulating PSN MUST define its own session header,
clearly identifying the format of the header and parameters necessary
to setup the session. Section 4.1 defines two session headers, one
for transport over UDP and one for transport over IP.
The L2 Specific Sublayer is an intermediary layer between the L2TP
session header and the start of the tunneled frame. It contains
control fields that are used to facilitate the tunneling of each
frame (e.g. sequence numbers or flags). The default L2-Specific
Sublayer for L2TPv3 is defined in Section 4.6.
The Data Message Header is followed by the Tunnel Payload, including
any necessary L2 framing as defined in the payload-specific companion
documents.
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,
Explicit Acknowledgement (ACK) messages may be sent after any of the
control messages indicated in the exchanges below if an
acknowledgment is not piggybacked on a later control message.
LCCEs are identified during control connection establishment either
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by the Host Name AVP, the Router ID AVP, or a combination of the two
(see Section 5.4.3). The identity of a peer LCCE is central to
selecting the proper configuration parameters (i.e. Hello interval,
window size, etc.) for a control connection, determination of how to
setup associated sessions within the control connection, password
lookup for control connection authentication, and control connection
level tie-breaking.
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 an ACK message 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 ACK message 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)
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.
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3.4 Session 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 Session Establishment for an Incoming Call
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
(Call
Accepted)
ICCN ->
3.4.2 Session Establishment for an Outgoing Call
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 ->
(Call Operation
Completed
Successfully)
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
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session is cleared, the control connection MAY be torn down as well
(and typically is). The following is an example of a typical control
message exchange:
LCCE A LCCE B
------ ------
CDN ->
(Clean up)
(Clean up)
4. Protocol Operation
This section defines the control and data protocol operation for
L2TP.
4.1 L2TP Over Specific Packet-Switched Networks (PSN)
If necessary, L2TP may operate over a variety of Packet Switched
Networks. While the L2TP Session Header encapsulation MAY vary for a
given PSN, the required formats for operation over IPv4 are defined
in this document.
There are two modes described for operation over IPv4, 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, and easier migration from
L2TPv2.
L2TP over other PSNs may be defined, but the specifics are outside
the scope of this document. Examples of L2TPv2 over other PSNs
include [RFC3070] and [RFC3355].
The following field definitions are defined for use in all L2TP
Session 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 control connection
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.
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Cookie
The optional Cookie field contains a variable length (maximum 64
bits), value used to check the association of a received data
message with the session identified by the Session ID. The Cookie
MUST be set to a random value utilizing all bits in the field.
The Cookie provides an additional level of guarantee that a data
message has been directed to the proper session by the Session ID.
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 session creation.
The Cookie provides some protection against specific malicious
packet insertion attacks, as described in section 8.2.
4.1.1 L2TPv3 over IP
L2TPv3 over IP utilizes the IANA assigned IP protocol ID 115.
4.1.1.1 L2TPv3 Session Header Over IP
Unlike L2TP over UDP, the L2TPv3 session 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 session header format is utilized when
operating L2TPv3 over IP:
Figure 4.1.1.1: L2TPv3 Session Header Over IP
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,
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which are present in L2TP over UDP, limits version extensibility for
data messages. However, given the freedom of setting the first 32
bits in the data message header (i.e. the Session ID field), an
acceptable workaround to this limitation can be devised if an
extension to the demultiplexing capabilities of L2TP is ever in need
of further revision.
4.1.1.2 L2TP Control and Data Traffic over IP
Unlike L2TP over UDP which uses the common T bit to distinguish
between L2TP control and data packets, L2TP over IP uses the reserved
Session ID of all zeros when sending control messages. It is
presumed that checking for the zero Session ID is more efficient --
both in header size for data packets and in processing speed for
distinguishing between control and data messages -- than checking for
the presence of a given single bit.
The entire control message header over IP, including the zero session
ID, appears as follows:
Figure 4.1.1.2: L2TPv3 Control Message Header Over IP
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 T bit. It does NOT include the "(32 bits of
zeros)" depicted above.
4.1.2 L2TP over UDP
L2TPv3 over UDP must consider 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,
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there are possible side effects as well. For instance, L2TP over IP
is not as NAT-friendly as L2TP over UDP. Also, control messages
transmitted over IP are not protected by a network-layer checksum as
they are with UDP.
4.1.2.1 L2TP Session Header Over UDP
The following session header format is utilized when operating L2TPv3
over UDP:
Figure 4.1.2.1: L2TPv3 Session Header over UDP
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 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 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.
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Any subsequent traffic associated with this control connection
(either control traffic or data traffic from a session established
through this control connection) must use these same UDP ports.
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 capability before
choosing an arbitrary source port. A NAT device that can pass TFTP
traffic with variant UDP ports should be able to pass L2TP UDP
traffic since both protocols 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 that enabling checksums on
data packets may significantly increase the data 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
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
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received and ignored from processing. However, in order to ensure
that all messages are acknowledged properly (particularly in the case
of a lost ACK message), receipt of duplicate messages MUST be
acknowledged by the reliable delivery mechanism. This acknowledgment
may either piggybacked on a message in queue or sent explicitly via
an ACK message.
All control messages take up one slot in the control message sequence
number space, except the ACK message. Thus, Ns is not incremented
after an ACK 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-
ACK message received plus 1, modulo 65536). While the Nr in a
received ACK message 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 ACK message. 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, the control message 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
but SHOULD be configurable) passes without acknowledgment, 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
place a cap upon the maximum interval between retransmissions. This
cap SHOULD be no less than 8 seconds per retransmission. If no peer
response is detected after several retransmissions (a recommended
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default is 10, but MUST be configurable), the control connection and
all associated sessions MUST be cleared. As it is the first message
to establish a control connection, the SCCRQ MAY employ a different
retransmission maximum than other control messages in order to help
facilitate failover to alternate LCCEs in a timely fashion.
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 (e.g. 1 + 2 + 4 + 8 + 8...
seconds).
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 control of 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 actions.
Appendix B contains examples of control message transmission,
acknowledgment, and retransmission.
4.3 Control Connection and Control Message Authentication
L2TP incorporates an optional authentication and integrity check for
all control messages. This mechanism consists of a computed one-way
hash over the entire contents of the L2TP control message, a pre-
configured shared secret, and a local and remote nonce (random value)
exchanged during control connection establishment. This per-message
authentication and integrity check is designed to perform a mutual
authentication between L2TP nodes, integrity checking of all control
messages, and guard against control message spoofing and replay
attacks that would otherwise be trivial to mount.
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A shared secret (password) MUST exist between communicating L2TP
nodes to obtain the benefit of message or peer authentication. If a
shared secret is not configured on either node, the per-message
integrity check may still be performed using an empty shared secret
of zero length. See Section 5.4.3 for details on calculation of the
Message Digest and construction of the Random Vector and Message
Digest AVPs.
L2TPv3 Control Connection and Control Message Authentication is
similar to L2TPv2 [RFC2661] Tunnel Authentication in its use of a
shared secret for peer authentication, use of a one-way hash
calculation, and exchange of a random value. The principal difference
is that, instead of computing the hash over selected contents of a
received control message (e.g. the Challenge AVP and Message Type) as
in L2TPv2, the entire message is used in the hash in L2TPv3. In
addition, instead of including the hash digest in just the SCCRP and
SCCCN messages, it is now included in all L2TP messages.
The Control Message Authentication mechanism is optional, and may be
disabled if both peers agree. For example, if IPsec is already being
used for security and integrity checking between the LCCEs, the L2TP
mechanism defined here becomes redundant and may be disabled.
Presence of the Message Digest AVP in an SCCRQ or SCCRP message
serves as an indication to a peer that Control Message Authentication
is enabled. If an SCCRQ or SCCRP contains a Message Digest AVP, the
receiver of the message MUST respond with a Message Digest AVP in all
subsequent messages sent. If an SCCRQ or SCCRP is received with a
missing or incorrect Message Digest AVP value, a StopCCN MAY be sent
with the Result Code set to 4 (see Section 5.4.2). Care should be
taken to limit such responses as to not end up in a denial of service
situation responding to rogue SCCRQ or SCCRP control messages. All
other control messages with missing or incorrect Message Digest AVPs
MUST be dropped.
4.4 Keepalive (Hello)
A keepalive mechanism is employed by L2TP to detect loss of
connectivity between a pair of LCCEs. This detection 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 with any other control message, if the
Hello message is not reliably delivered, the sending LCCE declares
that the control connection is down and resets its state for the
control connection. This behavior ensures that a connectivity
failure between the LCCEs is detected independently by each end of a
control connection
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If the control channel is operated in-band with data traffic over the
PSN, this single mechanism can be used to infer basic data
connectivity between a pair of LCCEs for all sessions associated with
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. An LCCE sending Hello messages
across multiple control connections between the same LCCE endpoints
SHOULD employ a jittered timer mechanism.
4.5 Forwarding Session Data Frames
Once session establishment is complete, circuit frames are received
at an LCCE, encapsulated in L2TP (with appropriate attention to
framing as described in documents for the particular pseudowire
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 a given 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. It is important for implementers to
note that the Cookie field check occurs after looking up the session
context by the Session ID, and may consist merely of a value match.
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 Default L2-Specific Sublayer
This document defines a default L2-Specific Sublayer (see Section
3.2.2) format that a pseudowire may use for features such as basic
sequencing support, marking of packets with a single high-priority
bit, or other general PW-specific per-packet control operations. The
default L2-Specific Sublayer SHOULD be used by a given PW type to
support these features if it is adequate, and its presence is
requested by a peer during session negotiation. Alternative
sublayers MAY be defined (e.g. an encapsulation with a larger
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Sequence Number field or timing information) and identified for use
via the L2-Specific Sublayer Type AVP.
Figure 4.6: Default L2-Specific Sublayer 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P|S|x|x|x|x|x|x| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The P (Priority) bit is used to identify a data packet that should be
dropped only as a last resort after being received by an L2TP peer.
This bit should be set to 1 for any traffic that should be given
higher priority than other data traffic in a congested environment.
For example, end-to-end L2 keepalive packets (e.g. LCP keepalives) or
other control packets vital to the life of the circuit may need
special handling by an LCCE upon receipt. This is not a replacement
for, or to be used as, a per-hop QoS method of any sort. It is only
to be used by the L2TP receiving node to prioritize incoming traffic.
The S (Sequence) bit is set to 1 when the Sequence Number contains a
valid number for this sequenced frame. If the S bit is set to zero,
the Sequence Number contents are undefined and MUST be ignored by the
receiver.
The Sequence Number field contains a free-running counter of 2^24
sequence numbers. If the number in this field is valid, the S bit
MUST be set to 1. The Sequence Number begins at zero, which is a
valid sequence number. (In this way, implementations inserting
sequence numbers do not have to "skip" zero when incrementing.) 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 (2^23-1)
values, inclusive.
4.6.1 Sequencing Data Packets
The Sequence Number field may be used to detect lost, duplicate, or
out-of-order packets within a given session.
When L2 frames are carried over an L2TP-over-IP or L2TP-over-UDP/IP
data channel, this part of the link has the characteristic of being
able to reorder, duplicate, or silently drop packets. Reordering may
break some non-IP protocols or L2 control traffic being carried by
the link. Silent dropping or duplication of packets may break
protocols that assume per-packet indications of error, such as TCP
header compression. While a common mechanism for packet sequence
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detection is provided, the sequence dependency characteristics of
individual protocols are outside the scope of this document.
If any protocol being transported by over L2TP data channels cannot
tolerate misordering, packet duplication or silent packet loss,
sequencing may be enabled on some or all packets by using the S bit
and Sequence Number field defined in the default L2-Specific
Sublayer(see Section 4.6). For a given L2TP session, each LCCE is
responsible for communicating to its peer the level of sequencing
support that it requires of data packets that it receives.
Mechanisms to advertise this information during session negotiation
are provided (see, in particular, the Data Sequencing AVP in Section
5.4.4).
When determining whether a packet is in or out of sequence, an
implementation SHOULD utilize a method that is resilient to temporary
dropouts in connectivity coupled with high per-session packet rates.
The recommended method is outlined in Appendix C.
4.7 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 at least be mindful of
these issues in order to interoperate with implementations that
support both versions.
4.7.1 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, without normative reference
to RFC2661.
If one wishes to tunnel PPP over L2TPv3, and fallback to L2TPv2 only
if it is not available, then L2TPv3 over UDP with the automatic
fallback as described in section 4.7.3 MUST be used. There is no
deterministic method for automatic fallback from L2TPv3 over IP to
either L2TPv2 or L2TPv3 over UDP. One could infer whether L2TPv3 over
IP is supported by sending an SCCRQ and waiting for a response, but
this could be problematic during periods of packet loss between L2TP
nodes.
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4.7.2 L2TPv3 over UDP
The format of the L2TPv3 over UDP header is defined in Section
4.1.2.1.
When operating over UDP, L2TPv3 uses the same port (1701) as L2TPv2
and shares the first two octets of header format with L2TPv2. The Ver
field is used to distinguish L2TPv2 packets from L2TPv3 packets. If
an implementation is capable of operating in L2TPv2 or L2TPv3 modes,
it is possible to automatically detect whether a peer can support
L2TPv2 or L2TPv3 and operate accordingly. The details of this
fallback capability is defined in the following section.
4.7.3 Automatic L2TPv2 Fallback
When running over UDP, an implementation may detect whether a peer is
L2TPv3-capable by sending an SCCRQ that is properly formatted for
L2TPv2 and L2TPv3. This is accomplished by sending the SCCRQ with
the Ver field set to 2, and ensuring that any L2TPv3-specific AVPs
within the message are sent with each M-bit set to 0 (so that they
may be safely 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
in L2TPv3, and the 16-bit Tunnel ID and 16-bit Session ID in L2TPv2,
is always 0 for an SCCRQ. This effectively hides the fact that there
are a pair of 16-bit fields in L2TPv2, and a single 32-bit field in
L2TPv3.
If the peer implementation is L2TPv3-capable, 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, it must be assumed that the
peer implementation is L2TPv2-only and fallback to L2TPv2 mode may
occur if local policy and capability permit.
The L2TPv2/v3 auto-detection mode requires that all L2TPv3
implementations be liberal in acceptance of an SCCRQ control message
with the Ver field set to 2 or 3.
5. Control Message Attribute Value Pairs
To maximize extensibility while permitting interoperability, a
uniform method for encoding message types is used throughout L2TP.
This encoding will be termed AVP (Attribute Value Pair) for the
remainder of this document.
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5.1 AVP Format
Each AVP is encoded as follows:
Figure 5.1: AVP 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H| rsvd | Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Type | Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(until Length is reached) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first six bits comprise a bit mask that describes 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 unrecognized or malformed AVP. 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: Contains 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
Enterprise Codes" [RFC1700] value. The value 0, corresponding to
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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 consequence is
intended.
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 receiving 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
connection teardown simply because its M bit was set to a value that
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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:
Figure 5.3: Hidden AVP Subformat
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
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placed in the Length of Original Attribute Value field, but does
alter the length of the resultant AVP that is being created. For
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 [RFC2865], 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
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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
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 General Control Message AVPs
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.
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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
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.
Message Digest (All Messages)
The Message Digest AVP, Attribute Type AVP-TBA-1, is used as an
integrity check and authentication of the L2TP Control Message
header and body.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest Type | Message Digest ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where Digest Type is a one octet integer indicating the Digest
calculation algorithm:
0 HMAC-MD5 1 HMAC-SHA-1
Digest type 0 (HMAC-MD5) MUST be supported, Digest Type 1 (HMAC-
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SHA-1) MAY be supported.
The Message Digest is of variable length and contains the result
of the control message authenticity and integrity calculation. For
Digest Type 0 (HMAC-MD5) the length of the digest MUST be 160
bits. The nonce is advertised via the Random Vector AVP also
defined in this section.
Calculation of the digest is as follows for all messages other
than the SCCRQ:
Digest = Hash (local_nonce | remote_nonce | password |
control_message)
Hash: Hashing algorithm identified by the Digest Type
local_nonce: Nonce chosen locally and advertised to the remote
LCCE.
remote_nonce: Nonce received from the remote LCCE
password: Configured shared secret for this Control Connection
control_message: The entire contents of the L2TP Control
Message, including the Control Message header and all AVPs.
Note that the Control Message header in this case begins after
the 0 Session ID when running over IP, and after the UDP header
when running over UDP.
When calculating the Message Digest, the Message Digest AVP itself
MUST be present within the control message with the Digest Type
set to its proper value, but the digest itself set to zeros. It
may be beneficial for an implementation to position the Message
Digest AVP just after the Message Type AVP when building a control
message so that the Digest may be inserted at a well-known offset
within the message.
When receiving a control message, the contents of the Message
Digest AVP MUST be compared against the expected digest value
based on local calculation. This is done by performing the same
digest calculation above, with the local_nonce and reverse_nonce
reversed. This message authenticity and integrity checking MUST be
performed before utilizing any information contained within the
control message. If the calculation fails, the message MUST be
dropped.
The SCCRQ has special treatment as it is the initial message
commencing a new Control Connection. As such, there is only one
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nonce available, and it is present within the message itself (so
there is no need to use it in the calculation explicitly).
Calculation of the SCCRQ Digest is performed as follows:
Digest = Hash (password | control_message)
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP MUST be set to 1. The Length is 28 for Digest Type 1
(HMAC-MD5), and may vary for other digest types.
Random Vector (All Messages)
The Random Vector AVP, Attribute Type 36, contains a
cryptographically random value [RFC1750]. This value is used as
a nonce for AVP Hiding and Control Message Authentication
calculations.
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 ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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), and as a nonce for the Control Message
Authentication calculation (see Message Digest AVP definition
in this section).
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, and the final instance used for Control Message
Authentication calculations. 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.
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5.4.2 Result and Error Codes
Result Code (StopCCN, CDN)
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, arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 plus 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.
General Result Code values for the CDN message are as follows:
0 - Reserved.
1 - Session disconnected due to loss of carrier or circuit disconnect.
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2 - Session disconnected for the reason indicated in Error Code.
3 - Session disconnected for administrative reasons.
4 - Session establishment failed due to lack of appropriate
facilities being available (temporary condition).
5 - Session establishment failed due to lack of appropriate
facilities being available (permanent condition).
6 - 11 Reserved (PPP-specific codes defined outside this document).
RC-TBA-1 - Session not established due to losing tie breaker.
RC-TBA-2 - Session not established due to unsupported PW type.
RC-TBA-3 - Session not established, sequencing required without valid
L2-Specific Sublayer.
Additional service-specific Result Codes are defined outside this
document.
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 shut down 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 (e.g. "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.
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When a General Error Code of 6 is used, additional information about
the error SHOULD be included in the Error Message field. A vendor-
specific AVP MAY be sent to more precisely detail a vendor-specific
problem.
5.4.3 Control Connection Management AVPs
Control Connection Tie Breaker (SCCRQ)
The Control Connection Tie Breaker AVP, Attribute Type 5, indicates
that the sender desires a single control connection to exist between
a given pair of LCCEs.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Control Connection Tie Breaker Value is an 8-octet random 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; if so, a tie
has been detected. In this case, the LCCE must compare its Control
Connection Tie Breaker value with the one received in the SCCRQ. The
lower value "wins", and the "loser" MUST discard its control
connection, sending a StopCCN if the SCCRQ that it had sent was
acknowledged by the receiving peer. 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 with a new, random tie breaker value.
If a tie breaker is received and an outstanding SCCRQ has no tie
breaker value, the initiator that included the Control Connection Tie
Breaker AVP "wins". If neither side issues a tie breaker, then two
separate control connections are opened.
Applications which employ a distinct and well-known initiator have no
need for tie-breaking, and this AVP MAY be omitted and the tie-
breaking functionality disabled. Applications which require tie-
breaking also require that an LCCE be uniquely identifiable upon
receipt of an SCCRQ. For L2TP over IP, this MUST be accomplished via
the Router ID AVP.
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Note that in [RFC2661], this AVP was referred to as the "Tie-Breaker
AVP". Here, the AVP serves the same purpose and has the same
attribute value and composition.
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.
Host Name (SCCRQ, SCCRP)
The Host Name AVP, Attribute Type 7, indicates the name of the
issuing LAC or LNS, encoded in the US-ASCII charset.
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 ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 host name with fully qualified
domain would be appropriate. The Host Name AVP and/or Router ID AVP
MUST be used to identify an LCCE as described in Section 3.3.
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.
Router ID (SCCRQ, SCCRP)
The Router ID AVP, Attribute Type AVP-TBA-2, is an identifier used to
identify an LCCE for control connection setup, tie breaking, and/or
tunnel authentication.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Router Identifier is a 4-octet unsigned integer. Its value is
unique for a given LCCE, per Section 8.1 of [RFC2072]. The Host Name
AVP and/or Router ID AVP MUST be used to identify an LCCE as
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described in Section 3.3.
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 10.
Vendor Name (SCCRQ, SCCRP)
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 ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Vendor Name is the indicated number of octets representing the
vendor string. Human-readable text for this AVP MUST be provided in
the US-ASCII charset [RFC1958, 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 (SCCRQ, SCCRP, StopCCN)
The Assigned Control Connection ID AVP, Attribute Type AVP-TBA-3,
contains 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
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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 earlier (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 4.7.3). The Length (before
hiding) of this AVP is 10.
Receive Window Size (SCCRQ, SCCRP)
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:
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. See Section 4.2 for more
information on reliable control message delivery.
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.
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Pseudowire Capabilities List (SCCRQ, SCCRP)
The Pseudowire Capabilities List (PW Capabilities List) AVP,
Attribute Type AVP-TBA-4, indicates the L2 payload types the sender
can support. The specific payload type of a given session is
identified by the Pseudowire 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type 0 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | PW Type N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Defined PW types that may appear in this list are managed by IANA and
MUST appear in associated pseudowire-specific documents for each PW
type.
If a sender includes a given PW type in the PW Capabilities List AVP,
the sender assumes full responsibility for supporting that particular
payload, such as any payload-specific AVPs, L2-Specific Sublayer, or
control messages that may be defined in the appropriate companion
document.
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 4.7.3). The Length (before
hiding) of this AVP is 8 octets with one PW type specified, plus 2
octets for each additional PW type.
Preferred Language (SCCRQ, SCCRP)
The Preferred Language AVP, Attribute Type AVP-TBD-14, provides a
method for an LCCE to indicate to the peer the language in which
human- readable messages it sends SHOULD be composed. This AVP
contains a single language tag or language range [RFC3066].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
Preferred Language... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The Preferred Language is the indicated number of octets representing
the language tag or language range, encoded in the US-ASCII charset.
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 Preferred Language.
5.4.4 Session Management AVPs
Local Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)
The Local Session ID AVP (analogous to the Assigned Session ID in
L2TPv2), Attribute Type AVP-TBA-5, contains the identifier 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 two identifiers used to
multiplex and demultiplex sessions between two LCCEs. Each LCCE
chooses any free value it desires, and sends it to the remote LCCE
using this AVP. The remote LCCE MUST then send all data packets
associated with this session using this value. Additionally, for all
session-oriented control messages sent after this AVP is received
(e.g. ICRP, ICCN, CDN, SLI, etc.), the remote LCCE MUST echo this
value in the Remote Session ID AVP.
Note that a Session ID value is unidirectional. Because each LCCE
chooses its Session ID independent of its peer LCCE, the value does
not have to match in each direction for a given session."
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 4.7 for L2TPv2 migration issues). The Length (before hiding)
of this AVP is 10.
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Remote Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)
The Remote Session ID AVP, Attribute Type AVP-TBA-6, contains the
identifier 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Remote Session ID is a 4-octet non-zero unsigned integer.
The Remote Session ID AVP MUST be present in all session-level
control messages. The AVP's value echoes the session identifier
advertised by the peer via the Local Session ID AVP. It is the same
value that will be used in all transmitted data messages by this side
of the session. In most cases, this identifier is sufficient for the
peer to look up session-level context for this control message.
When a session-level control message must be sent to the peer before
the Local Session ID AVP has been received from the peer, the value
of the Remote Sesson ID AVP MUST be set to zero. Additionally, the
Local Session ID AVP (sent in a previous control message for this
session) MUST be included in the control message. The peer must then
use the Local Session ID AVP to perform a "reverse lookup" to find
its session context. Session-level control messages defined in this
document that might be subject to a reverse lookup by a receiving
peer include the CDN, WEN, and SLI.
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 4.7 for L2TPv2 migration issues). The Length (before hiding)
of this AVP is 10.
Assigned Cookie (ICRQ, ICRP, OCRQ, OCRP)
The Assigned Cookie AVP, Attribute Type AVP-TBA-7, contains the
Cookie value being assigned to this session by the sender.
The Attribute Value field for this AVP has the following format:
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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 missing Assigned Cookie AVP or Assigned Cookie Value of zero length
indicates that the Cookie field should not be present in any data
packets sent to the LCCE sending this AVP.
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 4.7 for L2TPv2 migration issues). The Length (before hiding)
of this AVP may be 6, 10, or 14 octets.
Serial Number (ICRQ, OCRQ)
The Serial Number AVP, Attribute Type 15, contains an identifier
assigned by the LAC or LNS to this 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Serial Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Serial Number is a 32-bit value.
The Serial Number is intended to be an easy reference for
administrators on both ends of a control connection to use when
investigating session failure problems. 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.
Note that in RFC 2661, this value was referred to as the "Call Serial
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Number AVP". It serves the same purpose and has the same attribute
value and composition.
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.
Remote End ID (ICRQ, OCRQ)
The Remote End ID AVP, Attribute Type AVP-TBA-8, contains an
identifier used to bind L2TP sessions to a given circuit, interface,
or bridging instance. It also may be used to detect session-level
ties.
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 End Identifier ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Remote End Identifier field is a variable-length field whose
value is unique for a given LCCE peer, as described in Section 3.3.
A session-level tie is detected if an LCCE receives an ICRQ or OCRQ
with an End ID AVP whose value matches that which was just sent in an
outgoing ICRQ or OCRQ to the same peer. If the two values match, an
LCCE recognizes that a tie exists (e.g. both LCCEs are attempting to
establish sessions for the same circuit). The tie is broken by the
Session Tie Breaker AVP.
By default, the LAC-LAC cross-connect application (see section
2.0(b)) of L2TP over an IP network MUST utilize the Router ID AVP and
Remote End ID AVP to associate a circuit to an L2TP session. Other
AVPs MAY be used for
LCCE or circuit identification as specified in a companion documents.
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 Remote End Identifier value.
Application Code (ICRQ, OCRQ)
The Application Code AVP, Attribute Type AVP-TBA-9, is a 2 octet
value for enumerating application types for a given L2TP session.
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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 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Application Code is a 2 octet value used to identify a specific
application for an L2TP session, perhaps causing certain values
within AVPs defined in this document to be interpreted or acted upon
in a different manner dictated by the Application Code. For example,
a given Application Code could instruct an LCCE to perform a specific
directory lookup on the Hostname and/or Router ID AVP information
associated with this session (perhaps even encoding the destination
address of the given directory server).
An Application Code of 0, or absence of this AVP in any control
message, indicates that all AVPs should be interpreted as defined in
this document.
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 8
plus the length of the Application Info field.
Session Tie Breaker (ICRQ, OCRQ)
The Session Tie Breaker AVP, Attribute Type TBD, is used to break
ties when two peers concurrently attempt to establish a session 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Session Tie Breaker Value is an 8-octet random value that is used
to choose a session when two LCCEs concurrently request a session for
the same circuit. A tie is detected by examining the peer's identity
(described in Section 3.3) plus the per-session shared value
communicated via the End ID AVP. In the case of a tie, the recipient
of an ICRQ or OCRQ must compare the received tie breaker value with
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the one that it sent earlier. The LCCE with the lower value "wins",
and the "loser" MUST send a CDN with result code set to RC-TBA-1 (as
defined in Section 5.4.2) to tear down the session it instigated. In
the case in which a tie is detected, tie breakers are sent by both
sides, and the tie breaker values are equal, both sides MUST discard
their sessions and restart session negotiation with new random tie
breaker values.
If a tie is detected but only one side sends a Session Tie Breaker
AVP, the session initiator that included the Session Tie Breaker AVP
"wins". If neither side issues a tie breaker, then both sides MUST
tear down the session.
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.
Pseudowire Type (ICRQ, OCRQ)
The Pseudowire Type (PW Type) AVP, Attribute Type AVP-TBA-10,
indicates the L2 payload type of the packets that will be tunneled
using this L2TP session.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A peer MUST NOT request an incoming or outgoing call with a PW Type
AVP specifying a value not advertised in the PW Capabilities List AVP
it received during control connection establishment. Attempts to do
so MUST result in the call being rejected via a CDN with the Result
Code set to RC-TBA-2 (see Section 5.4.2).
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
4.7 for L2TPv2 migration issues). The Length (before hiding) of this
AVP is 8.
L2-Specific Sublayer (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The L2-Specific Sublayer AVP, Attribute Type AVP-TBA-11, indicates
the L2-Specific Sublayer the sender of this AVP requires to be
present on all incoming data packets for this L2TP session.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-Specific Sublayer Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The L2-Specific Sublayer Type is a 2-octet unsigned integer with the
following values defined in this document:
0 - There is no L2-Specific Sublayer present.
1 - The default L2-Specific Sublayer (defined in Section 4.6)
is used.
If this AVP is received and has a value other than zero, the
receiving LCCE MUST include the identified L2-Specific Sublayer in
its outgoing data messages. If the AVP is not received, it is
assumed that there is no sublayer present.
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 8.
Data Sequencing (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Data Sequencing AVP, Attribute Type AVP-TBA-12, indicates that
the sender requires some or all of the data packets that it receives
to be sequenced.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sequencing Level |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Data Sequencing Level is a 2-octet unsigned integer indicating
the degree of incoming data traffic that the sender of this AVP
wishes to be marked with sequence numbers.
The following values are valid data sequencing levels:
0 - No incoming data packets require sequencing.
1 - Only non-IP data packets require sequencing.
2 - All incoming data packets require sequencing.
If a data sequencing level of 0 is specified, there is no need to
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send packets with sequence numbers. If sequence numbers are sent,
they will be ignored upon receipt. If no Data Sequencing AVP is
received, a data sequencing level of 0 is assumed.
If a data sequencing level of 1 is specified, only non-IP traffic
carried within the given PW-specific framing should have sequence
numbers applied. Non-IP traffic here refers to any packets that
cannot be classified as an IP packet within their respective L2
framing (i.e., a PPP control packet or NETBIOS frame encapsulated by
Frame Relay before being tunneled). All traffic that can be
classified as IP MUST be sent with no sequencing (e.g. the S bit in
the L2-Specific Sublayer is set to zero). If a packet is unable to be
classified at all (e.g. due to it being compressed or encrypted at
layer 2) or if an implementation is unable to perform such
classification within L2 frames, all packets MUST be provided with
sequence numbers (essentially falling back to a data sequencing level
of 2).
If a data sequencing level of 2 is specified, all traffic MUST be
sequenced.
Data sequencing may only be requested when there is an L2-Specific
Sublayer present that can provide sequence numbers. If sequencing is
requested
without requesting a L2-Specific Sublayer AVP, the session MUST be
disconnected with a Result Code of RC-TBA-3.
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.
Tx Connect Speed (ICRQ, ICRP, ICCN)
The Tx Connect Speed BPS AVP, Attribute Type 24, contains 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tx Connect Speed BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Tx Connect Speed 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.
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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 (ICRQ, ICRP, ICCN)
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rx Connect Speed BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Rx Connect Speed 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, ICRP, OCRP)
The Physical Channel ID AVP, Attribute Type 25, contains the vendor-
specific physical channel number used for a call.
The Attribute Value field for this AVP has the following format:
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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.
5.4.5 Circuit Status AVPs
Circuit Status (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, SLI)
The Circuit Status AVP, Attribute Type AVP-TBA-13, indicates the
initial status of or a status change in the circuit to which the
session is bound.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The A (Active) bit indicates whether the circuit is up/active/ready
(1) or down/inactive/not-ready (0).
The N (New) bit indicates whether the circuit status indication is
for a new circuit (1) or an existing circuit (0). Links which have a
similar mechanism available (e.g. Frame Relay) MUST map the setting
of this bit to the associated signaling for that link. Otherwise, the
New bit SHOULD still be set the first time the L2TP session is
established after provisioning.
The remaining bits are reserved for future use. Reserved bits MUST
be set to 0 when sending and ignored upon receipt.
The Circuit Status AVP is used to advertise whether a circuit or
interface bound to an L2TP session is up and ready to send and/or
receive traffic. Different circuit types have different names for
status types. For example, HDLC primary and secondary stations refer
to a circuit as being "Receive Ready" or "Receive Not Ready", while
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Frame Relay refers to a circuit as "Active" or "Inactive". This AVP
adopts the latter terminology, though the concept remains the same
regardless of the PW type for the L2TP session.
In the simplest case, the circuit referred by this AVP is a single
physical
interface, port, or circuit depending on application and how the
session
was setup. The status indication in this AVP may then be used to
provide simple ILMI interworking for a variety of circuit types. For
virtual or multipoint interfaces, the Circuit Status AVP is still
utilized, but effectively refers to the state of an internal
structure or a logical set of circuits. Each PW-specific companion
document MUST then specify precisely how this AVP is translated for
each circuit type.
Regardless of the interworking for each circuit type which may or may
not be necessary, if this AVP is received with a Not Active
notification for a given L2TP session, all data traffic for that
session MUST cease (or not begin) in the direction of the sender of
the Circuit Status AVP until the circuit is advertised as Active.
The Circuit Status MUST be advertised by this AVP in ICRQ, ICRP,
OCRQ, and OCRP messages. Often, the circuit type will be marked
Active when initiated, but MAY be advertised as Inactive, indicating
that an L2TP session is to be created but that the interface or
circuit is still not ready to pass traffic. The ICCN, OCCN, and SLI
control messages all MAY contain this AVP to update the status of the
circuit after establishment of the L2TP session is requested.
If additional circuit status information is needed for a given PW
type, PW-specific AVPs MUST be defined in a separate document for
that information. This AVP is only for general circuit status
information applicable to all circuit/interface types.
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 8.
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:
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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 packets 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
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
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establishment process.
The following AVPs MUST be present in the SCCRQ:
Message Type
Host Name
Router ID
Assigned Control Connection ID
Pseudowire Capabilities List
The following AVPs MAY be present in the SCCRQ:
Random Vector
Message Digest
Control Connection Tie Breaker
Vendor Name
Receive Window Size
6.2 Start-Control-Connection-Reply (SCCRP)
Start-Control-Connection-Reply (SCCRP) is the 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
Host Name
Router ID
Assigned Control Connection ID
Pseudowire Capabilities List
The following AVPs MAY be present in the SCCRP:
Random Vector
Message Digest
Vendor Name
Receive Window Size
6.3 Start-Control-Connection-Connected (SCCCN)
Start-Control-Connection-Connected (SCCCN) is the 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:
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Message Type
The following AVP MAY be present in the SCCCN:
Random Vector
Message Digest
6.4 Stop-Control-Connection-Notification (StopCCN)
Stop-Control-Connection-Notification (StopCCN) is the 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
The following AVPs MAY be present in the StopCCN:
Random Vector
Message Digest
Assigned Control Connection ID
Note that the Assigned Control Connection ID MUST be present if the
StopCCN is sent after an SCCRQ or SCCRP message has been sent.
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
The following AVP MAY be present in the HELLO:
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Random Vector
Message Digest
6.6 Incoming-Call-Request (ICRQ)
Incoming-Call-Request (ICRQ) is the 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
Serial Number
Pseudowire Type
Circuit Status
The following AVPs MAY be present in the ICRQ:
Random Vector
Message Digest
Assigned Cookie
End ID
Application ID
Session Tie Breaker
L2-Specific Sublayer
Data Sequencing
Tx Connect Speed
Rx Connect Speed
Physical Channel ID
6.7 Incoming-Call-Reply (ICRP)
Incoming-Call-Reply (ICRP) is the control message sent by an LCCE in
response to a received 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
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the peer should establish (i.e. 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
Circuit Status
The following AVPs MAY be present in the ICRP:
Random Vector
Message Digest
Assigned Cookie
L2-Specific Sublayer
Data Sequencing
Tx Connect Speed
Rx Connect Speed
Physical Channel ID
6.8 Incoming-Call-Connected (ICCN)
Incoming-Call-Connected (ICCN) is the control message sent by the
LCCE that originally sent an ICRQ upon receiving an ICRP from its
peer. It is the final message in the three-message exchange used for
establishing L2TP sessions.
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
Local Session ID
Remote Session ID
The following AVPs MAY be present in the ICCN:
Random Vector
Message Digest
L2-Specific Sublayer
Data Sequencing
Tx Connect Speed
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Rx Connect Speed
Circuit Status
6.9 Outgoing-Call-Request (OCRQ)
Outgoing-Call-Request (OCRQ) is the 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 call
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
Serial Number
Pseudowire Type
Circuit Status
The following AVPs MAY be present in the OCRQ:
Random Vector
Message Digest
Assigned Cookie
End ID
Application ID
Session Tie Breaker
L2-Specific Sublayer
Data Sequencing
6.10 Outgoing-Call-Reply (OCRP)
Outgoing-Call-Reply (OCRP) is the control message sent by an LAC to
an LCCE in response to a received 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
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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
Circuit Status
The following AVPs MAY be present in the OCRP:
Random Vector
Message Digest
Assigned Cookie
L2-Specific Sublayer
Data Sequencing
Physical Channel ID
6.11 Outgoing-Call-Connected (OCCN)
Outgoing-Call-Connected (OCCN) is the control message sent by an LAC
to another LCCE after 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.
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
Local Session ID
Remote Session ID
The following AVPs MAY be present in the OCCN:
Random Vector
Message Digest
L2-Specific Sublayer
Data Sequencing
Circuit Status
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
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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
Result Code
Local Session ID
Remote Session ID
The following AVP MAY be present in the CDN:
Random Vector
Message Digest
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
Local Session ID
Remote Session ID
Circuit Errors
The following AVP MAY be present in the WEN:
Random Vector
Message Digest
6.14 Set-Link-Info (SLI)
The Set-Link-Info control message is sent by an LCCE to convey link
or circuit status change information regarding the circuit associated
with this L2TP session. For example, if PPP renegotiates LCP at an
LNS or between an LAC and a Remote System, or if a forwarded Frame
Relay VC transitions to Active or Inactive at an LAC, an SLI message
SHOULD be sent to indicate this event. Precise details of when the
SLI is sent, what PW type-specific AVPs must be present, and how
those AVPs should be interpreted by the receiving peer are outside
the scope of this document. These details should be described in the
associated payload-specific documents that require use of this
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message.
The following AVPs MUST be present in the SLI:
Message Type
Local Session ID
Remote Session ID
The following AVPs MAY be present in the SLI:
Random Vector
Message Digest
Circuit Status
6.15 Explicit-Acknowledgement (ACK)
The Explicit Acknowledgement (ACK) message is used only to
acknowledge receipt of a message or messages on the Control
Connection (e.g. for purposes of updating Ns and Nr values). Receipt
of this message does not trigger an event for the L2TP protocol state
machine.
A message received without any AVPs (including the Message Type AVP),
is referred to as a Zero Length Body (ZLB) message, and serves the
same function as the Explicit Acknowledgement. ZLB messages are only
permitted when the Control Message Authentication defined in Section
4.3 is not enabled.
The following AVPs MAY be present in the ACK message:
Message Type
Message Digest
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
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
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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.
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
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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
Control Connection 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
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
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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.
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.
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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
wait-reply Receive ICRP, Send ICCN established
acceptable
wait-reply Receive ICRP, Send CDN, idle
Not acceptable clean up
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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 messages 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
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.
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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.
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
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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
idle, Receive CDN Clean up idle
wait-reply,
wait-connect,
established
established Receive OCRQ, Send CDN, idle
OCRP, OCCN clean up
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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
idle Receive OCRP Send CDN, idle
clean up
idle Receive OCCN, Clean up idle
CDN
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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.
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 full
control message retransmission cycle (e.g. 1 + 2 + 4 + 8 ... seconds)
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.
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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
This section addresses some of the security issues that L2TP
encounters in its operation.
8.1 Control Connection Endpoint and Message Security
The LCCEs may configure a shared secret (password) in order to
perform a mutual authentication of one another, and construct an
authentication and integrity check of all arriving Control Messages.
This mechanism is built-in to L2TPv3, and is described in section 4.3
and in the definition of the Message Digest and Nonce AVPs in section
5.4.3.
This mechanism provides strong mutual peer authentication, and
authentication and integrity checking for individual Control
Messages.
8.2 Data Channel Security
As described in section 4.1, the Assigned Cookie sent with each data
packet MUST be selected in an unpredictable manner (with the added
restriction that two same Cookie values not be selected within a
short period of time for a given Session ID).
A 64-bit Cookie provides effective protection against a blind packet
insertion attack on a given PE. This is useful as a security feature
only within networks where sniffing and correlating packets between
L2TP nodes is considered impossible, though inserting IP packets
destined to an LCCE may be considered possible (and perhaps trivial
by an individual armed with the proper hacking tools). In such cases,
the Cookie provides an effective against barrier against packet
insertion into a VPN by enforcing that a given Session ID match the
random 64 bit Cookie. Without specific knowledge of what these values
are, it is virtually impossible to guess. A 32 bit Cookie is
vulnerable to brute force guessing at high packet rates, and as such
should not be considered an effective barrier (it still provides an
additional integrity check for the Session ID, as described in
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section 4.1).
The L2TPv3 Cookie MUST not be regarded as a substitute for packet-
level security such as that of IPsec when operating over an open or
untrusted network where packets may be sniffed and values correlated
to spoofed packets. Further, L2TPv3 does not attempt to provide data
packet encryption of any kind without the aid of IPsec.
8.3 End-to-End Security
Protecting the L2TP packet stream does, in turn, also protect the
data within the tunneled session packets while transported from one
LCCE to the other. Such protection MUST 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 [RFC2401] provides packet-level security
via ESP [RFC3193]. All L2TP control and data packets for a
particular control connection appear as homogeneous UDP or 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
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.
8.5 Impact of L2TPv3 Features on RFC 3193
[RFC3193] defines the recommended method for securing L2TPv2. L2TPv3
possesses identical characteristics to IPsec as L2TPv2 when running
on UDP/IP. When operating over IP directly, the principles defined
in [RFC3193] still apply, though references to UDP port selection (in
particular Section 4 ^oIPsec Filtering details when protecting L2TPo)
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become far simpler as we have only a single IP protocol ID to worry
about. Specific details for operating L2TPv3 with IPsec will be
specified in an update to [RFC3193].
9. Internationalization Considerations
The Host Name and Vendor Name AVPs are not internationalized. The
Vendor Name AVP, although intended to be human-readable, would seem
to fit in the category of "globally visible names" [RFC3066] and so
is represented in US-ASCII.
The Preferred Language AVP is not mandatory. If an LCCE does not
signify a language preference by the inclusion of this AVP in the
SCCRQ or SCCRP, the Preferred Language AVP is unrecognized, or the
requested language is not supported by the peer LCCE, the default
language [RFC2277] MUST be used for all internationalized strings
sent by the peer.
10. 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.
10.1 Control Message Attribute Value Pairs (AVPs)
This number space is managed by IANA as per [RFC3438].
New AVPs requiring assignment in this document are defined in the
"AVP Summary," Section 5.4, with the encoding "AVP-TBA-x," where "x"
is 1, 2, 3 ...
A short summary of the new AVPs follows:
AVP-TBA-1 Message Digest AVP-TBA-2 Router ID, AVP-TBA-3 Assigned
Control Connection ID AVP-TBA-4 Pseudowire Capabilities List AVP-
TBA-5 Local Session ID AVP-TBA-6 Remote Session ID AVP-TBA-7
Assigned Cookie AVP-TBA-8 Remote End ID AVP-TBA-9 Application Code
AVP-TBA-10 Pseudowire Type AVP-TBA-11 L2-Specific Sublayer AVP-TBA-12
Data Sequencing AVP-TBA-13 Circuit Status AVP-TBA-14 Preferred
Language
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10.2 Message Type AVP Values
This number space is managed by IANA as per [RFC3438]. There is one
new message type, defined in section 3.1, necessary to be allocated
for this specification:
TBA-M1 (ACK) Explicit Acknowledgement
10.3 Result Code AVP Values
This number space is managed by IANA as per [RFC3438].
New Result Code values for the CDN message are defined in section
5.4. Following is a summary:
RC-TBA-1 - Session not established due to losing tie breaker. RC-
TBA-2 - Session not established due to unsupported PW type. RC-TBA-3
- Session not established, sequencing required without valid
L2-Specific Sublayer.
There are a few cases in Section 5 where these values are referred to
directly within the document text with the RC-TBA-x format. The
assigned values should be inserted within the text for these cases.
10.3.2 Error Code Field Values
This number space is managed by IANA as per [RFC3438].
10.4 AVP Header Bits
There are four remaining reserved bits in the AVP header. Additional
bits should only be assigned via a Standards Action [RFC2434].
10.5 L2TP Control Message Header Bits
There are nine remaining reserved bits in the control message header.
Additional bits should only be assigned via a Standards Action
[RFC2434].
Care should be taken before using reserved bits 6 and 7 in the L2TPv3
control message header since these bits have meaning for L2TPv2 data
messages. Using these two bits in L2TPv3 MAY trigger an unforeseen
interoperability problem with L2TPv3 implementations based on L2TPv2.
Therefore, it is recommended that these two bits be utilized last,
after the other reserved bits have been assigned roles.
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10.6 Pseudowire Types
The Pseudowire Type (PW Type, Section 5.4) is a two-octet value used
in the Pseudowire Type AVP and Pseudowire Capabilities List AVP
defined in Section 5.4.3. 0 to 32767 are assignable by Expert Review
[RFC2434], 32768 to 65535 by a First Come First Served policy
[RFC2434]. There are no specific pseudowire types assigned within
this document. Each pseudowire-specific document MUST allocate its
own PW types from IANA as necessary.
10.7 Application Code
The Application Code (Section 5.4) is a two-octet value used in the
Application Code AVP. Value 0 is assigned to the base application
defined in this document. Additional Application Codes may be
assigned by IETF Consensus [RFC2434].
10.8 Circuit Status Bits
The Circuit Status (Section 5.4) field is a 16 bit mask, with the two
high order bits assigned.
Bit 15 - A (Active) bit Bit 16 ^u N (New) bit
Additional bits may be assigned by IETF Consensus [RFC2434].
10.9 Default L2-Specific Sublayer bits
The Default L2 Specific Sublayer defined in Section 4.6 contains 8
bits in the low-order portion of the header, two of which have been
assigned and 6 remain.
Bit 0 - P (Priority) bit Bit 1 - S (Sequence) bit
Additional values may be assigned by IETF Consensus [RFC2434].
10.10 L2-Specific Sublayer Type
The L2-Specific Sublayer Type is a 2 octet unsigned integer of which
two values have been assigned.
0 - No L2-Specific Sublayer 1 - Default L2-Specific Sublayer present
Additional values may be assigned by Expert Review [RFC2434].
10.11 Data Sequencing Level
The Data Sequencing Level is a 2 octet unsigned integer of which
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three values have been assigned.
0 - No incoming data packets require sequencing. 1 - Only non-IP
data packets require sequencing. 2 - All incoming data packets
require sequencing.
Additional values may be assigned by Expert Review [RFC2434].
13. Acknowledgments
Many of the protocol constructs were originally defined in, and the
text of this document began with, RFC 2661, "L2TPv2". RFC 2661
authors are W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn and
B. Palter.
The basic concept for L2TP and many of its protocol constructs were
adopted from L2F [RFC2341] and PPTP [RFC2637]. Authors of these
drafts are A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G. Pall,
W. Verthein, J. Taarud, W. Little, and G. Zorn.
Danny Mcpherson and Suhail Nanji published the first "L2TP Service
Type" draft which defined the use of L2TP for tunneling of various L2
payload types (initially, Ethernet and Frame Relay).
The team for splitting RFC 2661 into this base document and the
companion PPP document consisted of Ignacio Goyret, Jed Lau, Bill
Palter, Mark Townsley, and Madhvi Verma. Skip Booth also provided
very helpful review and comment.
Some constructs of L2TPv3 were based in part on UTI (Universal
Transport Interface), which was originally conceived by Peter
Lothberg and Tony Bates.
Stewart Bryant and Simon Barber provided valuable input for the
L2TPv3 over IP header.
Juha Heinanen provided helpful review, and input for the Application
ID AVP.
A number of people provided valuable input and effort for RFC2661, on
which this document was based:
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.
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Thomas Narten provided a great deal of critical review and
formatting. He originally 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 RFC
2661.
Steve Cobb and Evan Caves redesigned the state machine tables.
Barney Wolff provided a great deal of design input on the endpoint
authentication mechanism.
11. References
11.1 Normative References
[RFC1750] D. Eastlake III, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994
[RFC1958] Carpenter, B., "Architectural Principles of the Internet",
RFC 1958, June 1996.
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC2072] Berkowitz, H., "Router Renumbering Guide", RFC 2072,
January 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2661] Townsley, W., et al., "Layer Two Tunneling Layer Two Tunneling
Protocol (L2TP)", RFC 2661, August 1999.
[RFC2865] Rigney, C., Rubens, A., Simpson, W., and Willens, S.,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3066] Alvestrand, H., "Tags for the Identification of Languages",
RFC 3066, January 2001.
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INTERNET DRAFT L2TPv3 June 2003
[RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and Booth, S.,
"Securing L2TP using IPsec", RFC 3193, November 2001.
11.2 Informative References
[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.
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC1700] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2,
RFC 1700, October 1994. See also:
http://www.iana.org/numbers.html.
[RFC2138] Rigney, C., Rubens, A., Simpson, W., and Willens, S.,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2138, April 1997.
[RFC2341] Valencia, A., Littlewood, M., and Kolar, T.,
"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.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W.,
and Zorn, G., "Point-to-Point Tunneling Protocol (PPTP)",
RFC 2637, July 1999.
[RFC2809] Aboba, B., and Zorn, G., "Implementation of L2TP Compulsory
Tunneling via RADIUS", RFC 2809, April 2000.
[RFC3070] Rawat, V., Tio, R., Nanji, S., and Verma, R.,
"Layer Two Tunneling Protocol (L2TP) over Frame Relay",
RFC 3070, February 2001.
[RFC3355] Singh, A., Turner, R., Tio, R., Nanji, S., "Layer Two
Tunnelling Protocol (L2TP) Over ATM Adaptation
Layer 5 (AAL5)", RFC 3355, August 2002
[STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I: The
Protocols", Addison-Wesley Publishing Company, Inc.,
March 1996, ISBN 0-201-63346-9.
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12. Editors' Addresses
Jed Lau
cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
jedlau@cisco.com
W. Mark Townsley
cisco Systems
mark@townsley.net
Ignacio Goyret
Lucent Technologies
igoyret@lucent.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].
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 acknowledgment (either explicit or
piggybacked). When the acknowledgment 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 ACK message 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.
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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 an ACK
message. An alternative would be to piggyback the acknowledgment
within a message sent as a reply to the ICRQ or OCRQ that will likely
follow from the side that initiated the control connection.
LCCE A LCCE B
------ ------
SCCRQ ->
Nr: 0, Ns: 0
<- SCCRP
Nr: 1, Ns: 0
SCCCN ->
Nr: 1, Ns: 1
<- ACK
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 effects: 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.
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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 an ACK message)
<- ACK
Nr: 3, Ns: 2
(LCCE B's retransmit timer fires)
<- ICRP
Nr: 3, Ns: 1
ICCN ->
Nr: 2, Ns: 3
<- ACK
Nr: 4, Ns: 2
Appendix C: Processing Sequence Numbers
The Default L2-Specific Sublayer, defined in Section 4.6, provides a
24-bit field for sequencing of data packets within an L2TP session.
L2TP data packets are never retransmitted, so this sequence is used
only to detect packet order, duplicate packets, or lost packets.
The 24-bit field of the Default L2-Specific Sublayer contains a free-
running counter, including zero. Each sequenced data packet that is
sent must contain the sequence number, incremented by one, of the
previous sequenced packet sent on a given L2TP session. Upon receipt,
any packet with a sequence number equal to or greater than the
current expected packet (the last received in-order packet plus one)
should be considered "new" and accepted. All other packets are
considered "old" and discarded.
Larger or smaller sequence number fields are possible with L2TP if an
alternative format to the Default L2-Specific Sublayer defined in
this document is used. While 24 bits may be adequate in a number of
circumstances, a larger sequence number space will be less
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susceptible to sequence number wrapping problems for very high
session data rates across long dropout periods. The sequence number
processing recommendations below should hold for any size sequence
number field.
When detecting whether a packet sequence number is "greater" or
"less" than a given sequence number value, wrapping of the sequence
number must be considered. This is typically accomplished by keeping
a window of sequence numbers beyond the current expected sequence
number for determination of whether a packet is "new" or not. The
window may be sized based on the link speed and sequence number space
and SHOULD be configurable with a default equal to one half the size
of the available number space (e.g. 2^(n-1), where n is the number of
bits available in the sequence number).
Upon receipt, packets which exactly match the expected sequence
number are processed immediately and the next expected sequence
number incremented. Packets that fall within the window for new
packets may either be processed immediately and the next expected
sequence number updated to one plus that received in the new packet,
or held for a very short period of time in hopes of receiving the
missing packet(s). This 'very short period' should be configurable,
with a default corresponding to a time lapse which is at least an
order of magnitude less than the retransmission timeout periods of
higher layer protocols such as TCP.
For typical transient packet mis-orderings, dropping out-of-order
packets alone should suffice and generally requires far less
resources than actively reordering packets within L2TP. An exception
is a case where a pair of packet fragments are persistently
retransmitted and sent out-of-order. For example, if an IP packet has
been fragmented into a very small packet followed by a very large
packet before being tunneled by L2TP, it is possible (though
admittedly wrong) that the two resulting L2TP packets may be
consistently mis-ordered by the PSN in transit between L2TP nodes. If
sequence numbers were being enforced at the receiving node without
any buffering of out-of-order packets, then the fragmented IP packet
may never reach its destination. It may be worth noting here that
this condition is true for any tunneling mechanism of IP packets
which include sequence number checking on receipt (i.e. GRE
[RFC2890]).
Utilization of a Data Sequencing Level (see Section 5.4.3) of 1 (only
non-IP data packets require sequencing) allows IP data packets being
tunneled by L2TP to not utilize sequence numbers, while utilizing
sequence numbers and enforcing packet order for any remaining non-IP
data packets. Depending on the requirements of the link-layer being
tunneled, and the network data traversing the data-link, this is
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sufficient in many cases to enforce packet order on frames which
require it (such as end-to-end data-link control messages), while not
on IP packets which are known to be resilient to packet reordering.
If a large number of packets (e.g. more than one new packet window)
are dropped due to an extended outage, or loss of sequence number
state on one side of the connection (perhaps as part of a forwarding
plane reset or failover to a standby node), it is possible that a
large number of packets will be sent in-order, but be wrongly
detected by the peer as out-of-order. This can be generally
characterized for a window size, w, sequence number space, s, and
number of packets lost in transit between L2TP endpoints, p, as
follows:
If s > p > w, then an additional (s - p) packets that were otherwise
received in-order, will be incorrectly classified as out-of-order and
dropped. Thus, for a sequence number space, s = 128, window size, w =
64, and number of lost packets, p = 70; 128 - 70 = 58 additional
packets would be dropped after the outage until the sequence number
wrapped back to the current expected next sequence number.
To mitigate this additional packet loss, one MUST inspect the
sequence numbers of packets dropped due to being classified as "old"
and reset the expected sequence number accordingly. This may be
accomplished by counting the number of "old" packets dropped that
were in sequence among themselves and upon reaching a threshold,
resetting the next expected sequence number to that seen in the
arriving data packets. Packet timestamps may also be used as an
indicator to reset the expected sequence number by detecting a period
of time over which "old" packets have been received in-sequence. The
ideal thresholds will vary depending on link speed, sequence number
space, and link tolerance to out-of-order packets, and MUST be
configurable.
Appendix D: 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
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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.
Appendix E: Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Lau, Townsley, Goyret Standards Track [Page 86]
