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Network Working Group W. Townsley

Request for Comments: 2661 A. Valencia

Category: Standards Track cisco Systems

A. Rubens

Ascend Communications

G. Pall

G. Zorn

Microsoft Corporation

B. Palter

Redback Networks

August 1999

Layer Two Tunneling Protocol "L2TP"

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document describes the Layer Two Tunneling Protocol (L2TP). STD

51, RFC 1661 specifies multi-protocol access via PPP [RFC1661]. L2TP

facilitates the tunneling of PPP packets across an intervening

network in a way that is as transparent as possible to both end-users

and applications.

Table of Contents

1.0 Introduction.......................................... 3

1.1 Specification of Requirements......................... 4

1.2 Terminology........................................... 4

2.0 Topology.............................................. 8

3.0 Protocol Overview..................................... 9

3.1 L2TP Header Format.................................... 9

3.2 Control Message Types................................. 11

4.0 Control Message Attribute Value Pairs................. 12

4.1 AVP Format............................................ 13

4.2 Mandatory AVPs........................................ 14

4.3 Hiding of AVP Attribute Values........................ 14

Townsley, et al. Standards Track [Page 1]

RFC 2661 L2TP August 1999

4.4 AVP Summary........................................... 17

4.4.1 AVPs Applicable To All Control Messages.......... 17

4.4.2 Result and Error Codes........................... 18

4.4.3 Control Connection Management AVPs............... 20

4.4.4 Call Management AVPs............................. 27

4.4.5 Proxy LCP and Authentication AVPs................ 34

4.4.6 Call Status AVPs................................. 39

5.0 Protocol Operation.................................... 41

5.1 Control Connection Establishment...................... 41

5.1.1 Tunnel Authentication............................ 42

5.2 Session Establishment................................. 42

5.2.1 Incoming Call Establishment...................... 42

5.2.2 Outgoing Call Establishment...................... 43

5.3 Forwarding PPP Frames................................. 43

5.4 Using Sequence Numbers on the Data Channel............ 44

5.5 Keepalive (Hello)..................................... 44

5.6 Session Teardown...................................... 45

5.7 Control Connection Teardown........................... 45

5.8 Reliable Delivery of Control Messages................. 46

6.0 Control Connection Protocol Specification............. 48

6.1 Start-Control-Connection-Request (SCCRQ).............. 48

6.2 Start-Control-Connection-Reply (SCCRP)................ 48

6.3 Start-Control-Connection-Connected (SCCCN)............ 49

6.4 Stop-Control-Connection-Notification (StopCCN)........ 49

6.5 Hello (HELLO)......................................... 49

6.6 Incoming-Call-Request (ICRQ).......................... 50

6.7 Incoming-Call-Reply (ICRP)............................ 51

6.8 Incoming-Call-Connected (ICCN)........................ 51

6.9 Outgoing-Call-Request (OCRQ).......................... 52

6.10 Outgoing-Call-Reply (OCRP)........................... 53

6.11 Outgoing-Call-Connected (OCCN)....................... 53

6.12 Call-Disconnect-Notify (CDN)......................... 53

6.13 WAN-Error-Notify (WEN)............................... 54

6.14 Set-Link-Info (SLI).................................. 54

7.0 Control Connection State Machines..................... 54

7.1 Control Connection Protocol Operation................. 55

7.2 Control Connection States............................. 56

7.2.1 Control Connection Establishment................. 56

7.3 Timing considerations................................. 58

7.4 Incoming calls........................................ 58

7.4.1 LAC Incoming Call States......................... 60

7.4.2 LNS Incoming Call States......................... 62

7.5 Outgoing calls........................................ 63

7.5.1 LAC Outgoing Call States......................... 64

7.5.2 LNS Outgoing Call States......................... 66

7.6 Tunnel Disconnection.................................. 67

8.0 L2TP Over Specific Media.............................. 67

8.1 L2TP over UDP/IP...................................... 68

Townsley, et al. Standards Track [Page 2]

RFC 2661 L2TP August 1999

8.2 IP.................................................... 69

9.0 Security Considerations............................... 69

9.1 Tunnel Endpoint Security.............................. 70

9.2 Packet Level Security................................. 70

9.3 End to End Security................................... 70

9.4 L2TP and IPsec........................................ 71

9.5 Proxy PPP Authentication.............................. 71

10.0 IANA Considerations.................................. 71

10.1 AVP Attributes....................................... 71

10.2 Message Type AVP Values.............................. 72

10.3 Result Code AVP Values............................... 72

10.3.1 Result Code Field Values........................ 72

10.3.2 Error Code Field Values......................... 72

10.4 Framing Capabilities & Bearer Capabilities........... 72

10.5 Proxy Authen Type AVP Values......................... 72

10.6 AVP Header Bits...................................... 73

11.0 References........................................... 73

12.0 Acknowledgments...................................... 74

13.0 Authors’ Addresses................................... 75

Appendix A: Control Channel Slow Start and Congestion

Avoidance..................................... 76

Appendix B: Control Message Examples...................... 77

Appendix C: Intellectual Property Notice.................. 79

Full Copyright Statement.................................. 80

1.0 Introduction

PPP [RFC1661] defines an encapsulation mechanism for transporting

multiprotocol packets across layer 2 (L2) point-to-point links.

Typically, a user obtains a L2 connection to a Network Access Server

(NAS) using one of a number of techniques (e.g., dialup POTS, ISDN,

ADSL, etc.) and then runs PPP over that connection. In such a

configuration, the L2 termination point and PPP session endpoint

reside on the same physical device (i.e., the NAS).

L2TP extends the PPP model by allowing the L2 and PPP endpoints to

reside on different devices interconnected by a packet-switched

network. With L2TP, a user has an L2 connection to an access

concentrator (e.g., modem bank, ADSL DSLAM, etc.), and the

concentrator then tunnels individual PPP frames to the NAS. This

allows the actual processing of PPP packets to be divorced from the

termination of the L2 circuit.

One obvious benefit of such a separation is that instead of requiring

the L2 connection terminate at the NAS (which may require a

long-distance toll charge), the connection may terminate at a (local)

circuit concentrator, which then extends the logical PPP session over

Townsley, et al. Standards Track [Page 3]

RFC 2661 L2TP August 1999

a shared infrastructure such as frame relay circuit or the Internet.

From the user’s perspective, there is no functional difference between

having the L2 circuit terminate in a NAS directly or using L2TP.

L2TP may also solve the multilink hunt-group splitting problem.

Multilink PPP [RFC1990] requires that all channels composing a

multilink bundle be grouped at a single Network Access Server (NAS).

Due to its ability to project a PPP session to a location other than

the point at which it was physically received, L2TP can be used to

make all channels terminate at a single NAS. This allows multilink

operation even when the calls are spread across distinct physical

NASs.

This document defines the necessary control protocol for on-demand

creation of tunnels between two nodes and the accompanying

encapsulation for multiplexing multiple, tunneled PPP sessions.

1.1 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].

1.2 Terminology

Analog Channel

A circuit-switched communication path which is intended to carry

3.1 kHz audio in each direction.

Attribute Value Pair (AVP)

The variable length concatenation of a unique Attribute

(represented by an integer) and a Value containing the actual

value identified by the attribute. Multiple AVPs make up Control

Messages which are used in the establishment, maintenance, and

teardown of tunnels.

Call

A connection (or attempted connection) between a Remote System and

LAC. For example, a telephone call through the PSTN. A Call

(Incoming or Outgoing) which is successfully established between a

Remote System and LAC results in a corresponding L2TP Session

within a previously established Tunnel between the LAC and LNS.

(See also: Session, Incoming Call, Outgoing Call).

Townsley, et al. Standards Track [Page 4]

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hsyglk

2014-09-16

原始RFC文档，格式有点乱
benben3611

2016-03-09

rfc文档，详细介绍了l2tp相关内容，谢谢分享。

alexander_libo

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