Network Working Group J. Lau, Ed.
Request for Comments: 3931 M. Townsley, Ed.
Category: Standards Track Cisco Systems
I. Goyret, Ed.
Lucent Technologies
March 2005
Layer Two Tunneling Protocol - Version 3 (L2TPv3)
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.
Copyright Notice
Copyright (C) The Internet Society (2005).
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 nodes. Additional documents detail the specifics for each data
link type being emulated.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Changes from RFC 2661. . . . . . . . . . . . . . . . . . 4
1.2. Specification of Requirements. . . . . . . . . . . . . . 4
1.3. Terminology. . . . . . . . . . . . . . . . . . . . . . . 5
2. Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Protocol Overview. . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Control Message Types. . . . . . . . . . . . . . . . . . 10
3.2. L2TP Header Formats. . . . . . . . . . . . . . . . . . . 11
3.2.1. L2TP Control Message Header. . . . . . . . . . . 11
3.2.2. L2TP Data Message. . . . . . . . . . . . . . . . 12
3.3. Control Connection Management. . . . . . . . . . . . . . 13
3.3.1. Control Connection Establishment . . . . . . . . 14
3.3.2. Control Connection Teardown. . . . . . . . . . . 14
3.4. Session Management . . . . . . . . . . . . . . . . . . . 15
3.4.1. Session Establishment for an Incoming Call . . . 15
3.4.2. Session Establishment for an Outgoing Call . . . 15
Lau, et al. Standards Track [Page 1]
RFC 3931 L2TPv3 March 2005
3.4.3. Session Teardown . . . . . . . . . . . . . . . . 16
4. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 16
4.1. L2TP Over Specific Packet-Switched Networks (PSNs) . . . 16
4.1.1. L2TPv3 over IP . . . . . . . . . . . . . . . . . 17
4.1.2. L2TP over UDP. . . . . . . . . . . . . . . . . . 18
4.1.3. L2TP and IPsec . . . . . . . . . . . . . . . . . 20
4.1.4. IP Fragmentation Issues. . . . . . . . . . . . . 21
4.2. Reliable Delivery of Control Messages. . . . . . . . . . 23
4.3. Control Message Authentication . . . . . . . . . . . . . 25
4.4. Keepalive (Hello). . . . . . . . . . . . . . . . . . . . 26
4.5. Forwarding Session Data Frames . . . . . . . . . . . . . 26
4.6. Default L2-Specific Sublayer . . . . . . . . . . . . . . 27
4.6.1. Sequencing Data Packets. . . . . . . . . . . . . 28
4.7. L2TPv2/v3 Interoperability and Migration . . . . . . . . 28
4.7.1. L2TPv3 over IP . . . . . . . . . . . . . . . . . 29
4.7.2. L2TPv3 over UDP. . . . . . . . . . . . . . . . . 29
4.7.3. Automatic L2TPv2 Fallback. . . . . . . . . . . . 29
5. Control Message Attribute Value Pairs. . . . . . . . . . . . . 30
5.1. AVP Format . . . . . . . . . . . . . . . . . . . . . . . 30
5.2. Mandatory AVPs and Setting the M Bit . . . . . . . . . . 32
5.3. Hiding of AVP Attribute Values . . . . . . . . . . . . . 33
5.4. AVP Summary. . . . . . . . . . . . . . . . . . . . . . . 36
5.4.1. General Control Message AVPs . . . . . . . . . . 36
5.4.2. Result and Error Codes . . . . . . . . . . . . . 40
5.4.3. Control Connection Management AVPs . . . . . . . 43
5.4.4. Session Management AVPs. . . . . . . . . . . . . 48
5.4.5. Circuit Status AVPs. . . . . . . . . . . . . . . 57
6. Control Connection Protocol Specification. . . . . . . . . . . 59
6.1. Start-Control-Connection-Request (SCCRQ) . . . . . . . . 60
6.2. Start-Control-Connection-Reply (SCCRP) . . . . . . . . . 60
6.3. Start-Control-Connection-Connected (SCCCN) . . . . . . . 61
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