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IETF RFC 5571
Softwire Hub and Spoke Deployment Framework with Layer Two Tunneling Protocol Version 2 (L2TPv2)
Last modified on Friday, June 12th, 2009
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Network Working Group B. Storer
Request for Comments: 5571 C. Pignataro, Ed.
Category: Standards Track M. Dos Santos
Cisco Systems
B. Stevant, Ed.
L. Toutain
TELECOM Bretagne
J. Tremblay
Videotron Ltd.
June 2009
Softwire Hub and Spoke Deployment Framework
with Layer Two Tunneling Protocol Version 2 (L2TPv2)
 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) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Storer, et al. Standards Track PAGE 1 
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
Abstract
This document describes the framework of the Softwire "Hub and Spoke"
solution with the Layer Two Tunneling Protocol version 2 (L2TPv2).
The implementation details specified in this document should be
followed to achieve interoperability among different vendor
implementations.
Table of Contents
1. Introduction ....................................................4
1.1. Abbreviations ..............................................5
1.2. Requirements Language ......................................5
1.3. Considerations .............................................6
2. Applicability of L2TPv2 for Softwire Requirements ...............6
2.1. Traditional Network Address Translation (NAT and NAPT) .....6
2.2. Scalability ................................................7
2.3. Routing ....................................................7
2.4. Multicast ..................................................7
2.5. Authentication, Authorization, and Accounting (AAA) ........7
2.6. Privacy, Integrity, and Replay Protection ..................7
2.7. Operations and Management ..................................8
2.8. Encapsulations .............................................8
3. Deployment Scenarios ............................................8
3.1. IPv6-over-IPv4 Softwires with L2TPv2 .......................9
3.1.1. Host CPE as Softwire Initiator ......................9
3.1.2. Router CPE as Softwire Initiator ...................10
3.1.3. Host behind CPE as Softwire Initiator ..............11
3.1.4. Router behind CPE as Softwire Initiator ............12
3.2. IPv4-over-IPv6 Softwires with L2TPv2 ......................14
3.2.1. Host CPE as Softwire Initiator .....................14
3.2.2. Router CPE as Softwire Initiator ...................15
3.2.3. Host behind CPE as Softwire Initiator ..............16
3.2.4. Router behind CPE as Softwire Initiator ............16
4. References to Standardization Documents ........................17
4.1. L2TPv2 ....................................................18
4.2. Securing the Softwire Transport ...........................18
4.3. Authentication, Authorization, and Accounting .............18
4.4. MIB .......................................................18
4.5. Softwire Payload Related ..................................19
4.5.1. For IPv6 Payloads ..................................19
4.5.2. For IPv4 Payloads ..................................19
5. Softwire Establishment .........................................20
5.1. L2TPv2 Tunnel Setup .......................................22
5.1.1. Tunnel Establishment ...............................22
5.1.1.1. AVPs Required for Softwires ...............25
5.1.1.2. AVPs Optional for Softwires ...............25
5.1.1.3. AVPs Not Relevant for Softwires ...........26
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
5.1.2. Tunnel Maintenance .................................26
5.1.3. Tunnel Teardown ....................................27
5.1.4. Additional L2TPv2 Considerations ...................27
5.2. PPP Connection ............................................27
5.2.1. MTU ................................................27
5.2.2. LCP ................................................27
5.2.3. Authentication .....................................28
5.2.4. IPCP ...............................................28
5.2.4.1. IPV6CP ....................................28
5.2.4.2. IPv4CP ....................................28
5.3. Global IPv6 Address Assignment to Endpoints ...............28
5.4. DHCP ......................................................29
5.4.1. DHCPv6 .............................................29
5.4.2. DHCPv4 .............................................29
6. Considerations about the Address Provisioning Model ............30
6.1. Softwire Endpoints' Addresses .............................30
6.1.1. IPv6 ...............................................30
6.1.2. IPv4 ...............................................31
6.2. Delegated Prefixes ........................................31
6.2.1. IPv6 Prefixes ......................................31
6.2.2. IPv4 Prefixes ......................................31
6.3. Possible Address Provisioning Scenarios ...................31
6.3.1. Scenarios for IPv6 .................................32
6.3.2. Scenarios for IPv4 .................................32
7. Considerations about Address Stability .........................32
8. Considerations about RADIUS Integration ........................33
8.1. Softwire Endpoints ........................................33
8.1.1. IPv6 Softwires .....................................33
8.1.2. IPv4 Softwires .....................................33
8.2. Delegated Prefixes ........................................34
8.2.1. IPv6 Prefixes ......................................34
8.2.2. IPv4 Prefixes ......................................34
9. Considerations for Maintenance and Statistics ..................34
9.1. RADIUS Accounting .........................................35
9.2. MIBs ......................................................35
10. Security Considerations .......................................35
11. Acknowledgements ..............................................36
12. References ....................................................37
12.1. Normative References .....................................37
12.2. Informative References ...................................38
Storer, et al. Standards Track PAGE 3
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
1. Introduction
The Softwires Working Group has selected Layer Two Tunneling Protocol
version 2 (L2TPv2) as the phase 1 protocol to be deployed in the
Softwire "Hub and Spoke" solution space. This document describes the
framework for the L2TPv2 "Hub and Spoke" solution, and the
implementation details specified in this document should be followed
to achieve interoperability among different vendor implementations.
In the "Hub and Spoke" solution space, a Softwire is established to
provide the home network with IPv4 connectivity across an IPv6-only
access network, or IPv6 connectivity across an IPv4-only access
network. When L2TPv2 is used in the Softwire context, the voluntary
tunneling model applies. The Softwire Initiator (SI) at the home
network takes the role of the L2TP Access Concentrator (LAC) client
(initiating both the L2TP tunnel/session and the PPP link) while the
Softwire Concentrator (SC) at the ISP takes the role of the L2TP
Network Server (LNS). The terms voluntary tunneling and compulsory
tunneling are defined in Section 1.1 of [RFC 3193]. Since the L2TPv2
compulsory tunneling model does not apply to Softwires, it SHOULD NOT
be requested or honored. This document identifies all the voluntary
tunneling related L2TPv2 attributes that apply to Softwires and
specifies the handling mechanism for such attributes in order to
avoid ambiguities in implementations. This document also identifies
the set of L2TPv2 attributes specific to the compulsory tunneling
model that does not apply to Softwires and specifies the mechanism to
ignore or nullify their effect within the Softwire context.
The SI and SC MUST follow the L2TPv2 operations described in
[RFC 2661] when performing Softwire establishment, teardown, and
Operations, Administration, and Management (OAM). With L2TPv2, a
Softwire consists of an L2TPv2 Control Connection (also referred to
as Control Channel), a single L2TPv2 Session, and the PPP link
negotiated over the Session. To establish the Softwire, the SI first
initiates an L2TPv2 Control Channel to the SC, which accepts the
request and terminates the Control Channel. L2TPv2 supports an
optional mutual Control Channel authentication that allows both SI
and SC to validate each other's identity at the initial phase of
hand-shaking before proceeding with Control Channel establishment.
After the L2TPv2 Control Channel is established between the SI and
SC, the SI initiates an L2TPv2 Session to the SC. Then the PPP/IP
link is negotiated over the L2TPv2 Session between the SI and SC.
After the PPP/IP link is established, it acts as the Softwire between
the SI and SC for tunneling IP traffic of one Address Family (AF)
across the access network of another Address Family.
Storer, et al. Standards Track PAGE 4
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
During the life of the Softwire, both SI and SC send L2TPv2 keepalive
HELLO messages to monitor the health of the Softwire and the peer
L2TP Control Connection Endpoint (LCCE), and to potentially refresh
the NAT/NAPT (Network Address Translation / Network Address Port
Translation) entry at the CPE or at the other end of the access link.
Optionally, Link Control Protocol (LCP) ECHO messages can be used as
keepalives for the same purposes. In the event of keepalive timeout
or administrative shutdown of the Softwire, either the SI or the SC
MAY tear down the Softwire by tearing down the L2TPv2 Control Channel
and Session as specified in [RFC 2661].
1.1. Abbreviations
AF Address Family, IPv4 or IPv6.
CPE Customer Premises Equipment.
LCCE L2TP Control Connection Endpoint, an L2TP node that exists at
either end of an L2TP Control Connection. (See [RFC 3931].)
LNS L2TP Network Server, a node that acts as one side of an L2TP
tunnel (Control Connection) endpoint. The LNS is the logical
termination point of a PPP session that is being tunneled from
the remote system by the peer LCCE. (See [RFC 2661].)
SC Softwire Concentrator, the node terminating the Softwire in
the service provider network. (See [RFC 4925].)
SI Softwire Initiator, the node initiating the Softwire within
the customer network. (See [RFC 4925].)
SPH Softwire Payload Header, the IP headers being carried within a
Softwire. (See [RFC 4925].)
STH Softwire Transport Header, the outermost IP header of a
Softwire. (See [RFC 4925].)
SW Softwire, a shared-state "tunnel" created between the SC and
SI. (See [RFC 4925].)
1.2. Requirements Language
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 [RFC 2119].
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
1.3. Considerations
Some sections of this document contain considerations that are not
required for interoperability and correct operation of Softwire
implementations. These sections' titles are marked beginning with
"Considerations".
2. Applicability of L2TPv2 for Softwire Requirements
A list of Softwire "Hub and Spoke" requirements has been identified
by the Softwire Problem Statement [RFC 4925]. The following sub-
sections describe how L2TPv2 fulfills each of them.
2.1. Traditional Network Address Translation (NAT and NAPT)
A "Hub and Spoke" Softwire must be able to traverse Network Address
Translation (NAT) and Network Address Port Translation (NAPT, also
referred to as Port Address Translation or PAT) devices [RFC 3022] in
case the scenario in question involves a non-upgradable, preexisting
IPv4 home gateway performing NAT/NAPT or some carrier equipment at
the other end of the access link performing NAT/NAPT. The L2TPv2
Softwire (i.e., Control Channel and Session) is capable of NAT/NAPT
traversal since L2TPv2 can run over UDP.
Since L2TPv2 does not detect NAT/NAPT along the path, L2TPv2 does not
offer the option of disabling UDP. The UDP encapsulation is present
regardless of NAT/NAPT presence. Both NAT/NAPT "autodetect" and UDP
"bypass" are optional requirements in Section 2.3 of [RFC 4925].
As mentioned in Section 8.1 of [RFC 2661] and Section 4 of [RFC 3193],
an L2TP Start-Control-Connection-Reply (SCCRP) responder (SC) can
chose a different IP address and/or UDP port than those from the
initiator's Start-Control-Connection-Request (SCCRQ) (SI). This may
or may not traverse a NAT/NAPT depending on the NAT/NAPT's Filtering
Behavior (see Section 5 of [RFC 4787]). Specifically, any IP address
and port combination will work with Endpoint-Independent Filtering,
but changing the IP address and port will not work through Address-
Dependent or Address-and-Port-Dependent Filtering. Given this,
responding from a different IP address and/or UDP port is NOT
RECOMMENDED.
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
2.2. Scalability
In the "Hub and Spoke" model, a carrier must be able to scale the
solution to millions of Softwire Initiators by adding more hubs
(i.e., Softwire Concentrators). L2TPv2 is a widely deployed protocol
in broadband services, and its scalability has been proven in
multiple large-scale IPv4 Virtual Private Network deployments, which
scale up to millions of subscribers each.
2.3. Routing
There are no dynamic routing protocols between the SC and SI. A
default route from the SI to the SC is used.
2.4. Multicast
Multicast protocols simply run transparently over L2TPv2 Softwires
together with other regular IP traffic.
2.5. Authentication, Authorization, and Accounting (AAA)
L2TPv2 supports optional mutual Control Channel authentication and
leverages the optional mutual PPP per-session authentication. L2TPv2
is well integrated with AAA solutions (such as RADIUS) for both
authentication and authorization. Most L2TPv2 implementations
available in the market support the logging of authentication and
authorization events.
L2TPv2 integration with RADIUS accounting (RADIUS Accounting
extension for tunnel [RFC 2867]) allows the collection and reporting
of L2TPv2 Softwire usage statistics.
2.6. Privacy, Integrity, and Replay Protection
Since L2TPv2 runs over IP/UDP in the Softwire context, IPsec
Encapsulating Security Payload (ESP) can be used in conjunction to
provide per-packet authentication, integrity, replay protection, and
confidentiality for both L2TPv2 control and data traffic [RFC 3193]
and [RFC 3948].
For Softwire deployments in which full payload security is not
required, the L2TPv2 built-in Control Channel authentication and the
inherited PPP authentication and PPP Encryption Control Protocol can
be considered.
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
2.7. Operations and Management
L2TPv2 supports an optional in-band keepalive mechanism that injects
HELLO control messages after a specified period of time has elapsed
since the last data or control message was received on a tunnel (see
Section 5.5 of [RFC 2661]). If the HELLO control message is not
reliably delivered, then the Control Channel and its Session will be
torn down. In the Softwire context, the L2TPv2 keepalive is used to
monitor the connectivity status between the SI and SC and/or as a
refresh mechanism for any NAT/NAPT translation entry along the access
link.
LCP ECHO offers a similar mechanism to monitor the connectivity
status, as described in [RFC 1661]. Softwire implementations SHOULD
use L2TPv2 Hello keepalives, and in addition MAY use PPP LCP Echo
messages to ensure Dead End Detection and/or to refresh NAT/NAPT
translation entries. The combination of these two mechanisms can be
used as an optimization.
The L2TPv2 MIB [RFC 3371] supports the complete suite of management
operations such as configuration of Control Channel and Session,
polling of Control Channel and Session status and their traffic
statistics and notifications of Control Channel and Session UP/DOWN
events.
2.8. Encapsulations
L2TPv2 supports the following encapsulations:
o IPv6/PPP/L2TPv2/UDP/IPv4
o IPv4/PPP/L2TPv2/UDP/IPv6
o IPv4/PPP/L2TPv2/UDP/IPv4
o IPv6/PPP/L2TPv2/UDP/IPv6
Note that UDP bypass is not supported by L2TPv2 since L2TPv2 does not
support "autodetect" of NAT/NAPT.
3. Deployment Scenarios
For the "Hub and Spoke" problem space, four scenarios have been
identified. In each of these four scenarios, different home
equipment plays the role of the Softwire Initiator. This section
elaborates each scenario with L2TPv2 as the Softwire protocol and
Storer, et al. Standards Track PAGE 8
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
other possible protocols involved to complete the solution. This
section examines the four scenarios for both IPv6-over-IPv4
(Section 3.1) and IPv4-over-IPv6 (Section 3.2) encapsulations.
3.1. IPv6-over-IPv4 Softwires with L2TPv2
The following sub-sections cover IPv6 connectivity (SPH) across an
IPv4-only access network (STH) using a Softwire.
3.1.1. Host CPE as Softwire Initiator
The Softwire Initiator (SI) is the host CPE (directly connected to a
modem), which is dual-stack. There is no other gateway device. The
IPv4 traffic SHOULD NOT traverse the Softwire. See Figure 1.
IPv6 or dual-stack IPv4-only dual-stack
|------------------||-----------------||----------|
I SC SI
N +-----+ +----------+
T | | | v4/v6 |
E <==[ IPv6 ]....|v4/v6|....[IPv4-only]....| host CPE |
R [network] | | [ network ] | |
N | LNS | |LAC Client|
E +-----+ +----------+
T _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _() <-- IPv6 traffic
PPP o L2TPv2 o UDP o IPv4 (SPH)
Softwire
<------------------>
IPV6CP: capable of /64 Intf-Id assignment or
uniqueness check
|------------------>/64 prefix
RA
|------------------>DNS, etc.
DHCPv6
Figure 1: Host CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, the IPv6 Control Protocol (IPV6CP)
negotiates IPv6-over-PPP, which also provides the capability for the
ISP to assign the 64-bit Interface-Identifier to the host CPE or
perform uniqueness validation for the two interface identifiers at
the two PPP ends [RFC 5072]. After IPv6-over-PPP is up, IPv6
Storer, et al. Standards Track PAGE 9
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
Stateless Address Autoconfiguration / Neighbor Discovery runs over
the IPv6-over-PPP link, and the LNS can inform the host CPE of a
prefix to use for stateless address autoconfiguration through a
Router Advertisement (RA) while other non-address configuration
options (such as DNS [RFC 3646] or other servers' addresses that might
be available) can be conveyed to the host CPE via DHCPv6.
3.1.2. Router CPE as Softwire Initiator
The Softwire Initiator (SI) is the router CPE, which is a dual-stack
device. The IPv4 traffic SHOULD NOT traverse the Softwire. See
Figure 2.
IPv6 or dual-stack IPv4-only dual-stack
|------------------||-----------------||---------------------|
I SC SI
N +-----+ +----------+
T | | | v4/v6 | +-----+
E <==[ IPv6 ]....|v4/v6|....[IPv4-only]....| CPE |----|v4/v6|
R [network] | | [ network ] | | | host|
N | LNS | |LAC Client| +-----+
E +-----+ +----------+
T _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _() <-------- IPv6 traffic
PPP o L2TPv2 o UDP o IPv4 (SPH)
Softwire
<------------------>
IPV6CP: capable of /64 Intf-Id assignment or
uniqueness check
|------------------>/64 prefix
RA
|------------------>/48 prefix,
DHCPv6 DNS, etc.
|------->/64 prefix
RA
|-------> DNS, etc.
DHCPv4/v6
Figure 2: Router CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, IPV6CP negotiates IPv6-over-PPP,
which also provides the capability for the ISP to assign the 64-bit
Storer, et al. Standards Track PAGE 10
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
Interface-Identifier to the router CPE or perform uniqueness
validation for the two interface identifiers at the two PPP ends
[RFC 5072]. After IPv6-over-PPP is up, IPv6 Stateless Address
Autoconfiguration / Neighbor Discovery runs over the IPv6-over-PPP
link, and the LNS can inform the router CPE of a prefix to use for
stateless address autoconfiguration through a Router Advertisement
(RA). DHCPv6 can be used to perform IPv6 Prefix Delegation (e.g.,
delegating a prefix to be used within the home network [RFC 3633]) and
convey other non-address configuration options (such as DNS
[RFC 3646]) to the router CPE.
3.1.3. Host behind CPE as Softwire Initiator
The CPE is IPv4-only. The Softwire Initiator (SI) is a dual-stack
host (behind the IPv4-only CPE), which acts as an IPv6 host CPE. The
IPv4 traffic SHOULD NOT traverse the Softwire. See Figure 3.
IPv6 or dual-stack IPv4-only dual-stack
|------------------||----------------------------||----------|
I SC SI
N +-----+ +----------+
T | | +-------+ | v4/v6 |
E <==[ IPv6 ]....|v4/v6|....[IPv4-only]....|v4-only|--| host |
R [network] | | [ network ] | CPE | | |
N | LNS | +-------+ |LAC Client|
E +-----+ +----------+
T _ _ _ _ _ _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _ _ _ _ _ _() <-- IPv6
PPP o L2TPv2 o UDP o IPv4 traffic
Softwire (SPH)
<------------------------------>
IPV6CP: capable of /64 Intf-Id assignment or
uniqueness check
|------------------------------>/64 prefix
RA
|------------------------------>DNS, etc.
DHCPv6
Figure 3: Host behind CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, IPV6CP negotiates IPv6-over-PPP,
which also provides the capability for the ISP to assign the 64-bit
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
Interface-Identifier to the host or perform uniqueness validation for
the two interface identifiers at the two PPP ends [RFC 5072]. After
IPv6-over-PPP is up, IPv6 Stateless Address Autoconfiguration /
Neighbor Discovery runs over the IPv6-over-PPP link, and the LNS can
inform the host of a prefix to use for stateless address
autoconfiguration through a Router Advertisement (RA) while other
non-address configuration options (such as DNS [RFC 3646]) can be
conveyed to the host via DHCPv6.
3.1.4. Router behind CPE as Softwire Initiator
The CPE is IPv4-only. The Softwire Initiator (SI) is a dual-stack
device (behind the IPv4-only CPE) acting as an IPv6 CPE router inside
the home network. The IPv4 traffic SHOULD NOT traverse the Softwire.
See Figure 4.
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
IPv6 or dual-stack IPv4-only dual-stack
|------------------||-------------------------||-------------|
I SC SI
N +-----+ +----------+
T | | +-------+ | v4/v6 |
E <==[ IPv6 ]....|v4/v6|..[IPv4-only]..|v4-only|---| router |
R [network] | | [ network ] | CPE | | | |
N | LNS | +-------+ | |LAC Client|
E +-----+ | +----------+
T |
---------+-----+
|v4/v6|
| host|
_ _ _ _ _ _ _ _ _ _ _ _ _ +-----+
()_ _ _ _ _ _ _ _ _ _ _ _ _() <-- IPv6
PPP o L2TPv2 o UDP o IPv4 traffic
Softwire (SPH)
<--------------------------->
IPV6CP: capable of /64 Intf-Id assignment or
uniqueness check
|--------------------------->/64 prefix
RA
|--------------------------->/48 prefix,
DHCPv6 DNS, etc.
|----> /64
RA prefix
|----> DNS,
DHCPv6 etc.
Figure 4: Router behind CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, IPV6CP negotiates IPv6-over-PPP,
which also provides the capability for the ISP to assign the 64-bit
Interface-Identifier to the v4/v6 router or perform uniqueness
validation for the two interface identifiers at the two PPP ends
[RFC 5072]. After IPv6-over-PPP is up, IPv6 Stateless Address
Autoconfiguration / Neighbor Discovery runs over the IPv6-over-PPP
link, and the LNS can inform the v4/v6 router of a prefix to use for
stateless address autoconfiguration through a Router Advertisement
Storer, et al. Standards Track PAGE 13
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
(RA). DHCPv6 can be used to perform IPv6 Prefix Delegation (e.g.,
delegating a prefix to be used within the home network [RFC 3633]) and
convey other non-address configuration options (such as DNS
[RFC 3646]) to the v4/v6 router.
3.2. IPv4-over-IPv6 Softwires with L2TPv2
The following sub-sections cover IPv4 connectivity (SPH) across an
IPv6-only access network (STH) using a Softwire.
3.2.1. Host CPE as Softwire Initiator
The Softwire Initiator (SI) is the host CPE (directly connected to a
modem), which is dual-stack. There is no other gateway device. The
IPv6 traffic SHOULD NOT traverse the Softwire. See Figure 5.
IPv4 or dual-stack IPv6-only dual-stack
|------------------||-----------------||----------|
I SC SI
N +-----+ +----------+
T | | | v4/v6 |
E <==[ IPv4 ]....|v4/v6|....[IPv6-only]....| host CPE |
R [network] | | [ network ] | |
N | LNS | |LAC Client|
E +-----+ +----------+
T _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _() <-- IPv4 traffic
PPP o L2TPv2 o UDP o IPv6 (SPH)
Softwire
<------------------>
IPCP: capable of global IP assignment
and DNS, etc.
Figure 5: Host CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, the IP Control Protocol (IPCP)
negotiates IPv4-over-PPP, which also provides the capability for the
ISP to assign a global IPv4 address to the host CPE. A global IPv4
address can also be assigned via DHCP. Other configuration options
(such as DNS) can be conveyed to the host CPE via IPCP [RFC 1877] or
DHCP [RFC 2132].
Storer, et al. Standards Track PAGE 14
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
3.2.2. Router CPE as Softwire Initiator
The Softwire Initiator (SI) is the router CPE, which is a dual-stack
device. The IPv6 traffic SHOULD NOT traverse the Softwire. See
Figure 6.
IPv4 or dual-stack IPv6-only dual-stack Home
|------------------||-----------------||-------------------|
I SC SI
N +-----+ +----------+
T | | | v4/v6 | +-----+
E <==[ IPv4 ]....|v4/v6|....[IPv6-only]....| CPE |--|v4/v6|
R [network] | | [ network ] | | | host|
N | LNS | |LAC Client| +-----+
E +-----+ +----------+
T _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _() <--------- IPv4 traffic
PPP o L2TPv2 o UDP o IPv6 (SPH)
Softwire
<------------------>
IPCP: capable of global IP assignment
and DNS, etc.
|------------------>
DHCPv4: prefix, mask, PD
private/
|------> global
DHCP IP, DNS,
etc.
Figure 6: Router CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, IPCP negotiates IPv4-over-PPP, which
also provides the capability for the ISP to assign a global IPv4
address to the router CPE. A global IPv4 address can also be
assigned via DHCP. Other configuration options (such as DNS) can be
conveyed to the router CPE via IPCP [RFC 1877] or DHCP [RFC 2132]. For
IPv4 Prefix Delegation for the home network, DHCP [SUBNET-ALL] can be
used.
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3.2.3. Host behind CPE as Softwire Initiator
The CPE is IPv6-only. The Softwire Initiator (SI) is a dual-stack
host (behind the IPv6 CPE), which acts as an IPv4 host CPE. The IPv6
traffic SHOULD NOT traverse the Softwire. See Figure 7.
IPv4 or dual-stack IPv6-only dual-stack
|------------------||----------------------------||----------|
I SC SI
N +-----+ +----------+
T | | +-------+ | v4/v6 |
E <==[ IPv4 ]....|v4/v6|....[IPv6-only]....|v6-only|--| host |
R [network] | | [ network ] | CPE | | |
N | LNS | +-------+ |LAC Client|
E +-----+ +----------+
T _ _ _ _ _ _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _ _ _ _ _ _() <-- IPv4
PPP o L2TPv2 o UDP o IPv6 traffic
Softwire (SPH)
<------------------------------>
IPCP: capable of global IP assignment
and DNS, etc.
Figure 7: Host behind CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, IPCP negotiates IPv4-over-PPP, which
also provides the capability for the ISP to assign a global IPv4
address to the host. A global IPv4 address can also be assigned via
DHCP. Other configuration options (such as DNS) can be conveyed to
the host CPE via IPCP [RFC 1877] or DHCP [RFC 2132].
3.2.4. Router behind CPE as Softwire Initiator
The CPE is IPv6-only. The Softwire Initiator (SI) is a dual-stack
device (behind the IPv6-only CPE) acting as an IPv4 CPE router inside
the home network. The IPv6 traffic SHOULD NOT traverse the Softwire.
See Figure 8.
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IPv4 or dual-stack IPv6-only dual-stack
|------------------||-------------------------||------------|
I SC SI
N +-----+ +----------+
T | | +-------+ | v4/v6 |
E <==[ IPv4 ]....|v4/v6|..[IPv6-only]..|v6-only|---| router |
R [network] | | [ network ] | CPE | | | |
N | LNS | +-------+ | |LAC Client|
E +-----+ | +----------+
T |
--------+-----+
|v4/v6|
| host|
_ _ _ _ _ _ _ _ _ _ _ _ _ +-----+
()_ _ _ _ _ _ _ _ _ _ _ _ _() <--- IPv4
PPP o L2TPv2 o UDP o IPv4 traffic
Softwire (SPH)
<--------------------------->
IPCP: assigns global IP address and DNS, etc.
|--------------------------->
DHCPv4: prefix, mask, PD
private/
|----> global
DHCP IP, DNS,
etc.
Figure 8: Router behind CPE as Softwire Initiator
In this scenario, after the L2TPv2 Control Channel and Session
establishment and PPP LCP negotiation (and optionally PPP
Authentication) are successful, IPCP negotiates IPv4-over-PPP, which
also provides the capability for the ISP to assign a global IPv4
address to the v4/v6 router. A global IPv4 address can also be
assigned via DHCP. Other configuration options (such as DNS) can be
conveyed to the v4/v6 router via IPCP [RFC 1877] or DHCP [RFC 2132].
For IPv4 Prefix Delegation for the home network, DHCP [SUBNET-ALL]
can be used.
4. References to Standardization Documents
This section lists and groups documents from the Internet
standardization describing technologies used to design the framework
of the Softwire "Hub and Spoke" solution. This emphasizes the
motivation of Softwire to reuse as many existing standards as
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possible. This list contains both Standards Track (Proposed
Standard, Draft Standard, and Standard) and Informational documents.
The list of documents and their status should only be only used for
description purposes.
4.1. L2TPv2
RFC 2661 "Layer Two Tunneling Protocol 'L2TP'" [RFC 2661].
* For both IPv4 and IPv6 payloads (SPH), support is
complete.
* For both IPv4 and IPv6 transports (STH), support is
complete.
4.2. Securing the Softwire Transport
RFC 3193 "Securing L2TP using IPsec" [RFC 3193].
RFC 3948 "UDP Encapsulation of IPsec ESP Packets" [RFC 3948].
* IPsec supports both IPv4 and IPv6 transports.
4.3. Authentication, Authorization, and Accounting
RFC 2865 "Remote Authentication Dial In User Service (RADIUS)"
[RFC 2865].
* Updated by [RFC 2868], [RFC 3575], and [RFC 5080].
RFC 2867 "RADIUS Accounting Modifications for Tunnel Protocol
Support" [RFC 2867].
RFC 2868 "RADIUS Attributes for Tunnel Protocol Support" [RFC 2868].
RFC 3162 "RADIUS and IPv6" [RFC 3162].
4.4. MIB
RFC 1471 "The Definitions of Managed Objects for the Link Control
Protocol of the Point-to-Point Protocol" [RFC 1471].
RFC 1473 "The Definitions of Managed Objects for the IP Network
Control Protocol of the Point-to-Point Protocol"
[RFC 1473].
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
RFC 3371 "Layer Two Tunneling Protocol "L2TP" Management
Information Base" [RFC 3371].
RFC 4087 "IP Tunnel MIB" [RFC 4087].
* Both IPv4 and IPv6 transports are supported.
4.5. Softwire Payload Related
4.5.1. For IPv6 Payloads
RFC 4861 "Neighbor Discovery for IP version 6 (IPv6)" [RFC 4861].
RFC 4862 "IPv6 Stateless Address Autoconfiguration" [RFC 4862].
RFC 5072 "IP Version 6 over PPP" [RFC 5072].
RFC 3315 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"
[RFC 3315].
RFC 3633 "IPv6 Prefix Options for Dynamic Host Configuration
Protocol (DHCP) version 6" [RFC 3633].
RFC 3646 "DNS Configuration options for Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)" [RFC 3646].
RFC 3736 "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6" [RFC 3736].
4.5.2. For IPv4 Payloads
RFC 1332 "The PPP Internet Protocol Control Protocol (IPCP)"
[RFC 1332].
RFC 1661 "The Point-to-Point Protocol (PPP)" [RFC 1661].
RFC 1877 "PPP Internet Protocol Control Protocol Extensions for
Name Server Addresses" [RFC 1877].
RFC 2131 "Dynamic Host Configuration Protocol" [RFC 2131].
RFC 2132 "DHCP Options and BOOTP Vendor Extensions" [RFC 2132].
DHCP Subnet Allocation "Subnet Allocation Option".
* Work in progress, see [SUBNET-ALL].
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
5. Softwire Establishment
A Softwire is established in three distinct steps, potentially
preceded by an optional IPsec-related step 0 (see Figure 9). First,
an L2TPv2 tunnel with a single session is established from the SI to
the SC. Second, a PPP session is established over the L2TPv2 session
and the SI obtains an address. Third, the SI optionally gets other
information through DHCP such as a delegated prefix and DNS servers.
SC SI
| |
|<-------------IKEv1------------->| Step 0
| | IPsec SA establishment
| | (optional)
| |
|<-------------L2TPv2------------>| Step 1
| | L2TPv2 Tunnel establishment
| |
|<--------------PPP-------------->| Step 2
|<-----Endpoint Configuration---->| PPP and Endpoint
| | configuration
| |
|<------Router Configuration----->| Step 3
| | Additional configuration
| | (optional)
Figure 9: Steps for the Establishment of a Softwire
Figure 10 depicts details of each of these steps required to
establish a Softwire.
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
SC SI
| |
| | Step 0
|<------------IKEv1-------------->| = IKEv1 (Optional)
| |
| | Step 1
|<------------SCCRQ---------------| -
|-------------SCCRP-------------->| |
|<------------SCCCN---------------| |
|<------------ICRQ----------------| | L2TPv2
|-------------ICRP--------------->| |
|<------------ICCN----------------| -
| |
| | Step 2
|<-----Configuration-Request------| -
|------Configuration-Request----->| | PPP
|--------Configuration-Ack------->| | LCP
|<-------Configuration-Ack--------| -
| |
|-----------Challenge------------>| - PPP Authentication
|<----------Response--------------| | (Optional - CHAP)
|------------Success------------->| -
| |
|<-----Configuration-Request------| -
|------Configuration-Request----->| | PPP NCP
|--------Configuration-Ack------->| | (IPV6CP or IPCP)
|<-------Configuration-Ack--------| -
| |
|<------Router-Solicitation-------| - Neighbor Discovery
|-------Router-Advertisement----->| | (IPv6 only)
| | -
| |
| | Step3
| | DHCP (Optional)
|<-----------SOLICIT--------------| -
|-----------ADVERTISE------------>| | DHCPv6
|<---------- REQUEST--------------| | (IPv6 SW, Optional)
|-------------REPLY-------------->| -
| | or
|<---------DHCPDISCOVER-----------| -
|-----------DHCPOFFER------------>| | DHCPv4
|<---------DHCPREQUEST------------| | (IPv4 SW, Optional)
|------------DHCPACK------------->| -
Figure 10: Detailed Steps in the Establishment of a Softwire
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The IPsec-related negotiations in step 0 are optional. The L2TPv2
negotiations in step 1 are described in Section 5.1. The PPP Network
Control Protocol (NCP) negotiations in step 2 use IPV6CP for IPv6-
over-IPv4 Softwires, and IPCP for IPv4-over-IPv6 Softwires (see
Section 5.2.4). The optional DHCP negotiations in step 3 use DHCPv6
for IPv6-over-IPv4 Softwires, and DHCPv4 for IPv4-over-IPv6 Softwires
(see Section 5.4). Additionally, for IPv6-over-IPv4 Softwires, the
DHCPv6 exchange for non-address configuration (such as DNS) can use
Stateless DHCPv6, the two-message exchange with Information-Request
and Reply messages (see Section 1.2 of [RFC 3315] and [RFC 3736]).
5.1. L2TPv2 Tunnel Setup
L2TPv2 [RFC 2661] was originally designed to provide private network
access to end users connected to a public network. In the L2TPv2
incoming call model, the end user makes a connection to an L2TP
Access Concentrator (LAC). The LAC then initiates an L2TPv2 tunnel
to an L2TP Network Server (LNS). The LNS then transfers end-user
traffic between the L2TPv2 tunnel and the private network.
In the Softwire "Hub and Spoke" model, the Softwire Initiator (SI)
assumes the role of the LAC Client and the Softwire Concentrator (SC)
assumes the role of the LNS.
In the Softwire model, an L2TPv2 packet MUST be carried over UDP.
The underlying version of the IP protocol may be IPv4 or IPv6,
depending on the Softwire scenario.
In the following sections, the term "Tunnel" follows the definition
from Section 1.2 of [RFC 2661], namely: "The Tunnel consists of a
Control Connection and zero or more L2TP Sessions".
5.1.1. Tunnel Establishment
Figure 11 describes the messages exchanged and Attribute Value Pairs
(AVPs) used to establish a tunnel between an SI (LAC) and an SC
(LNS). The messages and AVPs described here are only a subset of
those defined in [RFC 2661]. This is because Softwires use only a
subset of the L2TPv2 functionality. The subset of L2TP Control
Connection Management AVPs that is applicable to Softwires is grouped
into Required AVPs and Optional AVPs on a per-control-message basis
(see Figure 11). For each control message, Required AVPs include all
the "MUST be present" AVPs from [RFC 2661] for that control message,
and Optional AVPs include the "MAY be present" AVPs from [RFC 2661]
that are used in the Softwire context on that control message. Note
that in the Softwire environment, the SI always initiates the tunnel.
L2TPv2 AVPs SHOULD NOT be hidden.
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SC SI
|<--------SCCRQ---------|
Required AVPs:
Message Type
Protocol Version
Host Name
Framing Capabilities
Assigned Tunnel ID
Optional AVPs:
Receive Window Size
Challenge
Firmware Revision
Vendor Name
|---------SCCRP-------->|
Required AVPs:
Message Type
Protocol Version
Framing Capabilities
Host Name
Assigned Tunnel ID
Optional AVPs:
Firmware Revision
Vendor Name
Receive Window Size
Challenge
Challenge Response
|<--------SCCCN---------|
Required AVPs:
Message Type
Optional AVPs:
Challenge Response
Figure 11: Control Connection Establishment
In L2TPv2, generally, the tunnel between an LAC and LNS may carry the
data of multiple users. Each of these users is represented by an
L2TPv2 session within the tunnel. In the Softwire environment, the
tunnel carries the information of a single user. Consequently, there
is only one L2TPv2 session per tunnel. Figure 12 describes the
messages exchanged and the AVPs used to establish a session between
an SI (LAC) and an SC (LNS). The messages and AVPs described here
are only a subset of those defined in [RFC 2661]. This is because
Softwires use only a subset of the L2TPv2 functionality. The subset
of L2TP Call Management (i.e., Session Management) AVPs that is
applicable to Softwires is grouped into Required AVPs and Optional
AVPs on a per-control-message basis (see Figure 12). For each
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control message, Required AVPs include all the "MUST be present" AVPs
from [RFC 2661] for that control message, and Optional AVPs include
the "MAY be present" AVPs from [RFC 2661] that are used in the
Softwire context on that control message. Note that in the Softwire
environment, the SI always initiates the session. An L2TPv2 session
setup for a Softwire uses only the incoming call model. No outgoing
or analog calls (sessions) are permitted. L2TPv2 AVPs SHOULD NOT be
hidden.
SC SI
|<--------ICRQ---------|
Required AVPs:
Message Type
Assigned Session ID
Call Serial Number
|---------ICRP-------->|
Required AVPs:
Message Type
Assigned Session ID
|<--------ICCN---------|
Required AVPs:
Message Type
(Tx) Connect Speed
Framing Type
Figure 12: Session Establishment
The following sub-sections (5.1.1.1 through 5.1.1.3) describe in more
detail the Control Connection and Session establishment AVPs (see
message flows in Figures 11 and 12, respectively) that are required,
optional and not relevant for the L2TPv2 Tunnel establishment of a
Softwire. Specific L2TPv2 protocol messages and flows that are not
explicitly described in these sections are handled as defined in
[RFC 2661].
The mechanism for hiding AVP Attribute values is used, as described
in Section 4.3 of [RFC 2661], to hide sensitive control message data
such as usernames, user passwords, or IDs, instead of sending the AVP
contents in the clear. Since AVPs used in L2TP messages for the
Softwire establishment do not transport such sensitive data, L2TPv2
AVPs SHOULD NOT be hidden.
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5.1.1.1. AVPs Required for Softwires
This section prescribes specific values for AVPs that are required
(by [RFC 2661]) to be present in one or more of the messages used for
the Softwire establishment, as they are used in the Softwire context.
It combines all the Required AVPs from all the control messages in
Section 5.1.1, and provides Softwire-specific use guidance.
Host Name AVP
This AVP is required in SCCRQ and SCCRP messages. This AVP MAY be
used to authenticate users, in which case it would contain a user
identification. If this AVP is not used to authenticate users, it
may be used for logging purposes.
Framing Capabilities AVP
Both the synchronous (S) and asynchronous (A) bits SHOULD be set
to 1. This AVP SHOULD be ignored by the receiver.
Framing Type AVP
The synchronous bit SHOULD be set to 1 and the asynchronous bit to
0. This AVP SHOULD be ignored by the receiver.
(Tx) Connect Speed AVP
(Tx) Connect Speed is a required AVP but is not meaningful in the
Softwire context. Its value SHOULD be set to 0 and ignored by the
receiver.
Message Type AVP, Protocol Version AVP, Assigned Tunnel ID AVP, Call
Serial Number AVP, and Assigned Session ID AVP
As defined in [RFC 2661].
5.1.1.2. AVPs Optional for Softwires
This section prescribes specific values for AVPs that are Optional
(not required by [RFC 2661]) but used in the Softwire context. It
combines all the Optional AVPs from all the control messages in
Section 5.1.1, and provides Softwire-specific use guidance.
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Challenge AVP and Challenge Response AVP
These AVPs are not required, but are necessary to implement tunnel
authentication. Since tunnel authentication happens at the
beginning of L2TPv2 tunnel creation, it can be helpful in
preventing denial-of-service (DoS) attacks. See Section 5.1.1 of
[RFC 2661].
The usage of these AVPs in L2TP messages is OPTIONAL, but SHOULD
be implemented in the SC.
Receive Window Size AVP, Firmware Revision AVP, and Vendor Name AVP
As defined in [RFC 2661].
5.1.1.3. AVPs Not Relevant for Softwires
L2TPv2 specifies numerous AVPs that, while allowed for a given
message, are irrelevant to Softwires. They can be irrelevant to
Softwires because they do not apply to the Softwire establishment
flow (e.g., they are only used in the Outgoing Call establishment
message exchange, while Softwires only use the Incoming Call message
flow), or because they are Optional AVPs that are not used. L2TPv2
AVPs that are relevant to Softwires were covered in Sections 5.1.1,
5.1.1.1, and 5.1.1.2. Softwire implementations SHOULD NOT send AVPs
that are not relevant to Softwires. However, they SHOULD ignore them
when they are received. This will simplify the creation of Softwire
applications that build upon existing L2TPv2 implementations.
5.1.2. Tunnel Maintenance
Periodically, the SI/SC MUST transmit a message to the peer to detect
tunnel or peer failure and maintain NAT/NAPT contexts. The L2TPv2
HELLO message provides a simple, low-overhead method of doing this.
The default values specified in [RFC 2661] for L2TPv2 HELLO messages
could result in a dead-end detection time of 83 seconds. Although
these retransmission timers and counters SHOULD be configurable (see
Section 5.8 of [RFC 2661]), these values may not be adapted for all
situations, where a quicker dead-end detection is required, or where
NAT/NAPT context needs to be refreshed more frequently. In such
cases, the SI/SC MAY use, in combination with L2TPv2 HELLO, LCP ECHO
messages (Echo-Request and Echo-Reply codes) described in [RFC 1661].
When used, LCP ECHO messages SHOULD have a re-emission timer lower
than the value for L2TPv2 HELLO messages. The default value
recommended in Section 6.5 of [RFC 2661] for the HELLO message
retransmission interval is 60 seconds. When used, a set of suggested
values (included here only for guidance) for the LCP ECHO message
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RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
request interval is a default of 30 seconds, a minimum of 10 seconds,
and a maximum of the lesser of the configured L2TPv2 HELLO
retransmission interval and 60 seconds.
5.1.3. Tunnel Teardown
Either the SI or SC can tear down the session and tunnel. This is
done as specified in Section 5.7 of [RFC 2661], by sending a StopCCN
control message. There is no action specific to Softwires in this
case.
5.1.4. Additional L2TPv2 Considerations
In the Softwire "Hub and Spoke" framework, L2TPv2 is layered on top
of UDP, as part of an IP-in-IP tunnel; Section 8.1 of [RFC 2661]
describes L2TP over UDP/IP. Therefore, the UDP guidelines specified
in [RFC 5405] apply, as they pertain to the UDP tunneling scenarios
carrying IP-based traffic. Section 3.1.3 of [RFC 5405] specifies that
for this case, specific congestion control mechanisms for the tunnel
are not necessary. Additionally, Section 3.2 of [RFC 5405] provides
message size guidelines for the encapsulating (outer) datagrams,
including the recommendation to implement Path MTU Discovery (PMTUD).
5.2. PPP Connection
This section describes the PPP negotiations between the SI and SC in
the Softwire context.
5.2.1. MTU
The MTU of the PPP link presented to the SPH SHOULD be the link MTU
minus the size of the IP, UDP, L2TPv2, and PPP headers together. On
an IPv4 link with an MTU equal to 1500 bytes, this could typically
mean a PPP MTU of 1460 bytes. When the link is managed by IPsec,
this MTU SHOULD be lowered to take into account the ESP encapsulation
(see [SW-SEC]). The value for the MTU may also vary according to the
size of the L2TP header, as defined by the leading bits of the L2TP
message header (see [RFC 2661]). Additionally, see [RFC 4623] for a
detailed discussion of fragmentation issues.
5.2.2. LCP
Once the L2TPv2 session is established, the SI and SC initiate the
PPP connection by negotiating LCP as described in [RFC 1661]. The
Address-and-Control-Field-Compression configuration option (ACFC)
[RFC 1661] MAY be rejected.
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5.2.3. Authentication
After completing LCP negotiation, the SI and SC MAY optionally
perform authentication. If authentication is chosen, Challenge
Handshake Authentication Protocol (CHAP) [RFC 1994] authentication
MUST be supported by both the Softwire Initiator and Softwire
Concentrator. Other authentication methods such as Microsoft CHAP
version 1 (MS-CHAPv1) [RFC 2433] and Extensible Authentication
Protocol (EAP) [RFC 3748] MAY be supported.
A detailed discussion of Softwire security is contained in [SW-SEC].
5.2.4. IPCP
The only Network Control Protocol (NCP) negotiated in the Softwire
context is IPV6CP (see Section 5.2.4.1) for IPv6 as SPH, and IPCP
(see Section 5.2.4.2) for IPv4 as SPH.
5.2.4.1. IPV6CP
In the IPv6-over-IPv4 scenarios (see Section 3.1), after the optional
authentication phase, the Softwire Initiator MUST negotiate IPV6CP as
defined in [RFC 5072]. IPV6CP provides a way to negotiate a unique
64-bit Interface-Identifier to be used for the address
autoconfiguration at the local end of the link.
5.2.4.2. IPv4CP
In the IPv4-over-IPv6 scenarios (see Section 3.2), a Softwire
Initiator MUST negotiate IPCP [RFC 1332]. The SI uses IPCP to obtain
an IPv4 address from the SC. IPCP MAY also be used to obtain DNS
information as described in [RFC 1877].
5.3. Global IPv6 Address Assignment to Endpoints
In several scenarios defined in Section 3.1, global IPv6 addresses
are expected to be allocated to Softwire endpoints (in addition to
the Link-Local addresses autoconfigured using the IPV6CP negotiated
interface identifier). The Softwire Initiator assigns global IPv6
addresses using the IPV6CP negotiated interface identifier and using
Stateless Address Autoconfiguration [RFC 4862], and/or using Privacy
Extensions for Stateless Address Autoconfiguration [RFC 4941], (as
described in Section 5 of [RFC 5072]), and/or using DHCPv6 [RFC 3315].
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The Softwire Initiator of an IPv6 Softwire MUST send a Router
Solicitation message to the Softwire Concentrator after IPV6CP is
completed. The Softwire Concentrator MUST answer with a Router
Advertisement. This message MUST contain the global IPv6 prefix of
the PPP link if Neighbor Discovery is used to configure addresses of
Softwire endpoints.
If DHCPv6 is available for address delegation, the M bits of the
Router Advertisement SHOULD be set. The Softwire Initiator MUST then
send a DHCPv6 Request to configure the address of the Softwire
endpoint.
Duplicate Address Detection ([RFC 4861]) MUST be performed on the
Softwire in both cases.
5.4. DHCP
The Softwire Initiator MAY use DHCP to get additional information
such as delegated prefix and DNS servers.
5.4.1. DHCPv6
In the scenarios in Section 3.1, if the SI supports DHCPv6, it SHOULD
send a Solicit message to verify if more information is available.
If an SI establishing an IPv6 Softwire acts as a router (i.e., in the
scenarios in Sections 3.1.2 and 3.1.4) it MUST include the Identity
Association for Prefix Delegation (IA_PD) option [RFC 3633] in the
DHCPv6 Solicit message [RFC 3315] in order to request an IPv6 prefix.
When delegating an IPv6 prefix to the SI by returning a DHCPv6
Advertise message with the IA_PD and IP_PD Prefix options [RFC 3633],
the SC SHOULD inject a route for this prefix in the IPv6 routing
table in order to forward the traffic to the relevant Softwire.
Configuration of DNS MUST be done as specified in [RFC 3646] and
transmitted according to [RFC 3315] and [RFC 3736]. In general, all
DHCPv6 options MUST be transmitted according to [RFC 3315] and
[RFC 3736].
5.4.2. DHCPv4
An SI establishing an IPv4 Softwire MAY send a DHCP request
containing the Subnet Allocation option [SUBNET-ALL]. This practice
is not common, but it may be used to connect IPv4 subnets using
Softwires, as defined in Sections 3.2.2 and 3.2.4.
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One Subnet-Request suboption MUST be configured with the 'h' bit set
to '1', as the SI is expected to perform the DHCP server function.
The 'i' bit of the Subnet-Request suboption SHOULD be set to '0' the
first time a prefix is requested and to '1' on subsequent requests,
if a prefix has been allocated. The Prefix length suboption SHOULD
be 0 by default. If the SI is configured to support only specific
prefix lengths, it SHOULD specify the longest (smallest) prefix
length it supports.
If the SI was previously assigned a prefix from that same SC, it
SHOULD include the Subnet-Information suboption with the prefix it
was previously assigned. The 'c' and 's' bits of the suboption
SHOULD be set to '0'.
In the scenarios in Section 3.2, when delegating an IPv4 prefix to
the SI, the SC SHOULD inject a route for this prefix in the IPv4
routing table in order to forward the traffic to the relevant
Softwire.
6. Considerations about the Address Provisioning Model
This section describes how a Softwire Concentrator may manage
delegated addresses for Softwire endpoints and for subnets behind the
Softwire Initiator. One common practice is to aggregate endpoints'
addresses and delegated prefixes into one prefix routed to the SC.
The main benefit is to ease the routing scheme by isolating on the SC
succeeding route injections (when delegating new prefixes for SI).
6.1. Softwire Endpoints' Addresses
6.1.1. IPv6
A Softwire Concentrator should provide globally routable addresses to
Softwire endpoints. Other types of addresses such as Unique Local
Addresses (ULAs) [RFC 4193] may be used to address Softwire endpoints
in a private network with no global connectivity. A single /64
should be assigned to the Softwire to address both Softwire
endpoints.
Global addresses or ULAs must be assigned to endpoints when the
scenario "Host CPE as Softwire Initiator" (described in
Section 3.1.1) is considered to be deployed. For other scenarios,
link-local addresses may also be used.
Storer, et al. Standards Track PAGE 30
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6.1.2. IPv4
A Softwire Concentrator may provide either globally routable or
private IPv4 addresses. When using IPv4 private addresses [RFC 1918]
on the endpoints, it is not recommended to delegate an IPv4 private
prefix to the SI, as it can lead to a nested-NAT situation.
The endpoints of the PPP link use host addresses (i.e., /32),
negotiated using IPCP.
6.2. Delegated Prefixes
6.2.1. IPv6 Prefixes
Delegated IPv6 prefixes should be of global scope if the IPv6
addresses assigned to endpoints are global. Using ULAs is not
recommended when the subnet is connected to the global IPv6 Internet.
When using IPv6 ULAs on the endpoints, the delegated IPv6 prefix may
be either of global or ULA scope.
Delegated IPv6 prefixes are between /48 and /64 in length. When an
SI receives a prefix shorter than 64, it can assign different /64
prefixes to each of its interfaces. An SI receiving a single /64 is
expected to perform bridging if more than one interface is available
(e.g., wired and wireless).
6.2.2. IPv4 Prefixes
Delegated IPv4 prefixes should be routable within the address space
used by assigned IPv4 addresses. Delegate non-routable IPv4 prefixes
(i.e., private IPv4 prefix over public IPv4 addresses or another
class of private IPv4 addresses) is not recommended as a practice for
provisioning and address translation should be considered in these
cases. The prefix length is between /8 and /30.
6.3. Possible Address Provisioning Scenarios
This section summarizes the different scenarios for address
provisioning with the considerations given in the previous sections.
Storer, et al. Standards Track PAGE 31
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6.3.1. Scenarios for IPv6
This table describes the possible combination of IPv6 address scope
for endpoints and delegated prefixes.
+------------------+-----------------------+------------------------+
| Endpoint IPv6 | Delegated Global IPv6 | Delegated ULA IPv6 |
| Address | Prefix | Prefix |
+------------------+-----------------------+------------------------+
| Link Local | Possible | Possible |
| | | |
| ULA | Possible | Possible |
| | | |
| Global | Possible | Possible, but Not |
| | | Recommended |
+------------------+-----------------------+------------------------+
Table 1: Scenarios for IPv6
6.3.2. Scenarios for IPv4
This table describes the possible combination of IPv4 address scope
for endpoints and delegated prefixes.
+-------------+-----------------+-----------------------------------+
| Endpoint | Delegated | Delegated Private IPv4 Prefix |
| IPv4 | Public IPv4 | |
| Address | Prefix | |
+-------------+-----------------+-----------------------------------+
| Private | Possible | Possible, but Not Recommended |
| IPv4 | | when using NAT (cf. |
| | | Section 6.1.2) |
| | | |
| Public IPv4 | Possible | Possible, but NAT usage is |
| | | recommended (cf. Section 6.2.2) |
+-------------+-----------------+-----------------------------------+
Table 2: Scenarios for IPv4
7. Considerations about Address Stability
A Softwire can provide stable addresses even if the underlying
addressing scheme changes, by opposition to automatic tunneling. A
Softwire Concentrator should always provide the same address and
prefix to a reconnecting user. However, if the goal of the Softwire
service is to provide a temporary address for a roaming user, it may
be provisioned to provide only a temporary address.
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The address and prefix are expected to change when reconnecting to a
different Softwire Concentrator. However, an organization providing
a Softwire service may provide the same address and prefix across
different Softwire Concentrators at the cost of a more fragmented
routing table. The routing fragmentation issue may be limited if the
prefixes are aggregated in a location topologically close to the SC.
This would be the case, for example, if several SCs are put in
parallel for load-balancing purpose.
8. Considerations about RADIUS Integration
The Softwire Concentrator is expected to act as a client to a AAA
server, for example, a RADIUS server. During the PPP authentication
phase, the RADIUS server may return additional information in the
form of attributes in the Access-Accept message.
The Softwire Concentrator may include the Tunnel-Type and Tunnel-
Medium-Type attributes [RFC 2868] in the Access-Request messages to
provide a hint of the type of Softwire being configured.
8.1. Softwire Endpoints
8.1.1. IPv6 Softwires
If the RADIUS server includes a Framed-Interface-Id attribute
[RFC 3162], the Softwire Concentrator must send it to the Softwire
Initiator in the Interface-Identifier field of its IPV6CP
Configuration Request message.
If the Framed-IPv6-Prefix attribute [RFC 3162] is included, that
prefix must be used in the router advertisements sent to the SI. If
Framed-IPv6-Prefix is not present but Framed-IPv6-Pool is, the SC
must choose a prefix from that pool to send RAs.
8.1.2. IPv4 Softwires
If the Framed-IP-Address attribute [RFC 2865] is present, the Softwire
Concentrator must provide that address to the Softwire Initiator
during IPCP address negotiation. That is, when the Softwire
Initiator requests an IP address from the Softwire Concentrator, the
address provided should be the Framed-IP-Address.
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8.2. Delegated Prefixes
8.2.1. IPv6 Prefixes
If the attribute Delegated-IPv6-Prefix [RFC 4818] is present in the
RADIUS Access-Accept message, it must be used by the Softwire
Concentrator for the delegation of the IPv6 prefix. Since the prefix
delegation is performed by DHCPv6 and the attribute is linked to a
username, the SC must associate the DHCP Unique Identifier (DUID) of
a DHCPv6 request to the tunnel it came from and its user.
Interaction between RADIUS, PPP, and DHCPv6 server may follow the
mechanism proposed in [RELAY-RAD]. In this case, during the Softwire
authentication phase, PPP collects the RADIUS attributes for the user
such as Delegated-IPv6-Prefix. A specific DHCPv6 relay is assigned
to the Softwire. The DHCPv6 relay fills in these attributes in the
Relay agent RADIUS Attribute Option (RRAO) DHCPv6 option, before
forwarding the DHCPv6 requests to the DHCPv6 server.
8.2.2. IPv4 Prefixes
RADIUS does not define an attribute for the delegated IPv4 Prefix.
Attributes indicating an IPv4 prefix and its length (for instance the
combination of the Framed-IP-Address and Framed-IP-Netmask attributes
[RFC 2865]) may be used by the Softwire Concentrator to delegate an
IPv4 prefix to the Softwire Initiator. The Softwire Concentrator
must add a corresponding route with the Softwire Initiator as next-
hop.
As this practice had been used, the inclusion of the Framed-IP-
Netmask attribute along with the Framed-IP-Address attribute tells
the Softwire Concentrator to delegate an IPv4 prefix to the Softwire
Initiator (e.g., in the IPv4-over-IPv6 scenarios where the Softwire
Initiator is a router, see Sections 3.2.2 and 3.2.4), as the SC
should forward packets destined to any IPv4 address in the prefix to
the SI.
9. Considerations for Maintenance and Statistics
Existing protocol mechanics for conveying adjunct or accessory
information for logging purposes, including L2TPv2 and RADIUS
methods, can include informational text that the behavior is
according to the Softwire "Hub and Spoke" framework (following the
implementation details specified in this document).
Storer, et al. Standards Track PAGE 34
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
9.1. RADIUS Accounting
RADIUS Accounting for L2TP and PPP are documented (see Section 4.3).
When deploying Softwire solutions, operators may experience
difficulties to differentiate the address family of the traffic
reported in accounting information from RADIUS. This problem and
some potential solutions are described in [SW-ACCT].
9.2. MIBs
MIB support for L2TPv2 and PPP are documented (see Section 4.4).
Also, see [RFC 4293].
10. Security Considerations
One design goal of the "Hub and Spoke" problem is to very strongly
consider the reuse of already deployed protocols (see [RFC 4925]).
Another design goal is a solution with very high scaling properties.
L2TPv2 [RFC 2661] is the phase 1 protocol used in the Softwire "Hub
and Spoke" solution space, and the L2TPv2 security considerations
apply to this document (see Section 9 of [RFC 2661]).
The L2TPv2 Softwire solution adds the following considerations:
o L2TP Tunnel Authentication (see Sections 5.1.1 and 9.1 of
[RFC 2661]) provides authentication at tunnel setup. It may be
used to limit DoS attacks by authenticating the tunnel before L2TP
and PPP resources are allocated.
o In a Softwire environment, L2TPv2 AVPs do not transport sensitive
data, and thus the L2TPv2 AVP hiding mechanism is not used (see
Section 5.1.1).
o PPP CHAP [RFC 1994] provides basic user authentication. Other
authentication protocols may additionally be supported (see
Section 5.2.3).
L2TPv2 can also be secured with IPsec to provide privacy, integrity,
and replay protection. Currently, there are two different solutions
for security L2TPv2 with IPsec:
o Securing L2TPv2 using IPsec "version 2" (IKEv1) is specified in
[RFC 3193], [RFC 3947], and [RFC 3948]. When L2TPv2 is used in the
Softwire context, the voluntary tunneling model applies.
[RFC 3193] describes the interaction between IPsec and L2TPv2, and
Storer, et al. Standards Track PAGE 35
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
is deployed. [RFC 3193] MUST be supported, given that deployed
technology must be very strongly considered [RFC 4925] for this
'time-to-market' solution.
o [SW-SEC] also specifies a new (incompatible) solution for securing
L2TPv2 with IPsec "version 3" (IKEv2). Section 3.5 of [SW-SEC]
describes the advantages of using IKEv2, and this solution needs
to be considered for future phases.
Additional discussion of Softwire security is contained in [SW-SEC].
11. Acknowledgements
The authors would like to acknowledge the following contributors who
provided helpful input on this document: Florent Parent, Jordi Palet
Martinez, Ole Troan, Shin Miyakawa, Carl Williams, Mark Townsley,
Francis Dupont, Ralph Droms, Hemant Singh, and Alain Durand.
The authors would also like to acknowledge the participants in the
Softwires interim meetings held in Hong Kong, China, and Barcelona,
Spain. The minutes for the interim meeting at the China University -
Hong Kong (February 23-24, 2006) are at
<http://www.ietf.org/proceedings/06mar/isoftwire.html>. The minutes
for the interim meeting at Polytechnic University of Catalonia -
Barcelona (September 14-15, 2006) are reachable at
<http://www.ietf.org/proceedings/06nov/isoftwire.html>. The
Softwires auxiliary page at <http://bgp.nu/~dward/softwires/>
contains additional meeting information.
During and after the IETF Last Call, useful comments and discussion
were provided by Jari Arkko, David Black, Lars Eggert, Pasi Eronen,
and Dan Romascanu.
Storer, et al. Standards Track PAGE 36
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
12. References
12.1. Normative References
[RFC 1332] McGregor, G., "The PPP Internet Protocol Control
Protocol (IPCP)", RFC 1332, May 1992.
[RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC 1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol
"L2TP"", RFC 2661, August 1999.
[RFC 2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC 3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
RFC 3162, August 2001.
[RFC 3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and S.
Booth, "Securing L2TP using IPsec", RFC 3193,
November 2001.
[RFC 3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC 3371] Caves, E., Calhoun, P., and R. Wheeler, "Layer Two
Tunneling Protocol "L2TP" Management Information Base",
RFC 3371, August 2002.
[RFC 3633] Troan, O. and R. Droms, "IPv6 Prefix Options for
Dynamic Host Configuration Protocol (DHCP) version 6",
RFC 3633, December 2003.
Storer, et al. Standards Track PAGE 37
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
[RFC 3736] Droms, R., "Stateless Dynamic Host Configuration
Protocol (DHCP) Service for IPv6", RFC 3736,
April 2004.
[RFC 3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[RFC 3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and
M. Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC 4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
Attribute", RFC 4818, April 2007.
[RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC 5072] S.Varada, Haskins, D., and E. Allen, "IP Version 6 over
PPP", RFC 5072, September 2007.
12.2. Informative References
[RELAY-RAD] Lau, W., "DHCPv6 Relay agent RADIUS Attribute Option",
Work in Progress, February 2006.
[RFC 1471] Kastenholz, F., "The Definitions of Managed Objects for
the Link Control Protocol of the Point-to-Point
Protocol", RFC 1471, June 1993.
[RFC 1473] Kastenholz, F., "The Definitions of Managed Objects for
the IP Network Control Protocol of the Point-to-Point
Protocol", RFC 1473, June 1993.
[RFC 1877] Cobb, S. and F. Baker, "PPP Internet Protocol Control
Protocol Extensions for Name Server Addresses",
RFC 1877, December 1995.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[RFC 2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, October 1998.
Storer, et al. Standards Track PAGE 38
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
[RFC 2867] Zorn, G., Aboba, B., and D. Mitton, "RADIUS Accounting
Modifications for Tunnel Protocol Support", RFC 2867,
June 2000.
[RFC 2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J.,
Holdrege, M., and I. Goyret, "RADIUS Attributes for
Tunnel Protocol Support", RFC 2868, June 2000.
[RFC 3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC 3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575,
July 2003.
[RFC 3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[RFC 3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC 3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
March 2005.
[RFC 4087] Thaler, D., "IP Tunnel MIB", RFC 4087, June 2005.
[RFC 4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC 4293] Routhier, S., "Management Information Base for the
Internet Protocol (IP)", RFC 4293, April 2006.
[RFC 4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-
to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
August 2006.
[RFC 4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP",
BCP 127, RFC 4787, January 2007.
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
Storer, et al. Standards Track PAGE 39
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
[RFC 4925] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
Problem Statement", RFC 4925, July 2007.
[RFC 4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC 5080] Nelson, D. and A. DeKok, "Common Remote Authentication
Dial In User Service (RADIUS) Implementation Issues and
Suggested Fixes", RFC 5080, December 2007.
[RFC 5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage
Guidelines for Application Designers", BCP 145,
RFC 5405, November 2008.
[SUBNET-ALL] Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
"Subnet Allocation Option", Work in Progress,
March 2009.
[SW-ACCT] Stevant, B., Toutain, L., Dupont, F., and D. Binet,
"Accounting on Softwires", Work in Progress,
April 2009.
[SW-SEC] Yamamoto, S., Williams, C., Parent, F., and H. Yokota,
"Softwire Security Analysis and Requirements", Work
in Progress, May 2009.
Authors' Addresses
Bill Storer
Cisco Systems
170 W Tasman Dr
San Jose, CA 95134
USA
EMail: bstorer@cisco.com
Carlos Pignataro (editor)
Cisco Systems
7200 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709
USA
EMail: cpignata@cisco.com
Storer, et al. Standards Track PAGE 40
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009
Maria Alice Dos Santos
Cisco Systems
170 W Tasman Dr
San Jose, CA 95134
USA
EMail: mariados@cisco.com
Bruno Stevant (editor)
TELECOM Bretagne
2 rue de la Chataigneraie CS17607
Cesson Sevigne, 35576
France
EMail: bruno.stevant@telecom-bretagne.eu
Laurent Toutain
TELECOM Bretagne
2 rue de la Chataigneraie CS17607
Cesson Sevigne, 35576
France
EMail: laurent.toutain@telecom-bretagne.eu
Jean-Francois Tremblay
Videotron Ltd.
612 Saint-Jacques
Montreal, QC H3C 4M8
Canada
EMail: jf@jftremblay.com
Storer, et al. Standards Track PAGE 41
Softwire Hub and Spoke Deployment Framework with Layer Two Tunneling Protocol Version 2 (L2TPv2)
RFC TOTAL SIZE: 91379 bytes
PUBLICATION DATE: Friday, June 12th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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