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IETF RFC 4925
Softwire Problem Statement
Last modified on Thursday, July 26th, 2007
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Network Working Group X. Li, Ed.
Request for Comments: 4925 CERNET
Category: Informational S. Dawkins, Ed.
Huawei
D. Ward, Ed.
Cisco Systems
A. Durand, Ed.
Comcast
July 2007
Softwire Problem Statement
 Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright © The IETF Trust (2007).
Abstract
This document captures the problem statement for the Softwires
Working Group, which is developing standards for the discovery,
control, and encapsulation methods for connecting IPv4 networks
across IPv6-only networks as well as IPv6 networks across IPv4-only
networks. The standards will encourage multiple, inter-operable
vendor implementations by identifying, and extending where necessary,
existing standard protocols to resolve a selected set of "IPv4/IPv6"
and "IPv6/IPv4" transition problems. This document describes the
specific problems ("Hubs and Spokes" and "Mesh") that will be solved
by the standards developed by the Softwires Working Group. Some
requirements (and non-requirements) are also identified to better
describe the specific problem scope.
Li, et al. Informational PAGE 1 
RFC 4925 Softwire Problem Statement July 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Hubs and Spokes Problem . . . . . . . . . . . . . . . . . . . 6
2.1. Description . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Non-Upgradable CPE Router . . . . . . . . . . . . . . . . 9
2.3. Network Address Translation (NAT) and Port Address
Translation (PAT) . . . . . . . . . . . . . . . . . . . . 10
2.4. Static Prefix Delegation . . . . . . . . . . . . . . . . . 10
2.5. Softwire Initiator . . . . . . . . . . . . . . . . . . . . 11
2.6. Softwire Concentrator . . . . . . . . . . . . . . . . . . 11
2.7. Softwire Concentrator Discovery . . . . . . . . . . . . . 12
2.8. Scaling . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.9. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.10. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 12
2.11. Security . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.11.1. Authentication, Authorization, and Accounting
(AAA) . . . . . . . . . . . . . . . . . . . . . . . . 12
2.11.2. Privacy, Integrity, and Replay Protection . . . . . . 13
2.12. Operations and Management (OAM) . . . . . . . . . . . . . 13
2.13. Encapsulations . . . . . . . . . . . . . . . . . . . . . . 13
3. Mesh Problem . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1. Description . . . . . . . . . . . . . . . . . . . . . . . 14
3.2. Scaling . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3. Persistence, Discovery, and Setup Time . . . . . . . . . . 16
3.4. Multicast . . . . . . . . . . . . . . . . . . . . . . . . 17
3.5. Softwire Encapsulation . . . . . . . . . . . . . . . . . . 17
3.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . 17
4. Security Considerations . . . . . . . . . . . . . . . . . . . 18
5. Principal Authors . . . . . . . . . . . . . . . . . . . . . . 18
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Normative References . . . . . . . . . . . . . . . . . . . 20
7.2. Informative References . . . . . . . . . . . . . . . . . . 20
Li, et al. Informational PAGE 2
RFC 4925 Softwire Problem Statement July 2007
1. Introduction
The Softwires Working Group is specifying the standardization of
discovery, control, and encapsulation methods for connecting IPv4
networks across IPv6-only networks and IPv6 networks across IPv4-only
networks in a way that will encourage multiple, inter-operable vendor
implementations. This document describes the specific problems
("Hubs and Spokes" and "Mesh") that will be solved by the standards
developed by the Softwires Working Group. Some requirements (and
non-requirements) are also identified to better describe the specific
problem scope. A few generic assumptions are listed up front:
o Local Area Networks will often support both protocol families in
order to accommodate both IPv4-only and IPv6-only applications, in
addition to dual-stack applications. Global reachability requires
the establishment of softwire connectivity to transit across
portions of the network that do not support both address families.
Wide area networks that support one or both address families may
be separated by transit networks that do not support all address
families. Softwire connectivity is necessary to establish global
reachability of both address families.
o Softwires are to be used in IP-based networks to forward both
unicast and multicast traffic.
o Softwires are assumed to be long-lived in nature.
o Although Softwires are long-lived, the setup time of a softwire is
expected to be a very small fraction of the total time required
for the startup of the Customer Premise Equipment (CPE)/Address
Family Border Router (AFBR).
o The nodes that actually initiate softwires should support dual-
stack (IPv4 and IPv6) functionality.
o The goal of this effort is to reuse or extend existing technology.
The 'time-to-market' requirement for solutions to the stated
problems is very strict and existing, deployed technology must be
very strongly considered in our solution selection.
The solution to the stated problem should address the following
points:
o Relation of the softwire protocols to other host mechanisms in the
same layer of the network stack. Examples of mechanisms to
consider are tunneling mechanisms, VPNs (Virtual Private
Networks), mobility, multihoming (SHIM6 (Level 3 Shim for
IPv6)),...
Li, et al. Informational PAGE 3
RFC 4925 Softwire Problem Statement July 2007
o Operational brittleness introduced by softwire, e.g., potential
single point of failure or difficulties to deploy redundant
systems.
o Effects of softwires on the transport layer. Issue like packet
losses, congestion control, and handling of QoS (Quality of
Service) reservation and usage of on-path protocols such as RSVP
(Resource Reservation Protocol).
The history of IPv4 and IPv6 transition strategies at the IETF is
very long and complex. Here we list out some steps we have taken to
further the effort and it has lead to the creation of this document
and a few 'working rules' for us to accomplish our work:
o At the IETF 63 "LightWeight Reachability softWires" (LRW) BOF
meeting, attendees from several operators requested a very tight
timeframe for the delivery of a solution, based on time-to-market
considerations. This problem statement is narrowly scoped to
accommodate near-term deployment.
o At the Paris Softwires interim meeting in October, 2005,
participants divided the overall problem space into two separate
"sub-problems" to solve based on network topology. These two
problems are referred to as "Hubs and Spokes" (described in
Section 2) and "Mesh" (described in Section 3).
As stated, there are two scenarios that emerged when discussing the
traversal of networks composed of differing address families. The
scenarios are quite common in today's network deployments. The
primary difference between "Spokes and Hubs" and "Mesh" is how many
connections and associated routes are managed by each IPv4 or IPv6
"island". "Hubs and Spokes" is characterized with one connection and
associated static default route, and "Mesh" is characterized by
multiple connections and routing prefixes. In general, the two can
be categorized as host or LAN connectivity and network (or VPN)
connectivity problems. Looking at the history of multi-address
family networking, the clear delineation of the two scenarios was
never clearly illustrated but they are now the network norm, and both
must be solved. Later, during the solution phase of the Work Group
(WG), these problems will be treated as related, but separate,
problem spaces. Similar protocols and mechanisms will be used when
possible, but different protocols and mechanisms may be selected when
necessary to meet the requirements of each given problem space.
1.1. Terminology
Address Family (AF) - IPv4 or IPv6. Presently defined values for
this field are specified in
Li, et al. Informational PAGE 4
RFC 4925 Softwire Problem Statement July 2007
http://www.iana.org/assignments/address-family-numbers.
Address Family Border Router (AFBR) - The router that interconnects
two networks that use different address families.
Customer Premise Equipment (CPE) - Under the scope of this document,
this refers to terminal and associated equipment and inside wiring
located at a subscriber's premises and connected with a carrier's
communication channel(s) at the demarcation point ("demarc"). The
demarc is a point established in a building or complex to separate
customer equipment from telephone, cable, or other service provider
equipment. CPE can be a host or router, depending on the specific
characteristics of the access network. The demarc point for IPv4 may
or may not be the same as the demarc point for IPv6, thus there can
be one CPE box acting for IPv4 and IPv6 or two separate ones, one for
IPv4 and one for IPv6.
Home gateway - Existing piece of equipment that connects the home
network to the provider network. Usually act as CPE for one or both
address families.
Softwire (SW) - A "tunnel" that is created on the basis of a control
protocol setup between softwire endpoints with a shared point-to-
point or multipoint-to-point state. Softwires are generally dynamic
in nature (they may be initiated and terminated on demand), but may
be very long-lived.
Softwire Concentrator (SC) - The node terminating the softwire in the
service provider network.
Softwire Initiator (SI) - The node initiating the softwire within the
customer network.
Softwire Transport Header AF (STH AF) - the address family of the
outermost IP header of a softwire.
Softwire Payload Header AF (SPH AF) - the address family of the IP
headers being carried within a softwire. Note that additional
"levels" of IP headers may be present if (for example) a tunnel is
carried over a softwire - the key attribute of SPH AF is that it is
directly encapsulated within the softwire and the softwire endpoint
will base forwarding decisions on the SPH AF when a packet is exiting
the softwire.
Subsequent Address Family (SAF) - Additional information about the
type of Network Layer Reachability Information (e.g., unicast or
multicast).
Li, et al. Informational PAGE 5
RFC 4925 Softwire Problem Statement July 2007
2. Hubs and Spokes Problem
The "Hubs and Spokes" problem is named in reference to the airline
industry where major companies have established a relatively small
number of well connected hubs and then serve smaller airports from
those hubs.
Manually configured tunnels (as described in [RFC 4213]) can be a
sufficient transition mechanism in some situations. However, cases
where Network Address Translation (NAT) traversal is a concern (see
Section 2.3), or dynamic IP address configuration is required,
another solution is necessary.
There are four variant cases of the "Hubs and Spokes" problem which
are shown in the following figures.
+-------+ +------------+ +--------+
| | |Softwire | | IPv6 |
+---------+ | IPv4 |--|concentrator|--| Network|=>Internet
|v4/v6 |--| | +------------+ +--------+
|Host CPE | | |
+---------+ |Network|
+-------+
_ _ _ _ _ _ __
()_ _ _ _ _ _ __() IPv6 SPH
"softwire"
|--------------||-------------------------|
IPv4-only IPv6 or dual-stack
Case 1: IPv6 connectivity across an IPv4-only access network (STH).
Softwire initiator 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.
Figure 1: Case 1
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RFC 4925 Softwire Problem Statement July 2007
+-------+ +-------------+ +--------+
| | | Softwire | | v6 |
+-----+ +------+ | v4 |--| concentrator|--| Network|=>Internet
|v4/v6|--|v4/v6 |--| | +-------------+ +--------+
|Host | |Router| |Network|
+-----+ |v4/v6 | | |
| CPE | +-------+
+------+
_ _ _ _ _ _ __
()_ _ _ _ _ _ __() IPv6 SPH
"softwire"
|--------------||--------------||-------------------------|
Dual-stack IPv4-only IPv6 or dual-stack
Case 2: IPv6 connectivity across an IPv4-only access network (STH).
Softwire initiator is the router CPE, which is a dual-stack device.
The IPv4 traffic should not traverse the softwire.
Figure 2: Case 2
+-------+ +-------------+ +--------+
| | | Softwire | | v6 |
+------+ +------+ | v4 |--| concentrator|--| Network|=>Internet
|v4/v6 |--|v4 |--| | +-------------+ +--------+
|Host | |Router| |Network|
|v6 CPE| |v4 CPE| | |
+------+ | | +-------+
+------+
_ _ _ _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _ _ _ _() IPv6 SPH
"softwire"
|-----------------------||-------------------------|
IPv4 only IPv6 or dual-stack
Case 3: IPv6 connectivity across an IPv4-only access network (STH).
The CPE is IPv4-only. Softwire initiator is a host, which act as an
IPv6 host CPE. The IPv4 traffic should not traverse the softwire.
Figure 3: Case 3
Li, et al. Informational PAGE 7
RFC 4925 Softwire Problem Statement July 2007
+-----+
|v4/v6| +-------+ +------------+ +-------+
|Host | | | |Softwire | | v6 |
+-----+ +------+ | v4 |--|concentrator|--|Network|=>Internet
| |v4 |--| | +------------+ +-------+
|---------|Router| |Network|
| |v4 CPE| +-------+
+---------+ +------+
|Softwire |
|Initiator|
|v6 Router|
| CPE |
+---------+
_ _ _ _ _ _ _ _ _ _ _ _
()_ _ _ _ _ _ _ _ _ _ _ _() IPv6 SPH
"softwire"
|--------||-----------------------||----------------------|
Dual IPv4 only IPv6 or dual-stack
stack
Case 4: IPv6 connectivity across an IPv4-only access network (STH).
The routing CPE is IPv4-only. Softwire initiator is a device acting
as an IPv6 CPE router inside the home network. The IPv4 traffic
should not traverse the softwire.
Figure 4: Case 4
The converse cases exist, replacing IPv4 by IPv6 and vice versa in
the above figures.
2.1. Description
In this scenario, carriers (or large enterprise networks acting as
carriers for their internal networks) have an infrastructure that in
at least one device on any given path supports only one address
family, with customers who wish to support applications bound to an
address family that cannot be routed end-to-end. The address family
that must be "crossed" is called the Softwire Transport Header, or
STH AF (which could be either IPv4 or IPv6).
In order to support applications bound to another address family (the
Softwire Payload Header Address Family, or SPH AF), it is necessary
to establish a virtual dual-stack infrastructure (end-to-end),
typically by means of automatically-established tunnels (Softwires).
The traffic that can traverse the network via its native AF must not
be forced to take the softwire path. Only the traffic that otherwise
would not be able to be forwarded due to the AF mismatch should be
Li, et al. Informational PAGE 8
RFC 4925 Softwire Problem Statement July 2007
forwarded within the softwire. The goal is to avoid overwhelming the
softwire concentrator (SC).
A network operator may choose to enable a single address family in
one or several parts of this infrastructure for policy reasons (i.e.,
traffic on the network is dominant in one of the address families, a
single address family is used to lower Operations and Management
(OAM) cost, etc.) or for technical reasons (i.e., because one or more
devices are not able to support both address families).
There are several obstacles that may preclude support for both
address families:
a) One or more devices (routers or some other media-specific
aggregation point device) being used across the infrastructure (core,
access) that supports only one address family. Typically the reasons
for this situation include a lack of vendor support for one of the
address families, the (perceived) cost of upgrading them, the
(perceived) complexity of running both address families natively,
operation/management reasons to avoid upgrades (perhaps temporarily),
or economic reasons (such as a commercially insignificant amount of
traffic with the non-supported address family).
b) The home gateway (CPE router or other equipment at the demarc
point), cannot be easily upgraded to support both address families.
Typically the reason for this is the lack of vendor support for one
of the address families, commercial or operational reasons for not
carrying out the upgrade (i.e., operational changes and/or cost may
need to be supported by the carrier for all the customers, which can
turn into millions of units), or customer reluctance to change/
upgrade CPE router (cost, "not broken, so don't change it"). Note
that the impracticality of systematic upgrades of the CPE routers is
also hindering the deployment of 6to4 based solutions [RFC 3056] in
IPv4 networks.
2.2. Non-Upgradable CPE Router
Residential and small-office CPE equipment may be limited to support
only one address family. Often, they are owned by a customer or
carrier who is unwilling or unable to upgrade them to run in dual
stack mode (as shown in Figure 3 and Figure 4).
When the CPE router cannot run in dual-stack mode, a softwire will
have to be established by a node located behind that CPE router.
This can be accomplished either by a regular host in the home running
softwire software (Figure 1 or Figure 3) or by a dedicated piece of
hardware acting as the "IPv6 router" (Figure 4). Such a device is
fairly simple in design and only requires one physical network
Li, et al. Informational PAGE 9
RFC 4925 Softwire Problem Statement July 2007
interface. Again, only the traffic of the mismatched AF will be
forwarded via the softwire. Traffic that can otherwise be forwarded
without a softwire should not be encapsulated.
2.3. Network Address Translation (NAT) and Port Address Translation
(PAT)
A typical case of non-upgradable CPE router is a pre-existing IPv4/
NAT home gateway, so the softwire solution must support NAT
traversal.
Establishing a Softwire through NAT or PAT must be supported without
an explicit requirement to "autodetect" NAT or PAT presence during
softwire setup. Simply enabling NAT traversal could be sufficient to
meet this requirement.
Although the tunneling protocol must be able to traverse NATs,
tunneling protocols may have an optional capability to bypass UDP
encapsulation if not traversing a NAT.
2.4. Static Prefix Delegation
An important characteristic of this problem in IPv4 networks is that
the carrier-facing CPE IP address is typically dynamically assigned.
(The IP address of the node establishing the softwire behind the CPE
router can also be dynamically assigned.)
Solutions like external dynamic DNS and dynamic NAT port forwarding
have been deployed to deal with ever changing addresses, but it would
be simpler if, in IPv6 networks, a static prefix was delegated to
customers. Such a prefix would allow for the registration of stable
addresses in the DNS and enable the use of solutions like RFC 3041
[RFC 3041] privacy extension or cryptographically generated addresses
(CGA) [RFC 3972].
The softwire protocol does not need to define a new method for prefix
delegation; however, the Dynamic Host Configuration Protocol for IPv6
(DHCPv6) prefix delegation [RFC 3633] must be able to run over a
softwire.
Link local addresses allocated at both ends of the tunnel are enough
for packet forwarding, but for management purpose like traceroute,
global addresses can be allocated using existing protocols such as
stateless address auto-configuration [RFC 2462] or DHCPv6 [RFC 3315].
The IP addresses of the softwire link itself do not need to be
stable, the desire for stability only applies to the delegated
prefix. Even if there is a single node attached behind a softwire
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RFC 4925 Softwire Problem Statement July 2007
link, nothing prevents a softwire concentrator to delegate it a /64
prefix.
Similarly, in the case of an IPv4 softwire, the address could be
provided by means of DHCP [RFC 2131]. In the case of an IPv4
softwire, a mechanism should be available in order to delegate an
IPv4 prefix [SUBNET].
Note about 6to4: This is one of the main differences between
Softwires and 6to4. 6to4 addresses will change every time the CPE
router gets a new external address, where a DHCPv6 delegated prefix
through a softwire link could be stable.
2.5. Softwire Initiator
In the "Hubs and Spokes" problem, softwires are always initiated by
the customer side. Thus, the node hosting the end of the softwire
within the customer network is called the softwire initiator. It can
run on any dual-stack node. As noted earlier, this can be the CPE
access device, another dedicated CPE router behind the original CPE
access device, or actually any kind of node (host, appliance, sensor,
etc.).
The softwire initiator node can change over time and may or may not
be delegated the same IP address for the softwire endpoint. In
particular, softwires should work in the nomadic case (e.g., a user
opening up his laptop in various Wi-Fi hot-spots), where the softwire
initiator could potentially obtain an IP address of one address
family outside its original carrier network and still want to obtain
the other address family addresses from its carrier.
If and when the IPv4 provider periodically changes the IPv4 address
allocated to the gateway, the softwire initiator has to discover in a
reasonable amount of time that the tunnel is down and restart it.
This re-establishment should not change the IPv6 prefix and other
parameters allocated to the site.
2.6. Softwire Concentrator
On the carrier side, softwires are terminated on a softwire
concentrator. A softwire concentrator is usually a dual-stack router
connected to the dual-stack core of the carrier.
A carrier may deploy several softwire concentrators (for example one
per POP) for scalability reasons.
Softwire concentrators are usually not nomadic and have stable IP
addresses.
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RFC 4925 Softwire Problem Statement July 2007
It may be the case that one of the address families is not natively
supported on the interface facing the core of the carrier.
Connectivity must then be provided by other tunnels, potentially
using the softwire mesh model.
Softwire concentrator functionality will be based on existing
standards for tunneling, prefixes, and addresses allocation,
management. The working group must define a softwire concentrator
architecture and interaction between these protocols and recommend
profiles. These recommendations must take into account the
distributed nature of the Softwires Concentrator in the provider
network and the impact on core IPv6 networks (for instance: prefix
aggregation).
2.7. Softwire Concentrator Discovery
The softwire initiator must know the DNS name or IP address of the
softwire concentrator. An automated discovery phase may be used to
return the IP address(s) or name(s) of the concentrator.
Alternatively, this information may be configured by the user, or by
the provider of the softwire initiator in advance. The details of
this discovery problem are outside the scope of this document,
however previous work could be taken in consideration. Examples
include: [SERVICE-DIS], [RFC 4891], and [TUN-AD].
2.8. Scaling
In a "Hubs and Spokes model", a carrier must be able to scale the
solution to millions of softwire initiators by adding more hubs
(i.e., softwire concentrators).
2.9. Routing
As customer networks are typically attached via a single link to
their carrier, the minimum routing requirement is a default route for
each of the address families.
2.10. Multicast
Softwires must support multicast.
2.11. Security
2.11.1. Authentication, Authorization, and Accounting (AAA)
The softwire protocol must support customer authentication in the
control plane, in order to authorize access to the service, and
provide adequate logging of activity (accounting). However, a
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RFC 4925 Softwire Problem Statement July 2007
carrier may decide to turn it off in some circumstances, for
instance, when the customer is already authenticated by some other
means, such as closed networks, cellular networks, etc., in order to
avoid unnecessary overhead.
The protocol should offer mutual authentication in scenarios where
the initiator requires identity proof from the concentrator.
The softwire solution, at least for "Hubs and Spokes", must be
integrable with commonly deployed AAA solutions (although extensions
to those AAA solutions may be needed).
2.11.2. Privacy, Integrity, and Replay Protection
The softwire Control and/or Data plane must be able to provide full
payload security (such as IPsec or SSL (Secure Socket Layer)) when
desired. This additional protection must be separable from the
tunneling aspect of the softwire mechanism itself. For IPsec,
default profiles must be defined. [RFC 4891] provides guidelines on
this.
2.12. Operations and Management (OAM)
As it is assumed that the softwire may have to go across NAT or PAT,
a keepalive mechanism must be defined. Such a mechanism is also
useful for dead peer detection. However in some circumstances (i.e.,
narrowband access, billing per traffic, etc.) the keepalive mechanism
may consume unnecessary bandwidth, so turning it on or off, and
modifying the periodicity, must be supported administrative options.
Other needed OAM features include:
- Logging
- Usage accounting
- End-point failure detection (the detection mechanism must operate
within the tunnel)
- Path failure detection (the detection mechanism must operate
outside the tunnel)
2.13. Encapsulations
IPv6/IPv4, IPv6/UDP/IPv4, and IPv4/IPv6 are on the critical path for
"Hubs and Spokes" softwires. There is no intention to place limits
on additional encapsulations beyond those explicitly mentioned in
this specification.
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RFC 4925 Softwire Problem Statement July 2007
3. Mesh Problem
3.1. Description
We use the term "Mesh Problem" to describe the problem of supporting
a general routed topology in which a backbone network that does not
support a particular address family can be used as part of the path
for packets that belong to that address family. For example, the
path for an IPv4 packet might include a transit network that supports
only IPv6. There might (or might not) be other paths that the IPv4
packet could take that do not use the IPv6 transit network; the
actual path chosen will be determined by the IPv4 routing procedures.
By saying that the transit network supports only a single address
family, we mean that the "core" routers of that network do not
maintain routing information for other address families, and they may
not even be able to understand the packet headers of other address
families. We do suppose though that the core will have "edge
routers" or "border routers", which maintain the routing information
for both address families, and which can parse the headers of both
address families. We refer to these as "Address Family Border
Routers" (AFBRs).
The following figure shows an AF2-only network connected to AF1-only
networks, AF2-only networks, and dual stack networks. Note that in
addition to paths through the AF2-only core, other paths may also
exist between AF1 networks. The AFBRs that support AF1 would use BGP
to exchange AF1 routing information between themselves, but such
information would not be distributed to other core routers. The
AFBRs would also participate in the exchange of AF2 routing
information with the core nodes.
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RFC 4925 Softwire Problem Statement July 2007
+----------+ +----------+
|AF1 only | |AF1 only |
| | | |
+----------+ +----------+
| |
| |
Dual-Stack Dual-Stack
"AFBR" "AFBR"
| |
| |
+----------------------------+
| |
+-------+ | | +-------+
|AF2 | | AF2 only | |AF2 |
|only |-------| (but also providing |-------|only |
+-------+ | transit for AF1) | +-------+
| |
+----------------------------+
| / \ |
| / \ |
Dual-Stack Dual-Stack
"AFBR" "AFBR"
| | |
| | |
+--------+ +--------+
|AF1 and | |AF1 and |
|AF2 | |AF2 |
+--------+ +--------+
Figure 5: Mesh Topology
The situation in which a pair of border routers use BGP to exchange
routing information that is not known to the core routers is
sometimes known, somewhat misleadingly, as a "BGP-free core". In
this sort of scenario, the problems to be solved are (a) to make sure
that the BGP-distributed routing updates for AF1 allow a given AFBR,
say AFBR1, to see that the path for a given AF1 address prefix is
through a second AFBR, say AFBR2, and (b) to provide a way in which
AFBR1 can send AF1 packets through the AF2-only core to AFBR2. Of
course, sending AF1 packets through an AF2-only core requires the AF1
packets to be encapsulated and sent through "tunnels"; these tunnels
are the entities known as "softwires".
One of the goals of the mesh problem is to provide a solution that
does not require changes in any routers other than the AFBRs. This
would allow a carrier (or large enterprise networks acting as carrier
for their internal resources) with an AF2-only backbone to provide
AF1 transit services for its clients, without requiring any changes
Li, et al. Informational PAGE 15
RFC 4925 Softwire Problem Statement July 2007
whatsoever to the clients' routers, and without requiring any changes
to the core routers. The AFBRs are the only devices that perform
dual-stack operations, and the only devices that encapsulate and/or
decapsulate the AF1 packets in order to send and/or receive them on
softwires.
It may be recognized that this scenario is very similar to the
scenario handled by the Layer 3 Virtual Private Network (L3VPN)
solution described in RFC 4364 [RFC 4364]. The AFBRs correspond to
the "Provider Edge Routers" (PE) of RFC 4364. In those L3VPN
scenarios, the PEs exchange routing information in an address family
(e.g., the VPN-IPv4 address family), but they send VPN data packets
through a core which does not have the VPN routing information.
However, the softwire problem is NOT focused on the situation in
which the border routers maintain multiple private and/or overlapping
address spaces. Techniques which are specifically needed to support
multiple address spaces are in the domain of L3VPN, rather than in
the domain of Softwires.
Note that the AFBRs may be multiply connected to the core network,
and also may be multiply connected to the client networks. Further,
the client networks may have "backdoor connections" to each other,
through private networks or through the Internet.
3.2. Scaling
In the mesh problem, the number of AFBRs that a backbone network
supporting only AF2 will need is approximately on the order of the
number of AF1 networks to which it connects. (This is really an
upper limit, since a single AFBR can connect to many such networks).
An AFBR may need to exchange a "full Internet's" worth of routing
information with each network to which it connects. If these
networks are not VPNs, the scaling issues associated with the amount
of routing information are just the usual scaling issues faced by the
border routers of any network which is providing Internet transit
services. (If the AFBRs are also attached to VPNs, the usual L3VPN
scaling issues apply, as discussed in RFC 4364 [RFC 4364] and RFC 4365
[RFC 4365].) The number of BGP peering connections can be controlled
through the usual methods, e.g., use of route reflectors.
3.3. Persistence, Discovery, and Setup Time
AFBRs may discover each other, and may obtain any necessary
information about each other, as a byproduct of the exchange of
routing information (essentially in the same way that PE routers
discovery each other in L3VPNs). This may require the addition of
new protocol elements or attributes to existing protocols.
Li, et al. Informational PAGE 16
RFC 4925 Softwire Problem Statement July 2007
The softwires needed to allow packets to be sent from one AFBR to
another should be "always available", i.e., should not require any
extended setup time that would impart an appreciable delay to the
data packets.
3.4. Multicast
If the AF2 core does not provide native multicast services, multicast
between AF1 client networks should still be possible, even though it
may require replication at the AFBRs and unicasting of the replicated
packets through Softwires. If native multicast services are enabled,
it should be possible to use these services to optimize the multicast
flow.
3.5. Softwire Encapsulation
The solution to the mesh problem must not require the use of any one
encapsulation. Rather, it must accommodate the use of a variety of
different encapsulation mechanisms, and a means for choosing the one
to be used in any particular circumstance based on policy.
In particular, the solution to the mesh problem must allow for at
least the following encapsulations to be used: Layer 2 Tunneling
Protocol version 3 (L2TPv3), IP-in-IP, MPLS (LDP-based and RSVP-TE-
based), Generic Routing Encapsulation (GRE), and IPsec. The choice
of encapsulation is to be based on policy, and the policies
themselves may be based on various characteristics of the packets, of
the routes, or of the softwire endpoints themselves.
3.6. Security
In the mesh problem, the routers are not advertising routes for
individual users. So the mesh problem does not require the fine-
grained authentication that is required by the "Hub and Spoke"
problem. There should however be a way to provide various levels of
security for the data packets being transmitted on a softwire. The
softwire solution must support IPsec and an IPsec profile must be
defined (see recommendations in [USEIPSEC]).
Security mechanisms for the control protocols are also required. It
must be possible to protect control data from being modified in
flight by an attacker, and to prevent an attacker from masquerading
as a legitimate control protocol participant.
The verification of the reachability information exchanged and issues
surrounding the security of routing protocols themselves is outside
the scope of the specification.
Li, et al. Informational PAGE 17
RFC 4925 Softwire Problem Statement July 2007
4. Security Considerations
Security considerations specific to the "Hubs and Spokes" and "Mesh"
models appear in those sections of the document.
As with any tunneling protocol, using this protocol may introduce a
security issue by circumventing a site security policy implemented as
ingress filtering, since these filters will only be applied to STH AF
IP headers.
5. Principal Authors
These are the principal authors for this document.
Xing Li
CERNET
Room 225 Main Building, Tsinghua University
Beijing 100084
China
Phone: +86 10 62785983
Fax: +86 10 62785933
Email: xing@cernet.edu.cn
Alain Durand
Comcast
1500 Market st
Philadelphia
PA 19102
USA
Email: Alain_Durand@cable.comcast.com
Shin Miyakawa
NTT Communications
3-20-2 TOC 21F, Nishi-shinjuku, Shinjuku
Tokyo
Japan
Phone: +81-3-6800-3262
Fax: +81-3-5365-2990
Email: miyakawa@nttv6.jp
Li, et al. Informational PAGE 18
RFC 4925 Softwire Problem Statement July 2007
Jordi Palet Martinez
Consulintel
San Jose Artesano, 1
Alcobendas - Madrid
E-28108 - Spain
Phone: +34 91 151 81 99
Fax: +34 91 151 81 98
Email: jordi.palet@consulintel.es
Florent Parent
Hexago
2875 boul. Laurier, suite 300
Sainte-Foy, QC G1V 2M2
Canada
Phone: +1 418 266 5533
Email: Florent.Parent@hexago.com
David Ward
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
USA
Phone: +1-408-526-4000
Email: dward@cisco.com
Eric C. Rosen
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA, 01716
USA
Email: erosen@cisco.com
6. Contributors
The authors would like to acknowledge the following contributors who
provided helpful inputs on earlier versions of this document: Alain
Baudot, Hui Deng, Francis Dupont, Rob Evans, Ed Koehler Jr, Erik
Nordmark, Soohong Daniel Park, Tom Pusateri, Pekka Savola, Bruno
Stevant, Laurent Totain, Bill Storer, Maria (Alice) Dos Santos, Yong
Cui, Chris Metz, Simon Barber, Skip Booth, Scott Wainner, and Carl
Williams.
Li, et al. Informational PAGE 19
RFC 4925 Softwire Problem Statement July 2007
The authors would also like to acknowledge the participants in the
Softwires interim meeting in Paris, France (October 11-12, 2005)
(minutes are at
http://bgp.nu/~dward/softwires/InterimMeetingMinutes.htm).
The authors would also like to express a special acknowledgement and
thanks to Mark Townsley. Without his leadership, persistence,
editing skills, and thorough suggestions for the document, we would
have not have been successful.
Tunnel-based transition mechanisms have been under discussion in the
IETF for more than a decade. Initial work related to softwire can be
found in RFC 3053 [RFC 3053]. The earlier "V6 Tunnel Configuration"
BOF problem statement [GOALS-TUN] a reasonable pointer to prior work.
The authors would like to acknowledge the work and support of Dr
Jianping Wu of Tsinghua university.
7. References
7.1. Normative References
[RFC 3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6",
RFC 3041, January 2001.
[RFC 3053] Durand, A., Fasano, P., Guardini, I., and D. Lento,
"IPv6 Tunnel Broker", RFC 3053, January 2001.
[RFC 3056] Carpenter, B. and K. Moore, "Connection of IPv6
Domains via IPv4 Clouds", RFC 3056, February 2001.
[RFC 3972] Aura, T., "Cryptographically Generated Addresses
(CGA)", RFC 3972, March 2005.
[RFC 4213] Nordmark, E. and R. Gilligan, "Basic Transition
Mechanisms for IPv6 Hosts and Routers", RFC 4213,
October 2005.
7.2. Informative References
[GOALS-TUN] Palet, J., "Goals for Tunneling Configuration", Work
in Progress, February 2005.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
Li, et al. Informational PAGE 20
RFC 4925 Softwire Problem Statement July 2007
[RFC 2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC 3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T.,
Perkins, C., and M. Carney, "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3315,
July 2003.
[RFC 3633] Troan, O. and R. Droms, "IPv6 Prefix Options for
Dynamic Host Configuration Protocol (DHCP) version 6",
RFC 3633, December 2003.
[RFC 4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC 4365] Rosen, E., "Applicability Statement for BGP/MPLS IP
Virtual Private Networks (VPNs)", RFC 4365,
February 2006.
[RFC 4891] Graveman, R., Parthasarathy, M., Savola, P., and H.
Tschofenig, "Using IPsec to Secure IPv6-in-IPv4
Tunnels", RFC 4891, May 2007.
[SERVICE-DIS] Durand, A., "Service Discovery using NAPTR records in
DNS", Work in Progress, October 2004.
[SUBNET] Johnson, R., "Subnet Allocation Option", Work in
Progress, June 2007.
[TUN-AD] Palet, J. and M, "Analysis of IPv6 Tunnel End-point
Discovery Mechanisms", Work in Progress, January 2005.
[USEIPSEC] Bellovin, S., "Guidelines for Mandating the Use of
IPsec", Work in Progress, February 2007.
Li, et al. Informational PAGE 21
RFC 4925 Softwire Problem Statement July 2007
Authors' Addresses
Xing Li (editor)
CERNET
Room 225 Main Building, Tsinghua University
Beijing, 100084
China
Phone: +86 10 62785983
Fax: +86 10 62785933
EMail: xing@cernet.edu.cn
Spencer Dawkins (editor)
Huawei Technologies (USA)
1700 Alma Drive, Suite 100
Plano, TX 75075
US
Phone: +1 972 509 0309
Fax: +1 469 229 5397
EMail: spencer@mcsr-labs.org
David Ward (editor)
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: 1-408-526-4000
EMail: dward@cisco.com
Alain Durand (editor)
Comcast
1500 Market St
Philadelphia, PA 19102
US
EMail: alain_durand@cable.comcast.com
Li, et al. Informational PAGE 22
RFC 4925 Softwire Problem Statement July 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Softwire Problem Statement
RFC TOTAL SIZE: 49299 bytes
PUBLICATION DATE: Thursday, July 26th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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