Scenarios and Analysis for Introducing IPv6 into ISP Networks
draft-ietf-v6ops-isp-scenarios-analysis-03
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4029.
|
|
|---|---|---|---|
| Authors | Pekka Savola , Vladimir Ksinant , Soohong Daniel Park , Mikael Lind , Alain Baudot | ||
| Last updated | 2020-01-21 (Latest revision 2004-06-18) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4029 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | David Kessens | ||
| Send notices to | pekkas@netcore.fi, jonne.Soininen@nokia.com |
draft-ietf-v6ops-isp-scenarios-analysis-03
Internet Draft M. Lind
<draft-ietf-v6ops-isp-scenarios-analysis-03.txt> TeliaSonera
V. Ksinant
Thales
Communications
S. Park
Samsung Electronics
A. Baudot
France Telecom
P. Savola
CSC/Funet
Expires: December 2004 June 2004
Scenarios and Analysis for Introducing IPv6 into ISP Networks
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than a "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Abstract
This document first describes different scenarios for the
introduction of IPv6 into an ISP's existing IPv4 network without
disrupting the IPv4 service. Then, the scenarios for introducing IPv6
are analyzed and the relevance of already defined transition
mechanisms are evaluated. Known challenges are also identified.
Lind, et al. Expires - December 2004 [Page 1]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
Table of Contents
1. Introduction................................................2
1.1 Goal and Scope of the Document...........................2
2. Brief Description of a Generic ISP Network..................3
3. Transition Scenarios........................................4
3.1 Identification of Stages and Scenarios...................4
3.2 Stages...................................................5
3.2.1 Stage 1 Scenarios: Launch..............................5
3.2.2 Stage 2a Scenarios: Backbone...........................6
3.2.3 Stage 2b Scenarios: Customer Connection................6
3.2.4 Stage 3 Scenarios: Complete............................7
3.2.5 Stages 2a and 3: Combination Scenarios.................7
3.3 Transition Scenarios.....................................7
3.4 Actions Needed When Deploying IPv6 in an ISP's network...8
4. Backbone Transition Actions.................................9
4.1 Steps in the Transition of Backbone Networks.............9
4.1.1 MPLS Backbone.........................................10
4.2 Configuration of Backbone Equipment.....................10
4.3 Routing.................................................10
4.3.1 IGP...................................................11
4.3.2 EGP...................................................12
4.3.3 Transport of Routing Protocols........................12
4.4 Multicast...............................................13
5. Customer Connection Transition Actions.....................13
5.1 Steps in the Transition of Customer Connection Networks.13
5.1.1 Small end sites.......................................14
5.1.2 Large end sites.......................................15
5.2 User Authentication/Access Control Requirements.........15
5.3 Configuration of Customer Equipment.....................16
5.4 Requirements for Traceability...........................16
5.5 Ingress Filtering in the Customer Connection Network....17
5.6 Multihoming.............................................17
5.7 Quality of Service......................................17
6. Network and Service Operation Actions......................18
7. Future Stages..............................................18
8. Requirements for Follow-On Work............................19
9. Example Networks...........................................19
9.1 Example 1...............................................20
9.2 Example 2...............................................22
9.3 Example 3...............................................22
10. Security Considerations....................................23
11. Acknowledgements...........................................23
12. Informative References.....................................24
1. Introduction
1.1 Goal and Scope of the Document
Lind, et al. Expires - December 2004 [Page 2]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
When an ISP deploys IPv6, its goal is to provide IPv6 connectivity
and global address space to its customers. The new IPv6 service must
be added to an already existing IPv4 service, and the introduction of
IPv6 must not interrupt this IPv4 service.
An ISP offering IPv4 service will find different ways to add IPv6 to
this service. This document discusses a small set of scenarios for
the introduction of IPv6 into an ISP's IPv4 network. It evaluates the
relevance of the existing transition mechanisms in the context of
these deployment scenarios, and points out the lack of essential
functionality in these methods to the ISP's operation of an IPv6
service.
The document is focused on services that include both IPv6 and IPv4
and does not cover issues surrounding IPv6-only service. It is also
outside the scope of this document to describe different types of
access or network technologies.
2. Brief Description of a Generic ISP Network
A generic network topology for an ISP can be divided into two main
parts: the backbone network and customer connection networks. It
includes, in addition to these, some other building blocks such as
network and service operations. The additional building blocks used
in this document are defined as follows:
"CPE" : Customer Premises Equipment
"PE" : Provider Edge equipment
"Network and service operation"
: This is the part of the ISP's network that hosts the
services required for the correct operation of the
ISP's network. These services usually include
management, supervision, accounting, billing, and
customer management applications.
"Customer connection"
: This is the part of the network used by a customer
when connecting to an ISP's network. It includes the
CPE, the last hop link and the parts of the PE
interfacing to the last hop link.
"Backbone" : This is the rest of the ISP's network infrastructure.
It includes the parts of the PE interfacing to the
core, the core routers of the ISP, and the border
routers used to exchange routing information with
other ISPs (or other administrative entities).
Lind, et al. Expires - December 2004 [Page 3]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
"Dual-stack network":
A network that supports natively both IPv4 and
IPv6.
It is noted that, in some cases (e.g., incumbent national or
regional operators), a given customer connection network may have
to be shared between or among different ISPs. According to the type
of customer connection network used (e.g., one involving only layer 2
devices or one involving non-IP technology), this constraint may
result in architectural considerations relevant to this document.
The basic components in the ISP's network are depicted in Figure 1.
------------ ----------
| Network and| | |
| Service |--| Backbone |
| Operation | | |\
------------ ---------- \
/ | \ \
/ | \ \_Peering( Direct and
/ | \ exchange points)
/ | \
/ | \
---------- / ---------- \ ----------
| Customer | / | Customer | \ | Customer |
|Connection|--/ |Connection| \--|Connection|
| 1 | | 2 | | 3 |
---------- ---------- ----------
| | | ISP's Network
-------------------------------------------------------
| | | Customers' Networks
+--------+ +--------+ +--------+
| | | | | |
|Customer| |Customer| |Customer|
| | | | | |
+--------+ +--------+ +--------+
Figure 1: ISP Network Topology.
3. Transition Scenarios
3.1 Identification of Stages and Scenarios
This section describes different stages an ISP might consider when
introducing IPv6 connectivity into its existing IPv4 network and the
different scenarios that might occur in the respective stages.
The stages here are snapshots of the ISP's network with respect to
IPv6 maturity. Because the ISP's network is continually evolving, a
stage is a measure of how far along the ISP has come in terms of
Lind, et al. Expires - December 2004 [Page 4]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
implementing the functionality necessary to offer IPv6 to its
customers.
It is possible for a transition to occur freely between different
stages. Although a network segment can only be in one stage at a
time, the ISP's network as a whole can be in different stages.
Different transition paths can be followed from the first to the
final stage. The transition between two stages does not have to be
instantaneous; it can occur gradually.
Each stage has different IPv6 properties. An ISP can, therefore,
based on its requirements, decide which set of stages it will follow
and in what order to transform its network.
This document is not aimed at covering small ISPs, hosting providers,
or data centers; only the scenarios applicable to ISPs eligible for
at least a /32 IPv6 prefix allocation from a RIR are covered.
3.2 Stages
The stages are derived from the generic description of an ISP's
network in Section 2. Combinations of different building blocks
that constitute an ISP's environment lead to a number of scenarios
from which the ISP can choose. The scenarios most relevant to this
document are those that maximize ISP's ability to offer IPv6 to its
customers in the most efficient and feasible way. The assumption in
all stages is that the ISP's goal is to offer both IPv4 and IPv6 to
the customer.
The four most probable stages are:
o Stage 1 Launch
o Stage 2a Backbone
o Stage 2b Customer connection
o Stage 3 Complete
Generally, an ISP is able to upgrade a current IPv4 network to an
IPv4/IPv6 dual-stack network via Stage 2b, but the IPv6 service can
also be implemented at a small cost by adding simple tunnel
mechanisms to the existing configuration. When designing a new
network, Stage 3 might be the first and last step, because there are
no legacy concerns. Nevertheless, the absence of IPv6 capability in
the network equipment can still be a limiting factor.
Note that in every stage except Stage 1, the ISP can offer both IPv4
and IPv6 services to its customers.
3.2.1 Stage 1 Scenarios: Launch
Lind, et al. Expires - December 2004 [Page 5]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
The first stage is an IPv4-only ISP with an IPv4 customer. This is
the most common case today and the natural starting point for the
introduction of IPv6. From this stage, the ISP can move (undergo a
transition) from Stage 1 to any other stage with the goal of offering
IPv6 to its customer.
The immediate first step consists of obtaining a prefix allocation
(typically a /32) from the appropriate RIR (e.g. AfriNIC, APNIC,
ARIN, LACNIC, RIPE, ...) according to allocation procedures.
The ISP will also need to establish IPv6 connectivity to its upstream
providers and peers; it is of utmost importance to require IPv6
transit when negotiating IP transit deals with the upstream ISPs. If
the upstream is not providing IPv6 connectivity at the moment, it may
be possible to temporarily obtain connectivity from some other ISP
nearby, possibly using a short configured tunnel. However, the
longer-term goal must be requiring and obtaining IPv6 connectivity
from the transit ISPs, because otherwise the quality of IPv6
connectivity will likely be poor.
Connectivity to peers can be typically established either directly or
at Internet Exchange Points (IX). Most IXs are using techniques
where IPv6 is easy to use, and many IXs already provide
infrastructure for IPv6 peerings. Such peerings can be done natively
using IPv6. Peerings over IPv6-in-IPv4 tunnels is also possible but
not recommended at least in the long term. Direct connectivity to
peers may be feasible when there is direct connectivity to the peer
for IPv4.
3.2.2 Stage 2a Scenarios: Backbone
Stage 2a deal with an ISP with IPv4-only customer connection networks
and a backbone that supports both IPv4 and IPv6. In particular, the
ISP has the possibility of making the backbone IPv6-capable through
either software upgrades, hardware upgrades, or a combination of
both.
Since the customer connections have not yet been upgraded, a
tunneling mechanism has to be used to provide IPv6 connectivity
through the IPv4 customer connection networks. The customer can
terminate the tunnel at the CPE (if it has IPv6 support) or at some
set of devices internal to its network. That is, either the CPE or a
device inside the network could provide global IPv6 connectivity to
the rest of the devices in the customer's network.
3.2.3 Stage 2b Scenarios: Customer Connection
Stage 2b consists of an ISP with an IPv4 backbone network and a
customer connection network that supports both IPv4 and IPv6. Because
Lind, et al. Expires - December 2004 [Page 6]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
the service to the customer is native IPv6, the customer is not
required to support both IPv4 and IPv6. This is the biggest
difference in comparison to the previous stage. The need to exchange
IPv6 traffic still exists but might be more complicated than in the
previous case, because the backbone is not IPv6-enabled. After
completing Stage 2b, the original IPv4 backbone is unchanged. This
means that the IPv6 traffic is transported either by tunneling over
the existing IPv4 backbone, or in an IPv6 overlay network more or
less separated from the IPv4 backbone.
Normally the ISP will continue to provide IPv4 connectivity using
private (NATted by the ISP) or public IPv4 address; in many cases,
the customer also has a NAT of his/her own, and if so, this likely
continues to be used for IPv4 connectivity.
3.2.4 Stage 3 Scenarios: Complete
Stage 3 can be said to be the final step in introducing IPv6, at
least within the scope of this document. This stage consists of
ubiquitous IPv6 service with native support for IPv6 and IPv4 in both
backbone and customer connection networks. It is identical to the
previous stage from the customer's perspective, because the customer
connection network has not changed. The requirement for exchanging
IPv6 traffic is identical to Stage 2.
3.2.5 Stages 2a and 3: Combination Scenarios
Some ISPs may use different access technologies of varying IPv6
maturity. This may result in a combination of the Stages 2a and 3:
some customer connections do not support IPv6, but others do; in both
cases the backbone is dual-stack.
This scenario is equivalent to Stage 2a, but it requires support for
native IPv6 customer connections on some access technologies.
3.3 Transition Scenarios
Given the different stages, it is clear that an ISP has to be able
to make a transition from one stage to another. The initial stage in
this document is IPv4-only service and network. The end stage is dual
IPv4/IPv6 service and network.
The transition starts with an IPv4 ISP and then moves in one of
three directions. This choice corresponds to the different
transition scenarios. Stage 2a consists of upgrading the backbone
first. Stage 2b consists of upgrading the customer connection
network. Finally, Stage 3 consists of introducing IPv6 in both the
backbone and customer connections as needed.
Lind, et al. Expires - December 2004 [Page 7]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
Because most ISP backbone IPv4 networks continually evolve (firmware
replacements in routers, new routers, etc.), they can be made ready
for IPv6 without additional investment (except staff training). This
may be a slower but still useful transition path, because it allows
for the introduction of IPv6 without any actual customer demand. This
process may be superior to doing everything at the last minute, which
may entail a higher investment. However, it is important to consider
(and to request from vendors) IPv6 features in all new equipment from
the outset. Otherwise, the time and effort required to remove non-
IPv6-capable hardware from the network may be significant.
3.4 Actions Needed When Deploying IPv6 in an ISP's network
Examination of the transitions described above reveals that it
is possible to split the work required for each transition into a
small set of actions. Each action is largely independent of the
others, and some actions may be common to multiple transitions.
Analysis of the possible transitions leads to a small list of
actions:
* Actions required for backbone transition:
- Connect dual-stack customer connection networks to other
IPv6 networks through an IPv4 backbone.
- Transform an IPv4 backbone into a dual-stack one. This
action can be performed directly or through intermediate
steps.
* Actions required for customer connection transition:
- Connect IPv6 customers to an IPv6 backbone through an IPv4
network.
- Transform an IPv4 customer connection network into a dual-
stack one.
* Actions required for network and service operation transition:
- Set up IPv6 connectivity to upstream providers and peers.
- Configure IPv6 functions into network components.
- Upgrade regular network management and monitoring
applications to take IPv6 into account.
- Extend customer management (e.g., RADIUS) mechanisms
to be able to supply IPv6 prefixes and other information
Lind, et al. Expires - December 2004 [Page 8]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
to customers.
- Enhance accounting, billing, etc. to work with IPv6
as needed. (Note: if dual-stack service is offered, this
may not be necessary.)
- Implement security for network and service operation.
Sections 4, 5, and 6 contain detailed descriptions of each action.
4. Backbone Transition Actions
4.1 Steps in the Transition of Backbone Networks
In terms of physical equipment, backbone networks consist mainly of
high-speed core and edge routers. Border routers provide peering
with other providers. Filtering, routing policy, and policing
functions are generally managed on border routers.
In the beginning, an ISP has an IPv4-only backbone. In the end, the
backbone is completely dual-stack. In between, intermediate steps may
be identified:
Tunnels Tunnels Dual Full
IPv4-only ----> or ---> or + Stack --> Dual Stack
dedicated IPv6 dedicated IPv6 routers
links links
Figure 2: Transition Path.
The first step involves tunnels or dedicated links but leaves
existing routers unchanged. Only a small set of routers then have
IPv6 capabilities. The use of configured tunnels is adequate during
this step.
In the second step, some dual-stack routers are added, progressively,
to this network.
The final step is reached when all or almost all routers are dual-
stack.
For many reasons (technical, financial, etc.), the ISP may progress
step by step or jump directly to the final one. One important
criterion in planning this evolution is the number of IPv6 customers
the ISP expects during its initial deployments. If few customers
connect to the original IPv6 infrastructure, then the ISP is likely
to remain in the initial steps for a long time.
In short, each intermediate step is possible, but none is mandatory.
Lind, et al. Expires - December 2004 [Page 9]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
4.1.1 MPLS Backbone
If MPLS is already deployed in the backbone, it may be desirable
to provide IPv6-over-MPLS connectivity. However, setting up an IPv6
Label Switched Path (LSP) requires signaling through the MPLS
network; both LDP and RSVP-TE can set up IPv6 LSPs, but this might
require upgrade/change in the MPLS core network.
An alternative approach is to use BGP for signaling or to perform,
for example IPv6-over-IPv4/MPLS, as described in [BGPTUNNEL]. Some of
the multiple possibilities are preferable to others depending on the
specific environment under consideration; the approaches seem to be:
1) Require that MPLS networks deploy native IPv6 routing and
forwarding support.
2) Require that MPLS networks support native routing and setting
up of IPv6 LSPs, used for IPv6 connectivity.
3) Use only configured tunneling over IPv4 LSPs.
4) Use [BGPTUNNEL] to perform IPv6-over-IPv4/MPLS encapsulation
for IPv6 connectivity.
Approaches 1) and 2) are clearly the best target approaches. However,
approach 1) may not be possible if the ISP is not willing to add IPv6
support in the network, or if the installed equipment is not capable
of high performance native IPv6 forwarding. Approach 2) may not be
possible if the ISP is not willing or able to add IPv6 LSP set-up
support in the MPLS control plane.
Approach 4) can be used as an interim mechanism where other options
are unfeasible or undesirable for the reasons discussed above.
Approach 3) is roughly equivalent to approach 4) except that it does
not require additional mechanisms but may lack scalability in the
larger networks especially if IPv6 is widely deployed.
4.2 Configuration of Backbone Equipment
In the backbone, the number of devices is small and IPv6
configuration mainly deals with routing protocol parameters,
interface addresses, loop-back addresses, access control lists, etc.
These IPv6 parameters need to be configured manually.
4.3 Routing
Lind, et al. Expires - December 2004 [Page 10]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
ISPs need routing protocols to advertise reachability and to
find the shortest working paths, both internally and externally.
Either OSPFv2 or IS-IS is typically used as the IPv4 IGP. RIPv2 is
not usually used in service provider networks due to OSPF and IS-IS
being far superior IGPs. BGP is the only IPv4 EGP. Static routes also
are used in both cases.
Note that it is possible to configure a given network so that it
results in having an IPv6 topology different from its IPv4 topology.
For example, some links or interfaces may be dedicated to IPv4-only
or IPv6-only traffic, or some routers may be dual-stack whereas
others may be IPv4- or IPv6-only. In this case, routing protocols
must be able to understand and cope with multiple topologies.
4.3.1 IGP
Once the IPv6 topology has been determined, the choice of IPv6 IGP
must be made: either OSPFv3 or IS-IS for IPv6. RIPng is not
appropriate in most contexts, due to RIPv2 not being appropriate
for IPv4 either, and is therefore not discussed here. The IGP
typically includes the routers' point-to-point and loop-back
addresses.
The most important decision is whether one wishes to have separate
routing protocol processes for IPv4 and IPv6. Separating them
requires more memory and CPU for route calculations, e.g., when the
links flap. On the other hand, separation provides a certain measure
of assurance that if problems arise with IPv6 routing, they will not
affect the IPv4 routing protocol at all. In the initial phases, if
it is uncertain whether joint IPv4-IPv6 networking is working as
intended, running separate processes may be desirable and easier to
manage.
Thus the possible combinations are:
- with separate processes:
o OSPFv2 for IPv4, IS-IS for IPv6 (only)
o OSPFv2 for IPv4, OSPFv3 for IPv6, or
o IS-IS for IPv4, OSPFv3 for IPv6
- with the same process:
o IS-IS for both IPv4 and IPv6
Note that if IS-IS is used for both IPv4 and IPv6, the IPv4/IPv6
topologies must be "convex," unless the multiple-topology IS-IS
extensions [MTISIS] have been implemented (note that using IS-IS for
only IPv4 or only IPv6 requires no convexity). In simpler networks or
with careful planning of IS-IS link costs, it is possible to keep
Lind, et al. Expires - December 2004 [Page 11]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
even incongruent IPv4/IPv6 topologies "convex." The convexity problem
is explained in more detail with an example in Appendix A.
When deploying full dual-stack in the short-term, using single-
topology IS-IS is recommended. This may be applicable particularly
for some larger ISPs. In other scenarios, determining between one or
two separate processes often depends on the perceived risk to the
IPv4 routing infrastructure, i.e., whether one wishes to keep them
separate for the time being. If that is not a factor, using a single
process is usually preferable due to operational reasons: not having
to manage two protocols and topologies.
The IGP is typically only used to carry loopback and point-to-point
addresses and doesn't include customer prefixes or external routes.
Internal BGP (iBGP), as described in the next section, is most often
deployed in all routers (PE and core) to distribute routing
information about customer prefixes and external routes.
Some of the simplest devices, e.g., CPE routers, may not implement
routing protocols other than RIPng. In some cases, therefore, it may
be necessary to run RIPng in addition to one of the above IGPs, at
least in a limited fashion, and then, by some mechanism, to
redistribute routing information between the routing protocols.
4.3.2 EGP
BGP is used for both internal and external BGP sessions.
BGP with multiprotocol extensions [RFC 2858] can be used for IPv6
[RFC 2545]. These extensions enable the exchange of IPv6 routing
information as well as the establishment of BGP sessions using TCP
over IPv6.
It is possible to use a single BGP session to advertise both IPv4
and IPv6 prefixes between two peers. However, the most common
practice today is to use separate BGP sessions.
4.3.3 Transport of Routing Protocols
IPv4 routing information should be carried by IPv4 transport and
similarly IPv6 routing information by IPv6 for several reasons:
* IPv6 connectivity may work when IPv4 connectivity is down
(or vice-versa).
* The best route for IPv4 is not always the best one for IPv6.
* The IPv4 and IPv6 logical topologies may be different,
because the administrator may want to assign different metrics
to a physical link for load balancing or because tunnels may be
in use.
Lind, et al. Expires - December 2004 [Page 12]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
4.4 Multicast
Currently, IPv6 multicast is not a major concern for most ISPs.
However, some of them are considering deploying it. Multicast is
achieved using the PIM-SM and PIM-SSM protocols. These also work with
IPv6.
Information about multicast sources is exchanged using MSDP in IPv4,
but MSDP is intentionally not defined for IPv6. Instead, one should
use only PIM-SSM or an alternative mechanism for conveying the
information [EMBEDRP].
5. Customer Connection Transition Actions
5.1 Steps in the Transition of Customer Connection Networks
Customer connection networks are generally composed of a small set of
PEs connected to a large set of CPEs, and may be based on different
technologies depending on the customer type or size, as well as the
required bandwidth or even quality of service. Basically, public
customers or small/unmanaged networks connection networks usually
rely on a different technologies (e.g. dial-up or DSL) than the ones
used for large customers typically running managed networks.
Transitioning these infrastructures to IPv6 can be accomplished in
several steps, but some ISPs, depending on their perception of the
risks, may avoid some of the steps.
Connecting IPv6 customers to an IPv6 backbone through an IPv4 network
can be considered as a first careful step taken by an ISP to provide
IPv6 services to its IPv4 customers. In addition, some ISPs may also
choose to provide IPv6 service independently from the regular IPv4
service.
In any case, IPv6 service can be provided by using tunneling
techniques. The tunnel may terminate at the CPE corresponding to the
IPv4 service or in some other part of the customer's infrastructure
(for instance, on IPv6-specific CPE or even on a host).
Several tunneling techniques have already been defined: configured
tunnels with tunnel broker, 6to4 [RFC3056], Teredo [TEREDO], etc.
Some of them are based on a specific addressing plan independent of
the ISP's allocated prefix(es), while some others use a part of the
ISP's prefix. In most cases using ISP's address space is preferable.
A key factor is the presence or absence of NATs between the two
tunnel end-points. In most cases, 6to4 and ISATAP are incompatible
with NATs, and UDP encapsulation for configured tunnels has not been
specified.
Lind, et al. Expires - December 2004 [Page 13]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
Dynamic and non-permanent IPv4 address allocation is another factor a
tunneling technique may have to deal with. In this case the tunneling
techniques may be more difficult to deploy at the ISP's end,
especially if a protocol including authentication (like PPP for IPv6)
is not used. This may need to be considered in more detail.
However, NAT traversal can be avoided if the NAT supports forwarding
protocol-41 [PROTO41] and is configured to do so.
Firewalls in the path can also break tunnels of these types. The
administrator of the firewall needs to create a hole for the tunnel.
This is usually manageable, as long as the firewall is controlled by
either the customer or the ISP, which is almost always the case.
When the CPE is performing NAT or firewall functions, terminating the
tunnels directly at the CPE typically simplifies the scenario
considerably, avoiding the NAT and firewall traversal. If such an
approach is adopted, the CPE has to support the tunneling mechanism
used, or be upgraded to do so.
5.1.1 Small end sites
Tunneling considerations for small end sites are discussed in
[UNMANEVA]. These identify solutions relevant to the first category
of unmanaged networks. The tunneling requirements applicable in these
scenarios are described in [TUNREQS]
The connectivity mechanisms can be categorized as "managed" or
"opportunistic." The former consist of native service or a
configured tunnel (with or without a tunnel broker); the latter
include 6to4 and, e.g., Teredo -- they provide "short-cuts" between
nodes using the same mechanisms and are available without contracts
with the ISP.
The ISP may offer opportunistic services, mainly a 6to4 relay,
especially as a test when no actual service is offered yet. At the
later phases, ISPs might also deploy 6to4 relays and Teredo servers
(or similar) to optimize their customers' connectivity to 6to4 and
Teredo nodes.
It is to be noticed that opportunistic services are typically based
on techniques that don't use IPv6 addresses out of the ISP's
allocated prefix(es), and that the services have very limited
functions to control the origin and the number of customers connected
to a given relay.
Most interesting are the managed services. When dual-stack is not an
option, a form of tunneling must be used. When configured tunneling
is not an option (e.g., due to dynamic IPv4 addressing), some form of
Lind, et al. Expires - December 2004 [Page 14]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
automation has to be used. Basically, the options are either to
deploy an L2TP architecture (whereby the customers would run L2TP
clients and PPP over it to initiate IPv6 sessions) or to deploy a
tunnel configuration service. The prime candidates for tunnel
configuration are STEP [STEP] and TSP [TSP], which both also work in
the presence of NATs. Neither is analyzed further in this document.
5.1.2 Large end sites
Large end sites usually have a managed network.
Dual-stack access service is often a realistic possibility since the
customer network is managed (although CPE upgrades may be necessary).
Configured tunnels, as-is, are a good solution when a NAT is not in
the way and the IPv4 end-point addresses are static. In this
scenario, NAT traversal is not typically required. If fine-grained
access control is needed, an authentication protocol needs to be
implemented.
Tunnel brokering solutions, to help facilitate the set-up of a bi-
directional tunnel, have been proposed. Such mechanisms are typically
unnecessary for large end-sites, as simple configured tunneling or
native access can be used instead. However, if such mechanisms would
already be deployed, large sites starting to deploy IPv6 might be
able to benefit from them in any case.
Teredo is not applicable in this scenario as it can only provide IPv6
connectivity to a single host, not the whole site. 6to4 is not
recommended due to its reliance on the relays and provider-
independent address space, which make it impossible to guarantee the
required service quality and manageability large sites typically
want.
5.2 User Authentication/Access Control Requirements
User authentication can be used to control who can use the IPv6
connectivity service in the first place or who can access specific
IPv6 services (e.g., NNTP servers meant for customers only, etc.).
The former is described at more length below. The latter can be
achieved by ensuring that for all the service-specific IPv4 access
lists, there are also equivalent IPv6 access lists.
IPv6-specific user authentication is not always required. An example
is a customer of the IPv4 service automatically having access to the
IPv6 service. In this case the IPv4 access control also provides
access to the IPv6 services.
Lind, et al. Expires - December 2004 [Page 15]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
When a provider does not wish to give its IPv4 customers
automatic access to IPv6 services, specific IPv6 access control must
be performed in parallel with the IPv4 access control. This does not
imply that different user authentication must be performed for IPv6,
but merely that the authentication process may lead to different
results for IPv4 and IPv6 access.
Access control traffic may use IPv4 or IPv6 transport. For instance,
RADIUS [RADIUS] traffic related to IPv6 service can be transported
over IPv4.
5.3 Configuration of Customer Equipment
The customer connection networks are composed of PE and CPE(s).
Usually, each PE connects multiple CPE components to the backbone
network infrastructure. This number may reach tens of thousands or
more. The configuration of CPE is a difficult task for the ISP, and
it is even more difficult when it must be done remotely. In this
context, the use of auto-configuration mechanisms is beneficial, even
if manual configuration is still an option.
The parameters that usually need to be provided to customers
automatically are:
- The network prefix delegated by the ISP,
- The address of the Domain Name System server (DNS),
- Possibly other parameters, e.g., the address of an NTP
server.
When user identification is required on the ISP's network, DHCPv6 may
be used to provide configurations; otherwise, either DHCPv6 or a
stateless mechanism can be used. This is discussed in more detail in
[DUAL ACCESS].
Note that when the customer connection network is shared between the
users or the ISPs, and not just a point-to-point link, authenticating
the configuration of the parameters (especially prefix delegation)
requires further study.
As long as IPv4 service is available alongside IPv6 it is not
required to auto configure IPv6 parameters in the CPE, except the
prefix, because the IPv4 settings may be used.
5.4 Requirements for Traceability
Most ISPs have some kind of mechanism to trace the origin of traffic
in their networks. This also has to be available for IPv6 traffic,
meaning that a specific IPv6 address or prefix has to be tied to a
certain customer, or that records of which customer had which
Lind, et al. Expires - December 2004 [Page 16]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
address or prefix must be maintained. This also applies to the
customers with tunneled connectivity.
This can be done, for example, by mapping a DHCP response to a
physical connection and storing the result in a database. It can also
be done by assigning a static address or prefix to the customer. A
tunnel server could also provide this mapping.
5.5 Ingress Filtering in the Customer Connection Network
Ingress filtering must be deployed towards the customers, everywhere,
to ensure traceability, to prevent DoS attacks using spoofed
addresses, to prevent illegitimate access to the management
infrastructure, etc.
Ingress filtering can be done, for example, by using access lists or
Unicast Reverse Path Forwarding (uRPF). Mechanisms for these are
described in [RFC3704].
5.6 Multihoming
Customers may desire multihoming or multi-connecting for a number of
reasons [RFC3582].
Mechanisms for multihoming to more than one ISP are still under
discussion. One working model could be to deploy at least one prefix
per ISP, and to choose the prefix from the ISP to which traffic is
sent. In addition, tunnels may be used for robustness [RFC3178].
Currently, there are no provider-independent addresses for end-sites.
Such addresses would enable IPv4-style multihoming, with associated
disadvantages.
Multi-connecting more than once to just one ISP is a simple
practice, and this can be done, e.g., by using BGP with public or
private AS numbers and a prefix assigned to the customer.
5.7 Quality of Service
In most networks, quality of service in one form or another is
important.
Naturally, the introduction of IPv6 should not impair existing
Service Level Agreements (SLAs) or similar quality assurances.
During the deployment of the IPv6 service, the service could be best-
effort or similar even if the IPv4 service has a SLA. In the end both
IP version should be treated equally.
Lind, et al. Expires - December 2004 [Page 17]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
IntServ and DiffServ are equally applicable to IPv6 as to IPv4 and
work in a similar fashion independent of IP version. Of these,
typically only DiffServ has been implemented.
Many bandwidth provisioning systems operate with IPv4 assumptions,
e.g., taking an IPv4 address or (set of) prefixes for which traffic
is reserved or preferred. These systems require special attention
when introducing IPv6 support in the networks.
6. Network and Service Operation Actions
The network and service operation actions fall into different
categories as listed below:
- Set up IPv6 connectivity to upstream providers and peers.
- IPv6 network device configuration: for initial configuration
and updates
- IPv6 network management
- IPv6 monitoring
- IPv6 customer management
- IPv6 network and service operation security
Some of these items will require an IPv6 native transport layer to
be available, whereas others will not.
As a first step, network device configuration and regular network
management operations can be performed over an IPv4 transport,
because IPv6 MIBs are also available over IPv4 transport.
Nevertheless, some monitoring functions require the availability of
IPv6 transport. This is the case, for instance, when ICMPv6 messages
are used by the monitoring applications.
The current inability on many platforms to retrieve separate IPv4 and
IPv6 traffic statistics from dual-stack interfaces for management
purposes by using SNMP is an issue.
As a second step, IPv6 transport can be provided for any of these
network and service operation facilities.
7. Future Stages
At some point, an ISP might want to transition to a service that is
IPv6 only, at least in certain parts of its network. Such a
transition creates many new cases into which continued maintenance of
the IPv4 service must be factored. Providing an IPv6-only service is
not much different from the dual IPv4/IPv6 service described in stage
3 except for the need to phase out the IPv4 service. The delivery of
IPv4 services over an IPv6 network and the phase out of IPv4 are left
for a subsequent document. Note that there are some services which
Lind, et al. Expires - December 2004 [Page 18]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
will need to maintain IPv4 connectivity (e.g., authorative and some
recursive DNS servers [DNSGUIDE]).
8. Requirements for Follow-On Work
This section tries to summarize the potential items requiring
specification in the IETF.
Work items for which concrete specifications for which an approach
was not yet apparent as of this writing are:
- A tunnel server/broker mechanisms for the cases where the customer
connection networks cannot be upgraded needs to be specified
[TUNREQS].
- An IPv6 site multihoming mechanism (or multiple ones) need to be
developed.
Work items which were already fast in progress, as of this writing,
are:
- 6PE for MPLS was identified as a required mechanism, and this is
already in progress [BGPTUNNEL]
- IS-IS for Multiple Topologies was noted as a helpful mechanism in
certain environments; however, it is possible to use alternative
methods to achieve the same end, so specifying this is not strictly
required.
- Any-source Multicast can be accomplished using Embedded-RP
[EMBEDRP], already in progress.
9. Example Networks
In this section, a number of different example networks are
presented. These will not necessarily match any existing networks but
are intended to be useful even in cases in which they do not
correspond to specific target networks. The purpose of these examples
is to exemplify the applicability of the transition mechanisms
described in this document to a number of different situations with
different prerequisites.
The sample network layout will be the same in all network examples.
These should be viewed as specific representations of a generic
network with a limited number of network devices. A small number of
routers have been used in the examples. However, because the network
examples follow the implementation strategies recommended for the
generic network scenario, it should be possible to scale the examples
to fit a network with an arbitrary number, e.g. several hundreds or
thousands, of routers.
Lind, et al. Expires - December 2004 [Page 19]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
The routers in the sample network layout are interconnected with each
other as well as with another ISP. The connection to another ISP can
be either direct or through an exchange point. In addition to these
connections, a number of customer connection networks are also
connected to the routers. Customer connection networks can be, for
example, xDSL or cable network equipment.
ISP1 | ISP2
+------+ | +------+
| | | | |
|Router|--|--|Router|
| | | | |
+------+ | +------+
/ \ +-----------------------
/ \
/ \
+------+ +------+
| | | |
|Router|----|Router|
| | | |
+------+ +------+\
| | \ | Exchange point
+------+ +------+ \ +------+ | +------+
| | | | \_| | | | |--
|Router|----|Router|----\|Router|--|--|Switch|--
| | | | | | | | |--
+------+ /+------+ +------+ | +------+
| / | |
+-------+/ +-------+ |
| | | |
|Access1| |Access2|
| | | |
+-------+ +-------+
||||| ||||| ISP Network
----|-----------|----------------------
| | Customer Networks
+--------+ +--------+
| | | |
|Customer| |Customer|
| | | |
+--------+ +--------+
Figure 3: ISP Sample Network Layout.
9.1 Example 1
Example 1 presents a network built according to the sample network
layout with a native IPv4 backbone. The backbone is running IS-IS and
IBGP as routing protocols for internal and external routes,
respectively. Multiprotocol BGP is used to exchange routes over the
Lind, et al. Expires - December 2004 [Page 20]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
connections to ISP2 and the exchange point. Multicast using PIM-SM
routing is present. QoS using DiffServ is deployed.
Access 1 is xDSL connected to the backbone through an access router.
The xDSL equipment, except for the access router, is considered to be
layer 2 only, e.g., Ethernet or ATM. IPv4 addresses are dynamically
assigned to the customer using DHCP. No routing information is
exchanged with the customer. Access control and traceability are
performed in the access router. Customers are separated into VLANs or
separate ATM PVCs up to the access router.
Access 2 is "fiber to the building or home" (FTTB/H) connected
directly to the backbone router. This connection is considered to be
layer-3-aware, because it is using layer 3 switches and it performs
access control and traceability through its layer 3 awareness by
using DHCP snooping. IPv4 addresses are dynamically assigned to the
customers using DHCP. No routing information is exchanged with the
customer.
The actual IPv6 deployment might start by enabling IPv6 on a couple
of backbone routers, configuring tunnels between them (if not
adjacent), and connecting to a few peers or upstream providers
(either through tunnels or at an internet exchange).
After a trial period, the rest of the backbone is upgraded to dual-
stack, and IS-IS without multi-topology extensions (the upgrade order
is considered with care) is used as an IPv6 and IPv4 IGP. When
upgrading, it's important to note that until IPv6 customers are
connected behind a backbone router, the convexity requirement is not
critical: the routers will just not be reachable using IPv6. That
is, software supporting IPv6 could be installed even though the
routers would not be used for (customer) IPv6 traffic yet. That way,
IPv6 could be enabled in the backbone on a need-to-enable basis when
needed.
Separate IPv6 BGP sessions are built, similar to IPv4. Multicast
(through SSM and Embedded-RP) and DiffServ are offered at a later
phase of the network, e.g., after a year of stable IPv6 unicast
operations.
Offering native service as quickly as possible is considered most
important. However, a 6to4 relay may be provided in the meantime for
optimized 6to4 connectivity and it may also be combined with a tunnel
broker for extended functionality. xDSL equipment, operating as
bridges at Layer 2 only, does not require changes in CPE: IPv6
connectivity can be offered to the customers by upgrading the PE
router to IPv6. In the initial phase, only Router Advertisements are
used; DHCPv6 Prefix Delegation can be added as the next step if no
other mechanisms are available.
Lind, et al. Expires - December 2004 [Page 21]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
The FTTB/H access has to be upgraded to support access control and
traceability in the switches, probably by using DHCP snooping or a
similar IPv6 capability, but it also has to be compatible with prefix
delegation and not just address assignment. This could, however, lead
to the need to use DHCPv6 for address assignment.
9.2 Example 2
In example 2 the backbone is running IPv4 with MPLS and is using OSPF
and IBGP are for internal and external routes respectively. The
connections to ISP2 and the exchange point run BGP to exchange
routes. Multicast and QoS are not deployed.
Access 1 is a fixed line, e.g., fiber, connected directly to the
backbone. Routing information is in some cases exchanged with CPE at
the customer's site; otherwise static routing is used. Access 1 can
also be connected to a BGP/MPLS-VPN running in the backbone.
Access 2 is xDSL connected directly to the backbone router. The xDSL
is layer 2 only, and access control and traceability are achieved
through PPPoE/PPPoA. PPP also provides address assignment. No routing
information is exchanged with the customer.
IPv6 deployment might start with an upgrade of a couple of PE routers
to support [BGPTUNNEL], because this will allow large-scale IPv6
support without hardware or software upgrades in the core. In a later
phase native IPv6 traffic or IPv6 LSPs would be used in the whole
network. In that case IS-IS or OSPF could be used for the internal
routing, and a separate IPv6 BGP session would be run.
For the fixed-line customers, the CPE has to be upgraded and prefix
delegation using DHCPv6 or static assignment would be used. An IPv6
MBGP session would be used when routing information has to be
exchanged. In the xDSL case the same conditions for IP-tunneling as
in Example 1 apply. In addition to IP-tunneling an additional PPP
session can be used to offer IPv6 access to a limited number of
customers. Later, when clients and servers have been updated, the
IPv6 PPP session can be replaced with a combined PPP session for both
IPv4 and IPv6. PPP has to be used for address and prefix assignment.
9.3 Example 3
A transit provider offers IP connectivity to other providers, but
not to end users or enterprises. IS-IS and IBGP are used internally
and BGP externally. Its accesses connect Tier-2 provider cores. No
multicast or QoS is used.
Lind, et al. Expires - December 2004 [Page 22]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
As this type of transit provider has a number of customers, which in
turn have a large number of customers, it obtains an address
allocation from a RIR. The whole backbone can be upgraded to dual-
stack in a reasonably rapid pace after a trial with a couple of
routers. IPv6 routing is performed using the same IS-IS process and
separate IPv6 BGP sessions.
The ISP provides IPv6 transit to its customers for free, as a
competitive advantage. It also provides, at the first phase only, a
configured tunnel service with BGP peering to the significant sites
and customers (those with an AS number) which are the customers of
its customers whenever its own customer networks are not offering
IPv6. This is done both to introduce them to IPv6 and to create a
beneficial side-effect: a bit of extra revenue is generated from its
direct customers as the total amount of transited traffic grows.
10. Security Considerations
This document analyses scenarios and identifies some transition
mechanisms that could be used for the scenarios. It does not
introduce any new security issues. Security considerations of each
mechanism are described in the respective documents.
However, a few generic observations are in order.
o introducing IPv6 adds new classes of security threats or
requires adopting new protocols or operational models
than with IPv4; typically these are generic issues, and
to be further discussed in other documents, for example,
[V6SEC].
o the more complex the transition mechanisms employed become,
the more difficult it will be to manage or analyze their
impact on security. Consequently, simple mechanisms are to
be preferred.
o this document has identified a number of requirements for
analysis or further work which should be explicitly considered
when adopting IPv6: how to perform access control over
shared media or shared ISP customer connection media,
how to manage the configuration management security on such
environments (e.g., DHCPv6 authentication keying), and
how to manage customer traceability if stateless address
autoconfiguration is used.
11. Acknowledgements
This draft has greatly benefited from inputs by Marc Blanchet, Jordi
Palet, Francois Le Faucheur, Ronald van der Pol and Cleve Mickles.
Lind, et al. Expires - December 2004 [Page 23]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
Special thanks to Richard Graveman and Michael Lambert for
proofreading the document.
12. Informative References
[EMBEDRP] Savola, P., Haberman, B., "Embedding the Address of
RP in IPv6 Multicast Address" -
Work in progress
[MTISIS] Przygienda, T., Naiming Shen, Nischal Sheth, "M-
ISIS: Multi Topology (MT) Routing in IS-IS"
Work in progress
[RFC2858] Bates, T., Rekhter, Y., Chandra, R., Katz, D.,
"Multiprotocol Extensions for BGP-4"
RFC 2858
[RFC2545] Marques, P., Dupont, F., "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing"
RFC 2545
[BCP3704] Baker, F., Savola, P., "Ingress Filtering for
Multihomed Networks"
RFC 3704
[RFC3582] Abley, J., Black, B., Gill, V., "Goals for IPv6 Site-
Multihoming Architectures"
RFC 3582
[RFC3178] Hagino, J., Snyder, H., "IPv6 Multihoming Support at
Site Exit Routers"
RFC 3178
[RFC3056] Carpenter, B., Moore, K., "Connection of IPv6 Domains
via IPv4 Clouds"
RFC 3056
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865
[BGPTUNNEL] De Clercq, J., Gastaud, G., Ooms, D., Prevost, S.,
Le Faucheur, F., "Connecting IPv6 Islands across IPv4
Clouds with BGP"
Work in progress
[DUAL ACCESS] Shirasaki, Y., Miyakawa, S., Yamasaki, T.,
Takenouchi, A.
"A Model of IPv6/IPv4 Dual Stack Internet Access
Lind, et al. Expires - December 2004 [Page 24]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
Service"
Work in progress
[STEP] Savola, P. "Simple IPv6-in-IPv4 Tunnel Establishment
Procedure (STEP)"
Work in progress
[TSP] Blanchet, M. "IPv6 Tunnel broker with Tunnel Setup
Protocol (TSP)"
Work in progress
[TUNREQS] Durand, A., Parent, F. "Requirements for assisted
tunneling"
Work in progress
[UNMANEVA] Huitema, C., Austein, R., Satapati, S.,
van der Pol, R.
"Evaluation of Transition Mechanisms for Unmanaged
Networks"
Work in progress
[PROTO41] Palet, J., Olvera, C., Fernandez, D. "Forwarding
Protocol 41 in NAT Boxes"
Work in progress
[V6SEC] Savola, P. "IPv6 Transition/Co-existence Security
Considerations"
Work in progress
[DNSGUIDE] Durand, A., Ihren, J. "DNS IPv6 transport operational
guidelines"
Work in progress
[TEREDO] Huitema, C. "Teredo: Tunneling IPv6 over UDP through
NATs"
Work in progress
Authors' Addresses
Mikael Lind
TeliaSonera
Vitsandsgatan 9B
SE-12386 Farsta, Sweden
Email: mikael.lind@teliasonera.com
Vladimir Ksinant
Thales Communications
160, boulevard de Valmy
Lind, et al. Expires - December 2004 [Page 25]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
92704 Colombes, France
Email: vladimir.ksinant@fr.thalesgroup.com
Soohong Daniel Park
Mobile Platform Laboratory, SAMSUNG Electronics.
416, Maetan-3dong, Paldal-Gu,
Suwon, Gyeonggi-do, Korea
Email: soohong.park@samsung.com
Alain Baudot
France Telecom R&D
42, rue des coutures
14066 Caen - FRANCE
Email: alain.baudot@rd.francetelecom.com
Pekka Savola
CSC/FUNET
Espoo, Finland
EMail: psavola@funet.fi
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Full Copyright Statement
Lind, et al. Expires - December 2004 [Page 26]
INTERNET DRAFT ISP Networks IPv6 Scenarios June 2004
Copyright (C) The Internet Society (2004). 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."
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
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 AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Appendix A: Convexity Requirements in Single Topology IS-IS
The single-topology IS-IS convexity requirements could be summarized,
from IPv4/6 perspective, as follows:
1) "any IP-independent path from an IPv4 router to any other IPv4
router must only go through routers which are IPv4-capable", and
2) "any IP-independent path from an IPv6 router to any other IPv6
router must only go through routers which are IPv6-capable".
As IS-IS is based upon CLNS, these are not trivially accomplished.
The single-topology IS-IS builds paths which are agnostic of IP
versions.
Consider an example scenario of three IPv4/IPv6-capable routers
and an IPv4-only router:
cost 5 R4 cost 5
,------- [v4/v6] -----.
/ \
[v4/v6] ------ [ v4 ] -----[v4/v6]
R1 cost 3 R3 cost 3 R2
Here the second requirement would not hold. IPv6 packets from R1 to
R2 (or vice versa) would go through R3, which does not support IPv6,
and the packets would get discarded. By reversing the costs between
R1-R3, R3-R2 and R1-R4,R4-R2 the traffic would work in the normal
case, but if a link fails and the routing changes to go through R3,
the packets would start being discarded again.
Lind, et al. Expires - December 2004 [Page 27]