V6OPS Working Group P. Matthews
Internet-Draft Alcatel-Lucent
Intended status: Informational October 22, 2012
Expires: April 25, 2013
Design Guidelines for IPv6 Networks
draft-matthews-v6ops-design-guidelines-01
Abstract
This document presents advice on the design choices that arise when
designing IPv6 networks (both dual-stack and IPv6-only). The
intended audience is someone designing an IPv6 network who is
knowledgeable about best current practices around IPv4 network
design, and wishes to learn the corresponding practices for IPv6.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Design Choices . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Mix IPv4 and IPv6 on the Same Link? . . . . . . . . . . . 4
2.2. Links with Only Link-Local Addresses? . . . . . . . . . . 4
2.3. Link-Local Next-Hop in a Static Route? . . . . . . . . . . 6
2.4. Separate or Combined eBGP Sessions? . . . . . . . . . . . 6
2.5. eBGP Endpoints: Global or Link-Local Addresses? . . . . . 7
3. General Observations . . . . . . . . . . . . . . . . . . . . . 8
3.1. Use of Link-Local Addresses . . . . . . . . . . . . . . . 8
3.2. Separation of IPv4 and IPv6 . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. History . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This document presents advice on the design choices that arise when
designing IPv6 networks (both dual-stack and IPv6-only). The
intended audience is someone designing an IPv6 network who is
knowledgeable about best current practices around IPv4 network
design, and wishes to learn the corresponding practices for IPv6.
The focus of the document is on design choices where there are
differences between IPv4 and IPv6, either in the range of possible
alternatives (e.g. the extra possibilities introduced by link-local
addresses in IPv6) or the recommended alternative. The document
presents the alternatives and discusses the pros and cons in detail.
Where consensus currently exists around the best practice, this is
documented; otherwise the document simply summarizes the current
state of the discussion. Thus this document serves to both to
document the reasoning behind best current practices for IPv6, and to
allow a designer to make an intelligent choice where no such
consensus exists.
This document does not present advice on strategies for adding IPv6
to a network, nor does it discuss transition mechanisms. For advice
in these areas, see [RFC6180] for general advice,
[I-D.ietf-v6ops-wireline-incremental-ipv6] for wireline service
providers, [RFC6342] for mobile network providers, [RFC5963] for
exchange point operators, [I-D.ietf-v6ops-icp-guidance] for content
providers, and both [RFC4852] and
[I-D.ietf-v6ops-enterprise-incremental-ipv6] for enterprises. Nor
does the document cover the ins and outs of creating an IPv6
addressing plan; for advice in this area, see [RFC5375].
The current version of this document focuses on unicast network
design only. It does not cover multicast,, nor supporting
infrastructure such as DNS. This may change in future versions.
The current version is still work in progress, and it is expected
that the presentation and discussion of additional design choices
will be added as the document matures.
2. Design Choices
This section consists of a list of specific design choices a network
designer faces when designing an IPv6-only or dual-stack network,
along with guidance and advice to the designer when making a choice.
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2.1. Mix IPv4 and IPv6 on the Same Link?
Should IPv4 and IPv6 traffic be logically separated on a link? That
is:
a. Mix IPv4 and IPv6 traffic on the same layer 2 connection, OR
b. Separate IPv4 and IPv6 by using separate physical or logical
links (e.g., two physical links or two VLANs on the same link)?
Option (a) implies a single layer 3 interface at each end with both
IPv4 and IPv6 addresses; while option (b) implies two layer 3
interfaces, one for IPv4 addresses and one with IPv6 addresses.
The advantages of option (a) include:
o Requires only half as many layer 3 interfaces as option (b), thus
providing better scaling;
o May require fewer physical ports, thus saving money;
o Can make the QoS implementation much easier (for example, rate-
limiting the combined IPv4 and IPv6 traffic to or from a
customer);
o Provides better support for the expected future of increasing IPv6
traffic and decreasing IPv4 traffic;
o And is generally conceptually simpler.
For these reasons, there is a pretty strong consensus in the operator
community that option (a) is the preferred way to go.
However, there can be times when option (b) is the pragmatic choice.
Most commonly, option (b) is used to work around limitations in
network equipment. One big example is the generally poor level of
support today for individual statistics on IPv4 traffic vs IPv6
traffic when option (a) is used. Other, device-specific, limitations
exist as well. It is expected that these limitations will go away as
support for IPv6 matures, making option (b) less and less attractive
until the day that IPv4 is finally turned off.
Most networks today use option (a) wherever possible.
2.2. Links with Only Link-Local Addresses?
Should the link:
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a. Use only link-local addresses ("unnumbered"), OR
b. Have global or unique-local addresses assigned in addition to
link-locals?
There are two advantages of unnumbered links. The first advantage is
ease of configuration. In a network with a large number of
unnumbered links, the operator can just enable an IGP on each router,
without going through the tedious process of assigning and tracking
the addresses for each link. The second advantage is security.
Since link-local addresses are unroutable, the associated interfaces
cannot be attacked from an off-link device. This implies less effort
around maintaining security ACLs.
Countering this advantage are various disadvantages to unnumbered
links in IPv6:
o It is not possible to ping an interface that has only a link-local
address from a device that is not directly attached to the link.
Thus, to troubleshoot, one must typically log into a device that
is directly attached to the device in question, and execute the
ping from there.
o A traceroute passing over the unnumbered link will return the
loopback or system address of the router, rather than the address
of the interface itself.
o On some devices, by default the link-layer address of the
interface is derived from the MAC address assigned to interface.
When this is done, swapping out the interface hardware (e.g.
interface card) will cause the link-layer address to change. In
some cases (peering config, ACLs, etc) this may require additional
changes. However, many devices allow the link-layer address of an
interface to be explicitly configured, which avoids this issue.
o The practice of naming router interfaces using DNS names is
difficult-to-impossible when using LLAs only.
o It is not possible to identify the interface or link (in a
database, email, etc) by just giving its address.
For more discussion on the pros and cons, see
[I-D.ietf-opsec-lla-only].
Today, most operators use numbered links (option b).
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2.3. Link-Local Next-Hop in a Static Route?
What form of next-hop address should one use in a static route?
a. Use the far-end's link-local address as the next-hop address, OR
b. Use the far-end's GUA/ULA address as the next-hop address?
Recall that the IPv6 specs for OSPF [RFC5340] and ISIS [RFC5308]
dictate that they always use link-locals for next-hop addresses. For
static routes, [RFC4861] section 8 says:
A router MUST be able to determine the link-local address for each
of its neighboring routers in order to ensure that the target
address in a Redirect message identifies the neighbor router by
its link-local address. For static routing, this requirement
implies that the next-hop router's address should be specified
using the link-local address of the router.
This implies that using a GUA or ULA as the next hop will prevent a
router from sending Redirect messages for packets that "hit" this
static route. All this argues for using a link-local as the next-hop
address in a static route.
However, there are two cases where using a link-local address as the
next-hop clearly does not work. One is when the static route is an
indirect (or multi-hop) static route. The second is when the static
route is redistributed into another routing protocol. In these
cases, the above text from RFC 4861 notwithstanding, either a GUA or
ULA must be used.
Furthermore, many network operators are concerned about the
dependency of the default link-local address on an underlying MAC
address, as described in the previous section.
Today most operators use GUAs as next-hop addresses.
2.4. Separate or Combined eBGP Sessions?
For a dual-stack peering connection where eBGP is used as the routing
protocol, then one can either:
a. Use one BGP session to carry both IPv4 and IPv6 routes, OR
b. Use two BGP sessions, a session over IPv4 carrying IPv4 routes
and a session over IPv6 carrying IPv6 routes.
The main advantage of (a) is a reduction in the number of BGP
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sessions compared with (b).
However, there are three main concerns with option (a). First, on
most existing implementations, adding or removing an address family
to an established BGP session will cause the router to tear down and
re-establish the session. Thus adding the IPv6 family to an existing
session carrying just IPv4 routes will disrupt the session, and the
eventual removal of IPv4 from the dual IPv4/IPv6 session will also
disrupt the session. This disruption problem will persist until
something similar to [I-D.ietf-idr-dynamic-cap] is widely deployed.
Second, there is the question of which protocol to use to carry the
dual IPv4/IPv6 session: over IPv4 or over IPv6? Carrying it over
IPv4 makes sense initially from a stability and troubleshooting
perspective, but will eventually seem out-of-date. Third, carrying
(for example) IPv6 routes over IPv4 means that route information is
transported over a different transport plane than the data packets
themselves. If the IPv6 data plane was to fail, then IPv6 routes
would still be exchanged, but any IPv6 traffic resulting from these
routes would be dropped.
Given these disadvantages, option (b) is the better choice in most
situations, and this is the choice selected in most networks today.
2.5. eBGP Endpoints: Global or Link-Local Addresses?
When running eBGP over IPv6, there are two options for the addresses
to use at each end of the eBGP session (or more properly, the
underlying TCP session):
a. Use link-local addresses for the eBGP session, OR
b. Use global addresses for the eBGP session.
Note that the choice here is the addresses to use for the eBGP
sessions, and not whether the link itself has global (or unique-
local) addresses. In particular, it is quite possible for the eBGP
session to use link-local addresses even when the link has global
addresses.
The big attraction for option (a) is security: an eBGP session using
link-local addresses is impossible to attack from a device that is
off-link. This provides very strong protection against TCP RST and
similar attacks. Though there are other ways to get an equivalent
level of security (e.g. GTSM [RFC5082], MD5 [RFC5925], or ACLs),
these other ways require additional configuration which can be
forgotten or potentially mis-configured.
However, there are a number of small disadvantages to using link-
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local addresses:
o Using link-local addresses only works for single-hop eBGP
sessions; it does not work for multi-hop sessions.
o One must use "next-hop self" at both endpoints, otherwise
redistributing routes learned via eBGP into iBGP will not work.
(Some products enable "next-hop self" in this situation
automatically).
o Operators and their tools are used to referring to eBGP sessions
by address only, something that is not possible with link-local
addresses.
o If one is configuring parallel eBGP sessions for IPv4 and IPv6
routes, then using link-local addresses for the IPv6 session
introduces an extra difference between the two sessions which
could otherwise be avoided.
o On some products, an eBGP session using a link-local address is
more complex to configure than a session that use a global
address.
o Finally, a strict interpretation of RFC 2545 can be seen as
forbidding running eBGP between link-local addresses, as RFC 2545
requires the BGP next-hop field to contain at least a global
address.
For these reasons, most operators today choose to have their eBGP
sessions use global addresses.
3. General Observations
There are two themes that run though many of the design choices in
this document. This section presents some general discussion on
these two themes.
3.1. Use of Link-Local Addresses
The proper use of link-local addresses is a common theme in the IPv6
network design choices. Link-layer addresses are, of course, always
present in an IPv6 network, but current network design practice
mostly ignores them, despite efforts such as
[I-D.ietf-opsec-lla-only].
There are three main reasons for this current practice:
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o Network operators are concerned about the volitility of link-local
addresses based on MAC addresses, despite the fact that this
concern can be overcome by manually-configuring link-local
addresses;
o It is impossible to ping a link-local address from a device that
is not on the same subnet. This is a troubleshooting
disadvantage, though it can also be viewed as a security
advantage.
o Most operators are currently running networks that carry both IPv4
and IPv6 traffic, and wish to harmonize their IPv4 and IPv6 design
and operational practices where possible.
3.2. Separation of IPv4 and IPv6
Currently, most operators are running or planning to run networks
that carry both IPv4 and IPv6 traffic. Hence the question: To what
degree should IPv4 and IPv6 be kept separate? As can be seen above,
this breaks into two sub-questions: To what degree should IPv4 and
IPv6 traffic be kept separate, and to what degree should IPv4 and
IPv6 routing information be kept separate?
The general consensus around the first question is that IPv4 and IPv6
traffic should generally be mixed together. This recommendation is
driven by the operational simplicity of mixing the traffic, plus the
general observation that the service being offered to the end user is
Internet connectivity and most users do not know or care about the
differences between IPv4 and IPv6. Thus it is very desirable to mix
IPv4 and IPv6 on the same link to the end user. On other links,
separation is possible but more operationally complex, though it does
occasionally allow the operator to work around limitations on network
devices. The situation here is roughly comparable to IP and MPLS
traffic: many networks mix the two traffic types on the same links
without issues.
By contrast, there is more of an argument for carrying IPv6 routing
information over IPv6 transport, while leaving IPv4 routing
information on IPv4 transport. By doing this, one gets fate-sharing
between the control and data plane for each IP protocol version: if
the data plane fails for some reason, then often the control plane
will too.
4. IANA Considerations
This document makes no requests of IANA.
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5. Security Considerations
(TBD)
6. Acknowledgements
Many, many people in the V6OPS working group provided comments and
suggestions that made their way into this document. A partial list
includes: Rajiv Asati, Fred Baker, Michael Behringer, Marc Blanchet,
Ron Bonica, Randy Bush, Cameron Byrne, Brian Carpenter, Tim Chown,
Lorenzo Colitti, Gert Doering, Bill Fenner, Kedar K Gaonkar, Chris
Grundemann, Steinar Haug, Ray Hunter, Joel Jaeggli, KK, Victor
Kuarsingh, Alexandru Petrescu, Mark Smith, Jean-Francois Tremblay,
Tina Tsou, Dan York, and Xuxiaohu. There are probably others which
are not listed here, likely because they made a helpful comment at
the mic during a WG session and I didn't catch the name.
I would also like to thank Pradeep Jain and Alastair Johnson for
helpful comments on a very preliminary version of this document.
7. History
Version -01
Many, many changes from version -00, too many to document
individually. Most of these changes are due to the many helpful
comments and suggestions received by email or at the mic during
the lengthy discussion at IETF 84 in Vancouver.
Version -00
Initial, very preliminary, version.
8. Informative References
[I-D.ietf-idr-dynamic-cap]
Ramachandra, S. and E. Chen, "Dynamic Capability for
BGP-4", draft-ietf-idr-dynamic-cap-14 (work in progress),
December 2011.
[I-D.ietf-opsec-lla-only]
Behringer, M. and E. Vyncke, "Using Only Link-Local
Addressing Inside an IPv6 Network",
draft-ietf-opsec-lla-only-01 (work in progress),
September 2012.
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[I-D.ietf-v6ops-enterprise-incremental-ipv6]
Chittimaneni, K., Chown, T., Howard, L., Kuarsingh, V.,
Pouffary, Y., and E. Vyncke, "Enterprise IPv6 Deployment
Guidelines",
draft-ietf-v6ops-enterprise-incremental-ipv6-01 (work in
progress), September 2012.
[I-D.ietf-v6ops-icp-guidance]
Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
Content and Application Service Providers",
draft-ietf-v6ops-icp-guidance-04 (work in progress),
September 2012.
[I-D.ietf-v6ops-wireline-incremental-ipv6]
Kuarsingh, V. and L. Howard, "Wireline Incremental IPv6",
draft-ietf-v6ops-wireline-incremental-ipv6-06 (work in
progress), September 2012.
[RFC4852] Bound, J., Pouffary, Y., Klynsma, S., Chown, T., and D.
Green, "IPv6 Enterprise Network Analysis - IP Layer 3
Focus", RFC 4852, April 2007.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
October 2008.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[RFC5375] Van de Velde, G., Popoviciu, C., Chown, T., Bonness, O.,
and C. Hahn, "IPv6 Unicast Address Assignment
Considerations", RFC 5375, December 2008.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC5963] Gagliano, R., "IPv6 Deployment in Internet Exchange Points
(IXPs)", RFC 5963, August 2010.
[RFC6180] Arkko, J. and F. Baker, "Guidelines for Using IPv6
Transition Mechanisms during IPv6 Deployment", RFC 6180,
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May 2011.
[RFC6342] Koodli, R., "Mobile Networks Considerations for IPv6
Deployment", RFC 6342, August 2011.
Author's Address
Philip Matthews
Alcatel-Lucent
600 March Road
Ottawa, Ontario K2K 2E6
Canada
Phone: +1 613-784-3139
Email: philip_matthews@magma.ca
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