V6OPS Working Group P. Matthews
Internet-Draft Alcatel-Lucent
Intended status: Informational June 29, 2012
Expires: December 31, 2012
Design Guidelines for IPv6 Networks
draft-matthews-v6ops-design-guidelines-00
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 as "work in progress."
This Internet-Draft will expire on December 31, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Matthews Expires December 31, 2012 [Page 1]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. General Observations . . . . . . . . . . . . . . . . . . . . . 3
2.1. Link-Local Addresses . . . . . . . . . . . . . . . . . . . 3
2.2. Separation of IPv4 and IPv6 . . . . . . . . . . . . . . . 4
3. Point-to-Point Links . . . . . . . . . . . . . . . . . . . . . 4
3.1. Mix IPv4 and IPv6? . . . . . . . . . . . . . . . . . . . . 4
3.2. Addressing? . . . . . . . . . . . . . . . . . . . . . . . 5
4. Static Routing . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Next-Hop Address? . . . . . . . . . . . . . . . . . . . . 6
5. eBGP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. One or Two eBGP Sessions? . . . . . . . . . . . . . . . . 7
5.2. eBGP Endpoints: Global or Link-Local Addresses? . . . . . 7
6. iBGP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. LDP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10. RSVP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. Security Considerations . . . . . . . . . . . . . . . . . . . 9
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
14. Informative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Matthews Expires December 31, 2012 [Page 2]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
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, or [RFC4852] for enterprises.
The current preliminary version of this document focuses on unicast
network design only. It does not cover multicast, nor IPv6
addressing plan development, nor supporting infrastructure such as
DNS. Some of these deficiencies may be lifted in future versions.
2. General Observations
There are two themes that run though most of the design questions in
this document. These themes are so pervasive that it seems
worthwhile to have a bit of meta discussion on them before
considering individual cases in the rest of the document.
2.1. Link-Local Addresses
The proper use of link-local addresses is a common theme in the IPv6
network design questions below. Link-layer addresses are, of course,
always present in an IPv6 network, but current network design
practice seems to view them more as a necessary evil rather than as a
feature to exploited as much as possible. For the most part, their
Matthews Expires December 31, 2012 [Page 3]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
presence is ignored in most IPv6 network designs.
It remains unclear whether they are viewed this way because of
inherent deficiencies, or are currently viewed this way because today
most operators need to operate networks that run both IPv4 and IPv6
and wish to utilize common procedures.
2.2. 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 end user wants Internet or VPN connectivity and often knows
nothing about IPv4 vs. 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 doesn't give a lot of advantages, except
perhaps ease of measuring traffic volumes. The situation here is
roughly comparable to IP and MPLS traffic: many networks mix the two
traffic types on the same links without issues.
However, 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.
The rest of this document is organized as a list of specific network
design questions, the choices a network designer has in answering the
questions, and a discussion of the choices.
3. Point-to-Point Links
3.1. Mix IPv4 and IPv6?
Should IPv4 and IPv6 traffic be logically separated on the link, or
should the two traffic types be mixed? That is:
a. Mix IPv4 and IPv6 traffic on the same logical link between the
two routers, OR
Matthews Expires December 31, 2012 [Page 4]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
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.
An advantage of option (a) is in producing traffic measurements.
From time-to-time, operators to want to separately measure IPv4 and
IPv6 traffic on a link, and this can be difficult to do with option
(a). Some routers today will only provide aggregate measurements for
traffic (i.e. IPv4 and IPv6 combined) if option (a) is used.
An advantage of option (b) is there is only half as many layer 3
interfaces, which can help with scaling. If physical links are used,
then there is also a capex advantage.
Most networks today use option (a).
3.2. Addressing?
Should the link:
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
Matthews Expires December 31, 2012 [Page 5]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
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 It is not possible to identify the interface or link (in a
database, email, etc) by just giving its address
For more discussion on these points see [recent I-D].
4. Static Routing
4.1. Next-Hop Address?
What next-hop should one use in a (one-hop) 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?
RFC 4861 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 option (b) will prevent the router from sending
Redirect messages for packets that "hit" this static route. This is
typically only an issue when there are two or more routers plus one
or more hosts attached to a LAN, and the static route, which is
installed on one of the routers and points at a second router on the
LAN, could reroute packets coming from the hosts to the second
router. See RFC 4861 for the exact conditions under which a router
sends and a host accepts an ICMPv6 Redirect. In cases where
Redirects are not a concern, then either option (a) or (b) can be
used.
Matthews Expires December 31, 2012 [Page 6]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
5. eBGP
5.1. One or Two 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
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 draft-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.
5.2. eBGP Endpoints: Global or Link-Local Addresses?
If eBGP running over IPv6 is used for the peering connection, then
depending on the addresses used on the link, 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
Matthews Expires December 31, 2012 [Page 7]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
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 and MD5), 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-
local addresses:
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.
6. iBGP
(TBD)
Matthews Expires December 31, 2012 [Page 8]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
7. IS-IS
(TBD)
8. OSPF
(TBD)
9. LDP
(TBD)
10. RSVP
(TBD)
11. IANA Considerations
This document makes no requests of IANA.
12. Security Considerations
This document introduces no new security concerns. Some pre-existing
security concerns are discussed in the sections above.
13. Acknowledgements
Thanks to Alastair Johnson and Pradeep Jain for helpful comments on a
preliminary version of this document.
14. Informative References
[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-01 (work in progress),
June 2012.
[I-D.ietf-v6ops-wireline-incremental-ipv6]
Kuarsingh, V. and L. Howard, "Wireline Incremental IPv6",
draft-ietf-v6ops-wireline-incremental-ipv6-04 (work in
Matthews Expires December 31, 2012 [Page 9]
Internet-Draft Design Guidelines for IPv6 Networks June 2012
progress), May 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.
[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,
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
Matthews Expires December 31, 2012 [Page 10]