V6OPS Working Group                                          P. Matthews
Internet-Draft                                            Alcatel-Lucent
Intended status: Informational                              V. Kuarsingh
Expires: September 7, 2014                                           Dyn
                                                           March 6, 2014

                    Design Choices for IPv6 Networks


   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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 7, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Design Choices  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Mix IPv4 and IPv6 on the Same Link? . . . . . . . . . . .   3
     2.2.  Links with Only Link-Local Addresses? . . . . . . . . . .   4
     2.3.  Link-Local Next-Hop in a Static Route?  . . . . . . . . .   5
     2.4.  Separate or Combined eBGP Sessions? . . . . . . . . . . .   6
     2.5.  eBGP Endpoints: Global or Link-Local Addresses? . . . . .   7
     2.6.  IGP Choice  . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  General Observations  . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Use of Link-Local Addresses . . . . . . . . . . . . . . .   9
     3.2.  Separation of IPv4 and IPv6 . . . . . . . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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, [RFC6782] for
   wireline service providers, [RFC6342] for mobile network providers,
   [RFC5963] for exchange point operators, [RFC6883] for content
   providers, and both [RFC4852] and

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   [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].

   This document focuses on unicast network design only.  It does not
   cover multicast, nor supporting infrastructure such as DNS.

   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.

2.1.  Mix IPv4 and IPv6 on the Same Link?

   Should IPv4 and IPv6 traffic be logically separated on a link?  That

   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

   o  Provides better support for the expected future of increasing IPv6
      traffic and decreasing IPv4 traffic;

   o  And is generally conceptually simpler.

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   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:

   a.  Use only link-local addresses ("unnumbered"), OR

   b.  Have global or unique-local addresses assigned in addition to

   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.

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      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

   Today, most operators use numbered links (option b).

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.

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   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
   sessions compared with (b).

   However, there are a number of concerns with option (a):

   o  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]
      or [I-D.ietf-idr-bgp-multisession] is widely deployed.

   o  Whatever selection you make for the underlying transport protocol
      (v4 or v6) will likely look funny at some date.  Using two
      sessions is appropriate both now and in the future.

   o  Carrying (for example) IPv6 routes over IPv4 means that route
      information is transported over a different transport plane than
      the data packets themselves.  If v6 connectivity goes down locally
      without v4 also going down, then v6 routes will still be
      exchanged, thus leading to a blackhole.

   o  In some implementations, carrying v4 routes in a BGP session over
      v6 transport (or vica-versa) results in the BGP next-hops in the
      wrong address family, which must be fixed using routing policy
      before the routes can be used.

   Given these disadvantages, option (b) is the better choice in most
   situations, and this is the choice selected in most networks today.

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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

   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-
   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

   o  Operators and their tools are used to referring to eBGP sessions
      by address only, something that is not possible with link-local

   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

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   o  If hardware or other issues cause one to move the cable to a
      different local interface, then reconfiguration is required at
      both ends: at the local end because the interface has changed (and
      with link-local addresses, the interface must always be specified
      along with the address), and at the remote end because the link-
      local address has likely changed.  (Contrast this with using
      global addresses, where less re-configuration is required at the
      local end, and no reconfiguration is required at the remote end).

   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

   For these reasons, most operators today choose to have their eBGP
   sessions use global addresses.

2.6.  IGP Choice

   One of the main decisions for an IPv6 implementor is the choice of
   IGP (Interior Gateway Protocol) within the network.  The primary
   choices are the IETF protocols of RIP [RFC2080], OSPF [RFC2328]
   [RFC5340] and IS-IS [RFC5120] [RFC5308], though some operators may
   consider non-IETF protocols.  Here we limit our discussion to the
   pros and cons of OSPF vs. IS-IS.

   Considering just OSPF vs. IS-IS, the options are:

   a.  Use OSPFv2 for IPv4 and OSPFv3 for IPv6, OR

   b.  Use OSPFv3 for both IPv4 and IPv6, OR

   c.  Use OSPFv2 for IPv4, and IS-IS for IPv6, OR

   d.  Use IS-IS for IPv4 and IPv6, OR

   e.  Use IS-IS for IPv4 and OSPFv3 for IPv6.

   Note that options (a), (c), and (e) involve running two different
   routing protocols, while options (b) and (d) involve running just one
   routing protocol.

   o  A big factor in the choice is the protocol the operator is
      currently using for routing IPv4.  Option (e) is unlikely to be a
      good choice for an operator currently using OSPF for IPv4 routing,
      and similarly option (a) is unlikely to be a good choice for an
      operator currently using IS-IS.

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   o  A pro for options (a), (c), and (e), which use two routing
      protocols, is that they give a hard separation between IPv4 and
      IPv6 routing.  Thus a problem with one protocol or one set of
      routes is unlikely to affect the other.

   o  There are two cons for options (a), (c), and (e).  One con is that
      two sets of all the protocol mechanisms need to be maintained.  On
      a larger modern router, this is unlikely to be a problem, but on
      some edge devices this might be an issue.  The second con is that
      some operational staff must be familiar with both protocols.  For
      many routing problems, the protocols are sufficiently similar that
      they can be considered identical, but some problems require a
      detailed knowledge of the differences.

   o  Option (b) requires the use of new protocol extensions that allow
      OSPFv3 to also route IPv4.  At the time of writing, these
      extensions are still quite new.

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

   There are three main reasons for this current practice:

   o  Network operators are concerned about the volatility of link-local
      addresses based on MAC addresses, despite the fact that this
      concern can be overcome by manually-configuring link-local

   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

   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.

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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.

5.  Security Considerations


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, KK
   Chittimaneni, Tim Chown, Lorenzo Colitti, Gert Doering, Bill Fenner,
   Kedar K Gaonkar, Chris Grundemann, Steinar Haug, Ray Hunter, Joel
   Jaeggli, Victor Kuarsingh, Ivan Pepelnjak, Alexandru Petrescu, Rob
   Shakir, Mark Smith, Jean-Francois Tremblay, Tina Tsou, Dan York, and

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   The authors would also like to thank Pradeep Jain and Alastair
   Johnson for helpful comments on a very preliminary version of this

7.  Informative References

              Scudder, J., Appanna, C., and I. Varlashkin, "Multisession
              BGP", draft-ietf-idr-bgp-multisession-07 (work in
              progress), September 2012.

              Ramachandra, S. and E. Chen, "Dynamic Capability for
              BGP-4", draft-ietf-idr-dynamic-cap-14 (work in progress),
              December 2011.

              Behringer, M. and E. Vyncke, "Using Only Link-Local
              Addressing Inside an IPv6 Network", draft-ietf-opsec-lla-
              only-07 (work in progress), February 2014.

              Chittimaneni, K., Chown, T., Howard, L., Kuarsingh, V.,
              Pouffary, Y., and E. Vyncke, "Enterprise IPv6 Deployment
              Guidelines", draft-ietf-v6ops-enterprise-incremental-
              ipv6-05 (work in progress), January 2014.

   [RFC2080]  Malkin, G. and R. Minnear, "RIPng for IPv6", RFC 2080,
              January 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [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.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

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   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October

   [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,
              May 2011.

   [RFC6342]  Koodli, R., "Mobile Networks Considerations for IPv6
              Deployment", RFC 6342, August 2011.

   [RFC6782]  Kuarsingh, V. and L. Howard, "Wireline Incremental IPv6",
              RFC 6782, November 2012.

   [RFC6883]  Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
              Content Providers and Application Service Providers", RFC
              6883, March 2013.

Authors' Addresses

   Philip Matthews
   600 March Road
   Ottawa, Ontario  K2K 2E6

   Phone: +1 613-784-3139
   Email: philip_matthews@magma.ca

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   Victor Kuarsingh
   150 Dow Street
   Manchester, NH  03101

   Email: victor@jvknet.com

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