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]