Network Working Group F. Baker Internet-Draft Cisco Systems Intended status: Standards Track B. Carpenter Expires: February 6, 2016 Univ. of Auckland August 5, 2015 Host routing in a multi-prefix network draft-baker-6man-multi-homed-host-00 Abstract This note describes expected host behavior in a network that has more than one prefix, each allocated by an upstream network that implements BCP 38 filtering. 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 February 6, 2016. Copyright Notice Copyright (c) 2015 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. Baker & Carpenter Expires February 6, 2016 [Page 1] Internet-Draft Host routing in a multi-prefix network August 2015 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 2. Expectations the host has of the network . . . . . . . . . . 2 3. Reasonable expectations of the host . . . . . . . . . . . . . 3 4. Expectations of multihomed networks . . . . . . . . . . . . . 4 5. Residual issues . . . . . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 9.1. Normative References . . . . . . . . . . . . . . . . . . 4 9.2. Informative References . . . . . . . . . . . . . . . . . 5 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction This note describes the expected behavior of an IPv6 [RFC2460] host in a network that has more than one prefix, each allocated by an upstream network that implements BCP 38 [RFC2827] filtering. It expects that the network will implement some form of egress routing, so that packets sent to a host outside the local network from a given ISP's prefix will go to that ISP. If the packet is sent to the wrong egress, it is liable to be discarded by the BCP 38 filter. However, the mechanics of egress routing once the packet leaves the host are out of scope. The question here is how the host interacts with that network. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Expectations the host has of the network A host receives on-link prefixes in a Router Advertisement [RFC4861], which goes on to identify preference order among them and whether they are usable by SLAAC [RFC4862] [RFC4941] [RFC7217], or whether they must be assigned using DHCPv6 [RFC3315]. The simplest multihomed network implementation might be a LAN with one or more hosts on it and two or more routers, one for each upstream network. In such a network, there is no routing protocol, and the two routers may not even know that the other is a router as opposed to a host, apart from the fact that it is emitting Router Baker & Carpenter Expires February 6, 2016 [Page 2] Internet-Draft Host routing in a multi-prefix network August 2015 Advertisements (RAs). One might expect that the routers may or may not receive each other's RAs and form an address in the other router's prefix. However, all hosts in such a network might be expected to create an address in each prefix so advertised. Because there is no routing protocol among those routers, there is no mechanism by which packets can be deterministically forwarded between the routers (as described in BCP 84 [RFC3704]) in order to avoid BCP 38 filters. Even if there was, it would be an indirect route, rather than a direct route originating with the host. Therefore the host needs to select the appropriate router itself. Since the host derives fundamental default routing information from the RA, this implies that, on any network with multiple prefixes, each prefix SHOULD be advertised by one of the attached routers, even if addresses are being assigned using DHCPv6. 3. Reasonable expectations of the host Modern hosts maintain a fair bit of history, in terms of what has historically worked or not worked for a given address or prefix and in some cases the effective window and MSS values for TCP. This includes a next hop address for use when a packet is sent to the indicated address. A host SHOULD include the prefix it used in a successful exchange with a remote address or prefix in such history. On subsequent attempts to communicate with that remote address, if it has such an address at that time, a host MAY use its address in the remembered prefix for the exchange. A host SHOULD select a "default gateway" for each prefix it uses to obtain one of its own addresses. That router SHOULD be one of the routers advertising the prefix in its RA. As a result of doing so, when a host emits a datagram using a source address in one of those prefixes and has no history directing it otherwise, it SHOULD send it to the indicated "default gateway". In the "simplest" network described in Section 2, that would get it to the only router that is capable of getting it to the right ISP. This will also apply in more complex networks, even when more than one physical or virtual interface is involved. There is an interaction with Default Address Selection [RFC6724]. Rule 5.5 of that specification states that the source address used to send to a given destination address should if possible be chosen from a prefix known to be advertised by the next-hop router for that destination. This selection rule would be applicable in a host following the recommendation in the previous paragraph. Baker & Carpenter Expires February 6, 2016 [Page 3] Internet-Draft Host routing in a multi-prefix network August 2015 4. Expectations of multihomed networks The direct implication of Section 2 is that routing protocols used in multihomed networks SHOULD be capable of source-prefix based egress routing, and that multihomed networks SHOULD deploy them. 5. Residual issues The assumption that packets will be forwarded to the appropriate egress by the local routing system might cause at least one extra hop in the local network (from the host to the wrong router, and from there to another router on the same LAN but in a different subnet). In some scenarios, where the local network is a highly constrained or lossy wireless network, this extra hop may be a significant performance handicap. In a slightly more complex situation, which happens to be one of the authors' home plus corporate home-office configuration, the two upstream routers might be on different LANs and therefore different subnets (e.g., the host is itself multi-homed). In that case, there is no way for the "wrong" router to detect the existence of the "right" router, or to route to it. In such a case it is particularly important that hosts take the responsibility to memorize and select the best first-hop as described in Section 3. 6. IANA Considerations This memo asks the IANA for no new parameters. 7. Security Considerations This document does not create any new security or privacy exposures. 8. Acknowledgements 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . Baker & Carpenter Expires February 6, 2016 [Page 4] Internet-Draft Host routing in a multi-prefix network August 2015 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, . 9.2. Informative References [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May 2000, . [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, . [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March 2004, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/ RFC4862, September 2007, . [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, . [RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012, . [RFC7217] Gont, F., "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)", RFC 7217, DOI 10.17487/ RFC7217, April 2014, . Baker & Carpenter Expires February 6, 2016 [Page 5] Internet-Draft Host routing in a multi-prefix network August 2015 Appendix A. Change Log Initial Version: date Authors' Addresses Fred Baker Cisco Systems Santa Barbara, California 93117 USA Email: fred@cisco.com Brian Carpenter Department of Computer Science University of Auckland PB 92019 Auckland 1142 New Zealand Email: brian.e.carpenter@gmail.com Baker & Carpenter Expires February 6, 2016 [Page 6]