Network Working Group J. Bi Internet-Draft CERNET Intended status: Standards Track G. Yao Expires: September 15, 2011 Tsinghua University J. Halpern Newbridge Networks Inc E. Levy-Abegnoli, Ed. Cisco Systems March 14, 2011 SAVI for Mixed Address Assignment Methods Scenario Abstract This document reviews how multiple address discovery methods can coexist in a single savi device and collisions are resolved when the same binding entry is discovered by two or more methods. 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 September 15, 2011. Copyright Notice Copyright (c) 2011 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 Bi, et al. Expires September 15, 2011 [Page 1] Internet-Draft SAVI mix March 2011 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Mixed Address Assignment Methods Scenario . . . . . . . . . . . 3 3. Problem Scope, Statement and Solution . . . . . . . . . . . . . 4 3.1. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Recommendations for preventing collisions . . . . . . . . . 4 3.3. Binding on the Same Address . . . . . . . . . . . . . . . . 4 3.3.1. Same Address on Different Binding Anchors . . . . . . . 5 3.3.1.1. Basic preference . . . . . . . . . . . . . . . . . 5 3.3.1.2. Multiple SAVI Device Scenario . . . . . . . . . . . 7 3.3.1.3. Conflict Announcement . . . . . . . . . . . . . . . 7 3.3.2. Same Address on the Same Binding Anchor . . . . . . . . 7 4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Normative References . . . . . . . . . . . . . . . . . . . 8 4.2. Informative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Contributors and Acknowledgments . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Bi, et al. Expires September 15, 2011 [Page 2] Internet-Draft SAVI mix March 2011 1. Introduction There are currently several documents [I-D.ietf-savi-fcfs], [I-D.ietf-savi-dhcp], [I-D.ietf-savi-send] that describe the different methods by which a switch can discover and record bindings between a node's layer3 address and a binding anchor and use that binding to perform Source Address Validation. The method used by nodes to assign the address drove the break down into these multiple documents, whether StateLess Autoconfiguration (SLAAC), Dynamic Host Control Protocol (DHCP), Secure Neighbor Discovery (SeND) or manual. Each of these documents describes separately how one particular discovery method deals with address collisions. While multiple assignment methods can be used in the same layer2 domain, a savi-switch might have to deal with a mix of binding discovery methods. The purpose of this document is to provide recommendations to avoid collisions and to review collisions handling when two or more such methods come up with competing bindings. 2. Mixed Address Assignment Methods Scenario There are four address assignment methods identified and reviewed in one of the SAVI document: 1. StateLess Address AutoConfiguration (SLAAC) - reviewed in [I-D.ietf-savi-fcfs] 2. Dynamic Host Control Protocol address assignment (DHCP) - reviewed in [I-D.ietf-savi-dhcp] 3. Secure Neighbor Discovery (SeND) address assignment, reviewed in [I-D.ietf-savi-send] 4. Manually address configuration - reviewed in [I-D.ietf-savi-fcfs] and [I-D.ietf-savi-framework] Each address assignment method corresponds to a binding discovery method: SAVI-FCFS, SAVI-DHCP and SAVI-SeND. Any combination of address assignment methods can be potentially mixed within a layer2 domain, and a savi device will have to implement the corresponding savi discovery method (referred to as a "savi solution") to enable Source Address Validation. If more than one SAVI solution is enabled on a SAVI device, the method is referred to as "mix address assignment method" in this document. Bi, et al. Expires September 15, 2011 [Page 3] Internet-Draft SAVI mix March 2011 3. Problem Scope, Statement and Solution 3.1. Problem Scope Different savi solutions are independent from each other, each one handling its own entries. In the absence of a reconciliation, each solution will reject packets sourced with an address it did not discovered. To prevent addresses discovered by one solution to be filtered out by another, the binding table should be shared by all the solutions. However this could create some conflict when the same entry is discovered by two different methods: the purpose of this document is of two folds: provide recommendations to avoid conflicts, and resolve conflicts if and when they happen. Collisions happening within a given solution is outside the scope of this document. 3.2. Recommendations for preventing collisions If each solution has a dedicated address space, collisions won't happen. Thus, it is recommended to avoid overlap in the address space across SAVI solutions enabled on any particular savi switch. More specifically: 1. DHCP/Static: exclude the static address from the DHCP pool. 2. DHCP/SLAAC: separate the prefix scope of DHCP and SLAAC. Set the A bit in Prefix information option of Router Advertisement for SLAAC prefix. And set the M bit in Router Advertisement for DHCP prefix. [RFC4861] [RFC4862]. 3. SLAAC/Static: separate the prefix scope of SLAAC and Static. It may be impossible in practice. SAVI device can perform DAD proxy for static address to hold the address from SLAAC node. 4. SEND/non-SEND: In an environment where SeND is deployed, the only way to avoid collisions in the SAVI devices is to have SeND-only nodes. In a mixed environment, two nodes, SeND and non-SeND, could configure the same address and the SAVI-device will have to deal with a collision. 3.3. Binding on the Same Address In situations where collisions could not be avoided, two cases should be considered: 1. The same address is bound on two different binding anchors by different SAVI solutions. 2. The same address is bound on the same binding anchor by different SAVI solutions. Bi, et al. Expires September 15, 2011 [Page 4] Internet-Draft SAVI mix March 2011 3.3.1. Same Address on Different Binding Anchors This is the very case of collision that could not be prevented by separating the assignment address spaces. For instance, an address is assigned by SLAAC on node X, installed in the binding table using SAVI-FCFS, anchored to "anchor-X". Later, the same address is assigned by DHCP to node Y, as a potential candidate in the same binding table, anchored to "anchor-Y". 3.3.1.1. Basic preference Within the SAVI perimeter, one address bound to a binding anchor by one SAVI solution could also be bound by another SAVI solution to a different binding anchor. If the DAD procedure is not performed, the same address will also be bound to the new binding anchor. Both bindings are legitimate within the corresponding solution. Though it is possible that the hosts and network can still work in such scenario, the uniqueness of address is not assured. The SAVI device must decide whom the address should be bound with. A binding preference level based solution is proposed here. To determine a proper preference level, following evidences are used: 1. "Duplicate Address Detection MUST be performed on all unicast addresses prior to assigning them to an interface, regardless of whether they are obtained through stateless autoconfiguration, DHCPv6, or manual configuration,..." [RFC4862] 2. "A tentative address that is determined to be a duplicate as described above MUST NOT be assigned to an interface,..." [RFC4862] 3. "The client SHOULD perform duplicate address detection on each of the addresses in any IAs it receives in the Reply message before using that address for traffic." [RFC3315] 4. A SEND node that uses the CGA authorization method to protect Neighbor Solicitations SHOULD perform Duplicate Address Detection as follows. If Duplicate Address Detection indicates that the tentative address is already in use, the node generates a new tentative CGA. If after three consecutive attempts no non-unique address is generated, it logs a system error and gives up attempting to generate an address for that interface. When performing Duplicate Address Detection for the first tentative address, the node accepts both secured and unsecured Neighbor Advertisements and Solicitations received in response to the Neighbor Solicitations. When performing Duplicate Address Detection for the second or third tentative address, it ignores unsecured Neighbor Advertisements and Solicitations." [RFC3971] Bi, et al. Expires September 15, 2011 [Page 5] Internet-Draft SAVI mix March 2011 5. "The node MAY have a configuration option whereby it ignores unsecured advertisements, even when performing Duplicate Address Detection for the first tentative address. This configuration option SHOULD be disabled by default. This is a recovery mechanism for cases in which attacks against the first address become common." [RFC3971] From the above materials, "First-Come First-Serve" should be the default behavior for choosing between two competing bindings. There can however be some exceptions, one of them being CGA addresses, another one controlled by the configuration of the switch: 1. When CGA addresses are used, and a collision is detected, preference should be given to the anchor that carries the CGA credentials once they are verified, in particular the CGA parameters and the RSA options. 2. The switch configuration should allow an address range (including a single address) to be configured together with a given anchor or constrained to be discovered by a particular savi-solution. If a DAD message for a target within that range is received on the savi-switch from an anchor, or via a discovery method different from the one configured, the switch should defend the address by responding to the DAD message. This is especially useful to protect well known bindings such as a static address of a server over anybody, even when the server is down. It is also a way to give priority to a binding learnt from SAVI-DHCP over a binding for the same address, learnt from SAVI-FCFS. Note that no binding shall be created in the binding table until an "acceptable" address owner shows up, either from the configured anchor or using the savi solution associated with that address. The following preference level can be inferred from listed materials and above analysis: 1. By default, SLAAC, DHCP and manually configured address by user have the same priority. 2. SEND can have higher priority because it may configure an address bound by non-SEND node. 3. Static binding configured on the switch (admin) will have the highest priority 4. Address range configured on the switch (admin) constrained to DHCP discovery will de-facto be given a higher priority over FCFS, by defending the address until it is is effectively learnt from DHCP Combined solution preference with binding sequence, there will be 16 Bi, et al. Expires September 15, 2011 [Page 6] Internet-Draft SAVI mix March 2011 scenarios (Denote solutions by FCFS, DHCP, SEND, and Admin correspondingly): Existing Candidate Default PREFERENCE FCFS FCFS In the scope of SAVI-SLAAC FCFS DHCP FCFS FCFS SEND SEND FCFS Admin Admin DHCP FCFS DHCP DHCP DHCP In the scope of SAVI-DHCP DHCP SEND SEND DHCP Admin Admin SEND FCFS SEND SEND DHCP SEND SEND SEND In the scope of SAVI-SEND SEND Admin Admin Admin FCFS Admin Admin DHCP Admin Admin SEND Admin Admin Admin Candidate binding 3.3.1.2. Multiple SAVI Device Scenario A single SAVI device doesn't have the information of all bound addresses on the perimeter. Therefore it is not enough to lookup local bindings to identify a collision. However, assuming DAD is performed throughout the security perimeter for all addresses regardless of the assignment method, then DAD response will inform all SAVI switches about any collision. In that case, FCFS will apply the same way as in a single switch scenario. If the admin configured on one the switches a range of addresses (or a single static binding) to defend, the DAD response generated by this switch will also prevent the binding to be installed on other switches of the perimeter. 3.3.1.3. Conflict Announcement If a host is prohibited from using a bound address, the violation MUST be announced to it, through delivering one (or more) Neighbor Advertisement message to the host. 3.3.2. Same Address on the Same Binding Anchor A binding may be set up on the same binding anchor by multiple solutions. Generally, the binding lifetimes of different solutions are different. Potentially, if one solution requires to remove the binding, the node using the address may be taken the use right. Bi, et al. Expires September 15, 2011 [Page 7] Internet-Draft SAVI mix March 2011 For example, a node performs DAD procedure after being assigned an address from DHCP, then the address will also be bound by SAVI-FCFS. If the SAVI-FCFS lifetime is shorter than DHCP lifetime, when the SAVI-FCFS lifetime expires, it will request to remove the binding. If the binding is removed, the node will not be able to use the address even the DHCP lease time doesn't expire. The solution proposed is to keep a binding as long as possible. A binding is kept until it has been required to be removed by all the solutions that ever set up it. 4. References 4.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 4.2. Informative References [I-D.ietf-savi-dhcp] Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for DHCP", draft-ietf-savi-dhcp-07 (work in progress), November 2010. [I-D.ietf-savi-fcfs] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS- SAVI: First-Come First-Serve Source-Address Validation for Locally Assigned Addresses", draft-ietf-savi-fcfs-05 (work in progress), October 2010. [I-D.ietf-savi-framework] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, "Source Address Validation Improvement Framework", draft-ietf-savi-framework-03 (work in progress), March 2011. [I-D.ietf-savi-send] Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source- Address Validation Implementation", draft-ietf-savi-send-04 (work in progress), October 2010. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for Bi, et al. Expires September 15, 2011 [Page 8] Internet-Draft SAVI mix March 2011 IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. Appendix A. Contributors and Acknowledgments Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun and Jari Arkko for their valuable contributions. Authors' Addresses Jun Bi CERNET Network Research Center, Tsinghua University Beijing 100084 China Email: junbi@cernet.edu.cn Guang Yao Tsinghua University Network Research Center, Tsinghua University Beijing 100084 China Email: yaoguang.china@gmail.com Joel M. Halpern Newbridge Networks Inc Email: jmh@joelhalpern.com Bi, et al. Expires September 15, 2011 [Page 9] Internet-Draft SAVI mix March 2011 Eric Levy-Abegnoli (editor) Cisco Systems Village d'Entreprises Green Side - 400, Avenue Roumanille Biot-Sophia Antipolis - 06410 France Email: elevyabe@cisco.com Bi, et al. Expires September 15, 2011 [Page 10]