SAVI J. Bi Internet-Draft G. Yao Intended status: Standards Track Tsinghua Univ. Expires: November 14, 2015 J. Halpern Newbridge E. Levy-Abegnoli, Ed. Cisco May 13, 2015 SAVI for Mixed Address Assignment Methods Scenario draft-ietf-savi-mix-08 Abstract In case that multiple IP address assignment methods are allowed in a network, the corresponding SAVI methods should be enabled to prevent spoofing in the network. This document reviews how multiple SAVI 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 November 14, 2015. 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 Bi, et al. Expires November 14, 2015 [Page 1] Internet-Draft SAVI MIX May 2015 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 3 5. Recommendations for preventing collisions . . . . . . . . . . 5 6. Resolving binding collisions . . . . . . . . . . . . . . . . 5 6.1. Same Address on Different Binding Anchors . . . . . . . . 5 6.1.1. Basic preference . . . . . . . . . . . . . . . . . . 6 6.1.2. Overwritten preference . . . . . . . . . . . . . . . 6 6.1.3. Multiple SAVI Device Scenario . . . . . . . . . . . . 7 6.2. Same Address on the Same Binding Anchor . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction There are currently several SAVI documents ([RFC6620], [savi-dhcp] and [RFC7219]) that describe the different methods by which a switch can discover and record bindings between a node's IP address and a binding anchor and use that binding to perform source address validation. Each of these documents specifies how to learn on-link addresses, based on the method used for their assignment, respectively: StateLess Autoconfiguration (SLAAC), Dynamic Host Control Protocol (DHCP) and Secure Neighbor Discovery (SeND). Each of these documents describes separately how one particular method deals with address collisions (same address, different binding anchor). While multiple IP assignment methods can be used in the same layer-2 domain, a SAVI device might have to deal with a mix of SAVI 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. Bi, et al. Expires November 14, 2015 [Page 2] Internet-Draft SAVI MIX May 2015 2. 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 RFC 2119 [RFC2119]. 3. Problem Scope There are three IP address assignment methods identified and reviewed in one of the SAVI document: 1. StateLess Address AutoConfiguration (SLAAC) - reviewed in SAVI- FCFS[RFC6620] 2. Dynamic Host Control Protocol address assignment (DHCP) - reviewed in SAVI-DHCP[savi-dhcp] 3. Secure Neighbor Discovery (SeND) address assignment, reviewed in SAVI-SEND[RFC7219] In addition, there is a fourth method for installing a bindings on the switch, referred to as "manual". It is based on manual (address or prefix) binding configuration and is reviewed in [RFC6620] and [RFC7039]. All combinations of address assignment methods can coexist within a layer-2 domain. A SAVI device will have to implement the corresponding binding setup methods (referred to as a "SAVI method") to enable Source Address Validation. If more than one SAVI method is enabled on a SAVI device, the method is referred to as "mix address assignment method" in this document. SAVI methods are independent from each other, each one handling its own entries. In the absence of reconciliation, each method will reject packets sourced with an address it did not discovered. To prevent addresses discovered by one method 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 and method to avoid conflicts, and resolve conflicts if and when they happen. Collisions happening within a given method are outside the scope of this document. 4. Architecture A SAVI device may enable multiple SAVI methods. This mechanism, called SAVI-MIX, is proposed as a arbiter of the binding generation algorithms, generating the final working binding entries Figure 1. Bi, et al. Expires November 14, 2015 [Page 3] Internet-Draft SAVI MIX May 2015 Once a SAVI method generates a candidate binding, it will request SAVI-MIX to set up a corresponding entry in the binding table. Then SAVI-MIX will check if there is any conflict in the binding table. A new binding will be generated if there is no conflict. If there is a conflict, SAVI-MIX will determine whether to replace the existing binding or reject the candidate binding based on the policies specified in Section 6. The packet filtering will not be performed by each SAVI method separately. Instead, SAVI-MIX will perform filtering based on the entries in the binding table. +--------------------------------------------------------+ | | | SAVI Device | | | | | | +------+ +------+ +------+ | | | SAVI | | SAVI | | SAVI | | | | | | | | | | | | FCFS | | DHCP | | SEND | | | +------+ +------+ +------+ | | | | | Binding | | | | | setup | | v v v requests | | +------------------------------+ | | | | | | | SAVI-MIX | | | | | | | +------------------------------+ | | | | | v Final Binding | | +--------------+ | | | Binding | | | | | | | | Table | | | +--------------+ | | | +--------------------------------------------------------+ Figure 1: SAVI-Mix Architecture Each entry in the binding table will contain the following fields: 1. IP source address Bi, et al. Expires November 14, 2015 [Page 4] Internet-Draft SAVI MIX May 2015 2. Binding anchor 3. Lifetime 4. Creation time 5. Binding methods: the methods which request the binding setup. 5. Recommendations for preventing collisions If each solution has a dedicated address space, collisions won't happen. Using non overlapping address space across SAVI solutions is therefore recommended. To that end, one should: 1. DHCP/SLAAC: use non-overlapping prefix for 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. For detail explanations on these bits, refer to [RFC4861][RFC4862]. 2. SeND/non-SeND: avoid mixed environment (where SeND and non-SeND nodes are deployed) or separate the prefixes announced to SeND and non-SenD nodes. One way to separate the prefixes is to have the router(s) announcing different (non-overlapping) prefixes to SeND and to non-SeND nodes, using unicast Router Advertisements, in response to SeND/non-SeND Router Solicit. 6. Resolving binding collisions 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 methods. 2. The same address is bound on the same binding anchor by different SAVI methods. 6.1. Same Address on Different Binding Anchors This would typically occur in case assignment address spaces could not be separated. 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, and SAVI-DHCP will generate a candidate binding entry, anchored to "anchor-Y". Bi, et al. Expires November 14, 2015 [Page 5] Internet-Draft SAVI MIX May 2015 6.1.1. Basic preference The SAVI device must decide whom the address should be bound with (anchor-X or anchor-Y in this example). Current standard documents of address assignment methods have implied the prioritization relationship (first-come). In the absence of any configuration or protocol hint (see Section 6.1.2) the SAVI device should choose the first-come binding anchor, whether it was learnt from SLACC, SeND or DHCP. 6.1.2. Overwritten preference There are two identified exceptions to the general prioritization model, one of them being CGA addresses, another one controlled by the configuration of the switch. 6.1.2.1. CGA preference 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. Note that if an attacker was trying to replay CGA credentials, he would then compete on the base of fcfs (first-come, first-serve). 6.1.2.2. configuration preference For configuration driven exceptions, the SAVI device may allow the configuration of a triplet ("prefix", "anchor", "method") or ("address", "anchor", "method"), where at least one of ("anchor", "method") should be specified. Later, if a DAD message is received with the following conditions verified: 1. The target in the DAD message does not exist in the binding table 2. The target is within configured "prefix" (or equal to "address") 3. The anchor bound to target is different from the configured anchor, when specified 4. The configured method, if any, is different from SAVI-FCFS the switch should defend the address by responding to the DAD message. It should not at this point install the entry into the binding table. It will simply prevent the node to assign the address, and will de-facto prioritize the configured anchor. 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 Bi, et al. Expires November 14, 2015 [Page 6] Internet-Draft SAVI MIX May 2015 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. 6.1.3. 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 devices 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 prefix (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. 6.2. Same Address on the Same Binding Anchor A binding may be set up on the same binding anchor by multiple methods, typically SAVI-FCFS and SAVI-DHCP. If the binding lifetimes obtained from the two methods are different, priority should be given to 1) Manual configuration 2) SAVI-DHCP 3) SAVI-FCFS as the least authoritative. The binding will be removed when the prioritized lifetime expires, even if a less authoritative method had a longer lifetime. 7. Security Considerations SAVI MIX does not eliminate the security problems of each SAVI method. Thus, the potential attacks, e.g., the DoS attack against the SAVI device resource, can still happen. In deployment, the security threats from each enabled SAVI methods should be prevented by the corresponding proposed solutions in each document. SAVI MIX is only a binding setup/removal arbitration mechanism. It does not introduce additional security threats only if the principle of decision is reasonable. However, there is a slight problem. SAVI MIX is more tolerant about binding establish than each SAVI method alone. As long as one of the enabled SAVI method generates a binding, the binding will be applied. As a result, the allowed binding number limitation or allowed binding setup rate limitation will be the sum of all the enabled SAVI methods. In deployment, whether a SAVI device is capable to support that resource requirement should be evaluated. Bi, et al. Expires November 14, 2015 [Page 7] Internet-Draft SAVI MIX May 2015 8. IANA Considerations This memo asks the IANA for no new parameters. 9. Acknowledgment Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun and Jari Arkko for their valuable contributions. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, "Source Address Validation Improvement (SAVI) Framework", RFC 7039, October 2013. [RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS SAVI: First-Come, First-Served Source Address Validation Improvement for Locally Assigned IPv6 Addresses", RFC 6620, May 2012. [RFC7219] Bagnulo, M. and A. Garcia-Martinez, "SEcure Neighbor Discovery (SEND) Source Address Validation Improvement (SAVI)", RFC 7219, May 2014. [savi-dhcp] Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for DHCP", draft-ietf-savi-dhcp-34 (work in progress), Feb 2015. 10.2. Informative References [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. Authors' Addresses Bi, et al. Expires November 14, 2015 [Page 8] Internet-Draft SAVI MIX May 2015 Jun Bi Tsinghua University Network Research Center, Tsinghua University Beijing 100084 China EMail: junbi@tsinghua.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 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 November 14, 2015 [Page 9]