Network Working Group J. Wu Internet-Draft J. Bi Intended status: Informational Tsinghua Univ. Expires: August 15, 2011 M. Bagnulo UC3M F. Baker Cisco C. Vogt, Ed. Ericsson February 11, 2011 Source Address Validation Improvement Framework draft-ietf-savi-framework-02 Abstract The Source Address Validation Improvement method was developed to complement ingress filtering with finer-grained, standardized IP source address validation. This document describes and motivates the design of the SAVI method. 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/. 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Expires August 15, 2011 [Page 2] Internet-Draft SAVI Framework February 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Deployment Options . . . . . . . . . . . . . . . . . . . . . . 6 4. Scalability Optimizations . . . . . . . . . . . . . . . . . . 7 5. Reliability Optimizations . . . . . . . . . . . . . . . . . . 9 6. Mix Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 Wu, et al. Expires August 15, 2011 [Page 3] Internet-Draft SAVI Framework February 2011 1. Introduction Since IP source addresses are used by hosts and network entities to determine the origin of a packet and as a destination for return data, spoofing of IP source addresses can enable impersonation, concealment, and malicious traffic redirection. Unfortunately, the Internet architecture does not prevent IP source address spoofing. Since the IP source address of a packet generally takes no role in forwarding the packet, it can be selected arbitrarily by the sending host without jeopardizing packet delivery. Extra methods are necessary for IP source address validation, to augment packet forwarding with an explicit check of whether a given packet's IP source address is legitimate. IP source address validation can happen at different granularity: Ingress filtering [BCP38], a widely deployed standard for IP source address validation, functions at the coarse granularity of networks. It verifies that the prefix of an IP source address routes to the network from which the packet was received. An advantage of ingress filtering is simplicity: The decision of whether to accept or to reject an IP source address can be made solely based on the information available from routing protocols. However, the simplicity comes at the cost of not being able to validate IP source addresses at a finer granularity, due to the aggregated nature of the information available from routing protocols. Finer-grained IP source address validation would be helpful to enable IP-source- address-based authentication, authorization, and host localization, as well as to efficiently identify misbehaving hosts. Partial solutions [BA2007] exist for finer-grained IP source address validation, but are proprietary and hence often unsuitable for corporate procurement. The Source Address Validation Improvement method was developed to complement ingress filtering with standardized IP source address validation at the maximally fine granularity of individual IP addresses: It prevents hosts attached to the same link from spoofing each other's IP addresses. To facilitate deployment in networks of various kinds, the SAVI method was designed to be modular and extensible. This document describes and motivates the design of the SAVI method. 2. Model To enable network operators to deploy fine-grained IP source address validation without a dependency on supportive functionality on hosts, the SAVI method was designed to be purely network-based. A SAVI instance is located on the path of hosts' packets, enforcing the Wu, et al. Expires August 15, 2011 [Page 4] Internet-Draft SAVI Framework February 2011 hosts' use of legitimate IP source addresses according to the following three-step model: 1. Identify which IP source addresses are legitimate for a host, based on monitoring packets exchanged by the host. 2. Bind a legitimate IP address to a link layer property of the host's network attachment. This property, called a "binding anchor", must be verifiable in every packet that the host sends, and harder to spoof than the host's IP source address itself. 3. Enforce that the IP source addresses in packets match the binding anchors to which they were bound. This model allows a SAVI instance to be located anywhere on the link to which the hosts attach, hence enabling different locations for a SAVI instance. One way to locate a SAVI instance is in the hosts' default router. IP source addresses are then validated in packets traversing the default router, yet the IP source addresses in packets exchanged locally on the link may bypass validation. Another way to locate a SAVI instance is in a switch between the hosts and their default router. Thus, packets may undergo IP source address validation even if exchanged locally on the link. The closer a SAVI instance is located to the hosts, the more effective the SAVI method is. This is because each of the three steps of the SAVI model can best be accomplished in a position close to the host: o Identifying a host's legitimate IP source addresses is most efficient close to the host, because the likelihood that the host's packets bypass a SAVI instance, and hence cannot be monitored, increases with the distance between the SAVI instance and the host. o Selecting a binding anchor for a host's IP source address is easiest close to the host, because many link layer properties are unique for a given host only on a link segment directly attaching to the host. o Enforcing a host's use of a legitimate IP source address is most reliable when pursued close to the host, because the likelihood that the host's packets bypass a SAVI instance, and hence do not undergo IP source address validation, increases with the distance between the SAVI instance and the host. The preferred location of SAVI instances is therefore close to hosts, such as in switches that directly attach to the hosts whose IP source Wu, et al. Expires August 15, 2011 [Page 5] Internet-Draft SAVI Framework February 2011 addresses are being validated. 3. Deployment Options The model of the SAVI method, as explained in Section 2, is deployment-specific in two ways: o The identification of legitimate IP source addresses is dependent on the IP address assignment method in use on a link, since it is through assignment that a host becomes the legitimate user of an IP source address. o Binding anchors are dependent on the technology used to build the link on which they are used, as binding anchors are link layer properties of a host's network attachment. To facilitate the deployment of the SAVI method in networks of various kinds, the SAVI method is designed to support different IP address assignment methods, and to function with different binding anchors. Naturally, both the IP address assignment methods in use on a link and the available binding anchors have an impact on the functioning and the strength of IP source address validation. The following two sub-sections explain this impact, and describe how the SAVI method accommodates this. 3.1. IP Address Assignment Methods Since the SAVI method traces IP address assignment packets, it necessarily needs to incorporate logic that is specific to particular IP address assignment methods. However, developing SAVI method variants for each IP address assignment method is alone not sufficient, since multiple IP address assignment methods may co-exist on a given link. The SAVI method hence comes in multiple variants: for links with Stateless Address Autoconfiguration, for links with DHCP, for links with Secure Neighbor Discovery, and for links that use any combination of IP address assignment methods. The reason to develop SAVI method variants for each single IP address configuration method, in addition to the variant that handles all IP address assignment methods, is to minimize the complexity of the common case: Many link deployments today either are constrained to a single IP address assignment methods or, equivalently from the perspective of the SAVI method, separate IP address assignment methods into different IP address prefixes. The SAVI method for such links can be simpler than the SAVI method for links with multiple IP address assignment methods per IP address prefix. Wu, et al. Expires August 15, 2011 [Page 6] Internet-Draft SAVI Framework February 2011 3.2. Binding Anchors The SAVI method supports a range of binding anchors: o The IEEE extended unique identifier, EUI-48 or EUI-64, of a host's interface. o The port on an Ethernet switch to which a host attaches. o The security association between a host and the base station on wireless links. o The combination of a host interface's link-layer address and a customer relationship in cable modem networks. o An ATM virtual channel, a PPPoE session identifier, or an L2TP session identifier in a DSL network. o A tunnel that connects to a single host, such as an IP-in-IP tunnel, a GRE tunnel, or an MPLS label-switched path. The various binding anchors differ significantly in the security they provide. IEEE extended unique identifiers, for example, fail to render a secure binding anchor because they can be spoofed with little effort. And switch ports alone may be insufficient because they may connect to more than a single host, such as in the case of concatenated switches. Given this diversity in the security provided, one could define a set of possible binding anchors, and leave it up to the administrator to choose one or more of them. Such a selection of binding anchors would, of course, have to be accompanied by an explanation of the pros and cons of the different binding anchors. In addition, SAVI devices may have a default binding anchor depending on the lower layers. Such a default could be to use switch ports when available, and MAC addresses otherwise. Or to use MAC addresses, and switch ports in addition if available. 4. Scalability Optimizations The preference to locate a SAVI instance close to hosts implies that multiple SAVI instances must be able to co-exist in order to support large links. Although the model of the SAVI method is independent of the number of SAVI instances per link, co-existence of multiple SAVI instances without further measures can lead to higher-than-necessary memory requirements: Since a SAVI instance creates bindings for the IP source addresses of all hosts on a link, bindings are replicated Wu, et al. Expires August 15, 2011 [Page 7] Internet-Draft SAVI Framework February 2011 if multiple SAVI instances co-exist on the link. High memory requirements, in turn, increase the cost of a SAVI instance. This is problematic in particular for SAVI instances that are located on a switch, since it may significantly increase the cost of such a switch. To reduce memory requirements for SAVI instances that are located on a switch, the SAVI method enables the suppression of binding replication on links with multiple SAVI instances. This requires manual disabling of IP source address validation on switch ports that connect to other switches running a SAVI instance. Each SAVI instance is then responsible for validating IP source addresses only on those ports to which hosts attach either directly, or via switches without a SAVI instance. On ports towards other switches running a SAVI instance, IP source addresses are not validated. The switches running SAVI instances thus form a "protection perimeter". The IP source addresses in packets passing the protection perimeter are validated by the ingress SAVI instance, but no further validation takes place as long as the packets remain within, or leave the protection perimeter. .............. protection perimeter --> : +--------+ : +---+ +---+ : | SAVI | : | A | | B | <-- hosts : | switch | : +---+ +---+ : +--------+ : ...|......|.............................: | : : +--------+ +--------+ +--------+ : : | SAVI |----------| legacy | | SAVI | : : | switch | | switch |----------| switch | : : +--------+ +--------+ +--------+ : : | ...............................|........: : +--------+ : +--------+ : | SAVI | : | legacy | : | switch | : | switch | : +--------+ : +--------+ :............: | | +---+ +---+ hosts --> | C | | D | +---+ +---+ Figure 1: Protection perimeter concept Figure 1 illustrates the concept of the protection perimeter. The figure shows a link with six switches, of which four, denoted "SAVI Wu, et al. Expires August 15, 2011 [Page 8] Internet-Draft SAVI Framework February 2011 switch", run a SAVI instance. The protection perimeter created by the four SAVI instances is shown as a dotted line in the figure. IP source address validation is enabled on all switch ports on the protection perimeter, and it is disabled on all other switch ports. Four hosts, denoted A through D in the figure, attach to the protection perimeter. In the example of figure Figure 1, the protection perimeter encompasses one of the legacy switches, located in the middle of the depicted link topology. This enables a single, unpartitioned protection perimeter. A single protection perimeter minimizes memory requirements for the SAVI instances because every binding is kept only once, namely, by the SAVI instance that attaches to the host being validated. Excluding the legacy switch from the protection perimeter would result in two smaller protection perimeters to the left and to the right of the depicted link topology. The memory requirements for the SAVI instances would then be higher: Since IP source address validation would be activated on the two ports connecting to the legacy switch, the SAVI instances adjacent to the legacy switch would replicate all bindings from the respectively other protection perimeter. The reason why it is possible to include the legacy switch in the protection perimeter is because the depicted link topology guarantees that packets cannot enter the protection perimeter via this legacy switch. Without this guarantee, the legacy switch would have to be excluded from the protection perimeter in order to ensure that packets entering the protection perimeter undergo IP source address validation. 5. Reliability Optimizations The explicit storage of legitimate IP addresses in the form of bindings implies that failure to create a binding, or the premature removal of bindings, can lead to loss of legitimate packets. There are three situations in which this can happen: o Legitimate IP address configuration packets, which should trigger the creation of a binding in a SAVI instance, are lost before reaching the SAVI instance. o A SAVI instance loses a binding, for example, due to a restart. o The link topology changes, resulting in hosts to communicate through SAVI instances that do not have a binding for those hosts' IP addresses. To limit the disruption that missing bindings for legitimate IP addresses can have, the SAVI method includes a mechanism for reactive Wu, et al. Expires August 15, 2011 [Page 9] Internet-Draft SAVI Framework February 2011 binding creation based on regular packets. This mechanism supplements the proactive binding creation based on IP address configuration packets. Reactive binding creation occurs when a SAVI instances recognizes excessive drops of regular packets originating from the same IP address. The SAVI instance then verifies whether said IP address is unique on the link. How the verification is carried out depends on the IP address configuration method that the SAVI instance supports: The SAVI method variant for Stateless Address Autoconfiguration and for Secure Neighbor Discovery verifies an IP address through the Duplicate Address Detection procedure. The SAVI method variant for DHCP verifies an IP address through a DHCP Lease Query message exchange with the DHCP server. If verification indicates that the IP address is unique on the link, the SAVI instance creates a binding for the IP address. Otherwise, no binding is created, and packets sent from the IP address continue to be dropped. 6. Mix Scenario While multiple assignment methods can be used on the same link, the SAVI device may have to deal with a mix of binding discovery methods. if the address prefix used for each assignment method is different, mix scenario can handle the same as scenario with only one assignment method. If different address assignment methods are used to assign addresses from the same prefix, additional considerations are needed because one binding mechanism may create a binding violating an existing binding from another binding mechanism, e.g., binding from SAVI-FCFS may violate binding from SAVI-DHCP. Thus, the collision between different SAVI mechanisms in mix scenario must be handled in case more than one address assignment method is used to assign addresses from the same prefix. Prioritization relationship between different address assignment methods is used as the basis to solve possible collisions. Current standard documents of address assignment methods have implied the prioritization relationship in general cases. However, considering in some scenarios, default prioritization level may not be quite suitable. Configurable prioritization level should be supported in a document of SAVI solution for the mix scenario. 7. Acknowledgment The author would like to thank the SAVI working group for a thorough technical discussion on the design and the framework of the SAVI method, as captured in this document, in particular Erik Nordmark, Guang Yao, Eric Levy-Abegnoli, and Alberto Garcia. Thanks also to Wu, et al. Expires August 15, 2011 [Page 10] Internet-Draft SAVI Framework February 2011 Torben Melsen for reviewing this document. This document was generated using the xml2rfc tool. 8. References [BA2007] Baker, F., "Cisco IP Version 4 Source Guard", IETF Internet draft (work in progress), November 2007. [BCP38] Paul, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", RFC 2827, BCP 38, May 2000. Authors' Addresses Jianping Wu Computer Science, Tsinghua University Beijing 100084 China Email: jianping@cernet.edu.cn Jun Bi Network Research Center, Tsinghua University Beijing 100084 China Email: junbi@tsinghua.edu.cn Marcelo Bagnulo Universidad Carlos III de Madrid Avenida de la Universidad 30 Leganes, Madrid 28911 Spain Email: marcelo@it.uc3m.es Wu, et al. Expires August 15, 2011 [Page 11] Internet-Draft SAVI Framework February 2011 Fred Baker Cisco Systems Santa Barbara, CA 93117 United States Email: fred@cisco.com Christian Vogt (editor) Ericsson 200 Holger Way San Jose, CA 95134 United States Email: christian.vogt@ericsson.com Wu, et al. Expires August 15, 2011 [Page 12]