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RFC 7211

Operations Model for Router Keying

Pages: 18
Informational

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Internet Engineering Task Force (IETF)                        S. Hartman
Request for Comments: 7211                             Painless Security
Category: Informational                                         D. Zhang
ISSN: 2070-1721                             Huawei Technologies Co. Ltd.
                                                               June 2014


                   Operations Model for Router Keying

Abstract

The IETF is engaged in an effort to analyze the security of routing protocol authentication according to design guidelines discussed in RFC 6518, "Keying and Authentication for Routing Protocols (KARP) Design Guidelines". Developing an operational and management model for routing protocol security that works with all the routing protocols will be critical to the deployability of these efforts. This document gives recommendations to operators and implementors regarding management and operation of router authentication. These recommendations will also assist protocol designers in understanding management issues they will face. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7211.
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Copyright Notice

   Copyright (c) 2014 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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3 3. Breakdown of KARP Configuration . . . . . . . . . . . . . . . 3 3.1. Integrity of the Key Table . . . . . . . . . . . . . . . 5 3.2. Management of Key Table . . . . . . . . . . . . . . . . . 5 3.3. Interactions with Automated Key Management . . . . . . . 6 3.4. Virtual Routing and Forwarding Instances (VRFs) . . . . . 6 4. Credentials and Authorization . . . . . . . . . . . . . . . . 6 4.1. Preshared Keys . . . . . . . . . . . . . . . . . . . . . 8 4.1.1. Sharing Keys and Zones of Trust . . . . . . . . . . . 9 4.1.2. Key Separation and Protocol Design . . . . . . . . . 10 4.2. Asymmetric Keys . . . . . . . . . . . . . . . . . . . . . 10 4.3. Public Key Infrastructure . . . . . . . . . . . . . . . . 11 4.4. The Role of Central Servers . . . . . . . . . . . . . . . 12 5. Grouping Peers Together . . . . . . . . . . . . . . . . . . . 12 6. Administrator Involvement . . . . . . . . . . . . . . . . . . 14 6.1. Enrollment . . . . . . . . . . . . . . . . . . . . . . . 14 6.2. Handling Faults . . . . . . . . . . . . . . . . . . . . . 15 7. Upgrade Considerations . . . . . . . . . . . . . . . . . . . 16 8. Security Considerations . . . . . . . . . . . . . . . . . . . 17 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . 17 10.2. Informative References . . . . . . . . . . . . . . . . . 18
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1. Introduction

The Keying and Authentication of Routing Protocols (KARP) working group is designing improvements to the cryptographic authentication of IETF routing protocols. These improvements include enhancing how integrity functions are handled within each protocol as well as designing an automated key management solution. This document discusses issues to consider when thinking about the operational and management model for KARP. Each implementation will take its own approach to management; this is one area for vendor differentiation. However, it is desirable to have a common baseline for the management objects allowing administrators, security architects, and protocol designers to understand what management capabilities they can depend on in heterogeneous environments. Similarly, designing and deploying the protocol will be easier when thought is paid to a common operational model. This will also help with the design of NETCONF schemas or MIBs later. This document provides recommendations to help establish such a baseline. This document also gives recommendations for how management and operational issues can be approached as protocols are revised and as support is added for the key table [RFC7210]. Routing security faces interesting challenges not present with some other security domains. Routers need to function in order to establish network connectivity. As a result, centralized services cannot typically be used for authentication or other security tasks; see Section 4.4. In addition, routers' roles affect how new routers are installed and how problems are handled; see Section 6.

2. Requirements Notation

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].

3. Breakdown of KARP Configuration

Routing authentication configuration includes configuration of key material used to authenticate routers as well as parameters needed to use these keys. Configuration also includes information necessary to use an automated key management protocol to configure router keying. The key table [RFC7210] describes configuration needed for manual keying. Configuration of automated key management is a work in progress.
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   There are multiple ways of structuring configuration information.
   One factor to consider is the scope of the configuration information.
   Several protocols are peer-to-peer routing protocols where a
   different key could potentially be used for each neighbor.  Other
   protocols require that the same group key be used for all nodes in an
   administrative domain or routing area.  In other cases, the same
   group key needs to be used for all routers on an interface, but
   different group keys can be used for each interface.

   Within situations where a per-interface, per-area, or per-peer key
   can be used for manually configured long-term keys, that flexibility
   may not be desirable from an operational standpoint.  For example,
   consider OSPF [RFC2328].  Each router on an OSPF link needs to use
   the same authentication configuration, including the set of keys used
   for reception and the set of keys used for transmission, but it may
   use different keys for different links.  The most general management
   model would be to configure keys per link.  However, for deployments
   where the area uses the same key, it would be strongly desirable to
   configure the key as a property of the area.  If the keys are
   configured per link, they can get out of sync.  In order to support
   generality of configuration and common operational situations, it
   would be desirable to have some sort of inheritance where default
   configurations are made per area unless overridden per interface.

   As described in [RFC7210], the cryptographic keys are separated from
   the interface configuration into their own configuration store.  Each
   routing protocol is responsible for defining the form of the peer
   specification used by that protocol.  Thus, each routing protocol
   needs to define the scope of keys.  For group keying, the peer
   specification names the group.  A protocol could define a peer
   specification indicating the key had a link scope and also a peer
   specification for scoping a key to a specific area.  For link-scoped
   keys, it is generally best to define a single peer specification
   indicating the key has a link scope and to use interface restrictions
   to restrict the key to the appropriate link.

   Operational Requirements: implementations of this model MUST support
   configuration of keys at the most general scope for the underlying
   protocol; protocols supporting per-peer keys MUST permit
   configuration of per-peer keys, protocols supporting per-interface
   keys MUST support configuration of per-interface keys, and so on for
   any additional scopes.  Implementations MUST NOT permit configuration
   of an inappropriate key scope.  For example, configuration of
   separate keys per interface would be inappropriate to support for a
   protocol requiring per-area keys.  This restriction can be enforced
   by rules specified by each routing protocol for validating key table
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   entries.  As such, these implementation requirements are best
   addressed by care being taken in how routing protocols specify the
   use of the key tables.

3.1. Integrity of the Key Table

The routing key table [RFC7210] provides a very general mechanism to abstract the storage of keys for routing protocols. To avoid misconfiguration and simplify problem determination, the router MUST verify the internal consistency of entries added to the table. Routing protocols describe how their protocol interacts with the key table including what validation MUST be performed. At a minimum, the router MUST verify: o The cryptographic algorithms are valid for the protocol. o The key derivation function is valid for the protocol. o The direction is valid for the protocol. For example, if a protocol requires the same session key be used in both directions, the direction field in the key table entry associated with the session key MUST be specified as "both". o The peer specification is consistent with the protocol. Other checks are possible. For example, the router could verify that if a key is associated with a peer, that peer is a configured peer for the specified protocol. However, this may be undesirable. It may be desirable to load a key table when some peers have not yet been configured. Also, it may be desirable to share portions of a key table across devices even when their current configuration does not require an adjacency with a particular peer in the interest of uniform configuration or preparing for fail-over. For these reasons, these additional checks are generally undesirable.

3.2. Management of Key Table

Several management interfaces will be quite common. For service provider deployments, the configuration management system can simply update the key table. However, for smaller deployments, efficient management interfaces that do not require a configuration management system are important. In these environments, configuration interfaces (such as web interfaces and command-line interfaces) provided directly by the router will be important for easy management of the router.
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   As part of adding a new key, it is typically desirable to set an
   expiration time for an old key.  The management interface SHOULD
   provide a mechanism to easily update the expiration time for a
   current key used with a given peer or interface.  Also, when adding a
   key, it is desirable to push the key out to nodes that will need it,
   allowing use for receiving packets and then later for enabling
   transmit.  This can be accomplished automatically by providing a
   delay between when a key becomes valid for reception and
   transmission.  However, some environments may not be able to predict
   when all the necessary changes will be made.  In these cases, having
   a mechanism to enable a key for sending is desirable.  The management
   interface SHOULD provide an easy mechanism to update the direction of
   an existing key or to enable a disabled key.

   Implementations SHOULD permit a configuration in which if no
   unexpired key is available, existing security associations continue
   using the expired key with which they were established.
   Implementations MUST support a configuration in which security
   associations fail if no unexpired key is available for them.  See
   Section 6.2 for a discussion of reporting and managing security
   faults including those related to key expiration.

3.3. Interactions with Automated Key Management

Consideration is required for how an automated key management protocol will assign key IDs for group keys. All members of the group may need to use the same key ID. This requires careful coordination of global key IDs. Interactions with the peer key ID field may make this easier; this requires additional study. Automated key management protocols also assign keys for single peers. If the key ID is global and needs to be coordinated between the receiver and transmitter, then there is complexity in key management protocols that can be avoided if key IDs are not global.

3.4. Virtual Routing and Forwarding Instances (VRFs)

Many core and enterprise routers support multiple routing instances. For example, a router serving multiple VPNs is likely to have a forwarding/routing instance for each of these VPNs. Each VRF will require its own routing key table.

4. Credentials and Authorization

Several methods for authentication have been proposed for KARP. The simplest is preshared keys used directly as traffic keys. In this mode, the traffic integrity keys are directly configured. This is the mode supported by most of today's routing protocols.
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   As discussed in [RTG-AUTH], preshared keys can be used as the input
   to a key derivation function (KDF) to generate traffic keys.  For
   example, the TCP Authentication Option (TCP-AO) [RFC5925] derives
   keys based on the initial TCP session state.  Typically, a KDF will
   combine a long-term key with public inputs exchanged as part of the
   protocol to form fresh session keys.  A KDF could potentially be used
   with some inputs that are configured along with the long-term key.
   Also, it's possible that inputs to a KDF will be private and
   exchanged as part of the protocol, although this will be uncommon in
   KARP's uses of KDFs.

   Preshared keys could also be used by an automated key management
   protocol.  In this mode, preshared keys would be used for
   authentication.  However, traffic keys would be generated by some
   key-agreement mechanism or transported in a key encryption key
   derived from the preshared key.  This mode may provide better replay
   protection.  Also, in the absence of active attackers, key-agreement
   strategies such as Diffie-Hellman can be used to produce high-quality
   traffic keys even from relatively weak preshared keys.  These key-
   agreement mechanisms are valuable even when active attackers are
   present, although an active attacker can mount a man-in-the-middle
   attack if the preshared key is sufficiently weak.

   Public keys can be used for authentication within an automated key
   management protocol.  The KARP design guide [RFC6518] describes a
   mode in which routers have the hashes of peer routers' public keys.
   In this mode, a traditional public-key infrastructure is not
   required.  The advantage of this mode is that a router only contains
   its own keying material, limiting the scope of a compromise.  The
   disadvantage is that when a router is added or deleted from the set
   of authorized routers, all routers in that set need to be updated.
   Note that self-signed certificates are a common way of communicating
   public keys in this style of authentication.

   Certificates signed by a certification authority or some other PKI
   could be used for authentication within an automated key management
   protocol.  The advantage of this approach is that routers may not
   need to be directly updated when peers are added or removed.  The
   disadvantage is that more complexity and cost are required.

   Each of these approaches has a different set of management and
   operational requirements.  Key differences include how authorization
   is handled and how identity works.  This section discusses these
   differences.
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4.1. Preshared Keys

In the protocol, manual preshared keys are either unnamed or named by a key ID (which is a small integer -- typically 16 or 32 bits). Implementations that support multiple keys for protocols that have no names for keys need to try all possible keys before deciding a packet cannot be validated [RFC4808]. Typically key IDs are names used by one group or peer. Manual preshared keys are often known by a group of peers rather than just one other peer. This is an interesting security property: unlike with digitally signed messages or protocols where symmetric keys are known only to two parties, it is impossible to identify the peer sending a message cryptographically. However, it is possible to show that the sender of a message is one of the parties who knows the preshared key. Within the routing threat model, the peer sending a message can be identified only because peers are trusted and thus can be assumed to correctly label the packets they send. This contrasts with a protocol where cryptographic means such as digital signatures are used to verify the origin of a message. As a consequence, authorization is typically based on knowing the preshared key rather than on being a particular peer. Note that once an authorization decision is made, the peer can assert its identity; this identity is trusted just as the routing information from the peer is trusted. Doing an additional check for authorization based on the identity included in the packet would provide little value: an attacker who somehow had the key could claim the identity of an authorized peer, and an attacker without the key should be unable to claim the identity of any peer. Such a check is not required by the KARP threat model: inside attacks are not in scope. Preshared keys used with key derivation work similarly to manual preshared keys. However, to form the actual traffic keys, session- or peer-specific information is combined with the key. From an authorization standpoint, the derivation key works the same as a manual key. An additional routing protocol step or transport step forms the key that is actually used. Preshared keys that are used via automatic key management have not yet been specified for KARP, although ongoing work suggests they will be needed. Their naming and authorization may differ from existing uses of preshared keys in routing protocols. In particular, such keys may end up being known only by two peers. Alternatively, they may also be known by a group of peers. Authorization could potentially be based on peer identity, although it is likely that knowing the right key will be sufficient. There does not appear to
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   be a compelling reason to decouple the authorization of a key for
   some purpose from the authorization of peers holding that key to
   perform the authorized function.

4.1.1. Sharing Keys and Zones of Trust

Care needs to be taken when symmetric keys are used for multiple purposes. Consider the implications of using the same preshared key for two interfaces: it becomes impossible to cryptographically distinguish a router on one interface from a router on another interface. So, a router that is trusted to participate in a routing protocol on one interface becomes implicitly trusted for the other interfaces that share the key. For many cases, such as link-state routers in the same routing area, there is no significant advantage that an attacker could gain from this trust within the KARP threat model. However, other protocols, such as BGP and RIP, permit routes to be filtered across a trust boundary. For these protocols, participation in one interface might be more advantageous than another. Operationally, when this trust distinction is important to a deployment, different keys need to be used on each side of the trust boundary. Key derivation can help prevent this problem in cases of accidental misconfiguration. However, key derivation cannot protect against a situation where a system was incorrectly trusted to have the key used to perform the derivation. This question of trust is important to the KARP threat model because it is essential to determining whether a party is an insider for a particular routing protocol. A customer router that is an insider for a BGP peering relationship with a service provider is not typically an insider when considering the security of that service provider's IGP. Similarly, to the extent that there are multiple zones of trust and a routing protocol is determining whether a particular router is within a certain zone, the question of untrusted actors is within the scope of the routing threat model. Key derivation can be part of a management solution for having multiple keys for different zones of trust. A master key could be combined with peer, link, or area identifiers to form a router- specific preshared key that is loaded onto routers. Provided that the master key lives only on the management server and not the individual routers, trust is preserved. However, in many cases, generating independent keys for the routers and storing the result is more practical. If the master key were somehow compromised, all the resulting keys would need to be changed. However, if independent keys are used, the scope of a compromise may be more limited.
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4.1.2. Key Separation and Protocol Design

More subtle problems with key separation can appear in protocol design. Two protocols that use the same traffic keys may work together in unintended ways permitting one protocol to be used to attack the other. Consider two hypothetical protocols. Protocol A starts its messages with a set of extensions that are ignored if not understood. Protocol B has a fixed header at the beginning of its messages but ends messages with extension information. It may be that the same message is valid both as part of protocol A and protocol B. An attacker may be able to gain an advantage by getting a router to generate this message with one protocol under situations where the other protocol would not generate the message. This hypothetical example is overly simplistic; real-world attacks exploiting key separation weaknesses tend to be complicated and involve specific properties of the cryptographic functions involved. The key point is that whenever the same key is used in multiple protocols, attacks may be possible. All the involved protocols need to be analyzed to understand the scope of potential attacks. Key separation attacks interact with the KARP operational model in a number of ways. Administrators need to be aware of situations where using the same manual traffic key with two different protocols (or the same protocol in different contexts) creates attack opportunities. Design teams should consider how their protocol might interact with other routing protocols and describe any attacks discovered so that administrators can understand the operational implications. When designing automated key management or new cryptographic authentication within routing protocols, we need to be aware that administrators expect to be able to use the same preshared keys in multiple contexts. As a result, we should use appropriate key derivation functions so that different cryptographic keys are used even when the same initial input key is used.

4.2. Asymmetric Keys

Outside of a PKI, public keys are expected to be known by the hash of a key or (potentially self-signed) certificate. The Session Description Protocol provides a standardized mechanism for naming keys (in that case, certificates) based on hashes (Section 5 of [RFC4572]). KARP SHOULD adopt this approach or another approach already standardized within the IETF rather than inventing a new mechanism for naming public keys. A public key is typically expected to belong to one peer. As a peer generates new keys and retires old keys, its public key may change. For this reason, from a management standpoint, peers should be
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   thought of as associated with multiple public keys rather than as
   containing a single public-key hash as an attribute of the peer
   object.

   Authorization of public keys could be done either by key hash or by
   peer identity.  Performing authorizations by peer identity should
   make it easier to update the key of a peer without risk of losing
   authorizations for that peer.  However, management interfaces need to
   be carefully designed to avoid making this extra level of indirection
   complicated for operators.

4.3. Public Key Infrastructure

When a PKI is used, certificates are used. The certificate binds a key to a name of a peer. The key management protocol is responsible for exchanging certificates and validating them to a trust anchor. Authorization needs to be done in terms of peer identities not in terms of keys. One reason for this is that when a peer changes its key, the new certificate needs to be sufficient for authentication to continue functioning even though the key has never been seen before. Potentially, authorization could be performed in terms of groups of peers rather than single peers. An advantage of this is that it may be possible to add a new router with no authentication-related configuration of the peers of that router. For example, a domain could decide that any router with a particular keyPurposeID signed by the organization's certificate authority is permitted to join the IGP. Just as in configurations where cryptographic authentication is not used, automatic discovery of this router can establish appropriate adjacencies. Assuming that self-signed certificates are used by routers that wish to use public keys but that do not need a PKI, then PKI and the "infrastructure-less" mode of public-key operation described in the previous section can work well together. One router could identify its peers based on names and use certificate validation. Another router could use hashes of certificates. This could be very useful for border routers between two organizations. Smaller organizations could use public keys and larger organizations could use PKI. A PKI has significant operational concerns including certification practices, handling revocation, and operational practices around certificate validation. The Routing PKI (RPKI) has addressed these concerns within the scope of BGP and the validation of address ownership. Adapting these practices to routing protocol authentication is outside the scope of this document.
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4.4. The Role of Central Servers

An area to explore is the role of central servers like RADIUS or directories. Routers need to securely operate in order to provide network routing services. Routers cannot generally contact a central server while establishing routing because the router might not have a functioning route to the central service until after routing is established. As a result, a system where keys are pushed by a central management system is an undesirable result for router keying. However, central servers may play a role in authorization and key rollover. For example, a node could send a hash of a public key to a RADIUS server. If central servers do play a role, it will be critical to make sure that they are not required during routine operation or a cold-start of a network. They are more likely to play a role in enrollment of new peers or key migration/compromise. Another area where central servers may play a role is for group key agreement. As an example, [OSPF-AUTO] discusses the potential need for key-agreement servers in OSPF. Other routing protocols that use multicast or broadcast such as IS-IS are likely to need a similar approach. Multicast key-agreement protocols need to allow operators to choose which key servers will generate traffic keys. The quality of random numbers [RFC4086] is likely to differ between systems. As a result, operators may have preferences for where keys are generated.

5. Grouping Peers Together

One significant management consideration will be the grouping of management objects necessary to determine who is authorized to act as a peer for a given routing action. As discussed previously, the following objects are potentially required: o Key objects are required. Symmetric keys may be preshared, and knowledge of the key may be used as the decision factor in authorization. Knowledge of the private key corresponding to asymmetric public keys may be used directly for authorization as well. During key transitions, more than one key may refer to a given peer. Group preshared keys may refer to multiple peers. o Peer objects are required. A peer is a router that this router might wish to communicate with. Peers may be identified by names or keys. o Objects representing peer groups are required. Groups of peers may be authorized for a given routing protocol.
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   Establishing a management model is difficult because of the complex
   relationships between each set of objects.  As discussed, there may
   be more than one key for a peer.  However, in the preshared key case,
   there may be more than one peer for a key.  This is true both for
   group security association protocols such as an IGP or one-to-one
   protocols where the same key is used administratively.  In some of
   these situations, it may be undesirable to explicitly enumerate the
   peers in the configuration; for example, IGP peers are auto-
   discovered for broadcast links but not for non-broadcast multi-access
   links.

   Peers may be identified either by name or key.  If peers are
   identified by key, it is strongly desirable from an operational
   standpoint to consider any peer identifiers or names to be a local
   matter and not require the identifiers or names to be synchronized.
   Obviously, if peers are identified by names (for example, with
   certificates in a PKI), identifiers need to be synchronized between
   the authorized peer and the peer making the authorization decision.

   In many cases, peers will explicitly be identified in routing
   protocol configuration.  In these cases, it is possible to attach the
   authorization information (keys or identifiers) to the peer's
   configuration object.  Two cases do not involve enumerating peers.
   The first is the case where preshared keys are shared among a group
   of peers.  It is likely that this case can be treated from a
   management standpoint as a single peer representing all the peers
   that share the keys.  The other case is one where certificates in a
   PKI are used to introduce peers to a router.  In this case, rather
   than configuring peers, the router needs to be configured with
   information on which certificates represent acceptable peers.

   Another consideration is which routing protocols share peers.  For
   example, it may be common for LDP peers to also be peers of some
   other routing protocol.  Also, RSVP - Traffic Engineering (RSVP-TE)
   may be associated with some TE-based IGP.  In some of these cases, it
   would be desirable to use the same authorization information for both
   routing protocols.

   Finally, as discussed in Section 7, it is sometimes desirable to
   override some aspect of the configuration for a peer in a group.  As
   an example, when rotating to a new key, it is desirable to be able to
   roll that key out to each peer that will use the key, even if in the
   stable state the key is configured for a peer group.

   In order to develop a management model for authorization, the working
   group needs to consider several questions.  What protocols support
   auto-discovery of peers?  What protocols require more configuration
   of a peer than simply the peer's authorization information and
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   network address?  What management operations are going to be common
   as security information for peers is configured and updated?  What
   operations will be common while performing key transitions or while
   migrating to new security technologies?

6. Administrator Involvement

One key operational question is what areas will administrator involvement be required. Likely areas where involvement may be useful include enrollment of new peers. Fault recovery should also be considered.

6.1. Enrollment

One area where the management of routing security needs to be optimized is the deployment of a new router. In some cases, a new router may be deployed on an existing network where routing to management servers is already available. In other cases, routers may be deployed as part of connecting or creating a site. Here, the router and infrastructure may not be available until the router has securely authenticated. In general, security configuration can be treated as an additional configuration item that needs to be set up to establish service. There is no significant security value in protecting routing protocol keys more than administrative password or Authentication, Authorization, and Accounting (AAA) secrets that can be used to gain login access to a router. These existing secrets can be used to make configuration changes that impact routing protocols as much as disclosure of a routing protocol key. Operators already have procedures in place for these items. So, it is appropriate to use similar procedures for routing protocol keys. It is reasonable to improve existing configuration procedures and the routing protocol procedures over time. However, it is more desirable to deploy KARP with security similar to that used for managing existing secrets than to delay deploying KARP. Operators MAY develop higher assurance procedures for dealing with keys. For example, asymmetric keys can be generated on a router and never exported from the router. Operators can evaluate the cost vs. security and the availability tradeoffs of these procedures.
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6.2. Handling Faults

Faults may interact with operational practice in at least two ways. First, security solutions may introduce faults. For example, if certificates expire in a PKI, previous adjacencies may no longer form. Operational practice will require a way of repairing these errors. This may end up being very similar to repairing other faults that can partition a network. Notifications will play a critical role in avoiding security faults. Implementations SHOULD use appropriate mechanisms to notify operators as security resources are about to expire. Notifications can include messages to consoles, logged events, Simple Network Management Protocol (SNMP) traps, or notifications within a routing protocol. One strategy is to have increasing escalations of notifications. Monitoring will also play an important role in avoiding security faults such as certificate expiration. Some classes of security fault, including issues with certificates, will affect only key management protocols. Other security faults can affect routing protocols directly. However, the protocols MUST still have adequate operational mechanisms to recover from these situations. Also, some faults, such as those resulting from a compromise or actual attack on a facility, are inherent and may not be prevented. A second class of faults is equipment faults that impact security. For example, if keys are stored on a router and never exported from that device, failure of a router implies a need to update security provisioning on the replacement router and its peers. One approach, recommended by work on securing BGP [KEYING] is to maintain the router's keying material so that when a router is replaced the same keys can be used. Router keys can be maintained on a central server. These approaches permit the credentials of a router to be recovered. This provides valuable options in case of hardware fault. The failing router can be recovered without changing credentials on other routers or waiting for keys to be certified. One disadvantage of this approach is that even if public-key cryptography is used, the private keys are located on more than just the router. A system in which keys were generated on a router and never exported from that router would typically make it more difficult for an attacker to obtain the keys. For most environments, the ability to quickly replace a router justifies maintaining keys centrally. More generally, keying is another item of configuration that needs to be restored to reestablish service when equipment fails. Operators typically perform the minimal configuration necessary to get a router
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   back in contact with the management server.  The same would apply for
   keys.  Operators who do not maintain copies of key material for
   performing key recovery on routers would need to perform a bit more
   work to regain contact with the management server.  It seems
   reasonable to assume that management servers will be able to cause
   keys to be generated or distributed sufficiently to fully restore
   service.

7. Upgrade Considerations

It needs to be possible to deploy automated key management in an organization without either having to disable existing security or disrupting routing. As a result, it needs to be possible to perform a phased upgrade from manual keying to automated key management. This upgrade procedure needs to be easy and have a very low risk of disrupting routing. Today, many operators do not update keys because the perceived risk of an attack is lower than the cost of an update combined with the potential cost of routing disruptions during the update. Even when a routing protocol has technical mechanisms that permit an update with no disruption in service, there is still a potential cost of service disruptions as operational procedures and practices need to correctly use the technical mechanisms. For peer-to-peer protocols such as BGP, upgrading to automated key management can be relatively easy. First, code that supports automated key management needs to be loaded on both peers. Then, the adjacency can be upgraded. The configuration can be updated to switch to automated key management when the second router reboots. Alternatively, if the key management protocols involved can detect that both peers now support automated key management, then a key can potentially be negotiated for an existing session. The situation is more complex for organizations that have not upgraded from TCP MD5 [RFC2385] to the TCP Authentication Option [RFC5925]. Today, routers typically need to understand whether a given peer supports TCP MD5 or TCP-AO before opening a TCP connection. In addition, many implementations support grouping configuration (including security configuration) of related peers together. Implementations make it challenging to move from TCP MD5 to TCP-AO before all peers in the group are ready. Operators perceive it as high risk to update the configuration of a large number of peers. One particularly risky situation is upgrading the configuration of Internal BGP (iBGP) peers. The situation is more complicated for multicast protocols. It's typically not desirable to bring down an entire link to reconfigure it as using automated key management. Two approaches should be considered. One is to support key table rows that enable the
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   automated key management and manually configured keying for the same
   link at the same time.  Coordinating this may be challenging from an
   operational standpoint.  Another possibility is for the automated key
   management protocol to actually select the same traffic key that is
   being used manually.  This could be accomplished by having an option
   in the key management protocol to export the current manual group key
   through the automated key management protocol.  Then after all nodes
   are configured with automated key management, manual key entries can
   be removed.  The next re-key after all nodes have manual entries
   removed will generate a new fresh key.  Group key management
   protocols are RECOMMENDED to support an option to export existing
   manual keys during initial deployment of automated key management.

8. Security Considerations

This document does not define a protocol. It does discuss the operational and management implications of several security technologies. Close synchronization of time can impact the security of routing protocols in a number of ways. Time is used to control when keys MAY begin being used and when they MUST NOT be used any longer as described in [RFC7210]. Routers need to have tight enough time synchronization that receivers permit a key to be utilized for validation prior to the first use of that key for generation of integrity-protected messages; otherwise, availability will be impacted. If time synchronization is too loose, then a key can be used beyond its intended lifetime. The Network Time Protocol (NTP) can be used to provide time synchronization. For some protocols, time synchronization is also important for replay detection.

9. Acknowledgments

Funding for Sam Hartman's work on this memo is provided by Huawei. The authors would like to thank Bill Atwood, Randy Bush, Wes George, Gregory Lebovitz, and Russ White for valuable reviews.

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. [RFC7210] Housley, R., Polk, T., Hartman, S., and D. Zhang, "Database of Long-Lived Symmetric Cryptographic Keys", RFC 7210, April 2014.
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10.2. Informative References

[KEYING] Turner, S., Patel, K., and R. Bush, "Router Keying for BGPsec", Work in Progress, May 2014. [OSPF-AUTO] Liu, Y., "OSPFv3 Automated Group Keying Requirements", Work in Progress, July 2007. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC4572] Lennox, J., "Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP)", RFC 4572, July 2006. [RFC4808] Bellovin, S., "Key Change Strategies for TCP-MD5", RFC 4808, March 2007. [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, June 2010. [RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for Routing Protocols (KARP) Design Guidelines", RFC 6518, February 2012. [RTG-AUTH] Polk, T. and R. Housley, "Routing Authentication Using A Database of Long-Lived Cryptographic Keys", Work in Progress, November 2010.

Authors' Addresses

Sam Hartman Painless Security EMail: hartmans-ietf@mit.edu Dacheng Zhang Huawei Technologies Co. Ltd. EMail: zhangdacheng@huawei.com