Integrity Protection for the Neighborhood Discovery Protocol (NHDP) and Optimized Link State Routing Protocol Version 2 (OLSRv2)

This specification updates and by defining mandatory-to-implement security mechanisms (for integrity and replay protection). A deployment of these protocols may choose to employ an alternative(s) to these mechanisms; in particular, it may choose to protect packets rather than messages, it may choose to use an alternative Integrity Check Value (ICV) with preferred properties, and/or it may use an alternative timestamp. A deployment may choose to use no such security mechanisms, but this is not recommended. The mechanisms specified are the use of an ICV for protection of the protocols' control messages and the use of timestamps in those messages to prevent replay attacks. Both use the TLV mechanism specified in to add this information to the messages. These ICV and TIMESTAMP TLVs are defined in . Different ICV TLVs are used for HELLO messages in NHDP and TC (Topology Control) messages in OLSRv2, the former also protecting the source address of the IP datagram that contains the HELLO message. This is because the IP datagram source address is used by NHDP to determine the address of a neighbor interface, and it is not necessarily otherwise contained in the HELLO message, while OLSRv2's TC message is forwarded in a new packet; thus, it has no single IP datagram source address. The mechanism specified in this document is placed in the packet/message processing flow as indicated in . It exists between the packet parsing/generation function of and the message processing/generation function of NHDP and OLSRv2.

| | Incoming | /|\ Outgoing packet \|/ | packet | | +--------------------------------+ | | | RFC 5444 packet | | parsing/generation | | | +--------------------------------+ | | Messages | /|\ Messages with \|/ | added TLVs | | D +--------------------------------+ R /__________________ | | O \ Messages | Mechanism specified in | P (failed check) | this document | | | +--------------------------------+ | | Messages | /|\ Messages (passed check) \|/ | | | +--------------------------------+ | | | NHDP/OLSRv2 message | | processing/generation | | | +--------------------------------+

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . Additionally, this document uses the terminology and notation of , , , and .

and enable specifications of extensions to recognize additional reasons for rejecting a message as "badly formed and therefore invalid for processing", and mention security (integrity protection) as an explicit example. This document specifies a mechanism that provides this functionality. Implementations of and MUST include this mechanism, and deployments of and SHOULD use this mechanism, except when a different security mechanism is more appropriate. The applicability of this mechanism is determined by its characteristics, which are that it: Specifies a security mechanism that is required to be included in conforming implementations of and . Specifies an association of ICVs with protocol messages, and specifies how to use a missing or invalid ICV as a reason to reject a message as "badly formed and therefore invalid for processing". Specifies the implementation of an ICV Message TLV, defined in , using a SHA-256-based Hashed Message Authentication Code (HMAC) applied to the appropriate message contents (and for HELLO messages also including the IP datagram source address). Implementations of and MUST support an HMAC&nbhy;SHA&nbhy;256 ICV TLV, and deployments SHOULD use it except when use of a different algorithm is more appropriate. An implementation MAY use more than one ICV TLV in a message, as long as they each use a different algorithm or key to calculate the ICV. Specifies the implementation of a TIMESTAMP Message TLV, defined in , to provide message replay protection. Implementations of and using this mechanism MUST support a timestamp based on POSIX time, and deployments SHOULD use it if the clocks in all routers in the network can be synchronized with sufficient precision. Assumes that a router that is able to generate correct integrity check values is considered trusted. This mechanism does not: Specify which key identifiers are to be used in a MANET in which the routers share more than one secret key. (Such keys will be differentiated using the <key-id> field defined in an ICV TLV in .) Specify how to distribute cryptographic material (shared secret key(s)). Specify how to detect compromised routers with valid keys. Specify how to handle (revoke) compromised routers with valid keys.

The mechanism specified in this document provides the following functionalities for use with messages specified by and : Generation of ICV Message TLVs (as defined in ) for inclusion in an outgoing message. An implementation of and MAY use more than one ICV TLV in a message, even with the same type extension, but these ICV TLVs MUST each use different keys or they MUST use a different algorithm to calculate the ICV, e.g., with different hash and/or cryptographic functions when using type extension 1 or 2. An implementation of and MUST at least be able to generate an ICV TLV using HMAC&nbhy;SHA&nbhy;256 and one or more secret keys shared by all routers. Generation of TIMESTAMP Message TLVs (as defined in ) for inclusion in an outgoing message. An implementation of and MAY use more than one ICV TLV in a message, but it MUST NOT use the same type extension. An implementation of and that is able to synchronize the clocks in all routers in the network with sufficient precision MUST at least be able to generate a TIMESTAMP TLV using POSIX time. Verification of ICV Message TLVs contained in a message, in order to determine if this message MUST be rejected as "badly formed and therefore invalid for processing" . An implementation of and MUST at least be able to verify an ICV TLV using HMAC/SHA-256 and one or more secret keys shared by all routers. Verification of TIMESTAMP Message TLVs (as defined in ) contained in a message, in order to determine if this message MUST be rejected as "badly formed and therefore invalid for processing" . An implementation of and that is able to synchronize the clocks in all routers in the network with sufficient precision MUST at least be able to verify a TIMESTAMP TLV using POSIX time. ICV Packet TLVs (as defined in ) MAY be used by a deployment of the multiplexing process defined in , either as well as or instead of the protection of the NHDP and OLSRv2 messages. (Note that in the case of NHDP, the packet protection is equally good, and also protects the packet header. In the case of OLSRv2, the packet protection has different properties than the message protection, especially for some forms of ICV. When packets contain more than one message, the packet protection has lower overheads in space and computation time.) When a router generates a message on a MANET interface, this mechanism: Specifies how to calculate an ICV for the message. Specifies how to include that ICV using an ICV Message TLV. and allow for the rejection of incoming messages prior to processing by NHDP or OLSRv2. This mechanism, when used, specifies that a message MUST be rejected if the ICV Message TLV is absent, or its value cannot be verified. Note that this means that routers whose implementation of NHDP and/or OLSRv2 does not include this specification will be ignored by routers using this mechanism, and these two sets of routers will, by design, form disjoint MANETs. (The unsecured MANET will retain some information about the secured MANET, but be unable to use it, not having any recognized symmetric links with the secured MANET.)

The following router parameters are specified for use by the two protocols; the first is required only by NHDP, but may be visible to OLSRv2, the second is required only by OLSRv2: MAX_HELLO_TIMESTAMP_DIFF - The maximum age that a HELLO message to be validated may have. If the current POSIX time of the router validating the HELLO message, minus the timestamp indicated in the TIMESTAMP TLV of the HELLO message, is greater than MAX_HELLO_TIMESTAMP_DIFF, the HELLO message MUST be silently discarded. MAX_TC_TIMESTAMP_DIFF - The maximum age that a TC message to be validated may have. If the current POSIX time of the router validating the TC message, minus the timestamp indicated in the TIMESTAMP TLV of the TC message, is greater than MAX_TC_TIMESTAMP_DIFF, the TC message MUST be silently discarded. The following constraints apply to these parameters: MAX_HELLO_TIMESTAMP_DIFF > 0 MAX_TC_TIMESTAMP_DIFF > 0 However, these bounds are insufficient: MAX_HELLO_TIMESTAMP_DIFF and MAX_TC_TIMESTAMP_DIFF MUST be least as great as the maximum expected "age" of a message (i.e., the time difference between a message has been sent by a router and received by all intended destinations). For HELLO messages, this needs only cover a single hop, but TC messages may have been forwarded a number of times. In particular, for TC messages, if using jitter as specified in and , the largest contribution the age may be a delay of up to F_MAXJITTER per hop (except the final hop) that the message has traveled. Other factors in the delay of both message types, per hop, may include the link-layer that is used in the MANET, and CPU and memory resources of routers (e.g., queuing delays, and delays for processing ICVs). An implementation MAY set lower and/or upper bounds on these parameters, if so, then these MUST allow values meeting these requirements. An implementation MAY make its value of MAX_TC_TIMESTAMP_DIFF dependent on the number of hops that a TC message has traveled. The above constraints assume ideal time synchronization of the clock in all routers in the network. The parameters MAX_HELLO_TIMESTAMP_DIFF and MAX_TC_TIMESTAMP_DIFF (and any constraints on them) MAY be increased to allow for expected timing differences between routers (between neighboring routers for MAX_HELLO_TIMESTAMP_DIFF, allowing for greater separation, but usually not per hop, for MAX_TC_TIMESTAMP_DIFF). Note that excessively large values of these parameters defeats their objectives, so these parameters SHOULD be as large as is required, but not significantly larger. Using POSIX time allows a resolution of no more than one second. In many MANET use cases, time synchronization much below one second is not possible because of unreliable and high-delay channels, mobility, interrupted communication, and possible resource limitations. In addition, when using the default message intervals and validity times as specified in and , where the shortest periodic message interval is 2 seconds, repeating the message within a second is actually beneficial rather than harmful (at a small bandwidth cost). Also, the use of jitter can cause a message to take that long or longer to traverse the MANET, thus even in a perfectly synchronized network, the TC maximum delay would usually be greater than 1 second. A finer granularity than 1 second, and thus the use of an alternative timestamp, is however RECOMMENDED in cases where, possibly due to fast moving routers, message validity times are below 1 second.

This section specifies how messages are generated and processed by and when using this mechanism.

Messages MUST have the content specified in and , respectively. In addition, messages that conform to this mechanism MUST contain: At least one ICV Message TLV (as specified in ), generated according to . Implementations of and MUST support the following version of the ICV TLV, but other versions MAY be used instead, or in addition, in a deployment, if more appropriate: For TC messages: type-extension := 1 For HELLO messages: type-extension := 2 hash-function := 3 (SHA-256) cryptographic-function := 3 (HMAC) The ICV Value MAY be truncated as specified in ; the selection of an appropriate length MAY be administratively configured. A message MAY contain several ICV Message TLVs. At least one TIMESTAMP Message TLV (as specified in ), generated according to . Implementations of and using this mechanism MUST support the following version of the TIMESTAMP TLV, but other versions MAY be used instead, or in addition, in a deployment, if more appropriate: type-extension := 1

After message generation (Section 11.1 of and Section 16.1. of ) and before message transmission (Section 11.2 of and Section 16.2 of ), the additional TLVs specified in MUST (unless already present) be added to an outgoing message when using this mechanism. The following processing steps (when using a single timestamp version and a single ICV algorithm) MUST be performed for a cryptographic algorithm that is used for generating an ICV for a message: All ICV TLVs (if any) are temporarily removed from the message. Any temporarily removed ICV TLVs MUST be stored, in order to be reinserted into the message in step 5. The message size and Message TLV Block size are updated accordingly. <msg-hop-count> and <msg-hop-limit>, if present, are temporarily set to 0. A TLV of type TIMESTAMP, as specified in , is added to the Message TLV Block. The message size and Message TLV Block size are updated accordingly. A TLV of type ICV, as specified in , is added to the Message TLV Block. The message size and Message TLV Block size are updated accordingly. All ICV TLVs that were temporary removed in step 1, are restored. The message size and Message TLV Block size are updated accordingly. <msg-hop-count> and <msg-hop-limit>, if present, are restored to their previous values. An implementation MAY add either alternative TIMESTAMP and/or ICV TLVs or more than one TIMESTAMP and/or ICV TLVs. All TIMESTAMP TLVs MUST be inserted before adding ICV TLVs.

Both and specify that: On receiving a ... message, a router MUST first check if the message is invalid for processing by this router and proceed to give a number of conditions that, each, will lead to a rejection of the message as "badly formed and therefore invalid for processing". When using a single timestamp version, and a single ICV algorithm, add the following conditions to that list, each of which, if true, MUST cause NHDP or OLSRv2 (as appropriate) to consider the message as invalid for processing when using this mechanism: The Message TLV Block of the message does not contain exactly one TIMESTAMP TLV of the selected version. This version specification includes the type extension. (The Message TLV Block may also contain TIMESTAMP TLVs of other versions.) The Message TLV Block does not contain exactly one ICV TLV using the selected algorithm and key identifier. This algorithm specification includes the type extension, and for type extensions 1 and 2, the hash function and cryptographic function. (The Message TLV Block may also contain ICV TLVs using other algorithms and key identifiers.) Validation of the identified (in step 1) TIMESTAMP TLV in the Message TLV Block of the message fails, as according to . Validation of the identified (in step 2) ICV TLV in the Message TLV Block of the message fails, as according to . An implementation MAY check the existence of, and verify, either an alternative TIMESTAMP and/or ICV TLVs or more than one TIMESTAMP and/or ICV TLVs.

For a TIMESTAMP Message TLV with type extension 1 (POSIX time) identified as described in : If the current POSIX time minus the value of that TIMESTAMP TLV is greater than MAX_HELLO_TIMESTAMP_DIFF (for a HELLO message) or MAX_TC_TIMESTAMP_DIFF (for a TC message), then the message validation fails. Otherwise, the message validation succeeds. If a deployment chooses to use a different type extension from 1, appropriate measures MUST be taken to verify freshness of the message.

For an ICV Message TLV identified as described in : All ICV Message TLVs (including the identified ICV Message TLV) are temporarily removed from the message, and the message size and Message TLV Block size are updated accordingly. The message's <msg-hop-count> and <msg-hop-limit> fields are temporarily set to 0. Calculate the ICV for the parameters specified in the identified ICV Message TLV, as specified in . If this ICV differs from the value of <ICV&nbhy;data> in the ICV Message TLV, then the message validation fails. If the <ICV-data> has been truncated (as specified in , the ICV calculated in the previous step MUST be truncated to the TLV length of the ICV Message TLV before comparing it with the <ICV-data>. Otherwise, the message validation succeeds. The message's <msg&nbhy;hop&nbhy;count> and <msg&nbhy;hop&nbhy;limit> fields are restored to their previous value, and the ICV Message TLVs are returned to the message, whose size is updated accordingly.

Before a router using this mechanism is able to generate ICVs or validate messages, it MUST acquire the shared secret key(s) to be used by all routers that are to participate in the network. This specification does not define how a router acquires secret keys. Once a router has acquired suitable key(s), it MAY be configured to use, or not use, this mechanism. Section 23.6 of provides a rationale based on why no key management is specified for OLSRv2.

This document specifies a security mechanism for use with NHDP and OLSRv2 that allows for mitigating several security threats.

This section briefly summarizes security threats that are mitigated by the mechanism presented in this document.

As only routers possessing the selected shared secret key are able to add a valid ICV TLV to a message, identity spoofing, where an attacker falsely claims an identity of a valid router, is countered. When using one or more shared keys for all routers in the MANET, it is only possible to determine that it is a valid router in the network, not to discern particular routers. Therefore, a malicious router in possession of valid keys (e.g., a compromised router) may still spoof the identity of another router using the same key.

Link spoofing, where an attacker falsely represents the existence of a nonexistent link, or otherwise misrepresents a link's state, is countered by the mechanism specified in this document, using the same argument as in .

Replay attacks are partly countered by the mechanism specified in this document, but this depends on synchronized clocks of all routers in the MANET. An attacker that records messages to replay them later can only do so in the selected time interval after the timestamp that is contained in message. As an attacker cannot modify the content of this timestamp (as it is protected by the identity check value), an attacker cannot replay messages after this time. Within this time interval, it is still possible to perform replay attacks; however, the limits on the time interval are specified so that this will have a limited effect on the operation of the protocol.

If no synchronized clocks are available in the MANET, replay attacks cannot be countered by the mechanism provided by this document. An alternative version of the TIMESTAMP TLV defined in , with a monotonic sequence number, may have some partial value in this case, but will necessitate adding state to record observed message sequence number information. The mechanism provided by this document does not avoid or detect security attacks by routers possessing the shared secret key that is used to generate integrity check values for messages. This mechanism relies on an out-of-band protocol or mechanism for distributing the shared secret key(s) (and if an alternative integrity check value is used, any additional cryptographic parameters). This mechanism does not provide a key management mechanism. Refer to Section 23.6 of for a detailed discussion why the automated key management requirements specified in do not apply for OLSRv2 and NHDP.

The authors would like to gratefully acknowledge the following people: Justin Dean (NRL) and Henning Rogge (Frauenhofer FKIE).