| 6lo | P. Thubert, Ed. |
| Internet-Draft | Cisco |
| Updates: 6775 (if approved) | B. Sarikaya |
| Intended status: Standards Track | |
| Expires: March 7, 2019 | M. Sethi |
| Ericsson | |
| September 3, 2018 |
Address Protected Neighbor Discovery for Low-power and Lossy Networks
draft-ietf-6lo-ap-nd-07
This document defines an extension to 6LoWPAN Neighbor Discovery (ND) [I-D.ietf-6lo-rfc6775-update] called Address Protected ND (AP-ND); AP-ND protects the owner of an address against address theft and impersonation inside a low-power and lossy network (LLN). Nodes supporting this extension compute a cryptographic Owner Unique Interface ID and associate it with one or more of their Registered Addresses. The Cryptographic ID identifies the owner of the Registered Address and can be used for proof-of-ownership. It is used in 6LoWPAN ND in place of the EUI-64-based unique ID that is associated with the registration. Once an address is registered with a Cryptographic ID, only the owner of that ID can modify the registration information of the Registered Address, and Source Address Validation can be enforced.
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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 March 7, 2019.
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"Neighbor Discovery Optimizations for 6LoWPAN networks" (6LoWPAN ND) adapts the IPv6 ND (NDv6) protocol [RFC4861][RFC4862] (IPv6 ND) for operations over a constrained low-power and lossy network (LLN). In particular, 6LoWPAN ND introduces a unicast host address registration mechanism that reduces the use of multicast messages that are present in the NDv6 protocol. 6LoWPAN ND defines a new Address Registration Option (ARO) that is carried in the unicast Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages exchanged between a 6LoWPAN Node (6LN) and a 6LoWPAN Router (6LR). It also defines the Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC) messages between the 6LR and the 6LoWPAN Border Router (6LBR). In LLN networks, the 6LBR is the central repository of all the registered addresses in its domain.
The registration mechanism in 6LoWPAN ND prevents the use of an address if that address is already registered in the subnet (first come first serve). In order to validate address ownership, the registration mechanism enables the 6LR and 6LBR to validate the association between a registered address and a Registration Ownership Verifier (ROVR). 6LoWPAN ND specifies that the ROVR is derived from the MAC address of the device (using the 64-bit Extended Unique Identifier EUI-64 address format specified by IEEE), which can be spoofed. Therefore, any node connected to the subnet and aware of a registered-address-to-ROVR mapping could effectively fake the ROVR, steal the address and redirect traffic for that address towards a different 6LN. The "Registration Extensions for 6LoWPAN Neighbor Discovery" defines an Extended ARO (EARO) option that allows to transport alternate forms of ROVRs, and is a prerequisite for this specification.
According to this specification, a 6LN generates a cryptographic ID (Crypto-ID) and places it in the ROVR field in the registration of one (or more) of its addresses with the 6LR(s) that the 6LN uses as default router(s). Proof of ownership of the cryptographic ID (Crypto-ID) is passed with the first registration exchange to a new 6LR, and enforced at the 6LR. The 6LR validates ownership of the cryptographic ID before it can create a registration, or a change the information, that is the Link-Layer Address and associated parameters, in an existing registration state.
The protected address registration protocol proposed in this document enables Source Address Validation (SAVI) [RFC7039], which ensures that only the owner uses a registered address in the source address field in IPv6 packets. Consequently, a 6LN that sources a packet has to use a 6LR to which the source address of the packet is registered to forward the packet. The 6LR maintains state information for the registered addressed, including the MAC address, and a link-layer cryptographic key associated with the 6LN. In SAVI-enforcement mode, the 6LR allows only packets from a connected Host if the connected Host owns the registration of the source address of the packet.
The 6lo adaptation layer framework ([RFC4944], [RFC6282]) specifies that a device forms its IPv6 addresses based on Layer-2 address, so as to enable a better compression. This is incompatible with "Secure Neighbor Discovery (SeND)" and "Cryptographically Generated Addresses (CGAs)", which derive the Interface ID (IID) in the IPv6 addresses from key material. "Privacy Considerations for IPv6 Address Generation Mechanisms" places additional recommendations on the way addresses should be formed and renewed.
This document specifies that a device may form and register addresses at will, without a constraint on the way the address is formed or the number of addresses that are registered in parallel, Multiple addresses with a single ROVR, which only needs to be sent once to a given 6LR for multiple addresses and registration updates.
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
In this document, readers will encounter terms and concepts that are discussed in the following documents:
This document often uses the following acronyms:
This document defines a new Crypto-ID as an identifier of variable size which is 64 to 256 bits long. It is generated using cryptographic means explained later in this document Section 4.2. "Elliptic Curves for Security" [RFC7748] and "Edwards-Curve Digital Signature Algorithm (EdDSA)" [RFC8032] provides information on Elliptic Curve Cryptography (ECC) and a (twisted) Edwards curve, Ed25519, which can be used with this specification. "Alternative Elliptic Curve Representations" [I-D.struik-lwig-curve-representations] provides additional information on how to represent Montgomery curves and (twisted) Edwards curves as curves in short-Weierstrass form and illustrates how this can be used to implement elliptic curve computations using existing implementations that already implement, e.g., ECDSA and ECDH using NIST [FIPS-186-4] prime curves.
This specification defines a cryptographic identifier (Crypto-ID) that can be used as a replacement to the MAC address in the ROVR field of the EARO option; the computation of the Crypto-ID is detailed in Section 4.2. A node in possession of the necessary cryptographic material SHOULD use Crypto-ID by default as ROVR in its registration. Whether a ROVR is a Crypto-ID is indicated by a new "C" flag in the NS(EARO) message.
In order to prove its ownership of a Crypto-ID, the registering node needs to supply certain parameters including a nonce and a signature that will prove that the node has the private key corresponding to the public key used to build the Crypto-ID. This specification adds the capability to carry new options in the NS(EARO) and the NA(EARO). The NS(EARO) carries a variation of the CGA Option (Section 4.4), a Nonce option and a variation of the RSA Signature option (Section 4.6) in the NS(EARO). The NA(EARO) carries a Nonce option.
In order to avoid the need for new ND option types, this specification reuses / extends options defined in SEND [RFC3971] and 6LoWPAN ND [RFC6775] [I-D.ietf-6lo-rfc6775-update]. This applies in particular to the CGA option and the RSA Signature Option. This specification provides aliases for the specific variations of those options as used in AP-ND. The presence of the EARO option in the NS/NA messages indicates that the crypto options are to be processed as specified in this document, not as a SEND message.
A 6LN provides its public key in an NS message. The public key could be in uncompressed form or in compressed form where the first octet of the OCTET STRING is 0x04 and 0x02 or 0x03, respectively. Point compression can further reduce the key size by about 32 octets.
Each 6LN using a Crypto-ID for registration MUST have a public/private key pair.
The Crypto-ID is computed as follows:
This specification updates the EARO option as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Status | Opaque |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Rsvd |C| I |R|T| TID | Registration Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
... Registration Ownership Verifier (ROVR) ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Enhanced Address Registration Option
6LoWPAN ND. No other new Status values is defined.
This specification uses Status values "Validation Requested" and "Validation Failed", which are defined in
This specification defines the Crypto-ID Parameters Option (CIPO), as a variation of the CGA Option that carries the parameters used to form a Crypto-ID. In order to provide cryptographic agility [RFC7696], AP-ND supports two possible signature algorithms, indicated by a Crypto-Type field. Elliptic Curve Cryptography (ECC) is used to calculate the Crypto-ID. NIST P-256 [FIPS186-4] MUST be supported by all implementations. The Edwards-Curve Digital Signature Algorithm (EdDSA) curve Ed25519ph (pre-hashing) [RFC8032] MAY be supported as an alternate.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Pad Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Crypto-Type | Modifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
. .
. Public Key (variable length) .
. .
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Crypto-ID Parameters Option
This document reuses the Nonce Option defined in section 5.3.2. of SEND without a change.
This document reuses the RSA Signature Option (RSAO) defined in section 5.2. of SEND. Admittedly, the name is ill-chosen since the option is extended for non-RSA Signatures and this specification defines an alias to avoid the confusion.
The description of the operation on the option detailed in section 5.2. of SEND apply, but for the following changes:
The scope of the present work is a 6LoWPAN Low Power Lossy Network (LLN), typically a stub network connected to a larger IP network via a Border Router called a 6LBR per [RFC6775]. A 6LBR has sufficient capability to satisfy the needs of DAD.
The 6LBR maintains registration state for all devices in its attached LLN. Together with the first-hop router (the 6LR), the 6LBR assures uniqueness and grants ownership of an IPv6 address before it can be used in the LLN. This is in contrast to a traditional network that relies on IPv6 address auto-configuration [RFC4862], where there is no guarantee of ownership from the network, and each IPv6 Neighbor Discovery packet must be individually secured [RFC3971].
---+-------- ............
| External Network
|
+-----+
| | 6LBR
+-----+
o o o
o o o o
o o LLN o o o
o o o (6LR)
o (6LN)
Figure 3: Basic Configuration
In a mesh network, the 6LR is directly connected to the host device. This specification mandates that the peer-wise layer-2 security is deployed so that all the packets from a particular host are securely identifiable by the 6LR. The 6LR may be multiple hops away from the 6LBR. Packets are routed between the 6LR and the 6LBR via other 6LRs. This specification mandates that a chain of trust is established so that a packet that was validated by the first 6LR can be safely routed by the next 6LRs to the 6LBR.
The 6LR/6LBR ensures first-come/first-serve by storing the EARO information including the Crypto-ID associated to the node being registered. The node can claim any address as long as it is the first to make such a claim. After a successful registration, the node becomes the owner of the registered address and the address is bound to the Crypto-ID in the 6LR/6LBR registry.
This specification enables the 6LR to verify the ownership of the binding at any time assuming that the "C" flag is set. The verification prevents other nodes from stealing the address and trying to attract traffic for that address or use it as their source address.
A node may use multiple IPv6 addresses at the same time. The node may use a same Crypto-ID, or multiple crypto-IDs derived from a same key pair, to protect multiple IPv6 addresses. The separation of the address and the cryptographic material avoids the constrained device to compute multiple keys for multiple addresses. The registration process allows the node to use the same Crypto-ID for all of its addresses.
A 6LN registers to a 6LR that is one hop away from it with the "C" flag set in the EARO, indicating that the ROVR field contains a Crypto-ID. The on-link (local) protocol interactions are shown in Figure 4 If the 6LR does not have a state with the 6LN that is consistent with the NS(EARO), then it replies with a challenge NA (EARO, status=Validation Requested) that contains a Nonce Option. The Nonce option MUST contain a Nonce value that was never used with this device.
The 6LN replies to the challenge with an NS(EARO) that includes the echoed Nonce option, the CIPO Section 4.4, and the NDPSO with the signature. The information associated to a crypto-ID stored by the 6LR on the first NS exchange where it appears. The 6LR SHOULD store the CIPO parameters associated with the crypto-ID so it can be used for more than one address.
6LN 6LR
| |
|<------------------------- RA -------------------------|
| | ^
|---------------- NS with EARO (Crypto-ID) ------------>| |
| | option
|<- NA with EARO (status=Validation Requested), Nonce --| |
| | v
|-------- NS with EARO, CIPO, Nonce and NDPSO --------->|
| |
|<------------------- NA with EARO ---------------------|
| |
...
| |
|--------------- NS with EARO (Crypto-ID) ------------->|
| |
|<------------------- NA with EARO ---------------------|
| |
...
| |
|--------------- NS with EARO (Crypto-ID) ------------->|
| |
|<------------------- NA with EARO ---------------------|
| |
Figure 4: On-link Protocol Operation
The steps for the registration to the 6LR are as follows:
In a multihop 6LoWPAN, the registration with Crypto-ID is propagated to 6LBR as described in this section. If the 6LR and the 6LBR maintain a security association, then there is no need to propagate the proof of ownership to the 6LBR.
A new device that joins the network auto-configures an address and performs an initial registration to a neighboring 6LR with an NS message that carries an Address Registration Option (EARO) [RFC6775]. The 6LR validates the address with an 6LBR using a DAR/DAC exchange, and the 6LR confirms (or denies) the address ownership with an NA message that also carries an Address Registration Option.
Figure 5 illustrates a registration flow all the way to a 6LowPAN Backbone Router (6BBR).
6LN 6LR 6LBR 6BBR
| | | |
| NS(EARO) | | |
|--------------->| | |
| | Extended DAR | |
| |-------------->| |
| | | |
| | | proxy NS(EARO) |
| | |--------------->|
| | | | NS(DAD)
| | | | ------>
| | | |
| | | | <wait>
| | | |
| | | proxy NA(EARO) |
| | |<---------------|
| | Extended DAC | |
| |<--------------| |
| NA(EARO) | | |
|<---------------| | |
| | | |
Figure 5: (Re-)Registration Flow
In a multihop 6LoWPAN, a 6LBR sends RAs with prefixes downstream and the 6LR receives and relays them to the nodes. 6LR and 6LBR communicate using ICMPv6 Duplicate Address Request (DAR) and Duplicate Address Confirmation (DAC) messages. The DAR and DAC use the same message format as NS and NA, but have different ICMPv6 type values.
In AP-ND we extend DAR/DAC messages to carry cryptographically generated ROVR. In a multihop 6LoWPAN, the node exchanges the messages shown in Figure 5. The 6LBR must identify who owns an address (EUI-64) to defend it, if there is an attacker on another 6LR.
Observations regarding the following threats to the local network in [RFC3971] also apply to this specification.
The threats discussed in 6LoWPAN ND [RFC6775] and its update [I-D.ietf-6lo-rfc6775-update] also apply here. Compared with SeND, this specification saves about 1Kbyte in every NS/NA message. Also, this specification separates the cryptographic identifier from the registered IPv6 address so that a node can have more than one IPv6 address protected by the same cryptographic identifier. SeND forces the IPv6 address to be cryptographic since it integrates the CGA as the IID in the IPv6 address. This specification frees the device to form its addresses in any fashion, thereby enabling not only 6LoWPAN compression which derives IPv6 addresses from Layer-2 addresses but also privacy addresses.
A collision of Registration Ownership Verifiers (ROVR) (i.e., the Crypto-ID in this specification) is possible, but it is a rare event. The formula for calculating the probability of a collision is 1 - e^{-k^2/(2n)} where n is the maximum population size (2^64 here, 1.84E19) and K is the actual population (number of nodes). If the Crypto-ID is 64-bits, the chance of a collision is 0.01% when the network contains 66 million nodes. Moreover, the collision is only relevant when this happens within one stub network (6LBR). In the case of such a collision, an attacker may be able to claim the registered address of an another legitimate node. However for this to happen, the attacker would also need to know the address which was registered by the legitimate node. This registered address is never broadcasted on the network and therefore providing an additional 64-bits that an attacker must correctly guess. To prevent address disclosure, it is RECOMMENDED that nodes derive the address being registered independently of the ROVR.
This document defines a new 128-bit value under the CGA Message Type [RFC3972] namespace, 0x8701 55c8 0cca dd32 6ab7 e415 f148 84d0.
IANA is requested to create a new subregistry "Crypto-Type Subregistry" in the "Internet Control Message Protocol version 6 (ICMPv6) Parameters". The registry is indexed by an integer 0..255 and contains a Signature Algorithm and a Hash Function as shown in Table 1. The following Crypto-Type values are defined in this document:
| Crypto-Type value | Signature Algorithm | Hash Function | Defining Specification |
|---|---|---|---|
| 0 | NIST P-256 [FIPS186-4] | SHA-256 [RFC6234] | RFC THIS |
| 1 | Ed25519ph [RFC8032] | SHA-256 [RFC6234] | RFC THIS |
Assignment of new values for new Crypto-Type MUST be done through IANA with "Specification Required" and "IESG Approval" as defined in [RFC8126].
Many thanks to Charlie Perkins for his in-depth review and constructive suggestions. We are also especially grateful to Rene Struik and Robert Moskowitz for their comments that lead to many improvements to this document, in particular WRT ECC computation and references.
In this section we state requirements of a secure neighbor discovery protocol for low-power and lossy networks.