Supporting Authentication Trailer for OSPFv3

Abstract

Currently, OSPF for IPv6 (OSPFv3) uses IPsec as the only mechanism for authenticating protocol packets. This behavior is different from authentication mechanisms present in other routing protocols (OSPFv2, Intermediate System to Intermediate System (IS-IS), RIP, and Routing Information Protocol Next Generation (RIPng)). In some environments, it has been found that IPsec is difficult to configure and maintain and thus cannot be used. This document defines an alternative mechanism to authenticate OSPFv3 protocol packets so that OSPFv3 does not depend only upon IPsec for authentication.

The OSPFv3 Authentication Trailer was originally defined in RFC 6506. This document obsoletes RFC 6506 by providing a revised definition, including clarifications and refinements of the procedures.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on Internet Standards is available in 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/rfc7166.

Copyright Notice

Copyright © 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
      1.1. Requirements ...............................................4
      1.2. Summary of Changes from RFC 6506 ...........................4
   2. Proposed Solution ...............................................5
      2.1. AT-Bit in Options Field ....................................5
      2.2. Basic Operation ............................................6
      2.3. IPv6 Source Address Protection .............................6
   3. OSPFv3 Security Association .....................................7
   4. Authentication Procedure ........................................9
      4.1. Authentication Trailer .....................................9
           4.1.1. Sequence Number Wrap ...............................11
      4.2. OSPFv3 Header Checksum and LLS Data Block Checksum ........11
      4.3. Cryptographic Authentication Procedure ....................12
      4.4. Cross-Protocol Attack Mitigation ..........................12
      4.5. Cryptographic Aspects .....................................12
      4.6. Message Verification ......................................15
   5. Migration and Backward Compatibility ...........................16
   6. Security Considerations ........................................17
   7. IANA Considerations ............................................18
   8. References .....................................................19
      8.1. Normative References ......................................19
      8.2. Informative References ....................................19
   Appendix A. Acknowledgments .......................................22

1. Introduction

Unlike Open Shortest Path First version 2 (OSPFv2) [RFC2328], OSPF for IPv6 (OSPFv3) [RFC5340] does not include the AuType and Authentication fields in its headers for authenticating protocol packets. Instead, OSPFv3 relies on the IPsec protocols Authentication Header (AH) [RFC4302] and Encapsulating Security Payload (ESP) [RFC4303] to provide integrity, authentication, and/or confidentiality.

[RFC4552] describes how IPv6 AH and ESP extension headers can be used to provide authentication and/or confidentiality to OSPFv3.

However, there are some environments, e.g., Mobile Ad Hoc Networks (MANETs), where IPsec is difficult to configure and maintain; this mechanism cannot be used in such environments.

[RFC4552] discusses, at length, the reasoning behind using manually configured keys, rather than some automated key management protocol such as Internet Key Exchange version 2 (IKEv2) [RFC5996]. The primary problem is the lack of a suitable key management mechanism, as OSPFv3 adjacencies are formed on a one-to-many basis and most key management mechanisms are designed for a one-to-one communication model. This forces the system administrator to use manually configured Security Associations (SAs) and cryptographic keys to provide the authentication and, if desired, confidentiality services.

Regarding replay protection, [RFC4552] states that:

Since it is not possible using the current standards to provide complete replay protection while using manual keying, the proposed solution will not provide protection against replay attacks.

Since there is no replay protection provided, there are a number of vulnerabilities in OSPFv3 that have been discussed in [RFC6039].

While techniques exist to identify ESP-NULL packets [RFC5879], these techniques are generally not implemented in the data planes of OSPFv3 routers. This makes it very difficult for implementations to examine OSPFv3 packets and prioritize certain OSPFv3 packet types, e.g., Hello packets, over the other types.

This document defines a mechanism that works similarly to OSPFv2 [RFC5709] to provide authentication to OSPFv3 packets and solves the problems related to replay protection and deterministically disambiguating different OSPFv3 packets as described above.

This document adds support for the Secure Hash Algorithms (SHAs) defined in the US NIST Secure Hash Standard (SHS), which is specified by NIST FIPS 180-4. [FIPS-180-4] includes SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512. The Hashed Message Authentication Code (HMAC) authentication mode defined in NIST FIPS 198-1 [FIPS-198-1] is used.

1.1. Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Summary of Changes from RFC 6506

This document includes the following changes from RFC 6506 [RFC6506]:

1. Sections 2.2 and 4.2 explicitly state that the Link-Local Signaling (LLS) block checksum calculation is omitted when an OSPFv3 Authentication Trailer is used for OSPFv3 authentication. The LLS data block is included in the authentication digest calculation, and computation of a checksum is unnecessary. Clarification of this issue was documented in an erratum.

2. Section 3 previously recommended usage of an expired key for transmitted OSPFv3 packets when no valid keys existed. This statement has been removed.

3. Section 4.5 includes a correction to the key preparation to use the Protocol-Specific Authentication Key (Ks) rather than the Authentication Key (K) as the initial key (Ko). This problem was also documented in an erratum.

4. Section 4.5 also includes a discussion of the choice of key length to be the hash length (L) rather than the block size (B). The discussion of this choice was included to clarify an issue raised in a rejected erratum.

5. Sections 4.1 and 4.6 indicate that sequence number checking is dependent on OSPFv3 packet type in order to account for packet prioritization as specified in [RFC4222]. This was an omission from RFC 6506 [RFC6506].

6. Section 4.6 explicitly states that OSPFv3 packets with a nonexistent or expired Security Association (SA) will be dropped.

7. Section 5 includes guidance on the precise actions required for an OSPFv3 router providing a backward-compatible transition mode.

2. Proposed Solution

To perform non-IPsec Cryptographic Authentication, OSPFv3 routers append a special data block, henceforth referred to as the Authentication Trailer, to the end of the OSPFv3 packets. The length of the Authentication Trailer is not included in the length of the OSPFv3 packet but is included in the IPv6 payload length, as shown in Figure 1.

    +---------------------+ --              --  +----------------------+
    | IPv6 Payload Length | ^               ^   | IPv6 Payload Length  |
    | PL = OL + LL        | |               |   | PL = OL + LL + AL    |
    |                     | v               v   |                      |
    +---------------------+ --              --  +----------------------+
    | OSPFv3 Header       | ^               ^   | OSPFv3 Header        |
    | Length = OL         | |               |   | Length = OL          |
    |                     | |    OSPFv3     |   |                      |
    |.....................| |    Packet     |   |......................|
    |                     | |    Length     |   |                      |
    | OSPFv3 Packet       | |               |   | OSPFv3 Packet        |
    |                     | v               v   |                      |
    +---------------------+ --              --  +----------------------+
    |                     | ^               ^   |                      |
    | Optional LLS        | |    LLS Data   |   | Optional LLS         |
    | LL = LLS Data       | |    Block      |   | LL = LLS Data        |
    |      Block Length   | v    Length     v   |      Block Length    |
    +---------------------+ --              --  +----------------------+
    
                                            ^   |                      |
                       AL = PL - (OL + LL)  |   | Authentication       |
                                            |   | AL = Fixed Trailer + |
                                            v   |      Digest Length   |
                                            --  +----------------------+

Figure 1: Authentication Trailer in OSPFv3

The presence of the Link-Local Signaling (LLS) [RFC5613] block is determined by the L-bit setting in the OSPFv3 Options field in OSPFv3 Hello and Database Description packets. If present, the LLS data block is included along with the OSPFv3 packet in the Cryptographic Authentication computation.

2.1. AT-Bit in Options Field

RFC 6506 introduced the AT-bit ("AT" stands for "Authentication Trailer") into the OSPFv3 Options field. OSPFv3 routers MUST set the AT-bit in OSPFv3 Hello and Database Description packets to indicate that all the packets on this link will include an Authentication Trailer. For OSPFv3 Hello and Database Description packets, the

AT-bit indicates that the AT is present. For other OSPFv3 packet types, the OSPFv3 AT-bit setting from the OSPFv3 Hello/Database Description setting is preserved in the OSPFv3 neighbor data structure. OSPFv3 packet types that don't include an OSPFv3 Options field will use the setting from the neighbor data structure to determine whether or not the AT is expected.

            0                   1                      2
            0 1 2 3 4 5 6 7 8 9 0 1 2 3  4 5  6 7 8  9 0 1  2 3
           +-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+--+-+-+--+-+-+--+-+--+
           | | | | | | | | | | | | | |AT|L|AF|*|*|DC|R|N|MC|E|V6|
           +-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+--+-+-+--+-+-+--+-+--+

Figure 2: OSPFv3 Options Field

The AT-bit, as shown in the figure above, MUST be set in all OSPFv3 Hello and Database Description packets that contain an Authentication Trailer.

2.2. Basic Operation

The procedure followed for computing the Authentication Trailer is much the same as those described in [RFC5709] and [RFC2328]. One difference is that the LLS data block, if present, is included in the Cryptographic Authentication computation.

The way the authentication data is carried in the Authentication Trailer is very similar to how it is done in the case of [RFC2328]. The only difference between the OSPFv2 Authentication Trailer and the OSPFv3 Authentication Trailer is that information in addition to the message digest is included. The additional information in the OSPFv3 Authentication Trailer is included in the message digest computation and is therefore protected by OSPFv3 Cryptographic Authentication as described herein.

Consistent with OSPFv2 Cryptographic Authentication [RFC2328] and Link-Local Signaling Cryptographic Authentication [RFC5613], checksum calculation and verification are omitted for both the OSPFv3 header checksum and the LLS data block when the OSPFv3 authentication mechanism described in this specification is used.

2.3. IPv6 Source Address Protection

While OSPFv3 always uses the Router ID to identify OSPFv3 neighbors, the IPv6 source address is learned from OSPFv3 Hello packets and copied into the neighbor data structure [RFC5340]. Hence, OSPFv3 is susceptible to Man-in-the-Middle attacks where the IPv6 source address is modified. To thwart such attacks, the IPv6 source address will be included in the message digest calculation and protected by OSPFv3 authentication. Refer to Section 4.5 for details. This is different than the procedure specified in [RFC5709] but consistent with [MANUAL-KEY].

3. OSPFv3 Security Association

An OSPFv3 Security Association (SA) contains a set of parameters shared between any two legitimate OSPFv3 speakers.

Parameters associated with an OSPFv3 SA are as follows:

• Security Association Identifier (SA ID)

This is a 16-bit unsigned integer used to uniquely identify an OSPFv3 SA, as manually configured by the network operator.

The receiver determines the active SA by looking at the SA ID field in the incoming protocol packet.

The sender, based on the active configuration, selects an SA to use and puts the correct Key ID value associated with the SA in the OSPFv3 protocol packet. If multiple valid and active OSPFv3 SAs exist for a given interface, the sender may use any of those SAs to protect the packet.

Using SA IDs makes changing keys while maintaining protocol operation convenient. Each SA ID specifies two independent parts: the authentication algorithm and the Authentication Key, as explained below.

Normally, an implementation would allow the network operator to configure a set of keys in a key chain, with each key in the chain having a fixed lifetime. The actual operation of these mechanisms is outside the scope of this document.

Note that each SA ID can indicate a key with a different authentication algorithm. This allows the introduction of new authentication mechanisms without disrupting existing OSPFv3 adjacencies.

• Authentication Algorithm

This signifies the authentication algorithm to be used with this OSPFv3 SA. This information is never sent in clear text over the wire. Because this information is not sent on the wire, the implementer chooses an implementation-specific representation for this information.

Currently, the following algorithms are supported:

• HMAC-SHA-1,

• HMAC-SHA-256,

      *  HMAC-SHA-384, and

• HMAC-SHA-512.

• Authentication Key

This value denotes the Cryptographic Authentication Key associated with this OSPFv3 SA. The length of this key is variable and depends upon the authentication algorithm specified by the OSPFv3 SA.

• KeyStartAccept

This value indicates the time that this OSPFv3 router will accept packets that have been created with this OSPFv3 SA.

• KeyStartGenerate

This value indicates the time that this OSPFv3 router will begin using this OSPFv3 SA for OSPFv3 packet generation.

• KeyStopGenerate

This value indicates the time that this OSPFv3 router will stop using this OSPFv3 SA for OSPFv3 packet generation.

• KeyStopAccept

This value indicates the time that this OSPFv3 router will stop accepting packets generated with this OSPFv3 SA.

In order to achieve smooth key transition, KeyStartAccept SHOULD be less than KeyStartGenerate, and KeyStopGenerate SHOULD be less than KeyStopAccept. If KeyStartGenerate or KeyStartAccept is left unspecified, the time will default to 0, and the key will be used immediately. If KeyStopGenerate or KeyStopAccept is left unspecified, the time will default to infinity, and the key's lifetime will be infinite. When a new key replaces an old key, the KeyStartGenerate time for the new key MUST be less than or equal to the KeyStopGenerate time of the old key.

Key storage SHOULD persist across a system restart, warm or cold, to avoid operational issues. In the event that the last key associated with an interface expires, the network operator SHOULD be notified, and the OSPFv3 packet MUST NOT be transmitted unauthenticated.

4. Authentication Procedure

4.1. Authentication Trailer

The Authentication Trailer that is appended to the OSPFv3 protocol packet is described below:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Authentication Type      |        Auth Data Len          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Reserved            |   Security Association ID     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Cryptographic Sequence Number (High-Order 32 Bits)  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Cryptographic Sequence Number (Low-Order 32 Bits)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                Authentication Data (Variable)                 |
    ~                                                               ~
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Authentication Trailer Format

The various fields in the Authentication Trailer are as follows:

• Authentication Type

This 16-bit field identifies the type of authentication. The following values are defined in this specification:

0 - Reserved.
1 - HMAC Cryptographic Authentication as described herein.

• Auth Data Len

This is the length in octets of the Authentication Trailer (AT), including both the 16-octet fixed header and the variable-length message digest.

• Reserved

This field is reserved. It SHOULD be set to 0 when sending protocol packets and MUST be ignored when receiving protocol packets.

• Security Association Identifier (SA ID)

This 16-bit field maps to the authentication algorithm and the secret key used to create the message digest appended to the OSPFv3 protocol packet.

Though the SA ID implies the algorithm, the HMAC output size should not be used by implementers as an implicit hint, because additional algorithms may be defined in the future that have the same output size.

• Cryptographic Sequence Number

This is a 64-bit strictly increasing sequence number that is used to guard against replay attacks. The 64-bit sequence number MUST be incremented for every OSPFv3 packet sent by the OSPFv3 router. Upon reception, the sequence number MUST be greater than the sequence number in the last accepted OSPFv3 packet of the same OSPFv3 packet type from the sending OSPFv3 neighbor. Otherwise, the OSPFv3 packet is considered a replayed packet and dropped. OSPFv3 packets of different types may arrive out of order if they are prioritized as recommended in [RFC4222].

OSPFv3 routers implementing this specification MUST use available mechanisms to preserve the sequence number's strictly increasing property for the deployed life of the OSPFv3 router (including cold restarts). One mechanism for accomplishing this would be to use the high-order 32 bits of the sequence number as a wrap/boot count that is incremented anytime the OSPFv3 router loses its sequence number state. Sequence number wrap is described in Section 4.1.1.

• Authentication Data

This field contains variable data that is carrying the digest for the protocol packet and optional LLS data block.

4.1.1. Sequence Number Wrap

When incrementing the sequence number for each transmitted OSPFv3 packet, the sequence number should be treated as an unsigned 64-bit value. If the lower-order 32-bit value wraps, the higher-order 32-bit value should be incremented and saved in non-volatile storage. If by some chance the OSPFv3 router is deployed long enough that there is a possibility that the 64-bit sequence number may wrap, all keys, independent of their key distribution mechanism, MUST be reset to avoid the possibility of replay attacks. Once the keys have been changed, the higher-order sequence number can be reset to 0 and saved to non-volatile storage.

4.2. OSPFv3 Header Checksum and LLS Data Block Checksum

Both the checksum calculation and verification are omitted for the OSPFv3 header checksum and the LLS data block checksum [RFC5613] when the OSPFv3 authentication mechanism described in this specification is used. This implies the following:

• For OSPFv3 packets to be transmitted, the OSPFv3 header checksum computation is omitted, and the OSPFv3 header checksum SHOULD be set to 0 prior to computation of the OSPFv3 Authentication Trailer message digest.

• For OSPFv3 packets including an LLS data block to be transmitted, the OSPFv3 LLS data block checksum computation is omitted, and the OSPFv3 LLS data block checksum SHOULD be set to 0 prior to computation of the OSPFv3 Authentication Trailer message digest.

• For received OSPFv3 packets including an OSPFv3 Authentication Trailer, OSPFv3 header checksum verification MUST be omitted. However, if the OSPFv3 packet does include a non-zero OSPFv3 header checksum, it will not be modified by the receiver and will simply be included in the OSPFv3 Authentication Trailer message digest verification.

• For received OSPFv3 packets including an LLS data block and OSPFv3 Authentication Trailer, LLS data block checksum verification MUST be omitted. However, if the OSPFv3 packet does include an LLS data block with a non-zero checksum, it will not be modified by the receiver and will simply be included in the OSPFv3 Authentication Trailer message digest verification.

4.3. Cryptographic Authentication Procedure

As noted earlier, the SA ID maps to the authentication algorithm and the secret key used to generate and verify the message digest. This specification discusses the computation of OSPFv3 Cryptographic Authentication data when any of the NIST SHS family of algorithms is used in the Hashed Message Authentication Code (HMAC) mode.

The currently valid algorithms (including mode) for OSPFv3 Cryptographic Authentication include:

• HMAC-SHA-1,

• HMAC-SHA-256,

• HMAC-SHA-384, and

• HMAC-SHA-512.

Of the above, implementations of this specification MUST include support for at least HMAC-SHA-256 and SHOULD include support for HMAC-SHA-1 and MAY also include support for HMAC-SHA-384 and HMAC-SHA-512.

Implementations of this specification MUST use HMAC-SHA-256 as the default authentication algorithm.

4.4. Cross-Protocol Attack Mitigation

In order to prevent cross-protocol replay attacks for protocols sharing common keys, the two-octet OSPFv3 Cryptographic Protocol ID is appended to the Authentication Key prior to use. Other protocols using Cryptographic Authentication as specified herein MUST similarly append their respective Cryptographic Protocol IDs to their keys in this step. Refer to the IANA Considerations (Section 7).

4.5. Cryptographic Aspects

In the algorithm description below, the following nomenclature, which is consistent with [FIPS-198-1], is used:

H is the specific hashing algorithm (e.g., SHA-256).

K is the Authentication Key from the OSPFv3 Security Association.

Ks is a Protocol-Specific Authentication Key obtained by appending Authentication Key (K) with the two-octet OSPFv3 Cryptographic Protocol ID.

Ko is the cryptographic key used with the hash algorithm.

B is the block size of H, measured in octets rather than bits. Note that B is the internal block size, not the hash size.

      For SHA-1 and SHA-256: B == 64
      
      For SHA-384 and SHA-512: B == 128

L is the length of the hash, measured in octets rather than bits.

XOR is the exclusive-or operation.

Opad is the hexadecimal value 0x5c repeated B times.

Ipad is the hexadecimal value 0x36 repeated B times.

Apad is a value that is the same length as the hash output or message digest. The first 16 octets contain the IPv6 source address followed by the hexadecimal value 0x878FE1F3 repeated (L-16)/4 times. This implies that hash output is always a length of at least 16 octets.

1. Preparation of the Key

The OSPFv3 Cryptographic Protocol ID is appended to the Authentication Key (K), yielding a Protocol-Specific Authentication Key (Ks). In this application, Ko is always L octets long. While [RFC2104] supports a key that is up to B octets long, this application uses L as the Ks length consistent with [RFC4822], [RFC5310], and [RFC5709]. According to [FIPS-198-1], Section 3, keys greater than L octets do not significantly increase the function strength. Ks is computed as follows:

If Ks is L octets long, then Ko is equal to Ks. If Ks is more than L octets long, then Ko is set to H(Ks). If Ks is less than L octets long, then Ko is set to the value of Ks, with zeros appended to the end of Ks such that Ko is L octets long.

2. First-Hash

First, the OSPFv3 packet's Authentication Data field in the Authentication Trailer is filled with the value Apad. This is very similar to the appendage described in [RFC2328], Appendix D.4.3, Items (6)(a) and (6)(d)).

Then, a First-Hash, also known as the inner hash, is computed as follows:

         First-Hash = H(Ko XOR Ipad || (OSPFv3 Packet))

When XORing Ko and Ipad, Ko will be padded with zeros to the length of Ipad.

Implementation Note: The First-Hash above includes the Authentication Trailer as well as the OSPFv3 packet as per [RFC2328], Appendix D.4.3, and the LLS data block, if present [RFC5613].

The definition of Apad (above) ensures that it is always the same length as the hash output. This is consistent with RFC 2328. Note that the "(OSPFv3 Packet)" referenced in the First-Hash function above includes both the optional LLS data block and the OSPFv3 Authentication Trailer.

The digest length for SHA-1 is 20 octets; for SHA-256, 32 octets; for SHA-384, 48 octets; and for SHA-512, 64 octets.

3. Second-Hash

Then a Second-Hash, also known as the outer hash, is computed as follows:

         Second-Hash = H(Ko XOR Opad || First-Hash)

When XORing Ko and Opad, Ko will be padded with zeros to the length of Opad.

4. Result

The resulting Second-Hash becomes the authentication data that is sent in the Authentication Trailer of the OSPFv3 packet. The length of the authentication data is always identical to the message digest size of the specific hash function H that is being used.

This also means that the use of hash functions with larger output sizes will also increase the size of the OSPFv3 packet as transmitted on the wire.

Implementation Note: [RFC2328], Appendix D specifies that the Authentication Trailer is not counted in the OSPF packet's own Length field but is included in the packet's IP Length field. Similar to this, the Authentication Trailer is not included in the OSPFv3 header length but is included in the IPv6 header payload length.

4.6. Message Verification

A router would determine that OSPFv3 is using an Authentication Trailer (AT) by examining the AT-bit in the Options field in the OSPFv3 header for Hello and Database Description packets. The specification in the Hello and Database Description options indicates that other OSPFv3 packets will include the Authentication Trailer.

The AT is accessed using the OSPFv3 packet header length to access the data after the OSPFv3 packet and, if an LLS data block [RFC5613] is present, using the LLS data block length to access the data after the LLS data block. The L-bit in the OSPFv3 options in Hello and Database Description packets is examined to determine if an LLS data block is present. If an LLS data block is present (as specified by the L-bit), it is included along with the OSPFv3 Hello or Database Description packet in the Cryptographic Authentication computation.

Due to the placement of the AT following the LLS data block and the fact that the LLS data block is included in the Cryptographic Authentication computation, OSPFv3 routers supporting this specification MUST minimally support examining the L-bit in the OSPFv3 options and using the length in the LLS data block to access the AT. It is RECOMMENDED that OSPFv3 routers supporting this specification fully support OSPFv3 Link-Local Signaling [RFC5613].

If usage of the AT, as specified herein, is configured for an OSPFv3 link, OSPFv3 Hello and Database Description packets with the AT-bit clear in the options will be dropped. All OSPFv3 packet types will be dropped if the AT is configured for the link and the IPv6 header length is less than the amount necessary to include an Authentication Trailer.

The receiving interface's OSPFv3 SA is located using the SA ID in the received AT. If the SA is not found, or if the SA is not valid for reception (i.e., current time < KeyStartAccept or current time >= KeyStopAccept), the OSPFv3 packet is dropped.

If the cryptographic sequence number in the AT is less than or equal to the last sequence number in the last OSPFv3 packet of the same OSPFv3 type successfully received from the neighbor, the OSPFv3 packet MUST be dropped, and an error event SHOULD be logged. OSPFv3 packets of different types may arrive out of order if they are prioritized as recommended in [RFC4222].

Authentication-algorithm-dependent processing needs to be performed, using the algorithm specified by the appropriate OSPFv3 SA for the received packet.

Before an implementation performs any processing, it needs to save the values of the Authentication Data field from the Authentication Trailer appended to the OSPFv3 packet.

It should then set the Authentication Data field with Apad before the authentication data is computed (as described in Section 4.5). The calculated data is compared with the received authentication data in the Authentication Trailer. If the two do not match, the packet MUST be discarded, and an error event SHOULD be logged.

After the OSPFv3 packet has been successfully authenticated, implementations MUST store the 64-bit cryptographic sequence number for each OSPFv3 packet type received from the neighbor. The saved cryptographic sequence numbers will be used for replay checking for subsequent packets received from the neighbor.

5. Migration and Backward Compatibility

All OSPFv3 routers participating on a link SHOULD be migrated to OSPFv3 authentication at the same time. As with OSPFv2 authentication, a mismatch in the SA ID, Authentication Type, or message digest will result in failure to form an adjacency. For multi-access links, communities of OSPFv3 routers could be migrated using different Interface Instance IDs. However, at least one router would need to form adjacencies between both the OSPFv3 routers including and not including the Authentication Trailer. This would result in sub-optimal routing as well as added complexity and is only recommended in cases where authentication is desired on the link and migrating all the routers on the link at the same time isn't feasible.

In support of uninterrupted deployment, an OSPFv3 router implementing this specification MAY implement a transition mode where it includes the Authentication Trailer in transmitted packets but does not verify this information in received packets. This is provided as a transition aid for networks in the process of migrating to the authentication mechanism described in this specification. More specifically:

1. OSPFv3 routers in transition mode will include the OSPFv3 Authentication Trailer in transmitted packets and set the AT-bit in the Options field of transmitted Hello and Database Description packets. OSPFv3 routers receiving these packets and not having authentication configured will ignore the Authentication Trailer and AT-bit.

2. OSPFv3 routers in transition mode will also calculate and set the OSPFv3 header checksum and the LLS data block checksum in transmitted packets so that they will not be dropped by OSPFv3 routers without authentication configured.

3. OSPFv3 routers in transition mode will authenticate received packets that either have the AT-bit set in the Options field for Hello or Database Description packets or are from a neighbor that previously set the AT-bit in the Options field of successfully authenticated Hello and Database Description packets.

4. OSPFv3 routers in transition mode will also accept packets without the Options field AT-bit set in Hello and Database Description packets. These packets will be assumed to be from OSPFv3 routers without authentication configured, and they will not be authenticated. Additionally, the OSPFv3 header checksum and LLS data block checksum will be validated.

6. Security Considerations

This document proposes extensions to OSPFv3 that would make it more secure than OSPFv3 as defined in [RFC5340]. It does not provide confidentiality, as a routing protocol contains information that does not need to be kept secret. It does, however, provide means to authenticate the sender of packets that are of interest. It addresses all the security issues that have been identified in [RFC6039] and [RFC6506].

It should be noted that the authentication method described in this document is not being used to authenticate the specific originator of a packet but rather is being used to confirm that the packet has indeed been issued by a router that has access to the Authentication Key.

Deployments SHOULD use sufficiently long and random values for the Authentication Key so that guessing and other cryptographic attacks on the key are not feasible in their environments. Furthermore, it is RECOMMENDED that Authentication Keys incorporate at least 128 pseudorandom bits to minimize the risk of such attacks. In support of these recommendations, management systems SHOULD support hexadecimal input of Authentication Keys.

Deployments that support a transitionary state but interoperate with routers that do not support this authentication method may be exposed to unauthenticated data during the transition period.

The mechanism described herein is not perfect and does not need to be perfect. Instead, this mechanism represents a significant increase in the effort required for an adversary to successfully attack the OSPFv3 protocol, while not causing undue implementation, deployment, or operational complexity.

Refer to [RFC4552] for additional considerations on manual keying.

7. IANA Considerations

This document obsoletes RFC 6506; thus, IANA has updated the references in existing registries that pointed to RFC 6506 to point to this document. This is the only IANA action requested by this document.

IANA previously allocated the AT-bit (0x000400) in the "OSPFv3 Options (24 bits)" registry as described in Section 2.1.

IANA previously created the "Open Shortest Path First v3 (OSPFv3) Authentication Trailer Options" registry. This registry includes the "OSPFv3 Authentication Types" registry, which defines valid values for the Authentication Type field in the OSPFv3 Authentication Trailer. The registration procedure is Standards Action [RFC5226].

           +-------------+-----------------------------------+
           |Value        | Designation                       |
           +-------------+-----------------------------------+
           | 0           | Reserved                          |
           |             |                                   |
           | 1           | HMAC Cryptographic Authentication |
           |             |                                   |
           | 2-65535     | Unassigned                        |
           +-------------+-----------------------------------+

OSPFv3 Authentication Types

Finally, IANA previously created the "Keying and Authentication for Routing Protocols (KARP) Parameters" registry. This registry includes the "Cryptographic Protocol ID" registry, which provides unique protocol-specific values for cryptographic applications, including but not limited to prevention of cross-protocol replay attacks. Values can be assigned for both native IPv4/IPv6 protocols and UDP/TCP protocols. The registration procedure is Standards Action.

                  +-------------+----------------------+
                  | Value/Range | Designation          |
                  +-------------+----------------------+
                  | 0           | Reserved             |
                  |             |                      |
                  | 1           | OSPFv3               |
                  |             |                      |
                  | 2-65535     | Unassigned           |
                  +-------------+----------------------+

Cryptographic Protocol ID

8. References

8.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
   
   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.
   
   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, October 2009.

8.2. Informative References

[FIPS-180-4]

US National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, March 2012.

[FIPS-198-1]

US National Institute of Standards and Technology, "The Keyed-Hash Message Authentication Code (HMAC)", FIPS PUB 198-1, July 2008.

[MANUAL-KEY]

              Bhatia, M., Hartman, S., and D. Zhang, "Security Extension
              for OSPFv2 when using Manual Key Management", Work in
              Progress, February 2011.
   
   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.
   
   [RFC4222]  Choudhury, G., Ed., "Prioritized Treatment of Specific
              OSPF Version 2 Packets and Congestion Avoidance", BCP 112,
              RFC 4222, October 2005.
   
   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.
   
   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.
   
   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, June 2006.
   
   [RFC4822]  Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
              Authentication", RFC 4822, February 2007.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   
   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, February 2009.
   
   [RFC5613]  Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
              Yeung, "OSPF Link-Local Signaling", RFC 5613, August 2009.
   
   [RFC5879]  Kivinen, T. and D. McDonald, "Heuristics for Detecting
              ESP-NULL Packets", RFC 5879, May 2010.
   
   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.
   
   [RFC6039]  Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
              with Existing Cryptographic Protection Methods for Routing
              Protocols", RFC 6039, October 2010.
   
   [RFC6506]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 6506,
              February 2012.

Appendix A. Acknowledgments

First and foremost, thanks to the US National Institute of Standards and Technology for their work on the SHA [FIPS-180-4] and HMAC [FIPS-198-1].

Thanks also need to go to the authors of the HMAC-SHA authentication RFCs, including [RFC4822], [RFC5310], and [RFC5709]. The basic HMAC-SHA procedures were originally described by Ran Atkinson in [RFC4822].

Also, thanks to Ran Atkinson for help in the analysis of RFC 6506 errata.

Thanks to Srinivasan K L and Marek Karasek for their identification and submission of RFC 6506 errata.

Thanks to Sam Hartman for discussions on replay mitigation and the use of a 64-bit strictly increasing sequence number. Also, thanks to Sam for comments during IETF last call with respect to the OSPFv3 SA and the sharing of keys between protocols.

Thanks to Michael Barnes for numerous comments and strong input on the coverage of LLS by the Authentication Trailer (AT).

Thanks to Marek Karasek for providing the specifics with respect to backward-compatible transition mode.

Thanks to Michael Dubrovskiy and Anton Smirnov for comments on document revisions.

Thanks to Rajesh Shetty for numerous comments, including the suggestion to include an Authentication Type field in the Authentication Trailer for extendibility.

Thanks to Uma Chunduri for suggesting that we may want to protect the IPv6 source address even though OSPFv3 uses the Router ID for neighbor identification.

Thanks to Srinivasan K L, Shraddha H, Alan Davey, Russ White, Stan Ratliff, and Glen Kent for their support and review comments.

Thanks to Alia Atlas for comments made under the purview of the Routing Directorate review.

Thanks to Stephen Farrell for comments during the IESG review. Stephen was also involved in the discussion of cross-protocol attacks.

Thanks to Brian Carpenter for comments made during the Gen-ART review.

Thanks to Victor Kuarsingh for the OPS-DIR review.

Thanks to Brian Weis for the SEC-DIR review.

Authors' Addresses

Manav Bhatia
Alcatel-Lucent
Bangalore
India

   EMail: manav.bhatia@alcatel-lucent.com
   
   Vishwas Manral
   Ionos Corp.
   4100 Moorpark Ave.
   San Jose, CA  95117
   USA

EMail:

          vishwas@ionosnetworks.com
   
   Acee Lindem
   Ericsson
   301 Midenhall Way
   Cary, NC  27513
   USA
   
   EMail: acee.lindem@ericsson.com