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Secure Neighbor Discovery (SEND)
by Lakshmi


Neighbor Discovery Protocol (NDP) is defined in RFCs 2461[7] and 2462[8]. It has specific functions like Neighbor Discovery (ND), Address Auto-configuration, Router Discovery (RD), Neighbor Un-reachability Detection (NUD), Address Resolution, Duplicate Address Detection (DAD), Redirection etc.


The NDP message format is given below.


Cryptographic Generated Addresses (CGA)

In basic CGA, 62 bits are used to store cryptographic hash of a public key.

host ID = HASH62(public_key)

By embedding security parameter, "sec" in the two rightmost bits of 128-bit Ipv6 address, the hash length can be increased to gain strong security.

In this case, the CGA will have the 64 + 20 x Sec rightmost bits of the hash value equal the concatenation of 20 x Sec zero bits and the interface identifier of the address. While comparing, the two rightmost bits and the universal and group bits are ignored.

Hash = HASH(public_key)



The NDP specifications emphasize the use of IPsec to protect NDP messages. But RFCS do not describe the instructions of using IPsec in NDP.

IPsec AH can be used with NDP messages to enhance security. Also, the hosts can verify through AH that Neighbor Advertisements and Router Advertisements do contain proper and accurate information.

IPsec can be used in NDP only through manual configuration of Security Associations (SA) and this can be a tedious or impractical task considering the volume. The main reasons for manual configuration are:

  • SAs can be created only through using the Internet Key Exchange (IKE). But IKE requires a functional IP stack in order to function and this result in a bootstrapping problem.

  • Even if SAs were established, it is not possible to verify the ownership of dynamically generated IP addresses.

    SEND Protocol

    As NDP is used by both hosts and routers, it is more vulnerable to various attacks unless secured. To encounter the threats to NDP, Secure Neighbor Discovery (SEND) protocol is designed. Various protocol options are given below.

  • Cryptographically Generated Addresses (CGA) Option

    The CGA ensures that the sender of an NDP message is the owner of the claimed address. Before claiming an address, each node generates a public-private key pair and the CGA option verifies this key.

    The format of the CGA option is:


  • RSA Signature Option

    The public key signatures maintain the integrity of the messages and authenticate the sender identity. The RSA Signature option protects messages by requiring public-key based signatures attached to every NDP message.

    The format of the RSA Signature option is:

  • Timestamp Option

    The Timestamp option provides replay protection and ensures that unsolicited advertisements and redirects have not been replayed.

    The format of Timestamp option is: Ipv6
  • Nonce Option

    The Nonce option protects messages when used in solicitation-advertisement pairs. It ensures that an advertisement is a fresh response to a solicitation sent earlier by the node.

    The format of nonce option is: Ipv6
  • Certification Path Solicitation

    Authorization is provisioned for both routers and hosts with routers getting certificates from a trust anchor and hosts getting configured to authorize routers. Separate certification path solicitation and advertisement messages are used to know the certification path to the trust anchor. Hosts will send the Certification Path Solicitations. Ipv6

    Routers will send the Certification Path Advertisement messages.


    Threats Countered by SEND