As illustrated in Figure 1, this work makes use the state machine of IKEv2. Assume a network device and its peer device are in State 1. That is, the device has not authenticated its peer device and does not have the keys to secure the routing protocol packets which it would like to exchange with the peer. Before sending any routing protocol packets, the two devices need to perform a IKE_SA_INIT exchange. If the IKE_SA_INIT exchange succeeds, both network devices are transferred to State 2 where they have agreed upon certain keying material but have decided how to use the material to derive keys to secure routing protocols. To achieve this objective, the two network devices perform an IKE_AUTH exchange, in which both endpoints try to authenticate each other and generate security associations for the routing protocol they intend to support. If the IKE_AUTH exchange succeeds, the network devices transfer their state to State 3 where both endpoints are authenticated and keys for securing the routing protocols are generated. If the endpoints intend to generate new SAs for routing protocols by using the keying material already generated, they can just perform an CREATE_CHILD_SA exchange. A discussion in more details can be found in Section 4.

                 -----------------------
      =======> |   Not Authenticated   |==========
     ||        |   No RP Keys          |          ||
     ||         -----------------------       IKE_SA_INIT exchange
     ||                State 1                    ||
     ||                                           ||
INFORMATIONAL                                     VV
     ||                                 --------------------------
     ||                                 | Privacy Keys Exchanged | 
     ||                                 | No RP Keys             |
     ||                                 --------------------------
     ||                                           ||  State 2
     ||                                           ||   
     ||         |--------------------          IKE_AUTH exchange
       =========| Authenticated     | <============                   
                | RP Keys Derived   | ==== 
                 --------------------   ||
                 State 3 ^^             ||
                         ||        CREATE_CHILD_SA
                         ||             ||
                          =============== 

Figure 1: State Diagram

2.1. Types of Keys

Three types of keys mentioned the discussion of this memo are listed as follows:

• PSK (Pre-Shared Key) : a PSK is a pair-wise unique key, which can be used for securing the routing protocol exchanges or be used for authenticating a network device by a KMP. These keys are configured by some mechanism such as manual configuration or a management application outside of the scope of KMP.
• Protocol master key: A protocol master key is a key exported by a KMP for use by a routing protocol. This is the key that is shared in the KMDB between the routing protocol and KMP. A routing protocol may use a protocol master key directly or derive traffic keys from it.
• Traffic key: A traffic key is the key actually used to protect the integrity of the routing messages exchanged in a routing protocol. In existing cryptographic authentication mechanisms for routing protocols, the traffic key can be the same as or derived from the protocol master key. If there is no KMP provided, a traffic key can be the same as or derived from a pre-shared key.

3. Protocol Exchanges

The KARP analysis in BGP, LDP, PCEP, and MSDP indicates that all of these routing protocols need a dedicated key management protocol [I-D.ietf-karp-routing-tcp-analysis] to confidentially exchange keying material between endpoints. There is no need to define an entirely new protocol for this purpose, when existing mature protocol exchanges and methods have been vetted. This draft makes use of the IKEv2 protocol exchanges, state machine, and policy definitions to define a dedicated key management protocol.

The notations contained in the IKEv2 message are defined as follows.

3.1. IKE_SA_INIT

A network device desiring to negotiate a key and other associated parameters for a pair-wise routing protocol to a peer initiates an IKE_SA_INIT exchange defined in IKEv2 [RFC5996]. The IKE_SA_INIT exchange is a two-message exchange that allows the network devices to negotiate cryptographic algorithms, exchange nonce information, and do a Diffie-Hellman (DH) [DH] exchange, for their routing protocols, after which protocols on these network devices can communicate privately. Note that at the end of a IKE_SA_INIT exchange the endpoints on the both sides have not authenticated each other yet. For the details of this exchange, refer to IKE_SA_INIT in IKEv2 [RFC5996].

 Peer (Initiator)                   Peer (Responder)
 --------------------               ------------------
 HDR, SAi1, KEi, Ni        -->
                           <--      HDR, SAr1, KEr, Nr, [CERTREQ,]

IKE_SA_INIT

Up to this step, this work introduces no change to IKEv2.

3.2. IKE_AUTH

Next, the network devices perform an IKE_AUTH exchange defined in IKEv2 [RFC5996]. The SA payloads contain the security policies for a key and the associated parameters (as defined in Header and Payload Formats [PAYLOAD-FORMATS]), and the TS payloads contains traffic selectors as defined in IKEv2 [RFC5996]. For the details of the exchange please refer to IKE_AUTH in IKEv2 [RFC5996].

Peer (Initiator)                         Peer (Responder)
--------------------                     ------------------
HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2, TSi, TSr}     -->
                                 <--     HDR, SK {IDr, [CERT,] AUTH,
                                         SAr2, TSi, TSr}

IKE_AUTH

3.3. CREATE_CHILD_SA

The network devices may then destroy the state associated with the IKEv2 SA, continuing to use the RP policy and keying material, or they may choose to retain them for further usages. Note that this policy differs from IKEv2/IPsec, where the deletion of the IKEv2 SA necessitates the deletion of the IPsec SAs. If both the network devices choose to retain them, they may use the IKEv2 SA to subsequently agree upon replacement policy for the same RP, or agree upon the policy and keying material for another routing protocol. Either case will require the use of the IKEv2 CREATE_CHILD_SA exchange as defined in IKEv2 [RFC5996].

A CREATE_CHILD_SA exchange therefore can be triggered in order to

1. Rekey an antique RP master key and establish a new equivalent one,
2. Generate needed keying material for a newly executed routing protocol based on an existing SA, or
3. Rekey an IKEv2 SA and establish a new equivalent IKEv2 SA.

Peer (Initiator)                      Peer (Responder)
--------------------                  ------------------
HDR, SK {[N], SA, Ni, [KEi],
[TSi, TSr]}                   -->   
                              <--    HDR, SK {SA, Nr, [KEr],
                                     [TSi, TSr]}

CREATE_CHILD_SA

A CREATE_CHILD_SA exchange MAY be initiated by either end of the SA after the initial exchanges are completed. All messages in a CREATE_CHILD_SA exchange are cryptographically protected using the cryptographic algorithms and keys negotiated in the initial exchange.

For details on the exchange, refer to the CREATE_CHILD_SA exchange as defined in IKEv2 [RFC5996].

3.4. INFORMATIONAL

Peer (Initiator)                   Peer (Responder)
-------------------                ------------------
HDR, SK {[N,] [D,] ... }    -->
                            <--    HDR, SK {[N,] [D,] ... }

INFORMATIONAL

The IKEv2 INFORMATIONAL exchange is also useful for deleting specific IKEv2 SAs or sending status information. The Notify (N) and Delete (D) payloads are as those defined by IKEv2 [IKEV2-PARAMS]. For example, if the Responder refused to accept one of Proposals sent by the Initiator, it would return an INFORMATIONAL exchange of type NO_PROPOSAL_CHOSEN instead of the response to CREATE_CHILD_SA.

4. Operation Details

4.1. General

IKEv2 is used to dynamically derive keying material information between the two network devices trying to establish or maintain a routing protocol neighbor adjacency. Typically network devices running the routing protocols establish neighbor adjacencies at the routing protocol level. These routing protocols may run different security algorithms that provide transport level security for the protocol neighbor adjacencies. Depending on the security algorithm used, the routing protocols are configured with security algorithm specific keys that are either long term keys or short term session keys. These keys are specific to the security algorithms used to enforce transport level security for the routing protocols.

A routing protocol causes IKEv2 to execute when it needs keying material to establish neighbor adjacency. This can be as a result of the routing protocol neighbor being configured, neighbor changed or updated, a local rekey policy decision, or some other event dictated by the implementation. The keying material would allow the network devices to then independently generate the same key and establish an IKEv2 session between them. This is typically done by the Initiator (IKEv2 speaker) initiating an IKEv2 IKE_SA_INIT exchange mentioned in the section 2.1 towards its IKEv2 peer. As part of IKEv2_INIT exchange, IKEv2 will send a message to the peer's IKEv2 port. The format of the message is explained in Section 6. The procedure to exchange key information is explained in Section 6. Once the keying material information is successfully exchanged by both of the IKEv2 speakers, the IKEv2 neighbor adjacency may be torn down or kept around as explained in Section 6.

The master key data received from IKEv2 peers is stored in the separate Key Management Database known as KMDB. KMDB follows the guidelines in Database of Long Lived Symmetric Cryptographic Keys [I-D.ietf-karp-crypto-key-table], and each entry consists of Key specific information, Security algorithm to which the Key is applicable to, Routing Protocol Clients of interest, and the announcing KMP Peer. KMDB is also used to notify the routing protocols about the key updates. Typically keying material information is exchanged whenever a routing protocol is about to create a new neighbor adjacency. This is considered as an Initial Key exchange mode. Keying material information is also exchanged to refresh existing key data on an already existing neighbor adjacency. This is considered as Key rollover exchange mode. The following sections describes their detail behavior.

4.2. Initial Key Specific Data Exchange

Routing protocols informs IKEv2 of its new neighbor adjacency. It does so by creating a local entry in KMDB which consists of a Security algorithm, Key specific information, routing protocol client and the routing protocol neighbor. Upon a successful creation of such an entry IKEv2 initiates KMP peering with the neighbor and starts an initial IKE_SA_INIT exchange explained in Section 3.1 followed by the RP_AUTH exchanged explained in Section 3.2. Once the key related information is successfully exchanged, KMDB may invoke the routing protocol client to provide key specific information updates if any.

4.3. Key Selection, Rollover and Protocol Interaction

A routing protocol may need to perform the key selection and rollover in cooperation with KMDB. Such a procedure is described in Section 3 of Database of Long-Lived Symmetric Cryptographic Keys [I-D.ietf-karp-crypto-key-table]. Details of how RP interact with KMDB and deals with multiple keys during rollover are also described in that section. When a routing protocol uses TCP-AO to secure its message exchanges, conditions could be a little more complex. Typically, a TCP-AO implementation has its own key tables. TCP-AO may only carry out key management operations on the key tables if the key information maintained in KDMB needs not to be updated. In [I-D.chunduri-karp-using-ikev2-with-tcp-ao], a Gatekeeper (GK) mechanism is provided to orchestrate the key management operations on the TCP-AO key tables and KMDB.

5. Key Management Database

Protocol interaction between KMP and its client routing protocols is typically done using KMDB. Routing protocols may be able to update KMDB by performing key selection and rollover operations. During a key selection, if there is no appropriate key found in the conceptual database, as a part of the KMDB update, IKEv2 is initiated to connect with its appropriate IKEv2 peer so as to generate a new key. When a key needs to be revoked, it is also the responsibility of IKEv2 to inform its peer to guarantee the synchronization of the databases on the both sides. In addition, when a key is obsoleted for some reasons when it is being used by a client routing protocol, the routing protocol may need to be informed of this update. For the routing protocols which using TCP-AO to secure their message exchanges, a Gatekeeper mechanism is provided to trigger the update of keys and manage the key revocation [I-D.chunduri-karp-using-ikev2-with-tcp-ao].

6. Header and Payload Formats

The protocol defined in this memo uses IKEv2 payload definitions. However, new security policy definitions are described to support security transforms and policy defined by routing protocol documents.

6.1. Header and Payload Formats for TCP-AO

6.1.1. Security Association Payload for TCP-AO

The TCP Authentication Option (TCP-AO) [RFC5925] is primarily intended for BGP and other TCP-based routing protocols. In order for IKEv2 to negotiate TCP-AO policy, a new Security Protocol Identifier needs to be defined in the IANA registry for "IKEv2 Security Protocol Identifiers" Magic Numbers' for ISAKMP Protocol [IKEV2-PROTOCOL-IDS]. This memo proposes adding a new Protocol Identifier to the table, with a Protocol Name of "TCP_AO" and a value of 6.

The Security Association (SA) payload contains a list of Proposals, which describe one or more sets of policies that a network device is willing to use to protect a routing protocol. In the Initiator's message, the SAi2 payload contains a list of Proposal payloads (as defined in the next sections), each of which contains a single set of policy that can be applied to the packets described in the Traffic Selector (TS) payloads in the same exchange. Each set of policy is given a particular "Proposal Number" uniquely identifying this set of policy.

The responder includes a single Proposal payload in it's SA policy, which denotes the choice it has made amongst the initiator's list of Proposals. Any attributes of a selected transform MUST be returned unmodified as explained in IKEv2 [RFC5996] section 3.3.6. The initiator of an exchange MUST check that the accepted offer is consistent with one of its proposals, and if not MUST terminate the exchange.

6.1.1.1. Transforms Substructures for TCP-AO

Each Proposal has a list of Transform (T) substructures, each of which describe a particular set of cryptographic policy choices. A TCP-AO proposal uses the INTEG transform to negotiate the MKT Message Authentication Code (MAC) algorithm. Cryptographic Algorithms for TCP-AO [RFC5926] describes HMAC-SHA-1-96, AES-128-CMAC-96, which map to the existing INTEG transform IDs of AUTH_HMAC_SHA1_96 and AUTH_AES_CMAC_96 respectively. The use of each INTEG algorithm implies the use of a specific KDF (deriving session keys from a master key), and so the choice of a particular INTEG transform ID also specifies the required KDF transform. This will be true for every transform ID used with TCP-AO, as required in RFC 5926 (see Section 3.2 where the "KDF_Alg" is a fixed element of a MAC algorithm definition for TCP-AO).

+-------+---------------------------------+
|Number |            Name                 |
+-------+---------------------------------+
|  0    |Options Not Integrity Protected  |
|  1    |Options Integrity Protected      |
+-------+---------------------------------

Protocol    Mandatory Types          Optional Types
---------------------------------------------------
TCP-AO      INTEG, TCP               D-H

   SA Payload
      |
      +--- Proposal #1 ( Proto ID = TCP-AO(T6), SPI size = 1,
            |            4 transforms,      SPI = 0x01 )
            |
            +-- Transform INTEG ( Name = AUTH_HMAC_SHA1_96 )
            +-- Transform INTEG ( Name = AUTH_AES_CMAC_96 )
            +-- Transform TCP ( Name = PROTECT_OPTIONS )
            +-- Transform TCP ( Name = NO_PROTECT_OPTIONS )

   SA Payload
      |
      +--- Proposal #1 ( Proto ID = TCP-AO(6), SPI size = 1,
            |            2 transforms,      SPI = 0x11 )
            |
            +-- Transform INTEG ( Name = AUTH_HMAC_SHA1_96 )
            +-- Transform TCP ( Name = PROTECT_OPTIONS )

+-------+---------------------------------+
|Number |            Name                 |
+-------+---------------------------------+
|  0    |Base Authentication              |
|  1    |Base Meticulous Authentication   |
|  2    |Generic Authentication           |
|  3    |Generic Meticulous Authentication|
+-------+---------------------------------+

Protocol    Mandatory Types          Optional Types
---------------------------------------------------
BFD         BFD, INTEG                 D-H

+-------+---------------------------------+
|Number |            Name                 |
+-------+---------------------------------+
|  0    |Not Allowed                      |
|  1    |Allowed                          |
+-------+---------------------------------+

Protocol    Mandatory Types          Optional Types
---------------------------------------------------
RSVP-TE     RSVP-TE,               D-H