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Internet Engineering Task Force (IETF)
Request for Comments: 6079
Category: Experimental
ISSN: 2070-1721
G. Camarillo
P. Nikander
J. Hautakorpi
A. Keranen
Ericsson
A. Johnston
Avaya
January 2011

HIP BONE: Host Identity Protocol (HIP)

Based Overlay Networking Environment (BONE)

Abstract

This document specifies a framework to build HIP-based (Host Identity Protocol) overlay networks. This framework uses HIP to perform connection management. Other functions, such as data storage and retrieval or overlay maintenance, are implemented using protocols other than HIP. These protocols are loosely referred to as "peer protocols".

Status of This Memo

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.

This document defines an Experimental Protocol for the Internet community. 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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see 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/rfc6079.

Copyright Notice

Copyright © 2011 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
   2. Terminology .....................................................3
   3. Background on HIP ...............................................4
      3.1. ID/Locator Split ...........................................4
           3.1.1. Identifier Format ...................................5
           3.1.2. HIP Base Exchange ...................................5
           3.1.3. Locator Management ..................................6
      3.2. NAT Traversal ..............................................6
      3.3. Security ...................................................7
           3.3.1. DoS Protection ......................................7
           3.3.2. Identifier Assignment and Authentication ............7
           3.3.3. Connection Security .................................9
      3.4. HIP Deployability and Legacy Applications ..................9
   4. Framework Overview .............................................10
   5. The HIP BONE Framework .........................................13
      5.1. Node ID Assignment and Bootstrap ..........................13
      5.2. Overlay Network Identification ............................14
      5.3. Connection Establishment ..................................15
      5.4. Lightweight Message Exchanges .............................15
      5.5. HIP BONE Instantiation ....................................16
   6. Overlay HIP Parameters .........................................17
      6.1. Overlay Identifier ........................................17
      6.2. Overlay TTL ...............................................17
   7. Security Considerations ........................................18
   8. Acknowledgements ...............................................19
   9. IANA Considerations ............................................19
   10. References ....................................................19
      10.1. Normative References .....................................19
      10.2. Informative References ...................................20

1. Introduction

The Host Identity Protocol (HIP) [RFC5201] defines a new name space between the network and transport layers. HIP provides upper layers with mobility, multihoming, NAT (Network Address Translation) traversal, and security functionality. HIP implements the so-called identifier/locator (ID/locator) split, which implies that IP addresses are only used as locators, not as host identifiers. This split makes HIP a suitable protocol to build overlay networks that implement identifier-based overlay routing over IP networks, which in turn implement locator-based routing.

Using HIP-Based Overlay Networking Environment (HIP BONE), as opposed to a peer protocol, to perform connection management in an overlay has a set of advantages. HIP BONE can be used by any peer protocol. This keeps each peer protocol from defining primitives needed for connection management (e.g., primitives to establish connections and to tunnel messages through the overlay) and NAT traversal. Having this functionality at a lower layer allows multiple upper-layer protocols to take advantage of it.

Additionally, having a solution that integrates mobility and multihoming is useful in many scenarios. Peer protocols do not typically specify mobility and multihoming solutions. Combining a peer protocol including NAT traversal with a separate mobility mechanism and a separate multihoming mechanism can easily lead to unexpected (and unpleasant) interactions.

The remainder of this document is organized as follows. Section 3 provides background information on HIP. Section 4 gives an overview of the HIP BONE (HIP-Based Overlay Networking Environment) framework and architecture and Section 5 describes the framework in more detail. Finally, Section 6 introduces new HIP parameters for overlay usage.

2. Terminology

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].

The following terms are used in context of HIP BONEs: 

   Overlay network:  A network built on top of another network.  In case
   
      of HIP BONEs, the underlying network is an IP network and the
      overlay can be, e.g., a peer-to-peer (P2P) network.
   
   Peer protocol:  A protocol used by nodes in an overlay network for
      performing, e.g., data storage and retrieval or overlay
      maintenance.
   
   HIP BONE instance:  A HIP-based overlay network that uses a
      particular peer protocol and is based on the framework presented
      in this document.
   
   Node ID:  A value that uniquely identifies a node in an overlay
      network.  The value is not usually human-friendly.  As an example,
      it may be a hash of a public key.
   
   HIP association:  An IP-layer communications context created using
      the Host Identity Protocol.
   
   Valid locator:  A locator (as defined in [RFC5206]; usually an IP
      address or an address and a port number) at which a host is known
      to be reachable, for example, because there is an active HIP
      association with the host.
   
   Final recipient:  A node is the final recipient of a HIP signaling
      packet if one of its Host Identity Tags (HITs) matches to the
      receiver's HIT in the HIP packet header.

3. Background on HIP

This section provides background on HIP. Given the tutorial nature of this section, readers that are familiar with what HIP provides and how HIP works may want to skip it. All descriptions contain references to the relevant HIP specifications where readers can find detailed explanations on the different topics discussed in this section.

3.1. ID/Locator Split

In an IP network, IP addresses typically serve two roles: they are used as host identifiers and as host locators. IP addresses are locators because a given host's IP address indicates where in the network that host is located. IP networks route based on these locators. Additionally, IP addresses are used to identify remote hosts. The simultaneous use of IP addresses as host identifiers and locators makes mobility and multihoming complicated. For example, when a host opens a TCP connection, the host identifies the remote end of the connection by the remote IP address (plus port). If the remote host changes its IP address, the TCP connection will not survive, since the transport layer identifier of the remote end of the connection has changed.

Mobility solutions such as Mobile IP keep the remote IP address from changing so that it can still be used as an identifier. HIP, on the other hand, uses IP addresses only as locators and defines a new identifier space. This approach is referred to as the ID/locator split and makes the implementation of mobility and multihoming more natural. In the previous example, the TCP connection would be bound to the remote host's identifier, which would not change when the remote hosts moves to a new IP address (i.e., to a new locator). The TCP connection is able to survive locator changes because the remote host's identifier does not change.

3.1.1. Identifier Format

HIP uses 128-bit ORCHIDs (Overlay Routable Cryptographic Hash Identifiers) [RFC4843] as identifiers. ORCHIDs look like IPv6 addresses but cannot collide with regular IPv6 addresses because ORCHID spaces are registered with the IANA. That is, a portion of the IPv6 address space is reserved for ORCHIDs. The ORCHID specification allows the creation of multiple disjoint identifier spaces. Each such space is identified by a separate Context Identifier. The Context Identifier can be either drawn implicitly from the context the ORCHID is used in or carried explicitly in a protocol.

HIP defines a native socket API [HIP-NATIVE-API] that applications can use to establish and manage connections. Additionally, HIP can also be used through the traditional IPv4 and IPv6 TCP/IP socket APIs. Section 3.4 describes how an application using these traditional APIs can make use of HIP. Figure 1 shows all these APIs between the application and the transport layers. 

            +-----------------------------------------+
            |               Application               |
            +----------------+------------------------+
            | HIP Native API | Traditional Socket API |
            +----------------+------------------------+
            |             Transport Layer             |
            +-----------------------------------------+

Figure 1: HIP API

3.1.2. HIP Base Exchange

Typically, before two HIP hosts exchange upper-layer traffic, they perform a four-way handshake that is referred to as the HIP base exchange. Figure 2 illustrates the HIP base exchange. The initiator sends an I1 packet and receives an R1 packet from the responder. After that, the initiator sends an I2 packet and receives an R2 packet from the responder. 

            Initiator                    Responder
            
                |            I1             |
                |-------------------------->|
                |            R1             |
                |<--------------------------|
                |            I2             |
                |-------------------------->|
                |            R2             |
                |<--------------------------|

Figure 2: HIP Base Exchange

Of course, the initiator needs the responder's locator (or locators) in order to send its I1 packet. The initiator can obtain locators for the responder in multiple ways. For example, according to the current HIP specifications the initiator can get the locators directly from the DNS [RFC5205] or indirectly by sending packets through a HIP rendezvous server [RFC5204]. However, HIP is an open- ended architecture. The HIP architecture allows the locators to be obtained by any means (e.g., from packets traversing an overlay network or as part of the candidate address collection process in a NAT traversal scenario).

3.1.3. Locator Management

Once a HIP connection between two hosts has been established with a HIP base exchange, the hosts can start exchanging higher-layer traffic. If any of the hosts changes its set of locators, it runs an update exchange [RFC5206], which consists of three messages. If a host is multihomed, it simply provides more than one locator in its exchanges. However, if both of the endpoints move at the same time, or through some other reason both lose track of the peers' currently active locators, they need to resort to using a rendezvous server or getting new peer locators by some other means.

3.2. NAT Traversal

HIP's NAT traversal mechanism [RFC5770] is based on ICE (Interactive Connectivity Establishment) [RFC5245]. Hosts gather address candidates and, as part of the HIP base exchange, hosts perform an ICE offer/answer exchange where they exchange their respective address candidates. Hosts perform end-to-end STUN-based [RFC5389] connectivity checks in order to discover which address candidate pairs yield connectivity.

Even though, architecturally, HIP lies below the transport layer (i.e., HIP packets are carried directly in IP packets), in the presence of NATs, HIP sometimes needs to be tunneled in a transport protocol (i.e., HIP packets are carried by a transport protocol such as UDP).

3.3. Security

Security is an essential part of HIP. The following sections describe the security-related functionality provided by HIP.

3.3.1. DoS Protection

HIP provides protection against DoS (denial-of-service) attacks by having initiators resolve a cryptographic puzzle before the responder stores any state. On receiving an I1 packet, a responder sends a pre-generated R1 packet that contains a cryptographic puzzle and deletes all the state associated with the processing of this I1 packet. The initiator needs to resolve the puzzle in the R1 packet in order to generate an I2 packet. The difficulty of the puzzle can be adjusted so that, if a receiver is under a DoS attack, it can increase the difficulty of its puzzles.

On receiving an I2 packet, a receiver checks that the solution in the packet corresponds to a puzzle generated by the receiver and that the solution is correct. If it is, the receiver processes the I2 packet. Otherwise, it silently discards it.

In an overlay scenario, there are multiple ways in which this mechanism can be utilized within the overlay. One possibility is to cache the pre-generated R1 packets within the overlay and let the overlay directly respond with R1s to I1s. In that way, the responder is not bothered at all until the initiator sends an I2 packet, with the puzzle solution. Furthermore, a more sophisticated overlay could verify that an I2 packet has a correctly solved puzzle before forwarding the packet to the responder.

3.3.2. Identifier Assignment and Authentication

As discussed earlier, HIP uses ORCHIDs [RFC4843] as the main representation for identifiers. Potentially, HIP can use different types of ORCHIDs as long as the probability of finding collisions (i.e., two nodes with the same ORCHID) is low enough. One way to completely avoid this type of collision is to have a central authority generate and assign ORCHIDs to nodes. To secure the binding between ORCHIDs and any higher-layer identifiers, every time the central authority assigns an ORCHID to a node, it also generates and signs a certificate stating who is the owner of the ORCHID. The owner of the ORCHID then includes the corresponding certificate in its R1 (when acting as responder) and I2 packets (when acting initiator) to prove that it is actually allowed to use the ORCHID and, implicitly, the associated public key.

Having a central authority works well to completely avoid collisions. However, having a central authority is impractical in some scenarios. As defined today, HIP systems generally use a self-certifying ORCHID type called HIT (Host Identity Tag) that does not require a central authority (but still allows one to be used).

A HIT is the hash of a node's public key. A node proves that it has the right to use a HIT by showing its ability to sign data with its associated private key. This scheme is secure due to the so-called second-preimage resistance property of hash functions. That is, given a fixed public key K1, finding a different public key K2 such that hash(K1) = hash(K2) is computationally very hard. Optimally, a preimage attack on the 100-bit hash function used in ORCHIDs will take an order of 2^100 operations to be successful, and can be expected to take in the average 2^99 operations. Given that each operation requires the attacker to generate a new key pair, the attack is fully impractical with current technology and techniques (see [RFC4843]).

HIP nodes using HITs as ORCHIDs do not typically use certificates during their base exchanges. Instead, they use a leap-of-faith mechanism, similar to the Secure Shell (SSH) protocol [RFC4251], whereby a node somehow authenticates remote nodes the first time they connect to it and, then, remembers their public keys. While user- assisted leap-of-faith mechanism (such as in SSH) can be used to facilitate a human-operated offline path (such as a telephone call), automated leap-of-faith mechanisms can be combined with a reputation management system to create an incentive to behave. However, such considerations go well beyond the current HIP architecture and even beyond this proposal. For the purposes of the present document, we merely want to point out that, architecturally, HIP supports both self-generated opportunistic identifiers and administratively assigned ones.

3.3.3. Connection Security

Once two nodes complete a base exchange between them, the traffic they exchange is encrypted and integrity protected. The security mechanism used to protect the traffic is IPsec Encapsulating Security Payload (ESP) [RFC5202]. However, there is ongoing work to specify how to use other protection mechanisms.

3.4. HIP Deployability and Legacy Applications

As discussed earlier, HIP defines a native socket API [HIP-NATIVE- API] that applications can use to establish and manage connections. New applications can implement this API to get full advantage of HIP. However, in most cases, legacy (i.e., non-HIP-aware) applications [RFC5338] can use HIP through the traditional IPv4 and IPv6 socket APIs.

The idea is that when a legacy IPv6 application tries to obtain a remote host's IP address (e.g., by querying the DNS), the DNS resolver passes the remote host's ORCHID (which was also stored in the DNS) to the legacy application. At the same time, the DNS resolver stores the remote host's IP address internally at the HIP module. Since the ORCHID looks like an IPv6 address, the legacy application treats it as such. It opens a connection (e.g., TCP) using the traditional IPv6 socket API. The HIP module running in the same host as the legacy application intercepts this call somehow (e.g., using an interception library or setting up the host's routing tables so that the HIP module receives the traffic) and runs HIP (on behalf of the legacy application) towards the IP address corresponding to the ORCHID. This mechanism works well in almost all cases. However, applications involving referrals (i.e., passing of IPv6 addresses between applications) present issues, which are discussed in Section 5 below. Additionally, management applications that care about the exact IP address format may not work well with such a straightforward approach.

In order to make HIP work through the traditional IPv4 socket API, the HIP module passes an LSI (Local Scope Identifier), instead of a regular IPv4 address, to the legacy IPv4 application. The LSI looks like an IPv4 address, but is locally bound to an ORCHID. That is, when the legacy application uses the LSI in a socket call, the HIP module intercepts it and replaces the LSI with its corresponding ORCHID. Therefore, LSIs always have local scope. They do not have any meaning outside the host running the application. The ORCHID is used on the wire; not the LSI. In the referral case, if it is not possible to rewrite the application level packets to use ORCHIDs instead of LSIs, it may be hard to make IPv4 referrals work in Internet-wide settings. IPv4 LSIs have been successfully used in existing HIP deployments within a single corporate network.

4. Framework Overview

The HIP BONE framework combines HIP with different peer protocols to provide robust and secure overlay network solutions.

Many overlays typically require three types of operations:

o overlay maintenance,
o data storage and retrieval, and
o connection management.

Overlay maintenance operations deal with nodes joining and leaving the overlay and with the maintenance of the overlay's routing tables. Data storage and retrieval operations deal with nodes storing, retrieving, and removing information in or from the overlay. Connection management operations deal with the establishment of connections and the exchange of lightweight messages among the nodes of the overlay, potentially in the presence of NATs.

The HIP BONE framework uses HIP to perform connection management. Data storage and retrieval and overlay maintenance are to be implemented using protocols other than HIP. For lack of a better name, these protocols are referred to as peer protocols.

One way to depict the relationship between the peer protocol and HIP modules is shown in Figure 3. The peer protocol module implements the overlay construction and maintenance features, and possibly storage (if the particular protocol supports such a feature). The HIP module consults the peer protocol's overlay topology part to make routing decisions, and the peer protocol uses HIP for connection management and sending peer protocol messages to other hosts. The HIP BONE API that applications use is a combination of the HIP Native API and traditional socket API (as shown in Figure 1) with any additional API a particular instance implementation provides. 

                       Application
            -------------------------------- HIP BONE API
             +---+   +--------------------+
             |   |   |    Peer Protocol   |
             |   |   +--------+ +---------+
             |   |<->|Topology| |(Storage)|
             |   |   +---------+----------+
             |   |             ^
             |   |             v
             |   +------------------------+
             |                HIP         |
             +----------------------------+

Figure 3: HIP with Peer Protocol

Architecturally, HIP can be considered to create a new thin "waist" layer on top of the IPv4 and IPv6 networks; see Figure 4. The HIP layer itself consists of the HIP signaling protocol and one or more data transport protocols; see Figure 5. The HIP signaling packets and the data transport packets can take different routes. In the HIP BONE scenarios, the HIP signaling packets are typically first routed through the overlay and then directly (if possible), while the data transport packets are typically routed only directly between the endpoints. 

            +--------------------------------------+
            |    Transport (using HITs or LSIs)    |
            +--------------------------------------+
            |                 HIP                  |
            +------------------+-------------------+
            |      IPv4        |       IPv6        |
            +------------------+-------------------+

Figure 4: HIP as a Thin Waist 

            +------------------+-------------------+
            |  HIP signaling   |  data transports  |
            +------------------+-------------------+

Figure 5: HIP Layer Structure

In HIP BONE, the peer protocol creates a new signaling layer on top of HIP. It is used to set up forwarding paths for HIP signaling messages. This is a similar relationship that an IP routing protocol, such as OSPF, has to the IP protocol itself. In the HIP BONE case, the peer protocol plays a role similar to OSPF, and HIP plays a role similar to IP. The ORCHIDs (or, in general, Node IDs if the ORCHID prefix is not used) are used for forwarding HIP packets according to the information in the routing tables. The peer protocols are used to exchange routing information based on Node IDs and to construct the routing tables.

Architecturally, routing tables are located between the peer protocol and HIP, as shown in Figure 6. The peer protocol constructs the routing table and keeps it updated. The HIP layer accesses the routing table in order to make routing decisions. The bootstrap of a HIP BONE overlay does not create circular dependencies between the peer protocol (which needs to use HIP to establish connections with other nodes) and HIP (which needs the peer protocol to know how to route messages to other nodes) for the same reasons as the bootstrap of an IP network does not create circular dependencies between OSPF and IP. The first connections established by the peer protocol are with nodes whose locators are known. HIP establishes those connections as any connection between two HIP nodes where no overlays are present. That is, there is no need for the overlay to provide a rendezvous service for those connections. 

            +--------------------------------------+
            |            Peer protocol             |
            +--------------------------------------+
            |            Routing table             |
            +--------------------------------------+
            |                 HIP                  |
            +--------------------------------------+

Figure 6: Routing Tables

It is possible that different overlays use different routing table formats. For example, the structure of the routing tables of two overlays based on different DHTs (Distributed Hash Tables) may be very different. In order to make routing decisions, the HIP layer needs to convert the routing table generated by the peer protocol into a forwarding table that allows the HIP layer select a next hop for any packet being routed.

In HIP BONE, the HIP usage of public keys and deriving ORCHIDs through a hash function can be utilized at the peer protocol side to better secure routing table maintenance and to protect against chosen-peer-ID attacks.

HIP BONE provides quite a lot of flexibility with regards to how to arrange the different protocols in detail. Figure 7 shows one potential stack structure. 

            +-----------------------+--------------+
            | peer protocols        |     media    |
            +------------------+----+--------------+
            | HIP signaling    |   data transport  |
            |                                      |
            +------------------+-------------------+
            | NAT    | non-NAT |                   |
            |                  |                   |
            |      IPv4        |       IPv6        |
            +------------------+-------------------+

Figure 7: Example HIP BONE Stack Structure

5. The HIP BONE Framework

HIP BONE is a generic framework that allows the use of different peer protocols. A particular HIP BONE instance uses a particular peer protocol. The details on how to implement HIP BONE using a given peer protocol need to be specified in a, so-called, HIP BONE instance specification. Section 5.5 discusses what details need to be specified by HIP BONE instance specifications. For example, the HIP BONE instance specification for RELOAD [P2PSIP-BASE] is specified in [HIP-RELOAD-INSTANCE].

5.1. Node ID Assignment and Bootstrap

Nodes in an overlay are primarily identified by their Node IDs. Overlays typically have an enrollment server that can generate Node IDs, or at least some part of the Node ID, and sign certificates. A certificate generated by an enrollment server authorizes a particular user to use a particular Node ID in a particular overlay. The way users are identified is defined by the peer protocol and HIP BONE instance specification.

The enrollment server of an overlay using HITs derived from public keys as Node IDs could just authorize users to use the public keys and HITs associated to their nodes. Such a Node ID has the same self-certifying property as HITs and the Node ID can also be used in the HIP and legacy APIs as an ORCHID. This works well as long as the enrollment server is the one generating the public/private key pairs for all those nodes. If the enrollment server authorizes users to use HITs that are generated directly by the nodes themselves, the system is open to a type of chosen-peer-ID attack.

If the overlay network or peer protocol has more specific requirements for the Node ID value that prevent using HITs derived from public keys, each host will need a certificate (e.g., in their HIP base exchanges) provided by the enrollment server to prove that they are authorized to use a particular identifier in the overlay. Depending on how the certificates are constructed, they typically also need to contain the host's self-generated public key. Depending on how the Node IDs and public keys are attributed, different scenarios become possible. For example, the Node IDs may be attributed to users, there may be user public key identifiers, and there may be separate host public key identifiers. Authorization certificates can be used to bind the different types of identifiers together.

HITs, as defined in [RFC5201], always start with the ORCHID prefix. Therefore, there are 100 bits left in the HIT for different Node ID values. If an overlay network requires a larger address space, it is also possible to use all the 128 bits of a HIT for addressing peer layer identifiers. The benefit of using ORCHID prefix for Node IDs is that it makes possible to use them with legacy socket APIs, but in this context, most of the applications are assumed to be HIP aware and able to use a more advanced API supporting full 128-bit identifiers. Even larger address spaces could be supported with an additional HIP parameter giving the source and destination Node IDs, but defining such a parameter, if needed, is left for future documents.

Bootstrap issues, such as how to locate an enrollment or a bootstrap server, belong to the peer protocol.

5.2. Overlay Network Identification

It is possible for a HIP host to participate simultaneously in multiple different overlay networks. It is also possible that some HIP traffic is not intended to be forwarded over an overlay. Therefore, a host needs to know to which overlay an incoming HIP message belongs and the outgoing HIP messages need to be labeled as belonging to a certain overlay. This document specifies a new generic HIP parameter (in Section 6.1) for the purpose of directing HIP messages to the right overlay.

In addition, an application using HIP BONE needs to define, either implicitly or explicitly, the overlay to use for communication. Explicit configuration can happen, e.g., by configuring certain local HITs to be bound to certain overlays or by defining the overlay identifier using advanced HIP socket options or other suitable APIs. On the other hand, if no explicit configuration for a HIP association is used, a host may have a configured default overlay where all HIP messages without a valid locator are sent. The specification for how to implement this coordination for locally originated messages is out of scope for this document.

5.3. Connection Establishment

Nodes in an overlay need to establish connections with other nodes in different cases. For example, a node typically has connections to the nodes in its forwarding table. Nodes also need to establish connections with other nodes in order to exchange application-layer messages.

As discussed earlier, HIP uses the base exchange to establish connections. A HIP endpoint (the initiator) initiates a HIP base exchange with a remote endpoint by sending an I1 packet. The initiator sends the I1 packet to the remote endpoint's locator. Initiators that do not have any locator for the remote endpoint need to use a rendezvous service. Traditionally, a HIP rendezvous server [RFC5204] has provided such a rendezvous service. In HIP BONE, the overlay itself provides the rendezvous service.

Therefore, in HIP BONE, a node uses an I1 packet (as usual) to establish a connection with another node in the overlay. Nodes in the overlay forward I1 packets in a hop-by-hop fashion according to the overlay's routing table towards its destination. This way, the overlay provides a rendezvous service between the nodes establishing the connection. If the overlay nodes have active connections with other nodes in their forwarding tables and if those connections are protected (typically with IPsec ESP), I1 packets may be sent over protected connections between nodes. Alternatively, if there is no such an active connection but the node forwarding the I1 packet has a valid locator for the next hop, the I1 packets may be forwarded directly, in a similar fashion to how I1 packets are today forwarded by a HIP rendezvous server.

Since HIP supports NAT traversal, a HIP base exchange over the overlay will perform an ICE [RFC5245] offer/answer exchange between the nodes that are establishing the connection. In order to perform this exchange, the nodes need to first gather candidate addresses. Which nodes can be used to obtain reflexive address candidates and which ones can be used to obtain relayed candidates is defined by the peer protocol.

5.4. Lightweight Message Exchanges

In some cases, nodes need to perform a lightweight query to another node (e.g., a request followed by a single response). In this situation, establishing a connection using the mechanisms in Section 5.3 for a simple query would be an overkill. A better solution is to forward a HIP message through the overlay with the query and another one with the response to the query. The payload of such HIP packets is integrity protected [RFC6078].

Nodes in the overlay forward this HIP packet in a hop-by-hop fashion according to the overlay's routing table towards its destination, typically through the protected connections established between them. Again, the overlay acts as a rendezvous server between the nodes exchanging the messages.

5.5. HIP BONE Instantiation

As discussed in Section 5, HIP BONE is a generic framework that allows using different peer protocols. A particular HIP BONE instance uses a particular peer protocol. The details on how to implement a HIP BONE using a given peer protocol need to be specified in a, so-called, HIP BONE instance specification. A HIP BONE instance specification needs to define, minimally:

o the peer protocol to be used,
o what kind of Node IDs are used and how they are derived,
o which peer protocol primitives trigger HIP messages, and
o how the overlay identifier is generated.

Additionally, a HIP BONE instance specification may need to specify other details that are specific to the peer protocol used. 

   As an example, the HIP BONE instance specification for RELOAD
   [P2PSIP-BASE] is specified in [HIP-RELOAD-INSTANCE].

The areas not covered by a particular HIP BONE instance specification are specified by the peer protocol or elsewhere. These areas include:

o the algorithm to create the overlay (e.g., a DHT),
o overlay maintenance functions,
o data storage and retrieval functions,
o the process for obtaining a Node ID,
o bootstrap function, and
o how to select STUN and TURN servers for the candidate address

collection process in NAT traversal scenarios.

Note that the border between a HIP BONE instance specification and a peer protocol specifications is fuzzy. Depending on how generic the specification of a given peer protocol is, its associated HIP BONE instance specification may need to specify more or less details. Also, a HIP BONE instance specification may leave certain areas unspecified in order to leave their configuration up to each particular overlay.

6. Overlay HIP Parameters

This section defines the generic format and protocol behavior for the Overlay Identifier and Overlay Time-to-Live (TTL) HIP parameters that can be used in HIP based overlay networks. HIP BONE instance specifications define the exact format and content of the Overlay Identifier parameter, the cases when the Overlay TTL parameter should be used, and any additional behavior for each packet.

6.1. Overlay Identifier

To identify to which overlay network a HIP message belongs, all HIP messages that are sent via an overlay, or as a part of operations specific to a certain overlay, MUST contain an OVERLAY_ID parameter with the identifier of the corresponding overlay network. Instance specifications define how the identifier is generated for different types of overlay networks. The generation mechanism MUST be such that it is unlikely to generate the same identifier for two different overlay instances and any other means possible for preventing collisions SHOULD be used.

The generic format of the OVERLAY_ID parameter is shown in Figure 8. Instance specifications define valid length for the parameter and how the identifier values are generated. 

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Identifier                          /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                                               |    Padding    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      
      Type        4592
      Length      Length of the Identifier, in octets
      Identifier  The identifier value
      Padding     0-7 bytes of padding if needed

Figure 8: Format of the OVERLAY_ID Parameter

6.2. Overlay TTL

HIP packets sent in an overlay network MAY contain an Overlay Time- to-live (OVERLAY_TTL) parameter whose TTL value is decremented on each overlay network hop. When a HIP host receives a HIP packet with an OVERLAY_TTL parameter, and the host is not the final recipient of the packet, it MUST decrement the TTL value in the parameter by one before forwarding the packet.

If the TTL value in a received HIP packet is zero, and the receiving host is not the final recipient, the packet MUST be dropped and the host SHOULD send HIP Notify packet with NOTIFICATION error type OVERLAY_TTL_EXCEEDED (value 70) to the sender of the original HIP packet.

The Notification Data field for the OVERLAY_TTL_EXCEEDED notifications SHOULD contain the HIP header and the TRANSACTION_ID [RFC6078] parameter (if one exists) of the packet whose TTL was exceeded.

Figure 9 shows the format of the OVERLAY_TTL parameter. The TTL value is given as the number of overlay hops this packet has left and it is encoded as an unsigned integer using network byte order. 

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             TTL               |            Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      
      Type        64011
      Length      4
      TTL         The Time-to-Live value
      Reserved    Reserved for future use

Figure 9: Format of the OVERLAY_TTL Parameter

The type of the OVERLAY_TTL parameter is critical (as defined in Section 5.2.1 of [RFC5201]) and therefore all the HIP nodes forwarding a packet with this parameter MUST support it. If the parameter is used in a scenario where the final recipient does not support the parameter, the parameter SHOULD be removed before forwarding the packet to the final recipient.

7. Security Considerations

This document provides a high-level framework to build HIP-based overlays. The security properties of HIP and its extensions used in this framework are discussed in their respective specifications. Those security properties can be affected by the way HIP is used in a particular overlay. However, those properties are mostly affected by the design decisions made to build a particular overlay; not so much by the high-level framework specified in this document. Such design decisions are typically documented in a HIP BONE instance specification, which describes the security properties of the resulting overlay.

8. Acknowledgements

HIP BONE is based on ideas coming from conversations and discussions with a number of people in the HIP and P2PSIP communities. In particular, Philip Matthews, Eric Cooper, Joakim Koskela, Thomas Henderson, Bruce Lowekamp, and Miika Komu provided useful input on HIP BONE.

9. IANA Considerations

This section is to be interpreted according to [RFC5226].

This document updates the IANA Registry for HIP Parameter Types [RFC5201] by assigning HIP Parameter Type values for the new HIP Parameters OVERLAY_ID (defined in Section 6.1) and OVERLAY_TTL (defined in Section 6.2). This document also defines a new HIP Notify Message Type [RFC5201], OVERLAY_TTL_EXCEEDED in Section 6.2. This value is allocated in the error range.

10. References

10.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC4843]  Nikander, P., Laganier, J., and F. Dupont, "An IPv6 Prefix
              for Overlay Routable Cryptographic Hash Identifiers
              (ORCHID)", RFC 4843, April 2007.
   
   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
              Henderson, "Host Identity Protocol", RFC 5201, April 2008.
   
   [RFC5202]  Jokela, P., Moskowitz, R., and P. Nikander, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 5202, April 2008.
   
   [RFC5770]  Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
              Keranen, Ed., "Basic Host Identity Protocol (HIP)
              Extensions for Traversal of Network Address Translators",
              RFC 5770, April 2010.
   
   [RFC6078]  Camarillo, G. and J. Melen, "Host Identity Protocol (HIP)
              Immediate Carriage and Conveyance of Upper-Layer Protocol
              Signaling (HICCUPS)", RFC 6078, January 2011.

10.2. Informative References

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.
   
   [RFC5204]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 5204, April 2008.
   
   [RFC5205]  Nikander, P. and J. Laganier, "Host Identity Protocol
              (HIP) Domain Name System (DNS) Extensions", RFC 5205,
              April 2008.
   
   [RFC5206]  Nikander, P., Henderson, T., Ed., Vogt, C., and J. Arkko,
              "End-Host Mobility and Multihoming with the Host Identity
              Protocol", RFC 5206, April 2008.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   
   [RFC5338]  Henderson, T., Nikander, P., and M. Komu, "Using the Host
              Identity Protocol with Legacy Applications", RFC 5338,
              September 2008.
   
   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

[HIP-NATIVE-API] 

              Komu, M. and T. Henderson, "Basic Socket Interface
              Extensions for Host Identity Protocol (HIP)", Work in
              Progress, January 2010.
   
   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

[P2PSIP-BASE] 

              Jennings, C., Lowekamp, B., Ed., Rescorla, E., Baset, S.,
              and H. Schulzrinne, "REsource LOcation And Discovery
              (RELOAD) Base Protocol", Work in Progress, November 2010.

[HIP-RELOAD-INSTANCE] 

              Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
              Protocol-Based Overlay Networking Environment (HIP BONE)
              Instance Specification for REsource LOcation And Discovery
              (RELOAD)", Work in Progress, January 2011.

Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland
   
   EMail: Gonzalo.Camarillo@ericsson.com
   
   Pekka Nikander
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland
   
   EMail: Pekka.Nikander@ericsson.com
   
   Jani Hautakorpi
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland
   
   EMail: Jani.Hautakorpi@ericsson.com

Ari Keranen
Ericsson
Hirsalantie 11
Jorvas 02420
Finland 

   EMail: Ari.Keranen@ericsson.com
   
   Alan Johnston
   Avaya
   St. Louis, MO  63124
   
   EMail: alan.b.johnston@gmail.com