RFC 8046
Host Mobility with the Host Identity Protocol
Internet Engineering Task Force (IETF) T. Henderson, Ed. Request for Comments: 8046 University of Washington Obsoletes: 5206 C. Vogt Category: Standards Track Independent ISSN: 2070-1721 J. Arkko Ericsson February 2017 Host Mobility with the Host Identity ProtocolAbstract
This document defines a mobility extension to the Host Identity Protocol (HIP). Specifically, this document defines a "LOCATOR_SET" parameter for HIP messages that allows for a HIP host to notify peers about alternate addresses at which it may be reached. This document also defines how the parameter can be used to preserve communications across a change to the IP address used by one or both peer hosts. The same LOCATOR_SET parameter can also be used to support end-host multihoming (as specified in RFC 8047). This document obsoletes RFC 5206. 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 7841. 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/rfc8046.
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Table of Contents
1. Introduction and Scope . . . . . . . . . . . . . . . . . . . 4 2. Terminology and Conventions . . . . . . . . . . . . . . . . . 4 3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Operating Environment . . . . . . . . . . . . . . . . . . 7 3.1.1. Locator . . . . . . . . . . . . . . . . . . . . . . . 9 3.1.2. Mobility Overview . . . . . . . . . . . . . . . . . . 9 3.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 10 3.2.1. Mobility with a Single SA Pair (No Rekeying) . . . . 10 3.2.2. Mobility with a Single SA Pair (Mobile-Initiated Rekey) . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.3. Mobility Messaging through the Rendezvous Server . . 13 3.2.4. Network Renumbering . . . . . . . . . . . . . . . . . 14 3.3. Other Considerations . . . . . . . . . . . . . . . . . . 14 3.3.1. Address Verification . . . . . . . . . . . . . . . . 14 3.3.2. Credit-Based Authorization . . . . . . . . . . . . . 15 3.3.3. Preferred Locator . . . . . . . . . . . . . . . . . . 16 4. LOCATOR_SET Parameter Format . . . . . . . . . . . . . . . . 16 4.1. Traffic Type and Preferred Locator . . . . . . . . . . . 18 4.2. Locator Type and Locator . . . . . . . . . . . . . . . . 19 4.3. UPDATE Packet with Included LOCATOR_SET . . . . . . . . . 19 5. Processing Rules . . . . . . . . . . . . . . . . . . . . . . 19 5.1. Locator Data Structure and Status . . . . . . . . . . . . 19 5.2. Sending the LOCATOR_SET . . . . . . . . . . . . . . . . . 21 5.3. Handling Received LOCATOR_SETs . . . . . . . . . . . . . 22 5.4. Verifying Address Reachability . . . . . . . . . . . . . 24 5.5. Changing the Preferred Locator . . . . . . . . . . . . . 26 5.6. Credit-Based Authorization . . . . . . . . . . . . . . . 26 5.6.1. Handling Payload Packets . . . . . . . . . . . . . . 27 5.6.2. Credit Aging . . . . . . . . . . . . . . . . . . . . 29 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29 6.1. Impersonation Attacks . . . . . . . . . . . . . . . . . . 30 6.2. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 31 6.2.1. Flooding Attacks . . . . . . . . . . . . . . . . . . 31 6.2.2. Memory/Computational-Exhaustion DoS Attacks . . . . . 32 6.3. Mixed Deployment Environment . . . . . . . . . . . . . . 32 6.4. Privacy Concerns . . . . . . . . . . . . . . . . . . . . 33 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 8. Differences from RFC 5206 . . . . . . . . . . . . . . . . . . 33 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 9.1. Normative References . . . . . . . . . . . . . . . . . . 35 9.2. Informative References . . . . . . . . . . . . . . . . . 35 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction and Scope
The Host Identity Protocol (HIP) [RFC7401] supports an architecture that decouples the transport layer (TCP, UDP, etc.) from the internetworking layer (IPv4 and IPv6) by using public/private key pairs, instead of IP addresses, as host identities. When a host uses HIP, the overlying protocol sublayers (e.g., transport-layer sockets and Encapsulating Security Payload (ESP) Security Associations (SAs)) are instead bound to representations of these host identities, and the IP addresses are only used for packet forwarding. However, each host needs to also know at least one IP address at which its peers are reachable. Initially, these IP addresses are the ones used during the HIP base exchange. One consequence of such a decoupling is that new solutions to network-layer mobility and host multihoming are possible. There are potentially many variations of mobility and multihoming possible. The scope of this document encompasses messaging and elements of procedure for basic network-level host mobility, leaving more complicated mobility scenarios, multihoming, and other variations for further study. More specifically, the following are in scope: This document defines a LOCATOR_SET parameter for use in HIP messages. The LOCATOR_SET parameter allows a HIP host to notify a peer about alternate locators at which it is reachable. The locators may be merely IP addresses, or they may have additional multiplexing and demultiplexing context to aid with the packet handling in the lower layers. For instance, an IP address may need to be paired with an ESP Security Parameter Index (SPI) so that packets are sent on the correct SA for a given address. This document also specifies the messaging and elements of procedure for end-host mobility of a HIP host. In particular, message flows to enable successful host mobility, including address verification methods, are defined herein. The HIP rendezvous server (RVS) [RFC8004] can be used to manage simultaneous mobility of both hosts, initial reachability of a mobile host, location privacy, and some modes of NAT traversal. Use of the HIP RVS to manage the simultaneous mobility of both hosts is specified herein.
The following topics are out of scope:
While the same LOCATOR_SET parameter supports host multihoming
(simultaneous use of a number of addresses), procedures for host
multihoming are out of scope and are specified in [RFC8047].
While HIP can potentially be used with transports other than the
ESP transport format [RFC7402], this document largely assumes the
use of ESP and leaves other transport formats for further study.
We do not consider localized mobility management extensions (i.e.,
mobility management techniques that do not involve directly
signaling the correspondent node); this document is concerned with
end-to-end mobility.
Finally, making underlying IP mobility transparent to the
transport layer has implications on the proper response of
transport congestion control, path MTU selection, and Quality of
Service (QoS). Transport-layer mobility triggers, and the proper
transport response to a HIP mobility or multihoming address
change, are outside the scope of this document.
The main sections of this document are organized as follows.
Section 3 provides a summary overview of operations, scenarios, and
other considerations. Section 4 specifies the messaging parameter
syntax. Section 5 specifies the processing rules for messages.
Section 6 describes security considerations for this specification.
2. Terminology and Conventions
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 [RFC2119].
LOCATOR_SET. A HIP parameter containing zero or more Locator fields.
locator. A name that controls how the packet is routed through the
network and demultiplexed by the end host. It may include a
concatenation of traditional network addresses such as an IPv6
address and end-to-end identifiers such as an ESP SPI. It may
also include transport port numbers or IPv6 Flow Labels as
demultiplexing context, or it may simply be a network address.
Locator. When capitalized in the middle of a sentence, this term
refers to the encoding of a locator within the LOCATOR_SET
parameter (i.e., the 'Locator' field of the parameter).
Address. A name that denotes a point of attachment to the network.
The two most common examples are an IPv4 address and an IPv6
address. The set of possible addresses is a subset of the set of
possible locators.
Preferred locator. A locator on which a host prefers to receive
data. Certain locators are labeled as preferred when a host
advertises its locator set to its peer. By default, the locators
used in the HIP base exchange are the preferred locators. The use
of preferred locators, including the scenario where multiple
address scopes and families may be in use, is defined more in
[RFC8047] than in this document.
Credit-Based Authorization (CBA). A mechanism allowing a host to
send a certain amount of data to a peer's newly announced locator
before the result of mandatory address verification is known.
3. Protocol Model
This section is an overview; a more detailed specification follows this section.3.1. Operating Environment
HIP [RFC7401] is a key establishment and parameter negotiation protocol. Its primary applications are for authenticating host messages based on host identities and establishing SAs for the ESP transport format [RFC7402] and possibly other protocols in the future. +--------------------+ +--------------------+ | | | | | +------------+ | | +------------+ | | | Key | | HIP | | Key | | | | Management | <-+-----------------------+-> | Management | | | | Process | | | | Process | | | +------------+ | | +------------+ | | ^ | | ^ | | | | | | | | v | | v | | +------------+ | | +------------+ | | | IPsec | | ESP | | IPsec | | | | Stack | <-+-----------------------+-> | Stack | | | | | | | | | | | +------------+ | | +------------+ | | | | | | | | | | Initiator | | Responder | +--------------------+ +--------------------+ Figure 1: HIP Deployment Model The general deployment model for HIP is shown above, assuming operation in an end-to-end fashion. This document specifies an extension to HIP to enable end-host mobility. In summary, these extensions to the HIP base protocol enable the signaling of new addressing information to the peer in HIP messages. The messages are authenticated via a signature or keyed Hash Message Authentication Code (HMAC) based on its Host Identity (HI). This document specifies the format of this new addressing (LOCATOR_SET) parameter, the procedures for sending and processing this parameter to enable basic host mobility, and procedures for a concurrent address verification mechanism.
--------- | TCP | (sockets bound to HITs) --------- | --------- ----> | ESP | {HIT_s, HIT_d} <-> SPI | --------- | | ---- --------- | MH |-> | HIP | {HIT_s, HIT_d, SPI} <-> {IP_s, IP_d, SPI} ---- --------- | --------- | IP | --------- Figure 2: Architecture for HIP Host Mobility and Multihoming Figure 2 depicts a layered architectural view of a HIP-enabled stack using the ESP transport format. In HIP, upper-layer protocols (including TCP and ESP in this figure) are bound to Host Identity Tags (HITs) and not IP addresses. The HIP sublayer is responsible for maintaining the binding between HITs and IP addresses. The SPI is used to associate an incoming packet with the right HITs. The block labeled "MH" corresponds to the function that manages the bindings at the ESP and HIP sublayers for mobility (specified in this document) and multihoming (specified in [RFC8047]). Consider first the case in which there is no mobility or multihoming, as specified in the base protocol specification [RFC7401]. The HIP base exchange establishes the HITs in use between the hosts, the SPIs to use for ESP, and the IP addresses (used in both the HIP signaling packets and ESP data packets). Note that there can only be one such set of bindings in the outbound direction for any given packet, and the only fields used for the binding at the HIP layer are the fields exposed by ESP (the SPI and HITs). For the inbound direction, the SPI is all that is required to find the right host context. ESP rekeying events change the mapping between the HIT pair and SPI, but do not change the IP addresses. Consider next a mobility event, in which a host moves to another IP address. Two things need to occur in this case. First, the peer needs to be notified of the address change using a HIP UPDATE message. Second, each host needs to change its local bindings at the HIP sublayer (new IP addresses). It may be that both the SPIs and IP addresses are changed simultaneously in a single UPDATE; the protocol described herein supports this. Although internal notification of transport-layer protocols regarding the path change (e.g., to reset
congestion control variables) may be desired, this specification does not address such internal notification. In addition, elements of procedure for traversing network address translators (NATs) and firewalls, including NATs and firewalls that may understand HIP, may complicate the above basic scenario and are not covered by this document.3.1.1. Locator
This document defines a generalization of an address called a "locator". A locator specifies a point of attachment to the network but may also include additional end-to-end tunneling or a per-host demultiplexing context that affects how packets are handled below the logical HIP sublayer of the stack. This generalization is useful because IP addresses alone may not be sufficient to describe how packets should be handled below HIP. For example, in a host multihoming context, certain IP addresses may need to be associated with certain ESP SPIs to avoid violating the ESP anti-replay window. Addresses may also be affiliated with transport ports in certain tunneling scenarios. Locators may simply be traditional network addresses. The format of the Locator fields in the LOCATOR_SET parameter is defined in Section 4.3.1.2. Mobility Overview
When a host moves to another address, it notifies its peer of the new address by sending a HIP UPDATE packet containing a single LOCATOR_SET parameter and a single ESP_INFO parameter. This UPDATE packet is acknowledged by the peer. For reliability in the presence of packet loss, the UPDATE packet is retransmitted as defined in the HIP specification [RFC7401]. The peer can authenticate the contents of the UPDATE packet based on the signature and keyed hash of the packet. When using the ESP transport format [RFC7402], the host may, at the same time, decide to rekey its security association and possibly generate a new Diffie-Hellman key; all of these actions are triggered by including additional parameters in the UPDATE packet, as defined in the base protocol specification [RFC7401] and ESP extension [RFC7402]. When using ESP (and possibly other transport modes in the future), the host is able to receive packets that are protected using a HIP- created ESP SA from any address. Thus, a host can change its IP address and continue to send packets to its peers without necessarily rekeying. However, the peers are not able to send packets to these new addresses before they can reliably and securely update the set of addresses that they associate with the sending host. Furthermore,
mobility may change the path characteristics in such a manner that reordering occurs and packets fall outside the ESP anti-replay window for the SA, thereby requiring rekeying.3.2. Protocol Overview
In this section, we briefly introduce a number of usage scenarios for HIP host mobility. These scenarios assume that HIP is being used with the ESP transform [RFC7402], although other scenarios may be defined in the future. To understand these usage scenarios, the reader should be at least minimally familiar with the HIP specification [RFC7401] and with the use of ESP with HIP [RFC7402]. According to these specifications, the data traffic in a HIP session is protected with ESP, and the ESP SPI acts as an index to the right host-to-host context. More specification details are found later in Sections 4 and 5. The scenarios below assume that the two hosts have completed a single HIP base exchange with each other. Therefore, both of the hosts have one incoming and one outgoing SA. Further, each SA uses the same pair of IP addresses, which are the ones used in the base exchange. The readdressing protocol is an asymmetric protocol where a mobile host informs a peer host about changes of IP addresses on affected SPIs. The readdressing exchange is designed to be piggybacked on existing HIP exchanges. In support of mobility, the LOCATOR_SET parameter is carried in UPDATE packets. The scenarios below at times describe addresses as being in either an ACTIVE, UNVERIFIED, or DEPRECATED state. From the perspective of a host, newly learned addresses of the peer need to be verified before put into active service, and addresses removed by the peer are put into a deprecated state. Under limited conditions described below (Section 5.6), an UNVERIFIED address may be used. The addressing states are defined more formally in Section 5.1. Hosts that use link-local addresses as source addresses in their HIP handshakes may not be reachable by a mobile peer. Such hosts SHOULD provide a globally routable address either in the initial handshake or via the LOCATOR_SET parameter.3.2.1. Mobility with a Single SA Pair (No Rekeying)
A mobile host sometimes needs to change an IP address bound to an interface. The change of an IP address might be needed due to a change in the advertised IPv6 prefixes on the link, a reconnected PPP link, a new DHCP lease, or an actual movement to another subnet. In order to maintain its communication context, the host needs to inform
its peers about the new IP address. This first example considers the case in which the mobile host has only one interface, one IP address in use within the HIP session, a single pair of SAs (one inbound, one outbound), and no rekeying occurring on the SAs. We also assume that the new IP addresses are within the same address family (IPv4 or IPv6) as the previous address. This is the simplest scenario, depicted in Figure 3. Note that the conventions for message parameter notations in figures (use of parentheses and brackets) is defined in Section 2.2 of [RFC7401]. Mobile Host Peer Host UPDATE(ESP_INFO, LOCATOR_SET, SEQ) -----------------------------------> UPDATE(ESP_INFO, SEQ, ACK, ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> Figure 3: Readdress without Rekeying but with Address Check The steps of the packet processing are as follows: 1. The mobile host may be disconnected from the peer host for a brief period of time while it switches from one IP address to another; this case is sometimes referred to in the literature as a "break-before-make" case. The host may also obtain its new IP address before losing the old one ("make-before-break" case). In either case, upon obtaining a new IP address, the mobile host sends a LOCATOR_SET parameter to the peer host in an UPDATE message. The UPDATE message also contains an ESP_INFO parameter containing the values of the old and new SPIs for a security association. In this case, both the OLD SPI and NEW SPI parameters are set to the value of the preexisting incoming SPI; this ESP_INFO does not trigger a rekeying event but is instead included for possible parameter-inspecting firewalls on the path ([RFC5207] specifies some such firewall scenarios in which the HIP-aware firewall may want to associate ESP flows to host identities). The LOCATOR_SET parameter contains the new IP address (embedded in a Locator Type of "1", defined below) and a lifetime associated with the locator. The mobile host waits for this UPDATE to be acknowledged, and retransmits if necessary, as specified in the base specification [RFC7401].
2. The peer host receives the UPDATE, validates it, and updates any
local bindings between the HIP association and the mobile host's
destination address. The peer host MUST perform an address
verification by placing a nonce in the ECHO_REQUEST parameter of
the UPDATE message sent back to the mobile host. It also
includes an ESP_INFO parameter with both the OLD SPI and NEW SPI
parameters set to the value of the preexisting incoming SPI and
sends this UPDATE (with piggybacked acknowledgment) to the mobile
host at its new address. This UPDATE also acknowledges the
mobile host's UPDATE that triggered the exchange. The peer host
waits for its UPDATE to be acknowledged, and retransmits if
necessary, as specified in the base specification [RFC7401]. The
peer MAY use the new address immediately, but it MUST limit the
amount of data it sends to the address until address verification
completes.
3. The mobile host completes the readdress by processing the UPDATE
ACK and echoing the nonce in an ECHO_RESPONSE, containing the ACK
of the peer's UPDATE. This UPDATE is not protected by a
retransmission timer because it does not contain a SEQ parameter
requesting acknowledgment. Once the peer host receives this
ECHO_RESPONSE, it considers the new address to be verified and
can put the address into full use.
While the peer host is verifying the new address, the new address is
marked as UNVERIFIED (in the interim), and the old address is
DEPRECATED. Once the peer host has received a correct reply to its
UPDATE challenge, it marks the new address as ACTIVE and removes the
old address.
3.2.2. Mobility with a Single SA Pair (Mobile-Initiated Rekey)
The mobile host may decide to rekey the SAs at the same time that it
notifies the peer of the new address. In this case, the above
procedure described in Figure 3 is slightly modified. The UPDATE
message sent from the mobile host includes an ESP_INFO with the OLD
SPI set to the previous SPI, the NEW SPI set to the desired new SPI
value for the incoming SA, and the KEYMAT Index desired. Optionally,
the host may include a DIFFIE_HELLMAN parameter for a new Diffie-
Hellman key. The peer completes the request for a rekey as is
normally done for HIP rekeying, except that the new address is kept
as UNVERIFIED until the UPDATE nonce challenge is received as
described above. Figure 4 illustrates this scenario.
Mobile Host Peer Host UPDATE(ESP_INFO, LOCATOR_SET, SEQ, [DIFFIE_HELLMAN]) -----------------------------------> UPDATE(ESP_INFO, SEQ, ACK, [DIFFIE_HELLMAN,] ECHO_REQUEST) <----------------------------------- UPDATE(ACK, ECHO_RESPONSE) -----------------------------------> Figure 4: Readdress with Mobile-Initiated Rekey3.2.3. Mobility Messaging through the Rendezvous Server
Section 6.11 of [RFC7401] specifies procedures for sending HIP UPDATE packets. The UPDATE packets are protected by a timer subject to exponential backoff and resent UPDATE_RETRY_MAX times. It may be, however, that the peer is itself in the process of moving when the local host is trying to update the IP address bindings of the HIP association. This is sometimes called the "double-jump" mobility problem; each host's UPDATE packets are simultaneously sent to a stale address of the peer, and the hosts are no longer reachable from one another. The HIP Rendezvous Extension [RFC8004] specifies a rendezvous service that permits the I1 packet from the base exchange to be relayed from a stable or well-known public IP address location to the current IP address of the host. It is possible to support double-jump mobility with this rendezvous service if the following extensions to the specifications of [RFC8004] and [RFC7401] are followed. 1. The mobile host sending an UPDATE to the peer, and not receiving an ACK, MAY resend the UPDATE to an RVS of the peer, if such a server is known. The host MAY try the RVS of the peer up to UPDATE_RETRY_MAX times as specified in [RFC7401]. The host MAY try to use the peer's RVS before it has tried UPDATE_RETRY_MAX times to the last working address (i.e., the RVS MAY be tried in parallel with retries to the last working address). The aggressiveness of a host replicating its UPDATEs to multiple destinations, to try candidates in parallel instead of serially, is a policy choice outside of this specification. 2. An RVS supporting the UPDATE forwarding extensions specified herein MUST modify the UPDATE in the same manner as it modifies the I1 packet before forwarding. Specifically, it MUST rewrite the IP header source and destination addresses, recompute the IP header checksum, and include the FROM and RVS_HMAC parameters.
3. A host receiving an UPDATE packet MUST be prepared to process the
FROM and RVS_HMAC parameters and MUST include a VIA_RVS parameter
in the UPDATE reply that contains the ACK of the UPDATE SEQ.
4. An Initiator receiving a VIA_RVS in the UPDATE reply should
initiate address reachability tests (described later in this
document) towards the end host's address and not towards the
address included in the VIA_RVS.
This scenario requires that hosts using RVSs also take steps to
update their current address bindings with their RVS upon a mobility
event. [RFC8004] does not specify how to update the RVS with a
client host's new address. Section 3.2 of [RFC8003] describes how a
host may send a REG_REQUEST in either an I2 packet (if there is no
active association) or an UPDATE packet (if such association exists).
According to procedures described in [RFC8003], if a mobile host has
an active registration, it may use mobility updates specified herein,
within the context of that association, to readdress the association.
3.2.4. Network Renumbering
It is expected that IPv6 networks will be renumbered much more often
than most IPv4 networks. From an end-host point of view, network
renumbering is similar to mobility, and procedures described herein
also apply to notify a peer of a changed address.
3.3. Other Considerations
3.3.1. Address Verification
When a HIP host receives a set of locators from another HIP host in a
LOCATOR_SET, it does not necessarily know whether the other host is
actually reachable at the claimed addresses. In fact, a malicious
peer host may be intentionally giving bogus addresses in order to
cause a packet flood towards the target addresses [RFC4225].
Therefore, the HIP host needs to first check that the peer is
reachable at the new address.
Address verification is implemented by the challenger sending some
piece of unguessable information to the new address and waiting for
some acknowledgment from the Responder that indicates reception of
the information at the new address. This may include the exchange of
a nonce or the generation of a new SPI and observation of data
arriving on the new SPI. More details are found in Section 5.4 of
this document.
An additional potential benefit of performing address verification is to allow NATs and firewalls in the network along the new path to obtain the peer host's inbound SPI.3.3.2. Credit-Based Authorization
CBA allows a host to securely use a new locator even though the peer's reachability at the address embedded in the locator has not yet been verified. This is accomplished based on the following three hypotheses: 1. A flooding attacker typically seeks to somehow multiply the packets it generates for the purpose of its attack because bandwidth is an ample resource for many victims. 2. An attacker can often cause unamplified flooding by sending packets to its victim, either by directly addressing the victim in the packets or by guiding the packets along a specific path by means of an IPv6 Routing header, if Routing headers are not filtered by firewalls. 3. Consequently, the additional effort required to set up a redirection-based flooding attack (without CBA and return routability checks) would pay off for the attacker only if amplification could be obtained this way. On this basis, rather than eliminating malicious packet redirection in the first place, CBA prevents amplifications. This is accomplished by limiting the data a host can send to an unverified address of a peer by the data recently received from that peer. Redirection-based flooding attacks thus become less attractive than, for example, pure direct flooding, where the attacker itself sends bogus packets to the victim. Figure 5 illustrates CBA: Host B measures the amount of data recently received from peer A and, when A readdresses, sends packets to A's new, unverified address as long as the sum of the packet sizes does not exceed the measured, received data volume. When insufficient credit is left, B stops sending further packets to A until A's address becomes ACTIVE. The address changes may be due to mobility, multihoming, or any other reason. Not shown in Figure 5 are the results of credit aging (Section 5.6.2), a mechanism used to dampen possible time-shifting attacks.
+-------+ +-------+ | A | | B | +-------+ +-------+ | | address |------------------------------->| credit += size(packet) ACTIVE | | |------------------------------->| credit += size(packet) |<-------------------------------| do not change credit | | + address change | + address verification starts | address |<-------------------------------| credit -= size(packet) UNVERIFIED |------------------------------->| credit += size(packet) |<-------------------------------| credit -= size(packet) | | |<-------------------------------| credit -= size(packet) | X credit < size(packet) | | => do not send packet! + address verification concludes | address | | ACTIVE |<-------------------------------| do not change credit | | Figure 5: Readdressing Scenario This document does not specify how to set the credit limit value, but the goal is to allow data transfers to proceed without much interruption while the new address is verified. A simple heuristic to accomplish this, if the sender knows roughly its round-trip time (RTT) and current sending rate to the host, is to allow enough credit to support maintaining the sending rate for a duration corresponding to two or three RTTs.3.3.3. Preferred Locator
When a host has multiple locators, the peer host needs to decide which to use for outbound packets. It may be that a host would prefer to receive data on a particular inbound interface. HIP allows a particular locator to be designated as a preferred locator and communicated to the peer (see Section 4).4. LOCATOR_SET Parameter Format
The LOCATOR_SET parameter has a type number value that is considered to be a "critical parameter" as per the definition in [RFC7401]; such parameter types MUST be recognized and processed by the recipient. The parameter consists of the standard HIP parameter Type and Length fields, plus zero or more Locator sub-parameters. Each Locator sub-
parameter contains a Traffic Type, Locator Type, Locator Length,
preferred locator bit ("P" bit), Locator Lifetime, and a Locator
encoding. A LOCATOR_SET containing zero Locator fields is permitted
but has the effect of deprecating all addresses.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Type | Locator Type | Locator Length | Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Type | Locator Type | Locator Length | Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: LOCATOR_SET Parameter Format
Type: 193
Length: Length in octets, excluding Type and Length fields, and
excluding padding.
Traffic Type: Defines whether the locator pertains to HIP signaling,
user data, or both.
Locator Type: Defines the semantics of the Locator field.
Locator Length: Defines the length of the Locator field, in units of
4-byte words (Locators up to a maximum of 4*255 octets are
supported).
Reserved: Zero when sent, ignored when received.
P: Preferred locator. Set to one if the locator is preferred for
that Traffic Type; otherwise, set to zero.
Locator Lifetime: Lifetime of the locator, in seconds.
Locator: The locator whose semantics and encoding are indicated by
the Locator Type field. All sub-fields of the Locator field are
integral multiples of four octets in length.
The Locator Lifetime (lifetime) indicates how long the following
locator is expected to be valid. The lifetime is expressed in
seconds. Each locator MUST have a non-zero lifetime. The address is
expected to become deprecated when the specified number of seconds
has passed since the reception of the message. A deprecated address
SHOULD NOT be used as a destination address if an alternate
(non-deprecated) is available and has sufficient address scope.
4.1. Traffic Type and Preferred Locator
The following Traffic Type values are defined:
0: Both signaling (HIP control packets) and user data.
1: Signaling packets only.
2: Data packets only.
The "P" bit, when set, has scope over the corresponding Traffic Type.
That is, when a "P" bit is set for Traffic Type "2", for example, it
means that the locator is preferred for data packets. If there is a
conflict (for example, if the "P" bit is set for an address of Type
"0" and a different address of Type "2"), the more specific Traffic
Type rule applies (in this case, "2"). By default, the IP addresses
used in the base exchange are preferred locators for both signaling
and user data, unless a new preferred locator supersedes them. If no
locators are indicated as preferred for a given Traffic Type, the
implementation may use an arbitrary destination locator from the set
of active locators.
4.2. Locator Type and Locator
The following Locator Type values are defined, along with the associated semantics of the Locator field: 0: An IPv6 address or an IPv4-in-IPv6 format IPv4 address [RFC4291] (128 bits long). This Locator Type is defined primarily for non-ESP-based usage. 1: The concatenation of an ESP SPI (first 32 bits) followed by an IPv6 address or an IPv4-in-IPv6 format IPv4 address (an additional 128 bits). This IP address is defined primarily for ESP-based usage.4.3. UPDATE Packet with Included LOCATOR_SET
A number of combinations of parameters in an UPDATE packet are possible (e.g., see Section 3.2). In this document, procedures are defined only for the case in which one LOCATOR_SET and one ESP_INFO parameter are used in any HIP packet. Any UPDATE packet that includes a LOCATOR_SET parameter SHOULD include both an HMAC and a HIP_SIGNATURE parameter. The UPDATE MAY also include a HOST_ID parameter (which may be useful for HIP-aware firewalls inspecting the HIP messages for the first time). If the UPDATE includes the HOST_ID parameter, the receiving host MUST verify that the HOST_ID corresponds to the HOST_ID that was used to establish the HIP association, and the HIP_SIGNATURE MUST verify with the public key associated with this HOST_ID parameter. The relationship between the announced Locators and any ESP_INFO parameters present in the packet is defined in Section 5.2. This document does not support any elements of procedure for sending more than one LOCATOR_SET or ESP_INFO parameter in a single UPDATE.
(page 19 continued on part 2)
