HIP Working Group A. Keranen
Internet-Draft J. Melen
Intended status: Standards Track M. Komu, Ed.
Expires: September 28, 2017 Ericsson
March 27, 2017
Native NAT Traversal Mode for the Host Identity Protocol
draft-ietf-hip-native-nat-traversal-19
Abstract
This document specifies a new Network Address Translator (NAT)
traversal mode for the Host Identity Protocol (HIP). The new mode is
based on the Interactive Connectivity Establishment (ICE) methodology
and UDP encapsulation of data and signaling traffic. The main
difference from the previously specified modes is the use of HIP
messages for all NAT traversal procedures.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on September 28, 2017.
Copyright Notice
Copyright (c) 2017 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
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Description . . . . . . . . . . . . . . . . . . . . 7
4.1. Relay Registration . . . . . . . . . . . . . . . . . . . 7
4.2. Transport Address Candidate Gathering . . . . . . . . . . 10
4.3. NAT Traversal Mode Negotiation . . . . . . . . . . . . . 12
4.4. Connectivity Check Pacing Negotiation . . . . . . . . . . 13
4.5. Base Exchange via HIP Relay Server . . . . . . . . . . . 14
4.6. Connectivity Checks . . . . . . . . . . . . . . . . . . . 17
4.6.1. Connectivity Check Procedure . . . . . . . . . . . . 17
4.6.2. Rules for Connectivity Checks . . . . . . . . . . . . 20
4.6.3. Rules for Concluding Connectivity Checks . . . . . . 22
4.7. NAT Traversal Alternatives . . . . . . . . . . . . . . . 23
4.7.1. Minimal NAT Traversal Support . . . . . . . . . . . . 23
4.7.2. Base Exchange without Connectivity Checks . . . . . . 23
4.7.3. Initiating a Base Exchange both with and without UDP
Encapsulation . . . . . . . . . . . . . . . . . . . . 24
4.8. Sending Control Packets after the Base Exchange . . . . . 25
4.9. Mobility Handover Procedure . . . . . . . . . . . . . . . 26
4.10. NAT Keepalives . . . . . . . . . . . . . . . . . . . . . 28
4.11. Closing Procedure . . . . . . . . . . . . . . . . . . . . 28
4.12. Relaying Considerations . . . . . . . . . . . . . . . . . 29
4.12.1. Forwarding Rules and Permissions . . . . . . . . . . 29
4.12.2. HIP Data Relay and Relaying of Control Packets . . . 30
4.12.3. Handling Conflicting SPI Values . . . . . . . . . . 30
5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 31
5.1. HIP Control Packets . . . . . . . . . . . . . . . . . . . 32
5.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 32
5.3. Keepalives . . . . . . . . . . . . . . . . . . . . . . . 33
5.4. NAT Traversal Mode Parameter . . . . . . . . . . . . . . 33
5.5. Connectivity Check Transaction Pacing Parameter . . . . . 34
5.6. Relay and Registration Parameters . . . . . . . . . . . . 35
5.7. LOCATOR_SET Parameter . . . . . . . . . . . . . . . . . . 36
5.8. RELAY_HMAC Parameter . . . . . . . . . . . . . . . . . . 38
5.9. Registration Types . . . . . . . . . . . . . . . . . . . 38
5.10. Notify Packet Types . . . . . . . . . . . . . . . . . . . 38
5.11. ESP Data Packets . . . . . . . . . . . . . . . . . . . . 38
5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters . . . . . . 39
5.13. PEER_PERMISSION Parameter . . . . . . . . . . . . . . . . 40
5.14. HIP Connectivity Check Packets . . . . . . . . . . . . . 41
5.15. NOMINATE parameter . . . . . . . . . . . . . . . . . . . 42
6. Security Considerations . . . . . . . . . . . . . . . . . . . 42
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6.1. Privacy Considerations . . . . . . . . . . . . . . . . . 42
6.2. Opportunistic Mode . . . . . . . . . . . . . . . . . . . 43
6.3. Base Exchange Replay Protection for HIP Relay Server . . 43
6.4. Demultiplexing Different HIP Associations . . . . . . . . 43
6.5. Reuse of Ports at the Data Relay . . . . . . . . . . . . 44
6.6. Amplification attacks . . . . . . . . . . . . . . . . . . 44
6.7. Attacks against Connectivity Checks and Candidate
Gathering . . . . . . . . . . . . . . . . . . . . . . . . 44
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 45
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
10.1. Normative References . . . . . . . . . . . . . . . . . . 46
10.2. Informative References . . . . . . . . . . . . . . . . . 47
Appendix A. Selecting a Value for Check Pacing . . . . . . . . . 48
Appendix B. Differences with respect to ICE . . . . . . . . . . 49
Appendix C. Differences to Base Exchange and UPDATE procedures . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
The Host Identity Protocol (HIP) [RFC7401] is specified to run
directly on top of IPv4 or IPv6. However, many middleboxes found in
the Internet, such as NATs and firewalls, often allow only UDP or TCP
traffic to pass [RFC5207]. Also, especially NATs usually require the
host behind a NAT to create a forwarding state in the NAT before
other hosts outside of the NAT can contact the host behind the NAT.
To overcome this problem, different methods, commonly referred to as
NAT traversal techniques, have been developed.
The HIP experiment report [RFC6538] mentions that Teredo based NAT
traversal for HIP and related ESP traffic (with double tunneling
overhead). Two HIP specific NAT traversal techniques for HIP are
specified in [RFC5770]. One of them uses only UDP encapsulation,
while the other uses also the Interactive Connectivity Establishment
(ICE) [I-D.ietf-ice-rfc5245bis] protocol, which in turn uses Session
Traversal Utilities for NAT (STUN) [RFC5389] and Traversal Using
Relays around NAT (TURN) [RFC5766] protocols to achieve a reliable
NAT traversal solution.
The benefit of using ICE and STUN/TURN is that one can re-use the NAT
traversal infrastructure already available in the Internet, such as
STUN and TURN servers. Also, some middleboxes may be STUN-aware and
may be able to do something "smart" when they see STUN being used for
NAT traversal. However, implementing a full ICE/STUN/TURN protocol
stack results in a considerable amount of effort and code which could
be avoided by re-using and extending HIP messages and state machines
for the same purpose. Thus, this document specifies an alternative
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NAT traversal mode that uses HIP messages instead of STUN for the
connectivity check keepalives and data relaying. This document also
specifies how mobility management works in the context of NAT
traversal, which was missing from [RFC5770].
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 [RFC2119].
This document borrows terminology from [RFC5770], [RFC7401],
[RFC8046], [RFC4423], [I-D.ietf-ice-rfc5245bis], and [RFC5389]. The
following terms recur in the text:
HIP relay server:
A host that forwards any kind of HIP control packets between the
Initiator and the Responder.
HIP data relay:
A host that forwards HIP data packets, such as Encapsulating
Security Payload (ESP) [RFC7402], between two hosts.
Registered host:
A host that has registered for a relaying service with a HIP data
relay.
Locator:
As defined in [RFC8046]: "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_SET (written in capital letters):
Denotes a HIP control packet parameter that bundles multiple
locators together.
ICE offer:
The Initiator's LOCATOR_SET parameter in a HIP I2 control packet.
Corresponds to the ICE offer parameter, but is HIP specific.
ICE answer:
The Responder's LOCATOR_SET parameter in a HIP R2 control packet.
Corresponds to the ICE answer parameter, but is HIP specific.
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HIP connectivity checks:
In order to obtain a non-relayed communication path, two
communicating HIP hosts try to "punch holes" through their NAT
boxes using this mechanism. Similar to the ICE connectivity
checks, but implemented using HIP return routability checks.
Controlling host:
The controlling host nominates the candidate pair to be used with
the controlled host.
Controlled host:
The controlled host waits for the controlling to nominate an
address candidate pair.
Checklist:
A list of address candidate pairs that need to be tested for
connectivity.
Transport address:
Transport layer port and the corresponding IPv4/v6 address.
Candidate:
A transport address that is a potential point of contact for
receiving data.
Host candidate:
A candidate obtained by binding to a specific port from an IP
address on the host.
Server reflexive candidate:
A translated transport address of a host as observed by a HIP
relay server or a STUN/TURN server.
Peer reflexive candidate:
A translated transport address of a host as observed by its peer.
Relayed candidate:
A transport address that exists on a HIP data relay. Packets that
arrive at this address are relayed towards the registered host.
Permission:
In the context of HIP data relay, permission refers to a concept
similar to TURN's channels. Before a host can use a relayed
candidate to forward traffic through a HIP data relay, the host
must activate the relayed candidate with a specific peer host.
Base:
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The base of an candidate is the local source address a host uses
to send packets for the associated candidate. For example, the
base of a server reflexive address is the local address the host
used for registering itself to the associated HIP relay. The base
of a host candidate is equal to the host candidate itself.
3. Overview of Operation
+-------+
| HIP |
+--------+ | Relay | +--------+
| Data | +-------+ | Data |
| Relay | / \ | Relay |
+--------+ / \ +--------+
/ \
/ \
/ \
/ <- Signaling -> \
/ \
+-------+ +-------+
| NAT | | NAT |
+-------+ +-------+
/ \
/ \
+-------+ +-------+
| Init- | | Resp- |
| iator | | onder |
+-------+ +-------+
Figure 1: Example Network Configuration
In the example configuration depicted in Figure 1, both Initiator and
Responder are behind one or more NATs, and both private networks are
connected to the public Internet. To be contacted from behind a NAT,
the Responder must be registered with a HIP relay server reachable on
the public Internet, and we assume, as a starting point, that the
Initiator knows both the Responder's Host Identity Tag (HIT) and the
address of one of its relay servers (how the Initiator learns of the
Responder's relay server is outside of the scope of this document,
but may be through DNS or another name service). The Responder may
have also registered to a data relay that can forward the data plane
in case NAT penetration fails. It is worth noting that the HIP relay
and data relay functionality may be offered by two separate servers
or the same one.
The first steps are for both the Initiator and Responder to register
with a relay server (need not be the same one) and gather a set of
address candidates. The hosts may use HIP relay servers (or even
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STUN or TURN servers) for gathering the candidates. Next, the HIP
base exchange is carried out by encapsulating the HIP control packets
in UDP datagrams and sending them through the Responder's relay
server. As part of the base exchange, each HIP host learns of the
peer's candidate addresses through the HIP offer/answer procedure
embedded in the base exchange, which follows closely the ICE
[I-D.ietf-ice-rfc5245bis] protocol.
Once the base exchange is completed, two HIP hosts have established a
working communication session (for signaling) via a HIP relay server,
but the hosts still have to find a better path, preferably without a
HIP data relay, for the ESP data flow. For this, connectivity checks
are carried out until a working pair of addresses is discovered. At
the end of the procedure, if successful, the hosts will have
established a UDP-based tunnel that traverses both NATs, with the
data flowing directly from NAT to NAT or via a HIP data relay server.
At this point, also the HIP signaling can be sent over the same
address/port pair, and is demultiplexed from IPsec as described in
the UDP encapsulation standard for IPsec [RFC3948]. Finally, the two
hosts send NAT keepalives as needed in order keep their UDP-tunnel
state active in the associated NAT boxes.
If either one of the hosts knows that it is not behind a NAT, hosts
can negotiate during the base exchange a different mode of NAT
traversal that does not use HIP connectivity checks, but only UDP
encapsulation of HIP and ESP. Also, it is possible for the Initiator
to simultaneously try a base exchange with and without UDP
encapsulation. If a base exchange without UDP encapsulation
succeeds, no HIP connectivity checks or UDP encapsulation of ESP are
needed.
4. Protocol Description
This section describes the normative behavior of the protocol
extension. Most of the procedures are similar to what is defined in
[RFC5770] but with different, or additional, parameter types and
values. In addition, a new type of relaying server, HIP data relay,
is specified. Also, it should be noted that HIP version 2 [RFC7401]
(instead of [RFC5201] used in [RFC5770]) is expected to be used with
this NAT traversal mode.
4.1. Relay Registration
In order for two hosts to communicate over NATted environments, they
need a reliable way to exchange information. HIP relay servers as
defined in [RFC5770] support relaying of HIP control plane traffic
over UDP in NATted environments. A HIP relay server forwards HIP
control packets between the Initiator and the Responder.
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To guarantee also data plane delivery over varying types of NAT
devices, a host MAY also register for UDP encapsulated ESP relaying
using Registration Type RELAY_UDP_ESP (value [TBD by IANA: 3]). This
service may be coupled with the HIP relay server or offered
separately on another server. If the server supports relaying of UDP
encapsulated ESP, the host is allowed to register for a data relaying
service using the registration extensions in Section 3.3 of
[RFC8003]). If the server has sufficient relaying resources (free
port numbers, bandwidth, etc.) available, it opens a UDP port on one
of its addresses and signals the address and port to the registering
host using the RELAYED_ADDRESS parameter (as defined in Section 5.12
in this document). If the relay would accept the data relaying
request but does not currently have enough resources to provide data
relaying service, it MUST reject the request with Failure Type
"Insufficient resources" [RFC8003].
A HIP relay server MUST silently drop packets to a HIP relay client
that has not previously registered with the HIP relay. The
registration process follows the generic registration extensions
defined in [RFC8003]. The HIP control plane relaying registration
follows [RFC5770], but the data plane registration is different. It
is worth noting that if the HIP control and data plane relay services
reside on different hosts, the relay client has to register
separately to each of them. In the example shown in Figure 2, the
two services are coupled on a single host.
HIP HIP
Relay [Data] Relay
Client Server
| 1. UDP(I1) |
+---------------------------------------------------------------->|
| |
| 2. UDP(R1(REG_INFO(RELAY_UDP_HIP,[RELAY_UDP_ESP]))) |
|<----------------------------------------------------------------+
| |
| 3. UDP(I2(REG_REQ(RELAY_UDP_HIP),[RELAY_UDP_ESP])) |
+---------------------------------------------------------------->|
| |
| 4. UDP(R2(REG_RES(RELAY_UDP_HIP,[RELAY_UDP_ESP]), REG_FROM, |
| [RELAYED_ADDRESS])) |
|<----------------------------------------------------------------+
| |
Figure 2: Example Registration with a HIP Relay
In step 1, the relay client (Initiator) starts the registration
procedure by sending an I1 packet over UDP to the relay. It is
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RECOMMENDED that the Initiator select a random port number from the
ephemeral port range 49152-65535 for initiating a base exchange.
Alternatively, a host MAY also use a single fixed port for initiating
all outgoing connections. However, the allocated port MUST be
maintained until all of the corresponding HIP Associations are
closed. It is RECOMMENDED that the HIP relay server listen to
incoming connections at UDP port 10500. If some other port number is
used, it needs to be known by potential Initiators.
In step 2, the HIP relay server (Responder) lists the services that
it supports in the R1 packet. The support for HIP control plane over
UDP relaying is denoted by the Registration Type value RELAY_UDP_HIP
(see Section 5.9). If the server supports also relaying of ESP
traffic over UDP, it includes also Registration type value
RELAY_UDP_ESP.
In step 3, the Initiator selects the services for which it registers
and lists them in the REG_REQ parameter. The Initiator registers for
HIP relay service by listing the RELAY_UDP_HIP value in the request
parameter. If the Initiator requires also ESP relaying over UDP, it
lists also RELAY_UDP_ESP.
In step 4, the Responder concludes the registration procedure with an
R2 packet and acknowledges the registered services in the REG_RES
parameter. The Responder denotes unsuccessful registrations (if any)
in the REG_FAILED parameter of R2. The Responder also includes a
REG_FROM parameter that contains the transport address of the client
as observed by the relay (Server Reflexive candidate). If the
Initiator registered to ESP relaying service, the Responder includes
RELAYED_ADDRESS paramater that describes the UDP port allocated to
the Initiator for ESP relaying. It is worth noting that this client
must first activate this UDP port by sending an UPDATE message to the
relay server that includes a PEER_PERMISSION parameter as described
in Section 4.12.1 both after base exchange and handover procedures.
After the registration, the relay client sends periodically NAT
keepalives to the relay server in order to keep the NAT bindings
between the initiator and the relay alive. The keepalive extensions
are described in Section 4.10.
The registered host MUST maintain an active HIP association with the
data relay as long as it requires the data relaying service. When
the HIP association is closed (or times out), or the registration
lifetime passes without the registered host refreshing the
registration, the data relay MUST stop relaying packets for that host
and close the corresponding UDP port (unless other registered hosts
are still using it).
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The data relay MAY use the same relayed address and port for multiple
registered hosts, but since this can cause problems with stateful
firewalls (see Section 6.5) it is NOT RECOMMENDED.
When a registered client sends an UPDATE (e.g., due to host movement
or to renew service registration), the relay server MUST follow the
general guidelines defined in [RFC8003], with the difference that all
UPDATE messages are delivered on top of UDP. In addition to this,
the relay server MUST include the REG_FROM parameter in all UPDATE
responses sent to the client. This applies both renewals of service
registration but also to host movement, where especially the latter
requires the client to learn its new server reflexive address
candidate.
4.2. Transport Address Candidate Gathering
A host needs to gather a set of address candidates before contacting
a non-relay host. The candidates are needed for connectivity checks
that allow two hosts to discover a direct, non-relayed path for
communicating with each other. One server reflexive candidate can be
discovered during the registration with the HIP relay server from the
REG_FROM parameter.
The candidate gathering can be done at any time, but it needs to be
done before sending an I2 or R2 in the base exchange if ICE-HIP-UDP
mode is to be used for the connectivity checks. It is RECOMMENDED
that all three types of candidates (host, server reflexive, and
relayed) are gathered to maximize the probability of successful NAT
traversal. However, if no data relay is used, and the host has only
a single local IP address to use, the host MAY use the local address
as the only host candidate and the address from the REG_FROM
parameter discovered during the relay registration as a server
reflexive candidate. In this case, no further candidate gathering is
needed.
If a host has more than one network interface, additional server
reflexive candidates can be discovered by sending registration
requests with Registration Type CANDIDATE_DISCOVERY (value [TBD by
IANA: 4]) from each of the interfaces to a HIP relay server. When a
HIP relay server receives a registration request with
CANDIDATE_DISCOVERY type, it MUST add a REG_FROM parameter,
containing the same information as if this were a relay registration,
to the response. This request type SHOULD NOT create any state at
the HIP relay server.
ICE guidelines for candidate gathering MUST be followed as described
in section 4.1.1 in [I-D.ietf-ice-rfc5245bis]. A number of host
candidates (loopback, anycast and others) should excluded as
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described in section 4.1.1.1 of the ICE specification
[I-D.ietf-ice-rfc5245bis]. Relayed candidates SHOULD be gathered in
order to guarantee successful NAT traversal. It is RECOMMENDED for
implementations to support this functionality even if it will not be
used in deployments in order to enable it by software configuration
update if needed at some point. A host SHOULD employ only a relay
server for gathering the candidates for a single HIP association;
either a one server providing both HIP and data relay functionality,
or one HIP relay server and another one for data relaying if the
functionality is offered by another server. When the relay service
is split between two hosts, the server reflexive candidate from the
HIP relay SHOULD be used instead of the one provided by the data
relay. If a relayed candidate is identical to a host candidate, the
relayed candidate must be discarded. NAT64 considerations in section
4.1.1.2 of [I-D.ietf-ice-rfc5245bis] apply as well.
HIP based connectivity can be utilized by IPv4 applications using
LSIs and by IPv6 based applications using HITs. The LSIs and HITs of
the local virtual interfaces MUST be excluded in the candidate
gathering phase as well to avoid creating unnecessary loopback
connectivity tests.
Gathering of candidates MAY also be performed as specified in
Section 4.2 of [RFC5770] if STUN servers are available, or if the
host has just a single interface and no STUN or HIP data relay
servers are available.
Each local address candidate MUST be assigned a priority. The
recommended formula in [I-D.ietf-ice-rfc5245bis] SHOULD be used:
priority = (2^24)*(type preference) + (2^8)*(local preference) +
(2^0)*(256 - component ID)
In the formula, type preference follows the ICE specification section
4.1.2.2 guidelines: the RECOMMENDED values are 126 for host
candidates, 100 for server reflexive candidates, 110 for peer
reflexive candidates, and 0 for relayed candidates. The highest
value is 126 (the most preferred) and lowest is 0 (last resort). For
all candidates of the same type, the preference type value MUST be
identical, and, correspondingly, the value MUST be different for
different types. For peer reflexive values, the type preference
value MUST be higher than for server reflexive types. It should be
noted that peer reflexive values are learned later during
connectivity checks, so a host cannot employ it during candidate
gathering stage yet.
Following the ICE specification, the local preference MUST be an
integer from 0 (lowest preference) to 65535 (highest preference)
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inclusive. In the case the host has only a single address candidate,
the value SHOULD be 65535. In the case of multiple candidates, each
local preference value MUST be unique. Dual-stack considerations for
IPv6 in section 4.1.2.2 in ICE apply also here.
Unlike ICE, this protocol only creates a single UDP flow between the
two communicating hosts, so only a single component exists. Hence,
the component ID value MUST always be set to 1.
As defined in ICE (in section 11.3), the retransmission timeout (RTO)
for address gathering from a relay SHOULD be calculated as follows:
RTO = MAX (500ms, Ta * (Num-Of-Pairs))
where Ta is the value used for Ta is the value used for the
connectivity check pacing and Num-Of-Pairs is number of pairs of
candidates with relay servers (e.g. in the case of a single relay
server, it would be 1). A smaller value than 500 ms for the RTO MUST
NOT be used.
4.3. NAT Traversal Mode Negotiation
This section describes the usage of a new non-critical parameter
type. The presence of the parameter in a HIP base exchange means
that the end-host supports NAT traversal extensions described in this
document. As the parameter is non-critical (as defined in
Section 5.2.1 of [RFC7401]), it can be ignored by an end-host, which
means that the host is not required to support it or may decline to
use it.
With registration with a HIP relay, it is usually sufficient to use
the UDP-ENCAPSULATION mode of NAT traversal since the relay is
assumed to be in public address space. Thus, the relay SHOULD
propose the UDP-ENCAPSULATION mode as the preferred or only mode.
The NAT traversal mode negotiation in a HIP base exchange is
illustrated in Figure 3. It is worth noting that the HIP relay could
be located between the hosts, but is omitted here for simplicity.
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Initiator Responder
| 1. UDP(I1) |
+--------------------------------------------------------------->|
| |
| 2. UDP(R1(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), ..)) |
|<---------------------------------------------------------------+
| |
| 3. UDP(I2(.., NAT_TRAVERSAL_MODE(ICE-HIP-UDP), LOC_SET, ..)) |
+--------------------------------------------------------------->|
| |
| 4. UDP(R2(.., LOC_SET, ..)) |
|<---------------------------------------------------------------+
| |
Figure 3: Negotiation of NAT Traversal Mode
In step 1, the Initiator sends an I1 to the Responder. In step 2,
the Responder responds with an R1. As specified in [RFC5770], the
NAT_TRAVERSAL_MODE parameter in R1 contains a list of NAT traversal
modes the Responder supports. The mode specified in this document is
ICE-HIP-UDP (value [TBD by IANA: 3]).
In step 3, the Initiator sends an I2 that includes a
NAT_TRAVERSAL_MODE parameter. It contains the mode selected by the
Initiator from the list of modes offered by the Responder. If ICE-
HIP-UDP mode was selected, the I2 also includes the "Transport
address" locators (as defined in Section 5.7) of the Initiator in a
LOCATOR_SET parameter (denoted here LOC_SET). The locators in I2 are
the "ICE offer".
In step 4, the Responder concludes the base exchange with an R2
packet. If the Initiator chose ICE NAT traversal mode, the Responder
includes a LOCATOR_SET parameter in the R2 packet. The locators in
R2, encoded like the locators in I2, are the "ICE answer". If the
NAT traversal mode selected by the Initiator is not supported by the
Responder, the Responder SHOULD reply with a NOTIFY packet with type
NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER and abort the base exchange.
4.4. Connectivity Check Pacing Negotiation
As explained in [RFC5770], when a NAT traversal mode with
connectivity checks is used, new transactions should not be started
too fast to avoid congestion and overwhelming the NATs. For this
purpose, during the base exchange, hosts can negotiate a transaction
pacing value, Ta, using a TRANSACTION_PACING parameter in R1 and I2
packets. The parameter contains the minimum time (expressed in
milliseconds) the host would wait between two NAT traversal
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transactions, such as starting a new connectivity check or retrying a
previous check. The value that is used by both of the hosts is the
higher of the two offered values.
The minimum Ta value SHOULD be configurable, and if no value is
configured, a value of 50 ms MUST be used. Guidelines for selecting
a Ta value are given in Appendix A. Hosts SHOULD NOT use values
smaller than 5 ms for the minimum Ta, since such values may not work
well with some NATs, as explained in [I-D.ietf-ice-rfc5245bis]. The
Initiator MUST NOT propose a smaller value than what the Responder
offered. If a host does not include the TRANSACTION_PACING parameter
in the base exchange, a Ta value of 50 ms MUST be used as that host's
minimum value.
4.5. Base Exchange via HIP Relay Server
This section describes how the Initiator and Responder perform a base
exchange through a HIP relay server. Connectivity pacing (denoted as
TA_P here) was described in Section 4.4 and is neither repeated here.
Similarly, the NAT traversal mode negotiation process (denoted as
NAT_TM in the example) was described in Section 4.3 and is neither
repeated here. If a relay receives an R1 or I2 packet without the
NAT traversal mode parameter, it MUST drop it and SHOULD send a
NOTIFY error packet with type NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER
to the sender of the R1 or I2.
It is RECOMMENDED that the Initiator send an I1 packet encapsulated
in UDP when it is destined to an IPv4 address of the Responder.
Respectively, the Responder MUST respond to such an I1 packet with a
UDP-encapsulated R1 packet, and also the rest of the communication
related to the HIP association MUST also use UDP encapsulation.
Figure 4 illustrates a base exchange via a HIP relay server. We
assume that the Responder (i.e. a HIP relay client) has already
registered to the HIP relay server. The Initiator may have also
registered to another (or the same relay server), but the base
exchange will traverse always through the relay of the Responder.
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Initiator HIP relay Responder
| 1. UDP(I1) | |
+--------------------------------->| 2. UDP(I1(RELAY_FROM)) |
| +------------------------------->|
| | |
| | 3. UDP(R1(RELAY_TO, NAT_TM, |
| | TA_P)) |
| 4. UDP(R1(RELAY_TO, NAT_TM, |<-------------------------------+
| TA_P)) | |
|<---------------------------------+ |
| | |
| 5. UDP(I2(LOC_SET, NAT_TM, | |
| TA_P)) | |
+--------------------------------->| 6. UDP(I2(LOC_SET, RELAY_FROM, |
| | NAT_TM, TA_P)) |
| +------------------------------->|
| | |
| | 7. UDP(R2(LOC_SET, RELAY_TO)) |
| 8. UDP(R2(LOC_SET, RELAY_TO)) |<-------------------------------+
|<---------------------------------+ |
| | |
Figure 4: Base Exchange via a HIP Relay Server
In step 1 of Figure 4, the Initiator sends an I1 packet over UDP via
the relay server to the Responder. In the HIP header, the source HIT
belongs to the Initiator and the destination HIT to the Responder.
The initiator sends the I1 packet from its IP address to the IP
address of the HIP relay over UDP.
In step 2, the HIP relay server receives the I1 packet. If the
destination HIT belongs to a registered Responder, the relay
processes the packet. Otherwise, the relay MUST drop the packet
silently. The relay appends a RELAY_FROM parameter to the I1 packet,
which contains the transport source address and port of the I1 as
observed by the relay. The relay protects the I1 packet with
RELAY_HMAC as described in [RFC8004], except that the parameter type
is different (see Section 5.8). The relay changes the source and
destination ports and IP addresses of the packet to match the values
the Responder used when registering to the relay, i.e., the reverse
of the R2 used in the registration. The relay MUST recalculate the
transport checksum and forward the packet to the Responder.
In step 3, the Responder receives the I1 packet. The Responder
processes it according to the rules in [RFC7401]. In addition, the
Responder validates the RELAY_HMAC according to [RFC8004] and
silently drops the packet if the validation fails. The Responder
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replies with an R1 packet to which it includes RELAY_TO and NAT
traversal mode parameters. The responder MUST include ICE-HIP-UDP in
the NAT traversal modes. The RELAY_TO parameter MUST contain the
same information as the RELAY_FROM parameter, i.e., the Initiator's
transport address, but the type of the parameter is different. The
RELAY_TO parameter is not integrity protected by the signature of the
R1 to allow pre-created R1 packets at the Responder.
In step 4, the relay receives the R1 packet. The relay drops the
packet silently if the source HIT belongs to an unregistered host.
The relay MAY verify the signature of the R1 packet and drop it if
the signature is invalid. Otherwise, the relay rewrites the source
address and port, and changes the destination address and port to
match RELAY_TO information. Finally, the relay recalculates
transport checksum and forwards the packet.
In step 5, the Initiator receives the R1 packet and processes it
according to [RFC7401]. The Initiator MAY use the address in the
RELAY_TO parameter as a local peer-reflexive candidate for this HIP
association if it is different from all known local candidates. The
Initiator replies with an I2 packet that uses the destination
transport address of R1 as the source address and port. The I2
packet contains a LOCATOR_SET parameter that lists all the HIP
candidates (ICE offer) of the Initiator. The candidates are encoded
using the format defined in Section 5.7. The I2 packet MUST also
contain a NAT traversal mode parameter that includes ICE-HIP-UDP
mode.
In step 6, the relay receives the I2 packet. The relay appends a
RELAY_FROM and a RELAY_HMAC to the I2 packet similarly as explained
in step 2, and forwards the packet to the Responder.
In step 7, the Responder receives the I2 packet and processes it
according to [RFC7401]. It replies with an R2 packet and includes a
RELAY_TO parameter as explained in step 3. The R2 packet includes a
LOCATOR_SET parameter that lists all the HIP candidates (ICE answer)
of the Responder. The RELAY_TO parameter is protected by the HMAC.
In step 8, the relay processes the R2 as described in step 4. The
relay forwards the packet to the Initiator. After the Initiator has
received the R2 and processed it successfully, the base exchange is
completed.
Hosts MUST include the address of one or more HIP relay servers
(including the one that is being used for the initial signaling) in
the LOCATOR_SET parameter in I2 and R2 if they intend to use such
servers for relaying HIP signaling immediately after the base
exchange completes. The traffic type of these addresses MUST be "HIP
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signaling" and they MUST NOT be used as HIP candidates. If the HIP
relay server locator used for relaying the base exchange is not
included in I2 or R2 LOCATOR_SET parameters, it SHOULD NOT be used
after the base exchange. Instead, further HIP signaling SHOULD use
the same path as the data traffic.
4.6. Connectivity Checks
When the Initiator and Responder complete the base exchange through
the HIP relay, both of them employ the IP address of the relay as the
destination address for the packets. This address MUST NOT be used
as a destination for ESP traffic unless the HIP relay supports also
ESP data relaying. When NAT traversal mode with ICE-HIP-UDP was
successfully negotiated and selected, the Initiator and Responder
MUST start the connectivity checks in order to attempt to obtain
direct end-to-end connectivity through NAT devices. It is worth
noting that the connectivity checks MUST be completed even though no
ESP_TRANSFORM would be negotiated and selected.
The connectivity checks MUST follow the ICE methodology [MMUSIC-ICE],
but UDP encapsulated HIP control messages are used instead of ICE
messages. Only normal nomination MUST be used for the connectivity
checks, i.e., aggressive nomination MUST NOT be employed. As stated
in the ICE specification, the basic procedure for connectivity checks
has three phases: sorting the candidate pairs according their
priority, sending checks in the prioritized order and acknowledging
the checks from the peer host.
The Initiator MUST take the role of controlling host and the
Responder acts as the controlled host. The roles MUST persist
throughout the HIP associate lifetime (to be reused in the possibly
mobility UPDATE procedures). In the case both communicating nodes
are initiating the communications to each other using an I1 packet,
the conflict is resolved as defined in section 6.7 in [RFC7401]: the
host with the "larger" HIT changes to its Role to Responder. In such
a case, the host changing its role to Responder MUST also switch to
controlling role.
The protocol follows standard HIP UPDATE sending and processing rules
as defined in section 6.11 and 6.12 in [RFC7401], but some new
parameters are introduced (CANDIDATE_PRIORITY, MAPPED_ADDRESS and
NOMINATE).
4.6.1. Connectivity Check Procedure
Figure 5 illustrates connectivity checks in a simplified scenario,
where the Initiator and Responder have only a single candidate pair
to check. Typically, NATs drop messages until both sides have sent
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messages using the same port pair. In this scenario, the Responder
sends a connectivity check first but the NAT of the Initiator drops
it. However, the connectivity check from the Initiator reaches the
Responder because it uses the same port pair as the first message.
It is worth noting that the message flow in this section is
idealistic, and, in practice, more messages would be dropped,
especially in the beginning. For instance, connectivity tests always
start with the candidates with the highest priority, which would be
host candidates (which would not reach the recipient in this
scenario).
Initiator NAT1 NAT2 Responder
| | 1. UDP(UPDATE(SEQ, CAND_PRIO, | |
| | ECHO_REQ_SIGN)) | |
| X<-----------------------------------+----------------+
| | | |
| 2. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | |
+-------------+------------------------------------+--------------->|
| | | |
| 3. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | |
|<------------+------------------------------------+----------------+
| | | |
| 4. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO)) | |
|<------------+------------------------------------+----------------+
| | | |
| 5. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) | |
+-------------+------------------------------------+--------------->|
| | | |
| 6. Other connectivity checks using UPDATE over UDP |
|<------------+------------------------------------+---------------->
| | | |
| 7. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO, NOMINATE)) |
+-------------+------------------------------------+--------------->|
| | | |
| 8. UDP(UPDATE(SEQ, ACK, ECHO_REQ_SIGN, ECHO_RESP_SIGN, |
| NOMINATE)) | |
|<------------+------------------------------------+----------------+
| | | |
| 9. UDP(UPDATE(ACK, ECHO_RESP_SIGN)) | |
+-------------+------------------------------------+--------------->+
| | | |
| 10. ESP data traffic over UDP | |
+<------------+------------------------------------+--------------->+
| | | |
Figure 5: Connectivity Checks
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In step 1, the Responder sends a connectivity check to the Initiator
that the NAT of the Initiator drops. The message includes a number
of parameters. As specified in [RFC7401]), the SEQ parameter
includes a running sequence identifier for the connectivity check.
The candidate priority (denoted "CAND_PRIO" in the figure) describes
the priority of the address candidate being tested. The
ECHO_REQUEST_SIGNED (denoted ECHO_REQ_SIGN in the figure) includes a
nonce that the recipient must sign and echo back as it is.
In step 2, the Initiator sends a connectivity check, using the same
address pair candidate as in the previous step, and the message
traverses successfully the NAT boxes. The message includes the same
parameters as in the previous step. It should be noted that the
sequence identifier is locally assigned by the Responder, so it can
be different than in the previous step.
In step 3, the Responder has successfully received the previous
connectivity check from the Initiator and starts to build a response
message. Since the message from the Initiator included a SEQ, the
Responder must acknowledge it using an ACK parameter. Also, the
nonce contained in the echo request must be echoed back in an
ECHO_REQUEST_SIGNED (denoted ECHO_REQUEST_SIGN) parameter. The
Responder includes also a MAPPED_ADDRESS parameter (denoted
MAPPED_ADDR in the figure) that contains the transport address of the
Initiator as observed by the Responder (i.e. peer reflexive
candidate). This message is successfully delivered to the Initiator,
and upon reception the Initiator marks the candidate pair as valid.
In step 4, the Responder retransmits the connectivity check sent in
the first step, since it was not acknowledged yet.
In step 5, the Initiator responds to the previous connectivity check
message from the Responder. The Initiator acknowledges the SEQ
parameter from the previous message using ACK parameter and the
ECHO_REQUEST_SIGN parameter with ECHO_RESPONSE_SIGNED. In addition,
it includes MAPPED_ADDR parameter that includes the peer reflexive
candidate. This response message is successfully delivered to the
Responder, and upon reception the Initiator marks the candidate pair
as valid.
In step 6, despite the two hosts now having valid address candidates,
the hosts still test the remaining address candidates in a similar
way as in the previous steps (due to the use of normal nomination).
It should be noted that each connectivity check has a unique sequence
number in the SEQ parameter.
In step 7, the Initiator has completed testing all address candidates
and nominates one address candidate to be used. It sends an UPDATE
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message using the selected address candidates that includes a number
of parameters: SEQ, ECHO_REQUEST_SIGN, CANDIDATE_PRIORITY and the
NOMINATE parameter.
In step 8, the Responder receives the message with NOMINATE parameter
from the Initiator. It sends a response that includes the NOMINATE
parameter in addition to a number of other parameters. The ACK and
ECHO_RESPONSE_SIGNED parameters acknowledge the SEQ and
ECHO_REQUEST_SIGN parameters from previous message from the
Initiator. The Responder includes SEQ and ECHO_REQUEST_SIGN
parameters in order to receive an acknowledgment from the Responder.
In step 9, the Initiator completes the candidate nomination process
by confirming the message reception to the Responder. In the
confirmation message, the ACK and ECHO_RESPONSE_SIGNED parameters
correspond to the SEQ and ECHO_REQUEST_SIGN parameters in the message
sent by the Responder in the previous step.
In step 10, the Initiator and Responder can start sending application
payload over the successfully nominated address candidates.
It is worth noting that if either host has registered a relayed
address candidate from a data relay, the host MUST activate the
address before connectivity checks by sending an UPDATE message
containing PEER_PERMISSION parameter as described in Section 4.12.1.
Otherwise, the relay drops ESP packets using the relayed address.
4.6.2. Rules for Connectivity Checks
The HITs of the two communicating hosts MUST be used as credentials
in this protocol (in contrast to ICE which employs username-password
fragments). A HIT pair uniquely identifies the corresponding HIT
association, and a SEQ number in an UPDATE message identifies a
particular connectivity check.
All of the connectivity check packets MUST be protected with HMACs
and signatures (even though the illustrations omitted them for
simplicity). Each connectivity check sent by a host MUST include a
SEQ parameter and ECHO_REQUEST_SIGN parameter, and correspondingly
the peer MUST respond to these using ACK and ECHO_RESPONSE_SIGNED
according to the rules specified in [RFC7401].
The host sending a connectivity check MUST validate that the response
uses the same pair of UDP ports, and drop the packet if this is not
the case.
A host may receive a connectivity check before it has received the
candidates from its peer. In such a case, the host MUST immediately
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generate a response, and then continue waiting for the candidates. A
host MUST NOT select a candidate pair until it has verified the pair
using a connectivity check as defined in section Section 4.6.1.
[RFC7401] states that UPDATE packets have to include either a SEQ or
ACK parameter (or both). According to the RFC, each SEQ parameter
should be acknowledged separately. In the context of NATs, this
means that some of the SEQ parameters sent in connectivity checks
will be lost or arrive out of order. From the viewpoint of the
recipient, this is not a problem since the recipient will just
"blindly" acknowledge the SEQ. However, the sender needs to be
prepared for lost sequence identifiers and ACKs parameters that
arrive out of order.
As specified in [RFC7401], an ACK parameter may acknowledge multiple
sequence identifiers. While the examples in the previous sections do
not illustrate such functionality, it is also permitted when
employing ICE-HIP-UDP mode.
In ICE-HIP-UDP mode, a retransmission of a connectivity check SHOULD
be sent with the same sequence identifier in the SEQ parameter. Some
tested address candidates will never produce a working address pair,
and thus may cause retransmissions. Upon successful nomination an
address pair, a host MAY immediately stop sending such
retransmissions.
ICE procedures for prioritizing candidates, eliminating redundant
candidates and forming check lists (including pruning) MUST be
followed as specified in [I-D.ietf-ice-rfc5245bis], with the
exception that the foundation, frozen candidates and default
candidates are not used. From viewpoint of the ICE specification
[I-D.ietf-ice-rfc5245bis], the protocol specified in this document
operates using Component ID of 1 on all candidates, and the
foundation of all candidates is unique. This specification defines
only "full ICE" mode, and the "lite ICE" is not supported. The
reasoning behind the missing features is described in Appendix B.
The connectivity check messages MUST be paced by the Ta value
negotiated during the base exchange as described in Section 4.4. If
neither one of the hosts announced a minimum pacing value, a value of
20 ms SHOULD be used.
Both hosts MUST form a priority ordered checklist and begin to check
transactions every Ta milliseconds as long as the checks are running
and there are candidate pairs whose tests have not started. The
retransmission timeout (RTO) for the connectivity check UPDATE
packets SHOULD be calculated as follows:
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RTO = MAX (500ms, Ta * (Num-Waiting + Num-In-Progress))
In the RTO formula, Ta is the value used for the connectivity check
pacing, Num-Waiting is the number of pairs in the checklist in the
"Waiting" state, and Num-In-Progress is the number of pairs in the
"In-Progress" state. This is identical to the formula in
[I-D.ietf-ice-rfc5245bis] when there is only one checklist. A
smaller value than 500 ms for the RTO MUST NOT be used.
Each connectivity check request packet MUST contain a
CANDIDATE_PRIORITY parameter (see Section 5.14) with the priority
value that would be assigned to a peer reflexive candidate if one was
learned from the corresponding check. An UPDATE packet that
acknowledges a connectivity check request MUST be sent from the same
address that received the check and delivered to the same address
where the check was received from. Each acknowledgment UPDATE packet
MUST contain a MAPPED_ADDRESS parameter with the port, protocol, and
IP address of the address where the connectivity check request was
received from.
Following ICE guidelines [I-D.ietf-ice-rfc5245bis], it is RECOMMENDED
to restrict the total number of connectivity checks to 100 for each
host association. This can be achieved by limiting the connectivity
checks to the 100 candidate pairs with the highest priority.
4.6.3. Rules for Concluding Connectivity Checks
The controlling agent may find multiple working candidate pairs. To
conclude the connectivity checks, it SHOULD nominate the pair with
the highest priority. The controlling agent MUST nominate a
candidate pair essentially by repeating a connectivity check using an
UPDATE message that contains a SEQ parameter (with new sequence
number), a ECHO_REQUEST_SIGN parameter, the priority of the candidate
in a CANDIDATE_PRIORITY parameter and NOMINATE parameter to signify
conclusion of the connectivity checks. Since the nominated address
pair has already been tested for reachability, the controlled host
should be able to receive the message. Upon reception, the
controlled host SHOULD select the nominated address pair. The
response message MUST include a SEQ parameter with a new sequence id,
acknowledgment of the sequence from the controlling host in an ACK
parameter, a new ECHO_REQUEST_SIGNED parameter, ECHO_RESPONSE_SIGNED
parameter corresponding to the ECHO_REQUEST_SIGNED parameter from the
controlling host and the NOMINATE parameter. After sending this
packet, the controlled host can create IPsec security associations
using the nominated address candidate for delivering application
payload to the controlling host. Since the message from the
controlled host included a new sequence id and echo request for
signature, the controlling host MUST acknowledge this with a new
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UPDATE message that includes an ACK and ECHO_RESPONSE_SIGNED
parameters. After this final concluding message, the controlling
host also can create IPsec security associations for delivering
application payload to the controlled host.
It is possible that packets are delayed by the network. Both hosts
MUST continue to respond to any connectivity checks despite an
address pair having been nominated.
If all the connectivity checks have failed, the hosts MUST NOT send
ESP traffic to each other but MAY continue communicating using HIP
packets and the locators used for the base exchange. Also, the hosts
SHOULD notify each other about the failure with a
CONNECTIVITY_CHECKS_FAILED NOTIFY packet (see Section 5.10).
4.7. NAT Traversal Alternatives
4.7.1. Minimal NAT Traversal Support
If the Responder has a fixed and publicly reachable IPv4 address and
does not employ a HIP relay, the explicit NAT traversal mode
negotiation MAY be omitted, and thus even the UDP-ENCAPSULATION mode
does not have to be negotiated. In such a scenario, the Initiator
sends an I1 message over UDP and the Responder responds with an R1
message over UDP without including any NAT traversal mode parameter.
The rest of the base exchange follows the procedures defined in
[RFC7401], except that the control and data plane use UDP
encapsulation. Here, the use of UDP for NAT traversal is agreed
implicitly. This way of operation is still subject to NAT timeouts,
and the hosts MUST employ NAT keepalives as defined in Section 4.10.
4.7.2. Base Exchange without Connectivity Checks
It is possible to run a base exchange without any connectivity checks
as defined in section 4.8 in [RFC5770]. The procedure is applicable
also in the context of this specification, so it is repeated here for
completeness.
In certain network environments, the connectivity checks can be
omitted to reduce initial connection set-up latency because a base
exchange acts as an implicit connectivity test itself. For this to
work, the Initiator MUST be able to reach the Responder by simply UDP
encapsulating HIP and ESP packets sent to the Responder's address.
Detecting and configuring this particular scenario is prone to
failure unless carefully planned.
In such a scenario, the Responder MAY include UDP-ENCAPSULATION NAT
traversal mode as one of the supported modes in the R1 packet. If
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the Responder has registered to a HIP relay server, it MUST also
include a LOCATOR_SET parameter in R1 that contains a preferred
address where the Responder is able to receive UDP-encapsulated ESP
and HIP packets. This locator MUST be of type "Transport address",
its Traffic type MUST be "both", and it MUST have the "Preferred bit"
set (see Table 1). If there is no such locator in R1, the source
address of R1 is used as the Responder's preferred address.
The Initiator MAY choose the UDP-ENCAPSULATION mode if the Responder
listed it in the supported modes and the Initiator does not wish to
use the connectivity checks defined in this document for searching
for a more optimal path. In this case, the Initiator sends the I2
with UDP-ENCAPSULATION mode in the NAT traversal mode parameter
directly to the Responder's preferred address (i.e., to the preferred
locator in R1 or to the address where R1 was received from if there
was no preferred locator in R1). The Initiator MAY include locators
in I2 but they MUST NOT be taken as address candidates, since
connectivity checks defined in this document will not be used for
connections with UDP-ENCAPSULATION NAT traversal mode. Instead, if
R2 and I2 are received and processed successfully, a security
association can be created and UDP-encapsulated ESP can be exchanged
between the hosts after the base exchange completes. However, the
Responder SHOULD NOT send any ESP to the Initiator's address before
it has received data from the Initiator, as specified in Sections
4.4.3. and 6.9 of [RFC7401] and in Sections 3.2.9 and 5.4 of
[RFC8046].
Since an I2 packet with UDP-ENCAPSULATION NAT traversal mode selected
MUST NOT be sent via a relay, the Responder SHOULD reject such I2
packets and reply with a NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER NOTIFY
packet (see Section 5.10).
If there is no answer for the I2 packet sent directly to the
Responder's preferred address, the Initiator MAY send another I2 via
the HIP relay server, but it MUST NOT choose UDP-ENCAPSULATION NAT
traversal mode for that I2.
4.7.3. Initiating a Base Exchange both with and without UDP
Encapsulation
It is possible to run a base exchange in parallel both with and
without UDP encapsulation as defined in section 4.9 in [RFC5770].
The procedure is applicable also in the context of this
specification, so it is repeated here for completeness.
The Initiator MAY also try to simultaneously perform a base exchange
with the Responder without UDP encapsulation. In such a case, the
Initiator sends two I1 packets, one without and one with UDP
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encapsulation, to the Responder. The Initiator MAY wait for a while
before sending the other I1. How long to wait and in which order to
send the I1 packets can be decided based on local policy. For
retransmissions, the procedure is repeated.
The I1 packet without UDP encapsulation may arrive directly, without
any relays, at the Responder. When this happens, the procedures in
[RFC7401] are followed for the rest of the base exchange. The
Initiator may receive multiple R1 packets, with and without UDP
encapsulation, from the Responder. However, after receiving a valid
R1 and answering it with an I2, further R1 packets that are not
retransmissions of the original R1 message MUST be ignored.
The I1 packet without UDP encapsulation may also arrive at a HIP-
capable middlebox. When the middlebox is a HIP rendezvous server and
the Responder has successfully registered with the rendezvous
service, the middlebox follows rendezvous procedures in [RFC8004].
If the Initiator receives a NAT traversal mode parameter in R1
without UDP encapsulation, the Initiator MAY ignore this parameter
and send an I2 without UDP encapsulation and without any selected NAT
traversal mode. When the Responder receives the I2 without UDP
encapsulation and without NAT traversal mode, it will assume that no
NAT traversal mechanism is needed. The packet processing will be
done as described in [RFC7401]. The Initiator MAY store the NAT
traversal modes for future use, e.g., in case of a mobility or
multihoming event that causes NAT traversal to be used during the
lifetime of the HIP association.
4.8. Sending Control Packets after the Base Exchange
The same considerations of sending control packets after the base
exchange described in section 5.10 in [RFC5770] apply also here, so
they are repeated here for completeness.
After the base exchange, the end-hosts MAY send HIP control packets
directly to each other using the transport address pair established
for a data channel without sending the control packets through the
HIP relay server. When a host does not get acknowledgments, e.g., to
an UPDATE or CLOSE packet after a timeout based on local policies,
the host SHOULD resend the packet through the relay, if it was listed
in the LOCATOR_SET parameter in the base exchange.
If control packets are sent through a HIP relay server, the host
registered with the relay MUST utilize the RELAY_TO parameter as in
the base exchange. The HIP relay server SHOULD forward HIP packets
to the registered hosts and forward packets from a registered host to
the address in the RELAY_TO parameter. The relay MUST add a
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RELAY_FROM parameter to the control packets it relays to the
registered hosts.
If the HIP relay server is not willing or able to relay a HIP packet,
it MAY notify the sender of the packet with MESSAGE_NOT_RELAYED error
notification (see Section 5.10).
4.9. Mobility Handover Procedure
A host may move after base exchange and connectivity checks.
Mobility extensions for HIP [RFC8046] define handover procedures
without NATs. In this section, we define how two hosts interact with
handover procedures in scenarios involving NATs. The specified
extensions define only simple mobility using a pair of security
associations, and multihoming extensions are left to be defined in
later specifications. The procedures in this section offer the same
functionality as "ICE restart" specified in
[I-D.ietf-ice-rfc5245bis]. The example described in this section
shows only a relay server for the peer host for the sake of
simplicity, but also the mobile host may also have a relay server.
The assumption here is that the two hosts have successfully
negotiated and chosen the ICE-HIP-UDP mode during the base exchange
as defined in Section 4.3. The Initiator of the base exchange MUST
store information that it was the controlling host during the base
exchange. Similarly, the Responder MUST store information that it
was the controlled host during the base exchange.
Prior to starting the handover procedures with all peer hosts, the
mobile host SHOULD first send UPDATE messages to its HIP and data
relays if it has registered to such. It SHOULD wait for all of them
to respond for two minutes and then continue with the handover
procedure without information from the relays that did not respond.
As defined in section Section 4.1, a response message from a relay
includes a REG_FROM parameter that describes the server reflexive
candidate of the mobile host to be used in the candidate exchange
during the handover. Similarly, an UPDATE to a data relay is
necessary to make sure the data relay can forward data to the correct
IP address after a handoff.
The mobility extensions for NAT traversal are illustrated in
Figure 6. The mobile host is the host that has changed its locators,
and the peer host is the host it has a host association with. The
mobile host may have multiple peers and it repeats the process with
all of its peers. In the figure, the HIP relay belongs to the peer
host, i.e., the peer host is a relay client for the HIP relay. Next,
we describe the procedure in the figure in detail.
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Mobile Host HIP relay Peer Host
| 1. UDP(UPDATE(ESP_INFO, | |
| LOC_SET, SEQ)) | |
+--------------------------------->| 2. UDP(UPDATE(ESP_INFO, |
| | LOC_SET, SEQ, |
| | RELAY_FROM)) |
| +------------------------------->|
| | |
| | 3. UDP(UPDATE(ESP_INFO, ACK, |
| | ECHO_REQ_SIGN)) |
| 4. UDP(UPDATE(ESP_INFO, ACK, |<-------------------------------+
| ECHO_REQ_SIGN, | |
| RELAY_TO)) | |
|<---------------------------------+ |
| | |
| 5. connectivity checks over UDP |
+<----------------------------------------------------------------->+
| | |
| 6. ESP data over UDP |
+<----------------------------------------------------------------->+
| | |
Figure 6: HIP UPDATE procedure
In step 1, the mobile host has changed location and sends a location
update to its peer through the HIP relay of the peer. It sends an
UPDATE packet with source HIT belonging to itself and destination HIT
belonging to the peer host. In the packet, the source IP address
belongs to the mobile host and the destination to the HIP relay. The
packet contains an ESP_INFO parameter, where, in this case, the OLD
SPI and NEW SPI parameters both contain the pre-existing incoming
SPI. The packet also contains the locators of the mobile host in a
LOCATOR_SET parameter. The packet contains also a SEQ number to be
acknowledged by the peer. As specified in [RFC8046], the packet may
also include a HOST_ID (for middlebox inspection) and DIFFIE_HELLMAN
parameter for rekeying.
In step 2, the HIP relay receives the UPDATE packet and forwards it
to the peer host (i.e. relay client). The HIP relay rewrites the
destination IP address and appends a RELAY_FROM parameter to the
message.
In step 3, the peer host receives the UPDATE packet, processes it and
responds with another UPDATE message. The message is destined to the
HIT of mobile host and to the IP address of the HIP relay. The
message includes an ESP_INFO parameter where, in this case, the OLD
SPI and NEW SPI parameters both contain the pre-existing incoming
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SPI. The peer includes a new SEQ and ECHO_REQUEST_SIGNED parameters
to be acknowledged by the mobile host. The message acknowledges the
SEQ parameter of the earlier message with an ACK parameter. After
this step, the peer host can initiate the connectivity checks.
In step 4, the HIP relay receives the message, rewrites the
destination IP address, appends an RELAY_TO parameter and forwards
the modified message to the mobile host. When mobile host has
processed the message successfully, it can initiate the connectivity
checks.
In step 5, the two hosts test for connectivity across NATs according
to procedures described in Section 4.6. The original Initiator of
the communications is the controlling and the original Responder is
the controlled host.
In step 6, the connectivity checks are successfully completed and the
controlling host has nominated one address pair to be used. The
hosts set up security associations to deliver the application
payload.
If either host has registered a relayed address candidate from a data
relay, the host MUST reactivate the address before connectivity
checks by sending an UPDATE message containing PEER_PERMISSION
parameter as described in Section 4.12.1. Otherwise, the relay drops
ESP packets using the relayed address.
4.10. NAT Keepalives
To prevent NAT states from expiring, communicating hosts send
periodic keepalives to other hosts with which they have established a
host associating. Both a registered client and relay server SHOULD
send HIP NOTIFY packets to each other every 15 seconds (the so called
Tr value in ICE) unless they have exchange some other traffic over
the used UDP ports. Other values MAY be used, but a Tr value smaller
than 15 seconds MUST NOT be used. The keepalive message encoding
format is defined in Section 5.3. If the base exchange or mobility
handover procedure occurs during an extremely slow path, a host MAY
also send HIP NOTIFY packet every 15 seconds to keep the path active
with the recipient.
4.11. Closing Procedure
The two-way procedure for closing a HIP association and the related
security associations is defined in [RFC7401]. One host initiates
the procedure by sending a CLOSE message and the recipient confirms
it with CLOSE_ACK. All packets are protected using HMACs and
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signatures, and the CLOSE messages includes a ECHO_REQUEST_SIGNED
parameter to protect against replay attacks.
The same procedure for closing HIP associations applies also here,
but the messaging occurs using the UDP encapsulated tunnel that the
two hosts employ. A host sending the CLOSE message SHOULD first send
the message over a direct link. After a number of retransmissions,
it MUST send over a HIP relay of the recipient if one exists. The
host receiving the CLOSE message directly without a relay SHOULD
respond directly. If CLOSE message came via a relay, the host SHOULD
respond using the same relay.
4.12. Relaying Considerations
4.12.1. Forwarding Rules and Permissions
The HIP data relay uses a similar permission model as a TURN server:
before the data relay forwards any ESP data packets from a peer to a
registered host (or the other direction), the client MUST set a
permission for the peer's address. The permissions also install a
forwarding rule for each direction, similar to TURN's channels, based
on the Security Parameter Index (SPI) values in the ESP packets.
Permissions are not required for HIP control packets. However, if a
relayed address (as conveyed in the RELAYED_ADDRESS parameter from
the data relay) is selected to be used for data, the registered host
MUST send an UPDATE message to the data relay containing a
PEER_PERMISSION parameter (see Section 5.13) with the address of the
peer, and the outbound and inbound SPI values the registered host is
using with this particular peer. To avoid packet dropping of ESP
packets, the registered host SHOULD send the PEER_PERMISSION
parameter before connectivity checks both in the case of base
exchange and a mobility handover. It is worth noting that the UPDATE
message includes a SEQ parameter (as specified in [RFC7401]) that the
data relay must acknowledge, so that the registered host can resend
the message with PEER_PERMISSION parameter if it gets lost.
When a data relay receives an UPDATE with a PEER_PERMISSION
parameter, it MUST check if the sender of the UPDATE is registered
for data relaying service, and drop the UPDATE if the host was not
registered. If the host was registered, the relay checks if there is
a permission with matching information (address, protocol, port and
SPI values). If there is no such permission, a new permission MUST
be created and its lifetime MUST be set to 5 minutes. If an
identical permission already existed, it MUST be refreshed by setting
the lifetime to 5 minutes. A registered host SHOULD refresh
permissions 1 minute before the expiration when the permission is
still needed.
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When a data relay receives an UPDATE from a registered client but
without a PEER_PERMISSION parameter and with a new locator set, the
relay can assume that the mobile host has changed its location and,
thus, is not reachable in its previous location. In such an event,
the data relay SHOULD deactivate the permission and stop relaying
data plane traffic to the client.
The relayed address MUST be activated with the PEER_PERMISSION
parameter both after a base exchange and after a handover procedure
with another ICE-HIP-UDP capable host. Unless activated, the data
relay MUST drop all ESP packets. It is worth noting that a relay
client does not have to renew its registration upon a change of
location UPDATE, but only when the lifetime of the registration is
close to end.
4.12.2. HIP Data Relay and Relaying of Control Packets
When a HIP data relay accepts to relay UDP encapsulated ESP between a
registered host and its peer, the relay opens a UDP port (relayed
address) for this purpose as described in Section 4.1. This port can
be used for delivering also control packets because connectivity
checks also cover the path through the data relay. If the data relay
receives a UDP encapsulated HIP control packet on that port, it MUST
forward the packet to the registered host and add a RELAY_FROM
parameter to the packet as if the data relay were acting as a HIP
relay server. When the registered host replies to a control packet
with a RELAY_FROM parameter via its relay, the registered host MUST
add a RELAY_TO parameter containing the peer's address and use the
address of its data relay as the destination address. Further, the
data relay MUST send this packet to the peer's address from the
relayed address.
If the data relay receives a UDP packet that is not a HIP control
packet to the relayed address, it MUST check if it has a permission
set for the peer the packet is arriving from (i.e., the sender's
address and SPI value matches to an installed permission). If
permissions are set, the data relay MUST forward the packet to the
registered host that created the permission. The data relay MUST
also implement the similar checks for the reverse direction (i.e.
ESP packets from the registered host to the peer). Packets without a
permission MUST be dropped silently.
4.12.3. Handling Conflicting SPI Values
The inbound SPI values of the registered clients should be unique so
that a data relay can properly demultiplex incoming packets from peer
hosts to the correct registered clients. Likewise, the inbound SPIs
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of the peer hosts should be unique for the same reason. These two
cases are discussed in this section separately.
In first case, the SPI collision problem occurs when two Initiators
run a base exchange to the same Responder (i.e. registered host), and
both the Initiators claim the same inbound SPI. This is not a
problem for Responder since the two Initiators can be distinguished
by their transport addresses. However, it is an issue for the data
relay because the it cannot demultiplex packets from the Initiator to
the correct Responder. Thus, upon receiving an I2 with a colliding
SPI, the Responder MUST NOT include the relayed address candidate in
the R2 message because the data relay would not be able demultiplex
the related ESP packet to the correct Initiator. The same applies
also the handover procedure; the registered host MUST NOT include the
relayed address candidate when sending its new locator set in an
UPDATE to its peer if it would cause a SPI conflict with another
peer. Since the SPI space is 32 bits and the SPI values should be
random, the probability for a conflicting SPI value is fairly small.
However, a registered host with many peers MAY proactively decrease
the odds of a conflict by registering to multiple data relays. The
described collision scenario can be avoided if the Responder delivers
a new relayed address candidate upon SPI collisions. Each relayed
address has a separate UDP port reserved to it, so the relay can
demultiplex properly conflicting SPIs of the Initiators based on the
SPI and port number towards the correct Responder.
In the second case, the SPI collision problems occurs if two hosts
have registered to the same data relay and a third host initiates
base exchange with both of them. In this case, the data relay has
allocated separate UDP ports for the two registered hosts acting now
as Responders. When the Responders send identical SPI values in
their I2 messages via the relay, the relay can properly deliver the
message to the correct Responder because the UDP ports are different.
5. Packet Formats
The following subsections define the parameter and packet encodings
for the HIP and ESP packets. All values MUST be in network byte
order.
It is worth noting that most of the parameters are shown for the sake
of completeness even though they are specified already in [RFC5770].
New parameters are explicitly described as new.
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5.1. HIP Control Packets
Figure 7 illustrates the packet format for UDP-encapsulated HIP. The
format is identical to [RFC5770].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32 bits of zeroes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ HIP Header and Parameters ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format of UDP-Encapsulated HIP Control Packets
HIP control packets are encapsulated in UDP packets as defined in
Section 2.2 of [RFC3948], "IKE Header Format for Port 4500", except
that a different port number is used. Figure 7 illustrates the
encapsulation. The UDP header is followed by 32 zero bits that can
be used to differentiate HIP control packets from ESP packets. The
HIP header and parameters follow the conventions of [RFC7401] with
the exception that the HIP header checksum MUST be zero. The HIP
header checksum is zero for two reasons. First, the UDP header
already contains a checksum. Second, the checksum definition in
[RFC7401] includes the IP addresses in the checksum calculation. The
NATs that are unaware of HIP cannot recompute the HIP checksum after
changing IP addresses.
A HIP relay server or a Responder without a relay SHOULD listen at
UDP port 10500 for incoming UDP-encapsulated HIP control packets. If
some other port number is used, it needs to be known by potential
Initiators.
5.2. Connectivity Checks
HIP connectivity checks are HIP UPDATE packets. The format is
specified in [RFC7401].
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5.3. Keepalives
The RECOMMENDED encoding format for keepalives is HIP NOTIFY packets
as specified in [RFC7401] with Notify message type field set to
NAT_KEEPALIVE [TBD by IANA: 16384] and with an empty Notification
data field. It is worth noting that sending of such a HIP NOTIFY
message MAY be omitted if the host is actively (or passively) sending
other traffic to the peer host over the UDP tunnel associate with the
host association (and IPsec security associations since the same port
pair is reused) during the Tr period. For instance, the host MAY
actively send ICMPv6 requests (or respond with an ICMPv6 response)
inside the ESP tunnel to test the health of the associated IPsec
security associations. Alternatively, the host MAY use UPDATE
packets as a substitute. A minimal UPDATE packet would consist of a
SEQ and ECHO_REQ_SIGN parameters, and a more complex would involve
rekeying procedures as specified in section 6.8 in [RFC7402]. It is
worth noting that a host actively sending periodic UPDATE packets to
a busy server may increase the computational load of the server since
it has to verify HMACs and signatures in UPDATE messages.
5.4. NAT Traversal Mode Parameter
The format of NAT traversal mode parameter is borrowed from
[RFC5770]. The format of the NAT_TRAVERSAL_MODE parameter is similar
to the format of the ESP_TRANSFORM parameter in [RFC7402] and is
shown in Figure 8. This specification defines the traversal mode
identifier for ICE-HIP-UDP and reuses the UDP-ENCAPSULATION mode from
[RFC5770]. The identifier named RESERVED is reserved for future use.
Future specifications may define more traversal modes.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Mode ID #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mode ID #2 | Mode ID #3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mode ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 608
Length length in octets, excluding Type, Length, and padding
Reserved zero when sent, ignored when received
Mode ID defines the proposed or selected NAT traversal mode(s)
The following NAT traversal mode IDs are defined:
ID name Value
RESERVED 0
UDP-ENCAPSULATION 1
ICE-HIP-UDP 3
Figure 8: Format of the NAT_TRAVERSAL_MODE Parameter
The sender of a NAT_TRAVERSAL_MODE parameter MUST make sure that
there are no more than six (6) Mode IDs in one NAT_TRAVERSAL_MODE
parameter. Conversely, a recipient MUST be prepared to handle
received NAT traversal mode parameters that contain more than six
Mode IDs by accepting the first six Mode IDs and dropping the rest.
The limited number of Mode IDs sets the maximum size of the
NAT_TRAVERSAL_MODE parameter. The modes MUST be in preference order,
most preferred mode(s) first.
Implementations conforming to this specification MUST implement UDP-
ENCAPSULATION and SHOULD implement ICE-HIP-UDP modes.
5.5. Connectivity Check Transaction Pacing Parameter
The TRANSACTION_PACING is a new parameter, and it shown in Figure 9
contains only the connectivity check pacing value, expressed in
milliseconds, as a 32-bit unsigned integer.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min Ta |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 610
Length 4
Min Ta the minimum connectivity check transaction pacing
value the host would use (in milliseconds)
Figure 9: Format of the TRANSACTION_PACING Parameter
5.6. Relay and Registration Parameters
The format of the REG_FROM, RELAY_FROM, and RELAY_TO parameters is
shown in Figure 10. All parameters are identical except for the
type. REG_FROM is the only parameter covered with the signature.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Port | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type REG_FROM: 950
RELAY_FROM: 63998
RELAY_TO: 64002
Length 20
Port transport port number; zero when plain IP is used
Protocol IANA assigned, Internet Protocol number.
17 for UDP, 0 for plain IP
Reserved reserved for future use; zero when sent, ignored
when received
Address an IPv6 address or an IPv4 address in "IPv4-Mapped
IPv6 address" format
Figure 10: Format of the REG_FROM, RELAY_FROM, and RELAY_TO
Parameters
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REG_FROM contains the transport address and protocol from which the
HIP relay server sees the registration coming. RELAY_FROM contains
the address from which the relayed packet was received by the relay
server and the protocol that was used. RELAY_TO contains the same
information about the address to which a packet should be forwarded.
5.7. LOCATOR_SET Parameter
This specification reuses the format for UDP-based locators specified
in [RFC5770] to be used for communicating the address candidates
between two hosts. The generic and NAT-traversal-specific locator
parameters are illustrated in Figure 11.
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 | Loc Type = 2 | Locator Length| Reserved |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Port | Transp. Proto| Kind |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: LOCATOR_SET Parameter
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The individual fields in the LOCATOR_SET parameter are described in
Table 1.
+-----------+----------+--------------------------------------------+
| Field | Value(s) | Purpose |
+-----------+----------+--------------------------------------------+
| Type | 193 | Parameter type |
| Length | Variable | Length in octets, excluding Type and |
| | | Length fields and padding |
| Traffic | 0-2 | Is the locator for HIP signaling (1), for |
| Type | | ESP (2), or for both (0) |
| Locator | 2 | "Transport address" locator type |
| Type | | |
| Locator | 7 | Length of the fields after Locator |
| Length | | Lifetime in 4-octet units |
| Reserved | 0 | Reserved for future extensions |
| Preferred | 0 or 1 | Set to 1 for a Locator in R1 if the |
| (P) bit | | Responder can use it for the rest of the |
| | | base exchange, otherwise set to zero |
| Locator | Variable | Locator lifetime in seconds |
| Lifetime | | |
| Transport | Variable | Transport layer port number |
| Port | | |
| Transport | Variable | IANA assigned, transport layer Internet |
| Protocol | | Protocol number. Currently only UDP (17) |
| | | is supported. |
| Kind | Variable | 0 for host, 1 for server reflexive, 2 for |
| | | peer reflexive or 3 for relayed address |
| Priority | Variable | Locator's priority as described in |
| | | [I-D.ietf-ice-rfc5245bis]. It is worth |
| | | noting that while the priority of a single |
| | | locator candidate is 32-bits, but an |
| | | implementation should use a 64-bit integer |
| | | to calculate the priority of a candidate |
| | | pair for the ICE priority algorithm. |
| SPI | Variable | Security Parameter Index (SPI) value that |
| | | the host expects to see in incoming ESP |
| | | packets that use this locator |
| Address | Variable | IPv6 address or an "IPv4-Mapped IPv6 |
| | | address" format IPv4 address [RFC4291] |
+-----------+----------+--------------------------------------------+
Table 1: Fields of the LOCATOR_SET Parameter
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5.8. RELAY_HMAC Parameter
As specified in [RFC5770], the RELAY_HMAC parameter value has the TLV
type 65520. It has the same semantics as RVS_HMAC [RFC8004].
5.9. Registration Types
The REG_INFO, REG_REQ, REG_RESP, and REG_FAILED parameters contain
Registration Type [RFC8003] values for HIP relay server registration.
The value for RELAY_UDP_HIP is 2 as specified in [RFC5770].
5.10. Notify Packet Types
A HIP relay server and end-hosts can use NOTIFY packets to signal
different error conditions. The NOTIFY packet types are the same as
in [RFC5770].
The Notify Packet Types [RFC7401] are shown below. The Notification
Data field for the error notifications SHOULD contain the HIP header
of the rejected packet and SHOULD be empty for the
CONNECTIVITY_CHECKS_FAILED type.
NOTIFICATION PARAMETER - ERROR TYPES Value
------------------------------------ -----
NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER 60
If a HIP relay server does not forward a base exchange packet due
to missing NAT traversal mode parameter, or the Initiator selects
a NAT traversal mode that the Responder did not expect, the relay
or the Responder may send back a NOTIFY error packet with this
type.
CONNECTIVITY_CHECKS_FAILED 61
Used by the end-hosts to signal that NAT traversal connectivity
checks failed and did not produce a working path.
MESSAGE_NOT_RELAYED 62
Used by a HIP relay server to signal that is was not able or
willing to relay a HIP packet.
5.11. ESP Data Packets
The format for ESP data packets is identical to [RFC5770].
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[RFC3948] describes the UDP encapsulation of the IPsec ESP transport
and tunnel mode. On the wire, the HIP ESP packets do not differ from
the transport mode ESP, and thus the encapsulation of the HIP ESP
packets is same as the UDP encapsulation transport mode ESP.
However, the (semantic) difference to Bound End-to-End Tunnel (BEET)
mode ESP packets used by HIP is that IP header is not used in BEET
integrity protection calculation.
During the HIP base exchange, the two peers exchange parameters that
enable them to define a pair of IPsec ESP security associations (SAs)
as described in [RFC7402]. When two peers perform a UDP-encapsulated
base exchange, they MUST define a pair of IPsec SAs that produces
UDP-encapsulated ESP data traffic.
The management of encryption/authentication protocols and SPIs is
defined in [RFC7402]. The UDP encapsulation format and processing of
HIP ESP traffic is described in Section 6.1 of [RFC7402].
5.12. RELAYED_ADDRESS and MAPPED_ADDRESS Parameters
While the type values are new, the format of the RELAYED_ADDRESS and
MAPPED_ADDRESS parameters (Figure 12) is identical to REG_FROM,
RELAY_FROM and RELAY_TO parameters. This document specifies only the
use of UDP relaying, and, thus, only protocol 17 is allowed.
However, future documents may specify support for other protocols.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Port | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type [TBD by IANA;
RELAYED_ADDRESS: 4650
MAPPED_ADDRESS: 4660]
Length 20
Port the UDP port number
Protocol IANA assigned, Internet Protocol number (17 for UDP)
Reserved reserved for future use; zero when sent, ignored
when received
Address an IPv6 address or an IPv4 address in "IPv4-Mapped
IPv6 address" format
Figure 12: Format of the RELAYED_ADDRESS and MAPPED_ADDRESS
Parameters
5.13. PEER_PERMISSION Parameter
The format of the new PEER_PERMISSION parameter is shown in
Figure 13. The parameter is used for setting up and refreshing
forwarding rules and the permissions for data packets at the data
relay. The parameter contains one or more sets of Port, Protocol,
Address, Outbound SPI (OSPI), and Inbound SPI (ISPI) values. One set
defines a rule for one peer address.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Port | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OSPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ISPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type [TBD by IANA; 4680]
Length length in octets, excluding Type and Length
Port the transport layer (UDP) port number of the peer
Protocol IANA assigned, Internet Protocol number (17 for UDP)
Reserved reserved for future use; zero when sent, ignored
when received
Address an IPv6 address, or an IPv4 address in "IPv4-Mapped
IPv6 address" format, of the peer
OSPI the outbound SPI value the registered host is using for
the peer with the Address and Port
ISPI the inbound SPI value the registered host is using for
the peer with the Address and Port
Figure 13: Format of the PEER_PERMISSION Parameter
5.14. HIP Connectivity Check Packets
The connectivity request messages are HIP UPDATE packets containing a
new CANDIDATE_PRIORITY parameter (Figure 14). Response UPDATE
packets contain a MAPPED_ADDRESS parameter (Figure 12).
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type [TBD by IANA; 4700]
Length 4
Priority the priority of a (potential) peer reflexive candidate
Figure 14: Format of the CANDIDATE_PRIORITY Parameter
5.15. NOMINATE parameter
Figure 15 shows the NOMINATE parameter that is used to conclude the
candidate nomination process.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type [TBD by IANA; 4710]
Length 4
Reserved Reserved for future extension purposes
Figure 15: Format of the NOMINATE Parameter
6. Security Considerations
The security considerations are the same as in [RFC5770], but are
repeated here for the sake of completeness.
6.1. Privacy Considerations
The locators are in plain text format in favor of inspection at HIP-
aware middleboxes in the future. The current document does not
specify encrypted versions of LOCATOR_SETs, even though it could be
beneficial for privacy reasons to avoid disclosing them to
middleboxes.
It is also possible that end-users may not want to reveal all
locators to each other. For example, tracking the physical location
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of a multihoming end-host may become easier if it reveals all
locators to its peer during a base exchange. Also, revealing host
addresses exposes information about the local topology that may not
be allowed in all corporate environments. For these two reasons, an
end-host may exclude certain host addresses from its LOCATOR_SET
parameter. However, such behavior creates non-optimal paths when the
hosts are located behind the same NAT. Especially, this could be
problematic with a legacy NAT that does not support routing from the
private address realm back to itself through the outer address of the
NAT. This scenario is referred to as the hairpin problem [RFC5128].
With such a legacy NAT, the only option left would be to use a
relayed transport address from a TURN server.
The use of HIP and data relays can be also useful for privacy
purposes. For example, a privacy concerned Responder may reveal only
its HIP relay server and Relayed candidates to Initiators. This same
mechanism also protects the Responder against Denial-of-Service (DoS)
attacks by allowing the Responder to initiate new connections even if
its relays would be unavailable due to a DoS attack.
6.2. Opportunistic Mode
A HIP relay server should have one address per relay client when a
HIP relay is serving more than one relay client and supports
opportunistic mode. Otherwise, it cannot be guaranteed that the HIP
relay server can deliver the I1 packet to the intended recipient.
6.3. Base Exchange Replay Protection for HIP Relay Server
In certain scenarios, it is possible that an attacker, or two
attackers, can replay an earlier base exchange through a HIP relay
server by masquerading as the original Initiator and Responder. The
attack does not require the attacker(s) to compromise the private
key(s) of the attacked host(s). However, for this attack to succeed,
the Responder has to be disconnected from the HIP relay server.
The relay can protect itself against replay attacks by becoming
involved in the base exchange by introducing nonces that the end-
hosts (Initiator and Responder) are required to sign. One way to do
this is to add ECHO_REQUEST_M parameters to the R1 and I2 packets as
described in [HIP-MIDDLE] and drop the I2 or R2 packets if the
corresponding ECHO_RESPONSE_M parameters are not present.
6.4. Demultiplexing Different HIP Associations
Section 5.1 of [RFC3948] describes a security issue for the UDP
encapsulation in the standard IP tunnel mode when two hosts behind
different NATs have the same private IP address and initiate
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communication to the same Responder in the public Internet. The
Responder cannot distinguish between two hosts, because security
associations are based on the same inner IP addresses.
This issue does not exist with the UDP encapsulation of HIP ESP
transport format because the Responder uses HITs to distinguish
between different Initiators.
6.5. Reuse of Ports at the Data Relay
If the data relay uses the same relayed address and port (as conveyed
in the RELAYED_ADDRESS parameter) for multiple registered hosts, it
appears to all the peers, and their firewalls, that all the
registered hosts using the relay are at the same address. Thus, a
stateful firewall may allow packets pass from hosts that would not
normally be able to send packets to a peer behind the firewall.
Therefore, a HIP data relay SHOULD NOT re-use the port numbers. If
port numbers need to be re-used, the relay SHOULD have a sufficiently
large pool of port numbers and select ports from the pool randomly to
decrease the chances of a registered host obtaining the same address
that a another host behind the same firewall is using.
6.6. Amplification attacks
A malicious host may send an invalid list of candidates for its peer
that are used for targeting a victim host by flooding it with
connectivity checks. To mitigate the attack, this protocol adopts
the ICE mechanism to cap the total amount of connectivity checks as
defined in section Section 4.7.
6.7. Attacks against Connectivity Checks and Candidate Gathering
Section 13.1 in [I-D.ietf-ice-rfc5245bis] discusses about attacks
against ICE connectivity checks. HIP bases its control plane
security on Diffie-Hellman key exchange, public keys and Hashed
Message Authentication codes, meaning that the mentioned security
concerns do not apply to HIP either. The mentioned section discusses
also of man-in-the-middle replay attacks that are difficult to
prevent. The connectivity checks in this protocol are immune against
replay attacks because a connectivity request includes a random nonce
that the recipient must sign and send back as a response.
Section 13.2 in [I-D.ietf-ice-rfc5245bis] discusses attacks on server
reflexive address gathering. Similarly here, if the DNS, a HIP relay
or a HIP data relay server has been compromised, not much can be
done. However, the case where attacker can inject fake messages
(located on a shared network segment like Wifi) does not apply here.
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HIP messages are integrity and replay protected, so it is not
possible inject fake server reflexive address candidates.
Section 13.3 in [I-D.ietf-ice-rfc5245bis] discusses attacks on
relayed candidate gathering. Similarly to ICE TURN servers, data
relays require an authenticated base exchange that protects relayed
address gathering against fake requests and responses. Further,
replay attacks are not possible because the HIP base exchange (and
also UPDATE procedure) is protected against replay attacks.
7. IANA Considerations
This section is to be interpreted according to [RFC5226].
This document updates the IANA Registry for HIP Parameter Types
[RFC7401] by assigning new HIP Parameter Type values for the new HIP
Parameters: RELAYED_ADDRESS, MAPPED_ADDRESS (defined in
Section 5.12), and PEER_PERMISSION (defined in Section 5.13).
This document updates the IANA Registry for HIP NAT traversal modes
[RFC5770] by assigning value for the NAT traversal mode ICE-HIP-UDP
(defined in Section 5.4) This specification introduces a new
keepalive Notify message type field NAT_KEEPALIVE.
This document defines additional registration types for the HIP
Registration Extension [RFC8003] that allow registering with a HIP
relay server for ESP relaying service: RELAY_UDP_ESP (defined in
Section 4.1; and performing server reflexive candidate discovery:
CANDIDATE_DISCOVERY (defined in Section 4.2).
ICE specifications discuss "Unilateral Self-Address Fixing" in
section 17 in [I-D.ietf-ice-rfc5245bis]. This protocol is based on
ICE, and thus the same considerations apply also here with one
exception: this protocol does not hide binary IP addresses from
application-level gateways.
8. Contributors
Marcelo Bagnulo, Philip Matthews and Hannes Tschofenig have
contributed to [RFC5770]. This document leans heavily on the work in
the RFC.
9. Acknowledgments
Thanks to Jonathan Rosenberg and the rest of the MMUSIC WG folks for
the excellent work on ICE. In addition, the authors would like to
thank Andrei Gurtov, Simon Schuetz, Martin Stiemerling, Lars Eggert,
Vivien Schmitt, and Abhinav Pathak for their contributions and Tobias
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Heer, Teemu Koponen, Juhana Mattila, Jeffrey M. Ahrenholz, Kristian
Slavov, Janne Lindqvist, Pekka Nikander, Lauri Silvennoinen, Jukka
Ylitalo, Juha Heinanen, Joakim Koskela, Samu Varjonen, Dan Wing, Tom
Henderson, Alex Elsayed and Jani Hautakorpi for their comments to
[RFC5770], which is the basis for this document.
This work has been partially funded by CyberTrust programme by
Digile/Tekes in Finland.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
RFC 7401, DOI 10.17487/RFC7401, April 2015,
<http://www.rfc-editor.org/info/rfc7401>.
[RFC8003] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Registration Extension", RFC 8003, DOI 10.17487/RFC8003,
October 2016, <http://www.rfc-editor.org/info/rfc8003>.
[RFC8004] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
October 2016, <http://www.rfc-editor.org/info/rfc8004>.
[RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
with the Host Identity Protocol", RFC 8046,
DOI 10.17487/RFC8046, February 2017,
<http://www.rfc-editor.org/info/rfc8046>.
[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, DOI 10.17487/RFC5770, April 2010,
<http://www.rfc-editor.org/info/rfc5770>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
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[RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 7402,
DOI 10.17487/RFC7402, April 2015,
<http://www.rfc-editor.org/info/rfc7402>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[I-D.ietf-ice-rfc5245bis]
Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", draft-ietf-ice-
rfc5245bis-08 (work in progress), December 2016.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<http://www.rfc-editor.org/info/rfc2475>.
10.2. Informative References
[RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May
2006, <http://www.rfc-editor.org/info/rfc4423>.
[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
Henderson, "Host Identity Protocol", RFC 5201,
DOI 10.17487/RFC5201, April 2008,
<http://www.rfc-editor.org/info/rfc5201>.
[RFC5207] Stiemerling, M., Quittek, J., and L. Eggert, "NAT and
Firewall Traversal Issues of Host Identity Protocol (HIP)
Communication", RFC 5207, DOI 10.17487/RFC5207, April
2008, <http://www.rfc-editor.org/info/rfc5207>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<http://www.rfc-editor.org/info/rfc5766>.
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[RFC6538] Henderson, T. and A. Gurtov, "The Host Identity Protocol
(HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538,
March 2012, <http://www.rfc-editor.org/info/rfc6538>.
[MMUSIC-ICE]
Rosenberg, J., "Guidelines for Usage of Interactive
Connectivity Establishment (ICE) by non Session Initiation
Protocol (SIP) Protocols", Work in Progress, July 2008.
[RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March
2008, <http://www.rfc-editor.org/info/rfc5128>.
[HIP-MIDDLE]
Heer, T., Wehrle, K., and M. Komu, "End-Host
Authentication for HIP Middleboxes", Work in Progress,
February 2009.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, DOI 10.17487/RFC3948, January 2005,
<http://www.rfc-editor.org/info/rfc3948>.
Appendix A. Selecting a Value for Check Pacing
Selecting a suitable value for the connectivity check transaction
pacing is essential for the performance of connectivity check-based
NAT traversal. The value should not be so small that the checks
cause network congestion or overwhelm the NATs. On the other hand, a
pacing value that is too high makes the checks last for a long time,
thus increasing the connection setup delay.
The Ta value may be configured by the user in environments where the
network characteristics are known beforehand. However, if the
characteristics are not known, it is recommended that the value is
adjusted dynamically. In this case, it is recommended that the hosts
estimate the round-trip time (RTT) between them and set the minimum
Ta value so that only two connectivity check messages are sent on
every RTT.
One way to estimate the RTT is to use the time that it takes for the
HIP relay server registration exchange to complete; this would give
an estimate on the registering host's access link's RTT. Also, the
I1/R1 exchange could be used for estimating the RTT, but since the R1
can be cached in the network, or the relaying service can increase
the delay notably, this is not recommended.
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Appendix B. Differences with respect to ICE
The protocol specified in this document follows the semantics of ICE
as close as possible, and most of the differences are syntactical due
to the use of a different protocol. In this section, we describe the
differences to the ICE protocol.
o ICE operates at the application layer, whereas this protocol
operates between transport and network layers, thus hiding the
protocol details from the application.
o The STUN protocol is not employed. Instead, this protocol reuses
the HIP control plane format in order simplify demultiplexing of
different protocols. For example, the STUN binding response is
replaced with a HIP UPDATE message containing an ECHO_REQUEST_SIGN
parameter and the STUN binding response with a HIP UPDATE message
containing an ECHO_RESPONSE_SIGNED parameter as defined in section
Section 4.6.
o The TURN protocol is not utilized. Instead, this protocol reuses
HIP relay servers for the same purpose.
o ICMP errors may be used in ICE to signal failure. In this
protocol, HIP NOTIFY messages are used instead.
o Instead of the ICE username fragment and password mechanism for
credentials, this protocol uses the HIT, derived from a public
key, for the same purpose. The username fragments are "transient
host identifiers, bound to a particular session established as
part of the candidate exchange" [I-D.ietf-ice-rfc5245bis]. In
HIP, a local public key and the derived HIT are considered long-
term identifiers, and invariant across different host associations
and different transport-layer flows.
o In ICE, the conflict when two communicating end-points take the
same controlling role is solved using random values (so called
tie-breaker value). In this protocol, the conflict is solved by
the standard HIP base exchange procedure, where the host with the
"larger" HIT switches to Responder role, thus changing also to
controlled role.
o The ICE-CONTROLLED and ICE-CONTROLLING attributes are not included
in the connectivity checks.
o The foundation concept is unnecessary in this protocol because
only a single UDP flow for the IPsec tunnel will be negotiated.
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o Frozen candidates are omitted for the same reason as foundation
concept is excluded.
o Components are omitted for the same reason as foundation concept
is excluded.
o This protocol supports only "full ICE" where the two communicating
hosts participate actively to the connectivity checks, and the
"lite" mode is not supported. This design decision follows the
guidelines of ICE which recommends full ICE implementations.
However, it should be noted that a publicly reachable Responder
may refuse to negotiate the ICE mode as described in
Section 4.7.2. This would result in a [RFC7401] based HIP base
exchange tunneled over UDP followed ESP traffic over the same
tunnel, without the connectivity check procedures defined in this
document (in some sense, this mode corresponds to the case where
two ICE lite implementations connect since no connectivity checks
are sent).
o As the "ICE lite" is not adopted here and both sides are capable
of ICE-HIP-HIP mode (negotiated during the base exchange), default
candidates are not employed here.
o If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its
connectivity checks.
o Unlike in ICE, the addresses are not XOR-ed in this protocol in
order to avoid middlebox tampering.
o This protocol does not employ the ICE related address and related
port attributes (that are used for diagnostic or SIP purposes).
Appendix C. Differences to Base Exchange and UPDATE procedures
This section gives some design guidance for implementers how the
extensions in this protocol extends and differs from [RFC7401] and
[RFC8046].
o Both control and data plane are operated on top of UDP, not
directly on IP.
o A minimal implementation would conform only to Section 4.7.1 or
Section 4.7.2, thus merely tunneling HIP control and data traffic
over UDP. The drawback here is that it works only in the limited
cases where the Responder has a public address.
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o It is worth noting that while a rendezvous server [RFC8004] has
not been designed to be used in NATted scenarios because it just
relays the first I1 packet and does not employ UDP encapsulation,
the HIP relay forwards all control traffic and, hence, is more
suitable in NATted environments. Further, the data relay concept
guarantees forwarding of data plane traffic also in the cases when
the NAT penetration procedures fail.
o Registration procedures with a relay server are similar as with
rendezvous server. However, a relay has different registration
parameters than rendezvous because it offers a different service.
Also, the relay includes also a REG_FROM parameter that informs
the client about its server reflexive address. In the case of a
data relay, it includes also a RELAYED_ADDRESS containing the
relayed address for the client.
o In [RFC7401], the Initiator and Responder can start to exchange
application payload immediately after the base exchange. While
exchanging data immediately after a base exchange via a data relay
would be possible also here, we follow the ICE methodology to
establish a direct path between two hosts using connectivity
checks. This means that there will be some additional delay after
the base exchange before application payload can be transmitted.
The same applies for the UPDATE procedure as the connectivity
checks introduce some additional delay.
o In HIP without NAT traversal support, the base exchange acts as an
implicit connectivity check, and the mobility and multihoming
extensions support explicit connectivity checks. After a base
exchange or UPDATE based connectivity checks, a host can use the
associated address pair for transmitting application payload. In
this extension, we follow the ICE methodology, where one end-point
acting in the controlled role chooses the used address pair also
on behalf of the other end-point acting in controlled role, which
is different from HIP without NAT traversal support. Another
difference is that the process of choosing an address pair is
explicitly signaled using the nomination packets. The nomination
process in this protocol supports only single address pair, and
multihoming extensions are left for further study.
o The UPDATE procedure resembles the mobility extensions defined in
[RFC8046]. The first UPDATE message from the mobile host is
exactly the same as in the mobility extensions. The second UPDATE
message from the peer host and third from the mobile host are
different in the sense that they merely acknowledge and conclude
the reception of the candidates through the relay. In other
words, they do not yet test for connectivity (besides reachability
through the HIP relay) unlike in the mobility extensions. The
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idea is that connectivity check procedure follows the ICE
specification, which is somewhat different from the HIP mobility
extensions.
o The connectivity checks as defined in the mobility extensions
[RFC8046] are triggered only by the peer of the mobile host.
Since successful NAT penetration requires that both end-points
test connectivity, both the mobile host and its peer host have to
test for connectivity. In addition, this protocol validates also
the UDP ports; the ports in the connectivity check must match with
the response, as required by ICE.
o In HIP mobility extensions [RFC8046], an outbound locator has some
associated state: UNVERIFIED mean that the locator has not been
tested for reachability, ACTIVE means that the address has been
verified for reachability and is being used actively, and
DEPRECATED means that the locator lifetime has expired. In the
subset of ICE specifications used by this protocol, an individual
address candidate has only two properties: type and priority.
Instead, the actual state in ICE is associated with candidate
pairs rather than individual addresses. The subset of ICE
specifications utilized by this protocol require the following
attributes for a candidate pair: valid bit, nominated bit, base
and the state of connectivity check. The connectivity checks have
the following states: Waiting, In-progress, Succeeded and Failed.
Handling of this state attribute requires some additional logic
when compared to the mobility extensions since the state is
associated with a local-remote address pair rather just a remote
address, and, thus, the mobility and ICE states do not have an
unambiguous one-to-one mapping.
o Credit-based authorization as defined in [RFC8046] could be used
before candidate nomination has been concluded upon discovering
working candidate pairs. However, this may result in the use of
asymmetric paths for a short time period in the beginning of
communications (similarly as in aggressive ICE nomination). Thus,
support of credit-based authorization is left for further study.
Authors' Addresses
Ari Keranen
Ericsson
Hirsalantie 11
02420 Jorvas
Finland
Email: ari.keranen@ericsson.com
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Jan Melen
Ericsson
Hirsalantie 11
02420 Jorvas
Finland
Email: jan.melen@ericsson.com
Miika Komu (editor)
Ericsson
Hirsalantie 11
02420 Jorvas
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Email: miika.komu@ericsson.com
Keranen, et al. Expires September 28, 2017 [Page 53]