HIP P. Nikander
Internet-Draft Ericsson
Expires: September 7, 2006 H. Tschofenig
Siemens
X. Fu
Univ. Goettingen
T. Henderson
The Boeing Company
J. Laganier
DoCoMo Euro-Labs
March 6, 2006
Preferred Alternatives for Tunnelling HIP (PATH)
draft-nikander-hip-path-01.txt
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Abstract
With the extensions defined in this document Host Identity Protocol
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(HIP) can traverse legacy Network Address Translators (NATs) and
certain firewalls. The extension will be useful as part of the base
exchange and with the HIP Registration Extension. By using a
rendezvous server an additional entity inside the network is
utilized, which not only allows but also supports more restrictive
NATs to be traversed.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 6
3.1. UDP Encapsulation of HIP . . . . . . . . . . . . . . . . . 6
3.2. UDP-REA parameter . . . . . . . . . . . . . . . . . . . . 6
3.3. S-UDP-REA parameter . . . . . . . . . . . . . . . . . . . 7
4. Message Handling Rules . . . . . . . . . . . . . . . . . . . . 10
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. HIP Initiator behind a NAT . . . . . . . . . . . . . . . . 11
5.2. PATH Server Registration and Keep Alive . . . . . . . . . 11
5.3. Message flow for data receiver behind a NAT . . . . . . . 12
5.4. Mobility and multihoming message flow . . . . . . . . . . 15
6. New Requirements for IPsec . . . . . . . . . . . . . . . . . . 17
6.1. Association with server inside/outside NAT . . . . . . . . 17
6.2. Mobility Scenarios . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Third Party Bombing . . . . . . . . . . . . . . . . . . . 18
7.2. Black hole . . . . . . . . . . . . . . . . . . . . . . . . 19
7.3. Man-in-the-middle attack . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Problem Definition . . . . . . . . . . . . . . 22
9.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 22
9.3. Brittleness Introduced by PATH . . . . . . . . . . . . . . 23
9.4. Requirements for a Long Term Solution . . . . . . . . . . 24
9.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 25
9.6. In Closing . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Normative References . . . . . . . . . . . . . . . . . . . 29
12.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . . . 33
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1. Introduction
This document defines extensions and allows the Host Identity
Protocol (HIP) to be used in an environment where legacy NATs or
Firewalls are present. To support this functionality it is necessary
to provide
o UDP encapsulation for HIP signaling messages
o UDP encapsulation for IPsec traffic
The problems of allowing IPsec protected traffic and the
corresponding signaling protocol (IKEv1) to traverse a NA(P)T are
well described in [5]. A proposal for UDP encapsulation of IPsec
protected traffic is described in [6]. It is possible to design an
optimized version of it for usage with HIP. This aspect is, however,
outside the scope of this document.
This document tries to accomplish the following goals:
o Make HIP work through legacy NATs (and possibly through some
firewalls)
o Make HIP hosts reachable behind NATs
HIP signaling consists of (for static and bootstrapping) base
exchange [1], which establish a HIP association state, and (in
particullar for mobilility and multi-homing scenarios) an Update
packet containing a LOCATOR parameter [2] which allows a HIP host to
notify a peer about alternate locator(s) at which it is reachable. A
third party in the network infrastructure, the rendezvous server
(RVS) is typically used to allow a HIP initiator to learn a
responder's (present) locator before initializing HIP base exchange.
The interaction of HIP hosts with the rendezvous server is described
in [3]. Currently, two possible ESP transport formats are being
defined for carrying HIP user data, namely the standard ESP [7] and
the BEET mode [8].
This document builds on the RVS concept to allow HIP signaling and
ESP-mode data traffic to successfully traverse legacy NA(P)Ts (and if
necessary, firewalls). There are two possible approaches to achieve
this:
o First, it is possible to combine a HIP rendezvous server and a
STUN server [9], TURN server [10] or NSIS NATFW [11] node. Here,
the STUN, TURN or NSIS NATFW protocol is used to allow the PATH
client to learn the public IP address (and port number) created at
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the NAT. The client obviously needs to support the client part of
the protocol as well.
o Alternatively, the HIP registration protocol can be extended to
integrate a NAT detection check. This ensembles NAT traversal
support in IKEv2 [12] and corresponding extensions for IKEv1 (see
[5] and [13]).
This document employs the latter approach to avoid the complexity of
integrating another protocol and additional message exchanges, while
still taking some advantages of the former approach. In contrast, an
integration with STUN or TURN may not bring better security features
in the protocol exchange.
In the proposed approach, a new parameter UDP-REA (UDP encapsulated
REAddress packet) is introduced to support NAT detection. When a HIP
message needs to be sent from a host to RVS (for registration
messages) or another HIP (HIP signaling and data traffic), with a
UDP-REA it is now possible to detect the existence of NATs and thus
retrieve or create only the preferred IP address and port number,
instead of the private address and port number for the host. Thus,
this approach is called "Preferred Alternatives for Tunnelling HIP",
or PATH.
To allow the client to inform the PATH server about its public IP
address and port in a secure fashion (where this is possible and
appropriate), another parameter is also introduced: S-UDP-REA, a
secure version of the UDP-REA parameter. By using this parameter a
secure traversal of legacy NATs is supported. The S-UDP-REA
parameter information can be obtained for example by interacting with
NSIS or MIDCOM. This provides better security properties, however
the details of interaction with NSIS or MIDCOM are outside the scope
of this document.
Please note that the goal of this document is different from that of
[14], where middleboxes (such as NATs and firewalls) are assumed to
be HIP-aware and participate in the HIP message exchange. As a
result, the security properties of these protocols are different as
well.
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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 [4].
For an interaction between a HIP host with a rendezvous server, two
communicating entities are also denoted as PATH client and PATH
server in this document. The PATH server always resides on the same
entity as the rendezvous server. A PATH client is a HIP-aware device
which supports the extensions defined in this document in addition to
the HIP Registration Extension [15]. The PATH client might be
located behind a legacy NAT and initiates the protocol exchange with
the PATH server. The PATH server interacts with the client in the
way specified in this document.
Different types of NATs (e.g., full cone, restricted NAT) are being
deployed today. [9] assigns these NAT boxes to certain categories
based on their data traffic forwarding or blocking behaviors. The
existence of different NAT types has an impact on the protocol.
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3. Protocol Extensions
This section explains the necessary protocol extensions to support
the above-mentioned functionality.
3.1. UDP Encapsulation of HIP
In order to deal with NA(P)Ts, it is necessary that the HIP signaling
messages are UDP encapsulated. Moreover, the source port and the
destination port MUST NOT be expected at a fixed port number. This
aspect of NAT traversal is known from IPsec/IKE and also reflected in
the design of IKEv2.
It is a policy issue whether to enable UDP encapsulation immediately
when the first HIP base message is sent (i.e., the I1 message).
For IPv4, the packet format is shown in Appendix E of [1]. The same
specification states that UDP encapsulation is forbidden for IPv6 but
might still be necessary, particularly for IPv4-IPv6 transition.
3.2. UDP-REA parameter
This section defines the UDP-REA parameter which will be used in the
traversal of legacy NATs.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Lifetime | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ HASH ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 bytes):
This parameter has the value of TBD.
Length (2 bytes)
Represents the length in octets,
excluding Type, Length and Padding.
Address Lifetime (2 bytes):
This field represents the address lifetime, in seconds.
HASH (variable):
This field of variable length contains the hash of
IP address and port information.
Padding (variable):
Padding information following the HASH value
The HASH is calculated as follows:
HASH = PRF(RANDOM | Source IP | Destination IP | Source Port |
Destination Port)
Where, PRF is a hash algorithm negotiated through HIP_TRANSFORM (see
Section 5.2.7 of [1]); the IP address is 4 octets for an IPv4 address
and 16 octets for an IPv6 address; the port numbers are encoded in
network byte-order. A RANDOM value is included to prevent pre-
computation attacks. The puzzle mechanism could be used for this
purpose.
The UDP-REA parameter is zero-padded to 8 bytes. The length field
contains the length of the payload without padding.
3.3. S-UDP-REA parameter
This section defines the S-UDP-REA parameter, the secure version of
the UDP-REA parameter. An end host might be able to retrieve address
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information securely using some protocols, such as MIDCOM or the NSIS
NATFW NSLP. These protocols enable the PATH client to create and
retrieve a NAT binding in a secure fashion. This information is then
communicated from the PATH client to the PATH server experiencing
integrity protection, thus it is called secured UDP-REA (S-UDP-REA).
Furthermore, when there is a stateful packet filter firewall along
the path, S-UDP-REA may be used to allow UDP encapsulation. UDP-REA
Section 3.2 would not be able to detect or act accordingly in such a
situation.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Lifetime | Reserved |T|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Source Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Destination Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 bytes):
This parameter has the value of TBD.
Length (2 bytes)
Represents the length in octets,
excluding Type, Length and Padding.
Address Lifetime (2 bytes):
This field represents the address lifetime, in seconds.
Type (T) Flag (1 bit):
If this bit is set to 1 then the values in the Address
fields are IPv6 addresses otherwise a IPv4 addresses.
Source Port (2 bytes):
This field contains the source port.
Destination Port (2 bytes):
This field contains the destination port.
Source Address (4 or 16 bytes):
This field contains either an IPv4 or an IPv6 address.
Destination Address (4 or 16 bytes):
This field contains either an IPv4 or an IPv6 address.
Padding (variable):
Padding information following the HASH value.
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4. Message Handling Rules
The PATH client attaches the UDP-REA payload to indicate support for
legacy NAT traversal. Thereby it generates a hash value over the
source IP address, source port, destination IP and destination port
from the IP header of the HIP message. When the HIP message
traverses a NAT along the path between the client and the server, its
IP header will be modified. When the server receives the HIP
message, it will compare the hash value carried in the HASH field of
the UDP-REA parameter and the value computed on the IP address header
information. If the two values do not match, then the server
determines that someone along the path modified the IP header (and
hopefully it is a NAT but not an adversary). The server will then
use the information in the IP header to return a response to the
client. If the two values are equal then it is assumed that no NAT
is located along the path and UDP encapsulation is not necessary.
If a PATH client is able to obtain S-UDP-REA related information, the
S-UDP-REA parameter in an integrity protected fashion instead of the
plain UDP-REA should be used. This offers better security and
additional capability of traversing firewalls.
This section provides further information on the message handling.
o Checksum and length field are provided in the UDP header and might
not need to be repeated in the HIP header.
o HIP version (either normal or secured) is determined by the used
destination port when sending the I1 packet.
o The digital signature and the keyed message digest is computed
over the original payload. First, a "normal" HIP packet is
constructed, then the HMAC and the digital signatures are
computed. Afterwards the HIP packet is encapsulated into the UDP
format.
o Short timeout (e.g., 200ms) after first packet and therefore
encourage NAT-less operation.
o If preferred source address is in RFC 1918 address space, then I1
is UDP encapsulated.
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5. Examples
5.1. HIP Initiator behind a NAT
This figure shows the usage of the UDP-REA parameter by the Initiator
and the Responder to detect the presence of a NAT along the path. In
this example, we assume the HIP Initiator is behind a NAT and the HIP
Initiator initially starts with UDP encapsulation.
Private Public
Network Network
HIP Network Address HIP
Initiator Translator Responder
| | |
| I1: Trigger exchange | I1: Trigger exchange |
| over UDP | over UDP |
| --------------------------> | --------------------------> |
| | |
| R1: {Puzzle,DH(R),HI(R) | R1: {Puzzle,DH(R),HI(R) |
| HIP Transform}SIG | HIP Transform}SIG |
| UDP-REA(R) | UDP-REA(R) |
| <-------------------------- | <-------------------------- |
| | |
| I2: {Solution,DH(I), | I2: {Solution,DH(I), |
| HIP Transform | HIP Transform |
| {H(I)},CERT(I)}SIG | {H(I)},CERT(I)}SIG |
| UDP-REA(I) | UDP-REA(I) |
| --------------------------> | --------------------------> |
| | |
| R2: {HMAC}SIG | R2: {HMAC}SIG |
| <-------------------------- | <-------------------------- |
| | |
Figure 3: HIP Initiation behind a NAT: Message Flow
5.2. PATH Server Registration and Keep Alive
This section illustrates the message exchange for a PATH client
registering with a PATH server, as introduced with [15]. After the
protocol exchange is finalized, both peers are mutually authenticated
and authorized by each other and a security association for HIP has
been established.
When the PATH client starts to interact with the PATH server, the
client can detect the presence of the legacy NAT along the path, by
including UDP-REA parameter in the registration messages and making
the computation in the client and server.
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Figure 4 shows such a message exchange.
Private Public
Network Network
PATH Network Address PATH
Client Translator Server
| | |
| | |
| I1: Trigger exchange | I1: Trigger exchange |
| over UDP | over UDP |
| --------------------------> | --------------------------> |
| | |
| R1: {Puzzle,DH(R),HI(R) | R1: {Puzzle,DH(R),HI(R) |
| HIP Transform}SIG | HIP Transform}SIG |
| UDP_REA(R), REG_INFO | UDP_REA(R), REG_INFO |
| <-------------------------- | <-------------------------- |
| | |
| I2: {Solution,DH(I), | I2: {Solution,DH(I), |
| HIP Transform | HIP Transform |
| {H(I)},CERT(I)}SIG | {H(I)},CERT(I)}SIG |
| UDP_REA(I), REG_REQ | UDP_REA(I), REG_REQ |
| --------------------------> | --------------------------> |
| | |
| R2: {HMAC}SIG, REG_RESP | R2: {HMAC}SIG, REG_RESP |
| <-------------------------- | <-------------------------- |
| | |
| | |
Figure 4: Registration Protocol Message Flow
Here, the HIP Registration messages are extended to not only carry
REG_INFO (in the R1 message), REG_REQ (in the I2 message), REG_RESP
(in the R2 message) and HIP_SIGNATURE, but also contain UDP_REQ for
the detection of NATs and behaving properly.
Note that this protocol exchange implicitly indicates that the PATH
client will use the source IP address of the I1 and I2 messages as
the preferred address when it needs to send out packets. The PATH
server will use the source IP address of the incoming packet as the
preferred address even though it was not authenticated (i.e.,
integrity protected). This extended HIP registration protocol, which
comprises a 4-way message exchange including a return routability
test, ensures that the PATH server can reach the PATH client and that
the message has not been crafted by an off-path adversary.
5.3. Message flow for data receiver behind a NAT
This section shows two approaches for a message flow where one HIP
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node acting as the data receiver is behind a NAT. The registration
with the PATH server is not shown in the figure. Figure 5 only shows
the HIP base exchange between the HIP Initiator and the HIP Responder
interacting with the PATH server. Figure 5 shows such a protocol
exchange taken from [2].
Figure 5 shows that the HIP base exchange between the HIP Initiator
and the PATH server does not use UDP encapsulation. UDP
encapsulation for HIP signaling messages and for the IPsec data
traffic is only enabled between the PATH server and the HIP Responder
which is enabled with this extension to the HIP registration
protocol. Note that IPsec data traffic will traverse the PATH server
to experience UDP encapsulation. The main advantage of this approach
is two-fold: (1) the HIP Initiator does not need to support the
extension defined in this document and (2) traversal of more
restrictive NATs can be supported when the PATH server also changes
IP address information.
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Public Private
Network Network
HIP PATH Network Address HIP
Initiator Server Translator Responder
| | | |
| I1 over IP | | |
| ----------------> | I1 over UDP | I1 over UDP |
| | ----------------> | ----------------> |
| | | |
| | R1 over UDP | R1 over UDP |
| R1 over IP | with UDP-REA | with UDP-REA |
| without UDP-REA | <---------------- | <---------------- |
| <---------------- | | |
| | | |
| I2 over IP | | |
| without UDP-REA | I2 over UDP | I2 over UDP |
| ----------------> | without UDP-REA | without UDP-REA |
| | ----------------> | ----------------> |
| | | |
| | R2 over UDP | R2 over UDP |
| R2 over IP | <---------------- | <---------------- |
| <---------------- | | |
| | | |
| IPsec ESP | IPsec ESP | IPsec ESP |
| <===============> | over UDP | over UDP |
| | <================ | ================> |
| | | |
| | | |
Legend:
-->: HIP signaling messages
==>: Data traffic
Figure 5: Establishing contact (1/3)
Figure 6 modifies the message flow described in Figure 5 whereby R2
is already sent from the HIP Responder to the HIP Initiator directly.
The responder thereby creates the necessary NAT binding at the NAT to
potentially allow IPsec protected traffic from the initiator towards
the responder to traverse the NAT. IPsec protected data traffic is
sent only directly between the HIP Initiator and the HIP Responder.
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Public Private
Network Network
HIP PATH Network Address HIP
Initiator Server Translator Responder
| | | |
| I1 over IP | | |
| ----------------> | I1 over UDP | I1 over UDP |
| | ----------------> | ----------------> |
| | | |
| | R1 over UDP | R1 over UDP |
| R1 over IP | with UDP-REA | with UDP-REA |
| with UDP-REA | <---------------- | <---------------- |
| <---------------- | | |
| | | |
| I2 over IP | | |
| without UDP-REA | I2 over UDP | I2 over UDP |
| ----------------> | without UDP-REA | without UDP-REA |
| | ----------------> | ----------------> |
| | | |
| R2 over UDP | R2 over UDP | R2 over UDP |
| <------------------------------------ | <---------------- |
| | | |
| IPsec ESP | IPsec ESP | IPsec ESP |
| over UDP | over UDP | over UDP |
| <==================================== | ================> |
| | | |
| | | |
Legend:
-->: HIP signaling messages
==>: Data traffic
Figure 6: Establishing contact (2/3)
Sending the IPsec protected data traffic via the PATH server is
useful if a NAT is very restrictive. This method also addresses
privacy and denial of service issues as raised in the rendezvous
server discussion. As symmetrical NATs are rare and an additional
proxy host should be avoided, the second approach is recommended as
the default method. The selection of the approach is a policy
decision.
5.4. Mobility and multihoming message flow
After the PATH client has registered itself to the PATH server, as
described in Figure 4, the PATH client might roam within a network or
roam outside a network. Whenever the PATH client obtains a new IP
address (either due to mobility, IP address reconfiguration or
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switching of interfaces), a UPDATE (containing UDP-REA) message will
be sent towards the PATH server to update the stored IP address
information. Note that the initial registration procedure might be
executed without a NAT along the path. Hence, the messages may be
carried over IP and do not require UDP encapsulation. When the PATH
client roams to a new network, UDP encapsulation should be used to
detect the presence of a NAT. Hence, it is required to have the
capability to enable UDP encapsulation for the HIP base exchange (and
for the IPsec protected data traffic) in addition to the registration
messages.
Figure 7 shows such a protocol exchange which ensembles the work in
[2] but applies it in the PATH/RVS registration. Note UPP_REA is
used for NAT traversal.
Private Public
Network Network
PATH Network Address PATH
Client Translator Server
| | |
| UPDATE(UDP_REA(S), SEQ) | UPDATE(UDP_REA(S), SEQ) |
| over UDP | over UDP |
| --------------------------> | --------------------------> |
| | |
| UPDATE(UDP_REA(C), SPI, SEQ,| UPDATE(UDP_REA(C), SPI, SEQ,|
| ACK, ECHO_REQUEST) | ACK, ECHO_REQUEST) |
| <-------------------------- | <-------------------------- |
| | |
| UPDATE(ACK, ECHO_RESPONSE) | UPDATE(ACK, ECHO_RESPONSE) |
| --------------------------> | --------------------------> |
| | |
| | |
Figure 7: Mobility Message Flow
Further issues with mobility and multihoming are being investigated
and will be detailed in next versions of the document.
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6. New Requirements for IPsec
The text in this section focuses on Dynamic UDP Encapsulation for
IPsec. By dynamic UDP encapsulation we mean UDP encapsulation per
security association. Before describing the approach we describe
some of the scenarios where dynamic UDP encapsulation is needed.
6.1. Association with server inside/outside NAT
The association of a client with a server outside NAT should have UDP
encapsulation on while an association with a server within the same
NAT should normal HIP association without any UDP encapsulation.
This identification is done during base exchange. Dynamic UDP
encapsulation based on security association could achieve this.
Figure 8 shows difference in association between different servers.
Private Public
Network Network
PATH PATH Network Address PATH
Server Client Translator Server
| | | |
| HIP association | HIP association | |
| No UDP encapsulation | UDP encapsulated | |
|<-----------------------| ------------------------------------>|
| | | |
Figure 8: Dynamic NAT Associations
6.2. Mobility Scenarios
If a HIP client moves from behind the NAT to outside it then it would
not need any more UDP encapsulation as it can have an HIP association
without any UDP encapsulation. So when the client moves out of NAT
it should reset all the NAT variables that are in security
associations.
To achieve dynamic UDP encapsulation for Legacy NAT traversal we need
to define it per Security Association basis. The SA would contain an
indicator which would indicate whether or not the particular HIP
association needs UDP encapsulation or not. By default this
indicator would be off. During base exchange if NAT is detected on
the way then this indicator should be turned on. The UDP
encapsulation in the kernel should also be based on this variable.
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7. Security Considerations
Currently this text in this section focuses on the attacks between
the PATH client and the PATH server since they differ from the
description of threats provided in the past about NAT traversal for
mobility protocols. The latter one have been investigated in context
of IKE, IKEv2 and various other protocols and will be summarized in a
future version of the document.
Attacks on the interaction between the PATH client and the PATH
server can be classified as denial of service and might be launched
against the PATH server itself, against third parties or against the
PATH client.
PATH servers create state through the HIP registration protocol. A
number of counter-measures are built-in into HIP registration
protocol. A PATH server might use the client-puzzle mechanism to
prevent a certain degree of DoS attacks. Additionally, it might be
reasonable to limit the number of registrations at a PATH server
itself. Since the PATH server needs to be discovered somehow it
needs to be ensured that some security mechanisms are provided for
this procedure. For example, if the PATH server is discovered using
DNS SRV records then an attacker can compromise the DNS, it can
inject fake records which map a domain name to the IP address of a
PATH server run by the attacker. This will allow it to inject fake
responses to launch a number of the attacks. This discovery
procedure might, however, be part of the HIP Registration protocol.
A detailed discussion about the security properties of the HIP
registration protocol is outside the scope of this document. Even
though the base HIP registration protocol is outside the scope of
this document some of its security properties are highly relevant and
applicable for this discussion. This document extends the
capabilities of the registration protocol that might raise security
concerns. This section mostly focuses on the security properties of
the UDP-REA parameter and it's semantic.
7.1. Third Party Bombing
Threat:
Third party bombing is also of concern when legacy NAT traversal
mechanisms are in place. These attacks have been discovered in
the context of Mobile IP and a threat description can be found in
[16]. The main problem described in [16] is caused by the missing
integrity protection of the IP address communicated from the PATH
client to the PATH server. The PATH client cannot protect the IP
address (without relying on additional protocol) since a NA(P)T is
supposed to change the header's IP address (source, possibly
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destination IP address and transport protocol identifiers).
Instead of using the protected IP address inside the signaling
message the PATH server is supposed to use IP header information.
An adversary might provide change the IP header address to point
to the intended target. Data sent to the PATH server will be send
to the target rather than to the true IP address of the client.
Countermeasures:
To prevent third party bombing, the address provided by the PATH
client via the IP header needs to be verified using a return-
routability check. This check might either be provided as part of
the base exchange (which involves two roundtrips) or as part of
the REA message exchange which also provides mechanisms to execute
such a test. This return-routability test MUST be performed in
order to ensure that this and other attacks can be thwarted. A
third party entity cannot respond to any of these HIP messages due
to the cryptographic properties of the HIP base protocol and the
multi-homing and mobility extensions.
7.2. Black hole
Threat:
This attack again exploits the ability for an adversary to act as
a NAT and to modify the IP address information in the header.
This information will then be used by the PATH server to sent
traffic towards the indicated address. If this address is not
used by any entity (and particularly by the legitimate PATH
client) then the traffic will be dropped. This attack is a denial
of service attack.
Countermeasures:
This threat can be avoided using the same counter measures as
third party bombing.
7.3. Man-in-the-middle attack
Threat:
This attack again requires the adversary to modify the IP header
of the HIP registration protocol messages exchanged between the
PATH server and the PATH client. Instead of pointing to a black
hole or to a third party the adversary provides his address. This
allows the adversary to eavesdrop the data traffic. However, in
order to launch the attack, the adversary must have already been
able to observe packets from the PATH client to the PATH server.
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In most cases (such as when the attack is launched from an access
network), this means that the attacker could already observe
packets sent to the client.
Countermeasures:
It is possible that an adversary modifies the IP address
information in such a way that it will receive the all traffic for
a particular PATH client. Therefore, it is necessary for the
adversary to be along the path to mount the initial attack. This
will allow the adversary to eavesdrop both the HIP message
exchange and the subsequent data traffic. However, the HIP
exchange is a cryptographic protocol which is resistant against
these types of attack. The data traffic is IPsec protected and
therefore the adversary will gain very little profit from this
attack. To make things worse for the adversary, if the PATH
client roams and uses the HIP registration protocol or the REA
message to update state at the PATH server the adversary needs to
be located somewhere along the path where it can observe this
exchange and to modify it. As a consequence, this attack is not
particular useful for the adversary.
The S-UDP-REA parameter does not suffer from the same threats as the
UDP-REA parameter since it aims to provide a secure mechanism for the
PATH server and the PATH client to communicate addressing
information. Still, the PATH server might want to authorize the
parameters provided by the PATH client by either executing a return-
routability check or by using other techniques (e.g., authorization
certificates) to ensure that the PATH client is indeed reachable at
the indicated addresses. A malicious PATH client might add wrong
addressing information to redirect traffic to a black hole or a third
party. This threat has a different degree than the previously
discussed threats in the sense that the PATH server will most likely
know the identity of the PATH client, if we assume that only
authenticated and authorized clients are allowed to use the PATH
server. If the PATH server is able to detect the malicious behavior
it can act accordingly.
Finally, it is necessary to add a remark on the usage of NAT/Firewall
signaling protocols in relationship with the S-UDP-REA parameter
usage. If the PATH client uses these protocols in an insecure or
inadequate way then the envisioned security of the S-UDP-REA
parameter is seriously affected. A discussion of the security
properties of various NAT/Firewall signaling protocols is outside the
scope of the document (in the same way as these protocols are outside
the scope of this document).
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8. IANA Considerations
This document extends the HIP registration protocol by defining a new
parameter (the UDP-REA and the S-UDP-REA parameter). These
parameters need IANA registration:
TBD:
Changes to the PATH protocol are made through a standards track
revision of this specification. This document does not create new
IANA registries.
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9. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing
(UNSAF)", which is the general process by which a client attempts to
determine its address in another realm on the other side of a NAT
through a collaborative protocol reflection mechanism (RFC 3424
[17]). PATH is an example of a protocol that performs this type of
function. The IAB has mandated that any protocols developed for this
purpose document a specific set of considerations. This section
meets those requirements.
The text in this section heavily borrows from [9].
9.1. Problem Definition
From RFC 3424 [17], any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term fixes
usually aren't".
The specific problem being solved by PATH is to provide a means for a
PATH client to detect the presence of one or more NATs between it and
a PATH server. The purpose of such detection is to determine the
need for UDP encapsulation by the PATH server (i.e., rendezvous
server).
PATH affect both UDP encapsulation of data traffic (which is IPsec
protected) and HIP signaling messages.
9.2. Exit Strategy
From [17], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
PATH comes with its own built in exit strategy. This strategy is the
detection operation that is performed as a precursor to the actual
UNSAF address-fixing operation. The discovery operation, described
in Section 3.2, attempts to discover the existence of, and type of,
any NATS between the client and the PATH server. PATH does not aim
to detect the type of NAT (due to known deficiencies) and the
discovery of the existence of NAT is itself quite robust. As NATs
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are phased out through the deployment of IPv6, the discovery
operation will return immediately with the result that there is no
NAT, and no further operations are required. Indeed, the discovery
operation itself can be used to help motivate deployment of IPv6; if
a user detects a NAT between themselves and the public Internet, they
can call up their access provider and complain about it.
PATH can also help to facilitate the introduction of MIDCOM or NSIS.
As MIDCOM or NSIS-capable NATs are deployed, HIP end hosts will,
instead of using UDP-REA, first allocate an address binding using
MIDCOM or NSIS and use S-UDP-REA. However, it is a well-known
limitation of MIDCOM that it only works when the agent knows the
middleboxes through which its traffic will flow. This issue is fixed
with the path-coupled approach followed in NSIS. Once bindings have
been allocated from those middleboxes, a PATH detection procedure can
validate that there are no additional middleboxes on the path from
the PATH server to the PATH client. If this is the case, the HIP end
host can continue operation using the address bindings allocated from
MIDCOM or NSIS. If it is not the case, PATH provides a mechanism for
self-address fixing through the remaining MIDCOM or NSIS-unaware
middleboxes. Thus, PATH provides a way to help transition to full
MIDCOM or NSIS-aware networks.
9.3. Brittleness Introduced by PATH
From [17], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
PATH has its own limitations in several ways:
[EDITOR's NOTE: Depending on the signaling flow and the
involvement of the PATH server some behavior is assumed by NATs.
There could be other types of NATs that are deployed that would
not work well with some of the proposed signaling message flows.
For some of the message flows the binding acquisition usage of
PATH does not work for all NAT types. It will work for any
application running across full cone NATs only. For restricted
cone and port restricted cone NAT, it may work for some cases.
For symmetric NATs, the binding acquisition will not yield a
usable address (in case that not all the signaling messages and
the entire data traffic is routed through the PATH server). The
tight dependency on the specific type of NAT may limit the
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protocol application scenarios.]
PATH assumes that the server exists on the public Internet. If
the server is located in another private address realm, the HIP
end host may or may not be able to use the established state at
the PATH server. This heavily depends on the protocol interaction
between the other HIP end host and possibly other PATH servers
than are cascaded.
The bindings allocated from the NAT need to be continuously
refreshed. Since the timeouts for these bindings is
implementation specific, the refresh interval cannot easily be
determined. When the binding is not being actively used to
receive traffic, but to wait for an incoming message, the binding
refresh will needlessly consume network bandwidth.
The use of the PATH server as an additional network element
introduces another point of potential security attack. These
attacks are largely prevented by the security measures provided
the HIP registration protocol, but not entirely.
The use of the PATH server as an additional network element
introduces another point of failure. If the client cannot locate
a PATH server, or if the server should be unavailable due to
failure, no interaction can be performed.
The use of PATH to enable UDP encapsulation for IPsec protected
data traffic and for HIP messages introduces an additional
bandwidth consumption which might be problematic in certain
wireless networks. The modified packet forwarding through the
PATH server, which might be necessary to ensure traversal of
certain NAT types, might represent a non-optimal route and may
increase latency for some applications (depending on the location
of the PATH server).
9.4. Requirements for a Long Term Solution
From [17], any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions -
contribute to the process of finding the right longer term
solution.
Our experience with PATH has led to the following requirements for a
long term solution to the NAT problem:
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Requests for bindings and control of other resources in a NAT need
to be explicit. Much of the brittleness in PATH derives from its
guessing at the parameters of the NAT, rather than telling the NAT
what parameters to use.
Control needs to be "in-band". There are far too many scenarios
in which the client will not know about the location of
middleboxes ahead of time. Instead, control of such boxes needs
to occur in-band, traveling along the same path as the data will
itself travel. This guarantees that the right set of middleboxes
are controlled. NSIS exactly provides a solution for this
purpose. Third-party controls are best handled using the MIDCOM
framework.
Control needs to be limited. Users will need to communicate
through NATs which are outside of their administrative control.
In order for providers to be willing to deploy NATs which can be
controlled by users in different domains, the scope of such
controls needs to be extremely limited - typically, allocating a
binding to reach the address where the control packets are coming
from.
Simplicity is paramount. The control protocol will need to be
implement in very simple clients. The servers will need to
support extremely high loads. The protocol will need to be
extremely robust, being the precursor to a host of application
protocols. As such, simplicity is key.
9.5. Issues with Existing NAPT Boxes
From [17], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA(P)Ts and experience reports.
Several of the practical issues with PATH involve future proofing -
breaking the protocol when new NAT types get deployed. Fortunately,
this is not an issue at the current time, since most of the deployed
NATs are of the types assumed by PATH. The primary usage PATH has
been found in the area of VoIP, to facilitate allocation of addresses
for receiving RTP [12] traffic. In that application, the periodic
keepalives are provided by the RTP traffic itself. However, several
practical problems arise for RTP. First, RTP assumes that RTCP
traffic is on a port one higher than the RTP traffic. This pairing
property cannot be guaranteed through NATs that are not directly
controllable. As a result, RTCP traffic may not be properly
received. Protocol extensions to SDP have been proposed which
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mitigate this by allowing the client to signal a different port for
RTCP [18]. However, there will be interoperability problems for some
time.
For VoIP, silence suppression can cause a gap in the transmission of
RTP packets. This could result in the loss of a binding in the
middle of a call, if that silence period exceeds the binding timeout.
This can be mitigated by sending occasional silence packets to keep
the binding alive. However, the result is additional brittleness;
proper operation depends on the silence suppression algorithm in use,
the usage of a comfort noise codec, the duration of the silence
period, and the binding lifetime in the NAT.
9.6. In Closing
Some of the limitations of PATH are not design flaws. Due to the
properties of HIP, PATH is fairly secure and robust form of legacy
NAT traversal compared to other approach such as STUN. Some
limitations are, however, related to the lack of standardized
behaviors and controls in NATs. The result of this lack of
standardization has been a proliferation of devices whose behavior is
highly unpredictable, extremely variable, and uncontrollable. PATH
does the best it can in such a hostile environment. Ultimately, the
solution is to make the environment less hostile, and to introduce
controls and standardized behaviors into NAT. However, until such
time as that happens, PATH provides a good short term solution given
the terrible conditions under which it is forced to operate. PATH
also offers a long-term solution if NATs are NSIS or MIDCOM aware.
The main benefit is increased secure and a less brittle protocol
operation since the NAT (or even firewalls) can be controlled and
should then behave according to respective middlebox signaling
protocol. Ultimately, NAT boxes might be HIP aware.
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10. Acknowledgements
The authors would like to thank Aarthi Nagarajan, Abhinav Pathak, and
Murugaraj Shanmugam for their helpful feedbacks on this document.
The authors would like to specially thank Lars Eggert for his
contribution to previous versions of the draft.
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11. Open Issues
Open issues can be found here:
http://www.tschofenig.com:8080/hip-nat/
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12. References
12.1. Normative References
[1] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-05
(work in progress), March 2006.
[2] Nikander, P., "End-Host Mobility and Multihoming with the Host
Identity Protocol", draft-ietf-hip-mm-03 (work in progress),
March 2006.
[3] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", draft-ietf-hip-rvs-04 (work in progress),
October 2005.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
12.2. Informative References
[5] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004.
[6] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC 3948,
January 2005.
[7] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.
[8] Melen, J. and P. Nikander, "A Bound End-to-End Tunnel (BEET)
mode for ESP", draft-nikander-esp-beet-mode-05 (work in
progress), February 2006.
[9] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[10] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-08 (work in progress),
September 2005.
[11] Stiemerling, M., "NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", draft-ietf-nsis-nslp-natfw-09 (work in progress),
February 2006.
[12] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
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[13] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[14] Tschofenig, H. and M. Shanmugam, "Traversing HIP-aware NATs and
Firewalls: Problem Statement and Requirements",
draft-tschofenig-hiprg-hip-natfw-traversal-03 (work in
progress), October 2005.
[15] Laganier, J., "Host Identity Protocol (HIP) Registration
Extension", draft-ietf-hip-registration-01 (work in progress),
December 2005.
[16] Dupont, F., "A note about 3rd party bombing in Mobile IPv6",
draft-dupont-mipv6-3bombing-03 (work in progress),
December 2005.
[17] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
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Authors' Addresses
Pekka Nikander
Ericsson Research Nomadic Lab
Hirsalantie 11
Turku FIN FIN-02420 JORVAS
Finland
Phone: +358 9 299 1
Email: pekka.nikander@nomadiclab.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
Xiaoming Fu
University of Goettingen
Institute for Informatics
Lotzestr. 16-18
Goettingen 37083
Germany
Email: fu@cs.uni-goettingen.de
Thomas R. Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
Email: thomas.r.henderson@boeing.com
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Julien Laganier
DoCoMo Communications Laboratories Europe GmbH
DoCoMo Communications Laboratories Europe GmbH
Munich 80687
Germany
Phone: +49 89 56824 231
Email: julien.ietf@laposte.net
URI: http://www.docomolab-euro.com/
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