HIP P. Nikander
Internet-Draft Ericsson
Expires: August 18, 2005 H. Tschofenig
Siemens
T. Henderson
The Boeing Company
L. Eggert
NEC
J. Laganier
SUN
February 14, 2005
Preferred Alternatives for Tunnelling HIP (PATH)
draft-nikander-hip-path-00.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 18, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Nikander, et al. Expires August 18, 2005 [Page 1]
Internet-Draft PATH February 2005
With the extensions defined in this document Host Identity Protocol
(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 . . . . . . . . . . . . . . . . . . . 8
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 . . . . . . . 13
5.4 Mobility and multihoming message flow . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . 18
6.1 Third Party Bombing . . . . . . . . . . . . . . . . . . . 18
6.2 Black hole . . . . . . . . . . . . . . . . . . . . . . . . 19
6.3 Man-in-the-middle attack . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
8. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 22
8.1 Problem Definition . . . . . . . . . . . . . . . . . . . . 22
8.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 22
8.3 Brittleness Introduced by PATH . . . . . . . . . . . . . . 23
8.4 Requirements for a Long Term Solution . . . . . . . . . . 24
8.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . 25
8.6 In Closing . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1 Normative References . . . . . . . . . . . . . . . . . . 29
11.2 Informative References . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . 32
Nikander, et al. Expires August 18, 2005 [Page 2]
Internet-Draft PATH February 2005
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 presents. 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 IPsec protected traffic and also the problems for a
signaling protocol (namely IKEv1) traversing 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
By using a rendezvous server an additional entity inside the network
is introduced which interacts with the HIP message exchange. The
interaction of HIP with the rendezvous server is described in [1].
This allows a complete NAT/Firewall traversal to be accomplished.
Two approaches are possible with respect to this rendezvous server
concept.
o First, it is possible to combine a HIP rendezvous server (i.e.,
PATH server) and a STUN server [7], TURN server [8] or NSIS NATFW
[9] node. The client obviously needs to support the clide part of
the protocol as well. 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 the NAT.
o Second, the HIP registration protocol can integrate a NAT
detection check. NAT-T support is provided by IKEv2 [10] and the
corresponding extensions have been designed for IKEv1 (see [5] and
[11]).
This document uses the later approach to avoid the integration of
another protocol and additional message exchanges but does not
rule-out the former approach. An integration with STUN and TURN
would not add more security to the protocol exchange. To support NAT
detection a new parameter UDP-REA is introduced.
To also allow the client to inform the server about its public IP
address and port in a secure fashion (where this is possible and
appropriate) another parameter has been defined S-UDP-REA, a secure
version of the UDP-REA parameter. Using this parameter a secure
Nikander, et al. Expires August 18, 2005 [Page 3]
Internet-Draft PATH February 2005
traversal of legacy NATs is supported, for example by interworking
with NSIS or MIDCOM. Both, MIDCOM and the NSIS NATFW NSLP provide
better security properties. The interaction with these protocols is
outside the scope of this document.
Please note that this document tries to accomplish a different goal
than [12] where middleboxes (such as NATs and firewalls) are assumed
to be HIP-aware and participate in the HIP message exchange. As a
consequence, the security properties of these protocols are different
as well.
Nikander, et al. Expires August 18, 2005 [Page 4]
Internet-Draft PATH February 2005
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 [2].
When interaction with a rendezvous server might be possible then we
denote these entities as the PATH client and the PATH server
traversing some type of legacy NAT. A PATH client is a HIP-aware
device which supports the extensions defined in this document in
addition to the HIP Registration Extension [13]. 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
deployed today and [7] assigns these NAT boxes to certain categories
based on their data traffic forwarding or blocking behavior. The
existence of different NAT types has an impact on the protocol.
Nikander, et al. Expires August 18, 2005 [Page 5]
Internet-Draft PATH February 2005
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 and 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 [3]. The same
specification states that UDP encapsulation is forbidden for IPv6 but
might still be necessary particuarly 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.
Nikander, et al. Expires August 18, 2005 [Page 6]
Internet-Draft PATH February 2005
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ HASH ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Padding ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 bytes):
This parameter has the value of TBD.
Length (2 bytes)
Represents the length in octets,
excluding Type and Length fields.
Address Lifetime (4 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)
using the negotiated hash algorithm (denoted as PRF) as part of the
HIP_TRANSFORM payload (see Section 6.2.8 of [3]). The hashed data is
in the network byte-order. The IP address is 4 octets for an IPv4
address and 16 octets for an IPv6 address. The port number is
encoded as a 2 octet number in network byte-order.
The necessary padding length depends on the selected hash algorithm.
It MUST be ensured that the total length (including padding) of the
UDP-REA parameter is 11 + Length - (Length + 3) % 8.
The RANDOM value is used to prevent precomputation attacks. The
puzzle mechanism could be used for this purpose.
Nikander, et al. Expires August 18, 2005 [Page 7]
Internet-Draft PATH February 2005
3.3 S-UDP-REA parameter
This section defines the S-UDP-REA parameter, a secure UDP-REA
parameter version. An end host might be able to retrieve address
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. Furthermore, this extension might also be used
when a stateful packet filtering firewall is known to be along the
path which requires UDP encapsulation in order to perform properly.
UDP-REA Section 3.2 would not be able to detect or act accordingly in
such a situation.
Nikander, et al. Expires August 18, 2005 [Page 8]
Internet-Draft PATH February 2005
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 |T|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Source Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Destination Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 bytes):
This parameter has the value of TBD.
Length (2 bytes)
Represents the length in octets,
excluding Type and Length fields.
Address Lifetime (4 bytes):
This field represents the address lifetime, in seconds.
Type (T) Flag (1 bit):
If this bit is set to 1 then the value in the Address
field is an IPv6 address otherwise an IPv4 address.
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.
Nikander, et al. Expires August 18, 2005 [Page 9]
Internet-Draft PATH February 2005
4. Message Handling Rules
The PATH client attaches the UDP-REA payload to indicate support for
legacy NAT traversal. Thereby it creates a hash value over the
source IP address, source port, destination IP and destination port
from the IP header of the HIP packet. When the HIP message traverse
a NAT along the path between the client and the server, the 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 is
assured that someone along the path modified the IP header (and
hopefully a NAT and 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 might not be necessary.
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 determined by the destination port used 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 the HMAC and the digital signatures are
computed. Afterwards the 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 send
I1 over IP and I1 over UDP a few milliseconds apart.
Nikander, et al. Expires August 18, 2005 [Page 10]
Internet-Draft PATH February 2005
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 case the HIP Initiator is behind a NAT. In this example, the
HIP Initiator is behind a NAT and we assume the HIP Initiator
immediately starts with UDP encapsulation.
Private Public
Addressing Addressing
HIP Realm Network Address Realm 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 [13]. After the
protocol exchange is finalized both peers will be mutually
authenticated and authorized each other and a security association
for HIP has been established.
When the PATH client starts to interact with the PATH server the
client might not be aware of the presence of the legacy NAT along the
Nikander, et al. Expires August 18, 2005 [Page 11]
Internet-Draft PATH February 2005
path. The I1 registration messages will most likely be dropped by
the NA[P]T. After some retransmissions the PATH client will switch
to an UDP encapsulated registration protocol exchange.
Figure 4 shows such a message exchange.
PATH Network Address PATH
Client Translator Server
| | |
| I1: Trigger exchange | |
| over IP | |
| --------------------------> | ...I1 dropped |
| | |
| ..retransmissions.. | |
| --------------------------> | ...I1 dropped |
| | |
| 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 |
| <-------------------------- | <-------------------------- |
| | |
| I2: {Solution,DH(I), | I2: {Solution,DH(I), |
| HIP Transform | HIP Transform |
| {H(I)},CERT(I)}SIG | {H(I)},CERT(I)}SIG |
| --------------------------> | --------------------------> |
| | |
| R2: {HMAC}SIG | R2: {HMAC}SIG |
| <-------------------------- | <-------------------------- |
| | |
| | |
Figure 4: Registration Protocol Message Flow
Additional payloads defined with the HIP Registration Extension are:
REG_INFO (carried in the R1 message), REQ_REQ (carried in the I2
message) and REQ_RESP (carried in the R2 message). These payloads
are not shown in the above message exchange.
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. 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). The HIP
middlebox registration protocol exchange already ensures that this
address is authorized via a return routability test.
Nikander, et al. Expires August 18, 2005 [Page 12]
Internet-Draft PATH February 2005
5.3 Message flow for data receiver behind a NAT
This section shows a message flow where one HIP 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 [4].
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
Nikander, et al. Expires August 18, 2005 [Page 13]
Internet-Draft PATH February 2005
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.
Nikander, et al. Expires August 18, 2005 [Page 14]
Internet-Draft PATH February 2005
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 |
| <==================================== | ================> |
| | | |
| | | |
Figure 6: Establishing contact (2/3)
Figure 7 extends Figure 6 further by allowing I2 to be sent directly
to the HIP Responder. This is only possible if the NAT forwards
packets with a different source IP address and source port than the
packets seen from the PATH server towards the HIP Responder.
Nikander, et al. Expires August 18, 2005 [Page 15]
Internet-Draft PATH February 2005
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 UDP | I2 over UDP | I2 over UDP |
| with UDP-REA | with UDP-REA | with UDP-REA |
| ------------------------------------> | ----------------> |
| | | |
| R2 over UDP | R2 over UDP | R2 over UDP |
| with UDP-REA | with UDP-REA | with UDP-REA |
| <------------------------------------ | <---------------- |
| | | |
| IPsec ESP | IPsec ESP | IPsec ESP |
| over UDP | over UDP | over UDP |
| <==================================== | ================> |
| | | |
| | | |
Figure 7: Establishing contact (3/3)
FOR DISCUSSION: It needs to be decided which approach is the best one
and if there are multiple preferred ways how we
(a) can detect which approach is applicable and
(b) what protocol extensions are required.
The answer most likely depends on the types of NATs we are willing to
support. For example, sending the IPsec protected data traffic via
the PATH server is useful if a NAT is very restrictive but, as a
default approach, probably not the best choice -- except if the PATH
server is along the path anyway (see discussion about discovery
exchange in the HIP registration protocol). Finally, the message
flows need to add info about the information conveyed to the end
hosts about the public IP address and port of it respective peer.
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
switching of interfaces) a REA message will be sent towards the PATH
Nikander, et al. Expires August 18, 2005 [Page 16]
Internet-Draft PATH February 2005
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 are carried over IP and do not require
UDP encapsulation. When the PATH client roams to a new network UDP
encapsulation might be required due to the presence of a NAT. Hence,
it is required to have the capability to enable UDP encapsulation for
the HIP exchange (and for the IPsec protected data traffic) not only
during the initial protocol exchange.
Figure 8 shows such a protocol exchange which is taken from [4].
PATH Network Address PATH
Client Translator Server
| | |
| UPDATE(REA, SEQ) | |
| over IP | |
| --------------------------> | ...UPDATE/REA dropped |
| | |
| ..retransmissions.. | |
| --------------------------> | ...UPDATE/REA dropped |
| | |
| UPDATE(REA, SEQ) | UPDATE(REA, SEQ) |
| over UDP | over UDP |
| --------------------------> | --------------------------> |
| | |
| UPDATE(SPI, SEQ, | UPDATE(SPI, SEQ, |
| ACK, ECHO_REQUEST) | ACK, ECHO_REQUEST) |
| <-------------------------- | <-------------------------- |
| | |
| UPDATE(ACK, ECHO_RESPONSE) | UPDATE(ACK, ECHO_RESPONSE) |
| --------------------------> | --------------------------> |
| | |
| | |
Figure 8: Mobility Message Flow
Nikander, et al. Expires August 18, 2005 [Page 17]
Internet-Draft PATH February 2005
6. 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.
6.1 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
[14]. The main problem described in [14] 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
Nikander, et al. Expires August 18, 2005 [Page 18]
Internet-Draft PATH February 2005
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.
6.2 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.
6.3 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.
In most cases (such as when the attack is launched from an access
network), this means that the attacker could already observe
Nikander, et al. Expires August 18, 2005 [Page 19]
Internet-Draft PATH February 2005
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).
Nikander, et al. Expires August 18, 2005 [Page 20]
Internet-Draft PATH February 2005
7. 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.
Nikander, et al. Expires August 18, 2005 [Page 21]
Internet-Draft PATH February 2005
8. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
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 [15]). 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 [7].
8.1 Problem Definition
From RFC 3424 [15], 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.
8.2 Exit Strategy
From [15], 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
are phased out through the deployment of IPv6, the discovery
operation will return immediately with the result that there is no
Nikander, et al. Expires August 18, 2005 [Page 22]
Internet-Draft PATH February 2005
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 MICOM 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.
8.3 Brittleness Introduced by PATH
From [15], 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 introduces brittleness into the system 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 for 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 makes the protocol brittle.]
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
Nikander, et al. Expires August 18, 2005 [Page 23]
Internet-Draft PATH February 2005
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).
8.4 Requirements for a Long Term Solution
From [15], 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:
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.
Nikander, et al. Expires August 18, 2005 [Page 24]
Internet-Draft PATH February 2005
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.
8.5 Issues with Existing NAPT Boxes
From [15], 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
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.
8.6 In Closing
Some of the limitations of PATH are not design flaws. Due to the
Nikander, et al. Expires August 18, 2005 [Page 25]
Internet-Draft PATH February 2005
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.
Nikander, et al. Expires August 18, 2005 [Page 26]
Internet-Draft PATH February 2005
9. Acknowledgements
The authors would like to thank Julien Laganier, Aarthi Nagarajan and
Murugaraj Shanmugam for their feedback on this document.
Nikander, et al. Expires August 18, 2005 [Page 27]
Internet-Draft PATH February 2005
10. Open Issues
This document is a first attempt in defining HIP NAT/Firewall
traversal extensions. The document raises some questions with regard
to the usage of the PATH server and the later flow of data packets.
The document still lacks a number of details which would benefit from
hands-on experience.
Nikander, et al. Expires August 18, 2005 [Page 28]
Internet-Draft PATH February 2005
11. References
11.1 Normative References
[1] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extensions", Internet-Draft draft-ietf-hip-rvs-00,
October 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[3] Moskowitz, R., "Host Identity Protocol",
Internet-Draft draft-ietf-hip-base-01, October 2004.
[4] Nikander, P., "End-Host Mobility and Multi-Homing with Host
Identity Protocol", Internet-Draft draft-ietf-hip-mm-00, October
2004.
11.2 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] 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.
[8] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
Internet-Draft draft-rosenberg-midcom-turn-06, October 2004.
[9] Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", Internet-Draft draft-ietf-nsis-nslp-natfw-04, October
2004.
[10] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
Internet-Draft draft-ietf-ipsec-ikev2-17, October 2004.
[11] Kivinen, T., "Negotiation of NAT-Traversal in the IKE",
Internet-Draft draft-ietf-ipsec-nat-t-ike-08, February 2004.
[12] Tschofenig, H., Nagarajan, A., Torvinen, V. and J. Ylitalo,
"NAT and Firewall Traversal for HIP",
Internet-Draft draft-tschofenig-hiprg-natfw-traversal-01.txt,
February 2005.
Nikander, et al. Expires August 18, 2005 [Page 29]
Internet-Draft PATH February 2005
[13] Laganier, J., Koponen, T. and L. Eggert, "A Middlebox
Registration Protocol for HIP",
Internet-Draft draft-koponen-hip-registration-00.txt, February
2005.
[14] Dupont, F., "A note about 3rd party bombing in Mobile IPv6",
Internet-Draft draft-dupont-mipv6-3bombing-01, January 2005.
[15] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[16] Kivinen, T., Swander, B., Huttunen, A. and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947, January
2005.
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
Thomas R. Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
Email: thomas.r.henderson@boeing.com
Nikander, et al. Expires August 18, 2005 [Page 30]
Internet-Draft PATH February 2005
Lars Eggert
NEC Network Laboratories
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 6221 90511 43
Email: lars.eggert@netlab.nec.de
Julien Laganier
Sun Labs (Sun Microsystems) and LIP (CNRS/INRIA/ENSL/UCBL)
180, Avenue de l'Europe
Saint Ismier CEDEX 38334
France
Phone: +33 476 188 815
Email: ju@sun.com
URI: http://research.sun.com
Nikander, et al. Expires August 18, 2005 [Page 31]
Internet-Draft PATH February 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Nikander, et al. Expires August 18, 2005 [Page 32]