One document matched: draft-tschofenig-hiprg-hip-natfw-traversal-03.txt
Differences from draft-tschofenig-hiprg-hip-natfw-traversal-02.txt
HIPRG H. Tschofenig
Internet-Draft Siemens
Expires: April 27, 2006 M. Shanmugam
TUHH
October 24, 2005
Traversing HIP-aware NATs and Firewalls: Problem Statement and
Requirements
draft-tschofenig-hiprg-hip-natfw-traversal-03.txt
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This Internet-Draft will expire on April 27, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The Host Identity Protocol is a signaling protocol which adds another
layer to the Internet model and (optionally) establishes IPsec ESP
SAs to protect subsequent data traffic. HIP is designed to be a
middlebox friendly protocol, it allows the middleboxes (such as NATs
and Firewalls) to participate in the base exchange messages in order
to learn the flow identifier and thereby, relying the data traffic.
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Adding authentication and authorization mechanisms can help the
middlebox decide which end hosts are allowed to traverse a firewall.
This can potentially prevent waste of network resources and limit
unwanted traffic. This document gives a problem statement and
requirements for HIP-aware middlebox traversal techniques.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview of HIP Base Exchange with Middleboxes . . . . . . . . 7
4.1. HIP Base Exchange with NAT . . . . . . . . . . . . . . . . 7
4.2. HIP Base Exchange with Firewall . . . . . . . . . . . . . 8
5. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Same Firewall at Initiator for both outgoing and
incoming packets . . . . . . . . . . . . . . . . . . . . . 10
5.2. Different Firewalls at Initiator for outgoing and
incoming packets . . . . . . . . . . . . . . . . . . . . . 11
5.3. Different Firewalls at Initiator and Receiver . . . . . . 12
6. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
An IP address serves the dual role of a locator and an identifier for
every host on the Internet. Since, the transport layer connections
are bound to the IP address, end systems that use IP addresses as
identifiers cannot support dynamic changes in the mapping between the
identifier and the locator in case of multi-homing and mobility.
The Host Identity Protocol (HIP) [I-D.ietf-hip-base] proposes to
separate the identifier from the locator by adding an additional
layer between the transport layer and the network layer. The
transport layer uses a new, mobility-unrelated identifier called as
Host Identity Tags (HITs), in place of IP addresses, while the
network layer uses conventional IP addresses for routing. IPsec
security associations are bound to the HITs and are not modified with
IP address changes. In other words, a host despite being mobile or
multi-homed can use a single transport layer connection associated to
one HIT and multiple IP addresses.
The Host Identity Protocol offers also the functionality to establish
IPsec ESP SAs which are subsequently used to encrypt data traffic
between the two end hosts. HIP is liable to all known
incompatibilities of IPsec with middleboxes such as NATs [RFC3022]
and firewalls. The problem statement for dealing with legacy NATs is
described in [I-D.irtf-hiprg-nat]. The main goal of the draft is to
present a problem statement and requirements in order to aim for a
NAT/FW traversal solution using the Host Identity Protocol.
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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 [RFC2119].
This draft used the terminology defined in [NATTerminology],
[I-D.ietf-hip-base], [I-D.ietf-HIP-esp] and
[draft-moskowitz-hip-arch] and [RFC2401].
The term SPI refers to the Security Parameter Index value used in
IPsec packets. The initiator selects one SPI(I) that can be found in
the ESP_info parameter, which is then used by the responder to create
an IPsec packet (ESP packet in this case) for traffic sent to the
initiator. The responder selects one SPI(R)(using ESP_info(R)
parameter) which is used by the initiator to encrypt all data sent to
the responder.
Other relevant abbreviations can be found in [I-D.ietf-hip-base] and
[I-D.ietf-HIP-esp].
The concept of a flow identifier is described in [RFC4080].
We use the following notation throughout this document:
o [x] indicates that x is optional.
o {x} indicates that x is encrypted.
o <x>y indicates that "x" is encrypted with the key "y".
o --> signifies "Initiator to Responder" communication (requests).
o <-- signifies "Responder to Initiator" communication (replies).
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3. Problem Statement
This document assumes that the data traffic following the HIP base
exchange is IPsec protected using the mechanism described in
[I-D.ietf-HIP-esp] for exchanging the IPsec parameters. A future
version of this draft might also be extended to support other
mechanisms for data traffic protection including no protection at
all.
Besides the communicating hosts in the Internet, the entities such as
NATs and Firewalls play a major role in the event of delivering
packets to an appropriate host, and each meant for specific
functionality. For instance, NATs are used to combat the IPv4
address depletion problem, and Firewalls are erected to protect
unsolicited information flowing in and out of a corporate network.
NATs use <src IP ,dst IP, src port, dst port, protocol> as an flow
identifier to identify a particular traffic or connection. Because
of this, protocols like IPsec suffers from well-known NAT related
problems. The NAT traversal approach described in [RFC3947] and
[I-D.ietf-ipsec-ikev2] allows the end hosts to detect one or more
NATs in between them and [RFC3948] proposes to use the UDP
encapsulation of IPsec ESP packets to traverse NATs.
Since HIP uses IPsec protection for the data traffic, the flow
identifier takes the shape of a <destination IP address, SPI and ESP>
(in order to support smooth traversal of the middleboxes) and the
middleboxes should learn this flow id in order to relay the data
packets. To achieve this, HIP aims to interact with middleboxes
actively whereby these devices need to understand the HIP protocol
and they need to be involved in the protocol exchange. HIP also
provides a way to deal with legacy NATs, as described in
[draft-nikander-hip-path-00.txt]. To support this functionality, it
is necessary to provide UDP encapsulation for both HIP signaling and
IPsec packets. Legacy NAT traversal does not require NATs to be HIP
aware or to understand the HIP message exchange.
Even though HIP allows the middleboxes to participate in the base
exchange, but scenarios like routing asymmetry poses a serious
challenge for the HIP to traverse a middlebox. Section 5 explains
some possible scenarios which have routing asymmetry. The inability
of HIP to handle routing asymmetry motivates to use an explicit
signaling mechanism for the HIP hosts in order to support secure and
smooth traversal of the middleboxes.
Although HIP is described as a two-party protocol, middle boxes are
supposed to intercept these messages in order to learn the flow
identifier and to process them correctly. In other words, a multi
party protocol is created such that the flow identifier is available
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to middle boxes between the HIP hosts. To provide proper security,
middleboxes should not be subject to denial of service attacks and
might want to authenticate or authorize entities which create state.
Note that the IPsec SA is unidirectional and therefore two IPsec SAs
(with two different SPIs, ESP_info contains the SPI value) have to be
established.
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4. Overview of HIP Base Exchange with Middleboxes
This section explains the HIP base exchange together with the
middleboxes and describes how the middleboxes behave during the base
exchange.
4.1. HIP Base Exchange with NAT
Figure 1 shows the HIP base exchange traversing a NAT.
I1 +-----------+ I1
+-------------------->| |----------------------+
| | | |
| | | |
R1 | Intercept | R1 v
+---------+ <-------------| the flow |<---------------- +---------+
|Initiator| I2 | identifer | I2 |Responder|
+---------+ ------------->| <Dest IP, |----------------> +---------+
^ | SPI,ESP> |
| | | |
| R2 | | R2 |
+---------------------| |<----------------------+
+-----------+
NAT
Figure 1: NAT and HIP Base Exchange
Subsequently, the HIP base exchange is described in more detail.
I -> R: I1: Trigger exchange
I <- R: R1: {Puzzle, D-H(R), HI(R), ESP Transform,
HIP Transform }SIG
I -> R: I2: {Solution, LSI(I), ESP_info(I), D-H(I),
ESP_Transform, HIP Transform, {H(I)}SK }SIG
I <- R: R2: {LSI(R), ESP_info(R), HMAC}SIG
A potential responsibility of the NAT, as shown in Figure 1, can be
the following
o Intercept the signaling messages
o Authenticate and authorize the HIP nodes by verifying the
signatures.
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o Process the flow identifier information
o Perform actions according to the state machine
o Create state based on the content of message I2 with ESP_info(I)
and R2 with ESP_info(R). Additionally, it might be necessary to
include support for storing the respective HITs and host
identities.
4.2. HIP Base Exchange with Firewall
In case of a firewall traversal, the routing asymmetry needs to be
considered i.e., the fact that the messages I1 and I2 do not
necessarily traverse the same devices as R1 and R2. The same is true
with more complex network topologies with a mixture of NATs and
Firewalls. This is an assumption made in the NSIS working group (and
therefore also with NAT/Firewall traversal). Pure NAT traversal is
therefore simpler to handle in comparison to middlebox traversal
which also includes devices such as Firewalls. Figure 3 shows this
circumstance graphically:
I1 +----------+ I1
+--------------------> | Firewall | -----------------------+
| I2 | 1 | I2 |
| +-----------------> | | ------------------+ |
| | +----------+ v v
+---------+ +---------+
|Initiator| |Responder|
+---------+ +---------+
^ ^ R1 +----------+ R1 | |
| +------------------ | Firewall | <-------------------+ |
| R2 | 2 | R2 |
+--------------------- | | <-----------------------+
+----------+
............... IPsec ESP protected traffic (SPI(R)).........>
<.............. IPsec ESP protected traffic (SPI(I))..........
Legend:
--- = HIP signaling
... = IPsec protected data traffic
Figure 3: Firewall and HIP Base Exchange
With one single NAT between the HIP nodes, all messages of the base
exchange are forced through it. With firewalls, it becomes obvious
that the nice property of a NAT with respect to the symmetric
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forwarding path is lost and the individual firewalls (Firewall 1 and
Firewall 2) are unable to create the necessary firewall pinholes.
SPI(I) is exchanged in I2 message (ESP_info(I)) through firewall 1,
however firewall 2 only needs it. Similarly firewall 2 needs SPI (R)
which is sent in message R2 (ESP_info(I)) through firewall 1.
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5. Scenarios
The following section describes some sample scenarios, in the context
of involving middleboxes, to learn the flow identifier:
5.1. Same Firewall at Initiator for both outgoing and incoming packets
This scenario assumes that the initiator I alone is behind a firewall
named FW(I). This firewall is both for the outgoing and incoming
packets and hence can look into all the base exchange messages. This
scenario is also applicable for NATs as well. This is illustrated in
Figure 4
FW(I)
I1 +-----+ I1
+----------> | |--------------------------------------+
| I2 | | I2 |
| +-----> | |---------------------------------+ |
| | | | | |
| | | | v v
---------+ | | +--------+
Initiator| | | |Receiver|
---------+ | | +--------+
^ ^ | |
| | R2 | | R2 | |
| +------ | |< --------------------------------+ |
| R1 | | R1 |
+---------- | |< -------------------------------------+
+-----+
Figure 4: One FW only at initiator end
1. I1 packet is sent from the initiator I to receiver R.
2. FW(I) forward the packet to the Receiver.
3. R sends R1 message with puzzle,D-H key protected with the
signature of R.
4. FW(I) forward the packet to the Initiator.
5. On receiving I2, FW(I) verifies the signature of I and learns the
SPI value form the ESP_info parameter and forwards it to the
Receiver
6. R sends the message R2 to I.
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7. On receiving R2, FW(I) verifies the signature of R. Accordingly,
it earns the SPI value form the ESP_info parameter and forwards
it to the Initiator.
8. The base exchange continues until complete. Since all messages
I1,R1,I2 and R2 traverse through FW(I), it can look into these
messages to learn the flow identifier information.
5.2. Different Firewalls at Initiator for outgoing and incoming packets
This scenario assumes that both the initiator I and the receiver R is
situated behind firewalls named FW(I) and FW(R) respectively. FW(I)
is for the incoming packets to I and FW(R) is for the incoming
packets to R. It is necessary that both the firewalls must learn the
flow identifier information and should store the state <SPI,IP,HIT>
to forward IPsec protected payload packets. This scenario is
illustrated in Figure 5
FW(R)
I1 +-----+ I1
+------------------------>| |--------+
| I2 | | I2 |
| +------------------->| |---+ |
| | +-----+ | |
| | v v
+---------+ +--------+
|Initiator| |Receiver|
+---------+ FW(I) +--------+
^ ^ +-----+ | |
| | R2 | | R2 | |
| +--------| |<--------------+ |
| R1 | | R1 |
+------------| |<-------------------+
+-----+
Figure 5: End hosts behind FWs
1. I1 packet is sent from the initiator I to receiver R.
2. FW(R) forwards the packet to the Receiver.
3. Then, R sends R1 message with puzzle,D-H key protected with the
signature of R.
4. FW(I) forward the packet to the Initiator.
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5. Now, I sends the I2 packet, on receiving I2, FW(R) verifies the
signature of I and learns the SPI value form the ESP_info
parameter and forwards it to the Receiver
6. To complete the base exchange, R sends the message R2 to I.
7. On receiving R2, FW(I) verifies the signature of R. Accordingly,
it earns the SPI value from the ESP_info parameter and forwards
it to the Initiator.
Here, the problem with this asymmetric base exchange is that the SPI
needed for the FW(I) is sent through the I2 message, which flows
through the FW(R) and the SPI needed for for the FW(I) is sent to
FW(R).
5.3. Different Firewalls at Initiator and Receiver
This scenario looks into a more complicated situation. Initiator I
is behind multiple firewalls FW1(I) for outgoing packets and FW2(I)
and FW3(I) are for incoming packets. Similarly, receiver R is behind
FW1(R) and FW2(R) for incoming packets and FW3(R) for outgoing
packets. The incoming firewalls are chosen based on the type of the
application and the hosts are unaware from which firewall they
receive packets. Here, however for our scenario we assume that
FW2(R) and FW2(I) are chosen about which also the hosts are unaware
of. This scenario is illustrated in Figure 6
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+-----+
| |
|FW1-R|
| |
+-----+ +-----+
I1 | | I1 +-----+
+------------| | -------------------> | |---------+
| I2 |FW1-I| I2 |FW2-R| |
| +-------| | -------------------> | |----+ |
| | | | +-----+ | |
| | +-----+ v v
+---------+ +--------+
|Initiator| |Receiver|
+---------+ +--------+
^ ^ +-----+
| | R2 | | R2 +-----+ | |
| +------ |FW2-I| <--------------------| |-----+ |
| R1 | | R1 |FW3-R| |
+---------- | | <--------------------| |----------+
+-----+ | |
+-----+ | |
| | +-----+
|FW3-I|
| |
+-----+
Figure 6: Multiple FWs at initiator's and receiver's end
1. I1 packet is sent from the initiator I to receiver R.
2. FW1(I) and FW2(R) forwards the packet to the Receiver.
3. Then, R sends R1 message with puzzle,D-H key protected with the
signature of R.
4. Now, FW3(R) and FW2(I) forward the packet to the Initiator.
5. Now, the I sends the I2 packet, on receiving I2, FW1(I) and
FW2(R) can verify the signature of I and can learn the SPI value
from the ESP_info parameter and forward it to the Receiver.
6. To complete the base exchange, R sends the message R2 to I.
7. On receiving R2, FW3(R) and FW2(I) can verify the signature of R.
Accordingly, they learn the SPI value form the ESP_info parameter
and forwards it to the Initiator.
Here, the problems are :
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1. With this asymmetric base exchange is that the SPI needed for the
Firewall(s) on the receiver side is sent through the I2 message,
Which is actually sent through FW1(I) and FW2(R) and the SPI
needed for for the Firewall(s) on the Initiator side is sent to
FW3(R) and FW2(I).
2. When hosts are behind multiple incoming firewalls, there are
unable to decide to which firewall they have to inform their SPI
values to.
3. The second problem is to secure the SPI signalling message from
the end host to the FW. Since the end hosts authenticate and
authorize to the FW that lets outgoing packets, they share keys
only with them. However, as mentioned earlier, they, somehow,
need to signal the SPI value to the FW on the other end which
forwards incoming packets.
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6. Requirements
In the context of middlebox signaling, a few high-level requirements
have to be accomplished:
o Add some authentication and authorization capabilities to NAT
traversal. Many NAT/Firewall traversal solutions do not allow the
end host to interact with the middlebox. As a consequence, some
security vulnerabilities are introduced.
o Add secure firewall traversal functionality as another type of
middlebox signaling by using <destination IP address, SPI and
protocol> triplet. as a substitute for the typical < source IP,
destination IP, source port, destination port, transport protocol>
information.
Such a solution for HIP-based middlebox signaling needs to have the
following properties:
o A HIP-aware NAT/FW MUST be able to authenticate the entity
requesting a NAT binding or a firewall pinhole.
o A HIP-aware NAT/FW MUST authorize the entity requesting a NAT
binding or a firewall pinhole before storing state information.
This requirement might be accomplished by identity based
authorization or an identity independent authorization mechanism.
o A HIP-aware NAT/FW MUST be able to intercept HIP messages in order
to extract the flow identifier information and other related
information. HIP messages are base exchange messages during
context establishment and readdressing messages during IP address
changes. A NAT/FW MUST be able to distinguish these messages.
o A NAT/FW node MUST NOT introduce new denial of service attacks
based on authentication or key management mechanisms.
o A potential solution MUST respect the property of some middleboxes
which do not allow traffic (data and signaling traffic) to
traverse this middlebox without proper authorization.
Some requirements are taken from [I-D.ietf-nsis-nslp-natfw].
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7. Security Considerations
This document analyzes the traversal of HIP-aware middleboxes. A
problem statement is given and scenarios are described that lead to a
number of requirements.
This document therefore lists a number of security aspects throughout
the document. Care should be taken when solutions are developed and
the security solution must not introduce new vulnerabilities to the
middlebox.
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8. Contributors
We would like to thank Aarthi Nagarajan, Vesa Torvinen, Jochen
Grimminger and Jukka Ylitalo for their help with initial versions of
this document.
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9. Acknowledgements
The authors would like to thank Pekka Nikander, Dieter Gollmann and
Thomas Aura for their feedback to this document.
This document is a byproduct of the Ambient Networks Project,
partially funded by the European Commission under its Sixth Framework
Programme. It is provided "as is" and without any express or implied
warranties, including, without limitation, the implied warranties of
fitness for a particular purpose. The views and conclusions
contained herein are those of the authors and should not be
interpreted as necessarily representing the official policies or
endorsements, either expressed or implied, of the Ambient Networks
Project or the European Commission.
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10. References
10.1. Normative References
[I-D.ietf-HIP-esp]
Moskowitz, R., Nikander, P., and P. Jokela, "Using ESP
transport format with HIP", draft-ietf-hip-esp-00 (work in
progress), June 2005.
[I-D.ietf-hip-base]
Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
"Host Identity Protocol", draft-ietf-hip-base-03 (work in
progress), June 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
10.2. Informative References
[I-D.ietf-ipsec-ikev2]
Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress),
September 2004.
[I-D.ietf-nsis-nslp-natfw]
Stiemerling, M., Tschofenig, H., and C. Aoun, "A NAT/
Firewall NSIS Signaling Layer Protocol (NSLP)",
draft-ietf-nsis-nslp-natfw-07 (work in progress),
July 2005.
[I-D.irtf-hiprg-nat]
Stiemerling, M., Quittek, J., and L. Eggert, "Middlebox
Traversal Issues of Host Identity Protocol (HIP)
Communication", draft-irtf-hiprg-nat-00.txt (work in
progress) (work in progress), October 2005.
[NATTerminology]
Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", Request
For Comments RFC 2663, August 1999.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
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[RFC3947] Kivinen, T., A. Huttunen, A., Swander, B., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[RFC3948] A. Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and
M. Stenberg, "UDP Encapsulation of IPsec Packets",
RFC 3948, January 2005.
[RFC4080] Hancock, R., "Next Steps in Signaling: Framework",
RFC 4080, November 2004.
[draft-ietf-hip-mm]
Henderson (Editor), T., "End-Host Mobility and Multi-
Homing with Host Identity Protocol",
draft-nikander-hip-mm-02.txt (work in progress) (work in
progress), July 2005.
[draft-ietf-ipsec-esp-v3-08]
Kent, S., "IP Encapsulating Security Payload (ESP)",
draft-ietf-ipsec-esp-v3-10 (work in progress) (work in
progress), March 2005.
[draft-moskowitz-hip-arch]
Moskowitz, R. and P. Nikander, "Host Identity Protocol
Architecture", draft-ietf-hip-arch-03 (work in progress)
(work in progress), August 2005.
[draft-nikander-hip-path-00.txt]
Nikander, P., Tschofenig, H., Henderson, T., Eggert, L.,
and J. Laganier, "Preferred Alternatives for Tunnelling
HIP (PATH)", draft-nikander-hip-path-00.txt (work in
progress) (work in progress), February 2005.
[rfc3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", Request For
Comments RFC 3022, January 2001.
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Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
Email: Hannes.Tschofenig@siemens.com
Murugaraj Shanmugam
Technical Univeristy Hamburg-Harburg
Schwarzenbergstrasse 95
Harburg, Hamburg 21073
Germany
Email: murugaraj.shanmugam@tuhh.de
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Internet-Draft NAT and Firewall Traversal for HIP October 2005
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