Network Working Group M. Stiemerling

Network Working Group M. Stiemerling
Request for Comments: 5207 J. Quittek
Category: Informational NEC
L. Eggert
Nokia
April 2008

NAT and Firewall Traversal Issues of Host Identity Protocol (HIP)
Communication

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

IESG Note

This RFC is a product of the Internet Research Task Force and is not
a candidate for any level of Internet Standard. The IRTF publishes
the results of Internet-related research and development activities.
These results might not be suitable for deployment.

Abstract

The Host Identity Protocol (HIP) changes the way in which two
Internet hosts communicate. One key advantage over other schemes is
that HIP does not require modifications to the traditional network-
layer functionality of the Internet, i.e., its routers. In the
current Internet, however, many devices other than routers modify the
traditional network-layer behavior of the Internet. These
"middleboxes" are intermediary devices that perform functions other
than the standard functions of an IP router on the datagram path
between source and destination hosts. Whereas some types of
middleboxes may not interfere with HIP at all, others can affect some
aspects of HIP communication, and others can render HIP communication
impossible. This document discusses the problems associated with HIP
communication across network paths that include specific types of
middleboxes, namely, network address translators and firewalls. It
identifies and discusses issues in the current HIP specifications
that affect communication across these types of middleboxes. This
document is a product of the IRTF HIP Research Group.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. HIP across NATs . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Phase 1: HIP Base Exchange . . . . . . . . . . . . . . . . 4
2.1.1. IPv4 HIP Base Exchange . . . . . . . . . . . . . . . . 4
2.1.2. IPv6 HIP Base Exchange . . . . . . . . . . . . . . . . 5
2.2. Phase 2: ESP Data Exchange . . . . . . . . . . . . . . . . 5
3. HIP Across Firewalls . . . . . . . . . . . . . . . . . . . . . 6
3.1. Phase 1: HIP Base Exchange . . . . . . . . . . . . . . . . 6
3.1.1. IPv4 HIP Base Exchange . . . . . . . . . . . . . . . . 6
3.1.2. IPv6 HIP Base Exchange . . . . . . . . . . . . . . . . 6
3.2. Phase 2: ESP Data Exchange . . . . . . . . . . . . . . . . 7
4. HIP Extensions . . . . . . . . . . . . . . . . . . . . . . . . 7
5. NAT Extensions . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Legacy NAT and Firewall Traversal . . . . . . . . . . . . . . 8
7. HIP across Other Middleboxes . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 10

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1. Introduction

The current specification of the Host Identity Protocol (HIP)
[RFC4423] assumes simple Internet paths, where routers forward
globally routable IP packets based on their destination address
alone.

In the current Internet, such pure paths are becoming increasingly
rare. For a number of reasons, several types of devices modify or
extend the pure forwarding functionality the Internet's network layer
used to deliver. [RFC3234] coins the term middleboxes for such
devices: "A middlebox is (...) any intermediary device performing
functions other than the normal, standard functions of an IP router
on the datagram path between a source host and destination host".

Middleboxes affect communication in a number of ways. For example,
they may inspect the flows of some transport protocols, such as TCP,
and selectively drop, insert, or modify packets. If such devices
encounter a higher-layer protocol they do not support, or even a
variant of a supported protocol that they do not know how to handle,
communication across the middlebox may become impossible for these
kinds of traffic.

There are many different variants of middleboxes. The most common
ones are network address translators and firewalls. [RFC3234]
identifies many other types of middleboxes. One broad way of
classifying them is by behavior. The first group operates on
packets, does not modify application-layer payloads, and does not
insert additional packets. This group includes NAT, NAT-PT, SOCKS
gateways, IP tunnel endpoints, packet classifiers, markers,
schedulers, transport relays, IP firewalls, application firewalls,
involuntary packet redirectors, and anonymizers.

Other middleboxes exist (such as TCP performance-enhancing proxies,
application-level gateways, gatekeepers, session control boxes,
transcoders, proxies, caches, modified DNS servers, content and
applications distribution boxes, and load balancers) that divert or
modify URLs, application-level interceptors, and application-level
multicast systems. However, NATs and firewalls are the most frequent
middleboxes that HIP traffic can encounter in the Internet.
Consequently, this memo focuses on how NAT and firewall middleboxes
can interfere with HIP traffic.

Middleboxes can cause two different kinds of communication problems
for HIP. They can interfere with the transmission of HIP control
traffic or with the transmission of the HIP data traffic carried
within the Encapsulating Security Payload (ESP) [RFC4303].

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This document serves mainly as a problem description that solution
proposals can reference. But it also discusses known approaches to
solving the problem and gives recommendations for certain approaches
depending on the specific scenario. It does not promote the use of
any of the discussed types of middleboxes.

This memo was discussed and modified in the Host Identity Protocol
Research Group, was reviewed by the Internet Research Steering Group
(IRSG), and represents a consensus view of the research group at the
time of its submission for publication.

2. HIP across NATs

This section focuses on the traversal of HIP across network address
translator (NAT) middleboxes. This document uses the term NAT for a
basic translation of IP addresses, whereas it uses the term NAPT for
NATs that additionally perform port translation [RFC2663], if a
differentiation between the two is important.

HIP operates in two phases. It first performs a HIP "base exchange"
handshake before starting to exchange application data in the second
phase. This section describes the problems that occur in each of the
two phases when NATs are present along the path from the HIP
initiator to the responder.

2.1. Phase 1: HIP Base Exchange

The HIP base exchange uses different transport mechanisms for IPv6
and IPv4. With IPv6, it uses a HIP-specific IPv6 extension header,
whereas it uses the IP payload with IPv4 [RFC5201].

2.1.1. IPv4 HIP Base Exchange

The HIP protocol specification [RFC5201] suggests encapsulating the
IPv4 HIP base exchange in a new IP payload type. The chances of NAT
traversal for this traffic are different, depending on the type of
NAT in the path. The IPv4 HIP base exchange traverses basic NATs
(that translate IP addresses only) without problems, if the NAT only
interprets and modifies the IP header, i.e., it does not inspect the
IP payload.

However, basic NATs are rare. NAPT devices that inspect and
translate transport-layer port numbers are much more common. Because
the IP payload used for the IPv4 base exchange does not contain port
numbers or other demultiplexing fields, NAPTs cannot relay it.

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A second issue is the well-known "data receiver behind a NAT"
problem. HIP nodes behind a NAT are not reachable unless they
initiate the communication themselves, because the necessary
translation state is otherwise not present at the NAT.

2.1.2. IPv6 HIP Base Exchange

The IPv6 HIP base exchange uses empty IPv6 packets (without a
payload). New HIP extension headers carry the base exchange
information. This approach can cause problems if NAT middleboxes
translate or multiplex IP addresses.

At this time, IPv6 NATs are rare. However, when they exist, IPv6
NATs operate similarly to IPv4 NATs. Consequently, they will likely
block IP payloads other than the "well-known" transport protocols.
This includes the IPv6 HIP base exchange, which does not contain any
IP payload.

2.2. Phase 2: ESP Data Exchange

HIP uses ESP to secure the data transmission between two HIP nodes
after the base exchange completes. Thus, HIP faces the same
challenges as IPsec with regard to NAT traversal. [RFC3715]
discusses these issues for IPsec and describes three distinct problem
categories: NAT-intrinsic issues, NAT implementation issues, and
helper incompatibilities.

This section focuses on the first category, i.e., NAT-intrinsic
issues. The two other problem categories are out of this document's
scope. They are addressed in the BEHAVE working group or in
[RFC3489].

With ESP-encrypted data traffic, all upper-layer headers are
invisible to a NAT. Thus, changes of the IP header during NAT
traversal can invalidate upper-layer checksums contained within the
ESP-protected payload. HIP hosts already avoid this problem by
substituting Host Identity Tags (HITs) for IP addresses during
checksum calculations [RFC5201].

Although the traversal of ESP-encrypted packets across NATs is
possible, [RFC3715] notes that the Security Parameter Index (SPI)
values of such traffic have only one-way significance. NATs can use
SPI values to demultiplex different IPsec flows, similar to how they
use port number pairs to demultiplex unencrypted transport flows.
Furthermore, NATs may modify the SPIs, similar to how they modify
port numbers, when multiple IPsec nodes behind them happen to choose

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identical SPIs. However, NATs can only observe the SPIs of outgoing
IPsec flows and cannot determine the SPIs of the corresponding return
traffic.

3. HIP Across Firewalls

This section focuses on the traversal of HIP across IP firewalls and
packet filters. These types of middleboxes inspect individual
packets and decide whether to forward, discard, or process them in
some special way, based on a set of filter rules and associated
actions.

Firewalls are not inherently problematic for HIP, as long as their
policy rules permit HIP base exchange and IPsec traffic to traverse.
The next sections discuss specific issues for HIP in typical firewall
configurations.

3.1. Phase 1: HIP Base Exchange

3.1.1. IPv4 HIP Base Exchange

A common and recommended configuration for IPv4 firewalls is to block
all unknown traffic by default and to allow well-known transport
protocols only and often just on specific ports and with specific
characteristics ("scrubbed" traffic). This common configuration
blocks the HIP base exchange.

3.1.2. IPv6 HIP Base Exchange

The configuration of IPv6 firewalls is similar to IPv4 firewalls.
Many IPv4 firewalls discard any IP packet that includes an IP option.
With IPv6, the expectation is that firewalls will block IPv6
extension headers in general or will at least block unknown extension
headers. Furthermore, payloads other than specific, well-known
transport protocols are likely to be blocked as well. Like IPv4,
this behavior blocks the HIP base exchange.

A problem similar to the "data receiver behind a NAT" issue (see
Section 2.1.1) applies to both IPv4 and IPv6 firewalls. Typically,
firewalls block all traffic into the protected network that is not
identifiable return traffic of a prior outbound communication. This
means that HIP peers are not reachable outside the protected network,
because firewalls block base exchange attempts from outside peers.

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3.2. Phase 2: ESP Data Exchange

Firewalls are less problematic than NATs with regard to passing ESP
traffic. The largest concern is commonly used firewall
configurations that block ESP traffic, because it is not a well-known
transport protocol and ports cannot be used to identify return flows.
However, firewalls could use mechanisms similar to Security Parameter
Index (SPI) multiplexed NAT (SPINAT) to use SPIs as flow identifiers
[YLITALO].

4. HIP Extensions

This section identifies possible changes to HIP that attempt to
improve NAT and firewall traversal, specifically, the reachability of
HIP peers behind those middleboxes and traversal of the HIP base
exchange. Sections 2 and 3 describe several problems related to
encapsulation schemes for the HIP base exchange in IPv4 and IPv6.

UDP may improve HIP operation in the presence of NATs and firewalls.
It may also aid traversal of other middleboxes. For example, load
balancers that use IP- and transport-layer information can correctly
operate with UDP-encapsulated HIP traffic.

HIP nodes located behind a NAT must notify their communication peers
about the contact information. The contact information is the NAT's
public IP address and a specific UDP port number. This measure
enables the peers to send return traffic to HIP nodes behind the NAT.
This would require a new HIP mechanism.

To be reachable behind a NAT, a rendezvous point is required that
lets HIP nodes behind a NAT register an IP address and port number
that can be used to contact them. Depending on the type of NAT, use
of this rendezvous point may be required only during the base
exchange or throughout the duration of a communication instance. A
rendezvous point is also useful for HIP nodes behind firewalls,
because they suffer from an analogous problem, as described in
Section 3.

The proposed mobility management packet exchange [RFC5206] [NIKANDER]
can support this method of NAT traversal. The original intention of
this extension is to support host mobility and multihoming. This
mechanism is similar to the Alternate Network Address Types (ANAT)
described in [RFC4091]. However, HIP peers use mobility management
messages to notify peers about rendezvous points, similar to
[RFC4091]. HIP peers must determine their contact address before
they can announce it to their peers.

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5. NAT Extensions

IPsec SPIs have only one-way significance, as described in
Section 2.2. Consequently, NATs and firewalls can observe the SPI
values of outgoing packets, but they cannot learn the SPI values of
the corresponding inbound return traffic in the same way. Two
methods exist:

First, NATs can observe the HIP base exchange and learn the SPI
values that HIP peers agree to use. Afterwards, NATs can map
outgoing and incoming IPsec flows accordingly. This approach is
called architectured NAT, or SPINAT [YLITALO], and can be used by
firewalls as well. It requires HIP-specific NAT modifications.

Second, HIP peers can use a generic NAT or firewall signaling
protocol to explicitly signal appropriate SPI values to their NATs
and firewalls. This approach does not require HIP-specific changes
at the middlebox, but does require integration of HIP with the
signaling protocol at the end systems.

Possible solutions for signaling SPI values are the mechanisms
proposed in the IETF NSIS WG (NATFW NSLP) and MIDCOM MIB module
[RFC5190]. Using MIDCOM in the context of HIP requires additional
knowledge about network topology. For example, in multihomed
environments with different border NATs or firewalls, a host must
know which of the multiple NATs/firewalls to signal. Therefore, this
solution can be problematic.

By using the NSIS NAT/FW traversal (NATFW NSLP) mechanism HIP nodes
can signal the used SPI values for both directions. NATFW NSLP
ensures that signaling messages will reach all NATs and firewalls
along the data path (path-coupled signaling). Although NSIS is
generally supported at both peers, the NATFW NSLP offers a "proxy
mode" for scenarios where only one end supports NSIS. This has
deployment advantages.

6. Legacy NAT and Firewall Traversal

The solutions outlined in Section 5 require that NATs and firewalls
are updated to support new functions, such as HIP itself or NSIS
NATFW signaling. NATs and firewalls are already widely deployed. It
will be impossible to upgrade or replace all such middleboxes with
HIP support. This section explores how HIP operates in the presence
of legacy NATs and firewalls that are not HIP-aware. Because the
vast majority of deployed NATs currently support IPv4 only, this
section focuses on them.

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For HIP over IPv4, UDP encapsulation of HIP traffic already solves
some NAT traversal issues. Usually, UDP packets can traverse NATs
and firewalls when communication was initiated from the inside.
However, traffic initiated outside a NAT is typically dropped,
because it cannot be demultiplexed to the final destination (for
NATs) or is prohibited by policy (for firewalls).

Even when UDP encapsulation enables the HIP base exchange to succeed,
ESP still causes problems [RFC3715]. Some NAT implementations offer
"VPN pass-through", where the NAT learns about IPsec flows and tries
to correlate outgoing and incoming SPI values. This often works
reliably only for a small number of nodes behind a single NAT, due to
the possibility of SPI collisions.

A better solution may be to use UDP encapsulation for ESP [RFC3948],
enabled through a new parameter in the base exchange. It is for
further study whether to mandate UDP encapsulation for all HIP
traffic to reduce the complexity of the protocol.

HIP may also consider other NAT/firewall traversal mechanisms, such
as the widely deployed Universal Plug and Play (UPNP) [UPNP]. UPNP
can be used to configure middleboxes on the same link as a HIP node.

7. HIP across Other Middleboxes

This document focuses on NAT and firewall middleboxes and does not
discuss other types identified in [RFC3234]. NATs and firewalls are
the most frequently deployed middleboxes at the time of writing.
However, future versions of this document may describe how HIP
interacts with other types of middleboxes.

8. Security Considerations

Opening pinholes in firewalls (i.e., loading firewall rules allowing
packets to traverse) and creating NAT bindings are highly security-
sensitive actions. Any mechanism that does so in order to support
HIP traversal across middleboxes should be well protected. Detailed
discussion of the related security issues can be found in the
security considerations sections of the corresponding standards
documents, such as [RFC3715] and [RFC5190].

This document has not considered whether some of the options listed
above pose additional threats to security of the HIP protocol itself.

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9. Acknowledgments

The following people have helped to improve this document through
thoughtful suggestions and feedback: Pekka Nikander, Tom Henderson,
and the HIP research group. The authors would like to thank the
final reviewers, Kevin Fall, Mark Allman, and Karen Sollins.

Lars Eggert and Martin Stiemerling are partly funded by Ambient
Networks, a research project supported by the European Commission
under its Sixth Framework Program. 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.

10. References

10.1. Normative References

[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.

[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.

[RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, May 2006.

[RFC5201] Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
Henderson, "Host Identity Protocol", RFC 5201,
April 2008.

10.2. Informative References

[NIKANDER] Nikander, P., Ylitalo, J., and J. Wall, "Integrating
Security, Mobility, and Multi-Homing in a HIP Way", Proc.
Network and Distributed Systems Security Symposium
(NDSS) 2003, February 2003.

[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.

[RFC3489] 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.

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[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address
Translation (NAT) Compatibility Requirements", RFC 3715,
March 2004.

[RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session
Description Protocol (SDP) Grouping Framework", RFC 4091,
June 2005.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.

[RFC5190] Quittek, J., Stiemerling, M., and P. Srisuresh,
"Definitions of Managed Objects for Middlebox
Communication", RFC 5190, March 2008.

[RFC5206] Henderson, T., Ed., "End-Host Mobility and Multihoming
with the Host Identity Protocol", RFC 5206, April 2008.

[UPNP] UPNP Web Site, "Universal Plug and Play Web Site", Web
Site http://www.upnp.org/, July 2005.

[YLITALO] Ylitalo, J., Melen, J., Nikander, P., and V. Torvinen,
"Re-Thinking Security in IP-Based Micro-Mobility", Proc.
7th Information Security Conference (ISC) 2004,
September 2004.

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Authors' Addresses

Martin Stiemerling
NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany

Phone: +49 6221 4342 113
Fax: +49 6221 4342 155
EMail: [email protected]
URI: http://www.nw.neclab.eu/

Juergen Quittek
NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany

Phone: +49 6221 4342 115
Fax: +49 6221 4342 155
EMail: [email protected]
URI: http://www.nw.neclab.eu/

Lars Eggert
Nokia Research Center
P.O. Box 407
Nokia Group 00045
Finland

Phone: +358 50 48 24461
EMail: [email protected]
URI: http://research.nokia.com/people/lars_eggert/

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