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RFC 5207 - NAT and Firewall Traversal Issues of Host Identity Pr

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

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

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.


   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.


   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.

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

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

   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].

   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.

   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

   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

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

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

   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

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.

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

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.

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.

   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.

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.

   [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.

Authors' Addresses

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

   Phone: +49 6221 4342 113
   Fax:   +49 6221 4342 155
   EMail: stiemerling@nw.neclab.eu
   URI:   http://www.nw.neclab.eu/

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

   Phone: +49 6221 4342 115
   Fax:   +49 6221 4342 155
   EMail: quittek@nw.neclab.eu
   URI:   http://www.nw.neclab.eu/

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

   Phone: +358 50 48 24461
   EMail: lars.eggert@nokia.com
   URI:   http://research.nokia.com/people/lars_eggert/

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