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Network Working Group                                     M. StiemerlingRequest for Comments: 5207                                    J. QuittekCategory: Informational                                              NEC                                                               L. Eggert                                                                   Nokia                                                              April 2008NAT and Firewall Traversal Issues of Host Identity Protocol (HIP)CommunicationStatus 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.Stiemerling, et al.          Informational                      [Page 1]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  HIP across NATs  . . . . . . . . . . . . . . . . . . . . . . .42.1.  Phase 1: HIP Base Exchange . . . . . . . . . . . . . . . .42.1.1.  IPv4 HIP Base Exchange . . . . . . . . . . . . . . . .42.1.2.  IPv6 HIP Base Exchange . . . . . . . . . . . . . . . .52.2.  Phase 2: ESP Data Exchange . . . . . . . . . . . . . . . .53.  HIP Across Firewalls . . . . . . . . . . . . . . . . . . . . .63.1.  Phase 1: HIP Base Exchange . . . . . . . . . . . . . . . .63.1.1.  IPv4 HIP Base Exchange . . . . . . . . . . . . . . . .63.1.2.  IPv6 HIP Base Exchange . . . . . . . . . . . . . . . .63.2.  Phase 2: ESP Data Exchange . . . . . . . . . . . . . . . .74.  HIP Extensions . . . . . . . . . . . . . . . . . . . . . . . .75.  NAT Extensions . . . . . . . . . . . . . . . . . . . . . . . .86.  Legacy NAT and Firewall Traversal  . . . . . . . . . . . . . .87.  HIP across Other Middleboxes . . . . . . . . . . . . . . . . .98.  Security Considerations  . . . . . . . . . . . . . . . . . . .99.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .1010. References . . . . . . . . . . . . . . . . . . . . . . . . . .1010.1. Normative References . . . . . . . . . . . . . . . . . . .1010.2. Informative References . . . . . . . . . . . . . . . . . .10Stiemerling, et al.          Informational                      [Page 2]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 20081.  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].Stiemerling, et al.          Informational                      [Page 3]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008   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.Stiemerling, et al.          Informational                      [Page 4]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008   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 chooseStiemerling, et al.          Informational                      [Page 5]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008   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 Exchange3.1.1.  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 (seeSection 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.Stiemerling, et al.          Informational                      [Page 6]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 20083.2.  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.  Sections2 and3 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 inSection 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.Stiemerling, et al.          Informational                      [Page 7]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 20085.  NAT Extensions   IPsec SPIs have only one-way significance, as described inSection 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 inSection 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.Stiemerling, et al.          Informational                      [Page 8]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008   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.Stiemerling, et al.          Informational                      [Page 9]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 20089.  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.  References10.1.  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.Stiemerling, et al.          Informational                     [Page 10]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008   [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               Sitehttp://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.Stiemerling, et al.          Informational                     [Page 11]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008Authors' Addresses   Martin Stiemerling   NEC Europe Ltd.   Kurfuersten-Anlage 36   Heidelberg  69115   Germany   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   Germany   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   Finland   Phone: +358 50 48 24461   EMail: lars.eggert@nokia.com   URI:http://research.nokia.com/people/lars_eggert/Stiemerling, et al.          Informational                     [Page 12]

RFC 5207           HIP NAT/Firewall Traversal Issues          April 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78 and athttp://www.rfc-editor.org/copyright.html,   and except as set forth therein, the authors retain all their rights.   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, THE IETF TRUST 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Stiemerling, et al.          Informational                     [Page 13]