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Network Working Group                                          P. SavolaRequest for Comments: 4459                                     CSC/FUNETCategory: Informational                                       April 2006MTU and Fragmentation Issues with In-the-Network TunnelingStatus 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.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   Tunneling techniques such as IP-in-IP when deployed in the middle of   the network, typically between routers, have certain issues regarding   how large packets can be handled: whether such packets would be   fragmented and reassembled (and how), whether Path MTU Discovery   would be used, or how this scenario could be operationally avoided.   This memo justifies why this is a common, non-trivial problem, and   goes on to describe the different solutions and their characteristics   at some length.Table of Contents1. Introduction ....................................................22. Problem Statement ...............................................33. Description of Solutions ........................................43.1. Fragmentation and Reassembly by the Tunnel Endpoints .......43.2. Signalling the Lower MTU to the Sources ....................53.3. Encapsulate Only When There is Free MTU ....................63.4. Fragmentation of the Inner Packet ..........................84. Conclusions .....................................................95. Security Considerations ........................................106. Acknowledgements ...............................................117. References .....................................................117.1. Normative References ......................................117.2. Informative References ....................................12Savola                       Informational                      [Page 1]

RFC 4459         Packet Size Issues in Network Tunnels        April 20061.  Introduction   A large number of ways to encapsulate datagrams in other packets,   i.e., tunneling mechanisms, have been specified over the years: for   example, IP-in-IP (e.g., [1] [2], [3]), Generic Routing Encapsulation   (GRE) [4], Layer 2 Tunneling Protocol (L2TP) [5], or IP Security   (IPsec) [6] in tunnel mode -- any of which might run on top of IPv4,   IPv6, or some other protocol and carrying the same or a different   protocol.   All of these can be run so that the endpoints of the inner protocol   are co-located with the endpoints of the outer protocol; in a typical   scenario, this would correspond to "host-to-host" tunneling.  It is   also possible to have one set of endpoints co-located, i.e.,   host-to-router or router-to-host tunneling.  Finally, many of these   mechanisms are also employed between the routers for all or a part of   the traffic that passes between them, resulting in router-to-router   tunneling.   All these protocols and scenarios have one issue in common: how does   the source select the maximum packet size so that the packets will   fit, even encapsulated, in the smallest Maximum Transmission Unit   (MTU) of the traversed path in the network; and if you cannot affect   the packet sizes, what do you do to be able to encapsulate them in   any case?  The four main solutions are as follows (these will be   elaborated inSection 3):   1.  Fragmenting all too big encapsulated packets to fit in the paths,       and reassembling them at the tunnel endpoints.   2.  Signal to all the sources whose traffic must be encapsulated, and       is larger than fits, to send smaller packets, e.g., using Path       MTU Discovery (PMTUD)[7][8].   3.  Ensure that in the specific environment, the encapsulated packets       will fit in all the paths in the network, e.g., by using MTU       bigger than 1500 in the backbone used for encapsulation.   4.  Fragmenting the original too big packets so that their fragments       will fit, even encapsulated, in the paths, and reassembling them       at the destination nodes.  Note that this approach is only       available for IPv4 under certain assumptions (seeSection 3.4).   It is also common to run multiple layers of encapsulation, for   example, GRE or L2TP over IPsec; with nested tunnels in the network,   the tunnel endpoints can be the same or different, and both the inner   and outer tunnels may have different MTU handling strategies.  InSavola                       Informational                      [Page 2]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006   particular, signalling may be a scalable option for the outer tunnel   or tunnels if the number of innermost tunnel endpoints is limited.   The tunneling packet size issues are relatively straightforward in   host-to-host tunneling or host-to-router tunneling where Path MTU   Discovery only needs to signal to one source node.  The issues are   significantly more difficult in router-to-router and certain   router-to-host scenarios, which are the focus of this memo.   It is worth noting that most of this discussion applies to a more   generic case, where there exists a link with a lower MTU in the path.   A concrete and widely deployed example of this is the usage of PPP   over Ethernet (PPPoE) [11] at the customers' access link.  These   lower-MTU links, and particularly PPPoE links, are typically not   deployed in topologies where fragmentation and reassembly might be   unfeasible (e.g., a backbone), so this may be a slightly easier   problem.  However, this more generic case is considered out of scope   of this memo.   There are also known challenges in specifying and implementing a   mechanism that would be used at the tunnel endpoint to obtain the   best suitable packet size to use for encapsulation: if a static value   is chosen, a lot of fragmentation might end up being performed.  On   the other hand, if PMTUD is used, the implementation would need to   update the discovered interface MTU based on the ICMP Packet Too Big   messages and originate ICMP Packet Too Big message(s) back to the   source(s) of the encapsulated packets; this also assumes that   sufficient data has been piggybacked on the ICMP messages (beyond the   required 64 bits after the IPv4 header).  We'll discuss using PMTUD   to signal the sources briefly inSection 3.2, but in-depth   specification and analysis are described elsewhere (e.g., in [4] and   [2]) and are out of scope of this memo.Section 2 includes a problem statement,section 3 describes the   different solutions with their drawbacks and advantages, andsection4 presents conclusions.2.  Problem Statement   It is worth considering why exactly this is considered a problem.   It is possible to fix all the packet size issues using solution 1,   fragmenting the resulting encapsulated packet, and reassembling it by   the tunnel endpoint.  However, this is considered problematic for at   least three reasons, as described inSection 3.1.   Therefore, it is desirable to avoid fragmentation and reassembly if   possible.  On the other hand, the other solutions may not beSavola                       Informational                      [Page 3]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006   practical either: especially in router-to-router or router-to-host   tunneling, Path MTU Discovery might be very disadvantageous --   consider the case where a backbone router would send ICMP Packet Too   Big messages to every source that would try to send packets through   it.  Fragmenting before encapsulation is also not available in IPv6,   and not available when the Don't Fragment (DF) bit has been set (seeSection 3.4 for more).  Ensuring a high enough MTU so encapsulation   is always possible is of course a valid approach, but requires   careful operational planning, and may not be a feasible assumption   for implementors.   This yields that there is no trivial solution to this problem, and it   needs to be further explored to consider the trade offs, as is done   in this memo.3.  Description of Solutions   This section describes the potential solutions in a bit more detail.3.1.  Fragmentation and Reassembly by the Tunnel Endpoints   The seemingly simplest solution to tunneling packet size issues is   fragmentation of the outer packet by the encapsulator and reassembly   by the decapsulator.  However, this is highly problematic for at   least three reasons:   o  Fragmentation causes overhead: every fragment requires the IP      header (20 or 40 bytes), and with IPv6, an additional 8 bytes for      the Fragment Header.   o  Fragmentation and reassembly require computation: splitting      datagrams to fragments is a non-trivial procedure, and so is their      reassembly.  For example, software router forwarding      implementations may not be able to perform these operations at      line rate.   o  At the time of reassembly, all the information (i.e., all the      fragments) is normally not available; when the first fragment      arrives to be reassembled, a buffer of the maximum possible size      may have to be allocated because the total length of the      reassembled datagram is not known at that time.  Furthermore, as      fragments might get lost, or be reordered or delayed, the      reassembly engine has to wait with the partial packet for some      time (e.g., 60 seconds [9]).  When this would have to be done at      the line rate, with, for example 10 Gbit/s speed, the length of      the buffers that reassembly might require would be prohibitive.Savola                       Informational                      [Page 4]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006   When examining router-to-router tunneling, the third problem is   likely the worst; certainly, a hardware computation and   implementation requirement would also be significant, but not all   that difficult in the end -- and the link capacity wasted in the   backbones by additional overhead might not be a huge problem either.   However, IPv4 identification header length is only 16 bits (compared   to 32 bits in IPv6), and if a larger number of packets are being   tunneled between two IP addresses, the ID is very likely to wrap and   cause data misassociation.  This reassembly wrongly combining data   from two unrelated packets causes data integrity and potentially a   confidentiality violation.  This problem is further described in   [12].   IPv6, and IPv4 with the DF bit set in the encapsulating header,   allows the tunnel endpoints to optimize the tunnel MTU and minimize   network-based reassembly.  This also prevents fragmentation of the   encapsulated packets on the tunnel path.  If the IPv4 encapsulating   header does not have the DF bit set, the tunnel endpoints will have   to perform a significant amount of fragmentation and reassembly,   while the use of PMTUD is minimized.   AsAppendix A describes, the MTU of the tunnel is also a factor on   which packets require fragmentation and reassembly; the worst case   occurs if the tunnel MTU is "infinite" or equal to the physical   interface MTUs.   So, if reassembly could be made to work sufficiently reliably, this   would be one acceptable fallback solution but only for IPv6.3.2.  Signalling the Lower MTU to the Sources   Another approach is to use techniques like Path MTU Discovery (or   potentially a future derivative [13]) to signal to the sources whose   packets will be encapsulated in the network to send smaller packets   so that they can be encapsulated; in particular, when done on   routers, this includes two separable functions:   a.  Forwarding behaviour: when forwarding packets, if the IPv4-only       DF bit is set, the router sends an ICMP Packet Too Big message to       the source if the MTU of the egress link is too small.   b.  Router's "host" behaviour: when the router receives an ICMP       Packet Too Big message related to a tunnel, it (1) adjusts the       tunnel MTU, and (2) originates an ICMP Packet Too Big message to       the source address of the encapsulated packet. (2) can be done       either immediately or by waiting for the next packet to trigger       an ICMP; the former minimizes the packet loss due to MTU changes.Savola                       Informational                      [Page 5]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006   Note that this only works if the MTU of the tunnel is of reasonable   size, and not, for example, 64 kilobytes: seeAppendix A for more.   This approach would presuppose that PMTUD works.  While it is   currently working for IPv6, and critical for its operation, there is   ample evidence that in IPv4, PMTUD is far from reliable due to, for   example firewalls and other boxes being configured to inappropriately   drop all the ICMP packets [14], or software bugs rendering PMTUD   inoperational.   Furthermore, there are two scenarios where signalling from the   network would be highly undesirable.  The first is when the   encapsulation would be done in such a prominent place in the network   that a very large number of sources would need to be signalled with   this information (possibly even multiple times, depending on how long   they keep their PMTUD state).  The second is when the encapsulation   is done for passive monitoring purposes (network management, lawful   interception, etc.) -- when it's critical that the sources whose   traffic is being encapsulated are not aware of this happening.   When desiring to avoid fragmentation, IPv4 requires one of two   alternatives [1]: copy the DF bit from the inner packets to the   encapsulating header, or always set the DF bit of the outer header.   The latter is better, especially in controlled environments, because   it forces PMTUD to converge immediately.   A related technique, which works with TCP under specific scenarios   only, is so-called "MSS clamping".  With that technique or rather a   "hack", the TCP packets' Maximum Segment Size (MSS) is reduced by   tunnel endpoints so that the TCP connection automatically restricts   itself to the maximum available packet size.  Obviously, this does   not work for UDP or other protocols that have no MSS.  This approach   is most applicable and used with PPPoE, but could be applied   otherwise as well; the approach also assumes that all the traffic   goes through tunnel endpoints that do MSS clamping -- this is trivial   for the single-homed access links, but could be a challenge   otherwise.   A new approach to PMTUD is in the works [13], but it is uncertain   whether that would fix the problems -- at least not the passive   monitoring requirements.3.3.  Encapsulate Only When There is Free MTU   The third approach is an operational one, depending on the   environment where encapsulation and decapsulation are being   performed.  That is, if an ISP would deploy tunneling in its   backbone, which would consist only of links supporting high MTUsSavola                       Informational                      [Page 6]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006   (e.g., Gigabit Ethernet or SDH/SONET), but all its customers and   peers would have a lower MTU (e.g., 1500, or the backbone MTU minus   the encapsulation overhead), this would imply that no packets (with   the encapsulation overhead added) would have a larger MTU than the   "backbone MTU", and all the encapsulated packets would always fit   MTU-wise in the backbone links.   This approach is highly assumptive of the deployment scenario.  It   may be desirable to build a tunnel to/from another ISP, for example,   where this might no longer hold; or there might be links in the   network that cannot support the higher MTUs to satisfy the tunneling   requirements; or the tunnel might be set up directly between the   customer and the ISP, in which case fragmentation would occur, with   tunneled fragments terminating on the ISP and thus requiring   reassembly capability from the ISP's equipment.   To restate, this approach can only be considered when tunneling is   done inside a part of specific kind of ISP's own network, not, for   example, transiting an ISP.   Another, related approach might be having the sources use only a low   enough MTU that would fit in all the physical MTUs; for example, IPv6   specifies the minimum MTU of 1280 bytes.  For example, if all the   sources whose traffic would be encapsulated would use this as the   maximum packet size, there would probably always be enough free MTU   for encapsulation in the network.  However, this is not the case   today, and it would be completely unrealistic to assume that this   kind of approach could be made to work in general.   It is worth remembering that while the IPv6 minimum MTU is 1280 bytes   [10], there are scenarios where the tunnel implementation must   implement fragmentation and reassembly [3]: for example, when having   an IPv6-in-IPv6 tunnel on top of a physical interface with an MTU of   1280 bytes, or when having two layers of IPv6 tunneling.  This can   only be avoided by ensuring that links on top of which IPv6 is being   tunneled have a somewhat larger MTU (e.g., 40 bytes) than 1280 bytes.   This conclusion can be generalized: because IP can be tunneled on top   of IP, no single minimum or maximum MTU can be found such that   fragmentation or signalling to the sources would never be needed.   All in all, while in certain operational environments it might be   possible to avoid any problems by deployment choices, or limiting the   MTU that the sources use, this is probably not a sufficiently good   general solution for the equipment vendors.  Other solutions must   also be provided.Savola                       Informational                      [Page 7]

RFC 4459         Packet Size Issues in Network Tunnels        April 20063.4.  Fragmentation of the Inner Packet   A final possibility is fragmenting the inner packet, before   encapsulation, in such a manner that the encapsulated packet fits in   the tunnel's path MTU (discovered using PMTUD).  However, one should   note that only IPv4 supports this "in-flight" fragmentation;   furthermore, it isn't allowed for packets where the Don't Fragment   bit has been set.  Even if one could ignore IPv6 completely, so many   IPv4 host stacks send packets with the DF bit set that this would   seem unfeasible.   However, there are existing implementations that violate the standard   that:   o  discard too big packets with the DF bit not set instead of      fragmenting them (this is rare);   o  ignore the DF bit completely, for all or specified interfaces; or   o  clear the DF bit before encapsulation, in the egress of configured      interfaces.  This is typically done for all the traffic, not just      too big packets (allowing configuring this is common).   This is non-compliant behaviour, but there are certainly uses for it,   especially in certain tightly controlled passive monitoring   scenarios, and it has potential for more generic applicability as   well, to work around PMTUD issues.   Clearing the DF bit effectively disables the sender's PMTUD for the   path beyond the tunnel.  This may result in fragmentation later in   the network, but as the packets have already been fragmented prior to   encapsulation, this fragmentation later on does not make matters   significantly worse.   As this is an implemented and desired (by some) behaviour, the full   impacts e.g., for the functioning of PMTUD (for example) should be   analyzed, and the use of fragmentation-related IPv4 bits should be   re-evaluated.   In summary, this approach provides a relatively easy fix for IPv4   problems, with potential for causing problems for PMTUD; as this   would not work with IPv6, it could not be considered a generic   solution.Savola                       Informational                      [Page 8]

RFC 4459         Packet Size Issues in Network Tunnels        April 20064.  Conclusions   Fragmentation and reassembly by the tunnel endpoints are a clear and   simple solution to the problem, but the hardware reassembly when the   packets get lost may face significant implementation challenges that   may be insurmountable.  This approach does not seem feasible,   especially for IPv4 with high data rates due to problems with   wrapping the fragment identification field [12].  Constant wrapping   may occur when the data rate is in the order of MB/s for IPv4 and in   the order of dozens of GB/s for IPv6.  However, this reassembly   approach is probably not a problem for passive monitoring   applications.   PMTUD techniques, at least at the moment and especially for IPv4,   appear to be too unreliable or unscalable to be used in the   backbones.  It is an open question whether a future solution might   work better in this aspect.   It is clear that in some environments the operational approach to the   problem, ensuring that fragmentation is never necessary by keeping   higher MTUs in the networks where encapsulated packets traverse, is   sufficient.  But this is unlikely to be enough in general, and for   vendors that may not be able to make assumptions about the operators'   deployments.   Fragmentation of the inner packet is only possible with IPv4, and is   sufficient only if standards-incompliant behaviour, with potential   for bad side-effects (e.g., for PMTUD), is adopted.  It should not be   used if there are alternatives; fragmentation of the outer packet   seems a better option for passive monitoring.   However, if reassembly in the network must be avoided, there are   basically two possibilities:   1.  For IPv6, use ICMP signalling or operational methods.   2.  For IPv4, packets for which the DF bit is not set can be       fragmented before encapsulation (and the encapsulating header       would have the DF bit set); packets whose DF bit is set would       need to get the DF bit cleared (though this is non-compliant).       This also minimizes the need for (unreliable) Internet-wide       PMTUD.   An interesting thing to explicitly note is that when tunneling is   done in a high-speed backbone, typically one may be able to make   assumptions on the environment; however, when reassembly is not   performed in such a network, it might be done in software or with   lower requirements, and there exists either a reassemblySavola                       Informational                      [Page 9]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006   implementation using PMTUD or using a separate approach for passive   monitoring -- so this might not be a real problem.   In consequence, the critical questions at this point appear to be 1)   whether a higher MTU can be assumed in the high-speed networks that   deploy tunneling, and 2) whether "slower-speed" networks could cope   with a software-based reassembly, a less capable hardware-based   reassembly, or the other workarounds.  An important future task would   be analyzing the observed incompliant behaviour about the DF bit to   note whether it has any unanticipated drawbacks.5.  Security Considerations   This document describes different issues with packet sizes and in-   the-network tunneling; this does not have security considerations on   its own.   However, different solutions might have characteristics that may make   them more susceptible to attacks -- for example, a router-based   fragment reassembly could easily lead to (reassembly) buffer memory   exhaustion if the attacker sends a sufficient number of fragments   without sending all of them, so that the reassembly would be stalled   until a timeout; these and other fragment attacks (e.g., [15]) have   already been used against, for example, firewalls and host stacks,   and need to be taken into consideration in the implementations.   It is worth considering the cryptographic expense (which is typically   more significant than the reassembly, if done in software) with   fragmentation of the inner or outer packet.  If an outer fragment   goes missing, no cryptographic operations have been yet performed; if   an inner fragment goes missing, cryptographic operations have already   been performed.  Therefore, which of these approaches is preferable   also depends on whether cryptography or reassembly is already   provided in hardware; for high-speed routers, at least, one should be   able to assume that if it is performing relatively heavy   cryptography, hardware support is already required.   The solutions using PMTUD (and consequently ICMP) will also need to   take into account the attacks using ICMP.  In particular, an attacker   could send ICMP Packet Too Big messages indicating a very low MTU to   reduce the throughput and/or as a fragmentation/reassembly   denial-of-service attack.  This attack has been described in the   context of TCP in [16].Savola                       Informational                     [Page 10]

RFC 4459         Packet Size Issues in Network Tunnels        April 20066.  Acknowledgements   While the topic is far from new, recent discussions with W. Mark   Townsley on L2TP fragmentation issues caused the author to sit down   and write up the issues in general.  Michael Richardson and Mika   Joutsenvirta provided useful feedback on the first version.  When   soliciting comments from the NANOG list, Carsten Bormann, Kevin   Miller, Warren Kumari, Iljitsch van Beijnum, Alok Dube, and Stephen   J. Wilcox provided useful feedback.  Later, Carlos Pignataro provided   excellent input, helping to improve the document.  Joe Touch also   provided input on the memo.7.  References7.1.  Normative References   [1]   Perkins, C., "IP Encapsulation within IP",RFC 2003, October         1996.   [2]   Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for         IPv6 Hosts and Routers",RFC 4213, October 2005.   [3]   Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6         Specification",RFC 2473, December 1998.   [4]   Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,         "Generic Routing Encapsulation (GRE)",RFC 2784, March 2000.   [5]   Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling         Protocol - Version 3 (L2TPv3)",RFC 3931, March 2005.   [6]   Kent, S. and K. Seo, "Security Architecture for the Internet         Protocol",RFC 4301, December 2005.   [7]   Mogul, J. and S. Deering, "Path MTU discovery",RFC 1191,         November 1990.   [8]   McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for         IP version 6",RFC 1981, August 1996.   [9]   Braden, R., "Requirements for Internet Hosts - Communication         Layers", STD 3,RFC 1122, October 1989.   [10]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)         Specification",RFC 2460, December 1998.Savola                       Informational                     [Page 11]

RFC 4459         Packet Size Issues in Network Tunnels        April 20067.2.  Informative References   [11]  Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and         R. Wheeler, "A Method for Transmitting PPP Over Ethernet         (PPPoE)",RFC 2516, February 1999.   [12]  Mathis, M.,"Fragmentation Considered Very Harmful", Work in         Progress, July 2004.   [13]  Mathis, M. and J. Heffner,"Path MTU Discovery", Work in         Progress, March 2006.   [14]  Medina, A., Allman, M., and S. Floyd, "Measuring the Evolution         of Transport Protocols in the Internet", Computer         Communications Review, Apr 2005, <http://www.icir.org/tbit/>.   [15]  Miller, I., "Protection Against a Variant of the Tiny Fragment         Attack (RFC 1858)",RFC 3128, June 2001.   [16]  Gont, F.,"ICMP attacks against TCP", Work in Progress,         February 2006.Savola                       Informational                     [Page 12]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006Appendix A.  MTU of the Tunnel   Different tunneling mechanisms may treat the tunnel links as having   different kinds of MTU values.  Some might use the same default MTU   as for other interfaces; some others might use the default MTU minus   the expected IP overhead (e.g., 20, 28, or 40 bytes); some others   might even treat the tunnel as having an "infinite MTU", e.g., 64   kilobytes.   As [2] describes, having an infinite MTU, i.e., always fragmenting   the outer packet (and never the inner packet) and never performing   PMTUD for the tunnel path, is a very bad idea, especially in   host-to-router scenarios.  (It could be argued that if the nodes are   sure that this is a host-to-host tunnel, a larger MTU might make   sense if fragmentation and reassembly are more efficient than just   sending properly sized packets -- but this seems like a stretch.)Author's Address   Pekka Savola   CSC/FUNET   Espoo   Finland   EMail: psavola@funet.fiSavola                       Informational                     [Page 13]

RFC 4459         Packet Size Issues in Network Tunnels        April 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, 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 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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Savola                       Informational                     [Page 14]