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Internet Engineering Task Force (IETF)                         R. BonicaRequest for Comments: 7588                              Juniper NetworksCategory: Informational                                     C. PignataroISSN: 2070-1721                                            Cisco Systems                                                                J. Touch                                                                 USC/ISI                                                               July 2015A Widely Deployed Solution to the Generic Routing Encapsulation (GRE)Fragmentation ProblemAbstract   This memo describes how many vendors have solved the Generic Routing   Encapsulation (GRE) fragmentation problem.  The solution described   herein is configurable.  It is widely deployed on the Internet in its   default configuration.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7588.Bonica, et al.                Informational                     [Page 1]

RFC 7588                    GRE Fragmentation                  July 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .31.2.  Requirements Language . . . . . . . . . . . . . . . . . .52.  Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .52.1.RFC 4459 Solutions  . . . . . . . . . . . . . . . . . . .52.2.  A Widely Deployed Solution  . . . . . . . . . . . . . . .53.  Implementation Details  . . . . . . . . . . . . . . . . . . .63.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .63.2.  GRE MTU (GMTU) Estimation and Discovery . . . . . . . . .63.3.  GRE Ingress Node Procedures . . . . . . . . . . . . . . .73.3.1.  Procedures Affecting the GRE Payload  . . . . . . . .73.3.2.  Procedures Affecting the GRE Deliver Header . . . . .83.4.  GRE Egress Node Procedures  . . . . . . . . . . . . . . .94.  Security Considerations . . . . . . . . . . . . . . . . . . .95.  References  . . . . . . . . . . . . . . . . . . . . . . . . .105.1.  Normative References  . . . . . . . . . . . . . . . . . .105.2.  Informative References  . . . . . . . . . . . . . . . . .11   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .12   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .12Bonica, et al.                Informational                     [Page 2]

RFC 7588                    GRE Fragmentation                  July 20151.  Introduction   Generic Routing Encapsulation (GRE) [RFC2784] [RFC2890] can be used   to carry any network-layer protocol over any network-layer protocol.   GRE has been implemented by many vendors and is widely deployed in   the Internet.   The GRE specification does not describe fragmentation procedures.   Lacking guidance from the specification, vendors have developed   implementation-specific fragmentation solutions.  A GRE tunnel will   operate correctly only if its ingress and egress nodes support   compatible fragmentation solutions.  [RFC4459] describes several   fragmentation solutions and evaluates their relative merits.   This memo reviews the fragmentation solutions presented in [RFC4459].   It also describes how many vendors have solved the GRE fragmentation   problem.  The solution described herein is configurable and has been   widely deployed in its default configuration.   This memo addresses point-to-point unicast GRE tunnels that carry   IPv4, IPv6, or MPLS payloads over IPv4 or IPv6.  All other tunnel   types are beyond the scope of this document.1.1.  Terminology   The following terms are specific to GRE:   o  GRE delivery header - an IPv4 or IPv6 header whose source address      represents the GRE ingress node and whose destination address      represents the GRE egress node.  The GRE delivery header      encapsulates a GRE header.   o  GRE header - the GRE protocol header.  The GRE header is      encapsulated in the GRE delivery header and encapsulates the GRE      payload.   o  GRE payload - a network-layer packet that is encapsulated by the      GRE header.  The GRE payload can be IPv4, IPv6, or MPLS.      Procedures for encapsulating IPv4 in GRE are described in      [RFC2784] and [RFC2890].  Procedures for encapsulating IPv6 in GRE      are described in [IPv6-GRE].  Procedures for encapsulating MPLS in      GRE are described in [RFC4023].  While other protocols may be      delivered over GRE, they are beyond the scope of this document.   o  GRE delivery packet - a packet containing a GRE delivery header, a      GRE header, and the GRE payload.Bonica, et al.                Informational                     [Page 3]

RFC 7588                    GRE Fragmentation                  July 2015   o  GRE payload header - the IPv4, IPv6, or MPLS header of the GRE      payload.   o  GRE overhead - the combined size of the GRE delivery header and      the GRE header, measured in octets.   The following terms are specific to MTU discovery:   o  Link MTU (LMTU) - the maximum transmission unit, i.e., maximum      packet size in octets, that can be conveyed over a link.  LMTU is      a unidirectional metric.  A bidirectional link may be      characterized by one LMTU in the forward direction and another      LMTU in the reverse direction.   o  Path MTU (PMTU) - the minimum LMTU of all the links in a path      between a source node and a destination node.  If the source and      destination nodes are connected through an Equal-Cost Multipath      (ECMP), the PMTU is equal to the minimum LMTU of all links      contributing to the multipath.   o  GRE MTU (GMTU) - the maximum transmission unit, i.e., maximum      packet size in octets, that can be conveyed over a GRE tunnel      without fragmentation of any kind.  The GMTU is equal to the PMTU      associated with the path between the GRE ingress and the GRE      egress nodes minus the GRE overhead.   o  Path MTU Discovery (PMTUD) - a procedure for dynamically      discovering the PMTU between two nodes on the Internet.  PMTUD      procedures for IPv4 are defined in [RFC1191].  PMTUD procedures      for IPv6 are defined in [RFC1981].   The following terms are introduced by this memo:   o  Fragmentable Packet - a packet that can be fragmented by the GRE      ingress node before being transported over a GRE tunnel.  That is,      an IPv4 packet with the Don't Fragment (DF) bit equal to 0 and      whose payload is larger than 64 bytes.  IPv6 packets are not      fragmentable.   o  ICMP Packet Too Big (PTB) message - an ICMPv4 [RFC792] Destination      Unreachable message (Type = 3) with code equal to 4 (fragmentation      needed and DF set) or an ICMPv6 [RFC4443] Packet Too Big message      (Type = 2).Bonica, et al.                Informational                     [Page 4]

RFC 7588                    GRE Fragmentation                  July 20151.2.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [RFC2119].2.  Solutions2.1.RFC 4459 SolutionsSection 3 of [RFC4459] identifies several tunnel fragmentation   solutions.  These solutions define procedures to be invoked when the   tunnel ingress router receives a packet so large that it cannot be   forwarded through the tunnel without fragmentation of any kind.  When   applied to GRE, these procedures are:   1.  Discard the incoming packet and send an ICMP PTB message to the       incoming packet's source.   2.  Fragment the incoming packet and encapsulate each fragment within       a complete GRE header and GRE delivery header.   3.  Encapsulate the incoming packet in a single GRE header and GRE       delivery header.  Perform source fragmentation on the resulting       GRE delivery packet.   As perRFC 4459, Strategy 2 is applicable only when the incoming   packet is fragmentable.  Also as perRFC 4459, each strategy has its   relative merits and costs.2.2.  A Widely Deployed Solution   Many vendors have implemented a configurable GRE fragmentation   solution.  In its default configuration, the solution behaves as   follows:   o  When the GRE ingress node receives a fragmentable packet with      length greater than the GMTU, it fragments the incoming packet and      encapsulates each fragment within a complete GRE header and GRE      delivery header.  Fragmentation logic is as specified by the      payload protocol.   o  When the GRE ingress node receives a non-fragmentable packet with      length greater than the GMTU, it discards the packet and sends an      ICMP PTB message to the packet's source.Bonica, et al.                Informational                     [Page 5]

RFC 7588                    GRE Fragmentation                  July 2015   o  When the GRE egress node receives a GRE delivery packet fragment,      it silently discards the fragment without attempting to reassemble      the GRE delivery packet to which the fragment belongs.   In non-default configurations, the GRE ingress node can execute any   of the procedures defined inRFC 4459.   The solution described above is widely deployed on the Internet in   its default configuration.  However, the default configuration is not   always appropriate for GRE tunnels that carry IPv6.   IPv6 requires that every link in the Internet have an MTU of 1280   octets or greater.  On any link that cannot convey a 1280-octet   packet in one piece, link-specific fragmentation and reassembly must   be provided at a layer below IPv6.   Therefore, the default configuration is appropriate for tunnels that   carry IPv6 only if the network is engineered so that the GMTU is   guaranteed to be 1280 bytes or greater.  In all other scenarios, a   non-default configuration is required.   In the non-default configuration, when the GRE ingress router   receives a packet lager than the GMTU, the GRE ingress router   encapsulates the entire packet in a single GRE and delivery header.   It then fragments the delivery header and sends the resulting   fragments to the GRE egress node, where they are reassembled.3.  Implementation Details   This section describes how many vendors have implemented the solution   described inSection 2.2.3.1.  General   The GRE ingress nodes satisfy all of the requirements stated in   [RFC2784].3.2.  GRE MTU (GMTU) Estimation and Discovery   GRE ingress nodes support a configuration option that associates a   GMTU with a GRE tunnel.  By default, GMTU is equal to the MTU   associated with the next hop toward the GRE egress node minus the GRE   overhead.   Typically, GRE ingress nodes further refine their GMTU estimate by   executing PMTUD procedures.  However, if an implementation supports   PMTUD for GRE tunnels, it also includes a configuration option thatBonica, et al.                Informational                     [Page 6]

RFC 7588                    GRE Fragmentation                  July 2015   disables PMTUD.  This configuration option is required to mitigate   certain denial-of-service attacks (seeSection 4).   The GRE ingress node's estimate of the GMTU will not always be   accurate.  It is only an estimate.  When the GMTU changes, the GRE   ingress node will not discover that change immediately.  Likewise, if   the GRE ingress node performs PMTUD procedures and interior nodes   cannot deliver ICMP feedback to the GRE ingress node, GMTU estimates   may be inaccurate.3.3.  GRE Ingress Node Procedures   This section defines procedures that GRE ingress nodes execute when   they receive a packet whose size is greater than the relevant GMTU.3.3.1.  Procedures Affecting the GRE Payload3.3.1.1.  IPv4 Payloads   By default, if the payload is fragmentable, the GRE ingress node   fragments the incoming packet and encapsulates each fragment within a   complete GRE header and GRE delivery header.  Therefore, the GRE   egress node receives several complete, non-fragmented delivery   packets.  Each delivery packet contains a fragment of the GRE   payload.  The GRE egress node forwards the payload fragments to their   ultimate destination where they are reassembled.   Also by default, if the payload is not fragmentable, the GRE ingress   node discards the packet and sends an ICMPv4 Destination Unreachable   message to the packet's source.  The ICMPv4 Destination Unreachable   message code equals 4 (fragmentation needed and DF set).  The ICMPv4   Destination Unreachable message also contains a next-hop MTU (as   specified by [RFC1191]), and the next-hop MTU is equal to the GMTU   associated with the tunnel.   The GRE ingress node supports a non-default configuration option that   invokes an alternative behavior.  If that option is configured, the   GRE ingress node fragments the delivery packet.  SeeSection 3.3.2   for details.3.3.1.2.  IPv6 Payloads   By default, the GRE ingress node discards the packet and sends an   ICMPv6 [RFC4443] Packet Too Big message to the payload source.  The   MTU specified in the Packet Too Big message is equal to the GMTU   associated with the tunnel.Bonica, et al.                Informational                     [Page 7]

RFC 7588                    GRE Fragmentation                  July 2015   The GRE ingress node supports a non-default configuration option that   invokes an alternative behavior.  If that option is configured, the   GRE ingress node fragments the delivery packet.  SeeSection 3.3.2   for details.3.3.1.3.  MPLS Payloads   By default, the GRE ingress node discards the packet.  As it is   impossible to reliably identify the payload source, the GRE ingress   node does not attempt to send an ICMP PTB message to the payload   source.   The GRE ingress node supports a non-default configuration option that   invokes an alternative behavior.  If that option is configured, the   GRE ingress node fragments the delivery packet.  SeeSection 3.3.2   for details.3.3.2.  Procedures Affecting the GRE Deliver Header3.3.2.1.  Tunneling GRE over IPv4   By default, the GRE ingress node does not fragment delivery packets.   However, the GRE ingress node includes a configuration option that   allows delivery packet fragmentation.   By default, the GRE ingress node sets the DF bit in the delivery   header to 1 (Don't Fragment).  However, the GRE ingress node also   supports a configuration option that invokes the following behavior:   o  When the GRE payload is IPv6, the DF bit on the delivery header is      set to 0 (Fragments Allowed).   o  When the GRE payload is IPv4, the DF bit is copied from the      payload header to the delivery header.   When the DF bit on an IPv4 delivery header is set to 0, the GRE   delivery packet can be fragmented by any router between the GRE   ingress and egress nodes.   If the GRE egress node is configured to support reassembly, it will   reassemble fragmented delivery packets.  Otherwise, the GRE egress   node will discard delivery packet fragments.Bonica, et al.                Informational                     [Page 8]

RFC 7588                    GRE Fragmentation                  July 20153.3.2.2.  Tunneling GRE over IPv6   By default, the GRE ingress node does not fragment delivery packets.   However, the GRE ingress node includes a configuration option that   allows this.   If the GRE egress node is configured to support reassembly, it will   reassemble fragmented delivery packets.  Otherwise, the GRE egress   node will discard delivery packet fragments.3.4.  GRE Egress Node Procedures   By default, the GRE egress node silently discards GRE delivery packet   fragments without attempting to reassemble the GRE delivery packets   to which the fragments belongs.   However, the GRE egress node supports a configuration option that   allows it to reassemble GRE delivery packets.4.  Security Considerations   In the GRE fragmentation solution described above, either the GRE   payload or the GRE delivery packet can be fragmented.  If the GRE   payload is fragmented, it is typically reassembled at its ultimate   destination.  If the GRE delivery packet is fragmented, it is   typically reassembled at the GRE egress node.   The packet reassembly process is resource intensive and vulnerable to   several denial-of-service attacks.  In the simplest attack, the   attacker sends fragmented packets more quickly than the victim can   reassemble them.  In a variation on that attack, the first fragment   of each packet is missing so that no packet can ever be reassembled.   Given that the packet reassembly process is resource intensive and   vulnerable to denial-of-service attacks, operators should decide   where the reassembly process is best performed.  Having made that   decision, they should decide whether to fragment the GRE payload or   GRE delivery packet accordingly.   Some IP implementations are vulnerable to the Overlapping Fragment   Attack [RFC1858].  This vulnerability is not specific to GRE and   needs to be considered in all environments where IP fragmentation is   present.  [RFC3128] describes a procedure by which IPv4   implementations can partially mitigate the vulnerability.  [RFC5722]   mandates a procedure by which IPv6-compliant implementations are   required to mitigate the vulnerability.  The procedure described inBonica, et al.                Informational                     [Page 9]

RFC 7588                    GRE Fragmentation                  July 2015RFC 5722 completely mitigates the vulnerability.  Operators SHOULD   ensure that the vulnerability is mitigated to their satisfaction on   equipment that they deploy.   PMTUD is vulnerable to two denial-of-service attacks (seeSection 8   of [RFC1191] for details).  Both attacks are based upon on a   malicious party sending forged ICMPv4 Destination Unreachable or   ICMPv6 Packet Too Big messages to a host.  In the first attack, the   forged message indicates an inordinately small PMTU.  In the second   attack, the forged message indicates an inordinately large MTU.  In   both cases, throughput is adversely affected.  In order to mitigate   such attacks, GRE implementations include a configuration option to   disable PMTUD on GRE tunnels.  Also, they can include a configuration   option that conditions the behavior of PMTUD to establish a minimum   PMTU.5.  References5.1.  Normative References   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,RFC 792, DOI 10.17487/RFC0792, September 1981,              <http://www.rfc-editor.org/info/rfc792>.   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery",RFC 1191,              DOI 10.17487/RFC1191, November 1990,              <http://www.rfc-editor.org/info/rfc1191>.   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security              Considerations for IP Fragment Filtering",RFC 1858,              DOI 10.17487/RFC1858, October 1995,              <http://www.rfc-editor.org/info/rfc1858>.   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery              for IP version 6",RFC 1981, DOI 10.17487/RFC1981, August              1996, <http://www.rfc-editor.org/info/rfc1981>.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.              Traina, "Generic Routing Encapsulation (GRE)",RFC 2784,              DOI 10.17487/RFC2784, March 2000,              <http://www.rfc-editor.org/info/rfc2784>.Bonica, et al.                Informational                    [Page 10]

RFC 7588                    GRE Fragmentation                  July 2015   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",RFC 2890, DOI 10.17487/RFC2890, September 2000,              <http://www.rfc-editor.org/info/rfc2890>.   [RFC3128]  Miller, I., "Protection Against a Variant of the Tiny              Fragment Attack (RFC 1858)",RFC 3128,              DOI 10.17487/RFC3128, June 2001,              <http://www.rfc-editor.org/info/rfc3128>.   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,              "Encapsulating MPLS in IP or Generic Routing Encapsulation              (GRE)",RFC 4023, DOI 10.17487/RFC4023, March 2005,              <http://www.rfc-editor.org/info/rfc4023>.   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet              Control Message Protocol (ICMPv6) for the Internet              Protocol Version 6 (IPv6) Specification",RFC 4443,              DOI 10.17487/RFC4443, March 2006,              <http://www.rfc-editor.org/info/rfc4443>.   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",RFC 5722, DOI 10.17487/RFC5722, December 2009,              <http://www.rfc-editor.org/info/rfc5722>.5.2.  Informative References   [IPv6-GRE] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support              for Generic Routing Encapsulation (GRE)", Work in              Progress,draft-ietf-intarea-gre-ipv6-10, June 2015.   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-              Network Tunneling",RFC 4459, DOI 10.17487/RFC4459, April              2006, <http://www.rfc-editor.org/info/rfc4459>.Bonica, et al.                Informational                    [Page 11]

RFC 7588                    GRE Fragmentation                  July 2015Acknowledgements   The authors would like to thank Fred Baker, Fred Detienne, Jagadish   Grandhi, Jeff Haas, Brian Haberman, Vanitha Neelamegam, Masataka   Ohta, John Scudder, Mike Sullenberger, Tom Taylor, and Wen Zhang for   their constructive comments.  The authors also express their   gratitude to Vanessa Ameen, without whom this memo could not have   been written.Authors' Addresses   Ron Bonica   Juniper Networks   2251 Corporate Park Drive   Herndon, Virginia  20170   United States   Email: rbonica@juniper.net   Carlos Pignataro   Cisco Systems   7200-12 Kit Creek Road   Research Triangle Park, North Carolina  27709   United States   Email: cpignata@cisco.com   Joe Touch   USC/ISI   4676 Admiralty Way   Marina del Rey, California  90292-6695   United States   Phone: +1 (310) 448-9151   Email: touch@isi.edu   URI:http://www.isi.edu/touchBonica, et al.                Informational                    [Page 12]