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RFC 8086 - GRE-in-UDP Encapsulation

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Internet Engineering Task Force (IETF)                      L. Yong, Ed.
Request for Comments: 8086                           Huawei Technologies
Category: Standards Track                                      E. Crabbe
ISSN: 2070-1721                                                   Oracle
                                                                   X. Xu
                                                     Huawei Technologies
                                                              T. Herbert
                                                              March 2017

                        GRE-in-UDP Encapsulation


   This document specifies a method of encapsulating network protocol
   packets within GRE and UDP headers.  This GRE-in-UDP encapsulation
   allows the UDP source port field to be used as an entropy field.
   This may be used for load-balancing of GRE traffic in transit
   networks using existing Equal-Cost Multipath (ECMP) mechanisms.
   There are two applicability scenarios for GRE-in-UDP with different
   requirements: (1) general Internet and (2) a traffic-managed
   controlled environment.  The controlled environment has less
   restrictive requirements than the general Internet.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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 Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................5
      1.2. Requirements Language ......................................5
   2. Applicability Statement .........................................6
      2.1. GRE-in-UDP Tunnel Requirements .............................6
           2.1.1. Requirements for Default GRE-in-UDP Tunnel ..........7
           2.1.2. Requirements for TMCE GRE-in-UDP Tunnel .............8
   3. GRE-in-UDP Encapsulation ........................................9
      3.1. IP Header .................................................11
      3.2. UDP Header ................................................11
           3.2.1. Source Port ........................................11
           3.2.2. Destination Port ...................................11
           3.2.3. Checksum ...........................................12
           3.2.4. Length .............................................12
      3.3. GRE Header ................................................12
   4. Encapsulation Procedures .......................................13
      4.1. MTU and Fragmentation .....................................13
      4.2. Differentiated Services and ECN Marking ...................14
   5. Use of DTLS ....................................................14
   6. UDP Checksum Handling ..........................................15
      6.1. UDP Checksum with IPv4 ....................................15
      6.2. UDP Checksum with IPv6 ....................................15
   7. Middlebox Considerations .......................................18
      7.1. Middlebox Considerations for Zero Checksums ...............19
   8. Congestion Considerations ......................................19
   9. Backward Compatibility .........................................20
   10. IANA Considerations ...........................................21
   11. Security Considerations .......................................21
   12. References ....................................................22
      12.1. Normative References .....................................22
      12.2. Informative References ...................................23
   Acknowledgements ..................................................25
   Contributors ......................................................25
   Authors' Addresses ................................................27

1.  Introduction

   This document specifies a generic GRE-in-UDP encapsulation for
   tunneling network protocol packets across an IP network based on
   Generic Routing Encapsulation (GRE) [RFC2784] [RFC7676] and User
   Datagram Protocol (UDP) [RFC768] headers.  The GRE header indicates
   the payload protocol type via an EtherType [RFC7042] in the protocol
   type field, and the source port field in the UDP header may be used
   to provide additional entropy.

   A GRE-in-UDP tunnel offers the possibility of better performance for
   load-balancing GRE traffic in transit networks using existing Equal-
   Cost Multipath (ECMP) mechanisms that use a hash of the five-tuple of
   source IP address, destination IP address, UDP/TCP source port,
   UDP/TCP destination port, and protocol number.  While such hashing
   distributes UDP and TCP [RFC793] traffic between a common pair of IP
   addresses across paths, it uses a single path for corresponding GRE
   traffic because only the two IP addresses and the Protocol or Next
   Header field participate in the ECMP hash.  Encapsulating GRE in UDP
   enables use of the UDP source port to provide entropy to ECMP

   In addition, GRE-in-UDP enables extending use of GRE across networks
   that otherwise disallow it; for example, GRE-in-UDP may be used to
   bridge two islands where GRE is not supported natively across the

   GRE-in-UDP encapsulation may be used to encapsulate already tunneled
   traffic, i.e., tunnel-in-tunnel traffic.  In this case, GRE-in-UDP
   tunnels treat the endpoints of the outer tunnel as the end hosts; the
   presence of an inner tunnel does not change the outer tunnel's
   handling of network traffic.

   A GRE-in-UDP tunnel is capable of carrying arbitrary traffic and
   behaves as a UDP application on an IP network.  However, a GRE-in-UDP
   tunnel carrying certain types of traffic does not satisfy the
   requirements for UDP applications on the Internet [RFC8085].
   GRE-in-UDP tunnels that do not satisfy these requirements MUST NOT be
   deployed to carry such traffic over the Internet.  For this reason,
   this document specifies two deployment scenarios for GRE-in-UDP
   tunnels with GRE-in-UDP tunnel requirements for each of them: (1)
   general Internet and (2) a traffic-managed controlled environment
   (TMCE).  Compared to the general Internet scenario, the TMCE scenario
   has less restrictive technical requirements for the protocol but more
   restrictive management and operation requirements for the network.

   To provide security for traffic carried by a GRE-in-UDP tunnel, this
   document also specifies Datagram Transport Layer Security (DTLS) for
   GRE-in-UDP tunnels, which SHOULD be used when security is a concern.

   GRE-in-UDP encapsulation usage requires no changes to the transit IP
   network.  ECMP hash functions in most existing IP routers may utilize
   and benefit from the additional entropy enabled by GRE-in-UDP tunnels
   without any change or upgrade to their ECMP implementation.  The
   encapsulation mechanism is applicable to a variety of IP networks
   including Data Center Networks and Wide Area Networks, as well as
   both IPv4 and IPv6 networks.

1.1.  Terminology

   The terms defined in [RFC768] and [RFC2784] are used in this
   document.  Below are additional terms used in this document.

   Decapsulator: a component performing packet decapsulation at tunnel

   ECMP: Equal-Cost Multipath.

   Encapsulator: a component performing packet encapsulation at tunnel

   Flow Entropy: The information to be derived from traffic or
   applications and to be used by network devices in the ECMP process

   Default GRE-in-UDP Tunnel: A GRE-in-UDP tunnel that can apply to the
   general Internet.

   TMCE: A traffic-managed controlled environment, i.e., an IP network
   that is traffic-engineered and/or otherwise managed (e.g., via use of
   traffic rate limiters) to avoid congestion, as defined in Section 2.

   TMCE GRE-in-UDP Tunnel: A GRE-in-UDP tunnel that can only apply to a
   traffic-managed controlled environment that is defined in Section 2.

   Tunnel Egress: A tunnel endpoint that performs packet decapsulation.

   Tunnel Ingress: A tunnel endpoint that performs packet encapsulation.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Applicability Statement

   GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks; in both
   cases, encapsulated packets are treated as UDP datagrams.  Therefore,
   a GRE-in-UDP tunnel needs to meet the UDP usage requirements
   specified in [RFC8085].  These requirements depend on both the
   delivery network and the nature of the encapsulated traffic.  For
   example, the GRE-in-UDP tunnel protocol does not provide any
   congestion control functionality beyond that of the encapsulated
   traffic.  Therefore, a GRE-in-UDP tunnel MUST be used only with
   congestion-controlled traffic (e.g., IP unicast traffic) and/or
   within a network that is traffic managed to avoid congestion.

   [RFC8085] describes two applicability scenarios for UDP applications:
   (1) general internet and (2) a controlled environment.  The
   controlled environment means a single administrative domain or
   bilaterally agreed connection between domains.  A network forming a
   controlled environment can be managed/operated to meet certain
   conditions, while the general Internet cannot be; thus, the
   requirements for a tunnel protocol operating under a controlled
   environment can be less restrictive than the requirements in the
   general Internet.

   For the purpose of this document, a traffic-managed controlled
   environment (TMCE) is defined as an IP network that is traffic-
   engineered and/or otherwise managed (e.g., via use of traffic rate
   limiters) to avoid congestion.

   This document specifies GRE-in-UDP tunnel usage in the general
   Internet and GRE-in-UDP tunnel usage in a traffic-managed controlled
   environment and uses "default GRE-in-UDP tunnel" and "TMCE GRE-in-UDP
   tunnel" terms to refer to each usage.

   NOTE: Although this document specifies two different sets of GRE-in-
   UDP tunnel requirements based on tunnel usage, the tunnel
   implementation itself has no ability to detect how and where it is
   deployed.  Therefore, it is the responsibility of the user or
   operator who deploys a GRE-in-UDP tunnel to ensure that it meets the
   appropriate requirements.

2.1.  GRE-in-UDP Tunnel Requirements

   This section states the requirements for a GRE-in-UDP tunnel.
   Section 2.1.1 describes the requirements for a default GRE-in-UDP
   tunnel that is suitable for the general Internet; Section 2.1.2
   describes a set of relaxed requirements for a TMCE GRE-in-UDP tunnel
   used in a traffic-managed controlled environment.  Both Sections
   2.1.1 and 2.1.2 are applicable to an IPv4 or IPv6 delivery network.

2.1.1.  Requirements for Default GRE-in-UDP Tunnel

   The following is a summary of the default GRE-in-UDP tunnel

   1.  A UDP checksum SHOULD be used when encapsulating in IPv4.

   2.  A UDP checksum MUST be used when encapsulating in IPv6.

   3.  GRE-in-UDP tunnel MUST NOT be deployed or configured to carry
       traffic that is not congestion controlled.  As stated in
       [RFC8085], IP-based unicast traffic is generally assumed to be
       congestion controlled, i.e., it is assumed that the transport
       protocols generating IP-based traffic at the sender already
       employ mechanisms that are sufficient to address congestion on
       the path.  A default GRE-in-UDP tunnel is not appropriate for
       traffic that is not known to be congestion controlled (e.g., most
       IP multicast traffic).

   4.  UDP source port values that are used as a source of flow entropy
       SHOULD be chosen from the ephemeral port range (49152-65535)

   5.  The use of the UDP source port MUST be configurable so that a
       single value can be set for all traffic within the tunnel (this
       disables use of the UDP source port to provide flow entropy).
       When a single value is set, a random port taken from the
       ephemeral port range SHOULD be selected in order to minimize the
       vulnerability to off-path attacks [RFC6056].

   6.  For IPv6 delivery networks, the flow entropy SHOULD also be
       placed in the flow label field for ECMP per [RFC6438].

   7.  At the tunnel ingress, any fragmentation of the incoming packet
       (e.g., because the tunnel has a Maximum Transmission Unit (MTU)
       that is smaller than the packet) SHOULD be performed before
       encapsulation.  In addition, the tunnel ingress MUST apply the
       UDP checksum to all encapsulated fragments so that the tunnel
       egress can validate reassembly of the fragments; it MUST set the
       same Differentiated Services Code Point (DSCP) value as in the
       Differentiated Services (DS) field of the payload packet in all
       fragments [RFC2474].  To avoid unwanted forwarding over multiple
       paths, the same source UDP port value SHOULD be set in all packet

2.1.2.  Requirements for TMCE GRE-in-UDP Tunnel

   The section contains the TMCE GRE-in-UDP tunnel requirements.  It
   lists the changed requirements, compared with a Default GRE-in-UDP
   tunnel, for a TMCE GRE-in-UDP tunnel, which corresponds to
   requirements 1-3 listed in Section 2.1.1.

   1.  A UDP checksum SHOULD be used when encapsulating in IPv4.  A
       tunnel endpoint sending GRE-in-UDP MAY disable the UDP checksum,
       since GRE has been designed to work without a UDP checksum
       [RFC2784].  However, a checksum also offers protection from
       misdelivery to another port.

   2.  Use of the UDP checksum MUST be the default when encapsulating in
       IPv6.  This default MAY be overridden via configuration of UDP
       zero-checksum mode.  All usage of UDP zero-checksum mode with
       IPv6 is subject to the additional requirements specified in
       Section 6.2.

   3.  A GRE-in-UDP tunnel MAY encapsulate traffic that is not
       congestion controlled.

   Requirements 4-7 listed in Section 2.1.1 also apply to a TMCE GRE-in-
   UDP tunnel.

3.  GRE-in-UDP Encapsulation

   The GRE-in-UDP encapsulation format contains a UDP header [RFC768]
   and a GRE header [RFC2890].  The format is shown as follows
   (presented in bit order):

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

   IPv4 Header:
   |Version|  IHL  |Type of Service|          Total Length         |
   |         Identification        |Flags|      Fragment Offset    |
   |  Time to Live | Prot.=17(UDP) |          Header Checksum      |
   |                       Source IPv4 Address                     |
   |                     Destination IPv4 Address                  |

   UDP Header:
   |  Source Port = Entropy Value  |  Dest. Port = 4754/4755       |
   |           UDP Length          |        UDP Checksum           |

   GRE Header:
   |C| |K|S| Reserved0       | Ver |         Protocol Type         |
   |      Checksum (optional)      |       Reserved1 (Optional)    |
   |                         Key (optional)                        |
   |                 Sequence Number (optional)                    |

                 Figure 1: UDP + GRE Headers in IPv4

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

   IPv6 Header:
   |Version| Traffic Class |           Flow Label                  |
   |         Payload Length        | NxtHdr=17(UDP)|   Hop Limit   |
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Outer Source IPv6 Address                 +
   |                                                               |
   +                                                               +
   |                                                               |
   |                                                               |
   +                                                               +
   |                                                               |
   +                  Outer Destination IPv6 Address               +
   |                                                               |
   +                                                               +
   |                                                               |

   UDP Header:
   |  Source Port = entropy value  |  Dest. Port = 4754/4755       |
   |           UDP Length          |        UDP Checksum           |

   GRE Header:
   |C| |K|S| Reserved0       | Ver |         Protocol Type         |
   |      Checksum (optional)      |       Reserved1 (Optional)    |
   |                         Key (optional)                        |
   |                 Sequence Number (optional)                    |

                 Figure 2: UDP + GRE Headers in IPv6

   The contents of the IP, UDP, and GRE headers that are relevant in
   this encapsulation are described below.

3.1.  IP Header

   An encapsulator MUST encode its own IP address as the source IP
   address and the decapsulator's IP address as the destination IP
   address.  A sufficiently large value is needed in the IPv4 TTL field
   or IPv6 Hop Count field to allow delivery of the encapsulated packet
   to the peer of the encapsulation.

3.2.  UDP Header

3.2.1.  Source Port

   GRE-in-UDP permits the UDP source port value to be used to encode an
   entropy value.  The UDP source port contains a 16-bit entropy value
   that is generated by the encapsulator to identify a flow for the
   encapsulated packet.  The port value SHOULD be within the ephemeral
   port range, i.e., 49152 to 65535, where the high-order two bits of
   the port are set to one.  This provides fourteen bits of entropy for
   the inner flow identifier.  In the case that an encapsulator is
   unable to derive flow entropy from the payload header or the entropy
   usage has to be disabled to meet operational requirements (see
   Section 7), to avoid reordering with a packet flow, the encapsulator
   SHOULD use the same UDP source port value for all packets assigned to
   a flow, e.g., the result of an algorithm that performs a hash of the
   tunnel ingress and egress IP address.

   The source port value for a flow set by an encapsulator MAY change
   over the lifetime of the encapsulated flow.  For instance, an
   encapsulator may change the assignment for Denial-of-Service (DoS)
   mitigation or as a means to effect routing through the ECMP network.
   An encapsulator SHOULD NOT change the source port selected for a flow
   more than once every thirty seconds.

   An IPv6 GRE-in-UDP tunnel endpoint SHOULD copy a flow entropy value
   in the IPv6 flow label field (requirement 6).  This permits network
   equipment to inspect this value and utilize it during forwarding,
   e.g., to perform ECMP [RFC6438].

   This document places requirements on the generation of the flow
   entropy value [RFC8085] but does not specify the algorithm that an
   implementation should use to derive this value.

3.2.2.  Destination Port

   The destination port of the UDP header is set to either GRE-in-UDP
   (4754) or GRE-UDP-DTLS (4755); see Section 5.

3.2.3.  Checksum

   The UDP checksum is set and processed per [RFC768] and [RFC1122] for
   IPv4 and per [RFC2460] for IPv6.  Requirements for checksum handling
   and use of zero UDP checksums are detailed in Section 6.

3.2.4.  Length

   The usage of this field is in accordance with the current UDP
   specification in [RFC768].  This length will include the UDP header
   (eight bytes), GRE header, and the GRE payload (encapsulated packet).

3.3.  GRE Header

   An encapsulator sets the protocol type (EtherType) of the packet
   being encapsulated in the GRE Protocol Type field.

   An encapsulator MAY set the GRE Key Present, Sequence Number Present,
   and Checksum Present bits and associated fields in the GRE header as
   defined by [RFC2784] and [RFC2890].  Usage of the reserved bits,
   i.e., Reserved0, is specified in [RFC2784].

   The GRE checksum MAY be enabled to protect the GRE header and
   payload.  When the UDP checksum is enabled, it protects the GRE
   payload, resulting in the GRE checksum being mostly redundant.
   Enabling both checksums may result in unnecessary processing.  Since
   the UDP checksum covers the pseudo-header and the packet payload,
   including the GRE header and its payload, the UDP checksum SHOULD be
   used in preference to the GRE checksum.

   An implementation MAY use the GRE Key field to authenticate the
   encapsulator.  (See the Security Considerations section.)  In this
   model, a shared value is either configured or negotiated between an
   encapsulator and decapsulator.  When a decapsulator determines that a
   presented key is not valid for the source, the packet MUST be

   Although the GRE-in-UDP encapsulation protocol uses both the UDP
   header and GRE header, it is one tunnel encapsulation protocol.  The
   GRE and UDP headers MUST be applied and removed as a pair at the
   encapsulation and decapsulation points.  This specification does not
   support UDP encapsulation of a GRE header where that GRE header is
   applied or removed at a network node other than the UDP tunnel
   ingress or egress.

4.  Encapsulation Procedures

   The procedures specified in this section apply to both a default GRE-
   in-UDP tunnel and a TMCE GRE-in-UDP tunnel.

   The GRE-in-UDP encapsulation allows encapsulated packets to be
   forwarded through "GRE-in-UDP tunnels".  The encapsulator MUST set
   the UDP and GRE headers according to Section 3.

   Intermediate routers, upon receiving these UDP encapsulated packets,
   could load-balance these packets based on the hash of the five-tuple
   of UDP packets.

   Upon receiving these UDP encapsulated packets, the decapsulator
   decapsulates them by removing the UDP and GRE headers and then
   processes them accordingly.

   GRE-in-UDP can encapsulate traffic with unicast, IPv4 broadcast, or
   multicast (see requirement 3 in Section 2.1.1).  However, a default
   GRE-in-UDP tunnel MUST NOT be deployed or configured to carry traffic
   that is not congestion-controlled (see requirement 3 in Section
   2.1.1).  Entropy may be generated from the header of encapsulated
   packets at an encapsulator.  The mapping mechanism between the
   encapsulated multicast traffic and the multicast capability in the IP
   network is transparent and independent of the encapsulation and is
   otherwise outside the scope of this document.

   To provide entropy for ECMP, GRE-in-UDP does not rely on GRE keep-
   alive.  It is RECOMMENDED not to use GRE keep-alive in the GRE-in-UDP
   tunnel.  This aligns with middlebox traversal guidelines in
   Section 3.5 of [RFC8085].

4.1.  MTU and Fragmentation

   Regarding packet fragmentation, an encapsulator/decapsulator SHOULD
   perform fragmentation before the encapsulation.  The size of
   fragments SHOULD be less than or equal to the Path MTU (PMTU)
   associated with the path between the GRE ingress and the GRE egress
   tunnel endpoints minus the GRE and UDP overhead, assuming the egress
   MTU for reassembled packets is larger than the PMTU.  When applying
   payload fragmentation, the UDP checksum MUST be used so that the
   receiving endpoint can validate reassembly of the fragments; the same
   source UDP port SHOULD be used for all packet fragments to ensure the
   transit routers will forward the fragments on the same path.

   If the operator of the transit network supporting the tunnel is able
   to control the payload MTU size, the MTU SHOULD be configured to
   avoid fragmentation, i.e., sufficient for the largest supported size
   of packet, including all additional bytes introduced by the tunnel
   overhead [RFC8085].

4.2.  Differentiated Services and ECN Marking

   To ensure that tunneled traffic receives the same treatment over the
   IP network as traffic that is not tunneled, prior to the
   encapsulation process, an encapsulator processes the tunneled IP
   packet headers to retrieve appropriate parameters for the
   encapsulating IP packet header such as Diffserv [RFC2983].
   Encapsulation endpoints that support Explicit Congestion Notification
   (ECN) must use the method described in [RFC6040] for ECN marking
   propagation.  The congestion control process is outside of the scope
   of this document.

   Additional information on IP header processing is provided in
   Section 3.1.

5.  Use of DTLS

   Datagram Transport Layer Security (DTLS) [RFC6347] can be used for
   application security and can preserve network- and transport-layer
   protocol information.  Specifically, if DTLS is used to secure the
   GRE-in-UDP tunnel, the destination port of the UDP header MUST be set
   to the IANA-assigned value (4755) indicating GRE-in-UDP with DTLS,
   and that UDP port MUST NOT be used for other traffic.  The UDP source
   port field can still be used to add entropy, e.g., for load-sharing
   purposes.  DTLS applies to a default GRE-in-UDP tunnel and a TMCE
   GRE-in-UDP tunnel.

   Use of DTLS is limited to a single DTLS session for any specific
   tunnel encapsulator/decapsulator pair (identified by source and
   destination IP addresses).  Both IP addresses MUST be unicast
   addresses -- multicast traffic is not supported when DTLS is used.  A
   GRE-in-UDP tunnel decapsulator that supports DTLS is expected to be
   able to establish DTLS sessions with multiple tunnel encapsulators,
   and likewise a GRE-in-UDP tunnel encapsulator is expected to be able
   to establish DTLS sessions with multiple decapsulators.  Different
   source and/or destination IP addresses will be involved; see
   Section 6.2 for discussion of one situation where use of different
   source IP addresses is important.

   When DTLS is used for a GRE-in-UDP tunnel, if a packet is received
   from the tunnel and that packet is not protected by the DTLS session
   or part of DTLS negotiation (e.g., a DTLS handshake message
   [RFC6347]), the tunnel receiver MUST discard that packet and SHOULD
   log that discard event and information about the discarded packet.

   DTLS SHOULD be used for a GRE-in-UDP tunnel to meet security
   requirements of the original traffic that is delivered by a GRE-in-
   UDP tunnel.  There are cases where no additional security is
   required, e.g., the traffic to be encapsulated is already encrypted
   or the tunnel is deployed within an operationally secured network.
   Use of DTLS for a GRE-in-UDP tunnel requires both tunnel endpoints to
   configure use of DTLS.

6.  UDP Checksum Handling

6.1.  UDP Checksum with IPv4

   For UDP in IPv4, when a non-zero UDP checksum is used, the UDP
   checksum MUST be processed as specified in [RFC768] and [RFC1122] for
   both transmit and receive.  The IPv4 header includes a checksum that
   protects against misdelivery of the packet due to corruption of IP
   addresses.  The UDP checksum potentially provides protection against
   corruption of the UDP header, GRE header, and GRE payload.  Disabling
   the use of checksums is a deployment consideration that should take
   into account the risk and effects of packet corruption.

   When a decapsulator receives a packet, the UDP checksum field MUST be
   processed.  If the UDP checksum is non-zero, the decapsulator MUST
   verify the checksum before accepting the packet.  By default, a
   decapsulator SHOULD accept UDP packets with a zero checksum.  A node
   MAY be configured to disallow zero checksums per [RFC1122]; this may
   be done selectively, for instance, disallowing zero checksums from
   certain hosts that are known to be sending over paths subject to
   packet corruption.  If verification of a non-zero checksum fails, a
   decapsulator lacks the capability to verify a non-zero checksum, or a
   packet with a zero checksum was received and the decapsulator is
   configured to disallow, the packet MUST be dropped and an event MAY
   be logged.

6.2.  UDP Checksum with IPv6

   For UDP in IPv6, the UDP checksum MUST be processed as specified in
   [RFC768] and [RFC2460] for both transmit and receive.

   When UDP is used over IPv6, the UDP checksum is relied upon to
   protect both the IPv6 and UDP headers from corruption.  As such, a
   default GRE-in-UDP tunnel MUST perform UDP checksum; a TMCE GRE-in-

   UDP tunnel MAY be configured with UDP zero-checksum mode if the
   traffic-managed controlled environment or a set of closely
   cooperating traffic-managed controlled environments (such as by
   network operators who have agreed to work together in order to
   jointly provide specific services) meet at least one of the following

   a.  It is known (perhaps through knowledge of equipment types and
       lower-layer checks) that packet corruption is exceptionally
       unlikely and where the operator is willing to take the risk of
       undetected packet corruption.

   b.  It is judged through observational measurements (perhaps of
       historic or current traffic flows that use a non-zero checksum)
       that the level of packet corruption is tolerably low and where
       the operator is willing to take the risk of undetected packet

   c.  Carrying applications that are tolerant of misdelivered or
       corrupted packets (perhaps through higher-layer checksum,
       validation, and retransmission or transmission redundancy) where
       the operator is willing to rely on the applications using the
       tunnel to survive any corrupt packets.

   The following requirements apply to a TMCE GRE-in-UDP tunnel that
   uses UDP zero-checksum mode:

   a.  Use of the UDP checksum with IPv6 MUST be the default
       configuration of all GRE-in-UDP tunnels.

   b.  The GRE-in-UDP tunnel implementation MUST comply with all
       requirements specified in Section 4 of [RFC6936] and with
       requirement 1 specified in Section 5 of [RFC6936].

   c.  The tunnel decapsulator SHOULD only allow the use of UDP zero-
       checksum mode for IPv6 on a single received UDP Destination Port,
       regardless of the encapsulator.  The motivation for this
       requirement is possible corruption of the UDP Destination Port,
       which may cause packet delivery to the wrong UDP port.  If that
       other UDP port requires the UDP checksum, the misdelivered packet
       will be discarded.

   d.  It is RECOMMENDED that the UDP zero-checksum mode for IPv6 is
       only enabled for certain selected source addresses.  The tunnel
       decapsulator MUST check that the source and destination IPv6
       addresses are valid for the GRE-in-UDP tunnel on which the packet
       was received if that tunnel uses UDP zero-checksum mode and
       discard any packet for which this check fails.

   e.  The tunnel encapsulator SHOULD use different IPv6 addresses for
       each GRE-in-UDP tunnel that uses UDP zero-checksum mode,
       regardless of the decapsulator, in order to strengthen the
       decapsulator's check of the IPv6 source address (i.e., the same
       IPv6 source address SHOULD NOT be used with more than one IPv6
       destination address, independent of whether that destination
       address is a unicast or multicast address).  When this is not
       possible, it is RECOMMENDED to use each source IPv6 address for
       as few GRE-in-UDP tunnels that use UDP zero-checksum mode as is

   f.  When any middlebox exists on the path of a GRE-in-UDP tunnel, it
       is RECOMMENDED to use the default mode, i.e., use UDP checksum,
       to reduce the chance that the encapsulated packets will be

   g.  Any middlebox that allows the UDP zero-checksum mode for IPv6
       MUST comply with requirements 1 and 8-10 in Section 5 of

   h.  Measures SHOULD be taken to prevent IPv6 traffic with zero UDP
       checksums from "escaping" to the general Internet; see Section 8
       for examples of such measures.

   i.  IPv6 traffic with zero UDP checksums MUST be actively monitored
       for errors by the network operator.  For example, the operator
       may monitor Ethernet-layer packet error rates.

   j.  If a packet with a non-zero checksum is received, the checksum
       MUST be verified before accepting the packet.  This is regardless
       of whether the tunnel encapsulator and decapsulator have been
       configured with UDP zero-checksum mode.

   The above requirements do not change either the requirements
   specified in [RFC2460] as modified by [RFC6935] or the requirements
   specified in [RFC6936].

   The requirement to check the source IPv6 address in addition to the
   destination IPv6 address and the strong recommendation against reuse
   of source IPv6 addresses among GRE-in-UDP tunnels collectively
   provide some mitigation for the absence of UDP checksum coverage of
   the IPv6 header.  A traffic-managed controlled environment that
   satisfies at least one of three conditions listed at the beginning of
   this section provides additional assurance.

   A GRE-in-UDP tunnel is suitable for transmission over lower layers in
   the traffic-managed controlled environments that are allowed by the
   exceptions stated above, and the rate of corruption of the inner IP

   packet on such networks is not expected to increase by comparison to
   GRE traffic that is not encapsulated in UDP.  For these reasons, GRE-
   in-UDP does not provide an additional integrity check except when GRE
   checksum is used when UDP zero-checksum mode is used with IPv6, and
   this design is in accordance with requirements 2, 3, and 5 specified
   in Section 5 of [RFC6936].

   Generic Router Encapsulation (GRE) does not accumulate incorrect
   transport-layer state as a consequence of GRE header corruption.  A
   corrupt GRE packet may result in either packet discard or packet
   forwarding without accumulation of GRE state.  Active monitoring of
   GRE-in-UDP traffic for errors is REQUIRED, as the occurrence of
   errors will result in some accumulation of error information outside
   the protocol for operational and management purposes.  This design is
   in accordance with requirement 4 specified in Section 5 of [RFC6936].

   The remaining requirements specified in Section 5 of [RFC6936] are
   not applicable to GRE-in-UDP.  Requirements 6 and 7 do not apply
   because GRE does not include a control feedback mechanism.
   Requirements 8-10 are middlebox requirements that do not apply to
   GRE-in-UDP tunnel endpoints.  (See Section 7.1 for further middlebox

   It is worth mentioning that the use of a zero UDP checksum should
   present the equivalent risk of undetected packet corruption when
   sending a similar packet using GRE-in-IPv6 without UDP [RFC7676] and
   without GRE checksums.

   In summary, a TMCE GRE-in-UDP tunnel is allowed to use UDP zero-
   checksum mode for IPv6 when the conditions and requirements stated
   above are met.  Otherwise, the UDP checksum needs to be used for IPv6
   as specified in [RFC768] and [RFC2460].  Use of GRE checksum is
   RECOMMENDED when the UDP checksum is not used.

7.  Middlebox Considerations

   Many middleboxes read or update UDP port information of the packets
   that they forward.  Network Address Port Translator (NAPT) is the
   most commonly deployed Network Address Translation (NAT) device
   [RFC4787].  A NAPT device establishes a NAT session to translate the
   {private IP address, private source port number} tuple to a {public
   IP address, public source port number} tuple, and vice versa, for the
   duration of the UDP session.  This provides a UDP application with
   the "NAT pass-through" function.  NAPT allows multiple internal hosts
   to share a single public IP address.  The port number, i.e., the UDP
   Source Port number, is used as the demultiplexer of the multiple

   internal hosts.  However, the above NAPT behaviors conflict with the
   behavior of a GRE-in-UDP tunnel that is configured to use the UDP
   source port value to provide entropy.

   A GRE-in-UDP tunnel is unidirectional; the tunnel traffic is not
   expected to be returned back to the UDP source port values used to
   generate entropy.  However, some middleboxes (e.g., firewalls) assume
   that bidirectional traffic uses a common pair of UDP ports.  This
   assumption also conflicts with the use of the UDP source port field
   as entropy.

   Hence, use of the UDP source port for entropy may impact middleboxes'
   behavior.  If a GRE-in-UDP tunnel is expected to be used on a path
   with a middlebox, the tunnel can be configured either to disable use
   of the UDP source port for entropy or to enable middleboxes to pass
   packets with UDP source port entropy.

7.1.  Middlebox Considerations for Zero Checksums

   IPv6 datagrams with a zero UDP checksum will not be passed by any
   middlebox that updates the UDP checksum field or simply validates the
   checksum based on [RFC2460], such as firewalls.  Changing this
   behavior would require such middleboxes to be updated to correctly
   handle datagrams with zero UDP checksums.  The GRE-in-UDP
   encapsulation does not provide a mechanism to safely fall back to
   using a checksum when a path change occurs that redirects a tunnel
   over a path that includes a middlebox that discards IPv6 datagrams
   with a zero UDP checksum.  In this case, the GRE-in-UDP tunnel will
   be black-holed by that middlebox.

   As such, when any middlebox exists on the path of a GRE-in-UDP
   tunnel, use of the UDP checksum is RECOMMENDED to increase the
   probability of successful transmission of GRE-in-UDP packets.
   Recommended changes to allow firewalls and other middleboxes to
   support use of an IPv6 zero UDP checksum are described in Section 5
   of [RFC6936].

8.  Congestion Considerations

   Section 3.1.9 of [RFC8085] discusses the congestion considerations
   for design and use of UDP tunnels; this is important because other
   flows could share the path with one or more UDP tunnels,
   necessitating congestion control [RFC2914] to avoid destructive

   Congestion has potential impacts both on the rest of the network
   containing a UDP tunnel and on the traffic flows using the UDP
   tunnels.  These impacts depend upon what sort of traffic is carried

   over the tunnel, as well as the path of the tunnel.  The GRE-in-UDP
   tunnel protocol does not provide any congestion control and GRE-in-
   UDP packets are regular UDP packets.  Therefore, a GRE-in-UDP tunnel
   MUST NOT be deployed to carry non-congestion-controlled traffic over
   the Internet [RFC8085].

   Within a TMCE network, GRE-in-UDP tunnels are appropriate for
   carrying traffic that is not known to be congestion controlled.  For
   example, a GRE-in-UDP tunnel may be used to carry Multiprotocol Label
   Switching (MPLS) traffic such as pseudowires or VPNs where specific
   bandwidth guarantees are provided to each pseudowire or VPN.  In such
   cases, operators of TMCE networks avoid congestion by careful
   provisioning of their networks, rate-limiting of user data traffic,
   and traffic engineering according to path capacity.

   When a GRE-in-UDP tunnel carries traffic that is not known to be
   congestion controlled in a TMCE network, the tunnel MUST be deployed
   entirely within that network, and measures SHOULD be taken to prevent
   the GRE-in-UDP traffic from "escaping" the network to the general
   Internet.  Examples of such measures are:

   o  physical or logical isolation of the links carrying GRE-in-UDP
      from the general Internet,

   o  deployment of packet filters that block the UDP ports assigned for
      GRE-in-UDP, and

   o  imposition of restrictions on GRE-in-UDP traffic by software tools
      used to set up GRE-in-UDP tunnels between specific end systems (as
      might be used within a single data center) or by tunnel ingress
      nodes for tunnels that don't terminate at end systems.

9.  Backward Compatibility

   In general, tunnel ingress routers have to be upgraded in order to
   support the encapsulations described in this document.

   No change is required at transit routers to support forwarding of the
   encapsulation described in this document.

   If a tunnel endpoint (a host or router) that is intended for use as a
   decapsulator does not support or enable the GRE-in-UDP encapsulation
   described in this document, that endpoint will not listen on the
   destination port assigned to the GRE-encapsulation (4754 and 4755).
   In these cases, the endpoint will perform normal UDP processing and
   respond to an encapsulator with an ICMP message indicating "port

   unreachable" according to [RFC792].  Upon receiving this ICMP
   message, the node MUST NOT continue to use GRE-in-UDP encapsulation
   toward this peer without management intervention.

10.  IANA Considerations

   IANA has allocated the following UDP destination port number for the
   indication of GRE:

         Service Name: GRE-in-UDP
         Transport Protocol(s): UDP
         Assignee: IESG <iesg@ietf.org>
         Contact: IETF Chair <chair@ietf.org>
         Description: GRE-in-UDP Encapsulation
         Reference: RFC 8086
         Port Number: 4754
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

   IANA has allocated the following UDP destination port number for the
   indication of GRE with DTLS:

         Service Name: GRE-UDP-DTLS
         Transport Protocol(s): UDP
         Assignee: IESG <iesg@ietf.org>
         Contact: IETF Chair <chair@ietf.org>
         Description: GRE-in-UDP Encapsulation with DTLS
         Reference: RFC 8086
         Port Number: 4755
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

11.  Security Considerations

   GRE-in-UDP encapsulation does not affect security for the payload
   protocol.  The security considerations for GRE apply to GRE-in-UDP;
   see [RFC2784].

   To secure traffic carried by a GRE-in-UDP tunnel, DTLS SHOULD be used
   as specified in Section 5.

   In the case that UDP source port for entropy usage is disabled, a
   random port taken from the ephemeral port range SHOULD be selected in
   order to minimize the vulnerability to off-path attacks [RFC6056].
   The random port may also be periodically changed to mitigate certain
   DoS attacks as mentioned in Section 3.2.1.

   Using one standardized value as the UDP destination port to indicate
   an encapsulation may increase the vulnerability to off-path attacks.
   To overcome this, an alternate port may be agreed upon to use between
   an encapsulator and decapsulator [RFC6056].  How the encapsulator
   endpoints communicate the value is outside the scope of this

   This document does not require that a decapsulator validate the IP
   source address of the tunneled packets (with the exception that the
   IPv6 source address MUST be validated when UDP zero-checksum mode is
   used with IPv6), but it should be understood that failure to do so
   presupposes that there is effective destination-based filtering (or a
   combination of source-based and destination-based filtering) at the

   Corruption of GRE headers can cause security concerns for
   applications that rely on the GRE Key field for traffic separation or
   segregation.  When the GRE Key field is used for this purpose, such
   as an application of a Network Virtualization Using Generic Routing
   Encapsulation (NVGRE) [RFC7637], GRE header corruption is a concern.
   In such situations, at least one of the UDP and GRE checksums MUST be
   used for both IPv4 and IPv6 GRE-in-UDP tunnels.

12.  References

12.1.  Normative References

   [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <http://www.rfc-editor.org/info/rfc6040>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <http://www.rfc-editor.org/info/rfc8085>.

12.2.  Informative References

   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,

   [RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <http://www.rfc-editor.org/info/rfc4787>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,

   [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042,
              October 2013, <http://www.rfc-editor.org/info/rfc7042>.

   [RFC7637]  Garg, P., Ed., and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,

   [RFC7676]  Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
              for Generic Routing Encapsulation (GRE)", RFC 7676,
              DOI 10.17487/RFC7676, October 2015,


   The authors would like to thank Vivek Kumar, Ron Bonica, Joe Touch,
   Ruediger Geib, Lars Eggert, Lloyd Wood, Bob Briscoe, Rick Casarez,
   Jouni Korhonen, Kathleen Moriarty, Ben Campbell, and many others for
   their reviews and valuable input on this document.

   Thanks to Donald Eastlake, Eliot Lear, Martin Stiemerling, and
   Spencer Dawkins for their detailed reviews and valuable suggestions
   during WG Last Call and the IESG process.

   Thanks to the design team led by David Black (members: Ross Callon,
   Gorry Fairhurst, Xiaohu Xu, and Lucy Yong) for efficiently working
   out the descriptions for the congestion considerations and IPv6 UDP
   zero checksum.

   Thanks to David Black and Gorry Fairhurst for their great help in
   document content and editing.


   The following people all contributed significantly to this document
   and are listed in alphabetical order:

   David Black
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748
   United States of America

   Email: david.black@emc.com

   Ross Callon
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States of America

   Email: rcallon@juniper.net

   John E. Drake
   Juniper Networks

   Email: jdrake@juniper.net

   Gorry Fairhurst
   University of Aberdeen

   Email: gorry@erg.abdn.ac.uk

   Yongbing Fan
   China Telecom

   Email: fanyb@gsta.com
   Phone: +86 20 38639121

   Adrian Farrel
   Juniper Networks

   Email: adrian@olddog.co.uk

   Vishwas Manral

   Email: vishwas@ionosnetworks.com

   Carlos Pignataro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   United States of America

   Email: cpignata@cisco.com

Authors' Addresses

   Lucy Yong
   Huawei Technologies, USA

   Email: lucy.yong@huawei.com

   Edward Crabbe

   Email: edward.crabbe@gmail.com

   Xiaohu Xu
   Huawei Technologies
   Beijing, China

   Email: xuxiaohu@huawei.com

   Tom Herbert
   1 Hacker Way
   Menlo Park, CA

   Email: tom@herbertland.com


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