Network Working Group                                        E. Crabbe
Intended status: Standard Track                                 L. Yong
                                                             Huawei USA
                                                                  X. Xu
                                                    Huawei Technologies
                                                             T. Herbert

Expires: September 2016                             March 1, 2016

                         GRE-in-UDP Encapsulation


   This document describes a method of encapsulating network protocol
   packets within GRE and UDP headers. In this encapsulation method,
   the source UDP port can be used as an entropy field for purposes of
   load balancing, while the protocol of the encapsulated packet in the
   GRE payload is identified by the GRE Protocol Type. This document
   specifies requirements for two applicability scenarios for the
   encapsulation: (1) General Internet; (2) Controlled Environment,
   e.g. well-managed operator networks. The controlled environment has
   less restrictive requirements than the general Internet.

Status of This Document

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF). Note that other groups may also distribute
   working documents as Internet-Drafts. The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 1,2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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...................................................3
      1.1. Applicability Statement...................................4
      1.2. GRE-in-UDP Tunnel Usage Requirements......................4
         1.2.1. Requirements for Default GRE-in-UDP Tunnel...........5
         1.2.2. Requirements Changes for WMON GRE-in-UDP Tunnel......5
   2. Terminology....................................................6
      2.1. Requirements Language.....................................6
   3. Encapsulation in UDP...........................................6
      3.1. IP Header.................................................9
      3.2. UDP Header................................................9
         3.2.1. Source Port..........................................9
         3.2.2. Destination Port.....................................9
         3.2.3. Checksum............................................10
         3.2.4. Length..............................................10
      3.3. GRE Header...............................................10
   4. Encapsulation Process Procedures..............................11
      4.1. MTU and Fragmentation....................................11
      4.2. Middlebox Considerations.................................12
      4.3. Differentiated Services and ECN Marking..................12
   5. UDP Checksum Handling.........................................13
      5.1. UDP Checksum with IPv4...................................13
      5.2. UDP Checksum with IPv6...................................13
         5.2.1. Middlebox Considerations............................16
   6. Congestion Considerations.....................................17
   7. Backward Compatibility........................................18
   8. IANA Considerations...........................................18
   9. Security Considerations.......................................19
   10. Acknowledgements.............................................20
   11. Contributors.................................................21
   12. References...................................................22
      12.1. Normative References....................................22
      12.2. Informative References..................................23
   13. Authors' Addresses...........................................24

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1. Introduction

   Load balancing, or more specifically statistical multiplexing of
   traffic using Equal Cost Multi-Path (ECMP) and/or Link Aggregation
   Groups (LAGs) in IP networks, is a widely used technique for
   creating higher capacity networks out of lower capacity links. Most
   existing routers in IP networks are already capable of distributing
   IP traffic flows over ECMP paths and/or LAGs on the basis of a hash
   function performed on flow invariant fields in IP packet headers and
   their payload protocol headers. Specifically, when the IP payload is
   a User Datagram Protocol (UDP)[RFC768] or Transmission Control
   Protocol (TCP) [RFC793] packet, router hash functions frequently
   operate on the five-tuple of source IP address, destination IP
   address, source port, destination port, and protocol/next-header

   GRE encapsulation has been widely used for many applications. For
   example, to redirect IP traffic to traverse a different path instead
   of the default path in an operator network, to tunnel private
   network traffic over a public network by use of public IP network
   addresses, to tunnel IPv6 traffic over an IPv4 network, tunnel
   Ethernet traffic over IP networks [RFC7637], etc. Unfortunately,
   using GRE encapsulated within IP may reduce the entropy available
   for use in load balancing compared to TCP/IP or UDP/IP, especially
   in cases where the GRE Key field [RFC2890] is not used for entropy
   purpose, i.e., the Key field is used for security authentication.

   This document defines a generic GRE-in-UDP encapsulation for
   tunneling network protocol packets across an IP network. The GRE
   header provides payload protocol type as an EtherType in the
   protocol type field [RFC2784][RFC7676], and the UDP header provides
   additional entropy by way of its source port. GRE-in-UDP offers the
   additional possibility of using GRE across networks that might
   otherwise disallow it; for instance GRE-in-UDP may be used to bridge
   two islands where GRE is not used natively across the Internet.

   This encapsulation method requires no changes to the transit IP
   network. Hash functions in most existing IP routers may utilize and
   benefit from the use of a GRE-in-UDP tunnel without needing any
   change or upgrade to their ECMP implementation. The encapsulation
   mechanism is applicable to a variety of IP networks including Data
   Center and wide area networks.

   GRE-in-UDP encapsulation may be used to encapsulate already tunneled
   traffic, i.e. tunnel-in-tunnel. In this case, GRE-in-UDP tunnel
   endpoints treat other tunnel endpoints as of the end hosts for the

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   traffic and do not differentiate such end hosts from other end

   1.1. Applicability Statement

   GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks including
   the Internet. When using GRE-in-UDP encapsulation, encapsulated
   traffic will be treated as a UDP application in an IP delivery
   network.  As such, GRE-in-UDP tunnel needs to meet UDP requirements
   specified in [RFC5405bis], which imposes limits on GRE-in-UDP tunnel
   usage. These limits may depend on both the 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, GRE-in-UDP MUST
   be used only with congestion controlled traffic (e.g., IP traffic)
   and/or within a network that has the congestion management.

   [RFC5405bis] considers two types of applicability where IETF
   applications utilize UDP: 1) General Internet and 2) Controlled
   Environment. The controlled environment means within a single
   administrative domain or bilaterally agreed connection between
   domains. A network under controlled environment can be
   managed/operated to meet certain conditions while the general
   Internet cannot be. Tunnel protocol requirements under controlled
   environment can be less restrictive than the requirements in the
   general Internet. This document specifies GRE-in-UDP tunnel usage in
   the general Internet and GRE-in-UDP tunnel usage in the well-managed
   operator network that is an example of controlled environment.

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

   This document refers to the GRE-in-UDP tunnel usage in the general
   Internet as Default GRE-in-UDP Tunnel; the GRE-in-UDP tunnel usage
   in a well-managed operator network as WMON GRE-in-UDP Tunnel.

   1.2. GRE-in-UDP Tunnel Usage Requirements

   The section summarizes GRE-in-UDP tunnel requirements. The
   requirements for Default GRE-in-UDP tunnel are listed in Section
   1.2.1, which applies to a GRE-in-UDP tunnel over the general
   Internet; the relaxed requirements for WMON GRE-in-UDP Tunnel are
   listed in Section 1.2.2, which applies to a GRE-in-UDP tunnel within
   a well-managed operator network. These networks can use IPv4 or IPv6.

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    1.2.1. Requirements for Default GRE-in-UDP Tunnel

   The following is a summary of the GRE-in-UDP requirements for use
   over the general Internet:

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

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

   3. GRE-in-UDP tunnel MUST NOT be used for traffic that has no
   congestion control. IP-traffic can be assumed to be congestion-
   controlled. GRE-in-UDP tunnels are not appropriate for other traffic
   that does not use congestion control.

   4. UDP source port that is used for flow entropy SHOULD be set to a
   UDP ephemeral port (49152-65535).

   5. UDP source port usage MUST be configurable so that a single value
   is used for all traffic in the tunnel (this disables use of the UDP
   source port to provide flow entropy).

   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 an MTU that is smaller than the packet
   SHOULD be performed before encapsulation [RFC7588]. 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, and SHOULD use the same source UDP port for all packet
   fragments to ensure the packet fragments traversing on the same path.

    1.2.2. Requirements Changes for WMON GRE-in-UDP Tunnel

   The following lists the changed requirements for WMON GRE-in-UDP
   Tunnel that is used in a well-managed operator network; they replace
   requirements 1-3 listed in section 1.2.1. The requirements 4-7 in
   that section are unchanged for WMON GRE-in-UDP Tunnel.

   1. UDP checksum MAY be used when encapsulating in IPv4.

   2. Use of 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 5.2.

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

2. Terminology

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

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

   WMON GRE-in-UDP Tunnel: A GRE-in-UDP tunnel that can only apply to a
   well-managed operator network that is defined in Section 1.1.

   2.1. Requirements Language

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

3. Encapsulation in UDP

   GRE-in-UDP encapsulation format is shown as follows:

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      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 |Protcol=17(UDP)|          Header Checksum      |
      |                       Source IPv4 Address                     |
      |                     Destination IPv4 Address                  |

      UDP Header:
      |       Source Port = XXXX      |       Dest Port = TBD         |
      |           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

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      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 = XXXX      |       Dest Port = TBD         |
      |           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

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   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. The TTL field in the IP header MUST be set to a value
   appropriate for delivery of the encapsulated packet to the peer of
   the encapsulation.

   3.2. UDP Header

    3.2.1. Source Port

   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 4.2), it SHOULD set a randomly selected constant value for
   UDP source port to avoid payload packet flow reordering, e.g., the
   port can be chosen as a hash of the tunnel ingress and egress IP

   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.

   For IPv6 delivery network, if IPv6 flow label load balancing is
   supported [RFC6438], the flow entropy SHOULD also be placed in the
   flow label field.

   How an encapsulator generates flow entropy from the payload is
   outside the scope of this document.

    3.2.2. Destination Port

   The destination port of the UDP header is set the GRE-in-UDP port or
   GRE-UDP-DTLS (TBD) (see Section 8).

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    3.2.3. Checksum

   The UDP checksum is set and processed per [RFC768] and [RFC1122] for
   IPv4, and [RFC2460] for IPv6. Requirements for checksum handling and
   use of zero UDP checksums are detailed in Section 5.

    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]. The reserved bits, i.e.,
   Reserved0, SHOULD be set zero.

   The GRE checksum MAY be enabled to protect the GRE header and
   payload. An encapsulator SHOULD NOT enable both the GRE checksum and
   UDP checksum simultaneously as this would be mostly redundant. Since
   the UDP checksum covers more of the packet including the GRE header
   and payload, the UDP checksum SHOULD have preference to using GRE
   checksum. The GRE checksum MAY be used for the payload integrity
   check when use of UDP zero-checksum.

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

   Although GRE-in-UDP encapsulation protocol uses both UDP header and
   GRE header, it is one tunnel encapsulation protocol. 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

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4. Encapsulation Process Procedures

   The procedures specified in this section apply to both Default GRE-
   in-UDP tunnel and WMON GRE-in-UDP tunnel.

   The GRE-in-UDP encapsulation allows encapsulated packets to be
   forwarded through "GRE-in-UDP tunnels".  When performing GRE-in-UDP
   encapsulation by the encapsulator, the entropy value is generated by
   the encapsulator and then be filled in the Source Port field of the
   UDP header. The Destination Port field is set to a value (TBD) to
   indicate that the UDP tunnel payload is a GRE packet. The Protocol
   Type header field in GRE header is set to the EtherType value
   corresponding to the protocol of the encapsulated packet.

   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 allows encapsulation of unicast, broadcast, or multicast
   traffic. Entropy may be generated from the header of encapsulated
   unicast or broadcast/multicast packets at an encapsulator. The
   mapping mechanism between the encapsulated multicast traffic and the
   multicast capability in the IP network is transparent and
   independent to 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 RECOMMENED not to use GRE keep-alive in the GRE-in-UDP
   tunnel. This aligns with middlebox traversal guidelines in Section
   3.5 of [RFC5405bis].

   4.1. MTU and Fragmentation

   Regarding packet fragmentation, an encapsulator/decapsulator SHOULD
   be compliant with [RFC7588] and perform fragmentation before the
   encapsulation. The size of fragments SHOULD be less or equal to the
   PMTU associated with the path between the GRE ingress and the GRE
   egress nodes minus the GRE and UDP overhead, assuming the egress
   resemble MTU is larger than PMTU. When applying payload fragment,
   the UDP checksum MUST be used so that the receiving endpoint can
   validate reassembly of the fragments; the same src UDP port SHOULD
   be used for all packet fragments to ensure the transit routers will
   forward the fragments on the same path.

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   If a tunnel operator is able to control the payload MTU size, the
   tunnel operator SHOULD factor in the additional bytes of tunnel
   overhead when considering the MTU size to avoid the likelihood of

   4.2. Middlebox Considerations

   The Source Port number of the UDP header is pertinent to the
   middlebox behavior. Network Address/Port Translator (NAPT) is the
   most commonly deployed Network Address Translation (NAT) device
   [RFC4787]. An 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 that the UDP source port number is used as entropy
   in GRE-in-UDP tunnel.

   Each UDP tunnel is unidirectional, as GRE-in-UDP traffic is sent to
   the GRE-in-UDP Destination Port (TBD), and in particular, is never
   sent back to any port used as a UDP Source Port (which serves solely
   as a source of entropy). It is common that a middlebox (e.g.,
   firewall) assume that bidirectional traffic uses a common pair of
   UDP ports. This assumption also conflicts with the use of the UDP
   source port number as entropy.

   Hence, use of the UDP src port for entropy may impact middlebox
   behavior. If a GRE-in-UDP tunnel is expected to pass a middlebox, to
   avoid the impact, the operator either disable UDP source port for
   entropy or configure the middlebox to deal with the UDP source port

   4.3. Differentiated Services and ECN Marking

   To ensure that tunneled traffic gets the same treatment over the IP
   network, prior to the encapsulation process, an encapsulator should
   process the payload to get the proper parameters to fill into the IP
   header such as DiffServ [RFC2983]. Encapsulation end points 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.

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5. UDP Checksum Handling

   5.1. UDP Checksum with IPv4

   Default GRE-in-UDP Tunnel SHOULD perform UDP checksum. WMON GRE-in-
   UDP Tunnel MAY perform UDP checksum.

   For UDP in IPv4, the UDP checksum MUST be processed as specified in
   [RFC768] and [RFC1122] for both transmit and receive. The IPv4
   header includes a checksum which protects against mis-delivery 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. Enabling or disabling the use of
   checksums is a deployment consideration that should take into
   account the risk and effects of packet corruption, and whether the
   packets in the network are protected by other, possibly stronger
   mechanisms such as the Ethernet CRC.

   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.

   5.2. 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,
   Default GRE-in-UDP Tunnel MUST perform UDP checksum; WMON GRE-in-UDP
   Tunnel MAY be configured with the UDP zero-checksum mode if the
   well-managed operator network or a set of closely cooperating well-
   managed operator networks (such as by network operators who have
   agreed to work together in order to jointly provide specific
   services) meet at least one of following conditions:

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   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 mis-delivered 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 WMON GRE-in-UDP Tunnel that use
   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 mis-delivered
        packet will be discarded.

     d. It is RECOMMENDED that UDP zero-checksum selectively be enabled
        for certain 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.

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     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 UDP zero-checksum mode GRE-in-UDP tunnels as is feasible.

     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 to
        be dropped.

     g. Any middlebox that allows UDP zero-checksum mode for IPv6 MUST
        comply with requirement 1 and 8-10 in Section 5 of [RFC6936].

     h. Measures SHOULD be taken to prevent IPv6 traffic with zero UDP
        checksums from "escaping" to the general Internet; see Section
        6 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, plus 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 well-managed operator network that satisfies at
   least one of three conditions listed above in this section provides
   additional assurance.

   GRE-in-UDP is suitable for transmission over lower layers in the
   well-managed operator networks 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

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   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].

   GRE does not accumulate incorrect state as a consequence of GRE
   header corruption. A corrupt GRE packet may result in either packet
   discard or forwarding of the packet without accumulation of GRE
   state. Active monitoring of GRE-in-UDP traffic for errors is
   REQUIRED as 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 5.2.1 for further middle
   box discussion).

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

   In summary, WMON 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 MUST be used for IPv6 as
   specified in [RFC768] and [RFC2460]. Use of GRE checksum is
   recommended when the UDP checksum is not used.

    5.2.1. Middlebox Considerations

   IPv6 datagrams with a zero UDP checksum will not be passed by any
   middlebox that validates the checksum based on [RFC2460] or that
   updates the UDP checksum field, such as NATs or 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 redirecting 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.

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   As such, when any middlebox exists on the path of GRE-in-UDP tunnel,
   it is RECOMMENDED to use the UDP checksum to reduce the chance that
   the encapsulated packets to be dropped. Recommended changes to allow
   firewalls, NATs and other middleboxes to support use of an IPv6 zero
   UDP checksum are described in Section 5 of [RFC6936].

6. Congestion Considerations

   Section 3.1.9 of [RFC5405bis] discussed the congestion implications
   of UDP tunnels. As discussed in [RFC5405bis], because other flows
   can share the path with one or more UDP tunnels, congestion control
   [RFC2914] needs to be considered.

   The impact of congestion must be considered both in terms of the
   effect on the rest of the network containing a UDP, and in terms of
   the effect on the flows using the UDP tunnels. The potential impact
   of congestion from a UDP tunnel depends upon what sort of traffic is
   carried over the tunnel, as well as the path of the tunnel.

   In many cases, GRE-in-UDP is used to carry IP traffic. IP traffic is
   generally assumed to be congestion controlled, and thus a tunnel
   carrying general IP traffic generally does not need additional
   congestion control mechanisms.

   GRE-in-UDP tunnel can be used in some cases to carry traffic that is
   not necessarily congestion controlled. For example, GRE-in-UDP may
   be used to carry MPLS that carries pseudowire or VPN traffic where
   specific bandwidth guarantees are provided to each pseudowire or to
   each VPN. In such cases, network operators may avoid congestion by
   careful provisioning of their networks, by rate limiting of user
   data traffic, and traffic engineering according to path capacity.
   For this reason, GRE-in-UDP tunnel MUST be used within a single
   operator's network that utilizes careful provisioning (e.g., rate
   limiting at the entries of the network while over-provisioning
   network capacity) to ensure against congestion, or within a limited
   number of networks whose operators closely cooperate in order to
   jointly provide this same careful provisioning.

   The default GRE-in-UDP tunnel can be used to carry IP traffic that
   is known to be congestion controlled on the Internet. Internet IP
   traffic is generally assumed to be congestion-controlled. The
   default GRE-in-UDP tunnel MUST NOT be used over the general Internet,
   or over non-cooperating network operators, to carry traffic that is
   not congestion-controlled.

   WMON GRE-in-UDP Tunnel is used within a well-managed operator
   network so that it can carry the traffic that is not necessarily

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   congestion controlled. Measures SHOULD be taken to prevent non-
   congestion-controlled GRE-in-UDP traffic from "escaping" to the
   general Internet, e.g.:

   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.

   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). For
      examples, a GRE-in-UDP tunnel only carries IP traffic or a GRE-
      in-UDP tunnel supports NVGRE encapsulation [RFC7637] only
      (Although the payload type is Ethernet in NVGRE, NVGRE protocol
      mandates that the payload of Ethernet is IP).

   o  Use of a "Circuit Breaker" for the tunneled traffic as described
      in [CB].

7. 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 router that is intended for use as a decapsulator does not
   support or enable GRE-in-UDP encapsulation described in this
   document, it should not be listening on the destination port (TBD).
   In these cases, the router will conform to 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.

8. IANA Considerations

   IANA is requested to make the following allocations:

   One UDP destination port number for the indication of GRE

         Service Name: GRE-in-UDP

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         Transport Protocol(s): UDP
         Assignee: IESG <>
         Contact: IETF Chair <>
         Description: GRE-in-UDP Encapsulation
         Reference: [This.I-D]
         Port Number: TBD
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

   One UDP destination port number for the indication of GRE with DTLS

         Service Name: GRE-UDP-DTLS
         Transport Protocol(s): UDP
         Assignee: IESG <>
         Contact: IETF Chair <>
         Description: GRE-in-UDP Encapsulation with DTLS
         Reference: [This.I-D]
         Port Number: TBD
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

9. Security Considerations

   GRE-in-UDP encapsulation does not affect security for the payload
   protocol. When using GRE-in-UDP, Network Security in a network is
   mostly equivalent to that of a network using GRE.

   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 an IANA-assigned value (TBD2) 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 usage 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 an GRE-in-UDP tunnel encapsulator is
   expected to be able to establish DTLS sessions with multiple

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   decapsulators (although different source and/or destination IP
   addresses may be involved -see Section 5.2 for discussion of one
   situation where use of different source IP addresses is important).

   In the case that UDP source port for entropy usage is disabled, a
   random port 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 denial of service
   attacks as mentioned in Section 3.2.

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

   This document does not require that a decapsulator validates 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 (or a
   combination of source-based and destination-based) filtering at the

   Corruption of GRE header can cause a privacy and security concern
   for some applications that rely on the key field for traffic
   segregation. When GRE key field is used for privacy and security,
   ether UDP checksum or GRE checksum SHOULD be used for GRE-in-UDP
   with both IPv4 and IPv6, and in particular, when UDP zero-checksum
   mode is used, GRE checksum SHOULD be used.

10. Acknowledgements

   Authors like to thank Vivek Kumar, Ron Bonica, Joe Touch, Ruediger
   Geib, Lar Edds, Lloyd Wood, Bob Briscoe, and many others for their
   review and valuable input on this draft.

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

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11. Contributors

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

   David Black
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748


   Ross Callon
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886


   John E. Drake
   Juniper Networks


   Gorry Fairhurst
   University of Aberdeen


   Yongbing Fan
   China Telecom
   Guangzhou, China.
   Phone: +86 20 38639121


   Adrian Farrel
   Juniper Networks


   Vishwas Manral

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   Hewlett-Packard Corp.
   3000 Hanover St, Palo Alto.


   Carlos Pignataro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC 27709 USA


12. References

   12.1. Normative References

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

   [RFC1122] Braden, R., "Requirements for Internet Hosts --
             Communication Layers", RFC1122, October 1989.

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

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

   [RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
             RFC2890, September 2000.

   [RFC5405bis] Eggert, L., "Unicast UDP Usage Guideline for
             Application Designers", draft-ietf-tsvwg-rfc5405bis, work
             in progress.

   [RFC6040] Briscoe, B., "Tunneling of Explicit Congestion
             Notification", RFC6040, November 2010.

   [RFC6347] Rescoria, E., Modadugu, N., "Datagram Transport Layer
             Security Version 1.2", RFC6347, 2012.

   [RFC6438] Carpenter, B., Amante, S., "Using the IPv6 Flow Label for
             Equal Cost Multipath Routing and Link Aggregation in
             tunnels", RFC6438, November, 2011.

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

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

   12.2. Informative References

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

   [RFC793] DARPA, "Transmission Control Protocol", RFC793, September

   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [RFC2914] Floyd, S.,"Congestion Control Principles", RFC2914,
             September 2000.

   [RFC2983] Black, D., "Differentiated Services and Tunnels", RFC2983,
             October 2000.

   [RFC4787] Audet, F., et al, "network Address Translation (NAT)
             Behavioral Requirements for Unicast UDP", RFC4787, January

   [RFC6056] Larsen, M. and Gont, F., "Recommendations for Transport-
             Protocol Port Randomization", RFC6056, January 2011.

   [RFC6438] Carpenter, B., Amante, S., "Using the Ipv6 Flow Label for
             Equal Cost Multipath Routing and Link Aggreation in
             Tunnels", RFC6438, November 2011.

   [RFC7588] Bonica, R., "A Fragmentation Strategy for Generic Routing
             Encapsulation (GRE)", RFC7588, July 2015.

   [RFC7637] Garg, P. and Wang, Y., "NVGRE: Network Virtualization
             Using Generic Routing Encapsulation", RFC7637, September

   [RFC7676] Pignataro, C., Bonica, R., Krishnan, S., "IPv6 Support for
             Generic Routing Encapsulation (GRE)", RFC7676, October

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   [CB]      Fairhurst, G., "Network Transport Circuit Breakers",
             draft-ietf-tsvwg-circuit-breaker-13, work in progress.

13. Authors' Addresses

   Edward Crabbe


   Lucy Yong
   Huawei Technologies, USA


   Xiaohu Xu
   Huawei Technologies,
   Beijing, China


   Tom Herbert
   1 Hacker Way
   Menlo Park, CA
   Email :

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