Network Working Group                                    Lucy Yong(Ed.)
Internet-Draft                                      Huawei Technologies
Intended status: Standard Track                               E. Crabbe
                                                                  X. Xu
                                                    Huawei Technologies
                                                             T. Herbert

Expires: February 2017                                September 5, 2016

                         GRE-in-UDP Encapsulation


   This document specifies a method of encapsulating network protocol
   packet 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 ECMP mechanisms. There are two applicability
   scenarios for GRE-in-UDP with different requirements: (1) general
   Internet; (2) a traffic-managed controlled environment. 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 February 5,2017.

Copyright Notice

   Copyright (c) 2016 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
   ( 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. Terminology...............................................4
      1.2. Requirements Language.....................................4
   2. Applicability Statement........................................5
      2.1. GRE-in-UDP Tunnel Requirements............................5
         2.1.1. Requirements for Default GRE-in-UDP Tunnel...........5
         2.1.2. Requirements for TMCE GRE-in-UDP Tunnel..............6
   3. GRE-in-UDP Encapsulation.......................................7
      3.1. IP Header................................................10
      3.2. UDP Header...............................................10
         3.2.1. Source Port.........................................10
         3.2.2. Destination Port....................................11
         3.2.3. Checksum............................................11
         3.2.4. Length..............................................11
      3.3. GRE Header...............................................11
   4. Encapsulation Process Procedures..............................12
      4.1. MTU and Fragmentation....................................12
      4.2. Differentiated Services and ECN Marking..................13
   5. Use of DTLS...................................................13
   6. UDP Checksum Handling.........................................14
      6.1. UDP Checksum with IPv4...................................14
      6.2. UDP Checksum with IPv6...................................14
   7. Middlebox Considerations......................................17
      7.1. Middlebox Considerations for Zero Checksums..............18
   8. Congestion Considerations.....................................18
   9. Backward Compatibility........................................19
   10. IANA Considerations..........................................20
   11. Security Considerations......................................21
   12. Acknowledgements.............................................22
   13. Contributors.................................................22
   14. References...................................................23
      14.1. Normative References....................................23
      14.2. Informative References..................................24
   15. Authors' Addresses...........................................25

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 Multi-Path (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.  While such hashing distributes UDP and
   Transmission Control Protocol (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
   protocol/next header fields participate in the ECMP hash.
   Encapsulating GRE in UDP enables use of the UDP source port to
   provide entropy to ECMP hashing.

   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. 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.

   In full generality with the capability to carry arbitrary traffic,
   GRE-in-UDP tunnels are not safe for general deployment in the public
   Internet.  Therefore GRE-in-UDP tunnel deployments are limited to
   two applicability scenarios for which requirements are specified in
   this document: (1) general Internet; (2) a traffic-managed
   controlled environment. The traffic-managed controlled environment
   scenario has less restrictive technical requirements for the
   protocol but more restrictive management and operation requirements
   for the network by comparison to the general Internet scenario.

   The document also specifies Datagram Transport Layer Security (DTLS)
   for GRE-in-UDP tunnels to be used where/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 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. Following are additional terms used in this draft.

   Decapsulator: a component performing packet decapsulation at tunnel

   ECMP: Equal-Cost Multi-Path.

   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 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

   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 end point that performs packet decapsulation.

   Tunnel Ingress: A tunnel end point that performs packet

   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 [RFC5405bis]. 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.

   [RFC5405bis] 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 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.

   2.1. GRE-in-UDP Tunnel Requirements

   This section states out 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 [RFC5405bis],
   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 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 fragments.

    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 the
   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 mis-delivery to
   another port.

   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 6.2.

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

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

      UDP Header:
      |  Source Port = Entropy Value  |  Dest. Port = TBD1/TBD2       |
      |           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 = TBD1/TBD2       |
      |           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 perform 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 [RFC5405bis] 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 either GRE-in-UDP
   (TBD1) or GRE-UDP-DTLS (TBD2) (see Section 5).

    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 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 using the GRE checksum.

   An implementation MAY use the GRE keyid to authenticate the
   encapsulator.(See Security Considerations 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

   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

4. Encapsulation Process 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 header 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 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
   perform fragmentation before the encapsulation. The size of
   fragments SHOULD be less 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 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 [RFC5405bis].

   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 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.

   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 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 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.

   If the traffic to be encapsulated is already encrypted, it is
   usually not necessary to encrypt it again. Applying DTLS to GRE-in-
   UDP tunnel requires both tunnel end points to configure use of DTLS.

6. UDP Checksum Handling

   6.1. UDP Checksum with IPv4

   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 that 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. 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 the 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 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 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 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 mis-delivered
        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 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
        will be dropped.

     g. Any middlebox that allows the UDP zero-checksum mode for IPv6
        MUST comply with requirement 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, 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 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 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 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 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 need to be used for IPv6
   as specified in [RFC768] and [RFC2460]. Use of GRE checksum is
   RECOMMENED 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]. 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 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., firewall) 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 to either disable use
   of the UDP source port for entropy or to configure 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 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.

   As such, when any middlebox exists on the path of 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 [RFC5405bis] 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
   distractive interference.

   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.

   A default GRE-in-UDP tunnel MAY be used to carry IP traffic that is
   known to be congestion controlled on the Internet. IP unicast
   traffic is generally assumed to be congestion-controlled. A default
   GRE-in-UDP tunnel is not appropriate for traffic that is not known
   to be congestion-controlled.

   A TMCE GRE-in-UDP tunnel can be used to carry traffic that is known
   not to be congestion controlled. For example, GRE-in-UDP may be used
   to carry Multiprotocol Label Switching (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.

   When a TMCE GRE-in-UDP tunnel carries traffic that is not known to
   be congestion controlled, the tunnel MUST be used within a traffic-
   managed controlled environment (e.g., single operator network that
   utilizes careful provisioning such as rate limiting at the entries
   of the network while over-provisioning network capacity) to manage
   congestion, or within a limited number of networks whose operators
   closely cooperate in order to jointly provide this same careful
   provisioning. When a TMCE GRE-in-UDP tunnel is used to carry the
   traffic that is not known to be congestion controlled, measures
   SHOULD be taken to prevent the 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) or by
      tunnel ingress nodes for tunnels that don't terminate at end

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

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
   (TBD1 and TBD2). 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 is requested to make the following allocations:

   One UDP destination port number for the indication of GRE,

         Service Name: GRE-in-UDP
         Transport Protocol(s): UDP
         Assignee: IESG <>
         Contact: IETF Chair <>
         Description: GRE-in-UDP Encapsulation
         Reference: [This.I-D]
         Port Number: TBD1
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

   Editor Note: replace "TBD1" in section 3 and 9 with IANA assigned

   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: TBD2
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

   Editor Note: replace "TBD2" in section 3, 5, and 9 with IANA
   assigned number.

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 original traffic, DTLS SHOULD be used as specified in
   Section 5.

   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 3.2.1.

   Using one standardized value as the UDP destination port to indicate
   an encapsulation 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 a GRE header can cause a privacy and security concern
   for some applications that rely on the key field for traffic
   segregation. When the GRE key field is used for privacy and security,
   either 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.

12. Acknowledgements

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

   Thank Donald Eastlake, Eliot Lear, Martin Stiemerling, and Spencer
   Dawkins for their detail reviews and valuable suggestions in WGLC
   and IESG process.

   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

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

13. 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
   Hewlett-Packard Corp.
   3000 Hanover St, Palo Alto.


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


14. References

   14.1. 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.

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

   [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.

   [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.

   14.2. 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.

   [RFC7042] Eastlake 3 , 3rd, D. and Abley, J., "IANA Considerations and
             IETF Protocol and Documentation Usage for IEEE 802
             Parameter", RFC7042, October 2013.

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

   [CB]      Fairhurst, G., "Network Transport Circuit Breakers",
             draft-ietf-tsvwg-circuit-breaker-15, work in progress.

15. Authors' Addresses

   Lucy Yong
   Huawei Technologies, USA


   Edward Crabbe


   Xiaohu Xu
   Huawei Technologies,
   Beijing, China

   Tom Herbert
   1 Hacker Way
   Menlo Park, CA
   Email :