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Versions: (draft-yong-tsvwg-gre-in-udp-encap) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 8086

Network Working Group                                    Lucy Yong (Ed.)
Internet-Draft                                               Huawei USA
Intended status: Standard Track                               E. Crabbe

                                                                  X. Xu
                                                    Huawei Technologies
                                                             T. Herbert
                                                               Facebook

Expires: September 2016                                  March 10, 2016


                         GRE-in-UDP Encapsulation
                   draft-ietf-tsvwg-gre-in-udp-encap-11

Abstract

   This document describes a method of encapsulating network protocol
   packets within GRE and UDP headers. The 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. This document specifies
   GRE-in-UDP tunnel requirements for two applicability scenarios: (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 http://datatracker.ietf.org/drafts/current/.

   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 10,2016.

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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................3
      1.1. Terminology...............................................3
      1.2. Requirements Language.....................................4
   2. Applicability Statement........................................4
      2.1. GRE-in-UDP Tunnel Usage Requirements......................5
         2.1.1. Requirements for Default GRE-in-UDP Tunnel...........5
         2.1.2. Requirements Changes for TMCE GRE-in-UDP Tunnel......6
   3. GRE-in-UDP Encapsulation.......................................6
      3.1. IP Header.................................................9
      3.2. UDP Header................................................9
         3.2.1. Source Port..........................................9
         3.2.2. Destination Port....................................10
         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. Differentiated Services and ECN Marking..................12
   5. Use of DTLS...................................................12
   6. UDP Checksum Handling.........................................12
      6.1. UDP Checksum with IPv4...................................12
      6.2. UDP Checksum with IPv6...................................13
   7. Middlebox Considerations......................................16
      7.1. Middlebox Considerations for Zero Checksums..............17
   8. Congestion Considerations.....................................17
   9. Backward Compatibility........................................18
   10. IANA Considerations..........................................19
   11. Security Considerations......................................20
   12. Acknowledgements.............................................20
   13. Contributors.................................................21
   14. References...................................................22
      14.1. Normative References....................................22
      14.2. Informative References..................................23
   15. Authors' Addresses...........................................24


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1. Introduction

   This document defines a generic GRE-in-UDP encapsulation for
   tunneling network protocol packets across an IP network. The
   encapsulation uses Generic Routing Encapsulation (GRE)
   [RFC2784][RFC7676] and User Datagram Protocol(UDP) [RFC768] headers.
   The GRE header provides payload protocol type as an EtherType in the
   protocol type field, and the source port field in the UDP header may
   be used to provide additional entropy that may be used for load
   balancing GRE traffic in transit networks using existing Equal-Cost
   Multi-Path (ECMP) mechanism. The existing ECMP mechanism is that,
   when the IP payload is a UDP or Transmission Control Protocol (TCP)
   [RFC793] packet, router hash functions frequently operate on the
   five-tuple of source IP address, destination IP address, UDP/TCP
   source port, UDP/TCP destination port, and protocol/next-header. A
   GRE-in-UDP tunnel 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 do
   not differentiate such end hosts from other end hosts, i.e.,
   applying the same treatment for traffic from hosts and tunnel
   endpoints.

   This document specifies GRE-in-UDP tunnel requirements for two
   applicability scenarios: (1) general Internet; (2) a traffic-managed
   controlled environment. The controlled environment has less
   restrictive requirements than the general Internet.

   1.1. Terminology

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

   A traffic-managed controlled environment: an IP network that is
   traffic-engineered and/or otherwise managed (e.g., via use of
   traffic rate limiters) to avoid congestion happening.




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   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.

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

   ECMP: Equal-Cost Multi-Path

   TMCE: Traffic-managed controlled environment (defined in Section 2)

   1.2. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


2. Applicability Statement

   GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks. When
   using GRE-in-UDP encapsulation, packets encapsulated by GRE-in-UDP
   are treated as UDP datagrams by an IP network. As such, GRE-in-UDP
   tunnel needs to meet the UDP requirements specified in [RFC5405bis],
   which imposes the requirements on GRE-in-UDP tunnel usage. 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 traffic) and/or within a network that has traffic management
   capability to avoid congestion.

   [RFC5405bis] considers two types of applicability where IETF
   applications utilize UDP: 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 happening. The document specifies GRE-in-UDP tunnel
   usage in the general Internet and specifies GRE-in-UDP tunnel usage


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   in a traffic-managed controlled environment. Furthermore, a default
   GRE-in-UDP tunnel described in this document refers to the usage
   over the general Internet; a TMCE GRE-in-UDP tunnel described in
   this document refers to the usage in a traffic-managed controlled
   environment.

   2.1. GRE-in-UDP Tunnel Usage Requirements

   This section provides a summary of the requirements for a GRE-in-UDP
   tunnel. Section 2.1.1 describes the default usage of 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 can be IPv4
   or IPv6.

    2.1.1. Requirements for Default GRE-in-UDP Tunnel

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

   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 used for traffic that does not
   implement 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 values that are used for 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 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; it MUST set the same DSCP value to all fragments. To



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   avoid unwanted forwarding over multiple paths the same source UDP
   port value SHOULD be set in all packet fragments.

    2.1.2. Requirements Changes for TMCE GRE-in-UDP Tunnel

   The section lists the changed requirements for a TMCE GRE-in-UDP
   Tunnel that applies to a traffic-managed controlled environment.
   This replaces requirements 1-3 listed in Section 2.1.1. The
   requirements 4-7 in Section 2.1.1 remain unchanged for a TMCE GRE-
   in-UDP Tunnel.

   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.

3. GRE-in-UDP Encapsulation

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




















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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 = 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)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IANA Note: Replace TBD1 and TBD2 with the IANA-assigned numbers


                    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 = 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)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IANA Note: Replace TBD1 and TBD2 with the IANA-assigned numbers

                    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. 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.

   Note: An IPv6 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 but does not specify the algorithm that an
   implementation should use to derive this value.






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    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). IANA Note: Please
   replace TBD1 and TBD2 with the IANA-assigned numbers.

    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 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



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   removed at a network node other than the UDP tunnel ingress or
   egress.

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 allows encapsulation of unicast, IPv4 broadcast, or
   multicast traffic. 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 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 tunnel endpoints minus the GRE and UDP overhead, assuming the
   egress resemble MTU 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 src 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



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   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, 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 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
   decapsulators (although different source and/or destination IP
   addresses may be involved (see Section 6.2) for discussion of one
   situation where use of different source IP addresses is important).

   IANA Note: Please replace TBD2 with the IANA-assigned numbers.

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



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   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. 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
   conditions:

   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
      corruption.





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   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
   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 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 to
        be dropped.





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     g. Any middlebox that allows the 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
        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 above in 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
   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].



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   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
   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, 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.




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   7.1. Middlebox Considerations for Zero Checksums

   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.

   As such, when any middlebox exists on the path of GRE-in-UDP tunnel,
   it is RECOMMENDED to use the UDP checksum to increase the
   probability of successful transmission of GRE-in-UDP packets.
   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].

8. 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, a GRE-in-UDP tunnel 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.

   A 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 usage MUST NOT be used over the general Internet, or over
   non-cooperating network operators, to carry traffic that is not
   congestion-controlled.

   A TMCE GRE-in-UDP tunnel can be used to carry traffic that is not
   necessarily congestion controlled. For example, GRE-in-UDP may be



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   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, when a TMCE GRE-in-UDP tunnel carries this type of
   traffic, the usage MUST be constrained to a traffic-managed
   controlled environment (e.g., single operator network that utilizes
   careful provisioning (e.g., 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 necessary 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].

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



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   encapsulation described in this document, it is not that an endpoint
   will 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.

   IANA NOTE: Please replace TBD1 and TBD2 with the IANA-assigned
   numbers.

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 <iesg@ietf.org>
         Contact: IETF Chair <chair@ietf.org>
         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

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

         Service Name: GRE-UDP-DTLS
         Transport Protocol(s): UDP
         Assignee: IESG <iesg@ietf.org>
         Contact: IETF Chair <chair@ietf.org>
         Description: GRE-in-UDP Encapsulation with DTLS
         Reference: [This.I-D]
         Port Number: TBD2
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A



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11. 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.

   To secure original traffic, DTLS SHOULD be used. (See 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 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
   boundaries.

   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 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.


12. 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



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   descriptions for the congestion considerations and IPv6 UDP zero
   checksum.

   Thank David Black and Gorry Fairhurst for their great help in
   document 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
   USA

   Email: david.black@emc.com

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

   Email: rcallon@juniper.net

   John E. Drake
   Juniper Networks

   Email: jdrake@juniper.net

   Gorry Fairhurst
   University of Aberdeen

   Email: gorry@erg.abdn.ac.uk

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

   Email: fanyb@gsta.com


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   Adrian Farrel
   Juniper Networks

   Email: adrian@olddog.co.uk

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

   Email: vishwas.manral@hp.com

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

   EMail: cpignata@cisco.com


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.

   [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.




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   [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
             1981.

   [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
             2007.

   [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.



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   [RFC7637] Garg, P. and Wang, Y., "NVGRE: Network Virtualization
             Using Generic Routing Encapsulation", RFC7637, September
             2015.

   [RFC7676] Pignataro, C., Bonica, R., Krishnan, S., "IPv6 Support for
             Generic Routing Encapsulation (GRE)", RFC7676, October
             2015.

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

15. Authors' Addresses

   Edward Crabbe

   Email: edward.crabbe@gmail.com

   Lucy Yong
   Huawei Technologies, USA

   Email: lucy.yong@huawei.com

   Xiaohu Xu
   Huawei Technologies,
   Beijing, China

   Email: xuxiaohu@huawei.com

   Tom Herbert
   Facebook
   1 Hacker Way
   Menlo Park, CA
   Email : tom@herbertland.com

















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