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

Expires: September 2015                                  March 6, 2015

                         GRE-in-UDP Encapsulation


   This document describes a method of encapsulating network protocol
   packets within GRE and UDP headers.  In this encapsulation, the
   source UDP port can be used as an entropy field for purposes of load
   balancing, while the protocol of the encapsulated packet in the GRE
   payload is identified by the GRE Protocol Type. Usage restrictions
   apply to GRE-in-UDP usage for traffic that is not congestion
   controlled and to UDP zero checksum usage with IPv6.

Status of This Document

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

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

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

   This Internet-Draft will expire on September 6, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document.  Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................3
      1.1. Applicability Statement...................................3
   2. Terminology....................................................4
      2.1. Requirements Language.....................................4
   3. Encapsulation in UDP...........................................4
      3.1. IP Header.................................................7
      3.2. UDP Header................................................7
         3.2.1. Source Port..........................................7
         3.2.2. Destination Port.....................................7
         3.2.3. Checksum.............................................7
         3.2.4. Length...............................................8
      3.3. GRE Header................................................8
   4. Encapsulation Process Procedures...............................8
      4.1. MTU and Fragmentation.....................................9
      4.2. Differentiated Services..................................10
   5. UDP Checksum Handling.........................................10
      5.1. UDP Checksum with IPv4...................................10
      5.2. UDP Checksum with IPv6...................................10
         5.2.1. Middlebox Considerations ...........................14
   6. Congestion Considerations.....................................14
   7. Backward Compatibility........................................16
   8. IANA Considerations...........................................16
   9. Security Considerations.......................................17
   10. Acknowledgements.............................................18
   11. Contributors.................................................18
   12. References...................................................20
      12.1. Normative References....................................20
      12.2. Informative References..................................20
   13. Authors' Addresses...........................................21

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1. Introduction

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

   Several encapsulation techniques are commonly used in IP networks,
   such as Generic Routing Encapsulation (GRE) [RFC2784], MPLS
   [RFC4023] and L2TPv3 [RFC3931]. GRE is an increasingly popular
   encapsulation choice. Unfortunately, use of common GRE endpoints may
   reduce the entropy available for use in load balancing, especially
   in environments where the GRE Key field [RFC2890] is not readily
   available for use as entropy in forwarding decisions.

   This document defines a generic GRE-in-UDP encapsulation for
   tunneling network protocol packets across an IP network. The GRE
   header provides payload protocol type as an EtherType in the
   protocol type field [RFC2784][GREIPV6], and the UDP header provides
   additional entropy by way of its source port.

   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.

   1.1. Applicability Statement

   GRE encapsulation is widely used for many applications. For example,
   to redirect IP traffic to traverse a different path instead of the
   default path in an operator network, to tunnel private network
   traffic over a public network by use of public IP network addresses,
   to tunnel IPv6 traffic over an IPv4 network, and etc.

   When using GRE-in-UDP encapsulation, encapsulated traffic will be
   treated as a UDP application, not as a GRE application, in an IP

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   network.  Thus GRE-in-UDP tunnel needs to meet UDP application
   guidelines specified in [RFC5405bis], which can constrain GRE-in-UDP
   tunnel usage to certain applications and/or environments.

   Here is the list of the UDP application guidelines in [RFC5405bis]
   and corresponding Sections to cover it in this document.

   o  Congestion Control: GRE-in-UDP does not have congestion control
      mechanism. The usage restrictions for traffic that is not
      congestion control is specified in Section 6.

   o  Message Size: Address in Section 4.1

   o  Reliability: not applicable to a GRE-in-UDP tunnel. GRE-in-UDP
      tunnel does not provide any reliable transport.

   o  Checksum:  Address in Section 5.

   o  Middlebox Traversal: Section 5.2.1.

   GRE-in-UDP encapsulation may be used to encapsulate already tunneled
   traffic, i.e. tunnel-in-tunnel. The tunneled traffic may use GRE-in-
   UDP or other tunnel encapsulation. In this case, GRE-in-UDP tunnel
   end points treat other tunnel endpoints as of the end hosts for the
   traffic and do not differentiate such end hosts from other end hosts.

2. Terminology

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

   2.1. Requirements Language

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

3. Encapsulation in UDP

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

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

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

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

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

                    Figure 1  UDP+GRE Headers in IPv4

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

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

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

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

                    Figure 2  UDP+GRE Headers in IPv6

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

   3.1. IP Header

   An encapsulator MUST encode its own IP address as the source IP
   address and the decapsulator's IP address as the destination IP
   address.  The TTL field in the IP header must be set to a value
   appropriate for delivery of the encapsulated packet to the peer of
   the encapsulation.

   3.2. UDP Header

   3.2.1. Source Port

   The UDP source port contains a 16-bit entropy value that is
   generated by the encapsulator to identify a flow for the
   encapsulated packet. The port value SHOULD be within the ephemeral
   port range. IANA suggests this range to be 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, it should set a randomly selected constant value for
   UDP source port to avoid payload packet flow reordering.

   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.

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

   3.2.2. Destination Port

   The destination port of the UDP header is set the GRE/UDP port (TBD)
   (see Section 8).

   3.2.3. Checksum

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

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   3.2.4. Length

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

   3.3. GRE Header

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

   An encapsulator may set the GRE Key Present, Sequence Number Present,
   and Checksum Present bits and associated fields in the GRE header as
   defined by [RFC2784] and [RFC2890].

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

   An implementation MAY use the GRE keyid to authenticate the
   encapsulator. In this model, a shared value is either configured or
   negotiated between an encapsulator and decapsulator. When a GRE-in-
   UDP packet is received with the keyid present, it is checked to see
   if it is valid for the source to have set for the tunnel packet was
   sent on. An implementation MAY enforce that a keyid be used for
   source authentication on selected tunnels. When a decapsulator
   determines a presented keyid is not valid for the source to send or
   the keyid is absent and is considered required for authenticating
   the encapsulator for a tunnel, the packet MUST be dropped.

4. Encapsulation Process Procedures

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

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

   Upon receiving these UDP encapsulated packets, the decapsulator
   would decapsulate them by removing the UDP and GRE headers and then
   process them accordingly.

   Note: Each UDP tunnel is unidirectional, as GRE-in-UDP traffic is
   sent to the IANA-allocated UDP Destination Port, and in particular,
   is never sent back to any port used as a UDP Source Port (which
   serves solely as a source of entropy). This is at odds with a common
   middlebox (e.g., firewall) assumption that bidirectional traffic
   uses a common pair of UDP ports.  As a result, arranging to pass
   bidirectional GRE-in-UDP traffic through middleboxes may require
   separate configuration for each direction of traffic.

   GRE-in-UDP allows encapsulation of unicast, broadcast, or multicast
   traffic. Entropy may be generated from the header of encapsulated
   unicast or broadcast/multicast packets at an encapsulator. The
   mapping mechanism between the encapsulated multicast traffic and the
   multicast capability in the IP network is transparent and
   independent to the encapsulation and is otherwise outside the scope
   of this document.

   To provide entropy for ECMP, GRE-in-UDP does not rely on GRE keep-
   alive. It is RECOMMENED no use of 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 fragmentation, an encapsulator SHOULD perform
   fragmentation [GREMTU] on a packet before encapsulation and factor
   in both GRE and UDP header bytes in the effective Maximum
   Transmission Unit (MTU) size. Not performing the fragmentation will
   cause the packets exceeding network MTU size to be dropped or
   fragmented in the network. An encapsulator MUST use the same source
   UDP port for all packet fragments to ensure that the transit routers
   will forward the packet fragments on the same path. An operator
   should factor in the additional bytes of overhead when considering
   an MTU size for the payload to avoid the likelihood of fragmentation.

   Fragmented packets MUST be reassembled at the decapsulator prior to
   being sent to a (payload) application. Packet fragmentation and
   reassembling process is outside the scope of the document.

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   4.2. Differentiated Services

   To ensure that tunneled traffic gets the same treatment over the IP
   network, prior to the encapsulation process, an encapsulator should
   process the payload to get the proper parameters to fill into the IP
   header such as DiffServ [RFC2983]. Encapsulation end points that
   support ECN must use the method described in [RFC6040] for ECN
   marking propagation.  This process is outside of the scope of this

5. UDP Checksum Handling

   5.1. 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. An
   encapsulator MAY set the UDP checksum to zero for performance or
   implementation considerations. The IPv4 header includes a checksum
   which protects against mis-delivery of the packet due to corruption
   of IP addresses. The UDP checksum potentially provides protection
   against corruption of the UDP header, GRE header, and GRE payload.
   Enabling or disabling the use of checksums is a deployment
   consideration that should take into account the risk and effects of
   packet corruption, and whether the packets in the network are
   already adequately protected by other, possibly stronger mechanisms
   such as the Ethernet CRC.

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

   5.2. UDP Checksum with IPv6

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

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   When UDP is used over IPv6, the UDP checksum is relied upon to
   protect both the IPv6 and UDP headers from corruption, and so MUST
   used with the following exceptions:

     a. Use of GRE-in-UDP in networks under single administrative
        control (such as within a single operator's network) where 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. Use of GRE-in-UDP in networks under single administrative
        control (such as within a single operator's network) where 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. Use of GRE-in-UDP for traffic delivery for 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.

   For these exceptions, the UDP zero-checksum mode can be used.
   However the use of the UDP zero-checksum mode must meet the
   requirements specified in [RFC6935] and [RFC6936] as well at the
   additional requirements stated below.

   These exceptions may also be extended to the use of GRE-in-UDP
   within a set of closely cooperating network administrations (such as
   network operators who have agreed to work together in order to
   jointly provide specific services).

   As such, for IPv6, the UDP checksum for GRE-in-UDP MUST be used as
   specified in [RFC768] and [RFC2460] for tunnels that span multiple
   networks whose network administrations do not cooperate closely,
   even if each non-cooperating network administration independently
   satisfies one or more of the exceptions for UDP zero-checksum mode
   usage with GRE-in-UDP over IPv6.

   The following additional requirements apply to implementation and
   use of UDP zero-checksum mode for GRE-in-UDP over IPv6:

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

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

   c. By default a decapsulator MUST disallow receipt of GRE-in-UDP
      packets with zero UDP checksums with IPv6. Zero checksums May
      selectively be enabled for certain source address. A 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.

   d. An 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.

   e. Any middlebox support for GRE-in-UDP with UDP zero-checksum mode
      for IPv6 MUST comply with requirements 1 and 8-10 in Section 5 of

   f. Measures SHOULD be taken to prevent IPv6 traffic with zero UDP
      checksums from "escaping" to the general Internet; see Section 6
      for examples of such measures.

   g. IPv6 traffic with zero UDP checksums MUST be actively monitored
      for errors by the network operator.

   h. If a packet with a non-zero checksum is received, the checksum
      MUST be verified before accepting the packet. This is regardless
      of whether a 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

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   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. Additional assurance is provided by the
   restrictions in the above exceptions that limit usage of IPv6 UDP
   zero-checksum mode to well-managed networks for which GRE
   encapsulated packet corruption has not been a problem in practice.

   Hence GRE-in-UDP is suitable for transmission over lower layers in
   the well-managed networks that are allowed by the exceptions stated
   above and the rate of corruption of the inner IP packet on such
   networks is not expected to increase by comparison to GRE traffic
   that is not encapsulated in UDP.  For these reasons, GRE-in-UDP does
   not provide an additional integrity check except when GRE checksum
   is used when UDP zero-checksum mode is used with IPv6, and this
   design is in accordance with requirements 2, 3 and 5 specified in
   Section 5 of [RFC6936].

   GRE does not accumulate incorrect state as a consequence of GRE
   header corruption. A corrupt GRE results in either packet discard or
   forwarding of the packet without accumulation of GRE state. GRE
   checksum MAY be used for protecting GRE header and payload. 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
   inapplicable to GRE-in-UDP.  Requirements 6 and 7 do not apply
   because GRE does not have a GRE-generic control feedback mechanism.
   Requirements 8-10 are middlebox requirements that do not apply to
   GRE-in-UDP tunnel endpoints, but see Section 5.2.1 for further
   middlebox discussion.

   It is worth to mention 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 and without GRE

   In summary, UDP zero-checksum mode for IPv6 is allowed to be used
   with GRE-in-UDP when one of the three exceptions specified above
   applies, provided that additional requirements stated above are
   complied with. Otherwise the UDP checksum MUST be used for IPv6 as
   specified in [RFC768] and [RFC2460].

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   5.2.1. Middlebox Considerations

   IPv6 datagrams with a zero UDP checksum will not be passed by any
   middlebox that validates the checksum based on [RFC2460] or that
   updates the UDP checksum field, such as NATs or firewalls. Changing
   this behavior would require such middleboxes to be updated to
   correctly handle datagrams with zero UDP checksums.  The GRE-in-UDP
   encapsulation does not provide a mechanism to safely fall back to
   using a checksum when a path change occurs redirecting a tunnel over
   a path that includes a middlebox that discards IPv6 datagrams with a
   zero UDP checksum.  In this case the GRE-in-UDP tunnel will be
   black-holed by that middlebox.  Recommended changes to allow
   firewalls, NATs and other middleboxes to support use of an IPv6 zero
   UDP checksum are described in Section 5 of [RFC6936].

6. Congestion Considerations

   Section 3.1.7 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.

   A major motivation for GRE-in-UDP encapsulation is to tunnel a
   network protocol over IP network and improve the use of multipath
   (such as ECMP) in cases where traffic is to traverse routers which
   are able to hash on UDP Port and IP address. As such, in many cases
   this may reduce the occurrence of congestion and improve usage of
   available network capacity. However, it is also necessary to ensure
   that the network, including applications that use the network,
   responds appropriately in more difficult cases, such as when link or
   equipment failures have reduced the available capacity.

   The impact of congestion must be considered both in terms of the
   effect on the rest of the network over which packets are sent in UDP
   tunnels, and in terms of the effect on the flows that are sent by
   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.

   GRE encapsulation is widely used to carry a wide range of network
   protocols and traffic. In many cases GRE encapsulation is used to
   carry IP traffic. IP traffic is generally assumed to be congestion
   controlled, and thus a tunnel carrying general IP traffic (as might
   be expected to be carried across the Internet) generally does not
   need additional congestion control mechanisms. As specified in RFC

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    "IP-based 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. Consequently, a tunnel carrying
   IP-based traffic should already interact appropriately with other
   traffic sharing the path, and specific congestion control mechanisms
   for the tunnel are not necessary."

   For this reason, where GRE-in-UDP tunneling is used to carry IP
   traffic that is known to be congestion controlled, the UDP tunnels
   MAY be used within a single network or across multiple networks,
   with cooperating network operators.  Internet IP traffic is
   generally assumed to be congestion-controlled.

   However, GRE-in-UDP tunneling can be also used to carry traffic that
   is not necessarily congestion controlled. In such cases network
   operators may avoid congestion by careful provisioning of their
   networks, by rate limiting of user data traffic, and/or by using
   Traffic Engineering tools to monitor the network segments and
   dynamically steers traffic away from potentially congested links.

   For this reason, where the GRE payload traffic is not congestion
   controlled, GRE-in-UDP tunnels MUST only be used within a single
   operator's network that utilizes careful provisioning (e.g., rate
   limiting at the entries of the network while over-provisioning
   network capacity) to ensure against congestion, or within a limited
   number of networks whose operators closely cooperate in order to
   jointly provide this same careful provisioning.

   As such, GRE-in-UDP MUST NOT be used over the general Internet, or
   over non-cooperating network operators, to carry traffic that is not

   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).

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

7. Backward Compatibility

   It is assumed that tunnel ingress routers must be upgraded in order
   to support the encapsulations described in this document.

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

   If a router that is intended for use as a decapsulator does not
   support or enable GRE-in-UDP encapsulation described in this
   document, it will not be listening on destination port (TBD).  In
   these cases, the router will conform to normal UDP processing and
   respond to an encapsulator with an ICMP message indicating "port
   unreachable" according to [RFC792].  Upon receiving this ICMP
   message, the node MUST NOT continue to use GRE-in-UDP encapsulation
   toward this peer without management intervention.

8. IANA Considerations

   IANA is requested to make the following allocations:

   One UDP destination port number for the indication of GRE

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

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

         Service Name: GRE-UDP-DTLS
         Transport Protocol(s): UDP
         Assignee: IESG <>

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

9. Security Considerations

   UDP and GRE encapsulation does not effect 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.

   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 implementation that supports DTLS is expected to be
   able to establish DTLS sessions with multiple tunnel encapsulators,
   and likewise an GRE-in-UDP tunnel encapsulator implementation 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 4.2 for discussion of one
   situation where use of different source IP addresses is important).

   Use of ICMP for signaling of the GRE-in-UDP encapsulation capability
   adds a security concern. Upon receiving an ICMP message and before
   taking an action on it, the ingress MUST validate the IP address
   originating against tunnel egress address and MUST evaluate the
   packet header returned in the ICMP payload to ensure the source port
   is the one used for this tunnel. The mechanism for performing this
   validation is out of the scope of this document.

   In an instance where the UDP source port is not set based on the
   flow invariant fields from the payload header, 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

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   to mitigate certain denial of service attacks. How the source port
   randomization occurs is outside scope of this document.

   Using one standardized value in 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 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

10. Acknowledgements

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

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

11. Contributors

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

   David Black
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748


   Ross Callon
   Juniper Networks

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   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


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12. References

   12.1. Normative References

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

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

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

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

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

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

   [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.

   12.2. Informative References

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

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   [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.

   [RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
             Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC4023] Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating
             MPLS in IP or Generic Routing Encapsulation (GRE)", RFC
             4023, March 2005.

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

   [GREIPV6] Pignataro, C., el al, "IPv6 Support for Generic Routing
             Encapsulation (GRE)", draft-ietf-intarea-gre-ipv6-02, work
             in progress.

   [GREMTU]  Bonica, R., "A Fragmentation Strategy for Generic Routing
             Encapsulation (GRE)", draft-ietf-intarea-gre-mtu, work in

   [CB]      Fairhurst, G., "Network Transport Circuit Breakers",
             draft-fairhurst-tsvwg-circuit-breaker-01, work in

13. Authors' Addresses

   Edward Crabbe


   Lucy Yong
   Huawei Technologies, USA


   Xiaohu Xu
   Huawei Technologies,
   Beijing, China


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   Tom Herbert
   1600 Amphitheatre Parkway
   Mountain View, CA
   Email :

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