GRE-in-UDP Encapsulation
draft-ietf-tsvwg-gre-in-udp-encap-08

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

Expires: April 2016                                    October 16, 2015

                         GRE-in-UDP Encapsulation
                   draft-ietf-tsvwg-gre-in-udp-encap-08

Abstract

   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. This encapsulation
   protocol can apply to IPv4 and IPv6 networks including the Internet.
   When applying it to a well-managed operator network, the tunnel
   implementation and usage can be less restrictive. The document
   specifies the tunnel implementations under both network scenarios.

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
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   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
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   This Internet-Draft will expire on April 16,2016.

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
   (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. Applicability Statement .............................. 3
         1.1.1. Requirements for Default GRE-in-UDP Tunnel
         Implementation over the Internet........................ 5
         1.1.2. Requirements for Conditional GRE-in-UDP Tunnel
         Implementation over a Well-Managed Operator Network..... 6
   2. Terminology ............................................... 7
      2.1. Requirements Language................................. 7
   3. Encapsulation in UDP ...................................... 7
      3.1. IP Header ........................................... 10
      3.2. UDP Header .......................................... 10
         3.2.1. Source Port .................................... 10
         3.2.2. Destination Port ............................... 10
         3.2.3. Checksum ....................................... 11
         3.2.4. Length ......................................... 11
      3.3. GRE Header .......................................... 11
   4. Encapsulation Process Procedures ......................... 11
      4.1. MTU and Fragmentation ............................... 12
      4.2. Differentiated Services ............................. 13
   5. UDP Checksum Handling  ................................... 13
      5.1. UDP Checksum with IPv4 .............................. 13
      5.2. UDP Checksum with IPv6 .............................. 14
         5.2.1. Middlebox Considerations ....................... 17
   6. Congestion Considerations ................................ 17
   7. Backward Compatibility  .................................. 18
   8. IANA Considerations ...................................... 19
   9. Security Considerations .................................. 19
   10. Acknowledgements   ...................................... 21
   11. Contributors    ......................................... 21
   12. References .............................................. 22
      12.1. Normative References ............................... 22
      12.2. Informative References ............................. 23
   13. Authors' Addresses ...................................... 24

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1. Introduction

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

   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. GRE-in-UDP offers the
   additional possibility of using GRE across networks that might
   otherwise disallow it; for instance GRE-in-UDP may be used to bridge
   two islands where GRE is 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.

   1.1. Applicability Statement

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

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   addresses, to tunnel IPv6 traffic over an IPv4 network, tunnel
   Ethernet traffic over IP networks [RFC7637], etc.

   GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks including
   the Internet. When using GRE-in-UDP encapsulation, encapsulated
   traffic will be treated as a UDP application in an IP network.  As
   such, GRE-in-UDP tunnel needs to meet UDP application requirements
   specified in [RFC5405bis], which requires additional tunnel
   functions besides the packet encapsulation/decapsulation at the
   tunnel endpoints. The required additional functions may be
   simplified according to the network operation condition. For
   example, if a GRE-in-UDP tunnel is used to carry IP payload only,
   tunnel congestion control function is not necessary.

   This document considers two network scenarios: 1) Use of GRE-in-UDP
   in a general IP network including the Internet, where a default GRE-
   in-UDP tunnel implementation specified in this draft can apply; 2)
   Use of GRE-in-UDP in a well-managed operator IP network, where a
   GRE-in-UDP tunnel implementation can be less restrictive than the
   default implementation. The implementation for a well-managed
   operator IP network is specified in this draft too and is referred
   to as conditional GRE-in-UDP tunnel implementation in the remaining
   document.

   A well-managed operator IP network (referred to Operator Network in
   the rest) is an IP network that meets at least one of following
   conditions:

   a. 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. 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 corruption.

   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.

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   As a result, use of GRE-in-UDP within a well-managed operator
   network, UDP zero-checksum in IPv6 may be used (see Section 5.2).

   Another characteristic that a well-managed operator network often
   has is a congestion control, i.e. the network is traffic-engineered
   and/or operated to avoid congestion.

   GRE-in-UDP tunnel implementation, either default or conditional,
   does not have congestion control capability. Therefore, it limits
   its usage for either tunneled traffic having congestion control
   and/or a well-managed operator network that provides traffic-
   engineering to avoid congestion.

   As a result, default GRE-in-UDP tunnel implementation MUST NOT apply
   to traffic that has no congestion control over the Internet;
   conditional GRE-in-UDP tunnel implementation can apply to a well-
   managed operator network that provides congestion control. (See
   Section 6)

   The following two sections summarize the requirements of GRE-in-UDP
   tunnel implementation for a generic IP network including the
   Internet and a well-managed operator network, respectively. The
   networks can be IPv4 or Ipv6.

   1.1.1. Requirements for Default GRE-in-UDP Tunnel Implementation
      over the Internet

   The following are the requirements for default GRE-in-UDP tunnel
   implementation that can apply to an IP network including Internet.

   1. SHOULD perform UDP checksum when over an IPv4 network.

   2. MUST perform UDP checksum when over an IPv6 network.

   3. IP-traffic can be assumed to be congestion-controlled; other
   tunneled protocol/payload SHOULD implement an appropriate congestion
   control method because the GRE/UDP tunnel does not itself provide
   any congestion control. If GRE-in-UDP tunnel MUST NOT to traffic
   that has no congestion control over the general Internet.

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

   5. For IPv6 delivery network, if IPv6 flow label load balancing is
   supported [RFC4638], the flow entropy SHOULD also be placed in the
   flow label field.

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   6. If a tunnel ingress fragments the incoming packet (before
   encapsulation), the UDP checksum MUST be used so that the receiving
   endpoint can validate reassembly of the fragments, and the same src
   UDP port SHOULD be used for all packet fragments to ensure that the
   transit routers will forward the packet fragments on the same path.

   7. If the incoming packet needs to be fragmented, it SHOULD be done
   before the encapsulation [RFC7588] and calculate the size of
   fragments based on the MTU and including the size of the UDP header.

   1.1.2. Requirements for Conditional GRE-in-UDP Tunnel Implementation
      over a Well-Managed Operator Network

   The following are the requirements for conditional GRE-in-UDP tunnel
   implementation that can apply to a well-managed IP network described
   above.

   1. When over an IPv4 network, SHOULD set UDP zero-checksum to
   improve the tunnel performance.

   2. When over an IPv6 network, MUST perform UDP checksum as default
   but MAY be configured with UDP zero-checksum with additional
   implementation requirements that are specified in Section 5.2.

   3. A tunnel may encapsulate a protocol/payload that does not provide
   congestion control if the delivery network is traffic-engineered
   and/or operated by the network operator to avoid congestion, e.g.
   use of pre-provision capacity or utilize a circuit breaker [CK].

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

   5. For IPv6 delivery network, if IPv6 flow label load balancing is
   supported [RFC4638], the flow entropy SHOULD also be placed in the
   flow label field.

   6. If a tunnel ingress fragments the incoming packet (before
   encapsulation), the UDP checksum MUST be used so that the receiving
   endpoint can validate reassembly of the fragments, and the same src
   UDP port SHOULD be used for all packet fragments to ensure that the
   transit routers will forward the packet fragments on the same path.

   7. If the incoming packet needs to be fragmented, it SHOULD be done
   before the encapsulation [RFC7588] and calculate the size of
   fragments based on the MTU and including the size of the UDP header.

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   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
   endpoints 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.

   Default GRE-in-UDP tunnel implementation: GRE-in-UDP tunnel
   implementation that can apply to an IP network including the
   Internet.

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

   2.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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, e.g. use of
   the system time to yield a value that is the range of entropy values.

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

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

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

   3.2.2. Destination Port

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

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   3.2.3. Checksum

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

   3.2.4. Length

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

   3.3. GRE Header

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

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

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

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

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

4. Encapsulation Process Procedures

   The GRE-in-UDP encapsulation allows encapsulated packets to be
   forwarded through "GRE-in-UDP tunnels".  When performing GRE-in-UDP

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   encapsulation by the encapsulator, the entropy value is generated by
   the encapsulator and then be filled in the Source Port field of the
   UDP header.  The Destination Port field is set to a value (TBD)
   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.

   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].

   The procedures specified in this section apply to default GRE-in-UDP
   tunnel implementation and conditional GRE-in-UDP tunnel
   implementation.

   4.1. MTU and Fragmentation

   Regarding packet fragmentation, an encapsulator/decapsulator SHOULD
   be compliant with [RFC7588]. For this case, the MTU is equal to the
   PMTU associated with the path between the GRE ingress and the GRE

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   egress nodes minus the GRE and UDP overhead. When applying payload
   fragment, the UDP checksum MUST be used so that the receiving
   endpoint can validate reassembly of the fragments; the same src UDP
   port SHOULD be used for all packet fragments to ensure the transit
   routers will forward the fragments on the same path. An operator
   should factor in the additional bytes of overhead when considering
   an MTU size for the payload to avoid the likelihood of fragmentation.

   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
   document.

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

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

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   Default GRE-in-UDP tunnel implementation SHOULD perform UDP checksum.
   Conditional GRE-in-UDP tunnel implementation MAY set UDP zero-
   checksum.

   5.2. UDP Checksum with IPv6

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

   When UDP is used over IPv6, the UDP checksum is relied upon to
   protect both the IPv6 and UDP headers from corruption. As such,
   default GRE-in-UDP tunnel implementation MUST perform UDP checksum;
   conditional GRE-in-UDP tunnel implementation MAY be configured with
   the UDP zero-checksum mode when the tunnel is used in a well-managed
   operator network and/or 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 the condition for UDP zero-checksum mode usage with GRE-
   in-UDP over IPv6.

   The use of the UDP zero-checksum mode must meet the requirements
   specified in [RFC6935] and [RFC6936], which conducts the following
   additional requirements for GRE-in-UDP tunnel implementation and use
   of UDP zero-checksum mode for GRE-in-UDP over IPv6:

     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. 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

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

     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.
        Note that if UDP checksum is used, such restriction is not
        necessary.

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

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

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

     i. IPv6 traffic with zero UDP checksums MUST be actively monitored
        for errors by the network operator. For example, Ethernet layer
        packet error rate or probe packet error rate.

     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

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   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
   middle box discussion.

   It is worth mentioning that the use of a zero UDP checksum should
   present the equivalent risk of undetected packet corruption when
   sending similar packet using GRE-in-IPv6 without UDP [GREIPV6] and
   without GRE checksums.

   In summary, conditional GRE-in-UDP tunnel implementation is allowed
   to use UDP-zero-checksum mode for IPv6, when additional
   implementation requirements stated above are provided. Otherwise the
   UDP checksum MUST be used for IPv6 as specified in [RFC768] and
   [RFC2460]. Use of GRE checksum favors non-use of the UDP checksum.

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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.

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

6. Congestion Considerations

   Section 3.1.3 of [RFC5405] discussed the congestion implications of
   UDP tunnels. As discussed in [RFC5405], 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 of a UDP tunnel that is consuming
   excessive capacity, and in terms of the effect on the flows using
   the UDP tunnels. The potential impact of congestion from a UDP
   tunnel depends upon what sort of traffic is carried over the tunnel,
   as well as the path of the tunnel.

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

   However, GRE-in-UDP tunnel can be used in some cases to carry
   traffic that is not necessarily congestion controlled. For example,
   GRE-in-UDP may be used to carry MPLS that carries pseudowire or VPN
   traffic where specific bandwidth guarantees are provided to each
   pseudowire or to each VPN. In such cases, network operators may
   avoid congestion by careful provisioning of their networks, by rate
   limiting of user data traffic, and traffic engineer according to
   path capacity. For this reason, GRE-in-UDP tunnel MUST be used
   within a single operator's network that utilizes careful

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   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.

   Default GRE-in-UDP tunnel implementation 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.
   GRE-in-UDP MUST NOT be used over the general Internet, or over non-
   cooperating network operators, to carry traffic that is not
   congestion-controlled.

   Conditional GRE-in-UDP tunnel implementation can be used within a
   well-managed operator network to carry 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 NVEGRE encapsulation only (Although the
      payload type is Ethernet in NVGRE, NVGRE protocol mandates that
      the payload of Ethernet is IP).

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

7. Backward Compatibility

   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

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

8. IANA Considerations

   IANA is requested to make the following allocations:

   One UDP destination port number for the indication of GRE

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

9. Security Considerations

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

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   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 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 5.2 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
   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

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   presupposes that there is effective destination-based (or a
   combination of source-based and destination-based) filtering at the
   boundaries.

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

10. Acknowledgements

   Authors like to thank Vivek Kumar, Ron Bonica, Joe Touch, Ruediger
   Geib, Lar Edds, Lloyd, 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
   checksum.

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
   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

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   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

   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

12. References

   12.1. Normative References

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

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

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

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

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

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

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

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

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

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

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

   12.2. Informative References

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

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

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

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

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

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

   [CB]      Fairhurst, G., "Network Transport Circuit Breakers",
             draft-ietf-tsvwg-circuit-breaker-04, work in progress.

   [GREIPV6] Pignataro, C., Bonica, R., Krishnan, S., "IPv6 Support for
             Generic Routing Encapsulation (GRE)", draft-ietf-intarea-
             gre-ipv6, work in progress.

13. 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

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   Tom Herbert
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA
   Email : tom@herbertland.com

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