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Internet Engineering Task Force (IETF)
Request for Comments: 8086
Category: Standards Track
ISSN: 2070-1721
L. Yong, Ed.
Huawei Technologies
E. Crabbe
Oracle
X. Xu
Huawei Technologies
T. Herbert
Facebook
March 2017

GRE-in-UDP Encapsulation

Abstract

This document specifies a method of encapsulating network protocol packets within GRE and UDP headers. This GRE-in-UDP encapsulation allows the UDP source port field to be used as an entropy field. This may be used for load-balancing of GRE traffic in transit networks using existing Equal-Cost Multipath (ECMP) mechanisms. There are two applicability scenarios for GRE-in-UDP with different requirements: (1) general Internet and (2) a traffic-managed controlled environment. The controlled environment has less restrictive requirements than the general Internet.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8086.

Copyright Notice

Copyright © 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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 ....................................................4
      1.1. Terminology ................................................5
      1.2. Requirements Language ......................................5
   2. Applicability Statement .........................................6
      2.1. GRE-in-UDP Tunnel Requirements .............................6
           2.1.1. Requirements for Default GRE-in-UDP Tunnel ..........7
           2.1.2. Requirements for TMCE GRE-in-UDP Tunnel .............8
   3. GRE-in-UDP Encapsulation ........................................9
      3.1. IP Header .................................................11
      3.2. UDP Header ................................................11
           3.2.1. Source Port ........................................11
           3.2.2. Destination Port ...................................11
           3.2.3. Checksum ...........................................12
           3.2.4. Length .............................................12
      3.3. GRE Header ................................................12
   4. Encapsulation Procedures .......................................13
      4.1. MTU and Fragmentation .....................................13
      4.2. Differentiated Services and ECN Marking ...................14
   5. Use of DTLS ....................................................14
   6. UDP Checksum Handling ..........................................15
      6.1. UDP Checksum with IPv4 ....................................15
      6.2. UDP Checksum with IPv6 ....................................15
   7. Middlebox Considerations .......................................18
      7.1. Middlebox Considerations for Zero Checksums ...............19
   8. Congestion Considerations ......................................19
   9. Backward Compatibility .........................................20
   10. IANA Considerations ...........................................21
   11. Security Considerations .......................................21
   12. References ....................................................22
      12.1. Normative References .....................................22
      12.2. Informative References ...................................23
   Acknowledgements ..................................................25
   Contributors ......................................................25
   Authors' Addresses ................................................27

1. Introduction

This document specifies a generic GRE-in-UDP encapsulation for tunneling network protocol packets across an IP network based on Generic Routing Encapsulation (GRE) [RFC2784] [RFC7676] and User Datagram Protocol (UDP) [RFC768] headers. The GRE header indicates the payload protocol type via an EtherType [RFC7042] in the protocol type field, and the source port field in the UDP header may be used to provide additional entropy.

A GRE-in-UDP tunnel offers the possibility of better performance for load-balancing GRE traffic in transit networks using existing Equal- Cost Multipath (ECMP) mechanisms that use a hash of the five-tuple of source IP address, destination IP address, UDP/TCP source port, UDP/TCP destination port, and protocol number. While such hashing distributes UDP and TCP [RFC793] traffic between a common pair of IP addresses across paths, it uses a single path for corresponding GRE traffic because only the two IP addresses and the Protocol or Next Header field participate in the ECMP hash. Encapsulating GRE in UDP enables use of the UDP source port to provide entropy to ECMP hashing.

In addition, GRE-in-UDP enables extending use of GRE across networks that otherwise disallow it; for example, GRE-in-UDP may be used to bridge two islands where GRE is not supported natively across the middleboxes.

GRE-in-UDP encapsulation may be used to encapsulate already tunneled traffic, i.e., tunnel-in-tunnel traffic. In this case, GRE-in-UDP tunnels treat the endpoints of the outer tunnel as the end hosts; the presence of an inner tunnel does not change the outer tunnel's handling of network traffic.

A GRE-in-UDP tunnel is capable of carrying arbitrary traffic and behaves as a UDP application on an IP network. However, a GRE-in-UDP tunnel carrying certain types of traffic does not satisfy the requirements for UDP applications on the Internet [RFC8085]. GRE-in-UDP tunnels that do not satisfy these requirements MUST NOT be deployed to carry such traffic over the Internet. For this reason, this document specifies two deployment scenarios for GRE-in-UDP tunnels with GRE-in-UDP tunnel requirements for each of them: (1) general Internet and (2) a traffic-managed controlled environment (TMCE). Compared to the general Internet scenario, the TMCE scenario has less restrictive technical requirements for the protocol but more restrictive management and operation requirements for the network.

To provide security for traffic carried by a GRE-in-UDP tunnel, this document also specifies Datagram Transport Layer Security (DTLS) for GRE-in-UDP tunnels, which SHOULD be used when security is a concern.

GRE-in-UDP encapsulation usage requires no changes to the transit IP network. ECMP hash functions in most existing IP routers may utilize and benefit from the additional entropy enabled by GRE-in-UDP tunnels without any change or upgrade to their ECMP implementation. The encapsulation mechanism is applicable to a variety of IP networks including Data Center Networks and Wide Area Networks, as well as both IPv4 and IPv6 networks.

1.1. Terminology

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

Decapsulator: a component performing packet decapsulation at tunnel egress.

ECMP:

Equal-Cost Multipath.

Encapsulator: a component performing packet encapsulation at tunnel egress.

Flow Entropy: The information to be derived from traffic or applications and to be used by network devices in the ECMP process [RFC6438].

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

TMCE: A traffic-managed controlled environment, i.e., an IP network that is traffic-engineered and/or otherwise managed (e.g., via use of traffic rate limiters) to avoid congestion, as defined in Section 2.

TMCE GRE-in-UDP Tunnel: A GRE-in-UDP tunnel that can only apply to a traffic-managed controlled environment that is defined in Section 2.

Tunnel Egress: A tunnel endpoint that performs packet decapsulation.

Tunnel Ingress: A tunnel endpoint that performs packet encapsulation.

1.2. Requirements Language

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

2. Applicability Statement

GRE-in-UDP encapsulation applies to IPv4 and IPv6 networks; in both cases, encapsulated packets are treated as UDP datagrams. Therefore, a GRE-in-UDP tunnel needs to meet the UDP usage requirements specified in [RFC8085]. These requirements depend on both the delivery network and the nature of the encapsulated traffic. For example, the GRE-in-UDP tunnel protocol does not provide any congestion control functionality beyond that of the encapsulated traffic. Therefore, a GRE-in-UDP tunnel MUST be used only with congestion-controlled traffic (e.g., IP unicast traffic) and/or within a network that is traffic managed to avoid congestion.

[RFC8085] describes two applicability scenarios for UDP applications: (1) general internet and (2) a controlled environment. The controlled environment means a single administrative domain or bilaterally agreed connection between domains. A network forming a controlled environment can be managed/operated to meet certain conditions, while the general Internet cannot be; thus, the requirements for a tunnel protocol operating under a controlled environment can be less restrictive than the requirements in the general Internet.

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

This document specifies GRE-in-UDP tunnel usage in the general Internet and GRE-in-UDP tunnel usage in a traffic-managed controlled environment and uses "default GRE-in-UDP tunnel" and "TMCE GRE-in-UDP tunnel" terms to refer to each usage.

NOTE: Although this document specifies two different sets of GRE-in- UDP tunnel requirements based on tunnel usage, the tunnel implementation itself has no ability to detect how and where it is deployed. Therefore, it is the responsibility of the user or operator who deploys a GRE-in-UDP tunnel to ensure that it meets the appropriate requirements.

2.1. GRE-in-UDP Tunnel Requirements

This section states the requirements for a GRE-in-UDP tunnel. Section 2.1.1 describes the requirements for a default GRE-in-UDP tunnel that is suitable for the general Internet; Section 2.1.2 describes a set of relaxed requirements for a TMCE GRE-in-UDP tunnel used in a traffic-managed controlled environment. Both Sections 2.1.1 and 2.1.2 are applicable to an IPv4 or IPv6 delivery network.

2.1.1. Requirements for Default GRE-in-UDP Tunnel

The following is a summary of the default GRE-in-UDP tunnel requirements:

  1. A UDP checksum SHOULD be used when encapsulating in IPv4.
  1. A UDP checksum MUST be used when encapsulating in IPv6.
  1. GRE-in-UDP tunnel MUST NOT be deployed or configured to carry traffic that is not congestion controlled. As stated in [RFC8085], IP-based unicast traffic is generally assumed to be congestion controlled, i.e., it is assumed that the transport protocols generating IP-based traffic at the sender already employ mechanisms that are sufficient to address congestion on the path. A default GRE-in-UDP tunnel is not appropriate for traffic that is not known to be congestion controlled (e.g., most IP multicast traffic).
  1. UDP source port values that are used as a source of flow entropy SHOULD be chosen from the ephemeral port range (49152-65535) [RFC8085].
  1. The use of the UDP source port MUST be configurable so that a single value can be set for all traffic within the tunnel (this disables use of the UDP source port to provide flow entropy). When a single value is set, a random port taken from the ephemeral port range SHOULD be selected in order to minimize the vulnerability to off-path attacks [RFC6056].
  1. For IPv6 delivery networks, the flow entropy SHOULD also be placed in the flow label field for ECMP per [RFC6438].
  1. At the tunnel ingress, any fragmentation of the incoming packet (e.g., because the tunnel has a Maximum Transmission Unit (MTU) that is smaller than the packet) SHOULD be performed before encapsulation. In addition, the tunnel ingress MUST apply the UDP checksum to all encapsulated fragments so that the tunnel egress can validate reassembly of the fragments; it MUST set the same Differentiated Services Code Point (DSCP) value as in the Differentiated Services (DS) field of the payload packet in all fragments [RFC2474]. To avoid unwanted forwarding over multiple paths, the same source UDP port value SHOULD be set in all packet fragments.

2.1.2. Requirements for TMCE GRE-in-UDP Tunnel

The section contains the TMCE GRE-in-UDP tunnel requirements. It lists the changed requirements, compared with a Default GRE-in-UDP tunnel, for a TMCE GRE-in-UDP tunnel, which corresponds to requirements 1-3 listed in Section 2.1.1.

  1. A UDP checksum SHOULD be used when encapsulating in IPv4. A tunnel endpoint sending GRE-in-UDP MAY disable the UDP checksum, since GRE has been designed to work without a UDP checksum [RFC2784]. However, a checksum also offers protection from misdelivery to another port.
  1. Use of the UDP checksum MUST be the default when encapsulating in IPv6. This default MAY be overridden via configuration of UDP zero-checksum mode. All usage of UDP zero-checksum mode with IPv6 is subject to the additional requirements specified in Section 6.2.
  1. A GRE-in-UDP tunnel MAY encapsulate traffic that is not congestion controlled.

Requirements 4-7 listed in Section 2.1.1 also apply to a TMCE GRE-in- UDP tunnel.

3. GRE-in-UDP Encapsulation

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   
   IPv4 Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version|  IHL  |Type of Service|          Total Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Identification        |Flags|      Fragment Offset    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Time to Live | Prot.=17(UDP) |          Header Checksum      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source IPv4 Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination IPv4 Address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   
   UDP Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Source Port = Entropy Value  |  Dest. Port = 4754/4755       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           UDP Length          |        UDP Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   
   GRE Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C| |K|S| Reserved0       | Ver |         Protocol Type         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Checksum (optional)      |       Reserved1 (Optional)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Key (optional)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Sequence Number (optional)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: UDP + GRE Headers in IPv4

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   
   IPv6 Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| Traffic Class |           Flow Label                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Payload Length        | NxtHdr=17(UDP)|   Hop Limit   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Outer Source IPv6 Address                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                  Outer Destination IPv6 Address               +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   
   UDP Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Source Port = entropy value  |  Dest. Port = 4754/4755       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           UDP Length          |        UDP Checksum           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   
   GRE Header:
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C| |K|S| Reserved0       | Ver |         Protocol Type         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Checksum (optional)      |       Reserved1 (Optional)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Key (optional)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Sequence Number (optional)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: UDP + GRE Headers in IPv6

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

3.1. IP Header

An encapsulator MUST encode its own IP address as the source IP address and the decapsulator's IP address as the destination IP address. A sufficiently large value is needed in the IPv4 TTL field or IPv6 Hop Count field to allow delivery of the encapsulated packet to the peer of the encapsulation.

3.2. UDP Header

3.2.1. Source Port

GRE-in-UDP permits the UDP source port value to be used to encode an entropy value. The UDP source port contains a 16-bit entropy value that is generated by the encapsulator to identify a flow for the encapsulated packet. The port value SHOULD be within the ephemeral port range, i.e., 49152 to 65535, where the high-order two bits of the port are set to one. This provides fourteen bits of entropy for the inner flow identifier. In the case that an encapsulator is unable to derive flow entropy from the payload header or the entropy usage has to be disabled to meet operational requirements (see Section 7), to avoid reordering with a packet flow, the encapsulator SHOULD use the same UDP source port value for all packets assigned to a flow, e.g., the result of an algorithm that performs a hash of the tunnel ingress and egress IP address.

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

An IPv6 GRE-in-UDP tunnel endpoint SHOULD copy a flow entropy value in the IPv6 flow label field (requirement 6). This permits network equipment to inspect this value and utilize it during forwarding, e.g., to perform ECMP [RFC6438].

This document places requirements on the generation of the flow entropy value [RFC8085] but does not specify the algorithm that an implementation should use to derive this value.

3.2.2. Destination Port

The destination port of the UDP header is set to either GRE-in-UDP (4754) or GRE-UDP-DTLS (4755); see Section 5.

3.2.3. Checksum

The UDP checksum is set and processed per [RFC768] and [RFC1122] for IPv4 and per [RFC2460] for IPv6. Requirements for checksum handling and use of zero UDP checksums are detailed in Section 6.

3.2.4. Length

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

3.3. GRE Header

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

An encapsulator MAY set the GRE Key Present, Sequence Number Present, and Checksum Present bits and associated fields in the GRE header as defined by [RFC2784] and [RFC2890]. Usage of the reserved bits, i.e., Reserved0, is specified in [RFC2784].

The GRE checksum MAY be enabled to protect the GRE header and payload. When the UDP checksum is enabled, it protects the GRE payload, resulting in the GRE checksum being mostly redundant. Enabling both checksums may result in unnecessary processing. Since the UDP checksum covers the pseudo-header and the packet payload, including the GRE header and its payload, the UDP checksum SHOULD be used in preference to the GRE checksum.

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

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

The procedures specified in this section apply to both a default GRE- in-UDP tunnel and a TMCE GRE-in-UDP tunnel.

The GRE-in-UDP encapsulation allows encapsulated packets to be forwarded through "GRE-in-UDP tunnels". The encapsulator MUST set the UDP and GRE headers according to Section 3.

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

Upon receiving these UDP encapsulated packets, the decapsulator decapsulates them by removing the UDP and GRE headers and then processes them accordingly.

GRE-in-UDP can encapsulate traffic with unicast, IPv4 broadcast, or multicast (see requirement 3 in Section 2.1.1). However, a default GRE-in-UDP tunnel MUST NOT be deployed or configured to carry traffic that is not congestion-controlled (see requirement 3 in Section 2.1.1). Entropy may be generated from the header of encapsulated packets at an encapsulator. The mapping mechanism between the encapsulated multicast traffic and the multicast capability in the IP network is transparent and independent of the encapsulation and is otherwise outside the scope of this document.

To provide entropy for ECMP, GRE-in-UDP does not rely on GRE keep- alive. It is RECOMMENDED not to use GRE keep-alive in the GRE-in-UDP tunnel. This aligns with middlebox traversal guidelines in Section 3.5 of [RFC8085].

4.1. MTU and Fragmentation

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

If the operator of the transit network supporting the tunnel is able to control the payload MTU size, the MTU SHOULD be configured to avoid fragmentation, i.e., sufficient for the largest supported size of packet, including all additional bytes introduced by the tunnel overhead [RFC8085].

4.2. Differentiated Services and ECN Marking

To ensure that tunneled traffic receives the same treatment over the IP network as traffic that is not tunneled, prior to the encapsulation process, an encapsulator processes the tunneled IP packet headers to retrieve appropriate parameters for the encapsulating IP packet header such as Diffserv [RFC2983]. Encapsulation endpoints that support Explicit Congestion Notification (ECN) must use the method described in [RFC6040] for ECN marking propagation. The congestion control process is outside of the scope of this document.

Additional information on IP header processing is provided in Section 3.1.

5. Use of DTLS

Datagram Transport Layer Security (DTLS) [RFC6347] can be used for application security and can preserve network- and transport-layer protocol information. Specifically, if DTLS is used to secure the GRE-in-UDP tunnel, the destination port of the UDP header MUST be set to the IANA-assigned value (4755) indicating GRE-in-UDP with DTLS, and that UDP port MUST NOT be used for other traffic. The UDP source port field can still be used to add entropy, e.g., for load-sharing purposes. DTLS applies to a default GRE-in-UDP tunnel and a TMCE GRE-in-UDP tunnel.

Use of DTLS is limited to a single DTLS session for any specific tunnel encapsulator/decapsulator pair (identified by source and destination IP addresses). Both IP addresses MUST be unicast addresses -- multicast traffic is not supported when DTLS is used. A GRE-in-UDP tunnel decapsulator that supports DTLS is expected to be able to establish DTLS sessions with multiple tunnel encapsulators, and likewise a GRE-in-UDP tunnel encapsulator is expected to be able to establish DTLS sessions with multiple decapsulators. Different source and/or destination IP addresses will be involved; see Section 6.2 for discussion of one situation where use of different source IP addresses is important.

When DTLS is used for a GRE-in-UDP tunnel, if a packet is received from the tunnel and that packet is not protected by the DTLS session or part of DTLS negotiation (e.g., a DTLS handshake message [RFC6347]), the tunnel receiver MUST discard that packet and SHOULD log that discard event and information about the discarded packet.

DTLS SHOULD be used for a GRE-in-UDP tunnel to meet security requirements of the original traffic that is delivered by a GRE-in- UDP tunnel. There are cases where no additional security is required, e.g., the traffic to be encapsulated is already encrypted or the tunnel is deployed within an operationally secured network. Use of DTLS for a GRE-in-UDP tunnel requires both tunnel endpoints to configure use of DTLS.

6. UDP Checksum Handling

6.1. UDP Checksum with IPv4

For UDP in IPv4, when a non-zero UDP checksum is used, the UDP checksum MUST be processed as specified in [RFC768] and [RFC1122] for both transmit and receive. The IPv4 header includes a checksum that protects against misdelivery of the packet due to corruption of IP addresses. The UDP checksum potentially provides protection against corruption of the UDP header, GRE header, and GRE payload. Disabling the use of checksums is a deployment consideration that should take into account the risk and effects of packet corruption.

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

6.2. UDP Checksum with IPv6

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

When UDP is used over IPv6, the UDP checksum is relied upon to protect both the IPv6 and UDP headers from corruption. As such, a default GRE-in-UDP tunnel MUST perform UDP checksum; a TMCE GRE-in-

UDP tunnel MAY be configured with UDP zero-checksum mode if the traffic-managed controlled environment or a set of closely cooperating traffic-managed controlled environments (such as by network operators who have agreed to work together in order to jointly provide specific services) meet at least one of the following conditions:

a. It is known (perhaps through knowledge of equipment types and

lower-layer checks) that packet corruption is exceptionally unlikely and where the operator is willing to take the risk of undetected packet corruption.

b. It is judged through observational measurements (perhaps of

historic or current traffic flows that use a non-zero checksum) that the level of packet corruption is tolerably low and where the operator is willing to take the risk of undetected packet corruption.

c. Carrying applications that are tolerant of misdelivered or

corrupted packets (perhaps through higher-layer checksum, validation, and retransmission or transmission redundancy) where the operator is willing to rely on the applications using the tunnel to survive any corrupt packets.

The following requirements apply to a TMCE GRE-in-UDP tunnel that uses UDP zero-checksum mode:

a. Use of the UDP checksum with IPv6 MUST be the default

configuration of all GRE-in-UDP tunnels.

b. The GRE-in-UDP tunnel implementation MUST comply with all

requirements specified in Section 4 of [RFC6936] and with requirement 1 specified in Section 5 of [RFC6936].

c. The tunnel decapsulator SHOULD only allow the use of UDP zero-

checksum mode for IPv6 on a single received UDP Destination Port, regardless of the encapsulator. The motivation for this requirement is possible corruption of the UDP Destination Port, which may cause packet delivery to the wrong UDP port. If that other UDP port requires the UDP checksum, the misdelivered packet will be discarded.

d. It is RECOMMENDED that the UDP zero-checksum mode for IPv6 is

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

e. The tunnel encapsulator SHOULD use different IPv6 addresses for

each GRE-in-UDP tunnel that uses UDP zero-checksum mode, regardless of the decapsulator, in order to strengthen the decapsulator's check of the IPv6 source address (i.e., the same IPv6 source address SHOULD NOT be used with more than one IPv6 destination address, independent of whether that destination address is a unicast or multicast address). When this is not possible, it is RECOMMENDED to use each source IPv6 address for as few GRE-in-UDP tunnels that use UDP zero-checksum mode as is feasible.

f. When any middlebox exists on the path of a GRE-in-UDP tunnel, it

is RECOMMENDED to use the default mode, i.e., use UDP checksum, to reduce the chance that the encapsulated packets will be dropped.

g. Any middlebox that allows the UDP zero-checksum mode for IPv6

MUST comply with requirements 1 and 8-10 in Section 5 of [RFC6936].

h. Measures SHOULD be taken to prevent IPv6 traffic with zero UDP

checksums from "escaping" to the general Internet; see Section 8 for examples of such measures.

i. IPv6 traffic with zero UDP checksums MUST be actively monitored

for errors by the network operator. For example, the operator may monitor Ethernet-layer packet error rates.

j. If a packet with a non-zero checksum is received, the checksum

MUST be verified before accepting the packet. This is regardless of whether the tunnel encapsulator and decapsulator have been configured with UDP zero-checksum mode.

The above requirements do not change either the requirements specified in [RFC2460] as modified by [RFC6935] or the requirements specified in [RFC6936].

The requirement to check the source IPv6 address in addition to the destination IPv6 address and the strong recommendation against reuse of source IPv6 addresses among GRE-in-UDP tunnels collectively provide some mitigation for the absence of UDP checksum coverage of the IPv6 header. A traffic-managed controlled environment that satisfies at least one of three conditions listed at the beginning of this section provides additional assurance.

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

Generic Router Encapsulation (GRE) does not accumulate incorrect transport-layer state as a consequence of GRE header corruption. A corrupt GRE packet may result in either packet discard or packet forwarding without accumulation of GRE state. Active monitoring of GRE-in-UDP traffic for errors is REQUIRED, as the occurrence of errors will result in some accumulation of error information outside the protocol for operational and management purposes. This design is in accordance with requirement 4 specified in Section 5 of [RFC6936].

The remaining requirements specified in Section 5 of [RFC6936] are not applicable to GRE-in-UDP. Requirements 6 and 7 do not apply because GRE does not include a control feedback mechanism. Requirements 8-10 are middlebox requirements that do not apply to GRE-in-UDP tunnel endpoints. (See Section 7.1 for further middlebox discussion.)

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

In summary, a TMCE GRE-in-UDP tunnel is allowed to use UDP zero- checksum mode for IPv6 when the conditions and requirements stated above are met. Otherwise, the UDP checksum needs to be used for IPv6 as specified in [RFC768] and [RFC2460]. Use of GRE checksum is RECOMMENDED when the UDP checksum is not used.

7. Middlebox Considerations

Many middleboxes read or update UDP port information of the packets that they forward. Network Address Port Translator (NAPT) is the most commonly deployed Network Address Translation (NAT) device [RFC4787]. A NAPT device establishes a NAT session to translate the {private IP address, private source port number} tuple to a {public IP address, public source port number} tuple, and vice versa, for the duration of the UDP session. This provides a UDP application with the "NAT pass-through" function. NAPT allows multiple internal hosts to share a single public IP address. The port number, i.e., the UDP Source Port number, is used as the demultiplexer of the multiple internal hosts. However, the above NAPT behaviors conflict with the behavior of a GRE-in-UDP tunnel that is configured to use the UDP source port value to provide entropy.

A GRE-in-UDP tunnel is unidirectional; the tunnel traffic is not expected to be returned back to the UDP source port values used to generate entropy. However, some middleboxes (e.g., firewalls) assume that bidirectional traffic uses a common pair of UDP ports. This assumption also conflicts with the use of the UDP source port field as entropy.

Hence, use of the UDP source port for entropy may impact middleboxes' behavior. If a GRE-in-UDP tunnel is expected to be used on a path with a middlebox, the tunnel can be configured either to disable use of the UDP source port for entropy or to enable middleboxes to pass packets with UDP source port entropy.

7.1. Middlebox Considerations for Zero Checksums

IPv6 datagrams with a zero UDP checksum will not be passed by any middlebox that updates the UDP checksum field or simply validates the checksum based on [RFC2460], such as firewalls. Changing this behavior would require such middleboxes to be updated to correctly handle datagrams with zero UDP checksums. The GRE-in-UDP encapsulation does not provide a mechanism to safely fall back to using a checksum when a path change occurs that redirects a tunnel over a path that includes a middlebox that discards IPv6 datagrams with a zero UDP checksum. In this case, the GRE-in-UDP tunnel will be black-holed by that middlebox.

As such, when any middlebox exists on the path of a GRE-in-UDP tunnel, use of the UDP checksum is RECOMMENDED to increase the probability of successful transmission of GRE-in-UDP packets. Recommended changes to allow firewalls and other middleboxes to support use of an IPv6 zero UDP checksum are described in Section 5 of [RFC6936].

8. Congestion Considerations

Section 3.1.9 of [RFC8085] discusses the congestion considerations for design and use of UDP tunnels; this is important because other flows could share the path with one or more UDP tunnels, necessitating congestion control [RFC2914] to avoid destructive interference.

Congestion has potential impacts both on the rest of the network containing a UDP tunnel and on the traffic flows using the UDP tunnels. These impacts depend upon what sort of traffic is carried over the tunnel, as well as the path of the tunnel. The GRE-in-UDP tunnel protocol does not provide any congestion control and GRE-in- UDP packets are regular UDP packets. Therefore, a GRE-in-UDP tunnel MUST NOT be deployed to carry non-congestion-controlled traffic over the Internet [RFC8085].

Within a TMCE network, GRE-in-UDP tunnels are appropriate for carrying traffic that is not known to be congestion controlled. For example, a GRE-in-UDP tunnel may be used to carry Multiprotocol Label Switching (MPLS) traffic such as pseudowires or VPNs where specific bandwidth guarantees are provided to each pseudowire or VPN. In such cases, operators of TMCE networks avoid congestion by careful provisioning of their networks, rate-limiting of user data traffic, and traffic engineering according to path capacity.

When a GRE-in-UDP tunnel carries traffic that is not known to be congestion controlled in a TMCE network, the tunnel MUST be deployed entirely within that network, and measures SHOULD be taken to prevent the GRE-in-UDP traffic from "escaping" the network to the general Internet. Examples of such measures are:

  • physical or logical isolation of the links carrying GRE-in-UDP from the general Internet,
  • deployment of packet filters that block the UDP ports assigned for GRE-in-UDP, and
  • imposition of restrictions on GRE-in-UDP traffic by software tools used to set up GRE-in-UDP tunnels between specific end systems (as might be used within a single data center) or by tunnel ingress nodes for tunnels that don't terminate at end systems.

9. Backward Compatibility

In general, tunnel ingress routers have to be upgraded in order to support the encapsulations described in this document.

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

If a tunnel endpoint (a host or router) that is intended for use as a decapsulator does not support or enable the GRE-in-UDP encapsulation described in this document, that endpoint will not listen on the destination port assigned to the GRE-encapsulation (4754 and 4755). In these cases, the endpoint will perform normal UDP processing and respond to an encapsulator with an ICMP message indicating "port unreachable" according to [RFC792]. Upon receiving this ICMP message, the node MUST NOT continue to use GRE-in-UDP encapsulation toward this peer without management intervention.

10. IANA Considerations

IANA has allocated the following 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: RFC 8086
         Port Number: 4754
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

IANA has allocated the following 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: RFC 8086
         Port Number: 4755
         Service Code: N/A
         Known Unauthorized Uses: N/A
         Assignment Notes: N/A

11. Security Considerations

GRE-in-UDP encapsulation does not affect security for the payload protocol. The security considerations for GRE apply to GRE-in-UDP; see [RFC2784].

To secure traffic carried by a GRE-in-UDP tunnel, DTLS SHOULD be used as specified in Section 5.

In the case that UDP source port for entropy usage is disabled, a random port taken from the ephemeral port range 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 DoS attacks as mentioned in Section 3.2.1.

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

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

Corruption of GRE headers can cause security concerns for applications that rely on the GRE Key field for traffic separation or segregation. When the GRE Key field is used for this purpose, such as an application of a Network Virtualization Using Generic Routing Encapsulation (NVGRE) [RFC7637], GRE header corruption is a concern. In such situations, at least one of the UDP and GRE checksums MUST be used for both IPv4 and IPv6 GRE-in-UDP tunnels.

12. References

12.1. Normative References

   [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <http://www.rfc-editor.org/info/rfc768>.
   
   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <http://www.rfc-editor.org/info/rfc1122>.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <http://www.rfc-editor.org/info/rfc2474>.
   
   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <http://www.rfc-editor.org/info/rfc2784>.
   
   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <http://www.rfc-editor.org/info/rfc2890>.
   
   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <http://www.rfc-editor.org/info/rfc6040>.
   
   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.
   
   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <http://www.rfc-editor.org/info/rfc6438>.
   
   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <http://www.rfc-editor.org/info/rfc6935>.
   
   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <http://www.rfc-editor.org/info/rfc6936>.
   
   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <http://www.rfc-editor.org/info/rfc8085>.

12.2. Informative References

   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <http://www.rfc-editor.org/info/rfc792>.
   
   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.
   
   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.
   
   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <http://www.rfc-editor.org/info/rfc2914>.
   
   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <http://www.rfc-editor.org/info/rfc2983>.
   
   [RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <http://www.rfc-editor.org/info/rfc4787>.
   
   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,
              <http://www.rfc-editor.org/info/rfc6056>.
   
   [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042,
              October 2013, <http://www.rfc-editor.org/info/rfc7042>.
   
   [RFC7637]  Garg, P., Ed., and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,
              <http://www.rfc-editor.org/info/rfc7637>.
   
   [RFC7676]  Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
              for Generic Routing Encapsulation (GRE)", RFC 7676,
              DOI 10.17487/RFC7676, October 2015,
              <http://www.rfc-editor.org/info/rfc7676>.

Acknowledgements

The authors would like to thank Vivek Kumar, Ron Bonica, Joe Touch, Ruediger Geib, Lars Eggert, Lloyd Wood, Bob Briscoe, Rick Casarez, Jouni Korhonen, Kathleen Moriarty, Ben Campbell, and many others for their reviews and valuable input on this document.

Thanks to Donald Eastlake, Eliot Lear, Martin Stiemerling, and Spencer Dawkins for their detailed reviews and valuable suggestions during WG Last Call and the IESG process.

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

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

Contributors

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

   David Black
   EMC Corporation
   176 South Street
   Hopkinton, MA  01748
   United States of America
   
   Email: david.black@emc.com
   
   Ross Callon
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States of America

Email: 

          rcallon@juniper.net

John E. Drake
Juniper Networks

Email: 

          jdrake@juniper.net

Gorry Fairhurst
University of Aberdeen 

   Email: gorry@erg.abdn.ac.uk

Yongbing Fan
China Telecom
Guangzhou
China 

   Email: fanyb@gsta.com
   Phone: +86 20 38639121

Adrian Farrel
Juniper Networks 

   Email: adrian@olddog.co.uk
   
   Vishwas Manral

Email: 

          vishwas@ionosnetworks.com
   
   Carlos Pignataro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   United States of America

Email: 

          cpignata@cisco.com

Authors' Addresses

Lucy Yong
Huawei Technologies, USA 

   Email: lucy.yong@huawei.com

Edward Crabbe
Oracle 

   Email: edward.crabbe@gmail.com

Xiaohu Xu
Huawei Technologies
Beijing, China

Email: 

          xuxiaohu@huawei.com

Tom Herbert
Facebook
1 Hacker Way
Menlo Park, CA

Email: 

          tom@herbertland.com