Internet Area WG                                              T. Herbert
Internet-Draft                                                Quantonium
Intended status: Standard track                                  L. Yong
Expires November 19, June 30, 2017                                         Huawei USA
                                                                  O. Zia
                                                            May 18,
                                                       December 30, 2017

                       Generic UDP Encapsulation

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   This specification describes Generic UDP Encapsulation (GUE), which
   is a scheme for using UDP to encapsulate packets of different IP
   protocols for transport across layer 3 networks. By encapsulating
   packets in UDP, specialized capabilities in networking hardware for
   efficient handling of UDP packets can be leveraged. GUE specifies
   basic encapsulation methods upon which higher level constructs, such
   as tunnels and overlay networks for network virtualization, can be
   constructed. GUE is extensible by allowing optional data fields as
   part of the encapsulation, and is generic in that it can encapsulate
   packets of various IP protocols.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1. Terminology and acronyms  . . . . . . . . . . . . . . . . .  5
     1.2.  Requirements Language  . . . . . . . . . . . . . . . . . .  6
   2. Base packet format  . . . . . . . . . . . . . . . . . . . . . .  7
     2.1. GUE version variant . . . . . . . . . . . . . . . . . . . . . . . .  7
   3. Version Variant 0 . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1. Header format . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2. Proto/ctype field . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1 Proto field  . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.2 Ctype field  . . . . . . . . . . . . . . . . . . . . . . 10
     3.3. Flags and extension fields  . . . . . . . . . . . . . . . . 10
       3.3.1. Requirements  . . . . . . . . . . . . . . . . . . . . . 10
       3.3.2. Example GUE header with extension fields  . . . . . . . 11
     3.4. Private data  . . . . . . . . . . . . . . . . . . . . . . . 12
     3.5. Message types . . . . . . . . . . . . . . . . . . . . . . . 12
       3.5.1. Control messages  . . . . . . . . . . . . . . . . . . . 12
       3.5.2. Data messages . . . . . . . . . . . . . . . . . . . . . 13
     3.6. Hiding the transport layer protocol number  . . . . . . . . 13
   4. Version Variant 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1. Direct encapsulation of IPv4  . . . . . . . . . . . . . . . 14
     4.2. Direct encapsulation of IPv6  . . . . . . . . . . . . . . . 14 15
   5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1. Network tunnel encapsulation  . . . . . . . . . . . . . . . 16
     5.2. Transport layer encapsulation . . . . . . . . . . . . . . . 16
     5.3. Encapsulator operation  . . . . . . . . . . . . . . . . . . 16
     5.4. Decapsulator operation  . . . . . . . . . . . . . . . . . . 17
       5.4.1. Processing a received data message  . . . . . . . . . . 17
       5.4.2. Processing a received control message . . . . . . . . . 18
     5.5. Router and switch operation . . . . . . . . . . . . . . . . 18
     5.6. Middlebox interactions  . . . . . . . . . . . . . . . . . . 18
       5.6.1. Inferring connection semantics  . . . . . . . . . . . . 19
       5.6.2. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     5.7. Checksum Handling . . . . . . . . . . . . . . . . . . . . . 19
       5.7.1. Requirements  . . . . . . . . . . . . . . . . . . . . . 19
       5.7.2. UDP Checksum with IPv4  . . . . . . . . . . . . . . . . 20
       5.7.3. UDP Checksum with IPv6  . . . . . . . . . . . . . . . . 20
     5.8. MTU and fragmentation . . . . . . . . . . . . . . . . . . . 21
     5.9. Congestion control  . . . . . . . . . . . . . . . . . . . . 21
     5.10. Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 22
     5.11. Flow entropy for ECMP  . . . . . . . . . . . . . . . . . . 22
       5.11.1. Flow classification  . . . . . . . . . . . . . . . . . 22
       5.11.2. Flow entropy properties  . . . . . . . . . . . . . . . 23
     5.12 Negotiation of acceptable flags and extension fields  . . . 24
   6. Motivation for GUE  . . . . . . . . . . . . . . . . . . . . . . 24
     6.1. Benefits of GUE . . . . . . . . . . . . . . . . . . . . . . 24
     6.2 Comparison of GUE to other encapsulations  . . . . . . . . . 25
   7. Security Considerations . . . . . . . . . . . . . . . . . . . . 26
   8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 27
     8.1. UDP source port . . . . . . . . . . . . . . . . . . . . . . 27
     8.2. GUE version variant number  . . . . . . . . . . . . . . . . . . . . 28
     8.3. Control types . . . . . . . . . . . . . . . . . . . . . . . 28
     8.4. Flag-fields . . . . . . . . . . . . . . . . . . . . . . . . 28
   9. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 28 29
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 29 30
   Appendix A: NIC processing for GUE . . . . . . . . . . . . . . . . 32
     A.1. Receive multi-queue . . . . . . . . . . . . . . . . . . . . 32
     A.2. Checksum offload  . . . . . . . . . . . . . . . . . . . . . 33
       A.2.1. Transmit checksum offload . . . . . . . . . . . . . . . 33
       A.2.2. Receive checksum offload  . . . . . . . . . . . . . . . 34
     A.3. Transmit Segmentation Offload . . . . . . . . . . . . . . . 34
     A.4. Large Receive Offload . . . . . . . . . . . . . . . . . . . 35
   Appendix B: Implementation considerations  . . . . . . . . . . . . 36
     B.1. Priveleged ports  . . . . . . . . . . . . . . . . . . . . . 36
     B.2. Setting flow entropy as a route selector  . . . . . . . . . 36
     B.3. Hardware protocol implementation considerations . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37

1. Introduction

   This specification describes Generic UDP Encapsulation (GUE) which is
   a general method for encapsulating packets of arbitrary IP protocols
   within User Datagram Protocol (UDP) [RFC0768] packets. Encapsulating
   packets in UDP facilitates efficient transport across networks.
   Networking devices widely provide protocol specific processing and
   optimizations for UDP (as well as TCP) packets. Packets for atypical
   IP protocols (those not usually parsed by networking hardware) can be
   encapsulated in UDP packets to maximize deliverability and to
   leverage flow specific mechanisms for routing and packet steering.

   GUE provides an extensible header format for including optional data
   in the encapsulation header. This data potentially covers items such
   as the virtual networking identifier, security data for validating or
   authenticating the GUE header, congestion control data, etc. GUE also
   allows private optional data in the encapsulation header. This
   feature can be used by a site or implementation to define local
   custom optional data, and allows experimentation of options that may
   eventually become standard.

   This document does not define any specific GUE extensions.
   specifies a set of core extensions.

   The motivation for the GUE protocol is described in section 6.

1.1. Terminology and acronyms

   GUE              Generic UDP Encapsulation

   GUE Header       A variable length protocol header that is composed
                    of a primary four byte header and zero or more four
                    byte words for optional header data

   GUE packet       A UDP/IP packet that contains a GUE header and GUE
                    payload within the UDP payload

   GUE variant      A version of the GUE protocol or an alternate form
                    of a version

   Encapsulator     A network node that encapsulates a packet packets in GUE

   Decapsulator     A network node that decapsulates and processes
                    packets encapsulated in GUE

   Data message     An encapsulated packet in the GUE payload that is
                    addressed to the protocol stack for an associated

   Control message  A formatted message in the GUE payload that is
                    implicitly addressed to the decapsulator to monitor
                    or control the state or behavior of a tunnel

   Flags            A set of bit flags in the primary GUE header

   Extension field
                    An optional field in a GUE header whose presence is
                    indicated by corresponding flag(s)

   C-bit            A single bit flag in the primary GUE header that
                    indicates whether the GUE packet contains a control
                    message or data message

   Hlen             A field in the primary GUE header that gives the
                    length of the GUE header

   Proto/ctype      A field in the GUE header that holds either the IP
                    protocol number for a data message or a type for a
                    control message

   Private data     Optional data in the GUE header that can be used for
                    private purposes

   Outer IP header  Refers to the outer most IP header or packet when
                    encapsulating a packet over IP

   Inner IP header  Refers to an encapsulated IP header when an IP
                    packet is encapsulated

   Outer packet     Refers to an encapsulating packet

   Inner packet     Refers to a packet that is encapsulated

1.2.  Requirements Language

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

2. Base packet format

   A GUE packet is comprised of a UDP packet whose payload is a GUE
   header followed by a payload which is either an encapsulated packet
   of some IP protocol or a control message such as an OAM (Operations,
   Administration, and Management) message. A GUE packet has the general

   |                               |
   |        UDP/IP header          |
   |                               |
   |                               |
   |         GUE Header            |
   |                               |
   |                               |
   |      Encapsulated packet      |
   |      or control message       |
   |                               |

   The GUE header is variable length as determined by the presence of
   optional extension fields.

2.1. GUE version variant

   The first two bits of the GUE header contain the GUE protocol version variant
   number. The rest of the fields after the GUE version variant number are
   defined based on can indicate the version number. Versions of the GUE
   protocol as well as alternate forms of a version.

   Variants 0 and 1 are described in this specification; versions variants 2 and
   3 are reserved.

3. Version Variant 0


   Variant 0 indicates version 0 of GUE GUE. This variant defines a generic
   extensible format to encapsulate packets by Internet protocol number.

3.1. Header format

   The header format for version variant 0 of GUE in UDP is:

    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
   |        Source port            |      Destination port         | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
   |           Length              |          Checksum             | |
   | 0 |C|   Hlen  |  Proto/ctype  |             Flags             |
   |                                                               |
   ~                  Extensions Fields (optional)                 ~
   |                                                               |
   |                                                               |
   ~                    Private data (optional)                    ~
   |                                                               |

   The contents of the UDP header are:

      o Source port: If connection semantics (section 5.6.1) are applied
        to an encapsulation, this is set to the local source port for
        the connection. When connection semantics are not applied, this
        is set to a flow entropy value for use with ECMP (Equal-Cost
        Mulit-Path [RFC2992]); the properties of flow entropy are
        described in section 5.11.

      o Destination port: If connection semantics (section 5.6.1) are
        applied to an encapsulation, this is set to the destination port
        for the tuple. If connection semantics are not applied this is
        set to the GUE assigned port number, 6080.

      o Length: Canonical length of the UDP packet (length of UDP header
        and payload).

      o Checksum: Standard UDP checksum (handling is described in
        section 5.7).

   The GUE header consists of:

      o Ver: Variant: 0 indicates GUE protocol version (0). 0 with a header.

      o C: C-bit: When set indicates a control message, not set
        indicates a data message.

      o Hlen: Length in 32-bit words of the GUE header, including
        optional extension fields but not the first four bytes of the
        header. Computed as (header_len - 4) / 4 4, where header_len is
        the total header length in bytes. All GUE headers are a multiple
        of four bytes in length. Maximum header length is 128 bytes.

      o Proto/ctype: When the C-bit is set, this field contains a
        control message type for the payload (section 3.2.2). When the
        C-bit is not set, the field holds the Internet protocol number
        for the encapsulated packet in the payload (section 3.2.1). The
        control message or encapsulated packet begins at the offset
        provided by Hlen.

      o Flags: Header flags that may be allocated for various purposes
        and may indicate presence of extension fields. Undefined header
        flag bits MUST be set to zero on transmission.

      o Extension Fields: Optional fields whose presence is indicated by
        corresponding flags.

      o Private data: Optional private data block (see section 3.4). If
        the private block is present, it immediately follows that last
        extension field present in the header. The private block is
        considered to be part of the GUE header. The length of this data
        is determined by subtracting the starting offset from the header

3.2. Proto/ctype field

   The proto/ctype fields either contains an Internet protocol number
   (when the C-bit is not set) or GUE control message type (when the C-
   bit is set).

3.2.1 Proto field

   When the C-bit is not set, the proto/ctype field MUST contain an IANA
   Internet Protocol Number. The protocol number is interpreted relative
   to the IP protocol that encapsulates the UDP packet (i.e. protocol of
   the outer IP header). The protocol number serves as an indication of
   the type of the next protocol header which is contained in the GUE
   payload at the offset indicated in Hlen. Intermediate devices MAY
   parse the GUE payload per the number in the proto/ctype field, and
   header flags cannot affect the interpretation of the proto/ctype

   When the outer IP protocol is IPv4, the proto field MUST be set to a
   valid IP protocol number usable with IPv4; it MUST NOT be set to a
   number for IPv6 extension headers or ICMPv6 options (number 58). An
   exception is that the destination options extension header using the
   PadN option MAY be used with IPv4 as described in section 3.6. The
   "no next header" protocol number (59) also MAY be used with IPv4 as
   described below.

   When the outer IP protocol is IPv6, the proto field can be set to any
   defined protocol number except that it MUST NOT be set to Hop-by-hop
   options (number 0). If a received GUE packet in IPv6 contains a
   protocol number that is an extension header (e.g. Destination
   Options) then the extension header is processed after the GUE header
   is processed as though the GUE header is an extension header.

   IP protocol number 59 ("No next header") can be set to indicate that
   the GUE payload does not begin with the header of an IP protocol.
   This would be the case, for instance, if the GUE payload were a
   fragment when performing GUE level fragmentation. The interpretation
   of the payload is performed through other means (such as flags and
   extension fields), and intermediate devices MUST NOT parse packets
   based on the IP protocol number in this case.

3.2.2 Ctype field

   When the C-bit is set, the proto/ctype field MUST be set to a valid
   control message type. A value of zero indicates that the GUE payload
   requires further interpretation to deduce the control type. This
   might be the case when the payload is a fragment of a control
   message, where only the reassembled packet can be interpreted as a
   control message.

   Control messages will be defined in an IANA registry. Control message
   types 1 through 127 may be defined in standards. Types 128 through
   255 are reserved to be user defined for experimentation or private
   control messages.

   This document does not specify any standard control message types
   other than type 0.

3.3. Flags and extension fields

   Flags and associated extension fields are the primary mechanism of
   extensibility in GUE. As mentioned in section 3.1, GUE header flags
   indicate the presence of optional extension fields in the GUE header.
   [GUEXTENS] defines a basic set of GUE extensions.

3.3.1. Requirements

   There are sixteen flag bits in the GUE header. Flags may indicate
   presence of an extension fields. The size of an extension field
   indicated by a flag MUST be fixed.

   Flags can be paired together to allow different lengths for an
   extension field. For example, if two flag bits are paired, a field
   can possibly be three different lengths-- that is bit value of 00
   indicates no field present; 01, 10, and 11 indicate three possible
   lengths for the field. Regardless of how flag bits are paired, the
   lengths and offsets of optional fields corresponding to a set of
   flags MUST be well defined.

   Extension fields are placed in order of the flags. New flags are to
   be allocated from high to low order bit contiguously without holes.
   Flags allow random access, for instance to inspect the field
   corresponding to the Nth flag bit, an implementation only considers
   the previous N-1 flags to determine the offset. Flags after the Nth
   flag are not pertinent in calculating the offset of the Nth flag.
   Random access of flags and fields permits processing of optional
   extensions in an order that is independent of their position in the
   packet. The processing order of extensions defined in [GUEEXTENS] [GUEEXTEN]
   demonstrates this property.

   Flags (or paired flags) are idempotent such that new flags MUST NOT
   cause reinterpretation of old flags. Also, new flags MUST NOT alter
   interpretation of other elements in the GUE header nor how the
   message is parsed (for instance, in a data message the proto/ctype
   field always holds an IP protocol number as an invariant).

   The set of available flags can be extended in the future by defining
   a "flag extensions bit" that refers to a field containing a new set
   of flags.

3.3.2. Example GUE header with extension fields

   An example GUE header for a data message encapsulating an IPv4 packet
   and containing the Group Identifier and Security extension fields
   (both defined in [GUEXTENS]) is shown below:

    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
   | 0 |0|    3    |      94       |1|0 0 1|          0            |
   |                        Group Identifier                       |
   |                                                               |
   +                           Security                            +
   |                                                               |
   In the above example, the first flag bit is set which indicates that
   the Group Identifier extension is present which is a 32 bit field.
   The second through fourth bits of the flags are paired flags that
   indicate the presence of a Security field with seven possible sizes.
   In this example 001 indicates a sixty-four bit security field.

3.4. Private data

   An implementation MAY use private data for its own use. The private
   data immediately follows the last field in the GUE header and is not
   a fixed length. This data is considered part of the GUE header and
   MUST be accounted for in header length (Hlen). The length of the
   private data MUST be a multiple of four and is determined by
   subtracting the offset of private data in the GUE header from the
   header length. Specifically:

      Private_length = (Hlen * 4) - Length(flags)

   where "Length(flags)" returns the sum of lengths of all the extension
   fields present in the GUE header. When there is no private data
   present, the length of the private data is zero.

   The semantics and interpretation of private data are implementation
   specific. The private data may be structured as necessary, for
   instance it might contain its own set of flags and extension fields.

   An encapsulator and decapsulator MUST agree on the meaning of private
   data before using it. The mechanism to achieve this agreement is
   outside the scope of this document but could include implementation-
   defined behavior, coordinated configuration, in-band communication
   using GUE control messages, or out-of-band messages.

   If a decapsulator receives a GUE packet with private data, it MUST
   validate the private data appropriately. If a decapsulator does not
   expect private data from an encapsulator, the packet MUST be dropped.
   If a decapsulator cannot validate the contents of private data per
   the provided semantics, the packet MUST also be dropped. An
   implementation MAY place security data in GUE private data which if
   present MUST be verified for packet acceptance.

3.5. Message types

3.5.1. Control messages

   Control messages carry formatted data that are implicitly addressed
   to the decapsulator to monitor or control the state or behavior of a
   tunnel (OAM). For instance, an echo request and corresponding echo
   reply message can be defined to test for liveness.

   Control messages are indicated in the GUE header when the C-bit is
   set. The payload is interpreted as a control message with type
   specified in the proto/ctype field. The format and contents of the
   control message are indicated by the type and can be variable length.

   Other than interpreting the proto/ctype field as a control message
   type, the meaning and semantics of the rest of the elements in the
   GUE header are the same as that of data messages. Forwarding and
   routing of control messages should be the same as that of a data
   message with the same outer IP and UDP header and GUE flags; this
   ensures that control messages can be created that follow the same
   path as data messages.

3.5.2. Data messages

   Data messages carry encapsulated packets that are addressed to the
   protocol stack for the associated protocol. Data messages are a
   primary means of encapsulation and can be used to create tunnels for
   overlay networks.

   Data messages are indicated in GUE header when the C-bit is not set.
   The payload of a data message is interpreted as an encapsulated
   packet of an Internet protocol indicated in the proto/ctype field.
   The packet immediately follows the GUE header.

3.6. Hiding the transport layer protocol number

   The GUE header indicates the Internet protocol of the encapsulated
   packet. A protocol number is either contained in the Proto/ctype
   field of the primary GUE header or in the Payload Type field of a GUE
   Transform extension field (used to encrypt the payload with DTLS,
   [GUEEXTEN]). If the transport protocol number needs to be hidden from
   the network, then a trivial destination options can be used.

   The PadN destination option [RFC2460] can be used to encode the
   transport protocol as a next header of an extension header (and
   maintain alignment of encapsulated transport headers). The
   Proto/ctype field or Payload Type field of the GUE Transform field is
   set to 60 to indicate that the first encapsulated header is a
   destination options extension header.

   The format of the extension header is below:

      | Next Header |    2      |     1     |      0    |

   For IPv4, it is permitted in GUE to used this precise destination
   option to contain the obfuscated protocol number. In this case next
   header MUST refer to a valid IP protocol for IPv4. No other extension
   headers or destination options are permitted with IPv4.

4. Version Variant 1


   Variant 1 of GUE allows direct encapsulation of IPv4 and IPv6 in UDP.
   In this version varinant there is no GUE header; a UDP packet carries an IP
   packet. The first two bits of the UDP payload for GUE are the GUE
   variant and coincide with the first two bits of the version number in
   the IP header. The first two version bits of IPv4 and IPv6 are 01, so
   we use GUE version variant 1 for direct IP encapsulation which makes two bits
   of GUE version variant to also be 01.

   This technique is effectively a means to compress out the version 0
   GUE header when encapsulating IPv4 or IPv6 packets and there are no
   flags or extension fields present. This method is compatible to use
   on the same port number as packets with the GUE header (GUE version variant 0
   packets). This technique saves encapsulation overhead on costly links
   for the common use of IP encapsulation, and also obviates the need to
   allocate a separate port number for IP-over-UDP encapsulation.

4.1. Direct encapsulation of IPv4

   The format for encapsulating IPv4 directly in UDP is:

    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
   |        Source port            |      Destination port         | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
   |           Length              |          Checksum             | |
   |0|1|0|0|  IHL  |Type of Service|          Total Length         |
   |         Identification        |Flags|      Fragment Offset    |
   |  Time to Live |   Protocol    |   Header Checksum             |
   |                       Source IPv4 Address                     |
   |                     Destination IPv4 Address                  |

   Note that the 0100 value IP version field express in the first four bits of the the UDP
   payload expresses the GUE version variant as 1 (bits 01) and IP version as 4
   (bits 0100).

4.2. Direct encapsulation of IPv6

   The format for encapsulating IPv6 directly in UDP is demonstrated

    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
   |        Source port            |      Destination port         | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
   |           Length              |          Checksum             | |
   |0|1|1|0| Traffic Class |           Flow Label                  |
   |         Payload Length        |     NextHdr   |   Hop Limit   |
   |                                                               |
   +                                                               +
   |                                                               |
   +                        Source IPv6 Address                    +
   |                                                               |
   +                                                               +
   |                                                               |
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination IPv6 Address                 +
   |                                                               |
   +                                                               +
   |                                                               |

   Note that the 0110 value IP version field in the first four bits of the the UDP
   payload expresses the GUE version variant as 1 (bits 01) and IP version as 6
   (bits 0110).

5. Operation

   The figure below illustrates the use of GUE encapsulation between two
   hosts. Host 1 is sending packets to Host 2. An encapsulator performs
   encapsulation of packets from Host 1. These encapsulated packets
   traverse the network as UDP packets. At the decapsulator, packets are
   decapsulated and sent on to Host 2. Packet flow in the reverse
   direction need not be symmetric; GUE encapsulation is not required in
   the reverse path.

   +---------------+                       +---------------+
   |               |                       |               |
   |    Host 1     |                       |     Host 2    |
   |               |                       |               |
   +---------------+                       +---------------+
          |                                        ^
          V                                        |
   +---------------+   +---------------+   +---------------+
   |               |   |               |   |               |
   | Encapsulator  |-->|    Layer 3    |-->| Decapsulator  |
   |               |   |    Network    |   |               |
   +---------------+   +---------------+   +---------------+

   The encapsulator and decapsulator may be co-resident with the
   corresponding hosts, or may be on separate nodes in the network.

5.1. Network tunnel encapsulation

   Network tunneling can be achieved by encapsulating layer 2 or layer 3
   packets. In this case the encapsulator and decapsulator nodes are the
   tunnel endpoints. These could be routers that provide network tunnels
   on behalf of communicating hosts.

5.2. Transport layer encapsulation

   When encapsulating layer 4 packets, the encapsulator and decapsulator
   should be co-resident with the hosts. In this case, the encapsulation
   headers are inserted between the IP header and the transport packet.
   The addresses in the IP header refer to both the endpoints of the
   encapsulation and the endpoints for terminating the transport
   protocol. Note that the transport layer ports in the encapsulated
   packet are independent of the UDP ports in the outer packet.

   Details about performing transport layer encapsulation are discussed
   in [TOU].

5.3. Encapsulator operation

   Encapsulators create GUE data messages, set the fields of the UDP
   header, set flags and optional extension fields in the GUE header,
   and forward packets to a decapsulator.

   An encapsulator can be an end host originating the packets of a flow,
   or can be a network device performing encapsulation on behalf of
   hosts (routers implementing tunnels for instance). In either case,
   the intended target (decapsulator) is indicated by the outer
   destination IP address and destination port in the UDP header.

   If an encapsulator is tunneling packets -- that is encapsulating
   packets of layer 2 or layer 3 protocols (e.g. EtherIP, IPIP, ESP
   tunnel mode) -- it SHOULD follow standard conventions for tunneling
   of one protocol over another. For instance, if an IP packet is being
   encapsualated in GUE then diffserv interaction [RFC2983] and ECN
   propagation for tunnels [RFC6040] SHOULD be followed.

5.4. Decapsulator operation

   A decapsulator performs decapsulation of GUE packets. A decapsulator
   is addressed by the outer destination IP address of a GUE packet.
   The decapsulator validates packets, including fields of the GUE

   If a decapsulator receives a GUE packet with an unsupported version, variant,
   unknown flag, bad header length (too small for included extension
   fields), unknown control message type, bad protocol number, an
   unsupported payload type, or an otherwise malformed header, it MUST
   drop the packet. Such events MAY be logged subject to configuration
   and rate limiting of logging messages. No error message is returned
   back to the encapsulator. Note that set flags in a GUE header that
   are unknown to a decapsulator MUST NOT be ignored. If a GUE packet is
   received by a decapsulator with unknown flags, the packet MUST be

5.4.1. Processing a received data message

   If a valid data message is received, the UDP header and GUE header
   are removed from the packet. The outer IP header remains intact and
   the next protocol in the IP header is set to the protocol from the
   proto field in the GUE header. The resulting packet is then
   resubmitted into the protocol stack to process that packet as though
   it was received with the protocol in the GUE header.

   As an example, consider that a data message is received where GUE
   encapsulates an IP packet. In this case proto field in the GUE header
   is set 94 for IPIP:

   |   IP header (next proto = 17,UDP)   |
   |                  UDP                |
   |         GUE (proto = 94,IPIP)       |
   |         IP header and packet        |
   The receiver removes the UDP and GUE headers and sets the next
   protocol field in the IP packet to IPIP, which is derived from the
   GUE proto field. The resultant packet would have the format:

   |   IP header (next proto = 94,IPIP)  |
   |         IP header and packet        |

   This packet is then resubmitted into the protocol stack to be
   processed as an IPIP packet.

5.4.2. Processing a received control message

   If a valid control message is received, the packet MUST be processed
   as a control message. The specific processing to be performed depends
   on the ctype value in the ctype field of the GUE header.

5.5. Router and switch operation

   Routers and switches SHOULD forward GUE packets as standard UDP/IP
   packets. The outer five-tuple should contain sufficient information
   to perform flow classification corresponding to the flow of the inner
   packet. A router does not normally need to parse a GUE header, and
   none of the flags or extension fields in the GUE header are expected
   to affect routing. In cases where the outer five-tuple does not
   provide sufficient entropy for flow classification, for instance UDP
   ports are fixed to provide connection semantics (section 5.6.1), then
   the encapsulated packet MAY be parsed to determine flow entropy.

   A router MUST NOT modify a GUE header when forwarding a packet. It
   MAY encapsulate a GUE packet in another GUE packet, for instance to
   implement a network tunnel (i.e. by encapsulating an IP packet with a
   GUE payload in another IP packet as a GUE payload). In this case, the
   router takes the role of an encapsulator, and the corresponding
   decapsulator is the logical endpoint of the tunnel. When
   encapsulating a GUE packet within another GUE packet, there are no
   provisions to automatically GUE copy flags or fields to the outer GUE
   header. Each layer of encapsulation is considered independent.

5.6. Middlebox interactions

   A middle box MAY interpret some flags and extension fields of the GUE
   header for classification purposes, but is not required to understand
   any of the flags or extension fields in GUE packets. A middle box
   MUST NOT drop a GUE packet merely because there are flags unknown to
   it. The header length in the GUE header allows a middlebox to inspect
   the payload packet without needing to parse the flags or extension

5.6.1. Inferring connection semantics

   A middlebox might infer bidirectional connection semantics for a UDP
   flow. For instance, a stateful firewall might create a five-tuple
   rule to match flows on egress, and a corresponding five-tuple rule
   for matching ingress packets where the roles of source and
   destination are reversed for the IP addresses and UDP port numbers.
   To operate in this environment, a GUE tunnel should be configured to
   assume connected semantics defined by the UDP five tuple and the use
   of GUE encapsulation needs to be symmetric between both endpoints.
   The source port set in the UDP header MUST be the destination port
   the peer would set for replies. In this case the UDP source port for
   a tunnel would be a fixed value and not set to be flow entropy as
   described in section 5.11.

   The selection of whether to make the UDP source port fixed or set to
   a flow entropy value for each packet sent SHOULD be configurable for
   a tunnel. The default MUST be to set the flow entropy value in the
   UDP source port.

5.6.2. NAT

   IP address and port translation can be performed on the UDP/IP
   headers adhering to the requirements for NAT with UDP [RFC4787]. In
   the case of stateful NAT, connection semantics MUST be applied to a
   GUE tunnel as described in section 5.6.1. GUE endpoints MAY also
   invoke STUN [RFC5389] or ICE [RFC5245] to manage NAT port mappings
   for encapsulations.

5.7. Checksum Handling

   The potential for mis-delivery of packets due to corruption of IP,
   UDP, or GUE headers needs to be considered. Historically, the UDP
   checksum would be considered sufficient as a check against corruption
   of either the UDP header and payload or the IP addresses.
   Encapsulation protocols, such as GUE, can be originated or terminated
   on devices incapable of computing the UDP checksum for packet. This
   section discusses the requirements around checksum and alternatives
   that might be used when an endpoint does not support UDP checksum.

5.7.1. Requirements

   One of the following requirements MUST be met:

  o UDP checksums are enabled (for IPv4 or IPv6).

  o The GUE header checksum is used (defined in [GUEEXTENS]). [GUEEXTEN]).

  o Use zero UDP checksums. This is always permissible with IPv4; in
    IPv6, they can only be used in accordance with applicable
    requirements in [RFC8086], [RFC6935], and [RFC6936].

5.7.2. UDP Checksum with IPv4

    For UDP in IPv4, the UDP checksum MUST be processed as specified in
    [RFC768] and [RFC1122] for both transmit and receive. An
    encapsulator MAY set the UDP checksum to zero for performance or
    implementation considerations. The IPv4 header includes a checksum
    that protects against mis-delivery of the packet due to corruption
    of IP addresses. The UDP checksum potentially provides protection
    against corruption of the UDP header, GUE header, and GUE payload.
    Enabling or disabling the use of checksums is a deployment
    consideration that should take into account the risk and effects of
    packet corruption, and whether the packets in the network are
    already adequately protected by other, possibly stronger mechanisms
    such as the Ethernet CRC. If an encapsulator sets a zero UDP
    checksum for IPv4, it SHOULD use the GUE header checksum as
    described in [GUEEXTENS] [GUEEXTEN] assuming there are no other mechanisms used
    to protect the GUE packet.

    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].
    Configuration of zero checksums can be selective. For instance, zero
    checksums might be disallowed from certain hosts that are known to
    be traversing 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.

5.7.3. UDP Checksum with IPv6

    In IPv6, there is no checksum in the IPv6 header that protects
    against mis-delivery due to address corruption. Therefore, when GUE
    is used over IPv6, either the UDP checksum or the GUE header
    checksum SHOULD be used unless there are alternative mechanisms in
    use that protect against misdelivery. The UDP checksum and GUE
    header checksum SHOULD not NOT be used at the same time since that would
    be mostly redundant.

    If neither the UDP checksum or the GUE header checksum is used, then
    the requirements for using zero IPv6 UDP checksums in [RFC6935] and
    [RFC6936] MUST be met.

    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 MUST only accept UDP packets with a zero checksum if
    the GUE header checksum is used and is verified. 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 and no
    GUE header checksum was received, the packet MUST be dropped.

5.8. MTU and fragmentation

    Standard conventions for handling of MTU (Maximum Transmission Unit)
    and fragmentation in conjunction with networking tunnels
    (encapsulation of layer 2 or layer 3 packets) SHOULD be followed.
    Details are described in MTU and Fragmentation Issues with In-the-
    Network Tunneling [RFC4459].

    If a packet is fragmented before encapsulation in GUE, all the
    related fragments MUST be encapsulated using the same UDP source
    port. An operator SHOULD set MTU to account for encapsulation
    overhead and reduce the likelihood of fragmentation.

    Alternative to IP fragmentation, the GUE fragmentation extension can
    be used. GUE fragmentation is described in [GUEEXTENS]. [GUEEXTEN].

5.9. Congestion control

    Per requirements of [RFC5405], if the IP traffic encapsulated with
    GUE implements proper congestion control no additional mechanisms
    should be required.

    In the case that the encapsulated traffic does not implement any or
    sufficient control, or it is not known whether a transmitter will
    consistently implement proper congestion control, then congestion
    control at the encapsulation layer MUST be provided per [RFC5405].
    Note that this case applies to a significant use case in network
    virtualization in which guests run third party networking stacks
    that cannot be implicitly trusted to implement conformant congestion

    Out of band mechanisms such as rate limiting, Managed Circuit
    Breaker [CIRCBRK], [RFC8084], or traffic isolation MAY be used to provide
    rudimentary congestion control. For finer-grained congestion control
    that allows alternate congestion control algorithms, reaction time
    within an RTT, and interaction with ECN, in-band mechanisms might be

5.10. Multicast

    GUE packets can be multicast to decapsulators using a multicast
    destination address in the encapsulating IP headers. Each receiving
    host will decapsulate the packet independently following normal
    decapsulator operations. The receiving decapsulators need to agree
    on the same set of GUE parameters and properties; how such an
    agreement is reached is outside the scope of this document.

    GUE allows encapsulation of unicast, broadcast, or multicast
    traffic. Flow entropy (the value in the UDP source port) can 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 of the encapsulation and is otherwise outside the scope
    of this document.

5.11. Flow entropy for ECMP

5.11.1. Flow classification

    A major objective of using GUE is that a network device can perform
    flow classification corresponding to the flow of the inner
    encapsulated packet based on the contents in the outer headers.

    Hardware devices commonly perform hash computations on packet
    headers to classify packets into flows or flow buckets. Flow
    classification is done to support load balancing of flows across a
    set of networking resources. Examples of such load balancing
    techniques are Equal Cost Multipath routing (ECMP), port selection
    in Link Aggregation, and NIC device Receive Side Scaling (RSS).
    Hashes are usually either a three-tuple hash of IP protocol, source
    address, and destination address; or a five-tuple hash consisting of
    IP protocol, source address, destination address, source port, and
    destination port. Typically, networking hardware will compute five-
    tuple hashes for TCP and UDP, but only three-tuple hashes for other
    IP protocols. Since the five-tuple hash provides more granularity,
    load balancing can be finer-grained with better distribution. When a
    packet is encapsulated with GUE and connection semantics are not
    applied, the source port in the outer UDP packet is set to a flow
    entropy value that corresponds to the flow of the inner packet. When
    a device computes a five-tuple hash on the outer UDP/IP header of a
    GUE packet, the resultant value classifies the packet per its inner

    Examples of deriving flow entropy for encapsulation are:

      o If the encapsulated packet is a layer 4 packet, TCP/IPv4 for
        instance, the flow entropy could be based on the canonical five-
        tuple hash of the inner packet.

      o If the encapsulated packet is an AH transport mode packet with
        TCP as next header, the flow entropy could be a hash over a
        three-tuple: TCP protocol and TCP ports of the encapsulated

      o If a node is encrypting a packet using ESP tunnel mode and GUE
        encapsulation, the flow entropy could be based on the contents
        of the clear-text packet. For instance, a canonical five-tuple
        hash for a TCP/IP packet could be used.

   [RFC6438] discusses methods to compute and set flow entropy value for
   IPv6 flow labels. Such methods can also be used to create flow
   entropy values for GUE.

5.11.2. Flow entropy properties

   The flow entropy is the value set in the UDP source port of a GUE
   packet. Flow entropy in the UDP source port SHOULD adhere to the
   following properties:

      o The value set in the source port is within the ephemeral port
        range (49152 to 65535 [RFC6335]). Since the high order two bits
        of the port are set to one, this provides fourteen bits of
        entropy for the value.

      o The flow entropy has a uniform distribution across encapsulated

      o An encapsulator MAY occasionally change the flow entropy used
        for an inner flow per its discretion (for security, route
        selection, etc). To avoid thrashing or flapping the value, the
        flow entropy used for a flow SHOULD NOT change more than once
        every thirty seconds (or a configurable value).

      o Decapsulators, or any networking devices, SHOULD NOT attempt to
        interpret flow entropy as anything more than an opaque value.
        Neither should they attempt to reproduce the hash calculation
        used by an encapasulator in creating a flow entropy value. They
        MAY use the value to match further receive packets for steering
        decisions, but MUST NOT assume that the hash uniquely or
        permanently identifies a flow.

      o Input to the flow entropy calculation is not restricted to ports
        and addresses; input could include flow label from an IPv6
        packet, SPI from an ESP packet, or other flow related state in
        the encapsulator that is not necessarily conveyed in the packet.

      o The assignment function for flow entropy SHOULD be randomly
        seeded to mitigate denial of service attacks. The seed SHOULD be
        changed periodically.

5.12 Negotiation of acceptable flags and extension fields

   An encapsulator and decapsulator need to achieve agreement about GUE
   parameters that will be used in communications. Parameters include
   supported GUE
   version, variants, flags and extension fields that can be used,
   security algorithms and keys, supported protocols and control
   messages, etc. This document proposes different general methods to
   accomplish this, however the details of implementing these are
   considered out of scope.

   General methods for this are:

      o Configuration. The parameters used for a tunnel are configured
        at each endpoint.

      o Negotiation. A tunnel negotiation can be performed. This could
        be accomplished in-band of GUE using control messages or private

      o Via a control plane. Parameters for communicating with a tunnel
        endpoint can be set in a control plane protocol (such as that
        needed for nvo3). network virtualization).

      o Via security negotiation. Use of security typically implies a
        key exchange between endpoints. Other GUE parameters may be
        conveyed as part of that process.

6. Motivation for GUE

   This section presents the motivation for GUE with respect to other
   encapsulation methods.

6.1. Benefits of GUE

      * GUE is a generic encapsulation protocol. GUE can encapsulate
        protocols that are represented by an IP protocol number. This
        includes layer 2, layer 3, and layer 4 protocols.

      * GUE is an extensible encapsulation protocol. Standardized
        optional data such as security, virtual networking identifiers,
        fragmentation are being defined.

      * For extensilbity, GUE uses flag fields as opposed to TLVs as
        some other encapsulation protocols do. Flag fields are strictly
        ordered, allow random access, and are efficient in use of header

      * GUE allows private data to be sent as part of the encapsulation.
        This permits experimentation or customization in deployment.

      * GUE allows sending of control messages such as OAM using the
        same GUE header format (for routing purposes) as normal data

      * GUE maximizes deliverability of non-UDP and non-TCP protocols.

      * GUE provides a means for exposing per flow entropy for ECMP for
        atypical protocols such as SCTP, DCCP, ESP, etc.

6.2 Comparison of GUE to other encapsulations

   A number of different encapsulation techniques have been proposed for
   the encapsulation of one protocol over another. EtherIP [RFC3378]
   provides layer 2 tunneling of Ethernet frames over IP. GRE [RFC2784],
   MPLS [RFC4023], and L2TP [RFC2661] provide methods for tunneling
   layer 2 and layer 3 packets over IP. NVGRE [RFC7637] and VXLAN
   [RFC7348] are proposals for encapsulation of layer 2 packets for
   network virtualization. IPIP [RFC2003] and Generic packet tunneling
   in IPv6 [RFC2473] provide methods for tunneling IP packets over IP.

   Several proposals exist for encapsulating packets over UDP including
   ESP over UDP [RFC3948], TCP directly over UDP [TCPUDP], VXLAN
   [RFC7348], LISP [RFC6830] which encapsulates layer 3 packets,
   MPLS/UDP [RFC7510], GENEVE [GENEVE], and Generic UDP Encapsulation
   for IP Tunneling (GRE over UDP)[RFC8086]. Generic UDP tunneling [GUT]
   is a proposal similar to GUE in that it aims to tunnel packets of IP
   protocols over UDP.

   GUE has the following discriminating features:

      o UDP encapsulation leverages specialized network device
        processing for efficient transport. The semantics for using the
        UDP source port for flow entropy as input to ECMP are defined in
        section 5.11.

      o GUE permits encapsulation of arbitrary IP protocols, which
        includes layer 2 3, and 4 protocols.

      o Multiple protocols can be multiplexed over a single UDP port
        number. This is in contrast to techniques to encapsulate
        protocols over UDP using a protocol specific port number (such
        as ESP/UDP, GRE/UDP, SCTP/UDP). GUE provides a uniform and
        extensible mechanism for encapsulating all IP protocols in UDP
        with minimal overhead (four bytes of additional header).

      o GUE is extensible. New flags and extension fields can be

      o The GUE header includes a header length field. This allows a
        network node to inspect an encapsulated packet without needing
        to parse the full encapsulation header.

      o Private data in the encapsulation header allows local
        customization and experimentation while being compatible with
        processing in network nodes (routers and middleboxes).

      o GUE includes both data messages (encapsulation of packets) and
        control messages (such as OAM).

      o The flags-field model facilitates efficient implementation of
        extensibility in hardware. For instance instance, a TCAM can be use to
        parse a known set of N flags where the number of entries in the
        TCAM is 2^N. By comparison, the number of TCAM entries needed to
        parse a set of N arbitrarily ordered TLVS is approximately e*N!.

7. Security Considerations

   There are two important considerations of security with respect to

      o Authentication and integrity of the GUE header.

      o Authentication, integrity, and confidentiality of the GUE

   GUE security is provided by extensions for security defined in
   [GUEEXTEN]. These extensions include methods to authenticate the GUE
   header and encrypt the GUE payload.

   The GUE header can be authenticated using a security extension for an
   HMAC. Securing the GUE payload can be accomplished use of the GUE
   Payload Transform. This extension can be used to perform DTLS in the
   payload of a GUE packet to encrypt the payload.

   A hash function for computing flow entropy (section 5.11) SHOULD be
   randomly seeded to mitigate some possible denial service attacks.

8. IANA Considerations

8.1. UDP source port

   A user UDP port number assignment for GUE has been assigned:

          Service Name: gue
          Transport Protocol(s): UDP
          Assignee: Tom Herbert <>
          Contact: Tom Herbert <>
          Description: Generic UDP Encapsulation
          Reference: draft-herbert-gue
          Port Number: 6080
          Service Code: N/A
          Known Unauthorized Uses: N/A
          Assignment Notes: N/A

8.2. GUE version variant number

   IANA is requested to set up a registry for the GUE version variant number.
   The GUE version variant number is 2 bits containing four possible values.
   This document defines version 0 and 1. New values are assigned in
   accordance with RFC Required policy [RFC5226].


      | Version Variant number | Description    | Reference     |
      | 0              | GUE Version 0  | This document |
      |                | with header    |               |
      |                |                |               |
      | 1              | GUE Version 1 0  | This document |
      |                | with direct IP |               |
      |                | encapsulation  |               |
      |                |                |               |
      | 2..3           | Unassigned     |               |

8.3. Control types

   IANA is requested to set up a registry for the GUE control types.
   Control types are 8 bit values.  New values for control types 1-127
   are assigned in accordance with RFC Required policy [RFC5226].

      |  Control type  | Description      | Reference     |
      | 0              | Need further     | This document |
      |                |  interpretation  |               |
      |                |                  |               |
      | 1..127         | Unassigned       |               |
      |                |                  |               |
      | 128..255       | User defined     | This document |

8.4. Flag-fields

   IANA is requested to create a "GUE flag-fields" registry to allocate
   flags and extension fields used with GUE. This shall be a registry of
   bit assignments for flags, length of extension fields for
   corresponding flags, and descriptive strings. There are sixteen bits
   for primary GUE header flags (bit number 0-15). New values are
   assigned in accordance with RFC Required policy [RFC5226]. New flags
   should be allocated from high to low order bit contiguously without
   holes. [GUEXTENS] requests in an initial set of flag assignments.

9. Acknowledgements

   The authors would like to thank David Liu, Erik Nordmark, Fred
   Templin, Adrian Farrel, Bob Briscoe, and Murray Kucherawy for
   valuable input on this draft.

10. References

10.1. Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
              10.17487/RFC0768, August 1980, <http://www.rfc-

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, DOI
              10.17487/RFC1122, October 1989, <http://www.rfc-

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434, DOI
              10.17487/RFC2434, October 1998, <http://www.rfc-

   [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
              2983, DOI 10.17487/RFC2983, October 2000, <http://www.rfc-

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <>.

   [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-

   [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,

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
              2006, <>.

10.2. Informative References

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
              and G. Fairhurst, Ed., "The Lightweight User Datagram
              Protocol (UDP-Lite)", RFC 3828, July 2004,

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, August 2014, <http://www.rfc-

   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <>.

   [RFC7637]  Garg, P., Ed., and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation", RFC
              7637, DOI 10.17487/RFC7637, September 2015,

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510, DOI
              10.17487/RFC7510, April 2015, <http://www.rfc-

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, DOI
              10.17487/RFC4340, March 2006, <http://www.rfc-

   [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, <>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008, <http://www.rfc-

   [RFC5285]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, DOI
              10.17487/RFC5245, April 2010, <http://www.rfc-

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, DOI
              10.17487/RFC5405, November 2008, <http://www.rfc-

   [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,

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003, DOI
              10.17487/RFC2003, October 1996, <http://www.rfc-

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
              3948, DOI 10.17487/RFC3948, January 2005, <http://www.rfc-

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830, DOI
              10.17487/RFC6830, January 2013, <http://www.rfc-

   [RFC3378]  Housley, R. and S. Hollenbeck, "EtherIP: Tunneling
              Ethernet Frames in IP Datagrams", RFC 3378, DOI
              10.17487/RFC3378, September 2002, <http://www.rfc-

   [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-

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, DOI 10.17487/RFC2661, August 1999,


   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers", BCP
              208, RFC 8084, DOI 10.17487/RFC8084, March 2017,

   [GUEEXTEN] Herbert, T., Yong, L., and Templin, F., "Extensions for
              Generic UDP Encapsulation" draft-herbert-gue-extensions-00

   [GUE4NVO3] Yong, L., Herbert, T., Zia, O., "Generic UDP Encapsulation
              (GUE) for Network Virtualization Overlay"
              draft-hy-nvo3-gue-4-nvo-03 draft-hy-nvo3-

   [GUESEC]   Yong, L., Herbert, T., "Generic UDP Encapsulation (GUE)
              for Secure Transport" draft-hy-gue-4-secure-transport-03

   [TCPUDP]   Chesire, S., Graessley, J., and McGuire, R.,
              "Encapsulation of TCP and other Transport Protocols over
              UDP" draft-cheshire-tcp-over-udp-00

   [TOU]      Herbert, T., "Transport layer protocols over UDP" draft-

   [GENEVE]   Gross, J., Ed., Ganga, I. Ed., and Sridhar, T., "Geneve:
              Generic Network Virtualization Encapsulation", draft-ietf-

   [GUT]      Manner, J., Varia, N., and Briscoe, B., "Generic UDP
              Tunnelling (GUT) draft-manner-tsvwg-gut-02.txt"

   [CIRCBRK]  Fairhurst, G., "Network Transport Circuit Breakers",

   [LCO]      Cree, E.,

Appendix A: NIC processing for GUE

   This appendix provides some guidelines for Network Interface Cards
   (NICs) to implement common offloads and accelerations to support GUE.
   Note that most of this discussion is generally applicable to other
   methods of UDP based encapsulation.

A.1. Receive multi-queue

   Contemporary NICs support multiple receive descriptor queues (multi-
   queue). Multi-queue enables load balancing of network processing for
   a NIC across multiple CPUs. On packet reception, a NIC selects the
   appropriate queue for host processing. Receive Side Scaling is a
   common method which uses the flow hash for a packet to index an
   indirection table where each entry stores a queue number. Flow
   Director and Accelerated Receive Flow Steering (aRFS) allow a host to
   program the queue that is used for a given flow which is identified
   either by an explicit five-tuple or by the flow's hash.

   GUE encapsulation is compatible with multi-queue NICs that support
   five-tuple hash calculation for UDP/IP packets as input to RSS. The
   flow entropy in the UDP source port ensures classification of the
   encapsulated flow even in the case that the outer source and
   destination addresses are the same for all flows (e.g. all flows are
   going over a single tunnel).

   By default, UDP RSS support is often disabled in NICs to avoid out-
   of-order reception that can occur when UDP packets are fragmented. As
   discussed above, fragmentation of GUE packets is be mostly avoided by
   fragmenting packets before entering a tunnel, GUE fragmentation, path
   MTU discovery in higher layer protocols, or operator adjusting MTUs.
   Other UDP traffic might not implement such procedures to avoid
   fragmentation, so enabling UDP RSS support in the NIC might be a
   considered tradeoff during configuration.

A.2. Checksum offload

   Many NICs provide capabilities to calculate standard ones complement
   payload checksum for packets in transmit or receive. When using GUE
   encapsulation, there are at least two checksums that are of interest:
   the encapsulated packet's transport checksum, and the UDP checksum in
   the outer header.

A.2.1. Transmit checksum offload

   NICs can provide a protocol agnostic method to offload transmit
   checksum (NETIF_F_HW_CSUM in Linux parlance) that can be used with
   GUE. In this method, the host provides checksum related parameters in
   a transmit descriptor for a packet. These parameters include the
   starting offset of data to checksum, the length of data to checksum,
   and the offset in the packet where the computed checksum is to be
   written. The host initializes the checksum field to pseudo header

   In the case of GUE, the checksum for an encapsulated transport layer
   packet, a TCP packet for instance, can be offloaded by setting the
   appropriate checksum parameters.

   NICs typically can offload only one transmit checksum per packet, so
   simultaneously offloading both an inner transport packet's checksum
   and the outer UDP checksum is likely not possible.

   If an encapsulator is co-resident with a host, then checksum offload
   may be performed using remote checksum offload (described in
   [GUEEXTEN]). Remote checksum offload relies on NIC offload of the
   simple UDP/IP checksum which is commonly supported even in legacy
   devices. In remote checksum offload, the outer UDP checksum is set
   and the GUE header includes an option indicating the start and offset
   of the inner "offloaded" checksum. The inner checksum is initialized
   to the pseudo header checksum. When a decapsulator receives a GUE
   packet with the remote checksum offload option, it completes the
   offload operation by determining the packet checksum from the
   indicated start point to the end of the packet, and then adds this
   into the checksum field at the offset given in the option. Computing
   the checksum from the start to end of packet is efficient if
   checksum-complete is provided on the receiver.

   Another alternative when an encapsulator is co-resident with a host
   is to perform Local Checksum Offload [LCO]. In this method, the inner
   transport layer checksum is offloaded and the outer UDP checksum can
   be deduced based on the fact that the portion of the packet covered
   by the inner transport checksum will sum to zero (or at least the bit
   wise "not" of the inner pseudo header).

A.2.2. Receive checksum offload

   GUE is compatible with NICs that perform a protocol agnostic receive
   checksum (CHECKSUM_COMPLETE in Linux parlance). In this technique, a
   NIC computes a ones complement checksum over all (or some predefined
   portion) of a packet. The computed value is provided to the host
   stack in the packet's receive descriptor. The host driver can use
   this checksum to "patch up" and validate any inner packet transport
   checksum, as well as the outer UDP checksum if it is non-zero.

   Many legacy NICs don't provide checksum-complete but instead provide
   an indication that a checksum has been verified (CHECKSUM_UNNECESSARY
   in Linux). Usually, such validation is only done for simple TCP/IP or
   UDP/IP packets. If a NIC indicates that a UDP checksum is valid, the
   checksum-complete value for the UDP packet is the "not" of the pseudo
   header checksum. In this way, checksum-unnecessary can be converted
   to checksum-complete. So, if the NIC provides checksum-unnecessary
   for the outer UDP header in an encapsulation, checksum conversion can
   be done so that the checksum-complete value is derived and can be
   used by the stack to validate checksums in the encapsulated packet.

A.3. Transmit Segmentation Offload

   Transmit Segmentation Offload (TSO) is a NIC feature where a host
   provides a large (>MTU size) TCP packet to the NIC, which in turn
   splits the packet into separate segments and transmits each one. This
   is useful to reduce CPU load on the host.

   The process of TSO can be generalized as:

      - Split the TCP payload into segments which allow packets with
        size less than or equal to MTU.

      - For each created segment:

        1. Replicate the TCP header and all preceding headers of the
           original packet.

        2. Set payload length fields in any headers to reflect the
           length of the segment.

        3. Set TCP sequence number to correctly reflect the offset of
           the TCP data in the stream.

        4. Recompute and set any checksums that either cover the payload
           of the packet or cover header which was changed by setting a
           payload length.

   Following this general process, TSO can be extended to support TCP
   encapsulation in GUE.  For each segment the Ethernet, outer IP, UDP
   header, GUE header, inner IP header (if tunneling), and TCP headers
   are replicated. Any packet length header fields need to be set
   properly (including the length in the outer UDP header), and
   checksums need to be set correctly (including the outer UDP checksum
   if being used).

   To facilitate TSO with GUE, it is recommended that extension fields
   do not contain values that need to be updated on a per segment basis.
   For example, extension fields should not include checksums, lengths,
   or sequence numbers that refer to the payload. If the GUE header does
   not contain such fields then the TSO engine only needs to copy the
   bits in the GUE header when creating each segment and does not need
   to parse the GUE header.

A.4. Large Receive Offload

   Large Receive Offload (LRO) is a NIC feature where packets of a TCP
   connection are reassembled, or coalesced, in the NIC and delivered to
   the host as one large packet. This feature can reduce CPU utilization
   in the host.

   LRO requires significant protocol awareness to be implemented
   correctly and is difficult to generalize. Packets in the same flow
   need to be unambiguously identified. In the presence of tunnels or
   network virtualization, this may require more than a five-tuple match
   (for instance packets for flows in two different virtual networks may
   have identical five-tuples). Additionally, a NIC needs to perform
   validation over packets that are being coalesced, and needs to
   fabricate a single meaningful header from all the coalesced packets.

   The conservative approach to supporting LRO for GUE would be to
   assign packets to the same flow only if they have identical five-
   tuple and were encapsulated the same way. That is the outer IP
   addresses, the outer UDP ports, GUE protocol, GUE flags and fields,
   and inner five tuple are all identical.

Appendix B: Implementation considerations

   This appendix is informational and does not constitute a normative
   part of this document.

B.1. Priveleged ports

   Using the source port to contain a flow entropy value disallows the
   security method of a receiver enforcing that the source port be a
   privileged port. Privileged ports are defined by some operating
   systems to restrict source port binding. Unix, for instance,
   considered port number less than 1024 to be privileged.

   Enforcing that packets are sent from a privileged port is widely
   considered an inadequate security mechanism and has been mostly
   deprecated. To approximate this behavior, an implementation could
   restrict a user from sending a packet destined to the GUE port
   without proper credentials.

B.2. Setting flow entropy as a route selector

   An encapsulator generating flow entropy in the UDP source port could
   modulate the value to perform a type of multipath source routing.
   Assuming that networking switches perform ECMP based on the flow
   hash, a sender can affect the path by altering the flow entropy.  For
   instance, a host can store a flow hash in its PCB for an inner flow,
   and might alter the value upon detecting that packets are traversing
   a lossy path. Changing the flow entropy for a flow SHOULD be subject
   to hysteresis (at most once every thirty seconds) to limit the number
   of out of order packets.

B.3. Hardware protocol implementation considerations

   Low level data path protocol, such is GUE, are often supported in
   high speed network device hardware. Variable length header (VLH)
   protocols like GUE are often considered difficult to efficiently
   implement in hardware. In order to retain the important
   characteristics of an extensible and robust protocol, hardware
   vendors may practice "constrained flexibility". In this model, only
   certain combinations or protocol header parameterizations are
   implemented in hardware fast path. Each such parameterization is
   fixed length so that the particular instance can be optimized as a
   fixed length protocol. In the case of GUE this constitutes specific
   combinations of GUE flags, fields, and next protocol. The selected
   combinations would naturally be the most common cases which form the
   "fast path", and other combinations are assumed to take the "slow

   In time, needs and requirements of the protocol may change which may
   manifest themselves as new parameterizations to be supported in the
   fast path. To allow allow this extensibility, a device practicing
   constrained flexibility should allow the fast path parameterizations
   to be programmable.

Authors' Addresses

   Tom Herbert
   4701 Patrick Henry
   Santa Clara, CA 95054


   Lucy Yong
   Huawei USA
   5340 Legacy Dr.
   Plano, TX 75024


   Osama Zia
   1 Microsoft Way
   Redmond, WA 98029