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Versions: (draft-herbert-gue) 00 01 02 03 04 05 draft-ietf-intarea-gue

Network Virtualization Overlays (nvo3)                        T. Herbert
Internet-Draft                                                  Facebook
Intended status: Standard track                                  L. Yong
Expires January 7, 2017                                       Huawei USA
                                                                  O. Zia
                                                               Microsoft
                                                            July 6, 2016

                       Generic UDP Encapsulation
                         draft-ietf-nvo3-gue-04

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   This Internet-Draft is submitted in full conformance with the
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   Copyright (c) 2016 IETF Trust and the persons identified as the
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   (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.












































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Abstract

   This specification describes Generic UDP Encapsulation (GUE), which
   is a scheme for using UDP to encapsulate packets of arbitrary 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
   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  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2. Base packet format  . . . . . . . . . . . . . . . . . . . . . .  5
     2.1. GUE version . . . . . . . . . . . . . . . . . . . . . . . .  5
   3. Version 0 . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1. Header format . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2. Proto/ctype field . . . . . . . . . . . . . . . . . . . . .  7
       3.2.1 Proto field  . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2 Ctype field  . . . . . . . . . . . . . . . . . . . . . .  8
     3.3. Flags and optional fields . . . . . . . . . . . . . . . . .  8
     3.4. Private data  . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5. Message types . . . . . . . . . . . . . . . . . . . . . . .  9
       3.5.1. Control messages  . . . . . . . . . . . . . . . . . . .  9
       3.5.2. Data messages . . . . . . . . . . . . . . . . . . . . . 10
     3.6. Hiding the transport layer protocol number  . . . . . . . . 10
   4. Version 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1. Direct encapsulation of IPv4  . . . . . . . . . . . . . . . 11
     4.2. Direct encapsulation of IPv6  . . . . . . . . . . . . . . . 12
   5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1. Network tunnel encapsulation  . . . . . . . . . . . . . . . 13
     5.2. Transport layer encapsulation . . . . . . . . . . . . . . . 13
     5.3. Encapsulator operation  . . . . . . . . . . . . . . . . . . 13
     5.4. Decapsulator operation  . . . . . . . . . . . . . . . . . . 14
     5.5. Router and switch operation . . . . . . . . . . . . . . . . 14
     5.6. Middlebox interactions  . . . . . . . . . . . . . . . . . . 14
       5.6.1. Inferring connection semantics  . . . . . . . . . . . . 14
       5.6.2. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.7. Checksum Handling . . . . . . . . . . . . . . . . . . . . . 15
       5.7.1. Checksum requirements . . . . . . . . . . . . . . . . . 15
       5.7.2. UDP Checksum with IPv4  . . . . . . . . . . . . . . . . 15
       5.7.3. UDP Checksum with IPv6  . . . . . . . . . . . . . . . . 16
     5.8. MTU and fragmentation . . . . . . . . . . . . . . . . . . . 17
     5.9. Congestion control  . . . . . . . . . . . . . . . . . . . . 17
     5.10. Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 17



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     5.11. Flow entropy for ECMP  . . . . . . . . . . . . . . . . . . 18
       5.11.1. Flow classification  . . . . . . . . . . . . . . . . . 18
       5.11.2. Flow entropy properties  . . . . . . . . . . . . . . . 19
   6. Motivation for GUE  . . . . . . . . . . . . . . . . . . . . . . 19
   7. Security Considerations . . . . . . . . . . . . . . . . . . . . 21
   8. IANA Consideration  . . . . . . . . . . . . . . . . . . . . . . 21
     8.1. UDP source port . . . . . . . . . . . . . . . . . . . . . . 21
     8.2. GUE version number  . . . . . . . . . . . . . . . . . . . . 22
     8.3. Control types . . . . . . . . . . . . . . . . . . . . . . . 22
     8.4. Flag-fields . . . . . . . . . . . . . . . . . . . . . . . . 22
   9. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A: NIC processing for GUE . . . . . . . . . . . . . . . . 26
     A.1. Receive multi-queue . . . . . . . . . . . . . . . . . . . . 27
     A.2. Checksum offload  . . . . . . . . . . . . . . . . . . . . . 27
       A.2.1. Transmit checksum offload . . . . . . . . . . . . . . . 27
       A.2.2. Receive checksum offload  . . . . . . . . . . . . . . . 28
     A.3. Transmit Segmentation Offload . . . . . . . . . . . . . . . 28
     A.4. Large Receive Offload . . . . . . . . . . . . . . . . . . . 29
   Appendix B: Implementation considerations  . . . . . . . . . . . . 30
     B.1. Priveleged ports  . . . . . . . . . . . . . . . . . . . . . 30
     B.2. Setting flow entropy as a route selector  . . . . . . . . . 30
     B.3. Hardware protocol implementation considerations . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31

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 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.




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   This document does not define any specific GUE extensions.
   [GUEEXTENS] specifies a set of core extensions and [GUE4NVO3] defines
   an extension for using GUE with network virtualization.

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

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 (like an OAM message). A GUE
   packet has the general format:

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

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

2.1. GUE version

   The first two bits of the GUE header contain the GUE protocol version
   number. The rest of the fields after the GUE version number are
   defined based on the version number. Versions 0x0 and 0x1 are
   described in this specification; versions 0x2 and 0x3 are reserved,

3. Version 0

   Version 0 of GUE defines a gereric extensible format to encapsulate
   packets by Internet protocol number.









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3.1. Header format

   The header format for version 0x0 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             | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
   |0x0|C|   Hlen  |  Proto/ctype  |             Flags             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Fields (optional)                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                    Private data (optional)                    ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The contents of the UDP header are:

      o Source port: This should be set to a flow entropy value for use
        with ECMP. The properties of flow entropy are described in
        section 5.11.

      o Destination port: 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 (see section 5.7).

   The GUE header consists of:

      o Ver: GUE protocol version (0x0).

      o C: Control flag. 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 fields but not the first four bytes of the header.
        Computed as (header_len - 4) / 4. 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



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        message type for the payload (section 3.2.2). When 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 optional fields. Undefined header
        flag bits MUST be set to zero on transmission.

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

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

3.2. Proto/ctype field

   When the C bit is not set, the proto/ctype field must be set to a
   valid Internet protocol number. The protocol number serves as an
   indication of the type of 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
   field.

3.2.1 Proto field

   When the C bit is not set the proto/ctype field contains an IANA
   Internet Protocol Number. The protocol number in interpreted relative
   to the IP protocol that encapsulates the UDP packet (i.e. protocol of
   the outer IP header).

   For an IPv4 header the protocol may be set to any number except for
   those that refer to 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) may be used with IPv4
   as described below.

   For an IPv6 header the protocol may be set to any defined protocol
   number except 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 as though the GUE header itself were an extension header.




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   IP protocol number 59 ("No next header") may 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 though other means (such as flags and
   optional fields), and intermediate devices must not parse packets the
   packet 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 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, for GUE.

3.3. Flags and optional fields

   Flags and associated optional fields are the primary mechanism of
   extensibility in GUE. There are sixteen flag bits in the GUE header.

   A flag may indicate presence of optional fields. The size of an
   optional field indicated by a flag must be fixed.

   Flags may be paired together to allow different lengths for an
   optional field. For example, if two flag bits are paired, a field may
   possibly be three different lengths. Regardless of how flag bits may
   be paired, the lengths and offsets of optional fields corresponding
   to a set of flags must be well defined.

   Optional 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.

   Flags (or paired flags) are idempotent such that new flags must not
   cause reinterpretation of old flags. Also, new flags should not alter
   interpretation of other elements in the GUE header nor how the



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

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 optional
   fields present in the GUE header. When there is no private data
   present, 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 optional 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, and 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 must
   be verified for packet acceptance.

3.5. Message types

3.5.1. Control messages

   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



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   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.

   Control messages can be defined for OAM type messages. For instance,
   an echo request and corresponding echo reply message may be defined
   to test for liveness.

3.5.2. Data messages

   Data messages are indicated in GUE header with C bit 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.

   Data messages are a primary means of encapsulation and can be used to
   create tunnels for overlay networks.

3.6. Hiding the transport layer protocol number

   The GUE header indicates the Internet protocol of the encapsulated
   packet. This is either contained in the Proto/ctype field of the
   primary GUE header, or is contained in the Payload Type field of a
   GUE Transform Field (used to encrypt the payload with DTLS). If the
   protocol number must be obfuscated, that is the transport protocol in
   use must be hidden from the network, then a trivial destination
   options can be used at the beginning of the payload.

   The PadN destination option 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



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   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 1

   Version 1 of GUE allows direct encapsulation of IPv4 and IPv6 in UDP.
   In this version 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
   version 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 1 for direct IP encapsulation which makes two bits
   of GUE version to also be 01.

   This technique is effectively a means to compress out the GUE header
   when encapsulating IPv4 or IPv6 packets and there are no flags or
   optional fields present. This method is compatible to use on the same
   port number as packets with the the GUE header (GUE version 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 demonstrated
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   |        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 0100 value IP version field express the GUE version as 1
   (bits 01) and IP version as 4 (bits 0100).





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4.2. Direct encapsulation of IPv6

   The format for encapsulating IPv4 directly in UDP is demonstrated
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   |        Source port            |      Destination port         | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ UDP
   |           Length              |          Checksum             | |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/
   |0|1|1|0| Traffic Class |           Flow Label                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Payload Length        |     NextHdr   |   Hop Limit   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                     Outer Source IPv6 Address                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                  Outer Destination IPv6 Address               +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note that 0110 value IP version field express the GUE version as 1
   (bits 01) and IP version as 6 (bits 0110).

5. Operation

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







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

   The encapsulator and decapsulator may be co-resident with the
   corresponding servers, 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 servers.

5.2. Transport layer encapsulation

   When encapsulating layer 4 packets, the encapsulator and decapsulator
   should be co-resident with the servers. 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
   the transport protocol.

5.3. Encapsulator operation

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

   An encapsulator may be an end host originating the packets of a flow,
   or may 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.

   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 IP protocol over another. Diffserv interaction with tunnels is



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   described in [RFC2983], ECN propagation for tunnels is described in
   [RFC6040].

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
   header. If a packet is acceptable, the UDP and GUE headers are
   removed and the packet is resubmitted for protocol processing or
   control message processing if it is a control message.

   If a decapsulator receives a GUE packet with an unsupported version,
   unknown flag, bad header length (too small for included optional
   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 GUE 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 dropped.

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 switch should not normally need to parse a GUE header, and
   none of the flags or optional fields in the GUE header should affect
   routing.

   A router should 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.

5.6. Middlebox interactions

   A middle box may interpret some flags and optional fields of the GUE
   header for classification purposes, but is not required to understand
   any of the flags or fields in GUE packets. A middle box must not drop
   a GUE packet 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 optional fields.

5.6.1. Inferring connection semantics



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   A middlebox may infer bidirectional connection semantics for a UDP
   flow. For instance a stateful firewall may 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 must assume connected semantics
   defined by the UDP five tuple and the use of GUE encapsulation must
   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.

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 [RFC478]. In
   the case of stateful NAT, connection semantics must be applied to a
   GUE tunnel as described in section 5.6.1.

5.7. Checksum Handling

   This section describes the requirements around the UDP checksum.
   Checksums are an important consideration in that that they can
   provide end to end validation and protect against packet mis-
   delivery. The latter is allowed by the inclusion of a pseudo header
   that covers the IP addresses and UDP ports of the encapsulating
   headers.

5.7.1. Checksum requirements

   The potential for mis-delivery of packets due to corruption of IP,
   UDP, or GUE headers must be considered. 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]).

  o Zero UDP checksums are used in accordance with applicable
    requirements in [GREUDP], [RFC6935], and [RFC6936].

5.7.2. UDP Checksum with IPv4

    For UDP in IPv4, the UDP checksum MUST be processed as specified in



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    [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].

    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.

5.7.3. 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. Unlike IPv4,
    there is no header checksum in IPv6 that protects against mis-
    delivery due to address corruption. Therefore, when GUE is used over
    IPv6, either the UDP checksum must be enabled or the GUE header
    checksum must be used. An encapsulator MAY set a zero UDP checksum
    for performance or implementation reasons, in which case the GUE
    header checksum MUST be used or applicable requirements for using
    zero UDP checksums in [GREUDP] MUST be met. If the UDP checksum is
    enabled, then the GUE header checksum should not be used since it is
    mostly redundant.

    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.



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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 may 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].

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 rather a transmitter will
    consistently implement proper congestion control, then congestion
    control at the encapsulation layer must be provided. Note 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 control.

    Out of band mechanisms such as rate limiting, Managed Circuit
    Breaker [CIRCBRK], or traffic isolation may 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 may
    warranted.

    DCCP [RFC4340] may be used to provide congestion control for
    encapsulated flows. In this case, the protocol stack for an IP
    tunnel may be IP-GUE-DCCP-IP. Alternatively, GUE can be extended to
    include congestion control (related data carried in GUE optional
    fields). Congestion control mechanisms for GUE will be elaborated in
    other specifications.

5.10. Multicast

    GUE packets may be multicast to decapsulators using a multicast
    destination address in the encapsulating IP headers. Each receiving



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    host will decapsulate the packet independently following normal
    decapsulator operations. The receiving decapsulators should 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) may be
    generated from the header of encapsulated unicast or
    broadcast/multicast packets at an encapsulator. The mapping
    mechanism between the encapsulated multicast traffic and the
    multicast capability in the IP network is transparent and
    independent to the encapsulation and is otherwise outside the scope
    of this document.

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 (statistical
    multiplexing) 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, the source port in the outer UDP packet is set to a flow
    entropy value that corresponds 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
    flow.

    Examples of deriving an 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.




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      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
        packet.

      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 clear-text packet. For instance, a canonical five-tuple hash
        for a TCP/IP packet could be used.

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 should be within the ephemeral
        port range. IANA suggests this range to be 49152 to 65535, where
        the high order two bits of the port are set to one. This
        provides fourteen bits of entropy for the value.

      o The flow entropy should have a uniform distribution across
        encapsulated flows.

      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 cannot 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 may be
        changed periodically.

6. Motivation for GUE



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

   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 [7510], and Generic UDP Encapsulation for IP Tunneling (GRE
   over UDP)[GREUDP]. 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 optional fields can be defined.

      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).



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7. Security Considerations

   Security for GUE can be provided by lower layers or through GUE
   security extensions.

   IPsec in transport mode may be used to authenticate or encrypt GUE
   packets (GUE header and payload). Existing network security
   mechanisms, such as address spoofing detection, DDOS mitigation, and
   transparent encrypted tunnels can be applied to GUE packets.

   Advanced security extensions for Generic UDP Encapsulation, including
   security for the GUE header and payload, are described in detail in
   [GUESEC].

   Encapsulation of IP protocols within GUE should not increase security
   risk, nor provide additional security in itself. A hash function for
   computing flow entropy (section 5.11) should be randomly seeded to
   mitigate some possible denial service attacks.

8. IANA Consideration

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 <therbert@google.com>
          Contact: Tom Herbert <therbert@google.com>
          Description: Generic UDP Encapsulation
          Reference: draft-herbert-gue
          Port Number: 6080
          Service Code: N/A
          Known Unauthorized Uses: N/A
          Assignment Notes: N/A
















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8.2. GUE version number

   IANA is requested to set up a registry for the GUE version number.
   The GUE version number is 2 bits containing four possible values.
   This document defines version 0 and 1. New values are assigned via
   Standards Action [RFC5226].

      +----------------+-------------+---------------+
      | Version number | Description | Reference     |
      +----------------+-------------+---------------+
      | 0              | Version 0   | This document |
      |                |             |               |
      | 1              | Version 1   | This document |
      |                |             |               |
      | 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 are assigned via
   Standards Action [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 optional fields for the GUE header flags and extension
   fields. This shall be a registry of bit assignments for flags, length
   of optional fields for corresponding flags, and descriptive strings.
   There are sixteen bits for primary GUE header flags (bit number 0-
   15). New values are assigned via Standards Action [RFC5226].









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      +-------------+--------------+-------------+--------------------+
      |  Flags bits | Field size   | Description | Reference          |
      +-------------+--------------+-------------+--------------------+
      | Bit 0       | 4 bytes      | VNID        | [GUE4NVO3]         |
      |             |              |             |                    |
      | Bit 1..2    | 01->8 bytes  | Security    | [GUEEXTENS]        |
      |             | 10->16 bytes |             |                    |
      |             | 11->32 bytes |             |                    |
      |             |              |             |                    |
      | Bit 3       | 4 bytes      | Checksum    | [GUEEXTENS]        |
      |             |              |             |                    |
      | Bit 4       | 8 bytes      | Fragmen-    | [GUEEXTENS]        |
      |             |              |  tation     |                    |
      |             |              |             |                    |
      | Bit 5       | 4 bytes      | Payload     | [GUEEXTENS]        |
      |             |              |  transform  |                    |
      |             |              |             |                    |
      | Bit 6       | 4 bytes      | Remote      | [GUEEXTENS]        |
      |             |              |  checksum   |                    |
      |             |              |  offload    |                    |
      |             |              |             |                    |
      | Bit 7..15   |              | Unassigned  |                    |
      +-------------+--------------+-------------+--------------------+

   New flags are to be allocated from high to low order bit contiguously
   without holes.

9. Acknowledgements

   The authors would like to thank David Liu, Erik Nordmark, Fred
   Templin, and Adrian Farrel 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-
              editor.org/info/rfc768>.

   [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-
              editor.org/info/rfc2434>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,



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              <http://www.rfc-editor.org/info/rfc2983>.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <http://www.rfc-editor.org/info/rfc6040>.

   [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>.

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
              2006, <http://www.rfc-editor.org/info/rfc4459>.


   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
              10.17487/RFC0768, August 1980, <http://www.rfc-
              editor.org/info/rfc768>.

   [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-
              editor.org/info/rfc2434>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
              2983, DOI 10.17487/RFC2983, October 2000, <http://www.rfc-
              editor.org/info/rfc2983>.

   [RFC6040]  Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", RFC 6040, DOI 10.17487/RFC6040, November
              2010, <http://www.rfc-editor.org/info/rfc6040>.

   [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>.

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
              2006, <http://www.rfc-editor.org/info/rfc4459>.

10.2. Informative References

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003, DOI
              10.17487/RFC2003, October 1996, <http://www.rfc-
              editor.org/info/rfc2003>.




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   [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-editor.org/info/rfc3948>.

   [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-
              editor.org/info/rfc6830>.

   [RFC3378]  Housley, R. and S. Hollenbeck, "EtherIP: Tunneling
              Ethernet Frames in IP Datagrams", RFC 3378, DOI
              10.17487/RFC3378, September 2002, <http://www.rfc-
              editor.org/info/rfc3378>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc4023>.

   [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,
              <http://www.rfc-editor.org/info/rfc2661>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <http://www.rfc-editor.org/info/rfc5925>.

   [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,
              <http://www.rfc-editor.org/info/rfc3828>.

   [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-
              editor.org/info/rfc7348>.

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



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              August 2015, <http://www.rfc-editor.org/info/rfc7605>.

   [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>.

   [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-
              editor.org/info/rfc7510>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, DOI
              10.17487/RFC4340, March 2006, <http://www.rfc-
              editor.org/info/rfc4340>.

   [GUEEXTENS] 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

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

   [GREUDP]   Crabbe, E., Yong, L., Xu, X., and Herbert, T.,Cpsest
              "Generic UDP Encapsulation for IP Tunneling" draft-ietf-
              tsvwg-gre-in-udp-encap-13

   [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", draft-ietf-tsvwg-circuit-breaker-15

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.

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




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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 must select
   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 should be 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 should be mitigated by
   fragmenting packets before entering a tunnel, path MTU discovery in
   higher layer protocols, or operator adjusting MTUs. Other UDP traffic
   may not implement such procedures to avoid fragmentation, so enabling
   UDP RSS support in the NIC should 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 may be of
   interest: the encapsulated packet's transport checksum, and the UDP
   checksum in the outer header.

A.2.1. Transmit checksum offload

   NICs may 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
   checksum.

   In the case of GUE, the checksum for an encapsulated transport layer



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   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. In this case
   setting UDP checksum to zero (per above discussion) and offloading
   the inner transport packet checksum might be acceptable.

   If an encapsulator is co-resident with a host, then checksum offload
   may be performed using remote checksum offload (described in
   [GUEEXTENS]). 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.

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 an checksums in the encapsulated packet.

A.3. Transmit Segmentation Offload

   Transmit Segmentation Offload (TSO) is a NIC feature where a host



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   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 optional fields
   should not contain values that must be updated on a per segment
   basis-- for example the GUE 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



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   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 may
   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 may store a flow hash in its PCB for an inner flow,
   and may 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

   A low level protocol, such is GUE, is likely interesting to being
   supported by high speed network devices. Variable length header (VLH)



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   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
   path".

   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
   Facebook
   1 Hacker Way
   Menlo Park, CA 94052
   US

   Email: tom@herbertland.com

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

   Email: lucy.yong@huawei.com

   Osama Zia
   Microsoft
   1 Microsoft Way
   Redmond, WA 98029
   US

   Email: osamaz@microsoft.com







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