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INTERNET-DRAFT                                                T. Herbert
Intended Status: Proposed Standard                            Quantonium
Expires: September 28, 2018                                      L. Yong
                                                                  Huawei
                                                              F. Templin
                                                                  Boeing

                                                          March 27, 2018


                Extensions for Generic UDP Encapsulation
                  draft-ietf-intarea-gue-extensions-04


Abstract

   This specification defines a set of the fundamental optional
   extensions for Generic UDP Encapsulation (GUE).

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   http://www.ietf.org/1id-abstracts.html

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Copyright and License Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  GUE header format with optional extensions . . . . . . . . . .  4
   3.  Group identifier option  . . . . . . . . . . . . . . . . . . .  6
     3.1.  Extension field format . . . . . . . . . . . . . . . . . .  6
     3.2.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Security option  . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Extension field format . . . . . . . . . . . . . . . . . .  7
     4.2.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Cookies  . . . . . . . . . . . . . . . . . . . . . . . . .  8
       4.4.1. Extension field format  . . . . . . . . . . . . . . . .  9
       4.3.1. Operation . . . . . . . . . . . . . . . . . . . . . . .  8
         4.3.1.1. Transmitter operation . . . . . . . . . . . . . . .  8
         4.3.1.2. Receiver operation  . . . . . . . . . . . . . . . .  9
     4.4.  HMAC . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       4.4.1. Extension field format  . . . . . . . . . . . . . . . .  9
       4.4.2. Selecting a hash algorithm  . . . . . . . . . . . . . . 10
       4.4.3. Pre-shared key management . . . . . . . . . . . . . . . 10
       4.4.4. Operation . . . . . . . . . . . . . . . . . . . . . . . 11
         4.4.4.1. Transmitter operation . . . . . . . . . . . . . . . 11
         4.4.4.2. Receiver operation  . . . . . . . . . . . . . . . . 11
     4.5.  Interaction with other optional extensions . . . . . . . . 12
   5.  Fragmentation option . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.3.  Extension field format . . . . . . . . . . . . . . . . . . 14
     5.4.  Fragmentation procedure  . . . . . . . . . . . . . . . . . 15
     5.5.  Reassembly procedure . . . . . . . . . . . . . . . . . . . 17
     5.6.  Security Considerations  . . . . . . . . . . . . . . . . . 19
   6.  Payload transform option . . . . . . . . . . . . . . . . . . . 19
     6.1.  Extension field format . . . . . . . . . . . . . . . . . . 19
     6.2.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.3.  Interaction with other optional extensions . . . . . . . . 21
     6.4.  DTLS transform . . . . . . . . . . . . . . . . . . . . . . 21
   7.  Remote checksum offload option . . . . . . . . . . . . . . . . 22
     7.1.  Extension field format . . . . . . . . . . . . . . . . . . 22
     7.2.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       7.2.1. Transmitter operation . . . . . . . . . . . . . . . . . 22



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       7.2.2. Receiver operation  . . . . . . . . . . . . . . . . . . 23
     7.3.  Security Considerations  . . . . . . . . . . . . . . . . . 24
   8.  Checksum option  . . . . . . . . . . . . . . . . . . . . . . . 24
     8.1.  Extension field format . . . . . . . . . . . . . . . . . . 24
     8.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . 25
     8.3.  GUE checksum pseudo header . . . . . . . . . . . . . . . . 25
     8.4.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . 26
       8.4.1. Transmitter operation . . . . . . . . . . . . . . . . . 27
       8.4.2. Receiver operation  . . . . . . . . . . . . . . . . . . 27
     8.5.  Corrupted flags  . . . . . . . . . . . . . . . . . . . . . 28
     8.6.  Security Considerations  . . . . . . . . . . . . . . . . . 28
   9.  NAT checksum address option  . . . . . . . . . . . . . . . . . 28
     9.1.  Extension field format . . . . . . . . . . . . . . . . . . 28
     9.2.  Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . 29
       9.2.1. Transmitter operation . . . . . . . . . . . . . . . . . 29
       9.2.2. Receiver operation  . . . . . . . . . . . . . . . . . . 30
   10. Alternative checksum option  . . . . . . . . . . . . . . . . . 30
     10.1. Extension field format . . . . . . . . . . . . . . . . . . 31
     10.2. Usage  . . . . . . . . . . . . . . . . . . . . . . . . . . 31
       10.2.2. Receiver operation . . . . . . . . . . . . . . . . . . 32
     10.3. Corrupted flags  . . . . . . . . . . . . . . . . . . . . . 32
     10.4. Security Considerations  . . . . . . . . . . . . . . . . . 33
   11. Processing order of options  . . . . . . . . . . . . . . . . . 33
     11.1. Processing order when sending  . . . . . . . . . . . . . . 33
     11.2. Processing order when receiving  . . . . . . . . . . . . . 34
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 34
   13. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 35
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     14.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40




















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1.  Introduction

   Generic UDP Encapsulation (GUE) [I.D.ietf-gue] is a generic and
   extensible encapsulation protocol. This specification defines a
   fundamental set of optional extensions for variant 0 of GUE. These
   extensions are the group identifier, security, fragmentation, payload
   transform, remote checksum offload, checksum, NAT address checksum,
   and alternate checksum.

2.  GUE header format with optional extensions

   The format of a variant 0 GUE header with optional extensions 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  |G| SEC |F|T|R|K|N|A|   Rsvd    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Group identifier (optional)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                      Security (optional)                      ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                     Fragmentation (optional)                  +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Payload transform (optional)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Remote checksum offload (optional)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Checksum (optional)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 NAT Address Checksum (optional)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                  Alternate checksum (optional)                +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                     Private data (optional)                   ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The contents of the UDP header are described in [I.D.ietf-gue].

   The GUE header consists of:

      o Variant: Set to 0 to indicate GUE encapsulation header. Note
        that variant 1 (direct IP encapsulation) does not allow options.

      o C: C-bit. Indicates the GUE payload is a control message when
        set, a data message when not set. GUE optional extensions can be
        used with either control or data messages unless otherwise
        specified in the extension definition.

      o Hlen: Length in 32-bit words of the GUE header, including
        optional extension fields and private data but not the first
        four bytes of the header. Computed as (header_len - 4) / 4. The
        length of the encapsulated packet is determined from the UDP
        length and the Hlen: encapsulated_packet_length = UDP_Length -
        12 - 4 * Hlen.

      o Proto/ctype: If the C-bit is not set this indicates IP protocol
        number for the packet in the payload; if the C bit is set this
        is the type of control message in the payload. The next header
        begins at the offset provided by Hlen. When the payload
        transform option or fragmentation option is used this field
        SHOULD be set to protocol number 59 for a data message, or zero
        for a control message, to indicate no next header for the
        payload.

      o G: Indicates the the group identifier extension field is
        present. The group identifier option is described in section 3.

      o SEC: Indicates security extension field is present. The security
        option is described in section 4.

      o F: Indicates fragmentation extension field is present. The
        fragmentation option is described in section 5.

      o T: Indicates payload transform extension field is present. The
        payload transform option is described in section 6.

      o R: Indicates the remote checksum extension field is present. The
        remote checksum offload option is described in section 7.

      o K: Indicates checksum extension field is present. The checksum
        option is described in section 8.

      o N: Indicates NAT address checksum field is present. The NAT
        address checksum option is described in section 9.



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      o A: Indicates alternative checksum field is present. The
        alternative checksum option is described in section 10.

      o Private data is described in [I.D.ietf-gue].

3.  Group identifier option

   A group identifier classifies packets that logically belong to the
   same group. Groups are arbitrarily defined for different purposes and
   their definition is shared between the communicating end nodes.

3.1.  Extension field format

   The presence of the GUE group identifier option is indicated in the G
   flag bit of the GUE header.

   The format of the group identifier option 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Group identifier                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o Group identifier: Identifier value of a group.

3.2.  Usage

   The group identifier is set by an encapsulator to indicate that a
   packet belongs to a group. Groups may be arbitrarily defined to
   classify packets. Specific use cases of the group identifier may be
   defined in other documents ([I.D.hy-nvo3-gue-4-nvo] defines a use of
   this field to contain a virtual networking identifier for
   implementing network virtualization).

   Intermediate nodes MAY apply semantics to group identifiers if group
   identifier information is shared and made global within a network.
   For instance, a firewall could block packets based on a group
   identifier that serves as a virtual identifier for a tenant.

4.  Security option

   The GUE security option provides origin authentication and integrity
   protection of the GUE header at tunnel end points to guarantee
   isolation between tunnels and mitigate Denial of Service attacks.




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4.1.  Extension field format

   The presence of the GUE security option is indicated in the SEC flag
   bits of the GUE header.

   The format of the security option 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                           Security                            ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o  Security (variable length). Contains the security information.
         The specific semantics and format of this field are expected to
         be negotiated between the two communicating nodes.

   To provide security capability, the SEC flags MUST be set. Different
   field sizes allow different methods and extensibility. The use of the
   security field is expected to be negotiated out of band between two
   tunnel end points.

   The values in the SEC flags are:

      o 000b - No security field

      o 001b - 64 bit security field

      o 010b - 128 bit security field

      o 011b - 256 bit security field

      o 100b - 320 bit security field (HMAC)

      o 101b, 110b, 111b - Reserved values

4.2.  Usage

   The GUE security option is used to provide integrity and
   authentication of the GUE header. Security parameters (interpretation
   of security field, key management, etc.) are expected to be
   negotiated out of band between two communicating hosts. Two security
   algorithms are defined below.




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   If the GUE security option is present in packet, the receiver MUST
   validate the security before processing other fields or accepting the
   packet. If the security option is not present but the encapsulator
   and decapsulator have agreed that security is required, the receiver
   MUST drop the packet as failing security checks. Note that this
   provision covers the case where the security flags bits are corrupted
   such that they are reset to zero which would be interpreted as no
   security field being present.

4.3.  Cookies

   The security field may be used as a cookie. This would be similar to
   the cookie mechanism described in L2TP [RFC3931], and the general
   properties should be the same. A cookie MAY be used to validate the
   encapsulation. A cookie is a shared value between an encapsulator and
   decapsulator which SHOULD be chosen randomly and MAY be changed
   periodically. Different cookies MAY be used for logical flows between
   the encapsulator and decapsulator; for instance packets sent with
   different VNIs in network virtualization [I.D.hy-nvo3-gue-4-nvo]
   might have different cookies. Cookies can be 64, 128, or 256 bits in
   size.

4.4.1. Extension field format

   The cookie security option is a 64, 128, or 256 bit field. The
   security flags are set to 001b, 010b, 011b respectively for the
   corresponding field size.

   The format of the field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                            Cookie                             ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields are:

      o Cookie: Shared cookie value between encapsulator and
                decapsulator

4.3.1. Operation

4.3.1.1. Transmitter operation

   The procedure for setting the GUE security cookie option on transmit



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   is:

      1) Create the GUE header including the security field with the
         selected length for the cookie. Set the cookie to the value
         that is shared with decapsulator.

4.3.1.2. Receiver operation

   The procedure for verifying the security cookie is:

      1) Compare the received cookie to the expected shared cookie. If
         both the lengths are equal and the cookie values are equal then
         that packet is accepted, if the lengths or values are not equal
         then verification failed and the packet MUST be dropped.

4.4.  HMAC

   Key-hashed message authentication code (HMAC) is a strong method of
   checking integrity and authentication of data. This sections defines
   a GUE security option for HMAC. Note that this is based on the HMAC
   TLV description in "IPv6 Segment Routing Header (SRH)" [I.D.previdi-
   6man-sr-header].

4.4.1. Extension field format

   The HMAC option is a 320 bit field (40 octets). The security flags
   are set to 100b to indicate the presence of a 320 bit security field.

   The format of the field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          HMAC Key-id                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Payload offset          |        Payload length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                        HMAC (256 bits)                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields are:

      o HMAC Key-id: opaque field to allow multiple hash algorithms or
                     key selection

      o Payload offset: offset in payload that indicates the first octet



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                     of payload data included in the HMAC.

      o Payload length: length of payload data starting from the payload
                     offset to be included in the HMAC calculation. Zero
                     indicates no payload data in used in the
                     calculation.

      o HMAC: Output of HMAC computation

   The HMAC field is the output of the HMAC computation (per [RFC2104])
   using a pre-shared key identified by HMAC Key-id and of the text
   which consists of the concatenation of:

      o The IP addresses

      o The GUE header including all optional extensions. For the
        purposes of calculating the HMAC value, the HMAC value is set
        all zeroes.

      o Optionally some or all of GUE payload. The portion of payload
        covered is indicated by an offset into the payload and a length
        relative to the offset.

   The purpose of the HMAC option is to verify the validity, the
   integrity and the authentication of the GUE header itself and
   optionally some or all of the GUE payload.

   The HMAC Key-id field allows for the simultaneous existence of
   several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
   well as pre-shared keys. The HMAC Key-id field is opaque, i.e., it
   has neither syntax nor semantic. Having an HMAC Key-id field allows
   for pre-shared key roll-over when two pre-shared keys are supported
   for a while when GUE endpoints converge to a fresher pre-shared key.

4.4.2. Selecting a hash algorithm

   The HMAC field in the HMAC option is 256 bit wide. Therefore, the
   HMAC MUST be based on a hash function whose output is at least 256
   bits. If the output of the hash function is 256 bits, then this
   output is simply inserted in the HMAC field. If the output of the
   hash function is larger than 256 bits, then the output value is
   truncated to 256 bits by taking the least-significant 256 bits and
   inserting them in the HMAC field.

   GUE implementations can support multiple hash functions but MUST
   implement SHA-2 [FIPS180-4] in its SHA-256 variant.

4.4.3. Pre-shared key management



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   The field HMAC Key-id allows for:

      o Key roll-over: when there is a need to change the key (the hash
        pre-shared secret), then multiple pre-shared keys can be used
        simultaneously.  A decapsulator can have a table of <HMAC Key-
        id, pre-shared secret> for the currently active and future keys.

      o Different algorithms: by extending the previous table to <HMAC
        Key-id, hash function, pre-shared secret>, the decapsulator can
        also support simultaneously several hash algorithms

   The pre-shared secret distribution can be done:

      o In the configuration of the endpoints

      o Dynamically using a trusted key distribution such as [RFC6407]

4.4.4. Operation

4.4.4.1. Transmitter operation

        The procedure for setting the GUE HMAC option on transmit is:

      1) Create the GUE header including the 320 bit security field to
         hold the HMAC option. Set the HMAC Key-Id, payload length, and
         payload offset appropriately. The 16 byte HMAC field is
         initialized to zero.

      2) Calculate the HMAC hash over the concatenation of the IP source
         and destination addresses. The particular hash and keys are
         agreed between the encpasulator and decapsulator out of band,
         and the key for input to the hash is the one indicated by the
         Key-Id amongst the set of shared keys.

      3) Continue the the HMAC hash calculation from the start of the
         GUE header through the its length as indicated in GUE Hlen.

      4) Continue the calculation to cover the payload portion if
         payload coverage is enabled (payload coverage field is non-
         zero). The calculation continues at the payload offset for
         payload length bytes.

      5) Set the resultant hash value in the HMAC field.

4.4.4.2. Receiver operation

   The procedure for verifying the HMAC security option is:




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      1) If the payload offset plus the payload coverage length is
         greater than the length of the encapsulated payload then drop
         the packet.

      2) Note value in the HMAC field and set the HMAC field to zero.

      3) Calculate the HMAC hash over the concatenation of the IP source
         and destination addresses. The particular hash and keys are
         agreed between the encpasulator and decapsulator out of band,
         and the key for input to the hash is the one indicated by the
         Key-Id amongst the set of shared keys.

      4) Continue the the HMAC hash calculation from the start of the
         GUE header through the its length as indicated in GUE Hlen.

      5) Continue the calculation to cover the payload portion if
         payload coverage is enabled (payload length field is non-zero).
         The calculation continues at the payload offset for payload
         length bytes.

      6) Compare the computed HMAC value with the original value of the
         HMAC field. If they are equal then the packet is accepted, if
         they are not equal then verification failed and the packet MUST
         be dropped.

      7) Restore the HMAC field to its original value.

4.5.  Interaction with other optional extensions

   If GUE fragmentation (section 5) is used in concert with the GUE
   security option, the security option processing is performed after
   fragmentation at the encapsulator and before reassembly at the
   decapsulator.

   The GUE payload transform option (section 6) may be used in concert
   with the GUE security option. The payload transform option could be
   used to encrypt the GUE payload to provide privacy for an
   encapsulated packet during transit. The security option provides
   authentication and integrity for the GUE header (including the
   payload transform field in the header). The two functions are
   processed separately at tunnel end points. A GUE tunnel can use both
   functions or use one of them. Section 6.3 details handling when both
   are used in a packet.

5.  Fragmentation option

   The fragmentation option allows an encapsulator to perform
   fragmentation of packets being ingress to a tunnel. Procedures for



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   fragmentation and reassembly are defined in this section. This
   specification adapts the procedures for IP fragmentation and
   reassembly described in [RFC0791] and [RFC8200]. Fragmentation can be
   performed on both data and control messages in GUE.

5.1.  Motivation

   This section describes the motivation for having a fragmentation
   option in GUE.

   MTU and fragmentation issues with In-the-Network Tunneling are
   described in [RFC4459]. Considerations need to be made when a packet
   is received at a tunnel ingress point which may be too large to
   traverse the path between tunnel endpoints.

   There are four suggested alternatives in [RFC4459] to deal with this:

      1) Fragmentation and Reassembly by the Tunnel Endpoints

      2) Signaling the Lower MTU to the Sources

      3) Encapsulate Only When There is Free MTU

      4) Fragmentation of the Inner Packet

   Many tunneling protocol implementations have assumed that
   fragmentation should be avoided, and in particular alternative #3
   seems preferred for deployment. In this case, it is assumed that an
   operator can configure the MTUs of links in the paths of tunnels to
   ensure that they are large enough to accommodate any packets and
   required encapsulation overhead. This method, however, may not be
   feasible in certain deployments and may be prone to misconfiguration
   in others.

   Similarly, the other alternatives have drawbacks that are described
   in [RFC4459]. Alternative #2 implies use of something like Path MTU
   Discovery which is not known to be sufficiently reliable. Alternative
   #4 is not permissible with IPv6 or when the DF bit is set for IPv4,
   and it also introduces other known issues with IP fragmentation.

   For alternative #1, fragmentation and reassembly at the tunnel
   endpoints, there are two possibilities: encapsulate the large packet
   and then perform IP fragmentation, or segment the packet and then
   encapsulate each segment (a non-IP fragmentation approach).

   Performing IP fragmentation on an encapsulated packet has the same
   issues as that of normal IP fragmentation. Most significant of these
   is that the Identification field is only sixteen bits in IPv4 which



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   introduces problems with wraparound as described in [RFC4963].

   The second possibility of alternative #1 follows the suggestion
   expressed in [RFC2764] and the fragmentation feature described in the
   AERO protocol [I.D.templin-aerolink]; that is for the tunneling
   protocol itself to incorporate a segmentation and reassembly
   capability. In this method, fragmentation is part of the
   encapsulation and an encapsulation header contains the information
   for reassembly. This differs from IP fragmentation in that the IP
   headers of the original packet are not replicated for each fragment.

   Incorporating fragmentation into the encapsulation protocol has some
   advantages:

      o At least a 32 bit identifier can be defined to avoid issues of
        the 16 bit Identification in IPv4.

      o Encapsulation mechanisms for security and identification, such
        as group identifiers, can be applied to each segment.

      o This allows the possibility of using alternate fragmentation and
        reassembly algorithms (e.g. fragmentation with Forward Error
        Correction).

      o Fragmentation is transparent to the underlying network so it is
        unlikely that fragmented packet will be unconditionally dropped
        as might happen with IP fragmentation.

5.2.  Scope

   This specification describes the mechanics of fragmentation in
   Generic UDP Encapsulation. The operational aspects and details for
   higher layer implementation must be considered for deployment, but
   are considered out of scope for this document. The AERO protocol
   [I.D.templin-aerolink] defines one use case of fragmentation with
   encapsulation.

5.3.  Extension field format

   The presence of the GUE fragmentation option is indicated by the F
   bit in the GUE header.










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   The format of the fragmentation option 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Fragment offset   |Res|M|  Orig-proto   |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
      |                        Identification                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o Fragment offset: This field indicates where in the datagram this
        fragment belongs. The fragment offset is measured in units of 8
        octets (64 bits).  The first fragment has offset zero.

      o Res: Two bit reserved field. MUST be set to zero for
        transmission. If set to non-zero in a received packet then the
        packet MUST be dropped.

      o M: More fragments bit. Set to 1 when there are more fragments
        following in the datagram, set to 0 for the last fragment.

      o Orig-proto: The control type (when the C-bit in the GUE header
        is set) or the IP protocol (when C-bit is not set) of the
        fragmented packet.

      o Identification: 40 bits. Identifies fragments of a fragmented
        packet.

   Pertinent GUE header fields to fragmentation are:

      o C-bit: This is set for each fragment based on the whether the
        original packet being fragmented is a control or data message.

      o Proto/ctype - For the first fragment (fragment offset is zero)
        this is set to that of the original packet being fragmented
        (either will be a control type or IP protocol). For other
        fragments, this is set to zero for a control message being
        fragmented, or to "No next header" (protocol number 59) for a
        data message being fragmented.

      o F bit - Set to indicate presence of the fragmentation extension
        field.

5.4.  Fragmentation procedure

   If an encapsulator determines that a packet must be fragmented (e.g.



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   the packet's size exceeds the Path MTU of the tunnel) it should
   divide the packet into fragments and send each fragment as a separate
   GUE packet, to be reassembled at the decapsulator (tunnel egress).

   For every packet that is to be fragmented, the source node generates
   an Identification value. The Identification MUST be different than
   that of any other fragmented packet sent within the past 60 seconds
   (Maximum Segment Lifetime) with the same tunnel identification-- that
   is the same outer source and destination addresses, same UDP ports,
   same orig-proto, and same group identifier if present.

   The initial, unfragmented, and unencapsulated packet is referred to
   as the "original packet". This will be a layer 2 packet, layer 3
   packet, or the payload of a GUE control message:

      +-------------------------------//------------------------------+
      |                        Original packet                        |
      |            (e.g. an IPv4, IPv6, Ethernet packet)              |
      +------------------------------//-------------------------------+

   Fragmentation and encapsulation are performed on the original packet
   in sequence. First the packet is divided up in to fragments, and then
   each fragment is encapsulated. Each fragment, except possibly the
   last ("rightmost") one, is an integer multiple of 8 octets long.
   Fragments MUST be non-overlapping. The number of fragments SHOULD be
   minimized, and all but the last fragment should be approximately
   equal in length.

   The fragments are transmitted in separate "fragment packets" as:

      +--------------+--------------+---------------+--//--+----------+
      |    first     |    second    |    third      |      |   last   |
      |   fragment   |   fragment   |   fragment    | .... | fragment |
      +--------------+--------------+---------------+--//--+----------+

   Each fragment is encapsulated as the payload of a GUE packet. This is
   illustrated as:

      +------------------+----------------+-----------------------+
      |  IP/UDP header   |   GUE header   |         first         |
      |                  | w/ frag option |        fragment       |
      +------------------+----------------+-----------------------+

      +------------------+----------------+-----------------------+
      |  IP/UDP header   |   GUE header   |         second        |
      |                  | w/ frag option |        fragment       |
      +------------------+----------------+-----------------------+
                                    o



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                                    o
      +------------------+----------------+-----------------------+
      |  IP/UDP header   |   GUE header   |          last         |
      |                  | w/ frag option |         fragment      |
      +------------------+----------------+-----------------------+

   Each fragment packet is composed of:

      (1) Outer IP and UDP headers as defined for GUE encapsulation. The
          IP addresses and UDP ports MUST be the same for all fragments
          of a fragmented packet.

      (2) A GUE header that indicates the fragmentation option is
          present. The C-bit and and proto/ctype are set appropriately
          as described above.

      (3) The GUE fragmentation option. Orig-protocol is set to the
          protocol of the original packet. The M-bit is set for all
          fragments except the last one. Fragment offset is set as the
          offset of each fragment in the original packet.

      (4) Other GUE extensions.

      (5) The fragment itself as payload of the GUE packet.

5.5.  Reassembly procedure

   At the destination, fragment packets are decapsulated and reassembled
   into their original, unfragmented form, as illustrated:

      +-------------------------------//------------------------------+
      |                        Original packet                        |
      |             (e.g. an IPv4, IPv6, Ethernet packet)             |
      +------------------------------//-------------------------------+

   The following rules govern reassembly:

        The IP/UDP/GUE headers of each packet are retained until all
        fragments have arrived. The reassembled packet is then composed
        of the decapsulated payloads in the GUE packets, and the
        IP/UDP/GUE headers are discarded.

        When a GUE packet is received with the fragment extension, the
        proto/ctype field in the GUE header MUST be validated. In the
        case that the packet is a first fragment (fragment offset is
        zero), the proto/ctype in the GUE header MUST equal the orig-
        proto value in the fragmentation option. For subsequent
        fragments, (fragment offset is non-zero) the proto/ctype in the



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        GUE header must be 0 for a control message or 59 (no-next-hdr)
        for a data message. If the proto/ctype value is invalid for a
        received packet it MUST be dropped.

        An original packet is reassembled only from GUE fragment packets
        that have the same outer source address, destination address,
        UDP source port, UDP destination port, GUE header C-bit, group
        identifier if present, orig-proto value in the fragmentation
        option, and Fragment Identification. The protocol type or
        control message type (depending on the C-bit) for the
        reassembled packet is the value of the GUE header proto/ctype
        field in the first fragment.

   The following error conditions can arise when reassembling fragmented
   packets with GUE encapsulation:

        If insufficient fragments are received to complete reassembly of
        a packet within 60 seconds (or a configurable period) of the
        reception of the first-arriving fragment of that packet,
        reassembly of that packet MUST be abandoned and all the
        fragments that have been received for that packet MUST be
        discarded.

        If the payload length of a fragment is not a multiple of 8
        octets and the M flag of that fragment is 1, then that fragment
        MUST be discarded.

        If the length and offset of a fragment are such that the payload
        length of the packet reassembled from that fragment would exceed
        65,535 octets, then that fragment MUST be discarded.

        If a fragment overlaps another fragment already saved for
        reassembly then the new fragment that overlaps the existing
        fragment MUST be discarded.

   If the first fragment is too small then it is possible that it does
   not contain the necessary headers for a stateful firewall. Sending
   small fragments like this has been used as an attack on IP
   fragmentation. To mitigate this problem, an implementation SHOULD
   ensure that the first fragment contains the headers of the
   encapsulated packet at least through the transport header.

   A GUE node MUST be able to accept a fragmented packet that, after
   reassembly and decapsulation, is as large as 1500 octets. This means
   that the node must configure a reassembly buffer that is at least as
   large as 1500 octets plus the maximum-sized encapsulation headers
   that may be inserted during encapsulation. Implementations may find
   it more convenient and efficient to configure a reassembly buffer



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   size of 2KB which is large enough to accommodate even the largest set
   of encapsulation headers and provides a natural memory page size
   boundary.

5.6.  Security Considerations

   Exploits that have been identified with IP fragmentation are
   conceptually applicable to GUE fragmentation.

   Attacks on GUE fragmentation can be mitigated by:

      o Hardened implementation that applies applicable techniques from
        implementation of IP fragmentation.

      o Application of GUE security (section 4) or IPsec [RFC4301].
        Security mechanisms can prevent spoofing of fragments from
        unauthorized sources.

      o Implement fragment filter techniques for GUE encapsulation as
        described in [RFC1858] and [RFC3128].

      o Do not accept data in overlapping segments.

      o Enforce a minimum size for the first fragment.

6.  Payload transform option

   The payload transform option indicates that the GUE payload has been
   transformed. Transforming a payload is done by running a function
   over the data and possibly modifying it (encrypting it for instance).
   The payload transform option indicates the method used to transform
   the data so that a decapsulator is able to validate and reverse the
   transformation to recover the original data. Payload transformations
   could include encryption, authentication, CRC coverage, and
   compression. This specification defines a transformation for DTLS.

6.1.  Extension field format

   The presence of the GUE payload transform option is indicated by the
   T bit in the GUE header.

   The format of Payload Transform Field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type       |     P_C_type  |            Info               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The fields of the option are:

      Type: Payload Transform Type or Code point. Each payload transform
            mechanism must have one code point registered in IANA.  This
            document specifies:

               0x01: for DTLS [RFC6347]

               0x80~0xFF: for private payload transform types

            A private payload transform type can be used for
            experimental purposes or proprietary mechanisms.

      P_C_type: Indicates the protocol or control type of the
            untransformed payload. When payload transform option is
            present, proto/ctype in the GUE header is set to 59 ("No
            next header") for a data message and zero for a control
            message. The IP protocol or control message type of the
            untransformed payload MUST be encoded in this field.

            The benefit of this rule is to prevent a middle box from
            inspecting the encrypted payload according to GUE next
            protocol. The assumption here is that a middle box may
            understand GUE base header but does not understand GUE
            option flag definitions.

      Info: A field that can be set according to the requirements of
            each payload transform type. If the specification for a
            payload transform type does not specify how this field is to
            be set, then the field MUST be set to zero.

6.2.  Usage

   The payload transform option provides a mechanism to transform or
   interpret the payload of a GUE packet. The Type field provides the
   method used to transform the payload, and the P_C_type field provides
   the protocol or control message type of the of payload before being
   transformed. The payload transformation option is generic so that it
   can have both security related uses (such as DTLS) as well as non
   security related uses (such as compression, CRC, etc.).

   An encapsulator performs payload transformation before transmission,
   and a decapsulator performs the reverse transformation before
   accepting a packet. For example, if an encapsulator transforms a
   payload by encrypting it, the peer decaspsulator MUST decrypt the
   payload before accepting the packet. If a decapsulator fails to
   perform the reverse transformation or cannot validate the
   transformation it MUST discard the packet and MAY generate an alert



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   to the management system.

6.3.  Interaction with other optional extensions

   If GUE fragmentation (section 5) is used in concert with the GUE
   transform option, the transform option processing is performed after
   fragmentation at the encapsulator and before reassembly at the
   decapsulator. If the payload transform changes the size of the data
   being fragmented this must be taken into account during
   fragmentation.

   If both the security option and the payload transform are used in a
   GUE packet, an encapsulator MUST perform the payload transformation
   first, set the payload transform option in the GUE header, and then
   create the security option. A decapsulator does processing in
   reverse-- the security option is processed (GUE header is validated)
   and then the reverse payload transform is performed.

   In order to get flow entropy from the payload, an encapsulator should
   derive the flow entropy before performing a payload transform.

6.4.  DTLS transform

   The payload of a GUE packet can be secured using Datagram Transport
   Layer Security [RFC6347]. An encapsulator would apply DTLS to the GUE
   payload so that the payload packets are encrypted and the GUE header
   remains in plaintext. The payload transform option is set to indicate
   that the payload is interpreted as a DTLS record.

   The payload transform option for DTLS 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      1        |    P_C_type   |              0                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   DTLS [RFC6347] provides a packet fragmentation capability. To avoid
   packet fragmentation being performed multiple times, a GUE
   encapsulator SHOULD use GUE fragmentation and not DTLS fragmentation.

   DTLS usage is limited to a single DTLS session for any specific
   tunnel encapsulator/decapsulator pair (identified by source and
   destination IP addresses). Both IP addresses MUST be unicast
   addresses - multicast traffic is not supported when DTLS is used. A
   GUE tunnel decapsulator implementation that supports DTLS can
   establish DTLS sessions with one or multiple tunnel encapsulators,
   and likewise a GUE tunnel encapsulator implementation can establish



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   DTLS sessions with one or multiple decapsulators.

7.  Remote checksum offload option

   Remote checksum offload is mechanism that provides checksum offload
   of encapsulated packets using rudimentary offload capabilities found
   in most Network Interface Card (NIC) devices. Many NIC
   implementations can only offload the outer UDP checksum in UDP
   encapsulation. Remote checksum offload is described in [UDPENCAP].

   In remote checksum offload the outer header checksum, that in the
   outer UDP header, is enabled in packets and, with some additional
   meta information, a receiver is able to deduce the checksum to be set
   for an inner encapsulated packet. Effectively this offloads the
   computation of the inner checksum. Enabling the outer checksum in
   encapsulation has the additional advantage that it covers more of the
   packet, including the encapsulation headers, than an inner checksum.

7.1.  Extension field format

   The presence of the GUE remote checksum offload option is indicated
   by the R bit in the GUE header.

   The format of remote checksum offload field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Checksum start         |       Checksum offset         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o Checksum start: starting offset for checksum computation
        relative to the start of the encapsulated payload. This is
        typically the offset of a transport header (e.g. UDP or TCP).

      o Checksum offset: Offset relative to the start of the
        encapsulated packet where the derived checksum value is to be
        written. This typically is the offset of the checksum field in
        the transport header (e.g. UDP or TCP).

7.2.  Usage

7.2.1. Transmitter operation

   The typical actions to set remote checksum offload on transmit are:




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      1) Transport layer creates a packet and indicates in internal
         packet meta data that checksum is to be offloaded to the NIC
         (normal transport layer processing for checksum offload). The
         checksum field is populated with the bitwise not of the
         checksum of the pseudo header or zero as appropriate.

      2) Encapsulation layer adds its headers to the packet including
         the remote checksum offload option. The start offset and
         checksum offset are set accordingly.

      3) Encapsulation layer arranges for checksum offload of the outer
         header checksum (e.g. UDP).

      4) Packet is sent to the NIC. The NIC will perform transmit
         checksum offload and set the checksum field in the outer
         header. The inner header and rest of the packet are transmitted
         without modification.

7.2.2. Receiver operation

   The typical actions a host receiver does to support remote checksum
   offload are:

      1) Receive packet and validate outer checksum following normal
         processing (i.e. validate non-zero UDP checksum).

      2) Validate the remote checksum option. If checksum start is
         greater than the length of the packet, then the packet MUST be
         dropped. If checksum offset is greater then the length of the
         packet minus two, then the packet MUST be dropped.

      3) Deduce full checksum for the IP packet. If a NIC is capable of
         receive checksum offload it will return either the full
         checksum of the received packet or an indication that the UDP
         checksum is correct. Either of these methods can be used to
         deduce the checksum over the IP packet [UDPENCAP].

      4) From the packet checksum, subtract the checksum computed from
         the start of the packet (outer IP header) to the offset in the
         packet indicted by checksum start in the meta data. The result
         is the deduced checksum to set in the checksum field of the
         encapsulated transport packet.

         In pseudo code:

           csum: initialized to checksum computed from start (outer IP
                 header) to the end of the packet
           start_of_packet: address of start of packet



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           encap_payload_offset: relative to start_of_packet
           csum_start: value from the checksum start field
           checksum(start, len): function to compute checksum from start
                 address for len bytes

           csum -= checksum(start_of_packet, encap_payload_offset +
                            csum_start)

      5) Write the resultant checksum value into the packet at the
         offset provided by checksum offset in the meta data.

         In pseudo code:

           csum_offset: value from the checksum offset field

           *(start_of_packet + encap_payload_offset +
             csum_offset) = csum

      6) Checksum is verified at the transport layer using normal
         processing. This should not require any checksum computation
         over the packet since the complete checksum has already been
         provided.

7.3.  Security Considerations

   Remote checksum offload allows a means to change the GUE payload
   before being received at a decapsulator. In order to prevent misuse
   of this mechanism, a decapsulator MUST apply security checks on the
   GUE payload only after checksum remote offload has been processed.

8.  Checksum option

   The GUE checksum option provides a checksum that covers the GUE
   header, a GUE pseudo header, and optionally all or part of the GUE
   payload. The GUE pseudo header includes the corresponding IP
   addresses as well as the UDP ports of the encapsulating headers. This
   checksum should provide protection against address corruption in IPv6
   when the UDP checksum is zero. Additionally, the GUE checksum
   provides protection of the GUE header when the UDP checksum is set to
   zero with either IPv4 or IPv6. In particular, the GUE checksum can
   provide protection for some sensitive data, such as the virtual
   network identifier ([I.D.hy-nvo3-gue-4-nvo]), which when corrupted
   could lead to mis-delivery of a packet to the wrong virtual network.

8.1.  Extension field format

   The presence of the GUE checksum option is indicated by the K bit in
   the GUE header.



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   The format of the checksum extension 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |        Payload coverage       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o Checksum: Computed checksum value. This checksum covers the GUE
        header (including fields and private data covered by Hlen), the
        GUE pseudo header, and optionally all or part of the payload
        (encapsulated packet).

      o Payload coverage: Number of bytes of payload to cover in the
        checksum. Zero indicates that the checksum only covers the GUE
        header and GUE pseudo header. If the value is greater than the
        encapsulated payload length, the packet MUST be dropped.

8.2.  Requirements

   The GUE header checksum SHOULD be set on transmit when using a zero
   UDP checksum with IPv6.

   The GUE header checksum SHOULD be used when the UDP checksum is zero
   for IPv4 if the GUE header includes data that when corrupted can lead
   to misdelivery or other serious consequences, and there is no other
   mechanism that provides protection (no security field that checks
   integrity for instance).

   The GUE header checksum SHOULD NOT be set when the UDP checksum is
   non-zero. In this case the UDP checksum provides adequate protection
   and this avoids convolutions when a packet traverses NAT that does
   address translation (in that case the UDP checksum is required).

8.3.  GUE checksum pseudo header

   The GUE pseudo header checksum is included in the GUE checksum to
   provide protection for the IP and UDP header elements which when
   corrupted could lead to misdelivery of the GUE packet. The GUE pseudo
   header checksum is similar to the standard IP pseudo header defined
   in [RFC0768] and [RFC0793] for IPv4, and in [RFC8200] for IPv6.








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   The GUE pseudo header for IPv4 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 Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Destination Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Source port            |      Destination port         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The GUE pseudo header for IPv6 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 Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Source port            |      Destination port         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The GUE pseudo header does not include payload length or protocol as
   in the standard IP pseudo headers. The length field is deemed
   unnecessary for inclusion because a corrupted length field should not
   cause mis-delivery, the GUE checksum is applied after GUE
   fragmentation, and without the length field the GUE pseudo header
   checksum is the same for all packets of flow.

8.4.  Usage

   The GUE checksum is computed and verified following the standard
   process for computing the Internet checksum [RFC1071]. Checksum
   computation may be optimized per the mathematical properties



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   including parallel computation and incremental updates.

8.4.1. Transmitter operation

   The procedure for setting the GUE checksum on transmit is:

      1) Create the GUE header including the checksum and payload
         coverage fields. The checksum field is initially set to zero.

      2) Calculate the 1's complement checksum of the GUE header from
         the start of the GUE header through the its length as indicated
         in GUE Hlen.

      3) Calculate the checksum of the GUE pseudo header for IPv4 or
         IPv6.

      4) Calculate checksum of payload portion if payload coverage is
         enabled (payload coverage field is non-zero). If the length of
         the payload coverage is odd, logically append a single zero
         byte for the purposes of checksum calculation.

      5) Add and fold the computed checksums for the GUE header, GUE
         pseudo header, and payload coverage.

      6) Set the bitwise not of the resultant value in the GUE checksum
         field.

8.4.2. Receiver operation

   If the GUE checksum option is present, the receiver MUST validate the
   checksum before processing any other fields or accepting the packet.

   The procedure for verifying the checksum is:

      1) If the payload coverage length is greater than the length of
         the encapsulated payload then drop the packet.

      2) Calculate the checksum of the GUE header from the start of the
         header to the end as indicated by Hlen.

      3) Calculate the checksum of the appropriate GUE pseudo header.

      4) Calculate the checksum of payload if payload coverage is
         enabled (payload coverage is non-zero). If the length of the
         payload coverage is odd logically append a single zero byte for
         the purposes of checksum calculation.

      5) Sum the computed checksums for the GUE header, GUE pseudo



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         header, and payload coverage. If the result is all 1 bits (-0
         in 1's complement arithmetic), the checksum is valid and the
         packet is accepted; otherwise the checksum is considered
         invalid and the packet MUST be dropped.

8.5.  Corrupted flags

   Note that the GUE checksum does not protect against the checksum flag
   (K flag) being corrupted. If an encapsulator sets the checksum flag
   and option but the K bit flips to be zero, then a decapsulator will
   incorrectly process the GUE packet as not having a checksum field.

   To mitigate this issue an encapsulator and depcapsulator might agree
   that checksum is always required. This agreement could be established
   by configuration or GUE capability negotiation.

8.6.  Security Considerations

   The checksum option is only a mechanism for corruption detection, it
   is not a security mechanism. To provide integrity checks or
   authentication of the GUE header, the GUE security option SHOULD be
   used.

9.  NAT checksum address option

   The NAT address checksum (NAC) option contains the checksum computed
   over the IP addresses of the packet. The computed value can be used
   by a receiver to adjust a transport layer checksum that was affected
   by NAT changing the IP addresses. This option is only useful when GUE
   encapsulates a transport layer packet that has a checksum with a
   pseudo header that includes the IP addresses (such as TCP or UDP).

9.1.  Extension field format

   The presence of the NAT checksum address option is indicated by the N
   bit in the GUE header.

   The format of the NAT checksum address extension 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o Checksum: Computed checksum value. This checksum covers the



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        outer IP addresses.

      o Reserved: Must be zero on transmit.

9.2.  Usage

   The NAT address extension SHOULD be set on transmit when
   encapsulating a transport layer packet whose checksum might be
   affected by NAT being performed on the outer IP header. If this
   option is used then the UDP checksum MUST be used (sent with non-zero
   value).

   The NAT address checksum is computed using the Internet checksum
   [RFC1071].

9.2.1. Transmitter operation

   The procedure for setting the GUE checksum on transmit is:

   An encapsulator computes the checksum value over the IP addresses in
   the IP header.

      1) Compute the ones complement checksum over the source and
         destination IPv4 or IPv6 addresses.

      2) Set the resultant value in the Checksum field.

   For IPv4 the checksum is computed over:

    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 Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Destination Address                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+















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   For IPv6 the checksum is computed over:

    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 Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Destination Address                      +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.2.2. Receiver operation

      1) Validate the UDP checksum is correct

      2) Compute the checksum over the IP address in the received packet

      3) Subtract the resultant from the checksum value in the NAC
         option. If the difference is non-zero then NAT has changed the
         addresses

      4) When processing a transport layer containing a checksum
         affected by NAT on the IP addresses, add the difference into
         the checksum calculation when verify the packet.

   In pseudo codes this is:

      actual = checksum(IP addresses)
      diff = actual - NAC_value
      verify = checksum(transport packet) + checksum(pseudo header)
               + diff

      if (verify == 0)
        packet is good

10. Alternative checksum option




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   The alternative checksum option contains a check over the GUE header
   and optionally all or part of the GUE payload. The algorithm used is
   CRC-32C which is the same as that used by iSCSI and SCTP. The
   alternative checksum can provide stronger protection against data
   corruption than provided by the one's complement checksum used in the
   UDP checksum or the GUE checksum (section 8).

10.1. Extension field format

   The presence of the GUE alternate checksum option is indicated by the
   A bit in the GUE header.

   The format of alternate checksum field 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |        Payload coverage       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              CRC                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of the option are:

      o CRC: Computed CRC value. This CRC covers the GUE header
        (including fields and private data covered by Hlen) and
        optionally all or part of the payload (encapsulated packet).

      o Payload coverage: Number of bytes of payload to cover in the CRC
        calculation. Zero indicates that the checksum only covers the
        GUE header. If the value is greater than the encapsulated
        payload length, the packet MUST be dropped.

10.2. Usage

   The 32-bit alternative checksum does not include a pseudo header
   containing IP addresses or ports.

   CRC-32C is calculated using the 0x1EDC6F41 polynomial:

      x^32 + x^28 + x^27 + x^26 + x^25 + x^23 + x^22 + x^20 + x^19 +
      x^18 + x^14 +x^13 + +x^11 + x^10 +x^9 +x ^8 +x^ + 1

   The procedure for setting the GUE alternative checksum on transmit
   is:

      1) Create the GUE header including the alternative checksum field.
         The CRC field is initialized to zero.



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      2) Calculate the CRC using the CRC-32C algorithm from the start of
         the GUE header through the its length as indicated in GUE Hlen.

      3) Continue the calculation to cover the payload portion if
         payload coverage is enabled (payload coverage field is non-
         zero). If the length of the payload coverage is not aligned to
         four bytes, logically append zero bytes up to the necessary
         alignment for the purposes of CRC calculation.

      4) Set the resultant value in the CRC field.

10.2.2. Receiver operation

   If the GUE alternative checksum option is present, the receiver MUST
   validate the checksum before processing any other fields or accepting
   the packet.

   The procedure for verifying the alternate checksum is:

      1) If the payload coverage length is greater than the length of
         the encapsulated payload then drop the packet.

      2) Note value in the CRC field and set the CRC field to zero.

      3) Calculate the CRC using the CRC-32C algorithm from the start of
         the GUE header through the its length as indicated in GUE Hlen.

      4) Continue the calculation to cover the payload portion if
         payload coverage is enabled (payload coverage field is non-
         zero). If the length of the payload coverage is not aligned to
         four bytes, logically append zero bytes up to the necessary
         alignment for the purposes of CRC calculation.

      5) Compare the computed value with the original value of the CRC
         field. If they are equal then that packet is accepted, if they
         are not equal then verification failed and the packet MUST be
         dropped.

      6) Restore the CRC field to its original value.

10.3. Corrupted flags

   Similar to GUE checksum properties, the GUE alternate checksum does
   not protect against the alternative checksum flag (A flag) being
   corrupted. If an encapsulator sets the alternative checksum flags and
   option but the A bit flips to be zero, then a decapsulator will
   incorrectly process that packet as not having an alternate checksum
   field.



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   To mitigate this issue an encapsulator and depcapsulator might agree
   that an alternate checksum is always required. This agreement could
   be established by configuration or GUE capability negotiation.

10.4. Security Considerations

   The alternate checksum option is only a mechanism for corruption
   detection, it is not a security mechanism. To provide integrity
   checks or authentication of the GUE header, the GUE security option
   SHOULD be used.

11. Processing order of options

   Options MUST be processed in a specific order for both transmission
   and reception. Note that some options, such as the checksum option,
   depend on other fields in the GUE header to be initialized.

11.1. Processing order when sending

   When setting the security option (HMAC option in particular), the
   checksum option, and the alternate checksum option-- all the GUE
   fields being used must be present and properly set in the header. The
   checksum value in the checksum option or alternate checksum option
   MUST be initialized to zero to ensure consistent HMAC and checksum
   calculation.

   The order of processing options to send a GUE packet are:

      1) Fragment if necessary and set fragmentation option. If the
         group identifier is present it is copied into each fragment. If
         payload transformation will increase the size of the payload
         that MUST be accounted for when deciding how to fragment. Apply
         processing below for each fragment

      2) Set group identifier option (to the same value for each
         fragment)

      3) Perform payload transform (potentially on a fragment) and set
         payload transform option.

      4) Set Remote checksum offload.

      5) Set NAT address checksum option.

      6) Set security option per cookie or HMAC calculation.

      7) Calculate GUE alternate checksum and set the alternate checksum
         option.



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      8) Calculate GUE checksum and set checksum option.

11.2. Processing order when receiving

   On reception the order of actions is:

      1) Verify GUE checksum.

      2) Verify alternate checksum. If the GUE checksum option is
         present, set its checksum fields to zero for computing the
         alternate checksum. After computation, restore the checksum
         value in the GUE checksum field.

      3) Verify security option. If the GUE checksum option or alternate
         checksum option are also present and HMAC computation is being
         done over the GUE header, then set the checksum fields to zero
         for computing the HMAC. After computation, restore the checksum
         values.

      4) Save the NAT address checksum value. It will be applied when
         processing the encapsulated packet.

      5) Adjust packet for remote checksum offload.

      6) Perform payload transformation (i.e. decrypt payload).

      7) Perform reassembly.

      8) Process packet (take group identifier into account if present).

   The relative processing order of GUE extensions and private fields is
   unspecified in this specification.

12. Security Considerations

   Encapsulation of a network protocol in GUE should not increase
   security risk, nor provide additional security in itself. GUE
   requires that the source port for UDP packets SHOULD be randomly
   seeded to mitigate some possible denial service attacks.

   If the integrity and privacy of data packets being transported
   through GUE is a concern, GUE security option and payload encryption
   using the the transform option SHOULD be used to remove the concern.
   If the integrity is the only concern, the tunnel may consider use of
   GUE security only for optimization. Likewise, if privacy is the only
   concern, the tunnel may use a GUE transform for encryption only.

   If GUE payload already provides secure mechanism, e.g., the payload



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   is IPsec packets, it is still valuable to consider use of GUE
   security.

   GUE may rely on other secure tunnel mechanisms such as DTLS [RFC6347]
   over the whole UDP payload for securing the whole GUE packet or IPsec
   [RFC4301] to achieve the secure transport over an IP network or
   Internet.

   IPsec [RFC4301] was designed as a network security mechanism, and
   therefore it resides at the network layer.  As such, if the tunnel is
   secured with IPsec, the UDP header would not be visible to
   intermediate routers in either IPsec tunnel or transport mode. This
   is a drawback since it prohibits intermediate routers to perform load
   balancing based on the flow entropy in UDP header. In addition, this
   method prohibits any middle box function on the path.

   By comparison, DTLS [RFC6347] was designed for application level
   security and can better preserve network and transport layer protocol
   information than IPsec [RFC4301]. Using DTLS over UDP to secure the
   GUE tunnel, both GUE header and payload will be encrypted. In order
   to differentiate plaintext GUE header from encrypted GUE header, the
   destination port of the UDP header between two must be different,
   which essentially requires another standard UDP port for GUE with
   DTLS. The drawback on this method is to prevent a middle box
   operation to GUE tunnel on the path.

   Use of two independent tunnel mechanisms such as GUE and DTLS over
   UDP to carry a network protocol over an IP network adds some overlap
   and complexity. For example, fragmentation will be done twice.

   As the result, a GUE tunnel SHOULD use the security mechanisms
   specified in this document to provide secure transport over an IP
   network or Internet when it is needed. GUE encapsulation can be used
   as a secure transport mechanism over an IP network and Internet.

13. IANA Consideration

   IANA is requested to assign flags for the extensions defined in this
   specification. Specifically, an assignment is requested for the Group
   Identfier, Security, Fragmentation, Payload Transform, Remote
   Checksum Offload, Checksum, NAT Checksum, and Alternate Checksum
   extensions in the "GUE flag-fields" registry.

      +-------------+---------------+-------------+--------------------+
      |  Flags bits | Field size    | Description | Reference          |
      +-------------+---------------+-------------+--------------------+
      | Bit 0       | 4 bytes       | Group       | This document      |
      |             |               |  identifier |                    |



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      |             |               |             |                    |
      | Bit 1..3    | 001->8 bytes  | Security    | This document      |
      |             | 010->16 bytes |             |                    |
      |             | 011->32 bytes |             |                    |
      |             | 100->40 bytes |             |                    |
      |             |               |             |                    |
      | Bit 4       | 8 bytes       | Fragmen-    | This document      |
      |             |               |  tation     |                    |
      |             |               |             |                    |
      | Bit 5       | 4 bytes       | Payload     | This document      |
      |             |               |  transform  |                    |
      |             |               |             |                    |
      | Bit 6       | 4 bytes       | Remote      | This document      |
      |             |               |  checksum   |                    |
      |             |               |  offload    |                    |
      |             |               |             |                    |
      | Bit 7       | 4 bytes       | Checksum    | This document      |
      |             |               |             |                    |
      | Bit 8       | 4 bytes       | NAT         | This document      |
      |             |               |  checksum   |                    |
      |             |               |  address    |                    |
      |             |               |             |                    |
      | Bit 9       | 4 bytes       | Alternate   | This document      |
      |             |               |  checksum   |                    |
      |             |               |             |                    |
      | Bit 10..15  |               | Unassigned  |                    |
      +-------------+---------------+-------------+--------------------+

   IANA is requested to set up a registry for the GUE payload transform
   types. Payload transform types are 8 bit values.  New values for
   control types 1-127 are assigned via Standards Action [RFC5226].




















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      +----------------+------------------+---------------+
      | Transform type | Description      | Reference     |
      +----------------+------------------+---------------+
      | 0              | Reserved         | This document |
      |                |                  |               |
      | 1              | DTLS             | This document |
      |                |                  |               |
      | 2..127         | Unassigned       |               |
      |                |                  |               |
      | 128..255       | User defined     | This document |
      +----------------+------------------+---------------+

14. References

14.1. Normative References

   [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
             1981.

   [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", STD 86, RFC 8200, DOI
             10.17487/RFC8200, July 2017, <https://www.rfc-
             editor.org/info/rfc8200>.

   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
             Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
             August 1980.

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI
             10.17487/RFC5226, May 2008, <http://www.rfc-
             editor.org/info/rfc5226>.

   [I.D.ietf-gue] T. Herbert, L. Yong, and O. Zia, "Generic UDP
             Encapsulation" draft-ietf-intarea-gue-01

14.2. Informative References

   [RFC3931] Lau, J., Townsley, W., et al, "Layer Two Tunneling Protocol
             - Version 3 (L2TPv3)", RFC3931, 1999

   [RFC2104] Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol" , RFC 2401, DOI 10.17487/RFC2401,
             November 1998, <http://www.rfc-editor.org/info/rfc2401>.

   [RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain of



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             Interpretation" , RFC 6407, DOI 10.17487/RFC6407, October
             2011, <http://www.rfc-editor.org/info/rfc6407>.

   [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>.

   [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
             Errors at High Data Rates", RFC 4963, DOI 10.17487/RFC4963,
             July 2007, <http://www.rfc-editor.org/info/rfc4963>.

   [RFC2764] B. Gleeson, A. Lin, J. Heinanen, G. Armitage, A. Malis, "A
             Framework for IP Based Virtual Private Networks", RFC2764,
             February 2000.

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, December 2005.

   [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
             Considerations for IP Fragment Filtering", RFC 1858,
             October 1995.

   [RFC3128] Miller, I., "Protection Against a Variant of the Tiny
             Fragment Attack (RFC 1858)", RFC 3128, June 2001.

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

   [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, DOI 10.17487/RFC0793, September 1981, <http://www.rfc-
             editor.org/info/rfc793>.

   [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>.

   [RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the
             Internet checksum", RFC1071, September 1988.

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

   [I.D.previdi-6man-sr-header] Previdi S. et al, "IPv6 Segment Routing
             Header (SRH) draft-ietf-6man-segment-routing-header-02

   [I.D.templin-aerolink] F. Templin, "Transmission of IP Packets over



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             AERO Links" draft-templin-aerolink-62

   [UDPENCAP] T. Herbert, "UDP Encapsulation in Linux",
             http://people.netfilter.org/pablo/netdev0.1/papers/UDP-
             Encapsulation-in-Linux.pdf

   [FIPS180-4] Secure Hash Standard (SHS), Nation Institute of Standards
             and Technology, 8/2015











































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Authors' Addresses

   Tom Herbert
   Quantonium
   4701 Patrick Henry Dr.
   Santa Clara, CA
   USA

   EMail: tom@herbertland.com


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

   Email: lucy.yong@huawei.com


   Fred L. Templin
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org
























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