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INTERNET-DRAFT                                                T. Herbert
Intended Status: Proposed Standard                              Facebook
Expires: April 2016                                           F. Templin
                                            Boeing Research & Technology
                                                        October 19, 2015


           Fragmentation option for Generic UDP Encapsulation
                   draft-herbert-gue-fragmentation-02


Abstract

   This specification describes a fragmentation and reassembly
   capability with an associated header option for Generic UDP
   Encapsulation.

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
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   Internet-Drafts.

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Copyright and License Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Motivation  . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  4
   2  Option format . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1 Fragmentation  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2 Reassembly . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4  Security Considerations . . . . . . . . . . . . . . . . . . . . 11
   5  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 11
   6  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 11
   7  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1  Normative References  . . . . . . . . . . . . . . . . . . . 11
     7.2  Informative References  . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12



























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1  Introduction

   This specification describes a fragmentation and reassembly
   capability in Generic UDP Encapsulation (GUE) [I.D.ietf-nvo3-gue].
   This entails adding a GUE option and procedures for fragmentation and
   reassembly in the encapsulation layer. This specification adapts the
   procedures for IP fragmentation and reassembly described in [RFC0791]
   and [RFC2460]. Fragmentation may be performed on both data and
   control messages in GUE.

1.1  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



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   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
   introduces problems with wraparound as described in [FRAGHRM].

   The second possibility 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 that operates at
   the tunnel level. In this method fragmentation is part of the
   encapsulation and an encapsulation header contains the information
   for reassembly. This is different 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 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
        virtual network 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.

1.2  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.

1.3  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].



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2  Option format

   Fragments in GUE are sent with a fragmentation option in the GUE
   header. The format of this 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   |    Reserved   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Identification                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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 C bit is set) or the IP
        protocol (when C bit is not set) of the fragmented packet.

      o Reserved: Must be set to 0 on transmission. If set to non-zero
        in a received packet then the packet MUST be dropped.

      o Identification: Identifies fragments of a fragmented packet.




















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   The format of the fragmentation option within the GUE header 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0x0|C|   Hlen  |  Proto/ctype  |V|SEC|K|F|     Flags         |E|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~            VNID, security fields, checksum (optional)         ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Fragment offset      |Res|M|   Orig-proto  |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Identification                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Extension flags(optional)                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                   Extension fields (optional)                 ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Pertinent fields to fragmentation are:


      o C: This bit 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 option
        field.

3  Procedures

3.1 Fragmentation

   If an encapsulator determines that a packet must be fragmented (eg.
   the packets size exceed the Path MTU of the tunnel) it may 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



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   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 virtual network 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         |
      |      header      | w/ frag option |        fragment       |
      +------------------+----------------+-----------------------+

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



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   Each fragment packet is composed of:

      (1) Outer IP and UDP headers as defined for GUE encapsulation.

          o The IP addresses and UDP destination port must be the same
            for all fragments of a fragmented packet.

          o The source port selected for the inner flow identifier must
            be the same value for all fragments of a fragmented packet.

      (2) A GUE header that contains:

          o The C bit which is set to the same value for all the
            fragments of a fragmented packet based on whether a control
            message or data message was fragmented.

          o A proto/ctype. In the first fragment this is set to the
            value corresponding to the next header of the original
            packet and will be either an IP protocol or a control type.
            For subsequent fragments, this field is set to 0 for a
            fragmented control message or 59 (no next header) for a
            fragmented data messages.

          o The F bit is set and fragment option is present. See below.

          o Other GUE options. Note that options apply to the individual
            GUE packet. For instance, the security option would be
            validated before reassembly.

      (2) The GUE fragmentation option. The option contents include:

          o Orig-proto that identifies the first header of the original
            packet.

          o A Fragment Offset containing the offset of the fragment, in
            8-octet units, relative to the start of the of the original
            packet.  The Fragment Offset of the first ("leftmost")
            fragment is 0.

          o An M flag value of 0 if the fragment is the last
            ("rightmost") one, else an M flag value of 1.

          o The Identification value generated for the original packet.

      (3) The fragment itself.


3.2 Reassembly



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   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 fragments, and the
        IP/UDP/GUE headers are discarded

        When a GUE packet is received with the fragment option, the
        proto/ctype 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 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 then the packet
        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, virtual
        network 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 may 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 length of a fragment, as derived from the GUE fragment
        packet's Payload Length field, is not a multiple of 8 octets and
        the M flag of that fragment is 1, then that fragment must be



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        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 portion of data in the new fragment that
        overlaps the existing fragment must be ignored.

        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.



































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4  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 [I.D.hy-gue-4-secure-transport] 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 accepted data in overlapping segments.

      o Enforce a minimum size for the first fragment.

5  IANA Considerations

   GUE fragmentation defines one flag bit in the GUE header and a
   corresponding 64-bit field.

6  Acknowledgements

   Motivations for including an encapsulation fragment header option
   were discussed on the int-area mailing list in the August 2015
   timeframe.

7  References

7.1  Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997,
             <http://www.rfc-editor.org/info/rfc2119>.

   [I.D.ietf-nvo3-gue] T. Herbert, L. Yong, and O. Zia, "Generic UDP
             Encapsulation" draft-ietf-nvo3-gue-01

7.2  Informative References

   [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
             1981, <http://www.rfc-editor.org/info/rfc791>.




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   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998,
             <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
             Malis, "A Framework for IP Based Virtual Private Networks",
             RFC 2764, February 2000, <http://www.rfc-
             editor.org/info/rfc2764>.

   [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
             Considerations for IP Fragment Filtering", RFC 1858,
             October 1995, <http://www.rfc-editor.org/info/rfc1858>.

   [RFC3128] Miller, I., "Protection Against a Variant of the Tiny
             Fragment Attack (RFC 1858)", RFC 3128, June 2001,
             <http://www.rfc-editor.org/info/rfc3128>.

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, December 2005,
             <http://www.rfc-editor.org/info/rfc4301>.

   [I.D.templin-aerolink] F. Templin, "Transmission of IP Packets over
             AERO Links" draft-templin-aerolink-62.txt

   [FRAGHRM] M. Mathis, J, Heffner, and B. Chandler, "Fragmentation
             Considered Very Harmful", draft-mathis-frag-harmful-00

   [I.D.hy-gue-4-secure-transport] L. Yong and T. Herbert, "Generic UDP
             Encapsulation (GUE) for Secure Transport" draft-hy-gue-4-
             secure-transport-02

Authors' Addresses

   Tom Herbert
   Facebook
   Menlo Park, CA
   USA

   Email: tom@herbertland.com

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

   Email: fltemplin@acm.org




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