Network Working Group                                    F. Templin, Ed.
Internet-Draft                                      Boeing Phantom Works
Intended status: Informational                              May 30, 2008
Expires: December 1, 2008

        The Subnetwork Encapsulation and Adaptation Layer (SEAL)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   Subnetworks are connected network regions bounded by border nodes
   that forward unicast and multicast packets over a virtual topology,
   often manifested by encapsulation and/or tunneling.  This virtual
   topology may span multiple IP- and/or sub-IP layer forwarding hops,
   and can introduce failure modes due to packet duplication and/or
   links with diverse Maximum Transmission Units (MTUs).  This document
   specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that
   accommodates such virtual topologies over diverse underlying link

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Requirements . . . . . . . . . . . . . . . . .  4
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  5
   4.  SEAL Protocol Specification - Tunnel Mode  . . . . . . . . . .  6
     4.1.  Model of Operation . . . . . . . . . . . . . . . . . . . .  6
     4.2.  ITE Specification  . . . . . . . . . . . . . . . . . . . .  7
       4.2.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . .  7
       4.2.2.  Accounting for Headers . . . . . . . . . . . . . . . .  8
       4.2.3.  Segmentation and Encapsulation . . . . . . . . . . . .  9
       4.2.4.  Packet Identification  . . . . . . . . . . . . . . . . 11
       4.2.5.  Sending SEAL Protocol Packets  . . . . . . . . . . . . 12
       4.2.6.  Sending Probes . . . . . . . . . . . . . . . . . . . . 12
       4.2.7.  Processing Raw ICMPv4 Messages . . . . . . . . . . . . 13
       4.2.8.  Processing SEAL-Encapsulated ICMPv4 Messages . . . . . 13
     4.3.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 14
       4.3.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 14
       4.3.2.  IPv4-Layer Reassembly  . . . . . . . . . . . . . . . . 14
       4.3.3.  Generating SEAL-Encapsulated ICMPv4 Fragmentation
               Needed Messages  . . . . . . . . . . . . . . . . . . . 14
       4.3.4.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 16
       4.3.5.  Decapsulation and Generating Other ICMPv4 Errors . . . 16
   5.  SEAL Protocol Specification - Transport Mode . . . . . . . . . 17
   6.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 17
   7.  End System Requirements  . . . . . . . . . . . . . . . . . . . 17
   8.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 18
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     12.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Historic Evolution of PMTUD (written 10/30/2002)  . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23

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1.  Introduction

   As Internet technology and communication has grown and matured, many
   techniques have developed that use virtual topologies (frequently
   tunnels of one form or another) over an actual IP network.  Those
   virtual topologies have elements which appear as one hop in the
   virtual topology, but are actually multiple IP or sub-IP layer hops.
   These multiple hops often have quite diverse properties which are
   often not even visible to the end-points of the virtual hop.  This
   introduces many failure modes that are not dealt with well in current

   The use of IP encapsulation has long been considered as an
   alternative for creating such virtual topologies.  However, the
   insertion of an outer IP header reduces the effective path MTU as-
   seen by the IP layer.  When IPv4 is used, this reduced MTU can be
   accommodated through the use of IPv4 fragmentation, but unmitigated
   in-the-network fragmentation has been shown to be harmful through
   operational experience and studies conducted over the course of many
   years [FRAG][FOLK][RFC4963].  Additionally, classical path MTU
   discovery [RFC1191] has known operational issues that are exacerbated
   by in-the-network tunnels [RFC2923][RFC4459].

   For the purpose of this document, subnetworks are defined as virtual
   topologies that span connected network regions bounded by
   encapsulating border nodes.  Examples include the global Internet
   interdomain routing core, Mobile Ad hoc Networks (MANETs) and
   enterprise networks.  Subnetwork border nodes support the Internet
   protocols [RFC0791][RFC2460] and forward unicast and multicast IP
   packets over the virtual topology across multiple IP- and/or sub-IP
   layer forwarding hops which may introduce packet duplication and/or
   traverse links with diverse Maximum Transmission Units (MTUs).

   This document proposes a Subnetwork Encapsulation and Adaptation
   Layer (SEAL) for tunnel-mode operation of IP over subnetworks that
   connect the Ingress- and Egress Tunnel Endpoints (ITEs/ETEs) of
   border nodes.  Operation in transport mode is also supported when
   subnetwork border node upper-layer protocols negotiate the use of
   SEAL during connection establishment.  SEAL accommodates links with
   diverse MTUs and supports efficient duplicate packet detection by
   introducing a minimal mid-layer encapsulation.  The SEAL
   encapsulation introduces an extended Identification field for packet
   identification and a mid-layer segmentation and reassembly capability
   that allows simplified cutting and pasting of packets without
   invoking in-the-network IPv4 fragmentation.  The SEAL encapsulation
   layer and protocol is specified in the following sections.

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2.  Terminology and Requirements

   The term "subnetwork" in this document refers to a virtual topology
   that is configured over a connected network region bounded by border

   The terms "inner", "mid-layer" and "outer" respectively refer to the
   innermost IP {layer, protocol, header, packet, etc.} before any
   encapsulation, the mid-layer IP {protocol, header, packet, etc.)
   after any mid-layer '*' encapsulation and the outermost IP {layer,
   protocol, header, packet etc.} after SEAL/*/IPv4 encapsulation.

   The notation IPvX/*/SEAL/*IPvY refers to an inner IPvX packet
   encapsulated in any mid-layer '*' encapsulations followed by the SEAL
   header followed by any outer '*' encapsulations followed by an outer
   IPvY header.  The notation "IP" means either IP protocol version
   (IPv4 or IPv6).

   The following abbreviations correspond to terms used within this
   document and elsewhere in common Internetworking nomenclature:

      Subnetwork - a connected network region bounded by border nodes

      SEAL - Subnetwork Encapsulation and Adaptation Layer

      ITE - Ingress Tunnel Endpoint

      ETE - Egress Tunnel Endpoint

      MTU - Maximum Transmission Unit

      ENCAPS - the length of any mid-layer '*' headers plus the length
      of the outer encapsulating SEAL/*/IPv4 headers

      S_MTU - the per-ETE SEAL Maximum Transmission Unit

      S_MRU- the per-ETE SEAL Maximum Reassembly Unit

      S_MSS - the SEAL Maximum Segment Size, derived from S_MTU

      PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "Fragmentation
      Needed" message

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      FLEN - the MTU value included in an ICMPv4 "Fragmentation Needed"

      DF - the IPv4 header "Don't Fragment" flag

      SEAL-ID - a 32-bit Identification value; randomly initialized and
      monotonically incremented for each SEAL protocol packet

      SEAL_PROTO - an IPv4 protocol number used for SEAL

      SEAL_PORT - a TCP/UDP service port number used for SEAL

      SEAL_OPTION - a TCP option number used for (transport-mode) SEAL

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

3.  Applicability Statement

   SEAL was motivated by the specific use case of subnetwork abstraction
   for Mobile Ad-hoc Networks (MANETs), however the domain of
   applicability also extends to subnetwork abstractions of enterprise
   networks, the interdomain routing core, etc.  The domain of
   application therefore also includes the map-and-encaps architecture
   proposals in the IRTF Routing Research Group (RRG) (see: http://

   SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation
   (e.g., as IPv6/SEAL/IPv4), and appears as a subnetwork encapsulation
   as seen by the inner IP layer.  SEAL can also be used as a sublayer
   for encapsulating inner IP packets within outer UDP/IPv4 header
   (e.g., as IP/SEAL/UDP/IPv4) such as for the Teredo domain of
   applicability [RFC4380].  When it appears immediately after the outer
   IPv4 header, the SEAL header is processed exactly as for IPv6
   extension headers.

   SEAL can also be used in "transport-mode", e.g., when the inner layer
   includes upper layer protocol data rather than an encapsulated IP
   packet.  For instance, TCP peers can negotiate the use of SEAL for
   the carriage of protocol data encapsulated as TCP/SEAL/IPv4.  In this
   sense, the "subnetwork" becomes the entire end-to-end path between
   the TCP peers and may potentially span the entire Internet.

   The current document version is specific to the use of IPv4 as the
   outer encapsulation layer, however the same principles apply when
   IPv6 is used as the outer layer.

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4.  SEAL Protocol Specification - Tunnel Mode

4.1.  Model of Operation

   SEAL supports the encapsulation of inner IP packets in mid-layer and
   outer encapsulating headers/trailers.  For example, an inner IP
   packet would appear as IP/*/SEAL/*/IPv4 after mid-layer and outer
   encapsulations, where '*' denotes zero or more additional
   encapsulation sublayers.  Ingres Tunnel Endpoints (ITEs) add mid-
   layer '*' and outer SEAL/*/IPv4 encapsulations to the inner packets
   they inject into a subnetwork, where the outermost IPv4 header
   contains the source and destination addresses of the subnetwork
   entry/exit points (i.e., the ITE/ETE), respectively.  SEAL defines a
   new IP protocol type and a new encapsulation sublayer for both
   unicast and multicast.  The ITE encapsulates an inner IP packet in
   mid-layer and outer encapsulations as shown in Figure 1:

                                            |                         |
                                            ~   Outer */IPv4 headers  ~
                                            |                         |
   I                                        +-------------------------+
   n                                        |       SEAL Header       |
   n      +-------------------------+       +-------------------------+
   e      ~ Any mid-layer * headers ~       ~ Any mid-layer * headers ~
   r      +-------------------------+       +-------------------------+
          |                         |       |                         |
   I -->  ~        Inner IP         ~  -->  ~        Inner IP         ~
   P -->  ~         Packet          ~  -->  ~         Packet          ~
          |                         |       |                         |
   P      +-------------------------+       +-------------------------+
   a      ~  Any mid-layer trailers ~       ~  Any mid-layer trailers ~
   c      +-------------------------+       +-------------------------+
   k                                        ~    Any outer trailers   ~
   e                                        +-------------------------+
           (After mid-layer encaps.)        (After SEAL/*/IPv4 encaps.)

                       Figure 1: SEAL Encapsulation

   where the SEAL header is inserted as follows:

   o  For simple IP/IPv4 encapsulations (e.g.,
      [RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
      the inner IP and outer IPv4 headers as: IP/SEAL/IPv4.

   o  For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
      SEAL header is inserted between the {AH,ESP} header and outer IPv4

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      headers as: IP/*/{AH,ESP}/SEAL/IPv4.

   o  For IP encapsulations over transports such as UDP, the SEAL header
      is inserted immediately after the outer transport layer header,
      e.g., as IP/*/SEAL/UDP/IPv4.

   SEAL-encapsulated packets include a 32-bit SEAL-ID formed from the
   concatenation of the 16-bit ID Extension field in the SEAL header as
   the most-significant bits, and with the 16-bit ID value in the outer
   IPv4 header as the least-significant bits.  (For tunnels that
   traverse IPv4 Network Address Translators, the SEAL-ID is instead
   maintained only within the 16-bit ID Extension field in the SEAL
   header.)  Routers within the subnetwork use the SEAL-ID for duplicate
   packet detection, and ITEs/ETEs use the SEAL-ID for SEAL segmentation
   and reassembly.

   SEAL enables a multi-level segmentation and reassembly capability.
   First, the ITE can use IPv4 fragmentation to fragment inner IPv4
   packets with DF=0 before SEAL encapsulation to avoid lower-level
   segmentation and reassembly.  Secondly, the SEAL layer itself
   provides a simple mid-layer cutting-and-pasting of mid-layer packets
   to avoid IPv4 fragmentation on the outer packet.  Finally, ordinary
   IPv4 fragmentation is permitted on the outer packet after SEAL
   encapsulation and used to detect and dampen any in-the-network
   fragmentation as quickly as possible.

   The following sections specifiy the SEAL-related operations of the
   ITE and ETE, respectively:

4.2.  ITE Specification

4.2.1.  Tunnel Interface MTU

   The ITE configures a tunnel virtual interface over one or more
   underlying links that connect the border node to the subnetwork.  The
   tunnel interface must present a fixed MTU to the inner IP layer
   (i.e., Layer 3) as the size for admission of inner IP packets into
   the tunnel.  Since the tunnel interface may support a potentially
   large set of ETEs, however, care must be taken in setting a greatest-
   common-denominator MTU for all ETEs while still upholding end system

   Due to the ubiquitous deployment of standard Ethernet and similar
   networking gear, the nominal Internet cell size has become 1500
   bytes; this is the de facto size that end systems have come to expect
   will either be delivered by the network without loss due to an MTU
   restriction on the path or a suitable PTB message returned.  However,
   the network may not always deliver the necessary PTBs, leading to

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   MTU-related black holes [RFC2923].  The ITE therefore requires a
   means for conveying 1500 byte (or smaller) packets to the ETE without
   loss due to MTU restrictions and without dependence on PTB messages
   from within the subnetwork.

   In common deployments, there may be many forwarding hops between the
   original source and the ITE.  Within those hops, there may be
   additional encapsulations (IPSec, L2TP, etc.) such that a 1500 byte
   packet sent by the original source might grow to a larger size by the
   time it reaches the ITE for encapsulation as an inner IP packet.
   Similarly, additional encapsulations on the path from the ITE to the
   ETE could cause the encapsulated packet to become larger still and
   trigger in-the-network fragmentation.  In order to preserve the end
   system expectations, the ITE therefore requires a means for conveying
   these larger packets to the ETE even though there may be links within
   the subnetwork that configure a smaller MTU.

   The ITE should therefore set a tunnel virtual interface MTU of 1500
   bytes plus extra room to accommodate any additional encapsulations
   that may occur on the path from the original source (i.e., even if
   the underlying links do not support an MTU of this size).  The ITE
   can set larger MTU values still (up to the maximum MTU size of the
   underlying links), but should select a value that is not so large as
   to cause excessive PTBs coming from within the tunnel interface (see:
   Sections 4.2.2 and 4.2.6).  The ITE can also set smaller MTU values,
   however care must be taken not to set so small a value that original
   sources would experience an MTU underflow.  In particular, IPv6
   sources must see a minimum path MTU of 1280 bytes, and IPv4 sources
   should see a minimum path MTU of 576 bytes.

   The inner IP layer consults the tunnel interface MTU when admitting a
   packet into the interface.  For inner IPv4 packets larger than the
   tunnel interface MTU and with the IPv4 Don't Fragment (DF) bit set to
   0, the inner IP layer uses IPv4 fragmentation to break the packet
   into IPv4 fragments no larger than the tunnel interface MTU then
   admits each fragment into the tunnel as an independent packet.  For
   all other inner packets (IPv4 or IPv6), the ITE admits the packet if
   it is no larger than the tunnel interface MTU; otherwise, it drops
   the packet and sends an PTB message with an MTU value of the tunnel
   interface MTU to the source.

4.2.2.  Accounting for Headers

   As for any upper-layer protocol, ITEs use the MTU of the underlying
   IPv4 interface and the length of the encapsulating SEAL/*/IPv4
   headers to determine the maximum-sized upper layer payload.  For
   example, when the underlying IPv4 interface advertises an MTU of 1500
   bytes and the ITE inserts a minimum-length (i.e., 20 byte) IPv4

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   header, the ITE sees an upper layer payload of 1480 bytes.  When the
   ITE inserts IPv4 header options, the size is further reduced by as
   many as 40 additional bytes (the maximum length for IPv4 options)
   such that as few as 1440 bytes may be available for the upper layer
   payload.  When other outer '*' encapsulations are inserted before the
   SEAL header, the available MTU for the upper layer payload is reduced
   further still.

   The ITE must additionally account for the length of the SEAL header
   as an extra encapsulation that further reduces the size available for
   the upper layer payload.  The length of the SEAL header is not
   incorporated in the IPv4 header length, therefore the network does
   not observe the SEAL header as an IPv4 option.  In this way, the SEAL
   header is inserted after the IPv4 options but before the upper layer
   payload in exactly the same manner as for IPv6 extension headers.

4.2.3.  Segmentation and Encapsulation

   The ITE maintains a SEAL Maximum Transmission Unit (S_MTU) value for
   each ETE as soft state within the tunnel interface (e.g., in the IPv4
   destination cache).  The ITE initializes S_MTU to the MTU of the
   underlying IPv4 interface, and decreases or increases S_MTU based on
   any ICMPv4 Fragmentation Needed messages received (see: Section
   4.2.6).  The ITE additionally maintains a SEAL Maximum Reassembly
   Unit (S_MRU) value for each ETE.  The ITE initializes S_MRU to a
   value no larger than 2KB (2048 bytes), and uses this value to
   determine when to set the "Dont Reassemble" bit (see below).

   The ITE performs segmentation and encapsulation on inner packets that
   have been admitted into the tunnel interface.  For each inner packet,
   the ITE stores the length of any mid-layer '*' encapsulation headers
   and trailers (e.g., for '*' = AH, ESP, NULL, etc.) plus the length of
   the outer SEAL/*/IPv4 encapsulation headers in a per-packet variable
   'ENCAPS' and sets a per-packet variable 'S_MSS" to (S_MTU-ENCAPS).
   Next, for inner IPv4 packets with the DF bit set to 0, if the length
   of the inner packet is larger than MIN(S_MSS, S_MRU) the ITE uses
   IPv4 fragmentation to break the packet into IPv4 fragments no larger
   than MIN(S_MSS, S_MRU).  For unfragmentable inner packets (e.g., IPv6
   packets, IPv4 packets with DF=1, etc.), if the length of the inner
   packet is larger than MAX(S_MSS, S_MRU) the ITE drops the packet and
   sends an PTB message with an MTU value of MAX(S_MSS, S_MRU) back to
   the original source.

   The ITE then encapsulates each inner packet/fragment in any mid-layer
   '*' headers and trailers.  For each such resulting mid-layer packet,
   if the packet is no larger than S_MRU but is larger than S_MSS, the
   ITE breaks it into N segments (N <= 16) that are no larger than S_MSS
   bytes each.  Each segment except the final one MUST be of equal

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   length, while the final segment MUST be no larger than the initial
   segment.  The first byte of each segment MUST begin immediately after
   the final byte of the previous segment, i.e., the segments MUST NOT

   Note that this SEAL segmentation is used only for packets that are no
   larger than S_MRU; packets that are larger than S_MRU (and also no
   larger than S_MSS) are instead encapsulated as a single segment.
   Note also that this SEAL segmentation ignores the fact that the mid-
   layer packet may be unfragmentable.  This segmentation process is a
   mid-layer (not an IP layer) operation employed by the ITE to adapt
   the mid-layer packet to the subnetwork path characteristics, and the
   ETE will restore the packet to its original form during
   decapsulation.  Therefore, the fact that the packet may have been
   segmented within the subnetwork is not observable after

   The ITE next encapsulates each segment in a SEAL header formatted as

       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
      |          ID Extension         |P|R|D|M|Segment|  Next Header  |

                       Figure 2: SEAL Header Format

   where the header fields are defined as follows:

   ID Extension (16)
      a 16-bit extension of the ID field in the outer IPv4 header;
      encodes the most-significant 16 bits of a 32 bit SEAL-ID value.

   P (1)
      the "Probe" bit.  Set to 1 if the ITE wishes to receive an
      explicit acknowledgement from the ETE.

   R (1)
      the "Report Fragmentation" bit.  Set to 1 if the ITE wishes to
      receive a report from the ETE if any IPv4 fragmentation occurs.

   D (1)
      the "Dont Reassemble" bit.  Set to 1 if the reassembled SEAL
      protocol packet is to be discarded by the ETE if any IPv4
      reassemly is required.

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   M (1)
      the "More Segments" bit.  Set to 1 if this SEAL protocol packet
      contains a non-final segment of a multi-segment mid-layer packet.

   Segment (4)
      a 4-bit Segment number.  Encodes a segment number between 0 - 15.

   Next Header (8)  an 8-bit field that encodes an IP protocol number
      the same as for the IPv4 protocol and IPv6 next header fields.

   For single-segment mid-layer packets, the ITE encapsulates the
   segment in a SEAL header with (M=0; Segment=0).  For N-segment mid-
   layer packets (N <= 16), the ITE encapsulates each segment in a SEAL
   header with (M=1; Segment=0) for the first segment, (M=1; Segment=1)
   for the second segment, etc., with the final segment setting (M=0;
   Segment=N-1).  For each SEAL-encapsulated packet with Segment=0, the
   ITE sets D=0 in the SEAL header if the ETE is permitted to reassemble
   the packet if it arrives as multiple IPv4 fragments and/or SEAL
   segments; in particular, the ITE sets D=0 in the SEAL header of each
   segment for all mid-layer packets no larger than S_MRU.  The ITE
   instead sets D=1 in the SEAL header if the ETE is to discard the
   packet if it arrives as multiple IPv4 fragments and/or SEAL segments;
   in particular, the ITE sets D=1 in the SEAL header of each segment
   for all mid-layer packets larger than S_MRU.

   The ITE next sets the P and R bits in the SEAL header of each segment
   as specified in Section 4.2.5, then writes the IP protocol number
   corresponding to the mid-layer packet in the SEAL 'Next Header'
   field.  Next, the ITE encapsulates the segment in the requisite
   */IPv4 outer headers according to the specific encapsulation format
   (e.g., [RFC2003], [RFC4213], [RFC4380], etc.), except that it writes
   'SEAL_PROTO' in the protocol field of the outer IPv4 header (when
   simple IPv4 encapsualtion is used) or writes 'SEAL_PORT' in the outer
   destination service port field (e.g., when UDP/IPv4 encapsulation is
   used).  The ITE finally sets packet identification values and sends
   the packets as described in the following sections.

4.2.4.  Packet Identification

   For the purpose of packet identification, the ITE maintains a 32-bit
   SEAL-ID value as per-ETE soft state, e.g. in the IPv4 destination
   cache.  The ITE randomly-initializes SEAL-ID when the soft state is
   created and monotonically increments it (modulo 2^32) for each
   successive SEAL protocol packet it sends to the ETE.  For each
   packet, the ITE writes the least-significant 16 bits of the SEAL-ID
   value in the ID field in the outer IPv4 header, and writes the most-
   significant 16 bits in the ID Extension field in the SEAL header.

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   For tunnels that may traverse IPv4 Network Address Translators
   (NATs), the ITE instead maintains SEAL-ID as a 16-bit value that it
   randomly-initializes when the soft state is created and monotonically
   increments (modulo 2^16) for each successive SEAL protocol packet.
   For each packet, the ITE writes SEAL-ID in the ID extension field of
   the SEAL header and writes a random 16-bit value in the ID field in
   the outer IPv4 header.  This requires that both the ITE and ETE
   participate in this alternate scheme.

4.2.5.  Sending SEAL Protocol Packets

   Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
   the outer IPv4 header of every outer packet it sends.  For
   "expendable" packets (e.g., for NULL packets used as probes - see:
   Section 4.2.6), the ITE may instead set DF=1.

   The ITE then sends each outer packet that encapsulates a segment of
   the same mid-layer packet into the tunnel in canonical order, i.e.,
   Segment 0 first, followed by Segment 1, etc. and finally Segment N-1.

4.2.6.  Sending Probes

   When S_MSS is larger than 128, the ITE sends ordinary data packets as
   implicit probes to detect in-the-network IPv4 fragmentation and to
   determine new values for S_MTU.  The ITE sets R=1 in the SEAL header
   and DF=0 in the outer IPv4 header of each segment of a SEAL-segmented
   packet to be used as an implicit probe, and will receive ICMPv4
   Fragmentation Needed messages from the ETE if any IPv4 fragmentation
   occurs.  When S_MSS=128, the ITE instead sets R=0 in the SEAL header
   to avoid generating fragmentation reports for unavoidable in-the-
   network fragmentation.

   The ITE may additionally send explicit probes periodically to manage
   a window of SEAL-IDs of outstanding probes as a means to validate any
   ICMPv4 messages it receives.  The ITE sets P=1 in the SEAL header of
   each segment of a SEAL-segmented packet to be used as an explicit
   probe, where the probe can be either an ordinary data packet or a
   NULL packet created by setting the 'Next Header' field in the SEAL
   header to a value of "No Next Header".

   The ITE should periodically probe to detect increases in S_MTU.  The
   ITE can 1) reset S_MTU to the MTU of the underlying IPv4 interface,
   and/or 2) send probes that are larger than the current S_MTU using
   either a NULL packet or an ordinary data packet that is padded at the
   end by setting the outer IPv4 length field to a larger value than the
   packet's true length.

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4.2.7.  Processing Raw ICMPv4 Messages

   The ITE may receive "raw" ICMPv4 error messages from routers within
   the subnetwork that comprise an outer IPv4 header followed by an
   ICMPv4 header followed by a portion of the SEAL packet that generated
   the error (also known as the "packet-in-error").  For such messages,
   the ITE can use the 32-bit SEAL ID encoded in the packet-in-error as
   a nonce to confirm that the ICMP message came from an on-path router
   within the subnetwork.  The ITE MAY process raw ICMPv4 messages as
   soft errors indicating that the path to the ETE may be failing, but
   it discards any raw ICMPv4 Fragmentation Needed messages for which
   the IPv4 header of the packet-in-error has the DF=0.

4.2.8.  Processing SEAL-Encapsulated ICMPv4 Messages

   In addition to any raw ICMPv4 messages, the ITE may receive SEAL-
   encapsulated ICMPv4 messages from subnetwork border nodes that
   comprise outer ICMPv4/*/SEAL/*/IPv4 headers followed by a portion of
   the packet-in-error.  The ITE can use the 32-bit SEAL ID encoded in
   the packet-in-error as well as the outer IPv4 and SEAL headers as
   nonces to confirm that the ICMP message came from a legitimate ETE.
   The ITE then verifies that the SEAL-ID encoded in the packet-in-error
   is within the current window of outstanding SEAL-IDs for this ETE.
   If the SEAL-ID is outside of the window, the ITE discards the
   message; otherwise, it advances the window and processes the message.

   The ITE processes SEAL-encapsulated ICMPv4 messages other than ICMPv4
   Fragmentation Needed exactly as specified in [RFC0792].  For SEAL-
   encapsulated ICMPv4 Fragmentation Needed messages, if the MTU value
   reported in the message is zero the ITE discards the message.
   Otherwise, if the MTU value is 576 or larger, the ITE sets S_MTU to
   this new value.  If the MTU value is smaller than 576, the ITE sets
   S_MTU to MAX(S_MTU/2, 128).

   Note that 576 is assumed as the nominal minimum MTU for common IPv4
   links, and accounts for normal-case IPv4 first fragments.  When an
   ETE reports an MTU smaller than 576, the ITE performs a "limited
   halving" of S_MTU that accounts for IPv4 links with unusually small
   MTUs or cases in which the ETE otherwise receives an undersized IPv4
   first-fragment [RFC1812].  This limited halving may require multiple
   iterations of sending probes and receiving ICMPv4 Fragmentation
   Needed messages, but will soon converge to a stable S_MTU value.

   After deterimining a new value for S_MTU, if the IPv4 header of the
   packet-in-error has M=1 and its SEAL header has D=1, the ITE discards
   the SEAL/*/IPv4 headers plus any mid-layer '*' headers/trailers (with
   a combined length of ENCAPS bytes).  The ITE then encapsulates the
   remaining inner IP packet portion in a PTB messsage to send back to

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   the original source, with the MTU field set to MAX((S_MTU-ENCAPS),

4.3.  ETE Specification

4.3.1.  Reassembly Buffer Requirements

   ETEs MUST be capable of using IPv4-layer reassembly to reassemble
   SEAL protocol outer IPv4 packets of at least 2KB plus the size of the
   maximum-length outer SEAL/*/IPv4 plus mid-layer '*' headers and
   trailers.  For example, for simple IP/SEAL/IPv4 encapsulation, the
   ETE must be capable of reassembling an outer IPv4 packet of 2KB + 4 +
   60 bytes

   The ETE MUST also be capable of using SEAL-layer reassembly to
   reassemble mid-layer packets of 2KB.

4.3.2.  IPv4-Layer Reassembly

   The ETE performs IPv4 reassembly as-normal, and should maintain a
   conservative high- and low-water mark for the number of outstanding
   reassemblies pending for each ITE.  When the size of the reassembly
   buffer exceeds this high-water mark, the ETE actively discards
   incomplete reassemblies (e.g., using an Active Queue Management (AQM)
   strategy) until the size falls below the low-water mark.  The ETE
   should also use a reduced IPv4 maximum segment lifetime value (e.g.,
   15 seconds), i.e., the time after which it will discard an incomplete
   IPv4 reassembly for a SEAL protocol packet.

   After reassembly, the ETE either accepts or discards the reassembled
   packet based on the current status of the IPv4 reassembly cache
   (congested vs uncongested).  The SEAL-ID included in the IPv4 first-
   fragment provides an additional level of reassembly assurance, since
   it can record a distinct arrival timestamp useful for associating the
   first-fragment with its corresponding non-initial fragments.  The
   choice of accepting/discarding a reassembly may also depend on the
   strength of the upper-layer integrity check if known (e.g., IPSec/ESP
   provides a strong upper-layer integrity check) and/or the corruption
   tolerance of the data (e.g., multicast streaming audio/video may be
   more corruption-tolerant than file transfer, etc.).  In the limiting
   case, the ETE may choose to discard all IPv4 reassemblies and process
   only the IPv4 first-fragment for SEAL-encapsulated error generation
   purposes (see the following sections).

4.3.3.  Generating SEAL-Encapsulated ICMPv4 Fragmentation Needed

   During IPv4-layer reassembly, the ETE determines whether the packet

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   belongs to the SEAL protocol by checking for SEAL_PROTO in the outer
   IPv4 header (i.e., for simple IPv4 encapsulation) or for SEAL_PORT in
   the outer */IPv4 header (e.g., for '*'=UDP).

   During reassembly, when the ETE processes the IPv4 first-fragment
   (i.e, one with DF=1 and Offset =0 in the IPv4 header) of a SEAL
   protocol packet with (R=1; Segment=0) in the SEAL header, it sends a
   SEAL-encapsulated ICMPv4 Fragmentation Needed message back to the ITE
   with the MTU value set to the length of the IPv4 first-fragment.

   Following reassembly, when the ETE processes a SEAL protocol packet
   with (P=1; Segment=0), it sends a SEAL-encapsulated ICMPv4
   Fragmentation Needed message back to the ITE with the MTU value set
   to 0 (i.e., while treating the reassembled SEAL protocol packet as an
   IPv4 first-fragment).

   The ETE prepares the ICMPv4 Fragmentation Needed message by
   encapsulating as much of the first fragment as possible in outer
   */SEAL/*/IPv4 headers without the length of the message exceeding 576
   bytes as shown in Figure 3:

   +-------------------------+ -
   |                         |   \
   ~ Outer */SEAL/*/IPv4 hdrs~   |
   |                         |   |
   +-------------------------+   |
   |      ICMPv4 Header      |   |
   |(Dest Unreach; Frag Need)|   |
   +-------------------------+   |
   |                         |    > Up to 576 bytes
   ~    IP/*/SEAL/*/IPv4     ~   |
   ~  hdrs of first-fragment ~   |
   |                         |   |
   +-------------------------+   |
   |                         |   |
   ~  Data of first-fragment ~   |
   |                         |   /
   +-------------------------+ -

      Figure 3: SEAL-encapsulated ICMPv4 Fragmentation Needed Message

   The ETE next sets D=0, P=0, R=0, M=0 and Segment=0 in the outer SEAL
   header, sets the SEAL-ID the same as for any SEAL packet, then sets
   the SEAL Next Header field and the fields of the outer */IPv4 headers
   the same as for ordinay SEAL encapsulation (see: Sections 4.2.3 and
   4.2.4).  The ETE then sets outer IPv4 destination address to the
   source address of the first-fragment and sets the outer IPv4 source
   address to the destination address of the first-fragment.  If the

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   destination address in the first-fragment was multicast, the ETE
   instead sets the outer IPv4 source address to an address assigned to
   the underlying IPv4 interface.  The ETE finally sends the SEAL-
   encapsulated ICMPv4 message to the ITE the same as specified in
   Section 4.2.5.

4.3.4.  SEAL-Layer Reassembly

   Following IPv4 reassembly of a SEAL protocol packet, the ETE adds the
   SEAL packet to a SEAL-Layer pending-reassembly queue (if necessary).
   If the packet arrived as multiple IPv4 fragments and with D=1 in the
   SEAL header, the ETE marks the packet and/or pending reassembly queue
   as "discard following reassembly".  The ETE also marks the packet as
   "discard following reassembly" if the (Next Header, P, R, D) fields
   of the packet's SEAL header differ from their respective values in
   other SEAL segments already in the queue, i.e., the (Next Header, P,
   R, D)-tuple serves as a reassembly nonce.

   The ETE performs SEAL-layer reassembly for multi-segment mid-layer
   packets through simple in-order concatenation of the encapsulated
   segments from N consecutive SEAL protocol packets from the same mid-
   layer packet.  SEAL-layer reassembly requires the ETE to maintain a
   cache of recently received SEAL packet segments for a hold time that
   would allow for reasonable inter-segment delays.  The ETE uses a SEAL
   maximum segment lifetime of 15 seconds for this purpose, i.e., the
   time after which it will discard an incomplete reassembly.  However,
   the ETE should also actively discard any pending reassemblies that
   clearly have no opportunity for completion, e.g., when a considerable
   number of new SEAL packets have been received before a packet that
   completes a pending reassembly has arrived.

   The ETE reassembles the mid-layer packet segments in SEAL protocol
   packets that contain Segment numbers 0 through N-1, with M=1/0 in
   non-final/final segments, respectively, and with consecutive SEAL-ID
   values.  That is, for an N-segment mid-layer packet, reassembly
   entails the concatenation of the SEAL-encapsulated segments with
   (Segment 0, SEAL-ID i), followed by (Segment 1, SEAL-ID ((i + 1) mod
   2^32)), etc. up to (Segment N-1, SEAL-ID ((i + N-1) mod 2^32)).  (For
   tunnels that may traverse IPv4 NATs, the ETE instead uses only a 16-
   bit SEAL-ID value, and uses mod 2^16 arithmetic to associate the
   segments of the same packet.)

4.3.5.  Decapsulation and Generating Other ICMPv4 Errors

   Following SEAL-layer reassembly, if the packet had the value "No Next
   Header" in the SEAL header's Next Header field, or if the packet was
   marked "discard following reassembly" the ETE silently discards the
   reassembled mid-layer packet; otherwise, the ETE decapsulates the

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   inner packet and processes it as normal.  If the ETE determines that
   the decapsulated inner packet cannot be processed further, it drops
   the packet and prepares an appropriate SEAL-encapsulated ICMPv4 error
   message and sends it back to the ITE exactly as for ICMPv4
   Fragmentation Needed messages (See: Section 4.3.3).

5.  SEAL Protocol Specification - Transport Mode

   Section 4 specifies the operation of SEAL in "tunnel mode", i.e.,
   when there is both an inner and outer IP layer and with a SEAL
   encapsulation layer between.  However, the SEAL protocol can also be
   used in a "transport mode" of operation in which the inner layer
   corresponds to an upper layer protocol (e.g., UDP, TCP, etc.) instead
   of an additional IP layer.

   For example, two TCP endpoints connected to the same subnetwork
   region can negotiate the use of transport-mode SEAL for a connection
   by inserting a 'SEAL_OPTION' TCP option during the connection
   establishment phase.  If both TCPs agree on the use of SEAL, their
   protocol messages will be carriaged as TCP/SEAL/IPv4 and the
   connection will be serviced by the SEAL protocol using TCP (nstead of
   an encapsulating tunnel endpoint) as the upper layer protocol.  The
   SEAL protocol for transport mode otherwise observes the same
   specifications as for Section 4.

6.  Link Requirements

   Subnetwork designers are strongly encouraged to follow the
   recommendations in [RFC3819] when configuring link MTUs, where all
   IPv4 links SHOULD configure a minimum MTU of 576 bytes.  Links that
   cannot configure an MTU of at least 576 bytes (e.g., due to
   performance characteristics) SHOULD implement transparent link-layer
   segmentation and reassembly such that an MTU of at least 576 can
   still be presented to the IP layer.

7.  End System Requirements

   SEAL provides robust mechanisms for returning PTB messages to the
   original source, however end systems that send unfragmentable IP
   packets larger than 1500 bytes are strongly encouraged to use
   Packetization Layer Path MTU Discovery per [RFC4821].

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8.  Router Requirements

   IPv4 routers within the subnetwork observe the requirements in
   [RFC1812], and are strongly encouraged to implement IPv4
   fragmentation such that the first fragment is the largest and
   approximately the size of the underlying link MTU.

9.  IANA Considerations

   SEAL_PROTO, SEAL_PORT and SEAL_OPTION are taken from their respective
   range of experimental values documented in [RFC3692][RFC4727].  These
   values are for experimentation purposes only, and not to be used for
   any kind of deployments (i.e., they are not to be shipped in any
   products).  This document therefore has no actions for IANA.

10.  Security Considerations

   Unlike IPv4 fragmentation, overlapping fragment attacks are not
   possible due to the requirement that SEAL segments be non-

   An amplification/reflection attack is possible when an attacker sends
   IPv4 first-fragments with spoofed source addresses to an ETE,
   resulting in a stream of ICMPv4 Fragmentation Needed messages
   returned to a victim ITE.  The encapsulated segment of the spoofed
   IPv4 first-fragment provides mitigation for the ITE to detect and
   discard spurious ICMPv4 Fragmentation Needed messages.

   The SEAL header is sent in-the-clear (outside of any IPsec/ESP
   encapsulations) the same as for the IPv4 header.  As for IPv6
   extension headers, the SEAL header is protected only by L2 integrity
   checks and is not covered under any L3 integrity checks.

11.  Acknowledgments

   Path MTU determination through the report of fragmentation
   experienced by the final destination was first proposed by Charles
   Lynn of BBN on the TCP-IP mailing list in May 1987.  An historical
   analysis of the evolution of path MTU discovery appears in and is
   reproduced in Appendix A of this document.

   The following individuals are acknowledged for helpful comments and
   suggestions: Jari Arkko, Fred Baker, Teco Boot, Iljitsch van Beijnum,
   Brian Carpenter, Steve Casner, Ian Chakeres, Remi Denis-Courmont,

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   Aurnaud Ebalard, Gorry Fairhurst, Joel Halpern, John Heffner, Bob
   Hinden, Christian Huitema, Joe Macker, Matt Mathis, Dan Romascanu,
   Dave Thaler, Joe Touch, Magnus Westerlund, Robin Whittle, James
   Woodyatt and members of the Boeing PhantomWorks DC&NT group.

12.  References

12.1.  Normative References

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

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",
              RFC 1812, June 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

12.2.  Informative References

   [FOLK]     C, C., D, D., and k. k, "Beyond Folklore: Observations on
              Fragmented Traffic", December 2002.

   [FRAG]     Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
              October 1987.

              Macker, J. and S. Team, "Simplified Multicast Forwarding
              for MANET", draft-ietf-manet-smf-07 (work in progress),
              February 2008.

              Templin, F., Russert, S., and S. Yi, "The MANET Virtual
              Ethernet (VET) Abstraction",
              draft-templin-autoconf-dhcp-14 (work in progress),
              April 2008.

   [MTUDWG]   "IETF MTU Discovery Working Group mailing list,
    , November
              1989 - February 1995.".

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   [RFC1063]  Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
              MTU discovery options", RFC 1063, July 1988.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC2004]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
              October 1996.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, September 2000.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692, January 2004.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

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

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

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

   [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
              ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

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   [TCP-IP]   "TCP-IP mailing list archives,
    , May
              1987 - May 1990.".

Appendix A.  Historic Evolution of PMTUD (written 10/30/2002)

   The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
   and numerous proposals in the late 1980's through early 1990.  The
   initial problem was posed by Art Berggreen on May 22, 1987 in a
   message to the TCP-IP discussion group [TCP-IP].  The discussion that
   followed provided significant reference material for [FRAG].  An IETF
   Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
   with charter to produce an RFC.  Several variations on a very few
   basic proposals were entertained, including:

   1.  Routers record the PMTUD estimate in ICMP-like path probe
       messages (proposed in [FRAG] and later [RFC1063])

   2.  The destination reports any fragmentation that occurs for packets
       received with the "RF" (Report Fragmentation) bit set (Steve
       Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal)

   3.  A hybrid combination of 1) and Charles Lynn's Nov. 1987 proposal
       (straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)

   4.  Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,

   5.  Fragmentation avoidance by setting "IP_DF" flag on all packets
       and retransmitting if ICMPv4 "fragmentation needed" messages
       occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191]
       by Mogul and Deering).

   Option 1) seemed attractive to the group at the time, since it was
   believed that routers would migrate more quickly than hosts.  Option
   2) was a strong contender, but repeated attempts to secure an "RF"
   bit in the IPv4 header from the IESG failed and the proponents became
   discouraged. 3) was abandoned because it was perceived as too
   complicated, and 4) never received any apparent serious
   consideration.  Proposal 5) was a late entry into the discussion from
   Steve Deering on Feb. 24th, 1990.  The discussion group soon
   thereafter seemingly lost track of all other proposals and adopted
   5), which eventually evolved into [RFC1191] and later [RFC1981].

   In retrospect, the "RF" bit postulated in 2) is not needed if a
   "contract" is first established between the peers, as in proposal 4)
   and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on

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   Feb 19. 1990.  These proposals saw little discussion or rebuttal, and
   were dismissed based on the following the assertions:

   o  routers upgrade their software faster than hosts

   o  PCs could not reassemble fragmented packets

   o  Proteon and Wellfleet routers did not reproduce the "RF" bit
      properly in fragmented packets

   o  Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
      "translucent" not "transparent" bridging)

   o  the 16-bit IP_ID field could wrap around and disrupt reassembly at
      high packet arrival rates

   The first four assertions, although perhaps valid at the time, have
   been overcome by historical events leaving only the final to
   consider.  But, [FOLK] has shown that IP_ID wraparound simply does
   not occur within several orders of magnitude the reassembly timeout
   window on high-bandwidth networks.

   (Authors 2/11/08 note: this final point was based on a loose
   interpretation of [FOLK], and is more accurately addressed in

Author's Address

   Fred L. Templin (editor)
   Boeing Phantom Works
   P.O. Box 3707
   Seattle, WA  98124


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Full Copyright Statement

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