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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                      Boeing Phantom Works
Intended status: Informational                             June 22, 2008
Expires: December 24, 2008

        The Subnetwork Encapsulation and Adaptation Layer (SEAL)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 24, 2008.


   For the purpose of this document, subnetworks are defined as virtual
   topologies that span connected network regions bounded by
   encapsulated border nodes.  These virtual topologies 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
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Approach . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology and Requirements . . . . . . . . . . . . . . . . .  5
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  6
   4.  SEAL Protocol Specification - Tunnel Mode  . . . . . . . . . .  7
     4.1.  Model of Operation . . . . . . . . . . . . . . . . . . . .  7
     4.2.  ITE Specification  . . . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . .  9
       4.2.2.  Accounting for Headers . . . . . . . . . . . . . . . . 10
       4.2.3.  Segmentation and Encapsulation . . . . . . . . . . . . 11
       4.2.4.  Sending Probes . . . . . . . . . . . . . . . . . . . . 13
       4.2.5.  Packet Identification  . . . . . . . . . . . . . . . . 14
       4.2.6.  Sending SEAL Protocol Packets  . . . . . . . . . . . . 14
       4.2.7.  Processing Raw ICMPv4 Messages . . . . . . . . . . . . 15
       4.2.8.  Processing SEAL-Encapsulated ICMPv4 Messages . . . . . 15
     4.3.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 16
       4.3.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 16
       4.3.2.  IPv4-Layer Reassembly  . . . . . . . . . . . . . . . . 16
       4.3.3.  Generating SEAL-Encapsulated ICMPv4 Fragmentation
               Needed Messages  . . . . . . . . . . . . . . . . . . . 17
       4.3.4.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 18
       4.3.5.  Decapsulation and Generating Other ICMPv4 Errors . . . 19
   5.  SEAL Protocol Specification - Transport Mode . . . . . . . . . 20
   6.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 20
   7.  End System Requirements  . . . . . . . . . . . . . . . . . . . 20
   8.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 21
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 21
   12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 22
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     14.2. Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  Historic Evolution of PMTUD . . . . . . . . . . . . . 25
   Appendix B.  Reliability . . . . . . . . . . . . . . . . . . . . . 27
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28

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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 network that suppors
   the Internet Protocol (IP) [RFC0791][RFC2460].  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 the means 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 found 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].  In the following
   subsections, we present further details on the motivation and
   approach for addressing these issues.

1.1.  Motivation

   Before discussing the approach, it is necessary to first understand
   the problems.  In both the Internet and private-use networks today,
   IPv4 is ubiquitously deployed as the Layer 3 protocol.  The two
   primary functions of IPv4 are to provide for 1) addressing, and 2) a
   fragmentation and reassembly capability used to accommodate links
   with diverse MTUs.  While it is well known that the addressing
   properties of IPv4 are limited (hence the larger address space
   provided by IPv6), there is a lesser-known but growing consensus that
   other limitations may be unable to sustain continued growth.

   First, the IPv4 header Identification field is only 16 bits in
   length, meaning that at most 2^16 packets pertaining to the same
   (source, destination, protocol, Identification)-tuple may be active
   in the Internet at a given time.  Due to the escalating deployment of
   high-speed links (e.g., 1Gbps Ethernet), however, this number may
   soon become too small by several orders of magnitude.  Furthermore,
   there are many well-known limitations pertaining to IPv4
   fragmentation and reassembly - even to the point that it has been
   deemed "harmful" in both classic and modern-day studies (cited
   above).  In particular, IPv4 fragmentation raises issues ranging from

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   minor annoyances (e.g., slow-path processing in routers) to the
   potential for major integrity issues (e.g., mis-association of the
   fragments of multiple IP packets during reassembly).

   As a result of these perceived limitations, a fragmentation-avoiding
   technique for discovering the MTU of the forward path from a source
   to a destination node was devised through the deliberations of the
   Path MTU Discovery Working Group (PMTUDWG) during the late 1980's
   through early 1990's (see: Appendix A).  In this method, the source
   node provides explicit instructions to routers in the path to discard
   the packet and return an ICMP error message if an MTU restriction is
   encountered.  However, this approach has several serious shortcomings
   that lead to an overall "brittleness".

   In particular, site border routers in the Internet are more and more
   being configured to discard ICMP error messages coming from the
   outside world.  This is due in large part to the fact that malicious
   spoofing of error messages in the Internet is made simple since there
   is no way to authenticate the source of the messages.  Furthermore,
   when a source node that requires ICMP error message feedback when a
   packets is dropped due to an MTU restriction does not receive the
   messages, a path MTU-related black hole occurs.  This means that the
   source will continue to send packets that are too large and never
   receive an indication from the network that they are being discarded.

   The issues with both IPv4 fragmentation and this "classical" method
   of path MTU discovery are exacerbated further when IP-in-IP tunneling
   is used.  For example, site border routers that are configured as
   ingress tunnel endpoints may be required to forward packets into the
   subnetwork on behalf of hundreds, thousands, or even more original
   sources located within the site.  If IPv4 fragmentation were used,
   this would quickly wrap the 16-bit Identification field and could
   lead to undetected data corruption.  If "classical" IPv4
   fragmentation were used instead, the site border router may be
   bombarded by ICMP error messages coming from the subnetwork which may
   be either untrustworthy or insufficiently provisioned to allow
   translation into error message to be returned to the original

   The situation is exacerbated further still by IPsec tunnels, since
   only the first IPv4 fragment of a fragmented packet contains the
   transport protocol selectors (e.g., the source and destination ports)
   required for identifying the correct security association rendering
   fragmentation useless under certain circumstances.  Even worse, there
   may be no way for a site border router the configures an IPsec tunnel
   to transcribe the encrypted packet fragment contained in an ICMP
   error message into a suitable ICMP error message to return to the
   original source.  Due to these many limitations, a new approach to

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   accommodate links with diverse MTUs is necessary.

1.2.  Approach

   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 some
   enterprise networks.  Subnetwork border nodes 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 introduces 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.
   Moreover, SEAL senses in-the-network IPv4 fragmentation as a "noise"
   indication that packet sizing parameters are "out of tune" with
   respect to the network path.  Instead of experiencing this
   fragmentation as a disasterous event, however, SEAL naturally tunes
   its packet sizing parameters to eliminate the in-the-network
   fragmentation and thereby squelch the noise.  The SEAL encapsulation
   layer and protocol is specified in the following sections.

2.  Terminology and Requirements

   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 term "IP" used throughout the document refers to either Internet
   Protocol version (IPv4 or IPv6).  Additionally, 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, where the

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   notation "IPvX" means either IP protocol version (IPv4 or IPv6).

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

      ITE - Ingress Tunnel Endpoint

      ETE - Egress Tunnel Endpoint

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

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

      MHLEN - the length of any mid-layer '*' headers and trailers

      OHLEN - the length of the outer encapsulating SEAL/*/IPv4 headers

      S_MRU- the per-ETE SEAL Maximum Reassembly Unit

      S_MSS - the SEAL Maximum Segment Size

      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 case of subnetwork abstraction for
   Mobile Ad-hoc Networks (MANETs), however the domain of applicability
   also extends to subnetwork abstractions of enterprise networks, the

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   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://www3.tools.ietf.org/group/

   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 IPv6/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.

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 IPv6
   packet would appear as IPv6/*/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 uses a new
   Internet 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:

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

   o  For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
      SEAL header is inserted between the {AH,ESP} header and outer IPv4
      headers as: IPvX/*/{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 IPvX/*/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 Identification 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.

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   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
   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.

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   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, 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 IPv4 layer uses IPv4 fragmentation to break the packet
   into 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 ICMP PTB message with an MTU value of the tunnel
   interface MTU to the source.

4.2.2.  Accounting for Headers

   As for any transport layer protocol, ITEs use the MTU of the
   underlying IPv4 interface, the length of any mid-layer '*' headers
   and trailers, and the length of the outer 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 header, the ITE
   sees a maximum payload size 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
   the ITE inserts additional '*' encapsulations, the available MTU for
   the upper layer payload is reduced further still.

   The ITE must additionally account for the length of the SEAL header
   itself 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

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4.2.3.  Segmentation and Encapsulation

   For each ETE, the ITE maintains the length of any mid-layer '*'
   encapsulation headers and trailers (e.g., for '*' = AH, ESP, NULL,
   etc.) in a variable 'MHLEN' and maintains the length of the outer
   SEAL/*/IPv4 encapsulation headers in a variable 'OHLEN'.  The ITE
   maintains a SEAL Maximum Reassembly Unit (S_MRU) value for each ETE
   as soft state within the tunnel interface (e.g., in the IPv4
   destination cache).  The ITE initializes S_MRU to a value no larger
   than 2KB and uses this value to determine the maximum-sized packet it
   will require the ETE to reassemble.  The ITE additionally maintains a
   SEAL Maximum Segment Size (S_MSS) value for each ETE.  The ITE
   initializes S_MSS to the maximum of (the underlying IPv4 interface
   MTU minus OHLEN) and S_MRU/8 bytes, and decreases or increases S_MSS
   based on any ICMPv4 Fragmentation Needed messages received (see:
   Section 4.2.6).

   The ITE performs segmentation and encapsulation on inner packets that
   have been admitted into the tunnel interface.  For inner IPv4 packets
   with the DF bit set to 0, if the length of the inner packet is larger
   than (S_MRU - MHLEN) the ITE uses IPv4 fragmentation to break the
   packet into IPv4 fragments no larger than (S_MRU - MHLEN).  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_MRU, S_MSS) - MHLEN), the ITE drops the packet and sends an
   ICMP PTB message with an MTU value of (MAX(S_MRU, S_MSS) - MHLEN)
   back to the original source.

   The ITE then encapsulates each inner packet/fragment in the MHLEN
   bytes of mid-layer '*' headers and trailers.  For each such resulting
   mid-layer packet, if the length of the mid-layer packet is no larger
   than S_MRU but is larger than S_MSS, the ITE breaks it into N
   segments (N <= 8) that are no larger than S_MSS bytes each.  Each
   segment except the final one MUST be of equal 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 overlap.  The ITE
   should generate the smallest number of segments possible, e.g., it
   should not generate 6 smaller segments when the packet could be
   accommodated with 5 larger segments.

   Note that this SEAL segmentation is used only for mid-layer packets
   that are no larger than S_MRU; mid-layer packets that are larger than
   S_MRU 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

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   packet to its original form during decapsulation.  Therefore, the
   fact that the packet may have been segmented within the subnetwork is
   not observable after decapsulation.

   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         |A|R|D|M|E| SEG |  Next Header  |

                       Figure 2: SEAL Header Format

   where the header fields are defined as follows:

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

   A (1)
      the "Acknowledgement Requested" 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.

   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.

   E (1)
      the "Extension" bit; reserved for future use.  Must be set to 0
      for the purpose of this specification.

   SEG (3)
      a 3-bit Segment number.  Encodes a segment number between 0 - 7.

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

   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 <= 8), 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 encapsulated segment, the ITE sets D=0 in the
   SEAL header if the ETE is to accept the packet even if it arrives as
   multiple IPv4 fragments; for example, the ITE may set 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; in
   particular, the ITE should set D=1 in the SEAL header of each segment
   for all mid-layer packets larger than S_MRU.

   The ITE next sets the A and R bits in the SEAL header of each segment
   according to whether the packet is to be used as an explicit/implicit
   probe as specified in Section 4.2.4, then writes the Internet
   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 as specified in Section 4.2.5 and sends the
   packets as specified in Section 4.2.6.

4.2.4.  Sending Probes

   When S_MSS is larger than S_MRU/8 bytes, the ITE sends ordinary
   encapsulated data packets as implicit probes to detect in-the-network
   IPv4 fragmentation and to determine new values for S_MSS.  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 the ITE has already reduced
   S_MSS to the minimum value, it instead sets R=0 in the SEAL header to
   avoid generating fragmentation reports for unavoidable in-the-network

   The ITE should 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 A=1 in the SEAL header of each

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   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" (see: [RFC2460], Section 4.7.

   The ITE should further send explicit probes periodically to detect
   increases in S_MSS by resetting S_MSS to the maximum of (the
   underlying IPv4 interface MTU minus OHLEN) and S_MRU/8 bytes, and/or
   sending explicit probes that are larger than the current S_MSS.

4.2.5.  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 Identification field in the outer IPv4 header, and
   writes the most-significant 16 bits in the ID Extension field in the
   SEAL header.

   For SEAL encapsulations specifically designed for the traversal of
   IPv4 Network Address Translators (NATs), e.g., for encapsulations
   that insert a UDP header between the SEAL header and outer IPv4
   header such as IPv6/SEAL/UDP/IPv4, 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 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 Identification field in the outer IPv4 header.  This is due to
   the fact that the ITE has no way to control IPv4 NATs in the path
   that coud rewrite the Identification value in the outer IPv4 header.

4.2.6.  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 optionally 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.

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4.2.7.  Processing Raw ICMPv4 Messages

   The ITE may receive "raw" ICMPv4 error messages from either the ETE
   or 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
   either the ETE or an on-path router.  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 DF=0.

   The ITE should specifically process raw ICMPv4 Protocol Unreachable
   messages as a hint that the ETE does not implement the SEAL protocol.

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 SEAL-encapsulated packet-in-error.  The ITE can use the 32-bit
   SEAL ID encoded in the packet-in-error as well as information in 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 transmitted 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 IPv4 length
   of the packet-in-error minus OHLEN is larger than S_MSS the ITE sets
   S_MSS to this new value.  Otherwise, if the packet-in-error is an
   IPv4 first-fragment (i.e., with MF=1; Offset=0) the ITE sets S_MSS to
   this new value if the value is no smaller than (576 - OHLEN) and sets
   S_MSS to MAX(S_MSS/2, S_MRU/8) if the value is smaller than (576 -

   Note that in the above, 576 accounts for the nominal minimum MTU for
   common IPv4 links.  When an ETE returns a packet-in-error with MF=1
   and with length smaller than 576, the ITE performs a "limited
   halving" of S_MSS to account for IPv4 links with unusually small MTUs
   or cases in which the ETE otherwise receives an undersized IPv4
   first-fragment.  This limited halving may require multiple iterations
   of sending probes and receiving ICMPv4 Fragmentation Needed messages,
   but will soon converge to a stable S_MSS value.  When performing this

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   limited having, it is important that the ITE adjust its S_MSS size
   based on the first ICMPv4 Fragmentation Needed message and refrain
   from reducing S_MSS until ICMPv4 Fragmentation Needed messages
   pertaining to packets sent under the new S_MSS are received.  For
   example, the ITE should not repeatedly halve the S_MSS based on a
   burst of ICMPv4 Fragmentation Needed messages all pertaining to
   packets sent under the same S_MSS.

   After deterimining a new value for S_MSS, if the packet-in-error is
   an IPv4 first fragment and its SEAL header has D=1 the ITE MAY
   transcribe the message into an ICMP PTB message to send back to the
   original source.  To do so, the ITE discards the SEAL/*/IPv4 headers
   plus any mid-layer '*' headers/trailers of the packet-in-error then
   encapsulates the remaining inner IP packet portion in a PTB message
   with the MTU field set to MAX((S_MRU, S_MSS) - MHLEN).  Note that
   this may not be possible when the inner IP packet portion was
   encrypted (e.g. via IPsec/ESP), and is otherwise not entirely
   necessary since the ITE will discard subsequent large packets and
   send back an ICMP PTB *before* encapsulating them and sending to the
   ETE.  Transcribing ICMPv4 Fragmentation Needed messages into ICMP
   PTBs is therefore offered only as an optional optimization.

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 (2KB + OHELN) and MUST also be
   capable of using SEAL-layer reassembly to reassemble mid-layer
   packets of (2KB + OHLEN).  The term OHLEN is included to account for
   the length of the SEAL/*/IPv4 header, which must be retained for the
   purpose of associating the fragments/segments of the same packet.
   Note that the term S_MRU used in section 4.2 omits OHLEN for the
   purpose of specification clarity.

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

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   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
   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).  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 IPv4 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 field set to 0.  (Note that
   setting a non-zero value in the MTU field of the ICMPv4 Fragmentation
   Needed message would be redundant with the length value in the IPv4
   header of the first fragment, since this value is set to the correct
   path MTU through in-the-network fragmentation.  Setting the MTU field
   to 0 therefore avoids the ambiguous case in which the MTU field and
   the IPv4 length field of the first fragment would record different
   non-zero values.)

   When the ETE processes a SEAL protocol IPv4 packet with A=1 for which
   no IPv4 reassembly was required, or for which IPv4 reassembly was
   successful and the R bit was not set, it sends a SEAL-encapsulated
   ICMPv4 Fragmentation Needed message back to the ITE with the MTU
   value set to 0.  Note therefore that when both the A and R bits are
   set and fragmentation occurs, the ETE only sends a single ICMPv4
   Fragmentation Needed message, i.e., it does not send two separate
   messages (one for the first fragment and a second for the reassembled
   whole SEAL packet).

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

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

      Figure 3: SEAL-encapsulated ICMPv4 Fragmentation Needed Message

   The ETE next sets D=0, A=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.  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 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, except that the ETE may send the
   messages subject to rate limiting since it is not entirely critical
   that all fragmentation be reported to the ITE.

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, A, 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, A,
   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

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   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
   SEAL encapsulations specifically designed for traversal of 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" and IPv4 fragmentation was
   experienced, the ETE silently discards the reassembled mid-layer
   packet.  In the same manner, the ETE silently discards any
   reassembled mid-layer packet larger than 2KB that did not arrive as a
   single, unfragmented packet.

   For inner packets other than ICMPv4 messages, if the 'E' bit was set
   in the SEAL header the ETE discards the packet and (subject to rate
   limiting) sends a SEAL-encapsulated ICMPv4 Parameter Problem message
   with pointer set to the SEAL header back to the ITE exactly as for
   SEAL-encapsulated ICMPv4 Fragmentation Needed messages (see: Section
   4.3.3).  If the ETE otherwise determines that the inner packet cannot
   be processed further, it drops the packet, prepares an appropriate
   SEAL-encapsulated ICMPv4 error message and sends the error message
   back to the ITE.

   Otherwise, the ETE decapsulates the inner packet and processes it as

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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 a transport layer protocol (e.g., UDP, TCP, etc.)
   instead of an inner 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 transport 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 IPv4 layer.

   In the case that a fast IPv4 link within the subnetwork configures an
   unusually small MTU, the ITE can sense this smaller value through
   implicit probing and should reduce S_MRU and S_MSS to sizes that
   would avoid IPv4 fragmentation.  However, the ITE must take care not
   to reduce S_MRU to such small a value that original sources
   experience an MTU underflow and hence an unusable path.  (For
   example, IPv6 sources must see a minimum path MTU of 1280 bytes.)

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 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.  Related Work

   Section 3.1.7 of [RFC2764] provides a high-level sketch for
   supporting large tunnel MTUs via a tunnel-level segmentation and
   reassembly capability to avoid IP level fragmentation, which is in
   part the same approach used by tunnel-mode SEAL.  SEAL could
   therefore be considered as a fully-functioned manifestation of the
   method postulated by that informational reference, however SEAL also
   supports other modes of operation including transport-mode and
   duplicate packet detection.

   Section 3 of[RFC4459] describes inner and outer fragmentation at the
   tunnel endpoints as alternatives for accommodating the tunnel MTU,

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   however the SEAL protocol specifies a mid-layer segmentation and
   reassembly capability that is distinct from both inner and outer

   Section 4 of [RFC2460] specifies a method for inserting and
   processing extension headers between the base IPv6 header and
   transport layer protocol data.  The SEAL header is in fact inserted
   and processed in exactly the same manner.

   The concepts of path MTU determination through the report of
   fragmentation and extending the IP Identification field were first
   proposed in deliberations of the TCP-IP mailing list and the Path MTU
   Discovery Working Group (MTUDWG) during the late 1980's and early
   1990's.  SEAL supports a report fragmentation capability using bits
   in an extension header (the original proposal used a spare bit in the
   IP header) and supports ID extension through a 16 bit field in an
   extension header (the original proposal used a new IP option).  An
   historical analysis of the evolution of these concepts as well as the
   development of the eventual path MTU discovery mechanism for IP
   appears in Appendix A of this document.

12.  SEAL Advantages over Classical Methods

   The SEAL approach offers a number of distinct advantages over the
   classical path MTU discovery methods[RFC1191] [RFC1981]:

   1.  Classical path MTU discovery *always* results in packet loss when
       an MTU restriction is encountered.  Using SEAL, IPv4
       fragmentation provides a short-term interim mechanism for
       ensuring that packets are delivered while SEAL adjusts its packet
       sizing parameters.

   2.  Classical path MTU discovery requires that routers generate an
       ICMP PTB message for *all* packets lost due to an MTU
       restriction; this situation is exacerbated at high data rates and
       becomes severe for in-the-network tunnels that service many
       communicating end systems.  Since SEAL ensures that packets no
       larger than S_MRU are delivered, however, it is sufficient for
       the ETE to return ICMPv4 Fragmentation Needed messages subject to
       rate limiting and not for every packet-in-error.

   3.  Classical path MTU may require several iterations of dropping
       packets and returning ICMP PTB messasges until an acceptable path
       MTU value is determined.  Under normal circumstances, SEAL
       determines the correct packet sizing parameters in a single

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   4.  Using SEAL, ordinary packets serve as implicit probes without
       exposing data to unnecessary loss.  SEAL also provides an
       explicit probing mode not available in the classic methods.

   5.  Using SEAL, ICMP error messages are encapsulated in an outer SEAL
       header such that packet-filtering network middleboxes can
       distinguish them from "raw" ICMP messages that may be generated
       by an attacker.

   6.  Most importantly, all SEAL packets have a 32 bit Identification
       value that can be used for duplicate packet detection purposes
       and to match ICMP error messages with actual packets sent without
       requiring per-packet state.  Moreover, the SEAL ITE can be
       configured to accept ICMP feedback only from the legitimate ETE,
       hence the packet spoofing-related denial of service attack
       vectors open to the classical methods are eliminated.

   In summary, the SEAL approach represents an architecturally superior
   method for ensuring that packets of various sizes are either
   delivered or deterministically dropped.  When end systems use their
   own end-to-end MTU determination mechanisms [RFC4821], the SEAL
   advantages are further enhanced.

13.  Acknowledgments

   The following individuals are acknowledged for helpful comments and
   suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Teco Boot,
   Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Remi Denis-
   Courmont, Aurnaud Ebalard, Gorry Fairhurst, Joel Halpern, John
   Heffner, Thomas Henderson, 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.

   Path MTU determination through the report of fragmentation was first
   proposed by Charles Lynn on the TCP-IP mailing list in 1987.
   Extending the IP identification field was first proposed by Steve
   Deering on the MTUDWG mailing list in 1989.

14.  References

14.1.  Normative References

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

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

   [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.

14.2.  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.

   [MTUDWG]   "IETF MTU Discovery Working Group mailing list,
              gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November
              1989 - February 1995.".

   [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.

   [RFC2764]  Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, February 2000.

   [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.

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              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.

   [TCP-IP]   "TCP-IP mailing list archives,
              http://www-mice.cs.ucl.ac.uk/multimedia/mist/tcpip, May
              1987 - May 1990.".

Appendix A.  Historic Evolution of PMTUD

   (Taken from 'draft-templin-v6v4-ndisc-01.txt'; 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

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       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
   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.

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

Appendix B.  Reliability

   The SEAL header includes a 1-bit Extension field that is set to zero
   for the purpose of this specification.  This field may be used by
   future updates to this specification for the purpose of improved
   reliability in the face of loss due to congestion, singal
   intermittence, etc.  Automatic Repeat-ReQuest (ARQ) mechanisms are
   used to ensure relaible delivery between the endpoints of physical
   links (e.g., on-link neighbors in an IEEE 802.11 network) as well as
   between the endpoints of an end-to-end transport (e.g., the endpoints
   of a TCP connection).  However, ARQ mechanisms are poorly suited to
   in-the-network elements such as the SEAL ITE and ETE, since
   retransmission of lost segments would require unacceptable state
   maintenance at the ITE and would result in packet reordering within
   the subnetwork.

   Instead, Forward Error Correction (FEC) mechanisms may be specified
   in future updates to this specification for the purpose of improved
   reliability.  Such mechanisms may entail the ITE performing proactive
   transmissions of redundant data, e.g., by sending multiple copies of
   the same data.  Signaling from the ETE (e.g., by sending SEAL-
   encapsulated ICMPv4 Source Quench messages) may be specified in a
   future document as a means for the ETE to dynamically update the ITE
   of changing FEC conditions.

Author's Address

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

   Email: fltemplin@acm.org

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