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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                         August 30, 2011
Expires: March 2, 2012


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-30.txt

Abstract

   For the purpose of this document, a subnetwork is defined as a
   virtual topology configured over a connected IP network routing
   region and bounded by encapsulating border nodes.  These virtual
   topologies are manifested by tunnels that 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 technologies.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 2, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Approach . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Terminology and Requirements . . . . . . . . . . . . . . . . .  7
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  9
   4.  SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  VET Interface Model  . . . . . . . . . . . . . . . . . . . 10
     4.2.  SEAL Model of Operation  . . . . . . . . . . . . . . . . . 10
     4.3.  SEAL Header Format . . . . . . . . . . . . . . . . . . . . 12
     4.4.  ITE Specification  . . . . . . . . . . . . . . . . . . . . 14
       4.4.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
       4.4.2.  Tunnel Interface Soft State  . . . . . . . . . . . . . 15
       4.4.3.  Submitting Packets for Encapsulation . . . . . . . . . 17
       4.4.4.  Mid-Layer Encapsulation  . . . . . . . . . . . . . . . 17
       4.4.5.  SEAL Segmentation  . . . . . . . . . . . . . . . . . . 18
       4.4.6.  SEAL Encapsulation . . . . . . . . . . . . . . . . . . 18
       4.4.7.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 19
       4.4.8.  Sending SEAL Protocol Packets  . . . . . . . . . . . . 19
       4.4.9.  Probing Strategy . . . . . . . . . . . . . . . . . . . 20
       4.4.10. Processing ICMP Messages . . . . . . . . . . . . . . . 20
       4.4.11. Black Hole Detection . . . . . . . . . . . . . . . . . 21
     4.5.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 21
       4.5.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 21
       4.5.2.  Tunnel Interface Soft State  . . . . . . . . . . . . . 22
       4.5.3.  IP-Layer Reassembly  . . . . . . . . . . . . . . . . . 22
       4.5.4.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 23
       4.5.5.  Decapsulation and Delivery to Upper Layers . . . . . . 24
     4.6.  The SEAL Control Message Protocol (SCMP) . . . . . . . . . 24
       4.6.1.  Generating SCMP Messages . . . . . . . . . . . . . . . 25
       4.6.2.  Processing SCMP Messages . . . . . . . . . . . . . . . 28
     4.7.  Tunnel Endpoint Synchronization  . . . . . . . . . . . . . 31
   5.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 32
   6.  End System Requirements  . . . . . . . . . . . . . . . . . . . 32
   7.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 32
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 33
   11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 34
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35



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     13.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     13.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Reliability . . . . . . . . . . . . . . . . . . . . . 39
   Appendix B.  Integrity . . . . . . . . . . . . . . . . . . . . . . 39
   Appendix C.  Transport Mode  . . . . . . . . . . . . . . . . . . . 40
   Appendix D.  Historic Evolution of PMTUD . . . . . . . . . . . . . 40
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 42












































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1.  Introduction

   As Internet technology and communication has grown and matured, many
   techniques have developed that use virtual topologies (including
   tunnels of one form or another) over an actual network that supports
   the Internet Protocol (IP) [RFC0791][RFC2460].  Those virtual
   topologies have elements that 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 that are often not
   even visible to the endpoints of the virtual hop.  This introduces
   failure modes that are not dealt with well in current approaches.

   The use of IP encapsulation (also known as "tunneling") 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 visible to the inner network 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].  The following subsections 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 IPv4 address
   space is rapidly becoming depleted, there is a lesser-known but
   growing consensus that other IPv4 protocol limitations have already
   or may soon become problematic.

   First, the IPv4 header Identification field is only 16 bits in
   length, meaning that at most 2^16 unique packets with the same
   (source, destination, protocol)-tuple may be active in the Internet
   at a given time [I-D.ietf-intarea-ipv4-id-update].  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 for high data rate packet sources such as tunnel endpoints
   [RFC4963].  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 (see above).  In particular, IPv4 fragmentation raises issues



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   ranging from minor annoyances (e.g., in-the-network router
   fragmentation [RFC1981]) to the potential for major integrity issues
   (e.g., mis-association of the fragments of multiple IP packets during
   reassembly [RFC4963]).

   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 D).  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" [RFC2923].

   In particular, site border routers in the Internet are being
   configured more and more 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 trivial since
   there is no way to authenticate the source of the messages [RFC5927].
   Furthermore, when a source node that requires ICMP error message
   feedback when a packet 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.  This behavior has been confirmed through documented
   studies showing clear evidence of path MTU discovery failures in the
   Internet today [TBIT][WAND][SIGCOMM].

   The issues with both IPv4 fragmentation and this "classical" method
   of path MTU discovery are exacerbated further when IP tunneling is
   used [RFC4459].  For example, ingress tunnel endpoints (ITEs) may be
   required to forward encapsulated packets into the subnetwork on
   behalf of hundreds, thousands, or even more original sources in the
   end site.  If the ITE allows IPv4 fragmentation on the encapsulated
   packets, persistent fragmentation could lead to undetected data
   corruption due to Identification field wrapping.  If the ITE instead
   uses classical IPv4 path MTU discovery, it may be inconvenienced by
   excessive ICMP error messages coming from the subnetwork that may be
   either suspect or contain insufficient information for translation
   into error messages to be returned to the original sources.

   Although recent works have led to the development of a robust end-to-
   end MTU determination scheme [RFC4821], this approach requires
   tunnels to present a consistent MTU the same as for ordinary links on
   the end-to-end path.  Moreover, in current practice existing
   tunneling protocols mask the MTU issues by selecting a "lowest common
   denominator" MTU that may be much smaller than necessary for most



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   paths and difficult to change at a later date.  Due to these many
   consideration, a new approach to accommodate tunnels over links with
   diverse MTUs is necessary.

1.2.  Approach

   For the purpose of this document, a subnetwork is defined as a
   virtual topology configured over a connected network routing region
   and bounded by encapsulating border nodes.  Example connected network
   routing regions include Mobile Ad hoc Networks (MANETs), enterprise
   networks and the global public Internet itself.  Subnetwork border
   nodes forward unicast and multicast packets over the virtual topology
   across multiple IP and/or sub-IP layer forwarding hops that 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 tunneling network layer protocols (e.g., IP, OSI,
   etc.) over IP subnetworks that connect Ingress and Egress Tunnel
   Endpoints (ITEs/ETEs) of border nodes.  It provides a modular
   specification designed to be tailored to specific associated
   tunneling protocols.  A transport-mode of operation is also possible,
   and described in Appendix C.  SEAL accommodates links with diverse
   MTUs, protects against off-path denial-of-service attacks, and can be
   configured to enable efficient duplicate packet detection through the
   use of a minimal mid-layer encapsulation.

   SEAL specifically treats tunnels that traverse the subnetwork as
   ordinary links that must support network layer services.  As for any
   link, tunnels that use SEAL must provide suitable networking services
   including best-effort datagram delivery, integrity and consistent
   handling of packets of various sizes.  As for any link whose media
   cannot provide suitable services natively, tunnels that use SEAL
   employ link-level adaptation functions to meet the legitimate
   expectations of the network layer service.  As this is essentially a
   link level adaptation, SEAL is therefore permitted to alter packets
   within the subnetwork as long as it restores them to their original
   form when they exit the subnetwork.  The mechanisms described within
   this document are designed precisely for this purpose.

   SEAL encapsulation provides extended identification fields as well as
   a mid-layer segmentation and reassembly capability that allows
   simplified cutting and pasting of packets.  Moreover, SEAL engages
   both tunnel endpoints in ensuring a functional path MTU on the path
   from the ITE to the ETE.  This is in contrast to "stateless"
   approaches which seek to avoid MTU issues by selecting a lowest
   common denominator MTU value that may be overly conservative for the
   vast majority of tunnel paths and difficult to change even when



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   larger MTUs become available.

   The following sections provide the SEAL normative specifications,
   while the appendices present non-normative additional considerations.


2.  Terminology and Requirements

   The following terms are defined within the scope of this document:

   subnetwork
      a virtual topology configured over a connected network routing
      region and bounded by encapsulating border nodes.

   Ingress Tunnel Endpoint
      a virtual interface over which an encapsulating border node (host
      or router) sends encapsulated packets into the subnetwork.

   Egress Tunnel Endpoint
      a virtual interface over which an encapsulating border node (host
      or router) receives encapsulated packets from the subnetwork.

   inner packet
      an unencapsulated network layer protocol packet (e.g., IPv6
      [RFC2460], IPv4 [RFC0791], OSI/CLNP [RFC1070], etc.) before any
      mid-layer or outer encapsulations are added.  Internet protocol
      numbers that identify inner packets are found in the IANA Internet
      Protocol registry [RFC3232].

   mid-layer packet
      a packet resulting from adding mid-layer encapsulating headers to
      an inner packet.

   outer IP packet
      a packet resulting from adding an outer IP header (and possibly
      other outer headers) to a mid-layer packet.

   packet-in-error
      the leading portion of an invoking data packet encapsulated in the
      body of an error control message (e.g., an ICMPv4 [RFC0792] error
      message, an ICMPv6 [RFC4443] error message, etc.).

   Packet Too Big (PTB)
      a control plane message indicating an MTU restriction, e.g., an
      ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
      "Fragmentation Needed" message [RFC0792], an SCMP "Packet Too Big"
      message (see: Section 4.5), etc.




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   IP, IPvX, IPvY
      used to generically refer to either IP protocol version, i.e.,
      IPv4 or IPv6.

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

      DF - the IPv4 header "Don't Fragment" flag [RFC0791]

      ETE - Egress Tunnel Endpoint

      HLEN - the sum of MHLEN and OHLEN

      ITE - Ingress Tunnel Endpoint

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

      MRU - Maximum Reassembly Unit

      MTU - Maximum Transmission Unit

      OHLEN - the length of any outer encapsulating headers and trailers

      S_IFT - SEAL Inner Fragmentation Threshold

      S_MRU - SEAL Maximum Reassembly Unit

      S_MSS - SEAL Maximum Segment Size

      SCMP - the SEAL Control Message Protocol

      SEAL - Subnetwork Encapsulation and Adaptation Layer

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

      SEAL_PROTO - an IPv4 protocol number used for SEAL

      TE - Tunnel Endpoint (i.e., either ingress or egress)

      VET - Virtual Enterprise Traversal

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  When used
   in lower case (e.g., must, must not, etc.), these words MUST NOT be
   interpreted as described in [RFC2119], but are rather interpreted as
   they would be in common English.




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3.  Applicability Statement

   SEAL was originally motivated by the specific case of subnetwork
   abstraction for Mobile Ad hoc Networks (MANETs), however it soon
   became apparent that the domain of applicability also extends to
   subnetwork abstractions over enterprise networks, ISP networks, SOHO
   networks, the global public Internet itself, and any other connected
   network routing region.  SEAL along with the Virtual Enterprise
   Traversal (VET) [I-D.templin-intarea-vet] tunnel virtual interface
   abstraction are the functional building blocks for a new
   Internetworking architecture based on Routing and Addressing in
   Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
   and the Internet Routing Overlay Network (IRON) [RFC6179].

   SEAL provides a network sublayer for encapsulation of an inner
   network layer packet within outer encapsulating headers.  For
   example, for IPvX in IPvY encapsulation (e.g., as IPv4/SEAL/IPv6),
   the SEAL header appears as a subnetwork encapsulation as seen by the
   inner IP layer.  SEAL can also be used as a sublayer within a UDP
   data payload (e.g., as IPv4/UDP/SEAL/IPv6 similar to Teredo
   [RFC4380]), where UDP encapsulation is typically used for Network
   Address Translator (NAT) traversal as well as operation over
   subnetworks that give preferential treatment to the "core" Internet
   protocols (i.e., TCP and UDP).  The SEAL header is processed the same
   as for IPv6 extension headers, i.e., it is not part of the outer IP
   header but rather allows for the creation of an arbitrarily
   extensible chain of headers in the same way that IPv6 does.

   SEAL supports a segmentation and reassembly capability for adapting
   the network layer to the underlying subnetwork characteristics, where
   the Egress Tunnel Endpoint (ETE) determines how much or how little
   reassembly it is willing to support.  In the limiting case, the ETE
   can avoid reassembly altogether and act as a passive observer that
   simply informs the Ingress Tunnel Endpoint (ITE) of any MTU
   limitations and otherwise discards all packets that arrive as
   multiple fragments.  This mode is useful for determining an
   appropriate MTU for tunneling between performance-critical routers
   connected to high data rate subnetworks such as the Internet DFZ, for
   unidirectional tunneling in which the ETE is stateless, and for other
   uses in which reassembly would present too great of a burden for the
   routers or end systems.

   When the ETE supports reassembly, the tunnel can be used to transport
   packets that are too large to traverse the path without
   fragmentation.  In this mode, the ITE determines the tunnel MTU based
   on the largest packet the ETE is capable of reassembling rather than
   on the MTU of the smallest link in the path.  Therefore, tunnel
   endpoints that use SEAL can transport packets that are much larger



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   than the underlying subnetwork links themselves can carry in a single
   piece.

   SEAL tunnels may be configured over paths that include not only
   ordinary physical links, but also virtual links that may include
   other tunnels.  An example application would be linking two
   geographically remote supercomputer centers with large MTU links by
   configuring a SEAL tunnel across the Internet.  A second example
   would be support for sub-IP segmentation over low-end links, i.e.,
   especially over wireless transmission media such as IEEE 802.15.4,
   broadcast radio links in Mobile Ad-hoc Networks (MANETs), Very High
   Frequency (VHF) civil aviation data links, etc.

   Many other use case examples are anticipated, and will be identified
   as further experience is gained.


4.  SEAL Specification

   The following sections specify the operation of SEAL:

4.1.  VET Interface Model

   SEAL is an encapsulation sublayer used within VET non-broadcast,
   multiple access (NBMA) tunnel virtual interfaces.  Each VET interface
   connects an ITE to one or more ETE "neighbors" via tunneling across
   an underlying enterprise network, or "subnetwork".  The tunnel
   neighbor relationship between the ITE and each ETE may be either
   unidirectional or bidirectional.

   A unidirectional tunnel neighbor relationship allows the near end TE
   to send data packets forward to the far end TE, while the far end
   only returns control messages when necessary.  A bidirectional tunnel
   neighbor relationship is one over which both TEs can exchange both
   data and control messages.

   Implications of the VET unidirectional and bidirectional models for
   SEAL will be discussed further throughout the remainder of the
   document.

4.2.  SEAL Model of Operation

   SEAL supports a multi-level segmentation and reassembly capability
   for the transmission of unicast and multicast packets across an
   underlying IP subnetwork with heterogeneous links.  First, the ITE
   can use IPv4 fragmentation to fragment inner IPv4 packets before SEAL
   encapsulation if necessary.  Secondly, the SEAL layer itself provides
   a simple cutting-and-pasting capability for mid-layer packets that



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   can be used to avoid IP fragmentation on the outer packet.  Finally,
   ordinary IP fragmentation is permitted on the outer packet after SEAL
   encapsulation and allows the TEs to detect and tune out any in-the-
   network fragmentation.

   SEAL-enabled ITEs encapsulate each inner packet in any mid-layer
   headers and trailers, segment the resulting mid-layer packet into
   multiple segments if necessary, then append a SEAL header and any
   outer encapsulations to each segment.  As an example, for IPv6 within
   IPv4 encapsulation a single-segment inner IPv6 packet encapsulated in
   any mid-layer headers and trailers, followed by the SEAL header,
   followed by any outer headers and trailers, followed by an outer IPv4
   header would appear as shown in Figure 1:

                                       +--------------------+
                                       ~  outer IPv4 header ~
                                       +--------------------+
   I                                   ~  other outer hdrs  ~
   n                                   +--------------------+
   n                                   ~    SEAL Header     ~
   e      +--------------------+       +--------------------+
   r      ~  mid-layer headers ~       ~  mid-layer headers ~
          +--------------------+       +--------------------+
   I -->  |                    |  -->  |                    |
   P -->  ~     inner IPv6     ~  -->  ~     inner IPv6     ~
   v -->  ~       Packet       ~  -->  ~       Packet       ~
   6 -->  |                    |  -->  |                    |
          +--------------------+       +--------------------+
   P      ~ mid-layer trailers ~       ~ mid-layer trailers ~
   a      +--------------------+       +--------------------+
   c                                   ~   outer trailers   ~
   k         Mid-layer packet          +--------------------+
   e      after mid-layer encaps.
   t                                      Outer IPv4 packet
                                     after SEAL and outer encaps.

               Figure 1: SEAL Encapsulation - Single Segment

   As a second example, for IPv4 within IPv6 encapsulation an inner IPv4
   packet requiring three SEAL segments would appear as three separate
   outer IPv6 packets, where the mid-layer headers are carried only in
   segment 0 and the mid-layer trailers are carried in segment 2 as
   shown in Figure 2:








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   +------------------+   +------------------+   +------------------+
   ~  outer IPv6 hdr  ~   ~  outer IPv6 hdr  ~   ~  outer IPv6 hdr  ~
   +------------------+   +------------------+   +------------------+
   ~ other outer hdrs ~   ~ other outer hdrs ~   ~ other outer hdrs ~
   +------------------+   +------------------+   +------------------+
   ~ SEAL hdr (SEG=0) ~   ~ SEAL hdr (SEG=1) ~   ~ SEAL hdr (SEG=2) ~
   +------------------+   +------------------+   +------------------+
   |  mid-layer hdrs  |   |    inner IPv4    |   | inner IPv4 Packet|
   ~ plus inner IPv4  ~   ~  Packet Segment  ~   ~   Segment plus   ~
   ~  Packet Segment  ~   ~   (Length = L)   ~   ~ mid-layer trails ~
   |   (Length = L)   |   |                  |   | (Len may be !=L) |
   +------------------+   +------------------+   +------------------+
   ~  outer trailers  ~   ~  outer trailers  ~   ~  outer trailers  ~
   +------------------+   +------------------+   +------------------+
   Segment 0 (includes     Segment 1 (no mid-    Segment 2 (includes
     mid-layer hdrs)         layer encaps)         mid-layer trails)

             Figure 2: SEAL Encapsulation - Multiple Segments

   The ITE inserts the SEAL header according to the specific tunneling
   protocol.  Examples include the following:

   o  For simple encapsulation of an inner network layer packet within
      an outer IPvX header (e.g., [RFC1070][RFC2003][RFC2473][RFC4213],
      etc.), the ITE inserts the SEAL header between the inner packet
      and outer IPvX headers as: IPvX/SEAL/{inner packet}.

   o  For encapsulations over transports such as UDP (e.g., [RFC4380]),
      the ITE inserts the SEAL header between the outer transport layer
      header and the mid-layer packet, e.g., as IPvX/UDP/SEAL/{mid-layer
      packet}.  Here, the UDP header is seen as an "other outer header".

   The SEAL header includes per-neighbor, per-link and per-packet
   identification values which routers within the subnetwork can use for
   duplicate packet detection and both TEs can use for SEAL
   segmentation/reassembly.

   The following sections specify the SEAL header format and SEAL-
   related operations of the ITE and ETE.

4.3.  SEAL Header Format

   The SEAL header is formatted as follows:








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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |VER|C|A|I|F|M|R|  NEXTHDR/SEG  |            LINK_ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             NBR_ID                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   PKT_ID (when necessary)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: SEAL Header Format

   where the header fields are defined as:

   VER (2)
      a 2-bit version field.  This document specifies Version 0 of the
      SEAL protocol, i.e., the VER field encodes the value 0.

   C (1)
      the "Control/Data" bit.  Set to 1 by the ITE in SEAL Control
      Message Protocol (SCMP) control messages, and set to 0 in ordinary
      data packets.

   A (1)
      the "Acknowledgement Requested" bit.  Set to 1 by the ITE in data
      packets for which it wishes to receive an explicit acknowledgement
      from the ETE.

   I (1)
      the "Identification Field Included" bit.  Set to 1 if the SEAL
      header includes a 32-bit packet Identification field (see below);
      set to 0 otherwise.

   F (1)
      the "First Segment" bit.  Set to 1 if this SEAL protocol packet
      contains the first segment (i.e., Segment #0) of a mid-layer
      packet.

   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.

   R (1)
      the "Reserved" bit.  Set to 0 by the ITE and ignored by the ETE.
      Future specifications may define different behaviors.






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   NEXTHDR/SEG (8)  an 8-bit field.  When 'F'=1, encodes the next header
      Internet Protocol number the same as for the IPv4 protocol and
      IPv6 next header fields.  When 'F'=0, encodes a segment number of
      a multi-segment mid-layer packet.  (The segment number 0 is
      reserved.)

   LINK_ID (16)
      a 16-bit link identification value, set to a unique value for each
      underlying link over which the ITE will send encapsulated packets
      to the ETE.  Used as a neighbor selector adjunct for the NBR_ID.

   NBR_ID (32)
      a 32-bit neighbor identification value.  Set to a random value by
      the ETE in an initial exchange with the ITE, and used as a
      neighbor selector in conjunction with the LINK_ID.

   PKT_ID (32)
      a 32-bit per-packet identification field.  Present only when the I
      bit is set to 1 (see above).

   Setting of the various bits and fields of the SEAL header is
   specified in the following sections.

4.4.  ITE Specification

4.4.1.  Tunnel Interface MTU

   The tunnel interface must present a constant MTU value to the inner
   network layer as the size for admission of inner packets into the
   interface.  Since VET NBMA tunnel virtual interfaces may support a
   large set of ETEs that accept widely varying maximum packet sizes,
   however, a number of factors should be taken into consideration when
   selecting a tunnel interface MTU.

   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 ICMP Packet Too Big (PTB)
   message returned.  When large packets sent by end systems incur
   additional encapsulation at an ITE, however, they may be dropped
   silently within the tunnel since the network may not always deliver
   the necessary PTBs [RFC2923].

   The ITE should therefore set a tunnel interface MTU of at least 1500
   bytes plus extra room to accommodate any additional encapsulations
   that may occur on the path from the original source.  The ITE can
   also set smaller MTU values; however, care must be taken not to set



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   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 ITE can alternatively set an indefinite MTU on the tunnel
   interface such that all inner packets are admitted into the interface
   without regard to size.  For ITEs that host applications that use the
   tunnel interface directly, this option must be carefully coordinated
   with protocol stack upper layers since some upper layer protocols
   (e.g., TCP) derive their packet sizing parameters from the MTU of the
   outgoing interface and as such may select too large an initial size.
   This is not a problem for upper layers that use conservative initial
   maximum segment size estimates and/or when the tunnel interface can
   reduce the upper layer's maximum segment size, e.g., by reducing the
   size advertised in the MSS option of outgoing TCP messages.

   The inner network layer protocol consults the tunnel interface MTU
   when admitting a packet into the interface.  For non-SEAL inner IPv4
   packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet
   is larger than the tunnel interface MTU the inner IPv4 layer uses
   IPv4 fragmentation to break the packet into fragments no larger than
   the tunnel interface MTU.  The ITE then admits each fragment into the
   interface as an independent packet.

   For all other inner packets, the inner network layer admits the
   packet if it is no larger than the tunnel interface MTU; otherwise,
   it drops the packet and sends a PTB error message to the source with
   the MTU value set to the tunnel interface MTU.  The message must
   contain as much of the invoking packet as possible without the entire
   message exceeding the network layer minimum MTU (e.g., 576 bytes for
   IPv4, 1280 bytes for IPv6, etc.).  For SEAL packets that would
   undergo recursive encapsulation, however, the inner layer must send a
   SEAL PTB message instead of a PTB of the inner network layer (see:
   Section 4.4.3).

   In light of the above considerations, the ITE SHOULD configure an
   indefinite MTU on tunnel *router* interfaces, since these may be
   required to carry recursively-nested SEAL encapsulations.  The ITE
   MAY instead set a finite MTU on tunnel *host* interfaces.  Any
   necessary tunnel adaptations are then performed by the SEAL layer
   within the tunnel interface as described in the following sections.

4.4.2.  Tunnel Interface Soft State

   The ITE maintains per-ETE soft state within the tunnel interface,
   e.g., in a neighbor cache.  The soft state includes the following:




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   o  a Mid-layer Header Length (MHLEN); set to the length of any mid-
      layer encapsulation headers and trailers that must be added before
      SEAL segmentation.

   o  an Outer Header Length (OHLEN); set to the length of the outer IP,
      SEAL and other outer encapsulation headers and trailers.

   o  a total Header Length (HLEN); set to MHLEN plus OHLEN.

   o  a SEAL Maximum Segment Size (S_MSS).  The ITE initializes S_MSS to
      the minimum MTU of the underlying interfaces if the underlying
      interface MTUs can be determined (otherwise, the ITE initializes
      S_MSS to "infinity").  The ITE decreases or increases S_MSS based
      on any SCMP "Packet Too Big (PTB)" messages received (see Section
      4.6).

   o  a SEAL Maximum Reassembly Unit (S_MRU).  If the ITE is not
      configured to use SEAL segmentation, it initializes S_MRU to the
      constant value 0 and ignores any S_MRU values reported by the ETE.
      Otherwise, the ITE initializes S_MRU to "infinity" (i.e., the
      largest possible inner packet size) and decreases or increases
      S_MRU based on any SCMP PTB messages received from the ETE (see
      Section 4.6).  When (S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256)
      as the effective S_MRU value.

   o  a SEAL Inner Fragmentation Threshold (S_IFT); used to determine a
      maximum fragment size for fragmentable IPv4 packets.  Required
      only for tunnels that support encapsulation with IPv4 as the inner
      network layer protocol.  The ITE should use a "safe" estimate for
      S_IFT that would be highly unlikely to trigger additional
      fragmentation on the path to the ETE.  This estimate SHOULD be
      selected such that S_IFT <= MAX(S_MSS, MS_MRU); more specifically,
      it is RECOMMENDED that the ITE set S_IFT to 512 unless it can
      determine a more accurate safe value (e.g., via probing).

   o  a NBR_ID value that is coordinated with the ETE and used to fill
      the SEAL header field of the same name for packets sent to this
      ETE.

   o  one or more LINK_ID values that are coordinated with the ETE and
      used to fill the SEAL header field of the same name for packets
      sent to this ETE.

   o  a PKT_ID value that is randomly-initialized and monotonically-
      incremented for each packet sent to this ETE.

   o  an outer IP address (and UDP port number when UDP encapsulation is
      used) for use as the destination addresses for each packet sent to



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      this ETE.

   Note that S_MSS and S_MRU include the length of the outer and mid-
   layer encapsulating headers and trailers (i.e., HLEN), since the ETE
   must retain the headers and trailers during reassembly.  Note also
   that the ITE maintains S_MSS and S_MRU as 32-bit values such that
   inner packets larger than 64KB (e.g., IPv6 jumbograms [RFC2675]) can
   be accommodated when appropriate for a given subnetwork.

4.4.3.  Submitting Packets for Encapsulation

   Once an inner packet/fragment has been admitted into the tunnel
   interface, it transitions from the inner network layer and becomes
   subject to SEAL layer processing.  The ITE then examines each packet
   to determine whether it is too large for SEAL encapsulation, then
   submits the packet for encapsulation according to whether it is
   "fragmentable" (discussed in the next paragraph) or "unfragmentable"
   (discussed in the following paragraph).

   For IPv4 packets with DF=0 in the IPv4 header, if the packet is no
   larger than S_IFT the ITE submits the packet for encapsulation.
   Otherwise, the ITE uses inner IPv4 fragmentation to break the packet
   into IPv4 fragments no larger than S_IFT bytes.  For non-SEAL IPv4
   packets, the ITE then submits each fragment for encapsulation
   separately.  For SEAL IPv4 packets, the ITE instead uses the first
   fragment to prepare an SCMP PTB message with Code=0 to return to the
   source (see: Section 4.6.1.1) then discards each fragment.

   For all other packets, if the packet is larger than (MAX(S_MRU,
   S_MSS) - HLEN), the ITE discards it and sends a PTB message to the
   source with an MTU value of (MAX(S_MRU, S_MSS) - HLEN); otherwise,
   the ITE submits the packet for encapsulation.  The ITE must include
   the length of the uncompressed headers and trailers when calculating
   HLEN even if the tunnel is using header compression.  The ITE is also
   permitted to submit inner packets for encapsulation if they can be
   accommodated in a single SEAL segment (i.e., no larger than S_MSS)
   even if they are larger than the ETE would be willing to reassemble
   if fragmented (i.e., larger than S_MRU) - see: Section 4.5.1.

4.4.4.  Mid-Layer Encapsulation

   After inner IP fragmentation (if necessary), the ITE next
   encapsulates each inner packet/fragment in the MHLEN bytes of any
   mid-layer headers and trailers.  The ITE then submits the mid-layer
   packet for SEAL segmentation and encapsulation.






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4.4.5.  SEAL Segmentation

   If the ITE is configured to use SEAL segmentation, it checks the
   length of the resulting packet after mid-layer encapsulation to
   determine whether segmentation is needed.  If the length of the
   resulting mid-layer packet plus OHLEN is larger than S_MSS but no
   larger than S_MRU the ITE performs SEAL segmentation by breaking the
   mid-layer packet into N segments (N <= 256) that are no larger than
   (S_MSS - OHLEN) bytes each.

   When the ITE performs SEAL segmentation, it MUST segment the mid-
   layer packet such that the first segment includes at least the mid-
   layer headers.  (When the inner packet header is available in-the-
   clear, the first segment MUST also include the inner header.)  Each
   segment except the final one MUST be of equal length, and 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 4 larger segments.

   This SEAL segmentation process ignores the fact that the mid-layer
   packet may be unfragmentable outside of the subnetwork.  The 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
   reassembly.  Therefore, the fact that the packet may have been
   segmented within the subnetwork is not observable outside of the
   subnetwork.

4.4.6.  SEAL Encapsulation

   Following SEAL segmentation, the ITE next encapsulates each segment
   in a SEAL header formatted as specified in Section 4.3.

   For the first segment, the ITE then sets F=1, and sets M=1 if there
   are more segments or sets M=0 otherwise.  The ITE then sets NEXTHDR
   to the Internet Protocol number corresponding to the encapsulated
   inner packet.  For example, the ITE sets NEXTHDR to the value '4' for
   encapsulated IPv4 packets [RFC2003], the value '41' for encapsulated
   IPv6 packets [RFC2473][RFC4213], the value '50' for encapsulated
   IPsec/ESP payloads [RFC4301][RFC4303], the value '80' for
   encapsulated OSI packets [RFC1070], etc.

   For each non-initial segment of an N-segment mid-layer packet (N <=
   256), the ITE instead sets (F=0; M=1; SEG=1) in the SEAL header of
   the first non-initial segment, sets (F=0; M=1; SEG=2) in the next
   non-initial segment, etc., and sets (F=0; M=0; SEG=N-1) in the final



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   segment.  (Note that the value SEG=0 is not used, since the initial
   segment encodes a NEXTHDR value and not a SEG value.)

   For each segment (i.e., both initial and non-initial), the ITE then
   sets C=0 and R=0.  For initial segments, the ITE also sets A=1 if an
   explicit acknowledgement is required (see Section 4.4.9).  The ITE
   then sets the NBR_ID and LINK_ID fields in order to identify itself
   to the ETE.

   Finally, for each SEAL segment of a multi-segment SEAL packet, the
   ITE sets I=1 and includes the current PKT_ID value in a trailing 32-
   bit Identification field in the SEAL header of each segment.  For
   each SEAL packet that will be sent as a single segment, however, the
   ITE MAY set I=0 and omit the trailing PKT_ID field.  Whether or not
   the PKT_ID field was included, the ITE then monotonically increments
   the PKT_ID value (modulo 2^32) for the next SEAL packet to be sent to
   the ETE.  (This allows the ETE to determine whether a large number of
   SEAL packets have been received since an incomplete reassembly was
   initiated.)

4.4.7.  Outer Encapsulation

   Following SEAL encapsulation, the ITE next encapsulates each SEAL
   segment in the requisite outer headers and trailers according to the
   specific encapsulation format (e.g., [RFC1070], [RFC2003], [RFC2473],
   [RFC4213], etc.), except that it writes 'SEAL_PROTO' in the protocol
   field of the outer IP header (when simple IP encapsulation is used)
   or writes 'SEAL_PORT' in the outer destination service port field
   (e.g., when IP/UDP encapsulation is used).  The ITE then writes the
   outer IP address for this ETE in the destination address of the outer
   IP header.

   When IPv4 is used as the outer encapsulation layer, the ITE finally
   sets the DF flag in the IPv4 header of each segment.  If the path to
   the ETE correctly implements IP fragmentation (see: Section 4.4.9),
   the ITE sets DF=0; otherwise, it sets DF=1.

   When IPv6 is used as the outer encapsulation layer, the "DF" flag is
   absent but implicitly set to 1.  The packet therefore will not be
   fragmented within the subnetwork, since IPv6 deprecates in-the-
   network fragmentation.

4.4.8.  Sending SEAL Protocol Packets

   Following outer encapsulation, the ITE sends each outer packet that
   encapsulates a segment of the same mid-layer packet over the same
   underlying link in canonical order, i.e., segment 0 first, followed
   by segment 1, etc., and finally segment N-1.



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4.4.9.  Probing Strategy

   When IPv4 is used as the outer encapsulation layer, the ITE can
   perform a qualification exchange over each underlying link to
   determine whether each subnetwork path to the ETE correctly
   implements IPv4 fragmentation.  This procedure could be employed,
   e.g., to determine whether there are any middleboxes on the path that
   violate the [RFC1812], Section 5.2.6 requirement that: "A router MUST
   NOT reassemble any datagram before forwarding it".

   To perform this qualification, the ITE prepares a probe packet that
   is no larger than 576 bytes (e.g., a NULL packet with A=1 and
   NEXTHDR="No Next Header" [RFC2460] in the SEAL header), then splits
   the packet into two outer IPv4 fragments and sends both fragments to
   the ETE over the same underlying link.  If the ETE returns an SCMP
   PTB message with Code=0 (see Section 4.6.1.1), then the subnetwork
   path correctly implements IPv4 fragmentation.  If the ETE returns an
   SCMP PTB message with Code=2, however, then a middlebox in the
   subnetwork is reassembling the IPv4 fragments before they are
   delivered to the ETE (i.e., in violation of [RFC1812]).

   In addition to any control plane probing, all SEAL encapsulated data
   packets sent by the ITE are considered implicit probes.  SEAL
   encapsulated packets that use IPv4 as the outer layer of
   encapsulation with DF=0 will elicit SCMP PTB messages from the ETE if
   any IPv4 fragmentation occurs in the path.  SEAL encapsulated packets
   that use either IPv6 or IPv4 with DF=1 as the outer layer of
   encapsulation may be dropped by a router on the path to the ETE which
   will also return an ICMP PTB message to the ITE.  If the message
   includes enough information (see Section 4.4.10), the ITE can then
   use the (NBR_ID, LINK_ID, PKT_ID)-tuple along with the destination
   addresses within the packet-in-error to determine whether the PTB
   message corresponds to one of its recent packet transmissions.

   The ITE should also send explicit probes, periodically, to verify
   that the ETE is still reachable.  The ITE sets A=1 in the SEAL header
   of the first segment of a SEAL packet to be used as an explicit
   probe, where the probe can be either an ordinary data packet segment
   or a NULL packet (see above).  The probe will elicit an SCMP PTB
   message with Code=2 from the ETE as an acknowledgement (see Section
   4.6.1.1).

4.4.10.  Processing ICMP Messages

   When the ITE sends outer IP packets, it may receive ICMP error
   messages [RFC0792][RFC4443] from either the ETE or routers within the
   subnetwork.  The ICMP messages include an outer IP header, followed
   by an ICMP header, followed by a portion of the outer IP packet that



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   generated the error (also known as the "packet-in-error").  The ITE
   can use the (NBR_ID, LINK_ID, PKT_ID)-tuple along with the source and
   destination addresses within the packet-in-error to confirm that the
   ICMP message came from either the ETE or an on-path router, and can
   use any additional information to determine whether to accept or
   discard the message.

   The ITE should specifically process raw ICMPv4 Protocol Unreachable
   messages and ICMPv6 Parameter Problem messages with Code
   "Unrecognized Next Header type encountered" as a hint that the ETE
   does not implement the SEAL protocol.  The ITE can also process other
   raw ICMPv4 messages as a hint that the path to the ETE may be
   failing.  Specific actions that the ITE may take in these cases are
   out of scope.

4.4.11.  Black Hole Detection

   In some subnetwork paths, ICMP error messages may be lost due to
   filtering or may not contain enough information due to a router in
   the path not observing the recommendations of [RFC1812].  The ITE can
   use explicit probing as described in Section 4.4.9 to determine
   whether the path to the ETE is silently dropping packets (also known
   as a "black hole").  For example, when the ITE is obliged to set DF=1
   in the outer headers of data packets it should send explicit probe
   packets, periodically, in order to detect path MTU increases or
   decreases.

4.5.  ETE Specification

4.5.1.  Reassembly Buffer Requirements

   The ETE SHOULD support the minimum IP-layer reassembly requirements
   specified for IPv4 (i.e., 576 bytes [RFC1812]) and IPv6 (i.e., 1500
   bytes [RFC2460]).  The ETE SHOULD also support SEAL-layer reassembly
   for inner packets of at least 1280 bytes in length and MAY support
   reassembly for larger inner packets.  The ETE records the SEAL-layer
   reassembly buffer size in a soft-state variable "S_MRU" (see: Section
   4.5.2).

   The ETE may instead omit the reassembly function altogether and set
   S_MRU=0, but this may cause ITEs to experience tunnel MTU underruns
   in some environments resulting in an unusable link.  When reassembly
   is supported, the ETE must retain the outer IP, SEAL and other outer
   headers and trailers during both IP-layer and SEAL-layer reassembly
   for the purpose of associating the fragments/segments of the same
   packet, and must also configure a SEAL-layer reassembly buffer that
   is no smaller than the IP-layer reassembly buffer.  Hence, the ETE:




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   o  SHOULD configure an outer IP-layer reassembly buffer of at least
      the minimum specified for the outer IP protocol version.

   o  SHOULD configure a SEAL-layer reassembly buffer S_MRU size of at
      least (1280 + HELN) bytes, and

   o  MUST be capable of discarding inner packets that require IP-layer
      and/or SEAL-layer reassembly and that are larger than (S_MRU -
      HLEN).

   The ETE is permitted to accept inner packets that did not undergo IP-
   layer and/or SEAL-layer reassembly even if they are larger than
   (S_MRU - HELN) bytes.  Hence, S_MRU is a maximum *reassembly* size,
   and may be less than the largest packet size the ETE is able to
   receive when no reassembly is required.

4.5.2.  Tunnel Interface Soft State

   The ETE maintains a per-interface default S_MRU value to be applied
   for all ITEs, and can optionally maintain individual per-ITE S_MRU
   values that override the default.

   The ETE also maintains per-ITE soft state to associate (NBR_ID,
   LINK_ID, PKT_ID)-tuples with the inner and/or mid-layer source
   addresses used by ITEs.

4.5.3.  IP-Layer Reassembly

   The ETE submits unfragmented SEAL protocol IP packets for SEAL-layer
   reassembly as specified in Section 4.5.4.  The ETE instead performs
   standard IP-layer reassembly for multi-fragment SEAL protocol IP
   packets as follows.

   The ETE should maintain conservative IP-layer reassembly cache high-
   and low-water marks.  When the size of the reassembly cache exceeds
   this high-water mark, the ETE should actively discard incomplete
   reassemblies (e.g., using an Active Queue Management (AQM) strategy)
   until the size falls below the low-water mark.  The ETE should also
   actively discard any pending reassemblies that clearly have no
   opportunity for completion, e.g., when a considerable number of new
   fragments have arrived before a fragment that completes a pending
   reassembly arrives.  Following successful IP-layer reassembly, the
   ETE submits the reassembled packet for SEAL-layer reassembly as
   specified in Section 4.5.4.

   When the ETE processes the IP first fragment (i.e., one with MF=1 and
   Offset=0 in the IP header) of a fragmented SEAL packet for which the
   (NBR_ID, LINK_ID, PKT_ID)-tuple belongs to a neighboring ITE, it



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   sends an SCMP PTB message with Code=0 back to the ITE (see Section
   4.6.1.1).  When the ETE processes an IP fragment that would cause the
   reassembled outer packet to be larger than the IP-layer reassembly
   buffer following reassembly, it discontinues the reassembly and
   discards any further fragments of the same packet.

4.5.4.  SEAL-Layer Reassembly

   Following IP reassembly (if necessary), the ETE examines each SEAL
   data packet (i.e., those with C=0 in the SEAL header) packet) to
   determine whether an SCMP error message is required.  If the packet
   has an incorrect value in the SEAL header the ETE discards the packet
   and returns an SCMP "Parameter Problem" message (see Section
   4.6.1.4).  Next, if the SEAL header has (A=1; F=1) and the packet did
   not arrive as multiple outer IP fragments, the ETE sends an SCMP PTB
   message with Code=2 back to the ITE (see Section 4.6.1.1).  The ETE
   next submits single-segment mid-layer packets for decapsulation and
   delivery to upper layers (see Section 4.5.5).  The ETE instead
   performs SEAL-layer reassembly for multi-segment mid-layer packets
   with I=1 in the SEAL header as follows.

   The ETE adds each segment of a multi-segment mid-layer packet with
   I=1 in the SEAL header to a SEAL-layer pending-reassembly queue
   according to the (NBR_ID, LINK_ID, PKT_ID)-tuple found in the SEAL
   header.  The ETE performs SEAL-layer reassembly through simple in-
   order concatenation of the encapsulated segments of the same mid-
   layer packet from N consecutive SEAL segments.  SEAL-layer reassembly
   requires the ETE to maintain a cache of recently received segments
   for a hold time that would allow for nominal inter-segment delays.
   When a SEAL reassembly times out, the ETE discards the incomplete
   reassembly and returns an SCMP "Time Exceeded" message to the ITE
   (see Section 4.6.1.4).  As for IP-layer reassembly, the ETE should
   also maintain a conservative reassembly cache high- and low-water
   mark and should 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.

   If the ETE receives a SEAL packet for which a segment with the same
   (NBR_ID, LINK_ID, PKT_ID)-tuple is already in the queue, it must
   determine whether to accept the new segment and release the old, or
   drop the new segment.  If accepting the new segment would cause an
   inconsistency with other segments already in the queue (e.g.,
   differing segment lengths), the ETE drops the segment that is least
   likely to complete the reassembly.  When the ETE has already received
   the SEAL first segment (i.e., one with F=1 and M=1 in the SEAL
   header) of a SEAL protocol packet that arrived as multiple SEAL
   segments, and accepting the current segment would cause the size of



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   the reassembled packet to exceed S_MRU, the ETE schedules the
   reassembly resources for garbage collection and sends an SCMP PTB
   message with Code=1 back to the ITE (see Section 4.6.1.1).

   After all segments are gathered, the ETE reassembles the packet by
   concatenating the segments encapsulated in the N consecutive SEAL
   packets beginning with the initial segment (i.e., SEG=0) and followed
   by any non-initial segments 1 through N-1.  That is, for an N-segment
   mid-layer packet, reassembly entails the concatenation of the SEAL-
   encapsulated packet segments with the same value in the
   Identification field and with (F=1, M=1) in the first SEAL header,
   followed by (F=0, M=1, SEG=1) in the next SEAL header, followed by
   (F=0, M=1, SEG=2), etc., up to (F=0, M=0, SEG=(N-1)) in the final
   SEAL header.  Following successful SEAL-layer reassembly, the ETE
   submits the reassembled mid-layer packet for decapsulation and
   delivery to upper layers as specified in Section 4.5.5.

   The ETE must not perform SEAL-layer reassembly for multi-segment mid-
   layer packets with I=0 in the SEAL header.  The ETE instead silently
   drops all segments with (I=0 && (F=0 || M=1)) in the SEAL header and
   sends an SCMP Parameter Problem message back to the ITE.

4.5.5.  Decapsulation and Delivery to Upper Layers

   Following any necessary IP- and SEAL-layer reassembly, the ETE
   performs ingress filtering on the mid-layer and/or inner source
   addresses (e.g., via a Reverse-Path Forwarding (RPF) lookup) to
   determine whether they are correct for the (NBR_ID, LINK_ID)-tuple
   encoded in the SEAL header.  (When the outer source address and/or
   port number for the ITE is known, they are also included in the
   ingress filtering check.)  If the source addresses are incorrect, the
   ETE silently drops the packet.

   Otherwise, the ETE performs any mid-layer transformations on the mid-
   layer packet and delivers the inner packet to the upper-layer
   protocol identified either in the SEAL NEXTHDR field or the next
   header field of the mid-layer packet (i.e., if the packet included
   mid-layer encapsulations).  The ETE instead silently discards the
   inner packet if it was a NULL packet (see Section 4.4.9).

4.6.  The SEAL Control Message Protocol (SCMP)

   SEAL uses a companion SEAL Control Message Protocol (SCMP) based on
   the same message format as the Internet Control Message Protocol for
   IPv6 (ICMPv6) [RFC4443].  Each SCMP message is embedded within an
   SCMP packet which begins with the same outer header format as would
   be used for outer encapsulation of a SEAL data packet (see: Section
   4.4.7).  The following sections specify the generation and processing



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   of SCMP messages:

4.6.1.  Generating SCMP Messages

   SCMP messages use the same message Type and Code values specified for
   ordinary ICMPv6 messages in [RFC4443].  SCMP is also used to carry
   Redirect messages [RFC4861] that include Route Information Options
   (RIOs) [RFC4191] for the purpose of dynamic route optimization.  The
   general format for SCMP messages is shown in Figure 4:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Code      |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                         Message Body                          ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  As much of invoking SEAL data                |
      ~                packet as possible without the SCMP            ~
      |                  packet exceeding 576 bytes (*)               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      (*) also known as the "packet-in-error"

                       Figure 4: SCMP Message Format

   TEs generate SCMP error messages and SCMP Redirect messages in
   response to certain SEAL data packets.  As for ICMP, TEs must not
   generate SCMP error messages in response to other SCMP messages.

   As for ordinary ICMPv6 messages, the SCMP message begins with a 4
   byte header that includes 8-bit Type and Code fields followed by a
   16-bit Checksum field followed by a variable-length Message Body.
   The TE sets the Type and Code fields to the same values that would
   appear in the corresponding ICMPv6 message and also formats the
   Message Body the same as for the corresponding ICMPv6 message.

   For SCMP error messages, the Message Body is followed by the leading
   portion of the invoking SEAL data packet as the "packet-in-error".
   For SCMP Redirect messages, the packet-in-error is instead included
   in a Redirected Header Option (RHO), which may or may not be the
   final option in the message.  The packet-in-error includes as much of
   the leading portion of the invoking SEAL data packet as possible
   beginning with the outer IP header and extending to a length that
   would not cause the entire SCMP packet following outer encapsulation
   to exceed 576 bytes (see: Figure 5).



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   The TE calculates the SCMP message Checksum the same as specified for
   ICMPv6 messages except that it does not prepend a pseudo-header of
   the outer IP header, i.e., the Checksum calculation procedure is
   identical to that used for ICMPv4 [RFC0792].  The TE then
   encapsulates the SCMP message in the outer headers as shown in
   Figure 5:

                                       +--------------------+
                                       ~  outer IPv4 header ~
                                       +--------------------+
                                       ~  other outer hdrs  ~
                                       +--------------------+
                                       ~    SEAL Header     ~
          +--------------------+       +--------------------+
          ~ SCMP message header~  -->  ~ SCMP message header~
          +--------------------+  -->  +--------------------+
          ~  SCMP message body ~  -->  ~  SCMP message body ~
          +--------------------+  -->  +--------------------+
          ~   packet-in-error  ~  -->  ~  packet-in-error   ~
          +--------------------+       +--------------------+
                                       ~   outer trailers   ~
               SCMP Message            +--------------------+
           before encapsulation
                                             SCMP Packet
                                         after encapsulation

                   Figure 5: SCMP Message Encapsulation

   When an ETE processes a SEAL data packet that passes ingress
   filtering (see: Section 4.5.5) but for which an error must be
   returned, it prepares an SCMP error message.  The ETE sets the outer
   destination address/port numbers of the SCMP message to the outer
   source address/port numbers of the SEAL data packet, and sets the
   outer source address/port numbers of the SCMP message to the outer
   destination address/port numbers of the data packet.  The ETE then
   sets the NBR_ID, LINK_ID and I flag in the SEAL header of the SCMP
   message to the same values that appeared in the SEAL header of the
   data packet; if the I flag is set, the ETE also includes the PKT_ID
   value copied from the SEAL header of the data packet.

   When an ITE forwards a SEAL data packet for which it is in a position
   to inform the previous hop of a more direct route, the ITE sends an
   SCMP "Predirect" message forward to the ETE (see: Section 4.6.1.3).
   The ETE in turn either proxies the Predirect message forward or
   returns a Redirect message.

   For all SCMP messages, the TE then sets the other flag bits in the
   SEAL header to C=1, A=0, F=1, M=0 and R=0.  It next sets the NEXTHDR/



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   SEG field to 0 and sends the SCMP packet to the tunnel neighbor.

4.6.1.1.  Generating SCMP Packet Too Big (PTB) Messages

   An ETE generates an SCMP "Packet Too Big" (PTB) message under one of
   the following cases:

   o  Case 0: when it receives the IP first fragment (i.e., one with
      MF=1 and Offset=0 in the outer IP header) of a SEAL protocol
      packet that arrived as multiple IP fragments, or:

   o  Case 1: when it has already received the SEAL first segment (i.e.,
      one with F=1 and M=1 in the SEAL header) of a SEAL protocol packet
      that arrived as multiple SEAL segments, and accepting the current
      segment would cause the size of the reassembled packet to exceed
      S_MRU, or:

   o  Case 2: when it receives a SEAL protocol data packet with A=1 in
      the SEAL header that did not arrive as multiple IP fragments
      (i.e., one that does not also match Case 0).

   The ETE prepares an SCMP PTB message the same as for the
   corresponding ICMPv6 PTB message, except that it writes the S_MRU
   value for this ITE in the MTU field (i.e., even if the S_MRU value is
   0).  For cases 0 and 2 above, the packet-in-error field includes the
   leading portion of the IP packet or fragment that triggered the
   condition.  For case 1 above, the packet-in-error field includes the
   leading portion of the SEAL first segment, beginning with the
   encapsulating outer IP header.

   Finally, the ETE writes the value 0, 1 or 2 in the Code field of the
   PTB message according to whether the reason for generating the
   message was due to the corresponding case number from the list of
   cases above.

   NB: Unlike cases 0 and 1 above, case 2 is not an error condition and
   does not necessarily signify packet loss.  Instead, it is a control
   plane acknowledgement of a data plane probe.  Also, if the ETE
   generates a Case 0 SCMP PTB message it MUST NOT also generate a Case
   2 PTB message on behalf of the same SEAL segment.

4.6.1.2.  Generating Other SCMP Error Messages

   An ETE generates an SCMP "Destination Unreachable - Communication
   with Destination Administratively Prohibited" message when its
   association with the ITE is bidirectional and it receives a SEAL
   packet with a (NBR_ID, LINK_ID, PKT_ID)-tuple or outer IP/UDP source
   address/port that does not correspond to this ITE (see: Section



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   4.5.5).

   An ETE generates an SCMP "Destination Unreachable" message with an
   appropriate code under the same circumstances that an IPv6 system
   would generate an ICMPv6 Destination Unreachable message using the
   same code.  The SCMP Destination Unreachable message is formatted the
   same as for ICMPv6 Destination Unreachable messages.

   An ETE generates an SCMP "Parameter Problem" message when it receives
   a SEAL packet with an incorrect value in the SEAL header, and
   generates an SCMP "Time Exceeded" message when it garbage collects an
   incomplete SEAL data packet reassembly.  The message formats used are
   the same as for the corresponding ICMPv6 messages.

   Generation of all other SCMP message types is outside the scope of
   this document.

4.6.1.3.  Generating SCMP Redirection Messages

   TEs generate SCMP Redirect/Predirect messages as part of a route
   optimization procedure as specified in [I-D.templin-intarea-vet].  An
   ITE generates an SCMP "Predirect" message when it sends a SEAL data
   packet forward to a next hop ETE which the previous hop could have
   instead reached directly, i.e., for route optimization purposes.  An
   ETE generates an SCMP "Redirect" message in response to a Predirect
   message received from an ITE.

   When an ETE generates an SCMP Redirect message for a unidirectional
   neighbor, it creates randomly-generated NBR_ID and LINK_ID values to
   return to the ITE.  The ETE then caches the values for ingress
   filtering purposes when decapsulating packets coming from the ITE.

4.6.2.  Processing SCMP Messages

   A TE processes any SCMP messages it receives as long as it can verify
   that the corresponding SCMP packet was sent from an on-path tunnel
   neighbor.  The TE can verify that the SCMP packet came from an on-
   path neighbor by checking that the (NBR_ID, LINK_ID, PKT_ID)-tuple in
   the SEAL header as well as the source addresses of the packet
   correspond to the neighbor.

   For each SCMP message it receives, the TE first verifies that the
   identifying information is acceptable, then verifies that the
   Checksum in the SCMP message header is correct.  If the identifying
   information and/or checksum are incorrect, the TE discards the
   message; otherwise, it processes the message the same as for ordinary
   ICMPv6 messages.




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4.6.2.1.  Processing SCMP PTB Messages

   An ITE may receive an SCMP PTB message after it sends a SEAL data
   packet to an ETE (see: Section 4.6.1.1).  The packet-in-error within
   the PTB message consists of the encapsulating IP/*/SEAL headers
   followed by the mid-layer/inner packet in the form in which the ITE
   received it prior to SEAL encapsulation.

   If the PTB message has Code=2 in the SCMP header, the ITE processes
   the message as both a response to an explicit probe request and an
   indication that the tunnel neighbor is responsive, i.e., in the same
   manner implied for IPv6 Neighbor Unreachability Detection "hints of
   forward progress" (see: [RFC4861]).  If the PTB has Code=0 or Code=1
   in the SCMP header, however, the ITE processes the message as an
   indication of an MTU limitation.

   if the PTB has Code =0, the ITE first verifies that the outer IP
   header in the packet-in-error encodes an IP first fragment, then
   examines the outer IP header length field to determine a new S_MSS
   value as follows:

   o  If the length is no less than 1280, the ITE records the length as
      the new S_MSS value.

   o  If the length is less than the current S_MSS value and also less
      than 1280, the ITE can discern that IP fragmentation is occurring
      but it cannot determine the true MTU of the restricting link due
      to the possibility that a router on the path is generating runt
      first fragments.

   In this latter case, the ITE may need to search for a reduced S_MSS
   value through an iterative searching strategy that parallels the IPv4
   Path MTU Discovery "plateau table" procedure in a similar fashion as
   described in Section 5 of [RFC1191].  This searching strategy may
   entail multiple iterations in which the ITE sends additional SEAL
   data packets using a reduced S_MSS and receives additional SCMP PTB
   messages, but the process should quickly converge.  During this
   process, it is essential that the ITE reduce S_MSS based on the first
   SCMP PTB message received under the current S_MSS size, and refrain
   from further reducing S_MSS until SCMP PTB messages pertaining to
   packets sent under the new S_MSS are received.

   For both Code=0 and Code=1 PTB messages, the ITE next records the
   value in the MTU field of the SCMP PTB message as the new S_MRU value
   for this ETE and examines the inner packet within the packet-in-
   error.  If the inner packet was unfragmentable (see: Section 4.4.3)
   and larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE then sends a
   transcribed PTB message appropriate for the inner packet to the



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   original source with MTU set to (MAX(S_MRU, S_MSS) - HLEN).  (In the
   case of nested SEAL encapsulations, the transcribed PTB message will
   itself be an SCMP PTB message).  If the inner packet is fragmentable,
   however, the ITE instead reduces its inner fragmentation S_IFT
   estimate to a size no larger than S_MSS for this ETE (see: Section
   4.4.3) and does not send a transcribed PTB.  In that case, some
   fragmentable packets may be silently discarded but future
   fragmentable packets will subsequently undergo inner fragmentation
   based on this new S_IFT estimate.

   The ITE may alternatively ignore the S_MSS and S_MRU values, thus
   disabling SEAL-layer segmentation.  In that case, the ITE sends all
   SEAL-encapsulated packets as single segments and implements stateless
   MTU discovery.  In that case, if the ITE receives an SCMP PTB message
   from the ETE with Code=0 and with a degenerate length value in the
   outer IP header, it can send a translated PTB message back to the
   source listing a slightly smaller MTU size than the length value in
   the inner IP header.  For example, if the ITE receives an SCMP PTB
   message with Code=0, outer IP length 256 and inner IP length 1500, it
   can send a PTB message listing an MTU of 1400 back to the source.  If
   the ITE subsequently receives an SCMP PTB message with Code=0, outer
   IP length 256 and inner IP length 1400, it can send a PTB message
   listing an MTU of 1300 back to the source, etc.

   Actual plateau table values for this "step-down" MTU determination
   procedure are up to the implementation, which may consult Section 7
   of [RFC1191] for non-normative example guidance.

4.6.2.2.  Processing Other SCMP Error Messages

   An ITE may receive an SCMP "Destination Unreachable - Communication
   with Destination Administratively Prohibited" message after it sends
   a SEAL data packet to a bidirectional neighbor.  The ITE processes
   the message as an indication that it needs to (re)synchronize with
   the ETE (see: Section 4.7).

   An ITE may receive other SCMP "Destination Unreachable" messages with
   an appropriate code under the same circumstances that an IPv6 node
   would receive an ICMPv6 Destination Unreachable message.  The ITE
   processes the message the same as for the corresponding ICMPv6
   Destination Unreachable messages.

   An ITE may receive an SCMP "Parameter Problem" message when the ETE
   receives a SEAL packet with an incorrect value in the SEAL header.
   The ITE should examine the incorrect SEAL header field setting to
   determine whether a different setting should be used in subsequent
   packets.




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   .An ITE may receive an SCMP "Time Exceeded" message when the ETE
   garbage collects an incomplete SEAL data packet reassembly.  The ITE
   should consider the message as an indication of congestion.

   Processing of all other SCMP message types is outside the scope of
   this document.

4.6.2.3.  Processing SCMP Redirection Messages

   TEs process SCMP Redirect/Predirect messages as part of a route
   optimization procedure as specified in [I-D.templin-intarea-vet].

4.7.  Tunnel Endpoint Synchronization

   The tunnel neighbor relationship between a pair of SEAL TEs can be
   either unidirectional or bidirectional.  A unidirectional
   relationship can be used when TE 'A' will tunnel data packets
   directly to TE 'B', but 'B' will not tunnel data packets directly to
   'A'.  A bidirectional relationship is necessary when both TEs will
   tunnel data packets directly to one another.

   In order to establish a bidirectional tunnel neighbor relationship,
   the initiating TE (call it "A") performs a reliable exchange (e.g., a
   short TCP transaction) with the responding TE (call it "B").  The
   application layer details of the transaction are out of scope for
   this document, and indeed need not be standardized as long as both
   TEs observe the same specifications.  Note that a short transaction
   instead of a persistent connection is advised if the outer network
   layer protocol addresses may change, e.g., due to a mobility event,
   due to loss of state in network middleboxes, etc.  If there is
   assurance that the outer network layer protocol addresses will not
   change, then a persistent connection may be used.

   During the transaction, "A" and "B" first authenticate themselves to
   each other, then exchange information regarding the inner network
   layer prefixes that will be used for conveying inner packets that
   will be forwarded over the tunnel.  Both TEs then select a randomly-
   generated NBR_ID and one or more randomly-generated LINK_IDs, where
   each LINK_ID represents a different underlying link over which the
   tunnel interface is configured.  Both TEs then register their
   (NBR_ID, LINK_ID)-tuples with each other to establish the appropriate
   bidirectional tunnel neighbor soft state (see Sections 4.4.2 and
   4.5.2).

   Following this bidirectional tunnel neighbor establishment, the TEs
   monitor the soft state for liveness, e.g., using Neighbor
   Unreachability Detection hints of forward progress.  When one of the
   TEs wishes to terminate the relationship, it performs another short



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   transaction to request the termination, then both TEs delete their
   respective tunnel soft state.

   Outbound and inbound traffic engineering between bidirectional tunnel
   neighbors is then coordinated by a link management agent that
   monitors the underlying link paths over which the tunnel is
   configured, and can remain continuous even if the paths through one
   or more of the underlying links has failed.  When one TE detects that
   most/all underlying link paths to the other TE have failed, however,
   it terminates the bidirectional tunnel neighbor relationship.

   This bidirectional tunnel neighbor establishment is most commonly
   initiated by a client TE in establishing a connection with a serving
   TE, e.g., when a customer router within a home network establishes a
   connection with a serving router in a provider network, when a mobile
   handset connects with a serving router in a cellular operator
   network, etc.


5.  Link Requirements

   Subnetwork designers are expected to follow the recommendations in
   Section 2 of [RFC3819] when configuring link MTUs.


6.  End System Requirements

   SEAL provides robust mechanisms for returning PTB messages; however,
   end systems that send unfragmentable IP packets larger than 1500
   bytes are strongly encouraged to implement their own end-to-end MTU
   assurance, e.g., using Packetization Layer Path MTU Discovery per
   [RFC4821].


7.  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, i.e.,
   they should avoid generating runt first fragments.

   IPv6 routers within the subnetwork are required to generate the
   necessary PTB messages when they drop outer IPv6 packets due to an
   MTU restriction.







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8.  IANA Considerations

   The IANA is instructed to allocate an IP protocol number for
   'SEAL_PROTO' in the 'protocol-numbers' registry.

   The IANA is instructed to allocate a Well-Known Port number for
   'SEAL_PORT' in the 'port-numbers' registry.

   The IANA is instructed to establish a "SEAL Protocol" registry to
   record SEAL Version values.  This registry should be initialized to
   include the initial SEAL Version number, i.e., Version 0.


9.  Security Considerations

   Unlike IPv4 fragmentation, overlapping fragment attacks are not
   possible due to the requirement that SEAL segments be non-
   overlapping.  This condition is naturally enforced due to the fact
   that each consecutive SEAL segment begins at offset 0 with respect to
   the previous SEAL segment.

   An amplification/reflection attack is possible when an attacker sends
   IP first fragments with spoofed source addresses to an ETE in an
   attempt to generate a stream of SCMP messages returned to a victim
   ITE.  The (NBR_ID, LINK_ID, PKT_ID)-tuple as well as the mid-layer
   and inner headers of the packet provide mitigation for the ETE to
   detect and discard SEAL segments with spoofed source addresses.

   The SEAL header is sent in-the-clear (outside of any IPsec/ESP
   encapsulations) the same as for the outer IP and other outer headers.
   In this respect, the threat model is no different than for IPv6
   extension headers.  As for IPv6 extension headers, the SEAL header is
   protected only by L2 integrity checks and is not covered under any L3
   integrity checks.

   SCMP messages carry the (NBR_ID, LINK_ID, PKT_ID)-tuple as well as
   the mid-layer and inner headers of the packet-in-error.  Therefore,
   when an ITE receives an SCMP message it can unambiguously associate
   it with the SEAL data packet that triggered the error.

   Security issues that apply to tunneling in general are discussed in
   [I-D.ietf-v6ops-tunnel-security-concerns].


10.  Related Work

   Section 3.1.7 of [RFC2764] provides a high-level sketch for
   supporting large tunnel MTUs via a tunnel-level segmentation and



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   reassembly capability to avoid IP level fragmentation, which is in
   part the same approach used by SEAL.  SEAL could therefore be
   considered as a fully functioned manifestation of the method
   postulated by that informational reference.

   Section 3 of [RFC4459] describes inner and outer fragmentation at the
   tunnel endpoints as alternatives for accommodating the tunnel MTU;
   however, the SEAL protocol specifies a mid-layer segmentation and
   reassembly capability that is distinct from both inner and outer
   fragmentation.

   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 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).  A
   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 D of this document.


11.  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, IP fragmentation
       provides a short-term interim mechanism for ensuring that packets
       are delivered while SEAL adjusts its packet sizing parameters.

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

   3.  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.




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   4.  Using SEAL, ETEs encapsulate SCMP error messages in outer and
       mid-layer headers such that packet-filtering network middleboxes
       will not filter them the same as for "raw" ICMP messages that may
       be generated by an attacker.

   5.  The SEAL approach ensures that the tunnel either delivers or
       deterministically drops packets according to their size, which is
       a required characteristic of any IP link.

   6.  Most importantly, all SEAL packets have Identification values
       that are sufficiently long to be used for duplicate packet
       detection purposes and to associate ICMP error messages with
       actual packets sent without requiring per-packet state; hence,
       SEAL avoids certain denial-of-service attack vectors open to the
       classical methods.


12.  Acknowledgments

   The following individuals are acknowledged for helpful comments and
   suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver
   Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner,
   Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph
   Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci,
   Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob
   Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt
   Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark
   Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin
   Whittle, James Woodyatt, and members of the Boeing Research &
   Technology NST 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.


13.  References

13.1.  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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



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              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

13.2.  Informative References

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

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

   [I-D.ietf-intarea-ipv4-id-update]
              Touch, J., "Updated Specification of the IPv4 ID Field",
              draft-ietf-intarea-ipv4-id-update-02 (work in progress),
              March 2011.

   [I-D.ietf-v6ops-tunnel-security-concerns]
              Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns With IP Tunneling",
              draft-ietf-v6ops-tunnel-security-concerns-04 (work in
              progress), October 2010.

   [I-D.templin-intarea-vet]
              Templin, F., "Virtual Enterprise Traversal (VET)",
              draft-templin-intarea-vet-24 (work in progress),
              March 2011.

   [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.

   [RFC1070]  Hagens, R., Hall, N., and M. Rose, "Use of the Internet as



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              a subnetwork for experimentation with the OSI network
              layer", RFC 1070, February 1989.

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

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

   [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.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [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.

   [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.

   [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
              link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
              August 2002.

   [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.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [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.



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   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, 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.

   [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.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, August 2007.

   [RFC5445]  Watson, M., "Basic Forward Error Correction (FEC)
              Schemes", RFC 5445, March 2009.

   [RFC5720]  Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720,
              February 2010.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

   [RFC6139]  Russert, S., Fleischman, E., and F. Templin, "Routing and
              Addressing in Networks with Global Enterprise Recursion
              (RANGER) Scenarios", RFC 6139, February 2011.

   [RFC6179]  Templin, F., "The Internet Routing Overlay Network
              (IRON)", RFC 6179, March 2011.

   [SIGCOMM]  Luckie, M. and B. Stasiewicz, "Measuring Path MTU
              Discovery Behavior", November 2010.

   [TBIT]     Medina, A., Allman, M., and S. Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              October 2004.

   [TCP-IP]   "Archive/Hypermail of Early TCP-IP Mail List,
              http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May
              1987 - May 1990.".

   [WAND]     Luckie, M., Cho, K., and B. Owens, "Inferring and
              Debugging Path MTU Discovery Failures", October 2005.



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Appendix A.  Reliability

   Although a SEAL tunnel may span an arbitrarily-large subnetwork
   expanse, the IP layer sees the tunnel as a simple link that supports
   the IP service model.  Since SEAL supports segmentation at a layer
   below IP, SEAL therefore presents a case in which the link unit of
   loss (i.e., a SEAL segment) is smaller than the end-to-end
   retransmission unit (e.g., a TCP segment).

   Links with high bit error rates (BERs) (e.g., IEEE 802.11) use
   Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] to increase
   packet delivery ratios, while links with much lower BERs typically
   omit such mechanisms.  Since SEAL tunnels may traverse arbitrarily-
   long paths over links of various types that are already either
   performing or omitting ARQ as appropriate, it would therefore often
   be inefficient to also require the tunnel to perform ARQ.

   When the SEAL ITE has knowledge that the tunnel will traverse a
   subnetwork with non-negligible loss due to, e.g., interference, link
   errors, congestion, etc., it can solicit Segment Reports from the ETE
   periodically to discover missing segments for retransmission within a
   single round-trip time.  However, retransmission of missing segments
   may require the ITE to maintain considerable state and may also
   result in considerable delay variance and packet reordering.

   SEAL may also use alternate reliability mechanisms such as Forward
   Error Correction (FEC).  A simple FEC mechanism may merely entail
   gratuitous retransmissions of duplicate data, however more efficient
   alternatives are also possible.  Basic FEC schemes are discussed in
   [RFC5445].

   The use of ARQ and FEC mechanisms for improved reliability are for
   further study.


Appendix B.  Integrity

   Each link in the path over which a SEAL tunnel is configured is
   responsible for link layer integrity verification for packets that
   traverse the link.  As such, when a multi-segment SEAL packet with N
   segments is reassembled, its segments will have been inspected by N
   independent link layer integrity check streams instead of a single
   stream that a single segment SEAL packet of the same size would have
   received.  Intuitively, a reassembled packet subjected to N
   independent integrity check streams of shorter-length segments would
   seem to have integrity assurance that is no worse than a single-
   segment packet subjected to only a single integrity check steam,
   since the integrity check strength diminishes in inverse proportion



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   with segment length.  In any case, the link-layer integrity assurance
   for a multi-segment SEAL packet is no different than for a multi-
   fragment IPv6 packet.

   Fragmentation and reassembly schemes must also consider packet-
   splicing errors, e.g., when two segments from the same packet are
   concatenated incorrectly, when a segment from packet X is reassembled
   with segments from packet Y, etc.  The primary sources of such errors
   include implementation bugs and wrapping IP ID fields.  In terms of
   implementation bugs, the SEAL segmentation and reassembly algorithm
   is much simpler than IP fragmentation resulting in simplified
   implementations.  In terms of wrapping ID fields, when IPv4 is used
   as the outer IP protocol, the 16-bit IP ID field can wrap with only
   64K packets with the same (src, dst, protocol)-tuple alive in the
   system at a given time [RFC4963] increasing the likelihood of
   reassembly mis-associations.  However, SEAL ensures that any outer
   IPv4 fragmentation and reassembly will be short-lived and tuned out
   as soon as the ITE receives an SCMP PTB message, and SEAL
   segmentation and reassembly uses a much longer Identification field.
   Therefore, reassembly mis-associations of IP fragments nor of SEAL
   segments should be prohibitively rare.


Appendix C.  Transport Mode

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

   If both TCPs agree on the use of SEAL, their protocol messages will
   be carried as IPv4/SEAL/TCP and the connection will be serviced by
   the SEAL protocol using TCP (instead 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.


Appendix D.  Historic Evolution of PMTUD

   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



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   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 (straw
       RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)

   4.  Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
       1990)

   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





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   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.  The final assertion is addressed
   by the mechanisms specified in SEAL.


Author's Address

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

   Email: fltemplin@acm.org































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