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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                      September 30, 2010
Expires: April 3, 2011


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-20.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 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 April 3, 2011.

Copyright Notice

   Copyright (c) 2010 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.















































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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 Protocol Specification  . . . . . . . . . . . . . . . . . 10
     4.1.  Model of Operation . . . . . . . . . . . . . . . . . . . . 10
     4.2.  SEAL Header Format . . . . . . . . . . . . . . . . . . . . 13
     4.3.  ITE Specification  . . . . . . . . . . . . . . . . . . . . 14
       4.3.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
       4.3.2.  Tunnel Interface Soft State  . . . . . . . . . . . . . 15
       4.3.3.  Admitting Packets into the Tunnel  . . . . . . . . . . 16
       4.3.4.  Mid-Layer Encapsulation  . . . . . . . . . . . . . . . 17
       4.3.5.  SEAL Segmentation  . . . . . . . . . . . . . . . . . . 17
       4.3.6.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 18
       4.3.7.  Probing Strategy . . . . . . . . . . . . . . . . . . . 19
       4.3.8.  Identification . . . . . . . . . . . . . . . . . . . . 19
       4.3.9.  Sending SEAL Protocol Packets  . . . . . . . . . . . . 20
       4.3.10. Processing Raw ICMP Messages . . . . . . . . . . . . . 20
     4.4.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 20
       4.4.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 20
       4.4.2.  IP-Layer Reassembly  . . . . . . . . . . . . . . . . . 21
       4.4.3.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 22
       4.4.4.  Decapsulation and Delivery to Upper Layers . . . . . . 23
     4.5.  The SEAL Control Message Protocol (SCMP) . . . . . . . . . 23
       4.5.1.  Generating SCMP Messages . . . . . . . . . . . . . . . 23
       4.5.2.  Processing SCMP Messages . . . . . . . . . . . . . . . 27
     4.6.  Tunnel Endpoint Synchronization  . . . . . . . . . . . . . 30
   5.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 32
   6.  End System Requirements  . . . . . . . . . . . . . . . . . . . 32
   7.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 32
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 32
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 33
   11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 34
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     13.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Appendix A.  Reliability . . . . . . . . . . . . . . . . . . . . . 38
   Appendix B.  Integrity . . . . . . . . . . . . . . . . . . . . . . 39
   Appendix C.  Transport Mode  . . . . . . . . . . . . . . . . . . . 40
   Appendix D.  Historic Evolution of PMTUD . . . . . . . . . . . . . 40
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41





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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 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 (cited above).  In particular, IPv4 fragmentation raises



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   issues ranging from minor annoyances (e.g., in-the-network router
   fragmentation) 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
   [I-D.ietf-tcpm-icmp-attacks].  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].

   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.  Examples include the
   global Internet interdomain routing core, Mobile Ad hoc Networks
   (MANETs) and enterprise networks.  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
   supports 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 introduces an extended Identification field for
   packet identification and a mid-layer segmentation and reassembly
   capability that allows simplified cutting and pasting of packets.
   Moreover, SEAL senses in-the-network fragmentation as a "noise"
   indication that packet sizing parameters are "out of tune" with
   respect to the network path.  As a result, SEAL can naturally tune
   its packet sizing parameters to eliminate the in-the-network
   fragmentation.  This approach is in contrast to existing tunneling



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   protocol practices which seek to avoid MTU issues by selecting a
   "lowest common denominator" MTU that may be overly conservative for
   many tunnels and difficult to change even when 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.).







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   Packet Too Big (PTB)
      a network layer 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.

   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_MRU - the SEAL Maximum Reassembly Unit

      S_MSS - the SEAL Maximum Segment Size

      SCMP - the SEAL Control Message Protocol

      SEAL_ID - a SEAL packet Identification/Nonce value

      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)

      THRESH - inner fragmentation threshold

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   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.


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 of enterprise networks, ISP networks, SOHO
   networks, the interdomain routing core, and any other networking
   scenario involving IP encapsulation.  SEAL and its associated
   technologies (including Virtual Enterprise Traversal (VET)
   [I-D.templin-intarea-vet]) are functional building blocks for a new
   Internetworking architecture based on Routing and Addressing in
   Networks with Global Enterprise Recursion (RANGER)
   [RFC5720][I-D.russert-rangers] and the Internet Routing Overlay
   Network (IRON) [I-D.templin-iron].

   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 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 tunnels between performance-critical routers
   connected to high data rate subnetworks such as the Internet DFZ, as
   well as for other uses in which reassembly would present too great of
   a burden for the routers or end systems.




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   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
   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 Protocol Specification

   The following sections specify the operation of the SEAL protocol.

4.1.  Model of Operation

   SEAL is an encapsulation sublayer that 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 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 is used 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-in-
   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



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   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-in-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  ~
   +------------------+   +------------------+   +------------------+
   ~ other outer hdrs ~   ~  outer IPv6 hdr  ~   ~ other outer hdrs ~
   +------------------+   +------------------+   +------------------+
   ~ SEAL hdr (SEG=0) ~   ~ other outer hdrs ~   ~ SEAL hdr (SEG=2) ~
   +------------------+   +------------------+   +------------------+
   ~  mid-layer hdrs  ~   ~ SEAL hdr (SEG=1) ~   |    inner IPv4    |
   +------------------+   +------------------+   ~      Packet      ~
   |    inner IPv4    |   |    inner IPv4    |   |    (Segment 2)   |
   ~      Packet      ~   ~      Packet      ~   +------------------+
   |    (Segment 0)   |   |    (Segment 1)   |   ~ mid-layer trails ~
   +------------------+   +------------------+   +------------------+
   ~  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 SEAL header itself is inserted 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 SEAL header is inserted 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 SEAL header is inserted 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".

   SEAL-encapsulated packets include a SEAL_ID that the TEs maintain as
   either a monotonically-incrementing packet identification number or
   as a static nonce to identify the tunnel.  When the SEAL_ID is
   maintained as a packet identifier, routers within the subnetwork can
   use it for duplicate packet detection and the TEs can use it for SEAL
   segmentation/reassembly.  TEs can also use the SEAL_ID to detect off-
   path attacks whether it is maintained as a packet identifier or a
   nonce.

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






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4.2.  SEAL Header Format

   The SEAL header is formatted as follows:

       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|R|F|M|  NEXTHDR/SEG  |    SEAL_ID (bits 48 - 32)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   SEAL_ID (bits 31 - 0)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       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 "Identifier" bit.  Set to 1 if the SEAL_ID contains a
      monotonically-incrementing packet identifier; set to 0 if the
      SEAL_ID contains a static nonce.

   R (1)
      the "Redirects Permitted" bit.  Set to 1 if the ITE is willing to
      accept SCMP redirects (see: Section 4.5); 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.




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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.)

   SEAL_ID (48)
      a 48-bit Identification or nonce field.

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

4.3.  ITE Specification

4.3.1.  Tunnel Interface MTU

   The ITE configures a point-to-(multi)point tunnel virtual interface
   over one or more underlying links that connect the border node to the
   subnetwork.  The tunnel interface must present a fixed MTU to the
   inner network layer as the size for admission of inner packets into
   the interface.  Since point-to-multipoint tunnel 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 the 1500 byte packets sent by end systems
   incur additional encapsulation at an ITE, however, they may be
   dropped silently since the network may not always deliver the
   necessary PTBs [RFC2923].

   The ITE should therefore set a tunnel virtual 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 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 virtual
   interface such that all inner packets are admitted into the interface
   without regard to size.  For ITEs that host applications that use the
   tunnel virtual interface directly, this option must be carefully



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   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, however, the
   inner layer must send a SEAL PTB message instead of a PTB of the
   inner network layer (see: Section 4.3.2).

   Note that when the tunnel interface sets a finite MTU the inner
   network layer must be made aware of the SEAL protocol; this may not
   be practical for some implementations.  When the interface sets an
   indefinite MTU, however, the inner network layer unconditionally
   admits all packets into the interface without fragmentation.  Once
   the packet has been admitted into the interface, it transitions from
   the inner network layer and becomes subject to SEAL layer processing.

   In light of the above considerations, it is RECOMMENDED that the ITE
   configure an indefinite MTU on the tunnel virtual interface such that
   the inner network layer unconditionally admits all inner packets into
   the tunnel and any necessary adaptations are performed by the SEAL
   layer within the tunnel virtual interface as described in the
   following sections.

4.3.2.  Tunnel Interface Soft State

   The ITE optionally maintains per-ETE soft state within the tunnel
   interface (e.g., in a neighbor cache) used to support inner
   fragmentation and SEAL segmentation for packets admitted into the
   tunnel interface.  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 underlying interface MTU if the underlying interface MTU can
      be determined (otherwise, the ITE initializes S_MSS to
      "infinity").  The ITE decreases or increased S_MSS based on any
      SCMP "MTU Report" messages received (see Section 4.5).

   o  a SEAL Maximum Reassembly Unit (S_MRU).  If the ITE is not
      configured to use SEAL segmentation, it initializes S_MRU to the
      static value 0.  Otherwise, it initializes S_MRU to "infinity" and
      decreases or increases S_MRU based on any SCMP MTU Report messages
      received (see Section 4.5).  When (S_MRU>(S_MSS*256)), the ITE
      uses (S_MSS*256) as the effective S_MRU value.

   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.

   When the ITE chooses to omit the segmentation and reassembly
   procedure, it may omit the per-ETE S_MSS and S_MRU soft state and
   simply use stateless translation to relay PTB messages coming from
   the ETE back to the original source.  In that case, the ITE can be
   considered as stateless.

4.3.3.  Admitting Packets into the Tunnel

   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 encapsulation, then prepares
   the packet for admission into the tunnel according to whether it is
   "fragmentable" (discussed in the next paragraph) or "unfragmentable"
   (discussed in the following paragraph).

   If the packet is a non-SEAL IPv4 packet with DF=0 in the IPv4 header
   (*), and the packet is larger than a constant inner fragmentation
   threshold value (THRESH), the ITE uses fragmentation to break the



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   packet into IPv4 fragments no larger than THRESH bytes then submits
   each fragment for encapsulation separately.  The THRESH value may be
   maintained as per-ETE soft state or as a single value that records a
   lowest common denominator value for all ETEs that are reached by the
   tunnel.  The ITE should use a "safe" estimate for THRESH that would
   be highly unlikely to trigger additional fragmentation within the
   current tunnel or within any additional tunnels that may occur along
   the path.  In particular, it is RECOMMENDED that the ITE set THRESH
   to 512 unless it can determine a more accurate safe value, e.g., via
   probing.

   For all other packets (*), if the packet is larger than (MAX(S_MRU,
   S_MSS) - HLEN), the ITE drops it and sends a PTB message to the
   original source with an MTU value of (MAX(S_MRU, S_MSS) - HLEN);
   otherwise, it submits the packet for encapsulation (**).  Note that
   the ITE must include the length of the uncompressed headers and
   trailers when calculating HLEN if the tunnel interface is using
   header compression.  Note also that the ITE is permitted to admit
   inner packets into the tunnel that 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 4.4.1.

   (*) In order to support nested encapsulations, inner SEAL-protocol
   IPv4 packets with DF=0 must be treated as unfragmentable.  In that
   case, a PTB message originating from outer nested SEAL encapsulations
   will be successively relayed to the ITEs of inner nested
   encapsulations.

   (**) When the tunnel interface omits per-ETE S_MRU and S_MSS soft
   state, it can alternatively encapsulate and submit all inner packets
   into the tunnel regardless of their size and use stateless
   translation to translate any resultant PTB messages.

4.3.4.  Mid-Layer Encapsulation

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

4.3.5.  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 SEAL 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



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   mid-layer packet into N segments (N <= 256) that are no larger than
   (S_MSS - OHLEN) bytes each.  Each segment, except the final one, MUST
   be of equal length.  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.

   Note that this SEAL segmentation ignores the fact that the mid-layer
   packet may be unfragmentable outside of the subnetwork.  This
   segmentation process is a mid-layer (not an IP layer) operation
   employed by the ITE to adapt the mid-layer packet to the subnetwork
   path characteristics, and the ETE will restore the packet to its
   original form during reassembly.  Therefore, the fact that the packet
   may have been segmented within the subnetwork is not observable
   outside of the subnetwork.

4.3.6.  Outer Encapsulation

   Following SEAL segmentation, the ITE next encapsulates each segment
   in a SEAL header formatted as specified in Section 4.2.  For the
   first segment, the ITE sets F=1, then sets NEXTHDR to the Internet
   Protocol number of the encapsulated inner packet, and finally sets
   M=1 if there are more segments or sets M=0 otherwise.  For each non-
   initial segment of an N-segment mid-layer packet (N <= 256), the ITE
   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 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, the ITE then sets C=0, sets R=1 if it is willing to
   accept SCMP redirects (see Section 4.5), sets A=1 if probing is
   necessary (see Section 4.3.7), and finally sets the I flag and packet
   identification values as specified in Section 4.3.8.  The ITE next
   encapsulates each 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 finally sends each encapsulated segment as a SEAL
   protocol packet as specified in Section 4.3.9.







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4.3.7.  Probing Strategy

   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 will elicit SCMP PTB messages from the
   ETE (see: Section 4.5) if any IPv4 fragmentation occurs in the path.
   SEAL encapsulated packets that use IPv6 as the outer layer of
   encapsulation may be dropped by an IPv6 router on the path to the ETE
   which will also return an ICMPv6 PTB message to the ITE.  The ITE can
   then use the SEAL_ID 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 a segment to be used as an explicit probe, where the probe can be
   either an ordinary data packet segment or a NULL packet created by
   setting the NEXTHDR field to a value of "No Next Header" (see Section
   4.7 of [RFC2460]).  The probe will elicit a solicited SCMP Neighbor
   Advertisement (NA) message from the ETE as an acknowledgement (see
   Section 4.5.1).

   Finally, the ITE MAY send "expendable" outer IP probe packets (see
   Section 4.3.9) as explicit probes in order to detect increases in the
   path MTU to the ETE.  One possible strategy is to send expendable
   packets with A=1 in the SEAL header and DF=1 in the IP header.  In
   all cases, the ITE MUST be conservative in its use of the A bit in
   order to limit the resultant control message overhead.

4.3.8.  Identification

   The ITE maintains a randomly-initialized SEAL_ID value as per-ETE
   soft state (e.g., in the neighbor cache).  If the SEAL_ID is to be
   used as a packet identifier, the ITE monotonically increments the
   value for each successive SEAL protocol packet it sends to the ETE.
   If the SEAL_ID is to be used as a tunnel identifier, the ITE instead
   maintains SEAL_ID as a static value.

   For each successive SEAL protocol packet, the ITE writes the current
   SEAL_ID value into the header field of the same name in the SEAL
   header.  It then sets I=1 if the SEAL_ID represents a packet
   identifier and I=0 if the SEAL_ID represents a tunnel identifier.

   Note that the ITE must be consistent in its setting of the I bit.
   For example, it must not set I=1 in some packets and I=0 in others
   since this may result in unpredictable behavior.





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4.3.9.  Sending SEAL Protocol Packets

   Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
   the header of each outer IPv4 packet to ensure that they will be
   delivered to the ETE even if they are fragmented within the
   subnetwork.  (The ITE can instead set DF=1 for "expendable" outer
   IPv4 packets (e.g., for NULL packets used as probes -- see Section
   4.3.7), but these may be lost due to an MTU restriction).  For outer
   IPv6 packets, the "DF" bit is always implicitly set to 1; hence, they
   will not be fragmented within the subnetwork.

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

4.3.10.  Processing Raw ICMP Messages

   The ITE may receive "raw" ICMP error messages [RFC0792][RFC4443] from
   either the ETE or routers within the subnetwork that comprise an
   outer IP header, followed by an ICMP header, followed by a portion of
   the SEAL packet that generated the error (also known as the "packet-
   in-error").  The ITE can use the SEAL_ID encoded in the packet-in-
   error as a nonce to confirm that the ICMP message came from either
   the ETE or an on-path router, 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; specific actions that the ITE
   may take in this case are out of scope.

4.4.  ETE Specification

4.4.1.  Reassembly Buffer Requirements

   The ETE SHOULD support IP-layer and 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".  (The ETE may instead
   omit the reassembly function altogether and set S_MRU=0, but this may
   cause tunnel MTU underruns in some environments.)  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 size of at
      least (1280 + HELN) bytes, and

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

   The ETE can maintain S_MRU either as a single value to be applied for
   all ITEs, or as a per-ITE value.  In that case, the ETE can manage
   each per-ITE S_MRU value separately (e.g., to reduce congestion
   caused by excessive segmentation from specific ITEs) but should seek
   to maintain as stable a value as possible for each ITE.

   Note that 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 ETE is able to receive without
   reassembly.

4.4.2.  IP-Layer Reassembly

   The ETE submits unfragmented SEAL protocol IP packets for SEAL-layer
   reassembly as specified in Section 4.4.3.  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 been received before a fragment that completes a
   pending reassembly has arrived.  Following successful IP-layer
   reassembly, the ETE submits the reassembled packet for SEAL-layer
   reassembly as specified in Section 4.4.3.

   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, it sends an
   SCMP PTB message back to the ITE (see Section 4.5.1).  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.  In the limiting case, the ETE may omit
   IP layer reassembly altogether and discard all IP fragments after
   sending an SCMP PTB message.





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4.4.3.  SEAL-Layer Reassembly

   Following IP reassembly (if necessary), the ETE examines each mid-
   layer data packet (i.e., one with C=0 in the SEAL header) packet) to
   determine whether an SCMP error message is required.  If the mid-
   layer data packet has an incorrect value in the SEAL header the ETE
   discards the packet and returns an SCMP "Parameter Problem" message
   (see Section 4.5.1).  Next, if the SEAL header has A=1, the ETE sends
   an SCMP Neighbor Advertisement (SNA) message back to the ITE (see
   Section 4.5.1).  The ETE next submits single-segment mid-layer
   packets for decapsulation and delivery to upper layers (see Section
   4.4.4).  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 (Source, Destination, SEAL_ID)-tuple found in the
   outer IP and SEAL headers.  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 4.5.1).  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
   (Source, Destination, SEAL_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.  If the ETE accepts a new SEAL
   segment that would cause the reassembled outer packet to be larger
   than S_MRU following reassembly, it schedules the reassembly
   resources for garbage collection and sends an SCMP PTB message back
   to the ITE (see Section 4.5.1).

   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-



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   encapsulated packet segments with (F=1, M=1, SEAL_ID=j) in the first
   SEAL header, followed by (F=0, M=1, SEG=1, SEAL_ID=(j+1)) in the next
   SEAL header, followed by (F=0, M=1, SEG=2, SEAL_ID=(j+2)), etc., up
   to (F=0, M=0, SEG=(N-1), SEAL_ID=(j + N-1)) in the final SEAL header.
   (Note that modulo arithmetic based on the length of the SEAL_ID field
   is used).  Following successful SEAL-layer reassembly, the ETE
   submits the reassembled mid-layer packet for decapsulation and
   delivery to upper layers as specified in Section 4.4.4.

   Note that 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 in the SEAL header
   and uses any segments with I=0; M=1 in the SEAL header to send an
   SCMP PTB message back to the ITE.  Note also that the ETE may set
   S_MRU=0 in order to omit SEAL layer reassembly altogether.

4.4.4.  Decapsulation and Delivery to Upper Layers

   Following any necessary IP- and SEAL-layer reassembly, the ETE
   discards the outer headers and trailers and performs any mid-layer
   transformations on the mid-layer packet.  The ETE next discards the
   mid-layer headers and trailers, and delivers the inner packet to the
   upper-layer protocol indicated 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
   4.3.9).

4.5.  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
   4.3.6).  The following sections specify the generation and processing
   of SCMP messages:

4.5.1.  Generating SCMP Messages

   SCMP messages may be generated by either ITEs or ETEs (i.e., by any
   TE) using the same message Type and Code values specified for
   ordinary ICMPv6 messages in [RFC4443].  SCMP is also used to carry
   other ICMPv6 message types and their associated options as specified
   in other documents (e.g., [RFC4191][RFC4861], etc.).  The general
   format for SCMP messages is shown in Figure 4:





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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     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 solicitation messages (e.g., an SCMP echo request, an
   SCMP router/neighbor solicitation, a SEAL data packet with A=1, etc.)
   for the purpose of triggering an SCMP response.  TEs generate
   solicited SCMP messages (e.g., an SCMP echo reply, and SCMP router/
   neighbor advertisement, etc.) in response to explicit solicitations,
   and generate SCMP error messages in response to errored SEAL data
   packets.  As for ICMP, TEs must not generate SCMP error message 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.

   The Message Body is followed by the leading portion of the invoking
   SEAL data packet (i.e., the "packet-in-error") IFF the packet-in-
   error would also be included in the corresponding ICMPv6 message.  If
   the SCMP message will include a packet-in-error, the TE 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).

   The TE then 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 since the SEAL_ID already gives
   sufficient assurance against mis-delivery.  (The Checksum calculation
   procedure is therefore identical to that used for ICMPv4 [RFC0792].)



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   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 a TE generates an SCMP message in response to an SCMP
   solicitation or an errored SEAL data packet (i.e., a soliciting
   packet), it sets the outer IP destination and source addresses of the
   SCMP packet to the soliciting packet's source and destination
   addresses (respectively).  (If the destination address in the
   solicitation was multicast, the TE instead sets the outer IP source
   address of the SCMP packet to an address assigned to the underlying
   IP interface.)  The TE then sets the SEAL_ID and I flag in the SEAL
   header of the SCMP packet to the same values that appeared in the
   soliciting or errored packet.

   When a TE generates an unsolicited SCMP message, it sets the outer IP
   destination and source addresses of the SCMP packet the same as it
   would for ordinary SEAL data packets.  The TE then sets the SEAL_ID
   and I flag in the SEAL header of the SCMP packet to the same values
   that it would use to send an ordinary SEAL data packet.

   For all SCMP messages, the TE then sets the other flag bits in the
   SEAL header to C=1, A=0, R=0, F=1, and M=0.  It next sets the
   NEXTHDR/SEG to an arbitrary value and sends the SCMP packet to the
   tunnel far end.






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4.5.1.1.  Generating SCMP Packet Too Big (PTB) Messages

   An ETE generates an SCMP PTB message 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 when
   it discontinues reassembly of a SEAL protocol packet that arrived as
   multiple IP fragments and/or multiple SEAL segments and would exceed
   S_MRU following reassembly.

   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.

4.5.1.2.  Generating SCMP Neighbor Discovery Messages

   An ITE generates an SCMP "Neighbor Solicitation" (SNS) or "Router
   Solicitation" (SRS) message when it needs to solicit a response from
   an ETE.  An ETE generates a solicited SCMP "Neighbor Advertisement"
   (SNA) or "Router Advertisement" (SRA) message when it receives an
   SNS/SRS message, and also generates a solicited SNA message when it
   receives a SEAL protocol data packet with A=1 in the SEAL header.
   Any TE may also generate unsolicited SNA/SRA messages that are not
   triggered by a specific solicitation event, but these may be
   discarded by the tunnel far-end.

   The TE generates SNS, SNA, SRS and SRA messages the same as described
   for the corresponding IPv6 Neighbor Discovery (ND) messages (see:
   [RFC4861]).  These messages may also be used in conjunction with the
   tunnel endpoint synchronization procedure specified in Section 4.6.

4.5.1.3.  Generating SCMP Redirect Messages

   An ETE generates an SCMP "Redirect" message when it receives a SEAL
   data packet with R=1 in the SEAL header and needs to inform the ITE
   of a better next hop.  The ETE generates SCMP Redirect messages the
   same as described in [RFC4861], except that it includes Route
   Information Options (RIOs) [RFC4191] to inform the ITE of a better
   next hop for an entire IP prefix instead of only a single
   destination.  The SCMP Redirect message therefore supports network
   redirection instead of host redirection.

4.5.1.4.  Generating Other SCMP Messages

   An ETE generates an SCMP "Destination Unreachable - Communication
   with Destination Administratively Prohibited" message when it is
   operating in synchronized mode and receives a SEAL packet with a
   SEAL_ID that is outside of the current window for this ITE (see:
   Section 4.6).



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   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.

4.5.2.  Processing SCMP Messages

   An ITE processes any solicited and error SCMP message it receives as
   long as it can verify that the corresponding SCMP packet was sent
   from an on-path ETE.  The ITE can verify that the SCMP packet came
   from an on-path ETE by checking that the SEAL_ID in the SEAL header
   of the packet corresponds to one of its recently-sent SEAL data
   packets or SCMP request packets.

   An ITE maintains a window of SEAL_IDs of packets that it has recently
   sent to each ETE.  For each solicited and error SCMP message it
   receives, the ITE first verifies that the SEAL_ID is within the
   window then verifies that the Checksum in the SCMP message header is
   correct.  If the SEAL_ID is outside of the window and/or the checksum
   is incorrect, the ITE discards the message; otherwise, it processes
   the message the same as for ordinary ICMPv6 messages.

   Any TE may also receive unsolicited SCMP messages (e.g., SNS, SRS,
   SNA, etc.) from the tunnel far end.  The TE sends SCMP response
   messages in response to solicitations, but does not otherwise process
   the unsolicited SCMP messages as an indication of tunnel far end
   liveness.

   Finally, TEs process solicited and error SCMP messages as an
   indication that the tunnel far end is responsive, i.e., in the same
   manner implied for IPv6 Neighbor Unreachability Detection "hints of
   forward progress" (see: [RFC4861]).

4.5.2.1.  Processing SCMP PTB Messages

   An ITE may receive an SCMP PTB message from an ETE after it sends a
   SEAL data packet (see: Section 4.5.1).  The packet-in-error within
   the PTB message consists of the "post-encapsulation headers" followed



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   by the "pre-encapsulation packet" in the form in which the ITE
   received it prior to SEAL encapsulation.

   When the ITE receives an SCMP PTB message, it first examines the
   post-encapsulation IP header of the packet-in-error.  If the packet-
   in-error is an IPv4 first fragment, the ITE determines a new S_MSS
   value according to the length recorded in the post-encapsulation IP
   header 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 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.

   After updating the S_MSS value if necessary (see above), 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 pre-encapsulation packet.
   If the pre-encapsulation packet was unfragmentable (see: Section
   4.3.3) and larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE then sends
   a transcribed PTB message appropriate for the pre-encapsulation
   packet to the original source with MTU set to (MAX(S_MRU, S_MSS) -
   HLEN).  (Note that in the case of nested SEAL encapsulations, the
   transcribed PTB message will itself be an SCMP PTB message).  If the
   pre-encapsulation packet is fragmentable, however, the ITE instead
   reduces its inner fragmentation THRESH estimate to a size no larger
   than S_MSS for this ETE (see: Section 4.3.3) and does not send a
   transcribed PTB.  In that case, future fragmentable packets will
   subsequently undergo inner fragmentation based on this new THRESH
   estimate.

   Note that the ITE may alternatively avoid stateful caching of per-ETE
   S_MSS values by implementing stateless MTU discovery plateau table



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   processing.  In particular, if the ITE receives and SCMP PTB message
   with a too-small length value in the post-encapsulation 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 pre-
   encapsulation IP header.  For example, if the ITE receives an SCMP
   PTB message with post-encapsulation length 256 and pre-encapsulation
   length 1500, it can send a PTB message listing an MTU of 1400 back to
   the source.  If the ITE then subsequently receives an SCMP PTB
   message with post-encapsulation length 256 and pre-encapsulation
   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.

4.5.2.2.  Processing SCMP Neighbor Discovery Messages

   An ETE may receive SNS/SRS messages from an ITE as the initial leg in
   a neighbor discovery exchange.  An ITE may also receive both
   solicited and unsolicited SNA/SRA messages from an ETE.

   The TE processes SNS/SRS and SNA/SRA messages the same as described
   for the corresponding IPv6 Neighbor Discovery (ND) messages (see:
   [RFC4861]).  The messages may also be used in conjunction with the
   tunnel endpoint synchronization procedure specified in Section 4.6.

4.5.2.3.  Processing SCMP Redirect Messages

   An ITE may receive SCMP redirect messages after sending a SEAL data
   packet with R=1 in the SEAL header to an ETE.  The ITE processes any
   RIO options in the SCMP redirect message and updates its Forwarding
   Information Base (FIB) accordingly.

4.5.2.4.  Processing Other SCMP Messages

   An ITE may receive an SCMP "Destination Unreachable - Communication
   with Destination Administratively Prohibited" message after it sends
   a SEAL data packet.  The ITE processes the message as an indication
   that it needs to (re)synchronize with the ETE (see: Section 4.6).

   An ITE may receive an SCMP "Destination Unreachable" message 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



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   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.

   .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.  Tunnel Endpoint Synchronization

   SEAL ITEs that maintain state retain a per-ETE window of SEAL_IDs of
   recently-sent packets, but by default the SEAL ETE does not retain
   inter-packet state.  When closer synchronization is required, SEAL
   TEs can exchange initial SEAL_IDs in a procedure that parallels IPv6
   neighbor discovery and the TCP 3-way handshake.  When the TEs are
   synchronized, the ETE can also maintain a per-ITE window of SEAL_IDs
   of its recently-received packets.

   When an initiating TE ("TE(A)") needs to synchronize with a new
   tunnel far end ("TE(B)"), it first chooses a randomly-initialized 48-
   bit SEAL_ID value that it would like TE(B) to use (i.e.,
   "SEAL_ID(B)").  TE(A) then creates a neighbor cache entry for TE(B)
   and records SEAL_ID(B) in the neighbor cache entry.  Next, TE(A)
   creates an SNS or SRS message that includes a Nonce option (see:
   [RFC3971], Section 5.3).  TE(A) then writes the value SEAL_ID(B) in
   the Nonce option, writes the value 0 in the SEAL_ID field of the SEAL
   header and sends the SNS/SRS message to TE(B).

   When TE(B) receives an SNS/SRS message with a Nonce option and with
   the value 0 in the SEAL_ID of the SEAL header, it considers the
   message as a potential synchronization request.  TE(B) first extracts
   the value SEAL_ID(B) from the Nonce option then chooses a randomly-
   initialized 48-bit SEAL_ID value that it would like TE(A) to use
   (i.e., "SEAL_ID(A)").  TE(B) then stores the tuple (ip_src,
   SEAL_ID(A), SEAL_ID(B)) in a minimal temporary fast path data
   structure, where "ip_src" is the outer IP source address of the SCMP
   message.  (For efficiency and security purposes, the data structure
   should be indexed, e.g., by a secret hash of the -tuple).  TE(B) then
   creates a solicited SNA or SRA message that includes a Nonce option.
   It then writes the value SEAL_ID(A) in the Nonce option, writes the
   value SEAL_ID(B) in the SEAL_ID field of the SEAL header and sends
   the SNA/SRA message back to TE(A).

   When TE(A) receives the SNA/SRA, it considers the message as a



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   potential synchronization acknowledgement.  TE(A) first verifies that
   the value encoded in the SEAL_ID of the SEAL header matches the
   SEAL_ID(B) in the neighbor cache entry.  If the values match, TE(A)
   extracts SEAL_ID(A) from the nonce option and records it in the
   neighbor cache entry; otherwise, it drops the packet.  If instead
   TE(A) does not receive a timely SNA/SRA response, it retransmits the
   initial SNS/SRS message for a total of 3 tries before giving up the
   same as for ordinary IPv6 neighbor discovery.

   After TE(A) receives the synchronization acknowledgement, it begins
   sending either unsolicited SNA/SRA messages or ordinary data packets
   back to TE(B) using SEAL_ID(A) as the initial sequence number.  When
   TE(B) receives these packets, it first checks its neighbor cache to
   see if there is a matching neighbor cache entry.  If there is a
   neighbor cache entry, and the SEAL_ID in the header of the packet is
   within the window of the SEAL_ID recorded in the neighbor cache
   entry, TE(B) accepts the packet.  If the SEAL_ID in the packet is
   newer than the SEAL_ID in the neighbor cache entry, TE(B) also
   updates the neighbor cache value.  If there is no neighbor cache
   entry, TE(B) instead checks the fast path cache to see if the packet
   is a match for an in-progress synchronization event.  If there is a
   fast path cache entry with a SEAL_ID(A) that is within the window of
   the SEAL_ID in the packet header, TE(B) accepts the packet and also
   creates a new neighbor cache entry with the tuple (ip_src,
   SEAL_ID(A), SEAL_ID(B)).  If there is no matching fast path cache
   entry, TE(B) instead simply discards the packet.

   By maintaining the fast path cache, each TE is able to mitigate
   buffer exhaustion attacks that may be launched by off-path attackers
   [RFC4987].  The TE will receive positive confirmation that the
   synchronization request came from an on-path tunnel far end after it
   receives a stream of in-window packets as the "third leg" of this
   three-way handshake as described above.  The TEs should maintain
   neighbor cache entries as long as they receive hints of forward
   progress from the tunnel far end, but should delete the neighbor
   cache entries after a nominal stale time (e.g., 30 seconds).  The TEs
   should also purge fast-path cache entries for which no window
   synchronization messages are received within a nominal stale time
   (e.g., 5 seconds).

   After synchronization is complete, when a TE receives a SEAL packet
   it checks in its neighbor cache to determine whether the SEAL_ID is
   within the current window, and discards any packets that are outside
   the window.  Since packets may be lost or reordered, and since SEAL
   presents only a best effort (i.e., and not reliable) link model, the
   TE should set a coarse-grained window size (e.g., 32768) and accept
   any packet with a SEAL_ID that is within the window.




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   Note that when the ITE sends SEAL packets with I=0, the window is
   trivial and a constant SEAL_ID nonce value instead of an incrementing
   sequence number is used.


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.


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



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   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, resulting
   in a stream of SCMP messages returned to a victim ITE.  The SEAL_ID
   in the encapsulated segment of the spoofed IP first fragment provides
   mitigation for the ITE to detect and discard spurious SCMP messages.

   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 SEAL_ID 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.  When the TEs
   are synchronized, the ETE can also detect off-path spoofing attacks.

   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
   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



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   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.

   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 an Identification field
       that is 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.





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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, 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
              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.





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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-00 (work in progress),
              March 2010.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-12 (work in progress),
              March 2010.

   [I-D.ietf-v6ops-tunnel-security-concerns]
              Hoagland, J., Krishnan, S., and D. Thaler, "Security
              Concerns With IP Tunneling",
              draft-ietf-v6ops-tunnel-security-concerns-02 (work in
              progress), August 2010.

   [I-D.russert-rangers]
              Russert, S., Fleischman, E., and F. Templin, "RANGER
              Scenarios", draft-russert-rangers-05 (work in progress),
              July 2010.

   [I-D.templin-intarea-vet]
              Templin, F., "Virtual Enterprise Traversal (VET)",
              draft-templin-intarea-vet-16 (work in progress),
              July 2010.

   [I-D.templin-iron]
              Templin, F., "The Internet Routing Overlay Network
              (IRON)", draft-templin-iron-12 (work in progress),
              September 2010.

   [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
              a subnetwork for experimentation with the OSI network



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              layer", RFC 1070, February 1989.

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

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

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

   [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.

   [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.



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   [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.

   [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.


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



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   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
   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 a Reassembly Repot, and SEAL segmentation
   and reassembly uses a much longer ID field.  Therefore, reassembly
   mis-associations of IP fragments nor of SEAL segments should be



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   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 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.

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

   For example, two TCP endpoints connected to the same subnetwork
   region can negotiate the use of transport-mode SEAL for a connection
   by inserting a 'SEAL_OPTION' TCP option during the connection
   establishment phase.  If both TCPs agree on the use of SEAL, their
   protocol messages will be carried as TCP/SEAL/IPv4 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
   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)




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   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

   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.







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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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