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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                           March 5, 2010
Expires: September 6, 2010


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-12.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 to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   This Internet-Draft will expire on September 6, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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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   the Trust Legal Provisions and are provided without warranty as
   described in the 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 . . . . . . . . . . . . . . . . . . . . 12
     4.3.  ITE Specification  . . . . . . . . . . . . . . . . . . . . 13
       4.3.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . . 13
       4.3.2.  Tunnel Interface Soft State  . . . . . . . . . . . . . 15
       4.3.3.  Admitting Packets into the Tunnel  . . . . . . . . . . 15
       4.3.4.  Mid-Layer Encapsulation  . . . . . . . . . . . . . . . 16
       4.3.5.  SEAL Segmentation  . . . . . . . . . . . . . . . . . . 17
       4.3.6.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 17
       4.3.7.  Probing Strategy . . . . . . . . . . . . . . . . . . . 18
       4.3.8.  Packet Identification  . . . . . . . . . . . . . . . . 18
       4.3.9.  Sending SEAL Protocol Packets  . . . . . . . . . . . . 18
       4.3.10. Processing Raw ICMP Messages . . . . . . . . . . . . . 19
     4.4.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 19
       4.4.1.  Reassembly Buffer Requirements . . . . . . . . . . . . 19
       4.4.2.  IP-Layer Reassembly  . . . . . . . . . . . . . . . . . 20
       4.4.3.  SEAL-Layer Reassembly  . . . . . . . . . . . . . . . . 20
       4.4.4.  Decapsulation and Delivery to Upper Layers . . . . . . 21
     4.5.  The SEAL Control Message Protocol (SCMP) . . . . . . . . . 22
       4.5.1.  Generating SCMP Messages . . . . . . . . . . . . . . . 23
       4.5.2.  Processing SCMP Messages . . . . . . . . . . . . . . . 25
     4.6.  TE Window Synchronization and Maintenance  . . . . . . . . 26
   5.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 28
   6.  End System Requirements  . . . . . . . . . . . . . . . . . . . 29
   7.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 29
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 30
   11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 30
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 31
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 32
     13.2. Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Reliability . . . . . . . . . . . . . . . . . . . . . 35
   Appendix B.  Integrity . . . . . . . . . . . . . . . . . . . . . . 35
   Appendix C.  Transport Mode  . . . . . . . . . . . . . . . . . . . 36
   Appendix D.  Historic Evolution of PMTUD . . . . . . . . . . . . . 37
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38





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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.  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 issues ranging from minor annoyances (e.g.,



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   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 made simple
   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.

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



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   ICMP error message into a suitable ICMP error message to return to
   the original source.

   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
   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
   unidirectional 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



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   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 IPv4 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
   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.






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   outer IP packet
      a packet resulting from adding an outer IP header 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.).

   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

      OHLEN - the length of the outer encapsulating headers and
      trailers, including the outer IP header, the SEAL header and any
      other outer headers and trailers.

      PTB - a Packet Too Big message recognized by the inner network
      layer, e.g., an ICMPv6 "Packet Too Big" message [RFC4443], an
      ICMPv4 "Fragmentation Needed" message [RFC0792], etc.

      S_MRU - the SEAL Maximum Reassembly Unit

      S_MSS - the SEAL Maximum Segment Size

      SCMP - the SEAL Control Message Protocol

      SEAL_ID - an Identification value, randomly initialized and
      monotonically incremented for each SEAL protocol packet

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

      SEAL_PROTO - an IPv4 protocol number used for SEAL





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

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


3.  Applicability Statement

   SEAL was motivated by the specific case of subnetwork abstraction for
   Mobile Ad hoc Networks (MANETs); however, the domain of applicability
   also extends to subnetwork abstractions of enterprise networks, ISP
   networks, SOHO networks, the interdomain routing core, and many
   others.  In particular, SEAL is a natural complement to the
   enterprise network abstraction manifested through the VET mechanism
   [I-D.templin-intarea-vet] and the RANGER architecture
   [I-D.templin-ranger][I-D.russert-rangers].

   SEAL can be used as a network sublayer for encapsulation of an inner
   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 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
   acts 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.

   When the ETE supports reassembly, the tunnel can be used to transport
   packets that are too large to traverse the path without



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   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, 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 SEAL 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 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 mid-layer headers
   and trailers, segment the resulting mid-layer packet into multiple
   segments if necessary, then append a SEAL header and (if necessary) a
   UDP header to each segment.  The ITE then adds the outer
   encapsulation headers to each segment.  For example, a single-segment
   inner IPv6 packet encapsulated in any mid-layer headers and trailers,
   the SEAL header, any outer headers and trailers and an outer IPv4
   header would appear as shown in Figure 1:






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                                       +--------------------+
                                       ~  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

   In a second example, an inner IPv4 packet requiring three SEAL
   segments would appear as three separate outer IPv4 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:
   +------------------+                          +------------------+
   ~  outer IPv4 hdr  ~                          ~  outer IPv4 hdr  ~
   +------------------+   +------------------+   +------------------+
   ~ other outer hdrs ~   ~  outer IPv4 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



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   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 IPsec encapsulations [RFC4301], the SEAL header is inserted
      between the {AH,ESP} headers and outer IP headers as: IPvX/SEAL/
      {AH,ESP}/{inner packet}.  Here, the {AH, ESP} headers and trailers
      are seen as mid-layer encapsulations.

   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 to uniquely identify each
   packet.  Routers within the subnetwork use the SEAL_ID for duplicate
   packet detection, and TEs use the SEAL_ID for SEAL segmentation/
   reassembly and protection against off-path attacks.  The following
   sections specify the SEAL header format and SEAL-related operations
   of the ITE and ETE, respectively.

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|A|S|P|F|M|R|  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.







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   A (1)
      the "Acknowledge" bit.  Set to 1 by the ETE to acknowledge a
      Synchronization event.

   S (1)
      the "Synchronize" bit.  Set to 1 by the ITE to request
      Synchronization.

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

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

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

   R (1)
      a Reserved bit.  Set to 0 for the purpose of this specification.

   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 field.

   Setting of the various bits and fields of the SEAL header is
   specified in the following sections.  Unless explicitly specified,
   each unspecified bit and field is assumed to be set to zero.

4.3.  ITE Specification

4.3.1.  Tunnel Interface MTU

   The ITE configures a tunnel virtual interface over one or more
   underlying links that connect the border node to the subnetwork.  The
   tunnel interface must present a fixed MTU to Layer 3 as the size for
   admission of inner packets into the tunnel.  Since the tunnel
   interface 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.



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   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 set larger MTU values still, but should select a value
   that is not so large as to cause excessive PTBs coming from within
   the tunnel interface.  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, this option
   must be carefully coordinated with protocol stack upper layers, since
   some upper layer protocols (e.g., TCP) derive their packet sizing
   parameters from the MTU of the outgoing interface and as such may
   select too large an initial size.  This is not a problem for upper
   layers that use conservative initial maximum segment size estimates
   and/or when the tunnel interface can reduce the upper layer's maximum
   segment size (e.g., the size advertised in the TCP MSS option) based
   on the per-neighbor MTU.

   The inner network layer protocol consults the tunnel interface MTU
   when admitting a packet into the interface.  For 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
   tunnel as an independent packet.

   For all other inner packets, the ITE 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.).



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   Note that when the tunnel interface sets an indefinite MTU the ITE
   unconditionally admits all packets into the interface without
   fragmentation.  In light of the above considerations, it is
   RECOMMENDED that the ITE configure an indefinite MTU on the tunnel
   virtual interface and handle any per-neighbor MTU mismatches within
   the tunnel virtual interface (e.g., by reducing the size advertised
   in the TCP MSS option).

4.3.2.  Tunnel Interface Soft State

   For each ETE, the ITE maintains 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:

   o  a Mid-layer Header Length (MHLEN); set to the length of any mid-
      layer encapsulation headers and trailers (e.g., AH, ESP, NULL,
      etc.) 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).  The ITE 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.

4.3.3.  Admitting Packets into the Tunnel

   After the ITE admits an inner packet/fragment into the tunnel
   interface, it uses the following algorithm to determine whether the
   packet can be accommodated and (if so) whether (further) inner IP



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   fragmentation is needed:

   o  if the inner packet is unfragmentable (e.g., an IPv6 packet, an
      IPv4 packet with DF=1, etc.), and the packet is larger than
      (MAX(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends a
      PTB message to the original source with an MTU value of
      (MAX(S_MRU, S_MSS) - HLEN); else,

   o  if the inner packet is fragmentable (e.g., an IPv4 packet with
      DF=0), and the packet is larger than 1280 bytes, the ITE uses
      inner fragmentation to break the packet into fragments no larger
      than 1280 bytes; else,

   o  the ITE processes the packet without inner fragmentation.

   In the above, the ITE must track whether the tunnel interface is
   using header compression.  If so, the ITE must include the length of
   the uncompressed headers and trailers when calculating HLEN.  Note
   also in the above 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).

   When the ITE uses inner fragmentation, it can optionally use a "safe"
   fragment size of 1280 bytes for initial packets while probing in
   parallel for a larger fragment size that would still avoid outer IP
   fragmentation within the tunnel.  If the ITE can determine a larger
   fragment size, it may use this larger size for inner fragmentation.

   If the inner packet is unfragmentable, and the packet will be sent
   in-the-clear with no mid-layer encryption, the ITE can instead employ
   a stateless strategy by simply encapsulating and sending the packet
   without regard to its length.  The ITE can then translate any SCMP
   MTU Report messages it receives from the ETE into PTB messages to
   return to the original source (where the translation is based on the
   packet-in-error within the SCMP MTU Report message).  In this method,
   the ITE need not maintain per-ETE S_MRU and S_MSS state.

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.  (For example, when IPsec ESP is used
   [RFC4301], the ITE performs the necessary security transformations on
   the inner packet/fragment then adds an ESP header and trailer.)  The
   ITE then presents the mid-layer packet for SEAL segmentation and
   outer encapsulation.



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4.3.5.  SEAL Segmentation

   After mid-layer encapsulation, if the length of the resulting mid-
   layer packet plus OHLEN is greater than S_MSS the ITE must
   additionally perform SEAL segmentation.  To do so, it breaks the 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.)

   The ITE next encapsulates each segment in the requisite outer headers
   and trailers according to the specific encapsulation format (e.g.,
   [RFC2003], [RFC2473], [RFC4213], [RFC4380], 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 sets the P bit to 1 if necessary as
   specified in Section 4.3.7, sets the packet identification values as
   specified in Section 4.3.8 and sends the packets as specified in
   Section 4.3.9.




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4.3.7.  Probing Strategy

   All SEAL encapsulated packets sent by the ITE are considered implicit
   probes, and will elicit SCMP MTU Report messages from the ETE (see:
   Section 4.5) with a new value for S_MSS if any IP fragmentation
   occurs in the path.  Thereafter, the ITE can periodically reset S_MSS
   to a larger value (e.g., the underlying IP interface MTU) to detect
   path MTU increases.

   The ITE also sends explicit probes, periodically, to verify that the
   ETE is still reachable.  The ITE sets P=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 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 an SCMP Neighbor Advertisement
   message from the ETE as an acknowledgement (see Section 4.5).
   Finally, the ITE MAY send "expendable" outer IP probe packets (see
   Section 4.3.9) as explicit probes in order to generate PTB messages
   from routers on the path to the ETE.

   In all cases, the ITE MUST be conservative in its use of the P bit in
   order to limit the resultant control message overhead.

4.3.8.  Packet Identification

   The ITE maintains a randomly-initialized SEAL_ID value as per-ETE
   soft state (e.g., in the neighbor cache) and monotonically increments
   it for each successive SEAL protocol packet it sends to the ETE.  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.

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.




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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 may optionally support no
   reassembly at all, but this may cause MTU underruns in some
   environments.  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:

   o  SHOULD configure an outer IP-layer reassembly buffer size of at
      least (1280 + HELN) bytes.

   o  MUST configure a SEAL-layer reassembly buffer size (i.e., S_MRU)
      that is no smaller than the IP-layer reassembly buffer size.

   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



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   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 MTU Report message back to the ITE with the MTU field set to
   S_MRU (see Section 4.5).  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.

4.4.3.  SEAL-Layer Reassembly

   Following IP reassembly (if necessary), if the SEAL 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).  The
   ETE next submits single-segment mid-layer packets for decapsulation
   and delivery to upper layers as specified in Section 4.4.4.  The ETE
   instead performs SEAL-layer reassembly for multi-segment mid-layer
   packets as follows.

   The ETE adds each segment of a multi-segment mid-layer packet 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



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   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).  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 discontinues the reassembly and
   sends an SCMP MTU Report message with the MTU field set to S_MRU (see
   Section 4.5).

   After all segments are gathered, the ETE reassembles the packet by
   concatenating the segments encapsulated in the N consecutive SEAL
   packets beginning with the initial segment (i.e., SEG=0) and followed
   by any non-initial segments 1 through N-1.  That is, for an N-segment
   mid-layer packet, reassembly entails the concatenation of the SEAL-
   encapsulated packet segments with (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.

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 (e.g., IPsec ESP) 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).




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4.5.  The SEAL Control Message Protocol (SCMP)

   SEAL uses a companion SEAL Control Message Protocol (SCMP) that
   implements the same message format as the Internet Control Message
   Protocol for IPv6 (ICMPv6) [RFC4443].  SCMP messages are further
   identified by the NEXTHDR value '58' the same as for ICMPv6 messages,
   however the SCMP message is *not* immediately preceded by an inner
   IPv6 header.  Instead, SCMP messages appear immediately following
   either the SEAL header or mid-layer header (i.e., if the packet
   included mid-layer encapsulations).  Therefore, this differing header
   arrangement is the sole means by which TEs differentiate SCMP
   messages from ordinary ICMPv6 messages.  Unlike ICMPv6 messages, SCMP
   messages are used only for the purpose of conveying information
   between TEs, i.e., they are used only for information sharing within
   the tunnel and not beyond the tunnel.

   SCMP messages use the same message types specified for ordinary
   ICMPv6 messages in [RFC4443][RFC4861].  SCMP can also be used to
   carry other ICMPv6 message types (e.g., [RFC4191], etc.) in manners
   that are outside the scope of this document.  SCMP messages are
   formatted as shown in Figure 4:

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

   (*) the invoking SEAL packet segment (i.e., the "packet-in-error")
   is only included for SCMP messages sent in response to SEAL data

                       Figure 4: SCMP Message Format

   As for ICMPv4 and 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.  The SCMP message header is followed by the
   message body which is followed by the leading portion of the invoking
   packet-in-error (when present) beginning with the packet's outer IP
   header.  The Checksum is calculated the same as specified for ICMPv4
   messages in [RFC0792], i.e., the checksum does not include a pseudo-



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   header of the outer IP header since the SEAL_ID gives sufficient
   assurance against mis-delivery.

4.5.1.  Generating SCMP Messages

   The TE prepares the SCMP message exactly as specified for the
   corresponding ICMPv6 message.  If the SCMP message will include a
   packet-in-error, the TE includes the leading portion of the invoking
   SEAL data packet beginning with the outer IP header, followed by the
   SEAL header, etc., and extending to a length that would not cause the
   entire SCMP message to exceed 576 bytes.  The TE then encapsulates
   the SCMP message in any mid-layer headers and trailers.  For example,
   if the TE uses IPsec ESP it encapsulates the SCMP message directly
   within the mid-layer ESP headers and trailers, i.e., it does not
   encapsulate the SCMP message within an inner header.  The TE next
   encapsulates the mid-layer packet in the SEAL header, any other outer
   headers and finally in the outer IP header.  The SCMP message format
   is shown in Figure 5.

                                       +--------------------+
                                       ~  outer IPv4 header ~
                                       +--------------------+
                                       ~  other outer hdrs  ~
                                       +--------------------+
                                       ~    SEAL Header     ~
   S      +--------------------+       +--------------------+
   C      ~  mid-layer headers ~       ~  mid-layer headers ~
   M      +--------------------+       +--------------------+
   P -->  ~ SCMP message header~  -->  ~ SCMP message header~
     -->  +--------------------+  -->  +--------------------+
   M -->  ~  SCMP message body ~  -->  ~  SCMP message body ~
   e -->  +--------------------+  -->  +--------------------+
   s -->  ~   packet-in-error  ~  -->  ~  packet-in-error   ~
   s      +--------------------+       +--------------------+
   a      ~ mid-layer trailers ~       ~ mid-layer trailers ~
   g      +--------------------+       +--------------------+
   e                                   ~   outer trailers   ~
               SCMP Message            +--------------------+
          after mid-layer encaps.
                                          SCMP Message after
                                        SEAL and outer encaps.

                   Figure 5: SCMP Message Encapsulation

   During outer encapsulation, the TE sets the outer IP destination and
   source addresses of the SCMP packet to the source and destination
   addresses (respectively) of the packet-in-error.  If the destination
   address in the packet-in-error was multicast, the TE instead sets the



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   outer IP source address of the SCMP packet to an address assigned to
   the underlying IP interface.  The TE finally sets the NEXTHDR field
   in either the SEAL header or the mid-layer header (if present) to the
   value '58', i.e., the official IANA protocol number for the ICMPv6
   protocol.

4.5.1.1.  Generating SCMP MTU Report Messages

   An ETE generates an SCMP MTU Report message in the following cases:

   o  Case 1: the ETE receives a SEAL data packet that would cause the
      reassembled outer packet to exceed S_MRU following reassembly.

   o  Case 2: the ETE receives a SEAL data packet with P=1 in the SEAL
      header.

   o  Case 3: the ETE receives the IP first fragment (i.e., one with
      MF=1 and Offset=0 in the IP header) of a fragmented SEAL data
      packet.

   The ETE prepares an SCMP MTU Report message the same specified for an
   ICMPv6 Packet Too Big message (see: [RFC4443], Section 3.2), and
   includes as much of the invoking SEAL data packet as possible in the
   packet-in-error field without the resulting SCMP packet exceeding 576
   bytes.  For Case 1 above, the ETE then writes the S_MRU value for
   this ITE in the MTU field and the value 0 in the Code field of the
   message.  For Cases 2 and 3 above, the ETE instead writes the value 0
   in the MTU field and the value 1 in the Code field of the message.
   The ETE then encapsulates the SCMP MTU Report message in any mid-
   layer and outer headers and trailers as shown in Figure 5 then sends
   the resulting SCMP message back to the ITE.

   After it sends the SCMP MTU Report message, the ETE next accepts or
   discards the SEAL data packet according to the specific case.  For
   Case 1, the ETE discards the SEAL data packet and schedules any
   reassembly resources for deletion.  For Cases 2 and 3, the ETE
   accepts the SEAL data packet even though it also returned an SCMP MTU
   Report message to the ITE.

4.5.1.2.  Generating SCMP Destination Unreachable Messages

   An ETE generates an SCMP "Destination Unreachable - Communication
   with Destination Administratively Prohibited" message when it
   receives a SEAL packet with a SEAL_ID that is outside of the current
   window for this ITE (see: Section 4.6).  The message is formatted the
   same as for ICMPv6 Destination Unreachable messages.

   Generation of SCMP Destination Unreachable messages with other codes



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   is outside the scope of this document.

4.5.2.  Processing SCMP Messages

   Each TE processes any SCMP messages it receives as long as it can
   verify that the message was sent from a legitimate tunnel far end.
   The TE can verify that the SCMP message came from a legitimate tunnel
   far end by checking that the SEAL_ID in the encapsulated packet-in-
   error corresponds to one of its recently-sent SEAL data packets.
   When the tunnel endpoints are synchronized, TE can also (or instead)
   check that the SEAL_ID in the SEAL header of the SCMP message is
   within the window of recently received packets from this tunnel far
   end (see Section 4.6).

   Each ITE maintains a window of outstanding SEAL_IDs of packets that
   it has recently sent to each ETE.  For each SCMP message it receives,
   the ITE first verifies that the SEAL_ID encoded in the packet-in-
   error is within 32768 of the SEAL_ID of the most recent packet that
   it has sent to the ETE.  The ITE 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.

4.5.2.1.  Processing SCMP MTU Report Messages

   An ITE may receive an SCMP MTU Report message after it sends a SEAL
   data packet (see: Section 4.5.1.1).  When the ITE receives an SCMP
   MTU Report message, it processes the message as follows:

   For SCMP MTU Report messages with Code=0, the ITE records the value
   in the MTU field as the new S_MRU value for this ETE.  The ITE then
   examines the packet-in-error to determine whether it can be
   translated into a PTB message to send back to the original source.
   If so, the ITE can optionally send a translated PTB message to the
   original source with MTU set to (S_MRU - HLEN).

   For SCMP MTU Report messages with Code=1, the ITE examines the IP
   header of the packet-in-error.  If the packet-in-error is not an IP
   fragment, and if the packet-in-error length is greater than the
   current S_MSS value, the ITE records the length as the new S_MSS
   value in its soft state for this ETE.  If the packet in-error is a
   first fragment, however, the ITE determines a new S_MSS value
   according to the packet-in-error length as follows:

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




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   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 must search for a reduced S_MSS value
   through an iterative searching strategy that parallels (Section 5 of
   [RFC1191]).  This searching strategy may require multiple iterations
   in which the ITE sends SEAL data packets using a reduced S_MSS and
   receives additional SCMP MTU Report messages.  During this process,
   it is essential that the ITE reduce S_MSS based on the first SCMP MTU
   Report message received under the current S_MSS size, and refrain
   from further reducing S_MSS until SCMP MTU Report messages pertaining
   to packets sent under the new S_MSS are received.

   Finally, the ITE examines the SEAL header of the packet-in-error to
   determine whether the message constitutes a reply to an explicit
   probe (see: Section 4.3.7) in order to facilitate neighbor
   unreachability detection "hints of forward progress".  The ITE then
   discards the SCMP message.

4.5.2.2.  Processing SCMP Destination Unreachable 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 this message as an indication
   that it needs to (re)synchronize with the ETE (see: Section 4.6).

   Processing of SCMP Destination Unreachable messages with other codes
   is outside the scope of this document.

4.6.  TE Window Synchronization and Maintenance

   SEAL Tunnel Endpoints (TEs) can optionally synchronize sequence
   numbers in an initial exchange that utilizes the IPv6 neighbor
   unreachability detection procedure and parallels the TCP 3-way
   handshake.  Each ITE can then use the SEAL_ID in the packets it sends
   not only to support the segmentation and reassembly procedures, but
   also as a sequence number of packets that it has recently sent to the
   ETE.  Similarly, each ETE can use the SEAL_ID in the packets it
   receives as a sequence number of packet that it has recently received
   from the ITE.  This arrangement requires an initial synchronization
   of sequence numbers between tunnel endpoints as specified below.

   SEAL ITEs should be operationally configured to operate in either
   synchronized or unsynchronized fashion.  When an ITE attempts to
   operate in unsynchronized fashion but the ETE requires synchronized



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   operation, the ETE will return an SCMP "Destination Unreachable -
   Communication with Destination Administratively Prohibited" message
   (see Section 4.5).  The ITE then verifies that the packet-in-error
   corresponds to a packet that it sent recently, and attempts to
   synchronize with the ETE so that future communications are not
   blocked.

   When an ITE needs to synchronize with a new ETE (i.e., one for which
   it has no neighbor cache entry), it first chooses a random 32-bit
   value.  The ITE then creates an initial 48-bit sequence number (i.e.,
   an initial "SEAL_ID(ITE)") with the random 32-bit value as the most
   significant 32-bits and the value 0 as the least significant 16 bits.
   The ITE then creates a neighbor cache entry for this ETE and records
   SEAL_ID(ITE) in the neighbor cache entry.  Next, the ITE creates an
   SCMP "Neighbor Solicitation (NS)" message and writes the value
   SEAL_ID(ITE) in the SCMP message SEAL header.  The ITE then sets the
   (A, S) bits in the SEAL header to (0, 1), then sends the NS message
   to the ETE.

   When the ETE receives the NS message, it notices that the (A, S) bits
   in the SEAL header are set to (0, 1), and considers the message as a
   potential window synchronization request.  The ETE then chooses a
   random 48-bit value to use as its initial sequence number (i.e., an
   initial "SEAL_ID(ETE)") which it stores in a minimal temporary fast
   path data structure that caches only the IP source address of the
   SCMP message, SEAL_ID(ITE) and SEAL_ID(ETE).  (For efficiency and
   security purposes, the data structure should be indexed, e.g., by a
   secret hash of the IP source address and SEAL_ID(ITE)).  The ETE then
   creates an SCMP "Neighbor Advertisement (NA)" message that includes a
   Nonce option (see: [RFC3971], Section 5.3) that encodes the value
   SEAL_ID(ITE).  The ETE then writes the value SEAL_ID(ETE) into the
   SEAL_ID field of the SCMP message SEAL header, sets the (A, S) bits
   in the SEAL header to (1, 1), and sends the NA message back to the
   ITE.

   When the ITE receives the NA, it notices that the (A, S) bits in the
   SEAL header are set to (1, 1) and considers the message as a
   potential window synchronization acknowledgement.  The ITE then
   verifies that the value encoded in the Nonce option matches the
   SEAL_ID(ITE) in the neighbor cache entry.  If so, the ITE records the
   value SEAL_ID(ETE) in the neighbor cache entry.  (If instead the ITE
   does not receive a timely NA response, it retransmits the initial NS
   message for a total of 3 tries before giving up the same as for
   ordinary IPv6 neighbor unreachability detection.)  After the ITE
   receives a matching NA message, it then uses SEAL_ID(ITE) as the SEAL
   _ID of subsequent SEAL packets that it sends to this ETE and uses
   SEAL_ID(ETE) as the SEAL_ID to match against subsequent SEAL packets
   that it receives from this ETE.



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   After the ITE receives the NA message, it begins sending either
   unsolicited NA messages or ordinary data packets back to the ETE
   using SEAL_ID(ITE) as the initial sequence number and with the S bit
   set to 0.  When the ETE 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 SEAL header of
   he packet is within +/- 32768 of the SEAL_ID recorded in the neighbor
   cache entry, the ETE accepts the packet and records this new SEAL_ID
   in the neighbor cache entry.  If there is no neighbor cache entry,
   the ETE instead checks the fast path cache to see if the packet is a
   match for an in-progress window synchronization event.  If the packet
   matches (i.e., if there is a fast path cache entry with a SEAL_ID
   (ITE) that matches the high-order 32 bits of the SEAL_ID in the
   packet header), the ETE accepts the packet and also creates a new
   neighbor cache entry.  If there is no matching fast path cache entry,
   the ETE instead discards the packet.

   By maintaining the fast path cache, the ETE is able to mitigate
   buffer exhaustion attacks that may be launched by off-path attackers
   [RFC4987].  The ETE will receive positive confirmation that the
   synchronization request came from an on-path ITE after it receives
   the "third leg" of this three-way handshake as described above.  The
   ITE and ETE should maintain neighbor cache entries as long as traffic
   is flowing through the tunnel, but should delete the neighbor cache
   entries after a nominal idle time (e.g., 30 seconds).  The ETE should
   also purge fast-path cache entries for which no window
   synchronization messages are received within a nominal idle 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 accept any packet with a SEAL_ID that is within +/- 32768
   of the most recently received SEAL_ID.  For this reason, the ITE must
   record the record the SEAL_ID of the most recently-received SEAL
   packet so that the window of SEAL_IDs advances with the flow of
   packets.


5.  Link Requirements

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






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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.


8.  IANA Considerations

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

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

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


9.  Security Considerations

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

   An amplification/reflection attack is possible when an attacker sends
   IP first fragments with spoofed source addresses to an ETE, 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



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   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.

   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
   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]:



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   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.


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



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   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.

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-lisp]
              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-06 (work in progress), January 2010.

   [I-D.ietf-tcpm-icmp-attacks]
              Gont, F., "ICMP attacks against TCP",
              draft-ietf-tcpm-icmp-attacks-11 (work in progress),
              February 2010.




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   [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-01 (work in
              progress), October 2008.

   [I-D.russert-rangers]
              Russert, S., Fleischman, E., and F. Templin, "RANGER
              Scenarios", draft-russert-rangers-01 (work in progress),
              September 2009.

   [I-D.templin-intarea-vet]
              Templin, F., "Virtual Enterprise Traversal (VET)",
              draft-templin-intarea-vet-09 (work in progress),
              February 2010.

   [I-D.templin-ranger]
              Templin, F., "Routing and Addressing in Next-Generation
              EnteRprises (RANGER)", draft-templin-ranger-09 (work in
              progress), October 2009.

   [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
              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.



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              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, February 2000.

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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              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
   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



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   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
   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.



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   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)

   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



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   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.


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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