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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                         December 19, 2011
Expires: June 21, 2012


        The Subnetwork Encapsulation and Adaptation Layer (SEAL)
                   draft-templin-intarea-seal-42.txt

Abstract

   For the purpose of this document, a subnetwork is defined as a
   virtual topology configured over a connected IP network routing
   region and bounded by encapsulating border nodes.  These virtual
   topologies are manifested by tunnels that may span multiple IP and/or
   sub-IP layer forwarding hops, and can introduce failure modes due to
   packet duplication, packet reordering, source address spoofing and
   traversal of links with diverse Maximum Transmission Units (MTUs).
   This document specifies a Subnetwork Encapsulation and Adaptation
   Layer (SEAL) that addresses these issues.

Status of this Memo

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

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

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

   This Internet-Draft will expire on June 21, 2012.

Copyright Notice

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

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



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















































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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 Specification . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  VET Interface Model  . . . . . . . . . . . . . . . . . . . 10
     4.2.  SEAL Model of Operation  . . . . . . . . . . . . . . . . . 10
     4.3.  SEAL Header and Trailer Format . . . . . . . . . . . . . . 12
     4.4.  ITE Specification  . . . . . . . . . . . . . . . . . . . . 14
       4.4.1.  Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
       4.4.2.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 15
       4.4.3.  Pre-Encapsulation  . . . . . . . . . . . . . . . . . . 16
       4.4.4.  SEAL Encapsulation . . . . . . . . . . . . . . . . . . 17
       4.4.5.  Outer Encapsulation  . . . . . . . . . . . . . . . . . 18
       4.4.6.  Path Probing and ETE Reachability Verification . . . . 19
       4.4.7.  Processing ICMP Messages . . . . . . . . . . . . . . . 19
       4.4.8.  IPv4 Middlebox Reassembly Testing  . . . . . . . . . . 21
       4.4.9.  Stateful MTU Determination . . . . . . . . . . . . . . 22
       4.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 23
     4.5.  ETE Specification  . . . . . . . . . . . . . . . . . . . . 23
       4.5.1.  Tunnel Neighbor Soft State . . . . . . . . . . . . . . 23
       4.5.2.  IP-Layer Reassembly  . . . . . . . . . . . . . . . . . 23
       4.5.3.  Decapsulation and Re-Encapsulation . . . . . . . . . . 24
     4.6.  The SEAL Control Message Protocol (SCMP) . . . . . . . . . 25
       4.6.1.  Generating SCMP Error Messages . . . . . . . . . . . . 26
       4.6.2.  Processing SCMP Error Messages . . . . . . . . . . . . 28
   5.  Link Requirements  . . . . . . . . . . . . . . . . . . . . . . 29
   6.  End System Requirements  . . . . . . . . . . . . . . . . . . . 30
   7.  Router Requirements  . . . . . . . . . . . . . . . . . . . . . 30
   8.  Nested Encapsulation Considerations  . . . . . . . . . . . . . 30
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 31
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 32
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 33
     13.2. Informative References . . . . . . . . . . . . . . . . . . 33
   Appendix A.  Reliability . . . . . . . . . . . . . . . . . . . . . 37
   Appendix B.  Integrity . . . . . . . . . . . . . . . . . . . . . . 37
   Appendix C.  Transport Mode  . . . . . . . . . . . . . . . . . . . 38
   Appendix D.  Historic Evolution of PMTUD . . . . . . . . . . . . . 38
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39






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1.  Introduction

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

   The use of IP encapsulation (also known as "tunneling") has long been
   considered as the means for creating such virtual topologies.
   However, the encapsulation headers often include insufficiently
   provisioned per-packet identification values.  This can present
   issues for duplicate packet detection and detection of packet
   reordering within the subnetwork.  IP encapsulation also allows an
   attacker to produce encapsulated packets with spoofed source
   addresses even if the source address in the encapsulating header
   cannot be spoofed.  A denial-of-service vector that is not possible
   in non-tunneled subnetworks is therefore presented.

   Additionally, the insertion of an outer IP header reduces the
   effective path MTU visible to the inner network layer.  When IPv6 is
   used as the encapsulation protocol, original sources will be informed
   of the MTU limitation through IPv6 path MTU discovery [RFC1981].
   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 IPv4 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,
   IP is ubiquitously deployed as the Layer 3 protocol.  The primary
   functions of IP are to provide for routing, addressing, and a
   fragmentation and reassembly capability used to accommodate links
   with diverse MTUs.  While it is well known that the IP address space
   is rapidly becoming depleted, there is a lesser-known but growing
   consensus that other IP protocol limitations have already or may soon



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   become problematic.

   First, the Internet historically provided no means for discerning
   whether the source addresses of IP packets are authentic.  This
   shortcoming is being addressed more and more through the deployment
   of site border router ingress filters [RFC2827], however the use of
   encapsulation provides a vector for an attacker to circumvent
   filtering for the encapsulated packet even if filtering is correctly
   applied to the encapsulation header.  Secondly, the IP header does
   not include a well-behaved identification value unless the source has
   included a fragment header for IPv6 or unless the source permits
   fragmentation for IPv4.  These limitations preclude an efficient
   means for routers to detect duplicate packets and packets that have
   been re-ordered within the subnetwork.

   For IPv4 encapsulation, when fragmentation is permitted the header
   includes a 16-bit Identification field, meaning that at most 2^16
   unique packets with the same (source, destination, protocol)-tuple
   can be active in the Internet at the same time
   [I-D.ietf-intarea-ipv4-id-update].  (When middleboxes such as Network
   Address Translators (NATs) re-write the Identification field to
   random values, the number of unique packets is even further reduced.)
   Due to the escalating deployment of high-speed links, however, these
   numbers have become too small by several orders of magnitude for high
   data rate packet sources such as tunnel endpoints [RFC4963].

   Furthermore, there are many well-known limitations pertaining to IPv4
   fragmentation and reassembly - even to the point that it has been
   deemed "harmful" in both classic and modern-day studies (see above).
   In particular, IPv4 fragmentation raises issues ranging from minor
   annoyances (e.g., in-the-network router fragmentation [RFC1981]) to
   the potential for major integrity issues (e.g., mis-association of
   the fragments of multiple IP packets during reassembly [RFC4963]).

   As a result of these perceived limitations, a fragmentation-avoiding
   technique for discovering the MTU of the forward path from a source
   to a destination node was devised through the deliberations of the
   Path MTU Discovery Working Group (PMTUDWG) during the late 1980's
   through early 1990's (see Appendix D).  In this method, the source
   node provides explicit instructions to routers in the path to discard
   the packet and return an ICMP error message if an MTU restriction is
   encountered.  However, this approach has several serious shortcomings
   that lead to an overall "brittleness" [RFC2923].

   In particular, site border routers in the Internet have been known to
   discard ICMP error messages coming from the outside world.  This is
   due in large part to the fact that malicious spoofing of error
   messages in the Internet is trivial since there is no way to



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   authenticate the source of the messages [RFC5927].  Furthermore, when
   a source node that requires ICMP error message feedback when a packet
   is dropped due to an MTU restriction does not receive the messages, a
   path MTU-related black hole occurs.  This means that the source will
   continue to send packets that are too large and never receive an
   indication from the network that they are being discarded.  This
   behavior has been confirmed through documented studies showing clear
   evidence of path MTU discovery failures in the Internet today
   [TBIT][WAND][SIGCOMM].

   The issues with both IPv4 fragmentation and this "classical" method
   of IPv4 path MTU discovery are exacerbated further when IP tunneling
   is used [RFC4459].  For example, an ingress tunnel endpoint (ITE) may
   be required to forward encapsulated packets into the subnetwork on
   behalf of hundreds, thousands, or even more original sources within
   the end site that it serves.  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 must rely
   on ICMP error messages coming from the subnetwork that may be
   suspect, subject to loss due to filtering middleboxes, or
   insufficiently provisioned for translation into error messages to be
   returned to the original sources.

   Although recent works have led to the development of a robust end-to-
   end MTU determination scheme [RFC4821], they do not excuse tunnels
   from delivering path MTU discovery feedback when packets are lost due
   to size restrictions.  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.  Therefore, a new
   approach to accommodate tunnels over links with diverse MTUs is
   necessary.

1.2.  Approach

   For the purpose of this document, a subnetwork is defined as a
   virtual topology configured over a connected network routing region
   and bounded by encapsulating border nodes.  Example connected network
   routing regions include Mobile Ad hoc Networks (MANETs), enterprise
   networks and the global public Internet itself.  Subnetwork border
   nodes forward unicast and multicast packets over the virtual topology
   across multiple IP and/or sub-IP layer forwarding hops that may
   introduce packet duplication and/or traverse links with diverse
   Maximum Transmission Units (MTUs).

   This document introduces a Subnetwork Encapsulation and Adaptation
   Layer (SEAL) for tunneling inner network layer protocol packets over



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   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 provides a mid-layer encapsulation that accommodates links with
   diverse MTUs, and allows routers in the subnetwork to perform
   efficient duplicate packet and packet reordering detection.  The
   encapsulation further ensures data origin authentication, packet
   header integrity and anti-replay in environments in which these
   functions are necessary.

   SEAL treats tunnels that traverse the subnetwork as ordinary links
   that must support network layer services.  Moreover, SEAL provides
   dynamic mechanisms to ensure a maximal path MTU over the tunnel.
   This is in contrast to static approaches which avoid MTU issues by
   selecting a lowest common denominator MTU value that may be overly
   conservative for the vast majority of tunnel paths and difficult to
   change even when 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.

   IP
      used to generically refer to either Internet Protocol (IP)
      version, i.e., IPv4 or IPv6.

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

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







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   ETE Link Path
      a subnetwork path from an ITE to an ETE beginning with an
      underlying link of the ITE as the first hop.  Note that, if the
      ITE's interface connection to the underlying link assigns multiple
      IP addresses, each address represents a separate ETE link path.

   inner packet
      an unencapsulated network layer protocol packet (e.g., IPv4
      [RFC0791], OSI/CLNP [RFC0994], IPv6 [RFC2460], etc.) before any
      outer encapsulations are added.  Internet protocol numbers that
      identify inner packets are found in the IANA Internet Protocol
      registry [RFC3232].  SEAL protocol packets that incur an
      additional layer of SEAL encapsulation are also considered inner
      packets.

   outer IP packet
      a packet resulting from adding an outer IP header (and possibly
      other outer headers) to a SEAL-encapsulated inner packet.

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

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

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

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

      ETE - Egress Tunnel Endpoint

      HLEN - the length of the SEAL header plus outer headers

      ICV - Integrity Check Vector

      ITE - Ingress Tunnel Endpoint

      MTU - Maximum Transmission Unit

      SCMP - the SEAL Control Message Protocol

      SDU - SCMP Destination Unreachable message




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      SPP - SCMP Parameter Problem message

      SPTB - SCMP Packet Too Big message

      SEAL - Subnetwork Encapsulation and Adaptation Layer

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

      VET - Virtual Enterprise Traversal

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


3.  Applicability Statement

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

   SEAL provides a network sublayer for encapsulation of an inner
   network layer packet within outer encapsulating headers.  SEAL can
   also be used as a sublayer within a transport layer protocol data
   payload, where transport layer encapsulation is typically used for
   Network Address Translator (NAT) traversal as well as operation over
   subnetworks that give preferential treatment to certain "core"
   Internet protocols (e.g., TCP, UDP, etc.).  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.

   To accommodate MTU diversity, the Egress Tunnel Endpoint (ETE) acts
   as a passive observer that simply informs the Ingress Tunnel Endpoint
   (ITE) of any packet size limitations.  This allows the ITE to return
   appropriate path MTU discovery feedback even if the network path



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   between the ITE and ETE filters ICMP messages.

   SEAL further provides mechanisms to ensure data origin
   authentication, packet header integrity, and anti-replay.  The SEAL
   framework is therefore similar to the IP Security (IPsec)
   Authentication Header (AH) [RFC4301][RFC4302], however it provides
   only minimal hop-by-hop authenticating services along a path while
   leaving full data integrity, authentication and confidentiality
   services as an end-to-end consideration.  While SEAL performs data
   origin authentication, the origin site must also perform the
   necessary ingress filtering in order to provide full source address
   verification [I-D.ietf-savi-framework].

   In many aspects, SEAL also very closely resembles the Generic Routing
   Encapsulation (GRE) framework [RFC1701].  SEAL can therefore be
   applied in the same use cases that are traditionally addressed by
   GRE, and can also provide additional capabilities as described in
   this document.


4.  SEAL Specification

   The following sections specify the operation of SEAL:

4.1.  VET Interface Model

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

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

   Implications of the VET unidirectional and bidirectional models are
   discussed in [I-D.templin-intarea-vet].

4.2.  SEAL Model of Operation

   SEAL-enabled ITEs encapsulate each inner packet in a SEAL header, any
   outer header encapsulations, and (in certain cases) a SEAL trailer as
   shown in Figure 1:



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                                +--------------------+
                                ~   outer IP header  ~
                                +--------------------+
                                ~  other outer hdrs  ~
                                +--------------------+
                                ~    SEAL Header     ~
   +--------------------+       +--------------------+
   |                    |  -->  |                    |
   ~        Inner       ~  -->  ~        Inner       ~
   ~       Packet       ~  -->  ~       Packet       ~
   |                    |  -->  |                    |
   +--------------------+       +----------+---------+
                                .  Trailer .
                                +..........+

                       Figure 1: SEAL Encapsulation

   The ITE inserts the SEAL header according to the specific tunneling
   protocol.  For simple encapsulation of an inner network layer packet
   within an outer IP header, the ITE inserts the SEAL header between
   the inner packet and outer IP headers as: IP/SEAL/{inner packet}.

   For encapsulations over transports such as UDP, the ITE inserts the
   SEAL header between the outer transport layer header and the inner
   packet, e.g., as IP/UDP/SEAL/{inner packet}.  In that case, the UDP
   header is seen as an "other outer header" as depicted in Figure 1.

   In certain cases, the ITE also appends a 16-bit trailer at the end of
   the SEAL packet.  In that case, the trailer is added after the final
   byte of the encapsulated packet and need not be aligned on an even
   word boundary.

   SEAL supports both "nested" tunneling and "re-encapsulating"
   tunneling.  Nested tunneling occurs when a first tunnel is
   encapsulated within a second tunnel, which may then further be
   encapsulated within additional tunnels.  Nested tunneling can be
   useful, and stands in contrast to "recursive" tunneling which is an
   anomalous condition incurred due to misconfiguration or a routing
   loop.  Considerations for nested tunneling are discussed in Section 4
   of [RFC2473].

   Re-encapsulating tunneling occurs when a packet arrives at a first
   ETE, which then acts as an ITE to re-encapsulate and forward the
   packet to a second ETE connected to the same subnetwork.  In that
   case each ITE/ETE transition represents a segment of a bridged path
   between the ITE nearest the source and the ETE nearest the
   destination.  Combinations of nested and re-encapsulating tunneling
   are also naturally supported by SEAL.



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   The SEAL ITE considers each {underlying interface, IP address} pair
   as the ingress attachment point to a subnetwork link path to the ETE.
   The ITE therefore maintains path MTU state on a per ETE link path
   basis, although it may instead maintain only the lowest-common-
   denominator values for all of the ETE's link paths in order to reduce
   state.

   Finally, the SEAL ITE ensures that the inner network layer protocol
   will see a minimum MTU of 1280 bytes over each ETE link path
   regardless of the outer network layer protocol version, i.e., even if
   a small amount of fragmentation and reassembly are necessary.

4.3.  SEAL Header and Trailer Format

   The SEAL header is formatted as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |VER|C|A|R|I|V|T|     NEXTHDR   |    PREFLEN    | LINK_ID |LEVEL|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Identification (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Integrity Check Vector (ICV) (optional)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: SEAL Header Format

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

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

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

   R (1)
      the "Redirects Permitted" bit.  For data packets, set to 1 by the
      ITE to inform the ETE that the source is accepting Redirects (see:
      [I-D.templin-intarea-vet]).





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   I (1)
      the "Identification Included" bit.

   V (1)
      the "ICV included" bit.

   T (1)
      the "Trailer included" bit for IPv4 ETE link paths.  Reserved for
      future use for IPv6 ETE link paths.

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

   PREFLEN (8)  an 8-bit field that encodes the length of the prefix to
      be applied to the source address of the inner packet.

   LINK_ID (5)
      a 5-bit link identification value, set to a unique value by the
      ITE for each link path over which it will send encapsulated
      packets to the ETE (up to 32 link paths per ETE are therefore
      supported).  Note that, if the ITE's interface connection to the
      underlying link assigns multiple IP addresses, each address
      represents a separate ETE link path that must be assigned a
      separate LINK_ID.

   LEVEL (3)
      a 3-bit nesting level; use to limit the number of tunnel nesting
      levels.  Set to an integer value up to 7 in the innermost SEAL
      encapsulation, and decremented by 1 for each successive additional
      SEAL encapsulation nesting level.  Up to 8 levels of nesting are
      therefore supported.

   Identification (32)
      an optional 32-bit per-packet identification field; present when
      I==1.  Set to a monotonically-incrementing 32-bit value for each
      SEAL packet transmitted to this ETE, beginning with 0.

   Integrity Check Vector (ICV) (32)
      an optional 32-bit header integrity check value; present when
      V==1.  Covers the leading 128 bytes of the packet beginning with
      the SEAL header.  The value 128 is chosen so that at least the
      SEAL header as well as the inner packet network and transport
      layer headers are covered by the integrity check.

   When (T==1), the SEAL encapsulation also includes a 16-bit trailing
   integrity check vector ("ICV2") formatted as follows:




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       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    ICV2(A)    |    ICV2(B)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: SEAL Trailer Format

   ICV2 (16)
      a 16-bit ICV2 value; present only when T==1.  The value is
      calculated by the 8-bit Fletcher's algorithm given in [RFC1146],
      where the "A" result is placed in the most significant byte and
      the "B" result is placed in the least significant byte.

4.4.  ITE Specification

4.4.1.  Tunnel Interface MTU

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

   Due to the ubiquitous deployment of standard Ethernet and similar
   networking gear, the nominal Internet cell size has become 1500
   bytes; this is the de facto size that end systems have come to expect
   will either be delivered by the network without loss due to an MTU
   restriction on the path or a suitable ICMP Packet Too Big (PTB)
   message returned.  When large packets sent by end systems incur
   additional encapsulation at an ITE, however, they may be dropped
   silently within the tunnel since the network may not always deliver
   the necessary PTBs [RFC2923].

   The ITE should therefore set a tunnel interface MTU of at least 1500
   bytes plus extra room to accommodate any additional encapsulations
   that may occur on the path from the original source.  The ITE can
   also set smaller MTU values; however, care must be taken not to set
   so small a value that original sources would experience an MTU
   underflow.  In particular, IPv6 sources must see a minimum path MTU
   of 1280 bytes, and IPv4 sources should see a minimum path MTU of 576
   bytes.

   The inner network layer protocol consults the tunnel interface MTU
   when admitting a packet into the interface.  For non-SEAL inner IPv4
   packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet
   is larger than the tunnel interface MTU the inner IPv4 layer uses



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   IPv4 fragmentation to break the packet into fragments no larger than
   the tunnel interface MTU.  The ITE then admits each fragment into the
   interface as an independent packet.

   For all other inner packets, the inner network layer admits the
   packet if it is no larger than the tunnel interface MTU; otherwise,
   it drops the packet and sends a PTB error message to the source with
   the MTU value set to the tunnel interface MTU.  The message contains
   as much of the invoking packet as possible without the entire message
   exceeding the network layer minimum MTU (e.g., 1280 bytes for IPv6,
   576 bytes for IPv4, etc.).

   The ITE can alternatively set an indefinite MTU on the tunnel
   interface such that all inner packets are admitted into the interface
   regardless of their size.  For ITEs that host applications that use
   the tunnel interface directly, this option must be carefully
   coordinated with protocol stack upper layers since some upper layer
   protocols (e.g., TCP) derive their packet sizing parameters from the
   MTU of the outgoing interface and as such may select too large an
   initial size.  This is not a problem for upper layers that use
   conservative initial maximum segment size estimates and/or when the
   tunnel interface can reduce the upper layer's maximum segment size,
   e.g., by reducing the size advertised in the MSS option of outgoing
   TCP messages (sometimes known as "MSS clamping").

   In light of the above considerations, the ITE should configure an
   indefinite MTU on tunnel *router* interfaces so that subnetwork
   adaptation is handled from within the interface.  The ITE can instead
   set a finite MTU on tunnel *host* interfaces.

4.4.2.  Tunnel Neighbor Soft State

   The tunnel virtual interface maintains a number of soft state
   variables for each ETE and for each ETE link path.

   When per-packet identification is required, the ITE maintains a per
   ETE window of Identification values for the packets it has recently
   sent to this ETE.  The ITE then sets a variable "USE_ID" to TRUE, and
   includes an Identification in each packet it sends to this neighbor;
   otherwise, it sets USE_ID to FALSE.

   When data origin authentication and integrity checking is required,
   the ITE also maintains a per ETE integrity check vector (ICV)
   calculation algorithm and a symmetric secret key to calculate the ICV
   in each packet it will send to this ETE.  The ITE then sets a
   variable "USE_ICV" to TRUE, and includes an ICV in each packet it
   sends to this ETE; otherwise, it sets USE_ICV to FALSE.




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   For each ETE link path, the ITE must also account for encapsulation
   header lengths.  The ITE therefore maintains the per ETE link path
   constant values "SHLEN" set to length of the SEAL header, "THLEN" set
   to the length of the outer encapsulating transport layer headers (or
   0 if outer transport layer encapsulation is not used), "IHLEN" set to
   the length of the outer IP layer header, and "HLEN" set to (SHLEN+
   THLEN+IHLEN).  (The ITE must include the length of the uncompressed
   headers even if header compression is enabled when calculating these
   lengths.)  In addition, the ETE maintains a constant value "MIN_MTU"
   set to (1280+HLEN) as well as a variable "PATH_MTU" initialized to
   the MTU of the underlying link.

   For ETE link paths that use IPv4 as the outer encapsulation protocol,
   the ITE also maintains the variables "USE_DF" set to FALSE, and
   "USE_TRAIL" set to TRUE if PATH_MTU is less than MIN_MTU (otherwise
   set to FALSE).

   The ITE may instead maintain the packet sizing variables and
   constants as per ETE (rather than per ETE link path) values.  In that
   case, the values reflect the lowest-common-denominator MTU across all
   of the ETE's link paths.

4.4.3.  Pre-Encapsulation

   For each inner packet admitted into the tunnel interface, if the
   packet is itself a SEAL packet (i.e., one with the port number for
   SEAL in the transport layer header or one with the protocol number
   for SEAL in the IP layer header) and the LEVEL field of the SEAL
   header contains the value 0, the ITE silently discards the packet.

   Otherwise, for IPv4 inner packets with DF==0 in the IPv4 header, if
   the packet is larger than 512 bytes and is not the first fragment of
   a SEAL packet (i.e., not a packet that includes a SEAL header) the
   ITE should fragment the packet into inner fragments no larger than
   512 bytes unless it has operational assurance that the path can
   support a larger inner fragment size.  The ITE then submits each
   inner fragment for SEAL encapsulation as specified in Section 4.4.4.

   For all other packets, if the packet is no larger than (MAX(PATH_MTU,
   MIN_MTU)-HLEN) for the corresponding ETE link path, the ITE submits
   it for SEAL encapsulation as specified in Section 4.4.4.  Otherwise,
   the ITE sends a PTB error message toward the source address of the
   inner packet.

   To send the PTB message, the ITE first checks its forwarding tables
   to discover the previous hop toward the source address of the inner
   packet.  If the previous hop is reached via the same tunnel
   interface, the ITE sends an SCMP PTB (SPTB) message to the previous



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   hop (see: Section 4.6.1.1) with the MTU field set to (MAX(PATH_MTU,
   MIN_MTU)-HLEN).  Otherwise, the ITE sends an ordinary PTB message
   appropriate to the inner protocol version with the MTU field set to
   (MAX(PATH_MTU, MIN_MTU)-HLEN).  (NB: for IPv4 SEAL packets with
   DF==0, the ITE should set DF=1 and re-calculate the IPv4 header
   checksum before generating the PTB message in order to avoid bogon
   filters.)

   After sending the (S)PTB message, the ITE discards the inner packet.

4.4.4.  SEAL Encapsulation

   The ITE next encapsulates the inner packet in a SEAL header formatted
   as specified in Section 4.3.  The SEAL header includes an
   Identification field when USE_ID is TRUE, followed by an ICV field
   when USE_ICV is TRUE.  When USE_TRAIL is TRUE, the ITE also leaves
   room for a trailing ICV2 field at the end of the packet.

   The ITE next sets C=0 in the SEAL header.  The ITE also sets A=1 if
   necessary for ETE reachability determination (see: Section 4.4.6) or
   for stateful MTU determination (see Section 4.4.9).  Otherwise, the
   ITE sets A=0.

   The ITE then sets R=1 if redirects are permitted (see:
   [I-D.templin-intarea-vet]) and sets PREFLEN to the length of the
   prefix to be applied to the inner source address.  The ITE's claimed
   PREFLEN is subject to verification by the ETE; hence, the ITE must
   set PREFLEN to the exact prefix length that it is authorized to use.
   (Note that if this process is entered via re-encapsulation (see:
   Section 4.5.4), PREFLEN and R are instead copied from the SEAL header
   of the re-encapsulated packet.  This implies that the PREFLEN and R
   values are propagated across a re-encapsulating chain of ITE/ETEs
   that must all be authorized to represent the prefix.)

   The ITE then sets LINK_ID to the value assigned to the underlying ETE
   link path, and sets NEXTHDR to the protocol number corresponding to
   the address family of the encapsulated inner packet.  For example,
   the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets
   [RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213],
   '80' for encapsulated OSI/CLNP packets [RFC1070], etc.

   Next, if the inner packet is not itself a SEAL packet the ITE sets
   LEVEL to an integer value between 0 and 7 as a specification of the
   number of additional layers of nested SEAL encapsulations permitted.
   If the inner packet is a SEAL packet that is undergoing nested
   encapsulation, the ITE instead sets LEVEL to the value that appears
   in the inner packet's SEAL header minus 1.  If the inner packet is
   undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL



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   value from the SEAL header of the packet to be re-encapsulated.

   The ITE then sets the (I, V, T) flags and initializes any header
   extension fields as follows:

   o  When USE_ID is TRUE, the ITE sets I=1 and writes a monotonically-
      increasing integer value for this ETE in the Identification field
      (beginning with 0 in the first packet transmitted).  Otherwise,
      the ITE sets I=0.

   o  When USE_ICV is TRUE, the ITE sets V=1 and initializes the ICV
      field to 0; otherwise, it sets V=0.

   o  When USE_TRAIL is TRUE, the ITE sets T=1; otherwise, it sets T=0.

   When USE_TRAIL is TRUE, the next calculates the trailing ICV2 value
   using the 8-bit Fletcher checksum algorithm given in Appendix I of
   [RFC1146].  Beginning with the SEAL header, the ITE calculates the
   checksum over the entire packet then places the "A" result in the
   first byte of the trailing ICV2 field and places the "B" result in
   the second byte.

   When USE_ICV is TRUE, the ITE then calculates the packet header ICV
   value using an algorithm agreed on by the ITE and ETE.  When data
   origin authentication is required, the algorithm uses a symmetric
   secret key so that the ETE can verify that the ICV was generated by
   the ITE.  Beginning with the SEAL header, the ITE calculates the ICV
   over the leading 128 bytes of the packet (or up to the end of the
   packet if there are fewer than 128 bytes) and places result in the
   ICV field.  (If the packet contains fewer than 128 bytes, the ITE
   does not include the trailing ICV2 field (if present) in the ICV
   calculation.)

   The ITE then adds the outer encapsulating headers and performs any
   necessary outer fragmentation as specified in Section 4.4.5.

4.4.5.  Outer Encapsulation

   Following SEAL encapsulation, the ITE next encapsulates the packet in
   the requisite outer transport (when necessary) and IP layer headers.
   When a transport layer header is included, the ITE writes the port
   number for SEAL in the transport destination service port field and
   writes the protocol number of the transport protocol in the outer IP
   header protocol field.  Otherwise, the ITE writes the protocol number
   for SEAL in the outer IP header protocol field.

   The ITE then sets the other fields of the outer transport and IP
   layer headers as specified in Sections 5.5.4 and 5.5.5



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   of[I-D.templin-intarea-vet].  If this process is entered via re-
   encapsulation (see: Section 4.5.4), the ITE instead follows the re-
   encapsulation procedures specified in Section 5.5.6 of
   [I-D.templin-intarea-vet].

   For IPv4 ETE link paths, if USE_DF is FALSE the ITE sets DF=0 in the
   IPv4 header to allow the packet to be fragmented within the
   subnetwork if it encounters a restricting link; otherwise, the ITE
   sets DF=1.  (For IPv6 ETE link paths, the "DF" flag is absent but
   implicitly set to 1.  The packet therefore will not be fragmented
   within the subnetwork, since IPv6 deprecates in-the-network
   fragmentation.)

   Next, the ITE uses IP fragmentation if necessary to fragment the
   encapsulated packet into outer IP fragments that are no larger than
   PATH_MTU.  By virtue of the pre-encapsulation packet size
   calculations specified in Section 4.4.3, fragmentation will therefore
   only occur for outer packets that are larger than PATH_MTU but no
   larger than MIN_MTU.  (Note that, for IPv6, fragmentation must be
   performed by the ITE itself, while for IPv4 the fragmentation could
   instead be performed by a router in the ETE link path.)

   The ITE then sends each outer packet/fragment via the underlying link
   corresponding to LINK_ID.

4.4.6.  Path Probing and ETE Reachability Verification

   All SEAL data packets sent by the ITE are considered implicit probes.
   SEAL data packets will elicit an SCMP message from the ETE if it
   needs to acknowledge a probe and/or report an error condition.  SEAL
   data packets may also be dropped by either the ETE or a router on the
   path, which will return an ICMP message.

   The ITE can also send an SCMP Router/Neighbor Solicitation message to
   elicit an SCMP Router/Neighbor Advertisement response (see:
   [I-D.templin-intarea-vet]) as verification that the ETE is still
   reachable via a specific link path.

   The ITE processes ICMP messages as specified in Section 4.4.7.

   The ITE processes SCMP messages as specified in Section 4.6.2.

4.4.7.  Processing ICMP Messages

   When the ITE sends SEAL packets, it may receive ICMP error messages
   [RFC0792][RFC4443] from an ordinary router within the subnetwork or
   from another ITE on the path to the ETE (i.e., in case of nested
   encapsulations).  Each ICMP message includes an outer IP header,



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   followed by an ICMP header, followed by a portion of the SEAL data
   packet that generated the error (also known as the "packet-in-error")
   beginning with the outer IP header.

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

   For other ICMP messages, the ITE should use any outer header
   information available as a first-pass authentication filter (e.g., to
   determine if the source of the message is within the same
   administrative domain as the ITE) and discards the message if first
   pass filtering fails.

   Next, the ITE examines the packet-in-error beginning with the SEAL
   header.  If the value in the Identification field (if present) is not
   within the window of packets the ITE has recently sent to this ETE,
   or if the value in the SEAL header ICV field (if present) is
   incorrect, the ITE discards the message.

   Next, if the received ICMP message is a PTB the ITE sets the
   temporary variable "PMTU" for this ETE link path to the MTU value in
   the PTB message.  If PMTU==0, the ITE consults a plateau table (e.g.,
   as described in [RFC1191]) to determine PMTU based on the length
   field in the outer IP header of the packet-in-error.  (For example,
   if the ITE receives a PTB message with MTU==0 and length 1500, it can
   set PMTU=1450.  If the ITE subsequently receives a PTB message with
   MTU==0 and length 1450, it can set PMTU=1400, etc.)

   If the ITE is performing stateful MTU determination for this ETE link
   path (see Section 4.4.9), the ITE next sets PATH_MTU=PMTU.  If PMTU
   is less than MIN_MTU, the ITE sets PATH_MTU=PMTU whether or not
   stateful MTU determination is used (and for IPv4 also sets
   (USE_TRAIL=TRUE; USE_DF=FALSE)), then discards the message.

   If the ICMP message was not discarded, the ITE then transcribes it
   into a message to return to the previous hop.  If the previous hop
   toward the inner source address within the packet-in-error is reached
   via the same tunnel interface the SEAL data packet was sent on, the
   ITE transcribes the ICMP message into an SCMP message.  Otherwise,
   the ITE transcribes the ICMP message into a message appropriate for
   the inner protocol version.

   To transcribe the message, the ITE extracts the inner packet from



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   within the ICMP message packet-in-error field and uses it to generate
   a new message corresponding to the type of the received ICMP message.
   For SCMP messages, the ITE generates the message the same as
   described for ETE generation of SCMP messages in Section 4.6.1.  For
   (S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field.

   The ITE finally forwards the transcribed message to the previous hop
   toward the inner source address.

4.4.8.  IPv4 Middlebox Reassembly Testing

   For IPv4 ETE link paths, the ITE can perform a qualification exchange
   to ensure that the subnetwork correctly delivers fragments to the
   ETE.  This procedure can be used, e.g., to determine whether there
   are middleboxes on the path that violate the [RFC1812], Section 5.2.6
   requirement that: "A router MUST NOT reassemble any datagram before
   forwarding it".

   The ITE should use knowledge of its topological arrangement as an aid
   in determining when middlebox reassembly testing is necessary.  For
   example, if the ITE is aware that the ETE is located somewhere in the
   public Internet, middlebox reassembly testing should not be
   necessary.  If the ITE is aware that the ETE is located behind a NAT
   or a firewall, however, then middlebox reassembly testing is
   recommended.

   The ITE can perform a middlebox reassembly test by selecting a data
   packet to be used as a probe.  While performing the test with real
   data packets, the ITE should select only inner packets that are no
   larger than 1280 bytes for testing purposes so that the reassembled
   packet will not be discarded by the ETE.  The ITE can also construct
   a NULL probe packet instead of using ordinary SEAL data packets.

   To generate a NULL probe packet, the ITE creates a packet buffer
   beginning with the same outer headers, SEAL header and an inner
   network layer header that would appear in an ordinary data packet.
   The ITE writes source address taken from the ITE's claimed prefix and
   a NULL destination address in the inner network layer header, then
   pads the packet with random data to a length that is at least 128
   bytes but no more than 1280 bytes.

   The ITE then sets (C=0; R=0; T=0) in the SEAL header of the probe
   packet, writes the length of the ITE's claimed prefix in the PREFLEN
   field and sets the NEXTHDR field to the inner network layer protocol
   type.  (The ITE may also set A=1 if it requires a positive
   acknowledgement; otherwise, it sets A=0.)  Next, the ITE sets LINK_ID
   and LEVEL to the appropriate values for this ETE link path, sets
   Identification and I=1 (when USE_ID is TRUE), then finally calculates



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   the ICV and sets V=1(when USE_ICV is TRUE).

   The ITE then encapsulates the probe packet in the appropriate outer
   headers, splits it into two outer IPv4 fragments, then sends both
   fragments over the same ETE link path.

   The ITE should send a series of probe packets (e.g., 3-5 probes with
   1sec intervals between tests) instead of a single isolated probe in
   case of packet loss.  If the ETE returns an SCMP PTB message with MTU
   != 0, then the ETE link path correctly supports fragmentation;
   otherwise, the ITE sets PATH_MTU=MIN_MTU and sets (USE_TRAIL=TRUE;
   USE_DF=FALSE).  The ITE may instead enable stateful MTU determination
   for this ETE link path as specified in Section 4.4.9 to attempt to
   discover larger MTUs.

   NB: Examples of middleboxes that may perform reassembly include
   stateful NATs and firewalls.  Such devices could still allow for
   stateless MTU determination if they gather the fragments of a
   fragmented IPv4 SEAL data packet for packet analysis purposes but
   then forward the fragments on to the final destination rather than
   forwarding the reassembled packet.

4.4.9.  Stateful MTU Determination

   SEAL supports a stateless MTU determination capability, however the
   ITE may in some instances wish to impose a stateful MTU limit on a
   particular ETE link path.  For example, when the ETE is situated
   behind a middlebox that performs IPv4 reassembly (see: Section 4.4.8)
   it is imperative that fragmentation of large packets be avoided.  In
   other instances (e.g., when the ETE link path includes performance-
   constrained links), the ITE may deem it necessary to cache a
   conservative static MTU in order to avoid sending large packets that
   would only be dropped due to an MTU restriction somewhere on the
   path.

   To determine a static MTU value, the ITE can send a series of probe
   packets of various sizes to the ETE with A=1 in the SEAL header and
   DF=1 in the outer IP header.  The ITE can then cache the size of the
   largest packet for which it receives a probe reply from the ETE as
   the PATH_MTU value this ETE link path.

   For example, the ITE could send NULL probe packets of 1500 bytes,
   followed by 1450 bytes, followed by 1400 bytes, etc. then set
   PATH_MTU for this ETE link path to the size of the largest probe
   packet for which it receives an SPTB reply message.  While probing
   with NULL probe packets, the ITE processes any ICMP PTB message it
   receives as a potential indication of probe failure then discards the
   message.



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   For IPv4 ETE link paths, if the largest successful probe is no larger
   than MIN_MTU the ITE then sets (USE_TRAIL=TRUE; USE_DF=FALSE);
   otherwise, the ITE sets (USE_TRAIL=FALSE; USE_DF=TRUE).

4.4.10.  Detecting Path MTU Changes

   When stateful determination is used, the ITE can periodically reset
   PATH_MTU to the MTU of the underlying link and/or re-probe the path
   to determine whether PATH_MTU has increased.  If the path still has a
   too-small MTU, the ITE will receive a PTB message that reports a
   smaller size.

   For IPv4 ETE link paths, when the path correctly implements
   fragmentation and USE_TRAIL is TRUE, the ITE can periodically reset
   USE_TRAIL=FALSE to determine whether the path still requires a
   trailing ICV2 field.  If the ITE receives an SPTB message for an
   inner packet that is no larger than 1280 bytes (see: Section
   4.6.1.1), the ITE should again set USE_TRAIL=TRUE.

4.5.  ETE Specification

4.5.1.  Tunnel Neighbor Soft State

   When data origin authentication and integrity checking is required,
   the ETE maintains a per-ITE ICV calculation algorithm and a symmetric
   secret key to verify the ICV.  When per-packet identification is
   required, the ETE also maintains a window of Identification values
   for the packets it has recently received from this ITE.

   When the tunnel neighbor relationship is bidirectional, the ETE
   further maintains a per ETE link path mapping of outer IP and
   transport layer addresses to the LINK_ID that appears in packets
   received from the ITE.

4.5.2.  IP-Layer Reassembly

   The ETE must maintain a minimum IP-layer reassembly buffer size of
   1500 bytes for both IPv4 [RFC0791] and IPv6 [RFC2460].

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



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   The ETE processes non-SEAL IP packets as specified in the normative
   references, i.e., it performs any necessary IP reassembly then
   discards the packet if it is larger than the reassembly buffer size
   or delivers the (fully-reassembled) packet to the appropriate upper
   layer protocol module.

   For SEAL packets, the ITE performs any necessary IP reassembly until
   it has received at least the first 1280 bytes beyond the SEAL header
   or up to the end of the packet.  The ETE then submits the (fully- or
   partially-reassembled) packet for SEAL decapsulation as specified in
   Section 4.5.3.

4.5.3.  Decapsulation and Re-Encapsulation

   For each SEAL packet submitted for decapsulation, when I==1 the ETE
   first examines the Identification field.  If the Identification is
   not within the window of acceptable values for this ITE, the ETE
   silently discards the packet.

   Next, if V==1 the ETE verifies the ICV value (with the ICV field
   itself reset to 0) and silently discards the packet if the value is
   incorrect.  For IPv4, if T==1 and the packet is no larger than 1280
   bytes the ITE next verifies the ICV2 value and silently discards the
   packet if the value is incorrect.  (Note that the ITE must verify the
   ICV2 value even if the packet arrives unfragmented in case a
   middlebox is performing reassembly.)

   Next, if the packet arrived as multiple IPv4 fragments and T==0, the
   ETE sends an SPTB message back to the ITE with MTU set to the size of
   the largest fragment received minus HLEN (see: Section 4.6.1.1).

   Next, if the packet arrived as multiple IP fragments and the inner
   packet is larger than 1280 bytes, the ETE then silently discards the
   packet; otherwise, it continues to process the packet.

   Next, if there is an incorrect value in a SEAL header field (e.g., an
   incorrect "VER" field value), the ETE discards the packet.  If the
   SEAL header has C==0, the ETE also returns an SCMP "Parameter
   Problem" (SPP) message (see Section 4.6.1.2).

   Next, if the SEAL header has C==1, the ETE processes the packet as an
   SCMP packet as specified in Section 4.6.2.  Otherwise, the ETE
   continues to process the packet as a SEAL data packet.

   Next, if the SEAL header has A==1, the ETE sends an SPTB message back
   to the ITE with MTU=0 (see: Section 4.6.1.1).

   Finally, the ETE discards the outer headers and processes the inner



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   packet according to the header type indicated in the SEAL NEXTHDR
   field.  If the inner destination address of the packet is NULL the
   ETE silently discards the packet.  Otherwise, if the next hop toward
   the inner destination address is via a different interface than the
   SEAL packet arrived on, the ETE discards the SEAL header and delivers
   the inner packet either to the local host or to the next hop
   interface if the packet is not destined to the local host.

   If the next hop is on the same interface the SEAL packet arrived on,
   however, the ETE submits the packet for SEAL re-encapsulation
   beginning with the specification in Section 4.4.3 above.  In this
   process, the packet remains within the tunnel interface (i.e., it
   does not exit and then re-enter the interface); hence, the packet is
   not discarded if the LEVEL field in the SEAL header contains the
   value 0.

4.6.  The SEAL Control Message Protocol (SCMP)

   SEAL provides a companion SEAL Control Message Protocol (SCMP) that
   uses the same message types and formats as for the Internet Control
   Message Protocol for IPv6 (ICMPv6) [RFC4443].  As for ICMPv6, each
   SCMP message includes a 32-bit header and a variable-length body.
   The TE encapsulates the SCMP message in a SEAL header and outer
   headers as shown in Figure 4:

                                       +--------------------+
                                       ~   outer IP header  ~
                                       +--------------------+
                                       ~  other outer hdrs  ~
                                       +--------------------+
                                       ~    SEAL Header     ~
          +--------------------+       +--------------------+
          | SCMP message header|  -->  | SCMP message header|
          +--------------------+       +--------------------+
          |                    |  -->  |                    |
          ~  SCMP message body ~  -->  ~  SCMP message body ~
          |                    |  -->  |                    |
          +--------------------+       +--------------------+

               SCMP Message                  SCMP Packet
           before encapsulation          after encapsulation

                   Figure 4: SCMP Message Encapsulation

   The following sections specify the generation, processing and
   relaying of SCMP messages.





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4.6.1.  Generating SCMP Error Messages

   ETEs generate SCMP error messages in response to receiving certain
   SEAL data packets using the format shown in Figure 5:

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Type-Specific Data                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      As much of the invoking SEAL data packet as possible     |
      ~       (beginning with the SEAL header) without the SCMP       ~
      |              packet exceeding 576 bytes (*)                   |

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

                    Figure 5: SCMP Error Message Format

   The error message includes the 32-bit SCMP message header, followed
   by a 32-bit Type-Specific Data field, followed by the leading portion
   of the invoking SEAL data packet beginning with the SEAL header as
   the "packet-in-error".  The packet-in-error includes as much of the
   invoking packet as possible extending to a length that would not
   cause the entire SCMP packet following outer encapsulation to exceed
   576 bytes.

   When the ETE processes a SEAL data packet for which the
   Identification and ICV values are correct but an error must be
   returned, it prepares an SCMP error message as shown in Figure 5.
   The ETE sets the Type and Code fields to the same values that would
   appear in the corresponding ICMPv6 message [RFC4443], but calculates
   the Checksum beginning with the SCMP message header using the
   algorithm specified for ICMPv4 in [RFC0792].

   The ETE next encapsulates the SCMP message in the requisite SEAL and
   outer headers as shown in Figure 4.  During encapsulation, the ETE
   sets the outer destination address/port numbers of the SCMP packet to
   the values associated with the ITE and sets the outer source address/
   port numbers to its own outer address/port numbers.

   The ETE then sets (C=1; A=0; R=0; T=0) in the SEAL header, then sets
   I, V, NEXTHDR, PREFLEN, and LEVEL to the same values that appeared in
   the SEAL header of the data packet.  If the neighbor relationship
   between the ITE and ETE is unidirectional, the ETE next sets the
   LINK_ID field to the same value that appeared in the SEAL header of
   the data packet.  Otherwise, the ETE sets the LINK_ID field to the



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   value it would use in sending a SEAL packet to this ITE.

   When I==1, the ETE next sets the Identification field to an
   appropriate value for the ITE.  If the neighbor relationship between
   the ITE and ETE is unidirectional, the ETE sets the Identification
   field to the same value that appeared in the SEAL header of the data
   packet.  Otherwise, the ETE sets the Identification field to the
   value it would use in sending the next SEAL packet to this ITE.

   When V==1, the ETE then calculates and sets the ICV field the same as
   specified for SEAL data packet encapsulation in Section 4.4.4.

   Finally, the ETE sends the resulting SCMP packet to the ITE the same
   as specified for SEAL data packets in Section 4.4.5.

   The following sections describe additional considerations for various
   SCMP error messages:

4.6.1.1.  Generating SCMP Packet Too Big (SPTB) Messages

   An ETE generates an SCMP "Packet Too Big" (SPTB) message when it
   receives a SEAL data packet that arrived as multiple outer IPv4
   fragments and for which T==0.  The ETE prepares the SPTB message the
   same as for the corresponding ICMPv6 PTB message, and writes the
   length of the largest outer IP fragment received minus HLEN in the
   MTU field of the message.

   The ETE also generates an SPTB message when it accepts a SEAL
   protocol data packet with A==1 in the SEAL header.  The ETE prepares
   the SPTB message the same as above, except that it writes the value 0
   in the MTU field.

4.6.1.2.  Generating Other SCMP Error Messages

   An ETE generates an SCMP "Destination Unreachable" (SDU) message
   under the same circumstances that an IPv6 system would generate an
   ICMPv6 Destination Unreachable message.

   An ETE generates an SCMP "Parameter Problem" (SPP) message when it
   receives a SEAL packet with an incorrect value in the SEAL header.

   TEs generate other SCMP message types using methods and procedures
   specified in other documents.  For example, SCMP message types used
   for tunnel neighbor coordinations are specified in VET
   [I-D.templin-intarea-vet].






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4.6.2.  Processing SCMP Error Messages

   An ITE may receive SCMP messages with C==1 in the SEAL header after
   sending packets to an ETE.  The ITE first verifies that the outer
   addresses of the SCMP packet are correct, and (when I==1) that the
   Identification field contains an acceptable value.  The ITE next
   verifies that the SEAL header fields are set correctly as specified
   in Section 4.6.1.  When V==1, the ITE then verifies the ICV value.
   The ITE next verifies the Checksum value in the SCMP message header.
   If any of these values are incorrect, the ITE silently discards the
   message; otherwise, it processes the message as follows:

4.6.2.1.  Processing SCMP PTB Messages

   After an ITE sends a SEAL data packet to an ETE, it may receive an
   SPTB message with a packet-in-error containing the leading portion of
   the packet (see: Section 4.6.1.1).  For IP SPTB messages with MTU==0,
   the ITE processes the message as confirmation that the ETE received a
   SEAL data packet with A==1 in the SEAL header.  The ITE then discards
   the message.

   For IPv4 SPTB messages with MTU != 0, the ITE instead processes the
   message as an indication of a packet size limitation as follows.  If
   the inner packet is no larger than 1280 bytes, the ITE sets
   (USE_TRAIL=TRUE; USE_DF=FALSE).  If the inner packet is larger than
   1280 bytes, the ITE instead examines the SPTB message MTU field.  If
   the MTU value is not less than (MIN_MTU-HLEN), the value is likely to
   reflect the true MTU of the restricting link on the path to the ETE;
   otherwise, a router on the path may be generating runt fragments.

   In that case, the ITE can consult a plateau table (e.g., as described
   in [RFC1191]) to rewrite the MTU value to a reduced size.  For
   example, if the ITE receives an IPv4 SPTB message with MTU==256 and
   inner packet length 1500, it can rewrite the MTU to 1450.  If the ITE
   subsequently receives an IPv4 SPTB message with MTU==256 and inner
   packet length 1450, it can rewrite the MTU to 1400, etc.  If the ITE
   is performing stateful MTU determination for this ETE link path, it
   then writes the new MTU value in PATH_MTU.

   The ITE then checks its forwarding tables to discover the previous
   hop toward the source address of the inner packet.  If the previous
   hop is reached via the same tunnel interface the SPTB message arrived
   on, the ITE relays the message to the previous hop.  In order to
   relay the message, the first writes zero in the Identification and
   ICV fields of the SEAL header within the packet-in-error.  The ITE
   next rewrites the outer SEAL header fields with values corresponding
   to the previous hop and recalculates the ICV using the ICV
   calculation parameters associated with the previous hop.  Next, the



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   ITE replaces the SPTB's outer headers with headers of the appropriate
   protocol version and fills in the header fields as specified in
   Sections 5.5.4-5.5.6 of [I-D.templin-intarea-vet], where the
   destination address/port correspond to the previous hop and the
   source address/port correspond to the ITE.  The ITE then sends the
   message to the previous hop the same as if it were issuing a new SPTB
   message.  (Note that, in this process, the values within the SEAL
   header of the packet-in-error are meaningless to the previous hop and
   therefore cannot be used by the previous hop for authentication
   purposes.)

   If the previous hop is not reached via the same tunnel interface, the
   ITE instead transcribes the message into a format appropriate for the
   inner packet (i.e., the same as described for transcribing ICMP
   messages in Section 4.4.7) and sends the resulting transcribed
   message to the original source.  (NB: if the inner packet within the
   SPTB message is an IPv4 SEAL packet with DF==0, the ITE should set
   DF=1 and re-calculate the IPv4 header checksum while transcribing the
   message in order to avoid bogon filters.)  The ITE then discards the
   SPTB message.

4.6.2.2.  Processing Other SCMP Error Messages

   An ITE may receive an SDU message with an appropriate code under the
   same circumstances that an IPv6 node would receive an ICMPv6
   Destination Unreachable message.  The ITE either transcribes or
   relays the message toward the source address of the inner packet
   within the packet-in-error the same as specified for SPTB messages in
   Section 4.6.2.1.

   An ITE may receive an SPP message when the ETE receives a SEAL packet
   with an incorrect value in the SEAL header.  The ITE should examine
   the SEAL header within the packet-in-error to determine whether a
   different setting should be used in subsequent packets, but does not
   relay the message further.

   TEs process other SCMP message types using methods and procedures
   specified in other documents.  For example, SCMP message types used
   for tunnel neighbor coordinations are specified in VET
   [I-D.templin-intarea-vet].


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

   End systems are encouraged to implement end-to-end MTU assurance
   (e.g., using Packetization Layer Path MTU Discovery per [RFC4821])
   even if the subnetwork is using SEAL.


7.  Router Requirements

   Routers within the subnetwork are expected to observe the router
   requirements found in the normative references, including the
   implementation of IP fragmentation and reassembly [RFC1812][RFC2460]
   as well as the generation of ICMP messages [RFC0792][RFC4443].


8.  Nested Encapsulation Considerations

   SEAL supports nested tunneling for up to 8 layers of encapsulation.
   In this model, the SEAL ITE has a tunnel neighbor relationship only
   with ETEs at its own nesting level, i.e., it does not have a tunnel
   neighbor relationship with other ITEs, nor with ETEs at other nesting
   levels.

   Therefore, when an ITE 'A' within an inner nesting level needs to
   return an error message to an ITE 'B' within an outer nesting level,
   it generates an ordinary ICMP error message the same as if it were an
   ordinary router within the subnetwork.  'B' can then perform message
   validation as specified in Section 4.4.7, but full message origin
   authentication is not possible.

   Since ordinary ICMP messages are used for coordinations between ITEs
   at different nesting levels, nested SEAL encapsulations should only
   be used when the ITEs are within a common administrative domain
   and/or when there is no ICMP filtering middlebox such as a firewall
   or NAT between them.  An example would be a recursive nesting of
   mobile networks, where the first network receives service from an
   ISP, the second network receives service from the first network, the
   third network receives service from the second network, etc.

   NB: As an alternative, the SCMP protocol could be extended to allow
   ITE 'A' to return an SCMP message to ITE 'B' rather than return an
   ICMP message.  This would conceptually allow the control messages to
   pass through firewalls and NATs, however it would give no more
   message origin authentication assurance than for ordinary ICMP
   messages.  It was therefore determined that the complexity of
   extending the SCMP protocol was of little value within the context of
   the anticipated use cases for nested encapsulations.




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9.  IANA Considerations

   The IANA is instructed to allocate a System Port number for "SEAL" in
   the 'port-numbers' registry for the TCP, UDP, DCCP and SCTP
   protocols.

   The IANA is further instructed to allocate an IP protocol number for
   "SEAL" in the "protocol-numbers" registry.

   Considerations for port and protocol number assignments appear in
   [RFC2780][RFC5226][RFC6335].


10.  Security Considerations

   SEAL provides a segment-by-segment data origin authentication and
   anti-replay service across the (potentially) multiple segments of a
   re-encapsulating tunnel.  It further provides a segment-by-segment
   integrity check of the headers of encapsulated packets, but does not
   verify the integrity of the rest of the packet beyond the headers
   unless fragmentation is unavoidable.  SEAL therefore considers full
   message integrity checking, authentication and confidentiality as
   end-to-end considerations in a manner that is compatible with
   securing mechanisms such as TLS/SSL [RFC5246].

   An amplification/reflection/buffer overflow attack is possible when
   an attacker sends IP fragments with spoofed source addresses to an
   ETE in an attempt to clog the ETE's reassembly buffer and/or cause
   the ETE to generate a stream of SCMP messages returned to a victim
   ITE.  The SCMP message ICV, Identification, as well as the inner
   headers of the packet-in-error, provide mitigation for the ETE to
   detect and discard SEAL segments with spoofed source addresses.

   The SEAL header is sent in-the-clear 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.  Unlike IPv6 extension
   headers, however, the SEAL header can be protected by an integrity
   check that also covers the inner packet headers.

   Security issues that apply to tunneling in general are discussed in
   [RFC6169].


11.  Related Work

   Section 3.1.7 of [RFC2764] provides a high-level sketch for
   supporting large tunnel MTUs via a tunnel-level segmentation and
   reassembly capability to avoid IP level fragmentation.  This



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   capability was implemented in the first edition of SEAL, but is now
   deprecated.

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

   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.

   IPsec/AH is [RFC4301][RFC4301] is used for full message integrity
   verification between tunnel endpoints, whereas SEAL only ensures
   integrity for the inner packet headers.  The AYIYA proposal
   [I-D.massar-v6ops-ayiya] uses similar means for providing message
   authentication and integrity.

   The concepts of path MTU determination through the report of
   fragmentation and extending the IPv4 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.  An historical analysis of the evolution of these concepts,
   as well as the development of the eventual path MTU discovery
   mechanism, appears in Appendix D of this document.


12.  Acknowledgments

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

   Discussions with colleagues following the publication of RFC5320 have
   provided useful insights that have resulted in significant
   improvements to this, the Second Edition of SEAL.

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



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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-intarea-ipv4-id-update]
              Touch, J., "Updated Specification of the IPv4 ID Field",
              draft-ietf-intarea-ipv4-id-update-04 (work in progress),
              September 2011.

   [I-D.ietf-savi-framework]
              Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
              "Source Address Validation Improvement Framework",
              draft-ietf-savi-framework-05 (work in progress),
              July 2011.

   [I-D.massar-v6ops-ayiya]
              Massar, J., "AYIYA: Anything In Anything",



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              draft-massar-v6ops-ayiya-02 (work in progress), July 2004.

   [I-D.templin-aero]
              Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", draft-templin-aero-05 (work in progress),
              December 2011.

   [I-D.templin-intarea-vet]
              Templin, F., "Virtual Enterprise Traversal (VET)",
              draft-templin-intarea-vet-32 (work in progress),
              November 2011.

   [I-D.templin-ironbis]
              Templin, F., "The Internet Routing Overlay Network
              (IRON)", draft-templin-ironbis-09 (work in progress),
              November 2011.

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

   [RFC0994]  International Organization for Standardization (ISO) and
              American National Standards Institute (ANSI), "Final text
              of DIS 8473, Protocol for Providing the Connectionless-
              mode Network Service", RFC 994, March 1986.

   [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.

   [RFC1146]  Zweig, J. and C. Partridge, "TCP alternate checksum
              options", RFC 1146, March 1990.

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

   [RFC1701]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701, October 1994.

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

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




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

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

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

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

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, March 2000.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 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.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.




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   [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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

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

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

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

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

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

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




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   [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.  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
   be inefficient to require the tunnel endpoints to also perform ARQ.


Appendix B.  Integrity

   The SEAL header includes an integrity check field that covers the
   SEAL header and at least the inner packet headers (or up to the end
   of the packet if IPv4 fragmentation is needed).  This provides for
   header integrity verification on a segment-by-segment basis for a
   segmented re-encapsulating tunnel path.

   Fragmentation and reassembly schemes must also consider packet-
   splicing errors, e.g., when two fragments from the same packet are
   concatenated incorrectly, when a fragment from packet X is
   reassembled with fragments from packet Y, etc.  The primary sources
   of such errors include implementation bugs and wrapping IPv4 ID
   fields.

   In particular, the IPv4 16-bit ID field can wrap with only 64K
   packets with the same (src, dst, protocol)-tuple alive in the system
   at a given time [RFC4963].  When the IPv4 ID field is re-written by a
   middlebox such as a NAT or Firewall, ID field wrapping can occur with
   even fewer packets alive in the system.

   When outer IPv4 fragmentation is unavoidable, SEAL therefore provides
   a trailing checksum as a first-pass filter to detect reassembly mis-
   associations.  Any reassembly mis-associations not detected by the
   checksum will very likely be detected later by upper layer checksums.





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Appendix C.  Transport Mode

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

   If both TCPs agree on the use of SEAL, their protocol messages will
   be carried as IP/SEAL/TCP and the connection will be serviced by the
   SEAL protocol using TCP (instead of an encapsulating tunnel endpoint)
   as the transport layer protocol.  The SEAL protocol for transport
   mode otherwise observes the same specifications as for Section 4.


Appendix D.  Historic Evolution of PMTUD

   The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
   and numerous proposals in the late 1980's through early 1990.  The
   initial problem was posed by Art Berggreen on May 22, 1987 in a
   message to the TCP-IP discussion group [TCP-IP].  The discussion that
   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



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

   In retrospect, the "RF" bit postulated in 2) is not needed if a
   "contract" is first established between the peers, as in proposal 4)
   and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
   Feb 19. 1990.  These proposals saw little discussion or rebuttal, and
   were dismissed based on the following the assertions:

   o  routers upgrade their software faster than hosts

   o  PCs could not reassemble fragmented packets

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

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

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

   The first four assertions, although perhaps valid at the time, have
   been overcome by historical events.  The final assertion is addressed
   by the mechanisms specified in SEAL.


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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